Method for improving the filtration and calcination yield of rare earth carbonates

Abstract

This invention relates to the field of rare earth hydrometallurgical processes and high-purity rare earth purification and extraction technology. It discloses a method for improving the yield of rare earth carbonates through pressure filtration, comprising: mixing rare earth mother liquor with a sodium bicarbonate aqueous solution to generate a rare earth carbonate slurry; subjecting the slurry to sedimentation in a sedimentation tank and settling in a product tank; discharging the upper layer of clear water while retaining the bottom layer of slurry and a protective layer; adding water to adjust the solid content and pH to obtain a pretreated slurry; sending the pretreated slurry to a transfer tank for buffering and pressure stabilization to dissociate aggregates and restore suspension; and pumping it to a plate and frame filter press for linear pressure increase and pressure holding filtration using a plunger pump. This invention controls sedimentation conditions and the water pressure interface to inhibit crystal hydrolysis, relaxes deformation stress in the transfer tank to restore uniform suspension, and utilizes a constant pressure difference to peel off the surface hydration shell and discharge deep water, thus eliminating the contradiction between high-pressure dehydration and the filter plate sealing limit.

Description

Technical Field

[0001] This invention relates to the field of rare earth hydrometallurgical processes and high-purity rare earth purification and extraction technology. More specifically, this invention relates to a method for improving the yield of rare earth carbonates by pressure filtration and calcination. Background Technology

[0002] Currently, in rare earth hydrometallurgical processes, carbonate precipitants are used to precipitate rare earth carbonate slurries, and filtration equipment is used to separate the solid and liquid phases to obtain high-purity rare earth compound precursors. This is a key unit operation in the extraction and purification chain of high-purity rare earth metals and compounds. Because rare earth carbonate particles have high surface energy and easily form hydrated colloidal networks, the solid-liquid distribution is conventionally controlled by extending the settling time or adjusting the slurry solid content. However, during subsequent continuous feeding in filter presses, the material undergoes strong shearing during pipeline transport. The high flow rate causes stress concentration and non-rigid deformation on the particle surface, causing the hydrated shells of adjacent particles to lock together and disrupting the uniform suspension state. When these agglomerates enter the filter press chamber, due to the abrupt change in fluid dynamics, the solid particles rapidly and irregularly accumulate on the filter cloth surface, forming a dense skin layer. Increasing mass transfer resistance and blocking fluid discharge locks the path for interstitial water and coordinating water adsorbed on the particle surface to migrate outward. To overcome the skin resistance and drive the precipitation of deep bound water, conventional attempts are made to increase pumping pressure or extend the pressing cycle. However, under actual working conditions, simply increasing the pushing pressure will induce inelastic compaction of the filter cake. Mechanical extrusion leads to the closure of fluid channels, resulting in a decrease in filtration flux. Moreover, the pressure pulse often exceeds the tolerance limit of the sealing surface of the reinforced polypropylene filter plate, increasing the risk of slurry leakage. If the cycle is extended, the processing efficiency will decrease and the hydrated shell on the particle surface cannot be destroyed, making it difficult for coordinating water to desorb. This results in a low yield during calcination conversion. This creates a process contradiction between the dehydration driving force, the sealing limit of the filter plate, and the unobstructed flow of the filter cake pores.

[0003] Traditional improvements to filtration equipment focus only on hardware limitations such as strengthening filter plate materials, locally improving sealing surfaces, or adjusting the rigidity of the pressing roller body. If breakthroughs cannot be achieved in process control methods, it remains difficult to regulate the intrinsic rheological characteristics of the slurry. For example, Chinese invention patent application CN105506287A discloses a process for recovering rare earths by precipitation of leaching mother liquor from southern rare earth mines. This process achieves rare earth enrichment and impurity removal by controlling the acidity and alkalinity of the neutralization reaction step by step and cooperating with multiple plate and frame filtrations. However, the process control method remains at the level of macroscopic chemical reactions and conventional continuous fluid propulsion. When faced with rare earth carbonate slurries with high surface energy under complex working conditions, this scheme lacks a relaxation mechanism for the inelastic deformation caused by the shear stress of the transport. It does not perform in-situ shear control of the surface potential of particles and the hydrated solvation shell. Solid particles cause irregular accumulation at the inlet of the filter chamber due to abrupt changes in the fluid dynamic state, and the resulting dense skin layer exacerbates the resistance to deep water migration.

