Oxidation of Pyrrhotite

3.2.2.1 Introduction

Pyrrhotite group of minerals are considered as all the iron monosulphides of the general formula, Fe(1.xlS (where 0 < x < 0.125) (Wang and Salveson, 2005). The pyrrhotite group of minerals often occurs in association with a variety of ore deposits including Ni-Cu, Pb-Zn, and platinum group elements and appears in different crystallographic forms and compositions (Becker et al.,

2010). Ideally, the pyrrhotite group of minerals is extremely complex (Wang and Salveson, 2005) with each type exhibiting subtly different physical and chemical properties (Ekmek^i et al., 2010). Pyrrhotite group includes troilite (FeS) which is hexagonal and pyrrhotite (Fea.xlS) which may be monoclinic or hexagonal (Wang and Salveson, 2005).

3.2.2.2 Oxidation Process

The oxidation of pyrrhotite is not a very well understood process compared to that of pyrite, and the rate of controls of the reactions and the oxidation products are also poorly known (Nicholson and Scharer, 1994; Fox et al., 1997). However, pyrite (FeS2) and pyrrhotite (Fea.x)S) are two of the most common iron-sulphide minerals in areas where AMD is prevalent (Fox et al., 1997). It is known that dissolved oxygen and Fe3+ are important oxidants of pyrrhotite. Therefore, pyrrhotite dissolution can proceed through oxidative or nonoxidative reactions (Blowes et al, 2003). The overall reaction when oxygen is the primary oxidant may be written as follows according to Blowes et al. (2003):

The stoichiometry of pyrrhotite affects the relative production of acid (Dold, 2010). For example, if x = 0 and the formula is FeS, no H+ will be produced in the oxidation reaction; at the other extreme (x = 0.125), the maximum amount of acid will be produced by the iron-deficient Fe7S8 phase (Dold, 2010).

As for nonoxidative dissolution of pyrrhotite, it occurs when predominant S2~ surface species are exposed to acidic solutions; and the reaction occurs as follows (Blowes et al., 2003):

Oxidation of Chalcopyrite

3.2.3.1 Introduction

Chalcopyrite is the most important copper-bearing ore mineral (Vaughan et al., 1995), comprising approximately 70% of copper reserves in the world (Baba et al., 2012). It is a mineral predominantly found in igneous and met- amorphic rock and in metalliferous veins (Baba et al. 2012; McGraw-Hill Encyclopedia, 1998). Chalcopyrite is a mineral with a brassy to golden yellow colour (Baba et al., 2012; Mamedov et al., 2012). It contains several minerals including copper, zinc, sulphur, and iron that were produced at different times (Baba et al., 2012).

3.2.3.2 Oxidation Process

Chalcopyrite dissolution is usually suggested to be an electrochemical corrosion activity with oxidants, such as Fe3+ or dissolved 02, being reduced at the mineral surface (Biegler and Swift, 1979; Li et al., 2017). The resulting Fe2+ may be re-oxidised either by iron-oxidizing microorganisms or, at a slower rate, by 02 (Li et al., 2017; Nazari and Asselin, 2009).

In the presence of ferric ions under acidic conditions, the oxidation of chal- copyrite can be represented as shown in reaction 3.7 (Blowes et al., 2003).

A study by Rimstidt et al. (1994) showed that, with an increase in Fe3+ concentration, the oxidation rate of chalcopyrite increases, though with an oxidation rate of 1-2 orders of magnitude less than that of pyrite. Other studies have shown that the combination of ferrous iron oxidation and ferrihydrate hydrolysis is the main acid producing process as shown in reaction 3.8 (Dold, 2010).

Dold (2010) noted that complete oxidation of chalcopyrite without acid production can be represented as reaction 3.9.

It has also been established that the dissolution of chalcopyrite can be greatly influenced by galvanic effects (Blowes et al., 2003). A study by Dutrizac and MacDonald (1973) reported that the presence of pyrite or molybdenite in association with chalcopyrite can cause accelerated rates of chalcopyrite dissolution, whereas according to Blowes et al. (2003) the presence of iron rich sphalerite and galena can slow the dissolution.

 
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