Inorganic Coated Sand and Method for Producing the Same, Mold for Casting, and Method for Improving Storage Stability of Inorganic Coated Sand

Abstract

To provide inorganic coated sand which has improved storage stability while reducing environmental loads in manufacture of casting sand for inorganic coated sand.SOLUTION: Inorganic coated sand in a dried state has casting sand having one or two kinds of aggregates selected from recycled sand (A) and a fire resistant aggregate (B) and a first coating layer which coats the aggregate and is positioned on the surface, and a second coating layer containing a metasilicate on the first coating layer of the casting sand, wherein the fire resistant aggregate (B) contains SiO2, and a cation exchange capacity (CEC) of the casting sand is 3 mmol(+) / kg or more and 40 mmol(+) / kg or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to inorganic coated sand, a method for producing the same, a casting mold for casting, and a method for improving the storage stability of inorganic coated sand.

Background Art

[0002] In recent years, from the perspective of sustainable development goals (SDGs), the importance of product design that takes into account people and the environment has been increasing, and the technological development of environmentally friendly casting processes with reduced CO2 emissions and VOCs has been accelerating. As an environmentally friendly casting process, for example, an inorganic binder process for wet sand using a refractory aggregate and an inorganic binder composed of liquid water glass is known. For example, the one described in Patent Document 1 (Japanese Patent Publication No. 2010-519042) can be mentioned.

[0003] Patent Document 2 (Japanese Patent Publication No. 2021-536367) describes that by mixing recycled sand and particulate amorphous oxide containing silicon dioxide to obtain a mixture and heat-treating the mixture at a temperature of 400°C or higher, the pot life can be improved, the surface quality of the casting can be improved, and significant energy savings can also be achieved.

[0004] Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2014-117740) describes a manufacturing method for dry coated sand having normal temperature fluidity, which is formed by mixing a specific water glass aqueous solution as a binder with a heated refractory aggregate and evaporating moisture to form a coating layer of the binder on the surface of the refractory aggregate.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] By the way, after casting, the coated sand used for the mold is usually reused as recycled sand obtained by subjecting the recovered sand obtained by breaking (crushing) the mold into single particles to various recycling processes. From the viewpoints of economy and waste reduction, etc., in foundries, it has become an indispensable situation to manufacture molds using recycled sand.

[0007] The inventors of the present invention newly formed a metasilicate hydrate layer to produce an inorganic coated sand using foundry sand made of recycled sand of an inorganic binder process using a conventional inorganic binder as described in Patent Documents 2 and 3, and found that it may rapidly become wet during storage and a desired mold may not be formed. Also, when using a refractory aggregate with a relatively high degree of amorphization, when forming a metasilicate hydrate layer to produce an inorganic coated sand, similarly, a tendency to become wet during storage was observed.

[0008] As a recycling method of a conventional inorganic binder process, roasting recycling shown in Patent Document 1 can be mentioned. However, sufficient suppression of wetting cannot be achieved, and since high-temperature treatment at 600 to 800°C is performed, there are also problems from the viewpoints of energy consumption and CO2 emissions. Further, Patent Document 1 relates to an inorganic binder process of wet sand using an inorganic binder composed of liquid water glass. That is, the problem of the present invention relates to suppressing wetting during storage of inorganic coated sand using recycled sand or the like, and reducing the environmental load during production of foundry sand for inorganic coated sand using the same.

MEANS FOR SOLVING THE PROBLEM

[0009] Therefore, the inventors of the present invention have found that applying a coating layer to recycled sand or a refractory aggregate containing SiO2 and setting the cation exchange capacity (CEC) of foundry sand to a specific value is effective for solving the above problems, and completed the present invention.

[0010] According to the present invention, a dry inorganic coated sand having a casting sand in which a first coating layer is disposed on the surface of one or two kinds of aggregates selected from recycled sand (A) and refractory aggregate (B), and a second coating layer containing a metasilicate on the first coating layer of the casting sand, wherein the refractory aggregate (B) contains SiO2, a dry inorganic coated sand is provided, wherein the cation exchange capacity (CEC) of the casting sand is 3 mmol(+) / kg or more and 40 mmol(+) / kg or less.

[0011] Also, according to the present invention, a casting mold made of the above inorganic coated sand is provided.

[0012] Also, according to the present invention, a step (1) of mixing one or two kinds of aggregates selected from recycled sand (A) and refractory aggregate (B) with an aluminate or aluminum hydroxide to form a first coating layer on the surface of the aggregate to obtain a casting sand; a step (2) of mixing the obtained casting sand with a melt of a metasilicate hydrate to obtain a mixture; a step (3) of cooling the mixture to a temperature below the melting point of the metasilicate hydrate; is included, a method for producing an inorganic coated sand is provided, wherein the refractory aggregate (B) contains SiO2.

[0013] Also, according to the present invention, a step (4) of mixing one or two kinds of aggregates selected from recycled sand (A) and refractory aggregate (B) with an aluminate or aluminum hydroxide to form a first coating layer on the surface of the aggregate to obtain a casting sand; a step (5) of mixing the obtained casting sand with a solution containing water glass, caustic alkali, and water to obtain a mixture containing a metasilicate hydrate; is included, A method for producing an inorganic-coated sand is provided, wherein the refractory aggregate (B) contains SiO2.

[0014] Further, according to the present invention, a casting sand containing one or two kinds of aggregates selected from recycled sand (A) and refractory aggregate (B), and a second coating layer containing a metasilicate for coating the casting sand, and a method for improving the storage stability of the inorganic-coated sand, wherein the refractory aggregate (B) contains SiO2, and a method for improving the storage stability of the inorganic-coated sand is provided, wherein the dry inorganic-coated sand is prepared so that the cation exchange capacity (CEC) of the casting sand satisfies 3 mmol(+) / kg or more and 40 mmol(+) / kg or less.

Effect of the Invention

[0015] According to the present invention, it is possible to improve the storage stability of the inorganic-coated sand while reducing the environmental load during the production of the casting sand for the inorganic-coated sand.

Mode for Carrying Out the Invention

[0016] Hereinafter, embodiments of the present invention will be described. In the present specification, "A to B" indicating a numerical range represents a range of A or more and B or less unless otherwise specified. Also, the components described in each embodiment can be appropriately combined as long as the effects of the invention are not impaired.

[0017] Also, "coating" and "coating layer" are not limited to being continuous, and may have partially discontinuous portions.

[0018] <Inorganic-coated sand> The inorganic-coated sand of this embodiment is a foundry sand comprising one or two types of aggregates selected from recycled sand (A) and refractory aggregates (B), and a first coating layer that covers the aggregates and is located on the surface, and a second coating layer containing a metasilicate on the first coating layer of the foundry sand, and is a dry inorganic-coated sand. Further, the refractory aggregate (B) contains SiO2, and the cation exchange capacity (CEC) of the foundry sand is 3 mmol(+) / kg or more and 40 mmol(+) / kg or less.

[0019] That is, when producing the inorganic-coated sand using one or two types of aggregates selected from recycled sand (A) and refractory aggregates (B), the inorganic-coated sand of this embodiment stacks a first coating layer and a second coating layer containing a silicate on the surface of the aggregates in this order, and controls the cation exchange capacity (CEC) of the foundry sand, thereby improving the storage stability of the inorganic-coated sand using one or two types of aggregates selected from recycled sand (A) and refractory aggregates (B).

