Method for obtaining aromatic dicarboxylic acids

Abstract

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Description

[0001] Process for the production of aromatic dicarboxylic acids

[0002] The present invention relates to a process for obtaining aromatic dicarboxylic acids from an effluent of a depolymerization reaction of polycondensates containing metal carboxylates of the aromatic dicarboxylic acids to be obtained.

[0003] This process is particularly suitable for use in or after processes that process polycondensates such as PET by depolymerization with the addition of metal hydroxides in order to recover the respective monomers of the polycondensates for reuse.

[0004] Polycondensates, which are generally produced directly from basic chemicals of fossil origin such as crude oil and / or natural gas, in particular polyethylene terephthalate plastics (PET), but also poly(ethylene naphthalates), poly(ethylene terephthalate)-poly(ethylene isophthalate) copolymers, poly(butylene terephthalate)-poly(ethylene terephthalate) copolymers with 1,4-cyclohexanedicarboxylate structural units, are used on a large scale, for example as materials for beverage bottles, food packaging, fibers, textiles, automotive components, or clear, opaque or colored bottles of liquid products such as detergents or oils.

[0005] Typically, polycondensates, and especially PET packaging, are mechanically shredded into so-called "flakes," cleaned of impurities, sorted by color, and processed into granules. A portion of the granules can be mixed with virgin material and used to produce new packaging materials. However, the production of new packaging materials exclusively from recycled material is not possible for technical and regulatory reasons.

[0006] Accordingly, for both economic and environmental reasons, it is desirable to recover the starting monomers of the polycondensates for reuse. For this purpose, the products made from these polycondensates or even production waste are recycled, particularly by alkaline depolymerization with the addition of alcohols and metal hydroxides such as sodium, potassium, magnesium, calcium, and / or barium hydroxide.

[0007] US Pat. No. 6,580,005 B1 describes such a recycling process for the recovery of terephthalic acid. In this process, shredded PET is mixed with polyethylene glycol and sodium carbonate to obtain metal carboxylate. This metal carboxylate is converted into terephthalic acid in several steps using sulfuric acid. After depolymerization, the steps of solid-liquid separation to remove impurities, including ethylene glycol, neutralization including crystallization of the terephthalic acid, and purification by washing, solid-liquid separation, and drying follow.

[0008] However, these processes are difficult to process non-segregated waste. The basic depolymerization reaction of non-segregated waste results in a feedstock mixture for subsequent process steps. This mixture contains, in addition to the main component of metal carboxylates in the form of one or more isomers of one or more monomers of the polycondensate to be recycled, residues of the added metal hydroxide, dyes, additives, degradation products, and other contaminants from the production, processing, and use of these polycondensates. Also present are organic solvents and, in particular, the corresponding monomer alcohols as hydrolysis products of the polycondensates, and ultimately also incompletely converted oligomers and polymers of the polycondensates. Due to the complex and varying composition of this feedstock mixture, its processing is difficult.

[0009] Based on this, it is an object of the invention to provide an improved process. Furthermore, it is an object of the invention to provide a process that is particularly suitable for obtaining aromatic dicarboxylic acids from an effluent from a depolymerization reaction of polycondensates comprising an isomer mixture of the aromatic dicarboxylic acids to be obtained.

[0010] This object is achieved by a process according to claim 1, with advantageous developments being specified in the subclaims. According to the invention, a process is provided for obtaining aromatic dicarboxylic acids from an effluent of a depolymerization reaction of polycondensates containing metal carboxylates of the aromatic dicarboxylic acids to be obtained, in which a first aromatic dicarboxylic acid underlying the polycondensate precipitates from this effluent at a first pH value, and after a subsequent solid-liquid separation, a further aromatic dicarboxylic acid underlying the polycondensate precipitates from a liquid phase obtained by the solid-liquid separation at a second pH value, which is lower than the first pH value.

[0011] Furthermore, the invention relates to the use of the above process for producing polycondensates from the obtained aromatic dicarboxylic acids, in particular for producing polyester and / or polyethylene terephthalate plastics.

[0012] In other words, one aspect of the invention is that after the first precipitation of the first aromatic dicarboxylic acid, the further aromatic dicarboxylic acid is precipitated again at a lower pH from the liquid phase obtained after the solid-liquid separation. Due to the two different pH values, in the first precipitation, with a pH value higher than the second, the precipitate - i.e. the solid precipitation product - predominantly contains the less soluble acid isomer of an acid isomer mixture, in the case of PET in particular terephthalic acid (also called para-phthalic acid), and in the second precipitation, with a pH value lower than the first, the precipitate contains larger proportions of the more soluble acid isomer, in the case of PET in particular isophthalic acid (also called mefa-phthalic acid).

[0013] It is therefore preferably also a process for obtaining a dicarboxylic acid isomer from the effluent of the depolymerization reaction of the polycondensates, wherein the effluent comprises the metal carboxylates from an isomer mixture of aromatic dicarboxylic acids. In other words, the process is suitable for separating an acid isomer present as a metal carboxylate from an acid isomer mixture. The acid isomer mixture preferably comprises several isomers of the dicarboxylic acid and particularly preferably terephthalic acid and isophthalic acid. The process according to the invention thus makes it possible to easily obtain a particularly pure product of a single dicarboxylic acid isomer, in particular terephthalic acid, with a simultaneous low loss of overall isomer yield.

[0014] Polycondensates are understood here as those polymers that can be produced by a polycondensation reaction, in particular polyesters. The process is particularly suitable for obtaining aromatic dicarboxylic acids from poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), poly(ethylene terephthalate)-poly(ethylene isophthalate) copolymers (PET-PEI-

[0015] Copolymer), and / or poly(butylene terephthalate)-poly(ethylene terephthalate) copolymers (PBT-PET copolymer) with 1,4-cyclohexanedicarboxylate structural units.

[0016] An aromatic dicarboxylic acid is a carboxylic acid that has two carboxyl groups in an aromatic system. It is particularly preferably a benzenedicarboxylic acid or a naphthalenedicarboxylic acid.

[0017] A depolymerization reaction is understood to be a reaction in which polymers are broken down into their monomers or other suitable building blocks whose molecular weight is smaller than the molecular weight of the polymer and which are suitable for the subsequent synthesis of new polymers. The depolymerization reaction of the polycondensates can preferably be an enzymatic depolymerization. More preferably, the depolymerization reaction of the polycondensates is a neutral or basic depolymerization reaction, and particularly preferably an alkaline hydrolysis.

[0018] A metal carboxylate is understood here as a system consisting of a deprotonated carboxyl group, i.e., R-COO, with a metal ion as a counterion. The counterion can be any metal ion and is preferably an alkali metal ion or alkaline earth metal.

[0019] According to a preferred development of the invention, the process comprises the steps of a) dissolving the metal carboxylates in the effluent, b) precipitating at the first pH value, c) separating a precipitate comprising the first dicarboxylic acid from the liquid phase by carrying out the solid-liquid separation, d) precipitating the liquid phase obtained by the solid-liquid separation at the second pH value, and e) separating a precipitate comprising the further dicarboxylic acid from the liquid phase by carrying out a further solid-liquid separation.

[0020] Preferably, in connection with step a), at least enough solvent, preferably water, is added to the effluent to ensure that the metal carboxylate isomer mixture is completely dissolved. For example, dissolving the metal carboxylate disodium terephthalate at 20 °C requires at least one liter of water per 130 g of disodium terephthalate. The dissolution process is preferably carried out advantageously in a stirred vessel, a static mixer, or a rotor-stator shear apparatus (inline disperser).

[0021] More preferably, step a) can be followed by one or more of the following steps before the precipitation is carried out in step b): separation of solid impurities from the effluent and / or separation of liquid and / or dissolved impurities from the effluent. Performing one or more of these steps preferably reduces discoloration, which reduces the quality of the subsequent product. It also preferably advantageously prepares the starting material mixture for the subsequent steps b) to e).

