Method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand

Abstract

The invention discloses a method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand, and relates to the field of mineral resource recycling, and the method comprises the following steps: adding a quartz sand raw material into a mechanical activation tank, simultaneously adding a strong alkali substance, stirring, grinding and screening, putting quartz powder into a reaction kettle, adding an alkaline solution, and stirring for 30-60 minutes; applying external field energy at a certain temperature to assist a stirring reaction to obtain an intermediate solution, carrying out suction filtration on the intermediate solution to remove precipitated impurities, and removing alkali metal impurities and iron impurities through strong acid type ion exchange resin and iron removal ion exchange resin; according to the method, the natural quartz graded fine sand is pretreated and purified, the impurity content of the quartz fine sand is effectively removed, the pressure of subsequent purification is reduced, meanwhile, a mechanochemical thermal activation technology is utilized, a mechanical grinding, alkali treatment and high-temperature calcination combined process is adopted, the reaction activity of the natural quartz graded fine sand is greatly improved, and the quality of the natural quartz graded fine sand is improved. The hydrolysis degree of the compound in a subsequent alkaline environment is promoted, and the yield of a final product is further improved.

Description

Technical Field

[0001] This invention relates to the field of mineral resource recycling technology, specifically to a method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand. Background Technology

[0002] High-purity quartz products are an indispensable core material for strategic emerging industries. Their applications span key technological links in the semiconductor, fiber optic communication, photovoltaic, and optics industrial chains, serving as the cornerstone of these industries. Historically, both domestically and internationally, high-purity natural quartz sand has been prepared from natural quartz resources such as crystal, vein quartz, and pegmatite through beneficiation, chemical purification, and deep purification. However, as the demand for high-purity quartz sand from strategic emerging industries gradually increases, high-quality natural quartz resources are becoming increasingly depleted. Furthermore, my country's reserves of high-quality natural quartz resources are extremely limited, resulting in a limited production capacity of high-quality quartz sand, which is insufficient to meet the needs of industrial development.

[0003] High-purity synthetic silica sand has gradually attracted attention in recent years. The raw materials for synthetic silica sand are mainly inorganic silicates and organosilicon, prepared through methods such as vapor deposition, precipitation, and sol-gel methods. Chinese patent CN202411725912.5 discloses a method for preparing synthetic silica sand using the sol-gel method, mainly including the following steps: first, mixing silanol salts and alcohol solvents, hydrolyzing under an acidic catalyst, then adding ammonia to promote polycondensation to produce silica gel, followed by sintering and pulverizing to obtain synthetic silica sand. Chinese patent CN202310758103.3 discloses a method for preparing low-carbon chemically synthesized silica sand. The preparation method mainly includes the following steps: preparing an acidic solution using ultrapure water and acid, adding organosilane to obtain silica sol, preparing precursor silicon source seed crystals using ultrapure water and precipitated silica, preparing silica gel by heating, removing alcohol, concentrating and aging, and preparing synthetic quartz sand by drying and sintering in a vacuum environment. All of the above methods obtain high-purity quartz sand through chemical synthesis. However, organosilicon has a high cost, and although inorganic silicates have a relatively low cost, their production process has high carbon emissions, which is extremely unfriendly to the environment. Therefore, the development of new low-carbon silicon sources is of great significance for the research and development and industrialization of synthetic quartz sand.

[0004] Natural quartz ore is a common resource for preparing silicon-based raw materials for photovoltaic and float glass production. However, the grading process during its production generates a large amount of fine sand, which typically has a particle size of ≤0.1mm, low silica content, and high impurity content, especially magnetic impurities, limiting its utilization rate. Current technologies usually use it as mine filler, and some studies have also applied it as fine aggregate or admixture in cement and concrete. However, these utilization methods result in low added value and limited consumption, leading to a waste of graded fine sand resources.

[0005] Therefore, we provide a method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand to solve the above problems. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand. The method utilizes natural quartz graded fine sand as a silicon source precursor and prepares high-purity synthetic quartz sand through a combination of pretreatment, mechanochemical thermal activation, hydrothermal synthesis, drying and aging, surface modification, and multi-atmosphere synergistic dehydroxylation technology.

