IL38841A - Recovery of hydrogen fluoride - Google Patents

Description

RECOVERY OF HYDROGEN FLUORIDE MD23 79 This invention relates to a process for the recovery of hydrogen fluoride, and especially for the production of substantially dry hydrogen fluoride from aqueous mixtures .

In a number of processes in which hydrogen fluoride is produced or used, hydrogen fluoride is obtained in the form of a dilute* gaseous mixture or is contaminated with water. Separation of hydrogen fluoride from water is made especially difficult by the fact that these two components form an azeotrope, and so cannot be separated readily by simple distillation. It is therefore desirable to have an effective method for recovering hydrogen fluoride and especially for recovering it in a substantially anhydrous condition from aqueous mixtures.

It is known that hydrogen fluori.de can be absorbed by solid sodium fluoride and can subsequently be recovered from it by heating, but this method presents great difficulties in practice; for example heat transfer problems make it necessary for the apparatus to be bulky and the breakdown of the solid absorbent (caused by expansion and contraction during the process) results in pressure build-up in the apparatus.

We have now found that these disadvantages may be obviated by using the absorbing solid in an inert liquid.

Thus accordincj to the present, invention we provide a process for the recovery of hydrogen fluoride from a gaseous mixture containing it which comprises passing the said gaseous mixture through a mixture of an alkali-metal fluoride in an inert liquid maintained at a temperature at which the hydrogen fluoride is absorbed and thereafter heating the mixture to a temperature sufficient to liberate hydrogen fluoride.

The alkal -metal fluoride is preferably sodium fluoride, but other alkali-metal fluorides or mixtures thereof may be used if desired.

The alkali-metal fluoride is usually used in the form of a suspension in the inert liquid and may be suspended in the liquid by any convenient means, for example by mechanical stirring, but adequate suspension may be achieved in some cases by the flow of the gaseous mixture through the suspension. The particle size of the alkali-metal fluoride is not critical, and may be chosen to achieve the desired degree of suspension in the system concerned.

The liquid may be any which is substantially inert under the conditions of use. Thus it should not be decomposed by contact with the hydrogen fluoride or the alkali metal fluoride or react to any substantial extent with either of them, and it should not be decomposed appreciably at the temperature of use.

Preferably it is one which has a boiling point of at least 250°C and, conveniently, it is one which is substantially immiscible with water. It is also desirable that the liquid is one which can dissolve hydrogen fluoride, even if only to a limited extent, and it is one which does not appreciably dissolve the alkali-metal fluoride.

Examples of suitable liquids include high-boiling aromatic compounds, for example diphenyl , diphenyl ether and mixtures of these, especially the eutectic mixture, and halogenated aromatic compounds, for example chlorinated diphenyl; esters , for example the phthalate esters, and in particular di-n-butyl phthalate; polyglycols for example those having a molecular weight of at least 1000, for example polypropylene glycol; mineral oils; long-chain carboxylic acids, for example oleic acid; and mixtures of such liquids.

The concentration of the alkali-metal fluoride in the liquid is not critical and may vary over a wide range for example between 1 and 50% by weight , preferably between 15 and 25% by weight, of alkali-metal fluoride in the mixture.

The absorption is generally carried out at a temperature in the range of 70-180°C, and preferably in the range 110-130°C. The further heating to liberate hydrogen fluoride, is preferably carried out at a temperature above 180°C, for example in the range 240-280°C.

The process of our present invention may be applied to the recovery of hydrogen fluoride from a wide range of gaseous mixtures. Thus it may be used to recover hydrogen fluoride from mixtures in which the hydrogen fluoride content may vary as widely as, for example, from 2 to 98% by weight, and the undesired component of the gas mixture may include for example one or more of nitrogen, oxygen, water-vapour, organic vapours, oxides of carbon and other acidic gases, for example sulphur dioxide, hydrogen sulphide, silicon tetrafluor ide and hydrogen chloride. The undesired components of the gaseous mixture pass on unabsorbed, and accordingly the temperature used for the absorption stage should be selected to minimise or avoid condensati of such components, for example steam. The process is especially useful for the recovery of hydrogen fluoride from mixtures containing water.

The process of the present invention may be applied to the recovery of hydrogen fluoride from the crude reaction vapours obtained for example in the pyrohydrolysi s of fluorspar (calcium fluoride). Since the process of the invention is highly efficient in separating hydrogen fluoride from a large variety of gases, the invention is particularly applicable in separating hydrogen fluoride from the vapours obtained by pyrohydrolysis of crude fluorspar and/or low grade fluorspar.

Thus hydrogen fluoride may be obtained by. pyrohydrolysis or other treatment of fluorspar varying in quality from crude ore as mined, which may contain as little as 20% by weight of calcium fluoride, through concentrated low grade ores to high grade concentrate containing at least 97% by weight of calcium fluoride.

Typical impurities in the fluorspar include silica, alumina, barytes and heavy metal carbonates and sulphides Thus one of the major contaminant gases from which the hydrogen fluoride is to be separated, may be sulphur dioxide which is obtained, for instance by decomposition of the barytes or oxidation of the sulphides.

