A method for improving lithium yield in lithium extraction from lepidolite ore

Abstract

The present invention relates to a method for improving the lithium yield in lithium extraction from lepidolite ore, belonging to the field of mineral processing technology. The method is used to solve the problems of harsh conditions, high energy consumption, and certain lithium loss in existing roasting methods for lithium extraction. The method comprises: crushing the lepidolite ore and sieving to obtain lepidolite powder; adding the lepidolite powder to a reactor, adding water and an alkaline activator thereto, and stirring; sealing the reactor, heating it to 75-85°C and keeping it warm to react; after the reaction is completed, opening the reactor, filtering the mixture therein to obtain a primary filtrate and a primary filter residue, and drying the primary filter residue; then uniformly mixing the primary filter residue with a composite sulfate and a calcium oxide auxiliary agent and roasting it; crushing the roasted primary filter residue and adding it to a reactor, adding water and sodium hydroxide thereto, and stirring to mix uniformly; heating it to 75-85°C and keeping it warm for 2-3 hours; after the reaction is completed, opening the reactor, filtering, and obtaining a secondary filtrate and a secondary filter residue. The method of the present invention can improve the lithium yield.

Description

Technical Field

[0001] The present invention relates to the technical field of mineral processing, and in particular to a method for improving the lithium yield in lithium extraction from lepidolite ore. Background Art

[0002] In recent years, with the widespread use of lithium-ion batteries, the demand for lithium and its compounds has been growing. Industrial methods for obtaining lithium can be divided into two main categories: one is to enrich lithium in brine and produce lithium salt products; the other is to extract lithium from ore, which mainly involves roasting to dissolve the lithium-containing compounds in the ore as soluble lithium salts, and then produce lithium salt products.

[0003] my country's domestic salt lake lithium resources are relatively poor, with low lithium content and a high magnesium-lithium ratio. Therefore, the main sources of lithium and its compounds are lithium-containing ores, usually spodumene and lepidolite.

[0004] The key to extracting lithium from lithium-containing ores is destroying the ore's original aluminosilicate structure. The traditional process for extracting lithium from spodumene involves roasting it at 1000°C to convert it into β-spodumene, which is then roasted with concentrated acid and then leached with water to extract the lithium. This method is not only complex but also significantly increases energy consumption and environmental pollution.

[0005] The commonly used process for lithium extraction from lepidolite is sulfate roasting, which requires multiple benefits during the sintering process, including fluorine capture, preventing sintering, and promoting lithium leaching. The sulfate roasting method typically uses complex recipes and high roasting temperatures (typically between 900-980°C). The lithium yield is highly dependent on the stability of the raw material composition. For example, a slight increase in the fluorine content of the lepidolite can result in insufficient calcium salt for fluorine capture, reducing the activation effect and generating additional fluoride ions that react with lithium to form volatile lithium fluoride, potentially damaging the kiln. This necessitates lithium content testing of each batch of lithium slag after leaching to monitor leaching performance and avoid waste due to low leaching rates. Furthermore, the crystal transformation process during high-temperature roasting is energy-intensive, lengthy, and costly, and results in a certain amount of lithium loss. This results in high production and management costs, hindering the widespread utilization of lepidolite resources. Summary of the Invention

[0006] In view of the above analysis, the present invention aims to provide a method for improving the lithium yield in lithium extraction from lepidolite ore, which is used to solve at least one of the following technical problems: the existing roasting method for lithium extraction has harsh conditions, high temperature, high energy consumption, and a certain amount of lithium loss.

[0007] The purpose of the present invention is mainly achieved through the following technical solutions:

[0008] In one aspect, the present invention provides a method for improving the lithium yield in lithium extraction from lepidolite ore, comprising the following steps:

[0009] S1, crushing the lepidolite ore and sieving it to obtain lepidolite powder;

[0010] S2, the lepidolite powder is added into the reactor, water and alkali activator are added thereto, and stirring is uniformly mixed;

[0011] S3, after sealing the reactor, raise the temperature to 75-85°C and keep it warm to react;

[0012] S4. After the reaction is completed, the reactor is opened and the mixture is filtered to obtain a primary filtrate and a primary filter residue, which is then dried; the primary filter residue is then mixed evenly with a composite sulfate and a calcium oxide additive, and calcined;

[0013] S5, crushing the primary filter residue after roasting, adding it to the reaction kettle, adding water and sodium hydroxide thereto, stirring and mixing evenly; heating to 75-85°C and keeping warm for 2-3h;

[0014] S6. After the reaction is completed, the reactor is opened and filtered to obtain a secondary filtrate and a secondary filter residue.

