Method for treating fluorine-containing water

Abstract

This method for treating fluorine-containing water includes: adding a flocculating agent to a dispersion liquid containing calcium fluoride and water to obtain a suspension liquid that contains calcium fluoride aggregate and water; depositing the calcium fluoride aggregate, in the suspension liquid that contains the calcium fluoride aggregate and the water, to separate the same into a supernatant and deposits; adding a portion of the separated deposits and a liquid that contains calcium ions to a liquid that contains fluoride ions and water so as to precipitate calcium fluoride in the presence of sulfate ions and obtain a dispersion liquid that includes the calcium fluoride and the water; and setting the abundance of sulfate ions to 35 mass% or less with respect to the amount of fluorine element included in the liquid that includes fluoride ions and water, and setting the sulfate ion concentration in the supernatant to 100 mg / L or more, when adding the portion of the deposits caused from the separation and the liquid containing calcium ions to the liquid that contains fluoride ions and the water. In the present invention, a fluorine-containing water treatment apparatus is constituted so that this treatment method can be performed.

Description

Method for treating fluoride-containing water

[0001] The present invention relates to a method for treating fluorine-containing water, and more particularly to a method for treating fluorine-containing water, which can stably obtain water having a low elemental fluorine concentration from a liquid containing fluoride ions and water.

[0002] As a method for treating fluorine-containing water, the coagulation sedimentation method and the HDS method are known. － ) to calcium compounds such as calcium hydroxide and calcium carbonate (Ca ２＋ ) was added to form calcium fluoride (CaF ２ The HDS method is a method comprising precipitating calcium fluoride (CaF), flocculating the precipitated calcium fluoride with a flocculating agent, separating the calcium fluoride aggregates and water by precipitation, and adding a calcium compound and a portion of the calcium fluoride aggregates separated above to fluoride-containing water to precipitate calcium fluoride. The HDS method can produce calcium fluoride aggregates with a higher density and a lower water content than the coagulation-sedimentation method. The resulting calcium fluoride (CaF ２ ) can be reacted with an acid such as sulfuric acid to obtain hydrofluoric acid.

[0003] In the HDS method, various proposals have been made to further improve the density and moisture content of sludge and the fluorine concentration of treated water.

[0004] For example, Patent Document 1 discloses a method for treating wastewater containing fluoride ions, which comprises reacting wastewater containing fluoride ions with a calcium compound in a reaction tank to produce insoluble matter, separating the insoluble matter, and returning the separated insoluble matter to the reaction tank, wherein the pH in the reaction tank is adjusted to 9.5 to 11, and the amount of insoluble matter returned to the reaction tank is set to 20 times or more by weight the amount of insoluble matter produced by the reaction of the calcium compound with the wastewater.

[0005] Patent Document 2 discloses a method for treating fluoride-containing water, which comprises adding a calcium compound to fluoride-containing water to produce calcium fluoride sludge and separating the produced calcium fluoride sludge, in which a portion of the produced calcium fluoride sludge is brought into contact with the calcium compound and then added to the fluoride-containing water, and further discloses adding sulfate ions to the fluoride-containing water in such a manner that the amount of sulfate ions relative to the amount of fluoride ions in the fluoride-containing water is 0.01 to 0.1 equivalents.

[0006] Patent Document 3 discloses a fluoride removal apparatus having a reaction tank for reacting fluoride-containing water with a calcium compound, a raw water channel for introducing fluoride-containing water into the reaction tank, a flocculation device for transferring the reaction liquid from the reaction tank and adding a polymer flocculant to perform flocculation, a solid-liquid separation tank for introducing a mixed liquid containing flocs formed in the flocculation device and forming a sludge blanket with upstream flowing water to perform solid-liquid separation, and a circulation path for adding a calcium compound to a part of the sludge separated in the solid-liquid separation tank and circulating the sludge to the reaction tank.

[0007] Patent Document 4 discloses a method for treating fluorine-containing wastewater, which is discharged from a semiconductor manufacturing process and is substantially free of sulfate ions, by adding a calcium compound to adjust the pH to 5 to 10, and then separating the resulting insoluble matter by precipitating, the method comprising adding sulfuric acid and / or a sulfate salt to the fluorine-containing wastewater before or during the addition of the calcium compound so that the sulfate ion concentration is 200 to 1,000 mg / L.