[0022] In the step of separating solid interfering components, the suspension obtained by dissolving the metal carboxylates is preferably separated from undissolved components by solid-liquid separation. Continuous and batch processes, such as inertial processes such as sink-float separation and filtration processes, are used for this purpose. When filtration processes are used, they are preferably carried out in multiple stages, typically with 2 to 8 stages and decreasing particle size, for example, from 150 pm down to 50 nm absolute particle size. Filtration preferably takes place at temperatures below 30°C to separate even substances with low solubility that would otherwise dissolve.It is also possible to separate the finest solid particles by flocculation with flocculants such as polyamines, sodium aluminates, magnesium chloride, iron sulfate and others into larger agglomerates, making solid-liquid separation easier and more efficient.

[0023] The liquid phase obtained in this way is preferably subsequently subjected to a step of separating dissolved interfering components, such as soluble dyes, UV stabilizers, and / or additives. The dissolved interfering components are separated by adsorption on, for example, activated carbon, zeolites, or iron hydroxide oxide, or by extraction processes. The activated carbon chosen is preferably that with low affinity to the dissolved metal carboxylates to avoid product losses. In addition, the pH can subsequently be adjusted by adding an acid to separate solid interfering components and improve purification by adsorption and / or extraction.

[0024] If extraction processes are used, either the undesirable interfering components or the metal carboxylates are transferred into the extract phase. Suitable extraction agents either have the lowest possible affinity for the dicarboxylic acid or a particularly high affinity for the selective extraction of the dicarboxylic acid. Separation of the interfering substances is preferred in order to leave the metal carboxylates in the aqueous medium.

[0025] As already mentioned, the step of separating dissolved interfering components is preferably followed by step b) precipitation at the first pH value. During this step, the metal carboxylates present in the purified solution are forced out of their metal salts and react to form the dicarboxylic acid. Due to the comparatively high pH value, the precipitate comprises a very high proportion, if not almost exclusively, of the less soluble acid isomer, which can be separated from the liquid phase in the subsequent step c) by solid-liquid separation. Using the isomer mixture of benzenedicarboxylic acids as an example, the more soluble isomers phthalic acid and isophthalic acid are precipitated to a significantly lesser extent than the less soluble terephthalic acid.

[0026] In this way, a specific isomer of the isomer mixture can be obtained in high isomeric purity at an early stage of the process for further processing by separating the precipitate containing this isomer from the liquid phase. Separation by solid-liquid separation is preferably carried out using continuous or batch filtration apparatus, particularly belt filters, indexing belt filters, inclined belt filters, or rotary pressure filters, as well as separation apparatus that utilize the different densities of the solid and liquid suspension components.

[0027] According to a further preferred embodiment of the invention, after the precipitation of the first dicarboxylic acid and the subsequent solid-liquid separation, the precipitate comprising the first dicarboxylic acid is washed, wherein the liquid phase obtained before washing the precipitate is collected separately for precipitating the further dicarboxylic acid. In other words, after the solid-liquid separation has taken place, the liquid phase is preferably collected separately before the precipitate is washed, thus preventing the washing liquid from mixing with the liquid phase. In other words, the washing liquid of the precipitate is also preferably collected separately and not mixed with the liquid phase from the solid-liquid separation of the precipitate comprising the first dicarboxylic acid.

[0028] The liquid phase obtained from step c), which comprises a high proportion of the more soluble isomer, is subjected to further precipitation in the following step d), as already mentioned, at a second pH value, which is comparatively lower than the first pH value, whereby the more soluble acid isomer also precipitates. Thus, in the subsequent step e), the precipitate comprising the additional dicarboxylic acid can be separated from the liquid phase by performing a further solid-liquid separation. The further solid-liquid separation preferably takes place analogously to the first solid-liquid separation. Therefore, continuous and batchwise apparatus for solid-liquid separation are preferably used, such as belt filters, indexing belt filters, rotary drum filters, filter presses, Nutsche filters, bag filters, candle filters, sieve filters, decanter screw centrifuges, or disc separators.Furthermore, according to a further preferred development of the invention, the further solid-liquid separation is followed by washing of the precipitate comprising the further dicarboxylic acid. With regard to the precipitation, according to a preferred development of the invention, the first pH is in a pH range from 3 to 5.5 and preferably in a pH range from 3.5 to 5. In other words, the first precipitation preferably takes place at the first pH in the range from 3 to 5.5 and preferably in the range from 3.5 to 5. It has been shown that this pH range is particularly suitable for obtaining a precipitate having a particularly high proportion of an acid isomer.

