BE1021481B1 - PROCESS FOR PURIFYING AQUEOUS LACTIC ACID SOLUTION FROM HIGH CARBON HYDRATE BY-PRODUCTS - Google Patents

Abstract

Process for the purification of an aqueous solution of lactic acid from high carbohydrate by-products The present invention relates to a process for the purification of an aqueous solution of lactic acid obtained by fermentation of lactic acid. 'a source of carbohydrate containing a part of non-fermentable sugars such as oligosaccharides (DPn). Figure 1. 2012/0673

Description

Process for purifying an aqueous solution of lactic acid from high-carbohydrate by-products

Field of the invention

The present invention relates to a process for purifying an aqueous lactic acid solution, obtained by fermentation of a source of carbohydrates containing a portion of non-fermentable sugars such as oligosaccharides (DPn).

State of the art

The current economic situation requires manufacturers to continually seek to reduce their production costs, especially those related to raw materials. Lactic acid is not immune to this market pressure and the use of by-products or waste from industry is seen as an alternative.

Currently, the raw materials used in the production of lactic acid by fermentation are rather noble and pure carbohydrates, such as, for example, sucrose or certain glucose syrups. The major disadvantage of these sources is obviously their high cost due mainly to their purity.

There are, however, other sources of carbohydrates of lesser purity, available in large quantities and at low costs, for example: - by-products from the starch industry (wheat, maize, potatoes, etc.); .) - By-products derived from the hydrolysis of cellulose; hemicellulose and lignin. These are contained in straw, wheat, maize, bagasse, wood, pulp, etc. - By-products of the sugar industry such as sucrose, contained in molasses, green cane juice and some effluents.

Patent EP0986532 describes a process for purifying lactic acid obtained by fermentation, however, these raw materials of lesser purity contain unfermented or non-fermentable polysaccharides which disturb the smooth running of the purification steps described. In particular: - Demineralization (irreversible fouling of ion exchange resins). - Evaporation (fouling of evaporators, loss of the thermal efficiency of the system and significant development of the color of the treated product). - Distillation (low yield and entrainment of impurities in the final product)

This process therefore does not make it possible to economically produce lactic acid of high purity from these substrates.

Patent EP1094054 describes a process for separating and purifying lactic acid by electrodialysis with a cationic demineralization step, however neither decationization nor electrodialysis can eliminate non-fermented or non-fermentable substances from fermentation .

Patent US5814498 describes the use of tangential filtration, particularly nanofiltration, coupled or not to a cationic demineralization step to remove divalent calcium and magnesium type cations. Patents CA2227398 and US5681728 also describe the production of lactic acid by electrodialysis with a possible nanofiltration to remove divalent cations.

Indeed, if these ions are not removed, they will form hydroxide metal films that will irreversibly block and seal the membranes of the electrodialysis unit required to allow the extraction of lactic acid. However, this process does not deal with the problem of eliminating unfermented or non-fermentable polysaccharides.

There is therefore a need to find a process for purifying lactic acid, obtained from carbohydrate sources, which achieves the ranks expected by the market.

Brief description of the invention

The present invention relates to a method for purifying an aqueous lactic acid solution, obtained by fermentation of a carbohydrate source containing a portion of non-fermentable sugars such as oligosaccharides (DPn), which comprises the following steps: a. A cationic demineralization with removal of multivalent cations capable of forming insoluble salts b. Membrane filtration of the partially demineralized solution to obtain a retentate containing the essential portion of DPn and a permeate containing the essential portion of lactic acid. vs. Anionic demineralization of the permeate to remove anions that could favor lactic acid condensation reactions. d. A concentration of permeate capable of reducing the polycondensation of lactic acid by using the thin film technique. e. A distillation of lactic acid capable of reducing its polycondensation.

DETAILED DESCRIPTION OF THE INVENTION The invention relates to a process for purifying an aqueous lactic acid solution obtained by fermentation of a carbohydrate source containing a portion of non-fermentable sugars such as oligosaccharides (DPn). .

Preferably, the starting substrates for fermentation will be impure substrates from the starch, cellulose or sucrose industry. They can also come from a sugar refining residue (cane or beet) such as molasses. However, the latter contains sugar, the content of which is variable, but also carbohydrates, organic compounds (carboxylic, aliphatic and olefinic acids), proteins and phenols.

