Method for preparing aqueous lactate solution

Abstract

In order to provide a method for producing a lactate solution which results in an economically attractive method for producing lactic acid, solid magnesium lactate or an aqueous suspension of magnesium lactate is reacted with KOH when the lactate solution is subjected to water-splitting electrodialysis to form a first suspension of solid magnesium hydroxide in a first potassium lactate solution comprising magnesium ions, and a second suspension of solid magnesium hydroxide in a second potassium lactate solution comprising magnesium ions. The amount of excess OH ions in the first suspension ranges from 20 ppm to 150 ppm. After solid-liquid separation, the first potassium lactate solution is subjected to a further reaction with KOH to form a second suspension of solid magnesium hydroxide in a second potassium lactate solution, the amount of excess OH ions in the second suspension being greater than 300 ppm. And separating the second suspension, wherein the magnesium ion content of the second potassium lactate solution is less than 50 ppm.

Description

[0001] The present invention relates to a method for preparing an aqueous lactate solution from a medium containing magnesium lactate. The invention also relates to the further processing of this solution to produce lactic acid.

[0002] Lactic acid (LA), also known as 2-hydroxypropionic acid, has many commercial applications, including use in the food industry and as a monomer for making biodegradable and / or renewable polymers.

[0003] Most commercial methods for producing lactic acid are based on the fermentation of carbohydrates by microorganisms. These methods require strict control of temperature and pH. A common feature of all fermentation processes is the need to neutralize the acids secreted by the microorganisms during the process. Depending on the microorganisms used in the process, a drop in pH below a critical value can impair the microorganism's metabolic processes and halt the fermentation process. Therefore, fermentation processes typically result in the formation of lactic acid in the form of salts, i.e., salts of bases that are added to maintain the pH of the fermentation medium within the pH range acceptable to the microorganism.

[0004] Despite numerous efforts in the field, producing high-purity lactic acid from aqueous media containing lactate salts remains a challenge, where the challenge lies not only in achieving high purity, but also in doing so reliably and consistently in an energy-efficient manner. The latter, among other characteristics, is required to make lactic acid an attractive monomer for polymer manufacturing.

[0005] An example of a method for producing lactic acid is described in WO98 / 22611. This patent publication describes a method for producing lactic acid by fermenting lactic acid, adding an alkaline earth metal base (e.g., a calcium base) to form an alkaline earth metal lactate (e.g., calcium lactate), removing biomass, reacting the alkaline earth metal base with an ammonium source to form ammonium lactate, and recovering lactic acid therefrom by salt decomposition electrodialysis.

[0006] WO2005 / 123647 describes a method for producing lactic acid from a culture medium containing magnesium lactate, which can be provided via fermentation. Magnesium lactate reacts with sodium, potassium, calcium, and / or ammonium hydroxides to form magnesium hydroxide and the corresponding monovalent and / or divalent lactate salts. The document suggests bipolar electrodialysis or the addition of a strong mineral acid as methods for converting lactate salts into lactic acid. However, such conversions are not included in the methods exemplified in this document.

[0007] WO2011 / 095631 describes a method for preparing lactic acid, comprising the following steps: a) providing an aqueous medium containing magnesium lactate; b) adding a monovalent base to the aqueous medium containing magnesium lactate to form an aqueous medium containing a water-soluble monovalent lactate and a solid magnesium base; c) separating the aqueous medium containing the water-soluble monovalent lactate and the magnesium base; d) adjusting the concentration of the monovalent lactate in the aqueous medium to a value between 10 wt.% and 30 wt.%; e) subjecting the aqueous medium containing the monovalent lactate to water-splitting electrodialysis to produce a first solution containing the monovalent base and a second solution containing lactic acid and the monovalent lactate, the electrodialysis being carried out to a partial conversion of 40 mol % to 98 mol %; f) separating the second solution containing lactic acid and the monovalent lactate into lactic acid and a solution containing the monovalent lactate by gas-liquid separation; g) recycling the solution containing the monovalent lactate of step f) to step d).

