Method for purifying magnesium chloride solution

Abstract

The present invention relates to a method for removing lactic acid from a magnesium chloride aqueous solution containing lactic acid, wherein the weight ratio of magnesium chloride to lactic acid in the magnesium chloride aqueous solution containing lactic acid is at least 1:1, the method comprising the steps of: subjecting the magnesium chloride aqueous solution containing lactic acid to an evaporation step, thereby forming MgCl 2 .MgL 2 .4H 2 O in an aqueous solution of magnesium chloride; subjecting the slurry to a solid-liquid separation step to separate solid MgCl from the aqueous solution of magnesium chloride 2 .MgL 2 .4H 2 O, thereby removing MgCl from the magnesium chloride aqueous solution containing lactic acid 2 .MgL 2 .4H 2 The method of the present invention can effectively remove lactic acid from a magnesium chloride aqueous solution containing lactic acid, thereby obtaining a magnesium chloride solution with a low lactic acid content, which can be further processed as needed. Solid MgCl can also be processed as needed. 2 .MgL 2 .4H 2 O, for example to provide it to an acidification step (wherein the magnesium lactate is reacted with HCl). It is attractive to integrate the process according to the invention into a process for the manufacture of lactic acid.

Description

[0001] The present invention relates to a method for purifying magnesium chloride solution, in particular magnesium chloride solution containing a limited amount of lactic acid. The present invention also relates to a method for producing lactic acid by a fermentation process.

[0002] Lactic acid can be produced by microbial fermentation of carbon sources such as carbohydrates or glycerol. In such a fermentation process, the carbohydrate source is usually fermented by microorganisms to form lactic acid. The liquid in which the carbohydrate source is fermented is called a fermentation broth or fermentation medium. The formation of lactic acid during the fermentation process can cause the pH value of the fermentation broth to decrease. Since this decrease in pH value can impair the metabolic process of the microorganism, it is common practice to add a neutralizing agent, i.e., an alkali, to the fermentation medium to neutralize the pH value. Therefore, the lactic acid produced in the fermentation medium is usually present in the form of lactate.

[0003] In order to recover lactic acid from the fermentation medium after fermentation, downstream processing is required. One of the steps in the downstream processing is an acidification step, in which the lactate is contacted with an inorganic acid in an aqueous medium, resulting in the formation of lactic acid and an inorganic salt. For example, if the lactate in the fermentation medium is a magnesium salt, acidification with HCl will result in the formation of a solution containing dissolved lactic acid and dissolved magnesium chloride.

[0004] The next step is to separate the lactic acid from the magnesium chloride solution. This separation is not simple because both lactic acid and magnesium chloride have high solubility in water.

[0005] WO00 / 17378 describes the preparation of lactic acid by fermentation, pH adjustment with Ca(OH)2 or Mg(OH)2, addition of HCl and extraction of lactic acid from a magnesium chloride solution using a solvent selected from amines, alcohols and ethers, preferably isoamyl alcohol, diisopropyl ether and Alamine 336. The solvent containing the lactic acid is then contacted with water to produce a lactic acid solution, which is further processed.

[0006] WO 2013 / 093028 describes the extraction of lactic acid from a magnesium chloride solution using an extract selected from C5+ ketones, diethyl ether and methyl tert-butyl ether, thereby obtaining an organic lactic acid solution and a spent magnesium chloride solution.

[0007] One problem that arises during this extraction process is that the extraction of lactic acid from the magnesium chloride solution will not be complete. A small amount of lactic acid will remain in the magnesium chloride solution. This is disadvantageous for two reasons. First, the presence of lactic acid in the magnesium chloride solution reduces the lactic acid yield of the entire process. Second, it has been found that the presence of lactic acid in the magnesium chloride solution interferes with the further processing of the magnesium chloride solution (where a high concentration of the solution is required). It has been found to be difficult to remove lactic acid, both because the amount of lactic acid in the magnesium chloride solution is relatively small compared to the amount of magnesium chloride in the solution, and because the solubility of both magnesium chloride and lactic acid in water is very high.

[0008] Therefore, there is a need in the art for a method of removing limited amounts of lactic acid from magnesium chloride solutions. The present invention provides such a method.

[0009] The present invention relates to a method for removing lactic acid from a magnesium chloride aqueous solution containing lactic acid, wherein the weight ratio of magnesium chloride to lactic acid in the magnesium chloride aqueous solution containing lactic acid is at least 1:1, and the method comprises the following steps:

[0010] - subjecting the aqueous magnesium chloride solution containing lactic acid to an evaporation step, thereby forming a slurry of MgCl2.MgL2.4H2O in the aqueous magnesium chloride solution,

[0011] - Subjecting the slurry to a solid-liquid separation step to separate solid MgCl2.MgL2.4H2O from the aqueous magnesium chloride solution. Separating solid MgCl2.MgL2.4H2O from the aqueous magnesium chloride solution results in the removal of lactic acid in the form of MgCl2.MgL2.4H2O from the aqueous magnesium chloride solution containing lactic acid.

