Method for reclaiming recovered foundry sand

Abstract

To provide a method for advantageously regenerating (reusing) recovered molding sand containing waste sand of a green sand mold using bentonite as a caking component as the molding sand in the case of producing a core (shell core) molded according to a shell mold method.SOLUTION: The recovered molding sand containing the molding sand to which the bentonite-containing binder is stuck is subjected to polishing treatment or washing treatment so that the methylene blue adsorption amount of the obtained regenerated molding sand is 10mmol / 100g or less. Preferably, the roasting treatment is performed after the polishing treatment or the water washing treatment.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for reclaiming recovered foundry sand, and more particularly to a method for advantageously reclaiming (reusing) waste foundry sand recovered after sand casting as foundry sand for shell molds. [Background technology]

[0002] Conventionally, when producing casting products made of various metallic materials such as iron or aluminum, sand casting, which uses a mold (sand mold) formed from foundry sand, has been widely adopted. In such sand casting, the sand mold after casting is usually crushed to a sand-like state and then recovered. Here, various binders are used in the molding of the sand mold used in sand casting. For example, in sand casting, which can suitably produce precision castings, a green sand mold made using bentonite, a clay mineral, as a binder is often used as the main mold, while an organic binder such as phenolic resin is widely used to form the core (shell core) used in the sand casting. This core molding method is called the shell molding method.

[0003] The sandy material recovered after sand casting (recovered foundry sand) contains various impurities and has various binders and their carbides adhered to the particle surfaces, making it difficult to reuse it as foundry sand without further treatment after recovery. For this reason, recovered foundry sand has traditionally been disposed of as industrial waste, but in recent years, from the perspective of environmental protection, various treatment methods (recycling methods) have been proposed to reuse recovered foundry sand as foundry sand.

[0004] For example, Patent Document 1 (JP 2006-68815 A) proposes a method for recycling waste green sand containing 0 to 40% by weight of shells, by subjecting the waste green sand containing shells to dry grinding to remove 80% or more by weight of impurities, mainly composed of clay, adhering to the surface of the waste green sand, and then subjecting the waste green sand to wet grinding so that the total clay content is 0.3% by weight or less, thereby recycling the waste green sand into foundry aggregate.

[0005] On the other hand, in recent years, there has been a demand for energy conservation and environmental considerations in the reclamation of recovered foundry sand, and the current situation is that further improvements are required for various conventional methods, including the reclamation method disclosed in Patent Document 1. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-68815 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made in light of the above circumstances, and the problem to be solved by the present invention is to provide a method for advantageously regenerating (reusing) recovered foundry sand, including waste sand from green sand molds that use bentonite as a binder, as foundry sand for producing cores (shell cores) molded by the shell molding method. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present invention can be suitably implemented in various aspects as listed below, and the aspects described below can be adopted in any combination. It should be understood that the aspects and technical features of the present invention are not limited to those described below, but can be recognized based on the inventive idea that can be grasped from the description of the entire specification.

[0009] (1) A method for reclaiming recovered foundry sand, comprising the steps of: polishing the recovered foundry sand to a binder containing bentonite as a binder component; and reducing the methylene blue adsorption of the resulting reclaimed foundry sand to 10 mmol / 100 g or less. (2) The method for recycling recovered foundry sand according to the aspect (1), wherein the ignition loss of the recycled foundry sand is 0.30 to 1.60%. (3) The method for recycling recovered foundry sand according to the aspect (1) or the aspect (2), wherein the recovered foundry sand after the polishing treatment is subjected to a baking treatment at a temperature of 550 to 700 °C. (4) The method for recycling recovered foundry sand according to the aspect (1) or the aspect (2), wherein the recovered foundry sand after the polishing treatment is subjected to a baking treatment according to the conditions satisfying the following formula (1). 150 < T×t < 500 ··· Formula (1) However, in the above formula (1), T is the treatment temperature (°C) of the baking treatment, and t is the treatment time (h) of the baking treatment. (5) The method for recycling recovered foundry sand according to the aspect (4), wherein the treatment temperature of the baking treatment is 550 °C or higher. (6) The method for recycling recovered foundry sand according to the aspect (4), wherein the treatment temperature of the baking treatment is 550 to 700 °C. (7) The method for recycling recovered foundry sand according to the aspect (1) or the aspect (2), wherein a water washing treatment is performed instead of the polishing treatment. (8) The method for recycling recovered foundry sand according to the aspect (7), wherein the recovered foundry sand after the water washing treatment is subjected to a baking treatment at a temperature of 550 to 700 °C. (9) The method for recycling recovered foundry sand according to the aspect (7), wherein the recovered foundry sand after the water washing treatment is subjected to a baking treatment according to the conditions satisfying the following formula (1). 150 < T×t < 500 ··· Formula (1) However, in the above formula (1), T is the treatment temperature (°C) of the baking treatment, and t is the treatment time (h) of the baking treatment. (10) The method for recycling recovered foundry sand according to the aspect (9), wherein the treatment temperature of the baking treatment is 550 °C or higher. (11) The method for recycling recovered foundry sand according to the aspect (9), wherein the treatment temperature of the baking treatment is 550 to 700 °C. (12) Recycled foundry sand made of recovered foundry sand after the recycling treatment, characterized in that the methylene blue adsorption amount is 10 mmol / 100 g or less. (13) The recycled foundry sand according to the above aspect (12), having an ignition loss of 0.30 to 1.60%. [Effects of the Invention]

