Preparation method of calcium fluoride

Abstract

The invention discloses a preparation method of calcium fluoride. Specifically, the invention discloses a preparation method of calcium fluoride, which comprises the following steps: carrying out ball milling treatment on calcium hydroxide and ammonium bifluoride to prepare calcium fluoride. According to the method, the calcium fluoride powder which is high in purity, free of water or low in oxygen content can be obtained, so that the requirements of optical-grade calcium fluoride crystal raw materials are met; the method disclosed by the invention is simple in process flow, mild in reaction condition and small in environmental pollution, and can be suitable for industrial large-scale production.

Description

Technical Field

[0001] The invention relates to a method for preparing calcium fluoride. Background Art

[0002] Calcium fluoride (CaF2) crystals are highly hard, resistant to mechanical and thermal shock, and exhibit excellent transmittance in the ultraviolet, visible, and infrared wavelengths. They are widely used in scientific and technological fields such as lasers, infrared optics, ultraviolet optics, and high-energy detectors. As the raw material for calcium fluoride crystals, the preparation of high-purity anhydrous calcium fluoride has been extensively studied both domestically and internationally. The mainstream methods for preparing high-purity anhydrous calcium fluoride include direct precipitation, microemulsion, and hydrothermal methods. Direct precipitation methods are categorized as aqueous, purely organic, and mixed solutions, depending on the solution system. These methods involve adding a precipitant to a metal salt solution under varying reaction conditions to precipitate the target product, which is then processed through washing, filtration, and other treatments. The microemulsion method is a common method for preparing nanoscale calcium fluoride. The hydrothermal method involves producing calcium fluoride through high-temperature and high-pressure reactions in a sealed container. These methods for preparing high-purity calcium fluoride suffer from varying degrees of harsh reaction conditions, the introduction of impurities, and the resulting calcium fluoride product, which is difficult to filter and has a high water and oxygen content.

[0003] Patent CN103088421A discloses a method for chemically synthesizing high-purity calcium fluoride. This involves reacting a mixture of calcium chloride and calcium nitrate with ammonium fluoride, which then uses ammonium bifluoride to grow calcium fluoride crystals while also purifying the material. This invention requires high pressure and a strictly sealed process, placing high demands on equipment and making it difficult to scale up.

[0004] Patent CN104773749A discloses a method for preparing high-purity calcium fluoride in an ethanol system. Calcium chloride is dissolved in a mixture of ethanol and water, large-particle calcium fluoride seed crystals are added, and ammonium fluoride solution is simultaneously added dropwise for crystallization. The product is then separated and dried to obtain a 99.9% purity calcium fluoride. However, the use of an ethanol-water mixture in this invention makes it difficult to avoid the introduction of moisture, resulting in a high water-oxygen content and a purity that does not meet optical-grade requirements.

[0005] Patent CN116062784A mixes calcium-containing ore raw materials with an acid solution, stirs, dissolves, and filters to obtain a filtrate. The oxidation-precipitation-extraction method can efficiently remove metal impurity ions to prepare high-purity calcium fluoride powder with a purity of 99.99%. This invention belongs to a wet preparation process and has problems such as unstable ore raw materials and high oxygen content in the product. Summary of the Invention

[0006] To address the shortcomings of the aforementioned prior art, the present invention provides a novel method for preparing calcium fluoride. This method avoids the harsh reaction conditions and corrosion-induced impurity introduction associated with traditional dry methods, as well as the high oxygen content and filtration difficulties associated with wet methods. It can produce high-purity, anhydrous or low-oxygen calcium fluoride powder, thus meeting the requirements for optical-grade calcium fluoride crystal raw materials. The method also features a simple process flow, mild reaction conditions, minimal environmental pollution, and is suitable for industrial scale-up production.

[0007] The present invention solves the above technical problems through the following technical solutions:

[0008] The invention provides a method for preparing calcium fluoride, which comprises the following steps: ball-milling calcium hydroxide and ammonium bifluoride to prepare calcium fluoride.

