Method for treating wastewater containing fluoride

Abstract

To provide a processing method for fluorine-containing waste water that is capable of sufficiently removing fluorine and also generates decreased amounts of sludge.SOLUTION: Provided is a processing method for fluorine-containing waste water by adding a cerium compound to the fluorine-containing waste water, such as desulfurization waste water of a thermal power plant. After adding a cerium compound, pH is adjusted to 4-7, followed by membrane separation. As the cerium compound, cerium chloride, etc., is used, and as a pH-adjusting agent, sulfuric acid, caustic soda, etc., is used. A membrane separation device is also included, which is, for example, one having a hollow fiber membrane having a pore diameter of 2 μm or under.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for treating fluorine-containing wastewater such as flue gas desulfurization wastewater, and more particularly to a method for treating fluorine-containing wastewater using a cerium compound. [Background technology]

[0002] As a method for treating fluorine-containing wastewater such as flue gas desulfurization wastewater from thermal power plants, an aluminum compound such as PAC is added to the fluorine-containing wastewater, the pH is adjusted to 6 to 8, and the wastewater is separated by membrane (Patent Document 1, etc.). When an aluminum compound is used in this way, a large amount of sludge is generated.

[0003] Patent Document 2 describes a method for treating harmful substances, which comprises the steps of adding a cerium compound to water to be treated that contains harmful substances, adjusting the pH to 8-10 using an alkali metal hydroxide, and forming the harmful substances as insoluble precipitates, wherein the harmful substances contain at least two or more selected from the group consisting of selenium, arsenic, hexavalent chromium, fluorine, boron, and phosphorus, and the method treats two or more of the harmful substances simultaneously. Thus, when a cerium compound is used, the amount of sludge is smaller than when an aluminum compound is used. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 3-118897 [Patent Document 2] JP 2016-198740 A Summary of the Invention [Problem to be solved by the invention]

[0005] As a result of extensive research, the present inventors have found that the fluorine removal performance is low within the pH adjustment range of Patent Document 2, and that a sufficient fluorine removal effect cannot be obtained particularly in the treatment of desulfurization wastewater from thermal power plants.

[0006] An object of the present invention is to provide a method for treating fluorine-containing wastewater which can sufficiently remove fluorine and produce a small amount of sludge.

[0007] Another object of the present invention, in one aspect, is to provide a method for treating fluorine-containing wastewater, which can increase the flux during membrane separation treatment. [Means for solving the problem]

[0008] The method for treating fluorine-containing wastewater of the present invention is a method for treating fluorine-containing wastewater by adding a cerium compound to the fluorine-containing wastewater, and is characterized in that the pH is adjusted to 4 to 7 after the addition of the cerium compound.

[0009] In one aspect of the present invention, the fluorine-containing wastewater is desulfurization wastewater from a thermal power plant.

[0010] In one embodiment of the present invention, the liquid after the pH adjustment is subjected to solid-liquid separation treatment by membrane separation. Effect of the Invention

[0011] The inventors have found that, among the harmful substances shown in Patent Document 2, the removal effect of fluorine is increased when a cerium compound is added in a neutral to acidic region of pH 7 or less. In the present invention, the amount of cerium compound added can be reduced by setting the pH at the time of adding the cerium compound to 4 to 7. Furthermore, in the desulfurization wastewater treatment of thermal power plants, replacing PAC with a cerium compound has the effect of reducing the amount of chemicals added, as well as reducing the amount of sludge generated and increasing the flux in membrane separation. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a flow chart showing an example of a method for treating fluorine-containing wastewater according to the present invention. [Diagram 2] 13 is a graph showing experimental results. [Diagram 3]13 is a graph showing experimental results. [Figure 4] 13 is a graph showing experimental results. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention will now be described in further detail.

[0014] 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 crystals, wastewater discharged from the pickling process of stainless steel, etc. The fluorine concentration in flue gas desulfurization wastewater from thermal power plants is usually 20 to 1,000 mg / L, particularly about 50 to 500 mg / L, and the pH is 3 to 8, particularly about 5 to 8, but is not limited thereto.

[0015] Examples of the cerium compound to be added to the fluorine-containing wastewater include cerium chloride, cerium carbonate, cerium nitrate, cerium acetate, etc. The cerium compound is preferably added in the form of an aqueous solution.

[0016] The amount of the cerium compound to be added depends on the fluorine concentration in the wastewater, but in general, the cerium concentration in the water after the addition is preferably about 10 to 5,000 mg / L, particularly about 20 to 1,000 mg / L. The ratio (b / a) of the fluorine concentration a (mg / L) in the fluorine-containing wastewater to the cerium concentration b (mg / L) in the fluorine-containing wastewater immediately after the addition is preferably about 0.5 to 10, particularly about 1 to 5.

[0017] After adding a cerium compound to the fluoride-containing wastewater, a pH adjuster is added to adjust the pH to 4 to 7, particularly 4 to 6.

[0018] Within the range of pH 4 to 7, the appropriate pH varies depending on the quality of the fluoride-containing wastewater, so the optimal value can be determined by a prior evaluation. However, if the amount of cerium compound to be added is determined at pH 7, the fluoride removal performance will not decrease even if the pH changes to the acidic side.

[0019] Suitable pH adjusters include sulfuric acid, caustic soda, hydrochloric acid, and hydrated lime.

[0020] By adding a cerium compound to fluorine-containing wastewater and adjusting the pH to 4 to 7, cerium hydroxide is precipitated, and fluoride ions are adsorbed to the precipitate, thereby capturing the fluorine in the water. The precipitate is then subjected to solid-liquid separation to remove the fluorine in the water. Membrane separation is suitable as a solid-liquid separation means, but precipitation, coagulation precipitation, etc. may also be used.

[0021] Generally, the cross-flow type membrane separation method is suitable. In order to recover a large amount of wastewater treatment water, the SS (suspended solids) on the primary side is highly concentrated, so a module consisting of bundled polymer hollow fiber membranes with an inner diameter of the raw liquid flow path of 3 mm or more is suitable as the membrane. The pore size of the membrane is preferably 2 μm or less. The raw liquid flow rate of the cross-flow is preferably 1 m / s or more relative to the membrane surface, and the treated water flux is preferably 5 m / d or more.

[0022] It is preferable to continue the membrane treatment for one week or more, stop the treatment once, and perform chemical cleaning with caustic soda and hydrochloric acid to wash away components that have contaminated the membrane during the filtration operation, and then perform the filtration operation again.

[0023] In this membrane filtration, by changing the fluoride removal agent from PAC to a cerium compound, the flux can be increased by more than 20% compared to when PAC was used, improving the treatment efficiency.Alternatively, without changing the flux, the cross-flow raw liquid circulation flow rate can be reduced by more than 20%, making it possible to reduce power costs.

[0024] An example of the flow of an apparatus for treating fluorine-containing wastewater suitable for carrying out the method of the present invention is shown in FIG.

[0025] Fluorine-containing wastewater is sent from raw water tank 1 to reaction tank 2, where an aqueous solution of a cerium compound is added and stirred. The wastewater is then transferred to pH adjustment tank 3, where a pH adjuster is added to adjust the pH to 4-7.

[0026] The cerium compound can be added using a storage tank and a pump. The amount of cerium compound added can be controlled by controlling the flow rate of the pump.

[0027] The amount of the cerium compound to be added can be adjusted as required by continuously monitoring the fluorine concentration in the fluorine-containing wastewater and calculating the optimum amount.

[0028] The liquid whose pH has been adjusted in the pH adjustment tank 3 is transferred to the membrane circulation tank 4, where a part of the produced solids precipitate and are taken out as sludge.

[0029] The liquid in the membrane circulation tank 4 is supplied to the membrane separation device 5 by a pump (not shown), where it is separated by membrane. The membrane permeated water is taken out as treated water and sent to the next process. The concentrated water that did not permeate the membrane is returned to the membrane circulation tank 4. EXAMPLES

[0030] [Example 1] In removing fluorine from desulfurization effluent by adding cerium chloride, the following experiment was conducted to demonstrate that removal performance at pH 4-7 is superior to that at pH 8-10. <Experimental conditions> The raw water used in the test was flue gas desulfurization wastewater from a thermal power plant using an ash mixing method. The fluorine concentration of the raw water was 32 mg / L and the pH was 7.

[0031] Cerium chloride (aqueous solution with a concentration of 10 wt%) was added to 100 mL of raw water at a concentration of 100 mg / L in terms of pure cerium, and the pH after addition was set to 3, 4, 5, 6, 7, 8, 9, or 10. After adjusting the pH, the mixture was mixed and reacted at room temperature for 10 minutes using a stirrer, the resulting precipitate was removed using 5A filter paper, and the fluorine concentration in the filtrate was measured. The results are shown in Figure 2.

