A method for improving the particle size of lithium hydroxide crystals

Abstract

The present application relates to lithium battery material preparation technical field, especially to a kind of method for improving lithium hydroxide crystal particle size, periodic crystal face reconfiguration process is introduced in crystal growth stage, and the physical stripping surface attached liquid is generated by negative pressure flash evaporation cold shock vapor flow, and cooperate with the reverse heat melting effect of superheated unsaturated clear liquid directional melting crystal corner and unclose cavity;Effectively alleviate the problem that mother liquor is hidden due to the too fast crystal growth rate in continuous crystallization process and downstream sintering is easy to burst;By using the dynamic mechanism of physical stripping-chemical melting-steady healing, the closed liquid cavity is forced to open and the internal residual liquid is released, and the loose skeleton is reshaped into a solid dense structure without hiding;Significantly improve the tap density and single crystal integrity of product, greatly alleviate the micro-explosion hazard during high-temperature sintering, meet the high-quality requirements of high-nickel positive material on precursor.

Description

Technical Field

[0001] This invention relates to the field of lithium battery material preparation technology, and in particular to a method for improving the particle size of lithium hydroxide crystals. Background Technology

[0002] Battery-grade lithium hydroxide monohydrate is a key lithium source for preparing high-nickel ternary cathode materials (such as NCM811 and NCA). In the current lithium battery industry chain, in order to meet the requirements of downstream precursor mixing processes for material flowability and reactivity, industrial production mainly adopts large-scale continuous evaporation crystallization process to ensure that the product has a specific particle size distribution and packing density.

[0003] However, in actual large-scale production and application scenarios, existing lithium hydroxide products generally suffer from a hidden quality defect that is difficult to detect through conventional testing—namely, the phenomenon of mother liquor inclusion inside the crystal. Although the appearance and main content indicators (such as LiOH content) of commercially available products often meet battery-grade standards, trace amounts of mother liquor are often physically encapsulated deep within their crystal lattice. Because these impurities are wrapped by a dense crystal shell, subsequent conventional processes such as centrifugation, spray washing, or hot air drying can only remove the physically attached water on the crystal surface and cannot remove the residual liquid inside.

[0004] Furthermore, this defect can lead to more serious process risks in the downstream cathode material preparation process. When lithium hydroxide containing internal inclusions is mixed with the precursor and calcined at high temperature (usually >700℃) in a roller kiln, the mother liquor sealed inside the crystal will instantly vaporize and expand, causing the lithium hydroxide particles to burst or pulverize at the microscopic level. This in-situ bursting not only destroys the microstructural integrity of the cathode material precursor, leading to a decrease in the cycle life of the finished battery, but the alkaline substances splashed out by the bursting will also cause serious chemical corrosion to the expensive kiln refractory materials, significantly increasing the equipment maintenance costs and production risks of battery material plants. Summary of the Invention

[0005] The main objective of this invention is to provide a method for improving the particle size of lithium hydroxide crystals, aiming to solve the problem of dead zones trapped in lithium hydroxide crystals by the solution in the prior art, thereby improving the particle size of lithium hydroxide crystals.

[0006] To achieve the above objectives, the present invention provides a method for improving the particle size of lithium hydroxide crystals, the method comprising the following steps: Constructing a thermally stable bottom solution: Dissolve lithium hydroxide raw material in water to prepare a saturated lithium hydroxide feed solution, preheat it to a first temperature T1, and establish a thermal cycle in the crystallization system to maintain the boiling point of the lithium hydroxide feed solution at the first temperature T1, so as to form a thermodynamic equilibrium state. Nucleus induction and framework growth: Reduce the vacuum level in the crystallization system to allow the lithium hydroxide feed liquid to enter the metastable region to generate crystal nuclei, and perform preliminary evaporation crystallization to form a crystal suspension; Performing a crystal surface reconstruction cycle: During crystal growth, the crystal suspension is periodically subjected to at least one surface reconstruction process, which includes: a negative pressure flash evaporation cooling stage and a superheated flow reverse thermal melting stage; wherein, the negative pressure flash evaporation cooling stage includes: reducing the pressure in the crystallization system to generate a vaporization gas flow on the crystal surface to peel off the surface liquid based on the temperature drop cooling effect generated by flash evaporation endothermic heat generation; the superheated flow reverse thermal melting stage is: injecting an unsaturated lithium hydroxide solution with a temperature higher than the first temperature T1 into the crystallization system to melt the crystal surface and obtain crystal slurry; Constant pressure healing and separation: After completing the surface reconstruction process, the vacuum degree of the crystallization system is restored, constant temperature crystal growth is performed, and then the crystal slurry is subjected to solid-liquid separation and drying.

[0007] Optionally, the concentration of the lithium hydroxide feed solution is 9.5 wt% to 10.5 wt%, and the first temperature T1 is in the range of 80°C to 90°C; in the nucleus induction and framework growth, when the crystal growth is controlled to reach an average particle size of 5 μm to 8 μm, the crystal plane reconstruction cycle is started.