[0008] It has been found that although water-splitting electrodialysis is in principle an attractive method for producing high-purity lactic acid, it is difficult to carry out in an economically attractive manner due to its sensitivity and inherent energy requirements. This is especially true if the starting material is a lactate solution derived from a fermentation process, which is associated with the possible presence of a variety of possible organic and inorganic contaminants, such as divalent cations, that may interfere with water-splitting electrodialysis.

[0009] There is a need in the art for a method of making a lactate solution which, when subjected to water-splitting electrodialysis, results in an economically attractive method for producing lactic acid. The present invention provides such a method.

[0010] The present invention relates to a method for preparing a potassium lactate aqueous solution having a magnesium ion content of less than 50 ppm, the method comprising the following steps:

[0011] - in a first reaction step, solid magnesium lactate in an aqueous magnesium lactate suspension is reacted with KOH to form a first suspension of solid magnesium hydroxide in a first potassium lactate solution containing magnesium ions, the reaction taking place under conditions where the amount of excess OH ions in the suspension is in the range of 20 ppm to 300 ppm,

[0012] - subjecting the first suspension to a solid-liquid separation step to separate the first potassium lactate solution from the solid magnesium hydroxide,

[0013] - in a second reaction step, the first potassium lactate solution resulting from the solid-liquid separation step is further reacted with KOH to form a second suspension of solid magnesium hydroxide in the second potassium lactate solution, the reaction occurring under conditions where the amount of excess OH ions in the suspension is greater than 300 ppm,

[0014] - subjecting the second suspension to a solid-liquid separation step to separate a second potassium lactate solution from solid magnesium hydroxide, wherein the second potassium lactate solution has a magnesium ion content of less than 50 ppm.

[0015] The present invention relies on a combination of several key features.

[0016] A first key feature of the process according to the present invention is that it is intended to produce a potassium lactate solution rather than a sodium lactate solution. Potassium lactate solutions have been found to be more attractive for processing in water-splitting electrodialysis than sodium lactate solutions. In chemistry, sodium salts are generally the preferred alkali metal salts for reasons of cost, safety, and availability.

[0017] Another key feature of the process is that the excess OH ions in the first reaction step is 20 ppm to 150 ppm, the excess OH ions in the second reaction step is greater than 300 ppm, and the two reaction steps are combined by intermediate product separation. It has been found that this combination of features produces a final KOH solution with a very low content of divalent alkaline earth metal components (particularly magnesium and calcium), while ensuring that magnesium hydroxide can be recovered from the process in an efficient and cost-effective manner.

[0018] The method according to the invention can be used in particular for the production of lactic acid. Therefore, the present invention also relates to the use of an aqueous potassium lactate solution having a magnesium content of less than 50 ppm obtained according to any of the preceding claims in the production of lactic acid.

[0019] Finally, the present invention relates to a method for producing lactic acid comprising the following steps:

[0020] - preparing an aqueous potassium lactate solution according to the invention having a magnesium content of less than 50 ppm,

[0021] - an aqueous potassium lactate solution having a magnesium content of less than 50 ppm is supplied to a reaction step in which the potassium lactate is converted into lactic acid, for example by reacting the potassium lactate with a strong mineral acid or by electrodialysis.

[0022] The present invention will be discussed in more detail below.

[0023] The starting material in the method according to the invention is solid magnesium lactate. The magnesium lactate can be provided in the form of a slurry, but it is also conceivable to provide it in solid form, for example in the form of a filter cake or in the form of dried crystals. The solid magnesium lactate can be suspended in water, wherein a portion of the magnesium lactate will dissolve in the water and a portion will remain solid. Alternatively, the solid magnesium lactate can also be suspended in the potassium lactate solution obtained from the first filtration or the second filtration.

[0024] Both forms (magnesium lactate in solid form or magnesium lactate in aqueous suspension form) will hereinafter be referred to as "magnesium lactate".

