Study On Reaction Mechanism Of Vanadium Slag With High MgO Content During High Temperature Roasting

Vanadium is an important strategic resource for China,and an important additive for modern industry,widely used in the steel,chemistry,aerospace,etc.Vanadium slag is the most predominant raw material for vanadium extraction in the world.The main methods of vanadium extraction widely used in industry are sodium roasting-water leaching and calcium roasting-acid leaching,but both have certain limitations.Sodium and calcium salt roasting additives present serious disposal problems in terms of waste water,gas or solid waste,with serious pollution and low resource utilisation.Existing roasting methods involve cooling the hot vanadium slag from 1300-1500℃ to room temperature for pre-treatment such as crushing,ball milling,magnetic separation and iron removal,mixing and then secondary heating to 800-1000℃ for roasting,making the waste heat of the vanadium slag completely wasted.Therefore,based on two considerations of making full use of the residual heat of vanadium slag and exploring a novel green vanadium extraction process,this paper investigates the evolutionary behaviour of oxidation roasting process of vanadium slag with high MgO content during high temperature roasting,ultimately providing new ideas for exploring a new clean vanadium extraction process.The main phases of vanadium slag are s（Fe,Mg）V₂O₄,（Mg,Fe）₂Ti O4,Fe Mn₂O₄and Fe₂Si O₄,where V is concentrated in the spinel phase.By adding a certain amount of MgO to the molten industrial vanadium slag,the recipe was adjusted to obtain vanadium slag with different MgO content composition ratios.The results show that the addition of MgO increases the starting melting temperature of the vanadium slag,increases the amount of liquid phase at 1450℃,and helps to reduce the viscosity of the slag and the degree of polymerisation of the melt.XRD and SEM/EDS analyses show that V in vanadium slag with high MgO content is always enriched in the spinel phase,however the added Mg²⁺gradually replaces Fe²⁺in the iron olivine phase,gradually producing magnesia olivine phase Mg₂Si O₄.The high temperature roasting process using waste heat of the vanadium slag was simulated by blowing oxygen into the molten vanadium slag with high MgO content.The results show that the sequence during high temperature roasting of the phases of vanadium slag with high MgO content is Fe₂Si O₄/Mg₂Si O₄,Fe₂Ti O₄/Fe V₂O₄ and Mg V₂O₄.The main phases produced after complete oxidation of the roasted vanadium slag are Fe₂Ti O₅,Mg Fe Al O₄ and（Mg,Mn）₂V₂O₇.The vanadium content of（Mg,Mn）₂V₂O₇ in roasted vanadium slag with high MgO content is essentially the same,remaining at around 20 at%,but the（Mg,Mn）₂V₂O₇ content increases as MgO content increases.Vanadium slag with less than 8.58 wt%MgO,the olivine phase dominated by Fe₂Si O₄ reacts preferentially to produce the intermediate phase Mg Si O₃,which continues to oxidise,and the Ti in the spinel phase oxidises to produce pseudobrookite Fe₂Ti O₅,a large amount of V entering the silicate matrix,and a small amount is solid-dissolved in the pseudobrookite Fe₂Ti O₅.In the late oxidation stage,The amorphous morphology of the solid solution of magnesium pyrovanadate（Mg,Mn）₂V₂O₇ began to incorporate from the silicate matrix.The vanadium slag with high MgO content of 12.74 wt%,Mg₂Si O₄ directly supplies Mg²⁺to the vanadium slag oxidation and magnesia processes,and Ti and V in the spinel rapidly dissociate from the interior,then Ti begins to form the ferroplate titanite phase Fe₂Ti O₅,and V directly forms the amorphous magnesium pyrovanate solid solution（Mg,Mn）₂V₂O₇.The effect of high temperature roasting of vanadium slag was measured by vanadium leaching rate in the leaching process.The roasted vanadium slag leached for30 min at a liquid to solid ratio of 5:1,temperature of 70℃,p H of 0.5 and stirring speed of 100 rpm.The phase of the leached residue is Fe₂Ti O₅,Mg Fe Al O₄,Fe₂O₃ and Si O₂.The leaching rate of vanadium slag increases obviously with the increase of oxidation roasting time and the content of MgO.The vanadium slag with high MgO content of 12.74 wt%can achieve a maximum vanadium leaching rate of 64.09%after30 min of oxidation.