Inorganic coated sand in a dry state, method for producing inorganic coated sand in a dry state, and method for producing a mold

Abstract

The method for manufacturing inorganic coated sand is a method for manufacturing dry inorganic coated sand having refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate. It includes a step of mixing refractory aggregate, inorganic coated sand for crystallization, and liquid metasilicate hydrate, wherein the amount of inorganic coated sand for crystallization is 0.1% by mass or more and 15% by mass or less relative to the total amount of the dry inorganic coated sand.

Description

Technical Field

[0001] This invention relates to dry inorganic coated sand, a method for manufacturing dry inorganic coated sand, and a method for manufacturing casting molds. Background Technology

[0002] As a casting used in casting, a mold is known to be obtained by molding inorganic coated sand having refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate into a target shape.

[0003] As for the technology involving this kind of inorganic coated sand, for example, the technology described in Patent Document 1 (Japanese Patent Application Publication No. 2020-11296) and Patent Document 2 (Japanese Patent Publication No. 53-025803) can be cited.

[0004] Patent document 1 discloses an inorganic coated sand having refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate, wherein the inorganic binder layer, as a metasilicate hydrate, includes one or more selected from sodium metasilicate 5-hydrate or sodium metasilicate 9-hydrate.

[0005] Patent document 2 describes a method for making a casting mold, characterized in that a metasilicic acid alkaline solution prepared by adding caustic alkali to water glass is added to refractory particles such as silica sand and then mixed, or alcohols are further added during mixing to precipitate crystalline silicates and alkaline alkali onto the surface of the refractory particles such as silica sand. Then, micro-dust generated during the refining of mixed Fe-Si with SiO2 as the main component is added, and the resulting powder particles are heated together with sand to at least above the melting temperature of the crystalline silicates and alkaline alkali to solidify them.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2020-11296

[0009] Patent Document 2: Japanese Patent Publication No. 53-025803 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] According to the inventor's research, the conventional inorganic coated sand disclosed in Patent Documents 1 and 2 still has room for improvement in the following aspects: during the process of forming an inorganic binder layer on the refractory aggregate, the time required for the inorganic binder to solidify and the inorganic coated sand to dry on the surface of the refractory aggregate is shortened.

[0012] Methods for solving problems

[0013] Therefore, the inventors conducted in-depth research to shorten the drying time during the manufacturing of inorganic coated sand. The new research focuses on using inorganic coated sand with an inorganic binder layer already formed on the surface of refractory aggregates as a crystallization promoter for the inorganic binder (for crystallization of the inorganic binder). Furthermore, further research revealed that by mixing a specific amount of crystallizing inorganic coated sand into the refractory aggregate during the formation of the inorganic binder layer, the crystallization of the inorganic binder can be efficiently promoted, allowing an inorganic binder layer to form on the surface of the refractory aggregate in a short time, thus completing this invention.

[0014] According to the present invention, a method for manufacturing dry inorganic coated sand can be provided, which is a method for manufacturing dry inorganic coated sand having refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate.

[0015] The above manufacturing method includes a step of mixing refractory aggregate, inorganic coated sand for crystallization, and liquid metasilicate hydrate.

[0016] The amount of inorganic coated sand used for crystallization is 0.1% by mass or more and 15% by mass or less relative to the total amount of the dry inorganic coated sand.

[0017] Furthermore, according to the present invention, a method for manufacturing a mold using dry inorganic coated sand obtained by the above-described method for manufacturing dry inorganic coated sand can be provided.

[0018] Furthermore, according to the present invention, a dry inorganic coated sand can be provided, which has refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate.

[0019] The particle group of the above-mentioned dry inorganic coated sand contains particles with a multi-layer structure of the above-mentioned inorganic binder layer.

[0020] Invention Effects

[0021] According to the present invention, a method for manufacturing dry inorganic coated sand can be provided, which can shorten the drying time during the manufacturing of inorganic coated sand. Detailed Implementation

[0022] The embodiments of the present invention will now be described. Furthermore, in this specification, unless otherwise specified, "A to B" indicating a numerical range refers to a range greater than A and less than B, including values ​​at both ends. Additionally, the components and elements described in each embodiment may be appropriately combined as long as they do not impair the effect of the invention.

[0023] It should be noted that the so-called dry inorganic coated sand in this application refers to inorganic coated sand with room temperature fluidity. More specifically, it means coated sand that can be measured when determining the dynamic angle of repose regardless of its moisture content.

[0024] <Method for manufacturing dry inorganic coated sand>

[0025] The method for manufacturing dry inorganic coated sand according to this embodiment is a method for manufacturing dry inorganic coated sand having refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate.

[0026] It includes a process of mixing refractory aggregate, inorganic coated sand for crystallization, and liquid metasilicate hydrate, wherein the amount of inorganic coated sand for crystallization is 0.1% by mass or more and 15% by mass or less relative to the total amount of the dry inorganic coated sand (the amount of inorganic coated sand obtained after the final process).

[0027] This allows for a reduction in the drying time during the manufacturing of dry inorganic coated sand.

[0028] Although the details of the reasons are unclear, they can be speculated as follows.

[0029] First, when forming an inorganic binder layer on the surface of refractory aggregate, it is necessary to solidify the inorganic binder on the surface of the refractory aggregate. Therefore, it can be considered that in this embodiment, by mixing an appropriate amount of crystallizing inorganic coated sand into the particle group of refractory aggregate, the crystallizing inorganic coated sand acts as a crystal nucleus, which can promote the crystallization of metasilicate hydrate (inorganic binder).

[0030] Furthermore, although it can be predicted that the strength of the mold using the inorganic coated sand will decrease due to the presence of particles other than the inorganic coated sand mixed in the particle group of the dry inorganic coated sand, in this embodiment, since the inorganic coated sand itself is mixed in as a crystallization promoter, good mold strength can be maintained.

[0031] In addition, although the method of directly adding metasilicate hydrate crystals as crystal nuclei can also be considered, compared with adding metasilicate hydrate crystals, using inorganic coated sand for crystallization can add finer crystals formed on the sand surface as crystal nuclei, thus further promoting the crystallization of metasilicate hydrates.

