DE2651630A1 - PROCESS FOR THE SEPARATION OF HYDROFLUOR FROM MIXTURES WITH ACID GASES - Google Patents

Description

RIEDEL-DE HAEN AKTIENGESELLSCHAFT 26 516

File number: HOE 7G / D 017

Date: H, November 1976 Dr.KO / Me

Process for the separation of hydrogen fluoride from mixtures with acid gases

The invention relates to a process for the separation of hydrogen fluoride (HF) from mixtures with acidic gases such as e.g. Sulfur dioxide and / or silicon tetrafluoride and / or hydrogen chloride.

Gas mixtures which, in addition to hydrogen fluoride, can be used individually or in Mixtures containing acidic gases are common in technology. For example, hydrofluoric acid is produced through conversion from sulfuric acid with fluorspar exhaust gases, which in addition to hydrogen fluoride also contain silicon tetrafluoride and sulfur dioxide. In the production of fluorocarbons from chlorinated hydrocarbons and hydrogen fluoride, waste gases are produced which, in addition to hydrogen fluoride, are accompanied by hydrogen chloride will. Even with the production of phosphoric acid and phosphates by the wet process, exhaust gases are obtained, which in addition to hydrogen fluoride still carry silicon tetrafluoride with you.

The separation of the hydrogen fluoride from the gas mixtures formed in the manner mentioned prepares z. T. considerable difficulties. So z. B. in those exhaust gases which are in hydrofluoric acid plants in the conversion of fluorspar with sulfur

809821/0066 _{/ 2}

Acid accumulate, proceed according to the state of the art so that the exhaust gases initially with that required for production practically 100% sulfuric acid are washed before this is added to the reaction with fluorspar. The in the Exhaust gases containing hydrogen fluoride are absorbed in the sulfuric acid and returned to the process, while silicon tetrafluoride and sulfur dioxide are not absorbed and thus separated. The disadvantage of this method is that the Hydrogen fluoride absorbed in practically 100% sulfuric acid is at least partially absorbed with it in a known manner converts to fluorosulfonic acid and water, which can give rise to severe corrosion.

In the German patent specification 1 567 504 it has already been proposed to use hydrogen fluoride in the production Gas mixture produced by chlorofluorocarbons, which essentially consists of hydrogen fluoride and hydrogen chloride consists of being separated by washing the gas mixture in a column with hydrochloric acid. The escapes at the head Column of hydrogen chloride, which is largely free of hydrogen fluoride, while at the foot of the column contains hydrogen fluoride Hydrochloric acid is removed. The acid mixture thus formed cannot, however, be used further without further ado.

Furthermore, it has already been proposed in German Patent 2 239 131 to divide hydrogen fluoride from hydrogen chloride to separate that one the hydrogen fluoride and hydrogen chloride containing gas mixture is reacted with hydroxyfluoroboric acid, the hydrogen fluoride content being converted into practically hydrogen chloride with this converts free fluoroboric acid. The resulting fluoroboric acid however, has only a limited market, so that the method can only be used to a limited extent.

In the production of phosphoric acid and phosphates by the wet process, considerable amounts of are produced as by-products Hydrogen fluoride and silicofluoride, their work-up was previously only possible in a complicated way. So z. B. Japanese Patent 71 20 134 reported in Chemical

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Abstracts 75. (1971) 153435u suggested both components to be absorbed in water, to precipitate with sodium sulfate and to precipitate this pyrolytically to sodium fluoride and To decompose silicon tetrafluoride. In this way, it is true that the fluoride content of the starting gas from the silicon tetrafluoride but it is obvious that this is a complicated process with numerous procedural steps such as absorbing, precipitating, drying, decomposing, etc.

The Polish patent specification 56 972 proposes, after a report in Chemical Abstracts 21 (1969) 23318q, hydrogen fluoride to be separated from silicon tetrafluoride by absorption in oleum. The resulting fluorosulfonic acid is then added by adding decomposed by water and expelled hydrogen fluoride at elevated temperature. The procedure requires because of the use of Oleum a complicated process. In addition, a constant supply of oleum is necessary.

