Foamed ceramic, foamed ceramic filter, method for its production and use

Abstract

The present invention relates generally to a foamed ceramic (3) and a filter comprising such a foamed ceramic, as well as to a method for producing a foamed ceramic and a filter comprising or made of such a foamed ceramic. Another aspect relates to the use of a foamed ceramic (3) and a filter comprising or made of such a foamed ceramic.

Description

Technical Field

[0001] This invention generally relates to foam ceramics and filters comprising such foam ceramics, as well as methods for producing foam ceramics and filters comprising or made of such foam ceramics. Another aspect relates to the use of foam ceramics and filters comprising or made of such foam ceramics. Background Technology

[0002] For example, foam ceramics are used as filter materials for molten metals. In particular, it has long been known that foam ceramics, including Al2O3, are used to filter molten non-ferrous metals, such as aluminum or aluminum alloys.

[0003] Within the scope of this invention, ceramics are generally understood to be inorganic, non-metallic, polycrystalline materials. Ceramics are typically obtained or produced in a method comprising the following steps: providing a raw material mixture comprising powdered inorganic materials, forming a generally aqueous slurry (or suspension or liquid), and forming a green body. The ceramic body is then fired. Firing the ceramic body to form the ceramic material typically involves a sintering process. In this case, the ceramics can be constructed in such a way that they comprise a base material, which constitutes the main part of the ceramic, i.e., at least 50% by weight, especially at least 60% by weight, and is generally composed of or includes particles, such as grains, crystals, or microcrystals, or aggregates of crystals or microcrystals, such as Al2O3, like α-Al2O3, corundum. In addition to the base material, the ceramic may also include another phase that at least partially surrounds and connects the individual particles of the base material to each other. The particles of the ceramic base material are embedded in the other phase, which is referred to as the matrix within the scope of this disclosure. The matrix may in particular include or be formed as a binder or adhesive. The matrix generally includes at least one binder. However, the matrix may also generally include a mixture of binders. The matrix may also be formed as a binder, or even composed of a binder. However, the matrix may also include components of the base material in addition to binders, for example, in the form of base material partially melted and incorporated into the matrix.

[0004] Within the scope of this invention, an adhesive or binder is understood as a substance in a mixture of substances that forms a bond with the other components of the mixture at the interface, in this case, for example, at the phase boundary or grain boundary of the grains of the base material, thereby enabling the components, in this case, the grains of the base material, to be connected and / or crosslinked to each other, for example, by adhesion and / or cohesive forces.

[0005] Within the scope of this invention, foam ceramics are understood to be ceramics having a foam-like structure. Foam ceramics can in particular be formed to include open, continuous porosity, allowing fluid materials to pass from a first side of an object made of or comprising such foam ceramics through the pores of the foam ceramic to a second side of the object, preferably the side opposite the first side. Such foam ceramics and corresponding foams are also referred to as "open-celled." Such foam ceramics can be defined by their density, typically given as a relative density, and their pore size. The relative density is given here as the ratio of the measured density of the foam ceramic to the theoretical density of a non-porous ceramic with the same composition in terms of solid material. Typical measured densities of foam ceramics can be from 0.25 to 0.50 g / cm³. 3 The pore size of foam ceramics is usually referred to as the number of pores based on length, such as pores per centimeter or ppi (pores per inch). The higher this value, the smaller the pores.

[0006] Foam ceramics have long been used for filtering molten aluminum or aluminum alloys, comprising alumina and aluminum phosphate as components. In this case, aluminum phosphate acts as a binder, at least partially surrounding and binding together the alumina particles of the base material. Therefore, such foam ceramics can also be described as comprising a core or base material containing crystalline alumina, typically in the form of α-Al₂O₃, such as calcined α-Al₂O₃, and a matrix containing a phosphate-containing binder or adhesive, wherein the matrix, i.e., the phosphate-containing matrix herein, at least partially surrounds or forms the core or base material, or the grains formed by or surrounded by that material, and binds them together. Thus, the matrix herein includes at least one phosphate-containing binder. Such foam ceramics are also called phosphate-bonded foam ceramics or phosphate-bonded alumina-based foam ceramics. In phosphate-bonded foam ceramics, aluminum dihydrogen phosphate (Al(H₂PO₄)₃) is typically added to the slurry, which is converted to aluminum aluminum phosphate (AlPO₄)₄ during ceramic firing.

[0007] Although such foam ceramics and filters made from or containing such phosphate-bonded foam ceramics have been on the market for many years, they have some drawbacks. For example, a disadvantage is that the coefficient of thermal expansion of ceramic materials is typically quite high, around 8.5 x 10⁻⁶. -6 / K and 8.9*10 -6Between / K, coupled with a high elastic modulus and only low thermal conductivity, resulting in low thermal shock resistance. Due to the porous, partially amorphous, and / or polycrystalline structure of the filter material, its thermal conductivity is very difficult to measure. During filtration, the phosphate matrix is ​​also eroded by the melt, further reducing the mechanical strength of the foam ceramic. Furthermore, monophosphine, a toxic gas, is generated or may be generated in this foam ceramic after use as a filter material, posing a particular problem for handling used filters. Additionally, particle discharge, also known as "snowflakes" or "pulverization," is common with conventional foam ceramics. Depending on the type of binder used, up to 1.2 grams of particles can be discharged in a 17” filter weighing approximately 3.3 kg to 4.0 kg, equivalent to a material loss of 0.36‰ due to snowflakes or pulverization. Even though this may seem like a small loss of filter material at first glance, it is highly detrimental because these discharged particles can, for example, enter the casting and then adversely affect its performance.

[0008] In light metal casting, such as the production of aluminum castings, another common problem is the appearance of porosity, particularly in the aluminum castings themselves. These are often caused by hydrogen, the only soluble gas in molten aluminum. Studies have shown that the choice of filter material can influence and purposefully reduce porosity formation. For example, Erzmetall et al. (Erzmetall 71 (2019), pp. 32 onwards) described how a filter containing a mixed oxide ceramic comprising 15 wt% spodumene (LiAlSi2O6) and 85 wt% Al2O3 can reduce porosity formation compared to pure Al2O3 filter materials. This is attributed to the formation of LiAlH4, which at least reduces the formation of gaseous hydrogen that contributes to porosity formation.

[0009] However, by The proposed filter materials containing spodumene are disadvantageous because a phase transition occurs in spodumene during the production of large-size foam ceramics. This leads to cracks forming in the foam ceramics after firing. Furthermore, spodumene has a very low coefficient of thermal expansion compared to Al₂O₃. Due to the difference in thermal expansion coefficients between this low-expansion phase and Al₂O₃, thermomechanical stresses occur in the material, ultimately causing mechanical failure of the filter due to breakage.

[0010] In addition, various foam ceramics have been proposed as alternatives to known phosphate-bonded foam ceramics. For example, US Patent 8,518,528 B2 describes a foam ceramic used as a filter material, for example for aluminum castings, wherein the foam ceramic comprises a core or base material containing aluminum silicate, namely Al2SiO5, especially kyanite, and a boron oxide-containing vitreous material surrounding the core or base material. The vitreous material at least partially surrounds the grains of the base material and binds them together, particularly in the form of a continuous, coherent material. The problem of hydrogen-induced porosity is not solved, but in this way, at least better environmental compatibility of the filter material can be achieved, as the adverse effects of phosphate from a treatment and environmental perspective are eliminated. However, these filter materials also exhibit significant pulverization / snowing phenomena. In particular, a comparison between the foam ceramic according to US 8,518,528 B2 and conventional phosphate-bonded foam ceramics shows that the latter has significantly more particle discharge. For example, one test showed that a 17” filter made of foam ceramic according to US 8,518,528 B2 had a particle discharge of 1.2 g. On the other hand, a 17” filter made of phosphate-bonded foam ceramic only lost 0.03 g. Particle discharge from the filter material or foam ceramic is not only disruptive, as it contaminates the casting and adversely affects its properties, but particle discharge is also directly related to the strength of the foam ceramic. The less particle discharge, the higher the strength of the foam ceramic. The problem of hydrogen discharge has been addressed in practice, for example, through degassing units, as compared in the study by Chesonis et al. in *Metal Quality Comparison of Alcan Compact Degasser and SNIF at Alcoa Mount Holly Casthouse*. The hydrogen concentration was measured once before and once after the degasser. The casting speed was approximately 36 kg / h. The hydrogen content at the ansicht / taphole determined in the above study was between 0.24 and 0.35 ml / 100g. After degassing, values ​​between 0.14 and 0.18 ml / 100g were obtained. A drawback of this method is that it necessitates the use of an additional component, the degassing machine, during the casting process. Furthermore, the hydrogen content in the resulting molten aluminum remains relatively high; therefore, further reduction of the hydrogen content is desirable.

