Method for removing calcium and magnesium from leaching solution of spodumene

Abstract

The application discloses a method for removing calcium and magnesium from leaching solution of spodumene, comprising the following steps: (1) adding liquid alkali into the lithium sulfate leaching solution obtained by spodumene sulfuric acid leaching under low temperature condition to form calcium sulfate-carbonate calcium composite seed crystal; (2) heating the reaction system and then adding liquid alkali, so that magnesium ions combine with hydroxyl ions to generate magnesium hydroxide precipitate, and the magnesium hydroxide precipitate is attached to the surface of the calcium sulfate-carbonate calcium composite seed crystal to grow in a heterogeneous nucleation mode; (3) adding sodium carbonate into the reaction system to precipitate the remaining calcium ions in the form of calcium carbonate, and the newly generated calcium carbonate fills the surface pores of the magnesium hydroxide coating layer to form dense composite precipitate particles; and (4) adding water into the reaction system of step (3) and stirring to separate solid and liquid. The application can solve the problems of the existing calcium and magnesium removal process for the spodumene leaching solution, such as the colloidal state of the magnesium hydroxide precipitate, the difficulty in filtration and the high loss of lithium entrainment.

Description

Technical Field

[0001] This invention belongs to the field of hydrometallurgical impurity removal technology in the lithium extraction process of spodumene, and particularly relates to a method for removing calcium and magnesium from lithium-containing leachate. Background Technology

[0002] Battery-grade lithium carbonate is a key raw material for manufacturing cathode materials for lithium-ion batteries. With the rapid development of the new energy market, lithium extraction from ore has become the dominant process in lithium carbonate production. Currently, the spodumene sulfuric acid process is the most widely used process route: after spodumene undergoes transformation roasting and sulfation roasting, lithium sulfate solution is obtained through leaching, followed by impurity removal, purification, and lithium precipitation to produce lithium carbonate products.

[0003] In the purification process of lithium sulfate solution, calcium and magnesium are the main impurity ions that need to be removed. Among them, magnesium removal is a recognized technical challenge in the industry. Current technologies typically remove magnesium by adding an alkali (such as NaOH) to reduce Mg content. 2+ Magnesium hydroxide precipitates out as magnesium hydroxide. However, the magnesium hydroxide precipitate is a colloidal flocculent substance with extremely fine particles, resulting in very slow settling and making filtration extremely difficult. This characteristic leads to problems such as low filtration efficiency, high equipment investment, and long operation cycles in the magnesium removal process.

[0004] To improve the filtration performance of magnesium hydroxide, some studies have attempted to add seed crystals to accelerate sedimentation and filtration rates. For example, patent application CN121976058A discloses a method for removing magnesium by simultaneously adding calcium hydroxide and magnesium hydroxide seed crystals to lithium leachate with high magnesium content.

[0005] However, the aforementioned method of adding seed crystals increases material costs, and the added seed crystals often lack good interfacial bonding with the in-situ generated precipitate, resulting in limited improvement in filtration performance. Furthermore, in existing calcium and magnesium removal processes, the formation sequence and morphology of calcium and magnesium precipitates lack effective control, making it difficult to fundamentally solve the problem of difficult magnesium hydroxide colloid filtration, while also presenting the problem of lithium entrainment. Summary of the Invention

[0006] This invention addresses the technical problems of colloidal magnesium hydroxide precipitate, difficult filtration, and high lithium entrainment loss in existing spodumene leaching processes. It provides a method for simultaneous removal of calcium and magnesium from spodumene leaching solutions by in-situ generation of composite seed crystals and control of the precipitation growth sequence to improve the filtration performance of magnesium hydroxide.

[0007] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0008] A method for removing calcium and magnesium from spodumene leachate includes the following steps: (1) Under low temperature conditions, liquid alkali is added to the lithium sulfate leaching solution obtained by spodumene sulfuric acid leaching, and the pH value of the reaction system is controlled to reach above 10. The sulfate ions and calcium ions in the leaching solution precipitate to form calcium sulfate precipitate. At the same time, the carbon dioxide dissolved in the alkaline low temperature environment reacts with calcium ions to form calcium carbonate precipitate, forming calcium sulfate-calcium carbonate composite seed crystals. (2) Heat the reaction system and add liquid alkali. Magnesium ions combine with hydroxide ions to form magnesium hydroxide precipitate, which is attached in situ to the surface of the calcium sulfate-calcium carbonate composite seed crystal generated in step (1) for heterogeneous nucleation growth. (3) Add sodium carbonate to the reaction system to precipitate the remaining calcium ions as calcium carbonate. At the same time, the newly generated calcium carbonate fills the surface pores of the magnesium hydroxide coating layer to form dense composite precipitate particles. (4) Add water to the reaction system of step (3) and stir to separate the solid and liquid, and obtain calcium magnesium slag with low lithium oxide content and purified lithium sulfate solution.

[0009] In the above method for removing calcium and magnesium from spodumene leaching solution, preferably, the low-temperature condition in step (1) is 45-60℃. 45-60℃ is the optimal temperature window for the "calcium sulfate-calcium carbonate composite seed crystal formation" in this invention. Below 45℃, the solubility of calcium sulfate decreases, which is beneficial for precipitation but increases the solution viscosity and limits mass transfer; above 60℃, the solubility of CO2 in the solution decreases sharply, reducing the amount of calcium carbonate generated, resulting in insufficient calcium carbonate proportion in the composite seed crystal, which affects the subsequent seed crystal induction effect. 45-60℃ balances the equilibrium precipitation of calcium sulfate and calcium carbonate with good solution fluidity.

[0010] In the above method for removing calcium and magnesium from spodumene leaching solution, preferably, in step (1), liquid alkali is added by spraying, and the mass of the liquid alkali added in the first round accounts for 40%-60% of the theoretical amount. The atomized spray disperses the liquid alkali into micron-sized droplets, which greatly increases the contact area between the liquid alkali and the leaching solution, avoiding local over-alkalinity (local pH spike) that leads to explosive homogeneous nucleation of magnesium hydroxide. The first round of liquid alkali should not be added in excess, otherwise it may cause magnesium precipitation. The above-mentioned spraying method and the control of the amount of liquid alkali added, together with the "low-temperature seed crystal generation", ensure that magnesium hydroxide does not precipitate prematurely during the seed crystal generation stage, which is a prerequisite for subsequent "in-situ attachment growth".

[0011] In the above method for removing calcium and magnesium from spodumene leaching solution, preferably, in step (1), the liquid alkali is added in 2-4 batches, with an interval of 5-15 minutes between adjacent batches. Adding in batches and setting an interval allows sufficient time for the system to reach reaction equilibrium and crystal growth after each batch of liquid alkali addition. The 5-15 minute interval ensures sufficient precipitation reaction while also considering production efficiency, avoiding continuously high supersaturation caused by continuous alkali addition. Furthermore, the stirring during the interval (45 rpm) promotes uniform dispersion of calcium sulfate and calcium carbonate seeds, providing better substrate conditions for the uniform adhesion of magnesium hydroxide after the next batch of liquid alkali addition.

[0012] In the above-mentioned method for removing calcium and magnesium from spodumene leaching solution, preferably, in step (2), the reaction system is heated to 75-90℃ at a heating rate of 0.5-3℃ / min. The control of the heating rate is one of the key parameters for "heterogeneous growth of magnesium hydroxide on the seed surface rather than homogeneous nucleation". Too fast a heating rate (>3℃ / min) will cause drastic changes in the thermodynamic state of the system, and a sudden increase in local supersaturation in the solution, which will trigger independent nucleation of magnesium hydroxide; too slow a heating rate (<0.5℃ / min) will reduce production efficiency. The mild heating of 0.5-3℃ / min allows the system to gradually cross the precipitation threshold of magnesium hydroxide, providing a sufficient kinetic window for magnesium ions to slowly crystallize on the existing seed surface, which is conducive to the growth and improvement of grains. 80-85℃ is the optimal temperature range for heterogeneous growth of magnesium hydroxide on the composite seed surface. At this temperature, the colloidal tendency of magnesium hydroxide is significantly reduced (high temperature destroys colloidal structure), which is conducive to the formation of precipitates with better crystallinity and excellent filtration performance. Meanwhile, this temperature range matches the temperature characteristics of the spodumene leachate itself, eliminating the need for additional significant heating or cooling, resulting in high energy utilization efficiency.

