Study On The Effect Of Na₃AlF₆ On The Roasting And Leaching Processes For Vanadium Extraction From Vanadium-bearing Shale

Vanadium-bearing shale is a unique vanadium ore resource in China,and its reserves is extremely abundant.In general,most of the vanadium in vanadium-bearing shale mainly exists as trivalent vanadium?V????which substitutes for trivalent alumium?Al????in the octahedral crystal lattice of the mica minerals as an isomorphism,and it is difficult to be extracted.The method used usually is to destroy the crystal structure of mica minerals by means of additives and high roasting temperature,which contributes to enhancing the leaching efficiency of vanadium.The introduction of sodium salt in the roasting process is beneficial to the formation of soluble sodium vanadate.And the fluoride can destroy the crystal structure of mica minerals in the leaching process.Na₃AlF₆ is often used as a flux in aluminum electrolysis industry,which can lower the melting point of alumina.Furthermore,in the abrasives manufacturing industry,Na₃AlF₆ can also reduce the refractoriness of low melting ceramic binders.In the course of the study,we find that a small amount of fluorine-containing sodium Na₃AlF₆ is used as a roasting additive in the roasting process,which can enhance the extraction of vanadium.The process of roasting with fluorine-containing sodium Na3AlF6 and leaching with sulfuric acid has been studied in this paper by using vanadium-bearing shale in Hubei province as raw material.And the influence of the main process parameters in the roasting and leaching process on the leaching efficiency of vanadium has been investigated by comparison of Na₃AlF₆roasting and blank roasting methods.The results showed that under the conditions that the roasting temperature was700°C,the roasting time was 1 h,Na₃AlF₆ dosage was 3%of raw ore quality,the sulfuric acid concentration was 20%,the liquid-to-solid ratio was 1.5 m L/g,the leaching temperature was 95°C,and leaching time was 3 h,the vanadium leaching efficiency reached 90.57%,which increased by 27.17%compared with that of blank roasting and sulfuric acid leaching process.The addition of Na3Al F6 had a significant effect on the leaching efficiency of vanadium.While adding Na₃AlF₆ in the roasting process,the roasting temperature could be reduced from 900°C to 700°C,and thus the energy consumption had been reduced.To achieve the same leaching result,under the condition of adding Na3AlF6 in the roasting process the amount of sulfuric acid required was lower than that of not adding Na3AlF6.Moreover,the leaching time could be shortened to 3 h by adding Na3AlF6 in the roasting process,and the reaction rate of vanadium was increased.Combining XRD,FTIR,SEM-EDS analysis and thermodynamic analysis,the mechanism of enhancing the vanadium extraction by using Na₃AlF₆ was studied.The results showed that with the increase of roasting temperature mica decomposed gradually and was transformed into feldspar minerals.During the roasting process Na₃AlF₆ participated in the decomposition reaction of mica,which reduced the Gibbs free energy of the decomposition reaction of mica.As a result,the decomposition of mica occurred at lower temperature and then the damage degree of mica structure was increased.Besides,the introduction of Na₃AlF₆ could promote the formation of soluble sodium vanadium.Thus,the vanadium was more easily to dissolve during sulfuric acid leaching process and the efficiency of vanadium was improved.Moreover,while the neighborite that generated during roasting process dissolved in sulfuric acid solution during the leaching process,hydrofluoric acid formed.Hydrofluoric acid could strengthened the break of the bond between Al-O in crystal structure of mica to form stable ions,which reduced the Gibbs free energy of the dissolution reaction of mica.Therefore,the damage degree of mica structure was increased,and vanadium was enhanced further released.