DE4007798C1 - Hardening sand mould members - comprises mixing catalyst with carrier gas, mixing with sand, pressing sand, retrieving gas and hardening - Google Patents

Abstract

A process for hardening sand mould members esp. mould cores made of synthetic resin bound sand, includes using a catalyst, mixing it with a carrier gas, mixing with the sand, pressing the sand, retrieving the gas and hardening. The catalyst is cleansed and collected in the gas phase, and the impurities are removed. The removal of impurities is carried out by a permeation process. ADVANTAGE - The process cleansing technique is simple and economical.

Description

The invention relates to a method for hardening sand moldings, in particular casting cores according to the preamble of Claim 1, as for example from DE-PS 37 36 775 is known.

The catalysts become light, liquid at room temperature volatile amines, most commonly triethylamine, Dimethylethylamine or dimethylisopropylamine. Because the amines are foul smelling and toxic, they must not harm the environment reach. That's why you try them out of the Isolate, clean and reuse exhaust airflow. If this is not possible, the rest of Amines are removed from the exhaust air using an acidic medium or afterburned thermally. According to the ge called publication is the amine by means of a gas separator driving under pressure difference on a selectively permeable membrane bran enriched in a permeate stream and then con densified so that the initially recovered amine in liquid Form again. This liquid catalyst still contains very many impurities, according to the pressure mentioned separated by fractional distillation and the pure amine is isolated therefrom, which finally again condensed and added to the fresh catalyst liquid becomes. The disadvantage of this cleaning process is that high equipment and energy expenditure because both that  Condensing and distilling energy-consuming Ver driving, which can also be mastered with the relatively large ones gas quantities are also voluminous and accordingly expensive Make plants necessary.

The object of the invention is that of the generic type Improve or simplify procedures in such a way that the cleaning process is simplified and cheaper.

This object is achieved by the characterizing Features of claim 1 solved. Practically, one is separated No cleaning stage. Because it has overrun shenderly pointed out that in the gas separation process the harmful impurities on the Enrich the retentate page to a reasonable extent. Process contaminations also get with the enriched catalyst also on the permeate side; it comes there to a stabilizing state of equilibrium a certain concentration level of these impurities. However, this concentration of contaminants is excessive way for the reusability of the recovered natal catalyst practically harmless. Despite this Recycled impurities remain the catalytic effect the amines to a practically fully satisfactory extent receive.

Advantageous embodiments of the invention can the Unteran sayings are taken. Otherwise, the invention is based a process scheme shown in the figure below still explained; the only figure shows a process scheme to treat the catalyst when curing sand mold bodies.  

The process diagram shown in the drawing contains a core mold 2 , which, after it is closed and filled with loose resin-wetted core sand mixture, is fed via the inlet line 1, a mixture of carrier gas and gaseous, catalytically active amine. The carrier gas / catalyst mixture introduced into the core mold 2 is suctioned off as completely as possible via the exhaust line 5 ; the core mold 2 is sealed to the outside, but false air 3 is also sucked in from the surroundings of the core mold. When the carrier gas / catalyst mixture is passed through the core mold, part of the catalyst contained is adsorbed on the surface of the sand grains and removed from the mold with the hardened core. The portion of the catalyst not adsorbed by the sand and the evaporation of the synthetic resin or its hardener and binder components as well as the evaporation of a separating oil applied to the wall of the core mold before filling with sand can be found in the exhaust gas mixture extracted. In addition, due to the sucked-in false air 3 , water moisture activity from the surroundings is continuously dragged into the extracted exhaust gas. Of course, there is also dust of various origins in the exhaust gas extracted.

After working in the core shooter in cycles and the sand molding is cured, there is also a correspondingly cyclical and only for a short time on the exhaust line 5 from a gas mixture. In the interruption times of the hardening process, which are due to work breaks, mold filling or core removal and the like. The mixture discharge line can be closed by means of the changeover valve 4 .

After passage of the extracted exhaust gas mixture through a filter 6 , in which dust and other particles are retained, the exhaust gas passes through the conveying action of a poet 7 via the feed stream line 8 into the gas separation system 9 .

