| Shale belongs to vanadium resource with low grades,the vanadium（III）in shale mainly exists in the crystal lattice of mica group minerals,making the extraction of vanadium difficult.Low reaction efficiency and separation difficulty in extraction process have been the technical bottleneck to restrict the development of vanadium extraction from shale.In this study,sulfuric acid roasting was employeed to enhance the dissociation reaction of V-bearing muscovite and a combined method of crystallization and synergetic extraction was proposed to optimize the separation process of vanadium in a leaching solution containing high concentrations of aluminum and impurities.Meanwhile,the mechanisms of sulfuric acid roasting and synergistic solvent extraction were studied deeply.The main conclusions are shown as below.（1）Determination of sulphuric acid roasting conditions and the mechanism of sulphuric acid roasting.For the sulphuric acid roasting process,90.1%of the vanadium leaching efficiency can be obtained under the optimum conditions:a sulphuric acid dosage of50 wt.%,a water dosage of 30 wt.%,a roasting temperature of 200°C and a roasting time of 90 min.SEM-EDS and XRD were used to analyze the changes of microstructures and phases of minerals during the sulphuric acid roasting process.The SEM-EDS analysis shows that sulphuric acid roasting sample had a honeycomb structure.The XRD analysis shows that the crystal structure of muscovite was destroyed and vanadium was released,and then the K and Al in the muscovite were converted into KAl（SO4）2 and Al2（SO4）3.Combined with the FT-IR analysis,the reaction process of sulphuric acid roasting can be described as:sulfuric acid diffusion,concentrated acid medium formation,muscovite dehydroxylation,metal ion sulphation and the vanadium transformation into free-state.Based on the shrinking core model and taking the phase conversion rate as the evaluation indicator,the sulphuric acid roasting was controlled by chemical reaction during the roasting temperature of120°C-140°C,and the control step was converted into internal diffusion control as the temperature rose to 160°C-220°C.（2）V-bearing leaching characteristics of sulphuric acid roasting under different leaching methods.The effects of leaching time and solid-liquid ratio on V,Al,K and Fe leaching behaviors under room temperature leaching and heating leaching was investigated in this part.The room temperature leaching had lower leaching rates and the results shows that a lower solid-liquid ratio can suppress the dissolution of Al,K and Fe in the intial stage,moreover,a higher solid-liquid ratio was in favor of V dissolution in the latter stage.On the contrary,the heating temperature leaching had higher leaching rates.At the beginning of the leaching process,soluble sulfates and free-state V were leached rapidly in the acidic solution system,resulting in the formation of mineral particle cracks and the diffusion channels,which can promote secondary reaction between the unreacted silicic acid and lixiviant.Therefore,the vanadium leaching efficiency can reach as high as 90.1%under the heating temperature leaching but massive Al,K and Fe impurities were also leached along with V.The ammonia-nitrogen wastewater from vanadium precipitation can be also used as lixiviant to extract V.（3）A combination process of crystallization and synergistic solvent extraction to separate and enrich vanadium from leaching solution and the mechanism study of synergistic solvent extraction.The Al was selectively separated in form of KAl（SO4）2·12H2O and NH4Al（SO4）2·12H2O by cooling crystallization.The comparative study on vanadium extraction from leaching solution and Al-removed leaching solution showed that the counter-current stage was decreased and aluminum co-extraction was suppressed,strengthening vanadium extraction.Meantime,the co-extraction coefficient k was put forward to quantitatively analyze the influence of aluminum co-extraction on vanadium extraction process.On the basis of extraction performance of D2EHPA and PC88A,the composite extraction system of D2EHPA-PC88A was proposed and its extraction ability was studied.The experimental results showed that the complex system has a synergistic effect on vanadium（IV）extraction,which belongs to the synergistic extraction system and can strengthen vanadium（IV）extraction and enrichment process.FT-IR was used to analyze the changes of the functional groups of extractant after vanadium（IV）loaded by comparison with fresh extractant.The results suggested that the cation exchange and coordination reaction contribute to vanadium（IV）synergistic extraction.From the point of molecular structure of extractant,a synergistic extraction strengthening mechanism of vanadium extraction was revealed.on the one hand,by comparison with the hydrogen bonds between the same extractant,a more stable hydrogen bonds can be formed between D2EHPA and PC88A,promoting H+dissociation from D2EHPA and VO2+exchange reaction.On the other hand,as a result of a strong coordination ability of P=O,PC88A can replace ligand molecule D2EHPA from extraction compound or be directly involved in the coordination reaction to generate a more stable extraction compound,which can strengthen vanadium extraction process.Based on the characteristics of high-aluminum and poly-impurities in the sulfuric leaching solution of vanadium-bearing shale,a combination process of crystallization and synergistic extraction was proposed.Vanadium was selectively separated and effectively enriched from sulfuric leaching solution by double reinforcement of aluminum co-extraction control with crystallization and synergistic effect of composite system.According to the above,a combination process of sulfuric acid roasting and synergistic solvent extraction was proposed to extract vanadium.The recovery of vanadium can reach 83.8%and the product V2O5 has a purity of 99.21%,via sulfuric acid roasting,heating leaching,cooling crystallization,synergistic solvent extraction and ammonium precipitation,realizing vanadium efficient extraction from shale. |
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