[0011] EP 3 508 461 A1 describes a foam ceramic comprising Al2O3 as a base material and colloidal SiO2 as a binder. The foam ceramic may further comprise borate glass and / or boron oxide as a further binder. This results in a filter material with low magnesium absorption after filtering magnesium-containing alloys. According to EP 3 508 461 A1, the foam ceramic material described therein also exhibits a lower tendency for particle discharge compared to filters with aluminosilicate-based materials. However, this document does not address the problem of porosity formation in castings due to hydrogen. Furthermore, due to the silicate binder, the cutting behavior of the foam ceramic described in EP 3 508 461 A1 can be considered worse than that of known phosphate-bonded foam ceramics.

[0012] Therefore, there is a need for foam ceramics used as filter materials in metal casting, especially in light metal casting, such as castings of aluminum or alloys containing aluminum and / or magnesium, and for processes that produce such foam ceramics, which can at least reduce porosity formation in metal castings and / or have lower particle discharge, and / or better mechanical strength and / or better environmental compatibility. Furthermore, there is a need for filters incorporating such foam ceramics for casting non-ferrous metals. Summary of the Invention

[0013] The object of this invention is to provide a foam ceramic that at least partially overcomes the problems of the prior art. Other aspects relate to providing filters comprising such foam ceramic, methods for manufacturing such foam ceramic, and the general use of the foam ceramic.

[0014] This objective is addressed by the subject matter of the independent claims. Specific or preferred embodiments can be found in the dependent claims and further disclosures.

[0015] Therefore, the present invention relates to a foam ceramic comprising a base material containing Al2O3 and preferably Li2O and a matrix containing SiO2 and / or B2O3 and / or P2O5 and / or Li2O and / or CaO, wherein the coefficients of thermal expansion of the preferred base material and the matrix differ from each other by a maximum of 6*10. -6 / K, preferably a maximum of 5*10 -6 / K, with a maximum of 4*10 as the preferred option. -6 / K, a very special and preferred option with a maximum of 3*10 -6 / K, and the most preferred option is at most 2*10 -6 / K.

[0016] This design scheme is very advantageous.

[0017] Because Al2O3-containing base materials generally possess excellent chemical stability, especially for foam ceramics suitable as filter materials in metal casting, such as in the casting of light metals like molten aluminum or aluminum-containing melts. Al2O3, for example in the form of calcined Al2O3, is known to have good resistance in contact with molten aluminum. The base material is preferably present in particulate form.

[0018] Foam ceramics further comprise a matrix consisting of SiO2 and / or B2O3 and / or P2O5 and / or Li2O and / or CaO. For example, the matrix may include silicic acid as a binder, for example, in the form of igneous silicic acid. However, the matrix may also, and even more preferably, include colloidal silica sol as a binder. In this case, the slurry can be produced particularly readily, especially using silica sols such as those marketed under the name Levasil, but also including the use of other commercially available silica sols. The matrix design described above is advantageous, particularly in terms of reducing firing temperatures, and is therefore advantageous from a cost and environmental perspective. For example, B2O3 can be used as a flux to reduce, for example, melting or sintering temperatures. Thus, B2O3 is another optional component of the matrix. However, the matrix may also be designed to alternatively or additionally include P2O5 (so-called phosphate-containing matrices). P2O5 is a known component of binders for ceramics such as foam ceramics, especially as a component of aluminum phosphate. Foam ceramics with a matrix that at least partially comprises phosphate exhibit low particle emission and therefore high strength. However, since phosphate-bonded ceramics are disadvantageous from an environmental and processing safety perspective, according to one embodiment, the foam ceramics are advantageously designed such that the matrix includes other components besides P2O5, particularly SiO2 and / or B2O3 and / or Li2O and / or CaO. A particularly preferred component of the matrix may be Li2O. Another particularly preferred matrix component may be CaO.

[0019] In other words, according to one embodiment, a foam ceramic comprising a base material containing Al2O3 and a matrix is ​​provided, wherein the foam ceramic includes Li2O. Li2O can exist as a component of the base material, as a component of the matrix, or as a phase forming the matrix. It has been shown that this design of the foam ceramic surprisingly and significantly improves its strength. In particular, even compared to standard foam ceramics, i.e., phosphate-bonded foam ceramics, particle discharge can be further reduced.

[0020] The reason for this increase in strength is currently unclear. It is possible that the presence of Li2O as a matrix component leads to better bonding between the particles of the base material, thus increasing the material's cohesion through stronger adhesion.

[0021] According to another embodiment, a foam ceramic comprising a base material containing Al2O3 and a matrix is ​​provided, wherein the foam ceramic includes CaO. The matrix particularly includes CaO. It has been shown that particularly good strength of the foam ceramic can also be achieved in such an embodiment. In particular, this can be achieved if the foam ceramic, in addition to including CaO, particularly includes B2O3 and / or SiO2, preferably B2O3 and SiO2.

[0022] It is further shown that a particularly uniform coefficient of thermal expansion can be achieved by utilizing a foam ceramic design comprising a base material containing Al2O3 and preferably Li2O, and a matrix containing SiO2 and / or B2O3 and / or P2O5 and / or Li2O and / or CaO. This design appears particularly achievable by including a certain amount of CaO in the matrix.

[0023] Preferably, the coefficients of thermal expansion of the base material and the matrix differ by a maximum of 6*10. -6 / K, preferably differing by a maximum of 5*10 -6 / K, with a maximum difference of 4*10 for optimal selection. -6 / K, the most specific and optimal values ​​differ by a maximum of 3*10. -6 / K, the most specific and optimal values ​​differ by a maximum of 2*10. -6 The result of this is that, at the interface between the base material and the matrix, or at the interface between the grains forming or surrounded by the base material and the matrix, the thermomechanical stress formed when the foam ceramic is subjected to temperature loads, for example, when a metal casting filter containing the foam ceramic is heated. In other words, the discharge of interfering particles, known in other filter materials, can be further reduced unexpectedly and easily in this way. As a result, when such foam ceramics are used as filter materials, metal castings with reduced particle input and thus improved quality can be obtained, especially castings of light metals, such as those made of aluminum or aluminum alloys.

[0024] In other words, according to one embodiment, a foam ceramic is provided in which the coefficients of thermal expansion of the base material and the matrix are mutually compatible, specifically, the difference between them is very small. Therefore, in this way, the coefficient of thermal expansion produced by the foam ceramic is very uniform.

[0025] Within the scope of this invention, the coefficient of thermal expansion, or α, is understood to be the linear coefficient of thermal expansion. Unless otherwise explicitly stated, this is an average value over a temperature range of 20°C to 700°C. The terms α and α' are used interchangeably. 20-700The coefficient of thermal expansion and the linear coefficient of thermal expansion are synonymous within the scope of this invention. The values ​​given are nominal average coefficients of thermal expansion. If the measurement is performed on glass within the scope of this invention, it is performed according to ISO 7991. For ceramics or foam ceramics, the measurement is performed using PU strips, which are immersed in the appropriate slurry and then fired. After firing, the ceramic strips are measured to determine the linear coefficient of thermal expansion.

[0026] To obtain a particularly uniform coefficient of thermal expansion for foamed ceramics, the smaller the difference in the coefficients of thermal expansion between the base material and the matrix, the better. In particular, within the range of measurement accuracy, the coefficients of thermal expansion of the base material and the matrix can ideally be the same.

