Process for the production of aromatic dicarboxylic acids

Abstract

The present invention relates to 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.

Description

The present invention relates to a process for recovering aromatic dicarboxylic acids from a effluent of a depolymerization reaction of polycondensates comprising metal carboxylates of the aromatic dicarboxylic acids to be recovered.This process is particularly suitable for use in or after processes which work up polycondensates, such as PET, for example, by means of depolymerization with addition of metal hydroxides, in order to recover the respective monomers of the polycondensates for renewed use.The polycondensates, in particular polyethylene terephthalate plastics (PET), but also poly(ethylene naphthalate), poly(ethylene terephthalate) poly(ethylene isophthalate) copolymers, poly(butylene terephthalate) poly(ethylene terephthalate) copolymers having 1,4-cyclohexanedicarboxylate structural units, which are generally produced directly from basic chemicals of fossil origin, such as petroleum and / or natural gas, are used to a large extent, for example as materials for beverage bottles, food packaging, fibers, textiles, automobile components, or clear, opaque or coloured bottles of liquid products, such as, for example, detergents or oils.As a rule, polycondensates and in particular PET packaging are mechanically comminuted into so-called "flakes", cleaned from foreign substances, sorted according to colors and processed to form granules. A portion of the granulate can be mixed with new material and used for producing new packaging materials. However, the production of new packaging materials exclusively from recycled material is not possible for technical and regulatory reasons.Accordingly, for economic as well as environmental reasons, it is desirable to recover the starting monomers of the polycondensates in order to supply them to reuse. For this purpose, the products produced from these polycondensates or else production waste are recycled, in particular by means of an alkaline depolymerization with addition of alcohols and metal hydroxides such as sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide and / or barium hydroxide.U.S. Pat. No. 6,580,005 B1 describes such a recycling process for recovering terephthalic acid, in which comminuted PET is added with polyethylene glycol and sodium carbonate to obtain metal carboxylate. This is reacted in several steps using sulfuric acid into terephthalic acid, wherein after the depolymerization the steps of a solid-liquid separation for separating impurities including ethylene glycol, a neutralization including a crystallization of terephthalic acid and its purification by washing, solid / liquid separation and drying are carried out.However, these processes make it difficult to process waste which is not pure in the type. This is because the basic depolymerization reaction of a non-pure waste leads to a starting material mixture for the further process steps which, in addition to the main component of the metal carboxylates, comprises, in the form of one or more isomers of one or different monomers of the polycondensate to be recycled, residues of the added metal hydroxide, dyes, additives, degradation products and other impurities from the production, processing and utilization 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 reacted oligomers and polymers of the polycondensates. Owing to the complex and varying composition of this starting material mixture, working up thereof is difficult.Proceeding from this, it is an object of the invention to specify an improved method. It is a further object of the invention to provide a process which is particularly suitable for obtaining aromatic dicarboxylic acids from a waste stream of a depolymerization reaction of polycondensates which comprise an isomer mixture of the aromatic dicarboxylic acids to be obtained.This object is achieved by a method according to claim 1, wherein advantageous developments are specified in the dependent claims.According to the invention, a process for obtaining aromatic dicarboxylic acids from a waste stream of a depolymerization reaction of polycondensates comprising metal carboxylates of the aromatic dicarboxylic acids to be obtained is thus provided, in which a first aromatic dicarboxylic acid on which the polycondensate is based precipitates from this waste stream at a first pH, and, after a solid-liquid separation connected thereto, a further aromatic dicarboxylic acid on which the polycondensate is based precipitates from a liquid phase obtained by the solid-liquid separation at a second pH which is lower than the first pH.