Palladium

Palladium has the lowest density, the lowest melting point and the highest reactivity of all platinum group elements. It does not tarnish in the air at room temperature, but does develop a thin layer of palladium oxide above 400°C, which decomposes again at 800°C and higher. Palladium is very ductile and can be rolled out into thin sheets, for example, but it is usually alloyed with other metals. Palladium has the unusual property of absorbing enormous amounts of hydrogen—over 1,000 times its own volume. This special characteristic makes it especially appealing as a medium for storing and purifying hydrogen. Today, palladium’s most widespread usage by far is as a catalyst for chemical reactions, particularly for those utilizing hydrogen. Palladium displays fine electrical contact properties, due to its resistance to oxidation and corrosion. Palladium normally occurs with a valence of +2 in compounds, although oxidation states 0, +3 and +4 are also possible.

Similar to platinum, palladium is found with other precious metals in nickel/copper sulfide ores. The most important reserves include nickel deposits in the Ural region of Russia (40–45% of global production), as well as the Bushveld Complex in South Africa (approx. 40%) and North American mines in the USA (Stillwater) and Canada. While the platinum-to-palladium ratio in the South African reserves lies at about 3:1, the deposits in Russia and North America show the opposite with a palladium surplus of 2.5 to 4 over platinum. The precious metals present in the ore as chalcogenides or alloys can be concentrated in an involved process using gravity separation, flotation, scorification in a smelter, and then bessemerization of the matte. Leaching the matte with sulfuric acid leaves a precious metal concentrate behind. Before separating the individual precious metals, this concentrate is dissolved in oxidized hydrochloric acid, and refined with selected process steps including reprecipitation, ion exchange and solvent extraction. After a reduction step, the palladium is processed with hydrogen to isolate a pure metal sponge. Key secondary sources for palladium include recycled carried catalysts containing the metal, such as industrial and exhaust emission catalysts. Heraeus processes these mainly in Germany and the USA.

The vast majority of palladium is used in exhaust emission catalytic converters and diesel particulate filters for the automotive industry, as well as catalysts for hydration and dehydration processes. The second-largest consumers are the electronics and electrotechnical industries, for which Heraeus provides conductive pastes and galvanized conductor ribbons containing palladium. Heraeus produces a broad portfolio of electrical contacts made of silver-palladium alloys, which display significantly better contact properties in sulfuric atmospheres than plain silver. Sliding contacts from Heraeus made from cold-hammered palladium alloys are far less costly than gold alloys. Palladium alloys are also popular as a jewelry material, for example as white gold, which has a palladium content of between 15 and 40%. The use of palladium-containing dental alloys is on the decline, although they are resistant to corrosion and have a relatively low melting point. Heraeus expects to see the use of palladium as a storage and separating membrane expand, particularly for future power supply systems that use H₂.