The major use of rhodium is in catalytic converters for cars (80-85%), where it reduces nitrogen oxides in exhaust gases.
In brief, exhaust gases contain three harmful pollutants, created by the fuel combustion process.: hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx). In order to minimize the effect of these pollutants on human health and on the environment, catalytic converters were introduced, with the key active components being platinum, palladium and rhodium. These platinum group metals have unique abilities, described by Dr Jeff Rieck of Johnson Matthey as facilitating “the reactions of HC and CO with oxygen to produce water and carbon dioxide (CO2) and to promote the reaction of CO with NOx to convert the NOx into harmless nitrogen gas”. Rieck states that it is theoretically possible to completely remove these pollutants. The below diagram demonstrates the process undergone within the converter.
Rhodium is the rarest of all non-radioactive metals. along with other platinum metals, in river sands in North and South America. It is also found in the copper-nickel sulfide ores of Ontario, Canada. Production comes from South Africa, Russia, North and South America and Canada. Rhodium is obtained commercially as a by-product of copper and nickel refining. World production is up to 30 tonnes per year.
The key property of rhodium is that it is most effective of the three metals in removing NOx from the exhaust, as well as contributing to the oxidation of HC and CO, as well as very good resistance to the poisons within the exhaust emissions. Cost has historically been one of the limiting factors as to how much rhodium has been used within the matrix of the three platinum group metals, however during 2015, reductions were seen in the values of all three. Despite ongoing efforts to find cheaper alternatives to the use of platinum, palladium and rhodium, tighter Original Equipment Manufacturer emissions standards mean that any potential loss of performance through use of substitutes would be deemed completely unacceptable. It seems therefore likely that they will continue to be key components of catalytic converters for some time to come.
As reported recently in Recycling International (Smaller scrap dealers ‘burnt’ by PGM market hiatus), one of the side effects of the drop in price of PGMs has been that scrap dealers have been holding onto their stocks of catalytic converters in the hope of seeing a reverse in the steep falls in prices of platinum, palladium and rhodium (drops of 25%, 29% and 50% respectively). The volume of recycled rhodium from spent catalytic converters in 2015 was 288,000 ounces, and some growth is predicted for the 2016 figure as the market adapts to the ‘new normal’ prices for PGM, and global vehicle sales continue to increase.
Other Uses of Rhodium
- Alloyed with platinum and iridium, it gives improved high-temperature strength and oxidization resistance, and is used, for example, in furnace windings, pen nibs, and electrodes for aircraft spark plugs.
- Due to its brilliance and tarnish resistance, rhodium is used to plate jewellery and the reflectors of headlights, as well as to coat optic fibres, optical mirrors, for crucibles and thermocouple elements.
- It is also used in the glass industry to produce fiberglass and flat-panel glass.
- It is used as an electrical contact material as it has a low electrical resistance and is highly resistant to corrosion.
- Rhodium is also used as catalysts in the chemical industry, for making nitric acid, acetic acid and hydrogenation reactions.
Catalyst Basics: Platinum, palladium, and rhodium – key ingredients that make converters tick, Dr. Jeff Rieck Senior Technology Manager, Johnson Matthey
Recycling International, March 2016, Smaller scrap dealers ‘burnt’ by PGM market haitus, Kirstin Linnenkoper
Special Metals Forum http://www.specialmetalsforum.com/rare-metals/5-most-expensive-metals-in-the-world/
Source: Royal Society of Chemistry/Chemistry World