Endogenous Calcium Participation and Multi-Host Phase Reconstruction During Additive-Free Roasting of High-Calcium Vanadium Slag

In high-calcium vanadium slag, calcium is already present in several mineral hosts, but its contribution to vanadate formation during roasting is not well resolved. We investigated additive-free oxidation roasting of an industrial slag by combining automated mineralogy, X-ray diffraction, thermal analysis, and SEM–EDS. In the untreated slag, 83.16% of the normalized V signal was associated with spinel, whereas Ca was distributed mainly among silicates and other Ca-bearing phases. With increasing roasting temperature, spinel developed reaction rims and decomposed into intergrown oxides, vanadate-bearing regions, and reconstructed silicates. The Ca/(Ca+Mn) atomic fraction measured in selected vanadate-bearing regions increased from 0.320–0.397 at 700–800 °C to 0.786 at 850 °C (1 h), reaching 0.898 after 2 h at 850 °C. Nevertheless, at 850 °C (1 h), reconstructed silicate still accounted for 21.05% of the normalized Ca signal, while three vanadate groups together accounted for 42.65% of the V signal. These observations indicate that local Ca enrichment and continued V association with other product groups occur simultaneously. The proposed multi-host framework therefore identifies the initial Ca-bearing phases and their evolving contacts as important context for vanadate formation; it does not establish calcium diffusion pathways or an optimum for vanadium extraction.