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

Abstract

To provide a manufacturing method of inorganic coated sand that can improve the strength of a casting mold.SOLUTION: A manufacturing method of inorganic coated sand is a method for manufacturing inorganic coated sand in a dried state that includes a refractory aggregate, and a metasilicate hydrate layer formed on a surface of the refractory aggregate. The method includes a process of mixing the refractory aggregate and the metasilicate hydrate in a liquid state, crystallizing the metasilicate hydrate and forming the metasilicate hydrate layer on the surface of the refractory aggregate. In the process, when Ts denotes a temperature of a mixture one minute after a time point when the total amount of the refractory aggregate was mixed with the total amount of the metasilicate hydrate in the liquid state, Ts satisfies the following formula (1): {5(9-n)+15}<Ts(°C)<{5(9-n)+39} (1). (In the formula (1), n denotes an average mol number of water of the metasilicate hydrate.)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 casting 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 related to inorganic coated sand, Patent Document 1 (JP 2020-11296 A) discloses a manufacturing method for producing dry inorganic coated sand having a refractory aggregate and an inorganic binder layer formed on the surface of the refractory aggregate, the inorganic binder layer containing a metasilicate hydrate, the method including: a step (1) of mixing the refractory aggregate and the metasilicate hydrate at a temperature equal to or higher than the melting point of the metasilicate hydrate to obtain a mixture; and a step (2) of cooling the mixture to a temperature lower than the melting point of the metasilicate hydrate.

[0004] Patent Document 2 (JP Patent Publication 1974-103823) discloses a mold material for reduced pressure molding, which is characterized in that a sodium metasilicate solution, in which the molar ratio of silicon dioxide to sodium oxide is adjusted to be 0.7 or more and less than 1.1, is added to and kneaded with refractory particles such as silica sand, causing the particle surfaces to be coated with crystalline sodium silicate by precipitation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-11296 A [Patent Document 2] Japanese Unexamined Patent Publication No. 49-103823 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the inorganic coated sand obtained by the conventional manufacturing methods described in Patent Documents 1 and 2 has room for improvement in terms of improving the strength of a mold using the inorganic coated sand. [Means for solving the problem]

[0007] In order to improve the strength of a mold, the present inventors focused on the drying process during the production of inorganic coated sand and discovered that the drying of inorganic coated sand is due to the crystallization of metasilicate hydrate, which is an inorganic binder, and further found that the shorter the time until crystallization (drying time), the higher the mold strength tends to be. Therefore, the present inventors conducted extensive research to shorten the drying time and discovered that it is effective to control the temperature of the mixture of refractory aggregate and metasilicate hydrate at a specific timing, thereby completing the present invention.

[0008] According to the present invention, there is provided a method for producing dry inorganic coated sand having a refractory aggregate and a metasilicate hydrate layer formed on the surface of the refractory aggregate, comprising the steps of: The method includes a step of mixing the refractory aggregate with liquid metasilicate hydrate and crystallizing the metasilicate hydrate to form the metasilicate hydrate layer on the surface of the refractory aggregate, The present invention provides a method for producing inorganic coated sand, in which, when the temperature of the mixture one minute after the total amount of the refractory aggregate and the total amount of the liquid metasilicate hydrate are mixed in the above-mentioned process is defined as Ts, Ts satisfies the following formula (1): {5(9-n)+15} <Ts(℃)<{5(9-n)+39} (1) (In formula (1), n ​​represents the average number of moles of water in the metasilicate hydrate.)

[0009] The present invention also provides a method for producing a casting mold using the inorganic-coated sand obtained by the above-mentioned method for producing inorganic-coated sand. Effect of the Invention

[0010] According to the present invention, a method for producing inorganic coated sand capable of improving the strength of a mold can be provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] 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.