[0025] Magnesium lactate is combined with KOH to form a reaction medium. For ease of processing, KOH is typically provided in the form of a solution. The concentration of the KOH solution will depend on the manner in which the magnesium lactate is provided. If the magnesium lactate is provided in the form of a filter cake or dried crystals, a more dilute KOH solution may be attractive. On the other hand, if the magnesium lactate is provided in the form of a relatively dilute slurry, a highly concentrated KOH solution will ideally be used. Alternatively, KOH can be provided in a solid form, such as pellets. The reaction of magnesium lactate with KOH then produces a first suspension of solid magnesium hydroxide in a first potassium lactate solution.

[0026] In one embodiment, the aqueous magnesium lactate suspension has a magnesium lactate content of at least 15 wt.% (calculated as anhydrous magnesium lactate). Preferably, the magnesium lactate content is at least 20 wt.%, and in particular at least 25 wt.%. Preferably, the magnesium lactate content of the aqueous magnesium lactate suspension (calculated as anhydrous magnesium lactate) is at most 50 wt.% of the total weight of the suspension.

[0027] In one embodiment, the solid magnesium lactate comprises at least 60 wt.%, preferably at least 70 wt.%, more preferably at least 80 wt.%, and especially at least 85 wt.% of magnesium lactate (calculated as anhydrous magnesium lactate) based on the total weight of the solid.

[0028] Preferably, the first reaction step is carried out under mixing. In this way, local hydroxide concentrations in the suspension that are too high can be avoided.

[0029] In a first reaction step, magnesium lactate reacts with KOH to form a first suspension of solid magnesium hydroxide in a first potassium lactate solution. The amount of KOH provided is such that the amount of excess OH ions in the first suspension is in the range of 20 ppm to 150 ppm. It has been found that selecting the amount of excess OH ions within a specified range in this reaction step ensures that crystals with good filterability are obtained. Such crystals can be easily removed from the potassium lactate solution without, for example, having to use oversized separation equipment and / or using large amounts of water to wash the potassium lactate from the Mg(OH)2 crystals.

[0030] Preferably, the amount of excess OH ions in the first suspension is at most 125 ppm, more preferably at most 100 ppm, in some embodiments at most 70 ppm. Additionally or in combination, the excess OH ions may preferably be at least 25 ppm, particularly at least 30 ppm, and more particularly at least 35 ppm.

[0031] The excess of OH ions in the solution is determined by potentiometric titration as follows: The liquid portion of the suspension is removed by using a syringe equipped with a 0.45 μm filter to remove any residual solids in the sample. 5 g-10 g of the filtered sample is weighed to the nearest 0.0001 g and placed in a titration vessel. 70 mL of milli-Q water is added to the sample. The sample is titrated with 0.01 M hydrochloric acid and the potential difference is measured at different volumes of hydrochloric acid added. The first equivalence point is determined by finding the maximum slope of the titration curve. The hydroxide concentration in the sample is then calculated using the following formula:

[0032]

[0033] Preferably, the vast majority of the magnesium present in the system is recovered from the process after the first reaction step. In particular, it is preferred that in the first reaction step, at least 75 wt.%, particularly at least 85 wt.%, more particularly at least 90 wt.%, still more particularly at least 95 wt.%, and even more particularly at least 98 wt.% of the total amount of magnesium lactate in the starting aqueous magnesium lactate suspension is converted into potassium lactate and magnesium hydroxide. This conversion can be achieved by controlling the pH and achieving an excess OH ion concentration as described in the current embodiment. The reaction time is typically 30 to 60 minutes, and at least 10 minutes.

[0034] The conversion rate can be calculated based on the amount of magnesium lactate present in the recovered solid magnesium hydroxide. This value can be determined based on the amount of magnesium lactate provided in the first reaction step and the amount of residual magnesium lactate present in the Mg(OH) filter cake recovered after separation of the first potassium lactate solution from the solid magnesium hydroxide.

[0035] The suspension of magnesium hydroxide in potassium lactate solution is subjected to a solid-liquid separation step to separate the first potassium lactate solution from the solid magnesium hydroxide. The solid-liquid separation step can be carried out in a manner known in the art. Because the magnesium hydroxide product has good filtering characteristics, conventional filtration using vacuum, high pressure and centrifugal force (e.g., horizontal belt filter, pressure belt filter or scraper discharge centrifuge) will be suitable for separating the first potassium lactate solution from the solid magnesium hydroxide.

