Method for treating wastewater containing fluorine

Abstract

Efficiently reduces fluorine in fluorine-containing wastewater. [Solution] A method for treating wastewater containing fluorine, comprising the steps of: adjusting the pH of the wastewater containing fluorine to 10-11; adding a phosphate-based agent to the wastewater whose pH has been adjusted; and adjusting the pH of the wastewater to which the phosphate-based agent has been added to 10-11 and removing the fluorine in the wastewater as fluoroapatite.

Description

[Technical Field]

[0001] The present invention relates to a method for treating wastewater containing fluorine. [Background technology]

[0002] Conventionally, various technologies have been proposed for removing fluorine from wastewater generated from exhaust gas treatment facilities of furnaces that process industrial waste and recycled materials (see, for example, Patent Documents 1 to 5).

[0003] A common method for removing fluorine from wastewater involves adding calcium salt agents to the wastewater to generate sparingly soluble calcium fluoride, which is then precipitated and separated. However, calcium fluoride is negatively charged in a colloidal state, and the particles repel each other, making it difficult to agglomerate (coagulate), thus making it difficult to remove from wastewater. Therefore, a method is known in which inorganic coagulants such as aluminum chloride, aluminum sulfate, or ferric chloride are added to the wastewater containing the generated calcium fluoride, and the pH is adjusted with slaked lime, etc., to cause coagulation by aluminum hydroxide or iron hydroxide. Finally, a polymer coagulant is added to cause the calcium fluoride particles to agglomerate and precipitate.

[0004] Since the solubility of calcium fluoride in water is 8 mg / L as fluoride ions, theoretically, fluorine removal by generating calcium fluoride can only reduce the fluorine concentration in wastewater to the wastewater standard of 8 mg / L. Furthermore, as described in Patent Document 1, the solubility of calcium fluoride increases as the concentration of coexisting salts, especially anions, increases, and due to the influence of reaction-inhibiting elements such as carbonate ions, silica, and sulfate ions, it is difficult to reduce the fluorine concentration in wastewater to 8 mg / L by removing fluorine through calcium fluoride generation in actual treatment.

[0005] As a method to further reduce the fluorine concentration in wastewater, as described in Patent Documents 6-8, there is a technique to add a drug containing phosphate ions such as phosphoric acid or phosphate salts (phosphate-based drug) to generate fluoroapatite, which has lower solubility than calcium fluoride. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2017-80722 [Patent Document 2] Japanese Patent Publication No. 2013-132601 [Patent Document 3] Japanese Patent Publication No. 2013-166120 [Patent Document 4] Japanese Patent Publication No. 2001-232373 [Patent Document 5] Japanese Patent Publication No. 2012-106226 [Patent Document 6] Japanese Patent Application Publication No. 52-088577 [Patent Document 7] Japanese Patent Publication No. 2019-181328 [Patent Document 8] Japanese Patent Publication No. 2019-181329 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, simply adding phosphate-based chemicals to remove fluorine would require large amounts of these chemicals, increasing operating costs.

[0008] The method for treating fluorine-containing wastewater described herein aims to efficiently remove fluorine from fluorine-containing wastewater. [Means for solving the problem]

[0009] The method for treating wastewater containing fluorine described in this specification includes a step of adjusting the pH of the wastewater containing fluorine to 10 - 11, a step of adding a phosphoric acid-based chemical to the wastewater whose pH has been adjusted, and a step of adjusting the pH of the wastewater to 10 - 11 after adding the phosphoric acid-based chemical and removing the fluorine in the wastewater as fluoroapatite.

Advantages of the Invention

[0010] The method for treating wastewater containing fluorine described in this specification has the effect that fluorine can be efficiently removed from the wastewater containing fluorine.

Brief Description of the Drawings

[0011] [Figure 1] Figure 1 is a diagram schematically showing the configuration of a wastewater treatment system according to an embodiment. [Figure 2] Figure 2 is a diagram for explaining the experimental procedure for verifying at which timing it is good to add a phosphoric acid-based chemical to the wastewater. [Figure 3] Figure 3 is a diagram for explaining the experimental procedure for verifying how much amount of a phosphoric acid-based chemical should be added to the wastewater.

Modes for Carrying Out the Invention

[0012] <​​​​​​​​​​The melting furnace recovers valuable metals such as copper, gold, and silver from raw materials such as ores and recycled materials, and at the same time encapsulates harmful heavy metals in slag to render them harmless and fixed. The treatment device for treating the exhaust gas generated in the melting furnace includes an electrostatic precipitator, a bag filter, a denitration catalyst tower, etc. In this treatment device, after leaching metals such as copper, lead, and zinc contained in the dust obtained by the electrostatic precipitator with sulfuric acid, neutralization and solid-liquid separation are repeated to recover the metal content. The finally remaining liquid is treated as wastewater, but it contains fluorine, and its fluorine concentration is about 20 mg / L.

[0015] The incinerator is a furnace that incinerates recycled materials such as electric component scraps and thermally decomposes liquid industrial wastes such as waste acids and waste alkalis. The treatment device for treating the exhaust gas generated by incineration, etc. includes a quench tower, a water-sealed scrubber, an ion scrubber, a mist cottrell, etc. The wastewater discharged from this treatment device contains fluorine, and its fluorine concentration is about 30 - 50 mg / L.

[0016] In order to finally discharge the fluorine-containing wastewater outside the smelter, it is necessary to remove fluorine from the wastewater and make the fluorine concentration in the wastewater below the discharge standard. Here, when treating the fluorine-containing wastewater with calcium salt chemicals and phosphoric acid-based chemicals, the fluorine concentration in the wastewater can be reduced to below the discharge standard. However, phosphoric acid-based chemicals are more expensive than calcium salt chemicals. For this reason, in order to reduce the usage amount of phosphoric acid-based chemicals, it is known to use a two-stage treatment method. Two-stage treatment means adding a first chemical to the wastewater to be treated to reduce the target component, then performing solid-liquid separation with a thickener, etc., adding a second chemical to the obtained liquid to further reduce the target component, and then performing solid-liquid separation (for the specific method of two-stage treatment, refer to, for example, Japanese Patent Laid-Open No. 11-333467). For example, if two-stage treatment is performed when reducing the fluorine concentration in the wastewater, as the primary treatment, calcium salt chemicals are added to the wastewater to generate calcium fluoride to reduce the fluorine concentration, then solid-liquid separation is performed, and for the fluorine remaining in the obtained liquid, as the secondary treatment, phosphoric acid-based chemicals are added to generate fluoroapatite to further reduce the fluorine. Such a method is adopted.

[0017] However, implementing the two-stage processing described above in actual operation requires two sets of reaction tanks, thickeners, and other processing equipment, necessitating large-scale facilities and installation space. For this reason, introducing two-stage processing into actual operation presents significant hurdles.

[0018] Therefore, in this embodiment, a wastewater treatment system as shown in Figure 1 is used to further reduce the fluorine concentration in the wastewater through a single-stage treatment (using a single-stage treatment facility). Here, "single-stage treatment" refers to a process in which, after adding a first chemical agent to the wastewater to be treated, a second chemical agent is added to the wastewater to further reduce the target component without performing solid-liquid separation using a thickener or the like, and then solid-liquid separation is performed. Furthermore, "single-stage treatment equipment" refers to equipment that does not have equipment such as a thickener for solid-liquid separation between the equipment that performs the step of adding chemical agents to the wastewater to be treated and the equipment that performs the step of adding another chemical agent. In other words, in this embodiment, the fluorine concentration in the wastewater is further reduced without installing equipment such as a thickener for solid-liquid separation between the equipment that performs the step of adding a calcium salt agent to the wastewater containing fluorine and the equipment that performs the step of adding a phosphate-based agent.

[0019] The wastewater treatment system in Figure 1 comprises a thickener (No. 1), a stirring tank, a reaction tank (No. 1), a reaction tank (No. 2), a coagulation tank, and a thickener (No. 2). The wastewater treatment using the wastewater treatment system in Figure 1 will be described below. The following treatment numbers (1) to (6) correspond to (1) to (6) in Figure 1. In this embodiment, the phosphate-based agent was phosphoric acid, and the calcium salt agent was calcium chloride solution.