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 the higher pH value compared to the second pH value in the precipitate--that is to say in the solid precipitation product--the less soluble acid isomer of an acid isomer mixture is mostly present, in the case of PET terephthalic acid (also called paraphthalic acid) in particular, and in the second precipitation with the lower pH value compared to the first pH value in the precipitate, larger portions of the more soluble acid isomer are present, in the case of PET isophthalic acid (also called metaphthalic acid).It is therefore preferably also a process for obtaining a dicarboxylic acid isomer from the effluent from the depolymerization reaction of the polycondensates, the effluent comprising the metal carboxylates of an isomer mixture of the aromatic dicarboxylic acids. In other words, the process is therefore suitable for separating an acid isomer present as metal carboxylate from an acid isomer mixture. The acid isomer mixture preferably comprises several isomers of dicarboxylic acid and more preferably terephthalic acid and isophthalic acid. The process according to the invention thus makes it possible in a simple manner to achieve a particularly pure product of a single dicarboxylic acid isomer, in particular terephthalic acid, with simultaneously little loss of total isomer yield.Polycondensates are understood in the present case to mean those polymers which can be prepared 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 copolymer), and / or poly(butylene terephthalate) poly(ethylene terephthalate) copolymers (PBT-PET copolymer) having 1,4-cyclohexanedicarboxylate structural units.An aromatic dicarboxylic acid is understood to mean a carboxylic acid which has two carboxyl groups on an aromatic system. Particular preference is given to a benzenedicarboxylic acid or a naphthalenedicarboxylic acid.A depolymerization reaction is understood to mean a reaction in which polymers are resolved into their monomers or into other suitable building blocks whose molar mass is less than the molar mass of the polymer and which are suitable for the subsequent synthesis of novel 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.A metal carboxylate is understood in the present case to mean a system of a deprotonated carboxyl group, i.e. R-COO -, with a metal ion as counterion. The counterion can be any metal ion, and preferably an alkali metal ion or alkaline earth metal ion.According to a preferred development of the invention, the method comprises the steps 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.In connection with step a), at least enough solvent, preferably water, is preferably added to the effluent that the metal carboxylate isomer mixture is present in completely dissolved form. Here, for example, for dissolving the metal carboxylate disodium terephthalate at 20° C., at least one liter of water is required per 130 g disodium terephthalate. The dissolving process is preferably advantageously carried out in terms of apparatus in a stirred vessel, a static mixer or by a rotor-stator shear apparatus (inline disperser).More preferably, step a) can be followed by one or more of the following steps before the precipitation is carried out in step b): removal of solid interfering constituents of the effluent and / or removal of liquid and / or dissolved interfering constituents of the effluent. Performing one or more of these steps preferably reduces discolorations which degrade the quality of the later product. It also preferably prepares the starting material mixture in an advantageous manner for the subsequent steps b) to e).In the solid impurity separating step, the suspension obtained by dissolving the metal carboxylates is preferably separated from undissolved components by solid-liquid separation. For this purpose, continuous and batch processes such as inertia-based processes such as float-sink separation and filtration processes are used. When using filtration processes, these are preferably carried out in multistage processes, typically with 2 to 8 stages and decreasing separation grain size of, for example. 150 μm up to 50 nm absolute separating grain size. The filtration preferably takes place at temperatures below 30° C. in order to also separate substances with low solubility, which would otherwise go into solution. It is likewise possible to use the separation of very fine solid particles by flocculation with flocculant materials such as polyamines, sodium aluminates, magnesium chloride, ferrous sulfate and others to form larger agglomerates, so that the solid-liquid separation becomes simpler and more efficient.The liquid phase obtained in this way is preferably subsequently subjected to a step of separating off dissolved interfering components, such as soluble dyes, UV stabilizers, and / or additives, the dissolved interfering components being separated off by adsorption on, for example, activated carbon, zeolites or iron hydroxide oxide, or by extraction methods. The activated carbon selected is preferably that activated carbon with low affinity for the dissolved metal carboxylates in order to avoid product losses. In addition, an adjustment of the pH by addition of an acid can be provided subsequently for the separation of solid interfering components and for improving the purification by means of adsorption and / or extraction.If extraction processes are used, either the unwanted interfering components or the metal carboxylates are converted into the extract phase. Extractants which can be used either have as low an affinity as possible for the dicarboxylic acid or have a particularly high affinity for the selective recovery of the