System and method for producing calcium fluoride

Abstract

The invention provides a system and method for producing calcium fluoride, and belongs to the field of chemical engineering. The system comprises a neutralizing tank, an ammonolysis tank, a silicon dioxide separator, a suspension preparation tank, a suspension delivery pump, a suspension spray gun, a calcium fluoride rotary kiln, a cyclone separator, a bag filter, a target product conveyor, an absorption tower, a circulating pump and a cooler. By adopting the system and the method, the calcium fluoride is produced by utilizing the by-product fluosilicic acid of the wet-process phosphoric acid device and the by-product calcium carbonate of the ammonium sulfate device, the by-product ammonium sulfate-ammonium carbonate solution enters the ammonium sulfate device to produce ammonium sulfate with phosphogypsum, and the by-product silicon dioxide is used for producing a silicon product. The invention aims to solve the storage problem of fluosilicic acid and the recycling problem of calcium carbonate and ammonium carbonate, waste is turned into wealth, and the environment is protected.

Description

Technical Field

[0001] The invention relates to the technical field of chemical production, and in particular to a system and method for producing calcium fluoride. Background Art

[0002] Calcium fluoride is mainly used as a raw material for the production of sodium fluoride, hydrofluoric acid and fluoride. It can also be used as a flux for smelting steel, a raw material for making opalescent glass, and a boiling stopper in the glass industry. It can also be used as an enamel glaze and insecticide. The active ingredient of fluorite is calcium fluoride. As a non-renewable resource, with the mining in recent years, my country's fluorite is facing the danger of depletion and is currently listed as a resource with restricted mining by the state.

[0003] Fluorosilicic acid is a by-product in the production of wet-process phosphoric acid. The main component of fluosilicic acid is H2SiF6. Every ton (calculated as P2O5) of phosphoric acid produced can produce about 0.5 to 0.6 tons (including H2SiF6 at a mass concentration of 18%) of fluosilicic acid. The annual by-product of fluosilicic acid in China (including H2SiF6 at a mass concentration of 18%) can reach about 7.5 million tons.

[0004] At present, the main preparation method of calcium fluoride is:

[0005] (1) Using the fluorine-containing mixed acid produced by lithium hexafluorophosphate production enterprises and calcium chloride as raw materials to prepare high-purity calcium fluoride and produce hydrochloric acid as a by-product.

[0006] (2) Using phosphogypsum as raw material, it was first washed with water, then mixed with activated carbon, and calcined at 700-800°C to obtain calcium oxide, and finally calcium fluoride was prepared with analytical pure ammonium fluoride. Summary of the invention

[0007] The purpose of the present invention is to provide a system and method for producing calcium fluoride, which utilizes fluorosilicic acid and liquid ammonia produced as byproducts of a wet-process phosphoric acid device and calcium carbonate produced as byproducts of an ammonium sulfate device to produce calcium fluoride, and the byproduct tail gas containing ammonia and carbon dioxide is absorbed by an ammonium sulfate solution produced as byproducts of an ammonium sulfate device to produce ammonium sulfate-ammonium carbonate solution, which is returned to the ammonium sulfate device to produce ammonium sulfate. The present invention aims to solve the problem of storing fluorosilicic acid and the problem of recycling calcium carbonate and ammonium carbonate, turn waste into treasure, and protect the environment.

[0008] The purpose of the present invention can be achieved through the following technical solutions:

[0009] A system for producing calcium fluoride comprises a neutralization tank, which is sequentially connected to an ammonia solution tank, a silicon dioxide separator, a suspension preparation tank, and a calcium fluoride rotary kiln; a suspension spray gun is installed on one side of the calcium fluoride rotary kiln connected to the suspension preparation tank, and a target product conveyor is connected to the bottom of the other side of the calcium fluoride rotary kiln.

[0010] As a preferred embodiment of the present invention, the top of one side of the calcium fluoride rotary kiln connected to the target product conveyor is connected to a cyclone separator, and the cyclone separator is connected to a bag filter, an absorption tower, a circulation pump and a cooler in sequence; the bottom of the cyclone separator and / or the bag filter is connected to the target product conveyor through a pipeline.

