Method for treating fluorine-containing waste water

Abstract

Provided is a method for treating fluorine-containing waste water, the method including a step in which fluorine is insolubilized and removed by adding a cerium salt to the fluorine-containing waste water, and the method being characterized in that the pH in the step for insolubilizing fluorine by adding the cerium salt is 3-6. The method additionally includes, subsequent to the step for insolubilizing fluorine by adding the cerium salt, an iron salt addition step and a filtration step subsequent to the iron salt addition step.

Description

Treatment method for fluoride-containing wastewater

[0001] The present invention relates to a method for treating fluorine-containing wastewater, and more particularly to a method for treating fluorine-containing wastewater using a cerium salt.

[0002] Patent Document 1 describes a two-stage coagulation and sedimentation method for treating fluoride-containing wastewater. In the first step, calcium salts such as slaked lime and calcium chloride are added to high-concentration fluoride, which is insolubilized and precipitated as sparingly soluble calcium fluoride. In the second step, aluminum salts such as PAC and aluminum sulfate are added to low-concentration fluoride remaining in the first step, and the fluorine is adsorbed by the precipitated aluminum hydroxide, which is insolubilized and precipitated.

[0003] This method can stably reduce the fluorine concentration of treated water to 8 mg / L or less, which is the discharge standard. However, the aluminum hydroxide flocs produced in the second step have a small density difference from water, which means that sedimentation takes time. This requires a large area for the coagulation and settling tank in the second step, which poses problems in terms of installation space and equipment cost. In addition, a large amount of aluminum hydroxide flocs is produced from the aluminum salt.

[0004] Patent Document 2 describes a method comprising the steps of adding a cerium salt to fluoride-containing wastewater, adjusting the pH to 8 to 10 with an alkali metal hydroxide, and forming an insoluble precipitate. Thus, when a cerium salt is used, the amount of sludge is smaller than when an aluminum compound is used.

[0005] JP 2007-768 A JP 2016-198740 A

[0006] An object of the present invention is to provide a method for treating fluoride-containing wastewater, which can optimize the pH for fluoride insolubilization by a cerium salt and reduce the amount of cerium salt required to be added.

[0007] The method for treating fluorine-containing wastewater of the present invention has the following gist.

[0008] [1] A method for treating fluorine-containing wastewater, comprising a step of adding a cerium salt to fluorine-containing wastewater to insolubilize and remove fluorine, characterized in that the pH in the step of insolubilizing fluorine by adding the cerium salt is 3 to 6.

[0009] [2] The method for treating fluoride-containing wastewater according to [1], further comprising a step of adding an iron salt after the step of adding a cerium salt to insolubilize fluoride, and a subsequent filtration step.

[0010] [3] The method for treating fluoride-containing wastewater according to [1] or [2], further comprising the steps of adding a calcium compound to the fluoride-containing wastewater, flocculating the wastewater, and performing solid-liquid separation, prior to the step of adding the cerium salt.

[0011] According to the method for treating fluoride-containing wastewater of the present invention, the pH for fluoride insolubilization by the cerium salt can be optimized, thereby reducing the concentration of the cerium salt required for addition.

[0012] According to one aspect of the present invention, it becomes possible to apply a filter to fluorine treatment, thereby making it possible to reduce the space required for wastewater treatment facilities.

[0013] 1 is a flow diagram of a method for treating fluorine-containing wastewater according to an embodiment of the present invention. 2 is a graph showing experimental results.

[0014] The present invention will be described in further detail below.

[0015] The method for treating fluorine-containing wastewater of the present invention comprises a step of adding a cerium salt to fluorine-containing wastewater to insolubilize and remove fluorine, and is characterized in that the pH in the step of insolubilizing fluorine by adding the cerium salt is 3 to 6.

[0016] Examples of fluorine-containing wastewater to be treated in the present invention include flue gas desulfurization wastewater from thermal power plants, wastewater generated in the manufacturing process of electronic parts such as semiconductors and liquid crystal displays, wastewater discharged from the pickling process of stainless steel, etc. The fluorine concentration in the fluorine-containing wastewater is preferably about 4 to 15 mg / L, particularly about 4 to 10 mg / L.

[0017] Examples of cerium salts to be added to fluoride-containing wastewater include cerium chloride, cerium carbonate, cerium nitrate, cerium acetate, etc. The cerium salt is preferably added in the form of an aqueous solution.

[0018] The amount of cerium salt to be added depends on the fluorine concentration in the wastewater, but generally, the cerium concentration in the water after addition is preferably about 1 to 100 mg / L, particularly about 1 to 50 mg / L.

[0019] When a cerium salt is added to fluoride-containing wastewater, the pH of the water is 3 to 6, preferably 4 to 5. If necessary, a pH adjuster is added before, during or after the addition of the cerium salt to adjust the pH to within this range.

[0020] Suitable pH adjusters include sulfuric acid, caustic soda, hydrochloric acid, and hydrated lime.

