Method for preparing high-purity lithium carbonate

Abstract

A method for preparing high-purity lithium carbonate includes: S1 purification: heating an 18-22 g / L lithium sulfate solution and a 70-80 g / L potassium-sodium mixed salt solution to 75-85°C, adding sodium hydroxide solution for preliminary purification; filtering or allowing to stand to remove solid impurities, then adding phosphate for secondary purification, filtering or allowing to stand to remove solid impurities to obtain a lithium sulfate solution; S2 alkali melting: adding sodium hydroxide solution to the purified lithium sulfate solution from S1, stirring and controlling the reaction temperature at 80-100°C; reaction time 2-8 h; obtaining a mother liquor after the reaction is complete; S3 lithium precipitation: cooling the mother liquor from S2 to 20°C, and continuously adding a precipitant at a rate of 0.10-0.15 mol / L·min; stopping the addition when the lithium content in the solution drops to a predetermined value; S4 separation: separating the precipitate in the solution after the S3 reaction to obtain high-purity lithium carbonate. This method can prepare high-purity lithium carbonate, improve the reaction rate and reduce production energy consumption; by controlling the cooling rate, it avoids uneven lithium carbonate crystallization caused by excessively rapid cooling.

Description

Technical Field

[0001] This invention relates to the field of lithium carbonate preparation technology, and in particular to a method for preparing high-purity lithium carbonate. Background Technology

[0002] With the rapid development of lithium battery technology, the demand for high-purity lithium carbonate continues to grow. Traditional lithium carbonate manufacturing processes suffer from numerous problems, such as low purity, high energy consumption, and severe environmental pollution. In particular, the removal efficiency of potassium and sodium mixed salts and trace metal impurities in the lithium sulfate solution is low during the purification process, leading to unstable final product quality.

[0003] The amount of sodium hydroxide added during the impurity removal process affects the yield of lithium carbonate. Insufficient sodium hydroxide reduces recovery efficiency, while excessive sodium hydroxide reacts with lithium sulfate to form water-soluble sodium sulfate, thus reducing the yield of lithium sulfate to lithium carbonate. Some excess sodium hydroxide remains in the solution, which may affect the crystal form and particle size distribution of the subsequent lithium carbonate precipitation. Repeated reaction-neutralization cycles will produce some byproducts, which may reduce the purity and quality of the final lithium carbonate product. Therefore, precise control of the amount of sodium hydroxide is essential.

[0004] Meanwhile, the control of reaction temperature has a significant impact on reaction time, while the concentration of reactants also affects reaction time and yield; moreover, some high-quality lithium carbonates have strict requirements on particle size. Therefore, the process for preparing high-quality, high-purity lithium carbonate is currently needed for the industry's development. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned technical defects and provide a method for preparing high-purity lithium carbonate.

[0006] In a first aspect, this application provides a method for preparing high-purity lithium carbonate, comprising:

[0007] S1 purification:

[0008] Heat 18-22 g / L lithium sulfate solution and 70-80 g / L potassium sodium mixed salt solution to 75-85℃, and add sodium hydroxide solution for preliminary impurity removal;

[0009] After removing solid impurities by filtration or settling, phosphate is added for secondary impurity removal. After removing solid impurities by filtration or settling, lithium sulfate solution is obtained.

[0010] S2 molten alkali:

[0011] Add sodium hydroxide solution to the purified lithium sulfate solution after removing impurities from S1, stir and control the reaction temperature at 80-100℃; react for 2-8 hours; after the reaction is complete, obtain the mother liquor;

[0012] S3 lithium deposition:

[0013] The mother liquor of S2 was cooled to 20°C, and a precipitant was continuously added at a rate of 0.10-0.15 mol / L·min; the addition was stopped when the lithium content in the solution dropped to a predetermined value.

[0014] S4 separation:

[0015] The precipitate in the solution after the S3 reaction was separated to obtain high-purity lithium carbonate.

[0016] The necessity of controlling the addition rate: if the addition rate is too fast, a large number of centers will be generated, resulting in small precipitate particles; if the addition rate is too slow, aggregation and suturing will occur, generating fragments with uneven particle size; only with an appropriate addition rate can high-quality precipitates with large volume and uniform distribution be obtained.

