EP3586995B1 - Method for preparing a foundry sand mixture - Google Patents

Description

The invention relates to a method for recovering molding sand from a foundry sand mixture, which comprises at least a portion of molded material fragments or loose molding material grains that arise from the molding sand when a casting is removed from a casting mold as a result of the destruction of the casting cores or moldings depicting the casting (F) and an inorganic binder and optionally one or more additives for adjusting the properties of the molding material have been molded. In this process, the foundry sand mixture is mixed with cleaning water to form a slurry in order to detach the inorganic binder residues contained in the foundry sand mixture and optionally present additives from the molding sand and to rinse them out of the foundry sand mixture. The cleaning water contaminated with the inorganic binder residues is then separated from the molding sand contained in the slurry.

Such a method is, for example, from WO 2007/082747 A1 known, whereby this method is said to be particularly suitable for the preparation of foundry sand mixtures in which an inorganic binder, in particular a water glass binder, is present. In the known method, in a first step, the casting cores or molded parts that arise when a cast part is removed from a so-called “lost” mold, ie, that is destroyed during demolding, are mechanically comminuted. A suspension is formed from the crushed fragments by adding water. This is followed by a separation of the components of the suspension. Out the molding sand obtained during the separation is provided with a new ready-to-use core or molding sand mixture. The first comminution of the core and molded part fragments is intended to separate impurities that adhere to the molding sand grains from which the relevant cores and molded parts were formed from the molding sand grains as far as possible. By then adding water to the fragments comminuted in this way, the impurities contained therein should be able to be removed and the individual constituents of the mixture can be fed to their respective intended further processing. Another known prior art document is that DE102005041519 .

Against the background of the prior art explained above, the task has arisen of specifying a method with which a preparation of foundry sand mixtures of the type specified at the beginning for further use can be carried out cost-effectively, in a resource-saving manner and with increased productivity.

The invention has achieved this object by the method specified in claim 1.

1. a) Vermischen der Gießereisandmischung mit Reinigungswasser zu einer Schlemme, um die in der Gießereisandmischung enthaltenen anorganischen Binderreste sowie optional vorhandenen Additive von dem Formsand zu lösen und aus der Gießereisandmischung auszuspülen,
   und
2. b) Trennen des mit den anorganischen Binderresten kontaminierten Reinigungswassers von dem in der Schlemme enthaltenen Formsand,

umfasst.In accordance with the prior art explained at the beginning, the method according to the invention is used to recover molding sand from a foundry sand mixture which comprises at least a proportion of molded material fragments or loose molded material grains that occur during the demolding of a casting from a casting mold as a result of the destruction of the casting cores depicting the casting or molded parts are obtained which have been molded from the molding sand and an inorganic binder and optionally one or more additives for adjusting the properties of the molding material, the method comprising the working steps

1. a) Mixing the foundry sand mixture with cleaning water to form a slurry to remove the inorganic contained in the foundry sand mixture To loosen binder residues and optionally existing additives from the molding sand and to rinse them out of the foundry sand mixture,
   and
2. b) Separating the cleaning water contaminated with the inorganic binder residues from the molding sand contained in the slurry,

includes.

According to the invention, the process temperature of the slurry formed from the cleaning water and the foundry sand mixture in step a) is 50-200 ° C.

Surprisingly, it has been shown that by setting a process temperature that is significantly higher than room temperature, the inorganic binder contained in a foundry sand mixture to be processed according to the invention can be largely completely dissolved in the cleaning water supplied. According to the knowledge on which the invention is based, this effect sets in when the process temperature of the slurry in step a) is at least 50 ° C, a process temperature of at least 70 ° C, in particular at least 80 ° C, being particularly favorable in practice affects the productivity and the completeness of the detachment of the inorganic binder from the molding sand. Temperatures of up to 120 ° C., in particular up to 100 ° C., have proven to be particularly advantageous with regard to the required energy input and the requirements that must be met by the required system technology.

The temperature window specified by the invention is set in such a way that the preparation of the foundry sand mixture can be integrated into a water- and energy-saving cycle.

The invention allows the process temperature and the process times to be coordinated with one another in such a way that an effective treatment of the foundry sand is possible with minimal costs. Practical tests have shown that at process temperatures that lie in a temperature window of 80-100 ° C, the molding sand can be separated from the inorganic binder in particularly short process times. Thus, the mixing of the foundry sand mixture with the cleaning water to form a slurry and the associated dissolving and rinsing out of the inorganic binder residues (step a) of the method according to the invention) can typically be done within 5 min-60 min.

In the method according to the invention, too, it can be expedient to mechanically comminute the fragments of molding material contained in the foundry sand mixture in a comminution device prior to mixing with the cleaning water. However, this comminution is not primarily used to separate binder residues from the molding sand grains, but rather to enlarge the area of attack for the cleaning water as much as possible in order to accelerate the dissolution of the binder when the foundry sand mixture is mixed with the cleaning water.

In the event that the foundry sand mixture intended for the preparation according to the invention contains predominantly coarse fragments, in order to accelerate the formation of a slurry intended in step a) it can be advantageous to mechanically remove the molding material fragments contained in the foundry sand mixture prior to mixing with the cleaning water (step a)) to isolate grain. For the mechanical comminution of the foundry sand mixture, all devices known from the prior art for this purpose, such as a lump crusher or the like, are suitable.

In particular, if waste heat is available from another process section of the method according to the invention or from a process in the plant in which the method according to the invention is also used, which would otherwise remain unused, it can, with regard to the minimization of the According to the invention provided temperature control necessary effort be expedient to preheat the foundry sand mixture before mixing with the cleaning water. In this way, the heating devices provided for heating the cleaning water or the slurry to the respective process temperature can be designed for small outputs and can be implemented and operated at correspondingly low costs.

For example, the contaminated cleaning water from work step b) can be used to heat the fresh water. For this purpose, the contaminated cleaning water can be passed through a heat exchanger in which heat is transferred from the contaminated cleaning water to fresh cleaning water without the contaminated and fresh cleaning water being mixed.

Depending on the degree of soiling of the contaminated cleaning water, it is also possible to use the contaminated water to run through work step a) again. This reuse can be repeated, for example, until the solubility of the binder in the contaminated water is reached, i.e. so much binder is dissolved in the water that no further binder can be dissolved, or the proportion of suspended matter predominates, i.e. the pollution of the water in it transported foreign bodies has increased so much that no cleaning effect is achieved when rinsing the molding sand with the contaminated water.

If mixing of the foundry sand mixture to be processed with contaminated cleaning water is to be avoided, the foundry sand mixture can pass through a heat exchanger before step a) through which contaminated and still warm cleaning water separated from the molding sand in step b) is passed in order to preheat the foundry sand mixture. This variant is particularly expedient if the cleaning water obtained in step b), possibly previously reused several times, is so contaminated that it is no longer sensible to reuse it.

Even when contaminated cleaning water is reused, it may be necessary to supply fresh cleaning water in work step a) in order to provide the required volume flow of cleaning water.

If necessary, the fresh cleaning water supplied in each case can also be heated by means of an additional heat source, for example by means of a flow heater or the like, so that the slurry formed in step a) by mixing with the optionally also preheated foundry sand mixture reaches a process temperature that in accordance with the invention prescribed range.

After the separation step (step b)), the molding sand obtained from the foundry sand mixture, freed from the binder and the other residues, can be dried in the usual way, dedusted if necessary and divided into different grain size classes.

The molding sand obtained in accordance with the invention generally has a pH of 9-13. Molding sands with this pH value can be used in molding materials intended for the production of casting cores and casting mold parts use to which an inorganic binder is added to bind the molding sand. However, if the molding sand is to be able to be used for a broader range of applications, in particular those in which molding materials with organic binders are to be provided, it is advisable to use the molding sand after the removal of the residues of the inorganic binder (step b)) according to the invention subject to additional treatment in which its pH is adjusted to values of 5-9, preferably 6.5-8.5, in particular when the molding sand obtained in step b) is used for molding materials with an organic binder should, the pH of the molding sand is optimally adjusted to 7 - 8.

To adjust its pH, the molding sand can be rinsed or wetted with a neutralizing solution. In the event that the molding sand obtained in step b) is strongly basic, water-diluted acids, such as, for example, water-diluted hydrochloric acid, sulfuric acid or organic acids (carbonic acid, citric acid), are suitable as the neutralizing solution. In addition, buffer substances such as carbonate buffers (e.g. sodium hydrogen carbonate) can also be used for neutralization.

To adjust the pH value, the molding sand can be stirred with the neutralizing solution. For this purpose, stirring devices and the like are available on the market.

After the pH value has been adjusted, the molding sand can go through a rinsing step to remove excess neutralizing solution.

The molding sand obtained in step b) and optionally adjusted with regard to its pH value can be subjected to mechanical dewatering. For this purpose, the molding sand can be applied to sieves, for example, through which liquid residues are present in the molding sand drip while the molding sand grains are retained, or presses, drying belts and the like are used, which are available in the prior art for this purpose, in order to mechanically drive out moisture from a free-flowing mass that is comparable to the molding sand obtained according to the invention. The mechanical dewatering can reduce the effort that, if necessary, has to be expended for drying the molding sand before it is further processed into molding material.

In order to be able to be processed into molding material, the molding sand obtained according to the invention must be sufficiently dry. For this purpose, the molding sand obtained in step b) can be dried by supplying heat, typical drying temperatures being in the range of 80-800.degree. In the event that the molding sand has been obtained from a foundry sand mixture, which consists of foundry cores and moldings, which have been molded exclusively from molding material that contained an inorganic binder, drying temperatures are less than 500 ° C, in particular 100-300 ° C , suitable, whereby temperatures of 200 - 250 ° C are particularly practical.

In many foundries, however, there are foundry sands in which, in addition to a proportion that comes from casting cores or moldings made from molding materials with inorganic binders, there is also a proportion of fragments or grains of casting cores or moldings that have been formed from a molding material have been formed from the molding sand and an organic binder and optionally one or more additives to adjust the properties of the molding material. The residues of the organic binder, which have not been detached from the molding sand grains in steps a) and b) of the method according to the invention, can be removed by an annealing treatment in which the molding sand present after step b) is heated to such an extent that that the organic binder residues burn. Temperatures of 500 ° C. or more are required for this, with a typical temperature window for this Treatment at 500 - 700 ° C. In the event that the molding sand obtained in step b) is thermally dried, this annealing treatment can also be carried out in the course of the drying step.

Finally, the molding sand obtained by the preparation according to the invention can be subjected to a classification in which it is divided depending on the size of its grains. At the same time, the molding sand can be dedusted to ensure that it is optimally suited for the production of molding material.

The invention is explained in more detail below with reference to a drawing showing an exemplary embodiment.

The figure shows schematically a workflow in the preparation of a foundry sand mixture, as it typically occurs in a casting operation, in which from a light metal melt, in particular an Al or Al alloy melt, not shown here cast parts, such as components for vehicles, in a conventional manner from casting molds, also not shown here, are produced by casting.

Some of the casting molds include casting cores or molded parts which are formed from a molding material which contain a molding sand that has been tried and tested in practice and an inorganic binder that has also proven itself, for example water glass. In the course of the production of the respective casting core or molded part, the binder is activated in the usual way by supplying heat in order to ensure the dimensionally stable cohesion of the grains of the molding sand.

Another part of the casting molds, on the other hand, contains casting cores or molded parts which are formed from a molding material, which has a molding sand that has been tried and tested in practice and an organic binder that has also proven itself contain. In the course of the production of the respective casting core or molded part, a chemical reaction of the binder is brought about by supplying a reaction medium, for example a gas, through which the binder develops its solidifying effect and ensures the dimensionally stable cohesion of the granules of the molding sand.

When the cast parts are removed from the mold, the casting cores or molded parts are destroyed in a known manner by thermal or mechanical treatments. The molded material fragments and loose molded material grains falling off the casting form a foundry sand mixture G in which molding sand F, hardened inorganic and organic binders and possibly also combustion residues are present, which are the result of the combustion that occurs as a result of the supply of heat during the casting process or the subsequent thermal treatment or decomposition of parts of the binder present in the respective core or molding. The foundry sand mixture G can also contain customary additives that are added in practice for the production of cores or molded parts, for example to ensure optimum flow behavior during the molding of the respective core or molded part ("core shooting").

