Tailings Management Consortium
Filtered
Stacked
Tailings
A Guide for Study Managers
© BHP RIO TINTO
Tailings Management Consortium.
First Edition, March 2024
Comments and feedback are welcome KnowledgeBaseFeedback@visionippm.com
3
Acknowledgments
Sponsor
Tailings Management Consortium
Editor
Rachel Jansen
Paterson & Cooke
Lead Authors
Mark Coghill
Rio Tinto
Waldo Dressel
Red Earth Engineering
Geoff Liggins
Consultant
Josh Rogers
Rio Tinto
Russell Staines
BHP
Todd Wisdom
Paterson & Cooke
Christopher Bareither
Colorado State University
Robert Cooke
Paterson & Cooke
Silvana Dal Pozzo
BHP
Amanda de Ruyter
Rio Tinto
Luke Dimech
BHP
Colleen English
Rio Tinto
Theo Gerritsen
Rio Tinto
Kaci Jenkins
Rio Tinto
Ognjen Kotur
Rio Tinto
Steve Liddell
BHP
Antonio Pucci
Rio Tinto
Ashley Rasmussen
Paterson & Cooke
Joseph Scalia
Colorado State University
Tony Tran
BHP
Kiron Unda
BHP
Nikk Vagias
Rio Tinto
Lourdes Valle
BHP
Contributors
Photography
Contributors
Metso, FLSmidth, Diemme Filtration, BHP, Rio Tinto,
Paterson & Cooke
© BHP RIO TINTO Tailings Management Consortium
Contents
1. Overview
6
1.1
7
Introduction
1.2 Filtered Tailings Study Approach
8
1.3 Filtered Tailings Study - Key Learnings
12
2. Tailings Characterization
14
2.1 Key Points
15
2.2 Introduction
16
2.3 Key Tailings Characterization Parameters
17
2.3.1
18
Preliminary Filtration Assessment
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests 20
2.4 Typical Characterization Issues
38
3. Site Closure
42
3.1 Key Points
43
3.2 Introduction
44
3.3 Filtered Tailings Stack Closure Design
45
3.3.1
Temporary Engineered Covers
45
3.3.2
Permanent Engineered Covers
46
3.3.3
Soil Covers
48
3.4 Potential Stack Closure Risks
50
© BHP RIO TINTO Tailings Management Consortium
5
4. Filtered Tailings Stack
52
6. Tailings Dewatering
92
4.1 Key Points
53
6.1 Key Points
93
4.2 Introduction
54
6.2 Introduction
94
4.3 Physical Stability
56
6.3 Typical Filtered Tailings Dewatering Flowsheet 96
4.4 Chemical Stability
60
6.4 Thickening
99
4.4.1
61
6.4.1
99
4.5 Water Management
64
6.4.2 Thickener Technology
100
4.5.1
66
6.5 Filtration
104
66
6.5.1
Basics
104
4.6 Design Considerations
68
6.5.2 Filtration Technology
106
4.6.1
68
6.5.3 Vertical Plate Pressure Filter
108
6.6 Considerations for Design
112
6.6.1
112
Geochemical Classification
High Rainfall Areas
4.5.2 Arid and Semi-Arid Regions
Tailings Compaction and Zonation
4.6.2 Filtered Tailings Stacking Over
Conventional Facilities
69
4.6.3
Water Management Considerations
70
4.6.4
Geochemical Considerations
70
Basics
Equipment Sizing
6.6.2 Planning for Maintenance
114
6.6.3 Filter and Building Design
116
7. Opportunities
118
7.1 Key Points
119
7.2 Introduction
120
4.7 Operational Considerations
71
4.8 Recommended Tasks by Study Level
72
5. Material Transport & Stacking
74
5.1 Key Points
75
7.3 Potential Project Justifications
121
5.2 Introduction
77
7.4 Opportunities to Reduce Project Costs
122
5.3 Design Parameters
78
5.3.1
78
7.4.1
Classification to Improve Filtration Characteristics
122
Tailings Filter Cake Properties
5.3.2 Tailings Production Volume
78
7.4.2
Improved Evaporative Drying using
Plough and Discs
5.3.3
Distance and Topography
79
7.4.3
Improved Thin Lift Filter Cake Deposition 125
5.3.4
Climate
79
7.4.4
Co-Disposal and Co-Mingling
5.3.5 Construction Requirements
79
5.3.6 Access Roads and Ramps
79
5.4 Tailings Loading, Transport, Deposition
and Conditioning Equipment
80
5.5 Tailings Transport, Deposition and
Conditioning
86
5.5.1
86
Transport Equipment Considerations
5.5.2 Deposition Equipment Considerations
88
5.5.3 Conditioning Equipment Considerations
90
5.5.4 Handling Off-Specification Tailings
91
8. References &
Recommended Reading
124
126
130
8.1 References
131
8.2 Recommended Reading
132
© BHP RIO TINTO Tailings Management Consortium
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1.
Overview
© BHP RIO TINTO Tailings Management Consortium
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1.1
Introduction
In sharing a commitment to adopt global best practices for
tailings management facilities, the International Council
on Mining and Metals, the United Nations Environment
Programme, and the Principles for Responsible Investment
co-convened the Global Tailings Review to establish an
international tailings standard.
The Global Tailings Standard published in August 2020 requires consideration
of alternative options to conventional tailings management facilities which
have the potential to deliver improved technical and environmental, social and
governance outcomes throughout the facility’s lifecycle. Such options include
in-pit disposal, underground tailings placement, and the application of
dewatering technologies to minimize the amount of water placed in surface
tailings facilities. As the mining industry, and society at large, places greater
scrutiny on tailings management, filtered tailings have come to the forefront of
options for consideration by mine operators to significantly reduce the amount
of water sent to, and stored on, a surface tailings facility.
Filtered stacked tailings (Figure 1) is an alternative tailings management system
that builds a geotechnically and geochemically stable structure from tailings that
have been filtered to produce an unsaturated, soil-like building material. Filtered
tailings management facilities have been successfully implemented across a
range of industries including gold, copper, iron ore, and aluminium processing
(red mud).
Historically, the approach to tailings management has been for the tailings
facility operators to accept the tailings delivered from the metallurgical plant,
and a typical wet tailings storage facility (Figure 2) designed to handle a wide
range of slurry solids concentrations if the water can be managed effectively.
The transition to lower risk, more socially acceptable dry tailings facilities require
a fundamental change to this practice.
It is critical to recognize that approaches, methodologies, and design parameters
that are applicable to conventional tailings system design may not be appropriate
for filtered tailings systems.
This document provides guidance to project study managers who are evaluating
filtered tailings systems, specifically using pressure filtration technology, to
produce a self-supported filtered tailings stack, also known as “dry” stacked
tailings. The term “dry”, widely adopted by the mining industry, is used to
describe unsaturated tailings (i.e., the voids between particles contain water
and air, usually at moistures less than 25% by mass), rather than truly dry
(i.e., the voids between particles contain only air). When surcharged, the dry
tailings stack base can become saturated. Thus, the term “dry” does not truly
reflect the stack’s moisture content and profile.
Figure 1 shows a filtered tailings
stack, while Figure 2 shows a typical
conventional wet tailings facility.
Figure 1
Figure 2
© BHP RIO TINTO Tailings Management Consortium
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1.2
Filtered Tailings
Study Approach
Figure 3 shows a recommended approach to filtered
tailings system design studies. The goal is to create a
geotechnically and geochemically stable landform that
meets closure requirements (i.e., regulatory, environmental,
etc.) and minimizes the associated capital and operating
costs by optimizing the upstream processes and mine plan.
To implement safe and reliable filtered stacked tailings facilities, the tailings
delivered to the facility must consistently comply with the stack’s specified
geotechnical moisture requirements (i.e., the generated tailings need to be
considered as a product). This iterative approach requires that the design
process starts with establishing the geotechnical requirements for the facility,
which in turn identifies the moisture target range for the tailings dewatering
plant. A holistic approach ensures that other considerations such as facility
siting, community and cultural heritage, water management, dust control and
closure requirements are included in the system design.
The goal is to use tailings to create a stable landform in
perpetuity and to develop the project starting with closure
and stepping back to the mine (ore body).
© BHP RIO TINTO Tailings Management Consortium
Site Closure
Define the stack (and site) closure
objectives, constraints and threats.
Assessment should include
community impact (i.e. visual),
usage, (recreation, farmland etc.)
and leachate management.
APPROACH
9
Site Closure
Review by-product opportunities
to reduce the stack size.
Filtered Tailings Stack
Define the compacted stacked
threshold moisture based on the
standard Proctor test, seismic and
slope stability tests.
Material Transport & Stacking
Filtered
Tailings
Stack
Material
Transport
& Stacking
Assess filter cake transport
requirements, analyses and project
costs, to optimize the filter plant
location (i.e., proximity to the
concentrator or stack site).
Conduct tests to define the filter
cake material handling properties,
and the Flow Moisture Point.
Tailings Dewatering
Conduct filter performance tests
to ensure the Flow Moisture Point
and Proctor moistures are achievable.
If not, then review the filter feed
conditioning (i.e. thickening, pH,
coagulation etc.), upstream process
changes, and post-filter solar or
thermally assisted drying.
Tailings
Dewatering
TA I L I N G S C H A R A C T E R I Z AT I O N
Develop a stack design that meets
closure plan based upon the facility
siting options study, selected site
surveys (topography, hydrology,
seismicity, geology etc.), community/
stakeholder engagement, and
filtered tailings characterization.
Concentrator
Size and cost the filtration plant.
Consider cost reduction opportunities
associated with feed pre-conditioning
and upstream process changes.
Concentrator and Mine
Alter upstream processes and
mine plan to optimize filtration
performance and costs. Examples
include ore discrimination, slimes
reduction and pyrite removal.
Mine
Figure 3: Recommended approach to
filtered tailings system design study.
© BHP RIO TINTO Tailings Management Consortium
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1.2
Filtered Tailings
Study Approach
(cont.)
A filtered tailings operation is essentially an earthworks
project, where the terrain is reshaped by placement of a
large volume of tailings. The final landform can either be
permanently closed and re-used for another purpose
(i.e., farming, recreational area), or reclaimed to recover
critical materials. To plan for closure, the total volume of
tailings that will be produced over the life of the mine
needs to be well understood, and opportunities to reduce
this volume through mine planning, processing (e.g., ore
sorting), or producing by-products should be considered.
Initial tailings facility siting
studies should be completed at
a high level to develop a short list
of preferred sites.
These selected sites are then evaluated in more detail considering a wide range
of factors from multiple disciplines, including but not limited to land ownership,
permitting, ground conditions, hydrology, dust generation and seismicity.
Filtered tailings transport costs are usually high compared to pumping slurry,
so efforts should be made to reduce the distances between the proposed filter
cake production area and the tailings facility, and review utility (e.g., electricity
and water) availability and costs.
© BHP RIO TINTO Tailings Management Consortium
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Meeting the required stability
of the stack.
The stack can be designed as a self-supporting structure of fully compacted
filtered tailings or some compacted filtered tailings and/or borrowed material
may be considered for embankment construction to contain non-compacted filter
cake. The costs associated with filtration, compaction and sourcing borrowed
material need to be assessed to identify opportunities for cost reduction.
Seepage and surface water management at the stack must be incorporated into
the design.
To create the desired landform,
construction material, in this
case tailings, needs to meet
certain specifications.
Geotechnical characterization of the tailings, including optimum standard
(or modified) Proctor dry density and moisture content inform the design of
the filtered tailings stack and cake specification. This cake specification also
needs to pass the requirements for effective materials handling and transport
by trucking or conveying. Flow moisture point, conveyability and material flow
properties testing will indicate if the filter cake will liquify during transport or is
sticky and a potential risk for plugging transfer points.
A range of options can be
considered for filter cake transport,
placement, and compaction.
For large tonnages, conveyors and mobile bridge stackers are usually the
more economical solution, however these technologies as they currently exist
do not allow flexibility for thinner deposition layers or maneuvering in tight areas
within the stack footprint. Trucks can place tailings in thinner lifts and more
complex geometries but can become uneconomical at large tailings operations.
Factors including topography, site weather conditions and travel distance all
need to be considered when optimizing the filtered tailings transport system.
Dewatering technologies to produce
filter cake should be assessed in
conjunction with any other upstream
processes after tailings production.
For example, it is common to install a relatively low-cost thickener ahead
of pressure filtration to reduce the filter cycle time and thus the plant size.
Optimizing thickener size and performance ensures that the filter feed yield
stress target can be maintained despite upstream variances.
As ore bodies around the world
become more finely disseminated
with lower grades and higher clay
contents, the trend is to grind finer
to achieve liberation and recovery
of value metals. Thus, pressure
filtration technology is typically
required to achieve target cake
moisture contents for stable stacking
and optimal compaction of fine
and/or high clay content tailings.
Pressure filtration, combined with tailings stacking and compacting, typically has
a higher capital and operating cost compared to conventional tailings thickening
operations. Also filtered tailings systems are very different to conventional
tailings systems, for example:
• A filtered tailings stack operation requires greater process control than a
thickener operation pumping tailings to a surface impoundment.
• Filtration performance and filter cake geotechnical behavior can be more
sensitive to upstream variations.
Opportunities can be pursued upstream to improve filtration rates and reduce
dewatering costs, such as coarsening the grind size, reducing fines generation,
or ore sorting. Filtration index testing of drill core samples could be incorporated
into the geo-metallurgical testing campaign for mine plan modelling.
An overarching requirement for implementing a successful filtered tailings system is the
characterization of the tailings expected over the life of the mine. Understanding the ore
variability and planned run-of-mine blending is crucial for selecting representative samples
for test work and designing a robust, economic solution that covers all aspects of the system.
© BHP RIO TINTO Tailings Management Consortium
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1.3 Filtered
Tailings Study Key Learnings
The following chapters will discuss in more detail tailings
characterization, designing for closure, stack geotechnical
parameters, geochemical requirements, and considerations
for dewatering and cake handling. To assist the study
manager the top key learnings that must be applied include:
CHAPTER
KEY LEARNING
PROJECT IMPACT
Tailings
Characterization
Only use representative tailings
and water samples for dewatering,
materials handling, geochemical
and geotechnical test work.
Tailings characterization is fundamental to developing
and costing the process flow sheet. Non-representative
tailings and water assessments can lead to false
positive or false negative study outcomes.
Complete standard (and/or modified)
Proctor Optimum Moisture Content
and Flow Moisture Point tests to
define the transport and stacking
moisture thresholds before embarking
on comprehensive filtration test work
and flow sheet development.
Transport and stacking moisture thresholds inform the
process flow sheet development and target filter cake
moisture range. The target moisture range is used to
define the comprehensive filtration test work program.
Site Closure
Company, stakeholder and regulatory
closure expectations and the capping
(cover) design drives the stack
location, design, surface runoff water
and leachate management, and
closure duration.
Allocate sufficient time and resources to fully engage
with community and regulatory representatives to
discuss the potential visual, noise, dust, environmental
and cultural impacts, and the final landform vegetation,
topography, and usage.
Filtered Tailings
Stack
Filtered stacked tailings operations
are more complex than conventional
slurry tailings management strategies
and have the potential to significantly
increase the project costs and
become the operational bottleneck.
Engage experienced processing and geotechnical
engineers to conduct failure and consequence
assessments (accounting for climate change), and
develop process designs and operating strategies
to manage:
If the transport and stacking moisture thresholds are
below what can be economically achieved by pressure
filtration, significant investment may be required for
post-filtration drying (such as mechanical mud farming
or a larger facility footprint for evaporative drying) to
reduce in-situ moistures before compaction.
• Ore body and upstream process variances.
• Inclement weather events that prevent tailings
placement.
• Handling and storage of non-specification filter
feed or filter cake.
• Relatively low filter utilization and unplanned
shutdowns requiring some redundancy in the
filter plant.
© BHP RIO TINTO Tailings Management Consortium
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1.3 Filtered
Tailings Study Key Learnings
(cont.)
Filtered Tailings
Stack (cont.)
Filtered tailings stacking can promote
oxidation of sulfides present in
tailings, lowering the pH of surface
runoff water and leachate.
Tailings with high sulfide content may preclude filtered
stacked tailings from the options being assessed
unless upstream process changes are implemented
to remove and separately manage the potentially acid
forming sulfides.
Filtered tailings stacks still require
surface runoff water and leachate
management, which may continue
long after the closure cap (cover)
is installed.
Allocate time and resources to:
• Conduct the appropriate laboratory surface runoff,
stack leaching and Acid Metalliferous Drainage
assessments, and develop surface and ground
water models.
• Model the process flow volume, suspended solids,
and solute balance to define the potential impact
on the upstream process water quality and the
project’s water management strategy.
Material Transport
and Stacking
Operating costs for filter cake
transport and placement can
be comparable to dewatering
operating costs.
Transport costs and the associated environmental,
cultural and community impacts may constrain the
stack site selection, and potentially require the
filtration plant to be located adjacent to the stack.
Tailings
Dewatering
Filtration rate, plant size and costs
are driven by tailings and process
properties, feed slurry solid
concentration and shear yield stress
(i.e., the lower and upper operating
boundaries, respectively).
Allocate time and resources to understand how the
tailings properties over the life of operation will impact
the filtration performance.
Filter utilization is typically less than
80% (driven by planned maintenance
requirements) and some redundancy
is required at the filter plant to match
the availability of the concentrator.
Major pressure filter maintenance requirements are
associated with filter cloth and filter plates. To facilitate
maintenance the filter plant design should:
• Allow plate exchange to occur without impacting
production.
• Allow sufficient floor space to safely perform plate
maintenance and cloth change outs.
• Include a roof to prevent ultra-violet radiation
degrading the plates, and walls to allow crane
usage at winds speeds above 10 km/h.
© BHP RIO TINTO Tailings Management Consortium
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2.
Tailings
Characterization
© BHP RIO TINTO Tailings Management Consortium
15
2.1
Key Points
Tailings characteristics can vary with ore source, upstream
processing steps and water quality. Thus, it is important to
validate the key filtration study assumptions by completing
a preliminary filtration assessment.
Delay comprehensive characterization programs until the
process flowsheet and tailings facility options are defined,
and regulatory data requirements are known.
Only characterize representative tailings, site raw water
and process water samples.
© BHP RIO TINTO Tailings Management Consortium
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2.2
Introduction
Tailings characterization is the ‘corner stone’ of a
tailings engineering study. It provides data that assists
with flowsheet development, reagent and equipment
selection, filtered tailings stack design, and the
identification of potential occupational health, safety
and environmental issues. Characterization data can
also be used to identify potential onsite and offsite
reuse (repurposing) opportunities.
This chapter outlines the key tailings laboratory
characterization parameters and assessment issues,
particularly during the early engineering study phases.
© BHP RIO TINTO Tailings Management Consortium
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2.3
Key Tailings
Characterization
Parameters
As illustrated in Table 1, the characterization of
tailings properties is a continuous process as the
project advances through the various engineering
study phases.
Table 1: Typical tailings
characterization performed at
each engineering study phase.
Laboratory characterization tests typically commence with the Order of
Magnitude study and are expanded during Prefeasibility and Feasibility
to address the study data requirements and the identified project risks.
CONCEPTUAL
STUDY
ORDER OF
MAGNITUDE STUDY
Conceptual study tailings data is usually inferred from available sources
rather than measured, particularly when expanding an existing mine,
or assessing comparable operations. However, a preliminary filtration
assessment is recommended to identify any potential fatal flaws
with advancing a filtered tailings stack flowsheet and to validate the
Conceptual study assumptions.
Tailings properties are usually inferred from available sources (e.g., publications,
experience with similar tailings, and comparable operations).
Recommend completing preliminary filtration assessment to valid Conceptual
Study assumptions.
Tailings processing and storage design criteria relevant to each option being
assessed should be based on laboratory characterization testing.
Major data acquisition occurs during this stage. The level of assessment must
be sufficient to confirm the scope of the preferred option(s).
PREFEASIBILITY
STUDY
Critical parameters required to characterize tailings (e.g., geochemistry,
consolidation and permeability, strength, rheology, dewatering properties etc.)
are determined with reasonable assurance, using at least bench scale tests.
If required, detailed or large-scale pilot plant tests on bulk samples are typically
undertaken during this study phase.
FEASIBILITY
STUDY
Supplementary process and geotechnical data acquisition to address specific
tailings management risks (e.g., design or operational) can occur during this
study phase.
This study phase can also include the completion of pilot plant test work to
confirm tailings and process design parameters.
The extensive list of tailings parameters that can be
characterized is beyond the scope of this chapter.
The key physical, chemical, geochemical, geotechnical,
materials handling, solid-liquid separation and rheology
characterization parameters that should be considered
are presented in Table 2 to Table 7.
© BHP RIO TINTO Tailings Management Consortium
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2.3.1
Preliminary Filtration
Assessment
One of the challenges with filtered tailings engineering
studies is the lack of first principles theory to define the
expected material-specific behavior, as the randomness
and interactions of mineralogy, particle size, particle shape,
slurry motion, and water chemistry are difficult to model.
