Method for producing calcium fluoride

Abstract

To provide a method for producing calcium fluoride with fewer impurities while reducing the concentration of fluoride ions in wastewater. [Solution] The present invention is a method for producing calcium fluoride, comprising the steps of (A) adding slaked lime to a fluoride ion-containing solution, (B) lowering the pH, and (C) adding a calcium-containing compound other than slaked lime, in this order.

Description

Technical Field

[0001] The present invention relates to a method for producing calcium fluoride.

Background Art

[0002] Wastewater such as wafer etching solutions for semiconductor applications and absorption solutions for combustion decomposition of fluorocarbon-based gases contains fluoride ions, so it must be treated to ensure the fluoride ion concentration in the effluent is 8 mg / L or less in accordance with the effluent standards under the Water Pollution Control Act. For example, as a treatment method for fluoride ion-containing wastewater, the calcium fluoride method is known, in which a calcium-containing compound such as slaked lime (Ca(OH)₂) is added to the wastewater, and calcium fluoride is precipitated, converted to sludge, and recovered. Although the calcium fluoride method can reduce the fluoride ion concentration in wastewater, the calcium fluoride produced by this method contains unreacted slaked lime, calcium carbonate and the like, making it difficult to reuse, and thus is disposed of by landfilling as sludge. For this reason, various treatment methods for fluoride ion-containing wastewater have been studied.

[0003] For example, Patent Document 1 discloses a treatment method for fluorine-containing wastewater, which allows a calcium compound to effectively act on fluorine to obtain stable treated water quality with fewer steps and reduce the water content of the generated sludge.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of Invention

Problems to be Solved by the Invention

[0005] Patent Document 1 describes a method for reducing the fluoride ion concentration in wastewater and lowering the water content of the resulting sludge compared to conventional methods. However, the process is complicated because return sludge is essential during treatment, and further improvements are needed to reduce the amount of sludge treated as waste from an environmental protection perspective.

[0006] The present invention aims to provide a method for producing calcium fluoride with fewer impurities while reducing the concentration of fluoride ions in wastewater. [Means for solving the problem]

[0007] As a result of diligent research, the inventors of the present invention have found that while conventional treatment of fluoride-containing wastewater involves adding a large excess amount of slaked lime to the wastewater to reduce the fluoride ion concentration, by reducing the amount of slaked lime added to the fluoride-containing wastewater and adjusting the pH of the wastewater by adding acid to the slaked lime, not only is the fluoride ion concentration in the wastewater reduced, but the purity of the calcium fluoride contained in the sludge is also increased. This has led to the discovery of the possibility of reusing calcium fluoride, and thus the completion of the present invention. Specifically, the present invention is configured as follows [1] to

[11] .

[0008] [1] A method for producing calcium fluoride, comprising the steps of (A) adding slaked lime to a fluoride ion-containing solution, (B) lowering the pH, and (C) adding a calcium-containing compound other than slaked lime, in this order.

[0009] [2] The method for producing calcium fluoride according to [1], wherein the fluoride ion-containing liquid mainly comprises sodium fluoride.

[0010] [3] The method for producing calcium fluoride according to [1] or [2], wherein the calcium-containing compound other than slaked lime is calcium chloride.

[0011] [4] The method for producing calcium fluoride according to any one of [1] to [3], wherein step (B) is a step of adjusting the pH of the fluoride ion-containing solution after step (A) to 4 to 7.

[0012] [5] A method for producing calcium fluoride according to any one of [1] to [4], wherein 0.8 to 1.2 equivalents of slaked lime are added relative to the number of moles of fluoride ions in the fluoride ion-containing solution.

[0013] [6] A method for producing calcium fluoride according to any one of [1] to [5], wherein 0.2 to 0.8 equivalents of a calcium-containing compound other than slaked lime are added relative to the number of moles of fluoride ions in the fluoride ion-containing solution.

[0014] [7] A method for producing calcium fluoride according to any one of [1] to [6], further comprising the step of (D) agglomerating the generated calcium fluoride particles after the step of (C).

[0015] [8] The method for producing calcium fluoride according to [7], wherein a flocculant is used in step (D).

