Occurrence and Production
Silver sometimes occurs free (chemically uncombined) in nature, but is usually found combined with other elements. The principal ores of silver are argentite, consisting of silver and sulfur; polybasite, stephanite, and pyrargyrite, consisting of silver, antimony, and sulfur; proustite, consisting of silver, arsenic, and sulfur; and cerargyrite, consisting of silver and chlorine. Small amounts of silver occur in most gold ores and in various base metal ores, chiefly those of copper, lead, and zinc.
Silver deposits that lie far underground are mined by using deep shafts. Sometimes uncombined silver or silver ores occur as small nuggets or flakes that lie on or near the surface of the earth. Such deposits are worked by placer mining methods.
Most of the world's silver is produced as a by-product in the refining of base metals, chiefly copper, lead, and zinc. Substantial amounts of silver are obtained from silver ores and in refining gold. The leading silver-mining countries are Mexico, the United States, and Peru. Silver is mined in about 20 states; Nevada, Idaho, Montana, and Arizona usually account for two-thirds to three-fourths of the United States output.
The largest percentage of silver is obtained from the electrolytic refining of copper. The silver collects in the anode slimes and is recovered by smelting. Silver in lead ores is recovered by the Parkes process. In this process molten lead that has been partially purified is treated with zinc. The zinc combines with the silver and any gold that may be present, forming an alloy that rises to the surface. The alloy is skimmed off, and then distilled to remove the zinc. Silver and gold are separated either by chemical processes or by electrolysis. Silver in zinc or gold ores may be recovered by various metallurgical processes.
Silver is usually extracted from its ores by the flotation process, and then refined by smelting. Silver is sometimes obtained from its ores by the cyanidation process and the amalgamation process.
Crushed ore is treated with a dilute cyanide solution to dissolve the silver. Lime is usually added to the mixture to control acidity and to reduce chemical reactions between the cyanide and the impurities. After the silver dissolves, the mixture is filtered to remove the solid impurities. Zinc powder is added to precipitate the silver. The silver is then refined by electrolysis.
Crushed ore is passed over copper plates coated with mercury. The mercury combines with the silver to form silver amalgam. The amalgam is washed, and then distilled to drive off the mercury. The silver is refined by electrolysis.
The recovery of silver from scrap is increasingly important to the world silver supply. Much silver is recovered from the wastes of manufacturing processes and the arts. Silver is also recovered from various used products, including batteries, mirrors, silverware, and jewelry. Also, considerable amounts of silver have been recovered from silver coins that have been withdrawn from circulation.