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Stabilization of sulphidic mine tailings for prevention of metal release and acid drainage using cementitious materials: a review Moncef Nehdi and Amjad Tariq
Abstract: Environmental contamination produced by acid generating mines currently represents the largest environmental liability of the mining industry and necessitates the development of novel techniques for its mitigation. This paper reviews existing literature on stabilization of sulphidic mine tailings and prevention of acid mine drainage (AMD). It is shown that stabilization using ordinary portland cement in combination with pozzolanic and cementitious materials could be a viable option. However, variation of mine waste constituents and their interactions with different binders thwart the formulation of a generalized recipe for stabilization and further necessitate research to explore the optimal waste-binder proportions of the stabilized system components for the particular mine tailings under consideration. The demonstrated effective utilization of industrial by-products (fly ash, slag, cement kiln dust, etc.) in the preparation of modified cementitious materials for stabilization of sulphidic mining waste reinforces further interest in this area, not only to cope with acid mine drainage, but also to utilize abundant discarded industrial by-products for various beneficial considerations. This paper critically examines various mine tailings stabilization techniques in the literature, identifies the fundamental mechanisms controlling their performance and the intrinsic parameters of stabilization systems, along with the tailings-binder interaction mechanisms and performance assessment tools for stabilized tailings. Key words: stabilization, mine tailings, acid mine drainage, metallic and metalloid elements, leaching, portland cement, pozzolanic binders. Résumé : La contamination de l’environnement causée par les mines générant de l’acide représente actuellement la plus grande responsabilité environnementale de l’industrie minière et demande le développement de techniques innovatrices pour l’atténuer. Cet article examine la littérature existante sur la stabilisation des résidus miniers sulfureux et la prévention du drainage minier acide (DMA). Il est démontré que la stabilisation par du ciment portland combiné à des liants pouzzolaniques et hydrauliques pourrait être une option viable. Toutefois, la variabilité de la composition des résidus miniers et leurs interactions avec les différents liants empêchent la formulation d’une « recette » généralisée de stabilisation et demandent une recherche plus poussée afin d’explorer les proportions optimales résidus-liants du système stabilisé pour les résidus miniers particuliers examinés. L’utilisation efficace de sous-produits industriels (cendres volantes, scories, poussière de four à ciment, etc.) dans la préparation de liants hydrauliques modifiés pour stabiliser les résidus miniers sulfureux augmente l’intérêt dans ce domaine non seulement pour régler le problème de drainage minier acide, mais aussi pour utiliser et valoriser la grande quantité de sous-produits industriels rejetés. Le présent article analyse attentivement les diverses techniques de stabilisation des résidus miniers dans la littérature, identifie les mécanismes de base contrôlant leur rendement et les paramètres intrinsèques des systèmes de stabilisation, en plus d’étudier les mécanismes d’interaction résidus-liants et le rendement des outils d’évaluation des résidus stabilisés. Mots-clés : stabilization, résidus miniers, drainage minier acide, éléments métalliques et métalloïdes, lixiviation, ciment portland, liants pouzzolaniques. [Traduit par la Rédaction]
Introduction Mining activities in many locations worldwide produce substantial amounts of sulphidic tailings, exposing previously buried natural ore to the atmosphere. The sulphide minerals, particularly pyrite (FeS2 ) and pyrrhotite (Fe1−x S), contained in sul-
phidic waste are oxidized in the presence of air and water. Bacteria (such as Tiobalcillus ferrooxidans) are a factor that can accelerate sulphide oxidation. Metals contained in metallic minerals are freed upon oxidation, which generates metallic oxides, soluble metals, and an increase in acidity. Such a phe-
Received 12 April 2005. Revision accepted 12 July 2006. Published on the NRC Research Press Web site at http://jees.nrc.ca/ on 23 June 2007. M. Nehdi1 and A. Tariq. Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON N6A 5B9, Canada. Written discussion of this article is welcomed and will be received by the Editor until 30 November 2007. 1
Corresponding author (e-mail:
[email protected]).
J. Environ. Eng. Sci. 6: 423–436 (2007)
doi: 10.1139/S06-060
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J. Environ. Eng. Sci. Vol. 6, 2007 Table 1. Common sulphide minerals and their products. Element
Sulphide mineral
Mineral name
Iron
FeS2 Fe1−x S ZnS CuFeS2 CuS CuFeS4 Cu2 S FeAsS AsS As2 S3 PbS NiAs (Fe, Ni)9 S8 HgS CoAsS
Pyrite or Marcasite Pyrrhotite Sphalerite Chalcopyrite Covellite Bornite Chalcocite Arsenopyrite Realgar Orpiment Galena Niccolite Pentlandite Cinnabar Cobaltite
Zinc Copper
Arsenic
Lead Nickel Mercury Cobalt
nomenon is commonly known as acid mine drainage (AMD). The combined effects of acidity and soluble elements increase the toxicity of effluent (water), affecting the health of adjacent ecosystems (Gray 1997). This environmental problem is often associated with base metal, gold and uranium mining operations, as well as coal and lignite mining (Xenidis et al. 2002). Common sulphide minerals and their oxidation products are shown in Table 1. Other inherit pollutions reported by the Environmental Council of British Columbia (1998) related to such mining operations include: (a) metal contamination and leaching, (b) processing chemicals pollution, and (c) erosion and sedimentation. Control techniques for sulphidic tailings sites are usually based on the anticipated future use of land and on technical, economic, and environmental considerations (Ciccu et al. 2003). These techniques which aim at restricting the principal components of the acid generation process, (i.e., sulphides, oxygen, and water) include (a) segregation of sulphides, (b) underwater storage of tailings such as in constructed surface impoundments, flooded pits, underground working and natural water bodies, and (c) application of covers constructed from low permeability soils, synthetic materials, organic substances, and composites (Xenidis et al. 2002). Additionally, there are preventive measures to control the factors that largely affect the nature and extent of acid formation, including the addition of alkaline minerals to control the pH of water and dissolved ferric ion, and application of bactericides to inhibit bacterial action (Steffen, Robertson and Kirsten Inc. 1989). This article focuses only on stabilization techniques that use cementitious materials and (or) recycled by-products to control pH, and provide cohesiveness and reduction of mass transport through sulphidic tailings. Various stabilization techniques detailed in the literature are discussed, their intrinsic parameters and fundamental mechanisms are examined, and their performance assessment tools are analyzed.
Stabilization of sulphidic mine tailings using cementitious materials Stabilization and solidification (S/S) is commonly used for the treatment of hazardous wastes before they are disposed (Poon and Lio 1997). In general, solidification can be simply defined as the conversion of a liquid-like material to a nonliquid material. The definition needs to be broadened when solids are treated to reduce available surface area. Solidification processes may not necessarily reduce leachablilty. On the other hand, stabilization generally refers to a purposeful chemical reaction intended to make waste constituents less leachable (Barth 1990). Stabilization technology often uses addition of binding agents and consists of incorporating industrial or mining waste in a matrix with reasonably good cohesive properties, providing encapsulation as well as chemical fixation of potential mobile elements (Conner 1990; Lange et al. 1996). The salient features of stabilization that led to its being described as the “best demonstrated available technology” (BDAT) for treatment of potentially hazardous wastes include low material and operating cost, ease of application, and capacity to contain waste and prevent its migration to the natural environment (Hamilton and Bowers 1997; Weitzman et al. 1990). Stabilization alters wastes to more physically and chemically stable forms, resulting in better environmental acceptance. Physical stabilization improves engineering properties of waste materials, such as their bearing capacity, trafficability, and impermeability. Chemical stabilization is the alteration of the chemical form of contaminants so that leachability is eliminated or substantially reduced (Mayers and Eappi 1992). Numerous formulations have been developed for stabilization processes depending on the type of waste, presence of heavy metals, etc. The most commonly used binder for stabilization is ordinary portland cement (OPC) (Bonen and Sarkar 1995). Ordinary portland cement (normally used as a major ingredient) can be combined with fly ash, lime, slag, soluble silicates, clay, and cement kiln dust, either to lower cost or to improve final product performance (Park 2000; Poon et al. 1983). Partial replacement of OPC with pozzolanic materials such as fly ash and ground blast furnace slag results in the consumption of calcium hydroxide and alkalis during pozzolanic reactions. This contributes to the production of secondary calcium silicate hydrates (CSH) to fill voids and cause further reduction in capillary pores, leading to reduced leaching of sulphidic wastes (Jiang and Roy 1995). Although, using such pozzolanic and cementitious by-products generally retards early-age strength gain, their incorporation considerably improves the long-term strength and impermeability of the stabilized system (Jones 1990). The utilization of commonly used binders namely, cement, fly ash and slag-based and some stabilization processes involving cement kiln dust have been underlined in this article along with the associated chemical processes (hydration mechanisms) in the stabilization of wastes containing metallic and metalloid elements.
