A method for recovering lithium carbonate from a lithium carbonate mother liquor

Abstract

The application discloses a method for preparing lithium carbonate from lithium precipitation mother liquor, and relates to the technical field of lithium salt production. The process comprises the following steps: filtering and impurity-removing pretreatment of the lithium precipitation mother liquor by using a nanofiltration membrane to obtain purified mother liquor; adding sodium phosphate into the purified mother liquor in a segmented under-amount mode to perform a lithium precipitation reaction, and performing solid-liquid separation to obtain crude lithium phosphate and a phosphorus precipitation mother liquor; performing causticization reaction of the crude lithium phosphate and water slurry and sodium hydroxide solution, performing solid-liquid separation to obtain a mixed filtrate, evaporating and concentrating the mixed filtrate to crystallize and precipitate sodium phosphate, and obtaining lithium hydroxide filtrate; and performing lithium precipitation and solid-liquid separation on the lithium hydroxide filtrate to obtain lithium carbonate and lithium precipitation liquid. The application realizes the recycling of sodium phosphate and sodium hydroxide by cooperation of nanofiltration impurity removal, lithium precipitation by sodium phosphate in a segmented under-amount mode, causticization transformation and lithium carbonate precipitation process, and has the advantages of high lithium recovery rate, high product purity, battery grade standard, low reagent consumption and green environmental protection.

Description

Technical Field

[0001] This invention relates to the field of lithium salt production technology, and in particular to a method for preparing lithium carbonate by recovering lithium precipitation mother liquor. Background Technology

[0002] In the industrial production process of lithium extraction from lepidolite roasting and lithium extraction from salt lake brine, the lithium precipitation mother liquor produced by the soda ash precipitation process still contains 2-3 g / L of soluble lithium ions. Due to the common ion effect in the solution and the interference of impurity ions, the conventional soda ash precipitation process cannot further recover the residual lithium resources.

[0003] The current mainstream sodium phosphate enrichment and lithium deposition process in the industry has many inherent technical defects: First, the traditional process involves adding an excessive amount of sodium phosphate at once, with the amount being 1.1 to 1.3 times the theoretical precipitation equivalent of lithium. Excessive phosphate remains in the mother liquor system, which not only causes serious waste of lithium precipitation agents but also requires a large amount of phosphorus removal agents, generating a large amount of phosphorus-containing solid waste, resulting in high costs for the treatment of the three wastes.

[0004] Second, most existing lithium phosphate conversion processes use hydrochloric acid dissolution and calcium salt dephosphorization, which generate a large amount of high-salt wastewater during the reaction and cannot achieve a closed-loop process. A few high-temperature roasting conversion processes require high-temperature conditions above 600°C, resulting in extremely high equipment energy consumption, high operation and maintenance costs, and poor industrialization economics.

[0005] Third, trace impurities such as calcium, magnesium, and boron in the mother liquor can easily cause co-precipitation and entrainment problems, resulting in high impurity content in crude lithium phosphate, and the final product can only meet the industrial-grade lithium carbonate standard.

[0006] Existing conventional processes mostly employ single-feed lithium precipitation and simple single-stage evaporation, which have drawbacks such as high reagent consumption, low product purity, and low resource utilization.

[0007] Therefore, it is of great significance to develop a resource-based process for the stable preparation of battery-grade lithium carbonate with low reagent consumption and a closed-loop process. Summary of the Invention

[0008] The main objective of this invention is to provide a method for preparing lithium carbonate from lithium precipitation mother liquor, thereby solving the technical problems of low lithium recovery rate, poor product purity, high sodium phosphate reagent consumption, and inability to achieve process closed-loop in existing lithium precipitation mother liquor recovery processes.

[0009] To achieve the above objectives, the present invention provides a method for preparing lithium carbonate by recovering lithium precipitation mother liquor, comprising the following steps: S1. Nanofiltration for impurity removal: The lithium precipitation mother liquor is pretreated by filtration and impurity removal using a nanofiltration membrane to obtain purified mother liquor; S2, Lithium phosphate precipitation: Sodium phosphate is added to the purified mother liquor in a segmented, insufficient manner, and lithium precipitation reaction is carried out under heating conditions. After the reaction is completed, solid and liquid are separated to obtain crude lithium phosphate and phosphate precipitation mother liquor. S3. Causticization reaction: The crude lithium phosphate is mixed with water to form a slurry, and then sodium hydroxide solution is added. A causticization reaction is carried out under heating conditions. After the reaction is completed, solid and liquid are separated to obtain a mixed filtrate. The mixed filtrate is evaporated, concentrated, and crystallized to precipitate sodium phosphate, thus obtaining lithium hydroxide filtrate. S4, Lithium carbonate precipitation: The lithium hydroxide filtrate is subjected to lithium precipitation and solid-liquid separation to obtain lithium carbonate and lithium precipitation liquid.

[0010] In some embodiments of the present invention, in step S1, the nanofiltration membrane includes an alkali-resistant nanofiltration membrane, and the molecular weight cutoff of the nanofiltration membrane is 150 Da to 200 Da. And / or, the pressure of the filtration and impurity removal pretreatment is 2.5 MPa to 3.5 MPa.

