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

Abstract

The invention claims the priority of the Chinese patent application CN202411900128.3 submitted on December 23, 2024, and the content (text and claims) of the application is incorporated into the application by reference. 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 comprises the following operation steps: carrying out lithium precipitation for the first time; freezing liquid after lithium precipitation to separate sodium sulfate; the frozen filtrate is subjected to hot melting crystallization to evaporate 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 contains sulfate ions, sodium ions, potassium ions and lithium ions; the filtrate is directly heated, residual lithium carbonate in the solution is further separated out, and the yield and purity of the lithium carbonate are improved.

Description

TECHNICAL FIELD

[0001] The present application claims priority to Chinese patent application CN202411900128.3, filed on December 23, 2024, the contents (text and claims) of which are incorporated herein by reference.

[0002] The present application belongs to the technical field of lithium sulfate solution recycling, and specifically relates to a lithium sulfate solution separation process of freezing and then evaporating and thermal crystallization after lithium precipitation. BACKGROUND

[0003] After industrial lithium extraction, the lithium-containing solution 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 been a problem that needs to be solved in the industry.

[0004] In the prior art, lithium hydroxide-containing mother liquor is first obtained by chemical impurity removal, and then high-purity lithium carbonate product is finally obtained through evaporation and extraction technology. However, this technology has problems such as large consumption of chemical reagents, and the extraction agent carbon tetrachloride is a toxic liquid.

[0005] Some processes use lithium precipitation, evaporation, freezing crystallization and other means to precipitate sodium sulfate and obtain lithium carbonate to achieve the purpose of lithium recovery. However, the purity of the recovered lithium carbonate is not high, and it still contains some sodium carbonate, and the recovery rate is low. SUMMARY

[0006] The purpose of the present application is to provide a lithium sulfate solution separation process of freezing and then evaporating and thermal crystallization after lithium precipitation. After lithium precipitation, sodium sulfate is first precipitated by freezing, then heated and warmed, and the remaining lithium carbonate in the system is precipitated by taking advantage of the property that the solubility of lithium carbonate decreases with increasing temperature. Then, decarburization is carried out by adding sulfuric acid. By taking advantage of the difference in solubility of lithium carbonate and sodium sulfate in water, the risk of sodium sulfate and lithium carbonate contamination is reduced, and the purity of the separated lithium carbonate and sodium sulfate is greatly improved.

[0007] The above technical purpose of the present application is achieved by the following technical scheme:

[0008] The present application provides a lithium sulfate solution separation process of freezing and then evaporating and thermal crystallization after lithium precipitation, which comprises the following operation steps:

[0009] First lithium precipitation: lithium-containing solution is added with sodium carbonate in an equimolar amount with lithium ions to obtain a first lithium precipitation solution and lithium carbonate;

[0010] Separation of sodium sulfate: the first lithium precipitation solution is frozen and crystallized to precipitate sodium sulfate, and a frozen filtrate is obtained;

[0011] Thermal melting and crystallization: the frozen filtrate is heated at a first temperature to evaporate part of the water, and lithium carbonate and a crystallization filtrate are obtained;

[0012] Decarburization: sulfuric acid is added to the filtrate after crystallization to obtain a solution after decarburization;

[0013] Evaporation: the solution after decarburization is heated and evaporated at a second temperature to obtain a potassium-sodium salt and a filtrate after evaporation;

[0014] Second lithium precipitation: sodium carbonate in an equimolar amount of lithium ions is added to the filtrate after evaporation to obtain a second lithium precipitation solution and lithium carbonate;

[0015] The second lithium precipitation solution is mixed with the solution after decarburization and is subjected to the evaporation process together;

[0016] The lithium-containing solution comprises sulfate ions, sodium ions, potassium ions and lithium ions.

[0017] In the present application, since the lithium-containing solution comprises 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 still exist in the filtrate, and if decarburization is performed at this time, the residual lithium carbonate in the system will be converted into lithium sulfate, and during the evaporation and crystallization, lithium sodium sulfate will be produced due to the common ion effect, which affects the yield and purity of lithium carbonate; in the present application, after the lithium precipitation mother liquor is frozen to precipitate sodium sulfate, the filtrate is directly heated by increasing the temperature, and the residual lithium carbonate is precipitated by using the principle that the solubility of lithium carbonate in water decreases with the increase of temperature, thereby further recovering lithium carbonate, avoiding the common ion effect and improving the purity and yield of lithium carbonate.

[0018] Further, the temperature of the lithium-containing solution during the first lithium precipitation is 60-95℃.

[0019] Further, the first temperature is 60-95℃.

[0020] When the temperature is increased to 60-95℃, the solubility of lithium carbonate in water is further decreased to below 1g / 100g of water, so that the residual lithium carbonate in the system is further precipitated.

[0021] Further, 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.

[0022] Further, the first temperature is preferably 70-80℃, and when the temperature is increased to above 80℃, the solubility of lithium carbonate will not continue to decrease, but with the evaporation of water, lithium carbonate continues to precipitate, and if the temperature is further increased, the water will evaporate too fast and the precipitation speed of lithium carbonate will be too fast, which will cause lithium carbonate to carry out part of sodium sulfate, therefore, the best interval of the first temperature is 70-80℃.

[0023] Further, the content of lithium carbonate in the filtrate after crystallization is 0.82-1.23%, and the content of Li is 2 g / L-3 g / L.

