Reaction Mechanism of Calcium Vanadate Formation in V-slag/CaO Diffusion System

Abstract

Previously, we found that the inward diffusion of oxygen from the atmosphere to the interior of vanadium slag (V-slag) has a significant influence on the roasting reaction and may possibly be the dominant mechanism of this reaction. However, the existing reaction mechanism does not reflect the role of oxygen in calcification roasting. In view of this, the proposed study aims to verify the influence of oxygen using the diffusion couple technology, and propose the reaction equations involving oxygen describing the calcium vanadate formation from the surface to the interior of V-slag. The V-slag/CaO diffusion couples were prepared by vacuum hot-pressing, and diffusion experiments were performed under different oxygen partial pressures at a temperature of 1083 K. First, the surfaces in contact with the atmosphere (S(xz)) during roasting were analyzed by electron probe microanalysis (EPMA), Fourier transform-infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD). Then, the diffusion couple was cut along a direction perpendicular to the V-slag/CaO interface and S(xz). The interior surface (S(xy)) was also analyzed by EPMA, FT-IR, and XRD. At S(xz), new phases were clearly observed in the vicinity of V-slag/CaO interface and characterized mainly as CaV₂O₆, Ca₂V₂O₇, and Ca₃V₂O₈. Moreover, the diffusion thickness of those new phases increased with the oxygen partial pressure. At S(xy), although relatively weak diffraction peaks and absorption bands of CaV₂O₆, Ca₂V₂O₇, and Ca₃V₂O₈ were detected, no distinct new phases were observed near the V-slag/CaO interface. Considering that the diffusion capacities of Ca and V at S(xz) and S(xy) cannot be evaluated based on the diffusion thickness, the use of interdiffusion coefficient was proposed to quantify the difference among the diffusion capacities. The average interdiffusion coefficients of Ca and V at S(xz) calculated as 1.02 × 10⁻⁸ and 0.91 × 10⁻⁸ cm² s⁻¹, respectively, were practically a hundred times these at S(xy). Following the conclusion that the calcium vanadate formation was governed by the inward oxygen diffusion, new reaction equations for describing the formation mechanism of calcium vanadate in calcification roasting were proposed. These equations, derived from the vacancy mechanism, express that the hoses and calcium vanadate are generated by Ca²⁺, V₂O₅, and O₂.