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.
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.