[0021] When a cerium compound is added to fluoride-containing wastewater and the pH is adjusted to 3 to 6, cerium hydroxide is produced, and this cerium hydroxide reacts with fluoride ions to produce cerium fluoride, thereby capturing fluorine in the water. Then, this product is subjected to solid-liquid separation, whereby fluorine is removed from the water.

[0022] In one embodiment of the present invention, an iron salt is added after the addition of a cerium salt. This causes the cerium fluoride to aggregate and become coarse, making it possible to capture it with a filter. The amount of iron salt added is preferably 10 to 40 mg / L, and more preferably 10 to 20 mg / L.

[0023] The cerium salt and iron salt may be mixed in a line by piping injection, or may be mixed in a tank.

[0024] In one aspect of the present invention, when the fluorine concentration of fluoride-containing wastewater is high, a calcium compound may be added to the fluoride-containing wastewater to form calcium fluoride, which is then separated to reduce the fluorine concentration, and then a cerium salt may be added. Suitable calcium compounds include calcium hydroxide and calcium chloride.

[0025] When the raw water fluoride concentration is 25 mg / L or less, the first step of insolubilization and precipitation with a calcium compound may be omitted.

[0026] FIG. 1 is a flow diagram showing an example of the method for treating fluorine-containing wastewater of the present invention.

[0027] Raw water (fluoride-containing wastewater) is introduced into the reaction tank 1, and calcium compounds and returned sludge are added and mixed. The pH in the reaction tank 1 is preferably about 4 to 5, and a pH adjuster is added as needed. In the reaction tank 1, the fluorine in the raw water reacts with the calcium compounds to produce calcium fluoride. By adding the returned sludge, most of the product is produced on the surface of the sludge, causing the sludge particles to grow.

[0028] The liquid in the reaction tank 1 is introduced into the coagulation tank 2, where a polymer coagulant is added to coagulate the calcium fluoride particles.

[0029] The liquid in the coagulation tank 2 is introduced into the settling tank 3 and subjected to solid-liquid separation treatment. A part of the precipitate is returned to the reaction tank 1 as returned sludge through the return line 4, and the remainder is discharged outside the system.

[0030] The supernatant water from the settling tank 3 is taken out through a pipe 5. A cerium salt is added to the pipe 5, followed by an iron salt. As a result, the fluorine component in the supernatant water reacts with the cerium salt, and the reaction product is coagulated by the iron salt. The water is then passed through a filter 6 and filtered, and the treated water is taken out.

[0031] Sand, anthracite, etc. are suitable as the filter material for the filter 6. In this way, solid-liquid separation using the filter 6 requires a significantly smaller installation area than a settling tank, so a smaller space is sufficient for the wastewater treatment facility.

[0032] Test examples are described below. Test Examples 1 to 21 (Jar Test) <Raw Water> Sodium fluoride was added and dissolved in Akishima city water to adjust the fluoride ion concentration to 10 mg / L, which was used as raw water.

[0033] <Test Procedure> 500 mL of raw water was taken, and the pH was adjusted to the predetermined value shown in Table 1 using hydrochloric acid.

[0034] In Test Examples 1 to 5, only PAC was added as a flocculant in the amount shown in Table 1, and the mixture was stirred at 150 rpm for 3 minutes. In Test Examples 6 to 15, only cerium chloride was added in the amount shown in Table 1, and the mixture was stirred at 150 rpm for 3 minutes. In Test Examples 16 to 21, cerium chloride was added first in the amount shown in Table 1, and then PAC or ferric chloride was added in the amount shown in Table 1, and the mixture was stirred at 150 rpm for 3 minutes. In Table 1, the amount of cerium chloride added is represented as Ce, and the amount of PAC added is represented as 10% Al. ２ O ３ As a product, the amount of ferric chloride added is 38% FeCl ３ Shown as a product.

[0035] The flocculated liquid was filtered through a 1 μm filter paper, and the SS (suspended solids) concentration in the flocculated liquid was determined from the amount of filtrate and the weight of the dried filtrate. The flocculated liquid was also filtered through a 0.45 μm filter paper, and the soluble fluorine concentration in the filtrate was measured using an autoanalyzer manufactured by BL-TEC.

[0036] [Results and Discussion] The results are shown in Table 1. As shown in Table 1, when PAC was used alone (Test Examples 1 to 5), adding as much as 800 mg / L was required to reduce soluble fluoride to 4 mg / L or less. As a result, the amount of SS generated was as high as 271 mg / L, raising concerns about clogging of the filter.

[0037] In the case of treatment with cerium salt alone (Test Examples 6 to 15), the soluble fluorine concentration decreases when the amount of cerium added is 11 mg / L or more and the pH is 6 or less. However, the fluorine-containing flocs that are produced are so fine that they are difficult to capture with a filter.

[0038] When cerium salt and PAC (aluminum salt) were used in combination (Test Examples 16 to 18), the soluble fluorine concentration increased with increasing PAC concentration. This is thought to be due to the formation of a complex between aluminum and fluorine in the low pH range.

