The resulting ammonium salt may then be treated with a mineral acid to release the ammonia. In this case, a large quantity of mineral acid is usually present in the product mixture which must be isolated and separated. This, of course, requires an additional step in the synthesis process.

Among the other processes which make it possible to obtain these a- hydroxy acids, there are many chemical processes which involve the hydrolysis of the nitrile, with strong inorganic acids such as sulphuric acid or hydrochloric acid.

In addition to the a-hydroxy acid, an ammonium salt of the acid used for the hydrolysis is obtained at the end of the reaction which must generally be treated or removed.

Among the processes which make it possible to release the a-hydroxy carboxylic acid from its ammonium salt, there may be mentioned the process described at the end of the enzymatic hydrolysis process of European Patent Application 0970913. This process carries out the release by an electrodialysis technique. This technique is perfectly feasible from a technical point of view but it is costly and substantially increases the price of obtaining the acid.

There is also described in European Patent Application 0884300, a process for the preparation of a-hydroxy acids from the ammonium salt without production of saline waste which consists, in a first step, of heating the aqueous mixture containing the ammonium salt in the presence or absence of organic solvents in order to remove the water and the ammonia and a second step, of adding water to the residue and heating the water. In the first step, the water is removed and the ammonia converts the intermediate a-hydroxy acid produced to low-molecular- weight poly-a-hydroxy acids. The addition of water in the second step is intended for the hydrolysis of the low-molecular-weight poly-a-hydroxy acids to give the free a-hydroxy acid. An additional step is further necessary which involves the evaporation of the water for hydrolysis in order to arrive at the desired concentration of methylthio-a-hydroxybutyric acid.

The present invention provides as a process which is different from the above mentioned processes because it avoids the intermediate formation of low- molecular-weight poly-a-hydroxy acids. We have found that ammonia can be easily removed from an aqueous solution of an ammonium salt of an a-hydroxy acid on the condition that a sufficient content of ammonium salt in the aqueous solution is reached.

Accordingly, the present invention provides a process for the preparation of a-hydroxy acids from an aqueous solution of an ammonium salt of an a-hydroxy acid which process comprises (a) a first step of concentrating the aqueous solution such that the concentration of the ammonium salt of the a-hydroxy acid is greater than 60% by weight, without converting more than 20% of the ammonium salt of the a-hydroxy acid, and (b) a second step of heating the solution obtained in step (a) in the presence of an entraining medium.

The present process substantially avoids the conversion of a-hydroxy acid to low-molecular-weight poly-a-hydroxy acid. Indeed, during the first step, the rate of formation of low-molecular-weight poly-a-hydroxy acid is preferably limited to 2% by weight and still more preferably limited to less than l %.

The present process is applicable to the conversion of ammonium salts of a-hydroxy acids to a-hydroxy acids of the following general formula: in which R represents a linear or branched alkyl group comprising 1 to 4 carbon atoms, a linear or branched alkoxy group comprising 1 to 4 carbon atoms, a linear or branched alkylthio group comprising 1 to 4 carbon atoms, an aryl radical containing 6 to 12 carbon atoms, an aryloxy radical containing 6 to 12 carbon atoms, an arylthio radical containing 6 to 12 carbon atoms, an aralkyl radical containing 7 to 16 carbon atoms, an arylalkoxy radical containing 7 to 16 carbon atoms, an arylalkylthio radical containing 7 to 16 carbon atoms, a heteroaryl radical containing 1 or 2 heteroatoms chosen from nitrogen, oxygen and sulfur, each of these radicals being optionally substituted with one or more halogen atoms, one or <BR> <BR> <BR> more C-C4 alkyl groups, one or more C-C4 alkoxy groups or one or more Cl-C4 alkylthio groups.

Among these derivatives, the use of the derivatives for which R represents an alkyl group containing 1 to 4 carbon atoms, an alkylthio group containing 1 to 4 carbon atoms or a phenyl group, is preferred, each group being optionally substituted with a methoxy or methylthio group. The use of the derivative for which R represents a C, alkyl group substituted with a methylthio group, that is to say ammonium 4-methylthio-2-hydroxybutyrate, is most particularly preferred.

The first step of the process consists of concentrating the aqueous solution containing the ammonium salt of a-hydroxy acid. This may be carried out by evaporation of the aqueous solution without degrading the ammonium salt. This step is carried out to a concentration by weight of ammonium salt of the a-hydroxy acid greater than 60% by weight, preferably this concentration is achieved so as to have a concentration by weight of ammonium salt of the a-hydroxy acid greater than 75% by weight. The formation of poly-a-hydroxy acids is suitably limited to 2% by weight, preferably less than 1% by weight.

During this first step, the conversion of the ammonium salt of the a- hydroxy acid to a-hydroxy acid is limited to 20%, that is to say that the quantity of ammonium salt of the a-hydroxy acid obtained after the concentration step is preferably at least equal to 80% by weight of the quantity of ammonium salt of the a-hydroxy acid present before the concentration step. The level of conversion of the ammonium salt of the a-hydroxy acid is still more preferably, according to a better means of carrying out the invention, limited to 10% by weight.

The concentration step is carried out in any type of apparatus which makes it possible to remove the water and not to convert the ammonium salt of the a- hydroxy acid. This type of apparatus may be chosen from rotary, plate, falling film and scraped falling film evaporators; it can also be carried out in a simple distiller.

The operation is carried out at atmospheric pressure or under reduced pressure. The temperature during this evaporation is preferably between 80 and 180°C.

The second step of the process according to the invention comprises heating the solution previously obtained in the presence of an entraining medium. <BR> <BR> <BR> <P> This medium is suitably a gas which is inert toward the ammonium salt of the a- hydroxy acid and toward the a-hydroxy acid and may be chosen from nitrogen, air and methane. The medium may also comprise steam. In a variant of the process, the entraining medium, namely steam, may be obtained by simply heating the solution obtained in the preceding step, when this solution still contains water.

This second entraining step may be carried out in a scraped film evaporator of the Luwa type, a falling film evaporator, a stirred reactor or a stripping column.

The use of a scraped film evaporator of the Luwa type or a stripping column, is preferred.

The temperature used during the carrying out of this step may be adjusted according to the apparatus used and the pressure used. When the pressure used is atmospheric pressure, the reaction temperature is preferably greater than 100°C.

When the reaction is carried out under reduced pressure, for example at 50 mbar, the temperature used will be preferably greater than 70°C and in particular between 110 and 200°C.

The flow rate of the entraining medium may be adjusted according to the desired reaction rate.

At the end of the second step of the process, most of the a-hydroxy acid is obtained as the free acid. Part of the acid may react with ammonia to provide the ammonium salt and remain as the ammonium salt of the acid. The product stream may therefore comprise the acid and the ammonium salt of the acid.

The process of the present invention is particularly preferred for the preparation of 4-methylthio-2-hydroxybutyric acid from the corresponding ammonium salt.

Where the process of the present invention produces 4-methylthio-2- hydroxybutyric acid, the product resulting from the second step comprising a mixture of 4-methylthio-2-hydroxybutyric acid and ammonium salt of this acid, exhibits completely unexpected properties. When the mixture obtained contains an acid to (acid+ammonium salt) ratio of 0.6 and about 90% of organic in water, it has an advantageous property; its liquid-solid transition point is less than 0°C and is in particular for this given composition-11°C. It has appeared, surprisingly, that when this ratio was 0.56, the solid-liquid transition temperature then increased to 24°C.

Thus, according to another aspect of the present invention there is provided a composition comprising 4-methylthio-2-hydroxybutyric acid and the corresponding ammonium salt of said acid obtainable by a process as herein before described in which the acid to (acid+ammonium salt) ratio is at least 0.6.

The compositions consisting of a mixture of 4-methylthio-2- hydroxybutyric acid and ammonium salt of said acid with a minimum ratio of 0.6 are very advantageous for their use in animal nutrition because they do not solidify

during storage under fairly severe atmospheric conditions such as those in countries where this type of composition is used. According to another aspect of the present invention there is provided a composition as herein before defined for use as an animal feed supplement.

