Foundry sand for mold casting with excellent collapsibility and method for producing the same

Abstract

To provide a foundry sand allowing for practically advantageously forming a mold whose mold collapsibility after casting could have been improved furthermore while achieving the improvement of mold properties, an advantageous method of producing the same, and furthermore a useful coated sand obtainable using such a foundry sand.SOLUTION: A foundry sand, consisting of a refractory aggregate, comprises a shape factor of equal to or smaller than 1.40, a filling rate of equal to or higher than 53%, a content of a clay component adhering to an aggregate surface of equal to or lower than 0.20 mass%, a content of a part soluble in salt acid of equal to or lower than 3 mass% and a thermal conductivity of equal to or greater than 0.25 W / m K.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a casting sand for mold casting excellent in collapsibility and a method for producing the same, and particularly to a casting sand capable of advantageously forming a mold excellent in collapsibility after casting while improving mold characteristics, a method capable of advantageously producing the same, and further to a useful coated sand obtained by using such a casting sand.

Background Art

[0002] In the production of castings by conventional sand mold casting, after cooling molten metal in a mold obtained by molding using casting sand composed of various refractory aggregates and allowing it to solidify, it is necessary to remove the main mold and chaplets constituting such a mold. As a method for removing such a mold, particularly chaplets, generally, after cooling the casting until the temperature of the casting reaches near room temperature after casting, vibration is applied to the casting using a vibrator such as a knockout machine to collapse the chaplets, and thus such chaplets are removed. However, since a resin such as a phenolic resin is used as a binder (adhesive) for binding the casting sands to each other in the chaplets, the casting sands are fixed by such a resin, and there are problems such as not easily collapsing simply by applying vibration and taking a long time to remove the casting sands from the casting.

[0003] Incidentally, in order to disintegrate sand cores fixed with such resin, one possible method is to use heat to thermally decompose the resin. However, in order to thermally decompose the resin contained in the sand core, it is necessary to apply a sufficient amount of heat to the sand core. For example, in the case of aluminum castings, by holding the material at a temperature close to the melting point of aluminum for 1 to 2 hours, the resin can be thermally decomposed, and the solidified casting sand that forms the sand core can be broken down and separated. After such heating, the material is cooled for 5 to 6 hours, and then vibration is applied to disintegrate the sand core and remove the casting sand. However, this method has the problem that it requires a large amount of thermal energy and a long processing time for the decomposition of the resin. If any undecomposed resin remains, the energy will be absorbed even if vibration is applied, and it will not work effectively to disintegrate the sand. Therefore, sufficient heat treatment was necessary to remove the sand core.

[0004] Furthermore, Japanese Patent Publication No. 9-182952 and others propose a method of removing sand cores from inside a casting by pulverizing them by spraying high-pressure water into the interior of the casting. However, when using high-pressure water, the water splashes out, flooding the work site, which necessitates waterproofing measures for the work site and workers, thus worsening work efficiency. In addition to equipment problems such as the need to prepare a device to spray high-pressure water, the removed sand becomes wet, and there is an inherent problem that requires time-consuming work such as drying the sand in order to reuse it.

[0005] Furthermore, Japanese Patent Publication No. 9-1285 proposes a mold material for shell molds that contains, along with foundry sand and a phenolic resin, a disintegration-improving agent consisting of a zinc oxide-based compound such as ZnO or ZnO·B2O3 as an essential component. It is revealed that by further combining such zinc oxide-based compounds with Br-containing organic compounds or halogen-containing compounds such as ZnBr2, the sand removal process after casting becomes even easier. However, the use of such halogen-based additives presents a problem in that, although the mold disintegration is good, it causes corrosion of the casting and the casting mold, making it difficult to adopt in practical use.

[0006] In addition, Japanese Patent Publication No. 62-124046 and Japanese Patent Publication No. 5-79423 have revealed that by including alkali metal salts of aromatic carboxylic acids or thermal decomposition products of alkali metal salts or alkaline earth metal salts of permanganate as disintegration enhancers in the mold material for shell molds, the disintegration properties and sand removal properties of the mold after casting can be improved. However, there are limits to the improvement in mold disintegration properties by adding these disintegration enhancers, and increasing the amount of such disintegration enhancers used in order to further enhance the disintegration properties of the mold has inherent problems such as adverse effects on the physical properties of the mold.

[0007] Thus, every known method for improving the collapse properties of a mold has its own inherent problems, and none of them can be directly adopted in practical terms to advantageously enhance the collapse properties of a mold after casting. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 9-182952 [Patent Document 2] Japanese Patent Application Publication No. 9-1285 [Patent Document 3] Japanese Patent Application Publication No. 62-124046 [Patent Document 4] Special Publication No. 5-79423 [Overview of the project] [Problems that the invention aims to solve]

[0009] Herein, the present invention has been made against the background of the above circumstances, and its objective is to provide a molding sand for mold making with excellent collapse properties and a method for manufacturing the same. Another objective is to provide a molding sand that can practically and advantageously create a mold in which mold collapse properties after casting are further enhanced while improving mold characteristics, an advantageous method for manufacturing the same, and a useful coated sand that can be obtained using such molding sand. [Means for solving the problem]

[0010] Furthermore, in order to solve the above-mentioned problems, the present invention can be suitably implemented in various embodiments as listed below, and each embodiment described below can be adopted in any combination. It should be noted that the embodiments or technical features of the present invention are not limited in any way to those described below, and should be understood based on the inventive concept grasped from the description of the entire specification.

[0011] Firstly, the first aspect of the present invention is a foundry sand for mold making that is made of refractory aggregate, characterized in that it has a shape coefficient of 1.40 or less, a filling rate of 53% or more, a clay content of 0.20% by mass or less attached to the aggregate surface, a hydrochloric acid soluble content of 3% by mass or less, and a thermal conductivity of 0.25 W / m·K or more, and is a foundry sand for mold making that is highly collapsible.

[0012] Furthermore, a second aspect of the present invention is characterized in that the fire-resistant aggregate is a natural aggregate.

[0013] Furthermore, a third aspect of the present invention is characterized in that the fire-resistant aggregate mainly consists of silica sand.

[0014] In addition, a fourth aspect of the present invention is characterized in that the refractory aggregate is recycled from recovered sand collected during the casting process.

[0015] Furthermore, in order to advantageously obtain the above-mentioned mold-forming sand with excellent collapsibility, the present invention provides a fifth aspect: (a) a magnetic separation step in which magnetic separation treatment is performed on the recovered sand recovered in the casting process to separate and remove magnetic metal components mixed in the recovered sand; (b) an eddy current separation step in which eddy current separation treatment is performed on the recovered sand to separate and remove non-magnetic metal components mixed in the recovered sand, thereby adjusting the content of hydrochloric acid-soluble components containing non-magnetic metal components in the recovered sand to 3% by mass or less; and (c) the material after the eddy current separation step. The present invention provides a method for producing mold-forming sand with excellent collapsibility, comprising at least: (d) a roasting step of roasting the recovered sand to modify or thermally decompose third components other than sand, including mold binders, that are attached to or mixed with the recovered sand; and a polishing step of polishing the roasted recovered sand to adjust the particle shape of the recovered sand, adjust its shape coefficient to 1.40 or less, and remove clay adhering to the surface of the sand particles of the recovered sand to obtain foundry sand with a clay content of 0.20% by mass or less.

[0016] Furthermore, a sixth aspect of the present invention is characterized in that the recovered sand is recovered from a mold obtained by molding using refractory aggregate made of natural aggregate.

[0017] Furthermore, a seventh aspect of the present invention is characterized in that the recovered sand is recovered from a mold obtained by molding using a refractory aggregate mainly composed of silica sand.

[0018] In addition, an eighth aspect of the present invention is characterized in that in the eddy current separation process, as the recovered sand passes through a rotating magnetic field, an eddy current is generated in the non-magnetic metal components in the recovered sand, and due to the interaction between this eddy current and the rotating magnetic field, such non-magnetic metal components are separated and removed from the recovered sand.

[0019] Further, a ninth aspect according to the present invention is that the magnetic separation process is repeatedly carried out a plurality of times at a magnetic flux density of 10,000 G or less, and the magnetic flux density employed in the final magnetic separation process is made larger than that in the first magnetic separation process. In the intermediate magnetic separation processes, a magnetic flux density that is not less than the magnetic flux density in the first magnetic separation process and smaller than the magnetic flux density in the final magnetic separation process is employed. Further, when there are a plurality of such intermediate magnetic separation processes, the subsequent magnetic separation process is carried out using a magnetic flux density equal to or higher than the magnetic flux density in the preceding magnetic separation process.

[0020] Furthermore, a tenth aspect of the present invention is characterized in that the roasting process is carried out at a temperature of 600 to 800°C.

[0021] And an eleventh aspect according to the present invention targets a coated sand characterized in that molding sand having the above-described characteristics is used, and the surface of such molding sand is coated with the mold binder by kneading the molding sand and the mold binder.

[0022] Also, a twelfth aspect according to the present invention targets a coated sand characterized in that molding sand obtained by the above-described manufacturing method is used, and the surface of such molding sand is coated with the mold binder by kneading the molding sand and the mold binder.

[0023] Furthermore, a thirteenth aspect of the present invention is characterized in that in the coated sand as described above, the mold binder is a resin binder.

[0024] Furthermore, a fourteenth aspect of the present invention is characterized in that, in the coated sand as described above, a metal oxide is further contained.

[0025] In addition, a fifteenth aspect of the present invention is characterized in that, in the coated sand as described above, at least one of an oxygen acid salt, a phosphate ester, and an aliphatic condensed phosphate ester is further contained as a disintegration improver.

[0026] And a sixteenth aspect of the present invention is characterized in that the thermal conductivity of the coated sand as described above is 0.30 W / m·K or more.

Advantages of the Invention

[0027] In the case of the casting sand for mold molding excellent in disintegration property according to the present invention as described above, it has excellent shape characteristics having a predetermined shape factor and packing ratio, the clay content is 0.20 mass% or less, and further, in addition to the content of the hydrochloric acid-soluble component containing a non-magnetic metal component being 3 mass% or less, it is composed of a refractory aggregate having an excellent thermal conductivity of 0.25 W / m·K or more. Therefore, the thermal conductivity of the mold molded using such casting sand can be effectively increased. As a result, during heating at the time of molding, the mold obtained by molding can be effectively cured to the inside thereof, and the presence of an uncured portion can be advantageously suppressed or eliminated. In short, due to such an improvement in thermal conductivity, during casting, the heat of the molten metal can be more easily transmitted uniformly by the entire mold. Therefore, by the heat of such molten metal, the bonding force of the mold binder that binds the refractory aggregate can be advantageously reduced, and thus the disintegration property of the mold can be effectively improved.

[0028] Moreover, in molds obtained by molding using the foundry sand according to the present invention, the excellent thermal conductivity allows for advantageously enhanced internal hardening, effectively reducing or eliminating the existence of unhardened areas that cannot be sufficiently hardened by conventional heating during molding. This leads to improved mold homogenization and enhanced mold characteristics, as well as an effective reduction in molding cycle time. Furthermore, it effectively reduces the amount of gas generated from the mold during casting, thus advantageously suppressing or preventing casting defects.

[0029] Furthermore, foundry sand having such characteristics can be manufactured industrially advantageously by using recovered sand collected in the casting process and subjecting it to a predetermined regeneration treatment, in accordance with the present invention. That is, by performing predetermined magnetic separation, eddy current separation, roasting, and polishing processes on the recovered sand collected in the casting process, foundry sand consisting of refractory aggregate with excellent thermal conductivity, possessing the desired shape coefficient, packing density, clay content, and hydrochloric acid soluble content, can be easily and industrially advantageously obtained. [Brief explanation of the drawing]

[0030] [Figure 1] This is a cross-sectional diagram illustrating the longitudinal section of the sand mold used for casting tests to measure the core's collapse properties in the example. [Figure 2] This is a cross-sectional diagram illustrating the vertical section of an aluminum alloy casting containing a waste core in the example. [Modes for carrying out the invention]

[0031] Incidentally, the foundry sand according to the present invention has a predetermined shape coefficient and packing density, and has a high thermal conductivity, with clay and hydrochloric acid-soluble content below a predetermined amount. In this invention, any non-magnetic inorganic particles with refractory properties that have been conventionally used as foundry sand can be used as the refractory aggregate constituting such foundry sand. However, in this invention, refractory particles mainly composed of silica sand, in particular natural particles (aggregates), are advantageous in improving the thermal conductivity of the foundry sand. In this invention, in addition to using only silica sand (including silica ore that has been crushed and sieved; the same applies hereinafter), there is no problem with using mixed sand which is a mixture of silica sand and various known foundry sands. Furthermore, recovered sand and recycled sand, in particular refractory particles recycled from recovered sand recovered in the casting process, can also be advantageously used. However, it is desirable that refractory aggregates consisting of such refractory particles generally contain 50% by mass or more of SiO2, and more preferably 60% by mass or more, and even more preferably 70% by mass or more of SiO2, as this can contribute even more favorably to improving the thermal conductivity of the foundry sand.

[0032] In the present invention, the fire-resistant particles used are those having a shape factor of 1.40 or less and a filling rate of 53% or more. Here, the shape factor of the fire-resistant particles (aggregate) used in the present invention is also called the particle shape factor or particle shape index, and is generally used as a measure of the external shape of the particles. The closer the value is to 1, the closer it is to a spherical shape (perfect sphere). Such a shape factor can be measured by various known methods. For example, as is clearly stated in Japanese Patent Publication No. 3253579, one method involves using a sand surface area measuring instrument (manufactured by George Fischer) to measure the surface area of ​​the actual sand grains per gram, and then dividing that value by the theoretical surface area, which is the surface area assuming that all sand grains are spherical, to obtain the shape factor.

[0033] Furthermore, the packing ratio refers to the ratio of the volume of the filled portion of sand particles to the apparent volume of the foundry sand. In this invention, such a packing ratio is defined as a value measured by the following method. First, 100 ml of a mixed solution of water and methanol in a weight ratio is placed in a 200 ml graduated cylinder. Then, 100 ml of foundry sand (refractory particles) measured by pouring method in another graduated cylinder is gradually added. After sealing the cylinder and confirming that no more bubbles have been released, the liquid level in the graduated cylinder is read. The difference between this value (M ml) and the 200 ml mark (200-M) is determined as the void ratio (V). Subtracting this void ratio (V) from 100 (100-V) gives the packing ratio (X). In addition, instead of a mixture of water and methanol, a mixture of water and a surfactant or other liquids can be used as the liquid placed in the graduated cylinder.

[0034] Furthermore, refractory particles (foundry sand) with a shape factor of 1.40 or less and a filling rate of 53% or more can be obtained by, for example, removing small particles, rounding the corners of the particles by polishing, spheroidizing them, or adjusting the particle size. However, if the shape factor exceeds 1.40, problems arise such as difficulty in increasing the filling rate. Moreover, if refractory particles with a filling rate lower than 53% are used, it becomes difficult to increase the thermal conductivity of the foundry sand. As a result, it becomes impossible to obtain coated sand (CS) with good thermal conductivity, and therefore it becomes difficult to improve the collapse resistance of molds formed using such CS.

[0035] Furthermore, the foundry sand according to the present invention is adjusted so that the clay content adhering to the surface of the refractory aggregate constituting it is 0.20% by mass or less, and the hydrochloric acid-soluble content in the foundry sand is 3% by mass or less. The clay content adhering to the surface of such refractory aggregate can be removed by ordinary polishing, and the hydrochloric acid-soluble content in the foundry sand, such as non-magnetic metal components, can be separated and removed from the refractory aggregate, which is a non-magnetic inorganic particle, by sieving or sorting using a conventionally known eddy current separator. If the content of these clay and hydrochloric acid-soluble components exceeds the values ​​specified in the present invention, it will lead to a decrease in the packing efficiency of the foundry sand and a deterioration in thermal conductivity, making it difficult to fully achieve the objectives of the present invention.

[0036] Thus, by specifying the clay content and hydrochloric acid soluble content along with the shape coefficient and filling rate, the foundry sand adjusted to have a thermal conductivity of 0.25 W / m·K or higher provides excellent thermal conductivity, thereby advantageously improving the collapse resistance of molds formed using such CS.

[0037] Furthermore, it is desirable that the refractory particles (refractory aggregates) described above have an AFS index of 30 to 90, preferably 35 to 80, and more preferably 40 to 70. If the AFS index is less than 30, the particle size of the refractory particles becomes too large, which negatively affects the thermal conductivity and may reduce the properties of the mold, as well as causing problems such as deterioration of the casting surface. On the other hand, if the AFS index exceeds 90, the particle size of the refractory particles becomes too small, which can lead to problems such as the formation of clumps (aggregates of refractory particles) when mixed with the mold binder, and difficulty in sufficiently improving the thermal conductivity.

[0038] Herein, the foundry sand according to the present invention can be easily manufactured to have the desired thermal conductivity by adjusting the shape coefficient and packing rate, as well as the clay content and hydrochloric acid soluble content, by using various new sands and used sands as refractory aggregate raw materials and subjecting them to predetermined treatments. In particular, in the present invention, it is of great practical significance to manufacture the foundry sand according to the present invention using recovered sand recovered in the casting process. This is because it is desirable to recycle the recovered sand obtained by recovering mold waste sand (old sand) generated by dismantling molds after casting and to reuse it as foundry sand, both as a measure to prevent environmental damage caused by disposal and from the viewpoint of resource conservation.

[0039] Furthermore, the recovered sand in such casting processes is mold waste sand generated from the dismantling of molds used in sand casting of non-ferrous castings such as aluminum alloys, magnesium alloys, and copper alloys, as well as iron castings such as cast iron and cast steel. Generally, this sand is collected from each casting site and accumulated for disposal or processing, and then utilized to its fullest potential.

[0040] According to the method for producing foundry sand preferably used in the present invention, the recovered sand is subjected to a magnetic separation treatment to remove magnetic metal (magnetic material) components such as iron, and then to an eddy current separation treatment to remove non-magnetic metal components such as aluminum and copper, in sequence. Although these two treatments can be performed in any order, it is advantageous that the magnetic separation treatment is carried out prior to the eddy current separation treatment.

[0041] Incidentally, in a magnetic separation process that involves performing such magnetic separation treatment on recovered sand, generally, a magnetic force is applied at a magnetic flux density of 10,000 G or less, preferably 1,000 to 8,000 G, to efficiently separate and remove magnetic metal components such as iron mixed in the recovered sand as magnetically attached components. In this process, it is recommended to repeat such magnetic separation treatment multiple times in order to effectively remove the mixed magnetic metal components (particles). Specifically, in multiple magnetic separation treatments, the magnetic flux density used in the final magnetic separation treatment is greater than that of the first magnetic separation treatment, and in intermediate magnetic separation treatments, a magnetic flux density greater than or equal to that of the first magnetic separation treatment but less than that of the final magnetic separation treatment is used. Furthermore, if there are multiple such intermediate magnetic separation treatments, the subsequent magnetic separation treatments are performed using a magnetic flux density equal to or greater than that of the preceding magnetic separation treatment. More specifically, for example, first, a magnetic separator with a magnetic flux density of about 1500G is used to remove large magnetic materials such as iron lumps, then a magnetic separator with a magnetic flux density of about 3000G is used, and further, if necessary, a magnetic separator with a magnetic flux density of about 7000G is used to remove small magnetic materials such as iron sand. Through this magnetic separation process, the content of magnetic metal components such as iron and magnetic sand particles in the recovered sand is generally reduced to 5% by mass or less. Various known magnetic separators are used for such magnetic separation processes, and separators such as suspended magnetic separators and drum-type magnetic separators are appropriately adopted.

