PROCESS FOR EXTRACTING AROMATIC DICARBOXYLIC ACIDS FROM THEIR METAL SALTS

Abstract

The present invention relates to a process for extracting aromatic dicarboxylic acids from an outlet stream of a basic depolymerization of polycondensates containing metal carboxylates of the aromatic dicarboxylic acid to be extracted, wherein a mineral or organic acid is added to this outlet stream, the pKs value of which acid being greater than or equal to that of the aromatic dicarboxylic acid, on which the polycondensate is based.

Claims

1. A method for producing aromatic dicarboxylic acids from an effluent of a basic depolymerization reaction of polycondensates comprising metal carboxylates of the aromatic dicarboxylic acid to be produced, wherein a mineral acid or an organic acid is added to this effluent, which 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; 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, wherein the step of purifying by way of recrystallization 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 claim 1, wherein step a) is carried out using such a small amount of solvent that only the better-soluble metal carboxylate is dissolved, or the better-soluble metal carboxylates are dissolved, the solvent preferably being water.

6. The method according to claim 1, wherein the acid of the precipitating agent, this being ethanoic acid, is present in pure or diluted form.

7. The method according to claim 1, wherein the acid is added directly to the solution, in particular via agitator paddles of an agitator.

8. The method according to claim 1, wherein 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 claim 1, wherein all method steps are carried out continuously.

10. The method according to claim 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.

11. The method according to claim 10, wherein the step of purifying by way of recrystallization 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.

12. The method according claim 11, wherein step a) is carried out using such a small amount of solvent that only the better-soluble metal carboxylate is dissolved, or the better-soluble metal carboxylates are dissolved, the solvent preferably being water.

13. The method according claim 12, wherein the acid of the precipitating agent, this being ethanoic acid, is present in pure or diluted form.

14. The method according to claim 13, wherein the acid is added directly to the solution, in particular via agitator paddles of an agitator.

15. The method according to claim 14, wherein 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.

16. The method according to claim 15, wherein all method steps are carried out continuously.

17. The method according to claim 2, wherein step a) is carried out using such a small amount of solvent that only the better-soluble metal carboxylate is dissolved, or the better-soluble metal carboxylates are dissolved, the solvent preferably being water.

18. The method according to claim 3, wherein step a) is carried out using such a small amount of solvent that only the better-soluble metal carboxylate is dissolved, or the better-soluble metal carboxylates are dissolved, the solvent preferably being water.

19. The method according to claim 4, wherein step a) is carried out using such a small amount of solvent that only the better-soluble metal carboxylate is dissolved, or the better-soluble metal carboxylates are dissolved, the solvent preferably being water.

20. The method according claim 19, wherein the acid of the precipitating agent, this being ethanoic acid, is present in pure or diluted form.

Description

EXEMPLARY EMBODIMENT 1

[0024] A pasty mixture made up of 124.5 g disodium terephthalate, 1.9 g disodium isophthalate, 2.4 g sodium hydroxide, 37.3 g ethylene glycol, 2.1 g polyethylene terephthalate flakes and 3.9 g polyethylene flakes from the alkaline ester hydrolysis of a PET/PE multilayer packaging material was mixed with 1000 g deionized water. At 20 C., the salts of the aromatic dicarboxylic acids dissolve completely, resulting in a suspension of PE flakes with PET residues having a pH value of 13.4. This suspension was filtered by means of a wedge wire screen having a slot width of 150 m, and subsequently by means of a sieve filter having a mesh size of 50 m. In this way, it was possible to remove 95% by weight of the undissolved polymer components of the suspension. 99.9% by weight of the remaining undissolved components was removed by subsequent microfiltration by means of a polypropylene absolute rated filter candle having a grade of filtration of 1 m. The resultant clear solution was adjusted to a pH value of 9.8 while stirring by adding 11 mL 25% acetic acid. Only the sodium hydroxide that is present is neutralized to yield sodium acetate. Thereafter, 13.8 g activated carbon granules having a lower affinity for the present aromatic dicarboxylic acids was added to this slightly yellowish solution, and the suspension was heated to 75 C. while stirring. Following a residence time of 50 min, the activated carbon was removed from the now colorless solution by means of filtration, and the solution was charged into a 1.5 L glass flask. There, the solution was heated to 80 C. and thereafter was mixed with 162.7 g 80% acetic acid via a peristaltic pump comprising a metering tube. The amount used corresponds to a stoichiometry factor of 3.6, based on the total resulting substance amount of aromatic dicarboxylic acids and precipitates 95% by weight thereof. The tip of the metering tube, in terms of the apparatus, was located directly above the paddles of the propeller agitator used so as to ensure optimal mixing of the reaction medium. The metering duration was 70 min, and the subsequent stirring duration was 50 min at 300 rpm. Thereafter, the suspension was cooled to 30 C., filtered by means of a vacuum Nutsche filter, and washed in two stages, using 200 ml deionized water in each case. The isophthalic acid content of the resulting crystallisate was 0.2% by weight (1.5% by weight in the starting substance), as ascertained by HPLC measurement using a UV-Vis detector.

