A method for defluorination and purification of fluoride-containing salt

Abstract

A method for removing fluorine from fluorine-containing salts comprises the following steps: (1) preparing a saturated salt solution containing fluorine, filtering to remove insoluble impurities; (2) selectively adding alkali to the filtrate of step (1), adding calcium chloride according to the fluorine content in the filtrate, heating and stirring to react; (3) adding a defluorinating agent, selectively adding alkali, heating and stirring to react; (4) selectively adding an acid and filtering to obtain a salt solution with a low fluorine content, and concentrating, dehydrating, crystallizing and drying the salt solution to obtain a solid salt. The method for removing fluorine from fluorine-containing salts of the present invention has a simple process flow, less three wastes, and low energy consumption, and solves the problems of difficult filtration of calcium fluoride, slow filtration speed, and unclear filtrate in traditional processes, and has the advantages of good defluorination effect and high filtration efficiency.

Description

Technical Field

[0001] The invention relates to a method for removing fluorine from fluorine-containing salts. Background Art

[0002] With the rapid development of new energy technologies, the research and development and application of new electrolyte salts have received widespread attention in recent years. In the process of preparing electrolyte salts, some by-products are inevitably produced. The treatment, discharge, recycling and reuse of these by-products deserve our attention. The by-products of preparing electrolyte salts mainly include a mixture of sodium fluoride and sodium chloride. Since such by-products contain high fluorine and have high COD values, it has always been an industrial problem to process them into non-toxic and harmless industrial products for discharge, or to purify and recycle them for reuse.

[0003] At present, the common method for purifying sodium chloride in this field is mainly to use evaporation, concentration and crystallization. This method can initially purify sodium chloride, but it cannot remove the higher impurity F (fluoride) ion content in sodium chloride. Adding calcium chloride to react with F ions in sodium chloride to form calcium fluoride, and then removing calcium fluoride by filtration can reduce the F content in sodium chloride, but the filtration speed of calcium fluoride is extremely slow, it is easy to clog the filter pores, and the defluorination effect is not good. Adding commercial defluoridation agent and stirring, then adding flocculant and letting it settle, and discharging the supernatant can also achieve the effect of defluorination, but this method has the following disadvantages: first, the treatment cycle is long; second, the lower layer of turbid liquid is difficult to handle; third, the filtrate is turbid. Summary of the invention

[0004] The object of the present invention is to provide a method for defluorination and purification of fluoride-containing salts with good defluorination effect and high efficiency.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A method for removing fluorine from fluorine-containing salts comprises the following steps:

[0007] (1) Prepare a saturated salt solution with fluoride-containing salt and filter to remove insoluble impurities;

[0008] (2) selectively adding alkali to the filtrate of step (1), adding calcium chloride according to the fluorine content in the filtrate, and heating and stirring to react;

[0009] (3) adding a defluorinating agent, optionally adding the base, heating and stirring to react;

[0010] (4) selectively adding an acid and filtering to obtain a salt solution with a low fluoride content, wherein the salt solution is concentrated, dehydrated, crystallized, and dried to obtain a solid salt;

[0011] Wherein, the defluorination agent comprises a mixed solution formed by aluminum chloride, ferric chloride, polyacrylamide and water.

[0012] Preferably, the amount of the defluorinating agent added is 0.05-0.15% of the mass of the filtrate in step (1).

[0013] According to some preferred embodiments, the defluorination agent comprises the following components in parts by weight:

[0014] 10-20 parts of aluminum chloride,

[0015] 80~90 parts of water,

[0016] 0.05~0.15 parts of ferric chloride,

[0017] Polyacrylamide 0.05~0.15 parts.

[0018] Further preferably, the defluorination agent comprises the following components in parts by weight:

[0019] 13-18 parts of aluminum chloride,

[0020] 82~87 parts of water,

[0021] 0.08~0.12 parts of ferric chloride,

[0022] Polyacrylamide 0.08~0.12 parts.

[0023] Preferably, the filtration time of step (4) is controlled to be less than or equal to 15 minutes.

[0024] Further preferably, the filtration time of step (4) is controlled to be less than or equal to 10 minutes.

[0025] More preferably, the filtration time of step (4) is controlled to be 3 to 10 minutes.

[0026] Preferably, the molar amount of the calcium chloride fed is 0.5 to 2 times the molar amount of fluorine in the filtrate.

[0027] Further preferably, the molar amount of the calcium chloride fed is 1 to 2 times the molar amount of fluorine in the filtrate.

[0028] Preferably, the reaction temperature of step (2) is controlled to be 60-120°C.

[0029] More preferably, the reaction temperature of step (2) is controlled to be 80-120°C.

[0030] Preferably, the reaction time of step (2) is controlled to be 1 to 4 hours.

[0031] Preferably, the reaction temperature of step (3) is controlled to be 60-120°C.

