FI61411C - FOER FLANGE FLEAR AVERAGE FLUORFOERENINGAR FRAON AVLOPPSGASER - Google Patents

Description

RSr ^ l [B] (11 AD 61411 lJ UTLÄGGNINGSSKRIFT ° '' C (45) Fatm'tl: * /!, 5 F3 ^ ^ (51) Kv.lk?/lnt.a.3 B 01 D 53 / 14 SU OM I —FI N LAND (21) P * Mnttlh »k * mu * - PKwitanteknlng 7031 ^ 7 (22) Hakamltpilvt - AiwMcnlngadag 03.11.76 (23) AikuplUvi— Glltlgh« t * d «g 03.11.76 (41) ) Gotten stuck - Bllvlt offantllg 05.05.77

National Board of Patents and Registration Nihaviiuip ^ on). kuui. | u.w ««, pvm.- 'nli' fi?

Patent · och r «gistarstyrelaen Antttktn utltgd och uti ^ krifMn pubucend (32) (33) (31) Requested utuolkuut — Begird priority 0U.11.75

France-France (FR) 7533659 (71) Rhöne-Poulenc-Industries, 22 Avenue Montaigne, 75 Paris (8eme), France

Frankrike (FR) (72) Claude Djololian, Maromme la Maine, France-Frajikrike (FR) (7 ^) Berggren Oy Ab (5 ^) Method for removing fluorine compounds from exhaust gases - Förfarande för avlägsnande av fluorföreningar frän avloppsgaser

The invention relates to a process for removing fluorine compounds from exhaust gases by washing the exhaust gas in the first section with water or an acidic solution and in the second section with a basic solution.

In particular, the invention relates to a process for the removal of fluorine compounds from exhaust gases found in production starting from a mineral containing significant amounts of fluorine.

This is the case with the wet road in the production of phosphoric acid, where the most common starting material is a fluoroapatite mineral containing a considerable amount of fluorine compounds. These are released by sulfuric acid and are generally distributed so that about 60% of the total is in the acid, about 35% in the gypsum by-product, and the remainder is present in the vapors from the etching vessel mainly in varying ratios of hydrofluoric acid HF and silicon trifluoride SiF4.

In particular, the invention relates to improvements in the emptying of the dissolution vessel, which often have to be done quite considerably. with amounts of vapors which, however, contain relatively little fluorine compounds; the absorption of fluorine into aqueous solutions 2 also depends on the method of cooling the dissolution basin. If cooling is carried out under reduced pressure, the solutions obtained are very dilute, containing less than 1 g / l, so that they cannot be used. In other methods, the washing and absorption of fluorine-containing vapors with water always causes a rather large liquid effluent. It is difficult to release this effluent into the environment and, in particular, there are costs involved in neutralizing it.

It is also known that high-efficiency absorption with water is difficult and that in order to obtain completely pure gases that can be released into the atmosphere, there must be several absorption devices.

It is also known that absorption with alkaline solutions gives good purification without many absorption devices, but consumes a lot of reagents without producing useful solutions.

The partial pressures of HF and SiF 3 as a function of the temperature and concentration of fluorosilicic acid H 2 SiFg have been studied, especially at concentrations of less than 10% by mass of f 2 SiF 2.

The liquid-vapor equilibria of the HF-SiF 2 -O 2 O system indicate that if the gas molar ratio HF / SiF 2, denoted by the letter R, is less than 2, excess SiO 2 is present in the liquid phase; otherwise, free HF is observed with SiF. It has been possible to observe variations in the ratio R, which, when starting from concentrated solutions, depend on the composition of the mineral, in particular its silicon content, as well as on the dissolution conditions. It is also known that the total concentration of fluorine compounds decreases in both the gas phase and the equilibrium solution in each absorption step of the multistage absorption, but difficulties are always observed in absorbing at dilute concentrations.

It is known that diluting the SiFg solution causes an increase in the molar ratio R = HF / SiF2 in the gas phase, which is known to shift the equilibrium towards HF formation.

The process according to the invention is characterized in that the exhaust gas is washed to remove hydrogen fluoride and silicon trifluoride in the first washing cycle with water or recycled washing water 3 61411 until the molar ratio R = HF / SiF 4> 4 and then a second washing cycle is performed.

The process according to the invention makes it possible to eliminate virtually completely the fluorine compounds HF and SiF2 present in the exhaust gases in any ratio.