[0017] In one embodiment, in step S1, the sodium hydroxide solution is a 5%-15% concentration sodium hydroxide solution, and the molar ratio of sodium hydroxide to lithium sulfate in the sodium hydroxide solution and the lithium sulfate solution is 2:1. When excess sodium hydroxide is added, the excess sodium hydroxide will react with lithium sulfate to form water-soluble sodium sulfate, thereby reducing the yield of lithium sulfate to lithium carbonate. Some excess sodium hydroxide remains in the solution, which may affect the crystal form and particle size distribution of the subsequent lithium carbonate precipitation. Repeated reaction-neutralization cycles will produce some byproducts, which may reduce the purity and quality of the final lithium carbonate product. Therefore, adding excess sodium hydroxide has a certain negative impact on the purity and yield of lithium carbonate. Precise control of the amount of sodium hydroxide is essential.

[0018] Preferably, the sodium hydroxide solution is a 5%-10% concentration sodium hydroxide solution;

[0019] Preferably, based on the molar amount of sodium hydroxide added, a 5% sodium hydroxide solution is added first, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1; then a 10% sodium hydroxide solution is added, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1.

[0020] Preferably, the addition rate of sodium hydroxide can also be guided by online monitoring of changes in the solution pH value; that is, when the pH value is low, the addition rate is increased; when it is close to neutral, the addition rate is decreased.

[0021] Preferably, in the lithium sulfate solution and the potassium-sodium mixed salt solution, the molar ratio of lithium sulfate to potassium-sodium mixed salt is 1:(0.1-0.5).

[0022] Preferably, the amount of phosphate to be added is determined based on the pH value of the solution. The pH value of the solution is measured simultaneously with the addition of the phosphate, and the addition of phosphate is stopped when the pH value of the solution is 6-7.

[0023] Preferably, the amount of phosphate added is determined based on the concentration of impurities in the solution, and the concentrations of impurity metal elements such as Fe, Al, Mn, Ca, and Mg in the solution are determined by standard methods such as atomic absorption spectrophotometry and ICP-OES to obtain the mass of impurities in the solution. The mass ratio of phosphate to impurities in the solution is (5-10):1.

[0024] In one embodiment, in step S2, the sodium hydroxide solution used for melting the alkali is a 10%-15% concentration sodium hydroxide solution, and the molar ratio of sodium hydroxide to lithium sulfate in the sodium hydroxide solution and the lithium sulfate solution is (2-2.5):1. The concentration of the reactants also affects the reaction time; the higher the concentration, the faster the reaction.

[0025] Preferably, the reaction temperature is 90-100℃. Controlling the reaction temperature has a significant impact on the reaction time; according to empirical data, the reaction rate approximately doubles for every 10℃ increase in temperature. Therefore, a higher reaction temperature (90-100℃) can shorten the reaction time. Considering reaction kinetics, empirical data, and equipment and energy factors, the suitable reaction time range for this reaction is: 5-8 hours at 80℃, 3-5 hours at 90℃, and 2-3 hours at 100℃. Within this range, efficient reaction completion and maximization of lithium hydroxide yield can be guaranteed.

[0026] In one embodiment, during step S2 (melting alkali), the reaction pressure is controlled to be 0.05-0.08 MPa.

[0027] Preferably, the reaction pressure is monitored by an online pressure sensor, with a reading frequency of 1 time per minute; the reaction temperature is monitored by a platinum resistance temperature probe, with a reading frequency of 1 time per 30 seconds.

[0028] In one embodiment, during step S2 (melting alkali), the pH value, conductivity, and lithium ion concentration in the solution are monitored to determine the end of the reaction. Preferably, the pH value is stable at 7.0 ± 0.2; the conductivity changes to PLATEAU; and the lithium content is detected to be only trace (<1%). When the above three parameters simultaneously meet the criteria and remain stable for a certain period of time (e.g., 0.5 h), the reaction can be determined to be completely finished.

[0029] Preferably, the pH value can be detected by a pH meter, the conductivity can be detected routinely online by a conductivity meter, and the lithium content can be detected by EDTA light titration method (GB / T 3683-2008).

[0030] Preferably, unreacted sodium hydroxide and sodium sulfate can be recovered and used for recycling or as byproducts.

