Fundamentals of partial pressure oxidation of gold bearing sulphide minerals

Abstract

Pressure oxidation (POX) technology has been used worldwide to treat refractory gold ores. Despite decades of operation, the chemistry inside the autoclave has not been fully established, and in particular the chemistry of iron is of critical importance. The aims of this thesis are to investigate the role of ferric as surrogate oxidant, determine the solubility of hematite, basic ferric sulphate (BFS) and potassium jarosite at 220°C and develop Eh-pH diagrams for the Fe-S-K-H2O and Fe-S-H2O systems which cover the range of potential operating conditions. The role of ferric as pyrite surrogate oxidant was investigated in an oxygen-deprived environment at 220°C using a microwave digester. The results showed that ferric was capable to act as pyrite surrogate oxidant in the high temperature system despite of the removal of ferric from solution via iron hydrolysis. Pyrite oxidation by ferric was found to be surface limited. The oxidation extent was found to be linearly dependent with ferric concentration when the reaction has not reached the surface limit. Ferrous was found to hinder pyrite oxidation but its negative impact was able to be offset to some extent with higher solid loading (i.e. surface area). The solubility studies were carried out in a titanium autoclave with high temperature sampling that allows rapid solid-liquid separation and in-line ORP measurement. The results from hematite solubility study indicated that aqueous FeHSO4SO4 0 species was the predominant species at higher acidity solution and it switched to aqueous Fe2(SO4)3 0 at lower acidity by considering the reaction stoichiometry. New Gibbs free energy of formation (ΔGf 0) data for BFS at 220°C was determined to be-919.4 ± 0.3 kJ mol-1. Similarly, the Gibbs free energy of formation data for potassium jarosite at 220°C was determined to be-3009.4 ± 0.7 kJ mol-1 which is within the range of values reported in the literature. Subsequently, Eh-pH diagram for Fe-S-H2O and Fe-S-K-H2O system at 220°C were developed based on this new data and validated. Using the developed Eh-pH diagram, a combination of potassium jarosite, hematite, BFS were predicted to precipitate in Lihir autoclave depending on the acidity and potassium concentration. Potassium jarosite was identified as the possible main lime consumers. Due to the small particle size (<6 µm), possible surface defect and crystallinity of the potassium jarosite precipitated at Lihir autoclave, its kinetic decomposition with lime could be relatively iii fast. Basic ferric sulphate was also predicted to form if the sulphuric acid concentration in Lihir autoclave is higher than approximately 35 g/L at autoclave temperature. xi 4.6 Solubility of basic ferric sulphate at 220°C .