[0009] In some embodiments of the present invention, the purity of the calcium hydroxide is ≥99.99%.

[0010] In some embodiments of the present invention, the purity of the ammonium bifluoride is ≥99.99%.

[0011] In some embodiments of the present invention, the molar ratio of calcium hydroxide to ammonium bifluoride is 1:(1-2), for example 1:(1.3-1.6), and for example 1:1.5.

[0012] In some embodiments of the present invention, the ball milling method is planetary ball milling or vibration ball milling, preferably planetary ball milling.

[0013] In some embodiments of the present invention, the ball mill is a planetary ball mill, and the rotation speed of the planetary ball mill is 400-800 rpm, such as 500 rpm, 600 rpm or 800 rpm, preferably 600 rpm.

[0014] In some embodiments of the present invention, the ball mill is a planetary ball mill, and the orbital speed of the planetary ball mill is 200-400 rpm, for example, 250 rpm.

[0015] In some embodiments of the present invention, the ball mill is a vibrating ball mill, and the rotation speed of the vibrating ball mill is 500-1000 rpm.

[0016] In some embodiments of the present invention, the ball mill is a vibrating ball mill, and the vibration frequency of the vibrating ball mill is 3-30 Hz, preferably 30 Hz.

[0017] In some embodiments of the present invention, the ball milling treatment time is 8-22 h, such as 10-20 h, such as 10 h, 12 h or 20 h, preferably 15 h.

[0018] In some embodiments of the present invention, the grinding balls subjected to ball milling are preferably polytetrafluoroethylene balls.

[0019] In some embodiments of the present invention, the weight ratio of the ball-milled grinding balls to the calcium hydroxide and ammonium bifluoride is (1-10):1, for example, 2.7:1.

[0020] In some embodiments of the present invention, the particle size of the ball-milled grinding balls is 5-10 mm, for example, 5 mm, 8 mm or 10 mm; preferably, the grinding balls are composed of 5 mm, 8 mm and 10 mm grinding balls, and more preferably, the weight ratio of 5 mm, 8 mm and 10 mm grinding balls is (1-2): (2-3): 1, for example, 1.5:2.5:1.

[0021] In some embodiments of the present invention, the method for preparing calcium fluoride further comprises the following post-processing steps: mixing the prepared calcium fluoride with an alcohol solvent, filtering to obtain a filter cake; and drying the filter cake.

[0022] In one embodiment of the present invention, the calcium fluoride is cooled before being mixed with the alcohol solvent.

[0023] The temperature after cooling is conventional in the art. In some embodiments of the present invention, the temperature after cooling is 18-35°C, such as 20-30°C.

[0024] In one embodiment of the present invention, the alcohol solvent is ethanol, such as anhydrous ethanol.

[0025] The mass ratio of the alcohol solvent to the calcium fluoride is conventional in the art. In some embodiments of the present invention, the mass ratio of the alcohol solvent to the calcium fluoride is (2-4):1, for example, 3:1.

[0026] In some embodiments of the present invention, the mixing is mixing and stirring; the mixing time is 1.5-2.5 hours, for example, 2 hours.

[0027] In one embodiment of the present invention, the drying is vacuum drying.

[0028] The vacuum drying temperature is conventional in the art. In some embodiments of the present invention, the vacuum drying temperature is 150-250°C, such as 200°C, 220°C, 240°C or 250°C.

[0029] The heating rate of the vacuum drying is conventional in the art. In some embodiments of the present invention, the heating rate of the vacuum drying is 8-12° C. / min, for example, 10° C. / min.

[0030] The vacuum drying time is conventional in the art. In some embodiments of the present invention, the vacuum drying time is 12-20 h, such as 15 h or 20 h.

[0031] In some embodiments of the present invention, the ball milling method includes the following steps: adding the calcium hydroxide, ammonium bifluoride and grinding balls into a ball milling jar, and performing ball milling on a ball mill.

[0032] The ball mill jar is conventional in the art. In some embodiments of the present invention, the ball mill jar is a polytetrafluoroethylene jar.

