Study On Hydrolysis Of Mixxed Chlorosilane Under Acidic Condition

During the process of the improved siemens producing polysilicon, chlorosilane residue and waste gas will be generated inevitably from trichlorosilane synthesis, trichlorosilane purification, trichlorosilane reduction and cold (or hot) hydrogenation processes. Both of chlorosilane residue and waste gas all contain SiCl4, SiHCl3, SiH2Cl2 and HCl, at present most of the domestic polysilicon plants adopt alkaline absorbent leaching process to achieve harmless disposal of chlorosilane residue and waste gas, and the generated solid waste with complex composition is difficult to utilize, it will result in a waste of resources. The treatment problems of chlorosilane residue and waste gas have become one of the bottlenecks for limiting the development of domestic polysilicon industry. To solve these problems, this paper proposes a method that mixing chlorosilane hydrolyses in hydrochloric acid to obtain simultaneously silica and concentrated hydrochloric acid (or HCl) to achieve the resource utilization of chlorosilane residue and waste gas. The main research work and results are as follows:The effects of the amount of chlorosilane residue, hydrolysis agent (water or hydrochloric acid), the final concentration of hydrochloric acid, the concentration of silica and the temperature of hydrolysis system on gel time of hydrolysate are studied by the experiments chlorosilane residue hydrolyses in water or high concentrations of hydrochloric acid, and the control method for gel time of hydrolyzate is obtained: With the increase of the final concentration of hydrochloric acid, or silica concentration, or hydrolysis reaction temperature, gel time of the hydrolyzate is shortened; With the decrease of the ratio of water (or hydrochloric acid) and chlorosilane residual, gel time of the hydrolyzate is shortened; During the mixing chlorosilane hydrolysing, the polymerization of silicic acid is inevitable, and the gelatinization of the hydrolyzate can be controlled, the ratio of chlorosilane residue and water (or hydrochloric acid) can be adjusted to control the final concentration of hydrochloric acid, the concentration of silica and temperature to control the gel time of hydrolyzate.Combined with the qualitative analysis of gas and infrared spectroscopy of silica, hydrolysis mechanism of mixed chlorosilane and gelation mechanism of hydrolyzate are proposed:Mixed chlorosilane hydrolyses under highly acidic condition, hydrolysis of SiCl4 generates orthosiliconic acid (Si(OH)4), Si—H bond hydrolyses slowly under highly acidic condition, hydrolysis of SiHCl3 and SiH2Cl2 generates orthosiliconic acid (Si(OH)4) and the Si—H bond-containing orthosiliconic acid (HSi(OH)3 and H2Si(OH)2), while hydrolysis of chlorosilane residue generates hydrochloric acid, simultaneous existance of orthosiliconic acid (Si(OH)4), the Si—H bond-containing orthosiliconic acid (HSi(OH)3 and H2Si(OH)2) and hydrochloric acid results in occurrence of the olation polymerization of orthosiliconic acid, and then the hydrolysate gelatinizes, the hydrolyzate loses fluidity. As the polymerization and gelation progress, the Si—H bonds also enter the gel, but the hydrolysis of Si-H bonds still occurs slowly.After studying hydrolysis mechanism of Mixed chlorosilane under highly acid condition and the gelation mechanism of hydrolyzate, combined with X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), particle size analysis, the structure and morphology characteristics of silica are studied: The silica, hydrolysis product of mixed chlorosilane under highly acidic condition, is randomly three-dimensional network structure and it is an amorphous substance, the inside of the silica is polymeride of silicon and oxygen atom, and the surface of the silica is a large number of Si-OH and Si-H, while there will be a large number of hydrogen bonding water on the Si-OH bond; Primary particle size of the silica is small (less than 5 microns), the primary particles are spherical particles, it generates irregular porous block after agglomeration, the particle size is 1～120μm.The hydrophilicity, reactivity, acidity and thermostability of the silica are studied, the results show that:The wet and dry silica show hydrophilicity in water, and show hydrophobicity or weak hydrophilicity in hydrochloric acid, the silica can release H2 when mixing with water or hydrochloric acid, alkali can promote the hydrolysis of Si —H bonds, the silica is the olation of polysilicon acid, and it can release acid group to make the aqueous solution be highly acid. It can remove most of free water of the silica after drying at 105℃, but the silica will adsorb moisture because of its strong adsorbability and hydrophilicity when it esposes to air. High temperature can convert Si—OH to Si—O—Si groups, and reduce the surface hydroxyl groups of the silica, thereby reduce hydroscopicity of the silica. The hydroxyl groups of the silica can be removed at 900～1000℃, the silica transforms from amorphous to crystalline state at 1063～1300℃, it loses the amorphous activity; Si—H bonds of the silica have better thermostability, and it is hard to removed at 105℃. The pyrolysis of Si—H bonds begins at 161.1 ℃ and finishes at 996.5 ℃, it releases H2 during the pyrolysis of Si— H bonds. A large number of HCl residues in the silica, it is hard to remove by washing and drying, it can be slowly released at high temperature.The method, mixed chlorosilane hydrolses in hydrochloric acid to obtain silica and concentrated hydrochloric acid (or hydrogen chloride), achieves the harmless disposal and resource utilization of chlorosilane residual and waste gas, reduces the pressure of waste water treatment, changes the unprofitable situation of chlorosilane waste treatment. This paper will provide theoretic foundation for the method and the basis for product positioning and reprocessing.