[0032] Further preferably, the reaction time of step (3) is controlled to be 1 to 4 hours.

[0033] Preferably, the base comprises sodium hydroxide or sodium hydroxide solution.

[0034] Preferably, the acid comprises hydrochloric acid.

[0035] Preferably, the base is added to the step (2) to adjust the pH to 9-14.

[0036] Further preferably, the base is added to the step (2) to adjust the pH to 10-14.

[0037] Preferably, the base is added to the step (3) to adjust the pH to 9-14.

[0038] Preferably, the acid is added to the step (4) to adjust the pH to 6-8.

[0039] Further preferably, the acid is added to the step (4) to adjust the pH to 6-7.

[0040] Preferably, the filtration comprises pressure filtration using a filter membrane with a filter diameter of 0.05 to 0.2 micrometers.

[0041] Preferably, the mass of water removed during the concentrated dehydration is controlled to be 45-55% of the total mass of the salt solution.

[0042] Preferably, the drying temperature is controlled to be 100-130°C.

[0043] Further preferably, the drying time is controlled to be 4 to 16 hours.

[0044] According to some preferred embodiments, the defluorination purification method further comprises adding activated carbon for decolorization after step (2) and before step (3), and the mass of the activated carbon is 0.1-1% of the mass of the filtrate of step (1).

[0045] Preferably, the fluoride-containing salt comprises fluoride-containing sodium chloride and / or potassium chloride.

[0046] Preferably, the fluorine-containing salt comprises a by-product obtained in a fluorine chemical production process, the color number of the by-product is greater than or equal to 5, the COD of the by-product is greater than 15000 ppm, and the by-product comprises the following components by mass percentage:

[0047] Sodium 35~45%,

[0048] Fluoride 5~12%,

[0049] Chlorine 45~60%.

[0050] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0051] The defluorination and purification method of fluorine-containing salts of the present invention has a simple process flow, less three wastes, and low energy consumption, and solves the problems of difficult filtration of calcium fluoride, slow filtration speed, unclear filtrate, etc. in traditional processes, and has the advantages of good defluorination effect and high filtration efficiency. DETAILED DESCRIPTION

[0052] The F content can be reduced by allowing fluorine to react with calcium to form calcium fluoride and then removing the calcium fluoride by filtration. However, for calcium fluoride filtration, either the filtration speed is extremely slow and the filter pores of the filter paper are clogged; or the filtrate obtained by filtration is still turbid, and a clear and transparent filtrate cannot be obtained no matter how it is filtered.

[0053] Based on the deficiencies of the prior art, the applicant has obtained the solution of the present application after long-term experiments and extensive research. The solution is further elaborated below.

[0054] A method for removing fluorine from fluorine-containing salts comprises the following steps:

[0055] (1) Prepare a saturated salt solution with fluoride-containing salt and filter to remove insoluble impurities;

[0056] (2) selectively adding alkali to the filtrate of step (1), adding calcium chloride according to the fluorine content in the filtrate, and heating and stirring to react;

[0057] (3) Adding a defluorinating agent and optionally adding a base, heating and stirring to react;

[0058] (4) After selectively adding an acid, filtering is performed to obtain a salt solution with a low fluoride content. The salt solution is concentrated, dehydrated, crystallized, and dried to obtain a solid salt.

[0059] In the present invention, the fluorine-containing salt includes by-products obtained in the fluorine chemical production process, such as fluorine-containing sodium chloride and / or fluorine-containing potassium chloride. The color number of the by-product is greater than or equal to 5, such as color numbers 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, etc., or the by-product with a color number greater than 80 can also be treated by the method of the present invention. The COD of the by-product is greater than 15000ppm, such as 16000ppm, 17000ppm, 18000ppm, 19000ppm, 20000ppm, etc.

[0060] Furthermore, by-products include the following components by mass percentage:

[0061] Sodium 35~45%,

[0062] Fluoride 5~12%,

[0063] Chlorine 45~60%.

[0064] In the present invention, before performing step (1), the fluorine-containing salt is tested to determine the fluoride ion content, chloride ion content, etc. in the fluorine-containing salt, and the purity of the fluorine-containing salt is calculated by the chloride ion content, so that a saturated salt solution is prepared according to the purity of the fluorine-containing salt.

[0065] In step (1), the saturated salt solution may be prepared by mixing a fluoride-containing salt with water. After the saturated salt solution is prepared, the insoluble impurities are removed by filtration to obtain a filtrate, and the filtrate is then tested to determine the F content therein, and then the content of calcium chloride to be added is calculated based on the fluoride content in the filtrate.

[0066] In some embodiments, in step (2), a base is added to the filtrate to adjust the pH of the filtrate to 9-14, for example, to 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, etc. The base includes sodium hydroxide or an aqueous sodium hydroxide solution. After the pH adjustment is completed, calcium chloride is added, and the molar amount of calcium chloride is 0.5-2 times the molar amount of fluorine in the filtrate, for example, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, etc.