[0027] The inventors believe that the very uniform coefficient of thermal expansion of the foam ceramic observed according to one embodiment may be due to the favorable composition of the foam ceramic. In particular, the content of Li₂O and / or CaO in the foam ceramic may be advantageous here.

[0028] According to one embodiment, the foam ceramic therefore includes Li₂O, wherein the Li₂O content of the foam ceramic is preferably at least 0.3% by weight, more preferably at most 5% by weight, and particularly preferably at most 0.5% by weight. On the one hand, as mentioned above, it has been found that the Li₂O content in the foam ceramic can minimize the occurrence of pores or bubbles in, for example, aluminum castings or aluminum alloys or aluminum alloy-containing castings. Therefore, the Li₂O content in the foam ceramic is at least 0.3% by weight. This ensures a significant reduction in bubbles in the casting, thereby allowing sufficient hydrogen to be drawn from the non-ferrous metal melt. However, the Li₂O content in the foam ceramic should not be too high, as Li₂O is an expensive raw material. Furthermore, Li₂O, as a basic oxide, is known to reduce the temperature stability of materials. Therefore, according to one embodiment, the Li₂O content in the foam ceramic is at most 5% by weight, preferably even at most 0.5% by weight. However, it has been surprisingly found that the Li₂O content in the foam ceramic not only reduces the formation of bubbles in the casting, but also further improves the strength of the foam ceramic in a very surprising way. The further reduction in pulverization / snowing of Li₂O-containing foam ceramics compared to known solid foam ceramics, such as phosphate-bonded foam ceramics, demonstrates this point. Therefore, it may even be possible to obtain high-strength foam ceramics that do not contain P₂O₅. However, the P₂O₅ content of foam ceramics can be reduced, at least by adding Li₂O. This is particularly surprising because it is known that foam ceramics containing Li₂O-containing minerals, namely spodumene, can reduce bubble formation in castings. However, at the same time, the mechanical stability of such foam ceramics is insufficient due to the phase transition of spodumene within the temperature range of the foam ceramic application.

[0029] According to one embodiment, the foam ceramic comprises at least 0.1% by weight of CaO, preferably up to 20% by weight of CaO, more preferably up to 10% by weight of CaO, and particularly preferably up to 2% by weight of CaO. This is particularly advantageous for forming very robust foam ceramics.

[0030] According to one embodiment, the foam ceramic comprises at least 67% by weight of Al2O3, preferably up to 95% by weight of Al2O3. The foam ceramic preferably comprises at least 72% by weight of Al2O3. Another embodiment sets the foam ceramic to comprise at least 75% by weight and up to 95% by weight of Al2O3. Al2O3 is a fundamental component of the foam ceramic according to the invention, and especially a fundamental component of the base material. As explained above, this is because Al2O3 has very good tolerance in typical applications of foam ceramics, such as filtering molten metals, like molten aluminum; however, the proportion of Al2O3 cannot be too high. In particular, to ensure sufficient mechanical strength of the foam ceramic, at least one binder needs to be added, which is included in the matrix and forms a binder for connecting the grains of the base material to each other during firing. Typically, this at least one binder comprises a substance capable of bonding with the grains of the base material at the firing temperature, and in particular may include at least one flux, i.e., a substance that lowers the melting or sintering temperature. However, since Al2O3 has a high melting temperature, it is not suitable as a flux, and therefore, an excessively high Al2O3 content in the foam ceramic is disadvantageous. While it is possible to obtain nearly pure Al2O3 ceramics at very high firing temperatures, this is economically difficult to achieve because high firing temperatures also lead to high manufacturing costs. Therefore, the Al2O3 content in foam ceramics is preferably limited, preferably not exceeding 95% by weight.

[0031] Whenever the composition of foam ceramics and / or the content of specific components and / or ingredients of foam ceramics are mentioned within the scope of this invention, the solid content of the foam ceramics is always referred to. Therefore, porosity is not considered for data on chemical and / or mineralogical-crystallographic composition expressed as weight % or volume % respectively.

[0032] According to one embodiment, the foam ceramic contains at least 5% by weight of SiO2, preferably up to 25% by weight of SiO2, for example up to 20% by weight of SiO2.

[0033] SiO2 is a component with high temperature resistance. SiO2 can be present, in particular, as a binder or a component of a binder, for example, when added to a slurry or suspension in the form of silica. To ensure sufficient mechanical strength of the foam ceramic by adequately bonding or cementing the grains of the base material, the SiO2 content in the foam ceramic should not be too low, preferably at least 5%, and most preferably at least 10% by weight. However, an excessively high SiO2 content in the foam ceramic can also be disadvantageous. In particular, SiO2 in contact with molten non-ferrous metals may be corroded by these metals and at least partially or completely dissolved. This can lead to contamination, for example, of the molten non-ferrous metal, and is therefore undesirable. Therefore, the SiO2 content in the foam ceramic is preferably limited, and according to one embodiment, the SiO2 content is at most 25% by weight, for example, about 20% by weight.

[0034] According to one embodiment, the foam ceramic comprises more than 15% by weight of SiO2, especially more than 18% by weight of SiO2, preferably more than 19% by weight of SiO2, and particularly preferably more than 20% by weight of SiO2.

[0035] Therefore, the present invention also relates to a foam ceramic comprising a base material containing Al2O3 and a matrix containing SiO2, particularly a foam ceramic according to an embodiment of the present invention, wherein the foam ceramic comprises more than 15% by weight of SiO2, particularly more than 18% by weight of SiO2, preferably more than 19% by weight of SiO2, and particularly preferably more than 20% by weight of SiO2, and preferably up to 25% by weight of SiO2.

[0036] According to one embodiment, the foam ceramic comprises at least 0.1% by weight and preferably up to 5% by weight of B2O3. The preferred range for the B2O3 content is at least 0.3% by weight, preferably up to 1.5% by weight. B2O3 is a known flux and is therefore advantageous for lowering the sintering temperature. Thus, if the foam ceramic contains B2O3, the firing temperature of the foam ceramic can be reduced. It may be advantageous if the B2O3 content in the foam ceramic is at least 0.1% by weight, preferably at least 0.3% by weight, and particularly preferably at least 0.5% by weight. However, the B2O3 content should not be too high, as otherwise the temperature resistance of the foam ceramic will be significantly affected. Therefore, the B2O3 content in the foam ceramic is preferably limited, and according to one embodiment, preferably not exceeding 5% by weight, preferably up to 1.5% by weight.

[0037] However, it is also possible, and may even be particularly preferred, if the foam ceramic is substantially boron-free. According to the invention, a substantially boron-free embodiment means that the B2O3 content in the foam ceramic is at most 500 ppm (0.05 wt%), preferably less, for example, up to 300 ppm (0.03 wt%), or up to 200 ppm (0.02 wt%), or up to 100 ppm (0.01 wt%). In this case, B2O3 is present as a trace component.

[0038] Such a design scheme for foam ceramics is possible, for example, by not adding B2O3-containing starting materials, such as borate glass, boron oxide, and / or boric acid slurries. In this context, B2O3-containing starting materials are generally understood to be those in which boron or B2O3 exists as a fundamental component, i.e., not merely as trace amounts and / or as unavoidable impurities. B2O3-containing starting materials are defined herein as starting materials with a B2O3 content exceeding 1% by weight.

[0039] B2O3 is not only a flux that can be used to lower the sintering temperature, but it can also protect any SiO2 matrix that may be present from the influence of the molten aluminum. Therefore, if the foam ceramic also includes SiO2, then the B2O3 content in the foam ceramic is particularly advantageous. Alternatively, it is also advantageous for the slurry used to produce the foam ceramic to include B2O3, especially when the foam ceramic is designed to include SiO2, but the resulting foam ceramic does not necessarily include B2O3.

