Method for producing dry inorganic coated sand and method for producing casting mold

Abstract

A method for producing dry inorganic coated sand includes: a roasting step (R) for roasting waste casting sand (A0) in which an inorganic binder (b0) is used; and a mixing step (M-I) for mixing the waste casting sand (A1) obtained in the roasting step (R) with an inorganic binder composition (b1) containing a metasilicate.

Description

Method for manufacturing dry inorganic coated sand and method for manufacturing casting mold

[0001] The present invention relates to a method for producing dry inorganic coated sand and a method for producing a casting mold.

[0002] Conventionally, molds used in casting include those obtained by molding a desired shape using inorganic-coated sand having a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate. Examples of technologies related to such inorganic-coated sand include the one described in Patent Document 1 (JP 2020-11296 A). Patent Document 1 describes dry inorganic-coated sand having a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate, where the inorganic binder layer contains metasilicate hydrate.

[0003] In recent years, the importance of product design that takes people and the environment into consideration has been increasing from the perspective of the Sustainable Development Goals (SDGs). ２ The development of environmentally friendly casting processes that reduce emissions and VOCs is accelerating. One such process involves producing molds using inorganic coated sand, casting, and then reusing the waste foundry sand obtained from the used molds by various regeneration methods.

[0004] Methods for regenerating waste foundry sand include those described in Patent Document 2 (JP 2010-519042 A) and Patent Document 3 (JP 2018-15814 A). Patent Document 2 describes a method for regenerating used foundry sand to which water glass has adhered, which method comprises preparing used foundry sand to which a binder based on water glass and containing added fine-grained metal oxide has adhered, and heat-treating the used foundry sand, heating the used foundry sand to a temperature of at least 200°C, thereby obtaining regenerated foundry sand. Patent Document 3 describes a method for reusing used core sand that uses water glass as a binder, the method comprising the steps of crushing the cores used in casting, heating the crushed cores after heat treatment so that the mass ratio of water-soluble water glass is 0.2% or less of the mass of the core sand and water glass, peeling the water glass from the core sand after the heating step, and separating and recovering the core sand from a mixture of the peeled water glass and the core sand.

[0005] Japanese Patent Application Laid-Open No. 2020-11296 Japanese Patent Application Laid-Open No. 2010-519042 Japanese Patent Application Laid-Open No. 2018-15814

[0006] However, inorganic-coated sand using water glass as an inorganic binder is generally used in a wet state, and is therefore known to have low fluidity and poor mold filling properties. Patent Document 2 focuses on the reuse of used foundry sand after casting using a mold made from a binder containing water glass and particulate metal oxides, and the reused foundry sand remains wet inorganic-coated sand. Similarly, the method for recycling used core sand described in Patent Document 3 focuses on the use of water glass, but does not consider the production of dry inorganic-coated sand.

[0007] In response to this, the present inventors have focused on a new method for obtaining dry inorganic-coated sand from waste foundry sand generated after casting in a mold made with a silicate compound such as water glass. Further intensive research has revealed that, when dry inorganic-coated sand is produced using waste foundry sand, there is room for improvement in terms of shortening the time required for forming an inorganic binder layer. In other words, in the method for obtaining dry inorganic-coated sand from waste foundry sand, there is a need to shorten the time required for the inorganic binder composition to crystallize and form a coating layer (hereinafter also referred to as the "drying time").

[0008] Therefore, the present inventors have conducted extensive research to shorten the drying time during the production of inorganic coated sand using waste foundry sand, and as a result have found that it is effective to subject the waste foundry sand to a roasting treatment, thereby completing the present invention.

[0009] According to the present invention, there is provided a method for producing dry inorganic-coated sand, which includes: a roasting step (R) of roasting waste foundry sand (A0) containing an inorganic binder (b0); and a mixing step (M-I) of mixing the waste foundry sand (A1) obtained in the roasting step (R) with an inorganic binder composition (b1) containing a metasilicate.

[0010] The present invention also provides a method for producing a casting mold, which comprises the step of forming a casting mold using the inorganic-coated sand obtained by the above-described method for producing dry inorganic-coated sand.

[0011] The present invention provides a method for producing inorganic-coated sand that can shorten the time required to produce dry inorganic-coated sand using waste foundry sand containing an inorganic binder, and also provides a method for producing a high-strength casting mold using the inorganic-coated sand obtained by the method.

[0012] In this specification, "a to b" indicating a numerical range represents a range from a to b unless otherwise specified. Furthermore, the components and elements described in each embodiment can be combined as appropriate as long as the effects of the invention are not impaired. Furthermore, in this specification, "coating" is not limited to being continuous, and may include discontinuous portions. Below, embodiments of the present invention will be described.

[0013] <Method for Producing Dry Inorganic Coated Sand> The method for producing dry inorganic coated sand of this embodiment (hereinafter also referred to as the production method of this embodiment) includes: a roasting step (R) of roasting waste foundry sand (A0) containing an inorganic binder (b0); and a mixing step (M-I) of mixing the waste foundry sand (A1) obtained in the roasting step (R) with an inorganic binder composition (b1) containing metasilicate. This shortens the production time for dry inorganic coated sand using waste foundry sand. Specifically, this shortens the time required for the inorganic binder composition (b1) containing metasilicate to crystallize and form an inorganic binder layer (B1) (coating layer) containing metasilicate, i.e., the drying time. Furthermore, this shortens the strength of casting molds formed using the inorganic coated sand obtained by this production method.

[0014] Although the details of the reason for this are not clear, it is speculated as follows. First, the waste foundry sand (A0) used after casting a mold formed using inorganic-coated sand coated with an inorganic binder (b0) typically contains residues such as silicate compounds derived from the inorganic binder (b0), their reaction products, and metal oxides. Therefore, if an attempt is made to reuse the waste foundry sand (A0) by using the waste foundry sand (A0) as aggregate and mixing it with a new inorganic binder composition (b1) containing metasilicate (hereinafter simply referred to as the inorganic binder composition (b1)) to coat it, it is believed that the residues contained in the waste foundry sand (A0) inhibit the crystallization of the inorganic binder composition (b1). As a result, it becomes difficult to form an inorganic binder layer (B1) containing metasilicate (hereinafter simply referred to as the inorganic binder layer (B1)), which requires additional time for drying, resulting in a decrease in the productivity of dry inorganic-coated sand. In contrast to this, in the present embodiment, by roasting the waste foundry sand (A0), the moisture content in the residue can be reduced, and structural changes occur in the silicate compounds and their reaction products, etc., which results in a decrease in the reactivity between the inorganic binder composition (b1) and the residue, promoting the crystallization of the inorganic binder composition (b1) and shortening the drying time. In addition, a uniform inorganic binder layer (B1) is formed, which is thought to improve the strength of a mold produced using the same.

[0015] As described above, the method for producing dry inorganic coated sand of this embodiment includes the roasting step (R) and the mixing step (M-I), but may further include any optional steps. For example, the method may include the polishing step (P-I), the roasting step (R), the mixing step (M-I), and the mixing step (M-II) in this order. Alternatively, the method may include the roasting step (R), the polishing step (P-II), the mixing step (M-I), and the mixing step (M-II) in this order. Each step will be described in detail below.

[0016] [Step of Preparing Waste Foundry Sand (A0)] First, the waste foundry sand (A0) to be used in the roasting step (R) is prepared. The waste foundry sand (A0) used in the manufacturing method of this embodiment can be one or more selected from a used mold formed from a refractory aggregate and inorganic-coated sand having a layer containing an inorganic binder (b0) on its surface, coarse fragments obtained by crushing the used mold, and particles obtained by pulverizing the coarse fragments. In other words, the waste foundry sand (A0) refers to used molds that have not been subjected to surface treatments such as polishing, roasting, or chemical treatment, and in which residues derived from the inorganic binder (b0) remain on the surface of the refractory aggregate that constitutes the waste foundry sand (A0).

[0017] In the manufacturing method of this embodiment, the inorganic binder (b0) refers to a binder containing an inorganic component such as a silicate compound. In the manufacturing method of this embodiment, the inorganic binder (b0) used in the waste foundry sand (A0) is preferably a silicate compound. In the manufacturing method of this embodiment, when the inorganic binder (b0) used in the waste foundry sand (A0) contains a silicate compound, specific examples of the silicate compound include sodium silicate, potassium silicate, sodium orthosilicate, potassium orthosilicate, sodium metasilicate, and potassium metasilicate, as well as hydrates thereof. That is, the waste foundry sand (A0) is preferably one or more of the following: a used mold formed from refractory aggregate and inorganic-coated sand having a layer containing the inorganic binder (b0) containing a silicate compound on the surface of the refractory aggregate; coarse fragments obtained by crushing the used mold; and particles obtained by pulverizing the coarse fragments.

