Electrowinning of Gold
Gold can be recovered from solution by electrolysis, a process that is known in the extractive metallurgy industry as electrowinning. When two electrodes (cathode and anode) are placed in a solution containing metal ions and an electric current is passed between them, the metal can be deposited on th e negative electrode. In the recovery of most metals, oxygen is evolved from water at the positive electrode. An electrolyte, and a current density, is generally chosen that gives a dense, compact electrodeposits, and some additives could be included in the electrolyte to further improve product quality.
All electrowinning cells with pervious, packed bed cathodes can be divided into two groups, those that operate with flow of the electrolyte at a right angle the current flow, and those that operate with parallel solution and current flows. Usually cathodes are of steel wool and anodes of stainless steel. It’s possible for 2 kg of gold to be deposited onto 0.5 kg of steel wool in each cathode compartment before the cell’s current efficiency drops or the cathode becomes blocked by the gold deposit. The loaded cathodes are calcined at 700 oC for 20 hours. The calcine mixed with 40% borax, 30% sodium carbonate, and 25% silica is melted at 1300 oC.
Electrowinning of gold from cyanide solutions onto steel wool cathodes has become a very strong competitor of zinc precipitation, and is the standard procedure of gold recovery for plants employing a CIP process. The gold laden cathodes are washed, dried, and smelted or treated with acid to remove the excess of iron and then smelted. Hence, iron slag with a high gold content is ground and gravity enriched, and the gravity tails are recycled into the mill.