Investigation of Reaction Mechanism on the Lime-Free Roasting of Chromium-Containing Slag

The lime-free roasting process of trivalent chromium-containing slag was investigated. The effect of Fe and liquid phase on the conversion reaction of chromium was discussed. The oxidation of trivalent chromium depends greatly on the diffusion of Na⁺ and O₂. Both the raw material Na₂CO₃ and the intermediate product NaFeO₂ serve as the carriers of Na⁺. The Na⁺ diffusion is improved by the binary liquid phase of Na₂CrO₄-Na₂CO₃, whereas excess liquid phase results in a low conversion rate of chromium by hindering the diffusion of oxygen towards the reaction interface. With the increasing of liquid volume, the controlled step of chromium oxidation changes from Na⁺ diffusion to oxygen diffusion. The mechanism study showed that the volume of liquid phase increased while raising the reaction temperature or prolonging the reaction time. Based on the role of both liquid phase and Fe, the oxidation process of chromium was summarized as a three-stage model: the Na⁺ diffusion-controlled stage, the O₂ diffusion-controlled stage, and the oxidation reaction halted stage.