EP2477766B1 - Foundry additive based on graphite - Google Patents

Description

The invention relates to a foundry additive, a molding material mixture containing the foundry additive, a foundry mold of a granular refractory molding material containing the foundry additive, and the use of the foundry mold for metal casting.

Most products of the iron and steel industry and the non-ferrous metal industry undergo casting processes for the first shaping. The molten materials, ferrous metals or non-ferrous metals, are converted into geometrically determined objects with specific workpiece properties. To shape the castings, very complicated casting molds for receiving the melt must first be produced. The casting mold essentially represents a negative mold of the casting to be produced. Cavities inside the casting are represented by cores, while the outer boundary of the casting is represented by molding. Molds are made of a refractory molding material, such as quartz sand, whose grains are connected after molding by a suitable binder to ensure sufficient mechanical strength of the mold. For the production of molds so you use a refractory molding material, which is first mixed with a suitable binder. The molding material mixture obtained from molding material and binder is preferably present in a free-flowing form, so that it can be filled into a suitable mold and compacted there. By the binder, a firm cohesion between the grains of the molding material is produced, so that the casting mold the required obtains mechanical stability. For cores, organic binders are often used, which are cured after shaping by a chemical reaction initiated, for example, by fumigation with amines. More recently, water glass based inorganic binders have been increasingly used for the production of cores. Here, however, care must be taken that the cores disintegrate again after the casting into a fine sand. For molds clay is mostly used as a binder. In particular, sodium bentonites are important because of their good water absorption capacity and swelling capacity. In order to achieve a sufficient binding effect, the clay must have a certain moisture content. The binding effect of the clay unfolds by compacting the molding material mixture.

During casting liquid metal is poured into the cavity of the mold. Upon contact with the mold, the liquid metal is quenched and forms a relatively stable peripheral shell, in which then the liquid metal is absorbed.

In order to achieve the most accurate possible representation of the shape of the casting predetermined by the cavity of the casting mold and to avoid post-processing of the casting, it must be ensured that the liquid metal does not penetrate into the porous material of the casting mold. Otherwise you get a rough surface, which must be reworked, for example, to remove sintered sand. If the metal penetrates deeper into the casting mold so that sand is trapped in the casting, we speak of mineralization that can render the casting unusable.

Thus, to obtain a satisfactory surface of the casting, the surface of the casting mold must be sealed against the metal. At the same time, however, the surface of the casting cavity must still be so permeable that gaseous products, which in the contact of metal and casting mold through the hot metal are released, such as water vapor, can escape through the mold to the outside and do not pass into the liquid metal.

Therefore, it often provides the cavity of the mold with a sizing. This is a slurry of finely ground inorganic ingredients in a suitable solvent, usually water or alcohol, which is applied to the wall of the cavity of the mold. However, the application of the size requires a separate operation. Another way to seal the pores of the mold from the casting cavity is to use lustrous carbon formers. For example, ground coal or synthetic and natural resins are used as the lustrous carbon formers. Upon contact with the liquid metal, these materials decompose in the then strongly reducing atmosphere, releasing low molecular weight fragments of organic compounds. These gaseous organic compounds condense on the surface of the grains of the refractory molding material and form a thin carbon layer. The carbon layer changes the wetting behavior of molten metals to quartz sand, thereby preventing the liquid metal between the grains of the refractory molding material from penetrating into the wall of the casting mold.

However, some of the organic volatiles formed on casting from the lustrous carbon formers have high toxicity. This is especially true for volatile aromatics, such as benzene and higher-condensed polycyclic hydrocarbons, which remain in the molding material. These toxic substances constitute a hazard at the workplace and must therefore be absorbed. Since they can not easily be released into the environment, the exhaust air must be post-treated and burned, for example. It is therefore an effort To provide molding material mixtures in which the emission of toxic substances is minimized.

In the WO 98/50181 are described additives for molding sands containing activated carbon. The activated carbon can also be formed in situ during the casting process. Molded sands, mixed with such additives, emit smaller amounts of volatile organic compounds during casting. In order to generate the activated carbon in situ , the molding sand is mixed with a humic acid-containing mineral, wherein the humic acid-containing mineral and activated carbon or graphite may be added. Leonardite is preferably used as humic acid-containing mineral. The humic acid-containing mineral and the carbon or graphite-containing additives preferably have a particle size of less than 1000 .mu.m, preferably less than 105 .mu.m and particularly preferably less than 74 .mu.m, in order to avoid surface defects in the castings. The proportion of the humic acid-containing mineral which is added to the foundry sand is preferably in the range from 0.1 to 10% by weight, preferably 0.1 to 2% by weight, particularly preferably 0.25 to 0.5% by weight. % selected, based on the total weight of foundry sand and additives. The examples describe a mixture containing graphite and leonardite. The graphite does not emit any organic compounds. The graphite contained in the mixture is to be activated according to the inventors by the lignite or the Leonardite during the casting, so that the activated graphite absorbs a surprisingly high proportion of the emitted from the oxidized Leonardite benzene.

In the DE 10 2007 027 621 A1 For example, a method for producing a core and / or foundry sand for foundry purposes is described. In this case, a mineral refractory molding material with an inorganic binder, for example Bentonite, as well as an inorganic Blähadditiv mixed. As an organic blowing additive, for example, expandable graphite can be used. As a further inorganic additive, the core or molding sand can also contain macrocrystalline graphite. The molding sand are added no organic additives. According to the inventors, the expanding graphite closes pores remaining in the casting mold during casting, whereby the roughness of the casting surface can be reduced. Furthermore, the blowing additive acts as an adsorbent, so that separating oils, condensates or benzene, which are released during the casting, are bound. Through the blowing additive binder bridges, which have formed between individual grains of sand, blown up during the casting, so that the casting mold decays back into a fine sand after casting. The grain size of the mixture particles is preferably selected between 5 and 500 μm, particularly preferably between 10 and 200 μm. The mean grain diameter can be, for example, about 65 microns. Such an inorganic masterbatch is obtained by premixing the inorganic binder and the inorganic swelling additive. The mixture can then be added to the mold. The grain size of the bulking additive is preferably selected in the range from 10 nm to 3000 nm. The mean grain diameter should be about 1 micron.

In the WO 03/066253 A1 For example, a method is described for producing a molding sand, in particular recycled, for foundry purposes. In this case, a porous, non-swellable in water material is added to the foundry sand, which has a very high specific surface area. Exemplary materials include framework or tectosilicates, pumice, allophane, imogolite, kieselguhr, palygorskites, sepiolites, diatomaceous earth, and acid and / or heat treated clays. As further additives, the molding sand may additionally be supplemented by carbon products, such as lustrous carbon formers,

Coal dust or graphite may be added. By adding graphite, a better and faster absorption of the water by the non-swellable material or the bentonite is to be achieved. Furthermore, the flowability of the molding sand is improved by the Grafitzugabe.

In the EP 0 279 031 A1 describes a process for accelerating the water adsorption of bentonite, in particular as an additive for molding sands. For this purpose, the bentonite graphite slammed. The graphite used may be natural graphite or an electro / synthetic graphite.

In the DE 32 46 324 A1 foundry molding sands are described, which include as an additive carbonaceous materials, which are characterized by a low proportion of volatile components. The carbonaceous material used is preferably a graphite mineral, in particular a natural graphite liberated from mineral constituents. Alternatively, synthetic graphites can also be used. The particle size is preferably chosen to be less than 1 mm, particularly preferably less than 0.15 mm.

