Other Methods Sulphide Passivation or Microencapsulation

Sulphide passivation or microencapsulation is amongst the new techniques of controlling AMD at source that falls under the category of chemical barriers (Sahoo et al., 2013; Nystrom, 2018). Passivation is a phenomenon that refers to the treatment of reactive sulphide-bearing waste materials and it results in the creation of a chemically inert and protective surface layer that prevents the material from oxygen and ferric iron attack (Zhang and Evangelou, 1998; Sahoo et al., 2013; Zhang et al., 2003; INAP, 2014; Nystrom, 2018). In other words, passivation or microencapsulation is a technique wherein a sulphidebearing material is coated with oxidatively stable materials that prevent the reaction of the sulphide mineral with water and oxygen or an attack by ferric iron (Evangelou, 1995; Evangelou, 2001; Park et al., 2018a). In addition, Bessho et al. (2011) also argue that the new technique inhibits the oxidation of sulphide-bearing materials through the formation of a coating on the surface of the sulphide, which, ideally, prevents both oxygen and ferric iron from oxidizing the sulphide-bearing waste. Research has shown that the existence of a chemically inert and protective surface coating resulting from the passivation treatment process inhibits the oxidation of sulphide-bearing waste materials and subsequent generation of AMD for much longer period (Zhang and Evangelou, 1998; Zhang et al., 2003) than other techniques.

Several additives that could enhance the inactivity of sulphide surfaces that include organic and inorganic materials have been studied (Sahoo et al., 2013; Nystrom, 2018; Park et al., 2019). These additives inhibit oxygen, water and ferric iron to diffuse to the surface of sulphide materials (Satur et al., 2007) and thus disrupt the oxidation of sulphide-bearing materials and generation of AMD through various mechanisms (Kleinmann, 1990).

Examples of organic materials that have been used as coatings on the surface of sulphide minerals to suppress oxidation of sulphide minerals include sodium oleate fatty acids (Jiang et al., 2000), natural organic matter (NOM) like humic acid and lignin (Lalvani et al., 1990; Khummalai and Boonam- nuayvitaya, 2005; Acai et al., 2009), lipids and phospholipids (Elsetinow et al., 2003; Zhang et al., 2003; Hao et al., 2006; Hao et al., 2009), oxalic acids (Sasaki et al., 1996) and many others such as diethylenetriamine (DETA), triethylene- tetramine (TETA), sodium triethylenetetramine-bisdithiocarbamate (DTC- TETA), methyl ethyl ketone formaldehyde resin modified carbazoles and

8-hydroxyquinoline as discussed in a review article by Park et al. (2019).

A number of inorganic materials are also available as passivating reagents. Readers are referred to Sahoo et al. (2013) for detailed information on research that directly used inorganic coatings for passivating sulphide-bearing waste materials. In summary some of the inorganic materials include phosphate (Nyavor and Egiebor, 1995; Evangelou, 2001), silica (Evangelou, 1996; Evangelou, 2001; Bessho et al., 2011) and permanganate (Ji et al., 2012). Alkaline materials such as sodium hydroxide (NaOH), sodium carbonate (Na2C03), sodium bicarbonate (NaHC03), lime (CaO, Ca(OH)2) and limestone (CaC03) have also been successfully used to prevent AMD (Nicholson et al., 1988; Nicholson et al., 1990; Evangelou, 1995; Sahoo et al., 2013). More recently, fly ash (from coal combustion) has been identified as a suitable alternative alkaline material because of its low cost, local availability and self-healing capacity (Perez-Lopez et al., 2007; Sahoo et al., 2013; Park et al., 2019). Ideally, fly ash has the potential to encapsulate sulphide-bearing materials with iron oxyhydroxide coatings by supplying a sufficient amount of alkalinity (Perez- Lopez et al., 2007). Perez-Lopez et al. (2007) also observed that the potential use of fly ash to attenuate the sulphide-bearing oxidation is not limited to its capacity to encapsulate the sulphide grains in a short term, but it can also promote a hardpan on the interface between the fly ash and the sulphide in a medium term that ensures the total neutralisation of the mine residues.

Other studies have shown that under suitable conditions such as near neutral pH in the presence of sufficient alkalinity, some sulphide materials such as pyrite can passivate on their own due to the formation of ferric oxyhydroxide coatings on the surfaces of such sulphide materials (Park et al., 2019). Ideally, the ferric oxyhydroxide coatings lower the rate of oxidation of sulphide materials (Nicholson et al., 1990). Huminicki and Rimstidt (2009) proposed the mechanism of iron oxyhydroxide coating formation on the sulphide as follows: (1) colloidal iron oxyhydroxide precipitates are deposited on the sulphide surface driven by electrostatic attraction between negatively charged sulphide material and positively charged iron oxyhydroxide and (2) the densification and inward propagation of the coating, i.e., the coating becomes thicker and denser making it impermeable and thus a more effective barrier to dissolved oxygen transport.

Unfortunately, the passivation or microencapsulation techniques do not have the ability to target specific problematic acid-generating sulphidebearing minerals in complex systems (Park et al., 2019). As a result, microencapsulation techniques have resulted in unnecessary consumption of large quantities of expensive reagents. A great improvement to the microencapsulation technique proposed by Satur et al. (2007) that specifically target potential acid-generating minerals is termed carrier-microencapsulation (CME). Since CME can target pyrite and arsenopyrite, for example, even in complex systems containing other minerals like silicates and aluminosilicates, unwanted consumption of chemicals during treatment is drastically reduced (Park et al., 2018a).

