A study of basic iron sulphate formation and control in the autoclaving of argentiferous sulphide gold ores

A pressure oxidation (POX)-hot cure (HC)-lime boil (LB) process is used to recover gold and silver from refractory sulphide ores containing minerals like pyrite and arsenopyrite. Pressure oxidation is used to oxidize minerals and liberate occluded precious metal particles. Iron that is present goes into solution and under acidic and high temperature conditions often precipitates as basic iron sulphate. Basic iron sulphate consumes excess lime during neutralization prior to cyanidation. Therefore, a hot cure stage is required to re-dissolve this precipitate. In the presence of silver, conditions in the hot cure favour the formation of silver jarosite, which is refractory to cyanidation and must be decomposed prior to cyanidation. This is done in the lime boil to produce cyanide-soluble silver hydroxide. This study was conducted to investigate parameters that affect the precipitates formed in each stage of the POX-HC-LB process. In the autoclave, at 225 °C, a range of ferric and acid concentrations were studied. It was determined that at low concentrations of free ferric and free acid, hematite was the favourable iron precipitate. Above a critical concentration of free acid, basic iron sulphate was favoured. Speciation modeling of solution complexes in the ferric-sulphate-water system pointed to the possibility of FeSO₄HSO₄⁰, FeHSO₄⁺² and/or FeSO₄⁺ as precursors to basic iron sulphate. Speciation models were also used to calculate ΔG° values of basic iron sulphate for the first time, at varying temperatures. Basic iron sulphate formed in the autoclave dissolved quickly in the hot cure and silver precipitation occurred slowly. Silver precipitation was limited in the absence of other cations, whereas in the presence of alkali salts, silver co-precipitated with alkali jarosites. Silver that precipitated by itself, was readily soluble in cyanide, whereas silver that co-precipitated with alkali jarosites, had low amenability to cyanidation. Alkali treatment of the jarosite phases using lime saw successful breakdown of alkali jarosites but limited silver recovery by cyanidation. It was hypothesized that conversion of jarosites to hydroxides through reaction with lime is diffusion-controlled by a calcium carbonate product layer. Treatment with sodium hydroxide as the alkalising agent saw higher silver recovery, and further supported this theory.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat