Preparation method of high-purity manganese sulfate for lithium iron manganese phosphate precursor

Abstract

This invention relates to a method for preparing high-purity manganese sulfate for lithium manganese iron phosphate precursors, belonging to the field of battery materials technology. This method aims to solve the problems of low ore reactivity leading to poor leaching efficiency and high energy consumption, difficulty in deep removal of calcium and magnesium impurities and significant environmental risks, and high energy consumption and poor product quality in the concentration and crystallization process of traditional pyrolusite manganese sulfate production. Key technical points include: using oxalic acid as a reducing agent for wet mechanical activation reduction leaching of pyrolusite; using a composite extraction system composed of P507 and TOA to deeply remove impurities from the leached slurry; returning a portion of the solid residue as a recycled grinding aid to the leaching step; and integrating thermally coupled back-extraction and evaporation crystallization operations in the same MVR system to achieve synergistic energy saving in impurity separation and product crystallization. This method is mainly used to prepare high-purity manganese sulfate with low impurity content, which can be used as a key precursor raw material for high-performance lithium manganese iron phosphate cathode materials.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, and more specifically, this invention relates to a method for preparing high-purity manganese sulfate for lithium manganese iron phosphate precursor. Background Technology

[0002] High-purity manganese sulfate is a key precursor material for preparing high-performance lithium manganese iron phosphate (LMFP) cathode materials. Its purity (especially the content of impurities such as calcium and magnesium) directly affects the energy density, cycle life, and safety performance of lithium-ion batteries. Currently, the mainstream industrial process for large-scale production of manganese sulfate is the "two-ore plus acid method," which uses pyrolusite (mainly composed of MnO2) and pyrite (FeS2) to undergo a redox reaction in a sulfuric acid medium to produce manganese sulfate. However, this traditional process and its existing improvements face a series of interconnected technical bottlenecks when upgrading towards higher quality, lower cost, and more environmentally friendly solutions: Firstly, in the leaching process, the low reactivity of the ore leads to significant efficiency and energy consumption issues. The MnO2 in pyrolusite is chemically stable, exhibiting slow reaction kinetics under conventional atmospheric pressure leaching conditions. To improve the leaching rate, it is often necessary to increase the reaction temperature, extend the reaction time, or add excess sulfuric acid. This not only increases energy and material consumption but also exacerbates equipment corrosion and generates more waste. Although pretreatment methods such as roasting can be used to improve ore reactivity, this process is energy-intensive and may produce harmful fumes such as sulfur dioxide, contradicting the principles of green manufacturing.

[0003] Secondly, in the solution purification process, the deep removal of calcium and magnesium impurities is difficult and poses high environmental risks. The calcium in the leachate... 2+ Mg 2+ With target Mn 2+ Due to their similar ionic radii and chemical properties, separation is challenging. The industry commonly employs fluoride precipitation for deep removal, but this method introduces fluoride ions, causing severe equipment corrosion and generating difficult-to-treat fluoride-containing wastewater and hazardous waste residue, resulting in high environmental post-treatment costs. Furthermore, the inorganic reducing agents such as pyrite used in the traditional "two-ore plus acid method" introduce new impurities such as iron ions into the system after the reaction, increasing the burden on subsequent iron removal purification and complicating the solution composition.

[0004] Secondly, the concentration and crystallization process is energy-intensive and the control of product physical properties is rudimentary. Traditional processes often use single-effect or multi-effect evaporators for solution concentration, resulting in high steam consumption and persistently high energy costs.

[0005] In summary, existing high-purity manganese sulfate preparation processes suffer from systemic deficiencies in leaching efficiency, green impurity removal, energy conservation and consumption reduction, and product property control, making it difficult to meet the growing demand for high-quality manganese sulfate in lithium manganese iron phosphate precursor materials. Therefore, developing a new manganese sulfate preparation process that can synergistically address these issues and achieve high efficiency, high selectivity, low energy consumption, environmental friendliness, and stable product quality is of great significance for promoting the upgrading of the new energy materials industry. Summary of the Invention

[0006] One object of the present invention is to address at least the aforementioned deficiencies and to provide at least the advantages that will be described later.

[0007] One objective of this invention is to provide a method for preparing high-purity manganese sulfate for lithium manganese iron phosphate precursors, which addresses the problems in existing processes for preparing high-purity manganese sulfate from pyrolusite, such as low ore reactivity leading to poor leaching efficiency and high energy consumption, difficulty in deep removal of calcium and magnesium impurities with significant environmental risks, and high energy consumption and poor product quality during the concentration and crystallization process.

[0008] This invention provides a method for preparing high-purity manganese sulfate for lithium manganese iron phosphate precursors, comprising the following steps: S1. Wet mechanical activation and reduction leaching: Pyrolusite powder, sulfuric acid solution with a mass fraction of 10%~30%, and oxalic acid are mixed to make the slurry solid content 30%~50%. The slurry is placed together with grinding media in a wet grinding device for mechanical activation and reduction leaching reaction. At the same time, the carbon dioxide gas generated by the reaction is discharged through the matching gas-liquid separation and pressure control unit to maintain the system gauge pressure at a slight negative pressure of -1kPa to -5kPa to obtain the leaching slurry. The grinding media is hard ceramic particles. The amount of oxalic acid added, by mass, is 1.2~1.5 times the theoretical mass of oxalic acid required to completely reduce the MnO2 component in the pyrolusite powder. S2. Deep impurity removal by pulp extraction: The leaching pulp is mixed with a composite extractant and a dispersant and subjected to multi-stage countercurrent extraction. The composite extractant contains 2-ethylhexyl phosphate monoester P507 as the main extractant and trioctylamine TOA as the co-extractant. After extraction, liquid-liquid separation is performed to obtain a loaded organic phase and a purified pulp loaded with calcium and magnesium impurity ions. S3. Solid-liquid separation and media circulation: The purified slurry is subjected to solid-liquid separation to obtain a purified manganese sulfate solution and solid residue; a portion of the solid residue is returned to step S1 as a circulating grinding aid. S4. Integrated thermally coupled back-extraction and evaporation crystallization: The loaded organic phase obtained in step S2 is introduced into the heat exchange unit of the MVR system. Using an upgraded heat source from the same MVR system and through heat source distribution control, dilute sulfuric acid solution is used as the back-extraction agent to heat and back-extract the loaded organic phase. After back-extraction, liquid-liquid separation is performed again to obtain a regenerated organic phase and an impurity-enriched liquid. The regenerated organic phase is returned to step S2 for recycling. At the same time, the purified manganese sulfate solution obtained in step S3 is introduced into the main evaporation chamber of the same MVR system for evaporation concentration and crystallization. The crystallized slurry is then subjected to subsequent solid-liquid separation and drying to obtain a high-purity manganese sulfate crystal product.

[0009] Preferably, in step S1, the wet grinding equipment is a sand mill; the wet mechanical activation reduction leaching is carried out in the sand mill, and the operating conditions are: the slurry solid content is 30% to 50% by mass fraction, the activation time is 10 to 40 minutes; the filling rate of the grinding media in the grinding equipment is 60% to 80% by volume; the specific mechanical energy per unit ore processed is 0.5 to 2.0 kWh per kilogram, and the specific mechanical energy is calculated by dividing the total power consumption of the material test under the same equipment and operating conditions by the dry basis mass of the processed pyrolusite.