[0010] Thus, in the method for reclaiming recovered foundry sand according to the present invention, recovered foundry sand, for example, after sand casting, containing foundry sand to which a binder containing bentonite as a caking component has adhered, is subjected to polishing (and / or other treatments) so that the methylene blue adsorption of the recovered foundry sand (reclaimed foundry sand) after the reclaiming treatment is below a predetermined value. In other words, the reclaimed foundry sand obtained according to the reclaiming method of the present invention has an effectively reduced amount of bentonite, an activated clay component, adhering to its particle surfaces. Therefore, when such reclaimed foundry sand is used to mold cores (shell cores) according to the shell molding method, the resulting shell cores have excellent physical properties, including bending strength. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an explanatory diagram showing the configuration of an apparatus used in a hot constant load test in the examples. [Figure 2] FIG. 1 is an explanatory diagram showing the measurement format in the method for measuring the ease of use employed in the Examples. DETAILED DESCRIPTION OF THE INVENTION

[0012] The recovered foundry sand targeted by the method for reclaiming recovered foundry sand according to the present invention includes sand to which a binder containing bentonite as a caking component has been adhered. Bentonite has been widely used as a binder in the production of green sand. For example, a method in which a mold material is prepared by mixing refractory aggregate (foundry sand), bentonite, and water, and then a mold is produced using the mold material and the resulting green sand is used for casting is a widely adopted typical sand casting method. The present invention is applicable not only to foundry sand recovered after a casting method using only a green sand, but also to foundry sand recovered in a casting method using a green sand made of bentonite as the master mold and a core made by the shell mold method using an organic binder such as a phenolic resin as the core. Such recovered foundry sand is generally used foundry sand obtained by dismantling (disassembling) molds that have been used to cast castings, specifically molds that have been filled with molten metal to cast objects. However, the present invention also covers other types of sand that have been disassembled from molds that have not been used for casting, or sand that has solidified or hardened as a mold material without being used for casting.

[0013] The refractory aggregate (molding sand) constituting the recycled foundry sand of the present invention is a refractory substance that functions as a base material for a foundry mold. Any refractory granular material conventionally used for foundry molds can be used without any particular limitation. Specific examples include general-purpose sands such as silica sand and recycled silica sand; specialty sands such as alumina sand, olivine sand, zircon sand, and chromium sand; slag-based particles such as ferrochrome slag, ferronickel slag, and converter slag; artificial particles such as alumina particles and mullite particles, as well as recycled particles thereof; alumina balls; magnesia clinker; and the like. These refractory aggregates (molding sands) may be new sand, recycled sand or recycled sand that has been used once or multiple times as foundry sand for mold making, or a mixed sand obtained by mixing such recycled sand or recycled sand with new sand. Such refractory aggregate is generally used with a particle size of about 40 to 130, preferably about 60 to 110, in terms of AFS index.

[0014] The regeneration method according to the present invention has a significant technical feature in that it subjects such recovered foundry sand to various treatments, including the polishing process described below, so that the resulting regenerated foundry sand has a methylene blue adsorption of 10 mmol / 100 g or less. Specifically, for recovered foundry sand containing bentonite-based binders (refractory aggregates) adhering to the foundry sand (refractory aggregate), the adhering materials can be effectively removed from the foundry sand (refractory aggregate) by, for example, extending the polishing time or increasing the roasting temperature or time. However, longer polishing and roasting times and higher roasting temperatures are undesirable in today's industrial environment, where energy conservation is required. Furthermore, longer roasting times and higher roasting temperatures result in increased carbon dioxide emissions, which is undesirable from an environmental perspective. Therefore, the present inventors have conducted extensive research into the active clay component (bentonite) remaining on the particle surfaces of recycled molding sand, with the aim of using the resulting recycled molding sand as molding sand for molding cores (shell cores) according to the shell molding method, and as a result have completed the present invention.

