A method for recycling high-concentration fluorine-containing waste liquid

Abstract

The application discloses a resource recycling method of high-concentration fluorine-containing waste liquid, and adopts two-stage mixed separation to recycle calcium fluoride, controls the Ca / F molar ratio in a first-stage mixed reaction tank to be 0.38-0.43, so that the system is in a high supersaturation region of calcium fluoride under the condition of fluorine ion excess; and controls the Ca / F molar ratio in a second-stage mixed reaction tank to be 0.50-0.60, so that the system is in a state of calcium ion excess, which is beneficial to homogeneous nucleation to form a large number of CaF2 crystal nuclei, and the fluorine removal rate is 98-99%. ‑ The method can not only make the high-concentration fluorine-containing waste liquid with F >10000mg / L reach the standard for discharge, but also recycle high-purity calcium fluoride, and realizes resource recycling of fluorine.

Description

Technical Field

[0001] This invention relates to a method for the resource recovery of high-concentration fluoride-containing waste liquid. Background Technology

[0002] The integrated circuit and semiconductor manufacturing processes utilize large quantities of fluorinated chemicals (such as hydrofluoric acid, ammonium fluoride, and other fluorinated compounds) in cleaning, etching, and polishing stages, generating substantial amounts of fluorinated wastewater. Especially in critical processes like etching and cleaning, the fluoride ion concentration in the wastewater can often exceed 10,000 mg / L, frequently accompanied by high salinity and coexisting ions such as silicon and sulfate. This results in significant acid-base fluctuations, complex composition, and high treatment difficulty. Direct discharge without effective treatment not only causes environmental pollution but also leads to substantial waste of fluorine resources due to the loss of the mother liquor.

[0003] Furthermore, calcium fluoride, as a key raw material in fluorine-containing chemicals (such as hydrofluoric acid production), can, to some extent, substitute for natural fluorite resources due to its high purity, possessing high industrial value and economic added value. Therefore, regarding F... - Achieving efficient defluorination and high-purity resource recovery of high-concentration fluoride-containing wastewater with concentrations above 10,000 mg / L has become a development goal in the environmental and resource recovery fields. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a method for the resource recovery of high-concentration fluoride-containing wastewater. This method can not only recover the fluoride... - High-concentration fluoride wastewater with a concentration of >10000mg / L can be discharged in compliance with standards, and high-purity calcium fluoride can be recovered, realizing the resource recovery of fluoride.

[0005] Technical solution: The present invention describes a method for the resource recovery of high-concentration fluoride-containing wastewater, wherein the method employs a two-stage chemical mixing and fractional recovery of calcium fluoride, and the specific steps are as follows:

[0006] (1) Fluorine-containing waste liquid with a fluorine concentration >10000mg / L after homogenization in the conditioning tank enters the primary chemical mixing reaction tank. The primary chemical mixing reaction tank includes at least a first stirring tank I and a second stirring tank I connected in sequence. An insufficient amount of alkaline calcium source is introduced into the first stirring tank I to make the Ca / F molar ratio 0.38~0.43. At this time, the system is in the supersaturated region of calcium fluoride under excess fluoride ions. The effluent from the first stirring tank I enters the second stirring tank I. The pH of the liquid in the second stirring tank I is adjusted to 4.5~5.5. By utilizing the ultra-low solubility of calcium fluoride molecules, the effective fluoride ions in the system preferentially react with calcium ions to inhibit the co-precipitation of impurities. The effluent from the primary chemical mixing reaction tank enters the primary sedimentation tank for solid-liquid separation.

[0007] (2) The effluent from the primary sedimentation tank enters the secondary chemical mixing reactor, which also adopts a three-stage series structure, namely the first stirring tank II, the second stirring tank II, and the third stirring tank II connected in sequence; an excess of neutral calcium source is introduced into the first stirring tank II to make the Ca / F molar ratio 0.50~0.60, so as to achieve rapid salt formation of residual fluoride; the effluent from the first stirring tank II enters the second stirring tank II, and the pH of the liquid in the second stirring tank II is adjusted to 4.5~5.5. Then the effluent from the second stirring tank II enters the third stirring tank II, and the pH of the liquid in the third stirring tank II is adjusted to 7. Then a polymer flocculant and coagulant are added to it, and low-shear stirring is used to promote the formation of larger flocs from fine calcium fluoride particles, thereby improving the sedimentation and filtration efficiency; the effluent from the secondary chemical mixing reactor enters the secondary sedimentation tank for solid-liquid separation, and the effluent from the secondary sedimentation tank F - Stable concentration less than 20 mg / L;

