EP2117749B1 - Thermal regeneration of foundry sand - Google Patents

Description

The invention relates to a method for producing casting molds with the reprocessing of foundry sands which are contaminated with water glass.

Casting molds for the production of metal bodies are essentially produced in two versions. The so-called cores or forms form a first group. The casting mold, which essentially represents the negative shape of the casting to be produced, is assembled from these. A second group consists of hollow bodies, so-called feeders, which act as compensation reservoirs. These absorb liquid metal, whereby appropriate measures are taken to ensure that the metal remains in the liquid phase longer than the metal that is in the casting mold that forms the negative mold. If the metal solidifies in the negative mold, liquid metal can flow in from the compensation reservoir in order to compensate for the volume contraction that occurs when the metal solidifies.

Casting molds consist of a refractory material, for example quartz sand, the grains of which are produced after the casting mold has been formed be connected by a suitable binding agent in order to ensure sufficient mechanical strength of the casting mold. For the production of casting molds, a foundry sand is used that has been treated with a suitable binding agent. The refractory basic molding material is preferably in a free-flowing form so that it can be filled into a suitable hollow mold and compacted there. The binding agent creates a firm bond between the particles of the basic molding material, so that the casting mold has the required mechanical stability.

Casting molds have to meet various requirements. During the casting process itself, they must first have sufficient stability and temperature resistance in order to accommodate the liquid metal in the hollow mold formed from one or more casting (part) forms. After the solidification process has started, the mechanical stability of the casting mold is ensured by a solidified metal layer that is formed along the walls of the hollow mold. The material of the casting mold must now decompose under the influence of the heat given off by the metal in such a way that it loses its mechanical strength, i.e. the cohesion between individual particles of the refractory material is broken. This is achieved in that, for example, the binder decomposes under the action of heat. After cooling, the solidified casting is shaken, and in the ideal case the material of the casting molds disintegrates again into fine sand, which can be poured out of the cavities of the metal mold.

Both organic and inorganic binders can be used to produce the casting molds, and they can be hardened by cold or hot processes. Cold processes are processes that are carried out essentially at room temperature without heating the casting mold become. The curing usually takes place through a chemical reaction, which is triggered, for example, when a gas as a catalyst is passed through the mold to be cured. In hot processes, the molding material mixture is heated to a sufficiently high temperature after molding, for example to drive out the solvent contained in the binder or to initiate a chemical reaction by which the binder is cured, for example by crosslinking.

At present, organic binders are often used for the production of casting molds in which the hardening reaction is accelerated by a gaseous catalyst or which are hardened by reaction with a gaseous hardener. These processes are referred to as "cold box" processes.

One example of the production of casting molds using organic binders is the so-called polyurethane cold box process. It is a two-component system. The first component consists of a solution of a polyol, usually a phenolic resin. The second component is a solution of a polyisocyanate. So according to the U.S. 3,409,579 A caused the two components of the polyurethane binder to react by passing a gaseous tertiary amine through the mixture of molding base material and binder after molding. The curing reaction of polyurethane binders is a polyaddition, ie a reaction without splitting off by-products such as water. Other advantages of this cold box process include good productivity, dimensional accuracy of the casting molds and good technical properties, such as the strength of the casting molds, the processing time of the mixture of mold base material and binding agent, etc.

The hot-curing organic processes include the hot-box process based on phenolic or furan resins and the warm-box process based on furan resins and the Croning process based on phenol novolak resins. In the hot box and warm box processes, liquid resins are processed into a molding material mixture with a latent hardener that only becomes effective at elevated temperatures. In the Croning process, basic mold materials such as quartz, chrome ore, zircon sands, etc. are coated at a temperature of approx. 100 to 160 ° C with a phenol novolak resin that is liquid at this temperature. Hexamethylenetetramine is added as a reaction partner for the subsequent hardening. In the case of the above-mentioned hot-curing technologies, the shaping and curing takes place in heatable tools that are heated to a temperature of up to 300 ° C.

Regardless of the hardening mechanism, what all organic systems have in common is that they thermally decompose when the liquid metal is poured into the casting mold, releasing harmful substances such as benzene, toluene, xylenes, phenol, formaldehyde and higher, partially unidentified cracking products. Various measures have succeeded in minimizing these emissions, but they cannot be completely avoided with organic binders. Such undesired emissions also occur in the case of inorganic- _{organic hybrid systems which, like the binders used, for example, in the resol-CO 2} process, contain a proportion of organic compounds.

In order to avoid the emission of decomposition products during the casting process, binders must be used that are based on inorganic materials or that contain at most a very small proportion of organic compounds. Such binder systems have been known for a long time. Binder systems have been developed which can be hardened by introducing gases. Such a system is for example in the GB 782 205 described in which an alkali water glass is used as a binder that can be cured _{by introducing CO 2.} In the DE 199 25 167 an exothermic feeder mass is described which contains an alkali silicate as a binder. Furthermore, binder systems have been developed which are self-curing at room temperature. Such a system based on phosphoric acid and metal oxides is, for example, in U.S. 5,582,232 described. Finally, inorganic binder systems are also known which are cured at higher temperatures, for example in a hot tool. Such thermosetting binder systems are, for example, from U.S. 5,474,606 known, in which a binder system consisting of alkali water glass and aluminum silicate is described.

During the production of castings, large quantities of used foundry sand are produced, which are contaminated with binding agent residues. These used sands must either be dumped or processed in a suitable manner so that they can be used again for the production of casting molds if necessary. The same applies to so-called overflow sand, i.e. sand that was mixed with binding agent but not hardened, as well as for cores or core lumps that did not reach the casting.

The most widespread is mechanical regeneration, in which the binder residues or decomposition products remaining on the used foundry sand after casting are removed by rubbing. For this purpose, the sand can be moved vigorously, for example, so that the binding agent residues adhering to them are detached when neighboring grains of sand collide. The binder residues can then be separated from the sand by sieving and dedusting.

However, mechanical regeneration often fails to completely separate the binder residues from the sand. Furthermore, due to the strong forces involved in mechanical regeneration act on the grains of sand, strong abrasion occurs or the grains of sand can splinter. The sand processed by mechanical regeneration is therefore usually not of the same quality as new sand. If the mechanically regenerated sand is therefore used for the production of casting molds, this can lead to castings of inferior quality being obtained.

To remove residues of organic binders, the used foundry sand can be heated with access to air, so that the binder residues burn. So in the DE 41 11 643 a device for the continuous regeneration of synthetic resin-bonded foundry sands described. After mechanical pre-cleaning, the used foundry sand is fed to a thermal regeneration stage in which the organic binder residues remaining on the sand grains are burned off. This thermal regeneration stage comprises a sand preheater, a cascade furnace that works continuously on the countercurrent principle with fluidized beds lying on top of one another on individual levels, and a sand cooler. The cooling air, which is forced through the sand cooler in coiled tubes, is fed to the furnace as hot air for swirling. It is also used as burner air. Furthermore, the hot air from the interior of the sand cooler is fed to the sand preheater for heating the sand. This achieves a temperature distribution in the furnace that does not lead to incomplete and therefore harmful exhaust gases at any point in combustion.

The used sand is usually separated from the casting before reprocessing. However, a method is also known in which the castings together with the cores and molds produced using organic binders are heated in an oven for a prolonged period of time to a temperature of about 400 to 550 ° C. immediately after casting. Due to the thermal Treatment, in addition to removing the organic binder, also brings about a metallurgical modification of the casting.

So in the EP 0 612 276 B2 a method for heat treatment of a casting with an adhering sand core comprising sand bound with a burn-out binder, with which the sand can be recovered from the sand core. The casting is inserted into a furnace and the furnace is heated so that parts of the sand core are detached from the casting. The loosened sand particles that are collected inside the furnace are recovered. The process step of recovery includes at least a fluidization of the detached sand core parts within the furnace. The detached sand core parts can be fluidized, for example, by introducing compressed air, with which the sand particles are kept in suspension.

Used foundry sands that are contaminated with inorganic binders, such as water glass, can be reprocessed by mechanical regeneration. A thermal pretreatment of the used sand can embrittlement the binder film surrounding the sand grain, so that the binder film can be rubbed off mechanically more easily.

In the DE 43 06 007 A1 a thermal treatment of foundry sand contaminated with water glass is described. The used foundry sand is obtained from molds that have been hardened with acid gases, usually carbon dioxide. The used foundry sand is first mechanically crushed and then heated to a temperature exceeding 200 ° C. The thermal treatment destroys disruptive components or transforms them in such a way that the foundry sand is suitable for a further molding process. The description does not include examples so that the exact implementation of the procedure remains unclear. In particular, it does not describe whether the binder is mechanically rubbed off the sand grains after the thermal treatment of the used sand.

In the DE 1 806 842 A a method for the regeneration of foundry used sands is also described, in which the used sand is first annealed and then specially treated to remove binder residues. All used foundry sands can be used, regardless of whether they have been bound by organic or inorganic binders. Processing by washing with water is only recommended for cement-bound foundry sands. To remove binding agent residues from the annealed used foundry sand, the annealed sand is first cooled and any remaining binding agent residues are removed from them by gently rubbing or impacting the sand grains. The sand is then sifted and dusted. Preferably, the annealed sand is shock-cooled by water to a temperature of a little over 100 ° C, whereby shrinkage stresses are triggered in the binder residues and the binder residues are burst open by the sudden formation of steam from the surface of the sand grains, whereby the binder residues can more easily be detached from the sand grains .

M. Ruzbehi, Giesserei 74, 1987, pp. 318 - 321 , reports on investigations into the thermo-mechanical regeneration of molding materials with a water glass ester binder system. Thermal treatment of the used sand embrittles the water glass ester system used as a binder and can therefore be rubbed off the sand grains mechanically more easily. _{The author assumes that the Na 2} O content is decisive for the regeneration of water-glass-bound sand. The fire resistance of the sand decreases with increasing Na _{2 O content.} When using the Water glass ester binder system remaining in the used sand lead to an uncontrolled hardening process when it is reused. Since the concentration of ester residues in the used sand is difficult to determine, the author uses the Na ₂ O content of the regenerated sand as a benchmark for reprocessing, ie the removal of the binding agent from the used sand. After the sand has been circulated several times, an equilibrium of the Na ₂ O content is established in the regenerated used sand from around the seventh circulation. During the thermal treatment, the used sand is heated to around 200 ° C. This prevents the grains of sand from sintering. In microscopic images of the thermally treated sand grains, embrittlement and tearing of the binder film can be observed, so that it can be mechanically rubbed off the sand grain.

