| Vanadium,as one of the important strategic resource,is widely used in metallurgy,chemical engineering,and aerospace.It is well known that vanadium mainly exists in the form of vanadium-titanium magnetite?VTM?,and the main route for the extraction of vanadium in converter is by oxidizing the vanadium-bearing hot metal that is formed by reducing VTM in a blast furnace to enrich vanadium into vanadium-bearing slag in China.With the gradual consumption of high quality VTM,the low-grade bearing chromium VTM has caused much attention in recent years,but its wide application has restricted by technological problems.According to the actual vanadium-extraction smelting process,the present work studied the phase diagrams and melting properties as well as structure characteristics of the main components of Cr2O3-bearing vanadium slag,established a kinetic model of vanadium extraction for describing the oxidation behavior of different elements.Meanwhile,considering the status of refractories used for vanadium-extraction converters,a novel refractory as lining refractories of vanadium-extraction converters was designed after studying the wear mechanism to provide theoretical basis for the vanadium extraction from the chromium-bearing hot metal,and the following conclusions have been drawn:Base on the correction of the phase diagrams of FeO-V2O3 and SiO2-V2O3 systems,the phase diagrams of the FeO-SiO2-V2O3 ternary system were established and studied the effect of Cr2O3 on the system to establish the phase diagrams of FeO-SiO2-V2O3-Cr2O3 quaternary system as the main components of Cr2O3-bearing vanddium slag.The results indicated that the liquid phase region of the isothermal section diagrams extended gradually when the temperature increased from 1300 ? to 1400 ? for the FeO-SiO2-V2O3 ternary system,but the phase regions with FeO were shrank,and the number of phase regions were reduced from seven to five according to the isothermal section diagrams and the projection diagram of the FeO-SiO2-V2O3 system.For the FeO-SiO2-V2O3-Cr2O3 quaternary system,by calculation the isothermal section diagrams at 1350 ? and 1400 ? according to the actual blowing temperature and content of Cr2O3 in Cr2O3-bearing vanadium slag,it is known that the isothermal section diagrams at 1350 ? and 1400 ? had three quaternary phase regions,three ternary phase regions,and one quarter-phase region,including the liquid+FeO+FeV2O4+FeCr2O4,liquid+tridymite+FeV2O4+FeCr2O4,tridymite+V2O3+FeV2O4+FeCr2O4,liquid+FeV2O4+ FeCr2O4,liquid+tridymite+FeCr2O4,liquid+FeO+FeCr2O4,and liquid+FeCr2O4,respectively.In addition,by comparing the FeO-SiO2-V2O3 system,it was known that each of phase region increased FeCr2O4 phase in the isothermal section diagrams with introduction of Cr2O3 in the system.Meanwhile,the phase regions with FeO were enlarged,but "liquid+FeCr2O4+SiO2" and "liquid+FeCr2O4" phase regions were shrank with the increasing of w?Cr2O3?in the system.In order to reveal the properties of melt and viscosity as well as the effect of Cr2O3 and TiO2 for FeO-SiO2-V2O3 system,the properties of melt and viscosity as well as melt structure of Cr2O3-bearing vanadium slag were studied to clarify the internal relation between components of melt and the properties of melt and viscosity.The results indicated that the melting temperature increased from 1308 ? to 1475 ? when w?Cr2O3?increased from 0 to 5 wt%in FeO-V2O3-SiO2-Cr2O3 system,and the viscosity increased from 0.68 Pa·s to 8.94 Pa·s at 1400 ?.Correspondingly,the polymerization degree of the system was enhanced drastically due to that monomer structure?Q0?and a chain structure?V-O-V?in the melt transformed into Q2 and Cr-O-Cr band in a chain structure as well as the formation of Q3 species in a sheet structure.Furthermore,part of the chromium existed in the form of the high melting point of spinel?FeCr2O4?to worsen the melting properties.With the introduction of 13 wt%TiO2 into the FeO-SiO2-V2O3-5 wt%Cr2O3 system,the melting temperature and viscosity decreased from 1475 ? and 8.94 Pa·s at 1400 ? to 1380 ? and 1.48 Pa·s at 1400 ? respectively,and the polymerization degree of the slag became weak due to the formation of discrete Si-O-Ti and Ti-O-Ti inhibiting the formation of sheet structure?Q3?and hampering the crystallization of FeCr2O4 as well as increasing the solubility of Cr2O3 in melts to improve the melting properties of vanadium slag.According to actual smelting condition,a kinetic model of vanadium extraction from the hot metal with chromium by converter was established to clarify the kinetic behaviors of elements oxidation in the hot metal.The results showed that the oxidations of[Si]and[Ti]in the metal bath were rapid at the initial stage of blowing?0-2 min?,and[V]started to be intensively oxidized and a small amount of[C]was oxidized at the intermediate stage of blowing?2-4.5 min?.At the final stage?4.5-5 min?,the oxidation order of[V]and[C]reversed,causing vigorous oxidation of[C].Besides,[Cr]was oxidized throughout the intermediate and final stages,competing with[V]for oxygen.The components evolution of slag and temperature evolution of the bath were related to the melting rate of the coolants and the exothermic oxidation rate of elements in the metal bath.It was found that the calculated values using the models were in good agreement with practical measured values.In order to obtain the high oxidation rate and yield rate of vanadium,the content of[Si]in hot metal should be controlled in the range of 0.15-0.25 wt%,and the combined coolants should be adopted,keeping the endpoint temperature of semi-steel below 1380 ?.Base on the revelation of the wear mechanism of refractory used as linings of vanadium-extraction converters,a novel refractory was designed and clarified the internal relation between given compositions of refractory and the properties of refractory.The results indicated that the poor service life of MgO-C bricks was due to the poor sinterability of the oxidized layer at 1400 ? to reduce the compactness and strength of MgO-C bricks,and the application of slag splashing technology had little influence on extending of service life due to that slag coating was shed under the washing resistance in the converter.With the introduction of 10 wt%Fe into MgO-16 wt%C bricks,the apparent porosity,compressive strength and thickness of oxidized layer after heating at 1400 ? for 3 h in air were changed from 23.4%,7.9 MPa and 4?5 mm to 18.2%,17.3 MPa and 3?4 mm,respectively.With the introduction of 10 wt%Fe into MgO-14 wt%C bricks,the thickness of oxidized layer was decreased from 1.95 mm to 1.07 mm after slag corrosion resistance?heating at 1400 ? for 7 h in air?and the compactness and strength of the oxidized layer were increased due to the well sinterability of oxidized layer to obtain excellent oxidation resistance and corrosion resistance against vanadium containing slag.Therefore,it is considered that MgO-Fe-C brick is a favorable substitute of MgO-C refractory to be used as linings of vanadium-extraction converters. |
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