Study On The Preparation Of Ultrafine Silica By Silicon Tetrachloride From Polysilicon Byproduct

With the increasing exhaustion of oil resources,alternative energy led by the solar energy had raised hopes of many scientists and environmentalists which promote the development of polysilicon industry.According to the predictive,the global polysilicon production will reach 400 thousand tons resulting in more than 3.2 million tons silicon tetrachloride which has the characteristics of high corrosion resistance,low boiling point,difficult transportation and storage.The problems of safety,environment and effective utilization of resources,caused by a huge of silicon tetrachloride,are becoming increasingly prominent and a bottleneck restricting the development of the polysilicon industry.In recent years,many scholars are devoted to exploring new ways to consume the silicon tetrachloride,has proposed some new techniques of hydrogenation to produce trichlorosilane and synthesis tetraethyl orthosilicate by silicon tetrachloride.However,those processes consume only a small amount of silicon tetrachloride.Precipitated silica is a very important of inorganic functional materials,has special physical and chemical properties that of common bulk materials do not have.It is widely used in the fields of new type of paint,food,medicine and catalytic materials,etc.Analysis revealed that high and mid-grade precipitated silica market have great potential for development according to China's import and export,and the application field of the distribution.Therefore,precipitated silica was prepared by chemical precipitation process in this paper with silicon tetrachloride as raw material.This provides a reference for the realization of resource efficiency,green and large-scale utilization in our country.This paper mainly involves the following six aspects:?1?silicon tetrachloride hydrolysis is discussed at different pH in methanol-water,ethanol-water and isopropanol-water and in the same alcohol water but different surfactant.?2?In isopropanol-water solvent,seven factors are investigate in order to finding the optimal reaction conditions by single factor method,including SiCl₄ volume,aging time,pH value,etc.?3?With the characterization analysis of particle size,effects on yield,particle size and particle dispersion of precipitated silica were studied under different washing solvent,solid-liquid separation method,drying method,drying time and drying temperature..?4?The effects on the yield and particle size of silicon dioxide were studied,which was caused by 5 kinds of feed sequence of direct addition in strong acid,parallel feed in weak acid,change parallel feed to anti-addition in weak alkaline,etc.?5?The effects of pH value on solution stability,dispersion medium and test methods on the particle size results are studied.?6?The corresponding physicals and chemical properties of samples is analyzed ordered by the technical standard of class ? food additives.The results reveal that:?1?silica particle size is smaller in acid isopropanol-water,in strong acid or alkaline methanol-water solvent and ethanol-water solvent with sodium dodecyl benzene sulfonate?SDBS?as the surfactant.In the three medium,yield of silicon dioxide is higher in acidic condition and density is lower under alkaline conditions.The acidic hydrolysis medium that consists of isopropanol-water and cetyltrimethylammonium bromide?CTAB?is better.?2?Precipitated silica of particle size is 6?m and the yield was up to 67.01% is prepared in isopropanol-water medium,the optimal condition is that the molar ratio of isopropanol to water is 3 to 3,adding 0.4g/L CTAB,adding 3mL SiCl₄,pH=5,reaction for 0.5 hour at 32? and then aging 0.5 hour.?3?Under centrifugal separation and ambient temperature drying,amorphous mesoporous silica spherical particle of diameter is 2.8 ? m,specific surface area is233.56 ? /g,and evenness index is 21.7 was synthesized with in isopropanol-water of the molar ratio is 3 to 3.Both the original particle and the second agglomeration particle after centrifugal-washing are nanoparticles.Drying process has a few affected on the yield.Both low temperature pre-drying before high-temperature drying and second drying after ultrasonic dispersion can reduce the size of silica particles,improve the uniformity of the particles?4?Feeding sequence and pH of hydrolysis process are keys for particle size and uniformity of silica.Compared with reciprocation change of pH influnced by feeding sequence,unidirectional change of pH obtains silica with smaller and uniformity particle size.Particle diameter is the smaller in change anti-addition to parallel feed in weak alkaline and in strong acid hydrolysis,but the later has the lowest yield.Parallel feed under weak acidity has the highest yield but largest particle size.?5?The stability of mother liquor is better with lower pH.Particle dispersed in ethanol was better than that of in pure water.Using laser particle to test the silica particle size has significant error while using scanning electron microscope is much better.?6?Precipitated silica prepared in paper conform to the technical standard of class ? food additives.