Method for producing calcium fluoride

Abstract

This invention provides a method for efficiently producing high-purity calcium fluoride. [Solution] A method for producing calcium fluoride, (1) A process for producing silica and ammonium fluoride from hydrofluorosilicic acid and ammonium components, (2) A step of reacting the ammonium fluoride and calcium carbonate at 50°C or higher to produce calcium fluoride and an ammonium component, (3) A step of recovering and recycling the ammonium component produced by step (2) above. A method for producing calcium fluoride, characterized by condensing the water vapor accompanying the ammonium component produced together with ammonium fluoride in step (2) at 70°C to 100°C and returning to step (2).

Description

Technical Field

[0001] The present invention relates to a method and an apparatus for continuously and stably producing calcium fluoride used as a raw material for producing hydrofluoric acid.

Background Art

[0002] When hexafluorosilicic acid, which is produced as a by-product in large quantities in the wet phosphoric acid industry and the hydrofluoric acid production industry, is neutralized and decomposed with ammonia and / or ammonium carbonate, silica with good filterability precipitates, and an aqueous solution of ammonium fluoride can be obtained. The silica obtained by this method has been used as one of the methods for producing wet silica since ancient times because ammonium fluoride volatilizes due to heating during drying and thus hardly remains in the silica.

[0003] Furthermore, a recycling technique is known in which calcium fluoride is synthesized and recovered from wastewater containing hexafluorosilicic acid discharged in the semiconductor industry, the chemical industry, etc., and the obtained calcium fluoride is used as a raw material for producing hydrofluoric acid.

[0004] As a method for producing such calcium fluoride, Japanese Patent Publication No. 2013-220956 (Patent Document 1) describes a method that focuses on the solubility in water of the fluoride salt used in the conversion of fluorine in hydrofluoric silica water to calcium fluoride, and states that calcium fluoride suitable as a raw material for hydrofluoric acid production can be produced by reacting an ammonium component, which has high solubility in water, with hydrofluoric silica water. Specifically, cited document 1 discloses a production method in which hydrofluoric acid-containing water and an ammonium compound are reacted to produce a first reaction product containing insoluble silica and an aqueous ammonium fluoride solution, the insoluble silica is separated from the first reaction product, the remaining first reaction product is reacted with calcium carbonate to produce a second reaction product containing calcium fluoride and an aqueous ammonium carbonate solution, and the aqueous ammonium carbonate solution is separated from the second reaction product. Patent Document 1 also describes the recovery and reuse of ammonia generated in the reaction of ammonium fluoride and calcium carbonate.

[0005] Furthermore, Japanese Patent Publication No. 2015-157734 (Patent Document 2) discloses a method for neutralizing exhaust gas containing hydrogen fluoride and at least one of hydrogen chloride and sulfur oxides, wherein the neutralization treatment is carried out in two stages, in which the exhaust gas is brought into contact with slaked lime powder at a temperature of 180°C to 400°C in the first stage of neutralization treatment to produce calcium fluoride, and in the second stage of neutralization treatment, the exhaust gas neutralized in the first stage of neutralization treatment is brought into contact with sodium bicarbonate.

[0006] U.S. Patent No. 4,093,706 (Patent Document 3) describes a method for producing calcium fluoride from ammonium fluoride and calcium carbonate. Furthermore, U.S. Patent No. 4,120,940 (Patent Document 4) discloses the production of calcium fluoride by reacting calcium carbonate with ammonium fluoride or hydrofluoric acid.

[0007] International Publication No. 2016 / 171535 (Patent Document 5) discloses a method in which fluorisic acid is reacted with ammonia to obtain an ammonium fluoride solution, which is then reacted with calcium carbonate to produce a slurry containing calcium fluoride and ammonium carbonate, and the by-product ammonium carbonate is partially decomposed into ammonia, which is then distilled and condensed to recover the liquid ammonia and recycled into the reaction. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2013-220956 [Patent Document 2] Japanese Patent Publication No. 2015-157734 [Patent Document 3] U.S. Patent No. 4093706 [Patent Document 4] U.S. Patent No. 4120940 [Patent Document 5] International Publication No. 2016 / 171535 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] Although Patent Document 1 discloses the recovery of ammonia, it does not suggest how to actually recover and reuse the ammonia from the ammonium carbonate produced. Furthermore, the manufacturing method in Patent Document 1 has the problem of low ammonia recovery efficiency due to the low concentration of ammonium carbonate when reused.