[0012] It has been found that the process of the present invention can effectively remove lactic acid from an aqueous magnesium chloride solution containing lactic acid, thereby producing a magnesium chloride solution with a low lactic acid content that can be further processed as required. The solid MgCl2.MgL2.4H2O can also be processed as required, for example by providing it to an acidification step in which magnesium lactate reacts with HCl. Other advantages of the present invention and specific embodiments thereof will become apparent from the further description.

[0013] The present invention will be discussed in more detail below.

[0014] The key to the present invention is the recognition that in order to separate lactic acid and magnesium chloride, both of which have high solubility in water, a double salt MgCl2.MgL2.4H2O can be produced which has low solubility in water and can actually be used as a carrier for removing lactic acid from magnesium chloride solution.

[0015] Therefore, the present invention can be expressed as a method for removing lactic acid from an aqueous magnesium chloride solution containing lactic acid, the method comprising the following steps:

[0016] - subjecting the aqueous magnesium chloride solution containing lactic acid to an evaporation step, thereby forming a slurry, and

[0017] - subjecting the slurry to a solid-liquid separation step,

[0018] The method is characterized in that

[0019] - the weight ratio of magnesium chloride to lactic acid in the magnesium chloride aqueous solution containing lactic acid is at least 1:1,

[0020] - the evaporation step results in the formation of a slurry of MgCl2.MgL2.4H2O in an aqueous solution of magnesium chloride, and

[0021] -The solid-liquid separation step is used to separate solid MgCl2.MgL2.4H2O from the magnesium chloride aqueous solution, thereby removing lactic acid in the form of MgCl2.MgL2.4H2O from the magnesium chloride aqueous solution containing lactic acid.

[0022] The aqueous magnesium chloride solution containing lactic acid to be treated according to the present invention has a weight ratio of magnesium chloride to lactic acid of at least 1:1. If the weight ratio of magnesium chloride to lactic acid is below this value, the process efficiency will be reduced. Since a higher ratio leads to an improved process efficiency, in particular the solid content of the product, it is preferred to operate at a higher weight ratio of magnesium chloride to lactic acid, for example at least 1.5:1. Alternatively, it is preferred that the weight ratio of magnesium chloride to lactic acid is at least 2:1. More specifically, it is preferred that the weight ratio of magnesium chloride to lactic acid is at least 4:1, in particular at least 5:1, and more particularly at least 6:1. If the ratio of magnesium chloride to lactic acid in the magnesium chloride solution to be treated is below the desired operating value, the solution can first be subjected to a lactic acid removal step, for example by extraction, membrane separation, ion exchange, ion absorption or adsorption, to increase the weight ratio of magnesium chloride to lactic acid to the desired operating value.

[0023] If the amount of lactic acid in the lactic acid-containing magnesium chloride medium is too low, then the removal of lactic acid by the process of the present invention may not be economically attractive. Therefore, the weight ratio of magnesium chloride to lactic acid is generally at most 70:1, in particular at most 50:1, more particularly at most 40:1, and in some embodiments at most 20:1.

[0024] The absolute concentrations of magnesium chloride and lactic acid in the magnesium chloride solution containing lactic acid to be treated according to the invention are not very relevant, since the first step of the process is the removal of water, resulting in an increase in concentration. Typically, the magnesium chloride concentration of the solution will be in the range of 5-35 wt.%, in particular in the range of 10-35 wt.%, more particularly in the range of 15-35 wt.%. The amount of lactic acid can be determined by the ratio specified above.

[0025] The magnesium chloride solution containing lactic acid is an aqueous solution. It may contain limited amounts of other compounds, such as other compounds from previous steps in the manufacture of the solution, but this is not necessary and is not desirable.

[0026] For example, in one embodiment, as will be discussed in more detail below, the magnesium chloride aqueous solution containing lactic acid used as starting material in the present invention can be obtained by treating a fermentation broth containing magnesium lactate. The fermentation broth containing magnesium lactate can be acidified with hydrochloric acid and separated to form lactic acid and a solution mainly containing magnesium chloride and a limited amount of lactic acid. The latter solution can be used as a starting material in the method according to the invention. In this case, it is preferred that the solution does not contain or contains a very limited amount of pollutants produced by the fermentation process, the acidification step or the separation step, because these impurities can also interfere with the further processing of the magnesium chloride solution prepared according to the method of the invention in an undesirable manner. Therefore, the magnesium chloride aqueous solution containing lactic acid preferably contains less than 10wt.% of other components (components except water, magnesium chloride, lactic acid and their salts), preferably less than 5wt.% and more preferably less than 1wt.%. Volatile organic compounds (i.e. compounds that evaporate from the solution under evaporation conditions), such as extracts obtained from the previous separation step by an extraction process, may be less harmful than other components because they will be removed in the evaporation step. Therefore, in one embodiment, the magnesium chloride solution containing lactic acid contains less than 8 wt.% of volatile organic compounds and less than 4 wt.% of other compounds (which are not lactic acid, magnesium chloride, double salts thereof or volatile organic compounds). Preferably, the amount of volatile organic compounds is at most 6 wt.%, in particular at most 4 wt.%, more in particular at most 2 wt.%. Preferably, the amount of other compounds is at most 3 wt.%, in particular at most 2 wt.%, more in particular at most 1 wt.%.