However, it has been shown that the abrasion of the binding agent occurs only very incompletely and the grains have a rough surface after the treatment. Compared to new sand, the regenerated used sand has a number of disadvantages. The regenerated used sand is more difficult to shoot on conventional core shooters. This can be seen, for example, in the lower density of the moldings made from regenerated waste sand. The moldings produced from regenerated waste sand also show a lower strength. Finally, the processing time of molding material mixtures made from regenerated used sand is shorter than that of mixtures made using new sand. The molding material mixtures made from mechanically regenerated waste sand encrust significantly faster.

The processing time of such molding material mixtures produced from mechanically regenerated waste sand can be improved by adding about 0.1 to 0.5% by weight of water, which may contain a surfactant, to the molding material mixture. By this measure can also improve the strength of the molded bodies produced from this molding material mixture. However, as a result of this measure, the regenerated used sand does not achieve the quality of new sand. Furthermore, the results are only reproducible to a limited extent, so that in the process of manufacturing casting molds, imponderables arise that cannot be accepted in industrial production.

Inorganic binders, in particular those based on water glass, are still largely water-soluble even after the casting mold has cured. The foundry sand can therefore also be worked up by washing off residues of the inorganic binder on the sand with water. The water can already be used to clean the casting of adhering used sand. For example, the EP 1 626 830 production line described before a wet core removal. However, the regeneration of the used sand is not discussed.

In the DE 10 2005 029 742 describes a method for treating foundry molding materials, in which part of the used foundry sand is washed with water. For this purpose, the used sand, which is bound with an inorganic binder, is separated dry from the casting after pouring. Lumpy parts are crushed dry. The crushed sand is sieved to a certain grain size and unwanted fine particles are removed. The screened sand is separated into two partial flows, one partial flow being fed to an interim storage facility. The other partial flow is washed with water until the grain surface is sufficiently cleaned of residues of the binding agent and products of the casting process. After washing, the washing water is separated off and the sand is dried. A portion of the screened used sand removed from the interim storage facility can then be added to the washed sand.

Wet cleaning of the used foundry sand is very effective in itself. The strengths of cores that are made from the washed reclaimed sand are roughly the same as those achieved when using new sand. However, the processing time of these molding mixtures made from regenerated used sand is somewhat shorter than when using new sand. The cleaning of the used sand, however, is very time-consuming, since large amounts of washing water arise that have to be cleaned again. Another disadvantage is that the wet sand has to be dried before it can be used again.

In the DE 38 15 877 C1 Finally, a method for the separation of inorganic binder systems in the regeneration of foundry used sand is described, in which a slurry of the used sand in, for example, water is treated with ultrasound. Bentonite, water glass and cement are given as exemplary binder systems. According to a preferred embodiment, the used sand can be subjected to a thermal treatment before the ultrasonic treatment. 400 to 1200 ° C., particularly preferably 600 to 950 ° C., are specified as preferred temperature ranges for the thermal pretreatment. The examples describe the processing of used sand to which bentonite / carbon adheres as binder residues. The thermal treatment is used to remove carbon, which accumulates in the bentonite in the form of polyaromatic carbons in a concentration that does not allow direct reuse.

T. Oehlerking (Foundry Verlag Düsseldorf, Vol. 80, No. 21, November 1, 1993, pages 721-728, ISSN: 0016-9765 ) investigated the regenerability of used molding sands bound with organic binders. These are exposed to heat and air and CO and CO ₂ , H ₂ O and or other pyrolysis gases are formed. In contrast, inorganic binders cannot be removed in this way. Water evaporates as a solvent, but the inorganic bridges remain intact.

As explained above, the importance of waterglass-based binders for the production of casting molds is increasing, since in this way harmful emissions during the casting process can be significantly reduced. Recently, very powerful binders based on water glass have been used in the foundry industry have been developed which contain proportions of a finely divided metal oxide, in particular finely divided silicon dioxide. These binders are cured while hot, ie by evaporation of the water contained in the water glass. The addition of the finely divided metal oxide, among other things, increases the strength immediately after removal from the hot tool, so that even very complex cores can be produced using this inorganic binder. Such a binder based on water glass is, for example, in WO 2006/024540 A described.

In the regeneration of used sands which had previously been hot-solidified with such a waterglass-based binder, it was observed, however, that the regenerated used sand has a shortened processing time when it is used again with a waterglass-based binder. In order to counteract this problem and to achieve a processing time suitable for industrial use, a larger amount of new sand can, for example, be added to the regenerated used sand in order to reduce the relative proportion of the binder carried over with the regenerated used sand. It is also possible to mix the regenerated used sand with other regenerated used sands that have different properties. The used sands are selected in such a way that a satisfactory processing time is achieved after adding a more water-glass-containing binding agent.

By using the newly developed binding agent based on water glass, as already described, it is also possible to manufacture cores and molds with very complex geometries. Since the increasingly stricter emission and occupational health and safety regulations can be expected to increase the importance of inorganic binders for the foundry industry, larger amounts of used sands with water glass will arise in the future. that have to be reprocessed. There is therefore a great need for methods for the regeneration of used molding sand, which should be easy to carry out and have to provide a reproducible quality of the regenerated used sand, ie the regenerated used sand should be able to be processed essentially in the same way as new sand.

The invention was therefore based on the object of providing a method for producing casting molds which comprises a method for reprocessing foundry sands contaminated with water glass, which can be carried out simply and cheaply, so that the sand is of high quality for the even after repeated reprocessing Has production of foundry molds. In particular, it should also be possible to regenerate those used sands with the process that have previously been solidified with a waterglass-based binder, including a particulate metal oxide to increase strength, the particulate metal oxide being synthetically produced amorphous silicon dioxide and, if subsequently, "particulate metal oxide "is mentioned, this term stands for" a particulate metal oxide that is synthetically produced amorphous silica ".

This object is achieved with a method having the features of claim 1. Advantageous embodiments of the method according to the invention are the subject of the dependent claims.

Surprisingly, it has been found that the cohesion between the grains of a foundry sand decreases significantly when the used casting mold, as it is after the metal casting, is heated to a temperature of at least 200 ° C. for a longer period of time. The molding sand reconditioned by thermal treatment shows no premature hardening when it is used again with a water glass-based binder. The processing time of regenerated used sand is comparable to the processing time of new sand. It is not necessary for the binder to be mechanically rubbed off the sand grains after the thermal treatment.

To remove oversized grain, a classification can also be carried out, for example by sieving or air sifting.

The inventors assume that when the used sand is regenerated by mechanical abrasion of the binding agent from the grain of sand or during at least partial wet processing, small amounts of the particulate metal oxide, in particular silicon dioxide, are dragged into a newly prepared molding material mixture with the regenerated used sand. The particulate metal oxide can presumably trigger premature hardening of the water glass, which significantly shortens the processing time of the molding material mixture.

However, if the used sand is thermally treated as in the process according to the invention, the particulate metal oxide present in the binder adhering to the grain of sand presumably causes the adhering water glass to be vitrified. A vitreous layer forms from the water glass on the grain of sand, which has only a low reactivity. This can also be seen, for example, in the fact that the amount of extractable sodium ions decreases during the regeneration of the sand and is very low in the regenerated sand.

Due to the thermal treatment, the strength of the used casting mold decreases significantly, so that it disintegrates even with little mechanical impact. The mechanism of the decay is unclear. The inventors assume, however, that the water glass adhering to the foundry sand at least partially reacts with the grain of sand and, under the influence of the particulate metal oxide, can form a thin glass envelope on its surface. The surface of the grain of sand is smoothed so that it can easily be incorporated into a molding material mixture after it has been incorporated again

Core shooters can be processed into moldings. The water glass remaining on the grain of sand only leads to an insignificant increase in grain size, so that the foundry sand can also go through several cycles before the reprocessed grains of sand are separated, for example in a classification following the thermal regeneration, such as a sieving step, due to excessive increase in size.

The progress of the regeneration of the used foundry sand can be followed, for example, by determining the acid consumption, which is a measure of the extractable sodium ions still present in the used sand. If the foundry sand includes larger aggregates, these are first crushed, for example with the help of a hammer. The foundry sand can then also be sifted through a sieve which, for example, has a mesh size of 1 mm. Then a certain amount of the foundry sand is suspended in water and reacted with a defined amount of hydrochloric acid. The amount of acid that has not reacted with the foundry sand or with the water glass adhering to the foundry sand can then be determined by back titration with NaOH. The acid consumption of the foundry sand can then be determined from the difference between the amount of acid used and the amount of acid that has been back-titrated.

In addition to the acid consumption, other parameters can also be used to monitor the progress of the thermal treatment. For example, the pH value or the conductivity of a slurry of the foundry sand can be used. The suspension can be prepared, for example, by slurrying 50 g of the foundry sand in one liter of distilled water. During the thermal treatment, the sand bodies are given a smooth surface. The flowability of the sand can therefore also be used as a parameter, for example.

In addition, properties of a molding material mixture produced from the regenerated foundry sand, for example its processing time, or properties of a molding produced from this molding material mixture, for example its density or flexural strength, can also be used to assess the thermal treatment of the used foundry sand.

When implementing the method according to the invention for an industrial application, it is possible, for example, to proceed in such a way that the parameters are determined by systematic series tests.

Samples of the used foundry sand can be thermally processed, with the treatment temperature and the treatment time being systematically varied. The acid consumption can then be determined for the thermally reprocessed samples. A molding material mixture is then produced from the individual samples and their processing time is determined. Furthermore, test specimens are produced from the molding material mixture and their density or flexural strength is determined. From the sample bodies, those whose properties meet the requirements are then selected and then, for example, the acid consumption of the relevant reprocessed foundry sand sample is used as a criterion for the thermal treatment on a larger scale.

The method according to the invention for reprocessing used foundry sands is simple to carry out and does not require any complicated devices per se. The reclaimed foundry sand obtained with the process according to the invention has approximately the same properties as new sand, ie the molded bodies produced from the reclaimed foundry sand have a comparable strength and a comparable density. Furthermore, one from the regenerated foundry sand Molding material mixture produced by adding water glass has approximately the same processing time as a molding material mixture based on new sand. With the method according to the invention, a simple and economical method is available with which used foundry sand that is adhered with waterglass-containing binder can be reprocessed, the molding material mixture or the used foundry sand containing the particulate metal oxide.

In detail, the method according to the invention for producing casting molds, which comprises a method for reprocessing used foundry sands which are contaminated with water glass, is carried out according to claim 1.