Thus, test work and industry experience play a significant
role in the design of filtered stacked tailings systems.
While inferred data is typically used during the Conceptual study, tailings
geotechnical, dewatering, and geochemical properties can vary significantly for
a given commodity. Hence, the inferred data may not be appropriate. Also, once
characterization commences, the study team can be overwhelmed by the list
of possible characterization parameters, test standards and data requests from
the process design team, consultancies, and equipment providers. Too often a
comprehensive, costly characterization program is initiated during the Order of
Magnitude study without first defining what is important from a tailings strategy
and flowsheet development viewpoint.
To address this, it is recommended that a limited set of characterization tests
are performed after acquiring representative samples. This limited assessment
program, labelled “preliminary filtration assessment” in this chapter, can occur
during the Conceptual study or commencement of the Order of Magnitude study.
The assessment’s purpose is to validate the Conceptual study assumptions and
demonstrate whether filtered tailings stacking is the appropriate flowsheet option.
The preliminary filtration assessment should include:
• Standard (and/or modified1) Proctor optimum moisture test
• Pressure2 filtration tests for minimum moisture content
• Shear yield stress versus solids concentration measurements
• Buchner funnel zero free water test.
A Proctor optimum moisture test is crucial to assessing whether a filtered
stacked tailings flow sheet is feasible. The test result provides an initial estimate
of the filter cake moisture threshold that must be met by the filtration process
facility to meet the specified stack compaction density for geotechnical stability.
Pressure filtration tests are conducted at commercially relevant feed flowrates
and operating conditions, and at the thinnest available chamber depths to
determine the minimum filter cake moisture.
The resultant filter cake moisture needs to be lower than optimum Proctor
moisture. Otherwise, stacking will be challenging, or impractical, and the study
team should consider:
• A more detailed laboratory characterization program to assess:
• Filter feed preconditioning and filtration equipment options to further
lower the cake moisture; or,
1
The standard Proctor measurements are typically
conducted to determine the optimum filtered
tailings stack compaction moisture. For tall stacks
the target moisture is more consistent with the
modified Proctor Optimum Moisture test.
Pressure filtration testing is recommended for
fine particle size distributions and tailings with
significant levels of clay. For sandy materials,
vacuum filtration testing should be performed.
2
© BHP RIO TINTO Tailings Management Consortium
• Specific geotechnical characteristics for input to a stack design that
can potentially handle cake moistures higher than the optimum Proctor
moisture for placement and compaction.
• Installing a thermal dryer prior to the filtered tailings stack to adjust the cake
moisture or enlarging the stack footprint to facilitate evaporative drying.
• Assessing alternative tailings management strategies, for example a
thickened slurry tailings storage facility.
19
2.3.1
Preliminary Filtration
Assessment (cont.)
The shear yield stress and Buchner funnel zero free water tests are important
for visualizing and communicating the dewatering step prior to filtration, or the
alternate thickened slurry tailings management option. It should be noted that
the Buchner funnel zero free water test is typically performed during material
characterization or vacuum filtration assessments. But when a viscometer is not
available to conduct shear yield stress tests then measuring the cake moisture
when the laboratory vacuum filter produces a ‘water free’ cake surface provides
an upper solids concentration threshold above which the tailings are difficult to
transport or feed into filter press chambers etc.
Figure 4 illustrates how the data from a preliminary filtration assessment can
be presented. The shear yield stress is plotted against the solid (or moisture)
concentration3. Using data provided by equipment vendors and published
journals it is possible to overlay the indicative maximum slurry shear yield stress
(and hence the solids concentration) that can be generated by different thickener
types (discussed in more detail in Chapter 6) or transported by different pump
types. The yield stress curve gradient can also be used to imply the level of
operational process control difficulty. The laboratory measured filter cake
moisture and Proctor optimum moisture results are then drawn on the graph.
The resultant graph will indicate whether filtration is viable and the maximum
slurry solids concentration that can potentially be produced by a thickener
and pumped to either a filter plant or a slurry tailings storage facility. Thus, the
preliminary filtration assessment can be used to identify the appropriate tailings
processing technology and storage strategy, highlight potential dewatering
performance risks that need to be investigated further, and assist with defining
the Order of Magnitude study dewatering, geotechnical, geochemical and
rheology tests required to meet the design and regulatory requirements.
Measured Vacuum or Pressure Filter Cake Moisture
SHEAR YIELD STRESS
Figure 4: Preliminary Filtration
Assessment - Tailings shear yield stress
(Pa) versus solids - moisture concentration
(%w/w) graph overlaid with moisture
measurements, plus the thickener and
pump operating shear yield stress limits.
Measured Proctor Optimum Moisture
Measured “Zero Free Water” Moisture
Paste Thickener Limit
Pressure Filter Feed Limit
Centrifugal Pump Limit
3
All mineral (tailings) slurries exhibit a shear
yield stress curve like the one shown in
Figure 4 that increases exponentially above a
critical solids mass concentration. The critical
solids mass concentration and the rate of
increase differs due to the tailings particle size,
mineralogy, and water chemistry.
Conventional Thickener Limit
INCREASING SOLIDS CONCENTRATION
INCREASING MOISTURE CONCENTRATION
© BHP RIO TINTO Tailings Management Consortium
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2.3.2
Order of Magnitude
and Prefeasibility
Study Laboratory
Characterization Tests
As outlined in Table 1 laboratory characterization testing typically commences
during the Order of Magnitude study phase and expands during the
Prefeasibility study.
Once characterization commences, the study team is usually overwhelmed
by the list of possible characterization parameters, available measurement
techniques and the data requests from consultancies and equipment providers.
Too often a compressive, costly characterization program is initiated without
first defining what is important from a project scheduling (i.e., regulator and
community discussions) and flowsheet development viewpoint.
To assist, Table 2 lists the key physical and chemical characterization tests
that should be performed on representative tailings.
Table 2: Key physical and chemical characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
Atomic absorption
spectroscopy.
Chemical
Composition
Solid and liquid
phases
Inductively coupled
plasma and
atomic emission
spectrometry / mass
spectrometry.
X-ray fluorescence
(solid samples only).
SIGNIFICANCE
Assist with reagent selection
(e.g., solution iron can be
determinantal to flocculant
activity [Witham et al., 2012]).
Identify occupational and
environmental threats.
Determine presence of
potential contaminants,
for example heavy metals,
which could potentially impact
runoff and ground water.
Data used to compare
measured concentrations
against regulatory thresholds.
Support mineralogical
assessment.
-
Conductivity
Slurry and
liquid phase
Assists flocculant / coagulant
selection.
Supports material of
construction selection.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
© BHP RIO TINTO Tailings Management Consortium
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
Consider sample collection
and storage protocols, and
test standards (e.g., filtering
water samples prior to
assaying).
Select appropriate solid
sample digestion method
prior to assessment. (e.g., the
Aqua Regia digestion method,
while applicable for some
environmental assessments,
may not dissolve all the solids
present, thus biasing the
assay results).
Seek expert occupational
health and safety advice
if toxic/heavy metals are
present.
Measurement can be affected
by viscosity, temperature, and
presence of solids.
Recommended that slurry and
liquid phases are measured at
a standard temperature.
comprehensive characterization conducted.
21
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 2: Key physical and chemical characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
Optical microscopy.
X-ray diffraction.
Mineralogy
Solid phase
Energy dispersive
X-ray analysis.
Mineral Liberation
Analyzer.
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
SIGNIFICANCE
Determines tailings mineral
composition and clay types
present to support (explain)
the solid-liquid separation,
rheology, and consolidation
behavior findings.
Identifies potential
occupational (e.g., asbestos)
and environmental
(e.g., sulfide) threats.
Identifies sulfide / sulfate
minerals and potential acid
neutralizing minerals.
Optical microscope.
Scanning electron
microscope.
Particle Shape
Correlate mineralogy with
chemical assay data to ensure
correct identification.
Use data to assist
interpretation of acid base
accounting results.
Estimate crystalline and
amorphous silica content
rather than just total silica.
Seek expert occupational
health and safety advice
if carcinogenic, asbestos
minerals and/or crystalline
silica is present.
Determine presence of fiber
shaped particles that pose an
occupational and community
health threat.
Particles with the following
dimensional parameters:
Support (explain) solidliquid separation, rheology,
and consolidation behavior
findings.
diameter <3 microns
length >5 microns,
<100 microns
Aspect ratio
(length:diameter) >3:1
Seek expert occupational
health and safety advice if
“fiber” particles are present.
Sieves.
Cyclo-sizer.
Hydrometer.
Particle Size
Distribution
Laser diffraction.
Can be used to compare
similar tailings with known
solid-liquid separation,
rheology, and consolidation
performance database.
Particle size distribution may
vary between measurement
techniques due to particle
shape, particle agglomeration
etc.
Supports (explains) the solidliquid separation, rheology,
and consolidation behavior
findings.
Testing technique needs to
capture the fines gradation
(-20 micron fraction).
Identifies potential respirable
(PM2.5) and thoracic (PM10)
particle threats.
Surfactant and high energy
ultrasonic dispersion is
required to determine the
particle size distribution.
Drying or diluting can
change particle dispersion
(or cause agglomeration),
altering the reported article
size distribution.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
comprehensive characterization conducted.
© BHP RIO TINTO Tailings Management Consortium
22
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 2: Key physical and chemical characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
pH Probe
Measures the acidity or
basicity.
-
Assists with coagulant and
flocculant selection.
pH
Slurry and
liquid phases
Supports material of
construction selection.
Marcy balance.
Pycnometer.
Slurry Density
SIGNIFICANCE
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
Required for mass balance
calculations.
Check protocol is appropriate
for:
Minimizing settling
(segregation) errors when
evaluating dilute (low
viscosity) slurries and/or
suspensions containing
coarse particles.
Vibrating U tube
densitometer.
Viscous suspensions.
Hydrometer density
measurement.
Solids Mass
Concentration
/ Moisture
Content by
Mass
Required for mass balance
calculations.
Oven or infrared
lamp drying weight
loss.
Ensure the drying
temperature does not
dehydrate minerals (e.g.,
gypsum) or salts present.
Report solids / moisture
values corrected for liquor
total dissolved solids content.
Ensure processing and
tailings teams use same
terminology and units.
Pycnometer.
Required for mass balance
calculations, storage volume
estimate, etc.
Ensure density measurement
suspension fluid wets the
solids particles and does not
dissolve the solids.
Specific
Gravity
Solid and
liquid phases
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
© BHP RIO TINTO Tailings Management Consortium
comprehensive characterization conducted.
23
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 2: Key physical and chemical characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
Oven or infrared
lamp drying weight
loss of a filtered
sample.
Total
Dissolved
Solids
Liquid phase
Total
Suspended
Solids
Liquid phase
SIGNIFICANCE
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
Used to correct solids/
moisture concentration data.
Ensure samples are filtered
prior to measurement.
Assists with coagulant and
flocculant selection.
Temperature change after
collection may alter the total
dissolved solids measurement
(e.g., salts precipitate).
Drying high total dissolved
solids tailings samples and
then resuspending can alter
the solids properties and the
reconstituted total dissolved
solids measurement.
Oven or infrared
lamp drying weight
loss measurement
of the solids
present on the total
dissolved solids test
filter paper.
Important when assessing:
Reagent dilution water
sources.
Overflow / filtrate quality as a
function of pre-conditioning
(e.g., pH adjustment) and
flocculant / coagulant
screening.
Tests are typically used to
quantify inorganic material
content. The suspended
organic material (e.g., algae
in raw water) content can
also cause operational
issues, such as flocculant
preparation.
Impact of return water on
upstream processing.
Bench scale
Buchner type filter
Zero Free
Water
Indicates the highest solids
mass concentration at which
the packed particle bed
is still fully saturated,
becoming unsaturated
because of ingress of air at
solids mass concentrations
beyond this point.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
The zero free water
concentration can be
considered as the theoretical
maximum concentration
achievable through dynamic
thickening processes.
When a viscometer is not
available to conduct shear
yield stress tests, the zero
free water moisture provides
an upper solids concentration
threshold above which
the tailings are difficult to
transport.
comprehensive characterization conducted.
© BHP RIO TINTO Tailings Management Consortium
24
2.3.2
Order of Magnitude
and Prefeasibility
Study Laboratory
Characterization Tests
(cont.)
Filter Cake Geochemical Characterization
Table 3 lists the key filter cake geochemical characterization tests:
• Kinetic tests, which provide information on the reactivity of sulfides and any
deleterious elements which are released as the sulfides oxidize, generate
acidity, etc. As kinetic tests are primarily designed to investigate sulfide
oxidation (the most common method of acid mine drainage generation in
mine waste) they are mainly used to assess materials which are known to
contain sulfides, as indicated by static testing.
• Static tests, which involve a core suite of industry standard procedures to
assess the potential for acid mine drainage, otherwise known as acid rock
drainage.
Information from the geochemical characterization program is used in
combination with inputs from other disciplines, such as physical properties,
climatic conditions, and the presence and nature of contaminant pathways and
receptors, to carry out risk assessments and provide input to facility and stack
water treatment design. If potentially acid generating minerals, such as pyrite,
are present in the tailings, the flowsheet may require a process to classify this
material from the bulk tailings for separate storage under a water cover.
When selecting and evaluating these tests the study team needs to consider:
• How the size of the filter cake pieces used could influence the dissolution/
leaching rate, for example in the bottle roll test; and,
• How best to pack the humidity cells and leaching columns to replicate the
filter cake stack compaction density.
The duration of the recommended humidity cell and column kinetic leaching
tests can extend from months to years due to placed filter cake’s low
permeability, so geochemical characterization tests should commence as soon
as possible to meet project schedule requirements.
Table 3: Key filter cake geochemical characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
SIGNIFICANCE
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
KINETIC
Column tests.
Humidity cell tests.
Leaching
(long-term)
Provides leaching kinetics,
water solute balance, pH and
chemical source term inputs
for geochemical modelling,
facility and treatment design,
material handling and storage
decisions.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
© BHP RIO TINTO Tailings Management Consortium
Test durations of months to
greater than one year are
common. Recommend that
these tests are scheduled
accordingly.
Test sample permeability
influenced by the packing
(consolidation) specified in the
test standard, which may not
represent the filtered tailings
stack permeability.
comprehensive characterization conducted.
25
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 3: Key filter cake geochemical characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
SIGNIFICANCE
STATIC
A set of laboratory
methods that vary
with jurisdiction
standards.
Used to predict whether the
tailings sample may either
generate or neutralize acidity.
Sample reacted
with hydrogen
peroxide to oxidize
sulfide minerals
present. The net
acid generation is
calculated based
on the sodium
hydroxidetitration
volume and
resulting liquor pH.
Acid generation and
neutralization reactions occur
simultaneously - net result
represents an indication of
the amount of acid a sample
may generate.
Bottle roll and
shaker flask tests.
Tests conducted
at various dilution
ratios and
durations. The
sequential leach
extractions are
assayed.
Identifies readily soluble
contaminants and
concentrations.
Acid Base
Accounting
Net Acid
Generation
Test
Leaching
(short-term)
-
Supports tailings classification,
material handling and storage
decisions.
Contributes to tailings
classification in terms of the
potential for acid generation,
and material handling /
storage decisions.
Sequential extractions may
indicate mineral phases with
contaminants.
Data used as input for
classifying the material
according to regulatory
frameworks.
Test techniques and
procedures vary depending
on the sample type,
equipment used and
jurisdiction.
Liquor may be analyzed
to indicate potential
contaminants released under
oxidizing / acid generating
conditions.
Consider performing the
tests using local rainwater
composition and pH in
addition to deionized water.
Combinations of short-term
leach tests may be sufficient
(i.e., kinetic leach testing not
required) for materials that do
not contain sulfides.
Test results can be influenced
by the size of the filter cake
pieces used.
Robust sampling/compositing
plans are important for
producing representative
samples for tests.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
comprehensive characterization conducted.
© BHP RIO TINTO Tailings Management Consortium
26
2.3.2
Order of Magnitude
and Prefeasibility
Study Laboratory
Characterization Tests
(cont.)
Filter Cake Geotechnical Characterization
Tailings geotechnical characterization is an important starting point of the
filtered stacked tailings design process. The data is used to design the storage
facility and help define the upstream processing specifications (e.g., filter cake
moisture target range).
The key filter cake geotechnical characterization tests recommended are
summarized in Table 4.
Table 4: Key filter cake geotechnical characterization tests recommended.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
Various techniques.
Air Drying
SIGNIFICANCE
Determines dry density of
sub-aerial tailings deposition
storage as a function of
time to size the storage and
estimate its operational life
span.
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
Methodology used should
simulate the seasons and
regional climate (e.g.,
temperature and humidity).
Used to estimate the surface
dry density as a function of
time.
Atterberg test
specialized
equipment.
Classifies soil type to allow
database comparison.
Oedometer.
The resultant density, void
fraction, and permeability
versus depth (pressure) data
is used in tailings storage
consolidation models to
predict settlement over
time, assess changes in the
stack strength and hydraulic
properties.
Atterberg
Limits
-
Provides useful solid to liquid
transition data as a function of
solids concentration.
(Plastic limit,
liquid limit,
shrinkage limit
and plasticity
index)
Rowe cell.
Consolidation
(onedimensional)
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
© BHP RIO TINTO Tailings Management Consortium
Test protocol should include
creep assessment.
For Rowe cell tests the
preference is to use large
diameter units.
comprehensive characterization conducted.
27
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 4: Key filter cake geotechnical characterization tests recommended.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
SIGNIFICANCE
Various techniques.
Drained and undrained
settling tests are typically
used to characterize slurry
tailings. These tests can also
be used to mimic and thus
quantify the stacked tailings
consolidation behavior
and permeability, and
provide useful solid – liquid
separation design data.
-
Various instruments
built to different
international
standards.
Data used to define filter cake
moisture target range and
stack compaction density
targets.
The Standard Proctor test
is typically used for stacked
filtered tailings projects.
Should consider using the
modified Proctor data for tall
stacks.
Triaxial
compression test
and ring shear test.
Test provides insight into:
Depending on the tailings
properties and stack location
etc., consider performing
drained and undrained triaxial
compression tests.
Drained and
Undrained
Settling Tests
Optimum
‘Standard’
and/or
‘Modified’
Proctor Dry
Density and
Moisture
Content
Critical State
(or Critical
Void Ratio)
Line Testing
The behavior of non-plastic
tailings during compaction
and shearing.
The stack stability and
liquefaction potential.
Data is used to optimize
the height and compaction
of filtered tailings stacks
ensuring dilative conditions
across the design stress
range.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
comprehensive characterization conducted.
© BHP RIO TINTO Tailings Management Consortium
28
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 4: Key filter cake geotechnical characterization tests recommended.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
Direct Simple
Shear
TYPICAL
ASSESSMENT
METHODS
Direct simple shear
instrument, sold by
various suppliers,
that compiles
with the regional
standard.
SIGNIFICANCE
Measures the shear strength
properties under confined
stress conditions to produce
tailings facility design data.
Simulates conditions along
the base of a circular or planar
shear surface.
Enables the review of the
susceptibility of liquefaction
under various degrees of
saturation, including the
influence of soil suction.
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
Recommend conducting
direct simple shear testing at:
Various confining stresses
on re-constituted samples at
various degrees of saturation.
Various particle size
distributions.
The test should involve two
phases: one cyclic phase at
various cyclic shear stress
ratios, and one monotonic,
post cyclic loading phase.
Recommend performing
monotonic direct simple shear
to large strains (30%).
Constant rate of
strain consolidation
testing.
Permeability
Soil Water
Characteristic
Curve
Data is used to help quantify
the stack seepage rate.
Large diameter tests are
desirable.
Conducted to understand the
unsaturated soil mechanics of
filtered tailings. For example,
evaluate if stacked filtered
tailings re-saturate during
rainfall events.
Specialized testing requiring
expertise.
Flexible wall
permeameter.
Conventional Soil
Water Characteristic
Curve technique
uses the pressure
plate method.
Various onedimensional
stress and triaxial
instruments.
When completing the tests
settlements should be
measured to calculate the
void ratio with increasing
suction.
Void ratio
measurements
conducted during
drying and wetting
cycles to determine
air entry value.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
© BHP RIO TINTO Tailings Management Consortium
comprehensive characterization conducted.
30
2.3.2
Order of Magnitude
and Prefeasibility
Study Laboratory
Characterization Tests
(cont.)
Filter Cake Transport Characterization
The filter cake transport and placement system can represent a significant
capital and operating cost to a filtered stacked tailings project, discussed
further in Chapter 5. To design a system that can manage normal and upset
filtration operating conditions, it is important to understand how bulk filter cake
at different moisture contents behaves in a truck, on a conveyor, and when
deposited and compacted.
