VERSION CORRIGÉE CORRECTED VERSION
The influence of kinetic test type on the geochemical response of low
acid generating potential tailings
Mathieu Villeneuve, Bruno Bussière and Mostafa Benzaazoua
Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Waste Management, UQAT, 445
University blvd, Rouyn-Noranda, Québec, Canada, J9X 5E4.
Michel Aubertin
Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Waste Management, École
Polytechnique, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, Canada, H3C 3A7
Marcos Monroy
ABSTRACT: This paper presents a comparison of the geochemical responses of five different tailings
submitted to five different kinetic tests. The five kinetic tests were done using column tests, humidity-cells,
shaking flasks, modified Soxhlet extractors and mini-alteration cells. The different tailings have all AP values
ranging from 23.9 to 178.8 kg CaCO3/t and NP values from 13.9 to 93.9 kg CaCO3/t. Evolution of the calcium, magnesium, manganese and sulfate loads were compared between each kinetic test and each of the tailings. It was found that, although the aggressiveness of the various kinetic tests was very different, the ratio of
the acid production rate to the neutralization rate was similar from test to test.
1 INTRODUCTION
The production of acid mine drainage (AMD) remains one of the most important environmental
problems facing the mining industry. Acid mine
drainage occurs when sulfide minerals contained in
mine wastes react with oxygen and water to produce
dissolved sulfuric acid. The production of acid lowers pH, which in turn increases the dissolution of
metals contained in the mine waste. Improper management of acid producing waste can be a source of
serious damage to the adjacent ecosystems. Many
different remedial techniques have been proposed to
control the production of AMD, such as sub-aquatic
storage, barriers that inhibit the infiltration of water
or gases, and collection of the effluents followed by
chemical or passive treatment (MEND, 2001).
Remediation costs alone for an acid generating tailings impoundment can range from 30 and 250 k
CAN $ per hectare (Aubertin et al., 2002). The rehabilitation strategy for closing a mine and the associated costs depend on the acid generating potential
(AGP) of the wastes.
The determination of a material’s AGP can be
done either by static or kinetic testing. A static test
consists of measuring the balance between acid producing and acid neutralizing potentials. By comparing the acidity potential (AP) to the neutralization
potential (NP), the net acid generating potential of a
specific waste can be estimated. However, these
tests have a relatively large uncertainty zone, where
it is difficult to determine if a specific waste has acid
generating potential or not (Miller et al. 1991, Adam
et al. 1997). In such cases, it is useful to run kinetic
tests to quantify evolution of water geochemistry
with time. Such measurements can help in predicting
the reaction rates of the acid producing and neutralizing minerals. Also, kinetic tests may be used to
predict mineral depletion, to determine if and when
AMD will occur.
The present work focuses on the comparison of 5
different kinetic tests that were performed on 5 tailings which either had low AP, or gave results in the
uncertainty zone for static testing. The first part of
this study describes the main properties of the five
tailings used in the experiments as well as the kinetic testing procedures used, i.e.: column tests, humidity-cells, shaking flasks, modified Soxhlet extractors and mini-alteration cells. Each of the kinetic
test procedures varies in both aggressiveness and its
ability to represent natural processes. Following
these descriptions, the cumulative results from the
different kinetic tests are presented and then analyzed.
2 MATERIALS AND METHODS
2.1 Methods
This section describes the different chemical and
physical characterization conducted on the tailings
and leachates.
2.1.1 Tailings characterization
2.1.2 Water chemistry
Physical properties
Humid tailings samples were oven dried at 45 °C for
24 hours to determine their water content and to
prepare dry samples for other analyses. The sample’s particle size distribution was determined using
a Malvern Mastersizer laser particle size analyzer.
The solid grain relative density was measured with a
Micromeritics Accupyc 1330 helium gas pycnometer.
pH and Eh
Leachate samples were filtered with a 0.45 µm nylon mesh filter immediately after collection. Sample
pH was read by a combination pH electrode with
temperature compensation. Redox potential was determined with a Pt/Ag/AgCl electrode. The results
were then corrected for the standard hydrogen electrode (SHE) to obtain Eh (expressed in mV).
Chemical composition and mineralogy
The chemical composition of the different tailings
was analyzed with a Perkin Elmer Optima 3100 RL
ICP-AES following a total HNO3/Br2/HF/HCl digestion. Dilute HCl was used to extract sulfates and the
solution obtained was analyzed by ICP-AES. Silica
content was determined by ICP-AES following a
Na2O2/NaOH fusion. The initial tailings mineralogy
was determined by a combination of visual observations, with a polarizing microscope and X-Ray diffraction spectroscopy.
