WO2016132408A1

WO2016132408A1 - Artificial sand and binder-containing foundry sand

Info

Publication number: WO2016132408A1
Application number: PCT/JP2015/005485
Authority: WO
Inventors: Norihiko KANEMOTO; Takayuki HYODA; Yoshikatsu Nishida; Sotomi TAKEDA; Ryuya OGUSU
Original assignee: Yamakawa Sangyo Co., Ltd.
Priority date: 2015-02-18
Filing date: 2015-10-30
Publication date: 2016-08-25
Also published as: MY187755A; JP2016150369A; JP5819551B1; CN107206470A; CN107206470B

Abstract

The present invention provides artificial sand comprising, for use as a material of binder-containing foundry sand, as a main component, synthetic mullite and/or synthetic corundum containing 40 to 90% by weight of alumina and 60 to 10% by weight of silica; having a particle size distribution of 30 to 1180 μm; having 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); and having a calcium ion elution in a 0.05 M HCl aqueous solution of 60 mg/L or less.

Description

ARTIFICIAL SAND AND BINDER-CONTAINING FOUNDRY SAND

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

One of the methods for manufacturing molds in the foundry industry is shell molding. This method consists of filling resin coated sand (RCS, binder-containing foundry sand) in a pre-heated metal mold followed by baking to manufacture a mold.
The binder-containing foundry sand contains an aggregate, a binder and a lubricant. As the binder, a resin obtained by hardening a thermosetting resin (for example, a phenolic resin) with a hardening agent (for example, hexamethylenetetramine) may be used, for example. The lubricant is used in order to facilitate shaping of the binder-containing foundry sand into a desired mold shape or to prevent blocking (aggregation) of a product. The lubricant generally used in the field of molds is calcium stearate.
In order to further reduce the amount of the binder and manufacture molds which can be easily collapsed after casting, Japanese Unexamined Patent Publication No. 2003-251434 (PTL 1) has proposed to use artificial sand which is almost spherical and has a smooth surface as an aggregate. In this document, an example of a lubricant disclosed is calcium stearate.

[PTL 1] Japanese Unexamined Patent Publication No. 2003-251434

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 the lubricant and carbides derived from a binder component which have been added to the binder-containing foundry sand during molding thereof into the desired shape. In order to recycle the waste mold sand, it is required to remove carbides. However, when a mold was manufactured by using the binder-containing foundry sand from which the carbides had been removed, 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 artificial sand from which a carbide is removed and is not yet mixed with a binder. Specifically, the inventors have learned that when the calcium ion elution is high, the mold strength is decreased and found that a reduction in the mold strength can be prevented by using the artificial sand as an aggregate, that has a certain level of the calcium ion elution. Thus the inventors have completed the present invention.

Thus the present invention provides the artificial sand comprising, for use as a material of the binder-containing foundry sand, as a main component, synthetic mullite and/or synthetic corundum containing 40 to 90% by weight of alumina and 60 to 10% by weight of silica;
having a particle size distribution of 30 to 1180 μm;
having 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); and
having a calcium ion elution in a 0.05 M HCl aqueous solution of 60 mg/L or less.
The present invention also provides the binder-containing foundry sand including the aggregate containing the artificial sand as a main component, the binder and the lubricant, wherein the artificial sand is the above described artificial sand.

According to the present invention, the binder-containing foundry sand which can provide the sufficient mold strength even after being recycled and the artificial sand which can be used therefor can be provided. The inventors of the present invention recognise that it is unexpected that, in the field of foundry industry, calcium ions correlate with a reduction in the mold strength.

Moreover, in any of the following cases, it is possible to provide the artificial sand for the binder-containing foundry sand which can provide further sufficient mold strength even after being recycled:
(a) the artificial sand derives from the waste mold sand generated after casting;
(b) the artificial sand has a particle shape factor of 1.2 or less; and
(c) the artificial sand uses for a shell mold process.
Further, 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:
(1) the binder is selected from a furan resin, a phenolic resin, an oil-modified urethane resin, a phenolic urethane resin, an alkaline phenolic resin, silicate of soda and bentonite, and the lubricant is calcium stearate; and
(2) the binder is contained in the range of 0.4 to 3 parts by weight relative to 100 parts by weight of the aggregate, and the lubricant is contained in the range of 0.01 to 0.2 parts by weight relative to 100 parts by weight of the sum of the aggregate and the binder.

Fig. 1 is a schematic illustration of a thermal reclamation furnace used in Examples; Fig. 2 is a schematic illustration of a trough attriting apparatus used in Examples; Fig. 3 is a graph showing a relation between bending strength and calcium ion elution of binder-containing foundry sand of Example 1; and Fig. 4 is a graph showing a relation between the number of how many times binder-containing foundry sand of Example 2 is recycled and bending strength and calcium ion elution thereof.

(Artificial sand)
Artificial sand is used as a raw material of binder-containing foundry sand containing an aggregate, a binder and a lubricant.
The artificial sand has the following composition, structure and physical properties.
- The artificial sand contains, as a main component, synthetic mullite and/or synthetic corundum containing 40 to 90% by weight of alumina and 60 to 10% by weight of silica.
- The artificial sand has a particle size distribution of 30 to 1180 μm.
- The artificial sand 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 artificial sand has calcium ion elution in a 0.05 M HCl aqueous solution of 60 mg/L or less.

(1) Composition
The artificial sand includes sand which contains the synthetic mullite and/or the synthetic corundum containing 40 to 90% by weight of alumina (Al₂O₃) and 60 to 10% by weight of silica (SiO₂). The proportions of alumina and silica may be 60 to 90% by weight and 40 to 10% by weight, respectively. The proportion of alumina may be 40% by weight, 45% by weight, 50% by weight, 55% by weight, 60% by weight, 65% by weight, 70% by weight, 75% by weight, 80% by weight, 85% by weight or 90% by weight. The artificial sand 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 weight or more of the artificial sand. The proportion may be 60% by weight or more, 70% by weight or more, 80% by weight or more, 90% by weight or more or 100% by weight. The artificial sand may contain either one of the synthetic mullite and the synthetic corundum or may contain both. For example, the artificial sand may contain 50 to 100 parts by weight of the synthetic mullite relative to 100 parts by weight of the sum of the synthetic mullite and the synthetic corundum.

(2) Structure
The artificial sand 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. The particle size distribution may be 30 μm, 53 μm, 75 μm, 106 μm, 150 μm, 212 μm, 300 μm, 425 μm, 500 μm, 600 μm, 700 μm, 820 μm, 900 μm, 1000 μm or 1180 μm. 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 artificial sand may contain sand of less than 30 μm at an amount that does not inhibit the effect of the invention (for example, 25% by weight 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.

Further, the artificial sand 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 artificial sand of, for example, 300 to 425 μm. When it is assumed that the artificial sand 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 artificial sand has higher surface roughness and may increase the amount of waste after the artificial sand is collapsed due to contact between the particles of the artificial sand. 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 per unit volume may be 6 x 10⁴/d, 1 x 10⁵/d, 2.5 x 10⁵/d, 5 x 10⁵/d, 7.5 x 10⁵/d, 1 x 10⁶/d, 1.1 x 10⁶/d, 1.3 x 10⁶/d, 1.45 x 10⁶/d, 1.5 x 10⁶/d, 1.6 x 10⁶/d or 1.8 x 10⁶/d. 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 artificial sand preferably has a round particle shape. Specifically, the artificial sand 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 binder-containing foundry sand 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 artificial sand may be reduced. The particle shape factor may be 1.2 or less, 1.15 or less, 1.1 or less, 1.05 or less, or 1.
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 sand particles by a theoretical surface area. The theoretical surface area refers to the surface area with assuming that all sand particles 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 artificial sand and the binder-containing foundry sand have almost the same surface area per unit volume, particle shape factor and particle size distribution.

(3) Physical properties
The artificial sand has calcium ion elution in a 0.05 M HCl aqueous solution of 60 mg/L or less. Due to the calcium ion elution in this range, a reduction in mold strength can be prevented. The inventors of the present invention infer the reason as follows. Namely, a binder is obtained by hardening a resin with a hardening agent. However, eluted calcium ions prevent hardening of the resin by binding to the resin or consuming the hardening agent. Therefore, a decreased calcium ion elution may not prevent hardening of the resin, resulting in certain mold strength. The calcium ion elution is desirably as low as possible and is preferably 40 mg/L or less, more preferably 30 mg/L or less, still more preferably 20 mg/L or less, and particularly preferably 10 mg/L or less. The calcium ion elution may be 60 mg/L or less, 55 mg/L or less, 50 mg/L or less, 45 mg/L or less, 40 mg/L or less, 35 mg/L or less, 30 mg/L or less, 25 mg/L or less, 20 mg/L or less, 15 mg/L or less, 10 mg/L or less, 5 mg/L or less, or 0 mg/L.

(4) Production method of artificial sand
The artificial sand having the above composition, structure and physical properties 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 artificial sand having the pre-determined composition, structure and physical properties 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 artificial sand may be adjusted to have desired composition, structure and physical properties by adjusting the composition of the synthetic mullite in the raw material, the melting temperature, the speed of air to be blown and the contact angle between the molten material and 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.

