Process for preparing lithium carbonate by low-grade lepidolite roasting, pressure-cooking and leaching comprehensive method

Abstract

The invention discloses a process for preparing lithium carbonate by a low-grade lepidolite roasting, pressure cooking and leaching comprehensive method. The process comprises the steps of preheating, defluorination roasting, crushing and screening, pressure cooking and burdening, pressure cooking and leaching, solid-liquid separation, resin calcium removal, extraction, reverse extraction, lithium precipitation, stirring and washing, drying and crushing. According to the method, defluorination roasting, pressure cooking leaching and extraction are combined, pressure cooking leaching is carried out in an alkaline environment, resin calcium removal is directly carried out after pressure cooking, the neutralization and impurity removal procedure is omitted, only lithium is extracted by an extracting agent in the extraction process, sodium and potassium ions are discharged along with raffinate, the quality of a lithium carbonate product is improved, the recovery rate of lithium is increased by 5% or above, and the method is suitable for industrial production. The leaching rate of lithium can reach 95%; fluorine in the lepidolite is recycled, and the utilization rate of valuable resources is increased; the auxiliary material added in the production process is soluble sodium sulfate, so that the amount of the generated lithium leaching residues is greatly reduced, the leaching toxicity of the lithium leaching residues reaches the general solid waste standard, and the lithium leaching residues can be sold as a glass ceramic raw material.

Description

Technical Field

[0001] The invention relates to the technical field of lithium carbonate preparation, and in particular to a process for preparing lithium carbonate by a comprehensive method of roasting, pressure boiling and leaching low-grade lepidolite. Background Art

[0002] Lepidolite is an important mineral resource, which contains abundant rare metal materials, such as lithium, sodium, potassium, rubidium, cesium, aluminum, etc. Lithium and its salts are the basic materials for lithium-ion new energy, and are praised by scientists as "industrial MSG, energy star". They are the best materials for the production of lithium-ion batteries and important metals for the development of new energy and new materials. my country's lithium mineral resources are mainly salt lake brine in Qinghai and Tibet, spodumene in Sichuan and lepidolite in Yichun, Jiangxi. As a common lithium ore, lepidolite currently accounts for more than 30% of the equivalent of lithium carbonate produced in China, and the reserves of low-grade lepidolite ore account for more than 40% of the total lepidolite ore.

[0003] The main method for extracting lithium from low-grade lepidolite in the industry is sulfate roasting, which is relatively costly. Since the price of sodium potassium sulfate is relatively high, and the addition of calcium sulfate can increase the extraction rate of lithium, the sulfates used in the existing sulfate roasting method are sodium potassium sulfate and calcium sulfate. In order to prevent kiln wall calcination, calcium carbonate is usually added. Therefore, the existing sulfate roasting method produces a large amount of lithium leaching slag, and the fluorine element in the lepidolite enters the lithium leaching slag, resulting in the lithium leaching slag toxicity failing to meet general solid waste standards, making it difficult to handle and causing environmental pollution. Fluorine is difficult to separate from the lithium leaching slag for recovery, reducing resource utilization.

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

[0005] In order to solve the problems that the existing methods for preparing lithium carbonate using low-grade lepidolite as raw material have low lithium recovery rate, high production cost, large amount of lithium leaching slag, difficult treatment, and fluorine not recovered, the present invention provides a process for preparing lithium carbonate by a comprehensive method of roasting, pressure cooking and leaching of low-grade lepidolite.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A process for preparing lithium carbonate by roasting and pressure-cooking leaching of low-grade lepidolite, characterized in that it comprises preheating, defluorination roasting, crushing and screening, pressure-cooking batching, pressure-cooking leaching, solid-liquid separation, resin decalcification, extraction, stripping, lithium precipitation, stirring and washing, drying, and crushing, and the preparation process comprises the following steps:

[0008] S1, preheating: add low-grade lithium mica ore powder into the preheating kiln through a quantitative screw feeder and heat it to 400-500 degrees;

[0009] S2, defluorination roasting: the lithium mica ore powder preheated in step S1 is sent to the rotary kiln from the tail of the rotary kiln, the material moves to the head of the rotary kiln as the rotary kiln rotates, and water vapor is introduced from the head of the rotary kiln to make the water vapor and the material move in opposite directions and fully mix, the temperature of the rotary kiln is controlled at 750-950±5°C, the water vapor generates superheated steam under high temperature environment, the fluorine in the lithium mica reacts with the superheated steam under high temperature conditions to generate hydrogen fluoride tail gas, and the lithium mica roasted material with fluorine removed is sent to a cooling kiln to be cooled to room temperature; the defluorination roasting time is 30-60min;

[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, and the material above 40 mesh is crushed by a double-roll crusher and then screened by a screening machine to control the material particle size to be less than 40 mesh;

[0011] S4, pressure cooking ingredients: put the screened material in step S3 into a pulping tank with pure water for slurry preparation, and add sodium sulfate at the same time, control the mass ratio of the screened material to sodium sulfate to be 1:0.5-0.8, and the mass ratio of the material to pure water to be 1:3-5, stir for 20-25 minutes, and then pump the slurry into the pressure cooking device through a pump;

[0012] S5, pressure cooking leaching: After adding the slurry to the pressure cooking device, add liquid alkali, adjust the pH>12, seal the pressure cooking device, open and heat to the reaction temperature of 230-250°C, control the pressure in the kettle to 2.1-3.0MPa, and the reaction time is 3-5 hours;

[0013] S6, solid-liquid separation: After the pressure cooking and leaching are completed, the temperature is lowered to 50±5°C, the discharge valve of the pressure cooking device is opened, and the slurry after the reaction is pumped into a plate and frame filter press for pressure filtration to obtain a filter cake and a lithium-containing filtrate;

