Reactivity and mineralogical evolution of an underground mine sulphidic cemented paste backfill☆
Introduction
The management of acid generating tailings is one of the most important environmental challenges for the mining industry. An interesting tailings management approach for underground mines consists of storing tailings in open stopes by using them as constituent of cemented paste backfill (CPB). CPB is prepared at the surface by mixing the total thickened mine tailings (at a pulp density usually between 75% and 85%) with binders (such as Portland cement, fly ash, blast furnace slag) and water. Mixture proportions are adjusted to achieve the required rheological and hardened strength characteristics. The CPB is usually placed in underground stopes via boreholes and pipelines. A survey conducted on 32 mines (Benzaazoua et al., 2005) showed that 44% of the surveyed underground mines in Canada, USA and Australia use CPB as backfilling technology.
Depending on the type of ore extracted at the mine, tailings used in the CPB mixture can contain sulphide minerals (such as pyrite and pyrrhotite) in a proportion varying between 1% and 70% (Goulet and Blais, 2001, Benzaazoua et al., 2003). These minerals are known to be reactive with water and oxygen to produce acidity and iron and sulphate ions (e.g. Lowson, 1982, Evangelou, 1995). Under theses conditions, sulphide oxidation reactions reduce the pH of the leachate which then increases the solubility and concentration of various elements contained in the tailings (this phenomenon is known as acid mine drainage, AMD or acid rock drainage, ARD). The combined effect of acidity and solubilised elements can adversely affect nearby waters (surface and underground).
The use of CPB improves ground support while reducing the amount of tailings that has to be sent to surface disposal facilities. This can decrease both the environmental impact and capital expenditures of the surface tailings facility (e.g. Hassani and Archibald, 1998). However, care must be taken not to simply displace the environmental impact, from the surface to the underground. For this, one must assess if tailings are prone to contaminate underground waters. Some authors have investigated the environmental impact of sulphidic backfill material when stored in underground mines. Thomson et al. (1986) concluded that the amount of metal ions released from backfill was negligible. Levens and Boldt, 1992, Levens and Boldt, 1994, Levens et al., 1996 mentioned some advantages related to the utilization of CPB in underground stopes such as the increase of the neutralization potential and the retention of metal ions due to the binder addition, and the decrease of its hydraulic conductivity. Nevertheless, laboratory tests performed by Bertrand et al. (2000) on CPB showed that addition of binders was not sufficient (in the long term) to neutralize the acid generated by the oxidation of sulphide minerals contained in the studied CPB. The ability of CPB to maintain a high degree of saturation (usually greater than 90%; Benzaazoua et al., 2000, Benzaazoua et al., 2002, Benzaazoua et al., 2004b, Belem et al., 2002, Ouellet et al., 2003) over time is a key aspect to minimize environmental impacts of sulphidic CPB. Indeed, the high degree of saturation reduces oxygen migration through the CPB and consequently minimizes sulphide oxidation reaction (e.g. Elberling et al., 1994, Elberling and Damgaard, 2001, Mbonimpa et al., 2003, Ouellet et al., 2003).
Oxidation of sulphide minerals in CPB is one of the main environmental concerns since it can affect the water quality. One way to evaluate the reactivity of sulphidic wastes (in laboratory and in the field) consists of measuring the consumption rate of oxygen by sulphide minerals. This approach, called the oxygen consumption (OC) test, was proposed by Elberling et al., 1994, Elberling and Nicholson, 1996. A few other techniques are available in order to measure (directly or indirectly) the reactivity of a sulphidic material, such as the sulphate release method (e.g. Elberling et al., 1994, Bussière et al., 2004) and the oxygen gradient method (Elberling et al., 1994, Yanful et al., 1999). Applicability of such methods is limited in the context of highly sulphidic CPB (especially for underground tests). For instance, the sulphate release method is inapplicable because Portland cement contains gypsum and the installation of oxygen ports in an underground environment (to measure oxygen concentration gradients) is difficult. The main advantages of the OC technique compared to other approaches are the rapid (quasi-instantaneous) estimation of the oxygen consumption, the relative simplicity of the technique, and its applicability to both laboratory and field conditions. Ouellet et al. (2003) performed a comparative laboratory OC study on five tailings containing different proportions of pyrite (4–74 wt.%) and on CPB prepared with these tailings using two binders: a 50:50 mixture of Portland cement type 10 and type 50 and a 20:80 mixture of Portland cement type 10 and ground granulated blast furnace slag. Results obtained on tailings showed that the degree of saturation played a significant role on OC; for instance, for a given sulphidic waste, the OC rate was 2 orders of magnitude higher at a degree of saturation of 50% compare to that at 90%. Results on CPB indicated that over the 60-days testing period (for all types of binder), the sulphidic CPB samples oxidized only near the exposed surface; the reacting layer typically has a thickness of less than 1 mm (as was also observed by Chapman et al. (2003) on CPB containing 40% of pyrite). CPB was shown to be effective to limit oxygen diffusion into the material, due to its ability to maintain a high degree of saturation (>85%) close to the surface. The CPB containing 74% of pyrite had an OC rate of less than 1 mol O2/m2/day at the end of the testing period (value much lower than similar sulphidic tailings without binder) (e.g. Tibble and Nicholson, 1997, Bussière et al., 2004).
