DE4223776C1 - Recycling amine from sand moulding process - by progressively increasing pH of used gases in solution and removing amine by gravity separation - Google Patents

Abstract

Recycling amine used in sand moulding as a hardening catalyst comprises collecting the gas mixture which has passed through the sand moulding to be hardened and dehydrating the aqueous mixt. by adding a strong alkali which renders the amine less soluble in water. By gravity separation the dehydrated amine is separated from the lead solution and distilled to remove residual processing contaminant. ADVANTAGE - The amount of chemical added is reduced to make the process more economical. The air released to the atmosphere is free from pollutants and waste water and salt solutions pose a significantly reduced disposal problem.

Description

The invention relates to a method for hardening sand Shaped bodies, in particular casting cores, according to the preamble of 1. Patent claim, as for example from DE-PS 25 50 588 or DE-PS 37 36 775 emerges as known.

According to the well-known cold curing process, this can be done sand moldings with good resistance to bending and abrasion speed, high dimensional accuracy and surface quality as well as good ter shelf life from cold molds in very short Establish cycle times. The sand mix for this cold curing process consists of a dry quartz sand and a liquid two-component synthetic resin binder system, which at Zu occurs a catalyst hardens.

As a catalyst when carrying out the process in space temperature liquid, volatile amines, mostly triethyl amine, dimethylethylamine or dimethylisopropylamine used. These amines are malodorous and toxic substances which is why you always tried, for health reasons to remove these amines from the core air exhaust stream nen. This was done by acid washing the exhaust air flow, by post-combustion of the exhaust air flow or by a Combustion of the exhaust air stream in a cupola furnace after feeding into the hot wind stream. With these procedures, the Amines lost, which at the relatively expensive purchase price is to be regarded as negative. One has therefore already tried  to remove the amines from the exhaust air flow during core production distant and at the same time for a renewed use at the end to recover the hardening process. So in DE-PS 25 50 588th a method and device for curing sand mold body specified, after which from the catalyst-containing exhaust gas core manufacturing the catalyst and other condensable Vapors are separated by condensation. The collected Condensate is subjected to fractional distillation, the catalyst fraction branched off and reused. After Part of this procedure is that the catalyst in the carrier gas mixture only in a relatively low con concentration is present. To remove the catalyst is therefore a strong cooling of the entire gas flow is required Lich. Because of the high costs involved, there could be not enforce the procedure in practice. From the DE-PS 37 36 775 is another method of curing sand mold bodies known, in which after the permeation of the catalyst / Carrier gas mixture through a semipermeable membrane Catalyst-enriched gas mixture is generated from which the Catalyst z. B. also recovered by condensation can be. This procedure is also not yet complete satisfactory, albeit with the processing costs for the exhaust air can be reduced.

In the applicant's core manufacturing practice, the Recovery of the amine from the form, catalyzed Sator-containing gas mixture without mixing with the surrounding air is collected as completely as possible and a sulfuric acid Subjected to laundry. For this, the gas mixture is diluted with Sulfuric acid contacted, the most volatile Amine is bound as amine sulfate. At a salary of around 10 % By weight of amine sulfate becomes the solution by adding sodium hydroxide solution neutralized and the pH increased. Thereby emerges from the Amine sulfate is again the volatile amine, which by a  simple rectification back as a pure amine compound is won. Although with such a sulfuric acid wash good cleaning of the catalyst-containing exhaust air at the Core production can be achieved and also a good return extraction of the amine is possible, but the process has decisive disadvantages. These consist in that Acid wash requires large amounts of sulfuric acid which, however, only used about 10% to form amine sulfate will. To release the amine from the amine sulfate is how therefore a large amount of caustic soda is required because the welding Rock acid completely new with the appropriate amount of lye must be tralized. So there is considerable knockout most for the sulfuric acid and sodium hydroxide and in addition, considerable costs for z. B. Depo nation of the sodium sulfate formed and the disposal of the salted wastewater.

In general, it is also from the prior art knows, for the purpose of isolation, drying and Reini amines these basic compounds in the form of alka lien add, the mixture by this addition of Hy droxid ions is brought into a strongly alkaline range. The mixtures thus obtained are then heated, de breastfeeding and the purified amine recovered (DE-OS 17 68 743, DE-AS 11 20 458, DE-OS 23 04 303, US-PS 33 37 630 and U.S. Patent 33 94 186).

