Comprehensive utilization method for step-by-step conversion of mother liquor in lithium salt production

Abstract

This invention discloses a comprehensive utilization method for the step-by-step conversion of mother liquor in lithium salt production, relating to the field of lithium salt preparation technology. The method includes: feeding lithium enrichment transformation liquid A (after liquid shaping) and sodium carbonate solution B1 into a micro-liquid membrane reactor MMR-101, where underequivalent transient nucleation occurs under conditions of η1 of 0.55-0.68; the resulting nucleated slurry enters the first-stage continuous crystallizer G1, where sodium carbonate solutions B2, B3, and B4 are distributedly added along the effective residence time axis; then it enters the second-stage continuous ripening crystallizer G2, where sodium carbonate solution B5 is added at the G2 inlet; and lithium carbonate is obtained through low-shear ripening, hot solid-liquid separation, hot displacement washing, and low-temperature vacuum drying. This method can suppress localized excessive nucleation and pulverization, reduce Na, S, and B entrainment, narrow particle size distribution, increase filtration flux, reduce washing water consumption, and extend continuous stable operation time.

Description

Technical Field

[0001] This invention relates to the field of lithium salt preparation technology, specifically to a comprehensive utilization method for the step-by-step conversion of mother liquor in lithium salt production. Background Technology

[0002] Lithium carbonate, a crucial base lithium salt for cathode materials in power and energy storage batteries, is widely used in the lithium extraction process from salt lake brine, hydrometallurgical conversion of lepidolite or spodumene, and hydrometallurgical recycling of decommissioned lithium batteries. This necessitates the recycling of lithium precipitation mother liquor, washing mother liquor, or intermediate solutions from salt-based conversion processes. These solutions are typically not single lithium salt solutions but rather complex salt systems characterized by high chloride, high sodium, and high ionic strength, often containing trace amounts of sulfate, boron, alkaline earth metals, and residual carbonate. Current production methods for these lithium enrichment and conversion solutions typically involve alkali addition and crystallization to convert lithium into solid lithium carbonate, followed by solid-liquid separation, washing, and drying to obtain the final product. The relevant processes have been widely used in industry and can meet the basic requirements of continuous production and lithium resource recovery. However, under the conditions of high-salt complex mother liquor, the crystallization process is simultaneously affected by local supersaturation, impurity adsorption, particle group evolution and differences in flow field within the equipment. In particular, when the composition of the feed liquid changes with mother liquor reuse, salt system switching and batch fluctuations, there are still large fluctuations between different production lines in terms of particle size distribution, filtration performance, impurity entrainment, residual lithium in mother liquor and operational stability.

[0003] In the aforementioned scenario, the core technical challenge faced by existing technologies lies in how to prevent the formation of persistent localized supersaturation in the front-end alkali contact zone during continuous lithium precipitation of high-salt lithium-enriched feed solutions derived from mother liquor through a stepwise conversion process. This supersaturation can lead to a chain reaction of instability, including concentrated nucleation of fine particles, entrainment of impurities in the bulk phase, and subsequent filtration deterioration. The underlying mechanism is that when the lithium-enriched feed solution comes into contact with the carbonate solution, the carbonate activity, temperature field, and micro-mixing state within the reaction zone undergo abrupt changes within a very short time. If the local mass transfer rate does not match the crystal surface growth rate, some areas will first enter the rapid homogeneous nucleation zone, forming a large number of fine crystal nuclei. Simultaneously, sodium salts, sulfates, and other trace impurities in the high ionic strength system are easily adsorbed, encapsulated, or retained inside the particles during the early stages of crystal nucleation and growth. This results in a wider particle size distribution, deterioration of the filter cake pore structure, and an increase in washing load, further inducing equipment wall adhesion, pressure drop fluctuations, prolonged filtration cycle time, and increased residual lithium in the mother liquor.

[0004] If this problem persists, it will not only weaken the stability of high-value mother liquor resource utilization, but also amplify batch quality fluctuations, increase the frequency of shutdowns for cleaning in continuous production, increase the difficulty of quality control, and increase the risk of releasing subsequent battery-grade products. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing technologies, this invention provides a comprehensive utilization method for the step-by-step conversion of mother liquor in lithium salt production. The lithium enrichment conversion liquid A, shaped by Na2CO3 solution, and sodium carbonate solution B1 are fed into a micro-liquid membrane reactor MMR-101, where underequivalent transient nucleation occurs under conditions of η1 of 0.55-0.68. The resulting nucleated slurry enters the first continuous crystallizer G1, where sodium carbonate solutions B2, B3, and B4 are added in a distributed manner along the effective residence time axis. It then enters the second continuous ripening crystallizer G2, where sodium carbonate solution B5 is added at the G2 inlet. After low-shear ripening, hot solid-liquid separation, hot displacement washing, and low-temperature vacuum drying, lithium carbonate is obtained. This method can suppress excessive local nucleation and pulverization, reduce Na, S, and B entrainment, narrow the particle size distribution, increase filtration flux, and reduce washing water consumption; thus solving the technical problems described in the background art.

[0007] (II) Technical Solution

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A comprehensive utilization method for the step-by-step conversion of mother liquor in lithium salt production includes: continuously feeding lithium enrichment conversion liquid A (after liquid shaping) and sodium carbonate solution B1 into a micro-liquid membrane reactor MMR-101 for underequivalent transient nucleation; the alkali equivalent coefficient η1 is the ratio of the actual number of moles of Na2CO3 added to the theoretically required number of moles of Na2CO3, and η1 is 0.55-0.68; feeding the resulting nucleated slurry into the first-stage continuous crystallizer G1, and adding sodium carbonate solution fed into G1 for distributed alkali replenishment at 2-5 alkali replenishment points along the effective residence time axis of G1, with the last alkali replenishment point located within the first 15-40% of the effective residence time of G1; feeding the slurry from G1 into the second-stage continuous ripening crystallizer G2, adding sodium carbonate solution B5 at the inlet of G2, and the power density of G2 being 0.05-0.16 kW / m³; and obtaining lithium carbonate from the overflow slurry of G2 through solid-liquid separation, washing, and drying.

[0010] Furthermore, the lithium enrichment transformation solution A after liquid shaping contains Li + Concentration of 10-30 g / L, pH of 6.5-8.2, SO4 2- The Li / Li molar ratio is not higher than 0.03, the total amount of Ca and Mg is not higher than 30 mg / L, the suspended matter is not higher than 20 mg / L, and the free CO3 content is... 2- With HCO3 - The total amount is calculated as Na2CO3, ranging from 0.1 to 1.5 g / L; the pH value is the measured value converted to 25°C after using a fixed quenching procedure.

[0011] Furthermore, sodium carbonate solution B1, sodium carbonate solution fed into G1 for distributed alkali replenishment, and sodium carbonate solution B5 are all obtained by diverting Na2CO3 mother liquor. The concentration of Na2CO3 mother liquor is 1.60-2.20 mol / L, the storage temperature is 28-35°C, and the CO2 in the gas phase of the storage tank is controlled below 500 ppm.

[0012] Furthermore, the rotor-stator gap of the micro liquid membrane reactor MMR-101 is 6-14μm, the rotation speed is 3600-5200rpm, the feed liquid temperature in the front section is 35-40°C, and the nominal average residence time in the front section is 8-16s.

