Friday, 11 February 2011 22:01

Zirconium and Hafnium

Written by

Gunnar Nordberg

Occurrence and Uses

It has been estimated that zirconium (Zr) constitutes about 0.017% of the lithosphere. Because of its very high chemical activity at temperatures only slightly above normal atmospheric temperature, the element occurs only in combined states. The most common ores are zircon (ZrO2) and baddeleyite (ZrSiO4). Zirconium is found in all animal tissues.

Hafnium (Hf) is found associated with zirconium in all its terrestrial occurrences. The amount of hafnium varies but averages about 2% of the total zirconium plus hafnium. In only one ore, low in both elements, has hafnium been found in greater quantity than zirconium. Spectrographic evidence indicates that the distribution is also about 2% hafnium in the total zirconium-plus-hafnium in the universe. These two elements are more closely identical in their chemical properties than are any other pair in the periodic table. The similarity is so great that no qualitative differences have yet been found which would permit their separation. For this reason, it can be assumed that most of the zirconium which has been used, and on the basis of which physiological effects have been reported, has contained 0.5 to 2% hafnium.

Zircon has been valued since the earliest times as a gem stone, since it occurs quite commonly in large single crystals; however, most of the commercially useful deposits of zirconium ore are in beach sands or other places where the relatively heavy and chemically inert zirconium minerals have been deposited while the lighter portions of the rocks in which they occurred have been disintegrated and washed away by the action of water. Substantial deposits of such beach sands are known in India, Malaya, Australia and the United States. Baddeleyite in commercially useful deposits was first observed in Brazil, and has since been found in a number of other locations including Sweden, India and Italy. Some zirconium ores have also been mined commercially in Madagascar, Nigeria, Senegal and South Africa.

Zircon is used as a foundry sand, an abrasive, and as a component of zircon and zirconia refractory compositions for laboratory crucibles. It is found in ceramic compositions where it acts as an opacifier in glazes and enamels. Zircon and zirconia bricks are used as linings for glass furnaces. Zirconia forms are also used as dies for extrusion of both ferrous and non-ferrous metals and as spout linings for pouring metals, particularly for continuous casting.

More than 90% of zirconium metal is now used in nuclear power generation because zirconium has a low absorption cross-section for neutrons and a high resistance to corrosion inside atomic reactors, provided it is free of hafnium. Zirconium is also used in the manufacture of cast iron, steel and surgical appliances. It is employed in arc lamps, pyrotechnics, in special welding fluxes, and as a pigment in plastics.

Powdered zirconium metal is used as a “getter” in thermionic tubes to absorb the last traces of gas after pumping and out-gassing of the tube elements. In the form of fine ribbon or wool, the metal is also used as the filter in photographic flash-bulbs. The massive metal is used either pure or in alloy form for the lining of reaction vessels. It is also used as a lining for pumps and piping systems for chemical processes. An excellent super-conducting alloy of zirconium and columbium has been used in a magnet with a field of 6.7 T.

Zirconium carbide and zirconium diboride are both hard, refractory, metallic compounds which have been used in cutting tools for metals. The diboride has also been used as a thermocouple jacket in open-hearth furnaces, providing very long-lived thermocouples. Zirconium tetrachloride is used in organic synthesis and in water repellents for textiles. It is also useful as a tanning agent.

Hafnium metal has been used as a cladding on tantalum for rocket engine parts which must operate in very high-temperature, erosive conditions. Because of its high thermal-neutron cross-section, it is also used as a control rod material for nuclear reactors. In addition, hafnium is used in the manufacture of electrodes and light-bulb filaments.

Hazards

It is inaccurate to state that zirconium compounds are physiologically inert, but the tolerance of most organisms to zirconium appears to be great in comparison to the tolerance for most heavy metals. Zirconium salts have been used in the treatment of plutonium poisoning to displace the plutonium (and yttrium) from its deposition in the skeleton and to prevent the deposition when treatment was started early. In the course of this study, it was determined that the diet of rats could contain as much as 20% of zirconia for comparatively long periods without harmful effects, and that the intravenous LD50 of sodium zirconium citrate for rats is about 171 mg/kg body weight. Other investigators have found an intraperitoneal LD50 of 0.67 g/kg for zirconium lactate and 0.42 g/kg for barium zirconate in rats and 51 mg/kg of sodium zirconium lactate in mice.

Zirconium compounds have been recommended and used for the topical treatment of Rhus (poison ivy) dermatitis and for body deodorants. Some compounds which have been used are carbonated hydrous zirconia, hydrous zirconia and sodium zirconium lactate. There have been a number of reports of the production of persistent granulomatous conditions of the skin as the result of these applications.

Of more direct interest in connection with occupational exposures is the effect of inhalation of zirconium compounds, and this has been less extensively investigated than the other routes of administration. There have, however, been several experiments and at least one report of human exposure. In this instance, a chemical engineer with seven years’ exposure in a zirconium and hafnium processing plant was found to have a granulomatous lung condition. Since examination of all the other employees revealed no comparable lesions, it was concluded that the condition was most probably to be attributed to a relatively heavy beryllium exposure prior to zirconium exposure.

Exposure of experimental animals to zirconium compounds showed that zirconium lactate and barium zirconate both produced severe, persistent, chronic interstitial pneumonitis at atmospheric zirconium concentrations of about 5 mg/m3. Much higher atmospheric sodium zirconium lactate concentrations of 0.049 mg/cm3 for shorter exposures have been found to produce peribronchial abscesses, peribronchiolar granulomas and lobular pneumonia. Although documentation of zirconium pneumoconiosis in humans has been lacking, authors of one study conclude that zirconium should be considered a likely cause of pneumoconiosis, and recommend taking appropriate precautions in the workplace.

The small number of investigations on the toxicity of hafnium compounds has indicated an acute toxicity slightly higher than that of zirconium salts. Hafnium and its compounds cause liver damage. Hafnyl chloride at 10 mg/kg produced cardiovascular collapse and respiratory arrest in a cat in the same manner as soluble zirconium salts; the intraperitoneal LD50 of 112 mg/kg for hafnium is not much smaller than that for zirconium.

Safety and Health Measures

Fire and explosion. Zirconium metal in the form of a fine powder burns in air, nitrogen or carbon dioxide. The powders are explosive in air in the range of 45 to 300 mg/l, and are self-igniting if disturbed, probably because of static electricity generated by separation of the grains.

