DE1022591B - Process for the purification of lactams of the ªÏ-aminocarboxylic acids - Google Patents

Description

Process for the post-purification of lactams of the c) -aminocarboxylic acids The lactams of the co-aminocarboxylic acids are used as starting materials for production used by polyamides. The quality of the products made from the polyamides depends to a large extent on the high purity of the raw materials used Lactams. The z. B. in the usual way by Beckmann rearrangement the alicyclic oximes obtained lactams by vacuum distillation or selective Extraction cleaned, probably normal lightfast, but none in sunlight stable lactams, d. that is, the pure white lactams yellow to a greater or lesser extent after a long time.

The polyamides made from it are especially shredded Condition, e.g. B. in the form of schnitzel, very sensitive to sharp drying Exposure to heat at higher temperatures in the presence of only traces of atmospheric oxygen, whereby they also turn yellow. The uncolored polyamide products also retain their bright white color does not last very long, but turns yellow after a while and show relatively rapid signs of aging in terms of weather resistance and lightfastness.

The cause of these deficiencies is that by the usual Purification methods of the lactams traces of certain impurities caused by the normal examination methods are difficult to detect and cannot be separated from the lactams are.

Attempts have already been made to distill the lactams by a Replace crystallization from organic solvents or this to an extraction to join the raw lactams. A crystallization of lactams from organic solvents, which usually dissolve the lactams less well, but has so far been used in technology no entry found yet. The reason is that on the one hand so far Crystallization of raw or still little prepurified lactams was carried out and on the other hand in the nature of the impurities, which are largely associated with the Lactams together excrete from the organic solvents, so that a high Cleaning effect could not be achieved. The last traces of the Organic solvents often stick very firmly to the crystallized lactams and interfere thereby further processing.

The lactams of the (o-aminocarboxylic acids are extremely soluble in water, and it can be at high lactam concentrations above the eutectic at Temperatures below the melting point of pure lactams saturated solutions produce, from which impure, caking and irregularly large lactam crystals that enclose a relatively large amount of mother liquor, retire.

It has now surprisingly been found that lactams of the o) -aminocarboxylic acids obtained of hitherto unknown highest purity and very good solubility are, if one in the process for the post-purification of lactams of the co-aminocarboxylic acids the largely purified aqueous solutions by crystallization from a solvent Supersaturation of lactams by evaporation of the water and by cooling dissipates the heat of crystallization while circulating the solution.

As largely purified lactams are z. B. those of co-aminocaproic acid, co-amino enanthic acid, co-aminoethyl acid, co-aminononylic acid, co-aminodecylic acid or co-aminoundecylic acid to be considered by at least one purification process from which raw lactams have been obtained. Such cleaning methods are z. B. vacuum distillation and selective extraction of the crude lactams. The one from it commonly obtained pure lactams contain despite their pure white color there are still traces of impurities that can only be detected spectrometrically. These Impurities are more soluble in water or aqueous lactam solutions than in organic solvents and remain the purest after crystallization Lactams in the mother liquor.

It is expedient to make concentrated lactams from the distilled aqueous solutions or uses the aqueous and concentrated lactam solutions, which one according to known methods by selective extraction with organic solvents obtained and the last remnants of solvent by steam distillation are exempt. By evaporating the water from the hydrous Solutions of the largely pure lactams below the melting point of the anhydrous Lactams are produced in this way supersaturated solutions of the lactams, followed by crystallization is effected by circulating and cooling this solution.

The heat generated during crystallization can be removed either by indirect cooling, e.g. B. by water, by evaporation of liquefied gases, or by direct cooling, whereby the concentration of the lactam solutions is reached to supersaturation at the same time with evaporation of water. This can be done either in a vacuum or under partial pressure while passing dry inert gases, e.g. B. air, nitrogen or carbon dioxide, can be carried out at temperatures below the melting point of the lactams. The supersaturation and crystallization is z. B. in the case of caprolactam expediently made under 50 ' .

Advantageously, depending on the temperature, the supersaturation becomes up to a lactam concentration of 75 to 95 ° o carried out. In this way, the Lactam crystals with a uniform grain size of about 1 mm and of the largest Purity obtained without the inclusion of mother liquors, which do not cake and unravel Let it spin off perfectly on centrifuges.

