Method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluosilicate

Abstract

A method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluosilicate, which comprises: preparing a slurry by adding water to potassium fluosilicate with a K2SiF6 content of 99.0 wt.%; heating to 85-95 DEG C; rapidly adding potassium hydroxide to rapidly increase the pH of the reaction system to 12.0-14.0; stopping the addition of potassium hydroxide after the potassium fluosilicate is completely hydrolyzed; continuing the reaction under stirring until the pH falls to 10.0-10.5, white precipitate appears in the system; continuing the reaction until the pH falls to 9.0-9.5; adding potassium hydroxide to adjust the pH to 9.5-10.0; continuing the reaction until the hydrolysis reaction is completed; filtering the slurry to obtain a clear filtrate; adding an industrial hydrofluoric acid solution to the filtrate after heating; controlling the pH of the reaction end point to 7.5-8.0; filtering after 30-60 min of reaction; obtaining a clear potassium fluoride solution; evaporating and concentrating the potassium fluoride solution; cooling and crystallizing; drying; and obtaining high-purity potassium fluoride with a KF content of 99.5 wt.%.

Description

Technical Field

[0001] This invention belongs to the field of fluorochemical technology, specifically relating to a method for preparing high-purity potassium fluoride using fluorosilicic acid, a byproduct of phosphate fertilizer production. Background Technology

[0002] Fluorosilicic acid is a major byproduct of wet-process phosphate fertilizer and fluorite-process hydrofluoric acid production. my country produces over one million tons of fluorosilicic acid annually as a byproduct. If not properly managed, it not only causes severe fluorine pollution but also leads to a huge waste of fluorine resources. Reacting fluorosilicic acid with potassium chloride to produce potassium fluorosilicate, and then converting it into high-value-added industrial potassium fluoride through alkaline hydrolysis, is one of the main pathways for the efficient utilization of byproduct fluorine resources.

[0003] Potassium fluoride is an important basic raw material in the fluorochemical industry, widely used in glass etching, electroplating, synthesis of pesticide and pharmaceutical intermediates, and organofluorination. The market demand for high-grade anhydrous potassium fluoride has been continuously growing. Currently, the core reaction for preparing potassium fluoride through the alkaline hydrolysis of potassium fluorosilicate is: K₂SiF₆ + 4KOH = 6KF + SiO₂↓ + 2H₂O. Researchers both domestically and internationally have conducted extensive studies on this reaction, resulting in two main technical routes. The first is the excess alkali complete dissolution-hydrofluoric acid neutralization process (representative patents CN101973580A and CN102153217A): This route is based on the understanding that "the excess potassium hydroxide coefficient determines the decomposition rate of potassium fluorosilicate." By adding an excess of 20%~50% potassium hydroxide, the pH of the system is always greater than 11, achieving complete dissolution and decomposition of potassium fluorosilicate. However, this route has the following drawbacks: to ensure complete decomposition, a large amount of excess potassium hydroxide must be added, causing all the generated silica to be converted into soluble potassium silicate. Subsequently, a large amount of hydrofluoric acid needs to be added to neutralize the excess alkali and precipitate silicon. The consumption of potassium hydroxide and hydrofluoric acid is extremely high, and the raw material cost far exceeds the selling price of potassium fluoride, resulting in a severe cost inversion and making industrialization impractical. Furthermore, the precipitated silica product is potassium fluorosilicate, making it impossible to obtain qualified silica byproducts, leading to low resource utilization. The second type is the low-alkali static pH control process (represented by patents CN106517324A and CN113754178A): This route is based on the understanding that "the final pH determines the precipitation form of silica." It controls the pH of the reaction system to ≤10 throughout the process, attempting to avoid the reaction of silica with strong alkali to form potassium silicate, thus achieving simultaneous decomposition of potassium fluorosilicate and precipitation of silica. However, this route has the following drawbacks: the decomposition kinetic barrier of potassium fluorosilicate is extremely high under low pH conditions, the decomposition rate is extremely slow, and it can never be completely decomposed. The reaction precipitate is a mixture of silica and unreacted potassium fluorosilicate, the yield of fluorine and potassium resources is extremely low, and the product purity cannot reach the standard of superior grade. At the same time, the reaction process will inevitably pass through the pH range of 10 to 11, in which hard polymeric silica precipitates will be generated, resulting in extremely poor filtration performance, severe clogging of filter cloth, and inability to achieve continuous industrial production.

