Abstract
The usual answer to the question stated in the title of this manuscript would be a definitive no due to the high stability of dissolved sulphide ions in water under strong alkaline conditions; nevertheless, experimental results presented in this research shows the opposite. Two arsenic-bearing copper concentrates coming from two different Chilean mine sites were used to carry out alkaline sulphide leach experiments. One of them (Concentrate 1) exhibits a P80 of 45 μm and 3.8% arsenic while the other (Concentrate 2) has a P80 of 98 μm and 2.3% arsenic. The arsenic extractions at 60 and 90 °C after 2 h reaction for concentrates 1 and 2 are around 10 and 80% and 14 and 93%, respectively.More relevantly, notwithstanding the strong alkaline conditions used in the experiments, the production of toxic hydrogen sulphide gas was detected and measured in tests performed at 90 °C. The molar ratio between the arsenic leached and the formation of hydrogen sulphide is 20:1 and 5:1 for concentrate 1 and 2, respectively. Stoichiometric computations associated to the mechanisms of carbonation of alkaline solutions and to the thermodynamic predominance of H2S at high pH values cannot fully explain the significant quantity of gas formed during the leaching process. One hypothesis to explain the formation of the gas is mineralogy related. It involves the occurrence of specific parallel secondary reactions taking place at the solid-liquid interface triggering the increase of the local acidity and producing the gas. Due to the high concentration of sulphide ions in solution and the high temperature conditions, the gas is not fully scrubbed by the strong alkaline solution. Looking at the mineralogy of the gangue material in both concentrates, the mass ratio of the gas produced between both concentrate matches the mass ratio of galena, haematite, goethite and kaolinite present in both concentrates suggesting that these minerals would be responsible for producing the gas. Reactions associated to each one of the mentioned species are presented suggesting two different mechanisms for local acidification. This finding would have a strong impact on the implementation of the process at large scale safety-wise.
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