Method for producing regenerated casting sand

Abstract

To provide a method for producing regenerated casting sand, which can produce high-quality regenerated casting sand capable of forming a highly strong mold from a mold formed through lamination molding for adding a resin containing alkaline, and ensure sand flowability during flowing of the sand into a roasting furnace.SOLUTION: This method for producing regenerated casting sand comprises: a lamination molding step of laying mixed sand mixing sand and a curative layer by layer and adding a resin containing alkaline to form a mold having the sand cured in uncured sand to which the resin is not added, and taking out the mold from the uncured sand; a mixing step of mixing the uncured sand with crushed sand obtained by crushing the mold to form mixed sand; a roasting step of introducing the mixed sand into a roasting furnace through flowing to roast the sand; and an acid addition step of adding acid to the mixed sand or the crushed sand so as to neutralize at least a part of the alkaline contained in the mold before the introduction into the mold.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing reclaimed foundry sand. [Background technology]

[0002] A method for producing recycled foundry sand is known in which recycled foundry sand is produced by destroying the mold after casting and roasting it (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3096477 Summary of the Invention [Problem to be solved by the invention]

[0004] In additive manufacturing (AM) processes such as alkaline phenol self-hardening molds, a mold is formed by adding an alkali-containing resin to a mixture of sand and a hardener layer by layer while adding an alkali-containing resin. This mold is then used for casting, and the recycled molding sand is produced by crushing and roasting it after use. However, when reused for AM, molds with sufficient strength have been found to be insufficient. This was found to be due to the alkali metals in the sand being difficult to volatilize during roasting and accumulating with each subsequent regeneration. This strength issue can be resolved by neutralizing the alkali metals by adding acid to the sand. However, when crushed sand obtained by crushing a mold is introduced directly into the inlet of a roasting furnace, adding sufficient acid to the crushed sand before introduction can reduce the flowability of the crushed sand and potentially clog the inlet.

[0005] Therefore, an object of the present invention is to provide a method for producing high-quality recycled foundry sand that can be used to form high-strength molds from molds formed by additive manufacturing using an alkali-containing resin, and that can ensure the fluidity of the sand when it is poured into a roasting furnace. [Means for solving the problem]

[0006] One aspect of the present invention is as follows.

[0007] [1] an additive manufacturing process in which mixed sand obtained by kneading sand and a hardener is layered one layer at a time while adding an alkali-containing resin to form a mold in unhardened sand to which no resin has been added, and the mold is then removed from the unhardened sand; a mixing step of mixing the unhardened sand with crushed sand obtained by crushing the mold to form a mixed sand; a roasting step of introducing the mixed sand into a roasting furnace and roasting it; and an acid addition step of adding acid to the mixed sand or the crushed sand so as to neutralize at least a portion of the alkali contained in the mold before the mixed sand or the crushed sand is introduced into the roasting furnace.

[0008] [2] The method for producing recycled foundry sand according to [1], wherein the acid addition step includes adding the acid to the mixed sand.

[0009] [3] The method for producing recycled foundry sand according to [1] or [2], wherein the acid addition step adds the acid to the crushed sand before mixing the unhardened sand and the crushed sand.

[0010] [4] The method for producing recycled foundry sand according to any one of [1] to [3], wherein the acid addition step adds the acid in an amount exceeding the amount required for neutralization, which is the amount of acid that can completely neutralize the amount of alkali contained in the sand immediately before the addition of the acid.

[0011] [5] The method for producing recycled foundry sand according to [4], wherein the acid addition step adds the acid in an amount 1.8 times or more the amount required for neutralization.

[0012] [6] The method for producing recycled foundry sand according to any one of [1] to [5], wherein the ratio of the mass of the acid added to the mixed sand in the acid addition step to the mass of the mixed sand immediately before being introduced into the roasting furnace is 1.6 mass% or less. [Effects of the Invention]

