Process for deeply removing fluorine from wastewater by means of induced crystallization method in sludge backflow

Abstract

Disclosed in the present invention is a process for deeply removing fluorine from wastewater by means of an induced crystallization method in sludge backflow. The process comprises the following steps: step 1. placing lime powder and deionized water in a seed crystal reactor to prepare lime milk and at the same time putting a calcite seed crystal into the seed crystal reactor; step 2. introducing high-concentration fluorine-containing acidic wastewater into a reactor; and step 3. introducing the lime milk obtained in step 1 into the reactor in step 2 to undergo a reaction, wherein during the reaction, the pH value of the mixed solution is controlled to remain neutral. The present invention is based on a high-density sludge method (HDS process). By utilizing the HDS process, the advantages of effectively improving the sludge density, reducing the lime consumption, facilitating the operation, and enabling the water quality of an effluent to be more stable are realized. In conjunction with induced crystallization technology, an induced crystallization process in a high-density sludge backflow is formed to promote precipitation and crystallization and allow sedimentation to proceed quickly, thereby shortening the process time and overcoming the defects of traditional lime precipitation methods.

Description

A deep defluorination process for wastewater using sludge reflux induced crystallization Technical Field

[0001] The invention relates to the technical field of fluorine-containing wastewater treatment, in particular to a process for deep defluorination of wastewater using a sludge reflux induced crystallization method. Background Art

[0002] At present, my country's high-purity graphite production and other industrial fields will produce a large amount of strongly acidic fluorine-containing wastewater. After treatment, the discharge of fluorine-containing solid waste is large and the effect is unstable, which seriously hinders the high-quality development of related industries. It is necessary to develop a reasonable and appropriate treatment plan. The main treatment methods for fluorine-containing wastewater currently include chemical precipitation, coagulation sedimentation, adsorption, reverse osmosis and induced crystallization.

[0003] The chemical precipitation method of fluoride removal is to add chemical substances such as calcium chloride, calcium hydroxide and calcium oxide to the fluoride-containing wastewater, so that they form calcium fluoride precipitates with fluoride ions to achieve the purpose of fluoride removal. At present, this method is generally applicable to the treatment of large-scale high-concentration fluoride-containing wastewater due to its simple operation, low investment and obvious fluoride removal effect. However, calcium fluoride itself has a certain solubility and will co-dissolve with calcium hydroxide, which often leads to the fluoride content in the treated wastewater still being 20-30 mg / L, which is difficult to meet the discharge standards. At the same time, there are problems such as large amount of sludge and serious secondary pollution. Calcium sulfate, calcium fluorosilicate and other precipitates are formed in the solution at the same time, adsorbed on the surface of the calcium fluoride precipitate, hindering the formation of calcium fluoride precipitate. The calcium fluoride particles generated by the reaction are wrapped on the surface of the lime particles, hindering the further reaction of the precipitant inside the particles, reducing the efficiency of calcium oxide use. Therefore, the wastewater often needs to be treated twice or even multiple times to meet the discharge requirements;

[0004] Fluoride removal by coagulation and sedimentation is a widely used method, suitable for large-scale wastewater treatment. The principle is to add a coagulant with a coagulation effect to the fluoride-containing wastewater, and then adjust the pH to an appropriate value. The fluoride in the wastewater is adsorbed by the formed colloid or precipitate, thereby achieving the purpose of removing fluoride ions. Flocculants can be divided into three categories: organic flocculants, inorganic flocculants and microbial flocculants. Among them, PAM, iron salts and aluminum salts are common flocculants. The advantage of the coagulation and sedimentation method is that the amount of flocculant added in the coagulation and sedimentation is small, and a large amount of wastewater can be treated at one time. The disadvantage is that the effluent water quality of this method is not stable enough, and the amount of sludge produced is large, which makes subsequent treatment more troublesome;

[0005] The adsorption method for treating fluoride-containing wastewater is a method that utilizes the adsorption effect of a series of adsorbents to remove fluoride from the wastewater. The quality of the adsorption effect depends largely on the adsorption material. The adsorption materials currently used to treat fluoride-containing wastewater include polymer adsorbents, such as chitosan, activated carbon fiber, fly ash, etc.; natural mineral adsorbents, such as bentonite, zeolite, etc.; rare earth adsorbents, such as zirconium-containing, lanthanum-containing, and cerium-containing adsorbents; as well as polyacrylamide grafts and biochar adsorbents. The key to using adsorbents to treat fluoride-containing wastewater is to improve treatment efficiency, enable the reuse of adsorbents and overcome the influence of interference factors. This requires in-depth research on the adsorption mechanism and the continuous development of new, efficient and inexpensive adsorbents. With the continuous deepening of research and the development of various modified adsorption materials, the adsorption method for treating fluoride-containing wastewater has good application prospects. However, given that most fluorine adsorbents are currently in the laboratory research stage and have certain limitations in actual treatment, the development of new, inexpensive and efficient adsorbents is still of great significance.

[0006] Reverse osmosis (RO) is a technique for treating fluoride-containing wastewater by separating fluoride ions from other molecules using a semipermeable membrane. This method requires pretreatment of the fluoride-containing wastewater to prevent membrane contamination by suspended solids. Lu Jilai et al. investigated the use of a nanofiltration (NF)-reverse osmosis (RO) process to treat low-fluoride wastewater. The results showed that when the fluoride content in the wastewater is less than 80 mg / L, under certain operating conditions, the NF effluent can meet the national Class I emission standard. Further RO treatment can also meet electroplating water requirements. This integrated process can be used for the separation and reuse of low-fluoride wastewater. However, RO is expensive, and the RO membrane is susceptible to contamination and clogging during the treatment process, resulting in limited practical application.