[0042] Furthermore, when an eddy current separation process is carried out following the magnetic separation process described above, which involves subjecting the recovered sand to eddy current separation to separate and remove non-magnetic metal components, such as aluminum alloys, copper alloys, brass, gunmetal, magnesium alloys, and other non-ferrous metals, that are mixed in the recovered sand, the recovered sand that has undergone magnetic separation will be classified as necessary, and if necessary, large non-magnetic metal components will be sieved out before the eddy current separation process is carried out. The eddy current separation process employed here utilizes eddy currents generated in non-magnetic metal components by a rotating magnetic field. Generally, as the recovered sand passes through a rotating magnetic field, eddy currents are generated in the non-magnetic metal components, which are hydrochloric acid-soluble, within the recovered sand. Through the interaction of these eddy currents and the magnetic field, these non-magnetic metal components are separated and removed from the recovered sand. Furthermore, the recovered sand subjected to eddy current separation treatment still contains magnetic materials such as iron, which are hydrochloric acid-soluble, that could not be completely removed by the magnetic separation treatment described above. However, at least a portion of these remaining magnetic materials is removed separately from the sand itself (aggregate) and non-magnetic metal components by this eddy current separation treatment. As a result of this eddy current separation treatment, the content of hydrochloric acid-soluble components such as non-magnetic metal components in the recovered sand is adjusted to 3% by mass or less, preferably 1% by mass or less, and more preferably 0.8% by mass or less. This makes it possible to further improve the packing ability of the recycled foundry sand and to improve the thermal conductivity of the foundry sand. On the other hand, if the removal of such hydrochloric acid-soluble components is insufficient, problems may arise such as the air outlet installed at the bottom of the roasting furnace becoming clogged with molten non-ferrous metal components and other hydrochloric acid-soluble metal components when a fluidized bed furnace is used as the roasting equipment in the subsequent roasting process, making it difficult for the recovered sand to be sufficiently roasted.

[0043] Furthermore, as a sorting device that performs such eddy current sorting, various known structures can be appropriately adopted as sorters for non-magnetic metals such as aluminum. For example, a sorting device can be used in which the recovered sand is placed on a belt, and this belt is driven by rotating a roll-type magnetic pole rotor incorporating a permanent magnet with a magnetic flux density of about 3000 to 10000 G at 1800 to 2500 rpm, and the recovered sand is dropped from the belt at the position where the magnetic pole rotor is installed, thereby separating the recovered sand on the belt into sand (aggregate) components, which are non-magnetic inorganic particles, and non-magnetic metal components mixed therein. Specifically, eddy currents are generated in the non-magnetic metallic components mixed in the recovered sand on the belt. The electromagnetic force generated between these eddy currents and the magnetic field of the magnetic pole rotor is used to cause the metallic components that generate eddy currents to fall away from the magnetic pole rotor in an arc, repelled by the electromagnetic force, while non-magnetic inorganic particles (sand particles) such as silica sand, which do not generate eddy currents, fall directly below the magnetic pole rotor in the vertical direction. In this way, non-magnetic metallic components and non-magnetic inorganic particles (sand particles) are separated. Any magnetic metallic components remaining in the recovered sand are magnetically attached to the belt by the magnetic force of the permanent magnets at the position where the magnetic pole rotor is located, and then fall off the belt as the belt moves away from the magnetic pole rotor.

[0044] Next, through such eddy current separation, hydrochloric acid-soluble components such as non-magnetic metal components are separated and removed. As a result, the recovered sand with a hydrochloric acid-soluble content of 3% by mass or less is subjected to a roasting treatment (heat treatment). This roasting process modifies (alteres) or thermally decomposes third components other than sand (aggregate) (including various mold binders) that are attached to or mixed in with the recovered sand. By employing this roasting process, organic components such as organic binders used as mold binders that are attached to or mixed in with the recovered sand are thermally decomposed and removed, while inorganic components such as inorganic binders used as mold binders are modified or altered, making them easily removable by polishing in the subsequent polishing process.

[0045] The roasting treatment of the recovered sand can be carried out as appropriate using conventionally known roasting furnaces. However, it is advantageous to use a fluidized bed roasting furnace that performs roasting (firing, incineration) treatment while the introduced recovered sand is fluidized, thereby making the roasting treatment of the recovered sand even more advantageous. The roasting temperature will be appropriately selected depending on the type of recovered sand to be treated and the type of third components other than sand that are attached to or mixed with it, but generally a temperature of about 500 to 900°C, preferably about 600 to 800°C, is used, and the treatment time is generally about 10 minutes to 3 hours, preferably about 30 minutes to 2 hours.

[0046] Furthermore, the recovered sand that has undergone such roasting treatment is cooled as appropriate, then classified as necessary, and further magnetically separated (for example, using a magnetic flux density of about 7000G) as necessary, followed by polishing. This process adjusts the particle shape of the recovered sand, adjusting its shape coefficient to 1.40 or less, and removes clay adhering to the surface of the sand particles, so that the clay content is 0.20% by mass or less, preferably 0.15% by mass or less, thereby forming the desired foundry sand.

[0047] Such polishing processes can be carried out by combining one or more of the various polishing devices that have been conventionally used for polishing recovered sand. Examples include the Sand Fresher (Kiyota Casting Co., Ltd.), the Mechanical Sand Recycler: USR (Shinto Kogyo Co., Ltd.), the Dry Foundry Sand Recycler: Rotary Reclaimer (NRR; Nippon Chuzo Co., Ltd.), and the Hybrid Sand Master (HSM; Nippon Chuzo Co., Ltd.). These polishing devices dry polish the recovered sand and are designed to simultaneously suction and remove the fine powder generated during the polishing process. This allows for the acquisition of foundry sand with a desirable shape and exhibits features such as a good yield after polishing.

[0048] The resulting foundry sand has had magnetic metal components such as iron efficiently removed, as well as non-magnetic metal components such as aluminum, bronze, and brass derived from the casting, effectively removed. Moreover, it exhibits a good sand shape, and clay components (fine powder) such as aulithic on the sand surface are also removed, improving its packing ability and favorably enhancing its thermal conductivity. By mixing such foundry sand with a mold binder and coating the surface of the foundry sand with the mold binder, coated sand (CS) with excellent properties can be advantageously obtained.

