A method for treating fluoride-containing wastewater and recovering calcium fluoride

Abstract

This invention discloses a method for treating fluoride-containing wastewater and recovering calcium fluoride, comprising the following steps: S1. Adding Ca(OH)2 clarifying solution to the fluoride-containing wastewater and then filtering to obtain a first filter residue and a first filtrate; washing and drying the first filter residue to obtain calcium fluoride; S2. Adding biochar adsorbent to the first filtrate for adsorption treatment, and then filtering to obtain a second filter residue and a second filtrate; discharging the second filtrate and reusing the second filter residue for adsorption treatment again; the method requires a small amount of precipitant, has a good fluoride removal effect, and can efficiently utilize solid waste.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a method for treating fluoride-containing wastewater and recovering calcium fluoride. Background Technology

[0002] High-purity silica sand (SiO2 content greater than 99.99%) is widely used in strategic emerging industries such as information technology, new energy, and high-end manufacturing. During the preparation of high-purity silica sand, hydrofluoric acid is often used for acid leaching to remove impurities, resulting in the generation of large amounts of fluoride-containing wastewater. Fluorine is an essential trace element for the human body, but excessive intake can cause fluorosis and a series of other health problems. Therefore, controlling the fluoride content in the environment is crucial.

[0003] Currently, common methods for treating fluoride-containing wastewater include chemical precipitation, coagulation sedimentation, adsorption, ion exchange, electrocoagulation, and membrane separation. When using lime precipitation to treat fluoride-containing wastewater, the extremely small particle size of the calcium fluoride precipitate generated easily coats the surface of calcium hydroxide, resulting in low lime utilization and reduced defluorination efficiency. Generally, lime precipitation alone can only reduce the fluoride ion concentration to around 20-40 mg / L. Related technologies often increase the lime dosage to more than twice the theoretical amount, or use other calcium-containing precipitants or add flocculants to improve the defluorination effect. This inevitably increases treatment costs and the yield of precipitated sludge, leading to higher impurity content and lower calcium fluoride purity in the precipitate, making it difficult to recycle and increasing the difficulty of subsequent sludge treatment, and even causing secondary pollution and resource waste.

[0004] Therefore, it is necessary to study a method that can reduce the amount of precipitant used, improve the defluorination effect and fluorine recovery rate, and make efficient use of solid waste and save treatment costs, while achieving both deep defluorination and resource utilization of solid waste. Summary of the Invention

[0005] In view of this, this application provides a method for treating fluoride-containing wastewater and recovering calcium fluoride, which requires a small amount of precipitant, has a good fluoride removal effect, and can efficiently utilize solid waste.

[0006] To achieve the above technical objectives, this application adopts the following technical solution:

[0007] In a first aspect, this application provides a method for treating fluoride-containing wastewater and recovering calcium fluoride, comprising the following steps:

[0008] S1. After adding Ca(OH)2 clarified solution to the fluoride-containing wastewater, filter it to obtain the first filter residue and the first filtrate; wash and dry the first filter residue to obtain calcium fluoride;

[0009] S2. Add biochar adsorbent to the first filtrate for adsorption treatment, and then filter to obtain a second filter residue and a second filtrate. Discharge the second filtrate and reuse the second filter residue for adsorption treatment again.

[0010] Preferably, the biochar adsorbent is a sunflower seed shell adsorbent.

[0011] Preferably, the method for preparing the sunflower seed shell adsorbent is as follows:

[0012] K1. The sunflower seed shell powder was heated and impregnated in a mixed solution of Ca(OH)2 and Na2HPO4, and after filtration, the third filter residue was obtained.

[0013] K2. The third filter residue is soaked in hydrogen peroxide and then filtered to obtain the fourth filter residue. The fourth filter residue is then microwave irradiated and cooled to obtain the sunflower seed shell adsorbent.

[0014] Preferably, the fluoride-containing wastewater is the liquid phase obtained by leaching quartz sand with hydrofluoric acid.

[0015] Preferably, the molar ratio of fluoride ions in the fluoride-containing wastewater to calcium ions in the Ca(OH)2 clarified solution is 0.35-0.5:1.

[0016] Preferably, the amount of biochar adsorbent used is 0.8-2 g / L.

[0017] Preferably, in step K1, the mass ratio of Ca(OH)2 to Na2HPO4 is (0.5-1):(1-2).

[0018] Preferably, in step K1, the heating and impregnation temperature is 35-60℃, and the heating and impregnation time is 8-12h.

[0019] Preferably, in step K2, the microwave irradiation power is 500-600W and the microwave irradiation time is 10-15min.

[0020] Preferably, in step K2, the concentration of hydrogen peroxide is 20-25%, and the soaking temperature is 50-80℃.