[0018] The refractory aggregate contained in the waste foundry sand (A0) is preferably the artificial sand described below, which makes the refractory aggregate less likely to break and improves durability against repeated use.

[0019] The form of the waste foundry sand (A0) is preferably particles obtained by pulverizing coarse fragments, since this facilitates the formation of an inorganic binder layer (B1) on the surface of each grain of the waste foundry sand (A1) by mixing the waste foundry sand (A1) with the inorganic binder composition (b1) containing a metasilicate in the mixing step (M-I) described below. In other words, the waste foundry sand (A0) is preferably in the form of particle groups.

[0020] The casting method, mold breaking method, and crushing method for the coarse fragments are not particularly limited, and any known method can be used. The size and shape of the coarse fragments are also not particularly limited.

[0021] Next, the refractory aggregate constituting the waste foundry sand (A0) and the residue remaining on the surface of the refractory aggregate will be described in detail.

[0022] (Refractory Aggregate) The refractory aggregate used in the manufacturing method of this embodiment may be natural sand or artificial sand. Examples of natural sand include silica sand, chromite sand, zircon sand, olivine sand, and alumina sand, which are primarily composed of quartz. Artificial sand is not naturally occurring foundry sand, but refers to foundry sand obtained by artificially preparing components such as metal oxides and then melting or sintering them. These may be used alone or in combination of two or more types.

[0023] The refractory aggregate in the manufacturing method of this embodiment, i.e., the refractory aggregate contained in the waste foundry sand (A0), is preferably artificial sand from the viewpoint of improving durability and mold strength. Among the artificial sands, synthetic mullite sand, SiO ２ Al-based foundry sand ２ O ３ Foundry sand of the SiO ２ / Al ２ O ３ Foundry sand of the SiO ２ / Al ２ O ３ / ZrO ２ System foundry sand and SiO ２ / Al ２ O ３ / Fe ２ O ３ It is preferable to use at least one type selected from the group consisting of foundry sands of the same type.

[0024] SiO in refractory aggregate ２ , Al ２ O ３ , Fe ２ O ３ The content of each component can be measured using known analytical methods, such as X-ray fluorescence. X-ray fluorescence can be analyzed by the following method. (X-ray fluorescence method) Refractory aggregate is adjusted to 0.1 μm or less using a vibration mill and heated at 1050°C for 1 hour. 5 g of lithium tetraborate and 0.5 g of refractory aggregate are then mixed, heated at 1200°C for 10 minutes to melt, and cooled to prepare a vitrified sample (glass bead method). The sample is subjected to X-ray fluorescence analysis using a ZSX Primus II X-ray fluorescence analyzer (manufactured by Rigaku Corporation) using the fundamental parameter (FP) method.

[0025] The amorphization degree of the refractory aggregate is preferably 10% or more, more preferably 20% or more, even more preferably 30% or more, and even more preferably 35% or more, from the viewpoint of making the surface of the aggregate smoother and further improving mold strength, and from the viewpoint of obtaining low thermal expansion. The upper limit of the amorphization degree of the refractory aggregate is not limited, but may be, for example, 100% or less, or may be 99% or less.

[0026] There are various methods for controlling the degree of amorphization of refractory aggregate, but it is generally preferable to use a manufacturing method that rapidly cools the molten material. For example, there is a method in which the raw material is melted and rapidly cooled by air crushing, or a method in which it is treated in a flame and rapidly cooled. In either case, the cooling method may be selected appropriately at various speeds depending on the material and particle size. Another possible method is to amorphize a material that has been crystallized by heat treatment and cooling treatment.

[0027] The degree of amorphization of a refractory aggregate can be determined by the X-ray diffraction method shown below. (X-ray Diffraction Method) The refractory aggregate is crushed in a mortar and pressed onto an X-ray glass holder of a powder X-ray diffractometer for measurement. The powder X-ray diffractometer is a MultiFlex (CuKα radiation source, 40 kV tube voltage, 40 mA tube current) manufactured by Rigaku Corporation, and the diffractometer is operated 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°. A straight line is drawn between the low-angle and high-angle X-ray intensities in the range of 2θ = 10° to 50°, and the area under the line is used as the background. The degree of crystallinity is calculated using the software provided with the instrument, and this is subtracted from 100 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, and the area of ​​each is determined, and the degree of amorphousness (%) is calculated using the following formula: degree of amorphousness (%) = (area of ​​halo) / (area of ​​crystalline component + area of ​​halo) × 100

[0028] The refractory aggregate is preferably spherical from the viewpoint of increasing the strength of a mold manufactured using the dry inorganic-coated sand obtained by the manufacturing method of this embodiment (hereinafter also referred to as the inorganic-coated sand of this embodiment). The preferred range of sphericity of the refractory aggregate is the same as the preferred range of sphericity of the inorganic-coated sand of this embodiment, which will be described later. The sphericity of the refractory aggregate can be measured by the same method as the measurement of the sphericity of the inorganic-coated sand of this embodiment, which will be described later.

[0029] The preferred range of the average particle size of the refractory aggregate is the same as the preferred range of the average particle size of the inorganic-coated sand of this embodiment, which will be described later. The average particle size of the refractory aggregate can be measured in the same manner as the measurement of the average particle size of the inorganic-coated sand of this embodiment, which will be described later.

[0030] (Residues) Components derived from the inorganic binder (b0) remain on the surface of the waste foundry sand (A0) in which the inorganic binder (b0) has been used. That is, when the inorganic binder (b0) used in the waste foundry sand (A0) in the manufacturing method of this embodiment contains a silicate compound, silica (silicic acid), silicate compounds, and their reaction products derived from the inorganic binder (b0) containing the silicate compound are present as residues on the surface of the waste foundry sand (A0). When the inorganic binder (b0) used in the waste foundry sand (A0) in this manufacturing method contains a silicate compound, the silicate compounds remaining as residues include the silicate compounds listed in the case where the inorganic binder (b0) used in the waste foundry sand (A0) contains a silicate compound, and these reaction products include reaction products of silica (silicic acid) with the silicate compounds and reaction products between the silicate compounds.

[0031] Methods for confirming the presence of silica (silicic acid), silicates, and their reaction products on the surface of the waste foundry sand (A0) include, for example, a method in which the waste foundry sand (A0) is stirred in a hydrochloric acid aqueous solution and the dissolved silicate ions, sodium ions, and other metal ions are analyzed using an ICP (inductively coupled plasma) emission spectrometer to determine the concentrations, etc.; a method in which the surface of the waste foundry sand (A0) is subjected to elemental analysis using a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) to confirm the presence of silicon, sodium, and other metals; ２３ Na, ２９ One method is to confirm the silicate-derived structure by Si solid-state NMR.

[0032] In this embodiment, the waste foundry sand (A0) is subjected to a roasting step (R) in which the waste foundry sand (A0) is roasted to become waste foundry sand (A1), which is then used as aggregate (aggregate in the dry inorganic coated sand of this embodiment) to be coated with an inorganic binder composition (b1) containing a metasilicate.

[0033] [Polishing Step (P)] The manufacturing method of this embodiment may include a polishing step (P) in which at least one of the waste foundry sand (A0) before the roasting step (R) described below and the waste foundry sand (A1) after the roasting step (R) and before the mixing step (M-I) described below is polished. That is, the manufacturing method of this embodiment preferably further includes at least one of a polishing step (P-I) in which the waste foundry sand (A0) is polished before the roasting step (R), and a polishing step (P-II) in which the waste foundry sand (A1) after the roasting step (R) and before the mixing step (M-I). By including the polishing step (P), the manufacturing method of this embodiment reduces residues on the surface of the waste foundry sand (A0) and residues of the inorganic binder (b0) on the surface of the waste foundry sand (A1) that have undergone structural changes due to the roasting step (R), and thus facilitates shortening the drying time. The polishing method is not particularly limited, but examples thereof include dry polishing (mechanical abrasion), wet polishing, and a combination of these methods.

[0034] In dry polishing processes, sand can be thrown onto a rotor that rotates at high speed, and the sand is ground by the collisions and friction that occur between the thrown sand generated by the centrifugal force and the falling thrown sand; a hybrid sand master, which is a combined reclaimer that combines a rotary reclaimer and a fluid classifier; or a sand fresher, which uses the grinding and polishing power of a grinding wheel.