[0008] (3) During operation, the calcium fluoride sludge formed in the primary sedimentation tank is periodically precipitated and discharged to obtain calcium fluoride sludge A, and the purity of calcium fluoride sludge A is ≥94%; the calcium fluoride sludge formed in the secondary sedimentation tank is periodically precipitated and discharged to obtain calcium fluoride sludge B; calcium fluoride sludge B is purified by alkali washing and acid washing in sequence, and after purification, the purity of calcium fluoride sludge B is ≥90%.

[0009] The fluoride-containing waste liquid is first homogenized in a regulating tank, and then pumped at a rate of 6.84 L / h to the first stirred tank I of the primary mixing reaction tank. Simultaneously, a 4% (mass fraction) calcium hydroxide solution is continuously added to the first stirred tank I by a metering pump at a rate of 2.7 L / h. An upward-flowing dosing pipe is preferred for the dosing pipeline to enhance micro-mixing and reduce localized dead zones. The Ca / F molar ratio is controlled at 0.38–0.43 in the first stirred tank I to maintain the system in a highly supersaturated calcium fluoride region under conditions of excess fluoride ions. Based on the calcium fluoride precipitation reaction, Ca… 2+ +2F - →CaF2(s), when the ion product of the ion activities (which can be approximated by concentration) in the solution [Ca 2+ ][F - ] 2 When the concentration exceeds the threshold corresponding to its dissolution equilibrium, supersaturation occurs and crystallization takes place. Higher supersaturation levels more readily trigger homogeneous nucleation, generating numerous fine crystal nuclei. Crystal growth and achieving good sedimentation and filtration performance require surface growth and maturation under low shear and stable chemical conditions. Therefore, this invention divides the primary mixing reaction tank into three sections: the first section involves rapid mixing for instantaneous salt formation and crystal nucleation; the effluent then enters the second section's stirred tank, where 40 wt.% NaOH solution is added via online pH meter feedback to stabilize the waste liquid's pH at 4.5-5.5, ensuring stable and effective filtration. -The liquid is leveled and unfavorable co-precipitation tendencies are suppressed. It then enters the third-stage stirred tank for low-shear, slow stirring to ripen and grow the crystals, preventing crystal breakage and improving separability. The waste liquid flows into the primary sedimentation tank for solid-liquid separation. The turbidity of the effluent from the primary sedimentation tank is consistently less than 100 NTU, and the F content in the effluent from the primary sedimentation tank is [not specified]. - The concentration, measured online by a fluoride meter, was 1500~3000 mg / L. The effluent from the primary sedimentation tank entered the secondary chemical mixing reactor. The secondary chemical mixing reactor also employed a three-stage series structure. The first stage used a 30 wt.% calcium chloride solution as the calcium source, controlling the Ca / F molar ratio at 0.50~0.60 to achieve rapid salt formation from residual fluoride. The second stage, also using online pH feedback, adjusted the pH to 4.5~5.5 with a 40 wt.% NaOH solution. The third stage further adjusted the pH to approximately 7 before adding 800 mg / L of PAC (polyaluminum chloride) and 5 ppm of CPAM (cationic polyacrylamide), followed by low-shear stirring to promote the formation of larger flocs from fine calcium fluoride particles, improving sedimentation and filtration efficiency. The effluent then entered the secondary sedimentation tank for solid-liquid separation. The effluent from the secondary sedimentation tank contained F... - The concentration of calcium fluoride sludge is stable at less than 20 mg / L. During operation, calcium fluoride sludge A is preferably collected through the sludge discharge pipe every 3 hours in the primary sedimentation tank. After washing with water, the CaF2 purity can reach over 94%. Calcium fluoride sludge B is preferably collected through the sludge discharge pipe every 6 hours in the secondary sedimentation tank. Its CaF2 purity can reach over 75%. For calcium fluoride sludge B with lower purity, after alkaline washing with sodium carbonate solution at 50 g / L and 70℃ (the alkaline washing process uses carbonate ions to replace sulfate ions in calcium sulfate), acid washing is performed using fluoride-containing wastewater in the equalization tank (the acid washing process uses fluoride ions to replace carbonate ions in calcium carbonate. Through alkaline washing and acid washing, calcium sulfate is removed, and calcium fluoride is generated). The HF concentration is preferably controlled at 0.5 mol / L. Under the condition of selectively removing impurities and surface impurities, calcium fluoride product C is obtained, and its CaF2 purity can be increased to over 90%.