[0067] In step (2), the reaction temperature is controlled to be 60-120°C, for example, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, etc. When the reaction temperature is relatively high (greater than or equal to 100°C), a condensation reflux device is installed on the reactor to cool the evaporated water back into the reaction system; when the reaction temperature is relatively low (lower than 100°C), a water bath is used for heating reaction. The reaction time is 1-4h, for example, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h.

[0068] In step (3), the defluorinating agent includes a mixed solution of aluminum chloride, ferric chloride, polyacrylamide and water. The amount of the defluorinating agent added is 0.05-0.15% of the mass of the filtrate, for example, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, etc.

[0069] Furthermore, the defluorination agent comprises the following components by weight:

[0070] 10-20 parts of aluminum chloride,

[0071] 80~90 parts of water,

[0072] 0.05~0.15 parts of ferric chloride,

[0073] Polyacrylamide 0.05~0.15 parts.

[0074] The preparation method of the defluorinating agent comprises the steps of preparing aluminum chloride and water into an aluminum chloride solution, and then adding polyacrylamide and ferric chloride into the aluminum chloride solution to obtain the defluorinating agent.

[0075] In step (3), the reaction temperature is controlled to be 60-120°C, for example, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, etc. When the reaction temperature is relatively high (greater than or equal to 100°C), a condensation reflux device is installed on the reactor to cool the evaporated water back into the reaction system; when the reaction temperature is relatively low (lower than 100°C), a water bath is used for heating reaction. The reaction time is 1-4h, for example, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h.

[0076] In some embodiments, if no alkali is added in step (2), alkali may be added after the defluorinating agent is added to adjust the pH to 9-14.

[0077] In some preferred embodiments, in step (4), the pH is adjusted to 6-8 by adding an acid before filtering, and the acid includes concentrated hydrochloric acid.

[0078] In step (4), the filtration includes pressure filtration using a filter membrane with a filter diameter of 0.05 to 0.2 microns, and the filtration time is less than or equal to 15 minutes, such as 15 minutes, 14 minutes, 13 minutes, 12 minutes, 11 minutes, 10 minutes, 9 minutes, 8 minutes, 7 minutes, 6 minutes, 5 minutes, 4 minutes, 3 minutes, 2 minutes, 1 minute, etc. After filtration in the present invention, a salt solution with a low fluoride content and a filter residue are obtained, the salt solution is basically clear, and the F content is less than 5ppm.

[0079] In step (4), the mass of water removed during the concentration and dehydration is 45-55% of the total mass of the salt solution, such as 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, etc. After the concentration and dehydration, the concentrated solution is cooled to room temperature, and the concentrated solution is filtered to obtain wet sodium chloride and concentrated filtrate. The concentrated filtrate can be further returned to step (1) to dissolve the fluoride-containing salt, thereby further extracting the remaining salt in the concentrated solution and improving the yield. The wet sodium chloride is dried to obtain the target product after defluorination and purification. If the F content in the product is higher than 10ppm, the above steps can be repeated for secondary purification.

[0080] The present invention is further described below in conjunction with the examples. However, the present invention is not limited to the following examples. The implementation conditions used in the examples can be further adjusted according to the different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in each embodiment of the present invention can be combined with each other as long as they do not conflict with each other.

[0081] Example 1

[0082] The sodium chloride raw material treated in this embodiment is a by-product obtained in the fluorine chemical production process. The by-product is tested to contain 38.17% Na (sodium); 5.42% F (fluorine); 53.4% ​​Cl (chlorine); color number: 10; COD: 18600ppm; based on the Cl content, the purity of the by-product is roughly estimated to be 88%.

[0083] (1) Weigh 200 g of the by-product sodium chloride, add 489 g of water based on the purity of 88%, stir for 5 min to prepare a saturated solution of sodium chloride, filter with filter paper to remove insoluble matter, and obtain 654 g of filtrate. The F content of the filtrate is 2131 ppm. Scrape 28 g of filter residue from the filter paper (a small amount of filter residue remains on the filter paper). After vacuum drying at 130°C for 4 h, 22 g of the filter residue remains. The Na content of the filter residue is 56% and the F content is 43%. It is preliminarily determined that most of the insoluble matter is NaF impurities.

[0084] (2) Add 1% of the mass of the filtrate in step (1) of solid sodium hydroxide to adjust the pH to 14. According to the theoretical content of F in the filtrate, 4.04 g of calcium chloride should be added to remove F. The actual amount of calcium chloride added was 12.1 g (200% excess). A large amount of white solid was produced upon addition. The mixture was refluxed and stirred at 110° C. using a heating mantle for 3 h.

[0085] (3) Prepare the defluoridation solution. Weigh 15g of aluminum chloride solid and add 85g of pure water to make an aluminum chloride solution with a mass concentration of 15%. Add 0.1g of PAM flocculant (polyacrylamide) and stir to dissolve. Add 0.1g of ferric chloride solid and stir to dissolve to prepare the defluoridation solution.