[0013] Furthermore, the nucleation intensity index Γ is calculated according to... Determined, where γ n τ is the average shear rate of the front section estimated based on the rotor outer edge linear velocity and rotor-stator clearance. n The nominal average dwell time in the first segment is Γ, which is 3.0 × 10⁻⁶. 6 -6.5×10 6 .

[0014] Furthermore, the total alkali amount is the sum of the molar amounts of Na2CO3 in sodium carbonate solution B1, sodium carbonate solution sent to G1 for distributed alkali replenishment, and sodium carbonate solution B5; the total amount of alkali replenishment formed by sodium carbonate solution sent to G1 for distributed alkali replenishment and sodium carbonate solution B5 accounts for 22-52% of the total alkali amount, of which the amount of alkali replenishment sent to G1 accounts for 65-90% of the total alkali replenishment.

[0015] Furthermore, three alkali replenishment points are set in G1. Sodium carbonate solutions B2, B3, and B4 are added from the first, second, and third alkali replenishment points, respectively. The third alkali replenishment point is located within the first 20-35% of the effective residence time of G1. The operating temperature of G1 is 82-89°C, the effective residence time is 40-90 min, the stirring power density is 0.25-0.70 kW / m³, and the blade tip linear velocity is 0.9-1.6 m / s.

[0016] Furthermore, the amount of sodium carbonate solution added in B5 accounts for 10-35% of the total alkali replenishment; the working temperature of G2 is 88-93.5°C, the effective residence time is 180-360 min, the power density is 0.05-0.16 kW / m³, and the solid content of the output slurry is maintained at 85-125 g / L.

[0017] Furthermore, the G2 overflow slurry undergoes hot solid-liquid separation at 80-90°C; after separation, it is subjected to two-stage heat exchange washing, with a total washing liquid to filter cake mass ratio of 0.80-1.30; the drying temperature is 60-72°C, and the vacuum degree is -0.050MPa to -0.085MPa.

[0018] Furthermore, the obtained lithium carbonate product has a D10 of 3.8-6.0 μm, a D50 of 8.8-13.0 μm, a D90 of 16-22 μm, a Span ≤ 1.50, a tap density of 0.90-1.10 g / mL, a BET of 0.35-0.90 m² / g, a total Na ≤ 80 ppm, and a total S ≤ 100 ppm.

[0019] (III) Beneficial Effects

[0020] This invention provides a comprehensive utilization method for the step-by-step conversion of mother liquor in lithium salt production, which has the following beneficial effects:

[0021] To address the technical problems of localized supersaturation, fine particle nucleation, impurity entrainment, and filtration deterioration in the high-salt lithium enrichment conversion solution obtained from the mother liquor step-by-step conversion during continuous lithium precipitation, a solution is set up with a feed liquid shaping section, an under-equivalent transient nucleation section, a distributed alkali supplementation growth section, and a low-shear ripening section. This decouples and reconstructs carbonate supply, micro-mixing intensity, and particle growth path in time and space.

[0022] The feed shaping section helps stabilize the feed within a salt system window suitable for continuous crystallization, reducing the interference of impurity fluctuations on crystal surface behavior; the underequilibrium transient nucleation section helps form controlled crystal nuclei in a confined space, thus suppressing continuous burst nucleation; the distributed alkali supplementation growth section allows the newly added solid phase to preferentially grow on the surface of existing crystal nuclei, reducing mother liquor encapsulation and bulk phase entrainment; and the low-shear ripening section promotes the uniformity of the particle population and improves the pore structure of the filter cake.

[0023] Therefore, the present invention can improve the controllability and operational stability of the continuous lithium precipitation process of high-salt complex liquids, reduce the burden of equipment wall adhesion, pressure drop fluctuations and subsequent washing, and is conducive to obtaining lithium carbonate products with more concentrated particle size distribution, easier removal of impurities and suitable for stable scale-up. Attached Figure Description

[0024] Figure 1 This is a topological flow diagram of the continuous lithium carbonate preparation process of the present invention.

[0025] Figure 2 This is a cross-sectional view of the microliquid membrane reactor of the present invention;

[0026] Figure 3 This is a flowchart illustrating the control and interlocking logic of underequivalent transient nucleation in this invention.

[0027] Figure 4 This is a structural layout diagram of the first continuous crystallizer of the present invention;

[0028] Figure 5 This is a structural layout diagram of the second continuous ripening crystallizer of the present invention;

[0029] Figure 6This is a schematic diagram illustrating the decoupling of the post-processing flow and the cycle time in this invention. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0031] Please see Figures 1-6 This invention provides a comprehensive utilization method for the step-by-step conversion of mother liquor in lithium salt production, comprising:

[0032] The present invention will be further described below in conjunction with embodiments and examples. Unless otherwise stated, concentration units are as listed in the text; pH values ​​refer to the measured values ​​converted to 25°C after using a fixed quenching / dilution procedure; online hot slurry pH is only used as a trend-based auxiliary signal and not as the sole control boundary; the listed preferred ranges can be used in combination without contradiction.

[0033] To facilitate understanding of the process steps and experimental setup of this invention, in this document, A-feed liquid represents the main feed of the lithium enrichment conversion liquid entering the underequilibrium transient nucleation step; B1 sodium carbonate solution represents the first sodium carbonate solution fed into this step and used for the front-end underequilibrium transient nucleation; B2, B3, and B4 represent sodium carbonate solutions fed into the first continuous crystallizer for distributed alkali replenishment; B5 represents sodium carbonate solution fed into the second continuous ripening crystallizer for polishing alkali replenishment. MMR-101 represents the microfilm reactor device used for controlled nucleation in the front end; G1 of the first continuous crystallizer represents the first continuous crystallization unit used for distributed alkali replenishment and crystal growth; G2 of the second continuous ripening crystallizer represents the second continuous crystallization unit used for low-shear ripening; and H1 represents the thermal crystallization unit used for single-stage thermal crystallization. The alkali equivalent coefficient η1 represents the ratio of the actual number of moles of sodium carbonate added in the front-stage nucleation unit to the theoretically required number of moles of sodium carbonate. η1 less than 1 indicates under-alkali addition in the front-stage, η1 equal to 0 indicates no alkali addition in the front-stage, η1 close to 1 indicates the alkali amount in the front-stage is close to stoichiometry, and η1 greater than 1 indicates over-alkali addition in the front-stage. Unless otherwise stated, pH values ​​in this paper refer to measurements converted to 25 degrees Celsius after using a fixed quenching procedure. Online hot slurry pH is only used as a trend-based auxiliary signal and not as the sole control boundary.

[0034] Implementation method: Method for continuous preparation of lithium carbonate

[0035] 1. Material liquid shaping

[0036] Before entering the underequivalent transient nucleation step, the feed undergoes homogenization, concentration adjustment, acid-base adjustment, impurity removal, fine filtration, and decarbonization treatment in the front-end feed shaping section to ensure the feed solution stably enters the set salt system window; among which, Li in the feed solution... + The concentration is controlled at 10-30 g / L, preferably 18-25 g / L; the pH is controlled at 6.5-8.2, preferably 7.1-7.8; SO4 2- The Li / Li molar ratio is not higher than 0.03, preferably not higher than 0.01; the total Ca and Mg content is not higher than 30 mg / L, preferably not higher than 8 mg / L; suspended solids are not higher than 20 mg / L, preferably not higher than 5 mg / L; free CO3 2- With HCO3 - The total amount, calculated as Na2CO3, is 0.1-1.5 g / L, preferably 0.3-0.9 g / L.

