A process for preparing lithium carbonate using a combined method of calcination, pressure cooking, and leaching of low-grade lepidolite.

Abstract

The application discloses a kind of low-grade lepidolite roasting pressure cooking leaching comprehensive method preparation lithium carbonate process, including preheating, defluorination roasting, crushing screening, pressure cooking ingredients, pressure cooking leaching, solid-liquid separation, resin calcium removal, extraction, stripping, lithium precipitation, stirring wash, drying, crushing.The application is combined by defluorination roasting, pressure cooking leaching, extraction, pressure cooking leaching is carried out in alkaline environment, resin calcium removal is directly carried out after pressure cooking, saves neutralization and impurity removal process, in the extraction process, extractant only extracts lithium, sodium and potassium ions are discharged with raffinate, improve the quality of lithium carbonate product, the recovery rate of lithium is increased by more than 5%, the leaching rate of lithium can be as high as 95%;Realize the fluorine recovery and utilization in lepidolite, improve the utilization rate of valuable resources;Auxiliary material added in production process is soluble sodium sulfate, lithium leaching residue generated is greatly reduced, and lithium leaching residue leaching toxicity reaches general solid waste standard, can be sold as glass ceramic raw material.

Description

Technical Field

[0001] This invention relates to the field of lithium carbonate preparation technology, and in particular to a process for preparing lithium carbonate using a combined method of roasting, pressure cooking, and leaching of low-grade lithium mica. Background Technology

[0002] Lepidolite is an important mineral resource containing abundant rare metals such as lithium, sodium, potassium, rubidium, cesium, and aluminum. Lithium and its salts are fundamental materials for lithium-ion batteries, hailed by scientists as "industrial MSG, the star of energy," and are considered the best material for producing lithium-ion batteries. They are also crucial metals for developing new energy and new materials. my country's lithium resources mainly consist of brine from salt lakes in Qinghai and Tibet, spodumene from Sichuan, and lepidolite from Yichun, Jiangxi. As a common lithium ore, lepidolite currently accounts for over 30% of the domestic lithium carbonate equivalent, while low-grade lepidolite reserves account for over 40% of the total lepidolite reserves.

[0003] In the industry, lithium extraction from low-grade lepidolite mainly relies on sulfate roasting, which is costly. Because sodium and potassium sulfate are expensive, and adding calcium sulfate can improve lithium extraction rates, the existing sulfate roasting method uses sodium and potassium sulfate and calcium sulfate. To prevent kiln wall buildup, calcium carbonate is usually added. Therefore, the existing sulfate roasting method produces a large amount of lithium leaching residue. Fluorine from the lepidolite enters this residue, causing the leaching toxicity to fall below general solid waste standards, making it difficult to treat and causing environmental pollution. Fluorine is also difficult to separate and recover from the lithium leaching residue, reducing resource utilization.

[0004] For the production of lithium carbonate from low-grade lepidolite ore, research on key technologies to improve lithium yield, reduce production costs, effectively control the amount of lithium leaching residue and harmful elements, and achieve comprehensive utilization of lepidolite is urgently needed and is of vital importance to the development of the lepidolite industry. Summary of the Invention

[0005] To address the shortcomings of existing methods for producing lithium carbonate from low-grade lepidolite, such as low lithium recovery rate, high production cost, large volume of lithium leaching residue that is difficult to handle, and lack of fluorine recovery, this invention provides a comprehensive process for preparing lithium carbonate using low-grade lepidolite through roasting, pressure cooking, and leaching.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A process for preparing lithium carbonate using a combined method of roasting, pressure cooking, and leaching of low-grade lepidolite is characterized by comprising the following steps: preheating, defluorination roasting, crushing and screening, pressure cooking and batching, pressure cooking and leaching, solid-liquid separation, resin calcium removal, extraction, back-extraction, lithium precipitation, stirring and washing, drying, and pulverization.

[0008] S1. Preheating: Low-grade lithium mica ore powder is added to the preheating kiln through a quantitative screw feeder and heated to 400-500 degrees Celsius.

[0009] S2, Defluorination Roasting: The preheated lepidolite powder from step S1 is fed into the rotary kiln from the kiln tail. As the rotary kiln rotates, the material moves towards the kiln head. Simultaneously, steam is introduced from the kiln head, causing the steam and material to move in opposite directions and mix thoroughly. The rotary kiln temperature is controlled at 750–950±5℃. The steam generates superheated steam under high temperature conditions. The fluorine in the lepidolite reacts with the superheated steam under high temperature conditions to generate hydrogen fluoride tail gas. The defluorinated lepidolite roasted material is sent to a cooling kiln to cool to room temperature. The defluorination roasting time is 30–60 minutes.

[0010] S3. Crushing and screening: The roasted material cooled in step S2 is screened by a screening machine to separate the material below 40 mesh. The material above 40 mesh is crushed by a double roll crusher and then screened by a screening machine to control the particle size of the material to be less than 40 mesh.

[0011] S4. Pressure Cooking and Ingredient Preparation: Put the sieved material from step S3 into a pulping tank with added purified water for pulping, and add sodium sulfate at the same time. Control the mass ratio of sieved material to sodium sulfate to be 1:0.5-0.8, and the mass ratio of material to purified water to be 1:3-5. After stirring for 20-25 minutes, pump the pulp into the pressure cooking device.

[0012] S5. Pressure cooking and leaching: After adding the slurry to the pressure cooking device, add liquid alkali and adjust the pH to >12. Seal the pressure cooking device, turn on the heater and heat to the reaction temperature of 230-250℃, control the pressure inside the vessel to 2.1-3.0MPa, and the reaction time to 3-5 hours.

[0013] S6. Solid-liquid separation: After the pressure cooking and leaching is completed, the temperature is lowered to 50±5℃, the discharge valve of the pressure cooking device is opened, and the reacted slurry is pumped into the plate and frame filter press for pressure filtration to obtain filter cake and lithium-containing filtrate.

[0014] S7. Resin calcium removal: Add chelating resin to the lithium-containing filtrate obtained in step S6 to remove calcium and magnesium ions from the lithium-containing filtrate, and obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate.

[0015] S8. Extraction: Add lithium extraction agent and diluent to the extraction tank and stir evenly. Then add the mixed mother liquor obtained in step S7, stir for 1-2 hours, let stand and separate into layers. First separate the aqueous phase and organic phase. The aqueous phase is the raffinate.

[0016] During the extraction process, the lithium extraction agent only extracts lithium, while sodium and potassium ions are discharged with the raffinate, thereby improving the lithium yield and the quality of lithium carbonate products.

