Method for removing fluorine from a sodium fluoroaluminate solution

Abstract

The present application belongs to the field of metallurgy, and discloses a method for removing fluorine from a sodium fluoroaluminate solution. A fluorine separation agent is added during evaporation and concentration of seed separation mother liquor in alumina production, and the mother liquor is evaporated and concentrated or cooled after evaporation and concentration to separate complex salt, and the fluorine-removed mother liquor and fluorine-removed residue are obtained through liquid-solid separation. Alternatively, a sodium fluoroaluminate evaporation mother liquor containing fluorine is obtained after evaporation and concentration of seed separation mother liquor in alumina production, and a fluorine separation agent is added to the sodium fluoroaluminate evaporation mother liquor to form complex salt from sodium fluoride in the solution and the fluorine separation agent, and the fluorine-removed mother liquor and fluorine-removed residue are obtained through liquid-solid separation. The method has a short process flow, simple equipment, low cost, and is easy to implement in industry, can efficiently remove fluorine in sodium aluminate solution, and does not introduce other impurity components.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a method for removing fluoride from sodium fluoroaluminate solution. Background Technology

[0002] Fluorine is an important trace impurity element in bauxite, typically ranging from 0.01% to 0.30%. During the Bayer process for alumina production, fluorine dissolves in the alkali solution to form NaF, which then enters the solution. Excessive accumulation of NaF in the sodium aluminate solution can cause various problems in the production process. Even small amounts of fluorine in the solution can lead to finer particle size in the aluminum hydroxide product. Fluorine accumulation can cause scaling on the surface of heat exchangers or pipes during the evaporation of the sodium aluminate solution or the Bayer leaching process, severely affecting the continuous and stable operation of production. Fluorine in the sodium aluminate solution can also affect the causticizing reaction efficiency of the mother liquor evaporation and salt residue removal.

[0003] In the traditional Bayer process for alumina production from gibbsite, 8-10% lime is added during the high-pressure leaching of bauxite to mitigate the harmful effects of titanium minerals on alumina leaching and reduce alkali consumption. Under lime-added conditions, some fluorine in the production system reacts with calcium-containing substances to form CaF2 (or replaces the hydroxyl groups in tricalcium aluminate hydrate) and enters the red mud, thus maintaining a low fluorine level in the solution. However, adding lime in the Bayer process not only results in alumina loss but also leads to a significant increase in red mud. In recent years, driven by the demand for red mud reduction, reducing or eliminating lime addition in the Bayer process of bauxite leaching has become a trend. Furthermore, for gibbsite, which is widely used in the alumina industry, lime is generally not added during the leaching process. Under Bayer leaching conditions without lime, the gradual accumulation of fluorine in the Bayer solution becomes one of the main problems affecting alumina production, requiring a more effective solution.

[0004] Currently, commonly used methods for removing fluoride from sodium aluminate solutions include: (1) Process of adding lime during leaching: When lime is added during leaching, lime preferentially forms hydrated calcium aluminate, resulting in a large loss of alumina and an increase in the amount of red mud. At the same time, it also makes the composition of red mud complex and makes it difficult to make comprehensive utilization of red mud.

[0005] (2) Evaporation-cooling crystallization method: As the evaporation depth increases, the concentration of caustic alkali in the solution increases, and sodium fluoride precipitates due to its decreased solubility in the solution. However, the equilibrium solubility of sodium fluoride in high-concentration sodium aluminate solution is not low enough, resulting in a still high concentration of fluoride in the defluorination solution, making it difficult to achieve deep defluorination. To achieve deep defluorination, the solution needs to be deeply evaporated or evaporated and then cooled for crystallization. However, the higher the concentration of sodium aluminate solution, the more difficult it is to evaporate, the higher the energy consumption of evaporation, the more difficult the liquid-solid separation during evaporation and crystallization, and the high-concentration sodium aluminate adsorbate carried away by the sodium fluoride crystallization is also high.

