FR2505671A1 - Removing fluoro cpds. from waste gases e.g. from phosphoric acid mfr. - by absorption with a liq. stream contg. hydro:fluoric or hexa:fluoro:silicic acid - Google Patents

Abstract

In the purificn. of gases of fluorine-contg. cpds. by treatment of the gas with a stream of a dispersion of hexafluorosilicic acid or hydrofluoric acid and subsequent sepn. of the liq. and gas phases, the treatment is carried out with a stream of dispersion into the interior of which there is introduced a stream of dispersed liquid so that across the cross-section of the stream the fluorine content varies from 0 to 40 wt.% and the temperature varies from -5 to +60 deg.C. The dispersed liquid is pref. water or an HF or HSiF6 solution of lower concn. than the main liquid phase. The process is suitably carried out in an absorption tower. Used for purification of fluorine-contg. waste gases from the prodn. of phosphoric acid and mineral fertilisers. Improved purification of the fluorine-contg. gas stream is obtd. without the use of a multistage process.

Description

 The present invention relates to the purification of gases resulting from the production of phosphoric acid and mineral fertilizers and, more specifically, to a method of purifying gases aimed at liberating them from fluorinated compounds.

 The proposed process will find wide application in the chemical industry.

 Processes are currently known for purifying fluorine-containing gases by phase contact in a stream consisting of the gas to be purified and dispersed solutions of absorbent acids.

 A process is known for removing fluorinated compounds from gases by treating fluorine-containing gases with a stream of dispersed aqueous solutions of hydrofluoric acid or hydrofluoric acid generated, for example, in a Venturi tube (cf. VN Ramm "Gas absorption", Moscow, Edt BKhimia ", 1976, pp. 536-537).

 The mentioned process is characterized by a low mass transfer value due to invariable absorbent concentrations (aqueous acid solution) and constant temperature in the cross section of the current.

 In order to achieve high degrees of fluorine extraction, this requires complicated multi-level production schemes. Industrial absorption units operating according to the given process make it possible to reach a content of fluorinated compounds in the gases to be purified from 40 to 3 50 mg / m3. These gases are released into the atmosphere which results in environmental pollution with toxic compounds.

 It has been proposed to develop a process for eliminating fluorinated compounds from gases by modifying the technological parameters, a process which would make it possible to increase the degree of purification of the gases and simultaneously simplify the implementation.

 The solution consists of a process for removing fluorinated compounds from the gases, treatment of the gases with a current consisting of a dispersed solution of hydrofluoric acid or hydrofluoric acid followed by a subsequent separation of the liquid and gaseous phases, process in which according to the invention, the gas treatment is carried out by the indicated current inside which at least one jet of dispersed liquid is introduced with formation in its cross section of variable fluorine concentrations from 0 to 40% by mass and temperatures from -5 C to + 600C.

 The process in question makes it possible to achieve a higher degree of elimination of fluorine from the gases without using multi-level systems, that is to say to increase the purification process.

 The high degree of elimination of fluorine from gases by the claimed process contributes to reducing the pollution of the environment by fluorine.

 It is advantageous, in order to raise the degree of elimination of the fluorinated compounds from the gases, to use, as dispersed liquid, dispersed water or a dispersed solution of hydrofluoric hydrofluoric acid or hydrofluoric acid at a concentration of fluorine lower than that in dispersed solution of hydrofluoric acid or hydrofluoric acid mentioned.

 The proposed process is carried out as follows.

 The gas from which the fluorinated compounds are to be removed is admitted into an absorber in which the gas undergoes treatment with an absorbent in the form of a stream of a dispersed solution of hydrofluoric hydrofluoric acid or hydrofluoric acid inside which introduces at least one jet of dispersed liquid to form in the current cross-section variable concentrations of fluorine and variable temperatures. Water or a solution of hydrofluoric acid or hydrofluoric acid, the fluorine concentration of which is lower than the concentration in solution mentioned of hydrofluoric hydrofluoric acid or hydrofluoric acid, can be used as liquid.