[0031] In one embodiment, during step S3 (lithium precipitation), the cooling rate is 0.2-0.5 °C / min. Preferably, when the solution temperature is greater than 60 °C, the cooling rate is 0.5 °C / min; when the solution temperature is less than or equal to 60 °C, the cooling rate is 0.2 °C / min. This stepped cooling mode can steadily and effectively reduce the temperature, ensuring a sufficient formation rate of lithium carbonate crystals and obtaining high-quality lithium carbonate precipitates with large volume and uniform distribution.

[0032] In one embodiment, in step S3 lithium precipitation, the precipitant is sodium carbonate.

[0033] Preferably, the sodium carbonate is Guangming anhydrous sodium carbonate, manufactured by Shanghai Fujie Chemical Co., Ltd.

[0034] In one embodiment, during step S3 lithium precipitation, the lithium content of the solution is detected, and the predetermined value is that the lithium content in the solution is ≤0.5%.

[0035] Preferably, the lithium content in the solution is ≤0.2%. Theoretically, to achieve complete lithium conversion, the residual lithium concentration in the solution should be close to 0; however, considering detection errors and trace amounts of dissolved residue, in actual operation, a lithium content of ≤0.5% in the solution can be considered as a complete reaction.

[0036] Preferably, electromagnetic stirring can be used to improve reaction efficiency during the continuous addition of the precipitant. Principle: Magnetic suspension: Precipitate particles can become magnetic under the influence of an electromagnetic field, thus suspending in the liquid. This suspension enhances the interaction between particles, promoting aggregation. Magnetic aggregation: By adjusting the parameters of the electromagnetic field, such as magnetic field strength and direction, the magnetism and interaction of the particles can be controlled, thereby promoting aggregation. When particles aggregate, their effective cross-sectional area increases, thus accelerating the sedimentation process.

[0037] The present invention has the following beneficial effects

[0038] The high-purity lithium carbonate preparation method provided by this invention involves purification, alkali melting, and lithium precipitation of a lithium sulfate solution to produce high-purity lithium carbonate, while simultaneously increasing the reaction rate and reducing production energy consumption. By controlling the concentration and amount of sodium hydroxide added in S1, the lithium carbonate yield and particle size uniformity are improved. By controlling the reaction temperature and sodium hydroxide concentration in S2, the reaction is completed efficiently, and the lithium hydroxide yield is maximized. By controlling the cooling rate, uneven lithium carbonate crystallization caused by excessively rapid cooling is avoided. By using a precipitant, the distribution of the precipitant and the reaction time can be more precisely controlled, thereby optimizing the entire precipitation process. Furthermore, by using electromagnetic stirring, the electromagnetic field parameters are precisely controlled, improving the efficiency and speed of separation between the lithium carbonate precipitate and the mother liquor. Detailed Implementation

[0039] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0040] The raw materials used in the embodiments and comparative examples of this invention are as follows:

[0041] Lithium sulfate solution: 20 g / L lithium sulfate solution;

[0042] Potassium-sodium mixed salt solution: 75 g / L potassium-sodium mixed salt solution, with potassium content of 10% and sodium content of 7%, measured by atomic absorption spectrophotometry;

[0043] Sodium hydroxide solution a: 5% sodium hydroxide solution;

[0044] Sodium hydroxide solution b: 10% sodium hydroxide solution;

[0045] Sodium hydroxide solution c: 15% sodium hydroxide solution;

[0046] Sodium hydroxide solution d: 20% sodium hydroxide solution;

[0047] Sodium carbonate: Anhydrous sodium carbonate from Guangming Chemical Co., Ltd., Shanghai Fujie Chemical Co., Ltd.; Sodium content (Na2CO3) ≥ 99.2%, Calcium content (Ca) ≤ 0.04%, Carbonate (CO3) 2- ≤0.03%

[0048] Phosphate: Water-soluble sodium sulfate, purity ≥95%;

[0049] Examples 1-3: Preparation method of high-purity lithium carbonate:

[0050] S1 purification:

[0051] A 20 g / L lithium sulfate solution and a 75 g / L potassium-sodium mixed salt solution were heated to 80 °C, and sodium hydroxide solution was added for preliminary impurity removal. The molar ratio of lithium sulfate to potassium-sodium mixed salt was 1:(0.1-0.5); the molar ratio of sodium hydroxide to lithium sulfate was 2:1. Based on the amount of sodium hydroxide added, a 5% sodium hydroxide solution was added first, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1; then a 10% sodium hydroxide solution was added, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1.

