Deep sewage defluorination method based on NH4F.LaF3 chemical precipitation

Abstract

The invention relates to a deep sewage defluorination method based on NH4F.LaF3 chemical precipitation, which comprises the following specific steps: adding a solution containing NH4 < + > and La < 3 + > into fluorine-containing water, adjusting the pH value of the sewage to 6-8, and carrying out NH4F.LaF3 chemical precipitation reaction; sediment is discharged, crystal particles in supernate are removed, deep defluorination of the sewage is achieved, and the concentration of fluorine ions in the water after defluorination can be stably lower than 1 mg / L. According to the method disclosed by the invention, by utilizing the characteristic that the solubility of double salt NH4F.LaF3 is very low, NH4 < + > ions (ammonium chloride or ammonia water and hydrochloric acid) and La < 3 + > ions (lanthanum chloride or lanthanum oxide and hydrochloric acid) are added into the fluorine-containing wastewater, so that equivalent reaction of fluorine, lanthanum ions, ammonium ions, fluorine ions and the like in the water is removed, and the addition of a medicament does not need to be excessive. The method has the characteristics of high efficiency and low cost, and has the advantages of small dosage of chemicals, high reaction speed, thorough reaction and low concentration of fluorine in effluent.

Description

Technical Field

[0001] The invention belongs to the technical field of sewage defluorination treatment, and in particular relates to a sewage deep defluorination method based on NH4F·LaF3 chemical precipitation. Background Art

[0002] In recent years, the rapid development of emerging industries such as electronic information, new energy, and semiconductors has led to a continuous increase in the amount of fluoride-containing wastewater. The types and components of industrial fluoride-containing wastewater are relatively complex, resulting in cumbersome processes, large amounts of reagents added, and high treatment costs in the wastewater defluorination process. How to treat fluoride-containing wastewater at low cost and high efficiency has become an important research topic in the field of environmental protection.

[0003] At present, common methods for removing fluoride from wastewater include precipitation, adsorption, ion exchange, membrane separation, electrochemical method, etc. The main treatment methods at home and abroad are still precipitation and adsorption. Other methods are relatively high in treatment cost and low in fluoride removal efficiency, so they are mostly in the experimental stage and difficult to be promoted and applied to the treatment of industrial fluoride-containing wastewater. Traditional chemical precipitation method is usually used to treat high-concentration fluoride-containing wastewater. It converts fluoride ions in wastewater into calcium fluoride (CaF2) for precipitation and removal. However, the fluoride removal effect of calcium fluoride precipitation method is restricted by the precipitation characteristics of calcium fluoride (solubility product constant and precipitation crystallization kinetics). As a result, the fluoride ions (F - ) concentration is too high, usually only the fluoride ions (F - ) concentration is controlled to about 10 mg / L. Even by significantly increasing the concentration of the reagent and combining it with calcium alkali, only the fluoride ion (F - ) concentration is controlled to about 5-8 mg / L, and it is very difficult to further reduce the fluoride concentration in the effluent.

[0004] In situations where environmental protection is required to be higher, the discharge standard limit of industrial fluoride-containing wastewater has been reduced to 1.5-2 mg / L, and some places even require a limit of 1.0 mg / L. The defluorination method based on calcium fluoride chemical precipitation can no longer achieve this technical goal. In order to achieve a lower fluoride concentration in the effluent, people often use adsorption technology or traditional defluoridation agents. The main component of traditional defluoridation agents is aluminum salt. When used, the dosage of the agent is large (often reaching more than 1g / L), and the amount of chemical sludge produced is huge, and the economic efficiency is poor. The adsorption method has the advantages of simple operation, high efficiency, low cost, and stable operation. At the same time, the adsorption material can be repeatedly regenerated and has a wide range of sources. It is suitable for the defluorination of fluoride-containing water with low concentrations of impurities. However, the adsorption capacity of commercially available adsorbents is limited. Currently, the best performance and the most widely used is activated alumina, which has a maximum defluorination adsorption capacity of only 4-4.5 mg / g. The low adsorption capacity and frequent regeneration required result in high operating costs. The total cost of treating a ton of water often reaches 5-6 yuan (initial fluoride concentration 10 mg / L). The groundwater quality is simple, and deep fluoride removal by adsorption is acceptable, but the industrial wastewater quality is complex, and if it is treated by adsorption, the efficiency is low and the cost is higher. Therefore, exploring and developing new fluoride removal processes and technologies has become a difficult problem that needs to be solved in the treatment of industrial fluoride-containing wastewater. Summary of the invention

[0005] The purpose of the present invention is to provide a method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation in order to solve the above problems.

[0006] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0007] A method for deep defluoridation of sewage based on NH4F·LaF3 chemical precipitation, the specific steps are as follows: adding NH4 to fluoride-containing water + ,La 3+ The solution is used to adjust the pH of the sewage to 6-8, and then NH4F·LaF3 chemical precipitation reaction is carried out; the precipitate is discharged and the crystal particles in the supernatant are removed to achieve deep defluorination of the sewage; the deep defluorination method of sewage of the present invention can achieve a fluorine concentration of / F － Stable below 1 mg / L; when the ammonium agent is appropriately excessive, the fluoride concentration in the effluent can be stably below 0.6 mg / L.

[0008] As a further optimization scheme of the present invention, when the fluorine concentration of the raw water is high, the fluorine concentration of the wastewater with high fluorine content is reduced to a reasonable concentration range (can reach below 10 mg / L) by calcium salt precipitation method; the calcium source used in the calcium salt precipitation method is preferably at least one of calcium chloride CaCl2, calcium oxide CaO or calcium hydroxide Ca(OH)2; when the fluorine-containing substance in the fluorine-containing water is not in the form of F - When the ionic state exists, the calcium source includes at least one of calcium oxide CaO or calcium hydroxide Ca(OH)2.

[0009] As a further optimization scheme of the present invention, containing NH4 + The solution is specifically ammonium chloride or a product obtained by the reaction of ammonia water and hydrochloric acid; containing La 3+ The solution is specifically lanthanum chloride or a product obtained by reacting lanthanum oxide and hydrochloric acid.

[0010] As a further optimization scheme of the present invention, when the emission fluorine concentration is required to be ≤0.6 mg / L, NH4 + The amount is greater than 25.0% of the molar concentration of fluoride ions in the sewage, and the NH4 + It is an overdose.

[0011] As a further optimization scheme of the present invention, the addition of NH4 + ,La 3+ The method of adding the solution includes adding synchronously or adding asynchronously;

[0012] Add simultaneously to first add NH4 + The solution contains La 3+ The solution is mixed evenly and then added to the fluoride-containing water; the asynchronous addition is to first add the NH4 + The solution was added to the fluoride-containing water, and then the La 3+ The solution was added to fluoride-containing water.

[0013] As a further optimization scheme of the present invention, when adding NH4 + ,La 3+ Before preparing the solution, NH4F·LaF3 seeds were added to the fluorine-containing water.

[0014] As a further optimization scheme of the present invention, NH4F·LaF3 seeds are added by refluxing the discharged precipitate, or by preparing NH4F·LaF3 seeds on site. The preparation method of NH4F·LaF3 seeds is as follows: - In the solution, add water-soluble ammonium chloride and lanthanum chloride in turn, stir, adjust the pH to 6-8, and stand to obtain seed crystals, wherein NH4 + and La 3 + The concentration is 25% of the molar concentration of fluoride ions.

[0015] As a further optimization scheme of the present invention, the method for removing crystal particles in water includes at least one of flocculation sedimentation or membrane separation.