[0024] In the present application, the content of lithium carbonate in the solution after freezing crystallization is further increased and approaches the mass fraction of the saturated solution at the freezing crystallization temperature, so that, during the heating process, a part of lithium carbonate is precipitated with the decrease of the solubility of lithium carbonate, and more lithium carbonate is precipitated after further heating and evaporation of part of water, so that the content of lithium carbonate in the filtrate after crystallization is reduced to 0.1-0.25%, which is far less than the mass fraction of the saturated solution of lithium carbonate at the first temperature. The present application uses the above method to maximize the precipitation of lithium carbonate and achieve the purpose of improving the yield of lithium carbonate.

[0025] In the present application, according to the solubility curve of lithium carbonate and sodium sulfate in water, when the temperature is increased to 50℃, the solubility of sodium sulfate in water tends to be constant with the change of temperature, so that, in the present application, the solubility of sodium sulfate and lithium carbonate in a certain temperature range is selected by comparing the mass fraction of each component before and after hot melt crystallization, so as to select a hot melt crystallization temperature which can ensure the precipitation of lithium carbonate and will not cause the precipitation of sodium sulfate.

[0026] Further, in the hot melt crystallization, the evaporation amount of water is calculated according to the content of lithium carbonate, the content of sodium sulfate and the mass percentage of the saturated solution of lithium carbonate and sodium sulfate at the first temperature before and after evaporation.

[0027] Further, the evaporation amount of water is determined according to the following formula:

[0028]

[0029] Wherein, M1 is the mass of water in the filtrate after crystallization, and the unit is t;

[0030] M is the evaporation amount of water in the hot melt crystallization process, and the unit is t;

[0031] A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %;

[0032] a is the mass fraction of sodium sulfate in the filtrate after crystallization, and the unit is %;

[0033] σ is a coefficient constant, and the value range is 0-1.

[0034] Further, σ is calculated according to the following formula:

[0035]

[0036] Wherein, A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %;

[0037] a is the mass fraction of sodium sulfate in the filtrate after freezing, unit is %;

[0038] B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, unit is %;

[0039] b is the mass fraction of lithium carbonate in the solution after the first lithium precipitation, unit is %.

[0040] Further, decarburization, the molar mass of added sulfuric acid and lithium carbonate in the filtrate after crystallization is equal.

[0041] Further, the second temperature is 40-60℃.

[0042] Further, according to the mass fraction, the lithium-containing solution contains the following components: sodium sulfate 3-10%, lithium sulfate 10-20%, potassium sulfate 0.5-5%, and the balance is water.

[0043] Further, the solution after the first lithium precipitation is frozen to 0℃ or below 0℃.

[0044] In summary, the present application has the following beneficial effects:

[0045] The lithium precipitation solution provided by the present application is first frozen and then evaporated and hot crystallized, and then decarburized. After lithium precipitation and freezing crystallization, the filtrate is directly heated to further precipitate the residual lithium carbonate in the solution, thereby improving the recovery rate of lithium carbonate. At the same time, the same ion effect of lithium ions and sodium ions and potassium ions in the presence of sulfate ions is avoided, which affects the purity of lithium carbonate. DETAILED DESCRIPTION

[0046] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined object of the application, the specific implementation, features and effects of the lithium precipitation solution provided by the present application are described in detail as follows.

[0047] Add: The content of this section is the mechanism explanation of the original technical scheme, the data quoted are from the literature published before the priority date, which does not constitute a limitation on the scope of protection of the claims, and does not require the priority effect.

[0048] It should be explained that the present application is based on the effect that the solubility of lithium carbonate decreases significantly with the increase of temperature, and the solubility of sodium sulfate slowly increases with the increase of temperature. The first lithium precipitation is carried out at 60-95℃ to realize physical separation, and ≥80% of lithium carbonate is extracted.

[0049] The solution after the first lithium precipitation mainly contains sodium ions, potassium ions, sulfate ions in the form of sodium sulfate and potassium sulfate, and a small amount of lithium ions that have not reacted completely due to incomplete precipitation, and lithium carbonate that has not been completely precipitated due to saturation.

[0050] The present application is directed to the solution after the first lithium precipitation being frozen to precipitate sodium sulfate decahydrate. At low temperatures, the solubility of lithium carbonate, lithium sulfate and potassium sulfate is relatively high and remains dissolved. After freezing, the filtrate mainly contains potassium ions, lithium ions, sulfate ions in the form of potassium sulfate and lithium sulfate, and a small amount of sodium ions and residual lithium carbonate due to incomplete precipitation of sodium sulfate. Specifically, by using direct cooling with a refrigerant to force freeze and crystallize, the rapid and low-cost precipitation of sodium sulfate decahydrate is achieved, reducing the total process cost.

[0051] In the hot melt crystallization step, the frozen filtrate is heated to 60-95°C to evaporate part of the water, and the concentration multiple can reach 4-8 times. During the process, the solubility of lithium carbonate decreases with the increase of temperature, and the evaporation of water increases the concentration, thereby causing the lithium carbonate that has not been completely precipitated due to saturation to precipitate. Assuming that the frozen filtrate contains a% of lithium carbonate before hot melt crystallization, after being concentrated by m times, the concentration of lithium carbonate is ma%, and the residual concentration in the system after crystallization is still a%, so the actual precipitation rate is (ma%-a%) / ma%. When m is 4, this step extracts at least 75% of the remaining 20% of lithium carbonate. That is, the first lithium precipitation and the hot melt crystallization step can achieve a recovery rate of at least 95%. It should be noted that this step does not involve new chemical reactions, but only physical precipitation of lithium carbonate solids. The filtrate after crystallization contains sodium sulfate, potassium sulfate, lithium sulfate, dissolved lithium carbonate and part of the free lithium ions. Since part of the lithium carbonate has been precipitated by hot melt crystallization, the amount of residual lithium carbonate in the filtrate after crystallization is reduced.