[0015] Furthermore, the alkali activator is calcium oxide or sodium hydroxide.

[0016] Furthermore, in S2, the mass concentration of the alkali activator is controlled to be 1.5% to 2.5%.

[0017] Furthermore, in S2, the liquid-to-solid ratio in the reactor is 4 to 10:1.

[0018] Furthermore, in S3, the reaction time is 1 to 2 hours.

[0019] Furthermore, in S4, the composite sulfate includes sodium sulfate and potassium sulfate.

[0020] Furthermore, the mass ratio of sodium sulfate to potassium sulfate is 1:1.9-2.1.

[0021] Furthermore, in S4, the mass ratio of the primary filter residue, the composite sulfate and the calcium oxide is 5:2.9 to 3.1:1.

[0022] Furthermore, in S4, the calcination temperature is 800°C to 860°C.

[0023] Furthermore, the reaction time in S5 is greater than the reaction time in S3.

[0024] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0025] The method of improving the lithium yield in lithium extraction from lepidolite of the present invention first uses an alkaline activator to treat the initial lepidolite powder, so that the components with relatively loose structure in the lepidolite powder are destroyed first, releasing and dissolving a portion of lithium into the alkaline solution. For components with compact structure that need to be activated by sulfate roasting, the pre-alkaline activation effectively improves the activation energy during the roasting activation process, while maintaining the lithium yield, reducing the roasting temperature; to a certain extent, it weakens the sintering phenomenon of other lithium-containing components that would otherwise occur due to the high roasting temperature, so that the lithium therein is no longer difficult to dissolve due to a large amount of conversion into a glass phase, thereby ensuring the lithium yield. Furthermore, in the method of the present invention, the lepidolite after roasting activation is subjected to secondary activation to break the glass phase that may still exist, thereby further improving the lithium yield.

[0026] The method of the present invention precisely controls each step and the process parameters within each step, fully considering the different process requirements of different lithium-containing components in lepidolite, separating them into different efficiencies, and reducing the activation process's high dependence on the stability of the raw material composition. The method of the present invention achieves targeted activation of different lithium-containing components in lepidolite, thereby significantly improving lithium yield and providing a new technical path for the efficient utilization of lepidolite resources.

[0027] In the method of the present invention, the temperature in the preliminary alkali activation reaction is relatively low, and the production safety is good. In the method of the present invention, the lithium yield is relatively high, which is above 89%, for example, 89.74% to 94.12%.

[0028] In the present invention, the above-mentioned technical solutions can be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages will become apparent from the description or be understood through practice of the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the contents pointed out in the embodiments of the description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings are only for purposes of illustrating particular embodiments and are not to be considered limiting of the invention.

[0030] Figure 1 The XRD patterns of lepidolite after calcination and after calcination with calcium oxide and salt are shown;

[0031] Figure 2 This is an optical photograph of lithium mica after single hydrothermal treatment and calcination with additives. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0033] The present invention provides a method for improving the lithium yield in lithium extraction from lepidolite ore, comprising the following steps:

[0034] S1, crushing the lepidolite ore and sieving it to obtain lepidolite powder;

[0035] S2, the lepidolite powder is added into the reactor, water and alkali activator are added thereto, and stirring is uniformly mixed;

[0036] S3, after sealing the reactor, raise the temperature to 75-85°C and keep it warm to react;

[0037] S4. After the reaction is completed, the reactor is opened and the mixture is filtered to obtain a primary filtrate and a primary filter residue, which is then dried; the primary filter residue is then mixed evenly with a composite sulfate and a calcium oxide additive, and calcined;

[0038] S5, crushing the primary filter residue after roasting, adding it to the reaction kettle, adding water and sodium hydroxide thereto, stirring and mixing evenly; heating to 75-85°C and keeping warm for 2-3h;

[0039] S6. After the reaction is completed, the reactor is opened and filtered to obtain a secondary filtrate and a secondary filter residue.

