Method for recovering sodium fluoride from sodium fluoride dilute solution

Abstract

The invention discloses a method for recovering sodium fluoride from a sodium fluoride dilute solution. The method comprises the following steps: 1) enriching a sodium fluoride dilute solution to obtain a sodium fluoride concentrated solution; 2) treating a part of the sodium fluoride concentrated solution through a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution; (3) adding the sodium hydroxide solution obtained in the step (2) into the rest sodium fluoride concentrated solution, carrying out salting-out reaction, and carrying out liquid-solid separation to obtain a sodium fluoride product and salting-out mother liquor; and 4) adding the hydrogen fluoride solution obtained in the step 2) into the salting-out mother liquor, carrying out a neutralization reaction, generating a sodium fluoride dilute solution, and returning to the step 1). According to the method, the fluorine resource in the sodium fluoride dilute solution can be recovered, the obtained sodium fluoride is high in recovery rate, the sodium fluoride product is high in purity, the energy consumption is low, the production cost is greatly reduced, the operation and the process are simple, the process is clean and pollution-free, and the method has remarkable environmental benefits and economic benefits and has a wide application prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of inorganic salt preparation and relates to a method for recovering sodium fluoride from a dilute sodium fluoride solution. Background Art

[0002] Sodium fluoride is an important chemical raw material, widely used in chemical industry, metallurgy, wood preservative, agricultural pesticide, brewing industry bactericide, pharmaceutical preservative, welding flux, alkaline zincate zinc plating additive, enamel, papermaking and other industries. At present, the main production methods of sodium fluoride are: melt leaching method, neutralization method, sodium fluorosilicate method, ion exchange method, etc.

[0003] The melt leaching method is to calcine fluorite, soda ash and quartz sand at a temperature of 800-1000°C, then leaching with water, and then evaporating, crystallizing and drying to obtain the finished sodium fluoride product; the neutralization method is to first neutralize hydrofluoric acid with soda ash or caustic soda solution, and then evaporate, crystallize and dry to obtain the finished sodium fluoride product; the sodium fluorosilicate method uses the waste gas containing fluorosilicic acid produced by the phosphate fertilizer plant as raw material, and can also use fluorosilicic acid or sodium fluorosilicate as raw materials to decompose in an alkaline solution to obtain white carbon black and sodium fluoride, but it is difficult to effectively separate white carbon black and sodium fluoride during the decomposition process, and the obtained sodium fluoride still needs to be washed with concentrated hot alkali solution. Even after washing with hot alkali solution, the sodium fluoride product usually still contains 0.4-0.5% silicon; the ion exchange method is to treat the sodium ion load exchange column with hydrofluoric acid to produce sodium fluoride.

[0004] The above sodium fluoride production methods all require that the solution contains F - The concentration of fluoride in industrial wastewater is more than 100g / L. - The concentration of sodium fluoride varies, and the composition is complex. The solubility of sodium fluoride in water is relatively large, and the solubility of NaF at room temperature is about 1 mol / L (42 g / L). Therefore, fluorine-containing wastewater is generally not used as a raw material for the production of sodium fluoride, especially low-concentration fluorine-containing wastewater. Because the fluorine concentration in the solution is low, the production process of sodium fluoride requires a lot of evaporation and concentration, which is economically uneconomical. At present, low-concentration fluorine-containing wastewater at home and abroad is generally treated by lime precipitation, which not only causes a serious waste of fluorine resources, but also produces a large amount of fluorine-containing solid waste.

[0005] In summary, it is urgent to develop a new method for recovering fluorine from low-concentration fluoride-containing wastewater with low cost, simple process and clean process, so as to achieve efficient extraction and recovery of fluorine. Summary of the invention

[0006] In view of the problems existing in the prior art, the object of the present invention is to provide a method for recovering sodium fluoride from a dilute sodium fluoride solution.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a method for recovering sodium fluoride from a dilute sodium fluoride solution, the method comprising the following steps:

[0009] (1) enriching the sodium fluoride dilute solution to obtain a sodium fluoride concentrated solution;

[0010] (2) treating a portion of the sodium fluoride concentrated solution obtained in step (1) through a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution;

[0011] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution to carry out a salting-out reaction, and after liquid-solid separation, obtaining a sodium fluoride product and a salting-out mother liquor;

[0012] (4) adding the hydrogen fluoride solution obtained in step (2) to the salting-out mother liquor of step (3) to carry out a neutralization reaction to generate a dilute sodium fluoride solution which is returned to step (1).

