Mark Doyle and Michael Turner Jones discuss the challenges and technological advances for recycling precious metals
IT IS increasingly important for companies to maximise the sustainability of their manufacturing processes, to reduce hazardous effects on the environment, and to ensure that we have sufficient natural resources for the future. Platinum group metals (PGMs) are widely used within consumer and industrial products, and include platinum, palladium, rhodium, iridium and ruthenium. They occur naturally but are scarce and, therefore, highly valuable.
Over the years, several industries have developed recycling and refining processes that allow the recovery and reuse of these precious metals. Such refining typically requires a mixture of advanced analytics and industrial-scale chemical processing to recover metal from process residues. However, demand for precious metals is expected to continue to grow over the coming years. This drives the need for greater metal recovery from more diverse product forms, through more cost-effective processing.
PGMs, with their special properties (eg low reactivity, high conductivity and high melting points), make them extremely valuable for a wide range of everyday consumer and industrial products. They are widely used in diverse industries including automotive, electronics, fuel cells, glass, jewellery, pharmaceuticals and healthcare, plus bulk and speciality chemicals production. In the automotive industry, they are used in autocatalysts that are fitted to cars, trucks and buses to control pollution, alongside spark plugs, engine sensors and many other parts of the vehicle. PGMs and platinum-based chemistries also make valuable contributions to healthcare, particularly in anti-cancer therapies as well as many dental and medical devices. PGM-based catalysts are used extensively in industrial manufacturing processes. Palladium is used in many of today’s electronic devices, including mobile phones and computers. Platinum and palladium are also used as jewellery materials, where their nobility makes them resistant to tarnishing, enabling them to maintain their attractive metallic lustre. PGMs are also used in aeroplane parts, including: the manufacture and coating of aero engine turbine blades; in manufacturing numerous glass materials, and essential bulk chemicals such as nitric acid; in sensors such as for detecting toxic and flammable gas, temperature monitoring and more.
Between one third and one fifth of all modern materials depend in some way on PGMs for their production, yet the total annual production of platinum from primary sources is just under 9 m3 (roughly the size of an SUV)
With such widespread use in many valuable everyday applications, demand for these PGMs is expected to increase. This is concerning because they are a finite resource, extracted through mining in areas known to be rich in PGMs, such as South Africa. Mining is an extremely costly enterprise, delivering at best around 10 g of PGM per metric ton of mined material. However, this is typically more likely to be just 3-6 g/t in a good deposit.
It’s thought that between one third and one fifth of all modern materials depend in some way on PGMs for their production, yet the total annual production of platinum from primary sources is just under 9 m3 (roughly the size of an SUV), and iridium about 4% of this volume. Because of their importance and relative rarity, PGMs feature on the European Commission’s critical strategic raw materials list, and on the US DoE list for critical and strategic raw materials.
To ensure industries’ continued access to PGMs in the future, it is essential that products containing them are recycled. There is already a strong recycling industry, with the current rates estimated to be >95% for closed loop industrial applications, such as in the glass or nitric acid industries, and around 65% for open loop autocatalyst recycling globally. The reported rates are better than for most other industrial materials.
Recycling and refining above-ground PGMs is a considerably more cost-effective source of PGMs when compared to primary mining. For example, they can be recovered from a scrapped car autocatalyst at concentrations of around 0.2%. This is significantly higher than the usual concentrations of PGMs in mined natural resources. Mobile phones and computer motherboards are rich in palladium, typically containing respectively around 130 g/t and 80 g/t.1 Recovering these materials through recycling is not only more cost-effective but also helps to protect limited natural resources while reducing the environmental impact of mining. Recovering one ounce of precious metals through recycling and refining uses about one fifth of the energy required to deliver the same amount of PGM through mining.
The nobility of PGMs, which describes how they remain unchanged by the reactions they catalyse, means these materials can potentially last forever – provided that they are recovered from the products in which they have been used. As a result, much effort has been put into recovering and recycling PGMs, regardless of their origin.