[0004] In summary, all existing technologies suffer from fatal flaws that cannot be resolved. The core root cause lies in the prevalent technological bias in these technologies: the assumption that the total amount / excess coefficient of potassium hydroxide is the core controlling factor determining the reaction effect. All process optimizations revolve around "statically controlling the excess coefficient of potassium hydroxide" and "statically controlling the final pH of the reaction." Limited by the understanding of "statically controlling the excess coefficient of potassium hydroxide / final pH," the dynamic changes in pH during the reaction process have been completely ignored, thus failing to find a process balance point that simultaneously achieves complete decomposition of potassium fluorosilicate, controllable precipitation of silica, good filtration performance, and low raw material consumption. This prevents the simultaneous resolution of the three major industry pain points: incomplete decomposition, difficult filtration, and high cost, thus hindering the large-scale industrial application of the potassium fluoride preparation route from potassium fluorosilicate. Summary of the Invention

[0005] The purpose of this invention is to solve the problems existing in the prior art and provide a method for preparing high-purity potassium fluoride by step-by-step alkaline hydrolysis of potassium fluorosilicate based on dynamic pH control. This method is characterized by complete decomposition of potassium fluorosilicate, easy filtration of precipitates, low raw material consumption, high resource utilization, green and environmentally friendly operation, strong process controllability, and high purity and stable quality of the prepared potassium fluoride product.

[0006] The technical solution adopted in this invention is as follows: A method for preparing high-purity potassium fluoride by step-by-step alkaline hydrolysis of potassium fluorosilicate, the method steps are as follows: S1. Slurry preparation: Potassium fluorosilicate with a dry basis K2SiF6 content ≥99.0wt.% is mixed with water to prepare a potassium fluorosilicate slurry with a solid content of 30%~50% and good fluidity; S2. Pre-dissolution and complete decomposition (pH rapid increase stage): Heat the potassium fluorosilicate slurry to 85~95℃, and rapidly add potassium hydroxide solution within 5~10min to rapidly increase the pH value of the reaction system to 12.0~14.0 in one go, so that the solid materials in the system are completely dissolved instantly, and a transparent homogeneous solution is obtained. Keep the reaction at the temperature for 20~40min to ensure that the potassium fluorosilicate is completely alkali-hydrolyzed, and solve the industry problem of incomplete decomposition in existing technologies from the source; S3. Stepwise pH Control Alkaline Hydrolysis (Dynamic pH Decline Phase): After complete alkaline hydrolysis of potassium fluorosilicate, stop adding alkali and maintain the reaction system temperature at 85-95℃ while continuing stirring. The pH value of the system will naturally decline with the reaction. When the pH value drops to 10.0-10.5, a white precipitate will begin to appear in the system. Control the pH value of the system to drop from 10.5 to 9.5 within 20-30 minutes, quickly passing through the difficult-to-filter pH range of 10-11 to avoid the formation of polymerized hard silica precipitate. Continue the reaction until the pH value drops to 9.0-9.5, add potassium hydroxide solution to fine-tune the pH value of the system to stabilize it at 9.5-10.0, and continue the reaction at the temperature to complete the alkaline hydrolysis reaction. At this time, the total amount of potassium hydroxide added to the system is 1.05-1.10 times the theoretical reaction amount of potassium fluorosilicate. S4. Solid-liquid separation: The liquid from which the alkaline hydrolysis reaction has been completed is subjected to vacuum filtration to obtain clear filtrate I and filter cake I. S5. Precise silicon precipitation: Transfer filtrate I into a reaction vessel, heat to 40~60℃, and slowly add 30~50% industrial hydrofluoric acid solution under stirring to carry out silicon precipitation reaction, remove trace amounts of dissolved silicon elements in the filtrate, control the pH value at the reaction endpoint to 7.5~8.0, and after reacting for 30~60 minutes, perform vacuum filtration to obtain pure potassium fluoride clear solution and filter cake II; S6. Concentration and Crystallization: Transfer the potassium fluoride solution to an evaporator and concentrate it under normal pressure until the potassium fluoride mass concentration is 45%~50%. Then transfer it to a crystallization vessel and stir and cool it to 30~35℃ for crystallization. After centrifugation, dry the crystals to obtain a high-purity potassium fluoride product with a potassium fluoride content ≥99.5wt.%.

[0007] Furthermore, in step S2, the mass concentration of the added potassium hydroxide solution is 45%~55%.

[0008] Furthermore, in step S3, after adding potassium hydroxide solution to finely adjust the pH value of the system to 9.5~10.0, the reaction is continued at this temperature for 1.5~2.5 hours.