[0029] According to a further preferred embodiment of the invention, the second pH value is in a pH range of 2 to 3. This achieves a high overall isomer yield. For example, the first pH value can be 3, and the second pH value, which is lower than the first pH value, can be correspondingly lower, for example, 2.5.

[0030] According to a preferred development, the first and / or second precipitation is preferably a reactive precipitation. This can preferably be an electrolytic precipitation, in which the corresponding H involved in the precipitation reaction are dissolved by electrolytic dissolution of a sacrificial electrode. + -ions dissolve. More preferably, the first and / or further dicarboxylic acid can be precipitated by adding a mineral or organic acid.

[0031] In connection with the latter variant, according to a preferred embodiment of the invention, the mineral or organic acid is sulfuric acid or hydrochloric acid in pure or diluted form. Particularly preferably, the mineral or organic acid is added directly to a reactor via the stirring blades of a stirrer for the precipitation reactions.

[0032] With regard to the precipitation of the first and the subsequent aromatic dicarboxylic acid, it is preferably provided that this is carried out continuously in the reactor, in which a constant fill level is established by simultaneous addition and discharge. In this way, a steady state with a constant pH value is established after a short time in the preferably stirred reactor, at which the dicarboxylic acids precipitate. In addition to the different pH values ​​for the precipitation reactions, the precipitation process parameters also contribute to the recovery of an aromatic dicarboxylic acid of high purity and with optimal physical properties.According to a further preferred development of the invention, the precipitation of the first dicarboxylic acid is carried out at a temperature between 40 °C and 200 °C, more preferably between 40 °C and 100 °C, and particularly preferably between 80 °C and 95 °C, and / or the precipitation of the further dicarboxylic acid is carried out at a temperature between 40 °C and 200 °C, more preferably between 40 °C and 100 °C, and particularly preferably between 80 °C and 95 °C. This leads to dicarboxylic acid crystals which have a morphology, in particular size and size distribution, suitable for subsequent processability. Larger dicarboxylic acid crystals allow for more effective and faster solid-liquid separation. At the same time, the addition of acid at these temperatures, and in particular at 80 °C to 95 °C, leads to a reduction in crystal discoloration.Finally, this process temperature also leads to a better separation of the isomers, especially during the first precipitation - in the example of the isomers of terephthalic acid, this leads to an equilibrium in the precipitate between this and isophthalic acid that is clearly shifted towards terephthalic acid.

[0033] The solid-liquid separation and / or the further solid-liquid separation is preferably carried out at a temperature between 40 °C and 200 °C, more preferably at a temperature between 40 °C and 100 °C, and particularly preferably at a temperature between 80 °C and 95 °C. Preferably, the solid-liquid separation and / or the further solid-liquid separation can be carried out at the same temperature as the preceding precipitation reaction.

[0034] To improve purity, the process can be followed by one or more purification steps, preferably in the form of recrystallization. During purification, the crystals obtained from reactive precipitation are preferably optimized for purity and morphological properties by recrystallization. Redissolving the precipitated aromatic dicarboxylic acid in a solvent followed by crystallization results in the removal of impurities trapped in the crystal. Depending on the temperature-dependent solubility of the dicarboxylic acid, cooling, evaporation, or flash crystallization processes are used. The entire recrystallization can be carried out batchwise or continuously, in one or in two to eight stages, with the temperature decreasing in each stage, starting from an initial maximum dissolution temperature; continuous recrystallization is preferred.

[0035] In this context, according to a further preferred development of the invention, the method comprises the steps

[0036] - purification of the precipitate comprising the first dicarboxylic acid by recrystallization at a temperature of up to 300 °C, preferably by a multi-stage recrystallization process, wherein the precipitate comprising the first dicarboxylic acid is at least partially dissolved in water; and

[0037] - Purification of the precipitate comprising the further dicarboxylic acid by recrystallization at temperatures up to 300 °C, preferably by a multi-stage recrystallization process, wherein the precipitate comprising the further dicarboxylic acid is at least partially dissolved in mother liquor obtained by the purification of the precipitate comprising the first dicarboxylic acid.