[0010] Furthermore, although Patent Document 5 discloses the recovery of ammonia, the high concentrations of fluorine and ammonia in the wastewater necessitate the separate removal of fluorine and ammonia, and a distillation condenser is required, resulting in the problem of a large amount of unreacted calcium carbonate remaining. Therefore, the object of the present invention is to provide a method for efficiently producing high-purity calcium fluoride. [Means for solving the problem]

[0011] The present invention was completed by discovering that the calcium fluoride production method, by adding a predetermined ammonium component recycling process, can resolve all of the above problems and produce calcium fluoride.

[0012] The gist of this invention is summarized in the following [1] to [5]. [1] A method for producing calcium fluoride, (1) A step (step (1)) of producing silica and ammonium fluoride from hydrofluorosilicic acid and ammonium components, (2) A step of reacting the ammonium fluoride and calcium carbonate at 50°C or higher to produce calcium fluoride and the ammonium component (step (2)), (3) A step (3) of recovering and recycling the ammonium component generated in step (2) above. A method for producing calcium fluoride, characterized by condensing the water vapor accompanying the ammonium component produced in step (2) together with ammonium fluoride at 70°C to 100°C and returning it to step (2). [2] A method for producing calcium fluoride according to [1], wherein the average particle size of calcium carbonate in (2) is 5 μm to 80 μm. [3] A method for producing calcium fluoride according to [1] or [2], wherein the molar ratio of ammonium fluoride to calcium carbonate in (2) is 1.90 to 2.05. [4] A method for producing calcium fluoride of [1], wherein condensation is carried out by reflux cooling. [5] A apparatus for producing calcium fluoride, (1) A tank for producing ammonium fluoride by reacting hydrofluoric acid and ammonium components, (2) A calcium fluoride production tank for reacting the ammonium fluoride and calcium carbonate at 50°C or higher. (3) The absorption tank for recycling the separated ammonium component into the ammonium fluoride production tank is provided. The calcium fluoride production tank is equipped with a means for condensing the accompanying water of the ammonium component at 70°C to 100°C, and is a production device for calcium fluoride.

Effects of the Invention

[0013] According to the production method of the present invention, high-purity calcium fluoride can be produced reactively and efficiently. According to the present invention, an apparatus suitable for such a production method can be provided. Since the recovered silica has few impurities, it can be used as various fillers, and the calcium fluoride also has high purity and can be used for optical parts and the like.

Brief Description of the Drawings

[0014] [Figure 1] It is a schematic process diagram showing the outline of the present invention. [Figure 2] It shows a schematic view of the implementation apparatus of step (1) used in the example. [[ID=二十六]] [Figure 3] It shows a schematic view of the implementation apparatus for evaluating the reaction temperature of step (2) used in the example. [Figure 4] It shows a schematic view of the implementation apparatus diagram of step (2). [Figure 5] It shows the particle size distribution of the raw material calcium carbonate and the obtained calcium fluoride in the example.

Modes for Carrying Out the Invention

[0015] The embodiments of the present invention will be described below. Figure 1 is a schematic process diagram of the method for producing calcium fluoride according to the present invention.

[0016] The method for producing calcium fluoride of the present invention is (1) A step of producing silica and ammonium fluoride from hydrofluosilicic acid and an ammonium component, and (2) A step of reacting the ammonium fluoride and calcium carbonate at 50°C or higher to produce calcium fluoride and an ammonium component, (3) A step of recovering and recycling the ammonium component produced by step (2) above. The process is characterized by condensing the water vapor accompanying the ammonium component produced together with ammonium fluoride in step (2) at 70°C to 100°C and returning it to step (2).

[0017] Process (1) Step (1) is a step in which hydrofluorosilicic acid and an ammonium component are reacted to produce a first reaction product containing silica and ammonium fluoride. In this step, the reaction is carried out in an aqueous solution system using water as the medium. In step (1), neutralization decomposition produces a first reaction product containing insoluble silica and an aqueous ammonium fluoride solution. In step (1), the following reaction, shown in formula (1), proceeds to produce silica and ammonium fluoride.