[0016] The beneficial effects of the present invention are:

[0017] 1) The present invention utilizes the low solubility of the double salt NH4F·LaF3 (ammonium fluoride lanthanum fluoride double salt, or ammonium lanthanum fluoride) to add NH4 + Ions (ammonium chloride or ammonia + hydrochloric acid) and La 3+ Ions (lanthanum chloride or lanthanum oxide + hydrochloric acid) are used to remove fluoride from water. The amount of lanthanum ions and ammonium ions added can meet the reaction requirements, that is, equivalent addition. In order to facilitate crystallization, the reaction is preferably carried out in a fluidized bed reaction tank, and the formed precipitate is refluxed as a crystal seed. The effluent is removed by flocculation to remove particles. This method is efficient and low-cost for low-concentration fluoride-containing wastewater, and has a fast reaction speed, a thorough reaction, and a low fluoride concentration in the effluent;

[0018] 2) When the inlet fluoride concentration is 10 mg / L, HRT (hydraulic retention time) ≤ 10 min, and the outlet fluoride concentration is required to be lower than 1 mg / L, the cost of the agent of the present invention is about 0.5 yuan / ton (market price in January 2025), which is much lower than the existing adsorption defluorination technology. DETAILED DESCRIPTION

[0019] The present application is described in further detail below. It is necessary to point out that the following specific implementation methods are only used to further illustrate the present application and cannot be understood as limiting the scope of protection of the present application. Technicians in this field can make some non-essential improvements and adjustments to the present application based on the above application content.

[0020] 1. Materials

[0021] Unless otherwise specified, the methods used in this application are conventional methods known to those skilled in the art, and the reagents and other materials used are commercially available products unless otherwise specified.

[0022] 2. Methods

[0023] A method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation, the specific steps are:

[0024] (1) Pretreatment: Use traditional chemical precipitation method to treat fluoride-containing wastewater to reduce the high concentration of fluoride in fluoride-containing wastewater to a reasonable range (technically, calcium salt precipitation can reduce fluoride to about 10 mg / L);

[0025] The traditional chemical precipitation method is to add calcium salt (i.e. calcium chloride CaCl2; calcium salt + calcium alkali can also be used) to high concentration fluoride water. 2+ +2F -=CaF2↓; 1.05 grams of calcium ions are required for each gram of fluoride ions. Since the reaction is not complete without pre-storage of seed crystals, and the fluoride ion concentration in water often reaches more than 20 mg / L, it is necessary to supplement seed crystals or reflux seed crystals, or control the reaction system to maintain seed crystals in the reaction tank, so that the fluoride ion concentration calculated based on the solubility product can be reached or approached;

[0026] If there is fluorosilicic acid (H2SiF6) and a small amount of ammonium hydrogen fluoride (NH4HF2) in the system, calcium oxide or calcium hydroxide needs to be added as a precipitation agent as appropriate. Please refer to the following reaction for details:

[0027] H2SiF6+Ca(OH)2→CaSiF6+2H2O;

[0028] CaSiF6+2Ca(OH)2→3CaF2↓+SiO2+2H2O;

[0029] NH4HF2+Ca(OH)2→CaF2↓+NH3+2H2O;

[0030] (2) Precipitation reaction: With the calcium salt precipitation pretreatment, the fluoride ion concentration in the effluent can basically be reduced to about 10 mg / L (or ≤10 mg / L). At this time, adding NH4 + (ammonium salt or ammonia water + hydrochloric acid); La also needs to be added to the water 3+ (Lanthanum chloride or lanthanum oxide + hydrochloric acid. If lanthanum oxide + hydrochloric acid is used, the two reagents cannot be directly added to the water. Lanthanum oxide and hydrochloric acid need to be reacted in equal amounts in advance until the reaction is completed and lanthanum salt is formed before adding. La2O3+6HCl=2LaCl3+3H2O, the effective ingredient is La 3+ In the chemical precipitation reaction of NH4F·LaF3, La 3+ The amount of lanthanum added is 25% of the molar concentration of fluoride ions in the water); if the raw water is acidic or alkaline, it is also necessary to adjust its pH value to neutral (pH = 6-8) with hydrochloric acid or sodium hydroxide, and carry out NH4F·LaF3 chemical precipitation in the reaction tank (taking LaCl3, NH4Cl and NaF as an example, LaCl3+NH4Cl+4NaF=NH4F·LaF3↓+4NaCl). The precipitated material in the reaction tank is white flocculent particles with fine particles, and a certain concentration needs to be maintained, and the loss from the supernatant should be minimized;

[0031] (3) Effluent treatment: The sediment is discharged from the bottom of the reaction tank, and the supernatant is discharged from the top of the reaction tank. The supernatant is treated by at least one of microfiltration, ultrafiltration, nanofiltration, direct filtration or flocculation precipitation to remove crystal particles in the water before it can be discharged.