This works with a pressure difference across a selectively permeable membrane 11 . Corresponding gas separation plants are already known, for which reference can be made, for example, to DE-OSs 38 39 984 and 38 39 985. The specifically selected membrane material has different diffusion rates for the individual components of the gas mixture. Due to a pressure gradient maintained by a vacuum pump 13 opposite the retentate side, a dif fusion occurs through the separating membrane 11 . Due to different diffusion rates similar to a reverse osmosis, the faster diffusing components on the permeate side and the slower diffusing mixture components accumulate on the retentate side. Through targeted selection of a membrane material, it is therefore possible to ensure that the catalyst gas accumulates on the permeate side and that the other constituents which are harmful to the process, in particular water moisture and hardener, binder and separating oil, accumulate on the permeate side. It goes without saying that the retentate also contains a certain amount of catalyst. This retentate is withdrawn via the retentate line 10 , about which further details are given below. The permeate obtained beyond the separating membrane 11 is drawn off via the permeate line 12 and the vacuum pump 13 and conveyed into an intermediate store 17 . This buffer is essentially used for averaging or smoothing fluctuations in the permeate stream with respect to pressure and / or concentration over time. By means of the concentration measuring device 18 , the concentration of the catalyst gas in the permeate within the intermediate storage 17 is measured. Depending on the level of this catalyst concentration, the permeate can be further treated under different.

It is initially assumed that the catalyst concentration in the intermediate store 17 is above a certain limit value, so that the permeate can be conveyed further into the store 21 by means of the compressor 19 . For this purpose or in this state, the valves 33 and 44 are open, whereas the valves 32 and 45 are kept closed. In the storage tank 21 - controlled by the pressure measuring device 29 - a sufficiently high process pressure is maintained, so that the pending gas mixture there, if necessary, is fed back into the curing process without additional feed pump, ie can be pressed through the core mold 2 . Assuming that the catalyst concentration in the carrier gas / catalyst mixture present in the storage tank 21 has a concentration which is sufficiently high for curing, this mixture can be shot by shot through the bleed line 36 , the switching valve 46 and the feed line 1 into the Core form can be fed.

In the case just described, the simplest possible design of the process flow is available, the following process cycle being formed, starting from core form 2 :

- exhaust line 5 ,
- changeover valve 4 ,
- filter 6 ,
- compressor 7 ,
- feed stream line 8 ,
- gas separation plant 9 ,
Separating membrane 11 ,
- permeate line 12 ,
- vacuum pump 13 ,
Buffer 17 ,
- valve 33 ,
- compressor 19 ,
- valve 44 ,
- storage memory 21 ,
- tap line 36 ,
- Changeover valve 46 and
- Inlet line 1 .

Regardless of this or other modes of operation of the process, the cleaning and collecting of the catalyst in the gas phase is carried out in the process and a complete elimination of contaminants harmful to the process is dispensed with. Rather, the contaminants are only partially de-enriched from the exhaust gas sucked off from the core mold 2 , this de-contamination of contaminants taking place at the same time as the separation of the collected gas mixture into carrier gas-enriched retentate and catalyst-enriched permeate. The impurities removed from the feed stream are carried away with the retentate; in part, however, they also remain in the permeate and accumulate in the cycle through the curing process, but stabilize at a certain level, which, however, is no longer harmful to the process. Stability tests on sample-hardened casting cores have shown that useful hardness results can be achieved. By dispensing with catalyst cleaning in the liquid phase, considerable energy costs and investment costs can be saved. The condensation of the catalyst in the liquid phase and the fractional distillation of this condensate and its subsequent recondensation, as mentioned, involve considerable energy and investment costs which can be avoided by the present invention. Although the catalyst can be cleaned very well by a fractional distillation, it has surprisingly been found that the catalytic hardening effect does not suffer as much as a result of the remaining impurities and that the hardness results which can be achieved with a residual contaminated catalyst are quite useful.

