Process for converting silicon-and-fluorine-containing waste gases into silicon dioxide and hydrogen fluoride

Claim 2

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.

Claim 3

A process according to claim 1 in which the hydrofluosilicic acid is decomposed with concentrated aqueous sulfuric acid, and the hydrogen fluoride separated, and silicon tetrafluoride recycled for hydrolysis in the vapor phase to form silica.

Claim 4

A process according to claim 1, in which hydrofluosilicic acid from the silicon-and fluorine-containing gases and from the hydrolysis of silicon tetrafluoride are combined and decomposed together with concentrated aqueous sulfuric acid to form silicon tetrafluoride.

Claim 5

A process according to claim 1, in which the silicon-and fluorine-containing compounds from the exhaust gases are absorbed in water.

Claim 6

A process according to claim 1, in which silicon dioxide is present in the waste gases and in the hydrofluosilicic acid solution formed therefrom, so that in the hydrolysis of hydrofluosilicic acid with sulfuric acid the by-product is water instead of hydrogen fluoride.

Claim 7

A process according to claim 1, which comprises employing molar proportions of hydrofluosilicic acid and silicon tetrafluoride in the hydrolysis with concentrated sulfuric acid adapted to obtain a selected proportion of hydrogen fluoride to silicon tetrafluoride in the reaction mixture.

Claim 8

A process according to claim 1, which comprises heating the blend of hydrofluosilicic acid and concentrated sulfuric acid to form the gaseous effluent comprising silicon tetrafluoride and hydrogen fluoride and passing the gaseous effluent through a scrubber, in which it is brought into contact with the entering concentrated sulfuric acid, thereby absorbing substantially all of the hydrogen fluoride and water vapour in the sulfuric acid, and recovering silicon tetrafluoride from the acid in the scrubber.

Claim 9

A process according to claim 1, in which silicon tetrafluoride is hydrolyzed in the gas phase with water by burning with combustion gases to form highly active silicon dioxide and gaseous hydrogen fluoride.

Claim 10

A process according to claim 1, in which silicon tetrafluoride is hydrolyzed in the gas phase with superheated steam at a temperature above about 500°C., the silicon tetrafluoride gas and water vapour being blended and passed through the hydrolysis zone together, with water vapour, and the silica which is formed separated from the effluent gas stream from the reaction zone.

Claim 11

A process according to claim 8, in which the temperature during the hydrolysis is maintained above the dew point of the gaseous reaction products, to prevent condensation of water from the reaction mixture on the silica.

Claim 12

A process according to claim 1, in which the gaseous effluent, comprising a mixture of hydrogen fluoride, unreacted silicon tetrafluoride, and water vapour, is absorbed in water or in acid to form a mixture of hydrofluoric acid and hydrofluosilicic acid, and residual gases, substantially free from fluorine, are vented into the atmosphere.

Claim 13

A process according to claim 11, in which the mixture of hydrofluoric acid and hydrofluosilicic acid is converted into silicon tetrafluoride and hydrogen fluoride by blending with concentrated sulfuric acid and heating at a temperature within the range from about 50° to about 150°C, at a proportion of acid such that the residual acid obtained comprises from about 25 to about 70 parts by weight of water per 100 parts by weight of sulfuric acid, vaporizing the silicon tetrafluoride and hydrogen fluoride, passing the gaseous effluent in countercurrent flow to the entering sulfuric acid, absorbing the steam and hydrogen fluoride present with the silicon tetrafluoride in the sulfuric acid and recycling the silicon tetrafluoride to the hydrolysis stage.

Claim 14

A process according to claim 12, in which an inert gas is introduced with the gaseous effluent to inhibit absorption of the silicon tetrafluoride in the sulfuric acid.

Claim 15

A process according to claim 12, in which the hydrofluosilicic acid-sulfuric acid solution is combined with the hydrofluosilicic acid-sulfuric acid solution obtained in the first stage from the waste gases, so that all of the silicon tetrafluoride is produced in that stage.

Claim 16

A process according to claim 12, in which the mixture of sulfuric acid and hydrogen fluoride is heated to 130° to 200°C, and the hydrogen fluoride vaporized.