The key filter cake transport characterization tests are listed in Table 5. It should
be noted that most transport characterization tests are based on material
handling test standards more applicable for powders or granular material. It is
recommended that the study team conducts tests using samples produced by
pilot filtration plants to understand how ‘large’ pieces of filter cake behave as a
function of cake moisture.
© BHP RIO TINTO Tailings Management Consortium
31
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization (cont.)
Table 5: Key filter cake transport characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
SIGNIFICANCE
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
Slump test
performed on a
tapping or vibrating
table.
Provides a preliminary
moisture threshold to
avoid unstable behavior
(liquefaction) on a conveyor
belt or truck.
Flow moisture point test is
derived from the transportable
moisture limit method
developed for transportation
of bulk material in ship-holds.
Belt incline.
The filter cake bulk density
and surcharge angle are
used to determine conveyor
transport capacity.
Preference is to use
representative pieces of
filter cake at various cake
moistures to perform the tests.
This requires specialized tests
and laboratories.
TYPICAL
ASSESSMENT
METHODS
Flow Moisture
Point
Surcharge angle.
Conveyance
Testing
Minimum
Angle for
Discharge
Angle of
Repose
Ground
Bearing
Pressure
Filter cake bulk
density.
The maximum belt incline
determines how steep an
angle a conveyor can be
designed at without material
slipping backwards.
Standard test
performed by
bulk material test
laboratories at
different cake
moistures.
Data is used to design
hoppers and chutes etc. to
ensure the filter cake flows
uniformly and continuously.
Preference is to use
representative pieces of
filter cake at various cake
moistures to perform the tests.
This requires specialized tests
and laboratories.
Various standard
techniques
performed by
bulk material test
laboratories.
Defines the maximum angle
the material can rest on an
inclined plane without sliding
down.
Various angle of repose (also
known as the critical angle
of repose) test standards
exist that pertain to granular
materials when piled or
heaped. Preference is to use
representative pieces of filter
cake which require specialized
tests and laboratories.
Cone penetrometer.
Quantifies the maximum
ground bearing pressure of
placed filter cake at different
moistures.
Data used to design hoppers
and conveyor belts, define
stockpile stability etc.
The California Bearing Ratio,
originally developed for
highway engineering, also
plays a crucial role in assessing
truck stability on landfills.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
Preference is to use
representative pieces of
filter cake which require
specialized tests and
laboratories.
comprehensive characterization conducted.
© BHP RIO TINTO Tailings Management Consortium
33
2.3.2
Order of Magnitude
and Prefeasibility
Study Laboratory
Characterization Tests
(cont.)
Solid – Liquid Separation Characterization
During the Order of Magnitude study, it is important to explore the potential
impact of tailings particle size, clay presence and types, water quality, upstream
reagents, coagulant and/or flocculant additions on the tailings thickening and
filtration behavior. Dewatering rates achieved for thickening and filtration dictate
the size and number of equipment units required and can have a significant
impact on capital and operating costs. Dewatering performance is sensitive to
tailings properties, and it is therefore essential that the expected variability in
these properties over the life of operation are understood.
Clays are notorious for creating challenges in the concentrator, and clays in
tailings are no exception. Clay particles are fine flakes with very large surface
areas and have high surface activity. This high activity can result in clay particles
staying suspended in liquid, which is detrimental to dewatering. Study managers
should look out for clays such as smectite, vermiculite and illite. Dewatering
properties of tailings with high clay content may be improved by manipulating
the slurry system's chemistry, such as increasing ionic concentration through the
addition of lime or dissolved salts, adjusting the pH, or increasing the ratio of
Ca2+ to Na+ ions.
Table 6 summarizes the key solid – liquid separation characterization tests used
to define the filtration and upstream thickening processes.
© BHP RIO TINTO Tailings Management Consortium
34
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 6: Key solid – liquid separation characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
Settling
Cylinder Tests
Raked Settling
Cylinder Tests
TYPICAL
ASSESSMENT
METHODS
SIGNIFICANCE
Laboratory
measuring
cylinders. Various
international /
industry standard
methods.
Sometimes called
the “static” settling
cylinder test
because fresh feed
is not continuously
added, nor the
settled material
removed.
Test used to:
Laboratory
measuring cylinder
with a suspended
motored rake to aid
compaction of the
settled material.
-
Estimate the optimum
thickener feed dilution.
Screen pre-conditioning
parameters (e.g., pH),
coagulant and flocculant
types and dosages.
Estimate the undrained
settled solids concentration
within a thickener and tailings
storage.
Various
international /
industry standard
methods.
Method has been
superseded by
the dynamic (or
continuous) settling
test.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
© BHP RIO TINTO Tailings Management Consortium
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
Preference is to use tall, wide
diameter laboratory cylinders.
Small diameter, short settling
cylinder results should not be
used to estimate thickener
performance. These cylinders
are only suitable for initial
pre-conditioning and reagent
screening test work.
Recommend settling tests
are conducted at free settling
rates that cover the expected
operating range.
The test provides initial
thickener sizing parameters
when sample quantities are
minimal.
Larger diameter, tall cylinders
are preferred for this test
work.
comprehensive characterization conducted.
35
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 6: Key solid – liquid separation characterization tests.
STUDY PHASE
PARAMETER
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
Equipment
produced by
various vendors.
Dynamic /
Continuous
Settling Tests
Similar to the raked
cylinder test except
the cylinder is
continuously fed
with coagulated /
flocculated feed
slurry and the
settled material
is continuously
removed.
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
SIGNIFICANCE
Standard test to provide
scalable thickener sizing and
performance (underflow solids
concentration and unsheared
yield stress, overflow total
suspended solids) data when
sufficient sample quantities
are available (preferred
method).
Recommend the tests are
conducted at feed flux and
rise rates that cover the
expected operating range.
Can use the test equipment
to produce samples for
laboratory rheology and
filtration tests.
Increase cylinder column
height to allow deeper (higher
compression) beds to be
assessed.
Less applicable during
Prefeasibility study if
extensive solids – liquid
separation pilot plant tests are
performed.
Various laboratory
benchtop and pilot
units manufactured
by vendors.
Quantifies the minimum filter
cake moisture achievable at
commercially relevant feed
flowrates and operating
conditions, and at the thinnest
available chamber depths.
This test is indicative only
and should not be used for
final equipment selection
and design. Further testing
at various chamber depths
and operating conditions
are needed to optimize the
filtration system design and
size filtration equipment.
Various laboratory
benchtop and pilot
units manufactured
by vendors.
Quantifies the filter cake
moisture and filtration
sizing and throughput
parameters over the expected
operating range (feed solids
concentration, flocculation,
chamber depth, pressure,
cycle time etc.).
Preference is to use pilot
filtration equipment when
sufficient test material
becomes available to improve
scale-up.
Minimum
Moisture
Content
Vacuum/
Pressure
Filtration Test
If testing with a vendor,
discuss test program to
ensure the test conditions
and data collection are
appropriate.
Laboratory vacuum Buchner
funnel tests are useful for
estimating the cake moisture
at zero free water point
(Table 2).
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
comprehensive characterization conducted.
© BHP RIO TINTO Tailings Management Consortium
36
2.3.2
Order of Magnitude
and Prefeasibility
Study Laboratory
Characterization Tests
(cont.)
© BHP RIO TINTO Tailings Management Consortium
Slurry Rheology
Table 7 summarizes the recommended rheology tests to characterize tailings
thickener underflow and filter feed slurry rheology. These measurements are
important because they are used to define the pumping and pipeline design,
filter filling efficiency, impact of upstream variances and whether rheology
modifiers are required.
37
2.3.2 Order of Magnitude and Prefeasibility Study Laboratory Characterization Tests (cont.)
Table 7: Key slurry rheology characterization tests.
STUDY PHASE
PARAMETER
Compression
Rheology
Concept
OoM
PFS
TYPICAL
ASSESSMENT
METHODS
Relatively
new rheology
measurement
technique using
specialized
equipment.
ASSESSMENT
CONSIDERATIONS
AND/OR ACTIONS
SIGNIFICANCE
Measures the tailings
compressive yield stress and
permeability as a function
of compressive load (or
gravitational force).
Useful screening method to
evaluate slurry dispersion and
agglomeration.
Only a limited number
of laboratories offer this
measurement.
Less applicable during
prefeasibility if extensive
laboratory and pilot plant
solids – liquid separation
testing is performed.
Provides initial thickener
bed, pressure filtration and
tailings management facility
consolidation / permeability
design data.
Rotational
viscometer.
Pipe loop.
Shear
Rheology
Rotational viscometer
provides ‘dynamic’ shear yield
stress and shear stress – rate
data to support slurry pump
selection and pipeline design.
The pipe loop, providing
the slurry is homogeneous,
provides scalable slurry pump
selection and pipeline design
data.
Preference is to conduct
pipe loop (pressure drop)
measurements when sample
volume permits.
Expert review required to
ensure slip, segregation,
turbulence etc. has not
affected the measurements.
Need to ensure the sample
shear history prior to
measurement reflects the
process conditions.
Pipe loops require significant
sample volumes.
Shear Yield
Stress
Slump cone or
cylinder.
Provides processing options
(constraints) insights.
Rotational
viscometer
fitted with vane
or concentric
cylinders.
Useful initial screening
measurement to evaluate
slurry dispersion and
coagulation / flocculation
before progressing to
detailed mineralogy, surface
charge (e.g., zeta potential)
measurements etc.
Slump test is a useful operator
method when operation
commences.
OoM = Order of Magnitude. PFS = Prefeasibility study.
Study phase markers: Typically not performed, limited assessment and
Shear yield stress estimated
by slump tests can differ from
the rotational viscometer
measurements.
Preference is to use rotational
viscometer fitted with vane
rather than the slump test.
Need to ensure the sample
shear history prior to
measurement reflects the
process conditions.
comprehensive characterization conducted.
© BHP RIO TINTO Tailings Management Consortium
38
2.4
Typical
Characterization
Issues
Typical sampling and assessment issues seen in filtered stacked tailings test
work programs are listed in Table 8. The most common issue is test work
completed using non-representative tailings and process (raw) water due to
time, money, and availability constraints. Testing non-representative samples
leads to incorrect design parameter definition, reagent selection, flowsheet
development and equipment sizing.
Another issue is developing a single set of project definitions for the multidisciplinary team, for example defining whether referring to tailings production
as wet or dry tonnes. Care must be taken when comparing density and moisture
data collated by mineral processing and geotechnical teams. The study
manager must ensure that the definition of tailings filter cake moisture content is
understood in terms of geotechnical or process engineering:
Process
moisture content:
mc=
Mw
Ms+Mw
Geotechnical
moisture content:
w=
Mw
Ms
Mw = mass of water (kg). Ms = mass of solids (kg)
Table 8: Common characterization sampling and assessment issues.
01
ISSUE
Limited or no characterization conducted due to ‘generalizations’ made by study
team based on another ore body, process facility, previous experience etc.
IMPACT
Incorrect flowsheet, equipment sizing and cost estimates.
CONTROL
Develop a limited characterization program to assess whether the tailings properties
and behavior are comparable to database.
ISSUE
02
IMPACT
Limited characterization data is available due to the time and cost required
to acquire, prepare, and conduct the test work. This is particularly an issue for
environmental tests which can take more than one year to complete.
Potential erroneous data due to having only a single measurement and/or
non-representative measurements.
Ore and process variance not measured and thus not included in the process design.
Identify the limited assessment in the project risk register as a threat.
CONTROL
Seek schedule and/or budget extension to characterize an appropriate number
of repeats, tailings property variance etc.
© BHP RIO TINTO Tailings Management Consortium
39
2.4 Typical Characterization Issues (cont.)
Table 8: Common characterization sampling and assessment issues.
03
ISSUE
Inappropriate characterization program for the engineering study level.
IMPACT
Increased study costs and schedule delays due to the magnitude of the
characterization program undertaken and the collection and/or generation of large
sample quantities.
Review company study guidance requirements.
CONTROL
Conduct a staged characterization program.
Assess the data before expanding the characterization assessment beyond the
study level requirement.
Collection and assessment of non-representative samples caused by:
• Limited or inappropriate process sampling or drill core campaign
ISSUE
• Mine plan
• Process stream sampling bias
• Non-representative laboratory or pilot plant production conditions
04
• Inappropriate laboratory sub-sampling equipment and procedures.
IMPACT
Incorrectly eliminate potential process, reagent, and storage options.
Incorrectly specify process equipment / storage size, costs etc.
Coordinate statistical sample program with mine planners / exploration drillers.
CONTROL
Demonstrate sampling is non-biased.
Document sampling points, periods, methods etc. and the resultant sample properties.
ISSUE
05
IMPACT
CONTROL
Using water non-representative of the site for reagent selection and characterization
assessments. The difference in pH and composition compared with the site process
and raw water can potentially change the reagent behavior, solid-liquid separation,
and rheology behavior.
Incorrectly eliminate potential process, reagent, and storage options.
Incorrectly specify process equipment / storage size, costs etc.
Collect and use site representative process and raw water samples.
Document the water properties.
© BHP RIO TINTO Tailings Management Consortium
40
2.4 Typical Characterization Issues (cont.)
Table 8: Common characterization sampling and assessment issues.
ISSUE
Adding biocides or thermal drying samples prior to storage and/or shipment.
Potentially:
06
IMPACT
• Alters the water phase pH and composition.
• Dehydrates (decomposes) minerals and salts present.
• Causes saline water ions to ‘bind’ to the mineral surfaces.
CONTROL
Determine whether the samples need to be stored under conditions that inhibit
oxidation etc.
Irradiate rather than thermally treat samples and store in a cool room.
ISSUE
07
08
ABC
Dehydration or decomposition of minerals (e.g., gypsum) and salts caused by drying
temperature specified in the test standard.
Incorrect mass balance.
IMPACT
Incorrect moisture versus solid-liquid separation and rheology behavior relationships.
Incorrect mineral / salt identification.
CONTROL
Refer to published literature to determine appropriate drying temperature for the
minerals and salts presumed to be present.
ISSUE A
Non-representative temperature and humidity conditions, excessive storage and/
or inappropriate shear history altering the flocculant polymer, slurry rheology and
filtration behavior.
ISSUE B
Use of inappropriate characterization assessment instruments or methods.
For example, screen options using cylinder slump measurements to design a
Feasibility Study pumping system.
ISSUE C
Data manipulation by test instrumentation software.
IMPACT
Eliminate potential flowsheet options.
Incorrectly specify flowsheet, process equipment sizing, costs etc.
Engage characterization experts to review sample preparation, measurement
methods and the resultant data.
CONTROL
Select the characterizing method appropriate for study level.
Employ characterization (rheology, filtration etc.) methods that minimize the
equipment scale-up factor.
© BHP RIO TINTO Tailings Management Consortium
3.
Site
Closure
APPROACH
42
Site Closure
Filtered
Tailings
Stack
Material
Transport
& Stacking
Tailings
Dewatering
Concentrator
Mine
© BHP RIO TINTO Tailings Management Consortium
43
3.1
Key Points
Company, stakeholder and regulatory closure expectations
and the cover (cap) design drives the stack location and
design, surface runoff and leachate management, closure
duration, and costs.
Progressive stack covers (caps) reduce dust emission and
rain infiltration and improve the stack’s visual appearance
but impact the project’s Net Present Value by introducing
closure costs earlier in the life of operation.
The stack closure strategy must consider climate change
and the relinquished land usage.
Stack leachate treatment can generate a brine/salt
stream for decades after closure, potentially requiring
offsite disposal.
© BHP RIO TINTO Tailings Management Consortium
44
3.2
Introduction
Like conventional slurry tailings storage facilities, the filtered
tailings stack represents the project’s legacy. The adopted
closure strategy will influence the stack’s site selection,
geometry, maximum height, cover (capping) design, and the
tailings transport and placement strategy.
As discussed in Chapter 1 (Overview)
it is recommended that the filtered
tailings study team define:
• Whether the tailings will be reclaimed in the future.
• The relinquishment parameters and thresholds.
• Tailings hazard classification and propensity to produce acidic leachate.
• The stack’s cap (cover) and water management requirements.
The closure strategy should focus
on reducing the time required to
rehabilitate and relinquish the site by:
• Applying international best practices.
• Ensuring the final landform and vegetation cover is compatible with the
intended land use.
• Considering potential stack design required to manage climate change.
• Minimizing rainfall ingress and stack basal saturation.
• Proper management of surface runoff and leachate to prevent water
contamination, ensure regulatory compliance, and reduce the water
treatment costs and duration.
© BHP RIO TINTO Tailings Management Consortium
45
3.3
Filtered Tailings
Stack Closure
Design
To relinquish the site the filtered tailings stack closure
design needs to minimize water and oxygen infiltration,
minimize surface erosion, and maintain the tailings in
an unsaturated state to ensure geotechnical, erosional,
and geochemical stability. These criteria are met by:
• The stack base liner, basal drainage system, leachate collection and runoff
water sedimentation ponds and water treatment facilities constructed when
the stack is commissioned.
• The buttresses, bunding, surface compaction and grading, spillways
and perimeter channels constructed when the stack is commissioned and
operated.
• Constructing an engineered cover over the stack surface after cessation
of tailings placement.
The engineered cover design is dependent on the regulatory requirements,
final landform usage, climate, and tailings composition. For filtered tailings
stacks, the engineered covers can be temporary or permanent.
3.3.1
Temporary
Engineered Covers
Temporary covers (caps) are constructed over the compacted
graded tailings surface during the stack’s operational phase.
Synthetic materials (e.g., high density polyethylene, geosynthetic clay liners,
geomembranes etc.), binders (e.g., polymers, fly ash), plants (e.g., grasses)
and/or crushed rock is laid over the inactive stack surfaces to curtail fugitive
dust emissions, minimize erosion maintenance and reduce rainfall infiltration,
which can reduce the water management costs.
Synthetic materials can degrade in direct sunlight or when exposed to extreme
cyclic weather conditions and are thus not suitable as a permanent cover unless
protected using clay, soil and/or rock cover.
When tailings placement recommences, the temporary cover is either left in
place, assuming the slope stability analysis shows an adequate safety factor, or
removed to facilitate slope contouring, maintain stack stability and/or the water
management strategy. Alternatively, the temporary cover can be incorporated
into a permanent cover design should the mine need to close unexpectedly.
While the temporary cap installation cost can be substantial, the operational,
maintenance, and water treatment cost savings over time can offset the
investment.
© BHP RIO TINTO Tailings Management Consortium
46
3.3.2
Permanent Engineered
Covers
Permanent engineered covers (caps) can either be
constructed progressively while the stack is operational or
after tailings placement ceases (Figure 5). Like temporary
covers, permanent covers are designed to minimize
fugitive dust, surface erosion, and rainfall infiltration.
Permanent covers also inhibit oxygen migration to minimize
acid generation1, support a vegetation cover, improve the
landform appearance, and ensure the final landform meets
the regulatory requirements and intended relinquished
site usage.
While the benefits of progressively constructing a permanent cap extend
beyond those of a temporary cap (e.g., demonstration of the closure design) it's
crucial to acknowledge that the closure costs are brought forward. These costs
may adversely impact a project's Net Present Value and present an ongoing
challenge to the stack’s design and execution. For example, the progressively
built permanent cap will dictate the stack design, stacking plan, borrow source
plan, liner procurement, and traffic interactions between the progressive closure
fleet and the operational fleet.
As illustrated in Figure 5, progressive
permanent caps will also shift the
stack construction to a staged
approach. The resultant footprint,
slope and height of these stages will
thus dictate, or be dictated by:
• The tailings transport requirements (i.e., breadth and gradient of mobile
equipment access ramps and placement areas).
• Slope erosion and surface runoff management.
• The vegetation planting program.
• The final landform topography.
The permanent cap tends to have a multi-layer construction design to achieve
different objectives. For example, Figure 6 illustrates a stack cap design that
includes a low permeability mineral or synthetic barrier layer placed over the
compacted filter cake, overlaid with a drainage layer (e.g., geocomposite, gravel)
to manage water infiltration, and a synthetic liner to reduce suspended solids
migration into the drain.
1
It should be noted that during placement unsaturated sulfate mineral tailings may oxidize, and on
contact with rainwater produce an acidic, metalliferous leachate. Covers cannot prevent this from
occurring but can minimize the production of acidic leachate during the closure phase.
© BHP RIO TINTO Tailings Management Consortium
47
Figure 5: Permanent engineered cover installation options.
Post operation closure cover.
The tailings are stacked in layers
across the entire footprint for
life of operation. The permanent
closure cover is then constructed
after tailings placement ceases.
Progressive closure cover.
The tailings are stacked on a portion of the
footprint and the permanent closure cover
is progressively constructed as each stage
reaches its final stack height.
Figure 6: Permanent closure cap (cover) design
that incorporates a low permeability barrier,
infiltration drainage and vegetation cover options.
