Refined lithium sulfate and production process thereof

Abstract

The invention discloses refined lithium sulfate and a production process thereof, and belongs to the technical field of refined lithium sulfate production, and the production process of the refined lithium sulfate comprises the steps of high-temperature roasting, acidification, low-temperature roasting, water adding and leaching, step-by-step pH value adjustment, impurity removal agent adding, solid-liquid separation and filter pressing, and finally the refined lithium sulfate is obtained. Wherein the impurity removal agent is cellulose nanocrystal composite nonionic PAM (Polyacrylamide). Different pH values are set step by step, and the function of the impurity removing agent is matched, so that the partitioned flocculation and sedimentation removal of impurity particles can be realized, the removal efficiency of impurity ions is improved, and the impurity ions are helped to be completely removed. The nonionic PAM takes cellulose nanocrystals as a carrier, so that the mechanical strength and the structural stability of the impurity removal agent can be improved, the impurity particle attraction and capture capability of the impurity removal agent is enhanced, the pH adaptability of the impurity removal agent is optimized, and the impurity particles are efficiently removed.

Description

Technical Field

[0001] The present invention belongs to the technical field of producing refined lithium sulfate, and particularly relates to a refined lithium sulfate and its production process. Background Art

[0002] Lithium materials are important representatives of green new energy materials. With the increasing demand for lithium in the lithium battery, grease, and tire rubber industries, as well as the demand for lithium alloys in the aerospace and aviation industries, the production scale and output of lithium have developed rapidly. Among them, lithium, as an essential component of new energy power batteries, refined lithium sulfate is a key precursor material for preparing new energy power batteries and has important applications in various links such as the synthesis of cathode materials, the preparation of electrolytes, and the recycling of batteries.

[0003] Lithium widely exists in salt lake brines and lithium-containing ores. Lithium ores mainly include lepidolite, spodumene, etc. Compared with lepidolite, spodumene has a high grade and is widely distributed, and is often used as a basic lithium source to produce high-end lithium salts such as lithium carbonate, lithium sulfate, and lithium hydroxide.

[0004] As the mainstream technology for extracting lithium from spodumene, the sulfuric acid method mainly involves high-temperature roasting, reacting with concentrated sulfuric acid to form a lithium sulfate solution, and then obtaining it through steps such as leaching, purification, and concentration. However, although the sulfuric acid method has a mature process, there are still problems such as incomplete impurity removal and large environmental pollution in the production of refined lithium sulfate. During the production process of the sulfuric acid method, it is often observed that a completely clear lithium sulfate purification solution often flocculates during the concentration process, and hydroxides of iron, aluminum, and magnesium precipitate out, indicating that in the production process of refining lithium sulfate by the sulfuric acid method, there is a low impurity precipitation removal rate and residual colloidal impurities remain in the lithium sulfate purification solution, which will affect the subsequent process and product quality. Summary of the Invention

[0005] The purpose of the present invention is to provide a refined lithium sulfate and its production process to solve the problem of low impurity removal rate in the production process of refined lithium sulfate.

[0006] The purpose of the present invention can be achieved through the following technical solutions: In the first aspect, the present invention provides a production process of refined lithium sulfate, including the following process steps: S1. After spodumene is crushed, it is preheated at 150 - 400 °C, and then subjected to high-temperature roasting at 900 - 1000 °C, and after cooling, it is ground to obtain a pretreated ore powder; S2. The pretreated ore powder is mixed with concentrated sulfuric acid and subjected to low-temperature roasting at 180 - 250 °C to obtain an acidified clinker; S3. Add the acidified clinker to deionized water to obtain a mixed solution. Stir and leach, add lime powder, adjust the pH value to 6 - 7, add a impurity remover, stir and mix for 30 - 60 min, continue to add lime powder, adjust the pH value to 10 - 11, stir for 20 - 40 min, and finally perform solid-liquid separation to remove the slag. Press-filter the obtained lithium sulfate solution to obtain refined lithium sulfate; The impurity remover is cellulose nanocrystal composite non-ionic PAM.

[0007] Preferably, the addition amount of the impurity remover is 1 - 3 wt% of the mixed solution.

[0008] Preferably, the particle size of the pretreated ore powder ≤ 200 mesh.

