The Betts electrolytic process is an industrial process for purification of lead from bullion. Lead obtained from its ores is famously impure because lead is a good solvent for many metals. Often these impurities are tolerated, but the Betts electrolytic process is used when high purity lead is required, especially for bismuth-free lead.
The electrolyte for this process is a mixture of lead fluorosilicate (PbSiF6) and hexafluorosilicic acid (H2SiF6) operating at 45 °C (113 °F). Cathodes are thin sheets of pure lead and anodes are cast from the impure lead to be purified. A potential of 0.5 volts is applied. At the anode, lead dissolves solution, as do metal impurities that are less noble than lead. Impurities that are more noble than lead, such as silver, gold, and bismuth, flake from the anode as it dissolves and settle to the bottom of the vessel as "anode mud." Pure metallic lead plates onto the cathode, with the less noble metals remaining in solution. Because of the high cost of electrolysis, this process is used only when very pure lead is needed. Otherwise pyrometallurgical methods are preferred, such as the Parkes process followed by the Betterton-Kroll process.
^Charles A. Sutherland, Edward F. Milner, Robert C. Kerby, Herbert Teindl, Albert Melin Hermann M. Bolt "Lead" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a15_193.pub2
^Samans, Carl H. Engineering Metals and their Alloys, 1949 MacMillan