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Cement-based binders Cement-based stabilization is a viable process for treating many potentially hazardous wastes, particularly that containing metallic and metalloid elements. In particular, it provides a very promising solution for the disposal of arsenic bearing wastes (Leist et al. 2003; Yukselen and Alpasalan 2001). In cement stabilization, water in the waste chemically reacts with OPC to form hydrated calcium silicates and aluminate compounds, while the solids act as aggregates to form a “concrete” (Conner 1993). The cohesiveness provided by OPC has a direct positive effect on limiting the mobility of arsenic, while the presence of excess lime favors trapping arsenic as calcium arsenate in the absence of CO2 (Picquet et al. 1995). Yilmaz et al. (2003) investigated stabilization and solidification of mining waste using two different levels of OPC (10 and 20%) and two different particle-size distributions of tailings (fine and coarse). Unconfined compression strength (UCS) tests were conducted on stabilized specimens under unconsolidated, undrained conditions. The unconfined compressive strength of 28 days cured samples varied between 1.1 and 3.3 MPa. The hydraulic conductivity and permeability of stabilized systems indicates their ability to permeate water. The hydraulic conductivity of stabilized specimens at the same age (28 days) was in the range of 1.04 × 10−7 to 2.1 × 10−7 cm/s, meeting the USEPA regulatory values for landfilling. The effect of aggregate size of tailings on the toxicity characteristic leaching procedure (TCLP) leachate quality was also investigated on solidified samples crushed into two different aggregate sizes. Test results showed that metal retaining efficiencies in the solidified mass were greater than 87%. Also, at the same cement/waste ratio, leachate concentrations of fine waste samples were generally lower than those of coarse waste samples. Benzaazoua et al. (2002) studied developing paste backfill using sulphidic-rich tailings (sulphur grade 5 to 32%) by preparing various mixtures using OPC, sulphate-resistant-cement, and aluminous cement. The effects of the binder type, binder proportion, and tailings properties on kinetics of compressive strength development of paste backfill were examined for various mixtures. It was found that dissolution of portlandite (in lime rich mixtures) was associated with the precipitation of arsenic compounds, thus limiting arsenic mobility. The calcium content of the binding agent was found to be the main factor in arsenic fixation. A cemented paste rich in portlandite establishes pH conditions and calcium activity favorable for the inhibition of arsenopyrite oxidation. The study concluded that for highsulphide tailings, neither slag nor fly-ash-based binders were effective, whereas sulphate-resistant cement (mixture of ASTM Type-I and Type-V portland cements) gave the highest strength values. A ternary plot of different cements (ASTM Type-I and Type-V) and pozzolanic additives (slag and fly ash) used in the study is shown in Fig. 1. The more the binder tends towards the SiO2 pole, the higher was the durability of the cement-based composite (Skalny and Daugherty 1972; Conner 1990; Benzaazoua et al. 2002). In another study, Benzaazoua et al. (2004a) investigated the hardening process of the cement and sulphidic mine tailings
425 Fig. 1. Ternary plot of Type-I and Type-V cements and supplementary cementitious materials (slag and fly ash) (after Benzaazoua et al. 2002).
SiO2
Ca0+MgO
Fe2O3+Al2O3
mixtures. Contrary to ordinary concrete or mortar, the hardening process of cement-tailings mixtures is not only due to the cement hydration, but also to the precipitation of hydrated phases from the pore water of the paste. The hardening occurs in two main stages. The first stage is dominated by dissolution reactions. The dissolution of tricalcium silicate (C3 S) or (3CaO·SiO2 ), which is the major anhydrous phase of cement, leads to the release of Ca2+ , H2 SiO4 2− , and OH− ions as shown by eq. [1] and Fig. 2. The concentration of calcium ions increases very rapidly during the first curing stages until super-saturation of Ca is reached. [1]
2Ca3 SiO5 + 6H2 O → 6Ca2+ + 8OH− + 2H2 SiO4 2−
The second stage (once super-saturation is reached) is characterized by precipitation reactions and direct hydration of the binder. Dissolution reactions decrease significantly at this stage. The precipitation of hydrated phases such as C–S–H (eq. [2]) contributes to the hardening of the tailings–cement paste. Portlandite (Ca(OH)2 ) can also precipitate under some conditions. In addition, if sulphates are present (sulphate content of the cement or sulphates produced by sulphide oxidation), gypsum or sulfo-aluminate phases can precipitate. Hydration reactions occur slowly at this stage. [2]
3Ca2+ + 2H2 SiO4 2− + 2OH− + 2H2 O → Ca3 H2 Si2 O7 (OH)2 · 3H2 O
The time-dependent importance of reactions of dissolution and precipitation of the binder in the hardening process of cement–sulphidic tailings mixtures is illustrated in Fig. 2. Yukselen and Alpaslan (2001) investigated the stabilization of a heavy metal contaminated soil originating from a mining and smelting area using lime and ordinary portland cement. A series of tests were conducted to optimize the binder/soil ratios for both lime and cement. Application of the USEPA toxicity characteristic leaching procedure (TCLP) on samples treated with lime at a binder/soil mixing ratio of 1/15 showed that the solubility of Cu and Fe was reduced by 94 and 90%, respectively. Results obtained using cement treatment at the © 2007 NRC Canada
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Fig. 2. Schematic diagram of the time-depending importance of reactions of dissolution and precipitation of the binder in the hardening process cement-sulphidic tailings mixtures (after Benzaazoua et al. 2004a).
same binder/soil ratio showed that the solubility was 48% and 71% for Cu and Fe, respectively. The optimum additive/soil ratio (1/15) was selected for more detailed studies, which were carried out at an acidic pH range. It was concluded that the degree of heavy metal leaching is highly dependent upon the pH. Catalan and Yin (2003) conducted a comparison between the effectiveness of calcite (CaCO3 ) and quick lime (CaO) for the treatment of partially oxidized sulphidic mine tailings before flooding. The study was carried out on zinc and copper sulphide ores incorporating total sulphur (S) and sulphate (SO4 2− ) contents of 96.3 and 18.9 mg S/g of tailings, respectively. Although higher initial pH values were obtained with quicklime, the pH of quicklime treated tailings decreased over time. This was attributed to the low buffering capacity of quicklime treated tailings and to the consumption of hydroxide ions by incongruent dissolution of water-insoluble iron oxyhydooxy – sulphate minerals (a phase containing 40–53 wt.% iron and 1.4–7.4 wt.% sulphur). In contrast, the pH of tailings treated with calcite increased initially and then remained stable at about 6.7. This was due to the lower reactivity of oxy-hydro-oxy-sulphates with calcite, the increased buffering capacity provided by bicarbonate ions, and the incomplete dissolution of calcite. Overall, calcite was found preferable to quicklime for maintaining long-term neutral pH conditions in the treated tailings. With the exception of zinc, acceptable dissolved heavy metal concentrations were maintained in calcite treated tailings. Jang et al. (1998) conducted batch screening, adsorption, and lab-scale continuous leaching experiments for the development of immobilization technology for toxic heavy metals typically found in soils of closed mine facilities. Ten candidate additives including quicklime (CaO), calcium hydroxide (Ca(OH)2 ), and sodium sulphide (Na2 S) were tested for heavy metal immobilization efficiency. Calcium oxide (CaO) and sodium sulphide (Na2 S) consistently out- performed all other additives in terms of immobilization efficiency and capacity for Cu, Ni, and Pb.
Fly ash-based binders Fly ash is a by-product of coal burning power plants, which is often composed of predominantly silt-sized, spherical, amorphous ferro-aluminosilicate minerals (Sale et al. 1997). Two major types of fly ash are specified in ASTM C 618 (1997) on the basis of their chemical composition, which is highly dependent upon the type of coal burned: Class C and Class F. Class C fly ash produced from burning bituminous coal has cementatious properties in addition to its pozzolanic properties, and is capable to counter act the acid potential of mine waste due to its high Ca content (Roy and Griffin 1982). Two mechanisms are involved, the first mechanism consists of the addition of alkalinity and neutralization of acidity. The second mechanism refers to the reduction of hydraulic conductivity, resulting in the inhibition of water penetration into sulphidic tailings, therefore, oxidation of sulphides is prevented. Xenidis et al. (2002) conducted experiments using lignite fly ash (Class C) for the control of acid generation from sulphidic tailings containing 27% S. Long-term laboratory kinetic tests were performed and drainage quality of test columns was monitored over a period of 600 days. Chemical and mineralogical characterization of column solids residues was performed after a 270 day test period. The addition of fly ash, even at low dosages, increased the pH of drainage to values of 8.6–10 and decreased the dissolved concentrations of contaminants to regulatory limits. Higher fly ash addition levels to tailings (31 and 63%) reduced the water permeability of the stabilized material from 1.2 × 10−5 cm/s to 3 × 10−7 and 2.5 × 10−8 cm/s, respectively. Mohamed et al. (2002) conducted hydro-mechanical evaluation of stabilized mine tailings using combinations of lime, fly ash Type C, and aluminum. The formation of ettringite and gypsum in the stabilized tailings samples was examined. Moreover, unconfined compressive strength, hydraulic conductivity, and cyclic freezing–thawing tests were performed to evaluate the hydro-mechanical properties of the stabilized samples. Ex© 2007 NRC Canada
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perimental results showed that the application of lime and Type C fly ash to high-sulphate content tailings improved plasticity, workability, and volume stability. Moreover, upon addition of aluminum to lime and fly ash in a sulphate-rich environment, ettringite and calcium sulfo-aluminate hydrate were formed. Application of 5% lime, 10% Type C fly ash, in combination with 110 ppm aluminum, resulted in the formation of a solid monolith capable of producing more than 1 MPa of unconfined compressive strength, and reduced tailings permeability to 1.96 × 10−6 cm/s, which is less than the maximum permissible limit of 1 × 10−5 cm/s specified by most environmental protection agencies. The permeability of the treated tailings samples remained below the recommended permeability, even after exposing the treated samples to 12 freeze–thaw cycles. Ciccu et al. (2003) used fly ash obtained from thermal power stations (containing 90.4% ash, 7.1% fixed carbon, and 2.5% volatile matter) and red mud from metallurgical treatment of bauxite ores for immobilizing metallic and metalloid elements contained in severely contaminated soil samples taken from a tailings pond. The results of column leaching tests demonstrated that a relatively small addition of fly ash (∼15%) or red mud (∼13%), or a combination of the two, to the contaminated soils, drastically reduced the heavy metal content in the effluent. Misra et al. (1996) evaluated the efficiencies of fly ash as a binder, either by itself or in combination with OPC, in the mitigation of acid mine drainage caused by reactive tailings. The compressive strength and the leachability of the cured agglomerates were evaluated. The use of fly ash as a sole binder was found to be inadequate in imparting the required strength, whereas the use of portland cement alone provided sufficient strength. Fly ash, however, when used in conjunction with cement increased the 7 days compressive strength beyond values achieved using cement alone. It was found that agglomeration of reactive tailings required 10% cement by weight. The addition of 20% fly ash was found to be equivalent to 5% cement in terms of controlling leaching. Slag-based binders Ground granulated blast furnace slag (GGBFS), a by-product of iron foundries, is commonly used for partial replacement of OPC, yet it is rarely used commercially in waste S/S processes. Rha et al. (2000) investigated the effectiveness of hardened slag paste for the S/S of wastes containing dissolved metallic element ions. The effects of different dissolved ions on the hydration of slag were investigated. Lead ions retarded the hydration of slag, whereas, chromium ions accelerated the initial hydration, generating aluminate hydrates. Benzaazoua et al. (2002) argued that slag-cement-based binders are appropriate for stabilizing low to medium sulphur-rich tailings (10 to 30% sulphides), but exhibit hydration inhibition until after 120 days of curing time. Cement kiln dust Cement kiln dust (CKD) is a by-product of cement manufacturing. The physical and chemical properties of CKD vary from one cement plant to another, depending upon the raw materials
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used in the manufacturing of cement and the type of dust collection process. However, the dust collected from the same kiln producing the same cement type will typically have a relatively consistent composition (Baghdadi et al. 1995). Cement kiln dust satisfies the requirements of a cementitious stabilizer having a hydration modulus [CaO/(Al2 O3 + SiO2 + Fe2 O3 )] between that of alite and belite; it behaves as a soil stabilizer in a manner similar to that of portland cement (Kamon and Nontananandh 1991). Heavy metals are known to retard the hydration reactions of cementitious materials. Some research suggests that using CKD as an admixture to OPC for stabilizing wastes containing metallic and metalloid elements accelerates hydration (Park 2000). Doye and Duchesne (2003) used a mixture of CKD and red mud bauxite (RMB) for the mitigation of acid mine drainage by neutralizing acid produced by Fe sulphides. A series of timecontrolled static leaching experiments were performed. Samples of reactive mine tailings were prepared containing 0, 2, 5, and 10% alkaline material (CKD, RMB, and a mixture of 0.5 CKD:0.5 RMB). The water/solid ratio was also varied (3:1, 6:1, and 20:1) in order to characterize the geochemical balance between liquid and solid phases. The pH results showed that the use of 5% CKD, 10% CKD, and 10% mixture of CKD and RMB allowed neutral pH conditions to be maintained in the reactive layer of tailings over the 365 days test duration. At these percentages, the concentrations ofAl, Fe, Cu, Zn, and SO4 in solution were significantly reduced compared with those obtained in reactive mine tailings alone. Aluminum concentrations were principally controlled by secondary phases like boehmite and gibbsite; Fe by goethite and ferrihydrite; Cu and Zn by hydroxides, while calcium and SO4 concentrations were controlled by precipitation of gypsum. Park (2000) investigated the stabilization of contaminated wastes from the steel industry containing heavy metals using OPC, CKD-modified OPC, and CKD and a quick setting agent (QSA)-modified OPC. For a mixture of 80% OPC and 20% CKD, alkalis in CKD accelerated the hydration of OPC at early age. The high specific surface area and alkali content of CKD contributed to high early development of compressive strength and better fixation of metallic and metalloid elements by filling voids, thus resulting in densification of the microstructure. The mixture incorporating CKD-modified cement developed higher strength and surface area of hydration products than that of the mixture made with OPC, and generated ettringite and Ca(OH)2 as major hydrates at early hydration time, and C3A·CaCO3 ·12H2 O after 7 days of curing. The QSA-modified and CKD-modified cementitious materials were most effective in fixing heavy metals, especially in wastes containing a combination of various metallic and metalloid elements. Fortin and Poulin (2001) used the co-mingling approach (Fig. 3) for the stabilization of sulphidic mine tailings with layers of compacted tailings incorporating 10% by mass of an alkaline additive (CKD). The laboratory study showed that using the CKD additive was effective in mitigating AMD. The fine CKD layers limited the mobility of water and air in the waste piles, while the use of alkaline residues provided a reser© 2007 NRC Canada
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Fig. 3. Layered co-mingling using cement kiln dust (CKD) at mine Doyan, Abitibi, Quebec, Canada (after Fortin and Poulin 2001). 10 m thick waste rock layers
1 m thick compacted layers of fine material using CKD
voir of alkalinity for neutralization, thus ensuring long-term prevention of AMD. Miller and Azad (2000) found that for soil stabilization using CKD the increase in unconfined compressive strength of the stabilized product is inversely proportional to the plasticity index (PI) of the untreated soil. Therefore, CKD can be particularly beneficial in stabilizing soils with a low PI. A correlation between unconfined compressive strength (UCS) of stabilized wastes (containing metallic and metalloid elements) using cementitious binders and contaminant leachability from such systems has not been established. However, in general, better strength provides better physical barriers for the containment of contaminants (Means et al. 1995) The effectiveness of the above cementitious binders in terms of UCS of stabilized matrices containing sulphidic mine tailings and binders can be categorized as follows: (1) Cement-based binders (ASTM Type-I and Type-V) produce lower strength with low sulphur grade tailings (5 wt.%) and medium sulphur grade tailings (16 wt.%). Higher UCS is attained using cement-based binders with high sulphur-rich tailings (32 wt.%). (2) Composite binders containing cement and fly ash are more suitable for lower and medium sulphur grade tailings than cement alone. However, for high-sulphur tailings cement used as a single binder develops higher compressive strength of the stabilized composite. (3) Composite binders containing cement and slag are more suitable for low to medium grade sulphur tailings and are not effective in stabilizing high-sulphur grade tailings. (4) The role of CKD in compressive strength development with sulphidic mine tailings is not clear in the literature. Therefore, it needs further research in view of the attractive cementitious properties of CKD, its abundance, and the environmental problems associated with its disposal. Table 2 summarizes the properties of stabilized waste materials, the stabilization techniques, and the associated results of various research studies in the literature on the use of alkaline additives to control acid mine drainage.
