Iridium (Ir) - Everyday Uses

Introduction to Iridium

Iridium is a chemical element with the symbol Ir and atomic number 77. It is a very hard, brittle, silvery-white transition metal of the platinum group. As one of the densest naturally occurring elements, it is also notable for its exceptional resistance to corrosion, even at high temperatures, making it a valuable material in various demanding applications.

Natural Occurrence and Extraction

Where is Iridium Found?

Iridium is one of the rarest elements in Earth’s crust, found in extremely low concentrations. It is often alloyed with osmium as the mineral osmiridium and is associated with other platinum group metals (PGMs) like platinum, palladium, rhodium, and ruthenium. The primary sources of iridium are magmatic sulfide deposits.

Major global reserves of iridium are concentrated in specific regions. South Africa’s Bushveld Igneous Complex is a prominent source, accounting for a significant portion of the world’s PGM production. Other notable locations include the Norilsk-Talnakh region in Russia and the Sudbury Basin in Canada, both rich in nickel-copper sulfide ores. The Stillwater Complex in Montana, United States, also contributes to the global supply.

Beyond terrestrial sources, unusually high concentrations of iridium are found in the K-Pg boundary layer (formerly known as the K-T boundary) in the geological record. This anomaly is widely cited as evidence of an extraterrestrial impact event, specifically a large asteroid, approximately 66 million years ago, which is believed to have contributed to the extinction of the dinosaurs. The presence of iridium in meteorites and asteroids is significantly higher than in Earth’s crust, supporting this hypothesis.

Extraction and Industrial Use

Due to its low concentration in ores, iridium is primarily obtained as a byproduct during the mining and refining of other metals, particularly nickel and copper. The extraction process is complex and labor-intensive, involving multiple stages of chemical separation to isolate iridium from the other platinum group metals and base metals.

Initially, nickel and copper sulfide ores are crushed, ground, and subjected to flotation to concentrate the metal-bearing minerals. The resulting concentrate then undergoes smelting and converting, producing matte that contains PGMs. Further refining, often involving electrolytic processes, separates nickel and copper. The remaining PGM-rich residue is then treated with various chemical methods, such as dissolution in aqua regia or molten salt fusion, followed by selective precipitation or solvent extraction techniques to separate individual PGMs, including iridium. The high melting point of iridium (2,466 °C) and its resistance to chemical attack make its processing challenging and energy-intensive.

Common Everyday Uses of Iridium

1. Spark Plugs: Iridium-tipped spark plugs are widely used in modern internal combustion engines across the global automotive industry. The extreme hardness, high melting point, and excellent corrosion resistance of iridium allow for smaller, more durable electrode tips. This design leads to a more consistent spark, improved fuel efficiency, and significantly extended service life compared to traditional copper or platinum spark plugs, reducing maintenance requirements for vehicles worldwide.

2. High-Temperature Crucibles: Due to its exceptional thermal stability and inertness, iridium is crucial for manufacturing crucibles and other equipment used in high-temperature processes. These are essential for growing high-purity single crystals, such as sapphire for LED substrates and specialized optical components. This application is particularly important in East Asian countries, where significant portions of the world’s semiconductor and LED manufacturing industries are located.

3. Electrical Contacts: In various electrical and electronic devices, iridium is utilized in alloys for electrical contacts. Its resistance to arc erosion and corrosion ensures reliable operation and longevity in switches, relays, and other contact points. This application is vital in telecommunications equipment, computer hardware, and critical control systems globally, where uninterrupted and precise electrical signaling is paramount.

4. Medical Radiotherapy: The radioactive isotope Iridium-192 ($^{192}$Ir) is extensively employed in brachytherapy, a form of internal radiation therapy used in oncology. Small, encapsulated sources of $^{192}$Ir are temporarily or permanently placed directly inside or next to the area requiring treatment, such as cancerous tumors. This targeted approach delivers high doses of radiation to cancer cells while minimizing exposure to surrounding healthy tissues, making it a critical tool in cancer treatment centers internationally.

5. Standards for Weights and Measures: Historically, a platinum-iridium alloy (90% platinum, 10% iridium) was used to construct the International Prototype Metre and the International Prototype Kilogram, the global standards for length and mass. While these standards have largely been redefined in terms of fundamental physical constants, the use of iridium in these original prototypes underscored its exceptional stability, resistance to wear, and unchanging physical properties, which were crucial for defining universal measurement units for over a century.