[0086] (4) Add 0.65 g of the defluorinating agent prepared in step (3) at a concentration of 0.1% by weight of the filtrate to the system of step (2), and reflux and stir at 110° C. using a heating mantle to react for 2 h.

[0087] (5) Concentrated hydrochloric acid was added to the system of step (4) to adjust the pH to 7, and the mixture was filtered using a 0.1 μm filter membrane for 4 min to obtain 13.6 g of white viscous wet residue (the main components of which were Ca, F, and Al) and 685 g of filtrate. The filtrate was clear and the F content was 1.96 ppm. The pH was 7.

[0088] (6) After the test, the remaining filtrate was 671 g, and 298.5 g was vacuum concentrated and dehydrated. The COD of the distilled water was 4059 ppm. The remaining material was cooled to room temperature and filtered to obtain 215.5 g of filtrate and 124.7 g of wet sodium chloride. The concentration ratio = (filtrate mass + wet sodium chloride mass) / filtrate mass before concentration = 0.51. The sodium chloride wet crystal F content was detected: 0.83 ppm.

[0089] (7) Wet sodium chloride crystals were dried at 105°C for 3 h. The purity was 98.6%, F: 2.83 ppm, moisture: 1.2%, color number: 5, Al and B were not detected, Ca metal ion content: 439 ppm, COD: 320 ppm.

[0090] Example 2

[0091] The sodium chloride raw material treated in this embodiment is a by-product obtained in the fluorine chemical production process. The by-product is tested to contain 41.13% Na, 11.17% F, 45.5% Cl, color number: 5; COD: 14500ppm. The purity of the by-product is roughly estimated to be 75% based on the Cl content.

[0092] (1) Weigh 200 g of the by-product sodium chloride, add 418 g of water based on the purity of 75%, stir for 1 h to prepare a saturated solution of sodium chloride, filter with filter paper to remove insoluble matter, and obtain 560 g of filtrate. The F content of the filtrate is 1665 ppm, and the B content is 35.7 ppm. Scrape off 55.7 g of filter residue from the filter paper (a small amount of filter residue remains on the filter paper). After vacuum drying at 130°C for 4 h, 45.6 g of the filter residue remains. The Na content of the filter residue is 55%, and the F content is 44%. It is preliminarily determined that most of the insoluble matter is NaF impurities.

[0093] (2) Add an appropriate amount of NaOH aqueous solution to the filtrate of step (1) to adjust the pH to about 10. According to the theoretical content of F in the filtrate, 2.7 g of calcium chloride should be added to remove F. In fact, 5.4 g of calcium chloride (100% excess) was added. A large amount of white solid was produced upon addition. The reaction was stirred in an 80°C water bath for 1 h.

[0094] (3) Prepare the defluoridation solution. Weigh 18g of aluminum chloride solid and add 82g of pure water to prepare an aluminum chloride solution with a mass concentration of 18%. Add 0.05g of PAM flocculant and stir to dissolve. Add 0.05g of ferric chloride solid and stir to dissolve to prepare the defluoridation solution.

[0095] (4) Add 0.28 g of a defluorinating agent (0.05% of the mass of the filtrate) to the system of step (2), and stir in a 60°C water bath to react for 1 h.

[0096] (5) Continue to add concentrated hydrochloric acid to adjust the pH to 6, and use a 0.1 μm filter membrane to filter. The filtration is completed for 5 minutes, and 11.5 g of white viscous wet filter residue is obtained (the main components are Ca, F, and Al), and 558 g of filtrate is obtained. The filtrate is basically clear; the F content is detected: 3.62 ppm.

[0097] (6) After the test, the remaining filtrate was 550 g, and 226.5 g was vacuum concentrated and dehydrated. The COD of the distilled water was detected to be 3805 ppm. The remaining material was cooled to room temperature and filtered to obtain 187.3 g of filtrate and 115.8 g of wet sodium chloride. The concentration ratio = (filtrate mass + wet sodium chloride mass) / filtrate mass before concentration = 0.55. The sodium chloride wet crystal F content was detected to be 3.2 ppm.

[0098] (7) Wet crystalline sodium chloride was vacuum dried at 130°C for 6 h. The purity was 99.8%, F: 6.1 ppm, moisture: 390 ppm, color number: 5, Al: 2.7 ppm, B: 5 ppm, Ca: 439 ppm, COD: 380 ppm.

[0099] Example 3

[0100] The sodium chloride raw material treated in this embodiment is a by-product obtained in a pharmaceutical production process. The by-product is tested to contain 40.74% Na, 7.1% F, 51.16% Cl, color number 80, and COD 16500ppm. The purity of the by-product is roughly estimated to be 84.3% based on the Cl content.