[0032] The details are explained below.

[0033] (Mixed process)

[0034] First, prepare inorganic coated sand for crystallization. Known inorganic coated sands can be used for crystallization; however, from the viewpoint of facilitating crystallization, sand containing metasilicate hydrates is preferred.

[0035] More specifically, the inorganic coated sand for crystallization can use known inorganic coated sand, which is inorganic coated sand in which the surface of the refractory aggregate is coated with an inorganic binder layer. Furthermore, the inorganic binder layer is obtained using an inorganic binder containing metasilicate hydrates. Details regarding the refractory aggregate and the inorganic binder layer are the same as those described later regarding the refractory aggregate and inorganic binder layer in the context of dry inorganic coated sand.

[0036] Alternatively, pre-manufactured inorganic coated sand remaining in the mixer can be used directly as inorganic coated sand for crystallization. This allows for the direct use of the same mixer, thereby improving continuous production capacity.

[0037] Then, refractory aggregate, inorganic coated sand for crystallization, and liquid metasilicate hydrate are mixed to obtain a mixture.

[0038] Regarding the amount of inorganic coated sand used for crystallization, from the viewpoint of easily promoting crystallization, it is 0.1% by mass or more, preferably 0.2% by mass or more, relative to the total amount of the final dry inorganic coated sand. From the viewpoint of reducing the amount of agglomeration (the inorganic coated sand agglomerates into large lumps), it is more preferably 0.4% by mass or more.

[0039] On the other hand, regarding the amount of inorganic coated sand for crystallization, from the viewpoint of easily promoting crystallization, it is 15% by mass or less, preferably 12% by mass or less, and more preferably 10% by mass or less, relative to the total amount of the finally obtained dry inorganic coated sand.

[0040] Regarding the amount of inorganic coated sand for crystallization, from the viewpoint of easily promoting crystallization, it is preferably 0.2% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more, relative to the refractory aggregate mixed together in this mixing process.

[0041] On the other hand, regarding the amount of inorganic coated sand for crystallization, from the viewpoint of easily promoting crystallization, it is preferably 15% by mass or less, and more preferably 12% by mass or less, relative to the refractory aggregate mixed together in this mixing process.

[0042] The process of obtaining the mixture may include the following steps (i) or (ii). In either case, crystallization may be promoted using inorganic coated sand for crystallization.

[0043] (i) The process includes, in sequence, the step of mixing refractory aggregate and crystallizing inorganic coated sand to obtain a mixture (hereinafter also referred to as "sand mixture"), and the step of further mixing the obtained mixture with liquid metasilicate hydrate.

[0044] (ii) The process includes, in sequence, a step of mixing refractory aggregate and liquid metasilicate hydrate to obtain a mixture, and a step of further mixing the obtained mixture with crystallizing inorganic coated sand (post-mixing step).

[0045] In the case of (i) above, the mixing conditions such as stirring speed and processing time when mixing refractory aggregate and inorganic coated sand for crystallization can be appropriately determined according to the processing volume of the mixture, for example, the following conditions can be set.

[0046] Regarding mixing time, it can be set to more than 5 seconds from the perspective of thorough mixing, or less than 180 seconds from the perspective of shortening manufacturing time.

[0047] Furthermore, in the case described in (i) above, regarding the temperature of the refractory aggregate during mixing and the temperature of the inorganic coated sand for crystallization, from the viewpoint of shortening the drying time, it is preferable to set them below the melting temperature of the inorganic binder layer present in the inorganic coated sand for crystallization. That is, in order for the inorganic binder layer present in the inorganic coated sand for crystallization to function as a crystallization promoter, it is crucial that the inorganic binder layer does not melt.

[0048] For example, when the inorganic binder layer of the inorganic coated sand for crystallization is formed by sodium metasilicate 9 hydrate, the temperature of the refractory aggregate and the temperature of the inorganic coated sand for crystallization are preferably set to be 47°C lower than the melting temperature of the sodium metasilicate 9 hydrate.

[0049] In addition, under the above (i) case, the particle group of refractory aggregate can be mixed with the particle group of inorganic coated sand for crystallization in one go, or the particle group of inorganic coated sand for crystallization can be added to the particle group of refractory aggregate in multiple batches.

[0050] In the case described in (ii), the mixing conditions, such as stirring speed and processing time, when mixing the refractory aggregate with the liquid metasilicate hydrate can be appropriately determined according to the amount of mixture to be processed, just as in the case described in (i).

[0051] Furthermore, in the case described in (ii) above, the particle group of inorganic coated sand for crystallization can be mixed all at once, or it can be added in multiple batches. Additionally, the mixing conditions during and after adding the inorganic coated sand for crystallization can be appropriately determined based on the amount of mixture to be processed, similar to the case described in (i) above.

[0052] In the cases (i) and (ii) above, when the average particle size (μm) of the refractory aggregate of the finally obtained dry inorganic coated sand is set as SA1 and the average particle size (μm) of the refractory aggregate of the crystallized inorganic coated sand is set as SA2, the average particle size ratio (SA2 / SA1) is preferably 1.6 or less, more preferably 1.4 or less, even more preferably 1.0 or less, and even more preferably 0.9 or less. Similarly, from the viewpoint of shortening the drying time, it is preferably 0.3 or more, and more preferably 0.4 or more.

[0053] In other words, a smaller average particle size ratio (SA2 / SA1) means that the average particle size of the inorganic coated sand for crystallization is smaller than the average particle size of the refractory aggregate of the final dry inorganic coated sand. It can be considered that by reducing the average particle size ratio (SA2 / SA1), it is easier to exert the function of the inorganic coated sand for crystallization as a crystallization promoter.

[0054] Furthermore, when the average particle size (μm) of the finally obtained dry inorganic coated sand is set as CS1 and the average particle size (μm) of the crystallized inorganic coated sand is set as CS2, for the same reasons as for refractory aggregates, the average particle size ratio (CS2 / CS1) is preferably 1.6 or less, more preferably 1.4 or less, even more preferably 1.0 or less, and even more preferably 0.9 or less. On the other hand, it is preferably 0.3 or more, and more preferably 0.4 or more.