[0033] The ball mill is conventional in the art. In some embodiments of the present invention, the ball mill is a planetary ball mill or a vibrating ball mill; preferably, the ball mill is a planetary ball mill.

[0034] In some embodiments of the present invention, the planetary ball mill is a YXQM-1L planetary ball mill produced by Changsha Miqi.

[0035] In some embodiments of the present invention, the vibrating ball mill is a Retsch MM400 vibrating ball mill produced by Germany.

[0036] The volume of the ball mill is determined according to the scale of the material. In some embodiments of the present invention, the ratio of the volume of the ball mill to the mass of the material is 5-8 mL / g, for example, 6.3 mL / g; the mass of the material is the total mass of calcium hydroxide and ammonium bifluoride.

[0037] In one embodiment of the present invention, the method for preparing calcium fluoride comprises the following steps:

[0038] Step 1: Calcium hydroxide, ammonium bifluoride and grinding balls are placed in a ball mill and subjected to ball milling treatment on a ball mill to allow the raw materials to undergo a mixed reaction to generate calcium fluoride;

[0039] Step 2: Cooling the prepared calcium fluoride and mixing it with an alcohol solvent;

[0040] Step 3: After filtering the mixed solution obtained in step 2, the filter cake is vacuum dried;

[0041] In the step 1, the calcium hydroxide, ammonium bifluoride, grinding balls, ball mill, ball mill and ball milling conditions are as described in any embodiment of the present invention;

[0042] In step 2, the alcohol solvent, the mass ratio of the alcohol solvent to calcium fluoride, the cooling, mixing, the temperature after cooling, and the mixing time are as described in any embodiment of the present invention;

[0043] In step 3, the vacuum drying temperature, vacuum drying heating rate and vacuum drying time are as described in any embodiment of the present invention.

[0044] In some embodiments of the present invention, the ammonium bifluoride is purified by the following method: the ammonium bifluoride is recrystallized, and the recrystallization solvent is water, such as pure water.

[0045] Preferably, the recrystallization comprises the following steps: mixing crude ammonium hydrofluoride with water, heating and dissolving until close to saturation, filtering while hot, cooling the filtrate, collecting crystals in the filtrate, and washing.

[0046] In some embodiments of the present invention, the mass ratio of ammonium bifluoride to water is 1:(1-1.5), for example, 1:1.2.

[0047] In some embodiments of the present invention, the dissolving temperature is 50-60°C.

[0048] In some embodiments of the present invention, the cooling temperature is -5°C to 5°C, for example, 0°C.

[0049] In some embodiments of the present invention, the washing solvent is water at a temperature between -5°C and 5°C or hydrofluoric acid at a temperature between -5°C and 5°C (eg, hydrofluoric acid with a mass concentration of 1-3%).

[0050] Without violating the common sense in the art, the above-mentioned preferred conditions can be arbitrarily combined to obtain preferred embodiments of the present invention.

[0051] The reagents and raw materials used in the present invention are commercially available.

[0052] The positive progress effect of the present invention is:

[0053] (1) The high-purity anhydrous calcium fluoride prepared by the ball milling method in the present invention avoids the harsh reaction conditions and the problem of easy corrosion and introduction of impurities in the traditional dry method, as well as the problems of high oxygen content and difficult filtration caused by the wet method aqueous system, thereby obtaining high-purity, anhydrous or low-oxygen calcium fluoride;

[0054] (2) The preparation method of the present invention also has the advantages of simple process, simple equipment, mild reaction conditions, and little pollution to the environment, and is suitable for industrial scale-up production. DETAILED DESCRIPTION

[0055] The present invention is further illustrated by way of examples below, but the present invention is not limited to the scope of the examples. Experimental methods in the following examples where specific conditions are not specified were performed according to conventional methods and conditions, or selected according to the product specifications.

[0056] The ball mills in the following examples are all YXQM-1L planetary ball mills produced by Changsha Miqi.

