GB2146011A

GB2146011A - Preparing sodium chloride solution and producing sodium bicarbonate

Info

Publication number: GB2146011A
Application number: GB08422321A
Authority: GB
Inventors: Leon Ninane; Claude Breton
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1983-09-05
Filing date: 1984-09-04
Publication date: 1985-04-11
Also published as: DE3432388A1; IT1178506B; BR8404405A; MA20222A1; LU85506A1; PT79156B; AU3255484A; IT8422439A0; SE8404185L; PT79156A; ATA283284A; NO843516L; AT380674B; FR2551428B1; IL72798A0; FR2551428A1; DE3432388C2; GB8422321D0; IL72798A; SE8404185D0

Abstract

Process for the preparation of an aqueous solution of sodium chloride, according to which sodium bicarbonate is decomposed in an aqueous solution by means of a chloride of an organic cation, the carbon dioxide formed is removed and rock salt is added to the resulting aqueous solution. The process applies to the preparation of an aqueous solution of sodium chloride from a mother liquor from the crystallisation of sodium bicarbonate and an organic solution of amine hydrochloride, which are obtained by treating an aqueous solution of sodium chloride, an organic solution of a water-insoluble amine and carbon dioxide.

Description

SPECIFICATION Preparing sodium chloride solution and producing sodium bicarbonate The present invention relates to a process for the preparation of an aqueous solution of sodium chloride from an aqueous solution of sodium bicarbonate.

A method for the manufacture of sodium bicarbonate is known, according to which an aqueous solution of sodium chloride is mixed with an organic solution of a water-insoluble amine, the resulting mixture is treated with a gas containing carbon dioxide, and is then decanted to separate an aqueous solution of sodium bicarbonate from an organic solution of amine hydrochloride, and the aqueous suspension of sodium bicarbonate is treated to separate the solid sodium bicarbonate which it contains from a mother liquor (Patent GB-A-1,082,436 (KAISER ALUMINUM & CHEMICAL CORPORATION), page 2, line 105 to page 3, line 5; Central Patents Index, Basic Abstracts Journal, Section E, Week T.49, Derwent Publication Ltd., London, abstract78430T-E: Patent Application JP-A-7241237 (ISRAEL MINING IND. INST. RES. & DEV.)).

In the following description, the expression "amines method" will indicate this known method for manufacturing sodium bicarbonate.

In the amines method, sodium chloride is usually added to the mother liquor from the crystallisation of sodium bicarbonate, in order to re-form the starting aqueous solution of sodium chloride. This practice makes it possible to recover all the sodium and the carbonate and bicarbonate ions in the mother liquor and thereby ensures a maximum degree of utilisation of the sodium employed. Until now, however, it has necessitated the use of sodium chloride of high purity and has prohibited, in particular, the use of rock salt.In fact, the presence of impurities in rock salt and, more particularly, of calcium sulphate results in precipitation of the carbonate and bicarbonate ions in the mother liquor, in the form of calcium carbonate and formation of sodium sulphate which goes into solution; the result is, on the one hand, a loss of carbonate and bicarbonate ions and, on the other hand, a progressive enrichment of the aqueous solution of sodium chloride in sodium sulphate, which leads to a decrease in the solubility of sodium chloride and to unwanted reactions with the amine, which are detrimental to the good operation of the method for the manufacture of sodium bicarbonate.

The invention aims at providing a process for the preparation of aqueous solutions of sodium chloride, from aqueous solutions of sodium bicarbonate, which permits the use of rock salt while avoiding the abovementioned disadvantages.

The invention consequently relates to a process for the preparation of an aqueous solution of sodium chloride from an aqueous solution of sodium bicarbonate, according to which the aqueous solution of sodium bicarbonate is treated with the chloride of an organic cation to decompose sodium bicarbonate, the carbon dioxide which is formed is removed and rock salt is added to the resulting aqueous solution.

