Efficient defluorination process for sedimentation tank

Abstract

The invention provides an efficient defluorination process for a sedimentation tank, and belongs to the technical field of wastewater defluorination, and the efficient defluorination process specifically comprises the following steps: introducing fluorine-containing wastewater into a premixing tank, adding an alkaline calcium compound to adjust the pH value, then adding cationic polyacrylamide, and carrying out a flocculation reaction to obtain a flocculation liquid after the reaction is completed; and enabling the flocculated liquid to flow into a sedimentation tank, adding hydrochloric acid to adjust the pH value, carrying out static settlement, then adding a composite fluorine removal agent, carrying out static settlement, carrying out solid-liquid separation in a sedimentation area, carrying out regular cleaning on settled sludge, and enabling effluent of supernate to enter a clean water tank, so as to obtain a purified water body. The defluorination process for the sedimentation tank has an excellent defluorination effect and meets the deep defluorination requirement that the concentration of effluent is lower than 1.0 mg / L. Meanwhile, the treated settled sludge has good sedimentation performance, the occurrence of sludge runout is reduced, and the treatment amount and treatment cost of the sludge are reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of fluoride removal from wastewater, and particularly relates to an efficient fluoride removal process for sedimentation tanks. Background Art

[0002] Fluoride-containing wastewater is a common type of wastewater in industrial production, characterized by large water volume and strong biological toxicity. With increasingly strict environmental protection requirements, the reduction treatment and resource utilization of fluoride-containing wastewater are currently hotspots and difficulties in research.

[0003] Commonly used methods for removing fluoride from fluoride-containing wastewater currently include chemical precipitation and coagulation precipitation. The chemical precipitation method generally uses lime water as a precipitant to separate and remove fluoride ions from the wastewater by generating CaF2 precipitation. This method has the advantages of large treatment water volume and low cost, but the fluoride ion treatment capacity is limited, and the fluoride concentration can only be reduced to 10 - 20 mg / L, making it difficult to meet the effluent fluoride concentration required by relevant national standards. The coagulation precipitation method is to add a coagulant to the fluoride-containing wastewater to form flocculent colloids. The colloids combine with fluoride ions in the water and finally form precipitates to be discharged, which can reduce the effluent F - concentration to below 1.5 mg / L. However, in the prior art, after multi-stage treatment of the coagulant, the generated flocs are loose and have poor sedimentation performance. During the discharge of the effluent, a large number of fine sludge particles are generated due to the impact and fluctuation of the sediment sludge, and the phenomenon of sludge running is likely to occur, increasing the difficulty of sludge treatment and affecting work efficiency.

[0004] In summary, developing a sedimentation tank fluoride removal process with good fluoride removal effect, simple process flow, and continuous and stable operation is of great practical significance for the efficient treatment of fluoride-containing wastewater and the sustainable development of industrial production. Summary of the Invention

[0005] To solve the problems in the background art, the present invention provides an efficient fluoride removal process for sedimentation tanks.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] An efficient fluoride removal process for sedimentation tanks specifically includes the following steps:

[0008] A1. Flow the fluoride-containing wastewater into a premixing tank, add an alkaline calcium compound to adjust the pH to 9 - 10, let it stand for 5 - 10 min, and then add 20 - 30 mg / L of cationic polyacrylamide and carry out a flocculation reaction for 20 - 40 min. After the reaction is completed, a flocculated liquid is obtained;

[0009] A2. Let the flocculant solution flow into the sedimentation tank, add hydrochloric acid to adjust the pH to 6.5 - 7.5, let it stand and precipitate for 30 - 60 min, then add a composite defluorinating agent at a dosage of 40 - 60 mg / L, stir for 1 - 2 h, adjust the pH to 7 - 8, let it stand for 30 - 60 min, conduct solid-liquid separation in the sedimentation area, regularly clean the sediment sludge, and the supernatant water enters the clear water tank to obtain the purified water body.

[0010] Further, the composite defluorinating agent described in step A2 is prepared from modified polyaluminum chloride and calcium oxide-doped hydroxyapatite.

