Kinetic studies of reduction of CoO and CoWO 4 by hydrogen
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I.
INTRODUCTION
g study of the kinetics of the reduction of metal oxides is of interest in the production of composite materials. A series of studies of the reduction of metal oxides has been carried out in the Division of Theoretical Metallurgyt~'2,3j at the Royal Institute of Technology (Stockholm, Sweden) in connection with Ni-W and NiA1203 composite materials. The present experiments were carried out with a view to understand the mechanism of reduction of cobalt tungstate in the formation of tungstenreinforced cobalt alloys. The theoretical interest in these studies is to establish correlations between the activation energies for the reduction of simple oxides and those for the complex ones under similar conditions of reaction. Some trends were established in the case of the reduction studies of NiO, t3] WO3, [11 and N i W O 4 .[2] The purpose of the present study is to find out if similar relationships could be established in the case of CoO, WO3, and C o W O 4.
II.
EXPERIMENTAL
The reduction studies were carried out by means of a thermogravimetric method. The samples were kept in the form of shallow loosely packed powder beds so that the reductant gas had access to all the individual particles and mass-transfer aspects did not influence the kinetics. In this way, it was possible to focus the studies to the chemical reaction in each small particle. A. Materials CoO was prepared by the decomposition of Co304 (Pro analysis grade, E. Merck, Darmstadt, Germany) by heating it in commercial argon for 2 hours at 1073 K. Trial thermogravimetric experiments showed that the material after the heat treatment had a composition corresponding to CO0.990. The heat-treated material was J.A. BUSTNES, Graduate Student, DU SICHEN, Associate Professor, and S. SEETHARAMAN, Professor, are with the Division of Theoretical Metallurgy, Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted May 24, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
ground to fine powder, which had an average particle size of about 5 /xm before being used in the reduction studies. CoWO4 powder (99.9 pct purity, Strem Chemicals, Newbury Port, MA) was dried in air at 623 K for 1 hour before the thermogravimetric measurements. The argon gas was supplied by AGA Special Gas, Stockholm and had a maximum of 2 ppm impurities. The hydrogen gas used for the reduction (Plus grade, maximum 10 ppm impurities) was obtained from the same supplier. B. Apparatus and Procedure The reduction experiments were carried out on a SETARAM TGA 92 (France), thermogravimetric instrument, which had a detection limit of 1 /zg (variation in mass to be detected). The system was controlled by an IBM PC* through a CS92 controller. Figure 1 shows the *IBM PC is a trademark of International Business Machines Corp., Armonk, NY.
experimental assembly. A detailed description of this apparatus was given in an earlier publication, t4j In both the reductions of CoO and CoWO4, a shallow alumina crucible (8-mm i.d. and 2-mm height) was employed as the container to ho
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