The effect of curing on arsenic precipitation and kinetic study of pressure oxidation of pyrite and arsenopyrite

In the treatment of arsenic-bearing refractory gold ores, pressure oxidation (POX) represents an attractive approach to arsenic immobilisation, as the conditions are suitable for both sulfide oxidation and the formation of stable arsenates. While extensive work has been conducted on the reactions of the arsenic species during POX, research on the behaviour in the subsequent curing stage has been limited. The focus of this study is to explore the effect of curing time and temperature on the properties of the arsenic-bearing product following POX, using a synthetic mixture of pyrite and arsenopyrite. Basic ferric arsenate sulfate (Fe(AsO4)1?x(SO4)x(OH)x.(1 ? x)H2O) was observed as the major arsenic-bearing phase for Fe:As feeds of 1:1 and 2:1, and an increase in curing time resulted in higher arsenate stability based on characteristic leaching studies. A transformation to scorodite occurred for the 1:1 sample after 18–24 h of curing at 90–120 °C, along with a 60% increase in arsenic removal from solution and increased stability during characteristic leaching. Scorodite was absent at curing temperatures of 60 °C and for samples with a feed Fe:As above 2:1. At a feed Fe:As of 10:1, the major arsenic-bearing phases were amorphous iron arsenates, and arsenic concentrations in the leach extract were two orders of magnitude higher than 1:1 samples. Worryingly, this suggests that high arsenic-to-iron ratios are required for the formation of stable precipitates, with unstable phases forming otherwise. Oxygen consumption data collected during POX was used to fit a shrinking core model for pyrite and arsenopyrite, and the reaction of pyrite in this work followed a mixed control of chemical and oxygen transport, while that of arsenopyrite was limited by surface chemical reaction. Pyrite oxidation was enhanced by the presence of arsenopyrite, likely due to the formation of Fe(III)-As(V) complexes that contribute to mineral oxidation, and the magnitude of the enhancement increased with temperature.