[0032] In order to further study the effects of the dosage and method of lanthanum ions (salts) and ammonium ions (salts) and whether to pre-add crystal seeds on the fluorine concentration in the treated sewage and the fluorine removal rate of the sewage, the present application also conducted the following experiments of Examples 1 to 4, as follows:

[0033] Example 1

[0034] It should be noted that the present embodiment adopts the ammonium chloride + lanthanum oxide, hydrochloric acid method, the reagents are equivalent, and the ammonium solution and the lanthanum solution are added asynchronously.

[0035] Preparation of ammonium solution: Prepare ammonium chloride solution with a concentration of 21.4g / L, NH4 + The equivalent concentration is 0.40 mol / L, which is called "ammonium solution".

[0036] Preparation of lanthanum solution 1#: Take 6.52g of lanthanum oxide (1 / 2La2O3, molecular weight 325.8, purity 99.95%), slowly add 1:10 hydrochloric acid, stirring continuously until all dissolved, dilute to 100mL, lanthanum oxide concentration 65.2g / L, La 3+ The equivalent concentration is 400mmol / L, called "Lanthanum Solution 1#".

[0037] Preparation of lanthanum solution 2#: Use lanthanum chloride to prepare a 98.1g / L solution with a purity of 99.95%. 3+ The equivalent is 0.40 mol / L, called "Lanthanum Solution 2#".

[0038] Preparation of simulated fluoride-containing water: Use NaF to prepare simulated industrial fluoride-containing water with a fluoride concentration of 11.4 mg / L. The actual fluoride concentration is 11.26 mg / L.

[0039] A method for deep defluorination of wastewater based on NH4F·LaF3 chemical precipitation. The specific steps of the precipitation reaction are: adding 0.19mL of "ammonium solution" and 0.19mL of "lanthanum solution 1#" (the theoretical required dosage is 0.1875mL, and 0.19mL is taken considering the test accuracy) to 0.50L of fluoride-containing water, stirring for 10 minutes, and NH4F·LaF3 chemical precipitation occurs in the reaction tank.

[0040] Experimental detection: Sampling was taken to test the fluoride concentration. After filtering with a 0.45μm filter membrane, the fluoride concentration in the filtered water was measured to be 0.98mg / L, and the fluoride removal rate was 91.3%. After using PAM flocculation precipitation, the fluoride concentration in the supernatant was measured to be 0.63mg / L, and the fluoride removal rate was 94.4%.

[0041] The reagent cost of lanthanum oxide + hydrochloric acid: lanthanum oxide is calculated at 4,000 yuan / ton, 31% hydrochloric acid is calculated at 400 yuan / ton, ammonium chloride is calculated at 1,000 yuan / ton, and the "ammonium solution" is added to convert the concentration of ammonium chloride to 8 mg / L, and the cost per ton of water is 0.008 yuan. The conversion price of lanthanum oxide (including hydrochloric acid) is 5,290 yuan / ton, and the conversion concentration of lanthanum oxide is 24.8 mg / L, and the cost per ton of water is 0.131 yuan. The concentration of PAM is 80 mg / L, 5,000 yuan / ton, and the cost per ton of water is 0.40 yuan. The total cost of the above reagents: 0.008+0.131+0.40=0.539 yuan / ton of water.