[0006] (3) Defluorination by adding lime / calcium aluminate to the solution: When lime is added to the sodium fluoroaluminate solution, the lime preferentially reacts with the aluminate ions in the solution to form hydrated calcium aluminate, resulting in a loss of alumina. The hydrated calcium aluminate formed by F substitution has a low F content, resulting in a large amount of slag formed during defluorination. In addition, the concentration of sodium aluminate solution in alumina production systems is generally high. Within this concentration range, the utilization efficiency of lime in defluorination reported in the literature is very low, resulting in low defluorination efficiency and large alumina loss. Summary of the Invention

[0007] In view of the problems existing in the prior art, the purpose of this invention is to provide a method for defluorinating sodium fluoroaluminate solution.

[0008] To achieve the objectives of this invention, the specific technical solution is as follows: A method for removing fluoride from a sodium fluoroaluminate solution involves adding a fluoride-removing agent during the evaporation and concentration of the mother liquor produced from alumina to carry out a fluoride removal reaction. During or after the evaporation and concentration of the mother liquor, a double salt is precipitated in the solution, and the defluorinated mother liquor and defluorinated residue are obtained through liquid-solid separation.

[0009] Adding a fluoride-releasing agent to a high-concentration sodium aluminate solution allows the agent to react with fluoride ions in the solution to form fluoride-containing double salts with lower solubility, thus precipitating out fluoride and achieving fluoride removal from the sodium aluminate solution. Utilizing the characteristic that the solubility of these double salts decreases with increasing alkali concentration in the sodium aluminate solution, this invention adds the fluoride-releasing agent before or during the evaporation and concentration of the seed mother liquor. As the alkali concentration of the solution increases during evaporation, fluoride precipitates out as double salts, resulting in a high fluoride removal rate. Adding the fluoride-releasing agent before evaporation and concentration allows a portion of the agent to fully dissolve and enter the solution. As evaporation proceeds, the agent reacts fully with fluorides to form double salts, resulting in good fluoride removal and relatively low agent dosage. Adding defluorination residue as seed crystals to the solution or the solution after evaporation and concentration accelerates the precipitation of fluoride-containing double salts, forming coarser defluorination residue particles, further reducing the fluoride concentration in the defluorinated purified solution.

[0010] A method for removing fluoride from a sodium fluoride aluminate solution involves evaporating and concentrating a seed mother liquor from alumina production to obtain a fluoride-containing sodium aluminate evaporation mother liquor. A fluoride-releasing agent is added to the fluoride-containing sodium aluminate evaporation mother liquor to precipitate a double salt. After liquid-solid separation, a defluorinated mother liquor and a defluorinated residue are obtained.

[0011] After the mother liquor is evaporated and concentrated, the solubility of sodium fluoride in the solution decreases. This invention chooses to add a fluoride-releasing agent at this point, which can form a double salt with fluoride in the solution. Accompanying the dissolution-re-precipitation reaction of the fluoride-releasing agent in the solution, fluoride in the solution precipitates out as a double salt under the induction of the fluoride-releasing agent, thus reducing the fluoride concentration in the solution. Adding the fluoride-releasing agent or seed crystals after the mother liquor has been evaporated and concentrated helps to avoid the large-scale precipitation of fluoride-containing double salts during the evaporation process, thereby reducing the risk of salt scaling and evaporator blockage in the evaporator.

[0012] Furthermore, the fluoride-releasing agent contains at least one of sulfate ions, phosphate ions, oxalate ions, and vanadate ions.

[0013] Further preferably, the fluoride-removing agent is at least one of sulfate, phosphate, vanadate, oxalate, and defluorination residue.

[0014] Further preferably, the fluoride-removing agent is at least one of sodium sulfate, sodium phosphate, sodium vanadate, sodium oxalate, and fluoride-removing residue.

[0015] Further preferred is that the fluoride-removing agent is sodium sulfate or a combination of sodium sulfate and fluoride-removing residue.

[0016] Furthermore, the molar amount of the fluoride-emitting agent added is 0.5 to 3.0 times the molar amount of fluorine in the solution.

[0017] Furthermore, after adding the fluoride-removing agent, a fluoride removal reaction is carried out by stirring. The reaction temperature is 40~120℃, and the reaction time is 0.5~6 h.

[0018] More preferably, the slurry after the defluorination reaction is cooled down, and the reaction is continued to be stirred for 0.5 to 4.0 h while cooling down, with the final temperature of the reaction being 40 to 70°C, and then solid-liquid separation is performed.