[0013] In the above method for removing calcium and magnesium from spodumene leaching solution, preferably, in step (2), liquid alkali is added by spraying, and the mass of the liquid alkali added in the second round accounts for 20%-40% of the theoretical amount. The atomized spray disperses the liquid alkali into micron-sized droplets, which greatly increases the contact area between the liquid alkali and the leaching solution, avoiding local over-alkalinity (local pH spike) that leads to explosive homogeneous nucleation of magnesium hydroxide. The addition of liquid alkali in the second round is mainly used to supplement alkalinity and promote the precipitation of magnesium ions. Adding too little will lead to insufficient alkalinity and incomplete magnesium precipitation. Although the sodium carbonate hydrolysis in step (3) will supplement some OH- - However, if the second round of alkali solution is insufficient, the total alkalinity of the system may be inadequate, and the final pH value of the reaction may not reach above 10.8, resulting in a large amount of residual magnesium ions in the solution and failure to meet purification standards. Furthermore, it may lead to an excessively thin coating layer, so even if magnesium is completely precipitated, the OH- ions may not be absorbed properly. -At low concentrations, the precipitation-driving force is weak, and the coating layer of magnesium hydroxide growing on the seed crystal surface may be too sparse or too thin to form an effective composite particle structure. Adding too much liquid alkali can cause localized supersaturation, leading to secondary nucleation. The solution is already highly reactive after heating; if excessive liquid alkali is added rapidly, extremely high supersaturation will occur locally. These excess magnesium ions do not have time to migrate to the existing seed crystal surface and instead spontaneously nucleate in the solution, regenerating a difficult-to-filter colloid. An excessively rapid growth rate leads to increased crystal defects and a loose, porous coating layer, which in turn increases the number of sites for subsequent lithium ion entrainment, resulting in a higher lithium loss rate.

[0014] In the above method for removing calcium and magnesium from spodumene leaching solution, preferably, in step (3), sodium carbonate is added by slow dripping over a period of 15-40 minutes. The essence of slow dripping of sodium carbonate is to control the rate of CaCO3 formation and avoid localized CO3 formation caused by a single addition. 2- Excessive concentration leads to explosive nucleation of CaCO3, which fails to effectively fill the pores of the magnesium hydroxide coating. A dropping time of 15-40 minutes ensures that calcium carbonate undergoes "filling-type growth" on the surface of existing composite particles, rather than independent nucleation, making it a core operating parameter for achieving "secondary filling and densification of calcium carbonate." Simultaneously, slow dropping also allows the OH- produced by the hydrolysis of sodium carbonate to... - Uniform release, with residual Mg in the system 2+ The reaction is mild, avoiding localized over-alkaliness.

[0015] In the above method for removing calcium and magnesium from spodumene leaching solution, preferably, in step (3), the amount of sodium carbonate added is 25%-40% of the theoretical amount of calcium precipitation in the leaching solution, and the hydroxide ions generated by the hydrolysis of sodium carbonate work together with the hydroxide ions provided by the liquid alkali in steps (1) and (2) to ensure that the total alkalinity in the system is sufficient to completely precipitate magnesium ions. When the amount of sodium carbonate added is controlled at 25%-40% of the theoretical amount of calcium precipitation, it ensures that the remaining calcium ions are fully precipitated to a qualified level (Ca in the solution). 2+ <0.03g / L), on the other hand, the amount of newly generated calcium carbonate is just enough to fill the pores of the magnesium hydroxide coating without producing excess free calcium carbonate precipitate. If it is less than 25%, the calcium removal depth is incomplete and the filling effect is insufficient; if it is more than 40%, the excess calcium carbonate may form nuclei independently, destroying the uniformity of the core-shell structure of the composite particles, which will affect the filtration performance and increase the cost.

[0016] In the above method for removing calcium and magnesium from spodumene leaching solution, preferably, in step (4), the mass-volume ratio of the composite particle precipitate and water in the reaction system is controlled to be 1:(6-10).