[0004] Therefore, the technical problems to be solved by this invention are how to eliminate shear stress during transport to restore the suspension state of particles, optimize the fluid dynamics balance in the filter press chamber to control the generation of skin resistance, and shear and peel off the hydrated shell on the surface of the particles within the sealing tolerance range of the filter plate to allow the interstitial moisture to be discharged. Summary of the Invention

[0005] To address the problems in the background art, the present invention provides a method for improving the yield of rare earth carbonates by pressure filtration calcination, comprising the following steps:

[0006] Step S1: Under the condition of aeration and stirring using an air compressor, a rare earth mother liquor with a mass concentration of 0.2 g / L to 0.5 g / L is mixed with a sodium bicarbonate aqueous solution with a mass concentration of 6% to 8% and reacted for 60 min to generate a rare earth carbonate slurry. The rare earth carbonate slurry is sent to a sedimentation tank for sedimentation. The sedimented bottom slurry is pumped through a pipeline to a product tank and allowed to stand for 3 h. The upper clear water is discharged, and the bottom slurry and a 5 cm clear water protective layer above the bottom slurry are retained. Clear water is added to the product tank to adjust the solid content of the bottom slurry to 120 g / L to 150 g / L and the pH value to 6.7 to 6.9, so that the solid particles are suspended and dispersed to obtain a pretreated slurry.

[0007] Step S2: The pretreated slurry is pumped into a transfer tank with a volume of 10m³ and equipped with a stirring blade via a feed pump with a power of 11kW and a flow rate of 18m³ / h. The rotation speed of the stirring blade is controlled at 15rpm to 20rpm. The pressure is buffered and stabilized in the transfer tank to reduce the slurry flow rate, dissociate the crystal agglomerates, and restore the suspension state of solid particles in the slurry.

[0008] Step S3: The slurry in the transfer tank is pumped to the reinforced polypropylene plate and frame filter press by a plunger pump. The plunger pump is started and the outlet pressure is linearly increased to 0.75 MPa within 5 minutes. After maintaining the outlet pressure at 0.75 MPa for 25 minutes, the filter cake is discharged to obtain rare earth carbonate filter press product.

[0009] Preferably, in step S1, the average grain size of the solid particles in the rare earth carbonate slurry is 5 μm to 10 μm.

[0010] Preferably, in step S1, the method of pumping the rare earth carbonate slurry to the product tank for static settling after sedimentation in the sedimentation tank includes the following sub-steps: Step S11, pumping the bottom slurry after sedimentation in the sedimentation tank to the product tank through a pipeline, controlling the feed flow rate to be 10 m³ / h to 12 m³ / h; Step S12, after static settling in the product tank for 3 hours, draining the upper layer of clear water through a siphon pipe, stopping the drainage when the liquid level drops to 5 cm from the surface of the bottom slurry, forming a clear water protective layer.

[0011] Preferably, in step S1, the absolute value of the Zeta potential corresponding to the solid particles in the pretreated slurry is less than or equal to 15mV.

[0012] Preferably, in step S2, the stirring blades inside the transfer tank are stainless steel low-shear blades.

[0013] Preferably, in step S3, the method of pumping the slurry in the transfer tank to the reinforced polypropylene plate and frame filter press by a plunger pump includes the following sub-steps: Step S31, start the plunger pump and linearly increase the outlet pressure from 0.1MPa to 0.75MPa within 5 minutes to push the slurry into the reinforced polypropylene plate and frame filter press; Step S32, maintain the pressure at the outlet pressure of 0.75MPa for 25 minutes and then discharge the filter cake.

[0014] Preferably, in step S3, the filter cloth used in the reinforced polypropylene plate and frame filter press is a polypropylene monofilament filter cloth, and the air permeability of the filter cloth is controlled to be 0.5 m³ / m²·min to 0.8 m³ / m²·min, and the filtration accuracy of the filter cloth is controlled to be 1 μm to 3 μm.

