A Morphological Study of the Carburization/Reduction of Tungsten Oxides with Carbon Monoxide
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1. INTRODUCTION
T HE present investigation of the direct gas phase carburization of various samples of tungsten trioxide and other lower oxides has been carried out as part of a continuing research program studying the chemical control of particle morphology of tungsten, its alloys and compounds. It can be predicted thermodynamically that the product of the reduction of tungsten trioxide by CO will most likely be tungsten carbide. 1 At 927°C, for a total pressure of one atm, CO/C02 mixtures ranging from 100 to 27 pet CO will yield WC as the product of reaction." At the same temperature tungsten metal will be produced with a CO content in the range of 27 to 21 pct. Below 21 pet CO the product of reaction may be predicted to be W0 2 • At the lower temperature of 827°C there is no range of col CO2 ratios that will yield tungsten as the end product. According to the Kellogg (predominance area) diagram for the W-C-O system at 927°C 2 (Fig. 1) the reduction/carburization of W0 3 proceeds through W2005S and then either via the phases WlS049 and W0 2 or via W1S049 alone before WC is produced. This prediction is in contradiction to the electron microscope observations of Fornwalt et aZ 3 who indicate that W0 3 produced from freeze-dried ammonium metatungstate is converted directly to WC. The changes that occur in the particle morphology of tungsten metal produced by the hydrogen reduction of blue tungsten oxide at various temperatures and H2/H 20 ratios have been shown by the present authors to be brought about by the lower oXides. 4,5 Consequently, if the predictions based on the Kellogg diagram are correct, there is the possibility that the morphological changes that can be controlled in the case of tungsten metal may also be controlled for a carbide product if two-stage reduction processes using HJH 20 and CO are used. The present study, therefore, has investigated the kinetics and mechanism of the CO reduction of W0 3 , W1S049 and W0 2 with a view to determining the possibility of producing A. K. BASU, formerly with the Metallurgy Department, Manchester University, Manchester, U.K., is now with the Regional Research Laboratory, Bhubaneswar, India. F. R. SALE is Lecturer in Metallurgy, Manchester University, Manchester, U.K. Manuscript submitted March 2, 1978. MET ALLURG1CAL TRANSACTIONS B
tungsten carbide of unusual morphologies by controlled reduction sequences. 2. EXPERIMENTAL PROCEDURE 2.1. Materials and Reduction Experiments Three samples of yellow tungsten trioxide were roduced from two conventionally- crystallized samples and one freeze-dried sample of ammonium paratungstate (APT) by decomposition in air at 600°C. These APT samples, and their decomposition processes, have been characterized and discussed previously.6,7 The three samples of tungsten trioxide of differing particle morphology were reduced with carbon monoxide in a glass spring thermobalance at temperatures of 600, 700, 800 and 900°C. The sample produced from the freeze-dried APT was also reduced at 1000 and llOO°C. A carbon monoxide flow rate of 250
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