[0042] Experimental conclusion: The goal of deep fluoride removal can be achieved by adding the reagent in equivalent amounts, and the fluoride concentration in the effluent can be lower than 1 mg / L. At present, the market price of lanthanum chloride is significantly higher than that of lanthanum oxide (containing hydrochloric acid). From the perspective of economic benefits, it is advisable to use lanthanum oxide + hydrochloric acid reaction to obtain lanthanum ions as an effective La for chemical precipitation. 3+ .

[0043] Example 2

[0044] It should be noted that the ammonium chloride + lanthanum chloride method is used in this embodiment, the reagent is excessive, and the ammonium solution and the lanthanum solution are added asynchronously.

[0045] The prepared "ammonium solution", "lanthanum solution 2#" and simulated fluorine-containing water are all the same as those in Example 1.

[0046] A method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation. The specific steps of the precipitation reaction are: adding 0.30mL of "ammonium solution" and 0.30mL of "lanthanum solution 2#" (the theoretical required dosage is 0.1875mL, but in this embodiment, the ammonium salt and lanthanum salt of the reagents are in excess of more than 50%) to 0.50L of fluoride-containing water, the fluoride ion concentration is 11.4mg / L, the actual raw water fluoride concentration is 11.26mg / L, stirring for 10 minutes, and performing NH4F·LaF3 chemical precipitation in the reaction tank.

[0047] Experimental detection: Sampling was taken to test the fluoride concentration. After filtering with a 0.45μm filter membrane, the fluoride concentration of the filtered water was measured to be 0.91mg / L, and the fluoride removal rate was 91.9%. After PAM flocculation precipitation (PAM dosage was 80mg / L), the fluoride concentration in the supernatant was measured to be 0.42mg / L, and the fluoride removal rate was 96.3%.

[0048] Experimental conclusion: Excessive addition of reagents can further reduce the fluoride ion concentration in the effluent, but the reduction is low, the reagent cost is high, and the economic efficiency becomes worse. Therefore, unless the fluoride concentration limit of the effluent is low, there is no need to overdose the reagents.

[0049] Example 3

[0050] It should be noted that the ammonium chloride + lanthanum chloride method is adopted in this embodiment, the reagents are equivalent, and the ammonium solution and the lanthanum solution are added simultaneously.

[0051] The prepared "ammonium solution", "lanthanum solution 2#" and simulated fluorine-containing water are all the same as those in Example 1.

[0052] A method for deep defluorination of wastewater based on NH4F·LaF3 chemical precipitation. The specific steps of the precipitation reaction are: taking 0.19mL and "ammonium solution" and 0.19mL and "lanthanum solution 2#" and mixing them evenly, then adding them into 0.5L of fluoride-containing water, stirring for 10 minutes, and then performing NH4F·LaF3 chemical precipitation in a reaction tank.

[0053] Experimental detection: Sampling was taken to test the fluoride concentration. After filtering with a 0.45μm filter membrane, the fluoride concentration of the filtered water was measured to be 0.92mg / L, and the fluoride removal rate was 91.8%. After PAM flocculation precipitation (PAM dosage was 80mg / L), the fluoride concentration in the supernatant was measured to be 0.44mg / L, and the fluoride removal rate was 96.1%.

[0054] Experimental conclusion: The effects of synchronous and asynchronous addition of the agent are basically the same.

[0055] Example 4

[0056] It should be noted that the ammonium chloride + lanthanum chloride method is used in this embodiment, the reagents are equivalent, seed crystals are pre-added, and the ammonium solution and lanthanum solution are added asynchronously.

[0057] The prepared "ammonium solution", "lanthanum solution 2#" and simulated fluorine-containing water are all the same as those in Example 1.