[0027] The aqueous magnesium chloride solution containing lactic acid is subjected to an evaporation step to form a slurry of MgCl2.MgL2.4H2O in the magnesium chloride solution. In the evaporation step, water is removed, resulting in an increase in the concentration of lactic acid and magnesium chloride to a value above the solubility of the product MgCl2.MgL2.4H2O.

[0028] Thus, the evaporation step will result in the formation of a slurry of MgCl2.MgL2.4H2O in a magnesium chloride solution. The magnesium chloride concentration at which MgCl2.MgL2.4H2O begins to precipitate will depend on the prevailing conditions, with higher lactic acid concentrations and magnesium chloride concentrations promoting precipitation. Typically, solid MgCl2.MgL2.4H2O will begin to precipitate even when the lactic acid concentration is less than 1 wt.%, but when the magnesium chloride concentration is above 31 wt.%, especially above 35 wt.%.

[0029] It is generally preferred that the temperature during the evaporation step is in the range of 50-200°C, in particular in the range of 80-150°C. Higher temperatures are preferred to increase the solubility of MgCl2, thereby increasing the concentration of MgCl2 in the product solution. It was surprisingly found that the selection of the above-mentioned higher temperature has only a limited effect on the solubility of MgCl2.MgL2.4H2O. Therefore, higher temperatures will lead to the formation of more solid MgCl2.MgL2.4H2O, because the higher magnesium chloride concentration will force more MgCl2.MgL2.4H2O to form in the solution. In addition, higher temperatures help the removal of water.

[0030] The evaporation step can be carried out at atmospheric pressure, or even under increased pressure. However, the evaporation step is preferably carried out under reduced pressure, as this will allow efficient evaporation at acceptable temperatures. Therefore, in one embodiment, the evaporation of water is carried out at a pressure of 0.01-0.9 bar, particularly in the range of 0.01-0.35 bar.

[0031] Evaporation is continued until the desired amount of MgCl2.MgL2.4H2O has precipitated. The first factor affecting this is the concentration of magnesium chloride dissolved in the liquid. Typically, evaporation will continue until the magnesium chloride concentration of the solution is in the range of 30-47 wt.%, particularly 33-45 wt.%, more particularly in the range of 38-45 wt.%. If evaporation is stopped at a lower concentration, precipitation of MgCl2.MgL2.4H2O will be insufficient. If evaporation is continued to too high a magnesium chloride concentration, there is a risk that magnesium chloride will precipitate, which is undesirable in the present process.

[0032] The evaporation step can be carried out in a single step or in multiple steps. In the case where the concentration of the starting solution is relatively low and / or the weight ratio of MgCL2 to lactic acid is relatively high, for example at least 6:1, more particularly at least 8:1, it is attractive to carry out a multiple step evaporation.

[0033] The product obtained after the evaporation step is a slurry of MgCl2.MgL2.4H2O in a magnesium chloride solution. Depending on the amount of lactic acid in the starting solution and the concentration of MgCl2 in the product solution, the slurry may contain at least 2 wt.% MgCl2.MgL2.4H2O, in particular at least 4 wt.%. Typically, the slurry will contain up to 50 wt.% MgCl2.MgL2.4H2O, in particular up to 40 wt.% MgCl2.MgL2.4H2O, in particular up to 30 wt.% MgCl2.MgL2.4H2O, in particular up to 25 wt.% MgCl2.MgL2.4H2O, and in some embodiments up to 20 wt.% MgCl2.MgL2.4H2O.

[0034] The slurry is subjected to solid-liquid separation, wherein MgCl2.MgL2.4H2O is separated from the magnesium chloride solution. During the solid-liquid separation, the slurry preferably has a temperature similar to that of the evaporation step. It is understood that similar temperature means that the temperature difference between the slurry during the evaporation step and the solid-liquid separation is less than 20°C, etc. The solid-liquid separation can be performed by methods known in the art, for example, by filtration or centrifugation, or by a combination thereof.

[0035] The magnesium chloride solution obtained from the separation step typically has a magnesium chloride concentration in the range of 35-47 wt.%. At concentrations below 35 wt.%, precipitation of lactic acid as MgCl2.MgL2.4H2O may not reach the desired extent, leaving too much lactic acid in solution. At concentrations above 47 wt.%, magnesium chloride may have precipitated out of solution. Preferably, the magnesium chloride concentration is at least 37 wt.%, in particular at least 39 wt.%, and / or at most 47 wt.%, in particular at most 45 wt.%.

[0036] The lactic acid concentration in the magnesium chloride solution obtained from the separation step is generally at most 1 wt.%, in particular at most 0.5 wt.%, more in particular at most 0.2 wt.%.

[0037] In one embodiment, the present invention relates to a method, wherein a magnesium chloride solution containing lactic acid having 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 to produce a slurry of MgCl2.MgL2.4H2O in a magnesium chloride solution, wherein MgCl2.MgL2.4H2O is 4-40 wt.%, and the magnesium chloride concentration is 35-47 wt.%; the slurry is subjected to a solid-liquid separation step to obtain solid MgCl2.MgL2.4H2O and a magnesium chloride solution having a lactic acid concentration of less than 0.5 wt.%, preferably less than 0.2 wt.%.