[0011] As a preferred embodiment of the present invention, a neutralization tank agitator and an ammonolysis tank agitator are installed on the top of the neutralization tank and the ammonolysis tank, respectively.

[0012] As a preferred embodiment of the present invention, a separator rake and an ammonium fluoride stirrer are respectively installed on the top of the silicon dioxide separator.

[0013] As a preferred embodiment of the present invention, a suspension agitator and a suspension delivery pump are installed on the top of the suspension preparation tank.

[0014] As a preferred embodiment of the present invention, the circulation pump is connected to the bottom of the cooler, and the top of the cooler is connected to the absorption tower.

[0015] As a preferred embodiment of the present invention, the rake arm of the separator rake is located at the bottom of the silica separator, wherein the bottom of the silica separator is funnel-shaped, and the height of the rake arm of the separator rake is lower than the paddle of the ammonium fluoride agitator.

[0016] As a preferred embodiment of the present invention, the height of the blades of the suspension agitator is higher than the bottom end of the suspension delivery pump.

[0017] The present invention also provides a method for producing calcium fluoride using the above system, comprising the following steps:

[0018] (1) Preparation of ammonium fluoride: Fluorosilicic acid and liquid ammonia are mixed and reacted in a neutralization tank to generate an ammonium fluosilicate solution, which is then mixed again with liquid ammonia in an ammonolysis tank to generate a silicon dioxide-ammonium fluoride solution. The silicon dioxide-ammonium fluoride solution is precipitated and separated in a silicon dioxide separator to generate silicon dioxide and ammonium fluoride solution;

[0019] (2) Preparation of calcium fluoride: ammonium fluoride solution is mixed with calcium carbonate in a suspension preparation tank to form a suspension, and the suspension is pressurized by a suspension spray gun and then sent into a calcium fluoride rotary kiln chamber for heating, conversion, and drying to obtain calcium fluoride;

[0020] (3) Calcium fluoride recovery: The tail gas from the calcium fluoride rotary kiln enters the cyclone separator and bag filter in sequence to continue separation to obtain calcium fluoride;

[0021] (4) Tail gas absorption: The tail gas from the bag filter enters the absorption tower and cooler in turn so that the NH3 and CO2 in the tail gas are completely absorbed.

[0022] As a preferred embodiment of the above method, in step (1), the molar ratio of fluorosilicic acid to liquid ammonia is 1:1.5-2.5, and the molar ratio of ammonium fluorosilicate solution to liquid ammonia is 1:3-5.

[0023] As a preferred embodiment of the above method, in step (2), the molar ratio of ammonium fluoride to calcium carbonate is 1.5-2.5:1, and the temperature in the calcium fluoride rotary kiln is 350-400°C.

[0024] As a preferred embodiment of the above method, in step (3), the temperature of the tail gas from the calcium fluoride rotary kiln is 200-300°C, and the temperature of the tail gas from the bag filter is 160-200°C.

[0025] As a preferred embodiment of the above method, the temperature of the absorption liquid at the bottom of the absorption tower in step (4) is 50-70°C.

[0026] Beneficial effects of the present invention:

[0027] Using fluorosilicic acid to produce calcium fluoride can solve the problem of fluorosilicic acid storage. Calcium fluoride can be used as an industrial raw material to reduce the mining of fluorite, which protects fluorite resources and the environment. Every ton of calcium fluoride produced can consume about 3.42 tons of fluorosilicic acid (containing H2SiF6 mass concentration of about 18%) produced as a byproduct of wet-process phosphoric acid, consume about 1.28 tons of calcium carbonate produced as a byproduct of ammonium sulfate, and produce about 1.23 tons of ammonium carbonate; 1.23 tons of ammonium carbonate can be used to produce about 1.69 tons of ammonium sulfate in the ammonium sulfate device, consume about 2.2 tons of phosphogypsum produced as a byproduct of wet-process phosphoric acid, and produce about 1.28 tons of calcium carbonate to return to the calcium fluoride device to produce 1 ton of calcium fluoride. The calcium fluoride and ammonium sulfate devices utilize each other's byproducts, while consuming the byproducts of wet-process phosphoric acid, fluorosilicic acid and phosphogypsum, to achieve a win-win situation, turn waste into treasure, and protect the environment. Using ammonium sulfate solution from an ammonium sulfate unit to absorb tail gas from a calcium fluoride unit is beneficial to the condensation of water vapor and the absorption of NH3 and CO2 gases to form an ammonium sulfate-ammonium carbonate solution; using ammonium sulfate-ammonium carbonate solution to produce ammonium sulfate in an ammonium sulfate unit is beneficial to increasing the initial concentration of ammonium sulfate and facilitating the crystallization of ammonium sulfate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The schematic diagram of the process flow in the present invention is