[0038] This makes it possible to reduce the content of the more soluble acid isomer, which was undesirably co-precipitated during the first precipitation, by recrystallization in the recrystallized product, since the more soluble acid isomer preferentially remains in the mother liquor. Because the mother liquor is also used as a solvent for the recrystallization of the other dicarboxylic acid, the proportion of the more soluble acid isomer in the recrystallized product of the other dicarboxylic acid is further increased.

[0039] In contrast to conventional purification and crystallization processes in industrial production processes for aromatic dicarboxylic acids, it is not necessary to select a temperature and concentration to completely dissolve all resuspended crystals, since the aromatic dicarboxylic acid resulting from the depolymerization process does not exhibit the typical impurities of conventional production processes for aromatic dicarboxylic acids. This procedure is particularly preferred for the recrystallization of the precipitate containing the remaining dicarboxylic acid. This advantageously reduces process costs and duration. After recrystallization, solid-liquid separation and washing of the recrystallized product can be carried out analogously to the method described above.Alternatively, it can also be provided that the crystals are completely dissolved, which is particularly preferred in the recrystallization of the precipitate comprising the first dicarboxylic acid.

[0040] According to a further preferred development of the invention, all process steps are carried out continuously. This has the advantage that the setup times for cleaning and refilling typical of a discontinuous process are eliminated, thus providing a more economical process.

[0041] The invention will now be explained by way of example with reference to the accompanying drawings using preferred embodiments, wherein the features presented below may represent an aspect of the invention, both individually and in combination. They show:

[0042] Fig. 1 is a schematic representation of a process for obtaining aromatic dicarboxylic acids from an effluent of a depolymerization reaction of polycondensates containing metal carboxylates of the aromatic dicarboxylic acids to be obtained, according to a preferred embodiment of the invention,

[0043] Fig. 2 is a schematic representation of a purification of the dicarboxylic acid following the process in Fig. 1, according to a further preferred embodiment of the invention.

[0044] Figure 1 schematically shows a simplified flow diagram with the steps of a process for obtaining aromatic dicarboxylic acids from an effluent of a depolymerization reaction of polycondensates containing metal carboxylates of the aromatic dicarboxylic acids to be obtained, according to a preferred embodiment of the invention. It is provided that a first aromatic dicarboxylic acid underlying the polycondensate precipitates from the effluent at a first pH value, and after a subsequent solid-liquid separation, a further aromatic dicarboxylic acid underlying the polycondensate precipitates from a liquid phase obtained by the solid-liquid separation at a second pH value, which is lower than the first pH value. In this embodiment, the first pH value is 4.5 and the second pH value is 2.

[0045] Not shown in Figure 1 is a possible step prior to precipitation at the first pH value for separating solid impurities from the effluent and / or separating liquid and / or dissolved impurities from the effluent. Furthermore, possible washing steps for the precipitates are also not shown in Figure 1.

[0046] Figure 2a) schematically shows a simplified flow diagram with the steps of purifying the dicarboxylic acid obtained by the process of Figure 1 and Figure 2b) schematically shows a flow diagram with the steps of purifying the further dicarboxylic acid obtained by the process of Figure 1, according to a preferred embodiment of the invention.

[0047] It is provided that a precipitate comprising the first dicarboxylic acid is purified by recrystallization at a temperature of up to 300 °C by a multi-stage recrystallization process, wherein the precipitate comprising the first dicarboxylic acid is completely dissolved in water, and that a precipitate comprising the further dicarboxylic acid is purified by recrystallization at temperatures of up to 300 °C by a multi-stage recrystallization process, wherein the precipitate comprising the further dicarboxylic acid is not completely dissolved in mother liquor obtained by the purification of the precipitate comprising the first dicarboxylic acid.

