Foundry sand

Abstract

To provide foundry sand that is less prone to blocking even in environments with relatively high humidity. [Solution] Foundry sand having a first coating layer containing aluminosilicate on a refractory aggregate, wherein the molar ratio of silicon (Si) to aluminum (Al) (Si / Al), measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the particles constituting the foundry sand, is 1.00 or more and 8.50 or less.

Description

Technical Field

[0001] The present invention relates to foundry sand.

Background Art

[0002] As a mold used for casting of castings, there is known a product obtained by filling a coated sand having a refractory aggregate and an inorganic binder layer containing a metasilicate hydrate formed on the surface of the refractory aggregate into a mold and curing the coated sand filled in the mold.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A mold manufactured using coated sand is usually broken (crushed) and recovered after being used for casting, and is subjected to a recycling process by various methods and reused as recycled sand. From the viewpoints of economy and waste reduction, it is common in foundry factories to manufacture molds using recycled sand as a refractory aggregate.

[0005] However, it has been revealed that recycled sand (foundry sand) obtained by recovering and subjecting to a recycling process after using coated sand as described in Patent Document 1 is likely to aggregate and block in an environment with a relatively high humidity. When the recycled sand (foundry sand) blocks, it becomes difficult to uniformly form an inorganic binder layer on the surface, which is not preferable from the viewpoint of the strength of the mold.

[0006] Furthermore, it has become clear that when coated sand, such as that described in Patent Document 1, is used, then recovered and subjected to a recycling process to obtain recycled sand (foundry sand), and a layer of inorganic binder containing metasilicate hydrate is formed on the recycled sand, a mold with the desired strength can be obtained if it is used immediately after production. However, as time passes after the production of the coated sand, the strength of the resulting mold tends to decrease.

[0007] The present invention aims to provide foundry sand that is less prone to blocking even in environments with relatively high humidity.

[0008] The present invention aims to provide coated sand that can suppress the decrease in the strength of a mold even when used in the manufacture of a mold a long time after its production. [Means for solving the problem]

[0009] The present invention relates to foundry sand having a first coating layer containing aluminosilicate on a refractory aggregate, wherein the molar ratio of silicon (Si) to aluminum (Al) (Si / Al), measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the particles constituting the foundry sand, is 1.00 or more and 8.50 or less.

[0010] The present invention relates to a coated sand containing the foundry sand, wherein the first coating layer has a second coating layer containing metasilicate hydrate. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide foundry sand that is less prone to blocking even in environments with relatively high humidity.

[0012] According to the present invention, it is possible to provide coated sand that can suppress the decrease in the strength of a mold even when used in the manufacture of a mold a long time after its manufacture. [Modes for carrying out the invention]

[0013] <Foundry sand> The foundry sand of this embodiment is a foundry sand having a first coating layer containing aluminosilicate on a refractory aggregate, wherein the molar ratio of silicon (Si) to aluminum (Al) (Si / Al), measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the particles constituting the foundry sand, is 1.00 or more and 8.50 or less. The reason why the foundry sand of this embodiment exhibits such effects is not clear, but it is thought to be as follows.

[0014] The blocking of recycled sand in relatively high humidity environments is thought to be due to moisture absorption by residues derived from metasilicate hydrate, which is used as an inorganic binder. Metasilicate hydrate becomes silicate during the casting process and remains on the surface of recycled sand (refractory aggregate). Silicate has silanol groups and readily adsorbs water molecules, adsorbing moisture from the environment and forming liquid crosslinks between refractory aggregates, which causes blocking. Therefore, blocking is further promoted when humidity is high. Since the cause of blocking is the formation of liquid crosslinks between refractory aggregates, it is expected that moisture absorption can be suppressed by reducing the amount of silanol groups. Conventional technology suppresses moisture absorption by high heat treatment at 400°C or higher to cause dehydration condensation of silanol groups and crystallization of silicate. However, it has been found that blocking occurs during the recycling process when such high heat treatment is performed. Therefore, we focused on reducing the amount of silanol groups on the surface of recycled sand (refractory aggregate) by forming a first coating layer containing aluminosilicate on the refractory aggregate. In particular, we found that blocking during the regeneration process can be suppressed by setting the molar ratio of Si derived from silanol groups and Al derived from aluminosilicate on the surface of the first coating layer to a specific range. This is thought to be because Al is incorporated into the crystalline structure of the surface of the recycled sand (refractory aggregate) in a specific proportion, forming a structure that can protect the silanol groups from moisture, thereby suppressing blocking of the recycled sand in environments with relatively high humidity.

[0015] The foundry sand in this embodiment is a group of particles. The sphericity of the foundry sand is preferably 0.75 or higher, more preferably 0.80 or higher, and even more preferably 0.82 or higher, from the viewpoint of improving fluidity and further enhancing the ability to fill into the molding die. Specifically, the upper limit of the sphericity is 1.

[0016] The sphericity of the foundry sand is consistent with that of the refractory aggregate described later. The method for measuring the sphericity of the foundry sand involves analyzing images (photographs) of the particles obtained using an optical microscope or digital scope (for example, Keyence VH-8000 model) to determine the area of ​​the particle projection cross-section and the perimeter of the cross-section, and then calculating the sphericity = [Area of ​​particle projection cross-section (mm²)]. 2 The value can be calculated by dividing the circumference of a perfect circle with the same area as the particle by the circumference of the particle projection cross-section (mm), and then averaging the obtained values ​​for any 50 particles.

[0017] The average particle size of the foundry sand is preferably 0.05 mm or more, and more preferably 0.10 mm or more, from the viewpoint of improving mold quality and mold strength, ease of mold making, and storage stability. The average particle size of the foundry sand is preferably 2.00 mm or less, more preferably 1.00 mm or less, and even more preferably 0.50 mm or less, from the viewpoint of improving mold quality and mold strength, and ease of mold making.

[0018] The average particle size of the foundry sand can be measured by the following method for measuring average particle size. (Method for measuring average particle size) When the sphericity from the particle projection cross-section of the particle is 1, measure the diameter (mm), while when the sphericity < 1, measure the major axis diameter (mm) and minor axis diameter (mm) of the randomly oriented particles, and obtain (major axis diameter + minor axis diameter) / 2. For any 100 particles, average the values obtained respectively to obtain the average particle diameter (mm). The major axis diameter and minor axis diameter are defined as follows. When the particle is stabilized on a plane and the projection image of the particle on the plane is sandwiched between two parallel lines, the width of the particle when the distance between the parallel lines is minimized is called the minor axis diameter, while when the particle is sandwiched between two parallel lines in a direction perpendicular to these parallel lines, the distance is called the major axis diameter. The major axis diameter and minor axis diameter of the particle can be obtained by taking a photograph (image) of the particle using an optical microscope or a digital scope (for example, VH-8000 type manufactured by Keyence Corporation) and performing image analysis on the obtained image.

[0019] [Refractory aggregate] The refractory aggregate contains one or more selected from the group consisting of natural sand and artificial sand.