For all these polymeric raw materials, it is obvious that the low-cost by-products result either from incomplete hydrolysis or from partial purification and therefore from the more marked presence of impurities of all kinds but especially polysaccharide molecules. usually reprint under the term of DPn.

The method of the invention (Figure 1) comprises the following steps: a. A cationic demineralization with removal of multivalent cations capable of forming insoluble salts b. Membrane filtration of the partially demineralized solution to obtain a retentate containing the essential portion of DPn and a permeate containing the essential portion of lactic acid. vs. Anionic demineralization of the permeate to remove anions that could favor lactic acid condensation reactions. d. A concentration of permeate capable of reducing the polycondensation of lactic acid by using the thin film technique. e. A distillation of lactic acid capable of reducing its polycondensation. at. Cationic demineralization

The lactic acid solution resulting from the fermentation is strongly charged with ions (contents up to 12000 ppm in cations and 22000 ppm in anions), it is thus treated by percolation on a cationic resin so as to eliminate the multivalent cations , mainly divalent, capable of forming insoluble salts during. the next step of membrane filtration. It is possible to push this decationization until the elimination of monovalent cationic species (sodium, potassium, ammonium) or to be confined to eliminating only the multivalent ions, this choice being left to the discretion of the user.

Although this is not its primary purpose in the context of the invention, the Applicant Company has found that the decationization treatment is accompanied by a bleaching effect of the lactic acid solution.

The term "cationic resin" means cation resins which are macroporous or non-cationic, cationic or weak; such as, but not limited to, products marketed by Rhom & Hass under the references Amberlite 252 Na, Amberlite 252 H, Amberlite 252RF H, IMAC HP1110 Na, Amberlite SR IL Na, IMAC C16P Na, Amberlite 200C Na, etc. .

The lactic acid solution obtained at the end of this treatment is generally characterized by a concentration of between 50 and 200 g / l and a total cationic charge of less than 50 ppm while the total charge with multivalent cations is less than 10 ppm. even at 5 ppm. Its coloration is less than 2000 Hazen and may even be less than 250 Hazen. b. Membrane filtration (nanofiltration-)

The partially demineralized lactic acid solution is then treated by membrane filtration, in particular by nanofiltration. The working pressure is between 1 MPa and 3 MPa.

The porosity of the membrane, sometimes expressed in terms of the retention threshold, as well as the design of the filtration modules and housings, are judiciously chosen according to, on the one hand, the size of the molecules to be removed (disaccharides, trisaccharides, polysaccharides, coloring molecules, proteins, colloids, ...), on the other hand depending on the size of the molecules whose elimination must be avoided (lactic acid), and finally depending on the operating parameters (temperature, pH, pressures) and transmembrane flow objectives. In the present invention, the porosities are between 100 and 1000 Dalton, preferably between 100 and 800 and more preferably between 100 and 400 Dalton.

Non-limiting mention may be made of the membranes marketed by OSMONICS DESAL under the references "DL4040F", "DL8040F", "DL1SF", "DL2540F", "DL2540C", "DL2540F", etc.

In the context of the present invention, the nanofiltration process can be conducted either continuously or discontinuously (batch). During the treatment, the molecules that pass through the membrane constitute a fraction called "permeate" containing all or the essential part of the lactic acid contained in the partially demineralized starting solution; while the molecules released by the membrane concentrate in a fraction generally called "retentate", which is recycled to the feed tray. Macromolecules such as polysaccharides, dyes, etc. are thus progressively concentrated in a smaller and smaller volume of retentate (the nature of the impurities, via the clogging of the membranes and the increase in the viscosity of the retentate, come set the limits). This concentration is controlled by the volume concentration factor (VCF) which is defined as the ratio of the initial volume to the retentate volume (average FCV) or the feed rate to the retentate flow rate (instantaneous FCV).

The collected permeate is characterized by a lactic acid concentration of between 50 and 185 g / l and a coloration of between 5 and 250 Hazen. The Applicant has also observed a slight decrease in the content of certain sulphate anions and the polysaccharide content of more than 3 glucose units is less than 0.5 g / l.

In the absence of the cationic demineralization step which eliminates the divalent ions (in particular calcium), the concentration factor, induced by nanofiltration, would give rise to the formation of insoluble compounds (in particular calcium sulphate, by combination of calcium ions with sulfate ions from the acidification stage of the fermentation juice), which would clog and clog the membranes. These clogging of ordinary little reversible, require, therefore, frequent and expensive washing of the membranes and their anticipated replacement.

