Method for removing and recycling fluorine ions by promoting calcium fluoride precipitation through sulfate doping

Abstract

The invention relates to the technical field of environmental science and engineering, and discloses a method for removing and recycling fluorine ions by promoting calcium fluoride precipitation through sulfate doping, and the method comprises the following steps: S1, preparing sodium fluoride solutions of sulfate with different concentrations, and analyzing the influence of different sulfate concentrations on calcium fluoride precipitation; s2, respectively adding calcium chloride solutions into the prepared sodium fluoride solutions of the sulfates with different concentrations; s3, the two solutions are subjected to a reaction within a set time, filtering is conducted after the reaction is conducted for a period of time, and the fluorine ion removal reaction is completed. By changing the concentration of sulfate in the sodium fluoride solution, the effect of the sulfate in calcium fluoride precipitation and the influence of the sulfate on the fluorine ion removal efficiency are researched; the positive effect and the action mechanism of the low-concentration sulfate in the calcium fluoride precipitation process are defined, and the problems that the influence of the sulfate in the high-fluoride wastewater on calcium fluoride precipitation is ignored, the calcium fluoride purity is low and the recovery difficulty is large are finally solved.

Description

Technical Field

[0001] The present invention relates to the field of environmental science and engineering technology, and in particular to a method for removing and recovering fluoride ions by promoting calcium fluoride precipitation through sulfate doping. Background Art

[0002] Fluoride contamination in drinking water poses a serious threat to public health. Excessive fluoride intake can lead to dental or bone diseases. Currently, approximately 200 million people worldwide rely on water sources with excessive fluoride ions (the World Health Organization stipulates that the fluoride concentration in drinking water should not exceed 1.5 mg / L). The development of 5G communications, semiconductors, photovoltaic power generation, and new energy electric vehicle materials has generated a large amount of high-fluoride wastewater, exacerbating fluoride pollution. Fluorite, a mineral primarily composed of calcium fluoride, has enormous application potential in new energy, atomic energy, and aerospace. However, as a non-renewable resource, high-quality fluorite deposits may be depleted within decades, and many countries around the world have designated it as a strategic mineral resource.

[0003] At present, fluoride removal technologies mainly include chemical precipitation, coagulation, adsorption, ion exchange, electrodialysis, reverse osmosis, etc. Chemical precipitation has become the most widely used method for treating high-fluoride wastewater due to its advantages of low cost, simple operation and high removal rate. Recovering sodium fluoride through calcium salt precipitation is an effective way to solve the pollution of high-fluoride wastewater and the depletion of fluorite resources. Among them, the purity and precipitation efficiency of calcium fluoride are the key to the removal and recovery of calcium fluoride. However, high-fluoride wastewater is a complex system containing a variety of coexisting anions, which affects the purity of calcium fluoride and increases the difficulty of recovery. Sulfate, as the main coexisting ion in high-fluoride wastewater, is the focus of research. However, the effect of sulfate on calcium fluoride precipitation under low saturation conditions has been ignored. In addition, the effect of sulfate on the crystal structure and properties of calcium fluoride is still unclear. Therefore, the present invention aims to provide a method for promoting calcium fluoride precipitation by sulfate doping to achieve fluoride ion removal and recovery, so as to solve the shortcomings of the existing technology. Summary of the Invention

[0004] (1) Technical problems solved

[0005] In response to the shortcomings of the existing technology, the present invention provides a method for removing and recovering fluoride ions by promoting calcium fluoride precipitation through sulfate doping. The method has the advantages of low cost, simple operation, high precipitation efficiency, and can effectively improve the purity of calcium fluoride. It solves the problems of neglected effects of sulfate on calcium fluoride precipitation in high-fluoride wastewater, low calcium fluoride purity, and difficulty in recovery.

[0006] (2) Technical solution

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for removing and recovering fluoride ions by promoting calcium fluoride precipitation by sulfate doping, comprising the following steps:

[0008] S1. Prepare sodium fluoride solutions with different sulfate concentrations and analyze the effect of different sulfate concentrations on calcium fluoride precipitation;

[0009] S2. Add calcium chloride solution to the prepared sodium fluoride solution of sulfate of different concentrations;

[0010] S3. Allow the two solutions to react within a set time, and filter after a period of reaction to complete the fluoride ion removal reaction.

[0011] Preferably, in step S1, the molar ratios of sodium fluoride to sulfate are 1000:1, 100:1 and 100:3, respectively.