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

[0015] S8, extraction: adding the lithium extraction agent and the diluent into the extraction tank and stirring evenly, then adding the mixed mother liquor obtained in step S7, stirring for 1 to 2 hours, standing and stratifying, first separating the aqueous phase and the organic phase, and the aqueous phase is the raffinate;

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

[0017] S9, stripping: the organic phase in step S8 is sent to the stripping tank, and the prepared 10% dilute sulfuric acid is added while stirring, and the pH is adjusted to 3-4, and the reaction is stirred for 1 hour, and the organic phase and the aqueous phase are separated. The aqueous phase is a lithium-containing mother liquor with a lithium oxide concentration of 20-40g / L, and the organic phase is returned to step S8 for repeated use as a lithium extraction agent;

[0018] S10, lithium precipitation: adding saturated sodium carbonate solution to the lithium-containing mother liquor of step S9, and precipitating lithium at 80-95°C for 3-5 hours. After the reaction is completed, the mother liquor is allowed to stand and settle, and then filtered and separated by a centrifuge to obtain 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 of the present technology, in step S2, the rotary kiln is equipped with a lifting plate, and the material is pushed from the kiln tail to the kiln head in combination with the rotation of the rotary kiln, so that the lithium mica and the water vapor are evenly mixed, and the mass ratio of the lithium mica ore powder to the water vapor is 1:0.08-0.12;

[0020] The lepidolite powder and water vapor move towards each other, which improves the mixing speed and uniformity of the lepidolite powder and water vapor, and makes the fluorine in the lepidolite fully contact with the superheated water vapor, so that it is completely converted into hydrogen fluoride gas, thus realizing the recovery of fluorine in the lepidolite and improving the resource recovery rate.

[0021] The preheating kiln in step S1 is arranged at the kiln tail side of the rotary kiln, and the rotary kiln is provided with an external heating jacket outside, and the external heating jacket is heated by natural gas combustion. The external heating jacket is provided with a natural gas combustion tail gas outlet pipe near the kiln tail, and the tail gas outlet pipe is connected with the interior of the preheating kiln, and the tail gas after the natural gas combustion is passed into the preheating kiln, so as to preheat the lithium mica ore powder in the preheating kiln.

[0022] The preheating kiln uses the tail gas of the heating jacket outside 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 of the present technology, the present invention further comprises step S11, fluorine recovery: the hydrogen fluoride tail gas and excess water vapor generated in step S2 are sucked into the condenser through a negative pressure pipe for condensation, and then are added with lime water with a concentration of 1 to 5% for treatment to generate calcium fluoride slurry. The slurry is then pressed by plate and frame 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 of the present technology, the filter cake obtained in step S6 is added with pure water for stirring and washing and then subjected to plate and frame pressing. The filter residue after pressing is treated as solid waste, and the stirring and washing water is returned to step S4 for pulp preparation.

[0025] As a further improvement of the present technology, the lithium extraction agent in step S8 includes alkyl diketone compounds, fluorine 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.

[0026] As a further improvement of the present technology, the alkyl diketone compound is preferably dodecylphenyl-methyl-β-diketone, the fluorine-based diketone compound is fluorododecylphenyl-methyl-β-diketone or fluorobis(1-phenylethyl)dione, and the phosphine oxide compound is preferably trioctyl / hexyl phosphine oxide.

[0027] As a further improvement of the present technology, step S12 is also included, removing carbonate: adding 98% concentrated sulfuric acid to the lithium precipitation mother liquor obtained in step S10 to remove carbonate to obtain sodium sulfate and lithium sulfate solution, returning to step S7 to be combined with the lithium-containing filtrate, performing resin decalcification, and performing cyclic lithium carbonate production.

[0028] As a further improvement of the present technology, the present invention further includes step S13, sodium and potassium sulfate recovery: the raffinate of step S8 is pumped into a high-efficiency MVR for evaporation to precipitate sodium sulfate crystals and potassium sulfate crystals, which are then centrifuged, and the sodium sulfate crystals and potassium sulfate crystals separated by centrifugation are returned to step S4 to participate in pressure cooking and batching, and the mother liquor after centrifugation is tested for lithium oxide concentration. When the concentration reaches 5 g / L, it is returned to step S7 to be combined with the lithium-containing filtrate, and the resin is used for decalcification to carry out cyclic lithium carbonate production.

[0029] As a further improvement of the present technology, the sodium sulfate crystals and potassium sulfate crystals recovered in step S13 are returned to step S4 to participate in the pressure cooking ingredients, 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 present invention has the following beneficial effects: 1) the present invention combines defluorination roasting, autoclave leaching, extraction and stripping, the autoclave temperature is low, the energy consumption is reduced, the autoclave leaching is carried out in an alkaline environment, and there is no need to carry out a neutralization and impurity removal process after autoclave. During the extraction process, the lithium extracting agent only extracts lithium, and sodium and potassium ions are discharged with the raffinate, thereby improving the quality of lithium carbonate products, and increasing the recovery rate of lithium in low-grade lepidolite by more than 5%, and the lithium leaching rate can be as high as 95%; 2) the present invention realizes the recovery and utilization of fluorine in lepidolite, and produces hydrogen fluoride or calcium fluoride as a byproduct, The utilization rate of valuable resources is improved; the only auxiliary material added in the production process is soluble sodium sulfate, which greatly reduces the amount of lithium leaching residue. The leaching toxicity of the lithium leaching residue reaches the general solid waste standard and can be sold as a glass ceramic raw material, saving the subsequent treatment process and treatment cost of the lithium leaching residue; 3) The lithium-containing mother liquor after stripping is directly used for lithium precipitation, and lithium precipitation after evaporation and concentration is not required, which shortens the process flow and saves the energy consumption of evaporation and concentration; 4) The preheating kiln of the present invention can recycle the waste heat of the natural gas combustion tail gas of the rotary kiln for preheating the lithium mica ore powder, thereby improving the heat utilization rate and reducing the energy consumption of the preheating kiln. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0033] In the figure: 1. preheating kiln, 2. rotary kiln, 201. external heating jacket, 3. tail gas outlet pipe, 4. cooling kiln. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] The low-grade lepidolite ore powder raw material used in the embodiment of the present invention has its main chemical composition as shown in Table 1 (wt%) after preliminary detection, and the balance is other small amounts of 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: 1000kg of low-grade lithium mica powder (lithium oxide content is 1.6%) is added into the preheating kiln through a quantitative screw feeder and heated to 400 degrees;