In the EP 0 337 080 A2 describes a method for the production of molds from clay-bonded molding sand. In this case, first of all a mold is produced from a molding sand which is essentially free from lustrous carbon formers and pyrolytically decomposable organic constituents. Onto those surfaces of the mold which come into contact with the cast metal, a sizing is then applied which contains the usual refractory inorganic constituents and is substantially free of pyrolytically decomposable organic constituents. As refractory inorganic constituents, for example, clays, talc, quartz, mica, zirconium silicate, magnesite, aluminum silicate and chamotte can be used. In the broader sense, as Inorganic constituents are also used graphite or coke. The particle size of the refractory inorganic constituents is less than 75 μm and is preferably less than about 60 μm. Down the particle size is no limit. For example, the primary particle size of bentonite and kaolin can reach down to about 0.1 μm, with a maximum of primary particles in the range of about 1 μm, for example, for bentonite.

In the GB 357,126 there is described a carbonaceous powdery release agent which is applied to the surface of the mold cavity during the manufacture of a mold. The carbonaceous material may comprise a core of graphite impregnated with a water-repellent high polymer residue such as that formed in the distillation of resin.

In the US 4,314,744 there is described a waterglass-based binder for the production of cores containing amorphous silica which facilitates disintegration of the cores after casting. In order to facilitate the disintegration of the cores, carbon-containing materials such as resins, pitch or activated carbon may also be added to the molding material mixture. Graphite is mentioned only at the edge to coat the surface of the cores. In this way, the surface of the metals or the storage stability of the cores can be increased. The grain size of graphite is not discussed.

In the WO 99/28064 describes a liquid composition which produces lustrous carbon during metal casting. The composition comprises particles of a lustrous carbon-forming agent in an aqueous carrier not coal. Graphite is described only as a possible additive, for example to improve the flowability of the molding sand. The particle size or the crystallinity of the graphite is not mentioned. Likewise, the expert can not refer to the description of the suitability of finely ground microcrystalline graphite.

In the DE 19 52 357 A1 an additive for molding compositions is described as a substitute for coal dust for the formation of lustrous carbon in molds, wherein the addition of a thermoplastic material in unfoamed form as unsubstituted polymerized carbon, in particular polystyrene, in a particle size of less than 0.3 mm. The use of graphite, especially in finely ground microcrystalline form, is disclosed in US Pat DE 19 52 357 A1 not described. Since graphite itself is not a lustrous carbon generator, would DE 19 52 357 A1 also lead the skilled person away from the use of finely ground graphite as a substitute for lustrous carbon described in the application.

In the DE 30 17 119 A1 describes a process for producing a foundry sand of quartz sand, bentonite and water for iron foundry purposes, wherein the molding sand contains a lustrous carbon formers to avoid the burning of the liquid iron on the surface of the molding sand. The lustrous carbonator is added to the mixture in the form of an agglomerate slurried in a liquid having a particle size of about 0.5 to 5 mm. For example, finely ground coal dust is used as a lustrous carbon former. For agglomeration, it may be mixed with further carbonaceous dust which does not contain a lustrous carbon generator. For example, charcoal dust, coke dust, wood flour, peat flour are suitable for this purpose or anthracite dust. Graphite as a possible component is not mentioned. Again, this is already true DE 19 52 357 A1 Said. The expert can by DE 30 17 119 A1 are not led to the registration subject, since graphite itself is not a lustrous carbon generator and receives only by very fine milling properties corresponding to, so that it can serve as a substitute for a lustrous carbon.

The use of graphite is widely used in the foundry industry. However, especially for cost reasons, relatively coarse-grained graphite powders are used. For example, graphite is used as a refractory material for the production of filters for the filtration of liquid metal, in particular aluminum. Graphite is also used to make refractory shapes.

The emission of toxic substances during casting remains a persistent problem, despite the recent progress made, and the reduction of these emissions remains an objective that needs to be constantly monitored in all aspects of the casting process.

The invention therefore an object of the invention to provide means that allow a further reduction of the emission of toxic substances during the casting process.

This object is achieved with a foundry additive having the features of patent claim 1. Preferred embodiments are subject of the dependent claims.

As part of the development of environmentally friendly alternatives, it has been found that the effect of graphite can be significantly increased if the graphite is provided in as finely ground form as possible. Without wishing to be bound by this theory, the inventors assume that the graphite particles because of their very small size envelop the grain of the refractory molding material, in particular quartz sand, and thus permanently change the surface properties of the refractory molding material, in particular its wettability by liquid metal , Thus, it can be demonstrated by experiments that the extent of sand buildup on the casting after demoulding can be significantly reduced if the sand was coated with a thin layer of graphite according to the invention. The result is the better, the finer the graphite was ground.

The uniformity of the graphite coating of the refractory base molding material is evident, for example, in the determination of the whiteness. Microscopic investigations of the graphite-coated refractory mold raw materials show that the lower the whiteness, the more uniform the graphite coating is. This can be explained by the fact that a large part of the incident in the determination of the degree of whiteness of light radiation is adsorbed by the graphite.

The invention therefore provides a foundry additive which comprises a finely ground, microcrystalline or amorphous graphite having an average particle size D ₅₀ of less than 100 μm and preferably an average crystallite size of less than 90 nm.

According to a further embodiment, the finely ground graphite has an average particle size D ₉₀ of less than 200 μm, according to another embodiment less than 50 μm, according to another embodiment less than 40 μm, according to another embodiment less than 30 μm, according to a further embodiment of less than 20 microns and according to yet another embodiment of less than 10 microns.

According to a further embodiment, the finely ground graphite has an average particle size D ₁₀ of less than 5 .mu.m, according to another embodiment of less than 3 .mu.m, according to another embodiment of less than 2 .mu.m, according to another embodiment of less than 1 .mu.m and according to yet another embodiment of less than 0.8 microns.

As such, the distribution of the graphite particles can be quite wide, since the finely divided portion is sufficient to observe the effect of avoiding mineralization observed according to the invention. According to one embodiment, the ratio D ₉₀ / D ₁₀ less than 20, in another embodiment less than 15, in another embodiment less than 10, in another embodiment less than 8, and in another embodiment less than. 6

By an average particle size D ₅₀ is meant a size at which 50% of the particles are larger and 50% of the particles are smaller than the D ₅₀ value. Accordingly, an average particle size D ₉₀ is understood to be a value at which 90% of the particles are smaller and 10% of the particles are larger. An average particle size D ₁₀ is understood to mean a value at which 10% of the particles are smaller and 90% of the particles are larger. The D ₅₀ value and all other values D _x for the description of the particle size distribution are related to the sample volume.

The size distribution of the particles may be monomodal or may comprise several maxima and be bimodal, for example. It is essential that a sufficient proportion of graphite particles with a diameter of less than 20 microns, according to an embodiment of less than 10 microns in the graphite additive is included in order to obtain the observed effect according to the invention.

The size distribution of the particles according to one embodiment corresponds to a Gaussian distribution. The standard deviation of the D ₅₀ value is less than 15 μm according to one embodiment, less than 10 μm in one embodiment and less than 8 μm according to another embodiment. The particle size is determined as the mean value of the expansion of the particles in all three spatial directions. A suitable method for determining the particle size distribution or the average particle sizes D ₉₀ , D ₅₀ , D ₁₀ is, for example, laser diffractometry.

As already explained, the finer the graphite contained in the foundry additive, the lower is the sand buildup or sanding sintering observed on the casting. According to one embodiment, it is therefore preferred that the mean particle size D _{50 is} less than 50 μm, and according to a further embodiment, less than 20 μm. According to one embodiment, the mean particle size D _{50 is} less than 10 μm, in accordance with a further embodiment the mean particle size D _{50 is} less than 5 μm.