The object on which the invention is based thus consisted in the preparation of gas mixtures obtained in the technical processes mentioned to separate the hydrogen fluoride contained therein, the process if possible

a) less labor-intensive than the known methods

b) applicable in continuous operation and

c) should be environmentally friendly.

The invention achieves the object by a method which is characterized in that in a first method stage the Gas mixture containing hydrogen fluoride and acidic gases, possibly also inert gases, at low temperature by means of sulfuric acid of 75-85% by weight, which largely and selectively absorbs the hydrogen fluoride contained in the gas mixture, and then In a second process stage, an inert gas stream is introduced into the hydrogen fluoride-containing sulfuric acid at an elevated temperature is blown in, which desorbs the hydrogen fluoride, which is free of acidic gases, from the sulfuric acid.

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According to the process according to the invention, gas mixtures of hydrogen fluoride with acid gases can be worked up, e.g. with hydrogen chloride HCl, sulfur dioxide SO- and silicon tetrafluoride SiF. which can also contain larger proportions of inert gas.

As low temperatures to be maintained in the first process stage, temperatures from 0 to 30 ^° C. have proven useful. Temperatures of 80 to 110 ^° C. are preferably maintained in the second process stage. It is possible to use even higher temperatures, but there are no advantages and there are even disadvantages in terms of energy consumption.

The concentration of the concentrated sulfuric acid used as an absorbent can only fluctuate within narrow limits, namely between concentrations of 75 to 85 percent by weight H ₂ SO .. At these concentrations no acidic gases are dissolved in it, while at the same time there is good dissolving power for hydrogen fluoride, without the formation of fluorosulfonic acid.

At lower concentrations, SiF becomes. with solved, so that Process loses selectivity. In addition, water is expelled in the desorption stage. With a concentration of over 85 percent by weight HpSO. hydrogen fluoride becomes bad dissolved or formed fluorosulfonic acid, which can be blown out schiecht.

To optimize the process, it is advantageous to largely reduce the amount of hydrogen fluoride obtained after the second process stage The released sulfuric acid can be recycled back into the first process stage. It can be the same after the second process stage accumulating hydrogen fluoride-containing inert gas by methods known per se, for. B. in one with Water or boric acid or alkali hydroxide solution operated washing system, largely freed from hydrogen fluoride and then recycled to the second process stage. Both Measures reduce the environmental pollution with pollutants.

As can be seen from the attached diagram, the outlay on equipment required to carry out the process is low. In this Scheme is = 809821/0066

1 absorber

2 heat exchangers

3 heaters

4 desorbers

5 pump -

6 coolers

7 fans

8 hydrogen fluoride separator

All apparatuses known as gas scrubbers can be used as absorber 1 can be used, e.g. jet scrubbers or multi-tray bell-bottom columns. However, a washing device with only one base is also sufficient.

The desorber 4 can be designed in the same way. In particular the use of a bubble cap column is favorable, because the residence time of the liquid in it is particularly long, so that the blowing-out process is as effective as possible. Also as A scrubber of the type mentioned comes into consideration.

All apparatus must be made of resistant to hydrofluoric acid and hydrofluoric acid Be material. Polytetrafluoroethylene, polypropylene, polyvinylidene chloride and graphite are suitable for this. It is expedient to use graphite for the heater 3 and for the remaining parts polypropylene.

According to the method of the invention, the gas mixture to be worked up is composed of hydrogen fluoride and acidic gases, e.g. B. hydrogen chloride and / or sulfur dioxide and / or silicon tetrafluoride, which can also contain larger proportions of inert gas, in absorber 1 at low temperatures, preferably at 0 to 30 ⁰ C with concentrated sulfuric acid, for example from 75 to 85 percent by weight H ₂ SO ₄ , which continuously in and out is washed. In this case, hydrogen fluoride dissolves selectively in the sulfuric acid, while the accompanying acid gases mentioned do not dissolve in it and emerge from the absorber 1 in gaseous form. These acid gases cannot be used in a downstream process, as shown in the diagram

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shown system are obtained or deposited.

