Stabilization-solidification process - Comparison of synthetic sludge and
volcanogenic massive sulfide tailings.
Mijno Violainea*, Martin Françoisa, Bollinger Jean-Claudeb and Catalan Lionelc
a
Laboratoire de Géosciences, UMR-CNRS 6532 HydrASA, Faculté des Sciences et Techniques,
Université de Limoges, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France
b
Laboratoire des Sciences de l'Eau et de l'Environnement, Faculté des Sciences et Techniques,
Université de Limoges, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France
c
Department of Chemical Engineering, Lakehead University,
955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada
Abstract
Stabilization-solidification (s/s) method is widely applied to immobilize mining and industrial wastes. This
technique uses cement chemical and physical properties to integrate Trace Elements (TE) during their hydration
reactions. Prediction of cement phases and pollutants evolutions during weathering is needed to predict risk of
pollution. TE can be integrated in cement phases like calcium silicate hydrate (CSH), Ca-Al hydroxy sulfate or
Ca-Zincate. The aim of this study is the determination of initial pollutant locations and TE-bearing phases in the
cement matrix. A comparison of hydration reaction between synthetic sludge and mining waste is provided.
Samples come from two sets of cement matrix, the first one is Ordinary Portland Cement (OPC) only and the
second one is a mix of OPC and Fly Ash (FA). Each set is subdivided as a function of the pollutant added that
are either synthetic sludges (Cd, Cu, Pb, Zn) or tailings (from a copper and zinc mine at Winston-Lake, Ontario,
Canada). Initial TE-bearing phases were determined using physical characterization (XRD, SEM, microprobe).
Sample with FA were depleted in portlandite and rich in CSH. This CSH and the gel are the main TE-bearing
phases in synthetic sludge samples, whereas pollutants from tailings are localised in sulfide minerals and iron
oxides.
Keywords: stabilization, solidification, tailings, chemical and physical characterization.
1. Introduction
The Winston Lake underground mine is located in northwesten Ontario, Canada, approximately 210 km east of
Thunder Bay. The Winston zinc-copper Volcanogenic Massive Sulphide (VMS) deposit has a typical
composition of 30-40% sphalerite, 3-5% chalcopyrite and 5-10% pyrite. Between 1985 and 1988, approximately
3.3 Millions tonnes of ore containing 14.3% Zn, 1% Cu, 32 gpt Ag and 1.4 gpt Au were mined and processed
onsite until January 1999. The Winston concentrator produced zinc concentrate at 54% Zn, copper concentrate at
27% Cu, 350 gpt Ag and 14 gpt Au, and an alkaline tailings slurry. This alkaline tailings slurry has pH = 9.3 due
to lime added during copper and zinc flotation, (Catalan et al., 2000). On the basis of ten whole-rock and
Inductive Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) analyses and three X-ray diffraction
analyses completed in 1996 and 1997, Eger et al. (2000) determined that non-oxidized Winston tailings contain
25 – 30% quartz, 15 – 20% feldspar, 22 – 27% pyrrhotite, 13% pyrite, and 1.8% sphalerite. Oxidation kinetics
tests and subsequent column leach tests were conducted as part of the Winston closer plan. Both types of tests
confirmed that the tailings were reactive and Potentially Acid Generating (PAG). Oxidation of iron sulfide
minerals presents in tailings induces Acid Mine Drainage (AMD) that causes important damage to the
environment. This reaction occurs with iron sulfides, oxygen and water, as the following equation:
2FeS 3H2O 9 O2 2FeOOH 4H  2SO4
2
2
[1]
Metal ions (e.g. Cu, Mn, Ni, Pb, and Zn) are often incorporate as solid solution in iron sulfate salts or may
occasionally form non iron-bearing sulfates. Sulfides dissolution releases these metal ions and decreases the pH
*
Author/s to whom correspondence should be addressed: Mail: violaine.mijno@unilim.fr - Phone: 33-(0)55-545-7413 - Fax
33-(0)55-545-7413
to acidic condition. Variations of chemical conditions influence metal speciation, and adsorbed metal will be
released in solution. Water covers of tailings process is used to limit oxygen diffusion but metal ions are still
released (Li et al., 1997; Aubertin et al., 1997). Cement based stabilization/solidification (s/s) processes use
cement chemical and physical properties to integrate TE during their hydration reactions. Paste backfill
technology developments are stakes to reduce costs associated with backfilling large open stopes or to fill up
underground mine, (Benzaazoua et al., 1999). Another attractive aspect is that a large amount of tailings are acid
generating and properties of cement (basic pH and high amount of calcium), decrease the risk of AMD.