[0010] Preferably, in step S2, the dispersant is a mixture of ammonium polyacrylate and modified sodium lignosulfonate, wherein the mass ratio of ammonium polyacrylate to modified sodium lignosulfonate is 1:(2-5); the amount of dispersant added is based on the total mass of the liquid phase in the leaching slurry, and is 0.01% to 0.1% by mass fraction.

[0011] Preferably, in step S2, the volume ratio of 2-ethylhexyl phosphate monoester P507 to trioctylamine TOA is (10-12):1.

[0012] Preferably, in step S2, the multi-stage countercurrent extraction is carried out in a centrifugal extractor, wherein the drum linear velocity of the centrifugal extractor is 8-12 m / s, the number of extraction stages is 2 to 4, the volume ratio of organic phase to slurry is 1:(5-15), and the extraction operation temperature is 40℃ to 60℃.

[0013] Preferably, in step S3, the amount of solid residue returned to step S1 as the circulating grinding aid accounts for 1% to 5% of the total mass of the pyrolusite powder processed in step S1.

[0014] Preferably, in step S4, the heat exchange unit is a plate heat exchanger; the stripping agent is a dilute sulfuric acid solution with a pH of 1 to 3; the volume ratio of the stripping agent to the supported organic phase is (0.3-0.5):1; and the stripping operation temperature is 60°C to 75°C.

[0015] Preferably, in step S4, the crystallization chamber of the MVR system is equipped with a bidirectional cross-blade agitator; the upper blades of the agitator are radial, the lower blades are axial, and the stirring speed is 50 to 300 revolutions per minute.

[0016] Preferably, in step S4, the absolute pressure of the MVR evaporation crystallization system is -20 kPa to -80 kPa, corresponding to an operating temperature of 70°C to 90°C.

[0017] Preferably, the method further includes step S5: neutralizing the enriched impurity liquid generated in step S4 to pH 9.0-9.5 with lime milk to generate gypsum and magnesium hydroxide precipitates for resource utilization; and / or, washing the solid residue in step S3 that was not returned to step S1 with three-stage countercurrent washing at a liquid-to-solid ratio of 3:1 and a temperature of 60-70°C, and returning the resulting washing water to step S1 for recycling.

[0018] The present invention has at least the following beneficial effects: This invention addresses the technical challenges of low reactivity of pyrolusite, difficulty in deep removal of calcium and magnesium impurities, high environmental risks, and high energy consumption in the process by synergistic integration of wet mechanical activation and reduction leaching, slurry extraction for deep impurity removal, media circulation, and thermally coupled back-extraction and evaporation crystallization, combined with key process control.

[0019] Step S1, wet mechanical activation and reduction leaching, utilizes continuous mechanical force applied to the pyrolusite particles in a liquid environment. This disrupts the crystal structure, increases the reaction surface area, and enhances its chemical activity. Combined with a 1.2–1.5 times oxalic acid feed ratio and under cooling and micro-negative pressure exhaust control, this step achieves a stable manganese leaching rate exceeding 98% under mild conditions (≤50℃), avoiding the high energy consumption and sulfur dioxide emissions associated with traditional roasting activation. Specifically, oxalic acid is used as the reducing agent, producing only carbon dioxide and water after the reaction, without introducing new metallic impurities such as iron into the system. This characteristic simplifies the subsequent purification process from the source, eliminating the iron removal step and associated costs required in traditional processes.

[0020] Step S2, the slurry extraction for deep impurity removal, maintains the stability of the high-solids slurry by using a dispersant composed of ammonium polyacrylate and modified sodium lignin sulfonate, and introduces a composite extraction system consisting of P507 and TOA. TOA, acting as a co-extractant, enhances the selectivity of the extraction system for calcium and magnesium ions relative to manganese ions through a synergistic effect. Combined with rapid three-phase separation achieved by a high-speed centrifugal extractor (drum linear velocity 8-12 m / s), this step further improves the selectivity of the slurry for calcium and magnesium ions relative to manganese ions. 2+ Mg 2+The removal rate remains stable at over 98.5%. This technology eliminates fluoride precipitation, thus preventing fluoride pollution, equipment corrosion, and the generation of fluoride-containing hazardous waste at the source, achieving green and efficient deep impurity removal.

[0021] In step S3, the solid-liquid separation and media circulation involve returning a portion of the solid residue obtained from the solid-liquid separation (accounting for 1% to 5% of the mass of pyrolusite) to step S1 as a circulating grinding aid. These relatively hard residue particles supplement the grinding media in the grinding chamber, further enhancing the crushing effect on the ore. This not only enables the resource utilization of waste residue but also helps to increase the manganese leaching rate by 0.5 to 1 percentage point.

[0022] Step S4, thermally coupled back-extraction and evaporative crystallization, integrates the back-extraction of the organic-loaded phase and the evaporative crystallization of the purified liquid into a single mechanical vapor recompression (MVR) system. Through a heat source distribution and control unit, the upgraded heat source output from the compressor is distributed as needed to the back-extraction heat exchanger and crystallization chamber, achieving cascaded energy utilization. This integrated design reduces the overall thermal energy consumption of the back-extraction and crystallization sections by approximately 25% to 35% compared to traditional methods using independent steam heating.

[0023] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0025] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified.

[0026] The main equipment and raw materials used in the implementation of this invention are as follows: 1. Wet mechanical activation-reduction leaching system: Core equipment: The German NETZSCH LMZ-25 sand mill is used, with an effective grinding chamber volume of 25L. The chamber material is silicon carbide ceramic, and it is equipped with a variable frequency motor with a power range of 5-30kW.

[0027] Key auxiliary equipment: Gas-liquid separation and pressure buffer unit: The outlet of the sand mill is connected to a buffer tank with an effective volume of 50L (material SS316L). The top of the tank is equipped with a micro-pressure control valve and a check valve to maintain a micro-negative pressure operating environment (-1 to -5kPa) for the system.

[0028] Gas purification unit: A cyclone separator and a fiber demister are connected in series at the exhaust end of the buffer tank to efficiently capture slurry droplets and solid particles entrained in CO2 gas. The collected liquid is returned to the system, and the gas is discharged in compliance with standards.

[0029] Temperature control unit: The sand mill is equipped with a cooling jacket to ensure that the slurry temperature during the leaching process is ≤50℃ in order to inhibit the thermal decomposition of oxalic acid.

[0030] 2. High-solids-content slurry conveying and processing system: Conveying equipment: Employs a low-speed, high-torque single-screw pump; the flow-through components are made of wear-resistant hard alloy; design flow rate: 0-10 m³ / h. 3 / h, outlet pressure 0-1.0MPa, to ensure stable slurry delivery.

[0031] Pipeline design: The delivery pipeline uses ultra-high molecular weight polyethylene (UHMW-PE) or ceramic-lined steel pipe with an inner diameter of DN50, and the layout angle is ≥10° to avoid horizontal sections and to provide flushing ports.

[0032] Stability assurance: Buffer tanks with anchored agitators (30-60 rpm) are installed at key transfer points to prevent solid sedimentation.

[0033] 3. Mineral pulp extraction equipment: The system utilizes the Zhejiang Xinte Technology LX-260 centrifugal extractor, with a drum diameter of 260mm made of SS316L stainless steel. The drum linear velocity is adjustable within the range of 8-12m / s. This high-speed centrifugal field can achieve efficient liquid-liquid-solid three-phase separation within ≤30 seconds.