Used foundry sand is understood to mean any foundry sand contaminated with water glass that is to be reprocessed, with a particulate metal oxide being added to the water glass in the previous production cycle to improve the initial strength of the casting mold. The binder coating adhering to the used foundry sand therefore still contains the particulate metal oxide. The used foundry sand comes from a used casting mold. The used foundry mold can be present in its entirety or it can also be broken into several parts or chunks. The used foundry mold can also have been crushed to the extent that that it is back in the form of a foundry sand with a water glass. A used mold can be a mold that has already been used for metal casting. A used casting mold can, however, also be a casting mold that was not used for metal casting, for example because it is redundant or defective. Partial forms of casting molds are also included. For example, permanent molds, so-called chill molds, can also be used for metal casting, which are used in combination with a casting mold made of foundry sand solidified with water glass. The latter can be processed again with the method according to the invention. Used foundry sand is also understood to mean overflow sand which, for example, has remained in the storage bunker or in feed lines of a core shooting device and has not yet hardened.

The water glass, which is contained as a binder in the used foundry sand, contains the particulate metal oxide. During the previous use of the foundry sand in the production of the molding mixture, this metal oxide was added to the waterglass binder in order to improve the initial strength of a mold produced from the molding mixture. The used foundry sand can consist entirely of foundry sand contaminated with such a binder. But it is also possible to regenerate other used foundry sands together with the used foundry sand described above. Such other used foundry sands can be, for example, foundry sands that are contaminated with an organic binder, or foundry sands that are contaminated with a waterglass-based binder to which no particulate metal oxide has been added. In order to be able to use the advantages of the method according to the invention, in particular the elimination of the need to mechanically rub the remaining binder from the grain of sand after the thermal regeneration, the proportion of the used foundry sand which is contaminated with a water glass-based binder to which the particulate metal oxide has been added is preferably greater than 20% by weight, preferably greater than 40% by weight, particularly preferably greater than 60% by weight , particularly preferably greater than 80% by weight, based on the amount of foundry sand to be regenerated.

A particulate metal oxide is understood to mean a very finely divided metal oxide, the primary particles of which preferably have an average diameter of less than 1.5 μm , particularly preferably between 0.10 μm and 1 μm . However, agglomeration of the primary particles can also result in larger particles.

In the practical implementation of the method according to the invention, the majority of the used foundry sand is obtained from the reprocessing of used casting molds. According to a preferred embodiment, the used foundry sand is therefore in the form of a used casting mold with which a metal casting has already been carried out.

If the used foundry sand is provided in the form of a casting mold that has already been used for metal casting, the used foundry mold can still contain the casting according to a first embodiment of the method according to the invention. For the thermal treatment, the used casting mold can therefore be used directly in the form in which it is obtained after the metal casting. The casting mold with the casting contained therein is subjected as a whole to a thermal treatment. For this purpose, the casting mold with the casting can be transferred to a suitably dimensioned furnace. The thermal treatment weakens the cohesion between the grains of the used foundry sand. The casting mold disintegrates and the foundry sand can, for example, by means of suitable devices collected in the oven. The disintegration of the casting mold in the furnace can be supported by machining the casting mold. For this purpose, the casting mold can be shaken, for example.

In order to carry out the method according to the invention, it is therefore not necessary to separate the casting mold from the casting. If necessary, a metallurgical improvement of the casting can also be achieved at the same time through the thermal treatment of the used casting mold. According to a further embodiment of the method according to the invention, however, the used casting mold is first separated from the casting and then the used casting mold is reprocessed separately from the casting.

The used foundry sand contaminated with water glass is produced in the usual process of manufacturing castings in foundries. The casting mold for metal casting, which is solidified with a binding agent based on water glass, can in itself have been produced in a known manner. The water-glass-containing binder, to which a particulate metal oxide is added, is thermally cured. The casting mold is solidified by removing water from the water-glass-containing binder. The casting mold can be constructed from a single molded body. But it is also possible that the

Casting mold is made up of several moldings, which may be produced in separate work steps and then put together to form a casting mold. The casting mold can also comprise sections that have not been solidified with water glass as a binder but, for example, with an organic binder, such as a cold box binder. It is also possible that the casting mold is partly formed from permanent molds. Those parts of the casting mold which consist of foundry sand solidified with water glass can then be reprocessed using the method according to the invention. It is also possible that the casting mold comprises, for example, only one core, which consists of foundry sand solidified with water glass as a binding agent, while the mold is made of so-called green sand. In the used casting mold, the parts which contain the foundry sand contaminated with water glass are then separated off and reprocessed using the method according to the invention.

The casting mold for metal casting is used in the usual way, with a used casting mold being obtained after the metal has cooled, which can be reprocessed using the method according to the invention.

For reprocessing, the casting mold is heated to a temperature of at least 200 ° C. The entire volume of the casting mold should reach this temperature so that a uniform disintegration of the casting mold is achieved. The duration for which the casting mold is subjected to a thermal treatment depends, for example, on the size of the casting mold or also on the amount of the water-glass-containing binder and can be determined by taking a sample. The sample taken should disintegrate into loose sand under slight mechanical impact, such as occurs when the casting mold is shaken. The cohesion between the grains of the foundry sand should be weakened so far be that the thermally treated foundry sand can be sieved without any problems in order to separate larger aggregates or impurities.

The duration of the thermal treatment can be chosen to be relatively short for small casting molds, especially if the temperature is chosen to be higher. For larger casting molds, especially if these still contain the casting, the treatment time can also be chosen to be significantly longer, up to several hours. The period of time within which the thermal treatment is carried out is preferably chosen between 5 minutes and 8 hours. The progress of the thermal regeneration can be followed, for example, by determining the acid consumption on samples of the thermally treated foundry sand. Foundry sands, such as chromite sand, can themselves have basic properties, so that the foundry sand influences acid consumption. However, the relative acid consumption can be used as a parameter for the progress of the regeneration. For this purpose, the acid consumption of the used foundry sand intended for reprocessing is first determined. To observe the regeneration, the acid consumption of the regenerated foundry sand is determined and related to the acid consumption of the used foundry sand. As a result of the thermal treatment carried out in the method according to the invention, the acid consumption for the regenerated foundry sand preferably decreases by at least 10%. The thermal treatment is preferably continued until the acid consumption compared to the acid consumption of the used foundry sand has decreased by at least 20%, in particular at least 40%, particularly preferably at least 60% and particularly preferably by at least 80%. The acid consumption is given in ml of acid consumed per 50 g of the foundry sand, the determination with 0.1 N hydrochloric acid analogous to the method given in VDG Merkblatt P 28 (May 1979) is determined. The method for determining the acid consumption is detailed in the examples.

The casting mold can be heated by any desired method. For example, it is possible to expose the casting mold to microwave radiation. However, other methods can also be used to heat the casting mold. It is also conceivable that an exothermic material is added to the used foundry sand, which provides the temperature required for the treatment alone or in combination with other heat sources. The duration of the thermal treatment can be influenced by the temperature to which the casting mold is heated. Decomposition can already be observed at temperatures of around 200 ° C. The temperature chosen is preferably higher than 250.degree. C., in particular higher than 300.degree. The upper limit for the temperature used in the thermal treatment corresponds to the sintering temperature of the sand. Usually, however, the temperature is limited by the design of the device in which the thermal treatment is carried out. The temperature for the thermal treatment is preferably selected to be less than 1300 ° C., particularly preferably less than 1100 ° C. and particularly preferably less than 1000 ° C. If the casting mold also contains organic impurities in addition to the water glass-containing binder, the temperature is preferably selected so high that the organic impurities burn.

The temperature can be kept constant during the thermal treatment. However, it is also possible that a temperature program is run through during the thermal treatment, in which the temperature is changed in a predetermined manner. For example, the thermal treatment can initially be carried out at a relatively high temperature, for example at a temperature of more than 500 ° C., in order to remove organic impurities to burn and accelerate the disintegration of the used casting mold. The temperature can then be gradually reduced, for example in order to adjust the acid consumption to the desired value.

As already explained further above, according to a first embodiment, the casting mold can be subjected to the thermal treatment in a state in which it has not yet been separated from the casting. In this case, both the casting mold and the casting experience a thermal treatment.

According to a second embodiment, the casting mold is separated from the casting before the thermal treatment. Customary procedures can be used for this purpose. For example, the casting mold can be smashed by mechanical action or the casting mold can be shaken so that it breaks up into several fragments.

In order to ensure uniform heating of the casting mold or the larger aggregates resulting from it during the thermal treatment, the casting mold is preferably broken into at least coarse fragments, which have a diameter of about 20 cm or less, for example. The fragments preferably have a largest dimension of less than 10 cm, particularly preferably less than 5 cm, particularly preferably less than 3 cm. Conventional devices, for example lump breakers, can be used to break the casting mold. Chunks of a corresponding size can also be obtained, for example, if the casting mold is separated from the casting by means of a jackhammer or a chisel or by shaking.

According to a further embodiment, a mechanical treatment of the foundry sand for grain separation is carried out before or after the thermal treatment. The mold can do this for example ground, crushed by rubbing or impacting and the sand obtained in this way can be sieved. Conventional devices can be used for this, such as those that have already been used, for example, in the mechanical processing of foundry sands. For example, the foundry sand can be passed through a fluidized bed in which the sand grains are held in suspension by means of a stream of compressed air. As a result of the collision of the grains of sand, the outer shell made of water glass binder is rubbed off. The sand grains can, however, also be directed against a baffle plate by means of an air stream, with the outer shell of the sand grain formed from water glass binding agent being removed upon impact on the baffle plate or other sand grains.

However, mechanical treatment of the thermally regenerated used sand is preferably dispensed with and only oversized grains are removed by means of an appropriate classification. This avoids mechanical damage to the sand, for example through splinters, and smooth, easily pourable sand grains are obtained. When using foundry sand regenerated in this way, essentially no shortening of the processing time is observed when compared to new sand when this is processed again to form a molding material mixture with water glass as a binder.

The temperature required for the thermal treatment can initially be set in any manner per se. In addition to methods such as treatment with microwaves, the thermal treatment is preferably carried out in such a way that the casting mold, possibly in comminuted form, is transferred to an oven for the thermal treatment.