[0017] S9. Back-extraction: The organic phase from step S8 is sent into the back-extraction tank. While stirring, 10% dilute sulfuric acid is added to adjust the pH to 3-4. The mixture is stirred and reacted for 1 hour. After standing and separating the layers, the organic phase and the aqueous phase are separated. The aqueous phase is a lithium-containing mother liquor with a lithium oxide concentration of 20-40 g / L. The organic phase is returned to step S8 as a lithium extraction agent for reuse.

[0018] S10, Lithium precipitation: Add saturated sodium carbonate solution to the lithium-containing mother liquor in step S9, and precipitate lithium at 80-95°C for 3-5 hours. After the reaction is completed, let it stand to settle, and then filter it with a centrifuge to separate the lithium precipitation mother liquor and crude lithium carbonate. The crude lithium carbonate is washed with circulating water, dried and crushed to obtain battery-grade lithium carbonate.

[0019] As a further improvement to this technology, in step S2, the rotary kiln is equipped with a lifting plate, which, combined with the rotation of the rotary kiln, pushes the material from the kiln tail to the kiln head, so that the lithium mica and water vapor are mixed evenly, and the mass ratio of lithium mica ore powder to water vapor is 1:0.08 to 0.12.

[0020] The movement of lepidolite powder and water vapor towards each other increases the mixing speed and uniformity of the lepidolite powder and water vapor, allowing the fluorine in the lepidolite to come into full contact with the superheated water vapor, thereby converting it all into hydrogen fluoride gas. This achieves the recovery of fluorine from the lepidolite and improves the resource recycling rate.

[0021] The preheating kiln in step S1 is located at the tail of the rotary kiln. An external heating jacket is provided outside the rotary kiln, and natural gas is used for heating inside the external heating jacket. A natural gas combustion exhaust pipe is provided near the tail of the kiln in the external heating jacket. The exhaust pipe is connected to the inside of the preheating kiln, and the exhaust gas after natural gas combustion is introduced into the preheating kiln to preheat the lithium mica ore powder inside the preheating kiln.

[0022] The preheating kiln utilizes the exhaust gas from the external heating jacket of the rotary kiln for preheating, which improves the heat utilization rate of the rotary kiln and reduces the energy consumption of the preheating kiln.

[0023] As a further improvement to this technology, step S11, fluorine recovery, is also included: the hydrogen fluoride tail gas and excess water vapor generated in step S2 are drawn into the condenser through the negative pressure pipe and condensed. After being treated with lime water of 1-5% concentration, calcium fluoride slurry is generated. The slurry is then pressed by plate and frame press to obtain calcium fluoride solid and filtrate. The calcium fluoride solid can be sold as a by-product, and the filtrate can be returned to prepare lime water.

[0024] As a further improvement to this technology, the filter cake obtained in step S6 is added to pure water for stirring and washing, and then pressed by plate and frame pressing. The filter residue after pressing is treated as solid waste, and the washing water is returned to step S4 to participate in pulp preparation.

[0025] As a further improvement to this technology, the lithium extraction extractant in step S8 includes alkyl diketone compounds, fluorodiketone compounds, phosphonium oxide compounds, and C6-C14 alcohol compounds, the diluent is sulfonated kerosene, and the mass ratio of the lithium extraction extractant to the mixed mother liquor is 1:5 to 8.

[0026] As a further improvement to this technology, the alkyl diketone compound is preferably dodecylphenyl-methyl-β-dione, the fluorodiketone compound is fluorododecylphenyl-methyl-β-dione or fluorobis(1-phenylethyl)dione, and the phosphine oxide compound is preferably trioctyl / hexylphosphine oxide.

[0027] As a further improvement to this technology, step S12, decarbonate removal, is also included: the lithium precipitation mother liquor obtained in step S10 is added to 98% concentrated sulfuric acid to remove carbonate to obtain sodium sulfate and lithium sulfate solution, which is then returned to step S7 and combined with lithium-containing filtrate for resin decalcification to carry out recycled lithium carbonate production.

[0028] As a further improvement to this technology, step S13, sodium and potassium sulfate recovery, is also included: the raffinate from step S8 is fed into a high-efficiency MVR for evaporation to precipitate sodium sulfate crystals and potassium sulfate crystals, which are then separated by centrifugation. The separated sodium sulfate crystals and potassium sulfate crystals are returned to step S4 to participate in pressure cooking. After centrifugation, the lithium oxide concentration of the mother liquor is detected. Once it reaches 5 g / L, it is returned to step S7 and combined with the lithium-containing filtrate for resin decalcification to produce recycled lithium carbonate.

[0029] As a further improvement to this technology, the sodium sulfate crystals and potassium sulfate crystals recovered in step S13 are returned to step S4 to participate in the pressure cooking process, and the mass ratio of the screened material to the recycled sodium sulfate and potassium sulfate is controlled to be 1:0.5-0.8.

[0030] Compared with the prior art, the beneficial effects of this invention are as follows: 1) This invention combines defluorination roasting, pressure leaching, extraction, and back-extraction. The pressure leaching temperature is low, reducing energy consumption. The pressure leaching is carried out in an alkaline environment, eliminating the need for neutralization and impurity removal after pressure leaching. During the extraction process, the lithium extraction agent only extracts lithium, while sodium and potassium ions are discharged with the raffinate, improving the quality of lithium carbonate products and increasing the lithium recovery rate in low-grade lepidolite by more than 5%, with a lithium leaching rate as high as 95%; 2) This invention achieves the recovery and utilization of fluorine in lepidolite to produce hydrogen fluoride or calcium fluoride as byproducts. The invention improves the utilization rate of valuable resources; the only auxiliary material added during the production process is soluble sodium sulfate, which greatly reduces the amount of lithium leaching residue. The leaching toxicity of the lithium leaching residue meets the general solid waste standard and can be sold as a glass ceramic raw material, saving the subsequent processing process and processing cost of lithium leaching residue; 3) After back-extraction, the lithium-containing mother liquor of this invention is directly precipitated for lithium precipitation, without the need for evaporation and concentration, which shortens the process flow and saves the energy consumption of evaporation and concentration; 4) The preheating kiln of this invention can recover the waste heat of the natural gas combustion tail gas of the rotary kiln for preheating lithium mica ore powder, which improves the heat utilization rate and reduces the energy consumption of the preheating kiln. Attached Figure Description

[0031] Figure 1 This is a process flow diagram of an embodiment of the present invention;

[0032] Figure 2 This is a schematic diagram of the connection structure between the preheating kiln and the rotary kiln in an embodiment of the present invention.