[0007] To achieve the above objectives, the present invention employs a method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand, comprising the following steps: Step 1: Select natural quartz graded fine sand with SiO2 content >90% as raw material, clean the mud on the surface and dry it for later use. Step 2: Remove iron impurities from the graded fine sand by passing it through a series of magnetic separation processes. Step 3: The magnetically separated and graded fine sand is placed in a reaction vessel, and an acidic solution is added for hot-press leaching at high temperature to further remove impurities and obtain quartz sand raw material. Step 4: Add the quartz sand raw material obtained in Step 3 into the mechanical activation tank, and add a strong alkaline substance at the same time, stir, grind and sieve. The sieve mesh sizes are 400, 800, and 1200 mesh, the grinding time is 1-3 hours, the quartz powder under the sieve is placed in a tube furnace for high-temperature calcination, and after the powder cools naturally, it is thoroughly cleaned to remove impurities and then dried for later use. Step 5: Place the quartz powder into the reaction vessel, add an alkaline solution, and apply external field energy to assist the stirring reaction at a certain temperature to obtain an intermediate solution; Step 6: Filter the intermediate solution from Step 5 to remove precipitated impurities, and then remove alkali metal impurities and iron impurities by passing it through a strong acid ion exchange resin and an iron removal ion exchange resin to obtain a sol. Step 7: Place the sol from step 6 into the reaction vessel, start stirring, and slowly heat to 100°C to gradually form a gel; Then sprinkle a hydrophobic agent on the surface of the gel, continue to keep it warm and age it to generate a dry silicone gel, and wash it thoroughly with ultrapure water and dry it. Step 8: Place the silica gel in a quartz crucible and put it into a tube furnace. First, slowly heat it to 300 degrees Celsius in air to remove unstable hydroxyl groups on the surface. Nitrogen gas was then introduced for protection, and the temperature was gradually increased to 1100 degrees Celsius and held for 3 hours to achieve deep dehydroxylation and calcination crystallization, thus obtaining quartz ingots. Step 9: Use a crusher to crush and then screen the quartz ingots, and use a screen to control the particle size range of quartz sand to 100 ~ 600 μm.

[0008] As a further optimization of the above scheme, in step 1, the cleaning includes: firstly, using a scrubbing machine to stir in a 0.5mol / L hydrochloric acid solution to clean the mud on the surface, then washing with ultrapure water, and finally drying in a vacuum drying oven at a temperature of 80~100℃ for 8~12h.

[0009] As a further optimization of the above scheme, in step 2, a high gradient magnetic separator is selected to perform three-stage stepped magnetic separation with field strengths of 0.4, 1.2, and 1.5T.

[0010] As a further optimization of the above scheme, in step 3, the acidic solution is one or more mixed acids selected from hydrofluoric acid, hydrochloric acid, sulfuric acid, and oxalic acid, the pickling temperature is 80℃, and the pickling time is 8~12h.

[0011] As a further optimization of the above scheme, in step 4, the strong alkaline substance is one of sodium hydroxide and potassium hydroxide or a mixture thereof, the mechanical stirring speed is 1000~2000 rpm / min, the calcination temperature is set to 1100℃, and the calcination time is 3h.

[0012] As a further optimization of the above scheme, in step 5, the alkaline solution is a 5 mol / L sodium hydroxide or potassium hydroxide solution, the external energy is an ultrasonic field environment, the reaction temperature is 80~100℃, and the reaction time is 12~24h.

[0013] As a further optimization of the above scheme, in step 6, the cation exchange column is filled with a strong acid cation exchange resin and an iron ion exchange resin.

[0014] As a further optimization of the above scheme, in step 7, the hydrophobic reagent is trimethylchlorosilane.

[0015] As a further optimization of the above scheme, in step 8, the heating rate of the tubular furnace is controlled at 50℃ / min, and the cooling rate after calcination is controlled at 50℃ / min.

[0016] The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand of the present invention has the following beneficial effects: This invention discloses a method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand. The invention pre-treats and purifies the natural quartz graded fine sand, effectively removing impurities and reducing the pressure of subsequent purification. At the same time, the invention utilizes mechanochemical thermal activation technology and adopts a combined process of mechanical grinding, alkali treatment, and high-temperature calcination to significantly improve the reactivity of the natural quartz graded fine sand, promote its hydrolysis degree in the subsequent alkaline environment, and further improve the yield of the final product. This is a completely new technical approach. This invention introduces a hydrophobic reagent to modify the silanol groups on the surface of the gel skeleton, changing them from hydrophilic to hydrophobic and reducing the surface tension of the solvent, thereby significantly reducing drying shrinkage. At the same time, during the high-temperature calcination process, this invention uses a multi-atmosphere gradient dehydroxylation technology, which uses air to pre-remove unstable hydroxyl groups on the surface in a medium-temperature environment, and then, under the protection of an inert gas, deeply removes the strong binding energy hydroxyl groups inside the quartz, greatly reducing the hydroxyl content of the quartz and significantly improving the internal density of the quartz. This invention is a green and low-carbon technology for preparing synthetic quartz sand, which not only reduces production costs but also improves the comprehensive utilization rate of natural quartz graded fine sand.

[0017] Specific embodiments of the present invention are disclosed in detail with reference to the following description and accompanying drawings, indicating how the principles of the present invention can be adopted. It should be understood that the embodiments of the present invention are not limited in scope as a result, and that the embodiments of the present invention include many changes, modifications and equivalents within the spirit and scope of the appended claims. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the process for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to the present invention. Figure 2 This is a particle size distribution diagram of the synthetic quartz sand of the present invention; Figure 3 This is an appearance diagram of the silica gel in Comparative Example 1 of the present invention; Figure 4 This is an optical microscope image of quartz sand in Comparative Example 2 of the present invention. Detailed Implementation

[0019] Natural quartz graded fine sand, due to its fine particle size and high impurity content, is traditionally used as a filling material in mines and an admixture in cement concrete, resulting in low added value and resource waste. Traditional synthetic quartz sand preparation processes typically use raw materials such as inorganic silicates and organosilicon, which are costly and extremely environmentally unfriendly.

[0020] Please refer to the instruction manual appendix. Figure 1-4 This invention provides a technical solution: a method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand, which uses natural quartz graded fine sand as a silicon source precursor, and prepares high-purity synthetic quartz sand through pretreatment-mechanical-chemical-thermal activation-hydrothermal synthesis-drying and aging-surface modification-multi-atmosphere synergistic dehydroxylation combined technology.

[0021] Example 1: like Figure 1 As shown, this embodiment uses natural quartz graded fine sand that has not been mechanically, chemically, or thermally activated as raw material to prepare synthetic quartz sand.

[0022] Step 1: Select natural quartz graded fine sand with SiO2 > 90% as raw material. First, use a scrubbing machine to stir in 0.5mol / L hydrochloric acid solution to clean the mud on the surface. Then, use ultrapure water for washing and dry in a vacuum drying oven at a temperature of 80~100℃ for 8~12h.

[0023] Step 2: Use a high gradient magnetic separator to remove iron impurities from the graded fine sand after passing it through three stages of magnetic separation; the field strength is 0.4, 1.2, and 1.5T. Step 3: Place the magnetically separated graded fine sand into a reaction vessel, add one or more mixed acids from hydrofluoric acid, hydrochloric acid, sulfuric acid, and oxalic acid, and hot-press leaching the solution at high temperature to further remove impurities and obtain quartz sand raw material; the acid washing temperature is 80℃ and the acid washing time is 8~12h. Step 4: Add the quartz sand raw material obtained in Step 3 into the mechanical activation tank, and add a strong alkaline substance at the same time, stir, grind and sieve. The sieve mesh sizes are 400, 800, and 1200 mesh, and the grinding time is 1-3 hours. The quartz powder that passes through the sieve is placed in a tube furnace for high-temperature calcination. After the powder cools naturally, it is thoroughly cleaned to remove impurities and then dried for later use. The strong alkali is sodium hydroxide, potassium hydroxide, or a mixture thereof. The mechanical stirring speed is 1000-2000 rpm / min. The calcination temperature is set to 1100℃ and the calcination time is 3 hours. Step 5: Place the quartz powder into the reaction vessel, add an alkaline solution, and apply external energy to assist the stirring reaction at a certain temperature; the alkaline solution is a 5 mol / L sodium hydroxide or potassium hydroxide solution, the external energy is an ultrasonic field environment, the reaction temperature is 80~100℃, and the reaction time is 12~24h to obtain an intermediate solution.