Acid-Base accounting
Neutralization potential (NP) was determined using
the modified static test proposed by Lawrence
(1990) for each different tailing sample. The NP
analyses were run in duplicate, and results were expressed in kg CaCO3/t. Acidity potential (AP), also
expressed in kg CaCO3/t, was calculated by using
the sulfide portion of total sulfur, obtained by subtracting the sulfate sulfur from the total sulfur (see
Equations 1 and 2). The net neutralization potential
(NNP) was calculated by subtracting the AP value
from the NP value, as shown by Equation (3). It is
postulated here that values of NNP < -20 kg
CaCO3/t indicate an acid producing material,
whereas materials with NNP > 20 kg CaCO3/t are
acid consuming. Hence, an uncertainty zone for this
technique would lie between 20 > NNP > -20 kg
CaCO3/t (Miller et al. 1991).
%Ssulfide = %S total - %Ssulfate
(1)
AP (kg CaCO 3 /t) = 31.25 × %Ssulfide
(2)
NNP (kg CaCO 3 /t) = NP (kg CaCO 3 /t) - AP (kg CaCO 3 /t)
(3)
Another useful way to evaluate the AMD production potential using static tests is the NP to AP ratio.
Typically, the material is considered non acidgenerating if NP/AP > 2.5, uncertain if 2.5 > NP/AP
> 1 and acid generating if NP/AP < 1 (Adam et al.
1997).
Metals and sulfates
Filtered leachates were acidified to 2 % HNO3 to insure metal solubilization. The resulting solutions
were analysed with a Perkin Elmer Optima 3100 RL
ICP-AES to determine metal and sulfate concentrations, results are expressed in mg/L.
Geochemical modeling
The aqueous geochemical equilibrium model Visual
MINTEQ version 2.14, which is a Visual Basic version of the previous version of MINTEQ (Felmy et
al., 1984), was used to evaluate the saturation indices (SI) of possible secondary minerals in the kinetic
tests leachates. A SI value greater than 0 for a given
mineral indicates that it would precipitate under
these conditions. A negative SI value means that the
mineral would dissolve. A SI equal to 0 indicates
equilibrium. The main model assumption is that
mineral dissolution or precipitation reactions occur
rapidly enough to be in equilibrium with the
leachate.
2.1.3 Kinetic tests procedures
Several different kinetic tests were used in this
study. The following section contains a brief description of each test used. A more detailed description of the protocols used can be found in Villeneuve (in prep).
Columns
Four of the five column tests were conducted in 1 m
tall Plexiglas tubes, 10 cm in diameter. Approximately 16 kg (dry weight) of GRE-M1, LAR-M3,
MAT-M1 and MAT-M2 tailings were placed in each
of these columns. Column UQ-8 was run in a separate investigation and used a Plexiglas column 0.7 m
tall by 10 cm in diameter filled with 8 kg (dry
weight) of tailings. The bottom of all columns included a porous ceramic plate to simulate a water
table 2 m below the column base. Two liters of deionized water were added to the top of each column
and allowed to drain, while the resulting leachate
was collected. This process was repeated twelve
times over a period of about one and a half year. The
top of each column was opened between flushes. A
detailed description of the column tests can be found
in Benzaazoua et al. (2001).
Humidity-cells
Testing in the humidity-cells was done following the
procedure described in Morin & Hutt (1997). Plexiglas cells, 14 cm in diameter and 30 cm in height,
were filled with approximately 1 kg (dry weight) of
fresh tailings. A full cycle consisted of 3 days of dry
air at 1 L/min, 3 days of humid air at 1 L/min and
leaching of the cells on the seventh day. The leaching was done by inundation and soaking of the tailings for 4 hours with 500 mL of deionized water.
The resulting leachate was then collected by overpressurization to about 35 kPa.
of the Buchner funnel. The solution was recovered
by applying suction on a filtering flask after 3 h of
contact with the tailings. Figure 2 shows a photograph of a typical mini-alteration cell used in this
study.
Shaking flasks
The shaking flasks procedure was inspired by the
work of Gleisner (2001). The ratio of solids to water
used was 1:10, sampling was periodic and compensated. Five 1 L Erlenmeyer flasks containing 50 g of
dry tailings and 500 mL of deionized water were
placed on a reciprocating shaker. 50 mL water samples were taken once a week and are compensated
by the addition of 50 mL of fresh deionized water.
The test flasks were adjusted to their original weight
weekly by addition of deionized water to compensate for evaporation. The dilution is taken into account in later load calculations.
Modified Soxhlet extractors
A specially designed modified Soxhlet extractor,
similar to the one used by Sullivan & Sobek (1982),
was used. This modified Soxhlet varies from the
classic design in that the extraction chamber is located outside the path of the rising water vapor (see
Figure 1). Also, there is no siphon tube, so the water
passes straight through the sample and back into the
boiling flask. The main reason for these modifications was to maintain a realistic temperature in the
sample. About 6 g of dry tailings were placed in a
glass extraction thimble and 200 mL of deionized
water were placed in the boiling flask. Sampling of
the extracted solution was done with a syringe
through a self-sealing septum placed on the boiling
flask. Sampling was done on days 1, 3, 7 and 14; 50
mL of the extracted solution was sampled and later
compensated by the addition of 50 mL of fresh deionized water. Again, the dilution is taken into account in later load calculations.