Graphite can be ground by conventional methods up to an average particle size D ₅₀ of about 1 micron. If the average particle size D _{50 is} to be lowered to values of less than 1 μm, this means a high outlay for the required devices and a high expenditure of time until the desired small particle size is reached. According to one embodiment, it is therefore provided that the mean particle size D _{50 is} more than 1 μm. However, it is also possible to use graphite with an average particle size D ₅₀ of less than 1 μm. However, the grinding of the graphite is then difficult and expensive, so speak economic reasons against the use of such a finely ground graphite.

The graphite contained in the foundry additive preferably has a very low average crystallite size. The mean crystallite size can be determined, for example, from the mean half-width of the reflections of the X-ray diffraction diagram. The average crystallite size is preferably less than 90 nm, in one embodiment less than 80 nm, according to another embodiment less than 70 nm and according to yet another embodiment less than 50 nm. According to one embodiment, the average crystallite size of the graphite is in the region of 20 up to 45 nm.

According to a further embodiment, microcrystalline or amorphous graphite is used, that is to say the graphite produces very broad reflections in the X-ray diffraction diagram.

The term "amorphous graphite" here refers to a microcrystalline graphite with an extremely small crystallite size, which thus shows very broad reflections in the X-ray diffraction pattern, so that the determination of the crystallite size from the X-ray diagram can be associated with a relatively large error. The transition between microcrystalline graphite and amorphous graphite is therefore fluid. As an amorphous graphite, for example, natural graphite can be referred to. Natural graphite may contain minor minerals as well as impurities, making it difficult to determine a crystallite size. The terms "amorphous graphite" and "microcrystalline graphite" are used in the following largely synonymous.

In one embodiment, the graphite is provided in the form of an aqueous suspension. It has been shown that sandane sinterings on the casting can be significantly reduced if the graphite is not added in dry form but in the form of an aqueous suspension to the granular refractory molding material, in particular quartz sand. The graphite is preferably present in the suspension in a proportion of between 20 and 50 parts by weight per 100 parts by weight of the suspension. In particular, if very finely divided graphite is used in the foundry additive, for example a graphite having an average particle size D ₅₀ of less than 10 μm, it is preferred that the suspension contains dispersing aids in order to achieve complete wetting of the graphite particles. The use of dispersing aids makes it possible to obtain graphite suspensions with a high proportion of graphite at comparatively low viscosities. It has also been found that the dispersing aid has an influence on the casting result, ie on the amount of sand sintered on the casting. Suitable dispersing aids are, for example, anionic or nonionic surfactants. Preferred anionic surfactants are, for example, alkali metal salts of polycarboxylic acids. Preferred nonionic dispersing aids are, for example, fatty alcohol ethoxylates. A suitable nonionic dispersing aid is, for example Pluronic ^® PE 10400 Fa. BASF SE. Based on the dry graphite content, the dispersing aids are preferably present in a proportion of from 2 to 10% by weight in the suspension.

In addition to dispersants, viscosity-adjusting additives may also be added to the suspension. Suitable viscosity-regulating additives are, for example high molecular weight polyacrylates or natural thickeners, such as xanthan or cellulose ethers. A suitable viscosity-regulating additive is, for example, sodium bentonite. The addition of thickeners effectively prevents sedimentation of the graphite during storage. Based on the dry graphite content, the thickeners are preferably present in a proportion of less than 10% by weight, in one embodiment in a proportion of less than 5% by weight in the suspension. In order to obtain a sufficient effect, the thickener according to one embodiment in a proportion of at least 0.5 wt .-%, according to another embodiment in a proportion of at least 1 wt .-% and according to yet another embodiment in a proportion of more contained as 2 wt .-%, each based on the dry graphite content. When organic, viscosity-regulating additives are used, the further addition of fungicides or biocides makes sense.

For molds, clay-bound sands, in particular quartz sands, are preferably used as the refractory molding material. According to one embodiment, it is provided that a bentonite is added to the foundry additive. The bentonite, especially sodium bentonite or calcium bentonite, may be wholly or partially equivalent to the amount of binder used in a molding material mixture to make clay bonded molds. This embodiment of the foundry additive according to the invention makes it possible to simultaneously introduce the binder bentonite and the finely ground graphite into the refractory molding material, usually quartz sand, in the production of a molding material mixture. According to one embodiment, the foundry additive contains an intimate mixture of bentonite and finely ground, preferably microcrystalline or amorphous, graphite, so that in the production of a molding material mixture the bentonite, which may act as a binder, and the finely ground, preferably microcrystalline or amorphous graphite added simultaneously and evenly distributed in the granular refractory molding material, in particular quartz sand during mixing.

The proportion of the finely ground microcrystalline or amorphous graphite, based on the anhydrous foundry additive, according to one embodiment is greater than 1 wt .-%, according to another embodiment in the range of 2 to 20 wt .-%, and according to yet another embodiment in the range of 5 to 15% by weight. The bentonite is preferably also added in finely ground form. According to one embodiment, the bentonite has a particle size D ₁₀₀ of less than 300 μm, according to one embodiment of less than 200 μm. The average particle size D _{50 of} the bentonite is preferably chosen to be less than 100 μm, and according to a further embodiment, less than 80 μm. According to one embodiment, the average particle size D _{50 of} the bentonite is greater than 10 .mu.m, chosen according to a further embodiment greater than 20 microns. The width of the particle size distribution can be set within wide limits. The standard deviation of the D ₅₀ value is, according to one embodiment, less than 50 μm, in one embodiment less than 30 μm. The proportion of bentonite, based on the anhydrous foundry additive, is preferably less than 99% by weight, in one embodiment in the range of 98 to 80% by weight and in another embodiment in the range of 95 to 85% by weight ,

As the bentonite, an alkali bentonite, particularly preferably a sodium bentonite, is preferably used. An alkali bentonite, in particular sodium bentonite, is understood as meaning a bentonite which contains at least 40%, preferably at least 50%, particularly preferably at least 60% of the cation exchange capacity in the form of exchangeable alkali ions, in particular sodium ions.

In the embodiment just described, the bentonite used to make a molding mixture may be fully contained in the foundry additive. According to a further embodiment, however, it is also possible to proceed in such a way that only a part of the bentonite which acts as a binder in the molding material mixture is contained in the foundry additive and the remaining amount of bentonite is added separately to the granular refractory molding material. In this case, the relative proportion of bentonite compared to the finely ground graphite can be chosen to be lower, so that the proportion of finely ground graphite in the anhydrous binder can assume values between 1 and 99 wt .-%. In particular, when using a graphite suspension, the bentonite, in particular sodium bentonite, also act as a thickener, which prevents settling of the graphite particles during storage. Since sodium bentonite has a strongly thickening effect even in small proportions, the proportion of bentonite, calculated as dry weight, in the suspension is preferably selected in the range from 1 to 10% by weight. In this embodiment, the bentonite is preferably used in a weight ratio of 10: 1 to 1: 10 to graphite.

According to a preferred embodiment, it is provided that the foundry additive is provided in the form of granules. For this purpose, the finely ground microcrystalline or amorphous graphite and optionally other constituents of the foundry additive, for example bentonite, in particular sodium bentonite or calcium bentonite, for example, intimately mixed and then formed into a granule. The finely ground graphite in this embodiment is preferably distributed homogeneously in the volume of the granule.

A granulate can be very easily and accurately dose because of its flowability.

