PROCESS FOR THE OBTAINING AROMATIC DICARBOXYLIC ACIDS FROM THEIR METAL SALTS

Description

[0001] The present invention relates to a process for obtaining aromatic dicarboxylic acids from their respective metal salts. This process is particularly suitable for use in or after processes involving basic depolymerization with the addition of metal hydroxides, with which polycondensates such as PET are reprocessed to recover the respective monomers for reuse.

[0002] Polycondensates, which are generally produced directly from basic chemicals of fossil origin, particularly polyethylene terephthalate plastics (PET), but also poly(ethylene naphthalates), poly(ethylene terephthalate)-poly(ethylene isophthalate) copolymers, and poly(butylene terephthalate)-poly(ethylene terephthalate) copolymers with 1,4-cyclohexanedicarboxylate structural units, are widely used, among other things, as materials for beverage bottles, food packaging such as salad trays, sausage and cheese packaging, fibers, textiles, automotive components, and clear, opaque, or colored detergent bottles. Therefore, for both economic and environmental reasons, it is desirable to recover the starting monomers of these polycondensates for reuse.For this purpose, the products manufactured 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 barium hydroxide. In these recycling processes, the alkaline depolymerization is followed by the steps of dissolving the metal carboxylates produced in the depolymerization, separating undissolved components and dissolved impurities, and then precipitating the metal carboxylates produced in the depolymerization by adding acid. The precipitate can then be purified as a crude product to produce the final product.

[0003] US Pat. No. 6,580,005 A 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 the ethylene glycol, neutralization including crystallization of the terephthalic acid, and purification by washing, solid / liquid separation, and drying follow.

[0004] US Pat. No. 6,031,128 A discloses a process for obtaining pure terephthalic acid, in which a very specific, single-grade PET plastic waste is recycled. Following basic depolymerization using NaOH, the resulting slurry is processed by solid / liquid separation and further processing of the metal carboxylate dissolved in the separated solvent, water. For this purpose, the latter, after dilution with a maximum of three times the required amount of solvent, is passed through an activated carbon adsorber to remove impurities. Following this, neutralization and precipitation using a strong acid such as HCl, H2SO4, HNO3, or H3PO4 takes place. The precipitated, moist terephthalic acid is then repeatedly recrystallized in successive tanks at decreasing temperatures in order to obtain larger and thus more easily separable particles.Finally, cooling under reduced pressure takes place, and the final product is obtained after filtration and drying.

[0005] 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.

[0006] The invention therefore has the object of providing a correspondingly improved method.

[0007] This object is achieved by a method according to claim 1, wherein advantageous further developments are specified in the subclaims.

[0008] A process according to the invention with all process steps proceeds as described in detail below, whereby at least the steps of dissolving and precipitating as well as separating the product must be present.

[0009] In a pretreatment step according to the invention of the starting material mixture originating from the basic depolymerization reaction, only a portion of the minimum amount of solvent required to completely dissolve all of the metal carboxylates contained therein is added, in particular only the amount that is matched to the content of the most soluble isomer of the metal carboxylate—or metal carboxylates in the case of copolymers—so that primarily this isomer dissolves and can thus be easily separated from the starting material mixture. In this advantageous manner, the more soluble isomer(s) in particular are available in the highest isomeric purity at a very early stage of the process for further processing by separating the solution containing only this isomer—in particular by filtration—from the remaining depolymerization effluent, and reprocessing the filtrate.Separation from the depolymerization effluent is preferably carried out using continuous or batch filtration equipment, particularly belt filters, indexing belt filters, inclined belt filters, melt filters, or rotary pressure filters, as well as separation equipment that utilizes the different densities of the solid and liquid suspension components. This results in a liquid phase that, in addition to the monomer alcohol—in the case of PET, this is ethylene glycol—as described above, contains almost exclusively the more soluble isomer of the metal carboxylate in the solvent, or the more soluble isomers, and the solvent, particularly water. In the further processing of this filtrate, the monomer alcohol is separated from the water, particularly by distillation, rectification, or membrane processes such as pervaporation.The separated, more soluble metal carboxylate remains undissolved in the monomer alcohol and is subsequently recovered by solid / liquid separation. Alternatively, the reactive precipitation step described below can follow.

