Molybdenum processing

Milling

Ball or rod mills crush and grind the mined ore to fine particles that may be only microns (10-3 mm) in diameter, releasing molybdenite from the gangue (worthless rock).

The grinding mills on the right reduce rocks the size of soccer balls to the size of gravel. Further ball milling reduces the material to the consistency of face powder.

Flotation

The milled ore/gangue powder is mixed with a liquid and aerated in the flotation step. The less dense ore rises in the froth to be collected, while the gangue sinks to be discarded. Flotation separates the metallic minerals from the gangue this way and – in the case of copper/ molybdenum ores – separates molybdenite from copper sulphide.

The resulting MoS₂ concentrate contains between 85% and 92% MoS₂. Further treatment by acid leaching can be used to dissolve impurities like copper and lead if necessary.

Roasters are multi-level hearth furnaces, in which molybdenite concentrates move from top to bottom against a current of heated air and gases blown from the bottom. The image on the right shows one of the levels in a typical roaster. Large rotary rakes move the molybdenite concentrate to promote the chemical reaction.  Desulfurisation systems such as sulfuric acid plants or lime scrubbers remove sulfur dioxide from the effluent roaster gases.

The resulting roasted molybdenite concentrate typically contains a minimum of 57% molybdenum, and less than 0.1% sulfur.

Smelting Ferromolybdenum

Between thirty and forty percent of tech oxide production is processed into ferromolybdenum (FeMo). The oxide is mixed with iron oxide and reduced by aluminium in a thermite reaction, producing a ferromolybdenum ingot weighing several hundred kilograms. The product contains between 60 and 75% molybdenum, balance essentially iron. After air cooling, the ingot is crushed and screened to meet specified ferromolybdenum particle size ranges.

The latter involves dissolution of the roasted concentrate in an alkaline medium (ammonium or sodium hydroxide), followed by removal of impurities by precipitation and filtration and/or solvent extraction. The resulting ammonium molybdate solution is then converted to any one of a number of molybdate products by crystallisation or acid precipitation. These can be further processed by calcination to pure molybdenum trioxide.

Molybdenum metal production

Molybdenum metal is produced by hydrogen reduction of pure molybdic oxide or ammonium molybdate.

The chemical reduction of pure molybdenum trioxide or ammonium dimolybdate to metal requires two stages because conversion directly to metal releases heat that inhibits the process. The first stage reduction to MoO₂ is performed in the 450-650°C range. Molybdenum dioxide is then reduced to molybdenum metal in second stage reduction, using temperatures in the 1,000-1,100°C range. Historically, both stages were accomplished by pushing boats loaded with powder through tube furnaces containing a flowing hydrogen atmosphere. Rotary furnaces, where powder is fed continuously through a rotating inclined tube in a flowing hydrogen atmosphere, are becoming common in first stage reduction operations, where they provide increased production efficiencies.