Preparation And Performance Research Of High Performance Mullite Ceramics Based On Associated Rare Earth Kaoli

China is rich in rare earth and kaolin reserves,and many kaolin clays are accompanied by associated minerals such as rare earths,which cause serious waste of mineral resources due to the low grade of rare earths and constant loss of tails in the beneficiation process.In order to promote the comprehensive utilization of associated mineral resources,this study synthesizes high-performance mullite ceramics with associated rare earth kaolin as raw material,which provides a new way to utilize the resources of associated rare earth kaolin,solves the difficulty of associated rare earth removal and extraction,and also reduces the addition of rare earth elements,which greatly reduces the production cost of high-performance ceramics and improves the cost effectiveness of the deep processing products of kaolin in China.In this paper,high-performance mullite was synthesized mainly from calcined associated rare-earth kaolin.Firstly,the chemical composition and mineral composition of associated rare-earth kaolin and calcined associated rare-earth kaolin were studied.Secondly,calcined associated rare-earth kaolin with particle sizes of 48μm and 38μm was used as raw material to study the effect of powder particle size and sintering temperature on mullite properties;calcined associated rare-earth kaolin and common commercially available calcined kaolin were used as raw material to synthesize mullite,and the effect of associated rare-earth elements in kaolin on mullite properties was compared and analyzed;Finally,0,1.0,2.0,and 4.0%micro/nano La₂O₃ were added to explore the effects of micro/nano La₂O₃ on the phase transformation,microstructure,sintering behavior,micro-Vickers hardness,flexural strength,and dielectric properties of mullite ceramics.The following conclusions were obtained from the above studies:（1）The process mineralogical analysis was carried out on the raw ore of associated rare earth kaolin and calcined associated rare earth kaolin.The results showed that the total content of rare earth TREO in the raw ore was about 0.311%,and the total content of associated rare earth TREO in the calcined kaolin was about 0.617%.The main minerals were kaolinite,quartz,and illite.The rare earth minerals are mainly cerium fluorocarbon,monazite,niobium-tantalum minerals,cerium-bearing rare earth ores,and thorite.The associated rare earth kaolinite undergoes phase change in high-temperature sintering and transforms from kaolinite to metakaolinite in the range of408.5 to 685.6°C,to aluminosilicate spinel at 953.3°C,and to mullite at 995.0°C.（2）Calcined associated rare-earth kaolin andγ-Al₂O₃ with particle sizes of 48μm and 38μm,respectively,were mixed and ball-milled in a ball mill for 6 h and sintered at 1320–1500°C for 4 h to synthesize mullite.The particle size of calcined associated rare earth kaolin decreased from 48μm to 38μm,the aspect ratio of mullite increased from 5.26 to 5.55,and the rate of mullite transformation and growth was faster.Mullite synthesized at 1500°C using calcined associated rare earth kaolin with a particle size of 38μm showed an increase in micro-Vickers hardness of 6.09%and an increase in flexural strength of 17.04%.（3）The mullite was synthesized by calcining associated rare earth kaolin with a particle size of 38μm,common commercially available kaolin,andγ-Al₂O₃,respectively,and mixing ball milling in a ball mill for 12 h and sintered at 1320–1500°C for 4 h.With the increase in sintering temperature,the relative content,bulk density,and micro Vickers hardness of mullite gradually increased,and at 1500°C sintered for4 h,the relative content of mullite was as high as 98.8%,the maximum aspect ratio was8.22,the relative density was 93.04%,and the micro Vickers hardness and flexural strength were 10.63 GPa and 184.24 MPa,respectively;at room temperature of 100k Hz,the Compared with the mullite samples prepared by calcination of common commercially available kaolin,the relative mullite content,bulk density,microscopic Vickers hardness,and flexural strength increased by 14.68%,10.88%,42.95%,and38.22%,respectively,and the mechanical properties were significantly improved in the relatively narrow temperature range of 1480°C to 1500°C.a relatively narrow temperature range,the mullite content increased sharply and produced a slight expansion in the axial direction.It indicates that the characteristic volume expansion is exhibited in this temperature range due to the mullite crystal growth process and may be related to the completion of secondary mullitization.（4）Mullite was prepared from calcined associated rare earth kaolin（38μm particle size）andα-Al₂O₃ with the addition of 0,1.0,2.0,and 4.0%micro/nano La₂O₃,mixed in a ball mill for 12 h,and sintered at 1320–1500°C for 4 h.The results showed that the addition of 2.0%La₂O₃ resulted in sharp and intense diffraction peaks of mullite and the lowest intensity of corundum,indicating that the synthesized mullite was highly crystalline.Micro/nano La₂O₃ addition was able to improve the densification and aspect ratio of mullite ceramics.With the addition of 4.0%micron La₂O₃ and 2.0%nano La₂O₃,the aspect ratios of mullite were 6.15 and 5.67,respectively,and interwoven interlocked columnar mullite grains were obtained.The bulk density was minimized at 1400°C and then gradually increased with increasing temperature,and the highest micro-Vickers hardness value was obtained at 1500°C.The addition of nano-La₂O₃ had a better effect on the densification,mechanical properties,and secondary mullitization of mullite ceramics than micron La₂O₃.The maximum flexural strength（157.82 MPa）was obtained with the addition of 2.0%nano-La₂O₃,and with the continued increase of nano-La₂O₃（4.0%）,an abnormal growth of some platelet or massive mullite grains was observed,leading to a further decrease.