[0010] In this way, the less soluble isomer in the depolymerization effluent is also purified at the same time. This advantageously also reduces impurities that could have a coloring effect on the subsequent aromatic dicarboxylic acid of the less soluble isomer. The invention accepts a loss of the other, less soluble monomer(s) according to the invention, provided that only this, most soluble, isomer, or more precisely its dicarboxylic acid, is to be obtained. This dissolution step, which only occurs for a single isomer with regard to the total content of metal carboxylates, is particularly advantageous because the additional metal carboxylates formed by the copolymers mentioned above are separated at this early stage of the process, so that they can either be processed separately or disposed of.According to the invention, this first separation step can be carried out simply by adding an appropriate amount of solvent to the depolymerization reaction itself. The solvent is preferably water or an aqueous solution of a metal sulfate / acetate, such as sodium sulfate or sodium acetate. The aqueous sodium sulfate and acetate solutions can be recycled to this step from subsequent washing processes of the process according to the invention.

[0011] As an alternative to this pretreatment step according to the invention, which specifically preferably separates only a single isomer, at least enough solvent is added to the starting material mixture, with the procedure otherwise being the same, to ensure that the metal carboxylate isomer mixture is completely dissolved. To dissolve the metal carboxylate disodium terephthalate at 20°C, at least one liter of water per 130 g of disodium terephthalate is required. The dissolution process is advantageously carried out in a stirred tank, a static mixer, or by means of a rotor-stator shear apparatus (inline disperser). In this alternative step, it is also possible according to the invention to directly add to the starting material mixture, in addition to adding the solvent - also in the form of an aqueous sulfate / acetate solution - a defined amount of the acid that is used in a precipitant in a subsequent process step of precipitation crystallization.According to the invention, the amount of acid to be used at this stage of the process is selected such that excess metal hydroxide is neutralized, but no aromatic dicarboxylic acid crystallizes. This advantageously reduces discoloration, which impairs the quality of the final product. It also advantageously reduces corrosion of downstream plant components and advantageously prepares the starting material mixture for subsequent purification, e.g., by adsorptive processes.

[0012] In a subsequent step of separating undissolved contaminants, the suspension obtained in the last step is separated from undissolved components by solid / liquid separation. Continuous and batch processes, such as inertia-based processes such as sink-float separation and filtration, are used for this purpose. When filtration processes are used, they are preferably carried out in multiple stages, typically with 2-8 stages and decreasing particle size distributions, for example, from 150 µm down to 0.5 µm absolute particle size distribution. Filtration preferably takes place at temperatures below 30°C to separate even substances with low solubility that would otherwise dissolve.Also usable according to the invention is the separation of very fine solid particles by flocculation with flocculants such as polyamines, sodium aluminates, magnesium chloride, iron sulfate and others to form larger agglomerates, so that the solid / liquid separation becomes simpler and more efficient.

[0013] The liquid phase obtained in this way is subsequently subjected to a step of separating dissolved interfering components such as soluble dyes, UV stabilizers, additives, etc., whereby, according to the invention, the dissolved interfering components are separated by adsorption on, for example, activated carbon, zeolites, iron hydroxide oxide, or by extraction processes. The activated carbon selected is one with low affinity for the dissolved metal carboxylates to avoid product losses. If necessary, the pH is adjusted by adding an acid to improve purification; this is preferably the acid later used as the precipitant. If extraction processes are used, either the undesired interfering components or the metal carboxylates are transferred to the extract phase. Suitable extraction agents have either the lowest possible affinity for the dicarboxylic acid or a particularly high affinity for the selective extraction of the dicarboxylic acid.It is preferred to separate the impurities in order to leave the metal carboxylates in the aqueous medium.

[0014] According to the invention, the step of separating dissolved interfering components is followed by a step of reactive precipitation by adding an acid. In this step, the metal carboxylate(s) present in the purified solution are displaced from their salts by the acid and react to form the respective dicarboxylic acid, which precipitates from the solution.

[0015] This particularly important step according to the invention leads to the precipitation of an aromatic dicarboxylic acid with optimized properties in terms of particle size and color through the selection of an acid with very specific properties, which significantly improves its processability in terms of filterability, washability, bulk density, and flow properties of the mixture with monomer alcohols. Furthermore, a particularly pure dicarboxylic acid is obtained because the acid according to the invention reacts selectively with the metal carboxylates of the less soluble aromatic dicarboxylic acid, and any other metal carboxylates of the more soluble aromatic dicarboxylic acids present in the solution are only slightly precipitated by the acid according to the invention.In other words, using the example of mixtures of benzenedicarboxylic acids, the more soluble isomers phthalic acid and isophthalic acid are precipitated to a much lesser extent by the acid according to the invention, but the less soluble terephthalic acid is precipitated to a greater extent.