Mini-alteration cells
Mini-alteration cells similar to those of Cruz et al.
(2001) were used. The aggressiveness of the method
lies in the use of a thin layer of sample and frequent
leaching-drying cycles. About 67 g (dry weight) of
tailings were placed in a 100 mm diameter Buchner
funnel equipped with a glass-fiber filter. A 7 day cycle consisted of 2 days of exposure to ambient air,
leaching on the 3rd day, 3 days of exposure to air and
finally, leaching on the last day. The flushes were
done by adding 50 mL of deionized water to the top
Figure 1. Photographs of the modified Soxhlet extractor used:
a) global view of the apparatus, b) modified extraction chamber.
Figure 2. Photograph of a typical mini-alteration cell.
2.2 Materials
Following is a description of the tailings characteristics, for more detailed results, see Villeneuve (2003,
Masters Thesis, to be submitted).
2.2.1 Tailings
UQ-8 tailings were sampled in an actual tailings impoundment from the Abitibi-Témiscamingue region
in the province of Québec, Canada. Tailings GREM1, LAR-M3, MAT-M1 and MAT-M2 were manufactured by mixing desulfurized and sulfurized fractions to obtain low AP tailings. Desulphurization of
the original fresh tailings was conducted with a laboratory flotation bench using a xanthate collector and
methylisobutyl carbinol (or MIBC) as frother (more
details on the desulphurization by flotation can be
found in Benzaazoua et al., 2000).
2.2.2 Physical properties
Table 1 presents, for each of the tailings, the solid
grain relative density (Dr) and the main characteristics of the particle size analysis.
Table 1. Physical properties of the five tailings studied.
Dr
% under
(-)
(%)
Material
UQ-8 GRE
-M1
3.04
2.90
94.6
94.0
LAR
-M3
2.78
89.2
MAT
-M1
3.02
90.3
MAT
-M2
3.07
83.6
(µm)
(µm)
(µm)
1.1
11.9
54.8
3.5
22.4
83.3
2.0
15.7
78.4
2.4
22.8
104
80 µm
D10
D50
D90
3.9
20.4
63.8
These tailings are typical of fine tailings from a
hard rock mines (Vick, 1983; Aubertin et al. 2002),
with D10 ranging from 1.1 to 3.9 µm and percent
passing 80 µm between 80 and 95 %. Relative
densities are very similar from one material to the
other, ranging from 2.78 to 3.07. Generally, a larger
sulfides content induces an increased density of the
tailings.
2.2.3 Chemical and mineralogical properties
Results from chemical analysis of the five tailings
are given in Table 2.
Table 2. Chemical composition of the five tailings studied
UQ-8
Al
As
B
Ba
Ca
Cd
Co
Cr
Cu
Fe
K
Mg
Mn
Na
Ni
Pb
SiO2
Stotal
Ssulfate
Ssulfide
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
(wt%)
3.81
0.027
0.068
0.006
3.43
0.015
0.004
0.008
0.012
17.1
n/d
1.65
0.456
1.8
0.005
0.008
n/d
7.09
1.37
5.72
GREM1
7.51
0
0.077
0.03
2.89
0.002
0.001
0.006
0.03
11.5
1.24
2.87
0.196
1.36
0.017
0.034
53.20
1.09
0.240
0.850
LARM3
5.87
0
0.129
0.031
1.68
0.002
0
0.003
0.023
2.83
1.25
0.730
0.043
1.37
0.008
0.046
68.81
0.816
0.050
0.766
MATM1
3.16
0
0.086
0.005
3.79
0.002
0.003
0.008
0.026
16.6
0.180
3.15
0.434
1.01
0.007
0.028
53.29
1.90
0.190
1.71
MATM2
2.99
0.001
0.081
0.006
3.60
0.002
0.004
0.008
0.028
17.9
0.170
2.94
0.409
1.01
0.007
0.028
46.82
2.85
0.324
2.53
Zn
(wt%)
0.007
0.159
0.108
0.234
0.276
As can be seen in Table 2, the present study focuses on relatively low-sulfide wastes, with sulfide
content varying from 0.766 to 5.72 %. All materials,
except UQ-8, had low sulfate contents (0.05 to 0.324
wt%), typical of fresh tailings. UQ-8 (1.37 wt% Ssulfate) was oxidized prior to being used in the present
work. Calcium, magnesium and manganese contents
of the tailings are tracers to their neutralization potential as these elements are generally found in carbonate minerals.
Table 3 shows the results of the mineralogical
characterization that was conducted by the Géoberex
Recherche firm in Montréal (Bernier, 2002) on the
five tailings. For each one, the principal minerals
found by the mineralogical analysis were quartz,
chlorite, feldspars, carbonates (mainly ankerite and
calcite) and micas. The main sulfide mineral was pyrite, with traces of pyrrhotite for tailings MAT-M1
and MAT-M2. The main Ca, Mg and Mn bearing
minerals found were carbonates.