[0015] Here, the "methylene blue adsorption amount of recycled foundry sand" in the present invention means the amount measured in accordance with the colorimetric method specified in JIS-Z-2451:2019 "Method for measuring methylene blue adsorption amount of bentonite, etc."

[0016] In the regeneration method of the present invention, various treatments such as polishing are preferably carried out so that the methylene blue adsorption of the resulting recycled foundry sand is 10 mmol / 100 g or less and its loss on ignition (IGross) is 0.30 to 1.60%. More preferably, various treatments such as polishing are carried out so that the methylene blue adsorption of the resulting recycled foundry sand is 5 mmol / 100 g or less and its loss on ignition (IGross) is 0.30 to 1.00%. When resin-coated sand (RCS) is produced using recycled foundry sand with a methylene blue adsorption of 10 mmol / 100 g or less and an ignition loss (IGross) of 0.30 to 1.60% as the refractory aggregate, and the RCS is used for molding, the resulting mold can exhibit superior properties. The ignition loss (igross) of the recycled foundry sand in the present invention is calculated in accordance with JACT test method: S-2 "Test method for ignition loss of foundry sand."

[0017] In the regeneration method according to the present invention, the following treatments and procedures for the recovered foundry sand can be presented as preferred embodiments: 1) performing only polishing, 2) performing polishing followed by roasting, 3) performing only water-washing, or 4) performing water-washing followed by roasting.

[0018] First, the polishing process involves polishing the recovered foundry sand to remove the solid binder (bentonite, etc.) that remains on the surface of the sand particles, and separating it from the foundry sand (aggregate). Specifically, the recovered foundry sand is fed into a conventional polishing device, where the binder, which contains bentonite as a binder component and has adhered to the surface of the foundry sand, is polished off. If the recovered foundry sand is in a nodular state, it is broken down into individual sand particles by the rotating rotor of the polishing device, and then the surfaces of the nodules are polished.

[0019] The polishing method used in this polishing process is not particularly limited, but dry polishing and wet polishing are commonly known. In the dry polishing process, a polishing method using, for example, a rotary cremator, sand freshener, sand shiner, Hybrid Sand Master, or USR-II is appropriately adopted. On the other hand, wet polishing processes include, for example, a rotary polishing method in which recovered foundry sand is immersed in water in a bladed drum and the mixture of recovered foundry sand and water is agitated by rotating the drum (blade) to remove stuck-on materials; a water spray polishing method in which recovered foundry sand is impacted with a water spray (shower) to remove stuck-on materials; and an ultrasonic vibration polishing method in which recovered foundry sand immersed in water is subjected to ultrasonic vibration to remove stuck-on materials. Each of these wet polishing processes is performed using various appropriate devices. The water used or added in the wet polishing method may contain an acid, alkali, surfactant, or the like to facilitate the polishing and removal of any solid matter remaining on the surfaces of the sand particles that make up the recovered foundry sand, and water heated to 30°C to 100°C may also be used. After polishing has been carried out using the wet polishing method described above, it is preferable to carry out a dehydration treatment to make the recovered foundry sand easier to handle. There are no particular restrictions on the dehydration treatment, and any method can be used as long as it can effectively remove the water used (added). Specific examples of dewatering methods include a method in which the ground recovered foundry sand containing water is placed in a sealable container equipped with a drain outlet fitted with a wire mesh finer than the grain size of the recovered foundry sand, and then compressed air is introduced into the sealed container to push out and remove the water between the sand particles, and a method in which the ground recovered foundry sand containing water is placed in a cylindrical container with holes on the side that are finer (smaller) than the grain size of the recovered foundry sand, and then the container is rotated to remove the water between the sand particles by centrifugal force. Furthermore, after carrying out the various wet grinding methods and dewatering methods described above, recycled foundry sand, which is the recovered foundry sand after the regeneration process, can be advantageously obtained by carrying out a drying process according to any of the various conventionally known techniques.

