WO2017068785A1

WO2017068785A1 - Binder-containing foundry sand and method of manufacturing the same

Info

Publication number: WO2017068785A1
Application number: PCT/JP2016/004644
Authority: WO
Inventors: Ryuya OGUSU; Yoshikatsu Nishida; Daisuke Tomomatsu; Yuuichi Noguchi
Original assignee: Yamakawa Sangyo Co., Ltd.; Hitachi Chemical Company, Ltd.
Priority date: 2015-10-20
Filing date: 2016-10-20
Publication date: 2017-04-27
Also published as: KR20180099639A; CN108367338A; JP2017077571A; MY194119A; JP5933800B1; CN108367338B

Abstract

A binder-containing foundry sand comprising an aggregate derived from artificial sand and/or natural sand, a binder and, as necessary, a hardening agent, and an antiblocking agent in which the antiblocking agent is a fatty acid amide.

Description

BINDER-CONTAINING FOUNDRY SAND AND METHOD OF MANUFACTURING THE SAME

The present invention relates to a binder-containing foundry sand and a method of manufacturing the same. More specifically, the present invention relates to the binder-containing foundry sand which can prevent a reduction in the mold strength even after repeated use and the method of manufacturing the same.

One of methods for manufacturing molds in the foundry industry is shell molding. This method consists of filling sand coated by a binder and, as necessary, a hardening agent (Resin Coated Sand, RCS; binder-containing foundry sand) in a pre-heated metal mold followed by baking to manufacture a mold.
Today, there are various aggregates of binder-containing foundry sand, including silica sand, used for methods for manufacturing molds such as shell molding. Binder-containing foundry sand contains an aggregate derived from artificial sand or natural sand, the binder, and, as necessary, the hardening agent and an antiblocking agent. As the binder, for example, a thermosetting resin (for example, a phenolic resin) may be used, and a hardening agent (for example, hexamethylenetetramine) may be also used as necessary.
The antiblocking agent is used in order to prevent blocking of a product or to facilitate shaping of the binder-containing foundry sand into a desired mold shape. The antiblocking agent generally used in the field of foundry industry is calcium stearate. For example, using calcium stearate as the antiblocking agent is described in Japanese Unexamined Patent Publication No. 2006-334612 (Patent Document 1).

Japanese Unexamined Patent Publication No. 2006-334612

Because of depletion of resources and regulations on industrial waste, it has been sought also in the field of foundry industry to reduce the amount of waste mold sand by reusing the waste mold sand for casting.
The waste mold sand contains carbides and inorganic substances derived from the antiblocking agent or a binder component. The waste mold sand is generally recycled as aggregates (hereinafter referred to as recycled sand) by performing thermal reclamation in a temperature range of 400 to 1000°C to obtain thermal reclaimed sand and then dry-polishing the thermal reclaimed sand. When the recycled sand was used, the mold did not have enough strength. Therefore, there is a need for providing the binder-containing foundry sand that can provide sufficient mold strength even when the sand is recycled.

The inventors of the present invention have studied the causes of reduction in the strength of a mold manufactured with the recycled binder-containing foundry sand, and found that the mold strength correlates with the amount of calcium ion elution into water from the recycled sand. Specifically, the inventors have learned that when the calcium ion elution is high, the mold strength is decreased. The inventors have found that the decrease in the mold strength can be suppressed from examination on types of antiblocking agents from a viewpoint that the calcium ion is derived from the calcium stearate used as the antiblocking agent. Thus the inventors have completed the present invention.

Thus, the present invention provides a binder-containing foundry sand comprising an aggregate derived from artificial sand and/or natural sand, a binder and, as necessary, a hardening agent, and an antiblocking agent in which the antiblocking agent is a fatty acid amide.
Furthermore, the present invention provides a method for manufacturing the binder-containing foundry sand comprising the steps:
thermally reclaiming waste mold sand generated after casting in a temperature range of 400 to 1000°C to obtain thermal reclaimed sand and then dry-polishing the thermal reclaimed sand to recycle the thermal reclaimed sand as an aggregate,
mixing the aggregate with a binder and, as necessary, a hardening agent, and
mixing the mixture of the aggregate, the binder and, as necessary, the hardening agent with an antiblocking agent,
in which the antiblocking agent is a fatty acid amide.

According to a recycling method of the present invention, the binder-containing foundry sand which can provide a sufficient mold strength even after being recycled can be provided. The inventors of the present invention recognize that it is unexpected that, in the field of foundry industry, calcium ions correlate with a reduction in the mold strength and recognize that fatty acid amide has not been generally used as an antiblocking agent in this field.

In addition, in any of the following cases, it is possible to provide the binder-containing foundry sand which can provide further sufficient mold strength even after being recycled:
(a) the antiblocking agent is a fatty acid amide having a melting point equal to or higher than 90°C;
(b) the antiblocking agent is selected from ethylene bis-stearamide, ethylene bis-behenamide, ethylene bis-lauramide, ethylene bis-caprinamide and methylene bis-stearamide;
(c) the antiblocking agent is contained in an amount of 0.01 to 10.0 parts by mass with respect to a total amount of 100 parts by mass of the aggregate, the binder and the hardening agent;
(d) the antiblocking agent is contained in an amount that exhibits a blocking rate equal to or lower than 15%; and
(e) the binder-containing foundry sand includes the aggregate derived from artificial sand, the binder, as necessary, the hardening agent, and the antiblocking agent in which the antiblocking agent is a fatty acid amide, and
the artificial sand satisfies the following:
(1) the artificial sand contains, as a main component, synthetic mullite and/or synthetic corundum containing 40 to 90% by mass of alumina and 60 to 10% by mass of silica; and
(2) a solution obtained by thermally reclaiming the binder-containing foundry sand for 1 hour at 600°C and stirring the binder-containing foundry sand for 1 hour in a 0.05M water solution of HCL after subjecting the binder-containing foundry sand to a dry-polishing step exhibits a calcium ion elution greater than 0.25 mg/L and less than 51.11 mg/L.