[0020] Although the details of such reasons are not clear, they are presumed as follows. (i) Residues of an inorganic binder that has been used once in casting may adhere to the surface of the recycled sand (A). When an inorganic-coated sand is produced by coating the surface of the recycled sand (A) having residues of the inorganic binder with a metasilicate hydrate, the components in the residues react with the metasilicate hydrate, melting the metasilicate hydrate crystals and causing the inorganic-coated sand to become wet. (ii) On the other hand, when producing an inorganic-coated sand by coating the surface of the refractory aggregate (B) containing SiO2 with a silicate hydrate, similar to (i), since the SiO2 component of the refractory aggregate (B) may react with the silicate hydrate, the silicate hydrate crystals may melt, and the inorganic-coated sand may become wet. (iii) Therefore, by forming the first coating layer so as to cover the surface of the recycled sand (A) or the refractory aggregate (B) and using molding sand having a specific CEC, the reaction with the residue can be suppressed. Subsequently, even if a second coating layer containing a metasilicate is laminated, the melting of the metasilicic acid hydrate in the second coating layer can be suppressed. As a result, it is presumed that the wetting of the inorganic-coated sand can be suppressed and the storage stability can be improved.

[0021] Further, when the first coating layer covering the surface of the recycled sand (A) or the refractory aggregate (B) is formed from an aluminosilicate-containing substance, it can be formed at a relatively low temperature. Therefore, in the regeneration of the conventional inorganic binder process, a baking step at 600 to 800 °C was taken, but this can be omitted, the consumption of thermal energy and CO2 emissions can be suppressed, and the environmental load can be reduced.

[0022] Hereinafter, the inorganic-coated sand of the present embodiment will be described more specifically.

[0023] The inorganic-coated sand is in a dry state. The dry-coated sand means a coated sand for which a measured value can be obtained when the dynamic angle of repose is measured regardless of the moisture content. The dynamic angle of repose is preferably 80° or less, more preferably 45° or less, and still more preferably 30° or less. Here, the dynamic angle of repose can be measured by the following method. (Method for measuring the dynamic angle of repose) Put half of the volume of the coated sand into a cylindrical transparent plastic bottle, hold it so that the axis is in the horizontal direction, and rotate it around the horizontal axis at a rotational speed of 60 rpm. The slope of the coated sand layer flowing in the cylinder becomes a flat surface. Measure the angle formed between such a slope and the horizontal plane. In addition, when the coated sand does not flow in the cylinder or even if it flows, the slope of the coated sand layer is not formed as a flat surface, and as a result, the dynamic angle of repose cannot be measured, it is in a wet state.

[0024] Specifically, the inorganic-coated sand is composed of a group of particles of the inorganic-coated sand.

[0025] From the viewpoint of improving fluidity and further enhancing the filling property into the molding die, the inorganic-coated sand is preferably spherical. Here, the inorganic-coated sand being spherical means having a round shape like a ball. More specifically, from the viewpoints of improving fluidity, mold quality, and mold strength, and the ease of molding the mold, the sphericity of the inorganic-coated sand is preferably 0.75 or more, more preferably 0.80 or more, and even more preferably 0.82 or more. Also, specifically, the upper limit value of the sphericity is 1. In this embodiment, the sphericity of the inorganic-coated sand specifically coincides with the sphericity of the refractory aggregate described later.

[0026] The sphericity of the inorganic-coated sand is obtained by image-analyzing the image (photo) of the particles obtained by an optical microscope or a digital scope (for example, VH-8000 type manufactured by Keyence Corporation) to obtain the area of the particle projection cross-section of the particles and the perimeter of the cross-section. Then, sphericity = [circumference of a perfect circle (mm) with the same area as the area of the particle projection cross-section (mm 2 )] / [perimeter of the particle projection cross-section (mm)] is calculated, and for any 50 particles, the values obtained for each can be averaged.

[0027] From the viewpoints of improving mold quality and mold strength, and the ease of molding the mold and storage stability, the average particle diameter of the inorganic-coated sand is preferably 0.05 mm or more, and more preferably 0.1 mm or more. Also, when the average particle diameter of the inorganic-coated sand is at least the above lower limit value, the amount of use of the second coating layer or the like can be reduced during the manufacture of the mold, which is also preferable in terms of making the regeneration of the inorganic-coated sand easier. From the viewpoints of improving mold quality and mold strength, and the ease of molding the mold, the average particle diameter of the inorganic-coated sand is preferably 2 mm or less, more preferably 1 mm or less, and even more preferably 0.5 mm or less. Also, when the average particle diameter of the inorganic-coated sand is at most the above upper limit value, the porosity becomes small during the manufacture of the mold, which is also preferable in terms of increasing the mold strength.

[0028] In this embodiment, the average particle diameters of the inorganic-coated sand and the refractory aggregate described below can be specifically measured by the following method.

[0029] (Method for measuring average particle diameter) When the sphericity = 1 from the particle projection cross-section of the particle, the diameter (mm) is measured. On the other hand, when the sphericity < 1, the major axis diameter (mm) and 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 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 minor axis diameter of the particle can be obtained by photographing an image (photo) of the particle with an optical microscope or a digital scope (for example, VH-8000 type manufactured by Keyence Corporation) and performing image analysis on the obtained image.

[0030] Hereinafter, each component of the inorganic-coated sand will be described.

[0031] [Recycled sand (A)] Recycled sand (A) is an aggregate and is a refractory material reused from a used casting mold or core formed from a refractory aggregate and a binder. Preferably, recycled sand (A) has a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate and containing one or more selected from silicates and reaction products of silicates. Recycled sand (A) is obtained, for example, by the manufacturing method described below.

[0032] (Refractory aggregate) Examples of the material of the refractory aggregate constituting recycled sand (A) include one or more selected from the group consisting of natural sand and artificial sand. Specifically, the refractory aggregate is composed of a group of particles of the refractory aggregate.

[0033] Examples of natural sand include one or more selected from the group consisting of silica sand mainly composed of quartz, chromite sand, zircon sand, olivine sand, and alumina sand.

[0034] Examples of artificial sand include one or more selected from the group consisting of synthetic mullite sand, SiO2-based casting sand mainly composed of SiO2, Al2O3-based casting sand mainly composed of Al2O3, SiO2 / Al2O3-based casting sand, SiO2 / MgO-based casting sand, SiO2 / Al2O3 / ZrO2-based casting sand, SiO2 / Al2O3 / Fe2O3-based casting sand, and slag-derived casting sand. Here, the main component refers to the component with the highest content among the components contained in the sand. Artificial sand refers to casting sand that is not produced naturally, but is artificially prepared with metal oxide components and melted or sintered. Also, recovered sand obtained by recovering used refractory aggregates, recycled sand obtained by subjecting the recovered sand to a recycling process, etc. can also be used.

[0035] Note that the content of each component such as SiO2, Al2O3, and Fe2O3 in the refractory aggregate can be measured using the following 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 is subjected to fluorescent X-ray analysis by the Fundamental Parameter (FP) method using a fluorescent X-ray analyzer ZSX Primus II (manufactured by Rigaku Corporation).

[0036] In this embodiment, since the sphericity of the refractory aggregate coincides with the sphericity of the inorganic-coated sand, the preferred conditions are the same. Also, from the viewpoints of improving mold quality and mold strength and the ease of molding the mold, the average particle size of the refractory aggregate is preferably 0.05 mm or more, more preferably 0.1 mm or more. Further, when the average particle size of the refractory aggregate is at or above the above lower limit value, the amount of the inorganic binder layer used as the second coating layer can be reduced during the production of the mold, which is also preferable in that the regeneration of the inorganic coated sand becomes easier. From the viewpoints of improving mold quality and mold strength and the ease of molding the mold, the average particle size of the refractory aggregate is preferably 2 mm or less, more preferably 1 mm or less, and even more preferably 0.5 mm or less. Further, when the average particle size of the refractory aggregate is at or below the above upper limit value, the porosity becomes small during the production of the mold, which is also preferable in that the mold strength can be increased.