[0019] Furthermore, the defluorinated residue can be returned to the seed mother liquor or the fluorine-containing sodium aluminate evaporation mother liquor as a fluorine-releasing agent.

[0020] Furthermore, the caustic soda concentration of the fluorine-containing sodium aluminate evaporation mother liquor is 185~300 g / L.

[0021] Furthermore, the sodium aluminate evaporation mother liquor obtained after evaporating and concentrating the seed liquor from alumina production is cooled before adding a fluoride-removing agent. A subsequent cooling step can be added to further reduce the solubility of fluorides and double salts, thereby increasing the depth of fluoride removal from the solution.

[0022] More preferably, the fluorine-containing sodium aluminate evaporation mother liquor is cooled to 40~80°C before a fluorine-releasing agent is added.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Compared with the traditional evaporation crystallization method for defluorination, this invention makes up for the deficiency of the evaporation-cooling crystallization method in that it cannot deeply defluorinate by forming a fluorine-containing complex salt with lower solubility, and can effectively and deeply defluorinate the mother liquor of the Bayer process. The solution evaporation depth of the defluorination process is small and can be carried out at a relatively high temperature, with low energy consumption and easy operation.

[0024] (2) Compared with adding lime to remove fluoride, the present invention forms aluminum-free double salts of F ions in the solution. The double salts have a high fluoride content, and theoretically there is no loss of aluminum oxide in the fluoride removal process.

[0025] (3) This invention is particularly suitable for industrial applications. In industry, mother liquor evaporation and salt removal are basic processes in the Bayer process for alumina production. The defluorination method for forming double salts described in this invention is easy to combine with mother liquor evaporation in the alumina production process and is easy to implement. Moreover, this invention does not use lime, which is beneficial to the significant reduction of red mud emissions in the alumina production process.

[0026] (4) This invention can effectively improve the defluorination depth and the concentration of fluoride ions in the solution (according to F). - The concentration of the volatile organic compound (VOC) can be reduced from 0.8~1.5 g / L to 0.2~0.5 g / L by the evaporation-cooling crystallization method. Attached Figure Description

[0027] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is the process flow of the present invention. Figure 1 .

[0028] Figure 2 This is the process flow of the present invention. Figure 2 . Detailed Implementation

[0029] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0030] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0031] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0032] Example 1 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 1 As shown, it includes the following steps: Sodium fluoroaluminate solution, caustic alkali concentration 147 g / L (with Na2O) k (Calculated, the same below), caustic ratio 2.92, initial fluoride ion concentration 1.88 g / L; Add sodium vanadate, a fluoride-releasing agent, to the above sodium aluminate solution. The amount added is 1.1 times the molar amount of fluorine in the solution. Evaporate and concentrate the solution after adding the fluoride-releasing agent. The concentration of caustic alkali in the mother liquor is 260 g / L. The above-mentioned mother liquor was stirred at 90°C for 2 hours to carry out further defluorination reaction; The slurry after the defluorination reaction was slowly cooled to 50°C and kept at that temperature for 2.0 h under stirring conditions. After solid-liquid separation, the slurry was defluorinated semen, and the fluoride ion concentration in the semen was reduced to 0.43 g / L.

[0033] Example 2 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 1 As shown, it includes the following steps: The sodium fluoroaluminate solution has a caustic alkali concentration of 147 g / L, a caustic ratio of 2.92, and an initial fluoride ion concentration of 1.88 g / L. Sodium sulfate, a fluoride-releasing agent, was added to the above sodium aluminate solution at an amount 1.05 times the molar amount of fluoride in the solution. After evaporation and concentration, the concentration of caustic alkali in the mother liquor was 265 g / L. The above-mentioned mother liquor was stirred at 80°C for 3 hours to carry out further defluorination reaction.

[0034] After solid-liquid separation, the slurry was used to obtain defluorinated semen, and the concentration of fluoride ions in the semen was reduced to 0.38 g / L.

[0035] Example 3 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 1 As shown, it includes the following steps: The sodium fluoroaluminate solution has a caustic alkali concentration of 150 g / L, a caustic ratio of 3.0, and an initial fluoride ion concentration of 1.81 g / L. Sodium sulfate and sodium carbonate, the fluoride-releasing agents, were added to the above sodium aluminate solution. The amount of sodium sulfate added was 1.25 times the molar amount of fluoride in the solution, and the amount of sodium carbonate added was 1.0 times the molar amount of fluoride in the solution. After evaporation and concentration, the concentration of caustic alkali in the mother liquor was 250 g / L. The above-mentioned mother liquor was stirred at 80°C for 0.5 h to carry out further defluorination reaction.

