PROCESS FOR RECOVERING LACTIC ACID FROM SOLUTIONS WHICH CONTAIN IT

Abstract: Process for recovering lactic acid from aqueous media which contain it, characterised in that said aqueous medium is treated at a pH value in the range from 1 to 6 with a system comprising a complexing agent and an organic extractant selective for lactic acid, said system being more specifically defined as comprising: a) a complexing agent comprising at least one macrocyclic octol with crown configuration, with substituents in axial configuration, dissolved in b) at least one organic solvent immiscible with water, selected from among the saturated, optionally halogenated, (cyclo)alkane hydrocarbons; optionally halogenated and/or alkylated aromatic hydrocarbons; petroleum ether, the organic phase, constituted by the lactic acid-octol complex dissolved in the organic solvent, is separated from the aqueous phase, and the organic phase is subsequently treated with an aqueous liquid, preferably water, at a pH value in the range from 7 to 10 or with methanol, with lactic acid being thus released from the complex and said lactic acid being then recovered by means of techniques known from the prior art.

Description: The present invention relates to a process for recovering lactic acid from solutions which contain it. Lactic acid constitutes an important product having interesting possibilities of application at an industrial level. In fact, it can be used in the production of dairy products; as an acidulating agent in the alimentary field; as an intermediate for the production of plasticizer agents; adhesives; pharmaceutical products; in the production of lactates; as a mordant in wool dyeing; and so forth. A particular application with considerable prospects of industrial development is the production of biocompatible and/or biodegradable polymers from lactic acid, useful for manufacturing bags; application films, in the sanitary field, and so on. These latter applications, which to date can be only accomplished at high costs, can find a cheap and simple alternative according to the present invention for recovering lactic acid from aqueous solutions which are extremely cheap byproducts from industrial processes e.g., from residual fermentation broths; extracts of vegetable origin; byproducts from the processing of agrarian raw materials, such as the byproducts from corn fermentation treatment, i.e., the so-said "corn-steep liquor", and so forth. The extraction of lactic acid from such aqueous solutions constitutes a serious problem owing to the extremely high solubility of lactic acid and the presence in the above said aqueous liquids, coming from industrial processes, of a large number of other substances and/or materials of organic and inorganic nature, in amounts exceeding the amount of lactic acid contained therein. Several routes are known for extraction of lactic acid from fermentation broths or from aqueous extracts. According to the most widely used route, lactic acid is salified in the presence of calcium carbonate, with calcium lactate being obtained. After fermentation residues have been filtered, the salt is displaced with sulfuric acid. So obtained lactic acid is further purified by filtration, treatment with activated charcoal and passage over ion-exchange resins, and so forth. According to another route, calcium lactate is transformed into zinc lactate, which is subsequently purified by means of a plurality of crystallizations in cascade. Lactic acid is subsequently released by means of the addition of hydrogen sulfide. Also used is the method consisting of steps of esterification, purification and saponification. More recently, the method was introduced which is based on the neutralisation with organic amines, followed by the extraction of the resulting salt with organic solvents. All of these purification methods are affected by considerable problems of industrial application (corrosion, residual salts, use of ion-exchange resins, etc.), to such an extent that they add considerably to the total product cost. Therefore, a purpose of the present invention is to provide a simple and cheap process for selectively recovering lactic acid from aqueous solutions, such as fermentation broths and liquors from vegetable origin, e.g., so-said "corn steep liquor", and the like, which is not affected by the above described drawbacks shown by the processes known from the prior art. Another purpose is to provide a process for recovering lactic acid, by starting from aqueous solutions obtained as processing byproducts, with a low content of lactic acid and/or containing massive amounts of other components and/or organic and/or inorganic products, without first concentrating such aqueous solutions. These purposes are achieved according to the present invention, by contacting an aqueous medium containing lactic acid with a system comprising a complexing agent, selective for lactic acid, dissolved in a water-immiscible extractant solvent selective for the formed complex. A new process according to the present invention for recovering lactic acid from aqueous media which contain it is characterised in that said aqueous medium is treated at a pH value in the range from 1 to 6, with a complexing agent selective for lactic acid and an organic extractant selective for the complex, wherein the system is more specifically defined as comprising: a) a complexing agent comprising at least one macrocyclic octol with crown configuration, with substituents in axial configuration, dissolved in b) at least one organic solvent immiscible with water, selected from among the saturated, optionally halogenated, (cyclo)alkane hydrocarbons; optionally halogenated and/or alkylated aromatic hydrocarbons; and petroleum ether, the organic phase, comprising the lactic acid-octol complex dissolved in the organic solvent is separated from the aqueous phase, and the organic phase is subsequently treated with an aqueous liquid at a pH value in the range from 7 to 10 or with methanol, to liberate lactic acid. The complexing agent is usually dissolved in the solvent before contact with the aqueous medium, so that in the process the aqueous medium is brought into contact with a solution of the complexing agent in the solvent. In that way, with the preliminary complexation of lactic acid with the octol, a selective extraction of lactic acid dissolved initially in the aqueous phase into the organic phase is obtained. Complexed lactic acid is subsequently liberated by means of a treatment with an aqueous liquid, preferably water, at a pH value in the range from 7 to 10 and preferably in the range from 7 to about 9, with a more or less concentrated aqueous solution of lactic acid being obtained, which can be used as such, or from which concentrated, pure lactic acid can be obtained by means of techniques known from the prior art (concentration, and so forth). As an alternative, instead of water, methanol can be used, provided that it is not miscible with the organic solvent used; in this case, lactic acid will be liberated as a methanolic solution. As the extractant agents, macrocyclic octols are used, which have the general formula (I): in crown configuration, in which: the "R" symbols represent linear or branched alkyl radical having from 5 to 18 carbon atoms, the R groups being in axial configuration. Among the groups of the octols of general formula (I), preferably "R" is an alkyl radical containing from 6 to 15 carbon atoms, and more preferably R = C₆H₁₃; C₉H₁₉; C₁₁H₂₃; C₁₅H₃₁: and their mixtures. Mixtures of octols can be used. Said macrocyclic octols are obtained by means of the catalysed acidic condensation of resorcinol with a suitable alkyl aldehyde, according to as described in J. Org. Chem. 1988, 53 , page 5475. According to such a process, the products of general formula (I) can be obtained as one single configurational isomer, called "crown isomer", as shown in the hereto attached Figure 1, in which a perspective view of the molecule is depicted. Only the compounds having a crown configuration, with all of the "R" groups standing in axial position, are used as the extractant agents according to the present invention. The solvents should display the following characteristics: be immiscible with water; if the process involves extraction with methanol in the second step, be immiscible with methanol; good solvents for the macrocyclic tetramers of general formula (I); and not form preferential bonds themselves (e.g., hydrogen bonds) with the extractant agents of general formula (I). Within the scope of the above definition, solvents can be used, which are selected from the following: saturated hydrocarbons, i.e., (cyclo)alkanes, which may also contain halogen substituents, such as hexane, cyclohexane, chloroform, methylene chloride, dichloroethane carbon tetrachloride; also alkylated and/or halogenated, aromatic hydrocarbons, such as benzenes, toluene, xylenes, alkylbenzenes, chlorobenzenes; hydrocarbon blends, such as gasoline or kerosene, petroleum ether; and so on; as well as their mixtures. Particularly advantageous results were obtained with the use of the carbon tetrachloride, toluene, benzene and their mixtures. The process is carried out with strong stirring by operating under pressure and temperature conditions substantially corresponding to room conditions. As already said, the aqueous solution used as the starting material is treated at a pH value in the range from 1 to 6, and preferably in the range from 1.4 to 5, with an amount of organic solution which contains the macromolecular octol (I) preferably in a molar excess relative to the lactic acid contained in said aqueous medium. However, said amount is not binding, and may depend on the solubility of the octol in the solvent, and on similar parameters. The obtained solution in the organic phase, containing the octol-lactic acid complex, is separated from the aqueous phase by means of known methods (decantation, etc.), and is treated with an aqueous liquid, preferably water, at a pH value in the range from 7 to 10, and preferably in the range from 7 to 9, or with methanol. As regards the amount, for example, if the starting aqueous medium is constituted by a corn steep liquor, the amount of water or methanol to be used is about two times as large as the volume of organic solution containing the complex to be treated. The released lactic acid remains in the aqueous or methanolic phase, whilst the immiscible organic solvent phase, containing all of the macrocyclic octol, is separated from said aqueous phase by decantation, centrifugation, and so forth, and, if so desired, is recycled. In order to complete the recovery process, times in the range from 1 to 12 hours for the initial step of contacting the aqueous medium with the organic solvent and complexing