[0055] The average particle size of the refractory aggregate was determined using the same method as that used for determining the average particle size of the inorganic coated sand, as described later.

[0056] By using liquid metasilicate hydrates, the surface of refractory aggregates can be coated with liquid metasilicate hydrates, forming an inorganic binder layer covering the surface of the refractory aggregates. "Liquid" can refer to a state that is both fluid and viscous.

[0057] Liquid metasilicate hydrates can be made into molten metasilicate hydrates by setting a temperature above (a) the melting temperature of metasilicate hydrates, or into (b) a mixture of water glass, caustic alkali and water in a specific ratio.

[0058] The temperature above the melting temperature of (a) metasilicate hydrates, specifically, can be set as 47°C to 100°C.

[0059] Furthermore, regarding (b), the mixture obtained by mixing water glass, caustic soda and water in a specific ratio refers to a mixture that remains in a liquid state even below the melting temperature of the metasilicate hydrate obtained from the mixture.

[0060] (Methosilicate hydrate)

[0061] Metasilicate hydrate is one of the components of the inorganic binder layer, and also one of the components of the aforementioned inorganic binder. Using metasilicate hydrate improves the crystallinity of the inorganic binder layer, and the inorganic coated sand becomes dry, exhibiting excellent fluidity at room temperature, making it preferable. Furthermore, by using metasilicate hydrate, an inorganic binder layer can be formed on the surface of the refractory aggregate in a water-insoluble state. That is, in the process of manufacturing inorganic coated sand, an aqueous solution of metasilicate hydrate is not required, thus eliminating the water removal process and simplifying the manufacturing method. Additionally, since the metasilicate in the inorganic binder layer is a hydrate, there is no need to circulate water vapor to solidify the mold, simplifying the equipment.

[0062] Alternatively, metasilicate hydrates can also be prepared by mixing water glass, caustic soda, and water in a specific ratio. Furthermore, the SiO2 / Na2O molar ratio of the metasilicate hydrate in this embodiment is 0.9 to 1.1.

[0063] As for water glass, specifically, one or more types selected from sodium silicate No. 1 to No. 5 can be cited. Here, sodium silicate is classified into No. 1 to No. 5 according to the molar ratio of SiO2 / Na2O, and the classification of sodium silicate No. 1 to No. 3 is specified by JIS-K-1408. The molar ratio of SiO2 / Na2O for each No. is as follows.

[0064] Sodium silicate No. 1: The molar ratio of SiO2 / Na2O is 2.0–2.3.

[0065] Sodium silicate No. 2: The molar ratio of SiO2 / Na2O is 2.4–2.6.

[0066] Sodium silicate No. 3: The molar ratio of SiO2 / Na2O is 2.8–3.3.

[0067] Sodium silicate No. 4: The molar ratio of SiO2 / Na2O is 3.3–3.5.

[0068] Sodium silicate No. 5: The molar ratio of SiO2 / Na2O is 3.6–3.8.

[0069] Alternatively, the molar ratio of SiO2 / Na2O can be adjusted to the desired level by mixing two or more types of sodium silicate.

[0070] The water glass is preferably selected from at least one of sodium silicate No. 1 and sodium silicate No. 2.

[0071] Salts of metasilicate hydrates can be alkali metals, and more preferably one or more selected from lithium, sodium, and potassium, more preferably at least one of sodium and potassium, and even more preferably sodium.

[0072] In the case of (i) above, as a method for mixing a sand mixture (a mixture of refractory aggregate and inorganic coated sand for crystallization) with liquid metasilicate hydrate, from the perspective of easily controlling the water content in the resulting inorganic binder layer and easily obtaining dry inorganic coated sand with excellent flowability, examples can be given such as (i)-1: a method of adding liquid metasilicate hydrate into a sand mixture with a melting temperature lower than that of metasilicate hydrate and mixing it, and (i)-2: a method of adding liquid metasilicate hydrate into a sand mixture with a melting temperature lower than that of metasilicate hydrate and mixing it, etc.

[0073] The temperature at which the liquid metasilicate hydrate is mixed is preferably 47°C or higher, and from the viewpoint of preventing water evaporation, it is preferably 100°C or lower, more preferably 90°C or lower, and even more preferably 80°C or lower.

[0074] Furthermore, the post-mixing process described above, which involves mixing the sand mixture with liquid metasilicate hydrate, is preferably carried out in such a way that the particles of the refractory aggregate, the inorganic coated sand for crystallization, and the resulting dry inorganic coated sand do not fuse together.

[0075] Regarding the amount of liquid metasilicate hydrate, from the viewpoint of obtaining a high-strength casting mold, it is, for example, 0.1 parts by mass or more, preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, further preferably 1 part by mass or more, and even more preferably 2 parts by mass or more, relative to 100 parts by mass of refractory aggregate.

[0076] Furthermore, regarding the amount of liquid metasilicate hydrate, from the viewpoint of obtaining a high-strength casting mold, it is, for example, 15 parts by mass or less, preferably 10 parts by mass or less, more preferably 8 parts by mass or less, further preferably 6 parts by mass or less, and even more preferably 5 parts by mass or less, relative to 100 parts by mass of refractory aggregate.

[0077] By forming an inorganic binder layer on the surface of refractory aggregate as described above, a dry inorganic coated sand with room-temperature fluidity can be obtained. Furthermore, the dry inorganic coated sand can contain particles with a multi-layered inorganic binder layer within the particle group.

[0078] It should be noted that in the above-mentioned post-mixing process, in order to reduce the fluidity of metasilicate hydrate and fix the metasilicate hydrate on the surface of refractory aggregate, it is also possible to cool to a temperature lower than the melting temperature of metasilicate hydrate.

[0079] Furthermore, from the viewpoint of improving mold strength, it is preferable to further sieve the recovered inorganic coated sand to remove agglomerates (clumps). Agglomerates (clumps) can include substances formed by the agglomeration of inorganic coated sand into lumps. As a sieve, a mesh size of 10 to 80 is preferred.

[0080] In this embodiment, the dry inorganic coated sand before agglomeration is removed is also referred to as untreated dry inorganic coated sand.