[0052] After removing solid impurities by filtration or settling, phosphate is added for secondary impurity removal. After removing solid impurities by filtration or settling, a lithium sulfate solution is obtained. The amount of phosphate to be added is determined according to the pH value of the solution. The pH value of the solution is measured at the same time as the phosphate is added. When the pH value of the solution is 7, the addition of phosphate is stopped.

[0053] S2 molten alkali:

[0054] Add sodium hydroxide solution to the purified lithium sulfate solution after S1 purification, with a molar ratio of sodium hydroxide to lithium sulfate of (2-2.5):1; stir and control the reaction temperature at 80-100℃; react for 2-8 hours; obtain the mother liquor after the reaction is complete;

[0055] The reaction pressure was controlled at 0.05 MPa;

[0056] The pH value is stable at 7.0±0.2; the conductivity changes to PLATEAU; the lithium content is detected to be only trace (<1%); when the above three parameters meet the standard simultaneously and remain stable for a certain period of time (e.g., 0.5h), the reaction can be judged to be completely finished.

[0057] S3 lithium deposition:

[0058] The mother liquor of S2 is cooled and a precipitant is continuously added at a rate of 0.10-0.15 mol / L·min; the addition is stopped when the lithium content in the solution drops to a predetermined value; the cooling rate is 0.2-0.5℃ / min.

[0059] The precipitant is sodium carbonate;

[0060] When the solution temperature is greater than 60℃, the cooling rate is 0.5℃ / min; when the solution temperature is less than or equal to 60℃, the cooling rate is 0.2℃ / min.

[0061] The lithium content of the solution during the lithium precipitation step is detected, wherein the predetermined value is a lithium content of ≤0.5% in the solution;

[0062] S4 separation:

[0063] The precipitate in the solution after the S3 reaction was separated to obtain high-purity lithium carbonate.

[0064] Example 4 differs from the above examples in that the amount of phosphate added in the S1 purification step is determined based on the impurity concentration.

[0065] S1 purification:

[0066] A 20 g / L lithium sulfate solution and a 75 g / L potassium-sodium mixed salt solution were heated to 80 °C, and sodium hydroxide solution a was added for preliminary impurity removal. The molar ratio of lithium sulfate to potassium-sodium mixed salt was 1:0.3; the molar ratio of sodium hydroxide to lithium sulfate was 2:1. Based on the molar amount of sodium hydroxide added, a 5% sodium hydroxide solution was added first, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1; then a 10% sodium hydroxide solution was added, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1.

[0067] After removing solid impurities by filtration or settling, phosphate is added for secondary impurity removal. After removing solid impurities by filtration or settling, a lithium sulfate solution is obtained. The amount of phosphate to be added is determined according to the concentration of impurities in the solution. The concentrations of impurity metal elements such as Fe, Al, Mn, Ca, and Mg in the solution are determined by standard methods such as atomic absorption spectrophotometry and ICP-OES to obtain the mass of impurities in the solution. The mass ratio of phosphate to impurities in the solution is (5-10):1.

[0068] S2 molten alkali:

[0069] Add sodium hydroxide solution b to the purified lithium sulfate solution (S1) with a molar ratio of sodium hydroxide to lithium sulfate of 2.5:1; stir and control the reaction temperature at 100℃; react for 3 hours; obtain the mother liquor after the reaction is complete.

[0070] The reaction pressure was controlled at 0.05 MPa;

[0071] The pH value is stable at 7.0±0.2; the conductivity changes to PLATEAU; the lithium content is detected to be only trace (<1%); when the above three parameters meet the standard simultaneously and remain stable for a certain period of time (e.g., 0.5h), the reaction can be judged to be completely finished.

[0072] S3 lithium deposition:

[0073] The mother liquor of S2 is cooled and a precipitant is continuously added at a rate of 0.13 mol / L·min; the addition is stopped when the lithium content in the solution drops to a predetermined value; the cooling rate is 0.2-0.5 °C / min.

[0074] The precipitant is sodium carbonate;

[0075] When the solution temperature is greater than 60℃, the cooling rate is 0.5℃ / min; when the solution temperature is less than or equal to 60℃, the cooling rate is 0.2℃ / min.

