Rhenium is one of the rarest metals in the Earth's crust; it is almost never mined from primary deposits but recovered as a by-product of molybdenite-copper ore processing, with tiny output and high price, and has long been classified as a strategic critical metal. Its largest use is in superalloys for the turbine blades, combustors and turbine disks of jet engines and gas turbines - rhenium-bearing nickel-based single-crystal superalloys can withstand enormous centrifugal forces and thermal shock for long periods at temperatures approaching melting, making rhenium an indispensable alloying element in the single-crystal blades of modern commercial aero-engines; the amount of rhenium in a large airliner engine is small but irreplaceable. Beyond aero-engines, rhenium-bearing alloys are also used in industrial gas turbines, rocket nozzles and hypersonic vehicle components. In chemicals, rhenium is used mainly in platinum-rhenium bimetallic catalysts for petroleum catalytic reforming, converting low-octane naphtha into high-octane gasoline and aromatics; these catalysts are highly active, long-lived and resistant to coke deposition, and are a key catalyst system in refining. Rhenium is also used in high-temperature thermocouples, X-ray targets and certain electronic components.
Rhenium heptoxide (Re2O7) is the most volatile oxide of rhenium, an orange-yellow solid and the key intermediate for extracting rhenium from the roaster dust of molybdenum ores, as well as the starting point for high-purity rhenium compounds and catalysts. Ammonium perrhenate (NH4ReO4) is the most industrially important rhenium salt, a white crystal used directly to prepare platinum-rhenium reforming catalysts and rhenium metal powder, and is also the main commercial form in which rhenium is traded and priced. Rhenium trioxide (ReO3) is a red oxide with metallic luster, used mainly in catalytic research and specialty thin-film materials. Rhenium disulfide (ReS2) is a layered transition-metal dichalcogenide with a graphene-like two-dimensional structure; it has drawn interest in catalytic hydrogenation, electrochemical energy storage and semiconductor research, but has not yet reached large-scale industrial use. Overall, the great majority of rhenium compounds serve refining catalysis and superalloy precursors, with negligible consumer markets.