[0040] For example, B₂O₃ can be added to ceramic powder in the form of boron oxide or boric acid to produce slurries or suspensions. However, this can be disadvantageous because, for example, B₂O₃ via boron oxide may cause silica sol gelation, thus negatively impacting the rheological properties of the slurry. Therefore, it may be advantageous to use other boron-containing substances instead of boron oxide or boric acid as starting materials, which, unlike the aforementioned boron compounds, will not cause the aforementioned undesirable reactions in the slurry, or if they do cause a reaction, to a much smaller extent.

[0041] B₂O₃, acting as a flux and thus lowering the sintering or firing temperature of ceramics, is advantageous from a cost-effectiveness standpoint and is typically a component of the matrix. Surprisingly, however, while it has been shown that slurries can also contain B₂O₃, foam ceramics derived from slurries containing B₂O₃ do not. The inventors believe this may be because B₂O₃ acts as a flux in this case, thus positively influencing the sintering of the ceramic, but volatilizes as the sintering process continues. This may also be related, in particular, to the reaction of other components or ingredients in the slurry or foam ceramic, for example, the formation of highly volatile or readily soluble borates in slurries containing both Li₂O and B₂O₃.

[0042] Therefore, this disclosure also relates to a foam ceramic comprising a base material containing Al2O3 and a matrix containing SiO2, particularly a foam ceramic according to an embodiment of the invention, wherein the B2O3 content of the foam ceramic is at most 500 ppm, based on weight.

[0043] According to a further embodiment, the foam ceramic is free of P2O5 except for unavoidable trace amounts. In other words, according to one embodiment, it is a non-phosphate-bonded foam ceramic. Within the scope of this invention, unavoidable trace amounts are understood to mean that the P2O5 content in the foam ceramic is at most 500 ppm P2O5.

[0044] As mentioned above, phosphate-bonded foam ceramics based on Al2O3 and using aluminum phosphate, such as monoaluminum orthophosphate, as a binder are well-established materials, for example, for filtering aluminum-containing melts, and high strength can also be achieved with these materials. However, during the filtration of these alloys, aluminum phosphide and / or magnesium phosphide may form in the case of magnesium-containing alloy melts, which may react with water, for example, to form monophosphine PH3 after use. To avoid the formation of such harmful substances, especially in the context of simplifying the handling of filter materials, it is therefore advantageous to design the foam ceramic as a non-phosphate-bonded foam ceramic.

[0045] According to another embodiment, the foam ceramic is designed as a phosphate-bonded foam ceramic, wherein the P2O5 content of the foam ceramic is at most 10% by weight, preferably at least 5% by weight. According to one embodiment, the P2O5 content is at most 7% by weight, preferably at least 5% by weight. In this way, a foam ceramic with good strength can be obtained, but the P2O5 content in this foam ceramic is reduced compared with conventional foam ceramics. Therefore, in this way, at least the processing problems caused by the formation of monophosphate can be reduced.

[0046] This could be advantageous if the foam ceramic still includes Li2O, especially as a matrix component.

[0047] According to one embodiment, the foam ceramic is formed such that the base material includes α-Al₂O₃. α-Al₂O₃, corundum, exhibits high heat resistance and is the most stable modifier of Al₂O₃. Furthermore, it is readily available, making it preferred from both a cost and availability perspective. α-Al₂O₃ is preferably present as calcined α-Al₂O₃.

[0048] As mentioned above, if foam ceramics are used as filter materials for casting non-ferrous metals, then the content of Li2O in the foam ceramics is very advantageous. Therefore, for example, a foam ceramic containing spodumene has been proposed.

[0049] Others believe that the absorption of hydrogen in lithium spodumene-containing foam ceramics proceeds according to the following formula:

[0050] LiAlSi2O6+4Al+4H→LiAlH4+2Al2O3+2Si

[0051] In other words, from a purely computational perspective, one unit of spodumene, LiAlSi₂O₆, can absorb four hydrogen atoms. LiAlH₄ is ​​a solid reaction product and therefore does not contribute to the formation of porosity in castings.

[0052] Despite this beneficial effect on porosity formation in castings, among which... Researchers not only demonstrated that the overall porosity was reduced, but also that the remaining pores were finer and more uniformly distributed. However, further research showed that foam ceramics containing spodumene cannot be used in practical applications. This is because spodumene undergoes a phase transition when the ceramic is sintered. This means that it is impossible to produce large-size filters. This is because during the phase transition process, a change from 3.2 g / cm³ occurs. 3 Up to 2.4 g / cm 3 Volume transition.

[0053] Surprisingly, this negative effect can be avoided if other lithium-containing starting materials are used, namely lithium-containing starting materials that do not exhibit phase jumps, especially at manufacturing temperatures, i.e., during the sintering of foam ceramics, and / or lithium-containing starting materials that do not form lithium-chain silicates.

[0054] Suitable lithium-containing starting materials include, for example, lithium-containing island silicates, such as nepheline, or lithium-containing layered silicates, such as petalite, or preferably inorganic non-silicate lithium compounds, such as mixed oxides comprising lithium oxide and at least one other metal oxide, such as lithium-aluminum spinel, or lithium salts, such as lithium carbonate. However, lithium-containing starting materials may also exist in the form of amorphous materials, for example as lithium-containing fluxes, such as lithium-containing glass fluxes, or as lithium-containing glass frits, wherein the glass can also be formed as silicate glass, for example, as borosilicate glass. It is preferred that the lithium-containing starting material be formed in such a way that it is free of fluorine except for unavoidable trace amounts, i.e., the content is at most 500 ppm by weight, preferably less.

[0055] This is surprising because these materials, such as lithium-containing layered silicates like scaly rock K(Li,Al)3[(F,OH)2](Si,Al)4O 10 Or through lithium feldspar LiAlSi4O 10 It has a layered structure and is known to be able to be produced from SiO4. 4 - The silicate anions generated by the corner bonding of the tetrahedrons contain molecules between the layers and have excellent expansion capacity. Therefore, it is believed that the use of this lithium-containing layered silicate is detrimental to the stability of foam ceramics, especially since water leakage during the firing process can cause the ceramics to break.

[0056] Furthermore, the reaction equation for hydrogen absorption using, for example, petalite as a lithium source is more unfavorable compared to the reaction for spodumene. For petalite, the calculated result for this reaction is:

[0057] 3LiAlSi4O 10 +20Al+12H→3LiAlH4+10Al2O3+12Si.

[0058] Therefore, purely from a calculation perspective, for one formula unit of lithium-bearing mineral, specifically spodumene, four formula units of hydrogen can be combined. However, the reaction equation is less favorable in terms of the byproducts. This is because the equation shows that in this reaction, one formula unit of spodumene consumes more than six formula units of metallic aluminum, compared to only four formula units in the corresponding reaction with spodumene. Furthermore, the amount of metallic silicon released is also twice as high.

[0059] Preliminary estimates suggest that the reaction with the scaly granules is more critical, as potassium may enter and contaminate the melt during the reaction. The fluorine content of the scaly granules is also unfavorable in this case.

[0060] However, surprisingly, actual experiments showed that the aforementioned difficulties did not actually occur despite using lithium-containing layered silicates as the starting material for producing foam ceramics and as a component of the base material. The inventors believe this is because only a small fraction of the lithium is available for the absorption reaction, especially the lithium located at the interface of the foam ceramic. In particular, the exact type of lithium-containing phase used, especially the exact type of crystalline phase or mineral used, may be less important to the formation mechanism of LiAlH4. The inventors believe this is likely due to at least a portion of the relatively mobile lithium transferring from the crystalline phase of the base material to the matrix during firing.

[0061] Therefore, advantageously, in addition to lithium-containing layered silicates, such as, in particular, lithium-bearing feldspar, other lithium-containing starting materials that do not form lithium-bearing chain silicates can also be used, such as materials that can act as fluxes, such as salts containing lithium oxide, or mixed oxides, such as lithium aluminate. Advantageously, no lithium-containing crystalline phase is formed here, or rather, no lithium-containing crystalline phase can be detected. Instead, in this case, lithium appears to form particularly as a component of the matrix at least partially surrounding the base material particles. However, it is also possible that lithium exists at least partially in a mixed crystalline form and therefore may also be a component of the base material.