[0101] (1) Weigh 200 g of the by-product sodium chloride, add 468 g of water based on the purity of 84.3%, stir for 10 min to prepare a saturated solution of sodium chloride, filter with filter paper to remove insoluble matter, and obtain 615 g of yellow filtrate. The filtrate is tested for F content: 2131 ppm, B content: 48.6 ppm. Scrape 32.7 g of yellow wet filter residue from the filter paper (a small amount of filter residue remains on the filter paper). After vacuum drying at 130°C for 4 h, 29.6 g of the filter residue remains. The filter residue is tested for Na content: 55.3%, F content: 44.7%. It is preliminarily determined that most of the insoluble matter is NaF impurities.

[0102] (2) 1.5% of the mass of the filtrate in step (1) was added with solid sodium hydroxide to adjust the pH to 14. According to the F content in the filtrate, 3.83 g of calcium chloride was theoretically required to remove F. The actual amount of calcium chloride added was 15.32 g (300% excess). A large amount of white solid was produced upon addition.

[0103] (3) Add 0.5% of the mass of the filtrate in step (1) activated carbon for decolorization (3.1 g), and reflux with stirring at 120°C using a heating mantle for 6 h.

[0104] (4) Prepare the defluoridation solution. Weigh 20g of aluminum chloride solid and add 80g of pure water to make an aluminum chloride solution with a mass concentration of 20%. Add 0.15g of PAM flocculant and stir to dissolve. Add 0.15g of ferric chloride solid and stir to dissolve to prepare the defluoridation solution.

[0105] (5) Add 0.923 g of a defluorinating agent (0.15% of the mass of the filtrate) to the system of step (3), and reflux and stir the mixture at 120° C. for 4 h using a heating mantle.

[0106] (6) Concentrated hydrochloric acid was added to adjust the pH to 6.5, and the mixture was refluxed and stirred at 120°C for 4 h using a heating mantle. The mixture was filtered using a 0.1 μm filter membrane for 3 min to obtain 26.5 g of black viscous wet residue and 590 g of filtrate. The filtrate was colorless, clear and transparent, and the F content was detected to be 2.8 ppm.

[0107] (7) After the test, the remaining filtrate was 580 g, and 288 g was vacuum concentrated and dehydrated. The COD of the distilled water was detected to be 1805 ppm. The remaining material was cooled to room temperature and filtered to obtain 135.4 g of filtrate and 125.6 g of wet sodium chloride. The concentration ratio = (filtrate mass + wet sodium chloride mass) / filtrate mass before concentration = 0.45. The sodium chloride wet crystal F content was detected to be 2.6 ppm.

[0108] (8) Wet sodium chloride crystals were dried at 105°C for 16 h. The purity was 99.2%, F: 3.5 ppm, moisture: 6508 ppm, color number: 5, Al: 1.9 ppm, B: 9 ppm, Ca: 389 ppm; COD: 630 ppm.

[0109] Example 4

[0110] The sodium chloride raw material treated in this embodiment is a by-product obtained in the process of producing fluorine chemical products. The by-product is tested to contain 38.17% Na, 5.42% F, 53.4% ​​Cl, color number: 10, COD: 18600ppm. The purity of the by-product is roughly estimated to be 88% based on the Cl content.

[0111] (1) Weigh 200 g of the by-product sodium chloride, add 489 g of water based on the purity of 88%, stir for 20 min to prepare a saturated solution of sodium chloride, filter with filter paper to remove insoluble matter to obtain 653 g of filtrate, the F content of the filtrate is 2163 ppm, scrape 25.3 g of filter residue from the filter paper (a small amount of filter residue remains on the filter paper), and dry the filter residue at 130°C in vacuum for 4 h to leave 21.8 g. The Na content of the filter residue is 55.5%, and the F content is 44%. It is preliminarily determined that most of the insoluble matter is NaF impurities.

[0112] (2) According to the theoretical content of F in the filtrate, 4.13 g of calcium chloride should be added to remove F. In fact, 8.26 g of calcium chloride (100% excess) was added. A large amount of white solid was produced upon addition. The reaction was stirred in a water bath at 90°C for 1 h.

[0113] (3) Prepare the defluoridation solution. Weigh 10g of aluminum chloride solid and add 90g of pure water to prepare an aluminum chloride solution with a mass concentration of 10%. Add 0.15g of PAM flocculant and stir to dissolve. Add 0.15g of ferric chloride solid and stir to dissolve to prepare the defluoridation solution.

[0114] (4) Add 0.647 g of a defluorinating agent (0.1% of the mass of the filtrate) to the system of step (2), add sodium hydroxide solution, adjust the pH to 9, and stir the reaction in a water bath at 70° C. for 1 h.

[0115] (5) Filtering was performed using a 0.1 μm filter membrane for 10 min, yielding 9.6 g of white viscous wet residue (the main components of which were detected to be Ca, F, and Al) and 629 g of filtrate. The filtrate was basically clear, and the F content was detected to be 1.69 ppm.

