Lithium carbonate separation process for freezing, evaporating, thermally crystallizing and decarburizing liquid after lithium precipitation

Abstract

The invention belongs to the technical field of recycling of lithium precipitation mother liquor, and particularly relates to a lithium carbonate separation process for freezing, evaporating, thermally crystallizing and decarburizing liquid after lithium precipitation, which is technically characterized by comprising the following operation steps: evaporating impurity-removed lithium liquid to precipitate lithium for the first time; freezing liquid after lithium precipitation to separate sodium sulfate; carrying out heating crystallization on the frozen filtrate, and simultaneously evaporating a part of water to obtain lithium carbonate crystals and crystallized filtrate; decarburization; performing evaporation; depositing lithium for the second time; the secondary lithium precipitation solution and the decarbonized solution are mixed and jointly enter an evaporation process; the lithium-containing solution comprises sulfate ions, sodium ions, potassium ions and lithium ions; the lithium-containing solution comprises sulfate ions, sodium ions, potassium ions and lithium ions. According to the method, the filtrate is directly heated, residual lithium carbonate in the solution is further separated out, the yield of lithium carbonate is improved, and meanwhile the phenomenon that in the presence of sulfate radicals, lithium ions, sodium ions and potassium ions generate the same ion effect, and the purity of lithium carbonate is affected is avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of recycling lithium precipitation mother liquor, and specifically relates to a lithium carbonate separation process in which the lithium precipitation liquid is first frozen and then evaporated for thermal crystallization before decarbonization. Background Art

[0002] The lithium-containing solution after industrial lithium extraction still contains a large amount of lithium ions, sodium ions and potassium ions. How to recover lithium under the interference of sodium ions and potassium ions has always been a problem that needs to be urgently solved in the industry.

[0003] In the prior art, usually chemical methods are used to remove impurities to obtain a mother liquor containing lithium hydroxide, and then evaporation and extraction technology are used to finally obtain a high-purity lithium carbonate product. However, this technology has problems such as high consumption of chemical agents and the extraction agent carbon tetrachloride is a toxic liquid.

[0004] There are also some processes that use lithium precipitation, evaporation, freezing crystallization and other means to precipitate sodium sulfate to obtain lithium carbonate, thereby achieving the purpose of recovering lithium. However, the purity of the recovered lithium carbonate is not high, and some sodium carbonate is still mixed in, and the recovery rate is low. Summary of the invention

[0005] The purpose of the present invention is to provide a lithium carbonate separation process in which the liquid after lithium precipitation is first frozen and then evaporated for thermal crystallization before decarbonization. After lithium precipitation, the sodium sulfate is first frozen to precipitate, and then heated to raise the temperature. The lithium carbonate remaining in the system is precipitated by utilizing the characteristic that the solubility of lithium carbonate decreases with increasing temperature, and then sulfuric acid is added for decarbonization. The difference in solubility of lithium carbonate and sodium sulfate in water is utilized to reduce the risk of doping with sodium sulfate and lithium carbonate, and greatly improve the purity of separated lithium carbonate and sodium sulfate.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions: The present invention provides a lithium carbonate separation process in which the liquid after lithium precipitation is first frozen and then evaporated for thermal crystallization before decarbonization, comprising the following steps: First lithium precipitation: adding sodium carbonate in an amount equimolar to lithium ions to the lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate; Separation of sodium sulfate: freezing and crystallizing the solution after the first lithium precipitation to precipitate sodium sulfate and obtain a frozen filtrate; Hot melt crystallization: heating the frozen filtrate at a first temperature to evaporate a portion of water to obtain lithium carbonate and a filtrate after crystallization; Decarbonization: add sulfuric acid to the filtrate after crystallization to obtain a decarbonized solution; Evaporation: heating the decarbonized solution at a second temperature to evaporate, to obtain potassium and sodium salts and a filtrate after evaporation; Second lithium precipitation: adding sodium carbonate in an amount equimolar to the lithium ions to the filtrate after evaporation to obtain a second lithium precipitation solution and lithium carbonate; The second lithium precipitation solution is mixed with the decarbonized solution and enters the evaporation process together; The lithium-containing solution comprises sulfate ions, sodium ions, potassium ions and lithium ions.