[0021] The beneficial effects of this application are as follows: This application combines precipitation and adsorption methods to treat fluoride-containing wastewater. The amount of precipitant used is small, the energy consumption is low, and high-grade calcium fluoride can be recovered and sold as a product, making it highly economical. The adsorbent is prepared using waste sunflower seed shells. The prepared adsorbent has a high content of oxygen groups (such as hydroxyl groups), which has the dual function of adsorption and ion exchange for fluoride removal. It has a large adsorption surface area and high adsorption capacity. The concentration of fluoride ions in the treated effluent can be reduced to below 1 mg / L, which meets the drinking water standard. This achieves waste-to-waste treatment and deep fluoride removal. The treated sunflower seed shell adsorbent can be reused without secondary pollution. Attached Figure Description

[0022] Figure 1 This is a process flow diagram for this application. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] like Figure 1 As shown, a method for treating fluoride-containing wastewater and recovering calcium fluoride includes the following steps:

[0025] S1. After adding Ca(OH)2 clarified solution to the fluoride-containing wastewater, filter the solution to obtain the first filter residue and the first filtrate; wash and dry the first filter residue to obtain calcium fluoride;

[0026] S2. Add biochar adsorbent to the first filtrate for adsorption treatment, then filter to obtain the second filter residue and the second filtrate. Discharge the second filtrate and reuse the second filter residue for adsorption treatment again.

[0027] The method for treating fluoride-containing wastewater and recovering calcium fluoride provided by this invention uses a small amount of lime to dissolve and obtain a clear Ca(OH)2 solution as a precipitant to precipitate acidic fluoride-containing wastewater. The amount of lime used in this process is lower than the theoretical value, saving reagent costs. This method not only neutralizes the wastewater but also recovers high-grade calcium fluoride. Collected waste sunflower seed shells are heated and impregnated in a mixed solution of Ca(OH)2 and Na2HPO4, and then modified by hydrogen peroxide soaking and microwave irradiation carbonization to prepare an adsorbent. The preparation method is simple and has high energy efficiency. The resulting sunflower seed shell adsorbent has high oxygen content, large specific surface area, and high adsorption capacity, realizing the resource utilization of agricultural solid waste. After precipitation treatment, the fluoride-containing wastewater is further treated with the sunflower seed shell adsorbent, and the fluoride ion concentration in the effluent is reduced to below 1 mg / L, meeting the standard for fluoride content in drinking water, and can be directly discharged. This method has good defluorination effect, is economical, green and pollution-free, and the second filter residue can be reused. At the same time, it realizes the recovery of fluorine resources and the resource utilization of agricultural solid waste, and has high application value.

[0028] In step S1, the fluoride-containing wastewater undergoes a precipitation reaction with the Ca(OH)2 clarified solution. The precipitation reaction is carried out at room temperature, with the stirring speed controlled at 50-200 r / min and the stirring time at 15-50 min.

[0029] The biochar adsorbent is a sunflower seed shell adsorbent.

[0030] The preparation method of sunflower seed shell adsorbent is as follows:

[0031] K1. The sunflower seed shell powder was heated and impregnated in a mixed solution of Ca(OH)2 and Na2HPO4, and after filtration, the third filter residue was obtained.

[0032] K2. Soak the third filter residue in hydrogen peroxide, then filter it to obtain the fourth filter residue. Irradiate the fourth filter residue with microwaves and cool it to obtain the sunflower seed shell adsorbent.

[0033] The content of oxygen-containing groups in sunflower seed shells can be increased by heating and impregnating them with Ca(OH)2 and Na2HPO4. By combining the modification of sunflower seed shells with hydrogen peroxide soaking and microwave irradiation carbonization, the resulting sunflower seed shell adsorbent has high oxygen-containing groups, large specific surface area, and high adsorption capacity. After precipitation treatment, fluoride-containing wastewater is further treated with sunflower seed shell adsorbent, and the concentration of fluoride ions in the effluent is greatly reduced.

[0034] Melon seed shell powder is obtained by grinding and sieving melon seed shells. The particle size of melon seed shell powder is 100-200 mesh. The raw material melon seed shells include one or more of the following: sunflower seed shells, watermelon seed shells, pumpkin seed shells, and loofah seeds.

[0035] Fluorine-containing wastewater is the liquid phase obtained after leaching quartz sand with hydrofluoric acid.

[0036] Before formally treating fluoride-containing wastewater, the content of fluoride ions in the wastewater is first tested. The molar ratio of fluoride ions in the fluoride-containing wastewater to calcium ions in the Ca(OH)2 clarified solution is 0.35-0.5:1.