[0039] When cerium salt and ferric chloride (iron salt) were used in combination (Test Examples 19 to 21), the soluble fluorine concentration was low and the flocs formed were large.

[0040] The capture ability of the filter was evaluated by the filter evaluation method described below.

[0041]

[0042] Test Examples 22 to 25 (Filtration Test) Raw Water: Water from Akishima City was placed in each of four tanks, numbered 1 to 4, and sodium fluoride was added to the tank so that the fluorine concentration was 10 mg / L. The water was then stirred and adjusted to a pH of 5 with hydrochloric acid to provide raw water.

[0043] <Test Procedure> In Test Example 22, ferric chloride was added to the first tank at a concentration of 38% FeCl ３ The product was added at 30 mg / L.

[0044] In Test Example 23, cerium chloride was added to the second tank at a concentration of 22 mg / L in terms of Ce.

[0045] In Test Examples 24 and 25, cerium chloride was first added to the third and fourth tanks at 22 mg / L as Ce, and then ferric chloride was added at 38% FeCl ３ The product was added in the amounts shown in Table 2.

[0046] The raw water in each tank was stirred and flocculated to produce a flocculated solution, which was then gently stirred at all times to prevent the flocs from settling.

[0047] Four 60 mm diameter columns (40 cm of filter sand packed at the bottom and 40 cm of anthracite packed above) were prepared, and the flocculated water in each raw water tank was passed through them in a downward flow at a LV of 10 m / h for 8 hours to obtain filtered water. The differential pressure was continuously measured using pressure gauges installed at the inlet and outlet of the column. In addition, the treated water was sampled every hour.

[0048] The treated water SS was determined by filtering a sample of treated water through a 1 μm filter paper and measuring the amount of filtrate and the weight of the filtrate after drying. The treated water was also filtered through a filter paper, and the fluorine concentration in the filtrate was measured using an autoanalyzer manufactured by BL-TEC. The measurement result after filtration through No. 5A filter paper was taken as the total fluorine concentration, and the measurement result after filtration through 0.45 μm filter paper was taken as the soluble fluorine concentration. The results are shown in Table 2.

[0049] [Results and Discussion] In the case of treatment with ferric chloride alone (Test Example 22), the fluorine concentration did not decrease.

[0050] In the case of treatment with cerium salt alone (Test Example 23), the soluble fluorine concentration of the treated water decreased, but the total fluorine concentration remained high. This is thought to be because, although the fluorine was insolubilized by the addition of the cerium salt, it did not form flocs at a level that could be captured by a filter.

[0051] In Test Examples 24 and 25, in which a cerium salt and ferric chloride were used in combination, the difference between the total fluorine concentration and the soluble fluorine concentration was small. This is thought to be because the fluorine that was insolubilized by the addition of the cerium salt was coagulated by the ferric chloride to a level that could be captured by the filter.

[0052]

[0053] Test Example 26 Cerium chloride was added to an aqueous solution of sodium fluoride, and the pH after the addition was adjusted in the range of 2 to 9. The solution was then filtered, and the fluorine concentration after filtration was measured.

[0054] That is, sodium fluoride was dissolved in Akishima city water to prepare an aqueous sodium fluoride solution with a fluoride ion concentration of 10 mg / L. Sulfuric acid was added to 500 mL of this aqueous sodium fluoride solution to adjust the pH to 2, 3, 4, 5, 6, 7, 8, or 9. Cerium chloride (Ce: 90 mg / L) was added to each aqueous sodium fluoride solution, and the solution was stirred with a stirrer for 3 minutes, filtered through 0.45 μm filter paper, and the soluble fluoride concentration in the filtrate was measured. The relationship between pH and soluble fluoride concentration is shown in Figure 2.

[0055] 2, it can be seen that the fluorine concentration of the treated water is low when the pH after the addition of cerium chloride is set to 3 to 6, particularly 3 to 5, and that the fluorine concentration of the treated water is high when the pH is around 2 or 7 to 9. In addition, when the pH is 6, the fluorine concentration of the treated water is about half of that of the raw water.

[0056] Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications are possible within the scope of the effects of the invention. This application is based on Japanese Patent Application No. 2024-111172 filed on July 10, 2024, the entire contents of which are incorporated by reference.

[0057] 1 Reaction tank 2 Coagulation tank 3 Sedimentation tank 6 Filter

Claims

1. A method for treating fluorine-containing wastewater, which comprises a step of adding a cerium salt to fluorine-containing wastewater to insolubilize and remove fluorine, characterized in that the pH of the wastewater treated in the step of insolubilizing fluorine by adding the cerium salt is between 3 and 6.

2. The method for treating fluoride-containing wastewater according to claim 1, further comprising a step of adding an iron salt after the step of adding a cerium salt to insolubilize fluoride, and a subsequent step of filtering.

3. The method for treating fluoride-containing wastewater according to claim 1 or 2, further comprising the steps of adding a calcium compound to the fluoride-containing wastewater, flocculating the fluoride-containing wastewater, and separating the solid from the liquid, prior to the step of adding the cerium salt.

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