[0029] Among them, neutralization tank 1, neutralization tank agitator 2, ammonolysis tank 3, ammonolysis tank agitator 4, silicon dioxide separator 5, separator rake 6, ammonium fluoride agitator 7, suspension preparation tank 8, suspension agitator 9, suspension delivery pump 10, suspension spray gun 11, calcium fluoride rotary kiln 12, cyclone separator 13, bag filter 14, target product conveyor 15, absorption tower 16, circulation pump 17, cooler 18. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but only represents the selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention. The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the drawings, where the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain the present invention, and cannot be understood as limitations on the present invention.

[0031] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0032] like Figure 1 A system for producing calcium fluoride comprises a neutralization tank 1, which is sequentially connected with an ammoniolysis tank 3, a silicon dioxide separator 5, a suspension preparation tank 8, a calcium fluoride rotary kiln 12, a cyclone separator 13, a bag filter 14, an absorption tower 16, a circulation pump 17 and a cooler 18; an ammonium fluoride solution inlet A4 and a calcium carbonate inlet B4 are arranged on the top of the suspension preparation tank 8, the calcium carbonate inlet B4 is connected to a calcium carbonate conveying device from an ammonium sulfate device, a suspension agitator 9 and a suspension delivery pump 10 are installed on the top of the suspension preparation tank 8, and the ammonium fluoride solution inlet A4 and the calcium carbonate inlet B4 are arranged away from the suspension delivery pump 10; a suspension spray gun 11 is installed on the side of the calcium fluoride rotary kiln 12 connected with the suspension preparation tank 8, a spray gun inlet A5 is arranged on one side of the suspension spray gun 11, the spray gun inlet A5 is connected to the outlet of the suspension delivery pump 10 through a pipeline, and a spray gun outlet B5 is arranged on the other side of the suspension spray gun 11.

[0033] The top of the neutralization tank 1 is provided with a fluorosilicic acid inlet A1 and a first liquid ammonia inlet B1, the fluorosilicic acid pipeline from the boundary zone is connected to the fluorosilicic acid inlet A1 at the top of the neutralization tank 1, the liquid ammonia pipeline from the boundary zone is connected to the first liquid ammonia inlet B1 at the top of the neutralization tank 1, a neutralization tank agitator 2 is installed on the top of the neutralization tank 1, and one side of the neutralization tank 1 is provided with an ammonium fluorosilicate outlet C1; the top of the ammoniolysis tank 3 is provided with an ammonium fluorosilicate inlet A2 and a second liquid ammonia inlet B2, and the liquid ammonia pipeline from the boundary zone is provided with a deammoniolysis tank 3. A pipeline for a di-liquid ammonia inlet B2 is provided, an ammonia solution tank stirrer 4 is provided on the top of the ammonia solution tank 3, and a silicon dioxide-ammonium fluoride outlet C2 is provided on one side of the ammonia solution tank 3; a silicon dioxide-ammonium fluoride inlet A3 and an ammonium fluoride solution outlet B3 are provided on both sides of the silicon dioxide separator 5, a separator rake 6 and an ammonium fluoride stirrer 7 are respectively installed on the top of the silicon dioxide separator 5, and a silicon dioxide outlet C3 is provided at the bottom of the silicon dioxide separator 5, and the silicon dioxide outlet C3 is connected to the silicon dioxide conveying equipment in the deboundary zone.

