Fundamental Applied Research On Resource Utilization Of Vanadium Tailings

Vanadium taillings are the residues of vanadium slags after vanadium extraction from iron and steel enterprise. In order to solve the environmental problems caused by stockpiling of tailings produced from vanadium produciton, much research has been done, mainly foused on the following three fields including secondary vanadium recovery, recovery of iron by reduction, and utilization as building materials. However, large-scale cleaner utilization of vanadium tailings is yet to be realized. In this research, alkali medium is utilized to efficiently decompose vanadium tailings, and vanadium and associated chromium can be recovered simultaneously accompanying the silicate decomposition process. After cleaner phase separation from the leaching solution, calcium silicate and vanadate products are obtained. The utilization of Fe-rich residues can be realized after sodium removal and fluidized reduction process. Further, this vanadium tailing comprehensive utilization process is a clean process due to zero discharge of waste residues, harmful kiln gas, and ammonia-nitrogen waste water. This dissertation systematically addresses topics including the vanadium tailing decomposition regularities, macro kinetics, and mechanisms in alkli medium, phase equilibrium investigatino with respect to vanadium, chromium, and silicon compounds in alkaline solution, and resourceful utilization of Fe-rich residues strategies.In this dissertation, the following results and progress were obtaineed:(1) Elements distribution rule of vanadium tailings have been discovered. Most of vanadium and chromium compounds are distributed in Fe-containing phasese which are randomly dispersed in the acmite phase. Above 92% of vanadium in vanadium tailings is in high valence states and soluble in alkaline media. Acmite can not be decomposed by roasting and acid leaching, however, can be efficiently decomposed by alkline medium. Accompanying the decomposition of acmite, vanadium can be efficiently leached out. The thermodynamic analysis suggests that vanadium tailing decomposition r in strong alkaline solution is energetically favorable, and the the formation of vanadate, chromate, and silicate soluble salts is exothermic.(2) The conditions for effective decomposition of vanadium tailing in sodium hydroxide solutions were systemically studies. The leaching of vanadium and silicon compounds is observed to follow unreacted core contraction model, and the controlling step is determined to be diffusion through product layer with vanadium leaching activation energy of 41.29 kJ/mol and silicon 59.35kJ/mol, respectively. The leaching kinetics equations for vanadium and silicon are 1+2(1-X)-3(1-x)2/3=6.0×102e 41290/RT·t and 1+2(1-X)-3(1-X)2/3=7.045×104 e 59350/RT·t, respectively. The vanadium and silicon leaching efficiencies could be strengthened by particle size reduction and agitation intensification. The chromium leaching processin NaOH-O2-H2O system is confirmed to follow unreacted core contraction model, and the control step is described by interfacial chemical reaction control with leaching activation energy of 41.73kJ/mol. The chromium leaching kinetics equation is 1-(1-X)1/3=50.34e 41730/RT·t. The key factors determing the chromium dissolution process are NaOH and O2 concentration and reaction temperature.(3) The phase equilibria of silicate, vanadate, and chromate salts in the leaching solution alkalis studies. It is concluded that low alkalinity, high temperature, and excessive Ca(OH)2 are beneficial for desilication. The optimal desilication conditions were determined to be NaOH concentration of 100-350 g/L, temperature above 80℃, and Ca(OH)2 to SiO2 molarratio of 1.5. Under these desilication conditions, the SiO2 concentraion could be controlled to be under 6 g/L. Further, via hydrothermal synthesis, the desilication product can be converted to xonotlite 6CaO·6SiO2·H2O by controlling the system Si/Ca ratio to be 1:1.(4) The phase diagrams of vanadate salts have been studied. From the NaOH-Na3VO4-Ca(OH)2-H2O quartenary phase diagram, it is concluded that 98% of Na3VO4 could be calcified when the solution NaOH concentration is below 150 g/L, and uniform rod shape Ca10V60O5 can be obtained.(5) A Fe-rich residue utilization process of has been investigated. The sodium content in the residue produced via sub-molten medium could be effectively reduced. However, if hydrothermal method is used to treat the vanadium taling, the sodium in the residue can only be reduced via alkaline solution treatment under hydrothermal conditions. And, the Na2O content in the residues can be controlled to be below 1%. Fe-rich residues after sodium removal could be easily reduced and black vitrified at 500-600℃ in the fluidized bed.(6) Three sets of processes for the treatment of vanadium tailings have been proposed. In seperate vanadium extraction process by sub-molten salt medium, vanadium extraction rate could reach to 93% at reaction temperature 170℃, reaction time 3 h, and alkali concentration 80 wt%. In hydrothermal seperate vanadium process, vanadium extraction rate could be stabilized at 87% at reaction temperature 270℃, reaction time 3 h, and alkali concentration 30 wt%. In vanadium and chromium co-extraction process by sub-molten salt medium, vanadium and chromium leaching rate could reach 97% and 89%, respectively, with reaction temperature 250℃, reaction time 3 h, alkali concentration 50 wt% and oxygen partial pressure 1 MPa. The mass and energy balances of these three sets of processes have been calculated uing software METSIM, and the corresponding energy consumption of three processes are 1326093 kcal/h,655770 kcal/h, and 793715 kcal/h, repectively. It is therefore more economical from energy saving point of view to select vanadium recovery via hydrothermal treatment process for pilot plant tests, and the capical cost for such process is less, showing attractive prospective.