[0013] The method of the present invention enriches and concentrates a sodium fluoride dilute solution to obtain a sodium fluoride concentrated solution, a part of the sodium fluoride concentrated solution is subjected to bipolar membrane electrodialysis to obtain a sodium hydroxide solution and a hydrogen fluoride solution, which are respectively reused as a salting-out agent and a neutralizing agent, and another part of the sodium fluoride concentrated solution is reacted with a sodium hydroxide solution to precipitate a sodium fluoride product, and the salting-out agent and the neutralizing agent are circulated in the system, thereby improving the recovery rate of sodium fluoride. The method of the present invention can realize the recovery of fluorine resources in the sodium fluoride dilute solution, the recovery rate of the obtained sodium fluoride is high, the purity of the obtained sodium fluoride product is high, the energy consumption is low, the production cost is greatly reduced, the operation and process are simple, the process is clean and pollution-free, it has significant environmental benefits and economic benefits, and will have broad application prospects.

[0014] In the method of the present invention, the sodium hydroxide solution obtained by bipolar membrane electrodialysis is used as a salting-out agent for salting out the sodium fluoride concentrated solution, and the sodium fluoride therein is precipitated by salting out; the salting-out mother liquor generated in the salting-out process is neutralized with hydrofluoric acid to obtain a dilute sodium fluoride solution, and the dilute sodium fluoride solution is further enriched and concentrated to achieve recycling.

[0015] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0016] Preferably, the concentration of the sodium fluoride solution in step (1) is 1 to 40 g / L, for example, 1 g / L, 2 g / L, 5 g / L, 8 g / L, 10 g / L, 14 g / L, 18 g / L, 20 g / L, 22 g / L, 25 g / L, 28 g / L, 30 g / L, 33 g / L, 36 g / L, 38 g / L or 40 g / L, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0017] Preferably, the sodium fluoride dilute solution in step (1) is fluorine-containing wastewater, for example, fluorine-containing wastewater generated by electrolytic aluminum solid waste.

[0018] Preferably, the enrichment method in step (1) includes but is not limited to any one of electrodialysis, reverse osmosis or evaporation concentration. Those skilled in the art can select the method as needed, as long as the enrichment and concentration effect can be achieved.

[0019] Preferably, the temperature of the electrodialysis is 15°C to 45°C, for example, 15°C, 16°C, 16.5°C, 16.8°C, 17°C, 20°C, 23°C, 26°C, 28°C, 30°C, 36°C, 40°C, 41°C, 43°C or 45°C, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0020] Preferably, the electrodialysis is performed under constant current conditions.

[0021] Preferably, during the electrodialysis process, the gauge pressure of the pump is 0.3 MPa to 0.8 MPa, for example, it can be 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa or 0.8 MPa, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0022] Preferably, the concentration of the sodium fluoride concentrated solution in step (1) is 40 g / L to 50 g / L, for example, 41 g / L, 42 g / L, 43 g / L, 44 g / L, 45 g / L, 46 g / L, 47 g / L, 48 g / L, 49 g / L or 50 g / L, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0023] As a preferred technical solution of the method of the present invention, the amount of the concentrated sodium fluoride solution in step (2) is 30% to 70% of the total amount, for example, 30%, 32%, 34%, 35%, 37%, 38%, 40%, 43%, 46%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68% or 70%, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable, preferably 40% to 60%.