The powdered metals should be transported and handled in the wet state; water is usually used for wetting. When the powder is dried prior to use, the quantities employed should be kept as small as possible and operations should be carried out in separate cubicles to prevent propagation in the event of an explosion. All sources of ignition, including static electric charges, should be eliminated from areas in which the powder is to be handled.

All surfaces in the area should be impervious and seamless so that they can be washed down with water and kept completely free from dust. Any spilled powder should be cleaned up immediately with water so that it has no chance to dry in place. Used papers and cloths which have become contaminated with the powders should be kept wet in covered containers until they are removed to be burned, which should be done at least daily. The dried powders should be disturbed and handled as little as possible, and then only with non-sparking tools. Rubber or plastic aprons, if worn over work clothes, should be treated with an anti-static compound. Work clothing should be made from non-synthetic fibres unless effectively treated with antistatic materials.

All processes using zirconium and or hafnium should be designed and ventilated to keep airborne contamination below the exposure limits.

 

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Friday, 11 February 2011 21:59

Zinc

Written by

Gunnar Nordberg

Occurrence and Uses

Zinc (Zn) is widely distributed in nature in quantities which amount to approximately 0.02% of the earth’s crust. It is found in nature as the sulphide (sphalerite), carbonate, oxide or silicate (calamine) in combination with many minerals. Sphalerite, the principal zinc mineral and the source of at least 90% of metallic zinc, contains iron and cadmium as impurities. It is almost always accompanied by galena, the sulphide of lead, and occasionally is found in association with ores containing copper or other base metal sulphides.

On exposure to air, zinc becomes covered with a tenacious film of oxide which protects the metal from further oxidation. This resistance to atmospheric corrosion forms the basis for one of the most common uses of the metal, the protection of steelwork by galvanizing. Zinc’s ability to protect ferrous metals against corrosion is reinforced by electrolytic action. It acts as an anode with respect to iron and other structural metals, except aluminium and magnesium, and is thus preferentially attacked by corrosive agents. This property is used in many other important applications of zinc—for example, in the use of zinc plates as anodes for cathodic protection of ships’ hulls, underground tanks and so on. Zinc metal is die cast for components in the automobile industry, electrical equipment industry, and in the light machine tool, hardware, toys and fancy goods industries. It is rolled into sheets in rolling mills for the manufacture of roofing, weather stripping, cases for dry batteries, printing plates and so on. Zinc is also alloyed with copper, nickel, aluminium and magnesium. When it is alloyed with copper, it forms the important groups of alloys known as the brasses.

Zinc oxide (ZnO), or zinc white (flowers of zinc) is produced by the oxidation of vaporized pure zinc or by the roasting of zinc oxide ore. It is used as a pigment in paints, lacquers and varnishes, as well as a filler for plastics and rubber. Zinc oxide is found in cosmetics, quick-setting cements, and in pharmaceuticals. It is useful in the manufacture of glass, automobile tyres, matches, white glue and printing inks. Zinc oxide is also used as a semiconductor in the electronics industry.

Zinc chromate (ZnCrO4), or zinc yellow, is produced by the action of chromic acid on slurries of zinc oxide, or on zinc hydroxide. It is used in pigments, paints, varnishes and lacquers, and in the manufacture of linoleum. Zinc chromate acts as a corrosion inhibitor for metals and epoxy laminates.

Zinc cyanide (Zn(CN)2) is produced by precipitation of a solution of zinc sulphate or chloride with potassium cyanide. It is used for metal plating and for gold extraction. Zinc cyanide acts as a chemical reagent and as a pesticide. Zinc sulphate (ZnSO4·7H2O), or white vitriol, is produced by roasting zinc blende or by the action of sulphuric acid on zinc or zinc oxide. It is used as an astringent, a preservative for hides and wood, a bleach for paper, a pesticide adjuvant and a fungicide. Zinc sulphate also serves as a fireproofing agent and as a depressant in froth flotation. It is used in water treatment and in textile dyeing and printing. Zinc sulphide is used as a pigment for paints, oilcloths, linoleum, leather, inks, lacquers, and cosmetics. Zinc phosphide (Zn3P2) is produced by passing phosphine through a solution of zinc sulphate. It is used mainly as a rodenticide.

Zinc chloride (ZnCl2), or butter of zinc, has numerous uses in the textile industry, including dyeing, printing, sizing and weighting fabrics. It is a component of cement for metals, dentifrices, and soldering fluxes. It is used alone or with phenol and other antiseptics for preserving railway ties. Zinc chloride is useful for glass etching and for the manufacture of asphalt. It is a vulcanizing agent for rubber, a flame retardant for wood, and a corrosion inhibitor in water treatment.

Hazards

Zinc is an essential nutrient. It is a constituent of metalloenzymes, which play an important role in nucleic acid metabolism and protein synthesis. Zinc is not stored in the body, and a minimum daily intake of zinc is recommended by nutritional experts. Absorption of zinc takes place more readily from animal protein sources than from plant products. The phytate content of plants binds zinc, rendering it unavailable for absorption. Zinc deficiency states have been reported from countries where cereals are the major source of protein consumed by the population. Some of the recognized clinical manifestations of chronic zinc deficiency in humans are growth retardation, hypogonadism in males, skin changes, poor appetite, mental lethargy and delayed wound healing.

In general, zinc salts are astringent, hygroscopic, corrosive and antiseptic. Their precipitating action on proteins forms the basis of their astringent and antiseptic effects, and they are absorbed relatively easily through the skin. The taste threshold for zinc salts is approximately 15 ppm; water containing 30 ppm of soluble zinc salts has a milky appearance, and a metallic taste when the concentration reaches 40 ppm. Zinc salts are irritating to the gastrointestinal tract, and the emetic concentrations for zinc salts in water range from 675 to 2,280 ppm.

The solubility of zinc in weakly acidic solutions, in the presence of iron, has led to accidental ingestion of large quantities of zinc salts when acid foods such as fruit drinks were prepared in worn galvanized iron vessels. Fever, nausea, vomiting, stomach cramps and diarrhoea occurred in 20 minutes to 10 hours following ingestion.

A number of zinc salts may enter the body by inhalation, through the skin or by ingestion and produce intoxication. Zinc chloride has been found to cause skin ulcers. A number of zinc compounds present fire and explosion hazards. The electrolytic manufacturing of zinc can produce mists containing sulphuric acid and zinc sulphate that can irritate the respiratory or digestive systems and lead to dental erosion. Metallurgic processes involving zinc can lead to arsenic, cadmium, manganese, lead and possibly chromium and silver exposures, with their associated hazards. Since arsenic is frequently present in zinc, it can be a source of exposure to highly toxic arsine gas whenever zinc is dissolved in acids or alkalis.

