| Today,with the rapid development of the global high-temperature industry,the highly efficient and comprehensive utilization of mineral resources has always been an important topic of most concern in economic development.The technological progress and development of high-temperature industry highly depend on the support of refractory industry.The two basic systems of China's refractory industry are aluminum-silica and magnesia materials.Their development from scratch to now high level is based on China's refractory resources with high quality.The mineral resources of Al2O3-SiO2 materials,high-alumina bauxite mainly locate in Shanxi,Henan,Guizhou Province in where an important natural raw material base for China's refractory industry is.Since the reform and opening up policy in China,high alumina bauxite clinker used as a raw material for aluminum-silicon refractories has developed rapidly,and based on this,refractory materials which suitable for various high-temperature furnaces/containers have been developed.However,in the development course,along with the unscientific disordered mining and unreasonable application of high-quality bauxite resources,a large number of fritter bauxite ore,mixed grade ore and low-grade ore have not been effectively utilized.Among them,a large number of high-iron low-alumina bauxite ore(Fe2O3?5.0 wt%)was abandoned in Yangquan,Shanxi Province,and Jiaozuo,Henan Province,China,and the natural environment of the mine was degraded.In this regard,this study is based on the application of the high-iron contenting bauxite ore,based on the effective comprehensive utilization of the mineral resources,through the adjustment and control on the compositions of the iron-containing aluminum-silicon raw materials,the high-performance y-mullite was successfully prepared by in-situ sintering method.Firstly,the mullite preparation by solid-state sintering of using high purity system of ultrafine a-alumina powder and fused silica powder was researched.After adding Fe2O3,the solid-state sintering system is converted into liquid-state sintering,which greatly reduces the mullite formation temperature and increases the conversion rate of the mullite.Meanwhile,the kinetic of the anisotropic growth of iron-containing columnar mullite crystals was studied.Based on these work,a-alumina and fused-silica powder were used as raw materials,and a certain amount of Fe2O3 and TiO2 were added in samples,high-performance y-mullite was successfully prepared on the basis of this high-purity raw materials,the occurrence state and stable condition of Fe+ and Ti+ ions in mullite solid solution was clarified.The results of this study provide a theoretical and scientific basis for the preparation of high-quality refractory raw material mullite from high-iron contenting bauxite,and a feasible industrialization approach for the scientific and highly efficient utilization of the iron-aluminum-silicon-based mineral resources.The results of preparation mullite by solid-state sintering with high purity raw materials show that the mullite is prepared by homogeneous mixing,compacting and sintering with a-Al2O3 powder and silica mirco-powder,cristobalite powder,natural quartz and fused silica as siliceous raw materials.It is proved from the experiments,that silica micro-powder has the highest activity,and the mullite crystal with complete needle/columnar morphology can be prepared.But the volume shrinkage of the sample is obvious during the sintering process.The preparation of mullite with fused silica and?-Al2O3 has the advantage of zero expansion.However,because the material system is a solid-solid sintering in high temperature,the mullite is difficult to nucleate.After firing at 1500 °C,cristobalite,corundum,and metastable phases coexist.Cristobalite(SiO2)reacts with corundum(Al2O3)to form mullite.The direct combination between cristobalite and corundum phase is typical morphology feature of solid-state reaction sintering.At the same time,corundum reacts with unconverted quartz glass to form a metastable amorphous liquid glass.Over firing at 1500 °C,with increase of temperature the cristobalite gradually reacts with the corundum and then disappears.The metastable liquid phase begins to soften and adheres on the surface of the mullite crystal.Due to the high viscosity of the liquid phase,mullite is difficult to grow and develop.The study on microstructure and properties of the iron-containing mullite solid solution shows that a-Al2O3 and fused silica are used as raw materials with different Fe2O3 contents(3wt.%,6wt.%and 9wt.%),after homogeneous mixing and molding,forming samples was sintered between 1300 0C?1700 ?C to prepare the iron-containing mullite solid solution.So the addition of Fe2O3 changes the Al2O3-SiO2 binary system to FeOn-Al2O3-SiO2 ternary system.Solid-liquid sintering replaces the solid-solid sintering in the high-purity system,the synthesis temperature of the mullite is reduced,and its conversion rate is increased due to the action of Fe2O3.The conversion rate of mullite in the sample after firing at 1600 °C reach 100%.After firing below 1400?,a large amount of Fe+ ions are distributed in the liquid phase.With an increase of temperature,Fe2+ ions are oxidized to Fe3+.After firing at 1600 ?,the contents of Fe3+and Fe2+ions are 84.1wt%and 15.9wt%,respectively.They are stably present in mullite solid solution to form an iron-rich mullite.Due to an influence of the iron ions,the crystal morphology of the iron-rich mullite develops into a spherical and short column shape.The mullite crystal grains are in contact with each other and are squeezed together,showing a stable structure Thus,the sample shows excellent high temperature mechanical properties.The sample with 9wt.