[0038] This method has been found to be commercially applicable.

[0039] It is particularly attractive to combine the method according to the present invention into a method for producing lactic acid using a fermentation step. Therefore, in this embodiment, a process for removing lactic acid from an aqueous solution of magnesium chloride containing lactic acid is used to separate lactic acid from an aqueous solution of magnesium chloride containing lactic acid obtained by a fermentation process. Preferably, the aqueous solution of magnesium chloride containing lactic acid is an effluent obtained by the following process:

[0040] - subjecting the carbon source to a fermentation step to form lactic acid, the fermentation step comprising the step of subjecting the carbon source to fermentation by a microorganism in a fermentation medium to form lactic acid,

[0041] - magnesium lactate is obtained by adding a magnesium base selected from magnesium oxide and magnesium hydroxide to the fermentation medium to neutralize at least part of the lactic acid,

[0042] - 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,

[0043] - subjecting the aqueous mixture comprising lactic acid and magnesium chloride to a separation step to form an effluent comprising lactic acid and an aqueous magnesium chloride solution comprising lactic acid, and recovering the effluent comprising lactic acid from the process.

[0044] Therefore, in one embodiment, the present invention therefore relates to a method for producing lactic acid comprising the following steps:

[0045] - subjecting the carbon source to a fermentation step to form lactic acid, the fermentation step comprising the step of subjecting the carbon source to fermentation by a microorganism in a fermentation medium to form lactic acid,

[0046] - magnesium lactate is obtained by adding a magnesium base selected from magnesium oxide and magnesium hydroxide to the fermentation medium to neutralize at least part of the lactic acid,

[0047] - 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,

[0048] - subjecting the aqueous mixture comprising lactic acid and magnesium chloride to a separation step to form an effluent comprising lactic acid and an aqueous magnesium chloride solution containing lactic acid, and recovering the effluent comprising lactic acid from the process,

[0049] - subjecting the aqueous magnesium chloride solution containing lactic acid, wherein the weight ratio of magnesium chloride to lactic acid in the aqueous magnesium chloride solution containing lactic acid is at least 1:1, to form a slurry of MgCl2.MgL2.4H2O in the aqueous magnesium chloride solution,

[0050] - Subjecting the slurry of MgCl2.MgL2.4H2O in magnesium chloride solution to a solid-liquid separation step to separate solid MgCl2.MgL2.4H2O from the aqueous magnesium chloride solution.

[0051] Solid MgCl2.MgL2.4H2O can be at least partially recycled to the acidification step, where it dissolves by reacting with HCl to form dissolved lactic acid and magnesium chloride.

[0052] The magnesium chloride solution obtained from the solid-liquid separation of the slurry has a high magnesium chloride content and a low lactic acid content. It is therefore suitable for processing by thermal decomposition. In thermal decomposition, magnesium chloride reacts with water (present in the aqueous solution) at high temperatures to form solid MgO and gaseous HCl. If desired, the solid MgO can be at least partially recycled to the fermentation step as is or after being converted into magnesium hydroxide. If desired, the gaseous HCl can be at least partially recycled to the acidification step. The gaseous HCl can be recycled as is, or recycled after being dissolved in water to form an aqueous HCl solution.

[0053] The individual steps of the integrated process, except for the purification of the magnesium chloride solution containing lactic acid, are discussed below.

[0054] In the first step, the carbon source is subjected to a fermentation step to form lactic acid, which comprises the steps of fermenting the carbon source to form lactic acid by means of microorganisms in a fermentation broth, and neutralizing at least part of the lactic acid by adding a magnesium base selected from magnesium oxide and magnesium hydroxide, thereby obtaining magnesium lactate.

[0055] Fermentation processes for producing lactic acid are known in the art and need no further explanation here. It is within the purview of those skilled in the art to select a suitable fermentation process, using common general knowledge, depending on the desired acid to be produced, the carbon source and the available microorganisms.

[0056] The product of the fermentation process is a fermentation broth, which is an aqueous liquid comprising magnesium lactate, biomass and optionally other components such as sugars, proteins and impurities such as salts.

[0057] If desired, the fermentation broth can be subjected to a biomass removal step, such as a filtration step, prior to further processing. This is often the first choice to improve product quality.

[0058] The magnesium lactate may be present in solution or solid form. Solid magnesium lactate may also be present, for example, in the form of a suspension or slurry. Another intermediate step may be to separate the solid magnesium lactate from the fermentation broth before, after or simultaneously with the biomass removal. The magnesium lactate may be present in the form of a wet cake, which may be obtained, for example, after the biomass removal step. Optionally, the magnesium lactate may be subjected to a washing step.

[0059] Another intermediate step may be to subject the fermentation broth to a concentration step to increase the concentration of magnesium lactate in the composition prior to acidification. This step may be performed before, after or simultaneously with biomass removal.

