Rhodium

Rhodium is currently the most expensive precious metal. It does not dissolve in hydrochloric, nitric or sulfuric acid, or even in aqua regia. It is quickly corroded by cyanide, alkali, soda and acid potassium sulfate melts. In its solid state, it is a silvery white and very shiny metal with a high surface luster. Its reflecting power (VIS >80%), thermal conductivity (RT 153 Wm_-1 K^-1) and electrical conductivity (specific electrical resistance at RT: 4.3 mWcm) are higher than in the other platinum-group metals. Rhodium has a face-centered cubic crystal structure and is easily ductile above 200°C. The elastic modulus at RT is 386 GPa. Its hardening properties (the hardness increases from HV 130 to HV 400 after a 50% deformation) mean that frequent annealing is necessary above 1000°C for shaping. At high temperatures (>1000°C) rhodium shows a very high oxidation resistance similar to platinum, although it begins to slowly oxidize into Rh₂O₃ in air above 700–800°C. In compounds rhodium occurs in oxidation states between 0 and +6, with +1 and +3 being the most frequent.

The most important rhodium deposits (approx. 85% of world production) are found in nickel/copper sulfide ores containing precious metals in South Africa’s Bushveld Complex. Less significant are deposits in Russia and North America. The precious metals present in the ore as chalcogenides or alloys can be concentrated in an involved process using gravity separation, flotation, scorification in a smelter, and then bessemerization of the matte. Leaching the matte with sulfuric acid leaves a precious metal concentrate behind. Before separating the individual precious metals, this concentrate is dissolved in oxidized hydrochloric acid. Selective separation of the other precious metals leaves a rhodium chloride solution, which is further concentrated by precipitation and ion exchange processes. The pure rhodium metal is finally obtained through wet chemical or electrolyte separation processes. Important secondary rhodium sources include the recycling of remnants from homogeneous catalytic processes, such as oxo processes and rhodium alloys.

Due to its distinct catalytic effects, rhodium is mainly used as a heterogeneous but also as a homogeneous catalyst. Approximately 90% of the annual production of rhodium is used in the automotive industry for exhaust catalytic converters. Heraeus supplies a large variety of catalytic converters for emission control. The chemical industry uses catalyst networks of platinum-rhodium alloys from Heraeus to produce synthetic fertilizers and hydrocyanic acid. Due to its pronounced hardening effect, rhodium is used in platinum alloys, which Heraeus supplies in large quantities to the glass industry for highly temperature resistant components (for example, fiber glass nozzles, crucibles, feeder systems and stirrers). Furthermore, rhodium from Heraeus is used as an alloy in platinum-rhodium thermocouples and iridium-rhodium spark plug electrodes. Thin layers of rhodium are also used in high quality jewelry as well as in electrical contacts (reed relays). Heraeus also supplies thin x-ray filters made of rhodium to obtain the optimum wave length in mammography devices.