[0081] <Dry Inorganic Coated Sand>

[0082] The dry inorganic coated sand of this embodiment is a particle group consisting of dry inorganic coated sand having refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate.

[0083] In this embodiment, the dry inorganic coated sand contains particles with a multi-layered inorganic binder layer within the particle group. This multi-layered structure is formed by the multiple crystallization processes of the inorganic binder, and can be identified by differences in crystal orientation or by observing interfaces in the cross-section of the inorganic binder layer.

[0084] The inorganic binder layer has a multi-layered particle content that is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more, relative to the total amount of dry inorganic coated sand.

[0085] On the other hand, the inorganic binder layer has a multi-layered particle content that is preferably 15% by mass or less relative to the total amount of dry inorganic coated sand, more preferably 12% by mass or less, and even more preferably 10% by mass or less.

[0086] The following is a more detailed explanation of dry inorganic coated sand.

[0087] In this embodiment, dry inorganic coated sand refers to coated sand for which a measurement value can be obtained when determining the dynamic angle of repose. The dynamic angle of repose is preferably 80° or less, more preferably 45° or less, and even more preferably 30° or less.

[0088] The dynamic angle of repose of dry inorganic coated sand can be determined using the following methods.

[0089] (Method for determining the dynamic angle of repose)

[0090] Half the volume of inorganic coated sand was added to a cylindrical transparent plastic bottle (diameter: 7.7 cm, height: 16 cm). Using a bottle mixer, with the bottle's axis horizontal, the mixture was rotated at 60 rpm around the horizontal axis for 10 seconds. The slope of the inorganic coated sand layer flowing inside the bottle became a flat surface. The angle formed between this slope and the horizontal plane was measured. It should be noted that the condition is considered wet if the inorganic coated sand does not flow inside the bottle, or if it does flow but does not form a flat slope, in which case the dynamic angle of repose cannot be measured.

[0091] Dry inorganic coated sand is specifically composed of inorganic coated sand particle groups.

[0092] From the viewpoint of ensuring good fluidity and further enhancing its filling properties into the molding die, dry inorganic coated sand is preferably spherical. Here, "spherical" in the context of dry inorganic coated sand refers to a round shape similar to a sphere.

[0093] Regarding the sphericity of dry inorganic coated sand, from the viewpoints of improving fluidity, mold quality, and mold strength, as well as the ease of molding, a sphericity of 0.75 or higher is preferred, 0.80 or higher is more preferred, and 0.82 or higher is even more preferred. Furthermore, the upper limit for sphericity is specifically 1.

[0094] In this embodiment, the sphericity of the dry inorganic coated sand is specifically consistent with the sphericity of the refractory aggregate described later.

[0095] The sphericity of dry inorganic coated sand can be determined as follows: Image analysis is performed on the particle image (photograph) obtained using an optical microscope or digital oscilloscope (e.g., Keyence VH-8000 model). The area of ​​the particle's projected cross-section and the perimeter of that cross-section are then calculated. Finally, the sphericity is calculated as: [Sphericity = (Area of ​​particle projected cross-section (mm²))] / [Sphericity = (Area of ​​particle projected cross-section (mm²)] 2 The value is calculated by averaging the values ​​obtained for any 50 particles: circumference (mm) of a circle with the same area / circumference (mm) of the particle's projected cross section.

[0096] Regarding the average particle size of dry inorganic coated sand, from the viewpoints of improving mold quality and strength, ease of molding, and preservation stability, it is preferably 0.05 mm or more, and more preferably 0.1 mm or more. Furthermore, if the average particle size of the inorganic coated sand in several states is at or above the aforementioned lower limit, the amount of coating layer used during mold manufacturing can be reduced, thus making the regeneration of dry inorganic coated sand easier; this is also preferable from this perspective.

[0097] Regarding the average particle size of the dry inorganic coated sand, from the viewpoints of improving mold quality and strength, and the ease of molding, it is preferably 2 mm or less, more preferably 1 mm or less, and even more preferably 0.5 mm or less. Furthermore, if the average particle size of the inorganic coated sand in several states is below the aforementioned upper limit, the porosity decreases during mold manufacturing, and the mold strength is improved; this is also preferable from this perspective.

[0098] In this embodiment, the average particle size of the dry inorganic coated sand and the inorganic coated sand for crystallization can be specifically determined using the following method.

[0099] (Method for determining average particle size)

[0100] When the sphericity is 1 based on the particle's projected cross section, the diameter (mm) is measured. Conversely, when the sphericity is <1, the major axis diameter (mm) and minor axis diameter (mm) of randomly oriented particles are measured, and (major axis diameter + minor axis diameter) / 2 is calculated. The average value obtained for any 100 particles is then taken as the average particle size (mm). The major axis diameter and minor axis diameter are defined as follows: With the particle stabilized on a plane, the width of the particle whose projection onto the plane is sandwiched between two parallel lines is called the minor axis diameter. Conversely, the distance between the particle sandwiched between two parallel lines perpendicular to the minor axis diameter is called the major axis diameter.

[0101] The major and minor axis diameters of a particle can be determined by taking an image (photograph) of the particle using an optical microscope or a digital oscilloscope (such as the Keyence VH-8000 model) and performing image analysis on the resulting image.

[0102] The following describes the composition of dry inorganic coated sand.

[0103] [Refractory Aggregates]

[0104] Refractory aggregates are specifically composed of particle groups of refractory aggregates.

[0105] The material used as refractory aggregate is selected from one or more types of natural sand and manufactured sand.

[0106] As natural sand, examples include one or more of the following: silica sand, chromite sand, zircon sand, olivine sand, and alumina sand, which are mainly composed of quartz.

[0107] Examples of artificial sand include, for instance, one or more of the following: synthetic mullite sand, SiO2-based casting sand with SiO2 as the main component, Al2O3-based casting sand with Al2O3 as the main component, SiO2 / Al2O3-based casting sand, SiO2 / MgO-based casting sand, SiO2 / Al2O3 / ZrO2-based casting sand, SiO2 / Al2O3 / Fe2O3-based casting sand, and casting sand derived from slag. Here, "main component" refers to the most abundant component in the sand.

[0108] Artificial sand refers to casting sand that is not naturally produced, but rather sand that is artificially prepared by melting or sintering metal oxides.

