Molybdenum Key Facts

  • Molybdenum is an essential trace element in humans, animals and plants
  • Molybdenum metal was first used in 1910 as a filament support for incandescent lamps
  • Molybdenum increases the strength of alloy steels
  • Molybdenum increases the corrosion resistance of stainless steels
  • Alloying in iron, steel and superalloys accounts for 80% of all Mo use
  • A wide range of high-technology products, including catalysts, jet engines, medical equipment and semi-conductors, rely on molybdenum metal and chemicals
  • Molybdenum transforms to molybdate in the freshwater & marine environment. The toxicity of molybdate in these environments is several orders of magnitude lower than that of most high-volume metallic elements that have been similarly assessed.

General

Molybdenum is a naturally occurring element identified in 1778 by Carl Wilhelm Scheele, the Swedish scientist who also discovered oxygen in the air.
Molybdenum has one of the highest melting points of all the elements yet its density is only 25% greater than that of iron.
Molybdenum is contained in various ores, but only molybdenite (MoS2) is used in the production of marketable molybdenum products.
Molybdenum’s coefficient of thermal expansion is the lowest of the engineering materials.
The principal molybdenum mines are found in Canada, USA, Mexico, Peru and Chile. Many of these mines are amongst the most productive in the world, with the largest capable of moving over 50,000 tonnes of ore per day.
About 20% of new molybdenum, produced from mined ore is used to make molybdenum grade stainless steel.
Engineering steels, tool and high speed steel, cast iron and superalloys taken together, account for an additional 60%.
The remaining 20% is used in upgraded products like lubricant grade molybdenum disulfide MoS2), molybdenum chemical compounds and molybdenum metal.
Ore reserves
In 2008 the ore reserve base totalled 19,000,000 tonnes (source: US Geological Survey). China has the largest reserves followed by USA and Chile.

Production

Molybdenite can occur as the sole mineralisation in an ore body, but is often associated with the sulfide minerals of other metals, notably copper.
There are three classes of molybdenum mines:

  • Primary mines – molybdenum recovery is sole objective
  • By-product mines – recovery of copper bearing ores is prime objective and recovery of molybdenite adds value
  • Co-product mines – commercial viability of mine requires molybdenite and copper bearing minerals to be recovered

Processing

The mined ore is crushed and ground to release the molybdenite from the gangue (worthless rock). The ground ore is then mixed with a liquid and aerated in a flotation process to separate the metallic minerals from the gangue.
The resulting MoS2 concentrate contains between 85% and 92% industrially useable MoS2.
Roasting molybdenite  in air at 500 to 650 °C converts molybdenum disulfide (MoS2) in the concentrate into RMC roasted molybdenite  concentrate (Mo03) which is also known as technical Mo oxide, or tech oxide. This contains a minimum of 57% molybdenum and less than 0.2% sulphur.  Some 40 to 50% of molybdenum is used in this form, mainly as an alloying element in steel products.
30-40% of RMC production is processed into ferromolybdenum (FeMo). The RMC is mixed with iron oxide and reduced with ferrosilicon and aluminium in a thermite reaction.  The resulting ingots which contain between 60 and 75% Mo (the balance as  iron), are crushed and screened to produce the desired FeMo particle size.
About 20% of the RMC produced worldwide is processed into a number of chemical products such as pure molybdic oxide Mo03 and molybdates. Ammonium molybdate solution can be converted to any number of molybdate products and further processing by calcinations produces pure molybdenum trioxide.
Molybdenum metal is produced by means of a two-stage hydrogen reduction process to give pure molybdenum powder.

Benefits and Uses

Stainless steel: molybdenum improves the corrosion resistance of all stainless steels and their high temperature strength. It has a particularly strong positive effect on pitting and crevice corrosion resistance in chloride-containing solutions, making it important in chemical and other processing applications. Molybdenum containing stainless steels are commonly used in Architecture Building and Construction. A wide range of molybdenum containing materials are available including structural components, roofing, curtain wall, handrails, swimming pool liners and accessories, doors, lighting and sunscreens.
The sustainability of stainless steel is demonstrated by the longevity of many well known buildings such as New York’s Chrysler building (1930). Increasingly today, sustainable design concepts are being incorporated in a wide range of projects to provide long service life.
Superalloys: these fall into two basic classes: corrosion resistant alloys and high temperature alloys. Corrosion resistant nickel-based alloys containing molybdenum are used in applications exposed to highly corrosive environments such as chemical processing, pharmaceutical, oil & gas, petrochemical and pollution control industries.
High temperature alloys can be sub-divided into two categories: solid-solution strengthened and age-hardenable. Solid solution strengthened superalloys provide resistance to damage caused by high temperature creep. This is because molybdenum diffuses very slowly in nickel and high temperature creep is generally diffusion controlled.
Age-hardenable superalloys provide additional strength without significantly reducing ductility and are also very effective in reducing the coefficient of thermal expansion.
Alloy steels: just a small amount of molybdenum improves hardenability, reduces temper embrittlement, boosts resistance to hydrogen attack and sulfide stress cracking. The added molybdenum also increases elevated temperature strength and improves weldability, especially in high strength low alloy steels (HSLA). In a synergetic way molybdenum enhances the metallurgical effects of other alloying elements and widens the processing window, particularly in the production of advanced high strength steels.
Molybdenum based alloys have excellent strength and mechanical stability at high temperatures (up to 1900°C) in non-oxidizing or vacuum environments. Their high ductility and toughness provide a greater tolerance for imperfections and brittle fracture than ceramics.
Specialised examples of molybdenum alloyed with other metals include molybdenum-tungsten alloys which are noted for exceptional resistance to molten zinc; molybdenum is clad with copper to provide low expansion and high conductivity electronic circuit boards and molybdenum-25% rhenium alloys are used for rocket engine components and liquid metal heat exchangers which must be ductile at room temperature.
Chemical molybdenum oxide is used in the production of catalysts for the petrochemical and chemical industries. Molybdenum based catalysts are widely used in the refining of crude oil. They make an important contribution to a cleaner environment by reducing the sulphur content of the refined products.
Chemical molybdenum products are also used in polymer compounding, corrosion inhibitors and high-performance lubricant formulations.