Rhenium, the last element discovered in nature


Rhenium is a kind of metallic element that can be used in producing electric lamp filaments, artificial satellites as well as the shell of rockets, preventer plates of atomic reactor and so on. Rhenium has the density of 21.04g/cm3, the melting point of 3440 degrees Celsius and the boiling point of 5627 degrees Celsius. It has hexagonal dense lattice type and same appearance with plantinum. Pure rhenium is soft and has excellent mechanical property. It is soluble in dilute nitric acid or hydrogen dioxide solution, but insoluble in the hydrochloric acid or hydrofluoric acid. Under high temperature conditions, it can compound with sulfur steam and form sulfide rhenium ReS2. It will not react with hydrogen or nitrogen, but it can absorb H2. Its valences are 3, 4, 6, and 7. It has a kind of special property that can be oxidized into very stable rhenium heptoxide Re2O7.

Rhenium is a kind of very rare and dispersed element which has only one over ten million content in the Earth’s crust and is mainly existed in molybdenite. It can be obtained by adding KCL into Re2O7 which is generated in the flue dust where the molybdenite is smelted and reducing it with hydrogen. Usually the content of molybdenite concentrate ranges between 0.001 percent to 0.003 percent. However, the molybdenum concentrate which is selected from porphyry copper can contain the rhenium up to 0.16 percent. The by-product which is generated in the smelting process of molybdenum becomes the main raw material in producing rhenium. Rhenium can be both recycled from some smelting smoke and residue of copper ores, platinum ores, niobium ores and even the blendes and the spent liquor of low grade molybdenum. When extracting rhenium, pure rhenium is firstly extracted to be made into rhenium powder by hydrogen reduction method or aqueous solution electrolysis before processing it into useful materials by powder metallurgy method.

Alloys composed by rhenium, tungsten and iron have high hardness and strong wear resistance and corrosion resistance. As rhenium has high selective catalytic function with many chemical reactions, it is often used as the catalyst in the oil industry. It also has very high electron emission performance that can be widely used in the wireless, television and vacuum technique. Besides, rhenium has high melting point, which is a kind of leading high-temperature instrument. Rhenium and rhenium alloy can be used as the valve components and ultra high temperature heater in the evaporation of metal. Tungsten-rhenium thermocouple will not soften at 3100 degrees Celsius, while the ductility of tungsten or molybdenum alloys will be enhanced by adding 25 percents of rhenium; rhenium can be used as the high temperature coating in rockets and missiles, for example, it can be used in the instruments and high-temperature components of spacecrafts such as heat shield, arc discharge, electric contactor and so on.

Rhenium is a kind of rare element. Its content in the Earth’s crust is smallest compared with all other rare earth element except for bigger than protactinium and radium. In addition, it cannot form stable ore and often associate with other metals, therefore, it is the last element discovered in the nature. As early as Dmitri Mendeleev established a periodic system of elements, he predicted the existence of rhenium which is a member of manganese subgroup and called it as dwi-manganese. Besides, another member in the subgroup which was also not discovered at that time was called eka-manganese. Later, Henry Moseley confirmed the atomic number of these two elements which are respectively as 75 and 43. As some unknown elements often can be found in the minerals that have similar element properties, scientists are devoting to find these two elements in the manganese ores, platinum ores and niobites (the minerals of tantalum and niobium). But it was not until 1925 that they were found in the concentrated products of large number of minerals and rocks by Nodack, Tucker and Bagley from Germany in using the X-ray spectrum. Later, the 75th element was named as rhenium and its symbol element was confirmed as Re.

Rhenium can simultaneously improve the strength and plasticity of tungsten, molybdenum and chrome, which can be called the “rhenium effect”, for example, perrhenic acid which is added with a small amount of rhenium (3 percents to 5 percents) can rise the initial recrystallization temperature of tungsten to 300 degrees Celsius-500 degrees Celsius. W-Re and Mo-Re alloys have excellent high-temperature strength and plasticity, which can be processed into plates, slices, lines, filaments and rods that used in the high-temperature structures (nozzles, effusers, heat shields and so on), elastic components and electronic components in the aerospace and aviation industries. Scientists have discovered the effects of rhenium on microstructure, mechanical property, unstable phase and single crystal defects of single crystal high-temperature alloy and also point out its future research direction.