[0057] Example 1 Preparation of ammonium bifluoride with a content of ≥99.99%

[0058] Slowly add 10 g of crude ammonium bifluoride to 12 ml of pure water at 50-60°C and stir until completely dissolved. Control the solution concentration to near saturation. Filter while hot to remove insoluble impurities. Place the filtrate on the upper shelf of a refrigerator and slowly cool to 0°C. Collect the crystals by suction, wash with a small amount of water at -5°C to 5°C or 1-3% hydrofluoric acid at -5°C to 5°C, and dry.

[0059] Example 2 Preparation method of anhydrous calcium fluoride

[0060] (1) Ball milling reaction: Weigh 7.40 g of calcium hydroxide with a content of ≥99.99% and 8.56 g of ammonium bifluoride (prepared by the method of Example 1, with a purity of ≥99.99%), place them in a 100 ml polytetrafluoroethylene ball mill vacuum tank, add polytetrafluoroethylene balls, and the ball-to-material ratio (mass ratio) is 2.7:1. The grinding balls include three particle sizes, namely 10 mm, 8 mm and 5 mm, and the weight ratio of the grinding balls of large, medium and small particle sizes is 3:5:2. Then seal and fix the ball mill tank, put it into a planetary ball mill, and grind it at a rotation speed of 500 rpm and a revolution speed of 250 rpm for 12 h;

[0061] (2) Ethanol washing: After the ball mill has cooled to room temperature, open it and add 30 mL of anhydrous ethanol. After stirring, transfer the mixture to a polytetrafluoroethylene flask and stir for 2 h for washing. Then filter the mixture to remove the water generated by the reaction, a small amount of ammonium fluoride, etc.

[0062] (3) Vacuum drying: Place the filter cake obtained by filtration in step (2) into a vacuum drying oven, heat it up at a rate of 10°C / min, heat it up to 200°C and keep it there for 15 hours, then turn off the power supply and allow the calcium fluoride product to cool to its natural temperature along with the drying oven;

[0063] (4) Quality analysis: After cooling, the high-purity anhydrous calcium fluoride powder was vacuum-packed in a polytetrafluoroethylene plastic bag and stored in a desiccator or glove box. The mass of the prepared high-purity anhydrous calcium fluoride was 7.66 g, with a yield of 98.1%. The purity was analyzed to be 99.993% and the oxygen content was 0.12%, which met the raw material index requirements for the preparation of optical-grade calcium fluoride crystals. The purity was determined by the reduction method, and the elements determined included iron, cobalt, nickel, copper, mercury, barium, sodium, potassium, titanium, beryllium, magnesium, aluminum, silicon, sulfur, chlorine, and rare earth elements. The oxygen content was tested using an O836 oxygen analyzer from LECO, USA.

[0064] Example 3 Preparation of anhydrous calcium fluoride

[0065] (1) Ball milling reaction: Weigh 7.40 g of calcium hydroxide with a content of ≥99.99% and 8.56 g of ammonium bifluoride (prepared by the method of Example 1, with a purity of ≥99.99%), place them in a 100 ml polytetrafluoroethylene ball mill vacuum tank, add polytetrafluoroethylene balls, and the ball-to-material ratio (mass ratio) is 2.7:1. The grinding balls include three particle sizes, namely 10 mm, 8 mm and 5 mm, and the weight ratio of the grinding balls of large, medium and small particle sizes is 3:5:2. Then seal and fix the ball mill tank, put it into a planetary ball mill, and grind it at a rotation speed of 600 rpm and a revolution speed of 250 rpm for 10 h;

[0066] (2) Ethanol washing: After the ball mill has cooled to room temperature, open it and add 30 mL of anhydrous ethanol. After stirring, transfer the mixture to a polytetrafluoroethylene flask and stir for 2 h for washing. Then filter the mixture to remove the water generated by the reaction, a small amount of ammonium fluoride, etc.