[0052] It should be noted that the first temperature is preferably 70-80°C because when the temperature rises above 80°C, the solubility of lithium carbonate will continue to decrease, and as the water evaporates, lithium carbonate continues to precipitate. However, as the water evaporates, the solubility of sodium sulfate approaches saturation, and if the temperature is further increased, the water will evaporate too quickly and the lithium carbonate will precipitate too quickly, causing some of the lithium carbonate to carry out part of the sodium sulfate. Therefore, the optimal temperature range for the first temperature is 70-80°C.

[0053] Preferably, the hot melt crystallization process uses a staged temperature control, which first rapidly heats to 60°C, and then heats at a rate of 1-2°C / min to the set temperature to evaporate part of the water, avoiding explosive precipitation of lithium carbonate and reducing impurity entrainment.

[0054] Then the present application carries out a decarburization step after hot melt crystallization, specifically adding sulfuric acid to the filtrate after crystallization to react with the lithium carbonate that has not been precipitated and remains dissolved in the filtrate, and the solution after decarburization mainly contains lithium sulfate, sodium sulfate, potassium sulfate, and sulfate. It should be noted here that because at least 90% of the lithium carbonate has been precipitated in the system, the lithium carbonate content is extremely low, so the amount of sulfuric acid to be added is significantly reduced.

[0055] The evaporation operation is to heat and evaporate the solution after decarburization at 40-60°C to supersaturation, and precipitate potassium and sodium salts (mainly a mixture of sodium sulfate and potassium sulfate). The residual components include concentrated lithium sulfate, and a small amount of sodium and potassium sulfate.

[0056] Then the second lithium precipitation operation adds sodium carbonate to the filtrate after evaporation in an amount equal to the molar amount of lithium ions to obtain lithium carbonate solids and a mother liquor containing sodium sulfate and potassium sulfate. Finally, the mother liquor after the second lithium precipitation is mixed with the solution after decarburization and recycled into the evaporation process, and the sulfate in the mother liquor after the second lithium precipitation is also precipitated in the evaporation step.

[0057] The core of the present application is that after the first lithium precipitation, instead of decarburization, the solution is directly subjected to a freeze crystallization process to precipitate sodium sulfate decahydrate, and then after the freeze crystallization of sodium sulfate decahydrate, part of the lithium carbonate is extracted by hot melt crystallization, and then decarburization is carried out, replacing the traditional multiple "lithium precipitation-decarburization" chemical cycles, thereby reducing the amount of sulfuric acid used from the root, eliminating the accumulation of sulfate, avoiding the problem of limited total lithium yield and low product purity caused by repeated co-crystallization of lithium sulfate and sodium sulfate, improving the total lithium yield in the lithium-containing sodium sulfate solution, and obtaining pure sodium sulfate decahydrate as a byproduct.

[0058] As a preferred embodiment, the filtrate after freezing can also be subjected to secondary freezing and centrifugation to further extract sodium sulfate decahydrate, avoid the same ion effect, and improve product purity. Although this step increases the operating cost, on the basis of the direct cooling process with refrigerant, the operating cost can be significantly reduced, and the filtrate after crystallization after extracting a large amount of lithium carbonate by the hot melt crystallization step can significantly reduce the amount of sulfuric acid added during decarburization, thereby offsetting the increased operating cost of the secondary freezing and centrifugation.

[0059] The hot melt crystallization step is not equivalent to conventional evaporation, and is not the evaporation step after the conventional decarburization step is advanced, because the components in the system are different, even if the results obtained by heating are different. In the present application, the hot melt crystallization at the first temperature physically precipitates the residual lithium carbonate by using the solubility difference, so that the amount of lithium sulfate generated by subsequent decarburization is greatly reduced, and only a small amount of dissolved lithium carbonate needs to be treated in the decarburization link, and the consumption of sulfuric acid is also reduced, thereby eliminating the co-crystallization from the source, achieving more than 90% lithium recovery in a single cycle. On this basis, through secondary lithium precipitation and cyclic evaporation, lithium carbonate is further precipitated, achieving a total lithium recovery of ≥98%, and the lithium content in the waste residue is <0.05%.

[0060] The lithium carbonate separation process provided by the embodiment first precipitates sodium sulfate after lithium precipitation, and then heats and warms up to precipitate the residual lithium carbonate in the system by using the property that the solubility of lithium carbonate decreases with the increase of temperature. Then, sulfuric acid is added for decarburization, and the solubility difference of lithium carbonate and sodium sulfate in water is used to reduce the risk of sodium sulfate and lithium carbonate contamination, thereby greatly improving the purity of the separated lithium carbonate.

