Aug. 19, 2010  3  US 2010/0210871 Al  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  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.  SUMMARY OF THE INVENTION 