[0017] In the above method for removing calcium and magnesium from spodumene leaching solution, preferably, in step (4), the stirring and washing is carried out for 0.5-2 hours at a stirring speed of 30-60 rpm. The low stirring speed of 30-60 rpm is sufficient to maintain the suspension uniformity of the slurry, while avoiding the breakage and wear of the already formed composite precipitate particles by high shear force. In contrast, the traditional impurity removal process often uses a higher stirring speed (80-120 rpm), which will destroy the crystalline structure of the precipitate, increase the proportion of fine particles, and deteriorate the filtration performance.

[0018] More specifically, the method for removing calcium and magnesium from spodumene leaching solution according to the present invention includes the following steps: Step 1: Low-Temperature Composite Seed Generation Stage The lithium sulfate leachate obtained from the spodumene sulfuric acid leaching process was placed in a reaction tank, and liquid alkali was added in batches via spraying under low-temperature conditions of 40-65℃. The first batch added 40%-60% of the theoretical total amount of liquid alkali, controlling the pH of the reaction system to be above 10. Under these pH and low-temperature conditions, sulfate ions and calcium ions in the leachate began to precipitate, forming calcium sulfate precipitate (reaction 1). Simultaneously, carbon dioxide dissolved in the alkaline low-temperature environment reacted with calcium ions to form calcium carbonate precipitate (reaction 2), forming calcium sulfate-calcium carbonate composite seed crystals. Ca 2+ + SO4 2- → CaSO4↓ Formula 1; Ca 2+ + CO3 2- → CaCO3↓ (Formula 2); Step Two: Temperature Induction Growth Stage The reaction system is heated to 75-90℃, and a second round of liquid alkali is added, with the amount added being 20%-40% of the theoretical total amount of liquid alkali. Under the heating condition, magnesium ions gradually combine with hydroxide ions to form magnesium hydroxide precipitate (reaction formula 3), which then adheres in situ to the surface of the calcium sulfate-calcium carbonate composite seed crystals generated in step one for heterogeneous nucleation and growth. Mg 2+ + 2OH - → Mg(OH)2↓ (Formula 3); Because the hydroxide concentration is relatively low at this stage, magnesium hydroxide crystallizes slowly on the seed surface with a low degree of supersaturation, allowing the grains to grow sufficiently.

[0019] Step 3: Deep Precipitation Stage of Calcium and Magnesium Add sodium carbonate at 20%-40% of the original calcium content in the leachate (based on theoretical calculations). This process precipitates the remaining calcium ions as calcium carbonate. Simultaneously, the hydroxide ions generated from the hydrolysis of sodium carbonate (reaction 4) work in conjunction with the hydroxide ions provided by the alkali solution in steps one and two, ensuring sufficient total alkalinity in the system to completely precipitate magnesium ions. The newly generated calcium carbonate also fills the surface pores of the magnesium hydroxide coating, forming dense composite precipitate particles.

[0020] CO3² - + H2O HCO3 - + OH - Formula 4.

[0021] Step 4: Washing and Separation Stage The calcium-magnesium composite slag obtained in step 3 is stirred and washed with water at a mass-volume ratio of 1:(6-10) for 0.5-2 hours, and the solid and liquid are separated to obtain calcium-magnesium slag with low lithium oxide content and purified lithium sulfate solution.

[0022] Compared with the prior art, the advantages of the present invention are as follows: 1. In-situ generation of composite seed crystals, no need for external seed crystals. This invention utilizes the inherent calcium ions and sulfate ions in the leachate to generate calcium sulfate-calcium carbonate composite seed crystals in situ under low-temperature alkaline conditions, eliminating the need for external seed materials and saving production costs and operational steps. Compared to the existing method of adding calcium sulfate seed crystals, this invention achieves in-situ self-generation of the seed crystals, resulting in better lattice matching and interfacial bonding between the seed crystals and subsequent precipitates.

[0023] 2. Temperature-feeding coordinated segmented control enables targeted regulation of sediment morphology. This invention achieves sequential control of calcium and magnesium precipitation through a temperature-feeding synergistic control strategy of "low-temperature seed crystal formation → temperature-induced growth". In the low-temperature stage, crystalline calcium sulfate and calcium carbonate (both crystalline precipitates, easily filtered) are first formed, providing a heterogeneous nucleation substrate for subsequent magnesium hydroxide formation. In the temperature-increasing stage, magnesium hydroxide is regenerated and grown on the seed crystal surface, avoiding the formation of a large amount of colloid through homogeneous nucleation of magnesium hydroxide alone. This stepwise strategy for controlling the precipitation sequence represents a significant technological advancement compared to existing one-step methods that simultaneously precipitate calcium and magnesium or simply add seed crystals.