[0007] In the present invention, since the lithium-containing solution contains sulfate ions, sodium ions and potassium ions, after the first lithium precipitation, a large amount of sulfate ions, sodium ions, lithium ions and potassium ions are still present in the filtrate. If decarbonization is performed at this time, the remaining lithium carbonate in the system will be converted into lithium sulfate. During evaporation and crystallization, lithium sodium salt of sulfate will be generated due to the common ion effect, which affects the yield and purity of lithium carbonate. In the present invention, the mother liquor after lithium precipitation is first frozen to precipitate sodium sulfate, and then the filtrate is directly heated. The residual lithium carbonate is precipitated by utilizing the principle that the solubility of lithium carbonate in water decreases as the temperature increases. While further recovering the lithium carbonate, the common ion effect is avoided, thereby improving the purity and yield of the lithium carbonate.

[0008] Furthermore, during the first lithium deposition, the temperature of the lithium-containing solution is 60-95°C.

[0009] Furthermore, the first temperature is 60~95°C.

[0010] When the temperature rises to 60-95°C, the solubility of lithium carbonate in water further decreases to below 1g / 100g water, causing the remaining lithium carbonate in the system to further precipitate.

[0011] Furthermore, the heating temperature of the hot melt crystallization is determined according to the solubility curve of lithium carbonate in water and the solubility curve of sodium sulfate in water.

[0012] Furthermore, the first temperature is preferably 70-80°C. When the temperature rises above 80°C, the solubility of lithium carbonate will not continue to decrease, but lithium carbonate will continue to precipitate as the water evaporates. If the temperature is further increased, the water will evaporate too quickly and the lithium carbonate will precipitate too quickly, causing the lithium carbonate to bring out some sodium sulfate. Therefore, the optimal range of the first temperature is 70-80°C.

[0013] Furthermore, in the filtrate after crystallization, the content of lithium carbonate is 0.82~1.23%, and the Li content is 2g / L~3g / L.

[0014] In the present invention, the content of lithium carbonate in the solution after freeze crystallization is further increased, approaching the mass fraction of the saturated solution at the freeze crystallization temperature. Therefore, during the heating process, as the solubility of lithium carbonate decreases, a portion of lithium carbonate is precipitated. After further heating to evaporate part of the water, more lithium carbonate will precipitate in the solution. Therefore, in the filtrate after crystallization, the content of lithium carbonate will be reduced to 0.1-0.25%, which is much smaller than the mass fraction of the saturated solution of lithium carbonate at the first temperature. The present invention adopts the above method to precipitate lithium carbonate to the maximum extent, thereby achieving the purpose of increasing the yield of lithium carbonate.

[0015] In the present invention, according to the solubility curves of lithium carbonate and sodium sulfate in water, when the temperature rises to 50°C, the solubility of sodium sulfate in water tends to remain unchanged as the temperature changes. Therefore, in the present invention, by comparing the solubility of sodium sulfate and lithium carbonate in a certain temperature range with the change in the mass fraction of each component before and after hot melt crystallization, a hot melt crystallization temperature that can ensure the precipitation of lithium carbonate but not cause the precipitation of sodium sulfate can be selected.

[0016] Furthermore, in the hot melt crystallization, the amount of water evaporated is calculated based on the content of lithium carbonate and sodium sulfate before and after evaporation and the mass percentage of the saturated solution of lithium carbonate and sodium sulfate at the first temperature.

[0017] Furthermore, the amount of water evaporation is determined according to the following formula: ; Where M1 is the mass of water in the filtrate after crystallization, in t; M is the amount of water evaporated during the hot melt crystallization process, in t; A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, in units of %; a is the mass fraction of sodium sulfate in the filtrate after crystallization, the unit is %; is a coefficient constant, ranging from 0 to 1.

[0018] Furthermore, Calculated according to the following formula: ; Wherein, A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %; a is the mass fraction of sodium sulfate in the filtrate after freezing, in units of %; B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, in units of %; b is the mass fraction of lithium carbonate in the solution after the first lithium precipitation, the unit is %.

[0019] Further, for decarbonization, the molar mass of the added sulfuric acid is equal to that of the lithium carbonate in the filtrate after crystallization.

[0020] Furthermore, the second temperature is 40-60°C.

[0021] Furthermore, calculated by mass fraction, the lithium-containing solution contains the following components: 3-10% sodium sulfate, 10-20% lithium sulfate, 0.5-5% potassium sulfate, and the remainder is water.

[0022] Furthermore, after the first lithium precipitation, the solution is frozen to 0°C or below.

[0023] In summary, the present invention has the following beneficial effects: The present invention provides a lithium carbonate separation process in which the solution after lithium precipitation is first frozen and then evaporated for thermal crystallization before decarbonization. After lithium precipitation and frozen crystallization, the filtrate is directly heated to further precipitate the residual lithium carbonate in the solution, thereby improving the recovery rate of lithium carbonate and avoiding the common ion effect between lithium ions, sodium ions and potassium ions in the presence of sulfate ions, thereby affecting the purity of lithium carbonate. DETAILED DESCRIPTION

[0024] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined purpose of the invention, a lithium carbonate separation process proposed by the present invention in which the liquid after lithium precipitation is first frozen and then evaporated and thermally crystallized before decarbonization, its specific implementation method, characteristics and effects are described in detail as follows.

[0025] The present specific embodiment provides a lithium carbonate separation process of first hot melt crystallization and then decarbonization. After lithium precipitation, sodium sulfate is first frozen to precipitate, and then heated to raise the temperature. The solubility of lithium carbonate decreases with increasing temperature to precipitate the remaining lithium carbonate in the system, and then sulfuric acid is added for decarbonization. The difference in solubility of lithium carbonate and sodium sulfate in water is used to reduce the risk of doping with sodium sulfate and lithium carbonate, thereby greatly improving the purity of the separated lithium carbonate.

[0026] The above technical objectives of the present invention are achieved through the following technical solutions: The present invention provides a lithium carbonate separation process of first hot melting and crystallization followed by decarburization, comprising the following steps: S1, first lithium precipitation: adding sodium carbonate in an amount equimolar to lithium ions to the lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate; S2, separation of sodium sulfate: freezing and crystallizing the solution after the first lithium precipitation to precipitate sodium sulfate and obtain a frozen filtrate; S3, hot melt crystallization: heating the frozen filtrate at a first temperature to obtain lithium carbonate and a crystallized filtrate; S4, decarbonization: adding sulfuric acid to the filtrate after crystallization to obtain a decarbonized solution; S5, evaporation: heating the decarbonized solution at a second temperature to evaporate, to obtain potassium and sodium salts and a filtrate after evaporation; S6, second lithium precipitation: adding sodium carbonate in an amount equimolar to the lithium ions to the filtrate after evaporation to obtain a second lithium precipitation solution and lithium carbonate; S7, mixing the second lithium precipitation solution with the decarbonized solution and entering the evaporation process together; The lithium-containing solution contains: sulfate ions, sodium ions, potassium ions and lithium ions.

[0027] There are a large number of sulfate ions, sodium ions and potassium ions in the lithium-containing solution. If decarbonization is performed directly after the first lithium precipitation, the remaining lithium carbonate in the solution will react with sulfuric acid to form lithium sulfate. Since lithium sulfate and sodium sulfate have similar solubility, a mixture will be generated due to the common ion effect during evaporation and crystallization, thereby reducing the purity of lithium carbonate and sodium sulfate. In this specific embodiment, by first heating to precipitate lithium carbonate and then performing the decarbonization process, lithium carbonate can be recovered to the maximum extent to avoid such problems.

[0028] In some preferred embodiments, during the first lithium precipitation, the temperature of the lithium-containing solution is 60-95°C.

[0029] In this specific embodiment, the solubility of lithium carbonate in water decreases with increasing temperature. In the temperature range of 60~95°C, the solubility of lithium carbonate is low, which is conducive to the precipitation and separation of lithium carbonate, which helps to maximize the recovery rate of lithium carbonate while reducing the residual lithium ions in the solution. At the same time, impurities such as sodium sulfate and potassium sodium salt have relatively high solubility at higher temperatures. Therefore, in the temperature range of 60~95°C, these impurities are more likely to remain in the solution, while lithium carbonate is more likely to precipitate, thereby improving the purity of lithium carbonate.

[0030] In some preferred embodiments, the temperature of the hot melt crystallization is 60-95°C.

