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Long-term neutralisation potential of red mud bauxite with brine amendment for the neutralisation of acidic mine tailings
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Applied Geochemistry Applied Geochemistry 22 (2007) 2326–2333 www.elsevier.com/locate/apgeochem
Long-term neutralisation potential of red mud bauxite with brine amendment for the neutralisation of acidic mine tailings M. Paradis a
a,b
, J. Duchesne
b,*
, A. Lamontagne a, D. Isabel
a
SNC-Lavalin Mine et Me´tallurgie Inc., 5500, des Galeries, Bureau 200, Que´bec, Qc, Canada G2K 2E2 b De´partement de Ge´ologie et de Ge´nie Ge´ologique Universite´ Laval, Que´bec, Qc, Canada G1K 7P4 Received 20 July 2004; accepted 18 April 2007 Editorial handling by B. Kimball Available online 8 June 2007
Abstract Acid mine drainage is an environmental problem produced when sulphide in mine tailings dams comes in contact with O2, bacteria and water. Oxidation occurs and a succession of reactions is responsible for producing acid and metals in the environment. One solution proposed is to use red mud bauxite (RMB) that is very alkaline to neutralise acidic tailings. Previous experiments showed that RMB has a good neutralisation capacity for a short time, but the long-term neutralisation potential is uncertain. So brine was added to RMB to verify if it can improve long-term alkalinity retention of RMB. Some authors have presented results where seawater or other natural or artificial Ca- and Mg-rich brines were used with RMB to convert the basicity (mainly NaOH) and other soluble alkalinity into low solubility hydroxide, carbonate and hydroxycarbonate minerals. Results showed that neutral pH conditions were maintained over the entire test for RMB and a mixture of RMB with brine. Addition of brine to RMB slightly lowered the pH compared to RMB alone. RMB alone lost a lot of dissolved alkalinity at the beginning of the test. Most of the alkalinity was lost in water after a few flushes for RMB samples. The addition of brine helped to keep neutralisation potential over more cycles of leaching. Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction Mining industries are facing a serious environmental problem with acid mine drainage (AMD). During mining operations, the end product discharged from an ore concentrator plant does not contain economic minerals in sufficient quantity and is rejected as tailings. These tailings can contain * Corresponding author. Tel.: +1 418 656 2177; fax: +1 418 656 7339. E-mail address: [email protected] (J. Duchesne).
different sulphide minerals that produce AMD when O2 comes in contact with these sulphides. Oxidation includes a succession of reactions that are responsible for acid and metals leaching in the environment. Many solutions have been proposed for this problem. One solution is to use alkaline material to neutralise oxidised tailings dams (Perry and Brady, 1995; Rose et al., 1995). Lime is the neutralising material often used for rehabilitation. But lime is expensive, and the pH of quicklime used to treat tailings decreases over time. The decrease is attributed to the low buffering capacity of quicklime
0883-2927/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2007.04.021
M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333
treated tailings and to the consumption of hydroxide ions by incongruent dissolution of water-insoluble Fe oxyhydroxysulfate minerals (Catalan and Yin, 2003). Fortin et al. (2000) proposed using red mud bauxite (RMB), an alkaline by-product from the Al industry, to neutralise acid sulphate tailings. Amendment with RMB may result in a more durable reduction in metal mobility than liming, and also a smaller risk of metal re-mobilisation if pH were to decrease (Lombi et al., 2002). The RMB containing a lot of hydroxides would play the role of a substrate on which metal ions would be adsorbed or coprecipitated with Fe-oxyhydroxides. Experiments show that RMB has a good neutralisation capacity on a short time scale but expected results are uncertain for the long-term neutralisation potential (Doye and Duchesne, 2003). The present study presents experiments made on RMB and on a mix of RMB and brine to characterise their long-term neutralisation potential on acidic mine tailings. The addition of brine to the RMB should improve the neutralization by improving the longterm alkalinity of RMB. McConchie et al. (1999) presented a study where seawater or other natural or artificial Ca- and Mg-rich brines were used with RMB to convert the basicity (mainly NaOH) and other soluble alkalinity into low solubility hydroxide, carbonate and hydroxycarbonate minerals. Paradis et al. (2006) presented a column leaching experiment where Ca–Na–Cl brine was added to the RMB to evaluate its influence on water quality. In this study, brine was added to RMB to verify if it can improve long-term alkalinity of RMB. 2. Materials
to fix metals in soils (Qiao and Goen, 1996). RMB amendments shifted metals from the exchangeable to the Fe-oxide fraction, and decreased acid extractability of metals (Lombi et al., 2002). RMB has a great specific surface area which creates a high reactivity with the solution it contacts. The water content of the RMB sample studied was 28.2% with a pH value of 12. X-ray diffraction analyses show that RMB samples were composed mainly of hematite (Fe2O3), goethite (FeO(OH)), boehmite (AlO(OH)), anatase (TiO2), gibbsite (Al(OH)3), sodalite (Na8Al6Si6O24Cl2), rutile (TiO2), calcite (CaCO3) and quartz (SiO2). The chemical composition of RMB is presented in Table 1. 2.2. Brine samples Natural brine samples come from an aquifer 1000-m deep in Becancour, Qc, Canada, and were provided by Junex-Solnat Inc. The physical and chemical descriptions of brine are shown in Table 2. The sample of brine has a salinity of 33% compared to seawater that has a salinity around 3%. 2.3. Natural acid sulphate tailings Samples were taken in tailings dams located in Abitibi, Qc, Canada. The samples were taken at the surface of tailings, in the most oxidised part of the site. Water content of the shallow 0–15 cm tailings sample was 12.5% with a pH value around 3.5. Chemical composition of the tailings is shown in Table 1. Table 1 Chemical composition of materials Elements