[0049] Furthermore, as a mold binder used to manufacture such CS, inorganic binders such as water glass and resin binders such as phenolic resins can be appropriately used, but in particular, resin binders are preferably used in the present invention. Various conventionally known resin binders can be used, such as phenolic resins, furan resins, urethane resins, amine polyol resins, unsaturated polyester resins, diallyl phthalate resins, and polyether polyol resins, which can be appropriately selected and used, but among these, phenolic resins are advantageously used.

[0050] Incidentally, in the present invention, the phenolic resin suitably used as a resin binder is, as is well known, a solid or liquid (including varnish or emulsion forms) condensation product obtained by reacting phenols and aldehydes in the presence of an acidic or basic catalyst, and is referred to as a novolac type or resol type depending on the type of catalyst used, and is a phenolic resin that exhibits thermosetting properties by heating in or without a predetermined curing agent or curing catalyst.

[0051] Novolac-type phenolic resins are formed by a condensation reaction using phenols and aldehydes with an acidic catalyst, as is well known. Resol-type phenolic resins are formed by a condensation reaction using phenols and aldehydes with a basic catalyst, as in the conventional method. These novolac-type and resol-type phenolic resins can be used individually or mixed in appropriate proportions without any problem. Furthermore, as is well known, modified phenolic resins obtained by changing some of the phenol to components such as bisphenol A and naphthol can also be used, and they can even be used as benzylic ether-type phenolic resins.

[0052] Furthermore, when mixing a resin binder such as the phenolic resin mentioned above into the foundry sand, the amount of such resin binder to be added is determined appropriately considering the type of resin used and the required strength of the mold, and cannot be uniquely defined. However, generally, it is in the range of 0.2 to 10 parts by mass per 100 parts by mass of foundry sand, preferably in the range of 0.5 to 8 parts by mass, and more preferably in the range of 1 to 5 parts by mass.

[0053] Furthermore, according to a preferred embodiment of the present invention, the CS (Clay Stabilizer) may contain metal oxides, thereby further improving the mold's ability to collapse. Such metal oxides generally exist in the form of particles or powder with an average particle diameter of 10 μm or less, and are incorporated into the CS during the mixing of the foundry sand and the mold binder. Specifically, such metal oxides are oxides of metals such as iron, copper, nickel, cobalt, and zinc, and examples include ferrous oxide, ferric oxide, iron(II,III) oxide, iron hydroxide, cobalt(III) oxide, cobalt(II) oxide, nickel(III) oxide, nickel(II) oxide, cuprous oxide, cupric oxide, and zinc oxide. The content of such metal oxides is generally about 0.1 to 3 parts by mass, preferably about 0.1 to 1 part by mass, and more preferably about 0.1 to 0.5 parts by mass, per 100 parts by mass of foundry sand.

[0054] Furthermore, according to another preferred embodiment of the present invention, the CS according to the present invention further contains, together with or in lieu of the above-mentioned metal oxide, at least one of oxyacid salts, phosphate esters, and aliphatic condensed phosphate esters as a disintegration enhancer, thereby effectively improving the disintegration properties of the mold formed using such CS. The oxyacid salt used here is generally preferably an alkali metal oxyacid salt, and specifically, examples include alkali metal nitrate, alkali metal permanganate, alkali metal molybdate, and alkali metal tungstate. Among these, alkali metal nitrate, alkali metal molybdate, and alkali metal tungstate are preferred from the standpoint of mold strength, and alkali metal nitric acid salts, such as potassium nitrate and sodium nitrate, are particularly preferred. The content of such oxyacid salts is generally about 0.1 to 50 parts by mass, preferably about 1 to 20 parts by mass, per 100 parts by mass of the mold binder.

[0055] Furthermore, examples of phosphate esters and / or aliphatic condensed phosphate esters that may be included together with or in place of such oxyacid salts include various aliphatic and aromatic phosphate esters such as trimethyl phosphate, triethyl phosphate, tributyl phosphate, tri-2-ethylhexyl phosphate, tributoxyethyl phosphate, triphenyl phosphate, diethyl-N,N-bis(2-hydroxyethyl)aminomethylphosphonate, and dibutyl butylphosphonate, as well as aliphatic condensed phosphate esters such as Fyrol PNX (manufactured by ICL JAPAN Co., Ltd.), and these will be appropriately selected and used. Generally, the content of these phosphate esters and / or aliphatic condensed phosphate esters is adopted at a ratio of 0.5 to 30 parts by mass, more preferably 1 to 10 parts by mass, per 100 parts by mass of the mold binder.

[0056] In addition to the disintegration enhancers mentioned above, halogen-based disintegrants can also be used. Examples of halogen-based disintegrants include tetrabromobisphenol A and trischloroethyl phosphate, but the process is not limited to these; various known halogen-based disintegrants can be used.

[0057] In addition to the above-mentioned compounding components, various conventionally used additives may also be appropriately blended and included in the CS as needed, for purposes such as improving the physical properties of the CS and mold. For example, lubricants that contribute to improving the fluidity of the CS include waxes such as paraffin wax, synthetic polyethylene wax, and montanic acid wax; fatty acid amides such as stearate amide, oleic acid amide, and erucic acid amide; and alkylene fatty acid amides such as methylenebisstearate amide and ethylenebisstearate amide. These can be included during the production of either the resin (binder) or the CS. Stearic acid, stearyl alcohol, metal stearate salts, lead stearate, zinc stearate, calcium stearate, magnesium stearate, monoglyceride stearate, stearyl stearate, and hardened oils can be added during the production of the CS. Furthermore, benzenecarboxylic acids such as benzoic acid, salicylic acid, para-aminobenzoic acid, anthranilic acid, phthalic acid, and terephthalic acid can be included as additives that contribute to improving the hardening speed of the mold during the production of either the resin or the CS. In addition, it is also effective to include coupling agents that strengthen the bond between the foundry sand and the mold binder; for example, silane coupling agents, zircon coupling agents, and titanium coupling agents can be included during the production of either the resin or the CS. In addition, paraffin, wax, light oil, machine oil, spindle oil, insulating oil, waste oil, vegetable oil, fatty acid esters, organic acids, graphite fine particles, mica, vermiculite, fluorine-based release agents, and silicone-based release agents can be used as release agents in the same manner as before.

