Process of Reduction and the Behavior of Carbon-Reducing Agent in the Kiln

Study on the pre-reduction of iron sand in a rotary kiln-III

Abstract

In the previous paper of this series the travel of raw material and the distribution of temperature in a reducing kiln had been discussed.The present study is to investigate the process of reduction and the behavior of carbon-reducing agent in the kiln.
By using chemical analysis, X-ray diffraction method and observation of microstructure on the specimens which are obtained through sampling holes fixed on the kiln shell, the author finds that the greater part of reduction occurs in the 20pct length from discharge end and the half-reducing reactions proceed as follows:-Magnetite is reduced to wüstite preferentially, ilmenite takes sintering wüstite into ulvöspinel, in the next stage partial reduction to the metallic state of iron from ulvöspinel progresses to some extent under slagging temperature.
Free wüstite is scarcely recognized throughout the kiln with the exception of 5-9m zone that exists in the early stage of reduction.
Generally the zone 2-3m from discharge end is approximately reoxidizing on account of the oxidizing atmosphere. This atmosphere is one of the great factors causing dam-ring troubles in a reducing kiln.
There are two types of consumption of the reducing agent in the burden material; that is, reduction and direct combustion including combustion of carbon monoxide which is a product from reduction.The reactivity of the reducing agent is an important factor for the reduction velocity concerning the consistency of carbon monoxide in the material bed and, at the same time, a factor influencing the consumption in direct combustion.