[0060] Other intermediate steps, such as purification steps, may be performed as desired, as will be apparent to the skilled person. An example of a suitable process is a process in which a fermentation broth comprising magnesium lactate and biomass is subjected to a biomass removal step, a step of removing solid magnesium lactate, a step of concentrating the remaining liquid medium to further form solid magnesium lactate, and removing additional solid magnesium lactate from the liquid medium. The two portions of solid magnesium lactate may then be subjected to an acidification step, as will be discussed below.

[0061] The next step of the integrated process according to the present invention is to subject 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. There are a variety of ways to achieve this step. The acidification step is typically performed by contacting the lactate with an acidic HCl solution. However, in some embodiments, the lactate may also be contacted with gaseous HCl.

[0062] The lactate can be in solid and / or dissolved form. In one embodiment, the lactate is provided in solid form. This solid magnesium lactate can also be in the form of a suspension or slurry. Or it can be in the form of a wet cake obtained after, for example, a filtering or centrifuging step, wherein the filter cake is optionally rediluted or re-slurried. In this case, the acidification step is carried out by contacting the lactate with an acidic solution. The advantage of preparing an aqueous mixture from a lactate in solid form is that a very high lactic acid concentration can be obtained, for example a concentration of at least 15 wt.%, particularly at least 25 wt.%, for example at most 50 wt.%, or at most, for example, 40 wt.%.

[0063] The lactate may also be in dissolved form, typically as part of an aqueous solution. In this case, the acidification step may be performed by contacting the lactate with an acidic solution or an acidic gas.

[0064] The acidification step can also be carried out with a mixture of lactic acid and lactate. Such a mixture can be obtained, for example, in a low pH fermentation. The mixture can, for example, be an aqueous suspension.

[0065] When the acidification of the lactate is carried out by contacting the lactate with an acidic HCl solution, it preferably has as high a HCl concentration as possible. Such a high HCl concentration will produce an aqueous mixture with a high lactic acid concentration, which is desirable. Thus, the HCl solution comprises at least 5 wt.%, more preferably at least 10 wt.%, even more preferably at least 20 wt.% HCl, based on the total weight of the HCl solution.

[0066] Acidification is usually carried out using an excess of HCl. Preferably the excess is small so that the aqueous mixture obtained is not highly acidic, which may be undesirable in view of the need to further process the mixture. For example, the use of an excess of acid can result in a pH value of 2 or less, preferably a pH value of 0-1, for the aqueous mixture obtained.

[0067] In the case of using gaseous HCl, the contacting can be carried out by contacting it with the lactate solution or suspension. In particular, the HCl gas can be blown through the solution or suspension.

[0068] Preferably, the acidification is carried out at a temperature of 75° C. or lower. At higher temperatures, it becomes uneconomical to subject the equipment to the harsh conditions of a high temperature acidic environment.

[0069] The acidification step results in the formation of an aqueous liquid comprising lactic acid and magnesium chloride. The aqueous liquid is subjected to a separation step, optionally after having been subjected to an intermediate treatment step such as a concentration step. Since the lactic acid will be dissolved in the aqueous liquid, any suitable separation technique may be used for separation, including extraction with a suitable extractant, membrane separation, ion exchange, ion absorption or adsorption.

[0070] In the case of extraction, it is important that the extractant, also referred to as extraction reagent, is substantially immiscible with water. The use of an extractant results in the formation of a two-phase system during the separation step, which comprises an organic liquid layer comprising the extraction reagent and lactic acid and an aqueous layer of magnesium chloride solution containing small amounts of lactic acid as a contaminant.

[0071] Examples of suitable extractants are aliphatic and aromatic hydrocarbons, such as alkanes and aromatic compounds, ketones and ethers. Mixtures of various compounds may also be used. Examples of suitable aliphatic alkanes are C5-C10 straight-chain, branched or cyclic alkanes, such as octane, hexane, cyclohexane, 2-ethyl-hexane and heptane. Examples of suitable aromatic compounds are C6-C10 aromatic compounds, such as toluene, xylene and ethylbenzene. Examples of suitable ketones are C5+ ketones, more particularly C5-C8 ketones in the present invention. C5+ represents a ketone having at least 5 carbon atoms. It is less preferred to use C9+ ketones. The use of methyl isobutyl ketone (MIBK) has been found to be particularly attractive. Examples of suitable ethers are C3-C6 ethers, such as methyl tert-butyl ether (MTBE) and diethyl ether (DEE).

[0072] The organic layer and the aqueous layer can be separated using conventional liquid-liquid separation methods, such as decantation, sedimentation, centrifugation, use of a plate separator, use of a coalescer, and use of a hydrocyclone. A combination of different methods and equipment can also be used.

[0073] The organic layer is a solution of lactic acid in an organic extractant. Lactic acid can be separated from the extractant as desired. In one embodiment, this can be accomplished by removing the extractant by evaporation. In another embodiment, the carboxylic acid can be recovered from the extractant by extraction with water or another aqueous liquid.

[0074] Lactic acid can be processed as required. Examples of further processing steps are purification steps, such as one or more of washing, activated carbon treatment, recrystallization, distillation and filtration. In the case where the carboxylic acid is lactic acid, it can be converted into lactide and polylactic acid (PLA). Methods for carrying out the various steps are known to those skilled in the art. The invention also relates to lactic acid, lactide and polylactic acid obtainable or obtained by the methods described herein.