[0109] Alternatively, recycled sand obtained by recycling used refractory aggregates, or recycled sand obtained by regenerating recycled sand, can also be used.

[0110] It should be noted that the contents of SiO2, Al2O3, Fe2O3, and other components in the refractory aggregate can be determined using the following fluorescence X-ray method. The refractory aggregate is adjusted to a size of approximately 0.1 μm or less using a vibratory mill and heated at 1050°C for 1 hour. Subsequently, 5 g of lithium tetraborate and 0.5 g of refractory aggregate are mixed and heated at 1200°C for 10 minutes to melt the mixture. After cooling, a glassy sample is prepared (glass bead method). The sample is analyzed using a ZSX Primus II fluorescence X-ray analyzer (RIGAKU Corporation) and the Fundamental Parameter (FP) method.

[0111] The sphericity of the refractory aggregate is consistent with that of the aforementioned dry inorganic coated sand. Specifically, regarding the sphericity of the refractory aggregate, from the viewpoints of fluidity, mold quality, improved mold strength, and ease of mold molding, a value of 0.75 or higher is preferred, more preferably 0.80 or higher, and even more preferably 0.82 or higher. Furthermore, the upper limit for sphericity is specifically 1.

[0112] The sphericity of refractory aggregates can be determined using the same method as that used for dry inorganic coated sand described above.

[0113] Regarding the average particle size of the refractory aggregate, from the viewpoints of improving mold quality and strength, and the ease of mold making, it is preferable to be 0.05 mm or more, and more preferably 0.1 mm or more. Furthermore, if the average particle size of the refractory aggregate is above the aforementioned lower limit, the amount of inorganic binder layer used as a coating layer can be reduced during mold manufacturing, thus making the regeneration of dry inorganic coated sand easier; this is also preferable from this perspective.

[0114] Regarding the average particle size of the refractory aggregate, from the viewpoints of improving mold quality and strength, and the ease of mold making, it is preferably 2 mm or less, more preferably 1 mm or less, and even more preferably 0.5 mm or less. Furthermore, if the average particle size of the refractory aggregate is below the aforementioned upper limit, the porosity decreases during mold manufacturing, and the mold strength is improved; this is also preferable from this perspective.

[0115] The average particle size of refractory aggregates can be determined using the same method as that used for dry inorganic coated sand described above.

[0116] Regarding the amorphism of refractory aggregates, from the viewpoints of making the aggregate surface smoother and further improving the mold strength, and from the viewpoints of obtaining low thermal expansion, it is preferably 20% or more, more preferably 30% or more, and even more preferably 40% or more.

[0117] There is no upper limit to the amorphousness of refractory aggregates; for example, it can be below 100% or below 99%.

[0118] The amorphism of refractory aggregates can be determined using the following X-ray diffraction method.

[0119] (X-ray diffraction method)

[0120] Refractory aggregates were pulverized in a mortar and pressed onto an X-ray glass support in a powder X-ray diffractometer for measurement. The powder X-ray diffractometer used was a Rigaku MultiFlex (source: CuKα rays, tube voltage: 40kV, tube current: 40mA) operating in the range of 2θ = 5–90° with a scan interval of 0.01°, a scan speed of 2° / min, and slits DS1, SS1, and RS0.3mm. In the range of 2θ = 10°–50°, the X-ray intensities on the low-angle and high-angle sides were connected by a straight line. The area under the line was taken as the background, and the crystallinity was calculated using the software provided with the equipment. This crystallinity was then subtracted from 100 to obtain the amorphousness. Specifically, for the area above the background, the amorphous peaks (halos) were separated from the crystalline components by curve fitting, and their respective areas were calculated. The amorphousness (%) was then calculated using the following formula.

[0121] Amorphousness (%) = (Area of ​​halo peak) / (Area of ​​crystalline component + Area of ​​halo peak) × 100

[0122] Various methods exist for controlling the amorphism of refractory aggregates; however, methods involving rapid cooling of the molten material are generally preferred. Examples include melting the raw material, agitating it with air, and then rapidly cooling it; or treating it in a flame and then rapidly cooling it. In either method, the cooling method can be appropriately selected based on the material and particle size at various rates. Alternatively, methods can be considered to amorphize temporarily crystallized refractory aggregates using heat treatment and cooling processes.

[0123] [Inorganic binder layer]

[0124] The inorganic binder layer is a layer formed on the surface of the refractory aggregate. In other words, the inorganic binder layer coats the surface of the refractory aggregate. It should be noted that this coating is not limited to a continuous process; there can also be localized discontinuous sections.

[0125] By utilizing an inorganic binder layer, molds can be formed in the form of dry inorganic coated sand.

[0126] Regarding the amount of inorganic binder layer contained in the dry inorganic coated sand, from the viewpoint of obtaining a high-strength casting mold, it is, for example, 0.1 parts by mass or more, preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, further preferably 1 part by mass or more, and even more preferably 2 parts by mass or more, relative to 100 parts by mass of refractory aggregate.

[0127] Furthermore, regarding the amount of inorganic binder layer contained in the dry inorganic coated sand, from the viewpoint of obtaining a high-strength casting mold, it is, for example, 15 parts by mass or less, preferably 10 parts by mass or less, more preferably 8 parts by mass or less, further preferably 6 parts by mass or less, and even more preferably 5 parts by mass or less, relative to 100 parts by mass of refractory aggregate.

[0128] Furthermore, the inorganic binder layer only needs to have a layer containing metasilicate hydrates; it can be a single layer or multiple layers. Additionally, the layer containing metasilicate hydrates is formed from an inorganic binder composition containing metasilicate hydrates.

[0129] Regarding the content of metasilicate in the inorganic binder layer, from the viewpoints of improving mold strength, excellent productivity, and ease of availability, it is preferably 80% by mass or more, more preferably 90% by mass or more, further preferably 95% by mass or more, even more preferably 98% by mass or more, and even more preferably substantially 100% by mass.

[0130] The content of metasilicate in inorganic binders refers to the content of metasilicate relative to all components in the inorganic binder layer except water.

