| Vanadium,titanium and chromium are significant strategic metal resources and distributed in high-chromium vanadium-titanium magnetite with relatively considerable amounts.High-chromium vanadium-titanium magnetite has great developing value with great quantities of reserves,and the comprehensive utilization of the ores is of great importance to the development of national economy and the guarantee of national security.In this paper with high-chromium vanadium-titanium magnetite as the fundamental raw materials,on the basis of preparing qualified high-chromium vanadium-titanium magnetite pellets(HCVTMP),the crushing strength,reduction.swelling,reducibility,reduction kinetics and softening-melting-dripping characteristics and mechanism were researched,in which effects of valuable components of TiO2,Cr2O3,B2O3 and CaO on the metallurgical properties of HCVTMP used in the blast furnace were investigated,and following conclusions can be drawn:(1)The crushing strength of high-chromium vanadium-titanium magnetite pellets decreased evidently with increasing the contents of TiO2 and Cr2O3,and the grinding treatment for high-chromium vanadium-titanium magnetite and titanium concentrate was beneficial to improving the crushing strength.The crushing strength of HCVTMP increased obviously with the increase of B2O3 content,and first increased and then decreased with the increase of CaO content,in which the crushing strength reached the maximum value when the mass percent of CaO addition was 2%.It is demonstrated that the crushing strength of HCVTMP has great relations with the phase compositions,pellet structure,porosity changes and microstructures and TiO2,Cr2O3,B2O3 and CaO have individual influencing mechanism.(2)It is obtained that TiO2,Cr2O3,B2O3 and CaO act as disincentives on the reduction swelling of HCVTMP,but with different restraining effects.The reduction swelling degree of HCVTMP decreased normally with the increase of TiO2 and Cr2O3 contents,while the reduction swelling degree decreased rapidly to nearly zero with increasing B2O3 content.The reduction swelling degree decreased as a whole with increasing CaO content,but special CaO content ranges may cause the increase of the reduction swelling degree.Effect of Cr2O3 content on the reduction of HCVTMP was obvious,and the reduction extents first increased slightly and then gradually decreased with increasing Cr2O3 mass percent from 0.28%to 8.22%.(3)On one hand,the non-isothermal reduction kinetics of HCVTMP at 400-1100? in the lumpy zone under simulated blast furnace conditions was studied.It is obtained that the reduction temperature and atmosphere have great effects on the reduction rates,and two different reduction stages existed with the infection point of 900?.The reaction order was in accordance with the first-order geometric convergence.It is calculated that the reaction activation energy was 42.6kJ/mol,and the foremost controlling steps were interfacial chemical reaction and gas internal diffusion through the product layer in non-isothermal reduction process.On the other hand,the isothermal reduction kinetics of HCVTMP at 600-900? was investigated.For the gas atmosphere of CO-CO2-N2,it is indicated that the reduction is controlled by a combined effect of gaseous internal diffusion through solid product layer and interfacial chemical reaction before 120min,and to be specific,at the initial stage the controlling step was predominantly interfacial chemical reaction;with the reduction proceeding before 120min the controlling step gradually became mixed control of interfacial chemical reaction and gas internal diffusion through solid product layer;after 120min,the controlling step became irregular.For the gas atmosphere of CO-N2,it is indicated that the reduction is mixed controlled by interfacial chemical reaction and gaseous internal diffusion through solid product layer,and to be specific,at the initial stage,the controlling step was mainly interfacial chemical reaction;With the reduction proceeding,the controlling step changed to be interfacial chemical reaction and gas internal diffusion through solid product layer when the reduction degree was lower than 30%;The controlling step was mainly gas internal diffusion through solid product layer when the reduction degree was above 35%;However,the controlling step percentage of interfacial chemical reaction reversely decreased to some extent due to the reduction restraining caused by the reaction between wustite and iron when the reduction degree achieved 60%.(4)With the increase of TiO2 mass percent in the range of 2.47%?12.14%,the softening start temperature and softening temperature gradually increased,and the softening zone gradually narrowed down;the melting start temperature and the dripping temperature increased,and the melting-dripping temperature zone was also widened.The permeability index became worsened as a whole.In the process of slag-iron's dripping and separating,it is proposed that Cr and V amounts that move to the melted iron are obviously more than those to the slag,while Ti amount that moves to slag is greatly more than that to the melted iron.It is demonstrated that Ti(C,N)generates increasingly with the increase of TiO2 contents,and accumulates as especial regular rigid granules on the surface of coke.With increasing Cr2O3 mass percent from 0.28%to 8.22%,the softening start temperature and the softening temperature gradually increased,and the softening zone increased as a whole.The melting start temperature increased as a whole.The dripping temperature gradually increased,the melting-dripping zone increased quickly to a relatively high value of above 230?,and the permeability index increased evidently,indicating the deterioration of melting-dripping index.The dripping difficulty increased with the increase of Cr2O3 contents is in accordance with the observed Cr-multicarbides and Cr-carbides and the microstructure and micro area composition of the not-dripped and dripped melted iron and slag.With increasing the mass percent of B2O3 additives at the range of 0%-4.5%,the softening start temperature and the softening zone increased obviously;the melting start temperature firstly decreased and then increased;the dripping temperature decreased evidently;the melting-dripping zone first increased and then rapidly decreased;the permeability index became better and better.It is demonstrated that the smelting properties with certain B2O3 range get improved and optimized greatly besides the softening stage.The formation of Ti(C,N)was greatly avoided and the slag and melted iron were separated greatly well in the smelting process with the addition of B2O3.B was great easily transformed to the slag,and this phenomenon became more and more evident with increasing B2O3 additives.The increase of boron oxides in the slag lowered the transformation ratio of titanium oxides to the slag.With increasing the mass percent of CaO additive from 0%to 8%,the softening start temperature and the softening end temperature increased as a whole with increasing CaO additives;the softening zone first increased and then decreased;the melting start temperature and the dripping temperature increased wholly;the melting zone was gradually widened;the gas permeability became improved.Although HCVTMP showed the highest crushing strength with CaO addition about 2 mass%,more CaO addition may be needed to achieve high permeability of the furnace burdens and good formation and separation condition of the slag and melted iron.It can be obtained that CaO could have relations with the transformation behavior of Cr,V and Ti in the formation process of the slag and melted iron to some extent.The restraining formation of Ti(C,N)and the promoting formation of CaTiO3 are in accordance with the improved melting-dripping index including the decrease of the maximum differential pressure and gas permeability and the increase of the melting-dripping zone and dripping difficulty.The fundamental research work and theoretical investigation system of high-chromium vanadium-titanium magnetite are effectively replenished and perfected and the development and utilization of this kind of beneficial polymetallic co-associated iron ore resources are promoted to be comprehensivized and efficient through the present study. |
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