Abstract:In recent years, with the development of the iron and steel industry, ordinary vanadium-titanium magnetite shows a decreasing trend. The high content of chromium vanadium titanium magnetite has gradually attracted the attention of researchers. During smelting using vanadium-containing titanium magnetite, chromium and vanadium are enriched in the vanadium slag to the same phase as the spinel phase, commonly referred to as high-chromium vanadium slag (HCVS, Cr mass fraction is >5%). Chromium, as a precious metal, is a resource to be reckoned with. However, chromium spinel is thermodynamically more stable than vanadium spinel and cannot be recovered synchronously at vanadium roasting temperatures. In the current vanadium extraction process for low chromium vanadium slag (LCVS), chromium is removed as an impurity. In the case of HCVS, due to its comparable content, chromium is no longer ignored and HCVS can be considered a valuable feedstock for simultaneous chromium extraction. The potential damage to the environment from the toxicity and carcinogenicity of chromium (Ⅵ) makes the comprehensive utilization of HCVS difficult. Vanadium residues currently used for large-scale industrial production and systematic laboratory studies have low chromium content, and high-chromium vanadium residues are gradually gaining attention because of their huge reserves and potential threat to the environment. However, little research has been done on the extraction behavior of chromium during calcification roasting of HCVS. Activation roasting plays an important role in the treatment of vanadium slag by destroying the structure of high vanadium slag and promoting the oxidation to produce high-valent vanadium. Calcification roasting is currently receiving attention as a more environmentally friendly pretreatment method, where calcium salts calcify vanadium spinel to calcium vanadate, which is insoluble in water but soluble in acid. Compared to conventional sodium roasting, calcification roasting avoids sintering and the generation of harmful gases, and the residual vanadium and chromium in the tailings are insoluble, avoiding the threat to water. Several studies have reported that vanadium extraction rates are influenced by a number of factors such as calcium salt addition, roasting temperature, holding time and particle size. In the calcareous roasting-HCVS acid leaching process, calcium salts can selectively calcify vanadium and retain chromium in the leach residue after leaching. Therefore, asynchronous extraction of vanadium and chromium by calcification roasting may be a new and environmentally friendly method for the separation of vanadium and chromium. However, the development of calcification roasting is hampered by the low vanadium recovery compared to sodium salt roasting. This is mainly due to the fact that vanadium recovery is highly dependent on the type of calcification roasting sample. During acid leaching, variations in pH value and lack of leaching power resulted in lower vanadium leaching rates. Therefore, in this paper, calcification roasting is used, and very little chromium can be extracted from the leach solution under conditions where the vanadium leaching ratio is maximized. Almost all of the chromium is retained in the leaching slag, which provides a feasible method for separating vanadium-chromium. At the same time, the threat to the environment can be effectively avoided. The XRF results show that the chromium in the vanadium slag is as high as 10.97%, while the vanadium content is only 8.75%. By examining the optimal conditions of the roasting and leaching regimes, the primary leaching rate of vanadium can be up to 88.99%, whereas the chromium is basically not leached out, and the vast majority of the chromium is retained in the leaching slag. XRD and SEM results show that vanadium slag is rich in iron and silicon elements, of which vanadium exists mainly in the trivalent ionic state in ferrovanadium spinel (FeV2O4) before roasting. The enriched forms of vanadium in vanadium slag are mainly in ferrovanadium spinel (FeV2O4) and vanadium oxide (V3O4), with small amounts of vanadium in ferrochromium spinel and ferrotitanium spinel, in which the calcium silicate phase is pyroxene, and together these constituents make up the mineral diversity of vanadium slag. After roasting, the silicate phase in the vanadium slag is destroyed and the vanadium-iron spinel gradually decomposes and releases vanadium oxides, mainly V2O4 and V2O5. The diffraction peaks of SiO2, (Mn,Fe)(V,Ti,Cr)2O4 and Ca(Fe,Mg)Si2O6 appear after vanadium slag roasting and leaching. It indicates that the mineral phases containing Mn, Fe, V, Ti, Cr and other elements are effectively dissociated during the leaching process, and some pyroxene phases are retained. The post-calcium roasting vanadium extraction liquid is basically free of chromium and can be properly disposed of. In addition, the slag produced in the leaching process can be used as a new raw material for extracting chromium, which further broadens its application field and market prospect. The results of the study can not only provide data support for vanadium-chromium extraction from high-chromium vanadium slag, but also provide some help in reducing the chromium leaching rate from vanadium slag and improving the direct recovery of vanadium in the enhanced smelting process.