Gunnar Nordberg

Occurrence and Uses

Molybdenum (Mo) is widely distributed throughout the earth’s crust, but it is mined in only a limited number of countries due to the rarity of bodies of sufficiently high quality molybdenite ore (MoSO₂). A certain amount of molybdenum is obtained as a by-product in the processing of copper ore. Coal-electrical power plants can be significant sources of molybdenum. Molybdenum is an essential trace element.

Molybdenum forms a large variety of commercially useful compounds in which it displays the valence numbers 0, +2, +3, +4, +5 and +6. It readily changes valence states (disproportionates) with only minor changes in external conditions. It has a strong tendency to form complexes; with the exception of the sulphides and halides, very few other simple compounds of molybdenum exist. The +6 molybdenum forms isopoly- and heteropoly- acids.

Over 90% of the molybdenum produced is used as an alloying element for iron, steel and non-ferrous metals, mainly because of its heat-resisting properties; the rest is used in chemicals and lubricants. As a steel alloy, molybdenum is utilized in the electric, electronics, military and automobile industries and in aeronautical engineering. Another important use of molybdenum is in the production of inorganic molybdenum pigments, dyes and lakes. Small but increasing amounts of molybdenum are used as trace elements in fertilizers.

The most important molybdenum chemical is molybdenum trioxide (MoO₃), made from roasting the sulphide ore. Pure molybdenum trioxide is used in chemical and catalyst manufacture. The technical product is added to steel as an alloying agent. Molybdenum trioxide also serves as a catalyst in the petroleum industry and as a component of ceramics, enamels and pigments. Molybdenum disulphide (MoS₂) is employed as a heat-resistant lubricant or a lubricant additive. Molybdenum hexacarbonyl (Mo(CO)₆) is the starting product for the manufacture of organomolybdenum dyes. It is increasingly used for molybdenum plating by thermal decomposition.

Molybdenum compounds are widely used as catalysts or catalyst activators or promoters, especially for hydrogenation-cracking, alkylation and reforming in the petroleum industry. They are employed as laboratory reagents (phosphomolybdates). In addition, molybdenum compounds are used in electroplating and in tanning.

Hazards

In the processing and industrial utilization of molybdenum and its compounds there may be exposure to dusts and fumes of molybdenum and its oxides and sulphides. This exposure may occur, especially where high-temperature treatment is being carried out as, for example, in an electric furnace. Exposure to molybdenum disulphide lubricant spray, molybdenum hexacarbonyl and its breakdown products during molybdenum plating, molybdenum hydroxide (Mo(OH)₃) mist during electroplating, and molybdenum trioxide fumes which sublime above 800 °C may all prove hazardous to health.

Molybdenum compounds are highly toxic based on animal experiments. Acute poisoning causes severe gastrointestinal irritation with diarrhoea, coma and deaths from heart failure. Pneumoconiosis-like effects in the lungs have been reported in animal studies. Workers exposed to pure molybdenum or to molybdenum oxide (MoO₃) (concentration of 1 to 19 mg Mo/m³) over a period of 3 to 7 years have suffered from pneumoconiosis. Inhalation of molybdenum dust from alloys or carbides can cause “hard metal lung disease”.

There is a wide degree of variation in the hazard resulting from exposure. Insoluble molybdenum compounds (e.g., molybdenum disulphide and many of the oxides and halides) are characterized by low toxicity; however, the soluble compounds (i.e., those in which molybdenum is an anion, such as sodium molybdenate—Na₂MoO₄·2H₂O) are considerably more toxic and should be handled with care. Likewise, precautions should be taken to prevent over-exposure to freshly generated molybdenum fumes as in the thermal decomposition of molybdenum hexacarbonyl.

Exposure to molybdenum trioxide produces irritation of the eyes and the mucous membranes of the nose and throat. Anaemia is a characteristic feature of molybdenum toxicity, with low haemoglobin concentrations and reduced red-cell counts.

High dietary levels of molybdenum in cattle were found to produce deformities in the joints of the extremities. Among chemists handling molybdenum and tungsten solutions, an abnormally high frequency of cases of gout have been reported, and a correlation has been found between the content of molybdenum in food, the incidence of gout, uricaemia and xanthine oxidase activity.

Safety Measures

While working with molybdenum in industry, proper local exhaust ventilation should be employed to collect fumes at their source. Respirators may be worn when engineering and work practices have failed, when such controls are in the process of being installed, for operations requiring entry into tanks or closed vessels, or in emergencies. In the paint, printing and coatings industries, local and general exhaust ventilation as well as safety glasses, protective clothing, face shields and acceptable respirators should be used to reduce exposure for workers handling molybdenum-based dry ingredients for inorganic and organic colours.

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