[0076] The lithium content of the solution during the lithium precipitation step is detected, wherein the predetermined value is a lithium content of ≤0.5% in the solution;

[0077] S4 separation:

[0078] The precipitate in the solution after the S3 reaction was separated to obtain high-purity lithium carbonate.

[0079] Preparation methods of high-purity lithium carbonate in Comparative Examples 1-4:

[0080] S1 purification:

[0081] A 20 g / L lithium sulfate solution and a 75 g / L potassium-sodium mixed salt solution were heated to 80 °C, and sodium hydroxide solution was added for preliminary impurity removal. The molar ratio of lithium sulfate to potassium-sodium mixed salt was 1:(0.1-0.5); the molar ratio of sodium hydroxide to lithium sulfate was 2:1. Based on the amount of sodium hydroxide added, a 5% sodium hydroxide solution was added first, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1; then a 10% sodium hydroxide solution was added, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1.

[0082] After removing solid impurities by filtration or settling, phosphate is added for secondary impurity removal. After removing solid impurities by filtration or settling, a lithium sulfate solution is obtained. The amount of phosphate to be added is determined according to the pH value of the solution. The pH value of the solution is measured at the same time as the phosphate is added. When the pH value of the solution is 7, the addition of phosphate is stopped.

[0083] S2 molten alkali:

[0084] Add sodium hydroxide solution to the purified lithium sulfate solution after S1 purification, with a molar ratio of sodium hydroxide to lithium sulfate of (2-2.5):1; stir and control the reaction temperature at 80-100℃; react for 2-8 hours; obtain the mother liquor after the reaction is complete;

[0085] The reaction pressure was controlled at 0.05 MPa;

[0086] The pH value is stable at 7.0±0.2; the conductivity changes to PLATEAU; the lithium content is detected to be only trace (<1%); when the above three parameters meet the standard simultaneously and remain stable for a certain period of time (e.g., 0.5h), the reaction can be judged to be completely finished.

[0087] S3 lithium deposition:

[0088] The mother liquor of S2 is cooled and a precipitant is continuously added, with the addition rate controlled. The addition is stopped when the lithium content in the solution drops to a predetermined value. The cooling rate is 0.2-0.5℃ / min.

[0089] The precipitant is sodium carbonate;

[0090] When the solution temperature is greater than 60℃, the cooling rate is 0.5℃ / min; when the solution temperature is less than or equal to 60℃, the cooling rate is 0.2℃ / min.

[0091] The lithium content of the solution during the lithium precipitation step is detected, wherein the predetermined value is a lithium content of ≤0.5% in the solution;

[0092] S4 separation:

[0093] The precipitate in the solution after the S3 reaction was separated to obtain high-purity lithium carbonate.

[0094] Comparative Example 5

[0095] S1 purification:

[0096] A 20 g / L lithium sulfate solution and a 75 g / L potassium-sodium mixed salt solution were heated to 80 °C, and sodium hydroxide solution a was added for preliminary impurity removal. The molar ratio of lithium sulfate to potassium-sodium mixed salt was 1:0.3; the molar ratio of sodium hydroxide to lithium sulfate was 3:1. Based on the molar amount of sodium hydroxide added, a 5% sodium hydroxide solution was added first, with a molar ratio of sodium hydroxide to lithium sulfate of 2:1; then a 10% sodium hydroxide solution was added, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1.

[0097] After removing solid impurities by filtration or settling, phosphate is added for secondary impurity removal. After removing solid impurities by filtration or settling, a lithium sulfate solution is obtained. The amount of phosphate to be added is determined according to the pH value of the solution. The pH value of the solution is measured at the same time as the phosphate is added. When the pH value of the solution is 7, the addition of phosphate is stopped.

[0098] S2 molten alkali:

[0099] Add sodium hydroxide solution b to the purified lithium sulfate solution (S1) with a molar ratio of sodium hydroxide to lithium sulfate of 2.5:1; stir and control the reaction temperature at 100℃; react for 3 hours; obtain the mother liquor after the reaction is complete.

[0100] The reaction pressure was controlled at 0.05 MPa;

[0101] The pH value is stable at 7.0±0.2; the conductivity changes to PLATEAU; the lithium content is detected to be only trace (<1%); when the above three parameters meet the standard simultaneously and remain stable for a certain period of time (e.g., 0.5h), the reaction can be judged to be completely finished.