[0020] (1) Wastewater (fluorine-containing wastewater) discharged as a result of treating exhaust gas generated in melting furnaces and incinerators is first sent to a thickener (No. 1), where wastewater residue (sludge) is separated by sedimentation. The reason for removing the sludge here is that components derived from the sludge may hinder fluorine removal in subsequent processes. However, if the fluorine-containing wastewater does not contain sludge, the thickener (No. 1) may be omitted in the wastewater treatment system.

[0021] (2) The overflow water (wastewater) from the thickener (No. 1) is sent to the stirring tank, and ferric chloride solution is added to the wastewater in the stirring tank and stirred. The reason for adding ferric chloride solution to the wastewater is to use it as an inorganic coagulant to co-precipitate trace amounts of heavy metals in the wastewater and to coagulate (congeal) the calcium fluoride fine particles described later.

[0022] (3) The wastewater to which ferric chloride solution has been added is sent from the stirring tank to the reaction tank (No. 1), but calcium chloride solution and slaked lime water are added along the way (in the trough between the tanks). Fluoride ions and calcium ions in the wastewater form sparingly soluble calcium fluoride (reaction equation: Ca 2+ +2F - →CaF2). This calcium fluoride is negatively charged in its colloidal state, so the particles repel each other and do not easily aggregate. In contrast, in (2) above, ferric chloride, an inorganic coagulant, is added, so by adjusting the pH of the wastewater to about 10 with slaked lime water, the charge is neutralized (coagulation occurs), and fine flocs are formed. In reaction tank (No. 1), stirring is performed so that the pH of the wastewater is uniformly around 10. Also, time is allocated in reaction tank (No. 1) for the production of calcium fluoride. The calcium chloride solution may also be added to the wastewater in the stirring tank together with the ferric chloride solution.

[0023] (4) The wastewater from reaction vessel (No. 1) (wastewater containing calcium fluoride) is sent to reaction vessel (No. 2), but phosphoric acid is added along the way (in the trough between the vessels), and slaked lime water is also added. When phosphoric acid is added to wastewater containing fluorine, Reaction equation: 3H3PO4 + 5Ca(OH)2 + F - →Ca5(PO4)3F+9H2O+OH - The reaction occurs, producing fluoroapatite (Ca5(PO4)3F).

[0024] In this embodiment, in (3) above, the pH of the wastewater is adjusted to about 10 (alkaline) before adding phosphoric acid, so trivalent iron ions (Fe) derived from ferric chloride are present. 3+ ) becomes a solid hydroxide (Fe(OH)3). Also, by adjusting the pH of the wastewater to about 10 (alkaline) before adding phosphoric acid, even if strongly acidic phosphoric acid is added, the pH after addition can be kept at a weakly acidic level of about 5, and iron hydroxide (Fe(OH)3) redissolves and trivalent iron ions (Fe 3+ This prevents the pH from dropping to the acidic range where phosphate ions become trivalent iron ions (Fe). 3+ This prevents the phosphate ions from being consumed in the formation of FePO4 through a reaction with other substances, and allows them to be utilized in the formation of fluoroapatite. The wastewater before the addition of phosphate only needs to be alkaline, and the pH should be, for example, 10 to 11.

[0025] Adding phosphoric acid to the wastewater makes it acidic, increasing the solubility of fluoroapatite and potentially reducing the fluorine removal effect. Therefore, in this embodiment, slaked lime water is added immediately after adding phosphoric acid to bring the pH of the wastewater to 10-11 (for example, 10). In the reaction vessel (No. 2), sufficient time is provided for the formation of fluoroapatite and the hydroxide or coprecipitation of heavy metals.

[0026] In this embodiment, the amount of phosphate ions added to the wastewater is set to 0.7 to 1 times the amount of phosphate ions needed to convert all of the fluoride ions in the wastewater into fluoroapatite. The reason for this will be explained later.

[0027] (5) The wastewater from the reaction tank (No. 2) is sent to the coagulation tank, and a polymer flocculant is added along the way. The wastewater is also stirred in the coagulation tank. This causes the solid matter generated in the wastewater to coagulate. In this embodiment, by adding calcium chloride solution to the fluorine-containing wastewater to produce calcium fluoride, the fluorine concentration in the wastewater can theoretically be reduced to about 8 mg / L, and by further adding phosphoric acid to produce fluoroapatite, the fluorine concentration in the wastewater can theoretically be reduced to 0.1 mg / L, which is the solubility of fluoroapatite.