[0009] Preferably, the mass ratio of the pretreated ore powder to concentrated sulfuric acid is (15 - 20) : 100.

[0010] By adopting the above technical solution, the main component of spodumene in nature is α-spodumene. α-spodumene belongs to the monoclinic system, has a dense structure, large chemical inertness, and hardly reacts with acids and alkalis. Therefore, first crush the spodumene to increase the specific surface area, and then perform crystal form transformation by roasting at high temperature to transform the monoclinic α-spodumene into tetragonal β-spodumene. The physical and chemical properties of the mineral will change significantly with the change of the crystal structure. Then mix with concentrated sulfuric acid, and under the condition of low-temperature roasting, an ion exchange reaction occurs between β-spodumene and hydrogen ions in concentrated sulfuric acid to generate soluble lithium sulfate. After water leaching, lithium sulfate is extracted from spodumene.

[0011] During the leaching and purification of lithium sulfate, first add lime powder for the first neutralization. On the one hand, increasing the solution pH value can neutralize the residual acid in the slurry of the acidified clinker, thereby reducing the corrosion of the equipment by the residual concentrated sulfuric acid. On the other hand, during the leaching process, impurities such as iron, aluminum, and magnesium in the ore also enter the solution. Under the condition of the first neutralization, iron ions and aluminum ions hydrolyze to form hydroxide colloids, and with the impurity remover, impurities such as iron ions and aluminum ions in the system are removed; further, add lime powder to adjust the pH value again. Under the condition of high pH value, magnesium ions will react to form hydroxide precipitates, and then be adsorbed by the impurity remover for removal. By using the two-step method to remove impurity ions in the lithium sulfate solution respectively, it can help to completely remove the impurity ions.

[0012] In the process of purifying the lithium sulfate solution, a impurity removing agent is also added. Specifically, the impurity removing agent is cellulose nanocrystal composite non-ionic PAM. There are no strong charge groups in the molecular chain of non-ionic PAM (i.e., non-ionic polyacrylamide). It mainly binds to hydroxide colloid particles through hydrogen bonds and van der Waals forces. Through the bridging effect, its long-chain molecules can connect multiple small colloid particles to form large flocs, so as to efficiently remove the hydroxide colloid in the lithium sulfate solution. And the low charge characteristic of non-ionic PAM can avoid competing with lithium ions for adsorption. The electrostatic force between non-ionic PAM and lithium ions is weak, and the dissolved lithium ions will not be directly adsorbed by non-ionic PAM. While ensuring the impurity removal and adsorption efficiency, the loss of lithium ions can be greatly reduced, thus increasing the recovery rate.

[0013] However, due to magnesium hydroxide, including calcium ions introduced from lime powder added for pH adjustment, the separation efficiency of these high-valent ions is high at higher pH values. However, when non-ionic PAM is directly added under high pH conditions, due to the lack of charge neutralization ability of non-ionic PAM, the formed flocs will be fluffy, the sedimentation rate will be slow, and the adsorption and impurity removal effect will be affected. And during the purification and impurity removal process, the long molecular chain segments of non-ionic PAM will break due to mechanical shearing, thus reducing the flocculation effect. But without mechanical shearing, it will lead to uneven dispersion of non-ionic PAM and the formation of agglomerated microparticles. To solve these problems, the present invention uses cellulose nanocrystals as a carrier to compound non-ionic PAM to form an impurity removing agent.

[0014] The rigid rod-like structure of cellulose nanocrystals can serve as a physical support framework, with good mechanical strength and shear resistance. Using it as the carrier material of non-ionic PAM can improve the mechanical stability of flocs, resist the water flow shear force, effectively prevent the flocs from breaking, accelerate the sedimentation process, and can also significantly extend the PAM molecular chain, increasing the contact area between the impurity removing agent and impurity particles. And the surface of cellulose nanocrystals contains a large number of hydroxyl groups and sulfate ester groups, which can enhance the attraction and capture ability of the impurity removing agent for impurity particles through electrostatic attraction. Moreover, the charge density of cellulose nanocrystals can adapt to different pH conditions and can remain stably dispersed in a high pH environment. After being compounded with non-ionic PAM, it can optimize the pH adaptability of the impurity removing agent, enhance the mechanical structure stability of the impurity removing agent itself and the adsorption ability for impurity particles.