[0016] If, according to the invention, only so little solvent is used in the first step of the pretreatment that primarily the most soluble isomer is present in the solution and has been separated, then primarily only this isomer can precipitate from its solution.

[0017] In addition to the inventive use of an acid with at least one higher pKa value, the special process parameters of the precipitation also contribute to the recovery of an aromatic dicarboxylic acid of high purity and with optimal physical properties. These include, in particular, a process temperature between 50°C and 100°C, which produces surprisingly larger aromatic dicarboxylic acid crystals, especially terephthalic acid crystals, than a higher temperature, in particular crystals up to 10 times larger. Larger dicarboxylic acid crystals, especially terephthalic acid crystals, allow for more effective and faster solid / liquid separation. At the same time, the addition of acid at these temperatures, and especially at 80°C - 95°C, leads to a reduction in crystal discoloration.Finally, using the example of terephthalic acid isomers, this process temperature also leads to a significantly shifted equilibrium in the crystals between terephthalic acid and isophthalic acid, should the latter still remain in the solution. The terephthalic acid:isophthalic acid ratio is at least 3:1.

[0018] According to the invention, an acid is used whose pKa value is higher than that of the less soluble aromatic dicarboxylic acid, in particular terephthalic acid. It was therefore surprising to the applicants that such a weak acid is not only suitable for the precipitation reaction at all, but is even particularly suitable, since it leads to terephthalic acid crystals of a particularly advantageous size for further processing. The ethanoic acid used according to the invention, in particular for the precipitation reaction, has a pKa value of 4.76, a lower acid strength than terephthalic acid, which has a pKa value of 3.54 and a pKa value of 4.46. The use and method of adding ethanoic acid to the solution of metal carboxylates of various benzenedicarboxylic acids also leads to the targeted, largely selective extraction of the less soluble acid, for example terephthalic acid compared to isophthalic acid, from its salt.In contrast, the sulfuric acid commonly used in the prior art, when added in equimolar amounts based on the number of available and required functional acid groups, leads to a higher yield of terephthalic acid, possibly in a mixture with isophthalic acid. In other words, the invention accepts a lower overall isomer yield compared to the use of sulfuric acid, which enables a yield of up to 99.9%, in order to obtain a purer and more easily processable product of a single dicarboxylic acid isomer. The yield reduction can be as much as 20%.

[0019] The concentration of the acid in the precipitant is generally between 1% and 100%. According to the invention, precipitation can also be carried out at a temperature between 100°C and 200°C and at a corresponding vapor pressure. Subsequent cooling of the resulting suspension in a suitable manner yields crystals of the aromatic dicarboxylic acid, which, in terms of their size and morphology, are suitable for direct use in conventional industrial polymerization processes, for example, terephthalic acid for the production of PET.

[0020] In a subsequent step of solid / liquid separation and washing of the precipitate and the recrystallization product, the suspension of crystals resulting from reactive precipitation or recrystallization is subjected to solid / liquid separation to separate the aromatic dicarboxylic acid from its mother liquor. Continuous and batchwise solid / liquid separation devices are used for this purpose, 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. The mother liquor obtained before washing is optionally collected separately and used partially or entirely as a solvent for the original starting material mixture, as explained above.The use of continuously operating filtration devices such as belt, indexing belt or rotary drum filters enables the direct washing of the moist filter cake with a washing medium, preferably water.