[0011] In some embodiments of the present invention, in step S1, the lithium ion content in the lithium precipitation mother liquor is 1.0-4.5 g / L, the sodium ion content is ≥80 g / L, the potassium ion content is 0.5-70 g / L, the total calcium and magnesium ion content is 5-80 ppm, and the boron content is ≤120 ppm. And / or, the lithium precipitation mother liquor is subjected to nanofiltration pretreatment to remove calcium, magnesium, and boron, while retaining potassium, sodium, and lithium; And / or, the total content of calcium and magnesium ions in the purified mother liquor is <5 ppm.

[0012] In some embodiments of the present invention, in step S2, the heating temperature is 35°C to 50°C; And / or, the lithium precipitation reaction time is 90 min to 120 min.

[0013] In some embodiments of the present invention, in step S2, the endpoint of the lithium precipitation reaction is a pH of 9.0 to 10.5.

[0014] In some embodiments of the present invention, in step S2, the segmented under-addition method is as follows: in the first stage, 60% to 70% of the total mass of sodium phosphate is added, and in the second stage, the remaining sodium phosphate is added.

[0015] In some embodiments of the present invention, in step S2, during the first stage, the reaction time is 40 min to 60 min.

[0016] In some embodiments of the present invention, in step S3, the mass ratio of crude lithium phosphate to water is (4-6):1.

[0017] In some embodiments of the present invention, in step S3, the temperature of the causticizing reaction is 45°C to 55°C, and the reaction time is 2.5h to 4h. And / or, the sodium phosphate is recycled to step S2.

[0018] In some embodiments of the present invention, in step S4, the lithium precipitation includes: adding soda ash to the lithium hydroxide filtrate, performing a first-stage low-temperature lithium precipitation at 50-60°C to obtain a first-stage lithium precipitation mother liquor, adding soda ash to the first-stage lithium precipitation mother liquor, and raising the temperature to 85-90°C to perform a second-stage high-temperature lithium precipitation.

[0019] In some embodiments of the present invention, the method for recycling and preparing lithium carbonate further includes the following steps: S5. Evaporation and salt precipitation: The phosphorus precipitation mother liquor from step S2 and the lithium precipitation liquid from step S4 are combined and subjected to a first-stage heating to precipitate sodium to obtain a first-stage mother liquor. The first-stage mother liquor is then subjected to a second-stage cooling to precipitate potassium to obtain an alkali-rich mother liquor. The alkali-rich mother liquor is returned to step S3 for recycling.

[0020] In some embodiments of the present invention, in step S5, the first-stage heating to precipitate sodium includes: high-temperature evaporation at 80°C to 90°C to precipitate sodium; the second-stage cooling to precipitate potassium includes: cooling the first-stage mother liquor to 30°C to 40°C for evaporation to precipitate potassium.

[0021] The beneficial effects that this invention can achieve are: This invention achieves the recycling of sodium phosphate and sodium hydroxide and the controllable open circuit of potassium resources through processes such as nanofiltration for impurity removal, segmented under-quantity sodium phosphate precipitation for lithium, causticization transformation, and precipitation of lithium carbonate. It has the advantages of high lithium recovery rate, high product purity (meeting battery-grade lithium carbonate standards), low reagent consumption, and green environmental protection, and has industrial promotion and industrial application value. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0023] Figure 1 This is a schematic flowchart of a method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to an embodiment of the present invention; Figure 2 This is a schematic flowchart of a method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to another embodiment of the present invention.

[0024] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0025] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] In this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. Furthermore, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. If the combination of technical solutions is contradictory or impossible to implement, such a combination should be considered non-existent and not within the scope of protection claimed by this invention.

[0028] This invention provides a method for preparing lithium carbonate by recovering lithium precipitation mother liquor, referring to... Figure 1 The method for preparing lithium carbonate by recovering lithium precipitation mother liquor includes the following steps: S1. Nanofiltration for impurity removal: The lithium precipitation mother liquor is pretreated by filtration and impurity removal using a nanofiltration membrane to obtain purified mother liquor; S2, Lithium phosphate precipitation: Sodium phosphate is added to the purified mother liquor in a segmented, insufficient manner, and lithium precipitation reaction is carried out under heating conditions. After the reaction is completed, solid and liquid are separated to obtain crude lithium phosphate and phosphate precipitation mother liquor. S3. Causticization reaction: The crude lithium phosphate is mixed with water to form a slurry, and then sodium hydroxide solution is added. A causticization reaction is carried out under heating conditions. After the reaction is completed, solid and liquid are separated to obtain a mixed filtrate. The mixed filtrate is evaporated, concentrated, and crystallized to precipitate sodium phosphate, thus obtaining lithium hydroxide filtrate. S4, Lithium carbonate precipitation: The lithium hydroxide filtrate is subjected to lithium precipitation and solid-liquid separation to obtain lithium carbonate and lithium precipitation liquid.

[0029] The lithium precipitation mother liquor used in this invention is derived from the mother liquor produced in the soda ash lithium precipitation process during the production of lithium-containing products.

[0030] In some embodiments, the process for producing lithium-containing products includes, but is not limited to, lithium extraction process by roasting lepidolite and lithium extraction process by salt lake brine.

[0031] In some embodiments, the lithium ion content in the lithium precipitation mother liquor is 1.0 g / L to 4.5 g / L, the sodium ion content is ≥80 g / L, the potassium ion content is 0.5 g / L to 70 g / L, the total calcium and magnesium ion content is 5 ppm to 80 ppm, and the boron content is ≤120 ppm. Calcium, magnesium, and boron are removed as impurities in step S1, while potassium, sodium, and lithium are retained in the purified mother liquor.

[0032] In some embodiments, the concentrated water containing high impurities such as calcium, magnesium, and boron is fed into the evaporation system to recover sodium salt resources, thereby reducing material waste.

[0033] In some embodiments, in step S1, the nanofiltration membrane includes an alkali-resistant nanofiltration membrane, which can accurately retain high-valence impurity ions such as calcium, magnesium, and boron, and avoid their subsequent co-precipitation and entrainment, thereby improving the purity of the lithium carbonate product.

[0034] In some embodiments, the pressure for pretreatment of lithium precipitation mother liquor by filtration and impurity removal using a nanofiltration membrane is 2.5 MPa to 3.5 MPa.

[0035] In some embodiments, the total content of calcium and magnesium ions in the purified mother liquor is <5 ppm.

[0036] In step S1 of this invention, a nanofiltration membrane is used to pretreat the lithium precipitation mother liquor to remove impurities. The nanofiltration membrane retains high-valence impurity ions such as calcium, magnesium, and boron by sieving effect, while potassium, sodium, and lithium ions pass through completely, resulting in a highly purified mother liquor. This effectively avoids the problem of co-precipitation and entrainment of impurity ions in the subsequent lithium precipitation process, laying the foundation for improving the purity of the final lithium carbonate product.

[0037] In step S2 of this invention, sodium phosphate is used for lithium precipitation. Potassium ions in the purified mother liquor do not react with phosphate ions to precipitate and are all retained in the phosphorus precipitation mother liquor, thus achieving the initial separation of lithium and potassium.

[0038] In some embodiments, in step S2, the heating temperature for the lithium deposition reaction is 35°C to 50°C.

[0039] In some embodiments, the lithium deposition reaction time in step S2 is 90 min to 120 min.

[0040] In some embodiments, the purified mother liquor is placed in a sealed reaction vessel with stirring and precise temperature control functions to carry out the lithium precipitation reaction. Carrying out the lithium precipitation reaction under stirring and heating conditions is conducive to the completeness of the reaction.

[0041] In some embodiments, in step S2, the endpoint of the lithium precipitation reaction is a pH of 9.0 to 10.5 to ensure a complete reaction and improve lithium recovery rate.

[0042] In some embodiments, in step S2, sodium phosphate is added in a staged, under-addition manner to carry out the lithium precipitation reaction. The staged, under-addition method is as follows: in the first stage, 60% to 70% of the total mass of sodium phosphate is added, and in the second stage, the remaining sodium phosphate is added. This embodiment uses a staged, under-addition method. In the first stage, 60% to 70% of the total mass of sodium phosphate is added to achieve rapid precipitation of most lithium ions. In the second stage, the remaining sodium phosphate is slowly and continuously added dropwise to prolong crystal growth time and reduce the entrainment of impurities through co-precipitation. Compared to the traditional method of one-time over-addition, this reduces reagent waste and phosphate residue from the source, eliminates the need for large amounts of phosphorus removal reagents, and lowers the cost of waste treatment.

[0043] In some embodiments, the response time of the first stage of the segmented under-quantity method is 40 min to 60 min.

[0044] In some embodiments, in step S2, the solid-liquid separation method includes pressure filtration to obtain crude lithium phosphate in the form of filter cake and phosphorus precipitation mother liquor.

[0045] In some embodiments, crude lithium phosphate is washed multiple times with pure water in a countercurrent manner to remove impurities such as sodium and potassium entrained on the surface. The wash water obtained after washing can be incorporated into the phosphorus precipitation mother liquor for unified treatment.

[0046] In step S3 of this invention, a causticizing process is used to transform crude lithium phosphate, which can achieve the conversion of lithium phosphate to lithium hydroxide at a relatively low temperature. The reaction is mild and energy-efficient. After the reaction, the solid-liquid separation yields a mixed filtrate of lithium hydroxide and sodium phosphate. Evaporating and concentrating the mixed filtrate allows for the crystallization of dissolved sodium phosphate crystals, and the lithium hydroxide filtrate is then separated, thus achieving the separation of sodium and lithium.

[0047] In some embodiments, in step S3, the mass ratio of crude lithium phosphate to water is (4~6):1, which can be 5:1.

[0048] In some embodiments, the mass concentration of the sodium hydroxide solution is 30% to 40%, and may be 32%.

[0049] In some embodiments, in step S3, the temperature of the causticizing reaction is 45°C to 55°C, and the reaction time is 2.5h to 4h. The present invention can realize the conversion of lithium phosphate to lithium hydroxide under low temperature conditions, and the reaction is mild and energy consumption is low.

[0050] In some embodiments, the reaction equation for the causticizing reaction is Li3PO4 + 3NaOH = 3LiOH + Na3PO4.

[0051] In some embodiments, in step S3, the solid-liquid separation method includes pressure filtration, where trace heavy metals, phosphates and other impurities form solid filter residue, which can be disposed of as solid waste in compliance with regulations.

[0052] In some embodiments, refer to Figure 2 Sodium phosphate is recycled to step S2. The sodium phosphate byproduct of the causticization reaction can be completely recycled to the lithium precipitation process after evaporation, concentration and crystallization, realizing a closed-loop cycle of sodium phosphate, avoiding the generation of high-salt wastewater, and improving the purity of the lithium hydroxide solution.

