The basic premise of most pyrometallurgical operations is simple: high-temperature chemistry is employed to segregate valuable metals in one phase while rejecting gangue and impurities in another phase. In most instances, both phases are molten (such as the matte and slag in a conventional copper smelting operation). The gas phase may also be used to advantage, either as a means of separating valuable volatile constituents or for removing unwanted volatile impurities. These separation techniques form the basis of thermal smelting and refining operations.
Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable metals. Pyrometallurgical treatment may produce saleable products such as pure metals, or intermediate compounds or alloys, suitable as feed for further processing. Examples of elements extracted by pyrometallurgical processes include the oxides of less reactive elements like Fe, Cu, Zn, Chromium, Tin, Manganese.
Calcining is thermal decomposition of a material. Examples include decomposition of hydrates such as ferric hydroxide to ferric oxide and water vapor, or decomposition of calcium carbonate to calcium oxide and carbon dioxide and or of iron carbonate to iron oxide. Calcination processes are carried out in a variety of furnaces, including shaft furnaces, rotary kilns, and fluidized bed reactors.
Roasting consists of thermal gas-solid reactions, which can include oxidation, reduction, chlorination, sulfation, and pyrohydrolysis. The most common example of roasting is the oxidation of metal sulfide ores. The metal sulfide is heated in the presence of air to a temperature that allows the oxygen in the air to react with the sulfide to form sulfur dioxide gas and solid metal oxide. The solid product from roasting is often called “calcine.” In sulfide roasting, if the temperature and gas conditions are such that the sulfide feed is completely oxidized, the process is known as “dead roasting.” Sometimes, as in the case of pre-treating reverberatory or electric smelting furnace feed, the roasting process is performed with less than the required amount of oxygen to fully oxidize the feed.
Refining is the removal of impurities from materials by a thermal process. This covers a wide range of processes, involving different kinds of furnace or other plant. The term, ‘refining’ can also refer to certain electrolytic processes. Accordingly, some kinds of pyrometallurgical refining are referred to as ‘fire refining’.
Smelting involves thermal reactions in which at least one product is a molten phase. Metal oxides can then be smelted by heating with coke or charcoal (forms of carbon), a reducing agent that liberates the oxygen as carbon dioxide leaving a refined mineral. Concern about the production of carbon dioxide is only a recent worry, following the identification of the enhanced greenhouse effect. Carbonate ores are also smelted with charcoal, but sometimes need to be calcined first. Other materials may need to be added as flux, aiding the melting of the oxide ores and assisting in the formation of a slag, as the flux reacts with impurities, such as silicon compounds.
Pyrometallurgy is a branch of extractive metallurgy.
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Cold working is the plastic deformation of metals below the recrystallization temperature.
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