In zinc metallurgy and manufacturing, welding and cutting of galvanized or zinc-coated metal, or melting and casting of brass or bronze, the most frequently encountered hazard from zinc and its compounds is exposure to zinc oxide fumes, which cause metal-fume fever. Symptoms of metal-fume fever include shivering attacks, irregular fever, profuse sweating, nausea, thirst, headache, pains in the limbs and a feeling of exhaustion. Attacks are of short duration (most cases are on the way to complete recovery within 24 hours of the onset of symptoms), and tolerance seems to be acquired. A significant increase in free erythrocyte protoporphyrin has been reported in zinc oxide packing operations.

Zinc chloride fumes are irritating to the eyes and mucous membranes. In an accident involving smoke generators, 70 exposed persons experienced varying degrees of irritation of the eyes, nose, throat and lungs. Of the 10 fatalities, some died within a few hours with pulmonary oedema, and others died later of bronchopneumonia. On another occasion, two firemen were exposed to zinc chloride fumes from a smoke generator during a firefighting demonstration, one briefly, the other for several minutes. The former recovered rapidly while the latter died after 18 days, due to respiratory failure. There was a rapid rise of temperature and marked upper respiratory tract inflammation soon after exposure. Diffuse pulmonary infiltrations were seen on the chest radiograph, and autopsy revealed active fibroblastic proliferation and cor pulmonale.

In an experiment primarily designed to evaluate carcinogenesis, groups of 24 mice received 1,250 to 5,000 ppm of zinc sulphate in drinking water for one year. Apart from severe anaemia in animals receiving 5,000 ppm, there were no adverse effects from zinc. Tumour incidence was not significantly different from that seen in the controls.

Zinc phosphide, which is used as a rodenticide, is toxic to humans whether swallowed, inhaled or injected, and, together with zinc chloride, is the most dangerous of the zinc salts; these two substances have been responsible for the only deaths definitely due to zinc poisoning.

Skin effects. Zinc chromate in primer paints used by car-body builders, tinsmiths and steel cupboard makers has been reported to cause nasal ulceration and dermatitis in exposed workers. Zinc chloride has a caustic action, which may result in ulceration of the fingers, hands and forearms of those who handle timber impregnated with it or use it as a flux in soldering. It has been reported that zinc oxide dust may block the ducts of the sebaceous glands and give rise to a papular, pustular eczema in humans packaging this compound.

Safety and Health Measures

Fire and explosion. Finely divided zinc powder, and other zinc compounds, can be fire and explosion hazards if stored in damp places, sources of spontaneous combustion. Residues from reduction reactions may ignite combustible materials. Zinc ammonium nitrate, zinc bromate, zinc chlorate, zinc ethyl, zinc nitrate, zinc permanganate and zinc picrate are all dangerous fire and explosion hazards. In addition, zinc ethyl will ignite spontaneously in contact with air. It should, therefore, be stored in a cool, dry, well-ventilated place away from acute fire risks, open flames and powerful oxidizing agents.

In all cases where zinc is heated to the point where fumes are produced, it is most important to ensure that adequate ventilation is provided. Individual protection is best ensured by education of the worker concerning metal-fume fever and the provision of local exhaust ventilation, or, in some situations, by wearing of a supplied-air hood or mask.

Workers who are none the less exposed to zinc chloride fumes should wear personal protective equipment including protective clothing, chemical eye and face protection and appropriate respiratory protective equipment. Exposure to zinc chloride fumes should be treated by copious irrigation of the exposed areas.

 

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Friday, 11 February 2011 21:58

Vanadium

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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.

Hazards

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.

 

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Friday, 11 February 2011 21:56

Tungsten

Written by

Gunnar Nordberg

Occurrence and Uses

Tungsten (W) never occurs free in nature and is found only in a few minerals as tungstate of calcium, iron or manganese. Of the known tungsten-bearing minerals, scheelite (CaWO4), wolframite ((Fe,Mn)WO4), hubnerite (MnWO) and ferberite (FeWO4) are commercially important. Total world reserves of tungsten trioxide (WO3 ) are estimated to be about 175,000,000 t. These tungsten minerals are mostly mined from underground workings, but open-cut operations and more primitive methods are also applied. The tungsten content of the ore mined is usually 0.5 to 2.0%. The more common impurities are gangue minerals such as quartz and calcite, and metallic minerals of copper, bismuth, tin and molybdenum.

Tungsten is a component in hard metals. It is used to increase the hardness, toughness, elasticity and tensile strength of steel. It is used in the production of tungsten steels for automobiles and high-speed cutting tools. Tungsten is also used in lamps, vacuum tubes, electric contacts, x-ray tubes and fluorescent light tubes. It serves as a flame retardant in the textile industry.

Tungsten carbide (WC) has replaced diamond in large drawing dies and rock drills because of its extreme hardness. Tungsten compounds are also used in lasers, dyes, inks and ceramic frits. Some tungsten alloys are used in the nuclear and space industries for nozzles of rocket motors and for protecting shields for spacecraft.

Hazards

Little is known of the toxicity of tungsten. The LD50 of sodium tungstate for 66-day-old rats was between 223 and 255 mg/kg and showed significant postprandial and age effect. Of three tungsten compounds, sodium tungstate is most toxic, tungstic oxide is intermediate, and ammonium paratungstate is least toxic. The feeding of 2.5 and 10% of diet as tungsten metal over a period of 70 days has been shown to be without marked effect upon the growth of male rats, as measured in terms of gain in weight, though it caused a 15% reduction in weight gain for female rats from that of control.

Industrial exposure is related chiefly to substances associated with the production and uses of tungsten, its alloys and compounds, rather than tungsten itself. In the mining and milling processes, the main hazards seem to be exposure to quartz-containing dust, noise, hydrogen sulphide, sulphur dioxide and chemicals such as sodium cyanide and sodium hydroxide. The exposure may be associated with other metals in the ore, such as nickel.

Hard metal is the mixture of tungsten carbide and cobalt, to which small amounts of other metals may be added. In the tool-cutting industry workers may be exposed to dust of tungsten carbide, cobalt fumes and dust, and carbides of nickel, titanium and tantalum. Following occupational exposure to tungsten carbide dust by inhalation, cases of pneumoconiosis or pulmonary fibrosis have been reported, but it is generally agreed that this “hard-metal disease” is more likely to be caused by the cobalt with which tungsten carbide is fused. Where machining and grinding of tungsten carbide tools is performed, the hard-metal workers may be at risk for the development of interstitial obstructive lung disease, a serious hazard associated with elevated air concentrations of cobalt. The effects of hard metals on the lungs are discussed elsewhere in this Encyclopaedia.