[0040] Specifically, in S1, the lepidolite ore includes lithium disilicate, which has a relatively compact lattice structure, and 1M-type lepidolite, which has a layered structure and interlayer bonding primarily due to van der Waals forces. Considering that excessively large powder particle size is detrimental to the activation reaction, the lepidolite powder is sieved through a 250-mesh sieve after crushing.

[0041] Specifically, the above S1 also includes testing and recording the lithium content in the lepidolite ore.

[0042] Specifically, in the above S2, the alkaline activator can be calcium oxide or sodium hydroxide. When the mass concentration of the alkaline activator increases to a certain amount, it will lose the effect of improving the lithium yield, and its mass concentration must be above a certain level to improve the lithium yield. Taking all factors into consideration, the mass concentration of the alkaline activator is controlled to be 1.5% to 2.5%, for example, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, and 2.4%.

[0043] Specifically, in S2 above, considering that a too large liquid-to-solid ratio (the mass ratio of water to lepidolite powder) will result in the use of too much alkali activator, which is not suitable for actual industrial production; a too small liquid-to-solid ratio will cause the lepidolite slurry to be too viscous, which is not conducive to its alkali activation reaction. Therefore, the liquid-to-solid ratio in the reactor is controlled to be (4-10):1, for example, 5:1, 6:1, 7:1, 8:1, and 9:1.

[0044] Specifically, in the above S3, after the reactor is sealed (a high-pressure environment is formed after sealing), the reaction is carried out at 75 to 85°C, which can cause the components with relatively loose structures in the lithium mica powder to be destroyed first, releasing and dissolving a portion of lithium into the alkaline solution; for components with compact structures that need to be calcined and activated, this step can effectively increase the activation energy during the calcination and activation process, while maintaining the lithium yield and reducing the calcination temperature. It should be noted that under alkaline conditions, the alkaline activator can provide a large amount of OH- ions. These OH- ions can react with the silicon-oxygen bonds (Si-O) in silicate minerals, resulting in the breaking of the Si-O bonds. The broken silicon-oxygen bonds will form silicate ions (such as [SiO4] 4- ) and other oligomers, which are highly active and can repolymerize into a more active silicate network. Therefore, this step can effectively increase the activation energy during the calcination process. Furthermore, under the above temperature conditions, the rate of this reaction process is significantly increased, and the solvent's solubility is greatly enhanced, accelerating the cleavage of silicon-oxygen bonds and the formation of oligomers while also increasing the degree of depolymerization of the silicate network.

[0045] Specifically, in the above S3, considering that the reaction temperature is too high, it will generate extremely high pressure, which places high demands on the reactor used and also poses a production safety hazard; if the reaction temperature is too low, the pressure generated is not high enough to achieve the destruction of the silicate structure; if the holding time is too long, it will increase production costs, and if the time is too short, the alkali activation cannot be completed. Therefore, the reaction temperature is controlled to be 75-85°C, for example, 76°C, 77°C, 78°C, 78°C, 80°C, 81°C, 82°C, 83°C, 84°C. The reaction time is controlled to be 1-2 hours, for example, 1 hour, 1.2 hours, 1.5 hours, 1.7 hours.

[0046] Specifically, in S4, the composite sulfate includes sodium sulfate and potassium sulfate. Considering that a large mass ratio of sodium sulfate to potassium sulfate and a low potassium sulfate content will reduce the effect of roasting activation, while a small mass ratio and a high potassium sulfate content will result in excessively high roasting activation costs, the mass ratio of sodium sulfate to potassium sulfate is controlled to be 1:1.9 to 2.1.

[0047] Specifically, if the amount of composite sulfate in S4 is too small, the calcination activation effect will be greatly reduced. However, if the amount of composite sulfate is increased to a certain level, sintering will occur, which is also detrimental to lithium leaching. After in-depth research, the mass ratio of primary filter residue, composite sulfate, and calcium oxide is controlled to be 5:2.9 to 3.1:1.

[0048] Specifically, in the above S4, if the calcination temperature is too low, the effect of calcination activation will be reduced, and if the time is too short, it will not be conducive to the completion of calcination activation; if the calcination temperature is too high and the time is too long, it is easy to cause a large amount of lithium to be converted into a glass phase and difficult to dissolve; therefore, the calcination temperature is controlled to 800℃~860℃, for example, 810℃, 820℃, 830℃, 840℃, 850℃; the calcination time is 1~2h, for example, 1.2h, 1.4h, 1.6h, 1.8h.