[0013] According to the present invention, it is possible to produce high-quality recycled foundry sand that can be used to form high-strength molds from molds formed by additive manufacturing using an alkali-containing resin, and it is also possible to provide a method for producing recycled foundry sand that can ensure the fluidity of the sand when it is poured into a roasting furnace. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is an explanatory diagram illustrating each step of a method for producing recycled foundry sand according to an embodiment of the present invention. [Figure 2] (a) is an explanatory diagram showing the step of stacking mixed sand in the additive manufacturing process of Figure 1, (b) is an explanatory diagram showing the step of adding resin to the stacked mixed sand, (c) is an explanatory diagram showing the step of lowering the stage after adding the resin, (d) is an explanatory diagram showing the state after the resin addition is completed by repeating steps (a) to (c), (e) is an explanatory diagram showing the state after the sand after the resin addition is completed is removed from the stage and heat treated, and (f) is an explanatory diagram showing the state after the hardened mold is removed from the unhardened sand in the state of (e). [Figure 3] FIG. 2 is an explanatory diagram illustrating an example of the roasting step in FIG. 1. [Figure 4] 1 is a graph of experimental results showing the effect of adding acid on the strength of molds made from recycled foundry sand. [Figure 5] 1 is a graph showing the results of an experiment to confirm the amount of acid added required for neutralization. [Figure 6](a) is a photograph showing the results of a sand fluidity experiment using a slump cone to confirm the relationship between the fluidity of the sand when it is poured into a roasting furnace and the amount of acid added. The experiment was conducted using an 87° slump cone to add 1.6% by mass of water. (b) is a photograph showing the results of an 87° slump cone to add 2% by mass of water. (c) is a photograph showing the results of an 80° slump cone to add 0.7% by mass of water. (d) is a photograph showing the results of an 80° slump cone to add 1% by mass of water. DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0016] As shown in Figures 1 to 3, a method for producing recycled foundry sand according to one embodiment of the present invention involves layering mixed sand 11, which is made by mixing sand and a hardener, one layer at a time (see Figure 2(a)), while adding (for example, by printing, see Figure 2(b)) a resin 1 containing alkali to form a mold 13 (see Figure 2(e)) made of hardened sand in unhardened sand 12 to which no resin 1 has been added, and then removing the mold 13 from the unhardened sand 12 (see Figure 2(f)). This is an additive manufacturing process (see Figures 1 and 2(c)). a) to (f)), a mixing step (see FIG. 1) of forming mixed sand 15 by mixing unhardened sand 12 with crushed sand 14 obtained by crushing molds 13, a roasting step (see FIGS. 1 and 3) of introducing the mixed sand 15 into a roasting furnace 30 and roasting it, and an acid addition step (see FIG. 1) of adding acid 2 to the mixed sand 15 or the crushed sand 14 so as to neutralize at least a portion of the alkali contained in the molds 13 before introduction into the roasting furnace 30.

[0017] According to the above configuration, acid 2 is added to sand to neutralize at least a portion of the alkali contained in the mold 13. This neutralizes alkali metals in the sand that are difficult to volatilize during roasting, making them harmless. This allows for the production of high-quality recycled foundry sand that can be used to form high-strength molds 13 from molds 13 formed by additive manufacturing using alkali-containing resin 1. Furthermore, according to the above configuration, instead of directly introducing crushed sand 14 into the roasting furnace 30, mixed sand 15 is formed by mixing with unhardened sand 12, and the mixed sand 15 is then introduced into the roasting furnace 30. This ensures the fluidity of the sand when it is introduced into the roasting furnace 30, even when a sufficient amount of acid 2 is added. As a result, clogging of the inlet 31 (see FIG. 3 ) of the roasting furnace 30 is suppressed. This is presumably because a portion of the acid 2 used for neutralization can be retained in the unhardened sand 12 in the mixed sand 15. Furthermore, since the alkali contained in the mold 13 is neutralized before being introduced into the roasting furnace 30, steps such as drying the sand, which are required when neutralizing the roasted sand by adding acid 2, can be eliminated.

[0018] Figure 2(a) is an explanatory diagram showing the process of stacking mixed sand 11 using a recoater 21 in the additive manufacturing process of Figure 1, Figure 2(b) is an explanatory diagram showing the process of adding resin 1 to the stacked mixed sand 11 using a resin supply unit (printer head) 22, Figure 2(c) is an explanatory diagram showing the process of lowering the stage 24 relative to the modeling box 23 after adding resin 1, Figure 2(d) is an explanatory diagram showing the state after the addition of resin 1 is completed by repeating the processes of Figures 2(a) to (c), Figure 2(e) is an explanatory diagram showing the state after the sand to which resin 1 has been added is removed from the stage 24 and heat-treated, and Figure 2(f) is an explanatory diagram showing the state after the hardened mold 13 has been removed from the unhardened sand 12 in the state of Figure 2(e).

[0019] The method for producing recycled molding sand of this embodiment is particularly useful when the recycled molding sand is reused for additive manufacturing because it can prevent alkali metals from accumulating in the sand with each roasting, as shown by the dashed arrow in Figure 1. For example, when the regeneration process using acid addition and roasting was repeated up to four times according to this embodiment and the bending strength of the mold 13 produced each time was measured, a significant improvement was confirmed compared to when the regeneration process was performed by roasting without adding acid.