The mean diameter D _{50 of} the granules is selected according to an embodiment between 0.05 and 5 mm, according to another embodiment between 0.1 and 3 mm. The size distribution of the granulate can be determined, for example, by sieve analysis. The information on the size distribution of the granulate relates in each case to the sample volume. The granules may have a narrow size distribution. The standard deviation of the D ₅₀ value, according to one embodiment, is less than 1 mm, in one embodiment less than 0.5 mm.

The granules can be produced by conventional means. A suitable device is for example a pelletizing plate or an intensive mixer.

According to one embodiment, it is provided that the granules are designed very fine-grained and preferably has a mean diameter D ₅₀ of less than 1 mm. Such fine granules can be prepared, for example, by spray drying. For this purpose, for example, a suspension can be prepared, which preferably has a proportion of 20 to 45 wt .-% finely ground graphite. Such a suspension is preferably prepared by means of a high shear agitator. Water is preferably used as the liquid phase of the suspension. The suspension may also be added to further constituents of the foundry auxiliary, for example bentonite, in particular sodium bentonite or calcium bentonite, which at the same time may also act as a binder for the granules.

According to a further embodiment, the suspension may also contain an organic binder, for example cellulose ethers, polyvinyl alcohol, hot or cold water-soluble starches or dextrins. The proportion of the binder, based on the dry graphite, is preferably selected in the range between 1 and 10 wt .-%.

The suspension can then be subsequently dried in a conventional spray dryer under customary conditions.

The foundry additive can be dosed with the usual devices for refractory molding material and mix with this. In the foundry additive according to the invention, it is therefore not necessary to change work processes in the production of the casting mold.

According to a particularly preferred embodiment, the foundry additive comprises a bentonite granules which is coated with the finely ground, preferably microcrystalline or amorphous graphite. In this embodiment, therefore, a bentonite granules is first prepared, including conventional devices can be used. The finely ground, preferably microcrystalline or amorphous graphite is then applied to the granules. This can be done in conventional mixers. The finely ground, preferably microcrystalline or amorphous graphite can be added in dry form to the bentonite granules. However, preference is given to proceeding in such a way that the finely ground microcrystalline or amorphous graphite is added in the form of a suspension to the bentonite granules. The proportion of graphite, based on the dry granules, is preferably selected in the range of less than 20 wt .-%. According to one embodiment, the proportion of finely ground microcrystalline or amorphous graphite in dry granules in a range of 0.1 to 10 wt .-% is selected according to another embodiment in proportion of 1 to 5 wt .-%.

According to a further preferred embodiment, it is provided that the foundry additive, in particular when used in the form of granules, has a moisture in the range from 15 to 35% by weight. In this way, the constituents of the foundry additive, in particular when used as granules, in the production of the Formstoffmischung be dispersed very quickly and efficiently in the refractory molding material. If a moisture content of less than 10 wt .-% is selected, the mixing time increases significantly during the production of the molding material mixture, so that the process loses economic efficiency. If, on the other hand, a foundry additive is used which has a very high moisture content, for example a moisture content of more than 40% by weight, the foundry additive is given a high tackiness so that the formation of lumps in the mixer can occur. A uniform distribution of the graphite in the granular refractory molding material, especially in the simultaneous presence of bentonite, then requires very long mixing times. The moisture content of the foundry additive, in particular if this is provided in the form of a granulate, can be adjusted, for example, by the water content of the graphite suspension, which according to one embodiment is used to produce the granules. The aqueous graphite suspension can be used directly as a granulating agent or the moisture content of the granules is adjusted by coating the previously prepared bentonite granules with the aqueous graphite suspension.

As such, the finely divided graphite can completely replace the lustrous carbon formers used in previous molding mixtures. Thus, according to one embodiment, the foundry additive does not contain a lustrous carbon generator.

However, it is also possible to replace only part of the carbon-containing compounds which are customarily contained in molding material mixtures as lustrous carbon formers by the finely divided graphite. According to one embodiment, it is provided that the foundry additive comprises a carbon support, preferably a finely ground carbon support. The carbon support according to one embodiment has an average particle size D ₅₀ of less than 300 microns, according to one embodiment, an average particle size D ₅₀ of less than 200 microns. According to one embodiment, the carbon support preferably has an average particle size D ₅₀ of at least 20 μm. According to one embodiment, the standard deviation of the D ₅₀ value is less than 100 μm, in one embodiment less than 60 μm and according to another embodiment less than 30 μm. As carbon supports, preference is given to using lustrous carbon formers. In one embodiment, the carbon support is selected from the group of coal, coke and activated carbon. Preferably, the proportion of the carbon support, based on the dry graphite, in the range of 10 to 100 wt .-%, selected according to a further embodiment in the range of 20 to 80 wt .-%.

In one embodiment, the foundry additive consists essentially of finely ground graphite and bentonite. "Substantially" means that the dry, i. Anhydrous foundry additive comprises less than 5 wt .-%, preferably less than 2.5 wt .-% and according to another embodiment, less than 1 wt .-% further constituents. Further constituents are, for example, the dispersants already mentioned or also impurities.

According to a further aspect, the invention relates to the use of a finely ground microcrystalline or amorphous graphite as a foundry additive, in particular for the production of casting molds. The properties of the graphite have already been described above with reference to the foundry additive. The finely ground graphite is particularly preferably used in the form of a suspension.

In another aspect, the invention relates to a molding material composition containing a granular refractory molding material and the foundry additive described above. As a granular refractory molding material can all molding materials used in the field of foundry technology. Especially for the production of molds quartz sand is preferably used. The granular refractory molding material has a grain size as commonly used for the production of molds. The granular refractory molding material preferably has a particle size in the range of 0.05 to 1 mm, preferably 0.1 to 0.7 mm. The adjustment of the particle size can be done, for example, by sieving or sifting. The foundry additive is evenly distributed in the molding material mixture. The proportion of finely ground, preferably microcrystalline or amorphous graphite, based on the dry weight of the molding material mixture, preferably in the range of 0.1 to 2 wt .-% and according to one embodiment in the range of 0.2 to 1.0 wt. % selected.

In one embodiment, the molding material mixture contains a clay as a binder, with sodium bentonite being preferred. According to one embodiment, the proportion of the binder (absolutely dry), in particular sodium bentonite, based on the dry molding material mixture, between 5 and 15 wt .-%, according to one embodiment, between 6 and 10 wt .-%.

According to one embodiment, the molding material mixture in addition to the graphite contains less than 2 wt .-%, according to another embodiment, less than 1 wt .-% carbon, in particular lustrous carbon. According to one embodiment, the molding material mixture is free of lustrous carbon formers. The percentages are based on the weight of the molding material mixture.

For the production of the finely ground, preferably microcrystalline or amorphous graphite, as it is contained in the foundry additive, each can itself microcrystalline or amorphous graphite can be used. The average crystallite size of the graphite used as starting material may be less than 90 nm in one embodiment and less than 60 nm in another embodiment. According to one embodiment, the average crystallite size of the graphite used as starting material is between 15 and 45 nm Grafits conventional equipment can be used. Suitable mills are, for example, air jet mills. The effectiveness of the graphite becomes better, the lower the particle size. To obtain graphite having a particle size of less than 5 microns, preferably the graphite is ground wet. The grinding can also be done in several stages. For example, the graphite can first be ground dry to an average particle size D ₅₀ of less than 100 .mu.m, preferably of less than 20 .mu.m, and then wet ground in a further step. Preferred embodiments of the graphite have already been described above.

Additional components can be added to the foundry additive. It is particularly preferred that the graphite is provided in the form of a suspension. The suspension can be obtained directly by wet milling the graphite, for example, and optionally diluting the resulting suspension so that the suspension has a proportion of preferably 20 to 50 parts by weight of graphite per 100 parts by weight of the suspension.

