Fundamental study of silver deportment during the pressure oxidation of sulphide ores and concentrates

Abstract

Effort to recover silver during the pressure oxidation requires an understanding of the behaviour of silver. In this work, pressure oxidation of silver sulphide was investigated in the temperature range of 110–150 °C and in the sulphate media. The subsequent deportment of silver was monitored to shed light on the path of aqueous silver. Silver sulphide reacts during the pressure oxidation and silver is released into the solution. Under the deoxygenated condition, aqueous silver can precipitate as silver sulphide. However, the presence of oxygen minimises this reaction and any silver sulphide precipitated is oxidised back to aqueous silver. Therefore, the major collector of aqueous silver is jarosite specie and its precipitation kinetics increases with increasing temperature and lead sulphate concentration. However, initial acidity has a negative effect on the rate of precipitation of silver and ferric ions. Silver precipitation is fast and finished before the ferric precipitation leading to the zoning of the silver in the core of the jarosite specie. During cyanidation, only the silver ion in the outer crystal planes of the jarosite is accessible to the cyanide resulting into low extraction of silver. Soluble iodide can be added to the pressure oxidation to precipitate silver iodide, which is soluble in cyanide. However, in the presence of ferric and iodide, some aqueous silver is sequenced preferentially into the jarosite and this is pronounced at high temperature because of the enhanced stability and the precipitation kinetics of jarosite.