In this paper the authors present the results of oxygen consumption tests performed in an underground stope to evaluate the in situ reactivity of the Laronde mine CPB. A detailed characterization of the superficial oxidized layer and the evolution of the CPB interstitial pore water chemistry over the testing period are also presented. The main objectives of this study were to investigate the evolving reactivity of a highly sulphidic CPB over time using OC tests, to understand the mechanisms explaining the variation of this reactivity, and ultimately to evaluate the environmental effects (and potential benefits) of incorporating sulphidic tailings into CPB. To the author’s knowledge, this is the first attempt to evaluate CPB reactivity directly in an underground stope.
Section snippets
Tailings properties
Field tests were performed at the Laronde mine, Quebec, Canada, one of deepest mine in North America at a depth of 2240 m below the surface. The daily production is approximately 7250 metric tonnes of ore (gold, silver, copper and zinc), resulting in a production of approximately 6150 tonnes of sulphidic tailings (dry mass) each day; nearly 25% of the tailings are returned underground as CPB.
Table 1 shows X-ray diffraction (XRD) analysis and ICP analysis results of the studied tailings. XRD
Oxygen consumption tests results
For all OC tests performed, oxygen concentration decreased significantly in the reservoir over a relative short period of 2–3 h. Fig. 2 (see the window on the upright) shows typical results from OC tests recorded at Day 41. The slope values −(KrDe)1/2 of the ln(C/C0) versus time plots are relatively similar for the three cylinders; they range between −1.39E−04 min−1 and −2.54E−04 min−1. For each in situ OC test, the slopes were used to calculate oxygen flux (or sulphide reactivity) with Eqs. (1),
Characterization of the oxidized zone
Oxidation mechanisms of pure sulphide minerals and of sulphidic mine tailings are relatively well understood (e.g. Lowson, 1982, Evangelou, 1995, Strömberg, 1997). However, oxidation of reactive tailings in the presence of hydrated cement has not been as extensively studied, and is hence poorly understood. As seen in the previous section, the studied CPB is almost fully saturated in the field. Nonetheless, local desaturation of the CPB is possible at the exposed surface and this can then induce
Discussion and conclusions
This study presented results of OC tests that were used to evaluate the reactivity of a sulphidic CPB. The tests were performed directly in an underground stope at the Laronde mine, Quebec, Canada. The in situ OC tests showed a high mean oxidation rate at the beginning of the testing period (2.4 mol O2/m2/day). The oxidation rate decreased progressively to reach a value near 0.3 mol O2/m2/day after 14 days, and then stabilized between 0.2 and 0.4 mol O2/m2/day afterward. The final mean oxygen flux
Acknowledgements
Funding of this work came from the Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Wastes Management (http://www.polymtl.ca/enviro-geremi). An NSERC Postgraduate Scholarship to the first author also supported this research. The first author would like to thank UQAT chemists and technicians (M. Bélanger, D. Bouchard, A. Perreault, B. Plante and M. Villeneuve) involved in field and laboratory measurements, and C. Goulet, P. Pépin, J.-F. St-Onge and D. Veillette from Laronde mine
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Paper presented at the PDMIW’05 Conference, Falmouth, UK.