CN112299453A

CN112299453A - Preparation method of high-purity lithium fluoride

Info

Publication number: CN112299453A
Application number: CN202011217443.8A
Authority: CN
Inventors: 谈宇清; 任琪
Original assignee: Shenzhen Maxchemtech Co ltd
Current assignee: Shenzhen Maxchemtech Co ltd
Priority date: 2020-11-04
Filing date: 2020-11-04
Publication date: 2021-02-02
Anticipated expiration: 2040-11-04
Also published as: CN112299453B

Abstract

The invention discloses a preparation method of high-purity lithium fluoride, which comprises the following steps: firstly, adding ammonium fluoride into a lithium bicarbonate aqueous solution, namely a refined solution A, which is obtained by carbonizing lithium carbonate by CO2 gas, to prepare high-purity lithium fluoride; or (ii) adding ammonium fluoride to a purified liquid B obtained from a mixed solution of lithium hydroxide and high-purity lithium formate to prepare high-purity lithium fluoride. The preparation method of the lithium fluoride can prepare the high-purity lithium fluoride with the purity of more than 99.5 percent, and has the advantages of high conversion rate of raw materials, high quality of products, low production cost and the like.

Description

Preparation method of high-purity lithium fluoride

Technical Field

The invention relates to a preparation method of high-purity lithium fluoride.

Background

The lithium fluoride has wide application, can be used as a fluxing agent in the enamel, glass and ceramic industries, is widely used in a flux and a soldering flux of aluminum and magnesium alloy, and also can be used as an additive for improving the electric efficiency in the electrolytic aluminum industry; the material is used as a neutron shielding material in the atomic energy industry, and is used as a solvent and a circulating working medium in a molten salt reactor (including but not limited to a thorium-based molten salt reactor of a fourth generation nuclear energy candidate technology); a transparent window (transmittance 77% -88%) used as ultraviolet ray in optical material; in addition, solar radiation thermal energy and the like are also stored as a heat receiver material in a space ship. With the wide application of lithium batteries in new energy automobiles, the demand of lithium hexafluorophosphate serving as an electrolyte component is continuously increased. And lithium fluoride is an important raw material for synthesizing lithium hexafluorophosphate.

Currently, the mature method for industrially producing lithium fluoride is to react lithium carbonate or lithium hydroxide with hydrofluoric acid to obtain lithium fluoride, and the reaction equation is as follows:

Li₂CO₃+HF→LF+H₂O+CO₂or

LiOH+HF→LF+H₂O

The disadvantages of the conventional method are:

the traditional method adopts hydrogen fluoride or hydrofluoric acid as raw materials, has high danger and extremely high requirements on equipment corrosion resistance, and is easy to generate environmental pollution.

For the wet synthesis route, the technical route of synthesizing lithium fluoride by reacting hydrofluoric acid with lithium hydroxide or lithium carbonate is mostly adopted, and raw material hydrofluoric acid (aqueous solution of HF) is used. As hydrofluoric acid has strong corrosivity, conventional reaction equipment and materials thereof (such as steel, stainless steel, glass and enamel) are not suitable for industrial mass production of the prior art route. Therefore, the conventional synthetic route can only adopt lead or platinum and other hydrofluoric acid corrosion-resistant materials, and small-batch preparation is completed in a laboratory. If the scale needs to be further enlarged, the corresponding industrial reaction kettle, material inlet and outlet pipelines and the like need to be specially designed and customized. Hydrofluoric acid is harmful to both human and the environment. The defects result in the limitation of large-scale industrialized production of the lithium fluoride and high production cost.

In conventional processes, a slight excess of hydrofluoric acid is required to obtain high purity lithium fluoride. The existing synthetic route is difficult to remove by a conventional method because of trace impurities brought by side reaction caused by excessive hydrofluoric acid.

The product of the traditional method also contains a trace amount of hydrofluoric acid, and can inevitably escape with water vapor or solvent in the drying process to cause damages such as corrosion to equipment and environment.

For the dry synthesis route, the problems of incomplete reaction, incomplete raw material conversion and the like exist in the reaction of gaseous HF and lithium carbonate solid powder and the like, and the high-purity lithium fluoride product is difficult to obtain.

In addition, the conventional metathesis reaction has the problems of low conversion rate, low product yield, difficulty in obtaining large-particle crystalline products and the like due to the dependence on raw materials such as battery-grade lithium carbonate, battery-grade lithium hydroxide and the like.

Metathesis is a reaction that exhibits a two-way dynamic equilibrium. Whether complete conversion from the left to the right of the reaction equation can be achieved depends on the reaction conditions and the removal of the products. In order to ensure the purity of the product, the purity of the raw materials is greatly improved, and the phenomena of coprecipitation, crystallization adsorption and the like generated in the crystallization process of the main product are avoided. The main product and the by-products should be easy to separate and purify, and avoid pollution and discharge. In summary, there is a conflict in the metathesis reaction route where three goals are difficult to achieve simultaneously, namely: it is difficult to achieve high conversion of raw materials, high quality of products and low cost of production at the same time.

The above background disclosure is only for the purpose of assisting understanding of the inventive concept and technical solutions of the present invention, and does not necessarily belong to the prior art of the present patent application, and should not be used for evaluating the novelty and inventive step of the present application in the case that there is no clear evidence that the above content is disclosed at the filing date of the present patent application.

Disclosure of Invention

The main purpose of the present invention is to overcome the above-mentioned drawbacks of the background art and to provide a method for preparing high-purity lithium fluoride.

In order to achieve the purpose, the invention adopts the following technical scheme:

a preparation method of high-purity lithium fluoride comprises the following steps: firstly, adding ammonium fluoride into a lithium bicarbonate aqueous solution, namely a refined solution A, which is obtained by carbonizing lithium carbonate by CO2 gas, to prepare high-purity lithium fluoride; or (ii) adding ammonium fluoride to a purified liquid B obtained from a mixed solution of lithium hydroxide and high-purity lithium formate to prepare high-purity lithium fluoride.

Further:

and controlling the feeding speed of the ammonium fluoride, and after the feeding is finished, controlling the double decomposition reaction process and the generation speed of the target product lithium fluoride in a closed reaction kettle by changing reaction temperature and pressure parameters.

The feeding is finished at normal temperature, after the materials are fully mixed by stirring, the air in the reaction kettle is replaced by nitrogen, and the nitrogen enables the pressure of the reaction kettle to be 0.01MPa-1 MPa; the temperature of the materials in the reaction kettle is gradually increased, and the removal speed of the gas generated in the reaction kettle is controlled by controlling the temperature of the reaction kettle and the pressure in the reaction kettle.

The double decomposition route is adopted, the mass fraction of ammonia is regulated and controlled in a solvent system of the reaction during or after the double decomposition reaction, and the solubility of lithium fluoride is improved, wherein the mass fraction of ammonia is regulated and controlled in the solvent system of the reaction, and the content and the escape speed of ammonia in the solvent are mainly regulated by changing temperature and pressure parameters.

Adding ammonia into the solvent when dissolving the raw materials; or by the reaction of lithium hydroxide with ammonium fluoride to form ammonia, namely: the ammonia is kept in the solvent through the pressurized reaction of the closed reaction kettle so as to control the precipitation process of the lithium fluoride crystal.

Also includes the preparation of the ammonium fluoride, which specifically includes:

a 1: dissolving industrial-grade ammonium bifluoride in excessive ammonia water to convert the ammonium bifluoride into ammonium fluoride, and precisely filtering to remove impurities;

a 2: adding reagent-grade oxalic acid and/or ammonium oxalate with the molar weight equivalent to 0.1-2% of ammonium fluoride, stirring uniformly, and then carrying out precise filtration to remove impurities;

a 3: evaporating, dehydrating, crystallizing, centrifuging, scrubbing with a small amount of electronic grade organic solvent, and drying to obtain refined ammonium fluoride;

or

Recrystallizing industrial ammonium bifluoride and/or industrial ammonium fluoride in an organic solvent to obtain the refined ammonium fluoride.