Vegetation
Soil/Growth Media
Drainage Layer
Low Permeability Barrier
Compacted Tailings
© BHP RIO TINTO Tailings Management Consortium
48
3.3.3
Soil Covers
Filtered tailings stacks are typically constructed above the surrounding terrain
and water table. The tailings are stacked over a basal drain, compacted, and
contoured to maintain the tailings in an unsaturated state which prevents
liquefaction during a seismic event and minimizes leachate seepage. Thus,
water covers are not implemented on stacks as they are on wet tailings facilities
to minimize (prevent) acidic leachate generation.
While synthetic turf membranes are available to improve the visual impact, stack
surfaces are typically covered with a layer of soil (growth media) and vegetation
(Figure 6) to ensure the final landform is compatible with the relinquished land
use and regulatory requirements.
• In sub-humid climates, water-shedding soil covers can be deployed to support
an overlying vegetation layer which controls surface erosion, limits rainfall
infiltration, and manages rainfall runoff.
As illustrated in Figure 7 the
appropriate soil cover type is based
on the following climatic conditions:
• In low rainfall regions, store and release soil covers are recommended.
This cover design retains rainwater during the wet season, releasing it via
evapotranspiration during the dry season to sustain the overlying vegetation
cover, which limits the net rainfall percolation and minimizes surface erosion
during heavy rainfall events.
50
0
3°C
4,0
00
2,0
00
1,0
00
ira
tio
6°C
8,0
00
sp
ap
otr
an
Ev
al
nti
Po
te
)
mm
n(
tio
Store and Release
(sustainability of vegetation)
12°C
16
32
,00
0
Sub-tropical
1.5°C
ita
cip
16
Warm Temperate
Pre
8
Cool Temperate
l
tro rs
on ove
nC C
tio ing
ltra dd
Infi r She
te
Wa
4
Boreal
Water Covers
Low Permeability
Oxygen Barriers
- Organic Covers
(erosion)
al
nu
2
Sub-polar
–8°C
An
nR
ati
o
Permafrost
Thermal Covers
(freeze thaw effects)
1
Polar
Tropical
25
0
5
0.2
0.5
Latitudinal Region
12
5
5
2
0.1
50
Figure 7: Global Acid Rock Drainage (GARD)
guide for cover type as a function of climate.
Credit: Global Acid Rock Drainage Guide,
Chapter 6 www.gardguide.com.
Super-arid
Per-arid
© BHP RIO TINTO Tailings Management Consortium
Arid
Semi-arid Sub-humid
Humid
Per-humid Super-humid
24°C
49
3.3.3
Soil Covers (cont.)
The soil (growth media) layer material can be sourced from local burrow
pits, site construction stockpiles or created using mine and municipal waste
material. To establish a sustainable vegetation layer ecosystem the closure
planting program needs to be staged to establish the vegetation canopy and
undergrowth. Thus, the project will need to develop an appropriately sized
seed collection and nursery program to raise the quantity of plants required.
To ensure success the closure cover planting program requires stringent quality
control and monitoring to prevent the introduction of invasive weeds.
Communities and regulators tend to specify the planting of native species or
horticultural crops. However, this requirement needs to be critically reviewed
due to:
• The native species may be difficult to source, and/or establish on the filtered
tailings stack due to its topography, elevation, soil (growth medium) layer
type and depth, and the stack’s surface water management strategy.
• Alternative vegetation types may produce a stable, sustainable low
maintenance cover that does not impact the cover’s integrity and achieves
the closure schedule.
Plant root penetration of underlying cap layers could cause water penetration,
gas exchange and contaminates reaching the surface. The underlying cap
layers can be protected by ensuring the soil (growth medium) layer is sufficiently
thick and by selecting lateral or fibrous rooted scrubs and trees in preference to
taproot plants (Figure 8).
Figure 8: Taproot plant versus fibrous
root plant.
TAPROOT SYSTEM
FIBROUS ROOT SYSTEM
© BHP RIO TINTO Tailings Management Consortium
50
3.4
Potential Stack
Closure Risks
When developing the stack closure strategy and design, the
study team should consider the following potential threats:
POTENTIAL THREAT
CAUSES
CONTROL/S
STACK SLUMP FAILURE
• Excessive stack height and/or gradient.
• Static and dynamic loading modelling.
• Saturating the stacked tailings.
• Tailings characterization.
• Proximity of seepage pond to stack toe
causing saturation during storm event.
TAILINGS SATURATION
• Inappropriate drainage design.
• Modelling.
• Basal drain mechanical failure, blockage,
or scaling.
• Leachate characterization.
• Damaged cover permitting water ingress.
VEGETATION COVER
DEGRADATION
• Invasive weeds, fire etc.
• Climate change.
• Emergency response plan.
• Soil quality control, vehicle inspections
and monitoring to prevent the
introduction of weeds.
• Emergency response plan.
COVER LAYER DAMAGE
• Excessive erosion caused by climate
change, extreme weather event,
inappropriate surface grading, channel –
spillway design.
• Monitoring and maintenance program.
• Emergency response plan.
• Plant root penetration.
ACIDIC (METALLIFEROUS)
LEACHATE
• Tailings saturation (see above).
FUGITIVE DUST
• Vegetation cover degradation.
• Oxygen ingress due to cover failure.
• Layer construction methodology,
supervision, and scheduling.
© BHP RIO TINTO Tailings Management Consortium
• Cover design appropriate for the
climate and vegetation layer.
• Dust monitoring and suppression
controls (e.g., windbreaks,
application of water or commercial
dust suppressants, restrict vehicle
movements on surfaces etc.).
51
3.4
Potential Stack
Closure Risks
(cont.)
Over time the leachate flow rate from the stack will decrease to a threshold
value agreed with the regulators, below which it does not need to be actively
managed. Likewise, the stack’s surface runoff sediment loading and composition
will change over time as the vegetation cover is established, eventually
permitting direct discharge into the environment. Thus, maintenance and
monitoring of the covered stack and its water management infrastructure is
crucial for ensuring the site meets the closure criteria and remains geochemically
and geotechnically stable. The stack closure strategy and design should include:
• Geotechnical and groundwater instrumentation to monitor the tailings pore
pressure, groundwater pH and composition, etc.
• Stack maintenance access roads and ramps to facilitate periodic visual
monitoring, gully and drainage repairs, and water sampling.
• An emergency response plan to address runoff and leachate collection dam
failures, vegetation fires, seismic events etc.
The brine stream produced by water treatment can represent a major threat.
The concentrated salts need to be contained within an appropriately designed
onsite or offsite repository to prevent ground water contamination. Alternatively,
the study team could assess offsite brine commercialization opportunities.
Considering the cost and potential threats associated with brine management,
it is recommended that a stack consolidation–infiltration model is developed
to predict the leachate flow rate over time to size the stack leachate collection,
treatment infrastructure and repository designs.
© BHP RIO TINTO Tailings Management Consortium
4.
Filtered
Tailings
Stack
APPROACH
52
Site Closure
Filtered
Tailings
Stack
Material
Transport
& Stacking
Tailings
Dewatering
Concentrator
Mine
© BHP RIO TINTO Tailings Management Consortium
53
4.1
Key Points
The tailings moisture content specifications should be
selected to achieve the performance objectives of the
filtered tailings stack. These performance objectives
are project-specific and account for items such as stack
stability, tailings transport and placement methodology,
climate, and seismicity.
Consideration must be made for tailings storage when “out
of specification” filtered tailings are produced and during
filter plant shutdown periods.
Surface and seepage water management and water
quality must be considered, as filtered tailings stacks may
produce seepage and exhibit enhanced acid generation
and metals leaching due to lower levels of saturation than
conventional tailings storage facilities.
© BHP RIO TINTO Tailings Management Consortium
54
4.2
Introduction
Following the 2014 Mount Polley tailings storage facility failure in British
Columbia, an Independent Expert Investigation and Review Panel (the Panel)
was formed, and the outcome of their investigation was the advocation of
migrating to best available technology for tailings management to achieve
physical stability of the tailings deposit using methods such as below ground
tailings storage (in pit storage) or filtered tailings storage (Morgenstern et
al., 2015). This recommendation aligns with the Global Tailings Standard
requirement of considering alternative options to conventional tailings
management facilities which have the potential to deliver improved technical
and environmental, social and governance outcomes throughout the facility’s
lifecycle (see Chapter 1).
The Panel also highlighted chemical stability as a fundamental consideration
for tailings storage. The most serious chemical stability problem relates to
sulfide minerals which are subject to acid generation and metals leaching in the
presence of oxygen. Other social and environmental impacts may occur through
losses from the facility by water or wind transport. Impacts may be physical,
(e.g., smothering of habitats by sediment or dust) or they may be chemical, (e.g.,
contamination of surface or sub-surface water resources). Additionally, chemical
changes within the tailings during mineral weathering and/or secondary mineral
precipitation may impact the physical characteristics and the resulting integrity
of the tailings storage facility.
Filtered stacked tailings represent a paradigm shift from
conventional wet tailings storage practice and introduces
a new set of challenges. As discussed in Chapter 1, the
design of a filtered tailings stack is an iterative process that
considers input from the mine and closure plan, materials
handling, ore and tailings processing, geotechnical stability,
and risk.
The study team needs to define the following for the filtered tailings stack:
• Detailed site characterization that includes tailings properties, site climate,
geology, geochemistry, hydrology and hydrogeology, and seismicity.
• Design basis and performance objectives, as informed by regulatory
requirements, industry practice, risk assessments, and site characterization.
• Required tailings moisture content range at the filter plant and placement
specifications to achieve the defined performance objectives.
• Approach to transport, placement, and storage of tailings during both normal
and upset operational conditions.
• Seepage and surface water quantity and quality estimates, and management
methods to achieve the design basis.
© BHP RIO TINTO Tailings Management Consortium
56
4.3
Physical Stability
Tailings dam failures are occurring at a rate relatively higher than for water
storage facility dams (Shahid & Li, 2010; Davies, 2002). This includes
catastrophic failures in recent years which have led to increased public,
regulator, and investor visibility, such as the Brumadinho failure (Figure 9) where
more than 270 people lost their lives. While there is evidence that tailings facility
dam failure rates have decreased since 2000 as more modern engineering
techniques are applied (Stark et al., 2022), the overall risk associated with
tailings storage may be continuing to increase as the size of tailings storage
facilities and volume of impounded tailings continues to increase in response to
growing resource consumption and the mining of increasingly lower grade ore
bodies (Oberle et al., 2020).
Figure 9: Mineral tailings mud after
dam rupture in Brumadinho.
Filtered tailings stacks present an opportunity to reduce tailings storage facility
failures by enhancing physical stability. Key physical stability concepts for
filtered tailings stacks include:
• The tailings moisture content and level of compaction at the stack placement
location plays a key role in the physical stability of the stack.
• Loose (contractive) tailings are likely to generate positive excess pore water
pressures when sheared under undrained conditions, potentially leading
to liquefaction under static or cyclic loads that are sufficient to trigger this
condition. However, dense (dilative) tailings with low moisture content are not
susceptible to liquefaction.
• The Critical State Line separates tailings that will dilate when sheared (below
the Critical State Line) from those that will contract when sheared (above
the Critical State Line) and is determined through a series of drained and
undrained triaxial tests.
• The tailings moisture content within the stack redistributes from the moment
of placement and can lead to saturation due to downward fluid flow under
gravity and compression of tailings under increasing loads as the stack
height builds.
• Saturated conditions can impact physical stability, allowing for positive pore
water pressure generation and undrained shear strength conditions, which
may control the stability of the stack.
© BHP RIO TINTO Tailings Management Consortium
57
4.3
Physical Stability
(cont.)
• Tailings placement specifications are typically defined as a function of the
maximum dry density optimum moisture content obtained from Proctor
testing. A typical Proctor curve is shown in Figure 10.
• To achieve and maintain dilative behavior as the stack is constructed, filtered
tailings may be placed in an unsaturated condition and compacted to
densify the tailings. Compaction occurs when particles are pressed together,
reducing the pore spaces between them (Figure 11). Heavily compacted
tailings contain few large pores and less total pore volume, and therefore
have a greater density. Compacted tailings have a reduced infiltration and
conductivity rate for both liquids and gases, and compaction increases the
strength of the material and the ability to resist movement when a force
is applied, making it more stable. Compaction equipment can be seen in
Figure 12 and is discussed in further detail in Chapter 5.
• Predicting the level of saturation and the dilative or contractive nature of the
tailings throughout the stack profile over time is critical to assess the risk
profile of the stack. The level of saturation and dilative or contractive nature
is a function of the tailings moisture content and compaction specifications,
and site-specific items such as climate, the presence of drainage zones, and
the facility’s rate of rise.
• Unsaturated conditions within the stack may lead to negative pore pressures
which can improve stability. These conditions, and their reliability with time
and varied climate conditions, should be considered in the design.
• Filtered tailings stack design should consider both global and local failure
modes, where global failure modes consist of deep-seated rotational failures
while local failure modes include sloughing of the advancing stack face
and bearing capacity failure below loads (such as transport and placement
equipment) on the stack surface (see Figure 13).
Figure 10: Proctor moisture curves.
Line of Optimums
Saturation Line Zero Air Voids
Ydmax : Maximum Dry Density
OMC : Optimum Moisture Content
DRY DENSITY (Yd)
Ydmax
MODIFIED
COMPACTION
Ydmax
STANDARD
PROCTOR
COMPACTION
OMC
OMC
WATER CONTENT % (w)
© BHP RIO TINTO Tailings Management Consortium
58
4.3
Physical Stability
(cont.)
Figure 11: Tailings mixture matrix
before and after compaction
showing the air void reduction that
can lead to a saturated condition.
AIR
WATER
After Compaction
AIR
WATER
After Compaction
Figure 12: Typical filtered tailings
compaction equipment – roller compactor.
© BHP RIO TINTO Tailings Management Consortium
59
4.3
Physical Stability
(cont.)
Figure 13: Schematic illustration of aspects of physical stability
of a filtered tailings stack, including (1) strength and deformation,
(2) volume change behavior, (3) fluid flow characteristics,
(4) unsaturated characteristics, and (5) foundation characteristics.
AS-PLACED TAILINGS (UNSATURATED)
MID-PLACED TAILINGS (TRANSITION
FROM UNSATURATED TO SATURATED)
BASE-OF-STACK TAILINGS
(POTENTIALLY SATURATED)
STACKER
BEARING CAPACITY FAILURE
GLOBAL FAILURE
FACE SLOUGHING
COMPACTION
(IF NECESSARY TO INCREASE INITIAL VOID RATIO)
DOWNWARD DRAINAGE OF PORE WATER
UNDER GRAVITY & COMPRESSION
(UNSATURATED FLOW)
PHREATIC SURFACE
FOUNDATION
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4.4
Chemical Stability
Achieving physical stability through the application of filtered tailings technology
can come at the expense of chemical stability for reactive tailings. Acid mine
drainage occurs when tailings containing sulfide minerals, such as pyrite, are
exposed to water and air. This exposure creates sulfuric acid which can cause
toxic metals to enter and dissolve into the water. Filtering and placing tailings in
an unsaturated state increases the potential for oxidation and the formation of
acid mine drainage compared to conventional wet slurry tailings storage, which
often relies on saturation and water covers to maintain chemical stability.
If improperly managed, impacted contact water can cause environmental and
health issues by altering the quality of surface and groundwater resources
which support environmental or human receptors (Figure 14). With continuing
increases in regulation and external disclosure, such waters also pose a
reputational risk and increase the likelihood of long-term liability retention.
Assessing chemical stability should utilize source-pathway-receptor
methodologies where:
• The source component is informed by geochemical characterization.
• The pathway component is informed by hydrological and hydrogeological
assessments.
• Receptors include the surrounding environment, human health, water
resources, reputational risk, and closure objectives.
Geochemical testing and classification results help to estimate whether
tailings contact water is likely to generate acid or leach metals. Initial material
classification is carried out using the results from acid base accounting and
net acid generation testing, and the resulting classifications should then be
refined following the completion of more detailed testing. Recommended key
geochemical characterization is summarized in Chapter 2.
Figure 14: Example of acid mine
drainage.
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4.4.1
Geochemical Classification
Results from geochemical characterization programs (summarized in
Chapter 2) are used to classify materials tested. In addition to being a useful
way to categorize different tailings streams, classifications will typically be
requested by regulatory bodies. Two key generic classification systems
are typically used to evaluate the preliminary acid forming potential of Acid
Metalliferous Drainage source material: the South Pacific or AMIRA method
(AMIRA, 2002) and the North American, or MEND method (Price, 2009).
The naming conventions used in each system are summarized further in this
chapter in Table 9 and Table 10 and the classification systems are included
in Table 11 and Table 12. Both methods identify materials which are likely
to generate acidity; however, site specific classification systems should be
developed for each operation once additional phases of detailed geochemical
testing have been completed.
Testing and classification help estimate whether contact water, which may be
considered as acid mine drainage, may be generated by the filtered tailings
stack. In addition to acid mine drainage, impacted contact water can also
include high total dissolved solids drainage, containing high metals or salinity
in non-acidic waters, and may be described as Neutral Metalliferous Drainage
and/or Saline Drainage.
Definitions in the literature vary, however the following criteria may be applied
as a guide:
• Acidity – A boundary of pH 6 is typically utilized (Acid Metalliferous Drainage
< pH6 < Neutral Metalliferous Drainage/Saline Drainage).
• Sulfate Content – Typically, Acid Metalliferous Drainage and Saline Drainage
have sulfate concentrations > 1,000 mg/L where Neutral Metalliferous
Drainage is < 1,000 mg/L.
• Metal Content – The term “metalliferous” includes metals, metalloids
(e.g., arsenic) and non-metals (e.g., selenium). Metalliferous concentration limits
are not typically defined; however, any drainage with concentrations elevated
with respect to site background/ baseline (i.e., pre-mining) concentrations
may be considered as potentially being problematic, particularly when being
assessed by regulatory organizations.
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4.4.1
Geochemical Classification (cont.)
Table 9: South Pacific Naming Conventions.
SOUTH PACIFIC CONVENTION
UNITS
Maximum Potential Acidity = 30.6 × sulfur (wt%)
kg H2SO4/t
Acid Neutralization Capacity
kg H2SO4/t
Net Acid Production Potential (=Maximum Potential Acidity – Acid Neutralization Capacity)
kg H2SO4/t
Acid Neutralization Capacity / Maximum Potential Acidity
No units
SOUTH PACIFIC
Table 11: South Pacific (AMIRA) material classification method
CLASSIFICATION
CRITERIA
COMMENTS
Potentially Acid
Forming
Net Acid Producing Potential > 0
Sample always has a significant sulfur content, the acid
generating potential of which exceeds the inherent Acid
Neutralizing Capacity of the material.
Net Acid Generation pH < 4.5
Net Acid Producing Potential < 0
Non-Acid Forming
Net Acid Generation pH ≥ 4.5
Net Acid Producing Potential > 0
Uncertain
Net Acid Generation pH ≥ 4.5
Net Acid Producing Potential < 0
Net Acid Generation pH < 4.5
© BHP RIO TINTO Tailings Management Consortium
Sample may, or may not, have a significant sulfur
content but the Acid Neutralizing Capacity availability
is more than adequate to neutralize the acid that could
be produced.
An uncertain classification is used when there is an
apparent conflict between the Net Acid Producing
Potential and Net Acid Generation results. Uncertain
samples are generally given a tentative classification that
is shown in brackets e.g. Uncertain (Non-Acid Forming).
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4.4.1
Geochemical Classification (cont.)
Table 10: North American Naming Conventions.
NORTH AMERICAN CONVENTION
UNITS
Acid Potential = 31.25 × sulfur (wt%)
kg CaCO3/t
Neutralization Potential
kg CaCO3/t
Net Neutralization Potential (=Neutralization Potential – Acid Potential)
kg CaCO3/t
Neutralization Potential Ratio (=Neutralization Potential / Acid Potential)
No units
NORTH AMERICA
Table 12: North American (MEND) material classification method
CLASSIFICATION
CRITERIA
COMMENTS
Potentially Acid
Generating
Neutralization Potential /
Acid production Potential < 1
Potentially Acid Generating material, unless sulfide
minerals are non-reactive, or Acid Neutralizing Capacity
is preferentially exposed on surfaces.
Non-Potentially
Acid Generating
Neutralization Potential /
Acid production Potential > 2
Non-Potentially Acid Generating material, unless
Acid Neutralizing Capacity is insufficiently reactive,
extremely reactive sulfides are present, or preferential
exposure of sulfides is found in the material.
Uncertain
1 < Neutralization Potential /
Acid production Potential < 2
Possibly Potentially Acid Generating if Neutralization
Potential is insufficiently reactive or is depleted at a
faster rate than sulfides.
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4.5
Water
Management
While filtered tailings stacks do not require the maintenance
of a reclaim water pond, surface and seepage water
management are still required to maintain the physical
and chemical stability of the facility. A conventional tailings
storage facility water management system is shown in
Figure 15 and a filtered tailings storage facility water
management system is shown in Figure 16. Note the
difference in pond water systems between the two tailings
storage designs.