[0075] As described above, the magnesium chloride solution containing lactic acid is subjected to the lactic acid removal step of the present invention.

[0076] The magnesium chloride solution from which lactic acid has been removed can be subjected to a thermal decomposition step in a thermal hydrolysis reactor, wherein magnesium chloride reacts with water to form magnesium oxide and HCl. Suitable apparatus for carrying out the thermal hydrolysis step (also referred to herein as the thermal decomposition step) are known in the art. For example, a spray roaster or a fluidized bed roaster can be used. Such equipment can be obtained, for example, at SMS Siemag, Andritz, Tenova and / or John Cockerill. A spray roaster is preferably used. The energy cost of the spray roaster is lower (also compared to the fluidized bed roaster) because it requires a relatively low temperature (as described below). It has further been found that the spray roaster can produce reactive MgO particles that are very suitable for use as a neutralizing agent in fermentation. The thermal decomposition is carried out at a temperature of at least 300° C., which is the minimum temperature at which MgCl2 decomposes. Preferably, the thermal decomposition is carried out at a temperature of at least 350° C. Due to energy costs, the temperature is preferably below 1000° C., more preferably below 800° C., and even more preferably below 600° C. Furthermore, it is undesirable to use too high a temperature in the thermal decomposition step, since it reduces the reactivity of the MgO formed, making it less suitable for use as a neutralizing agent in fermentation. For example, the temperature at which the thermal decomposition is carried out may be 350-600° C. or 400-500° C. The temperatures mentioned are the temperatures at which the gases are discharged from the apparatus.

[0077] The thermal decomposition applied in the present invention is preferably carried out at a pressure of 0.1-10 bar. However, the use of high pressure may be undesirable because the risk of corrosion of downstream equipment is increased due to the inability of HCl to condense. Preferably, the thermal decomposition is carried out at atmospheric pressure, especially when a roaster is used, to avoid unnecessary energy costs and the need for expensive high-pressure equipment. Pressures in the range of 0.9-1 bar may be preferred to prevent the discharge of HCl.

[0078] As will be clear to those skilled in the art, preferences for various aspects of the present invention may be combined unless they are mutually exclusive.

[0079] Figure 1 An embodiment of a method for producing lactic acid according to the present invention is described. Figure 1 In the fermentation step, the fermentation step is carried out in a fermentation reactor (1), which is provided with a carbon source and optionally other components, such as nutrients, via a pipeline not shown. In the fermentation step, the carbon source is fermented by microorganisms in a fermentation broth to form lactic acid carboxylic acid and at least part of the lactic acid is neutralized by adding a magnesium base, thereby obtaining magnesium lactate. The magnesium base is added via pipeline (14). The fermentation broth containing the magnesium lactate salt is provided to an acidification step (3) via pipeline (2). Intermediate steps, such as biomass removal, separation or concentration of solid products, may be performed, but are not shown. In the acidification step (3), the magnesium lactate is contacted with HCl in an aqueous environment to form an aqueous mixture containing lactic acid and magnesium chloride.

[0080] An aqueous mixture comprising carboxylic acid and magnesium chloride is provided to a separation step (5) via line (4). The separation step may be performed by extraction as described above. The separation step (5) produces an effluent comprising lactic acid and a magnesium chloride solution comprising lactic acid. The product lactic acid is removed via line (6), typically in the form of a lactic acid solution in an extractant. The lactic acid may be recovered from the extractant as described above and further processed as described above (not shown). The magnesium chloride solution containing lactic acid is removed via line (7) and provided to an evaporation step (8). In the evaporation step 8, water is evaporated, thereby forming a slurry of MgCl2.MgL2.4H2O in a magnesium chloride solution. The slurry is provided to a separation step (10) via line (9), wherein solid MgCl2.MgL2.4H2O is separated from the magnesium chloride solution. Solid MgCl2.MgL2.4H2O is removed via line (11) and recycled to the acidification step (3). The magnesium chloride solution is removed via line (12) and provided to a thermal decomposition step (13). In the thermal decomposition step, the magnesium chloride solution is converted into solid MgO, which can be supplied to the fermentation step as is via line (14) or after being converted into Mg(OH)2 by reaction with water. The thermal decomposition step also produces HCl, which can be supplied to the acidification step in gaseous form via line (15) or after being absorbed in an aqueous liquid in an absorption step (not shown).

[0081] As will be clear to the skilled person, preferences for various aspects of the invention may be combined unless they are mutually exclusive. In particular, the preferences described with respect to the process for removing lactic acid from a magnesium chloride solution containing lactic acid also apply to the corresponding steps in the process for producing lactic acid.

[0082] The present invention is further illustrated by the following examples, but is not limited thereto or thereby. Example

[0083] Embodiment 1:

[0084] A ternary solid-liquid phase diagram of the magnesium chloride-lactic acid-water system at 20°C, 50°C and 80°C was obtained. The diagram was constructed by measuring the solubility of a mixture of magnesium chloride hexahydrate (MgCl2.6H2O), twin (S)-lactic acid crystals and demineralized water. The solubility line was determined by adding an appropriate amount of the chemical to a glass container, heating the system to a suitable 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 for at least 10 minutes.