[0007] To this end, we proposed a sludge reflow induced crystallization method for deep defluoridation of wastewater to solve the above problems.

[0008] Summary of the Invention

[0009] The purpose of the present invention is to provide a deep fluoride removal process for wastewater by sludge reflow induced crystallization method, which is based on the high-density slurry method (HDS process). The HDS process can effectively increase sludge density, reduce lime consumption, facilitate operation, and make the effluent water quality more stable. Combined with induced crystallization technology, a high-density sludge reflow induced crystallization process is formed to promote precipitation and crystallization, make sedimentation proceed rapidly, shorten the process time, and overcome the defects of traditional lime precipitation method.

[0010] To achieve the above object, the present invention provides the following technical solutions:

[0011] A process for deep defluorination of wastewater by sludge reflow induced crystallization method comprises the following steps:

[0012] Step 1: lime powder and deionized water are placed in a seed crystal reactor to prepare lime milk, and calcite seeds are added into the seed crystal reactor;

[0013] Step 2: introducing high-concentration acidic fluorine-containing wastewater into the reactor;

[0014] Step 3, introducing the lime milk from step 1 into the reactor from step 2 to react, and controlling the pH value of the mixed solution to be kept neutral during the reaction;

[0015] Step 4: Based on step 3, after a period of reaction, PAM is added to the reactor and the stirring reaction is continued;

[0016] In step 5, the mixed liquid after the reaction in step 4 is discharged into a sedimentation tank for static sedimentation. The low-concentration fluorine-containing wastewater after precipitation is discharged, and the precipitated fluorine-containing sludge is detected using a sludge concentration meter. When the fluorine concentration is high, part of the precipitated fluorine-containing sludge is discharged into a seed reactor, and part of the sludge is filtered and discharged. When the concentration is low, the precipitated sludge is returned to the seed reactor. For example, if the sludge concentration is set at 30%, the sludge amount and concentration will definitely be small at the beginning. After the concentration increases, part of it will be returned and returned, which is equivalent to the system being balanced. Compared with a system without return flow, the sludge density will be much higher, more dense, and have a lower water content, which is a beneficial effect. The unreacted lime in the sludge is also returned to react again, which is equivalent to reducing lime consumption, improving treatment effect, and preventing scaling. The return of the already generated calcium fluoride crystals is equivalent to introducing seed crystals, which is conducive to crystal growth and density. The regenerated sludge also has high density and good particle settling performance.

[0017] In a further embodiment, in step 1, the weight ratio between lime powder and deionized water is 2:20.

[0018] In a further embodiment, in step 2, during the process of introducing the high-concentration acidic fluorine-containing wastewater into the reactor, a regulating tank is required to buffer the high-concentration acidic fluorine-containing wastewater, and the buffered high-concentration acidic fluorine-containing wastewater then enters the reactor.

[0019] In a further embodiment, in step 3, the weight ratio of lime milk to high-concentration acidic fluoride-containing wastewater is 27:20, and the reaction time of the two is 30-90 minutes.

[0020] In a further embodiment, in step 3, the pH value of the mixed solution is 7-9.

[0021] Preferably, in step 3, the pH value of the mixed solution is 7.

[0022] In a further embodiment, the weight ratio of the PAM, lime milk and high-concentration acidic fluoride-containing wastewater is 2:27:20.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This sludge return induced crystallization process for deep fluoride removal from wastewater is based on the high-density slurry method (HDS process). The HDS process can effectively increase sludge density, reduce lime consumption, facilitate operation, and achieve more stable effluent quality. Combined with induced crystallization technology, a high-density sludge return induced crystallization process is formed to promote precipitation and crystallization, enable rapid sedimentation, shorten process time, and overcome the shortcomings of traditional lime precipitation method:

[0025] ① The comprehensive treatment of "high-density sludge reflux induced crystallization + deep fluoride removal" is adopted, and the technical process is simple and efficient;

[0026] ② This project adopts advanced high-density sludge return induced crystallization treatment technology. Compared with the traditional lime neutralization method, this technology can reduce the amount of lime used by 10.2%, increase the sludge concentration, and reduce the output of fluorine-containing solid waste by 10.4%. At the same time, it can reduce fluorine to a lower level than the traditional lime method, which is conducive to subsequent deep fluoride removal. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 shows F - Schematic diagram of concentration changes;

[0028] Figure 2 shows SO4 2- and Ca 2+ Schematic diagram of concentration changes;

[0029] Figure 3 shows the effect of pH on F - Schematic diagram of the effect of concentration;

[0030] FIG4 is a schematic diagram showing the effect of CaO dosage changes at different pH values;

[0031] Figure 5 shows the Ca content of supernatants at different pH values. 2+ Concentration diagram;

[0032] Figure 6 shows the supernatant Fe, Si and SO4 at different pH values. 2+ Concentration diagram;

[0033] Figure 7 is a schematic diagram of the sludge volume and dry sludge amount produced when neutralized to different pH values;

[0034] Figure 8 is a schematic diagram of the sludge concentration produced when neutralized to different pH values;

[0035] Figure 9 is a schematic diagram of EDS analysis of sediment at pH 2, 5, 6, and 8 after grinding.