According to one embodiment, dispersants may be added to the suspension. These can also be added during grinding. Suitable dispersants are, for example, anionic or nonionic surfactants. Exemplary surfactants have already been described above. The dispersants are preferably in a proportion of 2 to 10 wt .-%, based on the dry graphite, added to the suspension.

It is also possible to add thickeners which prevent the graphite particles from sinking during storage. Suitable substances have already been explained above.

According to a preferred embodiment, the foundry additive contains bentonite, in particular sodium bentonite or calcium bentonite. The bentonite can be added to the graphite suspension, for example, or the graphite suspension can be added to a powdered bentonite, in particular calcium bentonite, wherein the moisture content of the mixture is adjusted according to one embodiment so that, for example, a granulate can be produced from the mixture. A calcium bentonite is understood as meaning a bentonite which contains at least 40%, preferably at least 50% and, according to one embodiment, at least 60% of the cation exchange capacity as exchangeable calcium ions.

According to one embodiment, the bentonite, in particular sodium bentonite or calcium bentonite, is first processed into a granulate. As already explained, this can be done with conventional devices, for example with a pelletizing plate or by means of an intensive mixer. The granules should not have too high strength. In the production of granules, therefore, the bentonite is preferably applied with medium energy. For example, the bentonite can be introduced into an intensive vortex mixer and then the water is metered in at preferably maximum swirling speed. A suitable intensive mixer is, for example, the R08 model from Eirich. The amount of water is based on the bentonite used, preferably selected in the range of 20 to 30 wt .-%. With continuing turntable and swirler forms in the course from about 30 seconds to 5 minutes from a uniform granules. The granules are then preferably sieved to the desired size. A suitable sieve has, for example, a mesh width of 2 mm. The sieved fraction can then be dried in a drying oven to the desired moisture, for example to a moisture in the range of 10 to 20 wt .-%.

On the granules, which may optionally also be dried before, then the graphite suspension is given up according to one embodiment. This can for example be done directly in the intensive mixer by the graphite suspension is applied to the moving granules, for example by the suspension is introduced in a thin stream into the mixing vessel of the intensive mixer.

After coating, the granules can be dried to set a moisture in the range specified above. According to a preferred embodiment, however, the water content of the suspension or the moisture content of the bentonite granules is adjusted prior to coating with the graphite such that the graphite-coated bentonite granules need not be dried after the coating in order to adjust the moisture content. When proceeding according to this embodiment, granules are obtained which can be distributed very easily and rapidly in the refractory molding material during the production of the molding material mixture.

In another aspect, the invention relates to a foundry mold of a granular refractory molding material containing the foundry additive described above. The foundry mold is preferably formed as a mold, that is as the part of the foundry mold, which images the outer contour of the casting.

The production of the foundry mold is carried out in a conventional manner. The foundry additive according to the invention is added while moving to the granular refractory molding material, preferably quartz sand being used as the refractory molding material. The foundry additive is preferably added in a proportion to the refractory molding material, so that the molding material mixture has a proportion of finely divided graphite in the range of 0.1 to 2 wt .-%, based on the dry molding material mixture. The molding material mixture is preferably a clay, in particular a bentonite, preferably sodium bentonite added. The proportion of sound is chosen in a common range. According to one embodiment, the proportion of the clay in the molding material mixture is 5 to 15 wt .-%, based on the dry weight of the molding material mixture. The molding material mixture has a conventional moisture content. According to one embodiment, the molding material mixture has a water content in the range of 2 to 4 wt .-%.

To produce the mold, the molding material mixture is placed in a corresponding molding box and compacted in the usual way. There then takes place an assembly of the casting mold, it being possible where appropriate to install cores in the casting cavity. Likewise, the mold is provided in a conventional manner with a feed system for the liquid metal and with feeders.

Furthermore, the invention relates to the use of the foundry mold for metal casting. The foundry mold according to the invention can in itself replace all previously known foundry molds. It is suitable both for steel and cast iron as well as for the casting of non-ferrous metals, such as aluminum casting. After casting, the casting is demoulded in the usual way. The molding sand can then be recycled in the usual way and used again for the production of molds.

Fig. 1:

eine Größenverteilungskurve eines in den Beispielen eingesetzten Grafits mit einer mittleren Partikelgröße D₅₀ von 10,78 µm;

Fig. 2:

eine Größenverteilungskurve eines in den Beispielen eingesetzten Grafits mit einer mittleren Partikelgröße D₅₀ von 4,53 µm;

Fig. 3:

eine Größenverteilungskurve eines in den Beispielen eingesetzten Grafits mit einer mittleren Partikelgröße D₅₀ von 1,55 µm;

Fig. 4:

eine Größenverteilungskurve eines in den Beispielen eingesetzten Grafits mit einer mittleren Partikelgröße D₅₀ von 2,17 µm.

The invention will be explained in more detail below by means of examples and with reference to the attached figures. Showing:

Fig. 1:

a size distribution curve of a graphite used in the examples with an average particle size D ₅₀ of 10.78 microns;

Fig. 2:

a size distribution curve of a graphite used in the examples with an average particle size D ₅₀ of 4.53 microns;

3:

a size distribution curve of a graphite used in the examples with an average particle size D ₅₀ of 1.55 microns;

4:

a size distribution curve of a graphite used in the examples with an average particle size D ₅₀ of 2.17 microns.

Used measuring methods: Determination of cation exchange capacity (CEC) and cation content

• Geräte: Sieb, 63 µm; Erlenmeyer-Schliffkolben, 300 ml; Analysenwaage; Membranfilternutsche, 400 ml; Cellulose-Nitrat-Filter, 0,15 µm (Fa. Sartorius); Trockenschrank;
• Rückflusskühler; Heizplatte; Destillationseinheit, VAPODEST-5 (Fa. Gerhardt, No. 6550); Messkolben, 250 ml; Flammen-AAS (FAAS)
• Chemikalien: 2N NH₄Cl-Lösung Neßlers-Reagenz (Fa. Merck, Art.Nr. 9028); Borsäure-Lösung, 2%-ig; Natronlauge, 32%-ig; 0,1 N Salzsäure; NaCl-Lösung, 0,1 %-ig; KCl-Lösung, 0,1%-ig
• Durchführung: 5 g Ton werden durch ein 63 µm-Sieb gesiebt und bei 110 °C getrocknet. Danach werden genau 2 g auf der Analysenwaage in Differenzwägung in den Erlenmeyer-Schliffkolben eingewogen und mit 100 ml 2N NH₄Cl-Lösung versetzt. Die Suspension wird unter Rückfluss eine Stunde lang gekocht. Bei stark CaCO₃-haltigen Tonen kann es zu einer Ammoniak-Entwicklung kommen. In diesem Fall muss solange NH₄Cl-Lösung zugegeben werden, bis kein Ammoniak-Geruch mehr wahrzunehmen ist. Eine zusätzliche Kontrolle kann mit einem feuchten Indikator-Papier durchgeführt werden. Nach einer Standzeit von ca. 16 h wird der NH₄ ⁺-Ton über eine Membranfilternutsche abfiltriert und bis zur weitgehenden Ionenfreiheit mit entionisiertem Wasser (ca. 800 ml) gewaschen. Der Nachweis der Ionenfreiheit des Waschwassers wird auf NH₄ ⁺-Ionen mit dem dafür empfindlichen Neßlers-Reagenz durchgeführt. Die Waschzahl kann je nach Tonsorte zwischen 30 Minuten und 3 Tagen variieren. Der ausgewaschene NH₄ ⁺-Ton wird vom Filter abgenommen, bei 110°C 2h lang getrocknet, gemahlen, gesiebt (63 µm-Sieb) und nochmals bei 110°C 2h lang getrocknet. Danach wird der NH₉ ⁺-Gehalt des Tons nach Kjeldahl bestimmt.

