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Smelting is the process of heating ore to extract base metals.It is a form of extractive metallurgy.It is used to extract a variety of metals from ore,including silver, iron, copper and other base metals.Smelting uses heat and chemical reducing agents to break down ore, expel other elements as gases or slag,and leave behind a metal matrix.The reducing agent is usually a fossil fuel source of carbon such as coke or, earlier,charcoal.Oxygen in ores combines with carbon at high temperatures because the chemical potential energy of bonds in carbon dioxide (CO2) is lower than those in ores.The carbon source acts as a chemical reactant to remove oxygen from the ore, resulting in purified metal elements as products.The carbon source is oxidized in two stages.First, carbon (C) burns with oxygen (O2) in the air to form carbon monoxide (CO).Second, the carbon monoxide reacts with the ore, such as Fe2O3, removing one of its oxygen atoms, releasing carbon dioxide (CO2).After continuous interaction with carbon monoxide,all oxygen in the ore will be removed, leaving the original metal elements (such as Fe).Since most ores are impure,it is often necessary to use fluxes such as limestone (or dolomite) to remove associated gangue stones as slag.This calcination reaction also often gives off carbon dioxide.The most prominent of smelting is the production of pig iron in blast furnaces,which is then converted into steel.Equipment for the electrolytic reduction of aluminum is also known as an aluminum smelter.
Smelting is more than melting metals out of ores.Most ores are compounds of metals and other elements,such as oxygen (as oxides),sulfur (as sulfides),or carbon and oxygen together (as carbonates).To extract the metals,workers must chemically react these compounds.Therefore,smelting consists of using suitable reducing substances in combination with those oxidizing elements to liberate the metals.
For sulfides and carbonates, a process called "roasting" removes unwanted carbon or sulfur, leaving oxides that can be directly reduced.Calcination is usually carried out in an oxidizing environment. A few practical examples:
Malachite, a common copper ore, is primarily copper carbonate hydroxide Cu2(CO3)(OH)2.This mineral undergoes thermal decomposition into 2CuO,CO2 and H2O in several stages between 250 °C and 350 °C .Carbon dioxide and water are vented to the atmosphere,leaving copper(II) oxide,which can be directly reduced to copper, as described below in the section entitled Reduction.Galena is the most common lead mineral, primarily lead sulfide (PbS).The sulfide is oxidized to sulfite (PbSO3),which is thermally decomposed to lead oxide and sulfur dioxide gas (PbO and SO2).Sulfur dioxide is expelled (like carbon dioxide in the previous example) and lead oxide is reduced as follows.
Reduction is the final high temperature step in smelting where the oxides are turned into elemental metals.A reducing environment (usually provided by carbon monoxide, created by incomplete combustion in a gas-starved furnace) extracts the final oxygen atoms from the raw metal.The required temperatures vary both in absolute value and in the melting point of the base metal.Iron oxide turns to metallic iron at about 1250 °C (2282 °F or 1523.15 K),nearly 300 degrees below iron's melting point of 1538 °C (2800.4 °F or 1811.15 K).Oxidized mercury becomes gaseous mercury at approximately 550 °C (1022 °F or 823.15 K), nearly 600 degrees above mercury's melting point of -38 °C (-36.4 °F or 235.15 K).After the reduction step is complete, the flux and slag can provide a secondary service: they provide a molten cover over the purified metal,protecting it from oxygen while still being hot enough to oxidize easily.This prevents impurities from forming in the metal.
Metalworkers use fluxes in smelting for a variety of purposes,the primary purpose of which is to catalyze desired reactions and chemically bind unwanted impurities or reaction products.Calcium oxide in the form of lime is often used for this purpose as it reacts with carbon dioxide and sulfur dioxide produced during roasting and smelting,keeping them out of the working environment.