[0032] [Example 2] In order to confirm the effect of membrane filtration, the following experiment was carried out. <Experimental conditions> The same desulfurization wastewater as in Example 1 was used for the test. Cerium chloride was added at 50 mg / L in terms of cerium purity, and the pH was adjusted to 5. A circulation-type cross-flow filtration test was performed using the pH-adjusted liquid as raw water. A hollow polypropylene membrane with a pore size of 0.2 μm and a flow path diameter of 6 mm was used as the membrane used in the test. A hollow membrane of 200 mm was attached to a test module, the raw liquid was circulated to the membrane module at a flow rate of 3.4 L / min, and the filtration test was performed at a filtration pressure of 0.2 MPa. 20 L of raw water was used, and the filtrate was discharged to the outside so that the SS and reaction precipitates in the raw water were concentrated. The raw water was periodically collected and filtered with 5A filter paper, and the solid matter separated was washed with pure water and the dry weight was measured, and the solid matter concentration of the raw water was measured. The fluorine concentration of the membrane filtrate was also measured. The relationship between the solid matter concentration of the raw water and the filtration speed was determined. The results are shown in Figure 3.

[0033] [Comparative Example 1] In removing fluorine from desulfurization wastewater, the following experiment was conducted to show that when cerium chloride was added or PAC was added, the amount of chemicals added was smaller and the amount of sludge generated was smaller when cerium chloride was used, when treatment was performed at the optimum pH for each. <Experimental conditions> Using the raw water of Example 1, the amount of cerium chloride and PAC added was changed to confirm the fluorine removal performance. The pH after the addition of cerium chloride was set to 5, and the pH after the addition of PAC was set to 7. After the pH adjustment, the mixture was mixed and reacted for 10 minutes, the resulting precipitate was removed with 5A filter paper, and the fluorine concentration in the filtrate was measured. The solid matter on the filter paper was washed with pure water and the dry weight was determined. The amount of cerium chloride added was evaluated based on the concentration of the pure cerium content added in the raw water, and the amount of PAC added was evaluated based on the concentration of the pure aluminum content of the added PAC in the raw water. The results are shown in Figure 4.

[0034] [Comparative Example 2] In the membrane separation of desulfurization effluent, the following experiment was carried out to reproduce the operating conditions when PAC was added and to compare with Example 2. <Experimental conditions> The same test as in Example 2 was carried out, except that cerium chloride was replaced with PAC. PAC was added at 100 mg / L in terms of pure aluminum, and the pH after addition was set at 7. The results are shown in FIG.

[0035] [Results and Discussion] Figure 2 shows the relationship between the amount of cerium added and the fluorine concentration after treatment in the test of Example 1. As shown in Figure 2, the fluorine removal performance of cerium chloride was superior at pH 7 or less than the pH range of 8 to 10 as described in the prior art.

[0036] Figure 3 shows the relationship between the flux and the dry weight of the raw water SS and the reaction precipitate in the tests of Example 2 and Comparative Example 2. The fluorine concentration of the membrane filtrate was less than 10 mg / L in both Example 2 and Comparative Example 2. As shown in Figure 3, a higher filtration rate was obtained when fluorine was removed using cerium and solid-liquid separation was performed by membrane separation compared to aluminum treatment.

[0037] Figure 4 shows the relationship between the amount of cerium and aluminum added and the fluorine concentration after treatment in the test of Comparative Example 1. As shown in Figure 4, the cerium treatment was able to remove fluorine with a smaller amount of added aluminum than the aluminum treatment.

[0038] The fluorine treatment target for this wastewater in the actual equipment is 10 mg / L. When comparing the minimum amount of chemicals added that can remove fluorine down to 10 mg / L, the amount of sludge generated by the cerium treatment was about half that of the aluminum treatment. [Explanation of symbols]

[0039] 1 Raw water tank 2. Reactor 3 pH adjustment tank 4 Membrane circulation tank 5 Membrane separation equipment

Claims

[Claim 1] A method for treating fluorine-containing wastewater by adding a cerium compound to the fluorine-containing wastewater, comprising the steps of: adjusting pH to 4 to 7 after adding the cerium compound; The fluorine-containing wastewater is desulfurization wastewater from a thermal power plant having a fluorine concentration of 50 to 500 mg / L, The liquid after the pH adjustment is subjected to solid-liquid separation by a cross-flow type membrane separation using a polymer hollow fiber membrane having an inner diameter of a raw liquid flow path of 3 mm or more, with the raw liquid flow rate of the cross-flow being 1 m / s or more relative to the membrane surface.

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