dicarboxylic acid. Preference is given to separating off the impurities in order to leave the metal carboxylates in the aqueous medium.As already mentioned, the step of separating off dissolved interfering components is preferably followed by step b) by precipitation at the first pH. In this process, the metal carboxylates present in the purified solution are forced from their metal salts and react to form the dicarboxylic acid. Owing to the comparatively high pH, the precipitate which precipitates comprises a very high proportion, to almost exclusively, of the sparingly soluble acid isomer, which can be separated from the liquid phase in the subsequent step c) by a solid-liquid separation. Using the example of the mixture of isomers of benzenedicarboxylic acids, the more soluble isomers phthalic acid and isophthalic acid are therefore precipitated to a significantly lesser extent than the less soluble terephthalic acid.In this way, a specific isomer of the isomer mixture in high isomer purity is thus obtainable at an early process time for further workup, in which the precipitate comprising this isomer is separated from the liquid phase. The separation by the solid-liquid separation is preferably effected by continuous as well as batchwise filtration apparatuses, in particular band filters, clocked band filters, helical band filters or rotary pressure filters, as well as separation apparatuses which utilize the different density of the solid and liquid suspension constituents.According to a further preferred development of the invention, it is provided that after the precipitation of the first dicarboxylic acid and the solid-liquid separation connected thereto, the precipitate comprising the first dicarboxylic acid is washed, wherein the liquid phase obtained before the washing of the precipitate is collected separately for the precipitation of the further dicarboxylic acid. In other words, after the solid-liquid separation has been completed, the liquid phase is thus preferably collected separately before the precipitate is washed and prevents the washing liquid from mixing with the liquid phase in such a way. In other words, the wash liquid of the precipitate is therefore preferably also collected separately and not mixed with the liquid phase from the solid-liquid separation of the precipitate comprising the first dicarboxylic acid.The liquid phase obtained in step c), which comprises a high proportion of more soluble isomer, is, as already mentioned, subjected to a further precipitation at the second pH which is lower compared with the first pH, as a result of which the more soluble acid isomer now also precipitates. In this way, in the subsequent step e), the precipitate comprising the further dicarboxylic acid can be separated from the liquid phase by carrying out a further solid-liquid separation. The further solid-liquid separation preferably takes place analogously to the first solid-liquid separation. Thus, preference is given to using continuous and batchwise apparatuses for solid-liquid separation, such as belt filters, clocked belt filters, rotary drum filters, filter presses, suction filters, bag filters, candle filters, screen filters, decanter screw centrifuges or plate separators. In addition, according to a further preferred development of the invention, it is provided that, after the further solid-liquid separation, a subsequent washing of the precipitate comprising the further dicarboxylic acid is carried out.With regard to precipitation, it is provided according to a preferred development of the invention that the first pH is in a pH range of 3 to 5.5 and preferably in a pH range of 3.5 to 5. In other words, the first precipitation thus 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 found that this pH range is particularly suitable for obtaining a precipitate which has a particularly high proportion of an acid isomer.According to a further preferred development of the invention, it is provided that the second pH value lies in a pH range from 2 to 3. A high total isomer yield is thus achieved. For example, the first pH value can thus be 3 and the second pH value, which is lower than the first pH value, can be correspondingly lower, for example 2.5.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 +- ions involved in the precipitation reaction pass into solution by electrolytic dissolution of a sacrificial electrode. More preferably, the first and / or further dicarboxylic acid can be precipitated by adding a mineral or organic acid.In connection with the latter variants, it is provided according to a preferred development of the invention that the mineral or organic acid is sulfuric acid or hydrochloric acid in pure or diluted form. Particularly preferably, the mineral or organic acid for the precipitation reactions is added directly to a reactor via stirrer blades of a stirrer.With regard to the precipitation of the first and also of the further aromatic dicarboxylic acids, 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, in the preferably stirred reactor, a steady state with a constant pH value is established after a short time, at which the dicarboxylic acids precipitate out.In