JP7215670B2 - Firing products containing calcium oxide - Google Patents

Description

TECHNICAL FIELD The present invention relates to a baked product containing calcium oxide, a method for producing the same, and the like. In particular, the present invention relates to a biological material baked product containing calcium oxide obtained from a biological material containing calcium carbonate, a method for producing the same, and the like.

In recent years, studies on these baked products have been conducted as a method of utilizing waste such as shells, eggshells, and sea urchin shells. The main component of these wastes is calcium carbonate, and it is said that calcium oxide, which is produced by liberating carbon dioxide from calcium carbonate by high-temperature firing, is the main component of the fired products.

Conventional calcium oxide (quicklime), which is made by burning limestone, instantly emits a high heat that exceeds 100 ° C when it comes into contact with water that is five times its mass. No exothermic reaction of hydration is observed. One of the factors is thought to be that calcium oxide was reduced by reacting with water vapor and carbon dioxide in the air during various processes.

While conventional limestone-derived calcium hydroxide (slaked lime) is highly toxic, calcium hydroxide derived from burned biological materials such as shells is highly safe and has low skin irritation, making it a food additive. approved as. On the other hand, it has been reported that calcium hydroxide derived from baked biological materials such as shells has an organic poison adsorption effect, a bactericidal effect, a virus inactivating effect, and the like (for example, Patent Documents 1 to 4). Thus, calcium hydroxide derived from baked products such as shells, eggshells, and sea urchin shells has already been reported and marketed as a material for sanitary environments.

Japanese Patent Application Laid-Open No. 2001-233720 JP 2008-179555 A JP 2012-062257 A Japanese Patent No. 5019123

In the past, the use of calcium hydroxide was intended for baked products such as shells, and it was possible to obtain a fine powder of the baked products containing a large amount of calcium hydroxide. On the other hand, baked products mainly composed of calcium oxide can be expected to exhibit novel properties such as oxidation/reduction/radical reactions in addition to strong alkaline properties. However, such a baked product mainly composed of calcium oxide could not actually be produced.

The present invention was made against the above background. A first object is to provide a baked product with a high content of calcium oxide. A second object is to provide the fired product which can be easily pulverized into fine powder.

The present inventors completed the present invention as a result of earnest research.

According to a first aspect of the present invention, a method for producing a calcined product containing calcium oxide from a starting material containing calcium carbonate and/or calcium hydroxide, comprising:
A primary firing step of firing the starting material at 1000° C. or higher for 4 hours or longer to obtain a primary fired product;
A fine pulverization step of finely pulverizing the primary fired product;
A secondary firing step of firing the primary fired product at 600° C. or higher for 1 hour or more to obtain a secondary fired product;
A secondary cooling step of cooling the secondary fired product to the outside temperature in a vacuum atmosphere or an inert gas atmosphere;
A method is provided comprising:

The method may not include the step of pulverizing the fired product after the secondary cooling step.

Part or all of the starting material may be shells. Also, part or all of the shell may be a scallop shell.

According to a second aspect of the present invention, there is provided a baked product produced by the above method. The average particle size of this baked product may be 2 μm or less. The weight loss ratio at 30 to 1000° C. measured by differential thermogravimetric analysis of the baked product may be 1% or less. The BET specific surface area of this baked product may be 0.5 m ² /g or more and 3.0 m ² /g or less.

According to a third aspect of the present invention, there is provided a suspension in which the baked product is suspended in an aqueous medium.

According to a fourth aspect of the present invention, there is provided a method of preserving the fired product under a vacuum atmosphere or an inert gas atmosphere.

The fired product of the present invention has a very high content of calcium oxide. Also, this fired product is a fine powder having a small average particle size. When this baked product was actually used, it was possible to obtain better effects than conventional products.

The test results of the adsorption removal effect evaluation for lubricating oil are shown. Test results of evaluation of adsorption and removal effects for odors are shown. The test result of bactericidal effect evaluation is shown.

The present invention will be described in detail below.

<<Definition of Terms>>

In the present invention, the term "outside temperature" means the temperature of the ambient environment in which the firing device (firing furnace) is placed. Although it cannot be uniformly defined, it may be interpreted as a temperature of less than 100°C, less than 80°C, less than 60°C, or less than 50°C.

In the present invention, the term "inert gas" means a gas that does not react with calcium oxide at temperatures below 100°C. It includes nitrogen gas, noble gases such as helium and argon, as well as oxygen.

In the present invention, the term "acid region" means a region with a pH of 6 or less. The term "neutral range" means the range of pH greater than 6 and less than 8. "Alkaline region" means a region with a pH of 8 or higher. A pH range of 10 or higher is sometimes expressed as a "strong alkaline range", and a pH range of 8 or more and less than 10 is sometimes referred to as a "weakly alkaline range". In addition, pH in this invention is a value measured at 25 degreeC.

<<starting material>>
The method of the present invention uses materials containing calcium carbonate and/or calcium hydroxide as starting materials. Calcium carbonate and calcium hydroxide change to calcium oxide when heated.

In the present invention, in particular, biological material containing calcium carbonate is used as starting material. Examples of such biological starting materials include seashells, eggshells, sea urchin shells, corals, and crustaceans. Seashells, eggshells and crustaceans are preferred from the viewpoint of availability, operability and workability, and shells are particularly preferred.

Seashell refers to the outer shell of a shellfish containing calcium carbonate. Shellfish are generally classified into monovalves, bivalves, and snails. Bivalve shells are preferred due to their simple construction and ease of cleaning. Abalone etc. are mentioned as a monovalve. Examples of bivalves include scallops, oysters, clams, clams, short-necked clams, and the like. Examples of snail shells include turban shells. Among shells, scallop shells and oyster shells are particularly preferred, and scallop shells are most preferred.

Eggshell refers to the outer shell of an egg containing calcium carbonate. It is a material produced by egg-laying organisms such as birds. Chicken or quail eggshells are preferred from the standpoint of availability.

Crustacean refers to the outer shell of a crustacean that contains calcium carbonate. Crustaceans include shrimp, crabs, barnacles, and the like. Crab shells are preferred because they are easier to wash.

<<Method>>
In a first aspect of the invention, a method is provided as described below.

(Primary firing process)
The method of the present invention includes a primary firing step of firing the starting materials described above in a firing furnace.

The atmosphere in the firing furnace is arbitrary, but an oxygen-containing atmosphere is preferred. The oxygen-containing atmosphere may be an atmosphere containing 1% by volume or more, 3% by volume or more, 5% by volume or more, 10% by volume or more, or 20% by volume or more of oxygen. Usually, it is air (atmospheric atmosphere). In order to increase the combustion removal efficiency, the atmosphere may contain 30% by volume or more, 40% by volume or more, 50% by volume or more, 60% by volume or more, 70% by volume or more, 80% by volume or more, 90% by volume or more, or a pure A low oxygen gas atmosphere (that is, an atmosphere with an oxygen content of 100%) may be used.

In the primary firing step, all or part of the calcium carbonate/calcium hydroxide contained in the starting material is thermally decomposed into calcium oxide. Biological materials may contain organic substances such as proteins. In this step, many elements derived from these organic substances are removed as gases by thermal decomposition or combustion.

The firing temperature in the primary firing step is 1000° C. or higher, 1050° C. or higher, 1100° C. or higher, 1150° C. or higher, 1200° C. or higher, 1250° C. or higher, 1300° C. or higher, 1350° C. or higher, 1400° C. or higher, 1450° C. or higher. By firing at these temperatures or higher, the organic matter can be sufficiently removed, and the purity of the fired product is increased. The firing temperature in the primary firing step is about 2600° C. (melting point of calcium oxide) or less, preferably 2000° C. or less, 1800° C. or less, or 1600° C. or less.

Although there is no particular limitation on the rate of temperature increase in the primary firing step, 1 to 20°C/min, 3 to 18°C/min, 5 to 16°C/min, 7 to 14°C/min, and 9 to 12°C/min are preferable. .

The firing time of the primary firing step is 4 hours or longer, 4.5 hours or longer, 5 hours or longer, 5.5 hours or longer, and 6 hours or longer. Firing for more than these times can sufficiently remove organic matter and increase the purity of the fired product. On the other hand, the upper limit of the firing time is not particularly limited, and is preferably 10 hours or less, 9 hours or less, or 8 hours or less.

As a matter of course, the firing temperature may be constant or variable within the above firing temperature range. The firing time means the total time during which the temperature in the firing furnace is within the above firing temperature range.

(Primary cooling process)
The method of the present invention may include a primary cooling step of cooling the primary fired product obtained by the primary firing step described above.

The cooling target temperature is arbitrary, but usually the baked product is cooled to ambient temperature for subsequent operations.

The cooling rate can be set arbitrarily. It may be rapidly cooled using any cooling means, or may be naturally cooled by heat radiation.

The primary cooling step may be performed under any atmosphere. The atmosphere may be the same as or different from that of the firing process. For example, it may be carried out in an inert gas atmosphere or in an air atmosphere.

The primary cooling step may be performed in the firing furnace, may be taken out of the firing furnace, or may be partially performed in the firing furnace and the rest may be performed outside the firing furnace.

(Preliminary pulverization step)
The method of the present invention may include a preliminary pulverization step of pulverizing the primary fired product.

The preliminary pulverization step is an optional step of pulverizing the primary fired product before pulverizing the primary fired product. In particular, the primary fired product is extremely fragile because the organic matter contained in the biological material has disappeared. Therefore, it can be easily pulverized. Any device and means can be used for the pre-grinding step. The preliminary pulverization step is not an essential step, but by pulverizing the primary fired product in advance, the efficiency of the fine pulverization step described later is improved, which leads to stabilization of the quality of the final product and pulverization.

The pre-grinding step may be performed under any atmosphere. The atmosphere may be the same as or different from that of other processes.

(Purification process)
The method of the present invention may include a purification step for purifying the primary fired product.

The purification step may be performed by passing the powder or fine powder of the primary fired product through one or more filters such as an air filter, a micromist filter, an activated carbon filter, or the like.

The purification step may be performed under any atmosphere. The atmosphere may be the same as or different from that of other processes.

(Fine pulverization process)
The method of the present invention may include a pulverization step of pulverizing the primary fired product.

The fine pulverization step is a step of finely pulverizing the primary fired product to a fine powder state to obtain a finely pulverized primary fired product. It is performed before the secondary firing described later, that is, between the primary firing and the secondary firing. By carrying out before the secondary firing, it was possible to make the average particle size of the fired product, which is the final product, smaller than after the secondary firing.

Any means can be used for pulverizing the primary fired product. For example, there is a device (Nano Jet Mizer; NJ-300-D, manufactured by Aisin Nano Technologies Co., Ltd.) that accelerates high-pressure gas particles with a special compressor and pulverizes an object by particle collision. As the high-pressure gas, dry air may be used, but an inert gas such as nitrogen, helium or argon is preferable.

The pulverization step may be performed under any atmosphere. The atmosphere may be the same as or different from that of other processes.

When the preliminary pulverization step, the fine pulverization step, and/or the purification step are performed, they are performed after the primary firing step and before the secondary firing step, which will be described later.

(Secondary baking process)
The method of the present invention includes a secondary firing step of firing the primary fired product in a firing furnace.

Calcium hydroxide and calcium carbonate remaining in the primary firing process or generated during each process after the primary firing are thermally decomposed into calcium oxide.

Unless otherwise specified, the description of the primary firing step also applies to this step.

The firing temperature in the secondary firing step is 600° C. or higher, 700° C. or higher, 800° C. or higher, 850° C. or higher, 900° C. or higher, and 950° C. or higher. Firing at these temperatures or higher can sufficiently convert calcium carbonate and calcium hydroxide into calcium oxide. The firing temperature in the secondary firing step is about 2600° C. (melting point of calcium oxide) or less, and usually 1500° C. or less, 1200° C. or less, or 1000° C. or less.

The firing time of the secondary firing step is 1 hour or longer, 1.5 hours or longer, or 2 hours or longer. Firing at these temperatures or higher can sufficiently convert calcium carbonate and calcium hydroxide into calcium oxide. On the other hand, the upper limit of the baking time is not particularly limited. 7 hours or less, 6 hours or less, 5 hours or less, 4 hours or less, and 3 hours or less are preferable from the viewpoint of the load on the firing furnace and the energy cost.

(Secondary cooling process)
The method of the present invention includes a secondary cooling step of cooling the secondary fired product obtained by the secondary firing step described above in a vacuum atmosphere or an inert gas atmosphere.

Unless otherwise specified, the description of the primary cooling step also applies to this step.

The vacuum atmosphere means that the pressure is sufficiently lower than the atmospheric pressure (approximately 100,000 Pa). Specifically, it means 1000 Pa or less, 100 Pa or less, 10 Pa or less, 1 Pa or less, 0.1 Pa or less, 0.01 Pa or less, 0.001 Pa or less, and 0.0001 Pa or less.

Under an inert gas atmosphere means under an inert gas atmosphere as described above. There is no limit to the atmospheric pressure, but it may be, for example, 10,000 Pa to 200,000 Pa, 50,000 Pa to 150,000 Pa, or 80,000 Pa to 120,000 Pa or more.

By cooling in a vacuum atmosphere or an inert gas atmosphere, the calcium oxide produced by firing can be preserved without reaction. In the case of a vacuum atmosphere, free gas generated in the secondary firing step is removed by the vacuumizing means, and calcium oxide in the fired product is preserved. In addition, it was found that the particle size of the fired product is particularly small. Therefore, a vacuum atmosphere is preferable.

It is preferable not to include other steps (for example, pulverization step) between the secondary cooling step and the sealing step described later. This is because the calcium oxide in the fired product may deteriorate during the other steps.

(Sealing process)
The method of the present invention may include a step of sealing the obtained secondary baked product in a sealed container.

Any sealing container can be used as long as it can shut off gas and seal the object. The secondary fired product is sealed in a sealed container under a vacuum atmosphere or an inert gas atmosphere. As a result, a package is completed in which the secondary baked product is enclosed in the sealed container.

<<Baked product>>
A second aspect of the present invention provides a fired product produced by the method of the first aspect.

The average particle size of the fired product of the present invention is usually 2.0 μm or less, 1.9 μm or less, or 1.8 μm or less.

The average particle size of the fired product may be measured using a particle size distribution analyzer. Such a device includes, for example, CILAS (available from Aisin Nanotechnologies Co., Ltd.).

The proportion of calcium element in the elements measurable by wavelength dispersive X-ray fluorescence spectroscopy (XRF) of the fired product of the present invention is usually 99.0 atom% or more, 99.1 atom% or more, and 99.2 atom%. 99.3 atom % or more, 99.4 atom % or more, 99.5 atom % or more, 99.6 atom % or more, 99.7 atom % or more, 99.8 atom % or more, 99.9 atom % or more. Carbon and oxygen are not measured by wavelength dispersive X-ray fluorescence spectroscopy (XRF).

In addition/apart from this, the peak intensity of elements other than calcium measured by X-ray fluorescence spectroscopy (XRF) of the fired product of the present invention is 1/1000 or less of the peak intensity of calcium. /1500 or less, 1/2000 or less, 1/2500 or less, or 1/3000 or less.

RIX3100 (manufactured by Rigaku Denki Kogyo Co., Ltd.) is exemplified as an apparatus for wavelength dispersive X-ray fluorescence spectroscopy (XRF).

The calcium oxide, calcium hydroxide, and calcium carbonate contents of the calcined product are estimated using a differential thermal calorimetric analyzer. When the fired product contains calcium hydroxide, a weight loss is observed between 200°C and 500°C. When the fired product contains calcium carbonate, a weight loss is observed at 500°C to 1000°C. No pretreatment is required prior to differential thermal thermogravimetric analysis.

The weight retention ratio at 30 to 1000 ° C. measured by differential thermogravimetric analysis of the fired product of the present invention is usually 99.0% or more, 99.1% or more, 99.2% or more, 99.3% Above, 99.4% or more, 99.5% or more, 99.6% or more. The upper limit is usually 100% or less. The weight retention rate is the percentage of the weight at 1000°C to the weight at 30°C.

In other words, the weight loss ratio at 30 to 1000° C. measured by differential thermogravimetric analysis of the baked product of the present invention is usually 1% or less, 0.9% or less, 0.8% or less, 0.8% or less. 7% or less, 0.6% or less, 0.5% or less, or 0.4% or less. The weight loss rate is the percentage of weight loss from 30°C to 1000°C relative to the weight at 30°C.

An example of such a device is TGA851e (manufactured by Mettler Toledo). Differential thermal thermogravimetric analysis is performed by heating from 30° C. to 1000° C. at a rate of 10° C./min in a nitrogen stream of 100 mL/min.

The BET specific surface area of the fired product of the present invention is usually 0.2 m ² /g or more, 0.3 m ² /g or more, 0.4 m ² /g or more, 0.5 m ² /g or more, 0.6 m ² /g or more, 0.7 m ² /g or more, 0.8 m ² /g or more, 0.9 m ² /g or more, 1.0 m ² /g or more, 1.1 m ² /g or more, 1.2 m ² /g or more 1.3 m ² /g or more, 1.5 m ² /g or more, 1.7 m ² /g or more, 1.9 m ² /g or more, 2.0 m ² /g or more. On the other hand, it is usually 3.0 m ² /g or less, 2.8 m ² /g or less, 2.6 m ² /g or less, 2.4 m ² /g or less, 2.2 m ² /g or less, 2.1 m ² /g or less. g or less.

An example of an apparatus for analyzing the BET specific surface area is ChemBET3000 manufactured by Quantachrome. The method for measuring the BET specific surface area is not particularly limited, and the measurement may be carried out under commonly used conditions.

The calcined product of the present invention has excellent adsorption ability and can be used for adsorption of harmful substances.

The calcined product of the present invention can be used by suspending it in an aqueous medium.

(aqueous medium)
90% by mass or more, 91% by mass or more, 92% by mass or more, 93% by mass or more, 94% by mass or more, 95% by mass or more, 96% by mass or more, 97% by mass or more, 98% by mass or more, 99% by mass of the aqueous medium % or more is water. Of course, 100 mass % of the aqueous medium may be water (that is, the aqueous medium may be pure water).

The medium other than water is not particularly limited as long as it is a liquid soluble in water. Alcohols such as methanol, ethanol, propanol and butyl alcohol are typical examples.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to these Examples.

(starting material)
Scallop shells were used as the starting material.

(Example)
The starting material was pre-calcined at 1450° C. for 6 hours. After the firing, the primary fired product was left to cool naturally to the ambient temperature. The primary fired material cooled to the ambient temperature was preliminarily pulverized and stirred to homogenize. After passing through an air filter, a micromist filter, and an activated carbon filter, the primary fired product was pulverized by a dry ultra-pulverization system (nano jetmizer). After that, the fired product was subjected to secondary firing at 950° C. for 2 hours. After the firing was completed, the secondary fired product was allowed to stand in a vacuum atmosphere (10 ⁻⁴ Pa or less) using a vacuumizing means to naturally cool to the ambient temperature. In this way, a powdery baked product of Example was obtained.

(Comparative example 1)
The starting material was pre-calcined at 1450° C. for 6 hours. After the firing, the primary fired product was left to cool naturally to the ambient temperature. The primary fired material cooled to the ambient temperature was preliminarily pulverized and stirred to homogenize. This baked product was secondarily baked at 1450° C. for 4 hours. This secondary fired product was allowed to stand and allowed to cool naturally to the ambient temperature. After secondary cooling, it was pulverized in the same manner as in the example. Thus, a powdery baked product of Comparative Example 1 was obtained.

(Comparative example 2)
The starting material was pre-calcined at 1450° C. for 6 hours. After the firing, the primary fired product was left to cool naturally to the ambient temperature. The primary fired material cooled to the ambient temperature was preliminarily pulverized and stirred to homogenize. In this way, a powdery baked product of Comparative Example 2 was obtained.

(Comparative Example 3)
The starting material was pre-calcined at 1100° C. for 4 hours. After the firing, the primary fired product was left to cool naturally to the ambient temperature. The primary fired material cooled to the ambient temperature was preliminarily pulverized and stirred to homogenize. Thus, a powdery baked product of Comparative Example 3 was obtained.

All steps were carried out under an atmospheric atmosphere and atmospheric pressure unless otherwise specified. Also, the rate of temperature increase in the firing process was set to 10° C./min.

(Comparative Example 4)
As the powder in Comparative Example 4, a commercially available baked scallop shell was used.

<<Evaluation of powder>>
The powders of Examples and Comparative Examples were measured by the measurement method described below.

(Particle size)
The average particle size of each powder was measured using a particle size analyzer (CILAS). Table 1 shows the measurement results.

(Calcium oxide content)
A differential thermal calorimetric gravimetric analyzer (TGA851e) was used to measure the calcium oxide content of each powder (analysis temperature ranged from 30°C to 1000°C). The weight loss (%) up to 200 to 500° C. was defined as calcium hydroxide content (%), and the weight (%) maintained at 30° C. to 1000° C. was defined as calcium oxide content (%). Table 1 shows the measurement results.

(Measurement of BET specific surface area)
The BET specific surface area of each powder was measured using ChemBET3000 manufactured by Quantachrome. Table 1 shows the measurement results.

The purity of the powders of the examples is extremely high. In addition, the fired products of Examples had a significantly smaller average particle size than the fired products of Comparative Examples. Therefore, by the method of the present invention, a fine powder having extremely high purity of calcium oxide could be obtained.