[0036] FIG10 is a schematic diagram showing the content of calcium fluoride in sediments at different pH values;

[0037] Figure 11 shows the effect of different sediment return rates on F - Schematic diagram of the processing effect;

[0038] Figure 12 is a schematic diagram showing the effect of sediment reflow on reaction rate;

[0039] Figure 13 is a schematic diagram showing the effect of sediment return flow on CaO consumption and sediment concentration;

[0040] Figure 14 shows the relationship between the number of sediment circulation and F - Schematic diagram of the impact of removal;

[0041] Figure 15 is a schematic diagram showing the effect of calcium oxide addition on sediment cycle times and sediment concentration;

[0042] Figure 16 is a schematic diagram showing the effect of sludge cycle times on sludge settling ratio;

[0043] FIG17 is a schematic diagram showing the effect of the number of sludge circulations on the proportion of calcium fluoride in the sludge;

[0044] FIG18 is a schematic diagram of a scanning electron microscope of sediments obtained by the conventional lime method and the sediment reflux induced crystallization method;

[0045] FIG19 is a schematic diagram of sediment particle size distribution based on the conventional lime method and the high-density sediment reflux induced crystallization method;

[0046] FIG20 is a schematic diagram showing the effect of adding seed crystals on sludge reflux-induced crystallization on fluoride removal in the step-by-step neutralization of Example 3;

[0047] FIG21 is a graph showing the effect of adding seed crystals on sludge reflux-induced crystallization on fluoride removal in the synchronous neutralization of Example 3;

[0048] FIG22 is a schematic diagram of a scanning electron microscope of the sediment sediment of Example 3;

[0049] FIG23 is a schematic diagram of the particle size distribution of sludge precipitated by the conventional lime process in Example 3;

[0050] FIG24 is a schematic diagram of the particle size distribution of precipitated sludge obtained by the high-density sludge recirculation induced crystallization method in Example 3;

[0051] FIG25 is a schematic diagram of the process structure of the present invention. DETAILED DESCRIPTION

[0052] In the description of the present invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "back," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended only to facilitate the description of the present invention and simplify the description. They do not indicate or imply that the devices or elements referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limiting the present invention. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0053] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0054] 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.

[0055] A sludge reflux induced crystallization method for deep defluoridation of wastewater comprises the following steps: firstly, lime powder and deionized water are placed in a seed crystal reactor to form lime milk; simultaneously, calcite crystal seeds are added into the seed crystal reactor; then, high-concentration acidic fluorine-containing wastewater is introduced into the reactor; then, the lime milk in step one is introduced into the reactor in step two for reaction; during the reaction, the pH value of the mixed solution is controlled to be kept neutral; on the basis of step three, after a period of reaction, PAM is added into the reactor for continued stirring reaction; in step four, the mixed solution after the reaction is discharged into a sedimentation tank for static sedimentation; the low-concentration fluorine-containing wastewater after precipitation is discharged, and the precipitated fluorine-containing sludge is detected by a sludge concentration meter; when the fluorine concentration is high, part of the precipitated fluorine-containing sludge is discharged into the seed crystal reactor, and part of the sludge is filtered and discharged; when the concentration is low, all the precipitated sludge is refluxed into the seed crystal reactor.

[0056] Example 1

[0057] Lime precipitation fluoride removal experiment

[0058] First, weigh 20g of lime powder and dissolve it in 200mL of deionized water to make 10% lime milk. Take 200mL of acidic fluoride-containing wastewater and place it in a 500mL beaker. Stir and slowly add 10% lime milk. Monitor the pH change. The reaction time is 1 hour. Finally, neutralize the mixture to neutral (about pH 7.0) and record the amount of lime milk added as 270mL. After that, 20 mL of PAM (0.5%) solution was added and stirred for 5 minutes. After standing and settling for 30 minutes, the supernatant was taken to test the concentrations of F-, Ca2+ and SO42-. The experimental results are shown in Figure 1-2. After the reaction, the concentration of F- decreased significantly, from 58290 mg / L to 14.41 mg / L, indicating that the lime precipitation method can effectively remove F-. Although it did not meet the national emission standards, most of the fluoride ions have been removed, and better treatment effects should be achieved after process adjustment. At the same time, the concentration of SO42- also decreased from 1960 mg / L to 70.08 mg / L, indicating that some Ca2+ combined with SO42- to form CaSO4 precipitate. It can be seen that the Ca2+ in the solution increased significantly before and after the reaction, indicating that a large amount of lime was used to neutralize the acidity.

[0059] The acidic wastewater generated during the graphite purification process was treated to investigate the effect of calcium oxide dosage on the fluoride ion concentration and pH of the effluent. Six groups of experiments were conducted. After determining the approximate CaO dosage range based on previous experiments, 200 mL of fluoride-containing acidic wastewater was taken and adjusted to different pH values. After sufficient stirring and precipitation under certain conditions, the fluoride ion concentration of the supernatant was measured. The results are shown in Figure 3. It can be seen from Figure 3 that as the pH gradually increases, the fluoride ion concentration first decreases linearly and then tends to be stable. When the calcium oxide dosage is too much, the fluoride ion concentration increases instead. Between pH 7 and 9, the fluoride ion concentration is not much different. At pH 8, the fluoride ion concentration reaches the lowest, which is 12.8 mg / L.