[0024] Preferably, the bipolar membrane electrodialysis device in step (2) comprises a cathode, an anode, a cathode chamber, an alkali chamber, a salt chamber, an acid chamber, an anode chamber, an anion exchange membrane, a cation exchange membrane, a first bipolar membrane and a second bipolar membrane, the cathode chamber is placed in a first conductive solution, the anode chamber is placed in a second conductive solution, the salt chamber is placed in the sodium fluoride concentrated solution, the alkali chamber is used to hold a sodium hydroxide solution, and the acid chamber is used to hold a hydrogen fluoride solution.

[0025] In one embodiment, in the bipolar membrane electrodialysis device, the order from left to right is anode→anode chamber→first bipolar membrane→acid chamber→anion exchange membrane→salt chamber→cation exchange membrane→alkali chamber→second bipolar membrane→cathode chamber→cathode.

[0026] In the method of the present invention, when a bipolar membrane electrodialysis device is used to treat a concentrated sodium fluoride solution, a direct current needs to be passed through the bipolar membrane electrodialysis device, and the OH generated by the electrodialysis of the first bipolar membrane and the second bipolar membrane is - and H + , driven by an electric field, OH - Flowing to the anode chamber and the alkali chamber, H + Specifically, the H flowing into the cathode chamber + and OH produced by the cathode reaction - Neutralization: the pH of the first conductive solution in the cathode chamber remains unchanged; the OH flowing into the alkali chamber - Na flows from the salt chamber through the cation exchange membrane + Sodium hydroxide is produced; OH flowing into the anode chamber - and H produced by the anode reaction + Neutralization: the pH of the second conductive solution in the anode chamber remains unchanged; the H + and F flowing from the salt chamber through the anion exchange membrane - The reaction generates hydrogen fluoride, thereby achieving charge balance and obtaining a hydrogen fluoride solution and a sodium hydroxide solution with a higher concentration.

[0027] Preferably, the first conductive solution and the second conductive solution are both aqueous solutions of water-soluble salts.

[0028] Preferably, the water-soluble salt is selected from any one of sulfate, nitrate or chloride, or a combination of at least two thereof.

[0029] Preferably, the sulfate includes any one of sodium sulfate, potassium sulfate or magnesium sulfate, or a combination of at least two of them.

[0030] Preferably, the nitrate includes any one of sodium nitrate, potassium nitrate, magnesium nitrate or calcium nitrate, or a combination of at least two of them.

[0031] Preferably, the chloride includes any one of sodium chloride, potassium chloride, magnesium chloride or calcium chloride, or a combination of at least two thereof.

[0032] Preferably, the concentration of the sodium hydroxide solution is 60 g / L to 80 g / L, for example 60 g / L, 63 g / L, 66 g / L, 68 g / L, 69 g / L, 71 g / L, 74 g / L, 78 g / L, 78 g / L or 80 g / L, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0033] Preferably, the concentration of the hydrogen fluoride solution is 1.5 mol / L to 2 mol / L, for example, it can be 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L or 2 mol / L.

[0034] As a preferred technical solution of the method of the present invention, in step (3), the volume ratio of the concentrated sodium fluoride solution to the sodium hydroxide solution is 1:(1-10), for example, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9 or 1:10, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0035] Preferably, the temperature of the salting-out reaction in step (3) is 25-60°C, for example, 25°C, 27.5°C, 30°C, 32.5°C, 35°C, 40°C, 47.5°C, 52°C, 57°C or 60°C, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0036] Preferably, the salting-out reaction in step (3) is 0.25 to 2 h, for example, 0.25 h, 0.3 h, 0.5 h, 0.75 h, 1 h, 1.2 h, 1.25 h, 1.3 h, 1.5 h, 1.75 h or 2 h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0037] In the present invention, the differences in solubility and crystallization properties of sodium fluoride in a solution in water and a sodium hydroxide mixture are studied and utilized, and a sodium fluoride precipitation crystallization reaction is carried out under the synergistic effect of a certain reagent ratio, reaction temperature and reaction time, so that a sodium fluoride product with a higher recovery rate and purity can be obtained.

[0038] In the present invention, optionally, the sodium fluoride product is washed with pure water, the washing liquid is combined with the filtrate, and the filter cake is dried.

