EP3586995B2 - 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 proportion of molding material fragments or loose molding material grains which accrue when a casting is removed from a mold as a result of the destruction of casting cores or molded parts which represent the casting and which have been formed from the molding sand (F) and an inorganic binder and one or more additives for adjusting the properties of the molding material. In this method, the foundry sand mixture is mixed with cleaning water to form a slurry in order to dissolve the inorganic binder residues contained in the foundry sand mixture and any additives present 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 procedure is known, for example, from WO 2007/082747 A1 known, whereby this method is 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 casting is demolded from a so-called "lost" mold, i.e. a mold that was destroyed during demolding, are mechanically crushed. A suspension is formed from the crushed fragments by adding water. This is followed by separation of the components of the suspension. A new ready-to-use core or molding sand mixture is prepared from the molding sand obtained during the separation. The initial crushing of the core and molded part fragments is intended to separate as far as possible from the molding sand grains any impurities that adhere to the molding sand grains from which the cores and molded parts in question were formed. By subsequently adding water to the crushed fragments, the impurities contained therein can be removed and the individual components of the mixture can be fed to their respective intended further processing. Another known document of the state of the art is the DE102005041519 .

Against the background of the state of the art explained above, the task arose of specifying a method with which foundry sand mixtures of the type specified at the beginning can be processed for further use in a cost-effective, resource-saving manner and with increased productivity.

The invention has solved this problem 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 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 outset, the method according to the invention serves to recover molding sand from a foundry sand mixture which comprises at least a proportion of molding material fragments or loose molding material grains which arise during the demolding of a casting from a casting mold as a result of the destruction of casting cores or moldings which represent the casting and which have been formed from the molding sand as well as an inorganic binder and one or more additives for adjusting the properties of the molding material, wherein the method comprises the steps

1. a) Mixing the foundry sand mixture with cleaning water to form a slurry in order to dissolve the inorganic binder residues contained in the foundry sand mixture and any additives present from the foundry sand and to rinse them out of the foundry sand mixture, and
2. b) Separation of the cleaning water contaminated with the inorganic binder residues from the moulding 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 now 80 - 100 °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 prepared according to the invention can be dissolved almost completely in the cleaning water supplied. According to the findings on which the invention is based, this effect occurs when the process temperature of the slurry in step a) is at least 80 °C, with a process temperature of at least 80 °C in practice having a particularly favorable effect on productivity and the completeness of the detachment of the inorganic binder from the molding sand. Temperatures of up to 100 °C have proven to be particularly advantageous in terms of the energy required and the requirements that the required plant technology must meet.

The temperature window specified by the invention is set in such a way that the processing 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 in such a way that the foundry sand produced can be effectively processed at minimal cost. Practical tests have shown that at process temperatures within a temperature range of 80 - 100 °C, the separation of the molding sand from the inorganic binder can be carried out in particularly short process times. For example, the mixing of the foundry sand mixture with the cleaning water to form a slurry and the associated dissolving and rinsing of the inorganic binder residues (step a) of the process according to the invention) can typically be completed within 5 min - 60 min.

In the method according to the invention, it can also be expedient to mechanically crush the molding material fragments contained in the foundry sand mixture in a crushing device before mixing with the cleaning water. However, this crushing does not primarily serve to separate binder residues from the molding sand grains, but rather to increase the attack surface 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 processing according to the invention contains predominantly coarse fragments, it may be advantageous to mechanically separate the molding material fragments contained in the foundry sand mixture into grains before mixing with the cleaning water (work step a)) in order to accelerate the formation of a slurry intended in work step a). All devices known from the state of the art for this purpose are suitable for the mechanical comminution of the foundry sand mixture, such as a lump breaker or the like.

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 be expedient, with a view to minimizing the effort required for the temperature control provided according to the invention, to preheat the foundry sand mixture before mixing it 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 low outputs and can be implemented and operated at correspondingly low costs.

For example, the contaminated cleaning water from step b) can be used to heat the fresh water. To do this, 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 mixing.

Depending on the degree of contamination of the contaminated cleaning water, the contaminated water is used for another run through step a). This reuse is repeated until the solubility of the binder in the contaminated water is reached, i.e. so much binder is dissolved in the water that no more binder can be dissolved, or the proportion of suspended matter predominates, i.e. the contamination of the water with foreign bodies carried in it has increased so much that rinsing the molding sand with the contaminated water no longer has a cleaning effect.

If the foundry sand mixture to be processed is to be avoided from mixing with contaminated cleaning water, 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 useful if the cleaning water obtained in step b), which may have been reused several times beforehand, is so contaminated that it is no longer advisable to reuse it.

Even in the case of reuse of contaminated cleaning water, it may be necessary to add fresh cleaning water in 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, such as by means of a continuous flow heater or the like, in such a way that the slurry formed in step a) by mixing with the foundry sand mixture, which is also optionally preheated, reaches a process temperature which lies in the range prescribed according to the invention.