[0018] [C1] H2SiF6+3(NH4)2CO3+H2O→6NH4F+SiO2+2H2O+3CO2 (1)

[0019] The ammonium component is not particularly limited as long as it has sufficient solubility in water, but common compounds such as ammonium carbonate, ammonium bicarbonate, and ammonium hydroxide can be used. These compounds can also be used as at least one or a mixture of two or more of them. In particular, ammonium carbonate may be used. In this invention, highly volatile components such as ammonia and ammonium carbonate are used.

[0020] The hydrofluorosilica-containing water is not particularly limited as long as it contains hydrofluorosilica. For example, hydrofluorosilica produced when mixing an aqueous solution of waste phosphoric acid containing the element Si with hydrogen fluoride to produce purified phosphoric acid, as described in Japanese Patent Publication No. 2024-129545 by the present applicant, can be used.

[0021] The concentration of the hydrofluoric acid silica solution is not particularly limited, but it is preferably in the range of 10 to 25% by mass, more preferably 11 to 20% by mass. The reaction conditions for step (1) are preferably a pH of 7 to 14 and a reaction temperature of 40 to 100°C, more preferably 40 to 60°C. Furthermore, the amount of ammonium component added is preferably adjusted to 1.1 to 10 equivalents, the amount necessary for the reaction with hydrofluorosilicic acid. There are no particular restrictions on the pressure during the reaction, and it is preferable to carry it out at atmospheric pressure. The reaction time is not particularly limited as long as the reaction proceeds; it should be 30 minutes or longer. Both batch and continuous reactions can be used, but in the case of a continuous reaction, the residence time should be considered the reaction time.

[0022] Furthermore, if the pH is less than 7, reaction (1) may not proceed easily, and the reaction that produces the ammonium salt ((NH4)2SiF6) as shown in reaction (1-2) may proceed instead. If the reaction temperature is lower than 40°C, the reaction in reaction (1) below may not proceed sufficiently, and reaction (1-2) may proceed instead, which is undesirable. On the other hand, if the temperature is higher than 100°C, the amount of water evaporation may increase or the water may boil. In addition, if the reaction temperature is higher than 100°C, the ammonium component (NH3) will volatilize more easily during the reaction, and the amount of NH4 ions in the reaction solution will decrease, making it easier for the ammonium salt ((NH4)2SiF6) as shown in reaction (1-2) to be formed.

[0023] [Case 2] H2SiF6+(NH4)2CO3→ (NH4F)2SiF6+ CO2+H2O (1-2)

[0024] The insoluble silica and aqueous ammonium fluoride solution are separated from the reaction product. There are no particular restrictions on the separation method, but the reaction product may be separated by filtration or by sedimentation.

[0025] The ammonium fluoride aqueous solution from which the silica has been separated is then subjected to the treatment in step (2). The recovered insoluble silica is washed with a washing solution such as water as appropriate, and then dehydrated and dried. The reaction in step (1) can be evaluated by elemental analysis of the filtrate from which silica has been removed.

[0026] Process (2) In step (2), the ammonium fluoride and calcium carbonate aqueous solution obtained by separating silica in step (1) are reacted at 50°C or higher to produce calcium fluoride. The synthesis of calcium fluoride proceeds as shown in the reaction equation (2) below.

[0027] [C3] CaCO3+2NH4F→CaF2+2NH3+CO2(2)

[0028] Furthermore, ammonium carbonate or ammonium bicarbonate may be formed from ammonia and carbon dioxide.

[0029] The reaction conditions for reaction equation (2) are such that the reaction temperature should be 50°C, more preferably 60°C or higher, and even more preferably 80-100°C. Within this temperature range, the reaction rate of calcium fluoride is high, and the amount of unreacted calcium carbonate is reduced. Furthermore, it becomes possible to recover the ammonium component from the product without any residue remaining in the calcium fluoride. The reaction time is not particularly limited as long as the reaction proceeds; it should be 30 minutes or longer. Both batch and continuous reactions can be used, but in the case of a continuous reaction, the residence time should be considered the reaction time.

