The effect of quench conditions on the properties of fumed silica produced using an arc process

Abstract

The effect of quench conditions on the properties of fumed silica generated using an arc process was studied. The process consisted of vaporizing quartz particles in a batch transferred arc reactor using the radiative energy from an Ar/NH$sb3$ thermal plasma. Ammonia was used to increase the vaporization rate of quartz. The hot gas stream exiting the reactor contained a mixture of the decomposition products of NH$sb3$ and SiO$sb2$. This included SiO$sb{ rm (g)}$, O$sb2$, H$sb2$ and N$sb2$. Rapid quenching of this gas stream with steam resulted in the production of a fumed silica aerosol. Collection of the product took place in a baghouse filter. The quench conditions studied included pre-quench temperature, T$sb{ rm q}$, and supersaturation, P$sb{ rm s}$, quench rate, R$sb{ rm q}$, and quench stoichiometry, S$sb{ rm q}$. P$sb{ rm s}$ was defined as the molar ratio of SiO$sb{ rm (g)}$ flow, calculated from the quartz weight loss during an experiment, to the SiO$sb{ rm (g)}$ flow predicted at equilibrium. S$sb{ rm q}$ was the molar ratio of steam to SiO$sb{ rm (g)}$ flow.In general, high pre-quench temperatures, high quench rates, and low pre-quench supersaturation ratios, produced high surface area powders. Quench stoichiometry did not have an effect here, but the use of a large excess of steam in the quench caused the degree of surface hydroxylation to be high. In all cases, the characteristic chain-like aggregate shape and an amorphous morphology were produced. The thickening and thixotropic ability of the powders was found to be significant compared to commercial brands of fumed silica. The best fumed silica produced during the study was found to be 46% as effective as Aerosil$sp circler$ 200 and 60% as effective as Cab-O-Sil$sp circler$ M-5. The experimentally generated powders were not equivalent to the commercial brands due to a lack of surface area. (Abstract shortened by UMI.