[0020] In the roasting process, also called a calcination process, for example, a rotary kiln, a tunnel kiln, or a fluidized bed roasting furnace is used. The recovered foundry sand is continuously charged into the roasting furnace (calcination furnace) to be roasted (calcined). When reclaiming recovered foundry sand according to the present invention, various conditions, such as the roasting temperature, can be appropriately determined as long as they do not impede the object of the present invention. In a preferred embodiment, the roasting process of the present invention is carried out at a roasting temperature (calcination temperature) of 550°C or higher, more preferably at a roasting temperature (calcination temperature) of 550 to 700°C. Since the roasting process of the present invention is carried out on recovered foundry sand after polishing or water washing, it is possible to carry out the roasting process at a lower temperature than conventional methods. Furthermore, by adopting a lower roasting temperature than conventional methods, it is possible to achieve energy savings in the reclaiming of recovered foundry sand. Furthermore, by setting the roasting temperature lower than that of conventional roasting processes, it becomes easier to keep the methylene blue adsorption amount of the recovered molding sand to 10 mmol / 100 g or less and to keep the ignition loss to 0.30 to 1.60%. Note that if the roasting temperature in the regeneration method of the present invention is set to less than 550°C, the methylene blue adsorption amount of the regenerated molding sand will be high, and there is a risk that the mold obtained using such regenerated molding sand will not be able to exhibit sufficient strength.

[0021] On the other hand, in another preferred embodiment of the method for recycling recovered foundry sand according to the present invention, the roasting treatment is carried out at a treatment temperature and treatment time that satisfy the following formula (1), where the treatment temperature (°C) of the roasting treatment is T and the treatment time (h: hours) of the roasting treatment is t. When the value (product) obtained by multiplying the treatment temperature (°C) and the treatment time (h: hours) is 150 or less, the loss on ignition of the obtained recycled foundry sand becomes high. When using such recycled foundry sand to mold a mold, the gas generation amount increases, and furthermore, the obtained mold may not exhibit sufficient strength. On the other hand, when the value (product) obtained by multiplying the treatment temperature (°C) and the treatment time (h: hours) is 500 or more, the loss on ignition of the obtained recycled foundry sand becomes too low, and the mold obtained using such recycled foundry sand may not exhibit sufficient moldability. In this aspect of the present invention, the treatment temperature and treatment time in the roasting treatment are appropriately selected to satisfy the following formula (1). Preferably, a treatment temperature of 550°C or higher is adopted, and more preferably, a treatment temperature of 550 to 700°C is adopted. 150 < T × t < 500 ··· Formula (1)

[0022] The water-washing method used in the present invention is not particularly limited, and can be any method capable of removing the adhered materials remaining on the surfaces of the sand particles by treating the recovered foundry sand with water. Examples include methods similar to the wet polishing method described above. Specifically, examples include a method in which the recovered foundry sand is immersed in water in a drum with blades and the mixture of recovered foundry sand and water is agitated by rotating the drum (blade) to remove the adhered materials; a method in which the recovered foundry sand is impacted by a water shower to remove the adhered materials; and a method in which ultrasonic vibrations are applied to the recovered foundry sand immersed in water to remove the adhered materials. Each of these water-washing methods requires the use of appropriate equipment. The water used or added in the water-washing process may contain an acid, alkali, surfactant, or the like to facilitate the removal of the adhered materials remaining on the surfaces of the sand particles. Furthermore, water heated to 30°C to 100°C may also be used. After the above-described water-washing treatment, it is preferable to perform a dehydration treatment to facilitate handling of the recovered foundry sand. Any method capable of effectively removing the added water can be used for this dehydration treatment. Specific examples of dehydration methods include a method in which the washed recovered foundry sand is placed in a sealable container equipped with a drain outlet fitted with a wire mesh finer than the particle size of the recovered foundry sand, and compressed air is then introduced into the sealed container to remove the water between the sand particles. Another method involves placing the washed recovered foundry sand in a cylindrical container with holes on the side that are finer (smaller) than the particle size of the recovered foundry sand, and then rotating the container to remove the water between the sand particles by centrifugal force. Furthermore, after the above-described various water-washing and dehydration treatments, a drying treatment according to any of the conventionally known methods can be performed to advantageously obtain recycled foundry sand.

[0023] The treatment conditions for the above-mentioned polishing, roasting and water-washing treatments are appropriately selected depending on, for example, the amount of recovered molding sand in one treatment run, the content of bentonite in the recovered molding sand, the state of adhesion of the deposits on the surfaces of the sand particles of the recovered molding sand, and the treatment capacity of each treatment device used.