[0067] (3) Vacuum drying: Place the filter cake obtained by filtration in step (2) into a vacuum drying oven, heat it up at a rate of 10°C / min, heat it up to 220°C and keep it there for 15 hours, then turn off the power supply and allow the calcium fluoride product to cool to its natural temperature in the drying oven;

[0068] (4) Quality analysis: After cooling, the high-purity anhydrous calcium fluoride powder was vacuum-packed in a polytetrafluoroethylene plastic bag and stored in a desiccator or glove box. The mass of the prepared high-purity anhydrous calcium fluoride was 7.68 g, with a yield of 98.3%. The purity was analyzed to be 99.991%, and the oxygen content was 0.14%, which met the raw material index requirements for the preparation of optical-grade calcium fluoride crystals. The purity was determined by the reduction method, and the elements determined included iron, cobalt, nickel, copper, mercury, barium, sodium, potassium, titanium, beryllium, magnesium, aluminum, silicon, sulfur, chlorine, and rare earth elements. The oxygen content was tested using an O836 oxygen analyzer from LECO, USA.

[0069] Example 4 Preparation method of anhydrous calcium fluoride

[0070] (1) Ball milling reaction: Weigh 7.40 g of calcium hydroxide with a content of ≥99.99% and 8.56 g of ammonium bifluoride (prepared by the method of Example 1, with a purity of ≥99.99%), place them in a 100 ml polytetrafluoroethylene ball mill vacuum tank, add polytetrafluoroethylene balls, and the ball-to-material ratio (mass ratio) is 2.7:1. The grinding balls include three particle sizes, namely 10 mm, 8 mm and 5 mm, and the weight ratio of the grinding balls of large, medium and small particle sizes is 3:5:2. Then, seal and fix the ball mill tank, put it into a planetary ball mill, and grind it at a rotation speed of 800 rpm and a revolution speed of 250 rpm for 10 h;

[0071] (2) Ethanol washing: After the ball mill has cooled to room temperature, open it and add 30 mL of anhydrous ethanol. After stirring, transfer the mixture to a polytetrafluoroethylene flask and stir for 2 h for washing. Then filter the mixture to remove the water generated by the reaction, a small amount of ammonium fluoride, etc.

[0072] (3) Vacuum drying: Place the filter cake obtained by filtration in step (2) into a vacuum drying oven, heat it up at a rate of 10°C / min, heat it up to 240°C and keep it there for 20 h, then turn off the power supply and allow the calcium fluoride product to cool to its natural temperature along with the drying oven;

[0073] (4) Quality analysis: After cooling, the high-purity anhydrous calcium fluoride powder was vacuum-packed in a polytetrafluoroethylene plastic bag and stored in a desiccator or glove box. The mass of the prepared high-purity anhydrous calcium fluoride was 7.7 g, with a yield of 98.6%. The purity was analyzed to be 99.993% and the oxygen content was 0.11%, which met the raw material index requirements for the preparation of optical-grade calcium fluoride crystals. The purity was determined by the reduction method, and the elements determined included iron, cobalt, nickel, copper, mercury, barium, sodium, potassium, titanium, beryllium, magnesium, aluminum, silicon, sulfur, chlorine, and rare earth elements. The oxygen content was tested using an O836 oxygen analyzer from LECO, USA.

[0074] Example 5 Preparation of anhydrous calcium fluoride

[0075] (1) Ball milling reaction: Weigh 7.40 g of calcium hydroxide with a content of ≥99.99% and 8.56 g of ammonium bifluoride (prepared by the method of Example 1, with a purity of ≥99.99%), place them in a 100 ml polytetrafluoroethylene ball mill vacuum tank, add polytetrafluoroethylene balls, and the ball-to-material ratio (mass ratio) is 2.7:1. The grinding balls include three particle sizes, namely 10 mm, 8 mm and 5 mm, and the weight ratio of the grinding balls of large, medium and small particle sizes is 3:5:2. Then seal and fix the ball mill tank, put it into a planetary ball mill, and grind it at a rotation speed of 500 rpm and a revolution speed of 250 rpm for 20 h;

[0076] (2) Ethanol washing: After the ball mill has cooled to room temperature, open it and add 30 mL of anhydrous ethanol. After stirring, transfer the mixture to a polytetrafluoroethylene flask and stir for 2 h for washing. Then filter the mixture to remove the water generated by the reaction, a small amount of ammonium fluoride, etc.

