Extractive Metallurgy of Molybdenum

To understand the Extractive Metallurgy of Molybdenum you must first know that Molybdenum is a transition element of group VI b of the periodic table and has similar properties to those of chromium and tungsten. The most important deposits of molybdenum are hydrothermal porphyries, but molybdenum can be found in contact metamorphic deposits, hydrothermal quartz vein and breccia deposits, pegmatite, and aplite deposits, and in some startabounds ores hosted in sedimentary rocks.

Porphyry deposits are perhaps the most important source of molybdenum. These deposits are formed from hydrothermal fluids by the intrusion of stocks of peralkaline granite to calc-alkaline diorite. The mineralization is present in stockwork quartz veins, in breccias and disseminated in the igneous rocks. These deposits are mined for their copper content, although many have significant by-product molybdenum, with molybdenum concentrations ranging up to equal those of copper. Grades of worked deposits are generally low, with ranges of 0.4 to 1.1% Cu and 0.016 to 0.060% Mo.

Molybdenite belongs to the minerals group of easy flotation which is related to its crystal structure. After grinding molybdenite particles present laminar structure that favors natural hydrophobicity. For this reason, its elevated hydrophobic capacity allows recover successfully molybdenum from ores with low grades.

The relative low grade of most molybdenum ores requires the use of high volume low cost mining extraction techniques. Mine ore is pulverized through crusher and grinding steps in order to liberate molybdenite from the host rock. The slurry is conditioned with flotation reagents (e.g. fuel oil) in order to concentrate molybdenite. If a plant treats a copper-molybdenum ore, then will be necessary to get a bulk concentrate formed by copper and molybdenum which will be treated by selective flotation in order to get a copper concentrate and a molybdenum concentrate. However, if a plan treats a molybdenum ore, the concentrate can be obtained directly by flotation. The final concentrate contains about 50 to 54% Mo.

The concentrate is roasted in order to get molybdenum oxide. The reactions are done at 600 to 800 ^oC in large multihearth furnaces. The concentrate reacts continuously with a steady supply of forced air for ten hours. The resultant oxide contains more than 57% molybdenum. Desulphurization systems remove sulphur dioxide from the roasters. Some of the by-product molybdenum concentrated from copper mines present rhenium which is employed in catalyst for the production of unleaded gasoline. Thus, some roaster can recover rhenium.

The oxide obtained is the principal product for adding molybdenum to alloys and stainless steel. Other important product is the ferromolybdenum which is obtained by the thermite reduction of initial oxide with iron. This alloy assays more than 60% Mo and the other part is iron. This product is used in melting processes such as induction melting that can not reduce the oxide.

Some technical oxide is processed into a number of chemical products, and into pure molybdenum metal. It is possible to upgrade the oxide by sublimation and by chemical processes to produce several chemical products (i.e. oxides and molybdates). Molybdates involve the initial dissolution in an alkaline medium followed by removal of impurities by precipitation or solvent extraction. Molybdenum chemical are used as lubricants, corrosion inhibitors, pigments, and catalyzers.