[0020] The results are shown in Figure 4. The bending strength after the fourth regeneration was approximately 0.5 MPa for roasting without acid addition, and approximately 8.5 MPa for roasting with acid addition. One cycle of forming mold 13 and regeneration by acid addition and roasting was performed by kneading sand (SCBX manufactured by Itochu Ceratec Co., Ltd.) with 0.15% by mass of ester (FA201J manufactured by Ex-One Co., Ltd.) as a hardener and 5.0% by mass of high-alkali resol (FB201J manufactured by Ex-One Co., Ltd.) as resin 1 with sand. The mixture was then molded to form a 20 x 20 x 170 mm rod. The rod was then demolded and left overnight. Then, it was heat-treated at 150 °C for 40 minutes, its bending strength was measured, and the material was crushed. Acid was added to the sand using a mixture of 55% by mass of xylene sulfonic acid, 10% by mass of sulfuric acid, and 35% by mass of water (1% by mass of the sand). The roasting was performed at 600 °C for 0.5 hours. The second and subsequent cycles were performed using only the previously regenerated sand. In contrast, the conditions for forming mold 13 and regenerating by roasting without acid addition were identical, except for the absence of acid addition and the roasting time of 1 hour for the third cycle and 2 hours for the fourth cycle. The roasting time was increased because the color of the sand indicated incomplete roasting even after 0.5 hours. This is thought to be due to the accumulation of alkali metals in the sand, which inhibits the effectiveness of roasting. Incompletely roasted sand has excess resin 1 and other substances adhering to the surface, which reduces the strength of the mold if reused. Neutralization with acid also contributes to preventing the deterioration of mold strength due to substances other than alkali metals adhering to the sand and to reducing the roasting time.

[0021] The alkaline strength of the reclaimed sand 16 after one cycle was also measured. The measurement was performed using the acid consumption test for foundry sand specified in the JACT test method. This test measures how much 0.1 mol / L hydrochloric acid 50 g of sand neutralizes. The amount of 0.1 mol / L hydrochloric acid is expressed in mL / 50 g. The value for the case of roasting without acid addition was 16.1 mL / 50 g, while the value for the case of roasting with acid addition was -0.6 mL / 50 g. While a negative value indicates that the sand has room to react with the base component, the value for the case of roasting with acid addition can be considered approximately neutral.

[0022] In this embodiment, the additive manufacturing process uses alkaline phenol self-hardening additive manufacturing. In alkaline phenol self-hardening additive manufacturing, an ester is used as the curing agent, and a high-alkali resol is used as the resin 1. Compared to furan self-hardening additive manufacturing, alkaline phenol self-hardening additive manufacturing has advantages such as less odor from pyrolysis gases during pouring, no contamination of the cast metal with sulfur-based gases, and good depowdering properties (the ease of removing unhardened sand from the hardened mold 13 after additive manufacturing). Since depowdering properties are important when casting products that require a deep and narrow shape in the mold 13, it is particularly advantageous to use alkaline phenol self-hardening additive manufacturing. Note that the additive manufacturing process is not limited to the configuration using alkaline phenol self-hardening additive manufacturing.

[0023] The acid 2 added to the sand in the acid addition step is preferably an acid that burns or volatilizes during roasting and is removed from the sand by roasting. For example, inorganic acids such as sulfuric acid and hydrochloric acid can be used. Sulfuric acid is particularly suitable because the melting point of the salt is relatively high and the equilibrium humidity of the salt is approximately 90%, making it less likely to deliquesce even if the salt remains in the sand. Organic acids such as alkyl sulfonic acids and alkyl benzene sulfonic acids are preferred. When these acids (sulfuric acid, hydrochloric acid, alkyl sulfonic acids, alkyl benzene sulfonic acids, etc.) are used, the salt generated on the sand surface after neutralization does not revert to an alkaline component during roasting, resulting in a neutral sand after roasting, which reduces the strength of the mold. Examples of alkyl sulfonic acids include methanoic acid and ethanoic acid, and examples of alkyl benzene sulfonic acids include toluene sulfonic acid and xylene sulfonic acid. Alternatively, a mixture of the above acids can be used as acid 2. The amount of acid used can be determined appropriately, taking into account factors such as the quality of the casting and the cost of sand reclamation.