From GB-PS 12 38 351 a method for cleaning Hexamethylenediamine, with sodium hydroxide and potassium hy hydroxide added to the solution of the amine and then the Lö solution is distilled and purified amine is obtained.

These amine purification procedures, in particular alone by distillation have the disadvantage that  due to the high affinity of the amine for water and bil formation of an azeotropic mixture, the implementation problematic that is. With the exhaust air from the core casting machines for sand Shaped body is also added that solvent and core Binder components are present, their share in the exhaust air Can be 50% or even more. For the sake of order The exhaust air from these components is also environmentally friendly clean, d. H. only a recovery of the amine from the Exhaust air without removing these components from the Air is unsatisfactory as this is only an incomplete one Exhaust air purification and therefore no improvement over would mean burning them.

The invention is therefore based on the object in a gat Appropriate method for hardening sand shaped bodies, the recovery of the catalyst used Amines to improve that a physical Ab separation process for the amine is used, the amount of the chemicals used can no longer be recovered Amine, but from the amount of impurities present is dependent, exhaust air is freed of all impurities and a disposal of waste water and / or waste salts clearly is reduced and that is easy and inexpensive to carry out.

The task set is based on the invention of the generic method with the characteristic features len of the 1st claim solved.

The solution to the problem is that the Rei Cleaning of the amine mixed with water and solvent in this way takes place that in the collected from the core hardening, amine-containing starting mixture the solution equilibrium of amine in water through strong alkalization of the starting mixture, preferably by adding OH ions towards one  low solubility of amine shifted in water and thereby gravity separation of - lighter, dewatered - Amine and - heavier, more water-containing - lye is forced and that the stripped, dewatered amine is then distilled is cleaned from the other process substances.

Thanks to the amine drainage and purification according to the invention a safe and simple as well as both in terms of inv stition costs as well as in terms of operating costs received procedure. Because the amine drainage and -Cleaning with relatively small amounts of the amine in the output mixture takes place, the process facilities ent talking small and inexpensive designed and the Ener Amounts of fuel and fuel are also small and therefore of Cost volume low. As a related to the amine Drainage essential point is that the amine forms an azeotrope with water and thus only on distillati vem ways cannot be completely drained, which is why here has been proposed for processes which have always been very complex the. A full drainage of the amine is for its how absolutely necessary to use; is against the other process-accompanying amine impurities are so high Purity, such as not necessary with regard to the absence of water, to reuse the amine in the curing of sand moldings to be able to.

Advantageous embodiments of the invention are the dependent claims Chen 1 to 7 can be seen. Otherwise, the invention is based of an embodiment shown in the drawing explained below.

In a preferred embodiment, the drainage of the starting mixture containing amine in several stages, each with a separate one Lye addition and subsequent gravity separation in case of  carried out increasing pH. It did found to be particularly advantageous that the drainage is carried out in three stages, one after the other stage a liquid alkali of sodium and / or potassium hy hydroxide, in the second stage solid sodium hydroxide and in the third stage also solid potassium hydroxide as an alkali agents are added. For environmental reasons and also for reasons of cost, in a preferred procedure the diluted alkali solution removed in the first stage Phase a remaining residue of water-containing starting amine and solvent distilled off and condensed and can Circulatory process are fed again. The rest of the rest lye is removed from the circulatory process.

In a further preferred embodiment, the abde Breast-fed water-containing starting amine together with solvent in a secondary circuit for drainage thrown, the phase separation in the second and / or third stage separation stage alkali phase is used.

Preferably, the dewatered in the secondary circuit is also amine-containing phase for further amine drainage in the second Stage of the main circuit and fed together with the in the first separation stage dewatered amine phase.

A method implementation is also preferred, in which the in the secondary phase separated aqueous phase in the form of a liquid lye or a lye mixture of sodium and / or Potassium hydroxide before the first stage as a liquid lye mix of sodium hydroxide and potassium hydroxide as a first stage Alkalizing agent is fed into the main circuit.