Table 3. Mineralogical analysis of the tailings (Bernier, 2002).
Mineral
Albite\Orthose
Amphibole
Anhydrite
Ankerite/dolo
mite
Barite
Biotite
Calcite
Chlorite
Clinopyroxène
Epidote
Gypsum
Magnetite
Muscovite\Illite
Pyrite
Pyrrhotite
Quartz
Sepiolite
Siderite
Talc
UQ-8
M
---
GREM1
L
L
--
LARM3
M
---
MATM1
L
L
--
MATM2
L
L
Tr
M
-L
-Tr
--Tr
--
L
--Tr
M
Tr
Tr
---
?
-Tr
Tr
M
L
----
M
-Tr
Tr
M
tr
Tr
-Tr
M
-Tr
Tr
M
Tr
Tr
-Tr
-M
-A
-L
--
L
L
-A
----
L
Tr
Tr
A
----
-L
?
A
L
---
-L
?
A
L
---
A: abundant (30-50 wt%); M: moderately abundant (10-30
wt%); L: low abundance (2-10 wt%); Tr: trace (<2 wt%); ?:
suspected
2.2.4 Acid-Base accounting
The results from acid-base accounting are shown in
Table 4.
Table 4. Acid-base accounting from the five tailings.
AP
UQ-8
NP
NNP
(kg CaCO3/t)
178.8
64.2
-115
NP/AP
(-)
0.359
2.7
0.568
1.76
1.15
Using the criteria of Miller et al. (1991), only the
UQ-8 tailings are considered to be acid generating,
with an NNP of -115 kg CaCO3/t. Materials LARM3 (-10.3 kg CaCO3/t) and MAT-M2 (12.1 kg
CaCO3/t) are in the uncertainty zone. Tailings GREM1 (45.1 kg CaCO3/t) and MAT-M1 (40.5 kg
CaCO3/t) are non acid generating.
Using the NP to AP ratio and the criteria of
Adam et al. (1997), UQ-8 (NP/AP = 0.359) and
LAR-M3 (NP/AP = 0.568) should be acid generating. Tailings MAT-M1 (NP/AP = 1.76) and MATM2 (NP/AP = 1.15) are in the uncertainty zone,
whereas GRE-M1 tailings (NP/AP = 2.7) are non
acid generating.
These results indicate that kinetic tests should be
run to better asses the nature of these tailings in
terms of their acid generating nature. Nevertheless,
the choice of which kinetic testing procedure to use
is not always evident. Therefore, the influence of the
kinetic test procedure on the geochemical response
of these low AP tailings was investigated.
3.1 pH and Eh measurements
For all of the kinetic tests conducted on all of the
tailings sample in this study, pH typically started
near neutrality (pH 7) and rose to alkaline values (up
to pH 9.8 for the GRE-M1 modified Soxhlet extractor test). In general, Eh readings oscillated between
around 300 and about 520 mV in all tests and for all
the tailings. The modified Soxhlet extractors run on
the five tailings had both decreasing Eh values in
time and consistently lower Eh readings than the
other kinetic tests results.
3.2 Cumulative sulfate, Ca, Mg and Mn loads
Figures 3 through 7 show the evolution of the cumulative sulfate load normalized by the dry weight of
sample for each kinetic test run on each of the tailings.
3 RESULTS
40000
35000
30000
25000
20000
15000
10000
5000
0
0
100
200
300
400
Time (d)
Figure 3. Evolution of the cumulative sulfate loads for the UQ8 tailing.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
4
2-
(mg/kg)
GRE-M1
Cumulative SO
The following results have been obtained from ongoing kinetic tests, except for the columns and
modified Soxhlet extractors which were run to completion (1 year and 14 days, respectively).
For each kinetic test run on the tailings, pH, Eh,
conductivity, punctual metal concentration and acidity and alkalinity were analyzed for each leachate
sample. This data was processed to compute instantaneous and cumulative loads as well as elemental
depletion curves based on the geochemistry of the
leachates. The apparatus used for each completed
kinetic test was dismantled at the end and a complete characterization of the post-tests tailings was
done. Detailed results are found in Villeneuve
(2003, Masters Thesis, to be submitted).
Results presented in this paper are expressed in
cumulative sulfate load and in cumulative calcium,
magnesium and manganese added loads. The former
represents the sulfide minerals oxidation products
and therefore acid production. The later represents
the by-products of acid neutralization by carbonates.
Results are normalized by kilogram (dry weight) of
tailings tested. This technique has been used in the
past to aid interpretation and comparison between
different kinetic testing protocols (Morin et Hutt,
1997; Benzaazoua et al. 2001).