[0053] In step S4 of this invention, high-purity lithium carbonate and lithium-precipitated liquid are obtained through lithium precipitation and solid-liquid separation. The lithium-precipitated liquid contains alkali, trace amounts of lithium ions, and potassium ions introduced into the system.

[0054] In some embodiments, in step S4, lithium precipitation includes: adding soda ash to the lithium hydroxide filtrate, performing a first-stage low-temperature lithium precipitation at 50-60°C to obtain a first-stage lithium precipitation mother liquor, adding soda ash to the first-stage lithium precipitation mother liquor, and raising the temperature to 85-90°C for a second-stage high-temperature lithium precipitation. This embodiment employs a two-stage temperature-controlled gradient lithium precipitation process. The first-stage low-temperature lithium precipitation utilizes the low solubility of lithium carbonate at low temperatures to rapidly precipitate the main lithium carbonate crystals, while the second-stage high-temperature lithium precipitation deeply precipitates residual trace lithium ions, maximizing the lithium recovery rate and improving the purity of the recovered lithium carbonate.

[0055] In some embodiments, in step S4, the solid-liquid separation method includes pressure filtration to separate solid lithium carbonate.

[0056] In some embodiments, the lithium carbonate separated in step S4 is further washed, dried, and pulverized, which is beneficial for obtaining high-purity battery-grade lithium carbonate.

[0057] In some embodiments, washing lithium carbonate with high-temperature pure water is beneficial for removing sodium and potassium salt impurities adsorbed on the surface.

[0058] In some embodiments, the temperature for drying the washed lithium carbonate is 115°C to 125°C.

[0059] In some embodiments, after step S4, refer to Figure 1 and Figure 2 The method for recycling and preparing lithium carbonate also includes the following steps: S5: The phosphorus precipitation mother liquor from step S2 and the lithium precipitation liquid from step S4 are combined to obtain a mixed lithium liquid. The mixed lithium liquid undergoes a first-stage heating process to precipitate sodium, yielding a first-stage mother liquor. The first-stage mother liquor undergoes a second-stage cooling process to precipitate potassium, resulting in an alkali-rich mother liquor. This alkali-rich mother liquor can be returned to step S3 for recycling. This embodiment designs a two-stage temperature difference evaporation salt separation system for the tail liquid. It utilizes the difference in solubility of sodium and potassium salts at different temperatures to achieve precise salt separation and directional potassium removal. The first-stage high-temperature sodium precipitation yields a low-potassium sodium salt byproduct, while the second-stage cooling potassium precipitation enables continuous open-loop removal of potassium ions from the closed-loop system, completely solving the problem of potassium accumulation in the recycling system. The alkali-rich mother liquor after potassium salt removal is rich in high-concentration sodium hydroxide, with a stable sodium hydroxide mass concentration of 15wt% to 25wt%. It can be entirely returned to the causticization reaction section in step S3 as an alkali source for recycling. Only a very small amount of fresh sodium hydroxide solution needs to be added to compensate for system mechanical losses. Furthermore, the entire process of washing water and condensate is recycled in a closed loop, achieving zero discharge of production wastewater.

[0060] In some embodiments, in step S5, the first-stage heating to precipitate sodium includes: high-temperature evaporation at 80°C to 90°C to precipitate sodium. Under these temperature conditions, the solubility of sodium salt is relatively stable, and the solubility of potassium salt is relatively high, which is conducive to the precipitation of sodium salt.

[0061] In some embodiments, the first-stage heating sodium precipitation step concentrates the mixed lithium solution to 30% to 40% of its original volume.

[0062] In some embodiments, in step S5, the secondary cooling potassium precipitation includes: cooling the primary mother liquor to 30°C~40°C for evaporation to precipitate potassium. Under this temperature condition, the solubility of potassium salt drops sharply, and a large amount of potassium salt crystallizes out, which can obtain high-purity potassium sulfate by-product, realize the continuous open-loop removal of potassium ions from the closed-loop system, and completely solve the problem of potassium accumulation in the circulating system.

[0063] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following specific embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0064] Example 1 Raw material: Lithium mica precipitation mother liquor, which contains Li + 2.8g / L, Na + 92g / L, K + 60g / L, Ca 2+ 18ppm, Mg 2+ 29ppm, B 86ppm.

[0065] The method for recovering and preparing lithium carbonate in this embodiment is as follows: S1: The lithium precipitation mother liquor was fed into an alkali-resistant nanofiltration membrane system for nanofiltration pretreatment at room temperature and 3.0 MPa pressure to obtain purified mother liquor. Testing showed that the total calcium and magnesium ion content in the purified mother liquor was 3.2 ppm.

[0066] S2: The purified mother liquor was transferred to a sealed reactor equipped with stirring and temperature control, and the temperature was maintained at 40℃. Sodium phosphate was added in stages in a short-term manner to carry out the lithium precipitation reaction. In the first stage, 65wt% of the total amount of sodium phosphate (based on 0.90 times the theoretical equivalent) was added, and the reaction was stirred continuously for 50 min. In the second stage, the remaining 35wt% of sodium phosphate was slowly and continuously added dropwise, and the reaction was continued for another 50 min, for a total reaction time of 100 min. The final pH of the reaction was 9.8. After the reaction was completed, the solid and liquid were separated by pressure filtration to obtain crude lithium phosphate filter cake and phosphorus precipitation mother liquor. The crude lithium phosphate was washed twice countercurrently with pure water, and the wash water was added to the phosphorus precipitation mother liquor.

