| Hongge ore,deposited in Panxi area,is the largest vanadium titanomagnetite resource in China.Besides the valuable elements of iron,titanium,and vanadium,it contains a high content of chromium.Because the reserve of Cr2O3 is 900 Mt,it presents a significant strategic position and requires comprehensive utilization.At present,vanadium titanomagnetite is mainly utilized in blast furnace-converter process,which leads to serious pollution and waste of resources due to the low recovery rates of the valuable elements.On the other hand,because of the high energy consumption,relative inefficiency and poor activity of titanium-containing slag,coal-based direct reduction method is still far from large-scale industrial production as well.The "12th Five-Year Plan for the Comprehensive Utilization and Industrial Development of Vanadium and Titanium Resources" points out that the south mining area of Hongge ore should be protected strictly as a national major strategic resource,before a large-scale recycling and utilization method is found.Based on this aspect,the present work aims at a clean and efficient utilization method for Hongge vanadium titanomagnetite by using gas-based shaft furnace process.The oxidation roasting and gas-based direct reduction behaviors of Hongge vanadium titanomagnetite pellets was investigated systematically by the means of X-ray diffraction analysis(XRD),scanning electron micros cope(SEM),and energy-dispersive spectroscopy(EDS),etc.The main contents and conclusions of the present work are given as follows.(1)The preparation of oxidized Hongge vanadium titanomagnetite pellets was carried out based on the study of fundamental characteristics of the ore.The results indicate that higher preheating and roasting temperatures,longer preheating and roasting time in a certain range could improve the compressive strength of preheated and roasted pellets.Good compressive strengths of preheated and roasted pellets,for instance 448 and 2893 N(roasting time of 15 min),can be obtained under the optimum parameters,namely preheating temperature of 900?,preheating time of 10 min,roasting temperature of 1200?,and roasting time of 15?20 min.(2)The oxidation induration process and non-isothermal oxidation kinetics of oxidized Hongge vanadium titanomagnetite pellets were studied.It is found that during the oxidation induration process,the phase transitions of iron-bearing,titanium-bearing,vanadium-bearing and chromium-bearing minerals are presented as "Fe3O4?Fe2O3","Fe2.75Ti0.25O4?FeTiO3?Fe9TiO15","Fe2VO4?(Cr0.15V0.85)2O3" and "FeCr2O4?(Fe0.6Cr0.4)2O3/Fe0.7Cr1.3O3/(Cr0.15V0.85)2O3",respectively.Three stages are identified during this process:(a)the oxidation of magnetite and titanomagnetite mainly at 300?500?;(b)besides the oxidation reaction the formation of Fe-Cr and Cr-V solid solutions at 500?900?;and(c)the evident increase of compressive strength at 900?1200?.The value of activation energy calculated by Flynn-Wall-Ozawa method increases first(conversion degree<0.4)and then decreases with the increase of conversion degree.The average value of activation energy is 69.33 kJ/mol.(3)The gas-based direct reduction kinetic behavior of oxidized Hongge vanadium titanomagnetite pellets was investigated.The isothermal reduction results are shown as follows:? higher reduction temperature,bigger ?(H2)/?(CO)ratio and smaller pellets are favorable for the reduction.The optimum reduction parameters in the present work are obtained,namely reduction temperature of 1050?,?(H2)/?(CO)ratio of 2.5 and pellet diameter of 8?11 mm.?During the gas-based direct reduction process,the phase transitions of iron-bearing,titanium-bearing,vanadium-bearing and chromium-bearing minerals are presented as "Fe2O3?Fe3O4?FeO?Fe","Fe9TiO15?Fe2.75Ti0.25O4?Fe2TiO4?FeTiO3?TiO2,","(Cr0.15V0.85)2O3?Fe2VO4" and "(Fe0.6Cr0.4)2O3/Fe0.7Cr1.3O3/(Cr0.15V0.85)2O3?FeCr2O4",respectively.? Under the same reduction conditions,higher preheating and roasting temperatures,longer preheating and roasting time in a certain range during oxidation roasting process could enhance the reduction process and inhibit reduction swelling with a higher reduction degree.Although preheating and roasting conditions are different,the reduced pellets still present the same phase composition.Besides,the reduction swelling also increases with the increase of porosity of roasted pellets.? Interfacial reaction is considered as the controlling step during the reduction process at the initial stage,while both interfacial reaction and internal diffusion control the reduction process at latter stage.The reduction swelling increases with the increase of reduction temperature and the decrease of ?(H2)/?(CO)ratio.The non-isothermal reduction results show that the phase transitions of iron,vanadium,and chromium under non-isothermal reduction are the same as isothermal reduction,but the titanium-bearing minerals are only reduced to Fe2TiO4 instead of TiO2.(4)The effects of valuable elements on the oxidation roasting and gas-based direct reduction behaviors of Hongge vanadium titanomagnetite pellets were focused on.It implies the increase of the addition of V2O5(0%?6%),TiO2(0%?9%)and Cr2O3(0%?12%)decreases the compressive strength and increases the porosity of roasted pellets,resulting in a detrimental effect on the oxidation induration.Due to different porosities,phase compositions,and micro structures of roasted pellets,the influential mechanisms of the addition of V2O5,TiO2 and Cr2O3 are different.Under the same reduction conditions,with the increase of V2O5 addition,the reduction is accelerated at the early stage of the reduction process,and then a slower reduction degree follows.The addition of TiO2 could retard the reduction and cause the decrease of reduction degree,especially at the latter stage of reduction.Cr2O3 has a complex effect on the reduction process,and it varies at different reduction stages.V2O5 and Cr2O3 restrict the further reduction of Ti-rich phases and the aggregation of metallic iron.In contrast to TiO2,they also aggravate reduction swelling and decrease compressive strength of the reduced pellets. |
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