The following sections detail the geochemical results of each of the five types of kinetic tests for
each of the tailings.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
UQ -8
(mg/kg)
45.1
-10.3
40.5
12.1
2-
71.7
13.6
93.9
91.0
4
26.6
23.9
53.4
78.9
Cumulative SO
GRE-M1
LAR-M3
MAT-M1
MAT-M2
8000
7000
6000
5000
4000
3000
2000
1000
0
0
100
200
300
400
500
Time (d)
Figure 4. Evolution of the cumulative sulfate loads for the
GRE-M1 tailing.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
(mg/kg)
LAR-M3
6000
4
4000
Cumulative SO
2-
5000
3000
fate load evolution is not linear and slope breaks are
often observed (Figures 4 to 7). In instances where a
break in slope is observed, it becomes difficult to
obtain relevant information from the slopes.
Figures 8 to 12 show the evolution of cumulative
Ca, Mg and Mn added loads for each test run on
each of the tailings.
2000
0
100
200
300
400
500
Time (d)
Figure 5. Evolution of the cumulative sulfate loads for the
LAR-M3 tailing.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
(mg/kg)
MAT-M1
10000
2-
8000
16000
14000
12000
10000
8000
6000
4000
2000
0
0
100
200
300
400
Time (d)
Figure 8. Evolution of the cumulative added Ca, Mg and Mn
loads for the UQ-8 tailing.
6000
4
Cumulative Ca+Mg+Mn
(mg/kg)
0
Cumulative SO
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
UQ -8
1000
4000
0
100
200
300
400
500
600
Time (d)
Figure 6. Evolution of the cumulative sulfate loads for the
MAT-M1 tailing.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
14000
12000
10000
8000
6000
4000
2000
0
4000
3000
2000
1000
0
0
100
200
300
400
500
Time (d)
Figure 9. Evolution of the cumulative added Ca, Mg and Mn
loads for the GRE-M1 tailing.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
LAR-M3
0
200
400
600
Time (d)
Figure 7. Evolution of the cumulative sulfate loads for the
MAT-M2 tailing
It can be observed on Figures 3 to 7 that many
similarities can be found from tailings to tailings.
Based on the total quantity of sulfates leached, the
following general gradation can be obtained: columns < mini-alteration cells < humidity-cells <
shake flasks < modified Soxhlet extractors. The order of increasing slope angle of the sulfates vs. time
plot is: columns < humidity-cells < mini-alteration
cells < shake flasks < Soxhlet extractors. It is also
apparent that for many kinetic tests (mainly humidity-cells, mini-alteration cells and shake flasks), sul-
Cumulative Ca+Mg+Mn
(mg/kg)
4
2-
(mg/kg)
MAT-M2
Cumulative Ca+Mg+Mn
(mg/kg)
0
Cumulative SO
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
GRE-M1
2000
2500
2000
1500
1000
500
0
0
100
200
300
400
500
Time (d)
Figure 10. Evolution of the cumulative added Ca, Mg and Mn
loads for the LAR-M3 tailing.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
Cumulative Ca+Mg+Mn
(mg/kg)
MAT-M1
4000
3000
2000
1000
0
0
200
400
600
Time (d)
Figure 11. Evolution of the cumulative added Ca, Mg and Mn
loads for the MAT-M1 tailing.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
Cumulative Ca+Mg+Mn
(mg/kg)
MAT-M2
5000
4000
3000
2000
1000
0
0
200
400
600
Time (d)
Figure 12. Evolution of the cumulative added Ca, Mg and Mn
loads for the MAT-M2 tailing.
The evolution of cumulative Ca, Mg and Mn
added loads for each test run on all tailings presented in Figures 8 to 12 shows the same overall
tendencies as for sulfates. Progressive increase of
the plotted slope angle is generally: columns < humidity-cells < mini-alteration cells < shake flasks <
modified Soxhlet extractors. Based on total quantity
of Ca, Mg and Mn, the same general order as for
sulfates is obtained: columns < mini-alteration cells
< humidity-cells < shake flasks < modified Soxhlet
extractors. Breaks in the slope of the curves representing the humidity-cells, mini-alteration cells and
shake flasks were also frequently observed (Figures
9 to 12).
Some exceptions to the general gradations presented earlier did occur. It was the case of the minialteration-cell test results run on the LAR-M3 tailings. This test extracted sulfates and neutralization
products at an uncharacteristically low rate relative
to the same test conducted on the other materials.
Also, the modified Soxhlet extractor conducted on
the MAT-M2 sample extracted a relatively small
quantity of reaction products per kilo versus other
Soxhlets.
The results presented here are assessed in more
detail in the following sections.
4 DISCUSSION
In this section, further analysis of the results is presented. A classical interpretation regarding each
sample’s AMD generation potential is first given.
Then, the cumulative loads graphs are evaluated. Finally, the global geochemical response of the five
tailings during the kinetic tests is discussed.
It should be recalled that results and interpretation (for depletion and precipitation) are based on
the geochemical responses and modeling from the
leachate collected. Most of these tests are still ongoing and therefore, it was not possible to validate precipitation and depletion by analyzing the final altered tailings sample.
4.1 AMD potential
The limit often used to characterize onset of AMD is
a pH lower than 5 (SRK, 1989). None of the kinetic
tests in the present study yielded such an acidic pH.