Hydration of cement results in the formation of multiple mineral phases, including portlandite (Ca(OH) 2),
Calcium
Silicate
Hydrate
(CSH),
ettringite
(Ca6Al2(SO4)2.26H2O),
and
monosulfate
([Ca2(Al,Fe)OH6]2(SO4).xH2O) according to Taylor (1997). When carbon is present calcite (CaCO3) can also
forms. Ettringite can immobilize contaminant metals by substitution of Ca 2+, Al3+, and SO42- (Gougar et al.,
1996; Albino et al., 1996). Metals can also be incorporated into CSH by sorption, mixing or substitution. X-ray
Absorption Fine Structure Spectroscopy (XAFS) and Electron Probe Micro-Analyses (EPMA) provide evidence
that incorporation into CSH is the main mode of Zn immobilization when it is added in cement based material as
synthetic sludge (Ziegler et al., 2001a and b; Johnson et al,. 1999). Determination of TE-bearing phases in s/s
wast will be provided in this study.
OPC is the most used cement matrix for s/s process. Mijno et al. (2004), show that portlandite is the first phase
to dissolve followed by CSH and Ca-Al hydroxy sulfates. Portlandite is a buffer and release hydroxyls ions
during its dissolution reported by Catalan et al. (DATE), and Mijno et al. (2004). Benzaazoua et al. (1999)
explain the loss of strength and the increase of porosity by portlandite dissolution. Addition of 25 to 35 wt. % fly
ash to OPC in the cement matrix increases strength of materials due to further precipitation of CSH that
consumes a part of portlandite according to Benzaazoua et al. (1999) and Conner and Hoeffner (1998).
Comparison of cement hydration reaction as a function of the pollutant added has not been completely resolved
albeit main chemical models used data provided from synthetic sludge for long term assessment. On the other
hand addition of FA in cement matrix decreases portlandite initial amount but CSH precipitation can be
dependant of the pollutant. Comparison of s/s waste, with mine tailings and synthetic sludge will be provided
with cement matrix set containing OPC or OPC and FA.
2. Materials and Methods
2.1 Sample preparation
First set of cement matrix is only OPC whereas the second one consists in FA and OPC with respectively 33.3
wt. % and 66.7 wt.%. Heavy metal sludges were prepared as a solution of 0.1 M Cd(NO 3)2.4H2O, 0.1 M
Cu(NO3)2.2.5H2O, 0.1 M Pb(NO3)2, 0.1 M Zn(NO3)2.6H2O and a 6M NaOH solution was added to reach an end
point pH of 9.0. Ultra pure water (Barnstread Nanopure Diamond) was used to prepare all solutions. Heavy
metals concentrations in the sludge were within the range of concentrations encountered in real wastes that are
treated by s/s, EPA/542-R-00-010 (2000) according to Catalan et al. (2002). Weight ratio of synthetic sludge to
cement matrix is fixed to 0.4 (Poon et al., 1997; Park, 2000).
To be representative of the whole tailings, samples were collected using a soil corer at four different locations.
At each location, the depth of oxidation was assessed visually and two composite tailing samples were collected:
one corresponds to the oxidized layer and the second one to the reduced layer (50 cm below the oxidized layer).
Thickness of the oxidized layer varied with location and ranged from 0.03 and 0.5 cm. Each of the four chosen
spots represent 25 wt. % of their respective sample (oxidized or reduced). Oxidized and reduced tailings form
two different pollutants. Before being used to prepare samples they were dehydrated using freeze-dryer
apparatus, Labconco Freezone 12, during 36 hours at –20°C. Amount of tailings, cement matrix and water was
in good agreement with data from Sanchez et al. (2003). Weight percentage of sample and name are reported in
table 1. Required amounts of OPC, FA, synthetic sludge, tailings and water were placed in a pre-washed with
nitric acid plastic bowl and thoroughly mixed with a plastic spatula. Mixtures were then poured in 5 layers into
PVC molds (4 cm diameter × 10 cm height) lined with parafilm. Each layer was rodded with a rounded end
plastic tamping rod at a rate of 25 strokes per layer to escape air bubbles formation. Samples were then closed
hermetically using a double coat of parafilm and duke-tape for at least twenty-eight days.