[0037] The feed liquid shaping process may include the following units: S0-1 is a homogenization and dehardening unit, which may employ homogenization tank mixing, softening sedimentation, chelating resin, membrane separation, or combinations thereof to control the Ca and Mg content within the aforementioned range; S0-2 is a sulfate removal or sulfur suppression unit, which may employ methods such as mixing with low-sulfate feed liquid, membrane separation, sedimentation impurity removal, or other methods that can reduce sulfate interference to remove SO42-. 2- The / Li molar ratio is controlled within the above range; SO-3 is the sodium carbonate mother liquor preparation and feedforward buffer unit, the Na2CO3 storage temperature is maintained at 28-35°C, preferably 30-32°C, and the CO2 in the gas phase of the storage tank is preferably controlled below 500 ppm, more preferably below 100 ppm.

[0038] The wet end portion in contact with the high-chlorinated thermal slurry is preferably lined with one of the following: Hastelloy C-276, titanium, zirconium, or fluoroplastic. Before start-up, it should be confirmed that there are no hard clumps of residual salt, no obvious wall adhesion, and no blockage from previous batches. Before entering the underequilibrium transient nucleation step, the feed solution should be subjected to two-stage fine filtration of five micrometers and 0.2 micrometers to reduce the risk of abnormal nucleation induced by fine suspended solids and wall adhesion in the early stage.

[0039] 2. Underequivalent transient nucleation

[0040] The underequilibrium transient nucleation steps sequentially include preheating and flow shaping, two-stream instantaneous mixing, and hydraulic residence quenching. The front-stage alkali equivalent coefficient η1 is defined as the ratio of the actual number of moles of Na2CO3 added in the front stage to the theoretical stoichiometric number of moles of Na2CO3. During continuous stable operation, η1 is controlled at 0.55-0.68, preferably 0.58-0.62; correspondingly, the front-stage Na2CO3 entering the microfilm reactor reacts with Li... + The molar ratio is controlled at 0.275-0.34, preferably 0.29-0.31.

[0041] The concentration of Na2CO3 mother liquor is controlled at 1.60-2.20 mol / L, with 1.70-1.90 mol / L being preferred in engineering applications. The rotor-stator gap of the microfilm reactor is controlled at 6-14 μm, preferably 8-12 μm. When the nominal average residence time of the current stage is 12-16 s, a gap of 10-12 μm is preferred, with a backwashing interlock; when the nominal average residence time of the current stage is reduced to 8-12 s, a gap of 6-10 μm can be used. The rotation speed is controlled at 3600-5200 rpm, preferably 4200-4700 rpm; the feed temperature of the current stage is controlled at 35-40°C, preferably 36-38°C. The outlet pH of the microfilm reactor is preferably within the trend range of 9.20-9.50, but the main control variables are η1, the online turbidity plateau, the pressure drop growth rate, and the fixed total alkalinity after quenching.

[0042] Preferably, the nucleation intensity index Γ of the front segment is calculated according to... Determined, where γ n τ is the average shear rate of the front section estimated based on the rotor outer edge linear velocity and rotor-stator clearance. n This represents the nominal average residence time in the initial stage. The nucleation intensity index Γ is controlled at 3.0 × 10⁻⁶. 6 -6.5×10 6 4.0×10 is preferred. 6 -5.5×10 6 Γ is only used as an indicator for flow field design and scale-up verification, and does not replace the closed-loop control of η1, turbidity, pressure drop and total alkalinity.

[0043] The microfilm nucleator preferably adopts a three-stage rotor-stator configuration, with two tangentially arranged feed nozzles at the front end. The inner diameter of the A-component nozzle can be 1.0-2.0 mm, and the inner diameter of the B1 sodium carbonate solution nozzle can be 0.5-1.2 mm. Both nozzles have their spray axes at a 20-40° angle to the shell tangent and are slightly downward inclined. This allows the two liquid streams to be first drawn into a thin layer by the outer edge of the rotor, and then stretched and mixed within the slit, rather than colliding head-on at the inlet. The effective volume of the nucleation chamber can be divided into a high-shear zone and a short quenching chamber where no alkali is added. Ideally, there should be no dead angle or shoulder between the two sections, and the inner wall chamfer should be no less than R1.5 mm to reduce the risk of the first batch of nuclei forming a long salt shell at the sharp angle. Before start-up, it is advisable to first circulate the preheated mother liquor through a bypass to confirm that the pressure difference across the nucleator is stable before increasing the speed and introducing the B1 sodium carbonate solution.

[0044] The upstream section does not use online pH as the sole control variable. Under optimal conditions, a thin, uniform opalescence appears in the quartz sight glass, the outlet turbidity enters a plateau region, and the pressure drop rate is less than 2-3 kPa / min. When star-shaped bright crystals, localized powdery white clumps, or a rapid increase in pressure drop occur, interlocking protection is achieved through B1 retraction, preheated mother liquor pulse backflushing, and bypass switching. A constant current bypass method is preferred for measuring online turbidity, and calibration is performed daily using the offline turbidity of a fixed quenched sample.

[0045] During short-term process exploration, start-up and shutdown transition, or boundary verification, η1 may fluctuate within the range of 0.48-0.78, but this is not considered the preferred control condition for continuous and stable operation. When η1 deviates from the above-mentioned stable window, it should be limited in conjunction with pressure drop, turbidity, and backwashing interlocks.

[0046] 3. Distributed alkali-supplemented growth

[0047] The nucleated slurry after underequilibrium transient nucleation enters the first continuous crystallizer G1. The subsequent total alkali replenishment is controlled at 22-52% of the total alkali, preferably 30-40%; of which the amount of alkali replenishment entering the first continuous crystallizer G1 accounts for 65-90% of the total alkali replenishment, preferably 72-82%.

[0048] The first-stage continuous crystallizer G1 has 2-5 alkali replenishment points, preferably 3-4; the last alkali replenishment point is located within the first 15-40% of the effective residence time of the first-stage continuous crystallizer G1, preferably within the first 20-35%. The operating temperature of the first-stage continuous crystallizer G1 is controlled at 82-89°C, preferably 84.5-87.0°C; the effective residence time is calculated based on the average volumetric flow rate of the section and controlled at 40-90 min, preferably 50-60 min; the stirring power density is controlled at 0.25-0.70 kW / m³, preferably 0.35-0.50 kW / m³; the blade tip linear velocity is controlled at 0.9-1.6 m / s, preferably 1.0-1.3 m / s; the outlet pH of the first-stage continuous crystallizer G1 is preferably within the trend range of 9.98-10.08, and is verified by a fixed total alkalinity after quenching.

[0049] The first-stage continuous crystallizer G1 employs a segmented alkali addition method along the residence time axis, ensuring that the alkali is primarily added in the first section of G1. Controlled, stepped carbonate supply promotes the growth of existing crystal nuclei. Short, translucent white plumes are permissible downstream of the alkali addition point, but continuously swirling bullseye-shaped white nuclei should not form. When a translucent plume downstream of a single alkali addition point exhibits a continuous length exceeding 100 mm and a duration exceeding 5 seconds under the observation window, the flow rate at that alkali addition point should be preferentially reduced, rather than simply increasing the stirring intensity.