[0021] An optional post-treatment step in the form of recrystallization of the precipitated dicarboxylic acid or the mixture of dicarboxylic acids can follow according to the invention. In this step, the crystals obtained from the reactive precipitation are optimized in terms of purity and morphological properties by recrystallization. Redissolving the precipitated aromatic dicarboxylic acid in a solvent followed by crystallization leads to the removal of the impurities trapped in the crystal. Depending on the temperature-dependent solubility behavior of the target dicarboxylic acid, cooling, evaporation, or flash crystallization processes are used. The entire recrystallization takes place batchwise or continuously, in one or in two to six stages, with the temperature decreasing in each stage starting from an initial maximum dissolution temperature, with continuous recrystallization being preferred.When using water as a solvent for the recrystallization of terephthalic acid, temperatures of 180°C–280°C at appropriate vapor pressures are advantageous for dissolving. In contrast to conventional purification and crystallization processes in industrial production processes for high-purity aromatic dicarboxylic acids, the invention advantageously eliminates the need to select a temperature and concentration for complete dissolution of all resuspended crystals, since the aromatic dicarboxylic acid resulting from the depolymerization process does not exhibit the impurities typical of conventional production processes for aromatic dicarboxylic acids. This advantageously reduces process costs and duration. The removal of impurities and morphological adaptation are dynamic processes that can also be based on crystal maturation processes.This dynamic dissolution and crystallization process can take between 5 and 300 minutes, depending on the required purity. Solid / liquid separation and washing of the crystallizate are carried out analogously to the method described above.

[0022] Finally, the washed and mechanically dehydrated crystals are dried. Various batch and continuous drying devices are used for this purpose, such as fluidized bed dryers, belt dryers, paddle dryers, spray dryers, or freeze dryers. Drying takes place in one to four stages. Example 1

[0023] A pasty mixture of 124.5 g of disodium terephthalate, 1.9 g of disodium isophthalate, 2.4 g of sodium hydroxide, 37.3 g of ethylene glycol, 2.1 g of polyethylene terephthalate flakes, and 3.9 g of polyethylene flakes from the alkaline ester hydrolysis of a PET / PE multilayer packaging material was mixed with 1000 g of deionized water. At 20°C, the salts of the aromatic dicarboxylic acids dissolve completely, forming a suspension of PE flakes with PET residues with a pH of 13.4. This suspension was filtered through a slotted sieve with a gap width of 150 µm and then through a sieve filter with a mesh width of 50 µm. 95 wt.% of the undissolved polymer components of the suspension were separated. By subsequent fine filtration using a polypropylene absolute filter cartridge with a filter fineness of 1 µm, 99.9 wt.% of the remaining undissolved components were separated.The resulting clear solution was adjusted to a pH of 9.8 by adding 11 mL of 25% acetic acid while stirring. Only the sodium hydroxide present is neutralized to sodium acetate. 13.8 g of activated carbon granules with a low affinity for the aromatic dicarboxylic acids present were then added to this slightly yellowish solution, and the suspension was heated to 75°C while stirring. After a residence time of 50 minutes, the now colorless solution was freed from the activated carbon by filtration and transferred to a 1.5 L glass flask. There, the solution was heated to 80°C, and then 162.7 g of 80% acetic acid was added via a peristaltic pump with a dosing tube. The amount used corresponds to a stoichiometry factor of 3.6 based on the total amount of aromatic dicarboxylic acids present, and 95% by weight of these precipitates.The tip of the dosing tube was positioned directly above the blades of the propeller stirrer to ensure optimal mixing of the reaction medium. The dosing time was 70 minutes, followed by stirring for 50 minutes at 300 rpm. The suspension was then cooled to 30°C, filtered through a vacuum filter, and washed in two stages with 200 mL of deionized water each time. The isophthalic acid content of the resulting crystals was 0.2 wt% (1.5 wt% in the starting material), determined by HPLC measurement with a UV / Vis detector.

[0024] 30 g of the moist crystals with a residual moisture content of 20 wt.% were slurried with 394 g of deionized water and added to a stirred 500 mL pressure reactor. The suspension was heated to 210°C at 300 rpm, resulting in a vapor pressure of 18.5 bar. The residence time at 210°C was 60 min. Under the stated process conditions, approximately 10 g of the 24 g of crystals dissolved. The suspension was then gradually cooled to ambient temperature over a period of 90 min. After further filtration using a vacuum suction filter and subsequent washing, the crystals were dried at 85°C for 10 h. The quality of the resulting crystals can only be achieved with the process described above. It meets the requirements for terephthalic acid for use in industrial plants for the production of polycondensates.In terms of flow properties, it achieves viscosities of < 5 Pa*s at 24°C in suspensions with ethylene glycol in molar ratios of 1:1.1 to 1:1.15. Regarding product purity, a color value of < 10°H is achieved. Example 2

