Casting sand

Abstract

The present invention pertains to a casting sand that has, on a refractory aggregate, a first coating layer containing an aluminosilicate. The mole ratio (Si / Al) of silicon (Si) and aluminum (Al) of particles forming the casting sand measured by X-ray photoelectron spectroscopy (XPS) in the surface of the first coating layer is 1.00-8.50. The present invention makes it possible to provide a casting sand that is unlikely to undergo blocking even in an environment that has a relatively high humidity.

Description

Foundry sand

[0001] This invention relates to foundry sand.

[0002] A known type of mold used in casting is obtained by filling a mold with coated sand, which has refractory aggregate and an inorganic binder layer containing metasilicate hydrate formed on the surface of the refractory aggregate, and then hardening the coated sand filled in the mold.

[0003] 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.

[0004] 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. Detailed description of the invention

[0005] Molds manufactured using coated sand are typically destroyed (crushed) and recovered after being used for casting, and then recycled through various methods for reuse as recycled sand. From an economic and waste reduction perspective, it is common practice in foundries to manufacture molds using recycled sand as refractory aggregate.

[0006] However, it has become clear that recycled sand (foundry sand) obtained by using coated sand as described in Japanese Patent Publication No. 2020-11296, then recovering and recycling it, tends to agglomerate and block in relatively humid environments. When recycled sand (foundry sand) blocks, it becomes difficult to uniformly form an inorganic binder layer on the surface, which is undesirable from the viewpoint of mold strength.

[0007] Furthermore, it has become clear that when coated sand, such as that described in Japanese Patent Publication No. 2020-11296, is used, then recovered and subjected to a recycling process to obtain recycled sand (foundry sand), and then an inorganic binder layer containing metasilicate hydrate is formed on the recycled sand, a mold with the desired strength can be obtained when used immediately after production. However, as time passes after the production of the coated sand, the strength of the resulting mold tends to decrease.

[0008] The present invention aims to provide foundry sand that is less prone to blocking even in environments with relatively high humidity.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] The following describes one embodiment of the present invention.

[0015] <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.

[0016] The blocking of recycled sand in relatively high humidity environments is thought to be due to the absorption of moisture from 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.

[0017] The foundry sand of this embodiment is a particle group. From the viewpoint of improving fluidity and further enhancing the filling property into the molding die, the sphericity of the foundry sand is preferably 0.75 or more, more preferably 0.80 or more, and still more preferably 0.82 or more. Specifically, the upper limit value of the sphericity is 1.

[0018] The sphericity of the foundry sand coincides with the sphericity of the refractory aggregate described later. The method for measuring the sphericity of the foundry sand is to perform image analysis on the image (photo) of the particles obtained by an optical microscope or a digital scope (for example, manufactured by Keyence Corporation, VH-8000 type) to obtain the area of the particle projection cross-section of the particles and the perimeter of the cross-section. Next, sphericity = [circumference (mm) of a perfect circle with the same area as the area (mm ２ 2) of the particle projection cross-section] / [perimeter (mm) of the particle projection cross-section], and for any 50 particles, the obtained values can be averaged.

[0019] From the viewpoints of improving mold quality and mold strength, ease of mold making, and storage stability, the average particle diameter of the foundry sand is preferably 0.05 mm or more, more preferably 0.10 mm or more. From the viewpoints of improving mold quality and mold strength and ease of mold making, the average particle diameter of the foundry sand is preferably 2.00 mm or less, more preferably 1.00 mm or less, and still more preferably 0.50 mm or less.

[0020] The average particle diameter of the foundry sand can be measured by the following method for measuring the average particle diameter. (Method for Measuring Average Particle Diameter) When the sphericity of the particle from the particle projection cross-section is 1, the diameter (mm) is measured. On the other hand, when the sphericity < 1, the major axis diameter (mm) and the minor axis diameter (mm) of the randomly oriented particles are measured, and (major axis diameter + minor axis diameter) / 2 is obtained. For any 100 particles, the values obtained are averaged to obtain the average particle diameter (mm). The major axis diameter and the minor axis diameter are defined as follows. The particle is stabilized on a plane, and when 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. On the other hand, 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 the minor axis diameter of the particle can be obtained by taking an image (photo) of the particle with an optical microscope or a digital scope (for example, manufactured by Keyence Corporation, VH-8000 type) and performing image analysis on the obtained image.

[0021] [Refractory aggregate] The refractory aggregate contains one or more selected from the group consisting of natural sand and artificial sand.

[0022] 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.

[0023] Examples of the artificial sand include, for example, synthetic mullite sand, SiO ２ -based sand having SiO ２ as the main component, Al ２ O ３ -based sand having Al ２ O ３ as the main component, SiO ２ / Al ２ O ３ -based sand, SiO ２ / MgO-based sand, SiO ２ / Al ２ O ３ / ZrO ２ -based sand, SiO ２ / Al ２ O ３ / Fe ２ O ３One or more selected from the group consisting of system sand and slag-derived sand may be mentioned. Here, the main component refers to the component with the largest mass among the components contained in the sand. The artificial sand refers not to naturally produced sand but to sand obtained by artificially preparing the components of metal oxides and melting or sintering them.

[0024] Incidentally, SiO ２ , Al ２ O ３ , Fe ２ O ３ The content of each component such as etc. can be measured by the following fluorescent X-ray method. [Method for measuring the components of the refractory aggregate by the 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 at 1200 °C for 10 minutes to be melted, and then cooled to prepare a sample in a glassy state (glass bead method). The sample can be used with a fluorescent X-ray analyzer ZSX Primus II (manufactured by Rigaku Corporation) to perform fluorescent X-ray analysis by the fundamental parameter (Fundamental Parameter: FP) method, thereby measuring the components of the refractory aggregate.

[0025] 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 inorganic binder after use on the refractory aggregate. The residue of the inorganic binder after use contains silicate. Examples of the cations constituting the silicate include monovalent cations such as sodium, potassium, lithium, and ammonium, and divalent cations such as magnesium, calcium, and zinc.

[0026] The recycled sand can be manufactured, for example, by the following method. [Method for manufacturing recycled sand] As a method for recycling mold waste sand after casting using coated sand, known methods (for example, "Mold Making Method", 4th edition, Japan Foundry Technology Association, November 18, 1996, pp. 327-330) can be followed. For example, methods such as dry polishing (mechanical wear), wet polishing, roasting, and combinations thereof are known.

[0027] 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.

[0028] 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.

[0029] 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 roasting is carried out in a range of 200 to 1000°C.

[0030] Any method can be used for regeneration, but since wet processing and roasting processes are complicated and energy-intensive, dry polishing is preferred.

[0031] 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 viewpoint of improving fluidity, mold quality, and mold strength, as well as 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.

[0032] 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 same measurement method as the measurement method for the average particle diameter of the foundry sand can be used to measure the average particle diameter of the refractory aggregate.

[0033] [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 there may be discontinuous parts. 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.

[0034] Aluminosilicates preferably contain at least one of a silicate and a reaction product of aluminate or aluminum hydroxide, as this improves 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.

[0035] Methods for confirming that the first coating layer contains aluminosilicate include, for example, a method of grinding foundry sand in a mill or other crushing machine 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.

[0036] Furthermore, the aluminosilicate content in the first coating layer is Al ２ O ３It can be calculated by conversion. The content of the aluminosilicate in the first coating layer is Al ２ O ３ In terms of conversion, from the viewpoint of storage stability, the amount 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, the amount 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.

[0037] Furthermore, the Al of the aluminosilicate in the first coating layer ２ O ３ The content obtained by conversion is determined by the following method. Using the analytical values ​​of the aggregate by the fluorescent X-ray method and the analytical values ​​of the foundry sand containing the first coating layer using those values, it is calculated from the following formula: Aluminosilicate content of the first coating layer (Al ２ O ３ (Conversion) [Parts by mass] = {Al of foundry sand} ２ O ３ [Mass portion] - {Al of fire-resistant aggregate} ２ O ３ [Parts by mass]}

[0038] 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.

[0039] 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.

[0040] 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 (hereinafter sometimes referred to as foundry sand particles) is preferably 1.00 or higher, more 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 preferably 8.50 or lower, more 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, from the viewpoint of suppressing blocking in a high-humidity environment and improving storage stability. 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.

[0041] 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, and preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less, from the viewpoint of obtaining a high-strength mold. The mole fraction is measured by the method described in the examples.

[0042] 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, from the viewpoint of improving the operability of the foundry sand. In this specification, the moisture content of the foundry sand particles is measured by the method described in the examples.

[0043] From the viewpoint of improving mold strength, the first coating layer preferably has sodium on its surface.

[0044] <Method for Manufacturing Foundry Sand> The method for manufacturing foundry sand according to this embodiment is a method for manufacturing 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.

[0045] In step (1) above, the method of mixing the refractory aggregate (A) and the aluminate is not particularly limited. For example, a known mixing device having a stirring section can be used, and the mixing can be performed by rotating the stirring section. Examples of such mixing devices include a kneader, ribbon mixer, Nauter mixer, Proscher mixer, Lödige mixer, and high-speed mixer. A Lödige mixer is preferred from the viewpoint of shortening the process because it can perform heating and mixing in one step.

[0046] In step (1) above, the temperature at which the refractory aggregate (A) and 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] <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 the 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.

[0053] 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 value 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.

[0054] The dynamic angle of repose of coated sand can be measured by the following method: (Method for measuring the 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 rotation 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, then the sand is in a wet state.

[0055] 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 molding dies. Here, "spherical" in the context of coated sand refers to a round shape, like a ball.

[0056] 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.

[0057] 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.

[0058] [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 coating layer and the second coating layer.

[0059] 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 parts 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.

[0060] 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.

[0061] 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.

[0062] 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 components may be 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.

[0063] 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 parts by mass or less, per 100.00 parts by mass of refractory aggregate.

[0064] 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.

[0065] [Measurement of hydration water content] (1) In a crucible that has been heated and weighed, amorphous SiO ２ 10 g of coated sand before the addition of additives such as fine particles is weighed and placed in the crucible, and the amount of mass loss (%) after heating at 900°C for 1 hour 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) Amorphous SiO ２ 100 g of coated sand before adding additives such as fine particles is weighed and immersed in 200 mL or more of water or hot water and stirred for 1 hour or more to extract metasilicate hydrate. The refractory aggregate is filtered out of the obtained extract, and water is removed by vacuum distillation using a rotary evaporator at 40°C and an internal pressure of 15 mmHg or less. Then, it is heated and dried at a temperature of 120°C to 180°C for 1 to 3 hours, and the weight of the dried product is weighed. The dry solid content (%) (B) of metasilicate hydrate in the coated sand is calculated. B = (M12 / M11) × 100 (M11: mass of coated sand (g), M12: weight of dried product (g)) (3) Amount of water in the hydration of metasilicate hydrate = [(A) / molecular weight of water] / [(B) / molecular weight of anhydrous metasilicate]

[0066] (Other) The second coating layer may further contain components other than metasilicate, for example amorphous SiO ２ Contains fine particles, amorphous SiO ２The composition may also contain inorganic fine particles other than the contained fine particles, humectants, moisture-resistant agents, coupling agents that strengthen the bond between the refractory aggregate and the inorganic binder composition, lubricants, surfactants, release agents, etc.

[0067] Amorphous SiO ２ The contained fine particles may be used because of their high reactivity with metasilicate hydrate. This makes it easier to improve the mechanical strength of the mold.

[0068] Amorphous SiO ２ The contained fine particles include precipitated silica, calcined silica produced in an electric arc or by flame hydrolysis, silica produced during the manufacture of Fe-Si, and ZrSiO ４ Examples include silica produced by thermal decomposition, silicon dioxide produced by oxidation of metallic silicon with an oxygen-containing gas, and spherical quartz glass powder produced from crystalline quartz by melting and subsequent rapid cooling. These can be used individually, or two or more can be used in combination.

[0069] Inorganic nanoparticles are amorphous SiO as described above. ２While not particularly limited to those containing fine particles, 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, lithium sulfate, magnesium sulfate Examples of fine particles include sulfates such as 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 of silicon, zinc, magnesium, aluminum, calcium, lithium, copper, iron, boron, zirconium, etc., selected from one or more types of fine particles.

[0070] The coupling agent is not limited to these, but examples include silane coupling agents, zircon coupling agents, and titanium coupling agents.