[0024] 3. Secondary filling with calcium carbonate to form a dense composite structure. In step three of this invention, the addition of sodium carbonate not only achieves deep removal of residual calcium, but more importantly, the newly generated calcium carbonate fills the surface pores of the magnesium hydroxide coating, making the composite particle structure more compact. This dense composite structure effectively reduces the porosity and specific surface area of ​​the particles, decreasing the entrainment and adsorption of lithium ions within the particles, thereby significantly improving lithium recovery. Simultaneously, the filter cake formed by the dense composite particles during filtration exhibits lower filtration resistance, further accelerating the filtration speed.

[0025] 4. Alkalinity is supplied in tandem to ensure complete magnesium precipitation. In this invention, liquid alkali and sodium carbonate hydrolysis together provide OH- - This ensures that even if the amount of liquid alkali added is less than 100% of the theoretical amount, the total alkalinity of the system is still sufficient to completely precipitate magnesium ions, avoiding the adverse effects of excessive local pH and magnesium hydroxide nucleation caused by excessive liquid alkali, while also saving the amount of liquid alkali used.

[0026] 5. Completes calcium and magnesium removal in one step, simplifying the process. This invention combines the traditional two-step magnesium and calcium removal process into a single step, shortening the process flow, reducing equipment investment and operating costs, and improving production efficiency. Detailed Implementation

[0027] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0028] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0029] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0030] Example 1: Lithium sulfate leachate (Li) obtained by leaching spodumene using the sulfuric acid method + Concentration 22 g / L, Mg 2+ Concentration 1.8 g / L, Ca 2+ Concentration 0.6 g / L, SO4 2- Place the mixture (concentration 120 g / L) into the reaction vessel, turn on the stirrer, and set the speed to 45 rpm.

[0031] Step 1: Control the reaction system temperature at 50℃. Add liquid alkali (32% NaOH solution by mass fraction) via atomized spray (spray pressure 0.3MPa). The first round adds 50% of the theoretical total amount of liquid alkali, with a spraying time of 10 minutes. After 15 minutes of reaction, the pH of the system is measured to be 10.5. Under these conditions, sulfate ions and calcium ions in the solution begin to precipitate to form calcium sulfate precipitate. Simultaneously, carbon dioxide dissolved in the alkaline low-temperature environment reacts with calcium ions to form calcium carbonate precipitate, forming calcium sulfate-calcium carbonate composite seed crystals.

[0032] Step Two: The reaction system is heated to 85°C at a rate of 1.5°C / min. In the second round, 30% of the theoretical total amount of liquid alkali is added via spraying for 8 minutes. Under the heating condition, magnesium ions gradually combine with hydroxide ions to form magnesium hydroxide precipitate, which then adheres in situ to the surface of the composite seed crystals generated in Step One for heterogeneous nucleation and growth. Samples are taken after 20 minutes of reaction, and a uniform coating layer of magnesium hydroxide has formed on the seed crystal surface.

[0033] Step 3: Add sodium carbonate at 30% of the original calcium content in the leachate (based on theoretical calculations), using a slow dropwise addition method over 25 minutes. After the addition is complete, continue stirring for 30 minutes. The OH- ions produced by the hydrolysis of sodium carbonate... - The reaction, in conjunction with the liquid alkali, brought the final pH of the reaction to 11.0. Simultaneously, the newly formed calcium carbonate filled the surface pores of the magnesium hydroxide coating, forming dense, composite precipitate particles.

[0034] Step 4: The reaction slurry is subjected to solid-liquid separation. The resulting calcium-magnesium composite slag is mixed with water at a mass-volume ratio of 1g:8mL, stirred and washed for 1 hour (45rpm), and then subjected to solid-liquid separation again to obtain calcium-magnesium slag with low lithium oxide content and purified lithium sulfate solution.

