Method for treating fluorine-containing waste water

Abstract

To optimize the pH of fluorine insolubilization by a cerium salt and to reduce the necessary addition amount of the cerium salt.SOLUTION: A method for treating fluorine-containing wastewater, comprising a step of adding a cerium salt to fluorine-containing wastewater to insolubilize and remove fluorine, wherein a pH in the step of adding a cerium salt to insolubilize fluorine is 3 to 6. The method further includes an iron salt addition step and a filtration step subsequent to the step of adding the cerium salt to insolubilize fluorine.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[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. [Background technology]

[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 the low-concentration fluoride remaining in the first step, and the fluorine is adsorbed onto the precipitated aluminum hydroxide, which is insolubilized and precipitated.

[0003] This method can stably reduce fluorine in treated water to below the discharge standard of 8 mg / L. However, the aluminum hydroxide flocs produced in the second process have a small density difference with the water, which means that settling takes time. This means that the area of ​​the flocculation and sedimentation tank in the second process needs to be large, which poses issues in terms of installation space and equipment costs. In addition, a large amount of aluminum hydroxide flocs is generated from the aluminum salts.

[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 using an alkali metal hydroxide, and forming an insoluble precipitate. As described above, when a cerium salt is used, the amount of sludge is smaller than when an aluminum compound is used. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-768 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-198740 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

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

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

[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 step of filtration.

[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 fluoride-containing wastewater, and performing solid-liquid separation, prior to the step of adding the cerium salt. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0013] [Figure 1]FIG. 1 is a flow diagram of a method for treating fluorine-containing wastewater according to an embodiment. [Figure 2] 10 is a graph showing experimental results. DETAILED DESCRIPTION OF THE INVENTION

[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 slaked 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, which reacts with fluoride ions to produce cerium fluoride, thereby capturing the fluorine in the water. Then, the product is subjected to solid-liquid separation, whereby the 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 coarsen, making it possible to capture it with a filter. The amount of iron salt added is preferably 10 to 40 mg / L, and particularly preferably about 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 flocculation tank provided.

[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] If the raw water fluoride concentration is 25 mg / L or less, the first step, the insolubilization and precipitation step using 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 this 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 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, causing the calcium fluoride particles to coagulate.

[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 via the return line 4, and the remainder is discharged outside the system.

[0030] The supernatant water from the settling tank 3 is taken out via a pipe 5. A cerium salt is added to the pipe 5, followed by an iron salt. This causes the fluorine components in the supernatant water to react 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. [Example]

[0032] Test examples will be described below. [Test Examples 1 to 21] (Jar Test) <Raw water> Sodium fluoride was added to Akishima city water and dissolved 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 shown as Ce, the amount of PAC added as a 10% Al2O3 product, and the amount of ferric chloride added as a 38% FeCl3 product.

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

[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), it was necessary to add as much as 800 mg / L 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 fluoride concentration decreases when the amount of cerium added is 11 mg / L or more and the pH is lower than 6. However, the fluoride-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 a 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 that formed were large.

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

[0041] [Table 1]

[0042] [Test Examples 22 to 25 (Filtration Test)] <Raw water> Akishima city water was stored in each of four tanks, numbered 1 to 4, and sodium fluoride was added to the water so that the fluorine concentration was 10 mg / L. The water was then stirred and adjusted to pH 5 with hydrochloric acid to prepare raw water.

[0043] <Test Procedure> In Test Example 22, 30 mg / L of ferric chloride was added to the first tank as a 38% FeCl3 product.

[0044] In Test Example 23, cerium chloride was added to the second tank at 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 in terms of Ce, and then ferric chloride was added in the amount shown in Table 2 as a 38% FeCl3 product.

[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 60mm diameter columns (40cm of filter sand packed at the bottom and 40cm of anthracite packed above that) were prepared, and the flocculated water from each raw water tank was passed through them in a downward flow at LV10m / h for 8 hours to obtain filtered water. The differential pressure was continuously measured using pressure gauges installed at the column inlet and outlet. Treated water samples were also taken every hour.

[0048] The treated water SS was calculated by filtering a sample of treated water through a 1 μm filter paper and measuring the volume of the filtered water and the weight of the dried filtrate. The treated water was also filtered through a filter paper, and the fluoride concentration in the filtrate was measured using a BL-TEC autoanalyzer. The measurement result after filtration through No. 5A filter paper was taken as the total fluoride concentration, and the measurement result after filtration through 0.45 μm filter paper was taken as the soluble fluoride 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 fluoride concentration does not decrease.

[0050] When treated 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 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 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 cerium salt was coagulated by ferric chloride to a level that could be captured by the filter.

[0052] [Table 2]

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

[0054] Specifically, sodium fluoride was dissolved in Akishima city water to prepare a sodium fluoride aqueous solution with a fluoride ion concentration of 10 mg / L. Sulfuric acid was added to 500 mL of this sodium fluoride aqueous 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 sodium fluoride aqueous solution, and the solution was stirred for 3 minutes using a stirrer. It was then 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 when the pH after addition of cerium chloride is set to 3 to 6, especially 3 to 5, the fluorine concentration of the treated water is low, and when the pH is around 2 or 7 to 9, the fluorine concentration of the treated water is high. At pH 6, the fluorine concentration of the treated water is about half that of the raw water. [Explanation of symbols]

[0056] 1 Reaction vessel 2 Coagulation tank 3 Settling tank 6 Filter

Claims

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

2. 2. The method for treating fluorine-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 fluorine, and a subsequent step of filtering.

3. 3. The method for treating fluorine-containing wastewater according to claim 1, further comprising the steps of adding a calcium compound to the fluorine-containing wastewater, flocculating the fluorine-containing wastewater, and separating the fluorine-containing wastewater into solids and liquids, prior to the step of adding the cerium salt.

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