Table 1: Amount of material used (all in weight %):
OPC
FA
synthetic sludge
water
Oxidized tailings
Reduced tailings
S-A
S-B
S-I
S-J
S-K
S-L
71.4
47.6
23.8
28.6
21.0
14.0
7.0
22.2
14.8
7.4
26.3
52.7
26.3
52.7
22.2
22.6
55.6
55.6
28.6
2.2 Samples analysis
Total composition of tailings (oxidized or reduced), OPC and FA were analysed using ICP-AES after microwave
digestion using EPA Method 3051 (1994). Thin section were prepared by impregnating tailings or thick slide of
s/s waste with epoxy resin and polishing them with a diamonded suspension to obtain a smooth grain section.
Observation using an optical microscope (Nikon) was used to ensure that samples were homogeneous in thin
section scale and to choose selected zones for Scanning Microscopic Electron (SEM). The thin sections were
then carbon-coated for observation with SEM and analysis with microprobe. Morphological and semiquantitative chemical analyses were carried out using SEM. The SEM apparatus is a Philips XL-30 model
equipped with an X-ray energy dispersion spectroscopy system (EDS), a backscattered electron (BSE) detector
and a secondary electron (SE) detector (accelerating voltage of 20 kV). A CAMECA SX-50 equipped with an
EDS system and 4 wavelength dispersive spectrometers (WDS, accelerating voltage of 15 kV, beam current 10
nA, 10s counting time, beam diameter up to 5 µm) was used for the electron microprobe analyses. Standard
reference materials used for the electron to calibrate the instrument for qualitative analysis include natural and
synthetic silicates, oxides and sulphide minerals. S/s wastes were characterized by X-ray diffraction using a Cu
K radiation on a Siemens D5000 diffractometer, equipped with a diffracted-beam graphite monochromator.
Prior to XRD analysis, s/s waste samples were finely powdered in an agate mortar, mounted on a powder holder,
and the powder surface was smoothed with a glass slide. XRD patterns were carried out in the range of 2° < 2θ <
120° with a 0.04° step and a counting time 15 s per step. The sample d-spacings were compared with data from
the Bruker Diffract plus EVA Search/Match Software, Bruker AXS Ltd., Congleton, UK.
3. Results and discussion
3.1 Tailings characterization
Tailings have a silty structure granulometry with a size distribution of 59 wt. % > 74 µm and 22 wt. % < 37 µm.
Optical observations of tailings phases show orthopyroxenes (enstatite and hypersthene spoiled), quartz, biotite
and opaque minerals. ICP-AES data from oxidized and non-oxidized tailings are presented in table 2.
Differences in total composition from oxidized and reduced tailings are minor, although oxidized samples seem
to contain less iron and sulfur, as expected from AMD.
Table 2. ICP-AES analyses from initial material (major elements, Al Ca, Fe, K, Mg, Na, S, and Si are
in wt. %, minor elements Cd, Co, Cr, Cu, Mn, Pb, Ti, and Zn are in ppm).
Reduced tailings
Oxidized tailings
Fly Ash
OPC
Detection limit
Al
1.46
1.40
9.22
2.62
2.5E-04
Ca
0.75
0.42
10.17
37.39
1.0E-04
Fe
20.97
16.77
3.32
1.34
5.0E-05
K
0.42
0.37
0.24
0.91
5.0E-04
Mg
1.94
1.76
2.04
1.36
1.0E-04
Na
0.05
0.04
5.66
0.20
1.0E-04
S
11.71
7.97
1.01
1.60
5.0E-04
Si
0.11
0.10
0.26
0.20
2.5E-04
Reduced tailings
Oxidized tailings
Fly Ash
OPC
Detection limit
Cd
30
14
1
0
1
Co
278
173
12
7
1
Cr
37
37
26
74
1
Cu
1 572
1 019
41
13
1
Mn
289
204
118
308
1
Pb
152
76
33
18
3
Ti
805
763
3 128
1 253
3
Zn
12 276
4 827
57
92
1
Table 3: Amount in ppm of TE in pyroxene, pyrite, sphalerite and iron oxide.
pyroxene
number of
analyses
As
Cd
Cu
Pb
Zn
a
10
Oxidized Tailings
pyrite
sphalerite
(FeS2)
(Fe,Zn)S
8
n.d.a
600 ± 5b
50 ± 1
530 ± 1
583 ± 1
4
1 487 ± 2
51
1 043
n.d.