[0037] The dosage of biochar adsorbent is 0.8-2 g / L, and the adsorption time is 20-60 min.

[0038] In step K1, the mass ratio of Ca(OH)2 to Na2HPO4 is (0.5-1):(1-2).

[0039] In step K1, the temperature for heating and impregnation is 35-60℃, and the heating and impregnation time is 8-12 hours.

[0040] In step K2, the microwave irradiation power is 500-600W, and the irradiation time is 10-15 minutes. The microwave irradiation device is a microwave oven, and microwave irradiation is the carbonization process.

[0041] In step K2, the concentration of hydrogen peroxide is 20-25%, and the soaking temperature is 50-80℃.

[0042] The proposed solution is a method for treating fluoride-containing wastewater that can reduce the amount of precipitant used, improve the defluorination effect and fluoride recovery rate, and efficiently utilize solid waste and save treatment costs. It also achieves deep defluorination and resource utilization of solid waste.

[0043] The following specific embodiments further illustrate this solution.

[0044] Example 1

[0045] A method for treating fluoride-containing wastewater and recovering calcium fluoride includes the following steps:

[0046] S1. The liquid phase obtained after leaching quartz sand with hydrofluoric acid is fluoride-containing wastewater. Calcium oxide is dissolved in water and allowed to stand. The supernatant is collected to obtain a clear Ca(OH)2 solution. After adding the clear Ca(OH)2 solution to the fluoride-containing wastewater, a precipitation reaction is carried out by stirring at room temperature. The stirring speed is controlled at 200 r / min and the stirring time is 50 min. After the reaction is completed, the mixture is filtered to obtain the first filter residue and the first filtrate. The molar ratio of fluoride ions in the fluoride-containing wastewater to calcium ions in the clear Ca(OH)2 solution is 0.35:1. The first filter residue is collected, washed, and dried to obtain calcium fluoride.

[0047] S2. Add 2 g / L of sunflower seed shell adsorbent to the first filtrate for adsorption treatment, and then filter to obtain the second filter residue and the second filtrate. Discharge the second filtrate and reuse the second filter residue by mixing it with the sunflower seed shell adsorbent for adsorption treatment again.

[0048] The preparation method of the sunflower seed shell adsorbent is as follows:

[0049] K1. Melon seed shell powder was heated and impregnated in a mixed solution of Ca(OH)2 and Na2HPO4. After filtration, the third filter residue was obtained. The mass ratio of Ca(OH)2 to Na2HPO4 was 0.5:1. The heating and impregnation temperature was 60℃ and the heating and impregnation time was 8h.

[0050] K2. Soak the third filter residue in hydrogen peroxide, then filter it to obtain the fourth filter residue. Irradiate the fourth filter residue with microwave power of 600W for 15 minutes. After cooling, the sunflower seed shell adsorbent is obtained.

[0051] Example 2

[0052] A method for treating fluoride-containing wastewater and recovering calcium fluoride is the same as in Example 1, except that the molar ratio of fluoride ions in the fluoride-containing wastewater to calcium ions in the Ca(OH)2 clarified solution is 0.5:1.

[0053] Example 3

[0054] A method for treating fluoride-containing wastewater and recovering calcium fluoride is the same as in Example 1, except that 0.8 g / L of sunflower seed shell adsorbent is added to the first filtrate.

[0055] Comparative Example 1

[0056] A method for treating fluoride-containing wastewater and recovering calcium fluoride is the same as in Example 1, except that the Ca(OH)2 clarified liquid is replaced with twice the molar amount of calcium oxide.

[0057] Comparative Example 2

[0058] A method for treating fluoride-containing wastewater and recovering calcium fluoride is the same as in Example 1, except that step S2 is not included.

[0059] Comparative Example 3

[0060] A method for treating fluoride-containing wastewater and recovering calcium fluoride is the same as in Example 1, except that in step K1, sunflower seed shell powder is heated and impregnated in Na2HPO4 solution.

[0061] Comparative Example 4

[0062] A method for treating fluoride-containing wastewater and recovering calcium fluoride is the same as in Example 1, except that the sunflower seed shell adsorbent is prepared by grinding sunflower seed shells into powder and sieving them.

[0063] Evaluation Test

[0064] The purity and defluorination efficiency of the calcium fluoride obtained in Examples 1-3 and Comparative Examples 1-4 were tested according to GB / T5195.1-2017; the fluoride ion concentration in the raw wastewater and discharged wastewater was tested according to GB / T 5750-2006, and the results are shown in Table 1:

[0065] Table 1 Test Results

[0066]

[0067] Examples 1-3 all treated fluoride-containing wastewater according to the treatment process of the present invention, and the effluent contained F - The concentrations of all samples were reduced to below 1 mg / L, and the recovered calcium fluoride had a high purity. In Comparative Example 1, solid calcium oxide was used as a precipitant for fluoride removal, and the removal efficiency was significantly lower compared to the Ca(OH)₂ clarified solution. After further adsorption treatment, the fluoride concentration in the effluent decreased. - The concentration only dropped to below 10 mg / L, and the purity of the recovered calcium fluoride decreased. The wastewater in Comparative Example 2 was not treated by adsorption, resulting in poor defluorination, but it did not affect the grade of calcium fluoride. The sunflower seed shell powder in Comparative Example 3 was only heated and impregnated in Na2HPO4 solution, which did not achieve a good modification effect, so the defluorination efficiency decreased. The sunflower seed shell powder in Comparative Example 4 was only ground and sieved, without modification, which is equivalent to the fluoride-containing wastewater only undergoing precipitation treatment, so the defluorination effect was poor.

[0068] This invention provides a method for treating fluoride-containing wastewater and recovering calcium fluoride. A small amount of lime is used to dissolve the fluoride in a clear Ca(OH)₂ solution, which is then used as a precipitant to precipitate the acidic fluoride-containing wastewater. The amount of lime used is lower than theoretically required, saving on reagent costs. This method not only neutralizes the wastewater but also recovers high-grade calcium fluoride. Collected waste sunflower seed shells are heated and impregnated in a mixed solution of Ca(OH)₂ and Na₂HPO₄, and then modified by hydrogen peroxide soaking and microwave irradiation carbonization to prepare an adsorbent. This preparation method is simple, energy-efficient, and yields a sunflower seed shell adsorbent with high oxygen-containing groups, large specific surface area, and high adsorption capacity, achieving resource utilization of agricultural solid waste. After precipitation treatment, the fluoride-containing wastewater is further treated with the sunflower seed shell adsorbent, reducing the fluoride ion concentration in the effluent to below 1 mg / L, meeting the standards for fluoride content in drinking water and allowing for direct discharge. This method offers good fluoride removal efficiency, is economical, and environmentally friendly, simultaneously achieving fluoride resource recovery and resource utilization of agricultural solid waste, demonstrating high application value.

[0069] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for treating fluoride-containing wastewater and recovering calcium fluoride, characterized in that, Includes the following steps: S1. After adding Ca(OH)2 clarified solution to the fluoride-containing wastewater, filter it to obtain the first filter residue and the first filtrate; wash and dry the first filter residue to obtain calcium fluoride; S2. Add biochar adsorbent to the first filtrate for adsorption treatment, and then filter to obtain a second filter residue and a second filtrate. Discharge the second filtrate and reuse the second filter residue for adsorption treatment again. The biochar adsorbent is a sunflower seed shell adsorbent; the preparation method of the sunflower seed shell adsorbent is as follows: K1. The sunflower seed shell powder was heated and impregnated in a mixed solution of Ca(OH)2 and Na2HPO4, and after filtration, the third filter residue was obtained. K2. The third filter residue is soaked in hydrogen peroxide and then filtered to obtain the fourth filter residue. The fourth filter residue is then microwave irradiated and cooled to obtain the sunflower seed shell adsorbent. The fluoride-containing wastewater is the liquid phase obtained after leaching quartz sand with hydrofluoric acid; the molar ratio of fluoride ions in the fluoride-containing wastewater to calcium ions in the Ca(OH)2 clarified solution is 0.35-0.5:1; The method for preparing Ca(OH)2 clear solution is as follows: dissolve calcium oxide in water and let it stand, take the supernatant to obtain Ca(OH)2 clear solution.

2. The method for treating fluoride-containing wastewater and recovering calcium fluoride according to claim 1, characterized in that, The amount of biochar adsorbent used is 0.8-2 g / L.

3. The method for treating fluoride-containing wastewater and recovering calcium fluoride according to claim 1, characterized in that, In step K1, the mass ratio of Ca(OH)2 to Na2HPO4 is (0.5-1):(1-2).

4. The method for treating fluoride-containing wastewater and recovering calcium fluoride according to claim 1, characterized in that, In step K1, the temperature for heating and impregnation is 35-60℃, and the heating and impregnation time is 8-12 hours.

5. The method for treating fluoride-containing wastewater and recovering calcium fluoride according to claim 1, characterized in that, In step K2, the microwave irradiation power is 500-600W, and the microwave irradiation time is 10-15min.

6. The method for treating fluoride-containing wastewater and recovering calcium fluoride according to claim 1, characterized in that, In step K2, the concentration of hydrogen peroxide is 20-25%, and the soaking temperature is 50-80℃.

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