RMB
Tailings (T(O)) 0–15 cm deep
Tailings (T(NO)) 15–35 cm deep
SiO2 (%) Al2O3 (%) Fe2O3 (%) MnO (%) MgO (%) CaO (%) Na2O (%) K2O (%) TiO2 (%) P2O5 (%) Stot (%) Cu (ppm) Pb (ppm) Zn (ppm) As (ppm)
12.63 20.65 35.26 0.025 0.24 4.86 7.16 0.11 6.152 0.17 0.232 38 51 31 33
73.87 10.03 6.81 0.031 2.37 0.11 0.45 1.94 0.589 0.16 0.199 128 12 416 15
72.14 10.03 6.81 0.031 2.37 0.11 0.45 1.94 0.589 0.16 7.04 935 40 1187 15
2.1. RMB samples RMB samples were from Alcan International, Arvida, Quebec. This residue was produced from alkaline extraction of alumina from bauxite by the Bayer process. In the Bayer process, NaOH and lime are first added to bauxite, and the mix is heated under pressure to separate the sodium alumina (NaAlO2). After the treatment, a solution of NaOH, Na2CO3 and sodium aluminate remains (Fortin, 1991). RMB is red because of its high Fe-oxide content, particularly because of the presence of hematite (Fe2O3). RMB is used for several of its properties. First, RMB is recognised for its capacity
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Table 2 Physical and chemical properties of brine Parameters
Units
Value
Total dissolved solid Chloride Calcium Sodium Magnesium Potassium pH Specific gravity Viscosity Resistivity Freezing point
mg/L mg/L mg/L mg/L mg/L mg/L
270,000–290,000 145,000–150,000 53,000–58,000 30,000–35,000 1400–1800 800–1000 5.5 1.1767 1.6 5 30
Cl Ca Na Mg K
(p/v) CSt Ohms/cm Celsius
Acid–base accounting results were compared between shallow 0–15 cm and deep 15–35 cm tailings (Table 3). TAP is the total acidification potential of the sample and SAP is the S acidification potential, i.e., if it is assumed that only the non-oxidised S could react. Tailings in the first 15 cm, named shallow tailings or oxidised tailings T(O), showed distinct signs of oxidation by their orange colour. Deeper samples (15–35 cm), named non-oxidised tailings T(NO), were grey. According to acid– base accounting results for the shallow 0–15 cm samples most of the sulphide has been oxidised. In total, 76% of the total S comes from SO4, the product of the acidification reaction. Mineral phases detected by XRD were quartz, chlorite ((Mg, Fe)3(Si, Al)4O10(OH)2 Æ (Mg, Fe)3(OH)6) and alkali feldspar (KAlSi3O8). No sulphide was observed. Scanning electron microscope observations showed that the sample was very oxidised. The neutralisation potential (NP) was measured in the laboratory by the Sobek method (Sobek et al., 1978). The net neutralisation potential (NNP) was represented by the NP–SAP. The sample had NNP of 2.34 kg CaCO3/t. According to Miller et al. (1991), a NNP value between 20 and 20 kg CaCO3/t, measured by the Sobek et al. (1978) method, reveals that the acidity generation potential is uncertain. According to Ferguson and Robertson
(1994), a value of the NP/AP ratio greater than 2.0 represents tailings which are not giving off acid. So, the 0–15 cm deep tailings samples needed to be neutralised, but it was not necessary to take into account future oxidation. On the other hand, the 15–35 cm deep tailings sample had a NNP value of 157.2 kg CaCO3/t which will likely result in acid drainage if not protected. 2.4. Mixture of brines and RMB The addition of brine to the RMB samples was responsible for the precipitation of secondary minerals as observed by X-ray diffraction. Hematite, boehmite, goethite and quartz were still present. Secondary minerals halite (NaCl) and aluminite (Al2(SO4)(OH)4 Æ 7(H2O)) were also observed. Most of the minerals present were Fe and Al oxide and hydroxide minerals (hematite, goethite, aluminite). 3. Experimental methods 3.1. Sample preparation A mixture of RMB and deionized water (ratio 2:1) and a mixture of RMB with brine (weight ratio 2:1) were stirred constantly for two months. Samples were put in 100-mL high density polyethylene bottles which were mounted horizontally on a Plexiglas carousel that was suspended in a temperature bath maintained to 20 °C. The carousel was rotated between 10 and 20 rpm for a period of two months. Samples were run in duplicate. Samples were agitated until equilibrium conditions were reached. Stable conductivity of the sample was used as an indicator of equilibrium. Tests were run with samples of RMB alone, RMB mixed with brine, and for samples with RMB mixed with tailings and RMB and brine mixed with tailings. The aim of these experiments was to characterise the long-term neutralisation potential of those mixtures. For the sample containing tailings, RMB
Table 3 Acid base accounting result Samples
pH
S(T)%
SO4%
Tailings (0–15 cm deep) Tailings (15–35 cm deep)
3.94
0.19
0.44
6.20
7.04
4.38
TAP kg CaCO3/t
SAP kg CaCO3/t
5.94
1.41
220
174.7
NP kg CaCO3/t 3.75 17.5
NP/SAP 2.66 0.10
NNP NP-SAP kg CaCO3/t 2.34 157.2
M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333
was mixed with tailings in a ratio of 10% RMB by mass with 3 g of RMB mixed with 30 g of 0–15 cm deep tailings from an acidic tailings dam and 3 g of RMB and brine (ratio 2:1) mixed with 30 g of 0– 15 cm deep tailings. The tests were repeated with the 15–35 cm deep samples from the same acidic tailings dam in the same ratio.
