Silver

I

Introduction

Silver, symbol Ag, white, lustrous metallic element that conducts heat and electricity better than any other metal. Silver is one of the transition elements of the periodic table. The atomic number of silver is 47.

Silver has been known and valued as an ornamental and coinage metal since ancient times. Silver mines in Asia Minor were probably worked before 2500 bc. The alchemists associated the metal with the Moon, perhaps because of the silvery appearance of the latter, and ascribed to it the symbol of a crescent moon.

II

Properties

With the exception of gold, silver is the most malleable and ductile of all metals. Its hardness ranges between 2.5 and 2.7; it is harder than gold but softer than copper. Silver melts at about 962° C (about 1764° F), boils at about 2212° C (about 4014° F), and has a relative density of 10.5. The atomic weight of silver is 107.868.

Chemically silver is not very active. It is insoluble in dilute acids and in alkalis but dissolves in concentrated nitric or sulphuric acid, and it does not react with oxygen or water at ordinary temperatures. Sulphur and sulphides attack silver, and tarnishing is caused by the formation of silver sulphide on the surface of the metal. Eggs, which contain a considerable quantity of sulphur as a constituent of protein, tarnish silver extremely quickly. Small amounts of sulphide, which occurs naturally in the atmosphere and which, as hydrogen sulphide (H₂S), is added to natural gas used domestically, tarnish silver. The black silver sulphide (Ag₂S) is among the most insoluble salts in aqueous solution, a property that is exploited for separating silver ions from other positive ions.

III

Occurrence

Silver ranks about 66th among elements in natural abundance in crustal rocks. It occurs in the pure state to a small extent; the most notable deposits of native silver are in Peru and Norway, where the mines have been worked for centuries. Pure silver is also found associated with pure gold in the form of an alloy known as electrum, and considerable amounts are recovered in the processing of gold. Silver is usually found combined with other elements (of which sulphur is the most predominant) in minerals and ores. Some of the important silver minerals are cerargyrite (or horn silver), pyrargyrite, sylvanite, and argentite. Silver also occurs as a constituent of lead, copper, and zinc ores, and half the world production of silver is obtained as a by-product in the processing of such ores. Practically all the silver produced in Europe is obtained from the lead sulphide ore, galena. In the United States relatively few mines are worked for their silver alone—the silver is mined in conjunction with lead, copper, and zinc. Most of the silver mined in the world comes from Mexico, Peru, Canada, the United States, and Australia. In 1988, about 444 million troy ounces were produced worldwide.

IV

Metallurgy

Silver is usually recovered from silver ores by roasting the ore in a furnace to convert the sulphides to sulphates and then chemically precipitating metallic silver. Several metallurgical processes are used to extract silver from ores of other metals. In the amalgamation process, liquid mercury, which forms an amalgam with the silver, is added to the crushed ore. After the amalgam is washed out of the ore the mercury is removed by distillation, leaving metallic silver. In lixiviation methods the silver is dissolved in a solution of a salt, usually sodium cyanide, after which metallic silver is precipitated by bringing the solution in contact with metallic zinc or aluminuim. For the Parkes process, which is used extensively in separating silver from lead ores, see lead. The impure silver obtained in the metallurgical processes is usually refined by electrolytic methods (see Electrochemistry) or by cupellation, a process that involves removing impurities by vaporization or absorption.