[0016] [9] A method for producing calcium fluoride according to any one of [1] to [8], further comprising the step of filtering and washing the aggregated calcium fluoride slurry after the step of (D).

[0017]

[10] A method for producing calcium fluoride according to any one of [1] to [9], wherein the fluoride ion-containing liquid is wastewater.

[0018]

[11] A water treatment method comprising, in this order: (a) adding slaked lime to fluoride ion-containing wastewater; (b) lowering the pH; (c) adding a calcium-containing compound other than slaked lime; (d) coagulating the generated calcium fluoride; and (e) filtering and washing the coagulated calcium fluoride slurry. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a method for producing calcium fluoride with few impurities while reducing the fluorine ion concentration in wastewater. [Mode for Carrying Out the Invention]

[0020] Hereinafter, specific embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and appropriate modifications can be made without departing from the gist of the present invention.

[0021] [Method for Producing Calcium Fluoride] The method for producing calcium fluoride includes, in this order: (A) a step of adding slaked lime to a fluorine ion-containing liquid, (B) a step of lowering pH, and (C) a step of adding a calcium-containing compound other than slaked lime. By performing the steps (A) to (C) in the above order, the fluorine ion concentration in wastewater can be reduced, and calcium fluoride with few impurities can be produced. Each step is described below.

[0022] <(A) Step of Adding Slaked Lime to Fluorine Ion-Containing Liquid> Examples of the step (A) of adding slaked lime to a fluorine ion-containing liquid (hereinafter also referred to as "step (A)") include a step of adding slaked lime to a fluorine ion-containing liquid to generate a slurry containing calcium fluoride (CaF₂) particles.

[0023] The above fluorine ion-containing liquid is wastewater that is generated from manufacturing factories in fields such as semiconductor manufacturing and its related fields, surface treatment fields for various metal materials or optical materials, combustion treatment processes for fluorocarbons, and laboratories, and contains fluorine ions as a main component as well as sodium ions, chloride ions and the like.

[0024] The fluorine ion concentration of the fluorine ion-containing liquid before step (A) is, for example, in the range of 1000 to 30000 mg / L.

[0025] Step (A) is a step of adding slaked lime to the fluorine ion-containing liquid having the above fluorine ion concentration to produce a slurry containing calcium fluoride particles. In this step, by preparing a slaked lime solution with a slaked lime concentration of 7 to 15 weight (wt)%, sedimentation of slaked lime is suppressed, and the solution can be added to the fluorine ion-containing liquid as a homogeneous solution. Further, the pH of the slurry containing calcium fluoride particles after the addition of slaked lime is preferably 11 or more, and the upper limit of the pH is more preferably 14 or less.

[0026] The addition amount of slaked lime is preferably 0.8 to 1.2 equivalents, more preferably 0.9 to 1.2 equivalents, relative to the number of moles of fluorine ions in the fluorine ion-containing liquid. By setting the addition amount of slaked lime within the above range, slaked lime will not be added in a large excess equivalent (for example, 1.5 equivalents or more), so unreacted slaked lime, calcium carbonate and the like are less likely to be contained in the resulting slurry containing calcium fluoride particles. Therefore, the purity of calcium fluoride contained in the filtered sludge can be increased.

[0027] <(B) Step of lowering pH> Examples of the step (B) of lowering pH (hereinafter also referred to as "(B) step") include a step of adding a pH adjuster to the fluorine ion-containing liquid after the step (A) to neutralize the pH of the fluorine ion-containing liquid from alkaline to neutral to weakly acidic. Specifically, a step of adjusting the pH of the slurry containing calcium fluoride particles with a pH of 11 or higher after the step (A) to 4 to 7 may be mentioned.

[0028] As the pH adjuster, an acid that forms a highly water-soluble calcium salt by neutralization, such as hydrochloric acid and nitric acid, can be used. Among these, hydrochloric acid is preferred from the viewpoint of ease of handling.

[0029] The concentration of the pH adjuster is not particularly limited, and for example, hydrochloric acid of 3.5 to 35 wt% can be used.