[0015] At the same time, compared with other carriers or coagulants, such as diatomite, bentonite, etc., cellulose nanocrystals have stronger tensile and deformation resistance, and are biodegradable. After being compounded with non-ionic PAM, the structure is stable, and it can also play a role in improving the extensibility of PAM molecules, and can better play a synergistic effect.

[0016] Preferably, the impurity removing agent is also compounded with active silicic acid.

[0017] By adopting the above technical solution, in order to reduce the loss rate of lithium ions, the impurity removing agent of the present invention uses non-ionic PAM. However, non-ionic PAM cannot reduce the potential of hydroxide colloid particles through charge neutralization and only relies on physical bridging action, resulting in a decrease in the flocculation removal efficiency.

[0018] Therefore, the impurity removing agent of the present invention is also compounded with active silicic acid. Active silicic acid can reduce the stability of hydroxide colloid particles in the lithium sulfate solution through electro-neutralization. After the colloid particles are destabilized, they can be adsorbed by cellulose nanocrystals compounded with non-ionic PAM, enhancing the impurity removing efficiency. Moreover, the network structure formed by active silicic acid can adsorb fine particles, wrap and precipitate some incompletely destabilized impurity particles and the added lime powder impurities, thereby improving the purity of refined lithium sulfate.

[0019] In addition, compounding active silicic acid can improve the structural stability of the impurity removing agent, enhance the shear resistance of the impurity removing agent during the purification process, and the formed composite system can further enhance the sedimentation effect and rate of the flocs, further increasing the environmental adaptability of the impurity removing agent.

[0020] Preferably, the raw materials for compounding the impurity removing agent with active silicic acid include microcrystalline cellulose, non-ionic PAM and active silicic acid in a mass ratio of 1:(0.3 - 0.4):(0.02 - 0.03).

[0021] Preferably, the impurity removing agent is prepared by the following method: Add microcrystalline cellulose to concentrated sulfuric acid, raise the temperature to 40 - 50 °C, stir and mix for 1 - 2 h, and then obtain cellulose nanocrystals through centrifugation, washing, neutralization and drying; Add sodium silicate to water, add inorganic acid, stir and react for 4 - 5 h, and then obtain active silicic acid through centrifugal separation and washing; Disperse cellulose nanocrystals in water, add a non-ionic PAM solution with a mass fraction of 4 - 6%, stir and react in an ice-water bath for 30 - 60 min, add active silicic acid, raise the temperature to 30 - 35 °C, continue to stir and react for 20 - 40 min, and finally obtain the product through filtration, washing and freeze-drying.

[0022] Preferably, the solid-liquid ratio of microcrystalline cellulose to concentrated sulfuric acid is 1 g:(35 - 45) mL.

[0023] Preferably, the inorganic acid includes one or a combination of two of hydrochloric acid and sulfuric acid; the mass ratio of sodium silicate to inorganic acid is 1:(3.5 - 4).

[0024] By adopting the above technical solution, under the action of concentrated sulfuric acid, the loose amorphous regions in microcrystalline cellulose will hydrolyze and break, improving the crystallinity of the obtained cellulose nanocrystals. At the same time, the cellulose chains break and finally form rod-shaped cellulose nanocrystals. This unique structure can help extend the molecular chains of non-ionic PAM and increase the contact area with impurity particles. And during the hydrolysis process, concentrated sulfuric acid reacts with the surface of microcrystalline cellulose to introduce sulfate groups, forming an electrostatic repulsion effect and improving the dispersibility of the impurity remover in the lithium sulfate solution.

[0025] The hydroxyl groups and sulfate groups on the surface of cellulose nanocrystals can bind to the polymer through hydrogen bonds and physical adsorption. The molecular chains of non-ionic PAM wind around the surface of cellulose nanocrystals, forming a wrapping structure with cellulose nanocrystals as the carrier. The high specific surface area of cellulose nanocrystals provides a large number of adsorption sites, and the rigid structure significantly improves the mechanical structure stability of the impurity remover.