agent and/or the back extraction with aqueous liquid or methanol are usually necessary. The time is a function of the operating conditions of stirring, temperature, and so on. As described above, the aqueous media containing lactic acid are generally constituted by fermentation broths, aqueous extracts of vegetable origin, the so said "corn steep liquors", and the like, which are available on the market at low cost. These are aqueous solutions with a molar titre of lactic acid generally in the range from 0.06 to about 12.1, besides a large number of other organic and inorganic components. Aqueous media can be treated, which have a molar titre of lactic acid in the range from 0.06 to 12.1 and preferably in the range from 0.1 to about 3.6. The process can also be used for aqueous solutions containing containing lactic acid in amounts above or below these ranges. The process can be applied to very diluted solutions of lactic acid (0.5%), as well as to concentrated solutions, practically up to about 90% by weight, without a preliminary concentration or dilution of the solution to be treated being necessary. The media can be subjected to one or more treatment cycles according to the present invention. According to a preferred, but non-exclusive, form of practical embodiment, the process of complexation-extraction of lactic acid according to the present invention, a corn steep liquor containing lactic acid is brought into contact, with stirring, at about 25°C for a period of about eight hours, with the organic phase formed by the solvent, selected from the group of the above mentioned solvents, in which the extractant agent of general formula (I) is dissolved in an excess molar ratio relative to the lactic acid. At the interface between the two phases, the selective complexation takes place, followed by the migration of the so-formed complex into the organic phase. The two phases are then separated from each other by decantation or centrifugation. The so-obtained organic phase contains the lactic acid-macrocyclic tetramer (I) complex. Lactic acid is liberated, e.g., by means of a back-extraction with distilled water, according to a volume ratio of about 2:1 (water:organic phase), which cleaves the complex and causes lactic acid to be released; lactic acid migrates into the new aqueous phase; the simultaneously released macrocyclic octol remains in the organic phase. If so desired, lactic acid can be obtained from the aqueous phase at the desired concentration by evaporation, and so on. Thanks to its simple and mild operating conditions, the process according to the present invention is particularly advantageous. The present invention is now disclosed in particular by referring to the following examples, which are given for the purpose of merely illustrating the invention without limiting it. Examples 1-5 Lactic acid was extracted from aqueous solutions at a known concentration and at a pH value in the range from 1.4 to 1.9 by means of a 1x10⁻² molar solution of octol 1 (R = C₁₁H₂₃) in carbon tetrachloride (see Table 1). A two-phase mixture (1:4 by volume) of 10 ml of aqueous phase and 40 ml of organic phase is kept vigorously stirred at 25°C for eight hours. After the complete de-mixing of the two phases from each other, the organic phase is separated and analysed. The quantitative determination of extracted lactic acid is carried out by HPLC analysis of the organic phase, vs. a standard reference solution [conditions: Lichrosorb RP18 column (by Merck); eluent mixture: 73:26:1 H₂O/CH₃CN/H₃P0₄-80%; flow rate: 1 ml/minute]. The so-obtained organic phase is then extracted once more with distilled water (pH 7). This latter is analysed in its turn via HLPC. The back-extraction in water of lactic acid is carried out as follows: 1 ml of organic phase is mixed with 2 ml of distilled water. After 8 hours of stirring at 25°C, the aqueous phase is centrifuged and analysed. The results are reported in Table 1. Example 6 By operating in the same way as of Examples 1 - 5, a 12.10 molar aqueous solution of lactic acid is extracted by means of a 5x10⁻² molar solution of octol 1 in carbon tetrachloride. The results are reported in Table 1. Examples 7- 11 Lactic acid was extracted from aqueous solutions at a known concentration and at a pH value in the range from 1.4 to 1.9 by means of a 1x10⁻² molar solution of octol 1 in toluene (see Table 2). A two-phase mixture (1:1 by volume) of 50 ml of aqueous phase and 50 ml of organic phase is kept vigorously stirred at 25°C for eight hours. After the complete de-mixing of the two phases from each other, the organic phase is separated and analysed by acidimetric titration. The quantitative determination of contained lactic acid is carried out by back-titration of the excess of base which is needed in order to neutralize the solution. A weighed amount of organic solution is titrated with an excess of 0.1 N solution of NaOH. The resulting two-phase mixture is kept stirred for 30 minutes and the excess of base is back-titrated with an 0.1 N aqueous solution of H₂SO₄. The potentiometric line makes it possible the concentration by weight of extracted lactic acid in the toluene phase to be determined with a very high precision. The results are reported in Table 2. The back-extraction with distilled water gave corresponding results. Examples 12 - 16 By operating in the same way as of Examples 7 - 11, a number of aqueous