Method for reducing toxicity of lithium residue in preparation of lithium carbonate from lepidolite

Abstract

The application provides a method for reducing the toxicity of lithium residue in the preparation of lithium carbonate from lepidolite, and relates to the technical field of preparation of lithium carbonate from lepidolite. The method for reducing the toxicity of lithium residue in the preparation of lithium carbonate from lepidolite comprises the following steps: mixing lepidolite, sodium sulfate and calcium sulfate, and then performing roasting, cooling and crushing to obtain crushed clinker; performing countercurrent washing on the crushed clinker to obtain filtrate and lithium residue; and performing purification and lithium precipitation on the filtrate to obtain lithium carbonate and lithium precipitation mother liquor. The method for reducing the toxicity of lithium residue in the preparation of lithium carbonate from lepidolite makes the content of toxic beryllium in the lithium residue less than 5 ppb, the content of toxic thallium less than 5 ppb, the content of toxic fluorine less than 10 ppm, and the content of soluble salt less than 2%, and reaches the standard of a solid waste, so that the influence on the ecological environment is small.

Description

Technical Field

[0001] This invention relates to the field of lithium carbonate preparation technology from lepidolite, and more particularly to a method for reducing the toxicity of lithium slag during the lithium carbonate preparation process from lepidolite. Background Technology

[0002] Yichun City in Jiangxi Province is leveraging its abundant lepidolite resources to vigorously develop lithium battery-related industries. According to incomplete statistics, from 2023 to 2025, Yichun City's lithium carbonate production from lepidolite accounted for 30-40% of the national total, averaging around 200,000 tons annually. However, Yichun's lepidolite ore is mostly lithium-bearing kaolin, with generally low grades, averaging 0.3-0.4%. After beneficiation, the average grade of the lepidolite concentrate is 1.5-2.5%. Each ton of lithium carbonate produces 40-60 tons of lithium slag, resulting in approximately ten million tons of lithium slag annually.

[0003] The production of lithium carbonate from lepidolite currently mainly employs the salt roasting-leaching process. In this process, elements such as beryllium, thallium, and fluorine contained in the raw ore are partially activated and leached out during roasting, becoming potential environmental risk factors in the lithium slag. Currently, lithium slag produced in Yichun area meets the control requirements for general industrial solid waste: beryllium ≤20 ppb, thallium ≤10 ppb, and fluorine ≤100 ppm in terms of toxicity leaching concentrations, while no limits are set for water-soluble salts. Despite this, large-scale resource utilization of this type of lithium slag has not been achieved; the vast majority can only be disposed of in stockpiles or landfills, forcing companies to invest significant effort and costs. Once lithium slag disposal channels are blocked, companies can only alleviate the pressure of overflowing storage by reducing or even halting production, severely restricting production and operations. To overcome this bottleneck, some companies have invested heavily in research and development, dedicated to developing production processes that meet Class I solid waste standards. This type of process can further reduce the toxicity of lithium slag to ≤5 ppb for beryllium and thallium, and ≤10 ppm for fluorine, while controlling the water-soluble salt content to within 2%. Once it meets the Class I solid waste standard, the application scenarios for lithium slag will be greatly expanded. It can be used in various ways, such as building material production, silicon-aluminum powder preparation, brick making, mine backfilling, and open-air storage. Not only will the usage of lithium slag no longer be limited, but it can also create considerable economic value for enterprises, truly realizing "turning waste into treasure."

[0004] Currently, the mainstream technical approach to reducing the toxicity of lithium slag is a high-ratio water washing process, which involves repeatedly rinsing with large amounts of clean water to remove soluble components such as beryllium, thallium, and fluorine adhering to the surface of the lithium slag. However, the effectiveness of this method is significantly time-sensitive: during the storage process, soluble ions in the internal pore fluid of lithium slag continuously migrate to the surface due to concentration gradients and re-accumulate on the surface, causing a "rebound" in leaching toxicity indicators (beryllium, thallium, and fluorine), making it difficult to achieve long-term stable compliance. This instability not only affects the environmental safety of lithium slag as a building material or backfill material but also increases the difficulty of compliant disposal and the cost of repeated treatment for enterprises. Summary of the Invention

[0005] Based on the technical problems existing in the background technology, the present invention proposes a method to reduce the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite.

[0006] The present invention proposes a method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite, comprising the following steps: mixing lepidolite, sodium sulfate, and calcium sulfate, followed by roasting, cooling, and crushing to obtain crushed clinker; washing the crushed clinker countercurrently to obtain filtrate and lithium slag; purifying the filtrate and precipitating lithium to obtain lithium carbonate and lithium precipitation mother liquor.