[0008] Patent Document 5 discloses a method for treating fluorine-containing wastewater, which comprises adding a calcium salt to the fluorine-containing wastewater and carrying out a coagulation and sedimentation treatment, and which comprises a fluoride ion concentration measuring step of measuring the fluoride ion concentration in the fluorine-containing wastewater by a fluoride ion electrode method, and a calcium concentration calculating step of calculating the calcium concentration of the calcium salt to be added by applying the fluoride ion concentration measured in the fluoride ion concentration measuring step to formula (I).１ -C ２ × Aw(Ca) / 2Aw(F)≦1500 Formula (I) where C １ represents the calcium concentration of the calcium salt [mg / L], and C ２ represents the fluoride ion concentration [mg / L] measured in the fluoride ion concentration measuring step, Aw(Ca) represents the atomic weight of calcium, and Aw(F) represents the atomic weight of fluorine. Furthermore, Patent Document 5 states that in the method for treating fluoride-containing wastewater, when sulfate ions are present in the fluoride-containing wastewater at a concentration of 1000 mg / L or more, calcium salt having a calcium concentration calculated by applying formula (II) is further added to the fluoride-containing wastewater to compensate for calcium lost from the calcium salt added by reacting with the sulfate ions and coagulating and precipitating. ３ = (C ４ -1000)×Aw(Ca) / Fw(SO4 2- ) Formula (II) where C ３ represents the calcium concentration of the supplemented calcium salt, and C ４ represents the concentration of sulfate ions in fluoride-containing wastewater, Aw(Ca) represents the atomic weight of calcium, and Fw(SO4 2- ) represents the formula weight of sulfate ions.

[0009] JP-A-5-293474 JP-A-2001-38368 JP-A-10-479 JP-A-2000-334470 JP-A-2017-64569

[0010] An object of the present invention is to provide a method for treating fluorine-containing water and an apparatus for treating fluorine-containing water, which can stably obtain water having a low elemental fluorine concentration from a liquid containing fluoride ions and water.

[0011] As a result of investigations to achieve the above-mentioned object, the present inventors have completed the present invention, which includes the following embodiments: [1] A method for treating fluoride-containing water, comprising: adding a flocculant to a dispersion containing calcium fluoride and water to obtain a suspension containing calcium fluoride aggregates and water; precipitating the calcium fluoride aggregates in the suspension containing calcium fluoride aggregates and water to separate the calcium fluoride aggregates into a supernatant liquid and a sediment; adding a portion of the separated sediment and a liquid containing calcium ions to a liquid containing fluoride ions and water to precipitate calcium fluoride in the presence of sulfate ions to obtain a dispersion containing calcium fluoride and water; and adjusting the amount of sulfate ions present when adding the portion of the separated sediment and the liquid containing calcium ions to the liquid containing fluoride ions and water to 35 mass % or less of the amount of elemental fluorine contained in the liquid containing fluoride ions and water, and adjusting the sulfate ion concentration in the supernatant liquid to 100 mg / L or more.

[0012] [2] The method for treating fluoride-containing water according to [1], further comprising passing the supernatant through a membrane filter with a pore size of 0.025 μm to obtain a liquid having a calcium element concentration of 20 mg / L or more.

[0013] [3] The method for treating fluoride-containing water according to [1] or [2], further comprising passing the supernatant through a membrane filter with a pore size of 0.45 μm to reduce the elemental fluorine concentration in the resulting liquid to 15 mg / L or less.

[0014] [4] The method for treating fluoride-containing water according to any one of [1] to [3], wherein the addition of the part of the separated sediment and the liquid containing calcium ions is the addition of a mixed liquid obtained by mixing the part of the separated sediment with the liquid containing calcium ions.

[0015] [5] A fluoride-containing water treatment device comprising: a coagulation tank, a settling tank, a reaction tank, and an adjusting mechanism; the coagulation tank is configured to add a coagulant to a dispersion containing calcium fluoride and water to obtain a suspension containing calcium fluoride aggregates and water; the settling tank is configured to separate the calcium fluoride aggregates from the suspension containing calcium fluoride aggregates and water by settling them into a supernatant liquid and a sediment; the reaction tank is configured to add a portion of the sediment separated in the settling tank and a liquid containing calcium ions to a liquid containing fluoride ions and water to precipitate calcium fluoride in the presence of sulfate ions, thereby obtaining a dispersion containing calcium fluoride and water; and the adjusting mechanism is configured to adjust the amount of sulfate ions present in the addition of the portion of the separated sediment and the liquid containing calcium ions to 35 mass % or less of the amount of elemental fluorine contained in the liquid containing fluoride ions and water, and to adjust the sulfate ion concentration in the supernatant to 100 mg / L or more.

