Device and method for preparing high-purity large-particle magnesium hydroxide by seawater continuous electrolysis

Abstract

The application discloses a device and method for preparing high-purity large-particle magnesium hydroxide by continuously electrolyzing seawater, and relates to the fields of comprehensive utilization of seawater resources and electrochemistry engineering.The application organically integrates continuous electrolysis extraction of seawater, electrochemical in-situ precipitation and crystallization kinetics regulation, and creatively combines ion directional migration, external circulation and heat exchange, multi-parameter crystallization regulation and crystal concentration dynamic balance, etc., so as to realize controllable growth and continuous separation of magnesium hydroxide crystallization by taking seawater as raw material, without adding an alkaline precipitant, and prepare the magnesium hydroxide with purity > 99.5% and D 50 ≥ 10 mu m, solve the problems of poor controllability of product quality, electrode passivation and great environmental protection pressure in the traditional process, and adapt to green electricity consumption, so as to provide a new path for high-value utilization of seawater magnesium resources.

Description

Technical Field

[0001] This invention relates to the field of comprehensive utilization of seawater resources and electrochemical engineering, and in particular to an apparatus and method for the continuous electrolysis of seawater to prepare high-purity, large-particle magnesium hydroxide. Background Technology

[0002] Seawater, as the most abundant liquid natural resource on Earth, contains a vast amount of mineral resources, with a magnesium ion concentration of approximately 1.3 g / L. It is a high-quality potential source for extracting magnesium and magnesium-based compounds, making the efficient extraction of magnesium hydroxide from seawater an important research direction in the comprehensive utilization of seawater resources. Magnesium hydroxide, as a multifunctional magnesium-based product, possesses high purity and large particle size characteristics, and has high-value applications in environmental protection, materials, and chemical industries. The market increasingly demands higher quality and greener, more continuous production processes. However, current traditional processes for preparing magnesium hydroxide from seawater or magnesium-containing solutions still have many technical shortcomings, failing to meet the actual needs of industrial development and becoming a key factor restricting the high-value utilization of seawater magnesium resources.

[0003] Existing methods for obtaining magnesium hydroxide from magnesium-containing systems are mainly divided into two categories: chemical precipitation and electrolysis. Both have inherent drawbacks that are difficult to overcome. Chemical precipitation is a commonly used method in industry due to its simplicity, but it requires the addition of alkaline precipitants such as lime milk, sodium hydroxide, and ammonia. This not only leads to low product purity, fine particle size, and uneven distribution due to impurities introduced by the alkali itself or pH loss during the precipitation process, but also causes problems such as difficult filtration and washing, and the generation of large amounts of saline wastewater. In addition, different alkalis have their own drawbacks, such as high carbon emissions from raw material extraction, high procurement costs, easy volatilization and environmental pollution, and the need for supporting anti-corrosion tail gas treatment equipment. Although electrolysis is a more promising technology due to its high atom economy, especially the electrolysis of magnesium chloride aqueous solution which can simultaneously produce magnesium hydroxide and chlorine, this method has long faced a core bottleneck in industrialization. Magnesium hydroxide easily deposits on the electrode surface, causing electrode passivation, which in turn leads to increased resistance, increased voltage, and a significant decrease in current efficiency in the electrolytic cell. At the same time, product separation is difficult, and the equipment cannot achieve continuous and stable operation, making it difficult to realize industrialization.

[0004] To address the aforementioned issues, researchers both domestically and internationally have conducted numerous studies on the electrochemical utilization of magnesium resources from seawater brine. However, no systematic solution has yet been achieved. Some technologies focus on the preparation of high-purity magnesium hydroxide, obtaining high-purity products through a coupling of reaction crystallization and electrolysis. However, these technologies use refined salt lake magnesium chloride solutions as raw materials, making it impossible to directly utilize the complex composition of seawater. Furthermore, each unit operation is relatively independent, resulting in insufficient system integration and continuity. Other technologies address the electrode deposition problem during electrolysis, mitigating mechanical issues such as electrode passivation and solid-liquid separation by modifying the electrode surface or designing scraping and filtering devices. However, these approaches only aim to protect the electrodes and maintain electrolysis efficiency, without addressing the control of electrode deposition. The combined approach of controlling magnesium hydroxide crystal growth cannot improve product quality. Some technologies have designed continuous electrolysis and product separation devices to achieve continuous electrolysis of magnesium chloride solution, but they still rely on refined magnesium chloride raw materials. The formation of magnesium hydroxide is merely a spontaneous precipitation caused by a local pH increase in the cathode area. The crystallization process is passive, and the product particle size and morphology are difficult to control, easily forming fine particles that exacerbate the difficulty of subsequent separation. In addition, there are seawater electrolysis technologies that involve the formation of magnesium hydroxide, but the core goal of these technologies is power generation or chlorine production. Magnesium hydroxide is merely an uncontrolled byproduct, and its crystallization process, product morphology, and separation and collection are not taken into account, making it impossible to obtain high-quality magnesium hydroxide products.

[0005] In summary, although existing technologies have improved the utilization of seawater magnesium resources or the preparation process of magnesium hydroxide in a single dimension, they still have many common core defects, making it difficult to meet the industrialization demand for the direct preparation of high-purity, large-particle magnesium hydroxide from seawater. These defects are mainly reflected in the following aspects: (1) The crystallization process lacks effective control methods. In most processes, the formation of magnesium hydroxide is a spontaneous process, and its crystal morphology, particle size distribution, and purity cannot be actively and precisely controlled. The products are mostly small, disordered particles with low added value and high subsequent processing costs; (2) The process continuity and product separation efficiency are low. Existing equipment is difficult to achieve continuous and stable operation of the entire process from raw material feeding to product output. Magnesium hydroxide is easily precipitated in the electric field. The accumulation on the electrode surface or inside the reactor affects the electrolysis efficiency, and the efficiency of the online separation and collection device is insufficient, which restricts the large-scale production; (3) The system design is complicated or the use of raw materials is limited. Some processes that can improve the quality of the product rely on the pre-prepared pure magnesium chloride raw material, which cannot directly process the complex composition of seawater. Some electrolysis devices that can achieve continuous operation have not solved the core problem of product quality control, and it is difficult to balance the compatibility of raw materials and the high quality of products; (4) In terms of electrode passivation, the existing anti-deposition technology focuses on protecting the electrode to maintain the electrolysis efficiency. It fails to transform the control of sediment into the guidance of the ideal product morphology, and cannot achieve the growth of magnesium hydroxide particles and active sedimentation collection by regulating the sedimentation process. Therefore, it is urgent to develop a new integrated device and process method that can directly and efficiently utilize seawater as a raw material, and accurately regulate the crystal growth of magnesium hydroxide in situ and actively during the electrolysis process, and achieve continuous and efficient separation of reaction products, so as to obtain high-purity, large-particle-size, and easily filterable magnesium hydroxide products stably and economically. Summary of the Invention

[0006] The purpose of this invention is to provide an apparatus and method for the continuous electrolysis of seawater to prepare high-purity, large-particle magnesium hydroxide, in order to solve the problems existing in the prior art.

[0007] To achieve the above objectives, the present invention provides the following solution: This invention provides an apparatus for the continuous electrolytic preparation of magnesium hydroxide from seawater, comprising: A dual-chamber diaphragm electrolyzer, wherein the anode chamber and the cathode chamber of the dual-chamber diaphragm electrolyzer are connected by a cation exchange membrane; The anode chamber is a cylindrical structure, containing an anode, an anode conductivity sensor, and an anode level sensor. The top of the anode chamber is equipped with a seawater inlet and an anode exhaust outlet, and the bottom is equipped with a discharge outlet. The seawater inlet is connected to an internal feed pipe extending to the lower part of the anode chamber. The cathode chamber is divided into a cylindrical crystallization zone and a conical collection zone. The conical collection zone is located below and connected to the cylindrical crystallization zone. The cathode chamber houses a cathode, a lifting agitator, a lifting cylinder, a cathode pH sensor, a cathode level sensor, and a cathode temperature sensor. The cathode chamber is equipped with a cathode chamber seawater inlet, a cathode acid inlet, and a cathode exhaust outlet. The conical collection zone is equipped with a slurry reflux circulation system and a temperature-controlled forced circulation system. The lifting agitator and the lifting cylinder are located in the middle of the cylindrical crystallization zone and extend into the conical collection zone. The lifting agitator is located inside the lifting cylinder. The slurry reflux circulation system includes a slurry reflux pipe, a slurry reflux pump, a reflux regulating valve, and a slurry discharge outlet. The reflux inlet and outlet of the slurry reflux pipe are respectively connected to the slurry reflux outlet at the bottom of the cathode chamber cone collection area and the bottom slurry reflux inlet. The slurry reflux pump is located near the bottom slurry reflux outlet of the slurry reflux pipe, and the reflux regulating valve is located at the connection between the slurry reflux pipe and the bottom slurry reflux inlet. The temperature-controlled forced circulation system includes a material circulation pipe, a material forced circulation pump, a pre-pump regulating valve, a post-pump regulating valve, and a temperature control device. The circulation inlet and outlet of the material circulation pipe are respectively connected to the material circulation outlet at the bottom of the cathode cone collecting area and the material circulation inlet at the top of the column crystallization area. The material forced circulation pump is installed on the material circulation pipe, and the temperature control device is connected to the cathode temperature sensor. And a crystallization control system, which includes a cathode pH sensor, a cathode liquid level sensor, and a cathode temperature sensor.

[0008] Furthermore, the inclination angle of the cone in the cone-shaped aggregate area is 45-75°.

[0009] Furthermore, the slurry reflux circulation system also includes a solids content sensor, which is connected to the bottom of the cathode chamber and electrically connected to the reflux regulating valve, and automatically adjusts the reflux regulating valve according to the solids content sensor signal.

[0010] Furthermore, the temperature control device is a pipe-type heat tracing structure or an external heat exchanger, wherein the external heat exchanger is a plate heat exchanger or a tubular heat exchanger.

[0011] This invention provides a method for the continuous electrolysis of seawater to prepare magnesium hydroxide using the above-described apparatus, comprising the following steps: (1) Open the seawater inlet of the anode chamber and the seawater inlet of the cathode chamber, and continuously introduce seawater into the anode chamber and the cathode chamber respectively. Monitor and control the seawater level in the two chambers to be no lower than the lower edge of the cathode and anode plates and no higher than 80% of the height of the cathode and anode chambers by the anode level sensor and the cathode level sensor. Open the regulating valve before the pump and the regulating valve after the pump of the temperature control forced circulation system, and start the lifting agitator and the material forced circulation pump of the temperature control forced circulation system. (2) The real-time monitoring data of the cathode pH sensor, cathode temperature sensor and cathode liquid level sensor are received through the crystallization control system. Based on this, the temperature control device is adjusted to maintain the temperature of the electrolyte in the cathode chamber at 30-80℃, and acid is added to the cathode chamber through the cathode acid inlet to maintain the pH value of the electrolyte in the cathode chamber at 9.5-11.0. (3) Apply direct current to the anode and cathode for electrolysis, and control the cathode current density to 10-350 mA / cm². Magnesium ions in the anode chamber migrate directionally to the cathode chamber through the cation exchange membrane and react with hydroxide ions generated by electroreduction on the cathode surface to generate magnesium hydroxide crystals. (4) During the electrolysis process, start the slurry return pump of the slurry return circulation system, and continuously draw out the slurry rich in magnesium hydroxide crystals in the cathode chamber cone collection area through the slurry return pipe. In the initial stage of startup, adjust the return regulating valve to fully open so that all the drawn-out slurry returns to the cathode chamber through the bottom slurry return inlet. After the system runs stably, monitor the solid content of the slurry at the bottom of the cathode chamber through the solid content sensor, and dynamically adjust the return regulating valve to keep the solid content of the slurry at the bottom of the cathode chamber in the range of 5-20%. The remaining slurry is discharged through the slurry discharge outlet. (5) During the electrolysis process, the gas generated in the chamber is discharged from the anode exhaust port and the cathode exhaust port in real time to maintain stable gas pressure. At the same time, the anolyte is continuously discharged from the anode chamber discharge port. The conductivity in the anode chamber is monitored in real time by the anode conductivity sensor. The discharge rate is adjusted to match the migration rate of magnesium ions to the cathode chamber to ensure that the fluctuation range of the data obtained by the conductivity sensor in the anode chamber does not exceed 30%. (6) Collect the product slurry discharged from the slurry outlet and obtain magnesium hydroxide after post-processing.

[0012] Furthermore, the cathode current density is controlled to be 50-200 mA / cm².

[0013] Furthermore, the post-processing includes washing and drying steps.

[0014] The present invention also provides magnesium hydroxide prepared by the above-described apparatus, which has high purity and large particle size. The purity of the magnesium hydroxide is greater than 99.5%, and the particle size D is [not specified]. 50 Not less than 10μm.

[0015] Furthermore, the particle size D of the magnesium hydroxide prepared by this invention is... 50 It is 10-40μm.

[0016] This invention abandons the traditional precipitation method that relies on the addition of chemical alkalis. Instead, it innovatively adopts an electrochemical in-situ alkali generation method coupled with dynamic crystallization control technology to achieve streamlined operation. It can directly and continuously produce magnesium hydroxide products with both high purity and large particle size from seawater with complex composition and low magnesium ion concentration. This invention aims to systematically solve the three core problems in the traditional seawater magnesium extraction process for magnesium hydroxide preparation: complex process composition, poor product performance, and heavy environmental burden.

[0017] Compared to traditional precipitation methods, the electrolysis method employed in this invention enables in-situ controllable production of alkaline ions, while simultaneously creating a stable crystallization environment with stable reaction process parameters. This simplifies the complex feeding and reaction systems required by precipitation methods, including raw material feeding devices, reaction equipment, and associated pump and tank sets, into a short-process, integrated electrolysis device. This invention replaces the production of chemical precipitants in precipitation methods with greener electricity, achieving zero-cost alkali source production. Furthermore, by precisely controlling electrolysis process parameters and optimizing the electrolysis tank structure, the prepared magnesium hydroxide crystals possess both large particle size and high purity. This process not only achieves green electricity utilization but also significantly reduces the procurement and transportation costs of chemical precipitants, substantially improving the economic benefits of the product.