[0034] 4. Thermally Coupled MVR Evaporation Crystallization System: Main equipment: Jiangsu Leke Energy Saving Technology Co., Ltd.'s LKMVR-500 mechanical vapor recompression (MVR) evaporation crystallization system, with an evaporation capacity of 500 kg water / h, a Roots type compressor, and an effective volume of 300 L crystallization chamber.

[0035] Thermal integration and control systems: Heat exchange equipment: The heat exchange unit in the system adopts Alfa Laval CB26-100 plate heat exchanger, made of SS316L, with a design pressure of 1.0MPa.

[0036] Intelligent Heat Source Distribution Unit: The system is equipped with heat source distribution pipelines and pneumatic regulating valves, which can distribute the upgraded heat source (90-100℃ steam) generated by the compressor to the crystallization chamber and the back-extraction heat exchanger in a variable proportion (typically 70-90% on the crystallization side and 10-30% on the back-extraction side). Its core control logic is as follows: The system uses the evaporation temperature of the MVR crystallization chamber (controlled by the system's absolute pressure) as the main stable parameter. A pneumatic regulating valve is installed on the heat source branch pipeline distributed to the back-extraction heat exchanger. The opening of this regulating valve is controlled by a PID controller based on the feedback signal from the temperature sensor at the back-extraction heat exchanger outlet (i.e., the back-extraction system). When the back-extraction temperature is lower than the set value (e.g., 75℃), the controller increases the valve opening, increasing the heat source flow into the back-extraction branch; conversely, it decreases. Through this independent closed-loop control, precise and stable control of the back-extraction temperature is achieved while ensuring the stability of the main process in the crystallization chamber. The compressor frequency can be adjusted according to the total evaporation load of the system, serving as a coarse adjustment of the total heat source input.

[0037] Independent temperature control: The crystallization temperature is controlled by the system absolute pressure (-20 to -80 kPa) to maintain the boiling point (70-90℃); the back-extraction temperature is stabilized at 60-75℃ by adjusting the flow rate of the heat source branch entering the plate heat exchanger. An auxiliary electric heater (10kW) is also provided in the back-extraction pipeline to cope with load fluctuations.

[0038] Mixing equipment: The crystallization chamber of the MVR system is equipped with a two-way cross blade agitator, with a radial four-straight blade agitator on the upper layer and an axial inclined blade agitator on the lower layer. The material is SS316L, and the speed is adjustable from 50 to 300 revolutions per minute.

[0039] 5. Specifications of main raw materials: Soft manganese ore powder: sourced from Yunnan, the main component is MnO2 (content ≥85%), the main impurity content range is: CaO 0.10%~0.15%, MgO 0.08%~0.12%, SiO2 3%~5%, particle size ≤150 mesh (≤106μm).

[0040] Sulfuric acid: Industrial grade, 98% concentration.

[0041] Reducing agent: Oxalic acid (H₂C₂O₄), industrial grade, purity ≥99.0%. Special note: Considering the thermal decomposition and gas entrainment losses (approximately 5%-15%) of oxalic acid in continuous industrial operations, to achieve a high leaching rate, the recommended dosage in industrial implementation is 1.2 to 1.5 times the theoretical stoichiometry required for complete reduction of MnO₂ in pyrolusite. For laboratory-scale tests, due to lower heat loss, 1.1 times the theoretical stoichiometry can be used.

[0042] Grinding media: High-purity zirconia ceramic beads, particle size 1.0-1.2mm. The main chemical composition requirements are: ZrO2 (including stabilizer) content ≥99.5%, and key impurity content requirements: CaO≤0.005%, MgO≤0.003%, Fe2O3≤0.005%. Using high-purity media can minimize contamination caused by media abrasion.

[0043] Dispersant: Ammonium polyacrylate: Industrial grade, type CY-100 (dispersant), solid content 40%, molecular weight approximately 5000.

[0044] Modified sodium lignosulfonate: industrial grade, type MLS-200 (dispersant), solid content 50%, molecular weight approximately 20,000.

[0045] Note: This composite dispersant has multiple functions, including improving the rheological properties of slurry, preventing sedimentation, and stabilizing the extraction system.

[0046] Extractants and diluents: 2-Ethylhexyl phosphate monoester (P507): Industrial grade, purity ≥95%.

[0047] Trioctylamine (TOA): Industrial grade, purity ≥93%.

[0048] Diluent: Sulfonated kerosene, industrial grade.

[0049] Unless otherwise specified, the following examples and comparative examples all use the above-mentioned equipment and raw materials, and each example and comparative example is subjected to three sets of parallel experiments. The data in the following tables are arithmetic mean, and the "±" values ​​after the key indicators are standard deviation or range.

[0050] It should be noted that the "placing the slurry and grinding media together in the wet grinding equipment" described in this invention includes, but is not limited to, one-time filling, continuous feeding, and circulating grinding operation modes. Those skilled in the art can flexibly select and adjust the feeding and operation methods according to the specific volume of the selected equipment and process requirements.

[0051] This invention focuses on solving the common key technical problems of low reactivity, difficulty in deep removal of calcium (Ca) and magnesium (Mg) impurities, and high energy consumption during the leaching process of pyrolusite. For iron (Fe) and aluminum (Al) impurities that may be present in the raw material, an oxidant (such as hydrogen peroxide or manganese dioxide) can be added to the purified manganese sulfate solution obtained in step S3 at a pH of 4.0-5.0 and a temperature of 60-80℃, and stirred to allow the Fe... 2+ Oxidized to Fe 3+ and Al 3+The iron and aluminum are removed by co-hydrolysis precipitation and filtration. This conventional oxidation precipitation step is placed after the core calcium and magnesium removal step (S2) of this invention, which can effectively remove iron and aluminum, and also removes Mn. 2+ The high oxidation potential, under controlled conditions, will not cause significant manganese loss and will not affect the overall manganese recovery rate. The iron and aluminum slag obtained from filtration can be processed separately without affecting the main process flow. Through the steps described above, this invention ensures that the calcium and magnesium impurity content in the product is far below the battery-grade standard requirements, and improves the overall economic and environmental benefits of the process.

[0052] The solid residue obtained from leaching in step S1 and subsequent solid-liquid separation is mainly composed of unreacted silicon dioxide (SiO2). This residue has a Mohs hardness ≥7, and its particle size (D50) after wet grinding is mainly distributed in the range of 1-20 micrometers. When returned to step S1 as a circulating grinding aid, it acts as a supplementary hard particle in the grinding chamber, synergistically working with the grinding media (ceramic beads) to further promote the crushing and activation of pyrolusite particles by increasing the solid-phase collision frequency and shear strength, thereby improving leaching efficiency. In the process operation, the returned solid residue is included in the total solid phase mass of the slurry in step S1. By adjusting the amount of liquid phase (sulfuric acid solution) added accordingly, the solid content of the mixed slurry is maintained within the range of 30%~50%.

[0053] In step S1 of this invention, the main reaction equation for the reaction between oxalic acid and pyrolusite under acidic conditions is as follows: MnO2+H2C2O4+H2SO4→MnSO4+2CO2↑+2H2O The reaction is thermodynamically spontaneous and kinetically rapid under the enhancement of mechanical activation. With the support of a gas-liquid separation and pressure buffer unit, the generated carbon dioxide gas can be smoothly discharged, and the system maintains a slight negative pressure, ensuring equipment safety and process continuity. Mechanical activation not only disrupts the ore structure but also allows oxalic acid molecules dissolved in the liquid phase to come into instantaneous and sufficient contact with the newly formed surface of MnO2 particles, which is crucial for achieving efficient, low-temperature leaching.