[0030] By adding a pH adjuster to set the pH of the fluoride ion-containing solution after step (A) to 4-7, the unreacted slaked lime added during step (A) can be dissolved. This significantly reduces the amount of unreacted slaked lime contained in the resulting calcium fluoride.

[0031] Furthermore, in step (B), the pH of the fluoride ion-containing solution after step (A) can be adjusted by providing a multi-stage pH adjustment tank and adding a pH adjusting agent (for example, 3.5-35 wt% hydrochloric acid) set to an arbitrary concentration in each tank. For example, first, 20-35 wt% hydrochloric acid can be added to adjust the pH of the fluoride ion-containing solution to 5-7.5, and then 3.5-20 wt% hydrochloric acid can be added to adjust the pH of the fluoride ion-containing solution to 4-7. By gradually lowering the pH of the fluoride ion-containing solution after step (A), it becomes easier to adjust it to the controllable pH range (pH 4-7).

[0032] <(C) Step of adding calcium-containing compounds other than slaked lime> (C) The step of adding a calcium-containing compound other than slaked lime (hereinafter also referred to as "step (C)") includes, for example, the step of adding a calcium-containing compound other than slaked lime to the slurry containing calcium fluoride particles after step (B) to react with the remaining fluoride ions. In this step, the remaining fluoride ions react with the calcium-containing compound other than slaked lime to produce calcium fluoride.

[0033] In step (A), by first reacting fluoride ions with slaked lime, which has low solubility in water, it is possible to suppress the formation of a large amount of fine calcium fluoride even when a calcium-containing compound other than slaked lime, which has high solubility in water, is used in this step. As a result, the generated calcium fluoride particles become a size that is easily aggregated, and the amount of suspended solids contained in the supernatant of the fluoride ion-containing liquid after step (D), described later, can be reduced.

[0034] Calcium-containing compounds other than slaked lime can be used in the pH range of 4 to 7 (an example of the pH range of the treatment solution after step (B)) and are not particularly limited as long as they are water-soluble. Examples of such water-soluble calcium-containing compounds include calcium chloride, calcium acetate, calcium lactate, and calcium nitrate, but calcium chloride is preferred because it is inexpensive and readily available.

[0035] The amount of calcium-containing compound other than slaked lime added is preferably 0.2 to 0.8 equivalents, and more preferably 0.3 to 0.6 equivalents, relative to the number of moles of fluoride ions in the fluoride ion-containing solution before step (A). By setting the amount of calcium-containing compound other than slaked lime within the above range, it reacts efficiently with the remaining fluoride ions, thereby reducing the fluoride ion concentration in the wastewater.

[0036] When using calcium chloride as a calcium-containing compound other than slaked lime, for example, it is preferable to use it within a range where the concentration of calcium chloride is below the saturation solubility.

[0037] <(D) Step of agglomerating the generated calcium fluoride particles> It is preferable that the process further includes a step (D) in which the generated calcium fluoride particles are agglomerated (hereinafter also referred to as "step (D)"). Specifically, step (D) is a step to improve the filterability of the agglomerated calcium fluoride slurry in step (E) (described later) by adding a flocculant to the calcium fluoride particle slurry generated after step (C) and agglomerating the calcium fluoride particles. In this invention, "agglomerated calcium fluoride slurry" refers to the agglomerated calcium fluoride slurry after the addition of a flocculant.

[0038] The type of flocculant is not particularly limited as long as it can flocculate calcium fluoride particles. Examples of flocculants include inorganic flocculants and polymer flocculants.

[0039] (Inorganic flocculant) Examples of inorganic flocculants include aluminum sulfate, polyaluminum chloride, ferric chloride, ferrous sulfate, and polyferric sulfate.

[0040] (Polymer flocculant) Examples of polymeric flocculants include cationic flocculants, anionic flocculants, and nonionic flocculants.

[0041] Examples of cationic flocculants include polyacrylic acid ester compounds, polyethyleneimine compounds, polyalkylaminoalkyl methacrylate quaternary salts, and vinylpyridine compound salts. Examples of anionic flocculants include polyacrylic acid compound salts and polyacrylamide compound salts. Examples of nonionic flocculants include polyacrylamide, polymethacrylamide, polyvinylformamide, polyvinylacetamide, polyethylene oxide, and polyvinyl alcohol.