In the process according to the invention, the aqueous solution of sodium bicarbonate may be, for example, a mother liquor from the crystallisation of sodium bicarbonate such as is obtained by carbonation of aqueous solutions of sodium hydroxide or in the amines method specified above.

The organic cation is, by definition, any organic cation which, in the form of a chloride, is capable of reacting with sodium bicarbonate to form sodium chloride, carbon dioxide and an organic base. In a preferred embodiment of the invention, the chloride of an organic cation is selected from those for which the said base has a basic character sufficient to make it capable of reforming the chloride of an organic cation by reaction with hydrogen chloride. In this embodiment of the invention, chlorides of nitrogenous organic cations are advantageously chosen, and more particularly those derived from water-insoluble nitrogenous organic bases such as, for example, water-insoluble amines and their derivatives, water-insoluble quarternary ammonium salts and water-insoluble amines and amine derivatives.Chlorides of organic cations which are particularly suitable within the scope of the invention are hydrochlorides of primary, secondary and tertiary amines; hydrochlorides of primary amines are particularly advantageous, especially hydrochlorides of primary alkylamines.

The chloride of an organic cation is employed in sufficient quantity to decompose at least a substantial part of sodium bicarbonate in the sodium bicarbonate solution. Preferably, a quantity of chloride of an organic cation is employed which suffices at least to decompose all of the sodium bicarbonate in the said solution.

In the process according to the invention, the chloride of an organic cation is usually employed in the form of an organic solution, so that, at the end of the reaction between sodium bicarbonate and the chloride of an organic cation, a dilute aqueous solution of sodium chloride and an organic solution containing the above-mentioned organic base, resulting from the said reaction, are collected separately. The choice of the nature and the quantity of organic solvent to be employed is consequently determined by the requirement that it should form, with the said base, an organic solution which is insoluble in aqueous solutions of sodium chloride. The separation of the aqueous solution of sodium chloride from the organic solution of the base can then be carried out by any suitable means, usually by gravity decanting or centrifuging.

The treatment of the aqueous solution of sodium bicarbonate with the chloride of an organic cation can be carried out in any appropriate chemical reactor designed to provide a uniform removal of the carbon dioxide formed. The removal of carbon dioxide as it is being formed is regulated so as to maintain continually a sufficiently low partial pressure of carbon dioxide in the reactor to promote the reaction of decomposition of sodium bicarbonate by the chloride of an organic cation. The choice of the optimum partial pressure of carbon dioxide in the reactor depends on various factors, particularly the temperature inside the reactor, the concentration of the aqueous solution of sodium bicarbonate and the chloride of an organic cation employed. A temperature of at least 15"C and a carbon dioxide partial pressure below 3 bars are usually produced in the reactor.Temperatures of at least 400C and carbon dioxide partial pressures not exceeding 0.9 bar are preferred, the temperatures of between 30 and 90"C and carbon dioxide partial pressures of between 1 and 0.3 bar being especially advantageous.

Any suitable known means can be employed to remove carbon dioxide from the reactor and to produce the desired partial pressure of carbon dioxide in the latter. A suitable means consists in bubbling an inert gas, for example nitrogen or argon, in the reaction mixture. It is also possible, in an alternative form, to subject the reactor to a regulated reduced pressure.

The means employed for adding rock salt to the aqueous solution resulting from the treatment of the solution of sodium bicarbonate with the chloride of an organic cation is not critical. For example, it is possible to use for this purpose a continuous dissolver supplied in a controlled manner with solid rock salt and with the said aqueous solution. Examples of dissolvers which may be employed in carrying out the process according to the invention are described in the Patent FR-A-2,471,350 (SOLVAY & Company).