[0048] A further exemplary embodiment of the invention is explained below: A pasty mixture of 7.64 kg of disodium terephthalate, 280 g of disodium isophthalate, 226 g of sodium hydroxide, 2.34 kg of ethylene glycol and 330 g of polyethylene terephthalate-isophthalate copolyester flake residues from the alkaline ester hydrolysis of a PET post-consumer packaging material was mixed with 67 kg of demineralized water. At 20°C, the salts of the aromatic dicarboxylic acids dissolve completely and a suspension of PET flakes with a pH of 13.4 is formed. This suspension was freed from undissolved components in a multi-stage filtration cascade down to a fineness of 50 nm. The resulting clear, yellowish solution was then neutralized with 25% sulfuric acid (pH 7) and mixed with 1.1 kg of activated carbon granules. After a residence time of 60 min, the now colorless solution was freed from the activated carbon by filtration.Subsequently, 9.5 L of demineralized water are added to a stirred stainless steel reactor and heated to 80°C. The two dosing lances of the pumps for the precipitated acid (P1) and the decolorized solution from the alkaline hydrolysis (P2) flow into this reactor, directly above the stirring blades. At the bottom of the reactor there is also the inlet for product removal via the product pump (P3), which completes the continuous setup. Subsequently, 13.68 kg / h of 25% sulfuric acid are pumped via the acid pump P1, and 77.5 kg / h of the decolorized solution, preheated to 80°C, are pumped into "Reactor A" filled with water via pump P2. At the same time, pump P3 is started and adjusted to maintain a constant fill level with a suspension volume of 9.8 L. The flows of pumps P1 and P2 represent a stoichiometric ratio of 0.85:1 based on the total amount of aromatic dicarboxylic acids contained.After a short time, a steady state with a constant pH of 4.7 was established in the stirred reactor, at which 85% of the dicarboxylic acids contained were precipitated. The resulting suspension was continuously passed through a pulsed "belt filter A" at a process temperature of 80 °C. Mother liquor and wash water were collected in separate separation vessels. In the first zone, the liquid phase, which still contained 15% of the total amount of dicarboxylic acids and the monomer ethylene glycol, was separated from the crystallizate "A". The crystallizate "A" was then washed with water and dehydrated in several steps. The liquid phase A with a pH of 4.7 was passed through a continuous stirred tank reactor "B" designed analogously to "Reactor A" and continuously precipitated at 80 °C with 2.5 kg / h of 25% sulfuric acid to a pH of 2.7. 99.7% of the remaining 15% of aromatic dicarboxylic acids precipitate as crystals.This suspension is first thickened using a hydrocyclone and then filtered through a batch-operated Nutsche filter. The resulting "crystallizate B" is washed and dehydrated. The mass and isophthalic acid content of the resulting crystallizates A and B were determined by HPLC measurement with a UV / Vis detector and are shown in Table 1 below.

[0049] The crystals thus obtained can then be dried directly or further processed using the following process steps to further adjust the morphology and purity.

[0050] For this purpose, the still-moist crystals A (65% residual moisture) were slurried with 41.8 kg of demineralized water and completely dissolved in a stirred reactor at 245 °C and 38 bar. After a residence time of 30 minutes, the solution was flashed into a second vessel at 16 bar and 200 °C, where it was subjected to flash evaporation, primarily resulting in the crystallization of terephthalic acid. The resulting suspension was then flashed into a third stirred reactor at 100 °C and 1 bar. The still-hot suspension contained terephthalic acid crystals A2, which had an isophthalic acid content of 0.01 g of A / 100 g of crystals and was filtered from the mother liquor A2 at 100 °C using a pressure filter and then washed. The resulting moist crystals A2 were subsequently dried and, in addition to their high purity, had a crystal size and morphology that fulfilled the properties of fossil terephthalic acid (PTA).

[0051] The moist crystals B were slurried with 6.3 kg of mother liquor A2 in a similar manner to crystals A and heated to 220 °C at 23 bar. Under these conditions, 30 mass% of the crystals were dissolved. After a residence time of 10 minutes, the suspension was flash evaporated in a second reactor at 195 °C and 13.5 bar and then in a third vessel at 100 °C and 1 bar. The suspension was then cooled to 25 °C and separated from the mother liquor using a vacuum filter. The resulting crystals B2 were then dried; they had an isophthalic acid content of 21.3 g / 100 g of crystals and, in addition, comparable properties to crystals A2. The described embodiments are merely examples that can be modified and / or supplemented in many ways within the scope of the claims.Any feature described for a particular embodiment can be used independently or in combination with other features in any other embodiment. Any feature described for an embodiment of a particular category can also be used in a corresponding manner in an embodiment of a different category.