[0131] Regarding the content of metasilicate (anhydrous equivalent) in the inorganic binder, from the viewpoint of ensuring good preservation stability of the dry inorganic coated sand, it is preferably 11% by mass or more, more preferably 40% by mass or more. On the other hand, from the viewpoint of improving the strength of the mold, it is preferably 58% by mass or less, more preferably 50% by mass or less.

[0132] As a method to confirm the presence of metasilicate hydrates in the inorganic binder layer, the following methods can be considered: Adding inorganic coated sand to a mill or other pulverizer to remove only the inorganic binder layer components, and then analyzing the inorganic binder layer components using XRD to confirm whether there are peaks showing the crystal structure of metasilicate hydrates; immersing the inorganic coated sand in water and stirring for a certain time to dissolve the inorganic binder layer components, drying the dissolved components, and analyzing the dried solid components using XRD to confirm whether there are peaks showing the crystal structure of metasilicates; simultaneously analyzing the amount of hydrated water using the following methods to confirm whether it is metasilicate hydrate.

[0133] <Determination of Hydration Quantity>

[0134] (1) Weigh 10g of inorganic coated sand containing additives such as amorphous SiO2 particles in a crucible after empty firing. Calculate the water content (%) in the inorganic coated sand by measuring the mass reduction (%) after 1 hour of exposure at 900°C (A).

[0135] A = [(M1-M2) / M3] × 100

[0136] (M1: Total mass of crucible and inorganic coated sand before firing (g), M2: Total mass of crucible and inorganic coated sand after firing (g), M3: Mass of inorganic coated sand before firing (g))

[0137] (2) Weigh 100g of inorganic coated sand before adding additives such as amorphous SiO2 particles, soak it in more than 200mL of water or hot water and stir for more than 1 hour to extract metasilicate hydrate. Filter the obtained extract to remove refractory aggregate, and then use a rotary evaporator to perform vacuum distillation at 40℃ and internal pressure below 15mmHg to remove water. Afterwards, heat and dry at 120℃~180℃ for 1~3 hours, and weigh the dried product. Calculate the dry solids content (%) of metasilicate hydrate in the inorganic coated sand (B).

[0138] B = (M12 / M11) × 100

[0139] (M11: Mass of inorganic coated sand (g), M12: Weight of dried product (g))

[0140] (3) The amount of water in metasilicate hydrate = [(A) / molecular weight of water] / [(B) / molecular weight of anhydrous metasilicate]

[0141] (other)

[0142] The inorganic binder layer may further contain components other than metasilicates, such as particles containing amorphous SiO2, inorganic particles other than particles containing amorphous SiO2, humectants, moisture-resistant enhancers, coupling agents for binding the reinforced refractory aggregate and the inorganic binder composition, lubricants, surfactants, release agents, etc.

[0143] From the perspective of high reactivity with metasilicate hydrates, particles containing amorphous SiO2 can also be used. This makes it easier to improve the mechanical strength of the mold.

[0144] Examples of particles containing amorphous SiO2 include precipitated silica, sintered silica formed in an electric arc or flame hydrolysis, silica formed by the thermal decomposition of ZrSiO4, silica formed by the oxidation of metallic silicon in an oxygen-containing gas, and quartz glass powder containing spherical particles formed from crystalline quartz due to melting and subsequent rapid cooling. These can be used individually or in combination of two or more substances.

[0145] The aforementioned inorganic particles are not particularly limited as long as they do not include the particles containing amorphous SiO2. Examples include carbonates selected from crystalline silicon dioxide and silicon; zinc carbonate, basic zinc carbonate, iron carbonate, manganese carbonate, copper carbonate, aluminum carbonate, barium carbonate, magnesium carbonate, calcium carbonate, lithium carbonate, potassium carbonate, sodium carbonate, etc.; sodium tetraborate, potassium tetraborate, lithium tetraborate, ammonium tetraborate, calcium tetraborate, strontium tetraborate, silver tetraborate, sodium metaborate, potassium metaborate, lithium metaborate, ammonium metaborate, calcium metaborate, silver metaborate, copper metaborate, lead metaborate, and boron metaborate. Magnesium borates, etc.; sulfates such as sodium sulfate, potassium sulfate, lithium sulfate, magnesium sulfate, calcium sulfate, strontium sulfate, barium sulfate, titanium sulfate, aluminum sulfate, zinc sulfate, copper sulfate, etc.; phosphates such as sodium phosphate, sodium hydrogen phosphate, potassium phosphate, potassium hydrogen phosphate, lithium phosphate, lithium hydrogen phosphate, magnesium phosphate, calcium phosphate, titanium phosphate, aluminum phosphate, zinc phosphate, etc.; hydroxides such as lithium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, aluminum hydroxide, zinc hydroxide, etc.; and one or more particles of oxides of silicon, zinc, magnesium, aluminum, calcium, lithium, copper, iron, boron, zirconium, etc.

[0146] There are no limitations on coupling agents; examples include silane coupling agents, zirconium coupling agents, and titanium coupling agents.

[0147] Examples of moisturizers include polyols, water-soluble polymers, hydrocarbons, sugars, proteins, and inorganic compounds other than those listed above.

[0148] Examples of moisture-resistant enhancers include metal oxides (excluding those mentioned above), carbonates, borates, sulfates, and phosphates.

[0149] Examples of lubricants include waxes; fatty amides; alkylene fatty amides; stearic acid; stearyl alcohol; metal stearate salts such as lead stearate, zinc stearate, calcium stearate, and magnesium stearate; monoglyceride stearate; stearate stearate; and hydrogenated oils.

[0150] Examples of mold release agents include paraffin wax, wax, light oil, machine oil, spindle oil, insulating oil, waste oil, vegetable oil, fatty acid esters, organic acids, graphite microparticles, mica, vermiculite, fluorinated mold release agents, and organosilicon mold release agents.

[0151] <Casting molds>

[0152] The casting mold of this embodiment will now be described.

[0153] The casting mold of this embodiment is manufactured using dry inorganic coated sand obtained by a method for manufacturing dry inorganic coated sand. The method for manufacturing the casting mold from dry inorganic coated sand is not particularly limited; for example, methods include filling a molding die with dry inorganic coated sand, using other components as needed, and then heating to cure it. Furthermore, various known molding methods can be applied.