[0035] As an example of the wet grinding treatment, there is a method using a trough grinder in which grinding is performed by friction between sand grains in a trough with rotating blades.

[0036] In this embodiment, the polishing treatment is preferably a dry polishing treatment. The dry polishing treatment time is preferably 5 minutes or more, more preferably 10 minutes or more, and even more preferably 15 minutes or more, from the viewpoint of removing the residue. On the other hand, the dry polishing treatment time is preferably 120 minutes or less, more preferably 90 minutes or less, and even more preferably 80 minutes or less, from the viewpoint of suppressing cracking of the waste mold sand.

[0037] [Roasting Step (R)] The roasting step (R) is a step of roasting the waste foundry sand (A0) containing the inorganic binder (b0) to obtain the waste foundry sand (A1). The roasting step (R) reduces the moisture content of the waste foundry sand (A0) and causes a structural change in the residue, thereby shortening the drying time without inhibiting the crystallization of the inorganic binder composition (b1) containing metasilicate, which will be described later. As described above, the waste foundry sand (A0) may be one that has been subjected to a polishing treatment (polishing step (P-I)).

[0038] The roasting temperature in the roasting step (R) is preferably 200°C or higher, more preferably 300°C or higher, and even more preferably 400°C or higher, from the viewpoint of shortening the drying time in the production method of this embodiment. On the other hand, the roasting temperature is preferably 1000°C or lower, more preferably 800°C or lower, and even more preferably 700°C or lower, from the viewpoint of melting the residue and suppressing aggregation of the sand particles. More specifically, the roasting temperature in the roasting step (R) is preferably 200°C or higher and 1000°C or lower, more preferably 300°C or higher and 800°C or lower, and even more preferably 400°C or higher and 700°C or lower.

[0039] The roasting time is preferably 10 minutes or more, more preferably 15 minutes or more, even more preferably 30 minutes or more, and even more preferably 45 minutes or more from the viewpoint of shortening the drying time, while the roasting time is preferably 180 minutes or less, more preferably 150 minutes or less, even more preferably 140 minutes or less, and even more preferably 120 minutes or less from the viewpoint of melting the residue and suppressing aggregation of the sand particles.

[0040] The roasting method is not particularly limited, and examples of the equipment include roasting furnaces such as fluidized bed roasters, rotary kilns, and muffle furnaces, microwave heating, etc. When roasting the waste foundry sand (A0) in a roasting furnace, the waste foundry sand (A0) may be charged into the furnace before roasting begins, or the waste foundry sand (A0) may be charged into a furnace heated to a predetermined temperature.

[0041] (Reduction in Acid Consumption) In the production method of this embodiment, the reduction in the acid consumption (AD1) of the waste foundry sand (A1) obtained after the roasting step (R) relative to the acid consumption (AD0) of the waste foundry sand (A0) is a value calculated by the following formula: Reduction (%) = {(AD0 - AD1) / AD0} × 100 That is, in the roasting step (R), a structural change occurs in the residue of the waste foundry sand, which changes the acid consumption calculated by the formula. In the production method of this embodiment, it is preferable that the acid consumption (AD1) of the waste foundry sand (A1) obtained after the roasting step (R) is less than the acid consumption (AD0) of the waste foundry sand (A0) before being roasted in the roasting step (R). In the production method of this embodiment, the reduction in the acid consumption (AD1) of the waste foundry sand (A1) relative to the acid consumption (AD0) of the waste foundry sand (A0) is preferably 18% or more, more preferably 30% or more, and even more preferably 50% or more. In the production method of this embodiment, if a polishing step (P-I) in which the waste foundry sand (A0) is polished is included before the roasting step (R), the acid consumption AD0 is measured before the polishing step (P-I). In the production method of this embodiment, if a polishing step (P-II) in which the waste foundry sand (A1) is polished is included after the roasting step (R) and before the mixing step (M-I), the acid consumption AD1 is measured after the polishing step (P-II).

[0042] The acid consumption (AD1) of the waste foundry sand (A1) can be controlled by controlling the roasting time and roasting temperature, or by adjusting the polishing treatment conditions of the waste foundry sand (A0) and / or the waste foundry sand (A1). The measurement of the acid consumption (AD1) and (AD0) will be described in the Examples.

[0043] (Amount of Sodium Ions Eluted) The amount of sodium ions eluted from 50 g of the waste foundry sand (A1) obtained after the roasting step (R) in 100 ml of a 0.05 mol / L hydrochloric acid solution (hereinafter also referred to as the amount of sodium ions eluted from the waste foundry sand (A1)) is preferably 420 mg / L or less, more preferably 400 mg / L or less, even more preferably 300 mg / L or less, and particularly preferably 200 mg / L or less, in order to reduce residues and facilitate a shortening of the drying time. When a polishing step (P-II) in which the waste foundry sand (A1) is polished is performed after the roasting step (R) and before the mixing step (M-I), the amount of sodium ions eluted from the waste foundry sand (A1) is measured after the polishing step (P-II).

[0044] The amount of sodium ions eluted from the waste foundry sand (A1) can be controlled by controlling the roasting time and roasting temperature, or by adjusting the polishing treatment conditions for the waste foundry sand (A0) and / or the waste foundry sand (A1). The measurement of the amount of sodium ions eluted from the waste foundry sand (A1) will be described in the Examples.

[0045] [Mixing Step (M-I)] Next, the roasted waste foundry sand (A1) is mixed with an inorganic binder composition (b1) containing a metasilicate in the mixing step (M-I). The mixing step (M-I) forms an inorganic binder layer (B1) on the waste foundry sand (A1). That is, the inorganic binder composition (b1) containing a metasilicate is crystallized to form an inorganic binder layer (B1) covering the waste foundry sand (A1), thereby obtaining dry inorganic-coated sand (B). As described above, the waste foundry sand (A1) may be subjected to the polishing treatment (P-II) before being mixed with the inorganic binder composition (b1).

[0046] The metasilicate contained in the inorganic binder composition (b1) in the mixing step (M-I) is preferably sodium metasilicate hydrate. From the viewpoint of improving the strength of the mold produced from the inorganic-coated sand of the present invention, the content of metasilicate in the inorganic binder composition (b1) in the mixing step (M-I) 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. The content of metasilicate in the inorganic binder composition (b1) refers to the content of metasilicate relative to all components other than water in the inorganic binder composition (b1). When a compound contained in the inorganic binder composition (b1) has water of hydration, the content of metasilicate is calculated by converting each of the compounds, including the metasilicate, into an anhydrous form.

[0047] Preferred methods for preparing the inorganic binder composition (b1) containing metasilicate include (i) a method of mixing metasilicate nonahydrate, metasilicate pentahydrate, and water, (ii) a method of mixing metasilicate anhydride with water, and (iii) a method of mixing water glass, caustic alkali, and water to obtain metasilicate hydrate.

[0048] (Procedure for performing the mixing step (M-1) using the preparation method (i) of the inorganic binder composition (b1) containing a metasilicate) Hereinafter, the procedure for performing the mixing step (M-1) using the preparation method (i) of the inorganic binder composition (b1) containing a metasilicate will be described. In particular, an example of the method (i) using the present metasilicate hydrate includes the following steps (1) and (2).

[0049] Step (1): A step of mixing waste foundry sand (A1) and metasilicate hydrate to obtain a mixture; and Step (2): A step of cooling the mixture.

[0050] In step (1), the metasilicate hydrate is metasilicate nonahydrate, metasilicate pentahydrate, a mixture of metasilicate pentahydrate and water such that the amount of water of hydration is 9 or less, or a mixture of metasilicate anhydrous and water such that the amount of water of hydration is 9 or less. Step (1) is preferably carried out at a temperature equal to or higher than the melting point of the metasilicate hydrate used. Specifically, when metasilicate nonahydrate is used as the metasilicate hydrate, the temperature is equal to or higher than the melting point of metasilicate nonahydrate; when metasilicate pentahydrate is used as the metasilicate hydrate, the temperature is equal to or higher than the melting point of metasilicate pentahydrate; and when a mixture of metasilicate nonahydrate and metasilicate pentahydrate is used as the metasilicate hydrate, the temperature is preferably equal to or higher than the melting point of at least one of the metasilicates, and more preferably equal to or higher than both of the melting points. By mixing the waste foundry sand (A1) with the metasilicate hydrate at a temperature equal to or higher than the melting point of the metasilicate hydrate used, a mixture of the waste foundry sand (A1) and the inorganic binder composition (b1) is obtained.