[0077] (3) Vacuum drying: Place the filter cake obtained by filtration in step (2) into a vacuum drying oven, heat it up at a rate of 10°C / min, heat it up to 250°C and keep it there for 20 h, then turn off the power supply and allow the calcium fluoride product to cool to its natural temperature along with the drying oven;

[0078] (4) Quality analysis: After cooling, the high-purity anhydrous calcium fluoride powder was vacuum-packed in a polytetrafluoroethylene plastic bag and stored in a desiccator or glove box. The mass of the prepared high-purity anhydrous calcium fluoride was 7.65 g, with a yield of 98.0%. The purity was analyzed to be 99.993% and the oxygen content was 0.12%, which met the raw material index requirements for the preparation of optical-grade calcium fluoride crystals. The purity was determined by the reduction method, and the elements determined included iron, cobalt, nickel, copper, mercury, barium, sodium, potassium, titanium, beryllium, magnesium, aluminum, silicon, sulfur, chlorine, and rare earth elements. The oxygen content was tested using an O836 oxygen analyzer from LECO, USA.

Claims

1. A method for preparing calcium fluoride, characterized in that: It includes the following steps: Calcium hydroxide and ammonium bifluoride are ball-milled to prepare calcium fluoride.

2. The method for preparing calcium fluoride according to claim 1, wherein: It meets one or more of the following conditions: (1) The purity of the calcium hydroxide is ≥99.99%; (2) The purity of the ammonium bifluoride is ≥99.99%; (3) The molar ratio of calcium hydroxide to ammonium bifluoride is 1:(1-2); (4) The ball milling method is planetary ball milling or vibration ball milling, preferably planetary ball milling; (5) The ball milling treatment time is 8-22 h; (6) The grinding balls used in the ball milling process are polytetrafluoroethylene balls; (7) The weight ratio of the ball-milled grinding balls to the calcium hydroxide and ammonium bifluoride is (1-10):1; and (8) The method for preparing calcium fluoride further includes the following post-processing steps: mixing the prepared calcium fluoride with an alcohol solvent, filtering to obtain a filter cake; and drying the filter cake.

3. The method for preparing calcium fluoride according to claim 2, wherein: It meets one or more of the following conditions: (1) The molar ratio of calcium hydroxide to ammonium bifluoride is 1:(1.3-1.6); (2) The ball mill is a planetary ball mill, and the rotation speed of the planetary ball mill is 400-800 rpm; (3) The ball mill is a planetary ball mill, and the revolution speed of the planetary ball mill is 200-400 rpm; (4) The ball mill is a vibrating ball mill, and the rotation speed of the vibrating ball mill is 500-1000 rpm; (5) The ball mill is a vibrating ball mill, and the vibration frequency of the vibrating ball mill is 3-30 Hz; (6) The ball milling treatment time is 10-20 h; (7) The weight ratio of the ball-milled grinding balls to the calcium hydroxide and ammonium bifluoride is 2.7:1; (8) The particle size of the grinding balls subjected to the ball milling treatment is 5-10 mm; (9) The calcium fluoride is cooled before being mixed with the alcohol solvent; (10) The alcohol solvent is ethanol; (11) The mass ratio of the alcohol solvent to the calcium fluoride is (2-4):1; (12) The mixing is mixing and stirring; and (13) The drying is vacuum drying.

4. The method for preparing calcium fluoride according to claim 3, wherein: It meets one or more of the following conditions: (1) The molar ratio of calcium hydroxide to ammonium bifluoride is 1:1.5; (2) The ball mill is a planetary ball mill, and the rotation speed of the planetary ball mill is 500 rpm, 600 rpm or 800 rpm; (3) The ball mill is a planetary ball mill, and the revolution speed of the planetary ball mill is 250 rpm; (4) The ball mill is a vibrating ball mill, and the vibration frequency of the vibrating ball mill is 30 Hz; (5) The ball milling treatment time is 10 h, 12 h or 20 h; (6) The particle size of the grinding balls subjected to ball milling is 5 mm, 8 mm or 10 mm; (7) The temperature after cooling is 18-35°C; (8) The alcohol solvent is anhydrous ethanol; (9) The mass ratio of the alcohol solvent to the calcium fluoride is 3:1; (10) The mixing time is 1.5-2.5 h; (11) The vacuum drying temperature is 150-250°C; (12) The heating rate of the vacuum drying is 8-12 °C / min; and (13) The vacuum drying time is 12-20 h.

