抄録
In order to make clear the accelerating effect of CaO on the wustite reduction, the dependence of the reduction rate of wustite containing CaO on the reduction temperature and the α-γ transformation of iron have been investigated. Pellets of pure wustite and wustite containing CaO at several quantity levels, some of which are the intermediate reaction product from magnetite, have been reduced by hydrogen in the temperature range from 873 to 1273 K. Gravimetric measurements, microscopic observations of cross section and reduction rate analyses have been made.
Each grain of pure wustite was covered with a dense layer of metallic iron, not allowing the gas phase diffusion above 973 K, although porous iron was formed at the temperature of 973 K or less. The CaO addition to wustite, however, can yield the metallic iron with fine pores open for reducing gas to the wustite surface at any temperature investigated in the present work. The number of fine pores in the metallic iron diminishes and the gain in the reduction rate decreases gradually with increasing temperature within the range of α-iron. However, when the temperature was further raised into the γ-iron range, the reduction rate increases to a great extent, probably because the sintering of fcc-iron is slower than that of bcc-iron. The above mentioned phenomena can be shown also by the change in the interfacial chemical reaction rate constant kr, which is estimated by analysing the reduction rate in terms of the mixed control equation.
The activation energy for the interfacial chemical reaction hardly changes by the CaO addition to wustite and practically does not depend on the quantity of CaO. This experimental results indicate that the reduction accelerating effect of CaO may not be catalytic, but it will be interpreted as being due to the iron formation that allows the reducing gas to diffuse to wustite.