[0040] S2, defluorination roasting: the lithium mica ore powder preheated in step S1 is sent from the tail of the externally heated rotary kiln to the externally heated rotary kiln, the externally heated rotary kiln is equipped with a lifting plate, the material moves to the kiln head as the externally heated rotary kiln rotates, and 80 kg of water vapor is introduced from the kiln head of the externally heated rotary kiln to make the water vapor and the material move in opposite directions and be fully mixed. The temperature of the externally heated rotary kiln is controlled at 750 ± 5 ° C, the water vapor generates superheated steam under high temperature conditions, and the fluorine in the lithium mica reacts with the superheated steam under high temperature conditions to generate hydrogen fluoride tail gas. The defluorination roasting time is 55 minutes, and the lithium mica roasted material with defluorination is sent to a cooling kiln to be cooled 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, and the material above 40 mesh is crushed by a double-roll crusher and then screened by a screening machine to control the material particle size to be less than 40 mesh;

[0042] S4, pressure cooking ingredients: put the screened material in step S3 into a stirring tank with pure water (including the stirring and washing water recycled in steps S6 and S10) for slurry mixing, and add sodium sulfate (after the initial feeding, the sodium sulfate and potassium sulfate recovered in step S13 later), initially control the mass ratio of the screened material to sodium sulfate to be 1:0.5 (later control the mass ratio of the screened material to sodium sulfate and potassium sulfate to be 1:0.5), and the mass ratio of the material to pure water to be 1:3. After stirring for 20 minutes, the slurry is pumped into the pressure cooking device;

[0043] S5, pressure cooking leaching: After adding the slurry to the pressure cooking device, add liquid alkali, adjust the pH>12, seal the pressure cooking device, open and heat to the reaction temperature of 230°C, control the pressure in the kettle to 2.1MPa, and the reaction time is 3.5 hours;

[0044] S6, solid-liquid separation: after the pressure cooking and leaching are completed, the temperature is lowered to 50±5°C, the discharge valve of the pressure cooking device is opened, and the slurry after the reaction is pumped into a plate-frame filter press for pressure filtration to obtain a filter cake and a lithium-containing filtrate; the filter cake is stirred and washed with pure water and then subjected to plate-frame pressing, and the filter residue after pressing is treated as solid waste, and the stirring and washing water is returned to step S4 to participate in slurry preparation;

[0045] S7, resin decalcification: adding a chelating resin to the combined solution of the lithium-containing filtrate obtained in step S6, the sodium sulfate and lithium sulfate solution obtained in step S12, and the centrifuged mother liquor with a lithium oxide concentration of ≥5 g / L obtained in step S13, removing calcium and magnesium ions in the lithium-containing filtrate to obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate;

[0046] S8, extraction: adding a lithium extraction agent and a diluent to an extraction tank and stirring evenly, then adding the mixed mother liquor obtained in step S7, stirring for 1 to 2 hours, standing and stratifying, first separating the aqueous phase and the organic phase, and the aqueous phase is the raffinate; wherein the lithium extraction agent is a combination of dodecylphenyl-methyl-β-diketone, fluorododecylphenyl-methyl-β-diketone, trioctyl / hexyl phosphine oxide and C6 alcohol, the diluent is sulfonated kerosene, and the mass ratio of the lithium extraction agent to the mixed solution is 1:5;

[0047] S9, stripping: the organic phase in step S8 is sent to the stripping tank, and the prepared 10% dilute sulfuric acid is added while stirring, and the pH is adjusted to 3-4, and the reaction is stirred for 1 hour, and the organic phase and the aqueous phase are separated. The aqueous phase is a lithium-containing mother liquor with a lithium oxide concentration of 20-40g / L, and the organic phase is returned to step S8 for repeated use as a lithium extraction agent;

[0048] S10, lithium precipitation: adding saturated sodium carbonate solution to the lithium-containing mother liquor in step S9, and reacting lithium precipitation at 80-95°C for 3-5 hours, allowing the reaction to settle after the reaction, and then filtering and separating the lithium precipitation mother liquor and crude lithium carbonate by centrifuge, washing the crude lithium carbonate with circulating water, drying, and crushing to obtain battery-grade lithium carbonate; the washing water is returned to step S4 for slurry preparation;

[0049] S11, tail gas absorption: the hydrogen fluoride tail gas and excess water vapor generated in step S2 are sucked into the condenser through a negative pressure pipe for condensation, and then treated with lime water with a concentration of 4% to generate calcium fluoride slurry. The slurry is then pressed by plate and frame 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, removing carbonate: adding 98% concentrated sulfuric acid to the lithium precipitation mother liquor obtained in step S10 to remove carbonate to obtain sodium sulfate and lithium sulfate solution, returning to step S7 to combine with the lithium-containing filtrate, performing resin decalcification, and performing cyclic lithium carbonate production;

[0051] S13, sodium and potassium sulfate recovery: the raffinate of step S8 is pumped into a high-efficiency MVR for evaporation to precipitate sodium sulfate crystals and potassium sulfate crystals, which are then centrifuged and separated. The separated sodium sulfate crystals and potassium sulfate crystals are returned to step S4 to participate in pressure cooking and recycling. The mother liquor after centrifugation is tested for lithium oxide concentration. When the concentration reaches 5 g / L, it is returned to step S7 to be combined with the lithium-containing filtrate, and the resin is used for decalcification to carry out cyclic lithium carbonate production.

[0052] This example produces about 33.70 kg of lithium carbonate, with a lithium yield of 85.38%.

[0053] Example 2

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

[0055] S2, defluorination roasting: the lithium mica ore powder preheated in step S1 is sent from the tail of the externally heated rotary kiln to the externally heated rotary kiln, the externally heated rotary kiln is equipped with a lifting plate, the material moves to the kiln head as the externally heated rotary kiln rotates, and at the same time, 100 kg of water vapor is introduced from the kiln head of the externally heated rotary kiln to make the water vapor and the material move in opposite directions and be fully mixed. The temperature of the externally heated rotary kiln is controlled at 850±5°C, the water vapor generates superheated steam under high temperature conditions, and the fluorine in the lithium mica reacts with the superheated steam under high temperature conditions to generate hydrogen fluoride tail gas. The defluorination roasting takes 40 minutes, and the lithium mica roasted material with defluorinated is sent to a cooling kiln to be cooled 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, and the material above 40 mesh is crushed by a double-roll crusher and then screened by a screening machine to control the material particle size to be less than 40 mesh;

[0057] S4, pressure cooking ingredients: put the screened material in step S3 into a stirring tank with pure water (including the stirring and washing water recycled in steps S6 and S10) for slurry mixing, and add sodium sulfate (after the initial feeding, the sodium sulfate and potassium sulfate recovered in step S13 later), the mass ratio of the screened material to sodium sulfate is initially controlled to be 1:0.65 (the mass ratio of the screened material to sodium sulfate and potassium sulfate is later controlled to be 1:0.65), and the mass ratio of the material to pure water is 1:4. After stirring for 25 minutes, the slurry is pumped into the pressure cooking device;

[0058] S5, pressure cooking leaching: After adding the slurry to the pressure cooking device, add liquid alkali, adjust the pH>12, seal the pressure cooking device, open and heat to the reaction temperature of 240°C, control the pressure in the kettle to 2.5MPa, and the reaction time is 4 hours;

[0059] S6, solid-liquid separation: after the pressure cooking and leaching are completed, the temperature is lowered to 50±5°C, the discharge valve of the pressure cooking device is opened, and the slurry after the reaction is pumped into a plate-frame filter press for pressure filtration to obtain a filter cake and a lithium-containing filtrate; the filter cake is stirred and washed with pure water and then subjected to plate-frame pressing, and the filter residue after pressing is treated as solid waste, and the stirring and washing water is returned to step S4 to participate in slurry preparation;

[0060] S7, resin decalcification: adding a chelating resin to the combined solution of the lithium-containing filtrate obtained in step S6, the sodium sulfate and lithium sulfate solution obtained in step S12, and the centrifuged mother liquor with a lithium oxide concentration of ≥5 g / L obtained in step S13, removing calcium and magnesium ions in the lithium-containing filtrate to obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate;

[0061] S8, extraction: adding a lithium extraction agent and a diluent to an extraction tank and stirring evenly, then adding the mixed mother liquor obtained in step S7, stirring for 1 to 2 hours, standing and stratifying, first separating the aqueous phase and the organic phase, and the aqueous phase is the raffinate; wherein the lithium extraction agent is a combination of dodecylphenyl-methyl-β-diketone, fluorobis(1-phenylethyl)dione, trioctyl / hexylphosphine oxide and C8 alcohol, the diluent is sulfonated kerosene, and the mass ratio of the lithium extraction agent to the mixed solution is 1:6;

[0062] S9, stripping: the organic phase in step S8 is sent to the stripping tank, and the prepared 10% dilute sulfuric acid is added while stirring, and the pH is adjusted to 3-4, and the reaction is stirred for 1 hour, and the organic phase and the aqueous phase are separated. The aqueous phase is a lithium-containing mother liquor with a lithium oxide concentration of 20-40g / L, and the organic phase is returned to step S8 for repeated use as a lithium extraction agent;

[0063] S10, lithium precipitation: adding saturated sodium carbonate solution to the lithium-containing mother liquor in step S9, and reacting lithium precipitation at 80-95°C for 3-5 hours, allowing the reaction to settle after the reaction, and then filtering and separating the lithium precipitation mother liquor and crude lithium carbonate by centrifuge, washing the crude lithium carbonate with circulating water, drying, and crushing to obtain battery-grade lithium carbonate; the washing water is returned to step S4 for slurry preparation;

[0064] S11, tail gas absorption: the hydrogen fluoride tail gas and excess water vapor generated in step S2 are sucked into the condenser through a negative pressure pipe for condensation, and then treated with lime water with a concentration of 5% to generate calcium fluoride slurry. The slurry is then pressed by plate and frame 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, removing carbonate: adding 98% concentrated sulfuric acid to the lithium precipitation mother liquor obtained in step S10 to remove carbonate to obtain sodium sulfate and lithium sulfate solution, returning to step S7 to combine with the lithium-containing filtrate, performing resin decalcification, and performing cyclic lithium carbonate production;

[0066] S13, sodium and potassium sulfate recovery: the raffinate of step S8 is pumped into a high-efficiency MVR for evaporation to precipitate sodium sulfate crystals and potassium sulfate crystals, which are then centrifuged and separated. The separated sodium sulfate crystals and potassium sulfate crystals are returned to step S4 to participate in pressure cooking and recycling. The mother liquor after centrifugation is tested for lithium oxide concentration. When it reaches 5g / L, it is returned to step S7 to be combined with the lithium-containing filtrate, and the resin is used for decalcification to carry out cyclic lithium carbonate production.

