Processing

Milling

Mined ore is crushed and ground in ball or rod mills to fine particles that may be only microns (10^-3 mm) in diameter.

These grinding mills reduce soccer ball size ore to the size of gravel. The ball mills further reduce the material to face powder consistency.

Flotation

Flotation takes place in aerated tanks to separate metallic minerals from gangue (worthless rock) and – in the case of copper/ molybdenum ores – to separate molybdenite from copper sulphide.

The resulting MoS₂ concentrate contains between 85% and 92% MoS₂.  If required, an acidic leach may be employed to dissolve impurities such as copper and lead.

Roasting

The roasting process converts MoS₂ concentrate into roasted molybdenite concentrate, (also known as Technical Mo Oxide) by the chemical reactions at temperatures between 500 and 650 °C:

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. A roaster consists of several levels like the one shown here.  The molybdenite concentrate is moved to promote the chemical reaction with large rakes.

The resulting roasted molybdenite concentrate typically contains a minimum of 57% molybdenum, and less than 0.1% sulfur.  Desulfurisation systems such as sulfuric acid plants or lime scrubbers remove sulfur dioxide from the effluent roaster gases.

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 the production of roasted molybdenite concentrate is further processed into Ferromolybdenum (FeMo). The roasted concentrate is mixed with iron oxide and reduced by aluminium in a thermite reaction, producing an ingot of several hundred kilograms of ferromolybdenum grading between 60 and 75% molybdenum, balance essentially iron. After air cooling the ingot is crushed and screened to meet specified Ferromolybdenum product size ranges.

Upgrading from Roasted Molybdenite Concentrates
(Mo -tech Oxide)

About 25% of the roasted molybdenite concentrate produced worldwide is further 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 wider range of pure molybdenum chemicals (mainly molybdic oxides and molybdates).

The latter involves the initial 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.

Overview of processes to upgrade Molybdenite Concentrate

Molybdenum Metal Production

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

Molybdenum Powder Production

The pure molybdenum powder produced by this process goes into various directions including:

• Mo metal & Base Alloys
• Mo metal pellets for alloying superalloys