Preparation Of Quasi-spherical Silica Colloids Via Ultrafine Grinding With Chemical-dissolution Assistance

Ultrafine spherical and quasi-spherical silica colloids（less than 1μm）is a kind of inorganic nonmetallic material in the modern industry.This material has good hydrophilicity,reinforcing,thickening,extinction and anti-cohesion due to the excellent particle characteristics,such as small particle size,large specific surface area,good fluidity,excellent dielectric properties and chemical stability.It is widely used in different fields like electronic packaging,polymer composites,ceramics and etc..In addition to the ultrafine particle size,the spheroidization is also critical for applications.Ultrafine spherical or quasi-spherical silica particles are usually prepared by different methods like flame ball formation,high-temperature melt spray,sol-gel and chemical precipitation.These conventional methods are complicated in process with higher costs.It is thus necessary for preparation of ultrafine quasi-spherical silica particles to develop an improved method with some features（i.e.,short process,low cost and commercial production）.The purpose of this dissertation is to prepare submicron-sized quasi-spherical silica（i.e.,fused silica and cristobalite）particles by chemical dissolution-assisted ultrafine grinding,and to analyze the preparation mechanism via the selection and breakage functions from population balance modeling.In this dissertation,submicron-sized fused silica particles with the quasi-sphericity were prepared via wet ultrafine grinding in a high energy-density stirred bead mill in the absence and presence of different solutions（i.e.,water,potassium chloride（KCl）,sodium chloride（NaCl）,magnesium chloride（MgCl₂）,calcium chloride（CaCl₂）,barium chloride（BaCl₂）and ammonia chloride（NH₄Cl））.The effects of salt solution types（i.e.,potassium chloride,sodium chloride,magnesium chloride,calcium chloride,barium chloride and ammonium chloride）,salt concentration（i.e.,0.01 mol/L,0.03 mol/L,0.05 mol/L and 0.10 mol/L）,solid content of fused silica particles（i.e.,20 wt.%,30 wt.%and 40 wt.%）and grinding time（i.e.,30 min,60 min and120 min）on the size/size distribution and sphericity of fused silica particles ground in the mill were investigated.The morphology of the ground particles was analyzed by scanning electron microscopy（SEM）,and the particle sphericity was evaluated by a software named Image-Pro Plus.The particle size and size distribution were characterized by laser particle size analysis,and the uniformity coefficient for particle size distribution was calculated based on the Rosin-Rammler-Bennett（RRB）model.The results show that submicron-sized quasi-spherical fused silica particles with the median size of 610 nm,uniformity coefficient of 2.98 and sphericity of0.89 can be obtained under the selected condition（i.e.,solution of BaCl₂,BaCl₂ concentration of 0.01 mol/L,solid content of 20 wt.%and grinding time of 30 min）.Besides the size reduction and size distribution improvement,the sphericity can enhance from 0.71 for the original particles to 0.76 for the ground particles in the absence of any salt solution.Also,the proper addition of BaCl₂ during ultrafine grinding can give a finer product with a steeper size distribution,and the particle quasi-sphericity increases from 0.76 to 0.89.It is revealed that ultrafine grinding can affect the spheroidization with and without chemical dissolution.In addition,the mechanism for ultrafine grinding of fused silica particles without and with chemically-dissolution assistance was also discussed via the selection and breakage functions from population balance modeling.The simulation results show that the grinding mechanism of fused silica particles in stirred bead mill is mainly shearing.Shear effect in the mill is beneficial to the spheroidization of particles in addition to the size reduction.Based on the calculated Kapur functions of particles ground in water and barium chloride solution,a coupling interaction of shear effect between beads and chemical dissolution on the particle surface in barium chloride solution can effectively reduce the product size,and improve the particle size distribution and sphericity.In addition,submicron-sized cristobalite particles with the quasi-sphericity were also prepared via wet ultrafine grinding in different solutions（i.e.,water,potassium chloride（KCl）,sodium chloride（NaCl）,magnesium chloride（MgCl₂）,calcium chloride（CaCl₂）,barium chloride（BaCl₂）and ammonia chloride（NH₄Cl））.The effects of salt solution types（i.e.,potassium chloride,sodium chloride,magnesium chloride,calcium chloride,barium chloride and ammonium chloride）,salt concentration（i.e.,0.01 mol/L,0.05 mol/L and 0.1 mol/L）and grinding time（i.e.,20 min,40 min and 60 min）on the particle size,particle size distribution and sphericity of ultrafine cristobalite particles were investigated.The morphology of the ground particles was analyzed by scanning electron microscopy（SEM）and the particle size/size distribution was characterized by laser particle size analysis.The results show that the median size,sphericity and uniformity coefficient of the product ground in water for 60 min are1.10μm,0.72 and 1.90,respectively.However,cristobalite particles ground in barium chloride solution with the concentration of 0.01 mol/L for 60 min have the median size of 840 nm,the uniformity coefficient of 2.359 and the sphericity of 0.80.The friction coefficient of cristobalite particles obtained by stirring or grinding for 60 min in different solutions（i.e.,barium chloride and water）was measured by a friction wear tester.The results show that the friction coefficient can be decreased for the increased sphericity of cristobalite particles ground in barium chloride solution for 60 min.Also,the mechanism for ultrafine grinding of cristobalite particles without and with chemically-dissolution assistance was discussed via the breakage functions.It is indicated that chemical dissolution in ultrafine grinding can reduce the product size and improve the size distribution.Finally,the conclusions obtained by experiments and analysis and the prospects for future work are given.