[0039] As a preferred technical solution of the method of the present invention, the pH of the neutralization reaction in step (4) satisfies: pH = 4 to 12, for example 4, 4.5, 5, 5.2, 5.6, 6, 6.8, 7, 7.5, 8, 9, 10, 11 or 12, etc., but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable. Preferably, pH = 6 to 10, and more preferably pH = 6 to 8.

[0040] Preferably, the neutralization reaction time in step (4) is 0.25 to 2 h, for example, 0.25 h, 0.3 h, 0.5 h, 0.75 h, 1 h, 1.2 h, 1.25 h, 1.3 h, 1.5 h, 1.75 h or 2 h, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0041] As a preferred technical solution of the method of the present invention, the method comprises the following steps:

[0042] (1) a sodium fluoride dilute solution with a concentration of 1 g / L to 40 g / L is subjected to electrodialysis to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 40 g / L to 50 g / L; wherein the electrodialysis temperature is 15° C. to 45° C., the electrodialysis is carried out under constant current conditions, the voltage value is 40 V, and the current value is 92 A;

[0043] (2) treating 30% to 70% of the concentrated sodium fluoride solution obtained in step (1) through a bipolar membrane electrodialysis device to obtain a 60 to 80 g / L sodium hydroxide solution and a 1.5 mol / L to 2 mol / L hydrogen fluoride solution, wherein the sodium hydroxide solution is used as a salting-out agent in step (3), and the hydrogen fluoride solution is used as a neutralizing agent in step (4);

[0044] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:(1-10), and salting out reaction is carried out at 25-60° C. for 0.25-2 h, and liquid-solid separation is performed to obtain a sodium fluoride product and a salting out mother liquor;

[0045] (4) adding the hydrogen fluoride solution obtained in step (2) to the salting-out mother liquor of step (3), adjusting the pH to 4-12, neutralizing for 0.25-2h, generating a dilute sodium fluoride solution, returning the dilute sodium fluoride solution to the dilute sodium fluoride solution of step (1), and continuing electrodialysis to increase the concentration.

[0046] The numerical range described in the present invention not only includes the point values ​​listed above, but also includes any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] The method of the present invention enriches and concentrates a sodium fluoride dilute solution to obtain a sodium fluoride concentrated solution, a part of the sodium fluoride concentrated solution is subjected to bipolar membrane electrodialysis to obtain a sodium hydroxide solution and a hydrogen fluoride solution, which are respectively reused as a salting-out agent and a neutralizing agent, and another part of the sodium fluoride concentrated solution is reacted with a sodium hydroxide solution to precipitate a sodium fluoride product, and the salting-out agent and the neutralizing agent are circulated in the system, thereby improving the recovery rate of sodium fluoride. The method of the present invention can realize the recovery of fluorine resources in the sodium fluoride dilute solution, the recovery rate of the obtained sodium fluoride is high, the purity of the obtained sodium fluoride product is high, the energy consumption is low, the production cost is greatly reduced, the operation and process are simple, the process is clean and pollution-free, it has significant environmental benefits and economic benefits, and will have broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 The present invention provides a process flow chart of a method for recovering sodium fluoride from a dilute sodium fluoride solution according to one embodiment of the present invention. DETAILED DESCRIPTION

[0050] In order to better illustrate the present invention and facilitate understanding of the technical solution of the present invention, the present invention is further described in detail below. However, the following embodiments are only simple examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0051] The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0052] The specific embodiment of the present invention provides a method for recovering sodium fluoride from a dilute sodium fluoride solution, and the process flow chart thereof is shown in Figure 1 , the method comprises the following steps:

[0053] (1) The dilute sodium fluoride solution is concentrated by electrodialysis to obtain a concentrated sodium fluoride solution (referred to as concentrated solution);

[0054] (2) treating a portion of the concentrated sodium fluoride solution obtained in step (1) by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the sodium hydroxide solution is used as a salting-out agent and the hydrogen fluoride solution is used as a neutralizing agent;

[0055] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (1) to carry out salting out reaction, and performing liquid-solid separation to obtain a sodium fluoride product and a salting out mother liquor;

[0056] (4) adding the hydrogen fluoride solution of step (2) to the salting-out mother liquor of step (3) to carry out a neutralization reaction to generate a dilute sodium fluoride solution which is returned to step (1).