5. The method for preparing calcium fluoride according to claim 4, wherein: It meets one or more of the following conditions: (1) The ball mill is a planetary ball mill, and the rotation speed of the planetary ball mill is 600 rpm; (2) The ball milling time is 15 h; (3) The grinding balls are composed of 5 mm, 8 mm and 10 mm grinding balls; preferably, the weight ratio of the 5 mm, 8 mm and 10 mm grinding balls is (1-2): (2-3): 1, for example, 1.5:2.5:1; (4) The temperature after cooling is 20-30°C; (5) The mixing time is 2 h; (6) The vacuum drying temperature is 200°C, 220°C, 240°C or 250°C; (7) The heating rate of the vacuum drying is 10°C / min; and (8) The vacuum drying time is 15 h or 20 h.

6. The method for preparing calcium fluoride according to claim 1, wherein: The ammonium bifluoride is purified by the following method: the ammonium bifluoride is recrystallized, wherein the recrystallization solvent is water; Preferably, the recrystallization comprises the following steps: mixing crude ammonium hydrofluoride with water, heating and dissolving until close to saturation, filtering while hot, cooling the filtrate, collecting crystals in the filtrate, and washing.

7. The method for preparing calcium fluoride according to claim 6, wherein: It meets one or more of the following conditions: (1) The water is pure water; (2) The mass ratio of ammonium bifluoride to water is: 1:(1-1.5), for example, 1:1.2; (3) The dissolution temperature is 50-60°C; (4) the cooling temperature is -5°C to 5°C, for example 0°C; and (5) The washing solvent is water at -5°C to 5°C or hydrofluoric acid at -5°C to 5°C; the mass concentration of the hydrofluoric acid is 1-3%.

8. The method for preparing calcium fluoride according to claim 1, wherein: The ball milling method comprises the following steps: adding the calcium hydroxide, ammonium bifluoride and grinding balls into a ball milling jar, and performing ball milling on a ball mill.

9. The method for preparing calcium fluoride according to claim 8, wherein: It meets one or more of the following conditions: (1) The ball mill jar is a polytetrafluoroethylene jar; (2) The ball mill is a planetary ball mill or a vibrating ball mill; preferably, the ball mill is a planetary ball mill; and (3) The ratio of the volume of the ball mill to the mass of the material is 5-8 mL / g, for example, 6.3 mL / g; the mass of the material is the total mass of calcium hydroxide and ammonium bifluoride.

10. The method for preparing calcium fluoride according to claim 1, wherein: The preparation method of calcium fluoride comprises the following steps: Step 1: Calcium hydroxide, ammonium bifluoride and grinding balls are placed in a ball mill and subjected to ball milling treatment on a ball mill to allow the raw materials to undergo a mixed reaction to generate calcium fluoride; Step 2: Cooling the prepared calcium fluoride and mixing it with an alcohol solvent; Step 3: After filtering the mixed solution obtained in step 2, the filter cake is vacuum dried; In step 1, the calcium hydroxide, ammonium bifluoride and grinding balls are as described in any one of claims 1 to 9; the ball mill and ball mill are as described in claim 8 or 9; and the ball milling conditions are as described in any one of claims 1 to 5; In step 2, the alcohol solvent and the mass ratio of the alcohol solvent to calcium fluoride are as described in claim 3 or 4; the cooling and mixing are as described in claim 3; the temperature after cooling and the mixing time are as described in claim 4 or 5; In step 3, the vacuum drying temperature, vacuum drying heating rate and vacuum drying time are as described in claim 4 or 5.