An intermittent crystallization deep fluorine removal system and method for treating low-concentration fluorine-containing wastewater

Abstract

The application discloses an intermittent crystallization deep fluorine removal system and method for treating low-concentration fluorine-containing wastewater, and belongs to the technical field of wastewater treatment. The application adopts the principles of induced crystallization and precipitation net trapping, so that fluorine ions form stable crystal structures on the surface of apatite, and the removal of the fluorine ions is realized. In the intermittent crystallization precipitation tank, part of the fine flocs formed by local homogeneous nucleation can be adsorbed and trapped by apatite particles in the precipitation step, so that the effluent is clear and does not contain suspended solids. The final product is a crystal with low water content, and can be directly used for secondary use. The process operation is simple, the treatment cost of the low-concentration fluorine-containing wastewater is greatly reduced, and the economic benefit is improved.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to an intermittent crystallization deep defluorination system and method for treating low-concentration fluoride-containing wastewater. Background Technology

[0002] In recent years, with the continuous development of new energy industries such as photovoltaic manufacturing, a large amount of fluoride-containing wastewater has been generated. Fluorine is toxic to humans and aquatic plants and animals, posing a high environmental risk and requiring proper disposal.

[0003] Industrial enterprises typically employ defluoridation methods such as precipitation, adsorption, and ion exchange. These methods are generally suitable for treating high-concentration fluoride-containing wastewater. However, due to significant variations in operating conditions during actual production, the resulting wastewater fluoride concentrations vary considerably. Faced with low-concentration fluoride wastewater, some enterprises resort to a method of enrichment followed by treatment, which greatly reduces wastewater treatment efficiency and economic benefits. While existing defluoridation technologies can relatively easily reduce fluoride levels to 10 mg / L, deep defluorination to below 1.5 mg / L is extremely difficult.

[0004] Therefore, it is evident that developing advanced treatment processes suitable for low-concentration fluoride-containing wastewater is of great practical significance. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides an intermittent crystallization-based deep defluorination system and method for treating low-concentration fluoride-containing wastewater. This system and method can be used for the deep treatment of effluent from conventional treatment processes, or directly applied to the deep treatment of low-concentration fluoride-containing wastewater, achieving rapid and efficient treatment of low-concentration fluoride-containing wastewater.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] One of the technical solutions of this invention is to provide an intermittent crystallization deep defluorination system for treating low-concentration fluoride-containing wastewater, the structure of which includes:

[0008] The water storage tank, regulating tank, intermittent crystallization sedimentation tank and crystal discharge tank are connected in sequence. The regulating tank is also connected to an alkali dosing tank and a phosphorus dosing tank. The intermittent crystallization sedimentation tank is also connected to a calcium dosing tank.

[0009] Both the regulating tank and the intermittent crystallization sedimentation tank are equipped with a stirrer;

[0010] The intermittent crystallization sedimentation tank is filled with apatite particles.

[0011] Preferably, the apatite particles have a particle size of 0.1 to 0.6 mm and a filling volume of 1 / 3 of the volume of the intermittent crystallization sedimentation tank.

[0012] The second technical solution of this invention: provides a method for treating low-concentration fluoride-containing wastewater based on the above-mentioned intermittent crystallization deep defluorination system for treating low-concentration fluoride-containing wastewater, the steps of which include:

[0013] The low-concentration fluoride-containing wastewater from the storage tank is introduced into the equalization tank. The stirring device in the equalization tank is activated, and under continuous stirring, the alkaline solution from the alkali dosing tank and the phosphate solution from the phosphorus dosing tank are added to the equalization tank, ensuring that the phosphorus concentration in the equalization tank is not less than 0.003 mol / L and the pH value is not less than 10, resulting in mixed wastewater. This mixed wastewater is then introduced into the intermittent crystallization sedimentation tank. The stirring device in the intermittent crystallization sedimentation tank is activated, and under continuous stirring, the calcium salt solution from the calcium dosing tank is added to the intermittent crystallization sedimentation tank, ensuring that the initial calcium concentration in the intermittent crystallization sedimentation tank is not less than 0.0075 mol / L, initiating a crystallization reaction. The mixture is then allowed to settle, and the supernatant is the treated wastewater. When the volume of the precipitate exceeds half the volume of the intermittent crystallization sedimentation tank, a portion of the precipitate is discharged to the crystal discharge tank.