[0009] Furthermore, the filter cake I obtained in step S4 is a mixture of unreacted potassium fluorosilicate and silicon dioxide. The filter cake I is returned to step S2 for recycling, thereby making full use of resources.

[0010] Furthermore, the filter cake II obtained in step S5 is a silicon-containing precipitate mainly composed of potassium fluorosilicate. The filter cake II is returned to step S2 for recycling, thereby achieving full utilization of resources.

[0011] Furthermore, the drying in step S6 involves drying the crystals at 105~110°C with forced air for 2~3 hours.

[0012] This invention, through systematic experimental verification for the first time, proposes a core technological discovery that overturns existing understanding: the dynamic change process of the pH value of the reaction system is the core controlling factor that determines the decomposition rate of potassium fluorosilicate, the precipitation form of silica, and the filtration performance, rather than the total amount / excess coefficient of potassium hydroxide. Based on this core discovery, this invention further clarifies three key technical laws: (1) The complete decomposition of potassium fluorosilicate is only related to whether the reaction system can quickly reach a strongly alkaline environment of pH ≥ 12, and has no direct correlation with the total amount of potassium hydroxide; as long as the pH value of the system is quickly raised to above 12 in the early stage of the reaction, the decomposition kinetic barrier of potassium fluorosilicate can be broken, and the potassium fluorosilicate can be instantly and completely dissolved and 100% decomposed without the need to add a large amount of excess potassium hydroxide. (2) The precipitation form of silica is only related to the rate at which the system pH value passes through the 10-11 range, and has no direct correlation with the final pH value. pH 10-11 is the irreversible range for the formation of polymeric silica. If the system stays in this range for more than 30 minutes, a hard and difficult-to-filter polymeric precipitate will be formed. Only by rapidly lowering the pH value through this range can the formation of hard and difficult-to-filter lumps be avoided. Static pH control cannot solve this problem. (3) The amount of potassium silicate formed in the system is only related to the time the system stays in the pH ≥ 11 range, and has no direct correlation with the total amount of potassium hydroxide used. By raising the pH value of the system to above 12 for a short time in the initial stage of the reaction and then rapidly lowering it back to the optimal range of 9.5-10, the amount of potassium silicate formed can be controlled at a very low level, which can significantly reduce the subsequent consumption of hydrofluoric acid.

[0013] Compared with the prior art, the present invention has the following advantages: 1. Breaking through industry technical biases, highlighting substantial features and significant progress. This invention is the first to propose and verify the core understanding that "dynamic pH change is the core control factor of the reaction, not the total amount of potassium hydroxide used," completely breaking the cognitive limitations of existing technology that relies on "static control of potassium hydroxide excess coefficient / final pH," and fundamentally solving the core industry contradiction that "complete decomposition of potassium fluorosilicate and controllable precipitation of silica cannot be achieved simultaneously" in existing technology.

[0014] 2. Completely solves three major industry pain points, with significant technological effects.

[0015] 2.1 Complete decomposition of potassium fluorosilicate: By rapidly raising the pH value to above 12 in the initial stage of the reaction, the decomposition kinetic barrier of potassium fluorosilicate is broken, achieving 100% complete decomposition of potassium fluorosilicate and solving the problem of incomplete decomposition in existing low-alkali processes.

[0016] 2.2 Excellent filtration performance of precipitates: By dynamically decreasing the pH value, the precipitate quickly passes through the difficult filtration range of 10-11, avoiding the formation of hard polymerized silica precipitates. The resulting precipitate is soft and easy to filter, and the filtration rate is more than 4 times higher than that of existing processes, solving the problems of difficult filtration and inability to produce continuously in existing processes.

[0017] 2.3 Raw material consumption is significantly reduced: The total amount of potassium hydroxide used is only 1.05 to 1.10 times the theoretical amount, which is more than 10% lower than the existing excess alkali process; the amount of potassium silicate generated in the system is extremely low, and the consumption of hydrofluoric acid in the silicon precipitation process is reduced by more than 80% compared with the existing full-dissolution process, which completely solves the problem of cost inversion in the existing process.

[0018] 3. High product purity and stable quality. The potassium fluoride product prepared by this invention has a main content of ≥99.5%, and all key indicators such as chloride, free alkali, and fluorosilicate consistently meet the GB / T 27813-2011 standard for superior grade industrial potassium fluoride, which can meet the needs of high-end fields such as high-end organic fluorination and electronic applications.

