GB1413333A - Process for producing silica and hydrogen fluoride from waste gases containing silicon- and fluorine containing compounds - Google Patents

Abstract

1413333 Hydrogen fluoride and SiO 2 from waste gases G L FLEMMERT 27 Oct 1972 [28 Oct 1971] 49703/72 Heading C1A Pure HF and SiO 2 are produced from waste gas containing SiF 4 and HF and optionally SiO 2 , by absorbing the gas in water when hydrofluorosilicic acid is formed, which is then mixed with conc. sulphuric acid so as to be decomposed and form a gaseous effluent comprising SiF 4 + HF ; the residual sulphuric acid is withdrawn from the decomposition stage, and the gaseous effluent is treated with cone. sulphuric acid to absorb water and HF, the SiF 4 being recovered and hydrolysed by blending it with combustible gas and air and burning to form SiO 2 and HF, the temperature of the reaction being maintained above the dew point to prevent the water condensing on the silica, separating the silica, and treating the effluent gases with water or aqueous HE to form hydrofluorosilicic acid, venting the remaining gas, and then decomposing the hydrofluoro-silicic acid with sulphuric acid, so liberating gases containing SiF 4 , HF and water vapour, removing the water vapour and HF, recycling the SiF 4 to the hydrolysis stage, separating the HF and recovering the sulphuric acid. The hydrofluorosilicic acid produced from the initial waste gas and that produced after the SiF 4 hydrolysis stage may be combined and decomposed together. When SiO 2 is also present in the waste gases, then conditions may be such that the reaction with conc. sulphuric acid produces only water and SiF 4 . The proportion of HF to SiF 4 formed in the decomposition may be controlled by adjusting the molar proportions of SiF 4 and H 2 SiF 6 in the reaction mixture. Steam may be added to the combustion zone to improve the hydrolysis of the SiF 4 . Particular concentrations and conditions of the various reaction stages are given in the Specification.