- Why it's so unique
- Strategic stockpiling?
- Production and uses
Back in 1870, had you scanned the periodic table below to find rhenium, you wouldn't have found it - it wasn't there. There were still gaps for predicted, but-as-yet-undiscovered, elements, and one of them was waiting for rhenium.
The Periodic Table
In fact the element (atomic number 75), was only discovered some 55 years later, in 1925, by a team of German scientists: Otto Berg, Walter Noddack and Ida Tacke. They subsequently named rhenium after the river Rhine. Indeed, rhenium was one of the two last naturally occurring chemical elements to be discovered. One of the reasons is that it occurs neither freely in nature nor as a compound in a distinct mineral species. Once it has been extracted, however, rhenium is a heavy, silvery white metal.
Son Of Moly
Rhenium is the "Son of Moly" for a couple of reasons. Not only is most rhenium today produced as a by-product of the copper and molybdenum mining industries, but like molybdenum, rhenium has some very special, if not unique, properties.
Like moly, also, there's not much of it around. In fact, there's considerably less rhenium than there is moly. In the Earth's crust, it is only to be found in single-digit parts in a billion. Only about 50 metric tonnes of it are produced each year, and total world reserves of the metal are probably not much more than 10,000 metric tonnes.
Why is rhenium so special? Here are just some of the reasons:
- Extraordinarily high melting point: Around 3,180 ºC, a melting point exceeded only by tungsten and carbon. It is, therefore, a refractory metal (extremely resistant to heat and wear), but the only such metal not to form carbides.
- Extremely dense: A density exceeded only by iridium, osmium and platinum.
- No ductile-to-brittle transition temperature: It remains ductile (i.e., malleable or being able to be deformed plastically without fracturing) from Absolute Zero (-273.15 ºC) to its melting point. A unique property.
- High modulus of elasticity: Extremely stable and rigid under stress (great tensile strength), rhenium has the third-highest modulus of elasticity of any metal.
- High electrical resistivity: This is true; however, rhenium-molybdenum alloys actually become super-conducting at 10 ºK (-263.15 ºC).
- Low friction
- High resistance to creep: i.e., the tendency to move slowly or deform permanently when under stress.
- Exceptionally resistant to chemical poisoning: Rhenium is particularly resistant to poisoning from nitrogen, sulfur and phosphorus and can be used very effectively for the hydrogenation (the addition of hydrogen - H2) of fine chemicals.
With all these very special characteristics and its industrial importance, rhenium truly can be defined as a "strategic" metal: one of the 40 mentioned at the beginning of the article on moly. (While its "strategic" nature really cannot be argued, it remains to be seen when, or even if, the U.S. government will include it in the nation's strategic stockpile of minor metals. At present the stockpile contains precisely none!)
Some everyday (albeit more often than not, industrial) demands for it, most usually in combination with another material as an alloy, include:
- Filaments in ion gauges, mass spectrographs and photoflashes
- Electron tubes & targets, and vacuum tubes for X-rays
- Temperature controls, high-temperature thermocouples, thermistors and heating elements
- Electrical contacts, semiconductors and electromagnets
- Treatment of liver cancer and restenosis following balloon angioplasty
Of the rhenium used in the U.S. in 2007, the USGS (United States Geological Survey) estimates that 20% (15% worldwide) was used in petroleum-reforming catalysts. Bimetallic platinum-rhenium catalysts are used in the production of high-octane hydrocarbons, which are, themselves, used in the production of lead-free gas.
Lastly and most importantly, however, the USGS estimates that of the rhenium consumed in the U.S. in 2007, 60% (77% worldwide) was used in the production of super-alloys. Essentially, when combined with other metals, it imparts to the resultant compounds, or super-alloys, those exceptional qualities it has itself. Indeed, it can augment these qualities.
Super-alloys containing rhenium are used not only in the nuclear power industry and ground-based gas turbines, but also to make vital components in both civil and military jet engines and rockets. In jet engines, one of their most significant uses is to make turbine blades.
While the metal has been used in military aircraft for decades, in, for example, both the old F-16s and the new F-22s and F-35s (Joint Strike Fighters), it has only been used relatively recently in civil jet engines. Indeed, Boeing 777s, with just two engines, rely upon super-alloys using rhenium. By being able to run at higher temperatures, jet engines using rhenium are much more fuel-efficient. Anything other than rhenium super-alloy turbine blades would melt at the temperatures at which some modern jet engines built by the likes of the Rolls-Royce, General Electric and Pratt & Whitney run.