[0067] S3: The washed crude lithium phosphate is slurried with pure water at a liquid-to-solid ratio of 5:1, mixed with 32wt% sodium hydroxide solution, and causticized at a constant temperature of 50℃ for 3 hours. After the reaction, the mixture is separated by pressure filtration, and the filtrate is a mixed solution of lithium hydroxide and sodium phosphate. The filtrate is then concentrated by low-temperature evaporation to crystallize sodium phosphate and obtain a lithium hydroxide solution. All the crystallized sodium phosphate is recycled to the lithium precipitation reaction process in step S2.

[0068] S4: The lithium hydroxide solution from step S3 is subjected to a first-stage low-temperature lithium precipitation process by adding a saturated soda ash solution at a constant temperature of 55°C. After solid-liquid separation, a first-stage lithium precipitation mother liquor is obtained. The first-stage lithium precipitation mother liquor is then heated to 88°C, and an appropriate amount of saturated soda ash solution is added to perform a second-stage high-temperature lithium precipitation process. After both stages of lithium precipitation, solid lithium carbonate and the lithium precipitation liquid are obtained by pressure filtration. The solid lithium carbonate is washed with high-temperature pure water, dried at a constant temperature of 120°C, and pulverized to obtain the finished lithium carbonate product.

[0069] S5: Combine the phosphorus precipitation mother liquor from step S2 with the lithium precipitation liquid from step S4, and perform a first-stage high-temperature evaporation at 85°C to concentrate it to 35% of its original volume, precipitating low-potassium sodium salt; continue to concentrate it to 12% of its original volume, then cool it to 35°C for a second-stage potassium precipitation, obtaining potassium salt byproducts. The alkali-rich mother liquor obtained after the above sodium and potassium precipitation processes is returned to step S3 for recycling.

[0070] Example 2 Raw material: lithium precipitation mother liquor from salt lake, which contains Li + 3.5g / L, Na + 98g / L, K + 8.2 g / L, Ca 2+ 22ppm, Mg 2+ 35ppm, B 95ppm.

[0071] The method for recovering and preparing lithium carbonate in this embodiment is as follows: S1: The above-mentioned lithium precipitation mother liquor was fed into an alkali-resistant nanofiltration membrane system for nanofiltration pretreatment at room temperature and 3.3 MPa pressure to obtain purified mother liquor. Testing showed that the total calcium and magnesium ion content in the purified mother liquor was 3.8 ppm.

[0072] S2: The purified mother liquor was transferred to a reaction vessel and kept at a constant temperature of 45°C. Sodium phosphate was added dropwise in stages to initiate the lithium precipitation reaction, with the sodium phosphate dosage being 0.95 times the theoretical amount. In the first stage, 65 wt% of the total sodium phosphate was added, and the reaction was stirred for 55 min. In the second stage, the remaining 35 wt% was added dropwise, and the reaction continued for another 55 min, for a total reaction time of 110 min. The final pH of the reaction was 10.2. After the reaction was completed, the solid and liquid phases were separated by pressure filtration, yielding a crude lithium phosphate filter cake and a phosphorus precipitation mother liquor. The crude lithium phosphate was washed twice countercurrently with pure water, and the wash water was added to the phosphorus precipitation mother liquor.

[0073] S3: The washed crude lithium phosphate is slurried with pure water at a liquid-to-solid ratio of 5:1, mixed with 32wt% sodium hydroxide solution, and causticized at a constant temperature of 52℃ for 3.5h. After the reaction, the mixture is separated by pressure filtration, and the filtrate is a mixed solution of lithium hydroxide and sodium phosphate. The filtrate is concentrated by low-temperature evaporation to crystallize sodium phosphate and obtain lithium hydroxide solution. All the crystallized sodium phosphate is recycled to the lithium precipitation reaction process in step S2.

[0074] S4: The lithium hydroxide solution from step S3 is subjected to a first-stage low-temperature lithium precipitation process by adding a saturated soda ash solution at a constant temperature of 58°C. After solid-liquid separation, a first-stage lithium precipitation mother liquor is obtained. The first-stage lithium precipitation mother liquor is then heated to 90°C, and an appropriate amount of saturated soda ash solution is added to perform a second-stage high-temperature lithium precipitation process. After both stages of lithium precipitation, solid lithium carbonate and the lithium precipitation liquid are obtained by pressure filtration. The solid lithium carbonate is washed with high-temperature pure water, dried at a constant temperature of 120°C, and pulverized to obtain the finished lithium carbonate product.

[0075] S5: Combine the phosphorus precipitation mother liquor from step S2 with the lithium precipitation liquid from step S4, and perform a first-stage high-temperature evaporation at 88°C to concentrate it to 32% of its original volume, precipitating low-potassium sodium salt (K). + (Content 0.12wt%); continue concentrating to 11% of the original volume, then cool to 35℃ for secondary potassium precipitation, obtaining a potassium salt byproduct with a purity of 93%. The alkali-rich mother liquor obtained after the above sodium and potassium precipitation processes is returned to step S3 for recycling.

[0076] Example 3 Raw materials: lithium spodumene precipitation mother liquor, Li + 2.6 g / L, Na + 115g / L, K + 3.1 g / L.