[0016] The method for treating fluorine-containing water and the apparatus for treating fluorine-containing water of the present invention can stably obtain water having a low elemental fluorine concentration from a liquid containing fluoride ions and water.

[0017] 1 is a conceptual diagram showing one embodiment of a water treatment device of the present invention, and FIG. 2 is a conceptual diagram showing another embodiment of a water treatment device of the present invention.

[0018] The method for treating fluoride-containing water of the present invention includes the steps of coagulation, precipitation, reaction and adjustment. The apparatus for treating fluoride-containing water of the present invention also includes a coagulation tank 2, a precipitation tank 3, a reaction tank 1 and an adjustment mechanism.

[0019] The coagulation stage involves adding a coagulant to a dispersion containing calcium fluoride and water to obtain a suspension containing calcium fluoride coagulates and water. The coagulation tank 2 is configured to add a coagulant 8 to a dispersion containing calcium fluoride and water to obtain a suspension containing calcium fluoride coagulates and water. The dispersion containing calcium fluoride and water can be transferred from the reaction tank 1 or the reaction stage to the coagulation tank 2 or the coagulation stage via a flow path. The coagulant 8 can be transferred from the coagulant preparation tank to the coagulation tank 2 via a flow path. The amount of each transferred can be adjusted using a level meter installed in the reaction tank 1 and / or the coagulation tank 2, as well as valves, flow meters, pumps, etc. installed in each flow path. A stirring means can be installed in the coagulation tank to ensure that the added coagulant is distributed throughout the coagulation tank. If necessary, a liquid containing sulfate ions may be added to the dispersion containing calcium fluoride and water to adjust the sulfate ion concentration in the dispersion containing calcium fluoride and water or the supernatant to a desired range.

[0020] Calcium fluoride (CaF ２ ) is produced by the reaction between fluoride ions and calcium ions. Calcium fluoride is poorly soluble in water, for example, with a solubility of 0.0016 g / 100 ml (20°C), and therefore precipitates in water. The precipitated calcium fluoride usually contains a large amount of suspended solid particles. Generally, suspended solid particles have a slow settling rate, and the precipitation process requires a long time.

[0021] The flocculant is not particularly limited as long as it promotes the formation of coarse flocs (settling particles) of suspended solid particles. The addition of a flocculant in the flocculation stage or flocculation tank causes calcium fluoride to flocculate into coarse flocs (calcium fluoride aggregates). The formation of calcium fluoride aggregates according to the present invention promotes gravitational settling and centrifugal settling, making it easier to obtain sludge with a low moisture content and water with a low elemental fluoride concentration.

[0022] Examples of the flocculant include polymer flocculants made of anionic polymers, cationic polymers, amphoteric polymers, nonionic polymers, etc. Such polymers preferably have a weight average molecular weight of more than 1 million, more preferably 5 million or more.

[0023] As the polymer flocculant made of a cationic polymer, it is preferable to use a cationic polymer solution or a cationic polymer dispersion (hereinafter, these may be collectively referred to as a "cationic polymer-containing liquid").

[0024] The cationic polymer solution is prepared by dissolving a cationic polymer in water or in a solvent having a high affinity for water, and the cationic polymer dispersion is prepared by dispersing a cationic polymer solution in a hydrophobic liquid (W / O type emulsion), etc. The weight-average molecular weight of the cationic polymer is preferably more than 1 million, more preferably 5 million or more, and even more preferably 6 million to 11 million.

[0025] Examples of the cationic polymer include polymers having cationic structural units derived from quaternary ammonium salts of (meth)acrylic acid esters (e.g., copolymers of acrylamide / [2-(acryloyloxy)ethyl]benzyldimethylammonium chloride / [2-(acryloyloxy)ethyl]trimethylammonium chloride, copolymers of acrylamide / [3-(acryloyloxy)propyl]benzyldimethylammonium chloride / [2-(acryloyloxy)ethyl]trimethylammonium chloride, copolymers of acrylamide / [2-(acryloyloxy)ethyl]benzyldimethylammonium chloride / [3-(acryloyloxy)propyl]trimethylammonium chloride, copolymers of acrylamide / [3-(acryloyloxy)propyl]benzyldimethylammonium chloride / [3-(acryloyloxy)propyl]trimethylammonium chloride), polyaminoalkyl acrylates, polyaminoalkyl methacrylates, polyethyleneimine, halogenated polydiallylammonium, chitosan, urea-formalin resins, and the like. These cationic polymers can be used alone or in combination of two or more. The addition of a cationic polymer can prevent re-dispersion of flocs and increase the efficiency of sedimentation, making it easier to obtain sludge with a lower moisture content.