1 756
3 550
n.d.
337
n.d.
266 095
iron oxide
pyroxene
5
8
1 243
463
2 385
138
8 485
Reduced Tailings
pyrite
sphalerite
(FeS2)
(Fe,Zn)S
7
n.d.
n.d.
120
n.d.
1 887
6
94
108
253
n.d.
115
134
716
504
n.d.
369 238
iron oxide
3
500
n.d.
n.d.
510
1 130
not detected.
b standard
deviation
Chlorite, jarosite, and goethite were identified with XRD analyses coming from an alteration of pyroxenes and
iron sulfides. Opaque minerals (pyrite, sphalerite, and iron oxide) are the main TE-bearing phases but pyroxenes
can also integrate copper, zinc and titanium. The average amounts of metal ions (e.g. As, Cd, Cu, Pb, and Zn) in
oxidized and reduced tailings are given in table 3. TE in oxidized tailings are associated as following: As with
sphalerite, pyrite and iron oxide, Cd with pyroxenes and pyrite, Cu with iron oxides and pyrite, Pb with
pyroxenes and iron oxides and Zn with sphalerite ((Zn,Fe)S), iron oxides, pyrite and pyroxenes. TE-bearing
phases in reduced tailings are sphalerite, iron oxides for As, sphalerite and pyrite for Cd, sphalerite pyrite and
pyroxene for Cu, iron oxide for Pb, and iron oxides and pyroxene for Zn.
3.2 S/s waste characterization
Figure 1 shows a Back-Scattered Electron (BSE) image of the sample S-K (oxidized tailings stabilized with only
OPC). The main phases were identified to be (a) pyroxene, (b) pyrite, (c) sphalerite, (d) quartz, (e) unhydrated
Ca3SiO5, (f) portlandite, and (g) CSH. XRD analyses confirm the presence of all these phases whereas no
ettringite was clearly identified. Calcium silicate hydrate coats pyroxene and TE-sulfur. Sample show and
homogeneous mix of tailings and cement matrix. Small cracks are present mainly situated in the few unhydrated
cement particle. Portlandite is not detected in sample with cement matrix being a mixture of OPC and FA,
neither with XRD, optical microscope or microprobe analyses. Portlandite is not detected in samples S-J and SL, OPC and FA compose their cement matrix.
BSE
f
e
c
b
b
d
a
g
a
Figure 1: Backscattered electron image of unleached S-K sample particle showing (a) pyroxene, (b) pyrite, (c) sphalerite, (d)
quartz, (e) unhydrated Ca3SiO5, (f) portlandite, and (g) calcium silicate hydrate.
a
a
g
BSE
a
c
e
d
d
b
a
Figure 2: Backscattered electron image of unleached S-A sample particle showing (a) sulfur TE-bearing phases, (b) gel, (c)
portlandite, (d) calcium silicate hydrate, and (e) ettringite.
3.3 Synthetic sludge s/s characterization
Figure 2 shows a backscattered electron image of the sample S-A (synthetic sludge and OPC). XRD analyses
indicated presence of portlandite, gel, CSH and ettringite. Matrix is well hydrated with only few unhydrated
phases and no cracks network. TE-bearing phases were identified as CSH and gel. Respective amount of TE in
gel and CSH are 3 778 and 584 ppm of Cd, 1 198 and 1 080 ppm of Cu, 3 778 and 380 ppm of Pb, and 2 895 and
1 892 ppm of Zn. Locally, small (diameter 2µm) TE-bearing phases appear brightly in BSE, they were identified
by microprobe analyses as iron sulfur (35.59 wt. % of Fe and 41.63 wt. % of S), with 85 ppm of As, 1 215 ppm
of Cd, 1 215 ppm of Cu, 9 563 ppm of Pb and 72 113 ppm of Zn. Comparison of sample S-A and S-B (cement
matrix is a mixture of OPC and FA) using XRD analyses, indicates a slightly decrease of portlandite amount.
4. Conclusion
During weathering, alteration products of oxidized tailings rich in pyroxene are mainly amorphous iron oxides
and chlorite. Iron oxides can incorporate significant levels of different metal ions in the crystal structure (Cornell
and Schwertmann, 1996). Main TE-bearing phase are sphalerite for As and Zn, pyroxene for Cd and Pb, and iron
oxide for As, Cu and Zn. Iron oxides are stable for pH close to neutral conditions and seem to be a good TEtrapper phase. Influence of dissolution of pyrite and formation of iron oxides in s/s matrix would be used for
long-term assessment of this remediation method. The CSH and the gel are the main TE-bearing phases in
sample using synthetic sludge. FA addition consuming portlandite to form further CSH is a positive point to
stabilized metal ions. Further studies on evolution of these TE-bearing phases during leaching process in batch
cells.