[0011] Further, the preparation of the composite defluorinating agent specifically includes the following steps:

[0012] Dissolve the modified polyaluminum chloride in water, stir evenly, adjust the pH value to 8 - 9 with a 0.5 - 1 mol / L sodium hydroxide solution to obtain a mixed solution, where the mass concentration of polyaluminum chloride is 20 - 30%, heat the mixed solution to 70 - 80 °C, and under stirring conditions, add calcium oxide-doped hydroxyapatite at a dosage of 40 - 60 g / L based on the volume of the mixed solution, stir for 2 - 3 h, let it stand for 6 - 10 h, evaporate and reflux, dry, grind, and pass through a 100 - 120 mesh sieve to obtain the composite defluorinating agent.

[0013] Further, the preparation of the modified polyaluminum chloride includes the following steps:

[0014] S1. Pour 1 kg of polyaluminum chloride with a particle size of 200 mesh into a reactor equipped with a reflux device, then add 5 - 6 kg of an ethanol / aqueous solution with a mass concentration of 40 - 60%, and start stirring.

[0015] S2. Place 5 - 10 g of carboxylated multi-walled carbon nanotubes with an average tube diameter of 2 - 10 nm and a length of 50 - 300 nm in a beaker, then add 1 kg of an ethanol / aqueous solution with a volume ratio of 1:1, and ultrasonically disperse for 10 - 20 min to obtain a carbon nanotube suspension.

[0016] S3. Pour the nanotube dispersion suspension in step S2 into the reactor in step S1, heat and reflux under stirring conditions, distill out the ethanol / aqueous solution, and dry to obtain the modified polyaluminum chloride.

[0017] Further, the preparation of the calcium oxide-doped hydroxyapatite includes the following steps:

[0018] Calcium oxide with a particle size of 200 - 325 mesh is added to a 0.1 mol / L hydroxyapatite dispersion. The mass ratio of calcium oxide to hydroxyapatite is 1:4 - 6. The pH value of the dispersion is adjusted to 8 - 9 by dropping ammonia water, and the mixture is stirred for 30 - 60 min, then left standing, and precipitated for 4 - 10 h. The precipitate is filtered, dried, ground, and passed through a 140 - 200 mesh sieve, and calcined at 550 - 650 °C for 3 - 5 h to obtain calcium oxide - doped hydroxyapatite.

[0019] Further, the basic calcium compound in step A1 is one or a mixture of two of calcium oxide and calcium hydroxide.

[0020] Further, the cationic polyacrylamide in step A1 has a molecular weight of 0.5×10 7 - 2×10 7 .

[0021] This application has the following beneficial effects:

[0022] 1. During the treatment of fluoride - containing wastewater, adding an appropriate amount of composite defluorination agent and, under suitable pH conditions, making the defluorination agent fully react with fluoride ions can improve the defluorination effect and maintain stability, meeting the deep defluorination requirement of the effluent being lower than 1 mg / L. At the same time, the composite defluorination agent enhances the flocculation and stability during the fluoride removal process of the wastewater, making the sludge have a lower water content, improving the sedimentation performance and dewatering performance of the sludge, effectively reducing the occurrence of sludge running, making the sludge easier to concentrate and dehydrate, reducing the sludge treatment volume and disposal cost.

[0023] 2. In the preparation of the composite defluorination agent, modifying polyaluminum chloride with carboxylated multi - walled carbon nanotubes. Since the polymer powder has a certain adsorption capacity for the nanotubes and this adsorption capacity is greater than the solvation capacity, it can improve the dispersion of the carbon nanotubes, thereby enhancing the suspension capacity of the defluorination agent. The carboxylated multi - walled carbon nanotubes have a large aspect ratio and a large number of carboxyl groups. When approaching calcium oxide - doped hydroxyapatite, a strong interaction will occur, and the two - phase synergistically enhances the flocculation and sedimentation effect of the defluorination agent and enhances the sedimentation performance of the sludge.

[0024] 3. The doping of calcium oxide into hydroxyapatite can effectively capture fluoride ions in water, obtaining many small calcium fluoride crystal nuclei, enabling the defluorination agent to quickly aggregate into blocks after adsorbing the surrounding calcium fluoride, forming a large - volume and high - density solid precipitate, making the structure of the sludge more compact, reducing the voids and porosity in the sludge, thereby maintaining a low SV30 value, making it not easy to resuspend or float during the sedimentation process, and enhancing the sedimentation and stability of the sludge. Specific embodiments

[0025] The following further elaborates on this application with reference to embodiments.

[0026] Unless otherwise specified, the raw materials used in the examples and comparative examples of this application are all commercially available.

[0027] Example 1

[0028] An efficient defluorination process for sedimentation tanks specifically includes the following steps:

[0029] A1. Flow the fluorine-containing wastewater into the premixing tank, add alkaline calcium compounds to adjust the pH to 9, let it stand for 5 minutes, then add 25 mg / L of cationic polyacrylamide and carry out a flocculation reaction for 30 minutes. After the reaction is completed, obtain the flocculated liquid.

[0030] A2. Flow the flocculated liquid into the sedimentation tank, add hydrochloric acid to adjust the pH to 6.5 - 7.5, let it stand and precipitate for 60 minutes, then add 50 mg / L of the composite defluorination agent, stir for 1 hour, adjust the pH to 7 - 8, let it stand for 60 minutes, carry out solid-liquid separation in the sedimentation area, regularly clean the sediment sludge, and the supernatant water enters the clear water tank to obtain the purified water body.

[0031] The composite defluorination agent described in step A2 is prepared from modified polyaluminum chloride and calcium oxide-doped hydroxyapatite, and specifically includes the following content:

[0032] Dissolve the modified polyaluminum chloride in water, stir evenly, adjust the pH value to 8 - 9 with 1 mol / L sodium hydroxide solution to obtain a mixed solution, where the mass concentration of polyaluminum chloride is 25%. Heat the mixed solution to 75°C, and under stirring conditions, add calcium oxide-doped hydroxyapatite at 50 g / L of the volume of the mixed solution, stir for 2.5 hours, let it stand for 8 hours, evaporate and reflux, dry, grind, and pass through a 100-mesh sieve to obtain the composite defluorination agent.

[0033] The preparation of the modified polyaluminum chloride includes the following content:

[0034] S1. Pour 1 kg of polyaluminum chloride with a particle size of 200 mesh into a reactor equipped with a reflux device, then add 5 kg of an ethanol / water solution with a mass concentration of 50%, and start stirring.

[0035] S2. Place 8 g of carboxylated multi-walled carbon nanotubes with an average tube diameter of 2 - 10 nm and a length of 50 - 300 nm in a beaker, then add 1 kg of an ethanol / water solution with a volume ratio of 1:1, and ultrasonically disperse for 15 minutes to obtain a carbon nanotube suspension.

[0036] S3. Pour the nanotube dispersion suspension in step S2 into the reactor in step S1, heat and reflux under stirring conditions, distill out the ethanol / water solution, and dry to obtain the modified polyaluminum chloride.

[0037] The preparation of the calcium oxide-doped hydroxyapatite includes the following content:

[0038] Calcium oxide with a particle size of 270 mesh is added to a 0.1 mol / L hydroxyapatite dispersion. The mass ratio of calcium oxide to hydroxyapatite is 1:5. The pH value of the dispersion is adjusted to 8 - 9 by dropping ammonia water, and the mixture is stirred for 40 min, then left standing, and precipitated for 6 h. The precipitate is filtered, dried, ground, passed through a 200 - mesh sieve, and calcined at 600 °C for 4 h to obtain calcium - oxide - doped hydroxyapatite;

[0039] The basic calcium compound described in step A1 is calcium oxide.

[0040] The molecular weight of the cationic polyacrylamide described in step A1 is 0.5×10 7 -2×10 7 。

[0041] Example 2

[0042] An efficient defluorination process for sedimentation tanks, specifically including the following steps:

[0043] A1. The fluorine - containing wastewater is passed into a premixing tank, and a basic calcium compound is added to adjust the pH to 9 - 10. After standing for 10 min, 25 mg / L of cationic polyacrylamide is added, and a flocculation reaction is carried out for 20 min. After the reaction is completed, a flocculated liquid is obtained;

[0044] A2. The flocculated liquid is flowed into a sedimentation tank, and hydrochloric acid is added to adjust the pH to 6.5 - 7.5. After standing and precipitating for 60 min, 40 mg / L of a composite defluorination agent is added, and it is stirred for 1 h. Then the pH is adjusted to 7 - 8, and after standing for 60 min, solid - liquid separation is carried out in the sedimentation area. The sediment sludge is regularly cleaned, and the supernatant water enters the clear water tank to obtain a purified water body.

[0045] The composite defluorination agent described in step A2 is prepared from modified polyaluminum chloride and calcium - oxide - doped hydroxyapatite, specifically including the following content:

[0046] The modified polyaluminum chloride is dissolved in water and stirred evenly. The pH value is adjusted to 8 - 9 with a 0.5 mol / L sodium hydroxide solution to obtain a mixed solution, where the mass concentration of polyaluminum chloride is 25%. The mixed solution is heated to 70 °C, and under stirring conditions, calcium - oxide - doped hydroxyapatite is added at 40 g / L of the volume of the mixed solution, stirred for 3 h, left standing for 8 h, evaporated and refluxed, dried, ground, and passed through a 120 - mesh sieve to obtain the composite defluorination agent.

[0047] The preparation of the modified polyaluminum chloride includes the following content:

[0048] S1. 1 kg of polyaluminum chloride with a particle size of 200 mesh is poured into a reactor equipped with a reflux device, and then 6 kg of an ethanol / water solution with a mass concentration of 60% is added, and stirring is started;

[0049] S2. Place 5 g of carboxylated multi-walled carbon nanotubes with an average tube diameter of 2 - 10 nm and a length of 50 - 300 nm in a beaker, then add 1 kg of an ethanol / water solution with a volume ratio of 1:1, and ultrasonically disperse for 20 min to obtain a carbon nanotube suspension;

[0050] S3. Pour the nanotube dispersion suspension in step S2 into the reactor in step S1, heat under reflux with stirring, distill out the ethanol / water solution, and dry to obtain modified polyaluminum chloride.

[0051] The preparation of the calcium oxide-doped hydroxyapatite includes the following:

[0052] Add calcium oxide with a particle size of 325 mesh to a 0.1 mol / L hydroxyapatite dispersion. The mass ratio of calcium oxide to hydroxyapatite is 1:5. Adjust the pH value of the dispersion to 8 - 9 by dropwise adding ammonia water, mix and stir for 60 min, let it stand, precipitate for 8 h, filter, dry, and grind the precipitate, pass through a 200-mesh sieve, and calcine at 550 °C for 5 h to obtain calcium oxide-doped hydroxyapatite.

[0053] The basic calcium compound in step A1 is a mixture of calcium oxide and calcium hydroxide with a mass ratio of 1:1.

[0054] The cationic polyacrylamide in step A1 has a molecular weight of 0.5×10 7 -2×10 7 .

[0055] Example 3

[0056] An efficient defluorination process for sedimentation tanks specifically includes the following steps:

[0057] A1. Flow the fluorine-containing wastewater into the premixing tank, add a basic calcium compound to adjust the pH to 9 - 10, let it stand for 5 min, then add 20 mg / L of cationic polyacrylamide and carry out a flocculation reaction for 20 min. After the reaction is completed, obtain a flocculated liquid;

[0058] A2. Flow the flocculated liquid into the sedimentation tank, add hydrochloric acid to adjust the pH to 6.5 - 7.5, let it stand and precipitate for 30 min, then add 40 mg / L of the composite defluorination agent, stir for 1 h, adjust the pH to 7 - 8, let it stand for 30 min, carry out solid-liquid separation in the sedimentation area, regularly clean the sediment sludge, and the supernatant water enters the clear water tank to obtain a purified water body.

