Electrowinning
Electrowinning (sometimes abbreviated to EW, also known as Electrorefining) the electrodeposition of metals from their ores that have been put in solution or liquefied. Electrowinning is electroplating writ large and is an important technique that allows purification of non-ferrous metals in an economical and straightforward step.
History
It is the oldest industrial electrolytic process. Electrorefining was first demonstrated experimentally by von Leuchtenberg in 1847. Later, the English chemist Humphrey Davy, obtained sodium metal in elemental form for the first time in 1807 by the electrolysis of molten sodium hydroxide. It was James Elkington, however who patented the commercial process in 1865 and opened the first successful plant in Pembrey, Wales in 1869.
Applications
The most common electrowon metals are lead, copper, gold, silver, zinc, aluminum, chromium, cobalt, manganese, and the rare-earth, alkali, and alkaline metals. For aluminum, this is the only practically used production process. Several industrially important active metals (which react strongly with water) are produced commercially by electrolysis of their pyrochemical molten salts. Experiments with using electrorefining to process spent nuclear fuel have been carried out. Electrorefining could separate heavy metals such as plutonium, cesium, and strontium from the less-toxic bulk of uranium. Many electroextraction systems are also avalable to remove toxic (and sometimes valuable) metals from industral wastestreams.
Process
Most metals occur in nature in oxidized form in their ores and thus must be reduced to their metalic forms. The ore is dissolved following some preprocessing in an aqueous electrolyte or in a molten salt and the resulting solution is electrolyzed. The metal is deposited on the cathode (either in solid or in liquid form), while the anodic reaction is usually oxygen evolution. Several metals are naturally present as metal sulfides; these include copper, lead, molybdenum, cadmium, nickel, silver, cobalt and zinc. In addition, gold and platinum group metals are associated with sulfidic base metal ores. Most metal sulfides or their salts, are electrically conductive and this allows electrochemical redox reactions to efficiently occur in the molten state or in aqueous solutions.
Some metals, including arsenic and nickel do not electrolyze out but remain in the electrolyte solution, these are then reduced by chemical reactons to refine the metal. Also after the process for the original target metal is complete, other metals which are reduced but not deposited at the cathode sink to the bottom of the electrolytic cell, where they form a substance refered to as anode sludge or anode slime. The metals in this sludge can be removed by standard pryrorefining methods.
Because metal deposition rates are related to available surface area, maintaining properly working cathodes is important. Two cathode types exist, flat-plate and reticulated cathodes, each with its own advantages. Flat-plate cathodes can be cleaned and reused, and plated metals recovered. Reticulated cathodes have a much higher deposition rate, compared to flat-plate cathodes. However, they are not reusable and must be sent off for recycling.