Molybdenum Production

The main feedstock for Molybdenum extraction consists of standard Molybdenum concentrates, which contain 47-50 % Mo, 28-32 % S, 1-9 % SiO₂ as well as other impurities. After ore mining, grinding and floatation, the obtained concentrate is undertaken to oxidizing roasting at 570-600°C in multiple-bedded furnace or in fluidized-bed kiln. The roasting process converts MoS₂ concentrate into roasted molybdenite concentrate, (also known as Technical Molybdenum Oxide) by the chemical reactions at temperatures between 500 and 650°C: The cinder in the furnaces contains contaminated MoO₃. Pure molybdic oxide MoO₃ is necessary for further molybdenum metal manufacture. For this reason the roasted molybdenite concentrate produced worldwide is further processed into a number of chemical products. Upgrading is performed either
1. by sublimation to produce pure (MoO₃) or
2. by wet chemical processes to produce a wider range of pure molybdenum chemicals (mainly molybdic oxides and molybdates).

The latter involves the oxide leaching by ammonium dissolving the Molibdenium trioxide, followed by removal of impurities (Cu, Fe) 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 and roasting at 450-500°C. The result of processing Molibdenium trioxide with less than 0.05 % impurities.

Molybdenum metal is produced by hydrogen reduction of pure molybdic in tube-type furnace, which is a two-step powder metallurgical process, first at 550-700°C and then at 900-1000°C. The first step results obtaining relatively small rods with cross-section 2-9 sm² and length 450-600 mm. Molybdenum powder is pressed in steel die set under 200-300 MN/m². Then, after preliminary sintering at 1000-1200°C the high-temperature sintering at 2200-2400°C follows. Metal forming, which consists of forging, drawing and rolling, is the next operation. The bigger rods 0-200 kg) is produced by hydrostatic (isostatic) pressing in elastic bags. Rods of 500-2000 kg are produced by arc melting in furnace with cooled copper crucible and sintered rods as consumable electrodes.

Although the purest Molybdenum Preparation is performed from wulfenite, the chief commercial source is molybdenite, which is converted into the trioxide by roasting in air either with or without the addition of sand, and, on dissolving the residue in ammonia, a solution of ammonium molybdate is obtained. This salt, freed from copper by treatment in ammoniacal solution with ammonium sulphide, and from aluminium by the addition of potassium carbonate, on ignition yields molybdenum dioxide; alternatively, heating with excess of sulphur yields pure molybdenum disulphide, MoS₂, which on roasting, or by treatment with nitric acid, is converted into the trioxide MoO₃.

Though usually prepared in the metallic condition by Goldschmidt's process, molybdenum may also be obtained by the reduction of the oxide, sulphide, or halide, by treatment under suitable conditions with carbon, hydrogen, or other reducing agent; by an electrolytic method; or by heating the nitride in vacuo.

Molybdenum Preparation by Reduction of the Oxide with Aluminium Powder (Goldschmidt's Process)

From the trioxide, MoO₃, a good yield of molybdenum free from air-bubbles can be obtained by Goldschmidt's method, it being preferable to moderate the reaction by the addition of a flux of 50 parts of calcium fluoride for every 100 parts of molybdenum trioxide, and 38 parts of aluminium, and to allow the metal to agglomerate by keeping the mass in a liquid condition for some time after the reaction. Fused molybdenum may also be conveniently obtained by reduction with aluminium powder of the dioxide MoO₂, the latter being prepared by the reduction in hydrogen of the trioxide, or by ignition of ammonium molybdate, (NH₄)₂MoO₄.

Molybdenum Preparation by Reduction of the Oxide or Sulphide with Carbon

Molybdenum containing 4 to 5 per cent, of carbon is prepared by reduction of oxides of molybdenum by means of carbon in brasqued crucibles. By reduction of 10 parts of the dioxide (obtained by ignition of ammonium molybdate) with 1 part of sugar charcoal in the electric furnace, using 800 amperes at 60 volts for six minutes, Moissan obtained the metal, from which carbon was removed by heating with molybdenum dioxide, in a fairly pure condition, while by a similar process molybdenite, MoS₂, yields a metallic product free from sulphur but containing a little iron. A mixture of molybdenite, lime, and fluorspar, heated in the electric furnace with a current of 70 to 100 amperes at 30 to 40 volts, produces a good yield of the homogeneous metal, though a purer product, containing 98.95 per cent, molybdenum, results if the mineral is first treated with concentrated hydrochloric acid.

Reduction of the Oxides, Halides, or Sulphides with Hydrogen

Molybdenum trioxide, purified by sublimation in a platinum tube, is heated in pure hydrogen first at as low a temperature as possible in order to convert it to the dioxide MoO₂, and finally in hydrogen in a quartz or platinum tube at a high temperature in order to complete the reduction. Certain modifications in the technique of the process have been described.

A brilliant steel-coloured form of the metal is obtained by reduction of chlorides of molybdenum at a red heat in a glass tube by hydrogen. It is found that molybdenum tetra- and penta-chlorides, on heating below 1330° C., yield metallic molybdenum.

Reduction of the sulphides MoS₂ or MoS₃ in a Rose crucible by means of hydrogen also yields the metal.

Other Methods of Molybdenum Preparation

Electrolysis of a solution of molybdic acid in hydrochloric acid, using a mercury cathode, yields an amalgam, from which an active form of molybdenum is obtained on distillation.

On heating molybdenum oxide or its mixture with the metal to 500° to 600° C. with equal parts of nitrogen and hydrogen under a pressure of 60 atmospheres, a nitride is obtained which, when heated in a vacuum, leaves a residue of pure metallic molybdenum.