Further, each powder was evaluated by the evaluation method described below.

(Adsorption performance: lubricating oil)
Lubricating oil was added to pure water so that the concentration was 1% by volume, and stirred to prepare a lubricating oil suspension (the lubricating oil suspension was pale yellow). To 100 parts by mass of this lubricating oil suspension, 0.04 parts by mass, 0.2 parts by mass and 1 part by mass of each powder were added and suspended. After that, the powder was precipitated by centrifugation at 3000 rpm for 10 minutes, and the turbidity of the supernatant was measured using a turbidimeter (Turbidimeter, TR-55, manufactured by Kasahara Chemical Industry Co., Ltd.). A smaller turbidity means that the lubricating oil has been removed. The results are shown in FIG. 1A. As is clear from the figure, the powders of Examples were found to be the most excellent.

A similar test was performed on supernatants obtained by blending 0.04 parts by mass, 0.2 parts by mass, and 1 part by mass of each powder with 100 parts by mass of pure water. The results are shown in FIG. 1B. As is clear from the figure, the supernatant of the powder of Example was found to be the best.

(Adsorption performance: Odor 1)
10 g of minced pork is sealed with 5 mL of supernatant obtained by blending 0.04 parts by mass, 0.2 parts by mass, and 1 part by mass of each powder with 100 parts by mass of pure water, and left at 37 ° C. for 3 days. bottom. After 3 days, the odor was measured using an odor meter (Handheld Odor Meter, OMX-SR, manufactured by Shinei Technology Co., Ltd.). The results are shown in Figure 2A. As is clear from the figure, the supernatant of the powder of Example was found to be the best.

(Adsorption performance: Odor 2)
Sawdust from used rat breeding was prepared as a deodorizing object. 0.04 g, 0.2 g and 1 g of each powder were thoroughly mixed with 10 g of this sawdust and allowed to stand for 30 minutes. The odor after 30 minutes was measured using the odor meter. The results are shown in Figure 2B. As is clear from the figure, the powders of Examples were found to be the most excellent.

(Bactericidal performance)
Pure water suspensions were prepared with mass concentrations of each powder of 1600 ppm, 800 ppm, 400 ppm, 200 ppm and 100 ppm. The bactericidal performance of this suspension was evaluated.

2% DMEM medium (D5796, Sigma Life Science, Sigma-Aldrich Japan, Tokyo) was added to the turbid water of the pond and left at room temperature for 18 hours to culture the general viable bacteria and coliforms to be sterilized. The viable and coliform counts were 4.6 and 4.2 (Log ₁₀ CFU/mL), respectively.

100 parts by volume of the above aqueous suspension was added to 100 parts by volume of the liquid containing viable bacteria and coliform bacteria, and the mixture was stirred well and then allowed to stand at room temperature for 30 minutes. Using culture medium kits for measuring the number of viable bacteria and coliform bacteria (compact dry "Nissui" TC and CF, respectively, manufactured by Nissui Pharmaceutical Co., Ltd.), the viable bacteria count and the coliform bacteria count were measured. The general viable count results are shown in FIG. 3A, and the coliform count results are shown in FIG. 3B (the horizontal axis is the final concentration of the powder). As is clear from the figure, it was found that the powder suspensions of Examples were the most excellent.

Claims (7)

A method for producing a fired product containing calcium oxide from a starting material containing calcium carbonate and/or calcium hydroxide, comprising:
A primary firing step of firing the starting material at 1000° C. or higher for 4 hours or longer to obtain a primary fired product;
A fine pulverization step of finely pulverizing the primary fired product;
A secondary firing step of firing the primary fired product at 600° C. or higher for 1 hour or more to obtain a secondary fired product;
A secondary cooling step of cooling the secondary fired product to the outside temperature in a vacuum atmosphere or an inert gas atmosphere;
method including. 2. The method according to claim 1, which does not include a step of pulverizing the fired product after the secondary cooling step. A method according to any one of claims 1 to 2, wherein part or all of the starting material is shell. 4. The method of claim 3, wherein part or all of the shell is scallop shell. A baked product produced by the method according to any one of claims 1 to 4. A suspension obtained by suspending the fired product according to claim 5 in an aqueous medium. A method of storing the fired product according to claim 5 in a vacuum atmosphere or an inert gas atmosphere.

Images