Abstract
The commercial extra-pure chromic oxide was reduced with powdered graphite (ash content <0.10%) over a temperature range of 990∼1170°C under Ar atmosphere. The reduction rate was measured by means of a thermobalance using a silica glass spring.
The results are as follows:
(1) The reduction took place in three orderly steps, i.e., in periods of incubation, acceleration and retardation.
(2) The incubation period became longer as the temperature was lowered and the particle size of the graphite was increased. In the incubation period, the process of nucleation was rate-determining.
(3) Based on the supposition that the Boudouard reaction was a rate-determining process in the acceleration period, the observed data were analysed kinetically and could be explained satisfactorily.
(4) The reduction products were Cr3C2 and Cr7C3.
(5) In the retardation period, the reaction rate was remarkably lowered. It seemed that the reaction was controlled by the counter-current diffusion of CO and CO2 through a relatively compact layer of chromium carbides.