[0026] Furthermore, the silicate radicals in sodium silicate react with hydrogen ions in inorganic acids to generate free silicic acid monomers, which then undergo polycondensation to form active silicic acid. The large number of hydroxyl groups contained in active silicic acid can form hydrogen bonds with the polar groups on the surface of cellulose nanocrystals, thus obtaining a composite. The introduction of active silicic acid can, under high pH conditions, make up for the defect of the decreased adsorption efficiency of non-ionic PAM and strengthen the adsorption of magnesium ions and calcium ions in the lithium sulfate solution through complexation. Under the action of the impurity remover, it is possible to simultaneously remove ionic impurities and flocculate and remove hydroxide colloid particles, significantly improving the purification effect of the lithium sulfate solution. And because lithium ions have a low charge density, a small hydration radius, and a weak interaction with the impurity remover, they will be preferentially retained in the solution during the adsorption process, achieving the selective separation of lithium ions from other impurities and avoiding the loss of lithium ions.

[0027] The beneficial effects of the present invention are as follows: 1. The present invention provides a production process for refining lithium sulfate. By setting different pH values step by step in the purification and impurity removal stage and combining with the action of the impurity remover, zonal flocculation and sedimentation removal of impurity particles are achieved, thereby improving the removal efficiency of impurity ions and helping to completely remove impurity ions.

[0028] 2. The impurity remover used in the production process of the refined lithium sulfate provided by the present invention is cellulose nanocrystal composite non-ionic PAM. The non-ionic PAM does not contain strong charge groups, which can significantly reduce the loss of lithium ions. At the same time, it can use the bridging effect to connect hydroxide colloid particles and efficiently remove impurity particles. The non-ionic PAM uses cellulose nanocrystals as a carrier, which can improve the mechanical strength and structural stability of the obtained impurity remover, and enhance the attraction and capture ability of the impurity remover for impurity particles, and optimize the pH adaptability of the impurity remover. Further, active silicic acid can be compounded on the impurity remover to enhance the impurity removal efficiency and further improve the purity of the refined lithium sulfate. Detailed implementation mode

[0029] The following will combine the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

[0030] Preparation example Preparation example 1, an impurity remover, is prepared according to the following method: Take 1 g of microcrystalline cellulose and add it to 40 mL of concentrated sulfuric acid. Raise the temperature to 45 °C and stir and mix for 1 h, then obtain cellulose nanocrystals through centrifugation, washing, neutralization, and drying. Disperse 10 g of the obtained cellulose nanocrystals in 50 mL of water, add a 5% non-ionic PAM solution (where the addition amount of non-ionic PAM is 3.5 g and the molecular weight of non-ionic PAM is 2 million to 14 million), stir and react in an ice-water bath for 40 min, and obtain the product through filtration, washing, and freeze-drying.

[0031] Preparation example 2, an impurity remover, is different from preparation example 1 only in that the addition amount of non-ionic PAM is 3 g.

[0032] Preparation example 3, an impurity remover, is different from preparation example 1 only in that the addition amount of non-ionic PAM is 4 g.

[0033] Preparation example 4, an impurity remover, is different from preparation example 1 only in that the addition amount of non-ionic PAM is 2 g.

[0034] Preparation example 5, an impurity remover, is different from preparation example 1 only in that the addition amount of non-ionic PAM is 5 g.

[0035] Preparation example 6, an impurity remover, is prepared according to the following method: 1 g of microcrystalline cellulose was added to 40 mL of concentrated sulfuric acid, the temperature was raised to 45 °C, and the mixture was stirred for 1 h. Then, cellulose nanocrystals were obtained through centrifugation, washing, neutralization, and drying. 1 g of sodium silicate was added to 50 mL of water, 3.7 g of concentrated sulfuric acid was added, and the mixture was stirred and reacted for 4 h. Then, active silicic acid was obtained through centrifugation and washing. 10 g of the obtained cellulose nanocrystals were dispersed in 50 mL of water, a 5% non-ionic PAM solution (where the addition amount of non-ionic PAM was 3.5 g and the molecular weight of non-ionic PAM was 2 million to 14 million) was added, and the mixture was stirred and reacted in an ice-water bath for 40 min. 0.2 g of the above-obtained active silicic acid was added, the temperature was raised to 30 °C, and the mixture was continuously stirred and reacted for 30 min. Finally, it was obtained through filtration, washing, and freeze-drying.