[0024] Step 6: The intermediate solution from Step 5 is filtered to remove insoluble precipitates and impurities, and then alkali metal and iron impurities are removed by passing it through a strong acid ion exchange resin and an iron removal ion exchange resin. The cation exchange column is filled with a strong acid cation exchange resin and an iron ion exchange resin to obtain a sol.

[0025] Step 7: Place the sol from Step 6 into the reaction vessel, start stirring, and slowly heat to 100°C to gradually form a gel; then sprinkle a hydrophobic reagent on the surface of the gel, continue to keep it warm and age to generate a dry silica gel, and thoroughly wash and dry it with ultrapure water; the hydrophobic reagent is trimethylchlorosilane.

[0026] Step 8: Place the silica gel in a quartz crucible and put it into a tube furnace. First, slowly heat it to 300 degrees Celsius in air to remove unstable hydroxyl groups on the surface. Then, introduce nitrogen gas for protection and gradually heat it to 1100 degrees Celsius. Hold it at this temperature for 3 hours to deeply remove hydroxyl groups and calcine to crystallize, obtaining a quartz ingot. The heating rate of the tube furnace is controlled at 50 degrees Celsius / min, and the cooling rate after calcination is controlled at 50 degrees Celsius / min.

[0027] Step 9: Use a crusher to crush and then screen the quartz ingots, and use a screen to control the particle size range of quartz sand to 100 ~ 600 μm.

[0028] The components of high-purity synthetic quartz sand are shown in Table 1: Table 1: Impurity component content of the synthetic quartz sand in Example 1 element Al B Fe Cu Cr K Mn Zn Na Ti P Ni Ma total content 2.8 0.6 0.2 0.3 0.2 0.3 0.2 0.4 1.6 0.3 0.2 0.2 0.2 9.3 Example 2: like Figure 1 As shown, this embodiment uses graded fine sand of natural quartz that has been mechanically, chemically, and thermally activated as raw material to prepare synthetic quartz sand.

[0029] Step 1: Select natural quartz graded fine sand with SiO2 > 90% as raw material. First, use a scrubbing machine to stir in 0.5mol / L hydrochloric acid solution to clean the mud on the surface. Then, use ultrapure water for washing and dry in a vacuum drying oven at a temperature of 80~100℃ for 8~12h.

[0030] Step 2: Use a high gradient magnetic separator to remove iron impurities from the graded fine sand after passing it through three stages of magnetic separation; the field strength is 0.4, 1.2, and 1.5T. Step 3: Place the magnetically separated graded fine sand into a reaction vessel, add one or more mixed acids from hydrofluoric acid, hydrochloric acid, sulfuric acid, and oxalic acid, and hot-press leaching the solution at high temperature to further remove impurities and obtain quartz sand raw material; the acid washing temperature is 80℃ and the acid washing time is 8~12h. Step 4: Add the quartz sand raw material obtained in Step 3 to a mechanical activation tank, and add a strong alkaline substance at the same time. Stir, grind and sieve thoroughly, with sieve mesh sizes of 400, 800 and 1200 mesh, and grinding time of 1~3 hours. Place the sieved quartz powder into a tube furnace for high-temperature calcination. After the powder cools naturally, wash thoroughly to remove impurities and dry for later use. The alkaline substance is one of sodium hydroxide and potassium hydroxide or a mixture thereof. The mechanical stirring speed is 1000~2000 rpm / min. The calcination temperature is set to 1100℃ and the calcination time is 3 hours.

