Molybdenum Processing

http://www.imoa.info/molybdenum/mining_processing/molybdenum_processing.php[22-03-2013 18:06:22]

(Photo courtesy of Kennecott UtahCopper Corporation, USA)

Processing

Molybdenum processing flowsheet

Milling

Ball or rod mills crush and grindthe mined ore to fine particles that

may be only microns (10-3 mm) indiameter, releasing molybdenitefrom the gangue (worthless rock).

The grinding mills on the rightreduce rocks the size of soccerballs to the size of gravel. Furtherball milling reduces the materialto 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 tobe discarded. Flotation separates the metallic minerals from the gangue this way and– in the case of copper/ molybdenum ores – separates molybdenite from coppersulphide.

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 ifnecessary.

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Molybdenum Processing

http://www.imoa.info/molybdenum/mining_processing/molybdenum_processing.php[22-03-2013 18:06:22]

Production of molybdenum concentrate Overview of banks of flotation cells(Photos courtesy of Kennecott UtahCopper Corporation, USA)

Interior of a roasting furnace(Courtesy of Molymet, Chile)

Close-up of flotation cell

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 movefrom top to bottom against a current ofheated air and gases blown from thebottom. The image on the right shows oneof the levels in a typical roaster. Largerotary rakes move the molybdeniteconcentrate to promote the chemicalreaction. Desulfurisation systems such assulfuric acid plants or lime scrubbersremove sulfur dioxide from the effluentroaster gases.

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

Rhenium recovery

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

Smelting Ferromolybdenum

Between thirty and forty percent of techoxide production is processed intoferromolybdenum (FeMo). The oxide ismixed with iron oxide and reduced byaluminium in a thermite reaction,producing aferromolybdenum ingot weighing several

Molybdenum Processing

http://www.imoa.info/molybdenum/mining_processing/molybdenum_processing.php[22-03-2013 18:06:22]

Ferromolybdenum smelting(Courtesy of Treibacher, Austria)

hundred kilograms. The product containsbetween 60 and 75% molybdenum,balance essentially iron. After air cooling,the ingot is crushed and screened to meetspecified ferromolybdenum particle sizeranges.

Upgrading from Tech Oxide

About 25% of the roasted molybdenite concentrate produced worldwide is processedinto 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 removal of impurities by precipitation andfiltration and/or solvent extraction. The resulting ammonium molybdate solution is thenconverted to any one of a number of molybdate products by crystallisation or acidprecipitation. These can be further processed by calcination to pure molybdenumtrioxide.

Overview of processes to upgrade Molybdenite Concentrate

Molybdenum Metal Production

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

Molybdenum Processing

http://www.imoa.info/molybdenum/mining_processing/molybdenum_processing.php[22-03-2013 18:06:22]

Molybdenum metal powder production

The chemical reduction of pure molybdenum trioxide or ammonium dimolybdate tometal requires two stages because conversion directly to metal releases heatthat 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 secondstage reduction, using temperatures in the 1,000-1,100 �C range. Historically,both stages were accomplished by pushing boats loaded with powder through tubefurnaces containing a flowing hydrogen atmosphere. Rotary furnaces, where powder isfed continuously through a rotating inclined tube in a flowing hydrogen atmosphere, arebecoming common in first stage reduction operations, where they provide increasedproduction efficiencies.

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