[0042] Among these, it is preferable to use an anionic flocculant that improves cohesiveness due to the potential characteristics of the generated calcium fluoride particles, which are positively charged at a pH of 10 or higher, and the neutralizing anions surrounding these particles. Among anionic flocculants, polyacrylamide-based flocculants can be used as the optimal choice.

[0043] <(E) Process of filtering and washing the aggregated calcium fluoride slurry> (D) Preferably, the process further includes filtering the aggregated calcium fluoride slurry after step (D) and washing the resulting solids (hereinafter also referred to as "(E) step").

[0044] Specifically, step (E) includes a step of filtering the flocculated calcium fluoride slurry obtained in step (D) above (the solid content obtained after the filtering step is called "sludge") and a step of washing the filtered sludge with hot water. The treated water generated after filtering and washing is neutralized and then discharged.

[0045] In the washing process, washing the sludge with hot water, for example, water at 30-70°C, can reduce chloride ions derived from water-soluble calcium-containing compounds such as sodium chloride and calcium chloride contained in the sludge. This is because chloride ions derived from water-soluble calcium-containing compounds have high solubility in water within this temperature range and can therefore be easily removed. It is preferable to wash the sludge with 10 to 20 times or more the amount of water.

[0046] The filtration method is not particularly limited, but the operation of redispersing the sludge in water and then filtration and recovery may be repeated, or filtration and washing may be repeated using a filtration device with a washing function (e.g., a filter press).

[0047] The cleaning status of the sludge can be evaluated, for example, by measuring its conductivity using an electromagnetic conductivity meter.

[0048] The purity (dry basis) of calcium fluoride contained in the obtained sludge can be calculated as follows. For example, assuming that the fluorine (F2) content [wt%] is entirely calcium fluoride (CaF2) and that a portion of the calcium (Ca) content [wt%] exists as CaF2, and that F2 [wt%] = 38 (molar mass of fluorine: 19 g / mol), then Ca [wt%] = (38 / 19) / 2 × 40.078 (molar mass of calcium: 40.078 g / mol) = 40.078 wt%, so CaF2 [wt%] = 38 + 40.078 = 78.078 wt%.

[0049] Furthermore, the fluoride ion-containing liquid (hereinafter referred to as "treated water") separated from the sludge by the filtration process is neutralized with an aqueous solution of an alkaline compound such as sodium hydroxide. Any concentration of sodium hydroxide can be used in the neutralization process. Through steps (A) to (E), the fluoride ion concentration, which was 1,000 to 30,000 mg / L, can be reduced to a level that can meet the wastewater standards, for example, 90 mg / L or less, through subsequent secondary treatment and dilution operations.

[0050] The fluoride ion concentration can be measured, for example, using the lanthanum-alizarin complexone absorbance method in accordance with JIS K0102 34.1.

[0051] By going through steps (A) to (E), it is possible to recover sludge with a low impurity content (valuable CaF2 sludge).

[0052] [Water treatment methods] The water treatment method comprises the following steps in this order: (a) adding slaked lime to fluoride ion-containing wastewater; (b) lowering the pH; (c) adding calcium-containing compounds other than slaked lime; (d) coagulating the generated calcium fluoride particles; and (e) filtering and washing the coagulated calcium fluoride slurry. By performing steps (a) to (e) in the above order, treated water with a reduced fluoride ion concentration in the fluoride ion-containing liquid can be obtained. Each step is described below.

[0053] (a) A step of adding slaked lime to wastewater containing fluoride ions. (a) The step of adding slaked lime to fluoride ion-containing wastewater (hereinafter also referred to as "step (a)") is, for example, the step of adding slaked lime to fluoride ion-containing wastewater to produce calcium fluoride particles.

