Due to the depletion of gold ore easily processed, the demand for processing high carbonate refractory gold ore is increasing worldwide, and accordingly, the interest in alkaline pressure oxidation (POX) is also increasing. This study was conducted with the aim of investigating the fundamental kinetics of alkaline pressure oxidation and testing new concepts to improve the pre-existing alkaline POX process. Magnesium carbonate (MgCO₃) was proposed as an alternative alkaline reagent instead of sodium carbonate (Na₂CO₃). Formation of kieserite (MgSO₄·H₂O) at high temperatures has been confirmed. Addition of MgCO₃ may have a positive effect on cyanidation by replacing a part of the iron oxide - calcium sulfate based product layer with kieserite, which is expected to re-dissolve at low temperature and leave pores in the oxidized solid. However, the direct addition of MgCO₃ showed limitations in terms of extent of sulfide oxidation and gold recovery. A further attempt was made to affect the POX chemistry by addition of magnesium sulfate as a pre-treatment. This method showed potential regarding reduction of calcium carbonate (CaCO₃) reactivity as well as formation of desired magnesium hydroxide precipitate in the MgSO₄ pre-treatment stage. Two stage tests (MgSO₄ pre-treatment followed by alkaline POX) verified that the magnesium sulfate addition to POX had a beneficial effect on the gold recovery (8-10% increased) with a concurrent increase of sulfide oxidation.
The kinetic study of pyrite oxidation showed the reaction rate can be modelled with the shrinking core model (SCM), and pyrite oxidation is under chemical reaction control in the initial stage regardless of key parameters (particle size, temperature, and Po2). However, rate control mechanism shifts to product layer diffusion with the effect of build-up of product layer. The activation energy for pyrite oxidation in the starting period was calculated at 36.1 kJ/mol which implied the reaction is controlled by chemical reaction, whereas the activation energy with a longer time span dramatically decreases to 15.9 kJ/mol (thought to be due to product layer diffusion rate control). In addition, it was found that excessive Na₂CO₃ addition to the POX system could have a detrimental effect on pyrite oxidation.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat