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

However, since the molding sand is to be used for a wider range of applications, in particular those for which molding materials with organic binders are to be provided, the molding sand is subjected to an additional treatment after the separation of the residues of the inorganic binder according to the invention (step b)) in which its pH value is adjusted to values of 7 - 8 -, whereby if the molding sand obtained in step b) is to be used for molding materials with an organic binder, the pH value of the molding sand is adjusted to 7 - 8.

To adjust its pH value, the molding sand is rinsed or wetted with a neutralizing solution. If the molding sand obtained in step b) is strongly basic, water-diluted acids such as water-diluted hydrochloric acid, sulfuric acid or organic acids (carbonic acid, citric acid) are suitable as neutralizing solutions. 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 mixed with the neutralizing solution. Stirring devices and the like are available on the market for this purpose.

After adjusting the pH, the molding sand can undergo a rinsing step to remove excess neutralizing solution.

The molding sand obtained in step b) and adjusted in terms of its pH value can be subjected to mechanical dewatering. For this purpose, the molding sand can be placed on sieves, for example, through which liquid residues 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 state of the art for this purpose, in order to mechanically expel moisture from a free-flowing mass that is comparable to the molding sand obtained according to the invention. Mechanical dewatering can reduce the effort that may be required to dry 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, with typical drying temperatures being in the range of 80 - 800 °C. In the case where the molding sand has been obtained from a foundry sand mixture consisting of foundry cores and molded parts that have been molded exclusively from molding material that contained an inorganic binder, drying temperatures of less than 500 °C, in particular 100 - 300 °C, are suitable, with temperatures of 200 - 250 °C being particularly practical.

However, many foundries produce foundry sands which, in addition to a portion that comes from casting cores or molded parts made of molding materials with inorganic binders, also contain a portion of fragments or grains of casting cores or molded parts that have been formed from a molding material that has been formed from the molding sand and an organic binder and one or more additives to adjust the properties of the molding material. The residues of the organic binder that have not been dissolved from the molding sand grains by the work 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 work step b) is heated to such an extent that the organic binder residues burn off. This requires temperatures of 500 °C or more, with a typical temperature window for this treatment being 500 - 700 °C. In the case that thermal drying of the molding sand obtained in step b) is carried out, this annealing treatment can also be carried out during the drying step.

Finally, the molding sand obtained by the processing 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 in order to ensure its optimal suitability for the production of molding material.

An embodiment not according to the invention is presented below.

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

Some of the casting molds include casting cores or molded parts that are made from a molding compound that contains a molding sand that has been tried and tested in practice and an equally tried and tested inorganic binder, such as water glass. During the production of the respective casting core or molded part, the binder is activated in the usual way by applying heat in order to ensure that the molding sand grains hold together in a dimensionally stable manner.

Another part of the casting molds, however, contains casting cores or molded parts that are made from a molding compound that contains a molding sand that has been tried and tested in practice and an organic binder that has also been tried and tested. During the production of the respective casting core or molded part, a chemical reaction of the binder is caused by the addition of a reaction medium, such as a gas, through which the binder develops its solidifying effect and ensures that the grains of the molding sand hold together in a dimensionally stable manner.

When the castings are demoulded, the casting cores or moulded parts are destroyed in a known manner by thermal or mechanical treatment. The moulding material fragments and loose moulding material grains falling off the casting form a foundry sand mixture G, in which moulding sand F, hardened inorganic and organic binders and possibly also combustion residues are present, which are the result of the combustion or decomposition of parts of the binder present in the respective core or molded part as a result of the heat supply during the casting process or the subsequent thermal treatment. The foundry sand mixture G may also contain common additives that are added in practice to molding materials intended for the production of cores or molded parts in order, for example, to ensure optimal flow behavior during the molding of the respective core or molded part ("core shooting").

To recover the moulding sand from the foundry sand mixture G, the mixture consisting of a proportion FAB of fragments or grains originating from mouldings or casting cores made of moulding material with an inorganic binder and a proportion FOB of fragments or grains originating from mouldings or casting cores made of moulding material with an organic binder is placed in the Fig.1 fed into 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 crushed in a manner known per se until only grains and smaller fragments remain.

The foundry sand mixture G, which has been separated into grains and optionally preheated in a heat exchanger (not shown here), is introduced into a mixing device 2 by means of gravity or, for example, by means of compressed air support.

In the mixing device 2, the foundry sand mixture G is flowed through or stirred with cleaning water RW, which has been previously heated in a continuous flow heater, for example using a fluidized bed or an agitator, in order to form a slurry S. In the slurry S, the inorganic binder residues adhering to the grains dissolve in the cleaning water RW. The slurry S formed in the mixing device 2 is intensively circulated in order to ensure turbulence that supports the detachment of the inorganic binder and other contaminants. If necessary, heat is added 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 contaminants, such as molding additives and combustion residues, is drained 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 the inorganic binder in particular 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 residence times of the slurry S in the mixing device 2 provided for this purpose are 5 - 60 minutes.

From the mixing device 2, the slurry S passes into a rinsing device 3, in which it is rinsed with cleaning water RW in order to rinse the inorganic binder residues dissolved in the slurry S from the molding sand grains and other impurities from the molding sand grains F of the slurry S. The rinsing device 3 can be designed as a conventional screening machine in which the slurry S is placed on a sieve and sprayed with cleaning water RW, which is applied 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 discharged 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 by feeding it to the mixing device 2 as cleaning water RW. The total volume flow of the cleaning water RW fed to the mixing device 2 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.