Fig. 1 is a schematic illustration of a thermal reclamation furnace used in Examples. Fig. 2 is a schematic illustration of a rotary reclaimer used in Examples. Fig. 3 is a graph showing a relation between the number of how many times binder-containing foundry sand of Example 1 is recycled, and bending strength and calcium ion elution thereof. Fig. 4 is a graph showing a relation between the number of how many times binder-containing foundry sand of Example 1 is recycled, and bending strength and calcium ion elution thereof. Fig. 5 is a graph showing a relation between the number of how many times binder-containing foundry sand of Example 1 is recycled and bending strength and calcium ion elution thereof. Fig. 6 is a graph showing a relation between the number of how many times binder-containing foundry sand of Example 1 is recycled, and bending strength and calcium ion elution thereof. Fig. 7 is a graph showing a relation between the number of how many times binder-containing foundry sand of Example 1 is recycled, and bending strength and calcium ion elution thereof. Fig. 8 is a graph showing a relation between bending strengths of binder-containing foundry sand of Example 1 at the time of using new sand and at the time of fifth recycle. Fig. 9 is a graph showing a relation between an amount of ethylene bis-stearamide added to binder-containing foundry sand of Example 2 and a blocking rate. Fig. 10 is a graph showing a relation between the number of how many times binder-containing foundry sand of Example 3 is recycled, and bending strength and calcium ion elution thereof. Fig. 11 is a graph showing a relation between the number of how many times binder-containing foundry sand of Example 3 is recycled, and bending strength and calcium ion elution thereof. Fig. 12 is a graph showing a relation between bending strengths of binder-containing foundry sand of Example 3 at the time of using new sand and at the time of fifth recycle.

(Binder-containing Foundry Sand)
Binder-containing foundry sand contains an aggregate, a binder and an antiblocking agent.
(1) Antiblocking Agent
The antiblocking agent is a fatty acid amide.
An amide of a fatty acid having 6 to 24 carbon atoms may be used as the fatty acid amide. Examples of a higher fatty acid include saturated fatty acids such as capric acid, undecylic acid, lauric acid, tridecylic acid, myristic acid, pentadecylic acid, palmitic acid, heptadecylic acid, stearic acid, nonadecanoic acid, arachic acid, behenic acid and lignoceric acid and unsaturated fatty acids such as oleic acid and linoleic acid.
The fatty acid amide preferably has a melting point equal to or higher than 90°C. A melting point equal to or higher than 120°C is more preferred. The melting point can be measured in accordance with JIS K 0064-1992.

Preferable examples of the fatty acid amid include ethylene bis-stearamide, ethylene bis-behenamide, ethylene bis-lauramide, ethylene bis-caprinamide and methylene bis-stearamide.
The antiblocking agent contains the fatty acid amide as a main component and may be used in combination with other antiblocking agents as long as the bending strength does not decrease in a case where thermal reclamation is performed in a temperature range of 400 to 1000°C to obtain thermal reclaimed sand and the binder-containing foundry sand is manufactured by using an aggregate obtained by dry-polishing the thermal reclaimed sand. The term “main component” means, for example, 50% by mass or more, 70% by mass or more, 90% by mass or more or 100% by mass.

The fatty acid amide described above is preferably present on an outermost surface of the aggregate derived from artificial sand and/or natural sand coated by the binder and, as necessary, a hardening agent.
The fatty acid amide described above is preferably contained in an amount that exhibits a blocking rate of equal to or lower than 15%. If the blocking rate is within this range, it will be judged that the amount of the fatty acid amide contained in the binder-containing foundry sand is appropriate for casting. In a case where the blocking rate is higher than 15%, a mold may not be manufactured properly and surface of a molded article may be rough due to an insufficient fluidity. Further, the binder-containing foundry sand may cause blocking in flexible container bags in which products are stored.
From a viewpoint of preventing blockings, a specific amount of the fatty acid amide content may be 0.01 to 10.0 parts by mass relative to the total amount of 100 parts by mass of the aggregate, the binder and the hardening agent. The content is more preferably 0.02 to 5.0 parts by mass and still more preferably 0.03 to 1.0 parts by mass.

A type of the antiblocking agent constituting the binder-containing foundry sand can be confirmed by, for example, a procedure below.
That is, first, the antiblocking agent is sampled from the surface of the binder-containing foundry sand by using a micro manipulator (for example, a micro manipulator system MMS-77 manufactured by Shimadzu Corporation). The obtained sample is subjected to infrared spectroscopic analysis (for example, SpectrumOne FT-IR/MultiScope microscope infrared microscopic system: manufactured by PerkinElmer) to obtain an IR chart. The obtained IR chart is compared with an IR chart of a known fatty acid amide to confirm the type of the antiblocking agent.

In addition, the amount of antiblocking agent constituting the binder-containing foundry sand can be measured by isolating the antiblocking agent using an appropriate solvent and analyzing the isolated matter by a well-known method such as spectroscopic analysis, gas chromatography or liquid chromatography.

(2) Aggregate
The aggregate is derived from artificial sand and/or natural sand. The artificial sand and the natural sand are not particularly limited and examples thereof include alumina sand, silica sand, zircon sand, chromite sand, MgO/SiO₂-containing sand and mixed sand of these.
The alumina sand may contain other components than Al₂O₃ such as SiO₂,Fe₂O₃, Cr₂O₃, CrO₂, MgO, CaO, K₂O and TiO₂. Moreover, the alumina sand may be artificial sand containing synthetic corundum and/or synthetic mullite containing Al₂O₃ and SiO₂.

The synthetic mullite and the synthetic corundum may be constituted by 40 to 90% by mass of alumina (Al₂O₃) and 60 to 10% by mass of silica (SiO₂). In addition, the proportions of alumina and silica may be 60 to 90% by mass and 40 to 10% by mass, respectively. The synthetic mullite and the synthetic corundum may contain other components than alumina and silica such as Fe₂O₃, Cr₂O₃, CrO₂, MgO, CaO, K₂O and TiO₂. The synthetic mullite and/or the synthetic corundum may account for 50% by mass or more of the artificial sand.

Further, the aggregate has a surface area per unit volume (cm²/cm³) of 6 x 10⁴/d to 1.8 x 10⁶/d (wherein d is an average particle size (μm) of the spherical substance). The following explanation is given for the aggregate of, for example, 300 to 425 μm. When it is assumed that the aggregate has an average particle size of 362.5 μm, which is an intermediate between 300 μm and 425 μm, then the surface area is in the range of 165.5 to 4965.5 cm²/cm³. When the surface area is 1.8 x 10⁶/d (cm²/cm³) or higher, the aggregate has higher surface roughness and may increase the amount of waste after the aggregate is collapsed due to contact between the particles of the aggregate. The surface area as used herein is a value obtained by measuring the specific surface area per gram on a specific surface area analyser (BELSORP 28SA AUTOMATIC GAS ADSORPTION APPARATUS: available from BEL Japan, Inc.) and multiplying the specific surface area by the true density. The surface area is preferably 1.6 x 10⁶/d or less, more preferably 1.45 x 10⁶/d or less, still more preferably 1.3 x 10⁶/d or less, and particularly preferably 1.1 x 10⁶/d or less.

The aggregate preferably has a round particle shape. Specifically, the aggregate preferably has a particle shape factor, which is an index of roundness, of 1.2 or less, and more preferably 1.1 or less. When the particle shape factor is 1.2 or less, the aggregate has an improved filling rate in a mold, and a resulting mold has improved permeability. Moreover, because of the shape which is almost spherical, the amount of the waste which may be generated due to contact between the particles of the aggregate may be reduced.
The particle shape factor as used herein means a value calculated on a sand surface area analyser (available from George Fischer Ltd.). Thus the particle shape factor means a value obtained by dividing an actual surface area of 1 g of aggregate by a theoretical surface area. The theoretical surface area refers to the surface area with assuming that all aggregates are spheres. Accordingly, the particle shape factor having a value closer to 1 means that a shape of the material is closer to a sphere.

The aggregate has a particle size distribution of 30 to 1180 μm. When the particle size distribution is less than 30 μm, a resulting mold may have decreased permeability. When the particle size distribution is higher than 1180 μm, a resulting casting may have a rough surface. Examples of a preferable particle size distribution include 212 to 1180 μm (corresponding to JIS #10 and #14), 150 to 820 μm (corresponding to JIS #20 and #28), 106 to 600 μm (corresponding to JIS #35 and #48), 75 to 425 μm (corresponding to JIS #65 and #100) and 53 to 300 μm (corresponding to JIS #150 and #200). The particle size distribution may be appropriately selected according to casting conditions such as a type of iron or steel castings (iron castings, ordinary steel castings, stainless steel castings, high-Mn steel castings, aluminium alloy castings, copper alloy castings, etc.), a size of the casting or a thickness of the casting. The aggregate may contain sand of less than 30 μm at an amount that does not inhibit the effect of the invention (for example, 25% by mass or less).

The particle size distribution refers to a value measured according to a method for determining particle size of foundry molding sand (JIS Z2601). The method is briefly described. For example, a sieve having a nominal dimension of 30 μm is overlaid on a sieve of 1180 μm. A raw material is placed on the sieve of 1180 μm and sifted on a sieve shaker such as a Ro-tap sieve shaker. The material left between the two sieves is referred to as sand having a particle size distribution of 30 to 1180 μm.
The aggregate and the binder-containing foundry sand have almost the same surface area per unit volume, particle shape factor and particle size distribution.

The aggregate containing the synthetic mullite and/or the synthetic corundum having the surface area described above can be obtained by melting a raw material of the synthetic mullite and/or the synthetic corundum containing alumina and silica and blowing air to the molten material. Thus, the molten material is ground by blown air to particles having the pre-determined particle size distribution and gives the aggregate having the pre-determined surface area by means of the surface tension of the molten particles. The method of melting is not particularly limited and may use an arc furnace, a crucible furnace, an induction electric furnace (a high-frequency furnace, a low-frequency furnace, etc.), a resistance electric furnace, a reverberatory furnace, a rotary furnace, a vacuum melting furnace or a cupola furnace. Among these, the arc furnace is preferable, which is relatively easy to operate.

The particle size distribution, appearance and so forth of the aggregate containing the synthetic mullite and/or the synthetic corundum can be adjusted by the composition of the raw material of the synthetic mullite and the synthetic corundum, the melting temperature, the speed of air at the time of the air blowing and the contact angle of the melting material and the air. The melting temperature is preferably in the range of 1600 to 2200°C. The speed of air is preferably 80 to 120 m/sec. The contact angle is preferably 60 to 90°.
It is preferable to carry out water cooling after blowing air.