[0060] It can be found from Figure 4 that as the pH gradually increases, the amount of calcium oxide added also gradually increases. It is known from Figure 4 that when the pH is between 7 and 9, the fluoride ion concentration is not much different. Therefore, after the pH rises to 7 (the amount of calcium oxide added is 27.7g), continuing to add calcium oxide after the pH rises to 7 does not have a significant effect.

[0061] The acidic fluoride-containing wastewater used in this example has complex components, so the effects of other ions on the removal of fluoride by calcium oxide need to be considered. As shown in Figure 5, when the acidic wastewater is neutralized to different pH values, the residual Ca2+ concentration in the supernatant reaches its maximum at pH 2 and remains almost unchanged between pH 5 and 9. As shown in Figure 6, with the increase of pH, the Fe, Si and SO42- contents in the acidic wastewater are greatly reduced. This indicates that only a portion of the added lime is used to remove fluoride ions, and a large amount of Ca2+ still exists in the supernatant after the reaction and precipitation. KspCaF2 = [Ca2+ ][F - ] 2 (1-1)

[0062] The equilibrium constant of CaF2 precipitation and dissolution Ksp = 1.7×10 -10 If pH = 7, when the reaction reaches equilibrium, the supernatant [Ca 2+ ] is 30970mg / L, which is about 0.75mol / L. According to formula 1-1, at this time [F - ] is 1.5×10 -5 mol / L, or 0.285 mg / L, is far lower than the measured value. This may be because, in addition to fluoride ions, other metals in the solution, such as iron and aluminum, can form fluorine-containing complexes with fluoride ions, increasing the fluoride content in the water. Calcium sulfate and calcium fluorosilicate precipitates also form in the solution and adsorb on the surface of the calcium fluoride precipitate, hindering its formation.

[0063] Example 2

[0064] Effects of different pH values ​​on the amount and composition of lime-precipitated sludge

[0065] Neutralizing fluorine-containing acidic wastewater to different pH values ​​results in different sludge volumes and compositions after adding calcium oxide. Therefore, it is necessary to investigate the characteristics of the sludge in order to provide guidance for sludge reduction and treatment and disposal. The sludge volume is measured after settling for 2 hours after the reaction. The bottom mud is then taken out, filtered, and dried until the weight does not decrease. The dry mud is weighed. The experimental results are shown in Figures 7 and 8. As the pH increases, the amount of sludge produced also gradually increases, and the bottom mud concentration (bottom mud concentration = dry mud amount / sludge volume) also tends to increase. When the pH rises to above 6, the sludge volume and dry mud amount remain basically stable. At this time, the bottom mud concentration is about 10.7%.

[0066] After grinding the dried sludge, the sludge at pH 2, 5, 6, and 8 was taken for energy spectrum spectrum (EDS) analysis. The analysis results are shown in Figure 9. It can be seen that the components of the sludge when the fluorine-containing acidic wastewater is neutralized to different pH values ​​are basically the same, mainly Ca, Si, Cl, F, Al, O, S, Fe, etc.

[0067] As shown in Figure 10, for the treatment of mixed wastewater, the calcium fluoride content in the sludge after the addition of calcium oxide is between 33.6% and 46.7%. As the amount of calcium oxide added increases, the calcium fluoride ratio tends to decrease. This is because the calcium oxide is not fully utilized. Excessive calcium oxide is added to ensure the full removal of fluoride ions. The precipitate contains a lot of unreacted calcium oxide and calcium hydroxide components. The sludge also contains calcium sulfate, calcium fluorosilicate, and other precipitates generated by calcium ions and other impurities. In addition, the generated calcium fluoride particles will also wrap around the surface of the lime particles, hindering the further reaction of the precipitant inside the particles, reducing the efficiency of the calcium oxide, and thus resulting in the presence of unreacted calcium oxide in the bottom mud.

[0068] Based on the above analysis, for the treatment of acidic fluoride-containing wastewater, the precipitation pH should be pH 7 and the dosage of calcium oxide should be 142.5 g / L.

[0069] Example 3

[0070] Effect of sediment reflow-induced crystallization on fluoride removal

[0071] 1. Effect of single sludge return on fluoride removal

[0072] According to the above experimental results, 3 portions of 1L acidic fluoride-containing wastewater were taken respectively and neutralized with lime to pH 7. After adding a certain amount of PAM for flocculation and sedimentation, the reaction liquid was separated into solid and liquid, and the supernatant was tested for F- concentration. 25%, 50% and 80% of each portion of sludge were mixed with a certain amount of lime milk, and the untreated acidic fluoride-containing wastewater was neutralized to pH 7 with the mixed solution. After adding a certain amount of PAM for flocculation and sedimentation, the reaction liquid was separated into solid and liquid, and the supernatant was tested for F- concentration. The sludge was collected in a beaker for use.

