METHOD FOR PURIFYING MAGNESIUM CHLORIDE SOLUTIONS

Abstract

A process for removing lactic acid from an aqueous lactic acid-containing magnesium chloride solution, the weight ratio of magnesium chloride to lactic acid in the aqueous lactic acid-containing magnesium chloride solution being at least 1:1, the process including the steps of subjecting the aqueous lactic acid-containing magnesium chloride solution to an evaporation step, resulting in the formation of a slurry of MgC12.MgL2.4H2O in an aqueous magnesium chloride solution, then subjecting the slurry to a solid-liquid separation step, to separate the solid MgC12.MgL2.4H2O from the aqueous magnesium chloride solution, resulting in the removal of lactic acid from the aqueous lactic acid-containing magnesium chloride solution in the form of MgC12.MgL2.4H2O. The process makes it possible to efficiently remove lactic acid from aqueous lactic acid-containing magnesium chloride solutions, resulting in magnesium chloride solutions with a low lactic acid content which can be further processed as desired.

Claims

1. Process for removing lactic acid from an aqueous lactic acid-containing magnesium chloride solution, the weight ratio of magnesium chloride to lactic acid in the aqueous lactic acid-containing magnesium chloride solution being at least 1:1, the process comprising the steps of subjecting the aqueous lactic acid-containing magnesium chloride solution to an evaporation step, resulting in the formation of a slurry of MgCl.sub.2.MgL.sub.2.4H.sub.2O in an aqueous magnesium chloride solution, subjecting the slurry to a solid-liquid separation step, to separate the solid MgCl.sub.2.MgL.sub.2.4H.sub.2O from the aqueous magnesium chloride solution, resulting in the removal of lactic acid from the aqueous lactic acid-containing magnesium chloride solution in the form of MgCl.sub.2.MgL.sub.2.4H.sub.2O.

2. Process according to claim 1, wherein the weight ratio of magnesium chloride to lactic acid is at least 1.5:1.

3. Process according to claim 1, wherein the lactic-acid containing magnesium chloride solution has a magnesium chloride concentration in the range of 5 to 35 wt.%.

4. Process according to claim 1, wherein the evaporation step is carried out in one or more steps, at a temperature in the range of 50-200° C., or at reduced pressure .

5. Process according to claim 1, wherein evaporation is continued until the solution has a magnesium chloride concentration in the range of 30-47 wt.%.

6. Process according to claim 1, wherein the slurry of MgCl.sub.2.MgL.sub.2.4H.sub.2O in a magnesium chloride solution comprises at least 2 wt.% of MgCl.sub.2.MgL.sub.2.4H.sub.2O and at most 50 wt.% of MgCl.sub.2.MgL.sub.2.4H.sub.2O.

7. Process according to claim 1, wherein the magnesium chloride solution obtained from the separation step has a magnesium chloride concentration in the range of 35-47 wt.%, and/or at most 47 wt.%.

8. Process according to claim 1, wherein the lactic acid concentration in the magnesium chloride solution obtained from the separation step is at most 1 wt.%.

9. Process according to claim 1, wherein a lactic acid-containing magnesium chloride solution with a lactic acid concentration of 0.5-7 wt.% and a magnesium chloride concentration of 15-25 wt.% is subjected to one or more evaporation steps, resulting in a slurry of 4-40 wt.% MgCl.sub.2.MgL.sub.2.4H.sub.2O in a magnesium chloride solution with a magnesium chloride concentration of 35-47 wt.%, and the slurry is subjected to a solid-liquid separation step resulting in solid MgCl.sub.2.MgL.sub.2.4H.sub.2O and a magnesium chloride solution with a lactic acid concentration of less than 0.5 wt.%.

10. Process for the manufacture of lactic acid comprising the steps of subjecting a carbon source to a fermentation step to form lactic acid, which fermentation step comprises the steps of fermenting a carbon source by means of a micro-organism in a fermentation medium to form lactic acid neutralizing at least part of the lactic acid by adding a magnesium base selected from magnesium oxide and magnesium hydroxide to the fermentation medium, thereby obtaining magnesium lactate, subjecting the magnesium lactate to an acidification step wherein the magnesium lactate is contacted with HCl in an aqueous environment to form an aqueous mixture comprising lactic acid and magnesium chloride, subjecting the aqueous mixture comprising lactic acid and magnesium chloride to a separation step, to form an effluent comprising lactic acid and an aqueous lactic acid-containing magnesium chloride solution, and recovering the effluent comprising lactic acid from the process, subjecting the aqueous lactic acid-containing magnesium chloride solution, the weight ratio of magnesium chloride to lactic acid in the aqueous lactic acid-containing magnesium chloride solution being at least 1:1, to an evaporation step, resulting in the formation of a slurry of MgCl.sub.2.MgL.sub.2.4H.sub.2O in an aqueous magnesium chloride solution, subjecting the slurry to a solid-liquid separation step, to separate the solid MgCl.sub.2.MgL.sub.2.4H.sub.2O from the aqueous magnesium chloride solution.

11. Process according to claim 10, wherein solid Mg MgCl.sub.2.MgL.sub.2.4H.sub.2O is recycled at least in part to the acidification step.