[0050] The first-stage continuous crystallizer G1 preferably adopts an inner diameter of approximately 210 mm and a four-baffle structure. The baffle width is approximately 1 / 10 of the vessel diameter, and the liquid level is stabilized at 360-370 mm, corresponding to an effective liquid volume of approximately 12.6 L. The agitator is preferably a 95 mm four-bladed, 45° downward-pressing impeller, with a distance of approximately 70 mm from the bottom. Three alkali replenishment ports are distributed along the main circulation streamline. The first alkali replenishment port faces the suction area under the impeller, and the second and third alkali replenishment ports are moved backward in sequence, so that the local carbonate pulse is sheared off by the impeller area within 3-5 seconds.

[0051] 4. Low-shear ripening

[0052] The slurry from the outlet of the first continuous crystallizer G1 enters the second continuous aging crystallizer G2. Polishing alkali can be introduced at the inlet of the second continuous aging crystallizer G2, accounting for 10-35% of the total alkali, preferably 15-25%.

[0053] The operating temperature of the second-stage continuous ripening crystallizer G2 is controlled at 88-93.5°C, preferably 89.5-91.5°C; the effective residence time, calculated based on the total inlet volumetric flow rate, is 180-360 min, preferably 220-280 min; the power density is controlled at 0.05-0.16 kW / m³, preferably 0.08-0.12 kW / m³; the overflow pH is preferably in the trend range of 10.10-10.30, preferably 10.14-10.22; and the solid content of the effluent slurry is maintained at 85-125 g / L, preferably 95-110 g / L.

[0054] The second stage continuous ripening crystallizer G2 is a low-shear but non-static ripening unit used for particle ripening and filter cake structure stabilization. If the supernatant continues to be grayish-white and cloudy, the previous stage should be checked first to see if there is excessive nucleation or fine pulverization.

[0055] The second-stage continuous aging crystallizer G2 preferably employs a low-shear axial flow field with an inner diameter of approximately 320 mm, an effective liquid level of approximately 700 mm, and a liquid volume of approximately 56.3 L. The agitator is preferably a hydrofoil or an equivalent low-shear axial flow impeller. A polishing and alkali replenishment point can be set at the inlet circulation line of the second-stage continuous aging crystallizer G2, but no alkali replenishment port is set in the dead zone of the main slurry. Under qualified operating conditions, no coarse bubbles or continuous white foam appear on the surface of the second-stage continuous aging crystallizer G2, and a relatively flat settling interface is formed after the measuring cylinder is left to stand for 40-60 seconds.

[0056] 5. Thermal separation, thermal displacement washing, and low-temperature vacuum drying

[0057] The overflow slurry from the second-stage continuous maturation crystallizer G2 undergoes hot solid-liquid separation at 80-90°C, preferably 84-88°C. The filter media is preferably heat-resistant polypropylene, polytetrafluoroethylene membrane filter cloth, or equivalent chlorine-resistant hot slurry filter media, with a nominal pore size of 0.5-5 μm; the filter cake thickness is controlled at 10-25 mm, preferably 15-20 mm; the cake pressure differential or purging pressure is controlled at 0.04-0.10 MPa, preferably 0.06-0.08 MPa. The filtration flux is uniformly defined as the filtrate mass flux during the first-stage cake formation and mother liquor removal phases.

[0058] After separation, a two-stage heat exchange washing process is employed, with the total washing liquid to filter cake mass ratio controlled at 0.80-1.30, preferably 0.90-1.10. The drying temperature is controlled at 60-72°C, preferably 63-68°C; the vacuum level is controlled at -0.050 MPa to -0.085 MPa, preferably -0.065 MPa to -0.080 MPa. In continuous processes, the thermal solid-liquid separation unit and the drying unit are preferably decoupled. This can be achieved using dual parallel Nutsche filters for alternating feeding, a continuous centrifuge, or a continuous pressure filtration unit, followed by an independent low-temperature vacuum dryer, to avoid mismatch between the single filter dryer and the continuous crystallization discharge cycle.

[0059] 6. Product property control and light elution test

[0060] The preferred microstructures of the resulting lithium carbonate product are: D10 = 3.8-6.0 μm, preferably 4.2-5.2 μm; D50 = 8.8-13.0 μm, preferably 9.8-11.8 μm; D90 = 16-22 μm, preferably 17-20 μm; Span ≤ 1.50, preferably 1.20-1.35; tap density 0.90-1.10 g / mL; BET 0.35-0.90 m² / g; total Na ≤ 80 ppm; total S ≤ 100 ppm.

[0061] The distribution of impurities can be characterized using a standardized light elution test: The sample is first equilibrated at 25°C and 30%RH for 24 h; 10.00 g of sample is weighed and 100.0 mL of deionized water at 25.0°C with a CO2 content below 20 ppm and an initial conductivity below 2 μS / cm is added. The mixture is stirred at 300 rpm for 30 s using a fixed-blade paddle, followed by rapid filtration through a 0.22 μm membrane; after filtration, the resulting filter cake is dried at 60°C for 2 h. The Na and S contents in the samples before and after light elution are then measured. The ratios of the decrease in the corresponding element content before and after light elution to the element content before light elution (F_Na and F_S) are used to characterize the sodium and sulfur light elution ratios. This index is used to evaluate the removability of residual impurities in the sample under light elution conditions. Preferably, F_Na is 0.50-0.70 and F_S is 0.55-0.70.

[0062] The online process sample employs a fixed quenching procedure: 5.0 mL of slurry is taken from the designated sampling port and injected into 45.0 mL of a fixed ionic strength quenching solution at 25.0°C, decarbonized by nitrogen, within 10 s. The preferred composition of the quenching solution is 1.0 wt% LiCl and 0.1 wt% NaCl. Filtration is initiated within 30 s from the injection of the quenching solution and immediately filtered through a 0.22 μm membrane. The filtrate after fixed quenching is used for total alkalinity titration and ion chromatography, while the filter cake is used for SEM-EDS or XRD-assisted characterization. For total alkalinity determination, preferably 10.0 mL of the fixed quenching filtrate is titrated with 0.1000 mol / L hydrochloric acid to a endpoint pH of 4.50, and the result is converted to total alkalinity based on Na₂CO₃.

[0063] In this embodiment, the stepwise conversion of the mother liquor refers to the primary or secondary resource recovery process of the lithium-containing mother liquor generated during lithium salt production, according to the needs of lithium recovery, impurity control, salt system adjustment, and concentration enhancement. This process gradually transforms the original complex mother liquor into a lithium-enriched transformation solution A that meets the subsequent continuous lithium precipitation feed window. This stepwise conversion of the mother liquor occurs before the underequilibrium transient nucleation, the first stage of continuous crystallization, and the second stage of continuous ripening in this invention. Its function is to provide a stable source of solution A and is not equivalent to the subsequent B1, B2, B3, B4, and B5 staged alkali precipitation lithium process of this invention.

[0064] The original lithium-containing mother liquor M0 used in this embodiment is a mixture of lithium carbonate precipitation mother liquor, washing and recovery mother liquor, and salt-based conversion intermediate liquor from the lithium salt production line. This type of mother liquor contains recoverable lithium, as well as chloride, sodium salt, potassium salt, a small amount of sulfate, boron, calcium and magnesium ions, suspended solids, and residual carbonate or bicarbonate ions. Due to fluctuations in the salt composition and impurity content of different batches of mother liquor, it cannot be directly used as a stable and continuous lithium precipitation feed and needs to undergo a step-by-step conversion process first.