The furnace can be designed as desired, provided that uniform heating of the material of the casting mold is guaranteed. The furnace can be designed so that the thermal Treatment is carried out discontinuously, so the furnace is charged, for example, batchwise with the, possibly crushed, casting mold and the thermally treated material is removed from the furnace before the furnace is then filled with the next batch. However, it is also possible to provide a furnace that enables the process to be carried out continuously. For this purpose, the furnace can be designed, for example, in the form of a road or a tunnel through which the used casting mold is transported, for example by means of a conveyor belt. For the treatment of the used foundry sand contaminated with water glass, ovens can be used, as are known from the thermal regeneration of used foundry sands contaminated with organic binders.

It is preferably provided that the used foundry sand is moved during the thermal treatment. The movement can take place, for example, by moving the casting mold or the chunks obtained from it around the three spatial axes, so that the casting mold or the chunks execute rolling movements through which a further comminution of the casting mold or the smaller casting sand aggregates resulting from it is achieved . Such a movement can be achieved, for example, by moving the smaller foundry sand aggregates produced from the casting mold by means of a stirrer or in a rotating drum. If the used foundry sand has already been comminuted to such an extent that it is in the form of sand, the movement can also take place by holding the sand in suspension in a fluidized bed by means of a heated stream of compressed air.

According to a preferred embodiment, a rotary kiln is used for the thermal treatment of the used foundry sand. It has been shown that even with a coarse pre-shredding the casting mold during the passage through the rotary kiln a substantial disintegration of the used casting mold can be achieved. If even larger aggregates remain in the regenerated foundry sand after leaving the rotary kiln, they can be separated by sieving, for example.

The thermal treatment can per se also be carried out under an inert gas atmosphere. However, the thermal treatment is advantageously carried out with admission of air. On the one hand, this reduces the effort involved in the thermal treatment, since no special measures have to be taken in order to exclude the ingress of oxygen. Another advantage of thermal treatment with access to air is that organic contaminants that contaminate the used foundry sand are burned so that further cleaning is achieved.

The method according to the invention for reprocessing foundry sand can per se also be combined with other reprocessing processes. For example, the thermal treatment can be preceded by a mechanical treatment in which part of the water glass is rubbed off the sand grains and removed by sieving and / or dedusting. It is also possible to carry out a wet treatment process before or after the thermal treatment according to the invention. For example, the used foundry sand can be washed with water before the thermal treatment in order to remove a portion of the water glass. Because of the considerable effort that such a wet treatment requires, the sand must be dried after washing and the contaminated washing water must be treated, the method according to the invention is, however, preferably carried out dry, that is to say without a wet step. Another advantage of dry reprocessing is that any contaminants that may still remain in the foundry sand after thermal processing can be removed from the The resulting layer of water glass can be firmly bound to the grain of sand. If the foundry sand is therefore discharged after several cycles, for example because the grain size has increased too much, a comparatively simple disposal of the sand is therefore possible.

After the thermal treatment or before it is used again as foundry sand for the production of a new casting mold, the reprocessed foundry sand is preferably sieved in order to separate larger aggregates and dedusted. Known devices can be used for this, as are known, for example, from the mechanical regeneration of used foundry sands or the thermal regeneration of organically bound foundry sands.

The result of the reconditioning can already be positively influenced by the process with which the casting mold for metal casting is produced.

• eine Formstoffmischung bereitgestellt wird, welche zumindest einen Gießereisand und zumindest ein wasserglashaltiges Bindemittel umfasst, welchem das teilchenförmige Metalloxid zugesetzt ist,
• die Formstoffmischung zu einer neuen Gießform verarbeitet und ausgehärtet wird, und
• mit der neuen Gießform ein Metallguss durchgeführt wird, sodass eine gebrauchte Gießform mit einem Gussstück erhalten wird.

When the process is carried out, water glass is essentially used as the binder, to which a portion of a particulate metal oxide has been added. In this embodiment, the used casting mold is therefore provided with the casting by

• a molding material mixture is provided which comprises at least one foundry sand and at least one water glass-containing binder to which the particulate metal oxide is added,
• the molding material mixture is processed into a new casting mold and cured, and
• a metal casting is carried out with the new casting mold, so that a used casting mold with a casting is obtained.

The production of the new casting mold and the subsequent metal casting take place according to known processes. The molding material mixture is produced by moving the foundry sand and then applying the particulate metal oxide or water glass in any order. The mixture is moved on until the grains of the foundry sand are evenly covered with the water glass.

Conventional materials for the production of casting molds can be used as foundry sand. For example, quartz or zircon sand are suitable. Fibrous refractory molding raw materials, such as fireclay fibers, are also suitable. Other suitable foundry sands are, for example, olivine, chrome ore sand, vermiculite.

Furthermore, artificial mold base materials can also be used as foundry sand, such as, for example, hollow aluminum silicate spheres (so-called microspheres) or spherical ceramic mold base materials known under the name "Cerabeads®" or "Carboaccucast®". For economic reasons, these artificial mold raw materials are preferably only added in part to the foundry sand. Based on the total weight of the foundry sand, the synthetic mold base materials are preferably used in a proportion of less than 80% by weight, preferably less than 60% by weight. These spherical ceramic mold base materials contain, for example, mullite, corundum and β-cristobalite in various proportions as minerals. They contain aluminum oxide and silicon dioxide as essential components. Typical compositions contain, for example, Al ₂ O ₃ and SiO ₂ in approximately equal proportions. In addition, other constituents can also be contained in proportions of <10%, such as TiO ₂ , Fe ₂ O ₃ . The diameter of the spherical Mold base materials are preferably less than 1000 μm, in particular less than 600 μm. Synthetically produced refractory mold base materials are also suitable, such as, for example, mullite (x Al ₂ O ₃ · y SiO ₂ , with x = 2 to 3, y = 1 to 2; ideal formula: Al ₂ SiO ₅ ). These artificial mold base materials do not go back to a natural origin and can also have been subjected to a special molding process, such as in the production of aluminum silicate micro hollow spheres or spherical ceramic mold base materials.

According to a further embodiment of the method according to the invention, glass materials are used as refractory, artificial basic molding materials. These are used in particular either as glass spheres or as glass granules. Conventional glasses can be used as the glass, glasses which have a high melting point being preferred. For example, glass beads and / or glass granules made from broken glass are suitable. Borate glasses are also suitable. The composition of such glasses is given by way of example in the table below. Table: Composition of glasses component Broken glass Borate glass SiO ₂ 50-80% 50-80% Al ₂ O ₃ 0 -15% 0-15% Fe ₂ O ₃ <2% <2% M ^II O 0 - 25% 0 - 25% M ^I ₂ O 5 - 25% 1 - 10% B ₂ O ₃ <15% Otherwise. <10% <10% M ^II : alkaline earth metal, e.g. Mg, Ca, Ba
M ^I : alkali metal, e.g. Na, K

In addition to the glasses listed in the table, it is also possible to use other glasses whose content of the above-mentioned compounds is outside the stated ranges. Likewise, special glasses can also be used which, in addition to the oxides mentioned, also contain other elements or their oxides.

The diameter of the glass spheres is preferably 1 to 1000 μm, more preferably 5 to 500 μm and particularly preferably 10 to 400 μm.

In casting tests with aluminum it was found that when using artificial mold base materials, especially with glass beads, glass granulate or microspheres, less used foundry sand adheres to the metal surface after casting than when using pure quartz sand. The use of artificial mold base materials therefore enables the production of smooth cast surfaces, with a complex post-treatment by blasting not or at least to a much lesser extent necessary.

It is not necessary to form all of the foundry sand from the artificial molding raw materials. The preferred proportion of artificial mold base materials is at least about 3% by weight, particularly preferably at least 5% by weight, especially preferably at least 10% by weight, preferably at least about 15% by weight, particularly preferably at least about 20% by weight , based on the total amount of foundry sand. The foundry sand preferably has a free-flowing state, so that the molding material mixture can be processed in conventional core shooters. The foundry sand can be formed by new sand, which has not yet been used for metal casting. However, the foundry sand which is used for the production of the molding material mixture preferably comprises at least a portion of reclaimed foundry sand, in particular a reclaimed foundry sand as obtained with the method according to the invention. The proportion of reclaimed foundry sand can be chosen anywhere between 0 and 100%. The method is particularly preferably carried out in such a way that only the portion of the foundry sand that is lost in the reprocessing according to the invention, for example during screening, is supplemented by new sand or another suitable sand. For example, thermally regenerated sand originally bound with an organic binder is also suitable. Mechanically regenerated foundry sands can also be used, provided that the organic binder still adhering to them does not accelerate the hardening of the waterglass binder. Mechanically regenerated foundry sands that are still contaminated with organic binders that have been acid-hardened are unsuitable, for example. The method according to the invention therefore does not necessarily require that a separate circuit be set up for foundry sand bound with water glass.

As a further component, the molding material mixture comprises a binder based on water glass. Conventional water glasses can be used as the water glass, as they have already been used up to now be used as binders in molding mixtures. These water glasses contain dissolved sodium or potassium silicates and can be made by dissolving vitreous potassium and sodium silicates in water. The water glass preferably has a module SiO ₂ / M ₂ O in the range from 1.6 to 4.0, in particular 2.0 to 3.5, where M stands for sodium and / or potassium. The water glasses preferably have a solids content in the range from 30 to 60% by weight. The solids content relates to the amount of SiO ₂ and M ₂ O contained in the water glass.

In the production of the molding material mixture, the procedure is generally such that first the foundry sand is initially introduced and then the binding agent and the particulate metal oxide are added with stirring. The binding agent can only consist of water glass. However, it is also possible to add additives to the water glass or the foundry sand, which have a positive effect on the properties of the casting mold or the regenerated foundry sand. The additives can be added in solid or also in liquid form, for example as a solution, in particular as an aqueous solution. Suitable additives are described below.

During the production of the molding material mixture, the foundry sand is placed in a mixer and then, if provided, the solid component (s) of the binder are preferably first added and mixed with the foundry sand. The mixing time is chosen so that the foundry sand and solid binder components are thoroughly mixed. The mixing time depends on the amount of molding material mixture to be produced and on the mixing unit used. The mixing time is preferably chosen between 5 seconds and 5 minutes. The liquid component of the binder is then added, preferably with further agitation of the mixture, and the mixture is then further mixed until the grains of the foundry sand become uniform Has formed a layer of the binder. Here, too, the mixing time depends on the amount of molding material mixture to be produced and on the mixing unit used. The duration for the mixing process is preferably chosen between 5 seconds and 5 minutes. A liquid component is understood to mean both a mixture of various liquid components and the entirety of all liquid individual components, the latter also being able to be added individually. A solid component is also understood to mean both the mixture of individual or all solid components and the entirety of all solid individual components, it being possible for the latter to be added to the molding material mixture together or one after the other.