[0031] When the temperature rises to 60-95°C, the solubility of lithium carbonate in water further decreases to below 1g / 100g water, causing the remaining lithium carbonate in the system to further precipitate.

[0032] In some preferred embodiments, the heating temperature of the hot melt crystallization is determined according to the solubility curve of lithium carbonate in water and the solubility curve of sodium sulfate in water.

[0033] Specifically, the first temperature is preferably 70-80°C. The solubility of lithium carbonate is about 1.5 g / L at 50°C, and drops to about 0.5 g / L at 80°C, while the solubility of sodium sulfate in the same temperature range is basically maintained at above 20 g / L; at the same time, when the temperature rises to above 80°C, the solubility of lithium carbonate will not continue to decrease, but lithium carbonate will continue to precipitate as the water evaporates. If the temperature is raised again, the water will evaporate too quickly, and the precipitation speed of lithium carbonate will be too fast, causing lithium carbonate to take out part of the sodium sulfate. Therefore, the optimal range of the first temperature is 70-80°C.

[0034] With respect to this specific embodiment, the content of lithium carbonate and sodium sulfate in the filtrate can be determined by selecting a moderate temperature in the optimal temperature range provided above, such as 70°C, 75°C, etc. At this time, the solubility of lithium carbonate has been significantly reduced, which is convenient for precipitation, while sodium sulfate has not reached a saturated solution at this temperature, so sodium sulfate still remains in the solution; through this method, the optimal hot melt crystallization heating temperature can be determined to ensure the efficient precipitation of lithium carbonate, while keeping sodium sulfate in the solution, thereby improving the efficiency of the overall process and the purity of the product.

[0035] In some preferred embodiments, the content of lithium carbonate in the filtrate after crystallization is 0.82~1.23%, and the Li content is 2g / L~3g / L. In the solution after frozen crystallization, the content of lithium carbonate further increases, approaching the mass fraction of the saturated solution at the frozen crystallization temperature. Therefore, during the heating process, as the solubility of lithium carbonate decreases, a part of lithium carbonate is precipitated. After further heating and evaporation of part of the water, more lithium carbonate will be precipitated in the solution. Therefore, in the filtrate after crystallization, the content of lithium carbonate will be reduced to 0.82~1.23%, which is much less than the mass fraction of the saturated solution of lithium carbonate at the first temperature. The present invention adopts the above method to precipitate lithium carbonate to the maximum extent, thereby achieving the purpose of increasing the yield of lithium carbonate.

[0036] In some preferred embodiments, the amount of water evaporated during hot melt crystallization is calculated based on the content of lithium carbonate and sodium sulfate before and after evaporation and the mass percentage of a saturated solution of lithium carbonate and sodium sulfate at the evaporation temperature.

[0037] Wherein, the evaporation amount of water is preferably determined according to the following formula: According to the above data, in step S3, the evaporation amount of water is determined according to the following formula: ; Where M1 is the mass of water in the filtrate after crystallization, in t; M is the amount of water evaporated during the hot melt crystallization process, in t; A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, in units of %; a is the mass fraction of sodium sulfate in the filtrate after freezing, in units of %; is a coefficient constant, ranging from 0 to 1.

[0038] According to the definition of the above formula, it can be ensured that during the evaporation process, the lithium carbonate after hot melt crystallization is evaporated and crystallized, while ensuring that sodium sulfate still exists in the filtrate, thereby improving the purity of the lithium carbonate.

[0039] Chinese The maximum evaporation of water can be calculated, but considering the stability of the control process and the problems caused by avoiding excessive evaporation, as well as the problem that some sodium ions are taken out by lithium ions due to the common ion effect during the evaporation process, the maximum evaporation of water is calculated. As a coefficient, the evaporation of water is further limited.

[0040] In some preferred embodiments, Calculated according to the following formula: ; Wherein, A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %; a is the mass fraction of sodium sulfate in the filtrate after freezing, in units of %; B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, in units of %; b is the mass fraction of lithium carbonate in the filtrate after freezing, in %.

[0041] The ratio between the actual evaporation and the maximum evaporation is calculated by formulating , can significantly optimize the purification process, not only ensuring that only lithium carbonate is precipitated and improving product purity, but also ensuring that as much lithium carbonate as possible is recovered and reducing losses.