The preparation method further comprises the following steps of preparing the high-purity lithium formate:

b 1: carbonizing industrial-grade lithium carbonate by using carbon dioxide, preferably, mixing the industrial-grade lithium carbonate with purified water, stirring to prepare lithium carbonate slurry with the weight percentage concentration of 10% -30%, continuously introducing CO2 gas into the slurry, carrying out carbonization reaction for 4-5 hours, and controlling the reaction temperature to be 30-40 ℃ until the reaction is complete to obtain suspension; precisely filtering and removing impurities to obtain a lithium bicarbonate aqueous solution, namely the refined solution A;

b 2: adding high-purity ammonium formate into the refined solution A according to the equal molar ratio of 1:1, fully dissolving, integrally heating to 40-90 ℃, and removing carbon dioxide and ammonia in a closed container under negative pressure to obtain a lithium formate aqueous solution;

b 3: adding tetramethylammonium hydroxide into the lithium formate aqueous solution, adjusting the pH value to 9-13, heating, evaporating, concentrating, removing part of water to a saturated state, performing precise filtration to remove impurities, cooling, crystallizing, centrifuging, removing liquid to obtain lithium formate crystals, and drying to obtain the high-purity lithium formate.

In the technical grade lithium carbonate: 99.0 wt% of lithium carbonate, 500PPM or more of sodium, 100PPM or more of magnesium, 50PPM or more of iron, 20PPM or more of potassium, aluminum and copper, 200PPM or more of calcium and 1000PPM or more of sulfate radical.

The preparation of the refined liquid B comprises the following steps: dissolving industrial-grade lithium hydroxide with purified water or ammonia water to prepare a saturated solution, adding the high-purity lithium formate, fully dissolving, and filtering to remove impurities to obtain the refined solution B.

The second step comprises the following steps:

s1: dissolving industrial-grade lithium hydroxide in pure water or ammonia water, adding high-purity lithium formate, fully dissolving, removing insoluble impurities, putting filtrate into a reaction kettle, starting the reaction kettle, and stirring;

s2: adding ammonium fluoride at a feeding speed of 1-10 kg per minute;

s3: replacing air in the reaction kettle with nitrogen, heating, and reacting for 1-4 hours at the temperature of 0-40 ℃; heating and raising the temperature, controlling the temperature raising speed per minute to be between 1 and 2 ℃, gradually raising the temperature of the materials in the reaction kettle to be between 40 and the boiling point, continuously stirring for 2 to 24 hours during the temperature raising period, and maintaining the pressure in the reaction kettle to be between 0.01 and 1.0MPa of positive pressure; after the reaction is fully completed, slowly reducing the air pressure in the kettle to the normal pressure within 30 minutes, and heating the materials in the kettle to a boiling state; condensing, refluxing and collecting steam in the kettle through a kettle top condenser; stopping heating until the PH value of the reflux liquid in the condenser is less than 8; continuously maintaining the stirring of the materials in the reaction kettle, cooling the materials in the reaction kettle to 20-40 ℃, and discharging;

s4: performing solid-liquid separation on the material discharged from the reaction kettle, separating liquid-phase material, collecting mother liquor, and washing a filter cake; collecting the filtrate;

s5: heating and drying, removing water and volatile matters in the filter cake, and cooling to obtain the high-purity lithium fluoride;

preferably, it further comprises S6: the mother liquor of the solid-liquid separation is collected in step S4, concentrated, filtered to remove impurities, crystallized to obtain ammonium formate, and recrystallized in electronic grade anhydrous methanol to obtain high purity ammonium formate, and preferably, the obtained high purity ammonium formate is used in step b 2.

The embodiment of the invention provides that materials such as lithium formate and lithium hydroxide and ammonium fluoride are subjected to double decomposition reaction in a water solvent, so that the effect of improving the single kettle yield of lithium fluoride, improving the granularity of a crystallized product and removing impurities is better than that of singly using lithium formate or lithium hydroxide; or the raw materials such as lithium hydroxide and lithium formate and ammonium fluoride are subjected to double decomposition reaction in an ammonia water solvent to generate lithium fluoride (main product). By-products of embodiments of the present invention are ammonia and ammonium formate. The main product lithium fluoride is slightly soluble in water and insoluble in alcohol. The byproducts such as ammonium formate and the like are not only easily dissolved in water and methanol, but also can maintain higher solubility in a wide alcohol-water binary solvent system (for example, the mass fraction of water is in a range of 5-95 percent), and are not easy to separate out, so that the byproducts are suitable for recrystallization purification in organic solvents such as electronic grade methanol and the like. Also provides a method for preparing high-purity lithium formate by reacting ammonium formate with lithium bicarbonate, and realizes the closed loop of recycling the byproduct ammonium formate. The other by-product ammonia can be used for treating fluosilicic acid and producing ammonium fluoride, thereby realizing the recycling of the by-product, reducing the material loss and lowering the production cost.

In the preferred embodiment of the invention, lithium formate and lithium hydroxide are mixed and then used as a lithium source to react with ammonium fluoride, so that the advantages of high conversion rate of raw materials (in the prior art, the high conversion rate can be ensured only by greatly excessive lithium compounds), high quality of products (crystallization of lithium fluoride can be regulated and controlled by controlling escape of ammonia), low production cost (by-products are recycled, and battery-grade lithium carbonate and battery-grade lithium hydroxide are not used) and the like can be obtained.

The method can utilize industrial-grade raw materials (industrial-grade ammonium bifluoride, industrial-grade lithium carbonate and industrial-grade lithium hydroxide) which are wide in sources and sufficient in domestic market supply to prepare the high-purity lithium fluoride crystal product with the purity of more than 99.9% on a large scale, and meets the urgent requirements of downstream products such as lithium battery electrolyte (including but not limited to lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorosulfonimide, lithium difluorooxalato borate and the like) and the like on high-quality raw materials which are growing day by day. The invention can prepare high-purity lithium fluoride (the purity is more than 99.9%), and can finally meet the extreme requirements (the content of transition elements is less than the minus ninth power of 10) of precision optical materials such as optical fibers on the quality of the lithium fluoride through further refining and purification.

The method can improve the purity (> 99.9%) of the lithium fluoride product so as to meet the strict requirements of downstream products such as necessary raw materials of lithium hexafluorophosphate and the like of lithium battery electrolyte on the quality of the lithium fluoride. The invention does not adopt hydrofluoric acid as raw material, reduces the requirement of corrosion resistance of production equipment, reduces the industrial investment cost, and does not contain free acid in the product. The invention provides a new process capable of controlling the crystallization process of a lithium fluoride product, and the lithium fluoride crystal product with larger granularity than that of the lithium fluoride crystal product obtained by the known method is obtained.

Because the invention does not adopt hydrofluoric acid as the synthetic raw material, but uses industrial ammonium bifluoride (the silicon tetrafluoride and fluosilicic acid which are produced as by-products in the phosphate fertilizer industry react with ammonia water) or industrial ammonium fluoride as the starting raw material, the latter can save precious hydrogen fluoride and fluorite resources at the upstream; the method comprises the steps of refining and removing impurities from industrial raw material ammonium fluoride, deeply evaluating a plurality of candidate double decomposition reaction technical routes and optimizing the process to determine the optimal process, so that high-purity lithium fluoride crystal products are prepared in a large scale.

Compared with the prior art, the invention has the following advantages:

the investment on equipment is saved, and the corrosion prevention requirement on the equipment is low; the novel process route has good safety, less pollution and more environmental protection because hydrofluoric acid is not used; the industrialization is easier to be scaled, the product yield is high, and the production cost is low; the quality of the product is better. The by-products can be recycled, and the raw material cost is reduced. Specifically, the method comprises the following steps:

first, technical grade ammonium bifluoride/ammonium fluoride, technical grade lithium carbonate and technical grade lithium hydroxide are used as raw materials. The raw materials are sufficient in domestic market supply and do not depend on high-purity battery grade raw materials imported overseas; second, the reaction involved comprises: 1. reacting ammonium fluoride with lithium formate to generate lithium fluoride and ammonium formate, reacting lithium hydroxide with ammonium fluoride to generate lithium fluoride and ammonia, and recovering ammonia; 2. reacting ammonium formate with lithium bicarbonate to synthesize lithium formate, and recovering ammonia; 3. the recovered ammonia reacts with fluosilicic acid to generate ammonium fluoride. The reaction realizes the cyclic reuse of by-products ammonium formate and ammonia and the refining of raw materials, improves the utilization rate of the raw materials, reduces the production cost, reduces the discharge amount and reduces the environmental pollution; 3. the slow pressure relief of the air pressure of the reaction kettle is controlled by controlling the feeding speed, the escape speed of the ammonia gas from the reaction system is controlled, the crystallization of the lithium fluoride is controlled, and the growth of crystals is realized to obtain products with larger particles.