[0014] The concentration of fluoride ions in the low-concentration fluoride wastewater is ≤20 mg / L.

[0015] Preferably, the phosphorus concentration in the regulating tank is 0.003–0.009 mol / L.

[0016] Preferably, the initial calcium concentration in the intermittent crystallization sedimentation tank is 0.0075–0.015 mol / L.

[0017] Preferably, the stirring speed of the stirring device in the regulating tank is 500-1200 rpm; the stirring speed of the stirring device in the intermittent crystallization sedimentation tank is 500-1200 rpm.

[0018] This stirring speed range allows the natural apatite particles to be fluidized and evenly distributed throughout the intermittent crystallization sedimentation tank. Too low a speed is not conducive to the fluidization of natural apatite particles and can easily cause blockage, while too high a speed will cause the reaction liquid to splash out, affecting equipment safety.

[0019] Preferably, the alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution.

[0020] Preferably, the calcium salt solution is a calcium chloride solution.

[0021] Preferably, the crystallization reaction takes 10 to 30 minutes.

[0022] This timeframe allows the crystallization reaction on the surface of natural apatite to occur fully.

[0023] Preferably, the settling time is 1 to 10 minutes.

[0024] The beneficial technical effects of the present invention are as follows:

[0025] This invention employs the principles of induced crystallization and sedimentation trapping to form a stable crystal structure of fluoride ions on the surface of apatite, thereby achieving fluoride ion removal. In the intermittent crystallization sedimentation tank, some fine flocs formed due to localized homogeneous nucleation are adsorbed and trapped by the apatite particles during the sedimentation process, resulting in clear effluent free of suspended solids. The final product is crystals with low water content, which can be directly reused. The process is simple to operate, significantly reducing the treatment cost of low-concentration fluoride wastewater and improving economic efficiency. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of an intermittent crystallization deep defluorination system for treating low-concentration fluoride-containing wastewater provided by the present invention; wherein, 1 is a water storage tank; 2 is an equalization tank; 3 is an intermittent crystallization sedimentation tank; 4 is a crystal discharge tank; 5 is an alkali dosing tank; 6 is a phosphorus dosing tank; and 7 is a calcium dosing tank. Detailed Implementation

[0027] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the present invention.

[0028] Furthermore, regarding the numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, are also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0029] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar to or equivalent to those described herein may be used in the implementation or testing of this invention.

[0030] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0031] A schematic diagram of the intermittent crystallization deep defluorination system for treating low-concentration fluoride wastewater provided by this invention is shown below. Figure 1 Among them, 1 is a water storage tank; 2 is a regulating tank; 3 is an intermittent crystallization sedimentation tank; 4 is a crystal discharge tank; 5 is an alkali dosing tank; 6 is a phosphorus dosing tank; and 7 is a calcium dosing tank.

[0032] The steps for treating low-concentration fluoride-containing wastewater using the intermittent crystallization depth defluorination system described above in the embodiments and comparative examples of the present invention are as follows:

[0033] Step S1: Low-concentration fluoride-containing wastewater (fluoride ion concentration ≤ 20 mg / L) in water storage tank 1 flows into equalization tank 2 until it reaches 2 / 3 of its volume, at which point the water inflow is stopped.

[0034] Step S2: Start the stirring device in the equalization tank 2. The stirring speed is 1000 rpm. Under the condition of continuous stirring, the alkaline solution in the alkali addition tank 5 and the phosphate solution in the phosphorus addition tank 6 are added to the equalization tank 2. The phosphorus concentration in the equalization tank is controlled to a specific value and the pH is eventually maintained at a specific value. After the pH stabilizes, mixed wastewater is obtained.

