Palladium is a soft, rare platinum group metal (PGM) that is valued for its catalytic properties.
- Atomic Symbol: Pd
- Atomic Number: 46
- Element Category: Transition metal
- Density: 12.02 g/cm³
- Melting Point: 2830 °F (1554 °C)
- Boiling Point: 5365 °F (2963 °C)
- Moh's Hardness: 4.75
Palladium is a soft, silvery-white metal that shares many of the characteristics common to the PGMs, such as a relatively high melting point and high density. Although high for a metal, palladium's melting point and density are the lowest of the PGMs.
Palladium, much like platinum, is very resistant to oxidation and corrosion, and has excellent catalytic properties. This is mainly due to the fact that palladium has an abnormal - and astounding - ability to absorb hydrogen gas at a rate of 900 times its own volume.
Soft and ductile when annealed, palladium increases with strength and hardness when cold annealed.
Palladium is also chemically stable and conductive, making it useful for applications in the electronics industry.
In 1803, William Hyde Wollaston was able to isolate palladium from other PGMs by dissolving platinum ore in aqua regia (hydrochloric and nitric acid) mixture along with ammonium chloride and iron. However, there were no commercial applications for palladium until the of the 20th century.
One of the first unique applications for palladium, which still continues, was in developing photographic prints. The process, known as 'platinotype', which can use either platinum or palladium, was used as early as the late 18th century.
But palladium, like all PGMs, was inhibited by its rarity and high price. This began to change after the discovery of large stores of the metals were discovered in South Africa in the 1920s and Canada in the 1930s. Not long after this, palladium and platinum began to be used in dental alloys.
By the 1960s, research into platinum and palladium's catalytic properties were resulting in new applications in chemical processining, including polyester production and fuel cracking.
The biggest breakthrough for palladium, however, came in the 1970s when automobile emmission standards were implemented in the USA. Demand for palladium increased dramatically. Palladium's ability to absorb carbon monoxide and purify hydrogen made it integral to automobile catalytic converters.
Growing interest in palladium as an investment tool has also led to exchange traded funds, which are backed by physical palladium, being traded on the London and New York Stock Exchanges. Palladium futures are traded on the New York Mercantile Exchange and palladium bullion is also one of only four metals to have an ISO currency code (the others are gold, silver and platinum).
Palladium is always found alongside other PGMs. PMGs naturally occur in placer deposits found in rocks such as dunite, chromite, and norite. In South Africa's Bushveld complex and a limited number of other ore bodies, PGMs occur in sufficient quantities so as to make it economical to exclusively extract these metals; whereas, at Russia's Norilsk and Canada's Sudbury deposits palladium and other PGMs are extracted as by-products of nickel and copper.
Palladium containing ores are first crushed and immersed in reagent containing water; a process known as 'froth flotation'. During flotation, air is pumped through the ore-water slurry. Palladium particles chemically attach on to the oxygen and rise to the surface in a froth that is skimmed off for further refining.
After the ore has been extracted, crushed, milled, and concentrated, it is taken to be smelted. The dried concentrated powder cake is heated to over 2732F° (1500C°) in electric furnaces and air is blown through again, removing iron and sulphur impurities. Electrolytic and chemical techniques are employed to extract nickel, copper and cobalt, resulting in a concentrate of 15-20% PGMs.
The refining process uses aqua regia to dissolve palladium metal from the mineral concentrate. The remaining solution is treated with ammonium chloride to precipitate out other PGMs.
In the final step, palladium, which has remained in the solution, is precipitated with ammonia to create a salt that undergoes a chemical reduction involving formic acid to be converted into palladium metal.
The US Geological Survey estimates that world mine production of palladium was 207,000 kilograms (about 455,000lbs) in 2011. Russia accounted for about 41% of this production, while South Africa accounted for over 38%. Canada, the US and Zimbabwe are also important sources of palladium.
Accourding to USGS statistics, about 150,000 kilograms (or 330,000 pounds) - or 30% - of palladium produced annually comes from recycled sources, primarily the electronics industry.
The largest producers of palladium are Norilsk Nickel (Russia), which was the source of all of Russia's roughly 85,000kgs (187,000lbs) of palladium in 2011, and Anglo American plc, whose production in South Africa was 45,100kgs (99,220lbs) in 2010.
Global sales of palladium were estimated to be around 300,000kgs (660,000lbs) in 2010. Autocatalysts are the largest application for the metal, accounting for an estimated 57% of palladium use in 2010.
Other major end-uses for palladium include (approximate percentage global use):
- electronics sector (15%)
- investments (11%)
- jewellry (6.5%)
- dental alloys (6%)
- chemicals (4%).
Some estimate the number of operational automobiles worldwide to be around 450 million. Increasingly stringent emmissions controls rely on autocatalysts to reduce carbon monoxide, sulfur dioxide and hydrocarbons. Palladium plays a crucial role in this process by oxidizing carbon before it is exhuasted.
Palladium is primarily used in electronics within multi-layer ceramic capacitors (MLCC), which help to control the flow of current to various parts of a circuit by storing and releasing charge as required. MLCCs are often made using palladium or an alloy of palladium and silver. Smaller quantities of palladium are used in hybrid integrated circuits and as a plating material.
Use of palladium in the dental industry is relatively new phenomenon and tends to fluctuate depending on the relative price of gold, platinum and palladium. Palladium is alloyed with gold or silver, copper and zinc to form dental inlays, crowns and bridges. The inclusion of PGMs is to increase strength and durability, while remaining malleable.
Palladium can also act as an economical substitute for platinum in catalysts that are used to produce a variety of chemicals, in particular, paints, adhesives, fibers and coatings. Palladium catchment gauzes, which consist of a fine wire mesh, filter gas flows and are used to make nitric acid.
Other uses for palladium are found in:
- Fuel Cells (hydrogen absorbotion)
- Ethanol fuel production (Wacker Process)
- Oil Refining (catalytic reforming and hydroprocessing)
- Polyester (in the production of purified terephthalic acid, PTA)
- Photography (Platinotype Process)
- Water treatment
- Medicine (palladium -103)
The PGM Database. Johnson Matthey plc. http://www.pgmdatabase.com/jmpgm/index.jsp
The International Platinum Group Metals Association (IPA).
The US Geological Survey. Minerals Yearbook: Platinum-Group Metals (2010) and Mineral Commodity Summaries (2012).
Stillwater Palladium. Palladium: Metal of the 21st Century.