[0036] Preparation Example 7, a deimpurant, which is only different from Preparation Example 6 in that the addition amount of inorganic silicic acid is 0.3 g.

[0037] Preparation Example 8, a deimpurant, which is only different from Preparation Example 6 in that the addition amount of inorganic silicic acid is 0.1 g.

[0038] Preparation Example 9, a deimpurant, which is only different from Preparation Example 6 in that the addition amount of inorganic silicic acid is 0.4 g.

[0039] Preparation Example 10, a deimpurant, was prepared according to the following method: 10 g of sodium-based bentonite (average particle size of 325 mesh) was dispersed in 50 mL of water, a 5% non-ionic PAM solution (where the addition amount of non-ionic PAM was 3.5 g and the molecular weight of non-ionic PAM was 2 million to 14 million) was added, and the mixture was stirred and reacted in an ice-water bath for 40 min. It was obtained through filtration, washing, and freeze-drying.

[0040] Examples Example 1, a refined lithium sulfate, was prepared according to the following process steps: S1. After spodumene was crushed, it was preheated at 250 °C and then subjected to high-temperature roasting at 1000 °C. After cooling, it was ground to obtain a pretreated ore powder, and the average particle size of the pretreated ore powder was 120 mesh. S2. The pretreated ore powder was mixed with concentrated sulfuric acid, and the mass ratio of the pretreated ore powder to concentrated sulfuric acid was 18:100. It was subjected to low-temperature roasting at 200 °C to obtain an acidified clinker. S3. Add the acidified clinker to deionized water, stir and leach it, add lime powder, adjust the pH value to 6.5, add the impurity remover prepared in Preparation Example 1 with a mass fraction of 2% of the total mass of the acidified clinker and deionized water, stir and mix for 40 min, continue to add lime powder, adjust the pH value to 11, stir for 30 min, and finally remove the slag by solid-liquid separation. Press-filter the obtained lithium sulfate solution to obtain refined lithium sulfate.

[0041] Example 2. A kind of refined lithium sulfate is prepared according to the following technological steps: S1. After spodumene is crushed, preheat it at 250 °C, then conduct high-temperature roasting at 1000 °C, cool and grind to obtain pretreated ore powder, and the average particle size of the pretreated ore powder is 120 mesh; S2. Mix the pretreated ore powder with concentrated sulfuric acid, where the mass ratio of the pretreated ore powder to concentrated sulfuric acid is 20:100, and conduct low-temperature roasting at 200 °C to obtain acidified clinker; S3. Add the acidified clinker to deionized water, stir and leach it, add lime powder, adjust the pH value to 6, add the impurity remover prepared in Preparation Example 1 with a mass fraction of 1% of the total mass of the acidified clinker and deionized water, stir and mix for 40 min, continue to add lime powder, adjust the pH value to 11, stir for 30 min, and finally remove the slag by solid-liquid separation. Press-filter the obtained lithium sulfate solution to obtain refined lithium sulfate.

[0042] Example 3. A kind of refined lithium sulfate is prepared according to the following technological steps: S1. After spodumene is crushed, preheat it at 250 °C, then conduct high-temperature roasting at 1000 °C, cool and grind to obtain pretreated ore powder, and the average particle size of the pretreated ore powder is 120 mesh; S2. Mix the pretreated ore powder with concentrated sulfuric acid, where the mass ratio of the pretreated ore powder to concentrated sulfuric acid is 15:100, and conduct low-temperature roasting at 200 °C to obtain acidified clinker; S3. Add the acidified clinker to deionized water, stir and leach it, add lime powder, adjust the pH value to 7, add the impurity remover prepared in Preparation Example 1 with a mass fraction of 3% of the total mass of the acidified clinker and deionized water, stir and mix for 40 min, continue to add lime powder, adjust the pH value to 10, stir for 30 min, and finally remove the slag by solid-liquid separation. Press-filter the obtained lithium sulfate solution to obtain refined lithium sulfate.