The quality of the main product lithium fluoride obtained by the invention completely meets the quality standard of LF-1 in GB/T22666-2008 lithium fluoride, meets various quality requirements of application scenes such as welding flux, soldering flux, lithium fluoride for electrolytic aluminum, special lithium fluoride for a new generation nuclear power station (such as thorium-based molten salt pile) and the like on lithium fluoride, and can also be directly used as a production raw material of necessary components of lithium battery electrolyte such as lithium hexafluorophosphate and the like; the by-product ammonium formate can reach or exceed the national quality standard of industrial grade and reagent grade products, and can be recycled internally and sold externally. Therefore, the industrialization route of the invention not only can meet various quality standards of main and auxiliary products, but also has the economy necessary for industrialized mass production, and has wide market prospect and industrialization prospect. The ammonia can also be recycled or used for producing products such as ammonium sulfate which are easy to sell.

Detailed Description

The embodiments of the present invention will be described in detail below. It should be emphasized that the following description is merely exemplary in nature and is not intended to limit the scope of the invention or its application.

In some embodiments, a method of preparing high purity lithium fluoride comprises: the high-purity lithium fluoride is prepared by adding ammonium fluoride to a lithium hydrogencarbonate aqueous solution, i.e., a purified solution a, obtained by carbonizing lithium carbonate with CO2 gas. In these examples, a lithium carbonate raw material with excellent quality is selected, and the content of other metal impurity ions after carbonization does not exceed 1 PPM.

In some embodiments, a method of preparing high purity lithium fluoride comprises: the high-purity lithium fluoride is prepared by adding ammonium fluoride to a purified liquid B obtained from a mixed solution of lithium hydroxide and high-purity lithium formate.

In a preferred embodiment, lithium formate and lithium hydroxide are mixed and used as a lithium source to react with ammonium fluoride, and the advantages of high raw material conversion rate, high product quality, low production cost and the like can be particularly obtained.

The inventor finds a set of optimized process technical route on a metathesis process route, and the specific embodiment comprises the following four core process flows:

one of the core process flows is as follows: the technical grade ammonium bifluoride (97%) is used as a raw material to prepare the high-purity ammonium fluoride, and the core process flow comprises but is not limited to the following three steps. 1. Dissolving in excessive ammonia water, converting into ammonium fluoride, and precisely filtering to remove impurities; 2. adding reagent-grade oxalic acid and/or ammonium oxalate with the molar weight equivalent to 0.1-2% of ammonium fluoride, stirring uniformly, and then carrying out precise filtration to remove impurities; 3. evaporating to dewater, crystallizing, centrifuging, scrubbing with small amount of electronic grade organic solvent (including but not limited to methanol, ethanol or acetone), and drying; or 4, recrystallizing in organic solvent (such as methanol) to obtain refined ammonium fluoride (purity > 99.9%);

the second core process flow is as follows: and preparing high-purity lithium formate by using industrial-grade lithium carbonate as a starting raw material. Including but not limited to the following steps. 1. Carbonizing industrial-grade lithium carbonate by using carbon dioxide (namely mixing and stirring the industrial-grade lithium carbonate with purified water to prepare lithium carbonate slurry with the weight percentage concentration of 10% -30%, continuously introducing CO2 gas into the slurry, carrying out carbonization reaction for 4-5 hours, controlling the reaction temperature to be 30-40 ℃ until the reaction is complete to obtain suspension liquid), carrying out precision filtration to obtain a lithium bicarbonate aqueous solution after impurity removal, and filtering and removing impurities by using ion exchange resin, an adsorption column and a precision filter to obtain a refined solution A; 2. adding high-purity ammonium formate (purity is more than 99.9%) into the refined liquid A according to an equimolar ratio (1:1), fully dissolving, integrally heating to 40-90 ℃, and removing gases such as carbon dioxide and ammonia in a closed container under negative pressure to obtain a lithium formate aqueous solution. 3. Adding a small amount of tetramethylammonium hydroxide into the lithium formate aqueous solution, adjusting the pH value to 9-13, heating, evaporating, concentrating (removing part of water) to a saturated state, performing thermal precision filtration to remove impurities, naturally cooling, crystallizing, centrifuging, removing liquid to obtain lithium formate crystals, scrubbing with electronic grade absolute ethyl alcohol, and drying in vacuum to obtain high-purity lithium formate (> 99.9%). (note: the raw material lithium carbonate is industrial grade lithium carbonate, wherein, the weight content of the lithium carbonate is 99.0 percent, the sodium is 500PPM or more, the magnesium is 100PPM or more, the iron content is 50PPM or more, the potassium, the aluminum and the copper are 20PPM or more, the calcium is 200PPM or more, the sulfate radical is 1000PPM or more)

The third core process flow is as follows: preparing a reaction kettle bottom material refining liquid B. Including but not limited to the following steps: 1. dissolving industrial-grade lithium hydroxide with purified water or recovered ammonia water to prepare a saturated solution, adding high-purity lithium formate (> 99.9%) and fully dissolving, and filtering and removing impurities through ion exchange resin, an adsorption column and a precision filter to obtain a refined solution B;

the fourth core process flow is as follows: and adding the solid ammonium fluoride obtained in the first core process flow into the refined liquid A or the refined liquid B prepared in the third core process flow through a hopper. The feeding speed is controlled by controlling the opening and closing degree of a gate valve at the bottom of the feeding hopper. After the charging is finished, the double decomposition reaction process and the generation speed of the target product lithium fluoride are controlled in a closed reaction kettle by changing reaction temperature and pressure parameters, so that the aim of harvesting the lithium fluoride product with larger crystal particles is fulfilled.

Through multiple rounds of experimental verification, the inventor provides a method for controlling the metathesis reaction speed, which is beneficial to the controllable growth of lithium fluoride crystals. Namely: at normal temperature, the feeding is completed by controlling the adding speed of the reaction materials, and after the materials are fully and uniformly mixed by stirring, the air in the reaction kettle is replaced by nitrogen, and the inlet and outlet valves of the reaction kettle are closed. Then nitrogen is used for pressurizing the reaction kettle to 0.01MPa-1 MPa. And opening the heating jacket of the reaction kettle to gradually increase the temperature of the materials in the kettle. The operator controls the speed of removing the gas (such as ammonia) generated in the reaction kettle by controlling the temperature of the reaction kettle and the pressure in the reaction kettle (such as slowly releasing the pressure by a throttle valve). After the pressure in the kettle is reduced to normal pressure, gas products such as ammonia gas and the like can be driven by modes of vacuumizing, negative pressure degassing, blowing of a nitrogen pipe into the material liquid, supplementing of pure water during boiling dehydration and the like.

Aiming at the fourth core process flow, the more specific process route is as follows:

the first step is as follows: dissolving 924kg of industrial-grade lithium hydroxide in 7500L of pure water or ammonia water, adding 2003kg of high-purity lithium formate, fully dissolving, removing insoluble impurities through a precision filter, putting filtrate into a reaction kettle, starting the reaction kettle, and stirring; the second step is that: controlling the feeding speed of 1428kg of refined ammonium fluoride to be 1-10 kg per minute through a discharge gate valve until the feeding is finished; the third step: and (4) completely closing the inlet valve and the outlet valve of the reaction kettle. After the air in the reaction kettle was replaced with nitrogen. Opening a jacket of the reaction kettle to heat and preserve the temperature of the materials in the kettle, and maintaining the temperature of the materials between 0 and 40 ℃ for reaction for 1 to 4 hours; and then heating to raise the temperature, controlling the temperature raising speed per minute to be between 1 and 2 ℃, gradually raising the temperature of the materials in the reaction kettle to be between 40 and the boiling point, continuously maintaining stirring for 2 to 24 hours during the temperature raising period, and maintaining the pressure in the reaction kettle to be positive pressure (between 0.01 and 1.0MPa, and automatically relieving the pressure if the pressure exceeds the pressure). After the reaction is fully completed, a pressure release valve at the top of the reaction kettle is opened, the air pressure in the kettle is slowly reduced to the normal pressure within thirty minutes, and then a heating jacket of the reaction kettle is opened to heat the materials in the kettle to a boiling state. And condensing and refluxing steam in the kettle through a condenser at the top of the kettle, and collecting ammonia-containing condensate to a storage tank for later use. With the reduction of the amount of the solvent in the kettle, supplementing pure water into the reaction kettle in a proper amount until the PH value of the reflux liquid in the condenser is less than 8, and stopping heating the jacket; then, continuously maintaining the stirring of the materials in the reaction kettle, cooling the materials in the reaction kettle to the normal temperature (20-40 ℃) and then discharging; the fourth step: the material discharged from the reaction kettle passes through solid-liquid separation equipment such as a centrifuge, etc., the liquid phase material is separated and the mother liquor is collected, and then a small amount of purified water is used for washing the filter cake (solid phase material) for 1-3 times. Collecting the filtrate; the fifth step: heating and drying, removing water in the filter cake, heating to about 400 ℃ again to completely remove volatile matters, and cooling to obtain about 2000kg of high-purity lithium fluoride (purity is more than 99.9%); and a sixth step: mother liquor is concentrated and recovered, after precise filtration and impurity removal, 2430kg of ammonium formate (by-product) can be recovered by cooling to normal temperature and natural crystallization. The recovered ammonium formate is recrystallized in electronic grade anhydrous methanol to obtain high-purity ammonium formate (purity is more than 99.9%), and the by-product can be sold to the outside and can also be returned to the core process flow for reuse.