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used as an indicator of the neutralising capacity of the sample. Mass balance was calculated on the 30 g samples. Results showed that the difference between solid alkalinity before and after the test was abnormally larger than the alkalinity lost in the liquid during the 30 cycles. This could be due to the presence of quartz in the sample and to the difficulty of leaching all 30 g of the sample. First, quartz was detected in XRD and SEM analyses. If the quantities of quartz (an inert material which did not add alkalinity to the sample) in the measurement of solid alkalinity was not the same in the samples tested before and after the cycles, then the alkalinity results are influenced because the total mass of material that input alkalinity was decreased by the mass of quartz. Secondly, for some samples part of the solid stuck to the side of the test tube, so not all the 30 g of sample was well leached with fresh water in all the cycles. For these reasons, the test was repeated with a smaller sample (3 g). All visible quartz grains were removed from the sample before the test. Mass balance was recalculated and alkalinity gained in the liquid was very close to the difference in alkalinity measured before and after the leaching cycles.
3.2. Measurements of neutralisation potential For the experiment of long-term alkalinity, two solid/liquid ratios were tested with 3 and 30 g of solid. Each solid sample was taken and put in a test container with 50 mL of deionized water. Samples were shaken for a minimum of 24 h. After shaking, samples were centrifuged for 1 h at 750 rpm or until the liquid and solid phases were distinctly separated. The liquid was analysed for alkalinity and pH. Solids were kept to repeat the procedure, and, a volume of 50 mL of deionized water was added to the solid. Each time, alkalinity and pH were measured on the liquid, and the solid was kept to repeat the test for a total of 31 cycles. Alkalinity was measured on the solid samples prior to and after the entire test to compare with the alkalinity measured in the liquid phase after each flush. After each cycle the volume of 50 mL of water was titrated with 0.01 N H2SO4 solution to an endpoint of 4.5. Solid alkalinity was measured by a titration on the solid sample. Two g of the solid sample were mixed with deoinized water. Alkalinity was expressed in mg CaCO3/L (ppm) for liquid and in mg CaCO3/kg (ppm) for solid. Alkalinity was
4. Results The pH evolution after each flush is shown in Fig. 1. RMB had a very high alkaline pH at the beginning of the test and pH stabilised between 11 and 12 after seven flushes. RMB with brine (RMB/ B) had a lower pH than RMB at the beginning of
31
29
27
25
23
21
19
17
15
13
9
11
7
5
3
14 13 12 11 10 9 8 7 6 5 4 1
pH
Evolution of pH
Flush Number RMB
RMB/B
RMB/T(NO)
RMB/B/T(NO)
RMB/T(O)
Fig. 1. pH evolution of mixtures after each flush.
RMB/B/T(O)
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M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333
the test and stabilised at the same pH as RMB after seven flushes. The mixture of 10% of RMB mixed with oxidised mining tailings (RMB/T(O) or shallow tailings) gave a pH of over 8 for all 30 flushes. The same
behaviour was obtained with the sample of RMB mixed with brines and tailings (RMB/B/T(O)). The most oxidised tailings (O) with a starting pH of 3.24 gave pH lower than mixtures with less oxidised tailing (NO or deep tailings) that had a pH
Alkalinity (mgCaCO3/L)
a 10000 1000 100 10 1 1
3
5
7
9
11 13 15
17 19 21 23 25 27
29 31
Number of cycles RMB
RMB/B
RMB/T(NO)
RMB/B/T(NO)
RMB/T(O)
RMB/B/T(O)
Alkalinity (mgCaCO3/L)
b 250 200 150 100 50 0 1
3
5
7
9
1 1 13 15 17 19 21 23 25 27 29 31
Number of cycles
Alkalinity (mg CaCO3/kg)
RMB/T(NO)
RMB/B/T(NO)
RMB/B/T(O)
RMB/T(O)
c 140000 120000 100000 80000 60000 40000 20000 0 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Number of cycles RMB
RMB/B
Fig. 2. Variation of alkalinity with the number of cycles for (a) liquid alkalinity of 30 g samples, (b) mine tailings mixed with RMB (10%) and (c) solid alkalinity of RMB and RMB and brine before and after the flushes.
M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333
6.2. All those samples kept neutral pH all through the test. In Fig. 2a dissolved alkalinity started around 10,000 mg CaCO3/L for the RMB sample and was leached mostly in the first three flushes. Dissolved alkalinity for the RMB with brine sample had low values (around 300 mg CaCO3/L) for all the flushes. Dissolved alkalinity of RMB matched the values of RMB with brine after seven flushes. Most of the dissolved alkalinity of RMB in the liquid part was gone after seven flushes (Fig. 2a) where the pH became stable (Fig. 1). Fig. 2b shows the alkalinity of the liquid in contact with a mixture of 10% RMB with tailings. Alkalinity of RMB (10%) mixed with deep 15–35 cm tailings (RMB/T(NO)) had a starting value of alkalinity greater than the same mixture with brine (RMB/B/T(NO)). Values became stable after 12 flushes for most of the mixtures. Mixes of RMB with shallow 0–15 cm tailing with (RMB/T(O)) or without (RMB/B/T(O)) brines showed the same alkalinity curves.
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Alkalinity on the solid was measured before and after the test (Figs. 2c, 3b) for the two liquid/solid ratios. It can be observed that the difference between the alkalinity of the solids before and after the flushes is largely due to the presence of brine. This remark is valid for the tests with a starting mass of 30 g (Fig. 2c) and those with 3 g (Fig. 3b). The sample of RMB showed an alkalinity level of 132,500 mg CaCO3/kg before the flushes and an alkalinity level of 38,200 mg CaCO3/kg after the flushes for 30 g RMB samples. The sample of RMB with brine showed an alkalinity level of 119,300 and 78,700 mg CaCO3/kg before and after the flushes, respectively. For the 3 g samples, the alkalinity level was 69,000 mg CaCO3/kg for RMB before the flushes and 17,100 mg CaCO3/kg after five cycles. RMB with brine had an alkalinity level of 55,500 mg CaCO3/kg before and 46,800 mg CaCO3/kg after five flushes. Fig. 2b shows that alkalinity of RMB (10%) mixed with deep 15–35 cm tailings (RMB/T(NO)) had a starting value of alkalinity greater than the
Alkalinity (mg CaCO3/ Litre)
a 1400 1200 1000 800 600 400 200 0
Alkalinity (mg CaCO3/kg)
1
2
3
4
5
b 80000 70000 60000 50000 40000 30000 20000 10000 0 0
1
2
3
4
5
6
Number of cycles RMB
RMB/B
Fig. 3. Variation of alkalinity of RMB and RMB/brine for a 3 g sample. (a) Dissolved alkalinity level in liquid and (b) solid alkalinity measured before and after the flushes.