(2) Binder
The binder is not particularly limited and examples thereof include a furan resin, a phenolic resin, an oil-modified urethane resin, a phenolic urethane resin, an alkaline phenolic resin, sodium silicate, bentonite and the like. The binder can be hardened with the hardening agent according to a type of the binder. Examples of the hardening agent for the furan resin include inorganic acids such as sulphuric acid, phosphoric acid, phosphate esters and pyrophosphoric acid and organic acids such as xylenesulphonic acid, toluenesulphonic acid and benzenesulphonic acid. Examples of the hardening agent for the alkaline phenolic resin include lactones (for example, propiolactone) and organic esters such as ethyl formate, methyl formate and triacetin. Examples of the hardening agent for the phenolic resin include hexamethylenetetramine and the like. Examples of the hardening agent for the phenolic urethane resin include triethylamine and pyridine-containing compounds. Examples of the hardening agent for the sodium silicate include carbon dioxide gas, dicalcium silicate and organic esters.

It is preferable that the binder is contained in the range of 0.4 to 10 parts by mass per 100 parts by mass of the aggregate. When the content is less than 0.4 parts by mass, bonding of the aggregate may not be sufficient, resulting in a reduction in the mold strength. When the content is more than 10 parts by mass, a component derived from the binder may adhere on the surface of a casting or the time required for recycle of the waste mold sand may be increased. The content is more preferably 0.2 to 2.0 parts by mass and still more preferably 0.4 to 3 parts by mass.
The aggregate having the surface area per unit volume in the range of 60,000/d to 1,800,000/d (d is the average particle diameter (μm) of a spherical matter) can reduce the amount of the binder to 0.2 to 2.0 parts by mass. When the amount of the binder is reduced, the effect of calcium ions on reduction of the mold strength is increased, and thus application of the present invention is particularly useful.

(4) Production method of binder-containing foundry sand
The binder-containing foundry sand can be produced by a well-known method. For example, while heating and mixing the aggregate in a mixer, the binder is charged into a mixer to obtain a mixture of the binder and the aggregate. In a case where the binder is a binder that gets hardened by the hardening agent, the hardening agent is mixed at this time. Subsequently, the antiblocking agent is charged into the mixer. This allows the antiblocking agent to be present on the outermost surface of the aggregate coated by the binder and, as necessary, the hardening agent. It is believed that the binder and the hardening agent coat all or a part of the surface of the aggregate. It is also believed that the antiblocking agent coats all or a part of the surface of the aggregate coated with the binder and the hardening agent.

It has been confirmed that the amount of charged antiblocking agent and the amount of antiblocking agent content in the binder-containing foundry sand are approximately the same.

In a case where the recycled sand derived from the waste mold sand generated after casting has been used by using the binder-containing foundry sand containing the fatty acid amide as the antiblocking agent, the waste mold sand can be turned into the recycled sand by, for example, going through the thermal reclamation process and the polishing process below.

(i) Thermal reclamation process
The thermal reclamation process can be carried out at a temperature within the range of 400 to 1000°C. By subjecting the waste mold sand to the thermal reclamation process, components derived from the antiblocking agent and the binder in the waste mold sand are carbonized and eliminated by burning. Remainder on the surface of the aggregate is removed in a subsequent attrition process and the recycled sand can be thus obtained. Here, a calcium component derived from the calcium stearate serving as a conventional antiblocking agent is accumulated on the surface of sand. This calcium component cannot be completely removed even by going through the attrition process.

When the temperature in the thermal reclamation process is lower than 400°C, carbonisation may not be carried out sufficiently, resulting in a reduction in the strength of the mold obtained with the recycled binder-containing foundry sand (recycled sand). When the temperature is higher than 1000°C, although sufficient carbonisation can be carried out, particles of sand may aggregate due to melting of the surface of recycled sand depending on the inorganic components contained in the recycled sand. The temperature range is more preferably 400 to 800°C and still more preferably 500 to 800°C.

The thermal reclamation time may be, for example, 0.5 to 2.5 hours. When the thermal reclamation time is less than 0.5 hours, sufficient carbonisation may not be carried out, resulting in a reduction in the strength of the mold obtained with the recycled sand. The reason that the upper limit of the thermal reclamation time is 2.5 hours is that even when the waste mold sand is thermal reclaimed for a longer time, an increase in the effect by thermal reclaiming may not be expected and recycling cost is increased with consumption of the fuel. The thermal reclamation time is more preferably 1.5 to 2.5 hours and still more preferably 1.75 to 2.25 hours.
The atmosphere in which the thermal reclamation process is carried out is not particularly limited as far as the binder in the waste mold sand can be carbonised, and is generally an atmosphere containing oxygen (for example, in air).

A thermal reclaimer (roaster) may have any structure without particular limitation as far as it can reclaime the waste mold sand. The waste mold sand in the thermal reclaimer may be fluidised or not; however in order to obtain uniformly thermal reclaimed sand, the waste mold sand is preferably fluidised. The thermal reclaimer may be of a batch type or a continuous type. In view of the processing efficiency, it is preferable to use a continuous fluidised thermal reclaimer.

Various structures are known for continuous fluidised thermal reclaimers. Examples thereof include a thermal reclaimer in which the flow direction of sand intersects with the flow direction of air for fluidisation of sand and a thermal reclaimer in which the directions are opposing and parallel. In view of the thermal efficiency, the latter thermal reclaimer is more preferable. Particularly, a thermal reclaimer in which the flow direction of sand is the same as the direction of the gravity and the flow direction of air is opposite to the direction of the gravity is preferable because it has high thermal efficiency, so that the amount of fuel for thermal reclaiming can be decreased.