addition to the different pH values for the precipitation reactions, the process parameters of the precipitation also contribute to the recovery of an aromatic dicarboxylic acid in high purity and with optimum physical properties. According to a further preferred development of the invention, these are that 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 that 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 suitable for subsequent processability, in particular size and size distribution. Larger crystals of dicarboxylic acid allow 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 the discoloration of the crystal. Finally, this process temperature, especially in the first precipitation, also leads to a better separation of the isomers--in the example of the isomers of terephthalic acid, thus to an equilibrium in the precipitate between this and isophthalic acid, which equilibrium is clearly shifted to terephthalic acid.The solid-liquid separation and / or the further solid-liquid separation is preferably carried out at a temperature between 40° C. and 200° C., further 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.To improve the purity, the process can be followed by one or more purification steps, preferably in the form of recrystallization. In the purification, the crystals obtained from the reactive precipitation are preferably optimized in purity and morphological properties by recrystallization. Redissolving the precipitated aromatic dicarboxylic acid in a solvent followed by crystallization results in the extraction of the impurities trapped in the crystal. Depending on the temperature-dependent solubility behavior of the dicarboxylic acid, cooling, evaporation or flash crystallization methods are used. The total recrystallization can be carried out batchwise or continuously, in one or in 2 to 8 stages, with a temperature falling per stage starting from an initial maximum dissolution temperature, with continuous recrystallization being preferred.In this context, according to a further preferred development of the invention, it is provided that the method includes the stepspurifying the precipitate comprising the first dicarboxylic acid by recrystallization at a temperature up to 300° C., preferably by a multistage recrystallization process, wherein the precipitate comprising the first dicarboxylic acid is at least partly dissolved in water; andpurifying the precipitate comprising the further dicarboxylic acid by recrystallization at temperatures up to 300° C., preferably by a multistage recrystallization process, the precipitate comprising the further dicarboxylic acid being at least partly dissolved in mother liquor which has been obtained by purifying the precipitate comprising the first dicarboxylic acid,comprising.This makes it possible to reduce the content of the more soluble acid isomer which has undesirably co-precipitated in the first precipitation by recrystallization in the recrystallizer, since the more soluble acid isomer preferably remains in the mother liquor. Because the mother liquor is also used as solvent for the recrystallization of the further dicarboxylic acid, the proportion of more readily soluble acid isomer in the recrystallizer of the further dicarboxylic acid is further increased.In contrast to conventional purification and crystallization processes of industrial aromatic dicarboxylic acid production processes, it is not necessary here to choose a temperature and concentration for completely dissolving all resuspended crystals, since the aromatic dicarboxylic acid originating from the depolymerization process does not contain the typical impurities of the conventional aromatic dicarboxylic acid production processes. This procedure is preferred, in particular in the recrystallization of the precipitate comprising the further dicarboxylic acid. This advantageously reduces process costs and process time. After recrystallization has taken place, a solid-liquid separation and washing of the recrystallisate can take place, which can be carried out analogously to the manner described above. Alternatively, however, it can also be provided that the crystals are completely dissolved, which is preferred in particular during the recrystallization of the precipitate comprising the first dicarboxylic acid.According to a further preferred development of the invention, all method steps are carried out continuously in the method. This has the advantage that the setup times for cleaning and refilling that are typical in a discontinuous process are omitted and an economical process is thus provided.The invention is explained below by way of example with reference to the attached drawings on the basis of preferred exemplary embodiments, wherein the features illustrated below can each represent an aspect of the invention both individually and in combination. The following are shown: FIG. 1 shows a schematic illustration of a process for recovering aromatic dicarboxylic acids from a effluent from a depolymerization reaction of polycondensates comprising metal carboxylates of the aromatic dicarboxylic acids to be recovered, according to a preferred exemplary embodiment of the invention, FIG. 2 