Selective recovery of gold from spent gold sulphite solutions by electrodeposition on graphite cloth electrodes

A flow-through cell, employing an array of twin graphite cloth electrodes and connected to a charge injector potentiostat was employed for the electroseparation of gold from gold sulphite, Au(SO₃)₂^3–, solution containing cadmium, copper and chloride ions. The high overvoltage for the reduction of gold ions in this medium prevents its selective deposition (copper codeposits and hydrogen gas evolves at the same time). Hydrogen peroxide, added judiciously, oxidizes sulphite to sulphate; the gold Au(I) ions complex with chloride, establishing a system similar in its electrochemical behaviour to one previously discussed. Gold chloride, [AuCl₂]^–, solutions disproportionate readily; to minimize this process, a double flow-through system was built, in which the sulphite and hydrogen peroxide solutions enter a small mixing chamber as separate streams, are mixed in the appropriate ratio and allowed to react only a few minutes before reaching the electrodes. Gold deposits on the first graphite screen, and cadmium and copper deposit partially on the next screen, providing an economical approach to the continuous recovery and/or separation of gold from gold sulphite effluents containing base metal admixtures.