[0052] The invention originates from a funded research project with the funding number: 16KN082928. Funding was provided within the framework of the "ZIM - Central Innovation Program for SMEs" funding measure.

Claims

Patent claims 1. A process for the recovery of aromatic dicarboxylic acids from an effluent of a depolymerization reaction of polycondensates containing metal carboxylates of the aromatic dicarboxylic acids to be recovered, in which a first aromatic dicarboxylic acid underlying the polycondensate precipitates from this effluent at a first pH value, and after a subsequent solid-liquid separation, a further aromatic dicarboxylic acid underlying the polycondensate precipitates from a liquid phase obtained by the solid-liquid separation at a second pH value which is lower than the first pH value.

2. Process according to the preceding claim comprising the steps of a) dissolving the metal carboxylates in the effluent, b) precipitating at the first pH, c) separating a precipitate comprising the first dicarboxylic acid from the liquid phase by carrying out the solid-liquid separation, d) precipitating the liquid phase obtained by the solid-liquid separation at the second pH, and e) separating a precipitate comprising the further dicarboxylic acid from the liquid phase by carrying out a further solid-liquid separation.

3. Process according to one of the preceding claims, wherein after the precipitation of the first dicarboxylic acid and the subsequent solid-liquid separation, a precipitate comprising the first dicarboxylic acid is washed, and wherein the liquid phase obtained before washing the precipitate is collected separately for the precipitation of the further dicarboxylic acid.

4. Process according to one of the preceding claims, wherein after the precipitation of the further dicarboxylic acids, the further solid-liquid separation is carried out with a subsequent washing of a precipitate comprising the further dicarboxylic acid.

5. The method according to any one of the preceding claims, wherein the first pH value is in a pH range of 3 to 5.5 and preferably in a pH range of 3.5 to 5.

6. The method according to any one of the preceding claims, wherein the second pH value is in a pH range of 2 to 3.

7. The process according to any one of the preceding claims, wherein the first and / or further dicarboxylic acid is precipitated by reactive precipitation; and / or wherein the first and / or further dicarboxylic acid is precipitated by electrolytic precipitation; and / or wherein the first and / or further dicarboxylic acid is precipitated by adding a mineral or organic acid.

8. Process according to the preceding claim, wherein the mineral or organic acid is sulfuric acid or hydrochloric acid in pure or diluted form.

9. The process according to any one of the preceding claims, wherein the precipitation of the first dicarboxylic acid is carried out at a temperature between 40 °C and 200 °C, preferably between 40 °C and 100 °C, and particularly preferably between 80 °C and 95 °C, and / or wherein the precipitation of the further dicarboxylic acid is carried out at a temperature between 40 °C and 200 °C, preferably between 40 °C and 100 °C, and particularly preferably between 80 °C and 95 °C.

10. The process according to any one of the preceding claims, wherein the solid-liquid separation and preferably the further solid-liquid separation is carried out at a temperature between 40 °C and 200 °C, preferably between 40 °C and 100 °C and particularly preferably between 80 °C and 95 °C.

11. A process according to any one of the preceding claims, comprising the steps of purifying a precipitate comprising the first dicarboxylic acid by Recrystallization at a temperature up to 300 °C, preferably by a multi-stage recrystallization process, wherein the precipitate comprising the first dicarboxylic acid is at least partially, and preferably completely, dissolved in water; and Purification of a precipitate comprising the further dicarboxylic acid by recrystallization at temperatures up to 300 °C, preferably by a multi-stage recrystallization process, wherein the precipitate comprising the further dicarboxylic acid is dissolved at least partially, and preferably not completely, in mother liquor obtained by the purification of the precipitate comprising the first dicarboxylic acid.

12. A method according to any one of the preceding claims, wherein all method steps are carried out continuously.

13. Use of a process according to one of claims 1 to 12 for the production of polycondensates from the obtained aromatic dicarboxylic acids, in particular for the production of polyester and / or polyethylene terephthalate plastics.