[0067] In this example, about 36.14 kg of lithium carbonate was produced, and the yield of lithium was 86.20%.

[0068] Embodiment 3:

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

[0070] S2, defluorination roasting: the lithium mica ore powder preheated in step S1 is sent from the tail of the externally heated rotary kiln to the externally heated rotary kiln, the externally heated rotary kiln is equipped with a lifting plate, and the material moves to the kiln head as the externally heated rotary kiln rotates, and at the same time, 120 kg of water vapor is introduced from the kiln head of the externally heated rotary kiln to make the water vapor and the material move in opposite directions and be fully mixed. The temperature of the externally heated rotary kiln is controlled at 950±5°C, and the water vapor generates superheated steam under high temperature conditions. The fluorine in the lithium mica reacts with the superheated steam under high temperature conditions to generate hydrogen fluoride tail gas, and the defluorination roasting is carried out for 30 minutes. The lithium mica roasted material with defluorination is sent to a cooling kiln to be cooled 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, and the material above 40 mesh is crushed by a double-roll crusher and then screened by a screening machine to control the material particle size to be less than 40 mesh;

[0072] S4, pressure cooking ingredients: put the screened material in step S3 into a stirring tank with pure water (including the stirring and washing water recycled in steps S6 and S10) for slurry mixing, and add sodium sulfate (after the initial feeding, the sodium sulfate and potassium sulfate recovered in step S13 later), the mass ratio of the screened material to sodium sulfate is initially controlled to be 1:0.8 (the mass ratio of the screened material to sodium sulfate and potassium sulfate is later controlled to be 1:0.8), and the mass ratio of the material to pure water is 1:5. After stirring for 25 minutes, the slurry is pumped into the pressure cooking device;

[0073] S5, pressure cooking leaching: After adding the slurry to the pressure cooking device, add liquid alkali, adjust the pH>12, seal the pressure cooking device, open and heat to the reaction temperature of 250°C, control the pressure in the kettle to 3.0MPa, and the reaction time is 5 hours;

[0074] S6, solid-liquid separation: after the pressure cooking and leaching are completed, the temperature is lowered to 50±5°C, the discharge valve of the pressure cooking device is opened, and the slurry after the reaction is pumped into a plate-frame filter press for pressure filtration to obtain a filter cake and a lithium-containing filtrate; the filter cake is stirred and washed with pure water and then subjected to plate-frame pressing, and the filter residue after pressing is treated as solid waste, and the stirring and washing water is returned to step S4 to participate in slurry preparation;

[0075] S7, resin decalcification: adding a chelating resin to the combined solution of the lithium-containing filtrate obtained in step S6, the sodium sulfate and lithium sulfate solution obtained in step S12, and the centrifuged mother liquor with a lithium oxide concentration of ≥5 g / L obtained in step S13, removing calcium and magnesium ions in the lithium-containing filtrate to obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate;

[0076] S8, extraction: adding a lithium extraction agent and a diluent to an extraction tank and stirring evenly, then adding the mixed mother liquor obtained in step S7, stirring for 1 to 2 hours, standing and stratifying, first separating the aqueous phase and the organic phase, and the aqueous phase is the raffinate; wherein the lithium extraction agent is a combination of dodecylphenyl-methyl-β-diketone, fluorobis(1-phenylethyl)dione, trioctyl / hexylphosphine oxide and c12 alcohol, the diluent is sulfonated kerosene, and the mass ratio of the lithium extraction agent to the mixed solution is 1:8;

[0077] S9, stripping: the organic phase in step S8 is sent to the stripping tank, and the prepared 10% dilute sulfuric acid is added while stirring, and the pH is adjusted to 3-4, and the reaction is stirred for 1 hour, and the organic phase and the aqueous phase are separated. The aqueous phase is a lithium-containing mother liquor with a lithium oxide concentration of 20-40g / L, and the organic phase is returned to step S8 for repeated use as a lithium extraction agent;

[0078] S10, lithium precipitation: adding saturated sodium carbonate solution to the lithium-containing mother liquor in step S9, and reacting lithium precipitation at 80-95°C for 3-5 hours, allowing the reaction to settle after the reaction, and then filtering and separating the lithium precipitation mother liquor and crude lithium carbonate by centrifuge, washing the crude lithium carbonate with circulating water, drying, and crushing to obtain battery-grade lithium carbonate; the washing water is returned to step S4 for slurry preparation;

[0079] S11, tail gas absorption: the hydrogen fluoride tail gas and excess water vapor generated in step S2 are sucked into the condenser through a negative pressure pipe for condensation, and then treated with lime water with a concentration of 5% to generate calcium fluoride slurry. The slurry is then pressed by plate and frame 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, removing carbonate: adding 98% concentrated sulfuric acid to the lithium precipitation mother liquor obtained in step S10 to remove carbonate to obtain sodium sulfate and lithium sulfate solution, returning to step S7 to combine with the lithium-containing filtrate, performing resin decalcification, and performing cyclic lithium carbonate production;

[0081] S13, sodium and potassium sulfate recovery: the raffinate of step S8 is pumped into a high-efficiency MVR for evaporation to precipitate sodium sulfate crystals and potassium sulfate crystals, which are then centrifuged and separated. The separated sodium sulfate crystals and potassium sulfate crystals are returned to step S4 to participate in pressure cooking and recycling. The mother liquor after centrifugation is tested for lithium oxide concentration. When it reaches 5g / L, it is returned to step S7 to be combined with the lithium-containing filtrate, and the resin is used for decalcification to carry out cyclic lithium carbonate production.

[0082] This example produces about 35.57 kg of lithium carbonate, with a lithium yield of 87.4%.