[0077] The method for recovering and preparing lithium carbonate in this embodiment is as follows: S1: The lithium precipitation mother liquor was fed into an alkali-resistant nanofiltration membrane system for nanofiltration pretreatment at room temperature and 3.5 MPa pressure to obtain purified mother liquor. Testing showed that the total calcium and magnesium ion content in the purified mother liquor was 3.2 ppm.

[0078] S2: The purified mother liquor was transferred to a sealed reactor equipped with stirring and temperature control, and the temperature was maintained at 38°C. Sodium phosphate was added in stages in a short-term manner to carry out the lithium precipitation reaction. In the first stage, 60 wt% of the total sodium phosphate (based on 0.84 times the theoretical equivalent) was added, and the reaction was stirred continuously for 40 min. In the second stage, the remaining 40 wt% of sodium phosphate was slowly and continuously added dropwise, and the reaction was continued for 50 min, for a total reaction time of 90 min. The final pH of the reaction was 9.58. After the reaction was completed, the solid and liquid were separated by pressure filtration to obtain crude lithium phosphate filter cake and phosphorus precipitation mother liquor. The crude lithium phosphate was washed twice countercurrently with pure water, and the wash water was added to the phosphorus precipitation mother liquor.

[0079] S3: The washed crude lithium phosphate is slurried with pure water at a liquid-to-solid ratio of 5:1, mixed with 32wt% sodium hydroxide solution, and causticized at a constant temperature of 48℃ for 4 hours. After the reaction, the mixture is separated by pressure filtration, and the filtrate is a mixed solution of lithium hydroxide and sodium phosphate. The filtrate is then concentrated by low-temperature evaporation to crystallize sodium phosphate and obtain a lithium hydroxide solution. All the crystallized sodium phosphate is recycled to the lithium precipitation reaction process in step S2.

[0080] S4: The lithium hydroxide solution from step S3 is subjected to a first-stage low-temperature lithium precipitation process by adding a saturated soda ash solution at a constant temperature of 55°C. After solid-liquid separation, a first-stage lithium precipitation mother liquor is obtained. The first-stage lithium precipitation mother liquor is then heated to 88°C, and an appropriate amount of saturated soda ash solution is added to perform a second-stage high-temperature lithium precipitation process. After both stages of lithium precipitation, solid lithium carbonate and the lithium precipitation liquid are obtained by pressure filtration. The solid lithium carbonate is washed with high-temperature pure water, dried at a constant temperature of 120°C, and pulverized to obtain the finished lithium carbonate product.

[0081] S5: Combine the phosphorus precipitation mother liquor from step S2 with the lithium precipitation liquid from step S4, and perform a first-stage high-temperature evaporation at 85°C to concentrate it to 35% of its original volume, precipitating low-potassium sodium salt (K). + (Content 0.12wt%); continue concentrating to 12% of the original volume, then cool to 32℃ for secondary potassium precipitation, obtaining potassium salt byproduct. The alkali-rich mother liquor obtained after the above sodium and potassium precipitation processes is returned to step S3 for recycling.

[0082] Example 4 Raw materials: lithium extraction and precipitation mother liquor from lepidolite, Li + 4.0 g / L, Na + 85g / L, K + 45g / L, Ca 2+ 25ppm, Mg 2+ 40ppm, B 60ppm.

[0083] The method for recovering and preparing lithium carbonate in this embodiment is as follows: S1: The lithium precipitation mother liquor was fed into an alkali-resistant nanofiltration membrane system for nanofiltration pretreatment at room temperature and 2.5 MPa pressure to obtain purified mother liquor. Testing showed that the total calcium and magnesium ion content in the purified mother liquor was 4.5 ppm.

[0084] S2: The purified mother liquor was transferred to a sealed reactor equipped with stirring and temperature control, and the temperature was maintained at 50°C. Sodium phosphate was added in stages in a short-term manner to carry out the lithium precipitation reaction. In the first stage, 70 wt% of the total sodium phosphate (based on 0.92 times the theoretical equivalent) was added, and the reaction was stirred continuously for 60 min. In the second stage, the remaining 30 wt% of sodium phosphate was slowly and continuously added dropwise, and the reaction was continued for another 60 min, for a total reaction time of 120 min. The final pH of the reaction was 10.5. After the reaction was completed, the solid and liquid were separated by pressure filtration to obtain crude lithium phosphate filter cake and phosphorus precipitation mother liquor. The crude lithium phosphate was washed twice countercurrently with pure water, and the wash water was added to the phosphorus precipitation mother liquor.

[0085] S3: The washed crude lithium phosphate is slurried with pure water at a liquid-to-solid ratio of 5:1, and then mixed with 32wt% sodium hydroxide solution. The mixture is then causticized at a constant temperature of 55℃ for 2.5 hours. After the reaction, the mixture is separated by pressure filtration, and the filtrate is a mixed solution of lithium hydroxide and sodium phosphate. The filtrate is then concentrated by low-temperature evaporation to crystallize sodium phosphate and obtain a lithium hydroxide solution. All the crystallized sodium phosphate is recycled to the lithium precipitation reaction process in step S2.