[0116] (6) After the test, the remaining filtrate was 623 g, and 322 g was vacuum concentrated and dehydrated. The COD of the distilled water was detected to be 4218 ppm. The remaining material was cooled to room temperature and filtered to obtain 144 g of filtrate and 137 g of wet sodium chloride. The concentration ratio = (filtrate mass + wet sodium chloride mass) / filtrate mass before concentration = 0.45. The sodium chloride wet crystal F content was detected to be 2.1 ppm.

[0117] (7) Wet sodium chloride crystals were dried at 105°C for 3 h. The purity was 98.7%, F: 4.5 ppm, moisture: 1.1%, color number: 5, Al and B were not detected, Ca metal ion content: 519 ppm, COD: 815 ppm.

[0118] The sodium chloride prepared by the method of Example 1 has high purity, contains a small amount of calcium chloride impurities, has a very low F content level that meets national standards, and has a low content of metal impurity ions. After treatment, the sodium chloride with high moisture content can be used as a snow melting agent. The moisture content of the sodium chloride can be further deeply dried to be reduced to below 500ppm, and the reduction of moisture content will also greatly improve the purity of the sodium chloride. The sodium chloride with higher purity, low moisture content, and low COD can also be used in manufacturing industries such as the chlor-alkali industry.

[0119] The products obtained by the methods of Examples 2 and 4 have high purity, low F content, low moisture content, low COD, simple process, low reaction temperature, short reaction time, low energy consumption, and low requirements for process equipment, and have high industrial value.

[0120] The method of Example 3 has high product quality and can also process byproduct sodium chloride with complex ingredients and high color number, thereby reducing the color number of the finished sodium chloride.

[0121] The method in the above embodiment has a fast filtering speed for calcium fluoride and a good filtering effect. The filtrate obtained after concentration can be repeatedly used to dissolve the next batch of sodium chloride, and the sodium chloride and excess calcium chloride in the filtrate can be recovered, which can greatly improve the yield of the product and is green, economical and environmentally friendly.

[0122] Comparative Example 1

[0123] The sodium chloride raw material treated in this embodiment is a by-product obtained in the fluorine chemical production process. The by-product is tested to contain 38.5% Na, 5.51% F, 53.6% Cl, color number: 10, COD: 18125ppm. The purity of the by-product is roughly estimated to be 88.3% based on the Cl content.

[0124] (1) Weigh 200 g of the by-product sodium chloride, add 492 g of water calculated based on the purity of 88.3%, stir for 5 min to prepare a saturated solution of sodium chloride, filter with filter paper to remove insoluble matter, and obtain 658 g of filtrate. The F content of the filtrate is 2195 ppm. Scrape 29 g of filter residue from the filter paper (a small amount of filter residue remains on the filter paper). After vacuum drying at 130°C for 4 h, 22 g of the filter residue remains. The Na content of the filter residue is 56% and the F content is 43%. It is preliminarily determined that most of the insoluble matter is NaF impurities.

[0125] (2) The obtained filtrate was directly evaporated, concentrated, crystallized, filtered and purified by conventional methods. 658 g of the filtrate was vacuum concentrated and dehydrated to 305 g. The COD of the evaporated water was detected to be 5159 ppm. The remaining material was cooled to room temperature and filtered to obtain 212 g of filtrate and 125.8 g of wet sodium chloride. The concentration ratio = (filtrate mass + wet sodium chloride mass) / filtrate mass before concentration = 0.51. The sodium chloride wet crystal F content was detected to be 4528 ppm.

[0126] (3) Wet sodium chloride crystals were dried at 105°C for 3 h. The purity was 88.6%, F: 12562 ppm, moisture: 1.3%, color number: 15.

[0127] It can be seen from Comparative Example 1 that the sodium chloride obtained by traditional evaporation, concentration, crystallization, filtration and purification has a very low purity, a high F content and a poor effect.

[0128] Comparative Example 2

[0129] The sodium chloride raw material treated in this embodiment is a by-product obtained in the fluorine chemical production process. The by-product is tested to contain 38.7% Na, 5.51% F, 53.5% Cl, color number: 10, COD: 15115ppm. The purity of the by-product is roughly estimated to be 88.2% based on the Cl content.

[0130] (1) Weigh 200 g of the by-product sodium chloride, add 490 g of water calculated based on the purity of 88.2%, stir for 5 min to prepare a saturated solution of sodium chloride, filter with filter paper to remove insoluble matter to obtain 656 g of filtrate, the F content of the filtrate is 2085 ppm, scrape 27 g of filter residue from the filter paper (a small amount of filter residue remains on the filter paper), and dry the filter residue at 130°C in vacuum for 4 h. The Na content of the filter residue is 56%, and the F content is 43%. It is preliminarily determined that most of the insoluble matter is NaF impurities.

