A method for direct solid-phase recovery of lithium sink mother liquor potassium salt

Abstract

This invention discloses a method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor, belonging to the field of lithium extraction by-product recovery technology. This method uses lithium precipitation mother liquor from mica ore and pyrolithium ore extracted by sodium sulfate roasting as raw material. After raw material pretreatment and high-temperature nucleation-induced sodium precipitation, sodium sulfate is densified by secondary temperature control using the characteristic that sodium sulfate solubility increases with temperature decrease (95-80℃). Then, potassium sulfate nuclei are added at 60℃ to form double salt nuclei and induce double salt precipitation. Finally, solid-phase stratification is achieved by high-speed stirring combined with gravity and mother liquor buoyancy, separating sodium salt and sodium-potassium sulfate double salt crystals. This invention operates entirely under normal pressure, requires no additional chemical reagents, achieves sodium sulfate purity ≥99.5%, recovery rate ≥85%, and potassium sulfate content in the double salt is 50-60%. Materials and water resources are recycled, it is suitable for industrial equipment, and has both economic and environmental benefits, contributing to the green upgrading of the lithium extraction industry.

Description

Technical Field

[0001] This invention relates to the field of raw material recovery technology, specifically to a method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor. Background Technology

[0002] Mica and lithium-ceramic ore are important sources of lithium resources in my country. Sodium sulfate roasting has become the mainstream process for lithium extraction from these ores due to its high lithium conversion and leaching rates. However, this process generates a large amount of lithium precipitation mother liquor. This mother liquor is high-salt wastewater, containing not only trace amounts of valuable metals such as Li, Rb, and Cs, but also high concentrations of Na+, K+, and SO42- ions. These ions originate from impurities in the ore itself and the roasting agent sodium sulfate. Direct discharge of this mother liquor not only results in a serious waste of sodium and potassium resources but also causes environmental problems such as water salinization, hindering the green development of the lithium extraction industry. Therefore, high-value, low-cost recovery of sodium and potassium resources from the lithium precipitation mother liquor is crucial for industry upgrading. Currently, most lithium precipitation mother liquor is recovered through direct evaporation, concentration, and crystallization of mixed salts, with potassium salts largely unrecovered. Some companies use traditional stepwise crystallization methods, which are costly and complex, failing to achieve high-value resource recovery. Therefore, a new technology needs to be developed to address this issue. Summary of the Invention

[0003] The problem to be solved by the present invention is to provide a method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor, so as to solve the above-mentioned background technical problems.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor uses lithium precipitation mother liquor produced by the lithium extraction process of mica ore and pyrolithium ore using sodium sulfate roasting as raw material. The method involves sequentially pretreatment of raw materials, high-temperature nucleation-induced sodium precipitation, secondary temperature-controlled sodium precipitation densification, nucleation-induced double salt crystallization, gravity-driven solid-phase separation, and product purification. This achieves efficient separation and recovery of sodium salt crystals and sodium-potassium sulfate double salt crystals. The specific steps are as follows: Step 1: Raw material pretreatment: Neutralize the lithium precipitation mother liquor, adjust the pH value to 7.5~8.0 to eliminate the interference of carbonate and hydroxide ions, and neutralize trace acidic impurities to obtain a neutralized mother liquor with a density of 1.240~1.243 g / L; Step 2: High-temperature crystal nucleus-induced sodium precipitation: The neutralized mother liquor is transferred to a concentration reaction device and heated to 95°C for atmospheric pressure concentration. In the initial stage of concentration, the stirring rate is maintained at 250 rpm. When the amount of water evaporated reaches 120~140g, sodium sulfate crystal nuclei are added, and the stirring rate is reduced to 150 rpm to continue concentration. Stirring is stopped when the total amount of water evaporated reaches 380~400g. After precipitation at a temperature above 80°C, solid-liquid separation is performed while hot to obtain high-purity sodium sulfate crystals and primary mother liquor. Step 3: Secondary Temperature Control for Sodium Precipitation and Density Adjustment: The primary mother liquor and the washing water from the sodium sulfate crystals are mixed to form a combined mother liquor. The combined mother liquor is divided into two groups, and their densities are adjusted separately. The first group is 1.340~1.342 g / L, and the second group is 1.345~1.346 g / L. Both groups of mother liquor are concentrated at 95℃ and atmospheric pressure, and sodium sulfate crystal nuclei are added. The stirring speed is maintained at 150 rpm. When the cumulative amount of water evaporated from each group reaches 180~220 g, the mixture is transferred to an 80℃ water bath heating environment, and the stirring speed is adjusted to 200 rpm for further concentration. Taking advantage of the characteristic that the solubility of sodium sulfate increases as the temperature decreases from 95℃ to 80℃, secondary sodium precipitation is achieved while increasing the density of the mother liquor, resulting in a specific density mother liquor system that meets the solid phase separation conditions. Step 4: Crystal nucleus-induced double salt crystallization: Mix the two sets of mother liquors at 80℃, and continue to concentrate under normal pressure at a constant temperature of 80℃ and stirring at 250 rpm until the evaporated water reaches 210~230g, so that the sodium / potassium molar ratio of the mother liquor is adjusted to 1.9~2.1; then allow the mother liquor to cool down naturally at a gradient. When the temperature drops to 60℃, add potassium sulfate crystal nuclei to the system to first form sodium potassium sulfate double salt crystal nuclei, then adjust the stirring rate to 150 rpm and continue to cool down at a gradient to 30℃ to achieve the directional precipitation of sodium potassium sulfate double salt crystals; Step 5: Gravity-induced solid-phase separation: After the mother liquor temperature drops to 30℃, adjust the stirring speed to 300 rpm and maintain it for 8~12 minutes. Then immediately stop stirring and keep it at 30℃ for 0.5~1 hour to allow it to settle. Under the initial effect of the specific stirring speed and the subsequent interaction of gravity and buoyancy of the mother liquor, the crystals in the system will undergo obvious stratification. The upper layer is a high-concentration concentrated mother liquor, the middle layer is loose sodium potassium sulfate double salt crystals, and the lower layer is dense sodium sulfate crystals. Step 6: Product purification: Without disturbing the lower layer of sodium sulfate crystals, the upper concentrated mother liquor and the middle layer of sodium sulfate-potassium double salt crystals are extracted by low-speed stirring. After solid-liquid separation, washing, and drying, the sodium sulfate-potassium double salt product is obtained. The lower layer of sodium sulfate crystals are separated, washed, and dried separately to obtain high-purity sodium sulfate product. The concentrated mother liquor can be used to extract valuable metals such as rubidium and cesium. The washing water is recycled back to the mother liquor merging stage.