[0033] In the diagram: 1. Preheating kiln, 2. Rotary kiln, 201. External heating jacket, 3. Exhaust gas outlet pipe, 4. Cooling kiln. Detailed Implementation

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] The low-grade lithium mica ore powder raw material used in this embodiment of the invention has been preliminarily analyzed, and its main chemical composition is shown in Table 1 (wt%). The remainder consists of other trace substances.

[0036] Table 1:

[0037] <![CDATA[Li2O]]> <![CDATA[K2O]]> <![CDATA[Na2O]]> <![CDATA[AL2O3]]> <![CDATA[SiO2]]> <![CDATA[Fe2O3]]> <![CDATA[Rb2O]]> <![CDATA[Cs2O]]> F <![CDATA[Na2o <!-- 3 -->]]> 1.5-1.8 5.86 0.64 27.04 49.93 2.79 0.71 0.16 2.85 0.72

[0038] Example 1

[0039] S1. Preheating: 1000 kg of low-grade lithium mica ore powder (lithium oxide content of 1.6%) is added to the preheating kiln through a quantitative screw feeder and heated to 400 degrees Celsius.

[0040] S2, Defluorination Roasting: The preheated lepidolite powder from step S1 is fed from the tail of the externally heated rotary kiln into the externally heated rotary kiln. The externally heated rotary kiln has a built-in lifting plate. The material moves towards the kiln head as the externally heated rotary kiln rotates. At the same time, 80 kg of steam is introduced from the kiln head of the externally heated rotary kiln, so that the steam and the material move towards each other and are fully mixed. The temperature of the externally heated rotary kiln is controlled at 750±5℃. The steam generates superheated steam in the high-temperature environment. The fluorine in the lepidolite reacts with the superheated steam under high-temperature conditions to generate hydrogen fluoride tail gas. The defluorination roasting time is 55 min. The defluorinated lepidolite roasted material is sent to a cooling kiln to cool to room temperature.

[0041] S3. Crushing and screening: The roasted material cooled in step S2 is screened by a screening machine to separate the material below 40 mesh. The material above 40 mesh is crushed by a double roll crusher and then screened by a screening machine to control the particle size of the material to be less than 40 mesh.

[0042] S4. Pressure Cooking and Ingredient Preparation: The sieved material from step S3 is placed into a mixing tank with purified water (including the washing water reused in steps S6 and S10) for slurry preparation. Sodium sulfate (sodium sulfate and potassium sulfate recovered in step S13 after the initial feeding) is added at the same time. The initial mass ratio of sieved material to sodium sulfate is controlled at 1:0.5 (the mass ratio of sieved material to sodium sulfate and potassium sulfate is controlled at 1:0.5 in subsequent feedings). The mass ratio of material to purified water is 1:3. After stirring for 20 minutes, the slurry is pumped into the pressure cooking device.

[0043] S5. Pressure leaching: After adding the slurry to the pressure leaching device, add liquid alkali to adjust the pH to >12. Seal the pressure leaching device, turn on the heater to the reaction temperature of 230℃, control the pressure inside the vessel to 2.1MPa, and the reaction time to 3.5 hours.

[0044] S6. Solid-liquid separation: After the pressure cooking and leaching is completed, the temperature is reduced to 50±5℃, the discharge valve of the pressure cooking device is opened, and the reacted slurry is pumped into the plate and frame filter press for filtration to obtain filter cake and lithium-containing filtrate; the filter cake is added to pure water for stirring and washing and then pressed in the plate and frame press. The filter residue after pressing is treated as solid waste, and the washing water is returned to step S4 to participate in slurry preparation.

[0045] S7. Resin removal of calcium: Add chelating resin to the combined liquid of the lithium-containing filtrate obtained in step S6, the sodium sulfate and lithium sulfate solution obtained in step S12, and the mother liquor after centrifugation with a lithium oxide concentration ≥5g / L obtained in step S13 to remove calcium and magnesium ions from the lithium-containing filtrate, and obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate.

[0046] S8. Extraction: Add the lithium extraction extractant and diluent to the extraction tank and stir evenly. Then add the mixed mother liquor obtained in step S7 and stir for 1-2 hours. Let it stand and separate into layers. First, separate the aqueous phase and the organic phase. The aqueous phase is the raffinate. The lithium extraction extractant is a combination of dodecylphenyl-methyl-β-dione, fluorododecylphenyl-methyl-β-dione, trioctyl / hexylphosphine oxide and C6 alcohol. The diluent is sulfonated kerosene. The mass ratio of lithium extraction extractant to mixed solution is 1:5.

[0047] S9. Back-extraction: The organic phase from step S8 is sent into the back-extraction tank. While stirring, 10% dilute sulfuric acid is added to adjust the pH to 3-4. The mixture is stirred and reacted for 1 hour. After standing and separating the layers, the organic phase and the aqueous phase are separated. The aqueous phase is a lithium-containing mother liquor with a lithium oxide concentration of 20-40 g / L. The organic phase is returned to step S8 as a lithium extraction agent for reuse.

[0048] S10, Lithium precipitation: Add saturated sodium carbonate solution to the lithium-containing mother liquor in step S9, and precipitate lithium at 80-95°C for 3-5 hours. After the reaction is completed, let it stand to settle, and then filter it with a centrifuge to separate the lithium precipitation mother liquor and crude lithium carbonate. The crude lithium carbonate is washed with circulating water, dried and crushed to obtain battery-grade lithium carbonate. The washing water is returned to step S4 to participate in slurry preparation.

[0049] S11, Tail Gas Absorption: The hydrogen fluoride tail gas and excess water vapor generated in step S2 are drawn into the condenser through the negative pressure pipe and condensed. Then, 4% lime water is added for treatment to generate calcium fluoride slurry. The slurry is then pressed by plate and frame press to obtain calcium fluoride solid and filtrate. The calcium fluoride solid can be sold as a by-product, and the filtrate can be returned to prepare lime water.

[0050] S12, Decarbonate removal: The lithium precipitation mother liquor obtained in step S10 is added to 98% concentrated sulfuric acid to remove carbonate to obtain sodium sulfate and lithium sulfate solution, which is returned to step S7 and combined with lithium-containing filtrate for resin decalcification and recycling of lithium carbonate production.