References
Albino V., R. Cioffi, M. Marroccoli, and L. Santoro, 1996, J. Hazard. Mater., 51, 241-252.
Aubertin M., J. Dionne, and L. Marches, 1997, Design guidelines and stability criteria if engineering works for water, In
Proceeding of the Fourth International Conference on Acid Rock Drainage, May 31-June 6, 1997, Vancouver, British
Columbia, Canada, p. 1849-1866.
Benzaazoua M., J. Ouellet, S. Servant, P. Newman and R. Verburg, 1999, Cement and Concrete Research, 29, 719-725.
Catalan L.J.J., E. Merlière, M. Bliss, 2000, Evaluating of Lime Requirements to Neutralize Pre-Oxidized Sulphidic Mine
Tailings Prior to Submergence, to be published.
Catalan L.J.J., E. Merlière and C.J. Chezick, 2002, J. Hazard. Mater., B94, 63-88.
Conner J.R. and S.L. Hoeffner, 1998, A Critical Review of Stabilization/Solidification Technology, Critical Reviews in
Environmental Science and Technology, 28 (4), 397-462.
Cornell R.M., and U. Schwertmann, 1996, The Iron Oxides, Structures, Properties, Reactions, occurrence and Uses, VCH,
Weiheim, 573 p.
Eger P., G. Melchert, D. Anderson, J. Wagner, and J. Folman, 2000, Creating Wetlands on Acid Generating Tailings –
Maintenance free Reclamation? Proceedings of the 5th International Conference on Acid Mine Drainage, Denver,
Colorado, 21-24 May, Vol. II, 1149-1158 Society for Mining Metallurgy and Exploration Inc.
Environmental Protection Agency (EPA), 2000, Solidification/Stabilization Use at Superfund Sites; EPA/542-R-00-010;
Office of Solid Waste and Emergency Response, Technology Innovation Office, U.S. EPA: Washington, DC.
Environmental Protection Agency (EPA), 1994. Method 3051: Microwave assisted acid digestions of sediments, sludges, and
oils. In SW-846 On-line, Tests Methods for Evaluating Solid Waste, Physical/Chemical Methods. Office of Solid Waste,
U.S. EPA, Athens, Georgia. Available from http://www.epa.gov/epaoswer/hazwaste/test/main.htm [cited July, 26, 2004].
Gougar M.L.D., B.E. Scheetz and D.M. Roy, 1996, Waste Manage., 16, 295-303.
Johnson C.A., and M. Kersten, 1999, Environ. Sci. Technol., 33, 2296-2298.
Li M.G., B. Aubé, L. St-Arnaud, 1997, Consideration in the use of shallow water covers for decommissioning reactive
tailings, in Proceeding of the Fourth International Conference on Acid Rock Drainage, May 31-June 6, Vancouver, British
Columbia, Canada, p. 115-130.
Mijno V., L.J.J. Catalan, F. Martin and J.-C. Bollinger, 2004, Journal of Colloid and Interface Science, 280, 465-477.
Park C.K., 2000, Cem. Concr. Res., 30, 429-435.
Poon C.S., K.W. Lio, 1997, Waste Manage., 17, 15-23.
Sanchez F., A.C. Garragrants, C. Vandecasteele, P. Moszkowicz and D.S. Kosson, 2003, J. Hazard. Mater., B96, 229-257.
Taylor H.F.W., 1997, Cement Chemistry, Telford, London.
Ziegler F., A.M. Scheidegger, C.A. Johnson, R. Dähn and E. Wieland, 2001a, Environ. Sci. Technol., 35, 1550-1555.
Ziegler F., R. Gieré and C.A. Johnson, 2001b, Environ. Sci. Technol., 35,4556-4561.
Acknowledgements
This work was funded by the Regional Council of the Limousin Region (France), and the European Social Funds
from the European Community. We wish to acknowledge gratefully the technical and chemical assistance of
Michel Peymirat of HydrASA (Limoges, France), Keith Pringnitz and Ain Raitsakas of LUIL (Thunder-Bay,
On, Canada).