[0010] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The present invention adopts a three-stage chemical mixing in the primary chemical mixing reaction tank. In the first stage (first stirring tank I), the Ca / F molar ratio is controlled at 0.38~0.43 by adding an under-equivalent amount of alkaline calcium source (preferably lime milk or calcium hydroxide), so that the system is in a state of calcium ion deficiency and fluoride ion excess, which is conducive to reducing the formation of impurities such as calcium sulfate and calcium phosphate. While meeting the requirement that the fluoride removal rate is 75~80%, the purity of sludge A (CaF2≥94wt.%) is improved. It can also achieve the initial pH adjustment and reduce the use of liquid alkali. In the third stage (third stirring tank I), the final pH is stabilized at about 5±0.5 and maintained at low shear. (1) Under the condition of shear mismatch, the tendency of crystal breakage and secondary nucleation caused by shear mismatch can be reduced, which is conducive to obtaining a solid phase morphology that is easier to settle and filter (to obtain larger calcium fluoride crystals); (2) In view of the problem that the secondary generated solid phase is easy to be fine particles and difficult to settle and filter, the present invention controls the Ca / F molar ratio at 0.50~0.60 by adding an excessive amount of neutral calcium source (preferably calcium chloride) in the first stage of the secondary chemical mixing reaction tank (first stirring tank II), so that the system is in a state of calcium ion excess, which is conducive to homogeneous nucleation to form a large number of CaF2 crystal nuclei, and the fluoride removal rate is 98~99%; in the third stage (third stirring tank II), the pH is first raised to a higher stage (preferably 7.0~9.0) and then PAC 800mg / L and CPAM 5 ppm are added, and flocculation is carried out under low shear conditions, which can promote the formation of larger flocs of fine CaF2 particles and suspended impurities, thereby improving the settling and filtration efficiency of the secondary sedimentation tank, and the generated sludge B has a CaF2 content ≥75wt.%. (3) Sludge B is first fully contacted with 50 g / L sodium carbonate solution at 70°C for alkaline washing. Desulfurization is achieved through the conversion and dissolution of sulfur-containing impurities by sodium carbonate, reducing the content of sulfur impurities in the solid phase and weakening the encapsulation and entrainment of sulfate impurities on calcium fluoride crystals. After solid-liquid separation by alkaline washing, fluorine-containing wastewater in the equalization tank is used as the acid washing medium, and the HF concentration is adjusted to 0.5 mol / L. The solid phase after alkaline washing is acid washed to further remove residual impurities and promote the enrichment of calcium fluoride phase. The CaF2 content of sludge C after purification is ≥90 wt.%. (4) The method of this invention treats fluorine-containing wastewater with a fluorine concentration of 10000 mg / L. The final fluorine recovery rate is greater than 98%, and the average size of the obtained calcium fluoride crystals is 29.586 μm, and the maximum particle size can reach 76.725 μm. Attached Figure Description

[0011] Figure 1 This is a flowchart of the method of the present invention;

[0012] Figure 2 EDS diagram of calcium fluoride (sludge A) recovered in Example 1;

[0013] Figure 3 SEM image of the calcium fluoride crystals (sludge A) recovered in Example 1;

[0014] Figure 4 EDS diagram of calcium fluoride (sludge B) recovered in Example 1;

[0015] Figure 5 SEM image of calcium fluoride crystals (sludge B) recovered in Example 1;

[0016] Figure 6 EDS diagram of calcium fluoride (sludge C) recovered in Example 1;

[0017] Figure 7 SEM image of calcium fluoride crystals (sludge C) recovered in Example 1. Detailed Implementation