Principle: The clay (eg bentonite) is treated with a large excess of aqueous NH ₄ Cl solution, washed out and the amount of NH ₄ ⁺ remaining on the clay determined according to Kjeldahl.

Me ⁺ (clay) ^- + NH ₄ ⁺ → NH4 ⁺ (clay) ^- + Me ⁺

(Me ⁺ = H ⁺ , K ⁺ , Na ⁺ , 1/2 Ca ²⁺ , 1/2 Mg ²⁺ ....)

• Equipment: sieve, 63 μm; Erlenmeyer grinding flasks, 300 ml; Analytical balance; Membrane filter chute, 400 ml; Cellulose nitrate filter, 0.15 μm (Sartorius); Drying oven;
• Reflux condenser; hot plate; Distillation unit, VAPODEST-5 (Gerhardt, No. 6550); Volumetric flask, 250 ml; Flame AAS (FAAS)
• Chemicals: 2N NH ₄ Cl solution Nessler's reagent (Merck, item No. 9028); Boric acid solution, 2%; Caustic soda, 32%; 0.1 N hydrochloric acid; NaCl solution, 0.1%; KCl solution, 0.1%
• Procedure: 5 g of clay are sieved through a 63 μm sieve and dried at 110 ° C. Then weigh exactly 2 g on the analytical balance in differential weighing into the Erlenmeyer grinding flask and add 100 ml of 2N NH ₄ Cl solution. The suspension is boiled under reflux for one hour. In the case of strongly CaCO ₃ -containing clays, ammonia development can occur. In this case, as long as NH ₄ Cl solution must be added until no ammonia smell is perceived. Additional control can be done with a wet indicator paper. After a standing time of about 16 h, the NH ₄ ⁺ clay is filtered through a membrane filter and washed until the substantial freedom from ion with deionized water (about 800 ml). The proof of the ionic freedom of the wash water is carried out on NH ₄ ⁺ ions with the sensitive Nessler's reagent. The washing rate may vary between 30 minutes and 3 days depending on the type of clay. The washed out NH ₄ ⁺ clay is removed from the filter, dried at 110 ° C for ₂ hours, ground, sieved (63 micron sieve) and dried again at 110 ° C for 2 hours. Thereafter, the NH ₉ ⁺ content of the clay is determined according to Kjeldahl.

• Beispiel: Stickstoff-Gehalt = 0,93%;
• Molekulargewicht: N = 14,0067 g/mol $$\mathit{CEC}=\frac{0,93\times 1000}{14,0067}=66,4\mathrm{meq}/100\mathrm{g}$$
  
  $$\mathrm{CEC}=66,4\mathrm{meq}/100{{\mathrm{<figure-callout\; id="4"\; label="g\; NH"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">g\; NH</figure-callout>}}_4}^{+}-\mathrm{Ton}$$
  

Calculation of the CEC: The CEC of the clay is the Kjeldahl NH ₄ ⁺ content of the NH ₄ ⁺ clay (CEC of some clay minerals, see Appendix). The data are given in meq / 100 g clay.

• Example: nitrogen content = 0.93%;
• Molecular weight: N = 14.0067 g / mol $$\mathit{CEC}=\frac{0.93\times 1000}{14.0067}=66.4\mathrm{meq}/100\mathrm{G}$$
  
  $$\mathrm{CEC}=66.4\mathrm{meq}/100{{\mathrm{<figure-callout\; id="4"\; label="NH"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">NH</figure-callout>}}_4}^{+}-\mathrm{volume}$$
  

Exchanged cations and their proportions:

The cations released by the exchange are in the wash water (filtrate). The proportion and the type of monovalent cations ("exchangeable cations") was determined spectroscopically in the filtrate according to DIN 38406, part 22. For example, the wash water (filtrate) is concentrated for AAS determination, transferred to a 250 ml volumetric flask and filled with deionized water to the measuring mark. Suitable measuring conditions for FAAS can be found in the following tables.

Calculation of cations:

Molmassen (g/mol): Ca=20,040; K=39,096; Li=6,94; Mg=12,156; Na=22,990; A1=8,994; Fe=18,616 $$\frac{\mathit{me}=\frac{\mathrm{me}-\mathrm{value}\left(\mathrm{mg}/\mathrm{l}\right)\times \mathrm{100}\times \mathrm{Verd}\ddot{\mathrm{u}}\mathrm{retr}}{}=\mathrm{meq}/100\mathrm{<figure-callout\; id="4"\; label="G"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">G</figure-callout>}}{\mathrm{4}\times \mathrm{weighing}\left(\mathrm{in\; g}\right)\times \mathrm{molar\; mass}\left(\mathrm{G}/\mathrm{mol}\right)}$$

Molar masses (g / mol): Ca = 20.040; K = 39.096; Li = 6.94; Mg = 12.156; Na = 22.990; A1 = 8.994; Fe = 18.616

Determination of moisture content:

The water content of the products at 105 ° C is determined using the method DIN / ISO-787/2.

Determination of wet tensile strength, green strength of shaped articles

The wet tensile strength (NZF) and the green strength (DF) of the moldings were measured according to the test specification P38 of the Bund Deutscher Gießer (BDG).

Determination of the compactability of molding material mixtures

The determination of the compressibility of molding material mixtures was measured in accordance with test specification P37 of the Bund Deutscher Gießer (BDG).

1. Preparation of the molding sand mixtures

In a vortex mixer, model R 08 of Eirich, 40 kg of quartz sand F 32 (quartz works Frechen) are presented. With stirring, 500-600 ml of water are then added, and it is mixed for 15 seconds. Subsequently, all further additives are added successively and mixed for 30 seconds. Thereafter, the compressibility is checked. It is then replenished so much water that sets a compressibility of 45% at the end of the mixing cycle. After the water has been dosed, it is mixed for a further 60 seconds. The molding sand mixture is removed from the mixer and stored in a closable plastic drum for 1 h at room temperature.

2. Production and casting of the green sand mold

After storage, the molding sand mixture is molded on a model plate with four discs of a diameter of 100 mm and a height of 40 mm in an APM S1 vibrating press plant from Künkel & Wagner. For this purpose, a box with 350 mm x 310 mm x 100 mm is used. Upper and lower box are then assembled into a mold. In an induction crucible furnace, a sufficient amount of lamellar graphite cast iron (GJL) is melted at about 3.5% C and poured into the casting mold at 1400 ° C. over a period of 10 to 12 seconds. The iron / sand ratio is 1: 3.

3. Determination of the amount of sintered sand

After a cooling time of 2 h upper and lower box are separated, and the Gusstraube with the 4 disc-shaped castings is removed. The Gusstraube is then dropped from a height of about 1 m on a stone floor, with the discs from the Angusssystem separate. Subsequently, the adhering sand is as completely as possible removed with a soft plastic brush from the bottom and the entire circumference of the discs, so that only the end face adhering sand has. To determine the amount of sintered sand, this face is first treated with a soft plastic brush. For this purpose, the iron disc is placed on a mortar mill from the company Retsch (type RMO), wherein between the casting and porcelain mortar, a plastic lid is inserted, which carries the metal disc. Subsequently, brushing is carried out at a contact pressure of 0.07 N / cm ² and a rotational speed of 90 rpm for 30 sec. This ensures that the loose and non-sintered sand is completely and evenly removed from the surface and not captured as sintered sand.

