In most cases, metals and their ores occur in the ground as part of complex mixtures that also contain rocks, sand, clay, silt and other impurities. The first step in producing the metal for commercial use, therefore, is to separate the ore from waste materials with which it occurs. The term ore is used to describe a compound of a metal that contains enough of that metal to make it economically practical to extract the metal from the compound.
One example of the way in which an ore can be purified is the froth flotation method used with ores of copper, zinc, and some other metals. In this method, impure ore taken from the ground is first ground into a powder and then mixed with water and a frothing agent such as pine oil. Then a stream of air is blown through the mixture, causing it to bubble and froth. In the frothing process, impurities such as sand and rock are wetted by the water and sink to the bottom of the container. The metal ore does not adsorb water but does adsorb the pine oil. The oil-coated ore floats to the top of the mixture, where it can be skimmed off.