The process of the invention may also be applied to recovering hydrogen fluoride obtained by pyrohydrolysis of fluorosilicic acid or its salts e.g. calcium fluorosilicate , which salts may optionally be mixed with a silicate e.g. calcium silicate. The process of the invention may also be applied to the recovery of hydrogen fluoride obtained as a by-product in other chemical reactions, or where it is recovered in reactions when it is used as a starting material, for instance in the fluorination of chlorinated hydrocarbons. It may also be recovered from aqueous solution, for example the aqueous condensates obtained from any of the above processes. If desired it may be applied to recover anhydrous hydrogen fluoride from the aqueous constant boiling acid, which contains about 38% of hydrogen fluoride.

The gaseous product containing hydrogen fluoride which is obtained as a result of the second heating stage of the process of the invention may be treated in conventional manner to recover the hydrogen fluoride therefrom. For example it may be cooled to a temperature below 19.5°C to condense the hydrogen fluoride or the gaseous stream of hydrogen fluoride may be used directly for any desired chemical reaction without intermediate isolation.

The process may be operated in batchwise or continuous manner, but is especially useful for continuous operation.

The accompanying drawing is a schematic representation of a process according to the present invention, adapted for continuous operation. A gaseous stream containing hydrogen fluoride enters along channel (3) into a reactor ( 1 ) which contains a slurry of alkali-metal fluoride in an inert liquid maintained at the absorption temperature, for example about 120°C, and unabsorbed gases leave the reactor along channel (4). The slurry of alkali-metal fluoride containing absorbed hydrogen fluoride is passed along channel (5) to the reactor (2) where it is heated to the evolution temperature, for example about 260°C; dry hydrogen fluoride is thus liberated and leaves along channel (6). The residual slurry is then returned to reactor (1) along channel (7) for re-cycle through the process.

The invention is further illustrated in the following Example.

Example Gaseous mixtures of steam, hydrogen fluoride and nitrogen were passed into a slurry comprising 55g of sodium fluoride in 188g of inert liquid maintained at 110-120°C. The slurry was analysed to determine how much hydrogen fluoride had been absorbed.

The slurry was then heated to 260°C over a period of 2 hours and the weicjht of hydrogen fluoride liberated was measured.

The results are shown in the following Table.

Table Gas Composition HF HF Liquid in which Inlet NaF is dispersed (% w/w) Absorbed Recovered Steam HF Nitrogen {%) {% of HF Absorbed ) Diphenyl/ Diphenyl ether eutectic mixture 23.1 5.2 71.7 56.0 91.5 Di-n-butyl phthalate 23.0 6.0 71. o 83.3 93.6 - Ditto - 21.9 7.1 61.0 93.1 89.6 Polypropylene glycol ( .W. 2025) 23.3 6.1 70.6 87.6 74.0

Claims (19)

1. What we claim is:- 1. A process for the recovery of hydrogen fluoride from a gaseous mixture containing it which comprises passing the said gaseous mixture through a mixture of an alkali-metal fluoride in an inert liquid maintained at a temperature at which the hydrogen fluoride is absorbed and thereafter heating the mixture to a temperature sufficient to liberate hydrogen fluoride.

2. A process according to claim 1 in which the alkali-metal fluoride is sodium fluoride.

3. A process according to claim 1 or 2 in which the alkali-metal fluoride comprises 1 to 50% by weight of the mixture with the liquid.

4. A process according to claim 3 in which the alkali-metal fluoride comprises 15 to 25% by weight of the mixture.

5. A process according to any one of the preceding claims in which the mixture is maintained at a temperature of 70 to 180°C whilst the hydrogen fluoride is absorbed.

6. A process according to claim 5 in w ich the temperature is 110 to 130 °C.

7. A process according to any one of the preceding claims in which the mixture is heated to a temperature above 180°C to liberate the hydrogen fluoride. * 11.

8. A process according to claim 7 in which the temperature is 240 to 280°C.

9. A process according to any one of the preceding claims in which the hydrogen fluoride is recovered from a gaseous mixture containing water.

10. A process according to claim 9 in which the gaseous mixture is obtained by the pyrohydrolysis of fluorspar.

11. A process according to any one of the preceding claims in which the liquid with which the alkali- metal fluoride is mixed is one having a boiling point of at least 250 °C.

12. A process according to any one of the preceding claims in which the liquid with which the alkali- metal fluoride is mixed is one which does not appreciably dissolve the alkali-metal fluoride.

13. A process accordini to any one of the preceding claims in which the liquid with which the alkali metal fluoride is mixed is one which is substantially immiscible with water.

14. A process according to any one of the preceding claims in which the liquid with which the alkali - metal fluoride is mixed is a high-boiling aromatic compound .

15. A process according to claim 14 in which the liquid is diphenyl, diphenyl ether or a mixture of these.

16. A process according to claim 14 in which the liquid is chlorinated diphenyl.

17. A process according to any one of claims 1 to 13 in which the liquid with which the alkali-metal fluoride is mixed is di-n-butyl phthalate, polypropylene glycol, mineral oil, oleic acid or a mixture of two or more of these.

18. A process according to claim 1 substantially as hereinbefore described with reference to the Example.

19. Hydrogen fluoride when obtained by the process claimed in any one of the preceding claims. S.HOROWITZ & CO. AGENT FOR THE APPLICANTS RDW/JMP