[0036] An excess OH ion amount of 20 ppm to 150 ppm in the first suspension is suitable for providing crystals with good filterability, but will also produce a first potassium lactate solution containing residual magnesium ions. To make the lactate solution more suitable for electrodialysis, the first potassium lactate solution is subjected to a further reaction with KOH to form a second suspension of solid magnesium hydroxide in a second potassium lactate solution. It has been found that when the amount of excess OH ions in the second suspension is greater than 300 ppm, a second potassium lactate solution having a magnesium ion content of less than 50 ppm is obtained. The second potassium lactate solution is separated from the solid magnesium hydroxide by a solid-liquid separation step, thereby providing a lactate solution more suitable for electrodialysis, resulting in a process for producing lactic acid with less downtime and, therefore, a more economically attractive process for producing lactic acid.

[0037] Preferably, in the second reaction step, the excess OH ion content in the second suspension is greater than 400 ppm, particularly greater than 500 ppm, more particularly greater than 650 ppm, and typically greater than 800 ppm. In this way, a second potassium lactate solution having a lower magnesium ion content is obtained. Preferably, the excess OH ion content in the second suspension in the second reaction step is at most 10,000 ppm, because the magnesium hydroxide crystals obtained at relatively high levels of excess OH ion have lower filterability and are therefore more difficult to remove from the suspension by filtration. With excess OH ion concentrations above this level, the magnesium ion content in the second potassium solution is hardly further reduced.

[0038] In one embodiment, the difference between the excess OH ions in the first suspension in the first reaction step and the excess OH ions in the second suspension in the second reaction step is at least 100 ppm, particularly at least 200 ppm, more particularly at least 300 ppm, more particularly at least 400 ppm, and at most 1200 ppm, particularly at most 1000 ppm, more particularly at most 850 ppm, further particularly at most 700 ppm, even more particularly at most 10000 ppm. In this way, the higher excess of OH ions in the second suspension in the second reaction step compared to the suspension in the first reaction step leads to a greater reduction in the magnesium ion content in the first potassium lactate solution. Thus, a second potassium lactate solution with a magnesium ion content of less than 50 ppm is obtained.

[0039] The OH ion concentration in the first suspension in the first reaction step and the second suspension in the second reaction step depends on both the temperature and pH of the respective suspensions. At higher temperatures, water ionization is enhanced, thereby increasing the OH ion concentration and the pH. On the other hand, adding OH ions to the suspension in the form of a base also increases the OH ion concentration and the pH. Therefore, by carrying out the second reaction step at a temperature higher than the temperature at which the first reaction step was carried out, by carrying out the second reaction step at a pH higher than the pH of the first reaction step, or by carrying out the second reaction step at a temperature and pH higher than both the temperature and pH of the first reaction step, a higher excess of OH ions in the second suspension than in the first suspension can be achieved.

[0040] In order to achieve a difference of at least 100 ppm between the excess OH ions in the first reaction step and the excess OH ions in the second reaction step, it is preferred that the second reaction step is carried out at a temperature of at least 20° C. higher than the temperature at which the first reaction step is carried out and / or the second reaction step is carried out at a pH of at least 0.5 higher than the pH at which the first reaction step is carried out.

[0041] In one embodiment, the second reaction step is carried out at a temperature that is at least 25° C. and at most 75° C. higher than the temperature at which the first reaction step is carried out. In this way, the excess OH ions in the second reaction is increased compared to the first reaction step, and a second potassium lactate solution having a magnesium ion content of less than 50 ppm is obtained.

[0042] Preferably, the first reaction step and the second reaction step are carried out at a temperature of 20° C. to 100° C., particularly 25° C. to 90° C., and more particularly 30° C. to 80° C., the temperatures in the two steps being selected independently. Preferably, the first reaction step is carried out at a temperature of 20° C. to 70° C., and the second reaction step is carried out at a temperature of 40° C. to 90° C. If the temperatures of the first reaction step and the second reaction step are selected such that the difference in excess OH ions between the first reaction step and the second reaction step is less than a desired value, the difference in excess OH ions can be increased by adding a base in the second reaction step.

