JPH0463008B2 - - Google Patents

Description

[Detailed description of the invention]

(1) Technical Field of the Invention The present invention relates to a method for producing high-purity cristobalite. More specifically, it relates to a method for producing high-purity cristobalite, which is used as a glass-forming material and refractory.In addition, it is highly pure, highly crystalline, and has low water absorption and is used as a mold material for metal casting. The present invention relates to a method for producing cristobalite, which requires a small water mixing ratio in order to provide sufficient thermal expansion and maintain sufficient strength as a molding material, and which exhibits sufficient fluidity at a low water mixing ratio. (2) Prior Art Silica has many types of transformations, but the main transformations are quartz, tridymite, cristobalite, and silica glass. The interrelationships of each metamorphosis can be summarized as follows.

[Table] The αβ type transformation occurs almost instantaneously, but the transition between different types of transformation is extremely slow and requires a sufficiently long hold at the transition temperature or the coexistence of a mineralizing agent to promote it. do. The applications of these metamorphoses are extremely wide-ranging;
Among these, cristobalite has α at 180℃ to 270℃.
Because it shows a remarkable volume change of 2% to 5% when the crystal transitions from the mold to the β-form, it is used as a mold material for metal casting, in precision casting of dentistry, ornaments, and other metals, and as a glass forming material. It is also used as a refractory material. Normally, natural silica occurs mostly in the form of quartz, and cristobalite is relatively rare in nature and is only found in volcanic silica along with amorphous silica. Furthermore, natural cristobalite often contains other metamorphoses such as tridymite and quartz, and impurities such as sulfur alumina. Therefore, high-purity cristobalite is produced from siliceous raw materials such as silica stone and silica sand by heat treatment utilizing the above-mentioned mutual relationships. However, it is difficult to industrially obtain pure cristobalite with a low content of quartz, tridymite, and glass from these siliceous raw materials, and at present, there are almost no conventional techniques for producing it. On the other hand, it is well known that crystal transition is promoted by adding a mineralizing agent to the formation of quartz or tridymite cristobalitization, and research on the conversion of quartz silica to cristobalitization when mineralizing agents are added has been conducted for a long time. It is being done. but,
Even if a mineralizing agent is added, it takes a considerable amount of time to complete the transition, and it has little utility value from an industrial standpoint. As described above, cristobalite has a wide variety of uses, and cristobalite, which is used as a mold material for metal casting, is required to have many physical properties. To list them first, it must have high purity; it must have thermal expansion sufficient to compensate for the shrinkage during casting of metal; this expansion is composed of both solidification expansion and thermal expansion; What matters is thermal expansion.
This thermal expansion depends not only on the purity of cristobalite but also on its crystallinity. In other words, not only silica glass but also cristobalite that does not contain other siliceous transformations cannot obtain a sufficient coefficient of thermal expansion unless it exhibits sufficient crystallinity; furthermore, it must have sufficient strength as a mold material; Usually, cristobalite is used after being kneaded and hardened with gypsum hemihydrate, water, or other binders, but in order to impart sufficient strength, the mixing ratio of water and water must be low. The water mixing ratio also affects the particle size composition of cristobalite, but the particle physical properties require low water absorption, which requires low porosity; in addition, low water miscibility. , and must have good workability during kneading. That is, good fluidity is required. As a mold material, cristobalite is required to have the above-mentioned physical properties. It has been mentioned above that it is difficult to obtain high purity cristobalite from other siliceous transformations. Of course, high-purity cristobalite can be obtained as long as time is spent at the temperature at which siliceous transformation transforms into cristobalite. However, in this case, it takes several hours or more than 10 hours in some cases, and even if the mineralizing agent is added, the effect cannot be expected to be great. Of course, high-purity cristobalite can be produced from various siliceous raw materials by long-term firing in a tunnel kiln or discontinuous furnace. Furthermore, it is expected that it will be possible to produce cristobalite that requires many properties so that it can be used as a molding material. (3) Summary of the invention However, in addition to the above-mentioned method of firing at high temperatures and over a long period of time, the present inventors have conducted various studies on methods for producing high-purity cristobalite in a short time from silica raw materials such as silica stone and silica sand. layered. As a result, high-purity cristobalite can be produced in an extremely short time (specifically, within a few minutes to 30 minutes) by firing the raw material powder, which has a fineness of 4000 cm ² /g or more in Blaine specific surface area, at a temperature that transforms it into cristobalite. It was discovered that it is possible to produce In addition, by firing the siliceous raw material powder having the above-mentioned fineness at a temperature that transforms it into cristobalite after molding with a binder, it not only becomes easier to handle the raw materials and fired products in the manufacturing process, but also makes it easier to handle the raw materials and fired products in the manufacturing process. In addition, by adding a mineralizing agent to the siliceous raw material powder having the above-mentioned fineness and then firing it at a temperature at which it transforms into cristobalite, the conversion of silica stone, silica sand, or amorphous silica to cristobalite is promoted. , high-purity cristobalite can be obtained in a shorter time, and furthermore, a mineralizing agent is added to the siliceous raw material having the above-mentioned fineness, and this raw material composition is molded in the presence of a binder or mixed with a mineralizing agent. By adding an additive that acts as both a binder and firing at a temperature that transforms into cristobalite after molding, it becomes easier to handle the raw materials and fired products during the manufacturing process, and it also takes less time to convert into cristobalite. We found that it is possible to shorten the time. Furthermore, we investigated various methods for producing cristobalite for use as mold material, which can be produced in a short time and which can use a rotary kiln as the firing furnace. As a result, when producing cristobalite for mold materials from quartz silica, the fineness of the quartz silica raw material is adjusted to a Blaine specific surface area of 5000 cm ² /g or more, and a binder or a Blaine specific surface area of 5000 cm ² /g or more is added. A raw material composition is formed by adding a binder and a mineralizer or an additive that acts as both a binder and a mineralizer to the raw material powder, and is fired in a rotary kiln whose burning point temperature is the temperature at which it transitions to cristobalite. By doing this, it was discovered that highly pure cristobalite could be obtained in an extremely short time, and by crushing it, it was possible to produce high-purity, high-performance cristobalite for mold materials that had various properties suitable for use as mold materials. did. (4) Detailed description of the invention The siliceous raw materials used in the production method of the present invention are silica stone,
Silica sand or amorphous silica can be used. As the raw material for the mold material, quartzite silica such as silica stone and silica sand is used. Therefore, the desired purity and color of the product,
Various raw materials may be selected depending on the purpose. A commonly considered method for obtaining cristobalite from these raw materials is to perform firing at a temperature at which the material transforms into cristobalite. The present inventors also conducted an experiment for producing cristobalite according to this method. Usually, silica silica such as silica stone or silica sand, or amorphous silica such as clay or proteinaceous silica is often obtained in the form of lumps or granules. These lumpy and granular raw materials were fired at a temperature that transforms them into cristobalite, that is, 1500°C or more and 1713°C or less. Of course, if it is quartz silica, it will be 1250℃.
A transition from β-quartz to β-cristobalite occurs at 1470°C, and tridymite also transforms to β-cristobalite at 1470°C, but these transitions are also called slow-purification type transitions, and even if the transition point is reached, it is difficult to transition. It is well known that it does not. Further, even if the transition point is exceeded, the transition speed does not increase significantly. However, in order to control these transitions only by temperature and time, it is best to not only make the time as long as possible, but also to bake at a temperature as high as possible before the transition to silica glass occurs. The above-mentioned lumpy or granular siliceous raw material is
As expected, cristobalite was fired at a temperature of 1500°C or higher but below the temperature at which it transitions to a glass phase, but the firing rate was extremely slow. For example, when massive silica stone with a particle size of 3.36 mm to 6.73 mm was heated at 1600°C for 1 hour, the conversion rate to cristobalite was less than 10%, and it took more than 5 hours to complete the conversion. In addition, in the case of silica sand with an average particle size of 0.5 mm, it is necessary to complete the transition at 1600℃.
It took time. In addition, assuming the case of firing in a rotary kiln, we have determined the average particle size, which is the particle size limit that will not scatter in the kiln.
For 0.15mm silica stone, it took about 2 hours at 1600℃ to convert to cristobalite. Furthermore, experiments were also conducted to convert silica powders of finer particle size, ie, average particle diameters of 70 μm and 40 μm, into cristobalite, and the time required for cristobalite conversion was approximately 2 hours for both cases. The above experimental examples are the results when using quartz silica, and the rate of cristobalite conversion varies depending on the raw material, but in all cases, the temperature is 1600℃.