[0020] Examples of the natural sand include one or more selected from the group consisting of silica sand, chromite sand, zircon sand, olivine sand, and alumina sand.

[0021] Examples of the artificial sand include one or more selected from the group consisting of synthetic mullite sand, SiO2-based sand with SiO2 as the main component, Al2O3-based sand with Al2O3 as the main component, SiO2 / Al2O3-based sand, SiO2 / MgO-based sand, SiO2 / Al2O3 / ZrO2-based sand, SiO2 / Al2O3 / Fe2O3-based sand, and slag-derived sand. Here, the main component refers to the component with the largest mass among the components contained in the sand. The artificial sand refers to sand that is not naturally produced but is artificially prepared with metal oxide components and melted or sintered.

[0022] Note that the content of each component such as SiO2, Al2O3, Fe2O3, etc. in the refractory aggregate can be measured using the following fluorescent X-ray method. [Method for measuring the components of the refractory aggregate by fluorescent X-ray method] The refractory aggregate is adjusted to a size of about 0.1 μm or less with a vibration mill and heated at 1050 °C for 1 hour. Then, 5 g of lithium tetraborate and 0.5 g of the refractory aggregate are mixed and heated and melted at 1200 °C for 10 minutes, and then cooled to form a glassy sample (glass bead method). The components of the refractory aggregate can be measured by performing fluorescent X-ray analysis on the sample using a fundamental parameter (FP) method with a fluorescent X-ray analyzer ZSX Primus II (manufactured by Rigaku Corporation).

[0023] The refractory aggregate is generally recycled sand. The recycled sand is obtained by recycling a used casting mold or core formed from a refractory aggregate and an inorganic binder containing metasilicate. The recycled sand contains the refractory aggregate and has a residue of the used inorganic binder on the refractory aggregate. The residue of the used inorganic binder contains silicate. Examples of the cation constituting the silicate include monovalent cations such as sodium, potassium, lithium, and ammonium, and divalent cations such as magnesium, calcium, and zinc.

[0024] The recycled sand can be produced, for example, by the following method. [Method for producing recycled sand] As a method for recycling the used sand of the mold after casting using coated sand, it can conform to a known method (for example, "Mold Molding Method", 4th edition, Japan Foundry Technology Association, November 18, 1996, pages 327 to 330). For example, methods such as dry grinding treatment (mechanical wear), wet grinding treatment, roasting treatment, and methods combining these treatments are known.

[0025] In the aforementioned dry polishing process, for example, a rotary reclaimer can be used, which polishes the sand by the collision and friction that occurs between the projected sand generated by centrifugal force and the falling input sand when sand is fed onto a high-speed rotating rotor; a hybrid sand master, which is a composite type of reclaimer that integrates a rotary reclaimer and a fluid classifier; and a sand freshener that utilizes the grinding and polishing force of a grinding wheel.

[0026] One example of the wet polishing treatment is a method using a trough polishing machine that polishes the sand by friction between sand grains in a trough with rotating blades.

[0027] The aforementioned roasting process includes, for example, a method in which sand is continuously added to a roasting furnace such as a fluidized bed furnace or a rotary kiln, and the mixture is roasted at a temperature in the range of 200 to 1000°C.

[0028] Any method can be used for regeneration, but since wet processing and roasting processes are complicated and energy-intensive, dry polishing is preferred.

[0029] The sphericity of the refractory aggregate is preferably 0.75 or higher, more preferably 0.80 or higher, and even more preferably 0.82 or higher, from the viewpoints of fluidity, mold quality, improved mold strength, and ease of mold formation. Specifically, the upper limit of sphericity is 1.00. The method for measuring the sphericity of the refractory aggregate is the same as the method for measuring the sphericity of the foundry sand.

[0030] The average particle diameter of the refractory aggregate is preferably 0.05 mm or more, more preferably 0.10 mm or more, from the viewpoint of improving mold quality and strength, and ease of mold formation. The average particle diameter of the refractory aggregate is preferably 2.00 mm or less, more preferably 1.00 mm or less, and even more preferably 0.50 mm or less, from the viewpoint of improving mold quality and strength, and ease of mold formation. The method for measuring the average particle diameter of the refractory aggregate is the same as the method for measuring the average particle diameter of the foundry sand.

[0031] [First coating layer] The foundry sand of this embodiment has a first coating layer containing aluminosilicate on the refractory aggregate. The first coating layer covers the refractory aggregate. Note that the coating is not limited to being continuous, and may have discontinuous portions. Furthermore, if the foundry sand of this embodiment contains the refractory aggregate as recycled sand, the foundry sand of this embodiment has the first coating layer on the residue of the used inorganic binder that the recycled sand has. If the foundry sand of this embodiment contains the refractory aggregate as recycled sand, the foundry sand of this embodiment has silicate between the refractory aggregate and the first coating layer.

[0032] Aluminosilicates preferably contain at least one of a silicate and a reaction product of aluminate or aluminum hydroxide, as this facilitates improved storage stability. Examples of cations constituting aluminates include monovalent cations such as sodium, potassium, and lithium, and divalent cations such as magnesium, calcium, and zinc.

[0033] Methods for confirming that the first coating layer contains aluminosilicate include, for example, a method of grinding foundry sand in a mill or other grinder to remove the components of the first coating layer, analyzing the components of the first coating layer by infrared spectroscopy to confirm absorption originating from aluminosilicate, or a method of analyzing by solid-state nuclear magnetic resonance spectroscopy (ssNMR) to confirm a signal originating from aluminosilicate.

[0034] The aluminosilicate content in the first coating layer can be determined in terms of Al2O3. From the viewpoint of storage stability, the aluminosilicate content in the first coating layer is preferably 0.005 parts by mass or more, more preferably 0.010 parts by mass or more, per 100 parts by mass of the refractory aggregate, and from the viewpoint of obtaining a high-strength mold, it is preferably 1.000 parts by mass or less, more preferably 0.500 parts by mass or less, and even more preferably 0.300 parts by mass or less, per 100 parts by mass of the refractory aggregate.

[0035] The content of aluminosilicate in the first coating layer, converted to Al2O3, can be determined by the following method. The following formula is used, based on the analytical values ​​of the aggregate obtained by the aforementioned X-ray fluorescence method and the analytical values ​​of the foundry sand containing the first coating layer using those values. Aluminosilicate content of the first coating layer (in terms of Al2O3) [parts by mass] ={Al2O3 [parts by mass] of foundry sand}-{Al2O3 [parts by mass] of refractory aggregate}

[0036] The content of the first coating layer is preferably 0.005 parts by mass or more, more preferably 0.010 parts by mass or more, and even more preferably 0.020 parts by mass or more, per 100 parts by mass of the refractory aggregate, from the viewpoint of improving storage stability and obtaining a high-strength casting mold, and preferably 1.000 parts by mass or less, and more preferably 0.500 parts by mass or less, per 100 parts by mass of the refractory aggregate, from the viewpoint of achieving both storage stability and strength.