[0043] In one embodiment, the pH in the second reaction step is at least 0.7, in particular at least 0.8, higher than the pH in the first reaction step. The pH in the second reaction step is typically at most 2.5, in particular at most 2, higher than the pH in the first reaction step. Preferably, the first and second reaction steps are performed to obtain a pH of 9.5-14, in particular 10-12.5, in the steps, the pH in the two steps being selected independently. If the pH of the first and second reaction steps is selected such that the difference in excess OH ions between the first and second reaction steps is less than the desired value, the difference in excess OH ions can be increased by performing the second reaction step at a higher temperature than the first reaction step.

[0044] Preferably, the second reaction step is conducted at a temperature that is at least 20°C higher than the temperature at which the first reaction step is conducted, and the pH of the second reaction step is less than 0.5 higher than the pH of the first reaction step. Alternatively, the pH in the second reaction step is at least 0.5 higher than the pH in the first reaction step, and the second reaction step is conducted at a temperature that is no more than 20°C higher than the temperature at which the first reaction step is conducted. In this manner, it is not necessary to significantly increase both the pH and the temperature in the second reaction step compared to the first reaction step in order to increase the excess OH ions in the first reaction step compared to the second reaction step. Since only one of the temperature and the pH is significantly changed between the first and second reaction steps, this creates a more economically attractive way to achieve an increased excess OH ions in the second reaction step compared to the first reaction step.

[0045] Preferably, the magnesium ion content of the second potassium lactate solution is less than 25 ppm, in particular less than 15 ppm, more in particular less than 10 ppm, even more in particular less than 5 ppm, or even less than 2 ppm. In this way, the lactate solution is more suitable for the subsequent electrodialysis step in the production of lactic acid.

[0046] Preferably, the process for preparing an aqueous potassium lactate solution having a magnesium ion content of less than 50 ppm is carried out in a continuous manner.

[0047] Aqueous potassium lactate solutions with a magnesium content of less than 50 ppm are particularly useful in the production of lactic acid. Therefore, the present invention further relates to the use of aqueous potassium lactate solutions with a magnesium content of less than 50 ppm according to the invention as a starting material in the production of lactic acid, and to a method for producing lactic acid.

[0048] Water-splitting electrodialysis is a common way to convert potassium lactate into lactic acid and potassium hydroxide. Water-splitting electrodialysis specifically allows the direct conversion of lactate into lactic acid and base. In this type of electrodialysis, bipolar membranes are typically used to split water into H and KOH, respectively. +And OH, which are combined with the anions and cations of lactate, respectively, to produce separate solutions of lactic acid and alkali. However, the presence of divalent cations (such as magnesium) during the dialysis process can cause membrane fouling and reduce efficacy efficiency. Therefore, it is necessary to regularly clean the electrodialysis equipment, resulting in electrodialysis interruption and reduced lactic acid production efficiency. The potassium lactate aqueous solution with a magnesium content of less than 50ppm according to the present invention is not easy to affect the electrodialysis equipment. Therefore, less equipment maintenance and therefore less electrodialysis interruption are required. Therefore, lactic acid and potassium hydroxide can be produced in an economically more attractive manner.

[0049] In one embodiment, the method for producing lactic acid comprises the following steps:

[0050] - preparing an aqueous potassium lactate solution according to the invention having a magnesium content of less than 50 ppm,

[0051] - an aqueous potassium lactate solution having a magnesium content of less than 50 ppm is supplied to a reaction step in which the potassium lactate is converted into lactic acid and potassium hydroxide by electrodialysis.

[0052] As an alternative to water-splitting electrodialysis, potassium lactate can be converted to lactic acid by reacting it with a strong mineral acid.