[0008] Optionally, during the negative pressure flash evaporation cooling stage, the reduction of pressure within the crystallization system specifically includes: reducing the pressure within the crystallization system by 10 kPa to 15 kPa within a time period of 0.5 to 2.0 seconds, and maintaining this pressure for 10 to 30 seconds.

[0009] Optionally, through the negative pressure flash evaporation cooling stage, the temperature of the crystal suspension is reduced by 3°C to 5°C within 1 to 3 seconds to form a supersaturated region on the crystal surface.

[0010] Optionally, during the superheated flow reverse melting stage, the temperature T2 of the unsaturated lithium hydroxide solution is 105°C to 115°C, and the injection method of the unsaturated lithium hydroxide solution is pulse injection, with a single pulse injection time of 5 to 15 seconds.

[0011] Optionally, the unsaturated lithium hydroxide solution has a saturation of less than 0.95 and the injection amount is 2% to 5% of the total liquid holding volume in the crystallization system.

[0012] Optionally, in the crystal plane reconstruction cycle, the surface reconstruction process is executed once every 15 to 25 minutes, and is executed a total of 5 to 8 times throughout the entire crystallization cycle.

[0013] Optionally, the isothermal crystal growth process specifically includes: The vacuum level of the crystallization system was restored to the level that maintains the boiling point at T1, and the supersaturation was controlled between 1.02 and 1.05. Stirring was maintained for 30 to 60 minutes to repair the hot melt defects on the crystal surface.

[0014] Optionally, in the superheated flow reverse melting stage, the injection direction of the unsaturated lithium hydroxide solution is opposite to the circulation flow direction of the crystal suspension.

[0015] Optionally, in the subsequent solid-liquid separation and drying of the crystal slurry, the temperature of the drying hot air is controlled at 100°C to 120°C, and the dried crystals are cooled to below 40°C for packaging, resulting in lithium hydroxide crystals with a tap density greater than 1.20 g / cm³.

[0016] The beneficial effects that this invention can achieve are as follows: This invention introduces a periodic crystal plane reconstruction cycle process by constructing a thermally stable bottom liquid and inducing framework growth in the conventional evaporation crystallization system of lithium hydroxide crystals. Specifically, it utilizes the sudden drop in system pressure and the endothermic effect of flash evaporation caused by the negative pressure flash evaporation cooling stage, combined with the high-temperature unsaturated clear liquid injected in the superheated flow reverse hot melting stage, to construct a dynamic growth regulation mechanism of physical peeling-chemical erosion-steady-state healing. This effectively solves the technical problems in the existing continuous evaporation crystallization process, such as the mother liquor being physically encapsulated inside the crystal lattice due to the excessively fast crystal framework growth rate and high viscosity of the surface liquid, resulting in liquid inclusion defects, as well as the resulting low product density and fragile crystal structure. Specifically, a negative pressure flash evaporation cooling process is used to form a locally highly supersaturated hardened layer on the crystal surface through the instantaneous temperature drop. At the same time, the intense vaporization gas flow physically forces the removal of residual liquid from the crystal surface, blocking the mother liquor encapsulation path at its source. The subsequent superheated flow reverses the thermal melting process, using unsaturated clear liquid at a temperature higher than the equilibrium temperature to directionally melt and repair dendrites, edges, and unclosed cavity edges on the crystal surface, forcibly opening partially closed liquid cavities and releasing internal residual liquid. Finally, combined with the isothermal crystal growth step after restoring vacuum, the Ostwald ripening effect is used to repair the microscopic defects left by the thermal melting, thereby obtaining high-quality lithium hydroxide crystals that are solid internally without inclusions, have a smooth and dense surface, and high mechanical strength, which significantly improves the efficiency of subsequent solid-liquid separation and the tap density after drying. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the method in Embodiment 1 of the present invention.

[0018] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0019] 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 seven embodiments described are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0021] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "fixed" can mean a fixed connection, a detachable connection, or an integral part; it can mean a mechanical connection or an electrical connection; it can mean a direct connection or an indirect connection through an intermediate medium; it can mean the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0022] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the meaning of "and / or" throughout the text includes three parallel solutions; for example, "A and / or B" includes solution A, solution B, or a solution where both A and B are satisfied simultaneously. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0023] As attached Figure 1 As shown, the present invention provides a method for improving the particle size of lithium hydroxide crystals, the method comprising the following steps: Constructing a thermally stable bottom solution: Dissolve lithium hydroxide raw material in water to prepare a saturated lithium hydroxide feed solution, preheat it to a first temperature T1, and establish a thermal cycle in the crystallization system to maintain the boiling point of the lithium hydroxide feed solution at the first temperature T1, so as to form a thermodynamic equilibrium state. Nucleus induction and framework growth: Reduce the vacuum level in the crystallization system to allow the lithium hydroxide feed liquid to enter the metastable region to generate crystal nuclei, and perform preliminary evaporation crystallization to form a crystal suspension; Performing a crystal surface reconstruction cycle: During crystal growth, the crystal suspension is periodically subjected to at least one surface reconstruction process, which includes: a negative pressure flash evaporation cooling stage and a superheated flow reverse thermal melting stage; wherein, the negative pressure flash evaporation cooling stage includes: reducing the pressure in the crystallization system to generate a vaporization gas flow on the crystal surface to peel off the surface liquid based on the temperature drop cooling effect generated by flash evaporation endothermic heat generation; the superheated flow reverse thermal melting stage is: injecting an unsaturated lithium hydroxide solution with a temperature higher than the first temperature T1 into the crystallization system to melt the crystal surface and obtain crystal slurry; Constant pressure healing and separation: After completing the surface reconstruction process, the vacuum degree of the crystallization system is restored, constant temperature crystal growth is performed, and then the crystal slurry is subjected to solid-liquid separation and drying.