[0054] (a) Step (a) is a step of adding slaked lime to a fluoride ion-containing liquid having the above-mentioned fluoride ion concentration to produce calcium fluoride particles. In this step, by using a slaked lime solution of 7 to 15 wt%, the generation of precipitated components is suppressed, and it can be added to the fluoride ion-containing liquid as a uniform solution. Furthermore, the pH of the slurry containing calcium fluoride particles after the addition of slaked lime is preferably 11 or higher, and the upper limit of the pH is more preferably 14 or lower.

[0055] The amount of slaked lime added is preferably 0.8 to 1.2 equivalents, and more preferably 0.9 to 1.2 equivalents, relative to the number of moles of fluoride ions in the fluoride ion-containing solution. By setting the amount of slaked lime added within the above range, it is not possible to add a large excess equivalent amount of slaked lime (e.g., 1.5 equivalents or more), so that the resulting calcium fluoride particles are less likely to contain unreacted slaked lime or calcium carbonate. Therefore, the purity of the calcium fluoride contained in the filtered sludge can be increased.

[0056] <(b) Step to lower the pH> (b) A step to lower the pH (hereinafter also referred to as "step (b)") is, for example, a step in which a pH adjusting agent is added to the fluoride ion-containing solution after step (a) to neutralize the pH of the fluoride ion-containing solution from alkaline to neutral to weakly acidic. Specifically, this is a step in which a slurry with a pH of 11 or higher after step (a) is adjusted to have a pH of 4 to 7.

[0057] The same pH adjusting agent as in step (B) above can be used. Also, as with step (B) above, the concentration of the pH adjusting agent is not particularly limited, but for example, 3.5 to 35 wt% hydrochloric acid can be used.

[0058] By adding a pH adjuster to set the pH of the fluoride ion-containing solution after step (a) to 4-7, the unreacted slaked lime added during step (a) can be dissolved. This significantly reduces the amount of unreacted slaked lime contained in the calcium fluoride particles.

[0059] Furthermore, in step (b), the pH adjustment of the fluoride ion-containing solution after step (a) can be performed in the same way as in step (B) above, by providing a multi-stage pH adjustment tank and adding a pH adjusting agent (for example, 3.5 to 35 wt% hydrochloric acid) set to an arbitrary concentration in each tank. For example, first, 20 to 35 wt% hydrochloric acid can be added to adjust the pH of the fluoride ion-containing solution to 5 to 7.5, and then 3.5 to 20 wt% hydrochloric acid can be added to adjust the pH of the fluoride ion-containing solution to 4 to 7.

[0060] <(c) Step of adding calcium-containing compounds other than slaked lime> (c) A step of adding a calcium-containing compound other than slaked lime (hereinafter also referred to as "step (c)") is, for example, a step of adding a calcium-containing compound other than slaked lime to the slurry after step (b) to react with the remaining fluoride ions. In this step, the remaining fluoride ions react with the calcium-containing compound other than slaked lime to produce calcium fluoride.

[0061] (a) In step (a), by first reacting fluoride ions with slaked lime, which has low solubility in water, it is possible to suppress the formation of a large amount of fine calcium fluoride even when a calcium-containing compound other than slaked lime, which has high solubility in water, is used in this step. As a result, the calcium fluoride particles are made to a size that is easily aggregated, and the amount of suspended solids contained in the supernatant of the fluoride ion-containing liquid after step (d), described later, can be reduced.

[0062] <(d) Step of agglomerating the generated calcium fluoride particles> (c) Preferably, the process further includes (d) a step of agglomerating the generated calcium fluoride particles (hereinafter also referred to as "(d) step"). Specifically, step (d) is a step to improve the filterability of the agglomerated calcium fluoride slurry in step (e) (described later) by adding a flocculant to the calcium fluoride particle slurry generated after step (c) and agglomerating the calcium fluoride particles.

[0063] Similar to step (D) described above, the type of flocculant is not particularly limited as long as it can flocculate and settle the calcium fluoride particles. Furthermore, the same flocculant as in step (D) can be used.

[0064] (e) The process of filtering and washing the aggregated calcium fluoride slurry. (e) Steps include, for example, a step of filtering the calcium fluoride slurry obtained after step (d) and washing the resulting solids (hereinafter also referred to as "step (e)").