[0037] From the viewpoints of the surface of the aggregate becoming smoother and the mold strength being further improved and obtaining low thermal expansibility, the degree of amorphization of the refractory aggregate is preferably 20% or more, more preferably 30% or more, and even more preferably 40% or more. The upper limit of the degree of amorphization of the refractory aggregate is not limited, but is, for example, 100% or less, and may be 99% or less.

[0038] The degree of amorphization of the refractory aggregate can be measured by the following X-ray diffraction method. (X-ray diffraction method) The refractory aggregate is pulverized in a mortar and pressed onto the X-ray glass holder of a powder X-ray diffractometer for measurement. The powder X-ray diffractometer uses MultiFlex manufactured by Rigaku Corporation (light source CuKα ray, tube voltage 40 kV, tube current 40 mA), and is performed at a scanning interval of 0.01°, a scanning speed of 2° / min, and slits DS1, SS1, and RS0.3 mm in the range of 2θ = 5 to 90°. In the range of 2θ = 10° to 50°, the X-ray intensities on the low-angle side and the high-angle side are connected by a straight line, the area under the straight line is taken as the background, the crystallinity is obtained using the software attached to the instrument, and 100 is subtracted to obtain the degree of amorphization. Specifically, for the area above the background, the amorphous peak (halo) and each crystalline component are separated by curve fitting, the area of each is obtained, and the degree of amorphization (%) is calculated by the following formula. Degree of amorphization (%) = Area of halo / (Area of crystalline component + Area of halo) × 100

[0039] There are various methods for controlling the degree of amorphization of refractory aggregates. Generally, it is preferable to use a manufacturing method that rapidly cools the melt. For example, there are methods such as melting the raw materials, crushing them with air, and rapidly cooling them, or treating them in a flame and rapidly cooling them. In any case, the cooling method may be appropriately selected at various rates depending on the material and particle size. Also, a method of amorphizing a once-crystallized material by heat treatment and cooling treatment is also conceivable. Among these, it is preferable to use the flame melting method in which heating and cooling can be easily controlled.

[0040] (Inorganic binder layer) The recycled sand (A) preferably has an inorganic binder layer. Specifically, the inorganic binder layer contains one or more selected from silicates and reaction products of silicates, and covers the surface of the refractory aggregate. Note that the covering is not limited to being continuous, and there may be partially discontinuous portions.

[0041] The above-mentioned inorganic binder layer is intended to be such that when inorganic coated sand having a refractory aggregate and an inorganic binder formed on the surface of the refractory aggregate is formed into a casting mold and used as a mold and then recycled into recycled sand, the inorganic binder exists on the surface of the refractory aggregate as a residual inorganic binder. That is, the silicates and reaction products of silicates contained in the above-mentioned inorganic binder layer are intended to have a residual inorganic binder present on the surface of the refractory aggregate. Specifically, the above-mentioned silicates and reaction products of silicates include, for example, silicates and metasilicates, etc. Examples of the reaction products include reaction products of silicate and amorphous silica, reaction products of silicate and amorphous silica, etc., and one or more of them are mixed. Also, examples of the cations constituting the salt include monovalent cations such as sodium, potassium, lithium, and ammonium, and divalent cations such as magnesium, calcium, and zinc.

[0042] As a method for confirming that the inorganic binder layer contains silicate or a reaction product of silicate, for example, a method of analyzing the components eluted by stirring the recycled sand in an aqueous hydrochloric acid solution with an ICP emission spectrometer to determine the concentrations of silicate ions, sodium ions, etc., a method of performing elemental analysis on the surface of the recycled sand by scanning electron microscope - energy dispersive X-ray spectroscopy (SEM-EDX) to confirm the presence of silicon, sodium, etc., or 23 Na, 29 a method of confirming the structure derived from silicate by Si solid NMR can be mentioned.

[0043] [Refractory aggregate (B)] From the viewpoints of the surface of the refractory aggregate (B) becoming smoother and the mold strength being further improved, and obtaining low thermal expansion, the degree of amorphization is at least 20% or more, preferably 30% or more. From the viewpoint of improving storage stability, 99% or less is preferable, more preferably 80% or less, still more preferably 70% or less, and even more preferably 60% or less.

[0044] Also, from the viewpoint of storage stability, the SiO2 content of the refractory aggregate (B) is preferably 10% or more, more preferably 20% or more, still more preferably 30% or more. From the viewpoint of further improving the mold strength, 99% or less is preferable, more preferably 80% or less, and still more preferably 70% or less. Also, from the viewpoint of improving storage stability, the SiO2 content of the refractory aggregate (B) is preferably 50% or less, and more preferably 40% or less.

[0045] The degree of amorphization of the refractory aggregate (B) is determined by the same X-ray diffraction method as that of the refractory aggregate of the recycled sand (A). Also, the SiO2 content in the refractory aggregate (B) can be measured using the same fluorescent X-ray method as that of the refractory aggregate of the recycled sand (A).

[0046] [Casting sand] The foundry sand has, on its outermost layer, a first coating layer formed on the surface of one or two types of aggregates selected from recycled sand (A) and refractory aggregates (B). The foundry sand can function as inorganic coated sand when a second coating layer is further formed on the first coating layer. The cation exchange capacity (CEC) of the foundry sand needs to be controlled to a specific value from the viewpoint of suppressing the wetting of the inorganic coated sand. Note that the control of the cation exchange capacity (CEC) can be performed by combining known methods. For example, it can be performed by selecting the material of the coating layer, adjusting the content, and adjusting the firing temperature and time. Specifically, the foundry sand is composed of a group of foundry sand particles.

[0047] The CEC of the foundry sand is determined by the ammonium acetate method (Fertilizer Analysis Method (Method of the National Institute of Agro-Environmental Sciences, Ministry of Agriculture, Forestry and Fisheries) 5.31.1) shown below. First, put a small piece of absorbent cotton at the lower part of the permeation tube as a support layer, and pack the filter paper pulp made by finely cutting the filter paper and stirring it in hot water to a thickness of about 5 mm to make a flat filtration surface. Next, plug the lower end of the permeation tube, put several mL of 1M ammonium acetate solution, weigh 2 - 4 g of the analysis sample (foundry sand), drop it in little by little for sedimentation filling, then remove the plug, assemble the device, and start washing with ammonium acetate solution. Use 100 mL of 1M ammonium acetate solution and adjust the dropping rate so that it finishes permeating in 4 - 20 hours. After the permeation is completed, wash the upper inner part of the permeation tube with a small amount of alcohol solution, and further wash the sample layer with 50 mL of 80% alcohol solution to remove the excess ammonium acetate solution. The analysis sample saturated with NH4 + is washed with 100 mL of 10% sodium chloride solution to exchange and leach NH4 + The ammoniacal nitrogen in this leachate is quantitatively determined by steam distillation and alkali titration, and is shown as milliequivalent per 1 kg of the analysis sample and taken as the cation exchange capacity.

[0048] From the viewpoint of enhancing storage stability, the cation exchange capacity (CEC) is 3 mmol(+) / kg or more, and preferably 6 mmol(+) / kg or more. On the one hand, from the perspective of enhancing storage stability, the cation exchange capacity (CEC) is 40 mmol(+) / kg or less, preferably 20 mmol(+) / kg or less, and more preferably 10 mmol(+) / kg or less.