[0073] The results of the sediment reflow experiment are shown in Figure 11. The reaction time is 30 minutes. When the sediment reflow rate is 25%, the F- concentration of the treated supernatant is 8.37 mg / L and the Ca2+ concentration is 22540 mg / L; when the sediment reflow rate is 50%, the F- concentration of the treated supernatant is 7.02 mg / L and the Ca2+ concentration is 20430 mg / L; when the sediment reflow rate is 80%, the F- concentration of the treated supernatant is 6.85 mg / L and the Ca2+ concentration is 20830 mg / L; while the F- concentration of the supernatant after treatment by ordinary lime precipitation method is 12.37 mg / L and the Ca2+ concentration is 24480 mg / L. It can be seen that sediment reflow has an improved effect on the removal of F-, and the slight decrease in the Ca2+ concentration in the supernatant indicates that the utilization rate of CaO has increased to a certain extent, but it is not obvious, which is related to the number of sediment reflows.

[0074] Since the treatment effect of 50% sediment return rate is slightly different from that of 80%, the 50% return rate was selected for subsequent experiments. As shown in Figure 12, there is basically no effect of sediment return or no return on the reaction equilibrium time, and both reach reaction equilibrium at 20 minutes. The large-scale removal of F- mainly occurs in the first 15 minutes of the reaction.

[0075] As shown in Figure 13, varying the amount of sediment recycle has no significant effect on CaO consumption or sediment concentration. This indicates that a single sediment cycle cannot change calcium oxide consumption and sediment concentration. The unused CaO in the sediment remains incompletely reacted, and the lack of an increase in sediment concentration indicates a weak effect of induced crystallization. Therefore, the goal of solid waste reduction cannot be achieved, and the number of sediment cycles must be increased.

[0076] 2. Effect of continuous sludge return on fluoride removal

[0077] 50% of the sludge from the previous cycle was mixed with lime milk and added to untreated acidic fluoride wastewater. Similarly, the supernatant F-concentration after different sludge cycles was obtained. As shown in Figure 14, after 7 cycles, the supernatant F-concentration was maintained at 6.24-7.62 mg / L, significantly improving the treatment effect compared to the conventional lime treatment method. The effect was relatively stable, meeting the F-<10 mg / L requirement of the Integrated Wastewater Discharge Standard and essentially meeting the F-<8 mg / L required by the project. The Ca2+ concentration in the supernatant remained at approximately 20,000 mg / L with increasing cycles, a decrease of approximately 20% compared to the case of no sludge recycling. This also demonstrates that the sludge recirculation-induced crystallization method can improve Ca utilization. This is because the sludge particles adsorb calcium hydroxide and then enter the reaction tank to react with the fluoride wastewater. The newly generated calcium fluoride, generated by the sludge recirculation, acts as a crystallization center, adsorbing and precipitating it, reducing its solubility.

[0078] As shown in Figures 14 and 15, as the number of sludge recirculations increases, the sludge concentration also increases, and the sludge settling ratio gradually decreases. The sludge concentration increases from 10.7% to approximately 12.5%, and the sludge volume percentage decreases from 85% to approximately 72%. The sludge settling performance is significantly improved, and the settled sludge has good water permeability, a dense structure, and is difficult to compress. The calcium oxide dosage gradually decreases from 14.25g / 100mL to approximately 12.8g / 100mL, reducing lime consumption by 10.2% year-on-year. Since sludge recirculation is an internal system cycle, the input and output materials of the entire system are equivalent. Therefore, adding 10.2% less lime, or 1.45g / 100mL, results in 1.45g / 100mL less sludge. As shown in Figures 5-8, when the fluorine-containing wastewater is neutralized to pH 7, the sludge production is 14g / 100mL, thereby reducing the fluorine-containing sludge by 10.4% (1.45 / 14), which has a more obvious effect on reducing the amount of fluorine-containing solid waste.

[0079] Due to the small particle size of calcium fluoride in the wastewater treatment process, after ordinary lime treatment, the turbidity of the supernatant is high within 30 minutes of sedimentation, it is not clear enough, and the sedimentation rate is slow. As can be seen from Figure 16, with the increase in the number of sediment circulation times, the turbidity of the supernatant is significantly reduced after 30 minutes of sedimentation, from 36NTU to about 8NTU. The above experimental results all show that sediment return has a positive effect on accelerating sludge sedimentation, reducing sludge moisture content, and reducing sludge volume.

[0080] As shown in Figure 17, with the increase in the number of sludge circulation times, the proportion of calcium fluoride increased slightly, from 35.5% to 36.5%~41.4%. This also indirectly reflects that the utilization of CaO is more sufficient than before. The reduction in the amount of CaO added leads to a decrease in the content of unreacted CaO in the sludge, which leads to a decrease in the total amount of sludge and an increase in the proportion of calcium fluoride in the sludge.

[0081] Figure 18 shows scanning electron microscope images of sediment from the conventional lime treatment method and the sediment recirculation-induced crystallization method. It can be seen that the sediment particles from the sediment recirculation-induced crystallization method have numerous fine particles formed on their surfaces, resulting in larger and denser sludge particles. Particle size analysis of the sediment from the conventional lime treatment method and the sediment recirculation-induced crystallization method was performed using a laser particle size analyzer. Figure 19 shows the sludge particle size distribution, showing that the particle size of sediment particles gradually increases with the number of sediment cycles. The average particle size of the sludge from the conventional lime treatment method is 7.715 μm, while that from the sediment recirculation-induced crystallization method is 9.941 μm, an increase of 28.9%. This demonstrates that sediment recirculation-induced crystallization can increase the size of CaF2 crystals. The increased sediment density accelerates sedimentation, resulting in significant fluoride removal and significantly improved sludge dewatering performance.