[0024] As described above, the molding sand regenerated by the method for regenerating recycled molding sand according to the present invention has an effectively reduced amount of bentonite adhering to its surface, and therefore can be suitably used as a refractory aggregate (molding sand) when producing cores (shell cores) by the shell molding method, and the resulting shell cores (molds) will exhibit excellent properties.

[0025] It should be understood that the method for reclaiming recovered foundry sand according to the present invention should not be construed in any way as being limited by the specific description of the above-mentioned exemplary embodiments, and that the present invention can be embodied in various forms with various changes, modifications, improvements, etc., based on the knowledge of those skilled in the art, and that all such embodiments fall within the scope of the present invention as long as they do not deviate from the spirit of the present invention.

[0026] For example, magnetic separation can be performed on the recovered foundry sand before or after the polishing and water-washing processes described above, or before or after. By performing such magnetic separation, it is possible to effectively remove impurities containing various metals contained in the recovered foundry sand. The magnetic separator used for the magnetic separation is not particularly limited, and any conventionally known type, such as a semi-magnetic outer ring type, a hanging type, or a magnetic pulley type, can be used. Furthermore, the magnetic force of such a magnetic separator should preferably have a magnetic flux density in the range of 500 to 10,000 gauss, preferably 1,000 to 8,000 gauss, and more preferably 1,500 to 6,000 gauss, in order to effectively separate the refractory aggregate from the solid binder powder containing iron-containing compounds. If the magnetic flux density is lower than 500 gausses, there is a risk that the separation of the binder powder by magnetic separation cannot be carried out effectively. On the other hand, if the magnetic flux density is higher than 10,000 gausses, the solid binder powder containing iron-containing compounds may be strongly magnetically attracted to the magnetic separator, making it difficult to separate and recover the powder from the magnetic separator. [Example]

[0027] Below, several examples of the present invention will be presented to clarify the present invention in more detail, but it goes without saying that the present invention is not limited in any way by the description of such examples. In the following description, "parts" and "%" mean "parts by mass" and "% by mass," respectively, unless otherwise specified. The recycled molding sand used in the following examples and comparative examples is waste foundry sand recovered by crushing molds used in casting, and the molds used in casting at least include molds formed using bentonite as a binder. Furthermore, the various properties of the recycled molding sand were measured according to the following test methods.

[0028] (1) AFS index (particle size index) Measurements were made in accordance with JACT test method: S-1 "Test method for particle size of molding sand."

[0029] (2) Methylene blue adsorption amount Measurements were made according to the colorimetric method specified in JIS-Z-2451:2019 "Method for measuring methylene blue adsorption amount on bentonite, etc."

[0030] (3)Ignition loss The calculation was performed in accordance with JACT Test Method S-2, "Test Method for Ignition Loss of Molding Sand." The specific calculation procedure is as follows: First, the free moisture contained in the recycled molding sand is removed according to the method specified in JIS-Z-2601:1993, "Test Methods for Molding Sand." After removing the moisture, 10 g (W1) of the recycled molding sand is accurately weighed and placed in a crucible. The crucible is then placed in an electric furnace preheated to 1000°C for 15 minutes, after which it is ignited in the electric furnace for 45 minutes. The crucible is then removed from the electric furnace and allowed to cool to room temperature in a desiccator. After cooling, the mass (W2) of the recycled molding sand in the crucible is measured. The ignition loss is then calculated according to the following formula (A). [Ignition loss (%)]=[(W1-W2) / W1]×100 (A)

[0031] Example 1 After magnetic separation (magnetic flux density: 1500 gauss) and drying (in a dryer kiln at approximately 200°C), 0.8 t of recovered molding sand was polished for 20 minutes (first treatment) in a sand freshener (manufactured by Kiyota Casting Co., Ltd.), and then 20 kg of the polished recovered molding sand was roasted for 30 minutes at 600°C (second treatment) in a Kanthal furnace to obtain recycled molding sand (Example 1). Note that in this example, the product of the temperature (°C) and the duration (h) of the roasting treatment was 300 (=600×0.5).

[0032] Example 2 After magnetic separation (magnetic flux density: 1500 gauss) and drying (in a dryer kiln at approximately 200°C), 0.8 t of recovered molding sand was polished for 20 minutes (first treatment) in a sand freshener (manufactured by Kiyota Casting Co., Ltd.), and then 20 kg of the polished recovered molding sand was roasted for 15 minutes at 700°C (second treatment) in a Kanthal furnace to obtain recycled molding sand (Example 2). Note that in this example, the product of the temperature (°C) and the roasting time (h) was 175 (=700 x 0.25).

