| The Bayan Obo mine is a great natural treasure because of its abundance of valuable elements However,at present,the utilization of Bayan Obo minerals is still dominated by the traditional method of beneficiation and metallurgy,resulting in a low utilization rate.For example,a large number of rare earths were discharged into the tailings,leading to the failure of effective recovery and utilization of valuable resources in Bayan Obo mine.Rare earth,fluorine,phosphorus,niobium and other resources are seriously wasted in the process of mineral processing.In addition,the decomposition of pellet sintering and rare earth concentrate produces a large amount of waste water,waste gas and waste residue,causing serious pollution to the environment.With the development of the research of materialized metallurgy,a lot of researches on the direct preparation of composite materials with minerals have been conducted The results show that this method is a new way for mineral utilization.Therefore,in the face of such abundant resources and the promotion of sustainable development strategy,it is urgent to develop a new method for comprehensive utilization of Bayan Obo minerals,which is of great significance for clean and high-value utilization of Bayan Obo mineralsIn this study,Bayan Obo iron concentrate was taken as the main raw material.And combined with the mineral advantages of China,some bauxite was added as ceramic phase High performance Fe-Al2O3 composite was prepared directly by carbothermal reduction from natural minerals.The reduction process and technological conditions of this method were studied deeply,and the optimum technological conditions for preparing Fe-Al2O3 composite were explored.The occurrence of "impurity" oxides in composite materials and their effects on material properties were studied.Meanwhile,the effects of trace elements in minerals on the structure and properties of composite materials were investigated.Finally,the effect of rare earth elements in Bayan Obo minerals on the structure and properties of composite materials was studied by adding CeO2.The thermodynamic calculation method,X-ray diffraction technology and DSC-TG analyzer were used to analyze the carbothermic reduction process.The effects of different reducing agent contents,sintering temperature and holding time on the microstructure and mechanical properties of Fe-Al2O3 composite were studied.The effects of trace elements and rare earth oxides in minerals on the structure and properties of composites were studied.The main results are as followsThe thermodynamic calculation results show that the reduction process of iron oxide was Fe2O3-Fe3O4-FeO-Fe.Due to the presence of SiO2 and Al2O3,the FeO obtained from the reaction in the system will produce Fe2SiO4 or FeAl2O4 with SiO2 or Al2O3.When the temperature elevated to about 1030℃,the reduction of Fe2SiO4 began,and the products were Fe and SiO2.When the temperature exceeded above 1100℃,FeAl2O4 was reduced by the activated carbon,and the products were Fe and Al2O3.For non-iron oxide impurities in the system,it is found that the reduction of MnO2 can be reduced at a low temperature.However,due to the low content of MnO2 in the system,no manganese-related diffraction peak can be detected in the XRD.The SiO2,TiO2 did not react with activated carbon under this conditionFe-Al2O3 composite was prepared by using Bay an Obo iron concentrate,alumina and activated carbon as main raw materials.The composite material is composed of alumina as the matrix and iron particles as the reinforcing phase.There are a few glass phases at the interface between iron particles and alumina,also at the grain boundary of alumina.On the one hand,the increase of carbon content will decrease the melting point of metal particles,further promote the sintering densification of composite materials,improving the properties of the materials.The lower sintering temperature is not conducive to the densification of materials,while the higher temperature is prone to sample expansion overburing,the best sintering temperature is 1400℃.The optimum properties of the composite were:density 4.14 g/cm3,linear shrinkage 18.74%,flexural strength 301 MPa,fracture toughness 5.01 MPa·m1/2,hardness 13.12 GPa,alkali resistance 98.20%,acid resistance 93.40%.The metal phase distributes in the matrix of aluminum oxide in granular form.Through the observation of crack development,it is found that the composite material is strengthened by two mechanisms.The one is that the cracks were clear deflected when the cracks encounter the metal particle,thereby increasing the path of crack development and increase the absorbed energy.The other is the bridging through the crack.When the crack encounters the metal particles,the crack is forced to pass through the metal particles and continue to extend forward,while the metal particles are plastically deformed and bridged in the middle of the crack.The sulfur and manganese elements in minerals will be precipitated around the metal phase in the form MnS when the carbon content is high,which can hinder the crack developmentIn the experiment of replacing part of alumina with bauxite,the microstructure and properties show that the best preparation conditions are reduction temperature 1160℃ and sintering temperature 1380℃.Under this process condition,the best performance of sample B3 is shown as follows:flexural strength up to 310 MPa,fracture toughness 5.21 MPa·m1/2,hardness 12.14 GPa,alkali resistance 98.32%and acid resistance 95.44%.With the increase of bauxite content in the raw material,the content of silicate liquid phase increases in the sintering process.Because of the heat capacity of the liquid phase is greater than that of the alumina crystal.Therefore,with the increase of bauxite,the cooling rate of the metal droplet in the sample is slower during the cooling process,resulting in a decrease in the undercooling of the metal droplet,a decrease in the thermal stress of the metal particle after solidification,and a decrease in the internal dislocation density.However,there are a lot of grain boundaries and dislocations in the metal particles of the samples with high undercooling,which affect their plastic deformation in the process of stress.By using EBSD to analyze the indentation of samples,it was found that during the stress process of the samples,the metal particles of sample B3 could obvious absorb more energy through plastic deformation,thus improving the mechanical properties of the samplesThe addition of CeO2 has direct impact on the microstructures of the composites.At 1 wt%addition,all CeO2 was dissolved in the glass phase.At 3 wt%CeO2 addition,crystalline precipitates of Ce2O3 were formed and distributed in the glass phase.Further increase of CeO2 addition to 5 wt%results in the formation of CeAl11O18 crystalline phases.The increase of fracture strength can be attributed to the increase of the density and Young’ s modulus of the composites by cerium oxide.The increase of Young’s modulus for glass phase due to the dissolution of CeO2 is contributed to the improvement of hardness and fracture toughness of composites.But excessive addition of CeO2 resulting in the reaction of Ce2O3 with Al2O3 to CeAl11O18 with a low Young’s modulus,which decreased the positive influence of the CeO2 The mechanical properties first improve and then decrease with the increase of CeO2 addition When the addition of CeO2 was 3%,the maximum value of density,fracture toughness,flexural strength,Vickers hardness,alkali resistance,acid resistance was 4.21 g/cm3,6.58 MPa·m1/2,401 MPa,13.07 GPa,98.71%,94.78%,respectively. |
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