[0034] A heating gas inlet C5 is provided below the spray gun inlet A5 of the calcium fluoride rotary kiln 12, a first tail gas outlet F is provided at the top of the other side of the calcium fluoride rotary kiln 12, and a first calcium fluoride outlet E is provided at the bottom, and the first calcium fluoride outlet E is connected to the feed end of the target product conveyor 15; a first tail gas inlet A6 is provided at the side of the cyclone separator 13, and the first tail gas inlet A6 is connected to the first tail gas outlet F; a second tail gas outlet C6 is provided at the top of the cyclone separator 13, and a second calcium fluoride outlet B6 is provided at the bottom of the cyclone separator 13, and the second calcium fluoride outlet B6 is connected to the feed end of the target product conveyor 15; a second tail gas inlet A7 is provided at the side of the bag filter 14, and the second tail gas inlet A7 is connected to the second tail gas outlet C6; a third tail gas outlet C7 is provided at the top of the bag filter 14, and a third calcium fluoride outlet B7 is provided at the bottom of the bag filter 14, and the third calcium fluoride outlet B7 is connected to the feed end of the target product conveyor 15.

[0035] The absorption tower 16 is provided with a third tail gas inlet A8, a first absorption liquid outlet B8 and a first absorption liquid inlet C8 on both sides, wherein the third tail gas inlet A8 is connected to the third tail gas outlet C7, the first absorption liquid outlet B8 is connected to the inlet of the circulation pump 17, and a non-condensable gas outlet D8 is provided on the top of the absorption tower 16, and the non-condensable gas outlet D8 is connected to the non-condensable gas pipeline of the de-boundary zone; the cooler 18 is provided with a second absorption liquid outlet D9 and a second absorption liquid inlet A9 on the top and bottom, wherein the second absorption liquid outlet D9 is connected to the first absorption liquid inlet C8, and an ammonium sulfate solution pipeline from the ammonium sulfate device is provided on the pipeline connected by the two, and the second absorption liquid inlet A9 is connected to the outlet of the circulation pump 17, and a circulating cooling water inlet C9 and a circulating cooling water outlet B9 are provided on one side of the absorption tower 16, and the two are respectively connected to the circulating water supply pipeline from the boundary zone and the circulating return water pipeline from the de-boundary zone.

[0036] The bottom of the silica separator 5 is funnel-shaped, and the silica outlet C3 is arranged at the bottom of the silica separator 5; the height of the ammonium fluoride solution outlet B3 is higher than the blade of the ammonium fluoride stirrer 7, and the height of the blade of the ammonium fluoride stirrer 7 is equal to or higher than the height of the silica-ammonium fluoride inlet A3; the rake arm of the separator rake 6 is located at the bottom of the silica separator 5.

[0037] The blade height of the suspension agitator 9 is higher than the bottom end of the suspension delivery pump 10 .

[0038] The first absorption liquid outlet B8 and the first absorption liquid inlet C8 are respectively located on the upper side and the lower side of the absorption tower 16, and the height of the third tail gas inlet A8 is higher than the height of the first absorption liquid outlet B8 and lower than the height of the first absorption liquid inlet C8.

[0039] The height of the circulating cooling water inlet C9 is higher than the height of the circulating cooling water outlet B9.

[0040] The method for producing calcium fluoride using the above system comprises the following steps:

[0041] (1) Ammonium fluoride preparation section: Fluorosilicic acid (containing H2SiF6 with a mass concentration of 18%) from the boundary zone and liquid ammonia (containing NH3 with a mass concentration of 99.8%) from the boundary zone enter the neutralization tank 1 from the fluosilicic acid inlet A1 and the first liquid ammonia inlet B1 respectively, and the molar ratio of H2SiF6 to NH3 is 1:2. The two solutions are mixed and reacted in the neutralization tank 1 by the action of the neutralization tank agitator 2 to generate an ammonium fluosilicate solution [containing (NH4)2SiF6 with a mass concentration of 21%). The ammonium fluorosilicate solution leaves the neutralization tank 1 from the ammonium fluorosilicate outlet C1 and enters the ammonolysis tank 3 through the ammonium fluorosilicate inlet A2. The liquid ammonia (containing NH3 with a mass concentration of 99.8%) from the boundary zone enters the ammonolysis tank 3 from the second liquid ammonia inlet B2. The molar ratio of (NH4)2SiF6 to NH3 is 1:4. The two solutions are mixed and reacted in the ammonolysis tank 3 by the action of the agitator 4 of the ammonolysis tank to generate a silicon dioxide-ammonium fluoride solution (containing SiO2 with a mass concentration of 6%). The silicon dioxide-ammonium fluoride solution enters the silicon dioxide separator 5 from the silicon dioxide-ammonium fluoride outlet C2 through the silicon dioxide-ammonium fluoride inlet A3 for sedimentation separation. The silicon dioxide goes to the boundary zone from the silicon dioxide outlet C3 to prepare silicon products. The ammonium fluoride solution (containing NH4F with a mass concentration of 26%) exits the silicon dioxide separator 5 from the ammonium fluoride solution outlet B3.