[0035] Step S3: The mixed wastewater flows into the intermittent crystallization sedimentation tank 3 and the stirring device in the intermittent crystallization sedimentation tank 3 is started. The stirring speed is 1000 rpm. The intermittent crystallization sedimentation tank 3 is filled with natural apatite particles with a particle size of 0.1 to 0.6 mm. The filling volume is 1 / 3 of the volume of the intermittent crystallization sedimentation tank 3. Under the condition of continuous stirring, the CaCl2 solution in the calcium dosing tank 7 is added to the intermittent crystallization sedimentation tank 3, and the initial calcium concentration is controlled at a specific value to carry out the crystallization reaction.

[0036] Step S4: Stop the stirring device of the intermittent crystallization sedimentation tank 3, and the particulate matter will settle under the action of gravity;

[0037] Step S5: Discharge of supernatant from intermittent crystallization sedimentation tank 3;

[0038] Step S6: When the volume of particulate matter in the intermittent crystallization sedimentation tank 3 exceeds 1 / 2 of the volume of the intermittent crystallization sedimentation tank 3, part of the particulate matter is discharged into the discharge tank 4 until the filling volume is restored to 1 / 3 of the volume of the intermittent crystallization sedimentation tank 3.

[0039] The parameters and treatment results of each embodiment and comparative example in the treatment of low-concentration fluoride-containing wastewater are as follows:

[0040] Example 1

[0041] The initial fluoride ion concentration of the low-concentration fluoride wastewater in reservoir 1 was 19 mg / L. The pH values ​​in regulating tank 2 were 10 and 11, respectively. The crystallization reaction lasted 30 min. The phosphate solution added to regulating tank 2 was a 2 mol / L NaH₂PO₄ solution, resulting in a phosphorus concentration of 0.006 mol / L after addition. The alkali solution in alkali addition tank 5 was a 1 mol / L NaOH solution. The calcium salt added to intermittent crystallization sedimentation tank 3 was a 2 mol / L CaCl₂ solution, resulting in an initial calcium concentration of 0.01 mol / L after addition. The crystallization reaction lasted 30 min, and the sedimentation time was 1 min.

[0042] Comparative Example 1

[0043] The initial fluoride ion concentration of the low-concentration fluoride wastewater in reservoir 1 was 19 mg / L. The pH in regulating tank 2 was set to 7, and the crystallization reaction lasted for 30 min. The phosphate solution added to regulating tank 2 was a 2 mol / L NaH₂PO₄ solution, resulting in a phosphorus concentration of 0.006 mol / L after addition. The alkali solution in alkali addition tank 5 was a 1 mol / L NaOH solution. The calcium salt added to intermittent crystallization sedimentation tank 3 was a 2 mol / L CaCl₂ solution, resulting in an initial calcium concentration of 0.01 mol / L after addition. The crystallization reaction lasted for 30 min, and the sedimentation time was 1 min.

[0044] The processing results of Example 1 and Comparative Example 1 are shown in Table 1.

[0045] Table 1

[0046] Example 1 10 19mg / L 0.67 mg / L 96.47％ Example 1 11 19mg / L 0.55 mg / L 97.11％ Comparative Example 1 7 19mg / L 4.30 mg / L 77.37％

[0047] Table 1 shows that when the pH in the equalization tank is within the required range, the residual fluoride concentration is below 1.5 mg / L. If it is outside this range, there is no deep fluoride removal effect. If the pH is greater than 10, the dosage will need to be increased significantly, but the treatment effect will not be significantly improved.