Tungsten carbonyl is a moderate fire hazard when exposed to flame. When heated to decomposition, it emits carbon monoxide. The incidence of accidents and diseases in tungsten mines and mills is not well documented. However, from the scarce data available it can be said that it is less than that of coal mines.

 

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Friday, 11 February 2011 21:55

Titanium

Written by

Gunnar Nordberg

Occurrence and Uses

Titanium (Ti) is contained in many minerals, but only a few of them are of industrial significance. These include ilmenite (FeTiO3), which contains 52.65% Ti and 47.4% FeO; rutile (TiO2), with admixtures of ferrous oxide; perovskite (CaTiO3), which contains 58.7% TiO2 and 41.3% CaO; and sphene, or titanite, (CaOTiO2·SiO2), which contains 38.8% TiO2. Some heterogeneous minerals, such as loparite, pyrochlor, and tailings from bauxite and copper ore processing may also be sources of titanium.

Titanium is used as a pure metal, in alloys, and in the form of various compounds. The bulk of titanium is needed in the iron and steel industry, in shipbuilding, for aircraft and rocket construction, and for the fabrication of chemical plants. Titanium is used as a protective surface on mixers in the pulp and paper industry. It is also found in surgical appliances. Titanium has been employed for the manufacture of electrodes, lamp filaments, paints, dyes and welding rods. Titanium powder is used in pyrotechnics and in vacuum engineering. Titanium is also used in dentistry and in surgery for implants or prostheses.

Titanium carbide and titanium nitride are used in powder metallurgy. Barium titanate is used for making heavy-duty capacitors. Titanium dioxide is utilized as a white pigment in paints, floor coverings, upholstery, electronics, adhesives, roofing, plastics and in cosmetics. It is also useful as a component of porcelain enamels and glazes, as a shrinking agent for glass fibres, and as a delustering agent for synthetic fibre. Titanium tetrachloride acts as an intermediate in the production of titanium metal and titanium pigments, and as a catalyst in the chemical industry.

Hazards

The formation of titanium dioxide (TiO2) and concentrate dust, pitch briquette dust arising from crushing, mixing and charging of bulk raw materials, and radiant heat from coking furnaces are hazards in titanium production. There may be chlorine, titanium tetrachloride (TiCl4) vapours and their pyrolysis products in the air of the chlorination and rectification plants, arising from leaking or corroded equipment. Magnesium oxide may be present in the air of the reduction area. Titanium dust becomes airborne when titanium sponge is knocked out, crushed, separated and bagged. Exposure to heat and infrared radiation occurs in the arc furnace area (up to 3 to 5 cal/cm2 per min).

Maintenance and repair of the chlorination and rectification installations, which includes disassembly and cleaning of the equipment and pipework, create particularly adverse conditions of work: high concentrations of TiCl4 vapours and hydrolysis products (HCl, Ti(OH)4), which are highly toxic and irritant. Workers in these plants often suffer from upper-airway disease and acute or chronic bronchitis. Liquid TiCl4 splashed on the skin causes irritation and burns. Even very short contact of the conjunctiva with TiCl4 leads to suppurative conjunctivitis and keratitis, which may result in corneal opacities. Animal experiments have shown that dust of metallic titanium, titanium concentrates, titanium dioxide and titanium carbide is slightly toxic. While titanium dioxide has not been found to be fibrogenic in animals, it seems to increase the fibrogenicity of quartz when given as combined exposure. Long-term exposure to titanium-containing dust may result in mild forms of chronic lung disease (fibrosis). There is radiological evidence that workers who have handled TiO2 for long periods develop lung changes resembling those observed in mild forms of silicosis. In one worker who had worked in contact with titanium dioxide for several years and died from brain cancer, the lungs displayed accumulations of TiO2 and changes analogous to anthracosis. Medical examinations of powder metallurgy workers in various countries have disclosed cases of chronic pneumonitis due to mixed dust including titanium carbide. The degree of this disease varied according to conditions of work, length of dust exposure and individual factors.

Workers who have been chronically exposed to titanium and titanium dioxide dust show a high incidence of chronic bronchitis (endobronchitis and peribronchitis). The early stages of the disease are characterized by impaired pulmonary respiration and ventilatory capacity, and by reduced blood alkalinity. Electrocardiographic tracings of these titanium workers revealed cardiac changes characteristic of pulmonary disease with hypertrophy of the right auricle. A considerable number of these cases presented myocardial hypoxia of various degrees, inhibited atrioventricular and intraventricular conductivity, and bradycardia.

Airborne metallic titanium dust is explosive.

Other hazards in titanium production are carbon monoxide exposures at the coking and arc furnaces, and burns.

Safety and Health Measures

Control dust during ore crushing by moistening the material to be processed (up to 6 to 8% moisture content), and by adopting a continuous process, which enables the equipment to be enclosed with exhaust devices at all points where dust may form; the dust-laden air exhausted should be filtered and the dust collected should be recycled. Dust exhaust systems must be provided at the knock-out stations; crushers, separators and baggers in the titanium sponge plant. Knocking out with pneumatic chipping hammers should be replaced by machining out on special milling or turning machines.

 

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Friday, 11 February 2011 21:48

Tin

Written by

Gunnar Nordberg

Tin has been used through the ages up to modern industrial times because it is pliable and easily shaped at normal temperatures, and it mixes readily with other metals to form alloys. One of its outstanding characteristics is its resistance to acids and atmospheric influences.

Occurrence and Uses

Although deposits of tin are widely distributed throughout the world, up to the eighteenth century the world’s supply of tin was mainly from England, Saxony and Bohemia. Today, except for some deposits in Nigeria, China, the Congo and Australia, the principal sources are found in Southeast Asia and Bolivia.

Of minerals containing tin, cassiterite (SnO2) or tinstone is of the greatest commercial importance. It is present in veins closely connected with granite or acid eruptive rocks, but five-sixths of the world’s total production is derived from secondary alluvial deposits resulting from the disintegration of the primary deposits. In Bolivia, sulphide ores, such as stannite (Cu2FeSnS2) and tealite (PbZnSnS2) are of commercial significance.