[0062] According to one embodiment, the matrix is ​​at least partially glassy.

[0063] The glassy formation of the matrix can be understood as the matrix forming an amorphous shape, preferably obtained through a melting process that is at least partially completed.

[0064] The at least partially glassy composition of the matrix can be particularly advantageous. On the one hand, the grains of the base material can be wetted particularly well when at least a portion of the binder phase melts. On the other hand, glass typically lacks any internal structure, such as grain boundaries, where penetration by corrosive substances can occur. In other words, the formation of at least a partially glassy matrix leads to greater stability of the foam ceramic as a whole. On the one hand, the formation of at least a partially glassy matrix results in better bonding of the base material, i.e., increased cohesion. On the other hand, corrosion is reduced due to the formation of at least a partially glassy matrix, which is preferably inert to materials in contact with it, such as liquid non-ferrous metals and further components or elements of such melts, such as corrosive gases, because the lack of an interface at least hinders the entry of corrosive media into the base material.

[0065] According to a preferred embodiment, the matrix comprises Li₂O. This design is advantageous because, in this way, lithium, which can be used for hydrogen absorption, is present at the interface between the foam ceramic and the contacting molten metal, preferably a non-ferrous metal, especially a melt of aluminum or an aluminum alloy.

[0066] According to one embodiment, the matrix comprises lithium-containing silicate glass and / or lithium-containing borosilicate glass, preferably lithium-containing borosilicate glass. This design is particularly advantageous because it not only ensures the advantages of glassy formation of the matrix and the availability of lithium at the foam ceramic interface, where it can contact any potential hydrogen-containing molten metal, but also, more importantly, facilitates the formation of foam ceramics with compatible coefficients of thermal expansion between the base material and the matrix. Therefore, the coefficients of thermal expansion of silicate and / or borosilicate glasses, as well as borosilicate glasses, can be varied by the alkali content, such as the lithium oxide content in the glass, and can reach 10 × 10⁻⁶. -6 / K or higher. However, this also allows for the selection of the composition of the glass phase, whose coefficient of thermal expansion can match or be compatible with the coefficient of thermal expansion of the base material or one or more of the materials contained in the base material, in this case, especially Al2O3. Since dibasic silicate glasses, such as pure lithium silicate glass, typically have very poor chemical resistance, it is advantageous to add boron oxide (B2O3) to the glass as a further component to increase chemical stability. In this way, the firing temperature of the ceramic can also be advantageously reduced.

[0067] According to another embodiment, the foam ceramic comprises the following components, expressed in weight percent based on oxides:

[0068] Al2O3 67 to 95, especially 75 to 95

[0069] Li₂O 0 to 5, preferably 0.3-5, more preferably 0.3 to 0.5

[0070] SiO2 0 to 25, preferably 5 to 25, more preferably 10 to 25

[0071] B2O3 0 to 5, preferably 0.1 to 5, more preferably 0.3 to 1.5 and / or

[0072] The B2O3 content is at most 500 ppm by weight.

[0073] CaO 0 to 20, preferably 0.1 to 20, more preferably 0.1 to 10, particularly preferably 0.1 to 20%.

[0074] P2O5 0 to 10, preferably up to 10% by weight, particularly preferably up to 7% by weight.

[0075] Very specifically, a maximum of 5% by weight is preferred.

[0076] According to another embodiment, the foam ceramic comprises the following components, in units of oxide-based weight percent:

[0077] Al2O3 75 to 95

[0078] Li₂O 0 to 5, preferably 0.3-5, more preferably 0.3 to 0.5

[0079] SiO2 0 to 25, preferably 5 to 25, more preferably 10 to 25

[0080] B2O3 0 to 5, preferably 0.1 to 5, more preferably 0.3 to 1.5 and / or

[0081] The B2O3 content is at most 500 ppm by weight.

[0082] CaO 0 to 20, preferably 0.1 to 20, more preferably 0.1 to 10, particularly preferably 0.1 to 20%.

[0083] P2O5 0 to 10, preferably up to 10% by weight, particularly preferably up to 7% by weight.

[0084] And very specifically, a maximum of 5% by weight is preferred.

[0085] Therefore, within the scope of this invention, the numerical values ​​of the components relate to the chemical composition of the material. Thus, within the scope of this invention, if the foam ceramic comprises, for example, 90% by weight of Al₂O₃, this is understood to relate to the total alumina content of the foam ceramic. In this case, the Al₂O₃ in the foam ceramic may include the form of Al₂O₃ and other compounds, such as aluminum silicate.

[0086] According to another embodiment, the foamed ceramic comprises the following components, expressed in volume percent, based on the solids percentage:

[0087] α-Al₂O₃ (corundum) 85 to 95

[0088] Quartz 0.8 to 2

[0089] Hematite 0 to 2.

[0090] In addition to the above-mentioned components, the foam ceramic may also include a further crystalline phase, such as AlPO4. According to one embodiment, the foam ceramic includes AlPO4 in crystalline form, wherein the content of crystalline AlPO4 in the foam ceramic is preferably at least 3.5% by volume, preferably not more than 9% by volume, particularly preferably not more than 8% by volume, and very particularly preferably not more than 7.5% by volume.

[0091] It has been shown that such an implementation plan is very advantageous.

[0092] Corundum is the main component of the foam ceramic according to this embodiment, which gives the foam ceramic very good chemical resistance, especially for use as a filter material in aluminum casting.

[0093] Furthermore, the foam ceramic according to this embodiment also includes quartz, particularly deep-layer quartz, in a content of 0.8 vol% to a maximum of 2 vol%, and selective cristobalite, wherein the cristobalite content in the foam ceramic is limited, preferably a maximum of 2 vol%. This is advantageous because the coefficient of thermal expansion of cristobalite is significantly greater than that of crystalline quartz, and in the case of cristobalite, a significant increase in thermal expansion occurs precisely in the range between 200°C and 300°C. Within this temperature range, the coefficient of thermal expansion of quartz is low, but significantly greater than, for example, that of amorphous SiO2 (silicic acid). The coefficient of thermal expansion of quartz is generally considered to be greater than 10 × 10⁻⁶. -6 / K, for example, in 12 to 16*10 -6 This is between / K. This is significantly greater than the coefficient of thermal expansion of corundum, which is approximately 8*10. -6 / K. The inventors believe that the greater coefficient of thermal expansion of corundum compared to quartz may result in a significant difference in the coefficients of thermal expansion between the base material and the matrix, which could lead to a reduction in the strength of foam ceramics, as shown in so-called powdering / snowing.

[0094] For example, as has already been shown, as will be demonstrated below using... Figures 1 to 3 As further explained by the diffraction pattern of the selected foam ceramic, in ordinary phosphate-bonded foam ceramics, there is more cristobalite than quartz. In other words, the high-elongation material dominates in the matrix. Although conventional foam ceramics are quite robust, some particle expulsion still occurs. This can be reduced, at least by using foam ceramics according to embodiments of the invention.

[0095] In contrast, silicate-bonded foam ceramics have a phase content of... Figure 2 For example, it shows a significantly increased quartz proportion compared to cristobalite. However, this foam ceramic contains very little corundum, and especially kyanite and boron mullite as the main crystalline phases. This is disadvantageous in terms of the chemical resistance of foam ceramics to aluminum melts, etc. Given the use of silica as a raw material, and considering the high amorphous content (visible as an elevated background at low 2θ values, also known as "amorphous bumps"), it can be further assumed that a high proportion of amorphous SiO2 should be present. This material has a very low coefficient of thermal expansion. These silicate-bonded foam ceramics have high particle expulsion, meaning only low strength. It is speculated that this is especially or possibly due to the base material (approximately 7*10). -6 The unfavorable ratio of the coefficient of thermal expansion of the matrix (which may have a high content of low elongation components, such as amorphous SiO2) to the coefficient of thermal expansion of the matrix (K) and the matrix (which may have a high content of low elongation components, such as amorphous SiO2).

[0096] Within the scope of this invention, the composition of foam ceramics is understood to be the solid phase contained within the foam ceramics, such as a glassy phase or a crystalline phase.