Subsequently, the thus pretreated iron disk is inserted into a metal ring, wherein the inner diameter of the metal ring is 11 cm. In the metal ring, a circular cut out sponge with a height of 1 cm is arranged on a base plate below the iron disk. Subsequently, the sample surface is brushed off with a file brush Type 533 720 from LUX, which is clamped via an adapter into a laboratory stirrer over a period of 120 sec at 70 rpm. The laboratory stirrer, which is firmly connected to the file brush, is connected by a hinge to a sturdy stand. As a result, the surface of the metal disc is loaded with the weight of the stirrer including the file brush. From the weight of the stirrer of about 2.6 kg and the bearing surface of the file brush on the 10 cm iron disc results in a bearing pressure of 0.8 N / cm ² , with which the iron disk is loaded by the file brush. The rotational speed of the brush is 70 rpm. From the weight difference of the iron disk before and after the Brush test results gravimetrically the part of the sand that was already sintered on the metal surface.

In order to fully grasp the sintered sand, the face of the iron disk is then blasted with a steel granulate (1.0 - 1.6 mm) for 30 sec. Again, the proportion of sintered sand is determined by the weight difference before and after the blasting process. The addition of the two individual sand amounts of jet and metal brush test results in the total amount of sand sintered on the front side of the iron disk.

4. Determination of wet tensile strength and wet compressive strength of foundry sand mixtures (VDG leaflet P38)

After final mixing and appropriate Maukzeit the molding material is adjusted by adjusting the water content to a compressibility of 45% and compressed on a rammer type PRA Georg Fischer with three ram strokes to cylinders with a diameter of 50 mm and a height of 50 mm. With the aid of the strength tester type PNZ from Georg Fischer, the wet compressive strength is determined on 5 specimens and the mean value is formed.

5. Measurement of the particle size distribution of the Grafite used

The graphites are as delivered on a laser diffraction Mastersizer ^® 2000 from Malvern in the continuous air stream related to the particle size distribution was measured (dry cell: Scirocco 2000)... In the case of graphite dispersion, the measurement is carried out in aqueous dispersion. For this purpose, the suspension is treated with ultrasound for 30 seconds and then measured in distilled water (wet cell: Hydro 2000 S). The measurement is carried out in accordance with ISO 13320 / DIN ISO 9276-1.

When measuring in air, the sample is blown through the measuring cell with the aid of compressed air (air pressure: 2 bar). The measuring time is 8 seconds. The particle size distribution is calculated according to Fraunhofer theory.

When measured in aqueous suspension, the sample is suspended in deionized water. Before starting the measurement, the sample is treated with an internal ultrasonic finger (intensity: 50%) for 30 seconds. The treatment with ultrasound is also continued during the measurement (intensity: 50%). The sample is pumped through the measuring cell at a pumping speed of 2000 rpm. The evaluation is carried out according to Mie theory, refractive index: 1.5295; Absorption index: 0.1.

6. Determination of the crystallinity of the graphites used

To determine the crystallinity of the graphites used, the 002 reflection from the X-ray diffraction analysis is used. With the help of the X-ray diffractometer Philipps X'Pert the X-ray diffraction pattern in the angular range 2 θ of 10 - 40 ° is recorded on a powder sample with the following parameters: Angular range (2 θ) 10 - 40 ° increment 0.02 ° Time per step 5 s Divergence diaphragm (primary) 1 mm Diffuser (secondary) 1 mm Detector panel (secondary) 0.1 mm Monochromator (secondary)

L_c

Kristallinität/Dicke [nm]

Λ

Wellenlänge CuKα = 0,154051 nm

K

dimensionslose Konstante

ΔH

Halbwertsbreite (FWHM) [rad]

Θ

Beugungswinkel [rad].

The underlying for the calculation of crystallite size 002-Reflex occurs between 26 and 27 2 θ. At this reflex the half width is first calculated. Subsequently, will this half-width for the calculation of the crystallinity is taken over into the Scherrer equation: $$L_c=\frac{K\cdot \lambda }{\mathrm{\Delta }\mathrm{<figure-callout\; id="2"\; label="H"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">H</figure-callout>}\left(2\theta \right)\cdot \cos \theta }$$

L _c

Crystallinity / thickness [nm]

Λ

Wavelength CuKα = 0.154051 nm

K

dimensionless constant

AH

Half-width (FWHM) [rad]

Θ

Diffraction angle [rad].

For graphite, a value of 0.89 is used as the Scherrer constant ( Determination of the crystallinity of calcined and graphite cokes by X-ray Diffraction Analyst, April 1998, Vol. 123 (pp 595-600 )).

7. Determination of the whiteness of the graphitic molding sand mixtures

First, the graphite-containing molding material mixture is prepared as described in paragraph 1. Subsequently, the still moist molding material mixture is pressed into the sample holder of the whiteness measuring device Gretag Macbeth CE 7000 A. Thereafter, the sample holder is dried together with molding material at 110 ° C for 12 h. Subsequently, the sample holder with the molding material is introduced into the measuring instrument and the whiteness is measured according to the TAPPI method T 452 at a color temperature of 6500 ° C. and at an angle of incidence of 10 °.

8. Grinding of graphite on an air jet mill

The dry milling of the graphite was carried out on an air jet mill AFG 100 from. Hosokawa with an attached classifier ATP 50. The air jet mill is with equipped with a 2 mm nozzle. By adjusting the number of revolutions of the classifier, the grinding pressure and the feed quantity, the average particle size distribution D ₅₀ between 1.5 and 20 μm could be set.