[0049] Specifically, in the above S4, the phase after calcination mainly includes lithium potassium sulfate that is easily soluble in water, and lithium silicate that is slightly soluble in water but has good solubility in alkaline solution.

[0050] Specifically, in the above S5, considering that the powder particle size is too large, it is not conducive to the leaching of lithium in the calcined lepidolite and the breaking of the glass phase (a small amount of glass phase may be formed during calcination. If a glass phase is present, this step can break the glass phase), the calcined lepidolite ore is crushed and passed through a 250 mesh sieve before being added to the reactor. The liquid-solid ratio of the reactor is (4-10):1, for example, 5:1, 6:1, 7:1, 8:1, 9:1, and the reaction temperature is 75-85°C, for example, 76°C, 77°C, 78°C, 78°C, 80°C, 81°C, 82°C, 83°C, 84°C. The reaction time is controlled to be greater than the reaction time in S3, for example, the reaction time is 2-3h, for example, 2.2h, 2.5h, 2.7h.

[0051] Specifically, in the above S5, the concentration of sodium hydroxide is controlled to be 2.5% to 3.5%, for example, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, 3.1%, 3.2%, 3.3%, or 3.4%.

[0052] Specifically, in above-mentioned S6, the lithium content therein is tested after the secondary filter residue is dried. The lithium content in the final secondary filter residue is multiplied by the material mass magnification (finding that material mass loss is less during the experiment, therefore choose to ignore) caused by introducing composite sulfate and calcium oxide, to obtain the lithium content of unit lepidolite. The ratio of the lithium content of unit lepidolite to the original lithium content is deducted from 1 to obtain the lithium leaching rate, for the purpose of evaluating the effect of the method of the present invention.

[0053] The method of improving the lithium yield in lithium extraction from lepidolite of the present invention first uses an alkaline activator to treat the initial lepidolite powder, so that the components with relatively loose structure in the lepidolite powder are destroyed first, releasing and dissolving a portion of lithium into the alkaline solution. For components with compact structure that need to be activated by sulfate roasting, the pre-alkaline activation effectively improves the activation energy during the roasting activation process, while maintaining the lithium yield, reducing the roasting temperature; to a certain extent, it weakens the sintering phenomenon of other lithium-containing components that would otherwise occur due to the high roasting temperature, so that the lithium therein is no longer difficult to dissolve due to a large amount of conversion into a glass phase, thereby ensuring the lithium yield. Furthermore, in the method of the present invention, the lepidolite after roasting activation is subjected to secondary activation to break the glass phase that may still exist, thereby further improving the lithium yield.

[0054] The method of the present invention precisely controls each step and the process parameters within each step, fully considering the different process requirements of different lithium-containing components in lepidolite, separating them into different efficiencies, and reducing the activation process's high dependence on the stability of the raw material composition. The method of the present invention achieves targeted activation of different lithium-containing components in lepidolite, thereby significantly improving lithium yield and providing a new technical path for the efficient utilization of lepidolite resources.

[0055] In the method of the present invention, the temperature in the preliminary alkali activation reaction is relatively low, and the production safety is good. In the method of the present invention, the lithium yield is relatively high, which is above 89%, for example, 89.74% to 94.12%.

[0056] Example 1

[0057] This embodiment provides a method for improving the lithium yield in lithium extraction from lepidolite ore, the steps of which are as follows:

[0058] S1. Crushing the lepidolite ore and passing it through a 250-mesh sieve to test the lithium content;

[0059] S2. Weigh 5 g of lepidolite powder and add it to a 100 mL reactor. Then, add 50 mL of water and 1.25 g of calcium oxide (2.5% by mass) and stir to mix.

[0060] S3. After the reactor is strictly sealed, the temperature is raised to 85°C and kept warm for 2 hours;

[0061] S4. After the reaction is completed, the reactor is opened, and the mixture therein is filtered to obtain a primary filtrate and a primary filter residue, and the primary filter residue is dried; then the primary filter residue is mixed evenly with a composite sulfate (sodium sulfate and potassium sulfate in a mass ratio of 1:1.9) and calcium oxide in a ratio of 5:3:1, and then calcined at 850° C. for 2 h;

[0062] S5, the primary filter residue after roasting was crushed and passed through a 250 mesh sieve, added to a reactor, and then 50 mL of water and 1.25 g of sodium hydroxide (mass concentration of 2.5%) were added thereto, and stirred to mix evenly; the temperature was raised to 85 ° C and kept warm for 2.5 h to carry out a lithium leaching process, and the lithium content in the secondary filter residue was tested after drying;

[0063] S6. The lithium yield was calculated to be 94.12%.