Figure 15: Conventional wet
slurry tailings facility schematic
(adapted from MEND, 2017).
DISCHARGE
WATER
TREATMENT
MAKEUP
WATER
POND RECLAIM
SURPLUS
POND WATER
UNDIVERTED
SURFACE RUNOFF
TAILINGS
DEPOSITION
DIRECT
PRECIPITATION
POND
EVAPORATION
TAILINGS
BEACH
TAILINGS
DEWATERING
DEWATERING
RECLAIM
SURFACE
EVAPORATION
TAILINGS
POND
TAILINGS FROM
PROCESS PLANT
PROCESS
PLANT
SEEPAGE RECLAIM
TAILINGS
DAM
WATER ENTRAINED
IN TAILINGS
POND
EVAPORATION
SEEPAGE
RECOVERY
DAM
UNDIVERTED
SURFACE
RUNOFF
MAJORITY
OF WATER
MANAGEMENT
CAPTURED
SEEPAGE
NON-CONTACT WATER
CONTACT WATER
TAILINGS
© BHP RIO TINTO Tailings Management Consortium
UNRECOVERED
SEEPAGE
SEEPAGE
RECOVERY POOL
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4.5
Water
Management
(cont.)
Considerations for water management include:
• The design should include developing a surface water management plan.
For more detailed design phases, a detailed surface water management plan
should be developed as part of the Operation and Maintenance plan of the
filtered tailings stack.
Surface and seepage water
management is site-specific, and
the design philosophy must be
evaluated during the design phases.
Understanding the site climate is
a critical component of successful
water management. Considerations
for water management at wet and
dry climates are provided below.
• Positive surface grading is critical to avoid surface water ponding on the
surface of the filtered tailings stack which can lead to increased seepage,
stability impacts through rising phreatic levels or difficulties in equipment
trafficability.
• Uncompacted stack slopes can erode quickly and form large gullies due
to rainfall and surface water runoff, and should be considered in stack and
surface water plans.
• Seepage modeling, which considers transient degree of saturation
conditions, can be used to assess seepage conditions during the design
stage and then further calibrated and updated during operations.
Figure 16: Filtered stacked tailings facility
schematic (adapted from MEND, 2017).
DISCHARGE
MAKEUP
WATER
WATER
TREATMENT
UNDIVERTED
SURFACE RUNOFF
TAILINGS
DEPOSITION
DIRECT
PRECIPITATION
TAILINGS
DEWATERING
FILTERED
TAILINGS PILE
TAILINGS FROM
PROCESS PLANT
DEWATERING
RECLAIM
SURFACE
EVAPORATION
SURPLUS POND
WATER
TAILINGS
SURFACE
RUNOFF
WATER ENTRAINED
IN TAILINGS
PROCESS
PLANT
UNDIVERTED
SURFACE
RUNOFF
POND
RECLAIM
POND
EVAPORATION
RUNOFF
COLLECTION
DAM
RUNOFF COLLECTION
& TREATMENT POND
MAJORITY
OF WATER
MANAGEMENT
CAPTURED
SEEPAGE
NON-CONTACT WATER
UNRECOVERED
SEEPAGE
CONTACT WATER
TAILINGS
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4.5.1
High Rainfall Areas
Stormwater management strategies may be challenging for filtered tailings
stacks in wet climates where annual rainfall exceeds annual evaporation, and
high-intensity storm events occur. Runoff from stacks in high rainfall areas is
likely to be high in suspended solids, and external water collection ponds are
required to collect and manage silt-laden runoff water. Stormwater drains may
also be required on the upper surface of the stack to convey 'dirty' water that
has come into contact with tailings. The surface water geochemistry may be
of poor quality and could require water treatment before discharging to the
downstream environment. A water balance is required to estimate the capacity
of the filtered tailings stack to absorb and evaporate the transient pond formed
after seasonal and extreme rainfall events.
Uncompacted stack slopes in high rainfall areas can erode quickly and form
large gullies. It is recommended that these areas be rolled-compacted to
minimize erosion. Sealed and compacted tailings are relatively erosion resistant
except in heavy rain and/or concentrated surface water flows.
4.5.2
Arid and Semi-Arid Regions
Operational experience with filtered tailings stacks in arid and semi-arid climatic
regions has shown that operational access and trafficability issues can be
addressed through surface water management controls, however seepage
from the tailings remains an important consideration. In addition to improve the
strength of the tailings, compaction will also reduce the hydraulic conductivity of
tailings and reduce infiltration.
Consolidation or desiccation may result in cracking on the upper surface of the
stack (Figure 17) which can concentrate surface water flows and lead to erosion
issues. Continuous monitoring is required, and cracks should be filled with
tailings or suitable fill to prevent water ingress that may develop sinkholes and
internal erosion.
Figure 17: Cracking of tailings
upper surface.
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4.6
Design
Considerations
4.6.1
Tailings Compaction
and Zonation
Filtered tailings may be placed in an unsaturated state and compacted to
achieve dilative behavior, and avoid the risk of flow liquefaction. Compaction
also provides the following benefits:
• Increased shear strength
• Improved filtered tailings stack trafficability
• Increased erosion resistance
• Reduced settlement potential
• Reduced hydraulic conductivity and potential for surface water infiltration
As a result, many filtered tailings stacks are constructed by placing filtered
tailings in relatively thin lifts and compacting them with typical earthworks
compaction equipment. However, this process can have significant impacts
on the feasibility of stacking due to the costs and operational constraints
associated with placing and compacting filtered tailings in thin lifts. As a result,
approaches to limit filtered tailings compaction have been considered. These
include placing the filtered tailings in zones, where a compacted structural zone
is placed to stabilize the adjacent non-structural zone (Figure 18). The nonstructural zone may constitute tailings with higher moisture contents and lower
compaction specifications. Thus, the non-structural zone can provide greater
operational flexibility and lower cost tailings storage. The structural zone can
be constructed similarly to a dam and raised using downstream, centerline, or
upstream methods of construction. The appropriate design is a function of the
performance objectives for the project and site-specific characteristics such as
climate, seismicity and the tailings shear strength.
Figure 18: Filtered stacked tailings
structural and non-structural zone
schematic.
COMPACTED
STRUCTURAL ZONE
NON-STRUCTURAL ZONE
(LOW TO NO COMPACTION)
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4.6.1
Tailings Compaction
and Zonation (cont.)
Alternatively, filtered tailings stacking with less stringent compaction and
moisture content specifications and no separate structural zone may also be
feasible to reduce costs. Key considerations include:
• Siting the facility in a location that supports the adoption of less restrictive
performance objectives.
• Understanding the potential behavior and consequences of facility failure of
the facility.
• Harnessing site characteristics such as atmospheric drying to reduce the
tailings moisture content and utilizing tailings transport/stacking equipment to
provide incidental compaction.
• Test fills, predictive engineering analyses, and surveillance during
construction are key to developing and assessing compliance with
performance objectives.
• Determining the seismic velocity ratio. The seismic velocity ratio
(compressional wave velocity (Vp) / shear wave velocity (Vs)) is used to
assess stability, specifically to assess unsaturated zones.
4.6.2
Filtered Tailings
Stacking Over
Conventional Facilities
In some instances, such as to reduce costs or environmental permitting issues,
it may be advantageous to construct a filtered tailings stack within an existing
conventional slurried tailings storage facility. This is known as “piggybacking”
and understanding the ground foundation in these cases is important as it is
likely weak and low strength. The following is required before placing filtered
tailings on an existing tailings facility:
• Undertaking a detailed geotechnical investigation, including Piezo Cone
Penetration testing, Seismic Cone Penetration testing, vane shear, etc.
• Retrieving high-quality samples of the existing deposited tailings (future stack
foundations) during geotechnical investigation for laboratory testing.
• Carrying out a comprehensive laboratory testing program to determine the
in-situ state of the tailings in the existing tailings facility that will form the
foundation for the filtered tailings stack.
• Undertaking the appropriate level of slope stability assessment that may
include limited equilibrium and more detailed deformation analyses to
determine if the existing tailings facility will provide short and long-term
stability under static and post-seismic loading conditions.
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4.6.3
Water Management
Considerations
Design considerations for filtered tailings stack water management include:
• Laboratory tailings characterization results should be reviewed to assess the
potential for re-saturation during rainfall or snowfall events.
• Surface water diversions should be designed upstream of the stack to
prevent 'clean' runoff water from entering the footprint of the stack, and to
reduce 'contact water' volumes that may require treatment prior to discharge
to the environment.
• A grading plan should be developed and maintained during operations to
avoid ponding of surface water on the stack and directing surface water flows
to erodable edges of the stack. Roller compaction of surfaces is useful to
reduce infiltration and erosion.
• If mobile stacking equipment is used on the filtered tailings stack, the active
stacking area should be shaped to shed water to assist with trafficability of
the equipment.
• Safety windrows/bunds are required along the edges of the stack. When
surface water management requires conveying runoff water from the
upper surface to the ground surface, stormwater discharge chutes must be
designed and appropriately lined (preventing erosion) to convey the peak
storm discharge volumes. Silt interception structures/areas are needed to
manage silt-laden runoff water.
4.6.4
Geochemical
Considerations
Although seepage from filtered tailings stacks may be less than from traditional
slurry deposited tailings, weathering of minerals such as pyrite may be
enhanced due to a greater abundance of oxygen in pore spaces.
Key considerations related to this are:
• Oxidizing conditions are greatest on the surface of the facility and this zone
is most likely to generate contact water which may lead to impacted runoff/
seepage water quality.
• Sulfide oxidation rates may be inhibited deeper within the tailings mass due
to oxygen consumption in outer layers of sulfidic tailings.
• Reducing conditions may develop at depth within the pile, limiting sulfide
oxidation rates.
• Contact water quality is influenced by reaction rates of various minerals and
the availability of water to transport these contaminants.
As the surface zone is most likely to be a source of generation of contaminants
through sulfide oxidation, options to minimize the facility surface area open at
any one time should be considered. Although it is noted that this is often difficult
for large facilities, depending on stacking method, where possible, progressive
closure may help to inhibit oxidation rates and/or flushing of tailings surfaces by
contact water.
Additionally, it is often possible to optimize placement and compaction methods
to reduce the hydraulic conductivity and therefore reduce the infiltration through
and seepage from tailings because of storm events. Parameters for different
construction scenarios may be built into hydrochemical modelling to assist with
optimization of operational methodology.
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4.7
Operational
Considerations
Operational considerations associated with filtered tailings stacks include:
• Careful planning is needed to determine how to manage out of specification
tailings or upset conditions at the stack to avoid impacting upstream
operations. Out of specification tailings may be generated due to changes in
the ore body characteristics or upset filter plant conditions while the ability
to access the stack may be impacted due to rainfall and poor surface water
management. Potential approaches to mitigate these conditions include
construction of a smaller conventional slurry storage facility to store out of
specification tailings, the use of structural and non-structural zones within
the filtered tailings stack, ripping wet tailings to allow for atmospheric drying,
and/or the use of additives such as cement to reduce the tailings moisture
content and/or strength.
• The allowable tailings moisture content at the filter plant may be higher than
the allowable moisture content at the stack due to drying during transport
and placement in arid climates.
• Dust generation from the tailings surface (Figure 19) can be a significant
concern and can impact the social license to operate the facility.
• The loading from tailings transport and placement equipment operating on
the stack surface and the loading from overlying stacked tailings should be
considered as a potential source of tailings compaction.
• The design should account for the site-specific climatic conditions. In arid
climates, it may be feasible to rely on evaporation to achieve required
moisture contents at the placement location while increasing the allowable
moisture contents at the filter plant. This may require a larger stack area to
allow for evaporation to occur.
• Operational practices should also account for seasonal variations in climate.
• Construction of a storage shed to ensure continued production during adverse
weather events that inhibit transportation, placement and compaction.
Figure 19: Dust generation from
haulage roads and stack ramps.
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4.8
Recommended
Tasks by Study
Level
Engineering analysis and characterization are critical components in successful
filtered tailings stack design. These tasks will vary by study level.
Preliminary filtration assessment focuses on flow sheet development and not
filtered tailings stack design.
Order of Magnitude and Prefeasibility Studies - The following should be
considered during this stage:
• A geotechnical engineering team with experience in filtered tailings stack
design should be engaged in the assessment of geotechnical stability.
• Complete the tailings geotechnical testing outlined in Chapter 2 for order of
magnitude and prefeasibility level studies.
• Geotechnical testing should be conducted on tailings produced from a pilot
operation or as close to the planned operations as possible to generate
relevant engineering parameters for design considerations. If representative,
site-specific tailings are not available, then data can be harvested from similar
operations to support the prefeasibility study and geotechnical testing moved
to the feasibility study.
• Conduct preliminary investigations and studies to characterize the site. These
may include field geologic mapping and climate studies and summarizing
tailings characteristics from completed testing.
Consider the following engineering analysis and assessments:
• Develop preliminary design basis and performance objectives for the filter
stack.
• Identification of potential failure modes and risk assessments to identify
controls and inform the instrumentation and monitoring plan.
• Slope stability analyses for both local and global stability. This should include:
• Evaluating bearing capacity to assess trafficability for transport and
placement equipment, local stability assessment to determine achievable
lift thicknesses, tolerable rate of rise, and stacker setback requirements.
• Assess overall stability of stack during and after construction to identify
placement conditions (e.g., slope angles, benches, number of lifts, overall
height, etc.).
• Assessment of the consequence of tailings facility failure. A rule of thumb
is that the filtered tailings stack will deform (slump) 10-times the stack
height during failure (KCB 2017).
• Develop tailings placement requirements based on analysis results and
site characterization and stack design data.
• Seepage modeling to assess unsaturated and saturated flow during and
after operations.
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4.8
Recommended
Tasks by Study
Level (cont.)
Feasibility Study - During this stage, the assumptions made and gaps identified
in the prior study stages should be addressed. Consider the following:
• Detailed (feasibility level) characterization of foundation conditions through
subsurface investigations and associated field and laboratory testing.
• Additional confirmatory tailings testing to address identified threats.
• Operational documents such as Operation, Maintenance and Surveillance
(OMS) Manual, surface water management plan, and trigger action response
plans (TARPs).
• For brownfield operations, it may be possible to conduct field scale testing to
evaluate densities and trafficability achieved during placement, compaction,
and stacking to confirm tailings and process design parameters.
• Update engineering analyses to incorporate newly acquired information.
• Perform additional engineering analyses such as considering multiple
seepage and stability analysis sections and more advanced analysis such as
finite element deformation analyses.
© BHP RIO TINTO Tailings Management Consortium
5.
Material
Transport
& Stacking
APPROACH
74
Site Closure
Filtered
Tailings
Stack
Material
Transport
& Stacking
Tailings
Dewatering
Concentrator
Mine
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5.1
Key Points
The placement and geometry of the filtered tailings stack
should be considered to assess the environmental and
cultural impacts, optimize the costs (capital and operating),
and provide operational flexibility.
Compatible tailings transport, deposition and conditioning
equipment should be assessed and combined to develop
flow sheet options for comparison through trade-off studies.
The scale of materials handling can be similar to the
mining operation, (particularly in copper production)
therefore, the impact of cost is significant and requires
significant attention to optimize. In some applications,
these costs can be higher than the dewatering costs.
© BHP RIO TINTO Tailings Management Consortium
© BHP RIO TINTO Tailings Management Consortium
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5.2
Introduction
Transport of filter cake starts at the point it is loaded onto
the system that will carry it from the filters to the filtered
tailings stack.
Filter cake from the filter plant can be deposited onto a temporary stockpile
from which it is reclaimed by the transport system, or it can be fed directly
into the system. The interface between transport and deposition is dependent
on the transport system and the stacking method; thus, it cannot be clearly
delineated. The completion of the process is when no more machinery is
needed to engage with the material in place. The design of this system must
be made in conjunction with the filtered tailings stack design.
There are many combinations of technologies and operating modes for the
transport and stacking of filtered tailings, and each technology and mode
have strengths and weaknesses to consider when designing the system. The
selection of technologies and operating modes is dependent on many factors,
including filter cake moisture, daily tonnage rate, distance to stack, topography,
climate, and stack design. Through careful analysis of the needs and constraints
of a given site, transport and stacking strategies can be selected to yield the
best compromise of cost, tolerable risk, effectiveness, and safe operation.
Considerations should also be made for automation to reduce risks to
personnel, and for electrical-powered equipment to reduce carbon emissions.
It is recommended to engage an automation specialist to optimize.
The transportation of filtered tailings is very site specific. Factors such as material
properties (which is dominated by the moisture of the material), climate conditions,
stack design (including topography, seismic conditions, lift height, and total stack
height), and trafficability of the material greatly affect the selection of the material
handling system.
Sites with higher tonnages (greater
than 15,000 tonne per day) and
regular shaped filtered tailings
stacks (such as rectangles, triangles
and circles) can lend themselves
to conveyance and large mobile
stackers.
Sites with low tonnages
(less than 15,000 tonne per day)
and have complex shapes, such as
multiple small valley fills, can lend
themselves toward mobile trucks.
Hybrid solutions may use conveyors
for long distance haulage of filtered
tailings and trucks for final placement.
Finally, the filtered tailings may need
to be conditioned after deposition,
such as spread into thin lifts,
evaporatively dried, and compacted.
All these issues must be evaluated and addressed to
design a safe, cost effective, reliable, material handling and
stacking system.
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5.3
Design
Parameters
Key parameters for tailings material transport and stacking
design include:
• Tailings filter cake properties
• Tailings production volume/tonnage
• Distance and topography between filter cake production and placement
• Climate
• Construction requirements
• Access.
There is no default design choice that is appropriate for all applications and
the designer will need to consider the whole system and not a collection of
individual components.
5.3.1
Tailings Filter Cake
Properties
The properties of the tailings filter cake and their expected ranges will influence
both the transport and placement strategies, with the moisture content being
the largest factor. Wet filter cake at moistures above its Flow Moisture Point can
be more difficult to transport, and the filtered tailings stack design may benefit
from trucks which can be more accommodating of wetter material. The tailings
particle size distribution will also influence the placement strategy. Finer material
may be more difficult to dewater, leading to higher moisture contents (which
are more difficult to traffic); therefore, the stacking method must allow for the
tailings angle of repose, and the load bearing capacity (both consolidated and
unconsolidated) will constrain the type of equipment that can be used to place
filter cake at the stack.
The number of transfer points filter cake passes through can reduce its moisture
content and increase dust generation, and should be considered in the material
handling system design.
Correct design of transportation equipment is vital for maintaining the availability
required for production. Equipment that is undersized or not adequate for
handling potentially wet material will affect production capabilities. Equipment
design is typically done based on data gathered from lab scale equipment which,
if done correctly, produces good results that have been proven to scale up to full
scale equipment. Data used includes angle of repose, surcharge angle (which
may decrease as the material is transported), bulk density (which may increase
as the material is transported), and minimum chute angle (which is dependent on
the chute construction material). Chapter 2 described these tests.
5.3.2
Tailings Production
Volume
© BHP RIO TINTO Tailings Management Consortium
The transport and stacking system must be scaled to accommodate the volume
of material generated. Certain transport technologies (e.g., conveyors) are
better suited to higher volumes, while others (such as trucks) are better suited to
smaller volumes. Additionally, other factors need to be taken into consideration
as mobile equipment may be better suited overall for the specific application.
79
5.3.3
Distance and
Topography
The distance that the tailings filter cake needs to be transported influences
the transport strategy. Longer distance increases the time to deliver material
to its intended destination. Trucks and mobile equipment must complete a
full cycle to move material. At the start of the cycle the vehicle is laden with
tailings, then after the material is discharged the vehicle must return the same
distance unladen to complete the cycle. Thus, for long hauls, trucks can be at
a disadvantage compared with conveyors.
The site topography greatly influences both the design of the filtered tailings
stack and the route to transport the tailings from the filters. If a site is relatively
flat and unconstrained by natural or man-made boundaries, the stack designer
can adopt a simple design. However, for complex topography with hills and
steep terrain (possibly compounded by other boundary constraints), the stack
designer may be forced to adopt more novel transport and deposition strategies,
often involving mobile equipment for its flexibility.
5.3.4
Climate
Arid climates are more forgiving as the final moisture content of the tailings filter
cake can be lowered at the filtered tailings stack using atmospheric evaporative
drying. Unfortunately, arid climates are also prone to dust generation and the
selection of equipment can impact significantly how much dust is generated.
Wet climates with significant precipitation can pose a challenge for tailings
management as the moisture level of the filtered tailings can be increased
in-situ. It will be important for the stack designer to consider water shedding
designs and drainage. Those sites with harsh winter conditions may have to
deal with snow and freezing, affecting both transport and deposition strategies.
Conveyors transporting wet material in freezing conditions can have issues,
particularly covered conveyors due to condensation and freezing of moisture
from the filter cake. As discussed in Section 4.7, a storage shed may be needed
to ensure continued production during adverse weather events.