Metallic tin is used for Babbitt type metals and for collapsible tubes in the pharmaceutical and cosmetic industries. Because of its resistance to corrosion, tin is used as a protective coating for other metals. Tinplate is sheet iron or steel which has been thickly coated with tin by dipping in a molten bath of that metal. It is used mainly for making household utensils and for utensils in food and beverage canning industries. It is often used for decorating purposes. Terneplate is sheet iron or steel coated with a lead-tin alloy containing 85% lead and 15% tin. It is used mainly for making roofing tile. Speculum is a tin-copper alloy containing 33 to 50% tin, that can be polished to a high degree of reflection. It is used as a coating applied by electrolytic deposition to impart brightness to silverware and similar articles, and for making telescope mirrors. A molten tin bath is also used in the production of window glass.

An important property of tin is its ability to form alloys with other metals, and it has a number of uses in this field. A tin-lead alloy known as soft solder is widely used for joining other metals and alloys in the plumbing, automobile, electrical and other industries, and as a filler in the finishing of car bodies. Tin is a constituent of a large number of non-ferrous alloys, including phosphor bronze, light brass, gun-metal, high-tensile brass, manganese bronze, die-casting alloys, bearing metals, type metal and pewter. The tin-niobium alloy is superconductive, and it is used in the manufacture of powerful electromagnets.

Stannic chloride (SnCl4), or tin chloride, is prepared by heating powdered tin with mercuric chloride or by passing a stream of chlorine over molten tin. It is used as a dehydrating agent in organic syntheses, a stabilizer for plastics, and as a chemical intermediate for other tin compounds. Stannic chloride is found in colours and perfumes in the soap industry. It is also employed in ceramics to produce abrasion-resistant or light-reflecting coatings. It is used for the bleaching of sugar and for the surface treatment of glass and other non-conductive materials. The pentahydrate of this salt is used as a mordant. It is also used in treating silk for the purpose of giving weight to the fabric.

Stannous chloride dihydrate (SnCl2·2H2O), or tin salt, is produced by dissolving metallic tin in hydrochloric acid and evaporating until crystallization begins. It is used in dye works as a mordant. It also serves as a reducing agent in the manufacture of glass, ceramics and inks.

The use of organotin (alkyl and aryl) compounds has greatly increased in recent years. Disubstituted compounds and, to a lesser degree, monosubstituted compounds, are used as stabilizers and catalysts in the plastics industry. Trisubstituted compounds are used as biocides, and tetrasubstitutes are intermediates in the production of other derivatives. Butyltin trichloride, or trichlorobutyltin; dibutyltin dichloride, or dichlorodibutyltin; trimethyltin; triethyltin chloride; triphenyltin chloride, or TPTC; tetraisobutyltin, or tetraisobutylstannane are among the most important.

Hazards

In the absence of precautions, mechanical injury can be caused by the heavy, powerful plant and machinery used in the dredging and washing operations. Serious burn hazards are present in the smelting processes when molten metal and hot slags are manipulated.

At the final stage of upgrading of cassiterite concentrate and during the roasting of sulphide ore, sulphur dioxide is evolved. Sulphur dioxide and stannous sulphide constitute a hazard when the rough molten tin is separated from the rest of the charge during refining. This work is done in a very hot environment, and heat exhaustion could arise. The noise on a dredger caused by the discharge from the dredging buckets to the primary washing plant may cause damage to the hearing of the workers.

Several studies report the hazards associated with exposure to radon, radon decay products and silica in tin mines. While most of the operations associated with the extraction and treatment of tin ore are wet processes, tin dust and oxide fumes may escape during bagging of concentrate, in ore rooms and during smelting operations (mixing-plant and furnace tapping), as well as during the periodic cleaning of bag filters used to remove particulate matter from smelter furnace flue gas before release to the atmosphere. The inhalation of tin oxide dust without silica leads to a benign nodular pneumoconiosis without pulmonary disability. The radiological picture is similar to baritosis. This benign pneumoconiosis has been called stannosis.

Tin powder is a moderate irritant to the eyes and airways; it is combustible and reacts violently with oxidants, strong acids, powdered sulphur and some extinguishing agents such as bicarbonate powder and carbon dioxide.

Tin ingested in small (mg) quantities is non-toxic (hence, the widespread use of tinplate in the food canning industry). The results of animal experiments indicate that the lethal dose by intravenous injection is about 100 mg/kg body weight, and that the ingestion of considerable quantities of powdered tin may cause vomiting but not permanent injury. It appears that humans can tolerate a daily intake of 800 to 1,000 mg without ill effect. The absorption of metallic tin or its inorganic salts from the alimentary tract seems to be small.

A number of tin alloys are injurious to health (particularly at high temperatures) because of the harmful characteristics of the metals with which may be alloyed (e.g., lead, zinc, manganese).

Organotin compounds are, in general, strong irritants, and acute conjunctivitis has been observed as a result of eye splashes, even when followed by immediate lavage; corneal opacities have also been reported. Prolonged contact of the skin with clothes moistened with vapour, or direct spillage on the skin, have been responsible for acute local burns, subacute diffuse erythematoid dermatitis with pruritus and some pustular eruption in the hair-covered areas. The irritation of the airways and pulmonary tissue can lead to lung oedema; the gastrointestinal tract can also be involved, and inflammatory reactions of the bile duct have been observed, mainly with the dialkyl compounds. Organotin compounds can injure liver and kidneys; they can depress the immune response and have haemolytic activity. In experimental animals they have been in some instances held responsible for reduction in fertility.

Tri- and tetralkyl compounds, in particular triethyltin chloride, cause encephalopathy and brain oedema, with clinical effects of depression, convulsions, flaccid paralysis and urinary retention, as seen in therapeutic use following oral administration.

Safety and Health Measures

Wherever possible, safer substitutes should be used in the place of alkyl tin compounds. When it is necessary to make and use them, the widest possible use should be made of enclosed systems and exhaust ventilation. Engineering control should ensure that exposure limits are not exceeded. Personal protective equipment should be worn, and in appropriate circumstances respiratory protection should be used. Emergency showers should be installed at workplaces in order to allow workers to wash immediately after splashes.

Medical surveillance should focus on eyes, skin and chest x rays in the exposure to inorganic tin compounds, and on eyes, skin, central nervous system, liver and kidney function, and blood in the exposure to organic tin compounds. Mercaprol has been reported as useful in the treatment of dialkyltin intoxications. Steroids have been suggested for the treatment of triethyltin poisoning; however only surgical decompression seems to be of value in encephalopathy and brain oedema provoked by tri- and tetraalkyl tin compounds.