[0083] like Figure 2As shown, the preheating kiln 1 in step S1 of Examples 1-3 of the present invention is arranged at the upper left side of the kiln tail side of the rotary kiln 2, and a cooling kiln 4 is arranged at the lower right side of the rotary kiln. The rotary kiln 2 is provided with an external heating jacket 201, and the external heating jacket 201 is heated by natural gas combustion. The external heating jacket is provided with a natural gas combustion exhaust gas outlet pipe 3 near the kiln tail, and the exhaust gas outlet pipe 3 is connected with the interior of the preheating kiln 1, and the exhaust gas after the natural gas combustion is passed into the preheating kiln 1, so as to preheat the lithium mica ore powder in the preheating kiln 1.

[0084] The preheating kiln 1 utilizes the tail gas of the external heating jacket 201 of the rotary kiln 2 for preheating, thereby improving the heat utilization rate of the rotary kiln 2 and reducing the energy consumption of the preheating kiln 1 .

[0085] The low-grade lepidolite raw material used in the following comparative examples is the same as that in the embodiments of the present invention. Its main chemical composition is shown in Table 1 (wt%) after preliminary detection, and the remainder is other small amounts of substances.

[0086] Comparative Example 1:

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

[0088] S2, the granulated material is conveyed to the rotary kiln for roasting by a belt, the roasting temperature of the rotary kiln is 900°C, the roasting time is 50 minutes, and the material after roasting is cooled to room temperature by a cooling kiln;

[0089] S3, the cooled calcined material is transported to the ball mill through a zipper machine, and circulating water is added to make slurry to ensure that the concentration of lithium oxide in the solution is stable at 15-20g / L, and wet-milled to a particle size of 60-100 mesh after ball milling;

[0090] S4, the material after ball milling is soaked in circulating water at a solid-liquid ratio of 1:0.8-1.2, the soaking temperature is kept at 70-95°C by jacket steam heating, the soaking time is 30-60min, and after soaking, a belt filter is used for vacuum filtration to obtain lithium sulfate mother liquor and leaching residue, and the leaching residue is washed with circulating water;

[0091] S5, lithium sulfate mother liquor 1 is transported to a neutralization and impurity removal tank through a pump through a pipeline, and a 27.5% hydrogen peroxide solution is first added to the mixed mother liquor to separate the iron ions, and then a 30-35% lime milk is added to control the pH value of the solution to pH 11.5 to produce precipitation, and heavy metals rubidium, cesium, aluminum, magnesium, manganese and calcium are separated, and then a chelating resin is added to further remove calcium and magnesium to obtain lithium sulfate mother liquor 2;

[0092] S6, the lithium sulfate mother liquor 2 is pumped into a lithium sulfate MVR high-efficiency evaporator for evaporation and concentration, and the Li2O concentration in the solution is controlled at 15-45 g / L to obtain a lithium sulfate concentrate;

[0093] S7, adding saturated sodium carbonate solution to the lithium sulfate concentrate, and reacting lithium precipitation at 80-95° C. for 3 hours. After the reaction is completed, the reaction is allowed to stand and settle, and then filtered and separated by a centrifuge to obtain lithium precipitation mother liquor and lithium carbonate crude product;

[0094] S8. The 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, and then dried and crushed to obtain battery-grade lithium carbonate.

[0095] S9: The lithium precipitation mother liquor of step S7 is adjusted to pH 5.5-6.5 by adding sulfuric acid, and the carbonate in the mother liquor is removed to obtain the lithium precipitation neutralized mother liquor, which is then concentrated in a sodium sulfate MVR high-efficiency evaporator. After concentration, the sodium sulfate potassium mixed salt and lithium sulfate mother liquor 3 are obtained by centrifugation. The sodium sulfate potassium mixed salt is added to step S1 for batching, and the lithium sulfate mother liquor 3 is subjected to secondary lithium precipitation.

[0096] Comparative Example 1 produced about 32.20 kg of lithium carbonate, and the yield of lithium was 79.12%.

[0097] Comparative Example 1 is a traditional sulfate roasting method. Auxiliary materials such as sodium potassium sulfate mixed salt, calcium sulfate, ferrous sulfate, magnesium sulfate and calcium carbonate are added during the process. During water leaching, in addition to soluble sodium potassium salts, calcium, iron, magnesium and sulfur ions will enter the lithium leaching slag, greatly increasing the amount of lithium leaching slag. In addition, fluoride ions are not recovered and enter the lithium leaching slag, which is difficult to handle and will pollute the environment. The calcium and sulfur trioxide contents in the water leaching slag are relatively high, affecting its application in the glass and ceramic industries.

[0098] Comparative Example 2:

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

[0100] S2, defluorination roasting: the lithium mica ore powder preheated in step S1 is sent from the tail of the externally heated rotary kiln to the externally heated rotary kiln, the externally heated rotary kiln is equipped with a lifting plate, the material moves to the kiln head as the externally heated rotary kiln rotates, and at the same time, 120 kg of water vapor is introduced from the kiln head of the externally heated rotary kiln to make the water vapor and the material move in opposite directions and be fully mixed. The temperature of the externally heated rotary kiln is controlled at 900° C. The water vapor generates superheated steam under high temperature conditions, and the fluorine in the lithium mica reacts with the superheated steam under high temperature conditions to generate hydrogen fluoride tail gas. The defluorination roasting is carried out for 30 minutes, and the lithium mica roasted material with defluorination is sent to a cooling kiln to be cooled 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, and the material above 40 mesh is crushed by a double-roll crusher and then screened by a screening machine to control the material particle size to be less than 40 mesh;