The exiting from the absorber 1, enriched with hydrogen fluoride sulfuric acid is passed through a heat exchanger 2 and a heater 3 and thereby, preferably 80 to 110 ⁰ C heated to higher temperature.

Then it is fed to the desorber 4, in which the hydrogen fluoride with an inert gas, e.g. air, from which sulfuric acid is blown.

The sulfuric acid exiting continuously from the desorber 4 and largely freed from hydrogen fluoride is fed via a pump 5 to the heat exchanger 2 mentioned, where part of the heat is given off to the hydrogen fluoride-containing sulfuric acid. In the subsequent cooler 6, the sulfuric acid is ^{cooled to preferably 0 to 30 °} C. and returned to the absorber 1.

The gas mixture of hydrogen fluoride emerging from the desorber 4 and inert gas is converted from hydrogen fluoride in the hydrogen fluoride separator 8 freed. For example, the latter can be reacted with water to form aqueous hydrofluoric acid. However, the gas mixture can also be used directly can be used for chemical reactions, e.g. by bringing it into contact with a base, e.g. with potassium hydroxide solution. It however, it can also, for example, be introduced into a slurry of boric acid in water, whereby fluoroboric acid is formed.

Finally, it is also possible to separate the hydrogen fluoride from the gas mixture by cooling, if necessary also by compression.

It is particularly advantageous to remove the hydrogen fluoride Inert gas circulated back into the desorber 4 via a fan 7, since the application is more complex in this way Absorption devices to free the inert gas from accompanying residual traces of hydrogen fluoride to avoid it of environmental damage is eliminated.

809821/0088. ^{/ 7}

The method according to the invention allows it. Hydrogen fluoride in a simple manner of accompanying acidic gases such. B. hydrogen chloride and / or sulfur dioxide and / or silicon tetrafluoride to separate. It can preferably be carried out continuously, although it can also be carried out batchwise is possible. There are no additional auxiliaries required, and there are also no waste products. The process is particularly environmentally friendly when the inert gas is in the Cycle is performed, as no harmful exhaust gases can arise.

It is also a particular advantage of the invention that hydrogen fluoride is also separated from such gas mixtures which, in addition to acidic impurities, can be considerable Contain proportions of inert gases, so that hydrogen fluoride and acid gases are present in strong dilution.

Execution examples of the method according to the invention are given below in an arrangement in which for absorbers 1 and Desorber 4 washing devices with only one base can be used. These examples are meant to be illustrative only and in no way limit the claimed method.

To carry out the examples, an apparatus according to the attached scheme was used, all parts of which were made of polypropylene. In the absorber 1 and in the desorber 4, the gases were introduced into the liquid initially introduced using a dip tube. The dimensions of the apparatus were such that the sulfuric acid circulation required an amount of 20 liters. Sulfuric acid with 80 percent by weight of H ₃ SO _{4 was} used. The temperature in the absorber 1 was maintained at 25 to 30 ⁰ C, in the desorber at 100 to 110 ⁰ C. The sulfuric acid was circulated at a rate of 10 1 / hour. 200 liters of air per hour were introduced into the desorber 4. This air was not circulated.

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example 1

200 liters of air per hour containing 7.2 g (- · 0.08 mol) of silicon tetrafluoride SiF ₄ and 36 g (= 1.80 mol) of hydrogen fluoride HF in gaseous form were introduced into the absorber 1. The one emerging from the absorber 1 is made of SiF. and air, the gas stream, which was present in a washing bottle (not shown in the scheme), was passed through initially introduced aqueous hydrofluoric acid, with hydrofluoric acid being formed. 7.2 g of SiF were obtained there per hour. in the form of hydrofluoric acid, which corresponds practically to 100% of the amount used, and 1 g of HF in the form of aqueous hydrofluoric acid, which corresponds to about 3% of the amount used.