[0030] In this invention, the water vapor accompanying the ammonium component produced in step (2) is condensed at 70°C to 100°C and returned to the reaction solution.

[0031] Specifically, a cooler is provided in the reactor where step (2) is carried out, and the water is condensed into steam at a temperature of 70 to 100°C, preferably 85 to 95°C. The condensed water is returned to the reactor in step (2). There are no particular restrictions on the cooler, but it is preferable to use a reflux cooler. By condensing at such a temperature, the amount of water used in the subsequent ammonium component recovery can be reduced, and the recovery efficiency of the ammonium component can be increased.

[0032] The calcium carbonate used in step (2) is not particularly limited, but it is generally preferable that the average particle size is 97 μm or less, more preferably 5 μm to 80 μm, and more preferably 10 to 40 μm. Since the resulting calcium fluoride particles correlate with the particle size of the calcium carbonate, the particle size of the calcium carbonate is almost maintained, so the particle size of the calcium carbonate used can be appropriately selected according to the desired particle size of calcium fluoride. If the particle size is too large, the reaction to the core of the calcium carbonate is difficult, but if the particle size is within the normal range, the purity of the resulting calcium fluoride can be increased.

[0033] In (2) above, the molar ratio of ammonium fluoride to calcium carbonate is preferably 1.90 to 2.50, and more preferably 2.00 to 2.05. At this ratio, high-purity calcium fluoride can be produced.

[0034] In this specification, "average particle diameter" refers to the particle diameter at 50% by mass (median diameter) of the integrated particle size distribution determined by laser diffraction and scattering. After the reaction, the resulting calcium fluoride and ammonium components are separated. Typically, the reaction solution is a slurry, and solid-liquid separation is performed by solid-liquid separation operations such as filtration and centrifugation. The recovered synthetic calcium fluoride may be washed with a washing solution such as water or ethanol as appropriate and then dried.

[0035] In step (2), the ion concentration can be measured using an electrical conductivity meter or similar device, and the degree of reaction progress can be confirmed from the amount of remaining ammonium fluoride.

[0036] Process (3) Step (3) involves recovering and recycling the ammonium component generated in step (2).

[0037] There are no particular restrictions on the recovery of the ammonium component, but it can be absorbed by contacting it with water. If the reaction temperature in step (2) is high, cooling may be performed as appropriate. In this invention, the recovered ammonia water, ammonium bicarbonate aqueous solution, or ammonium carbonate aqueous solution is recycled and reused as the ammonium component in step (1). For this purpose, the contact time with the ammonia absorbent and the temperature during recovery are adjusted to ensure that the solution contains a predetermined concentration of ammonium components.

[0038] Using the ammonium component recovered from step (2) increases the decomposition efficiency of hydrofluorosilicic acid in step (1), which in turn increases the silica recovery efficiency. This reduces the amount of silicon in the wastewater, making it possible to increase the purity of calcium fluoride. The reason for this is that the ammonium component recovered in step (2) undergoes a specific treatment, resulting in a higher pH than commercially available ammonium carbonate, which is thought to increase the decomposition efficiency of hydrofluorosilicic acid. When reagent-grade ammonium carbonate solution is used, the decomposition efficiency of hydrofluorosilicic acid is lower.

[0039] Undecomposed hydrofluorosilicic acid remains in wastewater and recovered calcium fluoride, which can affect yield and quality. However, according to the present invention, there are fewer unreacted substances in the wastewater, and the concentrations of fluorine and ammonium in the wastewater can also be reduced.

[0040] The manufacturing method of the present invention can be carried out as a series of continuous steps, enabling long-term continuous operation. Alternatively, the process may be carried out in batches, with the results supplied to each step.

[0041] According to the present invention, since the ammonium component can be recycled, the raw materials can be used effectively, and the production efficiency can be adjusted by adjusting the reaction temperature and condensation temperature in step (2).

[0042] According to the present invention, a calcium fluoride production apparatus is provided, (1) A tank for producing ammonium fluoride by reacting hydrofluoric acid and ammonium components, (2) A calcium fluoride production tank for reacting the ammonium fluoride and calcium carbonate at 50°C or higher. (3) The separated ammonium components are provided in an absorption tank for recycling into an ammonium fluoride production tank. The calcium fluoride production apparatus is provided, which includes a means for condensing the entrained water containing ammonium components at 70°C to 100°C in the calcium fluoride production tank.

