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Oxidation of Sphalerite

3.2.5.1 Introduction

Sphalerite with a chemical composition of (Zn,Fe)S is a sulphide mineral found in metamorphic, igneous, and sedimentary rocks in many areas globally (King, 2020b). Sphalerite compositions can vary among different sulphide-mineral deposit types (Stanton et al., 2006). For example, it contains variable amounts of iron up to 25% by weight that substitute for zinc in the mineral lattice (King, 2020b), and the structure of sphalerite can accommodate a wide variety of substitutions, such as Cu, Mn, and In (Jambor et al., 2005). Sphalerite also contains trace to minor amounts of rare earth elements such as cadmium, indium, germanium, or gallium, which, if present in large quantities, can be recovered as profitable by-products (King, 2020b).

3.2.5.2 Oxidation Process

Research has shown that the oxidation of sphalerite is dependent on a number of parameters including the concentration of oxidants, such as dissolved 02 or Fe3+ in solution, the temperature, and the pH (Bobeck and Su, 1985; Crundwell, 1988; Rimstidt et al., 1994; Blowes et al., 2003). The overall oxidation reaction for pure sphalerite, assuming that all sulphur is oxidised to sulphate, is given in reaction 3.14 (Blowes et al., 2003).

As can be seen from reaction 3.14, sphalerite falls in the category of non-acid producing sulphide minerals (Dold, 2010). However, if iron substitutes for zinc, sphalerite will be an acid generator in a similar way as pyrrhotite due to the hydrolysis of ferric iron (Blowes et al., 2003).

Oxidation of Galena

3.2.6.1 Introduction

Galena is a sulphide mineral with a chemical composition of PbS. The mineral occurs in igneous and metamorphic rocks in medium to low temperature hydrothermal veins (Ogwata and Onwughalu, 2019). In sedimentary rocks, it exits as veins, breccia cements, isolated grains, and as replacements of limestone and dolostone. Galena is also commonly associated with acidgenerating minerals, such as pyrite and pyrrhotite (Blowes et al., 2003).

3.2.6.2 Oxidation Process

Several researchers have studied the oxidation of galena and observed that in natural oxygenated environments, galena weathers to anglesite (PbS04), which is weakly soluble below the pH of 6 (Lin, 1997; Blowes et al., 2003; Shapter et al., 2000) according to reactions 3.15 and 3.16.

Jambor and Blowes (1998) found that as the secondary anglesite forms, it creates a layer on galena that can prevent the mineral from direct contact with oxidizing reagents because anglesite has a low solubility. A study by Fornasiero et al. (1994) showed that in the absence of oxygen, both lead and sulphide ions are released into solution in the form of free lead ions and hydrogen sulphide.

Rimstidt et al. (1994) also showed that, under acidic conditions, galena may also be oxidised by Fe3+ ions as follows:

In general, in the presence of Fe3+, the oxidation of MeS (where Me = divalent metal) produces acidity according to reaction schemes where part of the oxidation capacity of the system is derived from Fe3+ as given in reaction 3.18 (Dold, 2010).

Other studies have shown that the oxidation of galena in air may lead to the formation of lead hydroxide and lead oxide (Evans and Raftery, 1982; Buckley and Woods, 1984; Blowes et al., 2003). On the other hand, the exposure of galena to aqueous solutions may result in the formation of lead oxides and lead sulphate surface products (Fornasiero et al., 1994; Kartio et al., 1996; Kim et al., 1995; Blowes et al., 2003).

 
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