In the following, various process variants or process options that can be practiced with the process scheme shown, first of all on the retentate side, are discussed: depending on the pollutant concentration in the retentate, this can be done via the two opened ones, which will probably be less often the case Valves 31 and 43 and the clean air line 15 are blown over the roof. As a rule, however, the pollutant content in the retentate drawn off via the retentate line 10 will be higher than permitted by the TA-Luft. Exhaust air aftertreatment 14 is therefore provided, into which the retentate can be introduced with valve 42 open and valve 43 closed; It goes without saying that the valve 31 is then opened and the valve 30 is closed. In the exhaust air aftertreatment 14 , an acid wash is preferably provided, preferably with sulfuric acid. The aftertreated and cleaned exhaust air can finally be blown out through the roof. In the process diagram, a bypass line 34 branching from the switching valve 4 is provided, which opens up the possibility of introducing the exhaust air emerging at the core mold 2 directly into the exhaust air aftertreatment 14 via the valve 41 and the return line 35 . If only very little contaminated air escapes from the core mold, for example when the core mold is blown by means of compressed air, then this can be blown directly into the open air via the bypass line 34 via the open valve 40 .

A return line 16 branches off from the retentate line 10 , which can be switched by the valve 30 and opens into the feed flow line 8 in front of the compressor 7 . In connection with this retentate-side return line, a corresponding permeate-side return line must also be mentioned, namely the return line 37 , which branches off from the intermediate store 17 and can be connected by means of the valves 32 and 39 . These two permeate-side and retentate-side return lines 37 and 16 are provided for maintaining a continuous gas separation operation in the gas separation system 9 ; It should be remembered that the exhaust gas drawn off from the core mold 2 occurs only in shots, that is, intermittently and briefly, but that the gas separation process should be continued continuously. During the times in which exhaust gas is drawn off from the core mold 2 via the exhaust line 5 , the recirculation lines 37 and 16 mentioned are closed, whereas in the interruption times the gas separation process is maintained by permeate-side and retentate-side circulation; Here, the changeover valve 4 is closed, so as not to drag an unnecessarily large amount of fresh air into the process circuit.

Instead of feeding the permeate on the permeate side, it is also possible in the interruption times to feed the permeate back through the valve 38 and the return line 22 into the feed flow line 8 in front of the compressor 7 , which has the advantage that the concentration of catalyst gas on the permeate side is within the permeate flow elevated. This possibility of the method is indicated in particular if an insufficient concentration of the catalyst is found in the intermediate store 17 . It is also conceivable to feed the permeate back to both the permeate side of the gas separation unit via the return line 37 and the feed stream via the return line 22 , which would have the advantage that there is a constant flow on the permeate side within the gas separation unit, which is what Gas separation favors. The quantitative ratio of a permeate recirculation via the permeate-side return line 37 or feed-stream-side return line 22 can be provided by corresponding opening cross sections on the valves 38 and 39 respectively.

In the event of a certain minimum concentration of catalyst gas within the permeate present in the intermediate storage 17 , this can be passed on via the compressor in the direction of the storage tank 21 . Also in this continuation of the permeate, there are various method options, one, namely the simplest method, of direct, batchwise reuse of the stored carrier gas / catalyst mixture via the tap line 36 has already been mentioned above. This possibility is only given if the concentration of the mixture of catalyst present in the storage tank is sufficiently high for a curing process and if at the same time the storage pressure in the storage tank 21 is also sufficiently high. If one or the other of these two conditions is not given, the process scheme shown offers the following options: by opening the bypass lines 20 and 24 by means of the valve 45 and closing the valve 44 , the permeate conveyed by the compressor 19 can be used in a suitable manner Position of the switching valve 47 through the Do siergerät 27 are promoted. In the metering device, fed from the liquid supply 28 of catalyst, the carrier gas can be enriched with catalyst to defined final concentrations. The carrier gas / catalyst mixture enriched in this way to sufficiently high, process-compatible concentrations can now be fed into the storage tank 21 via the feedback line 25 with the valve 48 open by the action of the compressor 49 . The permeate withdrawn from the intermediate storage 17 is thus concentrated on the metering device 27 before it is collected in the storage 21 . In this way, it is possible to continuously maintain a processable concentration of catalyst in the mixture in the reservoir 21 .