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M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333
same mixture with brine (RMB/B/T(NO)). Values became stable after 12 flushes for most of the mixtures. Mixes of RMB with shallow 0–15 cm tailing with (RMB/T(O)) or without (RMB/B/T(O)) brines showed the same alkalinity curves. Deep T(NO) only needed 3% of RMB to have neutral pH, and 10% was applied, so non-used alkalinity for neutralisation with T(NO) was leached in water. But for the same mix with brine (RMB/B/T(NO)) non-used alkalinity remained in the sample and showed the same behaviour as the shallow T(O) sample that need 6% instead of 3% (T(NO)) RMB to have neutral pH. 5. Discussion
brine. It appears that the addition of brine may help to convert easily soluble alkalinity to less soluble alkalinity. The samples with oxidized tailings, RMB/T(O) and RMB/B/T(O) samples, showed similar results. These samples required more alkalinity to neutralise the acidity that was greater than in the unoxidized T(NO) sample. No surplus alkalinity was leached because all the neutralisation potential was used to neutralise the acidity. The deep 15–35 cm sample had less acidity to neutralise, so surplus alkalinity was leached out in the liquid rinse from the RMB sample (RMB/T(NO), Fig. 2). With addition of brine, however, alkalinity was not leached out (RMB/T/B(NO), Fig. 2) and could be available to neutralise future oxidation products.
5.1. Addition of brine to RMB 6. Conclusions Neutral pH conditions were maintained through the test for RMB and RMB with brine. Addition of brine to RMB initially lowered the pH compared to RMB alone. But pH of the two mixes reached the same value after seven flushes. RMB leachate had high dissolved alkalinity at the beginning of the test, but matched the value of RMB with brine leachates after seven flushes. Most of the alkalinity was lost in water during the few first flushes of RMB. For the 3 and 30 g samples, a part of the solid alkalinity was lost with the flushes for RMB. For RMB with brine samples, the disparity between solid alkalinity measured before and after the flushes was smaller than for the RMB sample.
RMB is a useful material for neutralizing acidity in tailings. Addition of brine to RMB increases the long-term effect and did not impair the short-term neutralisation and leaching properties of the shallow (0–15 cm deep) oxidised part of an acidic tailings dam. Thus, neutralisation of acidity can be done with less chance of a future resumption of oxidation. For the deep 15–35 cm unoxidized samples, where acidification potential was still present, the use of RMB treated with brine may provide a long-term alkalinity reserve to neutralise future acidification of tailings.
5.2. Test of RMB and RMB/brine with tailings
References
Neutral pH conditions were maintained all through the test for RMB and RMB with brine mixed with mine tailings. The shallow 0–15 cm sample, or oxidized tailings, had an initial pH 3.94 which was lower than the deep 15–35 cm sample, or unoxidized tailings (pH 6.2). Addition of 10% mass of RMB to tailings brought stable pH at values near 8 for the shallow 0–15 cm sample and 10 for the deep 15–35 cm samples. The deep 15–35 cm tailings sample mixed with RMB (RMB/T(NO)) showed a surplus of dissolved alkalinity leached in the liquid compared to RMB with brine for the same mix (RMB/B/T(NO)). Addition of brines seemed to decrease liquid alkalinity leached out and samples kept a neutral pH (Fig. 1). Alkalinity measured on solids after several flushes was higher for the sample of RMB with
Catalan, L.J.J., Yin, G., 2003. Comparison of Calcite to Quicklime For Amending Partially Oxidized Sulfidic Mine Tailings Before Flooding. Environ. Sci. Technol. 37, 1408– 1413. Doye, I., Duchesne, J., 2003. Neutralisation of acid mine drainage with alkaline industrial residues: laboratory investigation using batch-leaching tests. Appl. Geochem. 18, 1197– 1213. Ferguson, K., Robertson, J., 1994. Assessing the risk of ARD. In: International Land Reclamation and Mine Drainage Conference and third International Conference on Abatement of Acidic Drainage, vol. 1, U.S. Bureau of Mines Spec. Publ. SP 06A-94, pp. 2–11. Fortin, J., 1991. Caracte´risation mine´ralogique et chimique de boues rouges et leur mise en vegetation. Me´moire de maıˆtrise, Faculte´ des sciences de l’agriculture et de l’alimentation. Universite´ Laval, Que´bec, Canada. Fortin, S., Lamontagne, A., Poulin, R., Tasse´, N., 2000. The Use of Basic Additives to Tailings in Layered Co-Mingling to Improve Acid Mine Drainage Control. In: Singhal, R.K.,
M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333 Mehrotra (Eds.), 1978, Proceedings of the 6th International Symposium on Environmental Issues and Waste Management in Energy and Mineral Production SWEMP 2000, Calgary, Canada. A.A. Balkema, Rotterdam. Lombi, E., Zhao, F.-J., Zhang, G., Sun, B., Fitz, W., Zhang, H., McGrath, S., 2002. In situ fixation of metals in soils using bauxite residue: chemical assessment. Environ. Pollut. 118, 435–443. McConchie, D., Clark, M., Hanahan, C., Fawkes, R., 1999. The use of seawater-neutralised bauxite refinery residues (red mud) in environmental remediation programs. In: Gaballah, I., Hager, J., Solozabal, R. (Eds.), Proceedings of 1999 Global Symposium on Recycling, Waste Treatment and Clean Technology, San Sebastian, Spain, vol. 1. The Minerals, Metals and Materials Society, pp. 391–400. Miller, S.D., Jefferey, J.J., Wong, J.W.C., 1991. Use and misuse of the acid base account for AMD prediction. In: Proceedings of second International Conference Abatement of Acidic Drainage, vol. 3, CANMET, Ottawa, Ontario, pp. 489– 506.