[0048] Example 2

[0049] The initial fluoride ion concentration of the low-concentration fluoride wastewater in reservoir 1 was 19 mg / L. The pH in regulating tank 2 was set to 10, and the crystallization reaction lasted for 30 min. The phosphate solution added to regulating tank 2 was a 2 mol / L NaH₂PO₄ solution, resulting in a phosphorus concentration of 0.006 mol / L after addition. The alkali solution in alkali addition tank 5 was a 1 mol / L NaOH solution. The calcium salt added to intermittent crystallization sedimentation tank 3 was a 2 mol / L CaCl₂ solution, resulting in initial calcium concentrations of 0.0075 mol / L, 0.01 mol / L, 0.0125 mol / L, 0.015 mol / L, and 0.0175 mol / L after addition. The crystallization reaction lasted for 30 min, and the sedimentation time was 1 min.

[0050] Comparative Example 2

[0051] The initial fluoride ion concentration of the low-concentration fluoride wastewater in reservoir 1 was 19 mg / L. The pH in regulating tank 2 was set to 10, and the crystallization reaction lasted for 30 min. A 2 mol / L NaH₂PO₄ phosphate solution was added to regulating tank 2, resulting in a phosphorus concentration of 0.006 mol / L after addition. The alkali solution in alkali addition tank 5 was a 1 mol / L NaOH solution. A 2 mol / L CaCl₂ calcium salt solution was added to intermittent crystallization sedimentation tank 3, resulting in an initial calcium concentration of 0.005 mol / L after addition. The crystallization reaction lasted for 30 min, and the sedimentation time was 1 min.

[0052] The processing results of Example 2 and Comparative Example 2 are shown in Table 2.

[0053] Table 2

[0054] Example 2 10 0.0075 mol / L 0.41 mg / L 97.84％ Example 2 10 0.01 mol / L 0.23 mg / L 98.77％ Example 2 10 0.0125 mol / L 0.14 mg / L 99.25％ Example 2 10 0.015 mol / L 0.11 mg / L 99.42％ Example 2 10 0.0175 mol / L 0.097 mg / L 99.49％ Comparative Example 2 10 0.005 mol / L 2.09 mg / L 88.97％

[0055] As can be seen from Table 2, within the required range, the residual fluoride concentration can be stably maintained below 1.5 mg / L. Below this range, the above effect is not achieved. If the calcium concentration is further increased, the treatment efficiency will not be significantly improved.

[0056] Example 3

[0057] The initial fluoride ion concentration of the low-concentration fluoride wastewater in reservoir 1 was 19 mg / L. The pH in regulating tank 2 was set to 10, and the crystallization reaction lasted for 30 min. The phosphate solution added to regulating tank 2 was a 2 mol / L NaH₂PO₄ solution. After addition, the phosphorus concentrations in the regulating tank were 0.003 mol / L, 0.0045 mol / L, 0.006 mol / L, 0.0075 mol / L, 0.009 mol / L, and 0.0105 mol / L, respectively. The alkali solution in alkali addition tank 5 was a 1 mol / L NaOH solution. The calcium salt added to intermittent crystallization sedimentation tank 3 was a 2 mol / L CaCl₂ solution. After addition, the initial calcium concentration in intermittent sedimentation tank 3 was 0.0075 mol / L. The crystallization reaction lasted for 30 min, and the sedimentation time was 1 min.

[0058] Comparative Example 3

[0059] The initial fluoride ion concentration of the low-concentration fluoride wastewater in reservoir 1 was 19 mg / L. The pH in regulating tank 2 was set to 10, and the crystallization reaction lasted for 30 min. A 2 mol / L NaH₂PO₄ phosphate solution was added to regulating tank 2, resulting in a phosphorus concentration of 0.0015 mol / L after addition. The alkali solution in alkali addition tank 5 was a 1 mol / L NaOH solution. A 2 mol / L CaCl₂ calcium salt solution was added to intermittent crystallization sedimentation tank 3, resulting in an initial calcium concentration of 0.0075 mol / L after addition. The crystallization reaction lasted for 30 min, and the sedimentation time was 1 min.

[0060] The processing results of Example 3 and Comparative Example 3 are shown in Table 3.