[0131] (2) According to the theoretical content of F in the filtrate, 3.99 g of calcium chloride should be added to remove F. In fact, 4.01 g of calcium chloride was added, and a large amount of white solid was produced as soon as it was added.

[0132] (3) Filter pressure was performed using 0.1 μm filter paper for 2 h, and 649 g of filtrate was obtained with an F content of 565 ppm. The filtrate was turbid, especially at the beginning when a large amount of white solids penetrated the filter membrane. After a period of time, the pore size of the filter membrane was blocked and the filtration rate dropped sharply.

[0133] (4) The filtrate was evaporated, concentrated, crystallized and filtered to obtain 122.5 g of wet sodium chloride, which was dried at 105°C for 3 h. The purity was detected to be 91.6%, F: 8562 ppm, and moisture: 1.2%.

[0134] It can be seen from Comparative Example 2 that when calcium chloride is used alone to remove F, the filtrate obtained by filtration contains more calcium fluoride, calcium fluoride cannot be completely separated, and the filtration speed is very slow. The obtained product sodium chloride has a high F content and low purity, and the effect is not ideal.

[0135] Comparative Example 3

[0136] The sodium chloride raw material treated in this embodiment is a by-product obtained in the fluorine chemical production process. The by-product is tested to contain 39.5% Na, 5.49% F, 54.1% Cl, color number: 10, COD: 17215ppm. The purity of the by-product is roughly estimated to be 89.2% based on the Cl content.

[0137] (2) Weigh 200 g of the by-product sodium chloride, add 495 g of water based on the purity of 89.2%, stir for 5 min to prepare a saturated solution of sodium chloride, filter with filter paper to remove insoluble matter, and obtain 663 g of filtrate. The F content of the filtrate is 2115 ppm; scrape 30 g of filter residue from the filter paper (a small amount of filter residue remains on the filter paper), and dry the filter residue at 130°C in vacuum for 4 h to leave 24 g of residue. The Na content of the filter residue is 56% and the F content is 43%; it is preliminarily determined that most of the insoluble matter is NaF impurities.

[0138] (2) The commercial QKF-1 solid defluoridant produced by Suzhou Qingkong Environmental Protection Technology Co., Ltd. is used as follows: defluoridant dosage (mg) = fluorine concentration (mg / l) * wastewater volume (L) * 50; pH condition = 6-7, mixing reaction for 2-5 minutes; adding 0.1% concentration of PAM for flocculation and precipitation; taking the supernatant to determine the residual fluoride ion concentration.

[0139] (3) According to the theoretical F content in the filtrate, 70.11 g of QKF-1 solid defluorinating agent should be added. The actual amount of added was 71 g. A large amount of white solid was produced immediately after the addition. The defluorinating agent is weakly acidic. An appropriate amount of sodium hydroxide was added to adjust the pH to 6.5. 1 g of 0.1% PAM aqueous solution was added. The mixture was allowed to stand for 24 hours. The F content in the supernatant was measured: 8.8 ppm.

[0140] (4) After the precipitate is shaken evenly, it is filtered using 0.1 micron filter paper. The filtration speed is extremely slow and the filtration is completed in 8 hours. The filtrate is slightly turbid and the F content is detected to be 38 ppm.

[0141] (5) The filtrate was evaporated, concentrated, crystallized and filtered to obtain 130.5 g of wet sodium chloride, which was dried at 105°C for 3 h. The purity was detected to be 86.5%, F: 561 ppm, and moisture: 1.2%.

[0142] Comparative Example 4

[0143] The sodium chloride raw material treated in this embodiment is a by-product obtained in the fluorine chemical production process. The by-product is tested to contain 39.8% Na, 5.39% F, 55.1% Cl, color number: 10, COD: 16225ppm. The purity of the by-product is roughly estimated to be 90.8% based on the Cl content.

[0144] (1) Weigh 200 g of the by-product sodium chloride, add 504 g of water calculated based on the purity of 90.8%, and stir for 5 min to prepare a saturated solution of sodium chloride; use filter paper to filter out insoluble matter to obtain 671 g of filtrate, and the F content of the filtrate is 2065 ppm; scrape 31 g of filter residue from the filter paper (a small amount of filter residue remains on the filter paper), and after vacuum drying at 130°C for 4 h, 25 g of the filter residue remains. The Na content of the filter residue is 56% and the F content is 43%; it is preliminarily determined that most of the insoluble matter is NaF impurities.

[0145] (2) Use the commercial QKF-2 liquid defluoridant produced by Kunshan Ruoshui Wuji Environmental Protection Technology Co., Ltd. The usage is as follows: defluoridant dosage (mg) = fluorine concentration (mg / l) * wastewater volume (L) * 50; pH condition = 6-7, mixing reaction 2-5min; add 0.1% concentration of PAM for flocculation and precipitation; take the supernatant to determine the residual fluoride ion concentration.

