Rhenium has the honor of being the last stable (nonradioactive) element discovered. The discovery in 1915 of atomic number made the order of elements in the periodic table definite. There were four gaps among the low numbers, which were presumably stable. The gap at 72 would be filled by hafnium, and the gaps at 43 and 61 would eventually prove to hold radioactive short-lived elements. That leaves element 75.
One team pursuing the discovery of element 75 was the German chemists Walter Noddack and Ida Tacke. They estimated that the element is very rare in nature, similar to the platinum group of metals (PGMs). They therefore started with a large quantity of the mineral gadolinite and concentrated it down to a 0.01% mass of unknown substances. X-ray investigation in 1925 showed evidence of a new element, and a similar process with the mineral columbite confirmed this. Later, Noddack and Tacke would isolate one entire gram of the new element from 660 kilograms of molybdenite ore (right). They named it rhenium from the Latin for the Rhine River.
Mining and Production
Commercial rhenium is extracted along with molybdenum in copper ores. World production in 2014 was just under 50 tonnes, a very small amount. By far the largest amount, 53%, was produced in Chile. Rhenium is one of the rarest elements on earth, but it comprises around 0.5% of molybdenum minerals that occur alongside copper ore and must be separated anyway.
Properties and Uses
Rhenium is a silver metal with some of the same properties as the PGMs. It is very dense and has a high melting point. Rhenium resists many acids, but it is more reactive than the PGMs.
Although rhenium is considered a stable element, it's most common isotope is actually slightly radioactive. The half-life of rhenium-187 (62% of the total mass) is around 10 billion years or 70% of the age of the universe. Rhenium-185 is stable, yet comprises only 38% of the total mass.
Since rhenium is so uncommon, one of its most frequent uses is simply as a target of experiments in chemistry and metallurgy. Its biggest commercial use is in superalloys for jet engines. The very high melting point makes it very effective as a 3% additive to high-temperature nickel-steel alloys.
Rhenium alloyed with platinum is a catalyst surface in the same way that platinum or palladium works in catalytic converters. This rhenium-platinum catalyst in petroleum refineries converts some liquid products from low to high octane.
Rhenium also has some use as an alloying element for tiny metal parts that must resist corrosion or high temperature. Examples include electrical contacts, switches, and filaments.
Rhenium being useful yet in short supply, its price is fairly high. Shown is the price in troy ounces compared to platinum. Actually, the price is normally quoted per kilogram or per pound, which is currently $1150.