[0071] Examples of humectants include polyhydric alcohols, water-soluble polymers, hydrocarbons, sugars, proteins, and inorganic compounds other than those mentioned above.

[0072] Examples of moisture-resistant agents include metal oxides (excluding those listed above), carbonates, borates, sulfates, and phosphates.

[0073] 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.

[0074] 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.

[0075] <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.

[0076] [Step (3)] In step (3), 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 and 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.

[0077] 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.

[0078] 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.

[0079] <Casting mold> 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.

[0080] <Method for Manufacturing Casting Molds> The method for manufacturing casting molds according to this embodiment is a method for manufacturing molds using the coated sand. The method for manufacturing casting molds according to this embodiment can be manufactured by applying known methods other than using the coated sand. The method for manufacturing molds using coated sand is not particularly limited, but for example, 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, and a gas hardening method (CO2) ２ Methods for manufacturing molds include the following: the molding method, the 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.

[0081] In the aforementioned gas hardening method, the coated sand filled in the molding die is heated to carbon dioxide (CO2). ２ The inorganic binder may be gelled and hardened by treatment with gas, via a decrease in pH caused by dissolved carbon dioxide.

[0082] 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.

[0083] 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 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. <2> The foundry sand according to <1>, 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 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. <3> The foundry sand according to <1> or <2>, wherein 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. <4> The foundry sand according to any one of <1> to <3>, wherein the surface of the first coating layer contains sodium. <5> The foundry sand according to <4>, 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 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. <6> The foundry sand according to any one of <1> to <5>, wherein a silicate is present between the refractory aggregate and the first coating layer. <7> The content of the aluminosilicate in the first coating layer is Al ２ O ３The foundry sand according to any one of <1> to <6> above, wherein the amount 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, per 100 parts by mass of the refractory aggregate. <8> Coated sand having a second coating layer containing metasilicate hydrate on the first coating layer of the foundry sand according to any one of <1> to <7> above. <9> The coated sand according to <8>, wherein 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 refractory aggregate. <10> The coated sand according to <8> or <9>, which is in a dry state. <11> A casting mold comprising coated sand having a second coating layer containing metasilicate hydrate on the first coating layer of the foundry sand according to any one of <1> to <7>. <12> A method for producing foundry sand having a first coating layer containing aluminosilicate on a refractory aggregate, comprising the steps of: (1) mixing a refractory aggregate having silicate on its surface with an aluminate to obtain a mixture; and (2) heat-treating the mixture. <13> The method for producing foundry sand according to <12>, wherein 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. <14> The method for producing foundry sand according to <12> or <13>, wherein the time of the heat treatment 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.<15> A method for producing foundry sand according to any one of <12> to <14>, 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 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. <16> A method for producing foundry sand according to any one of <12> to <15>, wherein sodium is contained on the surface of the first coating layer. <17> The method for producing foundry sand according to <16>, 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 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. <18> The method for producing coated sand according to <18>, further comprising step (3) of forming a second coating layer containing metasilicate hydrate on the first coating layer of foundry sand according to any one of <1> to <7>. <19> The method for producing coated sand according to <18>, wherein step (3) comprises step (3-1) of mixing the foundry sand with an inorganic binder containing metasilicate hydrate. <20> The method for producing coated sand according to <18> or <19>, wherein step (3) is a step (3-2) of mixing the foundry sand with a solution containing water glass, caustic alkali, and water.