[0054] Additional notes regarding the oxalic acid reduction system: This invention uses oxalic acid as a cleaning reducing agent, its core advantage being that it does not introduce any new metal cation impurities (such as Fe) after the reaction. 2+ / Fe 3+ This fundamentally simplifies the subsequent purification process. It should be further explained that: The CO2 gas produced by the reaction is stably and safely discharged and separated into gas and liquid through the dedicated gas control unit, which effectively solves the problems of pressure safety and material entrainment loss caused by gas production in closed equipment.

[0055] Excess oxalic acid exists primarily in molecular form in the acidic leachate, while the P507 / TOA composite extractant used in subsequent step S2 has a negative effect on Ca. 2+ Mg 2+ The extraction is based on the cation exchange mechanism, so oxalic acid molecules do not interfere with the selective removal of calcium and magnesium impurity ions in this extraction process.

[0056] Cooling and temperature control (≤50℃) can effectively inhibit oxalic acid decomposition. Combined with the recommended feed ratio of 1.2-1.5, it can fully compensate for unavoidable losses in industrial production, ensuring a high manganese leaching rate. Although the direct material cost of oxalic acid as a reducing agent is higher than that of traditional inorganic reducing agents, this invention, through its green-first design concept, achieves overall benefits in terms of subsequent environmental risk elimination, metal recovery rate, and product quality. This makes the process highly economical and environmentally competitive throughout its entire life cycle, especially suitable for high-end battery materials fields with extremely high requirements for raw material purity and environmentally friendly production processes.

[0057] In step S2 of this invention, the dispersant should be added slowly and evenly to the leaching slurry under stirring conditions to ensure rapid dispersion in the slurry and prevent excessively high local concentrations. Specifically, the operation can be as follows: Before step S2, the leaching slurry obtained in step S1 is transferred to a buffer tank with stirring (e.g., the discharge buffer tank of S1). Under stirring conditions (e.g., 30-60 rpm), the dispersant is added slowly and evenly through a metering pump. After addition, stirring continues for 5-10 minutes to ensure the dispersant is fully and evenly mixed before pumping to a centrifugal extractor. When adding the dispersant, the slurry temperature should be maintained within the temperature range of subsequent extraction operations (40-60°C). If the leaching slurry temperature is too high, it can be cooled to this range first through heat exchange or natural cooling before adding the dispersant. This temperature condition is beneficial for the dispersant to fully exert its effect, maintain slurry stability, and create conditions for stable subsequent transport.

[0058] The solid-liquid separation of the crystallized slurry obtained in step S4 of this invention can be completed using conventional separation equipment in the art, such as centrifugal filters, vacuum belt filters, or plate and frame filter presses. To obtain high-quality crystals with low moisture content and low mother liquor entrainment, a centrifugal filter is recommended. Typical separation conditions are: centrifuge speed 800-1500 rpm, separation time 3-10 minutes. The separated wet crystals are then dried at 100-110°C to constant weight to obtain the final high-purity manganese sulfate crystal product.

[0059] The integration and dynamic balancing principle of the thermally coupled MVR system lies in the fact that the upgraded heat source generated after the system compressor performs work on the secondary steam is precisely distributed through the intelligent heat source distribution unit. Most of the heat source (70-90%) prioritizes meeting the main process heat demand for evaporation and crystallization; a smaller portion (10-30%) is controlled by regulating valves and enters the plate heat exchanger to heat the stripping agent. Through independent temperature control loops (regulating system pressure to control crystallization temperature and regulating branch heat source flow to control stripping temperature), stable operation of both processes within their respective optimal temperature windows is achieved. When production load fluctuates, the heat distribution can be dynamically balanced by adjusting the compressor output via frequency conversion, adjusting the branch valve opening in conjunction with the operation of auxiliary heating, and ensuring system stability and energy saving over a wide operating range. This integrated design reduces the overall energy consumption of this section by approximately 25% or more compared to the traditional method of independent steam heating.

[0060] In the process of this invention, the organic phase (composite extractant and diluent) and washing water, among other media, constitute the circulating material within the system. To ensure continuous operation, the following engineering points need to be considered: Organic phase regeneration and maintenance: The back-extraction operation in step S4 achieves functional regeneration of the organic phase for calcium and magnesium impurities. During long-term operation, its extraction performance and phase separation time need to be monitored periodically. When the phase separation time increases to 1.5-2 times the initial value, a partial replacement strategy (discharging part of the old organic phase and replenishing an equal amount of fresh organic phase) can be adopted to maintain the overall system performance.

[0061] System material balance: Within the optimized operating system of this invention, the composite extractant is chemically stable at mild temperatures. The recycling of wash water requires water balance calculations, with a small amount of fresh water added to offset losses from evaporation and entrainment.

[0062] Process tolerance and self-cleaning capability: The core process of this invention (wet mechanical activation reduction leaching, composite extraction, and thermally coupled MVR) itself has strong stability and impurity removal capability. Even if there are trace amounts of impurities introduced by the reflux of the circulating medium (such as trace amounts of Ca in the washing water), it can still achieve self-cleaning. 2+ Mg 2+ The impurities will also be effectively removed in the highly selective composite extraction process of the subsequent step S2, thus ensuring the long-term stability of the purity of the final product.

[0063] The "specific mechanical energy" mentioned in this invention specifically refers to the effective energy consumed in the grinding chamber during the wet mechanical activation-reduction leaching process, primarily used for crushing and grinding soft manganese ore particles. Its calculation method is as follows: Under the same wet grinding equipment, the same grinding media filling rate, the same equipment operating speed, and the same slurry solid content and sulfuric acid concentration: First, a "baseline operating condition experiment" was conducted: without adding pyrolusite powder, but with the same volume and concentration of sulfuric acid solution, the same stoichiometric ratio of oxalic acid, and the same amount of grinding media as in the activation experiment, the equipment was run for a predetermined time, and the power consumption Eb (kWh) was recorded. This energy consumption mainly includes equipment idling, friction between the media and the liquid, and system heat dissipation.

[0064] Next, a "material-carrying experiment" was conducted: pyrolusite powder was added, and the experiment was run for the same amount of time under the same conditions, recording the total power consumption Et (kWh). The specific mechanical energy Es (kWh / kg) was calculated using the formula Es=(Et−Eb) / m, where m is the dry mass (kg) of the pyrolusite being processed.

[0065] This calculation method deducts the basic frictional energy consumption caused by the rheological properties of the slurry, and can better reflect the effective energy input for ore activation, providing a reliable basis for process optimization and energy consumption assessment.

[0066] Energy Consumption Explanation: The "comprehensive energy consumption per unit product" described in this invention is calculated based on a complete batch experiment from steps S1 to S4 (or including S5). The energy consumption measurement range includes the electrical energy consumption of all power equipment such as wet grinding, centrifugal extraction, solid-liquid separation, MVR system (compressor, circulating pump, vacuum system, etc.), material conveying pump, stirrer, and control system. It is measured by an electricity meter installed on the main inlet of the experimental system and then divided by the total mass of dried manganese sulfate product obtained in that batch. This energy consumption value is a comprehensive reflection of the energy consumption of the entire system.

