[0092] It has been found that the temperature has a great influence on the extraction rate. The higher the temperature of the aqueous ammonium salt solution, the more rapidly the reactive extraction proceeds. 

[0093] In the case of use of a 10% aqueous 2-hydroxy-4- methylthiobutyric acid ammonium salt solution at 50° C. with isobutyl methyl ketone as the extractant and with 6 1 of nitrogen per hour as the entraining gas, after 90 hours of extraction time, 39% of the 2-hydroxy-4-methylthiobutyric acid used was found (Example 2). A temperature increase by 30° C. to 80° C. under otherwise identical conditions also increases the amount of 2-hydroxy-4-methylthiobutyric acid extracted within the same period to 93% (Example 1, FIG. 7). 

[0094] It has also been found that the concentration of the ammonium salt used has an influence on the extraction rate. The higher the concentration of the ammonium salt in the aqueous solution, the more slowly the reactive extraction proceeds. 

[0095] In the case of use of a 10% aqueous 2-hydroxy-4- methylthiobutyric acid ammonium salt solution at 80° C. with isobutyl methyl ketone as the extractant and with 6 1 of nitrogen per hour as the entraining gas, after 90 hours of extraction time, 93% of the 2-hydroxy-4-methylthiobutyric acid used was found (Example 1 ). When the concentration of the ammonium salt is increased to 20%, under otherwise identical conditions, 71 % of extracted 2-hydroxy-4-meth› ylthiobutyric acid is obtained within the same period (Ex› ample 3, FIG. 8). 

[0096] The reactive extraction is not restricted to the use of isobutyl methyl ketone as the extractant. It is possible to use all water-immiscible or only sparingly water-miscible organic solvents, such as alcohols, ethers, ketones or hydro› carbons, or mixtures thereof. 

[0097] In the case of use of a 10% aqueous 2-hydroxy-4- methylthiobutyric acid ammonium salt solution at 50° C. with isobutyl methyl ketone as the extractant and with 6 1 of nitrogen per hour as the entraining gas, after 90 hours of extraction time, 39% of the 2-hydroxy-4-methylthiobutyric acid used was found (Example 2). When, under identical conditions, methyl tent-butyl ether is used as the extractant, after 90 hours of extraction time, 38% of the 2-hydroxy-4- methylthiobutyric acid used is found in the solvent (Example 4, FIG. 9). 

[0098] In addition to 2-hydroxy-4-methylthiobutyric acid, reactive extraction using a stripping medium or entraining gas is also employable for other hydroxycarboxylic acids. 

[0099] Examples cited here include the commercially sig› nificant lactic acid, and 2-hydroxyisobutyric acid, which is used in plastics production as a precursor for MMA. 

[0100] In the case of use of a 10% aqueous lactic acid ammonium salt solution at 80° C. with 1-butanol as the extractant and with 61 of nitrogen per hour as the entraining gas, after 21 hours of extraction time, 88% of the lactic acid used was found (Example 8, FIG. 10). 

[0101] In the case ofuse ofa 10% aqueous 2-hydroxyisobu› tyric acid ammonium salt solution at 80° C. with isobutyl methyl ketone as the extractant and with 6 1 of nitrogen per hour as the entraining gas, after 21 hours of extraction time, 49% of the 2-hydroxyisobutyric acid used was found (Ex› ample 7, FIG. 11). 

[0102] The invention cited is not restricted only to the release ofhydroxycarboxylic acids from the ammonium salts thereof, but also includes other substituted or unsubsubsti› tuted carboxylic acids, e.g. valeric acid, and sulphonic acids, 

e.g. (+)-camphor-I 0-sulphonic acid, and phosphonic acids, for example toluenephosphonic acid. 

[0103] In the case of use of a 10% aqueous valeric acid ammonium salt solution at 80° C. with isobutyl methyl ketone as the extractant and with 6 1 of nitrogen per hour as the entraining gas, after 21 hours of extraction time, 90% of the valeric acid used was found (Example 9, FIG. 12). 

[0104] In the case of use of a 10% aqueous (+)-camphor› ! 0-sulphonic acid ammonium salt solution at 80° C. with isobutyl methyl ketone as the extractant and with 6 1 of nitrogen per hour as the entraining gas, after 66 hours of extraction time, 25% of the (+)-camphor- I 0-sulphonic acid used was found (Example 12). 

[0105] In the case of use of a 10% aqueous toluenephos› phonic acid ammonium salt solution, it was possible at 80° C. with isobutyl methyl ketone as the extractant and with 61 of nitrogen per hour as the entraining gas, after 46 hours of extraction time, to find 43% of the toluenephosphonic acid used (Example 13). 

[0106] The examples cited were performed in a specially developed perforator (FIG. 1). The extraction vessel of the perforator was half-filled with an aqueous ammonium salt solution of an organic acid and filled with an extractant until there is overflow to the reservoir. The reservoir itself is like› wise half-filled with extractant. The extraction vessel is equipped with an inserted distributor and a gas inlet tube with frit. The distributor is rotated by means of magnetic coupling. A stripping gas, for example nitrogen, is simultaneously introduced via the gas inlet tube. The extractant supplied to the distributor from the condenser above through a tube by distillation out of the reservoir is accelerated into the aqueous ammonium salt solution to be extracted as fine droplets out of small holes of a distributor ring by centrifugal force. This achieves fine distribution and intimate mixing of the extrac› tant with the material for extraction. At the same time, the gas stream drives the ammonia out of the aqueous phase. Caused by the co-rotation of the aqueous ammonium salt solution to be extracted, the finely distributed extractant laden with the free organic acid extracted reaches the deposition zone only after prolonged residence time in the aqueous phase and runs back into the reservoir (distillation flask), from which the solvent is recycled into the extraction circuit by re-evapora› tion. The free organic acid collects in the reservoir. The ammonia stream released is removed with the stripping gas via the attached jacketed coil condenser and collected in an aqueous sulphuric acid trap. 

[0107] An apparatus improvement is the countercurrent extractor (FIG. 2). In a reaction tube equipped with random packings, the aqueous ammonium salt solution of an organic acid at a controlled temperature is introduced from the top and pumped in circulation. The extractant is pumped into the reaction tube in countercurrent through a frit and the entrain› ing gas is introduced into the system. The finely distributed droplets of the extractant absorb the organic acid released. The lighter organic phase is removed via an outlet at the upper end of the reaction tube. After the separation of extractant and organic acid (for example crystallization, distillation, separa› tion by cooling, separation by back-washing with water), the extractant is introduced into the circuit again. The entraining gas and the released ammonia which has been driven out are removed via the top. Series connection of a plurality of coun› tercurrent extractors makes the process even more efficient and industrially employable. A cascade reactive extraction is obtained (FIG. 3). 