[0005] Specifically, the sodium sulfate crystal nuclei mentioned in step 2 are high-purity sodium sulfate crystals. After addition, sodium sulfate is oriented to crystallize and precipitate at a high temperature of 95°C. The obtained sodium sulfate crystals are washed and dried, and the purity is ≥99.5% with a recovery rate of ≥85%. They can be directly reused in the roasting process of mica ore and ceramic lithium ore for lithium extraction.

[0006] Specifically, in step 3, the amount of sodium sulfate crystal nuclei added is 0.05% to 0.1% of the corresponding mother liquor mass, and the crystal nuclei particle size is 80 to 100 mesh. The mother liquor obtained after the second sodium precipitation provides the core density conditions for achieving gravity stratification of sodium salt and complex salt crystals.

[0007] Specifically, the gradient cooling rate in step 4 is 1℃ / min in the 60℃ to 40℃ range and 0.5℃ / min in the 40℃ to 30℃ range. The potassium sulfate crystal nuclei are high-purity crystals with a particle size of 120~150 mesh. The sodium-potassium sulfate double salt crystal nuclei formed after addition can induce the directional combination of sodium ions, potassium ions and sulfate ions in the system, thereby achieving efficient precipitation of sodium-potassium sulfate double salt crystals.

[0008] Specifically, in step 5, the high-speed stirring at 300 rpm uses a turbine-type impeller. Through shearing action, a specific high-speed turbulent flow of the mother liquor is formed, which separates and aggregates sodium sulfate potassium double salt crystals and sodium sulfate crystals with small density differences, laying the foundation for subsequent gravity stratification. After the high-speed stirring is completed, the density of the upper mother liquor needs to be controlled at 1.353~1.359 g / L. If it does not reach this range, a small amount of anhydrous sodium sulfate can be added and stirring can continue for 2 minutes until the standard is met.

[0009] Specifically, during the heat preservation and sedimentation process described in step 5, if stirring is required, only a low-speed stirring of 40-50 rpm should be used to keep the middle layer of double salt crystals suspended in the upper mother liquor state, avoiding stirring the dense sodium sulfate crystals in the lower layer.

[0010] Specifically, in step 6, the sodium sulfate potassium double salt crystals are washed twice with deionized water, with the amount of washing solution used each time being 1 to 2 times the mass of the crystals. The drying temperature is 90°C, and the potassium sulfate content in the double salt after drying is 50 to 60%. The lower layer of sodium sulfate crystals is washed and dried at 100°C, and the purity of the crystals after drying is ≥99%.

[0011] The entire recycling process involves concentration operations at atmospheric pressure, requiring no additional chemical reagents other than crystal nuclei, resulting in no secondary pollution. Furthermore, all washing water and unprecipitated concentrated mother liquor are recycled, achieving dual recovery of resources and water.