[0065] Specifically, the original lithium-containing mother liquor M0 is fed into a homogenization tank and stirred and homogenized at 25-45°C for 2-8 hours. After removing obvious sediment, a primary homogenized mother liquor M1 is obtained. M1 is then concentrated or blended with a high-lithium, low-impurity mother liquor to achieve the desired lithium content. + The concentration was increased and stabilized within the range of 10-30 g / L to obtain secondary lithium enrichment mother liquor M2; M2 was then treated with dehardening, sulfur suppression, or desulfurization to ensure that the total Ca and Mg content did not exceed 30 mg / L and SO4 content was low. 2- The Li / Li molar ratio was not higher than 0.03, resulting in a tertiary purified mother liquor M3. M3 was then subjected to pH adjustment, decarbonization, fine filtration, and temperature control to stabilize its pH at 6.5-8.2, suspended solids at no more than 20 mg / L, and free CO3... 2- With HCO3 - The total amount, calculated as Na2CO3, is 0.1-1.5 g / L, to obtain lithium chloride-type lithium enrichment conversion solution A.

[0066] The hardening treatment can employ one or more of the following: softening sedimentation, chelating resin, membrane separation, and precision filtration; sulfur suppression or sulfate removal treatment can employ methods such as co-mixing with low-sulfate lithium enrichment solution, membrane separation, sedimentation to remove impurities, or other methods to reduce sulfate interference; decarbonization treatment can employ methods such as heating degassing, nitrogen purging, vacuum degassing, or a combination thereof; and fine filtration can employ two-stage filtration of 5 μm and 0.2 μm. The purpose of the above-mentioned treatments is to obtain a lithium enrichment conversion solution that meets the composition window of feed solution A of this invention, rather than limiting the mother liquor step-by-step conversion to a single equipment or a single operating sequence.

[0067] In this embodiment, the composition of the lithium chloride-type lithium enrichment conversion solution A obtained after the above-mentioned stepwise conversion at 25.0°C is as follows: Li + 22.40 g / L, Na + 0.43 g / L, K + 0.17 g / L, Ca 3.2 mg / L, Mg 1.8 mg / L, B 62mg / L, SO4 2- 0.86 g / L, Cl - 118.7 g / L, free CO3 2- With HCO3 - The total concentration (based on Na₂CO₃) is 0.74 g / L, suspended solids are 1.4 mg / L, and pH is 7.46. This feed solution A meets the requirements of the continuous lithium precipitation process of this invention for Li₂CO₃. + Concentration, pH, SO4 2- Feed control requirements for Li / Ca molar ratio, Ca / Mg content, suspended solids, and residual carbonate content.

[0068] Therefore, the lithium enrichment transformation liquid A obtained from the mother liquor step-by-step conversion in this embodiment specifically refers to the lithium chloride-type lithium enrichment transformation liquid formed by homogenization, lithium enrichment, impurity control, salt system adjustment, decarbonization, and fine filtration of the lithium-containing mother liquor produced from lithium salts. This transformation liquid enters MMR-101 as the main feed A in the underequilibrium transient nucleation step of this invention and undergoes a controlled nucleation reaction with the sodium carbonate solution B1.

[0069] 1. Raw material composition

[0070] This embodiment uses lithium chloride-type lithium enrichment conversion solution A obtained from the above-described mother liquor step-by-step conversion process. This solution A is obtained from lithium-containing mother liquor produced from lithium salts after homogenization, lithium enrichment, impurity removal, salt system adjustment, decarbonization, and fine filtration, and serves as the main feed for the underequilibrium transient nucleation step of this invention. The composition of this conversion solution A at 25.0°C is: Li + 22.40 g / L, Na + 0.43 g / L, K + 0.17 g / L, Ca 3.2 mg / L, Mg 1.8 mg / L, B 62 mg / L, SO4 2- 0.86 g / L, Cl - 118.7 g / L, free CO3 2- With HCO3 - The total amount, calculated as Na2CO3, was 0.74 g / L, the suspended matter was 1.4 mg / L, and the pH was 7.46.

[0071] Sodium carbonate mother liquor B is prepared using anhydrous Na₂CO₃ at a concentration of 1.800 mol / L and a storage temperature of 31.0°C. After filtration through a 0.20 μm pleated filter, it enters the metering manifold, and the storage tank is maintained at 0.02 MPa(g) under dry nitrogen sealing. This concentration is used to reduce the local carbonate activity in the front-end and the risk of crystal formation on the wall within the 8-12 μm gap. Feed solution A is the main feed for the lithium enrichment transformation solution entering the underequilibrium transient nucleation step; sodium carbonate solution B1 is the first sodium carbonate solution fed into this step and used for the front-end underequilibrium transient nucleation. Subsequent solutions B2, B3, etc., are distinct from B1.

[0072] 2. Main Equipment

[0073] The main equipment includes: a three-stage rotor-stator microfilm reactor MMR-101, with the wet end material being Hastelloy C-276, an effective working volume of 51.0 mL, of which approximately 34 mL is in the high-shear zone and approximately 17 mL is in the post-quenching chamber; the rotor-stator gap is 11.0 μm, and the rotor outer radius is 18.0 mm; of the two opposing tangential feed nozzles, the inner diameter of the A feed nozzle is 1.5 mm, and the inner diameter of the B1 sodium carbonate solution nozzle is 0.8 mm, with the spray axis at approximately 30° to the tangent of the shell and slightly inclined downwards. The first continuous crystallizer, G1, has an inner diameter of 210 mm, four baffles, and an effective liquid level of approximately 364 mm, with an effective working volume of 12.60 L. It uses a 95 mm four-bladed, 45° downward-pressing propeller with a bottom clearance of 70 mm. The second continuous curing crystallizer, G2, has an inner diameter of 320 mm, an effective liquid level of approximately 700 mm, and an effective working volume of 56.30 L. It uses a 115 mm low-shear hydrofoil propeller. Thermosolid-liquid separation employs dual parallel thermal filtration units F-301A and F-301B or equivalent continuous filtration units. Drying utilizes an independent low-temperature vacuum dryer, D-301. In pilot-scale verification, a combined filtration and drying unit can also be used for single-batch validation, but this is not the sole configuration for continuous operation.

[0074] 3. Feed flow rate and fixed operating parameters

[0075] The flow rate of feed solution A is set at 8.00 L / h; the concentration of sodium carbonate mother liquor B is 1.800 mol / L, and the total flow rate is 7.155 L / h. Among them, B1 has a flow rate of 4.320 L / h and enters MMR-101 as the front-stage nucleation alkali solution; B2, B3, and B4 enter the first-stage continuous crystallizer G1 at 0.815 L / h, 0.735 L / h, and 0.661 L / h respectively as the third-stage alkali replenishment; B5 enters the second-stage continuous ripening crystallizer G2 at 0.624 L / h as the polishing alkali replenishment.

[0076] Based on the above flow rate configuration, the initial nucleation alkali equivalent coefficient η1 is 0.602, and the total alkali equivalent coefficient ηΣ is 0.997. The MMR-101 rotational speed is fixed at 4500 rpm, corresponding to a rotor outer edge linear velocity of approximately 8.48 m / s and a rotor-stator clearance of 11.0 μm. The nominal average residence time in the initial stage, calculated based on the effective volume of MMR-101 plus the combined volumetric flow rates of feed A and B1, is approximately 14.9 s, and the nucleation strength index Γ is approximately 4.6 × 10⁻⁶. 6 .

