Electrorefining

Electrorefining is an electrolytic process that involves anodically dissolving a metal at the positive electrode in a cell and simultaneously re-depositing the same metal at the negative electrode. The first patents for metal electrorefining were for copper and were granted in 1865. The first successful plant was built in Pembrey, Wales in 1869 and this was followed in 1876 by the Norddeutsche plant in Hamburg, Germany, which is still operating today and is the largest copper refinery in Europe. The first commercial operation in the USA was constructed in Newark, New Jersey in 1883. The Newark plant produced 2-3 tonnes of electrolytic copper per day during its first year. This contrasts with the electrorefining capacity at Chuquicamata in Chile in 2009 of over 2,300 tonnes per day.
Electrorefining is the final step in the smelting process for the recovery of copper from sulfide ores, and this process accounted for nearly 80% of the world copper production of 16 million tones in 2009. Smelting produces blister copper, which contains about 99% copper and this is electrorefined to cathode copper that has a purity of around 99.99% copper. The positive electrodes for the electrorefining cell are cast from the blister produced by smelting copper sulfide flotation concentrates, and weigh 300 to 380 kg; the negative electrodes are the same as in copper electrowinning discussed in the previous section. The electrolyte contains 40 grams/liter of copper as copper sulfate and 150 to 200 g/l sulfuric acid and the refining cells operate at ~60oC (~140oF). Small quantities of glue, thiourea and chloride are added to the electrolyte; these compounds adsorb on the copper surface and help to form a dense electrodeposit. A current density of ~200 A/m2 is passed between the negative and positive electrodes and this results in a cell voltage of about 0.28 V. The electrical energy consumption in the copper electrorefining cell is ~0.25 kWh/kg.

Electrorefining is successful because metallic impurities that are more noble than copper, that is, are lower in the electrochemical series, do not enter the electrolyte when copper dissolves at the positive electrode. These impurities include precious metals – silver, gold and the platinum group metals – which are recovered by treatment of the sludge that falls to the bottom of the cell. Since impurities that do enter the electrolyte are less noble than copper, that is, are above copper in the electrochemical series, do not deposit on the negative electrode. Some metals, for example, nickel, accumulate in the electrolyte and are extracted in a separate stage.

Electrorefining of copper is a large operation. For example, an average copper refinery producing 500 tonnes of metal per day needs 0.2 km2 (kilometer = one thousand meters) of total electrode area. This corresponds to about 50,000 positive and 50,000 negative electrodes suspended in about 1,500 tanks occupying a total floor area of about 6,000 m2.


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.