A method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation

Abstract

The present invention relates to a method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation. The specific steps are as follows: Add a solution containing NH4 + , La 3+ to the fluoride-containing water. After adjusting the pH of the sewage to 6-8, carry out the NH4F·LaF3 chemical precipitation reaction; discharge the precipitate and remove the crystal particles in the supernatant to achieve deep fluoride removal from the sewage. The fluoride ion concentration in the water after fluoride removal can be stably lower than 1 mg / L. The present invention utilizes the characteristic that the double salt NH4F·LaF3 has a very low solubility, and adds NH4 + ions (ammonium chloride or ammonia water + hydrochloric acid) and La 3+ ions (lanthanum chloride or lanthanum oxide + hydrochloric acid) to remove fluoride in the water. The lanthanum ions, ammonium ions, and fluoride ions react in an equivalent amount, so the dosage of the reagent does not need to be excessive. The present invention has the characteristics of high efficiency and low cost, with less dosage of the reagent, fast reaction speed, complete reaction, and low fluoride concentration in the effluent.

Description

Technical Field

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

[0002] In recent years, with the rapid development of emerging industries such as electronic information, new energy, and semiconductors, the generation amount of fluorine-containing wastewater has been continuously increasing. The types and components of industrial fluorine-containing wastewater are relatively complex, resulting in problems such as cumbersome processes, large amounts of reagent addition, and high treatment costs in the wastewater defluorination process. How to efficiently treat fluorine-containing wastewater at low cost has become an important research topic in the environmental protection field.

[0003] Currently, common wastewater defluorination methods include precipitation method, adsorption method, ion exchange method, membrane separation method, electrochemistry method, etc. At present, the main treatment methods at home and abroad are still precipitation method and adsorption method, while other methods are mostly in the experimental stage due to relatively high treatment costs and low defluorination efficiency, and it is difficult to be popularized and applied to the treatment of industrial fluorine-containing wastewater. The traditional chemical precipitation method is usually used to treat high-concentration fluorine-containing wastewater, which converts fluoride ions in the wastewater into calcium fluoride (CaF2) precipitation for removal. However, the defluorination effect of the calcium fluoride precipitation method is restricted by the characteristics of calcium fluoride precipitation (properties such as solubility product constant and precipitation crystallization kinetics), resulting in a relatively high concentration of fluoride ions (F - ) in the effluent of the traditional chemical precipitation method. Usually, the concentration of fluoride ions (F - ) in the effluent can only be controlled to about 10 mg / L. Even by significantly increasing the reagent concentration and combining with calcium alkali, the concentration of fluoride ions (F - ) in the effluent can only be controlled to about 5 - 8 mg / L. It is very difficult to further reduce the fluorine concentration in the effluent.

[0004] In occasions with higher environmental protection requirements, the discharge standard limit of industrial fluoride-containing wastewater has been reduced to 1.5 - 2 mg / L, and in some places, the limit is even required to be 1.0 mg / L. The fluoride removal 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, adsorption technology or traditional fluoride removal agents are often used. The main component of traditional fluoride removal agents is aluminum salt. When used, the dosage of the agent is large (usually reaching more than 1 g / L), and the amount of chemical sludge generated is huge, resulting in poor economy. 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 regenerated repeatedly and has a wide range of sources, and is suitable for fluoride removal from fluoride-containing water with low concentration and less impurities. However, the adsorption capacity of currently commercially available adsorbents is limited. Currently, the most performant and widely used is activated alumina, whose maximum fluoride adsorption capacity is only 4 - 4.5 mg / g. The low adsorption capacity and the need for frequent regeneration lead to a high operating cost, and the total cost per ton of water treatment often reaches 5 - 6 yuan (initial fluoride concentration of 10 mg / L). The groundwater quality is simple, and it is acceptable to use the adsorption method for deep fluoride removal. However, the industrial wastewater quality is complex. If the adsorption method is used for treatment, 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 urgently 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 fluoride removal from sewage based on NH4F·LaF3 chemical precipitation in order to solve the above problems.

[0006] The present invention realizes the above purpose through the following technical solutions:

[0007] A method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation, the specific steps are as follows: Add a solution containing NH4 + , La 3+ to the fluoride-containing water. After adjusting the pH of the sewage to 6 - 8, carry out the NH4F·LaF3 chemical precipitation reaction; discharge the precipitate and remove the crystal particles in the supernatant to achieve deep fluoride removal from the sewage; Using the method for deep fluoride removal from sewage of the present invention, the fluoride concentration / F － can be stably lower than 1 mg / L; when the ammonium agent is appropriately in excess, the fluoride concentration in the effluent can be stably lower than 0.6 mg / L.

[0008] As a further optimized solution of the present invention, when the fluoride concentration of the raw water is high, the fluoride concentration of the sewage with a high fluoride content is reduced to a reasonable concentration range (can reach below 10 mg / L) through the 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 fluoride-containing water does not exist in the form of F - ions, the calcium source includes at least one of calcium oxide CaO or calcium hydroxide Ca(OH)2.

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

[0010] As a further optimized solution of the present invention, when the requirement for the discharged fluorine concentration is ≤ 0.6 mg / L, the amount of NH4 + added is greater than 25.0% of the molar concentration of fluoride ions in the sewage, and NH4 + in the sewage is in excess.

[0011] As a further optimized solution of the present invention, the method of adding the solution containing NH4 + , La 3+ includes synchronous addition or asynchronous addition;

[0012] Synchronous addition means first mixing the solution containing NH4 + and the solution containing La 3+ uniformly, and then adding it to the fluorine-containing water; asynchronous addition means first adding the solution containing NH4 + to the fluorine-containing water, and then adding the solution containing La 3+ to the fluorine-containing water.

[0013] As a further optimized solution of the present invention, before adding the solution containing NH4 + , La 3+ , add NH4F·LaF3 seeds to the fluorine-containing water.

[0014] As a further optimized solution of the present invention, the NH4F·LaF3 seeds are added by refluxing the discharged precipitate, or the NH4F·LaF3 seeds are prepared on-site. The preparation method of the NH4F·LaF3 seeds is: to the solution containing F - , successively add ammonium chloride and lanthanum chloride dissolved in water, stir, adjust the pH to 6 - 8, and let it stand to obtain the seeds. Among them, the concentrations of NH4 + and La 3 + are 25% of the molar concentration of fluoride ions.

[0015] As a further optimized solution of the present invention, the method for removing crystal particles in water includes at least one of flocculation precipitation or membrane filtration method.

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

[0017] 1) The present invention utilizes the characteristic that the double salt NH4F·LaF3 (ammonium fluoride lanthanum fluoride double salt, or ammonium lanthanum fluoride) has very low solubility, and adds NH4 + ions (ammonium chloride or ammonia + hydrochloric acid) and La 3+ ions (lanthanum chloride or lanthanum oxide + hydrochloric acid) to remove fluorine in water. The dosage of lanthanum ions and ammonium ions only needs to meet the reaction requirements, that is, equivalent dosing. For the sake of facilitating crystallization, the reaction is preferably carried out in a fluidized bed reaction tank, and the formed precipitate is refluxed as crystal seeds. The effluent is treated by flocculation to remove particles. This method has the characteristics of high efficiency, low cost, fast reaction speed, complete reaction, and low fluorine concentration in the effluent for low-concentration fluorine-containing wastewater;

[0018] 2) When the influent fluorine ion concentration is 10 mg / L, HRT (hydraulic retention time) ≤ 10 min, and the effluent is required to have a fluorine ion concentration lower than 1 mg / L, the chemical agent cost of the present invention is about 0.5 yuan / ton (market price in January 2025), and the cost is much lower than the existing adsorption defluorination technology. Specific Embodiments

[0019] The following further describes the present application in detail. It is necessary to point out here that the following specific embodiments are only used to further illustrate the present application and cannot be understood as limiting the protection scope of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0020] I. Materials

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

[0022] II. Methods

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

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

[0025] The traditional chemical precipitation method is to add calcium salt (that is, calcium chloride CaCl2; it can also be in the form of calcium salt + calcium base) to the high-concentration fluorine-containing water. According to Ca 2+ +2F -= CaF2↓; 1.05 grams of calcium ions are required per gram of fluoride ions. Since this reaction is incomplete without pre-existing seeds and the fluoride ion concentration in water often reaches above 20 mg / L, it is necessary to supplement seeds or through seed reflux, or control the reaction system to maintain the seeds in the reaction tank, so as to reach or approach the fluoride ion concentration calculated according to the solubility product;