[0043] Example 4. A kind of refined lithium sulfate, the difference from Example 1 is only that the impurity remover prepared in Preparation Example 2 is used to replace the impurity remover prepared in Preparation Example 1 in equal amount.

[0044] Example 5. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 3 is used to replace the impurity removing agent prepared in Preparation Example 1 in an equal amount.

[0045] Example 6. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 4 is used to replace the impurity removing agent prepared in Preparation Example 1 in an equal amount.

[0046] Example 7. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 5 is used to replace the impurity removing agent prepared in Preparation Example 1 in an equal amount.

[0047] Example 8. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 6 is used to replace the impurity removing agent prepared in Preparation Example 1 in an equal amount.

[0048] Example 9. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 7 is used to replace the impurity removing agent prepared in Preparation Example 1 in an equal amount.

[0049] Example 10. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 8 is used to replace the impurity removing agent prepared in Preparation Example 1 in an equal amount.

[0050] Example 11. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 9 is used to replace the impurity removing agent prepared in Preparation Example 1 in an equal amount.

[0051] Example 12. A refined lithium sulfate, which is different from Example 1 only in that the addition amount of the impurity removing agent prepared in Preparation Example 1 is 0.5%.

[0052] Example 13. A refined lithium sulfate, which is different from Example 1 only in that the addition amount of the impurity removing agent prepared in Preparation Example 1 is 4%.

[0053] Comparative Example Comparative Example 1. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 10 is used to replace the impurity removing agent prepared in Preparation Example 1 in an equal amount.

[0054] Comparative Example 2. A refined lithium sulfate, which is different from Example 1 only in that an equal amount of non-ionic PAM (the molecular weight of non-ionic PAM is 2 million to 14 million) is used to replace the impurity removing agent prepared in Preparation Example 1.

[0055] Comparative Example 3. A refined lithium sulfate, which is different from Example 1 only in that the impurity removing agent prepared in Preparation Example 1 is not added.

[0056] Comparative Example 4. A refined lithium sulfate is prepared according to the following process steps: S1. After spodumene is crushed, it is preheated at 250 °C and then subjected to high-temperature roasting at 1000 °C. After cooling, it is ground to obtain pretreated ore powder, and the average particle size of the pretreated ore powder is 120 mesh; S2. The pretreated ore powder is mixed with concentrated sulfuric acid, where the mass ratio of the pretreated ore powder to concentrated sulfuric acid is 15:100, and low-temperature roasting is carried out at 200 °C to obtain acidified clinker; S3. The acidified clinker is added to deionized water and stirred for leaching. Lime powder is added to adjust the pH value to 6.5. An impurity removal agent prepared in Preparation Example 1 with a mass fraction of 3% of the total mass of the acidified clinker and deionized water is added, and stirred and mixed for 40 min. Finally, through solid-liquid separation, the slag is removed, and the obtained lithium sulfate solution is pressure-filtered to obtain refined lithium sulfate.

[0057] Comparative Example 5, a kind of refined lithium sulfate, is prepared according to the following technological steps: S1. After spodumene is crushed, it is preheated at 250 °C and then subjected to high-temperature roasting at 1000 °C. After cooling, it is ground to obtain pretreated ore powder, and the average particle size of the pretreated ore powder is 120 mesh; S2. The pretreated ore powder is mixed with concentrated sulfuric acid, where the mass ratio of the pretreated ore powder to concentrated sulfuric acid is 15:100, and low-temperature roasting is carried out at 200 °C to obtain acidified clinker; S3. The acidified clinker is added to deionized water and stirred for leaching. Lime powder is added to adjust the pH value to 11. An impurity removal agent prepared in Preparation Example 1 with a mass fraction of 3% of the total mass of the acidified clinker and deionized water is added, and stirred and mixed for 40 min. Finally, through solid-liquid separation, the slag is removed, and the obtained lithium sulfate solution is pressure-filtered to obtain refined lithium sulfate.

[0058] Performance detection test According to the relevant records in the industry standard YS / T 1241-2018 "Lithium Sulfate", the lithium sulfate content, calcium, magnesium and iron contents of the refined lithium sulfate obtained in the examples and comparative examples are respectively tested, and the test results are shown in Table 1:

[0059] Note: "ND" means "not detected".