[0058] By the way, when manufacturing CS according to the present invention using the above-mentioned compound components, the mold binder and other compound components are mixed with the specified foundry sand (refractory aggregate) in accordance with conventional methods. The manufacturing method used is not particularly limited, and any conventionally known method such as the dry hot coat method, semi-hot coat method, cold coat method, or powder solvent method can be used. However, in the present invention, it is particularly recommended to use the so-called dry hot coat method, in which the preheated foundry sand and mold binder are mixed in a mixer such as a whirl mixer or speed mixer, an aqueous solution of a specified hardening agent or hardening accelerator such as hexamethylenetetramine, and other compound components are added, and the lump contents are separated into granules by forced air cooling, and then a lubricant such as calcium stearate is added. The timing for mixing mold binders, hardeners / hardening accelerators, etc., with the casting sand is to be appropriately selected based on the knowledge of those skilled in the art. They can be added and mixed individually, sequentially, or in combination as appropriate.

[0059] The CS obtained according to the present invention, when its granular form is measured for thermal conductivity, differs from conventional CS in that it has a thermal conductivity of 0.30 W / m·K or higher, preferably 0.33 W / m·K or higher, which can advantageously contribute to improving mold collapse properties, the objective of the present invention. Here, the thermal conductivity of CS can be measured in the same way as the thermal conductivity of foundry sand, and these are measured by the transient hot wire heating method (probe method) in accordance with JIS-R-2551-1:2007. Specifically, using a rapid thermal conductivity meter (QTM-710) manufactured by Kyoto Electronics Manufacturing Co., Ltd., the CS to be measured is placed in a predetermined powder container, and a probe (heating wire + thermocouple) is used to heat the heating wire by passing a constant current through it, while the temperature of the heating wire is measured with the thermocouple. The thermal conductivity of the CS to be measured is then determined from the slope of the resulting temperature rise graph (a graph with a logarithmic time axis).

[0060] Furthermore, it is desirable that the CS according to the present invention has a low content of so-called clumps, which are composite particles formed when multiple foundry sands are bonded (aggregated) during the manufacturing process. Generally, when the CS removed from the manufacturing process is sieved, it is recommended that the amount of clumps that do not pass through a 20-mesh sieve, in other words, the amount of clumps on the 20-mesh sieve, be 3% by mass or less, more preferably 1% by mass or less, relative to the total amount of CS. Conversely, if the amount of clumps in the CS is high, the filling ability into the molding cavity of the mold for forming the mold deteriorates, adversely affecting the hardening characteristics of the mold, making it difficult to sufficiently improve mold characteristics such as mold strength, and causing problems such as insufficient mold collapse after casting. The content of such clumps can be controlled by using foundry sand with a particle size index (AFS) within a preferred range related to the present invention and by limiting the amount of metal oxides, etc., with an average particle diameter of 10 μm or less added to a preferred range.

[0061] Furthermore, when forming a predetermined mold, such as a shell mold, using the CS obtained as described above, the mold is formed under heating in order to heat-harden the CS. However, there are no particular limitations on the heating method used, and any conventionally known method can be used to its advantage. For example, the CS described above can be filled into a mold preheated to about 150-300°C, which has a desired shape space to give the desired mold, by gravity drop or blowing, and after hardening, the hardened mold can be removed from the mold to obtain the desired casting mold.

[0062] Furthermore, using the mold obtained through such shaping, a predetermined molten metal, such as molten aluminum, is cast. After solidification, the casting is vibrated using a vibrator such as a knockout machine to collapse the mold, thereby removing it. In the case of a mold using CS according to the present invention, since CS itself has high thermal conductivity, the heat of the poured molten metal effectively accelerates the deterioration of the mold binder that binds the foundry sand, advantageously reducing the bonding strength. As a result, the mold can be easily collapsed by applying vibration for a short time, thereby making the removal of the mold (foundry sand) simple and easy. [Examples]

[0063] The present invention will be further clarified by showing some embodiments below, but it goes without saying that the present invention is not limited in any way by the description of such embodiments. It should be understood that, in addition to the embodiments below and the specific descriptions above, various changes, modifications, and improvements can be made to the present invention based on the knowledge of those skilled in the art, as long as they do not depart from the spirit of the present invention.

[0064] Furthermore, in the following description, parts and % refer to parts by mass and mass%, respectively, unless otherwise specified, except that the filling rate is volume%. In addition, the properties of the foundry sand and CS produced below, as well as the strength and collapse properties of the molds obtained from each CS, were evaluated according to the following methods.

[0065] -Measurement of AFS change rate- A crushing test of the foundry sand to be measured is conducted according to JACT Test Method S-6 "Test Method for Crushing Properties of Foundry Sand". The particle size distribution of the foundry sand (raw sand) before the test and the foundry sand after the test is measured, and the AFS particle size index is calculated. The crushing rate is obtained by dividing the particle size index after the crushing test by the particle size index of the raw sand, and this value is expressed as a percentage. This is defined as the AFS change rate (%).

[0066] -Measurement of shape factor- For each type of foundry sand, a sand surface area meter (manufactured by George Fischer) is used to measure the actual surface area of ​​the sand grains per gram. This value is then divided by the theoretical surface area, which is the surface area assuming all sand grains are spherical, and this value is defined as the shape factor.

[0067] - Measuring the filling density - For each type of foundry sand, the packing density is determined according to the measurement method described earlier in the main text of this specification.

[0068] -Measurement of clay content- 50g of the sample sand (foundry sand) is accurately weighed to determine the weight of the sand before treatment. This weight is then placed in a 500ml glass beaker, 300ml of water is added, and then one grain of sodium hydroxide (approximately 200mg) is added. Next, the mixture is boiled on an electric heater for 30 minutes, the boiling water is discarded, the mixture is washed several times with water, and then dried in a dryer. The weight of the dried sample sand is accurately weighed to determine the weight of the sand after treatment. The clay content (%) is then calculated based on the obtained sand weights according to the following formula. Clay content (%) = [(Mass of sand before treatment) - (Mass of sand after treatment)] / (Mass of sand before processing) × 100

[0069] -Measurement of hydrochloric acid soluble content- Approximately 10 g of sample sand is accurately weighed and placed in a Kjeldahl tube. Then, 50 ml of 20% hydrochloric acid is added to the Kjeldahl tube and heated at 180°C for approximately 20 minutes. After heating, the Kjeldahl tube is cooled, its contents are filtered, thoroughly washed with tap water, and then heated and dried. The weight of the hydrochloric acid-treated sample sand is then accurately weighed. The amount of sample sand lost due to the hydrochloric acid treatment is expressed as a percentage and is defined as the hydrochloric acid-soluble content.

[0070] -Measurement of thermal conductivity- The thermal conductivity of each sample (CS) is calculated from the temperature rise graph obtained according to the method described in the main text of this specification, using a rapid thermal conductivity meter: QTM-710 manufactured by Kyoto Electronics Manufacturing Co., Ltd.