[0102] S4, pressure cooking ingredients: put the screened material in step S3 into a stirring tank with pure water (including the stirring and washing water recycled in step S6 and step S9) for slurry mixing, and add sodium sulfate (after the initial feeding, the sodium sulfate and potassium sulfate recovered in step S11 later), the mass ratio of the screened material to sodium sulfate is initially controlled to be 1:0.8 (the mass ratio of the screened material to sodium sulfate and potassium sulfate is later controlled to be 1:0.8), and the mass ratio of the material to pure water is 1:5. After stirring for 25 minutes, the slurry is pumped into the pressure cooking device;

[0103] S5, pressure cooking leaching: After adding the slurry to the pressure cooking device, add liquid alkali, adjust the pH>12, seal the pressure cooking device, open and heat to the reaction temperature of 250°C, control the pressure in the kettle to 3.0MPa, and the reaction time is 5 hours;

[0104] S6, solid-liquid separation: after the pressure cooking and leaching are completed, the temperature is lowered to 50±5°C, the discharge valve of the pressure cooking device is opened, and the slurry after the reaction is pumped into a plate-frame filter press for pressure filtration to obtain a filter cake and a lithium-containing filtrate; the filter cake is stirred and washed with pure water and then subjected to plate-frame pressing, and the filter residue after pressing is treated as solid waste, and the stirring and washing water is returned to step S4 to participate in slurry preparation;

[0105] S7, resin decalcification: adding a chelating resin to the combined solution of the lithium-containing filtrate obtained in step S6 and the mother liquor after centrifugation obtained in step S1 to remove calcium and magnesium ions in the lithium-containing filtrate to obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate;

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

[0107] S9, lithium precipitation: adding saturated sodium carbonate solution to the lithium sulfate concentrate in step S8, and precipitating lithium at 80-95°C for 3-5 hours. After the reaction is completed, the solution is allowed to settle, and then filtered and separated by a centrifuge to obtain a lithium precipitation mother liquor and a crude lithium carbonate product. The crude lithium carbonate product is washed with circulating water, dried, and crushed to obtain battery-grade lithium carbonate; the washing water is returned to step S4 for slurry preparation;

[0108] S10, tail gas absorption: the hydrogen fluoride tail gas and excess water vapor generated in step S2 are sucked into the condenser through a negative pressure pipe for condensation, and then treated with lime water with a concentration of 5% to generate calcium fluoride slurry. The slurry is then pressed by plate and frame 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] S11. The mother liquor of lithium precipitation in step S9 is added with sulfuric acid to adjust the pH value to 5.5-6.5, and the carbonate in the mother liquor is removed to obtain the mother liquor of lithium precipitation and neutralization, and the mother liquor is put into a high-efficiency MVR for evaporation to precipitate sodium sulfate crystals and potassium sulfate crystals, which are then centrifuged and separated. The separated sodium sulfate crystals and potassium sulfate crystals are returned to step S4 to participate in pressure cooking and recycling. After centrifugation, the mother liquor is tested for lithium oxide concentration. When it reaches 5g / L, it is returned to step S7 to be combined with the lithium-containing filtrate, and the resin is used for decalcification to carry out cyclic lithium carbonate production.

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

[0111] In Comparative Example 2, after pressure cooking and leaching, no extraction-stripping process was used, which affected the lithium yield.

[0112] The chemical composition of the lithium leaching slag produced during the production process of Example 1-3 and Comparative Example 1-2 was tested by a certain analysis and testing company in Foshan City. The number of test samples was about 200 g. The test environment temperature and humidity were 25 degrees and 68% RH. The test was based on 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 can be seen from Table 2, Examples 1-3 of the present invention and Comparative Example 2 use a combination of defluorination roasting and pressure cooking leaching, so that the fluorine content in the lithium leaching residue is low, most of the fluorine is recovered as a calcium fluoride byproduct, and the contents of calcium and sulfur trioxide are very low, so that the toxicity of the leaching residue reaches the first-level solid waste standard, and can be directly used as glass and ceramic raw materials. However, the fluorine content in Comparative Example 1 is 1.93%, so that the toxicity of the leaching residue does not meet the first-level solid waste standard, and the calcium oxide content in Comparative Example 1 reaches 7.01, and the sulfur trioxide content reaches 8.85. The calcium and sulfur trioxide contents are relatively high, which affects its application in the glass and ceramic industries.

[0116] 2. The lithium leaching rate and lithium content of 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 Low-grade lithium oxide content of lithium mica % 1.6 1.7 1.65 1.65 1.65 Lithium leaching rate % (pressure cooking 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] From the test data in the above table, it can be seen that compared with Comparative Example 1, the lithium leaching rate of Examples 1-3 of the present invention is increased by 7.83%, the lithium yield is increased by 7.2%, and the lithium oxide content of the water leached slag is reduced 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 using pressure boiling leaching can reach 95%, but no extraction-stripping process is used later, which affects its final lithium yield.

