Rhenium is the second last element with a stable isotope ever discovered on Earth. Scientists discovered Rhenium in 1908. This chemical compound is named after the Rhine River in Europe. Rhenium is a heavy metal which belongs to the third row of transitional metal in group 7 in the periodic table. The symbol for its chemical element is Re. With a standard concentration of 1ppb (part per billion) this element is one of the uncommon metals on the crust with the 2nd highest boiling-point and the 3rd highest melting point of all elements. The metal chemically resembles technetium and manganese, and it is a byproduct of the refinement and extraction of copper and molybdenum ores.
Characteristics of Rhenium
Rhenium is silvery white in color, and it has the third highest melting points after carbon and tungsten. Re is the fourth densest element on earth after Osmium, Iridium, and platinum. Rhenium comes with a hexagonal closely-packed crystal structure plus a lattice parameter of c=445.6pm and a=276.1pm. Commercially, Rhenium exists in powder form, but it can be consolidated by sintering and pressing it in a hydrogen or vacuum atmosphere. The consolidation process produces a small solid whose density is over 90% of the metal. When heated this element tends to be flexible and easily rolled, coiled and even bent. Tungsten-rhenium and Rhenium-molybdenum alloys are excellent conductors. At room temperature, atmospheric pressure and in bulk form, this element can resist aqua regia, dilute nitric acid, sulfuric acid, alkalis, and hydrochloric acid.
How Many Stable Isotopes Does Rhenium Have?
Rhenium has only one stable isotope (Rhenium-185) which is one of the rarest elements found only in two elements of tellurium and indium. Naturally found rhenium-187 is only 62.6% and rhenium-185 is 37.4% which are very unstable with a very long half-life (1010 years) which is affected by the atomic charge state. Rhenium-186 isotope has the longest half-life of 200,000 years. Rhenium has more than 25 recognized radioactive isotopes.
Where Is Rhenium Mined?
With an average concentration of 1ppb (other sources quoting 0.5ppb), rhenium is the 77th rarest element in the Earth's crust. This metal is not found freely in nature; it occurs in quantities of 0.2% in molybdenite. Chile had the most significant rhenium reserves in their copper ore mines and was the top producer as of 2005. The first known rhenium sulfide mineral formed in Kudriavy volcano in Iturp Island Russia in 1994. Kudriavy volcano produces about 60kg of rhenium annually in the form of rhenium disulfide which condenses from fumaroles.
How is Rhenium Extracted from Molybdenite?
Commercial rhenium is mined from the molybdenum gas (roaster-flue) present in the copper-sulfide, and various molybdenite ores have a maximum of 0.2% rhenium. Perrhenic acid and rhenium VII oxide dissolves in water, and therefore they are filtered from the flue gasses with dust and then precipitated ammonium or potassium chloride as perrhenate salts which are purified by recrystallization. The total global annual production of this element is between 40-60tons/year with the primary producers being Poland, Peru, the United States, and Chile. About 10 tons of Rhenium is recovered annually from the salvaging of Pt-Re catalyst with various exceptional alloys. The metal structure of rhenium is created by merely tumbling the ammonium perrhenate through hydrogen gas at extremely high temperatures. The price of this metal rose from between $1000 and $2000 per kilogram in 2003-2006 to $10,000 per kilogram and above in 2008 which makes it one of the most expensive metals in the world.
Significant Applications of Rhenium
The high costs and the shortage of the element Rhenium limit its applications; nevertheless, due to its components of high melting point and high-temperature resistance, rhenium is indispensable in the production of the thermocouples for measuring high temperatures in the non-oxidized atmosphere. Approximately 70% of the global output of rhenium is used to produce parts of the jet engine. The platinum-rhenium catalysts assist in the production of high-octane gasoline which is lead-free.
The addition of this element in the nickel-based superalloys has helped improve its strength. The super-alloys have 3-6% of rhenium. The third generation superalloys with 6% rhenium are used in the building of F-35 and F-22 engine parts while the F-16 and F-15 engine parts have the second generation superalloys with 3% rhenium. Manufacturers add Rhenium to other superalloys like CMSX-10 and CMSX-4 which they use in making of various industrial gas-turbine engines like GE 7FA. Rhenium can make super alloys micro-structurally unstable thus forming an undesirable topologically close-packed (TCP) phases. For improved stability Ruthenium is added to 4th and 5th generation superalloys.
Addition of rhenium in tungsten helps improve its properties; thus making the tungsten-rhenium alloys ductile and easy to work with at low temperatures. The stability of tungsten at high temperatures grows with the addition of Rhenium. The balance of these tungsten-rhenium alloys improves with an increase in rhenium concentrations, and this is the reason why tungsten-rhenium alloy has 27% rhenium, which is their solubility limit. Addition of rhenium to tungsten makes it possible for the composites to meet specific functions like improved ductility, higher resistivity, and superior vibration resistance. Numerous X-ray sources use Tungsten-rhenium alloys. The higher melting point of these metals stabilizes their atoms against prolonged electron impact.
The catalytic reforming process (the process of transforming low-octane rating petroleum naphtha into high-octane products) uses Rhenium-platinum alloys as a catalyst. Globally over 30% of the catalysts used in the catalytic reforming process has rhenium. Olefin metathesis (an organic process which entails a redistribution of olefins by regeneration and scission of carbon-carbon double bond) uses rhenium based catalysts. Numerous hydrogenation processes use rhenium catalysts because they are resistant to all forms of chemical poisoning from phosphorus, sulfur, and nitrogen.
Re-188 and Re-186 isotopes are the radioactive rhenium isotopes used in liver cancer treatment. Although these isotopes have similar tissue penetration depth (Re-188 for 10mm and Re-186 for 5mm), Re-186 has a longer lifetime. Listeria monocytogenes helps deliver Re-188 into the body during the experimental treatment of pancreatic cancer. Rhenium works perfectly as a replacement for technetium, in the radio-pharmacy process, because it has a longer half-life.
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