The metals loads remained low in every test due to
the high pH. Heavy metals were often on or under
detection limits for the ICP analysis.
Therefore, we can conclude that evidence of the
AMD generating potential of the 5 tailings studied
was not observed during the course of the 5 different
kinetic testing protocols. However, these observations do not allow stipulation of the long term acid
generating potential of the tailings.
4.2 Geochemical data analysis
During a kinetic test, the main variables that influence the geochemical composition of the leachate
are: depletion of the reactants, depletion of ultra-fine
particles, precipitation of secondary minerals (Aubertin et al. 2002) or passivation of the reacting minerals surfaces (Cruz et al. 2001). Depletion of ultrafines can only be detected by the particle size distribution analysis of the post-dismantlement sample;
therefore this hypothesis will not be investigated in
this study, but will be the focus of future work.
Note that for all of the tests, the quantity of
cumulative sulfates collected exceeded the quantity
of soluble sulfates originally found in the tailings.
This means that sulfates found in leachates have
been mainly produced by oxidation of pyrite.
4.2.1 Elemental depletion
Table 5 lists the depletion of total sulfur and neutralizing elements (Ca+Mg+Mn) for all of the tailings
samples and kinetic tests. These results indicate that
modified Soxhlet extractors may be used to maximize depletion of minerals in the course of a laboratory kinetic test. Columns tests induced the lowest
depletion rates (% depleted vs. duration) of all five
tests. Generally, it has been found that the increasing
order of depletion rates is: columns < humidity-cells
< mini-alteration cells < shake flasks < modified
Soxhlet extractor. However, variations do occur.
They could be caused by precipitation of reaction
products which compromises the water quality
analysis or physical problems encountered during
the tests.
Table 5. Computed % remaining in total sulfur and
Ca+Mg+Mn for the kinetic tests run on the five tailings.
Kinetic test
Duration (days)
C
550
Tailings
UQ-8
Parameter
% remaining
Stotal
Ca+Mg+Mn
Stotal
Ca+Mg+Mn
Stotal
Ca+Mg+Mn
Stotal
Ca+Mg+Mn
Stotal
Ca+Mg+Mn
95.5
96.1
93.2
98.5
93.3
97.3
93.7
98.5
96.9
98.5
GREM1
LARM3
MATM1
MATM2
H.C.
225
83.3
86.0
77.8
94.9
83.9
92.2
85.8
96.0
85.8
94.0
M.C.
77
S.F.
105
Sox
14
88.0
83.4
80.1
95.5
94.5
97.1
88.9
97.0
89.6
96.1
82.7
74.4
79.2
94.9
87.5
93.3
87.9
95.9
82.9
94.5
81.0
71.2
76.4
93.8
73.6
92.7
83.6
94.9
82.3
94.9
C.: Column; H.C.: Humidity-Cell; M.C.: Mini-alteration cell;
S.F.: Shake flask; Sox.: Modified Soxhlet extractor.
Table 5 also shows that the total sulfur depletion
rates are greater than the neutralizing elements depletion rates for the tailings and tests evaluated here
(except for UQ-8).
4.2.2 Geochemical responses
UQ-8 tailings
Analysis of the pH, Eh and the curves on Figures 3
and 8 tend to indicate that the acidity was produced
by oxidation of the sulfide minerals. Acid was immediately neutralized by the carbonates in the sample. Both acid production and consumption reactions
followed an almost constant rate through all of the
kinetic tests, suggesting that neither the AP nor the
NP were fully consumed (see section 4.2.1). Similar
results have been observed by Benzaazoua et al.
(2001).
GRE-M1 tailings
Breaks in the slope, like the ones on curves shown in
Figures 4 and 9, are most important in the case of
the humidity-cell, the mini-alteration cells and the
shake flask.
Depletion alone can not explain the observed
slope changes, because neither acid producing nor
neutralizing minerals were found to be depleted (see
section 4.2.1).
Precipitation of metals inside the test chamber or
in the sample voids would reduce the amounts found
in the leachate. This would affect the shape of a resulting curve on a load vs. time graph. Geochemical
modeling was conducted based on the results. Modeling results show that, in all cases, calcite and
dolomite are close to equilibrium (SI ~ 0) throughout the tests. Also, gypsum was found to be close to
saturation in the first part of these tests, but not near
the end. Hence, it can be inferred that the breaks in
slope were not caused by precipitation.
From the combined facts that there is a greater
probability of precipitation happening before the
break in slope and that the reactants are not depleted,
it can be assumed that the reduction in reaction rate
may be related to some form of passivation. During
the first weeks of the tests, gypsum, iron hydroxides
and other secondary minerals slowly precipitated on
the surface of the pyrite and this coating may have
impeded further reaction with water and oxygen.
Acid production was slowed down and consequently, neutralization was reduced as well. This
would cause a reduction in the slopes of the SO42and Ca, Mg, Mn vs. time graphs. Such passivation
was measured using cyclic voltametry on altered pyrite by Cruz et al. (2001).
