Manufacturing method of inorganic coated sand, and manufacturing method of casting mold

Abstract

To provide a manufacturing method of inorganic coated sand that is improved in fluidity, and a manufacturing method of a casting mold that is improved in strength.SOLUTION: A manufacturing method of inorganic coated sand includes the step 1 and step 2 below. Step 1: A process of inputting a refractory aggregate and a metasilicate hydrate to an agitation vessel. Step 2: A process of covering a surface of the refractory aggregate with the metasilicate hydrate crystallized, in the agitation vessel, and obtaining a mixture in a dried state. The step 2 includes a process of agitating a mixture containing the refractory aggregate and the metasilicate hydrate at a peripheral speed of 1.5 m / s or higher.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] As a mold used for casting, for example, a mold obtained by molding a desired shape using dry inorganic coated sand having a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate is known. The dry inorganic coated sand is obtained by solidifying a liquid inorganic binder composition on the surface of the refractory aggregate to form a layer. Specifically, for example, the inorganic binder layer can be formed on the surface of the refractory aggregate by crystallizing metasilicate hydrate.

[0003] As a conventional technology relating to inorganic coated sand, Patent Document 1 (JP 2020-11296 A) describes dry inorganic coated sand having a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate, wherein the inorganic binder layer contains metasilicate hydrate.

[0004] Patent Document 2 (JP Patent Publication No. 2006-007319) describes foundry sand having an amorphization degree of 70 to 100% and a surface roughness Ra of 0.20 or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-11296 A [Patent Document 2] JP 2006-007319 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, the inorganic coated sand obtained by the conventional methods for producing inorganic coated sand as described in Patent Documents 1 and 2 was insufficient in terms of improving the fluidity of the inorganic coated sand and improving the strength of the mold. [Means for solving the problem]

[0007] The present inventors have conducted extensive research into improving the fluidity of inorganic coated sand and increasing the strength of the mold, and as a result have found that it is important to suppress the coarsening of the crystals of metasilicate hydrate, which is an inorganic binder. In other words, they have found that by refining the crystals of metasilicate hydrate, the smoothness of the inorganic coated sand surface is increased, the fluidity of the inorganic coated sand is improved, and the bulk specific gravity is increased, which results in an increase in the packing property of the inorganic coated sand and an improvement in the strength of the mold.

[0008] The present inventors have therefore discovered that an effective method for suppressing the coarsening of crystals of metasilicate hydrate, which is an inorganic binder, is to stir and mix the refractory aggregate and metasilicate hydrate at a speed faster than conventional speeds, and have completed an invention relating to a method for producing inorganic coated sand.

[0009] According to the present invention, the method includes the following steps 1 and 2: Step 1: A step of putting refractory aggregate and metasilicate hydrate into a mixing tank Step 2: A step of covering the surface of the refractory aggregate with the crystallized metasilicate hydrate in the mixing tank to obtain a dried mixture. There is provided a method for producing inorganic coated sand, which includes a step of stirring the mixture containing the refractory aggregate and the metasilicate hydrate at a peripheral speed of 1.5 m / s or more in step 2.

[0010] The present invention also provides a method for manufacturing a casting mold, comprising the steps of filling a mold with the inorganic coated sand produced by the above-mentioned method for producing inorganic coated sand, and hardening the inorganic coated sand in the mold. Effect of the Invention

[0011] According to the present invention, a method for producing inorganic coated sand having improved fluidity and a method for producing a mold having improved strength can be provided. [Brief description of the drawings]

[0012] [Figure 1] An example of a projected image of the surface of inorganic coated sand photographed by a laser microscope is shown. [Diagram 2] An example of a projected image of the surface of a refractory aggregate taken with a laser microscope is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In this specification, "a to b" indicating a numerical range means a to b range unless otherwise specified. In addition, the components and elements described in each embodiment can be appropriately combined as long as the effect of the invention is not impaired. In addition, in this specification, the "coating" is not limited to being continuous, and may have some discontinuous parts. Hereinafter, an embodiment of the present invention will be described.

[0014] <Inorganic coated sand> The inorganic coated sand of this embodiment is a dry inorganic coated sand having a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate.

[0015] In the inorganic coated sand of this embodiment, the ratio of the surface roughness Ra (μm) of the inorganic coated sand to the surface roughness Ra (μm) of the refractory aggregate is preferably 2.50 or less, in order to improve the strength of the mold. Although the details of why the mold strength can be improved by controlling the surface roughness ratio Ra are not clear, it is speculated as follows. Conventionally, the surface roughness of inorganic coated sand is largely dependent on the surface roughness of the refractory aggregate, which is the core of the inorganic coated sand. Therefore, by setting the ratio of the surface roughness Ra (μm) of the inorganic coated sand to the surface roughness Ra (μm) of the refractory aggregate to 2.5 or less, the irregularities of the inorganic binder layer covering the surface of the refractory aggregate are relatively reduced compared to conventional cases, and the surface of the inorganic coated sand becomes smoother. In other words, since the metasilicate hydrate constituting the inorganic binder layer is microcrystallized, the surface smoothness of the inorganic coated sand is increased, and as a result, the fluidity of the inorganic coated sand is improved and the bulk specific gravity is also increased, which is considered to improve the filling property of the inorganic coated sand and the strength of the mold.

[0016] Furthermore, according to the inorganic coated sand of this embodiment, the crystals of the inorganic binder can be prevented from becoming coarse, so that the crystallization of the inorganic binder is promoted and the drying time can be shortened.

[0017] The ratio of the surface roughness Ra (μm) of the inorganic coated sand to the surface roughness Ra (μm) of the refractory aggregate is preferably 2.5 or less, more preferably 2.3 or less, and even more preferably 1.8 or less, from the viewpoint of suppressing coarsening of the crystals of the inorganic binder and improving the mold strength.