[0012] Compared with the prior art, the advantages and positive effects of this invention are: This invention addresses the system characteristics of lithium extraction mother liquor from mica and lithium-bearing ore using sodium sulfate roasting. A staged recovery process is designed to achieve efficient and high-value recovery of sodium and potassium resources, demonstrating outstanding technical results. Firstly, it utilizes for the first time the gravitational stratification characteristics of sodium-potassium sulfate double salt and sodium sulfate in the lithium extraction mother liquor concentration system at 30℃, achieving direct solid-phase separation of the two without the need for any chemical reagents, eliminating secondary pollution, and significantly reducing process costs, thus meeting the low-cost industrialization needs of the lithium extraction industry. Secondly, through a concentration and crystallization process coupled with gradient temperature and multi-stage stirring rates, combined with precise sodium sulfate crystal nucleation induction, mixed crystal phenomena are effectively avoided. The sodium sulfate purity is ≥99.5% and the recovery rate is ≥85%, while the potassium sulfate content in the sodium-potassium sulfate double salt reaches 50-60%. The product purity and recovery rate are far higher than existing technologies, and the precipitated sodium sulfate can be recycled in the roasting process, achieving resource recycling. Third, the precise control of the sodium / potassium molar ratio to near 2 matches the crystallization ratio requirements of sodium sulfate-potassium double salt. Combined with high-speed stirring at 30℃ for densification and impurity removal, this not only enhances the stratification effect but also removes slag impurities from the crystal surface. The settling and maturation time is only 0.5 to 1 hour, significantly improving production efficiency. Fourth, all concentration processes are conducted at atmospheric pressure, adapting to existing equipment conditions in the lithium extraction industry without requiring additional equipment investment. The process steps are simple and highly repeatable. Both the intermediate concentration mother liquor and washing water can be recycled, achieving dual recovery of resources and water, reducing wastewater discharge, and possessing significant economic and environmental benefits, thus contributing to the green upgrading of the lithium extraction industry. Attached Figure Description

[0013] Figure 1 This is a flowchart of the present invention; Figure 2 This refers to the actual phenomena observed during the experimental process of this invention. Figure 1 ; Figure 3 This refers to the actual phenomena observed during the experimental process of this invention. Figure 2 ; Figure 4 This refers to the actual phenomena observed during the experimental process of this invention. Figure 3 . Detailed Implementation

[0014] To better understand the present invention, the present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0015] Examples of methods for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor I. Technical Content of the Implementation Examples like Figure 1-4As shown, this embodiment uses lithium-precipitated mother liquor from the sodium sulfate roasting process of mica ore and pyrolithium ore as raw material to conduct a direct solid-phase recovery experiment of potassium salt. The experiment processes 4 × 1 L of lithium-precipitated mother liquor per cycle. The initial detection indicators of the lithium-precipitated mother liquor are: density 1.224 g / L, pH=14, containing K⁺ 18.0325 g / L, Na⁺ 96.2875 g / L, SO₄²⁻ 208.6875 g / L, and also containing trace impurities of Li, Rb, and Cs, as well as a small amount of soluble slag impurities. The content of other metal impurities is all below 0.004 g / L. This embodiment strictly follows six steps: raw material pretreatment, high-temperature nucleus-induced sodium precipitation, secondary temperature-controlled sodium precipitation and densification, nucleus-induced double salt crystallization, gravity-separated solid-phase separation, and product purification. The entire process uses atmospheric pressure concentration, adding only sodium sulfate and potassium sulfate nuclei; no other chemical reagents are added. The specific operation is as follows: Step 1: Raw material pretreatment Four 1L portions of lithium precipitation mother liquor were taken and labeled as 1#, 2#, 3#, and 4#, respectively. 60mL of 98% sulfuric acid was added to each portion for neutralization treatment, adjusting the pH of mother liquors 1# and 2# to 7.5 and the pH of mother liquors 3# and 4# to 8.0. This eliminated the interference of carbonate and hydroxide ions in the mother liquor on subsequent crystallization and neutralized trace acidic impurities leached from the slag. The density of the mother liquor after neutralization was controlled within the range of 1.240~1.243g / L, with mother liquor 1# having a density of 1.242g / L, mother liquor 2# 1.243g / L, mother liquor 3# 1.241g / L, and mother liquor 4# 1.241g / L. The total mass of each portion of mother liquor was approximately 1291g, and the volume was 1.04L.

[0016] Step 2: High-temperature nucleation-induced sodium precipitation Four portions of neutralized mother liquor were transferred to a concentration reaction device and concentrated at atmospheric pressure to 95°C. During the initial concentration stage, a stirring rate of 250 rpm was maintained to ensure uniform heating and prevent localized scaling. When the evaporated water volume of mother liquor #1 reached 130g, #2 135g, #3 128g, and #4 132g, high-purity sodium sulfate crystal nuclei were added to each system. At this point, the solution became turbid and sodium sulfate crystals precipitated, and the density of the mother liquor reached 1.325~1.336g / L. The stirring rate was then reduced to 150 rpm. Concentration continued at atmospheric pressure and 95°C. When the total evaporated water volume of each portion of mother liquor reached 390g, stirring was stopped, and the system temperature was maintained at 85°C for precipitation. Subsequently, solid-liquid separation was performed while hot to obtain sodium sulfate crystals and the primary mother liquor. The precipitated sodium sulfate crystals were washed three times with 30 mL of pure water. After each wash, the crystals were filtered and then dried with hot air at 105℃ for 2 hours to obtain dried sodium sulfate crystals 1#175g, 2#178g, 3#171g, and 4#178g.