[0053] The potassium hydroxide produced by electrodialysis of potassium lactate is preferably recycled to the first reaction step and / or the second reaction step to react with magnesium lactate. In this way, a more cost-effective process is achieved. Since the temperature of potassium hydroxide produced by electrodialysis is generally about 40°C, the first reaction step and the second reaction step are preferably carried out at a temperature of at least 40°C to provide a more cost-effective process.

[0054] Lactic acid is typically produced by fermenting a carbohydrate source into lactic acid in the presence of a microorganism. To neutralize the fermentation medium, an alkali is added to the medium as a neutralizing agent, thereby forming an aqueous fermentation medium containing lactate. Therefore, the method for producing lactic acid preferably includes a step of fermenting the carbohydrate source into lactic acid in the presence of a microorganism, adding magnesium hydroxide as a neutralizing agent to form an aqueous fermentation medium containing magnesium lactate. This aqueous medium can then be used as a basis for preparing the potassium lactate aqueous solution according to the present invention. Optionally, before preparing the potassium lactate aqueous solution, the aqueous medium is subjected to a biomass removal step to substantially remove the microorganisms used in the fermentation process. Optionally, the aqueous medium is concentrated before preparing the potassium lactate aqueous solution to provide a magnesium lactate slurry that can be fed to the first reaction step. Optionally, a liquid-solid separation step is performed to obtain solid magnesium lactate. Optionally, the magnesium lactate dissolved in the aqueous fermentation medium is crystallized to obtain solid magnesium lactate. In this way, the yield of magnesium lactate from the fermentation is increased. The solid magnesium lactate can then be used to prepare an aqueous magnesium lactate suspension.

[0055] Preferably, magnesium hydroxide recovered from the first suspension and / or the second suspension, as obtained by subjecting the first potassium lactate suspension and / or the second potassium lactate suspension to solid-liquid separation, is used as a neutralizing agent in the fermentation step. This results in a more cost-effective method for producing lactic acid.

[0056] Preferably, the magnesium lactate slurry has a magnesium lactate content of at least 30 wt.% based on the total weight of the slurry (in terms of Anhydrous (Calculated as magnesium lactate). This magnesium lactate concentration is advantageous for preparing a potassium lactate solution with a relatively high potassium lactate concentration, which in turn can be used to prepare relatively large amounts of lactic acid. The magnesium lactate content can be achieved, for example, by heating the aqueous culture medium to remove water and concentrating the aqueous culture medium.

[0057] The following examples illustrate the practice of the present invention in some preferred embodiments. Other embodiments within the scope of the claims will be apparent to those skilled in the art. Example

[0058] Examples 1-2 illustrate the effect of a relatively low or relatively high excess of OH ions in both the first and second reaction steps on the filterability of magnesium hydroxide (Mg(OH)2) particles, as well as on the water content, potassium (K) content, and magnesium lactate (Mg) content of the Mg(OH)2 filter cake obtained after the first reaction step. In addition, Examples 3-5 illustrate the effect of a relatively low excess of OH ions in the first step and a relatively high excess of OH ions in the second step on the magnesium ion content of the second potassium lactate (KL) solution.

[0059] In the first reaction step, a 42 wt% to 47 wt% slurry of magnesium lactate (as anhydrous form) was reacted continuously with an 11 wt% to 12 wt% KOH solution in a first reactor at 50° C. and the pH shown in Table 1 below for one hour, thereby forming a first suspension of solid magnesium hydroxide (Mg(OH)2) and a first potassium lactate solution. After a residence time of one hour in the first reactor, the first potassium lactate solution was separated from the Mg(OH)2 by filtration with a mesh size of 7 μm.

[0060] The first potassium lactate solution was then transferred to a second reactor and reacted continuously with an 11 to 12 wt.% KOH solution in a second reaction step at a temperature or pH higher than that of the first reaction step for 0.5 hours, as shown in Table 1 below. The second reaction step resulted in the formation of a second suspension of solid Mg(OH)2 and a second potassium lactate solution. After a 0.5 hour residence time in the second reactor, the first potassium lactate solution was separated from the Mg(OH)2 by filtration through a filter cloth having a mesh size of 1 μm.