[0064] Example 2

[0065] This embodiment provides a method for improving the lithium yield in lithium extraction from lepidolite ore, the steps of which are as follows:

[0066] S1. Crushing the lepidolite ore and passing it through a 250-mesh sieve to test the lithium content;

[0067] S2. Weigh 5 g of lepidolite powder and add it to a 100 mL reactor. Then add 50 mL of water and 1 g of calcium oxide (2% by mass) and stir to mix.

[0068] S3. After the reactor is strictly sealed, the temperature is raised to 82°C and kept warm for 2 hours;

[0069] S4. After the reaction is completed, the reactor is opened and the mixture is filtered to obtain a primary filtrate and a primary filter residue, and the primary filter residue is dried; then the primary filter residue is mixed with a composite sulfate (sodium sulfate and potassium sulfate in a mass ratio of 1:2) and calcium oxide in a ratio of 5:2.9:1 and calcined at 830°C for 1.5h;

[0070] S5. Crushing the calcined primary filter residue and passing it through a 250-mesh sieve, adding it to a reactor, adding 50 mL of water and 1.5 g of sodium hydroxide (mass concentration of 3%), stirring and mixing evenly; after the reactor is strictly sealed, heating it to 85° C. and keeping it warm for 3 h;

[0071] S6. After the secondary reaction is completed, the reactor is opened, the mixture is filtered, and the lithium content in the secondary filter residue is tested after drying;

[0072] S7. The lithium yield was calculated to be 91.27%.

[0073] Example 3

[0074] This embodiment provides a method for improving the lithium yield in lithium extraction from lepidolite ore, the steps of which are as follows:

[0075] S1. Crushing the lepidolite ore and passing it through a 250-mesh sieve to test the lithium content;

[0076] S2, weigh 5g of lepidolite powder, add it to the middle of the reactor with a capacity of 100mL, then add 50mL of water and 0.75g of sodium hydroxide (mass concentration is 1.5%), stir and mix;

[0077] S3. After the reactor is strictly sealed, the temperature is raised to 85°C and kept warm for 2 hours;

[0078] S4. After the reaction is completed, the reactor is opened, and the mixture therein is filtered to obtain a primary filtrate and a primary filter residue, and the primary filter residue is dried; then the primary filter residue is mixed evenly with a composite sulfate (sodium sulfate and potassium sulfate in a mass ratio of 1:2) and calcium oxide in a ratio of 5:3:1, and then calcined at 850° C. for 2 h;

[0079] S5. Crush the calcined primary filter residue and pass it through a 250-mesh sieve. Add it to the reactor, then add 50 mL of water and 1.25 g of sodium hydroxide (2.5% by mass) and stir to mix evenly. After the reactor is tightly sealed, heat it to 85°C and keep it warm for 3 hours.

[0080] S6. After the secondary reaction is completed, the reactor is opened, the mixture is filtered, and the lithium content in the secondary filter residue is tested after drying;

[0081] S7. The lithium yield was calculated to be 89.74%.

[0082] Comparative Example 1

[0083] Comparative Example 1 discloses a method having the same overall steps as in Example 1, except that:

[0084] The alkaline activator used in S2 was sodium hydroxide, with an amount of 0.5 g and a mass concentration of 1%. The lithium yield was 84.31%, indicating that too low a mass concentration of the alkaline activator has a negative impact on the lithium yield.

[0085] Comparative Example 2

[0086] Comparative Example 2 discloses a method having the same overall steps as in Example 1, except that:

[0087] The reaction temperature in S3 was 60°C and the reaction time was 3 h. The lithium yield was 82.30%.

[0088] This comparative example shows that improper reaction temperature control will reduce the lithium yield.