[0019] 4. High resource utilization rate and environmentally friendly. All by-product filter cakes generated from the alkaline hydrolysis and silicon precipitation processes can be returned to the alkaline hydrolysis process for recycling, with no solid waste generated. After recycling, the total fluorine recovery rate is ≥95% and the total potassium recovery rate is ≥92%, significantly improving resource utilization and meeting the requirements of green chemical development.

[0020] 5. The process is highly controllable and easy to scale up industrially. The process steps of this invention are continuous and controllable, the reaction conditions are mild, there is no high temperature or high pressure operation, the equipment requirements are low, it can be directly adapted to existing fluorochemical plants for modification, the scale-up effect is small, and it has excellent prospects for industrial application. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to specific embodiments. All potassium fluorosilicate used in the embodiments meets the requirement of a dry basis K₂SiF₆ content ≥ 99.0 wt.%, with a representative batch of potassium fluorosilicate having a dry basis K₂SiF₆ content of 99.54%, free acid of 0.03%, loss on drying of 0.10%, and water-insoluble matter of 0.13%, which meets industrial raw material standards and experimental raw material standards. The following embodiments use representative batches of potassium fluorosilicate. Example 1

[0022] The method for preparing high-purity potassium fluoride by step-by-step alkaline hydrolysis of potassium fluorosilicate based on dynamic pH control in this embodiment is as follows: S1. Slurry preparation: Take 1000g of dry K2SiF6 and mix it with deionized water to prepare a potassium fluorosilicate slurry with a solid content of 40% and good fluidity. S2. Pre-dissolution and complete decomposition (rapid pH increase stage): Place the potassium fluorosilicate slurry in a sealed reactor equipped with a stirrer and online pH monitoring. Heat to 90°C, and rapidly add a 50% (w / w) potassium hydroxide solution within 8 minutes, instantly raising the pH of the reaction system to 13.2. The solid materials in the system dissolve instantly and completely, resulting in a transparent homogeneous solution. Maintain the reaction at 90°C for 30 minutes to ensure complete alkaline hydrolysis of the potassium fluorosilicate. The chemical reaction formula for this step is as follows: K₂SiF₆ + 4KOH = 6KF + SiO₂↓ + 2H₂O S3. Stepwise pH control in alkaline hydrolysis (dynamic pH decline phase): After complete alkaline hydrolysis of potassium fluorosilicate, stop adding alkali and maintain the reaction system temperature at 90℃ while continuing stirring. The pH value of the system will naturally decline with the reaction. When the pH value drops to 10.3, a white precipitate mainly composed of amorphous silica will begin to appear in the system. Control the system pH value to drop from 10.3 to 9.2 within 25 minutes to quickly pass through the difficult-to-filter pH range of 10-11 and avoid the formation of polymerized hard silica precipitate. Add potassium hydroxide solution to finely adjust the pH value of the system to stabilize at 9.7, and continue the reaction at the temperature for 2 hours to complete the alkaline hydrolysis reaction. At this time, the total amount of potassium hydroxide added to the system is 1.08 times the theoretical reaction amount of potassium fluorosilicate. S4. Solid-liquid separation: The liquid after the alkaline hydrolysis reaction is vacuum filtered at a rate of 120 mL / min. The filter cake is soft, non-sticky to the filter cloth, and does not cause clogging, resulting in clear filtrate I and filter cake I. The main component of filtrate I is potassium fluoride, containing trace amounts of soluble silicon impurities and excess potassium hydroxide. After drying, filter cake I weighs 152 g and is analyzed to contain 29.04% K2SiF6 and 28.46% SiO2. It is then returned to step S2 for recycling. S5. Precise Silicon Precipitation: Transfer filtrate I to a reaction vessel, heat to 50°C, and slowly add 40% (w / w) industrial hydrofluoric acid solution while stirring to carry out the silicon precipitation reaction, removing trace amounts of dissolved silicon from the filtrate. Control the pH value at the reaction endpoint to 7.8. After reacting for 40 minutes, perform vacuum filtration to obtain pure potassium fluoride solution and filter cake II. After drying, filter cake II weighs 48g and its composition is determined to be 75.85% K₂SiF₆ and 19.39% SiO₂. It is then returned to step S2 for recycling. The chemical reaction formula for the silicon precipitation reaction is as follows: K₂SiO₃ + 6HF = K₂SiF₆ + 3H₂O; KOH + HF = KF + H2O; S6. Concentration and Crystallization: The potassium fluoride solution was transferred to an evaporation kettle and concentrated under normal pressure to a potassium fluoride mass concentration of 48%. Then it was transferred to a crystallization kettle and stirred and cooled to 32°C for crystallization. After centrifugation, the crystals were dried at 108°C for 2.5 hours to obtain 782g of potassium fluoride product.