[0018] This invention is an integrated system based on the principle of "coordinated regulation of electrochemical in-situ precipitation and crystallization kinetics," aiming to achieve efficient extraction of magnesium ions and controllable growth of magnesium hydroxide crystals directly from low-concentration, multi-component seawater. Its core principle is not a simple superposition of single technologies, but rather a systematic solution to the coupling problems of mass transfer, reaction, crystallization, and separation in traditional processes through the synergistic design of multiple modules. The specific principles are explained below: 1) Electrochemical in-situ precipitation replaces external chemical precipitants: The traditional process is as follows: Mg 2+ (raw material solution) + 2OH - (Purchased alkali) → Mg(OH)2↓ Above OH -The magnesium hydroxide produced is derived from externally added chemical precipitants, primarily lime, caustic soda, and ammonia (or water). Firstly, while lime is inexpensive and easily mass-produced, its mining process mainly involves calcining mined ore (primarily CaCO3), resulting in relatively high impurities in the raw materials. Furthermore, because its cation is calcium ions, it readily reacts with sulfate ions in seawater to form insoluble calcium sulfate impurities, which cannot be effectively removed by washing. This leads to low purity in the product, magnesium hydroxide, making it unsuitable for high-end applications. Additionally, mining is a non-renewable process, accompanied by high carbon emissions, high energy consumption, significant pollution, and severe damage to the land surface. Secondly, caustic soda mainly originates from the chemical industry and is primarily produced as an electrolytic sodium chloride solution. The production process is complex, with high procurement costs. As a strong alkaline solvent, the pH controllability during precipitation is poor, often accompanied by explosive nucleation, resulting in fine products with poor filtration performance, affecting subsequent processing. Finally, since ammonia (water) is a weak base, the pH during precipitation is relatively controllable. Therefore, when using it as a raw material, it can usually form magnesium hydroxide products with high purity and controllable particle size. However, because it is volatile and forms irritating waste gas that pollutes the environment and harms human health, it requires supporting sealed reaction, tail gas absorption and anti-corrosion equipment, increasing initial investment. Moreover, the process parameter control is complex, and the stability of large-scale production is easily affected.

[0019] The process of this invention involves applying a potential to the cathode to drive the electroreduction reaction of water: 2H₂O + 2e⁻ → H₂↑ + 2OH⁻ - The generated OH - Immediately with Mg diffused to the cathode interface 2+ reaction: Mg 2+ + 2OH - → Mg(OH)2↓ In the electrolysis process of this invention, the cation exchange membrane is a key component for achieving the directional migration and enrichment of magnesium ions. Its mechanism of action is as follows: Under the action of a DC electric field, cations in the seawater in the anode chamber are driven by an electric field force pointing towards the cathode. The cation exchange membrane, with its inherent negative charge, provides a selective channel for the passage of cations while blocking anions. This allows Mg... 2+ Cations are able to undergo electromigration under the dominance of electric field forces, rather than relying solely on slow concentration diffusion.

[0020] Most importantly, the Mg(OH)₂ precipitation reaction occurring in the cathode chamber continuously consumes Mg. 2+ This creates an environment on both sides of the membrane that is conducive to the growth of Mg. 2+ The sustained concentration gradient transported. Therefore, Mg 2+Transmembrane transport is driven by both electric field and concentration gradient, resulting in a synergistic effect that greatly enhances transport flux. This effectively overcomes the low magnesium ion concentration in low-concentration magnesium-containing water systems, such as seawater / desalinated brine feedstock (Mg2+). 2+ The mass transfer limitation imposed by a feed concentration of approximately 1.3–2.4 g / L enables continuous extraction and concentration of target magnesium ions from large quantities of seawater. This principle ensures that the device maintains considerable production intensity and extraction efficiency even with low-concentration feeds.

[0021] Its advantages in terms of principle include: (1) Zero-cost alkali source: OH - Derived from water molecules, it consumes only electricity. In green power scenarios, it achieves a completely green and low-cost replacement for alkali agents.

[0022] (2) The precipitation process is precise and controllable: OH - The formation rate is directly and linearly controlled by the current density. This provides a more precise and responsive control condition for the "alkali addition acceleration rate" than the traditional stirring and alkali addition method, laying the foundation for subsequent control of crystallization.

[0023] 2) Principle of in-situ precipitation and multi-parameter crystallization kinetics control In the cathode chamber, Mg 2+ The precipitation process of Mg(OH)₂ is characterized by crystal morphology and particle size determined by the competitive relationship between nucleation and growth. This invention, through multi-dimensional precise control, finely manages the supersaturation (S) of Mg(OH)₂, guiding it to grow towards larger particles.

[0024] (1) Defects of traditional precipitation method: rapid mixing causes the solution to reach high supersaturation instantly, triggering "explosive homogeneous nucleation" and generating a large number of fine crystal nuclei, resulting in small particle size of the final product.

[0025] (2) Technical features of the present invention: It can effectively control OH - The generation rate (current density) should be adjusted to avoid instantaneous spikes in oversaturation and to maintain it at a moderate and stable level.

[0026] (3) Control the current density between 10-350 mA / cm² 2 Preferred 50-200 mA / cm 2 The selection of this range is based on the current density and OH - The relationship between current density and H2 gas production, as well as the energy consumption optimization and overall controllability of the magnesium hydroxide synthesis process, were investigated. The inventors discovered that increasing current density leads to increased energy consumption and also increases the OH content. - The output of H2, especially at excessively high current densities (≥350 mA / cm²), is also affected. 2The excessive and dense generation of H2 and the rapid and uncontrollable rise in pH value lead to a large number of bubbles in the system, which severely interfere with the nucleation, growth, and particle size classification of magnesium hydroxide crystals (small particles continue to grow, while large particles settle and are discharged). Furthermore, the poor controllability of pH value hinders crystallization and impurity removal. Within the aforementioned controlled and optimized current density range, stable and controllable synthesis of magnesium hydroxide crystals can be ensured, while energy consumption can be effectively controlled, achieving efficient, green, and stable preparation of high-purity, large-particle magnesium hydroxide.

[0027] Furthermore, this invention optimizes the crystal growth environment by controlling the temperature. Raising the temperature to 30-80℃ significantly accelerates ion diffusion and surface reaction rates, which is beneficial for crystal growth rather than the formation of new nuclei. This avoids the problems of excessively low temperatures (<30℃) which easily form colloids, and excessively high temperatures (>80℃) which result in high energy consumption and a tendency for the system to be in a slightly boiling state, which is not conducive to crystal growth. Maintaining the pH at 9.5-11.0 ensures the thermodynamic stability range of Mg(OH)2 and inhibits Ca2+. 2+ Co-precipitation of impurity ions.

[0028] 3) Fluid dynamics and crystal suspension growth To ensure that the crystallization process proceeds uniformly and stably in the solution phase and to prevent the product from depositing on the electrode surface and causing passivation, this invention designs a unique fluid environment.

[0029] (1) Forced Circulation Uniform Flow Field: By enhancing the stirring and forced circulation system, a high-intensity, uniform, and controllable flow field distribution is created in the cathode chamber. This flow field has three key functions: (i) Enhanced mass transfer: Rapidly renewing the liquid layer at the electrode / solution interface, ensuring the reactants (Mg) 2+ OH - (ii) Continuous supply of fluid; (iii) Crystal suspension: keeping the generated microcrystals and growing particles in suspension, forcing them to grow in the bulk phase of the solution, greatly reducing adhesion to the cathode surface; (iv) Providing moderate shear: uniform fluid shear force can limit the dendritic growth of crystals, promote the formation of dense particles, and prevent their excessive agglomeration.

[0030] (2) External high-efficiency heat exchange: The temperature control heat exchange process is integrated into the above-mentioned forced circulation system, which realizes efficient, uniform and scale-free temperature control of the reaction system, avoids the problem of "insulating scale layer" caused by crystal deposition on the surface of traditional internal heat exchange components, and ensures the stability of long-term operation.