[0049] (The first coating layer) The first coating layer that constitutes the foundry sand is a layer for coating the surface of one or two kinds of aggregates selected from recycled sand (A) and refractory aggregates (B). Further, the first coating layer constitutes the outermost layer of the foundry sand and is located between the second coating layer described later and the surface of the aggregate in the inorganic coated sand.

[0050] The first coating layer is made of a constituent material different from that of the second coating layer, preferably does not contain at least metasilicate, and more preferably is a layer formed from aluminosilicate. Thereby, it is possible to efficiently prevent the silicate in the second coating layer from acting on the aggregate and promoting the wetting of the inorganic coated sand.

[0051] Examples of the aluminosilicate include the reaction product of one or more selected from silicate and the reaction product of silicate, and aluminate or aluminum hydroxide. Here, the silicate refers to a substance containing an anion group having a structure in which one or several silicon atoms are centered and surrounded by electronegative ligands in the composition. Specifically, it refers to either the residual inorganic binder present on the surface of the refractory aggregate or the refractory aggregate (B), or both. From the viewpoint of easily improving the storage stability, it is preferable that the aluminosilicate contains at least one of the reaction product of one or more selected from silicate and the reaction product of silicate, and sodium aluminate or aluminum hydroxide. Note that the form of sodium aluminate is not particularly limited and may be either a powder or an aqueous solution.

[0052] The content of aluminum in the first coating layer, in terms of Al2O3 conversion, is preferably 0.1 part by mass or more, more preferably 0.4 part by mass or more, based on 100 parts by mass of the aggregate, from the viewpoint of enhancing storage stability. On the other hand, the content of aluminum in the first coating layer, in terms of Al2O3 conversion, is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, based on 100 parts by mass of the aggregate, from the viewpoint of enhancing mold strength while obtaining good storage stability.

[0053] Note that the content of aluminum in the first coating layer is determined by the following method. It is determined from the following formula using the analysis values of the aggregate by the fluorescent X-ray method and the analysis values of the casting sand containing the first coating layer using them. Al2O3 [parts by mass] of the first coating layer = {Al2O3 [parts by mass] of the casting sand} - {Al2O3 [parts by mass] of the aggregate}

[0054] The content of the first coating layer is preferably 0.02 part by mass or more, preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more, further preferably 1 part by mass or more, and even more preferably 2 parts by mass or more, based on 100 parts by mass of the aggregate, from the viewpoint of improving storage stability and obtaining a high-strength casting mold. The content of the first coating layer is preferably 10 parts by mass or less, more preferably 9.5 parts by mass or less, based on 100 parts by mass of the aggregate, from the viewpoint of achieving both storage stability and strength.

[0055] [Second Coating Layer] The second coating layer is a layer formed on the first coating layer and is the outermost layer of the inorganic coated sand.

[0056] The second coating layer contains metasilicate and is used to impart the function of a binder to the aggregate. It is preferable because it can improve the crystallinity of the second coating layer, and further because the inorganic coated sand becomes dry and has excellent normal-temperature fluidity.

[0057] Examples of the metasilicate include one or more selected from sodium metasilicate, potassium metasilicate, lithium metasilicate, ammonium metasilicate, etc., and hydrates thereof. Among them, sodium metasilicate and potassium metasilicate are preferable. Further, the amount of hydration of the metasilicate hydrate is not particularly limited, but is preferably 5 or more and 9 or less. Specifically, at least one selected from sodium metasilicate pentahydrate and sodium metasilicate nonahydrate is preferable, and sodium metasilicate nonahydrate is more preferable.

[0058] From the viewpoint of improving storage stability and obtaining a high-strength casting mold, the content of the second coating layer is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, still more preferably 0.5 parts by mass or more, even more preferably 1 part by mass or more, and even more preferably 2 parts by mass or more with respect to 100 parts by mass of the aggregate. On the other hand, from the viewpoint of achieving both storage stability and strength, the content of the second coating layer is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, and still more preferably 6 parts by mass or less with respect to 100 parts by mass of the aggregate.

[0059] From the viewpoints of improving mold strength, excellent productivity, and easy availability, the content of the metasilicate in the second coating layer is preferably 80% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more, even more preferably 98% by mass or more, and even more preferably substantially 100% by mass. Here, "substantially" means that it may contain components inadvertently included, for example, components other than the metasilicate contained in the raw material metasilicate. The total content of the metasilicate in the second coating layer refers to the total content of the metasilicate with respect to the entire components other than water in the second coating layer.

[0060] The content of the metasilicate in the second coating layer (in terms of anhydride) is preferably 0.03 parts by mass or more, more preferably 0.1 parts by mass or more, still more preferably 0.5 parts by mass or more, and even more preferably 0.9 parts by mass or more with respect to 100 parts by mass of the aggregate, from the viewpoint of improving storage stability and obtaining a high-strength casting mold. On the other hand, the content of the metasilicate is preferably 5 parts by mass or less, more preferably 4 parts by mass or less, still more preferably 3 parts by mass or less, and even more preferably 2 parts by mass or less with respect to 100 parts by mass of the aggregate, from the viewpoint of achieving both storage stability and strength.

[0061] (Inorganic fine particles) In the present embodiment, the inorganic-coated sand may further contain inorganic fine particles. When inorganic fine particles are included, it is preferable that the inorganic fine particles form part of the second coating layer. Specifically, it is preferable that the second coating layer further contains inorganic fine particles on at least one of the layer surface and within the layer, and it is more preferable that the second coating layer further contains inorganic fine particles on the layer surface. The inorganic fine particles may be contained both on the second coating layer and within the second coating layer. By doing so, the particles of the inorganic-coated sand can be more firmly bonded via the inorganic fine particles, and as a result, the strength of the obtained mold can be further improved. Note that the inorganic fine particles on the second coating layer may be partially embedded in the second coating layer.

[0062] The inorganic fine particles are not limited, and examples include one or more kinds of fine particles selected from silica, silicon, zinc oxide, aluminum hydroxide, tin oxide, and the like. Among them, silica particles are preferable from the viewpoint of improving the strength of the mold, and amorphous silica particles are more preferable from the viewpoint of having a large specific surface area and high reactivity with silicate. These inorganic fine particles may be used alone or in combination of two or more kinds.

[0063] (Other additives) In addition to the above components, the second coating layer may contain various additives as necessary. Examples of other additives include coupling agents, moisturizing agents, moisture resistance improvers, lubricants, surfactants, mold release agents, and the like. The coupling agent is not limited, and examples thereof include silane coupling agents, zircon coupling agents, titanium coupling agents, and the like. Examples of the moisturizing agent include polyhydric alcohols, water-soluble polymers, hydrocarbons, saccharides, proteins, and inorganic compounds other than those described above. Examples of the moisture resistance improver include metal oxides (excluding those listed above), carbonates, borates, sulfates, phosphates, and the like. Examples of the lubricant include waxes; fatty acid amides; alkylene fatty acid amides; stearic acid; stearyl alcohol; metal stearates such as lead stearate, zinc stearate, calcium stearate, magnesium stearate; monoglyceryl stearate; stearyl stearate; hydrogenated oils, and the like. Examples of the mold release agent include paraffin, wax, light oil, machine oil, spindle oil, insulating oil, waste oil, vegetable oil, fatty acid ester, organic acid, graphite fine particles, mica, vermiculite, fluorine-based mold release agents, silicone-based mold release agents, and the like.