In order to improve the efficiency of recovery of the lactic acid during the operation by decreasing its content in the retentate, diafiltration (dilution of the retentate with water) may optionally be carried out at the end of concentration. The diafiltration membrane may be, although it is not mandatory in the context of the invention, of the same nature as that used for nanofiltration. This step can be carried out continuously or discontinuously, possibly even within nanofiltration modules when it is conducted discontinuously or during the non-productive steps of its cycle (waiting step during rinsing / cleaning sequences). membranes). The concentration factor is a function of the concentration and the nature of the impurities present, the concentration of lactic acid present in the retentate and the possibility of optionally recycling the permeate obtained. This factor can vary from 0.5 to 10 although a value between 1 and 5 is generally applicable (a factor of 2 corresponds to 2 volumes of water for 1 volume of retentate resulting from nanofiltration). vs. Anionic demineralization

The permeate resulting from the nanofiltration step is treated with anionic resin in order to remove all traces of anions, such as mainly chlorides and sulphates, disruptive anions during the following stages of concentration and distillation.

The term "anionic resin" means strong or weak anion exchange resins, such as, in a non-exhaustive manner, the products marketed by Rhom & Hass under the references "Amberlite IRA67", "IMAC HP661", "DUOLITE A561", A568, etc.

The lactic acid solution obtained is characterized by a concentration of between 50 and 185 g / l and a total anion content of less than 10 ppm or even 5 ppm. The coloration may be less than 40 or even 20 Hazen.

If necessary, and although this is not essential for the proper performance of the invention but it may be required for consistency in production, the acid can be treated again by percolation on exchange resins cationic and anionic ions. This step will eliminate traces of residual ions that may come from among other chemical reagents (sodium hydroxide, hydrochloric or sulfuric acid ...) used during regeneration cycles of said resins. The resin (s) may / may be, although it is not mandatory in the context of the invention, of the same nature as that (s) used (s) for the previous demineralizations. d. Concentration and post-concentration

The lactic acid solution from the resin treatment is then concentrated in two stages.

The first step consists of the rapid and low-temperature concentration of the lactic acid solution, until a concentration of between 50 and 95% is reached, preferably between 70 and 95%. A preferred approach of the invention envisages conducting this evaporation under reduced pressure, maintained between 0.005 MPa and 0.05 MPa, in order to ensure a boiling temperature of the solution as low as possible.

This step of the invention is carried out by any technique known to those skilled in the art such as, for example, evaporation in trickling film, single or multiple effects, associated or not with mechanical recompression systems or thermal vapor .

The second step consists in the post-concentration of the lactic acid solution effiuting from the concentration stage to a content sufficient to allow distillation. The operation can advantageously be carried out with a minimum residence time and at a temperature as low as possible, in a thin-film apparatus and more particularly mechanically stirred thin film (thin-film) or using a short path evaporator. The pressure is between 0.001 and 0.05 MPa, preferably between 0.005 and 0.03 MPa and more preferably between 0.005 and 0.015 MPa. The temperature of the heating wall of the body of the evaporator is adjusted so as essentially to support the vaporization of the free water contained in the solution to be concentrated without overheating the latter, it is between 50 and 200 ° C. and more preferably between 80 and 150 ° C. e. Distillation

The concentrated lactic acid solution is subjected to a distillation carried out in a reactor that maximizes the vaporization surface relative to the volume of liquid, that is to say by a reactor that exploits the properties of the thin layer.

A preferred approach of the invention is to use, for the distillation of lactic acid, a mechanically stirred thin-film evaporator, outside of which purified lactic acid is condensed (thin-film) or a short-path evaporator. equipped with an internal condenser (short pass). The temperature of the wall is maintained between 50 and 180 ° C, preferably between 80 and 160 ° C, more preferably between 110 and 160 ° C. The pressure is between 10 "7 and 0.01 MPa absolute, preferably between 10'5 and 2.10'5 MPa absolute, more preferably between 10" 4 and 2.10'3 MPa.