[0061] After the first reaction step, the filterability of the Mg(OH)2 granules and the water content, potassium content and magnesium lactate content of the Mg(OH)2 filter cake were determined, as shown in Table 2.

[0062] After the second reaction step, the magnesium ion content of the second potassium lactate solution was determined, as shown in Table 3.

[0063] surface 1.

[0064]

[0065] result

[0066] The results show that when the amount of excess OH ions in the first reactor is relatively high, such as in Example 1, the filterability of Mg(OH)2 formed in the first reaction step is 109 kg dry solids / m 2 h, indicating that the Mg(OH)2 formed under these conditions is relatively difficult to separate from the potassium lactate solution.

[0067] For example, as shown in Example 2, when the amount of excess OH ions in the first reactor and the second reactor is less than 100 ppm, the filterability of Mg(OH)2 formed in the first reaction step is higher than 150 kg dry solids / m 2 h, while the second potassium lactate solution contained more than 50 ppm of Mg.

[0068] However, when the amount of excess OH ions in the first reactor is less than 150 ppm, as in Examples 3-5, the filterability of Mg(OH)2 formed in the first reaction step is >150 kg dry solids / m 2 h, indicating that the Mg(OH)2 formed in the first reaction step can be relatively easily separated from the potassium lactate solution. Since the first potassium lactate solution needs to be separated from the Mg(OH)2 before being sent to the second reactor to produce the second potassium lactate solution with a magnesium ion content of less than 50 ppm, the relatively easy separation of the first potassium lactate solution from the Mg(OH)2 after the first reactor helps to more efficiently prepare the second potassium lactate solution.

[0069] Furthermore, under the conditions of Experiments 3-5, the Mg(OH)2 filter cake contained a relatively low amount of potassium, making the Mg(OH)2 obtained after the second reaction step suitable for supplying to the fermentation step as a neutralizing agent.

[0070] surface 2

[0071]

[0072] Furthermore, as shown in Table 3 below, the second potassium lactate solution obtained in the process under the conditions of Examples 3-5 contained less than 50 ppm of magnesium ions. In fact, when the first reaction step was carried out with an excess of OH ions of less than 150 ppm, and the second reaction step was carried out with an excess of OH ions of greater than 150 ppm, the second potassium lactate solution contained between 0.7 ppm and 3.7 ppm of magnesium ions. These Mg concentrations are significantly lower than the 490 ppm and 100 ppm magnesium ion concentrations obtained in Example 6 of WO2005 / 052800.

[0073] Due to the low magnesium ion concentration, the second potassium lactate solution obtained according to the method of Examples 3-5 is very suitable for electrodialysis to produce lactic acid.

[0074] surface 3.

[0075] Example Mg in KL (ppm) 1 2.1 2 82 3 3.7 4 2.1 5 0.7

[0076] Thus, an excess of OH ions less than 150 ppm in the first reaction step and an excess of OH ions greater than 150 ppm in the second reaction step produce a second potassium lactate solution having a magnesium ion concentration less than 50 ppm, and the process is more efficient because the Mg(OH)2 formed in the first reaction step is more easily separated from the first potassium lactate solution.

Claims

1. A method for preparing an aqueous potassium lactate solution having a magnesium content of less than 50 ppm, the method comprising the following steps: - in a first reaction step, reacting solid magnesium lactate or an aqueous magnesium lactate suspension with KOH to form a first suspension of solid magnesium hydroxide in a first potassium lactate solution comprising magnesium ions, said reaction taking place under conditions where the amount of excess OH ions in the first suspension is in the range of 20 ppm to 150 ppm, - subjecting the first suspension to a solid-liquid separation step in order to separate the first potassium lactate solution from the solid magnesium hydroxide, - in a second reaction step, further reacting the first potassium lactate solution resulting from the solid-liquid separation step with KOH to form a second suspension of solid magnesium hydroxide in a second potassium lactate solution, said reaction occurring under conditions where the amount of excess OH ions in the second suspension is greater than 300 ppm, - subjecting the second suspension to a solid-liquid separation step to separate a second potassium lactate solution from solid magnesium hydroxide, wherein the second potassium lactate solution has a magnesium ion content of less than 50 ppm.