[0051] Furthermore, for example, by mixing metasilicate nonahydrate and metasilicate pentahydrate in a 50:50 (weight ratio) ratio and appropriately adjusting the mixing procedure and conditions, the amount of water of hydration of the metasilicate hydrate can be controlled to 7.

[0052] In addition, examples of the process for obtaining a mixture by mixing the waste foundry sand (A1) with the metasilicate hydrate at a temperature equal to or higher than the melting point of the metasilicate hydrate used include a process (1A) in which the waste foundry sand (A1) and the metasilicate hydrate are mixed at a temperature equal to or lower than the melting point of the metasilicate hydrate, and then the temperature is raised to a temperature equal to or higher than the melting point of the metasilicate hydrate; a process (1B) in which the mixture of metasilicate hydrate heated to a temperature equal to or higher than the melting point of the metasilicate hydrate is added to the waste foundry sand (A1) and mixed; and a process (1C) in which the waste foundry sand (A1) maintained at a temperature equal to or higher than the melting point of the metasilicate hydrate is added to the mixture of metasilicate hydrate heated to a temperature equal to or lower than the melting point of the metasilicate hydrate, and mixed. Among these, from the viewpoint of shortening the drying time, the method of adding metasilicate hydrate heated to a temperature equal to or higher than the melting point of metasilicate hydrate to the waste foundry sand (A1), i.e., step (1B), is more preferred.

[0053] From the same viewpoint, it is preferable that in step (1), the metasilicate hydrate is mixed without being previously converted into an aqueous solution. It is also preferable that step (1) does not include a step of intentionally adding water. The mixing conditions, such as the stirring speed and treatment time, when mixing the waste foundry sand (A1) and the metasilicate hydrate can be appropriately determined depending on the amount of the mixture to be treated.

[0054] In step (2), the mixture obtained in step (1) 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 to the surface of the waste foundry sand (A1), thereby forming an inorganic binder layer (B1) containing the metasilicate hydrate, thereby obtaining the inorganic coated sand (B) of this embodiment.

[0055] (Procedure for carrying out the mixing step (M-1) using the preparation method (iii) of an inorganic binder composition containing metasilicate) Next, the procedure for carrying out the mixing step (M-1) using the preparation method (iii) of an inorganic binder composition (b1) containing metasilicate will be described. The method (iii) includes a step of obtaining an inorganic binder composition (b1) containing metasilicate using a solution containing water glass, caustic alkali, and water. In the above step, water glass (SiO ２ : M ２ SiO (where M is an alkali metal) in a molar ratio of 2-4:1, caustic alkali, and water are mixed to produce SiO ２ : M ２ By preparing a composition in which the molar ratio of O (M is an alkali metal) to SiO is 1:1, the inorganic binder composition (b1) containing metasilicate in the production method of this embodiment can be produced.

[0056] Examples of caustic alkali include caustic soda (sodium hydroxide; NaOH) and caustic potash (potassium hydroxide; KOH). The conditions for mixing the water glass, caustic alkali, and water are not particularly limited, and known methods can be used. For example, mixing may be carried out at ambient temperature, or if heat is generated in the molten liquid, mixing may be continued and then allowed to cool to ambient temperature.

[0057] Furthermore, examples of a method for mixing the solution containing water glass, caustic alkali, and water obtained in the above step with the waste foundry sand (A1) include a method in which the solution is poured into the waste foundry sand (A1) and mixed, or a method in which the waste foundry sand (A1) is poured into the solution and mixed. When the production method of this embodiment is carried out at a temperature above room temperature, it is preferable to subsequently carry out a step of cooling the mixture. When the production method of this embodiment is carried out at a temperature below room temperature, the step (2) of cooling the mixture may not be carried out.

[0058] (Amount of Metasilicate-Containing Inorganic Binder Composition (b1) Added) In the production method of this embodiment, the amount of the metasilicate-containing inorganic binder composition (b1) added is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, even more preferably 0.5 parts by mass or more, still more preferably 1 part by mass or more, and still more preferably 2 parts by mass or more, per 100 parts by mass of the waste foundry sand (A1), from the viewpoint of obtaining a high-strength casting mold. The amount of the metasilicate-containing inorganic binder composition (b1) added is preferably 15 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 8 parts by mass or less, per 100 parts by mass of the waste foundry sand (A1), from the viewpoint of shortening the drying time and obtaining a high-strength casting mold. More specifically, the amount of the inorganic binder composition (b1) containing metasilicate added is preferably 0.1 parts by mass or more and 15 parts by mass or less, more preferably 0.2 parts by mass or more and 10 parts by mass or less, even more preferably 0.5 parts by mass or more and 8 parts by mass or less, still more preferably 1 part by mass or more and 8 parts by mass or less, and still more preferably 2 parts by mass or more and 8 parts by mass or less, relative to 100 parts by mass of the waste foundry sand (A1).

[0059] [Mixing Step (M-II)] The manufacturing method of this embodiment may further include a mixing step (M-II) in which the mixture obtained in the mixing step (M-I) is mixed with inorganic particles. The mixing step (M-II) allows inorganic-coated sand (B), which is formed by coating waste foundry sand (A1) with an inorganic binder layer (B1) containing metasilicate, to be further coated with inorganic particles, thereby obtaining inorganic-coated sand (C). As a result, the mechanical strength of a mold prepared using the inorganic-coated sand (C) of this embodiment can be further improved. Hereinafter, inorganic-coated sand (B), which is formed by forming an inorganic binder layer (B1) on waste foundry sand (A1), and further coated with inorganic particles, may be referred to as inorganic-coated sand (C). Furthermore, the term "inorganic-coated sand" may be used to refer to either or both of the inorganic-coated sand (B) and the inorganic-coated sand (C).

[0060] The method and conditions for mixing the mixture obtained in the mixing step (MI) with the inorganic particles in the production method of this embodiment, i.e., the method and conditions for the mixing step (M-II), are not particularly limited, and known methods and conditions can be used.

[0061] (Inorganic Particles) When the production method of this embodiment includes the mixing step (M-II), inorganic particles are used because they have high reactivity with metasilicate hydrate, which makes it easier to improve the mechanical strength.

[0062] The inorganic particles preferably contain silica. When the inorganic particles contain silica, the content of silica in the inorganic particles is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 85% by mass or more, still more preferably 90% by mass or more, still more preferably 95% by mass or more, and still more preferably 99% by mass or more.

[0063] When the inorganic particles contain silica, the silica may be precipitated silica, pyrogenic silica produced in an electric arc or by flame hydrolysis, ZrSiO ４ Examples of silica include silica produced by thermal decomposition of silica, silica produced during the production of Fe—Si, silica produced by oxidation of metallic silicon with an oxygen-containing gas, and spherical particles of quartz glass powder produced from crystalline quartz by melting and subsequent rapid cooling. These can naturally be used alone, or two or more types can be used in combination.

[0064] When the inorganic particles contain silica, the degree of amorphization of the silica is preferably 80% or more, more preferably 90% or more, even more preferably 93% or more, still more preferably 95% or more, and still more preferably 98% or more, from the viewpoint of more firmly binding the particles of the inorganic-coated sand via the silica. The upper limit of the degree of amorphization of the silica is not limited, but may be, for example, 100% or less, 99.8% or less, or may be 99% or less.

[0065] The degree of amorphization of the inorganic particles can be determined by the same X-ray diffraction method as in the case of the refractory aggregate.

[0066] The average particle diameter of the inorganic particles is preferably 0.1 μm or more, more preferably 0.3 μm or more, from the viewpoint of improving mold strength per unit mass and improving handleability. Furthermore, from the viewpoint of improving mold strength per unit mass, the average particle diameter of the inorganic particles is preferably 2.0 μm or less, more preferably 1.0 μm or less, even more preferably 0.8 μm or less, and even more preferably 0.6 μm or less. Here, the average particle diameter of the inorganic particles is determined from images observed with a scanning electron microscope, and various image analysis methods can be used. Irregular particle sorting may be performed as a pretreatment. For example, after determining the inorganic particles based on elements, 100 inorganic particles are selected at random, their particle diameters are measured, and the average particle diameter of 80 inorganic particles, excluding the 10 inorganic particles counting from the largest particle diameter and the 10 inorganic particles counting from the smallest particle diameter, a total of 20 inorganic particles, can be determined as the average particle diameter of the inorganic particles.

