Investigation of the carbothermal reduction process of chromium oxide by micro- and lab-scale methods

In this study the carbothermal reduction process of Cr2O3 was investigated by a micro-scale thermoanalytical method with quantitative in situ gas detection (CO) using three different types of carbon (carbon black, active carbon and graphite) in stoichiometric mixtures calculated for the formation of Cr3C2. The solid reaction products obtained in these investigations were characterized ex situ by chemical nonmetal analysis, X-ray diffraction, SEM and adsorption methods. In addition, mixtures with different Cr2O3/graphite ratios were reacted in a laboratory carbon tube furnace and subsequently characterized by the same methods. The results of both micro- and lab-scale methods are complementary and in very good agreement. Most probably, the oxide particles are the precursors of the carbide particles. The transfer of carbon to the surface of the oxide particles is realized by the CO/CO2 mass transport mechanism. Independently from the stoichiometry of the mixture, Cr3C2 is the first carbide formed, hypothetically as a layer around the Cr2O3 particles. The carbothermal reduction process of Cr2O3 consists of two subprocesses, firstly, the CO/CO2 transport reactions and, secondly, the reaction of the primarily formed Cr3C2 with the Cr2O3. The amount of carbon available in the mixture and the rate of the mass transfer from the carbon to the oxide particles determine the appearance of Cr3C2, Cr7C3 and Cr23C6 as intermediary or final reaction products. Carbon black was found as the carbon type with the highest reactivity. Pronounced grain growth is characteristic for reaction temperatures above 1200°C.