It is also possible to first add the liquid component of the binder to the foundry sand and only then, if provided, to add the solid component to the mixture. According to one embodiment, 0.05 to 0.3% water, based on the weight of the foundry sand, is first added to the foundry sand and only then are the solid and liquid components of the binder added. In this embodiment, a surprisingly positive effect on the processing time of the molding material mixture can be achieved. The inventors assume that the dehydrating effect of the solid components of the binder is reduced in this way and the hardening process is delayed as a result.

The molding material mixture is then brought into the desired shape. The usual methods are used for the shaping. For example, the molding material mixture can be shot into the molding tool by means of a core shooter with the aid of compressed air. The molded molding material mixture is then cured. All customary methods can be used for this. The mold can be gassed with carbon dioxide in order to solidify the molding material mixture. This fumigation is preferably carried out at room temperature, ie in a cold mold. The duration of the gassing depends, among other things, on the size of the molded part to be produced and is usually selected in the range from 10 seconds to 2 minutes. Longer gassing times, for example up to 5 minutes, are also possible for larger molded parts. However, shorter or longer gassing times are also possible.

The molding is cured by supplying heat, which means that the water contained in the binder is evaporated. The heating can take place in the molding tool, for example. For this purpose, the mold is heated, preferably to temperatures of up to 300.degree. C., particularly preferably to a temperature in the range from 100 to 250.degree. It is possible to cure the casting mold completely in the molding tool. However, it is also possible to cure the casting mold only in its edge area, so that it has sufficient strength to be able to be removed from the molding tool. If necessary, the casting mold can then be completely cured by removing further water from it. This can be done in an oven, for example, as described above. The removal of water can also take place, for example, by evaporating the water under reduced pressure.

The hardening of the casting molds can be accelerated by blowing heated air into the mold. In this embodiment of the process, the water contained in the binding agent is quickly transported away, as a result of which the casting mold is solidified in periods of time suitable for industrial use. The temperature of the blown air is preferably 100 ° C to 180 ° C, particularly preferably 120 ° C to 150 ° C. The flow rate of the heated air is preferably adjusted so that the casting mold is cured in periods of time suitable for industrial use. The time periods depend on the size of the molds made. The aim is to cure in less than 5 minutes, preferably less than 2 minutes. However, longer periods of time may be required for very large molds.

The water can also be removed from the molding material mixture in such a way that the molding material mixture is heated by irradiating microwaves. However, the irradiation of the microwaves is preferably carried out after the casting mold has been removed from the molding tool. For this, however, the casting mold must already have sufficient strength. As already explained, this can be achieved, for example, in that at least one outer shell of the casting mold is already cured in the molding tool.

If the casting mold consists of several partial molds, these are put together in a suitable manner to form the casting mold, it also being possible for supply lines and compensation reservoirs to be attached to the casting mold.

The casting mold is then used in the usual way for metal casting. The metal casting can be carried out with any metals. For example, cast iron or cast aluminum is suitable. After the metal has solidified or cooled, the casting mold is then reprocessed in the manner already described by thermal treatment.

The properties of the casting mold as well as the regenerated sand can be improved by adding additives to the molding material mixture.

As already explained, the particulate metal oxide is added to the water glass used as a binder. The particulate metal oxide does not match foundry sand. It also has a smaller mean particle size than foundry sand.

The molding material mixture contains a proportion of the particulate metal oxide. The strength of the casting mold can be influenced by adding this particulate metal oxide.

The average primary particle size of the particulate metal oxide can be between 0.10 µm and 1 µm. Because of the agglomeration of the primary particles, however, the particle size of the metal oxide is preferably less than 300 μm, more preferably less than 200 μm, particularly preferably less than 100 μm. It is preferably in the range from 5 to 90 μm, particularly preferably from 10 to 80 μm and very particularly preferably in the range from 15 to 50 μm. The particle size can be determined, for example, by sieve analysis. The sieve residue on a sieve with a mesh size of 63 μm is particularly preferably less than 10% by weight, preferably less than 8% by weight.

As the particulate metal oxide, synthetically produced amorphous silica is used.

Precipitated silica and / or fumed silica is preferably used as the particulate silicon dioxide. Precipitated silica is obtained by reacting an aqueous alkali silicate solution with mineral acids. The resulting precipitate is then separated off, dried and ground. Pyrogenic silicas are understood as meaning silicas which are obtained from the gas phase by coagulation at high temperatures become. Fumed silica can be produced, for example, by flame hydrolysis of silicon tetrachloride or in an electric arc furnace by reducing quartz sand with coke or anthracite to form silicon monoxide gas with subsequent oxidation to form silicon dioxide. The pyrogenic silicas produced by the electric arc furnace process can also contain carbon. Precipitated silica and pyrogenic silica are equally well suited for the molding material mixture according to the invention. These silicas are referred to below as "synthetic amorphous silicon dioxide".

The inventors assume that the strongly alkaline water glass can react with the silanol groups arranged on the surface of the synthetically produced amorphous silicon dioxide and that when the water evaporates, an intensive bond is established between the silicon dioxide and the then solid water glass.

According to a further embodiment, at least one organic additive is added to the molding material mixture.

An organic additive is preferably used which has a melting point in the range from 40 to 180 ° C., preferably 50 to 175 ° C., that is to say is solid at room temperature. Organic additives are understood to mean compounds whose molecular structure is predominantly made up of carbon atoms, for example organic polymers. The quality of the surface of the casting can be further improved by adding the organic additives. The mechanism of action of the organic additives has not been clarified. Without wishing to be bound by this theory, the inventors assume, however, that at least some of the organic additives burns during the casting process, creating a thin gas cushion between the liquid metal and the foundry sand forming the wall of the casting mold and thus a reaction between liquid metal and Foundry sand prevented becomes. The inventors also assume that some of the organic additives form a thin layer of so-called lustrous carbon under the reducing atmosphere prevailing during casting, which likewise prevents a reaction between metal and foundry sand. As a further advantageous effect, the addition of the organic additives can increase the strength of the casting mold after curing.

The organic additives are preferably used in an amount of 0.01 to 1.5% by weight, particularly preferably 0.05 to 1.3% by weight, particularly preferably 0.1 to 1.0% by weight, each based on the foundry sand, added.

The surface of the casting can be improved with very different organic additives. Suitable organic additives are, for example, phenol-formaldehyde resins such as novolaks, epoxy resins such as bisphenol A epoxy resins, bisphenol F epoxy resins or epoxidized novolaks, polyols such as polyethylene glycols or polypropylene glycols, polyolefins such as polyethylene or polypropylene, copolymers Olefins such as ethylene or propylene, and other comonomers such as vinyl acetate, polyamides such as polyamide-6, polyamide-12 or polyamide-6,6, natural resins such as gum resin, fatty acids such as stearic acid, fatty acid esters such as cetyl palmitate , Fatty acid amides, such as, for example, ethylenediamine bisstearamide, and metal soaps, such as, for example, stearates or oleates of mono- to trivalent metals. The organic additives can be contained either as a pure substance or as a mixture of various organic compounds.

According to a further embodiment, at least one carbohydrate is used as the organic additive. The addition of carbohydrates gives the casting mold a high level of strength, both immediately after production and during longer storage. Furthermore, after the metal casting, a casting with a very high surface quality is obtained, so that only a slight post-processing of the surface of the casting is required after the casting mold has been removed. This is an essential advantage, since in this way the costs for the production of a casting can be reduced significantly. If carbohydrates are used as an organic additive, significantly lower smoke development is observed during casting compared to other organic additives such as acrylic resins, polystyrene, polyvinyl esters or polyalkyl compounds, so that the workload for those employed there can be significantly reduced.

Both mono- or disaccharides and higher molecular weight oligo- or polysaccharides can be used. The carbohydrates can be used both as a single compound and as a mixture of different carbohydrates. No excessive demands are made on the purity of the carbohydrates used. It is sufficient if the carbohydrates, based on the dry weight, are present in a purity of more than 80% by weight, particularly preferably more than 90% by weight, particularly preferably more than 95% by weight, in each case based on the Dry weight. The monosaccharide units of the carbohydrates can be linked in any way. The carbohydrates preferably have a linear structure, for example an α- or β-glycosidic 1,4 linkage. The carbohydrates can also be fully or partially 1,6-linked, such as. B. amylopectin, which has up to 6% α-1,6 bonds.

The amount of carbohydrate can be chosen to be relatively small, in order to observe a clear effect on the strength of the casting molds before casting or a clear improvement in the quality of the surface. The proportion is preferred of the carbohydrate, based on the foundry sand, in the range from 0.01 to 10% by weight, particularly preferably 0.02 to 5% by weight, particularly preferably 0.05 to 2.5% by weight and very particularly preferred selected in the range from 0.1 to 0.5% by weight. Even small proportions of carbohydrates in the range of about 0.1% by weight lead to clear effects.

According to a further embodiment of the invention, the carbohydrate is used in underivatized form. Such carbohydrates can be obtained inexpensively from natural sources such as plants, for example cereals or potatoes. The molecular weight of such carbohydrates obtained from natural sources can be reduced, for example, by chemical or enzymatic hydrolysis in order, for example, to improve the solubility in water. In addition to underivatized carbohydrates, which are only made up of carbon, oxygen and hydrogen, it is also possible to use derivatized carbohydrates in which, for example, some or all of the hydroxyl groups are etherified with, for example, alkyl groups. Suitable derivatized carbohydrates are, for example, ethyl cellulose or carboxymethyl cellulose.

Already low molecular weight hydrocarbons, such as mono- or disaccharides, can be used per se. Examples are glucose or sucrose. The advantageous effects are observed in particular when using oligo- or polysaccharides. An oligo- or polysaccharide is therefore particularly preferably used as the carbohydrate.

It is preferred here that the oligo- or polysaccharide has a molar mass in the range from 1,000 to 100,000 g / mol, preferably 2,000 and 30,000 g / mol. In particular, when the carbohydrate has a molar mass in the range from 5,000 to 20,000 g / mol, a significant increase in the strength of the casting mold is observed, so that the casting mold is easy to manufacture can be removed from the mold and transported. Even after prolonged storage, the casting mold shows very good strength, so that storage of the casting molds, which is necessary for series production of castings, is also possible without any problems, even for several days with ingress of atmospheric moisture. The resistance to the action of water, as is unavoidable when applying a size to the casting mold, for example, is very good.

