Rhenium is a silvery, lustrous white metal. Its usual commercial form is a grey powder. It has one of the highest melting points of all known metals, exceeded by only tungsten. Its resistance value is 3.5 times as large than tungsten one. It also has one of the highest density , exceeded only by platinum, iridium and osmium. The existence of a yet undiscovered element at this position in the periodic table had been predicted by Dmitry Mendeleev in 1869. Rhenium have been discovered by W.C.F. Noddack and I. Noddack in 1925 in Germany. It was the next-to-last naturally occurring element to be discovered.
Rhenium has the lowest abundance on the Earth of any known natural element. Its clarke (average abundance in the earth's crust) is 7*10-7 by mass. Naturally occurring rhenium consist of two isotopes 185Re (37.4%, stable) and 187Re (62.6%, half-life ~1012 years). Rhenium is not found free in nature and it not form own deposit. It occur in molybdenite or copper sulfide ores.
Rhenium has the great range of oxidation states (see table below). The most common oxidation states are +7,+6,+4,+2 and -1. It dissolves in sulfuric and nitric acids. It is stable to corrosion and oxidation but slowly oxidizes in humid air to give HReO4. Vapour of rhenium oxide Re2O7 and other Re volatile compounds colour the flame into pale green tint.
||Re2O3*xH2O, Re3Cl9, complexes
||ReO2, ReS2, ReCl4, complexes
||Re2O5, ReBr5, ReF5, ReCl5, ReOCl3, complexes
||ReO3, ReCl6, ReF6, ReOCl4, ReF82-, complexes
||Re2O7, Re2S7, ReO3F, ReOF5, ReF7, ReSO4, complexes
Water soluble rhenium compounds have a low toxisity. Rhenium dust is also relatively innocuous. Nevertheless it it should be handled with care like other chemical.
There are two general sources for industrial rhenium production:
- Processing of molybdenite or copper sulfide ores. Molybdenum roaster-flue gas is absorbed by amines solution to form rhenium complex. The metal form is obtained by reducing ammonium perrhenate with hydrogen at high temperatures.
- Recycling of Re-containing catalysts and alloys.
Total world production is 40-50 tons per year.
- As a component of platinum-rhenium catalysts. These catalysts are used in oil processing to manufacture fuels with high octane number;
- As a catalysts in hydrogenation reaction. Re catalysts are stable to chemical poisoning;
- As a component of molibdenum-based alloys;
- In X-ray tubes, vacuum lamps;
- In thermocouples and electrical contacts;
- As a material for filaments manufacturing;
- To make rhenium diboride (this compound in known as extremely hard);
- To make Ni-Re alloys, used in jet engines;