[0067] When the manufacturing method of this embodiment includes the mixing step (M-II), the amount of inorganic particles mixed is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less, relative to 100 parts by mass of the aggregate in the inorganic-coated sand (B) of this embodiment, from the viewpoints of improving mold strength and mold surface shape while maintaining storage stability, and suppressing dust scattering. More specifically, the amount of inorganic particles mixed is preferably 0.1 parts by mass or more and 10 parts by mass or less, more preferably 0.2 parts by mass or more and 5 parts by mass or less, and even more preferably 0.2 parts by mass or more and 3 parts by mass or less, relative to 100 parts by mass of the aggregate in the inorganic-coated sand (B) of this embodiment.

[0068] Furthermore, when the production method of this embodiment includes the mixing step (M-II), the amount of inorganic particles mixed is preferably 25 parts by mass or more, more preferably 30 parts by mass or more, and preferably 200 parts by mass or less, more preferably 150 parts by mass or less, and even more preferably 145 parts by mass or less, relative to 100 parts by mass of the metasilicate (anhydride equivalent) in the inorganic-coated sand (B) of this embodiment. More specifically, the amount of inorganic particles mixed is preferably 25 parts by mass or more and 200 parts by mass or less, more preferably 30 parts by mass or more and 150 parts by mass or less, and even more preferably 30 parts by mass or more and 145 parts by mass or less, relative to 100 parts by mass of the metasilicate (anhydride equivalent) in the inorganic-coated sand (B) of this embodiment.

[0069] Furthermore, when the production method of this embodiment includes the mixing step (M-II), the amount of inorganic particles mixed is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less, relative to 100 parts by mass of the inorganic-coated sand (B) of this embodiment. More specifically, the amount of inorganic particles mixed is preferably 0.1 parts by mass or more and 10 parts by mass or less, more preferably 0.2 parts by mass or more and 5 parts by mass or less, and even more preferably 0.2 parts by mass or more and 3 parts by mass or less, relative to 100 parts by mass of the inorganic-coated sand of this embodiment.

[0070] <Inorganic Coated Sand of the Present Embodiment> The inorganic coated sand of the present embodiment is obtained by the above-described steps.

[0071] Next, the inorganic binder layer (B1) of the inorganic coated sand of this embodiment will be described in detail.

[0072] [Inorganic Binder Layer (B1)] The inorganic binder layer (B1) is formed by crystallizing an inorganic binder composition (b1) containing metasilicate and is formed on the surface of the waste foundry sand (A1). In other words, the inorganic binder layer (B1) covers the surface of the waste foundry sand (A1). The inorganic binder layer (B1) enables the formation of a mold as dry inorganic-coated sand.

[0073] (Coating Amount of Inorganic Binder Layer (B1) Included in Inorganic-Coated Sand) From the viewpoint of obtaining a high-strength casting mold, the coating amount of the inorganic binder layer (B1) included in the inorganic-coated sand of this embodiment is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, even 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, per 100 parts by mass of the waste foundry sand (A1). From the viewpoint of obtaining a high-strength casting mold, the coating amount of the inorganic binder layer (B1) included in the inorganic-coated sand is preferably 15 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 8 parts by mass or less, per 100 parts by mass of the waste foundry sand (A1).

[0074] (Metasilicate Hydrate) The inorganic binder layer (B1) may be a single layer or multiple layers as long as it has at least a layer containing metasilicate hydrate.

[0075] Metasilicate hydrate is one of the components constituting the inorganic binder layer (B1) and also one of the components constituting the inorganic binder composition (b1) containing the metasilicate hydrate. When the inorganic binder composition (b1) contains metasilicate hydrate, the crystallinity of the inorganic binder layer (B1) can be improved, and furthermore, the inorganic coated sand becomes a dry state, which is preferable because it has excellent fluidity at room temperature. Furthermore, since the metasilicate of the inorganic binder layer (B1) is a hydrate, there is no need to pass water vapor to harden the mold, and the equipment can be simplified. In this embodiment, the SiO of metasilicate hydrate ２ / Na ２ The O molar ratio is 0.9 to 1.1.

[0076] The salt of metasilicate hydrate is preferably an alkali metal, and more preferably one or more selected from lithium, sodium, and potassium, more preferably at least one of sodium and potassium, and even more preferably sodium.

[0077] From the viewpoints of improving mold strength, excellent productivity, and easy availability, the content of metasilicate in the inorganic binder layer (B1) is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, still more preferably 98% by mass or more, and still more preferably substantially 100% by mass. The content of metasilicate in the inorganic binder layer (B1) refers to the content of metasilicate relative to all components other than water (including hydration water) in the inorganic binder layer (B1).

[0078] Examples of methods for confirming that the inorganic binder layer (B1) contains metasilicate hydrate include a method in which inorganic-coated sand is subjected to a pulverizer such as a mill to separate only the inorganic binder layer (B1) component, and the inorganic binder layer (B1) component is analyzed by XRD to confirm a peak indicating the crystal structure of metasilicate hydrate; a method in which inorganic-coated sand is immersed in water and stirred for a certain period of time to elute the inorganic binder layer (B1) component, the eluted component is dried, and the dried solid content is analyzed by XRD to confirm a peak indicating the crystal structure of metasilicate, and the amount of water of hydration is analyzed by the following method to confirm that the inorganic binder layer (B1) is metasilicate hydrate.

[0079] (Measurement of Amount of Water of Hydration) When the inorganic-coated sand of this embodiment is not further coated with inorganic particles, the amount of water of hydration of the metasilicate hydrate in the inorganic binder layer (B1) contained in the inorganic-coated sand can be confirmed by the following method. [1] Measurement of Water Content (%) (A) in Inorganic-Coated Sand The water content (%) A in the inorganic-coated sand can be measured by either Method 1 or 2 below. (Method 1) 10 g of inorganic-coated sand is weighed and placed in a pre-baked and weighed crucible, and the amount of mass loss (%) after heating at 900°C for 1 hour is used to calculate the water content (%) (A) in the inorganic-coated sand. A = [(M1 - M2) / M3] x 100 (M1: total mass (g) of crucible and inorganic coated sand before firing, M2: total mass (g) of crucible and inorganic coated sand after firing, M3: mass (g) of inorganic coated sand before firing) (Method 2) The inorganic coated sand is measured under the following conditions using a simultaneous differential thermal and thermogravimetric analyzer (for example, Thermo plus EVO2 TG-DTA8122 (manufactured by Rigaku Corporation)), and the weight loss (%) (A) at 200°C is calculated. <Measurement conditions> Measurement atmosphere: Air Sample amount: Approximately 8 mg Heating rate: 10.0°C / min

[0080] [2] Measurement of the solid content (%) (B) of metasilicate hydrate The solid content (%) (B) of metasilicate hydrate in inorganic-coated sand can be measured by either Method 1 or 2 below. (Method 1) Weigh out 100 g of inorganic-coated sand, immerse in 200 mL or more of water or hot water, and stir for at least 1 hour to extract metasilicate hydrate. The refractory aggregate is removed from the resulting extract by filtration, and the water is removed by vacuum distillation using a rotary evaporator at 40°C and an internal pressure of 15 mmHg or less. The resulting mixture is then dried by heating 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 inorganic-coated sand is calculated. B = (M12 / M11) x 100 (M11: mass (g) of inorganic-coated sand, M12: dry weight (g)) (Method 2) Measurement is performed according to the following procedure. <Preparation of extract> 30 g of inorganic-coated sand is placed in a 200 ml tall beaker, 100 ml of pure water is added, and the mixture is stirred for 25 minutes with a magnetic stirrer to prepare an extract. The supernatant of the extract is then filtered through a membrane filter (pore size: 0.45 μm), and 10 ml of the filtrate is placed in a 100 ml beaker. <Titration> 20 ml of 0.1 mol / L HCl aqueous solution is added to the 100 ml beaker containing the 10 ml of filtrate above. Then, while stirring with a magnetic stirrer, 0.1 mol / L NaOH aqueous solution is titrated using an automatic titrator (e.g., Eco Titrator (Metrohm)). The titration amount X (ml) at which the pH reaches 7 is measured with a pH meter. <Calculation of metasilicate solid content> Acid consumption Y (mol) = ([amount of HCl aqueous solution (ml)] - [titer amount X (ml)]) / 1000 × [concentration of HCl aqueous solution (mol / l)] = (20 - X) / 1000 × 0.1 B = ([acid consumption Y (mol)] × 10 × [molecular weight of metasilicate anhydride] / [Na valence]) / (amount of inorganic coated sand (g)) × 100 = (Y × 10 × molecular weight of metasilicate anhydride / 2) / 30 × 100

[0081] [3] Calculation of the amount of water of hydration of metasilicate hydrate The amount of water of hydration of metasilicate hydrate is calculated by applying the amount of water (%) (A) in the obtained inorganic coated sand and the solid content (%) (B) of metasilicate hydrate to the following formula: Amount of water of hydration of metasilicate hydrate = [(A) / molecular weight of water] / [(B) / molecular weight of metasilicate anhydride]

[0082] (Components that the inorganic binder composition may contain) The inorganic binder composition (b1) may further contain components other than metasilicate hydrate, such as inorganic particles, a humectant, a moisture resistance improver, a coupling agent, a lubricant, a surfactant, and a release agent.