The polysaccharide is preferably built up from glucose units, these being particularly preferably linked α- or β-glycosidically 1,4. However, it is also possible to use carbohydrate compounds which contain other monosaccharides in addition to glucose, such as galactose or fructose, as an organic additive. Examples of suitable carbohydrates are lactose (α- or β-1,4-linked disaccharide from galactose and glucose) and sucrose (disaccharide from α-glucose and β-fructose).

The carbohydrate is particularly preferably selected from the group of cellulose, starch and dextrins and derivatives of these carbohydrates. Suitable derivatives are, for example, derivatives completely or partially etherified with alkyl groups. However, other derivatizations can also be carried out, for example esterifications with inorganic or organic acids.

A further optimization of the stability of the casting mold and the surface of the casting can be achieved if special carbohydrates and especially starches, dextrins (hydrolyzate product of starches) and their derivatives are used as additives for the molding material mixture. Naturally occurring starches such as potato, corn, rice, pea, banana, horse chestnut or wheat starch can be used as starches. But it is also possible to use modified starches, such as swelling starch, thin-boiling starch, oxidized starch, citrate starch, acetate starch, starch ethers, starch esters or starch phosphates. There is no restriction in the choice of strength per se. The starch can, for example, be of low viscosity, medium viscosity or high viscosity, cationic or anionic, soluble in cold water or soluble in hot water. The dextrin is particularly preferably selected from the group of potato dextrin, corn dextrin, yellow dextrin, white dextrin, borax dextrin, cyclodextrin and maltodextrin.

In particular when producing casting molds with very thin-walled sections, the molding material mixture preferably additionally comprises a phosphorus-containing compound. Both organic and inorganic phosphorus compounds can be used per se. In order not to trigger any undesired side reactions during metal casting, it is also preferred that the phosphorus in the phosphorus-containing compounds is preferably in the V oxidation state. The stability of the casting mold can be further increased by adding phosphorus-containing compounds. This is particularly important when, during metal casting, the liquid metal hits an inclined surface and there, due to the high metallostatic pressure, exerts a high erosion effect or can lead to deformations, particularly in thin-walled sections of the casting mold.

The phosphorus-containing compound is preferably in the form of a phosphate or phosphorus oxide. The phosphate can be in the form of an alkali metal phosphate or an alkaline earth metal phosphate, the sodium salts being particularly preferred. Ammonium phosphates or phosphates of other metal ions can also be used per se. The alkali metal or alkaline earth metal phosphates mentioned as preferred are, however, easily accessible and inexpensively available in any amount.

If the phosphorus-containing compound is added to the molding material mixture in the form of a phosphorus oxide, the phosphorus oxide is preferably in the form of phosphorus pentoxide. However, phosphorus trioxide and phosphorus tetroxide can also be used.

According to a further embodiment, the phosphorus-containing compound can be added to the molding material mixture in the form of the salts of the fluorophosphoric acids. The salts of monofluorophosphoric acid are particularly preferred here. The sodium salt is particularly preferred.

According to a preferred embodiment, organic phosphates are added to the molding material mixture as the phosphorus-containing compound. Alkyl or aryl phosphates are preferred here. The alkyl groups here preferably comprise 1 to 10 carbon atoms and can be straight-chain or branched. The aryl groups preferably contain 6 to 18 carbon atoms, it also being possible for the aryl groups to be substituted by alkyl groups. Phosphate compounds derived from monomeric or polymeric carbohydrates such as glucose, cellulose or starch are particularly preferred. The use of a phosphorus-containing organic component as an additive is advantageous in two respects. On the one hand, the necessary thermal stability of the casting mold can be achieved through the phosphorus content and, on the other hand, the surface quality of the corresponding casting is positively influenced by the organic content.

Both orthophosphates and polyphosphates, pyrophosphates or metaphosphates can be used as phosphates. The phosphates can be prepared, for example, by neutralizing the corresponding acids with a corresponding base, for example an alkali metal or an alkaline earth metal base such as NaOH, it not necessarily being necessary for all negative charges of the phosphate ion to be satisfied by metal ions. Both the metal phosphates and the metal hydrogen phosphates can be used and the metal dihydrogen phosphates can be used, such as Na ₃ PO ₄ , Na ₂ HPO ₄ and NaH ₂ PO ₄ . The anhydrous phosphates as well as hydrates of the phosphates can also be used. The phosphates can be introduced into the molding material mixture either in crystalline or in amorphous form.

Polyphosphates are understood to mean, in particular, linear phosphates which comprise more than one phosphorus atom, the phosphorus atoms each being connected via oxygen bridges. Polyphosphates are obtained by condensation of orthophosphate ions with elimination of water, so that a linear chain of PO ₄ tetrahedra is obtained, each of which is connected by corners. Polyphosphates have the general formula (O (PO ₃ ) _n ) ^{(n + 2) -} , where n corresponds to the chain length. A polyphosphate can comprise up to several hundred PO ₄ tetrahedra. However, preference is given to using polyphosphates with shorter chain lengths. N preferably has values from 2 to 100, particularly preferably 5 to 50. It is also possible to use more highly condensed polyphosphates, ie polyphosphates in which the PO ₄ tetrahedra are connected to one another via more than two corners and therefore show polymerization in two or three dimensions.

Metaphosphates are understood to mean cyclic structures that _{are built up from PO 4} tetrahedra, each of which is connected via corners. Metaphosphates have the general formula ((PO ₃ ) _n ) ^n- , where n is at least 3. N preferably has values from 3 to 10.

Both individual phosphates and mixtures of different phosphates and / or phosphorus oxides can be used.

The preferred proportion of the phosphorus-containing compound, based on the foundry sand, is between 0.05 and 1.0% by weight. If the proportion is less than 0.05% by weight, none is clearer Determine the influence on the dimensional stability of the casting mold. If the proportion of phosphate exceeds 1.0% by weight, the hot strength of the casting mold is greatly reduced. The proportion of the phosphorus-containing compound is preferably chosen between 0.10 and 0.5% by weight. The phosphorus-containing compound preferably contains between 0.5 and 90% by weight of phosphorus, calculated as P ₂ O ₅ . If inorganic phosphorus compounds are used, they preferably contain 40 to 90% by weight, particularly preferably 50 to 80% by weight, of phosphorus, calculated as P ₂ O ₅ . If organic phosphorus compounds are used, they preferably contain 0.5 to 30% by weight, particularly preferably 1 to 20% by weight, of phosphorus, calculated as P ₂ O ₅ .

The phosphorus-containing compound can itself be added to the molding material mixture in solid or dissolved form. The phosphorus-containing compound is preferably added to the molding material mixture as a solid. If the phosphorus-containing compound is added in dissolved form, water is preferred as the solvent.

The molding material mixture is an intensive mixture of water glass, foundry sand and possibly the above-mentioned constituents. The particles of the foundry sand are preferably coated with a layer of the binder. By evaporating the water present in the binding agent (approx. 40-70% by weight, based on the weight of the binding agent), firm cohesion between the particles of the foundry sand can then be achieved.

The binder, ie the water glass as well as the particulate metal oxide, in particular synthetic amorphous silicon dioxide, and / or the organic additive is preferably present in the molding material mixture in a proportion of less than 20% by weight, particularly preferably in a range from 1 to 15% by weight. -% contain. The proportion of the binder relates to the solid content of the binder. If pure foundry sand is used, such as quartz sand, for example, the binder is preferably used in a proportion of less than 10% by weight contain less than 8% by weight, particularly preferably less than 5% by weight. If the foundry sand also contains other refractory mold base materials which have a low density, such as hollow microspheres, the percentage of the binder increases accordingly.

The particulate metal oxide, in particular the synthetic amorphous silicon dioxide, is contained, based on the total weight of the binder, preferably in a proportion of 2 to 80% by weight, preferably between 3 and 60% by weight, particularly preferably between 4 and 50% by weight .-%.

The ratio of water glass to particulate metal oxide, in particular synthetic amorphous silicon dioxide, can be varied within wide limits. This offers the advantage of improving the initial strength of the casting mold, i.e. the strength immediately after removal from the hot tool, and the moisture resistance, without significantly increasing the final strength, i.e. the strength after the casting mold has cooled down, compared to a water glass binder without synthetically produced amorphous silicon dioxide influence. This is of particular interest in light metal casting. On the one hand, high initial strengths are desired so that after the casting mold has been produced, it can be easily transported or assembled with other casting molds. On the other hand, the final strength after hardening should not be too high in order to avoid difficulties in the case of binder disintegration after casting, i.e. the foundry sand should be able to be easily removed from cavities in the casting mold after casting.

In one embodiment of the invention, the foundry sand contained in the molding material mixture can contain at least a portion of hollow microspheres. The diameter of the hollow microspheres is usually in the range of 5 to 500 µm, preferably in the range of 10 to 350 µm, and the thickness of the shell is usually in the range of 5 to 15% of the diameter of the microspheres. These microspheres have a very low specific weight, so that the casting molds produced using hollow microspheres have a low weight. The insulating effect of the hollow microspheres is particularly advantageous. The hollow microspheres are therefore used in particular for the production of casting molds when these are to have an increased insulating effect. Such casting molds are, for example, the feeders already described in the introduction, which act as equalizing reservoirs and contain liquid metal, the metal being kept in a liquid state until the metal filled into the hollow mold has solidified. Another field of application of casting molds which contain hollow microspheres is, for example, sections of a casting mold which correspond to particularly thin-walled sections of the finished casting mold. The insulating effect of the hollow microspheres ensures that the metal in the thin-walled sections does not solidify prematurely and thus block the paths within the casting mold.

If hollow microspheres are used, the binder is used, due to the low density of these hollow microspheres, preferably in a proportion in the range of preferably less than 20% by weight, particularly preferably in the range of 10 to 18% by weight. The values relate to the solids content of the binder.

The hollow microspheres preferably consist of an aluminum silicate. These hollow aluminum silicate microspheres preferably have an aluminum oxide content of more than 20% by weight, but can also have a content of more than 40% by weight. Such hollow microspheres are available, for example, from Omega Minerals Germany GmbH, Norderstedt, under the names Omega-Spheres® SG with an aluminum oxide content of approx. 28-33%, Omega-Spheres® WSG with an aluminum oxide content of approx. 35 - 39% and E-Spheres® with an aluminum oxide content of approx. 43% are brought onto the market. Corresponding products are available from PQ Corporation (USA) under the name "Extendospheres®".

According to a further embodiment, hollow microspheres made of glass are used as the refractory basic molding material.