Instead of charging the supply store 21 from the permeate or from the intermediate store 17 , it is also possible to supply the supply store from an external compressed air supply 26 , which can be introduced into the system with a suitable position of the changeover valve 47 , and via the metering device 27 via the open one Charge the feed line 25 and the compressor 29 with a sufficiently highly concentrated carrier gas / catalyst mixture, this charging process being able to be continued up to any pressures tolerable by the storage reservoir.

Finally, the process diagram also provides a third possibility of charging the storage tank 21 with a sufficiently strongly concentrated carrier gas / catalyst mixture, this possibility being considered especially when there is sufficient carrier gas in the storage tank 21 , ie a sufficient amount High pressure is present but the catalyst concentration is too low: namely, the gas mixture stored in the storage tank 21 can be withdrawn via the bypass line 23 and the check valve 50 and via the bypass line 24 to the metering device 27 , enriched there and via the compressor 49 and the regenerative line 25 are fed back into the store.

Finally, there is also another operating option available: the curing process can also be supplied with a sufficiently highly enriched carrier gas / catalyst mixture without the reservoir 21 directly from the external compressed air supply 26 via the metering device 27 , whereby this mixture supply would also work in shots . However, this possibility will be limited to exceptional cases, because a shot-wise removal of carrier gas / catalyst mixture from the storage tank 21 via the tap 36 is more expedient than a discontinuous and only brief operation of the metering device 27 .

Finally, at the cost of a discontinuous operation of the metering device 27 and in the event that the storage tank 21 is filled with very high pressure of carrier gas but with a low catalyst concentration, the storage tank 21 is run empty by the fact that it is too lean Mixture by way of the bypass lines 23 and 24 in batches in the metering device 27 is enriched to process-suitable values and is immediately fed to the core mold 2 via the feed line 1 .

Since - apart from the liquid supply 28 of fresh catalyst - the catalyst within the system is only in gaseous form, even if it is present in different concentrations within a carrier gas, the process scheme offers a variety of options for process design in order to suitably deal with all eventualities during practical operation can. It is not only a constant production operation, but also in terms of malfunctions, longer interruptions such. B. at the end of the shift or at the weekend or to think of the starting operation at the beginning of the shift, although this list need not be complete. As mentioned, all of these different operating states can be flexibly dealt with in a simple manner.

Claims (6)

1. Process for hardening sand moldings, in particular casting cores, from synthetic resin-bonded sand by means of a catalyst, which is stored in liquid form and mixed in gas or vapor form with a carrier gas and is thus pressed through the mold filled with loose molding sand or core sand,

• - Then the gas mixture emerging again from the mold is collected as completely as possible and
• separated on the one hand by means of a gas separation process under pressure difference on a selectively permeable membrane into a permeate depleted of carrier gas and enriched with catalyst and on the other hand into a retentate depleted of catalyst,
• the catalyst contained in the permeate is cleaned and collected for reuse in the curing process,

characterized in that the cleaning and collection of the catalyst takes place in the gas phase and that the catalyst is only partially de-enriched without the complete elimination of contaminants harmful to the process, this de-contaminating at the same time as the separated gas mixture into carrier gas-enriched retentate and is carried out in catalyst-enriched permeate and the impurities released from the gas mixture are carried away with the retentate and the permeate is used again in the collected form or after metering in fresh catalyst and / or fresh carrier gas as catalyst / carrier gas mixture in the curing process. 2. The method according to claim 1, characterized, that depending on the catalyst concentration in Permeate this - at low concentration - on the inlet side the gas separation is returned or - at a high concentration tion - saved for reuse in the curing process becomes. 3. The method according to claim 1 or 2, characterized, that the permeate before storing fresh catalyst dosed and continuously a process-oriented concentration is maintained in memory. 4. The method according to claim 1 or 2, characterized, that the permeate taken from the storage is shot through Add fresh catalyst to one process-oriented concentration is enriched. 5. The method according to any one of claims 1 to 4, characterized, that in the work breaks, form filling, core removal, etc. conditional interruption times of the curing process the Ge mixed discharge line closed, the gas separation process but is still maintained by the permeate permeate side and / or inlet side and the retentate is fed back on the inlet side.