[0015] Preferably, after obtaining the rare earth carbonate filter press product, the rare earth carbonate filter press product is transferred into a drying kiln for drying and calcination. First, it is dried at a constant temperature of 105°C to 110°C for 3 to 4 hours to remove residual free water. Then, it is calcined at 850°C to 900°C for 2 to 3 hours to convert the rare earth carbonate filter press product into high-purity rare earth oxides. The final filter press yield is controlled to be 25% to 25.8%.

[0016] Preferably, during calcination, the heating rate inside the drying kiln is controlled to be 5°C / min to 8°C / min, and by controlling the discharge rate of carbon dioxide and water vapor in the exhaust gas generated during calcination, high-purity rare earth oxides with a purity greater than or equal to 99.99% are obtained.

[0017] The embodiments of the present invention have at least the following beneficial effects:

[0018] 1. In the method of improving the sintering yield of rare earth carbonates by pressure filtration, the airflow introduced by the air compressor is used to aerate and stir the flow field to balance the flow field, replacing the high-shear mechanical stirring. This eliminates the damage to the newly nucleated rare earth carbonate crystals caused by the local shear stress at the edge of the mechanical blades. The macroscopic micro-temperature mixing formed by the rising bubbles, combined with specific acid-base precipitation conditions and the upper hydrostatic pressure interface, stabilizes the crystal surface potential and double layer thickness, reduces uncontrolled secondary crystallization and disordered colloidal agglomeration between crystals, and keeps the bottom slurry in a uniformly dispersed suspension state. This provides a stable physical and rheological basis for the orderly arrangement of subsequent materials, thereby eliminating the mass transfer resistance of the dense filter cake layer caused by the sudden accumulation of materials at the front end of the filter press inlet.

[0019] 2. When the slurry is transported through a specific pipeline, it is subjected to instantaneous high shear stress, which causes the monomer particles with elastic hydrated shells to undergo inelastic deformation. As a result, significant rheological shear stress accumulates inside the material. By introducing the slurry into a buffer tank of a specific volume, the volume buffer environment provided by the specific geometric space is utilized to rapidly reduce the flow rate and shear rate of the slurry. The deformation stress accumulated inside the slurry is sufficiently relaxed by non-rigid rheology within a specific residence time, which drives the mutually locked hydrated crystal network structure to dissociate back into suspended particles with intrinsic solvation shells, restoring the spatially uniform suspension and dispersion state of the solid phase particles of the material.

[0020] 3. By continuously pushing the material into the filter chamber with a constant fluid pressure without pulses, the fluid dynamic pressure is precisely matched with the critical yield stress of the precipitated crystals, eliminating the mechanical fatigue damage caused by fluid pulses to the sealing surface of the chamber edge. This drives the rare earth carbonate particles to form a regular and compact crystalline orientation on the filter cloth surface, constructing a uniformly distributed through-pore transport channel across the entire cross-section. Thus, within a specific time frame after the free water is drained, the multi-layered hydrated shell adsorbed on the surface of the rare earth crystals is sheared and peeled off in situ by a constant pressure gradient, allowing the deep lattice coordinated water to migrate outward continuously and achieve highly efficient and ultimate dewatering of the deep bound water in the filter cake. Attached Figure Description

[0021] The above and other objects, features, and advantages of exemplary embodiments of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings, in which several embodiments of the invention are illustrated by way of example and not limitation, wherein:

[0022] Figure 1 This is a process flow diagram of a method for improving the yield of rare earth carbonates by pressure filtration calcination according to the present invention.

[0023] Figure 2 This is a structural diagram of the production system for a method of improving the yield of rare earth carbonates by pressure filtration calcination according to the present invention.

[0024] Figure 3 This is a schematic diagram of the control elements of a method for improving the yield of rare earth carbonates by pressure filtration and calcination according to the present invention. Detailed Implementation

[0025] The principles and spirit of the present invention will now be described with reference to several exemplary embodiments in conjunction with the accompanying drawings. It should be understood that these embodiments are provided merely to enable those skilled in the art to better understand and implement the present invention, and are not intended to limit the scope of the present invention in any way. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0026] A method for improving the yield of rare earth carbonates by pressure filtration calcination includes the following steps:

[0027] Step S1: Under the condition of aeration and stirring using an air compressor, a rare earth mother liquor with a mass concentration of 0.2 g / L to 0.5 g / L is mixed with a sodium bicarbonate aqueous solution with a mass concentration of 6% to 8% and reacted for 60 min to generate a rare earth carbonate slurry. The rare earth carbonate slurry is sent to a sedimentation tank for sedimentation. The sedimented bottom slurry is pumped through a pipeline to a product tank and allowed to stand for 3 h. The upper clear water is discharged, and the bottom slurry and a 5 cm clear water protective layer above the bottom slurry are retained. Clear water is added to the product tank to adjust the solid content of the bottom slurry to 120 g / L to 150 g / L and the pH value to 6.7 to 6.9, so that the solid particles are suspended and dispersed to obtain a pretreated slurry.

