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Friday, 11 February 2011 21:45

Tellurium

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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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