[0060] According to Table 1, in combination with Example 1, Example 6, and Example 7, it can be seen that the content of impurity ions in Example 6 and Example 7 increases, indicating that the impurity removal effect in Example 6 and Example 7 decreases and the impurity removal is incomplete. The reason is that in Example 6 and Example 7, the content of non-ionic PAM complexed with cellulose nanocrystals in the impurity remover is changed. When the content of non-ionic PAM decreases, correspondingly, the binding effect between the impurity remover and hydroxide colloid particles will decrease, and the adsorption effect with high-valence ions will also decrease, resulting in incomplete impurity removal; when the content of non-ionic PAM increases, the molecular chain segments in the impurity remover increase, and it is easy to occur molecular entanglement within the molecule, resulting in a decrease in the adsorption and flocculation ability. Its steric hindrance effect will also stabilize the hydroxide colloid particles in the solution, resulting in a decrease in the effect of impurity particle aggregation and sedimentation.

[0061] In combination with Example 1, Example 8, Example 10, and Example 11, it can be seen that the content of impurity ions in Example 8 is less than that in Example 1. The reason is that the impurity remover in Example 8 is also compounded with active silicic acid, which can improve the binding force between the impurity remover and high-valence ions such as calcium ions and magnesium ions under high pH conditions, making the removal of impurity particles more thorough; the removal effect of Example 10 and Example 11 is lower than that of Example 8. The reason is that the content of the compounded active silicic acid in Example 10 is reduced, and the removal efficiency decreases accordingly; the content of the compounded active silicic acid in Example 11 is increased, and the charge density contained in the impurity remover is high. Lithium ions in the lithium sulfate solution also begin to be adsorbed by the impurity remover, resulting in a decrease in the lithium sulfate content in the refined lithium sulfate.

[0062] In combination with Example 1, Example 12, Example 13, and Comparative Example 3, it can be seen that compared with Example 1, the contents of calcium, magnesium, and iron in the refined lithium sulfate in Example 12, Example 13, and Comparative Example 3 increase, indicating that the impurity removal effect decreases. The reason is that in Example 12, Example 13, and Comparative Example 3, the addition amount of the impurity remover is changed. The addition amount of the impurity remover in Example 12 is reduced, and the impurity removal effect decreases accordingly. In Comparative Example 3, no impurity remover is added, and the impurity removal is incomplete, and the impurity removal effect decreases significantly; the addition amount of the impurity remover in Example 13 is increased. The addition of excessive cellulose nanocrystal composite non-ionic PAM will reduce the diffusion efficiency, resulting in a decrease in the adsorption effect of impurity particles, and will also increase the loss rate of lithium ions, resulting in a decrease in the lithium sulfate content and an increase in the content of impurity ions in the finally obtained refined lithium sulfate.

[0063] Combined with Example 1, Comparative Example 1 and Comparative Example 2, it can be seen that the removal effects and lithium sulfate contents of Comparative Example 1 and Comparative Example 2 have both decreased. The reason is that in Comparative Example 1, bentonite and non-ionic PAM are compounded. Compared with cellulose nanocrystals, bentonite has low mechanical strength and the ability to resist water flow shear force decreases. During the purification and impurity removal process, the structural stability of the impurity remover decreases, thus affecting the impurity removal efficiency. Moreover, the unique rod-like structure of cellulose nanocrystals can also help improve the molecular extensibility of non-ionic PAM, thus playing a better synergistic effect. Compared with Example 1, Comparative Example 2 does not use cellulose nanocrystals as a carrier and directly adds non-ionic PAM. On the one hand, lacking the structural support of cellulose nanocrystals, the mechanical strength of non-ionic PAM is low. In an environment with a high pH value, the formed flocs are fluffy and the sedimentation speed is slow, resulting in a decrease in the impurity removal effect. On the other hand, lacking the compounding of cellulose nanocrystals, the adaptability of non-ionic PAM to the environmental pH value decreases, the dispersibility decreases, and agglomerated particles are easily formed, leading to a decrease in the impurity removal effect.