[0057] The following are typical but non-limiting embodiments of the present invention:

[0058] Example 1

[0059] This embodiment provides a method for recovering sodium fluoride from a dilute sodium fluoride solution, the method comprising the following steps:

[0060] (1) A sodium fluoride dilute solution with a concentration of 2 g / L is enriched and concentrated by electrodialysis to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 40 g / L; wherein the electrodialysis temperature is 15° C., the electrodialysis is performed under constant current conditions, and during the electrodialysis process, the gauge pressure of the pump is 0.3 MPa.

[0061] (2) 30% of the sodium fluoride concentrated solution obtained in step (1) is treated by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the concentration of the obtained sodium hydroxide solution is 60 g / L, and the concentration of the hydrofluoric acid solution is 1.5 mol / L, the sodium hydroxide solution is used as a salting-out agent, and the hydrogen fluoride solution is used as a neutralizing agent.

[0062] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the concentrated sodium fluoride solution in step (2), wherein the volume ratio of the remaining portion of the concentrated sodium fluoride solution to the sodium hydroxide solution is 1:1, reacting at 25° C. for 0.25 h, and performing liquid-solid separation to obtain a sodium fluoride product and a salting-out filtrate;

[0063] (4) Add hydrogen fluoride solution to the salting-out mother liquor of step (3) to adjust the pH to 4, and carry out neutralization reaction for 0.25 h to generate a dilute sodium fluoride solution, which is returned to step (1) to continue electrodialysis for concentration.

[0064] Example 2

[0065] (1) a sodium fluoride dilute solution with a concentration of 10 g / L is subjected to electrodialysis at a temperature of 35° C., a pump gauge pressure of 0.5 MPa, and a constant flow to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 45 g / L;

[0066] (2) 35% of the sodium fluoride concentrated solution obtained in step (1) is treated by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the concentration of the obtained sodium hydroxide solution is 70 g / L, and the concentration of the hydrofluoric acid solution is 1.5 mol / L, the sodium hydroxide solution is used as a salting-out agent, and the hydrogen fluoride solution is used as a neutralizing agent.

[0067] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:2.5, and salting out at 40° C. for 2 h, and liquid-solid separation is performed to obtain sodium fluoride and a salting out filtrate;

[0068] (4) Add hydrogen fluoride solution to the salting-out mother liquor of step (3) to adjust the pH to 7, and carry out neutralization reaction for 2 hours to generate a dilute sodium fluoride solution, which is returned to step (1) to continue electrodialysis for concentration.

[0069] Example 3

[0070] (1) a sodium fluoride dilute solution with a concentration of 10 g / L is subjected to electrodialysis at a temperature of 35° C., a pump gauge pressure of 0.3 MPa, and a constant flow to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 42 g / L;

[0071] (2) 50% of the sodium fluoride concentrated solution obtained in step (1) is treated by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the concentration of the obtained sodium hydroxide solution is 65 g / L, and the concentration of the hydrofluoric acid solution is 1.6 mol / L, the sodium hydroxide solution is used as a salting-out agent, and the hydrogen fluoride solution is used as a neutralizing agent.

[0072] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:1.5, reacting at 25° C. for 1.5 h, and performing liquid-solid separation to obtain sodium fluoride and a salting-out filtrate;

[0073] (4) Adding hydrogen fluoride solution to the salting-out mother liquor of step (3) to adjust the pH to 6, and carrying out neutralization reaction for 1 hour to generate a dilute sodium fluoride solution, and returning the dilute sodium fluoride solution to step (1) to continue electrodialysis for concentration.

