Most metals are chiefly useful in elemental form, but they usually occur in compounds on the earth’s surface. An ore is a naturally occurring material from which one or more useful elements or compounds may be obtained at a cost that is economically feasible. As can be seen from the table, most metal ores are oxides, carbonates, or sulfides. A few of the least electropositive metals occur as the element.
|Type of Ore||
|Native Metals||Cu, Ag, Au, As, Sb, Bi, Pd, Pt|
|Oxides||Al2O3, Fe2O3, Fe3O4, SnO2, MnO2, TiO2, FeO•Cr2O3, FeO•WO3, Cu2O, ZnO|
|Carbonates||CaCO3, CaCO3•MgCO3,MgCO3, FeCO3, PbCO3, BaCO3, SrCO3, ZnCO3, MnCO3, CuCO3•Cu(OH) 2|
|Sulfides||Ag2S, Cu2S, CuS, PbS, ZnS, HgS, FeS•CuS, FeS2, Sb2S3, Bi2S, MoS2, NiS, CdS|
|Halides||NaCl, KCl, AgCl, KCl•MgCl2•6H2O, NaCl and MgCl2 in seawater|
|Sulfates||BaSO4, SrSO4, PbSO4, CaSO4•2H2O, CuSO4•2Cu(OH)2|
|Silicates||Be3Al2Si6O18, ZrSiO4, Sc2Si2O7|
Whether a mineral can usefully be regarded as an ore or not depends on such factors as how concentrated it is, its exact location, and whether there is a suitable process for extracting the metal. As the more accessible higher-grade ores become exhausted, less accessible and lower-grade ores are becoming increasingly utilized with a consequent shift in the meaning of the word ore. It is conceivable that many silicates could become sources of metals, notably aluminum. Currently, though, silicates are expensive decompose chemically, and silicates form ores only for relatively expensive metals like zirconium and beryllium.
The processing of ores may be divided into three steps. Often concentration or other beneficiation is required to remove worthless material (gangue) or to convert the mineral into an appropriate form for subsequent processing. The second and most-important step is reduction of the metal from a positive oxidation state. This may involve elevated temperatures, chemical reducing agents, electrolysis, or some combination of these treatments. Usually a third step, refining, is required to achieve the purity (or precise mixed composition in the case of an alloy) desired in the final product.