Chromium recovery from chromium-containing slags

Abstract

Japan imports all of its chromium[ 1] to produce ferrochrome, which is used for the production of stainless, heat resistant and alloy steels. About 700 000 tonne of chromite was imported and 360 OOO[l] of ferrochrome was produced in 1977. However, the yield of chromium in ferrochrome production is limited to 92 94%[2], and the slags have been dumped as waste which may pollute the environment by means of the residual chromium, should it be oxidized to chromate. Accordingly, the recovery of chromium would be beneficial, not only in saving a large amount of chromite, (about 60 OOOt) but also in avoiding possible pollution by chromiumcontaining slags. In view of the present dependence on foreign sources and the increasing demand for chromium in steelmaking, the development of practical and economical methods for recovery of chromium from ferrochrome slags is highly desirable. Although the kinetics of the reduction of solid CraO, or chromite have been studied by many investigators[3 81, studies with molten chromite or chromium in molten slags are limited[9 111. The present authors have studied the thermodynamics of chromium in molten slags under strongly reducing conditions, such as with carbon or Fe C melts[ 12 141. The results indicate that most of the chromium in the slag can, in principle, be reduced and recovered as chromium carbide (Cr,C,) or Fe Cr C alloy. Consequently, in this study the kinetics of the reduction of chromium from industrial ferrochrome slags to carbon-saturated iron was studied. Since the original ore remains to some extent undissolved in industrial slags, slags were also microscopically examined. On the basis of the above, optimum conditions for the yield of chromium in ferrochrome production, leading to the development of an industrial recovery process, were determined.