Governance of intrinsic parameters of stabilization system components The performance of a stabilized system is based on the intrinsic parameters of its three main components, i.e., tailings, binder (type and proportions), and water. A detailed characterization program of physical properties (Atterberg limits, optimum moisture content for maximum dry density, specific gravity, particle size distribution of the mining waste and cement mixtures), chemical and mineralogical properties of tailings and binder is needed for devising a successful stabilization process. The particle size distribution of the tailings is an important factor in the determination of their reactivity. Finer size tailings, by virtue of their higher surface area, tend to be more reactive, both in ambient air oxidation and by microorganisms (Misra et al. 1996). Researchers have developed a correlation between the proportion of sulphides in sulphidic tailings and their density, which is partially due to the specific gravity (Gs) of pyrite. In a particular study (Benzaazoua et al. 2002), the specific gravity of pyrite was found to be ∼5, compared to the Gs of the gangue minerals, which had an average value of 2.7. The amount of sulphides in tailings has a direct impact on their density and consequently on the quantity of binder required for stabilization per unit volume since the proportions of added binder are calculated using the dry total mass of tailings. Furthermore, a higher sulphide content in tailings induces beneficial effects on the unconfined compressive strength of the stabilized material due to the larger proportion of binder required by volume (higher density of tailings), and better cohesion development as a result of the precipitation of sulphates available in higher sulphide tailings. It was also found that the type of binder has a pronounced effect on the short and long-term mechanical behavior of the stabilized system and its durability. Moreover, a sub-linear and proportional relationship exists between increasing the proportion of binder and the mechanical resistance achieved (Benzaazoua et al. 2004a). Pozzolanic S/S systems are useful for high sulphate wastes since they contain very limited free calcium hydroxide and, thus, have little reactivity with sulphates (Jones 1990). The chemistry of pore water also plays an important role in the hydration process depending on the binder type and curing procedure. Moreover, the proportion of water in a stabilized system has a strong effect since it influences, through the dilution rate, the reactions of the cementitious phases. Depending on their concentrations, sulphates can have a significant influence on the mechanical performance of the stabilized system and can also cause strength losses due to the precipitation of expansive species, since expansive sulphate crystallization (gypsum and ettringite) can cause cracking and fracturing of the hardened matrix (Benzaazoua et al. 2002). The trend of compressive strength gain and loss versus time, which is critical for the efficiency of cementitious binders in stabilizing sulphur-rich tailings, depends on the type of binder. For instance, although ordinary portland cement used as a single binder produces reasonable compressive strength in stabilization processes, it is
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Table 2. Acid mine drainage control using alkaline additives. Reference
Material stabilized
Stabilization technique
Results
Benzaazoua et al. (2002)
Sulphide-rich tailings from four Canadian mines. A1- Containing ≈60% sulphides (essentially pyrite) A2- ≈40% sulphides B- ≈30% sulphides C- ≈10% sulphides
OPC and fly ash binders proved suitable for high sulphide tailings, producing adequate strength after 28, 56, and 91 days of curing time. Slag based binders exhibited hydration inhibition until after 120 days of curing time and were found appropriate only for medium and low sulphur rich tailings.
Benzaazoua et al. (2004b)
Sulphidic mine tailings from milled gold ore containing 60% sulphide content. Fine-grained tailings containing pyrite and arsenopyrite in about equal amounts.
OPC Type-I and sulphate resistant cement Type-V; fly ash, and blast furnace slag. Mixing water types (lake water, municipal water, and sulphate rich process water). Paste backfill preparation using binders and sulphidic tailings. OPC, aluminous cement, ground blast furnace slag cement. Paste backfill preparation by mixing hydraulic binders with sulphidic tailings.
Calcite (CaCO3 ) and Zinc and copper sulphide ores with total sulfur (S) quicklime (CaO) addition and sulphate (SO4 2− ) to the tailings contents of 96.3 and 18.9 mg S/g of tailings, respectively. The sulphide content estimated to be 77.4 mg S/g tailings. Addition of fly ash Ciccu et al. (2003) Contaminated soil from containing 90.40% ash, tailings pond of lead and 7.08% fixed carbon, zinc mine, containing Zn 2.50% volatile matter, (3366 ppm), Pb (12245 and red mud from ppm), Cu (444 ppm), and metallurgical treatment of Cd (25 ppm). bauxite ores. Doye and Duchesne (2003) Reactive tailings rich in Fe, Cement kiln dust (CKD), S, Cu, Pb, and Zn. red mud bauxite (RMB)
Catalan and Yin (2003)
Fortin et al. (2000)
Mine tailings from copper, zinc, silver, and gold.
Cement kiln dust addition (10% by weight) using layered co-mingling technique.
Jang et al. (1998)
Mine soils containing Cu, Ni, and Pb, etc.
Misra et al. (1996)
Sample-1: Fine grained reactive mine tailings containing 4.33% iron and 8.5% sulphur. Sample-2: Tailings with comparatively lower fineness and having 3.0% sulphur.
CaO and Na2 S addition to soils containing heavy metals. Agglomerates preparation using OPC and fly ash with tailings. Fly ash : 5% to 20% by mass, and OPC: 2.5%, 5%, 7.5% and 10%.
Leaching experiments were done using shaked flask and soxhlet extractor. The ability of stabilized samples to retain arsenic was checked after 500 days of conditioning. The main factor in fixing arsenic was the calcium content of the binding agent. Cemented stabilized matrix rich in Ca(OH)2 maintained a neutral to alkaline environment with pH conditions and calcium activity favorable for the inhibition of arsenopyrite oxidation. Calcite was found preferable to quicklime for maintaining long-term neutral pH conditions in the treated tailings. With the exception of zinc, acceptable levels of dissolved metal concentrations were achieved with calcite treated tailings.
The addition of 15% fly ash and a mixture of 7.5% fly ash and 7.5% red mud by mass to contaminated soil drastically reduced the heavy metal contents of the soil leachates.
10% CKD and 10% mixture of CKD and RMB allowed neutral pH over 365 days of batch leaching tests. The compacted layers containing CKD acted as a trap for different metals favoring precipitation in addition to the alkaline reservoir to neutralize acidity and reduction in bacterial activity. Quick lime and sodium sulphide had good immobilization efficiencies for Cu, Ni, and Pb. Tests on tailings agglomerates prepared using fly ash and OPC resulted in much lower level of extractable ions concentration and well below the regulatory level than from agglomerates prepared using either fly ash or cement alone. Agglomeration of reactive tailings required 10% cement by mass, and addition of 20% fly ash was found to be equivalent to 5% cement in terms of leaching control. © 2007 NRC Canada
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Table 2. (concluded.) Reference
Material stabilized
Stabilization technique
Results
Mohamed et al. (2002)
High-sulphate content mine tailings Al2 O3 (6.6%), Fe2 O3 (41.07%), MgO (1.31%).
Lime, fly ash (Class-C) and aluminum (110 ppm) were used as additives to mine tailings.
Xenidis et al. (2002)
Sulphidic tailings containing 27% sulphur.
Lignite fly ash (Class-C) amount ranging from 10 to 63% by wt.
Yilmaz et al. (2003)
Metal enriched gold mining residue (Cr, Cu, Pb, Zn)
OPC mixture with tailings.
Yukselen and Alpasalan (2001) Soil contaminated with heavy metals (Pb (153 mg/kg), Cu (510 mg/kg), Fe (15.3%), S (14.63%) from an old mining and smelting area.
Lime and OPC. Lime/soil and cement/soil mixtures at ratios of 1/15, 1/20, and 1/25 by mass.