[0042] (2) Calcium fluoride preparation section: The ammonium fluoride solution from the silicon dioxide separator 5 and the calcium carbonate from the ammonium sulfate device enter the suspension preparation tank 8 from the ammonium fluoride solution inlet A4 and the calcium carbonate inlet B4 respectively. The molar ratio of NH4F to CaCO3 is 2:1. The two compounds are mixed in the suspension preparation tank 8 by the suspension agitator 9 to form a suspension. The suspension is sent to the suspension spray gun 11 installed in the calcium fluoride rotary kiln 12 by the suspension delivery pump 10, and the suspension is sprayed into the calcium fluoride rotary kiln 12 through the spray gun inlet A5. The slurry pressure at the spray gun outlet B5 is controlled at 80 kPa (gauge pressure). The heating gas (temperature 380°C) from the boundary area enters the calcium fluoride rotary kiln 12 from the heating gas inlet C5 to convert and dry the slurry. The dried calcium fluoride crystals are taken as the target product from the first calcium fluoride outlet E to the target product conveyor 15, and the target product conveyor 15 sends the target product-calcium fluoride (containing CaF2 mass concentration of 91%) to the boundary area. The tail gas (temperature 260° C.) in the calcium fluoride rotary kiln 12 leaves the calcium fluoride rotary kiln 12 from the first tail gas outlet F.

[0043] (3) Calcium fluoride recovery section: tail gas (containing CaF2, NH3, CO2, H2O) from the calcium fluoride rotary kiln 12 enters the cyclone separator 13 from the first tail gas inlet A6, the separated calcium fluoride is sent to the target product conveyor 15 from the second calcium fluoride outlet B6 of the cyclone separator 13, and the tail gas exits the cyclone separator 13 from the second tail gas outlet C6 of the cyclone separator 13. The tail gas from the cyclone separator 13 enters the bag filter 14 from the second tail gas inlet A7, the separated calcium fluoride is sent to the target product conveyor 15 from the third calcium fluoride outlet B7 of the bag filter 14, and the tail gas (temperature 170°C) exits the bag filter 14 from the third tail gas outlet C7 of the bag filter 14.

[0044] (4) Tail gas absorption section: Tail gas (containing NH3, CO2, H2O) from bag filter 14 enters absorption tower 16 from the third tail gas inlet A8. The tail gas in absorption tower 16 flows from the bottom to the top of the tower and contacts with the absorption liquid in the tower in countercurrent. The tail gas is cooled, and NH3 and CO2 in the tail gas are absorbed by the absorption liquid. The non-condensable gas in the tail gas goes to the boundary area from the non-condensable gas outlet D8 of absorption tower 16. The absorption liquid from cooler 18 is mixed with the ammonium sulfate solution [containing (NH4)2SO4 mass concentration of 45%] from the ammonium sulfate device and enters absorption tower 16 from the first absorption liquid inlet C8. The absorption liquid in absorption tower 16 flows from the top to the bottom of the tower and contacts with the tail gas in the tower in countercurrent. The absorption liquid absorbs the condensed water, NH3 and CO2 in the tail gas. The absorption liquid at the bottom of the absorption tower 16 (temperature 60°C) goes to the circulation pump 17 from the first absorption liquid outlet B8. A part of the absorption liquid leaving the circulation pump 17 is used as ammonium sulfate-ammonium carbonate solution [containing (NH4)2SO4 at a mass concentration of 30% and (NH4)2CO3 at a mass concentration of 11%) to go to the ammonium sulfate device to produce ammonium sulfate, and the rest goes to the cooler 18 for cooling.