[0031] 4) Dynamic equilibrium of crystal concentration and the principle of continuous separation To achieve continuous production and obtain easily separable products, this invention implements dynamic management of the crystal population within the system.

[0032] (1) Internal classification and enrichment: The cathode chamber collection area is cone-shaped, which facilitates material collection and, together with the internal upward flow field, achieves the separation effect of product particle size. Crystals that have grown to a certain size and whose settling velocity is greater than the upward flow velocity will naturally settle and be enriched in the bottom area of ​​the cone, while fine crystal nuclei will continue to grow in suspension with the flow field, thus achieving preliminary particle size screening.

[0033] (2) Adjustable reflux and controllable seed concentration: The slurry enriched at the bottom of the collecting zone cone is diverted through a slurry reflux circulation system consisting of pipelines, pumps, and valves. One part is continuously discharged as product, and the other part is refluxed back to the cathode chamber as "seed slurry". By dynamically adjusting the reflux ratio through sensors and regulating valves, the solid content of crystals in the reaction zone can be stabilized within the optimal range (e.g., 5-20%). This ensures that there are always sufficient growth sites in the system, inhibits secondary nucleation, and ensures continuous product output. In addition, continuous reflux circulation can prevent the slurry reflux outlet and slurry reflux inlet at the bottom of the collecting zone cone from being blocked by crystals, ensuring the stable operation of the system; (3) Material balance of the whole system: The independent continuous drainage design of the anode and cathode chambers can remove reaction byproducts, accumulated impurity ions and magnesium-poor solution after reaction in a timely manner, maintain a clean and stable chemical environment in the two chambers, and control the driving force of transmembrane ion migration. This is the key to achieving long-term stable operation of the system.

[0034] The specific process of this invention is as follows: 1) Continuous Circulation and Material Balance: This unit aims to establish and maintain a uniform, stable, and dynamically balanced macroscopic reaction and crystal growth environment, laying the foundation for subsequent precise control. First, through forced circulation constructed internally and externally, the ion concentration gradient (concentration polarization) and temperature gradient formed on the cathode surface due to reaction consumption and product formation are rapidly dispersed, ensuring a uniform reaction environment throughout the cathode chamber. This is a prerequisite for achieving stable and reproducible crystallization of magnesium hydroxide. Second, the circulating flow significantly enhances the absorption of Mg from the bulk seawater. 2+ The mass transfer rate to the cathode reaction interface effectively overcomes the problem of insufficient reaction kinetics at low-concentration raw materials, ensuring considerable production intensity of the unit. Secondly, the circulating fluid shear force provides gentle shearing and optimizes the crystal growth environment. This shear force has a dual function: on the one hand, it prevents the generated micro-nuclei or crystals from excessively adhering to the electrode surface and growing into a dense scale layer, forcing the crystals to grow in suspension in the bulk solution phase; on the other hand, moderate shearing promotes collision and growth between crystals, reduces dendritic growth, and promotes the formation of dense particles. Finally, the system achieves global material balance through independent continuous feeding and drainage of the anode and cathode chambers. Continuous feeding provides a continuous magnesium source and reaction medium for the reaction, while continuous drainage promptly removes the magnesium-poor mother liquor and accumulated impurity ions (such as Na+). + Ca2+ This prevents the concentration from being too high and affecting the crystallization purity or membrane performance. At the same time, by precisely coordinating the inlet and outlet rates, the magnesium ion concentration in the system can be maintained at the optimal steady-state concentration that is lower than the inlet water but higher than zero.

[0035] 2) Electrochemical in-situ precipitation (cathode region of a specially designed electrolytic cell): The core function of this unit is to generate OH- ions required for the precipitation reaction in situ and in a controlled manner. - Ions. First, under the drive of current, water molecules undergo a reduction reaction on the cathode surface (2H₂O + 2e⁻). - →H₂↑+2OH⁻ - ), continuously and in situ producing precipitated Mg 2+ Required OH - This replaces the purchased alkali agents [such as NaOH, Ca(OH)2] in traditional processes, achieving zero-cost and green alkali source; secondly, based on OH... - Based on the fundamental principle that the formation rate is proportional to the cathode current density (j), the linear, instantaneous, and precise control of the precipitation reaction rate can be achieved by adjusting the electrical parameters (current). This control precision is more reliable than that of the chemical precipitation and alkali addition process.

[0036] 3) Integrated Crystallization Environment Control: This unit provides real-time feedback and precise control of the parameters in the magnesium hydroxide crystallization process initiated by the electrochemical reaction. This guides the growth and controlled precipitation of magnesium hydroxide crystals, facilitating reduced mother liquor entrainment and improved washing efficiency during subsequent washing processes, ultimately yielding a high-purity product. First, the current density in the cathode region is controlled at 10-350 mA / cm². 2 (Preferred 50-200mA / cm) 2 Within this range, the pH and gas production in the cathode region are relatively stable and controllable. This avoids the interference of large amounts of bubbles on the growth of magnesium hydroxide crystals and the explosive nucleation problem caused by excessive pH changes, as well as the high energy consumption problem caused by excessive current density. Secondly, by precisely stabilizing the pH value of the cathode region at 9.5-11.0, the solubility product of Mg(OH)2 is the smallest within this pH range, resulting in the most complete precipitation reaction, while the main impurity ion in seawater, Ca2+, is also minimized. 2+ The formation of Ca(OH)₂ precipitate requires a higher pH (>12.5), therefore this pH window maximizes the precipitation of Mg. 2+ At the same time, minimize Ca 2+ Co-precipitation is key to obtaining high-purity magnesium hydroxide products. Furthermore, a stable pH range provides a stable crystallization environment and stable OH groups. -Activity becomes the basis for maintaining a stable supersaturation of the system. Finally, maintaining the temperature of the electrolysis process at 30-80℃ can effectively promote crystal growth while avoiding the adverse effects of excessively low temperatures. This can improve the ion diffusion coefficient and surface reaction rate, making it easier for dissolved Mg(OH)2 molecules to migrate to existing crystal nuclei and deposit, thereby promoting crystal growth and inhibiting the formation of new nuclei, which is beneficial for obtaining large-particle crystals. At the same time, the increase in temperature can also improve the solution properties, reduce the solution viscosity, and facilitate the mass transfer process and subsequent solid-liquid separation operations. In addition, the higher temperature can provide energy for crystal growth, improve crystal quality, and help obtain crystals with more complete structures and fewer defects.

[0037] 4) Product collection and continuous reflux: The function of this unit is to discharge the magnesium hydroxide solid product that meets the quality requirements from the system in a timely and selective manner in a certain proportion, while effectively controlling the solid content of the reaction system and realizing continuous production of the device. First, the conical collecting structure of the cathode chamber, combined with the rising circulating flow field, enables graded precipitation of magnesium hydroxide products with different particle sizes. Crystals reaching the target size (settling velocity greater than rising velocity) can naturally settle and accumulate at the bottom of the tank, and are continuously removed from the system through the discharge pipeline. Second, by controlling the reflux ratio through the reflux regulating valve, the average residence time of crystals in the reaction growth zone can be precisely controlled. This ensures that crystals have sufficient time to grow to the target size while avoiding overgrowth, breakage, or becoming secondary nucleation centers due to infinite circulation in the system, thus facilitating the production of products with uniform particle size and concentrated distribution. Third, refluxing a portion of the seed-rich slurry to the cathode chamber can maintain the crystal concentration (seed amount) within the optimal range under continuous production conditions, thereby stabilizing the crystallization process and ensuring controllable and adjustable product particle size. Finally, by continuously removing solid products and a portion of the mother liquor, the unlimited accumulation of solid concentration and impurity ions in the system can be effectively prevented, ensuring stable fluid properties and a clean reaction environment during long-term operation of the device.