5% lime, 10% Type C fly ash in combination with 110 ppm aluminum resulted in formation of a solid monolith capable of producing more than 1 MPa of unconfined compression strength and reduction in hydraulic conductivity to 1.96 × 10−6 cm/s. Fly ash addition at lower amount (10%) increased pH of leachates to values of 8.6–10 and decreased dissolved concentrations of contaminants, mainly Zn and Mn. Higher fly ash addition (31 and 63% w/w) reduced water permeability of the material from 1.2 × 10−5 cm/s to 3 × 10−7 and 2.5 × 10−8 cm/s, respectively. Long-term (600 days) laboratory column kinetic tests were performed. 10% to 20% addition of OPC, unconfined compressive strength values were 1.1 to 3.3 MPa and hydraulic conductivities were in the range of 1.04 × 10−9 to 2.1 × 10−9 m/s. Metal retaining efficiencies were greater than 87%. The addition of lime and cement to contaminated soil containing Cu, Pb, and Fe reduced the leachability of the contaminated metals. Additive to soil ratio of 1/15 was found superior for both lime and cement, giving rise to solubility reduction of Cu, Fe and Pb. Cu: Additive/soil mixture = 1/15; TCLP solubility decreased by 94% for lime/soil mixture and 48% for cement/soil mixture, respectively. Fe: Additive/soil mixture = 1/15; TCLP solubility decreased by 90% for lime/soil mixture and 71% for cement/soil mixture, respectively. Pb: Solubility found below the regulatory limit of 5mg/L.
often unable to maintain long-term stability of the matrix due to sulphate attack. On the other hand, using pozzolanic additives such as fly ash as partial replacement for portland cement mitigates the effects of strength loss (Hassani et al. 2001). In general, the tailings material characteristics (physical and chemical), cementitious binder type and proportion, and mixing water (quantity and chemistry) are fundamental factors that control the performance and properties of the stabilized system.
Matrix stability of stabilized systems containing metallic and metalloid elements Cement and (or) pozzolan-based stabilization of mine tailings containing metallic and metalloid elements owe their effec-
tiveness to the formation of calcium silicate hydrates (C–S–H) (Conner 1990). In such systems, heavy metals can interact in a number of ways including adsorption, chemical precipitation, ion exchange, surface complexation, micro-encapsulation, chemical incorporation in the hydrated neo-formed phases, diadochy and isomorphic substitution. The last two phenomena may preferentially take place when calcium tri-sulphoaluminate hydrate 6CaO·AL2 O3 ·3SO3 ·32H2 O (ettringite) is among the hydration products. This is illustrated by the occurrence of ettringite-like minerals (sturmanite Ca6 (Fe,Al)2 (SO4 )2 (B(OH)4 )(OH)12 ·26H2 O and bentorite Ca6 (Cr,Al)2 (SO4 )3 (OH)12 ·26H2 O), and also by the synthesis of ettringite derivatives (Bensted and Varma 1971; Pölmann and Kozel 1990; Pölmann et al. 1993). Ettringite has been re© 2007 NRC Canada
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ported to be an expansive mineral, which could cause physical and chemical instability (Mitchell and Dermatas 1992). Occurrence of expansion due to ettringite formation would not only result in structural instability, but also in possible release of absorbed and (or) adsorbed metals. Such disintegration can also lower the pH and produce excessive release of sulphate ions into the surrounding environment. In a low pH condition, the solubility of metallic and metalloid elements, which would otherwise precipitate, increases leading to harmful effects on the surrounding environment. Therefore, the factor that induces or prevents ettringite expansion is of great importance for stabilized matrices (Mohamed et al. 2002). Expansion and loss of structural integrity can take place in two forms; expansion caused by the formation of ettringite, and (or) loss of strength through softening attributed to gypsum formation (Cohen and Mather 1991). Berardi et al. (1997) studied a stabilized matrix with an ability to generate ettringite from calcium sulphoaluminate in the presence of CaSO4 ·2H2 O and Ca(OH)2 . They produced stabilized samples containing 10% of different heavy metal salts (Cd(NO3 )2 , Cr(NO3 )3 , Cu(NO3 )2 , Fe(NO3 )3 , Mn(NO3 )3 , Ni(NO3 )2 , Pb(NO3 )2 , Zn(NO3 )2 , K2 CrO4 and K2 MoO4 . It was found that the matrix stability and leaching behavior are strongly dependent on the dopant salt, as well as the nature of the leaching medium in contact with the stabilized matrix. Mohamed et al. (2002) used lime, fly ash, and aluminum, in the presence of a sulphate compound to promote the formation of the mineral ettringite as a means of S/S of reactive mine tailings. Upon addition of aluminum to lime and fly ash in a sulphate-rich environment, ettringite also formed and contributed to the high strength and stiffness of the treated tailings. Furthermore, the needle-like ettringite crystals that formed as a result of excess aluminum addition to the samples, blocked capillary spaces of the tailings and, therefore, reduced the permeability of the treated samples. Since sulphidic mine tailings are sulphur rich, understanding their interactions with cementious materials (binders) is important in predicting the long-term stability of the stabilized system. Ordinary portland cement contains tricalcium aluminate (C3A), which is vulnerable to sulphate attack in sulphate rich environments, thus jeopardizing matrix stability (Hassani et al. 2001). Nehdi (2001) argued that synergistic effects of individual components in binary and ternary binders can compensate for the mutual shortcomings and inherit deficiencies of single binders. Multi-component composite binders can be formulated to achieve “tailor-made” properties for more resilient stabilized systems, while reducing the dosage of portland cement. Moreover, an appropriate selection of binder or binder combination to reduce the C3A component can mitigate possible deterioration caused by available sulphates in the matrix. Such aspects need to be considered in designing stabilization operations to enhance the long-term performance of stabilized systems.
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Mine tailings – binder interaction and interference mechanisms There is a lack of comprehensive and consistent data on waste–binder interactions in the literature. Few links exist between research on stabilization mechanisms and formulation of binders for immobilization (Bonen and Sarkar 1995). The potential interference mechanisms between waste constituents and binders need to be identified in order to be able to adequately predict the effectiveness of stabilization techniques. The waste constituents undergo chemical interaction with cement and added cementitious materials, and metals can inhibit hydration reactions at the initial stages of mixing thereby retarding the setting (Park 2000). As hydration reactions proceed, metals in highly alkaline environments are precipitated as hydroxides, sulphides, silicates, carbonates, or phosphates and may also sorb onto the mineral phases of cement, clay, fly ash, activated carbon, and zeolites. Metals can affect the hydration process of cement as follows: • Lead — Lead retards cement hydration by precipitating onto the surface of calcium and aluminum silicates as insoluble lead sulphates and carbonates. These precipitates are thought to form an impermeable coating that acts as a barrier to the diffusion of water (Cocke et al. 1989). • Zinc — Zinc also impedes cement hydration and has been found to increase the permeability of zinc–cement matrix, probably by promoting ettringite formation (Poon et al. 1985, 1986). Increased permeability does not lead to a corresponding increase in zinc concentration in leachates, suggesting that zinc is retained in the matrix by a chemical mechanism. • Mercury — Mercury is associated with increased calcium carbonate formation via conversion of calcium hydroxide into calcium carbonate upon exposure to atmospheric CO2 . The increased CaCO3 may weaken the cementitious structure, however, no studies linked mercury to the retardation of cement setting (Whinney et al. 1990). • Chromium — Cr (VI), the most distinguished form of Cr in terms of toxicity, solubility, and mobility retards the initial and final setting times of cement by interfering with the hydration process. It was observed that a retardation mechanism in initial and final setting of cement occurs due to interference of Cr (VI) with the normal hydration process. The presence of 5% K2 CrO4 (equivalent to 14 000 ppm of Cr (VI)) increased the final setting time of cement by 14 h (Wang and Vipulanandan 2000). • Other metals — Cadmium and chromium are associated with increased formation of ettringite (Tashirio et al. 1979; Poon et al. 1986); a calcium sulphoaluminte compound that may cause expansion and cracking of hardened cement paste, leading to increased leaching of physically bound waste constituents (Trussell and Spence 1994). • Effect of anions — Sulphate in a waste material or in water in contact with stabilized waste can react with cement © 2007 NRC Canada
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hydrates to form ettringite. Intrusion of chloride ions makes cement more susceptible to sulphate attack in the disposal environment by dissolving calcium hydroxide thereby creating a more porous structure (Campball et al. 1987). Other possibilities of deleterious effects of strength loss of stabilized systems during curing include chemical weathering caused by aggressive media (presence of sulphates and production of acid) have been highlighted (e.g., Benzaazoua et al. 2002; Bernier et al. 1999). Despite the fact that retardation mechanisms impede hydration reactions of cement causing delayed setting, they do not necessarily decrease the long-term strength of the fully cured product. Cement, cement – fly ash, and cement–silicate mixtures have proven to be effective in chemically stabilizing metal wastes, and have been extensively used for this purpose (Conner 1990).