[0064] (Water content) From the viewpoint of obtaining a high-strength mold, the content of water including the crystal water in the second coating layer is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, still more preferably 20 parts by mass or more, based on 100 parts by mass of the silicate (anhydride conversion). Also, from the viewpoints of the filling property into the molding die and obtaining a high-strength mold, the water content in the second coating layer is preferably 180 parts by mass or less, more preferably 160 parts by mass or less, still more preferably 150 parts by mass or less, and even more preferably 140 parts by mass or less, based on 100 parts by mass of the silicate.

[0065] The water content including the crystal water in the second coating layer contained in the inorganic coated sand can be adjusted according to the type of the silicate.

[0066] When the metasilicate is sodium metasilicate, from the viewpoint of obtaining a high-strength mold and from the viewpoint of easily manufacturing the mold, the water content including the water of crystallization in the second coating layer is preferably 60 parts by mass or more, more preferably 65 parts by mass or more, still more preferably 90 parts by mass or more, and even more preferably 110 parts by mass or more with respect to 100 parts by mass (anhydrous equivalent) of sodium metasilicate. Also, from the viewpoint of improving the fluidity and further enhancing the filling property into the molding die, it is preferably 180 parts by mass or less, more preferably 160 parts by mass or less, still more preferably 150 parts by mass or less, and even more preferably 140 parts by mass or less. For example, when the silicate constituting the second coating layer is only sodium metasilicate pentahydrate, the water content is 74 parts by mass with respect to 100 parts by mass of sodium metasilicate, and when it is only sodium metasilicate nonahydrate, the water content is 133 parts by mass with respect to 100 parts by mass of sodium metasilicate.

[0067] <Method for manufacturing recycled sand (A) / Method for recycling used casting sand> As a method for recycling the mold waste sand after casting, it can conform to a known method (for example, "Molding Method", 4th Edition, Japan Foundry Technology Association, November 18, 1996, pages 327 - 330). For example, methods such as dry grinding treatment (mechanical wear), wet grinding treatment, roasting treatment, and methods combining these treatments are known.

[0068] In the dry grinding treatment, a part of the residue of the inorganic binder layer present on the surface of the refractory aggregate can be removed. For the removal, for example, a sand reclaimer that performs ore grinding treatment by causing the sand to rise in the apparatus by a high-speed air flow and collide with a collision plate, thereby causing collision and friction between the sand grains; a high-speed rotating rotary reclaimer that performs ore grinding treatment by the collision and friction occurring between the projected sand generated by its centrifugal force and the input sand that falls; a method using an agitator mill that performs ore grinding treatment by utilizing the friction between the sand grains can be used. In addition, as the wet grinding treatment, for example, a method using a trough mill that performs ore grinding by the friction between abrasive grains in a trough where blades are rotated can be mentioned.

[0069] As the roasting treatment, for example, a method of using a roasting furnace such as a fluidized roasting furnace or a rotary kiln, continuously charging sand into the roasting furnace, and firing in the range of 200 to 1000 °C can be mentioned. Also, it is known to roast at 600 to 800 °C to enhance the accuracy.

[0070] Any method can be used for regeneration, but wet treatment and roasting treatment have complicated processes and a large energy load. Therefore, among them, dry grinding treatment is preferable.

[0071] <Method for manufacturing inorganic coated sand> Next, the method for manufacturing the inorganic coated sand of the present embodiment will be described. The method for manufacturing the inorganic coated sand of the present embodiment is a dry inorganic coated sand including one or two kinds of aggregates selected from recycled sand (A) and refractory aggregate (B), and a first coating layer covering the aggregate and located on the surface, and a second coating layer containing metasilicate on the first coating layer of the foundry sand, wherein the refractory aggregate (B) contains SiO2, and the cation exchange capacity (CEC) of the foundry sand satisfies 3 mmol(+) / kg or more and 40 mmol(+) / kg or less. It is a method for manufacturing dry inorganic coated sand.

[0072] According to the manufacturing method of the present embodiment, by forming the first coating layer on the surface of the aggregate and controlling the cation exchange capacity (CEC) of the foundry sand, it is possible to improve the storage stability and omit the roasting process of the aggregate, thereby reducing the consumption of thermal energy and reducing the environmental load.

[0073] First, as an example of the method for manufacturing the inorganic coated sand of the present embodiment, the refractory aggregate (B) contains SiO2, and a manufacturing method including the following steps (1) to (3) can be mentioned. · Step (1): Mix one or two types of aggregates selected from recycled sand (A) and refractory aggregates (B) with an aluminate or aluminum hydroxide to form the first coating layer on the surface of the aggregates, thereby obtaining casting sand. · Step (2): Mix the obtained casting sand with a melt of metasilicate hydrate to obtain a mixture. · Step (3): Cool the obtained mixture to a temperature below the melting point of the metasilicate hydrate to form the second coating layer on the first coating layer.

[0074] In Step (1), by mixing both an aluminate or aluminum hydroxide and the aggregate, the first coating layer is formed so as to cover the surface of the aggregate. Among these, it is preferable to form the first coating layer by heating at 25°C or higher and lower than 400°C. As a method of forming the first coating layer by heating at 25°C or higher and lower than 400°C in Step (1), for example, a method of introducing an aluminate into the aggregate heated at 25°C or higher and lower than 400°C, or a method of introducing an aluminate or aluminum hydroxide into the aggregate and then heating at 25°C or higher and lower than 400°C can be mentioned. Among these, from the viewpoint of forming a uniform coating layer, the latter method is preferable. Also, from the viewpoint of improving storage stability, the heating temperature is preferably 40°C or higher and 300°C or lower, and more preferably 70°C or higher and 200°C or lower.

[0075] As a method of forming the first coating layer by heating at 25°C or higher and lower than 400°C in Step (1), for example, a method of introducing an aluminate or aluminum hydroxide into the aggregate heated at 25°C or higher and lower than 400°C, or a method of introducing an aluminate or aluminum hydroxide into the aggregate and then heating at 25°C or higher and lower than 400°C can be mentioned. Among these, from the viewpoint of forming a uniform coating layer, the latter method is preferable. Also, from the viewpoint of improving storage stability, the heating temperature is preferably 40°C or higher and 300°C or lower, and more preferably 70°C or higher and 200°C or lower. When mixing the aggregate with aluminate or aluminum hydroxide, the mixing conditions such as the stirring speed and treatment time can be appropriately determined according to the processing amount of the mixture. Thereby, foundry sand having a cation exchange capacity (CEC) satisfying 3 mmol(+) / kg or more and 40 mmol(+) / kg or less can be obtained.

[0076] In step (2), the foundry sand and the melt of the metasilicate hydrate are mixed to obtain a mixture. As a method of mixing the melt of the metasilicate hydrate and the foundry sand in step (2), for example, a method of introducing the metasilicate into the foundry sand heated to a temperature equal to or higher than the melting point of the metasilicate hydrate, or a method of introducing the metasilicate heated to a temperature equal to or higher than the melting point of the metasilicate hydrate into the foundry sand can be mentioned. Among them, from the viewpoint of being able to shorten the coating time, the latter method is preferable. When mixing the foundry sand and the metasilicate hydrate, the mixing conditions such as the stirring speed and treatment time can be appropriately determined according to the processing amount of the mixture.

[0077] In step (3), the mixture obtained in step (2) is cooled to a temperature below the melting point of the metasilicate hydrate to reduce the fluidity of the metasilicate hydrate and fix the metasilicate hydrate on the surface of the foundry sand, thereby forming a metasilicate hydrate layer, that is, a second coating layer. Thereby, the inorganic-coated sand according to the present embodiment can be obtained.