[0026] If there is fluorosilicic acid (H2SiF6) and a small amount of ammonium bifluoride (NH4HF2) in the system, calcium oxide or calcium hydroxide needs to be added as a precipitation agent as appropriate. The specific reactions are as follows:

[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, NH4 + (ammonium salt or ammonia water + hydrochloric acid) is added; it is also necessary to add La 3+ (lanthanum chloride or lanthanum oxide + hydrochloric acid. If lanthanum oxide + hydrochloric acid is used, these two agents cannot be directly added to water. It is necessary to react lanthanum oxide with hydrochloric acid in an equivalent amount in advance until the reaction is complete to form a lanthanum salt before adding. La2O3 + 6HCl = 2LaCl3 + 3H2O, and the active ingredient is La 3+ , in the chemical precipitation reaction of NH4F·LaF3, calculated by La 3+ , the amount of lanthanum added is 25% of the molar concentration of fluoride ions in water); if the raw water is acidic or alkaline, it is also necessary to adjust its pH value to neutral (pH = 6 - 8) through hydrochloric acid or sodium hydroxide, and conduct the chemical precipitation of NH4F·LaF3 in the reaction tank (taking LaCl3, NH4Cl, and NaF as examples, LaCl3 + NH4Cl + 4NaF = NH4F·LaF3↓ + 4NaCl). This precipitate is white flocculent particles in the reaction tank, with small particles. It is necessary to maintain a certain concentration and minimize the loss from the supernatant;

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

[0032] To further study the effects of the dosages and dosing methods of lanthanum ions (salts) and ammonium ions (salts), as well as whether to pre-add seeds on the fluoride concentration in the treated sewage and the fluoride removal rate of the sewage, the following experiments of Examples 1 to 4 were also carried out in this application, specifically as follows:

[0033] Example 1

[0034] It should be noted that in this example, the ammonium chloride + lanthanum oxide and hydrochloric acid method is adopted, with equivalent reagents, and the ammonium solution and lanthanum solution are added in a non-synchronous manner.

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

[0036] Preparation of lanthanum solution 1#: Take 6.52 g of lanthanum oxide (1 / 2La2O3, molecular weight 325.8, purity 99.95%), slowly add 1:10 hydrochloric acid dropwise, and stir continuously until completely dissolved, then dilute to 100 mL. The concentration of lanthanum oxide is 65.2 g / L, La 3+ with a normal concentration of 400 mmol / L, called "lanthanum solution 1#".

[0037] Preparation of lanthanum solution 2#: Prepare a solution with a concentration of 98.1 g / L using lanthanum chloride, and the purity of lanthanum chloride is 99.95%, La 3+ with an equivalent of 0.40 mol / L, called "lanthanum solution 2#".

[0038] Preparation of simulated fluorine-containing water: Use NaF to prepare simulated industrial fluorine-containing water with a fluorine concentration of 11.4 mg / L, and the measured fluorine concentration is 11.26 mg / L.

[0039] A method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation. The specific steps of the precipitation reaction are as follows: Add 0.19 mL of the "ammonium solution" and 0.19 mL of the "lanthanum solution 1#" (the theoretically required dosage is 0.1875 mL, and 0.19 mL is taken considering the test accuracy) to 0.50 L of fluorine-containing water, stir for 10 minutes, and NH4F·LaF3 chemical precipitation occurs in the reaction tank.

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

[0041] 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. The ammonium chloride concentration of the "ammonium solution" added is converted to 8 mg / L, and the cost per ton of water is 0.008 yuan. The converted price of lanthanum oxide (including hydrochloric acid) is 5,290 yuan / ton, the converted lanthanum oxide concentration is 24.8 mg / L, and the cost per ton of water is 0.131 yuan. The PAM concentration 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: When the reagents are added in equivalent amounts, the goal of deep fluoride removal can be achieved, and the fluoride concentration in the effluent can be lower than 1 mg / L. Currently, the market price of lanthanum chloride is significantly higher than that of lanthanum oxide (including hydrochloric acid). From the perspective of economic benefits, it is advisable to use lanthanum oxide + hydrochloric acid reaction to obtain lanthanum ions as the effective La for chemical precipitation 3+ 。

[0043] Example 2

[0044] It should be noted that in this example, the ammonium chloride + lanthanum chloride method is adopted, the reagents are in excess, and the ammonium solution and lanthanum solution are added in a non-synchronous manner.