[0007] This invention employs a carbonate coating technology, returning a portion of the lithium-precipitated mother liquor during the countercurrent washing process after calcination of lepidolite using the salt method. This forms a calcium carbonate mineral framework on the surface of the lithium slag, sealing and blocking the migration of soluble beryllium, thallium, and fluorine from deeper layers, thereby reducing the toxicity of the lithium slag. This invention can improve the stability of beryllium and thallium toxicity in lithium slag, reduce the impact on the ecological environment, and simultaneously reduce the production cost of lithium carbonate from lepidolite and the carbon emissions during the lithium extraction process from lepidolite, aligning with a green and low-carbon development strategy.

[0008] Preferably, the countercurrent washing process has at least two stages.

[0009] Preferably, the countercurrent washing method is as follows: the detergent enters from the last section, and between two adjacent washing sections, the washing liquid obtained after the latter washing section is used as the detergent for the former section. The detergent includes water and lithium precipitation mother liquor.

[0010] After being crushed, the roasted clinker enters a countercurrent washing process to leach soluble beryllium, thallium, fluorine, and other components into a solution. In the final countercurrent washing stage, a portion of the lithium carbonate-containing mother liquor is introduced for reuse to implement calcium carbonate coating treatment. Because the lithium mica feedstock is rich in calcium sulfate, the calcium sulfate in the clinker exists in dissolution equilibrium, resulting in a certain amount of calcium ions accumulating on the surface of the lithium slag. These calcium ions react with carbonate ions from the reused mother liquor to uniformly form a calcium carbonate mineral framework on the surface of the lithium slag. This effectively seals and blocks the migration of harmful substances such as soluble beryllium, thallium, and fluorine from deeper layers, ultimately reducing the toxicity of the lithium slag.

[0011] More preferably, the mass ratio of the crushed clinker, water, and lithium precipitation mother liquor is 0.5:(0.35-0.55):(0.1-0.3).

[0012] The lithium precipitation mother liquor is added to the countercurrent washing process. The carbonate ions in the lithium precipitation mother liquor react with the calcium ions on the surface of the filter residue to form a calcium carbonate mineral framework. This framework seals and blocks the migration of soluble beryllium, thallium, fluorine, and other substances from deeper layers, thereby reducing the toxicity of the lithium residue.

[0013] More preferably, the temperature of the countercurrent washing is 40-60°C, and the time of the countercurrent washing is 30-80 minutes.

[0014] Controlling the temperature and time of countercurrent washing within a certain range helps to coat calcium carbonate and wash away toxic beryllium and thallium.

[0015] Preferably, the mass ratio of lepidolite, sodium sulfate, and calcium sulfate is (60-65):(10-20):(15-30).

[0016] Controlling the mass ratio of lepidolite, sodium sulfate, and calcium sulfate within a certain range helps the reaction of lepidolite.

[0017] Preferably, the calcination temperature is 850-1000℃ and the calcination time is 60-180 min.

[0018] Controlling the roasting temperature and time within a certain range helps lithium ions in lepidolite to undergo a displacement reaction with metal ions in sodium sulfate and calcium sulfate.

[0019] Preferably, the particle size after crushing is less than 0.5 mm.

[0020] Preferably, the purification process includes removing calcium, fluoride, beryllium, and thallium from the mixed filtrate to obtain a purified solution.

[0021] Preferably, the lithium precipitation process involves mixing the purified solution with a sodium carbonate solution at 90-100°C to precipitate lithium, resulting in lithium carbonate and a lithium precipitation mother liquor.

[0022] More preferably, the concentration of the sodium carbonate solution is 200-300 g / L.

[0023] More preferably, the volume ratio of the purified solution to the sodium carbonate solution is 1:(0.35-0.5).

[0024] More preferably, the carbonate concentration of the lithium precipitation mother liquor is 15-20 g / L.

[0025] Within a certain range, the carbonate concentration in the lithium precipitation mother liquor helps to form a calcium carbonate mineral framework on the surface of the lithium slag, sealing and blocking substances such as beryllium and thallium from entering the solution, thereby reducing the toxicity of the lithium slag.

[0026] Preferably, the lithium precipitation mother liquor can be reacted with sulfuric acid and then concentrated and crystallized by MVR to obtain sodium sulfate.

[0027] Sulfuric acid is added to the lithium precipitation mother liquor to remove excess carbonate ions, which are then recycled as sodium sulfate.

[0028] Preferably, the lithium slag contains less than 5 ppb of toxic beryllium, less than 5 ppb of toxic thallium, less than 10 ppm of toxic fluorine, and less than 2% of soluble salts by mass.