The present invention will be described more fully with the aid of the following examples.

Example 1 Preparation of HMTBA from HMTBS The following definitions are used in the example: HMTBA: 4-methylthio-2-hydroxybutyric acid HMTBS: ammonium salt of 4-methylthio-2-hydroxybutyric acid 1.1. Concentration of the solution (1 st step): An aqueous solution of ammonium salt of 2-hydroxy-4-methylthiobutyric acid at 25% [w/w] was loaded into an unstirred 20 litre volume vessel. The vessel was placed under vacuum (50 mbar); a vacuum trap was fitted for ammonia (acetone + dry ice =-80°C) downstream of the condenser. The thermostat baths were switched to heat: The following reaction conditions were used: Tfeed (1) = 26°C TJ = 115°C (B) (jacket of the evaporator) TValve = 102°C (C) (drawing off heavy fraction) Theavy = 45°C (heavy fraction receiver) The coil of the condenser was fed with industrial water (at 15°C); the solution to be evaporated was fed into the body of the evaporator at a constant flow rate (2.17 kg/h). The stirring was started (400 rpm) and the distillate was drawn off at a constant flow rate (1.45 kg/h) while the heavy fraction was accumulated in the receiver (flow rate of the order of 0.71 kg/h).

The reaction was terminated after 5 hours 20minutes. The following weights were obtained: Heavy fraction: 3957 g; Distillate: 7657 g; Feed residue: 13 g

Vacuum trap: 50 g at-80°C, then 37 g at 20°C (= 13 g of NH3).

Analysis of the product showed 6.7 % conversion to conversion HMTBA [mol/mol]; 72.2% [w/w] of organic material [HMTBS+HMTBA] with sodium hydroxide; 72.4% [w/w] T. O. S. (Br03/Br analyses) : No dimers were detected by HPLC.

1.2. Reactive evaporation (2nd step): The same apparatus was used as above. A pre-concentrated solution (heavy fraction from the first step) was loaded into the unstirred 20 litre vessel. The vessel was placed under vacuum (50 mbar); a vacuum trap was fitted for ammonia (acetone + dry ice =-80°C), downstream of the condenser; the thermostat baths were switched to heat. The following reaction conditions were used: Tweed (1) = 37°C TJ = 180°C (jacket of the evaporator) TValve = 102°C (C) (drawing off heavy fraction) TheaVy = 80°C (3) (heavy fraction receiver) The coil of the condenser was fed with industrial water (at 15°C); the pre- concentrated solution was fed into the body of the evaporator at a constant flow rate (0.98 kg/h); the stirring was started (400 rpm); The distillate was drawn off at a constant flow rate of 0.33 kg/h while the heavy fraction was accumulated in the receiver (flow rate of the order of 0.65 kg/h). The reaction was terminated after 15 minutes. The following weights were obtained: Heavy fraction: 154 g; Distillate: 81 g; Analysis of the product showed 70 % conversion to conversion HMTBA [mol/mol]; 90% [w/w] of organic material [HMTBS+HMTBA] with sodium hydroxide and 15% dimers were detected by HPLC.

Characterization by ACD, of the solid/liquid transitions of the HMTBS/HMTBA system in water.

The characterisation test was carried out using a Perkin Elmer DSC 7 cell

Over the study range of-170 to 70°C using a programming speed of 5°/min. The sample port was a seamed aluminium capsule. The sample used was 10 mg and the content of organic material in the samples was about 90%.

The solid/liquid equilibria were studied during the rise in temperature.

The glass transition indicating the presence of the non-crystallized phase was determined as well as an exothermic phenomenon of re-crystallization determines the most probable temperature range for obtaining the crystallization Also obtained was an endothermic phenomenon relating to the dissolution; the temperature corresponding to the end of this phenomenon is that above which the system is liquid. HMTBA/(HMTBS + HMTBA) solid/liquid transition temperature 0.4+25°C 0.56+24°C 0.6-11°C 0.8-14°C