LAR-M3 tailings
In Figures 5 and 10, breaks in slope are most evident
in the case of the humidity-cell test and to a lesser
degree in the shake flask test.
Again, depletion calculations indicate that none
of the elements of interest were depleted during the
course of the different kinetic tests (see section
4.2.1).
As for the GRE-M1 tailing, the geochemical
modeling results show that precipitation is more
probable at the beginning of the humidity-cell test.
Various aluminum and iron hydroxides were found
to be either oversaturated or near equilibrium in the
first stages of the humidity-cell test. No sulfate bearing secondary minerals were found to precipitate by
the Visual MINTEQ model. This may indicate that
passivation of the pyrite surfaces could be responsible for the diminution in sulfate production rates.
Less acid production entails less neutralization and
as a result, less Ca, Mg and Mn in the flush waters.
The low reactivity observed in the mini-alteration
cells is believed to be due to a physical wetting
problem encountered during the course of the test
that lead to inadequate dissolution of reaction products.
MAT-M1 and MAT-M2 tailings
Both the MAT-M1 and the MAT-M2 tailings have
similar behavior on Figures 6, 7, 11 and 12. For both
samples, a change in the kinetic rate is observed
with the humidity-cell and the mini-alteration cell
tests results.
In both cases the reactants were far from complete depletion. Geochemical modeling of the rinse
waters of humidity-cells run on both samples
showed the same pattern. In the beginning, various
4.3 Comparison of the kinetic test protocols
4.3.1 Relative aggressiveness
Based on geochemical responses (reactive rates or
total cumulative loads extracted vs. test time) and
depletion data obtained in this study, a general trend
regarding the different kinetic tests relative aggressiveness becomes apparent. The following gradation
was observed: columns < humidity-cells < minialteration cells < shake flasks < modified Soxhlet
extractors.
4.3.2 Acid production to neutralization ratio
To evaluate the ability of a kinetic test to simulate
the acid production and the consequent neutralization, the cumulative loads of Ca, Mg and Mn (neutralization by-products) are plotted against the cumulative SO42- (acid production product). Therefore,
if neutralization is in response to the acid production, Ca+Mg+Mn vs. SO42- plots should be linear as
long as the reactants are not depleted and precipitation is not significant (Benzaazoua et al. 2001). The
main hypothesis behind this interpretation is that the
tailings contain a considerable amount of NP, the
Ca, Mg and Mn loads in the leachates are only attributable to dissolution of carbonate minerals. In the
case of near neutral drainage pH, it has been noted
that neutralization comes mainly from the carbonates and silicate minerals reaction rates are low
(Sverdrup, 1990; Kwong, 1993; Sherlock et al.
1995; Paktunc, 1999, Benzaazoua et al. 2001).
UQ-8 Tailings
Figure 13 shows the Ca+Mg+Mn cumulative and
normalized loads as a function of the cumulative
normalized Sulfate load for each of the kinetic tests
run on the UQ-8 tailings.
Cumulative Ca+Mg+Mn
(mg/kg)
UQ -8
16000
14000
12000
10000
8000
6000
4000
2000
0
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
0
10000
20000
30000
40000
50000
2-
Cumulative SO 4 (mg/kg)
Figure 13. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the UQ-8 tailing.
The fact that the curves shown on Figure 13 are
all linear and in the same axis implies that, no mater
how aggressive the kinetic testing procedure used to
alter the UQ-8 tailing, the ratio of the acid production rate to the neutralizing rate is constant. The column experiment is often considered to be the closest
to natural oxidation-neutralization conditions. As the
other curves are in line with the end portion of the
column’s curve, it can be tentatively stated that, for
the UQ-8 tailing, natural conditions are preserved
even in the more aggressive tests.
GRE-M1 tailings
The graph of normalized cumulative Ca, Mg and Mn
loads as a function of the cumulative normalized
SO42- for the different kinetic tests run on the GREM1 tailings is shown on Figure 14.
GRE-M1
Cumulative Ca+Mg+Mn
(mg/kg)
Al and Fe hydroxides, as well as various Ca, Mg,
Mn and Zn carbonates were found to be oversaturated or near equilibrium. Near the end of the
humidity-cell experiment, all the oversaturated
minerals were soluble and near equilibrium.
Modeling of the mini-cell results showed little or no
change in secondary minerals saturation indices
during the entire test. Various hydroxides remained
oversaturated and gypsum remained near
equilibrium throughout the tests.
On Figures 7 and 12, it can be noted that the
modified Soxhlet extractor yielded less total cumulative sulfates and Ca+Mg+Mn than the humidity-cell
(a phenomenon not observed for the other tailings).
This is probably due to water flow problem observed
with the MAT-M2 sample. In fact, the MAT-M2
sample was submerged with water through most of
the test, preventing oxygen intake.
In the case of the MAT-M1 and MAT-M2 tailings, the geochemical response was deemed to have
been influenced by both precipitation of metals as
secondary minerals and passivation of the primary
minerals’ surfaces.