[0024] It should be noted that in the traditional continuous evaporation crystallization process of lithium hydroxide, due to insufficient control of crystallization kinetics, internal dead zones are easily formed during crystal growth. This causes the mother liquor to be physically encapsulated by the dense crystal structure. Conventional centrifugation, spray washing, and hot air drying processes can only remove surface water and cannot effectively remove internal residual liquid. This phenomenon is due to the thermodynamic and kinetic imbalance during crystal growth, which causes trace amounts of mother liquor to remain inside the crystal, thereby affecting the purity and structural integrity of the product.

[0025] Based on the above problems, the method proposed in this embodiment first constructs a thermodynamically stable basic crystallization environment, namely, the thermally stable bottom liquid stage. In this process, industrial-grade lithium hydroxide monohydrate raw material is dissolved in deionized water or purified condensate to precisely prepare a near-saturated lithium hydroxide feed solution. To avoid the cold material entering the crystallization system and causing violent thermal shock leading to explosive nucleation, the feed solution needs to be preheated to a first temperature T1 using a plate heat exchanger or a shell-and-tube heater. This temperature is usually set within the main evaporation temperature range of the process design. Subsequently, the forced circulation pump of the crystallization system is started, so that the feed solution forms a continuous and stable fluid thermal circulation between the evaporation chamber and the heating chamber. By precisely adjusting the exhaust volume of the vacuum system, the boiling point in the crystallization system is strictly maintained at the first temperature T1. At this time, the entire system is in a thermodynamic equilibrium state where solubility and evaporation are relatively balanced. Although the solution is saturated, it has not yet entered the metastable region, laying a thermodynamic foundation for obtaining uniformly sized crystal nuclei.

[0026] After the system stabilizes, it enters the nucleation and framework growth stage. Operators or the control system adjust the vacuum valve opening to smoothly reduce the vacuum level in the crystallization system, or slightly increase the heating steam output under constant pressure to disrupt the original equilibrium state, driving the supersaturation of the lithium hydroxide feed solution across the metastable boundary and into the metastable region. Within this region, primary crystal nuclei begin to spontaneously form in the solution. As the solvent continues to evaporate, the nuclei adsorb surrounding solute molecules and gradually grow. In this initial stage, the crystals mainly undergo rapid framework growth, forming a preliminary crystal suspension. Understandably, although the crystal size is increasing at this stage, the rapid growth rate often leads to the formation of dendrites or irregular steps on the crystal surface.

[0027] Following this, before the crystal fully closes to form the final particles, a periodic crystal plane reconstruction cycle is initiated, comprising two core stages. The first is a negative pressure flash evaporation cooling stage, where the control system drastically reduces the pressure within the crystallization system within a very short time, lowering it below the saturated vapor pressure corresponding to the current temperature. This instantaneous pressure drop triggers a violent adiabatic flash evaporation phenomenon, with a large amount of solvent instantly vaporizing and carrying away a significant amount of latent heat. This causes a sharp drop in temperature at the solution bulk and crystal interface, producing a cooling effect. This cooling not only forms an instantaneous high-supersaturation layer on the crystal surface, promoting rapid surface hardening, but more importantly, the violent vaporization gas flow generated by flash evaporation creates strong turbulent shear forces at the microscale, effectively physically stripping away the high-viscosity liquid layer adsorbed on the crystal surface and grain boundary depressions, forcibly removing the mother liquor that might otherwise be covered by subsequent growth layers from the crystal surface.

[0028] The second stage is the reverse thermal melting stage of the superheated flow. At this time, an unsaturated lithium hydroxide solution with a temperature significantly higher than the first temperature T1 is pulsedly injected into the crystallization system through a nozzle or feeding line. Because this fluid has a high enthalpy value and is in an unsaturated state, when it comes into contact with the crystal suspension, it will preferentially perform selective melting on high surface energy areas such as protruding edges and dendrite ends on the crystal surface. This thermal melting effect is equivalent to a chemical polishing of the rough crystal skeleton. It not only melts through the walls of those cavities that are not completely closed and opens the sealed liquid holes, allowing the mother liquor hidden inside to be replaced, but also eliminates the structural defects in the crystal growth process, reshaping the micromorphology of the particles in the crystal slurry from a loose skeleton state to a dense solid state.