[0042] In the decarbonization step of some preferred embodiments, the molar mass of the added sulfuric acid is equal to the molar mass of the lithium carbonate in the filtrate after crystallization.

[0043] In this specific embodiment, since residual lithium carbonate is further precipitated by heating before decarbonization, the carbonate in the system is further reduced, and the amount of sulfuric acid added in the decarbonization step is reduced, thereby reducing the introduction of sulfate ions and facilitating the subsequent steps.

[0044] In some preferred embodiments, the second temperature is 40-60° C. If the evaporation temperature is higher than 60° C., the solubility of lithium sulfate in the system will decrease, and part of the lithium sulfate will precipitate together with potassium and sodium salts, affecting the yield of lithium carbonate.

[0045] In some preferred embodiments, the lithium-containing solution contains the following components, calculated by mass fraction: 3-10% sodium sulfate, 10-20% lithium sulfate, 0.5-5% potassium sulfate, and the remainder is water. Among the above components, the content of sodium sulfate and lithium sulfate is relatively high. The separation method provided in this specific embodiment can effectively separate the sodium ions and lithium ions in the system by freezing to precipitate sodium sulfate and then heating to precipitate sodium carbonate and lithium, respectively, to obtain lithium carbonate and sodium sulfate with higher purity.

[0046] In some preferred embodiments, the solution is frozen to 0° C. or below after the first lithium precipitation. When the freezing temperature is reduced to 0° C. or below, the solubility of lithium carbonate in the system gradually increases with the decrease in temperature, thereby preventing the sodium sulfate crystals precipitated by freezing from being doped with lithium carbonate, thereby greatly improving the purity of the sodium sulfate.

[0047] Example 1 In this embodiment, a lithium-containing solution is used as a raw material, and a lithium carbonate separation process of first hot melting and crystallization followed by decarbonization provided by the present invention is adopted to prepare pure lithium carbonate and sodium sulfate, wherein the lithium-containing solution comprises the following components calculated by mass percentage: 6% sodium sulfate, 15% lithium sulfate and 2.1% potassium sulfate, and the balance is water.

[0048] The specific process steps of the lithium carbonate separation process of hot melting crystallization followed by decarbonization are as follows: S1, first lithium precipitation: heating the lithium-containing solution to 90° C., and adding sodium carbonate in an amount equimolar to the lithium ions to the lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate; S2, separation of sodium sulfate: freezing the solution after the first lithium precipitation to 0°C for crystallization, precipitating sodium sulfate, and obtaining a frozen filtrate; S3, hot melt crystallization: heating the frozen filtrate to 70°C to evaporate a portion of water to obtain lithium carbonate and the filtrate after crystallization; S4, decarbonization: adding 98% sulfuric acid in an amount equal to the molar amount of lithium carbonate in the crystallization filtrate to the crystallization filtrate to obtain a decarbonized solution; S5, evaporation: heating the decarbonized solution to 60°C, evaporating to obtain potassium and sodium salts and a filtrate after evaporation; S6, second lithium precipitation: adding sodium carbonate in an amount equimolar to the lithium ions to the filtrate after evaporation to obtain a second lithium precipitation solution and lithium carbonate; S7, mixing the second lithium precipitation solution with the decarbonized solution and entering the evaporation process together.

[0049] Calculated by mass percentage, the solution after the first lithium precipitation contains: 19.13% sodium sulfate, 0.22% lithium carbonate and 1.6% potassium sulfate, and the balance is water; the filtrate after freezing contains: 5% sodium sulfate, 1.8% lithium carbonate and 2.55% potassium sulfate, and the balance is water.

[0050] According to the above data, in step S3, the evaporation amount of water is determined according to the following formula: ; Where M1 is the mass of water in the filtrate after crystallization, in t; M is the amount of water evaporated during the hot melt crystallization process, in t; A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, in units of %; a is the mass fraction of sodium sulfate in the filtrate after crystallization, the unit is %; is a coefficient constant, ranging from 0 to 1.

[0051] In this embodiment, Calculated according to the following formula: ; Wherein, A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %; a is the mass fraction of sodium sulfate in the filtrate after freezing, in units of %; B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, in units of %; b is the mass fraction of lithium carbonate in the solution after the first lithium precipitation, the unit is %.