According to a particularly preferred embodiment, the hollow microspheres consist of a borosilicate glass. The borosilicate glass has a boron content, calculated as B ₂ O ₃ , of more than 3% by weight. The proportion of hollow microspheres is preferably selected to be less than 20% by weight, based on the molding material mixture. If borosilicate glass hollow microspheres are used, a low proportion is preferably selected. This is preferably less than 5% by weight, preferably less than 3% by weight, and is particularly preferably in the range from 0.01 to 2% by weight.

As already explained, in one embodiment the molding material mixture contains at least a proportion of glass granulate and / or glass beads as the refractory molding base material.

It is also possible to design the molding material mixture as an exothermic molding material mixture which is suitable, for example, for the production of exothermic feeders. For this purpose, the molding material mixture contains an oxidizable metal and a suitable oxidizing agent. Based on the total mass of the molding material mixture, the oxidizable metals preferably make up a proportion of 15 to 35% by weight. The oxidizing agent is preferably added in a proportion of 20 to 30% by weight, based on the molding material mixture. Suitable oxidizable metals are, for example, aluminum or magnesium. Suitable oxidizing agents are, for example, iron oxide or potassium nitrate. Contains the used Foundry sand residues of exothermic feeders are preferably removed before the thermal treatment. If the exothermic risers are not completely burned down, there is otherwise the risk of ignition during the thermal treatment.

In comparison to binders based on organic solvents, binders which contain water have poorer flowability. This means that molds with narrow passages and multiple deflections are more difficult to fill. As a result, the casting molds have sections with insufficient compression, which in turn can lead to casting defects during casting. According to an advantageous embodiment, the molding material mixture contains a proportion of lubricants, preferably lamellar lubricants, in particular graphite, MoS ₂ , talc and / or pyrophillite. In addition to the flake-form lubricants, however, liquid lubricants such as mineral oils or silicone oils can also be used. It has been shown that when such lubricants, in particular graphite, are added, even complex shapes with thin-walled sections can be produced, the casting molds consistently having a uniformly high density and strength, so that essentially no casting defects are observed during casting. The amount of flake-form lubricant added, in particular graphite, is preferably 0.05% by weight to 1% by weight, based on the foundry sand.

In addition to the constituents mentioned, the molding material mixture can also comprise further additives. For example, internal release agents can be added, which facilitate the detachment of the casting molds from the mold. Suitable internal release agents are, for example, calcium stearate, fatty acid esters, waxes, natural resins or special alkyd resins. Furthermore, silanes can also be added to the molding material mixture according to the invention.

According to a further preferred embodiment, the molding material mixture contains a proportion of at least one silane. Suitable silanes are, for example, aminosilanes, epoxysilanes, mercaptosilanes, hydroxysilanes, methacrylsilanes, ureidosilanes and polysiloxanes. Examples of suitable silanes are γ-aminopropyltrimethoxysilane, γ-hydroxypropyltrimethoxysilane, 3-ureidopropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, β- (3,4-epoxycyclohexyl) -β- (3,4-epoxycyclohexyl) -β- 3-oxytrimethoxysilane, β- (3,4-epoxycyclohexyl) -β- 3-propyltrimethoxysilane, β- (3,4-epoxycyclohexyl) -β- 3-oxytrimethoxysilane, β- (3,4-epoxycyclohexyl) -β- (3,4-epoxycyclohexyl) -β- 3-oxytrimethoxysilane. aminopropyltrimethoxysilane.

Based on the particulate metal oxide, about 5-50% silane is typically used, preferably about 7-45%, particularly preferably about 10-40%.

The additives described above can be added to the molding material mixture in any form. They can be added individually or as a mixture. They can be added in the form of a solid, but also in the form of solutions, pastes or dispersions. If it is added as a solution, paste or dispersion, water is the preferred solvent. It is also possible to use the waterglass used as a binder as a solvent or dispersing medium for the additives.

According to a preferred embodiment, the binder is provided as a two-component system, a first liquid component containing the water glass and a second solid component containing the particulate metal oxide. The solid component can further include, for example, the phosphate and, if appropriate, a lubricant such as a platelet-shaped lubricant. If the carbohydrate is added to the molding material mixture in solid form, it can also be added to the solid component.

Water-soluble organic additives can be used in the form of an aqueous solution. If the organic additives are soluble in the binder and are stable in storage for several months without being decomposed, they can also be dissolved in the binder and thus added to the foundry sand together with it. Water-insoluble additives can be used in the form of a dispersion or a paste. The dispersions or pastes preferably contain water as a dispersing medium. As such, solutions or pastes of the organic additives can also be produced in organic solvents. However, if a solvent is used to add the organic additives, then preferably water is used.

The organic additives are preferably added as a powder or as short fibers, the mean particle size or the fiber length preferably being selected so that it does not exceed the size of the foundry sand particles. The organic additives can particularly preferably be sieved through a sieve with a mesh size of approx. 0.3 mm. In order to reduce the number of components added to the foundry sand, the particulate metal oxide and the organic additive (s) are preferably not added separately to the molding sand, but rather mixed beforehand.

If the molding material mixture contains silanes or siloxanes, they are usually added in such a way that they are incorporated into the binder beforehand. The silanes or siloxanes can also be added to the foundry sand as a separate component. However, it is particularly advantageous to silanize the particulate metal oxide, ie to mix the metal oxide with the silane or siloxane so that its surface is provided with a thin layer of silane or siloxane. If the particulate metal oxide pretreated in this way is used, the strengths are increased compared with the untreated metal oxide as well as improved resistance to high humidity. If, as described, an organic additive is added to the molding material mixture or the particulate metal oxide, it is expedient to do this before the silanization.

The reclaimed foundry sand obtained with the method according to the invention approximately achieves the properties of new sand and can be used for the production of moldings which have a density and strength comparable to moldings made from new sand. The invention therefore also relates to reconditioned foundry sand, as is obtained with the method described above. This sand consists of a grain of sand that is surrounded by a thin layer of glass. The layer thickness is preferably between 0.1 and 2 μm.

The invention is explained in more detail below on the basis of examples.

Measurement methods used:

AFS number: The AFS number was determined in accordance with VDG leaflet P 27 (Association of German Foundry Experts, Düsseldorf, October 1999).

Average grain size: The average grain size was determined in accordance with VDG leaflet P 27 (Association of German Foundry Experts, Düsseldorf, October 1999).

Acid consumption: The acid consumption was determined analogously to the regulation from the VDG leaflet P 28 (Association of German Foundry Experts, Düsseldorf, May 1979).

Reagents and equipment:

Hydrochloric acid 0.1 n
Sodium hydroxide solution 0.1 n
Methyl orange 0.1%
250 ml plastic bottles (polyethylene)
calibrated volumetric pipettes

Execution of the determination:

If the foundry sand contains even larger aggregates of bound foundry sand, these aggregates are crushed, for example with the aid of a hammer, and the foundry sand is sieved through a sieve with a mesh size of 1 mm.

50 ml of distilled water and 50 ml of 0.1N hydrochloric acid are pipetted into the plastic bottle. Then, using a funnel, 50.0 g of the foundry sand to be examined are added to the bottle and the bottle is closed. In the first 5 minutes, shake vigorously for 5 seconds every minute, then every 30 minutes again for 5 seconds each time. After each shaking, the sand is allowed to settle for a few seconds and the sand adhering to the bottle wall is rinsed off by swirling it briefly. The bottle remains at room temperature during the rest periods. After 3 hours, it is filtered through a medium filter (white tape, diameter 12.5 cm). The funnel and the beaker used to collect it must be dry. The first ml of the filtrate are discarded. 50 ml of the filtrate are pipetted into a 300 ml titration flask and 3 drops of methyl orange are added as an indicator. Then it is titrated from red to yellow with a 0.1 N sodium hydroxide solution.

Calculation:

(25.0 ml hydrochloric acid 0.1 n - ml sodium hydroxide solution 0.1 n used) x 2 = ml acid consumption / 50 g foundry sand

Examples 1. Production and curing of molding material mixtures bound with water glass 1.1 Molding material mixture 1

100 parts by weight (GT) of quartz sand H 32 (Quarzwerke GmbH, Frechen) were intensively mixed with 2.0 parts by weight of the commercially available alkali water glass binder INOTEC® EP 3973 (Ashland-Südchemie-Kernfest GmbH) and the molding material mixture was cured at a temperature of 200 ° C .

1.2 Molding material mixture 2

100 parts by weight of quartz sand H 32 were first intensively mixed with 0.5 part by weight of amorphous silicon dioxide (Elkem Microsilica 971) and then with 2.0 parts by weight of the commercially available alkali water glass binder INOTEC® EP 3973 and the molding material mixture was cured at a temperature of 200 ° C.

2. Regeneration of the hardened molding material mixtures bound with water glass. 2.1. Mechanical regeneration (comparison, not according to the invention)

The cured molding material mixtures produced according to 1.1 and 1.2 were first coarsely comminuted and then in a working according to the impact principle, with a dedusting regeneration system provided by Neuhof Gießerei- und Fördertechnik GmbH, Freudenberg, is mechanically regenerated and the dust that is created in the process is removed.

The analytical data, AFS number, mean grain size and acid consumption of the two regenerates are listed in Table 1. For comparison, the granulometric data of the starting molding material H 32 and the acid consumption of the two cured molding material mixtures before regeneration are given. The acid consumption is a measure of the alkalinity of a foundry sand. <u> Table 1 </u> H 32 Molding material mixture 1 Molding material mixture 2 mechan. Regenerate 1 ^(a) mechan. Reclaim 2 ^(b) AFS number 45 - - 44 45 Mean grain size (mm) 0.32 - - 0.34 0.32 Acid consumption (ml / 50 mg molding material) - 43.7 41.0 38.7 32.9 ^(a) starting from molding material mixture 1
^(b) starting from molding material mixture 2

2.2 Thermal regeneration

Approx. 6 kg each of the mechanical regenerates 1 and 2 were exposed to temperatures of 350 ° C. and 900 ° C. in a muffle furnace from Nabertherm GmbH, Lilienthal.

In the same way, the cured molding material mixtures 1 and 2 were thermally treated at 900 ° C. after coarse comminution without prior mechanical regeneration.

After cooling, the sands were used for further tests without sieving. For this reason, the AFS number and the mean grain size were not determined.

