In metallurgy, leach means to extract specific metals from other substances, usually by converting them into soluble salts in an aqueous solution. For example, gold is an excellent conductor of electricity, making it useful in circuit boards. Thus, leaching the gold from circuit boards, sometimes called urban mining, is a lucrative process because it allows the precious metal to be reused elsewhere.
The leaching process often utilizes chemicals, like nitric acid or sodium cyanide, that are toxic to humans. Protective gloves and a respirator are advised as exposure to bare skin or inhalation of the fumes can cause health issues.