[0029] The beneficial effects of this invention are as follows:

[0030] The method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite provided by this invention not only ensures that the lithium slag meets the standards for Class I solid waste, but also reduces the calcium concentration in the filtrate, alleviating scaling on pipes, equipment, and tanks during production and improving production efficiency. Simultaneously, since some of the lithium precipitation mother liquor is reused, it is expected to reduce the consumption of sulfuric acid in subsequent acidification processes and correspondingly reduce carbon dioxide emissions generated by carbonate removal, which has positive significance for reducing production costs and achieving green production. Attached Figure Description

[0031] Figure 1 This is a process flow diagram of the method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite proposed in this invention.

[0032] Figure 2 This is a schematic diagram of the countercurrent washing process proposed in this invention.

[0033] Figure 3 This is a schematic diagram illustrating the principle of the calcium carbonate coating technology of the present invention. Detailed Implementation

[0034] The technical solution of the present invention will be described in detail through specific embodiments.

[0035] In the following examples and comparative examples, the specific information regarding the raw materials used is as follows:

[0036] The lepidolite was sourced from Yichun City, Jiangxi Province. The chemical composition of the lepidolite is shown in Table 1.

[0037] Table 1

[0038]

[0039] Unless otherwise specified, all materials and reagents used in the following examples and comparative examples are commercially available.

[0040] Example 1

[0041] A method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite includes the following steps:

[0042] Lithium mica, sodium sulfate, and calcium sulfate were mixed in a mass ratio of 65:15:20 and calcined at 950℃ for 90 minutes. After cooling, clinker was obtained. The clinker was then crushed to obtain crushed clinker with a 0.25mm throughput of 90%. The crushed clinker was added to a detergent and subjected to two-stage countercurrent washing to obtain filtrate and lithium slag. The detergent entered from the second stage. The washing liquid obtained from the second stage was used as the detergent for the first stage between adjacent washing stages. The detergent consisted of water and lithium precipitation mother liquor. The mass ratio of crushed clinker, water, and lithium precipitation mother liquor was 0.5:0.5:0.3. The countercurrent washing temperature was 40℃ and the countercurrent washing time was 40 minutes. The lithium slag was tested, and the toxicity of beryllium, thallium, fluorine, and soluble salts were as shown in Table 1, meeting the requirements for Class I solid waste.

[0043] After purification, lithium is precipitated from the filtrate. After centrifugation, the solid is washed and dried to obtain lithium carbonate product. The liquid after centrifugation is the lithium precipitation mother liquor, and the carbonate concentration in the lithium precipitation mother liquor is 17 g / L. Part of the lithium precipitation mother liquor is added to the crushed clinker as a washing agent, and the other part of the lithium precipitation mother liquor is added with sulfuric acid to remove the carbonate. Then, it is concentrated and crystallized by MVR to obtain sodium sulfate, which is returned to the batching process.

[0044] Example 2

[0045] A method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite includes the following steps:

[0046] Lithium mica, sodium sulfate, and calcium sulfate were mixed in a mass ratio of 60:20:20 and calcined at 850℃ for 180 minutes. After cooling, clinker was obtained. The clinker was then crushed to obtain crushed clinker with a 0.25mm throughput of 82%. The crushed clinker was added to a detergent and subjected to a three-stage countercurrent washing process to obtain filtrate and lithium slag. The detergent entered from the third stage, and the washing liquid obtained from the later stage was used as the detergent for the previous stage. The detergent consisted of water and lithium precipitation mother liquor. The mass ratio of crushed clinker, water, and lithium precipitation mother liquor was 0.5:0.5:0.2. The countercurrent washing temperature was 50℃, and the countercurrent washing time was 60 minutes. The lithium slag was tested, and the toxicity of beryllium, thallium, fluorine, and soluble salts were as shown in Table 1, meeting the requirements for Class I solid waste.

[0047] After purification, lithium is precipitated from the filtrate. After centrifugation, the solid is washed and dried to obtain lithium carbonate product. The liquid after centrifugation is the lithium precipitation mother liquor, and the carbonate concentration in the lithium precipitation mother liquor is 15 g / L. Part of the lithium precipitation mother liquor is added to the crushed clinker as a washing agent, and the other part of the lithium precipitation mother liquor is added with sulfuric acid to remove the carbonate. Then, it is concentrated and crystallized by MVR to obtain sodium sulfate, which is returned to the batching process.

