Selective Separation and Blending of Sulphide-Rich Materials
In this technique, sulphide-rich waste materials are separated from the rest of other materials and disposed into specifically designed and prepared storage areas (Kuyucak, 2002). The primary strategy is to segregate and place the material in the storage areas in order to limit its exposure to water and air (Skousen et al., 1987; Skousen et al., 1998). It is also helpful to compact the material once placed in the storage areas (Skousen et al., 1998) and depending on the neutralisation potential, the wastes are mixed with alkaline amendments such as limestone in order to minimise acidity of the overall system (Skousen et al., 1998; Chowdhury et al., 2015). Indeed, co-disposal and/or mixing of sulphide materials along with some materials (waste rock, limestone) which are either potentially acid consuming or alkaline producing is the most common practice to reduce AMD production from mine waste (Skousen et al., 2000; Kuyucak, 2002; Johnson and Hallberg, 2005; RoyChowdhury et al., 2015; Park et al., 2019). In fact, research has shown that complete mixing of the benign and acid-producing mine wastes is more effective than placing the two materials separately in layers (Kuyucak, 2002). Ideally, this technique is meant to ensure that AMD-generating potential of a reactive mine waste is minimised by either reducing the net acid-producing potential of the reactive waste or by increasing the net alkaline neutralizing potential of the benign waste (Kuyucak, 2002).
As noted already, a number of materials are used for co-disposal or blending of reactive mine wastes. The most frequently used materials include lime, limestone and phosphate minerals (Renton et al., 1988; Mylona et al., 2000; Hakkou et al., 2009; Kastyuchik et al., 2016; Li et al., 2018). Other materials that include industrial by-products and residues have also become popular blending materials for preventing the generation of AMD due to their high neutralisation capabilities (Xenidis et al., 2002; Doye and Duchesne, 2003; Yeheyis et al., 2009; Alakangas et al., 2013;Park et al., 2019). These materials and many others, for example, are known to consume H+ ions generated by sulphidic waste materials, a property that does not only contribute to the formation of near-neutral AMD (reactions 6.1 and 6.2), but also immobilises soluble metals and metalloids via precipitation (Mylona et al., 2000; Hakkou et al., 2009; Lottermoser, 2003; Park et al., 2019).
It is also important to analyse the chemistry of how some of the blending materials such as limestone, in particular, and the rest, in general, control the oxidation of sulphide waste materials. A number of studies have shown that blending (e.g., by limestone) control the acid production of sulphide minerals through four mechanisms. The first mechanism involves precipitation of ferric iron into the hydroxide form, thus its further participation as an oxidizing agent in the oxidation of sulphide wastes is inhibited (Kelley and Tuovinen, 1988; Mylona et al., 2000; Park et al., 2019). The second mechanism involves the raising of the pH (pH 6.1-8.4) that significantly weakens the activities of the sulphide-oxidizing microorganisms (Nicholson et al., 1988; Mylona et al., 2000; Park et al., 2019). The third mechanism involves the precipitation of oxidised compounds and the formation of a protective layer such as ferric- oxy-hydroxide on the surface of sulphide minerals that reduce their reactive surface areas thus impairing their dissolution further (Nicholson et al., 1990; Mylona et al., 2000; Park et al., 2019). The fourth mechanism also involves the formation of a cemented layer (hardpan) consisting of ferric-oxy-hydroxide and gypsum which has very low permeability that limits the diffusion of 02 and infiltration of water into sulphide materials (Blowes et al., 1991; Lin, 1997; Mylona et al., 2000; Park et al., 2019). Hallberg et al. (2005) also argue that the formation of hardpans reduces permeability and oxygen diffusion and thus lowers the oxidation rate of sulphide waste materials. Besides, acting as a barrier, the cement-like hard substance also behaves as a stabilizing material (Stehouwer et al., 1995; Skousen et al., 2000; RoyChowdhury et al., 2015).
Other studies have also found that large acid-producing waste rocks can be mixed with fine materials that contain high moisture content such as tailings and disposed together (Kuyucak, 2002; RoyChowdhury et al., 2015). This process results in the filling of large pores of waste rock and can help in minimizing oxygen penetration into the acid-producing mine wastes thereby preventing the oxidation process (Kuyucak, 2002; RoyChowdhury et al., 2015). The effectiveness of tailings as oxygen barriers is influenced by the moisture content maintained in the tailings (Barton-Bridges and Robertson, 1989).