The artificial sand may be the one derived from waste mold sand generated after casting.
The artificial sand derived from the waste mold sand generated after casting can be obtained after, for example, the following thermal reclamation process and attrition process.
(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 lubricant and binder in the waste mold sand are carbonised as well as some of the components are eliminated by burning. By carbonising organic matters, carbides can be easily removed in the next attrition process to give the artificial sand.
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 reclamated for a longer time, an increase in the effect by thermal reclamating 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 reclamer (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 reclamer may be fluidised or not; however in order to obtain uniformly thermal reclaimed sand, the waste mold sand is preferably fluidised. The thermal reclamer 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 reclamer.
Various structures are known for continuous fluidised thermal reclamers. Examples thereof include a thermal reclamer in which the flow direction of sand intersects with the flow direction of air for fluidisation of sand and a thermal reclamer in which the directions are opposing and parallel. In view of the thermal efficiency, the latter thermal reclamer is more preferable. Particularly, a thermal reclamer 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 reclamating can be decreased.

In the thermal reclamer 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 reclamer and falls through the thermal reclamer. 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 reclamer. The sand retained at a certain position is thermal reclamated 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 reclamer as thermal reclaimed sand. This type of thermal reclamer 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, carbides on the surface of the thermal reclaimed sand are eliminated and thus the waste mold sand can be recycled as the artificial sand 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 (trough polisher) 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 wet method. By carrying out wet attrition, the calcium ion elution from the recycled artificial sand can be easily adjusted at or lower than 40 mg/L.
When conventional binder-containing foundry sand containing calcium stearate as the lubricant is repeatedly recycled from the waste mold sand by dry attriting, the mold strength is reduced, which is disadvantageous. The inventors of the present invention have measured the calcium ion elution of the artificial sand after repetitive use, and found that the calcium ion elution increases with an increase in the number of recycle and the calcium ion elution correlates with the mold strength. Thus, the inventors have found that prevention of accumulation of calcium in the recycled artificial sand by eliminating calcium ions by wet attriting can prevent a reduction in the mold strength.
The above calcium ion elution can be also obtained by using the lubricant devoid of calcium components such as a salt of a higher fatty acid with 10 to 24 carbon atoms and a monovalent metal (for example, potassium stearate, sodium stearate). Use of such the lubricant, which does not contain calcium, can decrease the calcium ion elution without undergoing the wet attrition.

(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 fluidising 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.

(Binder-containing foundry sand)
The binder-containing foundry sand includes the aggregate, the binder and the lubricant.
(1) Aggregate
The aggregate includes the artificial sand described above as a main component. The proportion of the contained artificial sand is preferably 50% by weight or more, more preferably 70% by weight or more and still more preferably 90% by weight or more. The aggregate may consist essentially of the artificial sand.
Examples of sand which may be contained other than the artificial sand include silica sand, zircon sand, chromite sand, MgO/SiO₂-containing sand and mixed sand thereof.

(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 a hardening agent according to a type of the binder. Examples of the hardening agent for a 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 an alkaline phenolic resin include lactones (for example, propiolactone) and organic esters such as ethyl formate, methyl formate and triacetin. Examples of a hardening agent for a phenolic resin include hexamethylenetetramine and the like. Examples of the hardening agent for a phenolic urethane resin include triethylamine and pyridine-containing compounds. Examples of the hardening agent for 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 3 parts by weight per 100 parts by weight of the aggregate. When the content is less than 0.4 parts by weight, bonding of the aggregate may not be sufficient, resulting in a reduction in the mold strength. When the content is more than 3 parts by weight, a component derived from the binder may adhere on the surface of a casting or the time required for recycle of the binder-containing foundry sand from the waste mold sand may be increased. The content is more preferably 0.4 to 2 parts by weight and still more preferably 0.4 to 1.5 parts by weight.
The artificial sand having the above-specified surface area may require less amount of the binder than natural sand. 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.