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Paradis, M., Duchesne, J., Lamontagne, A., Isabel, D., 2006. Using red mud bauxite for the neutralization of acid mine tailings: a column leaching test. Can. Geotech. J. 43, 1167– 1179. Perry, E.F., Brady, K.B., 1995. Influence of neutralization potential on surface mine drainage quality in Pennsylvania. In: Proceedings of 16th Surface Mine Drainage Task Force Symposium, April 4–5, 1995, Morgantown, WV. Qiao, L., Goen, H., 1996. The effect of clay amendment on speciation of heavy metals in sewage sludge. Water Sci. Technol. 34, 413–420. Rose, A.W., Phelps, L.B., Parizek, R.R., Evans, D.R., 1995. Effectiveness of lime kiln flue dust in preventing acid mine drainage at the Kauffman surface coal mine, Clearfield County, Pennsylvania. In: Proceedings of 12th American Society of Surface Mining and Reclamation Conference, June 3–8, 1995, Gillette, WY, pp. 159–171. Sobek, A.A., Schuller, W.A., Freeman, J.R., Smith, R.M., 1978. Field and laboratory methods applicable to overburden and mine soils. EPA 600/2-78-054.
Applied Geochemistry Applied Geochemistry 22 (2007) 2326–2333 www.elsevier.com/locate/apgeochem
Long-term neutralisation potential of red mud bauxite with brine amendment for the neutralisation of acidic mine tailings M. Paradis a
a,b
, J. Duchesne
b,*
, A. Lamontagne a, D. Isabel
a
SNC-Lavalin Mine et Me´tallurgie Inc., 5500, des Galeries, Bureau 200, Que´bec, Qc, Canada G2K 2E2 b De´partement de Ge´ologie et de Ge´nie Ge´ologique Universite´ Laval, Que´bec, Qc, Canada G1K 7P4 Received 20 July 2004; accepted 18 April 2007 Editorial handling by B. Kimball Available online 8 June 2007
Abstract Acid mine drainage is an environmental problem produced when sulphide in mine tailings dams comes in contact with O2, bacteria and water. Oxidation occurs and a succession of reactions is responsible for producing acid and metals in the environment. One solution proposed is to use red mud bauxite (RMB) that is very alkaline to neutralise acidic tailings. Previous experiments showed that RMB has a good neutralisation capacity for a short time, but the long-term neutralisation potential is uncertain. So brine was added to RMB to verify if it can improve long-term alkalinity retention of RMB. Some authors have presented results where seawater or other natural or artificial Ca- and Mg-rich brines were used with RMB to convert the basicity (mainly NaOH) and other soluble alkalinity into low solubility hydroxide, carbonate and hydroxycarbonate minerals. Results showed that neutral pH conditions were maintained over the entire test for RMB and a mixture of RMB with brine. Addition of brine to RMB slightly lowered the pH compared to RMB alone. RMB alone lost a lot of dissolved alkalinity at the beginning of the test. Most of the alkalinity was lost in water after a few flushes for RMB samples. The addition of brine helped to keep neutralisation potential over more cycles of leaching. Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction Mining industries are facing a serious environmental problem with acid mine drainage (AMD). During mining operations, the end product discharged from an ore concentrator plant does not contain economic minerals in sufficient quantity and is rejected as tailings. These tailings can contain * Corresponding author. Tel.: +1 418 656 2177; fax: +1 418 656 7339. E-mail address: [email protected] (J. Duchesne).
different sulphide minerals that produce AMD when O2 comes in contact with these sulphides. Oxidation includes a succession of reactions that are responsible for acid and metals leaching in the environment. Many solutions have been proposed for this problem. One solution is to use alkaline material to neutralise oxidised tailings dams (Perry and Brady, 1995; Rose et al., 1995). Lime is the neutralising material often used for rehabilitation. But lime is expensive, and the pH of quicklime used to treat tailings decreases over time. The decrease is attributed to the low buffering capacity of quicklime
0883-2927/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2007.04.021
M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333
treated tailings and to the consumption of hydroxide ions by incongruent dissolution of water-insoluble Fe oxyhydroxysulfate minerals (Catalan and Yin, 2003). Fortin et al. (2000) proposed using red mud bauxite (RMB), an alkaline by-product from the Al industry, to neutralise acid sulphate tailings. Amendment with RMB may result in a more durable reduction in metal mobility than liming, and also a smaller risk of metal re-mobilisation if pH were to decrease (Lombi et al., 2002). The RMB containing a lot of hydroxides would play the role of a substrate on which metal ions would be adsorbed or coprecipitated with Fe-oxyhydroxides. Experiments show that RMB has a good neutralisation capacity on a short time scale but expected results are uncertain for the long-term neutralisation potential (Doye and Duchesne, 2003). The present study presents experiments made on RMB and on a mix of RMB and brine to characterise their long-term neutralisation potential on acidic mine tailings. The addition of brine to the RMB should improve the neutralization by improving the longterm alkalinity of RMB. McConchie et al. (1999) presented a study where seawater or other natural or artificial Ca- and Mg-rich brines were used with RMB to convert the basicity (mainly NaOH) and other soluble alkalinity into low solubility hydroxide, carbonate and hydroxycarbonate minerals. Paradis et al. (2006) presented a column leaching experiment where Ca–Na–Cl brine was added to the RMB to evaluate its influence on water quality. In this study, brine was added to RMB to verify if it can improve long-term alkalinity of RMB. 2. Materials