[0074] Example 4

[0075] (1) a sodium fluoride dilute solution with a concentration of 10 g / L is subjected to electrodialysis at a temperature of 25° C., a pump gauge pressure of 0.4 MPa, and a constant flow to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 42 g / L;

[0076] (2) 70% of the sodium fluoride concentrated solution obtained in step (1) is treated by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the concentration of the obtained sodium hydroxide solution is 80 g / L, and the concentration of the hydrofluoric acid solution is 1.6 mol / L, the sodium hydroxide solution is used as a salting-out agent, and the hydrogen fluoride solution is used as a neutralizing agent.

[0077] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:2, reacting at 30° C. for 0.5 h, and performing liquid-solid separation to obtain sodium fluoride and a salting-out filtrate;

[0078] (4) Add hydrogen fluoride solution to the salting-out mother liquor of step (3) to adjust the pH to 6, and carry out neutralization reaction for 0.5 h to generate a dilute sodium fluoride solution, which is returned to step (1) to continue electrodialysis for concentration.

[0079] Example 5

[0080] (1) a sodium fluoride dilute solution with a concentration of 15 g / L is subjected to electrodialysis at a temperature of 25° C., a pump gauge pressure of 0.4 MPa, and a constant flow to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 45 g / L;

[0081] (2) 55% of the sodium fluoride concentrated solution obtained in step (1) is treated by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the concentration of the obtained sodium hydroxide solution is 65 g / L, and the concentration of the hydrofluoric acid solution is 2 mol / L, the sodium hydroxide solution is used as a salting-out agent, and the hydrogen fluoride solution is used as a neutralizing agent.

[0082] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:4, reacting at 30° C. for 1 h, and performing liquid-solid separation to obtain sodium fluoride and a salting-out filtrate;

[0083] (4) Adding hydrogen fluoride solution to the salting-out mother liquor of step (3) to adjust the pH to 6, and carrying out neutralization reaction for 1 hour to generate a dilute sodium fluoride solution, and returning the dilute sodium fluoride solution to step (1) to continue electrodialysis for concentration.

[0084] Example 6

[0085] (1) a sodium fluoride dilute solution with a concentration of 15 g / L is subjected to electrodialysis at a temperature of 30° C., a pump gauge pressure of 0.5 MPa, and a constant flow to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 45 g / L;

[0086] (2) 60% of the sodium fluoride concentrated solution obtained in step (1) is treated by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the concentration of the obtained sodium hydroxide solution is 70 g / L, and the concentration of the hydrofluoric acid solution is 1.7 mol / L, the sodium hydroxide solution is used as a salting-out agent, and the hydrogen fluoride solution is used as a neutralizing agent.

[0087] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:3, reacting at 35° C. for 1.5 h, and performing liquid-solid separation to obtain sodium fluoride and a salting-out filtrate;

[0088] (4) Adding hydrogen fluoride solution to the salting-out mother liquor of step (3) to adjust the pH to 7, and carrying out neutralization reaction for 1.5 hours to generate a dilute sodium fluoride solution, and returning the dilute sodium fluoride solution to step (1) to continue electrodialysis for concentration.

[0089] Example 7

[0090] (1) a sodium fluoride dilute solution with a concentration of 15 g / L is subjected to electrodialysis at a temperature of 35° C., a pump gauge pressure of 0.6 MPa, and a constant flow to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 48 g / L;

[0091] (2) 45% of the sodium fluoride concentrated solution obtained in step (1) is treated by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the concentration of the obtained sodium hydroxide solution is 75 g / L, and the concentration of the hydrofluoric acid solution is 2 mol / L, the sodium hydroxide solution is used as a salting-out agent, and the hydrogen fluoride solution is used as a neutralizing agent.

[0092] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:4, reacting at 50° C. for 2 h, and performing liquid-solid separation to obtain sodium fluoride and a salting-out filtrate;

[0093] (4) Add hydrogen fluoride solution to the salting-out mother liquor of step (3) to adjust the pH to 7, and carry out neutralization reaction for 2 hours to generate a dilute sodium fluoride solution, which is returned to step (1) to continue electrodialysis for concentration.