[0059] The composite defluorination agent in step A2 is prepared from modified polyaluminum chloride and calcium oxide-doped hydroxyapatite, and specifically includes the following:

[0060] Dissolve the modified polyaluminum chloride in water, stir evenly, adjust the pH value to 8 - 9 with 0.5mol / L sodium hydroxide solution to obtain a mixed solution, where the mass concentration of polyaluminum chloride is 20%. Heat the mixed solution to 70°C, and under stirring conditions, add calcium oxide-doped hydroxyapatite at 40g / L based on the volume of the mixed solution, stir for 2h, let it stand for 6h, evaporate and reflux, dry, grind, and pass through a 100-mesh sieve to obtain the composite defluorination agent.

[0061] The preparation of the modified polyaluminum chloride includes the following steps:

[0062] S1. Pour 1kg of polyaluminum chloride with a particle size of 200 mesh into a reactor equipped with a reflux device, then add 5kg of ethanol / aqueous solution with a mass concentration of 40%, and start stirring.

[0063] S2. Place 5g of carboxylated multi-walled carbon nanotubes with an average tube diameter of 2 - 10nm and a length of 50 - 300nm in a beaker, then add them to 1kg of ethanol / aqueous solution with a volume ratio of 1:1, and ultrasonically disperse for 10min to obtain a carbon nanotube suspension.

[0064] S3. Pour the nanotube dispersion suspension in step S2 into the reactor in step S1, heat and reflux under stirring conditions, distill out the ethanol / aqueous solution, and dry to obtain the modified polyaluminum chloride.

[0065] The preparation of the calcium oxide-doped hydroxyapatite includes the following steps:

[0066] Add calcium oxide with a particle size of 200 mesh to a 0.1mol / L hydroxyapatite dispersion. The mass ratio of calcium oxide to hydroxyapatite is 1:4. Adjust the pH value of the dispersion to 8 - 9 by dropping ammonia water, mix and stir for 30 - 60min, let it stand, and precipitate for 4h. Filter, dry, and grind the precipitate, pass through a 140-mesh sieve, and calcine at 550°C for 5h to obtain calcium oxide-doped hydroxyapatite.

[0067] The basic calcium compound in step A1 is calcium hydroxide.

[0068] The molecular weight of the cationic polyacrylamide in step A1 is 0.5×10 7 -2×10 7 .

[0069] Example 4

[0070] An efficient defluorination process for sedimentation tanks specifically includes the following steps:

[0071] A1. Flow the fluorine-containing wastewater into a premixing tank, add alkaline calcium compounds to adjust the pH to 9 - 10, let it stand for 10 min, then add 30 mg / L of cationic polyacrylamide and carry out a flocculation reaction for 40 min. After the reaction is completed, obtain a flocculated liquid.

[0072] A2. Flow the flocculated liquid into a sedimentation tank, add hydrochloric acid to adjust the pH to 6.5 - 7.5, let it stand and precipitate for 60 min, then add 60 mg / L of a composite defluorination agent, stir for 2 h, adjust the pH to 7 - 8, let it stand for 60 min, and carry out solid-liquid separation in the sedimentation area. Regularly clean the sediment sludge, and the supernatant effluent enters the clear water tank to obtain a purified water body.

[0073] The composite defluorination agent described in step A2 is prepared from modified polyaluminum chloride and calcium oxide-doped hydroxyapatite, and specifically includes the following content:

[0074] Dissolve the modified polyaluminum chloride in water, stir evenly, adjust the pH value to 8 - 9 with 1 mol / L sodium hydroxide solution to obtain a mixed solution, where the mass concentration of polyaluminum chloride is 30%. Heat the mixed solution to 80 °C, and under stirring conditions, add calcium oxide-doped hydroxyapatite at 60 g / L of the volume of the mixed solution, stir for 3 h, let it stand for 10 h, evaporate and reflux, dry, grind, and pass through a 120-mesh sieve to obtain the composite defluorination agent.

[0075] The preparation of the modified polyaluminum chloride includes the following content:

[0076] S1. Pour 1 kg of polyaluminum chloride with a particle size of 200 mesh into a reactor equipped with a reflux device, then add 6 kg of an ethanol / aqueous solution with a mass concentration of 60%, and start stirring.

[0077] S2. Place 10 g of carboxylated multi-walled carbon nanotubes with an average tube diameter of 2 - 10 nm and a length of 50 - 300 nm in a beaker, then add 1 kg of an ethanol / aqueous solution with a volume ratio of 1:1, and ultrasonically disperse for 20 min to obtain a carbon nanotube suspension.