In the thermal reclaimer described above in which the flow direction of sand is the same as the direction of the gravity, the waste mold sand is charged from the upper part of the thermal reclaimer and falls through the thermal reclaimer. The fallen sand is retained over a certain period of time as a fluidised bed at a certain position by means of air blown up from the lower part of the thermal reclaimer. The sand retained at a certain position is thermal reclaimed over a certain period of time by means of a heating means such as a burner. Sand at a lower part of the fluidised bed gradually descends because of sand fed from the upper part of the fluidised bed and falls to the bottom of the thermal reclaimer as thermal reclaimed sand. This type of thermal reclaimer is characterised in that it has high thermal efficiency because the heat of thermal reclaimed sand can be used for heating of the waste mold sand which will be charged next.

(ii) Attrition process
The thermal reclaimed sand obtained from the thermal reclamation process is subjected to the attrition process. In the attrition process, residue on the surface of the thermal reclaimed sand are eliminated and thus the waste mold sand can be recycled as the aggregate which is used as a raw material of molds.
Attriting may be dry attriting, wet attriting or a combination thereof.
Examples of dry methods include a method using a sand reclaimer in which sand is ground by means of collision between sand particles and friction by ascending sand in the machine by high speed air flow and allowing collision of sand to a collision plate, a method using a high speed rotary reclaimer in which sand is ground by means of collision between falling charged sand and projected sand generated by means of centrifugal force obtained by charging sand over a rotor rotating at high speed, and a method using an agitator mill in which sand is ground by friction between sand particles.

Examples of wet methods include a method using a trough attriting apparatus in which sand is ground by means of, for example, friction between sand particles in the trough containing rotating blades.
It is preferable that attrition is carried out by a dry method. The dry method enables manufacturing the binder-containing foundry sand even in a place short of water resources. Furthermore, the dry method does not require a draining process and thus can suppress influencing the environment.

When conventional binder-containing foundry sand containing calcium stearate as the antiblocking agent is repeatedly recycled from the waste mold sand by dry attrition, the mold strength is reduced, which is disadvantageous. The inventors of the present invention have measured the calcium content in the recycled sand, and found that the calcium content increases with an increase in the number of recycle and the content correlates with the mold strength. Thus, the inventors have found that decrease in the mold strength can be prevented by using a fatty acid amide not containing calcium as the antiblocking agent because the accumulation of calcium on the recycled sand is suppressed even when the attrition is performed by the dry method.
Conditions for the dry attrition is not particularly limited as long as the dry attrition can remove the carbides present on the surface of the thermal reclaimed sand.

(iii) Other embodiments
(1) The waste mold sand may be applied to a pulveriser before the thermal reclamation process. By applying to a pulveriser, aggregates of waste sand can be pulverised and thus the yield of the recycled sand from the waste mold sand can be increased.
(2) The waste mold sand may be applied to a magnetic separator before the thermal reclamation process. By applying to the magnetic separator, casting residues in the waste sand can be eliminated and thus the yield of the recycled sand from the waste mold sand can be increased.

(3) Thermal reclaimed sand obtained from the thermal reclamation process is preferably subjected to a cooling step before the attrition process. By subjecting to a cooling step, breaking of thermal reclaimed sand due to a sudden temperature change may be prevented and thus the yield of the recycled sand from the waste mold sand can be increased. The cooling step may be carried out while fluidizing thermal reclaimed sand, thereby uniformly cooling the thermal reclaimed sand.
(4) Sand after the attrition process may be subjected to a classification step to classify recycled sand according to a desired particle size distribution.

Example 1
As the antiblocking agent, calcium stearate, ethylene bis-stearamide, ethylene bis-behenamide, ethylene bis-lauramide, and ethylene bis-caprinamide were respectively used to produce the binder-containing foundry sand. The binder-containing foundry sand was produced through the following procedure and the bending strength and the calcium ion elution thereof were measured.

(1) Production of binder-containing foundry sand
As the aggregate, Espearl #60 (available from Yamakawa Sangyo Co., Ltd.; surface area per unit volume: 3300 cm²/cm³, particle size distribution: 53 to 600 μm, particle shape factor: 1.03; containing 40% by mass of the synthetic mullite and 10% by mass of the synthetic corundum with alumina and silica; being in total at 94% by mass (alumina : silica = 77% : 23% by mass)) that had never been used as the binder-containing foundry sand was used. This Espearl #60 will be referred to as new sand. The aggregate was heated to 160°C and placed in a mixer (available from Enshu Tekko K.K., Type NSC-1) to maintain the temperature of the aggregate at 150°C. While adding 0.8 parts by mass of a binder HP-333N (a novolac phenolic resin available from Hitachi Chemical Co., Ltd.) relative to 100 parts by mass of the aggregate, the aggregate was stirred for about 60 seconds to coat the aggregate with the binder. While stirring these, 15 parts by mass of hexamethylenetetramine (hardening agent) relative to 100 parts by mass of the binder and 1.3 parts by mass of water (dispersing medium of the hardening agent) relative to 100 parts by mass of the aggregate were then added and the stirring was performed for about 45 seconds to coat the aggregate with the mixture of the binder and the hardening agent. While stirring the mixture of the binder, the hardening agent and the aggregate, 0.06 parts by mass of the antiblocking agent relative to 100 parts by mass of the mixture of the binder, the hardening agent and the aggregate was then added and the stirring was performed for about 15 seconds to give binder-containing foundry sand (RCS). The resulting RCS was sifted on a sieve with the mesh size of 1180 μm to remove agglomeration.
The resulting RCS was measured for the bending strength according to the following procedure. The bending strength indicates the mold strength.

