Transition metal
Rhenium is a chemical element; it has symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust. It has one of the highest melting and boiling points of any element. It resembles manganese and technetium chemically and is mainly obtained as a by-product of the extraction and refinement of molybdenum and copper ores. It can assume a wide variety of oxidation states ranging from −3 to +7.
Rhenium was originally discovered in 1908 by Masataka Ogawa, but he mistakenly assigned it as element 43 (now known as technetium) rather than element 75 and named it nipponium. It was rediscovered in 1925 by Walter Noddack, Ida Tacke and Otto Berg, who gave it its present name. It was named after the river Rhine in Europe, from which the earliest samples had been obtained and worked commercially.
Nickel-based superalloys of rhenium are used in combustion chambers, turbine blades, and exhaust nozzles of jet engines. These alloys contain up to 6% rhenium, making jet engine construction the largest single use for the element. The second-most important use is as a catalyst: it is an excellent catalyst for hydrogenation and isomerization, and is used for example in catalytic reforming of naphtha for use in gasoline (rheniforming process). …
In 1908, Japanese chemist Masataka Ogawa announced that he had discovered the 43rd element and named it nipponium (Np) after Japan (Nippon in Japanese). In fact, he had found element 75 (rhenium) instead of element 43: both elements are in the same group of the periodic table. Ogawa's work was often incorrectly cited, because some of his key results were published only in Japanese; it is likely that his insistence on searching for element 43 prevented him from considering that he might have found element 75 instead. Just before Ogawa's death in 1930, Kenjiro Kimura analysed Ogawa's sample by X-ray spectroscopy at the Imperial University of Tokyo, and said to a friend that "it was beautiful rhenium indeed". He did not reveal this publicly, because under the Japanese university culture before World War II it was frowned upon to point out the mistakes of one's seniors, but the evidence became known to some Japanese news media regardless. As time passed with no repetitions of the experiments or new work on nipponium, Ogawa's claim faded away. The symbol Np was later used for the element neptunium, and the name "nihonium", also named after Japan, along with symbol Nh, was later used for element 113. Element 113 was also discovered by a team of Japanese scientists and was named in respectful homage to Ogawa's work. Today, Ogawa's claim is widely accepted as having been the discovery of element 75 in hindsight. …
Rhenium is a silvery-white metal with one of the highest melting points of all elements, exceeded by only tungsten (at standard pressure carbon sublimes rather than melts, though its sublimation point is comparable to the melting points of tungsten and rhenium). It also has one of the highest boiling points of all elements, and the highest among stable elements. It is also one of the densest, exceeded only by platinum, iridium, and osmium. Rhenium has a hexagonal close-packed crystal structure.
Its usual commercial form is a powder, but this element can be consolidated by pressing and sintering in a vacuum or hydrogen atmosphere. This procedure yields a compact solid having a density above 90% of the density of the metal. When annealed this metal is very ductile and can be bent, coiled, or rolled. Rhenium-molybdenum alloys are superconductive at 10 K; tungsten-rhenium alloys are also superconductive around 4–8 K, depending on the alloy. Rhenium metal superconducts at 1.697±0.006 K.
In bulk form and at room temperature and atmospheric pressure, the element resists alkalis, sulfuric acid, hydrochloric acid, nitric acid, and aqua regia. It will however, react with nitric acid upon heating.
Rhenium has one stable isotope, rhenium-185, which nevertheless occurs in minority abundance, a situation found only in two other elements (indium and tellurium). …
Rhenium is one of the rarest elements in Earth's crust with an average concentration of 1 ppb; other sources quote the number of 0.5 ppb making it the 77th most abundant element in Earth's crust. Rhenium is probably not found free in nature (its possible natural occurrence is uncertain), but occurs in amounts up to 0.2% in the mineral molybdenite (which is primarily molybdenum disulfide), the major commercial source, although single molybdenite samples with up to 1.88% have been found. Chile has the world's largest rhenium reserves, part of the copper ore deposits, and was the leading producer as of 2005. It was only recently (in 1994) that the first rhenium mineral was found and described, a rhenium sulfide mineral (ReS2) condensing from a fumarole on Kudriavy volcano, Iturup island, in the Kuril Islands. Kudriavy discharges up to 20–60 kg rhenium per year mostly in the form of rhenium disulfide. Named rheniite, this rare mineral commands high prices among collectors.
Rhenium is added to high-temperature superalloys that are used to make jet engine parts, using 70% of the worldwide rhenium production. Another major application is in platinum–rhenium catalysts, which are primarily used in making lead-free, high-octane gasoline.
Nickel-based superalloys have improved creep strength with the addition of rhenium. These alloys normally contain 3% or 6% rhenium. Second-generation alloys contain 3%; these alloys were used in the engines for the F-15 and F-16, whereas the newer single-crystal third-generation alloys contain 6% of rhenium; they are used in the F-22 and F-35 engines. Rhenium is also used in the superalloys, such as CMSX-4 (2nd gen) and CMSX-10 (3rd gen) that are used in industrial gas turbine engines like the GE 7FA. Rhenium can cause superalloys to become microstructurally unstable, forming undesirable topologically close packed (TCP) phases. In 4th- and 5th-generation superalloys, ruthenium is used to avoid this effect. Among others the new superalloys are EPM-102 (with 3% Ru) and TMS-162 (with 6% Ru), as well as TMS-138 and TMS-174.
For 2006, the consumption is given as 28% for General Electric, 28% Rolls-Royce plc and 12% Pratt & Whitney, all for superalloys, whereas the use for catalysts only accounts for 14% and the remaining applications use 18%. In 2006, 77% of rhenium consumption in the United States was in alloys. …
Rhenium has one stable isotope, rhenium-185, which nevertheless occurs in minority abundance, a situation found only in two other elements (indium and tellurium). Naturally occurring rhenium is only 37.4% 185Re, and 62.6% 187Re, which is unstable but has a very long half-life (41.6 billion years). A kilogram of natural rhenium emits 1.07 MBq of radiation due to the presence of this isotope. This lifetime can be greatly affected by the charge state of the rhenium atom. The beta decay of 187Re is used for rhenium–osmium dating of ores. The available energy for this beta decay (2.6 keV) is the second lowest known among all radionuclides, only behind the decay from 115In to excited 115Sn* (0.147 keV). The isotope rhenium-186m is notable as being one of the longest lived metastable isotopes with a half-life of around 200,000 years. There are 33 other unstable isotopes that have been recognized, ranging from 160Re to 194Re, the longest-lived of which is 183Re with a half-life of 70 days.
Very little is known about the toxicity of rhenium and its compounds because they are used in very small amounts. Soluble salts, such as the rhenium halides or perrhenates, could be hazardous due to elements other than rhenium or due to rhenium itself. Only a few compounds of rhenium have been tested for their acute toxicity; two examples are potassium perrhenate and rhenium trichloride, which were injected as a solution into rats. The perrhenate had an LD50 value of 2800 mg/kg after seven days (this is very low toxicity, similar to that of table salt) and the rhenium trichloride showed LD50 of 280 mg/kg.