[0094] Example 8

[0095] (1) a sodium fluoride dilute solution with a concentration of 20 g / L is subjected to electrodialysis at a temperature of 45° C., a pump gauge pressure of 0.8 MPa, and a constant flow to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 50 g / L;

[0096] (2) 55% of the sodium fluoride concentrated solution obtained in step (1) is treated by a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution, wherein the concentration of the obtained sodium hydroxide solution is 80 g / L, and the concentration of the hydrofluoric acid solution is 2 mol / L, the sodium hydroxide solution is used as a salting-out agent, and the hydrogen fluoride solution is used as a neutralizing agent.

[0097] (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:5, reacting at 60° C. for 2 h, and performing liquid-solid separation to obtain sodium fluoride and a salting-out filtrate;

[0098] (4) Adding hydrogen fluoride solution to the salting-out mother liquor of step (3) to adjust the pH to 8, and carrying out neutralization reaction for 2 hours to generate a dilute sodium fluoride solution, and returning the dilute sodium fluoride solution to step (1) to continue electrodialysis for concentration.

[0099] Comparative Example 1

[0100] This comparative example provides a method for recovering sodium fluoride from a dilute sodium fluoride solution. The raw materials and the method are the same as those in Example 1, except that there is no step (2) of treating with a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and hydrofluoric acid.

[0101] After multiple rounds of recycling treatment, the fluorine recovery efficiency of this comparative example is low and the reagent investment cost is high.

[0102] Comparative Example 2

[0103] This comparative example provides a method for recovering sodium fluoride from a dilute sodium fluoride solution. The raw materials and the method are referred to Example 1, except that the volume ratio of the remaining sodium fluoride concentrated solution to the sodium hydroxide solution is 1:0.5, the reaction is carried out at 25° C. for 0.25 h, and the liquid-solid separation is performed to obtain sodium fluoride and a salting-out filtrate.

[0104] The single-round fluorine recovery rate of this comparative example is low, and the fluorine recovery efficiency of multiple-round circulation treatment is low.

[0105] The fluorine recovery rates and sodium fluoride product purities of Examples 1-8 and Comparative Examples 1-2 are shown in Table 1 below:

[0106] Table 1

[0107]

[0108] In summary, the method of the present invention has a simple process, low production cost, clean process and no pollution, high fluorine recovery efficiency, and the obtained sodium fluoride product has high purity. It has broad application prospects in recovering fluorine resources.

[0109] The applicant declares that the present invention illustrates the detailed method of the present invention through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned detailed method, that is, it does not mean that the present invention must rely on the above-mentioned detailed method to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of various raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for recovering sodium fluoride from a dilute sodium fluoride solution, It is characterized in that The method comprises the following steps: (1) enriching the sodium fluoride dilute solution to obtain a sodium fluoride concentrated solution; (2) treating a portion of the sodium fluoride concentrated solution obtained in step (1) through a bipolar membrane electrodialysis device to obtain a sodium hydroxide solution and a hydrogen fluoride solution; (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution to carry out a salting-out reaction, and after liquid-solid separation, obtaining a sodium fluoride product and a salting-out mother liquor; (4) adding the hydrogen fluoride solution obtained in step (2) to the salting-out mother liquor of step (3) to carry out a neutralization reaction to generate a dilute sodium fluoride solution which is returned to step (1).

2. The method according to claim 1, It is characterized in that The concentration of the sodium fluoride dilute solution in step (1) is 1 to 40 g / L; Preferably, the dilute sodium fluoride solution in step (1) is fluoride-containing wastewater.

3. The method according to claim 1 or 2, It is characterized in that The enrichment method in step (1) includes any one of electrodialysis, reverse osmosis or evaporation concentration; Preferably, the temperature of the electrodialysis is 15°C to 45°C; Preferably, the electrodialysis is carried out under constant current conditions; Preferably, during the electrodialysis process, the gauge pressure of the pump is 0.3 MPa to 0.8 MPa.