As is known, potassium hydroxide is more capable of aqueous amines Drying as sodium hydroxide. In this respect, it is also a procedure  Re-implementation only possible with potassium hydroxide, d. H. so, that the 3rd and 2nd stages are combined in one stage. Because the cost of potassium hydroxide is higher than that of sodium hydroxide, is a decision to be made on whether to get higher Will accept the cost of a third drainage device or the higher cost of the alkali hydroxide to be used.

The procedure according to the invention is in accordance with the procedure the starting mixture is drained until there is no alkali metal droxid solves more. The amount of alkali hydroxide added is accordingly depending on the water content of the amine Starting mix.

The procedure - the stirring processes - takes place in the Practice - Use of Diethylamine Isopropylamine (DMIA) as Catalyst - at all stages at room temperature. Unless as Catalyst in the core hardening z. B. triethylamine is set, it is necessary because of the lower boiling point to work at lower temperatures.

The stirring time - or the residence time of the mixtures - in the mixing vessels upstream of the separation vessels everything depends on the intensity of the stirring process and on the Amount of alkali hydroxide fed. Will the individual Mixing vessels are only fed with as much lye as immediately in solution is the stirring time or the residence time of the mixture is long less than if the lye is added in excess, d. H. so in the latter case, more alkali is metered in than for absorption of the water present in the mixture is necessary.

The timing of the phase equilibrium is natural also depending on the constitution of the amine in the output mixture, d. H. as diethylamine or triethylamine etc. and also  on the type and amount of solvent present, e.g. B. Benzene, toluene or xylene.

In practice, there has been a quantitative addition revealed that drying of one liter of amine-containing gait mixture over the three drying stages - water content about 50% - about 10 g sodium hydroxide and 1 g potassium hydro xid are sufficient. Abge from the third separation container drawn amine and solvent phase began approximately boil at 65 ° C and the pure amine fraction (diethylamine) is distilled off up to 120 ° C in a known manner. Continuing was distilling off the solvent fraction at 160 ° C completed.

In the schematic drawing 1 means the feed for the condensed residue mixture containing amine, solvent and water (pH around 8), which is obtained when hardening sand moldings and which is introduced into a stirred tank 2 which is provided with a stirrer 5 . In the stirred tank 2 opens another feed line 3 , the diluted lye (heavier phase) 44 from a - arranged in a secondary circuit - separating tank 4 feeds the stirred tank 2 . After appropriate mixing with stirring, the starting mixture obtained is fed in the main circuit via a line 6 to the 1st separation tank 7 , where the alkalization of the mixture leads to gravity separation - lighter, dewatered amine and heavier water-containing alkali - and thus to phase separation . The lighter phase 8 formed from the 1st separation container 7 (dewatered amine) is fed via a feed line 9 to a 2nd stirred container 10 , into which 11 sodium hydroxide solution is introduced via a tube. From the separating tank 4 arranged in a secondary circuit, the amine-containing phase (lighter phase) 43 is also fed to the second separating tank 10 via a feed line 12 and the entire mixture 13 is stirred.

The separated from the first separation container 7 heavier Pha se 14 is fed via a derivative 15 into a further branch circuit of a distillation device 16 .

After appropriate mixing in the stirred tank 10 , the mixture 13 formed is fed via a feed line 17 to a second separating tank 18 , where it is again - due to the addition of alkali metal hydroxide, the pH of the mixture has been shifted even further into the alkaline range phase separation into a lighter, amine-containing phase 19 and a heavier, alkali-containing phase 20 , the lighter phase 19 is fed via a line 21 to a 3rd stirred tank 22 into which solid potassium hydroxide is added via a tube 23 . The heavier phase 20 from the second separation tank 18 (pH value approx. 14) is fed via a line 24 to a - the separation tank 4 upstream - stirred tank 25 supplied. After thorough mixing of phase 19 with the potassium hydroxide, the resulting mixture 26 is fed via a line 27 to a 3rd separation container 28 , where in turn there is a phase separation into an amine-containing, lighter phase 29 and heavier, alkali-containing phase 30 . The lighter phase 29 is fed from the 3rd separation container 28 via a line 31 to a distillation device 32 . The heavier phase 30 (pH <14) is in turn fed to the stirred tank 25 via a line 33 .