The acid consumption of the thermal regenerates was determined analytically (see Tab. 2) <u> Table 2 </u> thermal regenerate Source material Treatment time (h) Treatment temperature (° C) Acid consumption (ml / 50 g) 1 mechanical. Regenerate 1 3rd 900 2.8 2 mechanical. Regenerate 1 3rd 350 18.2 3rd mechanical. Regenerate 1 6th 350 9.9 4th hardened molding material mixture 1 * 3rd 900 4.3 5 mechanical. Regenerate 2 3rd 900 2.0 6th mechanical. Regenerate 2 3rd 350 14.4 7th mechanical. Regenerate 2 6th 350 7.8 8th hardened molding material mixture 2 * 3rd 900 3.7 * The sample was crushed, but not mechanically regenerated

3. Core production with the regenerated foundry sands 3.1 Mechanically regenerated foundry sands (comparison)

So-called Georg Fischer test bars were produced to test the mechanically regenerated foundry sands. Georg Fischer test bars are to be understood as cuboid test bars with dimensions of 150 mm x 22.36 mm x 22.36 mm.

The composition of the molding material mixtures is given in Tab. 3.

The following procedure was used to manufacture the Georg Fischer test bars:
The components listed in Table 3 were mixed in a laboratory paddle mixer (Vogel & Schemmann AG, Hagen). For this purpose, the regenerate was initially presented. Then, if indicated, the amorphous silicon dioxide (Elkem Mikrosilica 971) was added while stirring and, after a mixing time of about one minute, the commercially available water glass binder INOTEC® EP 3973 (Ashland-Südchemie-Kernfest GmbH) was added last. The mixture was then stirred for a further minute.

The freshly produced molding material mixtures were transferred to the storage bunker of an H 2.5 hot box core shooting machine from Röperwerk - Gießereimaschinen GmbH, Viersen, the mold of which was heated to 200.degree.

The molding material mixtures were introduced into the molding tool by means of compressed air (5 bar) and remained in the molding tool for a further 35 seconds.

To accelerate the hardening of the mixtures, hot air (2 bar, 120 ° C. when entering the tool) was passed through the tool for the last 20 seconds;
The mold was opened and the test bar removed.

To test the processing time of the molding material mixtures, the process was repeated three hours after the mixture was produced, the molding material mixture being kept in a closed vessel during the waiting time in order to prevent _{the mixture from drying out and the ingress of CO 2.}

To determine the flexural strengths, the test bars were placed in a Georg Fischer strength tester equipped with a 3-point bending device (DISA Industrie AG, Schaffhausen, CH) and the force that led to the breakage of the test bars was measured.

• 10 Sekunden nach der Entnahme (Heißfestigkeiten)
• ca. 1 Std. nach der Entnahme (Kaltfestigkeiten)

The flexural strengths were measured according to the following scheme:

• 10 seconds after removal (heat strength)
• approx. 1 hour after removal (cold strengths)

The measured strengths are summarized in Tab. 4.

The weight of the test bars was determined as a further test criterion. This is also listed in Tab. 4. Table 3: Composition of the molding material mixtures (comparative examples) sand amorphous silica ^(a) Binder ^(b) Ex. 1 100 GT H 32 ^(c) - 2.0 GT Ex. 2 100 GT H 32 ^(c) 0.5 GT 2.0 Gt Ex. 3 100 GT mechan. Regenerate 1 0.5 GT 2.0 GT Ex. 4 100 GT mechan. Regenerate 2 0.5 GT 2.0 GT ^(a) Elkem Microsilica 971
^(b) INOTEC® EP 3973 (Ashland-Südchemie-Kernfest GmbH)
^(c) Quarzwerke GmbH, Frechen Hot strength (fresh mix) Cold strength (fresh mix) Core weight (fresh mix) Hot strength (3 h old mixture) Cold strength (3 h old mixture) Core weight (3 h old mixture) Ex. 1 60 350 127.0 50 300 126.2 Ex. 2 155 440 127.6 140 420 126.9 Ex. 3 125 420 120.3 40 200 117.2 Ex. 4 120 410 117.9 (n) (n) (n) (n): no longer lockable

In the mechanically regenerated foundry sand used in Example 3, which was produced from a foundry sand that had been solidified with a water glass which did not contain any particulate amorphous silicon dioxide (mechanical regenerated material 1), a 3-hour-old mixture can still be used. However, test bars are obtained which, compared to Examples 1 and 2, have a poorer strength.

If the mechanically regenerated foundry sand contains a binding agent which contains amorphous silicon dioxide (example 4), the 3-hour-old mixture has hardened and can no longer be sealed. This shows that used foundry sands which contain a water glass as a binding agent to which a particulate metal oxide has been added are not suitable for mechanical regeneration.

3.2. Thermally regenerated foundry sand

When testing the thermally regenerated foundry sands, the procedure was analogous to that used for the mechanically regenerated foundry sands.

The composition of the molding mixtures is shown in Table 5, the strengths and core weights are summarized in Table 6. Tab. 5 Composition of the molding material mixtures (according to the invention) sand amorphous silica ^(a) Binder ^(b) Ex. 5 100 GT thermal regenerate 1 0.5 GT 2.0 GT Ex. 6 100 GT thermal regenerate 2 0.5 GT 2.0 GT Ex. 7 100 GT thermal regenerate 3 0.5 GT 2.0 GT Ex. 8 100 GT thermal regenerate 4 0.5 GT 2.0 GT Ex. 9 100 GT thermal regenerate 5 0.5 GT 2.0 GT Ex. 10 100 GT thermal regenerate 6 0.5 GT 2.0 GT Ex. 11 100 GT thermal regenerate 7 0.5 GT 2.0 GT Ex. 12 100 GT thermal regenerate 8 0.5 GT 2.0 GT ^(a) Elkem Microsilica 971
^(b) INOTEC® EP 3973 (Ashland-Südchemie-Kernfest GMBH) Hot strength (fresh mix) Cold strength (fresh mix) Core weight (fresh mix) Hot strength (3 h old mixture) Cold strength (3 h old mixture) Core weight (3 h old mixture) Ex. 5 145 450 124.4 135 410 123.6 Ex. 6 135 425 123.3 125 385 121.9 Ex. 7 140 435 123.4 125 390 122.2 Ex. 8 130 415 123.1 130 400 122.4 Ex. 9 150 445 123.1 135 405 122.7 Ex. 10 140 420 122.9 130 395 122.3 Ex. 11 140 430 123.1 125 405 122.6 Ex. 12 135 425 123.2 130 390 122.5

In Examples 5 to 8, thermal regenerates originating from molding material mixture 1 were used. A water glass that does not contain amorphous silicon dioxide was used as the binder for this molding material mixture. The molding material mixtures produced from the regrind can still be sealed very easily even after 3 hours. The test bars show very good strength.

The same result is achieved with thermal regenerates 5 to 8, as Examples 9 to 12 show. The regenerates used in these examples are based on the molding material mixture 2, which contains water glass as a binder to which amorphous silicon dioxide is added. Even after a standing time of 3 hours, the molding material mixture can be sealed very well. The test bars obtained show very good strength.

Result: Comparison of Tables 1 and 2:

It can be seen that the acid consumption of the molding materials is reduced considerably more by the supply of heat than with mechanical regeneration. The determination of the acid consumption is also a simple method to follow the progress of the thermal regeneration.

Comparison of Tables 4 and 6:

It can be seen that the processability of the molding material mixtures when using thermally regenerated foundry sands is significantly longer than when using mechanically regenerated foundry sands, regardless of whether or not mechanical regeneration was carried out before the thermal treatment.

It can also be seen that the weight of the test bars produced with the thermally regenerated foundry sands is higher than that of test bars produced with mechanically regenerated foundry sands, i.e. the flowability of the molding material mixtures has increased due to the thermal regeneration.

Claims (12)

1. A method for producing of casting moulds by application of used foundry sand comprising at least the following steps:

   (a) providing a first mould material mixture comprising at least a first foundry sand and at least a binder based on water glass and a particulate metal oxide, wherein the particulate metal oxide is synthetically produced amorphous silicon dioxide, processing the mould material mixture and curing to a first casting mould, conducting a metal casting within the first casting mould so that a used casting mould and a casting is obtained and the used foundry sand is present in the form of a casting mould,

   (b) subjecting the used foundry sand, which is adhered with a binder based on water glass and with the particulate metal oxide added, to a thermal treatment, in which the used foundry sand is heated to a temperature of at least 200°C, wherein the thermal treatment is carried out until the acid consumption of the foundry sand, measured by the consumption of 0.1 n HCl in a 50 g quantity of foundry sand, has decreased by at least 10%, whereby regenerated foundry sand is obtained,
   and

   (c) producing a second mould material mixture by using a second foundry sand, wherein at least a part of the second foundry sand is from the regenerated foundry sand and wherein at least a binder based on water glass and a particulate metal oxide is added to the second foundry sand and wherein the particulate metal oxide is synthetically produced amorphous silicon dioxide, and
   curing of the second mould material mixture to a second casting mould in a tool, wherein the curing is carried by heating the tool containing the second mould material mixture to a temperature of 100 to 300°C or by blowing heated air having a temperature of 100 to 180°C into the tool.
2. The method according to claim 1, wherein the thermal treatment consists inheating the used foundry sand to a temperature of above 300°C.
3. The method according to claim 1, wherein the first casting mould is fractured into coarse fragments before the thermal treatment.
4. The method according to claim 3, wherein the first casting mould is separated from the casting before the thermal treatment.
5. The method according to any one of claims 3 or 4, wherein the first casting mould is transferred to a furnace for the thermal treatment.
6. The method according to any one of the preceding claims, characterised by one or more of the following features:

   a) wherein the used foundry sand is agitated during the thermal treatment,

   b) wherein the thermal treatment is carried out under admission of air,

   c) wherein the regeneration is carried out dry or

   d) wherein before or after the thermal treatment a mechanical treatment is conducted to the moulding sand for grain singulation.
7. The method according to claim 1, wherein the curing of the first and the second mould is a thermal curing, wherein water is extracted from the binder based on water glass.
8. The method according to claim 1, wherein the water glass has an SiO₂/M₂O modulus in the range of 1.6 to 4.0, in particular 2.0 to 3.5, wherein M denotes sodium ions and potassium ions.
9. The method according to claim 8, wherein the water glass has a solid content of SiO₂ and M₂O in the range of 30 to 60 wt.%.
10. The method according to any one of claims 1, 8 or 9, wherein the particulate metal oxide is selected from the group of precipitated silicic acid and pyrogenic silicic acid.
11. The method according to any one of claims 1 or 8 to 10, wherein an organic additive is added to the second mould material mixture, and wherein the organic additive is particularly a carbohydrate.
12. The method according to any one of claims 1 or 8 to 11, wherein a phosphorus-containing additive is added to the second mould material mixture.