Affinage

Affinage 

a metallurgical process for obtaining noble metals of high purity by separating and recovering them from contaminating foreign matter. It is one of the ways of purifying metals.

Methods of affinage are divided into electrolytic, wet, and dry types. Electrolytic methods are used primarily for refining gold and silver; they involve the deposition of the pure metal on a cathode with the simultaneous separation of impurities in the form of sludge. Gold obtained by this method has a fineness of no less than 999.9. The advantage of electrolytic methods of affinage is in the lower cost of the process, the high degree of purity of the metals, the favorable working conditions, and the possibilities of obtaining metals in the platinum group as by-products by the addition of chemical reagents to the depleted electrolyte.

Wet affinage methods are used to obtain platinum, palladium, iridium, rhodium, and other metals of this group in a complex manner by dissolving the metals in aqua regia and subsequently separating them out of the solution with various reagents (such as ammonium chloride, ammonia, or sugar).

Dry gold affinage methods involve the treatment of the molten metal generally with chlorine. All the base metals then form chlorides and are volatilized, but silver chloride rises to the surface of the pure, molten gold. The fineness of the gold is 996.5, and the fineness of the silver (when recovered from the chloride) is 999.0.

REFERENCE

Plaksin, I. N. Metallurgiia blagorodnykh metallov. Moscow, 1958.