Rhodium is a Platinum Group Metal (PGM) known for its exceptional resistance to corrosion, extreme hardness, and high melting point. Its scarcity, as one of the rarest elements on Earth, contributes significantly to its high market value. Rhodium’s unique properties, particularly its function as an outstanding catalyst, make it indispensable in various industrial applications, leading to high demand for scrap materials containing it.
The Dominant Source: Automotive Catalytic Converters
The single largest source of rhodium scrap is the spent automotive catalytic converter, where the metal is instrumental in meeting modern emissions standards. Rhodium, alongside platinum and palladium, forms the active material in three-way converters, which simultaneously reduce three major pollutants. Specifically, rhodium is the component responsible for the reduction reaction, converting harmful nitrogen oxides into harmless nitrogen gas and oxygen.
Rhodium is not found in the metal casing of the converter but in a thin, porous coating called the washcoat, which is applied to the ceramic or metallic honeycomb substrate. This washcoat, primarily composed of aluminum and cerium oxides, provides a vast surface area where the rhodium particles are dispersed.
The concentration of rhodium in a catalytic converter varies significantly based on factors like the vehicle’s age, engine type, and the regulatory standards it was designed to meet. Generally, diesel and larger-displacement gasoline engines, especially those manufactured under stringent emissions regulations, tend to contain higher loadings of PGMs, including rhodium. Converters from older vehicles may contain less rhodium than newer ones, as the ratio of rhodium to platinum and palladium has changed over time to optimize the catalytic process.
Spent Industrial and Chemical Catalysts
Industrial processes are a highly concentrated source of rhodium scrap, often yielding materials with significantly greater metal content than automotive converters. A major application is in the large-scale production of nitric acid, a precursor for fertilizers and explosives.
In this chemical reaction, rhodium is alloyed with platinum to form specialized catalytic gauzes, typically containing 5 to 10 percent rhodium, which facilitate the conversion of ammonia into nitric oxide. These platinum-rhodium gauzes are used until they degrade, at which point the spent gauze material is collected for its high-value metal content.
Rhodium also plays a significant role in the petrochemical industry, particularly as a homogeneous catalyst used in the production of bulk chemicals. One prominent example is the synthesis of acetic acid and acetic anhydride, where rhodium is used in the methanol carbonylation process.
In these chemical applications, the rhodium scrap can take the form of spent catalyst solutions, contaminated filter cakes, or deactivated catalyst beds. These industrial scrap forms are attractive to refiners because the rhodium concentration is often much higher than the parts-per-million levels found in automotive scrap, streamlining the recovery process.
Specialized Electrical and Electronic Components
Rhodium’s exceptional properties make it a preferred material for specialized electrical and electronic components that require high reliability and durability. It is frequently employed as a plating material on high-reliability electrical contacts, particularly in specialized relay switches and connectors.
This application benefits from rhodium’s hardness, low electrical resistance, and stability, which ensure consistent performance over millions of switching cycles. The metal is typically applied as an electroplated thin film to protect underlying materials from wear and oxidation.
Another specialized application is in high-temperature measurement devices known as thermocouples, where rhodium is alloyed with platinum. These platinum-rhodium alloys are used to measure extreme temperatures, such as those in molten metals or high-temperature furnaces.
The scrap from this sector includes discarded thermocouple wires and specialized electronic assemblies containing plated contacts, which are collected for their concentrated PGM content. While less common, rhodium plating is also used in non-tarnishing surfaces for specialized optical instruments and high-end jewelry, though the rhodium content in these items represents a minor source for large-scale recycling operations.