Rhodium -- A Rare Element

Rhodium is a chemical element that is a rare, silvery-white, hard, and chemically inert transition metal and a member of the platinum group.  It has the chemical symbol Rh and atomic number 45. It is composed of only one naturally-occurring isotope, ¹⁰³Rh. Naturally occurring rhodium is usually found as the free metal, alloyed with similar metals, and rarely as a chemical compound in minerals such as bowieite and rhodplumsite. It is one of the rarest and most valuable precious metals.

Rhodium is a so-called noble metal, resistant to corrosion, found in platinum- or nickel ores together with the other members of the platinum group metals.  It was discovered in 1803 by William Hyde Wollaston in one such ore, and named for the rose color of one of its chlorine compounds, produced after it reacted with the powerful acid mixture aqua regia.

The element's major use (approximately 80% of world rhodium production) is as one of the catalysts in the three-way catalytic converters in automobiles. Because rhodium metal is inert against corrosion and most aggressive chemicals, and because of its rarity, rhodium is usually alloyed with platinum or palladium and applied in high-temperature and corrosion-resistive coatings.  White gold is often plated with a thin rhodium layer to improve its appearance while sterling silver is often rhodium plated for tarnish resistance.

Characteristics

Rhodium is a hard, silvery, durable metal that has a high reflectance.  Rhodium metal does not normally form an oxide, even when heated.  Oxygen is absorbed from the atmosphere only at the melting point of rhodium, but is released on solidification.  Rhodium has both a higher melting point and lower density than platinum.  It is not attacked by most acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia.

 

Mining and Price

The industrial extraction of rhodium is complex, as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold. It is found in platinum ores and extracted as a white inert metal which is very difficult to fuse. Principal sources are located in South Africa; in river sands of the Ural Mountains; and in North America, including the copper-nickel sulfide mining area of the Sudbury, Ontario, region. Although the quantity at Sudbury is very small, the large amount of processed nickel ore makes rhodium recovery cost-effective.

The main exporter of rhodium is South Africa (approximately 80% in 2010) followed by Russia.  The annual world production of this element is 30 tonnes and there are very few rhodium-bearing minerals. The price of rhodium is historically highly variable. In 2007, rhodium cost approximately eight times more than gold, 450 times more than silver, and 27,250 times more than copper by weight. In 2008, the price briefly rose above $10,000 per ounce ($350,000 per kilogram). The economic slowdown of the 3rd quarter of 2008 pushed rhodium prices sharply back below $1,000 per ounce ($35,000 per kilogram); they rebounded to $2,750 by early 2010 ($97,000 per kilogram) (over twice the gold price), but in late 2013, the prices were a bit lower than $1000

Applications

The primary use of this element is in automobiles as a catalytic converter, which changes harmful unburned hydrocarbons, carbon monoxide, and nitrogen oxide emissions from the engine into less noxious gases. Of 30,000 kg of rhodium consumed worldwide in 2012, some 24,300 kg (81%) went into and 8,060 kg recovered from this application. About 964 kg of rhodium was used in the glass industry, mostly for production of fiberglass and flat-panel glass, and 2,520 kg in the chemical industry.

Precautions

Being a noble metal, pure rhodium is inert. However, chemical complexes of rhodium can be reactive. Median lethal dose (LD₅₀) for rats is 198 mg of rhodium chloride (RhCl
3) per kilogram of body weight. Rhodium compounds can strongly stain human skin.  Like the other noble metals, all of which are too inert to occur as chemical compounds in nature, rhodium has not been found to play, or suspected to play, any biological role. If used in elemental form rather than as compounds, the metal is harmless.

http://en.wikipedia.org/wiki/Rhodium