[0043] A liquid transfer mechanism such as pipes or valves may be provided between each tank as appropriate. Furthermore, a solid-liquid separator, washing device, electrical conductivity meter, etc., may be provided between each tank. As the solid-liquid separator, filtration devices such as suction filtration, vacuum filtration, and filter presses, as well as sedimentation tanks and centrifugal separators, can be used. As the absorption tank, a scrubber that brings water and ammonia into contact inside can be used. Additionally, cooling and heating devices for heating and cooling the reaction solution and reaction tanks may be provided as appropriate. As the condensing means, a reflux type cooler that circulates a cooling medium inside is preferred because it allows for easy temperature control.

[0044] [Examples] The present invention will be described in detail below with reference to examples, but the present invention is not intended to be limited to these examples.

[0045] [Reference example 1] Step (1): A step to produce silica and ammonium fluoride by reacting hydrofluorosilicic acid with ammonium carbonate. The experimental apparatus diagram for step (1) shown in Figure 2 was used. This experimental apparatus consists of a 2L glass separable flask equipped with a Liebig condenser, a full-zone stirrer, and a thermometer, set in a heated stirrer and an oil bath. 824.0 g of an aqueous solution of ammonium carbonate, prepared by dissolving commercially available ammonium carbonate reagent in water to adjust the NH3 content to 6.20% by mass, was charged into the reactor. Next, the temperature was raised to 49°C while stirring at 90 rpm.

[0046] 623.2 g of an 11.56% by mass aqueous solution of fluorinated silica was added dropwise to a reactor (50°C) over 40 minutes using a rotary pump, and then aged at 51°C for 40 minutes. After maturation, the reactor temperature was cooled to 25°C, and solid-liquid separation was performed using a suction filter. A primary wash was then carried out with 200g of water, and a total of 1,385g of the filtrate and primary wash solution was recovered. Analysis of this recovered solution revealed a fluorine content of 3.99% by mass, a silicon content of 1,700 ppm by mass, and a pH of 7.60. Based on the fluorine content, the ammonium fluoride concentration was calculated to be 7.8% by mass.

[0047] The solid components were further washed with 2,000 ml of pure water, and dried at 105°C for 1 hour followed by 200°C for 3 hours to recover 27.9 g of silica. The recovered silica had an average particle size of 41.0 μm, a fluorine content of 20 ppm by mass or less, and an ammonia content of 10 ppm by mass or less.

[0048] [Reference examples 2~3] Step (1) was carried out in the same manner as in Preparation Example 1, except that the molar ratio of fluorine and ammonia, the reaction temperature, and the maturation temperature were as shown in Table 1.

[0049] [Example 1] Manufactured by reusing ammonium carbonate generated in process (2). Similar to Preparation Example 1, a 2L glass separable flask equipped with a Liebig condenser, a full-zone stirrer, and a thermometer was set up in a heated stirrer and oil bath. 823.2g of an aqueous solution of ammonium carbonate (NH3 content 12.4% by mass), recovered in Examples 6, 9, 10, 11, and 12 described later, was charged into the reactor as the ammonium component. Next, the temperature was raised to 30°C using the heated stirrer and oil bath while stirring at 90 rpm. After this, a 23.12% by mass aqueous solution of hydrofluorosilicic acid (F content 18.3% by mass) was placed in a 2L polypropylene bottle, placed on a weighing scale, and set up for quantitative supply using a rotor pump. The temperature inside the reactor was adjusted to around 30°C, and 623.2g of the hydrofluorosilicic acid solution was injected over 30 minutes while stirring at 90 rpm to allow the reaction to proceed. After the reaction, the liquid temperature was maintained at 30°C for 120 minutes while stirring to allow for maturation.