[0017] Step 3: Secondary temperature control for sodium precipitation and densification The primary mother liquors of 1# and 2# were mixed with the corresponding sodium sulfate crystal washing water at room temperature and a stirring speed of 80 rpm for 20 minutes to obtain the combined mother liquor of 5#, whose density was adjusted to 1.341 g / L. The primary mother liquors of 3# and 4# were mixed with the corresponding washing water under the same conditions to obtain the combined mother liquor of 6#, whose density was adjusted to 1.345 g / L. The mother liquors of 5# and 6# were placed in separate concentration equipment and concentrated at 95°C and atmospheric pressure. At the same time, sodium sulfate crystal nuclei with a particle size of 90 mesh and a mass of 0.08% of the mother liquor mass were added to each system to induce secondary crystallization, while maintaining a stirring speed of 150 rpm. When the cumulative water evaporated from each group of mother liquors reaches 200g, the solution becomes milky and fine crystals and larger crystal particles precipitate out. Then, it is transferred to an 80℃ water bath heating environment, and the stirring speed is adjusted to 200 rpm to continue concentration. Taking advantage of the characteristic that the solubility of sodium sulfate increases with temperature drop from 95 to 80℃, secondary sodium precipitation is achieved and the density of the mother liquor is increased, thus constructing a specific density mother liquor system that satisfies the requirement of solid phase stratification.

[0018] Step 4: Nucleus-induced crystallization of double salt Mother liquors #5 and #6, heated in an 80℃ water bath, were transferred to the same concentration equipment. They were mixed under constant temperature (80℃) and stirring at 250 rpm, and concentration continued at atmospheric pressure. Concentration was stopped when the evaporated water content of the mixed mother liquor reached 220g. At this point, the sodium / potassium molar ratio in the mother liquor was precisely controlled to 2.0, matching the crystallization ratio of sodium-potassium sulfate double salt. The mixed mother liquor was removed from the water bath system, and the temperature was gradually reduced with stirring. The cooling rate was controlled at 1℃ / min from 60℃ to 40℃ and 0.5℃ / min from 40℃ to 30℃. When the temperature dropped to 60℃, high-purity potassium sulfate crystal nuclei with a particle size of 130 mesh (0.075% of the total potassium sulfate in the solution) were added to the system to first form sodium-potassium sulfate double salt crystal nuclei. Simultaneously, the stirring speed was adjusted to 150 rpm, and the temperature was gradually reduced to 30℃ to achieve the directional precipitation of sodium-potassium sulfate double salt crystals.

[0019] Step 5: Gravity-based solid-phase separation Once the mother liquor temperature drops to 30℃, replace the agitator with a turbine agitator, adjust the stirring speed to 300 rpm and maintain it for 10 minutes. Utilize shearing action to create high-speed turbulence in the mother liquor, causing the sodium potassium sulfate double salt and sodium sulfate crystals to separate and aggregate. After stirring, the density of the upper mother liquor is measured at 1.357 g / L, reaching the set range of 1.353~1.359 g / L, eliminating the need for adding anhydrous sodium sulfate. Immediately stop stirring and maintain the system temperature at 30℃ for sedimentation treatment. The sedimentation time is controlled at 40 minutes. Under the combined action of gravity and the buoyancy of the mother liquor, the material undergoes clear stratification: the upper layer is a high-concentration concentrated mother liquor, the middle layer is loose sodium potassium sulfate double salt crystals, and the lower layer is dense sodium sulfate crystals.

[0020] Step 6: Product purification treatment Without disturbing the lower layer of sodium sulfate crystals, the middle layer of double salt crystals is suspended in the upper layer of mother liquor by low-speed stirring at 45 rpm. The upper concentrated mother liquor and the middle layer of double salt crystals are then extracted and transferred to a vacuum filtration device for solid-liquid separation. The sodium-potassium sulfate double salt crystals are washed twice with deionized water, with each washing solution being 1.5 times the mass of the crystals. They are then dried at 90°C to obtain sodium-potassium sulfate double salt crystals 1# 65g, 2# 66g, 3# 68g, and 4# 63g. After the upper layer material separation is completed, the lower layer of sodium sulfate crystals is separated separately, washed with pure water, and dried at 100°C to obtain dried sodium sulfate crystals 1# 80g, 2# 85g, 3# 78g, and 4# 78g. In this embodiment, the upper concentrated mother liquor is used to extract valuable metals such as rubidium and cesium, and all washing water is recycled back to the mother liquor merging stage, achieving zero waste of water resources.

[0021] II. Principle Analysis The core principle of this embodiment is based on the system characteristics of lithium extraction and precipitation mother liquor from mica ore and porcelain lithium ore. It combines three key mechanisms: nucleus-induced directional crystallization, temperature-rate coupling regulation of dissolution-crystallization balance, and solid-phase stratification through the synergy of gravity and mother liquor buoyancy, to achieve efficient solid-phase separation of sodium and potassium resources. The core principles of each step are as follows: The principle of pH and density control in raw material pretreatment: The initial lithium precipitation mother liquor is strongly alkaline and contains carbonate and hydroxide ions. These ions combine with Na⁺ and K⁺ to form carbonate and hydroxide impurities, which interfere with sulfate crystallization. By neutralizing with sulfuric acid to adjust the pH to 7.5~8.0, the interference of carbonate and hydroxide ions can be completely eliminated, while neutralizing trace acidic impurities, so that the mother liquor is in a stable sulfate system. The density is controlled at 1.240~1.243 g / L to set a stable initial material concentration for subsequent concentration and crystallization, and to avoid uneven crystallization due to concentration fluctuations.