[0102] S3 lithium deposition:

[0103] The mother liquor of S2 is cooled and a precipitant is continuously added at a rate of 0.13 mol / L·min; the addition is stopped when the lithium content in the solution drops to a predetermined value; the cooling rate is 0.2-0.5 °C / min.

[0104] The precipitant is sodium carbonate;

[0105] When the solution temperature is greater than 60℃, the cooling rate is 0.5℃ / min; when the solution temperature is less than or equal to 60℃, the cooling rate is 0.2℃ / min.

[0106] The lithium content of the solution during the lithium precipitation step is detected, wherein the predetermined value is a lithium content of ≤0.5% in the solution;

[0107] S4 separation:

[0108] The precipitate in the solution after the S3 reaction was separated to obtain high-purity lithium carbonate.

[0109] Comparative Example 6

[0110] S1 purification:

[0111] A 20 g / L lithium sulfate solution and a 75 g / L potassium-sodium mixed salt solution were heated to 80 °C, and sodium hydroxide solution a was added for preliminary impurity removal. The molar ratio of lithium sulfate to potassium-sodium mixed salt was 1:0.3; the molar ratio of sodium hydroxide to lithium sulfate was 2:1. Based on the molar amount of sodium hydroxide added, a 5% sodium hydroxide solution was added first, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1; then a 10% sodium hydroxide solution was added, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1.

[0112] After removing solid impurities by filtration or settling, phosphate is added for secondary impurity removal. After removing solid impurities by filtration or settling, a lithium sulfate solution is obtained. The amount of phosphate to be added is determined according to the pH value of the solution. The pH value of the solution is measured at the same time as the phosphate is added. When the pH value of the solution is 7, the addition of phosphate is stopped.

[0113] S2 molten alkali:

[0114] Add sodium hydroxide solution b to the purified lithium sulfate solution (S1) with a molar ratio of sodium hydroxide to lithium sulfate of 2.5:1; stir and control the reaction temperature at 100℃; react for 3 hours; obtain the mother liquor after the reaction is complete.

[0115] The reaction pressure was controlled at 0.05 MPa;

[0116] The pH value is stable at 7.0±0.2; the conductivity changes to PLATEAU; the lithium content is detected to be only trace (<1%); when the above three parameters meet the standard simultaneously and remain stable for a certain period of time (e.g., 0.5h), the reaction can be judged to be completely finished.

[0117] S3 lithium deposition:

[0118] The mother liquor of S2 is cooled and a precipitant is continuously added at a rate of 0.13 mol / L·min; the addition is stopped when the lithium content in the solution drops to a predetermined value; the cooling rate is 0.2-0.5 °C / min.

[0119] The precipitant is sodium carbonate;

[0120] When the solution temperature is greater than 60℃, the cooling rate is 1℃ / min; when the solution temperature is less than or equal to 60℃, the cooling rate is 0.5℃ / min.

[0121] The lithium content of the solution during the lithium precipitation step is detected, wherein the predetermined value is a lithium content of ≤0.5% in the solution;

[0122] S4 separation:

[0123] The precipitate in the solution after the S3 reaction was separated to obtain high-purity lithium carbonate.

[0124] Comparative Example 7

[0125] S1 purification:

[0126] A 20 g / L lithium sulfate solution and a 75 g / L potassium-sodium mixed salt solution were heated to 80 °C, and sodium hydroxide solution a was added for preliminary impurity removal. The molar ratio of lithium sulfate to potassium-sodium mixed salt was 1:0.3; the molar ratio of sodium hydroxide to lithium sulfate was 2:1. Based on the molar amount of sodium hydroxide added, a 5% sodium hydroxide solution was added first, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1; then a 10% sodium hydroxide solution was added, with a molar ratio of sodium hydroxide to lithium sulfate of 1:1.

[0127] After removing solid impurities by filtration or settling, phosphate is added for secondary impurity removal. After removing solid impurities by filtration or settling, a lithium sulfate solution is obtained. The amount of phosphate to be added is determined according to the pH value of the solution. The pH value of the solution is measured at the same time as the phosphate is added. When the pH value of the solution is 7, the addition of phosphate is stopped.

[0128] S2 molten alkali:

[0129] Add sodium hydroxide solution b to the purified lithium sulfate solution (S1) with a molar ratio of sodium hydroxide to lithium sulfate of 2.5:1; stir and control the reaction temperature at 100℃; react for 3 hours; obtain the mother liquor after the reaction is complete.