 The dispersion of the indicated acids and the liquid can be carried out using a group of injectors with the formation of a current covering the mixing chamber of a absorber with cross-section and length calculated.

 In order to increase the degree of elimination of the fluorinated compounds from the gases, it is essential to create a gradient of fluorine concentrations in the cross section of the current.

 For this purpose, a jet of dispersed water or solutions of dispersed acids with a concentration lower than that of the acids forming is introduced into the central part of the current formed by the dispersed solution of hydrofluoric acid or hydrofluoric acid. the current mentioned.

 The fluorine concentration in the cross section of the current formed then decreases from the periphery of the current towards its center. After the fluorine-containing gases have been treated with this stream and the phases have been separated, part of the acid is taken off as product and the other part is recycled to the dispersion.

 The temperature difference in the cross-section of the current is obtained by adjusting the temperatures of the acid and the liquid by means of a heat exchanger before their admission to the dispersion.

 The realization of the absorption of the gases containing fluorine by the indicated current having in cross section of different concentration of fluorine and different temperatures makes it possible to capture 85-90% of fluorine in its peripheral domains.

 The difference in fluorine concentrations and temperatures in the cross-section of the current also results in the vapor pressure of fluorinated compounds over the absorbent becoming non-uniform along the cross-section of the current (higher at the periphery of 11 flow and lower in the center). The part of the unabsorbed fluorinated compounds then begins to move towards the center of the current where it is absorbed almost completely.

 The indicated treatment of fluorine-containing gases makes it possible to obtain, by comparison with known methods, a higher degree of purification of the gases without the use of multi-level systems.

 Example 1.

 -We admit into the brewing chamber of an absorber 3 3 34 600 Nm / hour of gas containing 5.3 g / Nm of silicon tetrafluoride (when calculating with respect to fluorine), 204 g / Nm3 of water vapor at atmospheric pressure and an absorbent in the form of a current by which the gas containing fluorine is treated for 0.35 sec.

The stream consists of a dispersed solution of hydrofluoric acid containing 10% fluorine recycled and admitted at the rate of 200 t / h and 1.8 t / h of water dispersed in the form of three jets. Since water forms the central part of the current and the acid forms the peripheral part, a variable gradient of fluorine concentrations occurs.
0 to 10% by mass in the cross section of the current.

The temperature of the water admitted to the dispersion will be kept within the limits of between 20 and 300C, that of hydrofluoric acid: from +30 to 600C so that the temperature in the cross section of the flow varies from +20 to + 600C.

 When absorption is complete, the gas is separated from the acid which is admitted to the absorption stage and which undergoes dispersion.

 As a result of the absorption, 1800 kg of hydrofluoric acid containing 10% fluorine 34600 Nm3 / h of gas with fluorine content up to 0.01 g / Nm3 are obtained.

 The degree of elimination of fluorine is approximately 99.82%.

 After the treatment of 34,600 Nm3 / h of gas containing 5.3 g / Nm3 of silicon tetrafluoride (calculating with respect to fluorine) and 204 g / Nm3 of steam water for 0.35 sec with a dispersed solution of hydrofluoric acid at the rate of 200 t / h having an invariable concentration of fluorine, in particular 10%, and a constant temperature in cross section of the current, the degree of absorption of the fluorinated compounds is 90% and the fluorine content of the gases rejected is 0.53 g / Nm3.

 Example 2.

 In the brewing chamber of an absorber we admit 60,000 Nm3 / h of a gas containing 8.5 # g / Nm3 of silicon tetrafluoride (calculating with respect to fluorine), 3,230 g / Nm3 of water vapor at atmospheric pressure and an absorbent in the form of a current by which the gas containing fluorine is treated for 0.20 sec.

 The current is formed by a dispersed solution of hydrofluoric hydrofluoric acid with 15% fluorine continuously recycled at the rate of 360 t / h having a temperature of 50 to 600C and by two concentrically admitted jets of dispersed solution of hydrofluoric hydrofluoric acid at the content in 2% fluorine at a rate of 180 t / h at a temperature of 30 to 400C and by dispersed water at a rate of 2.8 t / h at a temperature of 20 to 300C.

