Re - Rhenium
|Chemical Element||Rhenium||Melting Point °C||3180|
|Chemical Symbol||Re||Boiling Point °C||5625|
|Atomic Number||75||Density g/cm3||21|
Rhenium is a silvery metal, and is a member of Group 7 of the Periodic Table. Rhenium is one of the rarest elements and its rarity, of no more than 0.4 parts per billion in the Earth’s crust, makes it the 77th most abundant element. Rhenium has the 3rd highest melting point after carbon and tungsten and the 4th highest density behind iridium, osmium and platinum. Due to its rarity, rhenium is also one of the most expensive metals. Rhenium resists corrosion, but when in moist air will slowly tarnish. It dissolves in concentrated nitric acid, but not in aqua regia.
Rhenium was discovered in 1925 by the German chemist, Ida Tacke, whilst analysing platinum ores. Up until 1925, Group 7 of the Periodic Table had two missing elements, technetium (43) and rhenium (75), both of which were discovered that year. The discovery of this element once again showed the power of the Periodic Table, as they were able to predict the position in which element number 75 should be, as well as many of its reactions. Rhenium was the last stable, non-radioactive, naturally occurring element to be discovered. It was not until May 1925 that rhenium was isolated by Walter Noddack and Ida Tacke. They achieved this by concentrating it from the ore gadolonite, where rhenium is an impurity. Tacke’s discovery was confirmed to be element number 75 when Otto Berg analysed its atomic spectrum, and confirmed that he had found new lines that could only be produced by an unfound element. Tacke confirmed his discovery later that year at a chemical meeting in Nuremburg.
A by-product of molybdenum, which is itself a by-product of the mining of copper-porphyry ores, Rhenium is also sometimes found within sedimentary copper ores. Its main recovery occurs where copper is mined along the Pacific fault, which stretches from the southern tip of Chile, north through Peru, New Mexico, and along the Rockies. It is also found where copper is mined in Mongolia, Iran, Armenia, Kazakhstan and, notably, in Poland.
The world’s largest producer, at about 28 mt (2009), is Molymet of Chile, whose toll molybdenum roasters draw in large quantities of by-product molybdenum sulphide concentrates (MoS2) from the world’s largest copper producing region. Here the MoS2 is roasted, in order to remove sulphur and produce molybdenum trioxide (MoO3) for the steel industry. During this process the rhenium particles form Re2O7, which passes up the flue where sulphur is wet scrubbed to produce sulphuric acid from which ammonium perrhenate (NH4ReO4) is precipitated.
Eighty percent of the world’s approximately 50-55 mt annual consumption of rhenium is as a 3% or 6% addition within complex nickel base alloys for the manufacture of single crystal turbine blades for high pressure aero and industrial gas turbine engines. Makers, such as GE, Pratt & Whitney and Rolls-Royce are thought to consume 45 mt per year where the alloys operate at up to 1,600 degrees centrigrade, allowing for more efficient fuel combustion and reduced emission of nitrous oxides (NOx). Rhenium also predominates in bi-metallic reforming catalysts where 0.3%Pt/0.3%Re on a substrate of alumina are used in production of high octane gasoline. About 15 mt per year rhenium is used for these catalysts, of which 80% remains in a closed recycle loop so consumption of virgin metal is relatively small.
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