[0018] The surface roughness Ra (μm) of the inorganic coated sand is preferably 0.15 μm or more, more preferably 0.18 μm or more, and even more preferably 0.20 μm or more, from the viewpoint of obtaining good adhesion between the refractory aggregate and the inorganic binder layer. On the other hand, the surface roughness Ra (μm) of the inorganic coated sand is preferably 0.85 μm or less, more preferably 0.80 μm or less, and even more preferably 0.75 μm or less, from the viewpoint of smoothing the surface and enhancing fluidity.

[0019] The surface roughness Ra (μm) is measured as follows. The measurement can be performed using a laser microscope (for example, VK-9710 manufactured by Keyence Corp.) Figure 1 shows an example of a projected image of the surface of inorganic coated sand photographed by the laser microscope, and Figure 2 shows an example of a projected image of the surface of refractory aggregate photographed by the laser microscope. The surface roughness Ra of refractory aggregate is determined by measuring the linear roughness at approximately 1 / 10 of the average particle size from the center of the projected refractory aggregate particle. Specifically, for a particle with a particle size of 300 μm, the Ra is measured at 5 points over a length of 30 μm from the approximate center of the particle, and the average value is calculated. This is carried out for 10 random particles, and the average value is calculated to be the surface roughness Ra of the refractory aggregate. The surface roughness Ra of inorganic coated sand is the surface roughness Ra on the inorganic binder layer that covers the surface of the refractory aggregate. The inorganic binder layer that covers the surface of the refractory aggregate can be found by comparing the projected image of the refractory aggregate (Fig. 2) which is the original sand, with the projected image of the inorganic coated sand (Fig. 1). Specifically, the inorganic binder layer is observed as shown in the black area in Fig. 1. To measure the surface roughness Ra on the inorganic binder layer, the same as for the refractory aggregate, the line roughness is measured at five points at about 1 / 10 of the average particle size of the inorganic coated sand, and the average value is calculated. This is performed for any 10 inorganic coated sand particles, and the average value is taken as the surface roughness Ra of the inorganic coated sand.

[0020] In the case of inorganic coated sand, the method for obtaining only the refractory aggregate from the refractory aggregate coated with the inorganic binder layer includes immersing the inorganic coated sand in a sufficient amount of water, stirring for one hour or more to dissolve the inorganic binder into the water, removing the eluate by filtration, and then drying.

[0021] The inorganic coated sand having the above ratio [surface roughness Ra (μm) of the inorganic coated sand / surface roughness Ra (μm) of the refractory aggregate] can be realized by using the manufacturing method of the inorganic coated sand described below. Specifically, the stirring speed of the mixture containing the refractory aggregate and the metasilicate hydrate can be adjusted.

[0022] Bulk density of inorganic coated sand (g / cm3 ) is preferably 1.20 g / cm from the viewpoint of improving the filling property and improving the mold strength. 3 More preferably, it is 1.30 g / cm 3 More preferably, it is 1.35 g / cm 3 That's all. On the other hand, the bulk density (g / cm 3 ) is preferably 2.50 g / cm from the viewpoint of obtaining good handling. 3 More preferably, it is 2.30 g / cm or less. 3 More preferably, it is 2.00 g / cm or less. 3 The following is the result.

[0023] In addition, the bulk density (g / cm 3 ) of inorganic coated sand (g / cm 3 ) ratio (bulk density of inorganic coated sand (g / cm 3 ) / bulk density of the refractory aggregate (g / cm 3 From the viewpoint of improving mold strength, the mould strength is preferably 0.90 or more, more preferably 0.95 or more, and even more preferably 0.98 or more.

[0024] Bulk density (g / cm 3 ) is measured as follows: Using a bulk density measuring device (JIS standard K6721), the refractory aggregate or inorganic coated sand is allowed to freely fall into a cylindrical container with an internal volume of 100 ml, and the container is filled with the refractory aggregate or inorganic coated sand until it overflows. After that, the excess refractory aggregate or inorganic coated sand is scraped off from the top of the cylindrical container, and the weight of the empty cylindrical container, which was measured beforehand, is subtracted from the weight of the cylindrical container filled with the refractory aggregate or inorganic coated sand to calculate the weight of the refractory aggregate or inorganic coated sand filled in the container. This operation is repeated three times, and the average of the weights obtained is divided by 100 to obtain the bulk density (g / cm3). 3 )

[0025] The slump flow value of the inorganic coated sand is preferably 280 mm or more, and more preferably 300 mm or more, from the viewpoints of enhancing the packing property and improving the mold strength. The slump flow value can be measured in accordance with JIS A 1101:2014.

[0026] The inorganic coated sand will be described in more detail below.

[0027] The inorganic coated sand is in a dry state having fluidity at room temperature. Dry coated sand means coated sand that can obtain a measured value 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 even more preferably 30° or less.

[0028] The dynamic angle of repose of the inorganic coated sand can be measured by the following method. (Method of measuring dynamic angle of repose) Put half the volume of coated sand into a cylindrical transparent plastic bottle, hold it so that its axis is horizontal, and rotate it around the horizontal axis at a rotation speed of 60 rpm. The inclined surface of the coated sand layer flowing inside the cylinder becomes flat. Measure the angle formed between this inclined surface and the horizontal plane. If the coated sand does not flow inside the cylinder, or if it does flow but the inclined surface of the coated sand layer does not form a flat surface, and as a result the dynamic angle of repose cannot be measured, it is in a wet state.

[0029] Specifically, the inorganic coated sand is composed of a group of inorganic coated sand particles.

[0030] From the viewpoint of improving the flowability and further improving the filling property into a molding die, the inorganic coated sand is preferably spherical. Here, spherical inorganic coated sand refers to sand having a round shape like a ball.