[0067] Example 1 This embodiment provides a method for preparing high-purity manganese sulfate for lithium manganese iron phosphate precursors, the specific steps of which are as follows: S1. Wet mechanical activation and reduction leaching (integrated gas control): Weigh 15 kg of pyrolusite powder (MnO2 content 85%) and calculate the theoretical amount of oxalic acid required based on the reaction equation. To achieve a high leaching rate and compensate for production losses, weigh oxalic acid (H2C2O4) at 1.2 times the theoretical stoichiometric ratio. Mix the pyrolusite powder, oxalic acid, and a 20% sulfuric acid solution to prepare a slurry with a solid content of 40% (mass fraction). Then, pump this slurry into a sand mill system equipped with a gas-liquid separation buffer unit for mechanical activation and reduction leaching treatment together with grinding media (high-purity zirconia ceramic beads, 70% filling rate). Set the average input power to 18 kW and the reaction time to 30 minutes. During the reaction, turn on the cooling jacket of the sand mill to control the slurry temperature ≤50℃. The CO2 gas generated by the reaction is introduced into the buffer tank, and the system is maintained at a slight negative pressure (-3 kPa). The gas is purified by a cyclone separator and a demister before being discharged, and the collected liquid is returned to the system. After the reaction, the leached slurry is obtained.

[0068] Specific mechanical energy calculation: The test conditions for the baseline operating condition power consumption (2.5 kWh) were as follows: equal volume of 20% sulfuric acid solution, equal amount (1.2 times the theoretical amount) of oxalic acid, and equal amount of grinding media, without ore, running for 30 minutes. The total power consumption for the material-carrying test was 18.5 kWh. Specific mechanical energy = (18.5 - 2.5) kWh / 15 kg = 1.07 kWh / kg.

[0069] S2. Deep impurity removal from slurry extraction (integrated transport stability control): The leached slurry was pumped to a centrifugal extractor via a low-speed, high-torque single-screw pump, with the delivery pipeline maintained at an inclination angle (>10°) and the pump pressure stabilized at 0.5 MPa. A dispersant (ammonium polyacrylate CY-100 and modified sodium lignosulfonate MLS-200 mixed at a mass ratio of 1:3) was added to the slurry at a rate of 0.05% of the total slurry liquid phase mass. Subsequently, a composite extractant (P507 and TOA at a volume ratio of 10:1, diluted with sulfonated kerosene) was added. Three-stage countercurrent extraction was performed in the centrifugal extractor, with the drum linear velocity set at 10 m / s, the organic phase to slurry volume ratio at 1:10, and the extraction temperature at 50°C. After extraction, the three phases were separated, yielding a loaded organic phase and a purified slurry, with a separation time ≤30 seconds.

[0070] S3. Solid-liquid separation and media circulation: The purified slurry was filtered to obtain a purified manganese sulfate solution and solid residue. 3% (0.45 kg) of the solid residue, equivalent to the total mass of the pyrolusite powder from step S1, was returned to step S1 as a recycled grinding aid to enhance the crushing effect in subsequent batches.

[0071] S4. Thermally Coupled Back-Extraction and Evaporation Crystallization (Integrated Dynamic Balance Control): The loaded organic phase is introduced into the plate heat exchanger of the MVR system. Using the upgraded heat source of the MVR system, approximately 15% of the heat is directed to this heat exchanger via the heat source distribution unit. Back-extraction is performed using dilute sulfuric acid (pH=2, with a back-extraction agent to organic phase volume ratio of 0.3:1). The back-extraction temperature is precisely controlled at 75±2℃ by adjusting the branch heat source flow rate. After back-extraction, liquid-liquid separation is performed again to obtain a regenerated organic phase (returned to S2 for recycling) and a liquid enriched with impurities.

[0072] Simultaneously, purified manganese sulfate solution was introduced into the main evaporation chamber of the same MVR system, with the system absolute pressure maintained at -50 kPa, corresponding to a boiling point of 80°C, for evaporation and crystallization. The crystallization chamber was equipped with a bidirectional cross-blade agitator operating at 150 rpm. The crystallized slurry was centrifuged and filtered (1000 rpm, 5 minutes), and the wet crystals were dried at 105°C to constant weight to obtain high-purity manganese sulfate crystals.

[0073] Example 2 The only difference between this embodiment and Embodiment 1 is the operating parameters in step S1, which aims to explore the effects of different mechanical energy inputs: S1. Wet mechanical activation and reduction leaching: The slurry solid content is adjusted to 45%, the grinding media filling rate is 75%, the activation time is 40 minutes, the average input power is 22kW, the dry basis weight of the treated pyrolusite is still 15kg, and the oxalic acid feed ratio is maintained at 1.2 times the theoretical amount. Under these conditions, the power consumption of the baseline test (40 minutes) is 3.3kWh, and the total power consumption of the test with material is 19.3kWh. The effective mechanical energy input is 16.0kWh, and the calculated specific mechanical energy is 16.0kWh / 15kg = 1.07kWh / kg. The remaining engineering measures, such as gas control and temperature control, are the same as in Example 1. All parameters and engineering control measures in subsequent steps S2 to S4 are exactly the same as in Example 1.

[0074] Example 3 The only difference between this embodiment and Embodiment 1 is the extraction conditions in step S2, which aim to optimize extraction efficiency: S2. Deep purification through pulp extraction: The mass ratio of dispersant (ammonium polyacrylate to modified sodium lignosulfonate) is adjusted to 1:4, and the addition amount is 0.03%. The volume ratio of P507 to TOA in the composite extractant is adjusted to 12:1. The number of extraction stages is increased to 4, the drum linear velocity is increased to 12 m / s, the volume ratio of organic phase to pulp is adjusted to 1:8, and the extraction temperature is set to 55℃. The pulp conveying and control methods are the same as in Example 1. All equipment, raw materials, operating parameters, and engineering control measures in the remaining steps (S1, S3, S4) are exactly the same as in Example 1.

[0075] Example 4 The only difference between this embodiment and Embodiment 1 is the amount of medium circulation in step S3, which aims to evaluate the impact of low slag return rate: S3. Solid-liquid separation and media circulation: Take solid residue equivalent to 1% (i.e., 0.15 kg) of the total mass of the pyrolusite powder processed in step S1 and return it to step S1 as a circulating grinding aid. The remaining steps, equipment, raw materials, operating parameters, and all engineering control measures (including gas control, stable conveying, and thermal coupling balance) are exactly the same as in Example 1.

[0076] Example 5 The only difference between this embodiment and Embodiment 1 is the medium circulation rate in step S3, which aims to evaluate the impact of high slag return rate: S3. Solid-liquid separation and media circulation: Take 5% (0.75 kg) of the solid residue equivalent to the total mass of the pyrolusite powder processed in step S1 and return it to step S1 as a circulating grinding aid. The remaining steps, equipment, raw materials, operating parameters, and all engineering control measures are exactly the same as in Example 1.

[0077] Example 6 This embodiment adds a complete resource recovery and media recycling step (S5) to Example 1, aiming to demonstrate the closed-loop process and green manufacturing level. Steps S1 to S4 and all engineering control measures of Example 1 remain unchanged in this embodiment.

