Properties and Overview of Rhenium

Overview:

Rhenium (Re) a rare, silvery-white, dense metallic element with the atomic number 75 and the symbol Re, is a member of the transition metals, located in group 7 of the periodic table. Discovered in 1925 by German chemists Ida Noddack, Walter Noddack, and Otto Berg, rhenium is one of the last naturally occurring elements to be found. It is among the rarest elements in the Earth's crust, with an estimated average concentration of about 1 part per billion. Rhenium typically occurs in trace amounts within ores of other metals, such as molybdenum and copper. Its rarity underscores its significance, as it is primarily obtained as a byproduct of the extraction and refinement of these ores. Physically, rhenium is characterized by its exceptional strength and stability, owing to its high density, melting point, and hardness. With a density of about 21.02g/cm³, it stands as one of the densest elements, surpassed only by platinum, iridium, and osmium. Rhenium's exceptionally high melting point of approximately 3,186°C and a boiling point of about 5,596°C, make it the third-highest melting and boiling point element after tungsten and carbon. These properties give rhenium unparalleled stability and resistance to thermal degradation. Its high resistance to corrosion and oxidation at high temperatures further enhance its stability. Rhenium's crystal structure is hexagonal close-packed, contributing to its high strength and hardness. These physical characteristics make rhenium an ideal material for high-temperature applications, such as in jet engine components and thermocouples, providing a sense of strength and stability in these applications.
Chemically, rhenium exhibits a wide range of oxidation states, from -1 to +7, with +4, +6, and +7 being the most stable and common. This flexibility in oxidation states allows rhenium to form various compounds, including oxides, halides, sulfides, and organometallic complexes. Rhenium is known for its catalytic properties, particularly in oxidation and hydrogenation reactions. The most notable rhenium compounds are rhenium heptoxide (Re₂O₇), a yellow crystalline solid that is highly soluble in water and forms perrhenic acid (HReO₄), and rhenium trioxide (ReO₃), which is a red solid. Rhenium also forms various halides, such as rhenium pentachloride (ReCl₅) and rhenium hexafluoride (ReF₅). Its chemical behavior is similar to its lighter homolog, technetium, and to a lesser extent, manganese.
Regarding safety, rhenium is considered relatively non-toxic and poses minimal health risks under normal handling conditions. However, rhenium compounds, particularly those in powder form, should be handled carefully to avoid inhalation or ingestion. The chemical toxicity of rhenium compounds could be better-studied, but they are generally regarded as having low toxicity. Standard safety precautions, such as using personal protective equipment (PPE) like gloves and masks and working in well-ventilated areas, should be observed when handling rhenium and its compounds. Due to its rarity and the specialized nature of its applications, exposure to rhenium is primarily limited to industrial and laboratory settings.

Production:

Rhenium production is typically achieved as a byproduct of the mining and refining of copper and molybdenum ores, which contain trace amounts of rhenium. The most common source of rhenium is molybdenite (MoS₂), a molybdenum sulfide mineral. During the processing of molybdenite concentrates to produce molybdenum, rhenium is extracted from the flue dusts that form while roasting molybdenum ores. The rhenium is then recovered as rhenium heptoxide, which is further purified and reduced to metallic rhenium powder. This powder can be consolidated into solid metal through powder metallurgy techniques or converted into other chemical forms for specific applications. Global rhenium production is limited due to its rarity, with only a few hundred metric tons produced annually.

Applications:

Rhenium has a range of specialized applications due to its unique properties, particularly its high melting point, thermal stability, and catalytic activity. One of the primary uses of rhenium is in high-temperature superalloys, which are employed in jet engines and gas turbine components. These superalloys, which contain rhenium in concentrations of up to 6%, provide exceptional strength and resistance to creep, fatigue, and oxidation at high temperatures, significantly extending the service life and efficiency of aerospace and industrial gas turbines. Rhenium is also used to produce filaments for mass spectrometers, ion gauges, and thermocouples due to its excellent electrical conductivity and resistance to high-temperature degradation. In catalysis, rhenium is utilized in platinum-rhenium catalysts for reforming hydrocarbons in producing high-octane gasoline and in other chemical processes, such as hydrogenation, dehydrogenation, and oxidation reactions.
Rhenium's unique properties also make it valuable in various other applications. It is used in the production of X-ray tubes and electronic components that require high resistance to electrical current and thermal shock. Rhenium-based catalysts are employed in the petrochemical industry for the production of lead-free, high-octane gasoline and other fuel additives. However, the most exciting aspect of rhenium is the ongoing research to explore additional uses, particularly in areas like nuclear medicine and advanced materials science. This research holds the promise of unlocking new applications for rhenium, where its unique combination of chemical and physical properties could offer significant benefits, creating a sense of anticipation and potential.

Summary:

Rhenium is a rare and valuable transition metal with unique physical and chemical properties, including a high melting point, density, and corrosion resistance. These properties make it well-suited for high-temperature aerospace, electronics, and catalysis applications. While rhenium is relatively non-toxic, standard safety precautions should be observed when handling its compounds. Due to its rarity and specialized uses, rhenium remains a critical material in industries that demand materials capable of withstanding extreme conditions and enhancing the performance of high-temperature systems.

See a comprehensive list of atomic, electrical, mechanical, physical and thermal properties for rhenium below: