Fundamental Study On Vanadium Titanium Magnetite Carbon Composite Briquettes Used As Innovative Blast Furnace Burden

Vanadium titanium magnetite has abundant reserve and extensive distribution in China and owns great comprehensive utilization value.Currently,BF-conventer process is the main process of the vanadium titanium magnetite utilization in the world,which has many advantages such as large capacity,stable production and mature technique.However,the traditional agglomeration processes are difficult and high energy consumption due to the drawbacks of vanadium titanium magnetite.Besides,due to the poor metallurgical properties of vanadium titanium sinter and vanadium titanium pellet and the imperfect burden structure,the technical indices of the BF should be improved further.As an innovative BF ironmaking material,carbon composite iron ore briquette exhibits excellent reducing performance,high-temperature compressive strength,and great material adaptability.It is one of the effective approaches to realize low-carbon,low-temperature and high-efficiency BF ironmaking by charging carbon composite iron ore briquette into mixed burden.Therefore,vanadium titanium magnetite carbon composite briquette(VTM-CCB)was proposed as innovative burden in this research to improve the BF smelting efficiency and technical indices and promote the utilization of vanadium titanium resources efficiently.In this work,to obtain the key preparation parameters and application theory of VTM-CCB,various researches from different aspects have been conducted,such as the preparation and optimization of VTM-CCB,metallurgical characteristics and evolvement mechanisms,reduction thermodynamics and dynamics,softening-melting behavior of VTM-CCB and its effect on the mixed burden,high-temperature interaction mechanisms between VTM-CCB and traditional vanadium titanium burden.Besides,the technical feasibility of BF smelting VTM-CCB is evaluated by comparing the influence of ore-coke mixing and VTM-CCB charging on the softening-melting performance of mixed burden.The preparation and consolidation mechanism of VTM-CCB were investigated firstly.The compressive strength of VTM-CCB mainly depended on the softening of coal and the gel precipitation in briquetting process and the gel solidification and semi-coke formation in carbonization process.The preparation parameaters of VTM-CCB included a briquetting temperature of 200?,a FC/O ratio of 1.4,a coal particle size of<0.075 mm,and a vanadium titanium magnetite particle size of<0.075 mm,a carbonization temperature of 500?,a carbonization duration of 3 h,and a heating rate of 3?/min.Based on the optimized parameters,the compressive strength of VTM-CCB reached 2588.8 N and the residual volatile matter content was 1.58%,which meets the requirement of BF ironmaking.The metallurgical characteristics and evolvement mechanism of VTM-CCB were studied.The VTM-CCB exhibited great reduction degradation property,favorable shrinking performance,and adequate compressive strength after reduction and high-temperature compressive strength.In the temperature range from 500? to 900?,the analogous coke structure formed due to the secondary degassing-coking of the part of coal in VTM-CCB and performed a certain strength,which could buffer the stress generated during the lattice transition of iron oxides.Therefore,the RDI+3.15 was always higher than 95%.In the temperature range from 900? to 1100?,the volumetric shrinking of VTM-CCB occurred as a combination of reduction swelling of iron oxides as well as sintering of iron oxide,coal consumption,and iron whiskers growth suppression.In the temperature range higher than 1100?,the volumetric shrinking was correlated to the formation and conglomeration of molten slag and iron and residual solid carbon.The shrinking process of VTM-CCB followed the linear shrinkage model of carbon composite iron ore pellet,and the shrinking formula of VTM-CCB was Sh=t2/5 exp(a-b/T).Under BF conditions,the compressive strength after reduction and high-temperature compressive strength of VTM-CCB were decreased with enhancing the reduction temperature.With a FC/O ratio of 1.4 and an reduction temperature of 1100?,the compressive strength after reduction and high-temperature compressive strength were 603.4 N and 348.0 N,respectively.Reduction thermodynamics and dynamics of VTM-CCB were analyzed.The reducibility sequence of iron-bearing phases in VTM-CCB from high to low was Fe2O3,Fe3O4,FeO,Fe2TiO4,Fe2VO4,FeTiO3,and FeTi2O5.The phase transition of iron-bearing minerals could be described as follows:Fe3O4?FeO?Fe,Fe2.75Ti0.25O4?Fe2.5Ti2.5O4?Fe2TiO4?FeTiO3?TiO2;FeTiO3?TiO2;Fe2VO4?V2O3;FeCr2O4?Cr2O3.The reduction process of VTM-CCB could be described by porous material model.In the temperature range lower than 1000?,the reduction was controlled by carbon gasification reaction,while in the temperature higher than 1000?,it was controlled by internal diffusion.In the initial reduction stage,the activation energy E was 40.26 kJ·mol-1,and it was mixed controlled by gas diffusion and interfacial chemical reaction.In the intermediate and final reduction stage,the activation energy E were 66.46 kJ·mol-1 and 90.96 kJ·mol-1,respectively.In this two stages,the restriction factor was interfacial chemical reaction.The VTM-CCB exhibited great reducibility and its metallization rate reached 91.95%at 1100? with a FC/O ratio of 1.4.The evlovement of VTM-CCB during softening-melting process and its effect on mixed burden were investigated.In the softening-melting process,with enhancing FC/O ratio,the softening behavior of VTM-CCB improved.When the FC/O ratio exceeded 1.2,a substantial precipitation of Ti(C,N)was observed at the slag-metal interface,which deteriorated the dripping behavior of VTM-CCB significantly.With increasing FC/O ratio from 0.8 to 1.4,the VTM-CCB tansformed from iron-slag mixed dense structure to concentric circularity structure with dense metallic iron shell outer side and slag,iron and residual carbon mixtures inner side.With charging 20%VTM-CCB into the mixed burden,the softening property and permeability of mixed burden improved,and(TD-Ts)reached the minimal value of 129.5?.When the VTM-CCB charging ratio exceeded 25%,the precipitation of Ti(C,N)would deteriorate the dripping behavior of mixed burden.Ore-coke mixing technique could improve the softening-melting-dripping behavior of mixed burden,strengthen the reduction of iron-bearing materials,reduce the resorted slag in cohesive zone,and promote the gas permeability of packed bed.When the ore-coke mixing ratio was up to 50%,Ti(C,N)precipitated at the slag-metal interface,thereby deteriorating the dripping behavior of slag and metal.The softening-melting-dripping performance and permeability of mixed burden with VTM-CCB charging was obviously better than that with ore-coke mixing.The interactions between VTM-CCB and traditional iron-bearing materials were investigated.Due to the coupling effect between the iron oxides reduction reaction and carbon gasification reaction,the reduction of sinter and pellet could be promoted by the charged VTM-CCB,and the reduction of sinter was promoted much more than that of pellet.With a temperature of 1100? and a FC/O ratio of 1.4,the increase in reduction degree of sinter and pellet were 11.70%and 2.03%,respectively.Therefore,in view of strengthening the reduction of BF burden,the VTM-CCB charged into mixed burden by replacing some part of pellet was more reasonable and realistic.During the softening-melting process of VTM-CCB and pellet mixtures,with enhancing FC/O ratio of VTM-CCB,the sticking layer between VTM-CCB and pellet transformed from fayalite molten slag to dense metallic iron shell,and the sticking behavior was suppressed,which results in T4 and T40 decreased first and then increased,and Ts increased gradually.During the softening process of VTM-CCB and sinter mixtures,with a FC/O ratio of 0.8,a slag-iron mixed structure,mainly consisting liquid slag and least amount of metallic iron,formed at the interface between the two iron-bearing materials based on the sinter edge substrate.The slag-iron mixed structure could decrease the T4 and T40 obviously.With increasing the FC/O ratio to 1.4,the iron joined crystal and the aggregation growth of liquid slag and iron were suppressed by the residual carbon,which results in the interface sticking was invalid and the softening and shrinking behavior of packed bed was inhibited,thereby increasing the T4 and T40.During the melting process of VTM-CCB and sinter mixtures,the sticking layer transformed from liquid slag-iron mixed structure based on the sinter edge substrate to dense metallic iron shell based on the sinter edge substrate with increasing FC/O ratio of charged VTM-CCB,which could alleviate the collapse of the packed bed,guarantee the gas permeability of packed bed,strengthen the reduction of iron-bearing burden,and enhance Ts.An reasonable increasing of FC/O ratio could promote the carburization of iron and decrease the TD.However,with a FC/O ratio of 1.4,large amounts Ti(C,N)precipitated at the slag-metal and slag-carbon interface,which would deteriorate the fluidity of molten slag and suppressed the congregating and dripping of molten slag and iron.As a result,the gas permeability of packed bed deteriorated and TD increased notably.Fully considering the interaction during reduction,softening-melting-dripping process and the precipitation of Ti(C,N),the appropriate FC/O ratio should not be higher than 1.2.The fundamental study of VTM-CCB as innovative burden in this work enriches the agglomeration method and theory of vanadium titanium magnetite BF burden,and also provides theoretical foundation for its practical application in BF ironmaking.All these would further promote vanadium titanium magnetite BF smelting efficiently and the comprehensive utilization level of vanadium titanium magnetite resources.