[0061] Table 3

[0062] Example 3 10 0.003 mol / L 0.74 mg / L 96.10％ Example 3 10 0.0045 mol / L 0.99 mg / L 94.76％ Example 3 10 0.006 mol / L 0.64 mg / L 96.62％ Example 3 10 0.0075 mol / L 0.41 mg / L 97.82％ Example 3 10 0.009 mol / L 0.43 mg / L 97.71％ Example 3 10 0.0105 mol / L 0.39 mg / L 97.95％ Comparative Example 3 10 0.0015 mol / L 1.62 mg / L 91.47％

[0063] As can be seen from Table 3, when the phosphorus concentration is within the required range, the residual fluoride concentration is below 1.5 mg / L. If it is below this range, the fluoride concentration is too high. If it is above this range, the fluoride removal rate will not increase significantly, but the phosphorus concentration in the effluent will increase.

[0064] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for treating low-concentration fluoride-containing wastewater, characterized in that, The system uses an intermittent crystallization deep defluorination system to treat low-concentration fluoride-containing wastewater. The structure of the intermittent crystallization deep defluorination system includes: a water storage tank, an equalization tank, an intermittent crystallization sedimentation tank, and a crystal discharge tank connected in sequence. The equalization tank is also connected to an alkali dosing tank and a phosphorus dosing tank. The intermittent crystallization sedimentation tank is also connected to a calcium dosing tank. Both the regulating tank and the intermittent crystallization sedimentation tank are equipped with a stirrer; The intermittent crystallization sedimentation tank is filled with apatite particles. The steps of the method for treating low-concentration fluoride-containing wastewater include: The low-concentration fluoride-containing wastewater from the storage tank is introduced into the equalization tank. The stirring device in the equalization tank is activated, and under continuous stirring, the alkaline solution from the alkali dosing tank and the phosphate solution from the phosphorus dosing tank are added to the equalization tank, ensuring that the phosphorus concentration in the equalization tank is not less than 0.003 mol / L and the pH value is not less than 10, resulting in mixed wastewater. This mixed wastewater is then introduced into the intermittent crystallization sedimentation tank. The stirring device in the intermittent crystallization sedimentation tank is activated, and under continuous stirring, the calcium salt solution from the calcium dosing tank is added to the intermittent crystallization sedimentation tank, ensuring that the initial calcium concentration in the intermittent crystallization sedimentation tank is not less than 0.0075 mol / L, initiating a crystallization reaction. The mixture is then allowed to settle, and the supernatant is the treated wastewater. When the volume of the precipitate exceeds half the volume of the intermittent crystallization sedimentation tank, a portion of the precipitate is discharged to the crystal discharge tank. The fluoride ion concentration in the low-concentration fluoride wastewater is ≤20 mg / L; The apatite particles have a particle size of 0.1~0.6mm and a filling volume of 1 / 3 of the volume of the intermittent crystallization sedimentation tank.

2. The method for treating low-concentration fluoride-containing wastewater according to claim 1, characterized in that, The phosphorus concentration in the regulating tank is 0.003~0.009 mol / L.

3. The method for treating low-concentration fluoride-containing wastewater according to claim 1, characterized in that, The initial calcium concentration in the intermittent crystallization sedimentation tank is 0.0075~0.015 mol / L.

4. The method for treating low-concentration fluoride-containing wastewater according to claim 1, characterized in that, The stirring speed of the stirring device in the regulating tank is 500~1200 rpm; the stirring speed of the stirring device in the intermittent crystallization sedimentation tank is 500~1200 rpm.

5. The method for treating low-concentration fluoride-containing wastewater according to claim 1, characterized in that, The alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution.

6. The method for treating low-concentration fluoride-containing wastewater according to claim 1, characterized in that, The calcium salt solution is a calcium chloride solution.

7. The method for treating low-concentration fluoride-containing wastewater according to claim 1, characterized in that, The crystallization reaction takes 10 to 30 minutes.

8. The method for treating low-concentration fluoride-containing wastewater according to claim 1, characterized in that, The settling time is 1 to 10 minutes.

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