[0082] 3. Effect of adding crystal seeds to sludge reflux induced crystallization on fluoride removal

[0083] On the basis of induced crystallization by sediment reflow, a certain amount of additional crystal seeds are added to enhance the induced crystallization effect in order to further improve the fluoride removal effect. In this experiment, low-cost natural crystal seeds calcite and quartz sand are selected and compared with calcium fluoride crystal seeds to examine the effects of the addition method and dosage of the three crystal seeds on the fluoride removal effect.

[0084] (1) Two-step neutralization

[0085] First, the sludge after multiple cycles was mixed with lime milk and added to 1L of untreated acidic fluoride-containing wastewater. When the pH was neutralized to 4 (to prevent the seed crystals from being dissolved by hydrofluoric acid), calcite, quartz sand, and calcium fluoride seed crystals were added at dosages of 0.5g, 1g, and 2g, respectively. After uniform stirring, the sludge / lime mixture was continued to be added to neutralize the acidic fluoride-containing wastewater to pH 7. After coagulation and precipitation with PAM, the supernatant was collected and tested for F- concentration. The experimental results are shown in Figure 20. All three types of added seed crystals improved the fluoride removal efficiency of sludge recirculation-induced crystallization. The optimal dosage of calcium fluoride seed crystals was 1g / L, resulting in an effluent fluoride concentration of 5.25mg / L. The optimal dosage of quartz sand seeds is 2g / L, and the fluorine concentration in the effluent is 5.81mg / L. The optimal dosage of calcite seeds is 1g / L, and the fluorine concentration in the effluent is 4.92mg / L. Compared with the case without added seeds, the fluorine removal by sludge reflux induced crystallization can be improved to 7mg / L to a certain extent. Among them, the use of calcite as the added seed has the best effect.

[0086] (2) Synchronous neutralization

[0087] Calcite, quartz sand, and calcium fluoride were first added to 1L of untreated acidic fluoride-containing wastewater at dosages of 0.5g, 1g, and 2g, respectively. After rapid stirring, the mixture of sludge and lime milk after multiple cycles was added. The acidic fluoride-containing wastewater was directly neutralized to pH 7. After adding PAM for coagulation and precipitation, the supernatant was collected to test the F- concentration. The experimental results are shown in Figure 21. All three types of added crystal seeds can improve the fluoride removal effect of sludge recirculation-induced crystallization. The optimal dosage of calcium fluoride crystal seeds is 1g / L, and the fluoride concentration in the effluent is 5.35mg / L. The optimal dosage of quartz sand crystal seeds is 2g / L, and the fluoride concentration in the effluent is 5.18mg / L. Compared with the case without added crystal seeds, the fluoride removal rate can be reduced to 7mg / L by sludge recirculation-induced crystallization, which is a certain degree of improvement. The optimal dosage of calcite crystal seeds is 1g / L, and the fluoride concentration in the effluent is 6.81mg / L. In this case, the use of calcite as an added crystal seed has little effect on the fluoride removal effect.

[0088] It can be seen that the addition of crystal seeds can improve the effect of fluoride removal by induced crystallization of sediment reflow. The optimal process is to first neutralize the acidic fluoride-containing wastewater to a certain degree (about pH 4) with a sediment / lime mixture, and then add calcite crystal seeds with an injection amount of 1g / L. The fluoride concentration in the effluent can be reduced to 4.92mg / L, which is 28% higher than that of the single sediment reflow induced crystallization defluoridation effect, and can stably meet the first-level standard in the "Integrated Sewage Discharge Standard".

[0089] Figure 22 is a scanning electron microscope image of the sedimentation sludge. It can be seen that the sludge particles with calcite as an external seed are significantly larger than the sludge particles without the external seed. The particle size of the two sludges was analyzed by a laser particle size analyzer. Figure 23 is a particle size distribution diagram of the sludge. It can be seen that the average particle size of the sludge particles without the external seed is 9.941μm, while the average particle size of the sludge with calcite as an external seed is 12.844μm, an increase of 29.2%. Compared with the sludge particle size of the ordinary lime method (7.741μm), it increases by 65.9%. This proves that the synergistic effect of the added calcite seeds and the sludge reflux can further induce the formation and growth of CaF2 crystals, and continue to improve the fluorine removal effect.

[0090] In summary, the high-density sludge recirculation induced crystallization defluoridation process is a new treatment process developed based on the calcium salt precipitation defluoridation process. It offers the advantages of improved defluoridation effectiveness, consistent effluent quality, increased utilization of added lime, and larger sludge particle size, resulting in faster sludge settling. Its process prototype, the high-density slurry method (HDS), is an acidic wastewater treatment process developed by the Mining and Metallurgical Technology Group Co., Ltd. It is an improvement on the traditional lime neutralization method. The main process flow is the same as the traditional lime method. The main difference is that the bottom sludge in the sedimentation tank is returned, pre-reacted with lime milk, and then neutralized with the acidic wastewater. It can effectively save lime consumption, increase sludge concentration, and accelerate sedimentation and separation. Its fluoride removal principle is that the bottom sludge containing calcium fluoride and other particulate fluoride is returned to the HDS reaction system to promote the growth of calcium fluoride crystals on the above-mentioned solid surface and the generation of new calcium fluoride crystals, and to make colloid adsorption, co-precipitation and induced crystallization occur simultaneously, and then through the bridging and capture of polymer flocculants, the characteristics of high HDS bottom sludge concentration and high specific gravity are used to quickly realize mud and water separation, thereby achieving the purpose of wastewater fluoride removal.