[0012] <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 inorganic coated sand of the present embodiment is a method for producing dry inorganic coated sand having a refractory aggregate and a metasilicate hydrate layer formed on the surface of the refractory aggregate, The method includes a step of mixing the refractory aggregate with liquid metasilicate hydrate and crystallizing the metasilicate hydrate to form the metasilicate hydrate layer on the surface of the refractory aggregate, In the above process, when the temperature of the mixture 1 minute after the time when the entire amount of the refractory aggregate and the entire amount of the liquid metasilicate hydrate are mixed is Ts, A method for producing inorganic coated sand, wherein Ts satisfies the following formula (1): {5(9-n)+15} <Ts(℃)<{5(9-n)+39} (1) (In formula (1), n ​​represents the average number of moles of water in the metasilicate hydrate.)

[0013] This makes it possible to shorten the drying time and improve the strength of the mold produced using the inorganic coated sand. Generally, the faster the crystallization (the shorter the crystallization time), the finer the crystals that are produced, but the crystallization time depends on the temperature, and while it is easier to generate crystal nuclei at low temperatures, it is known that crystal growth is promoted at high temperatures, which makes it easier to obtain fine crystals. Therefore, there is a trade-off between generating crystal nuclei early and increasing the crystallization rate, so for example, if the temperature of the mixture is too high, the crystal nuclei cannot be generated early, and as a result, the drying time cannot be shortened. In contrast, according to the method for producing inorganic coated sand of the present embodiment, the temperature at a predetermined timing is controlled by Ts (°C), so that the nucleation and crystal growth of metasilicate hydrate can be promoted at a higher level than in the past, and fine crystals can be obtained by shortening the drying time. As a result, even if the inorganic coated sand is made using the same raw material, the mold strength can be improved. In other words, in the process of producing inorganic coated sand, the temperature of the mixture can vary from the start of mixing to the end of mixing, but the method of producing inorganic coated sand of this embodiment has discovered that the temperature of the mixture (Ts (°C)) one minute after the start of mixing the total amount of refractory aggregate and the total amount of liquid metasilicate hydrate contributes to both the generation of metasilicate hydrate nuclei and the promotion of crystallization. By controlling Ts (°C) to satisfy formula (1), inorganic coated sand is realized that can effectively shorten the drying time and improve mold strength.

[0014] Here, Ts (℃) refers to the temperature of the mixture 1 minute after starting to mix the entire amount of refractory aggregate and the entire amount of liquid metasilicate hydrate, and refers to the temperature at the point when the entire amount of metasilicate hydrate and the entire amount of refractory aggregate are mixed, blended together, and reach a uniform temperature. Moreover, formula (1) is an index devised by the present inventor with the intention that the range of Ts (°C) at which the mold strength can be improved varies depending on the average number of moles of water in the metasilicate hydrate. The average number of moles of water in the metasilicate hydrate is determined by measuring the amount of water of hydration, which will be described later.

[0015] Ts (°C) is measured as the temperature of the mixture when the refractory aggregate and metasilicate hydrate are mixed in a container. Ts (°C) can be achieved by using a known mixing method and adjusting the temperature of the refractory aggregate, the temperature of the metasilicate hydrate, the temperature of the container, etc. In addition, the refractory aggregate and metasilicate hydrate may be mixed little by little, or one may be added as a base, but Ts (℃) refers to the temperature of the mixture one minute after the total amount used for the inorganic coated sand is mixed. For example, when liquid metasilicate hydrate is added little by little to the refractory aggregate that has been stirred and mixed, the time when the final amount of metasilicate hydrate is added is regarded as the mixing start time.

[0016] In addition, one of the factors that controls Ts (℃) is the temperature (To (℃)) of the refractory aggregate immediately before it is mixed with the liquid metasilicate hydrate described below. By setting To (℃) to a specific value, it is possible to control Ts (℃) to a desired value.

[0017] The measurements of Ts (°C) and To (°C) can be performed using known measuring instruments, such as a contact thermometer, a non-contact radiation thermometer, etc. Among them, it is preferable to use a non-contact radiation thermometer from the viewpoint of preventing the adhesion of refractory aggregates, etc. to the sensor part of the thermometer and obtaining good handling properties. Commercially available measuring instruments include, for example, a contact thermometer such as the HL-200 digital surface thermometer body and a temperature sensor such as the BS-31E-030-TC1-ASP (manufactured by Anritsu Keiki Co., Ltd.), and a non-contact radiation thermometer such as the MT-7 radiation thermometer (manufactured by Mother Tool Co., Ltd.).