5.3.5
Construction
Requirements
The filtered tailings stack design may require thin layers, called lifts, for
compaction or enhanced evaporative drying. The traditional deposition methods
used in other processes, such as heap leach pile construction, yield thick layers
of 10 m or more that are not amenable to compaction. Thin lifts are expensive
and may require different deposition equipment than is traditionally used for
other high tonnage applications.
5.3.6
Access Roads
and Ramps
The development of access roads and ramps for mobile equipment is an
on-going process as the stack is constructed over the life of operation. Access
roads and ramps are periodically incorporated into the stack, so their locations
should be well-planned to ensure minimal impact on the design and future lifts
of the stack. Stack access roads should be demarcated to reduce unnecessary
traffic and dust emissions.
© BHP RIO TINTO Tailings Management Consortium
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5.4
Tailings Loading,
Transport,
Deposition
and Conditioning
Equipment
LOADING
The movement of filtered tailings from the filters to their ultimate location at
the stack can be broken down into three steps; the transportation from the
filters to the stack location prior to discharge onto the stack; the deposition,
or discharge, of the filter cake; and finally, any post deposition conditioning,
which is any work performed on the filter cake after discharge from the final
transportation equipment.
There is a range of equipment that can transport, deposit and condition filter
cake. Each class of equipment has advantages and disadvantages that must be
weighed against the needs and constraints of the site. Table 13 shows the main
advantages and disadvantages of each major type of equipment.
ADVANTAGE
DISADVANTAGE
INDICATION OF COSTS
Lower operating costs.
Not suitable for large tonnages due to size.
Need liners and steep walls to reduce plugging and sticking issues.
DIRECT LOADING WITH
BINS AND HOPPERS
Cost increases significantly for tonnages greater the 10 ktpd.
Very flexible.
Maintaining a fleet of mobile equipment requires a workshop to
support them.
Not very suitable for high tonnages.
FRONT END LOADER
OR EXCAVATOR
Low to moderate capital costs and can be mobilized quickly.
They do require regular maintenance.
It is possible to lease this equipment
High volume with single pieces of equipment.
Suitable for large tonnages.
Electrically powered and can be automated.
Not mobile.
Wet material can cause fouling and cause increased maintenance.
STATIONARY APRON FEEDER
Moderate to high cost.
High volume with single pieces of equipment.
Suitable for large tonnages.
Designed so dozers can easily be used to feed the reclaim feeder.
Semi mobile.
Electrically powered and can be automated.
Wet material can cause fouling and cause increased maintenance.
RECLAIM FEEDER
Low to moderate cost.
Table 13: Filtered tailings loading, transport, deposition and conditioning equipment.
© BHP RIO TINTO Tailings Management Consortium
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5.4 Tailings Loading, Transport, Deposition and Conditioning Equipment (cont.)
TRANSPORT
ADVANTAGE
DISADVANTAGE
INDICATION OF COSTS
Well suited to transport material along a fixed corridor of moderate
distance that does not alter and has a long operating life.
Electrically powered and can be highly automated.
Wet material can cause fouling of rollers and the support frame.
If there is a real risk that the filter cake would have higher moisture levels
than planned for, this can reduce the effectiveness and increase the
maintenance demand, negatively impacting on the operating costs.
OVERLAND CONVEYOR
Initially, they require a higher capital investment but have a lower
operating cost.
This equipment is typically not leased.
Well suited for situations that require flexibility and the distance to be
travelled is short to moderate.
Maintaining a fleet of mobile equipment will require a workshop to
support them.
Not suitable for long haul distances as they will be expensive in both
capital and operating costs.
MOBILE EQUIPMENT (TRUCKS
- ARTICULATED, RIGID FRAME,
TRACTOR WITH TRAILER TYPE)
Low to moderate capital cost and can be mobilized quickly.
They do require regular maintenance.
It is possible to lease this equipment.
More accommodating than a conventional conveyor belt when
transporting off-specification material.
Electrically powered and can be highly automated.
Can go up vertically or across valleys.
The batch nature of the system requires a buffer to accumulate material
between batches, which adds the operational cost of placing and pickup up
material again, and the need for space to accommodate the buffer stockpile.
BATCH CONVEYOR (INCLUDING
ARIAL BUCKET CONVEYORS)
Like overland conveyors, they initially require a higher capital investment
and have a lower operating cost.
This equipment is typically not leased.
Can be electrically powered (but are often diesel) and can be highly
automated.
More accommodating than a conventional conveyor belt when
transporting off-specification material.
Only practical for larger volumes of material and longer haul distances.
RAIL HOPPER CAR
Requires a large capital investment but can be justified when transporting
large volumes (approximately 500,000 tonne per day) over long distances
greater than 10 km, or if there are existing rail assets that can be leveraged.
Table 13: Filtered tailings loading, transport, deposition and conditioning equipment.
© BHP RIO TINTO Tailings Management Consortium
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5.4 Tailings Loading, Transport, Deposition and Conditioning Equipment (cont.)
DEPOSITION
ADVANTAGE
DISADVANTAGE
INDICATION OF COSTS
Well suited for depositing 10 to 20 m lifts in a regular shape, such as a
rectangle or triangle, on a flat surface.
Can handle high tonnages at lower operational costs than trucks.
Not suited for placing material in irregular shapes or uneven surfaces.
Like overland conveyors, they initially require a higher capital investment
and have a lower operating cost.
MOBILE BRIDGE STACKER
This equipment is typically not leased.
Well suited for depositing 10 to 20 m lifts in regular and irregular shapes
(more flexibility), on a flat surface.
Can handle high tonnages at lower operational costs than trucks.
In event of unplanned shutdown or equipment failure, multiple individual
pieces of equipment can allow for quick replacement of equipment.
Has many transfer points which can plug and cause operational problems
for wet and sticky material.
Planned equipment movement occurs regularly (sometimes weekly),
so multiple lines are required to maintain high system availability.
Like overland conveyors, they initially require a higher capital investment
and have a lower operating cost.
MOBILE STACKER
This equipment is typically not leased.
If the topography is difficult or the geometry of the filtered tailings stack
design follows an irregular shape, then some form of mobile equipment is
the best choice.
Layers thinner than 5 m but subsequent spreading will still be needed
before compaction.
Maintaining a fleet of mobile equipment will require a workshop to
support them.
Not suitable for long haul distances as they will be expensive in both capital
and operating costs.
Low to moderate capital cost and can be mobilized quickly.
MOBILE EQUIPMENT (TRUCKS)
They do require regular maintenance. It is possible to lease this equipment.
Table 13: Filtered tailings loading, transport, deposition and conditioning equipment.
© BHP RIO TINTO Tailings Management Consortium
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5.4 Tailings Loading, Transport, Deposition and Conditioning Equipment (cont.)
CONDITIONING
ADVANTAGE
DISADVANTAGE
INDICATION OF COSTS
Can level tailings rows or piles to thin lifts required for compaction.
Dozers and graders can be used individual or in combination.
Maintaining a fleet of mobile equipment will require a workshop to
support them.
DOZERS AND GRADERS
Low to moderate capital cost and can be mobilized quickly.
They do require regular maintenance. It is possible to lease this equipment.
For shallow to moderate compaction/depths.
Available with a smooth (drum) roller for sealing the surface of deposited
filtered tailings to reduce ingress of surface water from precipitation.
Maintaining a fleet of mobile equipment will require a workshop to
support them.
ROLLER COMPACTOR AND
VIBRATING ROLL COMPACTOR
Low to moderate capital cost and can be mobilized quickly.
They do require regular maintenance. It is possible to lease this equipment.
For moderate compaction/depths.
Maintaining a fleet of mobile equipment will require a workshop to
support them.
Not typically used at mine sites but could be effective.
Typically found at landfills.
Not good at sealing the surface of the tailings.
New equipment at a mine site which may require operator training.
LANDFILL COMPACTOR
Low to moderate capital cost and can be mobilized quickly.
They do require regular maintenance. It is possible to lease this equipment.
For deep compaction/depths, potentially up to 3 m.
Maintaining a fleet of mobile equipment will require a workshop to
support them.
Not typically used at mine sites but could be effective.
Typically used for road or airstrip construction.
New equipment at a mine site which may require operator training.
Low to moderate capital cost and can be mobilized quickly.
DYNAMIC ROLLER
They do require regular maintenance. It is possible to lease this equipment.
Table 13: Filtered tailings loading, transport, deposition and conditioning equipment.
© BHP RIO TINTO Tailings Management Consortium
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5.4 Tailings Transport,
Deposition and
Conditioning Equipment
(cont.)
There are many combinations of equipment that can be used
to construct a filtered tailings stack, as shown in Figure 20.
While certain transport modes are more complementary to
specific deposition modes, virtually any transport mode can
be mixed with any deposition mode.
To navigate the potential combinations the more common,
or preferred, paths are shown in different colours in Figure 20.
BLUE PATH
The current state of the art for stacking high volumes of filtered tailings
which are not compacted; however, as will be discussed, it is not well suited
to all applications.
GREEN PATH
An alternative better suited to filtered tailings stack with a more complex stack
shape and topography, that require evaporative drying to achieve the moisture
content requirement for compaction.
BROWN PATH
A typical low volume filtered tailings solution.
CHARCOAL PATH
Connects the overland conveyor for the long-distance haul with mobile
equipment for the last section of the haul and deposition of the filter cake, forming
a hybrid solution which may currently be the optimum solution for many projects.
© BHP RIO TINTO Tailings Management Consortium
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CURRENT STATE OF ART FOR HIGH VOLUMES
POTENTIALLY SUITABLE FOR COMPLEX FILTERED TAILINGS STACKS
FILTER
TYPICAL LOW VOLUME FILTERED TAILINGS SOLUTION
HYBRID SOLUTION
PLANT MATERIALS HANDLING
COLLECTION
CONVEYOR
CONVEYOR
TRANSPORT
HOPPER CARS
(RAIL)
BATCH
CONVEYOR
PLANT RADIAL
STACKER
HOPPER CAR
UNLOADER
OVERLAND TRANSPORT
CONVEYOR
REMOTE RADIAL
STACKER
STATIONARY
APRON FEEDER
LOADING
MOBILE
EQUIPMENT
DIRECT FEED
CONVEYOR
DRIVE UNDER
HOPPER
MOBILE
EQUIPMENT
DIRECT FEED INTO
MOBILE EQUIPMENT
ARTICULATED
DUMP TRUCK
FRONT END LOADER
OR EXCAVATOR
RECLAIM FEEDER
WITH DOZER
OFF-ROAD TRACTOR
WITH TRAILER
ROAD TRACTOR
WITH TRAILER
LIGHT RIGID
FRAME TRUCK
PADDOCK TIP
INTEGRATED
SPREADER
EJECTION
SPREADING
HEAVY RIGID
FRAME TRUCK
DEPOSITION
BRIDGE
STACKER
CONDITIONING
DOZER
PLOUGH
DISC WITH ROLLING
BASKETS
DOZER
COMPACTOR
STACKED & COMPACTED
FILTERED TAILINGS STRUCTURE
Figure 20: Transport, deposition,
and conditioning pathways.
© BHP RIO TINTO Tailings Management Consortium
86
5.5
Tailings Transport,
Deposition and
Conditioning
5.5.1
Transport Equipment
Considerations
The moist nature of filtered tailings can cause issues with
conveyor and truck transportation. The higher the free
moisture content, and the stickier the material, the more
issues that can occur.
Conveyors
Poor belt tracking, belt slippage, pulley damage and idler damage caused
by wet and/or sticky material can increase demand for housekeeping and
maintenance. Scrapers and ploughs are commonly used to remove material
stuck to the conveyor belt to reduce these issues, as shown in Figure 21.
Transfer points (places where one conveyor offloads to another conveyor)
are also problematic with wet material as they can become fouled, as shown
in Figure 22. Liners to reduce cake build-up and chute design (including
engineering to minimize cake contact with the chute walls) can minimize this
issue. Vibrators or air cannons can also be employed to potentially discharge
cake adhered to the chute walls.
Figure 21: Belt scraper and plough.
V-PLOUGH
BELT SCRAPER
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5.5.1
Transport Equipment
Considerations (cont.)
Figure 22: Conveyors can become
fouled at transfer points.
Truck Haulage
Truck loading can be an issue, especially for large tonnages. Trucks may be
loaded using a front-end loader; however, this can be an expensive option.
To eliminate the need for a loader a hopper may be used. However, due to the
potentially sticky nature of filter cake (which requires very steep hopper walls to
minimize bridging and sticking) this method is not recommended for large-scale
filtered tailings facilities as the hoppers will be very tall. Self-loading mucking
into a truck is also used at smaller scales but the technology has not been
demonstrated viable on the productivity scale required for large-scale filtered
tailings handling.
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5.5.2
Deposition Equipment
Considerations
Deposition is the discharging of filter cake from the final transport system.
Depending on the type of transport that was used, the deposition produces
different pile shapes.
Trucks
Trucks may be used for transport as well as depositing. Wet sticky material
can cause discharge issues from the bed of the truck, and bed liners and
mechanical ejectors can be used to improve this. If trucks are used as the final
transport system, there are two different types of deposition and pile shapes.
Paddock tipping is the easiest and most straight forward deposition method,
as shown in Figure 23.
Ejection spreading, shown in Figure 24, is an improvement on paddock tipping
as the material is discharged from the carrier while the carrier is moving forward,
partially spreading the material out in thinner lifts. Unfortunately, ejectors are
typically not available on the largest trucks used by mines.
Figure 23: Dump truck paddock dumping.
Figure 24: Articulator truck with ejector.
Trucking options are also limited based on the load bearing capability of the
tailings and thus the pressure that can be exerted on it by mobile equipment
depositing and spreading filtered tailings. Trucks typically have higher load
bearing pressures than mobile conveying stacking equipment. Filter cake can
be analogous to silty sand so depending on its moisture level should be able
to support a vehicle with tracks or flotation tires. In some cases, it may support
quarry truck tires, but heavy mining trucks are unlikely to be supported at higher
moisture contents. The load bearing capacity of the filtered tailings is also a
concern while trucks are tipping, as the raised center of gravity could cause the
truck to roll on its side while tipping. Not tipping the bed, which raises the center
of gravity on a truck and can make it unstable, and using ejectors to discharge
reduces the potential tipping hazard.
© BHP RIO TINTO Tailings Management Consortium
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5.5.2
Deposition Equipment
Considerations (cont.)
Mobile Stackers
When conveyors are used as the final transport system (typically some form
of stacking equipment) rows of filtered tailings are created, as shown in
Figure 25. Again, care must be taken that the ground bearing pressure is
sufficient to support the equipment. However, these stackers can be designed
with much lower ground bearing pressures compared to trucks. Bridge stackers,
one type of conveyor stacking equipment, can be used for high tonnage
applications. They require relatively flat and uniform topography. The bridges
are moved using tracked crawlers and the material is placed with a moving
tripper. Movement of the feeding conveyors is required when the stack gets
too high, or the position needs to be moved. These types of movements occur
infrequently, typically on an annual basis for thick lifts. Movement can take
1 to 2 weeks, and when they occur the system is unavailable. Bridge stackers
require that the deposited material be levelled so that they can lay down the
next layer atop the previous one. Bridge stackers are most efficient depositing
thick lifts.
Figure 25: Bridge stacker.
Figure 26: Portable Conveyor.
Portable conveyors and stackers are tracked or wheeled conveyors that can
be moved as needed, allowing higher flexibility. The discharge of one portable
conveyor feeds the next conveyor, as shown in Figure 26, until the material is
placed at the filtered tailings stack.
One drawback of this type of equipment is its availability. When the conveyors
are moved that portion of the system is down and unavailable. To compensate
for this multiple stacking lines are needed. Movement frequencies can be
as often as daily, for thin lifts, or as long as monthly, for thick lifts. Another
drawback is the large number of transfer points that are required. Every transfer
point is a place where tailings can stick and eventually plug the system. For
wet material, the best transfer point is an eliminated transfer point. Portable
conveyors and stackers are also more efficient depositing thick lifts.
© BHP RIO TINTO Tailings Management Consortium
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5.5.3
Conditioning Equipment
Considerations
Once the filtered tailings are transported and deposited at the stack, additional
conditioning of the tailings may be needed. For example, the tailings may
require evaporative drying to reduce its moisture content (see Chapter 7 for
additional information). The tailings may also need to be leveled, with a dozer
or a grader (see Figure 27 and Figure 28) and compacted to form a dilatant,
structural zone. Or, in a wet environment, the tailings may be compacted with a
drum roller to make the surface of the tailings resistant to the ingress of surface
water (Figure 29).
Compaction
Compaction is the process of removing air and consolidating the tailings such
that it is dilatant, as described in Chapter 4. When compaction is completed
and meets the tailings geotechnical specification, the process can be repeated
using the compacted tailings as the base for the next layer of filter cake to
be deposited.
While some compaction can be completed using the tracks and wheels of
mobile equipment, this is not recommended due to the limited contacting
area of the tracks or wheels with the tailings and that the mobile equipment
is designed with low ground bearing pressures to reduce equipment tipping
and bogging. There are a few variations of specialized compactors, and all of
these can be either self-propelled or a pull type configuration. Selection of the
compaction equipment will depend on the filtered tailings stack design and
geotechnical specification.
Figure 27: Tracked dozer.
Figure 29: Smooth roll compactor.
© BHP RIO TINTO Tailings Management Consortium
Figure 28: Grader.
91
5.5.4
Handling
Off-Specification Tailings
When off-specification (typically wetter than design target)
tailings are produced it will be necessary to have a transport
strategy to deal with it. Figure 30 illustrates wet material
deposition by truck.
Equipment used for sticky, high moisture material must be designed to
discharge the material as completely as possible and to clean any surface in
contact with the wet cake. For transporting wet flowable material in a truck,
extra diligence is needed to avoid unexpected lateral forces that can potentially
move tailings in the bed of the truck and cause vehicle rollover. Deposited
wet tailings will have a decreased ground bearing pressure which can cause
bogging and tipping of mechanical equipment at the stack.
Figure 30: Transporting and deposition
of high moisture filtered tailings.
© BHP RIO TINTO Tailings Management Consortium
6.
Tailings
Dewatering
APPROACH
92
Site Closure
Filtered
Tailings
Stack
Material
Transport
& Stacking
Tailings
Dewatering
Concentrator
Mine
© BHP RIO TINTO Tailings Management Consortium
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6.1
Key Points
Dewatering performance is sensitive to tailings properties,
therefore test work is required to understand the impact
of ore variability over the life of operation. It is best to
understand the complete operating envelope and use ore
blending and a good mine plan to mitigate the effects of
the worst material rather than design for the worst case.
Most tailings applications require a pressure filter to
produce cake moisture contents low enough to allow
construction of a geotechnically stable filtered tailings stack.
Filter maintenance is not insignificant and needs to be
planned and designed for.
© BHP RIO TINTO Tailings Management Consortium
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6.2
Introduction
The key factors impacting
tailings dewatering costs are:
Tailings properties, in particular
particle size distribution and
mineralogy.
Target moisture content range,
based on the transport and
geotechnical requirements which are
related to the tailings’ properties
Selected dewatering technology.
© BHP RIO TINTO Tailings Management Consortium
The purpose of the tailings dewatering circuit is to create
an unsaturated product which is sufficiently dry to be
transported by mechanical conveyor and/or truck and
complies with the geotechnical moisture content specified for
the tailings facility. Different moisture target ranges may be
specified for structural and non-structural zones of the facility.
Tailings are a mixture of process water and solids with particle sizes ranging
from sub-micron to several hundred microns. To separate the solid and liquid
phases, a driving force is required. For tailings applications, forces resulting from
gravity (thickening) and pressure differential (filtration) are utilized. Centrifuges,
which use centrifugal forces for solid-liquid separation, can also be used.
Centrifuges and low-pressure differential filters are not suitable for most filtered
tailings applications as they do not produce a sufficiently dewatered product
which can be compacted without post-processing drying to create a structurally
stable stack.
In most applications, the effectiveness of the tailings solid-liquid separation
process is enhanced through the addition of reagents, such as flocculants and
coagulants. Laboratory tests to select the optimum reagent conditions are
presented in Chapter 2.
95
6.2
Introduction
(cont.)
Figure 31 illustrates that the costs for thickening are much lower than filtration.
Due to the relatively low cost, it is advantageous to primarily dewater the
tailings through thickening. This is followed by filtration to achieve the required
unsaturated tailings product. Finer particle size distributions and some
mineralogy profiles are problematic for solid-liquid separation and increase
costs, as more equipment capacity and energy are required for dewatering.
Figure 31 shows that varying the target cake moisture, even a few percentage
points, can have a significant impact on project costs. As such, in arid climates
there may be economic benefit to achieving the final moisture content through
post-filtration environmental drying on the tailings facility. Also as discussed
in Chapter 4, the non-structural zone of a filtered tailings stack may constitute
tailings with higher moisture contents (while still meeting the requirements for
transport), and this should be assessed in the overall design and economics.
