Re, a chemical element in group VII of Mendeleev’s periodic system. Atomic number, 75; atomic weight, 186.207. A light gray metal, natural rhenium occurs as a mixture of the stable isotope 185Re (37.07 percent) and the weakly radioactive isotope 187Re (half-life T½ = 1011 years).
In 1871, D. I. Mendeleev predicted the existence of an element with atomic weight 190—a manganese analog—and called it trimanganese. In the years following, there were many unconfirmed reports that the element had been discovered. It was only in 1925, however, that the German chemists I. Noddack and W. Noddack discovered rhenium in the mineral columbite by spectral analysis. The name “rhjenium” comes from the Latin name for the Rhine (Rhenus) River in Germany.
Rhenium is a typical dispersed element. Its average content in the earth’s crust is 7 × 10-8 percent by weight. The three known rhenium minerals are the oxide and sulfide of rhenium and copper rhenium sulfide (CuReS4), which forms the mineral dzhezkazganite. Rhenium occurs as an admixture in the minerals of other elements; higher Re concentrations are noted in columbites, tantalites, zirconates, rare-earth minerals, copper sulfides, and especially molybdenite, MoS2 (from 0.1 to 10-5 percent). The relationship between rhenium and molybdenite is determined by the isomorphism of MoS2 and ReS2. Certain copper sulfide concentrates (0.002-0.005 percent Re) are important sources of rhenium.
Rhenium has a hexagonal, close-packed structure (a = 2,760 angstroms [Å], c = 4,458 Å). The atomic radius is 1,373 Å, and the ionic radius of Re7+ is 0.56 Å. Rhenium has a density of 21.03 g/cm3, a melting point of 3180° ± 20°C, and a boiling point of 5900°C. Its specific heat capacity is 153 joules/kg/°K, or 0.03653 cal/g/deg in the range 0°—1200°C. The coefficient of linear thermal expansion is 6.7 × 10-6 in the range 20°-500°C, and the electrical resistivity is 19.3 × 10-6 ohm·cm (20°C). Rhenium has a superconductivity transition temperature of 1.699°K, a work function of 4.80 electron volts, and is paramagnetic.
The refractoriness of rhenium is second only to that of tungsten. Unlike tungsten, rhenium is ductile in both the cast and recrystallized states and undergoes deformation upon exposure to the cold. The elastic modulus of Re is 470 giganewtons/m2, or 47,000 kilograms-force/mm2, which is higher than that of all other metals except Os and Ir. This modulus accounts for rhenium’s high resistance to deformation and high-rate hardening in pressure treatment. Rhenium is characterized by its high endurance limit at temperatures of 1000°–2000°C.
The rhenium atom contains seven outer electrons; the higher energy levels that have the configuration 5d56s2. Rhenium is stable in the air at normal temperatures. Oxidation, with the formation of oxides (ReO3, Re2O7), begins at 300°C and intensifies at temperatures exceeding 600°C. Rhenium does not react with hydrogen at temperatures ranging to rhenium’s melting point and in general does not react at all with nitrogen. Rhenium, unlike other refractory metals, does not form carbides. Upon heating, fluorine and chlorine react with rhenium to form ReF6 and ReCl5, but rhenium will not interact directly with bromine or iodine. Sulfur vapor and rhenium produce the sulfide ReS2 at temperatures of 700°–800°C.
Rhenium does not corrode in hydrochloric or hydrofluoric acids of any concentration upon exposure to cold or heating to 100°C. It dissolves to form perrhenic acid in nitric acid, hot concentrated sulfuric acid, and hydrogen peroxide. It undergoes gradual corrosion in alkaline solutions upon heating, while fused alkalis are capable of dissolving it very rapidly.
Rhenium is known to display valences of +7 to - 1, which accounts for the abundance and diversity of its compounds. Heptavalent rhenium compounds are the most stable. Rhenium heptoxide (ReO7) is a light yellow substance that dissolves freely in water. Perrhenic acid (HReO4) is strong and colorless and a relatively weak oxidizing agent (unlike permanganic acid, HMnO4); it forms salts—perrhenates—upon interaction with the alkalis, oxides, and carbonates of metals. Rhenium compounds with other oxidation numbers include the reddish orange trioxide (ReO3), dark brown dioxide (ReO2), and volatile chlorides and oxychlorides (ReCl5, ReOCl4, ReO3Cl).
The primary sources of rhenium are molybdenite concentrates (Re content 0.01-0.04 percent) and the copper concentrates in certain copper deposits (Re content 0.002-0.003 percent). In the oxidative roasting of molybdenite concentrates, rhenium is removed with furnace gases in the form of Re2O7(boiling point, 360°C), which is concentrated in the products of dust-collecting systems (slurry, solutions). Rhenium is also removed with gases during various stages in the preparation of blister copper from concentrates. If the furnace gases are used in the preparation of sulfuric acid, rhenium is concentrated in the rinsing acid of the electrostatic precipitator. Leaching with weak H2SO4 and a pyrolusite oxidizing agent is used to extract Re from dust and slurry. Rhenium is removed from the solutions obtained and from the rinsing sulfuric acid by sorption or extraction. The final product, ammonium perrhenate (NH2ReO4), is reduced with hydrogen to produce rhenium powder, which is then converted into solid bars by the techniques of powder metallurgy. Rhenium can also be melted in electron-beam furnaces.
Both refractory Re and W-Re alloys are used in the manufacture of electronic devices. In addition, rhenium and W-Re alloys are used to manufacture electrical contact points, parts for precision instruments, and thermocouples for measuring temperatures up to 2500°C. Alloys of Re and W, Mo, and Ta are characterized by a high heat resistance and are used in aircraft and space technology. Rhenium and its compounds serve as effective catalysts in petroleum cracking.
A. N. ZELIKMAN