The word “rhodium” originates from the Greek word “rhodon,” which represents the lovely “rose.” When the metal was discovered, the scientist thought that the beautiful rose aptly describes its pinkish red color. Later on, however, the metal was found to have a shiny silver-white color in most cases.
Rhodium (Rh) belongs to the six member noble metals group that includes palladium, ruthenium, iridium, osmium and platinum. These metals exist in common rare metallic ores and exhibit some common characteristics. Rhodium is distinguished by its unique corrosion resistance, hardness, silvery-white metallic appearance and chemical inertness. It does not tarnish and is not prone to corrosion at normal room temperature. That is the secret of its durability, and it is one of the rarest precious metals.
Discovery of Pinkish Red Rhodium
After discovering the rare metal palladium, W. H. Wollaston discovered rhodium while he was trying to separate pure platinum from the ore. He used platinum ore from Peru as the raw material. After separating platinum and palladium from the ore sample, he was left with the residue of a red powder, which was later recognized as sodium rhodium chloride. Research studies state that he had used aqua regia, ammonium chloride and iron to separate palladium. By using hydrogen in the reduction process of the chloride salt of rhodium, he was able to separate rhodium, which had a pinkish hue. (Interested in discovering the properties of various materials? Be sure to read How to Get Started in a Career as a Materials Scientist.)
Similar studies confirm that rhodium is mainly found as a byproduct of platinum and nickel refining operations. It is sometimes found in the ores containing gold, silver and platinum.
Rhodium is a highly valued precious material. Most of the metal is used as a catalyst (along with other catalysts) for automobile catalytic converters.
Toxic Health Aspects of Rhodium Compounds
Rhodium compounds should be considered as toxic and carcinogenic. These can also cause strong stains on the skin. If the aerosol of this rare metal is inhaled it can be absorbed by the body, causing the risk of toxicity.
Because it is a rare mineral, there is insufficient data related to its safety. Hence, maximum precautions must be taken while handling, processing and using this material. (For more on safety, read The Dangers of Typical Corrosion Prevention Solutions.)
Properties of Rhodium
Rhodium displays many of the common properties of the rare platinum group metals (PGM), which generally have good chemical stability as well as catalytic properties. Additionally, rhodium is a good conductor of heat and electricity. It is corrosion resistant and stain resistant, and is one of the most reflective metals, which makes it a superior precious metal.
Low electrical contact resistance makes rhodium an ideal material for electrical contacts. It is durable, as it generally fails to oxidize even when heated. It absorbs oxygen while melting and releases the absorbed oxygen during solidification. It dissolves in aqua regia, but not in nitric acid.
It has a high melting point of 1,966°C (3,571°F). Thermocouples made of rhodium can accurately measure temperatures up to 1,800°C (3,272°F). This metal has a boiling point of 3,695°C (6,683°F). These properties ensure that it is suitable for high temperature applications.
Extraction of Rhodium
Most of the reserves of the platinum group of metals (PGMs) are found in South Africa. Production of these metals involves refining the base metals as well as finishing refinery of the precious metals. Steps involved include floatation, comminution, smelting and final conversion. Chrome and oxide ore content often creates challenges for the PGM metal extraction. Proprietary processes using low temperature roasting and bromine (acid) leaching process are able to maximize the yieldof oxide ores and mixed ores containing rhodium (improved to the tune of 65% for rhodium and 85% for platinum).
The normal production process of rhodium is briefly described as follows:
- Noble metals such as platinum, gold and palladium are first separated by precipitation from the PGM ores
- Sodium bisulfate is now added to the residue and melted; this step results in the production of rhodium sulfate
- Rhodium metal is precipitated from rhodium sulfate by the remaining steps
- The addition of sodium hydroxide causes precipitation of rhodium hydroxide
- The addition of hydrochloric acid to rhodium hydroxide results in H3RhCl6 (a purified acid solution of rhodium), which is further added to sodium nitrite and ammonium chloride, enabling the precipitation of rhodium
- The precipitate of rhodium is allowed to dissolve in hydrochloric acid and heated to remove contaminants by burning and thus the purified rhodium metal is finally produced
Recycling of Rhodium
Rhodium is reclaimed and recovered by recycling automobile catalytic converters. Almost 25% of the metal is provided by the recycling route.
Applications and Market Demand for Rhodium
Most of the market demand for rhodium is driven by the demand for automobile catalytic converters in Japan, Europe and the United States, and the glass industry demand in Asia. As it is one of the rarest metals, the price is determined by the demand. According to market research studies, increasing Asian demand for the rare metal is due to producers of flat display glass panels.
Rhodium is sometimes applied as a decorative coating on jewelry made of silver and on circuit components, making these products free from tarnish and corrosion. It is applied on decorative products and also used to obtain highly reflective shining surfaces for optical appliances. The electrodeposition process is used to create a durable rhodium coating with a presentable color on jewelry.
Rhodium is also used to produce palladium and platinum alloys that have high hardness and excellent corrosion resistance. These alloys are then used to manufacture catalytic converters and catalytic nets that catalyze chemical reactions. In 1976, three-way catalytic converters were developed by Volvo by using rhodium alloys. This breakthrough helped minimize nitrogen oxide (NOx) emissions from automobiles.
Rhodium-platinum catalysts are used by the chemical industry to produce nitric oxide, which is needed to produce explosives and fertilizers.
Other uses of rhodium include:
- As a coating on decorative products
- Finishing shiny mirrors in the optical industry
- Making components for optical instruments
- Making turbo engine parts
- For neutron flux detection in reactors
- Manufacturing of LCDs (liquid crystal displays) and cathode ray tubes (CRTs)
- Glass manufacturing
- Making thermocouples that ensure accurate measurement of high temperatures
- Making bushings for glass fiber production
Rhodium is a noble metal, known for its unique corrosion resistance, high temperature chemical stability, durability, shiny appearance and reflectance. It is one of the six rare metals of the platinum group. Rhodium does not tarnish and is not prone to corrosion. It is one of the rarest noble metals with very good durability.
Most of this rare metal is used by the automobile industry to make vehicle catalytic converters that accelerate and catalyze the reduction of nitrogen oxides into nitrogen gas, thus enabling regulatory engine exhaust compliance.