Influence of carbonation on cement-based stabilized systems For long-term leachability assessment, aging mechanisms such as carbonation of the cement matrix need to be taken into account (IAWG 1997; Rubin et al. 1997). Carbonation involves the chemical reaction of portlandite, Ca(OH)2 , and calcium silicate hydrate, C–S–H, in the cement matrix with atmospheric CO2 leading to formation of CaCO3 (Bin Shafique et al. 1998). This causes various effects including lowering the pH of pore water and production of carbonates from other metal hydroxides. When the leaching solution is “acidic” in comparison to the pH of the bulk matrix, two distinct compositional zones are formed within the portland cement matrix. The matrix can be described as an unleached core of high pH (i.e., pH ∼12.5) at initial constituent composition surrounded by a porous leached shell with a significantly lower pH (i.e., pH ∼9). The neutralized leached zone is essentially void of calcium bearing mineral phases and has high silica content. The progressive decalcification of cement-based materials leads to an increase in matrix porosity and a decrease in compressive strength. The spatial presence of the carbonated and uncarbonated phases and pH of leachate are important for carbonation and leaching. Decreasing the pH will turn carbonate into bicarbonate, with more solubility resulting in more leaching (Gerven et al. 2004). Thus, carbonation needs to be controlled to improve immobilization of stabilized waste (Rubin et al. 1997).
Performance assessment of stabilized systems A generalized testing program for evaluating the performance of stabilized mine tailings comprises (1) physical testing including hydraulic conductivity, porosity, unconfined compressive strength, durability under wetting and drying, freezing and thawing cycles; (2) leaching and extraction testing including the toxicity characteristic leaching procedure (TCLP), shake extraction, multiple extraction procedure, sequential extraction;
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and (3) chemical testing and analysis of stabilized systems before and after weathering processes (beyond the scope of this article and will not be discussed herein). Technical criteria for adequate performance of a stabilized system require high-unconfined compression strength, low hydraulic conductivity or permeability, and low leachate concentration (Yilmaz et al. 2003). The development and maintenance of high strength and stiffness is achieved by elimination of large pores, bonding particles and aggregates together, maintaining of a flocculated particle arrangement, and preventing swelling (Mohamed et al. 2002). The hydraulic conductivity of stabilized waste is an important factor because it indicates the ability to inhibit the passage of water and to limit the discharge of pollutants from stabilized waste to the environment. Alkaline materials used in stabilization of mine tailings tend to cause reduction in hydraulic conductivity through modification of the pore-size geometry and its distribution as a result of pozzolanic activity (Mohamed et al. 2002). Reduction in the mobility of contaminants is the most desirable property of stabilized systems and is a direct measure of its effectiveness. The degree of contaminant fixation in stabilized waste must be determined for all stabilization processes, so that individual processes (using various stabilization techniques, types and proportion of binders, etc.) and delisting petitions of technically inappropriate processes can be evaluated (Jones 1990). Leaching tests are indispensable characterization tools to evaluate the environmental threat of a miningderived waste system. Various countries have developed environmental regulations based on leaching tests (Gavasci et al. 1998). Leaching involves solubilization of species present in solid phases into the pore water and their transport through the network of connected pores within the solid into the bulk leachant by permeability, capillarity or diffusion. In cementbased stabilization processes, the presence of calcium hydroxide maintains high pH conditions in the pore water ensuring the insolubility of metal hydroxides (Cheng and Bishop 1996). The ingress of acid leachant into the pore water disturbs the chemical equilibrium formed with the surrounding solids, resulting in the solubilization of many insoluble metals. Calcium hydroxide, which is often the most readily available alkali material in stabilized wastes, is rapidly leached out (Revertegat et al. 1992). This dissolution of calcium hydroxide into the pore water results in an increasing degree of capillary pore connectivity, leading to further ingress and exposure of the binder and waste to the leachant. Factors affecting the leaching potential of waste include the type of ions present, pH, stoichiometry, temperature, electrostatic charges, oxidation-reduction (redox) potentials, along with other parameters (Josephson 1982). The USEPA toxicity characteristic leaching procedure (TCLP) is one of the most commonly used leaching tests for cementstabilized systems. As long as the stabilized waste forms have sufficient acid neutralizing capacity (ANC) to neutralize the acidity of leachant, the leaching of metals is usually small. Thus, the limitation of the TCLP is evident in comparing the performance of different cement-based forms, which is particularly © 2007 NRC Canada
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important for the assessment of long-term leaching potential after the ANC of the waste has been exhausted. The multiple extraction test (MEP) is relatively superior to the TCLP since it gradually removes excess alkalinity in the stabilized waste over time, resulting in continual consumption of alkalinity. The leaching behavior of metal contaminants can thus be evaluated as a function of decreasing pH, where the solubility of most metals increases. Normally, the pH is considered to be the most influential factor governing leaching of heavy metallic and metalloid elements from cement-based waste forms. However, other factors such as the redox potential also play an important role in leaching in real environments (Poon and Lio 1997; Means et al. 1995). Poon and Chen (1999) conducted a comparison of the characteristics of flow-through and flow-around leaching tests of cement based stabilized and solidified heavy metal wastes (Fig. 4). The flow-through (a) and the more common flow-around (b) (dynamic leaching) test methods were both carried out to compare the leaching behavior of solidified waste under different leaching environments. Solidified waste samples were prepared from five different metallic and metalloid elements (Pb2+ , Zn2+ , Cu 2+ , Ni 2+ , Cr 6+ ). The binders used were Type I OPC and pulverized fuel ash (PFA). A significant difference was observed in leaching behavior between the flow-through leaching test and flow-around dynamic leaching test. In flowthrough leaching tests the leachant came in close contact with the waste matrix (in different ways) resulting in the degradation of the solidified waste matrix. Therefore, the leaching rates of the contaminants were higher than in the dynamic leaching test. The flow-around leaching was expected to be higher than the flow-through leaching. However, the flow-through mechanism became more pronounced when the waste became degraded to a level that leachant flow-through resulted. The importance of both flow-through and flow-around tests was emphasized in assessing the long-term performance of solidified wastes. In cold regions, repeated freeze–thaw cycles can cause physical deterioration of exposed stabilized matrices through a progressive deterioration of hydration products due to pore-ice formation and associated pressures (Leroueil et al. 1991). Ice lenses formed during freeze–thaw can result in a network of cracks. When the temperature of the stabilized matrix falls below 0 ◦ C, ice crystals nucleate in the centre of large pores. When water changes to ice, its volume increases by about 9% exerting pressure on the surrounding soil. Thus, structural integrity of the stabilized matrix can be jeopardized due to the mechanical stresses caused by ice crystal formation (Yong and Mohamed 1992).
Economic considerations for stabilization operations Stabilization of sulphidic mine tailings technologies generally include surface disposal and underground disposal (backfill). The capital and operating costs associated with any stabilization technique for disposal are very much site and material specific. Since surface disposal costs are rising due to increas-