[0028] (6) The wastewater from the coagulation tank is then sent to the thickener (No. 2). In the thickener (No. 2), the solid components contained in the wastewater (called wastewater residue or fluorine-containing sludge) are settled and separated. The wastewater residue separated from the wastewater is put into a melting furnace and melted. In the melting furnace, the wastewater residue is heated to 1,000 to 1,700°C. This fixes the fluorine into the slag. Meanwhile, the wastewater after the wastewater residue has been separated in the thickener (No. 2) (overflow water) is sent to the next treatment (for example, integrated wastewater treatment). In integrated wastewater treatment, processes such as removing heavy metals from the wastewater are carried out.

[0029] (Regarding experiments on the timing of phosphate drug addition) The inventors conducted an experiment as shown in Figure 2 to verify the optimal timing for adding phosphate-based chemicals to wastewater. The experiment shown in Figure 2 will be described below.

[0030] As shown in Figure 2, in this experiment, fluorine-containing wastewater (200 mL) with a fluorine concentration of 24.6 mg / L was prepared. The wastewater temperature was set to 54°C and the stirring speed to 300 rpm. In this experiment, 85% phosphoric acid was used as the phosphate-based drug, and calcium chloride solution was used as the calcium salt drug.

[0031] (Experiment 1) In Experiment 1, 40 μL of 85% phosphoric acid was first added to 200 mL of fluorine-containing wastewater with a fluorine concentration of 24.6 mg / L, followed by 356 μL of ferric chloride solution, and the mixture was held for 15 minutes. Then, 535 μL of calcium chloride solution was added to the wastewater, and slaked lime water was added to adjust the pH to 7, and the mixture was held for 15 minutes. Next, slaked lime water was added to adjust the pH to 10, and the mixture was held for 15 minutes. Finally, 178 μL of polymer flocculant was added to the wastewater, and the mixture was held for 1 minute. After that, stirring was stopped, and the mixture was allowed to stand for 50 minutes while maintaining the temperature. The wastewater was then filtered, and the fluorine concentration of the filtrate was measured.

[0032] (Experiment 2) In Experiment 2, the timing of adding 40 μL of 85% phosphoric acid was changed to before adjusting the pH of the wastewater to 7 with slaked lime water, after adding calcium chloride solution to the wastewater. Otherwise, the experiment was the same as in Experiment 1.

[0033] (Experiment 3) In Experiment 3, the timing of adding 40 μL of 85% phosphoric acid was changed to before adjusting the pH of the wastewater to 10, after adjusting the pH to 7. Otherwise, the experiment was the same as in Experiment 1.

[0034] (Experimental results) As a result of conducting the above experiments 1-3, the fluorine concentration in Experiment 1 was 16.8 mg / L. In Experiment 2, the fluorine concentration was 18.2 mg / L. In contrast, in Experiment 3, the fluorine concentration was 6.9 mg / L. From these experimental results, it was found that adding the phosphate-based agent to the wastewater at the timing of Experiment 3 (when the pH of the wastewater before adding the phosphate-based agent was 7) enhances the fluorine removal effect of the phosphate-based agent.

[0035] Furthermore, in Experiment 3, the inventors also conducted an experiment in which the pH of the wastewater was adjusted to 10 before adding the phosphate-based agent. As a result, it was found that the fluorine concentration in the wastewater was reduced compared to Experiment 3 (where the pH of the wastewater before adding the phosphate-based agent was 7). Therefore, it was found that adjusting the pH to 10-11 before adding the phosphate-based agent enhances the fluorine removal effect of the phosphate-based agent.

[0036] (Regarding the verification experiment on the amount of phosphate-based drug added) The inventors conducted an experiment as shown in Figure 3 to verify the optimal amount of phosphate-based chemicals to add to wastewater. The experiment shown in Figure 3 will be described below.

[0037] In this experiment, we investigated the appropriate amount (X) of phosphoric acid-based chemical to add when treating fluorine-containing wastewater (200 mL) with a fluorine concentration of 30 mg / L. The wastewater temperature was set to 54°C and the stirring speed to 300 rpm. In this experiment, 85% phosphoric acid was used as the phosphate-based drug, and calcium chloride solution was used as the calcium salt drug.