[0083] Examples of inorganic particles include amorphous silica, crystalline silica, silicon; 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; 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; and hydroxides such as lithium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, aluminum hydroxide, and zinc hydroxide. The amorphous silica includes precipitated silica, pyrogenic silica produced in an electric arc or by flame hydrolysis, silica produced during the production of Fe—Si, ZrSiO ４ Examples of silica include silica produced by thermal decomposition of silica, silica produced by oxidation of metallic silicon with an oxygen-containing gas, and spherical particles of quartz glass powder produced from crystalline quartz by melting and subsequent rapid cooling.

[0084] Even when the inorganic binder composition (b1) contains the inorganic particles listed above as a component other than metasilicate hydrate, the production method of the present embodiment may include the mixing step (M-II).

[0085] Coupling agents include, but are not limited to, silane coupling agents, zirconium coupling agents, titanium coupling agents, etc. Moisturizing agents include, for example, polyhydric alcohols, water-soluble polymers, hydrocarbons, sugars, proteins, and inorganic compounds other than those mentioned above. Moisture resistance improvers include carbonates, borates, sulfates, phosphates, etc. Lubricants include, for example, waxes; fatty acid amides; alkylene fatty acid amides; stearic acid; stearyl alcohol; metal stearates such as lead stearate, zinc stearate, calcium stearate, and magnesium stearate; stearic acid monoglyceride; stearyl stearate; and hydrogenated oils. Mold release agents include, for example, paraffin, wax, oils and fats, fatty acid esters, organic acids, fluorine-based mold release agents, and silicone-based mold release agents.

[0086] (Dry Inorganic Coated Sand) The inorganic coated sand of this embodiment is in a dry state. Dry coated sand means coated sand for 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.

[0087] (Method for Measuring the Dynamic Angle of Repose) The dynamic angle of repose of inorganic-coated sand can be measured by the following method. Half the volume of inorganic-coated sand is placed in a cylindrical transparent plastic bottle (diameter: 7.7 cm, height: 16 cm), and the bottle is held so that its axis is horizontal and rotated around the horizontal axis at a rotation speed of 60 rpm. The inclined surface of the inorganic-coated sand layer flowing inside the cylinder becomes flat. The angle formed between this inclined surface and the horizontal plane is measured. Note that if the inorganic-coated sand does not flow inside the cylinder, or if it flows but the inclined surface of the inorganic-coated sand layer does not form a flat surface, and as a result the dynamic angle of repose cannot be measured, the inorganic-coated sand is in a wet state.

[0088] The inorganic coated sand of this embodiment is preferably spherical in shape, from the viewpoint of increasing the strength of a mold produced using the inorganic coated sand of this embodiment. Here, spherical refers to a round shape like a ball.

[0089] From the above viewpoint, the sphericity of the inorganic coated sand is preferably 0.80 or more, more preferably 0.82 or more, and even more preferably 0.85 or more. The upper limit of the sphericity is specifically 1.

[0090] The sphericity of the inorganic coated sand is determined by analyzing the image (photograph) of the particle obtained by an optical microscope or a digital scope (for example, VH-8000 model, manufactured by Keyence Corporation) to determine the area of ​​the projected cross section of the particle and the perimeter of the cross section, and then calculating the sphericity = [area of ​​the projected cross section of the particle (mm ２ The particle diameter can be determined by calculating the perimeter (mm) of a circle having the same area as the particle diameter (mm) / the perimeter (mm) of the projected cross section of the particle, and averaging the values ​​obtained for any 50 particles.

[0091] The average particle size of the inorganic-coated sand of this embodiment is preferably 0.05 mm or more, more preferably 0.1 mm or more, from the viewpoint of increasing the strength of the mold produced using the inorganic-coated sand of this embodiment. Furthermore, if the average particle size of the inorganic-coated sand is equal to or greater than the above-mentioned lower limit, the amount of inorganic binder composition (b1) used during mold production can be reduced, which is also preferable in that it makes it easier to regenerate the inorganic-coated sand. From the same viewpoint, the average particle size 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. If the average particle size is equal to or less than the above-mentioned upper limit, the number of contact points between the inorganic-coated sand particles can be ensured during mold production, which is also preferable in that it increases mold strength.

[0092] In this embodiment, the average particle size of inorganic-coated sand can be measured specifically by the following method. (Method for Measuring Average Particle Size) If the sphericity of a particle from a projected cross section is 1, the diameter (mm) is measured. On the other hand, if the sphericity is <1, the major axis diameter (mm) and minor axis diameter (mm) of randomly oriented particles are measured to calculate (major axis diameter + minor axis diameter) / 2, and the average value obtained for 100 randomly selected particles is used to determine the average particle size (mm). The major axis diameter and minor axis diameter are defined as follows: When a particle is stabilized on a flat surface and its projected image on the flat surface is sandwiched between two parallel lines, the width of the particle at the smallest distance between the parallel lines is called the minor axis diameter, and the distance between two parallel lines perpendicular to the parallel lines is called the major axis diameter. The major axis diameter and minor axis diameter of a particle can be determined by taking an image (photograph) of the particle using an optical microscope or digital scope and analyzing the resulting image.

[0093] <Method for Manufacturing a Casting Mold> The casting mold of this embodiment can be formed using the inorganic-coated sand obtained by the manufacturing method of this embodiment described above. That is, the present invention discloses a method for manufacturing a casting mold, which includes a step of forming a casting mold using the inorganic-coated sand obtained by the manufacturing method of this embodiment. Suitable methods for forming a casting mold include a method using a heated molding die, and a method in which water vapor is passed through a heated molding die and then hot air is passed through the die.

[0094] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations can also be adopted.

[0095] In relation to 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.