In other embodiments, lithium carbonate raw material with excellent quality (sampling detection after carbonization, content of other metal impurity ions is not more than 1PPM) can be directly used for reaction of the refined liquid A and ammonium fluoride to prepare lithium fluoride.

The technical principle of the preferred embodiment of the invention is as follows: the double decomposition reaction is carried out on materials such as lithium formate and lithium hydroxide and ammonium fluoride in a water solvent, so that the effect of improving the single kettle yield of lithium fluoride, improving the granularity of a crystallized product and removing impurities is better than that of singly using lithium formate or lithium hydroxide; or the raw materials such as lithium hydroxide and lithium formate and ammonium fluoride are subjected to double decomposition reaction in an ammonia water solvent to generate lithium fluoride (main product). The by-products are ammonia and ammonium formate. The main product lithium fluoride is slightly soluble in water and insoluble in alcohol. The byproducts such as ammonium formate and the like are not only easy to dissolve in water, but also can maintain higher solubility in a wide alcohol-water binary solvent system (for example, the mass fraction of water is in a range of 5-95 percent), and are not easy to separate out. Therefore, the by-product is suitable for recrystallization purification in organic solvents such as electronic grade methanol and the like. The inventor also provides a method for preparing high-purity lithium formate by reacting ammonium formate with lithium bicarbonate, and realizes the closed loop of recycling the byproduct ammonium formate. And the other by-product ammonia can be used for treating fluosilicic acid to produce ammonium fluoride. Thereby realizing the recycling of the by-products, reducing the material loss and lowering the production cost.

In general, lithium fluoride is produced by a metathesis reaction route, and the crystal size of the lithium fluoride product is smaller (the finer the particle is, the higher the amount of impurities adsorbed is, and purification of the product is not utilized) as compared with an acid-base neutralization reaction using hydrofluoric acid as a raw material. The reason is that: lithium fluoride is soluble in hydrofluoric acid (intermediate lithium hydrofluoride is generated), and products with larger particle sizes can be obtained by recrystallization and the like as long as the proportion of the hydrofluoric acid is adjusted. The above-mentioned effects are difficult to achieve by the conventional metathesis reaction. The inventors have surprisingly found that, by using a double decomposition route, if the mass fraction of ammonia can be controlled in a solvent system of a reaction during or after the double decomposition reaction, the solubility of lithium fluoride can be improved, and the content and the escape speed of ammonia in the solvent can be adjusted by changing temperature and pressure parameters, so that the preparation of a large-particle lithium fluoride product can be realized. Therefore, the method can perfectly replace a hydrofluoric acid method to obtain a high-quality lithium fluoride product. And ammonia is derived from two sources. One is adding into solvent (such as directly using ammonia water as solvent) while dissolving raw materials; the other is the generation of ammonia by the reaction of lithium hydroxide with ammonium fluoride, namely: the ammonia is kept in the solvent through the pressurized reaction of the closed reaction kettle so as to control the precipitation process of the lithium fluoride crystal.

The examples of the invention rely on the following metathesis reactions:

LiOH+NH₄F→LiF+H₂O+NH₃and LiCHO₂+NH₄F→LiF+NH₄CHO₂

And

LiHCO₃+NH₄CHO₂→LiCHO₂+H₂O+NH₃+CO₂

and the like.

The main technical process of the specific embodiment of the invention comprises the following steps:

firstly, dissolving and refining raw materials

Raw materials: ammonium fluoride (technical grade)

3000L of purified water is firstly injected into the batching kettle, 2855.8kg of ammonium fluoride is added, and the mixture is fully stirred and dissolved at normal temperature; then adding ammonia water or 1-2kg tetramethyl ammonium hydroxide, stirring, removing trace impurities by an adsorption column and a precision filter (adopting a PE sintering pipe with the filtering precision of 0.5 micron), and injecting the filtrate into a reaction kettle. A small amount of ammonium oxalate (0.5-3kg) was added. After fully and evenly stirring, precisely filtering again to remove impurities. Evaporating water until the ammonium fluoride saturated solution is obtained, stopping heating, and naturally cooling for crystallization. Scrubbing lithium fluoride crystals obtained by centrifugal separation for 1-3 times by using electronic grade anhydrous methanol, putting the lithium fluoride crystals into a vacuum drier, and removing a solvent to obtain high-purity anhydrous ammonium fluoride, wherein the purity of the product is more than 99.9%. Bagging for later use.

Secondly, feeding synthesis and reaction crystallization

The stirrer of the reaction vessel was opened, and a purified solution in which 923.3kg of lithium hydroxide and 2003.1kg of an aqueous solution of lithium formate (about 7500L in volume) were dissolved was gradually added. 2860kg of refined ammonium fluoride (purity > 99.9%) is added into a hopper, the reaction kettle is opened, stirring is carried out, and the feeding speed is controlled. After the feeding is finished, closing an emptying valve of the reaction kettle, replacing air in the reaction kettle with nitrogen, and increasing the pressure in the kettle to 0.01-1 MPa. Closing the nitrogen valve, opening a jacket heating valve of the reaction kettle, maintaining the temperature of the materials in the reaction kettle between 45 ℃ and 90 ℃, continuously stirring for 2 to 8 hours, opening a throttle valve of an emptying valve, and slowly releasing the air pressure of the reaction kettle to normal pressure. Naturally cooling for 8-12 hours. About 2000kg of lithium fluoride crystals were obtained. The mother liquor is further processed and, by evaporative crystallization, about 2431kg of ammonium formate as a by-product are obtained.

Raw materials: the purity of the industrial grade lithium formate is 99 percent, and the industrial grade lithium formate after being refined reaches various quality standards of battery grade lithium formate (99.9 percent), and the following table is referred to:

secondly, double decomposition reaction based on dropwise addition;

the synthesis process disclosed by the invention avoids impurity adsorption caused by excessive generation of early-stage crystal nuclei and excessively fine crystallization of the product. This is advantageous to ensure that the purity of the product meets the quality requirements.

Thirdly, separating and drying the main product;

the main product lithium fluoride is precipitated due to the insolubility in a solvent system of the reaction, a filter cake is obtained by centrifugal deliquoring, the filter cake is washed for one to three times by purified water, and after the filter cake is deliquored and dried, residual water is removed by drying (including but not limited to air flow drying or rotary dryer vacuum drying).

Fourthly, refining, separating and drying the by-products

In addition to the need for ammonia removal by negative pressure, the other by-products are relatively soluble and hygroscopic in the solvent, and require evaporation of a portion of the solvent before crystallization can begin. After crystallization, the water in the solvent is replaced by methanol or ethanol, which is helpful for complete crystallization. After centrifugal liquid removal, airflow drying or vacuum drying is adopted to obtain a finished product.

The lithium formate and the lithium hydroxide which are mixed and then used as a lithium source react with ammonium fluoride, so that the requirements of high conversion rate of raw materials (in the existing patent documents, the high conversion rate can be ensured only by greatly excessive lithium compounds), high quality of products (crystallization of the lithium fluoride is regulated and controlled by controlling the escape of ammonia) and low production cost (by-products are recycled, and expensive raw materials such as battery-grade lithium carbonate, battery-grade lithium hydroxide and the like are not used) are met.

