GOLD DISSOLUTION IN IODIDE ELECTROLYTES

Several oxidants (hypochloride, iodine and hydrogen peroxide) were used to evaluate the dissolution of gold in iodide electrolytes at ambient temperatures. Evans diagrams constructed for the two half cells involved in the dissolution process show that hydrogen peroxide is not a suitable oxidant for the iodide system. The gold dissolution rate in an iodine-iodide mixture is dependent upon the concentration of iodide, iodine and the solution pH. An optimum iodine-iodide mole ratio of 0.35-0.4 was found for 0.1 M KI solutions, with pH ranging from 2.7 to 11.5; these are capable of dissolving gold at a maximum rate of 17 mg/cm2h. If a small addition of hypochlorite is made (< 25 mM for 0.1 M KI solutions), gold will be dissolved faster than with iodine as oxidant. However, the gold dissolution rate in a hypochlorite-iodide mixture is strongly dependent upon the solution pH. The optimum hypochlorite-iodide mole ratio is 0.25 for KI solutions of 0.02 to 0.1 M at pH 2.7. Cyanidation, using the same concentration of cyanide yielded gold dissolution rates within the range of 1.3 (at pH 12.5) and 3.5 mg/cm2h (at pH 8.5).