Oxidation of Arsenopyrite

3.2.4.1 Introduction

Arsenopyrite (FeAsS) is the most common arsenic (As) bearing mineral (Saxe et al., 2005). Arsenopyrite and other primary arsenic minerals are formed only under high temperature conditions (Drewniak and Sklodowska, 2013) and are found in a variety of ore deposits, including magmatic, hydrothermal, and porphyry-style systems (Corkhill and Vaughan, 2009; Drewniak and Sklodowska, 2013). It is a common mineral constituent of refractory gold ores (Corkhill and Vaughan, 2009; Andrews and Merkle, 1999) and thus arsenopyrite is often mined, processed to extract the gold and discarded as solid waste (Corkhill and Vaughan,

2009). Other mineral constituents of arsenopyrite are copper and silver (Dos Santos et al., 2017). In addition, natural arsenopyrite samples are always associated with pyrite and are generally found with large domains of pyrite randomly inlaid in its structure (Fleet and Mumin, 1997; Dos Santos et al., 2017).

3.2.4.2 Oxidation Process

Arsenopyrite is stable under reducing conditions (Corkhill and Vaughan, 2009; Nesbitt et al., 1995). However, when arsenopyrite has been mined and exposed to the environment, it oxidises leading to the release of arsenite (As(III)), arsenate (As(V)) in addition to acid and heavy metals (Dos Santos et al., 2017). The oxidation of asernopyrite is a two-step process represented by reactions 3.10 and 3.11 (Drewniak and Sklodowska, 2013).

Combining reaction 3.10 and reaction 3.11 results in arsenopyrite oxidation process represented by the following reaction path (Dold, 2010; Mok and Wai, 1994; Simate and Ndlovu, 2014):

When ferrous iron generated in reaction 3.12 is oxidised forming ferric iron according to reaction 3.2, the ferric iron may hydrolyse at low pH generating ferrihydrate precipitation (reaction 3.4). Therefore, the overall arsenopyrite oxidation reaction can be written as follows:

Research studies have shown that the oxidation rate of arsenopyrite is similar to the oxidation rate of pyrite if ferric iron is the oxidant (Dold, 2010) whereas the oxidation rate of arsenopyrite is lower than that of pyrite if oxygen is the oxidant (Mok and Wai, 1994; Dold, 2010).

The galvanic effect between arsenopyrite and pyrite minerals has also been found to influence the dissolution of the two sulphide ores. For example, in the presence of arsenopyrite, the oxidation rate of pyrite is delayed whereas the oxidation rate of arsenopyrite increases (Dos Santos et al., 2017).

 
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