A synthesis process and a refining technology for preparing high-purity lithium fluoride and at least one byproduct based on double decomposition reaction; dissolving raw materials (preferably lithium formate and lithium hydroxide) in purified water or ammonia water, adjusting the pH value, filtering and removing impurities through an adsorption column and a precision filter, dropwise adding materials to complete double decomposition synthetic reaction to obtain main products of lithium fluoride and byproducts (ammonia and ammonium formate), refining and drying the main products and the byproducts, and recycling ammonia;

secondly, the solvent system involved in the double decomposition reaction comprises water and ammonia (added during feeding or generated in the reaction); in the synthesis process, along with the dropwise addition of the materials, the content of ammonia in a reaction solvent system changes along with the reaction process, the temperature of the materials and the pressure in a kettle, so that a main product (lithium fluoride) is controlled to be separated out to form crystals, and byproducts (ammonium formate and the like) are kept in the solvent system and are not separated out;

the contact part of the inner wall of the reaction kettle and the like and the material adopts an epoxy resin glass fiber reinforced plastic lining layer and is coated with a polytetrafluoroethylene surface layer; all pipelines and the inner wall of the container which are contacted with the materials are made of polytetrafluoroethylene materials or fluorinated HDPE materials;

the by-product ammonium formate is dissolved in anhydrous methanol, and anhydrous lithium chloride is added to carry out double decomposition reaction to generate ammonium chloride (precipitation) and lithium formate. The lithium formate can be reused after methanol solvent is removed, and the lithium formate is dissolved in water for recrystallization and purification.

The first embodiment is as follows:

the first step is as follows: reacting ammonium bifluoride with excessive ammonia water to generate ammonium fluoride (the purity is more than 99.9%);

the reaction principle of the first step of this example is:

NH₄HF+NH₃·H₂O→2NH₄F+H₂O

the main production flow is as follows:

step one, neutralization reaction

158.8kg of ammonium bifluoride (97 percent, industrial grade) is added into a neutralization reaction kettle through a feed inlet, then a bottom valve of a dosing tank is opened, 416L of ammonia water (20 percent) is slowly injected into the neutralization kettle, and after the feeding is finished, the stirring of the neutralization kettle is opened. After the materials are fully dissolved, adding 3kg of ammonium oxalate (reagent grade) into the kettle through a charging hole, and continuously stirring for 1-2 hours;

closing all the feed inlet valves, opening the jacket oil bath circulation of the neutralization reaction kettle to continuously raise the temperature in the kettle until the solvent is boiled, opening the circulating water of a condenser at the top of the kettle to make the solvent steam condensate and reflux for 1-2 hours, stopping heating, maintaining stirring, naturally cooling to normal temperature, adding 400L of anhydrous methanol (or 300L of anhydrous methanol plus 100L of recycled crystallization mother liquor plus 150L of cyclic crystallization mother liquor), and continuously stirring for one hour;

step two, precise filtration

And (3) passing the materials in the neutralization reaction kettle through a two-stage precision filter (respectively adopting a titanium rod filter core with the interception diameter of 1um and a ceramic membrane with the interception diameter of 20-100 nm) to remove insoluble impurities. Refluxing the filtered liquid to a neutralization reaction kettle, circulating for 1-2 hours, and discharging the filtered liquid into an evaporation crystallizer;

step three, refining the crystals

Opening a crystallization evaporator to heat steam, heating the filtrate obtained in the last step until the solvent is boiled, transferring the volatilized solvent steam and condensate obtained by heat exchange of a top condenser to a rectifying tower to remove water, refluxing methanol to the crystallization evaporator until the water content in the material in the crystallization evaporator is continuously reduced, and stopping heating and methanol refluxing until the water content in the condensate at the top of the kettle is lower than 10% in the solvent; the process removed about 350L of water.

Removing insoluble impurities from the materials in the crystallization evaporator through a precision filter again by heating, refluxing the filtrate to the evaporation crystallizer, and stopping filtering after circulating filtration for 1-2 hours;

and heating the evaporation solvent again, enabling the steam condensate on the top of the kettle to flow into a solvent recovery storage tank, continuously reducing the solvent amount in the evaporation crystallizer, concentrating the total volume of the materials from about more than 500L initially to 250-300L, stopping heating, discharging the materials into a crystallization kettle, standing for more than 60 hours, naturally cooling to enable the temperature of the materials to be reduced to below 30 ℃, basically completing crystallization, starting stirring, slowly stirring for 5 minutes, and preparing for discharging.

Step four, centrifugal liquid removal and vacuum drying

Sleeving a filter bag in a centrifuge, opening a valve at the bottom of the crystallization kettle, discharging materials into the centrifuge, and closing a discharge valve at the bottom of the kettle when the filter bag is full; and (5) starting the centrifugal machine, and discharging the crystallization mother liquor into a mother liquor tank for recycling. The product after centrifugal liquid removal is packaged by a plastic bag and a filter bag, and put into a rotary vacuum drier to remove residual solvent. The dryer is set to have a heating temperature of 40-90 ℃, the vacuum degree is gradually increased from the normal pressure to 0.1MPa, and the vacuum degree is maintained for 24-48 hours; and after drying, stopping heating, breaking vacuum by using nitrogen, sampling according to the requirements of 'drying and packaging post job duty', inspecting, and labeling each barrel with 25Kg of label for warehousing after passing inspection. About 200kg of high purity ammonium fluoride product was harvested per batch for use.

The high-purity ammonium fluoride (purity > 99.9%) obtained in the process meets the following quality standards (unit: PPM):

test item	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Maximum value	20	5	50	10	10	1	10	1	1

The second step is that: using battery-grade lithium carbonate as a starting material, carbonizing to prepare a lithium bicarbonate aqueous solution,

the reaction principle of the second step of this example is:

LiCO₃+H₂O+CO₂→2LiHCO₃(P_CO2＝0.4～0.45MPa)

step one, carbonization reaction.

2500L of purified water is injected into the carbonization kettle, and 200kg of industrial-grade lithium carbonate (the content is more than 99%) is added, and then a feeding port is closed. Starting stirring, opening jacket cooling water, slowly opening a carbon dioxide gas control valve, gradually increasing the air pressure in the carbonization kettle to 0.4MPa, continuously maintaining the air pressure in the kettle and stirring, and consuming about 120kg of carbon dioxide gas after full contact and reaction for 3-4 hours to obtain a lithium bicarbonate solution;

the quality specifications of the main raw materials of the reaction are as follows:

1. quality requirement of pure water (unit: PPM)

Detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe	Others
											Must not exceed	0.5	0.1	0.05	0.05	0.05	0.05	0.05	0.05	0.05	0.01

2. Quality requirement (unit: PPM) of battery-grade lithium carbonate (content > 99.9%)

Detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Must not exceed	10	10	10	5	5	1	5	1	1

3. Carbon dioxide: industrial or food grade, content > 99.9%

Step two, precisely filtering and removing impurities to obtain refined lithium bicarbonate solution

And (2) carrying out two-stage precise filtration on the reaction liquid obtained in the first step (a titanium rod filter element with the interception diameter of 1um and a ceramic membrane with the interception diameter of 50-100 nm) to obtain a filtrate (namely, lithium bicarbonate refined liquid), standing the filtrate in a crystallization tank for 2-8 hours, further removing trace impurity ions through an adsorption column and/or an adsorption resin refined column and the like, and injecting the refined liquid into a batching kettle.

The third step: and putting the high-purity ammonium fluoride into a lithium bicarbonate water solution for full reaction to prepare the high-purity lithium fluoride.

The reaction principle of the third step in this example is:

LiHCO₃+NH₄F→LiF+H₂O+CO₂

step one, double decomposition reaction

And (3) after all the refined liquid obtained in the last step is injected into the reaction kettle, opening the upper cover of the feeding hopper of the reaction kettle, feeding 200.5kg of refined ammonium fluoride (with the purity of more than 99.9%) into the feeding hopper, closing the top cover of the feeding hopper, opening a discharge gate valve, slowly adding the ammonium fluoride solid into the reaction kettle, and uniformly mixing the ammonium fluoride solid with the refined liquid until the feeding is finished. Closing the reaction kettle, opening a jacket oil bath to slowly heat materials in the kettle from normal temperature to 70-80 ℃ (ensuring that the temperature rise per hour is not more than 15 ℃), stopping heating, realizing condensate reflux for 3-4 hours under the condition of maintaining the positive pressure in the kettle, and opening an outlet throttle valve at the top of the kettle to discharge non-condensable gas (entering an absorption tower through a water-cooled heat exchanger, washing with cold methanol and the like to absorb carbon dioxide and ammonia) along with the increase of the pressure in the kettle; after the non-condensable gas is almost exhausted, the pressure in the kettle is reduced to normal pressure, and the reaction kettle is opened again for oil bath heating until boiling. Keeping the solvent boiling and the condensate refluxing for 2-8 hours, and stopping heating every time when the temperature of the material exceeds 105 ℃ until the double decomposition reaction is finished; and heating the evaporation solvent again, enabling the condensate to flow into the storage tank, supplementing pure water in a proper amount, and stopping heating after the ammonia gas and the carbon dioxide in the reaction kettle are almost completely discharged. Standing the material for more than 60 hours, naturally cooling to reduce the temperature of the material to below 30 ℃ to obtain about 140kg of lithium fluoride crystals, starting stirring, and slowly stirring for 5 minutes to prepare for discharging.

