Reduction of FeWO 4 -NiWO 4 solid solutions by hydrogen gas
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is not possible to obtain the isothermal oxidation energy value by other methods. (1) Aluminothermic reductions of Fe2O3, MnO2, and Cr2O3 are highly exothermic and fast processes. Prediction of their isothermal reduction kinetics by conventional methods is almost impossible. (2) Extrapolation of nonisothermal kinetics data provides a unique method for obtaining isothermal kinetics parameters. (3) Isothermal activation energy values obtained through extrapolation methods are smaller in comparison to nonisothermal values. (4) Kinetic data obtained using Coats and Redfern[5] and Dixit and Ray[6] approaches compare favorably. REFERENCES 1. B. Sarangi, H.S. Ray, S. Misra, and A. Sarangi: Trans. Ind. Inst. Met., 1991, vol. 44 (3), p. 279. 2. B. Sarangi, A. Sarangi, and H.S. Ray: Iron Steel Inst. Jpn. Int., 1996, vol. 36 (9), p. 1135. 3. H.S. Ray, B. Sarangi, and A. Sarangi: Scand. J. Metall., 1997, in press. 4. B. Sarangi, H.S. Ray, K.K. Tripathy, and A. Sarangi: J. Thermal Analysis, 1994, vol. 44, p. 441. 5. A.W. Coats and J.P. Redfern: Nature, 1964, vol. 68, p. 201. 6. S.K. Dixit and H.S. Ray: Thermochim. Acta, 1982, vol. 54, p. 245.
ides.[1] With a view to investigating the coreduction route, a series of reduction studies involving hydrogen gas has been carried out in this laboratory. This includes the reductions of tungsten oxides[2] and some transition metal tungstates, namely, nickel tungstate,[3] cobalt tungstate,[4] and iron tungstate.[5] As a continuation of the previous investigations, the present work aims at a study of the reduction of NiWO4-FeWO4 solid solutions, (Ni, Fe)WO4, by hydrogen. (Ni, Fe)WO4 solid solutions were prepared by mixing nickel tungstate powder (99 pct purity) and iron tungstate powder (99 pct purity), both supplied by Johnson Matthey, Germany in appropriate proportions thoroughly. The powder mixtures were pressed into pellets (about 14 mm in diameter and 5 mm in height) and sintered in a platinumlined alumina crucible under argon atmosphere (99.9990 pct pure, AGA Gas, Stockholm) at 1473 K for 7 days. Xray diffraction analyses of the samples carried out in a PHILIPS* X-ray diffraction instrument, ‘‘X-pert system,’’ *PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah, NJ.
showed the absence of pure NiWO4 and FeWO4, indicating thereby that the two tungstates had reacted completely forming solid solutions. Based on the X-ray diffraction measurements, the lattice parameter values for the two solid solutions as well as the two pure tungstates were evaluated. Figure 1 shows the lattice parameter as a function of composition of the solid solution. The linearity of the variation of lattice parameter with composition suggests that the NiWO4-FeWO4 system follows Vegard’s law, which further verifies the formation of the solid solutions. The thermogravimetric studies were carried out using a SETARAM TGA 92 unit (Setaram, Saint-Cloud, France) with a detection limit of 1 mg. The apparatus and experimental procedure have been described in detail in earlier publications.[2,3] In a gene
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