Taking into consideration the fact that most tin mines are located in developing countries, attention should also be paid to climatic and other factors influencing the health, well-being and productive capacity of the workers. Where mines are geographically isolated, good housing should be provided for all personnel. Nutritional standards should be upgraded by health education, and workers should be provided with adequate food supplies and good medical care.

 

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Friday, 11 February 2011 21:47

Thallium

Written by

Gunnar Nordberg

Occurrence and Uses

Thallium (Tl) is fairly widely distributed in the earth’s crust in very low concentrations; it is also found as an accompanying substance of other heavy metals in pyrites and blendes, and in the manganese nodules on the ocean floor.

Thallium is used in the manufacture of thallium salts, mercury alloys, low-melting glasses, photoelectric cells, lamps and electronics. It is used in an alloy with mercury in low-range glass thermometers and in some switches. It has also been used in semiconductor research and in myocardial imaging. Thallium is a catalyst in organic synthesis.

Thallium compounds are used in infrared spectrometers, crystals and other optical systems. They are useful for colouring glass. While many thallium salts have been prepared, few are of commercial significance.

Thallium hydroxide (TlOH), or thallous hydroxide, is produced by dissolving thallium oxide in water, or by treating thallium sulphate with barium hydroxide solution. It can be used in the preparation of thallium oxide, thallium sulphate or thallium carbonate.

Thallium sulphate (Tl2SO4), or thallous sulphate, is produced by dissolving thallium in hot concentrated sulphuric acid or by neutralizing thallium hydroxide with dilute sulphuric acid, followed by crystallization. Because of its outstanding efficacy in the destruction of vermin, particularly rats and mice, thallium sulphate is one of the most important of the thallium salts. However, some western European countries and the United States have prohibited the use of thallium on the grounds that it is inadvisable that such a toxic substance should be easily obtainable. In other countries, following the development of warfarin resistance in rats, the use of thallium sulphate has increased. Thallium sulphate is also used in semiconductor research, optical systems and in photoelectric cells.

Hazards

Thallium is a skin sensitizer and cumulative poison which is toxic by ingestion, inhalation or skin absorption. Occupational exposure may occur during the extraction of the metal from thallium-bearing ores. Inhalation of thallium has resulted from the handling of flue dusts and the dusts from roasting of pyrites. Exposure may also occur during the manufacture and use of thallium-salt vermin exterminators, the manufacture of thallium-containing lenses and separation of industrial diamonds. The toxic action of thallium and its salts is well documented from reports of cases of acute non-occupational poisoning (not infrequently fatal) and from instances of suicidal and homicidal use.

Occupational thallium poisoning is normally the result of moderate, long-term exposure, and the symptoms are usually far less marked than those observed in acute accidental, suicidal or homicidal intoxication. The course is usually unremarkable and characterized by subjective symptoms such as asthenia, irritability, pains in the legs, some nervous system disorders. Objective symptoms of polyneuritis may not be demonstrable for quite some time. The early neurologic findings include changes in the superficially provoked tendon reflexes and a pronounced weakness and fall-off in the speed of pupil reflexes.

The victim’s occupational history will usually give the first clue to the diagnosis of thallium poisoning since a considerable time may elapse before the rather vague initial symptoms are replaced by the polyneuritis followed by loss of hair. Where massive hair loss occurs, the likelihood of thallium poisoning is readily suspected. However, in occupational poisoning, where exposure is usually moderate but protracted, the loss of hair may be a late symptom and often noticeable only after the appearance of polyneuritis; in cases of slight poisoning, it may not occur at all.

The two principal criteria for the diagnosis of occupational thallium poisoning are:

  1. occupational history which shows that the patient has or may have been exposed to thallium in such work as rodenticide handling, thallium, lead, zinc or cadmium production, or the production or use of various thallium salts
  2. neurological symptoms, dominated initially by subjective changes in the form of paraesthesia (both hyperaesthesia and hypoaesthesia) and, subsequently, by reflex changes.

 

Concentrations of Tl in urine above 500 mg/l have been associated with clinical poisoning. At concentrations of 5 to 500 µg/l the magnitude of risk and severity of adverse effects on humans are uncertain.

Long-term experiments with radioactive thallium have shown marked excretion of thallium in both urine and faeces. On autopsy, the highest thallium concentrations are found in the kidneys, but moderate concentrations may also be present in the liver, other internal organs, muscles and bones. It is striking that, although the principal signs and symptoms of thallium poisoning originate from the central nervous system, only very low concentrations of thallium are retained there. This may be due to extreme sensitivity to even very small amounts of the thallium acting on the enzymes, the transmission substances, or directly on the brain cells.

Safety and Health Measures

The most effective measure against the dangers associated with the manufacture and use of this group of extremely toxic substances is the substitution of a less harmful material. This measure should be adopted wherever possible. When thallium or its compounds must be used, the strictest safety precautions should be taken to ensure that the concentration in the workplace air is kept below permissible limits and that skin contact is prevented. Continuous inhalation of such concentrations of thallium during normal working days of 8 hours may cause the urine level to exceed the above permissible levels.

Persons involved in work with thallium and its compounds should wear personal protective equipment, and respiratory protective equipment is essential where there is the possibility of dangerous inhalation of airborne dust. A complete set of working clothes is essential; these clothes should be washed regularly and kept in accommodation separate from that employed for ordinary clothes. Washing and shower facilities should be provided and scrupulous personal hygiene encouraged. Workrooms must be kept scrupulously clean, and eating, drinking or smoking at the workplace prohibited.

 

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Friday, 11 February 2011 21:45

Tellurium

Written by

Gunnar Nordberg

Tellurium (Te) is a heavy element with the physical properties and silvery lustre of a metal, yet with the chemical properties of a non-metal such as sulphur or arsenic. Tellurium is known to exist in two allotropic forms—the hexagonal crystalline form (isomorphous with grey selenium) and an amorphous powder. Chemically, it resembles selenium and sulphur. It tarnishes slightly in air, but in the molten state it burns to give the white fumes of tellurium dioxide, which is only sparingly soluble in water.

Occurrence and Uses

The geochemistry of tellurium is imperfectly known; it is probably 50 to 80 times more rare than selenium in the lithosphere. It is, like selenium, a by-product of the copper-refining industry. The anodic slimes contain up to 4% tellurium.

Tellurium is used to improve the machinability of “free-cutting” copper and certain steels. The element is a powerful carbide stabilizer in cast irons, and it is used to increase the depth of chill in castings. Additions of tellurium improve the creep strength of tin. The chief use of tellurium is, however, in the vulcanizing of rubber, since it reduces the time of curing and endows the rubber with increased resistance to heat and abrasion. In much smaller quantities, tellurium is used in pottery glazes and as an additive to selenium in metal rectifiers. Tellurium acts as a catalyst in some chemical processes. It is found in explosives, antioxidants and in infrared-transmitting glasses. Tellurium vapour is used in “daylight lamps”, and tellurium-radioiodinated fatty acid (TPDA) has been used for myocardial scanning.