Several technology initiatives are being pursued to reduce the cost of producing
filtered tailings and are discussed in Chapter 7; these include upstream process
changes to produce coarser tailings, improvements to thickener and filter
technology and performance, and post filtration drying techniques.
Figure 31: Impact of particle size and
required tailings residual moisture on
relative dewatering costs. Adapted from:
Svedala BASIC Pump & Process p7:42.
15x
D80 of 120μm
D80 of 80μm
D80 of 30μm
RELATIVE COST
10x
5x
FILTRATION
THICKENING
x
0%
10%
20%
30%
40%
50%
RESIDUAL MOISTURE (%m)
© BHP RIO TINTO Tailings Management Consortium
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6.3
Typical Filtered
Tailings
Dewatering
Flowsheet
A typical filtered tailings dewatering flowsheet is shown Figure 32. The tailings
are first dewatered using thickeners, recovering most of the process water
which is generally returned directly to the concentrator for re-use.
The thickened tailings are then filtered to produce an unsaturated filter cake.
The smaller portion of process water recovered during the filtration process
is termed filtrate. As the filtrate typically contains some solids, it is usually sent
to the thickener feed or a settling reclaim water pond where the solids are
recaptured, maintaining the quality of process water returned to the concentrator.
The thickener and filter plant locations are optimized for each project and are
not necessarily adjacent to each other, so in some cases a slurry pipeline is
required for thickened tailings delivery to the filters.
To achieve the required cake moisture contents needed to allow the tailings
to be compacted to a dilative stage, a batch type filter press is typically required
for tailings dewatering. As the overall dewatering circuit is continuous, filter feed
tank(s) are required to provide buffer capacity between batch and continuous
processes.
Due to their high level of operational reliability, standby thickeners are generally
not required in the flowsheet. As filters have high maintenance requirements,
and an average utilization of between 70% to 80%, filtration standby capacity
must be included in the design; this can be achieved by designing extra
capacity in the operating filters or having one or more full spare filters.
© BHP RIO TINTO Tailings Management Consortium
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PROCESS WATER
CLEAN WATER
FLOCCULANT
MAKE-UP PLANT
FLOCCULANT
TAILINGS
FEED
COLLECTION
BOX
RETURN WATER
THICKENER
COMPRESSED
AIR
PRESSURE
FILTER
COMPRESSED
AIR
FILTER FEED TANK
PRESSURE
FILTER
UNDER FILTER CONVEYOR
UNDER FILTER CONVEYOR
OVERLAND CONVEYOR
DEPOSITION
Figure 32: Typical filtered tailings dewatering flowsheet.
© BHP RIO TINTO Tailings Management Consortium
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6.4
Thickening
6.4.1
Basics
Thickeners are large settling tanks which use gravity
to separate solids and liquid in a continuous process.
The solid-liquid separation occurs in two zones within
the thickener (shown in Figure 33):
• In the upper zone, separation is achieved using gravity. This zone is
further divided into a free settling zone with low solids concentration and
hindered settling zone (where the solids concentration is high enough
that settling particle agglomerates interact with each other, reducing the
settling velocity). The settling rate is a function of the process water viscosity,
density, and chemistry, the particle size, shape, density, and surface charges,
the solids concentration, and the addition of reagents. The clarity of the
supernatant overflow is dependent on the effectiveness of separation in
the free settling zone.
• Below the settling zones, the particle agglomerates form a compact bed
zone with contact between the individual agglomerates. Further solid-liquid
separation is achieved through self-weight compression of the agglomerates
and the shearing action of the rotating thickener rake mechanism which
releases water entrained with the agglomerates. The solids concentration
of the thickener underflow in the compressed bed zone is related to bed
residence time and the design of the rake mechanism.
There are three main rates
associated with thickener
design and their corresponding
thickener underflow solids mass
concentrations are determined
from test work (see Chapter 2):
Solids loading rate ((t/h)/m²) =
solids in feed (t/h)
thickener cross sectional area (m²)
Rise rate (m/h) =
liquid in feed volumetric rate (m³/h)
thickener cross sectional area (m²)
Hydraulic loading rate (m/h) =
total slurry feed volumetric rate (m³/h)
thickener cross sectional area (m²)
SUPERNATANT OVERFLOW
FLOCCULANT
FEED
CLEAR SUPERNATANT
FREE SETTLING ZONE
HINDERED SETTLING ZONE
COMPACT BED ZONE
COMPRESSED BED ZONE
THICKENED UNDERFLOW
Figure 33:
Solid-liquid separation zones in thickener.
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6.4.2
Thickener Technology
Most thickeners are mechanically similar. A diagram
of a typical thickener is shown below in Figure 34.
COMPRESSION DEPTH
Higher bed heights (compact bed zone plus compressed bed zone) allow for
more residence time and higher thickener underflow solids concentrations.
SIDE WALL HEIGHT
Higher tank side walls increase the compression depth and residence time.
RAKE DRIVE TORQUE
The thickener drive applies torque to rotate the rake through the thickened
slurry bed.
FEEDWELL
The feedwell ensures good mixing of flocculant and feed slurry, and even
distribution of produced particle agglomerates over the cross-sectional area
of the thickener.
FEED SLURRY DILUTION
Feed slurry is diluted using clear supernatant to ensure good flocculation.
The amount of dilution required is a function of solids minerology and particle
size distribution.
RAKE ARM AND RAKES
Rakes move the settled slurry bed towards the central underflow discharge
cylinder. The number and style of rakes and rake arms can vary.
PICKETS
Pickets may be attached to the rake, providing channels in the settled bed to
induce water release and further bed compression.
TANK BOTTOM SLOPE
Cone bottom of the tank to help the settled slurry bed flow toward the central
underflow discharge cylinder. For Paste type thickeners the slope is increased.
DISCHARGE CYLINDER
Often mixed to reduce the slurry yield stress, thickened slurry underflow is
pumped out of the discharge cylinder volume.
SUPERNATANT OVERFLOW
SIDE WALL HEIGHT
COMPRESSION DEPTH
RAKE DRIVE TORQUE
FEEDWELL
FEED SLURRY DILUTION
PICKETS
RAKE ARM
TANK BOTTOM SLOPE
DISCHARGE CYLINDER
Figure 34:
Main components of a thickener.
© BHP RIO TINTO Tailings Management Consortium
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6.4.2
Thickener Technology
(cont.)
As shown in Figure 35, there are four basic thickener
categories with differing feed arrangements, geometries,
and mechanisms to suit their process targets.
Conventional thickener designs
Conventional thickeners are traditionally wide diameter tanks operated at slow
settling rates with or without flocculant. Many conventional thickeners have
been retrofitted with modern feedwells and are now operated at higher settling
rates.
High-rate thickeners
High-rate thickeners have feedwells designed to promote efficient mixing of
the feed tailings stream with added flocculant. The addition of flocculant results
in significantly smaller footprints than a conventional thickener, with typically
around 50% of the cross-sectional area required for the same tailings tonnage.
High-density and paste thickeners
High-density and paste thickeners employ the same feedwell technology used in
high-rate thickeners. For a given application they have a similar cross-sectional
area as a high-rate thickener, but produce higher underflow solids concentrations
by utilizing higher sidewall heights to achieve deeper beds, steeper floor slopes,
and specially designed rake mechanisms incorporating vertical dewatering
pickets. The mechanisms are equipped with high-torque drives to accommodate
the rheology of the high-density or paste material in the bed.
High-density thickeners are recommended for most filtered
tailings applications as these thickeners produce high solids
concentration underflow while maintaining a manageable
fully sheared yield stress1 of 40 to 60 Pa.
Higher yield stress values can be problematic during tailings pipeline transport
and filter feeding, and paste thickeners come with a significant increase in
cost, and mechanical and operational complexity. High-density thickeners are
currently available in sizes up to 125 m diameter. It should be noted that some
tailings may have dewatering properties that require a paste thickener to
produce underflow with a fully sheared yield stress of 40 to 60 Pa, and in these
cases paste thickeners should be considered.
Alternative names for high-density thickeners include deep cone (FLSmidth),
deep bed (WesTech), and high compression (Metso).
1
The fully-sheared yield stress refers to the equilibrium yield stress achieved after the slurry sample
has been subjected to shearing action until there is no further change in rheology.
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6.4.2
Thickener Technology (cont.)
Figure 35: Thickener technology classification.
CONVENTIONAL
HIGH RATE
HIGH DENSITY
PASTE
20 to 40 Pa (fully sheared)
20 to 40 Pa (fully sheared)
20 to 100 Pa (fully sheared)
>100 Pa (fully sheared)
30 to 60 Pa (un-sheared)
40 to 80 Pa (un-sheared)
100 to 250 Pa (un-sheared)
>300 Pa (un-sheared)
Diameter (maximum) 185 m
Diameter (maximum) 140 m
Diameter (maximum) 125 m
Diameter (maximum) 55 m
Low side wall height
Floor slope 9.5°
Sidewall 1.8 to 3.2m
Floor slope 9.5°
Sidewall 4.0 to 6.0m
Floor slope 14° to 18°
Sidewall 6.0 to 10.0m
Floor slope 30° to 45°
Because high-density thickeners
operate with a higher underflow
solids concentration, the tailings
rheology is higher compared to
high-rate thickeners. This requires
careful consideration when designing
and selecting the type of rake drive
for these thickeners since they
operate with a higher rake torque.
The Maximum Operating Torque (MOT) is defined as the highest continuous
torque the rake drive will experience during duty. The Normal Operating
Torque (NOT) is the torque that is expected when the thickener is run at normal
operating conditions and is usually assumed to be 25% of the MOT.
Rake Drive Sizing
MOT = k x D² x 14.63
Where:
MOT = Maximum operating torque (Nm)
k = Dimensionless empirical correlating factor, based on correlating imperial units.
D = Thickener diameter (m)
14.63 = Conversion factor from imperial units to metric units.
The k factor is an empirical number and is a function of several factors including
underflow solids concentration, application/material, particle size distribution,
solution chemistry, flocculant type, and rake design. A rough rule of thumb is
that the k factor should be greater than the thickened slurry un-sheared yield
stress measured in Pascals (Pa).
High-rate thickener k factor: 25 < k < 75
High-density thickener k factor: 75 < k < 200
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6.5
Filtration
Filtration involves using a porous medium, such as a filter
cloth, to separate a slurry’s solid and liquid phases.
As shown in Figure 36 a pressure differential is applied across the porous
medium and feed slurry, so that the solids form a cake on the medium while
the liquid passes through the cake and medium to produce filtrate. The source
of the pressure differential varies by filtration technology. Finer particle size
distributions, or problematic mineralogy, require higher pressure differentials
to produce competent filter cake.
6.5.1
Basics
Figure 36: Various types of pressure
differential that can produce filter cake.
POROUS MEDIUM
CAKE FORMATION
TAILINGS SLURRY
FILTRATE
∂P
GRAVITY
SUB-ATMOSPHERIC
SUPER-ATMOSPHERIC
CENTRIFUGAL
CAPILLARY
In copper tailings dewatering applications, the particle size distributions are
typically fine and usually require pressure filter technologies to achieve a
product moisture content that meets specification for compaction to produce a
dilative material (see Chapter 4). Vacuum filters must also be considered if the
tailings are classified into coarse and fine particle streams, this is discussed in
Chapter 7.
Filtration system design is driven by two factors, process factors which look at the
flow mechanisms of the slurry itself as it forms the cake, and equipment factors
that consider the equipment design to achieve the pressure differential, operating
mode, feed rate, and filter type. The process factors are tailings material specific,
whereas the equipment factors are driven by the selected technology.
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6.5.2
Filtration Technology
Figure 37 shows some example filtration technologies and their operating
windows. Dewatering screens use gravity and vibration to dewater coarse
sandy materials in the +0.10 mm particle size range, whereas vacuum filters
are suitable for dewatering materials with a particle size P80 of greater than
0.01 mm using sub-atmospheric pressure. Pressure filters are required for
materials with P80 values below 0.01mm or with high clay content and use
higher super-atmospheric pressure differentials.
PRESSURE FILTER
SuperAtmospheric
SOLID BOWL CENTRIFUGE
Centrifugal
SubAtmospheric
VACUUM FILTERS
VIBRATORY CENTRIFUGE
DEWATERING SCREEN
100
10
1
Gravity
0.1
0.01
DEWATERING PRESSURE DIFFERENTIAL ∆P
Figure 37: Tailings filtration technologies
and typical range of application.
0.001
PARTICLE SIZE (mm)
Currently the largest vacuum belt filter has 300 m² filtration area and applies
a maximum pressure differential of 85 kPa, compared to vertical hanging
plate pressure filters that can have over 2,000 m² of filtration area and apply
a pressure differential of 1,500 kPa or more. Orientating filter plates vertically
allows for a high number of plates in a single filter, whereas horizontal plates
stacked vertically are mechanically limited by the height of the stack.
The combination of high filtration area and high-pressure differential makes
vertical plate pressure filters especially effective for large tonnage tailings
applications. The remaining filter types, such as dewatering screens, and
vacuum filters, may be cost effective for certain low-tonnage or coarse particle
applications. Vertical plate pressure filters are complex machines. Each vendor
and filter model have unique features, but many components are common and
are shown in Figure 38.
Original equipment manufacturers are
currently developing or trialing larger
pressure filters to meet the copper tailings
market demand.
© BHP RIO TINTO Tailings Management Consortium
DIEMME FILTRATION
METSO
FILTER MODEL
GHT5000F DOMINO
Larox FFP3716
FILTRATION AREA
Approximately 2,850 m²
Approximately 1,980 m²
MAX PRESSURE
15 bar
16 bar
PLATE SIZE
4,120 mm x 4,850 mm
3,700 mm x 2,700 mm
NUMBER OF CHAMBERS
140
120
FEATURES
Recessed chamber or
membrane plate
Recessed chamber or
membrane plate
LEVEL OF MATURITY
Full-scale unit installed in
Peru copper tailings trial
since end of 2022.
Full-scale unit being
fabricated, no installations.
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6.5.2
Filtration Technology (cont.)
Figure 38: Main components of a vertical plate pressure filter.
FILTER CLOTH SUPPORTS
CLOTH WASH HEADER
CLOTH SHAKER MECHANISM
SIDE BEAM
FILTER PLATES
HYDRAULIC CLOSING CYLINDERS
DRIP TRAYS
FILTER PLATES
The filter plates are individually oriented vertically and stacked in a horizontal
direction. As a stack, they hold the filter cloth and membrane (optional) to
create a stack of cavities called filter chambers. The chamber thickness is
equal to the cake thickness.
FILTER PLATES WITH MEMBRANE
(OPTIONAL)
Inflates with compressed air or water to compress the cakes within the filter
chambers (see Figure 39).
FILTER CLOTH SUPPORTS
Holds the filtration media that stops the solids and lets water pass through.
CLOTH SHAKER MECHANISM
Shakes the cloth to remove large clumps of cake stuck on the filter media.
CLOTH WASH HEADER
Uses water to remove small pieces of cake stuck on the filter media.
HYDRAULIC CLOSING CYLINDERS
Opens, closes, and compresses the filter plate pack.
SIDE BEAM OR OVERHEAD BEAM
Holds the plate pack, filter cloth, and other components of the filter.
DRIP TRAYS OR BOMB BAY DOORS
Directly underneath the filter, these doors catch water drips that occur during
filtration. These doors open during cake discharge.
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6.5.3
Vertical Plate Pressure
Filter
There are two main types of vertical plate pressure filters:
membrane plate and recessed chamber. Both types of filters
incorporate a chamber fill and cake form step, and a cake
air blow dry step.
Membrane plate pressure filters have an inflatable membrane filled with
compressed air or water that squeezes the filter cake already formed in the
chamber (Figure 39). This membrane remains engaged during the air blow step,
before deflating for cake discharge. A membrane press step is often required
for the more difficult-to-filter applications, such as fine particle size distributions
or high clay content.
Recessed chamber filters do not have a membrane but are otherwise similar to
membrane plate pressure filters, as shown in Figure 40.
For both types of vertical plate pressure filters the more difficult a material is
to dewater, the thinner the cake and therefore the thinner the required filter
chambers are. Typical chamber thicknesses range from 25 mm to 60 mm.
Figure 39: Membrane plate press filter
filtration steps.
OPEN
CLOSED
FORM
STEP
PRESS
STEP
Figure 40: Recessed chamber filter
filtration steps.
OPEN
CLOSED
FORM
STEP
AIR BLOW
STEP
© BHP RIO TINTO Tailings Management Consortium
AIR BLOW
STEP
DISCHARGE
DISCHARGE
109
Filter press plates and cake discharge.
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6.5.3
Vertical Plate Pressure
Filter (cont.)
Figure 41: Typical pressure filtration cycle.
TOTAL
CYCLE TIME
10.5MIN
All pressure filters operate in batch mode. The time required for each batch
(start to start) is called the filter cycle. For a given filter size, the more batches
per hour the filter can achieve, the higher the production rate. The filter cycle
can be broken down into discrete steps, categorized as filtration steps and
mechanical steps. Filtration steps are when filtrate is being produced, and
these tailings material specific steps can be defined from laboratory test work.
Mechanical steps are required for the filter to operate, such as opening and
closing the plate pack. These steps are specific to the filter model and vendor
and are not dependent on tailings material characteristics.
FILTRATION
5MIN
MECHANICAL
5.5MIN
0.5MIN
CHAMBER FILL
Slurry feed is pumped into the filter and fills the empty chambers. The fill rate
is constrained by a pumped slurry feed velocity at which above plate wear
rates are significantly increased.
2MIN
CAKE FORM
Slurry feed continues to be pumped into the filter. Cake builds on the filter cloth.
The form rate can vary with chamber thickness, and slurry feed pressure.
1MIN
MEMBRANE PRESS
(OPTIONAL)
The membrane is inflated (using either air or water) to squeeze the cake in
the chamber. The membrane remains inflated until the cake air blow step
is completed. Some tailings do not see a significant filtration performance
improvement with a membrane press, and this is determined by test work.
2MIN
AIR BLOW
Compressed air is blown through the cake displacing interspatial water,
desaturating the formed filter cake.
0.25MIN
CORE WASH
Water is used to flush out the slurry in the feed line and the filter core.
0.25MIN
CORE BLOW
Compressed air flushes the water left in the feed line from the core wash.
0.25MIN
DRIP TRAY OPENING
Drip trays are opened to allow the filter cake to discharge.
1.5MIN
PLATE PACK OPENING
& CAKE DISCHARGE2
The plates open (either one at a time or all at once) to allow the cake to
discharge by gravity through the open drip trays.
0.5MIN
FILTER MEDIA SHAKING Helps discharge filter cake, typically only for full plate stack opening filters.
0.25MIN
DRIP TRAY CLOSING
Drip trays are closed to allow for cloth washing
0.5MIN
FILTER MEDIA FLOOD
WASHING
Helps keep filter media clean, typically only for full plate stack opening filters.
Some filters may also use a high-pressure cloth wash once per day.
1.5MIN
PLATE PACK CLOSING
The plates are closed to ready the filter for the next cycle.
2
Figure 41 shows a typical filtration cycle for a full plate stack opening filter assuming a plate pack opening and cake discharge time of 1.5 minutes.
For a single plate opening filter, this time can increase up to 15 minutes.
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6.5.3
Vertical Plate Pressure
Filter (cont.)
The method for opening the plates in a vertical plate
pressure filter has a significant impact on the mechanical
steps cycle time.
Single plate shifting filters have long mechanical times, as each plate is opened
individually for cake discharge and cloth washing.
Figure 42: Full plate stack opening style
pressure filter.
Full plate stack opening is where all the plates are connected to each other via
links and the plates open together. The advantage of this type of filter is a shorter
mechanical time, but it does require a longer filter and footprint. Most large filters
suitable for tailings use the full plate stack opening option. The plates on this style
of filter are supported on a sidebar that connects the two steel heads of the filter
(shown in Figure 42).
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6.6
Considerations
for Design
6.6.1
Equipment Sizing
Correct sizing of dewatering equipment and the addition of
some redundancy is vital for maintaining the availability to
meet production demands. Equipment that is undersized
will be operating at excessive rates and without the proper
downtime required for preventative maintenance.
Thickener tank sizing is relatively straight forward and derived directly from bench
scale test work, however selecting an appropriately sized thickener mechanism
and drive is often done without considering the full operating envelope and
torque requirements to save costs. An undersized drive results in difficulties
operating the thickener reliably, as spikes in bed rheology can cause the rakes to
fail or trip, or if equipped, lift until the torque requirements are reduced.
Filter sizing is typically done using bench scale filtration equipment which,
if done correctly, produces good results that have been proven to scale up to
full sized filter production. It is important to test a complete range of tailings
that are representative of the overall life-of-mine. Using a tailings composite
from this range to represent a “typical tailings” is not recommended. Figure 43
shows tailings filtration laboratory scale test results for the expected operating
envelope for a mine in South America. Each data set represents a core sample.