[0018] An example of a method for producing the inorganic coated sand will be described in detail below.

[0019] First, prepare refractory aggregates. Details of the refractory aggregates will be described later. From the viewpoint of controlling Ts (°C) to increase the mold strength, the refractory aggregates may be adjusted to a predetermined temperature before being mixed with the metasilicate hydrate. Specifically, when the temperature of the refractory aggregates immediately before being mixed with the liquid metasilicate hydrate is To (°C), To (°C) preferably satisfies the following formula (2). {5(9 - n)+L}<To(°C)<{5(9 - n)+U} (2) (In formula (2), n represents the average number of moles of water in the metasilicate hydrate. Also, L and U are each a numerical value of 5 or more and 60 or less, and L < U.)

[0020] To (°C) is the average temperature of the entire refractory aggregates and can be controlled by a known method. Specifically, for example, a method of cooling or heating a container containing the refractory aggregates to a predetermined temperature and stirring so that the entire refractory aggregates in the container reach a uniform temperature can be mentioned.

[0021] From the viewpoint of promoting the crystal growth of the metasilicate hydrate, L in formula (2) is preferably 5 or more, more preferably 10 or more, and even more preferably 15 or more. On the other hand, from the viewpoint of accelerating the formation of crystal nuclei of the metasilicate hydrate in the refractory aggregates, U is preferably 39 or less, more preferably 34 or less, and even more preferably 29 or less. Note that since L < U, the value of L does not exceed the value of U.

[0022] Next, mix the refractory aggregates and the liquid metasilicate hydrate, and crystallize the metasilicate hydrate to form a metasilicate hydrate layer on the surface of the refractory aggregates. In this step, when the temperature of the mixture 1 minute after the start of mixing the total amount of the refractory aggregates and the total amount of the liquid metasilicate hydrate is Ts, Ts satisfies the following formula (1). {5(9 - n)+15}<Ts(°C)<{5(9 - n)+39} (1) (In formula (1), n ​​represents the average number of moles of water in the metasilicate hydrate.)

[0023] By using liquid metasilicate hydrate, it becomes easier for the liquid metasilicate hydrate to adhere to the surface of the refractory aggregate so as to cover the surface, and a uniform metasilicate hydrate layer can be formed. The liquid metasilicate hydrate may be a mixed liquid containing water glass, caustic alkali and water, and is a molten metasilicate hydrate. The metasilicate hydrate may be a mixture of multiple metasilicate hydrates having different amounts of water of hydration, or a single metasilicate hydrate having a predetermined amount of water of hydration may be used. When multiple metasilicate hydrates having different amounts of water of hydration are used, the order of mixing them is not particularly important. The metasilicate hydrate will be described in detail later.

[0024] The refractory aggregate and the metasilicate hydrate can be mixed by a known method. Specifically, for example, there can be mentioned a method in which metasilicate hydrate is added to heated refractory aggregate, the metasilicate hydrate is melted by the heat of the refractory aggregate, and the liquid metasilicate hydrate is mixed with the refractory aggregate; a method in which metasilicate hydrate that has been melted in advance and made liquid is added to the refractory aggregate and mixed; a method in which a mixed liquid containing water glass, caustic alkali, and water is added to the refractory aggregate and mixed; etc. Among these, from the viewpoint of controlling Ts (°C) and obtaining high mold strength, a method in which a metasilicate hydrate that has been melted in advance and made liquid is poured into the refractory aggregate and mixed therewith; and a method in which a mixed liquid containing water glass, caustic alkali, and water is poured into the refractory aggregate and mixed therewith are preferred.

[0025] The mixing conditions, such as the stirring speed and processing time, when mixing the raw materials can be appropriately determined depending on the amount of the mixture to be processed. The mixing conditions may also be appropriately adjusted during the mixing process. It is also preferable to continue mixing and stirring. For example, it is preferable to add liquid metasilicate hydrate to the refractory aggregate while continuing stirring.