[0146] (3) According to the theoretical F content in the filtrate, 69.28g of QKF-2 solid defluorinating agent should be added, but 70g was actually added. A large amount of white solid was produced upon addition. The pH of the defluorinating agent was 2-3. 32g of sodium hydroxide solid was added to adjust the pH to 6.5. 1g of 0.1% PAM aqueous solution was added. After standing for 24h, the supernatant was taken to measure the F content: 5.9ppm.

[0147] (4) After the precipitate is shaken evenly, it is filtered using 0.1 micron filter paper. The filtration speed is extremely slow and the filtration is completed in 12 hours. The filtrate is slightly turbid and the F content is detected to be 25 ppm.

[0148] (5) The filtrate was evaporated, concentrated, crystallized and filtered to obtain 121.5 g of wet sodium chloride, which was dried at 105°C for 3 h. The purity was detected to be 89.2%, F: 495 ppm, and moisture: 1.3%.

[0149] As can be seen from Comparative Examples 3 and 4, the use of commercial solid defluorinating agent in large amounts will introduce a large amount of impurity ions, resulting in a very low purity product with a high F content. Although a supernatant with a low F content can be obtained by standing, and a certain low F content solution can be obtained by discharging the supernatant, the solution with a large amount of residual solids at the bottom is difficult to handle. In addition, filtration using commercial solid defluorinating agent still cannot obtain a clear sodium chloride solution, and the filtration speed is extremely slow.

[0150] The above detailed description of the present invention is intended to enable persons familiar with the art to understand the contents of the present invention and implement them. It does not limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for removing fluorine from fluorine-containing salts, characterized in that: The steps include: (1) preparing a saturated salt solution containing fluorine, and filtering to remove insoluble impurities, wherein the fluorine-containing salt is a by-product obtained in the fluorine chemical production process, and the by-product comprises the following components by mass percentage: 35-45% sodium, 5-12% fluorine, and 45-60% chlorine; (2) selectively adding alkali to the filtrate of step (1), adding calcium chloride according to the fluorine content in the filtrate, and heating and stirring to react; (3) adding a defluorinating agent, optionally adding the base, heating and stirring to react; (4) selectively adding an acid and filtering to obtain a salt solution with a low fluoride content, wherein the salt solution is concentrated, dehydrated, crystallized, and dried to obtain a solid salt; Wherein, the defluorination agent comprises a mixed solution formed by aluminum chloride, ferric chloride, polyacrylamide and water.

2. The method for defluorination and purification of fluorine-containing salt according to claim 1, characterized in that: The amount of the defluorinating agent added is 0.05-0.15% of the mass of the filtrate in step (1).

3. The method for defluorination and purification of fluorine-containing salt according to claim 1 or 2, characterized in that: The defluorination agent comprises the following components by weight: 10-20 parts of aluminum chloride, 80~90 parts of water, 0.05~0.15 parts of ferric chloride, Polyacrylamide 0.05~0.15 parts.

4. The method for defluorination and purification of fluorine-containing salt according to claim 1, characterized in that: The filtration time of step (4) is controlled to be less than or equal to 15 minutes.

5. The method for defluorination and purification of fluorine-containing salt according to claim 1, characterized in that: The molar amount of the calcium chloride added is 0.5 to 2 times the molar amount of fluorine in the filtrate.

6. The method for defluorination and purification of fluorine-containing salt according to claim 1, characterized in that: The reaction temperature of step (2) is controlled to be 60-120° C. and the reaction time is controlled to be 1-4 h; And / or, the reaction temperature of step (3) is controlled to be 60-120° C. and the reaction time is controlled to be 1-4 h.

7. The method for defluorination and purification of fluorine-containing salt according to claim 1, characterized in that: The alkali includes sodium hydroxide or sodium hydroxide solution; and / or, the acid comprises hydrochloric acid; and / or, adding the base to the step (2) to adjust the pH to 9-14; and / or, adding the base in step (3) to adjust the pH to 9-14; And / or, adding the acid to the step (4) to adjust the pH to 6-8.

8. The method for defluorination and purification of fluorine-containing salt according to claim 1, characterized in that: The filtration comprises pressure filtration using a filter membrane with a filter diameter of 0.05 to 0.2 microns; and / or, controlling the mass of water removed during the concentration and dehydration to be 45-55% of the total mass of the salt solution; And / or, the drying temperature is controlled to be 100-130° C., and / or, the drying time is controlled to be 4-16 hours.

9. The method for defluorination and purification of fluorine-containing salt according to claim 1, characterized in that: The defluorination and purification method further comprises adding activated carbon for decolorization after step (2) and before step (3), wherein the mass of the activated carbon is 0.1-1% of the mass of the filtrate of step (1).

10. The method for defluorination and purification of fluorine-containing salt according to claim 1, characterized in that: The color number of the by-product is greater than or equal to 5, and the COD of the by-product is greater than 15000 ppm.