[0077] The first-stage continuous crystallizer G1 has a fixed temperature of 85.5°C and a fixed stirring speed of 220 rpm. Based on a 95 mm impeller diameter, the tip velocity is approximately 1.09 m / s. Its effective residence time, calculated as the average of the effective liquid volume of G1 and the inlet and outlet volumetric flow rates, is approximately 56 min. The second-stage continuous ripening crystallizer G2 has a fixed temperature of 90.8°C and a fixed stirring speed of 105 rpm. Based on a 115 mm impeller diameter, the tip velocity is approximately 0.63 m / s. Its effective residence time, calculated as the effective liquid volume of G2 and the total inlet volumetric flow rate, is approximately 223 min. These parameters collectively define the basic operating cycle between the controlled nucleation in the first stage, the distributed alkali-supplemented growth in the second stage, and the low-shear ripening.

[0078] 4. Preparations before driving

[0079] Check that the filter cloth of the hot filter unit is intact and free of hard clumps of residual salt. MMR-101, the first continuous crystallizer G1, the second continuous ripening crystallizer G2, and all alkali replenishment branches are first rinsed with 60°C deionized water for 20 min, and then preheated A feed solution at 35-38°C and bypassed for 5 min to confirm that the pressure difference across the MMR is stable and the drift is less than 1 kPa / min.

[0080] The first continuous crystallizer G1 was pre-charged with 12.60 L of deionized water, and the second continuous aging crystallizer G2 was pre-charged with 56.30 L of deionized water. After opening the jacket, the first continuous crystallizer G1 was heated to 85.5°C at a rate of 1.6°C / min, and the second continuous aging crystallizer G2 was heated to 90.8°C at a rate of 1.1°C / min. The MMR-101 jacket was kept at a constant temperature of 37.0°C. After the temperatures of the three stages stabilized for 10 minutes with fluctuations not exceeding ±0.2°C, a slight positive pressure of dry nitrogen was applied to maintain a slight nitrogen sweep above each liquid level, and the hot slurry was vented for condensation or washed for collection.

[0081] 5. Underequivalent transient nucleation operation

[0082] Start the MMR-101 rotor and raise it to 4500 rpm. After confirming that it is running normally, first turn on the A feed pump and adjust it to 8.00 L / h; B1 is not opened all at once, but is raised to 4.320 L / h in a linear ramp over 60-90 seconds.

[0083] After stable operation, the online turbidity of the MMR-101 outlet bypass increases from 4-8 NTU to approximately 175-225 NTU, and stabilizes at 200±25 NTU after 2-3 minutes; the outlet pH trend value is usually between 9.25-9.45. At this time, the quartz sight glass should show a thin, uniform opalescence, without any dull, pinkish-white clumps or persistent bright spots on the wall. If star-shaped bright crystals appear within 10 mm downstream of the inlet, or if the pressure difference rises continuously by more than 8 kPa within 3 minutes, B1 will be automatically reduced by 6-10%, and a 0.5-0.8 s pulse backflushing will be performed with preheated mother liquor; if two backflushings still fail to restore stability, the bypass will be switched, B1 will be stopped, and only the A flow and rotor will be kept running for idling flushing to avoid continuous accumulation of localized oversaturation in the upstream section and induce crystal formation on the wall.

[0084] 6. Distributed alkali replenishment growth operation of the first stage continuous crystallizer G1

[0085] The MMR-101 feed enters the first continuous crystallizer G1 from top to bottom. The first continuous crystallizer G1 has three alkali replenishment points along the effective residence time axis, corresponding to equivalent residence times of approximately 1.8 min, 9.6 min, and 18.0 min, respectively. The flow rates of B2, B3, and B4 are fixed at 0.815 L / h, 0.735 L / h, and 0.661 L / h, respectively. The first alkali replenishment port is located approximately 55 mm below the liquid surface and approximately 15 mm from the vessel wall, facing the suction area under the impeller; the second and third alkali replenishment ports are sequentially moved backward along the main circulation flow line. The stirring speed of the first continuous crystallizer G1 is fixed at 220 rpm, and the temperature is fixed at 85.5°C.

[0086] After the first alkali replenishment point, the online pH trend value increased from approximately 9.35 to approximately 9.65; after the second alkali replenishment point, the pH trend value increased to approximately 9.88; after the third alkali replenishment point, the pH trend value at the outlet of the first continuous crystallizer G1 stabilized at 10.02±0.05. The step-by-step alkali replenishment trajectory should be verified at least once per shift by titrating the total alkalinity of the fixed quenched filtrate. When the continuous length of the translucent plume downstream of a single alkali replenishment point exceeds 100 mm and the duration exceeds 5 s under the observation window, the flow rate of that alkali replenishment point should be preferentially adjusted back, and the injection direction, local flow fluctuations, and impeller suction status of that point should be checked simultaneously, rather than simply correcting by increasing the stirring intensity.

[0087] 7. Second-stage continuous ripening crystallizer G2 low-shear ripening operation

[0088] The slurry from the outlet of the first continuous crystallizer G1 flows by gravity into the second continuous aging crystallizer G2. B5 is used as a polishing alkali supplement and is connected to the inlet circulation line of the second continuous aging crystallizer G2 at a fixed flow rate of 0.624 L / h, corresponding to an equivalent residence time of approximately 12.0 min. The temperature of the second continuous aging crystallizer G2 is fixed at 90.8°C, and the stirring speed is fixed at 105 rpm. Multiple alkali supplementation points are no longer set in the main slurry region of the second continuous aging crystallizer G2 to avoid the formation of new localized supersaturation pulses in the low-shear aging section.

[0089] After the system reached steady state, the online overflow density of the second-stage continuous ripening crystallizer G2 stabilized at 1.136±0.004 g / mL, and the pH trend value stabilized at 10.16±0.04. 100 mL of the slurry was placed in a graduated measuring cylinder and allowed to stand; a relatively flat clear liquid interface formed within 40-60 seconds. If the supernatant remained grayish-white and cloudy, the turbidity plateau and pressure drop history of the MMR upstream section should be checked first.

[0090] 8. Steady-state determination and material collection

[0091] The overflow from the second stage continuous aging crystallizer G2 in the first 5.0 h was entirely returned to the front-end balance tank and not included in product statistics. The material collection process was initiated after the following conditions were maintained for 40 min: MMR-101 outlet turbidity 200±25 NTU; MMR pressure drop growth rate less than 2 kPa / min with no continuous backflush alarm; first stage continuous crystallizer G1 outlet pH trend value 10.02±0.05 and fixed quenched total alkalinity within the set range; second stage continuous aging crystallizer G2 overflow pH trend value 10.16±0.04; second stage continuous aging crystallizer G2 overflow density 1.136±0.004 g / mL.

[0092] 9. Heat filtration, two-stage heat exchange washing and drying

[0093] After reaching steady state, every 120 minutes, a batch of overflow slurry from the second-stage continuous ripening crystallizer G2 is fed into the rotating F-301A / F-301B hot filtration unit or equivalent continuous filtration unit, with the slurry inlet temperature maintained at 86.5°C. A filter cake is first formed for 120 seconds using a nitrogen pressure differential of 0.07 MPa(g), resulting in a filter cake approximately 18 mm thick; the filtration flux is calculated based on the filtrate mass flux.