The first step of the process is generally performed using an arbitrary number of washing steps, for example only one step or a series of steps. Each washing step is carried out in an apparatus in which the gases can be washed countercurrently, hereinafter referred to as scrubber.

The first step of the process is preferably carried out in a scrubber at the outlet of which the ratio R of the gas mixture is at least 4, and the solution containing most of the fluorine compounds is recovered.

The first stage of the method is preferably performed in a series of scrubbers, each of which is supplied with gases from the previous scrubber and scrubbing solution from the next scrubber, the ratio R at each scrubber outlet being greater than its inlet, and the last scrubber at the outlet the fluorine compounds absorbed in their entirety.

In a variation of the embodiment performed in the series of scrubbers of the first stage of the method, the different washing solutions are recovered separately, whereby the concentration of these different solutions decreases from the first to the last in the series.

In a modification of the above modification, the washing liquid is distributed among a series of scrubbers so as to obtain detergents, all of which have the lowest possible concentration.

The washing liquids are recovered separately by any means known per se. It is sometimes advantageous to continuously circulate washing liquids 4 61411 and to remove part of the cycle as a final product and replace it with a corresponding amount of washing liquid.

The second cycle of washing is performed with a basic solution, such as an aqueous solution of an alkali metal or alkaline earth metal hydroxide, the concentration of which is adjusted so as to neutralize the acidic fluorine compounds. Preferably, soda solution or lime milk can be used for this.

The second cycle of washing is performed in any device that can perform countercurrent washing with water.

It is advantageous to continuously circulate the washing liquids and to remove from the cycle the part which forms the second end product and to replace it with a corresponding amount of washing liquid.

In this way, the total amount of water used can be adjusted.

Both cycles of the washing process are preferably carried out in a cyclone device of the type described in Finnish patent application 763146, which has the advantage of providing effective contact in a small space and at the same time obtaining a clean and drip-free gas. In this apparatus, liquids can be recycled to give a fluorosilicic acid solution after the first wash and a neutral fluoride and fluorosilicate solution in relatively small volumes after the second wash. Preferably, two water-cast cyclone columns are used in succession, in which a first washing cycle is carried out, in which the gas is purified purified of most of its fluorine compounds but retaining its vortex motion, and passed to a cyclone column of the same type in which the base washing is carried out.

The process according to the invention is particularly suitable for the production of phosphoric acid in the wet road and in particular for the emptying of dissolution vessels. The process recovers all gaseous fluorine compounds in liquid form and without releasing fluorine into the atmosphere; recovery of gaseous fluorine compounds in liquid form is preferred, since the liquid can be recycled to another part of the plant where its heat is recovered, and the fluorine compounds in a usable form.

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Example

The process according to the invention treats the air leaving the phosphoric acid wet road from the dissolution vessel of the Togo phosphate plant? the air is almost saturated with moisture and its temperature is close to the boiling point; it contains about 1 g of fluorine per m 2 of gas under mineral processing conditions. Its total composition at a flow rate of 267,753 kg / h at 58 ° C is as follows:

Air 233,669 kg / h HF 56.4

SiF4 147.2 C02 1.860 K20 32.020 and a small amount of entrained dust and small droplets.

This air flow is passed to the cyclone column described in Finnish patent application 763146 at the same time as the aqueous solution from another cyclone column of the same type. This aqueous solution contains 1.5 g / l of fluorine after washing the gaseous compounds from the first wash.

The gases after the first wash are passed to another cyclone column of the same type, where they are washed countercurrently with water.

At the outlet, the gases contain only 50 mg of fluorine per m 2 of gas mixture. Gas mixture ratio R = HF / SiF 2 = 10.

The gases are passed on the same type as in the above cyclone column countercurrently with the recycled NaOH solution. A portion of sodium fluoride containing 21 kg of sodium fluoride mixed with 9 kg of sodium fluorosilicate is removed from the resulting solution containing sodium fluoride, and this separated stream is passed to another part of the plant. The solution used for washing in the first absorber contains water and 5 g / l of fluorine in the form of H2SiFg, SiO2 and HF, as well as some phosphate dust and phosphoric acid droplets. It is continuously recycled, but the excess of this solution, which is not recycled, is utilized in another part of the plant. The exhaust gas flow at the outlet of the third column corresponds to the pollution standards.

