Rhenium is the 75th element on the periodic table of Mendeleev. The chemical symbol is Re (lat. Rhenium), a silvery-white metal similar in properties to tungsten and molybdenum. Two isotopes of rhenium are known: 185Re and 187Re. The heavy isotope is almost twice as abundant and, unlike the light one, it is radioactive. By emitting β-rays, Rhenium-187 turns into osmium within a billion years. Rhenium-185 was isolated in its pure form in 1925 by German chemists, the Noddak couple. It was the last nonradioactive element to be discovered.
This rare earth element is found in molybdenum and copper ores. Very rich deposits in Chile, USA. If annual consumption of rhenium will remain at 40-50 tons, the mankind will have enough reserves for 250-300 years, not taking into account secondary use of this metal. Depending on purity, the price of 1 kg of rhenium can range from $1,000 to $10,000.
|Atomic number Re||Atomic (molar) mass g/mol||Oxidation state||Density [g/cm3]||Melting point t ° C||Boiling point t ° C||Melting point kJ/kg|
|№ 75||186,2||-1, 0, 2, 3, 4, 5, 6, 7||21||3186°С||5596°С||34|
For decades, the demand for rhenium remained stable. The main consumer was the electrical industry. Mostly rhenium was used for production of thermocouples and filaments of vacuum devices. The need for rhenium began to grow in the second half of the last century, when the petrochemical industry began to use rhenium-platinum catalysts. Such catalysts made it possible to produce cheaper high-octane gasoline. Compared to old platinum catalysts, rhenium ones were 50% more effective and served four times longer. If earlier a considerable part of rhenium was used for alloying of heat-resistant alloys, since the last quarter of the XX century 75% of rhenium was used for production of catalysts. Today, this metal is important not only in metallurgy and electrical engineering, but also in petrochemistry.
It is refractory and heavy, and its properties are similar to those of molybdenum and tungsten. Rhenium is second only to tungsten in its melting point (3170°C). One cubic centimeter of rhenium weighs 21 grams, and only osmium, iridium and platinum are heavier. Pure rhenium is much more malleable than tungsten. It can be rolled and drawn into the thinnest wire under normal conditions. The "rhenium effect" was also discovered: it turned out that this metal increases both the strength and ductility of molybdenum and tungsten. Because of its high modulus of elasticity, the hardness of rhenium increases significantly after machining due to accretion. To restore ductility, it is annealed in hydrogen, vacuum or inert gas. Up to 1200 °C its strength is higher than that of tungsten and much higher than that of molybdenum. Rhenium withstands repeated cooling and heating without loss of strength. Its electrical resistance is four times that of tungsten and molybdenum.
Rhenium is more resistant to oxidation than tungsten; it never tarnishes when exposed to the air over the years, retaining its original luster; it is almost insoluble in hydrochloric and hydrofluoric acids.It is almost insoluble in hydrofluoric acids, weakly reacts with H2 SO4 even when heated, but easily soluble in nitric acid and in solution of H2 O2 it forms rhenic acid. With mercury, rhenium forms amalgam.
It is extracted from molybdenum and copper sulfide ores with minuscule amounts of rhenium salts, using pyrometallurgical methods (conversion, smelting, oxidation roasting). During roasting, the rhenium oxide is sublimed and then captured by special filters. Part of the rhenium is usually retained in the cinder, from which it is transferred to soda or ammonia solutions, from which the metal is later recovered by hydrogen. Smelting of copper concentrates usually removes 50-60% of Re. In the treatment of copper concentrates, sulfuric wash acid becomes the main source for obtaining this metal.
The use of rhenium is determined by its high electrical resistance and heat resistance, resistance to aggressive chemical factors, and high catalytic activity (close to platinoids). Modern nuclear power engineering cannot do without alloys containing rhenium. Back in the early days of the nuclear era, tungsten alloys with 26% rhenium were used for cladding fuel elements and other parts operating in reactors at t° 1600-3000°C. Rhenium and its alloys are gaining ground in aviation and space technology. In particular, tantalum alloy with 2.5% of rhenium and 8% of tungsten is indispensable for heat shields of modules returning from space to the Earth. The high physical and chemical properties (plus good weldability) determine the interest in rhenium for big-budget industries that can handle high costs. 2/3 of rhenium is used to alloy heat-resistant steels and coat other metals. Pure rhenium serves as a base for the most critical parts. Domestic tungsten-based alloys contain 5, 20, up to 27% Re (BP-5, BP-20, BP-27VP), molybdenum-based alloys contain 8, 20 to 47% rhenium. Tungsten-molybdenum-rhenium alloys are also used - ductile, high-tech, easily welded. They operate under the most demanding conditions: they can withstand high temperatures, shock loads, vibration and contact with aggressive substances.
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