[0065] Specifically, step (e) includes a step of filtering the flocculated calcium fluoride slurry obtained in step (d) above (the solid content obtained after the filtering step is called "sludge"), and a step of washing the filtered sludge with hot water, for example, water at 30-70°C. The treated water generated after filtering and washing is neutralized and discharged.

[0066] The filtration and washing processes can be carried out in the same manner as in process (E) described above. By going through these processes, sludge with a low impurity content (valuable CaF2 sludge) can be recovered.

[0067] The cleaning status of the sludge can be evaluated by measuring its conductivity in the same manner as in step (E) described above.

[0068] The purity of calcium fluoride contained in the obtained sludge can be calculated using the same method as in step (E) described above.

[0069] Furthermore, the treated water separated from the sludge by the filtration process is neutralized with an aqueous solution of an alkaline compound such as sodium hydroxide. In the neutralization process, sodium hydroxide of any concentration that allows for pH adjustment can be used. Through steps (a) to (e), the fluoride ion concentration, which was 1,000 to 30,000 mg / L, can be reduced to a level that can meet the wastewater standards, for example, 90 mg / L or less, through subsequent secondary treatment and dilution operations.

[0070] The above fluoride ion concentration can be measured in accordance with JIS K0102 34.1, in the same manner as in step (E) described above. [Examples]

[0071] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0072] [Example 1] To a fluoride ion-containing solution (liquid temperature 40°C, fluoride ion concentration 12000 mg / L, pH 9) obtained by absorbing a gas containing hydrogen fluoride (HF) into a 1.0 wt% sodium hydroxide (NaOH) aqueous solution, 10 wt% slaked lime was added in an amount equal to 1.0 equivalent relative to the number of moles of fluoride ions in the fluoride ion-containing solution. Next, 3.5 to 35 wt% hydrochloric acid was added in stages to adjust the pH of the fluoride ion-containing solution to 4 to 7, and then 10 wt% calcium chloride was added in an amount equal to 0.3 equivalent relative to the number of moles of fluoride ions in the fluoride ion-containing solution before treatment. Then, an anionic flocculant (polyacrylamide-based flocculant) was added to flocce the calcium fluoride particles, and the mixture was flocculated and settled in a flocculation and sedimentation tank. The flocculated calcium fluoride slurry after settling was filtered using a filter press and washed with 40°C water to obtain sludge. The purity of calcium fluoride in the obtained sludge was 78 wt% (dry basis), and the chlorine concentration in the sludge was less than 0.1 wt% (dry basis). The supernatant (treated water) of the fluoride ion-containing liquid after coagulation and sedimentation was neutralized by adding a 25 wt% sodium hydroxide aqueous solution. The fluoride ion concentration of the treated water after neutralization was 4.7 mg / L.

[0073] The purity (dry basis) of calcium fluoride in the obtained sludge was calculated as follows. It is assumed that the fluorine (F2) content [wt%] is entirely calcium fluoride (CaF2), and that a portion of the calcium (Ca) content [wt%] exists as CaF2. F2[wt%]=38 (molar mass of fluorine: 19g / mol) Ca[wt%] = (38 / 19) / 2 × 40.078 (molar mass of calcium: 40.078 g / mol) = 40.078 wt% CaF2[wt%] = 38 + 40.078 = 78.078 wt%

[0074] [Example 2] To a fluoride ion-containing solution (liquid temperature 40°C, fluoride ion concentration 12000 mg / L, pH 9) obtained by absorbing a gas containing hydrogen fluoride into a 1.0 wt% sodium hydroxide aqueous solution, 10 wt% slaked lime was added in an amount equal to 1.0 equivalent relative to the number of moles of fluoride ions in the fluoride ion-containing solution. Next, 3.5 to 35 wt% hydrochloric acid was added in stages to adjust the pH of the fluoride ion-containing solution to 4 to 7, and then 10 wt% calcium chloride was added in an amount equal to 0.6 equivalent relative to the number of moles of fluoride ions in the fluoride ion-containing solution. Furthermore, an anionic flocculant (polyacrylamide-based flocculant) was added to flocce the calcium fluoride particles, and then the mixture was flocculated and settled in a flocculation and sedimentation tank. The flocculated calcium fluoride slurry after settling was filtered using a filter press and washed with 40°C water to obtain sludge. The purity of calcium fluoride in the obtained sludge was 80 wt% (dry basis), and the chlorine concentration in the sludge was less than 0.1 wt% (dry basis). The supernatant (treated water) of the fluoride ion-containing liquid after coagulation and sedimentation was neutralized by adding a 25 wt% sodium hydroxide aqueous solution. The fluoride ion concentration of the treated water after neutralization was 4.7 mg / L.