[0026] As a polymer flocculant made of an anionic polymer, it is preferable to use an anionic polymer solution or an anionic polymer dispersion (hereinafter, these may be collectively referred to as "anionic polymer-containing liquid"). The anionic polymer solution is obtained by dissolving an anionic polymer in a solvent having a high affinity for water or in water, and the anionic polymer dispersion is, for example, obtained by dispersing the solution in a hydrophobic solvent (W / O type emulsion). The weight-average molecular weight of the anionic polymer is preferably more than 1 million, more preferably 5 million or more, and even more preferably 8 million to 15 million.

[0027] Examples of anionic polymers include sodium polyacrylate, polyacrylic acid sodium amide derivatives, polyacrylamide partial hydrolysates, partially sulfomethylated polyacrylamide, and poly(2-acrylamide)-2-methylpropane sulfate. These anionic polymers can be used alone or in combination of two or more. Anionic polymers with a degree of anionization of 10 to 30% by mass are preferred.

[0028] As a polymer flocculant made of an amphoteric polymer, it is preferable to use an amphoteric polymer solution or an amphoteric polymer dispersion (hereinafter, these may be collectively referred to as an "amphoteric polymer-containing liquid"). The amphoteric polymer solution is obtained by dissolving an amphoteric polymer in a solvent having a high affinity for water or in water, and the amphoteric polymer dispersion is, for example, obtained by dispersing the solution in a hydrophobic solvent (W / O type emulsion). The weight-average molecular weight of the amphoteric polymer is preferably more than 1 million, more preferably 5 million or more, and even more preferably 8 million to 10 million.

[0029] Examples of amphoteric polymers include copolymers of (meth)acrylamide, quaternary ammonium alkyl (meth)acrylate, and sodium (meth)acrylate, copolymers of acrylamide, aminoalkyl methacrylate, and sodium acrylate, etc. The molar ratio of anion / cation in the amphoteric polymer is preferably 0.2 to 2.0.

[0030] As a polymer flocculant made of a nonionic polymer, it is preferable to use a nonionic polymer solution or a nonionic polymer dispersion (hereinafter, these may be collectively referred to as a "nonionic polymer-containing liquid"). A nonionic polymer solution is obtained by dissolving a nonionic polymer in a solvent having a high affinity for water or in water, and a nonionic polymer dispersion is, for example, obtained by dispersing the solution in a hydrophobic solvent (W / O type emulsion). Examples of nonionic polymers include polyacrylamide. The weight-average molecular weight of the nonionic polymer is preferably more than 1 million, more preferably 5 million or more, and even more preferably 8 million to 20 million.

[0031] In the present invention, among these, the use of a polymer flocculant made of an anionic polymer is preferred.

[0032] The temperature of the dispersion containing calcium fluoride and water immediately before the addition of the flocculant is preferably 10 to 70°C, more preferably 10 to 50°C, and the pH is preferably 3 to 10, more preferably 5 to 9. The temperature of the suspension containing calcium fluoride aggregates and water immediately after the addition of the flocculant is preferably 10 to 70°C, more preferably 10 to 50°C, and the pH is preferably 3 to 10, more preferably 5 to 9. The amount of the flocculant (in terms of solid matter) to be added to the dispersion containing calcium fluoride and water is preferably 0.5 to 20 mg, more preferably 1 to 10 mg, per 1 L of the dispersion containing calcium fluoride and water. The flocculant to be added is preferably in the form of a liquid such as a solution or dispersion.

[0033] The precipitation step involves settling the calcium fluoride aggregates in a suspension containing calcium fluoride aggregates and water, thereby separating the calcium fluoride aggregates into a supernatant and a sediment. The precipitation tank 3 is configured to be able to settling the calcium fluoride aggregates in a suspension containing calcium fluoride aggregates and water, thereby separating the calcium fluoride aggregates into a supernatant and a sediment. The settling of the calcium fluoride aggregates can be carried out by a known unit operation utilizing gravity, centrifugal force, or the like. The suspension containing calcium fluoride aggregates and water can be transferred from the coagulation tank 2 or the coagulation step to the precipitation tank 3 or the precipitation step through a flow path. The amount transferred can be adjusted using a level gauge installed in the coagulation tank 2 and / or the precipitation tank 3, and a valve, flow meter, pump, or the like installed in the flow path. The temperature of the suspension containing calcium fluoride aggregates and water during precipitation is preferably 10 to 70°C, more preferably 10 to 50°C, and the pH is preferably 3 to 10, more preferably 5 to 9.

