Rhenium Recycling

Basic Properties

• Classification: A transition metal belonging to Group 7 (manganese group) and Period 6 of the periodic table.
• Atomic mass: ~186.21 u.
• Physical state: A silvery-white, hard, and brittle metal with a metallic luster.
• Melting point: ~3186 °C (one of the highest among elements, second only to tungsten).
• Boiling point: ~5596 °C, giving it an extremely wide liquid range.
• Density: ~21.02 g/cm³ (among the densest elements, comparable to platinum).
• Electrical conductivity: Moderate; it is also a good thermal conductor.

Chemical Behavior

• Relatively inert at room temperature, resistant to oxidation and corrosion by most acids (except concentrated nitric acid or aqua regia).
• Reacts with oxygen at high temperatures to form rhenium dioxide (ReO₂) or rhenium heptoxide (Re₂O₇), the latter being volatile.
• Forms compounds with halogens (e.g., rhenium hexafluoride, ReF₆) and can exhibit multiple oxidation states (most commonly +7, +6, +4).

Occurrence & Extraction

• One of the rarest elements in Earth’s crust, with an average abundance of only ~1 part per billion (ppb). It rarely occurs in its free form.
• Most commonly found as a trace impurity in molybdenum ores (e.g., molybdenite) and some copper ores, where it substitutes for molybdenum in crystal structures.
• Extracted as a byproduct during the processing of molybdenum or copper ores. The process involves roasting the ore to form soluble rhenium compounds, followed by solvent extraction or ion exchange to isolate rhenium.

Key Uses

• High-temperature alloys: The most critical application is in nickel-based superalloys for jet engine turbine blades and exhaust nozzles. Adding 2–6% rhenium to these alloys dramatically improves their creep resistance (resistance to deformation under high heat and stress), enabling engines to operate at higher temperatures for better efficiency.
• Catalysis: Rhenium catalysts are used in petroleum refining, particularly for reforming processes to produce high-octane gasoline, and in hydrogenation reactions.
• Electronics & instrumentation: Used in filaments for mass spectrometers, X-ray tubes, and thermocouples (due to its high melting point).
• Other uses: In specialty welding electrodes and as a coating to enhance wear resistance in high-stress components.

History

• Predicted in the 19th century based on periodic table trends but remained elusive due to its rarity.
• Discovered in 1925 by German chemists Walter Noddack, Ida Tacke, and Otto Berg, who isolated it from gadolinite and molybdenite ores.
• Named “rhenium” after the Rhine River (Latin: Rhenus), a major river in Europe, reflecting the discovery’s geographical connection.