[0130] The reaction pressure was controlled at 0.05 MPa;

[0131] The pH value is stable at 7.0±0.2; the conductivity changes to PLATEAU; the lithium content is detected to be only trace (<1%); when the above three parameters meet the standard simultaneously and remain stable for a certain period of time (e.g., 0.5h), the reaction can be judged to be completely finished.

[0132] S3 lithium deposition:

[0133] The mother liquor of S2 was cooled to 20°C, and a precipitant was continuously added at a rate of 0.13 mol / L·min; the addition was stopped when the lithium content in the solution dropped to a predetermined value; the cooling rate was 0.2-0.5°C / min.

[0134] The precipitant is sodium carbonate;

[0135] The cooling rate is 1℃ / min;

[0136] The lithium content of the solution during the lithium precipitation step is detected, wherein the predetermined value is a lithium content of ≤0.5% in the solution;

[0137] S4 separation:

[0138] The precipitate in the solution after the S3 reaction was separated to obtain high-purity lithium carbonate.

[0139] The differences between the preparation methods of the examples and the comparative examples are shown in the table below.

[0140]

[0141] The high-purity lithium carbonate prepared in each embodiment and comparative example was tested using the following various test methods:

[0142] 1. Lithium carbonate purity testing method: The purity of lithium carbonate is tested by X-ray diffraction analysis;

[0143] 2. Yield detection method: Calculate the lithium carbonate yield of the examples and comparative examples, that is, the ratio of the amount of lithium carbonate produced to the amount of raw materials consumed;

[0144] 3. Lithium carbonate particle size distribution test: The size and morphology of the crystals are evaluated using a microscope and a particle size analyzer.

[0145]

[0146] As can be seen from the above embodiments, the present invention can prepare high-purity lithium carbonate by controlling the cooling rate and the rate of adding precipitant, thereby increasing the reaction rate and reducing production energy consumption; by controlling the cooling rate, uneven crystallization of lithium carbonate caused by excessively rapid cooling can be avoided.

Claims

1. A method for preparing high-purity lithium carbonate, characterized in that, include: S1 purification: Heat 18-22 g / L lithium sulfate solution and 70-80 g / L potassium sodium mixed salt solution to 75-85℃, and add sodium hydroxide solution for preliminary impurity removal; After removing solid impurities by filtration or settling, phosphate is added for secondary impurity removal. After removing solid impurities by filtration or settling, lithium sulfate solution is obtained. S2 molten alkali: Add sodium hydroxide solution to the purified lithium sulfate solution after removing impurities from S1, stir and control the reaction temperature at 80-100℃; react for 2-8 hours; after the reaction is complete, obtain the mother liquor; S3 lithium deposition: The mother liquor of S2 was cooled to 20°C, and a precipitant was continuously added at a rate of 0.10-0.15 mol / L·min; the addition was stopped when the lithium content in the solution dropped to a predetermined value. S4 separation: The precipitate in the solution after the S3 reaction was separated to obtain high-purity lithium carbonate; In step S1, the sodium hydroxide solution is a 5%-15% concentration sodium hydroxide solution, and the molar ratio of sodium hydroxide to lithium sulfate in the sodium hydroxide solution and the lithium sulfate solution is 2:

1. In step S2, when the sodium hydroxide solution is melted, the sodium hydroxide solution is a 10%-15% concentration sodium hydroxide solution, and the molar ratio of sodium hydroxide solution to lithium sulfate solution is (2-2.5):

1. In step S3, lithium deposition, the cooling rate is 0.2-0.5℃ / min.

2. The method for preparing high-purity lithium carbonate according to claim 1, characterized in that, In step S2, when the alkali is melted, the reaction pressure is controlled at 0.05-0.08 MPa.

3. The method for preparing high-purity lithium carbonate according to claim 1, characterized in that, In step S2, during the alkali melting process, the pH value, conductivity, and lithium ion concentration in the solution are monitored to determine the end of the reaction.

4. The method for preparing high-purity lithium carbonate according to claim 1, characterized in that, In step S3, lithium precipitation, the precipitant is sodium carbonate.

5. The method for preparing high-purity lithium carbonate according to claim 1, characterized in that, In step S3, lithium precipitation, the predetermined value is that the lithium content in the solution is ≤0.5%.

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