[0037] The first coating layer is preferably solid at room temperature, from the viewpoint of improving fluidity and further enhancing its ability to fill into the molding die. Here, room temperature refers to 25°C.

[0038] The molar ratio of silicon (Si) to aluminum (Al) (Si / Al) measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the foundry sand particles (hereinafter sometimes referred to as foundry sand particles) is 1.00 or higher, preferably 1.50 or higher, more preferably 2.00 or higher, even more preferably 3.00 or higher, and even more preferably 3.50 or higher, from the viewpoint of improving the stability of the coated sand and obtaining a high-strength mold, and from the viewpoint of suppressing blocking in a high-humidity environment and improving storage stability, it is 8.50 or lower, preferably 7.00 or lower, more preferably 6.50 or lower, even more preferably 6.00 or lower, even more preferably 5.50 or lower, and even more preferably 5.00 or lower. The molar ratio of silicon (Si) to aluminum (Al) (Si / Al) on the surface of the first coating layer of the foundry sand particles is measured by the method described in the examples.

[0039] Furthermore, the ratio of sodium (Na) to the total of silicon (Si), aluminum (Al), and sodium (Na) (Na / (Si+Al+Na)×100(%)) measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the foundry sand particles is preferably 5% or more, more preferably 20% or more, and even more preferably 40% or more, from the viewpoint of obtaining a high-strength mold, and preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less. The mole fraction is measured by the method described in the examples.

[0040] From the viewpoint of improving the operability of the foundry sand particles, the moisture content of the foundry sand particles is preferably less than 1.50% by mass, more preferably 1.00% by mass or less, even more preferably 0.50% by mass or less, even more preferably 0.30% by mass or less, and even more preferably 0.15% by mass or less. In this specification, the moisture content of the foundry sand particles is measured by the method described in the examples.

[0041] The first coating layer preferably has sodium on its surface from the viewpoint of improving mold strength.

[0042] <Method for manufacturing foundry sand> The method for producing foundry sand according to this embodiment is a method for producing foundry sand, comprising the steps of (1) mixing a refractory aggregate having silicate on its surface (hereinafter referred to as refractory aggregate (A)) with an aluminate to obtain a mixture, and (2) heat-treating the mixture. The first coating layer containing aluminosilicate is formed by the reaction of silicate and aluminate. The refractory aggregate (A) is generally recycled sand.

[0043] In step (1) above, the method of mixing the refractory aggregate (A) and the aluminate is not particularly limited, and one example is a method of mixing by rotating the stirring part using a known mixing device having a stirring part. Examples of the mixing device include a kneader, ribbon mixer, Nauter mixer, Proscher mixer, Lödige mixer, high-speed mixer, etc., and a Lödige mixer is preferred from the viewpoint of shortening the process as it can perform heating and mixing in one step.

[0044] In step (1) above, the temperature at which the refractory aggregate (A) and the aluminate are mixed is preferably 15°C or higher, more preferably 30°C or higher, even more preferably 50°C or higher, and even more preferably 80°C or higher, from the viewpoint of forming a uniform first coating layer, and preferably 200°C or lower, more preferably 150°C or lower, and even more preferably 120°C or lower, from the viewpoint of reducing energy consumption.

[0045] In step (1) above, the mixing of the refractory aggregate (A) and the aluminate may be done by heating the refractory aggregate (A) to 20°C or 200°C or below, then adding the aluminate or aluminum hydroxide and mixing, or by adding the aluminate or aluminum hydroxide to the refractory aggregate (A) and then heating to 20°C or 200°C or below and mixing.

[0046] In step (1) above, the mixing time of the refractory aggregate (A) and the aluminate is preferably 0.5 minutes or more, more preferably 1 minute or more, from the viewpoint of uniformity, and preferably 10 minutes or less, more preferably 5 minutes or less, from the viewpoint of suppressing the crushing of the refractory aggregate.

[0047] In step (2) above, the method for heat-treating the mixture obtained in step (1) above is not particularly limited and can be heat-treated using a known heating device.

[0048] In step (2) above, the temperature of the heat treatment is preferably 60°C or higher, more preferably 70°C or higher, even more preferably 80°C or higher, even more preferably 90°C or higher, and even more preferably 100°C or higher, from the viewpoint of improving operability after modification, and preferably 200°C or lower, more preferably 150°C or lower, from the viewpoint of suppressing blocking.

[0049] In step (2) above, the time of the heat treatment is preferably 5 minutes or more, more preferably 15 minutes or more, from the viewpoint of improving operability after modification, and preferably 360 minutes or less, more preferably 240 minutes or less, even more preferably 180 minutes or less, and even more preferably 120 minutes or less, from the viewpoint of reducing energy load.

[0050] <Coated Sand> The coated sand of this embodiment is a coated sand having a second coating layer containing metasilicate hydrate on a first coating layer of foundry sand. The coated sand of this embodiment can suppress the decrease in the strength of the mold even when used in the manufacture of a mold a long time after its manufacture.

[0051] The coated sand of this embodiment is a group of particles. The coated sand of this embodiment is preferably in a dry state that is fluid at room temperature. Dry coated sand means coated sand from which a measurement can be obtained when measuring the dynamic angle of repose regardless of the moisture content. The dynamic angle of repose is preferably 80° or less, more preferably 45° or less, and even more preferably 30° or less.

[0052] The dynamic angle of repose of coated sand can be measured by the following method. (Method for measuring dynamic angle of repose) Half the volume of coated sand is placed in a cylindrical transparent plastic bottle (diameter: 7.7 cm, height: 16 cm). Using a bottle agitator, the bottle is held so that its axis is horizontal and rotated at a speed of 60 rpm around the horizontal axis. The slope of the flowing coated sand layer inside the plastic bottle becomes flat. The angle formed between this slope and the horizontal plane is measured. If the coated sand does not flow inside the cylindrical transparent plastic bottle, or if it flows but the slope of the coated sand layer does not form a flat surface, and as a result the dynamic angle of repose cannot be measured, the bottle is in a wet state.

[0053] In this embodiment, the coated sand is preferably spherical in shape, from the viewpoint of improving fluidity and further enhancing its ability to fill into the molding die. Here, "spherical" in the context of coated sand refers to a round shape, like a ball.

[0054] The sphericity of the coated sand is preferably 0.75 or higher, more preferably 0.80 or higher, and even more preferably 0.82 or higher, from the viewpoint of improving fluidity, mold quality, and mold strength, as well as ease of mold formation. Specifically, the upper limit of the sphericity is 1.00. The sphericity of the coated sand is the same as that of the refractory aggregate. The method for measuring the sphericity of the coated sand is the same as the method for measuring the sphericity of the foundry sand.