[0078] S3. Pour the nanotube dispersion suspension in step S2 into the reactor in step S1, heat and reflux under stirring conditions, distill out the ethanol / aqueous solution, and dry to obtain the modified polyaluminum chloride.

[0079] The preparation of the calcium oxide-doped hydroxyapatite includes the following content:

[0080] Calcium oxide with a particle size of 325 mesh is added to a 0.1 mol / L hydroxyapatite dispersion. The mass ratio of calcium oxide to hydroxyapatite is 1:6. The pH value of the dispersion is adjusted to 8 - 9 by dropping ammonia water, and the mixture is stirred for 60 min, then left standing. After precipitation for 10 h, the precipitate is filtered, dried, ground, passed through a 200 - mesh sieve, and calcined at 650 °C for 3 h to obtain calcium - oxide - doped hydroxyapatite.

[0081] The basic calcium compound described in step A1 is calcium oxide.

[0082] The molecular weight of the cationic polyacrylamide described in step A1 is 0.5×10 7 -2×10 7 。

[0083] Example 5

[0084] An efficient defluorination process for sedimentation tanks specifically includes the following steps:

[0085] A1. The fluorine - containing wastewater is introduced into a premixing tank, and a basic calcium compound is added to adjust the pH to 9 - 10. After standing for 8 min, 25 mg / L of cationic polyacrylamide is added, and a flocculation reaction is carried out for 40 min. After the reaction is completed, a flocculated liquid is obtained.

[0086] A2. The flocculated liquid flows into a sedimentation tank, and hydrochloric acid is added to adjust the pH to 6.5 - 7.5. After standing and precipitating for 30 min, 45 mg / L of a composite defluorination agent is added, and it is stirred for 1.5 h. Then the pH is adjusted to 7 - 8, and after standing for 45 min, solid - liquid separation is carried out in the sedimentation area. The sediment sludge is regularly cleaned, and the supernatant water enters the clear water tank to obtain a purified water body.

[0087] The composite defluorination agent described in step A2 is prepared from modified polyaluminum chloride and calcium - oxide - doped hydroxyapatite, and specifically includes the following content:

[0088] The modified polyaluminum chloride is dissolved in water and stirred evenly. The pH value is adjusted to 8 - 9 with a 0.8 mol / L sodium hydroxide solution to obtain a mixed solution, where the mass concentration of polyaluminum chloride is 27%. The mixed solution is heated to 75 °C, and under stirring conditions, calcium - oxide - doped hydroxyapatite is added at 50 g / L of the volume of the mixed solution, stirred for 2.5 h, left standing for 8 h, evaporated and refluxed, dried, ground, and passed through a 120 - mesh sieve to obtain the composite defluorination agent.

[0089] The preparation of the modified polyaluminum chloride includes the following content:

[0090] S1. 1 kg of polyaluminum chloride with a particle size of 200 mesh is poured into a reactor equipped with a reflux device, and then 5.5 kg of an ethanol / water solution with a mass concentration of 55% is added, and stirring is started.

[0091] S2. Place 8 g of carboxylated multi-walled carbon nanotubes with an average tube diameter of 2 - 10 nm and a length of 50 - 300 nm in a beaker, then add 1 kg of an ethanol / water solution with a volume ratio of 1:1, and ultrasonically disperse for 15 min to obtain a carbon nanotube suspension;

[0092] S3. Pour the nanotube dispersion suspension in step S2 into the reactor in step S1, heat under reflux with stirring, distill out the ethanol / water solution, and dry to obtain modified polyaluminum chloride.

[0093] The preparation of the calcium oxide-doped hydroxyapatite includes the following:

[0094] Add calcium oxide with a particle size of 200 mesh to a 0.1 mol / L hydroxyapatite dispersion. The mass ratio of calcium oxide to hydroxyapatite is 1:5. Adjust the pH value of the dispersion to 8 - 9 by dropwise addition of ammonia water, mix and stir for 45 min, let stand, and precipitate for 6 h. Filter, dry, and grind the precipitate, pass through a 140-mesh sieve, and calcine at 600 °C for 4 h to obtain calcium oxide-doped hydroxyapatite.