12. Process according to claim 10, wherein the magnesium chloride solution derived from the solid-liquid separation step is provided at least in part to a thermal decomposition step, wherein the magnesium chloride is reacted with water to form solid magnesium oxide and gaseous HCl, wherein the solid MgO can be recycled at least in part to the fermentation step, as such or after having been converted into magnesium hydroxide and the gaseous HCl can, if so desired, be recycled at least in part to the acidification step as such, or after having been dissolved in water to form an aqueous HCl solution.

13. Process according to claim 10, wherein lactic acid separated from the lactic acid-containing magnesium chloride solution is subjected to one or more steps selected from separation from an extractant, and purification, e.g. one or more purification steps selected from the group of washing, active carbon treatment, recrystallization, distillation, and filtration.

14. Process according to claim 10, wherein the lactic acid is converted to lactide or polylactic acid.

15. Lactic acid obtainable by the process of claim 10, or lactide or polylactic acid obtainable by the process according to claim 10, wherein the lactic acid is converted to lactide or polylactic acid.

Description

EXAMPLES

Example 1

[0083] A ternary solid-liquid phase diagram of the system magnesium chloride - lactic acid - water was obtained at 20° C., 50° C. and 80° C. This diagram was constructed by measuring the solubility of mixtures of magnesium chloride hexahydrate (MgCl.sub.2.6H.sub.2O) , double crystallized (S)-Lactic acid crystals and demineralized water. The solubility lines were determined by adding the appropriate amounts of said chemicals to a glass vessel, heating the system to the appropriate temperature and stirring for about 30 minutes. Next a small amount of water or lactic acid solution was added to change the overall composition and the new system was stirred at least 10 minutes.

[0084] When it was observed that the solids did not dissolve, another small amount of water was added followed by stirring for at least 10 minutes. This was repeated until all solids had dissolved. Care was taken to allow for additional dissolution time close to the dissolution point.

[0085] After establishing the dissolution point, the composition was back-calculated from the initial masses and added water/chemicals. The result is the ternary solid-liquid phase diagram of the system magnesium chloride - lactic acid -water as depicted in FIG. 2.

[0086] The crystals formed during the above experiment were examined and magnesium chloride hydrate crystals were only observed in lactic acid free samples. The solubility of magnesium chloride crystals decreased significantly when lactic acid was added and needle shaped crystals were formed.

[0087] The needle shaped crystals were isolated by filtration (twice with separate samples), the filter cake was washed with ethanol to remove residual liquid material and dried at room temperature. The solids obtained were analysed to determine their composition. The lactate/lactic acid content was determined by HPLC. The organic acids were quantified by HPLC with a column that has a stationary phase comprising a strong cation exchange resin in the calcium form (Bio-Rad Aminex HPX-87C, 300x7.8 mm at 85° C.) and an eluent comprising 3 mM Ca(H2PO4)2.Math.H2O and 0.015 M H3PO4 (pH=2.2). An UV3000 system was used as detector. The water content was determined using the Karl Fisher titration procedure (KF titration) which is extensively described in handbooks and literature. As base imidazole (Hydranal composite) was used. Magnesium was determined by atomic absorption spectroscopy (AAS) on a flame AAS Varian SpectrAA 300 (including PC with SpectrAA software). The solid crystals were dissolved in nitic acid solution. Chloride content was determined by titration with silver nitrate whereby the silver chloride precipitates quantitatively. The end point is determined potentiometrically using a silver ring electrode. The results confirmed that the composition of the crystals matched with that of MgCl.sub.2 .MgL.sub.2.4H.sub.2O.

Example 2

[0088] A lactic acid-containing magnesium chloride solution with a magnesium chloride concentration of 18.2 wt.% and a lactic acid concentration of 2.3 wt.% was prepared by mixing appropriate amounts of magnesium chloride hexahydrate (MgCl.sub.2.6H.sub.2O) , double crystallized (S)-Lactic acid crystals and demineralized water. Said solution was subjected to a first evaporation step where water was evaporated at a temperature of 65-80° C. and a pressure of 0.35 bar, to form a lactic acid-containing magnesium chloride solution with a magnesium chloride concentration of 28 wt.% and a lactic acid concentration of 3.5 wt.%. This solution was subjected to a further evaporation step, at a temperature of 80° C. and a pressure of 0.35 bar, resulting in the formation of a slurry of 9 wt.% of solid MgCl.sub.2.MgL.sub.2.4H.sub.2O in a magnesium chloride solution with a magnesium chloride concentration of 32 wt.% magnesium chloride.

[0089] The slurry was subjected in a centrifugation step and the obtained cake was washed with methanol (3x cake volume) and subsequently dried at 70° C. for 1 h. This isolated cake comprised many needle-shaped crystals, with a solids content of above 90 wt.%, and a lactic acid concentration of below 0.4 wt.%. The ratio of the weight of dried cake and weight of suspension was about 9%. The dried cake was analysed using the methods described in example 1 for magnesium (w%, AAS), lactic acid (w% as lactic acid, HPLC), and chloride (w%, titration) inclusion amount and said amounts were found to match with the theoretical values of MgCl2.MgL2.4H2O. This also proved that the double salt concentration obtained after evaporative crystallization step was about 9%.