High purity cristobalite could not be obtained without heating for several hours. On the other hand, it was found that converting white clay and proteinaceous amorphous silica containing tridymite in addition to quartz to cristobalite is more difficult than quartz silica, and requires higher temperatures and longer times. The present inventors investigated various methods for completing the metastasis in a shorter time, specifically within 30 minutes. First, we investigated the addition of mineralizing agents, which are effective in promoting metastasis. It is well known that alkali and alkaline earth metals are effective in promoting transition. As a result of examining alkali oxides such as soda carbonate and water glass, which were thought to be particularly effective as mineralizing agents, it was confirmed that they indeed promoted metastasis.
However, the transfer time is not drastically shortened, and is only shortened by about 10 to 30%. For example, it takes about 2 hours at 1600℃ to turn silica stone with an average particle size of 0.15 mm into cristobalite, but it takes about 2 hours at 1600℃, but
When 1.0% is added, the time to complete the transition is about 90 minutes.
On the other hand, if the amount of mineralizer added increases, the transition rate will increase, but a glass phase will tend to form, which will not only reduce the purity of cristobalite, but also lead to the formation of minerals other than cristobalite due to the reaction with the mineralizer component. There is a possibility. In particular, since cristobalite used as a mold material is required to have a high coefficient of thermal expansion, the formation of a glass phase with a low coefficient of thermal expansion must be avoided as much as possible. In order to produce high-purity cristobalite from various siliceous raw materials in a short period of time, we investigated the firing temperature and the addition of mineralizing agents as described above, but the results were not satisfactory. However, from numerous experimental examples, we have found that the conversion of siliceous raw materials to cristobalite is not simply influenced by the firing temperature, firing time, or the presence or absence of mineralizing agents. In other words, it was found that the fineness of the raw material has a considerable influence on the formation of cristobalite. The fineness in this case refers to powder in units of micrometers rather than millimeters, and when turning a fine powder raw material into cristobalite, the rate of transformation varies extremely depending on the degree of fineness. The relationship between cristobalite conversion of quartz silica such as silica stone and silica sand and raw material powder will be explained in detail using examples below. FIG. 1 shows the cristobalite conversion rate of quartz powder having a Blaine specific surface area of 1000 cm ² /g or more. The firing temperature was 1600°C, and the vertical axis was the cristobalite conversion rate of the fired product held at this temperature for 15 minutes. As is clear from the figure, cristobalite formation is greatly influenced by the fineness of the raw material. For example, when the Blaine specific surface area is 2000 cm ² /g, the cristobalite conversion rate is less than 20%, but when the Blaine specific surface area is 2000 cm 2 /g,
In terms of cm ² /g, it is approximately 80%. Next, Figure 2 shows the conversion of quartz powder with a Blaine specific surface area of 1000 cm ² /g or more to cristobalite.
This figure shows the firing time required to complete the crystal transition from quartz to cristobalite at a constant temperature of 1600°C. The fineness required to complete the transfer within 30 minutes is 4000 cm ² /g or more, and about 8000 cm ² /g or more within 15 minutes. In order to obtain high-purity cristobalite from bulk or granular quartz silica, it is necessary to heat and hold the material at a transition temperature for hours on end, and adding mineralizing agents could not be expected to have any significant effect. However, the raw material is 4000cm ² /g
The above powders can be transferred within 30 minutes. The above example is the case of quartz silica,
Similar results can be obtained with amorphous silica such as white clay or proteinaceous silica. The reaction in a normal multi-component solid phase reaction is a diffusion reaction, and the reaction progresses in the contacting parts of the reaction system. Therefore, it is a well-known fact based on many studies that the larger the contact area, that is, the finer the reaction system, the better the reactivity. However, with regard to a one-component rearrangement reaction, there is no research that mentions fine particles in which the particle size of the reaction system is on the order of μm as in the present invention. In addition, from the viewpoint of thermal conductivity, aggregates of molded or non-molded fine raw materials are disadvantageous because they are more porous and have poorer thermal conductivity than bulk or granular raw materials. However, even though it has the above-mentioned disadvantages, the present invention shows that the transition reaction from transformation of SiO ₂ to cristobalite changes significantly in the fine powder region with a Blaine specific surface area of 4000 cm ² /g or more in μm units. This became clear. The steps such as firing of the powdered siliceous raw material will be explained below. Powdered siliceous raw materials may be fired as they are without any treatment, or if you want to make handling of the raw materials and fired products easier, or if you use a rotary kiln for the firing furnace, you can use a binder. After molding,
The firing method is recommended. As the binder, any inorganic or organic glue or any other binder that is normally used for imparting moldability can be used. The amount of binder added varies depending on the type of binder, but adding about 0.1% to 1% usually provides sufficient moldability and strength to the molded product. The method of addition may be either in the form of a solid or an aqueous solution, but if it is added in solid form, mixing and pulverizing at the same time as pulverizing the raw materials will ensure that the binder is uniformly dispersed in the raw materials, improving both moldability and strength of the molded product. . Any of the rolling, extrusion, and compression molding methods can be used as the molding method. The transformation is further promoted by adding mineralizing agents to the powdered siliceous raw material. Compounds containing alkali metals or alkaline earth metals are effective as mineralizers. The amount of mineralizer added varies depending on its type, but a sufficient effect of promoting metastasis can be obtained if it is added in an amount of 0.1 to 5% by weight. The effect of the mineralizing agent is generally proportional to the amount added, but if the amount is 5% or more, the transition will be promoted, but vitrification will occur in part and the purity of the desired cristobalite will decrease, which is not preferable. Furthermore, there is a possibility that minerals other than cristobalite may be produced by reaction with mineralizer components. The mineralizing agent needs to be mixed uniformly into the raw material powder in order to spread its action throughout, and it is preferable to mix and crush the mineralizer at the same time as the raw material is crushed. The addition of a mineralizer promotes the crystal transition from quartz or tridymite to cristobalite, shortening the firing time and lowering the firing temperature. Compared to a product without a mineralizer, a product with 0.1 to 5% by weight of a mineralizer added will have a longer firing time if the same calcination temperature is used to complete the transformation, depending on the amount added. Time is approximately 20% to 50%
If the firing time is the same,
Firing temperatures can be reduced by 5% or more. When a binder and a mineralizing agent are used together, it is preferable in terms of the manufacturing process to use an additive that is the same substance and has the functions of both a binder and a mineralizing agent. That is, water glass and soda carbonate have both the function of a binder that imparts moldability of the powder and the strength of the molded body, and the function of a mineralizer that promotes crystal transition. Any kiln can be used for firing powdered siliceous raw materials or raw material compositions to which binders and mineralizers have been added. A rotary kiln can be used if the raw material composition is molded using a binder. In the present invention, since high-purity cristobalite can be obtained in a short period of time, firing in a rotary kiln is extremely advantageous in terms of manufacturing process and manufacturing cost. Firing conditions vary depending on the type of raw material and whether or not a mineralizing agent is added, but the Blaine specific surface area is approximately 4000.
cm ² /g of quartz silica powder at 1600℃ for 30 minutes, approx.
For 8000 cm ² /g of quartz silica powder, the heating time is about 15 minutes at 1600°C. When the mineralizer is added in an amount of 0.1 to 5% by weight, the firing time is shortened by about 20% to 50%. The cristobalite obtained through the above process is a high-purity cristobalite that can be used as a glass forming material or a refractory. Next, high purity, high crystallinity, and low water absorption are used as mold materials for metal casting, that is, water mixing ratios are adjusted to provide sufficient thermal expansion during metal casting and maintain sufficient strength as mold materials. A method for manufacturing cristobalite, which is small in size and exhibits sufficient fluidity at a low water mixing ratio, from a quartz-based silica raw material in a rotary kiln will be described. First, any quartz silica such as silica stone or silica sand can be used as the raw material. The cristobalite in the present invention is intended to be produced in a short time using a rotary kiln. For this reason, the raw material fineness is set to 5000 cm ² /g or more for the reason described above. The formation of cristobalite from quartz silica having different finenesses is shown in FIGS. 1 and 2. When firing in a regular rotary kiln, it varies depending on the size of the kiln and operating conditions, but
The residence time at the burning point is extremely short. Therefore, in order to obtain high-purity cristobalite in a short residence time, it is desirable that the raw material powder be as fine as possible, and for this reason, the fineness of the raw material must be at least 5000 cm ² /g in Blaine specific surface area. It is.
