Electrometallurgy uses electricity to refine or purify metals, including electroplating and electrowinning. Raw ore can be melted to separate desirable metals, but electrometallurgy is used to purify or separate remaining products. Electrorefining is used to extract metals like aluminum, while physical vapor deposition deposits a thin layer of metal on a surface.
Electrometallurgy is a term used for processes that refine or purify metals using electricity. It can also be a general term for the electrical processes used to plating one metal with another for decorative or corrosion resistance purposes. Metalworking by electricity is generally not the first step in purifying metals, but rather a later or final step used to create very pure metals for other industrial processes.
Raw ore mined from the ground may contain a precious metal such as gold, copper or aluminum with a large amount of impurities. Some processing can be done by melting the ores at high temperatures and separating the desirable metals. These processes give metals which may still contain an undesirable percentage of ores or other metals. Electrometallurgy can be used in several ways to purify or separate the remaining products.
Electrowinning is a process that uses electric current passing through a water-based bath, called a cell, to separate metal molecules, or ions, in a rod or plate. An electrical circuit consists of a positive and negative charge and a way that electric current flows between them. When two charged electrodes, called the cathode and anode, are inserted into the cell, metal ions collect on one of the electrodes. Careful control of voltage and current flow can create very pure metal deposits. This is a common process for purifying copper from less pure copper mixtures.
Another electrometallurgy process is electrorefining, which uses increased electric currents to heat and melt partially refined ores to extract metals. This is a common process for refining aluminum and is called the Hall process. In this process, partially refined aluminum oxide is first obtained from aluminum ore called bauxite. The aluminum oxide is then mixed with cryolite, a mineral composed of sodium, aluminum and fluorine, which melts at a much lower temperature than aluminum oxide. When the mixture is exposed to high electric currents, it melts and produces pure aluminum metal.
Electroplating is an electrometallurgy technique in which an electric current is passed through a bath of water and acid containing a dissolved mixture of metal and other minerals. An object placed in this bath and having an electrical charge can attract metal ions to its surface, which then stick or deposit as a thin layer on the object. This is a common process for creating plated items such as chrome parts used for automobiles, motorcycles, and household appliances. Galvanized metals not only have a lustrous decorative appearance, but the chrome plating acts as a corrosion-resistant coating to extend the life of the part.
Another process used in electrometallurgy is physical vapor deposition. This is similar to electroplating, where a metal is deposited on another surface. The metal is electrically vaporized to give it an electrical charge and the surface to be coated is given the opposite charge, which causes a very thin layer of metal to be deposited. A vacuum chamber in which air is removed is typically used to remove oxygen that can contaminate the product. Vapor deposition is used where acidic aqueous solutions could damage parts, such as in some electrical circuits. It also has the ability to form very thin layers which can be useful in electronics and some industrial processes.
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