Effect of dissolved metal species on arsenic precipitation and stability during the pressure oxidation of arsenopyrite

This work investigated the influence of dissolved Al3+, Ca2+, Cu2+ and Mg2+ cations on the behaviour of As during the acid pressure oxidation of arsenopyrite at 210C. Basic ferric arsenate sulfate was the major As-bearing product in the baseline after 1 h, and a similar result and degree of As precipitation was observed at up to 5,000 ppm of Al3+ and up to 500 ppm of Ca2+, Cu2+, and Mg2+. At 5,000 ppm of Cu2+, basic copper ferric arsenate was formed as a co-product along with improved As precipitation, while 5,000 ppm of Mg2+ resulted in scorodite as a minor product, potentially formed during cooling due to As co-precipitation following the dissolution and oxidation of Fe(II). All samples exhibited similar As stabilities, with As leachability near 10 ppm compared to the recommended limit of 5 ppm. The experimental and thermodynamic modelling results suggest that the dissolved cations at the studied concentrations have a beneficial impact on the removal of As under the investigated conditions. Subsequently, density functional theory calculations on doped scorodite indicated that ionic substitution of Fe with Al, Ca or Mg enhanced the stability of the crystal structure, while the opposite was observed with Cu due to increased reactivity of the Cu in the crystal.