Friday, 11 February 2011 21:31

Rhodium

Written by

Gunnar Nordberg

Occurrence and Uses

Rhodium is one of the rarest elements in the Earth’s crust (average concentration 0.001 ppm). It is found in small quantities associated with native platinum and some copper-nickel ores. It occurs in the minerals rhodite, sperrylite and iridosmine (or osmiridium).

Rhodium is used in corrosion-resistant electroplates for protecting silverware from tarnishing and in high-reflectivity mirrors for searchlights and projectors. It is also useful for plating optical instuments and for furnace winding. Rhodium serves as a catalyst for various hydrogenation and oxidation reactions. It is used for spinnerets in rayon production and as an ingredient in gold decorations on glass and porcelain.

Rhodium is alloyed with platinum and palladium to make very hard alloys for use in spinning nozzles.

Hazards

There have been no significant experimental data indicating health problems with rhodium, its alloys or its compounds in humans. Although toxicity is not established, it is necessary to handle these metals carefully. Contact dermatitis in a worker who prepared pieces of metal for plating with rhodium has been reported. The authors argue that the small number of reported cases of sensitization to rhodium may reflect the rarity of use rather than the safety of this metal. The American Conference of Governmental Industrial Hygienists (ACGIH) has recommended a low threshold limit value for rhodium and its soluble salts, based on analogy with platinum. The ability of soluble salts of rhodium to give rise to allergic manifestations in humans has not been completely demonstrated.

 

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Friday, 11 February 2011 21:33

Ruthenium

Written by

Gunnar Nordberg

Occurrence and Uses

Ruthenium is found in the minerals osmiridium and laurite, and in platinum ores. It is a rare element comprising about 0.001 ppm of the Earth’s crust.

Ruthenium is used as a substitute for platinum in jewellery. It is utilized as a hardener for pen nibs, electrical contact relays and electrical filaments. Ruthenium is also used in ceramic colours and in electroplating. It acts as a catalyst in the synthesis of long-chain hydrocarbons. In addition, ruthenium has been used recently in treating eye uveal malignant melanomas.

Ruthenium forms useful alloys with platinum, palladium, cobalt, nickel and tungsten for better wear resistance. Ruthenium red (Ru3Cl6H42N4O2) or ruthenium oxychloride ammoniated is used as a microscopy reagent for pectin, gum, animal tissues and bacteria. Ruthenium red is an eye inflammatory agent.

Hazards

Ruthenium tetraoxide is volatile and irritating to the respiratory tract.

Some ruthenium electroplating complexes may be skin and eye irritants, but documentation of this is lacking. Ruthenium radioisotopes, chiefly 103Ru and 106Ru, occur as fission products in the nuclear fuel cycle. Since ruthenium may transform to volatile compounds (it forms numerous nitrogen complexes as noted above), there has been concern about its uptake in the environment. The significance of radio-ruthenium as a potential radiation hazard is still largely unknown.

 

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Friday, 11 February 2011 21:34

Selenium

Written by

Gunnar Nordberg

Occurrence and Uses

Selenium (Se) is found in rocks and soils all over the world. There are no true deposits of selenium anywhere, and it cannot economically be recovered directly. Various estimates for selenium in the Earth’s crust range from 0.03 to 0.8 ppm; the highest concentrations known are in native sulphur from volcanoes, which contains up to 8,350 ppm. Selenium does, however, occur together with tellurium in the sediments and sludges left from electrolytic copper refining. The chief world supplies are from the copper-refining industries of Canada, the United States and Zimbabwe, where the slimes contain up to 15% selenium.

The manufacture of selenium rectifiers, which convert alternating current to direct current, accounts for over half the world’s production of selenium. Selenium is also used for decolourizing green glass and for making ruby glass. It is an additive in the natural and synthetic rubber industries and an insecticide. Selenium is used for alloying with stainless steel and copper.

75Se is used for the radioactive scanning of the pancreas and for photostat and x-ray xerography. Selenium oxide or selenium dioxide (SeO2) is produced by burning selenium in oxygen, and it is the most widely used selenium compound in industry. Selenium oxide is employed in the manufacture of other selenium compounds and as a reagent for alkaloids.