[0025] A pasty mixture of 121.3 g of disodium terephthalate, 6.3 g of disodium isophthalate, 2.4 g of sodium hydroxide, 37.4 g of ethylene glycol, 2.1 g of polyethylene terephthalate flakes, and 1.2 g of polyethylene terephthalate flakes from the alkaline ester hydrolysis of a post-consumer PET packaging material was mixed with 1,000 g of deionized water. At 20°C, the salts of the aromatic dicarboxylic acids dissolved completely, resulting in a suspension of PET flakes with a pH of 13.4. This suspension was filtered using a filter cloth with a fineness of 5-10 µm. 98 wt.% of the undissolved polymer components of the suspension were removed. Subsequent ultrafine filtration using a polypropylene absolute filter cartridge with a filter fineness of 1 µm removed 99.9 wt.% of the remaining undissolved components. Ultrafine clarification to a fineness of 50 nm was carried out in a subsequent polyethersulfone membrane filter cartridge.The resulting clear solution was adjusted to a pH of 8.9 by adding 12.5 mL of 25% citric acid while stirring. Only the sodium hydroxide present was neutralized, forming sodium citrate. 14 g of activated carbon granules with a low affinity for the aromatic dicarboxylic acids present were then added to this slightly yellowish solution, and the suspension was heated to 60°C while stirring. After 60 minutes, the now colorless solution was freed from the activated carbon by filtration and transferred to a 1.5 L glass flask. There, the solution was heated to 65°C and then, via a peristaltic pump with a dosing tube, 617 g of 25% citric acid were added, corresponding to a stoichiometry factor of 1.33 based on the total amount of aromatic dicarboxylic acids present. This resulted in the precipitation of 92 wt.% of the aromatic dicarboxylic acids present. The isophthalic acid content of the resulting precipitate was 2.8 wt.% (at 5 wt.-% in the starting material), determined by HPLC measurement with UV / Vis detector.

[0026] All further steps followed analogously to Example 1. Example 3

[0027] The particle-free and colorless solution from Example 2 was heated to 65°C and mixed with 472.3 g of 25% orthophosphoric acid in a stirred 1.5 L glass reactor, thus at a stoichiometry factor of 2. 87% of the aromatic dicarboxylic acids contained precipitated. The isophthalic acid content of the resulting precipitate was 2.7 wt.% (at 5 wt.% in the starting material, determined by HPLC measurement with a UV / Vis detector).

[0028] The process according to the invention for obtaining aromatic dicarboxylic acids from their metal carboxylates using an acid in the precipitant which is weaker or at most equally strong than the dicarboxylic acids surprisingly leads to a product which is highly pure and has optimized physical properties for subsequent use in polymerization reactions, to which an optional step of separating a more soluble metal carboxylate isomer early in the process also contributes.

Claims

1. A method for extracting aromatic dicarboxylic acids from an effluent of a basic depolymerization reaction of polycondensates containing metal carboxylates of the aromatic dicarboxylic acid to be extracted, in which a mineral acid or an organic acid is added to this effluent and has at least a pKs value that is greater than or equal to that of the underlying aromatic dicarboxylic acid of the polycondensate.

2. The method according to claim 1, comprising the steps of a) dissolving the metal carboxylates in the effluent; b) carrying out a reactive precipitation by adding the mineral and / or organic acid; and c) separating the precipitated dicarboxylic acid.

3. The method according to claim 1 or 2, furthermore comprising one or more of the following steps: separating solid impurities of the effluent; separating liquid and / or dissolved impurities of the effluent; and purifying the precipitated dicarboxylic acid.

4. The method according to claim 3, in which the step of purifying by way of recrystallisation at temperatures between 180°C and 280°C, preferably between 230°C and 275°C, is carried out in such a way that not all resuspended crystals are necessarily dissolved.

5. The method according to any one of the preceding claims, in which step a) is carried out using such a small amount of solvent that only the better-soluble metal carboxylate(s) is / are dissolved, wherein the solvent is preferably water.

6. The method according to any one of the preceding claims, in which the acid of the precipitating agent is ethanoic acid in pure or diluted form.

7. The method according to any one of the preceding claims, in which the acid is added directly to the solution, in particular via agitator paddles of an agitator.

8. The method according to any one of the preceding claims, in which the step of reactive precipitation is carried out at a temperature between 50°C and 200°C, and in particular between 50°C and 100°C.

9. The method according to any one of the preceding claims, in which all method steps are carried out continuously.