[0012] Preferably, in step S1, the sulfate is sodium sulfate, and the solution preparation temperature is 25±2°C.

[0013] Preferably, in step S2, the concentration of sodium fluoride solution is 0.1 mmol / L -1 , the concentration of calcium chloride solution is 1mmolL -1 .

[0014] Preferably, in step S2, the saturation index of the generated calcium fluoride is 5.8, while the saturation index of calcium sulfate is much less than 0, so no calcium sulfate precipitate is formed in the solution.

[0015] Preferably, in step S2, the calcium chloride solution is mixed with the sodium fluoride solution in a dropwise manner, and the stirring rate is maintained at 200-300 r / min during the mixing process.

[0016] Preferably, in step S3, the reaction time of the sodium fluoride solution with different sulfate concentrations and the calcium chloride solution is 0-30 min, and the chemical equation of the reaction is:

[0017] NaF + CaCl2 → CaF2↓ + 2NaCl

[0018] In the reaction formula, calcium fluoride will precipitate out in the form of precipitation.

[0019] Preferably, in step S3, the filter membrane used for filtration is a filter membrane with a pore size of 0.45 μm.

[0020] Preferably, in step S3, the size of the CaF2 aggregated particles formed under various conditions is greater than 500 nm.

[0021] Preferably, in step S3, the filtered calcium fluoride precipitate is dried at 75-80° C. for 1.5-3 hours to obtain a recyclable calcium fluoride solid.

[0022] Compared with the prior art, the present invention provides a method for removing and recovering fluoride ions by promoting calcium fluoride precipitation through sulfate doping, which has the following beneficial effects:

[0023] 1. This study explores the electrostatic bridging and lattice doping effects of low-concentration sulfate (fluoride to sulfate molar ratio ≤ 100:3) during the calcium fluoride crystallization process, thereby increasing the calcium fluoride particle size (from 650 nm to 1900 nm) and improving the sedimentation separation efficiency (turbidity reduction by 60%).

[0024] 2. The present invention controls the sulfate concentration to a low saturation condition (CaSO4 saturation index log 10 (Q / K)<0), avoiding the interference of calcium sulfate co-precipitation and ensuring the purity of calcium fluoride product (>98.7%). At the same time, it broke through the bottleneck of calcium ion competition consumption in the traditional high-concentration sulfate process (>1000mg / L), thereby improving the recovery rate of calcium fluoride. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a graph showing the effect of the molar ratio of fluoride to sulfate on the size of calcium fluoride aggregate particles during precipitation in Example 1;

[0026] Figure 2 This is a graph showing the change in the size of calcium fluoride aggregate particles during the precipitation process in Example 2 as a function of precipitation time;

[0027] Figure 3 This is a graph showing the changes in sulfate ion, calcium ion and fluoride ion concentrations with precipitation time in Example 4. DETAILED DESCRIPTION

[0028] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0029] See also Figure 1-Figure 3 A method for removing and recovering fluoride ions by promoting calcium fluoride precipitation by sulfate doping comprises the following steps:

[0030] S1. Prepare sodium fluoride (NaF) solutions with different sulfate concentrations and analyze the effect of different sulfate concentrations on calcium fluoride precipitation;

[0031] S2. Add calcium chloride (CaCl2) solution to the prepared sodium fluoride (NaF) solution with different sulfate concentrations;

[0032] S3. Allow the two solutions to react within a set time, and filter after a period of reaction to complete the fluoride ion removal reaction.

[0033] Specifically, in step S1, the molar ratios of sodium fluoride (NaF) to sulfate are 1000:1, 100:1, and 100:3, respectively.

[0034] Specifically, in step S1, the sulfate is sodium sulfate (Na2SO4), and the solution preparation temperature is 25±2°C.

[0035] Specifically, in step S2, the concentration of sodium fluoride (NaF) solution is 0.1 mmol / L -1 , the concentration of calcium chloride (CaCl2) solution is 1mmolL -1 .

[0036] Specifically, in step S2, the saturation index of calcium fluoride (CaF2) generated (defined as log 10 (Q / K)) is 5.8, and the saturation index of calcium sulfate (CaSO4) is much less than 0, so no calcium sulfate (CaSO4) precipitate will form in the solution.

[0037] Specifically, in step S2, the calcium chloride (CaCl2) solution is mixed with the sodium fluoride (NaF) solution in a dropwise manner, and the stirring rate is maintained at 200-300 r / min during the mixing process.

[0038] Specifically, in step S3, the reaction time of sodium fluoride (NaF) solution with different sulfate concentrations and calcium chloride (CaCl2) solution is 0-30 minutes, and the chemical equation of the reaction is:

[0039] NaF + CaCl2 → CaF2↓ + 2NaCl

[0040] In the reaction formula, calcium fluoride (CaF2) will precipitate out in the form of precipitation, thereby achieving the removal and recovery of fluoride ions.

[0041] Specifically, in step S3, the filter membrane used for filtration is a filter membrane with a pore size of 0.45 μm.

[0042] Specifically, in step S3, the size of the CaF2 aggregated particles formed under each condition is greater than 500 nm.

[0043] Specifically, in step S3, the filtered calcium fluoride (CaF2) precipitate is dried at 75-80°C for 1.5-3 hours to obtain a recyclable calcium fluoride solid.

[0044] In order to explore the role and mechanism of sulfate, the main coexisting ion in high-fluoride wastewater, in the precipitation of calcium fluoride, and to guide the removal and recovery of high-fluoride wastewater, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0045] Example 1

[0046] Effect of the Molar Ratio of Fluoride to Sulfate on the Aggregate Size of Calcium Fluoride during Precipitation

[0047] Experimental process: at 0.1mmolL -1 Na2SO4 was added to the NaF solution, and the molar ratio of fluoride to sulfate was 1000:1, 100:1 and 100:3 respectively. The NaF solution containing different concentrations of sulfate (0.1mmolL -1 ) into a beaker and add CaCl2 solution (1mmolL -1 After 20 min of reaction at 25°C, the solution was filtered through a 0.45 μm membrane. The calcium fluoride attached to the microfiltration membrane was dried and the particle size and Zeta potential of the calcium fluoride aggregates were measured using a nanoparticle size analyzer (ZS90, Malvern Panalytical). The effect of the molar ratio of fluoride to sulfate on the size of the calcium fluoride aggregates during precipitation was shown in Figure 2. Figure 1 shown.

[0048] from Figure 1 It can be seen that compared with the original solution, the size of the calcium fluoride aggregated particle clusters increases with the increase of sulfate concentration. When the molar ratio of fluoride to sulfate reaches 100:3, the size of the calcium fluoride aggregated particle clusters increases threefold, from 650 nanometers to 1900 nanometers. Therefore, in the presence of sulfate, the calcium fluoride aggregated particle clusters become larger and easier to settle and separate from the solution.

[0049] Example 2

[0050] Effect of precipitation time on the size of calcium fluoride aggregate particles

[0051] Experimental process: at 0.1mmolL -1 Na2SO4 was added to the NaF solution to keep the molar ratio of fluoride to sulfate at 100:3, and CaCl2 solution (1mmol / L -1 ), after the reaction temperature was 25°C for 0, 5, 10, 15 and 20 min, the solution was filtered with a 0.45 μm membrane, the calcium fluoride attached to the microfiltration membrane was dried, and the particle size of the calcium fluoride aggregates was measured using a nanoparticle size analyzer (ZS90, Malvern Panalytical). The change in the size of the calcium fluoride aggregates with precipitation time during the precipitation process is shown in Figure 2. Figure 2 shown.

[0052] from Figure 2 It can be seen that as the precipitation time gradually increases, the size of the calcium fluoride aggregated particles gradually increases.

[0053] Example 3

[0054] Effect of the Molar Ratio of Fluoride to Sulfate on the Turbidity of Mixed Systems During Precipitation

[0055] Experimental process: at 0.1mmolL -1 Na2SO4 was added to the NaF solution to make the molar ratio of fluoride to sulfate 1000:1, 100:1 and 100:3 respectively. Then, CaCl2 solution (1mmol / L -1 ), after reacting for 1 hour at a reaction temperature of 25°C, the solution was filtered with a 0.45 μm membrane and the turbidity of the mixed solution was measured using a turbidimeter. During the precipitation process, the turbidity of the mixed system changed with the molar ratio of fluoride to sulfate, as shown in Table 1.

[0056] Table 1 Changes in turbidity of mixed systems with the molar ratio of fluoride to sulfate

[0057]

[0058] As can be seen from Table 1, as the sulfate ion concentration increases, the turbidity of the solution gradually decreases. When the molar ratio of sulfate ion to fluoride ion increases to 3:100, the turbidity of the solution decreases by 60%, greatly promoting the precipitation of calcium fluoride.

[0059] Example 4

[0060] Effect of precipitation time on the concentrations of sulfate ions, calcium ions and fluoride ions in the system

[0061] Experimental process: at 0.1mmolL -1 Na2SO4 was added to the NaF solution to keep the molar ratio of fluoride to sulfate at 100:3. Then CaCl2 solution (1 mmol L -1 The reaction temperature was 25 ° C, and the reaction time was 0, 5, 10, 15 and 30 min respectively. The solution was then filtered with a 0.45 μm membrane. The concentrations of fluoride ions and sulfate ions were determined by ion chromatography (Aquion, Thermo Fisher Scientific, USA). The concentration of calcium ions was determined by inductively coupled plasma optical emission spectrometry (Optima-2000, PerkinElmer, USA). The changes in the concentrations of sulfate ions, calcium ions and fluoride ions in the mixed system with precipitation time are shown in Figure 2. Figure 3 shown.

[0062] from Figure 3 It can be seen that as the precipitation time gradually increases, the sulfate concentration in the solution decreases. In addition, the sulfate concentration tends to stabilize within 10 minutes, which is consistent with the time period when the size of the calcium fluoride aggregated particle clusters does not change. The fluoride ion concentration decreases linearly within the first 15 minutes and remains stable after 30 minutes. At the same time, compared with the original solution, the fluoride ion concentration of the sulfate ion solution decreases after 30 minutes of reaction, but the fluoride ion removal rate does not increase significantly. The change pattern of calcium ion concentration is basically the same as that of sulfate ion concentration. The calcium ion concentration basically decreases linearly within the first 15 minutes and reaches stability after 30 minutes of reaction.

[0063] Comparative Example 1

[0064] Inhibitory effect of high concentration sulfate on calcium fluoride precipitation

[0065] Experimental process: at 0.1mmolL -1 Na2SO4 was added to the NaF solution to make the molar ratio of fluoride to sulfate 100:10 (simulating the traditional high concentration sulfate condition), and CaCl2 solution (1 mmol L -1 ), reacted at 25℃ for 20min and then filtered, and the CaF2 particle size, F - Removal rate and sediment composition.

[0066] Results: The average size of CaF2 particles was 320 nm (much lower than the 1900 nm of the present invention); XRD showed that the precipitate contained a mixed phase of CaF2 and CaSO4 (the purity decreased); NTU = 35 ± 2.1 (the present invention was 10.9 ± 1.2), and high concentration of sulfate caused CaSO4 to co-precipitate, competing with CaF2 for CaSO4. 2+ , inhibiting particle growth and reducing removal efficiency.

[0067] The advantages are: the present invention achieves the goal of increasing the calcium fluoride particle size (from 650nm to 1900nm) and accelerating the sedimentation separation efficiency (turbidity reduced by 60%) by exploring the electrostatic bridging effect and lattice doping effect of low-concentration sulfate (fluoride to sulfate molar ratio ≤100:3) in the calcium fluoride crystallization process.

[0068] Comparative Example 2

[0069] Interference of calcium sulfate coprecipitation on fluoride removal

[0070] Experimental process: at 0.1mmolL -1 Add excess Na2SO4 (sulfate concentration 2000 mg / L) to the NaF solution and add CaCl2 solution (1 mmol / L -1), after 20 min of reaction, the Ca content in the solution was determined 2+ 、F - and SO4 2- concentration and calculate the CaSO4 saturation index.

[0071] Results: CaSO4 saturation index log 10 (Q / K)=0.5 (>0, precipitate formed); F - Removal rate: 58%; calcium ion consumption: 72% of Ca 2+ Used to generate CaSO4, only 28% participates in CaF2 precipitation. Under high concentration of sulfate, a large amount of CaSO4 precipitation is generated, which seriously consumes Ca 2+ , resulting in a decrease in fluoride ion removal rate.

[0072] Comparative Example 3

[0073] Comparison of reaction kinetics between traditional process and the present invention

[0074] The following two conditions were used to treat the same fluoride ion wastewater (F - =200mg / L) specifically:

[0075] (1) Traditional process: No sulfate is added, and CaCl2 (1mmolL -1 );

[0076] (2) The process of the present invention: adding Na2SO4 (SO4 2- :F - =3:100), CaCl2 (1mmolL -1 ). Monitor F under two conditions -1 Concentration changes over time (0-60min).

[0077] Results: The present invention increases the fluoride ion removal rate by sulfate doping, and reaches stability in 30 minutes, while the traditional process requires 60 minutes.

[0078] Comparative Example 4: Effect of sulfate concentration on calcium fluoride recovery purity

[0079] The following two conditions were used to treat the - Wastewater:

[0080] (1) High sulfate group: SO4 2- :F - =10:100, CaCl2 (1mmolL -1 );

[0081] (2) The present invention group: SO4 2- :F -=3:100, CaCl2 (1mmolL -1 The components of the recovered CaF2 precipitate were analyzed by X-ray fluorescence spectroscopy (XRF).

[0082] Results: The purity of CaF2 in the high sulfate group was 82.5%, containing 17.5% CaSO4 impurity; the purity of CaF2 in the present invention group was 98.7%, and no CaSO4 was detected. The present invention can significantly improve the purity of calcium fluoride recovery by controlling the low concentration of sulfate doping and avoiding the formation of impurity phases.

[0083] Table 2

[0084]

[0085] From Table 2, we can get: through SO4 2- The electrostatic bridging effect increases the CaF2 particle size, improves the sedimentation efficiency, and achieves the promoting effect of low-concentration sulfate. During thermodynamic control, it can ensure that CaSO4 does not precipitate (log(Q / K)<0), thereby avoiding competitive consumption of calcium ions. In order to adapt to complex wastewater systems (such as photovoltaic and semiconductor fluorine-containing wastewater), the method of the present invention can be directly used in industrial continuous treatment.

[0086] Summary: Through the above comparative examples, the advantages of the present invention in removal efficiency, reaction speed, product purity and process cost can be clearly verified.

[0087] The advantages are: the present invention controls the sulfate concentration to a low saturation condition (CaSO4 saturation index log 10 (Q / K)<0), avoiding the interference of calcium sulfate co-precipitation and ensuring the purity of calcium fluoride product (>98.7%). At the same time, it broke through the bottleneck of calcium ion competition consumption in the traditional high-concentration sulfate process (>1000mg / L), thereby improving the recovery rate of calcium fluoride.

[0088] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and alterations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for removing and recovering fluoride ions by promoting calcium fluoride precipitation by sulfate doping, characterized in that: The following steps are involved: S1. Prepare sodium fluoride solutions with different sulfate concentrations and analyze the effect of different sulfate concentrations on calcium fluoride precipitation; S2. Add calcium chloride solution to the prepared sodium fluoride solution of sulfate of different concentrations; S3. Allow the two solutions to react within a set time, and filter after a period of reaction to complete the fluoride ion removal reaction.

2. The method of claim 1, wherein: In step S1, the molar ratios of sodium fluoride to sulfate are 1000:1, 100:1 and 100:3 respectively.

3. The method of claim 1, wherein: In step S1, the sulfate is sodium sulfate, and the solution preparation temperature is 25±2°C.

4. The method of claim 1, wherein: In step S2, the concentration of sodium fluoride solution is 0.1 mmol / L -1 , the concentration of calcium chloride solution is 1mmolL -1 .

5. The method of claim 1 , wherein: In step S2, the saturation index of the generated calcium fluoride is 5.8, while the saturation index of calcium sulfate is much less than 0, so no calcium sulfate precipitate is formed in the solution.

6. The method of claim 1 for removing and recovering fluoride ions by promoting calcium fluoride precipitation through sulfate doping, characterized in that: In step S2, the calcium chloride solution is added dropwise and mixed with the sodium fluoride solution, and the stirring rate is maintained at 200-300 r / min during the mixing process.

7. The method of claim 1 for removing and recovering fluoride ions by promoting calcium fluoride precipitation through sulfate doping, characterized in that: In step S3, the reaction time of the sodium fluoride solution with different sulfate concentrations and the calcium chloride solution is 0-30 min, and the chemical equation of the reaction is: NaF + CaCl2 → CaF2↓ + 2NaCl In the reaction formula, calcium fluoride will precipitate out in the form of precipitation.

8. The method of claim 1 for removing and recovering fluoride ions by promoting calcium fluoride precipitation through sulfate doping, characterized in that: In step S3, the filter membrane used for filtration is a filter membrane with a pore size of 0.45 μm.

9. The method of claim 1 for removing and recovering fluoride ions by promoting calcium fluoride precipitation through sulfate doping, characterized in that: In step S3, the size of the CaF2 aggregated particles formed under each condition is greater than 500 nm.

10. The method of claim 1 for removing and recovering fluoride ions by promoting calcium fluoride precipitation through sulfate doping, characterized in that: In the step S3, the filtered calcium fluoride precipitate is dried at 75-80° C. for 1.5-3 hours to obtain a recyclable calcium fluoride solid.

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