[0154] The embodiments of the present invention have been described above; however, these are merely examples, and various configurations other than those described above may also be employed. Furthermore, modifications and alterations within the scope of achieving the objectives of the present invention are included in the present invention.

[0155] Regarding the above-described embodiments, the present invention further discloses the following dry inorganic coated sand, a method for manufacturing dry inorganic coated sand, and a method for manufacturing casting molds.

[0156] <1> A method for manufacturing dry inorganic coated sand, comprising a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate.

[0157] The above manufacturing method includes a step of mixing refractory aggregate, inorganic coated sand for crystallization, and liquid metasilicate hydrate.

[0158] The amount of inorganic coated sand for crystallization is 0.1% by mass or more and 15% by mass or less relative to the total amount of the dry inorganic coated sand, preferably 0.2% by mass or more, more preferably 0.4% by mass or more, and preferably 12% by mass or less, more preferably 10% by mass or less.

[0159] <2> According to the method for manufacturing dry inorganic coated sand described in <1>, wherein,

[0160] The above mixing process includes:

[0161] The process of mixing the above-mentioned refractory aggregate and the above-mentioned inorganic coated sand for crystallization to obtain a mixture, and

[0162] The process of further mixing the obtained mixture with the above-mentioned liquid metasilicate hydrate.

[0163] <3> According to the method for manufacturing dry inorganic coated sand described in <1> or <2>, wherein,

[0164] The aforementioned inorganic coated sand for crystallization has refractory aggregate and an inorganic binder layer formed on the surface of the aforementioned refractory aggregate.

[0165] <4> According to the method for manufacturing dry inorganic coated sand described in <3>, wherein,

[0166] The inorganic binder layer of the aforementioned inorganic coated sand for crystallization contains metasilicate hydrate.

[0167] <5> A method for manufacturing dry inorganic coated sand according to any one of <1> to <4>, wherein,

[0168] In the process of mixing the above-mentioned refractory aggregate and the above-mentioned inorganic coated sand for crystallization, the mixing is carried out under conditions lower than the melting temperature of the inorganic binder layer of the above-mentioned inorganic coated sand for crystallization.

[0169] <6> A method for manufacturing inorganic coated sand according to any one of <1> to <5>, wherein,

[0170] The average particle size ratio of the inorganic coated sand for crystallization to the average particle size of the dry inorganic coated sand (average particle size of the inorganic coated sand for crystallization / average particle size of the dry inorganic coated sand) is 0.3 or more and 0.9 or less, more preferably 0.4 or more, more preferably 1.4 or less, even more preferably 1.0 or less, and even more preferably 0.9 or less.

[0171] <7> A method for manufacturing inorganic coated sand according to any one of <1> to <6>, wherein,

[0172] The aforementioned liquid metasilicate hydrate is a metasilicate hydrate that is melted by heating.

[0173] <8> A method for manufacturing dry inorganic coated sand according to any one of <1> to <7>, wherein,

[0174] The aforementioned liquid metasilicate hydrate is a mixture of water glass, caustic alkali, and water.

[0175] <9> A method for manufacturing dry inorganic coated sand according to any one of <1> to <8>, wherein,

[0176] In the above-mentioned process of mixing the refractory aggregate, the inorganic coated sand for crystallization, and the liquid metasilicate hydrate,

[0177] The amount of inorganic coated sand for crystallization relative to the above-mentioned refractory aggregate is preferably 0.2% by mass or more, more preferably 0.3% by mass or more, even more preferably 0.4% by mass or more, and preferably 15% by mass or less, more preferably 12% by mass or less.

[0178] <10> A method for manufacturing a mold, wherein the mold is made using dry inorganic coated sand obtained by any of the methods for manufacturing dry inorganic coated sand described in <1> to <9>.

[0179] <11> A dry inorganic coated sand having refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate.

[0180] The particle group of the above-mentioned dry inorganic coated sand contains particles with a multi-layer structure of the above-mentioned inorganic binder layer.

[0181] <12> According to the dry inorganic coated sand described in <11>, among which,

[0182] The content of the aforementioned particles in the inorganic binder layer having a multi-layer structure is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less, relative to the total amount of the aforementioned dry inorganic coated sand.

[0183] Example

[0184] The present invention will now be described using examples and comparative examples; however, the present invention is not limited thereto.

[0185] (1) Materials

[0186] [Refractory Aggregates]

[0187] •ESPEARL 35L: ESPEARL#35L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 574μm, amorphousness 40%, sphericity 0.97)

[0188] •ESPEARL 50L: ESPEARL#50L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 417μm, amorphousness 40%, sphericity 0.97)

[0189] •ESPEARL 60L: ESPEARL#60L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 305μm, amorphousness 45%, sphericity 0.97)

[0190] •ESPEARL 75L: ESPEARL#75L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 269μm, amorphousness 50%, sphericity 0.97)

[0191] •ESPEARL 100L: ESPEARL#100L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 121μm, amorphousness 72%, sphericity 0.98)

[0192] • Sanhe Silica Sand R6: Sanhe Silica Sand R6 (manufactured by Sanhe Silica Company, average particle size: 200μm, amorphousness 0.2%, sphericity 0.85)

[0193] • Sanhe Silica Sand R8: Sanhe Silica Sand R8 (manufactured by Sanhe Silica Company, average particle size: 127μm, amorphousness 0.2%, sphericity 0.85)

[0194] [Inorganic binder: metasilicate hydrate]

[0195] • Metasilicate 1: A mixture of sodium metasilicate nonhydrate (Na2SiO3·9H2O, manufactured by Nippon Chemical Industries, Ltd., melting point 47℃, SiO2 / Na2O ratio = 0.9~1.1) and sodium metasilicate pentahydrate (Na2SiO3·5H2O, manufactured by Nippon Chemical Industries, Ltd., melting point 72℃, SiO2 / Na2O ratio = 0.9~1.1) in a weight ratio of 4:3.

[0196] • Metasilicate 2: An aqueous solution of water-containing glass prepared according to the following steps

[0197] (step)

[0198] Using the ratios (parts by mass) shown in Table 1 below, water glass, sodium hydroxide (NaOH), and water were mixed in a mixer for 10 minutes to obtain metasilicate 2 as shown in Table 1.