[0038] The present invention discloses the following technical effects: This invention can be used directly as raw material in seawater or inexpensive low-concentration magnesium brine, without the need for alkaline precipitants such as sodium hydroxide, lime, and ammonia. This greatly simplifies the production process, reduces raw material and process costs, and has good conditions for industrialization.

[0039] This invention achieves a significant improvement in the performance and precise quality control of magnesium hydroxide products. For the first time, it utilizes an electrolysis method combined with integrated flow field, chemical environment, and crystal concentration control techniques to directly prepare high-purity, large-particle magnesium hydroxide products from seawater. The product purity can reach over 99.5%, with an average particle size D... 50With a particle size of not less than 10μm and excellent filtration and washing performance, the product fundamentally solves the core pain point of difficult solid-liquid separation in the magnesium hydroxide industry, and provides convenience for simplifying subsequent processing procedures and high-value applications in the downstream industrial chain.

[0040] The production process of this invention combines continuity, greenness, stability, and high efficiency. On the one hand, it can achieve fully continuous automated production with high equipment utilization and stable product quality, making it suitable for large-scale industrial applications. At the same time, the process is adapted to green electricity consumption scenarios and can be carried out entirely using clean energy. Compared with the traditional magnesium hydroxide preparation process with added precipitants, it shows significant advantages in carbon footprint accounting and overall carbon emission reduction. On the other hand, the process is clean and controllable, eliminating the need for additional treatment of the large amount of chemical waste brine introduced by adding traditional chemical precipitants. The amount of mother liquor after solid-liquid separation of the reaction liquid is relatively small, making it more environmentally friendly. In addition, relying on the unique forced circulation flow field, external heat exchange, and adjustable reflux design, it effectively solves engineering and technical problems such as electrode scaling, difficulty in temperature control, and crystal concentration imbalance, achieving long-cycle, fully continuous, automated, and stable operation of the device.

[0041] This invention possesses strong technological integration and innovation, organically integrating low-concentration ion continuous electrolytic extraction, electrochemical in-situ precipitation, and precise control of crystallization kinetics. It also creatively integrates several key technologies such as ion directional migration, external circulation and heat exchange, multi-parameter crystallization control, and dynamic equilibrium of crystal concentration. The various technical modules work together and complement each other, ultimately forming a short-process, high-performance, and easily scalable innovative solution for seawater magnesium extraction, providing a brand-new technical path for the high-value comprehensive utilization of seawater magnesium resources. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0043] Figure 1 This is a flowchart of the apparatus and process for preparing high-purity, large-particle magnesium hydroxide by continuous electrolysis of seawater in Example 1 of the present invention.

[0044] Figure 2 This is a particle size distribution diagram of magnesium hydroxide prepared in Example 1 of the present invention.

[0045] Figure 3 This is a particle size distribution diagram of magnesium hydroxide prepared in Example 2 of the present invention.

[0046] Figure 4This is a particle size distribution diagram of magnesium hydroxide prepared in Comparative Example 1 of the present invention.

[0047] Figure 5 This is a particle size distribution diagram of magnesium hydroxide prepared in Comparative Example 4 of the present invention.

[0048] Figure 6 This is a particle size distribution diagram of magnesium hydroxide prepared in Comparative Example 5 of the present invention. Detailed Implementation

[0049] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0050] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0051] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0052] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0053] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0054] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0055] Figure 1 This is a flowchart of the apparatus and process for preparing high-purity, large-particle magnesium hydroxide by continuous electrolysis of seawater in Example 1 of the present invention.

[0056] Wherein: 1-Anode chamber, 2-Anode, 3-Anode conductivity sensor, 4-Anode chamber seawater inlet, 5-Anode chamber discharge port, 6-Anode level sensor, 7-Anode exhaust port, 8-Diaphragm (cation exchange membrane), 9-Anode / anion chamber connecting joint, 10-Cathode chamber (crystallization zone + conical collection zone), 11-Cathode chamber seawater inlet, 12-Variable frequency motor, 13-Cathode, 14-Lifting agitator, 15-Lifting cylinder, 16-Bottom slurry return outlet, 17-Bottom slurry return circulation pipe, 18-Slurry return pump, 19-Bottom slurry discharge outlet, 20-Bottom slurry return inlet, 21- 22-Material circulation outlet, 23-Forced material circulation pump, 24-Material circulation inlet, 25-Cathode temperature sensor, 26-Cathode pH sensor, 27-Cathode liquid level sensor, 28-Cathode exhaust port, 29-Cathode acid inlet, 30-Crystallization control system, 31-Anode seawater feed regulating valve, 32-Cathode seawater feed regulating valve, 33-Anode discharge valve, 34-Bottom slurry reflux discharge regulating valve, 35-Bottom slurry external discharge regulating valve, 36-Bottom slurry reflux inflow regulating valve, 37-Forced circulation pump front valve, 38-Forced circulation pump rear valve, 39-Solids content sensor.

[0057] Example 1 The raw material used in this embodiment is natural seawater that has been filtered through a 100-mesh sieve to remove suspended solids. Its composition index is Mg. 2+ 1.32 g / L, Ca 2+ 0.40 g / L, Na + 10.8 g / L, pH=8.1, conductivity 47.2 mS / cm.

[0058] This embodiment uses a dual-chamber diaphragm electrolyzer integrated device to conduct an experiment on the continuous electrolysis of seawater to prepare high-purity, large-particle magnesium hydroxide. The integrated device has an effective total electrolysis volume of 50L. The core consists of an anode chamber 1 and a cathode chamber 10, which are connected by a cathode / anode chamber connecting joint 9 and separated by a diaphragm (cation exchange membrane) 8 to achieve directional cation migration. The specifications, configuration, and connection relationships of each core component are as follows: The anode chamber 1 is a cylindrical structure with a seawater inlet 4 (connected to an internal feed pipe extending to the lower part of the anode chamber 1) and an anode exhaust port 7 at the top, and an anode discharge port 5 at the bottom. A seawater inlet regulating valve 31 is installed at the seawater inlet 4, and an anode discharge valve 33 is installed at the anode discharge port 5. The anode chamber 1 is equipped with a titanium mesh anode 2 coated with IrO2-Ta2O5, and also has an anode conductivity sensor 3 and an anode liquid level sensor 6 to monitor the conductivity and liquid level parameters inside the anode chamber in real time.

[0059] The cathode chamber 10 is divided into a columnar crystallization zone (300mm in diameter) and a conical collection zone (60° inclination). A seawater inlet 11 and a cathode vent 28 are located at the top of the chamber. A cathode seawater inlet regulating valve 32 is installed at the seawater inlet 11. A cathode acid inlet 29 is located on the side wall of the cathode chamber 10. A bottom slurry return outlet 16 and a material circulation outlet 21 are located at the bottom of the conical collection zone. A bottom slurry return inlet 20 is located on the side wall of the conical collection zone. A material circulation inlet 24 is located at the top of the columnar crystallization zone. The cathode chamber 10 contains a platinum electrode cathode 13, a lifting agitator 14, and a matching variable frequency motor 12 and lifting cylinder 15. The lifting agitator 14 is located inside the lifting cylinder 15, and both are situated in the middle of the columnar crystallization zone and extend into the conical collection zone. The cathode chamber 10 also contains a cathode temperature sensor 25, a cathode pH sensor 26, and a cathode level sensor 27 to monitor the internal temperature, pH, and level parameters in real time.