[0025] 30 g of the moist crystallisate having a residual moisture of 20% by weight was turned into a slurry with 394 g deionized water and charged into a stirred 500 mL pressurized reactor. The suspension was heated to 210 C. at 300 rpm and an accordingly resulting vapor pressure of 18.5 bar. The residence time at 210 C. was 60 min. Under the aforementioned process conditions, approximately 10 g of the resulting 24 g crystallisate is dissolved. Thereafter, the suspension was gradually cooled within 90 min to ambient temperature. After renewed filtration by means of a vacuum Nutsche filter and subsequent washing, the crystallisate was dried at 85 C. for 10 h. The quality of the crystallisate thus obtained can only be achieved by way of the above-described method. It meets the requirements with regard to terephthalic acid for use in industrial plants for producing polycondensates. In terms of the flow properties, in suspensions with ethylene glycol in molar ratios from 1:1.1 to 1:1.15, it reaches viscosities of <5 Pa*s at 24 C. With respect to the product purity, a tristimulus value of <10 H is achieved.

EXEMPLARY EMBODIMENT 2

[0026] A pasty mixture made up of 121.3 g disodium terephthalate, 6.3 g disodium isophthalate, 2.4 g sodium hydroxide, 37.4 g ethylene glycol, 2.1 g polyethylene terephthalate flakes and 1.2 g polyethylene terephthalate flakes from the alkaline ester hydrolysis of a PET post-consumer packaging material was mixed with 1,000 g deionized water. At 20 C., the salts of the aromatic dicarboxylic acids dissolved completely, resulting in a suspension of PET flakes having a pH value of 13.4. This suspension was filtered by means of a filter cloth having a grade of filtration of 5-10 m. In the process, 98% by weight of the undissolved polymer components of the suspension was removed. 99.9% by weight of the remaining undissolved components was removed by way of subsequent microfiltration by means of a polypropylene absolute rated filter candle having a degree of filtration of 1 m. The ultrafine clarification until a degree of filtration of 50 nm was reached was carried out in a downstream polyether sulfone membrane filter candle. The resultant clear solution was adjusted to a pH value of 8.9 while stirring by adding 12.5 mL 25% citric acid. Only the sodium hydroxide that was present was neutralized, resulting in the formation of sodium citrate. Thereafter, 14 g activated carbon granules having a lower affinity for the present aromatic dicarboxylic acids was added to this slightly yellowish solution, and the suspension was heated to 60 C. while stirring. After 60 min, the activated carbon was removed from the now colorless solution by means of filtration, and the solution was charged into a 1.5 L glass flask. There, the solution was heated to 65 C., and thereafter was mixed with 617 g of a 25% citric acid via a peristaltic pump comprising a metering tube, corresponding to a stoichiometry factor 1.33 based on the total present substance amount of aromatic dicarboxylic acids. This caused 92% by weight of the aromatic dicarboxylic acids contained therein to precipitate. The isophthalic acid content of the resulting precipitate was 2.8% by weight (at 5% by weight in the starting substance), as ascertained by HPLC measurement using a UV-Vis detector.

[0027] All further steps followed analogously to Exemplary Embodiment 1.

EXEMPLARY EMBODIMENT 3

[0028] The particle-free and colorless solution from Exemplary Embodiment 2 was heated to 65 C. and mixed, in a stirred 1.5 L glass reactor, with 472.3 g 25% ortho-phosporic acid, consequently at a stoichiometry factor of 2. In the process, 87% by weight of the aromatic dicarboxylic acids contained therein precipitated. The isophthalic acid content of the resulting precipitate was 2.7% by weight (at 5% by weight in the starting substance, as ascertained by HPLC measurement using a UV-Vis detector.

[0029] The method according to the invention for producing aromatic dicarboxylic acids from the metal carboxylates thereof, using an acid that, compared to the dicarboxylic acids, is weaker or at the most equally acidic, in the precipitating agent, surprisingly results in a highly pure product that is optimized, in terms of the physical properties thereof, for the use later on in polymerization reactions, to which an optional step of a step of separating a better-soluble metal carboxylate isomer carried out early during the course of the method.