Tungsten reduction

Figure 7-9. Tungsten reduction from WO3 by direct decomposition in atmospheric-pressure thermal plasma. Composition of products (1) W3O9 (2) W2O6 (3) W3O8 ...

Similarly, iodination of tetracarbonyl(2,5-dithiahexane)metal (Cr, Mo, W) complexes gives isolable seven-coordinate products [Ml2(CO)3(2,5-dithia-hexane)] only for molybdenum and tungsten. Reduction of these iodo compounds with lithium aluminum hydride forms unisolable materials with M—H bonds 45). 

Molybdenum is also recovered as a by-product of copper and tungsten mining operations. The metal is prepared from the powder made by the hydrogen reduction of purified molybdic trioxide or ammonium molybdate. 

Molybdenum hexafluoride is used in the manufacture of thin films (qv) for large-scale integrated circuits (qv) commonly known as LSIC systems (3,4), in the manufacture of metallised ceramics (see MetaL-MATRIX COMPOSITES) (5), and chemical vapor deposition of molybdenum and molybdenum—tungsten alloys (see Molybdenumand molybdenum alloys) (6,7). The latter process involves the reduction of gaseous metal fluorides by hydrogen at elevated temperatures to produce metals or their alloys such as molybdenum—tungsten, molybdenum—tungsten—rhenium, or molybdenum—rhenium alloys. 

Reduction to Solid Metal. Metals having very high melting points caimot be reduced in the Hquid state. Because the separation of a soHd metallic product from a residue is usually difficult, the raw material must be purified before reduction. Tungsten and molybdenum, for instance, are prepared by reduction of a purified oxide (WO, MoO ) or a salt, eg, (NH2 2 G4, using hydrogen. A reaction such as... 

The predominant process for manufacture of aniline is the catalytic reduction of nitroben2ene [98-95-3] ixh. hydrogen. The reduction is carried out in the vapor phase (50—55) or Hquid phase (56—60). A fixed-bed reactor is commonly used for the vapor-phase process and the reactor is operated under pressure. A number of catalysts have been cited and include copper, copper on siHca, copper oxide, sulfides of nickel, molybdenum, tungsten, and palladium—vanadium on alumina or Htbium—aluminum spinels. Catalysts cited for the Hquid-phase processes include nickel, copper or cobalt supported on a suitable inert carrier, and palladium or platinum or their mixtures supported on carbon. 

For pure carbon, this reduction can take place at temperatures as low as 1200°C (95). A similar reduction occurs with tungsten, tantalum, or molybdenum... 

CVD processing can be used to provide selective deposition on certain areas of a surface. Selective tungsten CVD is used to fill vias or holes selectively through siUcon oxide layers in siUcon-device technology. In this case, the siUcon from the substrate catalyzes the reduction of tungsten hexafluoride, whereas the siUcon oxide does not. Selective CVD deposition can also be accompHshed using lasers or focused electron beams for local heating. 

Reduction to Metal Powder. The metal powder is obtained from APT by stepwise reduction with carbon or hydrogen. The intermediate products are the yeUow oxide, WO blue oxide, (see Tungsten compounds) and brown oxide, WO2. Because carbon introduces impurities,... 

For the production of lamp-filament wire, aluminum, potassium, and siHcon dopants are added to the blue oxide. Some dopants are trapped in the tungsten particles upon reduction. Excess dopants are then removed by washing the powder in hydroflouric acid. Eor welding electrodes and some other appHcations, thorium nitrate is added to the blue oxide. After reduction, the thorium is present as a finely dispersed thorium oxide. 

High density tungsten alloy machine chips are recovered by oxidation at about 850°C, foUowed by reduction in hydrogen at 700—900°C. Typically, the resultant powders are about 3-p.m grain size and resinter readily. There can be some pickup of refractory materials used in furnace constmction, which must be controUed. This process is important commercially. Eor materials that may be contaminated with other metals or impurities, the preferred recovery process is the wet chemical conversion process used for recovery of tungsten from ores and process wastes. Materials can always be considered for use as additions in alloy steel melting. 

Tungsten pentafluofide [19357-83-6] prepared by the reduction of the hexafluoride on a hot tungsten filament in almost quantitative yield... 

Tungsten tetrafluofide [13766-47-7] WF, is a nonvolatile, hygroscopic, reddish-brown soHd. It has been prepared in low yields by the reduction of the hexafluoride with phosphoms trifluofide in the presence of Hquid anhydrous hydrogen fluoride at room temperature (6). 

Tungsten pentachlofide [13470-13-8], WCl, mp 243°C, bp 275.6°C, is a black, crystalline, deHquescent soHd. It is only slightly soluble in carbon disulfide and decomposes in water to the blue oxide, 200 2. Magnetic properties suggest that tungsten pentachlofide may contain trinuclear clusters in the soHd state, but this stmcture has not been defined. Tungsten pentachlofide may be prepared by the reduction of the hexachloride with red phosphoms (9). 

Tungsten tetrachloride [13470-14-9], WCl, is obtained as a coarse, crystalline, deHquescent soHd that decomposes upon heating. It is diamagnetic and maybe prepared by the thermal-gradient reduction of WCl with aluminum (10). 

Tungsten dichlofide [13470-12-7], WCI2, is an amorphous powder. It is a cluster compound and maybe prepared by the reduction of the hexachloride with aluminum in a sodium tetrachloroalurninate melt (11). 

Tungsten oxytrichlofide [14249-98-0], WOCl, a green soHd, is prepared by the aluminum reduction of WOCl in a sealed tube at 100—140°C (14). 

Tungsten tetrabromide [12045-94-2] WBr, black orthorhombic crystals, is formed by the thermal-gradient reduction of WBr with aluminum, similar to the reduction of WCI4 (10). 

Tungsten dibromide [13470-10-5] WBr2, formed by the partial reduction of the pentabromide with hydrogen, is a black powder that decomposes at... 

Tungsten dioxide [12036-22-5] WO2, is a brown powder formed by the reduction of WO3 with hydrogen at 575—600°C. Generally, this oxide is obtained as an intermediate in the hydrogen reduction of the trioxide to the metal. On reduction, first a blue oxide, then a brown oxide (WO2), is formed. The composition of the blue oxide was in doubt for a long time. However, it has since been resolved that W2Q03g and W are formed as intermediates, which may also be prepared by the reaction of tungsten with WO3. 

Salts containing pentavalent tungsten may be obtained by the reduction of alkaU tungstate in concentrated hydrochloric acid. Salts of types M(I)2(W0C1 ) (green), M(I)(WOCl (brown-yeUow), and M(I)(WOCl 2 ) h ve been isolated. Thiocyanato and bromo salts are also known. 

The only known trivalent tungsten complex is of the type M(I)2(W2Cl2). It is prepared by the reduction of strong hydrochloric acid solutions of K2WO4 with tin. If the reduction is not sufficient, a compound containing tetravalent tungsten, K2(WCl (OH)) [84238-10-0] is formed (57). 

Although distibenes, the antimony analogues of azo compounds, have never been isolated as free, monomeric molecules (130), a tungsten complex, tritungsten pentadecacarbonyl[p.2-Tj -diphenyldistibene] [82579-41-7] C2yH2Q025Sb2W2, has been prepared by the reductive dehalogenation of phenyldichlorostibine (131) ... 

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