[0024] The product cast using the mold 13 of this embodiment is made of, for example, an aluminum alloy. When salts (e.g., Na2SO4 or K2SO4) are removed from the sand by washing the sand with water after the roasting process, a high-melting-point metal such as cast iron or cast steel may be used instead of an aluminum alloy. Furthermore, washing with water after neutralization and roasting neutralizes the waste liquid generated by washing, thereby suppressing corrosion of containers used to store the waste liquid and reducing storage costs. Furthermore, neutralization work is not required when treating the waste liquid, reducing processing costs.

[0025] In the roasting step, the sand is roasted, for example, at 500 to 800°C (preferably 550 to 700°C) for 0.05 to 5 hours (preferably 0.1 to 3 hours, more preferably 0.3 to 1 hour). Under the above conditions, the resin 1 adhering to the sand and the excess acid 2 used for neutralization can be effectively removed from the sand by roasting. The roasting furnace 30 is, for example, a fluidized bed roasting furnace 30 as shown in Figure 3. In the fluidized bed roasting furnace 30, air is blown onto the sand flowing in through an inlet 31 within a furnace 32 to fluidize the sand, while heating the sand with a burner 33. The air is blown, for example, via a blower 34 and an air passage forming member 35 connected to the blower 34, as shown in Figure 3.

[0026] The acid addition step is preferably configured to add acid 2 to mixed sand 15 as shown by the solid line in Fig. 1. Alternatively, it is also preferable to add acid 2 to crushed sand 14 before mixing unhardened sand 12 and crushed sand 14 as shown by the two-dot chain line in Fig. 1. These configurations enable efficient neutralization.

[0027] Furthermore, the acid addition step is preferably configured to add acid 2 in an amount exceeding the required neutralization amount, which is the amount of acid 2 required to completely neutralize the amount of alkali contained in the sand (including not only the sand itself but also resin 1, etc.) immediately before the addition of acid 2. Because it is generally difficult to accurately determine the amount of resin 1 to be added in additive manufacturing, adding an excess amount of acid 2 greater than the required neutralization amount required for complete neutralization can reduce costs compared to analyzing the amount of alkali metal in the sand each time and adding only the necessary amount of acid 2. Furthermore, in order to neutralize the sand by adding acid 2 before roasting, it is necessary to react acid 2 with the alkali metal in resin 1 that covers the sand immediately before the addition of acid 2, so it is necessary to add an amount of acid 2 that exceeds the required neutralization amount. Specifically, it is preferable to add acid 2 in an amount that is 1.8 times or more the required neutralization amount.

[0028] The ratio of the mass of the acid 2 added to the mixed sand 15 in the acid adding step to the mass of the mixed sand 15 immediately before being introduced into the roasting furnace 30 is preferably 0.5 mass% or more. According to the above configuration, complete neutralization is possible by using, for example, concentrated sulfuric acid (H2SO4) as the acid 2, so that a particularly high-quality recycled foundry sand capable of forming a particularly high-strength mold 13 can be produced.

[0029] An experiment was conducted to confirm the amount of acid 2 required for neutralization. Sand (SCBX, manufactured by Itochu Ceratec Corporation) was mixed with 0.15% by mass of an ester (FA201J, manufactured by Ex-One Corporation) as a hardener and 5.0% by mass of a high-alkali resol (FB201J, manufactured by Ex-One Corporation) as a resin 1. The mixture was then molded to form a 20 × 20 × 170 mm rod. The rod was then demolded and left overnight. It was then heat-treated at 150 °C for 40 minutes, crushed, and roasted at 600 °C for 0.5 hours after adding 0 to 3.6% by mass of 1 M (=1 mol / L) sulfuric acid to the sand. Acid consumption and pH measurements were then performed. Acid consumption measurements were performed using the acid consumption test method for foundry sand specified in the JACT test method described above. pH measurements were not possible in the solid state, so the sand to be measured was mixed with purified water in a 1:1 mass ratio and the pH of the liquid was measured. The results are shown in Figure 5.

[0030] As shown in Figure 5, without the addition of Acid 2, the acid consumption was 14.4 mL / 50 g. Because sulfuric acid is a dibasic acid, 1 M H2SO4 can be neutralized with 1 / 20 the amount (compared to 0.1 M HCl). Therefore, the amount of 1 M H2SO4 required to neutralize the alkali content was 0.72 mL / 50 g. However, as shown in Figure 5, when adding acid before roasting (neutralization roasting), 1.30 mL / 50 g of 1 M H2SO4 was required to neutralize the roasted sand (pH 7). In other words, neutralization roasting required the addition of 1.8 times the amount of sulfuric acid required for neutralization. In this case, the amount of Acid 2 added to the sand was approximately 2.8 mass%. Note that zero acid consumption does not equate to neutrality. Even when neutral, the acid consumption can fluctuate between positive and negative depending on the presence of substances that react with acids or bases. In this experiment, the amount of acid consumed was approximately zero, but neutrality was determined by pH.