When firing raw material powder of 5000 cm ² /g or more in a rotary kiln, if it is fed in powder form, large lumps will form due to agglomeration or a coating will form on the inner wall of the kiln, making stable operation of the kiln impossible. Moreover, it is not possible to obtain a product that is satisfactory in terms of quality. Furthermore, it scatters in the kiln, reducing the yield. For this purpose, a binder is used to mold and then sinter. As the binder, any inorganic or organic glue or other binder that is normally used to impart moldability can be used. Details regarding the addition of binder are as described above. To complete the transition even more quickly, a mineralizing agent is added to the powder raw material. Type of mineralizer, amount added,
The addition method is the same as described above, but here special attention must be paid to the amount added. That is, if the amount added is 5% by weight or more, the transition is promoted, but vitrification occurs partially, reducing the purity of the desired cristobalite, and it is also possible that secondary substances are generated due to reaction with the mineralizing agent component. For this reason, the crystallinity of cristobalite decreases, and there is a possibility that sufficient thermal expansion cannot be obtained. The effect of the mineralizing agent varies depending on the amount added, but if the same firing temperature is used to complete the transition, the firing time will be shortened by about 20% to 50%, and if the firing time is the same, the firing time will be reduced by about 20% to 50%. The firing temperature can be lowered by 5% or more. Note that when a mineralizer is used in combination, it also has the effect of a binder. For example, it is preferable to use additives such as water glass and soda carbonate. A rotary kiln is used for firing a molded body made of powdered raw material or a molded body in which a mineralizing agent is added to powdered raw material. In this case, the firing temperature is not sufficient to be the temperature at which quartz transitions to cristobalite, that is, 1250° C., and in order to accelerate the transition rate, it is preferably as high as possible before silica glass is formed. Firing conditions vary depending on the fineness of the raw material and the presence or absence of mineralizing agents, but when the firing temperature is 1600°C, silica powder with a Blaine specific surface area of approximately 5000 cm ² /g will not accumulate during firing. The time is 20 to 30 minutes,
At about 8000 cm ² /g, it takes 10 to 15 minutes. mineralizer
When adding 0.1 to 5% by weight, the baking time is 20%.
It is possible to reduce the time by about 50%. After firing, the clinker is ground to the particle size required as a raw material for molding material. A normal crusher or ball mill can be used for clinker. (5) Effect As described above, in the past, in order to produce cristobalite from silica stone, silica sand, and amorphous silica, high-purity cristobalite could not be obtained unless it was maintained at high temperature for a long time, but with the present invention, cristobalite can be produced in an extremely short time. ,
Moreover, it has become possible to produce high-purity cristobalite, which can be used as a glass molding material and a refractory material. In addition, it has a high purity, high crystallinity, and low water mixing ratio that is used as a mold material for metal casting, and it is mixed with water to provide sufficient thermal expansion during metal casting and maintain sufficient strength as a mold material It has now become possible to produce cristobalite from quartz silica in an extremely short time, with a small water-to-water ratio and sufficient fluidity at a low water-mixing ratio. (6) Examples of the invention Example 1 Massive silica stone having the composition shown in Table 1 below is
By crusher and top grinder
After coarsely crushing to 0.21 mm or less, pot mill (volume: 7,
Grinding media amount: 6.2Kg) for 120 minutes. The Blaine specific surface area of the pulverized raw material was 5660 cm ² /g. The pulverized raw material was held in an electric furnace at 1600°C for 30 minutes. It was confirmed by powder X-ray diffraction that the fired product was almost pure cristobalite with no remaining quartz. Example 2 A block of proteinaceous silica having the composition shown in Table 2 below was crushed to 0.21 mm or less using a Joe crusher and a top grinder, and then 500 g was potmilled (volume: 7, amount of grinding medium: 6.2 kg) at 60
Pulverized for minutes. The Blaine specific surface area of the crushed material is 5100
cm ² /g. 500g of crushed raw materials and 5g of carbonated soda
The pellets were granulated using a pan pelletizer using an aqueous solution containing the following, and then kept in an electric furnace at 1680°C for 10 minutes. It was confirmed by powder X-ray diffraction that the fired product was pure cristobalite containing no other siliceous modifications. Example 3 Massive silica stone having the composition shown in Table 1 below was prepared by
After crushing into pieces with a diameter of 2 mm or less using a crusher and a bantam mill, 100 kg was crushed into a conical ball mill (volume:
1000, grinding media amount 750Kg) for 90 minutes.