Figure 6.3 is a schematic representation of CME. According to Park et al. (2018b), in CME, redox-sensitive organic compounds (e.g., catechol, 1,2-dihydroxybenzene) are used to transform relatively insoluble metal(loid) ions (e.g., Si4+ or Ti4+) into soluble metal(loid)-organic complexes (e.g., [Si(cat)3]2'


Schematic illustration of CME. (From Satur et al., 2007.1

and [Ti(cat)3]2'), which are stable in solution, but decompose selectively on the surfaces of pyrite and arsenopyrite, for example. The decomposition of metal(loid)-organic complex frees the insoluble metal(loid) ion, which is rapidly precipitated to form stable metal(loid)-oxyhydroxide coatings on the sulphide-bearing material. A number of researchers argue that the unique targeting ability of CME could be attributed to the decomposition of metal(loid)-organic complexes only on surfaces of minerals that dissolve electrochemically like pyrite and arsenopyrite, for example (Crundwell, 1988; Tabelin et al., 2017; Park et al., 2018b).

If successful, passivation is considered as a low-cost prevention technique, especially compared to traditional AMD treatments using alkaline additives (Sahoo et al., 2013; Nystrom, 2018). However, most of the materials used for passivation are either too expensive or potentially harmful to the environment (Sahoo et al., 2013; Nystrom, 2018). Thus, there is a need to find cost-effective materials able to passivate sulphide surfaces in a long-term perspective. Desulphurisation

Environmental desulphurisation is one technology that has gained popularity in the last couple of decades for the management of acid-generating sulphide materials (Bois et al., 2004; Sahoo et al., 2013; Nadeif et al., 2019). This technique consists of separating sulphide components from non-sulphide mine tailings using the principle of froth flotation (Nadeif et al., 2019). The success of the method relies on how well the sulphide minerals are isolated from the non-sulphide ones (Nystrom, 2018). Ideally, two different fractions are generated, the desulphurised tailings that do not generate AMD and a quantity of sulphide-enriched tailings which is acid generating (Bois et al., 2004). In principle, the low-sulphur tailings can be used as dry covers for existing high-sulphur materials (Bussiere et al., 2004; Demers et al., 2008; Fyfe and Martin, 2011; Sahoo et al., 2013); however, the low sulphur-bearing tailings should be evaluated to ensure that the cover material is not of an acidgenerating type (Sahoo et al., 2013). Alternatively, the resulting desulphurised fraction can be managed separately (Leppinen et al., 1997; Benzaazoua et al.,

  • 2000), for example, by backfilling it in underground mines (Benzaazoua and Kongolo, 2003). In view of the aforementioned examples, desulphurisation has demonstrated to be an economically and environmentally effective technique for decreasing the acid-generation potential of the sulphide-bearing materials (Bois et al., 2004; Demers et al., 2008; Nadeif et al., 2019). Furthermore, another advantage of desulphurisation is that, if successful, it can limit the amount of mine waste that needs treatment (Nystrom, 2018).
  • Electrochemical Protection Systems

It is established that sulphide minerals exhibit semi-conductor properties (Koch, 1975) and consequently are amenable to electrochemical manipulation (Lin et al., 2001). Therefore, the electrochemical protection systems are


Schematic illustration of an electrochemical protection system. (From Lin et al., 2001.)

premised on the fact that electronegative polarisation can be used to prevent the oxidation of sulphide-rich materials in a manner similar to that used in cathodic protection of steel structures (Lin et al., 2001). In this method, the cathode of the electrochemical cell is the tailings pile (or an exposed seam of sulphide-bearing rock), which must be partially submerged. A steel rod forms the electrical contact to the sulphide-bearing rock in the case of a seam, or a grid of metal mesh or graphite in the case of a pile of tailings. An external circuit connects the cathode to the anode (scrap iron), which is submerged in the water body to be protected, and the water also acts as its own supporting electrolyte (Bejan and Bunce, 2015).

Figure 6.4 is a schematic representation of the electrochemical protection system utilizing tailings as the cathode (Lin et al., 2001). The key to this technique involves negatively polarizing the tailing-electrolyte interface so that dissolved oxygen is reduced at the surface of the tailings (Sahoo et al., 2013). In this regard, electrons are transferred to the tailings through the electrical circuit, thus negatively polarizing the tailings/electrolyte interface. As a result, oxygen is reduced, in situ, at the tailings/electrolyte interface through the electrochemical reduction of oxygen as described by reaction 6.4 (Lin et al., 2001).

Lin et al. (2001) state that the rate at which the electrochemical reduction reaction proceeds is controlled predominantly by the applied current density that is related to the rate at which oxygen diffuses through the overburden (or soil cover) to the tailings. In addition to removing dissolved oxygen, hydroxyl ions are generated at the tailings, thereby inhibiting the oxidation of sulphide minerals and the associated release of metal ions (Lin et al.,

2001). According to Lin et al. (2001), the negative polarisation of tailings also changes the thermodynamic properties of the iron sulphide minerals in the tailings to more stable conditions.

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