[0097] According to another embodiment, the linear thermal expansion coefficient of the foamed ceramic is at least 7*10. -6 / K, preferably at least 7.5*10 -6 / K, preferably a maximum of 9*10 -6 / K, preferably up to 8.5*10 -6 / K.

[0098] The present invention also relates to a method for producing foamed ceramics, preferably according to embodiments of the present invention, comprising the following steps:

[0099] - Provides a preferred aqueous slurry comprising starting materials containing Al2O3 and starting materials containing SiO2 and / or B2O3 and / or P2O5 and / or Li2O and / or CaO.

[0100] - Open-cell foam, especially open-cell polymer foam, is impregnated with a slurry to obtain a slurry-coated foam. This can preferably be carried out during the impregnation process in a rolling mill, for example, using structured rolls. This is particularly advantageous because it allows for highly uniform impregnation of the foam. The uniform impregnation of the foam with a ceramic-forming material advantageously contributes to the formation of mechanically stable foam ceramics.

[0101] - Drying the foam yields the green body of the foamed ceramic. Drying can be carried out at a high temperature, for example, around 100°C. This allows the moisture present in the slurry to evaporate quickly. Temperatures above 100°C can also be chosen, but not too high, to avoid denaturation of the plastic foam. Drying temperatures below 140°C are particularly advantageous in this case.

[0102] - Preferably, the dried filter blank is coated, i.e., a viscous, sprayable slurry is sprayed onto the dried filter blank.

[0103] -Preferred method: burn-off polymer foam, and

[0104] - Sintering green bodies to obtain foamed ceramics can be carried out at a temperature between 850°C and 1300°C for a duration preferably at least one hour and preferably at most four hours. The heating and cooling times during the sintering process can preferably be at least 10 minutes and at most 100 minutes, respectively.

[0105] In the context of this disclosure, a slurry is understood as a mixture comprising one or more powdered starting materials and a liquid phase, particularly an aqueous phase, for the production of ceramics. A slurry may also be referred to as a paste, suspension, or liquid.

[0106] Within the scope of this invention, green body is understood as an unfired blank. Green body can be particularly understood as a blank obtained by slip casting (Schlickerguss) and bonded by an adhesive, wherein the adhesive can be particularly organic adhesive.

[0107] Silicate glass can be understood as an amorphous material obtained through a melting process, which includes SiO2 as a network-forming agent.

[0108] Borate glass can be understood as an amorphous material obtained through a melting process, which includes B2O3 as a web-forming agent.

[0109] Glasses that typically include SiO2 and B2O3 as network-forming agents are called borosilicate glasses. ) or borosilicate glass (Borsilikatg) ).

[0110] The term "network forming agent" is understood in the theoretical sense of Zachariasen within the scope of this invention.

[0111] According to another embodiment, the slurry comprises silicate glass or borosilicate glass, preferably borosilicate glass.

[0112] This is particularly advantageous because, in this way, B2O3, as a component of foam ceramics, as described above, lowers the firing temperature but avoids the use of starting materials that could cause the slurry to gel and thus lead to unfavorable rheological effects, such as a strong increase in the viscosity of the slurry.

[0113] Within the scope of this invention, glass frit is particularly understood as a powdered glass material obtained through a melting process and a final quenching of the liquid melt, followed by a crushing process, such as grinding. This glass powder can be used, in particular, as a binder, for example, in enamel pigments, or as a solder glass for producing connections between components to be joined.

[0114] Using glass frit as a starting material for the ceramic matrix can also be advantageous because the processes already carried out during the manufacture of the glass frit have resulted in a close mixing of the glass frit components at the molecular level, i.e., through the melting of the glass melt as a starting material and subsequent melt refining and homogenization. In other words, by using glass frit, such as molten glass, the occurrence of significant inhomogeneities in foam ceramics, such as in the foam ceramic matrix, can be reduced at least. This is because in molten glass, the homogenization and uniform distribution of the various components of the batch used for glass production have already occurred during the melting process.

[0115] According to a further embodiment, the slurry comprises a lithium-containing starting material, preferably not or not containing lithium-containing chain silicates, especially lithium-containing island silicates such as nepheline, or lithium-containing layered silicates such as petalite, or especially inorganic non-silicate lithium compounds, for example, mixed oxides comprising lithium oxide and at least one other metal oxide, such as lithium aluminum spinel, or lithium-containing salts, such as lithium carbonate, or lithium-containing fluxes, such as lithium-containing glass fluxes or lithium-containing glass frits, wherein the glass can be formed as silicate glass, for example, it can also be formed as borosilicate glass, wherein preferably the lithium-containing starting material is formed in such a way that it is free of fluorine except for unavoidable trace amounts, i.e., based on a weight content of at most 500 ppm, preferably less, and particularly preferably lithium-containing layered silicates and / or lithium-containing glass.

[0116] This is particularly advantageous for the production of lithium-containing foam ceramics. Preferably, the lithium-containing starting material is formed as layered silicates and / or lithium-containing glasses. In other words, it is possible, and may even be preferred, for the slurry to include more than one lithium-containing starting material.

[0117] Adding lithium-containing starting materials is particularly advantageous for producing foam ceramics suitable for reducing bubbles in non-ferrous metal molten castings.

[0118] The use of lithium-containing layered silicates and / or lithium-containing glasses is advantageous because it prevents the formation of phases with unfavorable coefficients of thermal expansion and / or phase transitions, such as spodumene, within the application temperature range. In this way, the formation of lithium-containing crystalline phases is particularly reduced. This can be understood as the lithium-containing crystalline phase not being identifiable in X-ray diffraction patterns. However, it is not excluded that lithium may at least partially form mixed crystals with other crystalline phases contained in the foam ceramic. However, this is not visible in the diffraction patterns. The inventors believe, however, that lithium exists primarily as a component of the amorphous matrix. This is at least confirmed by X-ray studies, in which foam ceramics with different compositions, particularly those related to the matrix, were investigated and compared. For example, while conventional phosphate-bonded foam ceramics have a very low amorphous content, the amorphous content of Li₂O-containing foam ceramics is increased. The corresponding X-ray diffraction patterns are shown in the figures below.

[0119] According to a further embodiment, the slurry does not include B2O3-containing starting materials, such as borate glass, boron oxide, and / or boric acid. In this case, B2O3-containing starting materials are generally understood to be starting materials in which boron or B2O3 is a fundamental component, i.e., not merely present as trace amounts and / or as unavoidable impurities. Starting materials are considered B2O3-containing starting materials if the weight of B2O3 in the starting material exceeds 1%.

[0120] The slurry may include other starting materials.

[0121] To selectively adjust the rheological properties of the slurry, the slurry may include one or more further substances. For example, the slurry may include clay minerals, such as bentonite, to adjust the rheological properties. Additives, such as liquefying agents, may also be added to improve the processability of the slurry. Such additives are known to those skilled in the art. They typically form organic compounds and decompose during firing.

[0122] To ensure sufficient mechanical stability of the green body, the slurry may also include further starting materials, such as organic and / or polymer binders, which can ensure sufficient green body strength in the unfired green body.

[0123] In addition, the slurry may include additives that improve the processing characteristics of the slurry, such as defoamers and / or degassing agents and / or additives that improve solid wetting.

[0124] The present invention also relates to a filter for filtering non-ferrous metal melts, especially light metal melts, preferably aluminum-containing melts, comprising foam ceramics according to the above embodiments and / or produced or producible by a method according to one of the aforementioned embodiments. Detailed Implementation

[0125] The invention will be further explained through examples below.

[0126] Example 1

[0127] The table below provides exemplary compositions of slurries used in the production of silicate-bonded foam ceramics, in weight percent:

[0128]

[0129] The borate glass material here preferably has the following composition, in units of weight % based on oxides:

[0130]

[0131] However, other compositions of the glass frit are also possible. For example, a glass frit without SiO2 can also be used. However, such a glass frit, for example, containing only CaO, Al2O2, and B2O3 as components, tends to be highly corrosive and therefore not well processed. Therefore, it is advantageous if the borate glass frit is formed into a silicate-borate glass frit. This also improves the processability of the slurry.