According to a particularly advantageous embodiment of the process according to the invention, in order to add the rock salt to the aqueous solution resulting from the treatment of the solution of sodium bicarbonate with the chloride of an organic cation, the said solution is caused to percolate through an underground deposit of rock salt; for this purpose, two pipelines are driven into the rock salt deposit, close to each other (usually a tubular pipeline and an annular pipeline which are co-axial), the aqueous solution is injected under pressure in one of the pipelines and an aqueous solution of sodium chloride is withdrawn through the other pipeline (Dale W. KAUFMANN "Sodium Chloride", American Chemical Society Monograph Series, 1960, Reinhold Publishing Corporation, Chapman & Hall, Ltd., London, pages 142 to 185).

The aqueous solutions of sodium chloride which are obtained by the process according to the invention can be employed in any industry which requires the use of concentrated, usually saturated, aqueous solutions of sodium chloride, and particularly in the process for the manufacture of ammonia soda, in the amines method specified above and in the electrolytic methods for the manufacture of chlorine and aqueous solutions of sodium hydroxide.

The process according to the invention finds an advantageous application in the amines method specified above, for regenerating simultaneously the aqueous solution of sodium chloride and the organic amine solution. Consequently, the invention also relates to a process for the manufacture of sodium bicarbonate, according to which an aqueous solution of sodium chloride is mixed with an organic solution of a water-insoluble amine, the resulting mixture is treated with a gas containing carbon dioxide to form an aqueous suspension of sodium bicarbonate and an organic solution of amine hydrochloride, the aqueous suspension of sodium bicarbonate is treated to separate the solid sodium bicarbonate which it contains from a mother liquor, the mother liquor is treated with at least a part of the organic solution of amine hydrochloride, the carbon dioxide formed is removed and a dilute aqueous solution of sodium chloride and an organic amine solution are collected separately, rock salt is added to the dilute solution of sodium chloride and the resulting concentrated aqueous solution of sodium chloride and the organic amine solution are recycled into the abovementioned mixture.

Particular features and details of the invention will emerge from the following description of the single Figure of the attached drawing which shows the general outline of a plant for the manufacture of sodium bicarbonate by the amines method described above, which incorporates a preferred embodiment of the process according to the invention.

The plant shown in the Figure includes a crystallisation chamber 1 which is supplied, continuously, with a substantially saturated aqueous solution of sodium chloride 2, an organic solution of a primary amine 3 which is insoluble in the aqueous solution of sodium chloride and a gas containing carbon dioxide 4. The organic solution of a primary amine can be, for example, a solution of 50% by weight of a primary alkylamine in xylene. The gas 4 is preferably an enriched gas, containing 60 to 80% by weight of carbon dioxide, produced, for example, by mixing appropriate proportions of a lime kiln gas and a residual gas from the calcination of sodium bicarbonate to sodium monocarbonate. The quantity of gas employed should be sufficient to convert a substantial part of the sodium chloride in the solution 2 to sodium bicarbonate.

Alternatively, the treatment in the crystallisation chamber 1 can be preceded by a partial carbonation of the mixture by means of a gas which is deleted in carbon dioxide, such as a lime kiln gas.

A liquid 5, consisting of a mixture of aqueous solution and organic solution, and containing sodium bicarbonate crystals in suspension, is drawn off from the crystallisation chamber 1. This liquid 5 is transferred to a decanting chamber 6 where an organic solution of amine hydrochloride 7 is separated by gravity from an aqueous slurry 8 of sodium bicarbonate crystals. The latter is conveyed to a liquid-separating or filtering device 9, where solid sodium bicarbonate 10 and a mother liquor 11 are collected separately.

According to the invention, the organic solution of primary amine hydrochloride 7 is divided into two fractions 12 and 13, a fraction 12 and the mother liquor 11 are introduced into a reaction chamber 14 and are subjected to vigorous stirring therein to cause a reaction between sodium bicarbonate and the amine hydrochloride, leading to the formation of sodium chloride which goes into aqueous solution, a primary amine which goes into organic solution and carbon dioxide. The size of the fraction 12 is sufficient to decompose all the sodium bicarbonate in the mother liquor. To promote the reaction between sodium bicarbonate and the amine hydrochloride the reaction chamber 14 is heated, for example to a temperature of the order of 60"C and the carbon dioxide formed therein is removed, by means ot a vacuum pump 15.The carbon dioxide 28 withdrawn from the reaction chamber 14 by the pump 15 is advantageously pumped into the crystallisation chamber 1.