(2) Measurement of bending strength
(a) Preparation of test pieces
A test piece was measured for bending strength according to JACT test method SM-1, a test method for bending strength (corresponding to JIS K 6910). Specific measurement conditions are as follows.
A lower mold having 5 recessed parts respectively of 10 mm in depth, 10 mm in width and 60 mm in length and an upper mold serving as a lid of the lower mold were prepared. The lower and upper molds were heated to 250°C ± 3°C and then the recessed parts were filled with about 50 g of the binder-containing foundry sand. The upper surface of the binder-containing foundry sand filling the recessed parts was smoothened with a sample scraping plate. The lower and upper molds were then joined and baked for 60 seconds. After baking, the upper mold was removed and baked articles were filed, so that the upper surface of the baked articles and the upper surface of the lower mold were levelled off. The baked articles were then removed from the lower mold to give test pieces. The time from the opening of the upper mold to the removal of test pieces from the lower mold was set to 30 seconds.

The obtained test pieces were cooled to room temperature (about 25°C) in a desiccator and maintained until the bending strength was measured.
Test pieces were prepared three times and thus 15 test pieces were obtained per the binder-containing foundry sand.

(b) Measurement of bending strength
A pair or projected members respectively having a tip angle of 60°, a curvature of the tip of 1.5 R and a length of 10 mm or more were placed with a gap of 50 mm therebetween on a test piece mounting stage, so that the members were parallel in length way. On the test piece mounting stage, test pieces were mounted, so that the filed surface of the test pieces was not placed on the stage or on the opposing side of the stage (top side).
Load was applied on the centre of the upper surface of the test piece with load wedges having a tip angle of 60° and a curvature of the tip of 1.5 R. The amount of load applied when the test piece was fractured was recorded. The load test was carried out for each of 15 test pieces.

From the resulting values of load, bending load was calculated according to the following equation.
σfb = 3 x l x P/2 x W x h²
wherein σfb is the bending load (kgf/cm²), l is a distance (5 cm) between a pair of projecting members on the test piece mounting stage, P is a value of load (kgf), W is a width (1 cm) of the test piece and h is a height (1 cm) of the test piece.
The bending strength (kgf/cm²) was obtained as an average of the bending load of 15 test pieces.

(3) Recycle of binder-containing foundry sand
The test pieces after measurement of the bending strength as above were pulverized and the pulverized material was then recycled into the aggregate by going through a thermal reclamation process and a dry attrition process.
The thermal reclamation process was carried out in a JFE Pipe Fitting Mfg. thermal reclaimer (available from JFE Pipe Fitting Mfg., Co., Ltd., Type JTR-G-1) as shown in Fig. 1 under the conditions of a thermal reclamation temperature of 600°C, a flowing differential pressure in the thermal reclaimer of 4.5 MPa and an input of sand of 2.5 t/hour. Under these conditions, the actual processing time of waste mold sand was about 1 hour. In Fig. 1, 1 denotes a thermal reclaimer, 2 denotes a pulverized material inlet, 3 denotes a burner, 4 denotes a fluidized bed, 5 denotes a heat exchanger, 6 denotes an air inlet for fluidizing sand, 7 denotes a cooling air inlet, 8 denotes a fluidized cooler, 9 denotes a sand discharge valve, 10 denotes an air nozzle, 11 denotes a flowing differential pressure gauge and 12 denotes an exhaust gas outlet.

The dry attrition was continuously performed by using an S-type rotary reclaimer (available from Nippon Chuzo K.K.) illustrated in Fig. 2 under conditions of a load current of 20 to 40 A and an input of sand of 2 to 3 t/hour. In Fig. 2, 21 denotes an orifice, 22 denotes a shelf, 23 denotes a shelf ring, 24 denotes a rotary drum, 25 denotes a fan, 26 denotes a motor and 27 denotes a cap.

(4) Measurement of bending strength for recycled RCS
The production of RCS in the same manner as in the step (1), the measurement of the bending strength in the same manner as in the step (2) and the recycle of RCS in the same manner as in the step (3) were repetitively performed for 5 times. The resulting bending strength is shown in Table 1.
Table 1 also shows the calcium ion (Ca²⁺ ion) elution of the new sand and the recycled aggregate.
(Calcium ion elution)
(1) Preparation of internal standard solution and standard solution
- Internal standard solution (Y: 50 mg/L)
A yttrium standard solution available from Kanto Chemical Co., Inc. (Y: 1000 mg/L, for atomic absorption analysis, 25 mL) was added to a 500-mL volumetric flask and pure water was added to the gauge line.
- Standard solution (Ca: 100 mg/L)
A standard solution IV for ICP emission spectrometric analysis available from Kanto Chemical Co., Inc. (Ca: 1000 mg/L, 10 mL) was added to a 100-mL volumetric flask and pure water was added to the gauge line.
- Standard solution (Ca: 10 mg/L)
Standard solution (Ca: 100 mg/L, 10 mL) was added to a 100-mL volumetric flask and pure water was added to the gauge line.
- Standard solution (Ca: 1 mg/L)
Standard solution (Ca: 10 mg/L, 10mL) was added to a 100-mL volumetric flask and pure water was added to the gauge line.
- Standard solution (Ca: 0.1 mg/L)
Standard solution (Ca: 1 mg/L, 10 mL) was added to a 100-mL volumetric flask and pure water was added to the gauge line.
- Standard solution (Ca: 0.01 mg/L)
Standard solution (Ca: 0.1mg/L, 10 mL) was added to a 100-mL volumetric flask and pure water was added to the gauge line.

