THE PASSIVATION OF IRON DURING THE LEACHING OF PRE-REDUCED LATERITE ORES IN AMMONIACAL SOLUTIONS

摘要

It is shown that iron spontaneously passivates in solutions typical of those used in the Caron process. In the absence of thiosulphate, the passivation requires very high concentrations of dissolved oxygen. A surface layer comprised of a cobaltickel sulphide forms as a result of reduction of thiosulfate in the presence of cobalt and nickel ions in solution. The combined cathodic processes of reduction of thiosulphate, cobalt (III) and oxygen generate relatively rapid initial rates of dissolution. The formation of the layer however, leads to a gradual, linear decrease in the rate of dissolution and eventually to passivation, hence an incomplete dissolution of the iron and loss of dissolved cobalt and nickel into the sulphide layer. Investigation, which involved measurements of the OCP of an iron electrode in the leaching circuit of an industrial operation, confirmed that passivation of iron also occurs in the aerated leaching tanks.