[0084] The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0085] [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 foundry sand] 1. Sample preparation Aluminum carbon double-sided tape (Nisshin EM Co., Ltd.) was attached to a φ13 mm copper SEM plate (Oken Shoji Co., Ltd.), and foundry sand particles were fixed on top of it so as to cover the entire surface of the tape. The sample was then attached to a measurement platen (sample stage; 75 mm x 75 mm) with carbon tape. 2. Measurement conditions An XPS instrument used was a Quantera SXM manufactured by ULVAC-PHI, Inc. The X-ray source was monochromatic Al Kα rays (tube voltage 15 kV, output 25 W, beam diameter 100 μm), the photoelectron extraction angle was 45°, the pass energy was 112 eV, and the energy step was 0.1 eV. Narrow scan spectra of C1s, O1s, Na1s, Al2p, Si2p, and Zn2p were acquired. Low-energy electron beams and low-energy argon ion beams were irradiated to neutralize the charge of the sample during XPS measurement. 3. Method for determining the Si / Al ratio on the sand surface The atomic concentrations of Al and Si were calculated using relative sensitivity coefficients from the integrated intensity obtained by subtracting the background from the narrow scan spectra of Al2p and Si2p acquired by XPS measurement, and the Si / Al ratio was determined by dividing the Si atomic concentration by the Al atomic concentration. Three points were measured randomly, 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. Method for evaluating Na, Al, and Si The atomic concentrations (%) of Na, Al, and Si were calculated using relative sensitivity coefficients from the integrated intensity obtained by subtracting the background from the narrow scan spectra of Na1s, Al2p, and Si2p acquired by XPS measurement. The proportion of each atom was calculated using the atomic concentrations. The proportion of Na = Atomic concentration of Na / (Atomic concentration of Na + Atomic concentration of Al + Atomic concentration of Si) × 100 The proportion of Al = 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

[0086] [Method for evaluating foundry sand] [Method for evaluating moisture content] 10 g of foundry sand particles were weighed into a crucible that had been preheated and weighed, and the amount of mass loss (mass%) after heating at 900°C for 1 hour was defined as the moisture content of the foundry sand particles.

[0087] [Evaluation of the sieve pass rate of foundry sand immediately after manufacturing, 3 minutes later] 1000g of each foundry sand sample from the examples and comparative examples was prepared and immediately subjected to a sieve with a mesh size of 1.7mm using a surface stability tester manufactured by Nakayama Co., Ltd., at a vibration rate of 300 times / minute for 3 minutes. The sieve pass rate was calculated from the mass of foundry sand that passed through the sieve using the following formula. A higher sieve pass rate indicates less blocked foundry sand. If the sieve pass rate after 3 minutes was below 50% by mass, the evaluation of the sieve pass rate of the foundry sand after subsequent storage in a high-humidity environment was not performed. Sieve pass rate (mass%) = (Foundry sand that passed through the sieve (g) / 1000g) × 100

[0088] [Evaluation of the sieve pass rate of foundry sand after storage in a high-humidity environment] 100g of each foundry sand from the examples and comparative examples was placed in a poly bottle, and stored for 6 hours at a temperature of 35°C and a relative humidity of 90% with the lid off and the bottle open. 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 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%) = (Foundry sand that passed through the sieve (g) / 100g) × 100 The smaller the difference between the sieve pass rate of the foundry sand immediately after manufacturing and the sieve pass rate of the foundry sand after storage in a high-humidity environment, the more effectively blocking can be suppressed in a high-humidity environment.

[0089] [Method for Determining the Wetting Time (Days) of 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. When the slope of the flowing coated sand layer in the plastic bottle became flat and the angle formed between this slope and the horizontal plane (dynamic angle of repose) could be measured, it was considered dry. When the coated sand did not flow in the plastic bottle, or even if it flowed, 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, it was considered wet. 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 wetting time (days). The evaluation results are shown in Table 2. If the wetting time (days) exceeded 10 days, the measurement was stopped at 10 days and indicated as ">10".

[0090] [Measurement of Mold Properties] 1. Storage of Coated Sand Immediately after preparing each coated sand according to Examples 4 to 10 and Comparative Example 3, 3 kg of the 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 at 35°C for 10 days. 2. Method for Determining the Dry or Wet State of Coated Sand After 10 Days of Storage at 35°C 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) at a speed of 60 rpm. The dry state was defined as the case where the slope of the coated sand layer flowing inside the cylinder became a flat surface, and the angle formed between this slope and the horizontal plane (dynamic angle of repose) could be measured. The wet state was defined as the case where the coated sand did not flow inside the cylinder, or even if it flowed, 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 case of the wet state, the fluidity was low, making it difficult to prepare the mold test specimen in the next step, so evaluation was not possible, meaning the value for evaluating the mold strength 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, was 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 obtained 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. The coated sand was then left to harden in the mold for 150 seconds to obtain mold test specimens. 4. Evaluation The bending strength (MPa) of each obtained mold test specimen was measured using an SVZ-201F tensile and compression testing machine (manufactured by Imada Seisakusho Co., Ltd.) under conditions of a span of 150 mm and a speed of 300 mm / min. The mold 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.