[0048] Example 3

[0049] A method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite includes the following steps:

[0050] Lithium mica, sodium sulfate, and calcium sulfate were mixed in a mass ratio of 60:10:30 and calcined at 1000℃ for 60 minutes. After cooling, clinker was obtained. The clinker was then crushed to obtain crushed clinker with a 0.25mm throughput of 95%. The crushed clinker was added to a detergent and subjected to a four-stage countercurrent washing process to obtain filtrate and lithium slag. The detergent entered from the fourth stage, and the washing liquid obtained from the next stage was used as the detergent for the previous stage. The detergent consisted of water and lithium precipitation mother liquor. The mass ratio of crushed clinker, water, and lithium precipitation mother liquor was 0.5:0.5:0.1. The countercurrent washing temperature was 60℃, and the countercurrent washing time was 80 minutes. The lithium slag was tested, and the toxicity of beryllium, thallium, fluorine, and soluble salts were as shown in Table 1, meeting the requirements for Class I solid waste.

[0051] After purification, the initial filtrate is precipitated with lithium. After centrifugation, the solid is washed and dried to obtain lithium carbonate product. The liquid after centrifugation is the lithium precipitation mother liquor, and the carbonate concentration in the lithium precipitation mother liquor is 20 g / L. Part of the lithium precipitation mother liquor is added to the crushed clinker as a washing agent, and the other part of the lithium precipitation mother liquor is added with sulfuric acid to remove the carbonate. Then, it is concentrated and crystallized by MVR to obtain sodium sulfate, which is returned to the batching process.

[0052] Comparative Example 1

[0053] The only difference between Comparative Example 1 and Example 3 is that lithium precipitation mother liquor is not added; otherwise, they are the same as in Example 3.

[0054] Each example and comparative example was repeated three times to obtain three sets of samples for testing, and the average value was taken. After 0 days (i.e., the day of testing), 20 days, and 100 days, lithium slag was leached according to the "Horizontal Oscillation Method for Leaching Toxicity of Solid Waste" in HJ557-2010 standard. The beryllium and thallium content of the leaching solution was detected by ICP-MS inductively coupled mass spectrometry, the fluorine content was detected by fluoride ion selective electrode method, and the soluble salts were detected by gravimetric method. The test results are shown in Table 2.

[0055] Table 2. Lithium slag toxicity test data

[0056]

[0057] As can be seen from the data in Table 2, the method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite provided by this invention results in the toxicity of beryllium and thallium in the lithium slag being within 5 ppb, the toxicity of fluorine being within 10 ppm, and the soluble salt content being within 2%, which meets the Class I solid waste standard and has a small impact on the ecological environment.

[0058] Figure 1 This is a process flow diagram of the method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite proposed in this invention. Figure 2This is a schematic diagram of the countercurrent washing process proposed in this invention. Figure 3 This is a schematic diagram illustrating the principle of the calcium carbonate coating technology of this invention. The calcium carbonate coating technology described in this invention refers to a technique in which, during the final stage of countercurrent washing, carbonate ions in the lithium precipitation mother liquor react with calcium ions enriched on the surface of the lithium slag to form a calcium carbonate precipitate layer in situ on the lithium slag surface, thereby physically preventing the migration of soluble harmful components from the interior.

[0059] In summary, the method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite provided by this invention helps to reduce the toxicity of lithium slag and has good stability.

[0060] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for reducing the toxicity of lithium slag during the preparation of lithium carbonate from lepidolite, characterized in that, Includes the following steps: Lithium mica, sodium sulfate, and calcium sulfate are mixed, roasted, cooled, and crushed to obtain crushed clinker. The crushed clinker is then washed countercurrently to obtain filtrate and lithium slag. The filtrate is purified and lithium is precipitated to obtain lithium carbonate and lithium precipitation mother liquor.

2. The method according to claim 1, characterized in that, The countercurrent washing method is as follows: the detergent enters from the last section, and between two adjacent washing sections, the washing liquid obtained after the latter washing section is used as the detergent for the former section.

3. The method according to claim 2, characterized in that, The detergent includes water and lithium precipitation mother liquor.

4. The method according to claim 3, characterized in that, The mass ratio of the crushed clinker, water, and lithium precipitation mother liquor is 0.5:(0.35-0.55):(0.1-0.3).

5. The method according to claim 2, characterized in that, The temperature of the countercurrent washing is 40-60℃, and the time of the countercurrent washing is 30-80 minutes.

6. The method according to claim 1, characterized in that, The countercurrent washing process has at least two stages.

7. The method according to claim 1, characterized in that, The mass ratio of lepidolite, sodium sulfate and calcium sulfate is (60-65):(10-20):(15-30).

8. The method according to claim 1, characterized in that, The roasting temperature is 850-1000℃, and the roasting time is 60-180min.

9. The method according to claim 1, characterized in that, The lithium precipitation process involves mixing the purified solution with a sodium carbonate solution at 90-100°C to precipitate lithium, resulting in lithium carbonate and a lithium precipitation mother liquor.

10. The method according to claim 1, characterized in that, The carbonate concentration of the lithium precipitation mother liquor is 15-20 g / L.

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