In the same way, another partial flow RWKV" of the pre-cleaned contaminated cleaning water RWK can be fed to the rinsing device 3 for rinsing the slurry S. Here too, the total volume flow of the cleaning water RW fed to the rinsing device 3 can be composed of a partial flow of fresh cleaning water RWF and the partial flow RWKV' of the pre-cleaned cleaning water RWV as well as 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 contaminated that it can no longer perform a cleaning function, is drained from the process and fed into a separate treatment plant.

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 in order to adjust its pH value to a value of 7 - 8 which is optimal for this purpose. The molding sand F, whose pH value has been adjusted in this way, is then rinsed in a rinsing device 6 with fresh cleaning water RWF in order to remove excess neutralizing solution NL. The resulting cleaning water RWN, which is contaminated with neutralization solution, is collected and disposed of.

The adjustment 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 comprising an inorganic binder.

The molding sand F, which is still loaded with cleaning water RW, is transported after rinsing in the rinsing device 3 or the optionally passed through stations "treatment device 5 and rinsing device 6" to a dewatering device 7, in which dewatering is carried out using 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 of the molding sand F to such an extent that significantly less energy is required during the subsequent thermal drying to achieve the required degree of dryness.

The contaminated cleaning water RWK arising during mechanical dewatering is, for example, fed to the rinsing device 3 as a further partial flow of the cleaning water RW fed there. For 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. If the foundry sand mixture G used includes a proportion of molding 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 organic binder residues still adhering to the corresponding portion of the molding sand F burn off.

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

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

After thermal drying in the drying device 8, the molding sand F passes through a dust removal device 9 in which the 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 deposited in the usual way or used for another purpose. The dust removal device 9 is based, for example, on the principle of stream classification, in which air is used as the separation medium (so-called "air classification"). The air used here can be reused or released into the environment.

The dedusted molding sand F finally reaches a classification device 10, in which the molding sand F is divided into at least two molding sand partial quantities Fk, Fm according to at least two molding sand classes, of which one molding sand partial quantity Fk comprises the part of the molding sand F whose grains do not exceed a certain limit size, while the other molding sand partial quantity Fm contains the part of the molding sand F whose grains have a size that is at least equal to this limit size. The classification step can also be carried out in combination with dedusting. For this purpose, fluid basins are usually used in which the molding sand F is fed in from above, air is passed through a sintering plate attached to the floor and is set into vibration using 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 extracted at opposite ends of the basin. The finer parts rise higher and have to overcome a barrier. The coarse parts do not rise as high and are therefore deducted below a barrier.

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

In Fig.1 The process flow followed by the foundry sand mixture G, the slurry S 40 formed from it and the molding sand F contained therein is shown in solid lines.

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

From the molding sand fractions FK and FM obtained after classification, new molding material FA, which contains inorganic binder, and new molding material FO, which contains organic binder, are produced by mixing with organic binder or inorganic binder and the required additives.

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

REFERENCE SIGNS

1

Grain separating device

2

Mixing device

3

Flushing device

4

Pre-cleaning device

5

Treatment facility

6

Flushing device

7

mechanical drainage device

8th

thermal drying device

9

Dust extraction system

10

Classification device

F

Moulding sand

FA

new molding material containing inorganic binder

FAB

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

FK,FM

Molding sand quantities

FO

new molding material containing organic binder

FOB

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

FS

particulate matter

G

Foundry sand mixture

NL

Neutralization solution

RW

Cleaning water

RWKE

contaminated cleaning water to be disposed of RW

RWF

fresh cleaning water

RWK

contaminated cleaning water

RWN

Cleaning water RW contaminated with neutralizing solution N

RWKV'

Partial flow of pre-cleaned contaminated cleaning water

RWKV"

Partial flow of pre-cleaned contaminated cleaning water

S

Feast

Claims (11)

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 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 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 80 to 100°C,

   the mixing of the foundry sand mixture (G) with the cleaning water (RW) to form a slurry (S) and the associated dissolving and rinsing out of the inorganic binder residues (AB) (working step a)) is carried out within 5 min - 60 min,

   the contaminated cleaning water (RWK) produced in step b) is reused at least once for step a),

   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, and

   the pH value of the moulding sand (F) obtained in step b) is adjusted to a pH value of 7 - 8 by rinsing or wetting with a neutralizing solution (NL).
2. Method according to claim 1, 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)).
3. 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).
4. 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.
5. Method according to claim 8, characterised in that a diluted acid is used as the neutralisation solution (NL).
6. Method according to any one of the preceding claims, characterised in that the moulding sand (F) obtained in work step b) is mechanically dewatered.
7. 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.
8. 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.
9. Method according to claim 8, characterized 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).
10. Method according to one of claims 7 and 9, characterised in that the burning of the organic binder residues takes place during the drying of the moulding sand (F).
11. Method according to 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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