[0018] Example 1

[0019] The method of this invention was used to treat the following fluoride-containing wastewater: fluoride ion concentration of 134380 mg / L, total silicon concentration of 23870 mg / L, nitrate ion concentration of 152 mg / L, chloride ion concentration of nd mg / L, sulfate ion concentration of 114 mg / L, water flow rate of 0.5 ton / h, and operating time of 24 hours. The specific steps are as follows:

[0020] (1) Introduce the fluoride-containing waste liquid into the equalization tank, add tap water to dilute to a fluoride concentration of 10000 mg / L; turn on the stirring to homogenize and stabilize the liquid level, and use a flow pump to continuously transport the waste liquid to the first stirring tank I of the primary chemical mixing reaction tank at a rate of 6.84 L / h; at the same time, prepare a 4% (mass fraction) calcium hydroxide solution and put it into the reagent storage tank, and continuously add it to the first stirring tank I of the primary chemical mixing reaction tank at a rate of 2.7 L / h through a metering pump. The addition pipeline adopts an upward distribution pipe to enhance the instantaneous mixing with the incoming water and reduce the local addition dead zone;

[0021] (2) In the first stirred tank I of the primary chemical mixing reactor, the Ca / F molar ratio is maintained at 0.38~0.43, so that the system is in the supersaturated region of calcium fluoride with excess fluoride ions, which is conducive to homogeneous nucleation and the formation of a large number of CaF2 crystal nuclei; then the effluent from the first stirred tank I enters the second stirred tank I, and 40wt.% NaOH solution is added to the waste liquid of the second stirred tank I through the online pH meter feedback linkage metering pump to adjust the pH of the waste liquid and stabilize it at 4.5~5.5; the effluent from the second stirred tank I enters the third stirred tank I, where the waste liquid is slowly stirred with low shear (stirring speed is 20rpm), and the pH at the end of the third stirred tank I is usually stable at about 5; the effluent from the primary chemical mixing reactor enters the primary sedimentation tank for solid-liquid separation, and the turbidity of the effluent from the primary sedimentation tank is less than 100. NTU, the fluoride ion concentration in the effluent (mother liquor) of the primary sedimentation tank is 1500~3000mg / L, and this mother liquor is continuously fed into the secondary chemical mixing reaction tank (flow rate is 0.8~1.0m / s).

[0022] (3) The secondary mixing reactor also adopts a three-stage series structure, namely the first stirring tank II, the second stirring tank II and the third stirring tank II connected in sequence. A 30 wt.% calcium chloride solution (flow rate of 0.5 L / h) is introduced into the first stirring tank II, and the Ca / F molar ratio is controlled at 0.50~0.60 to achieve rapid salt formation of residual fluoride. The effluent from the first stirring tank II enters the second stirring tank II, and a 40 wt.% NaOH solution is added to the second stirring tank II through an online pH meter feedback linkage metering pump. In the waste liquid of II, the pH of the waste liquid is adjusted to 4.5~5.5; the effluent from the second stirring tank II enters the third stirring tank II, the pH of the liquid in the third stirring tank II is first adjusted to 7, and then PAC and CPAM are added to it. The dosage of PAC is 800mg / L and the dosage of CPAM is 5ppm, and the mixture is stirred slowly with low shear (stirring speed is 20rpm). Subsequently, the effluent from the secondary chemical mixing reaction tank enters the secondary sedimentation tank for solid-liquid separation. The fluoride ion concentration in the effluent of the secondary sedimentation tank is less than 10mg / L.

[0023] (4) During operation, calcium fluoride sludge is collected by cyclical discharge. Calcium fluoride sludge A is discharged and collected through the sludge discharge pipe every 3 hours in the primary sedimentation tank, and calcium fluoride sludge B is discharged and collected through the sludge discharge pipe every 6 hours in the secondary sedimentation tank. After solid-liquid separation and water washing, it is dried and ground. The CaF2 purity in sludge A can reach more than 94wt.%, and the CaF2 purity in sludge B can reach more than 75wt.%.