In order to recover the molding sand from the foundry sand mixture G, a proportion of FAB of fragments or grains that originate from molded parts or casting cores made of molding material with an inorganic binder and a proportion of FOB of fragments or grains that originate from molded parts or casting cores made of molding material with an organic binder originate in the in Fig. 1 the processing process shown.

The foundry sand mixture G first passes through a grain separation device 1 in which the coarse fragments contained in the foundry sand mixture G are comminuted in a manner known per se until only grains and smaller fragments are present.

The granular foundry sand mixture G, which is optionally preheated in a heat exchanger (not shown here), is introduced into a mixing device 2 with the aid of gravity or, for example, with the aid of compressed air.

In the mixing device 2, the foundry sand mixture G is flowed through or stirred with the aid of a fluidized bed or a stirrer with cleaning water RW previously heated, for example in a flow heater, in order to form a slurry S. In the slurry S, the inorganic binder residues sticking to the grains dissolve in the cleaning water RW. The slurry S formed in the mixing device 2 is circulated intensively in order to ensure turbulence which supports the detachment of the inorganic binder and the other impurities. If necessary, heat is supplied to bring the slurry S to a process temperature that is in the optimal range of 80-100 ° C. Excess cleaning water RWK contaminated with inorganic binder residues and other soiling, such as molding material additives and combustion residues, is discharged from the mixing device 2.

Due to the increased process temperature, the mixing of the cleaning water RW with the foundry sand mixture G is so intensive that in particular the inorganic binder dissolves essentially completely in the cleaning water RW within a short time. At the same time, the combustion residues and any additive residues present are absorbed by the cleaning water RW from the foundry sand mixture G. The dwell times provided for this purpose for the slurry S in the mixing device 2 are 5 to 60 minutes.

The slurry S passes from the mixing device 2 into a rinsing device 3, in which it is rinsed with cleaning water RW in order to rinse off the inorganic binder residues and other impurities dissolved in the slurry S from the molding sand grains from the molding sand grains F of the slurry S. The rinsing device 3 can be designed as a conventional sieve machine, in which the slurry S is placed on a sieve and sprayed with cleaning water RW that is discharged by means of nozzles arranged above the sieve.

The resulting cleaning water RWK, which is contaminated with inorganic binder residues and other contaminants, is collected and fed to a pre-cleaning device 4 in which the insoluble inorganic binder residues are separated from the contaminated cleaning water RWK. The excess contaminated cleaning water RWK diverted from the mixing device 2 is also fed to the pre-cleaning device 4. A partial flow RWKV 'of the pre-cleaned contaminated cleaning water RWK can be reused in that it is fed to the mixing device 2 as cleaning water RW. The total volume flow of the cleaning water RW supplied to the mixing device 2 can be composed of a partial flow of fresh cleaning water RWF and the partial flow RWKV 'of the pre-cleaned cleaning water RWV.

Another partial flow RWKV "of the pre-cleaned contaminated cleaning water RWK for rinsing the slurry S can be fed to the flushing device 3. Here, too, the total volume flow of the cleaning water RW fed to the flushing device 3 can be made up of a partial flow of fresh cleaning water RWF and the partial flow RWKV 'of the pre-cleaned Cleaning water RWV and a further partial flow RWK 'of contaminated cleaning water RWK, which comes from one or more of the process steps explained below.

Contaminated cleaning water RWKE, which is so heavily soiled that it can no longer take on a cleaning function, is diverted from the process and fed to a separate treatment.

If the molding sand F separated from the slurry S in the rinsing device 3 is to be used for the production of molding material which comprises an organic binder, the molding sand F passes through a treatment device 5 in which it is wetted with an acidic neutralizing solution NL to reduce its pH -Value to be set to an optimal value of 7 - 8 for this purpose. Subsequently, the molding sand F adjusted in this way with regard to its pH value is rinsed in a rinsing device 6 with fresh cleaning water RWF in order to remove excess neutralizing solution NL. The resulting RWN cleaning water, which is contaminated with neutralizing solution, is collected and disposed of.

The setting of the pH value in the treatment device 5 and the subsequent rinsing in the rinsing device 6 can be skipped if the molding sand F is intended exclusively for the production of molding material which comprises an inorganic binder.