[0061] The above technical purposes of the present application are achieved by the following technical solutions:

[0062] The lithium carbonate separation process provided by the present application first heats and melts the lithium carbonate, and then decarburizes, and comprises the following operation steps:

[0063] S1, first lithium precipitation: adding sodium carbonate in an equal molar amount of lithium ions to a lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate;

[0064] S2, separate sodium sulfate: freeze crystallization of the first lithium precipitation solution to precipitate sodium sulfate, and obtain a freeze filtrate;

[0065] S3, hot melt crystallization: heating the freeze filtrate at a first temperature to obtain lithium carbonate and a crystallization filtrate;

[0066] S4, decarburization: adding sulfuric acid to the crystallization filtrate to obtain a decarburization solution;

[0067] S5, evaporation: heating and evaporating the decarburization solution at a second temperature to obtain a potassium-sodium salt and an evaporation filtrate;

[0068] S6, second lithium precipitation: adding sodium carbonate in an equal molar amount of lithium ions to the evaporation filtrate to obtain a second lithium precipitation solution and lithium carbonate;

[0069] S7, mixing the second lithium precipitation solution and the decarburization solution to enter the evaporation process together;

[0070] The lithium-containing solution comprises sulfate ions, sodium ions, potassium ions and lithium ions.

[0071] In the lithium-containing solution, there are a large number of sulfate ions, sodium ions and potassium ions. If the decarburization treatment is directly carried out after the first lithium precipitation, the residual lithium carbonate in the solution will react with sulfuric acid to generate lithium sulfate. Since lithium sulfate and sodium sulfate have similar solubility, when evaporative crystallization is carried out, a mixture will be generated due to the common ion effect, thereby reducing the purity of lithium carbonate and sodium sulfate. In the embodiment, by first precipitating lithium carbonate and then carrying out the decarburization process, lithium carbonate can be maximally recovered, and such problems can be avoided.

[0072] In some preferred embodiments, the temperature of the lithium-containing solution is 60-95°C during the first lithium precipitation.

[0073] In the embodiment, the solubility of lithium carbonate in water decreases with the increase of temperature. In the temperature range of 60-95°C, the solubility of lithium carbonate is relatively low, which is beneficial to the precipitation and precipitation 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, the solubility of sodium sulfate, potassium sodium salt and other impurities at a relatively high temperature is relatively high, so 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.

[0074] In some preferred embodiments, the temperature of the hot melt crystallization is 60-95°C.

[0075] When the temperature is increased to 60-95°C, the solubility of lithium carbonate in water is further reduced to below 1 g / 100 g of water, so that the residual lithium carbonate in the system is further precipitated.

[0076] 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.

[0077] Specifically, the first temperature is preferably 70-80°C. The solubility of lithium carbonate is about 1.5 g / L at 50°C, and decreases 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 is increased to above 80°C, the solubility of lithium carbonate will not continue to decrease, but with the evaporation of water, lithium carbonate continues to precipitate. If the temperature is further increased, the water will evaporate too fast, and the precipitation speed of lithium carbonate will be too fast, which will lead to the precipitation of part of sodium sulfate. Therefore, the best interval of the first temperature is 70-80°C.

[0078] For the content of lithium carbonate and sodium sulfate in the filtrate in the present specific embodiment, a moderate temperature can be selected in the above-mentioned optimal temperature range, for example, 70℃, 75℃, etc., at which the solubility of lithium carbonate has been significantly reduced, facilitating precipitation, while sodium sulfate remains in the solution as it has not reached the saturation solution at this temperature. In this way, the optimal heating temperature for hot melt crystallization can be determined to ensure 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.

[0079] 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. After freeze crystallization, the content of lithium carbonate in the solution is further increased, approaching the mass fraction of the saturation solution at the freeze crystallization temperature. Therefore, during the heating process, as the solubility of lithium carbonate decreases, a portion of lithium carbonate precipitates. After further heating and evaporation of part of the water, more lithium carbonate precipitates from the solution. Therefore, the content of lithium carbonate in the filtrate after crystallization is reduced to 0.82-1.23%, which is much lower than the mass fraction of the saturation solution of lithium carbonate at the first temperature. The present application uses the above method to maximize the precipitation of lithium carbonate, achieving the purpose of improving the yield of lithium carbonate.

[0080] In some preferred embodiments, the amount of water evaporated in hot melt crystallization is calculated based on the content of lithium carbonate and sodium sulfate before and after evaporation, and the mass percentage of the saturation solution of lithium carbonate and sodium sulfate at the evaporation temperature.

[0081] wherein the amount of water evaporated is preferably determined according to the following formula:

[0082] According to the above data, the amount of water evaporated in step S3 is determined according to the following formula:

[0083]

[0084] wherein M1 is the mass of water in the filtrate after crystallization, in t;

[0085] M is the amount of water evaporated during hot melt crystallization, in t;

[0086] A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, in %;

[0087] a is the mass fraction of sodium sulfate in the filtrate after freezing, in %;

[0088] σ is a coefficient constant, with a value range of 0-1.

[0089] According to the above formula, it can be ensured that the lithium carbonate after hot melt crystallization is evaporated and crystallized, and at the same time, the sodium sulfate still exists in the filtrate, thereby improving the purity of the lithium carbonate.

[0090] wherein formula The maximum evaporation amount of water can be calculated, but considering the stability of the control process and the problem of excessive evaporation, and the problem that some sodium ions are taken out by lithium ions due to the common ion effect in the evaporation process, σ is used as a coefficient to further limit the evaporation amount of water.

[0091] In some preferred embodiments, σ is calculated according to the following formula:

[0092]

[0093] wherein A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %;

[0094] a is the mass fraction of sodium sulfate in the filtrate after freezing, and the unit is %;

[0095] B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, and the unit is %;

[0096] b is the mass fraction of lithium carbonate in the filtrate after freezing, and the unit is %.

