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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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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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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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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