chemicals (entraining agents), which makes the process sig› nificantly more costly, specifically for use on the industrial scale. 

[0026] U.S. Pat. No. 6,291,708 Bl describes a process in which an aqueous solution of an ammonium salt is mixed with a suitable alcohol, and this alcohol-water mixture is then heated under elevated pressure in order to decompose the ammonium salt thermally to the free acid and ammonia. At the same time, a suitable gas as an entraining agent is con› tacted with the alcohol-water mixture, so as to drive out a gaseous product stream comprising ammonia, water and a portion of the alcohol, while at least 10% of the alcohol remains in the liquid phase and reacts with the free acid to give the corresponding ester. The disadvantages of this pro› cess include the necessity of additional chemicals (alcohol and a gas as an entraining agent) and the partial conversion of the free carboxylic acid formed to the ester, which in tum has to be hydrolysed in order to obtain the free carboxylic acid. 

[0027] In DE 10 2006 052 311 Al (published specifica› tion), the ammonium salt of an alpha-hydroxycarboxylic acid is heated in the presence ofa tertiary amine with release of the ammonia and formation of the salt of tertiary amine and alpha-hydroxycarboxylic acid in question. Subsequently, the salt is dissociated thermally and the tertiary amine formed is recovered by distillation. The free alpha-hydroxycarboxylic acid remains in the distillation bottoms. The purity of the alpha-hydroxycarboxylic acids obtained is 95%. 

[0028] In DE 10 2006 049 767 Al (published specifica› tion), this process is applied to the preparation of2-hydroxy- 4-methylthiobutyric acid from the corresponding 2-hydroxy- 4-methylthiobutyramide. With N-methylmorpholine, 2-hydroxy-4-methylthiobutyric acid is formed at 180° C. and 6 bar in a purity of 95% with 96% yield. The use of other tertiary amines gives similar results. 

[0029] In DE 10 2006 049 768 Al (published specifica› tion), the 2-hydroxy-4-methylthiobutyramide formed by mineral acid hydrolysis of 2-hydroxy-4-methylthiobutyroni› trile is extracted with a polar water-immiscible solvent. Pre› ferred solvents are ethers, ketones and trialkylphosphine oxides, also in mixtures with various hydrocarbons. The sol› vent is removed by distillation and the resulting 2-hydroxy- 4-methylthiobutyramide is base-hydrolysed. The bases used are tertiary amines, which can be removed again by distilla› tion from the salts formed with release of the 2-hydroxy-4- methylthiobutyric acid. The temperatures in this process are between 130 and 180° C. at 6 bar. 

[0030] The disadvantages of the latter processes are the use of tertiary amines as additives. These cannot be removed completely by distillation and thus remain in a small amount in the end product. The high temperatures of 130 to 180° C. employed are not very economic and the pressure range of 6 bar requires increased capital costs in the industrial imple› mentation. 

[0031] In U.S. Pat. No. 6,815,560 and the patent publica› tions cited there, the free 2-hydroxy-4-methylthiobutyric acid prepared by sulphuric acid hydrolysis is extracted from the hydrolysis solution with a water-immiscible solvent, prefer› ably isobutyl methyl ketone. The extractant is recovered by distillation; the 2-hydroxy-4-methylthiobutyric acid remains in the distillation bottoms in its monomeric and dimeric forms. The addition of water adjusts the thermodynamic equi› librium between the two forms. 

[0032] Against the background of the disadvantages of the prior art, it was an object of the present invention to find an 

## SUMMARY OF THE INVENTION 

inexpensive and environmentally compatible process for iso› lating free organic acids such as carboxylic acids, sulphonic acids, phosphonic acids and especially alpha- and beta-hy› droxycarboxylic acids from the ammonium salts thereof, which does not form a salt burden as a coproduct and is completely back-integrated by means of closed circuits. 

[0033] The technical object is achieved by a process for converting ammonium salts of organic acids to the particular free organic acid, wherein an aqueous solution of the ammo› nium salt is contacted with an organic extractant and the salt is dissociated at temperatures and pressures at which the aqueous solution and the extractant are in the liquid state, and a stripping medium or entraining gas is introduced in order to remove NH$_{3 }$ from the aqueous solution and transfer at least a portion of the free organic acid formed to the organic extrac› tant. 

[0034] The invention thus provides a process wherein the ammonium salt of organic acids is converted by means of reactive extraction using a stripping medium or entraining gas, for example by driving out ( stripping) the ammonia with steam or nitrogen, to the free organic acid which is then transferred into the organic extractant. It is preferred that at least 50%, preferably at least 80%, more preferably at least 90% and most preferably at least 95% of the free organic acid formed is transferred into the organic extractant. 

[0035] In a preferred process, the conversion is effected at pressures of0.01 bar to 200 bar, particularly of0.01 bar to 20 bar, very preferably of 0.1 bar to 5 bar. Moreover, it is pre› ferred that the salt dissociation is performed at temperatures of5° C. to 300° C., more preferably of20° C. to 300° C., even more preferably of 40° C. to 200° C., especially preferably of 50° C. to 130° C. 

[0036] The temperature has a high influence on the rate of formation of the free acid and the end yield thereof. The temperature is guided by the extractant used and is, according to the invention, below the boiling point of the aqueous solu› tion or of a possible azeotrope, the boiling point of the aque› ous solution or of any azeotrope which forms depending of course on the particular pressure applied. 

[0037] As already described above, the salt dissociation in the process according to the invention is performed at tem› peratures and pressures at which the aqueous solution and the extractant are in liquid form, not in solid form and not in gaseous form, i.e. below the boiling temperature, which depends on the particular pressure applied, of the aqueous solution or of any azeotropic mixture which forms. 

[0038] According to the invention, the starting concentra› tion of the ammonium salt of the organic acid in the aqueous solution used is preferably in the range from 90% by weight to 1 % by weight, more preferably from 7 5% by weight to 5% by weight and most preferably from 60% by weight to 10% by weight. In the course of the reaction of the salt dissociation, the corresponding concentration of the salt decreases. 

[0039] Moreover, it is preferred that the extractant used is a sparingly water-miscible or entirely water-immiscible sol› vent. The weight ratio of aqueous solution and organic extrac› tant is from 1: 100 to 100: 1, more preferably from 1: 10 to 10:1, most preferably from 1:5 to 5:1. According to the present invention, the organic acid may be selected from the group of monocarboxylic acid, dicarboxylic acid, tricarboxy› lic acid, ascorbic acid, sulphonic acid, phosphonic acid, hydroxycarboxylic acid, especially alpha-hydroxycarboxlic acid or beta-hydroxycarboxylic acid. 