(2) Preparation of calibration curve (measurement range: Ca: 0 to 10 mg/L)
The internal standard solution (Y: 50 mg/L, 20 mL) was added to 100-mL volumetric flasks and the standard solution (Ca: 10 mg/L), the standard solution (Ca: 1 mg/L), the standard solution (Ca: 0.1 mg/L) and the standard solution (Ca: 0.01 mg/L) were respectively added to the gauge line. As a blank, the internal standard solution (Y: 50 mg/L, 20 mL) was added to a 100-mL volumetric flask and pure water was added to the gauge line to prepare a standard solution. The solutions were measured on an ICP emission spectrometer (ICPS-8100) available from Shimadzu Corporation and a calibration curve of the relation between calcium ion concentrations and indicated values was prepared.

(3) Preparation of sample solutions
A sand specimen (50 g) was placed in a 300-mL polyethylene beaker, 50 mL of pure water and 50 mL of 0.1 mol/L hydrochloric acid solution were added thereto and the mixture was stirred on a magnetic stirrer for 1 hour. After stirring, the mixture was filtered through a glass fibre filter paper according to JIS P 3801 filter paper (for chemical analysis). After filtration, the solution was again vacuum filtered through a membrane filter (pore size: 0.45 μm) available from ADVANTEC to give a sample solution (neat). As a blank, 50 mL of pure water and 50 mL of a 0.1 mol/L hydrochloric acid solution were added to a 300-mL polyethylene beaker and subjected to the same procedure.

(4) Measurement of sample solutions
The internal standard solution (Y: 50 mg/L, 10 mL) was added to a 50-mL volumetric flask, the sample solution (neat) was added to the gauge line and the mixture was measured on an ICP emission spectrometer (ICPS-8100) available from Shimadzu Corporation. The calcium ion elution was calculated as the difference between the measured calcium ion concentration and the concentration obtained from the blank test. When the calcium ion concentration measured exceeded the measurement range of the calibration curve, the sample solution (neat) was diluted with pure water to obtain a sample solution (diluted) within the measurement range, the internal standard solution (Y: 50 mg/L, 10 mL) was added to a 50-mL volumetric flask, the sample solution (diluted) was added to the gauge line and the mixture was again measured on an ICP emission spectrometer (ICPS-8100) available from Shimadzu Corporation to determine the calcium ion concentration. When the sample solution (diluted) was measured, the calcium ion elution was calculated by subtracting the concentration obtained from the blank test from the product of the measured calcium ion concentration and the dilution factor.

The relation between the number of recycle (horizontal axis), the bending strength and Ca²⁺ ion elution (vertical axis) based on Table 1 is shown in Figs. 3 to 7 for respective antiblocking agents. Fig. 3 illustrates the result of the case where calcium stearate was used as the antiblocking agent. Fig. 4 illustrates the result of the case where ethylene bis-stearamide was used as the antiblocking agent. Fig. 5 illustrates the result of the case where ethylene bis-behenamide was used as the antiblocking agent. Fig. 6 illustrates the result of the case where ethylene bis-lauramide was used as the antiblocking agent. Fig. 7 illustrates the result of the case where ethylene bis-caprinamide was used as the antiblocking agent. In addition, the relation between the bending strength at the time of using the new sand and the bending strength at the time of fifth recycle is extracted for each antiblocking agent and summarized in Fig. 8. In Fig. 8, Ca-St represents calcium stearate, StA represents ethylene bis-stearamide, BeA represents ethylene bis-behenamide, LaA represents ethylene bis-lauramide and CpA represents ethylene bis-caprinamide.

From Table 1 and Figs. 3 to 7, it can be seen that the bending strength for calcium stearate decreases each time the recycle is repeated while hardly any change is seen for fatty acid amides. From Fig. 3, a tendency can be seen wherein the bending strength decreases with an increase in calcium ions, which are divalent ions that attach to the aggregate. Therefore, it is inferred that the calcium ions may adversely affect the binder (for example, viscosity is increased by chelating of the calcium ions with the binder). Meanwhile, from Figs. 4 to 7, it is assumed that the fatty acid amides do not have such adverse effects because the fatty acid amides do not include calcium.

As it is obvious from Fig. 8 illustrating the influence of each antiblocking agent on the decrease of bending strength, it can be understood that fatty acid amides exhibit remarkable superiority when used as the antiblocking agent.
It is believed that the calcium ions detected in Figs. 4 to 7 are derived from calcium ions included in the binder. The amount of calcium ions detected in these figures can be reduced further by using a binder that does not include calcium.

Example 2
The influence of the amount of added ethylene bis-stearamide on the blocking rate was confirmed by the following procedure.
(1) Production of binder-containing foundry sand
As the aggregate, Espearl #60 (available from Yamakawa Sangyo Co., Ltd.) that had never been used as the binder-containing foundry sand was used. The aggregate was heated to 160°C and placed in a mixer (available from Enshu Tekko K.K., Type NSC-1) to maintain the temperature of the aggregate at 150°C. While adding 0.8 parts by mass of a binder HP-333N (a novolac phenolic resin available from Hitachi Chemical Co., Ltd.) relative to 100 parts by mass of the aggregate, the aggregate was stirred for about 60 seconds to coat the aggregate with the binder. While stirring these, 24 parts by mass of hexamethylenetetramine (hardening agent) relative to 100 parts by mass of the binder and 1.6 parts by mass of water (dispersing medium of the hardening agent) relative to 100 parts by mass of the aggregate were then added and the stirring was performed for about 45 seconds to coat the aggregate with the mixture of the binder and the hardening agent. While stirring the mixture of the binder, the hardening agent and the aggregate, ethylene bis-stearamide of various amounts shown in Table 2 relative to 100 parts by mass of the mixture of the binder, the hardening agent and the aggregate was then added and the stirring was performed for about 15 seconds to give RCS. The resulting RCS was sifted on the sieve with the mesh size of 1180 μm to remove agglomeration.