[0055] The average particle size of the coated sand is preferably 0.05 mm or more, and more preferably 0.10 mm or more, from the viewpoint of improving mold quality and strength, ease of mold making, and storage stability. Furthermore, if the average particle size of the coated sand is above the lower limit, it is preferable that the amount of coating layer etc. used during mold manufacturing can be reduced, making it easier to regenerate the inorganic coated sand. The average particle size of the coated sand is preferably 2.00 mm or less, more preferably 1.00 mm or less, and even more preferably 0.50 mm or less, from the viewpoint of improving mold quality and strength, and ease of mold making. Furthermore, if the average particle size of the coated sand is below the upper limit, it is preferable that the porosity is reduced during mold manufacturing, thereby increasing mold strength. The method for measuring the average particle size of the coated sand can be the same as the method for measuring the average particle size of the foundry sand.

[0056] [Second coating layer] The second coating layer is a layer formed on the first coating layer, and from the viewpoint of improving mold strength, it is preferably formed to cover the surface of the first coating layer. The second coating layer is a layer obtained by the crystallization of metasilicate hydrate and is intended to function as coated sand. The second coating layer is not limited to being continuous, and may have discontinuous regions in part. The second coating layer may be formed directly on the first coating layer, or there may be other layers between the first and second coating layers.

[0057] From the viewpoint of obtaining a high-strength casting mold, the content of the second coating layer 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, even more preferably 1.0 part by mass or more, and even more preferably 1.5 parts by mass or more, per 100.0 parts by mass of refractory aggregate. From the viewpoint of obtaining a high-strength casting mold, the content of the second coating layer is, for example, 15.0 parts by mass or less, preferably 10.0 parts by mass or less, more preferably 8.0 parts by mass or less, even more preferably 6.0 parts by mass or less, even more preferably 4.0 parts by mass or less, and even more preferably 3.0 parts by mass or less, per 100.0 parts by mass of refractory aggregate.

[0058] The second coating layer may have at least a layer containing metasilicate hydrate, and may be a single layer or a multilayer. Furthermore, the layer containing at least metasilicate hydrate is formed by an inorganic binder composition containing metasilicate hydrate. The use of metasilicate hydrate is preferable because it can improve the crystallinity of the second coating layer and the coated sand exhibits excellent room-temperature fluidity. In addition, by using metasilicate hydrate, the second coating layer can be formed on the surface of the refractory aggregate without dissolving it in water.

[0059] Examples of cations constituting metasilicate hydrate salts include monovalent cations such as sodium, potassium, lithium, and ammonium, as well as divalent cations such as magnesium, calcium, and zinc.

[0060] The content of metasilicate in the second coating layer is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 98% by mass or more, and even more preferably substantially 100% by mass, from the viewpoint of improving mold strength, excellent productivity, and availability. In this specification, "substantially" means that it may include components that are included unintentionally. In this specification, the content of metasilicate in the second coating layer refers to the content of metasilicate relative to the total components other than water in the second coating layer.

[0061] From the viewpoint of obtaining a high-strength casting mold, the content of metasilicate in the second coating layer is, for example, 0.03 parts by mass or more, preferably 0.10 parts by mass or more, and more preferably 0.50 parts by mass or more, per 100.00 parts by mass of refractory aggregate. From the viewpoint of obtaining a high-strength casting mold, the content of metasilicate in the second coating layer is, for example, 5.00 parts by mass or less, preferably 4.00 parts by mass or less, more preferably 3.00 parts by mass or less, even more preferably 2.00 parts by mass or less, and even more preferably 1.00 part by mass or less, per 100.00 parts by mass of refractory aggregate.

[0062] Methods for confirming that the second coating layer contains metasilicate include, for example, a method of grinding the coated sand in a mill or other grinder to remove only the second coating layer component, analyzing the second coating layer component by XRD, and confirming the peak indicating the crystalline structure of metasilicate hydrate; a method of immersing the coated sand in water and stirring for a certain period of time to dissolve the second coating layer component, drying the dissolved component, analyzing the dried solid content by XRD, confirming the peak indicating the crystalline structure of metasilicate, and analyzing the amount of hydration water by the method described below to confirm that it is metasilicate hydrate.

[0063] [Measurement of hydration water volume] (1) A crucible that has been preheated and weighed is filled with 10 g of coated sand before the addition of additives such as amorphous SiO2 fine particles, and after heating at 900°C for 1 hour, the amount of mass loss (%) is used to calculate the moisture content (%) in the coated sand (A). A = [(M1 - M2) / M3] × 100 (M1: Total mass of crucible and coated sand before firing (g), M2: Total mass of crucible and coated sand after firing (g), M3: Mass of coated sand before firing (g)) (2) Weigh 100 g of coated sand before adding additives such as amorphous SiO2 fine particles, immerse it in 200 mL or more of water or hot water and stir for at least 1 hour to extract metasilicate hydrate. Filter the refractory aggregate from the obtained extract, and then remove water by vacuum distillation using a rotary evaporator at 40°C and an internal pressure of 15 mmHg or less. After that, heat dry at a temperature of 120°C to 180°C for 1 to 3 hours and weigh the dry product. Calculate the dry solid content (%) of metasilicate hydrate in the coated sand (B). B = (M12 / M11) × 100 (M11: Mass of coated sand (g), M12: Dry weight (g)) (3) Amount of water to hydrate metasilicate hydrate = [(A) / molecular weight of water] / [(B) / molecular weight of anhydrous metasilicate]

[0064] (others) The second coating layer may further contain components other than metasilicate, such as amorphous SiO2-containing fine particles, inorganic fine particles other than amorphous SiO2-containing fine particles, humectants, moisture-resistant agents, coupling agents that strengthen the bond between the refractory aggregate and the inorganic binder composition, lubricants, surfactants, mold release agents, etc.

[0065] Amorphous SiO2-containing fine particles may be used due to their high reactivity with metasilicate hydrates. This makes it easier to improve the mechanical strength of the mold.

[0066] Examples of amorphous SiO2-containing fine particles include precipitated silica, calcined silica produced in an electric arc or by flame hydrolysis, silica produced during the manufacture of Fe-Si, silica produced by the thermal decomposition of ZrSiO4, silicon dioxide produced by the oxidation of metallic silicon with an oxygen-containing gas, and spherical particles of quartz glass powder produced from crystalline quartz by melting and subsequent rapid cooling. These can be used individually, or two or more can be mixed and used together.

[0067] Inorganic fine particles are not particularly limited as long as they are not amorphous SiO2-containing fine particles as described above, but examples include crystalline silica, silicon; carbonates such as zinc carbonate, basic zinc carbonate, iron carbonate, manganese carbonate, copper carbonate, aluminum carbonate, barium carbonate, magnesium carbonate, calcium carbonate, lithium carbonate, potassium carbonate, and sodium carbonate; borates such as 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 magnesium metaborate; sodium sulfate, potassium sulfate, and sulfuric acid. Examples of fine particles include one or more types of fine particles selected from among sulfates such as lithium, magnesium sulfate, calcium sulfate, strontium sulfate, barium sulfate, titanium sulfate, aluminum sulfate, zinc sulfate, and copper sulfate; 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, and zinc phosphate; hydroxides such as lithium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, aluminum hydroxide, and zinc hydroxide; and oxides such as silicon, zinc, magnesium, aluminum, calcium, lithium, copper, iron, boron, and zirconium.