[0031] The ratio of the mass of acid 2 added to mixed sand 15 in the acid addition step to the mass of mixed sand 15 immediately before being introduced into roasting furnace 30 is preferably 1.6 mass% or less (preferably 0.7 mass% or less). According to the above configuration, a significant decrease in the fluidity of the sand due to the addition of acid 2 can be suppressed, thereby ensuring good fluidity of the sand when it is introduced into roasting furnace 30.

[0032] An experiment was conducted to confirm the relationship between the fluidity of sand when it was introduced into the roasting furnace 30 and the amount of acid 2 added. Sand fluidity is typically evaluated by measuring the angle of repose. When designing a sand tank, the angle of repose (the angle between the hopper portion that forms the bottom and the horizontal plane) is used to design the sand so that it will flow out under its own weight. Because measuring the angle of repose is difficult for wet sand, a slump cone test was used instead. In the slump cone test, sand (SCBX, manufactured by Itochu Ceratec Corporation) was poured into the container from the top, and the container was then pulled up to evaluate whether the sand would collapse. The slump cone had a 50 mm diameter at the top inlet and a 100 mm height. Two angles corresponding to the angle of repose were used: 87° and 80°. Water was added to the sand at 0.7%, 1%, 1.6%, and 2% by mass. The results are shown in Figures 6(a)–(d).

[0033] The 87° slump cone has a 3° draft angle for ease of removal, but is almost vertical and cylindrical. Because the addition of 1.6% by mass caused it to collapse under its own weight (see Figure 6(a)), it was confirmed that adding 1.6% by mass or less would result in an angle of repose of 90° or less, making it possible to design a hopper that would allow sand to flow out. The addition of 2% by mass prevented it from collapsing under its own weight (see Figure 6(b)).

[0034] In the 80° slump cone, the addition of 0.7% by mass caused it to collapse under its own weight (see Figure 6(c)), confirming that adding 0.7% by mass or less provides good fluidity with an angle of repose of 80° or less. The addition of 1% by mass prevented it from collapsing under its own weight (see Figure 6(d)).

[0035] The sand to be used is not particularly limited, and may be, for example, artificial sand produced by a sintering method, a melting method, a flame fusion method, or the like, natural silica sand, or a mixture thereof.

[0036] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be modified in various ways without departing from the gist of the present invention. [Explanation of symbols]

[0037] 1 resin 2. Acid 11 Mixed sand 12 Uncured sand 13 Mold 14 Crushed sand 15 Mixed sand 16 Recycled sand 20 Additive manufacturing equipment 21 Recoater 22 Resin supply unit (printer head) 23 Modeling Box 24 stages 30 Roasting furnace 31 Inlet 32 Furnace 33 Burner 34 Blower 35 Air passage forming member

Claims

1. an additive manufacturing process in which mixed sand obtained by kneading sand and a hardener is layered one layer at a time while adding an alkali-containing resin to form a mold in unhardened sand to which no resin has been added, and the mold is then removed from the unhardened sand; a mixing step of mixing the unhardened sand with crushed sand obtained by crushing the mold to form a mixed sand; a roasting step of introducing the mixed sand into a roasting furnace and roasting it; and an acid addition step of adding acid to the mixed sand or the crushed sand so as to neutralize at least a portion of the alkali contained in the mold before the mixed sand or the crushed sand is introduced into the roasting furnace.

2. The method for producing recycled foundry sand according to claim 1 , wherein the acid addition step includes adding the acid to the mixed sand.

3. 2. The method for producing recycled foundry sand according to claim 1, wherein the acid addition step adds the acid to the crushed sand before the unhardened sand and the crushed sand are mixed.

4. 2. The method for producing recycled foundry sand according to claim 1, wherein the acid addition step adds the acid in an amount exceeding a required neutralization amount, which is an amount of the acid that can completely neutralize the amount of the alkali contained in the sand immediately before the addition of the acid.

5. 5. The method for producing recycled foundry sand according to claim 4, wherein the acid addition step adds the acid in an amount 1.8 times or more the amount required for neutralization.

6. 6. The method for producing recycled foundry sand according to claim 1, wherein the ratio of the mass of the acid added to the mixed sand in the acid addition step to the mass of the mixed sand immediately before being introduced into the roasting furnace is 1.6 mass% or less.

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