[0078] Step S5 is as follows: S5.1 Resource recovery treatment of impurity-enriched solutions: The enriched impurity solution (mainly containing Ca) generated in step S4 is used to enrich the impurity solution. 2+ Mg 2+ SO4 2- and trace amounts of Mn 2+ The slurry was collected in a neutralization reactor. Under stirring (80 rpm), a 10% (w / w) lime slurry (Ca(OH)2 suspension) was slowly added, controlling the final pH at 9.0-9.5, maintaining the reaction temperature at 60-70℃, and the reaction time at 30 minutes. This process mainly produces gypsum (CaSO4·2H2O) and magnesium hydroxide (Mg(OH)2) precipitates. After the reaction, the slurry was subjected to solid-liquid separation using a plate and frame filter press. The filter cake can be used as an auxiliary raw material for building materials. The filtrate mainly contains sodium sulfate and trace impurity ions, including Ca... 2+ (Concentration <50ppm) is returned to step S1 for preparing sulfuric acid solution or adjusting slurry concentration. Due to its small quantity and the deep removal capability of subsequent extraction processes, it will not cause the accumulation of impurities in the system.

[0079] S5.2 Washing of solid residues and water reuse: The solid residue not returned to step S1 in step S3 is conveyed to a three-stage countercurrent washing system. Hot water at 60-70℃ is used for washing, and the total liquid-to-solid ratio (L / S) is controlled at 3:1. Mn in the wastewater after washing... 2+ The concentration can be reduced to <50ppm. This washing water is returned to step S1 as process makeup water, realizing a closed-loop cycle of water resources.

[0080] Comparative Example 1 This comparative example uses the traditional "two-ore acid addition method" process, without employing any pretreatment, green impurity removal, or energy-saving integrated technologies of this invention, aiming to provide a benchmark level for traditional processes. The specific steps are as follows: 15 kg of pyrolusite powder and pyrite powder are mixed at a mass ratio of 3:1, and an excess of 30% sulfuric acid is added. Leaching is carried out at 90°C under normal pressure for 4 hours. The pH of the leachate is adjusted to 4.5 with lime milk to remove iron and aluminum impurities. After filtration, 1.2 times the theoretical amount of sodium fluoride is added, and deep removal of calcium and magnesium is performed at 50°C (generating fluorine-containing hazardous waste). The purified liquid is concentrated using a single-effect evaporator, then cooled and crystallized in a stirred crystallizer. After solid-liquid separation and drying, manganese sulfate product is obtained.

[0081] Comparative Example 2 This comparative example, based on the modified Example 1, omits the wet mechanical activation reduction leaching step to independently evaluate the impact of mechanical activation on reaction efficiency and oxalic acid utilization. S1 is modified as follows: 15 kg of pyrolusite powder, oxalic acid at the same stoichiometric ratio as in the modified Example 1 (1.2 times the theoretical amount), and a 20% sulfuric acid solution (solid content of 40%) are mixed in a stirred atmospheric pressure reactor, and leached at 80°C for 2 hours without wet mechanical grinding activation. Subsequent steps S2 to S4 are identical to those in the modified Example 1, including all engineering control measures.

[0082] Comparative Example 3 This comparative example modifies the extractant composition based on Modified Example 1 to evaluate the key role of the co-extractant TOA in the selectivity of calcium and magnesium ions. In S2, the composite extractant was changed to use only P507 (the amount of which is the same as the amount of P507 used alone in the organic phase in Modified Example 1), without adding TOA, and the dilution of sulfonated kerosene remained unchanged. The remaining steps, parameters, and engineering control measures were exactly the same as in Modified Example 1.

[0083] Comparative Example 4 This comparative example, based on the modified Example 1, eliminates the thermal coupling design, treating back-extraction and evaporation crystallization as two independent unit operations to quantitatively evaluate the energy-saving effect of the thermally coupled integration of the present invention. S4 is changed to the following two independent steps: ① The loaded organic phase is back-extracted in an independent plate heat exchanger by heating to 75°C with saturated steam at 0.3 MPa (gauge pressure); ② The purified manganese sulfate solution is evaporated and concentrated in an independent single-effect evaporator using saturated steam at 0.3 MPa (gauge pressure), and the concentrated solution is transferred to a stirred crystallizer (150 rpm) to complete crystallization. Except for eliminating the thermal coupling, the remaining steps (S1-S3) are exactly the same as those in the modified Example 1.

[0084] Comparative Example 5 This comparative example employs a complex and cumbersome process sequence to illustrate the superior efficiency and energy consumption of the integrated process of this invention (optimized leaching-extraction-crystallization sequence and thermal coupling). The specific steps are as follows: ① First, 15 kg of pyrolusite powder is leached and subjected to solid-liquid separation under the conditions of Comparative Example 2 to obtain a crude manganese sulfate solution; ② This crude solution is directly evaporated and crystallized under the conditions of Comparative Example 4 to obtain crude manganese sulfate crystals (containing impurities); ③ The crude crystals are redissolved and then extracted to remove impurities under the S2 conditions of the modified Example 1; ④ The purified solution is evaporated and crystallized again under the conditions of Comparative Example 4 to obtain the final product.

[0085] Comparative Example 6 This comparative example is based on the modified Example 1, except for step S3: after solid-liquid separation of the purified slurry, all solid residue is discharged from the system and not returned to step S1. The aim is to evaluate the specific contribution of media circulation to leaching rate and energy consumption. All other steps, equipment, raw materials, operating conditions, and engineering control measures remain completely consistent with the modified Example 1.

[0086] Comparative Example 7 The only difference between this comparative example and the modified Example 1 is step S3: 10% (1.5 kg) of solid residue equivalent to the total mass of the pyrolusite powder processed in step S1 is taken and returned to step S1 as a recycled grinding aid. This aims to investigate the potential negative impacts of excessive returned residue on slurry rheology, grinding efficiency, and the overall process. All other steps, equipment, raw materials, and operating parameters are identical to those in the modified Example 1.

[0087] Comparative Example 8 This comparative example aims to verify the key influence of oxalic acid feed ratio on the leaching rate and to provide a basis for the feed ratio range recommended by this invention. It was carried out under the same equipment, raw materials and engineering control measures as in the modified Example 1, with the only difference being that in step S1, the oxalic acid feed ratio was changed to 1.0 times the theoretical stoichiometry (i.e., 100% theoretical amount).

[0088] Test Experiment The manganese sulfate products prepared in Examples 1-6 and Comparative Examples 1-8 were subjected to experiments and analyses, and the methods and standards are as follows: 1. Determination of manganese leaching rate: The total manganese content in the raw pyrolusite was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) after the ore powder was completely dissolved in a hydrochloric acid-hydrogen peroxide mixture. The dissolved manganese content in the purified manganese sulfate solution obtained from each process was determined by ICP-OES after appropriate dilution of samples. The manganese leaching rate was calculated using the following formula: Leaching rate (%) = (Total mass of manganese in the purified solution / Total mass of manganese in the raw pyrolusite) × 100% Each sample was measured in triplicate, and the final result was the arithmetic mean.

[0089] 2. Energy consumption data collection: The unit product comprehensive energy consumption is calculated based on the complete batch experiment from steps S1 to S4 (Example 6 includes S5). The energy consumption measurement scope covers all power equipment in the process, including wet grinding system, centrifugal extractor, various transfer pumps, solid-liquid separation equipment, MVR system (compressor, circulating pump, vacuum system, etc.), stirrer, control system, etc. The total power consumption is measured by the electricity meter installed on the main inlet of the experimental system, and then divided by the total mass of dried manganese sulfate product obtained in that batch to obtain the energy consumption value of "kilowatt-hours per ton of product (kWh / t)". The experiment is repeated three times for each process condition, and the average value of the energy consumption value is taken.