(3) Lubricant
The lubricant is not particularly limited and any lubricants generally used in the art may be used. Examples of the lubricant include a metal salt of a higher fatty acid with 10 to 24 carbon atoms. In view of availability and costs, calcium stearate can be generally used.
It is preferable that the lubricant is contained in the range of 0.01 to 0.2 parts by weight relative to 100 parts by weight of the sum of the aggregate and the binder. When the content is less than 0.01 parts by weight, the binder-containing foundry sand does not have sufficient flowability for mold manufacturing, resulting in an increased time required for mold manufacturing or castings having rough surfaces. Further, the binder-containing foundry sand may cause blocking in flexible container bags in which products are stored. When the content is more than 0.2 parts by weight, the mold strength may be reduced or the time required for recycle of the binder-containing foundry sand from the waste mold sand may be increased. The content is more preferably 0.02 to 0.16 parts by weight, still more preferably 0.03 to 0.12 parts by weight and particularly preferably 0.06 to 0.10 parts by weight.
The amount of lubricant in the binder-containing foundry sand can be measured by isolating the lubricant using an appropriate solvent and analysing the isolated matter by a well-known method such as infrared spectroscopic analysis, gas chromatography, liquid chromatography or NMR.

(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 artificial sand which serves as the aggregate in a mixer, the binder is charged into a mixer to obtain a mixture of the binder and the aggregate. When the binder is the one obtained by hardening a curable resin with the hardening agent, the curable resin is first charged into a mixer followed by the hardening agent. The lubricant is then charged into the mixer to mix the mixture of the binder and the aggregate with the lubricant. It is believed that the binder coats all or a part of the surface of the aggregate. It is also believed that the lubricant coats all or a part of the surface of the aggregate coated with the binder.

Example 1
(1) Production of binder-containing foundry sand
Artificial sand used which served as an aggregate was 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 weight of the synthetic mullite and 10% by weight of the synthetic corundum with alumina and silica; being in total at 94% by weight (alumina: 77% by weight and silica: 23% by weight)). The artificial sand which was not recycled was called “new sand”. The artificial sand 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 artificial sand at 150°C. While adding 0.8 parts by weight of resin (a novolac phenolic resin available from Hitachi Chemical Co., Ltd.) relative to 100 parts by weight of the artificial sand, the artificial sand was stirred to give a mixture of the resin and the aggregate. While stirring the mixture, 20 parts by weight of hexamethylenetetramine (hardening agent) relative to 100 parts by weight of the resin and 1.3 parts by weight of water (dispersing medium of the hardening agent) relative to 100 parts by weight of the aggregate were then added and obtain a mixture of the binder and the aggregate. Twenty seconds after addition of the hardening agent, cooling was started over about 20 seconds. While stirring the mixture of the binder and the aggregate, 0.06 parts by weight of a lubricant (containing 95% by weight or more of calcium stearate, 95% or more have a particle size of 75 μm or less: available from Kawamura Kasei Industry Co., Ltd.) relative to 100 parts by weight of the mixture of the binder and the aggregate was then added and stirred for about 15 seconds to give binder-containing foundry sand (RCS). The resulting binder-containing foundry sand was sifted on a sieve with the mesh size of 1180 μm to remove aggregates.
The resulting binder-containing foundry sand 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 pulverised and the pulverised material was divided into 10, referred to as n1 to n10. Ten pulverised materials were respectively recycled into the binder-containing foundry sand. The materials n1 to n5 and n9 to n10 were recycled only by a thermal reclamation process and n6 to n8 were recycled by a combination of the thermal reclamation process and a wet attrition process. After being recycled, the bending strength was measured. Before the thermal reclamation process, 10% by weight of new sand was added.
The thermal reclamation process was carried out in a JFE Pipe Fitting Mfg. thermal reclamer (available from JFE Pipe Fitting Mfg., Co., Ltd., Type JTR-G-1) as shown in Fig. 1 under the conditions of a thermal reclamation temperture of 600°C, a flowing differential pressure in the thermal reclamer 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 reclamer, 2 denotes a pulverised material inlet, 3 denotes a burner, 4 denotes a fluidised bed, 5 denotes a heat exchanger, 6 denotes an air inlet for fluidising sand, 7 denotes a cooling air inlet, 8 denotes a fluidised 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 wet attrition process was carried out on a trough attriting apparatus (diameter 609.4 mm x length 1517 mm, 8 impellers, available from Yamakawa Sangyo Co., Ltd.) as shown in Fig. 2 under the conditions of a load current of 130 A, an input of sand of 5 t/hour, water amount relative to input sand of 5 to 10% by weight, rotation of impellers of 86 rpm/min, an angle of impellers of 45° and an area of an impeller of 49100 mm². Under these conditions, the actual processing time of thermal reclaimed sand was 10 to 20 minutes. In Fig. 2, 21 denotes a trough attriting apparatus, 22 denotes a trough, 23 denotes an impeller, 24 denotes a thermal reclaimed sand inlet, 25 denotes an outlet of recycled sand, 26 denotes a spring, 27 denotes a lid, 28 denotes a joint, 29 denotes a rotation axis, 30 denotes a decelerator and 31 denotes a motor.