The Blaine specific surface area of the pulverized raw material was 7000 cm ² /g. After granulating the pulverized raw material with a No. 1 water glass aqueous solution with a solid content of 0.8% using a pan pelletizer, it was granulated with a diameter of 27 cm.
It was fed into a rotary kiln with a length of 470 cm at a rate of 15 kg/hour. The burning point temperature was about 1600°C, and the residence time at the burning point was about 15 minutes. The fired product is quartz.
It was confirmed by powder X-ray diffraction that it was high purity cristobalite containing 0.5%. The clinker was ground in the Konica Paul mill for 120 minutes. The Blaine specific surface area of the pulverized product was 11260 cm ² /g. After adding and mixing 30% by weight of calcined gypsum to the cristobalite powder obtained above, the mixture was subjected to various tests in accordance with JIST-6601 "Investment materials for dental casting". Postscript 3
The table summarizes the test results, and it was confirmed that each physical property value satisfied the JIS standard value, and that the cristobalite had a high coefficient of thermal expansion and good fluidity. Example 4 Massive silica stone having the composition shown in Table 1 below was
After being coarsely crushed to a diameter of 5 mm or less using a crusher and an impact crusher, it was sent to a continuous ball mill (inner diameter 1.4 m, length 5.7 m, amount of crushing media 6 tons) at a rate of 400 kg/hour. CMC was used as a binder, and 0.5% of this was added when the raw materials were crushed, and the mixture was mixed and crushed. Blaine specific surface area of pulverized raw material is approximately 8000
cm ² /g. The pulverized raw materials are granulated using a pan pelletizer and then sent every hour to a rotary kiln with a diameter of 1.25 mm and a length of 20 m.
Delivered at a rate of 500Kg. The burning point temperature was about 1600°C, and the residence time at the burning point was about 15 minutes. It was confirmed by powder X-ray diffraction that the fired product was high purity cristobalite containing no quartz. The clinker was fed into the ball mill at a rate of 200 kg/hour and pulverized. The Blaine specific surface area of the product is 10500cm ² /
It was hot at g. After adding and mixing 30% by weight of calcined gypsum to the cristobalite powder obtained in this way, JIST
-6601 was used for each test. Table 3 summarizes each test result, and each physical property value is JIS
It was confirmed that cristobalite satisfies the standard values, has a high coefficient of thermal expansion, and has good fluidity.

【table】

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a diagram showing the transition rate from quartz to cristobalite when powdered quartz silica is heated at 1600°C for 15 minutes. Figure 2 shows powdered quartz silica.
It is a diagram showing the time required to complete the quartz→cristobalite transition when heated and maintained at 1600°C.

Claims (1)

[Scope of Claims] 1. A method for producing cristobalite, which comprises firing a raw material powder having a Blaine specific surface area of 4000 cm ² /g or more at a temperature at which it transforms into cristobalite, in producing cristobalite from a siliceous raw material. 2. The manufacturing method according to claim 1, wherein a raw material composition containing a binder in raw material powder having a Blaine specific surface area of 4000 cm ² or more is fired at a temperature at which it transforms into cristobalite after molding. 3. The manufacturing method according to claim 1, wherein a raw material composition containing 0.1 to 5% by weight of a mineralizing agent in a raw material powder having a Blaine specific surface area of 4000 cm ² /g or more is fired at a temperature that transforms into cristobalite. 4. Containing 0.1 to 5% by weight of a mineralizing agent and a binder, or 0.1 to 5% by weight of an additive having the functions of both a mineralizing agent and a binding agent ^, in a raw material powder having a Blaine specific surface area of 4000 cm 2 /g or more. 2. The manufacturing method according to claim 1, wherein the raw material composition is molded and fired at a temperature at which it transforms into cristobalite. 5. The manufacturing method according to claim 3 or 4, wherein the mineralizing agent is a compound containing an alkali or alkaline earth metal. 6 Blaine specific surface area of siliceous raw material is 5000cm ² /
The manufacturing method according to any one of claims 2 to 5, which uses a rotary kiln for firing. 7. The manufacturing method according to claim 6, wherein the cristobalite is a cristobalite grade used as a mold material for metal casting.