An alternative (FIG. 2) to the process of the invention described by the steps "a" to "e" consists in eliminating, totally or partially, the "a" step of cationic demineralization. In this context, the lactic acid solution resulting from the fermentation is directly treated by nanofiltration (step b) and the collected permeate will then follow the steps "c" to "e". Nevertheless, since the nanofiltration induces a concentration factor in the retentate and the divalent cations are still present, the membrane will quickly become clogged following the precipitation of the salts (calcium sulfate in particular). In order to avoid this precipitation, it is necessary to cause crystallization outside the nanofiltration module. To do this, knowing that the calcium sulphate has an inverted solubility curve with respect to the temperature, the retentate leaving the nanofiltration module is warmed so as to reduce its calcium sulphate saturation concentration, which has the consequence that to cause crystallization in the maturation tray. The calcium sulfate is then removed by filtration or any other solid / liquid separation technique known to those skilled in the art (decantation, centrifugation, etc.).

The solution thus freed of its insoluble calcium sulphate content is then cooled sufficiently which increases the dissolution capacity of this salt and prevents the crystallization (due to the concentration) of the calcium sulphate still contained in dissolved form in a new passage in the nanofiltration module. The porosity of the filtration membrane will be judiciously chosen according to the size of the crystals to be removed but also the flow rates to be treated and may be less than 20 pm or even 10 pm. Similarly, the heating and cooling temperatures will be optimized in order to guarantee a coherent enthalpy balance while taking into account the influence of the impurities present in the streams to be treated. Other details and particularities of the invention, given hereinafter by way of non-limiting examples, emerge from the description as some possible forms of its implementation.

Example 1

A lactic acid solution obtained by fermentation from a dextrose containing 2% by weight of polysaccharides DPn (n> 3), is limed and filtered so as to extract the microorganisms before being acidified with sulfuric acid to a pH of 2 and a temperature of 72-75 ° C. The slurry thus obtained is then filtered so as to extract, on the one hand, a cake consisting essentially of calcium sulphate and, on the other hand, a filtrate. The composition of the filtrate is shown in Table 1.

Table 1: composition of the lactic acid solution after filtration

(*) DPn content (n> 3); Content of Glucose (3.6 gr / 1) and Maltose (3.4 gr / 1) remaining constant a. Cationic demineralization

The filtrate obtained above is fed at the rate of 3 BV / h on a column containing 1 BV of cationic macroporous resin "Rhom & Hass Amberlite 252 Na, previously cleansed by passing sodium hydroxide in the form of a 4% solution, as well as by passing hydrochloric acid in the form of a 7% solution to neutralize the previous solution. The resin is also preconditioned in H + form by passing 2 BV of a 10% hydrochloric acid solution.

The composition of the lactic acid solution after demineralization is shown in Table 2.

Table 2: Composition of the lactic acid solution after cationic demineralization

(*) DPn content (n> 3); Content of Glucose (3.6 gr / 1) and Maltose (3.4 gr / 1) remaining constant

In order to demonstrate the importance of the invention during the treatment of this type of raw material, the juice resulting from the cationic demineralization has been split into two fractions. The first (A) will follow the succession of the following purification steps: nanofiltration, anionic demineralization, concentration and distillation. The second fraction (B) will not be treated in nanofiltration and will therefore follow the following steps: anionic demineralization, concentration and distillation. b. Membrane filtration (nanofiltration)

Fraction A is treated by nanofiltration in a unit of the "SEPA CF Membrane Cell" type sold by the company OSMONICS SA This unit is provided with a "DESAL DL 1 SF" membrane with an area of 0.027 m2 and a porosity between 100 and 300 Dalton. The lactic acid solution is treated at ambient temperature and at a pressure set at 3 MPa. The permeate and retentate compositions obtained after nanofiltration of fraction A are shown in Table 3.

TABLE 3 Composition of the permeate and the retentate after nanofiltration of the fraction A n Glucose: 1.1 g / l; Maltose: 0 g / 1

During the test the average permeate flow through the membrane was determined at 13.65 l / hrm 2 and fluctuated between 26.15 and 2.65 l / hrm 2. It can be seen in the table above that maltose and all DPn are found in the retentate. vs. Anionic demineralization

The fraction B, non-nanofiltered, and the permeate resulting from the nanofiltration of the fraction A are demineralized by percolation on anionic resin at the rate of 3BV / h on the top of a column containing 1BV of weak anionic resin marketed by Applexion under the reference "XA 945". This resin is previously packaged in basic form by percolation of 2BV caustic soda at 5% concentration. The compositions of demineralized solutions are shown in Table 4.