6 61411

For comparison, it is noted that the dilute solutions obtained from the water absorption of gas mixtures starting from acid dissolution of phosphate minerals according to the prior art and gas mixtures cooled under reduced pressure generally contain less than 1 g of fluorine per liter and have not been utilized so far.

In previously known methods, the amount of neutralizing compounds used to remove dilute solutions containing the total amount of fluorine compounds formed during dissolution corresponds to a neutralization of about 5% of the total fluorine of the mineral.

In the process according to the invention, the amount of neutralizing compounds corresponds only to the amount necessary to neutralize a fluorine fraction of the order of 5% from the compounds treated in the dissolution gas mixture, i.e. about 0.25% of the total fluorine of the mineral.

Claims (8)

1. The use of fluorescent substances in the final genome as a result of the use of fluorescent substances in the form of fluorescent substances in the form of fluorinated substances cyclical tensile tills mol-förhällandet R = HF / SiF ^> 4 och därefter utföres andra tvättfasen. A method for removing fluorine compounds from exhaust gases by washing the exhaust gas in the first section with water or an acidic solution and in the second section with a basic solution, characterized in that the exhaust gas is washed to remove hydrogen fluoride and silicon trifluoride in the first wash cycle a second wash cycle is performed. 2. A method according to claim 1, which can be used as an alternative to a device according to the invention. 9,61411 A method according to claim 1, characterized in that the first cycle is performed using an arbitrary number of washing steps. 3. A patent according to claim 2, which is intended to be used in the manufacture of foodstuffs. A method according to claim 2, characterized in that each washing step is performed in a countercurrent gas scrubber. 4. For the purposes of this Regulation, the provisions of this Regulation shall apply to the application of the provisions of this Regulation to 4 months and to the date of application of the provisions of this Regulation. Process according to one of the preceding claims, characterized in that the first stage of the process is carried out in a scrubber at the outlet of which the ratio R of the gas mixture is at least = 4 and that a solution containing most of the fluorine compounds is recovered. 5. For the purposes of this Regulation, the provisions of this Regulation shall apply to the accounts of the Member States of the Community for the purposes of the provisions of this Regulation. the gas is stored in the same direction as in the case of gas, and in the case of gas so far R = min = 4 and in the case of the same gas is used. Method according to one of the preceding claims, characterized in that the first stage of the method is carried out in a series of scrubbers, each of which is supplied with gases from a previous scrubber and a scrubbing solution, the ratio R in the gases leaving each scrubber is greater than the ratio R in the incoming gases. at the scrubber outlet, the ratio R is at least = 4 (and that a wash liquor containing most of the fluorine compounds is recovered. 6. In the case of patent claim 5, the terms of reference for the application of the provisions of this Regulation apply to the application of the provisions of this Regulation. Method according to Claim 5, characterized in that the washing solution of each scrubber in the series is recovered separately and that the concentration of these different solutions decreases as the first to last scrubbers in the series are passed. Method according to Claim 5, characterized in that the washing liquid is distributed among a series of scrubbers and is 8 / /> 1 A, Λ A f; I * · τ I 'are made up of washing solutions, all of which have the lowest possible concentration. A method according to claim 1, characterized in that the washing liquids of each cycle are continuously circulated, that a part which forms the final product is removed from the cycle and that this part is replaced by a corresponding amount of washing liquid. Process according to Claim 1, characterized in that an aqueous solution of a basic compound, such as an alkali or alkaline earth metal hydroxide, is used in the second stage and its concentration is adjusted in such a way that the acidic fluorine compounds leaving the first stage are neutralized. Process according to Claim 9, characterized in that a NaOH solution is selected for use in the second cycle. Application of a process according to one of the preceding claims for the absorption of fluorine compounds contained in gas mixtures formed by dissolving phosphates in the wet path of phosphoric acid, characterized in that most of the fluorine compounds treated by the process are recovered as a fluorosilicose solution and the remainder recovered as a dilute solution. 7. For the purposes of claim 5, the terms of reference are set out in terms of the number of terms and conditions of use and the concentration of the active substance. 7 6141 1 8. For the purposes of claim 1, the provisions of this invention are based on the requirements of the circulating container, which may include certain products, and of the provisions of the present invention. 1 A particular form of claim 1, which is used in the manufacture of an alkali metal or alkaline metal, and the presence of hydroxides in the presence of Sura fluorine fractions in the gas phase, första fa '^ en, neutraliseras.