[0051] S13, Sodium and Potassium Sulfate Recovery: The raffinate from step S8 is pumped into a high-efficiency MVR for evaporation to precipitate sodium sulfate and potassium sulfate crystals. Then, it is centrifuged and separated. The separated sodium sulfate and potassium sulfate crystals are returned to step S4 to participate in pressure cooking and recycling. The lithium oxide concentration of the mother liquor after centrifugation is tested. After reaching 5 g / L, it is returned to step S7 and combined with the lithium-containing filtrate for resin calcium removal and recycling lithium carbonate production.

[0052] In this embodiment, approximately 33.70 kg of lithium carbonate was produced, with a lithium yield of 85.38%.

[0053] Example 2

[0054] S1. Preheating: 1000 kg of low-grade lithium mica ore powder (lithium oxide content of 1.7%) is added to the preheating kiln through a quantitative screw feeder and heated to 450 degrees Celsius.

[0055] S2, Defluorination roasting: The preheated lepidolite powder from step S1 is fed from the tail of the externally heated rotary kiln into the externally heated rotary kiln. The externally heated rotary kiln has a built-in lifting plate. The material moves towards the kiln head as the externally heated rotary kiln rotates. At the same time, 100 kg of steam is introduced from the kiln head of the externally heated rotary kiln, so that the steam and the material move towards each other and are fully mixed. The temperature of the externally heated rotary kiln is controlled at 850±5℃. The steam generates superheated steam in the high-temperature environment. The fluorine in the lepidolite reacts with the superheated steam under high-temperature conditions to generate hydrogen fluoride tail gas. The defluorination roasting takes 40 minutes. The defluorinated lepidolite roasted material is sent to a cooling kiln to cool to room temperature.

[0056] S3. Crushing and screening: The roasted material cooled in step S2 is screened by a screening machine to separate the material below 40 mesh. The material above 40 mesh is crushed by a double roll crusher and then screened by a screening machine to control the particle size of the material to be less than 40 mesh.

[0057] S4. Pressure Cooking and Ingredient Preparation: The sieved material from step S3 is placed into a mixing tank with purified water (including the washing water reused in steps S6 and S10) for slurry preparation. Sodium sulfate (sodium sulfate and potassium sulfate recovered in step S13 after the initial feeding) is added at the same time. The initial mass ratio of sieved material to sodium sulfate is controlled at 1:0.65 (the mass ratio of sieved material to sodium sulfate and potassium sulfate is controlled at 1:0.65 in subsequent feedings). The mass ratio of material to purified water is 1:4. After stirring for 25 minutes, the slurry is pumped into the pressure cooking device.

[0058] S5. Pressure cooking and leaching: After adding the slurry to the pressure cooking device, add liquid alkali to adjust the pH to >12. Seal the pressure cooking device, turn on the heater to the reaction temperature of 240℃, control the pressure inside the vessel to 2.5MPa, and the reaction time is 4 hours.

[0059] S6. Solid-liquid separation: After the pressure cooking and leaching is completed, the temperature is reduced to 50±5℃, the discharge valve of the pressure cooking device is opened, and the reacted slurry is pumped into the plate and frame filter press for filtration to obtain filter cake and lithium-containing filtrate; the filter cake is added to pure water for stirring and washing and then pressed in the plate and frame press. The filter residue after pressing is treated as solid waste, and the washing water is returned to step S4 to participate in slurry preparation.

[0060] S7. Resin removal of calcium: Add chelating resin to the combined liquid of the lithium-containing filtrate obtained in step S6, the sodium sulfate and lithium sulfate solution obtained in step S12, and the mother liquor after centrifugation with a lithium oxide concentration ≥5g / L obtained in step S13 to remove calcium and magnesium ions from the lithium-containing filtrate, and obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate.

[0061] S8. Extraction: Add the lithium extraction extractant and diluent to the extraction tank and stir evenly. Then add the mixed mother liquor obtained in step S7 and stir for 1-2 hours. Let it stand and separate into layers. First, separate the aqueous phase and the organic phase. The aqueous phase is the raffinate. The lithium extraction extractant is a combination of dodecylphenyl-methyl-β-dione, fluorobis(1-phenylethyl)dione, trioctyl / hexylphosphine oxide and C8 alcohol. The diluent is sulfonated kerosene. The mass ratio of lithium extraction extractant to mixed solution is 1:6.

[0062] S9. Back-extraction: The organic phase from step S8 is sent into the back-extraction tank. While stirring, 10% dilute sulfuric acid is added to adjust the pH to 3-4. The mixture is stirred and reacted for 1 hour. After standing and separating the layers, the organic phase and the aqueous phase are separated. The aqueous phase is a lithium-containing mother liquor with a lithium oxide concentration of 20-40 g / L. The organic phase is returned to step S8 as a lithium extraction agent for reuse.

[0063] S10, Lithium precipitation: Add saturated sodium carbonate solution to the lithium-containing mother liquor in step S9, and precipitate lithium at 80-95°C for 3-5 hours. After the reaction is completed, let it stand to settle, and then filter it with a centrifuge to separate the lithium precipitation mother liquor and crude lithium carbonate. The crude lithium carbonate is washed with circulating water, dried and crushed to obtain battery-grade lithium carbonate. The washing water is returned to step S4 to participate in slurry preparation.

[0064] S11, Tail Gas Absorption: The hydrogen fluoride tail gas and excess water vapor generated in step S2 are drawn into the condenser through the negative pressure pipe and condensed. After being condensed, they are treated with lime water with a concentration of 5% to generate calcium fluoride slurry. The slurry is then pressed by plate and frame press to obtain calcium fluoride solid and filtrate. The calcium fluoride solid can be sold as a by-product, and the filtrate can be returned to prepare lime water.

[0065] S12, Decarbonate removal: The lithium precipitation mother liquor obtained in step S10 is added to 98% concentrated sulfuric acid to remove carbonate to obtain sodium sulfate and lithium sulfate solution, which is returned to step S7 and combined with lithium-containing filtrate for resin decalcification and recycling of lithium carbonate production.

[0066] S13, Sodium and Potassium Sulfate Recovery: The raffinate from step S8 is pumped into a high-efficiency MVR for evaporation to precipitate sodium sulfate and potassium sulfate crystals. Then, it is centrifuged and separated. The separated sodium sulfate and potassium sulfate crystals are returned to step S4 to participate in pressure cooking and recycling. After centrifugation, the lithium oxide concentration of the mother liquor is tested. Once it reaches 5 g / L, it is returned to step S7 and combined with the lithium-containing filtrate for resin calcium removal and recycling lithium carbonate production.

[0067] In this embodiment, approximately 36.14 kg of lithium carbonate was produced, with a lithium yield of 86.20%.