Table 4: Composition of solutions A and B of lactic acid after anionic demineralization

(*) Content in DPn (n> 3)

It is observed in the table above that solution B (non-nanofiltered) still contains a high concentration of DPn. d. Concentration and post-concentration

Solutions A and B of lactic acid are then concentrated in a stainless steel film evaporator having an evaporation surface of 0.1 m 2. The concentrated lactic acid solution is extracted at the same rate as the system feed rate (3.5 l / h) in order to maintain a constant level. The heating of the wall is ensured by a circulation of heat-transfer oil in a double envelope. The pressure and temperature conditions are respectively 0.01 MPa and 100 ° C. The results are shown in Table 5.

Table 5: Composition of solutions A and B of lactic acid after concentration in a dripping film evaporator

After concentration, a strong coloration of solution B (non-nanofiltered) can be observed.

Both solutions are then post-concentrated in a mechanically stirred thin-film borosilicate glass evaporator with a heating area of 0.06 μm. The temperature of this evaporator is adjusted by circulating heat transfer oil in a double jacket, it also has an internal condenser (short path) cooled by circulation of water. During the test, the assembly is maintained under a pressure of 0.01 MPa, a wall temperature of 110 ° C., a condenser temperature of 15 ° C., a rotational speed of 600 rpm (rotations per minute) and a feed rate of 11 / h.

The results are shown in Table 6.

Table 6: Composition of solutions A and B of lactic acid after postconcentration in a mechanically stirred thin-film evaporator

After these post-concentration tests, we found with solution B, a significant fouling of the evaporator following the high viscosity of the solution from the previous concentration step. e. Distillation

The two concentrated lactic acid solutions, A and B, are then distilled in the same unit as that described for the post-concentration. The pressure was raised to 10'3 MPa and the temperature to 140 ° C. The coloring of the distillate obtained for each of the solutions will be considered as representative of the chemical purity of the final lactic acid solution. The results are shown in Table 7.

Table 7: Composition of solutions A and B of lactic acid after distillation in a mechanically stirred thin-film evaporator

For the lactic acid solution B, the distillation had to be stopped before completion due to clogging of the unit (blocking of the rotor).

This example proves the need to go through a nanofiltration step when using raw materials containing DPn.

Example 2

The retentate resulting from the nanofiltration of solution A of Example 1 undergoes a diafiltration step. This step will allow the recovery in the permeate of the lactic acid still contained in the retentate. This permeate can then be recycled in the process. The diafiltration is carried out in the same nanofiltration cell, and with the same pressure and temperature parameters as described in Example 1. The membrane used is the same as that used previously for the nanofiltration of the solution A. 2 3 liters of demineralized water are added to 1 liter of retentate. This solution is then passed through the membrane until an FCV of 2.3 is obtained.

The composition of the retentate is shown in Table 8.

Table 8: composition of permeate and retentate after diafiltration

Example 3

A lactic acid solution obtained by fermentation from a dextrose containing 2% by weight of polysaccharides DPn (n> 3), is limed and filtered so as to extract the microorganisms before being acidified with sulfuric acid to a pH of 2 and a temperature of 72-75 ° C. The slurry thus obtained is then filtered so as to extract, on the one hand, a cake consisting essentially of calcium sulphate and, on the other hand, a filtrate. The composition of the filtrate is shown in Table 9.

Table 9: Composition of the lactic acid solution after filtration

(*) DPn content (n> 3); Content of Glucose (3.6 gr / l) and Maltose (3.4 gr / 1) remaining constant At this stage, the lactic acid solution is saturated with lactic acid salt (CaSC> 4: gypsum). In contrast to Example 1, the cationic demineralisation step is removed and substituted by a gradual and controlled precipitation of this salt (gypsum) during the nanofiltration step. at. Membrane filtration (nanofiltration) with progressive precipitation of the gypsum 20 liters of solution are treated, until a FCV of 20, in the same nanofiltration cell and the same conditions as in Example 1.

In order to avoid clogging of the membranes, the incoming solution of lactic acid is desaturated beforehand by cooling. That is, the instant FCV in the membranes is limited to stay below the solubility limit. The solution then enters the nanofiltration cell, the outgoing retentate is raised above the solubility limit by heating. The delta temperature, between the hot and cold product, is about 20 ° C. This phenomenon of successive cooling and heating will promote the precipitation of the gypsum in the retentate which returns to the maturation tank. Then follows a filtration of the hot product, which allows the elimination of precipitated gypsum. The protective filter begins to retain the gypsum after 12 liters of treated permeate, ie an FCV of 2.5.