[0091] <Materials> [Refractory aggregate] ・Recycled sand (A1) and (A2): Prepared by the following method. [Method for preparing 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 put into a stirrer as refractory aggregate. Next, 2.00 parts by mass of sodium metasilicate heptahydrate, which had been heated to 80°C and melted, was put into 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 preparation of recycled sand (A1). (2) Preparation of mold 10 kg of the obtained coated sand was poured into a mold for test mold preparation and heated in a heating furnace at 230°C for 20 minutes to obtain a test mold. (3) Casting 10 kg of aluminum alloy AC4C material (720°C) was poured into the test mold obtained. (4) Preparation of recovered sand The casting was removed from the test mold after casting, the test mold was crushed with a hammer or the like, and further crushed using a mini crusher (manufactured by Taiyo Machinery Co., Ltd.) to obtain recovered sand. (5) Preparation of recycled sand 100 kg of recovered sand was put into a dry-type foundry sand recycling device equipped with a fluidized bed (Hybrid Sand Master manufactured by Nippon Chuzo Co., Ltd.), and batch processed for 60 minutes at a rotor rotation speed of 2400 rpm to obtain recycled sand (A1).

[0092] [Method for producing recycled sand (A2)] Recycled sand (A2) was obtained by performing the above production procedure (1) to (5) for obtaining recycled sand (A1) a total of five times.

[0093] [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 nonahydrate and 50 parts by mass of sodium metasilicate pentahydrate (Si / Na molar ratio 1.0, solids 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 amorphousness: 99.5% or higher)

[0094] <Manufacturing of Foundry Sand> [Materials for the 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

[0095] [Example 1] 0.051 parts by mass of a 50% sodium aluminate aqueous solution and 1.200 parts by mass of water were added to 100 parts by mass of recycled sand (A1), and the mixture was 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. Next, the mixture was removed from the calcination furnace, cooled to room temperature, and then aggregates in the mixture were removed by passing it through a sieve (28 mesh) to obtain foundry sand (B1).

[0096] [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.

[0097] [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.

[0098] [Comparative Example 1] 100 parts by mass of recycled sand (A1) was mixed with 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, and stirred for 2 minutes to obtain a mixture. The mixture was then treated in an electric furnace at a reaction temperature of 730°C for a regeneration treatment time of 1 hour. The heating of the electric furnace was then stopped, and the mixture was left in the furnace for 4 hours. The mixture was then removed from the electric furnace and cooled to room temperature. After that, 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.

[0099] [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.

[0100] Table 1 shows the evaluation results of the foundry sand for Examples 1 to 3 and Comparative Examples 1 and 2.

[0101]

[0102] <Manufacturing of Coated Sand and Mold> [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 an environment adjusted to 35°C. 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 an environment adjusted to 20-28°C for 2 minutes to obtain coated sand (C1) of Example 4.

[0103] [Example 5] Coated sand (C2) was obtained in the same manner as in Example 4, except that foundry sand (B1) was used.

[0104] [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.

[0105] [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.

[0106] [Example 8] Coated sand (C5) was obtained in the same manner as in Example 4, except that foundry sand (B2) was used.

[0107] [Example 9] Coated sand (C6) was obtained in the same manner as in Example 4, except that foundry sand (B3) was used.

[0108] [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.

[0109] [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.

[0110] [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.

[0111] [Comparative Example 3] Coated sand (C11) was obtained in the same manner as in Example 4, except that foundry sand (B12) was used.

[0112] 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.

[0113]

Claims

1. 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.

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 or 2, wherein the surface of the first coating layer contains sodium.

4. The foundry sand according to any one of claims 1 to 3, wherein a silicate is present 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 any one of claims 1 to 4, 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, comprising the steps of: (1) mixing a refractory aggregate having silicate on its surface with an aluminate to obtain a mixture; and (2) 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 or 11, 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 any one of claims 10 to 12, wherein the surface of the first coating layer contains sodium.

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) comprises 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 or 16, 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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