Electrorefining

Lead is also purified by electrorefining using the Betts process pioneered by Cominco at its smelter in Trail, BC, Canada in 1902. Lead from the smelter is melted and cast into slabs, which act as positive electrodes. Refined lead is cast into thin starter sheets for the negative electrodes on which lead is deposited. The electrolyte used is ~90 grams/liter hydrofluorosilicic acid containing ~70 g/l lead as lead hydrofluorosilicate and operates at 30 to 40oC (86-104oF). Glue and lignin sulfonate are included in the electrolyte to help form a smooth deposit. A current density of between 160 and 200 A/m2 is used and this gives a cell voltage of 0.35 to 0.65 V. The energy consumption is ~0.13 kWh/kg. Refining of lead is also carried out in sulfamide media. In 2009 the world production of lead was 4.0 million tonnes from mined ore and a further 4.8 million tonnes from secondary (recycled) sources. About 12 % of the world’s production of lead is electrorefined.

Nickel may also be purified by electrorefining; this process competes with the carbonyl process that exploits the volatility of nickel carbonyl.

Silver refining, which is carried out following the separation of this metal in copper and lead smelting, is especially interesting. While the aim in other metal electrorefining processes is to produce a massive, dense product, with silver, fine, tree-like dendrites are formed deliberately. This is achieved by plating the metal onto a substrate such as stainless steel or carbon from an electrolyte that does not complex with silver, plating from a quiescent electrolyte without the inclusion of additives that promote smooth deposits, and operating at a high current density. The silver dendrites formed on the negative electrodes can be easily harvested since they do not attach strongly to the electrode surface. The electrolyte typically contains 150 g/l of silver as silver nitrate and nitric acid to make the pH 1-1.5. The cell operates at ~35oC (~95oF) and a current density is ~400 A/m2. The positive electrodes are cast silver derived from the slimes formed in electrorefining of copper or lead. The electrorefined metal contains between 99.9% and 99.99% silver. The anodic slimes from silver refining are further treated to recover gold and platinum group metals.