[0022] The principle of directional crystallization of sodium sulfate induced by high-temperature crystal nuclei: Under high temperature conditions of 95℃, the solubility of sodium sulfate in the mother liquor is significantly reduced, making it a salt that is easy to crystallize and precipitate in the system; by adding sodium sulfate crystal nuclei, the supersaturated state of the solution is broken, providing growth nuclei for sodium sulfate crystallization, thus achieving directional precipitation of sodium sulfate at 95℃. At the same time, the stirring rate is controlled to be reduced from 250 rpm to 150 rpm to reduce the damage to crystal growth caused by stirring shear force and improve crystal particle size and purity.

[0023] The dissolution-crystallization equilibrium principle of secondary temperature-controlled sodium precipitation and density adjustment: Utilizing the unique property that the solubility of sodium sulfate increases with decreasing temperature from 95 to 80℃, sodium sulfate crystal nuclei are added a second time at 95℃ to induce partial crystallization of sodium sulfate. Then, the temperature is lowered to 80℃ to allow some sodium sulfate to dissolve back, establishing a new dissolution-concentration-crystallization sodium precipitation system. This process not only achieves secondary sodium precipitation but also gradually increases the density of the mother liquor, providing the core density conditions for subsequent solid phase stratification. At the same time, the combined mother liquor is divided into two groups to regulate density, which can avoid the problem of excessive or insufficient crystallization at a single concentration.

[0024] The principle of stoichiometry and directional precipitation of nucleus-induced double salt crystallization: There is an optimal sodium / potassium molar ratio (approximately 2:1) for the crystallization of sodium-potassium sulfate double salt. The sodium / potassium molar ratio of the mother liquor is precisely controlled to 1.9~2.1 through constant temperature concentration at 80℃ to match the stoichiometric ratio of double salt crystallization. When potassium sulfate nuclei are added at 60℃, potassium sulfate is not directly precipitated, but rather the potassium sulfate nuclei are rapidly combined with sodium sulfate in the solution to form sodium-potassium sulfate double salt nuclei. These nuclei are used as the core to induce the directional combination of Na⁺, K⁺, and SO₄²⁻ in the system to form double salt. At the same time, a gradient cooling method is used to avoid problems such as mixed crystals and uneven crystal particle size caused by excessively rapid cooling.

[0025] The principle of turbulent separation for high-speed stirring, densification, and impurity removal: High-speed stirring at 300 rpm uses a turbine-type impeller. The resulting high-speed turbulence can dissolve the partially separated sodium sulfate crystals using the principle of dissolution equilibrium, increasing the density of the upper mother liquor to 1.353~1.359 g / L. At the same time, it causes the slag impurities attached to the surface of the double salt crystals to fall off. More importantly, the high-speed turbulence can fully separate the sodium sulfate potassium double salt with small density differences from the sodium sulfate crystals and allow them to aggregate separately, laying the foundation for subsequent gravity stratification. If the density of the mother liquor does not meet the standard, adding anhydrous sodium sulfate can quickly adjust it to the set range to ensure the stratification conditions.

[0026] The density difference-driven principle of gravity-induced solid-phase separation: At 30℃, sodium-potassium sulfate double salt crystals, after being agglomerated by high-speed stirring, are loose and have a lower density, while sodium sulfate crystals are dense and have a higher density. In a mother liquor with a specific density (1.353~1.359 g / L), the two types of crystals naturally separate into layers during static settling by utilizing the synergistic effect of gravity and buoyancy of the mother liquor. The loose double salt crystals are suspended in the middle layer, while the dense sodium sulfate crystals settle to the lower layer. This principle is the core innovation of this method for achieving direct solid-phase separation.

[0027] The principle of resource conservation in product purification and recycling: Differentiated washing and drying processes are adopted based on the characteristics of double salts and sodium sulfate crystals to ensure product purity; the concentrated mother liquor is used to extract trace valuable metals such as Rb and Cs, and the washing water is recycled to the mother liquor merging stage, realizing the dual recovery of sodium, potassium, rubidium, cesium resources and water resources, which is in line with the principles of resource conservation and circular economy.

[0028] III. Technical Effects This embodiment designs a staged recovery process for lithium extraction and precipitation mother liquor from mica ore and lithium-bearing ore using sodium sulfate roasting. Verified through five repeated experiments, the process demonstrates strong stability and high repeatability, achieving efficient and high-value recovery of sodium and potassium resources. It also exhibits significant economic, environmental, and industrial applicability. Specific technical effects are as follows: The purity and recovery rate of the products are significantly improved: In this embodiment, the sodium sulfate crystals obtained by high-temperature nucleus-induced sodium precipitation have a purity of 99.6%, which is much higher than the 70% recovery rate of the existing technology. The total sodium sulfate recovery rate reaches 86.2%, and the dried sodium sulfate can be directly reused in the roasting process of mica ore and pyrite ore for lithium extraction, realizing the recycling of sodium salt resources. The potassium sulfate content in the sodium sulfate potassium double salt crystals reaches 55%, which is within the target range of 50-60%, which is significantly improved compared with the content of less than 40% in the existing technology, realizing the high-value recovery of potassium salt resources.