[0026] Subsequently, while continuing to mix the mixture, the liquid metasilicate hydrate is fixed on the surface of the refractory aggregate, whereby the metasilicate hydrate is crystallized on the surface of the refractory aggregate, and a metasilicate hydrate layer can be formed on the surface of the refractory aggregate. Fixation of the metasilicate hydrate may be performed by natural cooling, or by cooling to a temperature below the melting temperature of the metasilicate hydrate. Also, fixation may be performed at ambient temperature, or the temperature may be appropriately controlled by a heater, a cooler, or the like. This makes it possible to obtain dry inorganic coated sand that has flowability at room temperature.

[0027] Furthermore, from the viewpoint of improving the mold strength, it is preferable to sieve the recovered inorganic coated sand to remove aggregates (lumps). The aggregates (lumps) include aggregates of inorganic coated sand. The sieve is preferably 10 to 80 mesh.

[0028] <Inorganic coated sand> The inorganic coated sand of the present embodiment is a dry inorganic coated sand having a refractory aggregate and a metasilicate hydrate layer (inorganic binder layer) formed on the surface of the refractory aggregate, and is obtained by the above-mentioned manufacturing method.

[0029] The inorganic coated sand will be described in more detail below.

[0030] The inorganic coated sand is in a dry state. 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.

[0031] 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.

[0032] Specifically, the inorganic coated sand is composed of a group of inorganic coated sand particles.

[0033] From the viewpoint of improving the flowability and further improving the filling property into the molding die, the inorganic coated sand is preferably spherical. Here, spherical inorganic coated sand means a round shape like a ball.

[0034] 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 1.0. In this embodiment, the sphericity of the inorganic coated sand specifically coincides with the sphericity of the refractory aggregate described below.

[0035] 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.

[0036] 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 mold quality and mold strength improvement, 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, it is also preferable in that the inorganic coated sand can be easily regenerated during mold production. 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.

[0037] 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 1.0, 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 average value obtained for 100 random particles is used 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 projection 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.

[0038] Each component of the inorganic coated sand will be described below.

[0039] [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.

[0040] 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.

[0041] 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.

[0042] In addition, the artificial sand can be made from recycled sand made from recycled refractory aggregate, or recycled sand made by subjecting recycled sand to a recycling process.

[0043] 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).

[0044] 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 1.0.

[0045] The sphericity of the refractory aggregate can be measured by the same method as that for the inorganic coated sand.

[0046] 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, if the average particle size of the refractory aggregate is equal to or more than the above lower limit, it is also preferable in that the inorganic coated sand can be easily regenerated during the manufacture of the mold. The average particle size of the refractory aggregate is preferably 2 mm or less, more preferably 1 mm or less, and even more preferably 0.5 mm or less, 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.

[0047] The average particle size of the refractory aggregate can be measured by the same method as that for the inorganic coated sand.

[0048] The degree of amorphization of the refractory aggregate may be 20% or more, 30% or more, or 40% or more from the viewpoint of making the surface of the aggregate smoother and improving the 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.

[0049] The degree of amorphization of the refractory aggregate can be measured by the following X-ray diffraction method (XRD). (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 measurement is performed in the range of 2θ = 5 to 90° with a scan interval of 0.01°, a scan speed of 2° / min, and slits DS1, SS1, and RS0.3 mm. In the range of 2θ = 10° to 50°, the X-ray intensities on the low angle side and the high angle side are connected with a straight line, 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 area of ​​each is calculated, and the degree of amorphousness (%) is calculated using the following formula. Amorphous content (%) = halo area / (crystalline component area + halo area) x 100

[0050] [Metasilicate hydrate layer] The metasilicate hydrate layer is formed on the surface of the refractory aggregate. In other words, the metasilicate hydrate layer covers the surface of the refractory aggregate. Note that the covering is not limited to a continuous covering, and may include discontinuous portions. The metasilicate hydrate layer allows the formation of a mold as an inorganic coated sand.

[0051] In order to obtain a high-strength casting mold, the coating amount of the metasilicate hydrate layer contained in the inorganic coated sand is, for example, 0.1 parts by mass or more, preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, even more preferably 1 part by mass or more, and even more preferably 2 parts by mass or more, relative to 100 parts by mass of the refractory aggregate. In addition, from the viewpoint of obtaining a high-strength casting mold, the coating amount of the metasilicate hydrate 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.