[0023] Product testing results: Tested according to GB / T 27813-2011 "Industrial Anhydrous Potassium Fluoride" standard, the results are as follows: KF content 99.56%, chloride (as Cl) 0.03%, free alkali (as KOH) 0.05%, free acid (as HF) 0.00%, sulfate (as SO4) 0.00%, fluorosilicate (as SiO2) 0.05%, moisture 0.20%. All indicators meet the superior grade standard.

[0024] Recycling verification: Filter cake I and filter cake II of this embodiment are returned to step S2 to replace 10% of fresh potassium fluorosilicate. The above steps are repeated to finally obtain 795g of potassium fluoride product. All product test indicators still meet the superior product standard. The total fluorine recovery rate is 95.2% and the total potassium recovery rate is 92.6%. Example 2

[0025] The method for preparing high-purity potassium fluoride by step-by-step alkaline hydrolysis of potassium fluorosilicate based on dynamic pH control in this embodiment is as follows: S1. Slurry preparation: Take 1000g of potassium fluorosilicate, mix it with deionized water, and prepare a potassium fluorosilicate slurry with a solid content of 35% and good fluidity; S2. Pre-dissolution and complete decomposition: Place the potassium fluorosilicate slurry in a closed reactor equipped with a stirrer and online pH monitoring, heat to 88℃, and rapidly add a 50% potassium hydroxide solution within 10 minutes to quickly raise the pH of the reaction system to 12.5 in one go. The solid materials in the system will dissolve instantly and completely to obtain a transparent homogeneous solution. Keep the reaction at 88℃ for 35 minutes to ensure complete alkaline hydrolysis of potassium fluorosilicate. S3. Stepwise pH-controlled alkaline hydrolysis: After complete alkaline hydrolysis of potassium fluorosilicate, stop adding alkali and maintain the reaction system temperature at 88℃ while stirring. When the pH value of the system drops to 10.2, a white precipitate appears. Control the system pH value to drop from 10.2 to 9.3 within 20 minutes, quickly passing through the difficult-to-filter range. Add potassium hydroxide solution to fine-tune the system pH value to stabilize at 9.6, and continue the reaction at the temperature for 2 hours to complete the alkaline hydrolysis reaction. At this time, the total amount of potassium hydroxide added to the system is 1.06 times the theoretical reaction amount of potassium fluorosilicate. S4. Solid-liquid separation: The liquid from the alkaline hydrolysis reaction is vacuum filtered at a rate of 115 mL / min to obtain clear filtrate I and filter cake I; filter cake I is dried and returned to step S2 for recycling. S5. Precise silicon precipitation: Transfer filtrate I into the reaction vessel, heat to 55°C, and slowly add 40% industrial hydrofluoric acid solution with mass concentration while stirring until the final pH value of the reaction reaches 7.6. After reacting for 50 minutes, vacuum filter to obtain pure potassium fluoride solution and filter cake II; after drying filter cake II, return it to step S2 for recycling. S6. Concentration and Crystallization: The potassium fluoride solution was transferred to an evaporation kettle and concentrated under normal pressure until the potassium fluoride mass concentration was 47%. Then it was transferred to a crystallization kettle and stirred and cooled to 33°C for crystallization. After centrifugation, the crystals were dried at 105°C for 3 hours to obtain 775g of potassium fluoride product.

[0026] Product test results: KF content 99.52%, chloride 0.04%, free alkali 0.04%, free acid 0.00%, sulfate 0.00%, fluorosilicate 0.04%, moisture 0.18%, all indicators meet the superior grade standard; after recycling, the total fluorine recovery rate is 94.8%, and the total potassium recovery rate is 92.1%. Example 3