[0052] According to the solubility curve data, at 70°C, the mass fraction of the saturated solution of lithium carbonate is 0.91%; the mass fraction of the saturated solution of sodium sulfate is 31.3%; then, =0.9.

[0053] The calculation result is: M / M1=79.6%.

[0054] The filtrate after crystallization contains: 19.2% sodium sulfate, 0.82% lithium carbonate and 9.75% potassium sulfate, and the balance is water; the solution after decarbonization contains: 18.8% sodium sulfate, 0.82% lithium sulfate and 9.54% potassium sulfate, and the balance is water; the filtrate after evaporation contains: 5.2% sodium sulfate, 5% lithium sulfate and 2.5% potassium sulfate.

[0055] Example 2 In this embodiment, a lithium-containing solution is used as a raw material, and a lithium carbonate separation process of first hot melting and crystallization followed by decarbonization provided by the present invention is adopted to prepare pure lithium carbonate and sodium sulfate, wherein the lithium-containing solution comprises the following components calculated by mass percentage: 5% sodium sulfate, 17% lithium sulfate and 0.8% potassium sulfate, and the balance is water.

[0056] The specific process steps of the lithium carbonate separation process of hot melting crystallization followed by decarbonization are as follows: S1, first lithium precipitation: heating the lithium-containing solution to 95° C., and adding sodium carbonate in an amount equimolar to the lithium ions to the lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate; S2, separation of sodium sulfate: freezing the solution after the first lithium precipitation to 0°C for crystallization, precipitating sodium sulfate, and obtaining a frozen filtrate; S3, hot melt crystallization: heating the frozen filtrate to 80°C to evaporate a portion of water to obtain lithium carbonate and the filtrate after crystallization; S4, decarbonization: adding 98% sulfuric acid in an amount equal to the molar amount of lithium carbonate in the crystallization filtrate to the crystallization filtrate to obtain a decarbonized solution; S5, evaporation: heating the decarbonized solution to 60°C, evaporating to obtain potassium and sodium salts and a filtrate after evaporation; S6, second lithium precipitation: adding sodium carbonate in an amount equimolar to the lithium ions to the filtrate after evaporation to obtain a second lithium precipitation solution and lithium carbonate; S7, mixing the second lithium precipitation solution with the decarbonized solution and entering the evaporation process together.

[0057] Calculated by mass percentage, the solution after the first lithium precipitation contains: 16.1% sodium sulfate, 0.25% lithium carbonate and 0.61% potassium sulfate, and the balance is water; the filtrate after freezing contains: 4.1% sodium sulfate, 2.04% lithium carbonate and 0.65% potassium sulfate, and the balance is water.

[0058] According to the above data, in step S3, the evaporation amount of water is determined according to the following formula: ; Where M1 is the mass of water in the filtrate after crystallization, in t; M is the amount of water evaporated during the hot melt crystallization process, in t; A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, in units of %; a is the mass fraction of sodium sulfate in the filtrate after crystallization, the unit is %; is a coefficient constant, ranging from 0 to 1.

[0059] In this embodiment, Calculated according to the following formula: ; Wherein, A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %; a is the mass fraction of sodium sulfate in the filtrate after freezing, in units of %; B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, in units of %; b is the mass fraction of lithium carbonate in the solution after the first lithium precipitation, the unit is %.

[0060] According to the solubility curve data, at 80°C, the mass fraction of the saturated solution of lithium carbonate is 0.84%; the mass fraction of the saturated solution of sodium sulfate is 30.4%; then: =0.81; The calculation result is: M / M1=73.0%.

[0061] The filtrate after crystallization contains: 17.3% sodium sulfate, 1.23% lithium carbonate and 2.77% potassium sulfate, and the balance is water; the solution after decarbonization contains: 16.94% sodium sulfate, 1.23% lithium sulfate and 2.71% potassium sulfate, and the balance is water; the filtrate after evaporation contains: 5.36% sodium sulfate, 5.75% lithium sulfate and 0.71% potassium sulfate.

[0062] Comparative Example 1 In this embodiment, a lithium-containing solution is used as a raw material, and a lithium carbonate separation process of first freezing and then decarbonization provided by the present invention is adopted to prepare pure lithium carbonate and sodium sulfate, wherein the lithium-containing solution comprises the following components calculated by mass percentage: 6% sodium sulfate, 15% lithium sulfate and 2.1% potassium sulfate, and the balance is water.