The average permeate flow rate through the membrane is 10.3 l / h.m2. The permeate composition is shown in Table 10.

Table 10 permeate composition after nanofiltration with precipitation of gypsum

(*) Content in DPn (n> 3)

It is therefore possible to observe a decrease in the percentage of 99% staining and a reduction of the concentration of calcium cations and sulfate anions respectively by 97.8% and 78%. b. Anionic demineralization

The permeate resulting from the nanofiltration is demineralized by percolation on anionic resin under the same conditions as those described in Example 1. The composition of the demineralized solution is shown in Table 11.

Table 11: Composition of the lactic acid solution after anionic demineralization

(*) DPn content (n> 3) c. Concentration and post-concentration

The lactic acid solution is then concentrated in a trickle film evaporator under the same conditions as those described in Example 1. The results are shown in Table 12.

Table 12: Composition of the lactic acid solution after concentration in a dripping film evaporator

The solution is then post-concentrated in a thin-film borosilicate glass evaporator mechanically stirred under the same conditions as those described in Example 1.

The results are shown in Table 13.

Table 13: Composition of the lactic acid solution after post-concentration in a thin-film evaporator

d. Distillation

The concentrated lactic acid solution obtained is then distilled in the same unit as that described for the post-concentration and under the conditions of Example 1. The coloration of the distillate obtained will be considered as representative of the chemical purity of the final solution. lactic acid. The results are shown in Table 14.

Table 14: Composition of Solutions A and B of Lactic Acid After Distillation in a Thin Film Evaporator

This example thus proves that the nanofiltration step with progressive precipitation of the gypsum is an effective alternative of the process of the invention. Indeed, the distillation step is facilitated and the final product corresponds to the expected specifications.

Claims (9)

1. A method for purifying an aqueous lactic acid solution, obtained by fermentation of a carbohydrate source containing a portion of non-fermentable sugars such as oligosaccharides (DPn), characterized in that it comprises: at. A cationic demineralization with removal of multivalent cations capable of forming insoluble salts b. Membrane filtration of the partially demineralized solution to obtain a retentate containing the essential portion of DPn and a permeate containing the essential portion of lactic acid. vs. Anionic demineralization of the permeate to remove anions that could favor lactic acid condensation reactions. d. A concentration of permeate capable of reducing the polycondensation of lactic acid by using the thin film technique. e. A distillation of lactic acid capable of reducing its polycondensation. 2. Purification process according to claim 1 characterized in that the source of carbohydrate containing DPn is derived total or partial hydrolysis of products or by-products of the starch industry, sugar or cellulose. 3. purification process according to claims 1 and 2 characterized in that the cationic demineralization is carried out using one or more resin (s) cation exchange (s), macroporous (s) or not, cation type strong or weak. 4. Purification process according to claims 1 and 2 characterized in that the membrane filtration step is continuous or discontinuous nanofiltration, alone or coupled to a continuous or discontinuous diafiltration. 5. Purification process according to claim 4 characterized in that the nanofiltration requires a membrane porosity ranging between 100 and 1000 Dalton, preferably between 100 and 800 Dalton and more preferably between 100 and 400 Dalton. 6. Purification process according to claims 1 and 2 characterized in that the anionic demineralization is carried out using one or more anion exchange resin (s) strong or weak anions. 7. purification process according to claims 1 and 2 characterized in that the distillation is carried out by any technique known to those skilled in the art applicable to lactic acid such as, but not limited to, a thin film evaporator mechanically stirred. 8. A process for purifying an aqueous lactic acid solution, obtained by fermentation of a carbohydrate source containing a portion of non-fermentable sugars such as oligosaccharides (DPn) and neutralized with an oxide or a hydroxide of calcium and then acidified with sulfuric acid, characterized in that it comprises: a. Membrane filtration of the partially demineralized solution to obtain a retentate containing the essential portion of DPn and a permeate containing the essential portion of lactic acid. b. Elimination by gradual and controlled precipitation of calcium sulfate present in the retentate to prevent clogging of the membranes. vs. Anionic demineralization of the permeate to remove anions that could favor condensation reactions. d. A concentration of permeate capable of reducing the polycondensation of lactic acid by using the thin film technique. e. A distillation of lactic acid. 9. The method of claim 8 characterized in that the elimination by gradual and controlled precipitation is provided by successive heating and cooling of the solution to confine the crystallization and extraction of the crystals outside the filtration module.