[0068] The coupling agent is not limited to these, but examples include silane coupling agents, zircon coupling agents, and titanium coupling agents.

[0069] Examples of humectants include polyhydric alcohols, water-soluble polymers, hydrocarbons, sugars, proteins, and inorganic compounds other than those mentioned above.

[0070] Examples of moisture-resistant agents include metal oxides (excluding those listed above), carbonates, borates, sulfates, and phosphates.

[0071] Examples of lubricants include waxes; fatty acid amides; alkylene fatty acid amides; stearic acid; stearyl alcohol; metal stearate salts such as lead stearate, zinc stearate, calcium stearate, and magnesium stearate; monoglyceride stearate; stearyl stearate; and hydrogenated oils.

[0072] Examples of mold release agents include paraffin, wax, diesel fuel, machine oil, spindle oil, insulating oil, waste oil, vegetable oil, fatty acid esters, organic acids, graphite fine particles, mica, vermiculite, fluorine-based mold release agents, silicone-based mold release agents, and the like.

[0073] <Method for manufacturing coated sand> The manufacturing method of this embodiment is a method for manufacturing coated sand, comprising the step (3) of forming a second coating layer containing an inorganic binder containing metasilicate hydrate on the first coating layer of the foundry sand.

[0074] [Step (3)] In step (3) above, the method of mixing the foundry sand and the inorganic binder to form a second coating layer containing the inorganic binder on the first coating layer is not particularly limited. Examples include a method of mixing the foundry sand and the inorganic binder containing heated and melted metasilicate hydrate in a known manner to obtain a mixture, and then cooling the mixture to a temperature below the melting point of the inorganic binder to form the second coating layer on the first coating layer of the foundry sand, or a method of mixing the foundry sand with a solution containing water glass, caustic alkali, and water in a known manner to obtain a mixture, and then drying the mixture to form the second coating layer on the first coating layer of the foundry sand.

[0075] When a mixture is obtained by mixing the foundry sand and the inorganic binder containing heated and melted metasilicate hydrate in a known method, and then the mixture is cooled to a temperature below the melting point of the inorganic binder to form the second coating layer on the foundry sand, step (3) includes step (3-1) of mixing the foundry sand and the inorganic binder containing metasilicate hydrate.

[0076] When forming the second coating layer on the first coating layer of the foundry sand by mixing the foundry sand with a solution containing water glass, caustic alkali, and water in a known manner to obtain a mixture, and then drying the mixture, step (3) includes step (3-2) of mixing the foundry sand with a solution containing water glass, caustic alkali, and water.

[0077] <Casting molds> The casting mold of this embodiment includes coated sand having a second coating layer containing metasilicate hydrate on the first coating layer of the foundry sand. The casting mold of this embodiment is formed using the coated sand as the material.

[0078] <Method for manufacturing casting molds> The method for manufacturing a casting mold according to this embodiment is a method for manufacturing a mold using the coated sand. The method for manufacturing a casting mold according to this embodiment can be manufactured by applying known methods other than using the coated sand. The method for manufacturing a casting mold using coated sand is not particularly limited, but examples include a molding method using a heated molding die, a molding method in which steam is further passed through a heated molding die and then hot air is passed through, a gas hardening method (CO2 method), a room temperature self-hardening method (ester hardening method), or a combination thereof. The selection of these molding methods is determined appropriately within the scope that does not impair the effects of the present invention.

[0079] In the gas curing method described above, the coated sand filled in the molding die may be treated with carbon dioxide (CO2) gas to gel and harden the inorganic binder through a decrease in pH caused by dissolved carbon dioxide.

[0080] In the above-mentioned room-temperature self-hardening method, after mixing the inorganic binder with the foundry sand, an organic ester as a hardening agent may be added, and the inorganic binder may be hardened by the saponification of the ester and the resulting decrease in pH. The organic ester is not particularly limited, and examples include ethylene glycol diacetate, diacetin, triacetin, propylene carbonate, and γ-butyrolactone.

[0081] With respect to the embodiments described above, the present invention further includes the following embodiments. <1> Foundry sand having a first coating layer containing aluminosilicate on fire-resistant aggregate, Foundry sand in which the molar ratio of silicon (Si) to aluminum (Al) (Si / Al), measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the particles constituting the foundry sand, is 1.00 or more and 8.50 or less. <2> The molar ratio of silicon (Si) to aluminum (Al) (Si / Al), measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the particles constituting the foundry sand, is preferably 1.50 or more and 7.00 or less, more preferably 2.00 or more and 6.50 or less, even more preferably 3.00 or more and 6.00 or less, even more preferably 3.50 or more and 5.50 or less, and even more preferably 3.50 or more and 5.00 or less. <1> The foundry sand described above. <3> The moisture content of the foundry sand is preferably 0.04% by mass or more and less than 1.50% by mass, more preferably 1.00% by mass or less, even more preferably 0.50% by mass or less, even more preferably 0.30% by mass or less, and even more preferably 0.15% by mass or less. <1> or <2> The foundry sand described above. <4> The surface of the first coating layer contains sodium, <1> ~ <3> Foundry sand as described in any of the following. <5> The ratio of sodium (Na) to the total of silicon (Si), aluminum (Al), and sodium (Na) measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer (Na / (Si+Al+Na)×100(%)) is preferably 5% or more, more preferably 20% or more, even more preferably 40% or more, preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less. <4> The foundry sand described above. <6> The fire-resistant aggregate and the first coating layer have a silicate between them, <1> ~ <5> Foundry sand as described in any of the following. <7> The content of the aluminosilicate in the first coating layer is preferably 0.005 parts by mass or more and 1.000 parts by mass or less, more preferably 0.100 parts by mass or more and 1.000 parts by mass or less, and even more preferably 0.020 parts by mass or more and 0.500 parts by mass or less, based on Al2O3 equivalent, per 100 parts by mass of the refractory aggregate. <1> ~ <6> Foundry sand as described in any of the following. <8> The aforementioned <1> ~ <7> Coated sand having a second coating layer containing metasilicate hydrate on the first coating layer of the foundry sand described in any of the above. <9> The content of the second coating layer is preferably 0.1 parts by mass or more and 15.0 parts by mass or less, more preferably 0.2 parts by mass or more and 10.0 parts by mass or less, even more preferably 0.5 parts by mass or more and 8.0 parts by mass or less, even more preferably 1.0 parts by mass or more and 6.0 parts by mass or less, even more preferably 1.5 parts by mass or more and 4.0 parts by mass or less, and even more preferably 1.5 parts by mass or more and 3.0 parts by mass or less, per 100.0 parts by mass of the fire-resistant aggregate. <8> Coated sand as described. <10> The above is in a dry state. <8> or <9> Coated sand as described. <11> The aforementioned <1> ~ <7> A casting mold containing coated sand, wherein the first coating layer of the foundry sand described in any of the above has a second coating layer containing metasilicate hydrate. <12> A method for producing foundry sand having a first coating layer containing aluminosilicate on a refractory aggregate, (1) A step of mixing a refractory aggregate having silicate on its surface with an aluminate to obtain a mixture, and A method for producing foundry sand, comprising the step (2) of heat-treating the mixture. <13> The temperature of the heat treatment in step (2) is preferably 60°C or more and 200°C or less, more preferably 70°C or more and 200°C or less, even more preferably 80°C or more and 200°C or less, even more preferably 90°C or more and 150°C or less, and even more preferably 100°C or more and 150°C or less. <12> A method for producing foundry sand as described above. <14> The heating time in step (2) is preferably 5 minutes or more, more preferably 15 minutes or more, preferably 360 minutes or less, more preferably 240 minutes or less, even more preferably 180 minutes or less, and even more preferably 120 minutes or less. <12> or <13> A method for producing foundry sand as described above. <15> The molar ratio of silicon (Si) to aluminum (Al) (Si / Al) measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the particles constituting the foundry sand is preferably 1.00 or more and 8.50 or less, more preferably 1.50 or more and 7.00 or less, even more preferably 2.00 or more and 6.50 or less, even more preferably 3.00 or more and 6.00 or less, even more preferably 3.50 or more and 5.50 or less, and even more preferably 3.50 or more and 5.00 or less. <12> ~ <14> A method for manufacturing foundry sand as described in any of the following. <16> The surface of the first coating layer contains sodium, <12> ~ <15> A method for manufacturing foundry sand as described in any of the following. <17> The ratio of sodium (Na) to the total of silicon (Si), aluminum (Al), and sodium (Na) measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer (Na / (Si+Al+Na)×100(%)) is preferably 5% or more, more preferably 20% or more, even more preferably 40% or more, preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less. <16> A method for producing foundry sand as described above. <18> The aforementioned <1> ~ <7> A method for producing coated sand, comprising the step (3) of forming a second coating layer containing metasilicate hydrate on the first coating layer of the foundry sand described in any of the above. <19> The above step (3) includes a step (3-1) of mixing the foundry sand with an inorganic binder containing metasilicate hydrate. <18> The method for manufacturing coated sand as described above. <20> The aforementioned step (3) includes step (3-2) of mixing the foundry sand with a solution containing water glass, caustic alkali, and water. <18> or <19> The method for manufacturing coated sand as described above. [Examples]