[0029] For the first time, direct solid-phase separation without reagents and without secondary pollution has been achieved: This embodiment discovers and utilizes for the first time the gravity stratification characteristics of sodium sulfate potassium double salt and sodium sulfate in the lithium mother liquor concentration system at 30℃ to achieve direct solid-phase separation of the two. Only sodium sulfate and potassium sulfate crystal nuclei are added throughout the process, without the input of other chemical reagents. This avoids the increased raw material costs and secondary pollution problems of washing wastewater caused by the addition of reagents such as potassium chloride in the existing concentration crystallization + chemical conversion method, which significantly reduces process costs and meets the low-cost industrialization needs of the lithium extraction industry.

[0030] High process efficiency and significantly shortened settling and maturation time: By precisely controlling the sodium / potassium molar ratio to 2.0 and matching the sodium sulfate-potassium double salt crystallization ratio, combined with high-speed stirring at 30℃ to densify and remove impurities, not only is the solid phase stratification effect enhanced, but also the slag impurities on the crystal surface are effectively removed; in this embodiment, the heat preservation settling time is only 40 minutes, which is within the process range of 0.5 to 1 hour. Compared with the long settling time of the existing step crystallization method, this significantly improves production efficiency and is suitable for large-scale continuous production.

[0031] Atmospheric pressure operation is suitable for industrialization without the need for additional equipment investment: All concentration and crystallization operations in this embodiment are carried out at atmospheric pressure, without the need for high-pressure equipment. It can be directly adapted to the existing concentration, stirring, and solid-liquid separation equipment in the lithium extraction industry without the need for additional equipment investment, which greatly reduces the cost of industrialization transformation. At the same time, the process steps are clear, the operating parameters are well-defined, and the stirring rate, temperature, evaporation water volume and other indicators are all within a precise control range. It has strong repeatability and is suitable for large-scale industrial applications.

[0032] Dual recycling of resources and water yields significant environmental benefits: In this embodiment, the upper concentrated mother liquor can be further extracted to extract trace amounts of valuable metals such as rubidium and cesium, achieving comprehensive recycling of lithium ore associated resources; all washing water is recycled back to the mother liquor merging stage, achieving zero waste of water resources; the middle layer mother liquor is returned to the lithium extraction process for recycling, with no wastewater discharge, completely solving the water salinization problem caused by direct discharge of lithium precipitation mother liquor, and helping the lithium extraction industry to upgrade greenly.

[0033] Effectively avoids mixed crystal phenomenon and ensures stable product quality: By coupling and controlling gradient temperature (95℃→80℃→60℃→30℃) and multi-stage stirring rate (250→150→200→250→150→300 rpm), and with precise timing and dosage of crystal nucleation induction, the mixed crystal problem of sodium sulfate and potassium salt double salt in existing technologies is effectively avoided. In five repeated experiments, the purity of sodium sulfate crystals was ≥99.5%, and the potassium sulfate content in the double salt was in the range of 53~55%, ensuring stable and controllable product quality.

[0034] Experimental procedures and data related to the direct solid-phase recovery of potassium salt from lithium precipitate mother liquor. Raw material analysis The mother liquor has a density of 1.224, a pH of 14, and contains Na+ ions including sodium sulfate, sodium carbonate, and sodium hydroxide. Trial process and results I. Experimental Procedure 1. High-temperature concentration (4 samples: #1, #2, #3, #4) The neutral mother liquor after neutralization was heated to 95℃ and concentrated while stirring at 250 rpm. When the amount of water evaporated was m1=130g, sodium sulfate crystal nuclei were added, and the mixture began to become turbid and sodium sulfate crystals precipitated. At this point, the density ρ1=1.325-1.336g / l. The stirring speed was then reduced to 150 rpm.

[0035] 2. Continuous concentration and crystallization The concentration process continued until the water content of each sample evaporated to m2 = 390g. Stirring was then stopped, and the sample was allowed to settle. Sodium sulfate crystals and mother liquor were separated while still hot (temperature controlled above 80℃). The sodium sulfate crystals were washed three times with 30ml of water and dried. The sodium sulfate content was measured to be approximately m3 = 175g, with a purity of approximately 99.6%.

[0036] Mother liquor 1# + 2# combined + 1# sodium sulfate washing water + 2# sodium sulfate washing water = combined mother liquor 5#, density ρ2=1.341g / l; Mother liquor 3# + 4# combined + 3# sodium sulfate washing water + 4# sodium sulfate washing water = combined mother liquor 6#, density ρ3=1.345g / l.

[0037] 3. Combine mother liquor #5 and mother liquor #6 for further high-temperature concentration. The mother liquor from the combined No. 5 and No. 6 batches was further concentrated at 95°C. Sodium sulfate was added to induce crystallization, causing the solution to become turbid again. The stirring rate was 150 rpm. When the evaporation volume was m4 = 260 g, the solution began to become emulsified, containing both fine and larger crystal particles. Another 100 g of water was evaporated, and the solution was then heated and concentrated in an 80°C water bath at a stirring rate of 200 rpm.

[0038] Mother liquors #4, #5, and #6 are combined and concentrated. After the water bath heating continues to concentrate and evaporate water m4=100g, the mother liquor #5 and mother liquor #6 are combined into mother liquor #7 for concentration. The stirring speed is adjusted to 250 rpm, and the water volume of the further concentration and evaporation is m5=220g.

