Beneficiation may involve physical or chemical processes. Often, as in the case of panning for gold, the desired oreNaturally occurring minerals from which metals and other substances can be extracted commercially. or metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. is denser than the gangue. The latter can be suspended in a stream of water and flushed away. The iron ore magnetite, Fe3O4, is ferrimagnetic. It can be separated from abundant deposits of taconite by grinding to a fine slurry in water. Passing this suspension over powerful electromagnets removes the Fe3O4, more than doubling the concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of Fe. Both the physical beneficiation processes just described produce large quantities of tailings-water suspensions of the gangue. Usually the silicate and other particles are trapped in a settling basin, but in one case large quantities of taconite tailings have been dumped into Lake Superior for several decades.
Flotation This is a partly physical and partly chemical beneficiation used to concentrate ores of copper, lead, and zinc. These metals have considerable affinity for sulfur. (the table on the metallurgy section lists sulfide ores for each of them.) The ore is ground and suspended with water in large tanks. Flotation agents such as the xanthates are added and a stream of air bubbles passed up through the tank. The sulfur-containing end of the xanthate moleculeA set of atoms joined by covalent bonds and having no net charge. is attracted to the desired metal, while the hydrocarbonA compound containing only the elements carbon and hydrogen. end tends to avoid water (like the hydrocarbon end of a soap molecule). The metal-containing particles are swept to the top of the tank in a froth much akin to soap bubbles. The concentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). ore may then be skimmed from the water surface. By means of flotation, ores containing as little as 0.3% copper can be concentrated to 20 to 30% copper, making recovery of the metal economically feasible.
Chemical Processes Many ores can be chemically separated from their gangue by means of acidIn Arrhenius theory, a substance that produces hydrogen ions (hydronium ions) in aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond. or baseIn Arrhenius theory, a substance that increases the concentration of hydroxide ions in an aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) acceptor. In Lewis theory, a species that donates a pair of electrons to form a covalent bond.. Thus copper ore is often leached with acid which dissolves copper oxides and carbonates and leaves behind most silicates:
CuCO3 + H2SO4(aq) → CuSO4(aq) + H2O + CO2 (1)
In the Bayer process bauxite is dissolved in 30% NaOH:
Al2O3(s) + 2OH–(aq) + 3H2O(l) → 2Al(OH)4–(aq) (2)
and then reprecipitated by adding acid. Impurities such as Fe2O3 and SiO2 are eliminated in this way.
Many sulfide and carbonate ores are roasted in air in order to convert them into oxides, a form more suitable for further processing:
2ZnS(s) + 3O2(g) → 2ZnO(s) + SO2(g) (3)
PbCO3(s) → PbO(s) + CO2(g) (4)
The production of SO2 in Eq. (3) is a notorious source of air pollutionThe contamination of the air, water, and earth by personal, industrial, and farm waste..