Occurrence and Uses
The most important vanadium (V) ores are patronite (vanadium sulphide), found in Peru, and descloizite (lead-zinc vanadate), found in South Africa. Other ores, such as vanadinite, roscoelite and carnotite, contain vanadium in sufficient quantities for economic extraction. Crude petroleum may contain small amounts of vanadium, and flue-gas deposits from oil-fired furnaces may contain over 50% vanadium pentoxide. Slags from ferrovanadium are another source of the metal. One of the most important sources of human exposure to vanadium is vanadium oxides released when burning fuel oils.
Normally, small amounts of vanadium are found in the human body, particularly in adipose tissue and in the blood.
The larger amount of the vanadium produced is used in ferrovanadium, the most important direct use of which is in high-speed steel and tool steelmaking. Addition of 0.05 to 5% of vanadium removes occluded oxygen and nitrogen from the steel, enhances the tensile strength and improves the modulus of elasticity and the rust resistance of the final alloy. In the past vanadium compounds have been used as therapeutic agents in medicine. The vanadium-gallium alloy has shown interesting properties for production of high magnetic fields.
Certain vanadium compounds have a limited use in industry. Vanadium sulphate (VSO4·7H2O) and vanadium tetrachloride (VCl4) are used as mordants in the dyeing industry. Vanadium silicates are used as catalysts. Vanadium dioxide (VO2) and vanadium trioxide (V2O3) are employed in metallurgy. However, the most significant compounds in terms of industrial health hazards are vanadium pentoxide (V2O5) and ammonium metavanadate (NH4VO3).
Vanadium pentoxide is obtained from patronite. It has for a long time been an important industrial catalyst used in a number of oxidation processes such as those involved in the manufacture of sulphuric acid, phthalic acid, maleic acid and so on. It serves as a photographic developer and as a dyeing agent in the textile industry. Vanadium pentoxide is also used in ceramic colouring materials.
Ammonium metavanadate is employed as a catalyst in the same way as vanadium pentoxide. It is a reagent in analytical chemistry and a developer in the photography industry. Ammonium metavanadate is also used in dyeing and printing in the textile industry.
Experience has shown that vanadium oxides and, in particular, the pentoxide and its derivative ammonium metavanadate cause harmful effects in humans. Exposure to vanadium pentoxide is possible at the following points in industry: when vanadium pentoxide is used in particulate form in the production of metallic vanadium; during the repair of installations where vanadium pentoxide is used as a catalyst; and during the cleaning of oil-fired furnace flues in power stations, ships and so on. The presence of vanadium compounds in petroleum products is of particular significance and, because of the possibility of air pollution in the environment of oil-fired power stations, it receives attention from public health authorities as well as from those concerned with industrial health.
The inhalation of vanadium compounds may produce severe toxic effects. The severity of the effects depends on the atmospheric concentration of the vanadium compounds and the duration of exposure. Health impairment may occur after even brief exposure (e.g., 1 hour), and the initial symptoms are profuse lacrimation, burning sensation in the conjunctivae, serous or haemorrhageous rhinitis, sore throat, cough, bronchitis, expectoration and chest pain.
Severe exposure may result in pneumonia with fatal outcome; however, following one-time exposure, complete recovery usually occurs within 1 to 2 weeks; prolonged exposure may produce chronic bronchitis with or without emphysema. The tongue may present a greenish discolouration and also the cigarette ends of vanadium workers may show a greenish colour, resulting from chemical interactions.
Local effects in experimental animals are mainly observed in the respiratory tract. Systemic effects have been observed in the liver, kidney, nervous system, cardiovascular system and blood-forming organs. Metabolic effects include interference with biosynthesis of cystine and cholestrol, depression and stimulation of phospholipid synthesis. Higher concentrations have produced inhibition of serotonin oxidation. In addition, vanadate has been shown to inhibit several enzyme systems. In humans, systemic effects of vanadium exposure are less well documented, but reduction of serum cholestrol has been demonstrated. In the work environment, vanadium and its compounds are taken up in the human body by inhalation, mainly during production and boiler cleaning operations. Absorption of vanadium from the gastrointestinal tract is poor, not exceeding 1 to 2%; ingested vanadium compounds are largely eliminated with faeces.
A study was conducted to evaluate the level of bronchial responsiveness among workers recently exposed to vanadium pentoxide during periodic removal of ashes and clinker from boilers of an oil-fired power station. This study suggests that exposure to vanadium increases bronchial responsiveness even without the appearance of bronchial symptoms.
Safety and Health Measures
It is important to prevent the inhalation of airborne particulate vanadium pentoxide. For use as a catalyst, vanadium pentoxide can be produced in an agglomerated or pelleted form which is dust free; however, vibration in the plant may, in time, reduce a certain proportion to dust. In the processes associated with the manufacture of metallic vanadium, and in the sieving of used catalyst during maintenance operations, the escape of dust should be prevented by the enclosure of the process and by the provision of exhaust ventilation. In boiler cleaning in power stations and on ships, maintenance workers may have to enter the boilers to remove soot and to make repairs. These workers should wear adequate respiratory protective equipment with full face mask and eye protection. Wherever possible, on-load cleaning should be improved to reduce the need for workers to enter furnaces; where off-load cleaning proves essential, methods such as water lancing, which do not necessitate physical entry, should be tried.