4. The method according to any one of claims 1 to 3, It is characterized in that The concentration of the sodium fluoride concentrated solution in step (1) is 40 g / L to 50 g / L.

5. The method according to any one of claims 1 to 4, It is characterized in that The amount of the sodium fluoride concentrated solution in step (2) is 30% to 70% of the total amount, preferably 40% to 60%.

6. The method according to any one of claims 1 to 5, It is characterized in that The bipolar membrane electrodialysis device in step (2) comprises a cathode, an anode, a cathode chamber, an alkali chamber, a salt chamber, an acid chamber, an anode chamber, an anion exchange membrane, a cation exchange membrane, a first bipolar membrane and a second bipolar membrane, wherein the cathode chamber is provided with a first conductive solution, the anode chamber is provided with a second conductive solution, the salt chamber is provided with the sodium fluoride concentrated solution, the alkali chamber is used to contain a sodium hydroxide solution, and the acid chamber is used to contain a hydrogen fluoride solution; Preferably, the first conductive solution and the second conductive solution are both aqueous solutions of water-soluble salts; Preferably, the water-soluble salt is selected from any one of sulfate, nitrate or chloride, or a combination of at least two thereof; Preferably, the sulfate includes any one of sodium sulfate, potassium sulfate or magnesium sulfate, or a combination of at least two thereof; Preferably, the nitrate includes any one of sodium nitrate, potassium nitrate, magnesium nitrate or calcium nitrate, or a combination of at least two thereof; Preferably, the chloride includes any one of sodium chloride, potassium chloride, magnesium chloride or calcium chloride, or a combination of at least two thereof.

7. The method according to any one of claims 1 to 6, It is characterized in that The concentration of the sodium hydroxide solution is 60g / L to 80g / L; Preferably, the concentration of the hydrogen fluoride solution is 1.5 mol / L to 2 mol / L.

8. The method according to any one of claims 1 to 7, It is characterized in that In step (3), the volume ratio of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:(1-10); Preferably, the temperature of the salting-out reaction in step (3) is 25 to 60° C. Preferably, the salting-out reaction in step (3) lasts for 0.25 to 2 hours.

9. The method according to any one of claims 1 to 8, It is characterized in that The pH of the neutralization reaction in step (4) satisfies: pH=4-12, preferably pH=6-10, more preferably pH=6-8; Preferably, the neutralization reaction time in step (4) is 0.25 to 2 hours.

10. The method according to any one of claims 1 to 9, It is characterized in that The method comprises the following steps: (1) a sodium fluoride dilute solution with a concentration of 1 g / L to 40 g / L is subjected to electrodialysis to obtain a sodium fluoride concentrated solution with a sodium fluoride concentration of 40 g / L to 50 g / L; wherein the electrodialysis temperature is 15° C. to 45° C., the electrodialysis is carried out under constant current conditions, the voltage value is 40 V, and the current value is 92 A; (2) treating 30% to 70% of the concentrated sodium fluoride solution obtained in step (1) through a bipolar membrane electrodialysis device to obtain a 60 to 80 g / L sodium hydroxide solution and a 1.5 mol / L to 2 mol / L hydrogen fluoride solution, wherein the sodium hydroxide solution is used as a salting-out agent in step (3), and the hydrogen fluoride solution is used as a neutralizing agent in step (4); (3) adding the sodium hydroxide solution obtained in step (2) to the remaining portion of the sodium fluoride concentrated solution in step (2), wherein the volume ratio of the remaining portion of the sodium fluoride concentrated solution to the sodium hydroxide solution is 1:(1-10), and salting out reaction is carried out at 25-60° C. for 0.25-2 h, and liquid-solid separation is performed to obtain a sodium fluoride product and a salting out mother liquor; (4) adding the hydrogen fluoride solution obtained in step (2) to the salting-out mother liquor of step (3), adjusting the pH to 4-12, neutralizing for 0.25-2h, generating a dilute sodium fluoride solution, returning the dilute sodium fluoride solution to the dilute sodium fluoride solution of step (1), and continuing electrodialysis to increase the concentration.

Images