| Chinahas become a major steel producer and consumer,and the demand for iron ore is also growing.however,the average grade of iron ore resources in China is low,with the characteristics of less rich ore and more poor ore.It is subject to imports and is subject to other countries.In order to get rid of this dile mma,China began to explore the use of self-produced low-grade iron ore.Vanadium-titanium magnetite has a large number of strategic elements including titanium,vanadium and chromium in addition to iron.The development and utilization of such iron ore resources will play an increasingly important role in the development of steel industry of China.Since a large amount of components are present in the vanadium-titanium magnetite in the form of symbiotic or associated,the composition thereof is complicated,which may affect the sintering properties of the vanadium-titanium sintered ore.When the vanadium-titanium magnetite is sintered,ahigh-melting-point andhigh-brittle calcium titanate is precipitated in a large amount,and then the content of the calcium ferrite is reduced,resulting in deterioration of the sintering strength.The reason for the influence of the strength of the vanadium-titanium sinter is due to the fact that the precipitated calcium titanate destroys the calcium ferrite and causes the calcium ferrite content to decrease,which is not fully understood.At the same time,it is considered that the oxides such as TiO2,MgO and Al2O3 in the vanadium-titanium magnetite will react with CaO andhave an important influence on the properties of the sintered ore.Therefore,the reaction of Cr2O3 associated withhigh-chromium vanadium-titanium magnetite with CaO during sintering cannot be underestimated.In this thesis,calcium ferrite/calcium titanate diffusion couple and CaO/Cr2O3 diffusion couple were prepared firstly.Then,the diffusion mechanism affecting the reaction was explored based on the study of element diffusion and reaction process.The result shows:(1)The synthesis of calcium ferrite powder can be achieved by mixing Fe2O3 and CaO powder,pressing,and then ba king at a temperature of 1100℃ for 8 h,and then removing the impurities.(2)Preparation of calcium ferrite/calcium titanate diffusion couple can be achieved by combining calcium ferrite and calcium titanate powder into tablets and then sintering in an air atmosphere at 1000℃ for 2 h.(3)After the diffusion reaction occurs,a longitudinally distributed elongated line appears along the interface between the calcium ferrite and calcium titanate,and the position is biased toward the calcium titanate side.As the diffusion time increases,the elongated band becomes wider and wider.It is thick and longitudinally banded and distributed in a flocculent agglomeration.When the diffusion time is increased to 105 mins,there is a densehole distribution which is distinct from the calcium ferrite matrix near the interface of calcium ferrite.The pores are interlaced and the surface of the substrate collapses.This may be due to the fact that the affinity of Fe2+to vacancies is greater than that of Ti4+,resulting in a residual vacancy flow from calcium titanate to calcium ferrite.(4)Under the CO/N2 reducing atmosphere at 700℃,the growth of the diffusion layer of the calcium ferrite/calcium titanate system is controlled by diffusion in the time range of 0～1 20 mins.In the time range of 0～120 mins,the thic kness of the diffusion layer of calcium ferrite/calcium titanate system is about 17～48 μm,which conforms to the parabolic law of diffusion.(5)The diffusion coefficient of Ti4+ in calcium ferrite/calcium titanate system is related to its concentration.The value of diffusion coefficient of Ti4+increases with the increase of concentration,and the value is in the range of 10-10～10-12 cm2/s.(6)CaO/Cr2O3 diffusion couple with good structural properties can be prepared by applying a CaO and Cr2O3 powder in a vacuumhot pressing furnace at a temperature of 1200 to 1300℃ at a pressure of 0.4 MPa for 30 mins or more.(7)CaO/Cr2O3 diffusion couple may undergo obvious delamination after diffusion reaction,and it is divided into three layers:CaO,intermediate layer and Cr2O3.When the diffusion time increases to 90 mins,the Cr atoms are segreggated in the Cr2O3 layer,and the segregation zone moves toward the Cr2O3 layer with increasing time.The Ca atoms are densely segregated in the middle of the intermediate layer andhave a fixed concentration range,which may be caused by the rapid binding of Ca2+ in the region at the vacancies.(8)In the vertical vector direction from the CaO layer to the intermediate layer,the atomic ratio of Ca and Cr approaches the atomic ratio in CaCr2O4,and the diffusion reaction of CaCr2O4 is carried out by using the vacancy diffusion mechanism.The rate of reaction may be controlled by the interdiffusion of Ca2+ and Cr3+.(9)At 1300℃,the growth of the diffusion layer is controlled by diffusion in the CaO/Cr2O3 system over a period of 30 to 120 mins.In the time range of 30～120 mins,the thic kness of the diffusion layer of CaO/Cr2O3 system is about 49.98～100.01 μm,which conforms to the parabolic law of diffusion.(10)The diffusion coefficient of Cr3+in the CaO/Cr2O3 diffusion couple is in the range of 10-9～10-11 cm2/s,and is related Lo the molar ratio of Cr3+ and temperature.If the diffusion time is constant,the diffusion coefficient of Cr3+ is the largest at the NCr=0.286 position under the same temperature condition,and the line rule of increasing first and then decreasing along NCr=0.037→NCr=0.286→NCr=0.377.At the same molar ratio of Cr3+,the diffusion coefficient of Cr3+ increases with increasing temperature.(11)The diffusion activation energy of Cr3+calculated by Arrhenius equation is 310.51 KJ/mol on the CaO side,633.90 KJ/mol on the CaCr2O4 side,and 389.65 KJ/mol on the Cr2O3 side. |
PDF Full Text Request