to fix metals in soils (Qiao and Goen, 1996). RMB amendments shifted metals from the exchangeable to the Fe-oxide fraction, and decreased acid extractability of metals (Lombi et al., 2002). RMB has a great specific surface area which creates a high reactivity with the solution it contacts. The water content of the RMB sample studied was 28.2% with a pH value of 12. X-ray diffraction analyses show that RMB samples were composed mainly of hematite (Fe2O3), goethite (FeO(OH)), boehmite (AlO(OH)), anatase (TiO2), gibbsite (Al(OH)3), sodalite (Na8Al6Si6O24Cl2), rutile (TiO2), calcite (CaCO3) and quartz (SiO2). The chemical composition of RMB is presented in Table 1. 2.2. Brine samples Natural brine samples come from an aquifer 1000-m deep in Becancour, Qc, Canada, and were provided by Junex-Solnat Inc. The physical and chemical descriptions of brine are shown in Table 2. The sample of brine has a salinity of 33% compared to seawater that has a salinity around 3%. 2.3. Natural acid sulphate tailings Samples were taken in tailings dams located in Abitibi, Qc, Canada. The samples were taken at the surface of tailings, in the most oxidised part of the site. Water content of the shallow 0–15 cm tailings sample was 12.5% with a pH value around 3.5. Chemical composition of the tailings is shown in Table 1. Table 1 Chemical composition of materials Elements
RMB
Tailings (T(O)) 0–15 cm deep
Tailings (T(NO)) 15–35 cm deep
SiO2 (%) Al2O3 (%) Fe2O3 (%) MnO (%) MgO (%) CaO (%) Na2O (%) K2O (%) TiO2 (%) P2O5 (%) Stot (%) Cu (ppm) Pb (ppm) Zn (ppm) As (ppm)
12.63 20.65 35.26 0.025 0.24 4.86 7.16 0.11 6.152 0.17 0.232 38 51 31 33
73.87 10.03 6.81 0.031 2.37 0.11 0.45 1.94 0.589 0.16 0.199 128 12 416 15
72.14 10.03 6.81 0.031 2.37 0.11 0.45 1.94 0.589 0.16 7.04 935 40 1187 15
2.1. RMB samples RMB samples were from Alcan International, Arvida, Quebec. This residue was produced from alkaline extraction of alumina from bauxite by the Bayer process. In the Bayer process, NaOH and lime are first added to bauxite, and the mix is heated under pressure to separate the sodium alumina (NaAlO2). After the treatment, a solution of NaOH, Na2CO3 and sodium aluminate remains (Fortin, 1991). RMB is red because of its high Fe-oxide content, particularly because of the presence of hematite (Fe2O3). RMB is used for several of its properties. First, RMB is recognised for its capacity
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Table 2 Physical and chemical properties of brine Parameters
Units
Value
Total dissolved solid Chloride Calcium Sodium Magnesium Potassium pH Specific gravity Viscosity Resistivity Freezing point
mg/L mg/L mg/L mg/L mg/L mg/L
270,000–290,000 145,000–150,000 53,000–58,000 30,000–35,000 1400–1800 800–1000 5.5 1.1767 1.6 5 30
Cl Ca Na Mg K
(p/v) CSt Ohms/cm Celsius
Acid–base accounting results were compared between shallow 0–15 cm and deep 15–35 cm tailings (Table 3). TAP is the total acidification potential of the sample and SAP is the S acidification potential, i.e., if it is assumed that only the non-oxidised S could react. Tailings in the first 15 cm, named shallow tailings or oxidised tailings T(O), showed distinct signs of oxidation by their orange colour. Deeper samples (15–35 cm), named non-oxidised tailings T(NO), were grey. According to acid– base accounting results for the shallow 0–15 cm samples most of the sulphide has been oxidised. In total, 76% of the total S comes from SO4, the product of the acidification reaction. Mineral phases detected by XRD were quartz, chlorite ((Mg, Fe)3(Si, Al)4O10(OH)2 Æ (Mg, Fe)3(OH)6) and alkali feldspar (KAlSi3O8). No sulphide was observed. Scanning electron microscope observations showed that the sample was very oxidised. The neutralisation potential (NP) was measured in the laboratory by the Sobek method (Sobek et al., 1978). The net neutralisation potential (NNP) was represented by the NP–SAP. The sample had NNP of 2.34 kg CaCO3/t. According to Miller et al. (1991), a NNP value between 20 and 20 kg CaCO3/t, measured by the Sobek et al. (1978) method, reveals that the acidity generation potential is uncertain. According to Ferguson and Robertson
(1994), a value of the NP/AP ratio greater than 2.0 represents tailings which are not giving off acid. So, the 0–15 cm deep tailings samples needed to be neutralised, but it was not necessary to take into account future oxidation. On the other hand, the 15–35 cm deep tailings sample had a NNP value of 157.2 kg CaCO3/t which will likely result in acid drainage if not protected. 2.4. Mixture of brines and RMB The addition of brine to the RMB samples was responsible for the precipitation of secondary minerals as observed by X-ray diffraction. Hematite, boehmite, goethite and quartz were still present. Secondary minerals halite (NaCl) and aluminite (Al2(SO4)(OH)4 Æ 7(H2O)) were also observed. Most of the minerals present were Fe and Al oxide and hydroxide minerals (hematite, goethite, aluminite). 3. Experimental methods 3.1. Sample preparation A mixture of RMB and deionized water (ratio 2:1) and a mixture of RMB with brine (weight ratio 2:1) were stirred constantly for two months. Samples were put in 100-mL high density polyethylene bottles which were mounted horizontally on a Plexiglas carousel that was suspended in a temperature bath maintained to 20 °C. The carousel was rotated between 10 and 20 rpm for a period of two months. Samples were run in duplicate. Samples were agitated until equilibrium conditions were reached. Stable conductivity of the sample was used as an indicator of equilibrium. Tests were run with samples of RMB alone, RMB mixed with brine, and for samples with RMB mixed with tailings and RMB and brine mixed with tailings. The aim of these experiments was to characterise the long-term neutralisation potential of those mixtures. For the sample containing tailings, RMB
Table 3 Acid base accounting result Samples
pH
S(T)%
SO4%
Tailings (0–15 cm deep) Tailings (15–35 cm deep)
3.94
0.19
0.44
6.20
7.04
4.38
TAP kg CaCO3/t
SAP kg CaCO3/t
5.94
1.41
220
174.7
NP kg CaCO3/t 3.75 17.5
NP/SAP 2.66 0.10
NNP NP-SAP kg CaCO3/t 2.34 157.2
M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333
was mixed with tailings in a ratio of 10% RMB by mass with 3 g of RMB mixed with 30 g of 0–15 cm deep tailings from an acidic tailings dam and 3 g of RMB and brine (ratio 2:1) mixed with 30 g of 0– 15 cm deep tailings. The tests were repeated with the 15–35 cm deep samples from the same acidic tailings dam in the same ratio.