[0082] The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0083] [Method for measuring the molar ratio (Si / Al) of silicon (Si) to aluminum (Al) on the surface of the first coating layer of particles constituting the foundry sand] 1. Sample preparation Aluminum carbon double-sided tape (Nisshin EM Co., Ltd.) was attached to a φ13mm copper SEM plate (Oken Shoji Co., Ltd.), and foundry sand particles were fixed on top of the tape so that it covered the entire surface. The sample was then attached to a measurement platen (sample stage; 75mm x 75mm) with carbon tape. 2. Measurement conditions The XPS system used was a Quantera SXM manufactured by ULVAC-PHI, Inc. The X-ray source was monochromatic Al Kα (tube voltage 15kV, output 25W, beam diameter 100μm), with a photoelectron extraction angle of 45°, a pass energy of 112eV, and an energy step of 0.1eV. Narrow scan spectra of C1s, O1s, Na1s, Al2p, Si2p, and Zn2p were acquired. Low-energy electron beams and low-energy argon ion beams were used to neutralize the charge of the sample during XPS measurement. 3. How to determine the Si / Al ratio on the sand surface The atomic concentrations of Al and Si were calculated using relative sensitivity coefficients from the integrated intensities obtained by subtracting the background from the narrow scan spectra of Al2p and Si2p acquired by XPS measurement. The Si / Al ratio was then determined by dividing the Si atomic concentration by the Al atomic concentration. Three measurements were taken at random points, and the average value was calculated. The analysis software used was MultiPak (Ver.9) from ULVAC-FI, Inc., and the Shirley method was used for background subtraction. 4. Evaluation methods for Na, Al, and Si The atomic concentrations (%) of Na, Al, and Si were calculated using relative sensitivity coefficients from the integrated intensities obtained by subtracting the background from the narrow scan spectra of Na1s, Al2p, and Si2p acquired by XPS measurements. The proportion of each atom was then calculated using the atomic concentrations. The percentage of Na = Atomic concentration of Na / (Atomic concentration of Na + Atomic concentration of Al + Atomic concentration of Si) × 100 Al ratio = Atomic concentration of Al / (Atomic concentration of Na + Atomic concentration of Al + Atomic concentration of Si) × 100 The proportion of Si = Atomic concentration of Si / (Atomic concentration of Na + Atomic concentration of Al + Atomic concentration of Si) × 100

[0084] [Method for evaluating foundry sand] [Method for evaluating moisture content] A crucible that had been preheated and weighed was filled with 10g of foundry sand particles, and after being exposed to heat at 900°C for 1 hour, the mass loss (mass%) was taken as the moisture content of the foundry sand particles.

[0085] [Evaluation of the sieve pass rate of foundry sand immediately after manufacturing, 3 minutes later] 1000g of each foundry sand sample for the examples and comparative examples was prepared and immediately subjected to a surface stability test using a Nakayama Co., Ltd. surface stability tester with a mesh size of 1.7mm and a vibration frequency of 300 times / minute for 3 minutes. The sieve passability was calculated from the mass of foundry sand that passed through the sieve using the following formula. A higher sieve passability indicates less blocked foundry sand. If the sieve passability after 3 minutes was below 50% by mass, the sieve passability of the foundry sand after subsequent storage in a high-humidity environment was not evaluated. Sieve pass rate (mass %) = (Gold of foundry sand that passed through the sieve / 1000g) × 100

[0086] [Evaluation of the sieve pass rate of foundry sand after storage in a high-humidity environment] 100g each of the foundry sands from the examples and comparative examples were placed in a poly bottle, and stored open without a lid at a temperature of 35°C and a relative humidity of 90% for 6 hours. After that, the contents of the poly bottle were placed on a sieve with a mesh size of 1.7 mm, and the sieve pass-through rate of the foundry sand after storage in a high-humidity environment was calculated from the mass of the foundry sand that passed through the sieve using the following formula. Sieve pass rate (mass %) = (Gold of foundry sand that passed through the sieve / 100g) × 100 The smaller the difference between the sieve pass rate of foundry sand immediately after manufacturing and the sieve pass rate of foundry sand after storage in a high-humidity environment, the more effectively blocking can be suppressed in a high-humidity environment.