[0034] The supernatant 11 obtained by the present invention is water with a low concentration of elemental fluorine. In the present invention, the supernatant 11 can be subjected to a neutralization treatment, if necessary.

[0035] The sediment is an aqueous slurry with a high concentration of calcium fluoride aggregates. As will be described later, a portion of the sediment obtained in the settling tank 3 or precipitation stage (sediment 10) can be used in the reaction stage or reaction tank 1, and the remainder (sediment 9) can be used to recover calcium fluoride. To remove water from the sediment 9, it can be subjected to dehydration treatments such as centrifugal dehydration, filter press dehydration, roll press dehydration, and vacuum filter drum dehydration. The sediment 9 obtained by the method and apparatus of the present invention has the property of being easily dehydrated, has good efficiency in dehydration treatment, and can be made into sludge with a low moisture content.

[0036] The reaction step includes adding the deposit 10 and a liquid containing calcium ions to a liquid containing fluoride ions and water to precipitate calcium fluoride in the presence of sulfate ions, thereby obtaining a dispersion containing calcium fluoride and water. The reaction tank 1 is configured so that the deposit 10 and the liquid containing calcium ions can be added to a liquid containing fluoride ions and water to precipitate calcium fluoride in the presence of sulfate ions, thereby obtaining a dispersion containing calcium fluoride and water.

[0037] A liquid 12 containing fluoride ions and water can be transferred from its source to the reaction tank 1 or the reaction stage via a flow path. A liquid 4 containing calcium ions can be transferred from a preparation tank for the liquid to the reaction tank 1 via a flow path. A deposit 10 can be transferred from a settling tank 3 or the settling stage to the reaction tank 1 or the reaction stage via a flow path. A liquid 5 containing sulfate ions, which is added as needed, can be transferred from a preparation tank for the liquid to the reaction tank 1 via a flow path. The amount of each liquid transferred can be adjusted using a level meter installed in the reaction tank 1, as well as valves, flow meters, pumps, etc. installed in each flow path. A means for stirring the added calcium ion-containing liquid 4, the deposit 10, or the added sulfate ion-containing liquid 5 can be installed in the reaction tank 1 so that it is distributed throughout the reaction tank.

[0038] The liquid 12 containing fluoride ions and water may contain fluoride that has not completely dissolved in water. Examples of fluorides include ammonium fluoride, ammonium hydrogen fluoride, and sodium fluoride. Examples of liquids containing fluoride ions and water include wastewater from factories producing semiconductors, glass, or fluororesins; wastewater from mining sites for fluorite, cryolite, or the like; hot springs; mineral springs; river water; and groundwater.

[0039] The calcium ion-containing liquid 4 may contain calcium compounds that did not completely dissolve in water. Examples of the calcium ion-containing liquid 4 include aqueous solutions or dispersions of calcium compounds such as calcium carbonate, calcium bicarbonate, calcium chloride, calcium hydroxide, calcium oxide, calcium nitrate, and calcium sulfate. Of these calcium compounds, calcium chloride and calcium hydroxide are preferred, and calcium hydroxide is more preferred.

[0040] The amount of calcium ion-containing liquid added in the reaction stage or reaction tank 1 is an amount that causes the calcium element concentration in the liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.025 μm to be preferably 20 mg / L or more, more preferably 50 mg / L or more. There is no particular upper limit to the amount of calcium ion-containing liquid added in the reaction stage or reaction tank 1. However, increasing the amount of calcium ion-containing liquid tends to reduce the amount of sulfate ions by causing sulfate ions to react with calcium ions to produce calcium sulfate. Therefore, the amount of calcium ion-containing liquid added in the reaction stage or reaction tank 1 can be set, for example, to an amount that causes the calcium element concentration in the liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.45 μm to be preferably 500 mg / L or less, more preferably 300 mg / L or less, and even more preferably less than 200 mg / L.

[0041] The calcium fluoride aggregates contained in the precipitate 10 added in the reaction stage or reaction tank 1 function as seed crystals for precipitating calcium fluoride. The amount of solid content of the precipitate 10 added per hour in the reaction stage or reaction tank 1 is preferably 10 to 90 parts by mass, more preferably 20 to 50 parts by mass, per part by mass of solid content per hour that will be produced from the liquid 12 containing fluoride ions and water in the reaction stage or reaction tank 1.

