Molybdenum is a transition metal identified in early modern times. Historically, the main molybdenum ore now called molybdenite was often confused with graphite or galena, the ore of lead. (Molybdos is Greek for lead.) In 1778, Swedish chemist Carl Wilhelm Scheele determined that molybdenite contained the sulfide of a new metal. This metal was isolated five years later.
The new metal had no actual use for another hundred years. European factories began using molybdenum alloys to make armor plates before World War I. This application was expanded and refined in the 1930s, just before demand exploded in World War II, leading to the current molybdenum industry.
Mining and Production
World production of molybdenum is relatively small, about 290,000 tonnes per year, most of which comes from China, Chile, and the United States. Molybdenum ore is mined alone (40%) or as a secondary metal with copper (60%). In either case, it is always accompanied by small amounts of rhenium, which can also be extracted and sold.
Molybdenite is molybdenum sulfide. The ore is crushed, ground, and separated by flotation from any copper minerals. The metal is then converted to an oxide (MoO3) by roasting in oxygen to drive off sulfur dioxide. Some manufacturing applications can work from this, or the pure metal can be obtained by exposing molybdenum oxide to hydrogen.
In recent years, a large increase in demand for molybdenum has caused the roasting stage to be a bottleneck. Moly roasters are expensive, specialized power hogs that are not built very quickly.
Properties and Uses
Molybdenum is a hard silvery metal above tungsten on the periodic table. It shares with tungsten many characteristics, including a very high melting point, but has significantly lower density.
Molybdenum disulfide, the purified form of molybdenite, can be used in its own right as a high temperature, high pressure lubricant. It is used, for example, in aircraft engines. This is the mineral once confused with graphite, though unlike graphite, it is chemically inert and does not burn.
The greatest use of molybdenum is as a small (1-8%) alloying element with steel. Molybdenum steel encompasses a wide variety of alloys manufactured for different combinations of hardness, temperature tolerance, and corrosion resistance. And, although the molybdenum content is small, the steel market is very large, leading to very high demand for this fractional additive.
For example, stainless steel containing nickel and chromium may also include molybdenum to resist additional kinds of corrosive substances. High-temperature molybdenum steel is used in drive shafts, boilers, petroleum refineries, and various stress-critical components of automobiles, aircraft, and other vehicles. Case hardened steel is used in engine blocks, rollers, and construction cranes. Tool steel used in drill bits may contain molybdenum in place of tungsten.
Pure molybdenum is occasionally used for high-temperature elements that must be drawn or shaped, such as heating elements, electrodes, and heat sinks.