[0031] 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, from the viewpoints of improving fluidity, mold quality, and mold strength, and of ease of mold making. The upper limit of the sphericity is specifically 1. In this embodiment, the sphericity of the inorganic coated sand specifically coincides with the sphericity of the refractory aggregate described below.

[0032] The sphericity of the inorganic coated sand was determined by analyzing the image (photograph) of the particle taken with an optical microscope or a digital scope (e.g., Keyence VH-8000) 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 calculated by dividing the circumference (mm) of a perfect circle with the same area as the particle diameter by the circumference (mm) of the projected cross section of the particle, and then averaging the values ​​obtained for any 50 particles.

[0033] The average particle size of the inorganic coated sand is preferably 0.05 mm or more, more preferably 0.1 mm or more, from the viewpoints of improving mold quality and mold strength, ease of mold making, and storage stability. In addition, if the average particle size of the inorganic coated sand is equal to or more than the above lower limit, the amount of coating layer, etc. used during mold production can be reduced, which is also preferable in that the inorganic coated sand can be easily regenerated. From the viewpoints of improving mold quality and strength, and of ease of mold making, 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. In addition, when the average particle size of the inorganic coated sand is equal to or less than the above upper limit, it is also preferable in that the porosity is reduced during mold production, and the mold strength can be increased.

[0034] In this embodiment, the average particle size of the inorganic coated sand can be specifically measured by the following method. (Method of measuring average particle size) If the sphericity of the particle from the projected cross section is 1, the diameter (mm) is measured, whereas if the sphericity is <1, the long axis diameter (mm) and short axis diameter (mm) of the randomly oriented particles are measured to calculate (long axis diameter + short axis diameter) / 2, and the values ​​obtained for any 100 particles are averaged to determine the average particle size (mm). The long axis diameter and short axis diameter are defined as follows: A particle is stabilized on a flat surface, and when the projected image of the particle on the flat surface is sandwiched between two parallel lines, the width of the particle at which the distance between the parallel lines is the smallest is called the short axis diameter, whereas the distance when the particle is sandwiched between two parallel lines perpendicular to the parallel lines is called the long 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 a digital scope (for example, VH-8000 model, manufactured by Keyence Corporation) and subjecting the obtained image to image analysis.

[0035] Each component of the inorganic coated sand will be described below.

[0036] [Fire-resistant aggregate] The refractory aggregate is specifically composed of a group of refractory aggregate particles. The material of the fire-resistant aggregate is at least one selected from the group consisting of natural sand and artificial sand.

[0037] Examples of natural sand include one or more types selected from the group consisting of silica sand, which is mainly composed of quartz, chromite sand, zircon sand, olivine sand, and alumina sand.

[0038] Examples of artificial sand include one or more types selected from the group consisting of synthetic mullite sand, SiO2-based foundry sand mainly composed of SiO2, Al2O3-based foundry sand mainly composed of Al2O3, SiO2 / Al2O3-based foundry sand, SiO2 / MgO-based foundry sand, SiO2 / Al2O3 / ZrO2-based foundry sand, SiO2 / Al2O3 / Fe2O3-based foundry sand, and slag-derived foundry sand. Here, the term "major component" refers to the component contained in the sand in the greatest amount. Artificial sand is not found in nature, but is found in sand that has been artificially prepared from metal oxide components and then melted or sintered.

[0039] In addition, recycled sand made from recovered refractory aggregate and recycled sand made from recycled sand that has been regenerated can also be used.

[0040] The content of each component such as SiO2, Al2O3, and Fe2O3 in the refractory aggregate can be measured using the following X-ray fluorescence method. The refractory aggregate is adjusted to a size of approximately 0.1 μm or less using a vibration mill and heated at 1050°C for 1 hour. Then, 5 g of lithium tetraborate and 0.5 g of refractory aggregate are mixed and heated at 1200°C for 10 minutes to melt, and then cooled to prepare a glassy sample (glass bead method). The sample is subjected to X-ray fluorescence analysis using the Fundamental Parameter (FP) method using an X-ray fluorescence analyzer ZSX Primus II (manufactured by Rigaku Corporation).

[0041] The sphericity of the refractory aggregate is equal to that of the inorganic coated sand. Specifically, the sphericity of the refractory aggregate is preferably 0.75 or more, more preferably 0.80 or more, and even more preferably 0.82 or more, from the viewpoints of improving fluidity, mold quality, and mold strength, and from the viewpoints of ease of mold making. The upper limit of the sphericity is specifically 1.

[0042] The sphericity of the refractory aggregate can be measured by the same method as that for the inorganic coated sand.

[0043] The average particle size of the refractory aggregate is preferably 0.05 mm or more, more preferably 0.1 mm or more, from the viewpoints of improving the quality and strength of the mold and of ease of molding the mold. In addition, when the average particle size of the refractory aggregate is equal to or more than the above lower limit, the amount of inorganic binder layer used as a coating layer during mold production can be reduced, which is also preferable in that it makes it easier to regenerate the inorganic coated sand. 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, from the viewpoints of improving the quality and strength of the mold and of ease of molding the mold. In addition, if the average particle size of the refractory aggregate is equal to or less than the above upper limit, it is also preferable in that the porosity is reduced during mold production and the mold strength can be increased.

[0044] The average particle size of the refractory aggregate can be measured by the same method as that for the inorganic coated sand.

[0045] 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, from the viewpoint of obtaining a smoother surface of the aggregate and thus improving mold strength, and from the viewpoint of obtaining low thermal expansion. The upper limit of the degree of amorphization of the refractory aggregate is not limited, but may be, for example, 100% or less, and may be 99% or less.