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

Claims

1. A process for preparing lithium carbonate by roasting, pressure cooking and leaching of low-grade lepidolite, characterized in that: The preparation process includes preheating, defluorination roasting, crushing and screening, pressure cooking and batching, pressure cooking and leaching, solid-liquid separation, resin decalcification, extraction, stripping, lithium precipitation, stirring and washing, drying and crushing. The preparation process includes the following steps: S1, preheating: add low-grade lithium mica ore powder into the preheating kiln through a quantitative screw feeder and heat it to 400-500 degrees; S2, defluorination roasting: the lithium mica ore powder preheated in step S1 is sent to the rotary kiln from the tail of the rotary kiln, the material moves to the head of the rotary kiln as the rotary kiln rotates, and water vapor is introduced from the head of the rotary kiln to make the water vapor and the material move in opposite directions and fully mix, the temperature of the rotary kiln is controlled at 750-950±5°C, the water vapor generates superheated steam under high temperature environment, the fluorine in the lithium mica reacts with the superheated steam under high temperature conditions to generate hydrogen fluoride tail gas, and the lithium mica roasted material with fluorine removed is sent to a cooling kiln to be cooled to room temperature; the defluorination roasting time is 30-60min; S3, crushing and screening: the roasted material cooled in step S2 is screened by a screening machine to separate the material below 40 mesh, and the material above 40 mesh is crushed by a double-roll crusher and then screened by a screening machine to control the material particle size to be less than 40 mesh; S4, pressure cooking ingredients: put the screened material in step S3 into a pulping tank with pure water for slurry preparation, and add sodium sulfate at the same time, control the mass ratio of the screened material to sodium sulfate to be 1:0.5-0.8, and the mass ratio of the material to pure water to be 1:3-5, stir for 20-25 minutes, and then pump the slurry into the pressure cooking device through a pump; S5, pressure cooking leaching: After adding the slurry to the pressure cooking device, add liquid alkali, adjust the pH>12, seal the pressure cooking device, open and heat to the reaction temperature of 230-250°C, control the pressure in the kettle to 2.1-3.0MPa, 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°C, the discharge valve of the pressure cooking device is opened, and the slurry after the reaction is pumped into a plate and frame filter press for pressure filtration to obtain a filter cake and a lithium-containing filtrate; S7, resin decalcification: adding chelating resin to the lithium-containing filtrate obtained in step S6 to remove calcium and magnesium ions in the lithium-containing filtrate to obtain a mixed mother liquor of lithium sulfate, sodium sulfate and potassium sulfate; S8, extraction: adding the lithium extraction agent and the diluent into the extraction tank and stirring evenly, then adding the mixed mother liquor obtained in step S7, stirring for 1 to 2 hours, standing and stratifying, separating the aqueous phase and the organic phase, and the aqueous phase is the raffinate; S9, stripping: the organic phase in step S8 is sent to the stripping tank, and the prepared 10% dilute sulfuric acid is added while stirring, and the pH is adjusted to 3-4, and the reaction is stirred for 1 hour, and 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, and the organic phase is returned to step S8 for repeated use as a lithium extraction agent; S10, lithium precipitation: adding saturated sodium carbonate solution to the lithium-containing mother liquor of step S9, and precipitating lithium at 80-95°C for 3-5 hours. After the reaction is completed, the mother liquor is allowed to stand and settle, and then filtered and separated by a centrifuge to obtain 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.

2. The process for preparing lithium carbonate by roasting, pressure cooking and leaching low-grade lepidolite according to claim 1, characterized in that: In step S2, the rotary kiln is equipped with a lifting plate, and the material is pushed from the kiln tail to the kiln head in combination with the rotation of the rotary kiln, so that the lepidolite and the water vapor are evenly mixed, and the mass ratio of the lepidolite ore powder to the water vapor is 1:0.08-0.12; The preheating kiln in step S1 is arranged at the kiln tail side of the rotary kiln, and the rotary kiln is provided with an external heating jacket outside, and the external heating jacket is heated by natural gas combustion. The external heating jacket is provided with a natural gas combustion tail gas outlet pipe near the kiln tail, and the tail gas outlet pipe is connected with the interior of the preheating kiln, and the tail gas after the natural gas combustion is passed into the preheating kiln, so as to preheat the lithium mica ore powder in the preheating kiln.

3. The process for preparing lithium carbonate by roasting, pressure cooking and leaching low-grade lepidolite according to claim 1, characterized in that: The process also includes step S11, fluorine recovery: the hydrogen fluoride tail gas and excess water vapor generated in step S2 are sucked into the condenser through a negative pressure pipe for condensation, and then treated with lime water with a concentration of 1 to 5% to generate calcium fluoride slurry. The slurry is then pressed by plate and frame 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.

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

5. The process for preparing lithium carbonate by roasting, pressure cooking and leaching low-grade lepidolite according to claim 1, characterized in that: The lithium extraction agent in step S8 includes alkyl diketone compounds, fluorine 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 for preparing lithium carbonate by roasting, pressure cooking and leaching low-grade lepidolite according to claim 5, characterized in that: The alkyl diketone compound is preferably dodecylphenyl-methyl-β-diketone, the fluorine diketone compound is fluorinated dodecylphenyl-methyl-β-diketone or fluorinated bis(1-phenylethyl)dione, and the phosphine oxide compound is preferably trioctyl / hexyl phosphine oxide.

7. The process for preparing lithium carbonate by roasting, pressure cooking and leaching low-grade lepidolite according to claim 1, characterized in that: The process also includes step S12, removing carbonate: adding 98% concentrated sulfuric acid to the lithium precipitation mother liquor obtained in step S10 to remove carbonate to obtain sodium sulfate and lithium sulfate solutions, returning to step S7 to combine with the lithium-containing filtrate, performing resin decalcification, and performing cyclic lithium carbonate production.

8. The process for preparing lithium carbonate by roasting, pressure cooking and leaching low-grade lepidolite according to claim 1, characterized in that: The process also includes step S13, sodium and potassium sulfate recovery: the raffinate of step S8 is pumped into a high-efficiency MVR for evaporation to precipitate sodium sulfate crystals and potassium sulfate crystals, which are then centrifuged, and the sodium sulfate crystals and potassium sulfate crystals separated by centrifugation are returned to step S4 to participate in pressure cooking and batching, and the mother liquor after centrifugation is tested for lithium oxide concentration. When the concentration reaches 5 g / L, the mother liquor is returned to step S7 to be combined with the lithium-containing filtrate, and the resin is used to remove calcium to carry out cyclic lithium carbonate production.

9. The process for preparing lithium carbonate by roasting, pressure cooking and leaching low-grade lepidolite according to claim 8, characterized in that: The sodium sulfate crystals and potassium sulfate crystals recovered in step S13 are returned to step S4 to participate in the pressure cooking of ingredients, 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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