Step two, centrifugal liquid removal

Sleeving a filter bag on the centrifuge, opening a valve at the bottom of the reaction kettle, discharging materials into the A112 centrifuge, and closing a valve B505 when the filter bag is full; opening an inlet valve of a mother liquor reflux air pump, pumping the liquid discharged from A112 into a raw material tank or a preset crystallizing tank in time through the air pump, and discharging the mother liquor after harmless treatment; and packaging the centrifuged product by using a plastic bag and a filter bag, and putting the packaged product into a vacuum rotary dryer to remove residual moisture.

Step three, drying the product

And (3) washing the lithium fluoride filter cake obtained after spin-drying by a centrifuge for multiple times by using purified water, spin-drying again, packaging by using a plastic bag and a filter bag, and putting into a vacuum rotary dryer to remove residual water in the filter cake. Setting the temperature of the drying system at 90-110 ℃, gradually increasing the vacuum degree from normal pressure to 0.1MPa, maintaining for 24-48 hours until the drying is finished, stopping heating and breaking the vacuum by using nitrogen, then placing the drying system into a high-temperature oven, baking the drying system for 8-12 hours at the high temperature of 300-400 ℃, stopping heating, naturally cooling to normal temperature, sampling and inspecting according to the requirements of 'work duty of drying and packaging stations', and labeling each barrel for warehousing after the drying system is qualified according to 25 Kg. About 200kg of lithium fluoride crystal product can be obtained.

Example two:

the reaction principle of the first step of this example is:

NH₄HF+NH₃·H₂O→2NH₄F+H₂O

the main production flow is as follows:

step one, neutralization reaction

158.8kg of ammonium bifluoride (97%) is added into a neutralization reaction kettle through a feed inlet, then a bottom valve of a dosing tank is opened, 416L of ammonia water (20%) is slowly injected into the neutralization kettle, and after the feeding is finished, the stirring of the neutralization kettle is opened. After the materials are fully dissolved, adding 3kg of ammonium oxalate (reagent grade) into the kettle through a charging hole, and continuously stirring for 1-2 hours;

step two, precise filtration

step three, refining the crystals

Step four, centrifugal liquid removal and vacuum drying

test item	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Maximum value	20	5	50	10	10	1	10	1	1

The second step is that: preparing high-purity lithium formate by using industrial-grade lithium carbonate as a starting raw material through a two-step reaction;

the reaction principle of the second step of this example is:

LiCO₃+H₂O+CO₂→2LiHCO₃(P_CO2＝0.4～0.45MPa)

LiHCO₃+NH₄CHO₂→LiCHO₂+H₂O+NH₃+CO₂

the main production flow is as follows:

step one, carbonization reaction.

2500L of purified water is injected into the carbonization kettle, 75kg of industrial-grade lithium carbonate (the content is more than 99%) is added, and then a feeding port is closed. Starting stirring, opening jacket cooling water, slowly opening a carbon dioxide gas control valve, gradually increasing the air pressure in the carbonization kettle to 0.4MPa, continuously maintaining the air pressure in the kettle and stirring, and fully contacting and reacting for 3-4 hours to obtain a lithium bicarbonate solution;

1. quality requirement of pure water (unit: PPM)

2. Quality requirement (unit: PPM) of technical-grade lithium carbonate (content > 99%)

Detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Must not exceed	500	200	1000	500	200	100	20	20	50

3. Carbon dioxide: industrial or food grade, content > 99.9%

And (3) carrying out two-stage precise filtration (a titanium rod filter element with the interception diameter of 1um and a ceramic membrane with the interception diameter of 50-100 nm) on the reaction liquid obtained in the first step to obtain a filtrate (namely, the lithium bicarbonate refined liquid), and injecting the filtrate into a batching kettle.

Step three, double decomposition reaction

Opening a feeding port at the upper part of the reaction kettle, after 128kg of ammonium formate (with the purity of more than 99.5%) is added, opening a bottom valve of the batching kettle, adding all the filtrate prepared in the second step into the reaction kettle, and then starting stirring to dissolve all the ammonium formate;

after the materials are fully dissolved and uniformly mixed, a jacket of a reaction kettle is opened for heating, the temperature of the materials in the kettle is gradually increased to 80-95 ℃, the temperature is maintained for 2-8 hours, cooling water circulation of a condenser at the top of the kettle is kept, the evaporated solvent in the kettle is kept to be condensed and refluxed, and non-condensable gas enters an absorption tower; sampling, measuring the PH value of the condensate, dropping to 7.0-8.0, heating again to raise the temperature of the materials in the kettle to the boiling point, continuing to keep condensing and refluxing for 1-2 hours, stopping heating, and naturally cooling for 2-8 hours; opening a charging hole, adding 0.5kg of tetramethylammonium hydroxide and 100-200L of lithium formate mother liquor (if any), fully and uniformly stirring, and increasing the pH value of the materials in the kettle to be more than 10;

step four, fine filtering and impurity removal

And (3) carrying out two-stage precise filtration on the alkaline solution obtained in the previous step (a titanium rod filter element with the interception diameter of 1um and a ceramic membrane with the interception diameter of 50-100 nm) to obtain refined filtrate, and injecting the refined filtrate into an evaporative crystallization kettle.

Step five, cooling crystallization after evaporation concentration

Starting steam of the evaporation crystallizer, heating the material to boiling, continuously removing water, and stopping heating until a small amount of crystals are generated; and discharging the concentrated material into a crystallizing tank, standing for more than 60 hours, naturally cooling to reduce the temperature of the material to normal temperature, starting stirring, slowly stirring for 5 minutes, and then preparing to discharge the material to a centrifugal machine for liquid removal.

Step six, centrifugally dewatering, washing and drying

After filter cloth of the centrifuge is well loaded, a discharge valve of the crystallizing tank is opened, materials are discharged into the centrifuge, the discharge valve is closed when the filter bag is full, and the centrifuge is opened to spin-dry and remove liquid. And (4) refluxing the mother liquor to a lithium formate mother liquor storage tank for later use. The obtained lithium formate filter cake is washed by about 10L of anhydrous methanol and drained, then packaged by a plastic bag and a filter bag, and put into a vacuum rotary dryer to remove the residual solvent in the filter cake. Setting the temperature of the drying system at 60-98 ℃, gradually increasing the vacuum degree from normal pressure to 0.1MPa, maintaining for 24-48 hours until the drying is finished, stopping heating, breaking the vacuum by using nitrogen, sampling according to the requirements of 'work duty of drying and packaging post', inspecting, and labeling each barrel with 25Kg label for warehousing after the qualified inspection. About 100kg of lithium formate crystal product can be obtained for standby.

The lithium formate (purity > 99.9%) product obtained in the working procedure meets the following quality standards (unit: PPM):

detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Must not exceed	5	5	5	1	5	1	5	1	1

The third step: mixing the high-purity lithium formate with battery-grade lithium hydroxide, and then carrying out double decomposition reaction on the mixture and the high-purity ammonium fluoride to obtain high-purity lithium fluoride and a byproduct ammonium formate;

the reaction principle of the third step in this example is:

LiCHO₂+NH₄F→LiF+NH₄CHO₂

LiOH+NH₄F→LiF+NH₃+H₂O

step one, refining and impurity removal after feeding and dissolving

Opening a feeding port at the upper part of the reaction kettle, adding 200kg of high-purity lithium formate (purity is more than 99.9%) and 92.5kg of battery-grade lithium hydroxide (purity is more than 99.5%), injecting 1000L of purified water, starting the reaction kettle to stir, adding a small amount of adsorbent after completely dissolving the materials, starting a jacket to heat to 40-45 ℃, and continuing stirring for 2-4 hours; filtering materials in the reaction kettle by two-stage precise filtration (a titanium rod filter core with the interception diameter of 1um and a ceramic membrane with the interception diameter of 50-100 nm) to obtain a filtrate, and continuously returning the filtrate to the reaction kettle until the materials in the reaction kettle are completely clear and transparent and have no suspended matters;

the raw material battery-grade lithium hydroxide (purity > 99.5%) product in the process meets the following quality standard (unit: PPM):

detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe	CO₃
											Must not exceed	50	10	20	5	5	1	10	5	5	100