4000
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cells
3000
2000
1000
0
0
1000 2000 3000 4000 5000 6000 7000 8000
Cumulative SO 4 2- (mg/kg)
Figure 14. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the GRE-M1 tailing.
Figure 14 shows that, in the case of the column,
the humidity-cell and the mini-alteration cell tests,
the ratio of acid production to neutralization rates is
maintained. However, near the end of the humidity-
LAR-M3 tailings
Figure 15 presents the cumulative normalized
Ca+Mg+Mn vs. SO42- curves for all of the kinetic
tests run on the LAR-M3 samples.
Cumulative Ca+Mg+Mn
(mg/kg)
MAT-M1
4000
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
3000
2000
1000
0
0
2000
Cumulative Ca+Mg+Mn
(mg/kg)
6000
8000
10000
Figure 16. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the MAT-M1 tailing.
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
MAT-M2
5000
4000
3000
2000
1000
0
0
LAR-M3
4000
Cumulative SO 4 2- (mg/kg)
Cumulative Ca+Mg+Mn
(mg/kg)
cell test we see an upward shift towards the neutralization products. As mentioned before, modeling
showed that none of the sulfate bearing secondary
minerals was oversaturated. This response could be
explained by the passivation of the sulfide surfaces,
but the natural dissolution of the carbonates could
have still taken place.
In the case of the shake flask and the modified
Soxhlet extractor tests, the curves on Figure 14 are
shifted towards higher Ca+Mg+Mn values than the
end portion of the column experiment curve. In
these two tests, which are more aggressive, an exaggeration of the carbonates dissolution is observed.
Note that the GRE-M1 material has a very low sulfide content (0.850 %Ssulfide), which gives it an AP
of only 26.6 kg CaCO3/t and a relatively large NP of
71.7 kg CaCO3/t. It seems that, in the case of a low
AP-large NP sample, Soxhlets and shake flasks do
not adequately simulate the natural oxidationneutralization processes.
5000
10000
15000
Cumulative SO 4 2- (mg/kg)
2500
Figure 17. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the MAT-M2 tailing.
2000
1500
Column
Humidity-Cell
Shake Flask
Soxhlet
Mini-Cell
1000
500
0
0
1000
2000
3000
4000
5000
6000
2-
Cumulative SO 4 (mg/kg)
Figure 15. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the LAR-M3 tailing.
Again, in Figure 15, one can see that results from
the column and humidity-cell tests are well correlated. Solution in the boiling flask of the Soxhlet extraction has reached saturation and precipitation of
various Ca, Mg and Mn carbonates, whereas sulfate
bearing minerals were still soluble. This explains the
behavior of the Soxhlet test curve. Notice also the
shake flask’s tendency to exaggerate carbonate dissolution when the material has a low AP.
MAT-M1 and MAT-M2 tailings
Figures 16 and 17 show the different kinetic tests’
Ca+Mg+Mn cumulative normalized loads vs. the
SO42- cumulative normalized loads curves for the
MAT-M1 and MAT-M2 tailings.
Analysis of Figures 16 and 17 shows that most kinetic tests were affected by precipitation and redissolution. However, the main trend remains apparent for all the kinetic tests. Shake flasks tests show
signs of what is believed to be sulfate precipitation
in the first weeks, then re-dissolution. Both modified
Soxhlets and shake flasks tests show signs of carbonate over-dissolution as well. Again, one can see
that in the Soxhlet tests, sulfate becomes saturated in
the boiling flask near the end of the 14 day period.
Mini-alteration cells show a good correlation with
the column test results until near the end of the test
where neutralization products were found more saturated.
5 CONCLUSIONS
The main findings of this study can be summarized
as follows:
− Based on the geochemical behaviors (cumulative
curves and depletion data) observed in this study,
the following trend has been obtained for the relative aggressiveness of the kinetic testing procedures: columns < humidity-cells < mini-alteration
−
−
−
−
cells < shake flasks < modified Soxhlet extractors.
The plot of the cumulative and normalized Ca,
Mg and Mn loads vs. cumulative normalized load
in sulfate is a valuable tool to assess a kinetic
test’s ability to simulate the natural acid production to neutralization ratio.
For tailings with a medium AP and a medium
NP, the ratio of acid production to acid neutralization rates appears to be constant for all the kinetic test procedures used in this study.
The more aggressive kinetic test procedures
(shake flasks and modified Soxhlets) tend to exaggerate dissolution of carbonate minerals in the
case of low AP materials.
Mini-alteration cell tests are a valid alternative to
humidity cell tests, especially when: a small
quantity of the sample is available, one needs to
alter a sample faster than in the humidity-cell, a
large number of samples must be tested. Results
from the mini-cells are very similar to those of
the humidity-cells.
6 ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial
contribution of the Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine
Wastes Management (www.polymtl.ca/envirogeremi) and the “Unité de Recherche et de Service
en Technologie Minérale” for its technical support.
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