[0075] [Example 3] To a fluoride ion-containing solution (liquid temperature 40°C, fluoride ion concentration 12000 mg / L, pH 9) obtained by absorbing a gas containing hydrogen fluoride into a 1.0 wt% sodium hydroxide aqueous solution, 1.2 equivalents of 10 wt% slaked lime were added relative to the number of moles of fluoride ions in the fluoride ion-containing solution. Next, 3.5 to 35 wt% hydrochloric acid was added in stages to adjust the pH of the fluoride ion-containing solution to 6, and then 0.3 equivalents of 10 wt% calcium chloride were added relative to the number of moles of fluoride ions in the fluoride ion-containing solution. Furthermore, an anionic flocculant (polyacrylamide-based flocculant) was added to floccate the calcium fluoride particles, and then the mixture was flocculated and settled in a flocculation and sedimentation tank. The flocculated calcium fluoride slurry after settling was filtered using a filter press and washed with 40°C water to obtain sludge. The purity of calcium fluoride in the obtained sludge was 76 wt% (dry basis), and the chlorine concentration in the sludge was less than 0.1 wt% (dry basis). The supernatant (treated water) of the fluoride ion-containing liquid after coagulation and sedimentation was neutralized by adding a 25 wt% sodium hydroxide aqueous solution. The fluoride ion concentration of the treated water after neutralization was 28 mg / L.

[0076] [Comparative Example 1] To a fluoride ion-containing solution (liquid temperature 40°C, fluoride ion concentration 10,000 mg / L, pH approximately 9) obtained by absorbing a gas containing hydrogen fluoride into a 1.0 wt% sodium hydroxide aqueous solution, 10 wt% slaked lime was added in an amount equal to 1.5 equivalents relative to the number of moles of fluoride ions in the fluoride ion-containing solution. Next, an anionic coagulant (polyacrylamide-based coagulant) was added to coagulate the calcium fluoride particles, and then the solution was coagulated and settled in a coagulation and settling tank. The coagulated calcium fluoride slurry after settling was filtered using a filter press and washed with 40°C water to obtain sludge. The purity of the calcium fluoride contained in the obtained sludge was 50 wt% (dry basis), and the chlorine concentration in the sludge was 0.35 wt% (dry basis). The supernatant liquid (treated water) of the fluoride ion-containing solution after coagulation and settling was neutralized by adding a 35 wt% hydrochloric acid aqueous solution. The fluoride ion concentration of the treated water after neutralization was 80 mg / L.

[0077] [evaluation] Table 1 shows the evaluation results for Examples 1-3 and Comparative Example 1. The evaluation method for each result is as follows.

[0078] (cohesion) To evaluate the coagulation properties, a specified amount of coagulated calcium fluoride slurry was sampled into a volumetric flask, shaken five times, and then allowed to stand. The state of the slurry was then evaluated according to the following criteria. ○ (Good): The sludge is settling while a sludge interface is present. △(Acceptable): A sludge interface is present, and some turbidity is observed in the supernatant treated water, but this has little impact on the treatment process itself. × (Unacceptable): The sludge interface does not exist according to the simplified sedimentation evaluation. Alternatively, turbidity is observed in the supernatant after a certain period of time, indicating that treatment is not possible.

[0079] (Fluoride ion concentration) The fluoride ion concentration in the fluoride ion-containing solution was measured by the lanthanum-alizarin complexone absorbance method in accordance with JIS K0102 34.1.