Step two, double decomposition reaction

286kg of refined ammonium fluoride (purity > 99.9%) is loaded into a hopper at the upper part of the reaction kettle, an upper cover of the hopper is tightly closed, ammonium fluoride powder is slowly added into the reaction kettle by controlling a discharge gate valve, and the ammonium fluoride powder and materials in the kettle are fully and uniformly mixed by stirring. The feeding speed is controlled not to exceed 10 kg/min until the ammonium fluoride is completely added. Then the discharge gate valve and other inlet and outlet pipe valves of the reaction kettle are closed. And replacing the air in the reaction kettle with high-pressure nitrogen, raising the air pressure in the kettle to 0.2-0.5 MPa, starting the jacket of the reaction kettle for oil bath circulation, and slowly raising the temperature of the material. Controlling the temperature rising speed to be not more than 15 ℃/h until the materials are boiled, stopping heating, and continuously maintaining the stirring of the reaction kettle for 2-8 h. When the pressure in the kettle exceeds 0.5MPa, the pressure is released, so that the nitrogen in the kettle and the ammonia generated by the reaction are released continuously due to the pressure release in the kettle. And after the pressure in the kettle is reduced to normal pressure, starting the jacket of the reaction kettle again for heating, keeping the solution boiling and the steam at the top of the kettle for condensing and refluxing for 2-8 hours, sampling and detecting the pH value of the condensate, stopping heating when the pH value is reduced to below 8, keeping stirring for 24-48 hours, and naturally cooling the materials to normal temperature. In the process, the ammonia gas escaping from the reaction kettle is absorbed by the absorption tower and then recycled to the ammonia water storage tank for later use.

Step three, centrifugal separation, washing and drying

After filter cloth of the centrifuge is well loaded, a discharge valve of the crystallizing tank is opened, materials are discharged into the centrifuge, the discharge valve is closed when the filter bag is full, and the centrifuge is opened to spin-dry and remove liquid. And (5) refluxing the mother liquor to an ammonium formate mother liquor storage tank for later use.

And (3) washing the lithium fluoride filter cake obtained after spin-drying by a centrifuge for multiple times by using purified water, spin-drying again, packaging by using a plastic bag and a filter bag after detecting that no ammonium formate is left, and putting into a vacuum rotary dryer to remove residual water in the filter cake. Setting the temperature of the drying system at 90-110 ℃, gradually increasing the vacuum degree from normal pressure to 0.1MPa, maintaining for 24-48 hours until the drying is finished, stopping heating and breaking the vacuum by using nitrogen, then placing the drying system into a high-temperature oven, baking the drying system for 8-12 hours at the high temperature of 300-400 ℃, stopping heating, naturally cooling to normal temperature, sampling and inspecting according to the requirements of 'work duty of drying and packaging stations', and labeling each barrel for warehousing after the drying system is qualified according to 25 Kg. About 200kg of lithium fluoride crystal product can be obtained.

Step four, concentrating the mother liquor, refining and recrystallizing

And (2) putting the ammonium formate-containing mother liquor obtained in the previous procedure into an evaporation crystallization kettle to remove water until the concentration is close to the saturated concentration, filtering the mother liquor while the mother liquor is hot by two-stage precise filtration (a titanium rod filter core with the interception diameter of 1um and a ceramic membrane with the interception diameter of 50-100 nm), and injecting the filtrate into a crystallization tank to naturally cool for 60 hours to obtain ammonium formate crystals. And (3) removing the mother liquor by using a centrifugal machine, and removing water from the obtained ammonium formate filter cake by using a vacuum rotary dryer to obtain a high-purity ammonium formate product. And the centrifugal mother liquor flows back to the ammonium formate mother liquor storage tank. After multiple concentration, about 240kg of ammonium formate byproduct can be obtained. The ammonium formate product can be recrystallized and refined in absolute ethyl alcohol or absolute methyl alcohol again. Through detection:

the lithium fluoride (purity > 99.9%) product obtained in the working procedure meets the following quality standard (unit: PPM):

detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Must not exceed	10	5	10	5	10	0.5	2	0.5	1

The ammonium formate recovered in the working procedure is refined (purity is more than 99.9%) to meet the following quality standards (unit: PPM):

detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Must not exceed	10	2	20	5	1	1	5	1	1

Example three:

the first step is as follows: preparing high-purity ammonium fluoride (purity > 99.9%) by using industrial-grade ammonium fluoride (purity > 97%);

the industrial-grade ammonium fluoride is prepared by separating and recrystallizing ammonium fluoride and white carbon black which are generated by reacting fluorosilicic acid or ammonium fluorosilicate and the like which are byproducts of phosphate fertilizer industry with ammonia water. The quality of the product (ammonium fluoride, purity > 97%) meets the following quality standards:

detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Must not exceed	100	20	50	100	500	20	50	50	50

The main production flow is as follows:

procedure one, feeding and dissolving

205kg of industrial-grade ammonium fluoride (97%) is added into the refining reaction kettle through a charging hole, then a bottom valve of a batching tank is opened, 500L of purified water is slowly injected into the refining reaction kettle, and after the feeding is finished, the refining reaction kettle is opened for stirring. After the materials are fully dissolved, adding 5kg of tetramethylammonium hydroxide (reagent grade) and 2kg of ammonium oxalate (reagent grade) into the kettle through a charging opening, and continuously stirring for 1-2 hours; sampling and detecting the pH value of the solution, wherein the pH value is qualified if the pH value is more than 10. If the product is not qualified, adding tetramethylammonium hydroxide (reagent grade) and continuously stirring and mixing uniformly;

closing all the feed port valves, starting jacket oil bath circulation of the refining reaction kettle to continuously raise the temperature in the kettle until the solvent is boiled, starting circulating water of a condenser at the top of the kettle to make solvent steam condensate and reflux for 1-2 hours, stopping heating, maintaining stirring, naturally cooling to normal temperature, adding 400L of anhydrous methanol (or 300L of anhydrous methanol plus 100L of recycled crystallization mother liquor plus 150L of recycled crystallization mother liquor), and continuously stirring for one hour;

step two, precise filtration

And (3) passing the materials in the refining reaction kettle through a two-stage precision filter (respectively adopting a titanium rod filter element with the interception diameter of 1um and a ceramic membrane with the interception diameter of 20-100 nm) to remove insoluble impurities. Refluxing the filtered liquid to the refining reaction kettle, circulating for 1-2 hours, and discharging the filtered liquid into an evaporation crystallizer;

step three, refining the crystals

Opening a crystallization evaporator to heat steam, heating the filtrate obtained in the last step until the solvent is boiled, transferring the volatilized solvent steam and condensate obtained by heat exchange of a top condenser to a rectifying tower to remove water, refluxing methanol to the crystallization evaporator until the water content in the material in the crystallization evaporator is continuously reduced, and stopping heating and methanol refluxing until the water content in the condensate at the top of the kettle is lower than 10% in the solvent; in the process, about 400-450L of water is removed.

and heating again and further evaporating the solvent, enabling the steam condensate on the top of the kettle to flow into a solvent recovery storage tank, continuously reducing the solvent amount in the evaporation crystallizer, concentrating the total volume of the material from about 450-500L initially to 250-300L, stopping heating, discharging the material into a crystallization kettle, standing for more than 60 hours, naturally cooling to reduce the temperature of the material to below 30 ℃, basically completing crystallization, starting stirring, slowly stirring for 5 minutes, and preparing for discharging.

Step four, centrifugal liquid removal and vacuum drying

Sleeving a filter bag in a centrifuge, opening a valve at the bottom of the crystallization kettle, discharging materials into the centrifuge, and closing a discharge valve at the bottom of the kettle when the filter bag is full; and (5) starting the centrifugal machine, and discharging the crystallization mother liquor into a mother liquor tank for recycling. Washing the product after centrifugal liquid removal with about 10L of anhydrous methanol, draining, packaging with plastic bags and filter bags, and putting into a rotary vacuum drier to remove residual solvent. The dryer is set to have a heating temperature of 40-90 ℃, the vacuum degree is gradually increased from the normal pressure to 0.1MPa, and the vacuum degree is maintained for 24-48 hours; and after drying, stopping heating, breaking vacuum by using nitrogen, sampling according to the requirements of 'drying and packaging post job duty', inspecting, and labeling each barrel with 25Kg of label for warehousing after passing inspection. About 200kg of high purity ammonium fluoride product was harvested per batch for use.

test item	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Maximum value	20	5	50	10	10	1	10	1	1

The second step is that: preparing high-purity lithium formate by using industrial-grade lithium carbonate as a starting raw material;

the reaction principle of the second step of this example is:

LiCO₃+H₂O+CO₂→2LiHCO₃(P_CO2＝0.4～0.45MPa)

LiHCO₃+NH₄CHO₂→LiCHO₂+H₂O+NH₃+CO₂

the main production flow is as follows:

step one, carbonization reaction.