[0036] After solid-liquid separation, the slurry yielded defluorinated semen, with the fluoride ion concentration reduced to 0.30 g / L and SO42-. 2- Concentration 4.30 g / L.

[0037] Example 4 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 1 As shown, it includes the following steps: A certain alumina plant has a mother liquor with a caustic alkali concentration of 145 g / L, a caustic ratio of 3.02, an initial fluoride ion concentration of 0.92 g / L, and SO42-. 2-The concentration of Na₂CO₃ was 2.35 g / L, and the concentration of Na₂CO₃ was 19.20 g / L. Sodium sulfate, a fluoride-releasing agent, was added to the above sodium aluminate solution in an amount 1.1 times the molar amount of fluoride in the solution. After evaporation and concentration, the concentration of caustic alkali in the mother liquor was 268 g / L. The above-mentioned mother liquor was cooled to 65°C and stirred for 2 hours to carry out further defluorination reaction.

[0038] After solid-liquid separation, the slurry yielded defluorinated semen and defluorinated residue. The fluoride ion concentration in the semen was reduced to 0.42 g / L. X-ray diffraction analysis of the defluorinated residue showed that the main component was Na3SO4F double salt.

[0039] Example 5 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 1 As shown, it includes the following steps: A certain alumina plant has a mother liquor with a caustic alkali concentration of 145 g / L, a caustic ratio of 3.02, an initial fluoride ion concentration of 0.92 g / L, and SO42-. 2- The concentration of Na₂CO₃ was 2.35 g / L, and the concentration of Na₂CO₃ was 19.20 g / L. Sodium sulfate, a fluoride-releasing agent, was added to the above sodium aluminate solution in an amount 1.1 times the molar amount of fluoride in the solution. After evaporation and concentration, the concentration of caustic alkali in the mother liquor was 268 g / L. The above-mentioned evaporation mother liquor was added with defluorination slag seed crystals (the defluorination slag obtained in Example 4), and the amount of seed crystals added was 10 g / L; the slurry was cooled to 65°C and stirred for 2 hours to carry out further defluorination reaction.

[0040] After solid-liquid separation, the slurry was used to obtain defluorinated semen, and the fluoride ion concentration in the semen was reduced to 0.36 g / L.

[0041] Example 6 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 1 As shown, it includes the following steps: A certain alumina plant has a mother liquor with a caustic alkali concentration of 145 g / L, a caustic ratio of 3.02, an initial fluoride ion concentration of 0.92 g / L, and SO42-. 2- The concentration of Na₂CO₃ was 2.35 g / L, and the concentration of Na₂CO₃ was 19.20 g / L. Sodium sulfate, a fluoride-releasing agent, and fluoride-removing slag seed crystals (the fluoride-removing slag obtained in Example 4) were added to the above sodium aluminate solution. The amount of fluoride-releasing agent added was 1.1 times the molar amount of fluorine in the solution, and the amount of fluoride-removing slag seed crystals added was 10 g / L. After evaporation and concentration, the concentration of caustic alkali in the mother liquor was 272 g / L. The above-mentioned mother liquor was stirred at 65°C for 3 hours to carry out the defluorination reaction.

[0042] After solid-liquid separation, the slurry yielded defluorinated semen and defluorinated residue, with the fluoride ion concentration in the semen reduced to 0.29 g / L.

[0043] Example 7 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 2 As shown, it includes the following steps: The sodium fluoroaluminate solution has a caustic alkali concentration of 158 g / L, a caustic ratio of 3.0, and an initial fluoride ion concentration of 1.72 g / L. The above sodium fluoroaluminate solution was evaporated and concentrated to a caustic alkali concentration of 258 g / L; Add sodium sulfate, a fluoride-releasing agent, to the above-mentioned evaporated and concentrated sodium aluminate solution. The amount added is 1.1 times the molar amount of fluorine in the sodium fluoroaluminate solution. The above slurry was stirred at 80℃ for 0.5 h to carry out a defluorination reaction. After the defluorination reaction, the fluoride ion concentration in the defluorinated concentrate obtained by solid-liquid separation was reduced to 0.32 g / L.

[0044] After the above defluorination reaction, the slurry was cooled to 40°C and stirred for 2 hours. The concentration of fluoride ions in the semen after defluorination was reduced to 0.23 g / L.

[0045] Example 8 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 2 As shown, it includes the following steps: The sodium fluoroaluminate solution has a caustic alkali concentration of 158 g / L, a caustic ratio of 3.0, and an initial fluoride ion concentration of 1.72 g / L. The above sodium fluoroaluminate solution was evaporated and concentrated to a caustic alkali concentration of 262 g / L; Sodium sulfate, a fluoride-releasing agent, and fluoride-removing slag seed crystals (the fluoride-removing slag obtained in Example 7) were added to the above-mentioned evaporated and concentrated sodium aluminate solution. The amount of fluoride added was 1.1 times the molar amount of fluorine in the sodium fluoroaluminate solution, and the amount of fluoride-removing slag seed crystals added was 15 g / L. The above slurry was stirred at 80°C for 2.0 h to carry out the defluorination reaction.

[0046] After the defluorination reaction, the concentration of fluoride ions in the defluorinated concentrate obtained by solid-liquid separation was reduced to 0.27 g / L.

[0047] Example 9 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 2 As shown, it includes the following steps: A certain alumina plant has a mother liquor with a caustic alkali concentration of 150 g / L, a caustic ratio of 2.90, an initial fluoride ion concentration of 1.06 g / L, and SO42-. 2-The concentration of Na₂CO₃ was 2.68 g / L, and the concentration of Na₂CO₃ was 18.70 g / L. The above sodium fluoroaluminate solution was evaporated and concentrated to a caustic alkali concentration of 268 g / L; Add sodium sulfate, a fluoride-releasing agent, to the above-mentioned evaporated and concentrated sodium aluminate solution. The amount of fluoride-releasing agent added is 0.6 times the molar amount of fluorine in the solution. The above slurry was stirred at 65°C for 3 hours to carry out the defluorination reaction.

[0048] After the defluorination reaction, the concentration of fluoride ions in the defluorinated concentrate obtained by solid-liquid separation was reduced to 0.51 g / L. X-ray diffraction analysis of the defluorinated residue showed that the main component was Na3SO4F double salt.

[0049] Example 10 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 2 As shown, it includes the following steps: A certain alumina plant has a mother liquor with a caustic alkali concentration of 150 g / L, a caustic ratio of 2.90, an initial fluoride ion concentration of 1.06 g / L, and SO42-. 2- The concentration of Na₂CO₃ was 2.68 g / L, and the concentration of Na₂CO₃ was 18.70 g / L. The above sodium fluoroaluminate solution was evaporated and concentrated to a caustic alkali concentration of 272 g / L; Add sodium sulfate, a fluoride-releasing agent, and seed crystals (the fluoride-removing residue obtained in Example 9) to the above-mentioned evaporated and concentrated sodium aluminate solution. The amount of fluoride-releasing agent added is 1.0 times the molar amount of fluorine in the solution, and the amount of seed crystals added is 10 g / L. The above slurry was stirred at 80°C for 3 hours to carry out the defluorination reaction.

[0050] The concentration of fluoride ions in the defluorinated concentrate obtained after solid-liquid separation following the defluorination reaction was reduced to 0.29 g / L.

[0051] Example 11 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 2 As shown, it includes the following steps: The concentration of caustic alkali in the seed mother liquor was 145 g / L, the caustic ratio was 3.03, and the initial fluoride concentration was 1.60 g / L. After evaporation and concentration, the concentration of caustic alkali in the mother liquor is 255 g / L. Sodium phosphate and sodium oxalate seed crystals were added to the mother liquor of evaporation, with the addition amounts being 0.6 times and 0.5 times the molar amount of fluoride ions in the solution, respectively. The mixture was stirred and reacted at 85°C for 3 h. After solid-liquid separation, the slurry was used to obtain defluorinated semen, and the concentration of fluoride ions in the semen was reduced to 0.37 g / L.