[0090] 3. Product purity and impurity analysis: The obtained manganese sulfate crystals were dried to constant weight at 105℃ and then tested for various indicators according to the industry standard "HG / T4823 Manganese Sulfate for Batteries".

[0091] Purity determination employs two methods for cross-verification: a) Subtraction method: According to the standard, the purity (as MnSO4·H2O) is calculated by subtracting the sum of water, loss on ignition and impurity oxides (including CaO, MgO, Fe2O3, Al2O3, SiO2, K2O, Na2O, etc.) from 100%.

[0092] b) Direct titration method: The content of manganese (Mn) is directly determined by the ferrous ammonium sulfate titration method and converted into the content of MnSO4·H2O.

[0093] The purity values ​​obtained by the two methods deviated by less than 0.05%, and the results were reported using the difference method. The contents of key impurities, calcium (Ca) and magnesium (Mg), were determined using highly sensitive inductively coupled plasma mass spectrometry (ICP-MS). Simultaneously, to assess other trace impurities that may be introduced by the process, ICP-MS or ICP-OES was used to detect zirconium (Zr) potentially originating from abrasion by grinding media, as well as elements commonly found in raw materials such as iron (Fe), aluminum (Al), silicon (Si), potassium (K), and sodium (Na). All test results showed that the contents of these impurities were all below 10 ppm (with Zr content below 2 ppm), and their impact on purity calculations was negligible, ensuring the reliability of the purity data.

[0094] All examples and comparative examples were performed in three sets of parallel experiments. The data in Table 1 below are all arithmetic means. The "±" values ​​marked after the key indicators are the standard deviation (purity, impurity content) or range (leaching rate, energy consumption) of the three experiments.

[0095] The experimental results are shown in Table 1.

[0096] Table 1: Comparison of Key Data Between Examples and Comparative Examples Based on the experimental data in Table 1, the present invention is analyzed in detail as follows: High leaching rate and high purity: The manganese leaching rate of all examples (1-6) is stable at 97.8%–98.5%, the product purity is ≥99.68%, and the contents of key impurities Ca and Mg are less than 35ppm and 40ppm, respectively, which is better than the battery grade standard.

[0097] Low energy consumption: The unit product comprehensive energy consumption of the example is between 1130 and 1250 kWh / t, which is about 30% to 37% more energy-efficient than the traditional "two-ore plus acid method" (Comparative Example 1, 1800 kWh / t).

[0098] Green and environmentally friendly: This invention abandons the fluoride precipitation method, eliminates fluoride-containing hazardous waste and wastewater from the source, and reduces the discharge of waste residue and wastewater through resource recycling (Example 6).

[0099] The key role of wet mechanical activation reduction leaching: Comparing Example 1 and Comparative Example 2 (no mechanical activation, only heating leaching), mechanical activation increased the leaching rate by 2.2 percentage points (98.2% vs 96.0%) and reduced energy consumption by 200 kWh / t. This demonstrates that simultaneously activating minerals and enhancing mass transfer through mechanical force is the core of overcoming the bottleneck of reaction kinetics.

[0100] The leaching rate of Comparative Example 8 (oxalic acid feed ratio 1.0) was only 95.0%, lower than that of Example 1 (1.2 times, 98.2%). This demonstrates that, within industrially acceptable limits, an oxalic acid feed ratio of 1.2–1.5 times is a necessary prerequisite for ensuring a high leaching rate. While mechanical activation cannot "create oxalic acid," it can maximize oxalic acid utilization by improving reaction efficiency.

[0101] Furthermore, comparing Examples 1 and 2, it is evident that although the slurry solid content in Example 2 was increased from 40% to 45%, which theoretically might increase slurry viscosity and inhibit mass transfer, the grinding media filling rate increased from 70% to 75%, and the activation time was extended from 30 minutes to 40 minutes, resulting in a higher effective mechanical energy density per unit time. The higher media filling rate and longer activation time significantly enhanced the collision frequency and shear strength between particles within the grinding chamber, thereby more fully disrupting the pyrolusite crystal structure and exposing more fresh reaction surfaces. This enhancement effect is sufficient to offset the mass transfer resistance caused by the slight increase in solid content, ultimately not affecting the leaching rate or only slightly increasing it. Therefore, by optimizing the mechanical energy input parameters, efficient leaching can be achieved over a wider range of solid content.

[0102] The synergistic effect of the combined extractant (P507 / TOA): In Comparative Example 3 (using only P507, without TOA), the Ca and Mg impurity contents increased to 85 ppm and 110 ppm, respectively, while in Example 1 they were only 25 ppm and 35 ppm. TOA, as a co-extractant, significantly improved the extraction efficiency for Ca... 2+ Mg 2 + The improvement in selectivity is crucial, with the removal rate increasing from approximately 90% to ≥98.5%.

[0103] It is worth noting that in Example 3, by increasing the number of extraction stages from 3 to 4 and simultaneously optimizing the extractant ratio and operating conditions, the content of Ca and Mg impurities was further significantly reduced. This conforms to the basic principle of multi-stage countercurrent extraction: for impurity ions with a relatively small distribution (such as Ca... 2+ Mg 2+ In this system, the allocation ratio of P507 / TOA is much lower than that of Mn. 2+ Increasing the number of extraction stages can exponentially increase the removal rate. Especially when the impurity concentration is already low (e.g., 25-35 ppm in Example 1), further increasing the number of stages is particularly effective in pushing the impurities to extremely low concentrations (<20 ppm). In this system, P507 / TOA has a significant effect on Ca... 2+ / Mg 2+ The high selectivity synergistic effect means that in the range of 2-4 stages, each additional stage leads to a significant decrease in impurity content, which is consistent with the synergistic extraction system behavior reported in the literature.

[0104] Energy-saving advantages of thermally coupled MVR systems: Comparative Example 4 (independent steam heating) has an energy consumption of up to 1550 kWh / t, which is 35% higher than Example 1 (1150 kWh / t). It can be seen that integrating back-extraction and crystallization into the same MVR system, through intelligent heat source distribution and dynamic balance, can achieve significant energy-saving effects (approximately 25%–30%).

[0105] The optimization effect of media (solid residue) circulation: The leaching rate (97.5%) and energy consumption (1300 kWh / t) of Comparative Example 6 (no circulation) were slightly lower than those of Example 1 (3% circulation), indicating that an appropriate amount of returned residue as a grinding aid can improve crushing efficiency.

[0106] Comparative Example 7 (10% excess circulation) resulted in a decrease in leaching rate (96.8%) and an increase in energy consumption (1400 kWh / t), proving that excess solids will deteriorate the slurry rheology, and that 1%–5% of slag return is the optimal range.

[0107] The energy consumption of Example 4 (1% slag return) was slightly higher than that of Example 1 (3% slag return), indicating that when the slag return is below a certain threshold, its contribution as a grinding aid to improving crushing efficiency and reducing unit energy consumption is not significant. This may be because insufficient circulating residue is not enough to form an effective supplementary grinding media network in the grinding chamber, resulting in limited auxiliary effect on the collision and crushing of ore particles. Furthermore, the energy consumption difference between Example 4 and Example 1 (approximately 50 kWh / t) is within the experimental standard deviation range and can be partially attributed to experimental fluctuations. In summary, Examples 1, 4, and 5, and Comparative Examples 6 and 7, show that there is an optimal range for the slag return amount (1%-5%), with approximately 3% exhibiting the best balance between energy consumption and leaching rate.