Note the variability in the filtration performance even though all test
conditions and parameters were fixed. This variability is due to changes in
tailings material properties.
The charcoal line shows a cake
moisture of 15% by mass. At this
moisture the filtration rate ranges from
50 to 160 kg/m²/h across the various
tailings samples tested. The composite
sample (brown dots) was produced by
mixing the individual core samples and
has a filtration rate of 100 kg/m²/h.
Cake moisture of 15% by mass
Operating Envelope
300
FILTRATION RATE (kg/m2/hr)
Figure 43: Filtration rate versus cake
moisture for multiple core samples
from single ore body targeting a cake
moisture of 15% by mass.
250
200
150
100
50
0
10%
12%
14%
16%
18%
20%
22%
24%
CAKE MOISTURE (wt%)
There are several instances where a filtration plant has been sized based on
a composite sample without testing other samples. If that was the case here,
there would be times when the plant is less than half the size that is required.
Understanding the frequency of the various tailings types is critical for optimizing
filter plant sizing.
During the project design phase, it is tempting to use the best filtering material
as the design basis to save capital costs; however, this again leads to the risk of
under sizing the filters. Conversely, if the worst-case material is selected as the
basis of design, the capital and operating expenses may be too high resulting
in the project never proceeding. It is best to understand the complete operating
envelope and use ore blending and a good mine plan to mitigate the effects of
the worst material.
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6.6.2
Planning for Maintenance
Preventative maintenance is required to keep filters
operating at optimum performance and prevent
unscheduled shutdowns. Approximately 80% of
maintenance activities on a pressure filter are associated
with the filter cloth and filter plates. Designing the filter plant
such that the filter cloths, and to a lesser extent the plates,
are easy to remove is essential for successful long-term
maintenance and maintaining high equipment availability.
The expected cloth life is tailings material, cloth material, and site specific.
The cloth life should be determined after startup to minimize the number of
unplanned cloth failures. The goal is to have 99% of cloth changes planned
and built into the maintenance program. Filter cloth that has been appropriately
selected for the application will achieve between 1,000 and 4,000 cycles of
operation before failure, depending on the type of tailings being processed.
The worst type of tailings for cloth life typically contain a significant portion of
clay and clay-sized particles, whereas tailings with more quartz-type particles or
larger particles will have a longer cloth life.
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6.6.2
Planning for Maintenance
(cont.)
Unplanned cloth failures lead to a knock-on effect of failures:
• Solids build-up behind the cloth (Plate 2).
• Built-up solids behind the cloth passing through the filtrate ports in the plate
(path of least resistance) and building up behind the adjacent cloth on the
other side of the plate (Plate 4).
• Built-up solids on the sealing surface between plates causing the plates to
deform when the plate pack is closed under pressure.
• Plate deformation leading to premature slurry leakage at the sealing surface,
failure of the adjacent plate and failure of the next cloth.
Figure 44: The Knock-On Effect cloth failure progression.
PLATE 4
PLATE 3
PLATE 2
PLATE 1
PLATE 4
PLATE 3
PLATE 2
PLATE 1
SOLIDS BUILD-UP
BEHIND THE CLOTH
BUILT-UP SOLIDS BEHIND THE CLOTH PASSING
THROUGH THE FILTRATE PORTS IN THE PLATE
AND BUILDING UP BEHIND THE ADJACENT
CLOTH ON THE OTHER SIDE OF THE PLATE
Good maintenance procedures ensure your filtration system operates at peak
efficiency and include:
• Daily walk-arounds, inspections, tightening bolts and other preventative
maintenance. Some of this is completed as the filter is running (greasing, etc.).
• Filter cloth change-outs on a scheduled cycle count.
• Plate and cloth maintenance.
• Applying good housekeeping and keeping the area clear.
• 10-hour shutdown every two weeks for:
• More significant maintenance to the filter.
• Valve/seal replacements.
• Feed pump maintenance.
• Cloth wash pump maintenance.
• Pipe replacements.
• Hose replacements.
The time for this maintenance needs to be accounted for when sizing filtration
equipment and determining utilization and redundancy.
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6.6.3
Filter and Building Design
It is important that the filtration system be protected from the environment. At a
minimum, a roof over the filters must be provided to protect from precipitation
and sunlight. Ultra-violet (UV) radiation is harmful to the polypropylene filter
plates and some rubber components on the filter. Precipitation can make a filter
inoperable, especially in colder climates. If filters are operated in a cold climate
without a heated building, all water lines must have heat tracing. Depending
on wind speeds, building walls may need to be provided to allow for crane
operation during maintenance. Wind speeds of more than 10 km/h will make use
of the maintenance cranes difficult, if not impossible.
Building layout is important to allow access to filter components and ensure
there is adequate space available for cloth and plate maintenance. An adequate
area inside the building is necessary to ensure a sufficient inventory of filter
cloth and filter plates and allow for any maintenance or staging of these parts.
As filter cloth and plate maintenance are constant and can be time consuming,
efficiency is key when designing the areas associated with these activities.
Besides the filter cloth and plate maintenance, maintenance of other pressure
filter components is required. These items include slurry valves, instruments,
and hydraulic cylinders. Valves, particularly the pinch valves, require periodic
inspection and maintenance. The positioning of these valves should be carefully
considered so they are accessible and can be easily maintained. Pinch valves
should have a flexible connection (bellows) as an interface with the rigid piping
system, or a removable angled spool where a bellows is not practical or possible.
A lifting point must be provided above every valve with sufficient headroom to
allow easy extraction. Valves can be bulky, heavy, and not easily maneuvered.
Figure 45: Model of typical filter
plant showing the high aspect ratio
of the building.
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6.6.3
Filter and Building Design
(cont.)
There are two heads on a vertical plate pressure filter, one at each end of the
plate stack. One head moves while the other remains stationary. These heads
contain the high pressures required for pressure filtration. The stationary head
should be equipped with at least one walkway and hand-railings to enable
routine unrestricted access for the purpose of installation and maintenance.
An access ladder or stairway to the moving head, possibly at the filter’s open
position, should be incorporated into the building design.
Access to the drip tray components, particularly the hydraulic cylinders and
instrumentation can be challenging, as they will be located under the floor of the
filter. A walkway offering access to these areas is a critical item to avoid delays
for installation, inspection, or repair activities.
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7.
Opportunities
© BHP RIO TINTO Tailings Management Consortium
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7.1
Key Points
When evaluating opportunities to reduce project costs, the
study manager must consider optimizing the overall project
costs, not just the optimization of some individual unit
operation costs, which when combined may result in higher
global costs.
Tailings classification and post deposition evaporative
drying has the potential to reduce project dewatering
costs, however the impact on overall water recovery from
tailings should be considered.
There is a potential to reduce project dewatering costs by
increasing target deposition moisture contents through the
implementation of co-disposal/comingling of tailings.
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7.2
Introduction
The previous chapters of this guide described the building blocks for designing
a typical filtered stacked tailings flowsheet. Although results are site-specific,
assessments of filtered stacked tailings systems often show this option to be
more expensive than conventional wet tailings facilities, both for capital and
operating costs. Transition of a brownfield operation from a conventional wet
tailings facility to a filtered stacked tailings system will likely increase operating
costs, particularly if the conventional system has manageable embankment
construction costs.
This chapter discusses some potential project justifications
along with opportunities and variants which may allow for
project improvements and/or reduce the costs such that a
filtered stacked tailings project may be viable.
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7.3
Potential Project
Justifications
What can be done to justify, or offset, the capital and
operating costs such that a filtered stacked tailings project
has a positive Net Present Value? As each mine site is
different, it is important for the project study manager to
understand the considerations that may bring value to a
filtered tailings project. These considerations include:
WATER COST AND AVAILABILITY
Water can be more than USD $3/m³ (Herrera-León et al., 2019) in regions
where water is desalinated and pumped long distances and/or to higher
altitudes. Water costs at these rates may be high enough to offset filtered
stacked tailings operating costs.
STORAGE SPACE LIMITATIONS
Filtered tailings can have a higher placed density than other tailings
management options, reducing tailings storage volumes and footprints. This can
be further optimized if tailings and waste rock are deposited in the same facility.
REGULATORY REQUIREMENTS
Depending on jurisdiction, permitting times for filtered stacked tailings facilities
can be significantly less compared to conventional wet tailings facilities.
BROWNFIELD TAILINGS FACILITY
REMAINING LIFE
In some applications, costs associated with constructing a new conventional
wet tailings facility to maintain operations or future tailings facility embankment
raises can be used to offset some or all the costs of filtered tailings.
CLOSURE
While progressive closure introduces costs earlier which can negatively impact
the project’s Net Present Value, there may be other cost savings associated
with the significant reduction of time between end of operation and closure
completion, claiming closure bonds earlier, and reducing dust emission and
infiltration management costs.
STAGED IMPLEMENTATION
If a filtered tailings system can be implemented in stages over time this
allows costs to be deferred to the future, reducing the negative impact on
Net Present Value. Brownfield operations may be particularly suitable for
this approach as the existing conventional wet tailings facility could continue
operation with a reduced tailings stream. This has the additional benefit of
reducing the rise rate of the existing facility.
BUTTRESSING
There may be an option to use filtered tailings to buttress an existing tailings
facility instead of using borrowed material, offsetting buttressing costs.
TAILINGS RE-PURPOSING
Value generation opportunities may exist for reuse/repurposing of filtered
tailings, such as for underground paste backfill, or road construction material.
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7.4
Opportunities
to Reduce
Project Costs
There are various areas of opportunities for cost reduction
that may exist within a project, including dewatering,
transport and placement, and co-disposal and co-mingling
with available waste rock.
7.4.1
Classification to Improve
Filtration Characteristics
Tailings filtration characteristics can be improved by the removal of ultra-fine
particles or clays. This is achieved by sending whole tailings to a classification
process, such as cycloning. The cyclone underflow is then sent to vacuum
filters or dewatering screens both of which are potentially less expensive than
pressure filtration to produce a stackable cake. The cyclone overflow can be
sent to a thickener and then either to pressure filtration to produce cake or
deposited directly as a slurry.
However, when evaluating opportunities to reduce costs, care must be taken
that all other impacted costs are accounted for. For example, reducing target
filter cake moisture contents may allow for the use of lower cost filtration
technologies, but will likely increase transportation and stacking costs. Likewise,
relying on evaporative drying of placed filter cake to supplement or replace
filtration can reduce dewatering costs, but without capture, the cost of this
water loss may have a negative impact on the project’s economics and needs
to be considered. For each specific project, the overall project impacts must be
determined for each opportunity, not just the individual costs.
The philosophy behind this flowsheet is to separate the portion of the tailings
that contains fines or clays from the rest of the tailings, reducing the flow sent
to pressure filtration. Vacuum filters and screens are also continuous, which
simplifies this portion of the flowsheet. In some applications the quantity of
fines in the cyclone overflow may be low enough that some other dewatering
technology, such as a centrifuge, could be used to dewater that stream instead
of thickening or pressure filtration.
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7.4.1
Classification to Improve
Filtration Characteristics (cont.)
Figure 46: Potential
Classification Flowsheet.
THICKENER O/F
CYCLONE O/F
AUTOMATIC FILTER PRESS
PLANT TAILINGS
FILTRATE
THICKENER U/F
FILTER CAKE
HORIZONTAL BELT FILTER
CYCLONE U/F
FILTER CAKE
FILTRATE
Some considerations for this
flowsheet include:
• If more than 60% of the particles are greater than 75 µm, this flowsheet
should be considered for further investigation.
• If less than 60% of the particles are greater than 75 µm it is likely that
pressure filtration of the whole combined tailings will be more economical.
• Classifying solids by particle size distribution will change the slurry
streams’ geotechnical behavior and this change needs to be understood.
Typically, the coarse stream will have lower target cake moistures for
placement and compaction than the original whole tailings, and the fine
stream will have higher target cake moistures.
• Depositing the fines stream as a slurry can greatly decrease dewatering
costs. The disadvantages are reduced water recovery from tailings and
the necessity of a tailings facility that can support wet slurry disposal. In
arid environments, it may still be possible to have a “dry stack” from slurry
deposition using thin lifts with evaporative drying.
• The two separated dewatered tailings streams can be recombined before
deposition depending on the tailings management strategy. Attention must
be paid to the mixing process design to ensure a homogenous product
for deposition.
• Fine tailings streams with a significant fines fraction or clay mineralogy are
typically challenging and high cost to filter. In some cases, increasing the
solids content by 2% to 3% mass can double the number of filters required
and consequently drive the operating costs significantly higher. Recombining
a portion of the coarse tailings with the fine tailings is an option to help
lower the dewatering costs of the fines stream that has proven feasible in
some applications.
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7.4.2
Improved Evaporative
Drying using Plough
and Discs
Figure 47: Agricultural plough.
© BHP RIO TINTO Tailings Management Consortium
Environmental evaporative drying of tailings is not typically effective for depths
greater than 100 mm, and mechanical turning of the tailings may be required
to assist with achieving target moistures. Dozers can be used to rip into the
deposited tailings; however, ploughs (shown in Figure 47) and tandem discs
are agricultural implements specifically designed to do this. While these
technological innovations from the agricultural industry are promising, for
any technologies not currently proven in tailings processes, the risks of the
technology must be evaluated and mitigated.
125
7.4.3
Improved Thin Lift Filter
Cake Deposition
Creating a thin lift of filter cake using standard equipment (covered in
Chapter 5) takes two steps. First the filtered tailings are deposited from a
conveyor or truck in a paddock or row, then this row is leveled using a dozer
or grader. There is a potential opportunity to combine the deposition and
leveling into a single step, using equipment not traditionally found in mining
or tailings management. Figure 48 shows a manure spreader from the
agricultural industry that is designed to spread sticky material in a thin layer
over a wide area.
Figure 48: From farming to mining –
manure spreaders deposit thin layers
of sticky material.
Slinging belts, shown in Figure 49, could also be attached to a stacking conveying
system for thin lift deposition. Again, for any technologies not proven in tailings
processes, the risks of the technology must be evaluated and mitigated.
Figure 49: Slinging belt.
© BHP RIO TINTO Tailings Management Consortium
126
7.4.4
Co-Disposal and
Co-Mingling
Co-disposal and co-mingling both describe the deposition
of mine waste rock and tailings in the same footprint with
different amounts of mixing. Co-disposal typically means
deposition of waste rock and tailings in separate layers or
cells while co-mingling means the mixing of the waste rock
and non-segregating tailings into a homogeneous matrix.
Co-mingling has had several trade names associated with it, such as
Paste-Rock™, EcoTails™, GeoWaste™, and GeoStable™.
The co-mingling of waste rock and tailings has the following potential benefits:
• Improved stability of the tailings
• Reduced overall waste storage volumes
• Reduced acid rock drainage potential of the waste rock.
In general, co-mingling attempts to retain the high shear strength of the waste
rock matrix while also limiting exposure to oxygen.
Figure 50: Placement of co-mingled
tailings and waste rock.
© BHP RIO TINTO Tailings Management Consortium
127
7.4.4
Co-Disposal and
Co-Mingling (cont.)
CO-DISPOSAL OPTIONS
INCREASING
DEGREE OF
MIXING
HOMOGENEOUS MIXTURES
Waste rock and tailings are blended
to form a homogeneous mass.
LAYERED CO-MINGLING
Layers of waste rock and dewatered
tailings are alternated.
WASTE ROCK AND DEWATERED
TAILINGS ARE DISPOSED IN THE
SAME TOPOGRAPHIC DEPRESSION
Figure 51: Co-Disposal Options.
(Adapted from Wickland et al., 2006)
© BHP RIO TINTO Tailings Management Consortium
128
7.4.4
Co-Disposal and
Co-Mingling (cont.)
Co-mingling has been practiced for many years at smaller tonnages using
dozers and loaders to mix and place the tailings and waste rock. Typically, the
ratio of waste rock to tailings required is approximately 4:1. This results in a large
volume of material that needs to be mixed and transported. These types of
volumes are not attractive to high tonnage operations if traditional mixing and
truck transportation methods are used. There have been some advancements
at pilot scale using conveyors for the transportation of waste rock and filtered
tailings while also using the energy of conveyance transfer point drops for
continuous mixing, see Figure 52. Figure 53 shows a concept for a large scale
co-mingling system, for which the mixing efficiencies and product geotechnical
and geochemical performance still need to be proven at scale.
Figure 52: Pilot Testing.
Figure 53: Large Scale Co-Mingled Concept.
WATER RECYCLING
CONCENTRATOR
WASTE ROCK
TRANSFER
CHUTES
FILTERED
TAILINGS
TO BELT
CHUTE MIXING
STACKER
STACK
© BHP RIO TINTO Tailings Management Consortium
Waste Rock Pile
130
8.
References &
Recommended Reading
© BHP RIO TINTO Tailings Management Consortium
131
8.1
References
AMIRA 2002, ARD Test Handbook - Project P387A Prediction and Kinetic Control
of Acid Mine Drainage, AMIRA International Limited, Melbourne, Australia.
Davies, M. P. 2002, ‘Tailings Impoundment Failures: Are Geotechnical Engineers
Listening?’ Geotechnical News, 20, pp. 31-36.
Herrera-León, S, Lucay, F.A, Cisternas, L.A, Kraslawski, A 2019, ‘Applying a
multi-objective optimization approach in designing water supply systems for
mining industries. The case of Chile’, Journal of Cleaner Production, Volume 210
pp. 994-1004.
MEND/KCB 2017, Study of Tailings Management Technologies: MEND Report
2.50.1, 24th Annual BC MEND Metal Leaching/Acid Rock Drainage Workshop,
Vancouver, B.C. Ottawa, ON: MEND (Mine Environment Neutral Drainage), MAC
(Mineralogical Association of Canada).
Morgenstern, N, Vick, S, and Van Zyl, D 2015, Report on Mount Polley Tailings
Storage Facility Breach, prepared by the Independent Expert Engineering
Investigation and Review Panel, https://www.mountpolleyreviewpanel.ca.
Oberle, B, Brereton, D, Mihaylova, A (eds.) 2020, Towards Zero Harm:
A Compendium of Papers Prepared for the Global Tailings Review. St Gallen,
Switzerland: Global Tailings Review. https://globaltailingsreview.org/.
Price, W.A 2009, Prediction Manual for Drainage Chemistry from Sulphidic
Geologic Materials, MEND Report 1.20.1, pp. 579.
Shahid, A, Li, Q 2010, ‘Tailings Dam Failures: A Review of the Last One Hundred
Years’, Geotechnical News, 28.
Stark, T. D, Moya, L, Lin, J 2022, ‘Rates and Causes of Tailings Dam Failures’,
Advances in Civil Engineering.
Wickland, B.E, Wilson, G.W, Wijewickreme, D, Fredlund, D.G 2006, 'Unsaturated
Properties of Mixtures of Waste Rock and Tailings', Proceedings of the Fourth
International Conference on Unsaturated Soils Conference, Arizona.
Witham, M.I, Grabsch, A.F, Owen, A.T, Fawell, P.D 2012, ‘The effect of cations on
the activity of anionic polyacrylamide flocculant solutions’, International Journal
of Mineral Processing, Volumes 114-117, pp. 51-62.
© BHP RIO TINTO Tailings Management Consortium
132
8.2
Recommended
Reading
Paste and Thickened Tailings – A Guide (third edition), Australian Centre for
Geomechanics, ISBN 978-0-9924810-0-1.
Tailings management handbook: A life-cycle approach, Society for Mining,
Metallurgy & Exploration, ISBN 978‑0‑87335-490-5.
For more information about defining closure objectives:
• Landform Design Institute’s 2021 position paper titled Mining with the
end in mind: Landform design for sustainable mining, available at
https://landformdesign.com.
• Australian Government handbook titled The Leading Practice Sustainable
Development Program (LPSDP) for the Mining Industry promotes
sustainable mining practices – Mine Closure, available at https://www.
industry.gov.au/publications/leading-practice-handbooks-sustainablemining/mine-closure.
• Global Tailings Review Chapter VIII Closure and Reclamation, available at
https://globaltailingsreview.org/wp-content/uploads/2020/09/Ch-VIIIClosure-and-Reclamation.pdf.
• Global Acid Rock Drainage Guide, Chapter 6 www.gardguide.com.
For information about integrating tailings facility closure with the overall
mine closure:
• ICMM’s Good Practice Guide Integrated Mine Closure, available at
https://www.icmm.com/.
https://link.springer.com/book/10.1007/978-3-319-02484-4
https://shop.elsevier.com/books/solid-liquid-separation-equipment-selectionand-process-design/tarleton/978-1-85617-421-3
DIIS, 2016. Leading Practice Sustainable Development Program for
the Mining Industry, Leading Practice Handbook: Preventing Acid and
Metalliferous Drainage. Department of Industry, Innovation and Science,
Commonwealth of Australia. Canberra, Australia.
© BHP RIO TINTO Tailings Management Consortium
Tailings Management Consortium
Filtered Stacked Tailings
A Guide for Study Managers