shows a schematic illustration of a purification of the dicarboxylic acid subsequent to the process in FIG. 1, according to a further preferred exemplary embodiment of the invention.FIG. 1 schematically shows a simplified flow diagram with the steps of a process for obtaining aromatic dicarboxylic acids from a waste stream of a depolymerization reaction of polycondensates comprising metal carboxylates of the aromatic dicarboxylic acids to be obtained, according to a preferred embodiment of the invention. It is provided here that a first aromatic dicarboxylic acid on which the polycondensate is based precipitates out of the effluent at a first pH value, and after a solid-liquid separation connected thereto, a further aromatic dicarboxylic acid on which the polycondensate is based precipitates out of 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 is 4.5 and the second pH is 2.Not shown in FIG. 1 is a step for separating solid interfering constituents of the effluent and / or separating liquid and / or dissolved interfering constituents of the effluent before precipitation at the first pH value. In addition, possible washing steps of the precipitates are likewise not shown in FIG. 1.FIG. 2 a) schematically shows a simplified flow chart with the steps of purifying the dicarboxylic acid obtained by the method from FIG. 1 and FIG. 2 b) schematically shows a flow chart with the steps of purifying the further dicarboxylic acid obtained by the method from FIG. 1, according to a preferred exemplary embodiment of the invention.It is provided that a precipitate comprising the first dicarboxylic acid is purified by recrystallization at a temperature up to 300° C. by a multistage recrystallization process, wherein the precipitate comprising the first dicarboxylic acid is dissolved completely in water, and that a precipitate comprising the further dicarboxylic acid is purified by recrystallization at temperatures up to 300° C. by a multistage 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.A further exemplary embodiment of the invention is explained below: 67 kg of demineralized water were added to 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, 330 g of polyethylene terephthalate isophthalate copolyester Flake residues from the alkaline ester hydrolysis of a PET post-consumer packaging material. At 20° C., the salts of the aromatic dicarboxylic acids dissolve completely and a suspension of PET flakes having a pH of 13.4 is formed. This was freed from undissolved constituents in a multistage filtration cascade to a fineness of 50 nm. The resulting clear, yellowish solution was then neutralized with 25% strength sulfuric acid (pH 7) and 1.1 kg of activated carbon granules were added. 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 is placed in a stirred stainless steel reactor and heated to 80° C. Into this reactor, directly above the stirrer blades, open the two dosing lances of the pumps of the precipitation acid (P1) and the decolored solution from the alkaline hydrolysis (P2). At the bottom of the reactor there is also the inlet opening for the product draw by means of a product pump (P3), which completes the continuous construction. Subsequently, 25% sulfuric acid is conveyed via the acid pump P1, 13.68 kg / h and 77.5 kg / h of the decolored solution preheated to 80° C. are conveyed via pump P2 into the "reactor A" filled with water. At the same time, the pump P 3 is also started and set in such a way that a constant fill level with a suspension volume of 9.8 L is established. The streams of the pumps P1 and P2 represent a stoichiometric ratio of 0.85:1, based on the total amount of aromatic dicarboxylic acids present. After a short time, a steady state with a constant pH of 4.7 is established in the stirred reactor, at which 85% of the dicarboxylic acids present are present in precipitated form. The suspension obtained in this way was passed continuously at process temperature of 80° C. over a clocked "band filter A". Mother liquor and wash water are collected in separate separation containers. In the first zone, the liquid phase, which still contains 15% of the total amount of dicarboxylic acids and the monomer ethylene glycol, was separated from the crystals "A". The crystals "A" are then washed with water and dehumidified in a plurality of steps. The liquid phase A having a pH of 4.7 was passed into a continuous stirred tank reactor "B" designed analogously to "reactor A" and continuously precipitated at 80° C. with 2.5 kg / h of 25% strength sulfuric acid to a pH of 2.7. 