[0086] S4: The lithium hydroxide solution from step S3 is subjected to a first-stage low-temperature lithium precipitation process by adding a saturated soda ash solution at a constant temperature of 60°C. After solid-liquid separation, a first-stage lithium precipitation mother liquor is obtained. The first-stage lithium precipitation mother liquor is then heated to 90°C, and an appropriate amount of saturated soda ash solution is added to perform a second-stage high-temperature lithium precipitation process. After both stages of lithium precipitation, solid lithium carbonate and the lithium precipitation liquid are obtained by pressure filtration. The solid lithium carbonate is washed with high-temperature pure water, dried at a constant temperature of 125°C, and pulverized to obtain the finished lithium carbonate product.

[0087] S5: Combine the phosphorus precipitation mother liquor from step S2 with the lithium precipitation liquid from step S4, and perform a first-stage high-temperature evaporation at 90°C to concentrate it to 30% of its original volume, precipitating low-potassium sodium salt; continue to concentrate it to 15% of its original volume, then cool it to 40°C for a second-stage potassium precipitation, obtaining potassium salt byproducts. The alkali-rich mother liquor obtained after the above sodium and potassium precipitation processes is returned to step S3 for recycling.

[0088] Example 5 Raw material: lithium precipitation mother liquor from salt lake, which contains Li + 1.5g / L, Na + 90g / L, K + 15g / L, Ca 2+ 10ppm, Mg 2+15ppm, B 40ppm.

[0089] S1: The lithium precipitation mother liquor was fed into an alkali-resistant nanofiltration membrane system for nanofiltration pretreatment at room temperature and 2.8 MPa pressure to obtain purified mother liquor. Testing showed that the total calcium and magnesium ion content in the purified mother liquor was 2.5 ppm.

[0090] S2: The purified mother liquor was transferred to a sealed reactor equipped with stirring and temperature control, and the temperature was maintained at 35°C. Sodium phosphate was added in stages in a short-term manner to induce lithium precipitation. In the first stage, 65 wt% of the total sodium phosphate (based on 0.88 times the theoretical equivalent) was added, and the reaction was stirred continuously for 45 min. In the second stage, the remaining 35 wt% of sodium phosphate was slowly and continuously added dropwise, and the reaction was continued for another 55 min, for a total reaction time of 100 min. The final pH of the reaction was 9.0. After the reaction was completed, the solid and liquid were separated by pressure filtration to obtain crude lithium phosphate filter cake and phosphorus precipitation mother liquor. The crude lithium phosphate was washed twice countercurrently with pure water, and the wash water was added to the phosphorus precipitation mother liquor.

[0091] S3: The washed crude lithium phosphate is slurried with pure water at a liquid-to-solid ratio of 5:1, mixed with 32wt% sodium hydroxide solution, and causticized at a constant temperature of 45℃ for 3 hours. After the reaction, the mixture is separated by pressure filtration, and the filtrate is a mixed solution of lithium hydroxide and sodium phosphate. The filtrate is then concentrated by low-temperature evaporation to crystallize sodium phosphate and obtain a lithium hydroxide solution. All the crystallized sodium phosphate is recycled to the lithium precipitation reaction process in step S2.

[0092] S4: The lithium hydroxide solution from step S3 is subjected to a first-stage low-temperature lithium precipitation process by adding a saturated soda ash solution at a constant temperature of 50°C. After solid-liquid separation, a first-stage lithium precipitation mother liquor is obtained. The first-stage lithium precipitation mother liquor is then heated to 85°C, and an appropriate amount of saturated soda ash solution is added to perform a second-stage high-temperature lithium precipitation process. After both stages of lithium precipitation, solid lithium carbonate and the lithium precipitation liquid are obtained by pressure filtration. The solid lithium carbonate is washed with high-temperature pure water, dried at a constant temperature of 115°C, and pulverized to obtain the finished lithium carbonate product.

[0093] S5: Combine the phosphorus precipitation mother liquor from step S2 with the lithium precipitation liquid from step S4, and perform a first-stage high-temperature evaporation at 80°C to concentrate it to 40% of its original volume, precipitating low-potassium sodium salt; continue to concentrate it to 10% of its original volume, then cool it to 30°C for a second-stage potassium precipitation, obtaining potassium salt byproducts. The alkali-rich mother liquor obtained after the above sodium and potassium precipitation processes is returned to step S3 for recycling.

[0094] Comparative Example 1 The raw materials are the same as in Example 1.

[0095] The method for preparing lithium carbonate from recycled materials in Comparative Example 1 is as follows: S1: Same as Example 1.

[0096] S2: The purified mother liquor was transferred to a sealed reactor equipped with stirring and temperature control. The temperature was maintained at 40℃, and sodium phosphate was added in excess at once to initiate the lithium precipitation reaction. The amount of sodium phosphate added was 1.2 times the theoretical amount (excess addition), and the entire amount was added at once. The reaction was stirred continuously for 100 minutes. The final pH of the reaction was 11.2. After the reaction was completed, the solid and liquid were separated by pressure filtration, yielding crude lithium phosphate filter cake and phosphorus precipitation mother liquor. The crude lithium phosphate was washed twice countercurrently with pure water, and the wash water was added to the phosphorus precipitation mother liquor.

[0097] Steps S3 to S5 are the same as in Example 1.

[0098] Comparative Example 2 The raw materials are the same as in Example 1.

[0099] The method for preparing lithium carbonate from recycled materials in Comparative Example 2 is as follows: S1: Omit the nanofiltration impurity removal pretreatment step and directly introduce the original lithium precipitation mother liquor into step S2.