[0087] [How to determine the wetness time (days) for coated sand] Half the volume of coated sand was placed in a cylindrical transparent plastic bottle with a diameter of 76 mm and a height of 125 mm. Using a bottle agitator, the plastic bottle was held so that its axis was horizontal, and rotated around the horizontal axis at room temperature (25°C) and a speed of 60 rpm. The dry state was defined as when the slope of the flowing coated sand layer inside the plastic bottle became flat and the angle formed between this slope and the horizontal plane (dynamic angle of repose) could be measured. The wet state was defined as when the coated sand did not flow inside the plastic bottle, or when it flowed but the slope of the coated sand layer did not form a flat surface, and as a result the dynamic angle of repose could not be measured. The time until the dynamic angle of repose could no longer be measured, i.e., when the coated sand became wet, after being stored in an arbitrary environment after production was defined as the wetness time (days). The evaluation results are shown in Table 2. If the wetness time (days) exceeded 10 days, the measurement was stopped at 10 days and indicated as ">10".

[0088] [Measurement of the physical properties of the mold] 1. Storage of coated sand Immediately after preparing each coated sand according to Examples 4-10 and Comparative Example 3, 3 kg of each coated sand was placed in a poly bag (0.05 mm thick, 500 mm wide, 600 mm long), the air inside the poly bag was squeezed out by hand, and the bag was sealed and stored for 10 days at 35°C. 2. Method for determining the dry or wet state of coated sand after storage at 2.35°C for 10 days. Half the volume of coated sand was placed in a cylindrical transparent plastic bottle with a diameter of 76 mm and a height of 125 mm. Using a bottle agitator, the plastic bottle was held so that its axis was horizontal, and rotated around the horizontal axis at room temperature (25°C) and a speed of 60 rpm. The dry state was defined as when the slope of the flowing coated sand layer inside the cylinder became a flat surface, and the angle formed between this slope and the horizontal surface (dynamic angle of repose) could be measured. The wet state was defined as when the coated sand did not flow inside the cylinder, or when it flowed but the slope of the coated sand layer did not form a flat surface, and as a result the dynamic angle of repose could not be measured. In the wet state, the fluidity was low, making it difficult to prepare the mold test piece in the next step, so evaluation was not possible, and the value for mold strength evaluation was set to 0 MPa. 3. Preparation of mold test specimens 100 parts by mass of each coated sand, stored at 35°C for 10 days, were placed in a stirrer (Taiyo Machinery Co., Ltd., Mini-Mini Super Mixer type B), 0.70 parts by mass of amorphous silica fine particles were added, and the mixture was stirred for 1 minute. The resulting mixture was filled into a mold (for 5 test specimens, 22.3 mm × 22.3 mm × 180 mm) heated to 180°C at a blow pressure of 0.3 MPa using a CSR-43 blow molding machine, and the coated sand was left to harden in the mold for 150 seconds to obtain a molded test specimen. 4. Evaluation The bending strength (MPa) of each molded test specimen was measured using a tensile and compression testing machine SVZ-201F (manufactured by Imada Seisakusho Co., Ltd.) under conditions of a span of 150 mm and a speed of 300 mm / min. The molded test specimens were left for 24 hours in a constant temperature and humidity chamber at 25°C and 55% relative humidity after being removed from the mold. The evaluation results are shown in Table 2.

[0089] <Material> [Fire-resistant aggregate] • Recycled sand (A1) and (A2): Prepared using the following method. [Method for producing recycled sand (A1)] (1) Preparation of coated sand 100 parts by mass of refractory aggregate 1 (Espearl #60L, manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 241 μm) was added to a stirrer as a refractory aggregate. Next, 2.00 parts by mass of sodium metasilicate heptahydrate, which had been heated to 80°C and melted, was added to the stirrer and kneaded for 4 minutes. Then, amorphous silica fine particles (0.7 parts by mass) were added and kneaded for 2 minutes to obtain dry coated sand to be used in the production of recycled sand (A1). (2) Preparation of the mold Ten kg of the obtained coated sand was poured into a mold for test mold production, and a test mold was obtained by heating it in a heating furnace at 230°C for 20 minutes. (3) Casting Ten kg of aluminum alloy AC4C material (720°C) was poured into the resulting test mold. (4) Preparation of recovered sand After casting, the castings were removed from the test molds, the test molds were crushed using hammers and other tools, and the sand was further crushed using a mini crusher (manufactured by Taiyo Machinery Co., Ltd.) to obtain recovered sand. (5) Production of recycled sand 100 kg of recovered sand was fed into a dry-type foundry sand recycling device (Hybrid Sand Master, manufactured by Nippon Chuzo Co., Ltd.) equipped with a fluidized bed, and batch processed at a rotor speed of 2400 rpm for 60 minutes to obtain recycled sand (A1).

[0090] [Method for producing recycled sand (A2)] By performing the above manufacturing procedure (1) to (5) to obtain the recycled sand (A1) a total of five times, recycled sand (A2) was obtained.

[0091] [Inorganic binders] • Sodium metasilicate nonahydrate: Sodium metasilicate nonahydrate (manufactured by Nippon Chemical Industrial Co., Ltd., Si / Na molar ratio 1.0, solids content 42.9% by weight) • Sodium metasilicate pentahydrate: Sodium metasilicate pentahydrate (manufactured by Nippon Chemical Industrial Co., Ltd., Si / Na molar ratio 1.0, solids content 57.5% by weight) • Sodium metasilicate heptahydrate: A mixture of 50 parts by mass of sodium metasilicate 9hydrate and 50 parts by mass of sodium metasilicate pentahydrate (Si / Na molar ratio 1.0, solid content 50.2% by weight). [Inorganic particles] • Amorphous silica particles: Denka Fused Silica SFP-20M (manufactured by Denka Co., Ltd., average particle size: 0.4 μm, degree of amorphization: 99.5% or higher)

[0092] <Manufacturing of foundry sand> [Material for first coating layer] • 50% sodium aluminate aqueous solution: Sodium aluminate #2019 (50% sodium aluminate aqueous solution manufactured by Asada Chemical Industries Co., Ltd.) • Potassium aluminate trihydrate: Potassium aluminate trihydrate (Kanto Chemical Co., Ltd.) • Water: Ion-exchanged water

[0093] [Example 1] 100 parts by mass of recycled sand (A1) was mixed with 0.051 parts by mass of a 50% sodium aluminate aqueous solution and 1.200 parts by mass of water, and stirred for 1 minute to obtain a mixture. The mixture was treated in a calcination furnace at a reaction temperature of 120°C for a regeneration treatment time of 1 hour. The mixture was then removed from the calcination furnace, cooled to room temperature, and then sieved through a 28-mesh sieve to remove aggregates from the mixture to obtain foundry sand (B1).

[0094] [Example 2] Foundry sand (B2) was obtained in the same manner as in Example 1, except that 0.063 parts by mass of a 50% sodium aluminate aqueous solution was added and the reaction time was changed to 0.5 hours.

[0095] [Example 3] Foundry sand (B3) was obtained in the same manner as in Example 1, except that the reaction time was changed to 3 hours.