[0035] Tests showed that the purified lithium sulfate solution contained Mg. 2+ Concentration <0.05g / L, Ca 2+ Concentration < 0.03 g / L; Lithium oxide content in calcium-magnesium slag < 0.5%; filtration speed is about 60% higher than that of the conventional one-step method (directly adding alkali to pH 11).

[0036] Example 2: Take lithium sulfate leachate from the same source as in Example 1.

[0037] Step 1: Control the reaction system temperature at 45℃. Add liquid alkali by atomized spraying. The first round adds 55% of the theoretical total amount of liquid alkali, and the spraying time is 12 minutes. After the reaction, the pH of the system is 10.8, and calcium sulfate-calcium carbonate composite seed crystals are generated.

[0038] Step 2: Heat the reaction system to 80℃ at a rate of 1.0℃ / min. Add 25% of the theoretical total amount of liquid alkali in the second round, spraying for 6 minutes. Magnesium hydroxide undergoes heterogeneous nucleation and growth on the surface of the composite seed crystals.

[0039] Step 3: Add sodium carbonate at 40% of the original calcium content in the leachate, slowly dripping over 30 minutes. After the addition is complete, continue stirring for another 40 minutes. The final pH is 11.1.

[0040] Step 4: After solid-liquid separation, the calcium-magnesium composite slag is stirred and washed with water at a mass-volume ratio of 1:7 for 1.5 hours (40 rpm), and then solid-liquid separation is performed again.

[0041] Tests showed that the purified lithium sulfate solution contained Mg. 2+ Concentration <0.04 g / L, Ca 2+ Concentration < 0.02 g / L; Lithium oxide content in calcium-magnesium slag < 0.45%; filtration speed is increased by about 55% compared with conventional one-step method.

[0042] Example 3: Take lithium sulfate leachate from the same source as in Example 1.

[0043] Step 1: Control the reaction system temperature at 60℃. Add liquid alkali by atomized spraying. The first round of addition is 45% of the theoretical total amount of liquid alkali. The spraying time is 8 minutes. After the reaction, the pH of the system is 10.3, and calcium sulfate-calcium carbonate composite seed crystals are generated.

[0044] Step 2: Heat the reaction system to 90℃ at a rate of 2.0℃ / min. Add 35% of the theoretical total amount of liquid alkali in the second round, spraying for 10 minutes. Magnesium hydroxide undergoes heterogeneous nucleation and growth on the surface of the composite seed crystals.

[0045] Step 3: Add sodium carbonate at 25% of the original calcium content in the leachate, slowly dripping over 20 minutes. After the addition is complete, continue stirring for another 25 minutes. The final pH is 11.2.

[0046] Step 4: After solid-liquid separation, the calcium-magnesium composite slag is stirred and washed with water at a mass-volume ratio of 1:9 for 0.8 hours (50 rpm), and then solid-liquid separation is performed again.

[0047] Tests showed that the purified lithium sulfate solution contained Mg. 2+ Concentration <0.05g / L, Ca 2+ Concentration < 0.03 g / L; Lithium oxide content in calcium-magnesium slag < 0.48%; filtration speed is increased by about 65% compared to conventional one-step method.

[0048] Comparative Example 1: Lithium sulfate leachate from the same source as in Example 1 was used to remove calcium and magnesium using a conventional one-step method: the leachate was heated to 85°C in a reaction tank, and liquid alkali (100% of the theoretical amount) was added by spraying until the pH value was 11. After reacting at 45 rpm for 30 min, solid-liquid separation was performed. The calcium and magnesium slag was stirred at a mass-volume ratio of 1:8 for 1 h and then solid-liquid separation was performed again.

[0049] The results showed that the lithium oxide content in the magnesium slag obtained in the comparative example was approximately 1.4%; significant filter cloth clogging occurred during the filtration process, and the Mg content in the purified lithium sulfate solution was low. 2+ The concentration was approximately 0.14 g / L, and the magnesium removal effect was not as good as in Example 1.

[0050] Comparative Example 2: Take lithium sulfate leaching solution from the same source as in Example 1: heat the leaching solution to 85°C in the reaction tank, add liquid alkali (80% of the theoretical amount) by spraying until the pH value is 11, then add sodium carbonate according to 30% of the calcium content in the leaching solution, react at 45 rpm for 30 min, and then separate the solid and liquid. Stir the calcium and magnesium slag at a mass-volume ratio of 1:8 for 1 h and then separate the solid and liquid again.