[0042] The calcium ion-containing liquid 4 and the sediment 10 to be added in the reaction stage or reaction tank 1 can be added in the reaction stage or reaction tank 1 as a mixed liquid 14 obtained by mixing them (see FIG. 2).

[0043] The sulfate ions present when a portion of the separated sediment and the calcium ion-containing liquid are added to the liquid containing fluoride ions and water originate from those contained in the liquid containing fluoride ions and water 12, the liquid containing calcium ions 4, or the sediment 10 supplied to the reaction stage or reaction tank, and from those contained in the liquid containing sulfate ions 5, which may be added from outside the system as needed. Examples of the liquid containing sulfate ions 5 include sulfuric acid, an ammonium sulfate-containing liquid, a calcium sulfate-containing liquid, and an aluminum sulfate-containing liquid. Of these, sulfuric acid is preferred.

[0044] The dispersion containing calcium fluoride and water obtained in the reaction stage or reaction tank preferably has a pH of 3-10, more preferably 5-9, and even more preferably 6-8.

[0045] To adjust the pH of each solution, a basic compound such as sodium hydroxide or sodium carbonate, or an acidic compound such as hydrochloric acid or nitric acid can be used. To adjust the pH, a calcium compound can be used as the basic compound, and sulfuric acid or a sulfate can be used as the acidic compound. However, from the viewpoint of ease of adjusting the pH and controlling the precipitation reaction, it is preferable to use a basic compound other than a calcium compound and an acidic compound other than sulfuric acid or a sulfate.

[0046] To promote settling in the precipitation stage or settling tank, a coagulation stage or tank can be provided between the reaction stage or tank and the coagulation stage or tank. In the coagulation stage or tank, a coagulant is added to the dispersion containing calcium fluoride and water obtained in the reaction stage or tank to promote coagulation of calcium fluoride. The coagulant is not particularly limited as long as it is a substance that promotes the formation of microflocs, and examples of the coagulant include inorganic coagulants such as aluminum sulfate, PAC, aluminum chloride, polyiron, and ferric chloride, and organic coagulants such as polyamine and polydadomac. Of these, inorganic coagulants are preferred.

[0047] The adjusting step is performed by adjusting the ratio of the amount of sulfate ions present (SO ) to the amount of elemental fluorine contained in the liquid 12 containing fluoride ions and water when adding a part of the separated sediment and the liquid containing calcium ions to the liquid containing fluoride ions and water. ４ ２－ / F) to 35% by mass or less, preferably 31% by mass or less, and the sulfate ion concentration in the supernatant is 100 mg / L or more. ４ ２－ The lower limit of / F is not particularly limited, but the adjustment step is ４ ２－ / F is, for example, preferably 10 mass % or more, more preferably 15 mass % or more, and even more preferably 20 mass % or more. Furthermore, the adjusting step includes adjusting the sulfate ion concentration in the dispersion containing calcium fluoride and water to preferably 100 mg / L or more, adjusting the elemental fluorine concentration in the liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.45 μm to preferably 15 mg / L or less, more preferably 8.0 mg / L or less, and / or adjusting the elemental calcium concentration in the liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.025 μm to preferably 20 mg / L or more, more preferably 50 mg / L or more. The upper limit of the elemental calcium concentration in the liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.025 μm is not particularly limited, but is preferably 500 mg / L, more preferably 300 mg / L, from the viewpoint of scale generation, etc.

[0048] The adjusting mechanism is to adjust the ratio of the amount of sulfate ions present (SO ) to the amount of elemental fluorine contained in the liquid containing fluoride ions and water when adding a part of the separated sediment and the liquid containing calcium ions to the liquid containing fluoride ions and water. ４ ２－ / F) can be 35 mass % or less, and the sulfate ion concentration in the supernatant can be 100 mg / L or more. ４ ２－ The lower limit of / F is not particularly limited, but the adjusting mechanism is ４ ２－ The adjusting mechanism is configured to adjust the sulfate ion concentration in the dispersion containing calcium fluoride and water to preferably 100 mg / L or more, to adjust the elemental fluorine concentration in the liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.45 μm to preferably 15 mg / L or less, more preferably 8.0 mg / L or less, and / or to adjust the elemental calcium concentration in the liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.025 μm to preferably 20 mg / L or more, more preferably 50 mg / L or more.

