" DISCLAIMER: The ILO does not take responsibility for content presented on this web portal that is presented in any language other than English, which is the language used for the initial production and peer-review of original content. Certain statistics have not been updated since the production of the 4th edition of the Encyclopaedia (1998)."

Friday, 11 February 2011 21:08

Molybdenum

Written by
Rate this item
(0 votes)

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.

 

Back

Read 2527 times Last modified on Thursday, 19 May 2011 10:29
More in this category: « Mercury Nickel »

Contents

Preface
Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Using, Storing and Transporting Chemicals
Minerals and Agricultural Chemicals
Metals: Chemical Properties and Toxicity
Resources
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVII. Services and Trade
Part XVIII. Guides

Metals: Chemical Properties and Toxicity Additional Resources

Click the Button below to view additional resources for this topic.

button

Metals: Chemical Properties and Toxicity References

Agency for Toxic Substances and Disease Registry (ATSDR). 1995. Case Studies in Environmental Medicine: Lead Toxicity. Atlanta: ATSDR.

Brief, RS, JW Blanchard, RA Scala, and JH Blacker. 1971. Metal carbonyls in the petroleum industry. Arch Environ Health 23:373–384.

International Agency for Research on Cancer (IARC). 1990. Chromium, Nickel and Welding. Lyon: IARC.

National Institute for Occupational Safety and Health (NIOSH). 1994. NIOSH Pocket Guide to Chemical Hazards. DHHS (NIOSH) Publication No. 94-116. Cincinnati, OH: NIOSH.

Rendall, REG, JI Phillips and KA Renton. 1994. Death following exposure to fine particulate nickel from a metal arc process. Ann Occup Hyg 38:921–930.

Sunderman, FW, Jr., and A Oskarsson,. 1991. Nickel. In Metals and their compounds in the environment, edited by E Merian, Weinheim, Germany: VCH Verlag.

Sunderman, FW, Jr., A Aitio, LO Morgan, and T Norseth. 1986. Biological monitoring of nickel. Tox Ind Health 2:17–78.

United Nations Committee of Experts on the Transport of Dangerous Goods. 1995. Recommendations on the Transport of Dangerous Goods, 9th edition. New York: United Nations.