Background

Rhenium. (Source:University of Texas)

Rhenium is a rare, silvery-white metallic element. Its atomic number is 75 and its symbol is Re. Rhenium was discovered in 1925 by a team of German scientists named Walter Noddack, Ida Tacke-Noddack, and Otto Berg. They discovered rhenium as a trace element in platinum ores and the mineral columbite. It is very dense. It has a melting temperature of 3,186 degrees Celsius (5,767 degrees Fahrenheit). It is not known to have any health benefit for animals or plants. Rhenium does not form minerals of its own, but it does occur as a trace element in columbite, tantalite and molybdenite. These minerals are the principal sources of columbium (commonly called niobium), tantalum and molybdenum metals.

Rhenium is a very rare element that is produced principally as a by-product of the processing of porphry copper-molybdenum ores. Because it is scarce, very little rhenium is actually processed and isolated each year as compared to the millions of tons of copper and millions of pounds of molybdenum that are extracted from these same porphry copper deposits. As a result, the processing of rhenium poses no environmental threat. The equipment that reduces sulfur dioxide in these processing plants also removes any rhenium that may escape through the smokestacks.

Name

Rhenium was named after the Greek word for the Rhine River, Rhenus.

Sources

Rhenium is obtained almost exclusively as a by-product of the processing of a special type of copper deposit known as a porphyry copper deposit. Specifically, it is obtained from the processing of the mineral molybdenite (a molybdenum ore) that is found in porphyry copper deposits. A porphyry copper deposit is a valuable copper-rich deposit in which copper minerals occur throughout the rock. The copper in these deposits occurs as primary chalcopyrite (CuFeS₂) or the important secondary copper mineral chalcocite (Cu₂S).

The identified rhenium resources in the United States are estimated to total 5 million kilograms. These resources are found in the southwestern United States. The identified rhenium resources in the rest of the world are estimated to total 6 million kilograms. Countries producing rhenium include Armenia, Canada, Chile, Kazakhstan, Mexico, Peru, Russia, and Uzbekistan. Even though the United States has significant rhenium resources, the majority of the rhenium consumed in the U.S. is imported. Chile and Kazakhstan provide the majority of the imported rhenium. The rest is imported from Mexico and other nations.

Very small amounts are gathered by recycling molybdenum-rhenium and tungsten-rhenium scrap metals.

Uses

Because of its very high melting point, rhenium is used to make high temperature alloys (an alloy is a mixture of metals) that are used in jet engine parts. It is also used to make strong alloys of nickel-based metals. Rhenium alloys are used to make a variety of equipment and equipment parts, such as temperature controls, heating elements, mass spectrographs, electrical contacts, electromagnets, and semiconductors. An alloy of rhenium and molybdenum is a superconductor of electricity at very low temperatures. These superalloys account for the majority of the rhenium use each year.

Rhenium is also used in the petroleum industry to make lead-free gasoline. In this application, rhenium compounds act as catalysts. (A catalyst is a chemical compound that takes part in a chemical reaction, and can often make the reaction proceed more quickly, but the chemical is not consumed in the chemical reaction.)

Substitutes and Alternative Sources

Substitutes for rhenium as a catalyst are being researched. Iridium and tin have been found to be a good catalyst for at least one reaction. Cobalt, tungsten, platinum and tantalum can be used in some of the other applications for rhenium.

Further Reading

• Common Minerals and Their Uses, Mineral Information Institute.
• More than 170 Mineral Photographs, Mineral Information Institute.