[0097] By calculating the ratio σ between the actual evaporation amount and the maximum evaporation amount, the purification process can be significantly optimized, which not only ensures that only lithium carbonate is precipitated to improve product purity, but also ensures that as much lithium carbonate as possible is recovered to reduce loss.

[0098] In the decarburization step of some preferred embodiments, the added sulfuric acid has a molar mass equal to that of the lithium carbonate in the filtrate after crystallization.

[0099] In this specific embodiment, since the residual lithium carbonate is further precipitated by heating before decarburization, the carbonate in the system is further reduced, and the amount of sulfuric acid added in the decarburization step is also reduced, thereby reducing the introduction of sulfate ions, facilitating the subsequent steps.

[0100] 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 be precipitated together with the potassium-sodium salt, affecting the yield of lithium carbonate.

[0101] In some preferred embodiments, the lithium-containing solution comprises, by mass fraction, 3-10% of sodium sulfate, 10-20% of lithium sulfate, 0.5-5% of potassium sulfate, and the balance being water. In the above components, the contents of sodium sulfate and lithium sulfate are relatively high, and the separation method provided in the specific embodiment can effectively separate sodium ions and lithium ions in the system by freezing out sodium sulfate and then heating to separate out lithium carbonate, so as to obtain lithium carbonate and sodium sulfate with relatively high purity.

[0102] In some preferred embodiments, the solution is cooled 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 of temperature, so as to avoid the doping of lithium carbonate in the sodium sulfate crystals precipitated by freezing, and greatly improve the purity of sodium sulfate.

[0103] Embodiment 1

[0104] In this embodiment, lithium-containing solution is used as raw material, and lithium carbonate and sodium sulfate pure products are prepared by using the lithium carbonate separation process provided in the present application, which comprises the following steps: heating and melting crystallization, and then decarburization. In the lithium-containing solution, by mass fraction, 6% of sodium sulfate, 15% of lithium sulfate, and 2.1% of potassium sulfate are contained, and the balance is water.

[0105] The specific process steps of the lithium carbonate separation process of heating and melting crystallization and then decarburization are as follows:

[0106] S1, first lithium precipitation: heating the lithium-containing solution to 90°C, and adding sodium carbonate with equal molar amount of lithium ions to the lithium-containing solution to obtain a first lithium precipitation solution and lithium carbonate;

[0107] S2, separating sodium sulfate: freezing the first lithium precipitation solution to 0°C to crystallize sodium sulfate, and obtaining a frozen filtrate;

[0108] S3, heating and melting crystallization: heating the frozen filtrate to 70°C to evaporate part of the water, and obtaining lithium carbonate and a crystallization filtrate;

[0109] S4, decarburization: adding 98% sulfuric acid with equal molar amount of lithium carbonate in the crystallization filtrate to the crystallization filtrate to obtain a decarburization solution;

[0110] S5, evaporation: heating the decarburization solution to 60°C to evaporate, and obtaining a potassium-sodium salt and an evaporation filtrate;

[0111] S6, second lithium precipitation: adding sodium carbonate with equal molar amount of lithium ions to the evaporation filtrate to obtain a second lithium precipitation solution and lithium carbonate;

[0112] S7, mixing the second lithium precipitation solution and the decarburization solution to enter the evaporation process together.

[0113] Wherein, according to the mass percentage, the solution after the first lithium precipitation contains: sodium sulfate 19.13%, lithium carbonate 0.22% and potassium sulfate 1.6%, and the balance is water; the filtrate after freezing contains: sodium sulfate 5%, lithium carbonate 1.8% and potassium sulfate 2.55%, and the balance is water.

[0114] According to the above data, in step S3, the evaporation amount of water is determined according to the following formula:

[0115]

[0116] Wherein, M1 is the mass of water in the filtrate after crystallization, and the unit is t;

[0117] M is the evaporation amount of water in the hot melt crystallization process, and the unit is t;

[0118] A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %;

[0119] a is the mass fraction of sodium sulfate in the filtrate after crystallization, and the unit is %;

[0120] σ is a coefficient constant, and the value range is 0-1.

[0121] In this embodiment, σ is calculated according to the following formula:

[0122]

[0123] Wherein, A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %;

[0124] a is the mass fraction of sodium sulfate in the filtrate after freezing, and the unit is %;

[0125] B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, and the unit is %;

[0126] b is the mass fraction of lithium carbonate in the solution after the first lithium precipitation, and the unit is %.

[0127] According to the solubility curve data, it is found that at 70°C, the mass fraction of the saturated solution of lithium carbonate is 0.91%, and the mass fraction of the saturated solution of sodium sulfate is 31.3%; then, σ=0.9.

[0128] The calculation result is: M / M1=79.6%.

[0129] The filtrate after crystallization contains: sodium sulfate 19.2%, lithium carbonate 0.82% and potassium sulfate 9.75%, and the balance is water; the solution after decarburization contains: sodium sulfate 18.8%, lithium sulfate 0.82% and potassium sulfate 9.54%, and the balance is water; the filtrate after evaporation contains: sodium sulfate 5.2%, lithium sulfate 5% and potassium sulfate 2.5%.

[0130] Example 2

[0131] This embodiment uses a lithium-containing solution as raw material to prepare lithium carbonate and sodium sulfate pure products by the lithium carbonate separation process provided by the present application, which comprises the following steps: heating the lithium-containing solution to 95℃, adding sodium carbonate to the lithium-containing solution, and obtaining a first lithium precipitation solution and lithium carbonate.

