2. The extraction of metals from ores by treating them with aqueous chemical solutions, including extraction by electrolysis and ion exchange.
While some metals; such as, gold and silver can be found uncombined with other elements in nature, most metals occur as minerals; that is, in chemical combination with nonmetallic elements.
When the combination is such that the extraction of a metal from a mineral is economically attractive, the combination is called an ore. The technique of producing metals from ore is called "process metallurgy" or "extraction metallurgy".
The principles involved in hydrometallurgy are those of physical chemistry, inorganic chemistry, electrochemistry and analytical chemistry.
Hydrometallurgical processes are used commercially because hey are convenient to operate, the recovery of metals is high, and operating costs usually are low; and especially, because of the variety of quantitative chemical separations that can be made in aqueous solutions much more readily than is possible with the melts used in pyrometallurgical processes.