[0100] Steps S2 to S5 are the same as in Example 1.

[0101] Table 1

[0102] As shown in Table 1, the lithium recovery rate of Examples 1 to 5 is above 93%, and the main lithium carbonate content is above 99.5%. In addition, the contents of impurities Na, Ca, Mg and K are all low, which meets the national standard for battery-grade lithium carbonate.

[0103] Comparative Example 1 showed a lithium recovery rate of only 88.6% and a lithium carbonate content of 99.15%, but with high levels of impurities such as Na, Ca, Mg, and K, only meeting the industrial-grade lithium carbonate standard. Compared to Example 1, the lithium recovery rate decreased by approximately 5.7 percentage points, the product purity significantly decreased, and the sodium phosphate reagent consumption increased by approximately 33%.

[0104] Comparative Example 2 showed a lithium recovery rate of only 89.2% throughout the entire process, with a lithium carbonate content of 98.70%. However, the levels of impurities, Ca (28.5%) and Mg (22.3%), were both high, far exceeding battery-grade standard limits and only meeting industrial-grade lithium carbonate standards. Compared to Example 1, the lack of nanofiltration for impurity removal resulted in the co-precipitation and entrainment of calcium and magnesium impurities during lithium precipitation, leading to a significant decrease in product purity.

[0105] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A method for preparing lithium carbonate by recovering lithium precipitation mother liquor, characterized in that, Includes the following steps: S1. Nanofiltration for impurity removal: The lithium precipitation mother liquor is pretreated by filtration and impurity removal using a nanofiltration membrane to obtain purified mother liquor; S2, Lithium phosphate precipitation: Sodium phosphate is added to the purified mother liquor in a segmented, insufficient manner, and lithium precipitation reaction is carried out under heating conditions. After the reaction is completed, solid and liquid are separated to obtain crude lithium phosphate and phosphate precipitation mother liquor. S3. Causticization reaction: The crude lithium phosphate is mixed with water to form a slurry, and then sodium hydroxide solution is added. A causticization reaction is carried out under heating conditions. After the reaction is completed, solid and liquid are separated to obtain a mixed filtrate. The mixed filtrate is evaporated, concentrated, and crystallized to precipitate sodium phosphate, thus obtaining lithium hydroxide filtrate. S4, Lithium carbonate precipitation: The lithium hydroxide filtrate is subjected to lithium precipitation and solid-liquid separation to obtain lithium carbonate and lithium precipitation liquid.

2. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, In step S1, the nanofiltration membrane includes an alkali-resistant nanofiltration membrane, and the molecular weight cutoff of the nanofiltration membrane is 150 Da to 200 Da; And / or, the pressure of the filtration and impurity removal pretreatment is 2.5 MPa to 3.5 MPa.

3. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, In step S1, the lithium ion content in the lithium precipitation mother liquor is 1.0-4.5 g / L, the sodium ion content is ≥80 g / L, the potassium ion content is 0.5-70 g / L, the total calcium and magnesium ion content is 5-80 ppm, and the boron content is ≤120 ppm. And / or, the lithium precipitation mother liquor is subjected to nanofiltration pretreatment to remove calcium, magnesium, and boron, while retaining potassium, sodium, and lithium; And / or, the total content of calcium and magnesium ions in the purified mother liquor is <5 ppm.

4. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, In step S2, the heating temperature is 35℃~50℃; And / or, the lithium precipitation reaction time is 90 min to 120 min.

5. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, In step S2, the endpoint of the lithium precipitation reaction is a pH of 9.0 to 10.

5.

6. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, In step S2, the segmented shortfall method is as follows: in the first stage, 60% to 70% of the total mass of sodium phosphate is added, and in the second stage, the remaining sodium phosphate is added.

7. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 6, characterized in that, In step S2, during the first stage, the reaction time is 40 min to 60 min.

8. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, In step S3, the mass ratio of crude lithium phosphate to water is (4-6):

1.

9. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, In step S3, the temperature of the causticizing reaction is 45℃~55℃, and the reaction time is 2.5h~4h; And / or, the sodium phosphate is recycled to step S2.

10. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, In step S4, the lithium precipitation includes: adding soda ash to the lithium hydroxide filtrate, performing a first-stage low-temperature lithium precipitation at 50-60°C to obtain a first-stage lithium precipitation mother liquor, adding soda ash to the first-stage lithium precipitation mother liquor, and raising the temperature to 85-90°C to perform a second-stage high-temperature lithium precipitation.

11. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 1, characterized in that, The method for recycling and preparing lithium carbonate further includes the following steps: S5. Evaporation and salt precipitation: The phosphorus precipitation mother liquor from step S2 and the lithium precipitation liquid from step S4 are combined and subjected to a first-stage heating to precipitate sodium to obtain a first-stage mother liquor. The first-stage mother liquor is then subjected to a second-stage cooling to precipitate potassium to obtain an alkali-rich mother liquor. The alkali-rich mother liquor is returned to step S3 for recycling.

12. The method for preparing lithium carbonate by recovering lithium precipitation mother liquor according to claim 11, characterized in that, In step S5, the first-stage heating for sodium precipitation includes: high-temperature evaporation at 80°C to 90°C to precipitate sodium; the second-stage cooling for potassium precipitation includes: cooling the first-stage mother liquor to 30°C to 40°C for evaporation to precipitate potassium.

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