A liquid 16 consisting of a mixture of a dilute aqueous solution of sodium chloride and an organic amine solution is drawn off from the reaction chamber 14; this liquid 16 is conveyed to a decanting chamber 17 from which the dilute aqueous solution of sodium chloride 18 and the organic amine solution 19 are collected separately.

The dilute aqueous solution of sodium chloride 18 is then introduced into a deposit of rock salt 20, from which an equivalent flow of a saturated aqueous solution of sodium chloride 21 is withdrawn and combined with the starting sodium chloride stream 2.

The organic amine solution 19 collected from the decanting chamber 17 can be wholly combined with the starting organic amine solution 3 and introduced with the latter into the crystallisation chamber 1.

In an alternative form, the organic amine solution 19 collected from the decanting chamber 17, or a part 22 of the latter, can be combined with the fraction 13 of the organic solution of amine hydrochloride.

The fraction 13 of the organic solution of amine hydrochloride (mixed, if appropriate, with the organic amine solution 22) is then treated in a manner known per se in the amines method, to regenerate the amine.

For this purpose, it is conveyed to a reaction chamber 23, where it is stirred with an aqueous suspension of calcium hydroxide 24, and the resulting reaction mixture 25 is conveyed to a decanting chamber 26 from which an aqueous solution of calcium chloride 27 and an organic amine solution 3, which is recycled to the crystalliser 1, are collected separately.

The advantage of the invention becomes apparent from the description of the examples which follow.

Example 1 Into a reaction chamber are introduced: 315 g of an aqueous solution of sodium bicarbonate, and 700 g of an organic solution of a hydrochloride of a primary alkylamine containing from 18 to 24 carbon atoms, in an aromatic organic solvent known under the trade name SHELLSOLAB (SHELL).

The aqueous solution of sodium bicarbonate had the following composition by weight: NaHCO3 : 20.0 g/kg NaCI 173.9 g/kg The organic solution contained, per kg, 1.115 mole of amine hydrochloride and 0.246 mole of the corresponding amine.

In the reaction chamber, the two solutions were stirred with a mechanical stirrer and an upward stream of nitrogen was passed through them at a rate of 8 I/h; the temperature in the reaction chamber was maintained continuously at 40"C.

After 90 minutes' reaction, 98.7% of the initial weight of sodium bicarbonate has been converted to NaCI.

At the end of 135 minutes' reaction, the reaction chamber was drained and the organic phase and the aqueous phase were collected separately. The aqueous phase collected was found to be an aqueous solution of sodium chloride free from sodium bicarbonate; the organic phase collected contained, per kg, 0.356 mole of amine and 1.014 mole of amine hydrochloride.

Rock salt was added to the aqueous solution of sodium chloride collected, until it was saturated with sodium chloride.

Example 2 The test of Example 1 was repeated with the same operating conditions being reproduced, with the exception of the temperature which was fixed at 60"C. 60 minutes' reaction was sufficient for all the sodium bicarbonate to be converted to sodium chloride.

Example 3 The aqueous and organic solutions employed in the test of Example 1 were mixed in a reaction chamber.

The reaction chamber was maintained continuously at the temperature of 50"C and a reduced pressure of 0.1 bar was produced therein by means of a vacum pump.

At the end of 60 minutes' reaction, the reaction chamber was drained and the organic phase and the aqueous phase were collected separately. The aqueous phase collected was found to be an aqueous solution of sodium chloride free from sodium bicarbonate; it was saturated with sodium chloride by adding rock salt.