[0046] 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 crushed in a mortar and pressed against an X-ray glass holder of a powder X-ray diffractometer for measurement. The powder X-ray diffractometer is a Rigaku MultiFlex (CuKα light source, tube voltage 40 kV, tube current 40 mA), 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°. The X-ray intensities on the low angle side and the high angle side are connected with a straight line in the range of 2θ=10° to 50°, and the area under the straight line is taken as the background. The crystallinity is calculated using the software attached to the device, and the degree of amorphousness is calculated by subtracting it from 100. Specifically, for the area above the background, the amorphous peak (halo) and each crystalline component are separated by curve fitting, the areas of each are calculated, and the degree of amorphousness (%) is calculated using the following formula. Amorphous content (%) = halo area / (crystalline component area + halo area) x 100

[0047] 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 crushing it with air, or a method in which it is treated in a flame and rapidly cooled. In either case, the cooling method may be appropriately selected at various speeds depending on the material and particle size. In addition, a method in which a crystallized material is once made amorphic by heat treatment and cooling treatment may also be considered.

[0048] The surface roughness Ra (μm) of the refractory aggregate is preferably 0.85 μm or less, more preferably 0.80 μm or less, and further preferably 0.50 μm or less, from the viewpoint of smoothing the surface of the inorganic coated sand. The lower limit of the surface roughness Ra (μm) of the refractory aggregate is not particularly limited, since the smoother the surface, the more preferable it is in terms of obtaining high fluidity.

[0049] Bulk density of refractory aggregate (g / cm 3 ) is preferably 1.20 g / cm from the viewpoint of improving the filling property and improving the mold strength. 3 More preferably, it is 1.30 g / cm 3 More preferably, it is 1.35 g / cm 3 That's all. On the other hand, the bulk density of the refractory aggregate (g / cm 3 ) is preferably 2.50 g / cm from the viewpoint of obtaining good handling. 3 More preferably, it is 2.30 g / cm or less. 3 More preferably, it is 2.00 g / cm or less. 3 The following is the result.

[0050] [Inorganic binder layer] The inorganic binder layer is formed on the surface of the refractory aggregate. In other words, the inorganic binder layer covers the surface of the refractory aggregate. Note that the covering is not limited to a continuous covering, and may have a discontinuous portion. The inorganic binder layer allows the mold to be formed as inorganic coated sand.

[0051] From the viewpoint of obtaining a high-strength casting mold, the coating amount of the inorganic binder layer contained in the inorganic coated sand is, for example, 0.1 parts by mass or more relative to 100 parts by mass of the refractory aggregate, preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, even more preferably 1 part by mass or more, and even more preferably 1.5 parts by mass or more. In addition, the coating amount of the inorganic binder layer contained in the inorganic coated sand is, for example, 15 parts by mass or less, preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, relative to 100 parts by mass of the refractory aggregate, from the viewpoint of obtaining a high-strength casting mold.

[0052] The inorganic binder layer may be a single layer or multiple layers as long as it has at least a layer containing a metasilicate. The layer containing at least a metasilicate is formed from an inorganic binder composition containing a metasilicate. The use of metasilicate hydrate is preferable because it can improve the crystallinity of the inorganic binder layer, and furthermore, the inorganic coated sand becomes a dry state and has excellent fluidity at room temperature. In addition, the use of metasilicate hydrate allows the inorganic binder layer to be formed on the surface of the refractory aggregate in a state where it is not dissolved in water.

[0053] (Metasilicate) The inorganic binder layer contains metasilicate as an inorganic binder. Examples of cations constituting metasilicate salts include monovalent cations such as sodium, potassium, lithium, and ammonium, and divalent cations such as magnesium, calcium, and zinc. Specific examples include sodium metasilicate and potassium metasilicate. Of these, sodium metasilicate is more preferred. The metasilicate may be a hydrate. The metasilicate hydrate may be produced by using a mixed liquid in which water glass, caustic alkali, and water are mixed at a specific ratio. In the present embodiment, the SiO2 / Na2O molar ratio of the metasilicate hydrate is 0.9 to 1.1.

[0054] Specific examples of the water glass include one or more types selected from the group consisting of sodium silicate No. 1 to No. 5. Sodium silicate is classified into No. 1 to No. 5 based on the molar ratio of SiO2 / Na2O, and sodium silicate No. 1 to No. 3 are specified in JIS-K-1408. The molar ratio of SiO2 / Na2O for each type is specifically as follows: Sodium silicate No. 1: SiO2 / Na2O molar ratio = 2.0-2.3 Sodium silicate No. 2: SiO2 / Na2O molar ratio = 2.4-2.6 Sodium silicate No. 3: SiO2 / Na2O molar ratio = 2.8-3.3 Sodium silicate No. 4: SiO2 / Na2O molar ratio = 3.3-3.5 Sodium silicate No. 5: SiO2 / Na2O molar ratio = 3.6-3.8 Moreover, two or more kinds of sodium silicate may be mixed to adjust the molar ratio of SiO2 / Na2O to a desired level. The water glass is preferably at least one selected from sodium silicate No. 1 and sodium silicate No. 2.

[0055] From the viewpoints of improving mold strength, excellent productivity, and ease of availability, the content of metasilicate in the inorganic binder layer 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 even more preferably substantially 100% by mass. The content of metasilicate in the inorganic binder refers to the content of metasilicate relative to the total components other than water in the inorganic binder layer.

[0056] As a method for confirming that the inorganic binder layer contains metasilicate, for example, the following method can be mentioned. Possible methods include a method in which the inorganic coated sand is put into a grinding machine such as a mill to peel off only the inorganic binder layer components, which are then analyzed by XRD to confirm the peaks indicating the crystal structure of metasilicate hydrate, or a method in which the inorganic coated sand is immersed in water and stirred for a certain period of time to elute the inorganic binder layer components, the eluted components are dried, and the dried solids are analyzed by XRD to confirm the peaks indicating the crystal structure of metasilicate, and the amount of water of hydration is analyzed by the following method to confirm that it is metasilicate hydrate.