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used as an indicator of the neutralising capacity of the sample. Mass balance was calculated on the 30 g samples. Results showed that the difference between solid alkalinity before and after the test was abnormally larger than the alkalinity lost in the liquid during the 30 cycles. This could be due to the presence of quartz in the sample and to the difficulty of leaching all 30 g of the sample. First, quartz was detected in XRD and SEM analyses. If the quantities of quartz (an inert material which did not add alkalinity to the sample) in the measurement of solid alkalinity was not the same in the samples tested before and after the cycles, then the alkalinity results are influenced because the total mass of material that input alkalinity was decreased by the mass of quartz. Secondly, for some samples part of the solid stuck to the side of the test tube, so not all the 30 g of sample was well leached with fresh water in all the cycles. For these reasons, the test was repeated with a smaller sample (3 g). All visible quartz grains were removed from the sample before the test. Mass balance was recalculated and alkalinity gained in the liquid was very close to the difference in alkalinity measured before and after the leaching cycles.
3.2. Measurements of neutralisation potential For the experiment of long-term alkalinity, two solid/liquid ratios were tested with 3 and 30 g of solid. Each solid sample was taken and put in a test container with 50 mL of deionized water. Samples were shaken for a minimum of 24 h. After shaking, samples were centrifuged for 1 h at 750 rpm or until the liquid and solid phases were distinctly separated. The liquid was analysed for alkalinity and pH. Solids were kept to repeat the procedure, and, a volume of 50 mL of deionized water was added to the solid. Each time, alkalinity and pH were measured on the liquid, and the solid was kept to repeat the test for a total of 31 cycles. Alkalinity was measured on the solid samples prior to and after the entire test to compare with the alkalinity measured in the liquid phase after each flush. After each cycle the volume of 50 mL of water was titrated with 0.01 N H2SO4 solution to an endpoint of 4.5. Solid alkalinity was measured by a titration on the solid sample. Two g of the solid sample were mixed with deoinized water. Alkalinity was expressed in mg CaCO3/L (ppm) for liquid and in mg CaCO3/kg (ppm) for solid. Alkalinity was
4. Results The pH evolution after each flush is shown in Fig. 1. RMB had a very high alkaline pH at the beginning of the test and pH stabilised between 11 and 12 after seven flushes. RMB with brine (RMB/ B) had a lower pH than RMB at the beginning of
31
29
27
25
23
21
19
17
15
13
9
11
7
5
3
14 13 12 11 10 9 8 7 6 5 4 1
pH
Evolution of pH
Flush Number RMB
RMB/B
RMB/T(NO)
RMB/B/T(NO)
RMB/T(O)
Fig. 1. pH evolution of mixtures after each flush.
RMB/B/T(O)
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the test and stabilised at the same pH as RMB after seven flushes. The mixture of 10% of RMB mixed with oxidised mining tailings (RMB/T(O) or shallow tailings) gave a pH of over 8 for all 30 flushes. The same
behaviour was obtained with the sample of RMB mixed with brines and tailings (RMB/B/T(O)). The most oxidised tailings (O) with a starting pH of 3.24 gave pH lower than mixtures with less oxidised tailing (NO or deep tailings) that had a pH
Alkalinity (mgCaCO3/L)
a 10000 1000 100 10 1 1
3
5
7
9
11 13 15
17 19 21 23 25 27
29 31
Number of cycles RMB
RMB/B
RMB/T(NO)
RMB/B/T(NO)
RMB/T(O)
RMB/B/T(O)
Alkalinity (mgCaCO3/L)
b 250 200 150 100 50 0 1
3
5
7
9
1 1 13 15 17 19 21 23 25 27 29 31
Number of cycles
Alkalinity (mg CaCO3/kg)
RMB/T(NO)
RMB/B/T(NO)
RMB/B/T(O)
RMB/T(O)
c 140000 120000 100000 80000 60000 40000 20000 0 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Number of cycles RMB
RMB/B
Fig. 2. Variation of alkalinity with the number of cycles for (a) liquid alkalinity of 30 g samples, (b) mine tailings mixed with RMB (10%) and (c) solid alkalinity of RMB and RMB and brine before and after the flushes.