[0071] -Measuring mold strength- For each test specimen (molding temperature: 250°C) with dimensions of 10 mm width x 10 mm thickness x 60 mm length obtained using the CS, the fracture load is measured using a measuring instrument (Takachiho Seiki Co., Ltd.: Digital Foundry Sand Strength Tester). Then, using this measured fracture load, the flexural strength is calculated using the following formula and defined as the mold strength. Transverse bending strength (N / cm 2 ) = 1.5 × LW / ab 2 [However, L: distance between supports (cm), W: breaking load (N), a: width of the test specimen (cm), b: thickness of the test specimen (cm)]

[0072] -Evaluation of disintegration- First, as shown in Figure 1, a dogbone-shaped tensile strength test specimen 2 with dimensions of width: 40 mm, length: 75 mm, and thickness: 25 mm is prepared at each CS to serve as a core for the collapse test.

[0073] Next, an outer mold 4 with dimensions of 125 mm × 80 mm × 75 mm, having a space slightly larger than the core test piece 2, is separately prepared. The core test piece 2 is then placed inside this outer mold 4, and molten aluminum alloy at a temperature of 720°C is poured in at an S / M ratio of 0.383 to cast the target casting 6 (see Figure 2).

[0074] After the casting 6 has cooled, vibration is applied to one location on the casting 6 (indicated by the white arrow in Figure 2) using an air hammer at a chipping pressure of 0.3 MPa. The weight of the sand discharged from the casting's outlet (diameter: 16 mm) is measured at regular intervals, and this measured weight is divided by the total weight of the sand discharged. The resulting amount is then expressed as a percentage. A higher percentage indicates better disintegration properties.

[0075] -Example 1- First, the silica sand recovered and accumulated from the casting process (recovered silica sand) was subjected to magnetic separation, eddy current separation, roasting, and polishing treatments, each under the conditions shown in Table 1 below. Specifically, the magnetic separation treatment was performed using a commercially available magnetic separator, combining three magnetic separation treatments at a magnetic flux density of 1500G and one magnetic separation treatment at a magnetic flux density of 3000G, and repeated. The eddy current separation treatment was then performed on the recovered silica sand that had undergone such magnetic separation using a non-ferrous magnetic separator (ALS type) manufactured by Nippon Magnetics Co., Ltd., under conditions of magnetic flux density: 3000G and rotation speed: 2200rpm. Furthermore, the roasting treatment was performed on the recovered silica sand that had undergone the eddy current separation treatment using a conventional fluidized bed roasting furnace, under conditions of temperature: 800℃ and time: 60 minutes. The final polishing treatment was carried out on the recovered silica sand that had undergone the roasting treatment as described above, using a commercially available dry-type foundry sand recycling device: rotary reclaimer (NRR; manufactured by Nippon Chuzo Co., Ltd.), under the conditions shown in Table 1 below. The yield, along with the crushing rate, or in other words, the AFS change rate, was then calculated for the recovered silica sand that had undergone such polishing treatment, and these results are also shown in Table 1 below.

[0076] The foundry sand, consisting of the recovered silica sand after polishing, was measured for its shape factor, packing density, clay content, hydrochloric acid soluble content, AFS index, and thermal conductivity according to the methods described above, and the results are shown in Table 1 below.

[0077] -Examples 2-6- Using recovered silica sand, which is recovered sand from the casting process, the target foundry sand was obtained by sequentially performing magnetic separation, eddy current separation, roasting, and polishing treatments under the conditions shown in Table 1 below, in the same manner as in Example 1. In the polishing treatment, the "USR" used as the polishing device was a mechanical sand regenerator: USR-II manufactured by Shinto Kogyo Co., Ltd., and the "HSM" was a dry foundry sand regenerator: Hybrid Sand Master manufactured by Nippon Chuzo Co., Ltd. Furthermore, in Examples 4 and 6, the magnetic separation treatment was carried out using three magnetic flux densities: 1500G, 1800G, and 3000G.

[0078] Then, for the foundry sand obtained in each example, the shape factor, packing density, clay content, hydrochloric acid soluble content, AFS index, and thermal conductivity were measured according to the measurement method described above, and the results are shown in Table 1 below.

[0079] -Examples 7-11- In the same manner as in Example 1, the recovered silica sand, which is the recovered sand from the casting process, was subjected to magnetic separation, eddy current separation, roasting, and polishing treatments sequentially under the conditions shown in Table 2 below. In Example 10, a mixture of recovered sand and Mikawa silica sand (artificial silica sand) (90:10) was used as the raw material for refractory aggregate, and Mikawa silica sand was replenished before the polishing process. Furthermore, in Examples 7 to 9, the second stage of magnetic separation was repeated three times at a magnetic flux density of 3000G, and in Example 11, the magnetic separation was performed using three magnetic flux densities of 1500G, 3000G, and 7000G, and the polishing treatment was performed in two stages using two types of polishing equipment.

[0080] Then, for the foundry sand obtained in each example, the shape factor, packing density, clay content, hydrochloric acid soluble content, AFS index, and thermal conductivity were measured in the same manner as in Example 1, and the results obtained are shown in Table 2 below.

[0081] -Comparative Examples 1-7- Except for omitting at least one of the following: magnetic separation, eddy current separation, and polishing, the recovery of silica sand or a mixture of recovered sand and Mikawa silica sand (90:10) or recovered artificial sand (commercially available mullite-based artificial sand) was subjected to the regeneration treatments shown in Table 3 below to obtain the desired foundry sand. In Comparative Example 7, one of the polishing devices used in the polishing process was a sand freshener manufactured by Kiyota Casting Machinery Co., Ltd., which polishes the sand surface with a high-speed rotating grinding wheel.

[0082] Then, for the foundry sand obtained in each comparative example, the shape factor, packing density, clay content, hydrochloric acid soluble content, AFS index, and thermal conductivity were measured in the same manner as in the previous example, and the results are shown in Table 3 below.

[0083] [Table 1]

[0084] [Table 2]

[0085] [Table 3]

[0086] -Evaluation of resin-coated sand and molds- Using the various types of foundry sand obtained in Examples 1 to 11 and Comparative Examples 1 to 7 described above, resin-coated sand (RCS) was prepared, and then molds (cores) for disintegration testing were fabricated using each of the RCS.

[0087] Specifically, 100 parts of each type of foundry sand heated to 150°C were mixed with the amounts of commercially available novolac-type phenolic resin (SP610, manufactured by Asahi Organic Chemicals Co., Ltd., softening point: approximately 90°C) shown in Tables 4-6 below, and the proportions of metal oxides, oxyacid salts, or phosphate esters / condensed phosphate esters shown in Tables 4-6 below. The mixture was then kneaded in a speed mixer for 50 seconds. A solution of 0.23 parts of hexamethylenetetramine dissolved in 1.5 parts of water was then added, and the mixture was kneaded until the sand separated into individual particles. Finally, 0.1 parts of calcium stearate was added and mixed for 15 seconds. The mixture was then removed from the mixer, thereby obtaining the desired RCS, in which the surface of the foundry sand was coated with phenolic resin.