[0085] When solids are observed to not dissolve, add a small amount of water and then stir for at least 10 minutes. Repeat this operation until all solids are dissolved. Be careful to allow additional dissolution time close to the dissolution point.

[0086] After the dissolution point is determined, the composition is back-calculated from the initial mass and the water / chemicals added. The result is a ternary solid-liquid phase diagram for the magnesium chloride-lactic acid-water system, such as Figure 2 shown.

[0087] The crystals formed during the above experiments were examined, and magnesium chloride hydrate crystals were only observed in the sample without lactic acid. After adding lactic acid, the solubility of magnesium chloride crystals decreased significantly, and needle-shaped crystals were formed.

[0088] Needle-shaped crystals were separated by filtration (each sample was filtered twice), and the filter cake was washed with ethanol to remove residual liquid material and dried at room temperature. The solids obtained were analyzed to determine their composition. Lactate / lactic acid content was determined by HPLC. Organic acids were quantified by HPLC, and the column of HPLC had a stationary phase containing a strong cation exchange resin (Bio-Rad AminexHPX-87C, 300x 7.8mm, 85°C) in the form of calcium and an eluent (pH=2.2) containing 3mM Ca(H2PO4)2·H2O and 0.015M H3PO4. The UV3000 system was used as a detector. The water content was determined using the Karl Fisher titration procedure (KF titration) widely described in the manual and literature. Imidazole was used as a base (Hydranal composite material). Magnesium was determined by atomic absorption spectroscopy (AAS) on a flame AAS Varian SpectrAA300 (including a PC with SpectrAA software). Solid crystals were dissolved in a nitric acid solution. Chloride content was determined by titration with silver nitrate, whereby silver chloride was quantitatively precipitated. The endpoint was determined potentiometrically using a silver ring electrode. The results confirmed that the composition of the crystals matched that of MgCl2.MgL2.4H2O.

[0089] Embodiment 2:

[0090] A magnesium chloride solution containing lactic acid having a magnesium chloride concentration of 18.2 wt.% and a lactic acid concentration of 2.3 wt.% is prepared by mixing appropriate amounts of magnesium chloride hexahydrate (MgCl2.6H2O), twin (S)-lactic acid crystals and demineralized water. The solution is subjected to a first evaporation step, wherein water is evaporated at a temperature of 65-80°C and a pressure of 0.35 bar to form a magnesium chloride solution containing lactic acid having a magnesium chloride concentration of 28 wt.% and a lactic acid concentration of 3.5 wt.%. The solution is further evaporated at a temperature of 80°C and a pressure of 0.35 bar to form a slurry of solid MgCl2.MgL2.4H2O in a magnesium chloride solution (9 wt.%), wherein the magnesium chloride concentration is 32 wt.% of magnesium chloride.

[0091] The slurry was subjected to a centrifugation step and the filter cake obtained was washed with methanol (3 times the volume of the filter cake) and subsequently dried at 70°C for 1 hour. The separated filter cake contained many needle-shaped crystals, with a solid content of more than 90 wt.%, and a lactic acid concentration of less than 0.4 wt.%. The ratio of the weight of the dry filter cake to the weight of the suspension was about 9%. The amount of magnesium (w%, AAS), lactic acid (w%, HPLC as lactic acid) and chlorine (w%, titration) contained in the dry filter cake was analyzed using the method described in Example 1, and it was found that the amounts matched the theoretical values ​​of MgCl2.MgL2.4H2O. This also proves that the concentration of the double salt obtained after the evaporative crystallization step is about 9%.

Claims

1. A method for removing lactic acid from a magnesium chloride aqueous solution containing lactic acid, wherein the weight ratio of magnesium chloride to lactic acid in the magnesium chloride aqueous solution containing lactic acid is at least 1:1, the method comprising the following steps: - subjecting the aqueous magnesium chloride solution containing lactic acid to an evaporation step, thereby forming a slurry of MgCl2.MgL2.4H2O in the aqueous magnesium chloride solution, - Subjecting the slurry to a solid-liquid separation step to separate solid MgCl2.MgL2.4H2O from the aqueous magnesium chloride solution, thereby removing lactic acid in the form of MgCl2.MgL2.4H2O from the aqueous magnesium chloride solution containing lactic acid.

2. The method of claim 1, wherein the weight ratio of magnesium chloride to lactic acid is 1.5:1 to 70:

1.

3. The method according to claim 1 or 2, wherein the magnesium chloride solution containing lactic acid has a magnesium chloride concentration of 5-35 wt.%.

4. The process according to claim 1 or 2, wherein the evaporation step is carried out in one or more steps at a temperature in the range of 50-200°C and / or under reduced pressure.

5. A process according to claim 1 or 2, wherein evaporation is continued until the magnesium chloride concentration of the solution is in the range of 30-47 wt.%.

6. The method of claim 1 or 2, wherein the slurry of MgCl2.MgL2.4H2O in magnesium chloride solution contains 2 wt.% to 50 wt.% MgCl2.MgL2.4H2O.

