Iodide—thiocyanate leaching system for gold

Abstract

The leaching of Au by thiocyanate solutions is performed at pHs between 1-2, which allows the use of iron(III) as an oxidizing agent. The mechanism of dissolution of Au by iron(III) thiocyanate solutions is directly linked with the autoreduction process, in which Fe^III is spontaneously reduced to Fe^II while oxidizing SCN^-. This oxidation proceeds through the formation of several intermediate species, particularly (SCN)2 and (SCN)3^-, which act both as oxidants and, upon reduction, as complexants for Au. The production of (SCN)2 and (SCN)3^- must be continuous due to their fast decomposition by hydrolysis. The instability of these intermediate species towards hydrolysis is a major drawback of the iron(III) thiocyanate leaching system, which can be overcome by additions of small amounts of I^- and/or I2. Experiments revealed a synergistic effect of the thiocyanate-iodide mixture on the dissolution of Au. The dissolution rates in the mixed system were substantially higher than those obtained when either iron(III) thiocyanate or iron(III) iodide were used separately. The synergistic effect was attributed to the formation of relatively stable mixed iodine-thiocyanate species such as I2SCN^- and I(SCN)2^-, which participate in the mechanism of dissolution of Au. Graphs. 34 ref.