[0031] Step 5: Place the quartz powder into the reaction vessel, add an alkaline solution, and apply external energy to assist the stirring reaction at a certain temperature; the alkaline solution is a 5 mol / L sodium hydroxide or potassium hydroxide solution, the external energy is an ultrasonic field environment, the reaction temperature is 80~100℃, and the reaction time is 12~24h to obtain an intermediate solution.

[0032] Step 6: The intermediate solution from Step 5 is filtered to remove insoluble precipitates and impurities, and then alkali metal and iron impurities are removed by passing it through a strong acid ion exchange resin and an iron removal ion exchange resin. The cation exchange column is filled with a strong acid cation exchange resin and an iron ion exchange resin to obtain a sol.

[0033] Step 7: Place the sol from Step 6 into the reaction vessel, start stirring, and slowly heat to 100°C to gradually form a gel; then sprinkle a hydrophobic reagent on the surface of the gel, continue to keep it warm and age to generate a dry silica gel, and thoroughly wash and dry it with ultrapure water; the hydrophobic reagent is trimethylchlorosilane.

[0034] Step 8: Place the silica gel in a quartz crucible and put it into a tube furnace. First, slowly heat it to 300 degrees Celsius in air to remove unstable hydroxyl groups on the surface. Then, introduce nitrogen gas for protection and gradually heat it to 1100 degrees Celsius. Hold it at this temperature for 3 hours to deeply remove hydroxyl groups and calcine to crystallize, obtaining a quartz ingot. The heating rate of the tube furnace is controlled at 50 degrees Celsius / min, and the cooling rate after calcination is controlled at 50 degrees Celsius / min.

[0035] Step 9: Use a crusher to crush and then screen the quartz ingots, and use a screen to control the particle size range of quartz sand to 100 ~ 600 μm.

[0036] The components of high-purity synthetic quartz sand are shown in Table 2: Table 2: Impurity component content of the synthetic quartz sand in Example 2 element Al B Fe Cu Cr K Mn Zn Na Ti P Ni Ma total content 2.1 0.3 1.8 0.2 0.2 0.8 0.1 0.2 0.1 0.2 0.1 0.1 6.3 A comparison of Examples 1 and 2 shows that the synthetic quartz sand prepared from graded fine sand of natural quartz after mechanochemical thermal activation exhibits significantly reduced impurity content and greatly improved particle transparency. This is because high-speed mechanical grinding not only reduces quartz particle size, increases specific surface area, and enhances surface activation energy, but also promotes the propagation of microcracks within the quartz particles, which is beneficial for subsequent alkaline leaching. Furthermore, the addition of strong alkaline substances such as sodium hydroxide or potassium hydroxide during the activation process can disrupt the quartz crystal structure, promote the breakage of Si-O bonds on the quartz surface, and form unsaturated bonds and active sites, enhancing the compatibility and reactivity of quartz. High-temperature calcination provides sufficient energy to further promote the breakage of Si-O-Si bonds in quartz, forming amorphous silica, which is beneficial for subsequent hydrolysis. Therefore, the synthetic sand prepared in Example 2 can be used as a raw material for photovoltaic quartz crucibles.

[0037] Comparative Example 1: The difference between this comparative example and Example 2 is that trimethylchlorosilane hydrophobic agent is not added in step 6, while other steps and parameters remain unchanged; the image of the silica gel prepared in this comparative example is shown below. Figure 3 As shown, a large number of cracks and pores are generated on the surface and inside of the gel; hydrophobic reagents such as trimethylchlorosilane can modify the silanol groups on the surface of the gel skeleton, changing them from hydrophilic to hydrophobic, and reduce the surface tension of the solvent, thereby significantly reducing drying shrinkage.

[0038] Comparative Example 2: The difference between this comparative example and Example 2 is that in step 8, only nitrogen gas is introduced into the tube furnace for protection, and the heating rate is controlled at 100℃ / min, while the cooling rate is not controlled and natural cooling is performed. Other steps and parameters remain unchanged. A microscopic image of the synthetic quartz sand prepared in this comparative example is shown below. Figure 4 As shown, the prepared synthetic quartz sand produced a large number of fine particles with uneven particle size distribution.