[0064] Combined with Example 1, Comparative Example 4 and Comparative Example 5, it can be seen that the impurity ion contents in Comparative Example 4 and Comparative Example 5 are significantly increased compared with Example 1. The reason is that in Comparative Example 4 and Comparative Example 5, the stepwise method is not adopted to adjust the pH value of the lithium sulfate solution twice. Specifically, in Comparative Example 4, the adjusted pH value is relatively low. The hydroxide colloid particles formed by iron ions and aluminum ions in this pH value range are easy to sediment, which is suitable for the removal of iron ions and aluminum ions, but not suitable for the calcium ions introduced by lime powder and the magnesium ion impurities in the ore, resulting in a significant increase in the calcium ion and magnesium ion contents in the finally obtained refined lithium sulfate; Comparative Example 5 is the opposite of Comparative Example 4. The pH value range in Comparative Example 5 is suitable for the removal of calcium ions and magnesium ions and is not conducive to the removal of iron ions and magnesium ions. Moreover, under high pH value conditions, the remaining iron ions and aluminum ions may also competitively adsorb on the surface of cellulose nanocrystal composite non-ionic PAM, thus interfering with the bridging effect of non-ionic PAM and affecting the impurity removal effect of the impurity remover.

[0065] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0066] Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A production process for refined lithium sulfate, characterized in that: The process steps include: S1. After the spodumene is crushed, it is preheated at 150-400°C, then calcined at 900-1000°C, and then ground to obtain pretreated ore powder after cooling; S2. The pretreated ore powder is mixed with concentrated sulfuric acid and calcined at 180 to 250 ° C to obtain acidified clinker; S3. The acidified clinker is added to deionized water to obtain a mixed solution, stirred and leached, lime powder is added, the pH value is adjusted to 6 to 7, an impurity remover is added, and the mixture is stirred for 30 to 60 min, lime powder is continued to be added, the pH value is adjusted to 10 to 11, and stirred for 20 to 40 min, and finally the slag is removed by solid-liquid separation, and the obtained lithium sulfate solution is filtered to obtain refined lithium sulfate; The impurity remover is cellulose nanocrystal composite non-ionic PAM.

2. The production process of refined lithium sulfate according to claim 1, characterized in that: The addition amount of the impurity remover is 1 to 3 wt % of the mixed solution.

3. The production process of refined lithium sulfate according to claim 1, characterized in that: The impurity remover is also compounded with active silicic acid.

4. The production process of refined lithium sulfate according to claim 3, characterized in that: The raw materials of the impurity remover composite active silicate include microcrystalline cellulose, nonionic PAM and active silicate in a mass ratio of 1: (0.3-0.4): (0.02-0.03).

5. The production process of refined lithium sulfate according to claim 4, characterized in that: The impurity remover is prepared according to the following method: Add microcrystalline cellulose to concentrated sulfuric acid, raise the temperature to 40-50°C, stir and mix for 1-2 hours, and then centrifuge, wash, neutralize and dry to obtain cellulose nanocrystals; Sodium silicate is added to water, and then an inorganic acid is added, and the mixture is stirred for 4 to 5 hours, and then centrifuged and washed to obtain active silicate; Disperse cellulose nanocrystals in water, add 4-6% by mass of non-ionic PAM solution, stir and react in an ice-water bath for 30-60 minutes, add active silicic acid, raise the temperature to 30-35°C, continue stirring and reacting for 20-40 minutes, and finally filter, wash and freeze-dry to obtain the product.

6. The production process of refined lithium sulfate according to claim 5, characterized in that: The solid-liquid ratio of the microcrystalline cellulose to concentrated sulfuric acid is 1 g: (35-45) mL.

7. The production process of refined lithium sulfate according to claim 5, characterized in that: The inorganic acid includes one or a combination of hydrochloric acid and sulfuric acid; the mass ratio of the sodium silicate to the inorganic acid is 1:(3.5-4).

8. The production process of refined lithium sulfate according to claim 1, characterized in that: The particle size of the pretreated mineral powder is ≤200 mesh.

9. The production process of refined lithium sulfate according to claim 1, characterized in that: The mass ratio of the pretreated mineral powder to concentrated sulfuric acid is (15-20):

100.

10. A refined lithium sulfate, characterized in that: The refined lithium sulfate is prepared according to the production process of refined lithium sulfate according to any one of claims 1 to 9.

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