[0068] Example 3:

[0069] S1. Preheating: 1000 kg of low-grade lithium mica ore powder (lithium oxide content of 1.65%) is added to the preheating kiln through a quantitative screw feeder and heated to 500 degrees Celsius.

[0070] S2, Defluorination roasting: The preheated lepidolite powder from step S1 is fed from the tail of the externally heated rotary kiln into the externally heated rotary kiln. The externally heated rotary kiln has a built-in lifting plate. The material moves towards the kiln head as the externally heated rotary kiln rotates. At the same time, 120 kg of steam is introduced from the kiln head of the externally heated rotary kiln, so that the steam and the material move towards each other and are fully mixed. The temperature of the externally heated rotary kiln is controlled at 950±5℃. The steam generates superheated steam in the high-temperature environment. The fluorine in the lepidolite reacts with the superheated steam under high-temperature conditions to generate hydrogen fluoride tail gas. Defluorination roasting is carried out for 30 minutes. The defluorinated lepidolite roasted material is sent to a cooling kiln to cool to room temperature.

[0071] S3. Crushing and screening: The roasted material cooled in step S2 is screened by a screening machine to separate the material below 40 mesh. The material above 40 mesh is crushed by a double roll crusher and then screened by a screening machine to control the particle size of the material to be less than 40 mesh.

[0072] S4. Pressure Cooking and Ingredient Preparation: The sieved material from step S3 is placed into a mixing tank with purified water (including the washing water reused in steps S6 and S10) for slurry preparation. Sodium sulfate (sodium sulfate and potassium sulfate recovered in step S13 after the initial feeding) is added at the same time. The initial mass ratio of sieved material to sodium sulfate is controlled at 1:0.8 (the mass ratio of sieved material to sodium sulfate and potassium sulfate is controlled at 1:0.8 in subsequent feedings). The mass ratio of material to purified water is 1:5. After stirring for 25 minutes, the slurry is pumped into the pressure cooking device.

[0073] S5. Pressure cooking and leaching: After adding the slurry to the pressure cooking device, add liquid alkali to adjust the pH to >12. Seal the pressure cooking device, turn on the heater to the reaction temperature of 250℃, control the pressure inside the vessel to 3.0MPa, and the reaction time is 5 hours.

[0074] S6. Solid-liquid separation: After the pressure cooking and leaching is completed, the temperature is reduced to 50±5℃, the discharge valve of the pressure cooking device is opened, and the reacted slurry is pumped into the plate and frame filter press for filtration to obtain filter cake and lithium-containing filtrate; the filter cake is added to pure water for stirring and washing and then pressed in the plate and frame press. The filter residue after pressing is treated as solid waste, and the washing water is returned to step S4 to participate in slurry preparation.

[0075] S7. Resin removal of calcium: Add chelating resin to the combined liquid of the lithium-containing filtrate obtained in step S6, the sodium sulfate and lithium sulfate solution obtained in step S12, and the mother liquor after centrifugation with a lithium oxide concentration ≥5g / L obtained in step S13 to remove calcium and magnesium ions from the lithium-containing filtrate, and obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate.

[0076] S8. Extraction: Add the lithium extraction extractant and diluent to the extraction tank and stir evenly. Then add the mixed mother liquor obtained in step S7 and stir for 1-2 hours. Let it stand and separate into layers. First, separate the aqueous phase and the organic phase. The aqueous phase is the raffinate. The lithium extraction extractant is a combination of dodecylphenyl-methyl-β-dione, fluorobis(1-phenylethyl)dione, trioctyl / hexylphosphine oxide and C12 alcohol. The diluent is sulfonated kerosene. The mass ratio of lithium extraction extractant to mixed solution is 1:8.

[0077] S9. Back-extraction: The organic phase from step S8 is sent into the back-extraction tank. While stirring, 10% dilute sulfuric acid is added to adjust the pH to 3-4. The mixture is stirred and reacted for 1 hour. After standing and separating the layers, the organic phase and the aqueous phase are separated. The aqueous phase is a lithium-containing mother liquor with a lithium oxide concentration of 20-40 g / L. The organic phase is returned to step S8 as a lithium extraction agent for reuse.

[0078] S10, Lithium precipitation: Add saturated sodium carbonate solution to the lithium-containing mother liquor in step S9, and precipitate lithium at 80-95°C for 3-5 hours. After the reaction is completed, let it stand to settle, and then filter it with a centrifuge to separate the lithium precipitation mother liquor and crude lithium carbonate. The crude lithium carbonate is washed with circulating water, dried and crushed to obtain battery-grade lithium carbonate. The washing water is returned to step S4 to participate in slurry preparation.

[0079] S11, Tail Gas Absorption: The hydrogen fluoride tail gas and excess water vapor generated in step S2 are drawn into the condenser through the negative pressure pipe and condensed. After being condensed, they are treated with lime water with a concentration of 5% to generate calcium fluoride slurry. The slurry is then pressed by plate and frame press to obtain calcium fluoride solid and filtrate. The calcium fluoride solid can be sold as a by-product, and the filtrate can be returned to prepare lime water.

[0080] S12, Decarbonate removal: The lithium precipitation mother liquor obtained in step S10 is added to 98% concentrated sulfuric acid to remove carbonate to obtain sodium sulfate and lithium sulfate solution, which is returned to step S7 and combined with lithium-containing filtrate for resin decalcification and recycling of lithium carbonate production.

[0081] S13, Sodium and Potassium Sulfate Recovery: The raffinate from step S8 is pumped into a high-efficiency MVR for evaporation to precipitate sodium sulfate and potassium sulfate crystals. Then, it is centrifuged and separated. The separated sodium sulfate and potassium sulfate crystals are returned to step S4 to participate in pressure cooking and recycling. After centrifugation, the lithium oxide concentration of the mother liquor is tested. Once it reaches 5 g / L, it is returned to step S7 and combined with the lithium-containing filtrate for resin calcium removal and recycling lithium carbonate production.

[0082] This embodiment produced approximately 35.57 kg of lithium carbonate, with a lithium yield of 87.4%.

[0083] like Figure 2As shown, in step S1 of embodiments 1-3 of the present invention, the preheating kiln 1 is located on the upper left side of the kiln tail of the rotary kiln 2, and the cooling kiln 4 is located on the lower right side of the rotary kiln. An external heating jacket 201 is provided outside the rotary kiln 2. Natural gas is used for heating inside the external heating jacket 201. A natural gas combustion exhaust pipe 3 is provided near the kiln tail of the external heating jacket. The exhaust pipe 3 is connected to the inside of the preheating kiln 1, and the exhaust gas after natural gas combustion is introduced into the preheating kiln 1 for preheating the lithium mica ore powder inside the preheating kiln 1.

