Metal Profile: Iridium

Iridium is a hard, brittle, and lustrous platinum group metal (PGM) that is very stable at high temperatures as well as in chemical environments.

Properties

• Atomic Symbol: Ir
• Atomic Number: 77
• Element Category: Transition Metal
• Density: 22.56g/cm³
• Melting Point: 4471 F (2466 C)
• Boiling Point: 8002 F (4428 C)
• Mohs hardness: 6.5

Characteristics

Pure iridium metal is an extremely stable and dense transition metal.

Iridium is considered the most corrosion-resistant pure metal because of its resistance to attack from salts, oxides, mineral acids and aqua regia (a mixture of hydric and nitrochloric acids), while only being vulnerable to attack by molten salts such as sodium chloride and sodium cyanide.

The second most dense of all metal elements (behind only osmium, although this is debated), iridium, like other PGMs, has a high melting point and good mechanical strength at high temperatures.

Metallic iridium has the second-highest modulus of elasticity of all metal elements, meaning that it is very stiff and resistant to deformation, characteristics that make it difficult to fabricate into usable parts but which make it a valuable alloy-strengthening additive. Platinum, when alloyed with 50% iridium, for example, is nearly ten times harder than when in its pure state.

History

Smithson Tennant is credited with the discovery of iridium while examining platinum ore in 1804. However, crude indium metal was not extracted for another 10 years and a pure form of the metal was not produced until nearly 40 years after Tennant's discovery.

In 1834, John Isaac Hawkins developed the first commercial use for iridium. Hawkins had been searching for a hard material to form pen tips that would not wear out or break after repeated use. After hearing about the properties of the new element, he acquired some iridium-containing metal from Tennant's colleague William Wollaston and began producing the first iridium-tipped gold pens.

In the second half of the 19th century, the British firm Johnson-Matthey took the lead in developing and marketing iridium-platinum alloys. One of the initial uses of which was in Witworth cannons, which saw action during the American Civil War.

Prior to the introduction of iridium alloys, cannon vent pieces, which held the cannon's ignition, were notorious for deformation as a result of repeated ignition and high combustion temperatures. It was claimed that vent pieces made of iridium-containing alloys held their shape and form for over 3000 charges.

In 1908, Sir William Crookes designed the first iridium crucibles (vessels used for high-temperature chemical reactions), which he had produced by Johnson Matthey, and found had great advantages over pure platinum vessels.

The first iridium-ruthenium thermocouples were developed in the early 1930s and in the late 1960s, the development of dimensionally stable anodes (DSAs) significantly increased demand for the element.

Development of the anodes, which consist of titanium metal coated with PGM oxides, was a major advancement in the chloralkali process for producing chlorine and caustic soda and the anodes continue to be a major consumer of iridium.

Production

Like all PGMs, iridium is extracted as a by-product of nickel, as well as from PGM rich ores.

PGM concentrates are often sold to refiners that specialize in the isolation of each metal.

Once any existing silver, gold, palladium, and platinum are removed from the ore, the remaining residue is melted with sodium bisulfate in order to remove rhodium.

The remaining concentrate, which contains iridium, along with ruthenium and osmium, is melted with sodium peroxide (Na₂O₂) to remove ruthenium and osmium salts, leaving behind low purity iridium dioxide (IrO₂).

By dissolving iridium dioxide in aqua regia, the oxygen content can be removed while producing a solution known as ammonium hexachloroiridate. An evaporation drying process, followed by burning with hydrogen gas, finally results in pure iridium.

The global production of iridium is limited to roughly 3-4 tons per year. Most of this originates from primary ore production, although some iridium is recycled from spent catalysts and crucibles.

South Africa is the main source of iridium, but the metal is also extracted from nickel ores in Russia and Canada.

The largest producers include Anglo Platinum, Lonmin, and Norilsk Nickel.

Applications

Although iridium finds itself in a wide range of products, its end-uses can be generally categorized into four sectors:

1. Electrical
2. Chemical
3. Electrochemical
4. Other

According to Johnson Matthey, electrochemical uses accounted for nearly 30 percent of the 198,000 ounces consumed in 2013. Electrical applications accounted for 18 percent of total iridium consumption, while the chemical industry consumed approximately 10 percent. Other uses rounded out the remaining 42 percent of total demand. 

_Sources

_{Johnson Matthey. PGM Market Review 2012.}

_{http://www.platinum.matthey.com/publications/pgm-market-reviews/archive/platinum-2012}

_{USGS. Mineral Commodity Summaries: Platinum Group Metals. Source: http://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2010-plati.pdf}

_{Chaston, J.C. "Sir William Crookes: Investigations on Iridium Crucibles and the Volatility of the Platinum Metals". Platinum Metals Review, 1969, 13 (2).}