[0039] 5. Cooling to induce crystallization The mother liquor was transferred out of the water bath heating system, stirred, and cooled to 60°C. Potassium sulfate crystal nuclei were then added to induce crystallization. The stirring rate was adjusted to 150 rpm, and the temperature was continuously lowered to 30°C.

[0040] 6. Gravity settlement and stratification When the temperature drops to 30℃, the stirring speed is adjusted to 300 rpm and stirred for 10 minutes. Then, stirring is stopped, the mixture is kept warm and allowed to settle. The density of the upper mother liquor is measured to be ρ4=1.357.

[0041] 7. Layered separation The upper mother liquor and the middle crystals were separated by a vacuum filtration device to obtain sodium potassium sulfate double salt and concentrated mother liquor. The crystals were washed three times with pure water to obtain sodium potassium sulfate double salt. After drying, approximately 65g of sodium potassium sulfate double salt (potassium sulfate content approximately 55%) was obtained. The sodium sulfate crystals at the bottom were washed three times with pure water to obtain approximately 80g of sodium sulfate with a purity of 99%.

[0042] II. Experimental Results The above experiment was repeated five times, and the data and intermediate parameters are statistically analyzed as follows.

[0043] 2. During the experiment, the measured values ​​were m1-m7 and ρ1-ρ4. Table 3 Data Table of Measurement Values III. Regarding the circumstances of this experiment 1. The lithium extraction mother liquor needs to be neutralized to neutral, the sodium / potassium ratio increased, and the influence of carbonate and hydroxide ions removed. Concentration at 95℃ followed by nucleation-induced crystallization can precipitate high-purity sodium sulfate with a purity exceeding 99.5%. Secondary nucleation-induced crystallization is also possible. The recovery rate of sodium sulfate can reach approximately 85%.

[0044] 2. After sodium precipitation at 95℃, when the sodium / potassium molar ratio is close to 2, transfer to an 80℃ water bath for heating and concentration, and increase the stirring speed to precipitate sodium sulfate and sodium-potassium sulfate double salt simultaneously; cooling and adding potassium sulfate to induce crystallization can increase the amount of sodium-potassium sulfate double salt precipitated.

[0045] 3. After crystallization, at 30°C, accelerating stirring will dissolve some of the precipitated sodium sulfate, increasing the density of the mother liquor.

[0046] 4. Stop stirring. Under the influence of gravity, the sodium sulfate potassium double salt and sodium sulfate will separate into two layers and precipitate. The upper layer is the loose sodium sulfate potassium double salt region, while the lower layer is the denser sodium sulfate crystals. This is the essential finding of this experiment.

[0047] 5. Low-speed stirring allows the mother liquor and the upper layer of sodium-potassium sulfate double salt to be extracted, making it easier to separate from the lower layer of sodium sulfate crystals. Furthermore, the precipitation and maturation time is 0.5-1 hour, making separation even easier.

[0048] The embodiments of the present invention have been described in detail above, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of this patent.

Claims

1. A method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor, characterized in that: Using lithium precipitation mother liquor from the sodium sulfate roasting process of mica ore and lithium-ceramic ore as raw material, the process involves the following steps: raw material pretreatment, high-temperature nucleation-induced sodium precipitation, secondary temperature-controlled sodium precipitation densification, nucleation-induced double salt crystallization, gravity-driven solid-phase separation, and product purification. This achieves efficient separation and recovery of sodium salt crystals and sodium-potassium sulfate double salt crystals. The specific steps are as follows: Step 1: Raw material pretreatment: Neutralize the lithium precipitation mother liquor, adjust the pH value to 7.5~8.0 to eliminate the interference of carbonate and hydroxide ions, and neutralize trace acidic impurities to obtain a neutralized mother liquor with a density of 1.240~1.243 g / L; Step 2: High-temperature crystal nucleus-induced sodium precipitation: The neutralized mother liquor is transferred to a concentration reaction device and heated to 95°C for atmospheric pressure concentration. In the initial stage of concentration, the stirring rate is maintained at 250 rpm. When the amount of water evaporated reaches 120~140g, sodium sulfate crystal nuclei are added, and the stirring rate is reduced to 150 rpm to continue concentration. Stirring is stopped when the total amount of water evaporated reaches 380~400g. After precipitation at a temperature above 80°C, solid-liquid separation is performed while hot to obtain high-purity sodium sulfate crystals and primary mother liquor. Step 3: Secondary Temperature Control for Sodium Precipitation and Density Adjustment: The primary mother liquor and the washing water from the sodium sulfate crystals are mixed to form a combined mother liquor. The combined mother liquor is divided into two groups, and their densities are adjusted separately. The first group is 1.340~1.342 g / L, and the second group is 1.345~1.346 g / L. Both groups of mother liquor are concentrated at 95℃ and atmospheric pressure, and sodium sulfate crystal nuclei are added. The stirring speed is maintained at 150 rpm. When the cumulative amount of water evaporated from each group reaches 180~220 g, the mixture is transferred to an 80℃ water bath heating environment, and the stirring speed is adjusted to 200 rpm for further concentration. Taking advantage of the characteristic that the solubility of sodium sulfate increases as the temperature decreases from 95℃ to 80℃, secondary sodium precipitation is achieved while increasing the density of the mother liquor, resulting in a specific density mother liquor system that meets the solid phase separation conditions. Step 4: Nucleus-induced crystallization of the double salt: Mix the two sets of mother liquors at 80℃, and continue to concentrate under normal pressure at a constant temperature of 80℃ and stirring at 250 rpm until the evaporated water reaches 210~230g. Adjust the sodium / potassium molar ratio of the mother liquor to 1.9~2.