In the FIGS. 1 to 4 Particle size distributions for different degrees of grinding of the graphite are shown. The corresponding values are summarized in Tables 1a to 1d. Table 1a: Shares of different particle sizes in a finely ground graphite sample; D <sub> 10 </ sub>: 3.27 μm, D <50>: 10.78 μm, D _{90 </ sub>: 30.29 μm, D <100>}: 321.89 μm, measurement in air (Fig. 1) μm ¹ % ² μm ¹ % ² μm ¹ % ² μm ¹ % ² μm ¹ % ² 0.01 0.00 0.50 0.00 5.00 20.38 60,00 98.19 250.00 99.74 0.20 0.00 0.60 0.00 7.00 31.80 70,00 98.69 300.00 99.95 0.40 0.00 0,700 0.02 10.00 46.63 80,00 98.83 400.00 100.00 0.60 0.00 0.80 0.10 15.00 65.12 90.00 98.91 500.00 100.00 0.80 0.00 0.90 0.19 20.00 77.21 100.00 98.94 600.00 100.00 0.10 0.00 1.00 0.32 25,00 89,90 125.00 98.94 750.00 100.00 0.20 0.00 2.00 3.35 30.00 89.78 150.00 99.03 900.00 100.00 0.30 0.00 3.00 8.47 40,00 94.95 175.00 99.21 0.40 0.00 4.00 14,37 50,00 97.18 200.00 99.40 μm ¹ % ² μm ¹ % ² μm ¹ % ² μm ¹ % ² μm ¹ % ² 0.01 0.00 0.50 1.75 5.00 53.84 60,00 100.00 250.00 100.00 0.20 0.00 0.60 2.84 7.00 66.82 70,00 100.00 300.00 100.00 0.40 0.00 0.70 4.11 10.00 79.15 80,00 100.00 400.00 100.00 0.60 0.00 0.80 5.50 15.00 90.15 90.00 100.00 500.00 100.00 0.80 0.00 0.90 6.96 20.00 95.63 100.00 100.00 600.00 100.00 0.10 0.00 1.00 8.47 25,00 98,40 125.00 100.00 750.00 100.00 0.20 0.05 2.00 23.05 30.00 99.68 150.00 100.00 900.00 100.00 0.30 0.36 3.00 35.14 40,00 100.00 175.00 100.00 0.40 0.90 4.00 45,30 50,00 100.00 200.00 100.00 μm ¹ % ² μm ¹ % ² μm ¹ % ² μm ¹ % ² μm ¹ % ² 0.01 0.00 0.50 2.61 5.00 96.45 60,00 100.00 250.00 100.00 0.20 0.00 0.60 5.82 7.00 99.01 70,00 100.00 300.00 100.00 0.40 0.00 0.70 9.98 10.00 99.61 80,00 100.00 400.00 100.00 0.60 0.00 0.80 14.72 15.00 99.86 90.00 100.00 500.00 100.00 0.80 0.00 0.90 19.74 20.00 99.99 100.00 100.00 600.00 100.00 0.10 0.00 1.00 24.84 25,00 100.00 125.00 100.00 750.00 100.00 0.20 0.00 2.00 64.77 30.00 100.00 150.00 100.00 900.00 100.00 0.30 0.04 3.00 83,84 40,00 100.00 175.00 100.00 0.40 0.68 4.00 92,50 50,00 100.00 200.00 100.00 μm ¹ % ² μm ¹ % ² μm ¹ % ² μm ¹ % ² μm ¹ % ² 0.01 0.00 0.50 7.88 5.00 89,00 60,00 100.00 250.00 100.00 0.20 0.00 0.60 8.87 7.00 96.83 70,00 100.00 300.00 100.00 0.40 0.06 0.70 10.43 10.00 99.90 80,00 100.00 400.00 100.00 0.60 0.77 0.80 12.45 15.00 100.00 90.00 100.00 500.00 100.00 0.80 1.93 0.90 14.83 20.00 100.00 100.00 100.00 600.00 100.00 0.10 3.15 1.00 17.45 25,00 100.00 125.00 100.00 750.00 100.00 0.20 6.58 2.00 45.81 30.00 100.00 150.00 100.00 900.00 100.00 0.30 7.31 3.00 67.11 40,00 100.00 175.00 100.00 0.40 7.45 4.00 80.71 50,00 100.00 200.00 100.00

9. Preparation of an aqueous graphite dispersion

In a 3 1 plastic cup, 1.5 liters of demineralized water are initially charged. Subsequently, with the aid of a stirrer type LD 50 from Pendraulik, at a rotational speed of 2000 rpm and attached dissolver disc, so much dry-ground graphite is stirred in until, depending on the particle size, a concentration between 20 and 40% is reached. Subsequently, between 2 and 10% of a suitable dispersing aid is added with gentle stirring. The amount of dispersant here refers to the weight of the dry graphite. After the dispersant is evenly distributed in the dispersion, so much more graphite powder is added with slow stirring until the desired final concentration is reached. Foaming should be avoided as far as possible. Optionally, the suspension can be 0.1 - 0.3% Degressal ^® SD 20 BASF be added as a defoamer.

The conditions for the preparation of the finely ground graphite and their average particle size are summarized in Table 2.

The results of the examples are summarized in Table 3. Table 2: Grinding of graphite on an air jet mill sample Feed material Sizing wheel adjustment (rpm) Feed quantity (kg / h) Pressure (N / cm ² ) Average particle size (D ₅₀ , μm) Crystal size (nm) JM / K0 Crystalline powder graphite type GHL from Grafit Kropfmühl AG - - - 42 45 JM / K1 " 5000 3.5 25 18 JM / K2 " 8000 3.0 25 10 JM / K3 " 12000 2.4 30 4 JM / K4 " 17000 1.7 35 1.5 JM / T0 Amorphous natural graphite TG 80/85-S of Technografit - - - 38 24 JM / T1 " 5000 4.2 25 20 JM / T2 " 8000 3.7 25 9 JM / T3 " 12000 2.9 30 5 JM / T4 " 17000 2.1 35 2 Ex. Quartz sand F32 (%) Bentonite Geko B (%) Coal ² (%) Graphite (%) Einarb. graphite Whiteness (%) Compressibility (%) NZF (N / cm ² ) DF (N / cm ² ) Anges. Sand (g / m ² ) Bew. ³ See 90.5 7.5 2 - - 12.4 46 0.27 15.7 520 smooth 1 92 7.5 - 0.5% JM / KO powder 16.2 44 0.25 15.1 1350 smooth 2 92 7.5 - 0.5% JM / TO powder 16.8 45 0.27 14.9 1210 smooth 3 92 7.5 - 0.5% JM / K4 powder 13.2 47 0.28 16.3 1060 smooth 4 92 7.5 - 0.5% JM / T4 powder 13.8 45 0.25 17.0 1130 smooth 5 92 7.5 - 0.5% JM / KO powder ¹ 15.9 46 0.26 15.8 1460 smooth 6 92 7.5 - 0.5% JM / K1 Suspens. 11.3 45 0.28 16.4 1050 smooth 7 92 7.5 - 0.5% JM / K2 Suspens. 10.3 47 0.29 15.5 810 smooth 8th 92 7.5 - 0.5% JM / K3 Suspens. 8.4 45 0.25 14.7 580 smooth 9 92 7.5 - 0.5% JM / K4 Suspens. 6.3 44 0.27 15.0 440 smooth 10 92 7.5 - 0.5% JM / T1 Suspens. 10.9 46 0.25 16.0 990 smooth 11 92 7.5 - 0.5% JM / T2 Suspens. 8.8 45 0.30 17.1 690 smooth 12 92 7.5 - 0.5% JM / T3 Suspens. 6.9 46 0.28 14.9 460 smooth 13 92 7.5 - 0.5% JM / T4 Suspens. 5.5 44 0.25 15.2 350 smooth ¹ : Grind bentonite and graphite together and mix in dry
² : hard coal Ecosil ^®
3 : Visual evaluation of the casting surface

Claims (12)

1. Foundry additive comprising finely milled microcrystalline or amorphous graphite which has an average particle size D₅₀ of less than 100 µm.
2. Foundry additive according to Claim 1, characterized in that the finely milled microcrystalline or amorphous graphite has an average crystallite size of less than 90 nm.
3. Foundry additive according to Claim 1 or 2, characterized in that the graphite has been provided in the form of an aqueous suspension.
4. Foundry additive according to Claim 1, 2 or 3, characterized in that a bentonite has been added to the foundry additive.
5. Foundry additive according to Claim 4, characterized in that the bentonite has been provided in the form of granules.
6. Foundry additive according to Claim 5, characterized in that the bentonite granules have been coated with the finely milled microcrystalline or amorphous graphite.
7. Foundry additive according to either Claim 5 or 6, characterized in that the granules have a moisture content in the range from 15 to 35% by weight.
8. Foundry additive according to any of the preceding claims, characterized in that the foundry additive comprises a finely milled carbon support, preferably selected from the group consisting of coal, coke, activated carbon.
9. Mould material mixture containing a particulate refractory mould material and a foundry additive according to any of Claims 1 to 8.
10. Mould material mixture according to Claim 9, characterized in that the proportion of the finely milled microcrystalline or amorphous graphite is in the range from 0.1 to 2% by weight, based on the dry weight of the mould material mixture.
11. Foundry mould composed of a particulate refractory mould material which contains a foundry additive according to any of Claims 1 to 8.
12. Use of a foundry mould according to Claim 11 for metal casting.

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