[0199] [Table 1]

[0200]

[0201] • No. 1 50 water glass: Manufactured by Fuji Chemical Co., Ltd., SiO2 (%) = 30.0, Na2O (%) = 14.7, solid content 44.7% by mass

[0202] • NaOH: Sodium hydroxide, produced by Fujifilm and Kojun Pharmaceutical Co., Ltd., in granular form.

[0203] (2) Preparation of inorganic coated sand for crystallization

[0204] Using the materials described in (1) above, an inorganic coated sand for crystallization is prepared in such a manner that the refractory aggregate and inorganic binder are as shown in Table 2 (type, mass parts).

[0205] Specifically, the refractory aggregates shown in Table 2 are put into a mixer, and then the inorganic binder heated to 80°C and melted is put into the mixer and mixed for 4 minutes to obtain dry sand with room temperature fluidity, which is designated as inorganic coated sand for crystallization.

[0206] (3) Preparation of dry inorganic coated sand

[0207] Using the materials described in (1) above and the inorganic coated sand for crystallization obtained in (2) above, dry inorganic coated sand is prepared in such a manner that the refractory aggregate and inorganic binder are as shown in Table 2 (type, mass parts).

[0208] That is, as shown in Table 2, the total amount of refractory aggregate used in (3) and refractory aggregate in inorganic coated sand for crystallization is set to 100 parts by mass, and the total amount of sodium metasilicate 9 hydrate used in (3) and metasilicate 1 in inorganic coated sand for crystallization is set to 2 or 3 parts by mass.

[0209] The following describes each embodiment and comparative example.

[0210] <Example 1>

[0211] The amounts of refractory aggregate (ESPEARL 60L: 99.70 parts by mass) set at 35°C and inorganic coated sand for crystallization (ESPEARL 60L: 0.30 parts by mass, metasilicate 1: 0.006 parts by mass) set at 35°C, as shown in Table 2, were added to the mixer.

[0212] Then, while stirring, 1.994 parts by mass of molten metasilicate 1 heated to 80°C was added to the aforementioned mixer and homogenized mixing began. Stirring continued until the drying time shown in Table 2 was confirmed, at which point stirring was stopped, yielding 102 parts by mass of untreated, dry inorganic coated sand with room-temperature fluidity. Subsequently, the untreated, dry inorganic coated sand was sieved (20 mesh) to remove agglomerates (clumps), yielding 101 parts by mass of dry inorganic coated sand. Therefore, the amount of inorganic coated sand added for crystallization was 0.3% by mass relative to the total amount of dry inorganic coated sand (0.306 parts by mass of inorganic coated sand for crystallization ÷ 101 parts by mass of total dry inorganic coated sand × 100).

[0213] The ratio of the amount of agglomerates removed to the amount of untreated dry inorganic coated sand was calculated as 0.6 (mass%) of agglomerates removed (0.6 parts by mass ÷ 102 parts by mass of untreated dry inorganic coated sand × 100). The results are shown in Table 2.

[0214] <Examples 2-9, Comparative Examples 1-3>

[0215] Except for changing the refractory aggregates and inorganic binders to the contents shown in Table 2 (types, parts by mass), the dry inorganic coated sands shown in Table 2 were obtained in the same manner as in Example 1.

[0216] [Table 2]

[0217]

[0218] Give the meaning of the following statements in Table 2.

[0219] SA1: Average particle size (μm) of refractory aggregates used as raw materials for dry inorganic coated sand.

[0220] SA2: Average particle size (μm) of refractory aggregate in inorganic coated sand for crystallization.

[0221] CS1: Average particle size (μm) of dry inorganic coated sand.

[0222] CS2: Average particle size (μm) of inorganic coated sand used for crystallization.

Claims

1. A method for manufacturing dry inorganic coated sand, comprising a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate. The manufacturing method includes the step of mixing refractory aggregate, inorganic coated sand for crystallization, and liquid metasilicate hydrate. The amount of the inorganic coated sand used for crystallization is more than 0.1% by mass and less than 15% by mass relative to the total amount of the dry inorganic coated sand.

2. The method for manufacturing dry inorganic coated sand according to claim 1, wherein, The mixing process includes: The process of mixing the refractory aggregate and the crystallizing inorganic coated sand to obtain a mixture, and The process of further mixing the resulting mixture with the liquid metasilicate hydrate.

3. The method for manufacturing dry inorganic coated sand according to claim 1 or 2, wherein, The inorganic coated sand for crystallization has refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate.

4. The method for manufacturing dry inorganic coated sand according to any one of claims 1 to 3, wherein, The inorganic binder layer of the inorganic coated sand for crystallization contains metasilicate hydrate.

5. The method for manufacturing dry inorganic coated sand according to any one of claims 1 to 4, wherein, In the process of mixing the refractory aggregate and the inorganic coated sand for crystallization, the mixing is carried out under conditions lower than the melting temperature of the inorganic binder layer of the inorganic coated sand for crystallization.

6. The method for manufacturing dry inorganic coated sand according to any one of claims 1 to 5, wherein, The average particle size ratio of the inorganic coated sand for crystallization to the average particle size of the dry inorganic coated sand, i.e., the average particle size of the inorganic coated sand for crystallization / the average particle size of the dry inorganic coated sand, is 0.3 or more and 0.9 or less.

7. The method for manufacturing dry inorganic coated sand according to any one of claims 1 to 6, wherein, The liquid metasilicate hydrate is a metasilicate hydrate that is melted by heating.

8. The method for manufacturing dry inorganic coated sand according to any one of claims 1 to 6, wherein, The liquid metasilicate hydrate is a mixture of water glass, caustic alkali and water.

9. A method for manufacturing a mold, wherein the mold is manufactured using dry inorganic coated sand obtained by the method for manufacturing dry inorganic coated sand according to any one of claims 1 to 8.

10. A dry inorganic coated sand having refractory aggregate and an inorganic binder layer formed on the surface of said refractory aggregate, The particle group of the dry inorganic coated sand contains particles with a multi-layered structure of the inorganic binder layer.

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