[0027] The method for preparing high-purity potassium fluoride by step-by-step alkaline hydrolysis of potassium fluorosilicate based on dynamic pH control in this embodiment is as follows: S1. Slurry preparation: Take 1000g of potassium fluorosilicate, mix it with deionized water, and prepare a potassium fluorosilicate slurry with a solid content of 45% and good fluidity. S2. Pre-dissolution and complete decomposition: Place the potassium fluorosilicate slurry in a closed reactor, heat it to 92°C, and rapidly add a 50% potassium hydroxide solution within 5 minutes to quickly raise the pH of the reaction system to 13.5 in one go. The solid materials in the system will dissolve instantly and completely to obtain a transparent homogeneous solution. Keep the reaction at 92°C for 25 minutes to ensure that the potassium fluorosilicate is completely alkali-hydrolyzed. S3. Stepwise pH-controlled alkaline hydrolysis: After complete alkaline hydrolysis of potassium fluorosilicate, stop adding alkali and maintain the reaction system temperature at 92℃ while stirring. When the pH value of the system drops to 10.4, a white precipitate appears. Control the system pH value to drop from 10.4 to 9.1 within 22 minutes, quickly passing through the difficult-to-filter range. Add potassium hydroxide solution to finely adjust the system pH value to stabilize at 9.8, and continue the reaction at the temperature for 2.5 hours to complete the alkaline hydrolysis reaction. At this time, the total amount of potassium hydroxide added to the system is 1.10 times the theoretical reaction amount of potassium fluorosilicate. S4. Solid-liquid separation: The liquid from the alkaline hydrolysis reaction is vacuum filtered at a rate of 125 mL / min to obtain clear filtrate I and filter cake I; filter cake I is dried and returned to step S2 for recycling. S5. Precise silicon precipitation: Transfer filtrate I into the reaction vessel, heat to 45°C, and slowly add 40% industrial hydrofluoric acid solution with mass concentration while stirring until the reaction endpoint pH value is 7.7. After reacting for 60 minutes, vacuum filter to obtain pure potassium fluoride solution and filter cake II; after drying filter cake II, return it to step S2 for recycling. S6. Concentration and Crystallization: The potassium fluoride solution was transferred to an evaporation kettle and concentrated under normal pressure until the potassium fluoride mass concentration was 50%. Then it was transferred to a crystallization kettle and stirred and cooled to 30°C for crystallization. After centrifugation, the crystals were dried at 110°C for 2 hours to obtain 786g of potassium fluoride product.

[0028] Product test results: KF content 99.58%, chloride 0.02%, free alkali 0.05%, free acid 0.00%, sulfate 0.00%, fluorosilicate 0.05%, moisture 0.17%, all indicators meet the superior grade standard; after recycling, the total fluorine recovery rate is 95.5%, and the total potassium recovery rate is 92.8%. Example 4

[0029] The method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluorosilicate based on dynamic pH control in this embodiment is as follows: S1. Slurry preparation: Take 1000g of dry K2SiF6 and mix it with deionized water to prepare a potassium fluorosilicate slurry with a solid content of 30% and good fluidity; S2. Pre-dissolution and complete decomposition: Place the potassium fluorosilicate slurry in a closed reactor equipped with a stirrer and online pH monitoring, heat to 85°C, and rapidly add a 45% potassium hydroxide solution within 10 minutes to quickly raise the pH of the reaction system to 12.0 in one go. The solid materials in the system will dissolve instantly and completely, resulting in a transparent homogeneous solution. Keep the reaction at 85°C for 40 minutes to ensure complete alkaline hydrolysis of potassium fluorosilicate. S3. Stepwise pH control during alkaline hydrolysis: After complete alkaline hydrolysis of potassium fluorosilicate, stop adding alkali and maintain the reaction system temperature at 85℃ while continuing stirring. The pH value of the system will naturally decrease as the reaction proceeds. When the pH value drops to 10.0, a white precipitate mainly composed of amorphous silica will begin to appear in the system. Control the pH value of the system to drop from 10.0 to 9.0 within 20 minutes to quickly pass through the difficult-to-filter pH range of 10-11 and avoid the formation of polymerized hard silica precipitate. Add potassium hydroxide solution to finely adjust the pH value of the system to stabilize at 9.5, and continue the reaction at the temperature for 1.5 hours to complete the alkaline hydrolysis reaction. At this time, the total amount of potassium hydroxide added to the system is 1.05 times the theoretical reaction amount of potassium fluorosilicate. S4. Solid-liquid separation: The liquid after the alkaline hydrolysis reaction is vacuum filtered at a rate of 112 mL / min. The filter cake is soft, non-sticky to the filter cloth, and does not cause clogging, resulting in clear filtrate I and filter cake I. The main component of filtrate I is potassium fluoride, containing trace amounts of soluble silicon impurities and excess potassium hydroxide. After drying, filter cake I weighs 158 g and is found to contain 30.12% K2SiF6 and 27.85% SiO2. It is then returned to step S2 for recycling. S5. Precise Silicon Precipitation: Transfer filtrate I to a reaction vessel, heat to 40°C, and slowly add 30% (w / w) industrial hydrofluoric acid solution while stirring to carry out the silicon precipitation reaction, removing trace amounts of dissolved silicon in the filtrate. Control the pH value at the reaction endpoint to 7.5. After reacting for 30 minutes, perform vacuum filtration to obtain pure potassium fluoride solution and filter cake II. After drying, filter cake II weighs 52g and its composition is determined to be 74.68% K2SiF6 and 20.12% SiO2. Return to step S2 for recycling. S6. Concentration and Crystallization: The potassium fluoride solution was transferred to an evaporation kettle and concentrated under normal pressure to a potassium fluoride mass concentration of 45%. Then it was transferred to a crystallization kettle and stirred and cooled to 30°C for crystallization. After centrifugation, the crystals were dried at 105°C (with forced air) for 2 hours to obtain 768g of potassium fluoride product.