[0094] The first washing stage involves adding deionized water at 86.5°C, with a washing volume calculated based on 0.32-0.36 t / t of filter cake mass. The solution is evenly distributed from the top of the filter cake, and drained after 90 s of contact. The second washing stage involves adding deionized water at 86.5°C, with a washing volume calculated based on 0.20-0.24 t / t of filter cake mass, and drained after 180 s of contact. Subsequently, the filter cake is purged with nitrogen at 0.07 MPa(g) for 150 s to remove surface free liquid.

[0095] After draining the washing liquid, the filter cake is transferred to an independent low-temperature vacuum dryer D-301 for drying: the jacket is heated to 66.0°C, the vacuum is controlled at -0.074 MPa, the slow stirring speed is set to 12 rpm, and the stirring is intermittently rotated for 60 seconds every 20 minutes, with a total drying time of 6.5 hours. When using an integrated filter-dryer for single-batch validation, the cycle conflict with the continuous discharge from upstream should be eliminated through parallel equipment or buffer tanks.

[0096] 10. Target Steady-State Results

[0097] After stable operation and material collection are completed, the product quality indicators and engineering operation indicators of this embodiment shall be based on the data listed in the test results.

[0098] To avoid ambiguity caused by listing the target set values ​​and actual test results side by side, this embodiment will not list the target steady-state results separately. Indicators such as Li2CO3 content, Na, S, B, particle size distribution, tap density, BET, light elution ratio, single-pass lithium recovery rate, residual Li in mother liquor, filtration flux, washing water consumption, unit mechanical power consumption, and continuous operating time before unplanned shutdown are all measured using a unified detection method and statistical caliber, and are characterized by results obtained from parallel operation or parallel testing.

[0099] I. Experimental Samples and Variable Settings

[0100] This experiment included one example group and five comparative groups. Except for the control variables listed in the table, all comparable process conditions were kept consistent across groups to evaluate the impact of the initial underequilibrium transient nucleation, the distributed alkali replenishment in the first continuous crystallizer G1, and the low-shear ripening in the second continuous ripening crystallizer G2 on product quality and engineering operability. All results are expressed as the mean ± standard deviation of three parallel operations or parallel tests, n=3.

[0101]

[0102] II. Preparation conditions of the experimental example

[0103] 1. Example 1

[0104] The flow rate of lithium enrichment conversion solution A is 8.00 L / h; the concentration of Na2CO3 mother liquor B is 1.800 mol / L, and the total flow rate is 7.155 L / h. Among them, B1 enters the front-stage nucleation flow rate of MMR-101 at 4.320 L / h; B2, B3, and B4 are introduced at the three alkali replenishment points of the first-stage continuous crystallizer G1, with flow rates of 0.815 L / h, 0.735 L / h, and 0.661 L / h, respectively; B5 is used as polishing alkali replenishment at the inlet of the second-stage continuous ripening crystallizer G2, with a flow rate of 0.624 L / h.

[0105] Therefore, η1 = 0.602 and ηΣ = 0.997. The rotor-stator gap of MMR-101 is 11.0 μm, the rotational speed is 4500 rpm, the nominal average residence time in the first stage is 14.9 s, and the shell temperature is 37.0°C; the temperature of the first continuous crystallizer G1 is 85.5°C, and the residence time is approximately 53.0 min based on the average volumetric flow rate of the section; the temperature of the second continuous ripening crystallizer G2 is 90.8°C, and the residence time is approximately 223 min based on the total inlet volumetric flow rate. Hot filtration is carried out at 86.5°C and a pressure difference of 0.07 MPa(g), with a cake formation time of 120 s and a cake thickness of approximately 18 mm; subsequently, two-stage heat exchange washing and low-temperature vacuum drying are performed.

[0106] Filtration flux is uniformly defined as the filtrate mass flux during the initial cake formation and mother liquor removal stages, expressed in kg filtrate·m³. -2 ·h -1 The thermo-solid-liquid separation unit and the drying unit are configured in an alternating or decoupled manner.

[0107] 2. Comparative Examples 1-5

[0108] III. Test Items and Indicator Definitions

[0109] The single-pass lithium recovery rate was calculated based on the Li material balance in the feed, mother liquor, and washing liquid. The filtration flux was measured at 86.5°C, 0.07 MPa (g), and a fixed cake thickness, and was uniformly defined as the filtrate mass flux during the first stage of cake formation and mother liquor removal. The unit mechanical power consumption only includes MMR, circulating pump, and agitator, excluding steam, hot water, and drying heat consumption.

[0110] The continuous operating time before unplanned shutdown is defined as the cumulative continuous operating time when the MMR differential pressure increases by 30 kPa relative to the initial value, or the filtration flux decreases by 20% relative to the initial value; whichever comes first is the operating time of that group.

[0111] Light wash fraction F na F s The standard light washing procedure was followed: the sample was equilibrated at 25°C and 30%RH for 24 h; 10.00 g of sample was weighed and 100.0 mL of deionized water at 25.0°C, with CO2 < 20 ppm and initial conductivity < 2 μS / cm was added. The mixture was stirred at 300 rpm for 30 s using a fixed-blade paddle, and then rapidly filtered through a 0.22 μm membrane. After drying the filter cake at 60°C for 2 h, Na and S were measured. , calculate.

[0112] IV. Test Results

[0113] The following data are expressed as mean ± standard deviation (n=3).

[0114]

[0115]

[0116] V. Data Analysis Conclusions

[0117] Example 1 showed simultaneous improvement in multiple metrics compared to Comparative Example 1, which was closest to the process baseline: the single-pass lithium recovery rate increased from 93.96±0.17% to 94.42±0.19%, an increase of approximately 0.46 percentage points; the filtration flux increased from 374±19 kg filtrate·m -2 ·h -1 Increased to 612±26 kg filtrate·m -2 ·h -1 The efficiency was increased by approximately 63.6%; the washing water consumption decreased from 0.89±0.05 t / t dry product to 0.56±0.03 t / t dry product, a reduction of approximately 37.1%; and the continuous operating time before unplanned shutdown increased from 7.9±1.0 h to 18.6±1.8 h, approximately 2.35 times that of Comparative Example 1. Meanwhile, the unit mechanical power consumption of Example 1 was 46.8±1.9 kWh / t dry product, higher than that of Comparative Example 1, reflecting the engineering costs incurred in microliquid film nucleation and online control enhancement.

[0118] Regarding product quality, the Li₂CO₃ content of Example 1 was 99.68±0.03 wt%, and the Na, S, and B contents were 63±6 ppm, 87±8 ppm, and 9±2 ppm, respectively, which were lower than those of Comparative Example 1 (129±11 ppm, 183±15 ppm, and 17±3 ppm). Its span was 1.34±0.07, lower than that of Comparative Example 1 (1.93±0.11), indicating a narrower particle size distribution. The F content of Example 1... na and F s The values ​​were 0.58±0.03 and 0.61±0.04, respectively, which were higher than those of Comparative Example 1, indicating that the residual Na / S was more likely to be elutable surface residues rather than encapsulated in the bulk phase.