[0084] The preheating kiln 1 uses the exhaust gas from the external heating jacket 201 of the rotary kiln 2 for preheating, which improves the heat utilization rate of the rotary kiln 2 and reduces the energy consumption of the preheating kiln 1.

[0085] The low-grade lithium mica raw material used in the following comparative examples is the same as that used in the embodiments of the present invention. Its main chemical composition is shown in Table 1 (wt%) after preliminary testing, with the remainder being other trace substances.

[0086] Comparative Example 1:

[0087] S1. Add 1000 kg of low-grade lithium mica ore powder (lithium oxide content of 1.65%), sodium sulfate potassium salt mixture, additives and calcium carbonate in a ratio of 55:15:10:6 into a mixer and mix evenly. The mixture is then granulated by a granulator to form ellipsoids with a length of 39 mm, a width of 27 mm and a thickness of 15 mm. The additives are calcium sulfate, ferrous sulfate and magnesium sulfate.

[0088] S2. The granulated material is conveyed to the rotary kiln for roasting via a belt. The roasting temperature of the rotary kiln is 900℃ and the roasting time is 50 minutes. The material after roasting is cooled to room temperature through a cooling kiln.

[0089] S3. The cooled roasted material is conveyed to the ball mill via a zipper, and circulating water is added to prepare the slurry to ensure that the concentration of lithium oxide in the solution is stable at 15-20 g / L. The material is wet-milled, and the particle size after ball milling is 60-100 mesh.

[0090] S4. The material after ball milling is leached with circulating water at a solid-liquid ratio of 1:0.8 to 1.2. The leaching temperature is maintained at 70 to 95°C by jacket steam heating and the leaching time is 30 to 60 minutes. After leaching, the material is vacuum filtered using a belt filter to obtain lithium sulfate mother liquor and leaching residue. The leaching residue is rinsed with circulating water.

[0091] S5. Lithium sulfate mother liquor 1 is pumped through pipeline to neutralization and impurity removal tank. 27.5% hydrogen peroxide is added to the mixed mother liquor to separate iron ions. Then, 30-35% lime milk is added to control the pH of the solution to 11.5, producing precipitation and separating heavy metals rubidium, cesium, aluminum, magnesium, manganese, and calcium. Chelating resin is then added to further remove calcium and magnesium, yielding lithium sulfate mother liquor 2.

[0092] S6. The lithium sulfate mother liquor is fed into a lithium sulfate MVR high-efficiency evaporator for evaporation and concentration. The concentration of Li2O in the solution is controlled at 15-45 g / L to obtain lithium sulfate concentrate.

[0093] S7. Add saturated sodium carbonate solution to lithium sulfate concentrate, and react with lithium precipitation at 80-95℃ for 3 hours. After the reaction is completed, let it stand to settle, and then filter it with a centrifuge to obtain lithium precipitation mother liquor and crude lithium carbonate.

[0094] S8. Crude lithium carbonate is washed with circulating water at a solid-liquid ratio of 1:3 at a temperature above 90°C for 30 minutes. After washing, it is dried and pulverized to obtain battery-grade lithium carbonate.

[0095] S9. The mother liquor from step S7 is adjusted to pH 5.5-6.5 with sulfuric acid to remove carbonate ions from the mother liquor, resulting in a neutralized mother liquor for lithium precipitation. This mother liquor is then concentrated in a sodium sulfate MVR high-efficiency evaporator. After concentration, it is centrifuged to obtain a sodium sulfate-potassium mixed salt and lithium sulfate mother liquor III. The sodium sulfate-potassium mixed salt is added to step S1 for batching, and the lithium sulfate mother liquor III undergoes secondary lithium precipitation.

[0096] Comparative Example 1 produced approximately 32.20 kg of lithium carbonate, with a lithium yield of 79.12%.

[0097] Comparative Example 1 is a traditional sulfate roasting method. In this process, auxiliary materials such as sodium and potassium sulfate mixed salt, calcium sulfate, ferrous sulfate, magnesium sulfate, and calcium carbonate are added. During water leaching, in addition to soluble sodium and potassium salts, calcium, iron, magnesium, and sulfur ions all enter the lithium leaching residue, greatly increasing the amount of lithium leaching residue. Furthermore, fluoride ions are not recovered and enter the lithium leaching residue, making it difficult to handle and polluting the environment. The high calcium and sulfur trioxide content in the water leaching residue affects its application in the glass and ceramics industries.

[0098] Comparative Example 2:

[0099] S1. Preheating: 1000 kg of low-grade lithium mica ore powder (lithium oxide content of 1.65%) is added to the preheating kiln through a quantitative screw feeder and heated to 500 degrees Celsius.

[0100] S2, Defluorination Roasting: The preheated lepidolite powder from step S1 is fed from the tail of the externally heated rotary kiln into the externally heated rotary kiln. The externally heated rotary kiln has a built-in lifting plate. The material moves towards the kiln head as the externally heated rotary kiln rotates. At the same time, 120 kg of steam is introduced from the kiln head of the externally heated rotary kiln, so that the steam and the material move towards each other and are fully mixed. The temperature of the externally heated rotary kiln is controlled at 900℃. The steam generates superheated steam in the high-temperature environment. The fluorine in the lepidolite reacts with the superheated steam under high-temperature conditions to generate hydrogen fluoride tail gas. Defluorination roasting is carried out for 30 minutes. The defluorinated lepidolite roasted material is sent to a cooling kiln to cool to room temperature.

[0101] S3. Crushing and screening: The roasted material cooled in step S2 is screened by a screening machine to separate the material below 40 mesh. The material above 40 mesh is crushed by a double roll crusher and then screened by a screening machine to control the particle size of the material to be less than 40 mesh.

[0102] S4. Pressure Cooking and Ingredient Preparation: The sieved material from step S3 is placed into a mixing tank with purified water (including the washing water reused in steps S6 and S9) for slurry preparation. Sodium sulfate (sodium sulfate and potassium sulfate recovered in step S11 after the initial feeding) is added at the same time. The initial mass ratio of sieved material to sodium sulfate is controlled at 1:0.8 (the mass ratio of sieved material to sodium sulfate and potassium sulfate is controlled at 1:0.8 in subsequent feedings). The mass ratio of material to purified water is 1:5. After stirring for 25 minutes, the slurry is pumped into the pressure cooking device.