By using largely pure lactams for crystallization it is possible to achieve a high crystallization yield; so it is B. with caprolactam it is possible to obtain 50% or more with unchanged purity.

With the high concentration of lactams in water, this is enough even slight changes in concentration to make significant differences in the To achieve crystallization yield.

For better centrifugation of such difficult to flow crystal-rich Mixtures, it is useful to the separated mother liquor in the crystallizer returned, resulting in mixtures of lesser crystals and easier flow and can be thrown off more easily.

The separated mother liquor can be fed to a further crystallization which gives less pure lactam crystallizates and those of the first crystallization can be added again. When working up the mother liquor can before Crystallization also a purification z. B. be done by distillation or extraction. So the impurities are removed by vacuum distillation of the mother liquor in the forerun and residue largely withheld. Furthermore, the work-up of the mother liquor can take place in that they are in the production process of the lactams at a suitable point is returned.

The lactam crystals that accumulate on the centrifuge fall almost free of water and are expediently washed with a little water or with water-based Solutions of the pure lactams from the externally adhering residues of the mother liquor freed. In order to prevent the crystals from sticking together during storage, the crystals contain less than 0.5% water after being spun off. By drying afterwards they leave the crystals at temperatures below the melting point of the lactams win yourself anhydrous. However, this post-drying is for further processing not necessary for polyamides, since water is usually added during the polymerization must be added as a catalyst.

The crystallization process can be advantageous in a known manner perform continuously.

The lactams obtained by this crystallization keep their light color white color not only indefinitely in sunlight and air, but also deliver non-yellowing after further processing into polyamides and excellent weatherproof as well as lightfast products.

Examples 1. An aqueous solution of 92.5 parts by weight of enantholactam, obtained from the crude lactam by vacuum distillation and dissolving in water is concentrated to 7.5 parts by weight of water by evaporation, the solution is cooled while stirring with cold brine to 0` and lets them crystallize out at this temperature. After the crystals have been separated off and washed with very little ice water, 45 Parts by weight of the purest enantholactam with a melting point of 25 'are obtained.

2. 4.8 parts by volume of 85% aqueous caprolactam solution, which has been obtained by selective extraction of the crude lactam and removal of the solvent by steam distillation, are allowed to flow into a crystallizer at a temperature of 35 ° which has a content of 60 parts by volume and is filled with 12 parts by volume of crystallization mixture. Every hour, about 20,000 parts by volume flow through the crystal mixture in fine distribution, dried, oil-free and heated to 60 'air. Every hour 4.1 parts by volume of a crystal mixture are drawn off from the crystallizer, centrifuged off and washed with a little water. Thus, about 2.1 parts by weight of caprolactamic crystals of uniform grain size and high purity with about 0.307 water content are obtained per hour. F. 68 '.

3. 200 parts of oil flow into a vacuum evaporator every hour largely pre-cleaned, aqueous, about 80 ° own caprolactam solution, and this is evaporated at a temperature of 21 'and a vacuum of 18 mm until the Crystallization occurs. The resulting crystal mixture flows in continuously Barometrically submerged downpipe into the crystallizer, where by pumping over a cooler and a downpipe the temperature is kept at 21-. From the crystallizer the crystal mixture is continuously withdrawn, and the crystals «earth via a The centrifuge was removed by washing it slightly with a little water. A part of The mother liquor is continuously returned to the crystallizer. It “earths every hour 93 parts by weight of uniformly crystallized caprolactam with 0.2 l, ', water content preserved in excellent purity. F. 68.5 °.

Claims (3)

PATENT CLAIMS 1. A process for the post-purification of lactams of the c -) - aminocarboxylic acids by crystallization from a solvent, characterized in that aqueous solutions of the largely purified lactams are brought to supersaturation by evaporation of the water and the heat of crystallization is removed by cooling while the solution is circulated. 2. The method according to claim 1, characterized in that one saturation with simultaneous discharge the heat of crystallization by evaporating the water in a vacuum. 3. Procedure according to claim 1, characterized in that the supersaturation with simultaneous Dissipation of the heat of crystallization by evaporation of the '' water by passing it through inert gases. Documents considered: British Patent Specification No. 669 903.