[0132] The specific process steps of the lithium carbonate separation process are as follows:

[0133] S1, first lithium precipitation: heating the lithium-containing solution to 95℃, and adding sodium carbonate to the lithium-containing solution, and obtaining a first lithium precipitation solution and lithium carbonate;

[0134] S2, separating sodium sulfate: freezing the first lithium precipitation solution to 0℃ to crystallize sodium sulfate, and obtaining a frozen filtrate;

[0135] S3, hot melting and crystallization: heating the frozen filtrate to 80℃, and evaporating part of the water to obtain lithium carbonate and a crystallization filtrate;

[0136] S4, decarburization: adding 98% sulfuric acid to the crystallization filtrate to obtain a decarburized solution;

[0137] S5, evaporation: heating the decarburized solution to 60℃, and evaporating to obtain a potassium-sodium salt and an evaporated filtrate;

[0138] S6, second lithium precipitation: adding sodium carbonate to the evaporated filtrate to obtain a second lithium precipitation solution and lithium carbonate;

[0139] S7, mixing the second lithium precipitation solution with the decarburized solution and entering the evaporation process together.

[0140] According to the above data, in step S3, the amount of water evaporated is determined according to the following formula:

[0141] According to the above data, in step S3, the amount of water evaporated is determined according to the following formula:

[0142]

[0143] Wherein M1 is the mass of water in the crystallization filtrate, and the unit is t;

[0144] M is the amount of water evaporated during the hot melting and crystallization process, and the unit is t;

[0145] A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %;

[0146] a is the mass fraction of sodium sulfate in the filtrate after crystallization, and the unit is %;

[0147] σ is a coefficient constant, and the value range is 0-1.

[0148] In this embodiment, σ is calculated according to the following formula:

[0149]

[0150] A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %;

[0151] a is the mass fraction of sodium sulfate in the filtrate after freezing, and the unit is %;

[0152] B is the mass fraction of lithium sulfate in the saturated solution of lithium carbonate at the first temperature, and the unit is %;

[0153] b is the mass fraction of lithium carbonate in the solution after the first lithium precipitation, and the unit is %.

[0154] According to the solubility curve data, it is found that at 80℃, the mass fraction of the saturated solution of lithium carbonate is 0.84%, and the mass fraction of the saturated solution of sodium sulfate is 30.4%; then σ=0.81.

[0155] The calculation result is: M / M1=73.0%.

[0156] The filtrate after crystallization contains: 17.3% of sodium sulfate, 1.23% of lithium carbonate and 2.77% of potassium sulfate, and the balance is water; the solution after decarburization contains: 16.94% of sodium sulfate, 1.23% of lithium sulfate and 2.71% of potassium sulfate, and the balance is water; the filtrate after evaporation contains: 5.36% of sodium sulfate, 5.75% of lithium sulfate and 0.71% of potassium sulfate.

[0157] Comparative Example 1

[0158] In this embodiment, lithium-containing solution is used as raw material, and lithium carbonate and sodium sulfate pure products are prepared by using the lithium carbonate separation process provided by the present application, which comprises the following steps:

[0159] The specific process steps of the lithium carbonate separation process after freezing and decarburization are as follows:

[0160] S1, first lithium precipitation: heat the lithium-containing solution to 90℃, and add sodium carbonate with the same molar amount of lithium ions to the lithium-containing solution to obtain the solution after the first lithium precipitation and lithium carbonate;

[0161] S2, separating sodium sulfate: freezing the solution after the first lithium precipitation to 0°C to crystallize, precipitating sodium sulfate, to obtain the filtrate after freezing;

[0162] S3, decarburization: adding 98% sulfuric acid to the filtrate after crystallization in an equal molar amount of lithium carbonate in the crystallization filtrate, to obtain the solution after decarburization;

[0163] S4, evaporation: heating the solution after decarburization to 60°C, and evaporating, to obtain the potassium-sodium salt and the filtrate after evaporation;

[0164] S5, second lithium precipitation: adding sodium carbonate to the filtrate after evaporation in an equal molar amount of lithium ions, to obtain the second lithium precipitation solution and lithium carbonate;

[0165] S6, mixing the second lithium precipitation solution and the solution after decarburization, and jointly entering the evaporation process.

[0166] In the present example, the solution after the first lithium precipitation contains 19.13% sodium sulfate, 0.22% lithium carbonate, and 1.6% potassium sulfate, with the balance being water; the filtrate after freezing contains 5% sodium sulfate, 2.8% lithium carbonate, and 2.55% potassium sulfate, with the balance being water; the solution after decarburization contains 4.8% sodium sulfate, 2.8% lithium sulfate, and 2.45% potassium sulfate, with the balance being water; and the filtrate after evaporation contains 1.32% sodium sulfate, 0.5% lithium sulfate, and 0.67% potassium sulfate. In the present example, since one step of hot melting and crystallization is less than in Example 1, the amount of water evaporated is 79.2% less than in Example 1, resulting in a lower content of each solute in the evaporation filtrate before the second lithium precipitation, and less and lower-purity lithium carbonate obtained during the second lithium precipitation.

[0167] Comparative Example 2

[0168] The present example uses a lithium-containing solution as a raw material, and uses the lithium carbonate separation process provided by the present application to prepare lithium carbonate and sodium sulfate pure products. In the present example, the lithium-containing solution contains the following components: 6% sodium sulfate, 15% lithium sulfate, and 2.1% potassium sulfate, with the balance being water.