[0024] (5) The low-purity calcium fluoride sludge B was purified by sampling at a solid-liquid ratio of 1:10 (1g sludge powder: 10mL HF waste liquid or 10mL sodium carbonate solution). The sludge powder was first alkali-washed in 10mL sodium carbonate solution at 50g / L and 70℃ for 30min, and then acid-washed with fluoride-containing waste liquid in the equalization tank. The HF concentration in the acid washing solution was 0.5mol / L. After acid washing for 60min, it was allowed to stand for 2h for solid-liquid separation, and washed until near neutral and dried to obtain calcium fluoride product C. The CaF2 purity could be increased to over 90wt.%. The mass of sludge A obtained was 2.81kg / d; the mass of sludge C was 0.73kg / d; and the calculated fluorine recovery rate was 98.7%.

[0025] Table 1 shows the hydraulic residence time and stirring speed of each stirred tank in the primary mixing reactor for fluorine-containing waste liquid, as well as the hydraulic residence time and stirring speed of each stirred tank in the secondary mixing reactor.

[0026] Table 1

[0027]

[0028] The compositional analysis of sludge A, sludge B and sludge C recovered in Example 1 is shown in Table 2.

[0029] Table 2

[0030]

[0031] pass Figures 2-3 As can be seen, the fluoride and calcium peaks of sludge A obtained in Example 1 are prominent, indicating that the precipitate is mainly composed of calcium fluoride. The overall structure of the sludge particles is a super-large dense aggregate, with an overall size much larger than 1 μm, exhibiting continuous cross-linked flocculent lumps, and a very small amount of free, dispersed small particles. This indicates that in the high-concentration HF system, Ca... 2+ F - With extremely high supersaturation, a large number of nanocrystal nuclei burst forth instantaneously. The nuclei simultaneously and rapidly aggregate and cross-link, resulting in low impurities (high purity) and no interference from external flocs, forming continuous, dense, large flocs. The crystals do not have a clearly regular geometric shape and are mainly composed of amorphous nano-aggregates.

[0032] pass Figures 4-5 It can be seen that the fluoride peak of sludge B obtained in Example 1 weakened, the fluoride content in the precipitate decreased, while the calcium peak remained high, indicating that the precipitate was mainly composed of calcium-containing substances, but the proportion of calcium fluoride decreased. The sludge particles were highly dispersed and fragmented, without continuous large agglomerates. The particles were small, loose flocs of 0.2~0.8μm; the gaps between the particles were huge, loose and independent, without continuous cross-linking structures. This indicates that in the secondary reaction, F -With low concentration and low supersaturation, the formation rate of CaF2 crystal nuclei is slow and the number is small; at the same time, the system contains impurities such as sulfate, aluminosilicate, and calcium hydroxide, which break up the CaF2 crystals and prevent them from agglomerating, thus failing to form continuous large flocs; the product has many impurities and strong particle dispersibility.

[0033] like Figures 6-7 It can be seen that the fluorine content in the purified precipitate (sludge C) increased significantly, while the sulfur content decreased from 0.962% to 0.64%, indicating that a certain amount of calcium sulfate was converted into calcium fluoride. The crystal size increased significantly after purification, exhibiting a regular rhombic crystal form, with a small amount of nano-CaF2 microcrystals adhering to the crystal surface. This indicates that acid washing removed the amorphous impurities originally coated on the crystal surface, converting them into calcium fluoride molecules that adhered and grew on the original matrix, significantly improving the matrix purity and increasing the size of the impurities.

Claims

1. A method for resource recovery of high concentration fluorine-containing waste liquid, characterized in that, The method employs a two-stage chemical mixing and fractional recovery of calcium fluoride, with the specific steps as follows: (1) Fluorine-containing waste liquid with a fluorine concentration >10000mg / L after homogenization in the conditioning tank enters the primary chemical mixing reaction tank. The primary chemical mixing reaction tank includes at least a first stirring tank I and a second stirring tank I connected in sequence. An alkaline calcium source is introduced into the first stirring tank I to make the Ca / F molar ratio 0.38~0.

43. The effluent from the first stirring tank I enters the second stirring tank I. The pH of the liquid in the second stirring tank I is adjusted to 4.5~5.