M. Paradis et al. / Applied Geochemistry 22 (2007) 2326–2333
6.2. All those samples kept neutral pH all through the test. In Fig. 2a dissolved alkalinity started around 10,000 mg CaCO3/L for the RMB sample and was leached mostly in the first three flushes. Dissolved alkalinity for the RMB with brine sample had low values (around 300 mg CaCO3/L) for all the flushes. Dissolved alkalinity of RMB matched the values of RMB with brine after seven flushes. Most of the dissolved alkalinity of RMB in the liquid part was gone after seven flushes (Fig. 2a) where the pH became stable (Fig. 1). Fig. 2b shows the alkalinity of the liquid in contact with a mixture of 10% RMB with tailings. Alkalinity of RMB (10%) mixed with deep 15–35 cm tailings (RMB/T(NO)) had a starting value of alkalinity greater than the same mixture with brine (RMB/B/T(NO)). Values became stable after 12 flushes for most of the mixtures. Mixes of RMB with shallow 0–15 cm tailing with (RMB/T(O)) or without (RMB/B/T(O)) brines showed the same alkalinity curves.
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Alkalinity on the solid was measured before and after the test (Figs. 2c, 3b) for the two liquid/solid ratios. It can be observed that the difference between the alkalinity of the solids before and after the flushes is largely due to the presence of brine. This remark is valid for the tests with a starting mass of 30 g (Fig. 2c) and those with 3 g (Fig. 3b). The sample of RMB showed an alkalinity level of 132,500 mg CaCO3/kg before the flushes and an alkalinity level of 38,200 mg CaCO3/kg after the flushes for 30 g RMB samples. The sample of RMB with brine showed an alkalinity level of 119,300 and 78,700 mg CaCO3/kg before and after the flushes, respectively. For the 3 g samples, the alkalinity level was 69,000 mg CaCO3/kg for RMB before the flushes and 17,100 mg CaCO3/kg after five cycles. RMB with brine had an alkalinity level of 55,500 mg CaCO3/kg before and 46,800 mg CaCO3/kg after five flushes. Fig. 2b shows that alkalinity of RMB (10%) mixed with deep 15–35 cm tailings (RMB/T(NO)) had a starting value of alkalinity greater than the
Alkalinity (mg CaCO3/ Litre)
a 1400 1200 1000 800 600 400 200 0
Alkalinity (mg CaCO3/kg)
1
2
3
4
5
b 80000 70000 60000 50000 40000 30000 20000 10000 0 0
1
2
3
4
5
6
Number of cycles RMB
RMB/B
Fig. 3. Variation of alkalinity of RMB and RMB/brine for a 3 g sample. (a) Dissolved alkalinity level in liquid and (b) solid alkalinity measured before and after the flushes.
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same mixture with brine (RMB/B/T(NO)). Values became stable after 12 flushes for most of the mixtures. Mixes of RMB with shallow 0–15 cm tailing with (RMB/T(O)) or without (RMB/B/T(O)) brines showed the same alkalinity curves. Deep T(NO) only needed 3% of RMB to have neutral pH, and 10% was applied, so non-used alkalinity for neutralisation with T(NO) was leached in water. But for the same mix with brine (RMB/B/T(NO)) non-used alkalinity remained in the sample and showed the same behaviour as the shallow T(O) sample that need 6% instead of 3% (T(NO)) RMB to have neutral pH. 5. Discussion
brine. It appears that the addition of brine may help to convert easily soluble alkalinity to less soluble alkalinity. The samples with oxidized tailings, RMB/T(O) and RMB/B/T(O) samples, showed similar results. These samples required more alkalinity to neutralise the acidity that was greater than in the unoxidized T(NO) sample. No surplus alkalinity was leached because all the neutralisation potential was used to neutralise the acidity. The deep 15–35 cm sample had less acidity to neutralise, so surplus alkalinity was leached out in the liquid rinse from the RMB sample (RMB/T(NO), Fig. 2). With addition of brine, however, alkalinity was not leached out (RMB/T/B(NO), Fig. 2) and could be available to neutralise future oxidation products.
5.1. Addition of brine to RMB 6. Conclusions Neutral pH conditions were maintained through the test for RMB and RMB with brine. Addition of brine to RMB initially lowered the pH compared to RMB alone. But pH of the two mixes reached the same value after seven flushes. RMB leachate had high dissolved alkalinity at the beginning of the test, but matched the value of RMB with brine leachates after seven flushes. Most of the alkalinity was lost in water during the few first flushes of RMB. For the 3 and 30 g samples, a part of the solid alkalinity was lost with the flushes for RMB. For RMB with brine samples, the disparity between solid alkalinity measured before and after the flushes was smaller than for the RMB sample.
RMB is a useful material for neutralizing acidity in tailings. Addition of brine to RMB increases the long-term effect and did not impair the short-term neutralisation and leaching properties of the shallow (0–15 cm deep) oxidised part of an acidic tailings dam. Thus, neutralisation of acidity can be done with less chance of a future resumption of oxidation. For the deep 15–35 cm unoxidized samples, where acidification potential was still present, the use of RMB treated with brine may provide a long-term alkalinity reserve to neutralise future acidification of tailings.
5.2. Test of RMB and RMB/brine with tailings
References
Neutral pH conditions were maintained all through the test for RMB and RMB with brine mixed with mine tailings. The shallow 0–15 cm sample, or oxidized tailings, had an initial pH 3.94 which was lower than the deep 15–35 cm sample, or unoxidized tailings (pH 6.2). Addition of 10% mass of RMB to tailings brought stable pH at values near 8 for the shallow 0–15 cm sample and 10 for the deep 15–35 cm samples. The deep 15–35 cm tailings sample mixed with RMB (RMB/T(NO)) showed a surplus of dissolved alkalinity leached in the liquid compared to RMB with brine for the same mix (RMB/B/T(NO)). Addition of brines seemed to decrease liquid alkalinity leached out and samples kept a neutral pH (Fig. 1). Alkalinity measured on solids after several flushes was higher for the sample of RMB with
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