[0029] Finally, after completing the aforementioned drastic surface reconstruction process, the constant pressure healing and separation stage begins. The system slowly restores the vacuum level to the initially set stable value, allowing the crystallization system to return to a mild growth environment with low supersaturation. During this stage, continuous isothermal crystal growth is carried out, utilizing the Ostwald ripening mechanism to further smooth the reconstructed crystal surface under the action of surface tension. Tiny pits are filled, and the crystal structure becomes more compact. Once the crystals have grown to the target particle size and the particle size distribution meets the requirements, the crystal slurry is discharged to a centrifuge for solid-liquid separation. Due to the reconstruction treatment, the crystal surface is smooth and has no internal cavities, significantly improving the solid-liquid separation efficiency. Finally, after hot air drying, a high-quality battery-grade lithium hydroxide product with high tap density and no liquid inclusion defects can be obtained.

[0030] Further, the concentration of the lithium hydroxide feed solution is 9.5wt% to 10.5wt%, and the first temperature T1 is in the range of 80℃ to 90℃; in the crystal nucleus induction and framework growth, when the crystal growth is controlled to reach an average particle size of 5μm to 8μm, the crystal plane reconstruction cycle is started.

[0031] During the raw material preparation stage, the mass concentration of the lithium hydroxide feed solution is strictly limited to the range of 9.5wt% to 10.5wt%. This concentration is slightly lower than the saturation at room temperature, but remains unsaturated near the boiling point. This ensures that the feed solution does not experience pre-crystallization and blockage of the pipelines due to localized overcooling as it passes through the conveying pipes and preheater. It also ensures that once the feed solution enters the crystallization system, only a small amount of solvent needs to be evaporated to quickly reach the upper limit of the metastable region, thereby minimizing steam energy consumption. Simultaneously, the system's first temperature, T1, i.e., the main crystallization temperature range, is set between 80℃ and 90℃. This higher temperature range is chosen because lithium hydroxide has greater solubility at higher temperatures, which is beneficial for increasing the yield of a single batch of crystallization. More importantly, this temperature range provides sufficient enthalpy for the subsequent negative pressure flash evaporation cooling step. Only when the base liquid temperature is maintained above 80℃ can the adiabatic temperature drop caused by the subsequent instantaneous reduction of vacuum be sufficiently significant, thereby generating a cooling impact force sufficient to harden the crystal surface.

[0032] It should be noted that the system does not immediately initiate the cooling and melting cycle upon detecting crystal nucleus formation. This is because the newly formed crystal nuclei are too small at this stage and have poor thermodynamic stability. Premature and violent disturbances can easily lead to the complete dissolution or breakage of the crystal nuclei into a large number of useless fine crystals. Therefore, this invention uses a laser particle size analyzer or an online image monitoring system to monitor the crystal growth status in the suspension in real time. A crystal nucleus cultivation period is specifically set, which maintains mild evaporation conditions, allowing the crystal nuclei to grow freely and build a preliminary crystal framework. When the average particle size of the crystals in the suspension increases to the critical threshold of 5μm to 8μm, the control system determines that the crystals have sufficient structural strength and are at the turning point from a slow growth phase to a rapid growth phase. At this point, the preferential growth tendency of the crystal edges begins to appear, and the first crystal face reconstruction cycle is triggered. The drastic change in the physical field is used to promptly correct the newly formed, still imperfect three-dimensional framework, thereby eliminating defects before they are deeply buried, laying a solid core foundation for the subsequent growth of large-particle dense crystals.

[0033] Furthermore, in the negative pressure flash evaporation cooling stage, the reduction of pressure within the crystallization system specifically includes: reducing the pressure within the crystallization system by 10 kPa to 15 kPa within a time period of 0.5 to 2.0 seconds, and maintaining this pressure for 10 to 30 seconds.

[0034] Understandably, because the rate of pressure decrease is much faster than the rate of heat transfer, the explosive vaporization of the solvent is forced to absorb a large amount of latent heat of vaporization from itself and the surrounding microenvironment, thus forming a significant cooling layer on the crystal surface. It is also understandable that, accompanying this instantaneous flash evaporation, the viscous mother liquor layer originally adsorbed on the crystal surface, especially stagnant at crystal defects, steps, and unclosed cavity entrances, will generate a high-speed microscopic airflow ejected outward due to the dramatic volume expansion of the solvent (the volume increases more than a thousandfold from liquid to gas). This vaporizing airflow originating from within the interface overcomes the surface tension and viscous resistance of the mother liquor, forcibly stripping the remaining liquid from the crystal lattice structure like a high-pressure air gun.

[0035] After completing this momentary pressure shock and physical stripping, the system does not immediately restore pressure, but maintains the reduced low-pressure state for 10 to 30 seconds. On the one hand, this ensures that the cooling effect generated by flash evaporation has sufficient time to allow a dense, hard shell to rapidly grow on the crystal surface due to localized extremely high supersaturation, locking in the surface morphology that has just been cleaned and preventing the mother liquor from flowing back. On the other hand, this time is strictly limited to within 30 seconds, effectively avoiding large-scale explosive secondary nucleation of the solution due to prolonged exposure to high supersaturation. Thus, while ensuring the single-crystal repair effect, it prevents the generation of a large number of fine dust crystals, ensuring the particle size distribution concentration of the final product.

[0036] Furthermore, through the negative pressure flash evaporation cooling stage, the temperature of the crystal suspension drops by 3°C to 5°C within 1 to 3 seconds to form a supersaturated region on the crystal surface.