[0030] Product test results: KF content 99.51%, chloride (as Cl) 0.04%, free alkali (as KOH) 0.04%, free acid (as HF) 0.00%, sulfate (as SO4) 0.00%, fluorosilicate (as SiO2) 0.05%, moisture 0.21%. All indicators meet the superior grade standard. After recycling, the total fluorine recovery rate is 94.6%, and the total potassium recovery rate is 92.0%. Example 5

[0031] The method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluorosilicate based on dynamic pH control in this embodiment is as follows: S1. Slurry preparation: Take 1000g of dry K2SiF6 and mix it with deionized water to prepare a potassium fluorosilicate slurry with a solid content of 50% and good fluidity; S2. Pre-dissolution and complete decomposition: Place the potassium fluorosilicate slurry in a closed reactor equipped with a stirrer and online pH monitoring, heat to 95°C, and rapidly add a 45% potassium hydroxide solution within 8 minutes to quickly raise the pH of the reaction system to 14.0 in one go, obtaining a transparent homogeneous solution. Keep the reaction at 95°C for 20 minutes to ensure complete alkaline hydrolysis of potassium fluorosilicate. S3. Stepwise pH-controlled alkaline hydrolysis: After complete alkaline hydrolysis of potassium fluorosilicate, stop adding alkali and maintain the reaction system temperature at 95℃ while stirring. The pH value of the system will naturally decrease as the reaction proceeds. When the pH value drops to 10.5, a white precipitate begins to appear in the system. Control the pH value of the system to drop from 10.5 to 9.5 within 30 minutes to quickly pass through the difficult-to-filter pH range of 10-11 and avoid the formation of polymerized hard silica precipitate. Continue the reaction until the pH value drops to 9.5, then add potassium hydroxide solution to fine-tune the pH value of the system to stabilize it at 10.0. Keep the system at this temperature and continue the reaction for 2.5 hours to complete the alkaline hydrolysis reaction. At this point, the total amount of potassium hydroxide added to the system is 1.10 times the theoretical reaction amount of potassium fluorosilicate. S4. Solid-liquid separation: The liquid after the alkaline hydrolysis reaction is completed is vacuum filtered at a rate of 128 mL / min. The filter cake is soft, non-sticky to the filter cloth, and does not cause clogging, resulting in clear filtrate I and filter cake I. The main component of filtrate I is potassium fluoride, containing trace amounts of soluble silicon impurities and excess potassium hydroxide. After drying, filter cake I weighs 147 g and is analyzed to contain 28.15% K2SiF6 and 29.02% SiO2. It is then returned to step S2 for recycling. S5. Precise Silicon Precipitation: Transfer filtrate I to a reaction vessel, heat to 60°C, and slowly add 50% (w / w) industrial hydrofluoric acid solution while stirring to carry out the silicon precipitation reaction, removing trace amounts of dissolved silicon in the filtrate. Control the pH value at the final reaction point to 8.0. After reacting for 50 minutes, perform vacuum filtration to obtain pure potassium fluoride solution and filter cake II. After drying, filter cake II weighs 45g and its composition is determined to be 76.22% K2SiF6 and 18.95% SiO2. Return to step S2 for recycling. S6. Concentration and Crystallization: The potassium fluoride solution was transferred to an evaporation kettle and concentrated under normal pressure until the potassium fluoride mass concentration was 48%. Then it was transferred to a crystallization kettle and stirred and cooled to 35°C for crystallization. After centrifugation, the crystals were dried at 110°C for 3 hours to obtain 790g of potassium fluoride product.