[0063] The specific process steps of the lithium carbonate separation process of freezing first and then decarbonization are as follows: S1, first lithium precipitation: heating the lithium-containing solution to 90° C., and adding sodium carbonate in an amount equimolar to the lithium ions to the lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate; S2, separation of sodium sulfate: freezing the solution after the first lithium precipitation to 0°C for crystallization, precipitating sodium sulfate, and obtaining a frozen filtrate; S3, decarbonization: adding 98% sulfuric acid in an amount equal to the molar amount of lithium carbonate in the crystallization filtrate to the crystallization filtrate to obtain a decarbonized solution; S4, evaporation: heating the decarbonized solution to 60°C, evaporating to obtain potassium and sodium salts and a filtrate after evaporation; S5, second lithium precipitation: adding sodium carbonate in an amount equimolar to the lithium ions to the filtrate after evaporation to obtain a second lithium precipitation solution and lithium carbonate; S6. The second lithium precipitation solution is mixed with the decarbonized solution and enters the evaporation process together.

[0064] Among them, calculated by mass percentage, the solution after the first lithium precipitation contains: 19.13% sodium sulfate, 0.22% lithium carbonate and 1.6% potassium sulfate, and the balance is water; the filtrate after freezing contains: 5% sodium sulfate, 2.8% lithium carbonate and 2.55% potassium sulfate, and the balance is water; the solution after decarbonization contains: 4.8% sodium sulfate, 2.8% lithium sulfate and 2.45% potassium sulfate, and the balance is water; the filtrate after evaporation contains: 1.32% sodium sulfate, 0.5% lithium sulfate and 0.67% potassium sulfate. In this comparative example, since there is one less step of hot melt crystallization than in Example 1, the amount of water evaporated is 79.2% less than that in Example 1, resulting in the content of each solute in the evaporated filtrate before the second lithium precipitation being less than that in Example 1. During the second lithium precipitation, less lithium carbonate is obtained and the purity is lower.

[0065] Comparative Example 2 In this embodiment, a lithium-containing solution is used as a raw material, and a lithium carbonate separation process of first freezing and then decarbonization provided by the present invention is adopted to prepare pure lithium carbonate and sodium sulfate, wherein the lithium-containing solution comprises the following components calculated by mass percentage: 6% sodium sulfate, 15% lithium sulfate and 2.1% potassium sulfate, and the balance is water.

[0066] The specific process steps of the lithium carbonate separation process of freezing first and then decarbonization are as follows: S1, first lithium precipitation: heating the lithium-containing solution to 90° C., and adding sodium carbonate in an amount equimolar to the lithium ions to the lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate; S2, separation of sodium sulfate: freezing the solution after the first lithium precipitation to 0°C for crystallization, precipitating sodium sulfate, and obtaining a frozen filtrate; S3, hot melt crystallization: heating the frozen filtrate to 70°C to evaporate a portion of water to obtain lithium carbonate and the filtrate after crystallization; S4, decarbonization: adding 98% sulfuric acid in an amount equal to the molar amount of lithium carbonate in the crystallization filtrate to the crystallization filtrate to obtain a decarbonized solution; S5, evaporation: heating the decarbonized solution to 60°C, evaporating to obtain potassium and sodium salts and a filtrate after evaporation; S6, second lithium precipitation: adding sodium carbonate in an amount equimolar to the lithium ions to the filtrate after evaporation to obtain a second lithium precipitation solution and lithium carbonate; S7, mixing the second lithium precipitation solution with the decarbonized solution and entering the evaporation process together.

[0067] Calculated by mass percentage, the solution after the first lithium precipitation contains: 19.13% sodium sulfate, 0.22% lithium carbonate and 1.6% potassium sulfate, and the balance is water; the filtrate after freezing contains: 5% sodium sulfate, 1.8% lithium carbonate and 2.55% potassium sulfate, and the balance is water.

[0068] According to the above data, in step S3, the evaporation amount of water is determined according to the following formula: ; Where M1 is the mass of water in the filtrate after crystallization, in t; M is the amount of water evaporated during the hot melt crystallization process, in t; A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, in units of %; a is the mass fraction of sodium sulfate in the filtrate after crystallization, the unit is %; The calculation result is: M / M1=88.4%.