Selenium chloride (Se2Cl2) is a dark brownish-red stable liquid which hydrolyses in moist air to give selenium, selenious acid and hydrochloric acid. Selenium hexafluoride (SeF6) is used as a gaseous electric insulator.

Hazards

The elemental forms of selenium are probably completely harmless to humans; its compounds, however, are dangerous and their action resembles that of sulphur compounds. Selenium compounds may be absorbed in toxic quantities through the lungs, intestinal tract or damaged skin. Many selenium compounds will cause intense burns of skin and mucous membranes, and chronic skin exposure to light concentrations of dust from certain compounds may produce dermatitis and paronychia.

The sudden inhalation of large quantities of selenium fumes, selenium oxide or hydrogen selenide may produce pulmonary oedema due to local irritant effects on the alveoli; this oedema may not set in for 1 to 4 hours after exposure. Exposure to atmospheric hydrogen selenide concentrations of 5 mg/m3 is intolerable. However, this substance occurs in only small amounts in industry (for example, due to bacterial contamination of selenium-contaminated gloves), although there have been reports of exposure to high concentrations following laboratory accidents.

Skin contact with selenium oxide or selenium oxychloride may cause burns or sensitization to selenium and its compounds, especially selenium oxide. Selenium oxychloride readily destroys skin on contact, causing third-degree burns unless immediately removed with water. However, selenium oxide burns are rarely severe and, if properly treated, heal without a scar.

Dermatitis due to exposure to airborne selenium oxide dust usually starts at the points of contact of the dust with the wrist or neck and may extend to contiguous areas of the arms, face and upper portions of the trunk. It usually consists of discrete, red, itchy papules which may become confluent on the wrist, where selenium dioxide is liable to penetrate between the glove and sleeve of the overall. Painful paronychia may also be produced. However, one more frequently sees cases of excruciatingly painful throbbing nail beds, due to the selenium dioxide penetrating under the free edge of the nails, in workers handling selenium dioxide powder or waste red selenium fume powder without wearing impermeable gloves.

Splashes of selenium oxide entering the eye may cause conjunctivitis if not treated immediately. Persons who work in atmospheres containing selenium dioxide dust may develop a condition known among the workers as “rose eye”, a pink allergy of the eyelids, which often become puffy. There is usually also a conjunctivitis of the palpebral conjunctiva but rarely of the bulbar conjunctiva.

The first and most characteristic sign of selenium absorption is a garlic odour of the breath. The odour is probably caused by dimethyl selenium, almost certainly produced in the liver by the detoxication of selenium by methylation. This odour will clear quickly if the worker is removed from exposure, but there is no known treatment for it. A more subtle and earlier indication than the garlic odour is a metallic taste in the mouth. It is less dramatic and is often overlooked by the workers. The other systemic effects are impossible to evaluate accurately and are not specific to selenium. They include pallor, lassitude, irritability, vague gastrointestinal symptoms and giddiness.

The possibility of liver and spleen damage in people exposed to high levels of selenium compounds deserves further attention. In addition, more studies of workers are needed to examine the possible protective effects of selenium against lung cancer.

Safety and Health Measures

Selenium oxide is the main selenium problem in industry since it is formed whenever selenium is boiled in the presence of air. All sources of selenium oxide or fumes should be fitted with exhaust ventilation systems with an air speed of at least 30 m/min. Workers should be provided with hand protection, overalls, eye and face protection, and gauze masks. Supplied-air respiratory protective equipment is necessary in cases where good extraction is not possible, such as in the cleaning of ventilation ducts. Smoking, eating and drinking at the workplace should be prohibited, and dining and sanitary facilities, including showers and locker rooms, should be provided at a point distant from exposure areas. Wherever possible, operations should be mechanized, automated or provided with remote control.

 

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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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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 21:45

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 21:47

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 µg/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 21:48

    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 21:55

    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 21:56

    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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    Page 3 of 3

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

    Contents

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

    Metals: Chemical Properties and Toxicity References

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

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

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

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

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

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

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

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