[0096] [Comparative Example 1] A mixture was obtained by adding 0.25 parts by mass of calcined kaolin, 0.25 parts by mass of amorphous silica fine particles, and 0.50 parts by mass of water to 100 parts by mass of recycled sand (A1) and stirring for 2 minutes. The mixture was then treated in an electric furnace at a reaction temperature of 730°C for a regeneration treatment time of 1 hour. Next, the heating of the electric furnace was stopped and the mixture was left in the furnace for 4 hours. Subsequently, the mixture was removed from the electric furnace and cooled to room temperature, and then aggregates in the mixture were removed by passing it through a sieve (28 mesh) to obtain foundry sand (B11). When the "evaluation of the sieve pass rate of foundry sand immediately after production after 3 minutes" was performed on foundry sand (B11), the sieve pass rate was 35.9% by mass, so the evaluation of coated sand could not be performed.

[0097] [Comparative Example 2] Foundry sand (B12) was obtained in the same manner as in Example 1, except that a 50% sodium aluminate aqueous solution was not added.

[0098] Table 1 shows the evaluation results of the foundry sand for Examples 1-3 and Comparative Examples 1 and 2.

[0099] [Table 1]

[0100] <Manufacturing of coated sand and molds> [Example 4] Foundry sand (B4) was obtained in the same manner as in Example 1, except that 0.0315 parts by mass of a 50% sodium aluminate aqueous solution was added. 100 parts by mass of the foundry sand (B4) was left to stand at 35°C for 12 hours to adjust the temperature, and then added to a stirrer set up in a 35°C environment. Sodium metasilicate heptahydrate (2 parts by mass), which had been heated to 80°C and melted, was added to the stirrer and kneaded for 4 minutes to obtain coated sand. The obtained coated sand was then left to stand at 25°C and 55% RH for 12 hours to adjust the temperature and humidity, and then kneaded together with amorphous silica particles (0.70 parts by mass) in a stirrer set up in a 20-28°C environment for 2 minutes to obtain coated sand (C1) of Example 4.

[0101] [Example 5] Coated sand (C2) was obtained in the same manner as in Example 4, except that foundry sand (B1) was used.

[0102] [Example 6] Foundry sand (B5) was obtained in the same manner as in Example 1, except that 0.1260 parts by mass of a 50% sodium aluminate aqueous solution was added. Coated sand (C3) was obtained in the same manner as in Example 4, except that foundry sand (B5) was used.

[0103] [Example 7] Foundry sand (B6) was obtained in the same manner as in Example 1, except that the reaction temperature was changed to 160°C. Coated sand (C4) was obtained in the same manner as in Example 4, except that foundry sand (B6) was used.

[0104] [Example 8] Coated sand (C5) was obtained in the same manner as in Example 4, except that foundry sand (B2) was used.

[0105] [Example 9] Coated sand (C6) was obtained in the same manner as in Example 4, except that foundry sand (B3) was used.

[0106] [Example 10] Foundry sand (B7) was obtained in the same manner as in Example 1, except that recycled sand (A2) was used and 0.1600 parts by mass of a 50% sodium aluminate aqueous solution was added. Coated sand (C7) was obtained in the same manner as in Example 4, except that foundry sand (B7) was used.

[0107] [Example 11] Foundry sand (B8) was obtained in the same manner as in Example 1, except that 0.0389 parts by mass of potassium aluminate trihydrate was added instead of a 50% sodium aluminate aqueous solution. Coated sand (C8) was obtained in the same manner as in Example 4, except that foundry sand (B8) was used.

[0108] [Example 12] Foundry sand (B9) was obtained in the same manner as in Example 1, except that 0.0792 parts by mass of a 50% sodium aluminate aqueous solution was added. Coated sand (C9) was obtained in the same manner as in Example 4, except that foundry sand (B9) was used.

[0109] [Comparative Example 3] Coated sand (C11) was obtained in the same manner as in Example 4, except that foundry sand (B12) was used.

[0110] Table 2 shows the evaluation results of the strength of the molds manufactured using coated sand according to Examples 4 to 10 and Comparative Example 3.

[0111] [Table 2]

Claims

1. Foundry sand having a first coating layer containing aluminosilicate on fire-resistant aggregate, Foundry sand in which the molar ratio of silicon (Si) to aluminum (Al) (Si / Al), measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the particles constituting the foundry sand, is 1.00 or more and 8.50 or less.

2. The foundry sand according to claim 1, wherein the moisture content of the foundry sand is 0.04% by mass or more and less than 1.50% by mass.

3. The foundry sand according to claim 1, wherein the surface of the first coating layer contains sodium.

4. The foundry sand according to claim 1, wherein a silicate is provided between the refractory aggregate and the first coating layer.

5. The content of the aluminosilicate in the first coating layer is Al ２ O ３ The foundry sand according to claim 1, wherein the amount is 0.005 parts by mass or more and 1.000 parts by mass or less per 100 parts by mass of the refractory aggregate.

6. The foundry sand according to claim 3, wherein the ratio of sodium (Na) to the total of silicon (Si), aluminum (Al), and sodium (Na) measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer (Na / (Si+Al+Na)×100(%)) is 20% or more and 70% or less.

7. Coated sand having a second coating layer containing metasilicate hydrate on the first coating layer of the foundry sand according to any one of claims 1 to 6.

8. The coated sand according to claim 7, which is in a dry state.

9. A casting mold comprising coated sand, wherein the first coating layer of the foundry sand according to any one of claims 1 to 6 has a second coating layer containing metasilicate hydrate.

10. A method for producing foundry sand having a first coating layer containing aluminosilicate on a refractory aggregate, A step (1) of mixing a refractory aggregate having silicate on its surface with an aluminate to obtain a mixture, and A method for producing foundry sand, comprising the step (2) of heat-treating the mixture.

11. The method for producing foundry sand according to claim 10, wherein the temperature of the heat treatment in step (2) is 60°C or higher and 200°C or lower.

12. The method for producing foundry sand according to claim 10, wherein the molar ratio of silicon (Si) to aluminum (Al) (Si / Al), measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer of the particles constituting the foundry sand, is 1.00 or more and 8.50 or less.

13. A method for producing foundry sand according to claim 10, wherein sodium is contained on the surface of the first coating layer.

14. The method for producing foundry sand according to claim 13, wherein the ratio of sodium (Na) to the total of silicon (Si), aluminum (Al), and sodium (Na) measured by X-ray photoelectron spectroscopy (XPS) on the surface of the first coating layer (Na / (Si + Al + Na) × 100 (%)) is 20% or more and 70% or less.

15. A method for producing coated sand, comprising the step (3) of forming a second coating layer containing metasilicate hydrate on the first coating layer of the foundry sand according to any one of claims 1 to 6.

16. The method for producing coated sand according to claim 15, wherein step (3) is a step (3-1) of mixing the foundry sand with an inorganic binder containing metasilicate hydrate.

17. The method for producing coated sand according to claim 15, wherein step (3) comprises step (3-2) of mixing the foundry sand with a solution containing water glass, caustic alkali, and water.

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