[0069] The filtrate after crystallization contains: 20.3% sodium sulfate, 3.18% lithium carbonate and 10.5% potassium sulfate, and the balance is water; the solution after decarbonization contains: 19.6% sodium sulfate, 3.18% lithium sulfate and 9.94% potassium sulfate, and the balance is water; the filtrate after evaporation contains: 5.41% sodium sulfate, 4.5% lithium sulfate and 2.73% potassium sulfate.

[0070] The purity of the lithium carbonate and sodium sulfate obtained in each step of Examples 1-2 and Comparative Examples 1-2 was calculated, and the final yield of lithium carbonate was calculated. The results are shown in Table 1.

[0071] Table 1. Purity of lithium carbonate, sodium sulfate and lithium carbonate yield of Examples 1-2 and Comparative Examples 1-2 According to the above data, after reducing the hot melt crystallization step, the final yield of lithium carbonate decreases; when the hot melt crystallization adopts the maximum evaporation amount, the purity and yield of lithium carbonate obtained by hot melt crystallization are reduced.

[0072] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been presented as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A lithium carbonate separation process in which the solution after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: The steps are as follows: First lithium precipitation: adding sodium carbonate in an amount equimolar to lithium ions to the lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate; Separation of sodium sulfate: freezing and crystallizing the solution after the first lithium precipitation to precipitate sodium sulfate and obtain a frozen filtrate; Hot melt crystallization: heating the frozen filtrate to a first temperature to evaporate a portion of water to obtain lithium carbonate and a filtrate after crystallization; Decarbonization: add sulfuric acid to the filtrate after crystallization to obtain a decarbonized solution; Evaporation: heating the decarbonized solution at a second temperature to evaporate, to obtain potassium and sodium salts and a filtrate after evaporation; Second lithium precipitation: adding sodium carbonate in an amount equimolar to the lithium ions to the filtrate after evaporation to obtain a second lithium precipitation solution and lithium carbonate; The second lithium precipitation solution is mixed with the decarbonized solution and enters the evaporation process together; The lithium-containing solution comprises sulfate ions, sodium ions, potassium ions and lithium ions.

2. A lithium carbonate separation process according to claim 1, wherein the liquid after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: The first temperature is 60~95°C.

3. A lithium carbonate separation process according to claim 2, wherein the liquid after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: The first temperature is determined according to a solubility curve of lithium carbonate in water and a solubility curve of sodium sulfate in water.

4. A lithium carbonate separation process according to claim 2 or 3, wherein the liquid after lithium precipitation is first frozen and then evaporated and thermally crystallized before decarbonization, characterized in that: The first temperature is 70-80°C.

5. The lithium carbonate separation process according to claim 1, wherein the liquid after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: In the filtrate after crystallization, the content of lithium carbonate is 0.82-1.23%, and the Li content is 2 g / L-3 g / L.

6. A lithium carbonate separation process according to claim 5, wherein the liquid after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: The evaporation amount of the water is determined according to the following formula: ； Where M1 is the mass of water in the filtrate after crystallization, in t; M is the amount of water evaporated during the hot melt crystallization process, in t; A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, in units of %; a is the mass fraction of sodium sulfate in the filtrate after crystallization, the unit is %; is a coefficient constant, ranging from 0 to 1.

7. A lithium carbonate separation process according to claim 6, wherein the solution after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: Said Calculated according to the following formula: ； Wherein, A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %; a is the mass fraction of sodium sulfate in the filtrate after freezing, in units of %; B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, in units of %; b is the mass fraction of lithium carbonate in the solution after the first lithium precipitation, the unit is %.

8. The lithium carbonate separation process according to claim 1, wherein the solution after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: The second temperature is 40-60°C.

9. The lithium carbonate separation process according to claim 1, wherein the solution after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: Calculated by mass fraction, the lithium-containing solution contains the following components: 3-10% sodium sulfate, 10-20% lithium sulfate, 0.5-5% potassium sulfate, and the balance is water.

10. The lithium carbonate separation process according to claim 1, wherein the solution after lithium precipitation is first frozen and then evaporated and crystallized before decarbonization, characterized in that: After the first lithium precipitation, the solution is frozen to 0°C or below.

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