Mechanism study on solid‐state reduction in the Fe2O3–NiO–Cr2O3–C system using thermal analyses

Abstract

In order to clarify the interactions among metal oxides during reduction of pickling sludge, the reduction mechanism of the metal oxide mixture, Fe2O3–NiO–Cr2O3, which simulates the composition of these metal oxides in pickling sludge, was studied using graphite as a reductant, by means of thermal analysis techniques. X‐ray diffraction analysis was used for characterization of reduction products. In addition to the reduction test on the metal oxide mixture mentioned above, for comparison, separate tests on the reduction of Fe2O3, NiO and Cr2O3 were also conducted. The results demonstrate that through formation of a transitional compound ‘FeCr2O4’, Fe2O3 promotes the complete reduction of Cr2O3 at a temperature lower than that required for reducing pure Cr2O3. On the other hand, Cr2O3 hinders the reduction of Fe2O3 at lower temperatures. The whole reduction process may be classified as five distinguishable reduction stages in which possible reactions were proposed to represent the reduction mechanism. The reduced metals in the final product exist primarily in the form of solid solutions, such as ‘Cr–Fe–Ni’, which mainly containing ‘(Fe,Ni)’ and ‘(Fe,Cr)’, and small amounts of carbides, such as (Cr,Fe)7C3 and/or Cr7C3.