[0057] <Measurement of the amount of water of hydration> (1) 10 g of inorganic coated sand to which additives such as amorphous SiO2-containing fine particles have not yet been added is weighed and placed in a pre-baked and weighed crucible, and the moisture content (%) in the inorganic coated sand (A) is calculated using the mass loss (%) after heating at 900°C for 1 hour. A = [(M1-M2) / M3] x 100 (M1: total mass (g) of the crucible and inorganic coated sand before firing, M2: total mass (g) of the crucible and inorganic coated sand after firing, M3: mass (g) of the inorganic coated sand before firing) (2) Weigh out 100 g of inorganic coated sand before adding additives such as amorphous SiO2-containing fine particles, immerse in 200 mL or more of water or hot water, and stir for 1 hour or more to extract metasilicate hydrate. Filter the resulting extract to remove the refractory aggregate, and then use a rotary evaporator to remove moisture by vacuum distillation at 40°C and an internal pressure of 15 mmHg or less. Then heat and dry at a temperature of 120°C to 180°C for 1 to 3 hours, and weigh the weight of the dried material. Calculate the dry solid content (%) (B) of metasilicate hydrate in the inorganic coated sand. B = (M12 / M11) x 100 (M11: mass of inorganic coated sand (g), M12: dry weight (g)) (3) Amount of water of hydration of metasilicate hydrate = [(A) / molecular weight of water] / [(B) / molecular weight of metasilicate anhydride]

[0058] (others) The inorganic binder layer may further contain components other than metasilicate, such as amorphous SiO2-containing fine particles, inorganic fine particles other than amorphous SiO2-containing fine particles, a humectant, a moisture resistance improver, a coupling agent that strengthens the bond between the fire-resistant aggregate and the inorganic binder composition, a lubricant, a surfactant, a release agent, etc.

[0059] Amorphous SiO2-containing fine particles may be used because of their high reactivity with metasilicate hydrate, which makes it easier to improve the mechanical strength of the mold. Examples of amorphous SiO2-containing fine particles include precipitated silica, calcined silica produced in an electric arc or by flame hydrolysis, silica produced by thermal decomposition of ZrSiO4, silicon dioxide 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 be used alone, or two or more of them can be mixed together.

[0060] The inorganic fine particles are not particularly limited as long as they are not the above-mentioned amorphous SiO2-containing fine particles, and examples thereof include crystalline silica, silicon; carbonates such as zinc carbonate, basic zinc carbonate, iron carbonate, manganese carbonate, copper carbonate, aluminum carbonate, barium carbonate, magnesium carbonate, calcium carbonate, lithium carbonate, potassium carbonate, and sodium carbonate; borates such as sodium tetraborate, potassium tetraborate, lithium tetraborate, ammonium tetraborate, calcium tetraborate, strontium tetraborate, silver tetraborate, sodium metaborate, potassium metaborate, lithium metaborate, ammonium metaborate, calcium metaborate, silver metaborate, copper metaborate, lead metaborate, and magnesium metaborate; sodium sulfate, potassium sulfate, sulfate Examples of fine particles include one or more types selected from sulfates such as lithium, magnesium sulfate, calcium sulfate, strontium sulfate, barium sulfate, titanium sulfate, aluminum sulfate, zinc sulfate, and copper sulfate; phosphates such as sodium phosphate, sodium hydrogen phosphate, potassium phosphate, potassium hydrogen phosphate, lithium phosphate, lithium hydrogen phosphate, magnesium phosphate, calcium phosphate, titanium phosphate, aluminum phosphate, and zinc phosphate; hydroxides such as lithium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, aluminum hydroxide, and zinc hydroxide; and oxides such as silicon, zinc, magnesium, aluminum, calcium, lithium, copper, iron, boron, and zirconium.

[0061] The coupling agent is not limited, but examples thereof include silane coupling agents, zircon coupling agents, and titanium coupling agents. Examples of the moisturizing agent include polyhydric alcohols, water-soluble polymers, hydrocarbons, sugars, proteins, and inorganic compounds other than those mentioned above. Examples of the moisture resistance improver include metal oxides (other than those listed above), carbonates, borates, sulfates, phosphates, and the like. Examples of lubricants include 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 hardened oils. Examples of the release agent include paraffin, wax, light oil, machine oil, spindle oil, insulating oil, waste oil, vegetable oil, fatty acid ester, organic acid, graphite particles, mica, vermiculite, fluorine-based release agents, and silicone-based release agents.

[0062] <Manufacturing method of inorganic coated sand> Next, a method for producing the inorganic coated sand of this embodiment will be described. The method for producing the inorganic coated sand of this embodiment includes the following steps 1 and 2. Step 1: A step of putting refractory aggregate and metasilicate hydrate into a mixing tank Step 2: A step of covering the surface of the refractory aggregate with the crystallized metasilicate hydrate in the mixing tank to obtain a dried mixture. Furthermore, in step 2, the method includes a step of stirring the mixture containing the refractory aggregate and the metasilicate hydrate at a peripheral speed of 1.5 m / s or more.

[0063] Step 1 and step 2 are carried out in this order, preferably consecutively. The mixture containing the refractory aggregate and the metasilicate hydrate in step 2 is preferably the mixture obtained in the stirring tank by charging the refractory aggregate and the metasilicate hydrate into the same stirring tank in step 1.