solutions of lactic acid at a pH comprised within the range of from 1.9 to 2.5 are extracted by means of an 1x10⁻¹ molar solution of octol 1 in toluene. The results are reported in Table 3. The back-extraction with distilled water save corresponding results. Examples 17 - 18 By operating in the same way as of Examples 1 - 5, a 12.10 M aqueous solution of lactic acid is respectively extracted with an 1x10⁻² molar solution of octol 1 (R = C₁₁H₂₃) and with an 1x10⁻² molar solution of octol 2 (R = C₆H₁₃) in benzene. The results are reported in Table 4. The back-extraction with distilled water gave corresponding results. Examples 19 - 20 By operating in the same way as of Examples 1 - 5, a 12.10 M aqueous solution of lactic acid is respectively extracted with an 1x10⁻² molar solution of octols3 (R = C₉H₁₉) and with an 1x10⁻² molar solution of octols4 (R = C₁₅H₃₁) in carbon tetrachloride. The results are reported in Table 4. The back-extraction with distilled water gave corresponding results. Example 21 By operating in the same way as of Examples 1 - 5, a comparative extraction of a 12.10 M aqueous solution of lactic acid is carried out with carbon tetrachloride alone. 0.33x10⁻² mol/litre of lactic acid is extracted. Example 22 By operating in the same way as of Examples 7 - 11, a comparative extraction of lactic acid from an aqueous solution containing a 12.10 molar concentration thereof is carried out with toluene alone. 1.0x10⁻² mol/litre of lactic acid is extracted. Example 23 Lactic acid was extracted from corn steep liquor (containing about 8-15% by weight of lactic acid) at a pH value of 4.2, by means of a 1x10⁻² molar solution of octol 1 in toluene. A two-phase mixture (2:1 by volume) containing 50 ml of organic phase and 25 ml of corn steep liquor is kept vigorously stirred at 25°C for eight hours. After the separation of the two phases, lactic acid is back-extracted from the organic phase with a same volume of distilled water. The concentration of back-extracted lactic acid is determined via HPLC under the same operating conditions as disclosed in Example 1. A value of 5.7x10⁻² mol/litre is obtained, vs. a value of 1x10⁻² mol/litre which was obtained when corn steep liquor was extracted under the same conditions, with toluene alone. Example 24 Lactic acid was extracted from the same corn steep liquor, by means of an 1x10⁻² molar solution of octol 1 in carbon tetrachloride. By operating in the same way as of Example 23 with a two-phase mixture (4:1 V/V), containing 100 ml of organic phase and 25 ml of corn steep liquor, 5x10⁻² mol/litre of lactic acid are extracted, to be compared to an extracted concentration of 0.55x10⁻² mol/litre only when the extraction is carried out with carbon tetrachloride alone.

Claims: Process for recovering lactic acid from aqueous media which contain it, characterised in that said aqueous medium is treated at a pH value in the range from 1 to 6 with a complexing agent and an organic extractant, in which: a) the complexing agent comprises at least one macrocyclic octol with crown configuration, with axially arranged substituents, and b) the organic solvent is immiscible with water and selected from saturated, optionally halogenated, (cyclo)alkane hydrocarbons, optionally halogenated and/or alkylated aromatic hydrocarbons, and petroleum ether, the organic phase constituted by the lactic acid-octol complex dissolved in the organic solvent is separated from the aqueous phase and the organic phase is subsequently treated with an aqueous liquid at a pH value in the range from 7 to 10 or with methanol, to liberate lactic acid. Process according to claim 1, characterised in that the macrocyclic octol has the general formula (I): in crown configuration, in which: the "R" symbols represent linear or branched alkyl radical of from 5 to 18 carbon atoms, and are arranged in axial configuration. Process according to claim 2 characterised in that in the macrocyclic octol of formula (I) the "R" symbols represent alkyl radicals containing from 6 to 15 carbon atoms and are preferably selected from the following: C₆H₁₃; C₉H₁₉; C₁₁H₂₃; C₁₅H₃₁; and their mixtures. Process according to any preceding claim characterised in that the organic solvent immiscible with water is selected from the following: hexane, cyclohexane, chloroform, methylene chloride, dichloroethane, xylenes, alkylbenzenes, chlorobenzenes, hydrocarbon blends, preferably selected from gasoline, kerosene and petroleum ether; and, preferably, carbon tetrachloride, benzene, toluene, and their mixtures. Process according to any preceding claim, characterised in that the macromolecular octol (I) is present in a molar excess relative to the lactic acid contained in said aqueous medium. A process according to any preceding claim in which the aqueous medium contains 0.06-12.1M lactic acid and is preferably corn steep liquor. A process according to any preceding claim in which the aqueous medium has a pH in the range 1.4 to 5.0. A process according to any preceding claim in which the aqueous medium organic solvent and complexing agent are contacted for a period in the range 1-12 hours. A process according to any preceding claim which is carried out at a temperature of about 25°C.

Inventors: BONSIGNORE, STEFANIO, DALCANALE, ENRICO, DU VOSEL, ANNICK
Applicants: ISTITUTO GUIDO DONEGANI S.P.A.
Application year: 1989
Main classification: C07C59
IPC codes: C07C51, C07C59