[0132] Using such a slurry, foam ceramics with the following composition can be obtained (data in weight %):

[0133] Main materials 54％ <![CDATA[Calcined bauxite (Al2O3)]]> auxiliary materials 20％ petalite adhesives 9％ <![CDATA[SiO2]]> adhesives 7％ borate glass adhesives 6％ <![CDATA[B2O3]]> Rheological additives 4％ Bentonite

[0134] Calcined bauxite exists chemically as Al₂O₃, specifically as a modified form of corundum (α-Al₂O₃). Surprisingly, the presence of petalite itself is no longer detectable in X-ray diffraction. The inventors believe that during calcination, petalite transforms in such a way that it becomes part of an amorphous matrix. For example, it may constitute a lithium-containing matrix in this manner. Bentonite also transforms during calcination and becomes a component of the matrix.

[0135] An exemplary chemical composition, expressed in weight percent based on oxides, of the non-phosphate-bonded foamed ceramic obtained using the slurry according to the above composition is as follows:

[0136]

[0137]

[0138] The composition was determined by RFA and ICP analysis, calculated on the heated material. The contents of B2O3 and Li2O were determined by ICP-OES.

[0139] Example 1 relates to a foamed ceramic obtained from a slurry containing a B2O3-containing starting material; however, it contains only trace amounts of B2O3. The inventors therefore believe that a foamed ceramic of this composition could also be obtained from a slurry containing no B2O3-containing starting material. However, using a B2O3-containing starting material may offer advantages in the establishment and / or manufacturing process of the resulting foamed ceramic.

[0140] Example 2

[0141] The table below provides another exemplary composition of the slurry used to produce phosphate-bonded foamed ceramics:

[0142]

[0143] Through such a slurry, foamed ceramics with the following composition can be obtained:

[0144]

[0145]

[0146] During the firing process, bentonite undergoes a transformation and reacts with phosphates to form aluminum phosphate. Similarly, petalite itself can no longer be detected by X-ray diffraction.

[0147] An exemplary chemical composition, expressed in weight percent based on oxides, of the foamed ceramic obtained using the slurry according to the above composition is as follows:

[0148] Components Content in weight % <![CDATA[Al2O3]]> 81.0 <![CDATA[SiO2]]> 10.9 <![CDATA[TiO2]]> 0.01 <![CDATA[Fe2O3]]> 0.14 CaO 0.14 <![CDATA[K2O]]> 0.15 MgO 0.02 MnO <0.01 <![CDATA[Na2O]]> 0.48 <![CDATA[Li2O]]> 0.54 <![CDATA[Cr2O3]]> <0.01 <![CDATA[P2O5]]> 6.54 <![CDATA[SO3]]> <0.01 ZnO 0.01 <![CDATA[ZrO2]]> <0.01 Total 100 Heat loss 0.14

[0149] The composition was determined by RFA, calculated on heated material.

[0150] Example 3

[0151] The table below provides an example of a slurry for another type of foamed ceramic according to one embodiment, which comprises B2O3, Li2O, and SiO2:

[0152]

[0153]

[0154] Various casting tests were conducted using a filter according to an embodiment of the invention, during which the hydrogen content in the aluminum melt before and after the filter was determined. In particular, this was performed using a lithium-containing filter, designed to absorb hydrogen. The results of this absorption are briefly summarized.

[0155] as follows:

[0156] At the start of casting, the hydrogen content before the filter was 0.502 ml / 100g aluminum, and after the filter chamber it was 0.321 ml / 100g aluminum. Near the end of casting, the content before the filter chamber was 0.474 ml / 100g aluminum, and after the filter chamber it was 0.343 ml / 100g aluminum. The casting time was approximately 130 minutes, with a throughput of 26 kg / minute. The alloy being cast was alloy 5083 (Mg 4.5).

[0157] Therefore, the reduction in hydrogen achieved by using a lithium-ion filter is on the same order of magnitude as that achieved by using a degasser (see also the discussion above of the results of Chenisola et al.). Attached Figure Description

[0158] The invention will now be further explained with reference to the accompanying drawings. In the drawings:

[0159] Figures 1 to 3 XRD images of different foam ceramics are shown, and

[0160] Figure 4 The dilatometer curves of different foam ceramics are shown.

[0161] Figure 5 A schematic diagram of a cross-section of foam ceramic is shown.

[0162] Figure 1The first XRD image of a conventional phosphate-bonded foam ceramic is shown. Quantitative assessment of the diffraction pattern reveals a phase content of 90.5 vol% α-Al₂O₃ (corundum), 6.4 vol% SiO₂ (cristobalite), 2.8 vol% AlPO₄, and 0.2 vol% SiO₂ (quartz). The phase content of the amorphous material is very low here, estimated by the formation of a high background at X-ray reflections in the angular range of approximately 2θ to 30°.

[0163] Figure 2 XRD images of a commercially available SiO2-bonded filter material are shown. Evaluation results show that this material comprises only 5.8 vol% Al2O3 in the form of corundum and 0.4 vol% SiO2 in the form of cristobalite. Since this is a non-phosphate-bonded filter material, AlPO4 is also undetectable. The cristobalite SiO2 content is 6.7 vol%, which is increased compared to phosphate-bonded foam ceramics. Furthermore, the foam ceramic comprises 34.2 vol% Al2SiO5 in the form of kyanite and 52.9 vol% of a presumed composition of Al... 4.5 Si 0.9 B 0.9 O 9.4 Boron murylite. In addition to the very different phase content compared to traditional phosphate-bonded foam ceramics, and the presence of phases not found in phosphate-bonded foam ceramics, it can be noted that the content of amorphous material is also significantly increased (visible "amorphous protrusions", especially in the angular range of 16° to 30°2θ).

[0164] Figure 3 An XRD image of a foam ceramic corresponding to Example 2 of the present invention is shown. This is a foam ceramic comprising P2O5 and Li2O. Evaluation of the diffraction pattern shows a phase content of 90.4 vol% Al2O3, 1.6 vol% SiO2 (cristobalite), 6.9 vol% AlPO4, and 1.1 vol% SiO2 (quartz). Figure 1 Compared to the diffraction pattern, the background of the diffraction pattern is slightly higher within an angular range of up to 30°2θ. Therefore, the content of amorphous phase here is somewhat higher than that of conventional phosphate-bonded foam ceramics. However, the phase content transition is shown in the form that the material according to one embodiment of the invention has less kyanite, but slightly more quartz and significantly more crystalline AlPO4. Surprisingly, no crystalline phase containing Li2O is detected in the diffraction pattern. The inventors believe that Li2O is present as a component of the increased amorphous phase compared to conventional phosphate-bonded foam ceramics. Such filter materials exhibit particularly good strength, especially with a further reduction in snowfall compared to conventional phosphate-bonded foam ceramics.

[0165] Equally surprising is that, despite the high AlPO4 phase content detectable in this material crystallographically, it exhibits smaller volumetric transitions during production compared to conventional phosphate-bonded filter materials. This is particularly surprising because such volumetric transitions, typically around 2-3%, are due to the transformation of beryl or AlPO4 at around 200°C. Figure 4 As can be seen in the image, it shows a traditional phosphate-bonded foam ceramic (corresponding to...) Figure 1 The dilatation curves of the foam ceramics characterized in this paper (obtained according to DIN 51045-1:2005-08 and DIN 51045-2:2009-04, where the heating rate is 10 K / min, which differs from the standard) are denoted as 1 here. , and the dilatation curves of the foam ceramics according to one embodiment, which are related to the phase content... Figure 3 Corresponding to the foamed ceramic characterized in the figure, denoted here as 2.), the volumetric transition at approximately 200°C, indicating a phase change at 200°C, is significantly reduced in the foamed ceramic 2.) according to one embodiment.