[0052] Example 12 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 2 As shown, it includes the following steps: The concentration of caustic alkali in the seed mother liquor was 150 g / L, the caustic ratio was 2.99, and the initial fluoride concentration was 1.91 g / L. After evaporation and concentration, the concentration of caustic alkali in the mother liquor is 270 g / L; Sodium fluoride and sodium sulfate seed crystals were added to the mother liquor from evaporation. The amount of sodium fluoride added was 3 g / L, and the amount of sodium sulfate added was 1.0 times the total molar amount of fluorine in the solution and the added sodium fluoride.

[0053] The slurry was reacted at 80℃ for 4 h; after the reaction, the slurry was slowly cooled to 50℃ and stirred for another 1.5 h. After solid-liquid separation, the slurry was used to obtain defluorinated semen, and the concentration of fluoride ions in the semen was reduced to 0.27 g / L.

[0054] Example 13 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 2 As shown, it includes the following steps: A fluoride ion concentration of 1.81 g / L and a caustic alkali concentration of Na2O were used. K A sodium aluminate solution of 158 g / L with a caustic ratio of 3.01 was evaporated and concentrated to a caustic alkali concentration of Na₂O. K 252g / L.

[0055] Add sodium sulfate as a fluoride-releasing agent to the above-mentioned evaporated and concentrated solution, wherein the amount of sodium sulfate added is 1.05 times the molar amount of fluorine in the solution; After stirring and reacting at 80℃ for 0.5 h to remove fluoride, the fluoride ion concentration in the sodium aluminate solution decreased to 0.51 g / L; further lowering the temperature to 50℃ and stirring for another 1 h further reduced the fluoride ion concentration to 0.43 g / L.

[0056] Example 14 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 2 As shown, it includes the following steps: A fluoride ion concentration of 1.81 g / L and a caustic alkali concentration of Na2O were used. K A sodium aluminate solution of 158 g / L with a caustic ratio of 3.01 was evaporated and concentrated to a caustic alkali concentration of Na₂O. K 260g / L.

[0057] Add sodium sulfate as a fluoride-releasing agent to the above-mentioned evaporated and concentrated solution, wherein the amount of sodium sulfate added is 0.5 times the molar amount of fluorine in the solution; After stirring and reacting at 60℃ for 2 hours to remove fluoride, the fluoride ion concentration in the sodium aluminate solution decreased to 0.52 g / L.

[0058] Example 15 This embodiment is basically the same as embodiment 14, except that: Add sodium sulfate as a fluoride-releasing agent to the above-mentioned evaporated and concentrated solution, the amount of sodium sulfate added being 1.1 times the molar amount of fluorine in the solution; After stirring and reacting at 60℃ for 2 hours to remove fluoride, the fluoride ion concentration in the sodium aluminate solution decreased to 0.32 g / L.

[0059] Comparative Example 1 This comparative example is basically the same as Example 14, except that: The mother liquor after evaporation was cooled to 60°C, and without adding a fluoride-removing agent, it was stirred and crystallized for 2 hours to remove fluoride. The concentration of fluoride ions remaining in the mother liquor was 1.18 g / L.

[0060] Example 16 This embodiment provides a method for removing fluoride from a sodium fluoroaluminate solution, such as... Figure 1 As shown, it includes the following steps: The sodium fluoroaluminate solution has a caustic alkali concentration of 158 g / L, a caustic ratio of 3.01, and an initial fluoride ion concentration of 1.81 g / L. Add sulfuric acid, a fluoride-releasing agent, to the above sodium aluminate solution. The amount added is 1.0 times the molar amount of fluoride in the solution. After evaporation and concentration, the concentration of caustic alkali in the mother liquor is 259 g / L. The above-mentioned mother liquor was stirred at 50°C for 2 hours to carry out further defluorination reaction.

[0061] After solid-liquid separation, the slurry yielded defluorinated semen, with the fluoride ion concentration in the semen reduced to 0.40 g / L.

[0062] Comparative Example 2 This comparative example is basically the same as Example 16, except that comparative example 2 does not contain the fluoride-releasing agent sodium sulfate. Specifically: The temperature of the evaporated solution was lowered to 50°C and kept at this temperature for 2 hours under stirring. The slurry after stirring was subjected to liquid-solid separation, and the concentration of fluoride ions remaining in the mother liquor was 1.09 g / L.