[0091] Return sludge can enhance the contact flocculation effect:

[0092] In the reactor, F- reacts with Ca2+ to generate a large number of CaF2 colloidal particles. To make these CaF2 particles coagulate and flocculate, the collision speed and probability between them must be increased. Generally, the driving force for CaF2 particle collision in water mainly comes from two points: first, the Brownian motion of the particles in water. The collision caused by Brownian motion belongs to "heterotropic flocculation". According to Fick's law, the collision speed Np between particles can be expressed as: N p =8πdD B n 2 (1-2)

[0093] Where: Np is the collision velocity between particles, 1 / cm3·s;

[0094] n——number of particles, pieces / cm3;

[0095] d——diameter of the particle, cm;

[0096] DB - diffusion coefficient of Brownian motion, cm2 / s;

[0097] Secondly, there is the "co-directional flocculation" between particles due to stirring and other effects. Similarly, the collision velocity N0 between particles can be expressed as:

[0098] Where: N0——interparticle collision velocity, 1 / cm3·s;

[0099] n——the number of particles, pieces / cm3;

[0100] d——diameter of the particle, cm;

[0101] G——Velocity gradient, s-1.

[0102] From the above, we can see that no matter whether it is co-directional flocculation or hetero-directional flocculation, the number of collisions between particles is the same as that of particles.

[0103] In general, other conditions such as particle diameter, Brownian motion expansion,

[0104] The scatter coefficient cannot be changed. When the velocity gradient remains unchanged, increasing the number of particles can significantly increase the particle size.

[0105] The collision speed between particles is significantly promoted, which significantly promotes the coagulation and flocculation of particles. In the high-density sludge recirculation induced crystallization process, after mixing part of the sludge with lime, it is added to the acidic fluoride-containing wastewater. This can greatly increase the number of particles in the mixing reaction tank, increase the probability of collision between particles, and promote the coagulation of particles.

[0106] The sludge return process is equivalent to increasing the amount of lime added:

[0107] When treating acidic fluoride-containing wastewater with the traditional lime precipitation process, a large amount of lime is often added due to the high acidity and fluoride content of the wastewater. Lime generates calcium hydroxide when it comes into contact with water, which is a slightly soluble electrolyte. After a large amount of addition, the utilization efficiency will become low. During the reaction, calcium fluoride particles generated by the reaction of fluoride ions and calcium ions will adhere to the surface of the calcium hydroxide particles, hindering the continued dissolution of the calcium hydroxide particles. The undissolved lime particles cause a huge waste of lime and also increase the discharge of fluoride-containing sludge. When the high-density sludge return induced crystallization process is adopted, the internal dynamic circulation process formed by the sludge return in the reaction greatly increases the utilization rate of lime, thereby reducing the discharge of fluoride-containing sludge.

[0108] Return sludge contains a large number of calcium fluoride crystal nuclei, which can promote and adsorb the formation of calcium fluoride:

[0109] When the high-density sludge reflow process is used to treat acidic, high-concentration fluoride-containing wastewater, calcium fluoride crystal nuclei exist in the reflow sludge. The sludge is circulated in a certain proportion, and the precipitated sludge is coarsened and crystallized through induced crystallization and adherent crystallization. The crystal seeds brought by the circulating sludge can provide a carrier or place for the reactants to attach and deposit. These crystal nuclei have a strong adsorption nucleation effect on the calcium fluoride particles suspended in the water, promoting the further generation of calcium fluoride particles from fluoride ions and calcium ions. By adding external crystal seeds (calcite, etc.), the appearance of crystal nuclei can be accelerated, and the ions participating in the reaction are enriched on their surface, thereby causing the local ion concentration to increase to an oversaturated state, further improving the fluoride removal effect of the high-density sludge reflow induced crystallization process.

[0110] Experimental results: The high-density sludge reflux induced crystallization process can reduce the fluorine content in the acidic fluorine-containing wastewater produced by graphite purification from 58920 mg / L to 4.92 mg / L, with a removal rate of 99.99%, which has reached the first-level standard of the "Integrated Sewage Discharge Standard".

[0111] This patent develops a high-density sludge reflux induced crystallization defluorination process for high-concentration fluorine-containing and acid-containing wastewater in the graphite purification industry. The process flow chart is shown in Figure 24.

[0112] High-concentration acidic fluoride-containing wastewater is first collected into a regulating tank for homogenization and equalization, and then enters the HDS reactor, where a lime slurry mixture is added and calcite seeds are added to it. The pH of the HDS reactor is controlled at 7, and the stirring reaction time is 60 minutes. After that, it enters the HDS sedimentation tank to remove 99.99% of the fluoride in the wastewater. At this time, the supernatant of the sedimentation tank contains 5 mg / L of fluoride. The sedimentation sludge is partially returned to the seed pre-reactor with a reflux ratio of 50%. The remaining sludge is filtered by a filter press and disposed of as fluoride-containing solid waste. The residual fluoride in the precipitated water is reacted in a deep defluorination reaction tank by adding calcium chloride (400 mg / L), PAC (400 mg / L) and PFS (200 mg / L). After the fluoride and the defluoridation agent produce a complex precipitate, lime and PAM are added to adjust the pH and flocculation reaction. After that, it flows into the sedimentation tank for mud and water separation, and the clean water (fluoride can be reduced to ≤5.0 mg / L after defluoridation) meets the discharge standards.