[0166] The reasons for this are not entirely clear. However, the inventors believe it may be due to the chemical composition of the matrix, particularly, or it may be due to the matrix of the foam ceramic according to the invention having a greater content of amorphous phase than conventional phosphate-bonded foam ceramics. However, it seems that not only the presence of an amorphous phase is important, but also a suitable chemical composition. This is because the lower volumetric transitions in the foam ceramic according to the embodiment lead to improved strength, which is also reflected in the lower degree of pulverization of the foam ceramic, etc. Although Figure 2 The non-phosphate-bonded foam ceramics shown also have amorphous phases, especially compared to... Figure 3 or Figure 4 Curve 2 shows a higher proportion of foam ceramics. However, such foam ceramics are characterized by their rather low strength, which is also reflected in strong particle expulsion. It is precisely through the appropriate composition of the foam ceramic, especially the appropriate composition of the matrix, and the combined effect of the appropriate formation of crystalline phases, that the advantageous properties of the foam ceramics according to the embodiment are achieved.

[0167] Figure 5 A schematic cross-sectional view of a foam ceramic 3 according to one embodiment is shown. The foam ceramic 3 includes a solid phase 4 and pores 5.

Claims

1. Foam ceramics (3), which include - Containing at least 67% by weight and up to 95% by weight of Al2O3 and base materials containing Li2O, and - Containing at least 5% and at most 25% by weight of SiO2 and / or at least 0.1% and at most 5% or at most 500 ppm of B2O3 and / or 0 to 10% by weight of P2O5, and a matrix containing Li2O and CaO. The coefficients of thermal expansion of the base material and the matrix differ by a maximum of 6*10. -6 / K, The foam ceramic (3) therein comprises at least 0.3% by weight and up to 5% by weight of Li2O, and at least 0.1% by weight and up to 20% by weight of CaO.

2. The foam ceramic (3) according to claim 1, wherein the coefficients of thermal expansion of the base material and the matrix differ by a maximum of 5*10. -6 / K.

3. The foam ceramic (3) according to claim 1, wherein the coefficients of thermal expansion of the base material and the matrix differ by a maximum of 4*10. -6 / K.

4. The foam ceramic (3) according to claim 1, wherein the coefficients of thermal expansion of the base material and the matrix differ by a maximum of 3*10. -6 / K.

5. The foam ceramic (3) according to claim 1, wherein the coefficients of thermal expansion of the base material and the matrix differ by a maximum of 2*10. -6 / K.

6. The foam ceramic (3) according to claim 1. The foam ceramic (3) therein comprises more than 15% by weight of SiO2 and up to 25% by weight of SiO2.

7. The foam ceramic (3) according to claim 6, wherein the foam ceramic (3) comprises more than 18% by weight of SiO2.

8. The foam ceramic (3) according to claim 6, wherein the foam ceramic (3) comprises more than 19% by weight of SiO2.

9. The foam ceramic (3) according to claim 6, wherein the foam ceramic (3) comprises more than 20% by weight of SiO2.

10. The foam ceramic (3) according to claim 6. The B2O3 content of the foam ceramic (3) is at most 500 ppm by weight.

11. The foam ceramic (3) according to any one of claims 1 to 10, wherein the Li2O content of the foam ceramic (3) is at most 0.5% by weight.

12. The foam ceramic (3) according to any one of claims 1 to 10, comprising at least 75% by weight and at most 95% by weight of Al2O3.

13. The foam ceramic (3) according to any one of claims 1 to 10, comprising at least 10% by weight of SiO2 and at most 25% by weight of SiO2.

14. The foam ceramic (3) according to any one of claims 1 to 10. The foam ceramic (3) therein contains no P2O5 except for unavoidable trace amounts. or The foam ceramic (3) is designed as a phosphate-bonded foam ceramic, wherein the P2O5 content of the foam ceramic (3) is at most 10% by weight and at least 5% by weight.

15. The foam ceramic (3) according to any one of claims 1 to 10, wherein the base material comprises α-Al2O3.

16. The foam ceramic (3) according to any one of claims 1 to 10, wherein the matrix is ​​at least partially formed as glass.

17. The foam ceramic (3) according to any one of claims 1 to 10, wherein the base material exists in particulate form.

18. The foam ceramic (3) according to any one of claims 1 to 10, wherein the matrix comprises lithium-containing silicate glass and / or lithium-containing borate glass.

19. The foam ceramic (3) according to claim 18, wherein the matrix comprises lithium borosilicate glass.

20. The foam ceramic (3) according to any one of claims 1 to 10, comprising the following components, in weight percent: Al2O3 67 to 95 Li₂O 0.3 to 0.5 SiO2 5 to 25 B2O3 0.1 to 5, or The B2O3 content is based on a maximum of 500 ppm by weight. CaO 0.1 to 10 P2O5 0 to 10.

21. The foam ceramic (3) according to claim 20, comprising 75 to 95% by weight of Al2O3.

22. The foam ceramic (3) according to claim 20, comprising 10 to 25% by weight of SiO2.

23. The foam ceramic (3) according to claim 20, comprising 0.1 to 5% by weight of B2O3.

24. The foam ceramic (3) according to claim 20, comprising 0.3 to 1.5% by weight of B2O3.

25. The foam ceramic (3) according to claim 20, comprising 0.1 to 2% CaO by weight.

26. The foam ceramic (3) according to claim 20, comprising P2O5 at a maximum of 7% by weight.

27. The foam ceramic (3) according to claim 20, comprising P2O5 at a maximum of 5% by weight.

28. The foam ceramic (3) according to any one of claims 1 to 10, comprising the following components, in volume percent, based on solids percentage: α-Al₂O₃ 85 to 95 Quartz 0.8 to 2 Hematite 0 to 2.

29. The foam ceramic (3) according to any one of claims 1 to 10, wherein its linear coefficient of thermal expansion is at least 7*10. -6 / K, maximum 9*10 -6 / K.

30. The foam ceramic (3) according to claim 29, wherein its linear coefficient of thermal expansion is at least 7.5 × 10⁻⁶. -6 / K, maximum 8.5*10 -6 / K.

31. A method for producing foam ceramic (3) according to any one of claims 1 to 30, comprising the following steps - An aqueous slurry is provided, comprising starting materials containing Al2O3 and starting materials containing SiO2 and / or B2O3 and / or P2O5, as well as Li2O and CaO, wherein the slurry includes lithium-containing starting materials that are not or do not include lithium-containing chain silicate starting materials. - Impregnate open-cell polymer foam with slurry to obtain slurry-coated open-cell polymer foam. - Dry the foam to obtain a green body of foam ceramic. - Apply a coating to the dried filter blank by spraying a viscous, sprayable slurry onto the dried filter blank. - Completely scorched open-cell polymer foam, and - Sintering green bodies to obtain foam ceramics (3).

32. The method according to claim 31, wherein, The slurry includes a glass frit, wherein the glass frit includes Li2O as a component.

33. The method according to claim 32, wherein, The slurry includes silicate glass or borate glass material.

34. The method according to claim 33, wherein, The slurry includes borosilicate glass frit.

35. The method according to any one of claims 31 to 34, wherein the lithium-containing starting material is a lithium-containing island silicate, or a lithium-containing layered silicate, or an inorganic non-silicate lithium compound, or a lithium-containing salt or a lithium-containing flux, or a lithium-containing glass frit, wherein the glass can be formed as a silicate glass or as a borosilicate glass, wherein... The lithium-containing starting material is formed in such a way that it contains no fluorine except for unavoidable trace amounts, i.e., at most 500 ppm by weight.

36. The method of claim 35, wherein the lithium-containing starting material is a mixed oxide comprising lithium oxide and at least one other metal oxide.

37. The method of claim 36, wherein the lithium-containing starting material is a lithium-containing layered silicate and / or a lithium-containing glass.

38. A filter for filtering non-ferrous metal melts, comprising foam ceramic according to any one of claims 1 to 30.

39. A filter for filtering non-ferrous metal melts, comprising foam ceramic produced by the method according to any one of claims 31 to 37.

40. The filter according to claim 38 or 39, used for filtering light metal melts.

41. The filter according to claim 40, used for filtering aluminum-containing melt.

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