[0037] It should be noted that when the system pressure drops below the saturated vapor pressure corresponding to the current liquid temperature within milliseconds, the heat energy accumulated in the solution is released instantly, driving the solvent water to undergo violent adiabatic flash evaporation. This phase change process requires the plundering of a huge amount of latent heat of vaporization from the liquid phase bulk and the surface of the suspended crystal particles, resulting in a steep drop in the overall temperature of the crystal suspension within a very short period of 1 to 3 seconds, with the temperature drop precisely controlled between 3°C and 5°C.

[0038] By utilizing the property that the solubility of lithium hydroxide decreases significantly with decreasing temperature, an extreme thermodynamic environment is created at the microscale. Because the heat transfer response at the crystal solid-liquid interface is much faster than the solute diffusion rate, the temperature drop at the interface occurs before concentration equilibrium, instantly constructing a region of extremely high supersaturation within the micron-level boundary layer adjacent to the crystal surface. Within this locally supersaturated region, the supersaturation value far exceeds the upper limit of the metastable region in conventional growth, forcing lithium hydroxide solute molecules dissolved near the interface to deposit at an explosive rate onto the crystal lattice surface exposed by the airflow stripping, before they can diffuse outwards. This process is equivalent to performing an in-situ rapid coating on the crystal surface, quickly forming a dense and hard microcrystalline shell. This shell not only locks in the surface morphology after cleaning and effectively blocks the channel for external mother liquor to re-infiltrate the internal cavity of the crystal, but also provides the necessary structural strength protection to withstand subsequent overheating and flow impacts, ensuring that the crystal maintains the integrity of its single-crystal morphology even after undergoing drastic alternating heating and cooling processes.

[0039] Furthermore, during the superheated flow reverse melting stage, the temperature T2 of the unsaturated lithium hydroxide solution is 105°C to 115°C, and the injection method of the unsaturated lithium hydroxide solution is pulse injection, with a single pulse injection time of 5 to 15 seconds.

[0040] Furthermore, the saturation of the unsaturated lithium hydroxide solution is less than 0.95, and the injection amount is 2% to 5% of the total liquid holding volume in the crystallization system.

[0041] During the reverse thermal melting stage of the superheated flow, the control system will precisely adjust and inject a stream of unsaturated lithium hydroxide clear liquid with a specific thermodynamic state. The temperature T2 of this clear liquid is controlled between 105°C and 115°C, so that it can form a transient local high temperature field the moment it comes into contact with the crystal suspension. At the same time, the saturation of this clear liquid is limited to an unsaturated state, which means that the fluid itself has a strong resolution driving force.

[0042] Regarding the injection method, this invention employs a pulsed injection method that generates strong disturbances. The control valve opens with an extremely fast response speed, rapidly injecting superheated unsaturated clear liquid, equivalent to 2% to 5% of the total liquid holdup in the crystallization system, into the reactor within a short period of 5 to 15 seconds. If the time is too short, the hot melt fluid cannot fully disperse and contact all crystal particles; if the time is too long, it will cause a significant drop in the overall supersaturation in the reactor, leading to excessive dissolution of the crystal skeleton and even production loss. Controlling the injection volume to 2% to 5% of the total liquid holdup ensures that there is sufficient solvent to erode the defect layer on the crystal surface, while also ensuring that the system can quickly return to its original crystallization equilibrium state after the pulse ends, without disrupting the stability of continuous production.

[0043] Upon entering the system, this pulsed high-temperature unsaturated fluid preferentially undergoes intense heat and mass exchange with the crystal that has just undergone cold quenching. Because the edges, dendrite ends, and unclosed cavity edges of the crystal surface have higher surface energy than smooth crystal faces, their dissolution rate increases exponentially upon encountering the high-temperature unsaturated fluid. Therefore, this heat flow effectively performs a directional "peak-shaving" operation on the crystal, forcibly melting away excessively growing edges and using dissolution to erode through the thin-walled crystal shells covering the mother liquor inclusion points. This mechanism successfully reopens the closed liquid cavities inside the crystal, releasing not only the encapsulated mother liquor but also providing a clean, dense, and low-defect substrate for the next stage of crystal growth and smoothing, thus achieving a fundamental reshaping of the crystal structure at the microscopic level.

[0044] Furthermore, in the crystal plane reconstruction cycle, the surface reconstruction process is executed once every 15 to 25 minutes, and is executed a total of 5 to 8 times throughout the entire crystallization cycle.

[0045] It should be noted that a growth window of 15 to 25 minutes allows the crystal to complete a stage of lattice stacking in a relatively stable thermodynamic environment, resulting in a noticeable and substantial increase in grain size. It also allows for the accumulation of a certain thickness of new layer on the crystal edges. If the execution interval is too short, for example, less than 10 minutes, frequent flash evaporation and thermal shock will cause the crystal surface to remain in a state of fatigue. The newly formed crystal faces will be eroded again before they can solidify, which will not only significantly reduce the net growth rate and prolong the production cycle, but also easily induce a large number of secondary nucleations of fine grains. Conversely, if the execution interval is too long, for example, more than 30 minutes, the rapidly growing edges and dendrites may have already physically closed, deeply encapsulating the mother liquor within the crystal. At this point, further surface stripping and erosion will struggle to penetrate the thick crystal shell to reach internal defects, leading to the failure of the de-encapsulation effect.