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

[0046] A method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation. The specific steps of the precipitation reaction are as follows: Add 0.30 mL of the "ammonium solution" and 0.30 mL of the "lanthanum solution 2#" to 0.50 L of fluorine-containing water (the theoretically required dosage is 0.1875 mL, but in this example, the ammonium salt and lanthanum salt reagents are more than 50% in excess). The fluoride ion concentration is 11.4 mg / L, and the measured original water fluoride concentration is 11.26 mg / L. Stir for 10 minutes, and carry out NH4F·LaF3 chemical precipitation in the reaction tank.

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

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

[0049] Example 3

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

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

[0052] A method for deep fluorine removal from sewage based on NH4F·LaF3 chemical precipitation. The specific steps of the precipitation reaction are as follows: First, take 0.19 mL of the "ammonium solution" and 0.19 mL of the "lanthanum solution 2#", mix them evenly, and then add them to 0.5 L of fluorine-containing water. After stirring for 10 minutes, carry out NH4F·LaF3 chemical precipitation in the reaction tank.

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

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

[0055] Example 4

[0056] It should be noted that in this embodiment, the ammonium chloride + lanthanum chloride method is adopted, the reagents are in equivalent amounts, and crystal seeds are pre-added, and the ammonium solution and the lanthanum solution are added in an asynchronous manner.

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

[0058] A method for deep fluorine removal from sewage based on NH4F·LaF3 chemical precipitation. The specific steps of the precipitation reaction are as follows:

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

[0060] Step 2: Discard the supernatant and wash the crystal seeds with deionized water.

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

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

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

[0064] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present invention patent. As long as the core technology is based on using ammonium salts (NH4 + ) and lanthanum salts (La 3+ ) to chemically precipitate fluoride ions (F - ) to remove fluoride ions in water, regardless of the process flow, chemical agent dosing method, and reactor type used, they all fall within the protection scope of this application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A method for deep fluoride removal from 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; When the required fluorine emission concentration ≤ 1 mg / L, add NH4 + and La 3+ at a concentration of 25.0% of the molar concentration of fluoride ions in the sewage. The reaction between fluoride ions and NH4 + and La 3+ is an equivalent reaction, and the reagent is not in excess; When the required fluorine emission concentration ≤ 0.6 mg / L, add NH4 + with a concentration greater than 25.0% of the molar concentration of fluoride ions in the sewage. The added NH4 + is in excess; La 3+ is still equivalent.

2. The method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation according to claim 1 is characterized in that: When the fluoride concentration in the raw water is relatively high, the fluoride concentration in the sewage is reduced by the calcium salt precipitation method.

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

4. A method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: The method of adding the solution containing NH4 + , La 3+ includes synchronous addition or asynchronous addition; Synchronous addition means first mixing the solution containing NH4 + and the solution containing La 3+ evenly, and then adding it to the fluorine-containing water; asynchronous addition means first adding the solution containing NH4 + to the fluorine-containing water, and then adding the solution containing La 3+ to the fluorine-containing water.

5. A method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: Before adding the solution containing NH4 + , La 3+ , add NH4F·LaF3 seeds to the fluorine-containing water.

6. The method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation according to claim 5, characterized in that: Add NH4F·LaF3 seeds by refluxing the discharged precipitate, or prepare NH4F·LaF3 seeds in situ. The preparation method of NH4F·LaF3 seeds is as follows: into a solution containing F - , sequentially add ammonium chloride and lanthanum chloride after dissolving them in water, stir, adjust the pH to 6 - 8, and let it stand to obtain seeds. Among them, the amounts of NH4 + and La 3+ are 25% of the molar concentration of fluoride ions.

7. A method for deep fluoride removal from sewage based on NH4F·LaF3 chemical precipitation according to claim 1, characterized in that: The method for removing crystal particles in water includes at least one of the flocculation precipitation method or the membrane filtration method.