[0045] The results of the operation of the present invention are shown in Table 1.

[0046] Table 1 Performance evaluation results of 180 tons / day calcium fluoride production unit using fluorosilicic acid

[0047]

Claims

1. A system for producing calcium fluoride, characterized in that: The invention comprises a neutralization tank (1), wherein the neutralization tank (1) is connected to an ammonia decomposition tank (3), a silicon dioxide separator (5), a suspension preparation tank (8), and a calcium fluoride rotary kiln (12) in sequence; a suspension spray gun (11) is installed on one side of the calcium fluoride rotary kiln (12) connected to the suspension preparation tank (8), and a target product conveyor (15) is connected to the bottom of the other side of the calcium fluoride rotary kiln (12).

2. The system according to claim 1, characterized in that: The top of one side of the calcium fluoride rotary kiln (12) connected to the target product conveyor (15) is connected to the cyclone separator (13), and the cyclone separator (13) is connected to the bag filter (14), the absorption tower (16), the circulation pump (17) and the cooler (18) in sequence; the bottom of the cyclone separator (13) and / or the bag filter (14) is connected to the target product conveyor (15) through a pipeline.

3. The system according to claim 1, characterized in that: A neutralization tank agitator (2) and an ammonolysis tank agitator (4) are installed on the top of the neutralization tank (1) and the ammonolysis tank (3), respectively; a separator rake (6) and an ammonium fluoride agitator (7) are installed on the top of the silicon dioxide separator (5), respectively.

4. The system according to claim 1, characterized in that: A suspension agitator (9) and a suspension delivery pump (10) are installed on the top of the suspension preparation tank (8).

5. The system according to claim 2, characterized in that: The circulating pump (17) is connected to the bottom of the cooler (18), and the top of the cooler (18) is connected to the absorption tower (16).

6. The system according to claim 3, characterized in that: The rake arm of the separator rake (6) is located at the bottom of the silicon dioxide separator (5), wherein the bottom of the silicon dioxide separator (5) is funnel-shaped, and the height of the rake arm of the separator rake (6) is lower than the paddle of the ammonium fluoride stirrer (7).

7. The system according to claim 4, characterized in that: The blade height of the suspension agitator (9) is higher than the bottom end of the suspension delivery pump.

8. A method for producing calcium fluoride using the system according to any one of claims 1 to 7, characterized in that: The method includes: (1) Preparation of ammonium fluoride: Fluorosilicic acid and liquid ammonia are mixed and reacted in a neutralization tank (1) to generate an ammonium fluosilicate solution, which is then mixed with liquid ammonia in an ammonolysis tank (3) and reacted to generate a silicon dioxide-ammonium fluoride solution. The silicon dioxide-ammonium fluoride solution is precipitated and separated in a silicon dioxide separator (5) to generate silicon dioxide and ammonium fluoride solution; (2) Preparation of calcium fluoride: ammonium fluoride solution is mixed with calcium carbonate in a suspension preparation tank (8) to form a suspension, and the suspension is pressurized by a suspension spray gun (11) and then sent into a chamber of a calcium fluoride rotary kiln (12) for heating, conversion, and drying to obtain calcium fluoride; (3) Calcium fluoride recovery: the tail gas from the calcium fluoride rotary kiln (12) enters the cyclone separator (13) and the bag filter (14) in sequence to continue separation and obtain calcium fluoride; (4) Tail gas absorption: The tail gas from the bag filter (14) enters the absorption tower (16) and the cooler (18) in sequence so that NH3 and CO2 in the tail gas are completely absorbed.

9. The method according to claim 8, characterized in that: In the step (1), the molar ratio of fluorosilicic acid to liquid ammonia is 1:1.5-2.5, and the molar ratio of ammonium fluorosilicate solution to liquid ammonia is 1:3-5.

10. The method according to claim 8, characterized in that: In the step (2), the molar ratio of ammonium fluoride to calcium carbonate is 1.5-2.5:1, and the temperature in the calcium fluoride rotary kiln (12) is 350-400°C.

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