[0064] Step 2 is intended to mean that the method for producing the inorganic coated sand of this embodiment includes a step of stirring the metasilicate hydrate and the refractory aggregate at a predetermined speed when the liquid metasilicate hydrate is crystallized on the surface of the refractory aggregate to form an inorganic binder layer (i.e., dry the aggregate). That is, by including a step of stirring the refractory aggregate and metasilicate hydrate at a higher speed than in the past in step 2, the metasilicate hydrate is adhered more uniformly to the surface of the refractory aggregate, and the metasilicate hydrate crystals on the surface of the refractory aggregate are prevented from becoming coarse and are made finer. As a result, the surface of the inorganic coated sand becomes smoother, the flowability of the inorganic coated sand improves, and the bulk specific gravity increases, which in turn improves the packing ability of the inorganic coated sand and the strength of the mold. Although the details of why the above metasilicate hydrate crystals can be prevented from becoming coarse and made finer are not clear, it is believed that by increasing the stirring speed, the liquid metasilicate hydrate spreads more evenly over the surface of the refractory aggregate, and as the refractory aggregate collides with each other as it is stirred, crystal nuclei are formed and crystallization progresses more easily. Therefore, it is considered that the faster the stirring speed, the more collisions occur, the more sites there are for crystallization, and the more microcrystallization is promoted.

[0065] Furthermore, in the method for producing inorganic coated sand of the present embodiment, the mixture during crystallization of the liquid metasilicate hydrate is stirred at a higher speed than in the past, thereby suppressing the generation of lumps. By reducing the amount of lumps, the filling property of the inorganic coated sand can be improved, and the mold strength can be easily improved.

[0066] The term "lumps" refers to aggregates of refractory aggregates, etc. Specifically, the lumps refer to those that remain on the sieve and are removed when the inorganic coated sand obtained is sieved through a 20-mesh sieve. When the ratio of the removed lumps to the total amount of inorganic coated sand (weight of untreated inorganic coated sand + weight of lumps) is defined as the lump amount (mass %), from the viewpoint of improving the packing ability of the inorganic coated sand, the lump amount is preferably 1.0 mass % or less, more preferably 0.8 mass % or less, and even more preferably 0.5 mass % or less.

[0067] An example of a method for producing the inorganic coated sand will be described in detail below.

[0068] First, the refractory aggregate is prepared. The details of the refractory aggregate are as described above.

[0069] (Process 1) Step 1 is a step of putting refractory aggregate and metasilicate hydrate into a mixing tank. The mixing tank is a container or kettle capable of containing both the refractory aggregate and the metasilicate hydrate, and is a known one. The mixer is used to uniformly mix the refractory aggregate and the metasilicate hydrate and to suitably form an inorganic binder layer made of the metasilicate hydrate on the surface of the refractory aggregate. The form of the metasilicate hydrate when it is added is not important, and it may be liquid or solid. The order of adding the refractory aggregate and the metasilicate hydrate is not particularly limited, and they may be added simultaneously or separately. In addition, stirring may be performed intermittently or continuously in the stirring tank. Specifically, the following methods of adding the materials may be mentioned.

[0070] The metasilicate hydrate in step 1 is preferably in a liquid state when or after it is added, from the viewpoint of quickly homogenizing the refractory aggregate and the metasilicate hydrate. When liquid metasilicate hydrate is added in step 1, the method of adding it to the mixing tank may include adding the refractory aggregate to the mixing tank and then adding the liquid metasilicate hydrate, adding the liquid metasilicate hydrate to the mixing tank and then adding the refractory aggregate, or adding the refractory aggregate and liquid metasilicate hydrate simultaneously. In addition, when solid metasilicate hydrate is added in step 1, the following methods can be used: adding heated refractory aggregate to the stirring tank and then adding solid metasilicate hydrate; adding solid metasilicate hydrate to the stirring tank and then adding heated refractory aggregate; or adding heated refractory aggregate and solid metasilicate hydrate simultaneously. When heated refractory aggregate and solid metasilicate hydrate are added, the solid metasilicate hydrate melts due to the heat of the refractory aggregate and becomes liquid. Among the above-mentioned methods, it is preferable to add liquid metasilicate hydrate after adding refractory aggregate to the mixing tank, because it is easy to control the water content in the resulting inorganic binder layer and to obtain inorganic coated sand with excellent fluidity. It is also preferable to include stirring during the addition of the refractory aggregate and metasilicate hydrate from the viewpoints of early homogenization and promotion of crystallization of metasilicate hydrate.

[0071] In addition, in step 1, when liquid metasilicate hydrate is added while stirring after the refractory aggregate is added, it is presumed that the liquid metasilicate hydrate coats the surface of the refractory aggregate while promoting the crystallization of the metasilicate hydrate.

[0072] The liquid metasilicate hydrate refers to a state in which metasilicate hydrate is melted by heating (molten metasilicate hydrate) or a mixed liquid state containing water glass, caustic alkali, and water. By making the metasilicate hydrate liquid, it is possible to more uniformly cover the surface of the refractory aggregate, and it is possible to more uniformly form an inorganic binder layer on the surface of the refractory aggregate. Moreover, the solid metasilicate hydrate refers to one that does not have fluidity at room temperature and that retains its shape, such as granular, powdery, or lumpy form.

[0073] When stirring is performed in step 1, the stirring speed of the contents in the stirring tank is preferably a peripheral speed of 1.5 m / s or more, more preferably a peripheral speed of 3.0 m / s or more, even more preferably a peripheral speed of 4.5 m / s or more, even more preferably a peripheral speed of 5.0 m / s or more, and even more preferably a peripheral speed of 6.0 m / s or more, from the viewpoint of early homogenization. Furthermore, when stirring is performed in step 1, the stirring speed of the contents in the stirring tank is preferably a peripheral speed of 100 m / s or less, more preferably a peripheral speed of 50 m / s or less, and even more preferably a peripheral speed of 30 m / s or less, from the viewpoint of suppressing the generation of frictional heat and shortening the time for step 2.