The gas stream emerging from the desorber 4 was in a washing bottle with a Kasser serving as a hydrogen fluoride separator 8 washed. There were obtained 87.5 g per hour of an aqueous hydrofluoric acid with 40 percent by weight of HF, which is 35 g of HF or 97% of the the amount used. The H, SiF, content of hydrofluoric acid was very low, i.e. less than 0.1%.

200 liters of air per hour were introduced into absorber 1, containing 67 g (1.83 mol) of hydrogen chloride HCl and 36 g (1.80 mol) Containing hydrogen fluoride HF in gaseous form. The one from the absorber 1 Exiting gas stream consisting of HCl and air was passed through water in a washing bottle, whereby aqueous hydrochloric acid formed. 67 g of HCl in the form of aqueous hydrochloric acid and 1 g of HF in the form of aqueous hydrofluoric acid were obtained there per hour.

The gas stream emerging from the desover 4 was in a washing bottle washed with water. 140 per hour were obtained there g hydrofluoric acid with 25 percent by weight HF (35 g HF) and a chloride content of less than 0.01%.

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Example 3

200 liters of air containing 37 g (0.58 mol) of sulfur dioxide SO ₂ and 36 g (1.80 mol) of hydrogen fluoride HF in gaseous form were introduced into the absorber 1 per hour. The gas stream exiting from the absorber 1 and consisting of SO ₂ and air was passed through potassium hydroxide solution in a washing bottle. 37 g SO2 and 1 g HF in the form of potassium salt are increased there per hour.

The air stream emerging from the desorber 4 was washed with water in a wash bottle. 140 g of hydrofluoric acid with 25 percent by weight HF (= 35 gHF) with an SO ₂ content of less than 0.1% were obtained there per hour.

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first page

Claims (6)

Claims; ■ 1. Process for the separation of hydrogen fluoride from mixtures with acidic gases, characterized in that in a first process stage the gas mixture, which is hydrogen fluoride and acidic gases, possibly also inert gases, at low temperature by sulfuric acid is passed, which largely and selectively absorbs the hydrogen fluoride contained in the gas mixture, and then in a second process stage into the hydrogen fluoride-containing one Sulfuric acid at an elevated temperature, an inert gas stream is blown in, which consists of the sulfuric acid desorbs the hydrogen fluoride, which is free from acid gases 2. The method according to claim 1, characterized in that in the first process stage at temperatures of 0 to 30 ⁰ C and in the second process stage at temperatures of up to 110 ⁰ C is carried out. 3. The method according to claim 1 and 2, characterized in that the sulfuric acid used 75 to 85 percent by weight H ₂ SO. contains. 4. The method according to claim 1 to 3, characterized in that the sulfuric acid obtained after the second process stage returned to the first stage of the process in the cycle will. 5. The method according to claim 4, characterized in that the inert gas containing hydrogen fluoride obtained after the second process stage largely in a manner known per se freed from hydrogen fluoride and then circulated into the second process stage is recycled. 6. Process for the separation of hydrogen fluoride from mixtures with acid gases, characterized in that in a first process stage the gas mixture consists of 809821/0065 ORIGINAL HOE 76 / D 01.7 Hydrogen fluoride HF and hydrogen chloride, HCl and / or sulfur dioxide, SO ₃ and / or silicon tetrafluoride SiF _4, which may also contain large proportions of inert gas, at temperatures of 0 to 30 ⁰ C by sulfuric acid to 85 weight percent H "SO. is passed, which largely and selectively absorbs the hydrogen fluoride contained in the gas mixture, then in a second process stage ^{an inert gas stream is blown into the hydrogen fluoride-containing sulfuric acid at temperatures of 80 to 110 0} C, which desorbs the hydrogen fluoride free of acidic gases from the sulfuric acid Inert gas stream wins the hydrogen fluoride in a known manner and the inert gas largely freed from hydrogen fluoride is recycled to the second process stage, while the sulfuric acid obtained after the second process stage, largely freed from hydrogen fluoride, is recirculated to the first process stage. 809821 / 008S