[0078] Next, as another example of the method for producing the inorganic-coated sand of the present embodiment, the refractory aggregate (B) contains SiO2, and a production method including the following steps (4) to (5) can be mentioned. Step (4): A step of mixing one or two kinds of aggregates selected from the recycled sand (A) and the refractory aggregate (B) with an aluminate or aluminum hydroxide to form the first coating layer on the surface of the aggregate and obtain foundry sand Step (5): A step of mixing the foundry sand with a solution containing water glass, caustic alkali, and water to obtain a mixture containing sodium metasilicate hydrate

[0079] Step (4) can be carried out in the same manner as step (1) above.

[0080] In step (5), by setting the molar ratio of SiO2 / M2O (where M represents an alkali metal) / H2O in a solution containing water glass, caustic alkali, and water to 1:1:n (5 ≤ n ≤ 9), a metasilicate hydrate with a desired amount of hydration water can be formed. Furthermore, by mixing the above solution containing water glass, caustic alkali, and water with the foundry sand, the surface of the foundry sand can be coated with a metasilicate hydrate to form a second coating layer. The mixing conditions of water glass, caustic alkali, and water are not particularly limited, and known methods can be used. For example, the mixing may be carried out at ambient temperature, and when heat generation of the melt occurs, the mixing can be continued as it is, and then left until it reaches ambient temperature.

[0081] Thereby, the inorganic-coated sand according to this embodiment can be obtained.

[0082] <Mold> The casting mold of this embodiment includes the inorganic-coated sand in the above-described embodiment and is formed using the inorganic-coated sand. Examples of the molding method of the casting mold include a molding method using a heated mold, a molding method in which steam is further passed through a heated mold and then hot air is passed through.

[0083] <Method for improving storage stability> The method for improving the storage stability of the inorganic-coated sand of this embodiment is a foundry sand containing one or two kinds of aggregates selected from recycled sand (A) and refractory aggregates (B), and a second coating layer containing a metasilicate that coats the foundry sand, and is a method for improving the storage stability of inorganic-coated sand, wherein the refractory aggregate (B) contains SiO2, A step of preparing dry inorganic-coated sand is included so that the cation exchange capacity (CEC) of the foundry sand satisfies 3 mmol(+) / kg or more and 40 mmol(+) / kg or less. Thereby, the storage stability of the inorganic-coated sand can be improved. The step of adjusting the cation exchange capacity (CEC) of the foundry sand to 3 mmol(+) / kg or more and 40 mmol(+) / kg or less is performed by adjusting the manufacturing method of the inorganic-coated sand and the foundry sand. Details are the same as those described in the above-described manufacturing method of the inorganic-coated sand. Also, details such as the configuration of the inorganic-coated sand and the foundry sand are the same as those described above.

[0084] As described above, embodiments of the present invention have been described, but these are examples of the present invention, and various configurations other than the above can also be adopted.

[0085] Regarding the above-described embodiments, the present invention further discloses the following inorganic-coated sand, a method for manufacturing the inorganic-coated sand, and a method for manufacturing a casting mold.

[0086] <1> Foundry sand including one or two types of aggregates selected from recycled sand (A) and refractory aggregates (B), and a first coating layer covering the aggregates and located on the surface. A dry inorganic-coated sand having a second coating layer containing metasilicate on the first coating layer of the foundry sand. The refractory aggregate (B) contains SiO2. The cation exchange capacity (CEC) of the foundry sand is 3 mmol(+) / kg or more, preferably 6 mmol(+) / kg or more, 40 mmol(+) / kg or less, preferably 20 mmol(+) / kg or less, and more preferably 10 mmol(+) / kg or less. <2> The inorganic-coated sand according to <1>. The content of the first coating layer is 0.02 parts by mass or more, preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, still more preferably 1 part by mass or more, and even more preferably 2 parts by mass or more, with respect to 100 parts by mass of the aggregate, while it is 10 parts by mass or less, preferably 9.5 parts by mass or less. Inorganic coated sand. <3> The inorganic coated sand according to <1> or <2>, The content of the second coating layer is 0.05 parts by mass or more, preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, still more preferably 1 part by mass or more, and even more preferably 2 parts by mass or more, with respect to 100 parts by mass of the aggregate, while it is 10 parts by mass or less, preferably 8 parts by mass or less, and more preferably 6 parts by mass or less. Inorganic coated sand. <4> The inorganic coated sand according to any one of <1> to <3>, The first coating layer of the foundry sand contains aluminosilicate, and the aluminosilicate contains a reaction product of one or more selected from silicates and reaction products of silicates and an aluminate or aluminum hydroxide. Inorganic coated sand. <5> The inorganic coated sand according to any one of <1> to <4>, The aluminosilicate contains at least one of a reaction product of one or more selected from silicates and reaction products of silicates and sodium aluminate or aluminum hydroxide. Inorganic coated sand. <6> The inorganic coated sand according to any one of <1> to <5>, The dynamic angle of repose of the inorganic coated sand is preferably 80° or less, more preferably 45° or less, and still more preferably 30° or less. Inorganic coated sand. <7> The inorganic coated sand according to any one of <1> to <6>, The degree of amorphization of the refractory aggregate (B) is preferably 30% or more, more preferably 50% or more, still more preferably 65% or more, and even more preferably 80% or more. On the other hand, it is preferably 100% or less, more preferably 99% or less, an inorganic-coated sand. <8> The inorganic-coated sand according to any one of <1> to <7>, The content of SiO2 in the refractory aggregate (B) is preferably 10% or more, preferably 20% or more, more preferably 30% or more. On the other hand, it is preferably 99% or less, more preferably 80% or less, still more preferably 70% or less, even more preferably 50% or less, and even more preferably 40% or less, an inorganic-coated sand. <9> The inorganic-coated sand according to any one of <1> to <8>, The content of aluminum in the first coating layer in terms of Al2O3 is preferably 0.1 part by mass or more, more preferably 0.4 part by mass or more, based on 100 parts by mass of the aggregate. On the other hand, it is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, an inorganic-coated sand. <10> The inorganic-coated sand according to any one of <1> to <9>, The content of metasilicate in the second coating layer is preferably 80% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more, even more preferably 98% by mass or more, and even more preferably substantially 100% by mass, an inorganic-coated sand. <11> The inorganic-coated sand according to any one of <1> to <10>, The content of water including crystal water in the second coating layer is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, still more preferably 20 parts by mass or more, based on 100 parts by mass of the silicate (anhydrous basis). On the other hand, it is preferably 180 parts by mass or less, more preferably 160 parts by mass or less, still more preferably 150 parts by mass or less, and even more preferably 140 parts by mass or less, an inorganic-coated sand. A casting mold comprising the inorganic-coated sand according to any one of <1> to <11>. <13> A method for producing inorganic-coated sand, comprising: Step (1) of mixing one or two kinds of aggregates selected from recycled sand (A) and refractory aggregates (B) with an aluminate or aluminum hydroxide to form the first coating layer on the surface of the aggregate to obtain the casting sand; Step (2) of mixing the obtained casting sand with a melt of metasilicate hydrate to obtain a mixture; Step (3) of cooling the mixture to a temperature below the melting point of the metasilicate hydrate; and wherein the refractory aggregate (B) contains SiO2, a method for producing inorganic-coated sand. <14> A method for producing inorganic-coated sand, comprising: Step (4) of mixing one or two kinds of aggregates selected from recycled sand (A) and refractory aggregates (B) with an aluminate or aluminum hydroxide to form the first coating layer on the surface of the aggregate to obtain the casting sand; Step (5) of mixing the obtained casting sand with a solution containing water glass, caustic alkali, and water to obtain a mixture containing metasilicate hydrate; wherein the refractory aggregate (B) contains SiO2, a method for producing inorganic-coated sand. <15> The method for producing inorganic-coated sand according to <13> or <14>, comprising: In step (1) or step (4), heating at 25°C or higher and lower than 400°C, preferably 40°C or higher and 300°C or lower, more preferably 70°C or higher and 200°C or lower to form the first coating layer, a method for producing inorganic-coated sand. <16> The method for producing inorganic-coated sand according to <14>, comprising: In step (5), the molar ratio of SiO2 / M2O (M represents an alkali metal) / H2O in the solution containing water glass, caustic alkali, and water is 1:1:n (5 ≤ n ≤ 9), a method for producing inorganic-coated sand. <17> A foundry sand containing one or two kinds of aggregates selected from recycled sand (A) and refractory aggregate (B), A method for improving the storage stability of an inorganic coated sand, comprising a second coating layer containing a metasilicate coating the foundry sand, wherein the refractory aggregate (B) contains SiO2, A method for improving the storage stability of an inorganic coated sand, which prepares a dry inorganic coated sand so that the cation exchange capacity (CEC) of the foundry sand satisfies 3 mmol(+) / kg or more and 40 mmol(+) / kg or less.