In the distillation device 32 , the volatile constituents, that is to say the enriched amine, are distilled off, which is fed via a discharge line 33 to a cooling device 34 and condensed there in order to feed it back to a core hardening process. The remaining in the distillation device 32 higher-boiling fractions are fed via a line 35 to a second distillation unit 36 , the higher-boiling fractions - predominantly distilled off solvent - and fed via line 37 to a cooling device 38, where they are condensed and are also available for reuse to disposal. The bottom product remaining in the distillation device 36 is removed and discarded.

The heavier, alkali-containing phase 14 discharged from the first separation container 7 via the discharge line 15 is subjected to a distillation in a heating device 16 , the amine being distilled off and fed via a line 39 to a cooling device 40 and condensed there. The condensate is then fed to the stirred tank 25 via line 41 . The remaining in the distillation device 16 laugenhal term residue is conveniently discarded.

The heat removed from the second separation vessel 18 caustic phase 20, the heat removed from the 3rd separation vessel 28 caustic phase 20, the heat removed from the 3rd separation vessel 28 leach containing phase 30 and discharged from the condensing means 40 solvent-containing amine in the stirred tank 25 mixed and then fed via a line 42 to the separation container 4 , from which they the main circuit by feeding the lighter amine-containing phase 43 via line 12 into the 2nd stirred tank 10 or by feeding the heavier alkaline phase 44 via line 3 be fed back into the 1st mixing container.

The advantages of the method according to the invention are in particular special in that when curing sand moldings Recovery of the amine present in the exhaust air - in addition to Lö solvent and water vapor - by means of a simple and economical most economical separation process is to be carried out. From a be Already freed of water, dried amine-containing output the product becomes an amine by subsequent simple distillation of high purity. In addition to the technically simple Execution of the process is also only a small outlay on equipment  required. Disposal of the exhaust air from the curing device driving or of wastewater is only minimally required and there are almost no waste salts.

Claims (7)

1. A process for the catalytic hardening of sand moldings, in particular casting cores, made of synthetic resin-bonded sand using a gaseous or vaporous amine as a catalyst, which is added to a carrier gas, after which the catalyst / carrier gas mixture is pressed through the mold containing the loose sand moldings, the from the Form-escaping gas mixture is collected as completely as possible, from the gas mixture the amine, initially still strongly water-containing and contaminated with other process lubricants, is recovered as liquid pure amine, characterized in that for isolating, dewatering and purifying the amine in a reusable purity, this initially is dehydrated by in the aqueous amine and solvent-containing starting mixture, the solution equilibrium of amine in water by a strong alkali lization of the starting mixture, preferably by adding OH ions, in the direction of a lower solubility of amine in what water shifted and thereby a gravity separation of - lighter, dewatered - amine and - heavier, water-containing - lye is made possible and that the stripped ne, dewatered amine-containing phase is then distilled from the other process substances. 2. The method according to claim 1, characterized, that dewatering the starting mixture in several stages with each separate addition of alkali and subsequent gravity separation tion takes place with an increasingly reduced pH value. 3. The method according to claim 2, characterized, that with a three-stage drainage of the respective mixture in the first stage, a liquid alkali of sodium and / or Potassium hydroxide, solid sodium hydroxide in the second stage and in the third stage solid potassium hydroxide as an alkalizing agent medium is added. 4. The method according to claim 2, characterized, that from the aqueous phase obtained in the first separation stage (Lye) the amine still contained in a separate Separation stage by alkaline shift of the solution equilibrium is dewatered and thereby as an alkalization medium the aqueous separated in later separation stages Phase (lye) is used. 5. The method according to claim 2, characterized, that from the aqueous phase separated in the first stage (Alkali) a residue of amine still distilled off, kon densified and the remaining lye as excess lye from the Process is removed. 6. The method according to claim 2, characterized, that the separated in the separate separation stage, drained ester amine phase for further alkaline dewatering before  second separation stage and fed in together with that in which he most separation stage separated amine phase is dewatered. 7. The method according to claim 2, characterized, that the separated in the separate separation stage, in Form of a liquid lye or a mixture of lye from Na trium and / or potassium hydroxide resulting aqueous phase the first drainage stage as a first stage alkalization medium is returned to the main circle.