[0060] The device is equipped with a slurry reflux circulation system, a temperature-controlled forced circulation system, and a crystallization control system 30. Each system is precisely connected to the components of the cathode chamber 10 and the anode chamber 1. The slurry reflux circulation system consists of a bottom slurry reflux outlet 16, a bottom slurry reflux circulation pipe 17, a slurry reflux pump 18, a bottom slurry reflux inlet 20, a bottom slurry reflux discharge regulating valve 34, a bottom slurry discharge regulating valve 35, a bottom slurry reflux inlet regulating valve 36, and a bottom slurry discharge outlet 19. It is also equipped with a solid content sensor 39 connected to the bottom of the cone of the cathode chamber 10 to monitor the solid content of the slurry in real time. The temperature-controlled forced circulation system consists of a material circulation outlet 21, a material circulation pipe 22, a material forced circulation pump 23, a material circulation inlet 24, a forced circulation pump pre-valve 37, a forced circulation pump post-valve 38, and an external pipe heat exchanger (temperature control device). It works in conjunction with the cathode temperature sensor 25 in the cathode chamber 10 to form a closed-loop temperature control, precisely regulating the temperature of the cathode chamber 10. The internal reaction temperature; the crystallization control system 30 serves as the intelligent central hub for the control of parameters throughout the entire process. It is electrically connected to the anode conductivity sensor 3, anode liquid level sensor 6, cathode temperature sensor 25, cathode pH sensor 26, cathode liquid level sensor 27, and solid content sensor 39. At the same time, it is linked with various regulating valves, pumps, variable frequency motors 12, and other actuators to achieve precise control of the operating parameters of the entire device.

[0061] The operation process of this embodiment is as follows: (1) System startup and pre-loop: First, open the anode seawater feed regulating valve 31 and the cathode seawater feed regulating valve 32. Filtered seawater is continuously fed into the anode chamber 1 and cathode chamber 10 at the same rate of 2 L / min through the anode chamber seawater feed pipe 4 and the cathode chamber seawater feed pipe 11. The liquid level is monitored in real time by the anode level sensor 6 and the cathode level sensor 27. When the liquid level in both chambers rises to 50 mm above the lower edge of the cathode plate 13 (approximately 60% of the total height of the electrolytic cell), the feed regulating valves are closed to stop feeding. Then, the variable frequency motor 12 of the stirring and lifting device is started, driving the lifting agitator 14 to run at 150 rpm. This creates a flow field within the lifting cylinder 15 and allows the seawater in the cathode chamber 10 to be initially mixed. Next, open the forced circulation pump inlet valve 37 and the forced circulation pump outlet valve 38, start the forced circulation pump 23, and set the flow rate to 25 L / min. This allows the seawater in the cathode chamber 10 to pass through the material circulation outlet 21, material circulation pipe 22, external pipe heat exchanger, and material circulation inlet 24. A forced circulation is formed. The crystallization control system 30 receives real-time temperature data from the cathode temperature sensor 25 and controls the external pipe heat exchanger to heat the circulating seawater using hot water as a heat source, gradually raising the seawater temperature in the cathode chamber 10 to 40±2℃ and maintaining a constant temperature.

[0062] (2) Start electrolysis to achieve in-situ precipitation of magnesium ions: After the temperature of cathode chamber 10 stabilizes at 40±2℃, the electrolysis and crystallization parameter precision control process is started to achieve in-situ precipitation of magnesium ions. A DC regulated power supply is applied to anode 2 and cathode 13, and the cathode current density is set to 200mA / cm². 2 The electrolysis reaction is initiated, and an electroreduction reaction of water occurs on the surface of cathode 13 to generate OH. - Mg in anode chamber 1 2+ Driven by both electric field and concentration gradient, cations migrate directionally through cation exchange membrane 8 to cathode chamber 10 and react with OH-. -In-situ reaction generates Mg(OH)2 crystals. During the initial electrolysis phase, the bottom slurry discharge regulating valve 35 is closed, and the bottom slurry return discharge regulating valve 34 and the bottom slurry return inlet regulating valve 36 are fully opened. The slurry return pump 18 is started, returning all the slurry from the cone-shaped collection area of ​​the cathode chamber 10 to the bottom of the cathode chamber 10 through the bottom slurry return outlet 16, the bottom slurry return circulation pipe 17, and the bottom slurry return inlet 20. This constructs the initial seed crystal system while preventing the pipe interfaces from being blocked by crystals. The crystallization control system 30 receives real-time monitoring data from the cathode pH sensor 26. When the pH in the cathode chamber 10 exceeds 10.4, dilute hydrochloric acid is precisely added dropwise through the cathode acid addition port 29 to stabilize the electrolyte pH in the cathode chamber 10 at 10.2±0.2, thereby inhibiting Ca2+. 2+ Co-precipitation ensures the thermodynamic stability range for magnesium hydroxide crystallization. During electrolysis, the gas generated during electrolysis is discharged in real time through the anode exhaust port 7 and the cathode exhaust port 28 to maintain stable gas pressure inside the two chambers and prevent gas accumulation from interfering with the flow field and crystallization process.

[0063] (3) Maintain dynamic balance of seed concentration to achieve controllable crystal growth: When electrolysis continues until the solid content at the bottom of the cathode chamber 10 cone reaches 10% and the system pH and temperature fluctuations are both less than 5%, the system is considered to have entered a stable operating state. At this time, the dynamic balance process of slurry reflux and seed concentration is initiated to achieve controllable growth of magnesium hydroxide crystals. The crystallization control system 30 receives real-time data from the solid content sensor 39 and adjusts the bottom slurry reflux inflow regulating valve 36 and the bottom slurry outflow regulating valve 35 in conjunction, setting the reflux ratio to 75% (75% slurry reflux, 25% slurry outflow). That is, by controlling the opening and closing of the regulating valves, 75% of the slurry pumped by the slurry reflux pump 18 is refluxed back to the cathode chamber 10 through the bottom slurry reflux inlet 20, serving as seed crystals to provide a base point for magnesium hydroxide crystal growth and inhibit secondary nucleation, while 25% is discharged as crude product slurry through the bottom slurry outlet 19. The reflux ratio is controlled in a closed loop by the solid content sensor 39 throughout the process, ensuring the controllable growth of magnesium hydroxide crystals in the cathode chamber 10. The bottom slurry solid content is kept stable within the optimal crystallization range of 5-20%, ensuring that magnesium hydroxide crystals grow continuously and uniformly into large particles.

[0064] (4) Continuous and stable electrolysis: After the system is running stably, material balance control is implemented to achieve continuous and stable electrolysis. The anode conductivity sensor 3 in anode chamber 1 monitors the electrolyte conductivity in real time. The crystallization control system 30 adjusts the opening and closing of the anode discharge valve 33 based on the conductivity data, allowing the anolyte to be continuously discharged through the anode chamber discharge pipe 5 at a rate of 1.8 L / min, maintaining the conductivity fluctuation of anode chamber 1 to less than 20%, ensuring a stable migration rate of magnesium ions across the cation exchange membrane 8. The cathode level sensor 27 in cathode chamber 10 monitors the liquid level in real time. The crystallization control system 30, in conjunction with this, adjusts the opening and closing of the cathode seawater feed regulating valve 32, adding seawater to cathode chamber 10 to ensure the feed rate matches the slurry discharge rate, maintaining a constant liquid level in cathode chamber 10, and ensuring a stable electrolysis and crystallization space. During electrolysis, the stirring pump 14 and the forced material circulation pump 23 operate continuously, forming a uniform flow field within cathode chamber 10, ensuring that magnesium hydroxide crystals always grow suspended in the bulk solution phase, preventing crystal formation at cathode 13. Surface deposition leads to electrode passivation, while the flow field provides moderate shear force, promoting the formation of a dense crystal structure and avoiding dendritic growth and excessive aggregation.