[0033] Example 3 To 50 kg of recovered foundry sand after magnetic separation (magnetic flux density: 1500 gauss) and drying (in a dryer kiln at approximately 200°C), 50 parts water per 100 parts of recovered foundry sand was added, thoroughly stirred, and allowed to stand for approximately 1 minute. The supernatant was then removed. This process was repeated until the supernatant became clear, after which the remaining sand was removed and dried in a hot-air dryer set at 100-120°C (water-washing treatment: first treatment). Next, 20 kg of the recovered foundry sand after the water-washing treatment was roasted in a Kanthal furnace at 600°C for 30 minutes (second treatment), yielding recycled foundry sand (Example 3). Note that in this example, the product of the roasting temperature (°C) and the roasting time (h) was 300 (= 600 × 0.5).

[0034] Example 4 After magnetic separation (magnetic flux density: 1500 gauss) and drying (dryer kiln, approximately 200°C), 0.8 t of the recovered molding sand was polished for 20 minutes using a sand freshener (manufactured by Kiyota Casting Machinery Co., Ltd.) to obtain recycled molding sand (Example 4).

[0035] Example 5 After magnetic separation (magnetic flux density: 1500 gauss) and drying (dryer kiln, approximately 200°C), 0.8 t of the recovered foundry sand was continuously polished at a rate of 2 t / hr using a mechanical sand reclamation machine (product name: USR-II) manufactured by Shinto Kogyo Co., Ltd., to obtain recycled foundry sand (Example 5).

[0036] Example 6 After magnetic separation (magnetic flux density: 1500 gauss) and drying (in a dryer kiln at approximately 200°C), 50 parts of water per 100 parts of recovered foundry sand was added to 50 kg of recovered foundry sand, thoroughly stirred, and then allowed to stand for approximately 1 minute, after which the supernatant liquid was removed. This process was repeated until the supernatant liquid became clear, after which the remaining sand was removed and dried in a hot air dryer set at 100-120°C to obtain recycled foundry sand (Example 6).

[0037] -Comparative Example 1- The recovered foundry sand after magnetic separation (magnetic flux density: 1500 gauss) and drying (dryer kiln, approximately 200°C) was used as recycled foundry sand (Comparative Example 1) without any further treatment.

[0038] -Comparative Example 2- After magnetic separation (magnetic flux density: 1500 gauss) and drying (dryer kiln, approximately 200°C), 0.8 t of the recovered foundry sand was roasted in a roasting furnace at 750°C for 1 hour (first treatment), cooled, and then continuously polished (second treatment) at 4 t / hr using a mechanical sand reclamation machine (product name: USR-II) manufactured by Shinto Kogyo Co., Ltd., to obtain recycled foundry sand (Comparative Example 2).

[0039] -Comparative Example 3- After magnetic separation (magnetic flux density: 1500 gauss) and drying (dryer kiln, approximately 200°C), 20 kg of the recovered foundry sand was roasted in a Kanthal furnace at 700°C for 1 hour to obtain recycled foundry sand (Comparative Example 3).

[0040] -Comparative Example 4- After magnetic separation (magnetic flux density: 1500 gauss) and drying (dryer kiln, approximately 200°C), 20 kg of the recovered foundry sand was roasted in a Kanthal furnace at 600°C for 15 minutes to obtain recycled foundry sand (Comparative Example 4).

[0041] -Comparative Example 5- After magnetic separation (magnetic flux density: 1500 gauss) and drying (dryer kiln, approximately 200°C), 20 kg of the recovered molding sand was roasted in a Kanthal furnace at 500°C for 1 hour to obtain recycled molding sand (Comparative Example 5).

[0042] Resin-coated sand (RCS) was produced using each recycled foundry sand obtained as described above according to the following procedure. 100 parts of recycled foundry sand heated to 145°C and 2.5 parts of a commercially available novolac phenolic resin (manufactured by Asahi Organic Chemicals Co., Ltd., product name: SP615U) were mixed in a speed mixer for 40 seconds. A solution of 0.15 parts of hexamethylenetetramine in 1.5 parts of water was then added to the mixer and mixed until the sand particles separated into individual particles. After this mixing, 0.1 parts of calcium stearate was added to the mixer, mixed for 15 seconds, and then discharged from the mixer to obtain the desired RCS.

