Rhodium Recycling

Basic Properties

• Classification: A transition metal, part of the platinum group metals (PGMs), which also include platinum, palladium, ruthenium, iridium, and osmium.
• Atomic mass: ~102.91 u.
• Physical state: A hard, brittle metal with a brilliant silvery luster.
• Melting point: ~1966 °C (high, though lower than platinum or iridium).
• Boiling point: ~3695 °C.
• Density: ~12.41 g/cm³ (denser than platinum but less than iridium).
• Hardness: Relatively hard compared to other PGMs, making it resistant to wear.

Chemical Behavior

• Highly inert at room temperature, resistant to corrosion by most acids (including hydrochloric and sulfuric acid) and alkalis. It dissolves only in aqua regia (a mixture of concentrated nitric and hydrochloric acids).
• Oxidizes minimally in air, even at high temperatures, forming a thin, protective oxide layer that prevents further reaction.
• Exhibits a primary oxidation state of +3, though other states (e.g., +1, +4) are possible in some compounds.

Occurrence & Extraction

• One of the rarest elements on Earth, with an average crustal abundance of only ~0.001 parts per million (ppm). It rarely occurs in its free form.
• Most commonly found as a trace component in platinum ores (e.g., sperrylite) and nickel-copper sulfide ores (e.g., in South Africa, Russia, and Canada).
• Extracted as a byproduct during the processing of platinum or nickel ores. The complex extraction process involves crushing the ore, followed by froth flotation, smelting, and chemical separation (using solvents or ion exchange) to isolate rhodium from other PGMs.

Key Uses

• Automotive catalysts: The single largest application. Rhodium is a critical component in three-way catalytic converters, where it helps convert harmful exhaust gases (nitrogen oxides, NOₓ) into nitrogen and oxygen, reducing air pollution from gasoline-powered vehicles.
• Jewelry: Used as a plating material for white gold or silver jewelry. A thin rhodium coating enhances durability, adds a bright, reflective finish, and protects against tarnishing.
• Chemical catalysis: Employed in industrial processes such as hydrogenation, hydroformylation, and the production of nitric acid, where its high catalytic activity (even at low concentrations) drives efficient chemical reactions.
• Electronics: Used in electrical contacts, thermocouples, and as a coating for optical fibers, leveraging its conductivity and resistance to corrosion.
• Aerospace: In high-temperature alloys for jet engines and turbine components, though this is a smaller application compared to other PGMs.

History & Economics

• Discovered in 1803 by English chemist William Hyde Wollaston, who isolated it from a platinum ore residue. He named it “rhodium” from the Greek word rhodon (ῥόδον), meaning “rose,” due to the rose-red color of its chloride compounds.
• Due to its rarity, rhodium is one of the most expensive metals, with prices often fluctuating dramatically based on supply (primarily from South Africa) and demand (driven by automotive and industrial needs).