[0051] The results showed that the lithium oxide content in the magnesium slag obtained in the comparative example was approximately 1.2%; significant filter cloth clogging occurred during the filtration process, and the Mg content in the purified lithium sulfate solution was low. 2+ The concentration was approximately 0.12 g / L, and the magnesium removal effect was not as good as in Example 1.

[0052] Comparative Example 3: Lithium sulfate leaching solution from the same source as in Example 1 was used. The reaction system temperature was controlled at 60°C. Liquid alkali was added via atomized spraying, with 45% of the theoretical total amount of liquid alkali added in the first round, and the spraying time was 8 minutes. After the reaction, the pH of the system was 10.3, and calcium sulfate-calcium carbonate composite seed crystals were generated. The reaction system was heated to 90°C at a rate of 2.0°C / min. In the second round, 55% of the theoretical total amount of liquid alkali was added, and the spraying time was 10 minutes. Magnesium hydroxide underwent heterogeneous nucleation and growth on the surface of the composite seed crystals. After reacting at 45 rpm for 30 minutes, solid-liquid separation was performed. The calcium-magnesium slag was stirred at a mass-to-volume ratio of 1:8 for 1 hour, and then solid-liquid separation was performed again.

[0053] The results showed that the lithium oxide content in the magnesium slag obtained in the comparative example was approximately 1.3%, and the Mg content in the purified lithium sulfate solution was... 2+ The concentration is approximately 0.14 g / L.

Claims

1. A method for removing calcium and magnesium simultaneously from spodumene leachate, characterized in that, Includes the following steps: (1) Under low temperature conditions, liquid alkali is added to the lithium sulfate leaching solution obtained by spodumene sulfuric acid leaching, and the pH value of the reaction system is controlled to reach above 10. The sulfate ions and calcium ions in the leaching solution precipitate to form calcium sulfate precipitate. At the same time, the carbon dioxide dissolved in the alkaline low temperature environment reacts with calcium ions to form calcium carbonate precipitate, forming calcium sulfate-calcium carbonate composite seed crystals. (2) Heat the reaction system and add liquid alkali. Magnesium ions combine with hydroxide ions to form magnesium hydroxide precipitate, which is attached in situ to the surface of the calcium sulfate-calcium carbonate composite seed crystal generated in step (1) for heterogeneous nucleation growth. (3) Add sodium carbonate to the reaction system to precipitate the remaining calcium ions as calcium carbonate. At the same time, the newly generated calcium carbonate fills the surface pores of the magnesium hydroxide coating layer to form dense composite precipitate particles. (4) Add water to the reaction system of step (3) and stir to separate the solid and liquid, and obtain calcium magnesium slag with low lithium oxide content and purified lithium sulfate solution.

2. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (1), the temperature of the low-temperature condition is 45-60℃.

3. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (1), liquid alkali is added by spraying, and the mass of liquid alkali added in the first round accounts for 40%-60% of the theoretical amount.

4. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (1), the liquid alkali is added in 2-4 batches, with an interval of 5-15 minutes between adjacent batches.

5. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (2), the reaction system is heated to 75-90℃ at a rate of 0.5-3℃ / min.

6. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (2), liquid alkali is added by spraying, and the mass of liquid alkali added in the second round accounts for 20%-40% of the theoretical amount.

7. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (3), sodium carbonate is added slowly, with a dripping time of 15-40 minutes.

8. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (3), the amount of sodium carbonate added is 25%-40% of the theoretical amount of calcium precipitation in the leachate. The hydroxide ions generated by the hydrolysis of sodium carbonate work together with the hydroxide ions provided by the liquid alkali in steps (1) and (2) to make the total alkalinity in the system sufficient to completely precipitate magnesium ions.

9. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (4), the mass-volume ratio of the composite particle precipitate to water in the reaction system is controlled to be 1:(6-10).

10. The method for removing calcium and magnesium from spodumene leaching solution according to claim 1, characterized in that, In step (4), the stirring and washing is carried out for 0.5-2 hours at a stirring speed of 30-60 rpm.