7. The process according to claim 1 or 2, wherein the magnesium chloride concentration of the magnesium chloride solution obtained from the separation step is in the range of 35-47 wt.%.

8. The process according to claim 1 or 2, wherein the magnesium chloride solution obtained from the separation step has a lactic acid concentration of at most 1 wt.%.

9. The method according to claim 1 or 2, wherein a magnesium chloride solution containing lactic acid having 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 to obtain a slurry of MgCl2.MgL2.4H2O in a magnesium chloride solution, wherein the slurry has a MgCl2.MgL2.4H2O concentration of 4-40 wt.% and a magnesium chloride concentration of 35-47 wt.%; and the slurry is subjected to a solid-liquid separation step to obtain solid MgCl2.MgL2.4H2O and a magnesium chloride solution having a lactic acid concentration of less than 0.5 wt.%.

10. The method of claim 2, wherein the weight ratio of magnesium chloride to lactic acid is 2:1 to 50:

1.

11. The method of claim 2, wherein the weight ratio of magnesium chloride to lactic acid is 4:1 to 40:

1.

12. The method of claim 2, wherein the weight ratio of magnesium chloride to lactic acid is 4:1 to 20:

1.

13. The method according to claim 3, wherein the magnesium chloride solution containing lactic acid has a magnesium chloride concentration of 10-35 wt.%.

14. The method according to claim 3, wherein the magnesium chloride solution containing lactic acid has a magnesium chloride concentration of 15-35 wt.%.

15. The method according to claim 4, wherein the evaporation step is performed at a temperature of 80-150°C and / or at a pressure of 0.01-0.9 bar.

16. The method of claim 4, wherein the evaporation step is performed at a pressure of 0.01-0.35 bar.

17. The method of claim 5, wherein evaporation is continued until the magnesium chloride concentration of the solution is in the range of 38-45 wt.%.

18. The method of claim 6, wherein the slurry of MgCl2.MgL2.4H2O in magnesium chloride solution comprises 4 to 40 wt.% MgCl2.MgL2.4H2O.

19. The method of claim 6, wherein the slurry of MgCl2.MgL2.4H2O in magnesium chloride solution comprises 4 to 30 wt.% MgCl2.MgL2.4H2O.

20. The method of claim 6, wherein the slurry of MgCl2.MgL2.4H2O in magnesium chloride solution comprises 2 wt.% to 20 wt.% MgCl2.MgL2.4H2O.

21. The method according to claim 7, wherein the magnesium chloride solution obtained from the separation step has a magnesium chloride concentration of 37 wt.% to 47 wt.%.

22. The method according to claim 7, wherein the magnesium chloride solution obtained from the separation step has a magnesium chloride concentration of 39 wt.% to 45 wt.%.

23. The process of claim 8, wherein the magnesium chloride solution obtained from the separation step has a lactic acid concentration of at most 0.5 wt.%.

24. The process of claim 8, wherein the magnesium chloride solution obtained from the separation step has a lactic acid concentration of at most 0.2 wt.%.

25. A method for producing lactic acid, comprising the steps of: - subjecting the carbon source to a fermentation step to form lactic acid, the fermentation step comprising the step of subjecting the carbon source to fermentation by a microorganism in a fermentation medium to form lactic acid, - magnesium lactate is obtained by adding a magnesium base selected from magnesium oxide and magnesium hydroxide to the fermentation medium to neutralize at least part of the lactic acid, - 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 magnesium chloride solution containing lactic acid, and recovering the effluent comprising lactic acid from the process, - subjecting the aqueous magnesium chloride solution containing lactic acid, wherein the weight ratio of magnesium chloride to lactic acid is at least 1:1, to an evaporation step, thereby forming a slurry of MgCl2.MgL2.4H2O in the aqueous magnesium chloride solution, - Subjecting the slurry to a solid-liquid separation step to separate solid MgCl2.MgL2.4H2O from the aqueous magnesium chloride solution.

26. The process according to claim 25, wherein the solid Mg MgCl2.MgL2.4H2O is at least partially recycled to the acidification step.

27. The process according to claim 25 or 26, wherein the magnesium chloride solution originating from the solid-liquid separation step is at least partly supplied to a thermal decomposition step, wherein the magnesium chloride reacts with water to form solid magnesium oxide and gaseous HCl, wherein the solid MgO is at least partly recycled to the fermentation step as such or after conversion into magnesium hydroxide.

28. The method according to claim 25 or 26, wherein the lactic acid separated from the magnesium chloride solution containing lactic acid is subjected to one or more steps selected from separation from an extract and purification.

29. The method of claim 25 or 26, wherein lactic acid is converted to lactide or polylactic acid.

30. The process of claim 27, wherein the gaseous HCl is at least partially recycled to the acidification step as such or after being dissolved in water to form an aqueous HCl solution.

31. The method according to claim 28, wherein the lactic acid separated from the magnesium chloride solution containing lactic acid is subjected to one or more purification steps selected from washing, activated carbon treatment, recrystallization, distillation and filtration.

32. Lactic acid obtained by the method of any one of claims 25-28.

33. Lactide or polylactic acid obtained by the method of claim 29.

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