[0108] The advantage of these two apparatuses (FIGS. 2 and 3) over the perforator is that it is firstly possible here to use significantly higher entraining gas flows and there is secondly continuous removal of the organic acid released. In this way, the extractant can always be reused unburdened and thus dissolve more organic acid released. Redissolutionofthe organic acid by the water in the ammonium salt solution is thus prevented. The extraction can thus be conducted with high extraction rates. 

[0109] One example cited here is a reactive extraction with 2-hydroxy-4-methylthiobutyric acid and isobutyl methyl ketone. 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 30 1 of nitrogen per hour as the entraining gas, after 90 hours of extraction time, 73% of the 2-hydroxy-4-methylthiobutyric acid used was found (Example 6, FIG. 13). 

[0110] High influence is possessed here not only by the temperature in the extraction colunm and the amount of entraining gas introduced but also by the flow rates of the extractant and of the aqueous ammonium salt solution. The temperature is guided by the extractant used and should be below the boiling point of a possible azeotrope. 

[0111] 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 60 1 of nitrogen per hour as the stripping medium, after 90 hours of extraction time, 95% of the 2-hydroxy-4-methylthiobutyric acid used was found (Example 10, FIG. 14). 

[0112] Means oflndustrial Implementation 

[0113] An apparatus used on the industrial scale in a liquid› liquid extraction by the countercurrent principle is a mixer› settler apparatus. In countercurrent extraction, in a mixer› settler, the carrier and the extractant are conducted through the mixing battery in opposite directions. Thus, in the first stage, the highly laden carrier stream is contacted with already enriched extractant, which results in a first refining step. With each stage, the loading of the carrier stream decreases. The loading of the extractant stream contacted therewith decreases in the same direction, such that, ulti› mately, the already highly depleted raffinate is dispersed with fresh unladen extractant in the last stage. In the countercur› rent process, a high depletion of the raffinate is achieved with small amounts of extractant, which makes this variant very economically viable. 

[0114] The apparatuses shown (FIG. 4 for high boilers as the extractant and FIG. 5 for low boilers) serve to dissociate ammonium salts of organic acids into ammonia and the cor› responding organic acids, it being possible for this thermal dissociation to take place under mild conditions, such that there is no decomposition of the organic acids. 

[0115] The apparatus consists of a column with n trays, the trays of which are preferably configured as bubble-cap or valve trays, such that there is only a very minor degree of, if any, direct trickle of liquid phases from the upper trays through to the trays below. A stripping medium flows through the colunm from the bottom upwards, which is preferably introduced into the bottom of the colunm or below the low› ermost tray. The stripping medium may preferably be steam, which is obtained by the heating of the aqueous phase passed downwards, or the stripping medium or entraining gas may also consist of an inert gas, for example nitrogen or another gas which forms a mixture with ammonia by interactions, which is readily convertible to the gas phase. 

[0116] The configuration of the trays is preferably such that the aqueous and organic phases are conducted together from the entry to the tray to the exit by suitable baffles, in order to prevent back-mixing or short-circuit flows. On the trays, by virtue of the gas entering at the bottom, good mixing of all three phases takes place, such that the ammonia released by thermal decomposition, owing to the large phase interface, is converted easily to the gas phase and the organic acid which forms can be extracted rapidly out of the aqueous phase into the organic phase. 

[0117] Thus, the aqueous phase comprising the ammonium salt and the organic phase which absorbs the organic acid are firstly introduced together to the uppermost tray (No. 1) of the column and mixed. In the case of an extraction in countercur› rent, the aqueous phase with the greatest ammonium salt loading is combined on the uppermost tray with the organic phase which absorbs the organic acid downstream of the separation process belonging to the tray below (No. 2). As a result of the thermal elimination of ammonia, by virtue of the contact with the gas phase, it passes into the gas phase at each tray, and the organic acid passes from the aqueous phase into the organic phase. If ammonium salt is present in the aqueous phase, no thermal equilibrium will exist between the concen› tration of the organic acid in the aqueous phase and in the organic phase, and as a result there is always transfer of the organic acid which forms after the elimination of ammonia into the organic phase. After the aqueous and organic phases have left the uppermost tray, the two phases are separated from one another in a suitable separation process. In the case oflow mutual solubility of organic and aqueous phases, this separation process may be a phase separator. In order to promote the phase separation, a temperature change can be effected before the separation process. Further separation processes, such as a distillation, rectification, membrane pro› cess, crystallization, adsorption, chromatography, etc., are likewise possible. Thus, at the uppermost tray, the organic phase with the highest loading of the organic acid is obtained. The organic acid can be separated from the solvent by one or more further separation processes such as a distillation, rec› tification, membrane process, crystallization, adsorption, chromatography, etc. The solvent released can then be fed back into the colunm for extraction. 

[0118] After the separation process on the uppermost tray (No. 1), the aqueous phase passes to the tray below (No. 2) and is in turn combined with the organic phase which absorbs the organic acid downstream of the separation process belonging to the tray below (No. 3). The fresh organic solvent, or that recycled from preceding separation processes, is com› bined on the lowermost tray (No. N) together with the aque› ous phase from the tray above. 

[0119] To operate such a reactive extraction, it is necessary to decide whether water or the organic solvent possesses a higher boiling temperature. In the case of water as the lower› boiling component at the operating pressure of the colunm, and of utilization of steam as the entraining medium, the water is collected in the lower part of the colunm and evapo› rated by means of a heat exchanger operated in circulation. This evaporator may also be configured as a natural circula› tion evaporator. The excess water is conducted out of the column under fill level control. The steam is introduced again below the lowermost tray (No. N). Steam, ammonia and optionally a further gas and small amounts of solvent vapours are passed out at the top of the column and can optionally be separated in a downstream separation process. 

