Cadmium: batteries, TVs, plastics

Greenockite from Tsumeb, Namibia. 

Nobody mines cadmium. It comes from metal refineries, mostly zinc processors, where it is recovered as a trace component. It’s toxic, and not much goes a long way—only about 228 tons in the US in 2009 (down from 700 tons in 2005), with nickel-cadmium batteries (NiCd) leading the way. As lithium ion batteries, with greater energy density, take over in many small devices, NiCd batteries have declined in use, but they may return as storage batteries for on-grid solar energy systems that store electricity during the day and make it available at night.

China produces about a quarter of the world’s cadmium, and while the US is a net exporter, it ranks #9 in world cadmium production with about 4% of the total. A lot of US cadmium is exported to Asia where batteries are made.

Cadmium’s minor uses include photovoltaic devices such as photocopiers, where cadmium sulfide coats drums. Traditional uses include yellow, orange, and red pigments: yellow no-passing stripes on highways once contained cadmium. It also stabilizes plastics, makes lasers, and in phosphors gave the bluish tint to black-and-white TV sets in the 1950s. Cadmium was also once a low-melting component of solder and Wood’s metal—an alloy of bismuth, lead, tin, and cadmium sometimes used in the fusible valves found in automatic sprinkler systems. Wood’s metal melts at 158°F; when fires reach that temperature, the metal melts to open the valve, allowing water to flow.

The only noteworthy cadmium mineral is greenockite, cadmium sulfide, which forms pretty honey-colored crystals shaped like hexagonal barrels.

Photo by Christian Rewitzer, via Wikipedia under creative commons license.