[0038] (Experiment 1) In Experiment 1, 356 μL of ferric chloride solution was added to 200 mL of fluorine-containing wastewater with a fluorine concentration of 30 mg / L, and the mixture was held for 15 minutes. Then, 535 μL of calcium chloride solution was added to the wastewater, and slaked lime water was added to adjust the pH to 7, and the mixture was held for 15 minutes. Next, slaked lime water was added to adjust the pH to 10, 33 μL of 85% phosphoric acid was added, and slaked lime water was added again to adjust the pH to 10, and the mixture was held for 15 minutes. Finally, 178 μL of polymer flocculant was added to the wastewater, and the mixture was held for 1 minute. After that, stirring was stopped, and the mixture was allowed to stand for 50 minutes while maintaining the temperature. The wastewater was then filtered, and the fluorine concentration of the filtrate was measured.

[0039] (Experiment 2) In Experiment 2, the amount of 85% phosphoric acid added was 66 μL, but other than the amount of 85% phosphoric acid added, the experiment was the same as in Experiment 1. The amount of 85% phosphoric acid added in Experiment 2 (66 μL) is the same amount of phosphate ions required to convert all fluoride ions in the wastewater into fluoroapatite (1.0 times the standard amount). In other words, the amount of 85% phosphoric acid added in Experiment 1 is 0.5 times the amount of phosphate ions required to convert all fluoride ions in the wastewater into fluoroapatite (standard amount).

[0040] (Experiment 3) In Experiment 3, the addition amount of 85% phosphoric acid was 99 μL, and other conditions were the same as those in Experiment 1 except for the addition amount of 85% phosphoric acid. The addition amount of 85% phosphoric acid in Experiment 3 was 1.5 times the amount of phosphate ions (standard) required to convert all fluoride ions in the wastewater into fluoroapatite.

[0041] (Experimental results) As a result of conducting the above Experiments 1 to 3, in Experiment 1, the fluorine concentration in the wastewater was 5.0 mg / L. In Experiment 2, the fluorine concentration in the wastewater was 2.9 mg / L. In Experiment 3, the fluorine concentration in the wastewater was 1.9 mg / L. From the results of this experiment, it was found that even when the amount of phosphate ions added was 1.5 times the standard, a 1.5-fold fluorine removal effect could not be obtained, indicating poor efficiency. Also, it was found that when the amount of phosphate ions added was 0.5 times the standard, the fluorine removal effect was low. According to the inventor, the amount of phosphate ions at which the fluorine concentration in the wastewater meets the discharge standard and an effect commensurate with the added amount of phosphate ions can be obtained is 0.7 to 1 times the amount of phosphate ions (standard) required to convert all fluoride ions in the wastewater into fluoroapatite. Therefore, in the above embodiment, the amount of phosphate ions added was set to 0.7 to 1 times the standard.

[0042] As described in detail above, according to this embodiment, a step of adjusting the pH of the fluoride-containing wastewater to 10 to 11 (step (3) in FIG. 1) and a step of adding phosphoric acid to the wastewater with the adjusted pH to remove fluorine in the wastewater as fluoroapatite (steps (4) to (6) in FIG. 1) are performed. By adjusting the pH of the wastewater to 10 to 11 (alkaline) before adding phosphoric acid in this way, trivalent iron ions (Fe 3+ ) in the wastewater become solids of hydroxide (Fe(OH)3). Also, by adjusting the pH of the wastewater to about 10 (alkaline) before adding phosphoric acid, even when strongly acidic phosphoric acid is added, the pH can be kept weakly acidic at about 5 after addition, preventing the pH from decreasing to the acidic region where iron hydroxide (Fe(OH)3) redissolves into trivalent iron ions (Fe 3+ ). Thereby, phosphate ions and trivalent iron ions (Fe 3+This prevents the phosphate ions from reacting with other phosphates to produce FePO4, allowing them to be used for fluoroapatite production. As a result, fluorine can be removed even with a small amount of phosphate-based chemicals added. This reduces operating costs. Therefore, it becomes possible to efficiently remove fluorine from fluorine-containing wastewater.