Examples

[0087] Hereinafter, the present invention will be described with reference to examples and comparative examples, but the present invention is not limited thereto.

[0088] (1) Materials The materials used in the following examples and comparative examples will be described. [Refractory aggregate] · Refractory aggregate 1: Espar #60L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 241 μm, degree of amorphization: 45%) · Refractory aggregate (B1): Spherical fused silica (spheroidized natural silica sand by flame melting method, average particle size: 200 μm, degree of amorphization: 95% or more) · Refractory aggregate (B2): Nigai ceramic beads 60#650 (manufactured by Itochu Ceratec Co., Ltd., average particle size: 200 μm, degree of amorphization: 30%)

[0089] [Inorganic fine particles] · Amorphous silica fine particles: Denka fused silica SFP-20M (manufactured by Denka Co., Ltd., average particle size: 0.4 μm, degree of amorphization: 99.5% or more) · Calcined kaolin: SatintonW (manufactured by BASF Co., Ltd., average particle size: 0.4 μm)

[0090] [Recycled sand (A)] · Recycled sand (A1) was prepared according to the following procedure. (i) Preparation of inorganic coated sand 100 parts by mass of refractory aggregate 1 (Espal #60L) as a refractory aggregate was put into a stirrer. Next, 2.00 parts by mass of sodium metasilicate nonahydrate heated to 80°C and melted was put into the stirrer and kneaded for 4 minutes to obtain dry sand having normal temperature fluidity. Then, 0.6 parts by mass of amorphous silica fine particles were added and kneaded for 2 minutes to obtain dry inorganic coated sand used for producing recycled sand (A1). (ii) Production of mold A part of 10 kg of the obtained inorganic coated sand was poured into the center of a frustum-shaped mold with an upper diameter of 298 mm, a lower diameter of 205 mm, and a height of 265 mm for producing a test mold up to a height of 50 mm. Subsequently, a frustum-shaped metal core with an upper diameter of 280 mm, a lower diameter of 200 mm, and a height of 220 mm heated to 180°C was installed. The remaining inorganic coated sand was poured into the space between the mold and the metal core into the mold and heated in a heating furnace at 180°C for 20 minutes to obtain a test mold. (iii) Casting 10 kg of aluminum alloy AC4C material (720°C) was poured into the obtained test mold. After pouring, it was left to cool at room temperature. (iv) Production of recovered sand The casting was taken out from the test mold after casting, the test mold was crushed with a hammer or the like, and further pulverized with a mini crusher (manufactured by Taiyo Machinery Co., Ltd.) until the mold became single particles to obtain recovered sand. (v) Production of recycled sand 100 kg of the recovered sand was put into a dry casting sand recycling device (Hybrid Sand Master manufactured by Nippon Casting Co., Ltd.) equipped with a fluidized bed and batch-treated at a rotor rotation speed of 2400 rpm for 60 minutes to obtain recycled sand (A1). In addition, fine powder derived from the binder generated during the treatment was removed by a dust collector.

[0091] [Material for the first coating layer] · 50% sodium aluminate: Sodium aluminate #2019 (50% aqueous sodium aluminate solution manufactured by Asada Chemical Industry Co., Ltd.) · Aluminum hydroxide (Al(OH)3): Aluminum hydroxide (manufactured by Fujifilm Wako Pure Chemical Corporation, powdery, average particle size: 2.01 μm) · No. 2 water glass: Sodium silicate No. 2 (manufactured by Fuji Chemical Co., Si / Na molar ratio 2.4)

[0092] [Material for the second coating layer] · Sodium metasilicate nonahydrate: Sodium metasilicate nonahydrate (manufactured by Nippon Chemical Industry Co., Ltd.)

[0093] (2) Preparation of inorganic coated sand Each inorganic coated sand with the composition shown in Table 1 was prepared by the following procedure.

[0094] [Example 1] 100 parts by mass of recycled sand (A1) as a refractory aggregate was put into a stirrer. Next, 2.3 parts by mass of a 50% aqueous sodium aluminate solution was added and stirred for 2 minutes to obtain a mixture. Then, the obtained mixture was heat-treated in a firing furnace at a firing temperature of 80°C for 12 hours, cooled to room temperature, and then the aggregates in the mixture were removed through a sieve (20 mesh) to obtain casting sand with a first coating layer made of aluminosilicate formed on the surface of the recycled sand (A1). Next, the casting sand and 3.50 parts by mass of sodium metasilicate nonahydrate heated and melted at 80°C were put into a stirrer and kneaded for 4 minutes to form a second coating layer on the first coating layer, thereby obtaining the inorganic coated sand of Example 1. Table 1 shows the composition of the inorganic coated sand.

[0095] [Examples 2 - 3] The inorganic coated sands of Examples 2 - 3 shown in Table 1 were obtained in the same manner as in Example 1, except that the addition amount of the 50% aqueous sodium aluminate solution when forming the first coating layer made of aluminosilicate on the surface of the recycled sand (A1) was changed to the parts by mass shown in Table 1.

[0096] [Example 4] The inorganic coated sand of Example 4 shown in Table 1 was obtained in the same manner as in Example 1, except that 0.1 part by mass of No. 2 water glass was additionally added when adding the 50% aqueous sodium aluminate solution when forming the first coating layer made of aluminosilicate on the surface of the recycled sand (A1).

[0097] <Examples 5 to 8> Inorganic coated sand was obtained in the same manner as in Example 1, except that the baking temperature for forming the first coating layer made of aluminosilicate on the surface of the recycled sand (A1) was changed to the temperature shown in Table 1. Table 1 shows the composition of the inorganic coated sand of Examples 5 to 8.

[0098] <Example 9> Inorganic coated sand was obtained in the same manner as in Example 1, except that a suspension in which aluminum hydroxide was previously suspended in water (aluminum hydroxide: water = 1:1 mass ratio) was used instead of the 50% sodium aluminate aqueous solution. Table 1 shows the composition of the inorganic coated sand of Example 9.

[0099] <Examples 10 and 12> Inorganic coated sand was obtained in the same manner as in Example 2, except that the recycled sand (A1) as the refractory aggregate was changed to the refractory aggregate shown in Table 2. Table 2 shows the composition of the inorganic coated sand of Examples 10 and 12.

