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Friday, 11 February 2011 21:14

Niobium

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Gunnar Nordberg

Occurrence and Uses

Niobium (Nb) is found together with other elements including titanium (Ti), zirconium (Zr), tungsten (W), thorium (Th) and uranium (U) in ores such as tantalite-columbite, fergusonite, samarskite, pyrochlore, koppite and loparite. The largest deposits are located in Australia and Nigeria, and during the last few years extensive deposits have been discovered in Uganda, Kenya, Tanzania and Canada.

Niobium is widely used in the electrovacuum industry and also in the manufacture of anodes, grids, electrolytic condensers and rectifiers. In chemical engineering, niobium is used as a corrosion-proof material for heat exchangers, filters, needle valves and so on. High-quality cutting tools and magnetic materials are made from niobium alloys. Ferroniobium alloy is used in thermonuclear appliances.

Niobium and its refractory alloys are utilized in the field of rocket technology, in the supersonic aircraft industry, interplanetary flight equipment and in satellites. Niobium is also used in surgery.

Hazards

During the mining and concentration of niobium ore and processing of the concentrate, the workers may be exposed to general hazards, such as dust and fumes, which are typical for these operations. In the mines, the action of dust may be aggravated by exposure to radioactive substances such as thorium and uranium.

Toxicity

Much of the information about the behaviour of niobium in the body is based on studies of the radioisotope pair 95Zr-95Nb, a common nuclear fission product. 95Nb is the daughter of 95Zr. One study investigated cancer incidence among niobium mine workers exposed to radon and thoron daughters and found an association between lung cancer and cumulative alpha-radiation.

Intravenous and intraperitoneal injections of niobium (radioactive) and its compounds showed a fairly uniform distribution through the organism, with a tendency to accumulate in the liver, kidneys, spleen and bone marrow. The elimination of radioactive niobium from the organism can be hastened appreciably by the injection of massive doses of zirconium nitrate. After intraperitoneal injections of stable niobium in the form of potassium niobate, the LD50 for rats was 86 to 92 mg/kg and for mice 13 mg/kg. Metallic niobium is not absorbed from the stomach or intestines. The LD50 of niobium pentachloride in these organs was 940 mg/kg for rats, while the corresponding figure for potassium niobate was 3,000 mg/kg. Niobium compounds administered intravenously, intraperitoneally or per os produce a particularly pronounced effect on the kidneys. This effect can be attenuated by preventive medication with ascorbic acid. Oral intake of niobium pentachloride furthermore causes acute irritation of the mucous membranes of the gullet and stomach, and liver changes; chronic exposure during 4 months causes temporary blood changes (leukocytosis, prothrombin deficiency).

Inhaled niobium is retained in the lung, which is the critical organ for dust. Daily inhalation of niobium nitride dust at a concentration of 40 mg/m3 of air leads within a few months to signs of pneumoconiosis (while there are no noticeable signs of toxic action): thickening of the interalveolar septa, development of considerable amounts of collagenous fibres in the peribronchial and perivascular tissue, and desquamation of the bronchial epithelium. Analogous changes develop upon intratracheal administration of niobium pentoxide dust; in this case dust is found even in the lymph nodes.

Safety and Health Measures

Atmospheric concentrations of the aerosols of niobium alloys and compounds that contain toxic elements such as fluorine, manganese and beryllium, should be strictly controlled. During the mining and concentration of niobium ore containing uranium and thorium, the worker should be protected against radioactivity. Proper engineering design including adequate ventilation with fresh air is necessary to control dust in mine air. In the extraction of pure niobium from its compounds by powder metallurgy, the workplaces must be kept free from niobium dust and fumes, and workers must be protected against chemicals such as caustic alkalis and benzene. In addition, regular medical examinations which include lung function tests are recommended.

 

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Read 2840 times Last modified on Thursday, 19 May 2011 10:30