[0080] (Purity of calcium fluoride) The purity (dry basis) of calcium fluoride in sludge is calculated from the weight percentage fluorine concentration in the dried sludge. The weight percentage fluorine concentration was determined by pretreatment of the dried sludge by eluting it with acid and transferring fluoride ions to the liquid side, followed by measurement of the fluorine concentration value using the lanthanum-alizarin complexone method in accordance with JIS K0102 34.1. Furthermore, the obtained fluorine concentration value was converted to weight percentage fluorine for calculation.

[0081] (Chlorine concentration) The chlorine concentration (dry basis) in the sludge was measured by ion chromatography (IC method) using a solution prepared by dissolving a specified amount of sludge in a specified amount of nitric acid for a certain period of time.

[0082] (comprehensive evaluation) A comprehensive evaluation was conducted based on the following criteria, considering the results of coagulation, fluoride ion concentration, calcium fluoride (CaF2) purity, and chlorine concentration in the sludge. ○ (Good): The chlorine concentration in the sludge is 0.1 wt% or less (dry basis), the purity of CaF2 is 62 wt% or more (dry basis), the coagulation performance is ○ (Good), and the fluoride ion concentration in the treated water is less than 100 mg / L. × (Not acceptable): Does not meet at least one of the following conditions: chlorine concentration, CaF2 purity, coagulation properties, and fluoride ion concentration.

[0083] [Table 1]

[0084] (result) We confirmed that by adding step (B) to adjust the pH of the fluoride ion-containing solution between steps (A) and (C), the concentration of fluoride ions in the fluoride ion-containing solution can be reduced, and the purity of calcium fluoride contained in the sludge can be increased. Furthermore, we confirmed that the chlorine concentration contained in the sludge also decreases. [Industrial applicability]

[0085] The present invention's method for producing calcium fluoride can reduce the concentration of fluoride ions in wastewater and increase the calcium fluoride content in sludge. Therefore, this method is a useful process because it allows for the recycling of calcium fluoride sludge that has normally been disposed of in landfills, and is therefore useful in business fields where liquids containing fluoride ions (for example, in the treatment of wafer etching solutions during semiconductor manufacturing or in the treatment of liquids after combustion decomposition of fluorocarbon exhaust gases) are generated as wastewater.

Claims

1. (A) A step of adding slaked lime to a fluoride ion-containing solution, (B) A process to lower the pH, (C) A step of adding a calcium-containing compound other than slaked lime, A method for producing calcium fluoride in this order.

2. The method for producing calcium fluoride according to claim 1, wherein the fluoride ion-containing liquid mainly contains sodium fluoride.

3. The method for producing calcium fluoride according to claim 1, wherein the calcium-containing compound other than slaked lime is calcium chloride.

4. The method for producing calcium fluoride according to claim 1, wherein step (B) is a step of adjusting the pH of the fluoride ion-containing solution after step (A) to 4 to 7.

5. The method for producing calcium fluoride according to claim 1, wherein 0.8 to 1.2 equivalents of slaked lime are added relative to the number of moles of fluoride ions in the fluoride ion-containing solution.

6. A method for producing calcium fluoride according to claim 1, wherein 0.2 to 0.8 equivalents of a calcium-containing compound other than slaked lime are added relative to the number of moles of fluoride ions in the fluoride ion-containing solution.

7. After the step (C), (D) A method for producing calcium fluoride according to claim 1, further comprising the step of agglomerating the generated calcium fluoride particles.

8. The method for producing calcium fluoride according to claim 7, wherein a flocculant is used in step (D) above.

9. After step (D), (E) A method for producing calcium fluoride according to claim 7, further comprising the step of filtering and washing the aggregated calcium fluoride slurry.

10. The method for producing calcium fluoride according to claim 1 or 2, wherein the fluoride ion-containing liquid is wastewater.

11. (a) A step of adding slaked lime to fluoride ion-containing wastewater, (b) A step to lower the pH, (c) A step of adding a calcium-containing compound other than slaked lime, (d) A step of agglomerating the generated calcium fluoride particles, (e) A step of filtering and washing the aggregated calcium fluoride slurry, Water treatment methods in this order.

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