Metallurgy reduction

The metal is isolated commercially by a complex chemical process, the final stage of which is the hydrogen reduction of ammonium ruthenium chloride, which yields a powder. The powder is consolidated by powder metallurgy techniques or by argon-arc welding. 

Reduction of metal oxides with hydrogen is of interest in the metals refining industry (94,95) (see Metallurgy). Hydrogen is also used to reduce sulfites to sulfides in one step in the removal of SO2 pollutants (see Airpollution) (96). Hydrogen reacts directiy with SO2 under catalytic conditions to produce elemental sulfur and H2S (97—98). Under certain conditions, hydrogen reacts with nitric oxide, an atmospheric poUutant and contributor to photochemical smog, to produce N2 ... 

Re OPe . The final step in the chemical processing of rare earths depends on the intended use of the product. Rare-earth chlorides, usually electrolytically reduced to the metallic form for use in metallurgy, are obtained by crystallisation of aqueous chloride solutions. Rare-earth fluorides, used for electrolytic or metaHothermic reduction, are obtained by precipitation with hydrofluoric acid. Rare-earth oxides are obtained by firing hydroxides, carbonates or oxalates, first precipitated from the aqueous solution, at 900°C. 

The essential operations of an extractive metallurgy flow sheet are the decomposition of a metallic compound to yield the metal followed by the physical separation of the reduced metal from the residue. This is usually achieved by a simple reduction or by controlled oxidation of the nonmetal and simultaneous reduction of the metal. This may be accompHshed by the matte smelting and converting processes. 

The preparation, reduction, and refining operations are very much interdependent, and for a given metal must be considered as parts of a single flow sheet. To illustrate the principles of extractive metallurgy, however, it is convenient to discuss the various operations separately. 

Niobium is also important in nonferrous metallurgy. Addition of niobium to tirconium reduces the corrosion resistance somewhat but increases the mechanical strength. Because niobium has a low thermal-neutron cross section, it can be alloyed with tirconium for use in the cladding of nuclear fuel rods. A Zr—l%Nb [11107-78-1] alloy has been used as primary cladding in the countries of the former USSR and in Canada. A Zr—2.5 wt % Nb alloy has been used to replace Zircaloy-2 as the cladding in Candu-PHW (pressurized hot water) reactors and has resulted in a 20% reduction in wall thickness of cladding (63) (see Nuclear reactors). 

T. WHken, C. Wert,J. Woodhouse, and W. Morcom, "Reduction of Blue Tungsten Oxide," in H. H. Hausner and P. V. Taubenblat, eds.. Modem Developments in Powder Metallurgy, Vol. 9, Plenum Press, New York, 1977, pp. 161—169. 

An alternative commercial form of a metallic mixed lanthanide-containing material is rare-earth siUcide [68476-89-1/, produced in a submerged electric-arc furnace by the direct reduction of ore concentrate, bastnasite, iron ore, and quart2. The resulting alloy is approximately 1/3 mischmetal, 1/3 sihcon, and 1/3 iron. In addition there are some ferro-alloys, such as magnesium—ferrosilicons, derived from cerium concentrate, that contain a few percent of cerium. The consumption of metallic cerium is overwhelmingly in the mixed lanthanide form in ferrous metallurgy. 

In a chemical vapor deposition (CVD) variant of conventional powder metallurgy processing, fine chromium powder is obtained by hydrogen reduction of Crl2 and simultaneously combined with fine thorium(IV) oxide [1314-20-17, H1O2, particles. This product is isostaticaHy hot pressed to 70 MPa (700 atm) and 1100°C for 2 h. Compacts are steel clad and hot roUed to sheets (24). 

Developments in the metallurgy of copper or its alloys were mentioned in 1556 in De MetalBca where the process of copper ore was described by Agricola (see also Copper alloys). About that time, smelting operations commenced at Mansfield, Germany, and at the Swansea smelter in Wales. Both smelters employed successive oxidations and reductions to eliminate iron and sulfur. The process used in the Swansea smelter is similar to modern techniques. 

Metallurgy includes separation, conversion, reduction, and refining steps. The starting material is an impure ore, and the end product is pure metal. 

One of the most common chemical reducing agents for metallurgy is coke, a form of carbon made by heating coal at high temperature until all of the volatile impurities have been removed. Metals whose cations have moderately negative reduction potentials—Co, Ni, Fe, and Zn—are reduced by coke. For example, direct reaction with coke in a furnace frees nickel from its oxide NiO(.j) + C( ) Ni(/) -F CO(g)... 

Potassium perrhenate (KRe04) is reduced by hydrogen in two stages. The first operation is carried out at 500 to 550 °C. The reduced product is washed to remove the hydroxide. The powder is then subjected to a second reduction at a higher temperature (900 to 1000 °C). The product is washed, first with dilute hydrochloric acid and then with water, and dried in vacuum or in a current of hydrogen. Solid rhenium is made by powder metallurgy techniques. 

D. J. I. Evans, Production of Metals by Gaseous Reduction from Solution Processes and Chemistry in Advances in Extractive Metallurgy p. 831, IMM, London, 1968. 

Shinotake, A., and Takamoto, Y., Combustion and Heat Transfer Mechanism in Iron Bath Smelting Reduction Furnace, La Revue de Metallurgie - CIT, p. 965 (1993)... 

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