In the aerospace materials realm, the undisputed star of late has been composites. Composite materials are being used in unprecedented amounts, most notably on major aircraft projects such as the Boeing 787 Dreamliner and Airbus A380.
With regard to propulsion systems, much ado has been made of titanium and new metal alloys, advanced ceramic materials such as alumina, silicon nitride, and aluminum nitride, and even the development of nanostructured composites and other materials.
Rarely, it seems, does the metal rhenium (Re) get much mention when discussing alloys. That may be because rhenium is one of the rarest elements found in the Earth’s crust, or perhaps because it was the last naturally occurring stable element to be discovered, in 1925.
Even so, it is increasingly being used to boost the efficiency of jet engines.
One company that produces rhenium is KGHM Ecoren, which belongs to the Polish copper producer KGHM Polska Miedź S.A. The third largest company in this field, KGHM Ecoren claims to be the sole European producer of rhenium and ammonium perrhenate from its own raw materials.
The largest producer of rhenium is Molymet of Chile; the second-largest producer is U.S.-based Climax Molybdenum. KGHM Ecoren claims that about 85% of Molymet’s production is metallic rhenium, which is almost entirely used in the U.S. aviation industry.
Rhenium raises engine efficiency
The aviation industry is currently the largest consumer of rhenium, according to KGHM Ecoren, followed by the petrochemical industry for the production of catalysts when refining high-octane unleaded petrol.
“The connection between rhenium and the aviation industry is strong indeed,” said Grzegorz Bigosinski, Hydrometallurgical Plant Manager for KGHM Ecoren. In 2007, the company signed a supply contract with jet-engine producer Rolls-Royce plc, which manufactures engines used in, among others, Boeing 787 passenger airliners, the new Airbus 380, as well as in the Lockheed Martin F-35 Lightning.
GE and Pratt & Whitney also reportedly use the metal, for which new uses are constantly being found.
The factors that make the aerospace industry so interested in rhenium are its very high melting point, hardness, and scratch resistance, according to Bigosinski.
“Even a small amount (just 1.5%) of rhenium added to alloys—changing them into superalloys—makes them resistant to many factors, including very high temperature,” he explained. “Using rhenium in a jet engine results in its greater work efficiency—that is, higher work temperature, reduced fuel consumption, and as a result, reduced environmental pollution.”
Rhenium is primarily used in jet-engine turbines (monocrystal jet-engine blades) as well as in shielding for spaceships, which can thus be exposed to very high temperatures. Bigosinski noted that only a few years ago the maximum amount of rhenium in jet-engine blades was about 3%, whereas now it can reach as high as 6% for some blades.
KGHM Ecoren is only a rhenium producer; its specialists are not involved in airplane construction, Bigosinski admitted. And the company does not monitor how its materials are used by other companies. That said, he and the company’s other specialists do know the material’s properties and how those can be beneficial to the aerospace sector.
“The main advantage of rhenium is its chemical resistance, as it dissolves neither in hydrochloric nor in hydrofluoric acid, breaking down only in oxidizing acid,” he explained. “Also, the key advantage is its extremely high melting point, 3180°C, whereas its boiling point is over 5600°C.
“After tungsten, rhenium is the second hardest-to-melt metal. This extraordinary resistance is used first of all in jet-engine blades and other jet-engine elements exposed to high temperatures.”
Metallic rhenium is similar to platinum, according to KGHM Ecoren, with its thickness of 21.09 g/cm³ similar to metals from the platinum group. In pure form, it is soft and ductile, but it is said to take on its unique qualities when processed.
The process and supply
KGHM Polska Miedź has been mining massive copper ores located in Western Poland for 60 years. Copper found in Earth’s crust is accompanied by tens of other metals, known as “impurities.” Some of these can be recovered by applying suitable technologies, which is the case, for example, with silver, gold, and platinum group metals.
“As far as rhenium is concerned, for many years there was no suitable technology that would enable the recovery of rhenium during the copper production process,” Bigosinski shared. “What finally proved to be an effective idea was the production of ammonium perrhenate from metallurgical waste—first smelting of electrolytic copper in a metallurgical furnace and then conducting the electrorefining process.”
This technology was jointly developed by specialists from KGHM Ecoren and scientists from the Institute of Non-Ferrous Metals in Gliwice. The entire process is carried out where the waste is produced, which is at the premises of the Głogów Copper Smelter.
“At first, the effluent is filtrated and then run through special columns filled with ion-exchanger. It is precisely in these columns that the first stage of the entire process occurs, which is the ‘capturing’ of rhenium ions,” Bigosinski explained. “The next stage involves eluation of rhenium by ammonia solution, which results in doubling the element concentration. Only the solution that is thus enriched (rhenium eluate) is used as raw material for ammonium perrhenate production, ammonium perrhenate (NH4ReO4) being a commercial product which is later used in the production of pure rhenium metal.”
With rhenium being so rare, the question becomes, “Is there enough of it?” According to KGHM Ecoren, current annual rhenium consumption amounts to approximately 60 t.
“Rhenium producers that are currently in business have no problems supplying this amount,” said Bigosinski. “Furthermore, research on technologies that would enable rhenium recovery from scrap material—in other words, rhenium recycling—looks very promising. There are many other variables which have a much greater impact on the aviation industry than rhenium supply.”
Worldwide reserves of rhenium are estimated at a maximum of 17,000 t, according to the supplier. The largest deposits are found in Chile, the U.S., Canada, Kazakhstan, Russia, Uzbekistan, and Peru.