99.7% of the 15% of the aromatic dicarboxylic acids still present are thereby obtained as crystals. This suspension is first thickened via a hydrocyclone and then filtered through a batchwise suction filter and the resulting "crystals B" are washed and dehumidified. The mass and the isophthalic acid content of the crystals A and B formed were determined by HPLC measurement using a UV / Vis detector and are shown in Table 1 below.Crystal mass [g]62565318938Content of isophthalic acid [g IA / 100 g Kristallisat]3,540,918,5The crystals obtained in this way can subsequently be dried directly or processed further by means of the subsequent process steps in order to further adapt the morphology and purity.For this purpose, the still moist crystals A (65% residual moisture) were slurried with 41.8 kg of demineralized water and were dissolved completely at 245° C. and 38 bar in a stirred reactor. After a residence time of 30 min, the solution was flash evaporated (flashed) into a second vessel in which a pressure of 16 bar and a temperature of 200° C. prevail, with terephthalic acid being primarily crystallized. The suspension now obtained is then expanded in a third stirred reactor at 100° C. and 1 bar pressure. The still hot suspension contains terephthalic acid crystallisate A2, which had an isophthalic acid content of 0.01 g IA / 100 g Kristallisat and was filtered from the mother liquor A2at 100° C. by means of a pressure suction filter and then washed. The moist crystals A2 obtained in this way were then dried and, in addition to the high purity, had a crystal size and morphology which meets the properties of fossil terephthalic acid (PTA).The moist crystals B were slurried analogously to crystals A with 6.3 kg of the mother liquor A2 and heated to 220° C. at 23 bar. Under these conditions, 30 mass % of the crystals are dissolved. After a residence time of 10 min, the suspension is flash-evaporated in a second reactor at 195° C. and 13.5 bar and then in a third vessel to 100° C. and 1 bar. The suspension is then cooled down to 25° C. and separated from the mother liquor by means of a vacuum suction filter. The crystal B2 obtained in this way is now dried; it has an isophthalic acid content of 21.3 g IA / 100 g Kristallisat and additionally comparable properties to crystal A2.The described exemplary embodiments are merely examples which can be modified and / or supplemented in a wide variety of ways within the scope of the claims. Each feature described for a particular embodiment may be used independently or in combination with other features in any other embodiment. Each feature described for an embodiment of a particular category may also be used in a corresponding manner in an embodiment of a different category.The invention originates from a promoted research project having the delivery number: 16KN082928. The conveyance took place within the framework of the conveyance measure "ZIM-Central Innovation Program Midstand".References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedU.S. Pat. No. 6,580,005 B1

[0006]

Claims

Process for obtaining aromatic dicarboxylic acids from a waste stream of a depolymerization reaction of polycondensates comprising metal carboxylates of the aromatic dicarboxylic acids to be obtained, wherein a first aromatic dicarboxylic acid on which the polycondensate is based precipitates from this waste stream at a first pH, and after a solid-liquid separation connected thereto, a further aromatic dicarboxylic acid on which the polycondensate is based precipitates from a liquid phase obtained by the solid-liquid separation at a second pH which is lower than the first pH.A 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.The method according to any one of the preceding claims, wherein after the precipitation of the first dicarboxylic acid and the solid-liquid separation connected thereto, a precipitate comprising the first dicarboxylic acid is washed, and wherein the liquid phase obtained before washing the precipitate is collected separately for precipitating the further dicarboxylic acid.The method according to any 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.The method according to any one of the preceding claims, wherein the first pH is in a pH range of 3 to 5.5, and preferably in a pH range of 3.5 to 5.The method of any preceding claim, wherein the second pH is in a pH range of 2 to 3.The method according to any one of the preceding claims, wherein the first and / or further dicarboxylic acid is precipitated by a reactive precipitation; and / or wherein the first and / or further dicarboxylic acid is precipitated by an electrolytic precipitation; and / or wherein the first and / or further dicarboxylic acid is precipitated by adding a mineral or organic acid.Process according to the preceding claim, in which the mineral or organic acid is sulphuric acid or hydrochloric acid in pure or diluted form.The method 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.The method 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.Process according to any 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 multistage recrystallization process, wherein the precipitate comprising the first dicarboxylic acid is at least partly, and preferably completely, dissolved in water; and purifying a precipitate comprising the further dicarboxylic acid by recrystallization at temperatures up to 300°C, preferably by a multistage recrystallization process, wherein the precipitate comprising the further dicarboxylic acid is at least partly, and preferably not completely, dissolved in mother liquor obtained by purifying the precipitate comprising the first dicarboxylic acid.A process according to any preceding claim, wherein all process steps are carried out continuously.

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