(2) Measurement of blocking rate
The blocking rate was measured on the basis of a blocking test method of JACT test method C-3 according to the following procedure.
A sample was obtained by charging 100 g of RCS into a 50 ml glass beaker. The sample was heated for 60 minutes in a thermostat chamber (ONW-450S available from AS ONE Corporation) kept at 45 to 50°C. The humidity in the thermostat chamber was kept at 40 to 60% by putting an aluminum vat containing about 1 L of water in the thermostat chamber.
After 60 minutes, the heated RCS was sifted on a sieve of 6 mesh. The amount of RCS remaining on the sieve was measured and its percentage in the total amount of RCS was calculated as the blocking rate.
The results are shown in Table 2. The results of Table 2 is expressed as a graph in Fig. 9.

From Table 2 and Fig. 9, it can be seen that, when the amount of added ethylene bis-stearamide is in the range of 0.01 to 10.0 parts by mass relative to 100 parts by mass of the mixture of the binder, the hardening agent and the aggregate, the blocking rate can be reduced to equal to or lower than 15% compared with a case where ethylene bis-stearamide is not added.

Example 3
The bending strength was measured in the same way as in Example 1 other than that calcium stearate and ethylene bis-stearamide were each used as the antiblocking agent, that silica sand (flattery sand available from Yamakawa Sangyo Co., Ltd.; particle size distribution: 75 to 600 μm, particle shape factor: 1.43) that had never been used as binder-containing foundry sand was used as the aggregate, and that the amount of used binder was set to 1.0 parts by mass/100 parts by mass of the aggregate. The results of the measurement are shown in Figs. 10 and 11 for respective lubricants. Fig. 10 corresponds to the result of a case where calcium stearate was used as the antiblocking agent, and Fig. 11 corresponds to the result of a case where ethylene bis-stearamide was used as the antiblocking agent. In addition, the bending strength at the time of using the new sand and the bending strength at the time of fifth recycle are extracted for each antiblocking agent and summarized in Fig. 12.
From Figs. 10 to 12, it can be seen that the bending strength for calcium stearate decreases each time the recycle is repeated even in the case where the silica sand was used as the aggregate while hardly any change is seen for ethylene bis-stearamide.

References Signs List

1: thermal reclaimer, 2: pulverized material inlet, 3: burner, 4: fluidized bed, 5: heat exchanger, 6: air inlet for fluidizing sand, 7: cooling air inlet, 8: fluidized cooler, 9: sand discharge valve, 10: air nozzle, 11: flowing differential pressure gauge, 12: exhaust gas outlet, 21: orifice, 22: shelf, 23: shelf ring, 24: rotary drum, 25: fan, 26: motor, 27: cap

Claims

A binder-containing foundry sand comprising an aggregate derived from artificial sand and/or natural sand, a binder and, as necessary, a hardening agent, and an antiblocking agent in which the antiblocking agent is a fatty acid amide.
The binder-containing foundry sand according to claim 1, wherein the antiblocking agent is a fatty acid amide having a melting point equal to or higher than 90°C;
The binder-containing foundry sand according to claim 1, wherein the antiblocking agent is selected from ethylene bis-stearamide, ethylene bis-behenamide, ethylene bis-lauramide, ethylene bis-caprinamide and methylene bis-stearamide;
The binder-containing foundry sand according to claim 1, wherein the antiblocking agent is contained in an amount of 0.01 to 10.0 parts by mass with respect to a total amount of 100 parts by mass of the aggregate, the binder and the hardening agent;
The binder-containing foundry sand according to claim 1, wherein the antiblocking agent is contained in an amount that exhibits a blocking rate equal to or lower than 15%; and
The binder-containing foundry sand according to claim 1, wherein the binder-containing foundry sand includes the aggregate derived from artificial sand, the binder, as necessary, the hardening agent, and the antiblocking agent in which the antiblocking agent is a fatty acid amide, and
the artificial sand satisfies the following:
(1) the artificial sand contains, as a main component, synthetic mullite and/or synthetic corundum containing 40 to 90% by mass of alumina and 60 to 10% by mass of silica; and
(2) a solution obtained by thermally reclaiming the binder-containing foundry sand for 1 hour at 600°C and stirring the binder-containing foundry sand for 1 hour in a 0.05M water solution of HCL after subjecting the binder-containing foundry sand to a dry-polishing step exhibits a calcium ion elution greater than 0.25 mg/L and less than 51.11 mg/L.
A method for manufacturing the binder-containing foundry sand of claim 1 comprising :
thermally reclaiming waste mold sand generated after casting in a temperature range of 400 to 1000°C to obtain thermal reclaimed sand and then dry-polishing the thermal reclaimed sand to recycle the thermal reclaimed sand as an aggregate,
mixing the aggregate with a binder and, as necessary, a hardening agent, and
mixing the mixture of the aggregate, the binder and, as necessary, the hardening agent with an antiblocking agent,
in which the antiblocking agent is a fatty acid amide.