%Fe2O3 was fired after at 1700 °C,a part of the iron ions was de-dissolved from the mullite lattice,then the mullite crystals grew rapidly and develop completely,to form an iron-exsolution mullite.The dissolved iron ions exist in the voids around the mullite crystal,showing an island-like structure of ferro-crystalline phase.This structural characteristic has little effect on high temperature performance of the mullite.The prepared iron-rich mullite and iron-exsolution mullite samples have a room temperature compressive strength of 178 MPa and 169 MPa,respectively,and the RUL temperatures(T2.0)are 1653 °C and 1648 °C,respectively,showing excellent room/high temperature mechanical properties.The anisotropic growth and its kinetic behavior of long columnar mullite was studied.The growth index of iron-containing mullite with different Fe2O3 adding amount(3 wt.%,6 wt.%and 9 wt.%)was calculated by empirical formulas of anisotropic growth kinetics of the crystal.The surface energy of each crystal plane of the columnar crystal and the solid solution behavior of iron ions in the crystal column were simulated and calculated by Density functional theory(DFT).It shows that the Fe2O3 addition greatly promotes the anisotropic growth of the mullite.The length and aspect ratio of the synthesized iron-containing mullite columnar crystals increase with increase of the Fe2O3 addition.The sample after firing at 1700 °C has parameters got from the calculation as follows:the growth kinetic indices in the length direction are 2.22,2.50 and 3.85,respectively,and the reaction activation energies are 457.3,442.1 and 411.9 KJ·mol-1,respectively(corresponding to Fe2O3 addition amounts of 3 wt%,6 wt%and 9 wt%,respectively).According to the first principle the surface energy of mullite columnar crystal is calculated,the results show that the surface energy on the(001)crystal plane is the smallest and most stable,which is the preferred growth direction of the mullite.By comparing the surface energy of different Fe2O3 additions on the(001)surface,as the amount of Fe2O3 added increases and the surface energy of the mullite crystal decreases,the growth trend of mullite crystals increases and their aspect ratio increases.The study on preparation of y-mullite and its phase,microstructure and physical properties show that ?-Al2O3 and fused silica powders are used as raw materials,8 wt%Fe2O3 and 4 wt%TiO2 fine powder are added respectively for preparation of the samples.?-mullite can be obtained in the samples after sintering at 16000°C and 1700 °C.The results of XRD analysis shows that the mullite is the only stable crystalline phase in the sample.The morphology of the mullite crystal is short columnar and/or spherical shape,and the crystal grains are intertwined to from a dense net structure.The lattice constants of the y-mullite were calculated by XRD patterns,and the lattice constants of y-mullite obtained are a=7.570838A,b=7.706788 A,c=2.894456 A.The lattice constant value of the crystal in the sample increase compared with a-mullite.According to the experimental results,the formation mechanism of ?-mullite was analyzed and summarized as follows:in sintering process of the samples,the mullite crystal nucleus is firstly formed by the mullitization,and then it is the processes of crystallizing,developing and growing,Fe3+,Ti4+ions entrance and occupy the octahedral position of Al3+ in the mullite lattice.Finally,these ions are solid-dissolved in the lattice structure of the mullite to form ?-mullite.The testing results of physical property of the samples show that the y-mullite prepared by firing at 1600 °C and 1700 °C have good mechanical properties at room and high temperature and can be used in various high temperature industrial furnaces and/or metallurgical vessel.The microstructure and properties of y-mullite prepared by natural bauxite ore with high-iron content were investigated.Their chemical compositions are Al2O3=56.13wt.%,Fe2O3=8.04wt.%and Al2O3= 60.99wt.%;Fe2O3=7.16 wt.%from Shanxi's bauxite mine.There is other from high-iron bauxite ore after adjusting chemical composition and homogenization as raw material which shows Al2O3=65.01wt.%;Fe2O3=7.04.According to XRD analysis,the main crystalline phases of the samples after firing at 1600 °C are y-mullite with a small amount of aluminum-titanate solid solution.The SEM analysis shows that the y-mullite crystals in the sample are long columnar,well-developed,intertwined with a network structure.A small amount of aluminum titanate solid solution present between the voids of the mullite crystals.This structure is benefits for structural stability and high temperature mechanical properties of the material.Their physical properties and the RUL temperature were tested.The results show that the strength of the sample increase by 14 MPa and the RUL temperature increase by 40 °C after the homogenization on chemical compositions,indicating that controlling the ratio of(Fe+A1)/Si of the raw material can effectively improve the properties of the sample.The preparation technology of bauxite-based y-mullite with good room and high temperature mechanical properties provides a theoretical basis and application direction for the efficient use of such abandoned mineral resources. |
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