[0169] The specific process steps of the lithium carbonate separation process are as follows:

[0170] S1, first lithium precipitation: heating the lithium-containing solution to 90°C, and adding sodium carbonate to the lithium-containing solution in an equal molar amount of lithium ions, to obtain the solution after the first lithium precipitation and lithium carbonate;

[0171] S2, separating sodium sulfate: freezing the solution after the first lithium precipitation to 0°C to crystallize, precipitating sodium sulfate, to obtain the filtrate after freezing;

[0172] S3, hot melt crystallization: the filtrate after freezing is heated to 70℃, and a part of water is evaporated to obtain lithium carbonate and a filtrate after crystallization;

[0173] S4, decarburization: 98% sulfuric acid is added to the filtrate after crystallization in an amount equal to the amount of lithium carbonate in the filtrate after crystallization to obtain a solution after decarburization;

[0174] S5, evaporation: the solution after decarburization is heated to 60℃, and evaporated to obtain a potassium-sodium salt and a filtrate after evaporation;

[0175] S6, second lithium precipitation: sodium carbonate is added to the filtrate after evaporation in an amount equal to the amount of lithium ions to obtain a second lithium precipitation solution and lithium carbonate;

[0176] S7, the second lithium precipitation solution is mixed with the solution after decarburization, and both enter the evaporation process.

[0177] According to the 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.

[0178] According to the above data, in step S3, the evaporation amount of water is determined according to the following formula:

[0179]

[0180] Wherein M1 is the mass of water in the filtrate after crystallization, and the unit is t;

[0181] M is the evaporation amount of water in the hot melt crystallization process, and the unit is t;

[0182] A is the mass fraction of lithium sulfate in the saturated solution of sodium sulfate at the first temperature, and the unit is %;

[0183] a is the mass fraction of sodium sulfate in the filtrate after crystallization, and the unit is %;

[0184] Then the calculation result is: M / M1=88.4%.

[0185] 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 decarburization 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.

[0186] Comparative Example 3

[0187] The comparative example uses the same lithium-containing solution as Example 1, and after the first lithium precipitation, sulfuric acid is added to remove carbon first, and then frozen crystallization is carried out. Sodium sulfate is precipitated at 0°C, and then the filtrate after freezing is evaporated to obtain potassium sodium salt and the filtrate after evaporation. The filtrate after evaporation is subjected to a second lithium precipitation.

[0188] Among them, the solution after decarburization contains: sodium sulfate 19.13%, lithium sulfate 5.8%, potassium sulfate 1.6%, free sulfuric acid 0.3%, and the balance is water. The filtrate after freezing contains: sodium sulfate 5.0%, lithium sulfate 5.3%, potassium sulfate 2.55%, free sulfuric acid 0.28%, and the balance is water. The filtrate after evaporation contains: sodium sulfate 5.8%, lithium sulfate 4.2%, potassium sulfate 2.6%.

[0189] Comparative Example 4

[0190] The comparative example uses the same lithium-containing solution as Example 1, and the difference is that after the frozen crystallization step, 0.1% lithium carbonate seed is added to the filtrate after freezing, and then evaporated and concentrated at 70°C to precipitate lithium carbonate.

[0191] The filtrate after adding lithium carbonate seed contains: sodium sulfate 19.2%, lithium carbonate 0.52%, potassium sulfate 8.8%, and the balance is water.

[0192] The purity of lithium carbonate and sodium sulfate obtained in each step of Examples 1-2 and Comparative Examples 1-4 is calculated, and the yield of the final lithium carbonate is calculated. The results are shown in Table 1.

[0193] Table 1. Purity of lithium carbonate and sodium sulfate and yield of lithium carbonate in Examples 1-2 and Comparative Examples 1-4

[0194]

[0195]

[0196] According to the above data, after reducing the hot melt crystallization step, the yield of the final lithium carbonate decreases; when the maximum evaporation amount is used for hot melt crystallization, the purity and yield of lithium carbonate obtained by hot melt crystallization both decrease.

[0197] Among them, the filtrate after freezing in Comparative Example 1 contains 2.8% lithium carbonate, which is higher than the 1.8% in Example 1. The concentration of lithium sulfate after decarburization increases significantly, which is higher than the 0.82% in Example 1. In the evaporation step, lithium sulfate is precipitated with potassium sodium salt, which causes a bottleneck in the yield of lithium carbonate. In addition, since one step of hot melt crystallization is less than Example 1, the amount of water evaporated is less than Example 1, resulting in a lower content of each solute in the evaporation filtrate before the second lithium precipitation. In the second lithium precipitation, less lithium carbonate is obtained, and the purity is lower.

[0198] In the comparative example 2, the solution after the first lithium precipitation and the filtrate after the freezing are the same as those in the example 1, and the filtrate after the hot melt crystallization is co-precipitated with the impurities and lithium carbonate due to excessive concentration, and the purity of the product is reduced. The lithium sulfate content in the filtrate after evaporation is 4.5%, and the lithium residual amount is high.