Claim 17

A process according to claim 15, in which the hydrogen fluoride is dried by passing in countercurrent flow to concentrated sulfuric acid.

Claim 18

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 2 SiF 6 SiF 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 4 , HF and H 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.

Claim 19

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 2 SiF 6 SiF 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 4 , HF and H 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.

Claim 20

A cyclic process according to claim 17, which comprises decomposing the hydrofluosilicic acid with concentrated aqueous sulfuric acid containing from about 60 to about 90% sulfuric acid by weight at a temperature within the range from about 50 to about 150°C to form gaseous silicon tetrafluoride and hydrogen fluoride.

Claim 21

A cyclic process according to claim 17, in which the silicon-and fluorine-containing compounds from the exhaust gases are absorbed in water.

Claim 22

A cyclic process according to claim 17, in which silicon dioxide is present in the waste gases and in the hydrofluosilicic acid solution formed therefrom, so that in the hydrolysis of hydrofluosilicic acid with sulfuric acid, the by-product is water instead of hydrogen fluoride.

Claim 23

A cyclic process according to claim 17, in which silicon tetrafluoride is hydrolyzed in the gas phase with water by burning with combustion gases to form highly active silicon dioxide and gaseous hydrogen fluoride.

Claim 24

A cyclic process according to claim 17, in which silicon tetrafluoride is hydrolyzed in the gas phase with superheated steam at a temperature above about 500°C., the silicon tetrafluoride gas and water vapour being blended and passed through the hydrolysis zone together, with an excess of water vapour, and the silica which is formed separated from the effluent gas stream from the reaction zone.

Claim 25

A cyclic process according to claim 17, in which the mixture of hydrofluoric acid and hydrofluosilicic acid is converted into silicon tetrafluoride and hydrogen fluoride by blending with concentrated sulfuric acid and heating at a temperature within the range from about 50 to about 150°C, at a proportion of acid such that the residual acid obtained comprises from about 25 to about 70 parts by weight of water per 100 parts by weight of sulfuric acid, vaporizing silicon tetrafluoride and hydrogen fluoride, passing gaseous effluent in countercurrent flow to entering sulfuric acid and absorbing steam and hydrogen fluoride present with silicon tetrafluoride in said sulfuric acid and recycling silicon tetrafluoride to the hydrolysis stage.

Claim 26

A cyclic process according to claim 17, in which an inert gas is introduced with the gaseous effluent to inhibit absorption of the silicon tetrafluoride in the sulfuric acid.

Claim 27

A cyclic process according to claim 17, in which the hydrofluosilicic acid-sulfuric acid solution is combined with the hydrofluosilicic acid-sulfuric acid solution obtained in the first stage from the waste gases, so that all of the silicon tetrafluoride is produced in that stage.

Claim 28

A cyclic process according to claim 17, in which the mixture of sulfuric acid and hydrogen fluoride is heated to 130° to 200°C, and the hydrogen fluoride vaporized.

Claim 29

A cyclic process according to claim 17, in which the hydrogen fluoride is dried by passing in countercurrent flow to concentrated sulfuric acid.

Claim 30

A cyclic process according to claim 1, which comprises absorbing silicon tetrafluoride and hydrogen fluoride in a member selected from the group consisting of water and aqueous hydrogen fluoride to form a hydrofluosilicic acid-containing liquor; venting the remaining gases to the atmosphere, substantially free from fluorine-containing components; recycling and combining the hydrofluosilicic acid-containing liquor with the aqueous hydrofluosilicic acid-containing liquor obtained from the waste gases; combining the aqueous effluent from the sulfuric acid decomposition of said hydrofluosilicic acid with an additional proportion of sulfuric acid; volatilizing hydrogen fluoride and water vapour; removing the water vapour by scrubbing with concentrated sulfuric acid; and recovering dry hydrogen fluoride; and recovering the sulfuric acid in dilute aqueous form; and concentrating the sulfuric acid.

Claim 31

A cyclic process according to claim 1, which comprises blending the 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 the hydrofluosilicic acid with concentrated sulfuric acid, the acid having previously been passed in countercurrent flow to the 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 the hydrogen fluoride in a dry form.