Hazards

Cases of acute industrial poisoning have occurred as a result of metallic tellurium fumes being absorbed into the lungs.

A study of foundry workers throwing tellurium pellets by hand into molten iron with the emanation of dense white fumes showed that persons exposed to tellurium concentrations of 0.01 to 0.74 mg/m3 had higher urinary tellurium levels (0.01 to 0.06 mg/l) than workers exposed to concentrations of 0.00 to 0.05 mg/m3 (urinary concentrations of 0.00 to 0.03 mg/l). The most common sign of exposure was a garlic odour of the breath (84% of cases) and a metallic taste in the mouth (30% of cases). Workers complained of somnolence in the afternoons and loss of appetite, but suppression of sweat did not occur; blood and central nervous system test results were normal. One worker still had a garlic odour in his breath and tellurium in the urine after being away from the work for 51 days.

In laboratory workers who were exposed to fumes of melting tellurium-copper (fifty/fifty) alloy for 10 min, there were no immediate symptoms, but the effects of stinking breath were pronounced. Since tellurium forms a sparingly soluble oxide with no acidic reaction, there is no danger to the skin or to the lungs from tellurium dust or fumes. The element is absorbed through the gastrointestinal tract and lungs, and excreted in the breath, faeces and urine.

Tellurium dioxide (TeO2), hydrogen telluride (H2Te) and potassium tellurite (K2TeO3) are of industrial health significance. Because tellurium forms its oxide over 450 ºC and the dioxide formed is almost insoluble in water and body fluids, tellurium appears to be less of an industrial hazard than is selenium.

Hydrogen telluride is a gas which decomposes slowly to its elements. It has a similar smell and toxicity to hydrogen selenide, and is 4.5 times heavier than air. There have been reports that hydrogen telluride causes irritation to the respiratory tract.

One unique case is reported in a chemist who was admitted to hospital after accidently inhaling tellurium hexafluoride gas whilst engaged on making the tellurium esters. Streaks of blue-black pigmentation below the skin surface were seen on the webs of his fingers and to a lesser degree on his face and neck. The photographs show very clearly this rare example of true skin absorption by a tellurium ester, which was reduced to black elemental tellurium during its passage through the skin.

Animals exposed to tellurium have developed central nervous system and red blood cell effects.

Safety and Health Measures

Where tellurium is being added to molten iron, lead or copper, or being vaporized onto a surface under vacuum, an exhaust system should be installed with a minimum air speed of 30 m/min to control vapour emission. Tellurium should preferably be used in pellet form for alloying purposes. Routine atmospheric determinations should be made to ensure that the concentration is maintained below the recommended levels. Where no specific permissible concentration is given for hydrogen telluride; however, it is considered advisable to adopt the same level as for hydrogen selenide.

Scrupulous hygiene should be observed in tellurium processes. Workers should wear white coats, hand protection and simple gauze mask respiratory protection if handling the powder. Adequate sanitary facilities must be provided. Processes should not require hand grinding, and well-ventilated mechanical grinding stations should be used.

 

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Friday, 11 February 2011 21:44

Tantalum

Written by

Gunnar Nordberg

Occurrence and Uses

Tantalum (Ta) is obtained from the ores tantalite and columbite, which are mixed oxides of iron, manganese, niobium and tantalum. Although they are considered rare elements, the earth’s crust contains about 0.003% of niobium and tantalum together, which are similar chemically and usually occur in combination.

The chief use of tantalum is in the production of electric capacitators. Tantalum powder is compacted, sintered and subjected to anodic oxidation. The film of oxide on the surface serves as an insulator, and upon introduction of an electrolyte solution, a high-performance capacitator is obtained. Structurally, tantalum is used where its high melting point, high density and resistance to acids are advantageous. The metal is employed widely in the chemical industry. Tantalum has also been used in rectifiers for railway signals, in surgery for suture wire and for bone repair, in vacuum tubes, furnaces, cutting tools, prosthetic appliances, fibre spinnerets and in laboratory ware.

Tantalum carbide is used as an abrasive. Tantalum oxide finds use in the manufacture of special glass with a high index of refraction for camera lenses.

Hazards

Metallic tantalum powder presents a fire and explosion hazard, although not as serious as that of other metals (zirconium, titanium and so on). The working of tantalum metal presents the hazards of burns, electric shock, and eye and traumatic injuries. Refining processes involve toxic and hazardous chemicals such as hydrogen fluoride, sodium and organic solvents.

Toxicity. The systemic toxicity of tantalum oxide, as well as that of metallic tantalum, is low, which is probably due to its poor solubility. It does, however, represent a skin, eye and respiratory hazard. In alloys with other metals such as cobalt, tungsten and niobium, tantalum has been attributed an aetiological role in hard-metal pneumoconiosis and in skin affections caused by hard-metal dust. Tantalum hydroxide was found to be not highly toxic to chick embryos, and the oxide was non-toxic to rats by intraperitoneal injection. Tantalum chloride, however, had an LD50 of 38 mg/kg (as Ta) while the complex salt K2TaF7 was about one-fourth as toxic.

Safety and Health Measures

In most operations, general ventilation can maintain the concentration of the dust of tantalum and its compounds below the threshold limit value. Open flames, arcs and sparks should be avoided in areas where tantalum powder is handled. If workers are regularly exposed to dust concentrations approaching the threshold limit level, periodic medical examinations, with emphasis on pulmonary function, are advisable. For operations involving fluorides of tantalum, as well as hydrogen fluoride, the precautions applicable to these compounds should be observed.

Tantalum bromide (TaBr5), tantalum chloride (TaCl5) and tantalum fluoride (TaF5) should be kept in tightly stoppered bottles which are plainly labelled and stored in a cool, ventilated place, away from compounds which are affected by acids or acid fumes. Personnel involved should be cautioned about their hazards.

 

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Friday, 11 February 2011 21:42

Silver

Written by

Gunnar Nordberg

Occurrence and Uses

Silver (Ag) is found throughout the world, but most of it is produced in Mexico, the western United States, Bolivia, Peru, Canada and Australia. Much of it is obtained as a by-product from argentiferous lead, zinc and copper ores in which it occurs as the silver sulphide, argentite (Ag2S). It is also recovered during the treatment of gold ores and is an essential constituent of the gold telluride, calaverite ((AuAg)Te2).

