Cadmium and Cadmium Alloys

Abstract

AbstractCadmium (Cd), a Group 12 (IIB) element occurring between zinc and mercury, is a soft, ductile, and silver‐white metal with a distorted, hexagonal, and close‐packed crystal structure. Its crustal abundance is between 0.1 and 0.5 ppm, and several cadmium minerals have been identified, the most common being greenockite (CdS). Cadmium is found most often in zinc ores, zinc‐bearing lead ores, or complex copper‐lead‐zinc ores where it usually forms an isomorphic impurity in the zinc mineral, sphalerite (ZnS). For this reason, cadmium is almost invariably recovered as a by‐product from the processing of zinc, lead, and copper ores. Its oxidation state in almost all of its compounds is +2, although a few compounds have been reported in which cadmium exists in the +1 oxidation state. There are eight natural isotopes of cadmium, ranging in mass from 106 to 116. When heated in air, cadmium metal forms a brown‐colored cadmium oxide fume (CdO). Cadmium also will react readily to form stable compounds with the halogens, selenium, sulfur, phosphorus, and tellurium.Cadmium‐containing zinc, lead‐zinc, and copper‐lead‐zinc ores normally are processed by flotation or heavy‐media separation to yield concentrates, which then are processed further to recover the primary metals—zinc, lead, or copper. Because cadmium is associated closely with zinc in its chemical properties, it normally is processed in zinc recovery circuits. Most zinc, along with cadmium, is recovered today by electrolytic techniques rather than by the older pyrometallurgical methods. The level of primary cadmium production from zinc, lead‐zinc, and copper‐lead‐zinc ores is dependent on the level of cadmium in the ores and, hence, in the subsequent concentrate. Most cadmium‐containing ores have from 0.2% to 0.3% cadmium by weight.Cadmium exhibits known adverse human health and environmental effects in certain forms and in certain concentrations and is classified as a toxic metal. Occupational exposure to cadmium dust or fume may occur during certain heating or welding operations, and protection against inhalation of respirable particles must be provided either by properly designed exhaust ventilation systems or by personal respiratory protection.The three principal applications for cadmium are nickel‐cadmium (NiCd) batteries, cadmium pigments, and cadmium coatings. The use of cadmium as stabilizers in plastics, alloys, and other minor applications has now largely disappeared and was less than 1% of total consumption in 2009. In the past 20 years, the recycling of nickel‐cadmium has increased markedly in North America, Europe, and Japan, so that today, approximately 22% of total cadmium production comes from recycled NiCd batteries.