[0063] Example 17 This embodiment provides a method for defluorinating sodium fluoroaluminate solution, including the following steps: A certain alumina plant has a mother liquor with a caustic alkali concentration of 150 g / L, a caustic ratio of 2.90, an initial fluoride ion concentration of 1.06 g / L, and SO42-. 2- The concentration of Na₂CO₃ was 2.68 g / L, and the concentration of Na₂CO₃ was 18.70 g / L. Add sodium hydroxide to the above sodium fluoroaluminate solution to increase the caustic alkali concentration of the solution to 272 g / L; Add sodium sulfate, a fluoride-releasing agent, and seed crystals (the fluoride-removing residue obtained in Example 9) to the above-mentioned evaporated and concentrated sodium aluminate solution. The amount of fluoride-releasing agent added is 1.0 times the molar amount of fluorine in the solution, and the amount of seed crystals added is 10 g / L. The above slurry was stirred at 80°C for 3 hours to carry out a defluorination reaction. After the defluorination reaction, the fluoride ion concentration in the defluorinated concentrate obtained by solid-liquid separation was reduced to 0.31 g / L.

[0064] Example 18 This embodiment provides a method for defluorinating sodium fluoroaluminate solution, including the following steps: The evaporation mother liquor from an alumina plant has a caustic alkali concentration of 250 g / L, a caustic ratio of 3.0, an initial fluoride ion concentration of 1.30 g / L, and SO42-. 2- The concentration of Na₂CO₃ was 30.20 g / L. Add sodium sulfate, a fluoride-removing agent, and seed crystals (the fluoride-removing residue obtained in Example 9) to the above-mentioned mother liquor. The amount of fluoride-removing agent added is 1.0 times the molar amount of fluorine in the solution, and the amount of seed crystals added is 10 g / L. The above slurry was stirred at 80℃ for 3 hours to carry out a defluorination reaction. After the defluorination reaction, the fluoride concentration in the defluorinated concentrate obtained by solid-liquid separation was reduced to 0.27 g / L, and the SO42- content in the solution was also reduced. 2- Concentration 3.75 g / L.

[0065] Example 19 This embodiment provides a method for defluorinating sodium fluoroaluminate solution, including the following steps: The evaporation mother liquor from an alumina plant has a caustic alkali concentration of 250 g / L, a caustic ratio of 3.0, an initial fluoride ion concentration of 1.30 g / L, and SO42-. 2- Concentration 3.80 g / L, Na2CO3 concentration 30.20 g / L; Add sodium sulfate, a fluoride-releasing agent, to the above-mentioned mother liquor. The amount of fluoride-releasing agent added is 1.0 times the molar amount of fluorine in the solution. The above slurry was stirred at 65°C for 2 hours to carry out a defluorination reaction. After the defluorination reaction, the fluoride concentration in the defluorinated concentrate obtained by solid-liquid separation was reduced to 0.45 g / L, and the SO42- content in the solution was reduced. 2- Concentration 3.70 g / L.

[0066] Comparative Example 3 This comparative example is basically the same as Example 19, except that lime was added as a defluorinating agent in Comparative Example 3. Specifically: The evaporation mother liquor from an alumina plant has a caustic alkali concentration of 250 g / L, a caustic ratio of 3.0, an initial fluoride ion concentration of 1.30 g / L, and SO42-. 2- Concentration 3.80 g / L, Na2CO3 concentration 30.20 g / L; Lime was added to the above-mentioned mother liquor as a defluorinating agent, and the amount of lime added was 10 g / L; The above slurry was stirred at 80℃ for 2 hours to carry out a defluorination reaction. After the defluorination reaction, the fluoride ion concentration in the defluorinated concentrate obtained by solid-liquid separation was reduced to 0.76 g / L. XRD analysis of the defluorinated residue showed that the main phase in the residue was tricalcium aluminate hexahydrate. The alumina concentration in the evaporation mother liquor before and after defluorination was measured, and the alumina loss in the solution during the defluorination process was calculated to be 4 kg / m³. 3 above.

[0067] Comparative Example 3 shows that adding lime to sodium aluminate solution has a poor effect on fluoride removal and low fluoride removal efficiency. At the same time, it reacts with aluminate ions in the solution to form calcium aluminate precipitate, which causes the loss of alumina in the solution.

