Chromium, Molybdenum, and Tungsten

Abstract

The physical and chemical characteristics of chromium and some of its compounds are summarized. The term chromium is derived from the Greek word for color, because most chromium compounds are brightly pigmented. The element chromium was discovered in 1798 by N. L. Vauquelin, but it had already been used in swords by the Hittits about 1300 b.c. Chromium occurs in nature in bound-form chromite ore, which is the only chromium ore of any importance, and it makes up 0.1–0.3 ppm of the earth's crust. The red color of rubies and green color of emeralds, serpentine, and chrome mica are produced by chromium. Chromium metal is prepared by reducing the ore in a blast furnace with carbon (coke) or silicon to form an alloy of chromium and iron called ferrochrome, which is used as the starting material for the many iron-containing alloys that employ chromium. Chromium to be used in iron-free allloys is obtained by reduction or electrolysis of chromium compounds. Chromium is difficult to work in the pure metal form; it is brittle at low temperatures, and its high melting point makes it difficult to cast. The use of chromium in stainless steel (18%+) is a major use of the element. The U.S. National Occupational Exposure Survey estimated that a total of about 200,000 workers, including about 30,000 women, were potentially exposed to hexavalent chromium compounds. The typical airborne concentrations in various industrial operations are given; however, the combustion of coal and oil is the largest single source of air pollution. Chromium in the trivalent form is an essential trace element to humans. It is involved in the metabolism of glucose. Chromium deficiency may result in impaired glucose tolerance, peripheral neuropathy, and elevated serum insulin, cholesterol, and triglycerides, similar to those symptoms observed in diabetic patients. Molybdenum is a dark-gray, or a black powder with a metallic luster and a chemical element of the second transition series. The name is derived from the Greek molybdos, meaning “lead.” In 1778 Carl Scheele of Sweden recognized molybdenite as a distinct ore of a new element. Hjelm in 1782 prepared an impure form of the metal. Free molybdenum does not occur in nature, but it is extracted from molybdenite, wulfenite, and powellite and is recovered as a by-product of copper and tungsten mining operations. Molybdenum is found in many parts of the world, but relatively few deposits are rich enough to warrant recovery costs. By far the largest and richest deposits occur in the western hemisphere, with the United States contributing the major share. Molybdenite concentrates are roasted to produce technical-grade oxide, considerable amounts of which are used directly in steel; the rest is converted to other molybdenum products. MoO3 of higher purity is made by sublimation of the technical-grade oxide or from (NH4)2MoO4. FerroMo is made from the oxide by ignition with aluminum, iron ore, ferrosilicon, lime, and fluorspar. Work-related exposure during production and fabrication of Mo products are to dusts and fume of Mo, its oxides, and its sulfides, chiefly from electric furnace or other high-temperature treatment. MoS2 as a lubricant may be applied to metal surfaces at 700°F. Spraying of Mo may provide a hazard, and loss of Mo catalysts to the air adds to the metal burden of contaminated atmospheres. The sublimation characteristics of MoO3 (above 800°C) present a fume hazard. In addition to its industrial hygiene significance, Mo is of considerable biological importance as an essential trace element in the Mo-flavoprotein enzyme xanthine oxidase, in which it functions as an electron transport agent. It is also necessary for the fixation of nitrogen in the soil by bacteria; cattle and sheep can be poisoned feeding on herbage that has taken up Mo in abnormal quantities. The physical and chemical properties of tungsten and some of its compounds are listed. The chemistry of tungsten and its compounds is similar to that molybdenum. The properties of tungsten alloys offer more limited uses than those of most metals. The prime use of tungsten is in cutting and wear-resistant materials (65%), mill products (12%), specialty steels, tools, stainless, and alloys (9%), hard-facing rods (8%), super alloys (3%), and chemicals (2%). It can reasonably be inferred from the small usage of ferro and super alloys relative to other uses, that tungsten, unlike most metals, forms relatively few alloys with properties superior to those of others. Exposure to tungsten-containing compounds may occur during production and uses of tungsten, its alloys, and compounds, rather than to tungsten itself. It is, however, still not clear precisely what role tungsten plays in the exposures. Many investigations on the physiologic effects of tungsten followed the marketing of cobalt-cemented WC just before 1940. Hence most of the investigations concern the toxicity and health effects of cemented WC and its constituents, particulary in humans, rather than tungsten and its compounds themselves, all of which may blur the true toxicity of tungsten. The most significant exposure-related disease is mostly referred to as hard metal pneumoconiosis. The few determinations of toxicity of tungsten and its compounds made before 1950 clearly showed a difference between soluble and insoluble forms. Soluble compounds were distinctly more toxic than insoluble forms, resulting in two separate permissible limits for industrial exposure. Keywords: Skin; Gastrointestinal tract; Clinical cases; Welders; Ferrochromium; Air standards; Cutting materials; Chromium; Chromium compounds; Molybdenum; Molybdenum compounds; Tungsten; Tungsten compounds