[0050] The reaction slurry was filtered and washed with 200 g of water. A total of 1,351 g of the filtrate and primary washing solution was recovered, and analysis of this recovered solution revealed a fluorine content of 7.80 mass%, a silicon content of 190 mass ppm, and a pH of 8.70. The solids were further washed with 2,000 ml of pure water, and dried at 105°C for 1 hour followed by 200°C for 3 hours to recover 58.7 g of silica. The average particle size of the recovered silica was 48.0 μm, the fluorine content was 20 ppm by mass or less, and the ammonia content was 10 ppm by mass or less.

[0051] [Examples 2-5] Similar to Example 1, the aqueous ammonium carbonate solution (NH3 content 12.4% by mass) recovered in step (2) was used. Examples 2 and 3 were carried out with an NH3 / F molar ratio of 1.00 by equivalent, and the reaction temperature and aging temperature were varied. Examples 4 and 5 were carried out with an NH3 / F molar ratio of 1.10 by equivalent, under conditions of excess NH3, and the reaction temperature and aging temperature were varied. The experimental conditions and results are shown in Table 1.

[0052] The aqueous solution prepared from commercially available ammonium carbonate had a pH of 7.85. On the other hand, the aqueous solution of ammonium carbonate recovered in step (2) had a pH of 9.80. The reason the recovered ammonium carbonate aqueous solution has a high pH is thought to be that, in addition to the ammonium carbonate component, some of the ammonium carbonate produced during the reaction process decomposes and the carbon dioxide volatilizes, resulting in the presence of ammonia components with a high pH. There is a correlation between the pH of the reaction solution and the concentration of silicon remaining in the solution. When commercially available ammonium carbonate reagent with a pH not exceeding 8 was used, the silicon content in the recovered ammonium fluoride aqueous solution could not be reduced below 700 ppm by mass. On the other hand, when the ammonium carbonate solution recovered in step (2) was used, the pH was between 8 and 9, and the silicon content could be reduced to below 200 ppm by mass in all cases.

[0053] The main cause of residual silicon in the solution is thought to be undecomposed hydrofluorosilicic acid based on reaction equation (1). Therefore, a lower silicon residue is preferable, as it reduces the silica content in the calcium fluoride obtained in step (2). Based on these results, it is preferable to use the ammonium carbonate solution recovered in step (2).

[0054] [Table 1]

[0055] [Preparation Example 4] Step (2): Process for producing calcium fluoride from ammonium fluoride and calcium carbonate. The reaction apparatus shown in Figure 3 was used. This apparatus consisted of a 2L separable glass flask equipped with a full-zone stirrer and thermometer, set in a heated stirrer and water bath. 153g of commercially available ammonium fluoride reagent with a purity of 97% by mass was placed in the reactor and then dissolved in 900g of water. Next, 1g of 200g of 99.5% by mass CaCO3 was placed in the reactor and heated to the target temperature shown in Table 2 using a heated stirrer and water bath while stirring at 140 rpm. Once the target temperature was reached, the reaction was started and the temperature was maintained for each reaction time. After the reaction was complete, the reactor temperature was cooled to 23°C, solid separation was performed using a suction filter, and the calcium fluoride was recovered by drying.

[0056] [Table 2]

[0057] It was found that the reaction efficiency could be increased by setting the reaction temperature to 50°C or higher.

[0058] [Example 6] Figure 4 shows the apparatus diagram for process (2). The reactor used was a 3L container made of jacketed SUS316L, equipped with a full-zone stirrer, thermometer, and electrical conductivity measuring probe. Heating was performed using low-pressure steam, and the amount of heat supplied was controlled by controlling the steam flow rate. A reflux condenser made of SUS316L, whose temperature was controlled with hot water, was installed at the gas outlet to return some of the water in the gas coming out of the reactor, thereby preventing the concentration of the recovered ammonium carbonate aqueous solution from becoming diluted.