[0046] Based on the aforementioned time interval control, this surface reconstruction process is cumulatively executed 5 to 8 times within the complete crystallization cycle from nucleus formation to material output. This aims to construct a hierarchical, dense growth pattern. Through 5 to 8 repeated cycles of growth-peeling-melting-healing, it ensures that each growth ring from the core to the surface of the crystal undergoes a liquid-filled vesicle removal process. This multi-cycle cumulative effect results in lithium hydroxide particles that are not simply a stack of single crystals, but a solid entity composed of multiple densely nested growth rings. This range of execution counts balances energy economy and product quality, avoiding both the hollowing out of the crystal center due to too few counts and the energy waste and excessive etching of the crystal morphology caused by too many counts. This ensures that the final product meets the large particle size requirements while possessing extremely high internal density capable of withstanding downstream high-temperature sintering.

[0047] Furthermore, the isothermal crystal growth process specifically includes: The vacuum level of the crystallization system was restored to the level that maintains the boiling point at T1, and the supersaturation was controlled between 1.02 and 1.05. Stirring was maintained for 30 to 60 minutes to repair the hot melt defects on the crystal surface.

[0048] Understandably, by precisely adjusting the flow rate of heating steam and the feed rate, the supersaturation of the solution is strictly locked in the low supersaturation range of 1.02 to 1.05. Under this supersaturation, the driving force in the solution is insufficient to trigger new explosive nucleation, or even to support the rapid growth of crystal edges, but it just meets the thermodynamic requirements for repairing crystal surface defects.

[0049] While maintaining the aforementioned thermodynamic conditions, the stirring paddle was kept running stably, and the isothermal crystal growth time was set to 30 to 60 minutes. During this period, based on the principle of surface energy minimization and the Ostwald ripening effect, solute molecules in the solution preferentially diffuse and deposit at high-energy sites on the crystal surface—namely, pits, erosion holes, and grain boundary faults left from the thermal melting stage. This slow and precise mass transfer process is actually a spontaneous lattice filling and surface polishing mechanism, which gradually smooths and repairs the originally rough and uneven crystal surface without significantly increasing the crystal volume, until a smooth, regular, spherical appearance is formed. The lithium hydroxide crystals not only completely eliminate surface thermal damage traces, but also achieve optimal particle strength and packing density, providing a decisive quality guarantee for subsequent solid-liquid separation and the storage stability of the final product.

[0050] Furthermore, in the superheated flow reverse melting stage, the injection direction of the unsaturated lithium hydroxide solution is opposite to the circulation flow direction of the crystal suspension.

[0051] Understandably, when an unsaturated fluid with high enthalpy impacts a group of equally high-speed flowing crystal particles head-on, the relative velocity between the solid and liquid phases reaches its peak instantaneously, significantly increasing the local Reynolds number on the particle surface. This allows for the powerful tearing and thinning of the laminar boundary layer and concentration polarization layer surrounding the crystal surface. Under this high-intensity hydraulic shearing environment, the high-temperature liquid no longer flows gently across the crystal surface but preferentially impacts and erodes areas of the crystal surface with greater hydrodynamic resistance—namely, rapidly growing, outwardly protruding dendrite tips, rough edges, and edge structures of unclosed cavities.

[0052] Furthermore, in the subsequent solid-liquid separation and drying of the crystal slurry, the temperature of the drying hot air is controlled at 100°C to 120°C, and the dried crystals are cooled to below 40°C for packaging, resulting in lithium hydroxide crystals with a tap density greater than 1.20 g / cm³.

[0053] Understandably, after multiple rounds of negative pressure flash stripping and superheated flow melting trimming, the solid particles in the crystal slurry have eliminated internal liquid-encapsulated cavities, and their surfaces exhibit extremely high smoothness and density. When this slurry enters a centrifuge for solid-liquid separation, due to the smooth crystal surface and absence of dendrites, the adhesion between the mother liquor and the crystal surface is significantly reduced. The dewatering effect of the filter cake is significantly better than that of traditional coarse particles, thus obtaining wet crystals with extremely low moisture content.

[0054] Subsequently, the wet crystals are conveyed to a drying system, where the temperature of the hot air is strictly maintained within the range of 100°C to 120°C. This is sufficient to quickly remove free water from the crystal surface while preventing lithium hydroxide monohydrate from losing its water of crystallization or undergoing chemical decomposition due to excessively high temperatures. More importantly, because the present invention largely removes the mother liquor encapsulated inside the crystal in the preceding steps, making the crystal a solid and dense body, no microscopic cracking or pulverization caused by the vaporization and expansion of residual mother liquor will occur inside the crystal in the hot air environment of 100°C to 120°C. This thermal stability ensures that the crystal can completely retain its reconstructed single-crystal morphology during the drying process, avoiding high surface energy fine powder generated by thermal breakage.