The molding sand F still loaded with cleaning water RW is transported to a dewatering device 7 after rinsing in the rinsing device 3 or the optionally run through stations "treatment device 5 and rinsing device 6", in which dewatering is carried out by mechanical means. The dewatering machine 7 can be designed as a screening machine known for these purposes in the prior art, as a vacuum belt dryer or as a press. The mechanical dewatering reduces the moisture in the molding sand F to such an extent that significantly less energy is required to achieve the required degree of drying during the subsequent thermal drying.

The contaminated cleaning water RWK resulting from the mechanical dewatering is fed to the rinsing device 3, for example, as a further partial flow of the cleaning water RW fed in there. For the thermal drying, the mechanically dewatered molding sand F is fed to a drying device 8, which can be a rotary kiln, a belt dryer or the like. In the event that the foundry sand mixture G used comprises a proportion of molded material fragments and grains that contain organic binders or binder residues, the temperature Tw at which the thermal drying takes place is set to> 500-700 ° C., so that the temperature at which the Burn the corresponding portion of the molding sand F still adhering organic binder residues.

If, on the other hand, the molding sand F no longer contains any organic binder components, the thermal drying can be carried out at temperatures in the range of 100-300 ° C.

The water vapor produced during thermal drying is captured, condensed and fed into the process as fresh RWF cleaning water. The fresh cleaning water RWF obtained during the thermal drying also forms, for example, a partial flow of the cleaning water RW fed into the rinsing device 3.

After the thermal drying in the drying device 8, the molding sand F passes through a dedusting device 9 in which fine dust FS present in the molding sand F is separated from the remaining grains of the molding sand F. The fine dust FS can no longer be used for casting purposes and is therefore disposed of in the usual way or put to another use. The dedusting device 9 is based, for example, on the principle of current classification, in which air is used as the separating medium (so-called “air sifting”). The air used here can be reused or released into the environment.

The dust-free molding sand F finally arrives in a classification device 10 in which the molding sand F corresponds to at least two classes of molding sand is divided into at least two molding sand sub-quantities Fk, Fm, of which one molding sand sub-quantity Fk comprises the part of the molding sand F whose grains do not exceed a certain limit size, while the other molding sand sub-quantity Fm contains the part of the molding sand F whose grains have a size that is at least equal to this limit value. The classification step can also be carried out in combination with the dedusting. Fluid basins are usually used for this, in which the molding sand F is fed in from above, air flows through a sintered plate attached to the bottom and is set in vibration with the aid of unbalanced motors. At the same time, the fine dust FS is transported away by means of the air via a suction device. The grain classes are drawn off at opposite ends of the tank. The finer parts rise higher and have to overcome a barrier. The gross proportions do not rise that high and are therefore deducted under a barrier.

The fresh cleaning water RWF required in the treatment process according to the invention and reused contaminated cleaning water RWK or the cleaning water RW formed therefrom by mixing, if necessary, can be preheated via heat exchangers, not shown here, in which waste heat is used in the process according to the invention itself or in other processes, in order to heat the respective cleaning water RWF, RWK, RW to an optimal temperature for the respective process step.

In Fig. 1 the process run followed by the foundry sand mixture G, the melt S formed from it and the molding sand F contained therefrom are shown in solid lines.

In contrast, the run of the cleaning water RW, the fresh cleaning water RWF, the contaminated cleaning water RWK, the pre-cleaned contaminated cleaning water RWKV, the Neutralizing solution NL and the cleaning water RWL contaminated with neutralizing solution are shown in dashed lines.

From the partial quantities of molding sand FK, FM obtained after the classification, new molding material FA, which contains inorganic binders, and new molding material FO, which contains organic binders, are produced by mixing with organic binder or inorganic binder and the additives required in each case.

Cores or molded parts for casting molds can be produced in a conventional manner from the molding materials FA, FO.

REFERENCE MARK

1

Grain singling device

2

Mixing device

3

Flushing device

4th

Pre-cleaning device

5

Treatment facility

6th

Flushing device

7th

mechanical drainage device

8th

thermal drying device

9

Dust collector

10

Classification device

F.

