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Cd, A. Jonathan Buhalis
CADMIUM
by Jonathan Buhalis

History

Cadmium is found under zinc in the periodic table, and it has similar properties. In 1787, chemists Karl Hermann and Friedrich Stromeyer separately discovered the new metal as an impurity in zinc carbonate. The name cadmium comes from the Latin word for calamine, which is zinc carbonate.

Van Gogh self-portrait with cadmium sulfide yellow, A. Jonathan BuhalisCadmium was not produced in large quantities until the 1930s. At that time, it was mainly used to coat and protect steel, similar to galvanizing with zinc. Various cadmium compounds are brightly colored, and so its other use was in pigments (right).

Cadmium is a heavy metal and toxic. Being an environmental contaminant and a health concern, the metal has been phased out of its traditional roles in pigments, alloys, and coatings. The new uses that have arisen, as described below, are those for which no easy replacements are available.

Mining and Production

Cadmium is recovered as a byproduct of zinc mining. Zinc sulfides and other ores commonly contain between 0.25% and 0.5% as much cadmium as zinc. In 2014, 22,000 tonnes of cadmium were produced, with China (34%) and South Korea (18%) the largest refiners. (Compare zinc production at 13 million tonnes.) Cadmium is also recycled from nickel-cadmium batteries. This accounts for about 20% of world production.

Zinc is refined and purified when it is separated from sulfur by smelting or electrolysis. In either case, an extra step isolates the cadmium that accompanies the zinc. Vacuum distillation is one process, as the two metals have different boiling points.

Properties and Uses

Cadmium is a soft bluish-white metal. It is ductile and malleable. It is stable and corrosion-resistant, which is why it is still used sometimes to plate steel and iron.

Cadmium forms some particularly brightly-colored compounds. Cadmium pigments make brilliant yellow, orange, and red paints. Cadmium phosphors are used in TV picture tubes for the white, blue, and green dots.

An unusual use of cadmium is in nuclear fission plants. Cadmium absorbs neutrons, and so it is used in nuclear pressure vessels and control rods to contain radiation or dampen the reaction.

9-volt NiCd battery, A. Jonathan BuhalisBy far the largest current use of cadmium, though, is in rechargeable nickel-cadmium batteries (NiCd or NiCad). Cadmium forms the negative terminal of the battery. NiCd batteries were very popular in consumer electronics until about 1995, when nickel-metal hydrides began to replace them. NiCd batteries are still in wide use, however, because they have different cost and performance tradeoffs compared to NiMH (and compared to the much newer lithium-ion batteries).

Cadmium is a toxic heavy metal. This is the biggest tradeoff in using it. Batteries with cadmium must be disposed of properly. In the United States, this typically includes recycling the heavy metal for reuse.
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Content by Jonathan Buhalis
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