[0065] (5) Continuous product collection and post-processing: After the system is running stably, crude product slurry is continuously collected through the bottom slurry outlet 19. In this embodiment, the system runs stably for 12 hours and continuously collects crude product slurry. The collected crude product slurry is sent to a plate and frame filter press for filtration to obtain magnesium hydroxide filter cake. Then, deionized water is added at a mass ratio of 1:4 to remove soluble impurities by washing the filter cake 4 times. The washed filter cake is then placed in an oven and dried at a constant temperature of 105°C. After cooling, white powdered magnesium hydroxide product is obtained.

[0066] Tested by a laser particle size analyzer and a component analyzer, the magnesium hydroxide product prepared in this embodiment exhibits excellent core indicators, with a purity of 99.63% and a volume average particle size D. 50 =11.9μm, with a particle size distribution standard deviation SD=11.89, and the product has excellent filtration and washing performance, meeting the needs of high-value applications without additional complex processing.

[0067] Example 2 This embodiment is a continuous operation verification experiment of embodiment 1. It uses the same dual-chamber diaphragm electrolytic cell integrated device and raw materials as embodiment 1. The operation steps are the same as in embodiment 1. After the system has been running for a period of time according to the above steps, when the solid content of the slurry at the bottom of the cathode chamber 10 rises to 12%, the opening and closing degree of the bottom slurry discharge regulating valve 35, the bottom slurry return discharge regulating valve 34 and the bottom slurry return inflow regulating valve 36 are adjusted to control the slurry return ratio to 70%, that is, 70% of the slurry is returned to the cathode chamber 10 to continue to participate in the crystallization reaction, and 30% of the slurry is discharged as product slurry. At the same time, the anode chamber drain port 5 is controlled to continuously discharge the anolyte at a rate of 18L / h to maintain the magnesium ion concentration in the anode chamber 1 stable and ensure the directional migration efficiency of magnesium ions across the diaphragm 8 to the cathode chamber 10.

[0068] Under the above process parameters, the device operated continuously and stably for 120 hours. After the operation was completed, the discharged product slurry was collected and processed according to the same post-processing procedure as in Example 1. The resulting product was then tested and analyzed.

[0069] The test results showed that the magnesium hydroxide product prepared in this embodiment had a purity of 99.7%. Laser particle size analysis showed that the product's volume average particle size D... 50 The particle size distribution was 12.22 μm, and the standard deviation (SD) of the particle size distribution was 9.53 (see Table 1). The particle size distribution of the product was more uniform than that of Example 1, while still maintaining the excellent characteristics of high purity and large particles. This verifies the feasibility of this device and method for long-term continuous and stable production of high-purity large-particle magnesium hydroxide.

[0070] Comparative Example 1 (without forced loop) This comparative example is based on Example 1, except that the circulating motor 12 of the stirring and lifting device is turned off, the forced circulation flow field design of the cathode chamber is cancelled, and the other equipment configurations, raw materials and operating parameters are the same as in Example 1.

[0071] After one hour of testing, a large number of agglomerated crystals appeared in the system. Testing of the resulting product showed that its particle size D... 50 The particle size is approximately 15.12 μm, but the standard deviation of particle size distribution (SD) increases to 55.41 (see Table 1), indicating that the particle size distribution of the product is extremely uneven. At the same time, it is possible to directly observe the scaling phenomenon of aggregated magnesium hydroxide crystals on the surface of the electrode plate of cathode 13, indicating significant electrode passivation problems.

[0072] Comparative Example 2 (without bottom reflux circulation) Based on Example 1, this comparative example closes the bottom slurry discharge regulating valve 35, the bottom slurry return discharge regulating valve 34, and the bottom slurry return inflow regulating valve 36, and cancels the bottom slurry return circulation in the cathode chamber. The remaining equipment configuration, raw materials, and operating parameters are consistent with those in Example 1.

[0073] After 2 hours of test operation, the bottom slurry discharge regulating valve 35 and the bottom slurry return discharge regulating valve 34 were opened and adjusted to discharge the slurry. It was found that the initially discharged slurry contained a large number of crystal lumps. As the running time increased, the solid content of the discharged slurry showed a significant downward trend, from about 15% to 9%, indicating that the solid content of the slurry at the bottom of the cathode chamber could no longer be effectively controlled. Furthermore, obvious crystal blockage was observed at the bottom slurry return inlet 20, indicating that the pipeline operation stability was compromised.

[0074] Comparative Example 3 (replaced with a traditional internal coil heating design) Based on Example 1, this comparative example replaces the external heat exchange device of the crystallization control system 30 with a traditional built-in heat exchange coil, while the remaining equipment configuration, raw materials and operating parameters remain the same as in Example 1.

[0075] After 50 hours of test operation, severe crystal scaling was clearly observed on the outer wall of the internal heat exchange coil, resulting in a significant decrease in the heat exchange efficiency of the heat exchange device. This directly led to a significant increase in the reaction temperature fluctuation in the cathode chamber 10, and the stability of the crystallization environment was disrupted.

[0076] The test results of the above three sets of comparative examples prove that the external forced circulation loop, bottom slurry circulation reflux system and external heat exchange device designed in this invention are key designs to ensure the continuous and stable operation of the device and the preparation of high-quality magnesium hydroxide products, and are of great importance to ensuring the continuous and stable production of high-quality products.

[0077] To verify the superiority of the method of the present invention over the traditional chemical precipitation method for preparing magnesium hydroxide from seawater, two sets of comparative experiments were set up using the same batch and volume of filtered seawater as used in Example 1 as raw material, and lime milk and sodium hydroxide, commonly used in industry, as precipitants: Comparative Example 4 (lime milk precipitation method) and Comparative Example 5 (sodium hydroxide precipitation method). During the experiments, the reaction pH and temperature were controlled to be consistent with those in Example 1, and the post-processing of the product was carried out according to the operating standards of Example 1, as follows: Comparative Example 4 The experiment used a jacketed temperature-controlled glass crystallization reactor as the reaction device. Filtered seawater was added to the reactor, and the mixture was stirred at a constant speed of 150 rpm. Ca(OH)2 suspension (lime slurry) was slowly added to the reactor. The reaction conditions were strictly controlled so that the pH value at the end of the reaction was stable at 10.5±0.2. The temperature of the reaction system was maintained at 40℃ by temperature control through a jacketed water bath. After the reaction was completed, the system was aged for another hour.

[0078] The resulting slurry was vacuum filtered, washed, and dried according to the process standards of Example 1 to obtain magnesium hydroxide product. Composition analysis showed that the product had a purity of only 87.5%, with the main impurities being insoluble salts such as calcium sulfate, which could not be removed by conventional washing. Laser particle size analysis showed that the product's volume average particle size D... 50 The particle size was 9.82 μm, and the standard deviation of particle size distribution (SD) was 43.19 (see Table 1 and Figure 2). Compared with Example 1, the product had a smaller particle size and a wider particle size distribution range.