[0074] The temperature of the mixture of the refractory aggregate and the metasilicate hydrate in step 1 is preferably 80°C or lower, more preferably 70°C or lower, and even more preferably 60°C or lower, from the viewpoints of suppressing coarsening of the metasilicate hydrate crystals on the surface of the refractory aggregate and increasing the fluidity of the inorganic coated sand. The temperature of the mixture of the refractory aggregate and the metasilicate hydrate in step 1 is preferably equal to or higher than the melting point of the metasilicate hydrate, from the viewpoint of supplying the metasilicate to the refractory aggregate and increasing the fluidity of the inorganic coated sand.

[0075] In addition, the temperature of the metasilicate hydrate immediately before being added in step 1 is, from the viewpoint of supplying metasilicate to the refractory aggregate and increasing the fluidity of the inorganic coated sand, preferably equal to or higher than the melting point of the metasilicate hydrate, more preferably equal to or higher than 50°C, even more preferably equal to or higher than 60°C, and even more preferably equal to or higher than 70°C.

[0076] In addition, from the viewpoint of obtaining a high-strength casting mold, the amount of metasilicate hydrate added in step 1 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, particularly preferably 1 part by mass or more, and even more particularly preferably 1.5 parts by mass or more, relative to 100 parts by mass of the refractory aggregate. In addition, the amount of metasilicate hydrate added in step 1 is, for example, 15 parts by mass or less, preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, relative to 100 parts by mass of the refractory aggregate, from the viewpoint of obtaining a high-strength casting mold.

[0077] (Process 2) Step 2 is started after all of the refractory aggregate and metasilicate hydrate necessary for obtaining the inorganic coated sand have been charged into the mixing tank in step 1. The mixing may be started before the start of step 2, or may be continued from the mixing in step 1. In step 2, the liquid metasilicate hydrate is crystallized while stirring the contents in the stirring tank to form an inorganic binder layer. That is, by stirring and mixing the refractory aggregate and the liquid metasilicate hydrate, the crystallization of the liquid metasilicate hydrate can be promoted. As a result, an inorganic binder layer is formed on the surface of the refractory aggregate, and a dried mixture is obtained.

[0078] Step 2 includes a step of stirring the mixture containing the refractory aggregate and the metasilicate hydrate at a peripheral speed of 1.5 m / s or more. In this embodiment, the peripheral speed of the mixture can be the stirring speed of the refractory aggregate. The stirring speed of the refractory aggregate is set to a peripheral speed of 1.5 m / s or more from the viewpoint of suppressing coarsening of the crystals of the metasilicate hydrate, which is an inorganic binder, and refining them, and is preferably set to a peripheral speed of 3.0 m / s or more, more preferably set to a peripheral speed of 4.5 m / s or more, and even more preferably set to a peripheral speed of 6.0 m / s or more. On the other hand, the upper limit of the stirring speed of the refractory aggregate is preferably a peripheral speed of 100 m / s or less, more preferably a peripheral speed of 50 m / s or less, and even more preferably a peripheral speed of 30 m / s or less, from the viewpoint of stabilizing the particle size distribution of the inorganic coated sand and suppressing the generation of frictional heat.

[0079] The refractory aggregate can be stirred by a known method, and may be rotated by using a stirring blade, may be rotated in a stirring tank, or may be simultaneously performed. Any known stirrer can be used. The peripheral speed during stirring and mixing can be adjusted by changing the number of rotations or the size of the stirring blade. The peripheral speed during stirring and mixing can be calculated from the following formula. Circumferential speed (m / s) = Mixer rotation speed (rpm) × mixing blade diameter (m) × pi (π) ÷ 60

[0080] In step 2, the stirring speed of the refractory aggregate may be varied, provided that the time is at least such that the peripheral speed is 1.5 m / s or more. In other words, assuming that the end point is from the time when the entire amount of the refractory aggregate and metasilicate hydrate are charged into the stirring tank in step 1 (the start time of step 2) to the time when the dried mixture is obtained in step 2, the time (minutes) for stirring and mixing at a peripheral speed of 1.5 m / s or more may be controlled as follows: Specifically, in step 2, the time (minutes) for stirring and mixing at a peripheral speed of 1.5 m / s or more relative to the time (minutes) required for the step is preferably 5% or more, more preferably 10% or more in terms of increasing mold strength, and even more preferably 20% or more in terms of shortening the drying time. The time (minutes) for stirring and mixing at a peripheral speed of 1.5 m / s or more relative to the time (minutes) required for this step may be 100%.

[0081] Furthermore, in order to suppress the generation of frictional heat, a cooling means may be used during stirring. As the cooling means, a known method can be used, for example, blowing cold air into the stirring vessel or cooling the stirring vessel from the outside.

[0082] In step 2, in order to reduce the fluidity of the metasilicate hydrate and fix the metasilicate hydrate to the surface of the refractory aggregate, cooling may be performed to a temperature below the melting temperature of the metasilicate hydrate.

[0083] (Step 3) Furthermore, from the viewpoint of improving mold strength, it is preferable to further include the following step 3 after step 2 (after drying the mixture). Step 3: Removing aggregates from the dried mixture. That is, in step 2, it is preferable to sieve the dried mixture to remove aggregates (lumps) from the mixture. Examples of aggregates (lumps) include aggregates of inorganic coated sand. The sieve is preferably 10 to 80 mesh.

[0084] <Mold> The casting mold of this embodiment is formed from the inorganic coated sand of this embodiment described above. A method for making a casting mold includes a method having a step of filling a mold with the inorganic coated sand and hardening the inorganic coated sand in the mold. A method for hardening the inorganic coated sand includes a method of filling a preheated molding mold with the inorganic coated sand and leaving it as it is, and a method of filling a preheated molding mold with the inorganic coated sand and further ventilating it with water vapor and then ventilating it with hot air.

[0085] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various configurations other than those described above can also be adopted.