[0043] The properties of the resulting resin-coated sand (RCS) were measured according to the following test methods.

[0044] -Bending strength measurement- Using each resin-coated sand, JIS-type test pieces (10mm x 10mm x 60mm, baking conditions: 250°C x 60 seconds) were prepared in accordance with JIS-K-6910:2007 "Phenol Resin Test Methods." The resulting JIS-type test pieces were then tested for bending strength (N / cm2) in accordance with JACT test method: SM-1 "Bending Strength Test Method." 2 The higher the bending strength, the stronger the mold obtained.

[0045] -Measurement of gas generation rate- Measurements were performed at 850°C using a PGD-type gas pressure measuring device (manufactured by George Fischer). Specifically, after the furnace temperature was raised to 850°C, 1 g of a sample cut from the JIS-type test piece obtained in the "Bending Strength Measurement" section above was placed in a cylindrical copper sample tube (approximately 0.7 cm × 7.7 cm in diameter). The tube was then covered with Kaowool (a commercially available insulating material) and placed at the end of the furnace, creating a nitrogen atmosphere inside the furnace. The sample tube was then placed in the sealed furnace, and the generated gas pressure was detected by a pressure sensor. Pressure data was collected using a signal converter or other device until the pressure value became constant (i.e., until gas generation ceased). The amount of gas generated was then calculated from the obtained pressure data using a pressure-volume conversion calibration curve (a calibration curve utilizing the decomposition of calcium bicarbonate). [Gas generation amount (ml / g)] = [Total gas generation amount (ml)] / [Sample mass (g)]

[0046] -Measurement of time to fracture in hot constant load test- 1) First, a mold capable of being split into halves was prepared, with a molding cavity measuring 12 mm in diameter and 20 mm in height formed at the parting surface. The apparatus configuration shown in Figure 1 was also prepared for the hot constant-load test. The apparatus configuration shown in Figure 1 is capable of performing a hot exposure heat treatment while applying a constant load to a test piece (mold piece) in its axial direction using a weight, thereby measuring the time until the test piece (mold piece) fractures (fracture time) and its change in length in the axial direction. More specifically, a weight is placed on a movable mounting base to apply a predetermined constant load (e.g., 2.5 kg) to the test piece (mold piece) via an upper quartz rod. Furthermore, a Digimatic indicator, which is a displacement meter that detects the vertical position (displacement) of the weight, is arranged in a fixed position with its probe in contact with the weight, and this Digimatic indicator is capable of measuring the axial length of the test piece (mold piece), which changes over the course of hot heat treatment under a constant load. In measuring the time to fracture of a test piece (mold piece) in the apparatus configuration shown in Figure 1, the lower base is moved upward, and the test piece (mold piece) placed on the lower quartz rod is brought into close contact with the lower end surface of the upper quartz rod, and is sandwiched between the lower and upper quartz rods via a heat-resistant woven fabric and a heat-resistant cushioning material (neither of which are shown).The electric furnace is then raised, and heat exposure treatment (for example, at a temperature of 600°C to 1200°C) of the test piece (mold piece) is initiated under a constant load applied by a weight, and the time until the test piece (mold piece) fractures is measured.

[0047] 2) Each resin-coated sand (RCS) obtained above was filled into the previously prepared mold while vibrating at room temperature. Any RCS that had overflowed onto the mold surface was scraped off with a spatula, and the mold was then placed in an electric furnace preheated to 260°C for 6 minutes. After 6 minutes, the mold was removed from the electric furnace and opened to release the formed test pieces (mold pieces). After cooling to room temperature, they were subjected to a hot constant load test using the testing equipment described above. Twelve test pieces (mold pieces) were prepared for each RCS.

[0048] 3) Then, 12 test pieces (mold pieces) made from each resin-coated sand (RCS) were subjected to a heat treatment at 1000°C and a constant load of 2.5 kgf (mass of weight 26) in an air atmosphere using the apparatus configuration shown in Figure 1, and the fracture time of each of the 12 test pieces (mold pieces) was measured, and the average fracture time was calculated for each RCS constituting the test pieces (mold pieces). The calculated average fracture times are shown in Tables 1 and 2 below.

[0049] -Measurement of sexuality- 1) First, mold pieces (120 mm x 50 mm x 5 mm) made of each resin-coated sand (RCS) were prepared as molds for the evaluation of the ductility under firing conditions of 250°C for 40 seconds. After preparation, the mold pieces were left to cool to room temperature.