433 Fig. 4. (a) Flow-through and (b) common dynamic leaching modes (after Poon and Chen 1999).
ingly stringent environmental regulations, the cost of placing tailings backfill is decreasing as technology improves. The cost of binder required to solidify tailings is high; this makes waste disposal very costly because the extra operation cost is not being realized through better production or ore recovery. Methods to decrease cement consumption (other than using composite binders) when dumping waste can include waste rock or aggregate to increase the heterogeneity of the fill (Aref et al. 1987; Hassani et al. 1994). In general, cost increases in proportion to the percentage of binder materials (cement, fly ash, cement kiln dust, etc.) and the transport distance if the binder material is not locally available. Additionally, the implementation of stabilization technology requires a comparison between the feasibility of various approaches such as embankment construction, engineering and closure or reclamation costs (surface disposal), plant and operational costs (paste backfill). The long-term environmental liability issues are also required to be carefully examined before the selection of a stabilization technology. If the stabilization technique involves the use of other waste material (CKD, etc.), the cost of disposing such waste should be considered. Generally, a life cycle cost analysis approach with a holistic approach is needed to capture the various economic considerations of the stabilization technology. © 2007 NRC Canada
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Summary and conclusions Stabilization of sulphidic mine tailings using optimized cementitious blends transforms such waste into a matrix with adequate cohesive properties, providing encapsulation as well as chemical fixation of toxic mobile elements. Most research in the literature involves laboratory studies aiming at determining the efficient type and proportion of binder for stabilizing the specific tailings under consideration. The findings of such laboratory investigations reinforce the interest in using conventional binders for such stabilization applications. It was found that mass transfer mechanisms inside stabilized tailings systems are minimized due to reduced permeability and precipitation of calcium sulphates and arsenates. Stabilization systems using pozzolanic additives are useful for high-sulphate content wastes since they do not contain excess free calcium hydroxide, and thus are less reactive with sulphates. The type and composition of tailings influence the selection of the stabilization approach, not only due to their physical characteristics (such as their particle size distribution), but also their reactivity and the rate at which they deliver sulphates. The chemistry of pore water with regards to soluble sulphates (mine process or residual water) affects both the short- and long-term mechanical resistance. The binder chemistry along with the mixing water chemistry affect the formation of primary and secondary hydrates during the strengthening processes. The quasi-linearity of the relationship between the binder content and developed mechanical strength holds true for stabilization of sulphidic mine tailings with both high and low sulphide contents. For stabilizing high sulphide tailings, the effectiveness of a sulphate-resistant binder (mixture ofASTM Type I and Type V portland cements) is reported. However, for low to medium sulphide bearing tailings, slag based binders have generally shown best results. The effectiveness of a binder greatly varies depending upon the type of mine tailings and its composition. Since the cost of binder is the major material cost in tailings stabilization, the necessity of selection of relatively economical but effective binders such as industrial by-products and optimization of binder composition is evident in stabilization operations. The matrix stability and leaching behavior of stabilized tailings strongly depend on the nature of the dopant salt, as well as the leaching medium in contact with the stabilized system. Potential interferences (between sulphidic waste constituents and cement-based binders) from heavy metals or sulphates in high concentrations can be controlled by limiting the quantity of interferent stabilized per unit weight of binder. The presence of heavy metals impedes hydration reactions of cement, causing a delayed setting. However, they do not necessarily decrease the long-term strength of the fully cured product. Cement, cement – fly ash, and cement–silicate mixtures have proven to be effective in chemically stabilizing metal wastes. The long-term performance assessment of stabilized wastes should entail both flow-through and flow-around tests, since the flow-through mechanism may also become important when the waste has degraded to a level that flow-through of the leachant becomes possible at later stages. Performance evaluation tests carried out for the conformance to environmental regulatory
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standards are to be seen as a compromise between good simulation of reality (effects of oxidation, carbonation, etc.) and simplicity, robustness and acceptable cost. Finally, there is no universally accepted recipe for stabilization of sulphidic mine tailings. The variability and complexity of mine tailings preclude such a generalization. Moreover, the behavior of stabilized materials is quite different from that of cementitious mortars and concrete, and knowledge transposition between the two types of composites is not evident. Thus, thorough mixture optimization work is desired to define the most optimal stabilization method for specified tailings, which should account for technical constraints, environmental aspects, and cost management in stabilization operations.
Acknowledgement This research was supported by The Cement Association of Canada, The Canada Foundation for Innovation, The Ontario Innovation Trust, and The Ontario Premier Research Excellence Award to Moncef Nehdi.
References Aref, K., Hassani, F.P., and Churcher, D. 1987. A study in liquefaction potential of paste backfill. Proceedings of the 4th International Symposium on Mining with Backfill, Montreal, Quebec. ASTM C 618. 1997. Standard specification for coal fly ash and raw or calcined natural pozzolan for use as a mineral admixture in concrete. Annual Book of ASTM Standards 04.02, pp. 294–296. Baghdadi, Z.A., Fatani, M.N., and Sabban, M.A. 1995 Soil modification by cement kiln dust. J. Mater. Civil Eng. 7: 218–222. Barth, F.E. 1990. An overview of the history, present status, and future direction of solidification/stabilization technologies for hazardous waste treatment. J Hazard. Mater. 24: 103–109. Bensted, J., and Varma, P. 1971 Studies of ettringite and its derivatives. Cem. Technol. 2: 73–76. Benzaazoua, M., Belem, T., and Bussiere, B. 2002. Chemical factors that influence the performance of mine sulphidic paste backfill. Cem. Concr. Res. 32: 1133–1144. Benzaazoua, M., Fall, M., and Belem, T. 2004a. A contribution to understanding the hardening process of cemented paste. Mineral. Eng. 17: 141–152. Benzaazoua, M., Marion, P., Picquet, I., and Bussiere, B. 2004b. The use of pastefill as solidification and stabilization process for the control of acid mine drainage. Mineral. Eng. 17: 233–243. Berardi, R., Cioffi, R., and Santoro, L. 1997. Matrix stability and leaching behavior in Ettringite-based stabilization systems doped with heavy metals. Waste Manage. 17: 535–540. Bernier, R.L., Li, M.G., and Moerman, A. 1999. Effects of tailings and binder geochemistry on the physical strength of paste backfill. Edited by D. Goldsack, N. Belzile, P.Yearwood, and G. Hall. Mining and the Environment II, Laurentian University, Sudbury, Ont. 3: 1113–1122. Bin Shafique, M.S., Walton, J.C., Gutirrrez, N., Smith, R.W., and Tarquin, A.J. 1998. Influence of carbonation on leaching of cementitious waste forms. J. Environ. Eng. 124: 463–467. Bonen, D., and Sarkar, S.L. 1995. The effects of simulated environmental attack on immobilization of heavy metals doped in cement-based materials. J. Hazard. Mater. 40: 321 335. © 2007 NRC Canada
Nehdi and Tariq Campball, K.M., El-Korichi, T., Gress, D., and Bishop, P. 1987. Stabilization of cadmium and lead in portland cement paste using a synthetic seawater leachant. Environ. Prog. 6: 99–103. Catalan, L.J.J., and Yin, G. 2003. Comparison of calcite to quicklime for amending partially oxidized sulphidic mine tailings before flooding. Environ. Sci. Technol. 37: 1408–1413. Cheng, K.Y., and Bishop, P.L. 1996. Morphology and pH changes in leached solidified/stabilized waste forms. In Stabilization and solidification of hazardous, radioactive, and mixed wastes. Edited by T.M. Gilliam and C.C. Wiles. American Society for Testing and Materials (ASTM), Philadelphia, Pa. ASTM Special Tech. Pub. 1240: 73–79. Ciccu, R., Ghiani, M., Serci, A., Fadda, S., Peretti, R., and Zucca, A. 2003. Heavy metal immobilization in the mining-contaminated soils using various industrial wastes. Mineral. Eng. 16: 187–192. Cocke, D., Ortego, J.D., McWhinney, H., Lee, K., and Shukla, S. 1989. A model for lead retardation of cement setting. Cem. Concr. Res. 19: 156–159. Cohen, M., and Mather, B. 1991. Sulphate attack on concrete research needs. Am. Concr. Inst. Mater. J. 88: 62–69. Conner, J.R. 1990. Chemical fixation and solidification of hazardous wastes. Van Nostrand Reinhold, New York, N.Y. pp. 376–385. Conner, J.R. 1993. Chemistry and microstructure of solidified waste forms. Lewis Publishers, London, UK. p. 41. Doye, I., and Duchesne, J. 2003. Neutralization of acid mine drainage with alkaline industrial residues: laboratory investigation using batch-leaching tests. Appl. Geochem. 18: 1197–1213. Environmental Council of British Columbia. 1998. Mining and water pollution issues in British Columbia: acid mine drainage. Publication by Environmental Council of British Columbia, Broad Street, Victoria, B.C. pp. 4–5. Fortin, S., and Poulin, R. 2001. Layered co-mingling with industrial alkaline additives-technical and economic perspectives. InfoMine Inc., Vancouver, B.C. Fortin, S., Lamontagne, A., and Poulin, R. 2000. The use of basic additives to tailings in layered co-mingling to improve AMD control. InfoMine Inc., Vancouver, B.C. Gavasci, R., Lombardi, F., and Polletine, A. 1998. Leaching tests on solidified products. J. Solid Waste Technol. Manage. 25: 14–20. Gerven, T.V., Baelen, D.V., Dutre, V., and Vandecasteele, C. 2004. Influence of carbonation and carbonation methods on leaching of metals from mortars. Cem. Concr. Res. 34: 149–156. Gray, N.F. 1997. Environmental impact and remediation of acid mine drainage: a management problem. Environ. Geol. 30: 62–71. Hamilton, W.P., and Bowers, A.R. 1997. Determination of acute Hg emission from solidified/stabilized cement waste forms. Waste Manage. 17: 25–32. Hassani, F.P., Hossein, M., and Newman, P. 1994. Comparison of surface and underground disposal of tailings waste. CIM Bull. 94(January). Hassani, F.P., Quellet, J., and Hossein, M. 2001. Strength development in underground high sulphate paste backfill operation. CIM Bull. 94(1050): 57–62. IAWG. 1997. Municipal solid waste incineration residues. Stud. Environ. Sci. 67: 550–558. Jang, A., Choi, Y.S., and Kim, S. 1998. Batch and column tests for the development of an immobilization technology for toxic heavy metals in contaminated soils of closed mines. Water Sci. Technol. 37: 81–88.