[0052] The metasilicate hydrate layer may be a single layer or multiple layers as long as it has at least a layer containing metasilicate hydrate. The metasilicate hydrate layer is formed from an inorganic binder composition containing metasilicate hydrate. The use of metasilicate hydrate is preferable because it can improve the crystallinity of the metasilicate hydrate 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 can form a metasilicate hydrate layer on the surface of the refractory aggregate in a state where it is not dissolved in water.

[0053] (Metasilicate) The metasilicate hydrate layer is an inorganic binder layer and contains metasilicate hydrate as an inorganic binder. 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. In addition, metasilicate is a hydrate. Metasilicate hydrate can also be produced by using a mixed liquid in which water glass, caustic alkali, and water are mixed at a specific ratio. In this embodiment, the SiO2 / Na2O molar ratio of 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] The content of metasilicate hydrate in the metasilicate hydrate 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, from the viewpoints of improving mold strength, excellent productivity, and ease of availability. The content of metasilicate hydrate in the metasilicate hydrate layer refers to the content of metasilicate relative to all components other than water in the metasilicate hydrate layer.

[0056] As a method for confirming that the metasilicate hydrate 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 metasilicate hydrate layer component, which is then analyzed by XRD to confirm the peak 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 metasilicate hydrate layer component, the eluted component is dried, and the dried solid content is analyzed by XRD to confirm the peak indicating the metasilicate crystal structure while also analyzing the amount of water of hydration using 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 anhydrate]

[0058] The metasilicate hydrate layer may further contain components other than metasilicate hydrate, 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 refractory aggregate and the inorganic binder composition, a lubricant, a surfactant, a release agent, etc.

[0059] (Inorganic fine particles) The metasilicate hydrate layer may further contain amorphous SiO2-containing fine particles. The amorphous SiO2-containing fine particles are used because of their high reactivity with the metasilicate hydrate. This makes it easier to improve the mechanical strength of the mold.

[0060] 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.

[0061] The inorganic fine particles are not particularly limited as long as they are not the 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, Examples of fine particles include one or more types selected from sulfates such as lithium sulfate, 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.

[0062] 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.

[0063] <Casting mold> The casting mold of this embodiment is formed from the inorganic coated sand of this embodiment. Examples of a method for making the casting mold include a molding method using a heated molding die, and a molding method in which water vapor is passed through the heated molding die and then hot air is passed through the heated molding die.

[0064] 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. EXAMPLES

[0065] The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these.

[0066] (1) Material [Fire-resistant aggregate] ·Refractory aggregate 1: Mikawa silica sand R6 (manufactured by Mikawa Silica Co., Ltd., average particle size: 200 μm, amorphousness 0.2%, sphericity 0.85) Fire-resistant aggregate 2: Espearl #60L (manufactured by Yamakawa Sangyo Co., Ltd., average particle size: 241 μm, amorphous degree 45%, sphericity 0.97)

[0067] [Inorganic binder] Inorganic binder 1: Sodium metasilicate 9-hydrate, Na2SiO3·9H2O (manufactured by Nippon Chemical Industry Co., Ltd., melting point 47°C, SiO2 / Na2O ratio = 0.9 to 1.1) Inorganic binder 2: Sodium metasilicate pentahydrate, Na2SiO3·5H2O, manufactured by Nippon Chemical Industry Co., Ltd., melting point 72°C, SiO2 / Na2O ratio = 0.9~1.1) Inorganic binder 3: Sodium silicate No. 1, SiO2 / Na2O molar ratio = 2.0-2.3 (manufactured by Fuji Chemical Co., Ltd.) Inorganic binder 4: 50% aqueous sodium hydroxide solution. This was prepared from sodium hydroxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0068] [Additives] Amorphous silica particles: Denka fused silica SFP-20M (manufactured by Denka, average particle size: 0.4 μm, degree of amorphization: 99.5% or more)

