Molybdenum processing

Milling the mined ore through crushing and grinding

Ball or rod mills crush and grind the mined ore to fine particles, releasing molybdenite from the gangue (worthless rock). The grinding mills shown here reduce rocks from 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 through air bubbles in the flotation step. The less dense ore rises in the froth to be collected, while the heavier 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 sulfide.

The resulting MoS₂ concentrate contains between 85 and 92% MoS₂. Further treatment by acid leaching dissolves impurities like copper and lead, if necessary.

Banks of flotation cells. © Antofagasta

The floatation process separates molybdenite from copper sulfide. © Antofagasta

Roasting

Roasting in air at temperatures between 500 and 650°C converts MoS₂ concentrate into roasted molybdenite (MoO₃) concentrate (also known as technical mo oxide, or tech oxide) by the chemical reactions:

2MoS₂ + 7O₂   → 2MoO₃ + 4SO₂

MoS₂ + 6MoO₃   → 7MoO₂ + 2SO₂

2MoO₂ + O₂   → 2MoO₃

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 shows one of the levels in a typical roaster. Large rotary rakes move the molybdenite concentrate to promote the chemical reaction. Desulfurization 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.

Worker maintains the roasting furnace. © SeAH

Reaction inside the roasting furnace. © SeAH

Rhenium recovery

Some of the by-product molybdenite concentrates from copper mines contain small quantities (<0.10%) of rhenium. Molybdenum roasters equipped to recover rhenium are one of the principal commercial sources for this rare metal.

Smelting Ferromolybdenum

Between 30 and 40% of technical Mo oxide production is further processed into ferromolybdenum (FeMo). The oxide is mixed with iron oxide and reduced by aluminum in a thermite reaction, producing a ferromolybdenum ingot weighing several hundred kilograms. The product contains between 60 and 75% molybdenum, and the balance is essentially iron. After air cooling, the ingot is crushed and screened to meet specified ferromolybdenum particle size ranges.

Ferromolybdenum is produced by a thermite reaction. © SeAH

Ferromolybdenum separation process. © SeAH

Chemical production from technical Mo oxide

About 20% of the roasted molybdenite concentrate produced worldwide is processed into a number of chemical products. Upgrading is performed:

• By sublimation to produce pure molybdic oxide (MoO₃)
• By wet chemical processes to produce a wide range of pure molybdenum chemicals (mainly molybdic oxides and molybdates)

The latter involves dissolution of the roasted concentrate in an alkaline medium (ammonium or sodium hydroxide), followed by the 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 crystallization 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 of reduction to MoO₂ is done in temperatures ranging from 450–650°C. The second stage, where molybdenum dioxide is reduced to molybdenum metal is done in  temperatures ranging from 1,000–1,100°C. Historically, both stages were carried out 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 used at some operations, mainly in Asia, in first stage reduction operations, where they provide increased production efficiencies.