[0049] These adjustments can be made by, for example, adjusting the ratio of the amount of sulfate ions present to the amount of elemental fluorine contained in a liquid containing fluoride ions and water (SO ４ ２－ / F), and the sulfate ion concentration in the supernatant, and if necessary, the sulfate ion concentration in a dispersion containing calcium fluoride and water, the elemental fluorine concentration in a liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.025 μm, the elemental fluorine concentration in a liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.45 μm, the elemental calcium concentration in a liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.45 μm and / or the elemental calcium concentration in a liquid obtained by passing the supernatant through a membrane filter with a pore size of 0.025 μm, and based on the monitoring results, the amount of fluoride ions to be treated (for example, the flow rate of the liquid containing fluoride ions and water), the amount of calcium added in the reaction tank or reaction stage, and the like are monitored. This can be achieved by changing the amount of calcium ions (e.g., the flow rate of the calcium ion-containing liquid), the amount of sulfate ions added in the reaction tank or reaction stage (e.g., the flow rate of the sulfate ion-containing liquid), the amount of sediment 10 added in the reaction tank or reaction stage (e.g., the flow rate of the sediment 10), the amount of coagulant added in the coagulation tank or coagulation stage (e.g., the flow rate of the coagulant-containing liquid), the temperature of each liquid, the pH of each liquid, etc. Furthermore, if necessary, this can be achieved by changing the amount of coagulant added in the coagulation tank or coagulation stage (e.g., the flow rate of the coagulant-containing liquid), the amount of sulfate ions added to the dispersion containing calcium fluoride and water (e.g., the flow rate of the sulfate ion-containing liquid), etc. The flow rate can be changed by manually or automatically changing the valve opening. The temperature can be changed by manually or automatically changing the amount of energy supplied to or discharged from the heating means or cooling means. The pH can be changed by manually or automatically changing the valve opening in the supply flow path for the acidic compound or basic compound.

[0050] Next, the present invention will be described in more detail with reference to examples and comparative examples, but the following examples are not intended to limit the scope of the present invention.

[0051] (Experiment) Hydrogen fluoride was dissolved in pure water to prepare water to be treated with a fluorine concentration of 2000 mg / L. The water treatment system shown in Figure 1 was prepared. The water to be treated, calcium hydroxide solution 4, sulfuric acid 5, sediment 10, and sodium hydroxide solution 7 or hydrochloric acid 6 were continuously supplied to reaction tank 1. The liquid withdrawn from the reaction tank and a polymer flocculant-containing solution 8 containing an anionic polymer were continuously supplied to coagulation tank 2 so that the liquid in the reaction tank remained at a predetermined volume. The liquid withdrawn from the coagulation tank was continuously supplied to settling tank 3 so that the liquid in the settling tank remained at a predetermined volume. Supernatant 11 and sediment (slurry) were withdrawn from the top and bottom of the settling tank, respectively, so that the liquid in the settling tank remained at a predetermined volume. A portion of the withdrawn sediment (sediment 10) was supplied to reaction tank 1 as described above. The remaining part of the extracted sediment (sediment 9) was pressed at 0.3 MPa to obtain sludge.

[0052] The calcium ion concentration of the liquid in the reaction tank, the pH of the liquid in the reaction tank, and the sulfate ion concentration in the supernatant (supernatant SO ４ ２－ The calcium ion concentration in the liquid in the reaction tank, the amount of sulfate ions in the liquid in the reaction tank, and the amount of elemental fluorine in the liquid containing fluoride ions and water (water to be treated) are measured at appropriate times, and the ratio of the amount of sulfate ions in the reaction tank 1 to the amount of elemental fluorine in the liquid containing fluoride ions and water (SO ４ ２－ / F) and the sulfate ion concentration in the supernatant (supernatant SO ４ ２－ The amounts of water to be treated 12, calcium hydroxide aqueous solution 4, sulfuric acid 5, deposit 10, and sodium hydroxide aqueous solution 7 or hydrochloric acid 6 supplied were adjusted so that the concentration (%) of the treated water reached the values ​​shown in Table 1.

[0053] The elemental fluorine concentration in the sludge (sludge F concentration) was measured. The results are shown in Table 1. The amount of suspended solid particles in the supernatant 11 was estimated as follows. The supernatant 11 was filtered through a membrane filter with a pore size of 0.45 μm and a membrane filter with a pore size of 0.025 μm. The filtrates obtained through the 0.45 μm membrane filter and the 0.025 μm membrane filter were separately collected, and the elemental fluorine concentration (supernatant F concentration) and elemental calcium concentration (supernatant Ca concentration) in each filtrate were measured. The results are shown in Table 1. The difference between the elemental fluorine concentration in the filtrate obtained from the 0.45 μm pore size membrane filter (0.45 μm filtrate) and the elemental fluorine concentration in the filtrate obtained from the 0.025 μm pore size membrane filter (0.025 μm filtrate) corresponds to the concentration of elemental fluorine derived from solid suspended particles with a particle size of 0.025 μm to 0.45 μm, and the elemental fluorine concentration in the filtrate obtained from the 0.025 μm pore size membrane filter (0.025 μm filtrate) corresponds to the sum of the concentration of elemental fluorine derived from solid suspended particles with a particle size of less than 0.025 μm and the concentration of fluorine ions dissolved in the supernatant.