[0088] Then, the thermal conductivity of the obtained RCS was measured, and test specimens for evaluating mold strength were prepared from each RCS. Furthermore, dogbone-shaped tensile strength test specimens (cores for mold collapse testing) were fabricated for evaluating mold collapse resistance. Along with measuring mold strength, mold collapse tests were conducted as described above, and the results obtained are shown in Tables 4 to 6 below.

[0089] [Table 4]

[0090] [Table 5]

[0091] [Table 6]

[0092] As is clear from the comparison of the results in Tables 4 to 6, which evaluate the various foundry sands shown in Tables 1 to 3 above, the foundry sands obtained in Examples 1 to 11 all have a shape factor of 1.40 or less, a filling rate of 53% or more, a clay content of 0.20% by mass or less on the sand surface, a hydrochloric acid soluble content of 3% by mass or less, and a thermal conductivity of 0.25 W / m·K or more. Therefore, it can be seen that the collapse of the mold after casting proceeds immediately when vibration is applied, indicating extremely good collapse properties. In particular, the molds formed from RCS using the foundry sand obtained in Examples 4 to 9 all showed excellent results in terms of mold collapse resistance, as they contained at least one of metal oxides, oxyacid salts, and phosphate esters / condensed phosphate esters. Furthermore, the foundry sand obtained in Example 11 had an extremely high packing density and a further reduced hydrochloric acid soluble content, so even with an RCS using a small amount of phenolic resin, it exhibited high mold strength and excellent mold collapse resistance.

[0093] In contrast, in the case of the foundry sand obtained in Comparative Examples 1 to 7, at least one of the following was outside the specified range of the present invention: shape factor, packing density, clay content, hydrochloric acid soluble content, and thermal conductivity. Therefore, in the mold collapse test, if the knockout time for applying vibration was 10 seconds or less, no collapse of the mold was observed, or even if collapse was observed, it was not sufficiently collapsed. For this reason, it was found that the mold collapse properties were inferior. [Explanation of symbols]

[0094] 2 Core test specimens 4 Outer mold 6 Castings

Claims

1. Foundry sand for mold making, which is made of refractory aggregate, has a shape coefficient of 1.40 or less, a filling rate of 53% or more, a clay content attached to the aggregate surface of 0.20% by mass or less, a hydrochloric acid soluble content of 3% by mass or less, and has excellent disintegrability and a thermal conductivity of 0.25 W / m K or more.

2. 2. The molding sand for forming foundry molds having excellent disintegrability according to claim 1, wherein the refractory aggregate is a natural aggregate.

3. 3. Foundry sand for molding foundry molds having excellent disintegrability according to claim 1 or 2, characterized in that the refractory aggregate is mainly composed of silica sand.

4. 4. The molding sand for molding foundry molds having excellent disintegrability according to claim 1, wherein the refractory aggregate is recycled from recovered sand collected in a casting process.

5. a magnetic separation process in which magnetic separation is performed on recovered sand recovered in the casting process to separate and remove magnetic metal components mixed in the recovered sand; an eddy current sorting process in which the recovered sand is subjected to an eddy current sorting process to separate and remove non-magnetic metal components mixed in the recovered sand, thereby adjusting the content of hydrochloric acid soluble matter including non-magnetic metal components in the recovered sand to 3 mass% or less; a roasting step in which the recovered sand that has been subjected to the eddy current sorting step is roasted to denature or pyrolyze third components other than sand, including foundry binders, that are attached to or mixed in the recovered sand; a polishing process in which the roasted recovered sand is polished to adjust the particle shape of the recovered sand, adjusting the shape coefficient to 1.40 or less, and removing clay adhering to the surfaces of the recovered sand particles, thereby obtaining foundry sand with a clay content of 0.20 mass% or less; 1. A method for producing molding sand for molding a foundry mold having excellent disintegratability, comprising:

6. 6. The method for producing molding sand for foundry molding having excellent disintegrability according to claim 5, wherein the recovered sand is recovered from a mold obtained by molding using a refractory aggregate made of a natural aggregate.

7. 7. The method for producing molding sand for foundry molding having excellent disintegrability according to claim 5 or 6, wherein the recovered sand is recovered from a mold obtained by molding using a refractory aggregate mainly composed of silica sand.

8. 8. A method for producing molding sand for foundry molding with excellent disintegrability according to claim 5, wherein the eddy current sorting process generates eddy currents in non-magnetic metal components in the recovered sand by passing the recovered sand through a rotating magnetic field, and the non-magnetic metal components are separated and removed from the recovered sand by the interaction between the eddy currents and the rotating magnetic field.

9. 9. The method for producing molding sand for foundry molding having excellent disintegrability according to claim 5, wherein the magnetic separation treatment is repeated multiple times at a magnetic flux density of 10,000 G or less, the magnetic flux density used in the final magnetic separation treatment is higher than that used in the first magnetic separation treatment, and the magnetic flux density used in the intermediate magnetic separation treatment is equal to or higher than that used in the first magnetic separation treatment but lower than that used in the final magnetic separation treatment, and further, when multiple intermediate magnetic separation treatments are performed, the magnetic flux density used in the subsequent magnetic separation treatment is equal to or higher than that used in the previous magnetic separation treatment.

10. 10. The method for producing molding sand for forming foundry molds having excellent disintegrability according to claim 5, wherein the roasting treatment is carried out at a temperature of 600 to 800°C.

11. 5. Coated sand, characterized in that the foundry sand according to any one of claims 1 to 4 is used, and the surface of the foundry sand is coated with a foundry binder by kneading the foundry sand with the foundry binder.

12. 11. Coated sand, characterized in that foundry sand obtained by the manufacturing method according to any one of claims 5 to 10 is used, and the surface of the foundry sand is coated with a foundry binder by kneading the foundry sand with the foundry binder.

13. 13. The coated sand according to claim 11 or 12, wherein the foundry binder is a resin binder.

14. 14. The coated sand according to claim 11, further comprising a metal oxide.

15. 15. The coated sand according to claim 11, further comprising at least one of an oxygen acid salt, a phosphate ester, and an aliphatic condensed phosphate ester as a disintegration improver.

16. 16. The coated sand according to any one of claims 11 to 15, characterized in that the coated sand has a thermal conductivity of 0.30 W / m·K or more.

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