 In the cross section of the current the fluorine concentration varies from 0 to 15% by mass and the temperature varies from +20 to +60 C.

 After absorption, the gas is separated from the acid which is recycled to the absorption stage and subjected to dispersion.

 Following absorption, 3395 kg of hydrofluoric acid containing 15% fluorine and 60000nom3 of gas are obtained, with a fluorine content of up to 0.013 3 g / Nm3. The degree of elimination of fluorine from the gas constitutes 99.85%.

 After an analogous treatment of the fluorine-containing gas with hydrofluoric hydrofluoric acid with invariable fluorine concentration (at 15%) and at constant temperature in the cross-section of the flow, the degree of absorption is 85% and the fluorine content of the exhaust gas is 1.18 g / Nm3.

EXAMPLE 3
Admitted to the brewing chamber of an absorber 100,000 Nm3 / h of water vapor containing 5.6 g / Nm3 of fluorine (SiF4 + 2 HF), at an absolute pressure of 0.15 bar and an absorbent in the form a current by which the gas containing fluorine is treated for 0.15 s,
The current is formed by a dispersed solution of hydrofluoric acid containing 12% of recycled fluorine at the rate of 240 t / h having a temperature of 25 to 300C and admitted under the jet of a dispersed solution of hydrofluoric acid at 11.6 % fluorine at a rate of 120 t / h having a temperature of 20 to 250C. In the cross section of the current the fluorine concentration varies from 11.6 to 12% by mass and the temperature varies from 20 to 300C.

 After absorption the gas is separated from the acid which is recycled to the absorption stage and which is subjected to dispersion.

 Following absorption, 4,416 kg of hydrofluoric hydrofluoric acid containing 12% fluorine and 100,000 Nm3 / h of water vapor with fluorine content up to 0.3 g / Nm3 are obtained.

 The degree of absorption of fluorine is 94.7%.

 By an analogous treatment of water vapor with a dispersed solution of hydrofluoric acid, at an invariable fluorine concentration equal to 11.6% and at constant temperature in the cross section of the current, the degree of absorption is 85 %.

Example 4
A 3 3 10000 Nm3 / h absorber of a gas containing 112 g / Nm3 of fluorinated hydrogen (according to fluorine) is brought into the brewing chamber at an absolute pressure of 1 bar and an absorbent in the form of 'a current by which the gas containing fluorine is treated for 1.5 sec.

The stream is formed by a dispersed and continuously recycled solution of hydrofluoric acid containing 40% fluorine at a rate of 50 t / h having a temperature of 20 to 300C and admitted under the jet of a dispersed solution of hydrofluoric acid containing 38.5% of fluorine at a rate of 4.5 t / h at a temperature of -50C up to + 50C. The fluorine concentration varies in the cross section of the current from 38.5 to 40% by mass and the temperature varies from 3Q to -50C,
After absorption the gas is separated from the acid which is recycled to the absorption stage and which undergoes dispersion.

 Following the absorption and separation of the phases, 2800 kg of hydrofluoric acid are obtained at a concentration of 40% fluorine as product and 10000 Nm3 / h of gas with traces of HF.

Claims (3)

 1. Process for eliminating fluorinated compounds from gases by treating the gases with a current consisting of a dispersed solution of hydrofluoric acid or hydrofluoric acid followed by a subsequent separation of the liquid and gaseous phases, characterized in that gas treatment is carried out by the current mentioned inside which at least one jet of dispersed liquid is introduced with formation in its cross section of variable fluorine concentrations from 0 to 40% by mass and variable temperatures of -5 C at +60 C.  2. Method according to claim 1, characterized in that dispersed water is used as the dispersed liquid.  3. Method according to claim 1, characterized in that as dispersed liquid a dispersed solution of hydrofluoric acid or hydrofluoric acid with a fluorine concentration lower than the concentration of the dispersed solution of hydrofluoric acid or hydrofluoric acid mentioned.