[0058] A method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation, the specific steps of the precipitation reaction are:

[0059] Step 1: Take 500mL of a solution with a fluoride ion concentration of 0.19g / L, a fluoride ion molar concentration of 0.1mol / L, and a total of 0.005mol; first add 0.0669g of ammonium chloride (do not add solid powder directly, but add after dissolving in water), then add 0.3067g of lanthanum chloride (add after dissolving in water), stir and react for 1h, adjust the pH to neutral (about 6.5-7.5), and let it stand to retain the seed crystals;

[0060] Step 2: discard the supernatant and wash the seed crystals with deionized water;

[0061] Step 3: first add 15 mL of the above seed solution to 0.50 L of fluorine-containing water, then add 0.19 mL of "ammonium solution" and 0.19 mL of "lanthanum solution 2#" (the weighted calculation of the fluorine concentration of the original water is 10.88 mg / L, and the theoretical required dosage is 0.1789 mL. Considering the test accuracy, 0.18 mL is taken), stir for 10 minutes, and then carry out NH4F·LaF3 chemical precipitation in the reaction tank.

[0062] Experimental detection: Sampling was taken to test the fluoride concentration. After filtering with a 0.45μm filter membrane, the fluoride concentration in the filtered water was measured to be 0.48mg / L, and the fluoride removal rate was 95.6%. After PAM flocculation precipitation (PAM dosage was 80mg / L), the fluoride concentration in the supernatant was measured to be 0.36mg / L, and the fluoride removal rate was 96.7%.

[0063] Experimental conclusion: Pre-adding seed crystals is beneficial to the removal of fluorine, but the increase is not obvious.

[0064] The above-mentioned embodiments only express several implementation modes of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the present invention. + ) and lanthanum salts (La 3+ ) for fluoride ion (F - ) to remove fluoride ions from water by chemical precipitation, regardless of the process flow, reagent addition method, and reactor type, all fall within the scope of protection of this application. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all fall within the scope of protection of the present invention.

Claims

1. A method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation, characterized in that: The specific steps are as follows: add NH4 + ,La 3+ The solution is used to adjust the pH of the sewage to 6-8, and then the NH4F·LaF3 chemical precipitation reaction is carried out; the precipitate is discharged and the crystal particles in the supernatant are removed to achieve deep defluorination of the sewage.

2. The method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: When the fluoride concentration in the raw water is high, the fluoride concentration in the wastewater can be reduced by treating it with calcium salt precipitation.

3. The method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: Contains NH4 + The solution is specifically ammonium chloride or a product obtained by the reaction of ammonia water and hydrochloric acid; containing La 3+ The solution is specifically lanthanum chloride or a product obtained by reacting lanthanum oxide and hydrochloric acid.

4. The method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: When the emission fluorine concentration is required to be ≤1mg / L, add NH4 + and La 3+ The concentration is 25.0% of the molar concentration of fluoride ions in sewage. The fluoride ions in sewage and NH4 + and La 3+ There is an equivalent reaction between the two and the dosage is not excessive.

5. The method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: When the emission fluorine concentration is required to be ≤0.6mg / L, add NH4 + The concentration is greater than 25.0% of the molar concentration of fluoride ions in the sewage. + Belong to excessive amount; La 3+ Still equivalent.

6. The method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: The addition contains NH4 + ,La 3+ The method of adding the solution includes adding synchronously or adding asynchronously; Add simultaneously to first add NH4 + The solution contains La 3+ The solution is mixed evenly and then added to the fluoride-containing water; the asynchronous addition is to first add the NH4 + The solution was added to the fluoride-containing water, and then the La 3+ The solution was added to fluoride-containing water.

7. The method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: When adding NH4 + ,La 3+ Before preparing the solution, NH4F·LaF3 seeds were added to the fluorine-containing water.

8. The method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation according to claim 7, characterized in that: The NH4F·LaF3 seed crystals are added by refluxing the discharged precipitate, or by preparing the NH4F·LaF3 seed crystals on site. The preparation method of the NH4F·LaF3 seed crystals is as follows: - In the solution, add water-soluble ammonium chloride and lanthanum chloride in turn, stir, adjust the pH to 6-8, and stand to obtain seed crystals, wherein NH4 + and La 3+ The amount is 25% of the molar concentration of fluoride ions.

9. The method for deep defluorination of sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: The method of removing crystal particles in water includes at least one of flocculation precipitation or membrane separation.