[0086] In relation to the above-mentioned 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. EXAMPLES

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

[0088] (1) Material [Fire-resistant aggregate] Fireproof aggregate 1: Espearl #60L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 241 μm, degree of amorphization: 45%, sphericity: 0.97, surface roughness Ra: 0.21 μm) ·Refractory aggregate 2; Mikawa silica sand R6 (manufactured by Mikawa Silica Co., Ltd., average particle size: 200 μm, amorphousness: 0.2%, sphericity: 0.85, surface roughness Ra: 0.36 μm) [Inorganic binder] Sodium metasilicate nonahydrate: Na2SiO3·9H2O (manufactured by Nippon Chemical Industry Co., Ltd., melting point: 47°C, SiO2 / Na2O ratio = 0.9 to 1.1)

[0089] (2) Mixer Mixer 1: Super Mixer MS-I type (manufactured by Taiyo Machinery Co., Ltd., mixing blade diameter: 0.45 m) Mixer 2: Ribbon mixer R17-W type (manufactured by Hosokawa Micron Corporation, mixing blade diameter: 0.22 m) Mixer 3: High-speed mixer FS-GC-10 type (manufactured by Fukae Powtec Co., Ltd., mixing blade diameter: 0.39 m)

[0090] (3) Preparation of inorganic coated sand <Example 1> Using the stirrer and stirring conditions (rotation speed, peripheral speed) shown in Table 1, inorganic coated sand was obtained according to the following procedure. Refractory aggregate 1 (100 parts by mass) at room temperature was charged into mixer 1 and mixed at the peripheral speed of mixing shown in Condition-1 in Table 1. Next, while continuing stirring at the same stirring speed, liquid sodium metasilicate nonahydrate (3.0 parts by mass) that had been heated to 80°C and melted was further added to the above-mentioned mixer 1. Furthermore, stirring and mixing was continued at the same stirring speed, and after it was confirmed that the mixture had been dried within the drying time shown in Table 1 from the start of stirring and mixing, stirring and mixing was stopped, and untreated inorganic coated sand having room temperature fluidity was obtained. Thereafter, the untreated inorganic coated sand was sieved through a 20 mesh sieve to remove lumps, and the inorganic coated sand having the room temperature fluidity shown in Table 1 was obtained. The surface roughness Ra (μm), amount of lumps (mass%), and drying time (min) of the inorganic coated sand were measured and calculated, and are shown in Table 2.

[0091] <Examples 2 to 9, Comparative Examples 1 to 2> Inorganic coated sand was prepared in the same manner as in Example 1, except that the mixer and mixing conditions (rotation speed, peripheral speed) were changed to those shown in Table 1, and the type of fire-resistant aggregate and the amount of inorganic binder mixed were also changed to those shown in Table 1. The drying time (min), lump amount (mass%), and surface roughness Ra (μm) of each were measured. However, the mixing conditions in Example 2 were changed to "Condition-2" immediately after "Condition-1". The results are shown in Table 2.

[0092] (4) Evaluation and measurement The following evaluations and measurements were carried out using the obtained inorganic coated sand. The results are shown in Table 1.

[0093] <Liquidity> The slump flow value was measured and the filling property was evaluated according to the following procedure. The slump test was conducted in accordance with JIS A 1101:2014 using a slump cone with an upper inner diameter of 50 mm, a lower inner diameter of 100 mm, and a height of 150 mm. Specifically, in an environment of 25°C and relative humidity of 55%, the slump cone was filled with inorganic coated sand, and when the slump cone was lifted up in one go, the diameter of the inorganic coated sand that spread out in a circle was measured and recorded as the slump flow value (mm).

[0094] <Mold strength> Using each inorganic coated sand, a mold was made according to the following procedure, and the mold strength was measured. The evaluation results are shown in Table 1. (procedure) A mold for 22.3 mm × 22.3 mm × 180 mm test pieces (5 pieces) was heated to 180° C. Each of the inorganic coated sands of the above Examples and Comparative Examples was filled into the mold heated to 180° C. at a blow pressure of 0.3 MPa using a CSR-43 blow molding machine, and the inorganic coated sand was left to stand in the mold for 150 seconds to harden, thereby obtaining a mold test piece. (measurement) The mold strength (MPa) of each mold test piece was measured using a universal strength testing machine (PFG type) manufactured by George Fischer, which was previously equipped with a PBV flexural attachment. The mold test pieces were left in a constant temperature and humidity room (25°C / 55% RH) for 1 hour after being removed from the metal mold.

[0095] [Table 1]

[0096] [Table 2]

Claims

1. The following steps 1 and 2 are included: Step 1: Step of adding refractory aggregate and metasilicate hydrate to a stirring tank. Step 2: In the stirring tank, the surface of the refractory aggregate is covered with the crystallized metasilicate hydrate to obtain a dried mixture. A method for producing inorganic coated sand, comprising step 2, which involves stirring a mixture containing the refractory aggregate and the metasilicate hydrate at a peripheral speed of 1.5 m / s or more.

2. A method for producing inorganic coated sand according to claim 1, A method for producing inorganic coated sand, further comprising the following step 3 after step 2. Step 3: Step to remove aggregates from the dried mixture.

3. A method for producing inorganic coated sand according to claim 1, A method for producing inorganic coated sand, wherein the amount of metasilicate hydrate added in step 1 is 0.1 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the refractory aggregate.

4. A method for producing inorganic coated sand according to claim 1, A method for producing inorganic coated sand, wherein the mixture in step 2, which includes the refractory aggregate and the metasilicate hydrate, is a mixture obtained by adding the refractory aggregate and the metasilicate hydrate to a stirring tank in step 1.

5. A method for manufacturing a mold, comprising the steps of filling a mold with inorganic coated sand produced by the method for manufacturing inorganic coated sand described in any one of claims 1 to 4, and hardening the inorganic coated sand in the mold.

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