[0119] In Comparative Example 2, after the removal of the S1 underequilibrium transient nucleation, the single-pass lithium recovery rate decreased to 92.38±0.24%, the Na / S ratio increased to 158±14 ppm / 226±19 ppm, and the Span expanded to 2.48±0.20. This indicates that when there is a lack of high-quality crystal nuclei in the front stage, it is difficult to avoid delayed nucleation, bimodal particle size, and impurity entrainment when alkali is added in the back stage. Its D50 increases but its distribution becomes wider, which is consistent with the characteristics of non-uniform hysteretic nucleation.

[0120] Comparative Example 3 retained the front-end nucleation stage but eliminated the first-stage continuous crystallizer G1 distributed alkali replenishment, reducing the filtration flux to 262±17 kg filtrate·m -2 ·h -1 The continuous operating time before the unplanned shutdown was only 5.6±0.7 h, indicating that the addition of the remaining alkali at the inlet of G1 in the first continuous crystallizer would cause local secondary nucleation, soft agglomeration and filter cake skin formation. The recovery rate of this group was not the lowest, but the filtration and operational stability deteriorated the most significantly.

[0121] Comparative Example 4, which replaced the MMR-101 with a conventional Y-type mixer, a Kenics static mixer, and an insulated coil, did not achieve the same level as Example 1 in terms of filtration flux, wash water consumption, Na / S entrainment, and span. This indicates that conventional enhanced mixing can bring some improvement, but it cannot be equated to the synergistic effect of short residence time underequilibrium nucleation of microliquid membranes and subsequent distributed alkali replenishment.

[0122] In Comparative Example 5, after increasing η1 to 0.823, the following results were observed: decreased filtration flux, increased wash water consumption, shortened continuous operating time before unplanned shutdown, and increased Na / S / B entrainment. The recovery rate of this group did not drop sharply, but the pressure drop in the front end increased, the pulverization and filter cake skinning were more obvious, indicating that after η1 in the front end exceeded the control window, the system shifted from controlled nucleation to excessive nucleation and pulverization.

[0123] Based on the above data, Example 1 does not improve a single indicator, but rather improves the single-pass lithium recovery rate while increasing the filtration flux, reducing wash water consumption, reducing Na / S / B entrainment, extending the continuous operation time before unplanned shutdowns, and narrowing the particle size distribution. Comparative Examples 2-5 respectively demonstrate that there is a necessary synergistic relationship between underequilibrium transient nucleation, distributed alkali replenishment in the first continuous crystallizer G1, low-shear ripening in the second continuous ripening crystallizer G2, and the front-end η1 window.

[0124] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0125] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0126] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between apparatuses or units may be electrical, mechanical, or other forms.

[0127] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0128] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A comprehensive utilization method for the step-by-step conversion of mother liquor in lithium salt production, characterized in that: include, The lithium enrichment conversion solution A, after being shaped, and sodium carbonate solution B1 are continuously fed into the microfilm reactor MMR-101 for underequilibrium transient nucleation. The alkali equivalent coefficient η1 is the ratio of the actual number of moles of Na2CO3 added to the theoretically required number of moles of Na2CO3, and η1 is 0.55-0.

68. The resulting nucleated slurry is fed into the first-stage continuous crystallizer G1, and 2-5 alkali replenishment points are set along the effective residence time axis of G1 to add sodium carbonate solution fed into G1 for distributed alkali replenishment. The last alkali replenishment point is located within the first 15-40% of the effective residence time of G1. The slurry from G1 is fed into the second-stage continuous ripening crystallizer G2, and sodium carbonate solution B5 is added at the inlet of G2. The power density of G2 is 0.05-0.16 kW / m³. 3 Lithium carbonate was obtained by solid-liquid separation, washing and drying of the G2 overflow slurry.

2. The comprehensive utilization method according to claim 1, characterized in that: Lithium enrichment conversion solution A after liquid shaping. + Concentration of 10-30 g / L, pH of 6.5-8.2, SO4 2- The Li / Li molar ratio is not higher than 0.03, the total amount of Ca and Mg is not higher than 30 mg / L, the suspended matter is not higher than 20 mg / L, and the free CO3 content is... 2- With HCO3 - The total amount is calculated as Na2CO3, ranging from 0.1 to 1.5 g / L; the pH value is the measured value converted to 25°C after using a fixed quenching procedure.

3. The comprehensive utilization method according to claim 1, characterized in that: Sodium carbonate solution B1, sodium carbonate solution fed into G1 for distributed alkali replenishment, and sodium carbonate solution B5 are all obtained by diverting Na2CO3 mother liquor. The concentration of Na2CO3 mother liquor is 1.60-2.20 mol / L, the storage temperature is 28-35°C, and the CO2 in the gas phase of the storage tank is controlled below 500 ppm.

4. The comprehensive utilization method according to claim 1, characterized in that: The rotor-stator gap of the MMR-101 microfilm reactor is 6-14 μm, the rotation speed is 3600-5200 rpm, the feed temperature in the front section is 35-40°C, and the nominal average residence time in the front section is 8-16 s.

5. The comprehensive utilization method according to claim 4, characterized in that: Nucleation intensity index Γ Determined, where γ n τ is the average shear rate of the front section estimated based on the rotor outer edge linear velocity and rotor-stator clearance. n The nominal average dwell time in the first segment is Γ, which is 3.0 × 10⁻⁶. 6 -6.5×10 6 .

6. The comprehensive utilization method according to claim 1, characterized in that: The total alkali amount is the sum of the molar amounts of Na2CO3 in sodium carbonate solution B1, sodium carbonate solution sent to G1 for distributed alkali replenishment, and sodium carbonate solution B5. The total amount of alkali replenishment formed by the sodium carbonate solution sent to G1 for distributed alkali replenishment and sodium carbonate solution B5 accounts for 22-52% of the total alkali amount, of which the amount of alkali replenishment sent to G1 accounts for 65-90% of the total alkali replenishment.

7. The comprehensive utilization method according to claim 6, characterized in that: Three alkali replenishment points are set in G1. Sodium carbonate solutions B2, B3, and B4 are added from the first, second, and third alkali replenishment points, respectively. The third alkali replenishment point is located within the first 20-35% of the effective residence time of G1. The operating temperature of G1 is 82-89°C, the effective residence time is 40-90 min, the stirring power density is 0.25-0.70 kW / m³, and the blade tip linear velocity is 0.9-1.6 m / s.

8. The comprehensive utilization method according to claim 6, characterized in that: The amount of sodium carbonate solution added (B5) accounts for 10-35% of the total alkali replenishment; the operating temperature (G2) is 88-93.5°C, the effective residence time is 180-360 min, and the power density is 0.05-0.16 kW / m³. 3 The solid content of the discharged slurry is maintained at 85-125 g / L.

9. The comprehensive utilization method according to claim 1, characterized in that: The G2 overflow slurry undergoes hot solid-liquid separation at 80-90°C; after separation, it is subjected to two-stage heat exchange washing, with a total washing liquid to filter cake mass ratio of 0.80-1.30; the drying temperature is 60-72°C, and the vacuum degree is -0.050MPa to -0.085MPa.

10. The comprehensive utilization method according to any one of claims 1-6, characterized in that: The obtained lithium carbonate products have a D10 of 3.8-6.0 μm, a D50 of 8.8-13.0 μm, a D90 of 16-22 μm, a Span of ≤1.50, a tap density of 0.90-1.10 g / mL, a BET of 0.35-0.90 m² / g, a total Na content of ≤80 ppm, and a total S content of ≤100 ppm.

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