5. After the reaction is complete, the effluent from the primary chemical mixing reaction tank enters the primary sedimentation tank for solid-liquid separation. (2) The effluent from the primary sedimentation tank enters the secondary chemical mixing reaction tank, which includes a first stirring tank II, a second stirring tank II, and a third stirring tank II connected in sequence. An excess of neutral calcium source is introduced into the first stirring tank II to make the Ca / F molar ratio 0.50~0.

60. The effluent from the first stirring tank II enters the second stirring tank II, and the pH of the liquid in the second stirring tank II is adjusted to 4.5~5.

5. Then, the effluent from the second stirring tank II enters the third stirring tank II, and the pH of the liquid in the third stirring tank II is adjusted to neutral. Then, flocculants and coagulants are added to it. After sufficient settling, the effluent from the secondary chemical mixing reaction tank enters the secondary sedimentation tank for solid-liquid separation. The effluent F from the secondary sedimentation tank... - Stable concentration less than 20 mg / L; (3) During operation, the calcium fluoride sludge formed in the primary sedimentation tank is periodically precipitated and discharged to obtain calcium fluoride sludge A, and the purity of calcium fluoride sludge A is ≥94%; the calcium fluoride sludge formed in the secondary sedimentation tank is periodically precipitated and discharged to obtain calcium fluoride sludge B; calcium fluoride sludge B is purified by alkali washing and acid washing in sequence, and after purification, the purity of calcium fluoride sludge B is ≥90%.

2. The method of reclaiming high strength fluorochemical waste solutions of claim 1, wherein: In step (1), the calcium source is calcium hydroxide or lime milk; the flow rate of the fluoride-containing waste liquid introduced into the first stirring tank I is 6.84~7 L / h, the flow rate of the calcium hydroxide solution introduced into the first stirring tank I is 2.7~2.8 L / h, and the mass concentration of the calcium hydroxide is 3.5~4%.

3. The method of reclaiming high strength fluorochemical waste solutions of claim 1, wherein: In step (1), the primary mixing reaction tank also includes a third stirring tank I.

4. The method of reclaiming high strength fluorochemical waste solutions of claim 3, wherein: The stirring speeds of each stirring tank in the primary mixing reaction tank are as follows: the stirring speed of the first stirring tank I is 40~45 rpm, the stirring speed of the second stirring tank I is 40~45 rpm, and the stirring speed of the third stirring tank I is 20~25 rpm.

5. The method for resource recovery of high-concentration fluoride-containing wastewater according to claim 1, characterized in that: In step (2), the fluoride ion concentration in the effluent of the primary sedimentation tank is 1500~3000 mg / L.

6. The method for resource recovery of high-concentration fluoride-containing waste liquid according to claim 1, characterized in that: In step (2), the calcium source is calcium chloride; the flow rate of the effluent from the primary sedimentation tank is 0.8~1.0 m / s, the flow rate of the calcium chloride solution introduced into the first stirring tank II is 0.5~0.55 L / h, and the mass concentration of the calcium chloride is 30~35%.

7. The method for resource recovery of high-concentration fluoride-containing waste liquid according to claim 1, characterized in that: In step (2), the coagulant is PAC; the flocculant is CPAM; the dosage of the coagulant is 800~850mg / L; and the dosage of the flocculant is 5~10ppm.

8. The method of reclaiming high strength fluorochemical waste solutions of claim 1, wherein: In step (2), the stirring speeds of each stirring tank in the secondary mixing reaction tank are as follows: the stirring speed of the first stirring tank II is 50~60 rpm, the stirring speed of the second stirring tank II is 40~45 rpm, and the stirring speed of the third stirring tank II is 20~25 rpm.

9. The method of reclaiming high strength fluorochemical waste solutions of claim 1, wherein: In step (3), during operation, the primary sedimentation tank is sludge discharged once every 3 to 3.5 hours; the secondary sedimentation tank is sludge discharged once every 6 to 6.5 hours.

10. The method of reclaiming high strength fluorochemical waste solutions of claim 1, wherein: In step (3), the alkaline washing is performed by washing with a sodium carbonate solution at a temperature of not less than 70°C and a concentration of 50~55g / L for not less than 30 minutes; then acid washing is performed by using fluorine-containing waste liquid in the regulating tank for a time of not less than 60 minutes.

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