[0096] <1> A method for producing dry inorganic-coated sand, comprising: a roasting step (R) of roasting waste foundry sand (A0) containing an inorganic binder (b0); and a mixing step (M-I) of mixing the waste foundry sand (A1) obtained in the roasting step (R) with an inorganic binder composition (b1) containing a metasilicate. <2> The method for producing dry inorganic-coated sand according to <1>, wherein the roasting temperature in the roasting step (R) is preferably 200°C or higher, more preferably 300°C or higher, even more preferably 400°C or higher, and preferably 1000°C or lower, more preferably 800°C or lower, and even more preferably 700°C or lower. <3> The method for producing dry inorganic-coated sand according to <1> or <2>, further comprising at least one of a polishing step (P-I) of polishing the waste foundry sand (A0) before the roasting step (R), and a polishing step (P-II) of polishing the waste foundry sand (A1) after the roasting step (R) and before the mixing step (M-I). <4> The method for producing dry inorganic-coated sand according to any one of <1> to <3>, further comprising a polishing step (P-I) of polishing the waste foundry sand (A0) before the roasting step (R). <5> The method for producing dry inorganic-coated sand according to any one of <1> to <4>, wherein the waste foundry sand (A0) is one or more selected from a used mold after casting using a mold formed from a refractory aggregate and inorganic-coated sand having a layer containing an inorganic binder (b0) on its surface, coarse fragments obtained by crushing the used mold, and particles obtained by pulverizing the coarse fragments. <6> The method for producing dry inorganic-coated sand according to any one of <1> to <5>, wherein the amount of reduction in acid consumption (AD1) of the waste foundry sand (A1) relative to the amount of acid consumption (AD0) of the waste foundry sand (A0) is preferably 18% or more, more preferably 30% or more, and even more preferably 50% or more.<7> The method for producing dry inorganic-coated sand according to any one of <1> to <6>, wherein the amount of sodium ions eluted from 50 g of the waste foundry sand (A1) in 100 ml of a 0.05 mol / L hydrochloric acid solution is preferably 420 mg / L or less, more preferably 400 mg / L or less, even more preferably 300 mg / L or less, and particularly preferably 200 mg / L or less. <8> The method for producing dry inorganic-coated sand according to any one of <1> to <7>, wherein the metasilicate contained in the inorganic binder composition (b1) in the mixing step (M-I) is sodium metasilicate hydrate. <9> The method for producing dry inorganic-coated sand according to any one of <1> to <8>, further comprising a mixing step (M-II) of mixing the mixture obtained in the mixing step (M-I) with inorganic particles. <10> The method for producing dry inorganic-coated sand according to <9>, wherein the inorganic particles contain silica. <11> The method for producing dry inorganic-coated sand according to <9> or <10>, wherein the content of silica in the inorganic particles is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 85% by mass or more, still more preferably 90% by mass or more, still more preferably 95% by mass or more, and still more preferably 99% by mass or more. <12> The method for producing dry inorganic-coated sand according to any one of <1> to <11>, wherein the content of metasilicate contained in the inorganic binder composition (b1) in the mixing step (M-I) is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, still more preferably 98% by mass or more, and still more preferably substantially 100% by mass. <13> The method for producing dry inorganic coated sand according to any one of <1> to <12>, wherein the method for preparing the inorganic binder composition (b1) in the mixing step (M-I) is selected from the group consisting of (i) a method for mixing metasilicate nonahydrate, metasilicate pentahydrate, and water, (ii) a method for mixing anhydrous metasilicate and water, and (iii) a method for obtaining metasilicate hydrate by mixing water glass, caustic alkali, and water.<14> The method for producing dry inorganic-coated sand according to any one of <1> to <13>, wherein in the roasting step (R), the inorganic binder (b0) contains a silicate compound. <15> A method for producing a casting mold, comprising a step of forming a casting mold using the inorganic-coated sand obtained by the method for producing dry inorganic-coated sand according to any one of <1> to <14>.

[0097] The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited thereto. <Materials> The materials used in the following examples and comparative examples will be described. Refractory aggregate 1: Espearl #60L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 241 μm, amorphousness: 45%, sphericity: 0.97) Inorganic particles 1: Denka fused silica SFP-20M (average particle size: 0.4 μm, amorphousness: 99.5% or more, silica content: 99% by mass or more) manufactured by Denka Co., Ltd. Metasilicate 1: Sodium metasilicate nonahydrate (Na ２ SiO ３ ・9H ２ O, SiO ２ / Na ２ O molar ratio = 0.9 to 1.1), melting point 47°C. Metasilicate 2: Sodium metasilicate pentahydrate (Na ２ SiO ３ ・5H ２ O, SiO ２ / Na ２ O molar ratio = 0.9 to 1.1), melting point 72°C Metasilicate 3: Preparation from water glass and caustic soda

[0098] (Procedure for preparing metasilicate 3) No. 1 50 water glass, caustic soda (NaOH), and water were mixed in a mixer for 10 minutes in the ratio (parts by mass) shown in Table 1 below to obtain metasilicate 3.

[0099]

[0100] No. 1 50 water glass: manufactured by Fuji Chemical Co., Ltd., SiO ２ (%)=30.0, Na ２O (%) = 14.7, solid content 44.7 mass% Caustic soda (NaOH): sodium hydroxide, granular, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0101] <Preparation of Waste Foundry Sand (A0)> Waste foundry sand (A0) was prepared according to the following procedure. (1) Preparation of Inorganic Coated Sand (a) 100 parts by mass of refractory aggregate 1 (Espearl #60L) was added to a mixer. Next, a mixture (2.00 parts by mass) of metasilicate 1 and metasilicate 2 mixed at a mass ratio of 1:1 was heated to 80°C to melt, added to a mixer, and kneaded for 7 minutes. After that, inorganic particles 1 (0.6 parts by mass) were added, and kneaded for 1 minute to obtain dry inorganic coated sand (a) used to prepare waste foundry sand (A0). (2) Preparation of Mold 10 kg of the obtained inorganic coated sand (a) was poured into a mold for preparing a test mold and heated in a heating furnace at 180°C for 20 minutes to obtain a test mold. (3) Casting Ten kg of aluminum alloy AC4C material (720°C) was poured into the obtained test mold, and the casting was removed from the test mold after casting. (4) Preparation of waste foundry sand (A0) The test mold from which the casting was removed was crushed with a hammer and further crushed into particles using a mini crusher (manufactured by Taiyo Machinery Co., Ltd.) to obtain waste foundry sand (A0). (5) Measurement of acid consumption of waste foundry sand (A0) The acid consumption (AD0) of the waste foundry sand (A0) before roasting was measured using the method described below.

[0102] Examples 1-7 [Polishing Treatment (P-I)] First, the waste foundry sand (A0) was subjected to a polishing treatment (P-I) before the roasting step (R). 100 kg of the waste foundry sand (A0) was placed in a dry-type foundry sand reclamation 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 polished granular (sand-like) sand.

[0103] [Roasting Step (R)] Next, the ground granular (sand-like) sand was roasted to obtain waste foundry sand (A1). Specifically, 1 kg of the obtained sand was placed in an alumina firing container. Next, a muffle furnace (KBF894N2 model, manufactured by Koyo Thermo Systems Co., Ltd.) was heated to the temperature (°C) specified in the roasting conditions shown in Table 2, and the firing container was placed in the muffle furnace to perform the roasting process. The time when the firing container was placed was set to zero, and after the treatment time (minutes) specified in the roasting conditions shown in Table 2 had elapsed, the firing container was removed from the muffle furnace and allowed to cool to room temperature to obtain waste foundry sand (A1). If the roasted waste foundry sand (A1) in the firing container after cooling was found to be blocked, it was sieved through a 20-mesh sieve. (Measurement of roasted waste foundry sand (A1)) The acid consumption (AD1) and the amount of sodium ions eluted from the roasted waste foundry sand (A1) were measured by the methods described below. The results are shown in Table 2.

[0104] [Mixing Step (M-I)] Next, inorganic-coated sand (B) was produced using the roasted waste foundry sand (A1). Specifically, 100 parts by mass of the roasted waste foundry sand (A1) was first charged into a mixer. Separately, the amount (parts by mass) of the inorganic binder composition (a mixture of metasilicate 1 and metasilicate 2) shown in Table 2 was heated to 80°C to melt the mixture, producing a molten liquid. Next, the molten liquid was charged into the mixer, and uniform stirring and kneading were initiated together with the waste foundry sand (A1). While continuing stirring, the mixture was allowed to cool naturally below the melting point of the inorganic binder composition (a mixture of metasilicate 1 and metasilicate 2). Stirring was continued until the mixture was dried within the drying time (minutes) shown in Table 2, at which point stirring was stopped, yielding inorganic-coated sand (B) with room-temperature fluidity. The drying time of the inorganic-coated sand (B) was measured by setting the time at which the inorganic binder composition was completely charged as 0.

[0105] Example 3 Inorganic coated sand (B) was prepared in the same manner as in Example 1, except that metasilicate 3 was used instead of the mixture of metasilicate 1 and metasilicate 2 as the inorganic binder composition.

[0106] Example 8 Inorganic coated sand (B) was produced in the same manner as in Example 2, except that the order of the polishing treatment (P-I) and the roasting step (R) was reversed. Specifically, inorganic coated sand (B) was produced in the same manner as in Example 2, except that the waste foundry sand (A0) was subjected to a roasting treatment (roasting step (R)), and then the resulting waste foundry sand (A1) was subjected to a polishing treatment (i.e., polishing treatment (P-II)) under the same conditions as the polishing treatment (P-I). In Example 8, in the above-mentioned "Evaluation and measurement of the roasted waste foundry sand (A1)" and "Production of inorganic coated sand (B) using the roasted waste foundry sand (A1)," the waste foundry sand that had been subjected to the polishing treatment (P-II) was used, rather than the roasted waste foundry sand (A1).

[0107] Comparative Example 1 Inorganic coated sand (B) was produced in the same manner as in Example 1, except that the waste foundry sand (A0) was not subjected to the roasting treatment.

