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to this problem, large amounts of these salts even have to be disposed of with disposal costs. 

[0014] Another chemical process is the hydrolysis of cyanohydrin with inorganic bases, for example sodium hydroxide. It is likewise necessary here to release the alpha› hydroxycarboxylic acid by adding a mineral acid in stoichio› metric amounts. The hydrolysis of cyanohydrins likewise goes to the stage of the ammonium salt with titanium dioxide as the catalyst. The salt problem remains the same. 

[0015] Mono-, di- and tricarboxylic acids, and alpha- and beta-hydroxycarboxylic acids, can be prepared fermenta› tively with the aid of microorganisms or enzymatically. This affords the organic acids in the form of the ammonium salt. They are released by adding the stoichiometric amount of a mineral acid. In the case of di- or tricarboxylic acids, even twice or three times the stoichiometric amount of a mineral acid has to be added. This likewise gives rise to very large amounts of ammonium salts, which in tum have to be recycled in a complex manner or disposed of expensively. 

[0016] Processes in which no salt burden occurs are to date uneconomic on the industrial scale for reasons of cost. One example thereof is the esterification of an ammonium salt of an alpha-hydroxycarboxylic acid with an alcohol and subse› quent hydrolysis of the ester with an acid catalyst (JP7194387). 

[0017] In order to prepare free carboxylic acids from the ammonium salts, there are various processes based on the thermal decomposition of the ammonium carboxylates to release ammonia (Scheme 1 ): 

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[0018] According to GB967352, a small amount of water is added to an ammonium salt of an unsaturated fatty acid, and the mixture is heated at total reflux (80° C.) or higher in organic solvents in order to free or to remove ammonia to obtain the unsaturated fatty acid. 

[0019] According to JP54115317, an organic solvent which forms an azeotropic mixture with water is added to a 10-50% aqueous solution of ammonium methacrylate, and the solu› tion which arises is heated to 60-100° C. As a result, water is distilled off as an azeotropic mixture and ammonia is removed simultaneously in order to obtain free methacrylic acid. 

[0020] According to JP7330696, a 10-80% aqueous solu› tion of an ammonium salt of an acidic amino acid is heated with addition of water. Ammonia and water are distilled off and the amino acid is released. 

[0021] In these processes, ammonia is in principle removed easily, since the carboxylic acid has a high dissociation con› stant. In contrast, the degree of dissociation of ammonium ions from ammonium salts of carboxylic acids with pKa values less than 4, for example sulphonic acids and alpha› hydroxycarboxylic acids, is low. It is therefore very difficult to remove ammonia from the salts of strong acids. In order to remove the majority of ammonia, a long period is required, or it is necessary to add a large amount of water or of organic solvents. In the abovementioned processes, 50% or more of the corresponding carboxylic acid remains as the ammonium salt. 

[0022] U.S. Pat. No. 6,066,763 describes a process for pre› paring alpha-hydroxycarboxylic acids, which proceeds with› out the inevitable occurrence of large amounts of salts which can be disposed ofonly with difficulty, ifat all. In this process, the starting materials used are the ammonium salts of the corresponding alpha-hydroxycarboxylic acids, said salts being obtainable from the corresponding cyanohydrins with the aid of enzymes (nitrilases). The salt is heated in the pres› ence of water and a solvent. Preferred solvents have a boiling point of >40° C. and form an azeotrope with water. The distillative removal of the azeotropic mixture releases ammo› nia, which escapes in gaseous form via the condenser. The corresponding alpha-hydroxycarboxylic acid accumulates in the bottom of the distillation apparatus. However, the removal of the water at elevated temperature results in large amounts of the initially released alpha-hydroxycarboxylic acid being converted by intra- or else intermolecular esterification to dimers and polymers of the alpha-hydroxycarboxylic acid in question. These subsequently have to be converted back to the monomeric alpha-hydroxycarboxylic acid in question by heating with water under elevated pressure. Another disad› vantage is the long residence times in both process stages. They are 4 hours in the examples mentioned. Since the sol› vent is kept at boiling for the whole time in stage 1, the steam consumption is uneconomically high. The cause of this is the release of the alpha-hydroxycarboxylic acid, which becomes more difficult with increasing depletion of ammonia. It does not succeed 100%. After the end of the reaction, 3-4% bound ammonia still remains in the bottoms. Under the reaction conditions, another by-product which occurs is the corre› sponding amide of the alpha-hydroxycarboxylic acid, which is only partly converted to the corresponding ammonium salt by hydrolysis in stage 2 of the process (Scheme 2). 

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[0023] The alpha-hydroxycarboxylic acids obtained pos› sess only a purity of approx. 80%, and so a further purification by means ofliquid-liquid extraction or crystallization is usu› ally necessary. 

[0024] In patent publication WO 00/59847, the ammonium salt solutions of the alpha-hydroxycarboxylic acids are brought to a concentration of >60% under reduced pressure. The conversion to dimeric or polymeric esters of the corre› sponding alpha-hydroxycarboxylic acids is intended to be less than 20%. Passing a gas through, preferably steam, releases and drives out ammonia. Using the example of2-hy› droxy-4-methylthiobutyric acid, 70% free acid is achieved; the remainder consists of the unconverted ammonium salt of 2-hydroxy-4-methylthiobutyric acid and the corresponding dimeric ester. 

[0025] US 2003/0029711 Al describes a process for obtaining organic acids, including from aqueous solutions of the ammonium salts, with addition of a hydrocarbon as an entraining agent. Heating the mixture affords a gaseous prod› uct stream which comprises an azeotrope consisting of the organic acid and the entraining agent. In order to isolate the acid from this product stream, further steps such as conden› sation and additional distillations must be carried out. Fur› thermore, this process also requires the addition of additional 