[0054]

[0055] As shown in Table 1, the 0.45 μm filtrate and 0.025 μm filtrate of the supernatants obtained in Examples 1 and 2 have lower elemental fluorine concentrations than those of the supernatants obtained in Comparative Examples 1 to 4.

[0056] According to the present invention, by setting the ratio of the amount of sulfate ions present to the amount of elemental fluorine contained in a liquid containing fluoride ions and water to 35 mass % or less and setting the sulfate ion concentration in the supernatant to 100 mg / L or more, water having a low elemental fluorine concentration can be stably obtained from the liquid containing fluoride ions and water. According to the present invention, fluorine-containing solid suspended particles small enough to pass through a membrane filter with a pore size of 0.45 μm contained in the treated water can be significantly reduced, and the elemental fluorine concentration, i.e., calcium fluoride concentration, in the sludge can be maintained high.

[0057] 1: Reaction tank 2: Coagulation tank 3: Sedimentation tank 4: Calcium ion-containing liquid 5: Sulfate ion-containing liquid 6: Acidic compound-containing liquid 7: Basic compound-containing liquid 8: Coagulant-containing liquid 9: Deposit (discharge) 10: Deposit (return) 11: Supernatant 12: Fluoride ion-containing liquid 13: Mixing tank 14: Mixed liquid

Claims

1. A method for treating fluoride-containing water, comprising: adding a coagulant to a dispersion containing calcium fluoride and water to obtain a suspension containing calcium fluoride aggregates and water; precipitating the calcium fluoride aggregates in the suspension containing calcium fluoride aggregates and water to separate the calcium fluoride aggregates into a supernatant liquid and a sediment; adding a portion of the separated sediment and a liquid containing calcium ions to a liquid containing fluoride ions and water to precipitate calcium fluoride in the presence of sulfate ions to obtain a dispersion containing calcium fluoride and water; and adjusting the amount of sulfate ions present when adding the portion of the separated sediment and the liquid containing calcium ions to the liquid containing fluoride ions and water to 35 mass % or less of the amount of elemental fluorine contained in the liquid containing fluoride ions and water, and adjusting the sulfate ion concentration in the supernatant liquid to 100 mg / L or more.

2. The method for treating fluoride-containing water according to claim 1, further comprising passing the supernatant through a membrane filter having a pore size of 0.025 μm to obtain a liquid having a calcium element concentration of 20 mg / L or more.

3. The method for treating fluoride-containing water according to claim 1, further comprising passing the supernatant through a membrane filter having a pore size of 0.45 μm to reduce the elemental fluorine concentration in the resulting liquid to 15 mg / L or less.

4. A method for treating fluoride-containing water as described in claim 1, wherein the addition of a portion of the separated sediment and a liquid containing calcium ions is the addition of a mixed liquid obtained by mixing a portion of the separated sediment with a liquid containing calcium ions.

5. A fluoride-containing water treatment device comprising: a coagulation tank, a settling tank, a reaction tank, and an adjustment mechanism; the coagulation tank is configured to add a coagulant to a dispersion containing calcium fluoride and water to obtain a suspension containing calcium fluoride agglomerates and water; the settling tank is configured to separate the calcium fluoride agglomerates in the suspension containing calcium fluoride agglomerates and water into a supernatant liquid and a sediment; the reaction tank is configured to add a part of the sediment separated in the settling tank and a liquid containing calcium ions to a liquid containing fluoride ions and water to precipitate calcium fluoride in the presence of sulfate ions to obtain a dispersion containing calcium fluoride and water; and the adjustment mechanism is configured to adjust the amount of sulfate ions present when the part of the separated sediment and the liquid containing calcium ions are added to the liquid containing fluoride ions and water to 35 mass % or less relative to the amount of elemental fluorine contained in the liquid containing fluoride ions and water, and to adjust the sulfate ion concentration in the supernatant to 100 mg / L or more.

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