Rhenium — 75th element of the periodic table. Chemical symbol Re (lat. Rhenium) — silver-white metal similar in properties to tungsten and molybdenum. Two isotopes of rhenium are known: 185Re and 187Re. Heavy isotope is almost twice more and it, unlike light, is radioactive. Emitting β-rays, Rhenium-187 turns into osmium for a billion years. Rhenium-185 isolated in its pure form in 1925 by German chemists spouses Noddak. He became last discovered non-radioactive element.
This rare earth element is found in molybdenum and copper ores. Very rich deposits in Chile, USA and Russia. If annual rhenium consumption level of 40−50 tons remains, humanity will have enough world reserves for 250−300 years, excluding secondary use of this metal. Depending on purity, the price of 1 kg of rhenium can range from 1,000 to 10,000 dollars.
|Atomic Number Re||Atomic (molar) mass g/mol||Oxidation state||Density [g/cm3]||Melting point t°С||Boiling temperature t°С||Heat of Fusion kJ/kg|
|№ 75||186,2||-1, 0, 2, 3, 4, 5, 6, 7||21||3186°С||5596°С||34|
For decades, need for rhenium has remained stable. Main consumer was electrical industry. Rhenium was mainly used for manufacture of thermocouples and incandescent filaments of vacuum devices. The need for rhenium began to grow in second half of last century, when petrochemical industry began using rhenium-platinum catalysts. Such catalysts allowed production of cheaper high-octane gasoline. Compared to old platinum catalysts, rhenium was 50% more efficient and lasted 4 times longer. If previously significant part of rhenium was used for alloying heat-resistant alloys, then from last quarter of 20th century 75% of rhenium began to be used for production of catalysts. Today this metal is relevant not only in metallurgy and electrical engineering, but also in petrochemistry.
It is refractory and heavy metal, similar in properties to molybdenum and tungsten. Rhenium is second only to tungsten in melting point (3170 °C). One cubic centimeter of rhenium weighs 21 grams, only osmium, iridium and platinum are heavier than it. Pure rhenium is much more ductile than tungsten. It can be rolled and pulled into thinnest wire under normal conditions. A «rhenium effect» was discovered: it turned out that this metal simultaneously increases both strength and ductility of molybdenum and tungsten. Due to high modulus of elasticity, hardness of rhenium after treatment increases significantly due to hardening. 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 strength of molybdenum. Rhenium withstands repeated cooling and heating without loss of strength. In electrical resistance, it is four times superior to tungsten and molybdenum.
Rhenium is more resistant to oxidation than tungsten; over the years it does not fade in air, retaining its pristine luster. It is almost insoluble in hydrofluoric acids, weakly reacts with Н2 SO4 even when heated, but is easily soluble in nitric acid. In a solution of H2 O2 forms rhenium acid. Rhenium forms an amalgam with mercury.
It is extracted from molybdenum and copper sulfide ore with meager content of rhenium salts using pyrometallurgical methods (conversion, smelting, oxidative calcination). During firing, rhenium oxide is sublimated and then captured by special filters. Part of rhenium is usually stored in cinder, from where it passes into soda or ammonia solutions, from which metal is later reduced with hydrogen. When smelting copper concentrates with smoke, typically 50−60% Re. When processing copper concentrates, sulfuric wash acid becomes main source for obtaining this metal.
The use of rhenium is determined by high rates of electrical resistance and heat resistance, resistance to aggressive chemical factors, high catalytic activity (close to platinoids). Modern nuclear power cannot do without alloys containing rhenium. Even at dawn of atomic era, tungsten alloy with 26% rhenium began to be used for shells of fuel elements and other parts operating in reactors at t° 1600−3000 °C. Rhenium and its alloys occupy an increasingly strong position in aviation and space technology. In particular, tantalum alloy with 2.5% rhenium and 8% tungsten is indispensable for heat shields of modules returning from space to Earth. High physical and chemical advantages (and plus good weldability) determine interest in rhenium in large-budget industries, which can cost a lot. 2/3 of rhenium goes to alloying heat-resistant steels and coating of other metals. Pure rhenium serves as basis for most critical parts. Domestic tungsten-based alloys contain 5, 20, up to 27% Re (BP-5, BP-20, VR-27VP), based on molybdenum — 8, 20 to 47% rhenium. Also used are tungsten-molybdenum-rhenium alloys — malleable, high-tech, easily weldable. They work in most difficult conditions: withstand high temperatures, shock loads, vibration, contact with aggressive substances.
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