[0068] Comparative Example 4 This comparative example is basically the same as Example 16, except that the concentration of caustic alkali in the mother liquor of sodium aluminate evaporation concentration in Comparative Example 4 is reduced to 180 g / L. Specifically: The sodium fluoroaluminate solution has a caustic alkali concentration of 158 g / L, a caustic ratio of 3.01, and an initial fluoride ion concentration of 1.81 g / L. Add sulfuric acid, a fluoride-releasing agent, to the above sodium aluminate solution. The amount added is 1.0 times the molar amount of fluoride in the solution. After evaporation and concentration, the concentration of caustic alkali in the mother liquor is 180 g / L. The above-mentioned mother liquor was stirred at 50°C for 2 hours to carry out further defluorination reaction.

[0069] After solid-liquid separation, the slurry was defluorinated semen, and the fluoride ion concentration in the semen was reduced to 1.2 g / L.

[0070] Comparative Example 4 shows that when the concentration of caustic alkali in the evaporation mother liquor is low, the effect of adding fluoride removal agent becomes worse, and the fluoride removal efficiency decreases.

[0071] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of the present invention.

Claims

1. A method for removing fluoride from a sodium fluoroaluminate solution, characterized in that, In the process of evaporating and concentrating the mother liquor produced by alumina production, a fluoride-removing agent is added to carry out a fluoride removal reaction. During or after the evaporation and concentration of the mother liquor, double salts are precipitated. After liquid-solid separation, the defluorinated mother liquor and defluorinated residue are obtained. The fluoride-removing agent is at least one of sodium sulfate, sodium phosphate, sodium vanadate, sodium oxalate, and fluoride-removing residue; the molar amount of the fluoride-removing agent added is 0.5 to 3.0 times the molar amount of fluorine in the solution; after adding the fluoride-removing agent, the fluoride removal reaction temperature is 40 to 120°C. The seed mother liquor is evaporated and concentrated to a caustic soda concentration of 185~300g / L.

2. A method for removing fluoride from a sodium fluoroaluminate solution, characterized in that, The mother liquor from alumina production is evaporated and concentrated to obtain sodium aluminate evaporation mother liquor containing fluorine. A fluorine-releasing agent is added to the sodium aluminate evaporation mother liquor to precipitate double salts. After liquid-solid separation, defluorinated mother liquor and defluorinated residue are obtained. The fluoride-removing agent is at least one of sodium sulfate, sodium phosphate, sodium vanadate, sodium oxalate, and fluoride-removing residue; the molar amount of the fluoride-removing agent added is 0.5 to 3.0 times the molar amount of fluorine in the solution; after adding the fluoride-removing agent, the fluoride removal reaction temperature is 40 to 120°C. The concentration of caustic soda in the fluorine-containing sodium aluminate evaporation mother liquor is 185~300g / L.

3. The method for defluorinating sodium fluoroaluminate solution as described in claim 1 or 2, characterized in that, The fluoride removal agent is sodium sulfate or a combination of sodium sulfate and fluoride removal residue.

4. A method for defluorinating sodium fluoroaluminate solution as described in claim 1 or 2, characterized in that, After adding the fluoride removal agent, the fluoride removal reaction time is 0.5~6 h.

5. The method for defluorinating sodium fluoroaluminate solution as described in claim 4, characterized in that, The slurry after the defluorination reaction is cooled down while the reaction is stirred for 0.5 to 4.0 h. The final temperature of the reaction is 40 to 70 °C, and then the solid and liquid are separated.

6. A method for defluorinating sodium fluoroaluminate solution as described in claim 1 or 2, characterized in that, The defluorinated residue is returned to the mother liquor or the mother liquor containing fluorine-containing sodium aluminate as a fluorine release agent.

7. The method for defluorinating sodium fluoroaluminate solution as described in claim 2, characterized in that, The sodium aluminate evaporation mother liquor obtained after evaporation and concentration of the seed mother liquor from alumina production is cooled and then a fluorine-releasing agent is added.

8. The method for defluorinating sodium fluoroaluminate solution as described in claim 7, characterized in that, The fluorine-containing sodium aluminate evaporation mother liquor is cooled to 40-80°C, and then a fluorine-releasing agent is added.

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