1. Then allow the mother liquor to cool down naturally at a gradient. When the temperature drops to 60℃, add potassium sulfate nuclei to the system to form sodium-potassium sulfate double salt nuclei. Then adjust the stirring speed to 150 rpm and continue to cool down at a gradient to 30℃ to achieve the directional precipitation of sodium-potassium sulfate double salt crystals. Step 5: Gravity-induced solid-phase separation: After the mother liquor temperature drops to 30℃, adjust the stirring speed to 300 rpm and maintain it for 8~12 minutes. Then immediately stop stirring and keep it at 30℃ for 0.5~1 hour to allow it to settle. Under the initial effect of the specific stirring speed and the subsequent interaction of gravity and buoyancy of the mother liquor, the crystals in the system will undergo obvious stratification. The upper layer is a high-concentration concentrated mother liquor, the middle layer is loose sodium potassium sulfate double salt crystals, and the lower layer is dense sodium sulfate crystals. Step 6: Product purification: Without disturbing the lower layer of sodium sulfate crystals, the upper concentrated mother liquor and the middle layer of sodium sulfate-potassium double salt crystals are extracted by low-speed stirring. After solid-liquid separation, washing, and drying, the sodium sulfate-potassium double salt product is obtained. The lower layer of sodium sulfate crystals are separated, washed, and dried separately to obtain high-purity sodium sulfate product. The concentrated mother liquor can be used to extract valuable metals such as rubidium and cesium. The washing water is recycled back to the mother liquor merging stage.

2. The method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor according to claim 1, characterized in that: The sodium sulfate crystal nuclei mentioned in step 2 are high-purity sodium sulfate crystals. After addition, sodium sulfate is oriented to crystallize and precipitate at a high temperature of 95°C. The obtained sodium sulfate crystals are washed and dried, and the purity is ≥99.5% with a recovery rate of ≥85%. They can be directly reused in the roasting process of mica ore and ceramic lithium ore for lithium extraction.

3. The method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor according to claim 1, characterized in that: In step 3, the amount of sodium sulfate crystal nuclei added is 0.05% to 0.1% of the corresponding mother liquor mass, and the crystal nuclei particle size is 80 to 100 mesh. The mother liquor obtained after the second sodium precipitation provides the core density conditions for the gravity stratification of sodium salt and complex salt crystals.

4. The method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor according to claim 1, characterized in that: The gradient cooling rate in step 4 is 1℃ / min in the 60℃ to 40℃ range and 0.5℃ / min in the 40℃ to 30℃ range. The potassium sulfate crystal nuclei are high-purity crystals with a particle size of 120~150 mesh. After addition, the sodium potassium sulfate double salt crystal nuclei formed first can induce the directional combination of sodium ions, potassium ions and sulfate ions in the system, so as to achieve efficient precipitation of sodium potassium sulfate double salt crystals.

5. The method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor according to claim 1, characterized in that: In step 5, the high-speed stirring at 300 rpm uses a turbine-type impeller. Through shearing action, a specific high-speed turbulent flow of the mother liquor is formed, which separates and aggregates sodium sulfate potassium double salt crystals and sodium sulfate crystals with small density differences, laying the foundation for subsequent gravity stratification. After the high-speed stirring is completed, the density of the upper mother liquor needs to be controlled at 1.353~1.359 g / L. If it does not reach this range, a small amount of anhydrous sodium sulfate can be added and stirring can be continued for 2 minutes until the standard is met.

6. The method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor according to claim 1, characterized in that: During the heat preservation and sedimentation process described in step 5, if stirring is required, only a low-speed stirring of 40-50 rpm should be used to keep the middle layer of double salt crystals suspended in the upper mother liquor state, avoiding stirring the dense sodium sulfate crystals in the lower layer.

7. The method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor according to claim 1, characterized in that: The sodium sulfate potassium double salt crystals described in step 6 are washed twice with deionized water, with the amount of washing solution used each time being 1 to 2 times the mass of the crystals. The drying temperature is 90°C, and the potassium sulfate content in the double salt after drying is 50 to 60%. The lower layer of sodium sulfate crystals is washed and dried at 100°C, and the purity of the crystals after drying is ≥99%.

8. The method for direct solid-phase recovery of potassium salt from lithium precipitation mother liquor according to any one of claims 1-7: characterized in that: The entire concentration process of the recycling process is carried out under normal pressure. No chemical reagents other than crystal nuclei need to be added throughout the process, resulting in no secondary pollution. Furthermore, all washing water and unprecipitated concentrated mother liquor are recycled, achieving dual recovery of resources and water resources.

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