3000L of purified water is injected into the carbonization kettle, and after 100kg of industrial-grade lithium carbonate (the content is more than 99 percent) is added, the feeding port is closed. Starting stirring, opening jacket cooling water, slowly opening a carbon dioxide gas control valve, gradually increasing the air pressure in the carbonization kettle to 0.4MPa, continuously maintaining the air pressure in the kettle and stirring, and fully contacting and reacting for 3-4 hours to obtain a lithium bicarbonate solution;

1. quality requirement of pure water (unit: PPM)

3. Carbon dioxide: industrial or food grade, content > 99.9%

Step three, double decomposition reaction

Opening a feeding port at the upper part of the reaction kettle, after 170kg of ammonium formate (with the purity of more than 99.5%) is added, opening a bottom valve of the batching kettle, adding all the filtrate prepared in the second step into the reaction kettle, and then starting stirring to dissolve all the ammonium formate;

after the materials are fully dissolved and uniformly mixed, a jacket of a reaction kettle is opened for heating, the temperature of the materials in the kettle is gradually increased to 80-95 ℃, the temperature is maintained for 2-8 hours, cooling water circulation of a condenser at the top of the kettle is kept, the evaporated solvent in the kettle is kept to be condensed and refluxed, and non-condensable gas enters an absorption tower; sampling, measuring the PH value of the condensate, dropping to 7.0-8.0, heating again to raise the temperature of the materials in the kettle to the boiling point, continuing to keep condensing and refluxing for 1-2 hours, stopping heating, and naturally cooling for 2-8 hours; opening a charging hole, adding 1kg of tetramethylammonium hydroxide and 100-200L of lithium formate mother liquor (if any), fully and uniformly stirring, and increasing the pH value of the materials in the kettle to be more than 10;

step four, fine filtering and impurity removal

Step five, cooling crystallization after evaporation concentration

Step six, centrifugally dewatering, washing and drying

After filter cloth of the centrifuge is well loaded, a discharge valve of the crystallizing tank is opened, materials are discharged into the centrifuge, the discharge valve is closed when the filter bag is full, and the centrifuge is opened to spin-dry and remove liquid. And (4) refluxing the mother liquor to a lithium formate mother liquor storage tank for later use. The obtained lithium formate filter cake is washed by about 10L of anhydrous methanol and drained, then packaged by a plastic bag and a filter bag, and put into a vacuum rotary dryer to remove the residual solvent in the filter cake. Setting the temperature of the drying system at 60-98 ℃, gradually increasing the vacuum degree from normal pressure to 0.1MPa, maintaining for 24-48 hours until the drying is finished, stopping heating, breaking the vacuum by using nitrogen, sampling according to the requirements of 'work duty of drying and packaging post', inspecting, and labeling each barrel with 25Kg label for warehousing after the qualified inspection. About 134kg of lithium formate crystal product can be obtained for standby.

detecting items	Na	Ca	SO₄	Cl	Si	Mg	K	Al	Fe
										Must not exceed	5	5	5	1	5	1	5	1	1

the reaction principle of the third step in this example is:

LiCHO₂+NH₄F→LiF+NH₄CHO₂

LiOH+NH₄F→LiF+NH₃+H₂O

step one, refining and impurity removal after feeding and dissolving

Step two, double decomposition reaction

Step three, centrifugal separation, washing and drying

Step four, concentrating the mother liquor, refining and recrystallizing

And (2) putting the ammonium formate-containing mother liquor obtained in the previous procedure into an evaporation crystallization kettle to remove water until the concentration is close to the saturated concentration, filtering the mother liquor while the mother liquor is hot by two-stage precise filtration (a titanium rod filter core with the interception diameter of 1um and a ceramic membrane with the interception diameter of 50-100 nm), and injecting the filtrate into a crystallization tank to naturally cool for 60 hours to obtain ammonium formate crystals. And (3) removing the mother liquor by using a centrifugal machine, and removing water from the obtained ammonium formate filter cake by using a vacuum rotary dryer to obtain a high-purity ammonium formate product. And the centrifugal mother liquor flows back to the ammonium formate mother liquor storage tank. After multiple concentration, about 240kg of ammonium formate byproduct can be obtained. The ammonium formate product can be recrystallized and refined again in absolute ethyl alcohol or absolute methyl alcohol to obtain a high-purity product. Through detection:

The background of the present invention may contain background information related to the problem or environment of the present invention and does not necessarily describe the prior art. Accordingly, the inclusion in the background section is not an admission of prior art by the applicant.

The foregoing is a more detailed description of the invention in connection with specific/preferred embodiments and is not intended to limit the practice of the invention to those descriptions. It will be apparent to those skilled in the art that various substitutions and modifications can be made to the described embodiments without departing from the spirit of the invention, and these substitutions and modifications should be considered to fall within the scope of the invention. In the description herein, references to the description of the term "one embodiment," "some embodiments," "preferred embodiments," "an example," "a specific example," or "some examples" or the like are intended to mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Various embodiments or examples and features of various embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction. Although embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope of the claims.

Claims

1. A preparation method of high-purity lithium fluoride is characterized by comprising the following steps: firstly, adding ammonium fluoride into a lithium bicarbonate aqueous solution, namely a refined solution A, which is obtained by carbonizing lithium carbonate by CO2 gas, to prepare high-purity lithium fluoride; or (ii) adding ammonium fluoride to a purified liquid B obtained from a mixed solution of lithium hydroxide and high-purity lithium formate to prepare high-purity lithium fluoride.

2. The method of claim 1, wherein the rate of ammonium fluoride addition is controlled and the rate of metathesis reaction and the formation of the desired lithium fluoride is controlled in a closed reaction vessel after the addition is complete by varying the reaction temperature and pressure parameters.

3. The preparation method of claim 2, wherein the feeding is completed at normal temperature, and after the materials are fully and uniformly mixed by stirring, the air in the reaction kettle is replaced by nitrogen, and the nitrogen pressurizes the reaction kettle to 0.01MPa-1 MPa; the temperature of the materials in the reaction kettle is gradually increased, and the removal speed of the gas generated in the reaction kettle is controlled by controlling the temperature of the reaction kettle and the pressure in the reaction kettle.

4. The method of claim 1, wherein the metathesis route is used, and the mass fraction of ammonia is controlled in the solvent system of the reaction during or after the metathesis reaction, and the solubility of lithium fluoride is increased to optimize the crystal size of lithium fluoride, wherein the mass fraction of ammonia is controlled by adjusting the content and the escape rate of ammonia in the solvent by changing the temperature and pressure parameters.

5. The production method according to claim 4, wherein ammonia is added to the solvent at the time of dissolving the raw material; or by the reaction of lithium hydroxide with ammonium fluoride to form ammonia, namely: the ammonia is kept in the solvent through the pressurized reaction of the closed reaction kettle so as to control the precipitation process of the lithium fluoride crystal.

6. The method according to any one of claims 1 to 5, further comprising preparing the ammonium fluoride, in particular comprising:

or

7. The method according to any one of claims 1 to 5, further comprising preparing the high-purity lithium formate, specifically comprising:

8. The method of claim 7, wherein the technical grade lithium carbonate comprises: 99.0 wt% of lithium carbonate, 500PPM or more of sodium, 100PPM or more of magnesium, 50PPM or more of iron, 20PPM or more of potassium, aluminum and copper, 200PPM or more of calcium and 1000PPM or more of sulfate radical.

9. The production method according to any one of claims 1 to 8, wherein the production of the purified liquid B comprises: dissolving industrial-grade lithium hydroxide with purified water or ammonia water to prepare a saturated solution, adding the high-purity lithium formate, fully dissolving, and filtering to remove impurities to obtain the refined solution B.

10. The method of claim 1, wherein the second step (ii) comprises:

s2: adding ammonium fluoride at a feeding speed of 1-10 kg per minute;