(4) Measurement of bending strength for recycled binder-containing foundry sand
In the same manner as in the step (1), the binder-containing foundry sand was produced and the bending strength was measured in the same manner as in the step (2). The resulting bending strength is shown in Table 1.
Table 1 also shows the calcium ion elution of the new sand and the recycled artificial sand. The calcium ion elution was measured according to the following procedures.
Meanwhile, the surface area per volume, particle size distribution, particle shape factor and contents of alumina and silica were almost the same between the recycled aggregate and the new sand.

(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 bending strength and the calcium ion elution (in a 0.05 M HCl aqueous solution) based on Table 1 is shown in Fig. 3.
From Table 1 and Fig. 3, a tendency can be seen wherein the bending strength decreases with an increase in the calcium ion elution and becomes constant at the elution of 60 mg/L or lower. Therefore, it is inferred that calcium ions may adversely affect hardening of resin during preparation of the binder (for example, consumption of the hardening agent by the ions, or polymerisation of the resin by chelating the hardening sites of the resin with the ions).

Example 2
The relation between the calcium ion elution and the bending strength of repeatedly recycled artificial sand was investigated. Specifically, the calcium ion elution was measured in the same manner as in Example 1 for the artificial sand recycled only by thermal reclamating and the artificial sand recycled by thermal reclamating followed by wet attriting, and the bending strength of the binder-containing foundry sand was measured in the same manner as in Example 1. The results are shown in Table 2. Table 2 also shows the calcium ion elution of the new sand and the bending strength of the binder-containing foundry sand derived from the new sand. The conditions for thermal reclamating and wet attrition and the conditions for production of the binder-containing foundry sand were the same as those in Example 1.

The relation between the bending strength and calcium ion elution (in a 0.05 M HCl aqueous solution) (vertical axis) and the number of recycle (horizontal axis) based on Table 2 is shown in Fig. 4. The vertical axis of the bar graph in Fig. 4 represents the bending strength, a bar on left hand side of the recycle number represents the bending strength of the binder-containing foundry sand recycled only by thermal reclamating and a bar on the right hand side of the recycle number represents the bending strength of the binder-containing foundry sand recycled by thermal reclamating and wet attriting. The line plot represents the calcium ion elution, the plot with Δ represents the calcium ion elution of the artificial sand recycled only by thermal reclamating and the plot with O represents the calcium ion elution of the artificial sand recycled by thermal reclamating and wet attriting.
From Fig. 4, a tendency can be seen wherein the bending strength decreases with an increase in the calcium ion elution and becomes constant at the elution of 60 mg/L or lower. It is found that in order to maintain the elution at or lower than 60 mg/L, it is suitable to include wet attrition in recycle process of the binder-containing foundry sand.

References Signs List

1: Thermal reclamer, 2: pulverised material inlet, 3: burner, 4: fluidised bed, 5: heat exchanger, 6: air inlet for fluidising sand, 7: cooling air inlet, 8: fluidised cooler, 9: sand discharge valve, 10: air nozzle, 11: flowing differential pressure gauge, 12: exhaust gas outlet, 21: trough attriting apparatus, 22: trough, 23: impeller, 24: thermal reclaimed sand inlet, 25: outlet of recycled sand, 26: spring, 27: lid, 28: joint, 29: rotation axis, 30: decelerator, 31: motor.

Claims

Artificial sand comprising, for use as a material of binder-containing foundry sand, as a main component, synthetic mullite and/or synthetic corundum containing 40 to 90% by weight of alumina and 60 to 10% by weight of silica;
having a particle size distribution of 30 to 1180 μm;
having 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); and
having a calcium ion elution in a 0.05 M HCl aqueous solution of 60 mg/L or less.
The artificial sand according to claim 1, which is derived from waste mold sand generated after casting.
The artificial sand according to claim 1, which has a particle shape factor of 1.2 or less.
The artificial sand according to claim 1, which is used for shell molding process.
Binder-containing foundry sand comprising an aggregate containing artificial sand as a main component, a binder and a lubricant, wherein the artificial sand is the artificial sand according to claim 1.
The binder-containing foundry sand according to claim 5, wherein the binder is selected from a furan resin, a phenolic resin, an oil-modified urethane resin, a phenolic urethane resin, an alkaline phenolic resin, silicate of soda and bentonite, and the lubricant is calcium stearate.
The binder-containing foundry sand according to claim 6, wherein the binder is contained in the range of 0.4 to 3 parts by weight relative to 100 parts by weight of the aggregate, and the lubricant is contained in the range of 0.01 to 0.2 parts by weight relative to 100 parts by weight of the sum of the aggregate and the binder.