[0055] Example 1: This embodiment provides a method for improving the particle size of lithium hydroxide crystals, the specific steps of which are as follows: Constructing a thermally stable bottom solution: Dissolve lithium hydroxide monohydrate raw material to prepare a feed solution with a concentration of 10.0 wt%, preheat to T1=85℃, establish circulation in the DTB crystallization system, and control the vacuum to maintain the boiling point at 85℃; Crystal nucleation induction: Reduce the vacuum level to allow the feed liquid to enter the metastable region, control the crystal growth to an average particle size of about 6 μm, and then start the reconstruction cycle; Execute the crystal plane reconstruction cycle: Negative pressure flash evaporation cooling: The absolute pressure in the crystallization system is instantly reduced by 12 kPa within 1.0 second and maintained for 20 seconds. During this time, the temperature of the suspension is monitored to drop by approximately 4°C instantaneously. Reverse thermal melting of superheated flow: Immediately afterwards, unsaturated lithium hydroxide solution with a temperature of T2=110℃ and a saturation of 0.90 is injected into the crystallization system in a countercurrent pulse. The injection volume is 3% of the total liquid holding volume, and the single pulse time is 10 seconds.

[0056] The above loop executes once every 20 minutes, for a total of 6 times in the entire cycle; Constant pressure healing and separation: restore vacuum, grow crystals at a constant temperature of 1.03 for 45 minutes, then centrifuge, dry in hot air at 110℃, and cool to 35℃ for packaging.

[0057] Example 2: The difference between this embodiment and Embodiment 1 is that: The feed liquid concentration is 9.5 wt%, the first temperature T1 = 80 ℃, during the reconstruction cycle, the pressure reduction during negative pressure flash evaporation is 10 kPa, and it is maintained for 10 seconds; during the superheated flow reverse heat melting, the injected clear liquid temperature T2 = 105 ℃, the injection amount is 2% of the total liquid holding volume, the circulation frequency is once every 25 minutes, and it is executed 5 times in total. The remaining steps are the same as in Example 1.

[0058] Example 3: The difference between this embodiment and Embodiment 1 is that: The feed liquid concentration is 10.5 wt%, the first temperature T1 = 90℃, and during the reconstruction cycle, the pressure reduction during negative pressure flash evaporation is 15 kPa, maintained for 30 seconds; during the superheated flow reverse melting, the injected clear liquid temperature T2 = 115℃, and the injected amount is 5% of the total liquid holding volume. The cycle frequency is once every 15 minutes, for a total of 8 cycles, and the remaining steps are the same as in Example 1.

[0059] Comparative Example 1: The same crystallization equipment and basic thermodynamic parameters (T1=85℃) as in Example 1 were used, but the vacuum degree was kept constant throughout the crystallization process, and no surface reconstruction process was performed (no cold shock, no hot melting). The crystals grew naturally to the target particle size by relying on constant supersaturation, and then the same crystal growth and post-processing were performed.

[0060] Comparative Example 2: The only difference from Example 1 is that in the surface reconstruction process, only the negative pressure flash evaporation cooling step is performed, and the superheated flow reverse thermal melting step is not performed. That is, the original vacuum level is directly restored after flash evaporation to continue growth. The aim is to verify whether relying solely on physical peeling is sufficient to alleviate encapsulation.

[0061] Comparative Example 3: The only difference from Example 1 is that in the surface reconstruction process, the negative pressure flash evaporation cooling step is not performed, and the superheated unsaturated clear liquid is directly injected periodically. The purpose is to verify whether simple hot melting can effectively open the internal defects if the cold evaporation is not performed first.

[0062] Test method description: Tap density: Measured using a tap density meter according to national standards, the higher the tap density, the denser the crystal and the fewer cavities it contains. Content of inclusion solution inside the crystal: After drying, the crystal structure was destroyed by grinding, and the difference between the weight loss rate of heating and the surface weight loss rate before grinding was measured to characterize the water content inside the crystal lattice. High-temperature sintering cracking rate: Simulating the downstream process, the sample was placed in a high-temperature furnace at 750℃ for 4 hours. After being taken out, the percentage of particles that broke or cracked in the field of view was counted by SEM scanning electron microscope. D50 and particle size distribution Span value: Measured using a laser particle size analyzer. Span = (D90 - D10) / D50. The smaller the value, the more uniform the particle size.

[0063] The test results are shown in Table 1 below:

[0064] Table 1 Based on the data analysis in the table above, it can be seen that the tap density of Examples 1-3 all exceed 1.20 g / cm³, and the content of inclusion solution in the crystal is extremely low (<100 ppm). In contrast, the tap density of Comparative Example 1 (Prior Art) is only 0.88 g / cm³, and the inclusion solution content is as high as 1500 ppm. This directly proves that the crystal plane reconstruction process of the present invention successfully transforms the traditional loose and porous skeleton growth into solid and dense growth.

[0065] The sintering burst rate of the example group was controlled below 1.5%, and that of Example 3 was even as low as 0.3%, which basically solved the pain point of downstream application. In contrast, the burst rate of Comparative Example 1 was as high as 18.5%, indicating that conventional products are very easy to break due to the vaporization of the internal mother liquor at high temperature.

