Thermodynamic analysis of the immobilisation of arsenic during the pressure oxidation and curing processes

In recent decades, there has been increasing interest in the application of hydrothermal treatment as a pathway for the immobilisation of arsenic waste from the treatment of complex ores. These techniques can involve temperatures up to 250 °C, where thermodynamic data is often limited due to the challenges involved in the in-situ measurement of arsenic-bearing phases at elevated temperatures. A potential approach is the use of thermodynamic modelling to predict and understand the behavior of the aqueous arsenic complexes and arsenate minerals at high temperatures. Herein, the polyhedral method has been used to estimate the standard Gibbs free energy of formation (?Gfo) of 52 arsenate minerals and enthalpy of formation (?Hfo) of 26 arsenate minerals from 25 °C to 300 °C. The predicted values for the arsenate minerals showed good agreement with experimental results from literature, with average relative errors and absolute deviations of 0.32% and 6.77 kJ/mol for ?Gfo and 0.17% and 4.69 kJ/mol for ?Hfo. Subsequently, the polyhedral method and Helgeson-Kirkham-Flowers model were combined to estimate the solubility product constant (Ksp) of the arsenate minerals at 1.013 bar and 25, 50, 100 °C, and at saturation pressure and 150, 200, 250 and 300 °C. Based on the estimated values, the phase diagram for the Fe-As-S-H2O system demonstrated that scorodite is one of the most thermodynamically stable ferric arsenates at acidic conditions and temperatures below 130 °C, while ferric arsenate subhydrate (FAsH) and basic ferric arsenate sulfate (BFAS) were thermodynamically favoured at higher temperature. Under similar conditions, the model also predicts the preferential formation of BFAS over FAsH with increasing concentrations of SO42? and higher temperatures.