[0059] The gas discharged from the reflux condenser was mostly absorbed by a scrubber equipped with a condenser. The piping between the reflux condenser and the scrubber was wrapped with ribbon heaters set to 80°C, above the ammonium carbonate decomposition temperature, to prevent blockage due to ammonium carbonate precipitation. 1975.4 g of a 7.5% by mass aqueous solution of ammonium fluoride (NH4F: 4 mol), prepared by adding water to the ammonium fluoride aqueous solutions obtained in Examples 1-5, and 201.2 g of 99.5% by mass calcium carbonate (CaCO3: 2 mol) (NH4F / CaCO3 molar ratio 2.00) with an average particle size of 12.6 μm (particle size distribution shown in Figure 5, 1a) were charged into the reactor. The mixture was heated by flowing steam through the jacket while stirring at 90 rpm. Foaming began when the temperature inside the reactor exceeded 60°C, and the temperature at the reactor outlet rose. When the temperature at the reactor outlet reached 80°C, 90°C hot water was started to flow through the reflux condenser. The internal temperature reached 98°C in about 2 hours, and the reflux state was maintained for another 3 hours after reducing the amount of superheated steam. During this time, the electrical conductivity gradually decreased and finally stopped at 7.85 mS / cm. After stopping the heating steam and stirring, the contents of the reactor were filtered to obtain 1,476 g of filtrate and 178.9 g of wet solids.

[0060] The solids were dried at 105°C for 2 hours to recover 155.6 g of calcium fluoride. The filtrate contained 660 ppm of fluorine and 690 ppm of ammonia. The purity of the recovered calcium fluoride was 98.8% by mass, the unreacted calcium carbonate content was 1.03% by mass, and the loss on burning at 900°C for 1 hour was 2.55% by mass. The particle size distribution of the obtained calcium fluoride is shown in Figure 5-1b. It is almost similar to Figure 5-1a, but fine particles smaller than 2 μm have disappeared. The absence of fine particles smaller than 2 μm indicates improved filterability.

[0061] Based on the increase in the volume of scrubber liquid after the reaction and the change in the composition of the liquid, the recovered ammonium carbonate solution was 36.5% by mass and weighed 509.4 g, meaning that 96.7% by mass of ammonia was recovered.

[0062] [Example 7] 1975.4 g of a 7.5% by mass ammonium fluoride aqueous solution (NH4F: 4 mol), prepared by adding water to the ammonium fluoride aqueous solution obtained in step (1), and 201.2 g of calcium carbonate (CaCO3: 2 mol) (NH4F / CaCO3 molar ratio 2.00) with particle size 0.5-100 μm, average particle size 12.6 μm, and purity 99.5% by mass were charged into the reactor. The temperature was raised by flowing steam through the jacket while stirring at 90 rpm. Once the temperature reached 50°C, this temperature was maintained for 8 hours, and then it was allowed to cool while stirring until the next day. The next day, the contents of the reactor were filtered to obtain 1,976 g of filtrate and 189 g of wet solids. The solids were dried at 105°C for 2 hours to recover 157.4 g of calcium fluoride. The purity of the recovered calcium fluoride was 94.9% by mass, and the unreacted calcium carbonate was 3.48% by mass. Under reaction conditions of 50°C, the purity was 94.9% by mass, but the recovery efficiency was sufficient.

[0063] [Comparative Example 1, Examples 8-12] NH 4 F / CaCO 3 Effects of different molar ratios As shown in Table 3 below, the same procedure as in Example 6 was followed, but with the molar ratio of NH4F / CaCO3 varied to 1.95, 1.98, 2.02, and 2.04, and the temperature of the hot water flowing through the reflux condenser varied from 65°C to 95°C, and the same experiment was carried out.

[0064] This study confirmed that increasing the temperature of the hot water flowing through the reflux condenser to 70°C or higher, especially to 85°C or higher, increases the amount of ammonium that can be recovered and improves the recovery efficiency. Furthermore, it was found that calcium fluoride with a purity of 99% by mass or higher can be produced by setting the molar ratio of NH4F / CaCO3 to 2.00 or higher, and especially to 2.02 or higher.

[0065] [Table 3] % stands for mass%, ppm stands for mass ppm

[0066] [Examples 13-15] "Confirmation of the effects of different particle sizes of calcium carbonate" The same procedure as in Example 6 was performed, but with a different particle size of calcium carbonate. Example 13 uses commercially available calcium carbonate reagent (particle size 0.5-100 μm, average particle size 12.5 μm: particle size distribution is shown in Figure 5, 2a), and the particle size distribution after the reaction is shown in Figure 5, 2b. Example 14 uses calcium carbonate obtained by removing fine particles smaller than 20 μm from the calcium carbonate used in Example 6 using a sieve with a mesh size of 25 μm (particle size 20-100 μm, average particle size 43.0 μm: particle size distribution shown in Figure 5, 3a). The particle size distribution after the reaction is shown in Figure 5, 3b.