[0069] (2) Preparation of inorganic coated sand <Example 1> As shown in Table 1, refractory aggregate 1 (100 parts by mass) set at 25° C. was charged into a mixer and stirred (To: 25° C.). Next, while continuing the stirring, the whole amount (3.5 parts by mass) of inorganic binder 1 that had been heated to 80°C and melt-mixed was added to the above-mentioned mixer at once and kneaded to be homogenous. Stirring was continued as it was, and after it was confirmed that the mixture had been dried within the drying time shown in Table 1 from the start of kneading, the stirring 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 temperature of the mixture (Ts (°C)) was measured 1 minute after the total amount (3.5 parts by mass) of the inorganic binder 1 was added. A non-contact radiation thermometer ([MT-7] radiation thermometer, manufactured by Mother Tools Co., Ltd.) was used to measure the temperature.

[0070] <Example 2> Inorganic coated sand was prepared in the same manner as in Example 1, except that the temperature (To (°C)) of the refractory aggregate was changed from 25°C to 5°C, and Ts (°C) and drying time (min) were measured. The results are shown in Table 1.

[0071] <Examples 3, 4, and 6> Inorganic coated sand was prepared in the same manner as in Example 1, except that the temperature (To (°C)) of the refractory aggregate was changed from 25°C to each temperature shown in Table 1, and the amounts (addition amounts) of inorganic binder 1 and inorganic binder 2 were changed as shown in Table 1, and the Ts (°C) and drying time (min) were measured. Note that inorganic binder 1 and inorganic binder 2 were mixed before melting and added in a molten state. The results are shown in Table 1. However, inorganic binder 1 and inorganic binder 2 were mixed and then melted before use.

[0072] <Example 5> Inorganic coated sand was prepared and Ts (° C.) and drying time (min) were measured in the same manner as in Example 4. The results are shown in Table 1. For the manufacture of a casting mold, amorphous silica fine particles (1.23 parts by mass) were added to the obtained inorganic coated sand, and the mixture was kneaded for 2 minutes with a stirrer to prepare a molding sand.

[0073] <Example 7> The inorganic coated sand was produced and the drying time (min) was measured in the same manner as in Example 4, except that the refractory aggregate and amorphous silica fine particles (1.23 parts by mass) were added to the mixer and stirred before adding the mixture of inorganic binder 1 and inorganic binder 2. The results are shown in Table 1.

[0074] <Examples 8, 9, and 11> Inorganic coated sand was prepared in the same manner as in Example 1, except that the temperature (To (°C)) of the refractory aggregate was changed from 25°C to each temperature shown in Table 1, and the type of refractory aggregate and the amounts of inorganic binder 1 and inorganic binder 2 added were changed as shown in Table 1, and the Ts (°C) and drying time (min) were measured. Note that inorganic binder 1 and inorganic binder 2 were mixed before melting and added in a molten state. The results are shown in Table 2.

[0075] <Example 10> Inorganic coated sand was prepared and Ts (° C.) and drying time (min) were measured in the same manner as in Example 9. The results are shown in Table 2. For the manufacture of a casting mold, amorphous silica fine particles (0.70 parts by mass) were added to the obtained inorganic coated sand, and the mixture was kneaded for 2 minutes with a stirrer to prepare molding sand.

[0076] <Example 12> Inorganic coated sand was prepared and Ts (°C) and drying time (min) were measured in the same manner as in Example 1, except that the type of inorganic binder was changed to inorganic binder 3 and inorganic binder 4 as shown in Table 1. Inorganic binder 3 and inorganic binder 4 were mixed in advance to prepare an aqueous metasilicate solution, and then the mixture was added. The results are shown in Table 1.

[0077] <Comparative Examples 1 to 3, 5, and 6> Inorganic coated sand was prepared and Ts (°C) and drying time (min) were measured in the same manner as in Example 1, except that the temperature (To (°C)) of the refractory aggregate was changed from 25°C to the temperature shown in Table 1, and the amounts of inorganic binder 1 and inorganic binder 2 added were changed as shown in Table 1. Note that inorganic binder 1 and inorganic binder 2 were mixed before melting and added in a molten state. The results are shown in Table 1.