[0100] <Example 11> Inorganic coated sand was obtained in the same manner as in Example 3, except that the recycled sand (A1) as the refractory aggregate was changed to the refractory aggregate shown in Table 2. Table 2 shows the composition of the inorganic coated sand of Example 11.

[0101] <Example 13> Using the inorganic coated sand obtained in Example 2, recycled sand (A2) was obtained in the same manner as in steps (ii) to (v) of the above recycled sand (A1). Using this recycled sand (A2), inorganic coated sand was prepared in the same manner as in Example 2, and again, steps (ii) to (v) of the above recycled sand (A1) were repeated to obtain recycled sand (A3). Further, the process from the above recycled sand (A2) to obtaining recycled sand (A3) using recycled sand (A3) was repeated twice to obtain recycled sand (A5). Inorganic coated sand was obtained in the same manner as in Example 2 using the recycled sand (A5). The results of the storage stability are shown in Table 3.

[0102] <Comparative Example 1> An inorganic-coated sand containing the inorganic binder of Comparative Example 1 and not containing the first coating layer made of aluminosilicate was obtained in the same manner as in Example 1, except that neither 50% sodium aluminate nor heat treatment was performed. Table 1 shows the composition of the inorganic-coated sand of Comparative Example 1.

[0103] <Comparative Examples 2 - 3> Inorganic-coated sands of Comparative Examples 2 - 3 were obtained in the same manner as in Example 1, except that heat treatment was performed at the temperatures shown in Table 1 without adding 50% sodium aluminate. Table 1 shows the composition of the inorganic-coated sands of Comparative Examples 2 - 3.

[0104] <Comparative Example 4> 100 parts by mass of recycled sand (A1) as a refractory aggregate was prepared and put into a stirrer. Then, 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 were added and stirred for 2 minutes. Thereafter, the obtained mixture was heat-treated in an electric furnace at 730°C for 1 hour, then the heating of the electric furnace was stopped, and the mixture was left in the electric furnace for 4 hours. Subsequently, the mixture was taken out of the electric furnace, cooled to room temperature at room temperature, and then aggregates were removed from the mixture through a sieve (20 mesh). The obtained mixture and sodium metasilicate nonahydrate (3.50 parts by mass) heated to 80°C and melted were put into a stirrer and kneaded for 4 minutes to obtain the inorganic-coated sand of Comparative Example 4. Table 1 shows the composition of the inorganic-coated sand.

[0105] <Comparative Examples 5 - 6> Inorganic-coated sands of Comparative Examples 5 - 6 were obtained in the same manner as in Comparative Example 1, except that the recycled sand (A1) as the refractory aggregate was changed to the types of refractory aggregates shown in Table 2. Table 2 shows the composition of the inorganic-coated sands of Comparative Examples 5 - 6.

[0106] (3) Evaluation and Measurement Using the obtained inorganic-coated sand, the following evaluation and measurement were performed. The results are shown in Tables 1 and 2.

[0107] <Storage Stability; Measurement of Time to Wetting Immediately after preparing the inorganic-coated sand, the dynamic angle of repose of the inorganic-coated sand was measured according to the following procedure. Separately, immediately after preparing the inorganic-coated sand, 2 kg of the inorganic-coated sand was put into a plastic bag and sealed, and then left in a constant temperature room at 25°C / 55%RH. The dynamic angle of repose of the inorganic-coated sand was measured according to the following procedure every 24 hours, and the following confirmation was carried out regarding the state (dry state or wet state) of the inorganic-coated sand. When the inorganic-coated sand was in a wet state, the time from putting it into the plastic bag until it became wet was defined as the "time until wetting" (days).

[0108] (i) Procedure (Measurement of dynamic angle of repose) An inorganic-coated sand with a volume of half of a cylindrical transparent plastic bottle with a diameter of 76 mm and a height of 125 mm was put in, held so that the axis was in the horizontal direction, and rotated around the horizontal axis at a speed of 60 rpm at room temperature (25°C). The slope of the inorganic-coated sand layer flowing in the cylinder became a flat surface, and the angle (dynamic angle of repose) formed between such a slope and the horizontal plane was measured. (ii) Confirmation (Judgment of dry state or wet state) In the above procedure, the case where the angle could be measured was defined as the "dry state", and the case where the inorganic-coated sand did not flow in the cylinder, or even if it flowed, the slope of the inorganic-coated sand layer was not formed as a flat surface, and as a result, the dynamic angle of repose could not be measured was defined as the "wet state".

[0109] [Table 1]

[0110] [Table 2]

[0111] [Table 3]

Claims

1. A dry inorganic coated sand comprising: foundry sand in which a first coating layer is disposed on a surface of one or two types of aggregate selected from recycled sand (A) and refractory aggregate (B); and a second coating layer containing a metasilicate on the first coating layer of the foundry sand, The refractory aggregate (B) is SiO ２ and The cation exchange capacity (CEC) of the foundry sand is 3 mmol(+) / kg or more and 40 mmol(+) / kg or less, and the dry inorganic coated sand is thus obtained.

2. The inorganic coated sand according to claim 1, The content of the first coating layer is 0.02 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the aggregate.

3. The inorganic coated sand according to claim 1 or 2, The content of the second coating layer is 0.05 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the aggregate.

4. The inorganic coated sand according to claim 1 or 2, The inorganic coated sand, wherein the first coating layer contains an aluminosilicate, the aluminosilicate containing a reaction product of one or more selected from the group consisting of silicates and reaction products of silicates, and an aluminate or aluminum hydroxide.

5. The inorganic coated sand according to claim 4, The aluminosilicate comprises at least one of a reaction product of one or more selected from the group consisting of silicates and reaction products of silicates, and sodium aluminate or aluminum hydroxide.

6. A casting mold made of the inorganic coated sand according to claim 1 or 2.

7. A step (1) of mixing one or two aggregates selected from recycled sand (A) and refractory aggregate (B) with an aluminate or aluminum hydroxide, and forming a first coating layer on the surface of the aggregate to obtain molding sand; (2) mixing the obtained foundry sand with a molten liquid of metasilicate hydrate to obtain a mixture; (3) cooling the mixture to a temperature below the melting point of the metasilicate hydrate; Including, The refractory aggregate (B) is SiO ２ and The method for producing inorganic coated sand, wherein the cation exchange capacity (CEC) of the foundry sand is 3 mmol(+) / kg or more and 40 mmol(+) / kg or less.

8. (4) mixing one or two aggregates selected from the recycled sand (A) and the refractory aggregate (B) with an aluminate or aluminum hydroxide, and forming a first coating layer on the surface of the aggregate to obtain molding sand; (5) mixing the obtained foundry sand with a solution containing water glass, caustic alkali, and water to obtain a mixture containing metasilicate hydrate; Including, The refractory aggregate (B) is SiO ２ and The method for producing inorganic coated sand, wherein the cation exchange capacity (CEC) of the foundry sand is 3 mmol(+) / kg or more and 40 mmol(+) / kg or less.

9. A method for producing the inorganic coated sand according to claim 7 or 8, The method for producing inorganic coated sand, wherein in the step (1) or (4), the first coating layer is formed by heating to a temperature of 25° C. or higher and lower than 400° C.

10. A foundry sand containing one or two types of aggregate selected from recycled sand (A) and refractory aggregate (B); and a second coating layer containing a metasilicate coating the foundry sand, comprising: The refractory aggregate (B) is SiO ２ and The method for improving the storage stability of inorganic coated sand comprises preparing the dry inorganic coated sand so that the cation exchange capacity (CEC) of the foundry sand is 3 mmol(+) / kg or more and 40 mmol(+) / kg or less.

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