[0043] Furthermore, in this embodiment, the fluorine concentration in wastewater can be efficiently reduced by a single-stage treatment (using a single treatment facility) using the wastewater treatment system shown in Figure 1. In other words, since the two-stage treatment described above is not performed, two stages of treatment facilities such as reaction tanks and thickeners are unnecessary, eliminating the need for large-scale equipment and installation space. Therefore, the fluorine concentration in wastewater can be efficiently reduced at a low cost.

[0044] Furthermore, in this embodiment, the amount of phosphate ions added to the wastewater is set to 0.7 to 1 times the amount of phosphate ions (standard) required to convert all fluoride ions in the wastewater into fluoroapatite. This ensures that the fluorine concentration of the wastewater meets the wastewater standards, and that a fluorine removal effect commensurate with the amount of phosphate ions added can be obtained.

[0045] Furthermore, in this embodiment, the pH of the wastewater is adjusted to 10-11 by adding slaked lime water to the wastewater after adding phosphoric acid. By adjusting the wastewater, which has become acidic after adding phosphoric acid, back to alkaline, the solubility of fluoroapatite can be reduced, thereby improving the fluorine removal effect.

[0046] Furthermore, in this embodiment, since ferric chloride solution is added to the wastewater, trace amounts of heavy metals in the wastewater can be co-precipitated, and calcium fluoride can be coagulated (coagulated).

[0047] Furthermore, in this embodiment, calcium chloride solution (calcium salt agent in this embodiment) is added to the wastewater sent from the stirring tank to the reaction tank (No. 1). This allows calcium fluoride to be generated from fluoride ions in the wastewater, and the generated calcium fluoride can be separated from the wastewater in the thickener (No. 2), thereby reducing the fluorine concentration in the wastewater.

[0048] In the above embodiment, the case of adding phosphoric acid to wastewater was described, but it is not limited to this. Other phosphoric acid-based agents may be added to the wastewater, such as phosphorus oxoacids such as phosphorous acid and hypophosphorous acid, or alkali phosphate salts such as sodium phosphate and potassium phosphate.

[0049] In the above embodiment, the case in which wastewater discharged as a result of treating exhaust gas generated in an incinerator is treated using the wastewater treatment system shown in Figure 1 was described, but the invention is not limited to this. That is, wastewater discharged as a result of treating exhaust gas generated in a melting furnace or the like may also be treated using the wastewater treatment system shown in Figure 1.

[0050] According to one embodiment of the present disclosure, fluorine-containing wastewater can be treated efficiently. Therefore, one embodiment of the present disclosure may contribute to the United Nations Sustainable Development Goals (SDGs) Goal 3, "Ensure healthy lives and promote well-being for all at all ages," Goal 6, "Ensure availability and sustainable management of water and sanitation for all," and Goal 12, "Ensure sustainable consumption and production patterns."

[0051] The embodiments described above are preferred examples of the present invention. However, the invention is not limited thereto, and various modifications are possible without departing from the spirit of the invention.

Claims

1. A process to adjust the pH of wastewater containing fluorine to 10-11, The process involves adding a phosphate-based agent to the wastewater whose pH has been adjusted, A step of adjusting the pH of the wastewater to which the phosphate-based agent has been added to 10 to 11, A method for treating wastewater containing fluorine.

2. The method for treating fluorine-containing wastewater according to claim 1, characterized in that the amount of the phosphate-based agent added is 0.7 to 1 times the amount of phosphate ions necessary to convert the entire amount of fluoride ions in the wastewater into fluoroapatite.

3. A method for treating fluorine-containing wastewater according to claim 1, further comprising the step of adding ferric chloride to the wastewater before the step of adjusting the pH of the fluorine-containing wastewater to 10 to 11.

4. A method for treating fluorine-containing wastewater according to claim 1, further comprising the step of adding a calcium salt agent to the wastewater before the step of adjusting the pH of the fluorine-containing wastewater to 10 to 11.

5. The method for treating fluorine-containing wastewater according to claim 4, characterized in that, after the step of adding a calcium salt agent to the wastewater, the step of adding the phosphate-based agent without performing solid-liquid separation of the wastewater is carried out.

6. The method for treating fluorine-containing wastewater according to claim 1, characterized in that the phosphate-based agent is either a phosphorus oxoacid or an alkali phosphate salt.

7. The method for treating fluorine-containing wastewater according to any one of claims 1 to 6, characterized in that the wastewater is wastewater generated when treating gas generated from an incinerator.

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