[0095] The basic calcium compound in step A1 is calcium hydroxide.

[0096] The molecular weight of the cationic polyacrylamide in step A1 is 0.5×10 7 -2×10 7 。

[0097] Comparative Example 1

[0098] The difference between this comparative example and Example 1 is only that in the preparation of the composite defluorination agent, the modified polyaluminum chloride is replaced by polyaluminum chloride.

[0099] Comparative Example 2

[0100] The difference between this comparative example and Example 1 is only that in the preparation of the composite defluorination agent, the calcium oxide-doped hydroxyapatite is replaced by hydroxyapatite, which specifically includes the following: For a 0.1 mol / L hydroxyapatite dispersion, adjust the pH value of the dispersion to 8 - 9 by dropwise addition of ammonia water, mix and stir for 40 min, let stand, and precipitate for 6 h. Filter, dry, and grind the precipitate, pass through a 200-mesh sieve, and calcine at 600 °C for 4 h to obtain calcium oxide-doped hydroxyapatite.

[0101] Comparative Example 3

[0102] The difference between this comparative example and Example 1 is only that in the preparation of the composite defluorination agent, the modified polyaluminum chloride is replaced by polyaluminum chloride, and at the same time, the calcium oxide-doped hydroxyapatite is replaced by hydroxyapatite.

[0103] Effect Verification

[0104] 1. Fluoride ion content: In the examples and comparative examples, a certain amount of purified water was taken respectively to detect the fluoride ion content in the water;

[0105] 2. Sedimentation performance: In the examples and comparative examples, a certain amount of sludge mixture was placed in a 100 mL graduated cylinder. After standing for 30 minutes, the scale value at the junction of the precipitated sludge and the supernatant was observed and recorded. This value is the volume of the precipitated sludge, and the percentage of it to the volume of the original mixture is the sludge sedimentation ratio, i.e., SV30.

[0106] Table 1

[0107] Fluoride ion content (mg / L) SV30 (%) Example 1 0.78 23 Example 2 0.86 25 Example 3 0.84 24 Example 4 0.83 23 Example 5 0.81 24 Comparative Example 1 1.16 28 Comparative Example 2 1.29 37 Comparative Example 3 1.68 41

[0108] Result analysis

[0109] By analyzing Examples 1-5 and Comparative Examples 1-3 and combining with Table 1, it can be seen that an efficient defluorination process for sedimentation tanks provided by the present invention has excellent defluorination effect, meets the requirement of deep defluorination with the effluent being lower than 1.0 mg / L, and at the same time, the treated precipitated sludge has good sedimentation performance. The specific analysis is as follows:

[0110] Compared with Example 1, in the preparation of the composite defluorinating agent in Comparative Example 1, the modified polyaluminum chloride was replaced by polyaluminum chloride, and both the fluoride ion content and SV30 increased, indicating that the modification of polyaluminum chloride can improve the defluorination effect of the water body and the sedimentation performance of the sludge;

[0111] Compared with Example 1, in the preparation of the composite defluorinating agent in Comparative Example 2, the calcium oxide-doped hydroxyapatite was replaced by hydroxyapatite, and both the fluoride ion content and SV30 increased, indicating that the doping of calcium oxide into hydroxyapatite can improve the defluorination effect of the water body and the sedimentation performance of the sludge;

[0112] Compared with Example 1, in the preparation of the composite defluorinating agent in Comparative Example 3, the modified polyaluminum chloride was replaced by polyaluminum chloride, and at the same time, the calcium oxide-doped hydroxyapatite was replaced by hydroxyapatite. The fluoride ion content increased, and the sedimentation performance of the sludge was greatly improved, indicating that the modified polyaluminum chloride and the calcium oxide-doped hydroxyapatite can produce synergistic effects while improving the defluorination effect, and significantly improve the sedimentation performance of the sludge.

[0113] In addition, it should be noted that, among the various specific technical features described in the above specific embodiments, they can be combined in any suitable way without contradiction. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0114] In addition, any combination can be made among various different embodiments of the present invention as long as it does not violate the idea of the present invention, and it should also be regarded as the content disclosed by the present invention.

Claims

1. An efficient defluorination process for a sedimentation tank, characterized in that: The high-efficiency defluorination process comprises the following steps: A1. The fluoride-containing wastewater is passed into the premixing tank, and alkaline calcification is added to adjust the pH to 9-10. The mixture is allowed to stand for 5-10 minutes, and then 20-30 mg / L cationic polyacrylamide is added to carry out flocculation reaction for 20-40 minutes. After the reaction is completed, a flocculated liquid is obtained. A2. The flocculated liquid flows into the sedimentation tank, and hydrochloric acid is added to adjust the pH to 6.5-7.5, and the solution is allowed to settle for 30-60 minutes. Then 40-60 mg / L of composite defluoridant is added, stirred for 1-2 hours, and the pH is adjusted to 7-8, and the solution is allowed to settle for 30-60 minutes. Solid-liquid separation is performed in the sedimentation area, and the precipitated sludge is cleaned regularly. The supernatant water enters the clean water tank to obtain purified water.

2. The high-efficiency defluorination process for a sedimentation tank according to claim 1, characterized in that: The composite defluoridating agent in step A2 is prepared by doping hydroxyapatite with modified polyaluminium chloride and calcium oxide.

3. The high-efficiency defluorination process for a sedimentation tank according to claim 1, characterized in that: The preparation of the composite defluorinating agent described in step A2 specifically includes the following contents: dissolving the modified polyaluminium chloride in water, stirring evenly, adjusting the pH value to 8-9 with 0.5-1 mol / L sodium hydroxide solution to obtain a mixed solution, wherein the mass concentration of polyaluminium chloride is 20-30%, heating the mixed solution to 70-80°C, adding calcium oxide-doped hydroxyapatite according to the volume of the mixed solution of 40-60 g / L under stirring conditions, stirring for 2-3 hours, standing for 6-10 hours, evaporating and refluxing, drying, grinding, and passing through a 100-120 mesh sieve to obtain a composite defluorinating agent.

4. A high-efficiency defluorination process for a sedimentation tank according to claim 2 or 3, characterized in that: The preparation of the modified polyaluminium chloride comprises the following contents: S1, pour 1 kg of polyaluminium chloride with a particle size of 200 mesh into a reactor equipped with a reflux device, then add 5-6 kg of ethanol / water solution with a mass concentration of 40-60%, and start stirring; S2, placing 5-10 g of carboxylated multi-walled carbon nanotubes with an average diameter of 2-10 nm and a length of 50-300 nm in a beaker, then adding them to 1 kg of ethanol / water solution with a volume ratio of 1:1, and ultrasonically dispersing for 10-20 min to obtain a carbon nanotube suspension; S3, pouring the nanotube dispersion suspension in step S2 into the reactor in step S1, heating and refluxing under stirring, distilling out the ethanol / water solution, and drying to obtain modified polyaluminium chloride.

5. The high-efficiency defluorination process for a sedimentation tank according to claim 2 or 3, characterized in that: The preparation of calcium oxide doped hydroxyapatite comprises the following steps: adding calcium oxide with a particle size of 200-325 meshes into a 0.1 mol / L hydroxyapatite dispersion, wherein the mass ratio of calcium oxide to hydroxyapatite is 1:4-6, adding ammonia water dropwise to adjust the pH value of the dispersion to 8-9, mixing and stirring for 30-60 minutes, standing, and settling for 4-10 hours, filtering, drying, and grinding the precipitate, passing through a 140-200 mesh sieve, and calcining at 550-650° C. for 3-5 hours to obtain calcium oxide doped hydroxyapatite.

6. The high-efficiency defluorination process for a sedimentation tank according to claim 1, characterized in that: The alkaline calcification material in step A1 is calcium oxide, calcium hydroxide or a mixture of the two.

7. The high-efficiency defluorination process for a sedimentation tank according to claim 1, characterized in that: The cationic polyacrylamide in step A1 has a molecular weight of 0.5×10 7 -2×10 7 .