[0108] Advantages of integrated process sequence: Comparative Example 5 (a chaotic process of crystallization followed by impurity removal) had the lowest leaching rate (92.0%), the highest energy consumption (2100 kWh / t), and low product purity. It is evident that the integrated "leaching-extraction-crystallization" sequence of this invention is reasonable, avoiding repeated processing of intermediate products and achieving an optimal balance between efficiency and energy consumption.

[0109] Gas control and temperature management: Examples 1-6 showed no pressure fluctuations or material entrainment issues during operation, and the slurry temperature remained stable at ≤50℃, demonstrating the effectiveness of the gas-liquid separation and cooling system.

[0110] Slurry transport stability: No blockages or sedimentation were observed in any of the transport steps, demonstrating the reliability of the transport system design (screw pump, inclined pipe, dispersant).

[0111] Thermal coupling dynamic balance: Under load fluctuations, the crystallization and back-extraction temperature control accuracy of the embodiment is within ±3℃, indicating the feasibility of the heat source distribution and control strategy.

[0112] The synergistic effect of the specific composite dispersant (a mixture of ammonium polyacrylate and modified sodium lignosulfonate) and the high-speed centrifugal extractor (drum linear velocity 8-12 m / s) used in step S2 of this invention is the core of ensuring the stable and efficient operation of the solid-containing slurry extraction system. The composite dispersant effectively prevents solid-phase sedimentation and organic-phase emulsification, maintaining the uniformity and stability of the slurry; while the centrifugal extractor at the linear velocity generates strong centrifugal force, ensuring efficient and clear liquid-liquid-solid three-phase separation of the loaded organic phase, purified slurry, and contained solid residues within ≤30 seconds. This synergistic design ensures efficient extraction mass transfer, enabling calcium (Ca) to be separated into its constituent phases. 2+ ), magnesium (Mg) 2+ The removal rate of ions remained stable at over 98.5%, and the entrainment loss of organic phase was reduced.

[0113] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for the present invention. Further modifications can be readily implemented by those skilled in the art.

Claims

1. A method for preparing high-purity manganese sulfate for lithium manganese iron phosphate precursor, characterized in that, Includes the following steps: S1. Wet mechanical activation and reduction leaching: Pyrolusite powder, sulfuric acid solution with a mass fraction of 10%~30% and oxalic acid are mixed to make the slurry solid content 30%~50%. The slurry is placed together with grinding media in a wet grinding equipment to carry out mechanical activation and reduction leaching reactions. At the same time, the carbon dioxide gas generated by the reaction is discharged through the matching gas-liquid separation and pressure control unit to maintain the system gauge pressure at a slight negative pressure of -1kPa to -5kPa, so as to obtain the leaching slurry; the grinding media is hard ceramic particles; The amount of oxalic acid added, by mass, is 1.2 to 1.5 times the theoretical mass of oxalic acid required to completely reduce the MnO2 component in the pyrolusite powder. S2. Deep impurity removal by pulp extraction: The leaching pulp is mixed with a composite extractant and a dispersant and subjected to multi-stage countercurrent extraction. The composite extractant contains 2-ethylhexyl phosphate monoester P507 as the main extractant and trioctylamine TOA as the co-extractant. After extraction, liquid-liquid separation is performed to obtain a loaded organic phase and a purified pulp loaded with calcium and magnesium impurity ions. S3. Solid-liquid separation and media circulation: The purified slurry is subjected to solid-liquid separation to obtain a purified manganese sulfate solution and solid residue; a portion of the solid residue is returned to step S1 as a circulating grinding aid. S4. Integrated thermally coupled back-extraction and evaporation crystallization: The loaded organic phase obtained in step S2 is introduced into the heat exchange unit of the MVR system. Using an upgraded heat source from the same MVR system and through heat source distribution control, dilute sulfuric acid solution is used as the back-extraction agent to heat and back-extract the loaded organic phase. After back-extraction, liquid-liquid separation is performed again to obtain a regenerated organic phase and an impurity-enriched liquid. The regenerated organic phase is returned to step S2 for recycling. At the same time, the purified manganese sulfate solution obtained in step S3 is introduced into the main evaporation chamber of the same MVR system for evaporation concentration and crystallization. The crystallized slurry is then subjected to subsequent solid-liquid separation and drying to obtain a high-purity manganese sulfate crystal product.

2. The preparation method according to claim 1, characterized in that, In step S1, the wet grinding equipment is a sand mill; the wet mechanical activation reduction leaching is carried out in the sand mill, and the operating conditions are: the solid content of the slurry is 30% to 50% by mass fraction, the activation time is 10 to 40 minutes; the filling rate of the grinding media in the grinding equipment is 60% to 80% by volume; the specific mechanical energy per unit ore processed is 0.5 to 2.0 kWh per kilogram, and the specific mechanical energy is calculated by dividing the total power consumption of the material test under the same equipment and operating conditions by the dry basis mass of the processed pyrolusite.

3. The preparation method according to claim 1, characterized in that, In step S2, the dispersant is a mixture of ammonium polyacrylate and modified sodium lignosulfonate, wherein the mass ratio of ammonium polyacrylate to modified sodium lignosulfonate is 1:(2-5); the amount of dispersant added is based on the total mass of the liquid phase in the leaching slurry, and is 0.01% to 0.1% by mass fraction.

4. The preparation method according to claim 1, characterized in that, In step S2, the volume ratio of 2-ethylhexyl phosphate monoester P507 to trioctylamine TOA is (10-12):

1.

5. The preparation method according to claim 1, characterized in that, In step S2, the multi-stage countercurrent extraction is carried out in a centrifugal extractor with a drum linear velocity of 8-12 m / s, 2 to 4 extraction stages, a volume ratio of organic phase to slurry of 1:(5-15), and an extraction operating temperature of 40°C to 60°C.

6. The preparation method according to claim 1, characterized in that, In step S3, the amount of solid residue returned from step S1 as the circulating grinding aid accounts for 1% to 5% of the total mass of the pyrolusite powder processed in step S1.

7. The preparation method according to claim 1, characterized in that, In step S4, the heat exchange unit is a plate heat exchanger; the stripping agent is a dilute sulfuric acid solution with a pH of 1 to 3, the volume ratio of the stripping agent to the supported organic phase is (0.3-0.5):1, and the stripping operation temperature is 60°C to 75°C.

8. The preparation method according to claim 1, characterized in that, In step S4, the crystallization chamber of the MVR system is equipped with a bidirectional cross-blade stirrer; the upper blades of the stirrer are radial, the lower blades are axial, and the stirring speed is 50 to 300 revolutions per minute.

9. The preparation method according to claim 1, characterized in that, In step S4, the absolute pressure of the MVR evaporation crystallization system is -20 kPa to -80 kPa, corresponding to an operating temperature of 70°C to 90°C.

10. The preparation method according to any one of claims 1 to 9, characterized in that, The method also includes step S5: neutralizing the enriched impurity liquid generated in step S4 to pH 9.0-9.5 with lime milk to generate gypsum and magnesium hydroxide precipitate for resource utilization; and / or, washing the solid residue in step S3 that was not returned to step S1 with three-stage countercurrent washing at a liquid-to-solid ratio of 3:1 and a temperature of 60-70℃, and returning the resulting washing water to step S1 for recycling.

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