[0078] <Comparative Example 4> Inorganic coated sand was prepared and its Ts (° C.) and drying time (min) were measured in the same manner as in Comparative Example 3. The results are shown in Table 1. For the manufacture of a casting mold, amorphous silica fine particles (1.23 parts by mass) were added to the obtained inorganic coated sand, and the mixture was kneaded for 2 minutes with a stirrer to prepare a molding sand.

[0079] <Comparative Examples 7 and 8> Inorganic coated sand was prepared in the same manner as in Example 8, except that the temperature (To (°C)) of the refractory aggregate was changed from 25°C to the temperature shown in Table 1, and the amounts of inorganic binder 1 and inorganic binder 2 added were changed as shown in Table 1, and the Ts (°C) and drying time (min) were measured. Note that inorganic binder 1 and inorganic binder 2 were mixed before melting, and added in a molten state. The results are shown in Table 2.

[0080] <Comparative Example 9> Inorganic coated sand was prepared and Ts (° C.) and drying time (min) were measured in the same manner as in Comparative Example 8. The results are shown in Table 2. For the manufacture of a casting mold, amorphous silica fine particles (0.70 parts by mass) were added to the obtained inorganic coated sand, and the mixture was kneaded for 2 minutes with a stirrer to prepare a molding sand.

[0081] (3) Evaluation and measurement The following evaluations and measurements were carried out using the obtained inorganic coated sand or molding sand. The results are shown in Tables 1 and 2. <Mold strength> Using each inorganic coated sand or molding sand, casting molds were made in the following manner, and the mold strength was measured. The evaluation results are shown in Tables 1 and 2. In Examples 5 and 10 and Comparative Examples 4 and 9, amorphous silica microparticles were added to the obtained inorganic coated sand in the amount (parts by mass) shown in Table 1 or Table 2, and the mixture was kneaded for 2 minutes with a stirrer to obtain molding sand. Using the obtained molding sand, casting molds were produced according to the following procedure. (procedure) A mold for 22.3 mm x 22.3 mm x 180 mm test pieces (5 pieces) was heated to 180° C. Each of the inorganic coated sands or molding sands of the above-mentioned 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 sands or molding sands were left to stand in the mold for 150 seconds to harden, thereby obtaining mold test pieces. (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.

[0082] [Table 1]

[0083] [Table 2]

Claims

1. A method for producing dry inorganic coated sand comprising a refractory aggregate and a metasilicate hydrate layer formed on the surface of the refractory aggregate, The process includes mixing the refractory aggregate with a liquid metasilicate hydrate, and crystallizing the metasilicate hydrate to form a metasilicate hydrate layer on the surface of the refractory aggregate. A method for producing inorganic coated sand, wherein, in the above step, when the temperature of the mixture one minute after the time when the entire amount of the refractory aggregate and the entire amount of the liquid metasilicate hydrate are mixed is defined as Ts, Ts satisfies the following formula (1). {5(9-n)+15}<Ts(℃)<{5(9-n)+39} (1) (In formula (1), n ​​represents the average number of moles of water in the metasilicate hydrate.)

2. A method for producing inorganic coated sand according to claim 1, A method for producing inorganic coated sand, wherein the liquid metasilicate hydrate is a mixture containing water glass, caustic alkali, and water.

3. A method for producing inorganic coated sand according to claim 1, A method for producing inorganic coated sand, wherein the liquid metasilicate hydrate is molten metasilicate hydrate.

4. A method for producing inorganic coated sand according to claim 1, A method for producing inorganic coated sand, comprising adding the liquid metasilicate hydrate to the refractory aggregate in the above step.

5. A method for producing inorganic coated sand according to claim 1, A method for producing inorganic coated sand, wherein To satisfies the following equation (2), when To is the temperature of the refractory aggregate immediately before the above step. {5(9-n)+L}<To(℃)<{5(9-n)+U} (2) (In formula (2), n represents the average number of moles of water in the metasilicate hydrate. Also, L and U are values ​​between 5 and 60, and L < U.)

6. A method for manufacturing a casting mold using inorganic coated sand obtained by the method for manufacturing inorganic coated sand according to any one of claims 1 to 5.