Molding sand

FA

new molding material that contains inorganic binders

FAB

Proportion of fragments or grains with an inorganic binder in the foundry sand mixture G

FK, FM

Part quantities of molding sand

FO

new molding material that contains organic binders

FOB

Proportion of fragments or grains with an inorganic binder in the foundry sand mixture G

FS

particulate matter

G

Foundry sand mix

NL

Neutralizing solution

RW

Cleaning water

RWKE

Contaminated cleaning water RW to be disposed of

RWF

fresh cleaning water

RWK

contaminated cleaning water

RWN

RW cleaning water contaminated with neutralizing solution N

RWKV '

Partial flow of the pre-cleaned contaminated cleaning water

RWKV "

Partial flow of the pre-cleaned contaminated cleaning water

S.

Slumber

Claims (15)

1. Method for recovering moulding sand (F) from a foundry sand mixture (G), which comprises at least one proportion (FAB) of moulding material fragments or loose moulding material grains, which accumulates when a cast part is demoulded from a casting mould as a result of the destruction of casting cores or moulded parts representing the cast part which have been formed from the moulding sand (F) and an inorganic binder and optionally one or a plurality of additives to set the properties of the moulding material, wherein the method comprises the work steps

   a) mixing the foundry sand mixture (G) with cleaning water (RW) to form a slurry (S), in order to dissolve the inorganic binder residues (AB) contained in the foundry sand mixture (G) and optionally present additives from the moulding sand (F) and to rinse them out of the foundry sand mixture (G),
   and

   b) separating the cleaning water (RWK) contaminated with the inorganic binder residues from the moulding sand (F) contained in the slurry (S), characterised in that the process temperature of the slurry (S) formed from the cleaning water and the foundry sand mixture (G) (work step a)) is 50 to 200°C.
2. Method according to claim 1, characterised in that the process temperature of the slurry (S) is 70 to 120°C.
3. Method according to any one of the preceding claims, characterised in that the moulding material fragments contained in the foundry sand mixture (G) are mechanically separated into grains prior to mixing with the cleaning water (work step a)).
4. Method according to any one of the preceding claims, characterised in that the foundry sand mixture (G) passes through a heat exchanger prior to work step a), through which cleaning water (RWK) that is contaminated, still hot and separated from the moulding sand (F) in work step b) is channelled in order to pre-heat the foundry sand mixture (G).
5. Method according to any one of the preceding claims, characterised in that the contaminated cleaning water (RWK) separated from the moulding sand (F) in work step b) passes through a heat exchanger in which cleaning water (RW) flowing in for work step a) is heated.
6. Method according to any one of the preceding claims, characterised in that the contaminated cleaning water (RWK) accumulating in work step b) is reused at least once for work step a).
7. Method according to claim 6, characterised in that the reuse is repeated until the solubility of binder in the water is reached or the proportion of suspended materials contained in the water prevails.
8. Method according to any one of the preceding claims, characterised in that the pH value of the moulding sand (F) obtained in work step b) is set to a pH value of 5 to 9 by rinsing or wetting with a neutralisation solution (NL).
9. Method according to claim 8, characterised in that a diluted acid is used as the neutralisation solution (NL).
10. Method according to any one of the preceding claims, characterised in that the moulding sand (F) obtained in work step b) is mechanically dewatered.
11. Method according to any one of the preceding claims; characterised in that the moulding sand (F) obtained in work step b) is dried at a drying temperature of 80 to 800°C.
12. Method according to any one of the preceding claims, characterised in that the foundry sand mixture (G) contains a proportion (FOB) of fragments or grains of casting cores or moulded parts, which have been formed from a moulding material, which has been formed from the moulding sand (F) and an organic binder and optionally one or a plurality of additives to set the properties of the moulding material.
13. Method according to claim 12, characterised in that the moulding sand (F) obtained in work step b) is heated to a temperature of at least 500°C in order to burn organic binder residues adhering to the moulding sand (F).
14. Method according to any one of claims 11 and 13, characterised in that the burning of the organic binder residues takes place during the drying of the moulding sand (F).
15. Method according to any one of the preceding claims, characterised in that the moulding sand (F) obtained in work step b) is subjected to a classification.

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