Claims

1. A method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand, characterized in that, Includes the following steps: Step 1: Select natural quartz graded fine sand with SiO2 content >90% as raw material, clean the mud on the surface and dry it for later use. Step 2: Remove iron impurities from the graded fine sand by passing it through a series of magnetic separation processes. Step 3: The magnetically separated and graded fine sand is placed in a reaction vessel, and an acidic solution is added for hot-press leaching at high temperature to further remove impurities and obtain quartz sand raw material. Step 4: Add the quartz sand raw material obtained in Step 3 into the mechanical activation tank, and add a strong alkaline substance at the same time, stir, grind and sieve. The sieve mesh sizes are 400, 800, and 1200 mesh, the grinding time is 1-3 hours, the quartz powder under the sieve is placed in a tube furnace for high-temperature calcination, and after the powder cools naturally, it is thoroughly cleaned to remove impurities and then dried for later use. Step 5: Place the quartz powder into the reaction vessel, add an alkaline solution, and apply external field energy to assist the stirring reaction at a certain temperature to obtain an intermediate solution; Step 6: Filter the intermediate solution from Step 5 to remove precipitated impurities, and then remove alkali metal impurities and iron impurities by passing it through a strong acid ion exchange resin and an iron removal ion exchange resin to obtain a sol. Step 7: Place the sol from step 6 into the reaction vessel, start stirring, and slowly heat to 100°C to gradually form a gel; Then sprinkle a hydrophobic agent on the surface of the gel, continue to keep it warm and age it to generate a dry silicone gel, and wash it thoroughly with ultrapure water and dry it. Step 8: Place the silica gel in a quartz crucible and put it into a tube furnace. First, slowly heat it to 300 degrees Celsius in air to remove unstable hydroxyl groups on the surface. Nitrogen gas was then introduced for protection, and the temperature was gradually increased to 1100 degrees Celsius and held for 3 hours to achieve deep dehydroxylation and calcination crystallization, thus obtaining quartz ingots. Step 9: Use a crusher to crush and then screen the quartz ingots, and use a screen to control the particle size range of quartz sand to 100 ~ 600 μm.

2. The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to claim 1, characterized in that: In step 1, the cleaning process includes: first, using a scrubbing machine to stir in a 0.5 mol / L hydrochloric acid solution to clean the mud off the surface; then, washing with ultrapure water; and finally, drying in a vacuum drying oven at a temperature of 80-100℃ for 8-12 hours.

3. The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to claim 1, characterized in that: In step 2, a high gradient magnetic separator is selected for three-stage stepped magnetic separation with field strengths of 0.4, 1.2, and 1.5T.

4. The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to claim 1, characterized in that: In step 3, the acidic solution is one or more mixed acids selected from hydrofluoric acid, hydrochloric acid, sulfuric acid, and oxalic acid. The pickling temperature is 80°C and the pickling time is 8-12 hours.

5. The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to claim 1, characterized in that: In step 4, the strong alkaline substance is one of sodium hydroxide and potassium hydroxide or a mixture thereof, the mechanical stirring speed is 1000~2000 rpm / min, the calcination temperature is set to 1100℃, and the calcination time is 3h.

6. The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to claim 1, characterized in that: In step 5, the alkaline solution is a 5 mol / L sodium hydroxide or potassium hydroxide solution, the external energy is an ultrasonic field environment, the reaction temperature is 80~100℃, and the reaction time is 12~24h.

7. The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to claim 1, characterized in that: In step 6, the cation exchange column is filled with a strong acid cation exchange resin and an iron ion exchange resin.

8. The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to claim 1, characterized in that: In step 7, the hydrophobic reagent is trimethylchlorosilane.

9. The method for preparing low-carbon synthetic quartz sand from natural quartz graded fine sand according to claim 1, characterized in that: In step 8, the heating rate of the tubular furnace is controlled at 50℃ / min, and the cooling rate after calcination is controlled at 50℃ / min.

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