Manufacture of Chromium Metal

Alloys./ A substantial amount of chromic oxide is used in the manufacture of chromium metal and aluminum—chromium master alloys. 

The chromium(lll) oxide thus obtained has a low sulfur content, particularly when a substoichiometric amount of ammonium salt is used, and can be used for the aluminothermic manufacture of chromium metal. 

Chromium(IIf) oxide utilized in the manufacture of chromium metal by aluminothermic reduction, as a pigment and as a polishing agent. 

The use of stoichiometric quantities of ammonium chloride or slightly under stoichiometric quantities of ammonium sulfate in the reaction mixture yields low sulfur chromium(lll) oxide for the manufacture of chromium metal. 

Inc., Ward Hill, Massachusetts (16) chromium potassium sulfate McGean-Rohco, Inc., Cleveland, Ohio and (17) chromium-silicon monoxide Cerac Incorporated, Milwaukee, Wisconsin (SRI 1997). Besides these producers of chromium metal alloys and chromium compounds, Table 4-1 reports the number of facilities in each state that manufacture and process chromium, the intended use of the products, and the range of maximum amounts of chromium products that are stored on site. The data reported in Table 4-1 are derived from the Toxic Release Inventory (TRI) of EPA (TRI97 1999). The TRI data should be used with caution since only certain types of facilities were required to report. Hence, this is not an exhaustive list. 

Apart from the utilization of large quantities of ferrochrome (produced by reducing chromite with coal) in the manufacture of alloys containing iron and chromium, smaller quantities of chromium metal are used in, for example, the manufacture of turbine blades, the production of iron-free alloys and cermets (metal ceramics e.g. 23% by weight aluminum oxide, 77% by weight chromium). 

Cobalt is used in the manufacture of alloys with a high melting point, strength, and resistance to oxidation, in nuclear technology, and in the manufacture of hard metal alloys, e.g., for grinding wheels. Alloys with chromium, nickel, molybdenum, and copper are used in the electrical, aeronautical, and car industries. 

In 1799, Tassierf a German chemist active in France, discovered chromium in an iron mineral from the Var region of southeastern France. This interested Fourcroy, who anticipated that iron-chromium ores might be source materials for a diversified manufacture of chromium chemicals. Due to the richness of color of the metal s compounds the minerals could be used for painting and for manufacturing colored glass and china. 

Ferrochromium is an alloy of iron and chromium. More than 80% of all chromite ore is consumed in manufacturing this alloy. It is mainly used as the chromium source for the manufacture of stainless steels and other chromium steels. HC ferrochromium with high carbon content (5-6%) is manufactured in electric furnaces with coke as the reducing agent. The process has a high energy consumption, 4000 kWh per tonne of chromium metal. Plants for ferrochromium production have therefore been located in areas with low energy prices. This was one reason why Sweden, without its own chromium deposits, was a pioneer country in this field and in the production of stainless steels. 

Heterogeneous vapor-phase fluorination of a chlorocarbon or chlorohydrocarbon with HP over a supported metal catalyst is an alternative to the hquid phase process. Salts of chromium, nickel, cobalt or iron on an A1P. support are considered viable catalysts in pellet or fluidized powder form. This process can be used to manufacture CPC-11 and CPC-12, but is hampered by the formation of over-fluorinated by-products with Httle to no commercial value. The most effective appHcation for vapor-phase fluorination is where all the halogens are to be replaced by fluorine, as in manufacture of 3,3,3-trifluoropropene [677-21 ] (14) for use in polyfluorosiHcones. 

High density polyethylene (HDPE) is defined by ASTM D1248-84 as a product of ethylene polymerisation with a density of 0.940 g/cm or higher. This range includes both homopolymers of ethylene and its copolymers with small amounts of a-olefins. The first commercial processes for HDPE manufacture were developed in the early 1950s and utilised a variety of transition-metal polymerisation catalysts based on molybdenum (1), chromium (2,3), and titanium (4). Commercial production of HDPE was started in 1956 in the United States by Phillips Petroleum Company and in Europe by Hoechst (5). HDPE is one of the largest volume commodity plastics produced in the world, with a worldwide capacity in 1994 of over 14 x 10 t/yr and a 32% share of the total polyethylene production. 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Water-Soluble Trivalent Chromium Compounds. Most water-soluble Cr(III) compounds are produced from the reduction of sodium dichromate or chromic acid solutions. This route is less expensive than dissolving pure chromium metal, it uses high quaHty raw materials that are readily available, and there is more processing fiexibiHty. Finished products from this manufacturing method are marketed as crystals, powders, and Hquid concentrates. 

Chbihi, H., et ah, CVD of Chromium Oxide Coatings on Metal Substrates from Chromium Acetyl acetone, Materials and Manufacturing Processes, 6(3) 469-480 (1991)... 

Chemical precipitation. Chemical precipitation followed by solids separation is particularly useful for separating heavy metals. The heavy metals of particular concern in the treatment of wastewaters include cadmium, chromium, copper, lead, mercury, nickel and zinc. This is a particular problem in the manufacture of dyes and textiles and in metal processes such as pickling, galvanizing and plating. 

Heavy metals are widely used as catalysts in the manufacture of anthraquinonoid dyes. Mercury is used when sulphonating anthraquinones and copper when reacting arylamines with bromoanthraquinones. Much effort has been devoted to minimising the trace metal content of such colorants and in effluents from dyemaking plants. Metal salts are used as reactants in dye synthesis, particularly in the ranges of premetallised acid, direct or reactive dyes, which usually contain copper, chromium, nickel or cobalt. These structures are described in detail in Chapter 5, where the implications in terms of environmental problems are also discussed. Certain basic dyes and stabilised azoic diazo components (Fast Salts) are marketed in the form of tetrachlorozincate complex salts. The environmental impact of the heavy metal salts used in dye application processes is dealt with in Volume 2. 

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