Rhodium is a platinum group metal. It is silvery-white, hard, and fairly inert. In recent years, rhodium became for a while the most expensive of the precious metals.
Rhodium was isolated from platinum ore in 1804 by English chemist William Hyde Walliston, who had discovered palladium the same way two years earlier. After removing platinum and palladium from the ore, he was left with a reddish salt that yielded the new metal. Rhodium is named from the Greek word for "rose".
For the next two centures, rhodium was mainly a chemical footnote because of its rarity. In just the last few years, however, it has become indispensable for automobile catalytic converts. The price soared and then dipped to a still-pricey value similar to platinum.
Mining and Production
Rhodium is occasionally found as a pure metal, but it is more commonly mined from deposits containing the other platinum group metals. By far the most (84%) rhodium comes from platinum mines in South Africa, followed by Russia (12%). It can also be extracted in small quantities from nickel and copper ores such as the nickel mining in Ontario.
Actual isolation of rhodium uses the byproduct of the main smelting or refining operation. A series of chemical treatments will separate rhodium from the palladium and other chemically similar metals. For example, adding a hydroxide base will yield rhodium hydroxide in solution that is then precipitated out and exposed to a sequence of additional acids and bases.
Most byproduct metals don't change in supply because of price or demand changes, but instead track the production of the main metal being mined. Rhodium is expensive enough, though, to influence its own production. There is also a very healthy recycling program for rhodium based mainly on recovery of automobile catalytic converters.
Properties and Uses
Rhodium is a hard and inert metal much like the rest of the platinum group. Unlike related metals, it does not oxidize at all in air and remains shiny. Therefore, cheap jewelry (and even good jewelry) is sometimes plated with a thin layer of rhodium to enhance its attractiveness.
The surface of rhodium is a catalyst for various chemical reactions similar to platinum and palladium. Specifically, rhodium catalyzes the reduction of nitrous oxides to nitrogen and oxygen and, therefore, 80% of all rhodium goes into catalytic converters.
Rhodium is plated onto optical surfaces because of its hardness.
Otherwise, rhodium is mainly used as an alloying agent with platinum and palladium to improve hardness and corrosion resistance. For example, fine pen nibs may contain rhodium.
The rise of the price of rhodium has been astonishing, as was the subsequent fall. Rhodium is uniquely suited to performing a function mandated by law in the United States and other developed countries, that function being NOx removal in automobile catalytic converters. Yet, it is scarce, and so as the demand for automobiles pushed the price higher, palladium was found to be an adequate substitute.
Rhodium is difficult to invest in because of the tiny market compared to similar metals. For example, annual production is around 30 tonnes compared with 192 tonnes of platinum.
Rhodium spot prices are posted by Johnson Matthey, and rhodium futures trade thinly among institutional investors. One American dealer offers pool accounts for trading or physical delivery at a premium. The easiest access to rhodium is to invest in mining companies such as Stillwater Mining and Anglo American. Note that these are not primary rhodium miners – no one is.