[0066] Comparative Example 2: Using only negative pressure flash evaporation and cooling, although it can peel off the surface liquid, it lacks high-temperature hot melting and finishing, some deep cavities cannot be closed, and fine grains are easily generated (D50 is small, Span is large), resulting in limited improvement in the bursting rate (8.4%).

[0067] Comparative Example 3: Using only superheated flow for hot melting, although it can melt some of the edges, due to the lack of instantaneous high supersaturation hard shell protection generated by cold shock, the hot melting process is prone to excessive erosion of the crystal skeleton, and the mother liquor adhering to the surface is not removed first, resulting in a de-encapsulation effect (450ppm) that is not as good as the example with the combination of dual processes.

[0068] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A method for improving the particle size of lithium hydroxide crystals, characterized in that, The method includes the following steps: Constructing a thermally stable bottom solution: Dissolve lithium hydroxide raw material in water to prepare a saturated lithium hydroxide feed solution, preheat it to a first temperature T1, and establish a thermal cycle in the crystallization system to maintain the boiling point of the lithium hydroxide feed solution at the first temperature T1, so as to form a thermodynamic equilibrium state. Nucleus induction and framework growth: Reduce the vacuum level in the crystallization system to allow the lithium hydroxide feed liquid to enter the metastable region to generate crystal nuclei, and perform preliminary evaporation crystallization to form a crystal suspension; Performing a crystal surface reconstruction cycle: During crystal growth, the crystal suspension is periodically subjected to at least one surface reconstruction process, which includes: a negative pressure flash evaporation cooling stage and a superheated flow reverse thermal melting stage; wherein, the negative pressure flash evaporation cooling stage includes: reducing the pressure in the crystallization system to generate a vaporization gas flow on the crystal surface to peel off the surface liquid based on the temperature drop cooling effect generated by flash evaporation endothermic heat generation; the superheated flow reverse thermal melting stage is: injecting an unsaturated lithium hydroxide solution with a temperature higher than the first temperature T1 into the crystallization system to melt the crystal surface and obtain crystal slurry; Constant pressure healing and separation: After completing the surface reconstruction process, the vacuum degree of the crystallization system is restored, constant temperature crystal growth is performed, and then the crystal slurry is subjected to solid-liquid separation and drying.

2. The method for improving the particle size of lithium hydroxide crystals as described in claim 1, characterized in that, The concentration of the lithium hydroxide feed solution is 9.5wt% to 10.5wt%, and the first temperature T1 is in the range of 80℃ to 90℃; in the crystal nucleus induction and framework growth, when the crystal growth is controlled to reach an average particle size of 5μm to 8μm, the crystal plane reconstruction cycle is started.

3. The method for improving the particle size of lithium hydroxide crystals as described in claim 1, characterized in that, During the negative pressure flash evaporation cooling stage, the reduction of pressure within the crystallization system specifically includes: reducing the pressure within the crystallization system by 10 kPa to 15 kPa within a time period of 0.5 to 2.0 seconds, and maintaining this pressure for 10 to 30 seconds.

4. The method for improving the particle size of lithium hydroxide crystals as described in claim 3, characterized in that, Through the negative pressure flash evaporation cooling stage, the temperature of the crystal suspension drops by 3°C to 5°C within 1 to 3 seconds, so as to form a supersaturation region on the crystal surface.

5. The method for improving the particle size of lithium hydroxide crystals as described in claim 1, characterized in that, During the superheated flow reverse melting stage, the temperature T2 of the unsaturated lithium hydroxide solution is 105°C to 115°C, and the injection method of the unsaturated lithium hydroxide solution is pulse injection, with a single pulse injection time of 5 to 15 seconds.

6. The method for improving the particle size of lithium hydroxide crystals as described in claim 5, characterized in that, The unsaturated lithium hydroxide solution has a saturation of less than 0.95 and is injected at a rate of 2% to 5% of the total liquid holding capacity in the crystallization system.

7. The method for improving the particle size of lithium hydroxide crystals as described in claim 1, characterized in that, In the crystal plane reconstruction cycle, the surface reconstruction process is executed once every 15 to 25 minutes, and is executed a total of 5 to 8 times throughout the entire crystallization cycle.

8. The method for improving the particle size of lithium hydroxide crystals as described in claim 1, characterized in that, The isothermal crystal growth process specifically includes: The vacuum level of the crystallization system was restored to the level that maintains the boiling point at T1, and the supersaturation was controlled between 1.02 and 1.

05. Stirring was maintained for 30 to 60 minutes to repair the hot melt defects on the crystal surface.

9. The method for improving the particle size of lithium hydroxide crystals as described in claim 1, characterized in that, In the superheated flow reverse melting stage, the injection direction of the unsaturated lithium hydroxide solution is opposite to the circulation flow direction of the crystal suspension.

10. The method for improving the particle size of lithium hydroxide crystals as described in claim 1, characterized in that, In the subsequent solid-liquid separation and drying of the crystal slurry, the temperature of the drying hot air is controlled at 100°C to 120°C, and the dried crystals are cooled to below 40°C for packaging. The resulting lithium hydroxide crystals have a tap density greater than 1.20 g / cm³.

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