2. The method according to claim 1, wherein The magnesium ion content of the second potassium lactate solution is less than 25 ppm, in particular less than 15 ppm, more in particular less than 10 ppm, still more in particular less than 5 ppm, or even less than 2 ppm.

3. A method according to any one of the preceding claims, wherein The magnesium lactate content of the aqueous magnesium lactate suspension (calculated as anhydrous magnesium lactate) is at least 15 wt.%, preferably at least 20 wt.% and in particular at least 25 wt.%, based on the total weight of the suspension, and the magnesium lactate content of the aqueous magnesium lactate suspension (calculated as anhydrous magnesium lactate) is at most 50 wt.% based on the total weight of the suspension.

4. The method according to claim 1 or 2, wherein: The solid magnesium lactate comprises at least 60 wt.%, preferably at least 70 wt.%, more preferably at least 80 wt.% and in particular at least 85 wt.% of magnesium lactate (calculated as anhydrous magnesium lactate) based on the total weight of the solid.

5. A method according to any one of the preceding claims, wherein In the first reaction step, the excess OH ions in the first suspension is particularly 20 ppm to 125 ppm, more particularly 25 ppm to 100 ppm, still more particularly 30 ppm to 70 ppm.

6. A method according to any one of the preceding claims, wherein In the second reaction step, the excess OH ions in the second suspension is particularly higher than 400 ppm, in particular higher than 500 ppm, more particularly higher than 650 ppm, typically higher than 800 ppm and at most 10 000 ppm.

7. A method according to any one of the preceding claims, wherein - the second reaction step is carried out at a temperature at least 20°C higher than the temperature at which the first reaction step is carried out, or - The pH in the second reaction step is at least 0.5 higher than the pH in the first reaction step.

8. The method according to claim 7, wherein: The second reaction step is carried out at a temperature that is at least 25°C and at most 75°C higher than the temperature at which the first reaction step is carried out.

9. The method according to claim 7, wherein: The pH in the second reaction step is at least 0.7, in particular at least 0.8, higher than the pH in the first reaction step, and wherein the pH in the second reaction step is at most 2.5, in particular at most 2, higher than the pH in the first reaction step.

10. A method according to any one of the preceding claims, wherein The first reaction step and the second reaction step are carried out at a temperature of 20 to 100°C, in particular 20 to 90°C, more in particular 30 to 80°C, the temperatures in the two steps being selected independently.

11. Use of an aqueous potassium lactate solution obtained according to any one of the preceding claims and having a magnesium content of less than 50 ppm in the production of lactic acid.

12. A method for producing lactic acid, comprising the following steps: - an aqueous potassium lactate solution having a magnesium content of less than 50 ppm prepared according to any one of the preceding claims, - providing the potassium lactate aqueous solution having a magnesium content of less than 50 ppm to a reaction step in which the potassium lactate is converted into lactic acid and KOH by water-splitting electrodialysis.

13. The method for producing lactic acid according to claim 12, wherein The KOH obtained after the electrodialysis is recycled to the first reaction step and / or the second reaction step.

14. The method for producing lactic acid according to claim 13, wherein The potassium lactate aqueous solution having a magnesium content of less than 50 ppm is obtained by a method comprising the following steps - fermenting a carbohydrate source to lactic acid in the presence of a microorganism, wherein magnesium hydroxide is added as a neutralizing agent to form an aqueous fermentation medium comprising magnesium lactate, - optionally subjecting said aqueous culture medium to a biomass removal step, - optionally carrying out a concentration step to provide a magnesium lactate slurry, - optionally carrying out a liquid-solid separation step to obtain solid magnesium lactate, - optionally crystallizing the dissolved magnesium lactate to obtain solid magnesium lactate.

15. The method according to claim 13, wherein The method for producing lactic acid comprises the following steps - a step of recycling the magnesium hydroxide obtained from subjecting the first potassium lactate suspension and / or the second potassium lactate suspension to solid-liquid separation to the fermentation of the carbohydrate source.

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