Because pure silver is too soft for coins, ornaments, cutlery, plate and jewellery, silver is hardened by alloying with copper for all these applications. Silver is extremely resistant to acetic acid and, therefore, silver vats are used in the acetic acid, vinegar, cider and brewing industries. Silver is also used in busbars and windings of electrical plants, in silver solders, dental amalgams, high-capacity batteries, engine bearings, sterling ware and in ceramic paints. It is employed in brazing alloys and in the silvering of glass beads.

Silver finds use in the manufacture of formaldehyde, acetaldehyde and higher aldehydes by the catalytic dehydrogenation of the corresponding primary alcohols. In many installations, the catalyst consists of a shallow bed of crystalline silver of extremely high purity. An important use of silver is in the photography industry. It is the unique and instantaneous reaction of the halides of silver on exposure to light that makes the metal virtually indispensable for films, plates and photographic printing paper.

Silver nitrate (AgNO3) is used in photography, the manufacture of mirrors, silver plating, dyeing, colouring of porcelain, and etching ivory. It is an important reagent in analytical chemistry and a chemical intermediate. Silver nitrate is found in sympathetic and indelible inks. It also serves as a static inhibitor for carpets and woven materials and as a water disinfectant. For medical purposes silver nitrate has been used for the prophylaxis of ophthalmia neonatorum. It has been utilized as an antiseptic, an astringent, and in veterinary use for the treatment of wounds and local inflammations.

Silver nitrate is a powerful oxidizing agent and a fire hazard, in addition to being strongly caustic, corrosive and poisonous. In the form of dust or a solid it is dangerous to the eyes, causing burns of the conjunctiva, argyria and blindness.

Silver oxide (Ag2O) is used in the purification of drinking water, for polishing and colouring glass yellow in the glass industry, and as a catalyst. In veterinary medicine, it is used as an ointment or solution for general germicidal and parasiticidal purposes. Silver oxide is a powerful oxidizing material and a fire hazard.

Silver picrate ((O2N)3C6H2OAg·H2O) is used as a vaginal antimicrobial. In veterinary medicine it is used against granular vaginitis for cattle. It is highly explosive and poisonous.

Hazards

Silver exposure may lead to a benign condition called “argyria”. If the dust of the metal or its salts is absorbed, silver is precipitated in the tissues in the metallic state and cannot be eliminated from the body in this state. Reduction to the metallic state takes place either by the action of light on the exposed parts of the skin and visible mucous membranes, or by means of hydrogen sulphide in other tissues. Silver dusts are irritants and can lead to ulceration of the skin and nasal septum.

Occupations involving the risk of argyria can be divided into two groups:

  1. workers who handle a compound of silver, either the nitrate, fulminate or cyanide, which, broadly speaking, giving rise to generalized argyria from inhalation and ingestion of the silver salt concerned
  2. workers who handle metallic silver, small particles of which accidentally penetrate the exposed skin, giving rise to local argyria by a process equivalent to tattooing.

 

Generalized argyria is unlikely to occur at respirable silver concentrations in air of 0.01 mg/m3 or at oral cumulative doses lower than 3.8 g. Persons affected by generalized argyria are often called “blue men” by their fellow workers. The face, forehead, neck, hands and forearms develop a dark slatey-grey colour, uniform in distribution and varying in depth depending on the degree of exposure. Pale scars up to about 6 mm across may be found on the face, hands and forearms due to the caustic effects of silver nitrate. The fingernails are a deep chocolate-brown colour. The buccal mucosa is slatey-grey or bluish in colour. Very slight pigmentation may be detected in the covered parts of the skin. The toenails may show a slight bluish discolouration. In a condition called argyrosis conjunctivae, the colour of the conjunctivae varies from a slight grey to a deep brown, the lower palpebral portion being particularly affected. The posterior border of the lower lid, the caruncle and the plica semilunaris are deeply pigmented and may be almost black. Examination by means of the slit-lamp reveals a delicate network of faint grey pigmentation in the posterior elastic lamina (Descemet’s membrane) of the cornea, known as argyrosis corneae. In cases of long duration, argyrolentis is also found.

Where persons work with metallic silver, small particles may accidentally penetrate the exposed skin surface, giving rise to small pigmented lesions by a process equivalent to tattooing. This may occur in occupations involving the filing, drilling, hammering, turning, engraving, polishing, forging, soldering and smelting of silver. The left hand of the silversmith is more affected than the right, and the pigmentation occurs at the site of injuries from instruments. Many instruments, such as engraving tools, files, chisels and drills, are sharp and pointed and are liable to produce skin wounds. The piercing saw, an instrument resembling a fret saw, may break and run into the worker’s hand. If the file slips, the worker’s hand may be injured on the silver article; this is especially the case with the prongs of forks. A worker drawing silver wire through a hole in a silver draw-plate may get splinters of silver in his or her fingers. The pigmented points vary from tiny specks to areas 2 mm or more in diameter. They may be linear or rounded and in varying shades of grey or blue. The tattoo marks remain for life and cannot be removed. The use of gloves is usually impractical.

Safety and Health Measures

In addition to the engineering measures necessary to keep the airborne concentrations of silver fumes and dust as low as possible and in any case below the exposure limits, medical precautions for preventing argyria have been recommended. These include, in particular, the periodic medical examination of the eye, because the discolouration of the Descemet’s membrane is an early sign of the disease. Biological monitoring seems to be possible via the faecal excretion of silver. There is no recognized effective treatment of argyria. The condition seems to stabilize when exposure to silver is discontinued. Some clinical improvement has been achieved by use of chelating agents and intradermal injection of sodium thiosulphate or potassium ferrocyanide. Sun exposure should be avoided to prevent further discolouration of the skin.

The main incompatibilities of silver with acetylene, ammonia, hydrogen peroxide, ethyleneimine and a number of organic acids should be kept in mind in order to prevent fire and explosion hazards.

The most unstable silver compounds, such as silver acetylide, silver ammonium compounds, silver azide, silver chlorate, silver fulminate and silver picrate, should be kept in cool, well-ventilated places, protected from shock, vibration and contamination by organic or other readily oxidizable materials and away from light.

When working silver nitrate, personal protection should include the wearing of protective clothing to avoid skin contact as well as chemical safety goggles for the protection of the eyes where spillage may occur. Respirators should be available at workplaces in which engineering control cannot maintain an acceptable environment.

 

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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
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