[0089] Comparative Example 3

[0090] Comparative Example 3 discloses a method with the same overall steps as in Example 2, except that:

[0091] In S4, the calcination temperature was 950°C and the holding time was 1 hour. In S5, the amount of sodium hydroxide used was 2.5 g, the concentration was 5%, the temperature was 100°C, and the holding time was 4 hours. The resulting lithium yield was 71.23%. This was due to the excessively high calcination temperature, which resulted in a large amount of glassy phase within the primary filter residue. The addition of additives to the primary filter residue during calcination caused the residue to solidify, reducing the lithium yield.

[0092] Comparative Example 4

[0093] This comparative example discloses a method having the same overall steps as in Example 3, except that:

[0094] The amount of sodium hydroxide used in S2 is 0.25 g, and the concentration is 0.5%; the amount of sodium hydroxide used in S5 is 0.25 g, and the concentration is 0.5%. The insulation temperature is 70° C., the insulation time is 1 h, and the obtained lithium yield is 76.26%.

[0095] This comparative example shows that inappropriate material dosage and inappropriate reaction temperature will reduce the lithium yield.

[0096] Comparative Example 5

[0097] This comparative example provides a method for extracting lithium from lepidolite. The overall method is the same as that of Example 1, except that the reaction temperature in S3 is 150°C and the reaction time is 2 hours. The resulting lithium yield is 76.37%. This is because S3 simultaneously extracts lithium from some loosely structured silicates and activates some compactly structured silicates. If the temperature is too high, the amount of lithium extracted from the loosely structured silicates reaches the upper limit, but the activation of the remaining components is too strong, resulting in more glass phase under the same sintering process. Subsequent reactions are unable to break down this large amount of glass phase, thereby reducing the lithium yield.

[0098] Comparative Example 6

[0099] This comparative example provides a method for extracting lithium from lepidolite, and the method of this comparative example is as follows:

[0100] S1. Crushing the lepidolite ore and passing it through a 250 mesh sieve;

[0101] S2, weigh 5g of lepidolite powder, lepidolite powder and composite sulfate (the mass ratio of sodium sulfate and potassium sulfate is 1:1.9), calcium oxide are mixed in a ratio of 5:3:1 and roasted at 850 DEG C for 2h;

[0102] The lithium yield obtained in this comparative example was 68.72%.

[0103] Comparative Example 7

[0104] This comparative example provides a method for extracting lithium from lepidolite. The overall method is the same as that in Example 1, except that:

[0105] Only S1-S4 are performed.

[0106] The lithium yield obtained in this comparative example was 82.12%.

[0107] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for improving the lithium yield in lithium extraction from lepidolite ore, characterized in that: The steps include: S1, crushing the lepidolite ore and sieving it to obtain lepidolite powder; S2, the lepidolite powder is added into the reactor, water and alkali activator are added thereto, and stirring is uniformly mixed; S3. After sealing the reactor, heat it to 75-85°C and keep it warm to react; S4. After the reaction is completed, the reactor is opened and the mixture is filtered to obtain a primary filtrate and a primary filter residue, which is then dried; the primary filter residue is then mixed evenly with a composite sulfate and a calcium oxide additive, and calcined; S5. Crushing the primary filter residue after calcination, adding it to a reactor, adding water and sodium hydroxide thereto, stirring and mixing evenly; heating to 75-85° C. and keeping the temperature for 2-3 hours; S6. After the reaction is completed, the reactor is opened and filtered to obtain a secondary filtrate and a secondary filter residue.

2. The method according to claim 1, characterized in that In the step S2, the alkali activator is calcium oxide or sodium hydroxide.

3. The method according to claim 1, characterized in that In the S2, the mass concentration of the alkali activator is controlled to be 1.5% to 2.5%.

4. The method according to claim 1, wherein In S2, the liquid-to-solid ratio in the reactor is 4-10:

1.

5. The method according to claim 1, wherein In the step S3, the reaction time is 1 to 2 hours.

6. The method according to claim 1, characterized in that In the S4, the composite sulfate includes sodium sulfate and potassium sulfate.

7. The method according to claim 6, characterized in that The mass ratio of sodium sulfate to potassium sulfate is 1:1.9~2.

1.

8. The method according to claim 1, characterized in that In the S4, the mass ratio of the primary filter residue, the composite sulfate and the calcium oxide is 5:2.9 to 3.1:

1.

9. The method according to any one of claims 1 to 8, characterized in that In the step S4, the calcination temperature is 800°C to 860°C.

10. The method according to any one of claims 1 to 8, characterized in that The reaction time in S5 is greater than the reaction time in S3.

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