[0032] Product test results: KF content 99.59%, chloride (as Cl) 0.02%, free alkali (as KOH) 0.05%, free acid (as HF) 0.00%, sulfate (as SO4) 0.00%, fluorosilicate (as SiO2) 0.04%, moisture 0.16%. All indicators meet the superior grade standard. After recycling, the total fluorine recovery rate is 95.7%, and the total potassium recovery rate is 93.0%.

[0033] Unless otherwise stated, all percentages mentioned in this invention are mass percentages.

[0034] All equipment used in the method of this invention can be existing technology equipment.

Claims

1. A method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluorosilicate, characterized in that, The steps are as follows: S1. Slurry preparation: Potassium fluorosilicate with K2SiF6 content ≥99.0wt.% is mixed with water to prepare a potassium fluorosilicate slurry with a solid content of 30%~50%; S2. Pre-dissolution and complete decomposition: Heat the potassium fluorosilicate slurry to 85~95℃, and quickly add potassium hydroxide solution within 5~10min to rapidly raise the pH value of the reaction system to 12.0~14.0 in one go, so that the solid materials in the reaction system are completely dissolved instantly, and a transparent homogeneous solution is obtained. Keep the reaction at the temperature for 20~40min to ensure that the potassium fluorosilicate is completely alkaline hydrolyzed. S3. Stepwise pH control in alkaline hydrolysis: After complete alkaline hydrolysis of potassium fluorosilicate, stop adding alkali and maintain the reaction system temperature at 85-95℃ while stirring. The pH value of the reaction system will naturally decrease as the reaction proceeds. When the pH value drops to 10.0-10.5, a white precipitate will begin to appear in the system. Control the pH value of the system to drop from 10.5 to 9.5 within 20-30 minutes to quickly pass through the difficult-to-filter pH range of 10-11 and avoid the formation of polymerized hard silica precipitate. Continue the reaction until the pH value drops to 9.0-9.

5. Add potassium hydroxide solution to fine-tune the pH value of the system to stabilize it at 9.5-10.

0. Keep the system at the temperature and continue the reaction to complete the alkaline hydrolysis reaction. At this point, the total amount of potassium hydroxide added to the system is 1.05-1.10 times the theoretical reaction amount of potassium fluorosilicate. S4. Solid-liquid separation: The liquid from the alkaline hydrolysis reaction is filtered to obtain clear filtrate I and filter cake I. S5. Precise silicon precipitation: Transfer filtrate I into a reaction vessel, heat to 40~60℃, and slowly add 30~50% industrial hydrofluoric acid solution under stirring to carry out silicon precipitation reaction, remove trace amounts of dissolved silicon elements in the filtrate, control the pH value at the reaction endpoint to 7.5~8.0, filter after reacting for 30~60 minutes to obtain pure potassium fluoride clear solution and filter cake II; S6. Concentration and Crystallization: Transfer the potassium fluoride solution to an evaporator and concentrate it under normal pressure until the potassium fluoride mass concentration is 45%~50%. Then transfer it to a crystallization vessel and stir and cool it to 30~35℃ for crystallization. After centrifugation, dry the crystals to obtain a high-purity potassium fluoride product with a potassium fluoride content ≥99.5wt.%.

2. The method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluorosilicate according to claim 1, characterized in that, In step S2 above, the mass concentration of the added potassium hydroxide solution is 45%~55%.

3. The method for preparing high-purity potassium fluoride by step-by-step alkaline hydrolysis of potassium fluorosilicate according to claim 1, characterized in that, In step S3 above, after adding potassium hydroxide solution to finely adjust the pH value of the system to 9.5~10.0, the reaction time is maintained at the temperature for 1.5~2.5 hours.

4. The method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluorosilicate according to claim 1, 2, or 3, characterized in that, The filter cake I obtained in step S4 is a mixture of unreacted potassium fluorosilicate and silicon dioxide. The filter cake I is returned to step S2 for recycling.

5. The method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluorosilicate according to claim 1, 2, or 3, characterized in that, The filter cake II obtained in step S5 is a silicon-containing precipitate mainly composed of potassium fluorosilicate. The filter cake II is returned to step S2 for recycling.

6. A method for preparing high-purity potassium fluoride by stepwise alkaline hydrolysis of potassium fluorosilicate according to claim 1, 2, or 3, characterized in that, The drying process described in step S6 involves drying the crystals at 105-110°C with a forced airflow for 2-3 hours.