The organic phase collected contained, per kg, o.358 mole of amine and 1.015 mole of amine hydrochloride.

Example 4 The reaction chamber of the test in Example 3 was taken again and in it were placed: 642 g of an aqueous solution of sodium bicarbonate, and 168 g of an organic solution of the hydrochloride of a primary alkylamine containing from 18 to 24 carbon atoms, in the abovementioned aromatic organic solvent SHELLSOL AB.

The aqueous solution of sodium bicarbonate had the following composition: NaHCO3 : 18.6 g/kg NaCI : 170 g/kg The organic solution contained, per kg, 1.090 mole of amine hydrochloride and 0.258 mole of the corresponding amine.

The temperature in the reaction chamber was fixed at 80 C and the reduced pressure at 0.4 bar.

At the end of 60 minutes' reaction, the reaction chamber was drained and the organic phase and the aqueous phase were collected separately. The aqueous phase collected was found to be an aqueous solution of sodium chloride free from sodium bicarbonate; it was subsequently saturated with sodium chloride by adding rock salt to it. The organic phase collected contained, per kg, 1.214 mole of amine and 0.211 mole of amine hydrochloride

Claims

1. Processforthe preparation of an aqueous solution of sodium chloride from an aqueous solution of sodium bicarbonate, characterised in that the aqueous solution of sodium bicarbonate is treated with chloride of an organic cation, to decompose sodium bicarbonate, the carbon dioxide which is formed is removed and rock salt is added to the resulting aqueous solution.

2. Process according to Claim 1, characterised in that a chloride of an organic cation is selected which is insoluble in aqueous solutions of sodium chloride.

3. Process according to Claim 1 or 2, characterised in that a chloride of an organic cation is selected which can be obtained by reaction of an organic base with hydrogen chloride.

4. Process according to Claim 3, characterised in that the chloride of an organic cation is selected from the chlorides of nitrogenous organic cations.

5. Process according to Claim 4, characterised in that the chloride of an organic cation is selected from the hydrochlorides of primary amines.

6. Process according to any one of Claims 1 to 5, characterised in that the chloride of an organic cation is employed in the form of a water-insoluble organic solution.

7. Process according to any one of Claims 1 to 6, characterised in that the aqueous solution of sodium bicarbonate and the chloride of an organic cation are treated at a temperature of at least 15"C under a partial pressure of carbon dioxide below 3 bars.

8. Process according to Claim 7, characterised in that the aqueous solution of sodium bicarbonate and the chloride of an organic cation are treated at a temperature of between 30 and 90"C and under a partial pressure of carbon dioxide of between 0.3 and 1 bar.

9. Process according to any one of Claims 1 to 8, characterised in that, to dissolve the rock salt in the aqueous solution resulting from the treatment of the sodium bicarbonate solution with the chloride of an organic cation, the said resulting solution is percolated in a deposit of rock salt.

10. Process for the manufacture of sodium bicarbonate according to which an aqueous solution of sodium chloride and an organic solution of a water-insoluble amine are mixed, the resulting mixture is treated with a gas containing carbon dioxide to form an aqueous suspension of sodium bicarbonate and an organic solution of amine hydrochloride, and the aqueous suspension of sodium bicarbonate is treated to separate the solid sodium bicarbonate which it contains and a mother liquor, characterised in that the mother liquor is treated with at least a part of the organic solution of amine hydrochloride, the carbon dioxide formed is removed and a dilute aqueous solution of sodium chloride and an organic amine solution are collected separately, rock salt is added to the dilute solution of sodium chloride and the resulting concentrated aqueous solution of sodium chloride and the organic amine solution are recycled to the abovementioned mixture.

11. Process according to claim 1 substantially as described with reference to the accompanying drawing.

12. Process according to claim 1 substantially as described in any of the foregoing Examples.

13. Process according to claim 10 substantially as described with reference to the accompanying drawing.

14. Sodium bicarbonate produced by a process according to claim 10 or 13.