[0108] <Evaluation and Measurement> The following evaluations and measurements were performed on the waste foundry sand (A0) and the waste foundry sand (A1). [Acid Consumption (AD) of Waste Foundry Sand] (1) Procedure: 25 g of the waste foundry sand (A1) roasted under the conditions shown in Table 2 and the waste foundry sand (A0) before roasting were weighed into 100 ml beakers. 50 ml of 0.1 mol / L hydrochloric acid solution (standardized according to JIS K 8001) was added to each via pipette and stirred for 15 minutes. This liquid was then filtered, and 25 ml of the filtrate was pipetted and titrated with 0.1 mol / L sodium hydroxide solution (standardized according to JIS K 8001) until the pH reached 7. (2) Calculation of Acid Consumption (AD) The acid consumption (AD) was calculated using the following formula: Acid consumption (ml / 50g) = (25 - X) x 4 X: Titration amount (ml) of 0.1 mol / L NaOH in this test Note that the acid consumption of the waste foundry sand (A1) that had been roasted was designated AD1, and the acid consumption of the waste foundry sand (A0) before roasting was designated AD0. (3) Next, the loss was calculated from the obtained acid consumption using the following formula. The results are shown in Table 2. Loss (%) = {(AD0 - AD1) / AD0} x 100

[0109] [Amount of Sodium Ions Leached from Waste Foundry Sand (A1)] (1) Preparation of Internal Standard Solution and Standard Solution: Internal standard solution (Y: 50 mg / L): 25 mL of yttrium standard stock solution (Y: 1000 mg / L, for atomic absorption spectrometry) manufactured by Kanto Chemical Co., Inc. was placed in a 500 mL volumetric flask, and purified water was added up to the mark. Standard solution (Na: 100 mg / L): 10 mL of standard solution IV (Na: 1000 mg / L) for ICP atomic emission spectrometry manufactured by Kanto Chemical Co., Inc. was placed in a 100 mL volumetric flask, and purified water was added up to the mark. Standard solution (Na: 20 mg / L): 20 mL of standard solution (Na: 100 mg / L) was placed in a 100 mL volumetric flask, and purified water was added up to the mark. Standard solution (Na: 10 mg / L): 10 mL of standard solution (Na: 100 mg / L) was placed in a 100 mL volumetric flask, and purified water was added up to the mark. Standard solution (Na: 1 mg / L): 10 mL of standard solution (Na: 10 mg / L) was placed in a 100 mL volumetric flask, and purified water was added up to the mark. (2) Preparation of a calibration curve (measurement range: Na: 0-20 mg / L): 20 mL of internal standard solution (Y: 50 mg / L) was placed in a 100 mL volumetric flask, and standard solution (Na: 20 mg / L), standard solution (Na: 10 mg / L), and standard solution (Na: 1 mg / L) were added up to the mark, respectively. Separately, as a blank experiment, 20 mL of internal standard solution (Y: 50 mg / L) was placed in a 100 mL volumetric flask, and purified water was added up to the mark to prepare a standard solution. In addition, a blank test was conducted by placing 20 mL of an internal standard solution (Y: 50 mg / L) in a 100 mL volumetric flask and adding purified water up to the mark to prepare a standard solution. These were measured using an ICP emission spectrometer (ICPS-8100) manufactured by Shimadzu Corporation, and a calibration curve was created between the sodium ion elution concentration and the indicated value. (3) Sample Solution Preparation: 50 g of roasted waste foundry sand (A0) was placed in a 300 mL polyethylene beaker, and 50 mL of purified water and 50 mL of 0.1 mol / L hydrochloric acid solution were added. The mixture was stirred for 1 hour using a magnetic stirrer. After stirring, the mixture was filtered using glass fiber filter paper specified in JIS P 3801 Filter Paper (for Chemical Analysis). After filtration, the solution was again suction-filtered using a membrane filter (pore size 0.45 μm) to prepare the sample solution (stock solution).In addition, a blank test was conducted using 50 mL of purified water and 50 mL of 0.1 mol / L hydrochloric acid solution in a 300 mL polyethylene beaker. (4) Measurement of Sample Solution: 10 mL of internal standard solution (Y: 50 mg / L) was placed in a 50 mL volumetric flask, and the sample solution (stock solution) was added up to the marked line. Measurement was performed using an ICP emission spectrometer (ICPS-8100). The difference between the obtained sodium ion concentration and the concentration in the blank test was taken as the sodium ion elution amount (mg / L). If the sodium ion elution amount (mg / L) exceeded the measurement range of the calibration curve, the sample solution (stock solution) was diluted with purified water to obtain a sample solution (diluted solution) within the measurement range. 10 mL of internal standard solution (Y: 50 mg / L) was placed in a 50 mL volumetric flask, and the sample solution (diluted solution) was added up to the marked line. Measurement was again performed using the ICP emission spectrometer (ICPS-8100) to obtain the sodium ion elution amount (mg / L). When measuring the sample solution (diluted solution), the amount of sodium ion elution (mg / L) was determined by subtracting the blank test concentration from the product of the obtained sodium ion concentration and the dilution rate. The results are shown in Table 2.

[0110] [Mold Strength] Inorganic coated sand (C) was prepared according to the following procedure, and a mold was prepared using the inorganic coated sand (C) according to the following procedure, and the mold strength was measured. The evaluation results are shown in Table 2. (Procedure) Each of the inorganic coated sands (B) (100 parts by mass) from the above Examples and Comparative Examples was charged into a mixer. Next, inorganic particles 1 (0.6 parts by mass) were charged and kneaded for 1 minute to obtain inorganic coated sand (C) coated with inorganic particles 1. Next, the obtained inorganic coated sand (C) was filled into a mold for 22.3 mm × 22.3 mm × 180 mm test pieces (5 pieces) heated to 180°C at a blow pressure of 0.3 MPa using a CSR-43 blow molding machine, and the inorganic coated sand (C) was left to stand in the mold for 150 seconds to harden, thereby obtaining mold test pieces. (Measurement) The mold strength (MPa) of each of the obtained mold test pieces was measured using a universal strength testing machine PFG type manufactured by George Fischer, which was previously equipped with a PBV transverse attachment. The mold test pieces were left for 1 hour in a constant temperature and humidity chamber at 25°C / 55% RH after being removed from the mold.

[0111]

Claims

1. A method for producing dry inorganic-coated sand, comprising: a roasting step (R) of roasting waste foundry sand (A0) containing an inorganic binder (b0); and a mixing step (M-I) of mixing the waste foundry sand (A1) obtained in the roasting step (R) with an inorganic binder composition (b1) containing metasilicate.

2. The method for producing dry inorganic coated sand according to claim 1, wherein the roasting temperature in the roasting step (R) is 200°C or higher and 1000°C or lower.

3. The method for producing dry inorganic coated sand according to claim 1 or 2, further comprising at least one of a polishing step (P-I) in which the waste foundry sand (A0) is polished before the roasting step (R), and a polishing step (P-II) in which the waste foundry sand (A1) is polished after the roasting step (R) and before the mixing step (M-I).

4. A method for producing dry inorganic coated sand according to claim 1 or 2, further comprising a polishing step (P-I) of polishing the waste foundry sand (A0) before the roasting step (R).

5. A method for producing dry inorganic-coated sand according to any one of claims 1 to 4, wherein the waste foundry sand (A0) is one or more selected from a used mold after casting using a mold formed from refractory aggregate and inorganic-coated sand having a layer containing an inorganic binder (b0) on its surface, coarse fragments obtained by crushing the used mold, and particles obtained by pulverizing the coarse fragments.

6. A method for producing dry inorganic coated sand according to any one of claims 1 to 5, wherein the reduction in acid consumption (AD1) of the waste foundry sand (A1) relative to the acid consumption (AD0) of the waste foundry sand (A0) is 18% or more.

7. A method for producing dry inorganic-coated sand according to any one of claims 1 to 6, wherein the amount of sodium ions eluted from 50 g of waste foundry sand (A1) in 100 ml of 0.05 mol / L hydrochloric acid solution is 420 mg / L or less.

8. A method for producing dry inorganic coated sand according to any one of claims 1 to 7, wherein in the mixing step (M-I), the metasilicate contained in the inorganic binder composition (b1) is sodium metasilicate hydrate.

9. A method for producing dry inorganic-coated sand according to any one of claims 1 to 8, further comprising a mixing step (M-II) of mixing the mixture obtained in the mixing step (MI) with inorganic particles.

10. A method for manufacturing a casting mold, comprising the step of forming a casting mold using inorganic coated sand obtained by the method for manufacturing dry inorganic coated sand according to any one of claims 1 to 9.

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