Process for converting silicon-and-fluorine-containing waste gases into silicon dioxide and hydrogen fluoride
OwnerGosta Lennart Flemmertinventor
Life of this patent
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Abstract
A process is provided for converting silicon and fluorine-containing waste gases into silicon dioxide and hydrogen fluoride, absorbing the waste gases in water to form hydrofluosilicic acid, decomposing the hydrofluosilicic acid in the presence of concentrated sulfuric acid to form silicon tetrafluoride and hydrogen fluoride, converting the silicon tetrafluoride in the vapor phase to silica and hydrogen fluoride, and recovering the hydrogen fluoride.
Claims29
A cyclic process for producing hydrogen fluoride and highly active silicon dioxide from silicon-and fluorine-containing waste gases, the silicon and fluorine being present at least as silicon tetrafluoride and hydrogen fluoride, and also silicon dioxide, if present, which comprises combining water and silicon-and fluorine-containing waste gases and absorbing silicon tetrafluoride and hydrogen fluoride in water thereby forming hydrofluosilicic acid;
- blending hydrofluosilicic acid with concentrated sulfuric acid to decompose hydrofluosilicic acid to form a gaseous effluent comprising tetrafluoride, hydrogen fluoride and water vapour;
- passing gaseous effluent into contact with sulfuric acid to absorb water and hydrogen fluoride thereform;
- recovering silicon tetrafluoride from gaseous effluent and hydrolyzing silicon tetrafluoride by blending with combustible gas and with air, and burning to form silica and hydrogen fluoride;
- withdrawing residual sulfuric acid;
- separating silica formed in the hydrolysis;
- blending effluent gases from the combustion containing hydrogen fluoride, water vapour, unreacted silicon tetrafluoride and combustion products with a member selected from the group consisting of water and aqueous hydrofluoric acid solution to form aqueous hydrofluosilicic acid solution and venting unabsorbed exhaust gases, now relatively free from fluorine-containing materials, to the atmosphere;
- combining and decomposing hydrofluosilicic acid with concentrated sulfuric acid;
- liberating and volatilizing hydrogen fluoride and water vapour;
- removing water vapour;
- recovering hydrogen fluoride and recovering sulfuric acid.
A cyclic process which comprises combining water and silicon- and fluorine-containing gases comprising silicon tetrafluoride, hydrogen fluoride and silica if present thereby absorbing and dispersing silicon tetrafluoride, hydrogen fluoride and any silica in the water forming hydrofluosilicic acid;
- blending a member selected from the group consisting of hydrofluosilicic acid, high silica fluosilicic acid, H.sub.2 SiF.sub.6 SiF.sub.4 and mixtures thereof with a member selected from the group consisting of solid silicon dioxide and hydrogen fluoride with concentrated sulfuric acid to decompose hydrofluosilicic acid to SiF.sub.4, HF and H.sub.2 O, the sulfuric acid having previously been passed in countercurrent flow to the gaseous effluent from hydrofluosilicic acid decomposition to absorb water and hydrogen fluoride therefrom;
- recovering silicon tetrafluoride from gaseous effluent and hydrolyzing it by blending with combustible gas and with air, and burning to form silica and hydrogen fluoride;
- withdrawing residual sulfuric acid, and concentrating and recycling sulfuric acid;
- separating silica formed in the hydrolysis;
- blending effluent gases containing hydrogen fluoride, water vapour, unreacted silicon tetrafluoride and combustion products with a member selected from the group consisting of water and aqueous hydrofluoric acid solution, to form hydrofluosilicic acid and venting the unabsorbed exhaust gases, now relatively free from fluorine-containing materials, to the atmosphere;
- combining and decomposing hydrofluosilicic acid with concentrated sulfuric acid, the acid having previously been passed in countercurrent flow to effluent gases from the decomposition;
- liberating and volatilizing silicon tetrafluoride and recycling it for hydrolysis into hydrogen fluoride and silica;
- heating the residual solution, composed of a mixture of sulfuric acid, hydrogen fluoride and water, to volatilize hydrogen fluoride and water vapour;
- condensing the water vapour;
- and recovering hydrogen fluoride.
Dependencies are inferred from the claim text.