Reduction behavior of chromium oxide in molten slag using aluminum, ferrosilicon and graphite

Abstract

To reduce chromium in stainless steel slag and simultaneously recover chromium as a ferroalloy for the steel making industry, the direct smelting reduction of chromium oxide in molten slag was investigated by small furnace experiments. Thermodynamic equilibrium calculations were also carried out to suggest the limit of reduction of CrOx in the slags. Aluminum, ferrosilicon and graphite were used as reductants under conditions of 1 723-1 823 K and various slag compositions. In the experiments, the formation of high Al2O3 solid phase in the upper and middle parts of slag prevented uniform reduction when aluminum was added as a reductant, although FeO and CrOx were reduced drastically to a low content compared with the cases of ferrosilicon and graphite except for the lower part of slag melt. The low content of CrOx by aluminum was about 0.01 mass% which was about one order higher than calculated thermodynamic equilibrium one. The reductions of FeO and CrOx by aluminum were promoted to the lower part of slag with the increase in temperature and CaO/SiO2 ratio because of the increase in fluidity of slag. A liquid metal containing chromium was formed and precipitated on the bottom of slag during the reduction. The formation behavior of metal drop was observed directly by an X-ray fluoroscopy technique.