[0079] Comparative Example 5 The experiment used the same jacketed temperature-controlled glass crystallization reactor as Comparative Example 4. The same batch of filtered seawater was added to the reactor and stirred at a constant speed of 150 rpm. A 2 mol / L high-purity sodium hydroxide solution was added dropwise at a constant rate using a peristaltic pump. The feeding rate was precisely controlled to stabilize the pH value at the end of the reaction at 10.5. The temperature of the reaction system was maintained at 40°C by a jacketed water bath. After the addition was completed, stirring and aging continued for 1 hour.

[0080] The slurry obtained from the reaction was processed according to the steps of Comparative Example 4, including vacuum filtration, washing, and drying to obtain magnesium hydroxide product. Component analysis showed that, due to the presence of mostly soluble inorganic salts as impurities, its purity was improved compared to the lime milk precipitation method, reaching 92.1%, but still significantly lower than the product prepared in the embodiments of this invention. Laser particle size analysis results indicated that the product's volume average particle size D... 50 With a particle size of only 6.81 μm and a standard deviation (SD) of 50.19 (see Table 1 and Figure 2), the product contains a large number of tiny particles and agglomerated crystals. Furthermore, impurity salts are easily encapsulated in the crystals, resulting in low washing and impurity removal efficiency. The dried product is also prone to clumping.

[0081] The particle size distribution data of the products obtained from the embodiments and comparative examples of this invention are summarized in Table 1.

[0082] Table 1 The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. An apparatus for the continuous electrolytic preparation of magnesium hydroxide from seawater, characterized in that, include: A dual-chamber diaphragm electrolyzer, wherein the anode chamber and the cathode chamber of the dual-chamber diaphragm electrolyzer are connected by a cation exchange membrane; The anode chamber is a cylindrical structure, containing an anode, an anode conductivity sensor, and an anode level sensor. The top of the anode chamber is equipped with a seawater inlet and an anode exhaust outlet, and the bottom is equipped with a discharge outlet. The seawater inlet is connected to an internal feed pipe extending to the lower part of the anode chamber. The cathode chamber is divided into a columnar crystallization zone and a conical collection zone. The conical collection zone is located below and connected to the columnar crystallization zone. The cathode chamber houses a cathode, a lifting agitator, a lifting cylinder, a cathode pH sensor, a cathode level sensor, and a cathode temperature sensor. The cathode chamber is equipped with a cathode chamber seawater inlet, a cathode acid inlet, and a cathode exhaust outlet. The conical collection zone is equipped with a slurry reflux circulation system and a temperature-controlled forced circulation system. The lifting agitator and the lifting cylinder are located in the middle of the columnar crystallization zone and extend into the conical collection zone. The lifting agitator is located inside the lifting cylinder. The slurry reflux circulation system includes a slurry reflux pipe, a slurry reflux pump, a reflux regulating valve, and a slurry discharge outlet. The reflux inlet and outlet of the slurry reflux pipe are respectively connected to the slurry reflux outlet at the bottom of the cathode chamber cone collection area and the bottom slurry reflux inlet. The slurry reflux pump is located near the bottom slurry reflux outlet of the slurry reflux pipe, and the reflux regulating valve is located at the connection between the slurry reflux pipe and the bottom slurry reflux inlet. The temperature-controlled forced circulation system includes a material circulation pipe, a material forced circulation pump, a pre-pump regulating valve, a post-pump regulating valve, and a temperature control device. The circulation inlet and outlet of the material circulation pipe are respectively connected to the material circulation outlet at the bottom of the cathode cone collecting area and the material circulation inlet at the top of the column crystallization area. The material forced circulation pump is installed on the material circulation pipe, and the temperature control device is connected to the cathode temperature sensor. And a crystallization control system, which includes a cathode pH sensor, a cathode liquid level sensor, and a cathode temperature sensor.

2. The apparatus for continuous electrolytic preparation of magnesium hydroxide from seawater according to claim 1, characterized in that, The inclination angle of the cone in the cone-shaped aggregate zone is 45-75°.

3. The apparatus for continuous electrolytic preparation of magnesium hydroxide from seawater according to claim 1, characterized in that, The slurry reflux circulation system also includes a solids content sensor, which is connected to the bottom of the cathode chamber and electrically connected to the reflux regulating valve. The reflux regulating valve is automatically adjusted according to the solids content sensor signal.

4. The apparatus for continuous electrolytic preparation of magnesium hydroxide from seawater according to claim 1, characterized in that, The temperature control device is a pipe-type heat tracing structure or an external heat exchanger; the external heat exchanger is a plate heat exchanger or a tubular heat exchanger.

5. A method for continuously preparing magnesium hydroxide by seawater electrolysis using the apparatus described in any one of claims 1-4, characterized in that, Includes the following steps: (1) Open the seawater inlet of the anode chamber and the seawater inlet of the cathode chamber, and continuously introduce seawater into the anode chamber and the cathode chamber respectively. Monitor and control the seawater level in the two chambers to be no lower than the lower edge of the cathode and anode plates and no higher than 80% of the height of the cathode and anode chambers by the anode level sensor and the cathode level sensor. Open the pump regulating valve before the pump and the pump regulating valve of the temperature control forced circulation system, and start the lifting agitator and the material forced circulation pump of the temperature control forced circulation system. (2) The real-time monitoring data of the cathode pH sensor, cathode temperature sensor and cathode liquid level sensor are received through the crystallization control system. Based on this, the temperature control device is adjusted to maintain the temperature of the electrolyte in the cathode chamber at 30-80℃, and acid is added to the cathode chamber through the cathode acid inlet to maintain the pH value of the electrolyte in the cathode chamber at 9.5-11.

0. (3) Apply direct current to the anode and cathode for electrolysis, and control the cathode current density to be 10-350 mA / cm². Magnesium ions in the anode chamber migrate directionally to the cathode chamber through the cation exchange membrane and react with hydroxide ions generated by electroreduction on the cathode surface to generate magnesium hydroxide crystals. (4) During the electrolysis process, the slurry return pump of the slurry return circulation system is started, and the slurry rich in magnesium hydroxide crystals in the cathode chamber cone collection area is continuously drawn out through the slurry return pipe, so that all the drawn-out slurry is returned to the cathode chamber through the bottom slurry return inlet. After the system is running stably, the solid content of the slurry at the bottom of the cathode chamber is monitored by the solid content sensor, and the return regulating valve is dynamically adjusted to keep the solid content of the slurry at the bottom of the cathode chamber in the range of 5-20%, and the remaining slurry is discharged through the slurry discharge outlet. (5) During the electrolysis process, the gas generated in the chamber is discharged from the anode exhaust port and the cathode exhaust port in real time to maintain stable gas pressure. At the same time, the anolyte is continuously discharged from the anode chamber discharge port. The conductivity in the anode chamber is monitored in real time by the anode conductivity sensor. The discharge rate is adjusted to match the migration rate of magnesium ions to the cathode chamber to ensure that the fluctuation range of the data obtained by the conductivity sensor in the anode chamber does not exceed 30%. (6) Collect the product slurry discharged through the slurry outlet, and obtain magnesium hydroxide product after post-processing.

6. The method according to claim 5, characterized in that, The cathode current density is controlled to be 50-200 mA / cm².

7. The method according to claim 5, characterized in that, The post-processing includes washing and drying steps.

8. A magnesium hydroxide prepared by the method according to any one of claims 5-7, characterized in that, The magnesium hydroxide has a purity greater than 99.5% and a particle size D 50 Not less than 10μm.

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