[0050] 2) Next, as shown in Figure 2, the resulting mold pieces were placed on a support table, and the heating element (Erema rod) was gradually heated from 200°C to 800°C. During this process, a laser displacement meter was placed 10 mm from the tip of the mold piece, and data was directly imported into a computer. The deformation behavior of the mold piece was first observed as it expanded due to heating, and then gradually began to deflect, eventually resulting in fracture at approximately the center of the mold piece, i.e., the heated portion of the heating element. The "flexibility" here refers to the maximum deflection and the time to fracture (time to fracture). The larger these values, the more easily the mold deformed and the more flexible it was. This measurement was also performed with a measurement cycle in mind, such that measurement of the next mold piece was initiated when the heating element temperature reached approximately 200°C.

[0051] [Table 1]

[0052] [Table 2]

[0053] As is clear from the results in Tables 1 and 2, when foundry sand regenerated by the regeneration method of the present invention (regenerated foundry sand: Examples 1 to 6) is used as aggregate to produce resin-coated sand (RCS), and when this RCS is used to mold, the resulting mold exhibits excellent properties. More specifically, when foundry sand regenerated by the present invention (regenerated foundry sand) is used as aggregate to produce resin-coated sand (RCS), and when this RCS is used to mold, the resulting mold advantageously exhibits at least one of the following properties: 1) excellent bending strength, 2) excellent heat resistance, and 3) excellent flexibility. Therefore, it is clear that the regenerated foundry sand obtained by the regeneration method of the present invention can be advantageously used as aggregate (foundry sand) when producing cores (shell cores) by the shell molding method. In contrast, it was found that foundry sands recycled by recycling methods outside the scope of the present invention (recycled foundry sands: Comparative Examples 1 to 5) could be produced as RCS, but were difficult to mold into molds, and even if molds were successfully produced, the molds had poor properties.

Claims

1. As a method for regenerating recycled foundry sand, The recovered foundry sand contains foundry sand to which a binder containing bentonite as a binder component has adhered, The recovered foundry sand is subjected to a polishing treatment, and the methylene blue adsorption amount of the resulting recycled foundry sand is set to 10 mmol / 100 g or less. A method for reclaiming recycled foundry sand.

2. 2. The method for reclaiming recycled foundry sand according to claim 1, wherein the ignition loss of the reclaimed foundry sand is 0.30 to 1.60%.

3. 3. The method for reclaiming recovered foundry sand according to claim 1, wherein the recovered foundry sand after the polishing treatment is roasted at a temperature of 550 to 700°C.

4. 3. The method for reclaiming recovered foundry sand according to claim 1, wherein the recovered foundry sand after the polishing treatment is roasted under conditions that satisfy the following formula (1): 150<T×t<500...Formula (1) In the above formula (1), T is the roasting temperature (°C), and t is the roasting time (h).

5. 5. The method for reclaiming recovered foundry sand according to claim 4, wherein the roasting treatment is carried out at a temperature of 550° C. or higher.

6. 5. The method for reclaiming recovered foundry sand according to claim 4, wherein the roasting treatment is carried out at a temperature of 550 to 700°C.

7. 3. The method for reclaiming recovered foundry sand according to claim 1, wherein a water washing treatment is carried out instead of the polishing treatment.

8. 8. The method for reclaiming recovered foundry sand according to claim 7, wherein the recovered foundry sand after the water-washing treatment is subjected to a roasting treatment at a temperature of 550 to 700°C.

9. 8. The method for reclaiming recovered foundry sand according to claim 7, wherein the recovered foundry sand after the water-washing treatment is subjected to a roasting treatment under conditions that satisfy the following formula (1): 150<T×t<500...Formula (1) In the above formula (1), T is the roasting temperature (°C), and t is the roasting time (h).

10. 10. The method for reclaiming recovered foundry sand according to claim 9, wherein the roasting treatment is carried out at a temperature of 550° C. or higher.

11. The method for reclaiming recovered foundry sand according to claim 9, wherein the roasting treatment is carried out at a temperature of 550 to 700°C.

12. 1. Reclaimed foundry sand, comprising recovered foundry sand after reclaiming treatment, characterized in that the amount of methylene blue adsorbed is 10 mmol / 100 g or less.

13. The reclaimed foundry sand according to claim 12, wherein the loss on ignition is 0.30 to 1.60%.

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