[0199] The core contradiction between the comparative example 3 and the example 1 lies in the irreversibility of the accumulation of sulfate, wherein, in the example 1, the solution after lithium precipitation is subjected to freezing crystallization, and the filtrate after freezing contains dissolved sodium sulfate, then in the hot melt crystallization process, the part of sodium sulfate remains in the dissolved state, and since the hot melt crystallization removes most of the lithium carbonate, the amount of sulfuric acid added to the system in the decarburization step is reduced, and after decarburization, the trace amount of sulfuric acid remains in the system. In the comparative example 3, the solution after lithium precipitation is directly subjected to decarburization, a large amount of sulfuric acid is consumed to generate lithium sulfate, and the unreacted sulfuric acid remains in the system for freezing crystallization, then in the evaporation process, the high-concentration lithium sulfate is co-crystallized with the sodium sulfate that is not precipitated by freezing, the purity of sodium sulfate is reduced, the lithium loss is increased, and since the accumulation of sulfate in the system, the lithium precipitation cycle is forced to increase, the acid consumption and energy consumption are simultaneously increased, and the cost is higher.

[0200] In the comparative example 4, lithium carbonate seeds are added to the filtrate after the freezing crystallization step, the seeds adsorb impurity ions and free lithium ions, so that the purity of the precipitated lithium carbonate is reduced, and the seeds use the induced heterogeneous nucleation method to break through the balance of solubility, and the remaining mother liquor still contains a large amount of lithium carbonate, and still needs to be subjected to multiple lithium precipitation operations.

[0201] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Although the present application has been shown by the preferred embodiment as above, it is not intended to limit the present application, and any person skilled in the art can make some changes or modifications to the above disclosed technical content without departing from the scope of the technical solution of the present application, and any simple modification, equivalent change and modification of the above examples according to the technical essence of the present application are still within the scope of the technical solution of the present application.

Claims

1. A lithium carbonate separation process involving first freezing the lithium-precipitated liquid, then evaporating it to crystallize, and finally decarbonizing the resulting solution, characterized in that... The following steps are included: First lithium precipitation: Add sodium carbonate in an equimolar amount to lithium ions to the lithium-containing solution to obtain the solution after the first lithium precipitation and lithium carbonate; Separation of sodium sulfate: The solution after the first lithium precipitation is frozen to crystallize sodium sulfate, and the frozen filtrate is obtained. Hot melt crystallization: The frozen filtrate is heated to a first temperature to evaporate some of the water, yielding lithium carbonate and the crystallized filtrate. Decarbonization: Add sulfuric acid to the filtrate after crystallization to obtain a decarbonized solution; Evaporation: The decarbonized solution was heated and evaporated at a second temperature to obtain potassium and sodium salts and the filtrate after evaporation; Second lithium precipitation: Add sodium carbonate in an equimolar amount of 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 then introduced into the evaporation process together. The lithium-containing solution contains sulfate ions, sodium ions, potassium ions, and lithium ions.

2. The lithium carbonate separation process according to claim 1, which involves freezing the lithium-precipitated liquid followed by evaporative thermal crystallization and decarbonization, is characterized in that... The first temperature is 60–95°C.

3. The lithium carbonate separation process according to claim 2, which involves freezing the lithium-precipitated liquid followed by evaporative thermal crystallization and decarbonization, is characterized in that... The first temperature is determined based on the solubility curves of lithium carbonate in water and sodium sulfate in water.

4. A lithium carbonate separation process according to claim 2 or 3, characterized in that, after lithium precipitation, the liquid is first frozen, then evaporated to thermal crystallization, and finally decarbonized, the process involves the following steps: The first temperature is 70-80℃.

5. The lithium carbonate separation process according to claim 1, characterized in that, after lithium precipitation, the liquid is first frozen, then evaporated to thermal crystallize, and finally decarbonized, the process involves the following steps: The filtrate after crystallization contains 0.82-1.23% lithium carbonate and 2-3g / L Li.

6. The lithium carbonate separation process according to claim 5, which involves freezing the lithium-precipitated liquid followed by evaporative thermal crystallization and decarbonization, is characterized in that... The amount of water evaporated is determined according to the following formula: Where M1 is the mass of water in the filtrate after crystallization, in tons (t); M is the amount of water evaporated during the hot melt crystallization process, measured in tons (t). A is the mass fraction of lithium sulfate in a saturated sodium sulfate solution at the first temperature, expressed in %; a is the mass fraction of sodium sulfate in the filtrate after crystallization, in %; σ is a constant coefficient, and its value ranges from 0 to 1.

7. The lithium carbonate separation process according to claim 6, characterized in that, after lithium precipitation, the liquid is first frozen, then evaporated to thermal crystallization, and finally decarbonized, the process involves the following steps: The σ is calculated according to the following formula: Where A is the mass fraction of lithium sulfate in the saturated sodium sulfate solution at the first temperature, in %; a is the mass fraction of sodium sulfate in the filtrate after freezing, in %; B is the mass fraction of lithium sulfate in a saturated lithium carbonate solution at the first temperature, expressed as a percentage (%). b is the mass fraction of lithium carbonate in the solution after the first lithium precipitation, expressed as a percentage.

8. The lithium carbonate separation process according to claim 1, which involves freezing the lithium-precipitated liquid followed by evaporative thermal crystallization and decarbonization, is characterized in that... The second temperature is 40–60°C.

9. The lithium carbonate separation process according to claim 1, which involves freezing the lithium-precipitated liquid followed by evaporative thermal crystallization and decarbonization, is characterized in that... The lithium-containing solution contains the following components by mass fraction: 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, characterized in that, after lithium precipitation, the liquid is first frozen, then evaporated to thermal crystallize, and finally decarbonized, the process involves the following steps: The solution is frozen to 0°C or below after the first lithium precipitation.

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