[0103] S5. Pressure cooking and leaching: After adding the slurry to the pressure cooking device, add liquid alkali to adjust the pH to >12. Seal the pressure cooking device, turn on the heater to the reaction temperature of 250℃, control the pressure inside the vessel to 3.0MPa, and the reaction time is 5 hours.

[0104] S6. Solid-liquid separation: After the pressure cooking and leaching is completed, the temperature is reduced to 50±5℃, the discharge valve of the pressure cooking device is opened, and the reacted slurry is pumped into the plate and frame filter press for filtration to obtain filter cake and lithium-containing filtrate; the filter cake is added to pure water for stirring and washing and then pressed in the plate and frame press. The filter residue after pressing is treated as solid waste, and the washing water is returned to step S4 to participate in slurry preparation.

[0105] S7. Resin calcium removal: Add chelating resin to the combined liquid of lithium-containing filtrate obtained in step S6 and centrifuged mother liquor obtained in step S1 to remove calcium and magnesium ions from lithium-containing filtrate, and obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate.

[0106] S8. Concentration: The mixed mother liquor from step S7 is fed into a lithium sulfate MVR high-efficiency evaporator for evaporation and concentration. The concentration of Li2O in the solution is controlled at 15-45 g / L to obtain lithium sulfate concentrate.

[0107] S9, Lithium precipitation: Add saturated sodium carbonate solution to the lithium sulfate concentrate in step S8, and precipitate lithium at 80-95℃ for 3-5 hours. After the reaction is completed, let it stand to settle, and then filter it with a centrifuge to separate the lithium precipitation mother liquor and crude lithium carbonate. The crude lithium carbonate is washed with circulating water, dried and crushed to obtain battery-grade lithium carbonate. The washing water is returned to step S4 to participate in slurry preparation.

[0108] S10, Tail Gas Absorption: The hydrogen fluoride tail gas and excess water vapor generated in step S2 are drawn into the condenser through the negative pressure pipe and condensed. After being condensed, they are treated with lime water with a concentration of 5% to generate calcium fluoride slurry. The slurry is then pressed by plate and frame press to obtain calcium fluoride solid and filtrate. The calcium fluoride solid can be sold as a by-product, and the filtrate can be returned to prepare lime water.

[0109] In steps S11 and S9, sulfuric acid is added to adjust the pH of the lithium precipitation mother liquor to 5.5-6.5 to remove carbonate ions, resulting in a lithium precipitation neutralization mother liquor. This mother liquor is then fed into a high-efficiency MVR for evaporation, precipitating sodium sulfate and potassium sulfate crystals. These crystals are then separated by centrifugation. The separated sodium sulfate and potassium sulfate crystals are returned to step S4 for pressure cooking and recycling. After centrifugation, the lithium oxide concentration in the mother liquor is measured. Once it reaches 5 g / L, it is returned to step S7 and combined with the lithium-containing filtrate for resin decalcification, thus enabling the production of recycled lithium carbonate.

[0110] Comparative Example 2 produced approximately 33.32 kg of lithium carbonate, with a lithium yield of 81.88%.

[0111] In Comparative Example 2, the extraction-back-extraction process was not used after pressure leaching, which affected the lithium yield.

[0112] The chemical composition of the lithium leaching residue generated during the production process of Examples 1-3 and Comparative Examples 1-2 was tested by an analysis and testing company in Foshan City. The sample quantity was approximately 200g. The ambient temperature and humidity of the test were 25 degrees Celsius and 68% RH. The testing standards were GB / T 21114-2019, GB / T 4734-2022, YS / T509.1-2008, YS / T 509.2-2008, JC / T1021.7-2007, GB / T 30902-2014, and GB / T30903-2014.

[0113] Table 2:

[0114]

[0115] As shown in Table 2, Examples 1-3 and Comparative Example 2 of this invention combine defluorination roasting with pressure leaching, resulting in low fluorine content in the lithium leaching residue. Most of the fluorine is recovered as calcium fluoride byproduct, and the contents of calcium and sulfur trioxide are very low, bringing the leaching residue toxicity down to the first-class solid waste standard, making it suitable for direct use as a raw material for glass and ceramics. In contrast, Comparative Example 1 has a fluorine content of 1.93%, which fails to meet the first-class solid waste toxicity standard. Furthermore, Comparative Example 1 has a calcium oxide content of 7.01% and a sulfur trioxide content of 8.85%, indicating high calcium and sulfur trioxide content, which affects its application in the glass and ceramics industries.

[0116] 2. The lithium leaching rate and lithium content in the lithium leaching residue of Examples 1-3 and Comparative Examples 1-2 were tested, and the test data are shown in Table 1.

[0117] Table 3

[0118] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Lithium oxide content in low-grade lithium mica (%) 1.6 1.7 1.65 1.65 1.65 Lithium leaching rate % (pressure boiling or water leaching) 92.8 93.7 95.0 86 95 Lithium yield % 85.38 86.20 87.40 79.12 81.88 Lithium oxide content (%) in lithium leaching residue 0.12 0.11 0.08 0.17 0.08

[0119] As can be seen from the test data in the table above, compared with Comparative Example 1, the lithium leaching rate of Examples 1-3 of the present invention increased by 7.83%, the lithium yield increased by 7.2%, and the lithium oxide content of the water leaching residue decreased by 39.2%. From the lithium leaching rate and lithium yield of Comparative Example 2, it can be seen that the lithium leaching rate of Comparative Example 2 can reach 95% by pressure leaching, but the subsequent extraction-back-extraction process was not used, which affected its final lithium yield.