[0113] Process characteristics:

[0114] ① The comprehensive treatment of "high-density sludge reflux induced crystallization + deep fluoride removal" is adopted, and the technical process is simple and efficient;

[0115] ② This project adopts advanced high-density sludge return induced crystallization treatment technology. Compared with the traditional lime neutralization method, this technology can reduce the amount of lime used by 10.2%, increase the sludge concentration, and reduce the output of fluorine-containing solid waste by 10.4%. At the same time, it can reduce fluorine to a lower level than the traditional lime method, which is conducive to subsequent deep fluoride removal.

[0116] In summary

[0117] 1. An experiment on treating fluoride-containing wastewater by lime precipitation was carried out. The experimental results showed that when CaO was used to neutralize the wastewater to pH 7, most of the fluoride could be removed. Without sludge circulation, the fluoride ion concentration of the supernatant after the reaction could reach about 12-13 mg / L. The lime consumption was 142.5 kg / t water, and the output of fluoride-containing solid waste was 140 kg / t water.

[0118] 2. After treatment with the sediment recirculation-induced crystallization method, the fluoride ion concentration in the supernatant was 7 mg / L, meeting the comprehensive sewage discharge standard (F-<10 mg / L). Lime consumption was 128 kg / t water, a 10.2% decrease in lime consumption, and CaO utilization increased. Sludge concentration increased, sludge particle size increased, and sedimentation performance improved. The output of fluorine-containing solid waste was 125.5 kg / t water, a 10.4% decrease. The average particle size of the sludge treated with the sediment recirculation-induced crystallization method was 9.941 μm, an increase of 28.9%. On this basis, calcite was added as an external crystal seed, and the average sludge particle size was 12.844 μm, a 65.9% increase compared to the sludge particle size treated with the conventional lime method. This proves that sludge recirculation-induced crystallization can cause CaF2 crystals to grow.

[0119] 3. Returned sludge can increase the number of particles in the mixed reaction tank, increase the probability of collision between particles, and promote the cohesion of particles. When the high-density sludge return induced crystallization process is adopted, the internal dynamic circulation process formed by the sludge return in the reaction greatly increases the utilization rate of lime, thereby reducing the discharge of fluorine-containing sludge. When the high-density bottom sludge return process is used to treat acidic, high-concentration fluorine-containing wastewater, calcium fluoride crystal nuclei exist in the returned sludge. The bottom sludge is circulated at a certain ratio, and the precipitated bottom sludge is coarsened and crystallized through induced crystallization and adhesion crystallization. The crystal seeds brought by the circulating bottom sludge can provide a carrier or place for the reactants to attach and deposit. These crystal nuclei enrich the ions participating in the reaction on their surface, thereby causing the local ion concentration to increase to a supersaturated state, further enhancing the fluoride removal effect of the high-density bottom sludge return induced crystallization process.

[0120] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

[0121] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A process for deep defluorination of wastewater by sludge reflow induced crystallization method, comprising the following steps: Step 1, lime powder and deionized water are placed in a seed reactor to prepare lime milk, and calcite seeds are added into the seed reactor at the same time; Step 2, introducing high-concentration acidic fluoride-containing wastewater into the reactor; Step 3, introducing the lime milk of step 1 into the reactor of step 2 for reaction, and controlling the pH value of the mixed solution to be kept neutral during the reaction; Step 4: Based on step 3, after a period of reaction, PAM is added into the reactor to continue stirring the reaction; Step five, the mixed liquid after the reaction in step four is discharged into a sedimentation tank for static sedimentation, and the low-concentration fluorine-containing wastewater after precipitation is discharged, and the precipitated fluorine-containing sludge is detected by a sludge concentration meter. When the fluorine concentration is high, part of the precipitated fluorine-containing sludge is discharged into the seed reactor, and part of the sludge is discharged after filter pressing. When the concentration is low, the precipitated sludge is returned to the seed reactor.

2. A process for deep defluorination of wastewater by sludge reflow induced crystallization method according to claim 1, characterized in that: In the step 1, the weight ratio between lime powder and deionized water is 2:

20.

3. A process for deep defluorination of wastewater by sludge reflow induced crystallization according to claim 1, characterized in that: In the step 2, when the high-concentration acidic fluorine-containing wastewater is introduced into the reactor, a regulating tank is used to buffer the high-concentration acidic fluorine-containing wastewater, and the buffered high-concentration acidic fluorine-containing wastewater then enters the reactor.

4. A process for deep defluorination of wastewater by sludge reflow induced crystallization according to claim 1, characterized in that: In the step three, the weight ratio between the lime milk and the high-concentration acidic fluoride-containing wastewater is 27:20, and the reaction time of the two is 30-90 minutes.

5. The process for deep defluorination of wastewater by sludge reflow induced crystallization method according to claim 1, characterized in that: In the step 3, the pH value of the mixed solution is 7-9.

6. A process for deep defluorination of wastewater by sludge reflow induced crystallization according to claim 5, characterized in that: In step 3, the pH value of the mixed solution is 7.

7. The process for deep defluorination of wastewater by sludge reflow induced crystallization method according to claim 1, characterized in that: The weight ratio of PAM, lime milk and high-concentration acidic fluoride-containing wastewater is 2:27:20.

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