435 Jiang, W., and Roy, D.M. 1995. Microstructural aspects of highperformance cement-based materials. Mater. Res. Soc. Symp. Proc. 370: 115–124. Jones, L.W. 1990. Interference mechanisms in waste stabilization/solidification processes. J. Hazard. Mater. 24: 83–88. Josephson, J. 1982. Immobilization and leachability of hazardous wastes. Environ. Sci. Technol. 16: 219A–223A. Kamon, M., and Nontananandh, S. 1991. Combining industrial wastes with lime for soil stabilization. J. Geotech. Eng. 117: 1–17. Lange, L., Hills, C., and Poole, A. 1996. Preliminary investigation into the effects of carbonation on cement-solidified hazardous wastes. Environ. Sci. Technol. 30: 25–30. Leist, M., Casey, R.J., and Caridi, D. 2003. Evaluation of leaching tests for cement based immobilization of hazardous compounds. J. Environ. Eng. 129: 637–641. Leroueil, S., Tardif, J., Roy, M., La Rochelle, P., and Konrad, J.-M. 1991. Effects of frost on the mechanical behavior of Champlain Sea clays. Can. Geotech. J. 28: 690–697. Mayers, T.E., and Eappi, M.E. 1992. Laboratory evaluation of stabilization/solidification technology for reducing the mobility of heavy metals in New Bedford Harbor superfund site sediment. In Stabilization and solidification of hazardous, radioactive, and mixed wastes. 2nd ed. Edited by T.M. Gilliam. ASTM, Philadelphia, Pa. p. 304. Means, J.L., Smith, L.A., Nehring, K.W., Brauning, S.E., Gavaskar, A.R., Sass, B.M., Carlton C.W., and Mashni, C.I. 1995. The application of solidification/stabilization to waste materials. Lewis Publisher, Boca Raton, Fla. p. 118. Miller, A.G., and Azad, S. 2000. Influence of soil type on stabilization with cement kiln dust. Constr. Build. Mater. 14: 89–97. Misra, M., Yang, K., and Mehta, R.K. 1996. Application of fly ash in the agglomeration of reactive mine tailings. J. Hazard. Mater. 51: 181–192. Mitchell, J.K., and Dermatas, D. 1992. Clay soil heave caused by lime sulphate reaction. ASTM Standard Testing Procedures 1135, ASTM, Philadelphia, Pa. pp. 41–64. Mohamed, A.M.O., Hossein, M., and Hassani, F.P. 2002. Hydromechanical evaluation of stabilized mine tailings. Environ. Geol. 41: 749–759. Nehdi, M. 2001. Ternary and quaternary cements for sustainable development. Concr. Int. 2001(April): 35–42. Park, C.K. 2000. Hydration and solidification of hazardous wastes containing heavy metals using modified cementitious materials. Cem. Concr. Res. 30: 429–435. Picquet, I., Benzaazoua, M., Marion, P., and Videau, G. 1995. Inertage de stériles miniers arséno-sulfurés dans des liants hydrauliques. In Congrès International PSS Déchets, Nancy, France. Edited by J.M. Cases and F. Thomas. pp. 311–318. Pölmann, H., and Kuzel, H.J. 1990. Solid solution of ettringite. Part I: incorporation of OH− and CO3 2− in 3CaO·Al2 O3 ·3CaSO4 ·32H2 O. Cem. Concr. Res. 20: 941–947. Pöllmann, H., Auer, S., Kuzel, H.J., and Wenda, R. 1993. Solid solution of ettringite. Part II: incorporation of B(OH)4 − and CrO4 2− in 3CaO·Al2 O3 ·3CaSO4 ·32H2 O. Cem. Concr. Res. 23: 422–430. Poon, C.S., and Chen, Z.Q. 1999. Comparison of the characteristics of flow-through and flow-around tests of solidified heavy metal wastes. Chemosphere, 38: 663–680. Poon, C.S., and Lio, K.W. 1997. The limitation of the toxicity characteristic leaching procedure for evaluating cement-based stabilized/solidified waste forms. Waste Manage. 17: 15–23.
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436 Poon, C.S., Peter, C.J., and Perry, R. 1983. Use of stabilization processes in the control of toxic wastes. Effluent Water Treat. J. 23: 451–459. Poon, C.S., Peters, C.J., Perry, R., Barnes, P., and Baker, A.P. 1985. Mechanism of metal stabilization by cement based fixation processes. Sci. Total Environ. 41: 55–71. Poon, C.S., Clark, A.I., and Perry, R. 1986. Permeability study on the cement based solidification process for the disposal of hazardous wastes. Cem. Concr. Res. 16: 161–172. Revertegat, E., Richet, C., and Gegout, P. 1992. Effect of pH on the durability of cement pastes. Cem. Concr. Res. 22: 259–272. Rha, C.Y., Kang, S.K., and Kim, C.E. 2000. Investigation of the stability of hardened slag paste for the stabilization/solidification of wastes containing heavy metal ions. J. Hazard. Mater. B73: 255– 267. Roy, W.R., and Griffin, R.A. 1982. A proposed classification system for coal fly ash in multidisciplinary research. J. Environ. Qual. 11: 563–568. Rubin, J.B., Taylor, C.M.V., Sivlis, L.D., and Carey, J.W. 1997. Radioactive wastes dispersed in stabilized ash cements. International Ash Utilization Symposium, Lexington, Ky., 20–22 October 1997. Sale, L.Y., Chanasyk, D.S., and Naeth, M.A. 1997. Temporal influence of fly ash on selected soil physical properties. Can. J. Soil Sci. 77: 677–683. Skalny, J., and Daugherty, K.E. 1972. Everything you always wanted to know about Portland cement, but did not ask. Chem. Technol. 1972(January): 38. Steffen, Robertson and Kirsten Inc. 1989. Draft acid rock drainage technical guide. Vol 1. British Columbia Acid Mine Drainage Task Force Report, BiTech Publishers Ltd., Vancouver, B.C.
J. Environ. Eng. Sci. Vol. 6, 2007 Tashirio, C., Oba, J., and Akama, K. 1979. The effects of several heavy metals oxides on the formation of ettringite and the microstructure of hardened ettringite. Cem. Concr. Res. 9: 303–308. Trussell, S., and Spence, R.D. 1994. A review of solidification/stabilization interferences. Waste Manage. 14: 507–519. Wang, S., and Vipulanandan, C. 2000. Solidification/stabilization of Cr (VI) with cement leachability and XRD analyses. Cem. Concr. Res. 30: 385–389. Weitzman, L., Heamel, E.L., and Barth, F.E. 1990. Evaluation of solidification/stabilization as a best demonstrated available technology for contaminated soils. EPA-600/S2-89/013, Risk Reduction Laboratory, US Environmental Protection Agency, Cincinnati, Ohio. Whinney, H.G., Cocke, D.L., Balke, K., and Ortego, J.D. 1990. An investigation of mercury solidification and stabilization in portland cement using X-ray photoelectron spectroscopy and energy dispersivescopy. Cem. Concr. Res. 20: 79–91. Xenidis, A., Mylona, E., Paspaliaris, I. 2002. Potential use of lignite fly ash for the control of acid generation from sulphidic wastes. Waste Manage. 22: 631–641. Yilmaz, O., Unlu, K., and Cokca, E. 2003. Solidification/stabilization of hazardous wastes containing metals and organic contaminants. J. Environ. Eng. 129: 366–376. Yong, R.N., and Mohamed, A.M.O. 1992. Cyclic freeze thaw consideration in design of engineered soil covers for reactive tailings. Proceedings of the CSCE 2nd Environmental Specialty Conference, Québec, Que., 27–29 May 1992. Vol. 3. Canadian Society for Civil Engineering (CSCE), Montréal, Que. pp. 173–182. Yukselen, M.A., and Alpasalan, B. 2001. Case study-leaching of metals from soil contaminated by mining activities. J. Hazard. Mater. B87: 289–300.
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