[0120] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for the preparation of lithium carbonate from low-grade lepidolite by roasting-pressure digestion-leaching integrated method, characterized in that, The preparation process comprises the following steps: S1, preheating: the low-grade lepidolite powder is added into a preheating kiln through a quantitative screw feeder and heated to 400-500 DEG C; S2, defluorination roasting: the preheated lepidolite powder in step S1 is sent into a rotary kiln from the kiln tail, and the material moves to the kiln head along with the rotation of the rotary kiln, while water vapor is introduced from the kiln head to make the water vapor and the material move towards each other and mix fully, and the temperature of the rotary kiln is controlled at 750±5 DEG C-950±5 DEG C, the water vapor generates superheated steam in the high-temperature environment, and the fluorine in the lepidolite reacts with the superheated steam to generate hydrogen fluoride tail gas under the high-temperature condition, and the defluorinated lepidolite roasting material is sent into a cooling kiln and cooled to room temperature; the defluorination roasting time is 30-60 min; S3, crushing and screening: the cooled roasting material in step S2 is screened by a screening machine to remove the material below 40 mesh, and the material above 40 mesh is crushed by a double-roll crusher and then screened by the screening machine, and the particle size of the material is controlled to be less than 40 mesh; S4, pressure cooking and ingredient mixing: the screened material in step S3 is put into a pulp tank with added pure water and sodium sulfate, the mass ratio of the screened material to sodium sulfate is controlled to be 1:0.5-0.8, and the mass ratio of the material to pure water is controlled to be 1:3-5, the pulp is stirred for 20-25 min, and then pumped into a pressure cooking device; S5, pressure cooking and leaching: after the pulp is added into the pressure cooking device, liquid alkali is added to adjust the pH to be greater than 12, the pressure cooking device is sealed, heated to a reaction temperature of 230-250 DEG C, the pressure in the kettle is controlled to be 2.1-3.0 MPa, and the reaction time is 3-5 hours; S6, solid-liquid separation: after the pressure cooking and leaching are completed, the temperature is lowered to 50±5 DEG C, the discharge valve of the pressure cooking device is opened, the reacted pulp is pumped into a plate-and-frame filter press through a pump to perform pressure filtration, and a filter cake and a lithium-containing filtrate are obtained; S7, resin calcium removal: chelating resin is added into the lithium-containing filtrate obtained in step S6 to remove calcium and magnesium ions in the lithium-containing filtrate, and a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate is obtained; S8, extraction: lithium extraction reagent and diluent are added into an extraction tank and stirred uniformly, the mixed mother liquor obtained in step S7 is added, stirred for 1-2 h, and then separated into an aqueous phase and an organic phase, and the aqueous phase is a raffinate; S9, back extraction: the organic phase in step S8 is sent into a back extraction tank, 10% dilute sulfuric acid is added while stirring to adjust the pH to 3-4, stirred for 1 h, and then separated into an organic phase and an aqueous phase, and the aqueous phase is a lithium oxide solution with a lithium concentration of 20-40 g / L, and the organic phase is returned to step S8 as lithium extraction reagent for repeated use; S10, lithium precipitation: saturated sodium carbonate solution is added into the lithium-containing mother liquor in step S9, and lithium precipitation is performed at a temperature of 80-95 DEG C for 3-5 hours, after the reaction is completed, the mixture is allowed to stand and settle, and then separated by a centrifuge to obtain a lithium precipitation mother liquor and a crude lithium carbonate, the crude lithium carbonate is washed with circulating water, dried, and crushed to obtain battery-grade lithium carbonate.

2. The process for preparing lithium carbonate using a combined method of roasting, pressure cooking, and leaching of low-grade lepidolite according to claim 1, characterized in that: The rotary kiln in the step S2 is provided with a material lifting plate, and the material is pushed from the kiln tail to the kiln head by the rotation of the rotary kiln, so that the lepidolite is uniformly mixed with the water vapor, and the mass ratio of the lepidolite powder to the water vapor is 1:0.08-0.

12. The preheating kiln in the step S1 is arranged at the kiln tail side of the rotary kiln, and an outer heating jacket is arranged outside the rotary kiln, and natural gas combustion heating is adopted in the outer heating jacket, and a natural gas combustion tail gas outlet pipe is arranged at the kiln tail side of the outer heating jacket, and the tail gas outlet pipe is communicated with the inside of the preheating kiln, and the tail gas after the natural gas combustion is introduced into the preheating kiln, and is used for preheating the lepidolite powder in the preheating kiln.

3. The process according to claim 1, characterized in that: The step S11 further comprises a fluorine recovery step, the hydrogen fluoride tail gas and the excess water vapor generated in the step S2 are sucked into a condenser through a negative pressure pipe and are condensed, then are treated by adding lime water with a concentration of 1-5%, calcium fluoride slurry is generated, the slurry is further pressed by a plate frame to obtain calcium fluoride solids and filtrate, the calcium fluoride solids are sold as by-products, and the filtrate is returned to the lime water preparation.

4. The process according to claim 1, wherein the process for preparing lithium carbonate from low-grade lepidolite by roasting, autoclaving and leaching is characterized by: The filter cake obtained in the step S6 is stirred and washed by adding pure water, then is pressed by a plate frame, the filter residue after the pressing is treated as solid waste, and the stirring and washing water is returned to the step S4 to participate in the slurry preparation.

5. The process as claimed in claim 1, wherein the process for the preparation of lithium carbonate from low grade lepidolite by roasting, pressure digestion and leaching is characterized by: The lithium extraction agent in the step S8 comprises alkyl diketone compounds, fluorine-based diketone compounds, phosphine oxide compounds and c6-c14 alcohol compounds, the diluent is sulfonated kerosene, and the mass ratio of the lithium extraction agent to the mixed mother liquor is 1:5-8.

6. The process according to claim 1, wherein the process for the preparation of lithium carbonate from low-grade lepidolite by roasting-pressure digestion-leaching is characterized in that: The step S12 further comprises a step of removing carbonate, the lithium precipitation mother liquor obtained in the step S10 is added with 98% concentrated sulfuric acid to remove carbonate to obtain sodium sulfate and lithium sulfate solution, and the solution is returned to the step S7 to be combined with the lithium-containing filtrate, resin calcium removal is performed, and cyclic lithium carbonate production is performed.

7. The process according to claim 1, wherein the process is characterized by: The step S13 further comprises a step of recovering sodium sulfate and potassium sulfate, the raffinate in the step S8 is punched into a high-efficiency MVR to evaporate, sodium sulfate crystals and potassium sulfate crystals are precipitated, then are centrifuged, the centrifuged sodium sulfate crystals and potassium sulfate crystals are returned to the step S4 to participate in the pressure boiling preparation, the mother liquor after the centrifugation is detected for lithium oxide concentration, and when the concentration reaches 5g / L, the mother liquor is returned to the step S7 to be combined with the lithium-containing filtrate, resin calcium removal is performed, and cyclic lithium carbonate production is performed.

8. The process for preparing lithium carbonate using a combined method of roasting, pressure cooking, and leaching of low-grade lepidolite according to claim 7, characterized in that: The sodium sulfate crystals and the potassium sulfate crystals recovered in the step S13 are returned to the step S4 to participate in the pressure boiling preparation, and the mass ratio of the screened material to the recycled sodium sulfate and potassium sulfate is controlled to be 1:0.5-0.8.

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