[0067] Example 15 uses industrial calcium carbonate with coarse particle size, from which particles smaller than 30 μm were removed using a sieve with a mesh size of 45 μm (particle size 30-400 μm, average particle size 93 μm: particle size distribution shown in Figure 5, 4a). The particle size distribution after the reaction is shown in Figure 5, 4b. Figure 5 shows the particle size distribution of the four types of calcium carbonate used as raw materials (left) and the particle size distribution of the recovered fluorite obtained from the reaction (right).

[0068] In both Figure 51b and 52b, it is clear that particles smaller than 2 μm have been eliminated. While the average particle size of the recovered calcium fluoride was smaller than that of the raw material calcium carbonate, the distribution did not change significantly. Reaction (2) proceeds via salt exchange, but it is thought that the dissolution and extraction of calcium fluoride also occurs simultaneously. The disappearance of particles smaller than 2 μm is a significant advantage when considering the filterability after the reaction. Table 4 summarizes these results, including those from Example 6. It was confirmed that the larger the particle size of the calcium carbonate, the lower the purity of the calcium fluoride.

[0069] [Table 4] % represents mass %

[0070] [Examples 16, 17] NH 4 Confirmation of the effect of F concentration The same procedure as in Example 6 was carried out, but with the molar ratio of NH4F / CaCO3 fixed at 2.00, and the concentration of ammonium fluoride used varied to 3.75% by mass and 13.5% by mass. The reaction conditions and results are shown in Table 5. The effect of the NH4F concentration used was almost negligible, and good results were obtained in all cases.

[0071] [Table 5] % stands for mass%, ppm stands for mass ppm

[0072] [Example 18] 1,500 g of the filtrate recovered in Example 6, with a fluorine content of 3,700 ppm by mass and an ammonium content of 3,400 ppm by mass, and 58.4 g of calcium carbonate, equivalent to four times the amount of fluorine based on reaction equation (2), were placed in the reactor shown in Figure 3. The mixture was heated by stirring at 90 rpm while steam was flowed through the jacket. When the temperature at the reactor outlet reached 80°C, 90°C hot water was started flowing into the reflux condenser. The internal temperature reached 100°C in about 2 hours, during which time the electrical conductivity gradually decreased and eventually fell to 50 μS / cm or less. At this point, stirring was stopped, and after cooling and filtration, the fluorine content in the filtrate was 14 ppm by mass and the ammonium content was 18 ppm by mass.

Claims

1. A method for producing calcium fluoride, (1) A step of producing silica and ammonium fluoride from hydrofluorosilicic acid and ammonium components (step (1)), (2) A step of reacting the ammonium fluoride and calcium carbonate at 50°C or higher to produce calcium fluoride and an ammonium component (step (2)), (3) A step (3) of recovering and recycling the ammonium component generated in step (2) above. A method for producing calcium fluoride, characterized by condensing the water vapor accompanying the ammonium component produced in step (2) together with ammonium fluoride at 70°C to 100°C and returning it to step (2).

2. The method for producing calcium fluoride according to claim 1, wherein the average particle size of calcium carbonate in step (2) is 5 μm to 80 μm.

3. A method for producing calcium fluoride according to claim 1 or 2, characterized in that the molar ratio of ammonium fluoride to calcium carbonate in step (2) is 1.90 to 2.

05.

4. A method for producing calcium fluoride according to claim 1, wherein condensation is performed by reflux cooling.

5. A calcium fluoride manufacturing apparatus, (1) A tank for producing ammonium fluoride by reacting hydrofluorosilicic acid and ammonium components, (2) A calcium fluoride production tank for reacting the ammonium fluoride and calcium carbonate at 50°C or higher. (3) The separated ammonium components are provided with an absorption tank for recycling into an ammonium fluoride production tank, The calcium fluoride production apparatus is provided with a means for condensing the entrained water containing ammonium components at a temperature of 70°C to 100°C.

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