[0028] Step S2: The pretreated slurry is pumped into a transfer tank with a volume of 10m³ and equipped with a stirring blade via a feed pump with a power of 11kW and a flow rate of 18m³ / h. The rotation speed of the stirring blade is controlled at 15rpm to 20rpm. The pressure is buffered and stabilized in the transfer tank to reduce the slurry flow rate, dissociate the crystal agglomerates, and restore the suspension state of solid particles in the slurry.

[0029] Step S3: The slurry in the transfer tank is pumped to the reinforced polypropylene plate and frame filter press by a plunger pump. The plunger pump is started and the outlet pressure is linearly increased to 0.75 MPa within 5 minutes. After maintaining the outlet pressure at 0.75 MPa for 25 minutes, the filter cake is discharged to obtain rare earth carbonate filter press product.

[0030] Preferably, in step S1, the average grain size of the solid particles in the rare earth carbonate slurry is 5 μm to 10 μm.

[0031] Preferably, in step S1, the method of pumping the rare earth carbonate slurry to the product tank for static settling after sedimentation in the sedimentation tank includes the following sub-steps: Step S11, pumping the bottom slurry after sedimentation in the sedimentation tank to the product tank through a pipeline, controlling the feed flow rate to be 10 m³ / h to 12 m³ / h; Step S12, after static settling in the product tank for 3 hours, draining the upper layer of clear water through a siphon pipe, stopping the drainage when the liquid level drops to 5 cm from the surface of the bottom slurry, forming a clear water protective layer.

[0032] Preferably, in step S1, the absolute value of the Zeta potential corresponding to the solid particles in the pretreated slurry is less than or equal to 15mV.

[0033] Preferably, in step S2, the stirring blades inside the transfer tank are stainless steel low-shear blades.

[0034] Preferably, in step S3, the method of pumping the slurry in the transfer tank to the reinforced polypropylene plate and frame filter press by a plunger pump includes the following sub-steps: Step S31, start the plunger pump and linearly increase the outlet pressure from 0.1MPa to 0.75MPa within 5 minutes to push the slurry into the reinforced polypropylene plate and frame filter press; Step S32, maintain the pressure at the outlet pressure of 0.75MPa for 25 minutes and then discharge the filter cake.

[0035] Preferably, in step S3, the filter cloth used in the reinforced polypropylene plate and frame filter press is a polypropylene monofilament filter cloth, and the air permeability of the filter cloth is controlled to be 0.5 m³ / m²·min to 0.8 m³ / m²·min, and the filtration accuracy of the filter cloth is controlled to be 1 μm to 3 μm.

[0036] Preferably, after obtaining the rare earth carbonate filter press product, the rare earth carbonate filter press product is transferred into a drying kiln for drying and calcination. First, it is dried at a constant temperature of 105°C to 110°C for 3 to 4 hours to remove residual free water. Then, it is calcined at 850°C to 900°C for 2 to 3 hours to convert the rare earth carbonate filter press product into high-purity rare earth oxides. The final filter press yield is controlled to be 25% to 25.8%.

[0037] Preferably, during calcination, the heating rate inside the drying kiln is controlled to be 5°C / min to 8°C / min, and by controlling the discharge rate of carbon dioxide and water vapor in the exhaust gas generated during calcination, high-purity rare earth oxides with a purity greater than or equal to 99.99% are obtained.

[0038] Example 1: The method claimed in this invention is applicable to the solid-liquid separation process of rare earth carbonate slurry produced by the mixing and reaction of rare earth mother liquor and sodium bicarbonate aqueous solution in the manufacture of high-purity rare earth compounds. In the solid-liquid separation process, rare earth carbonate crystals have high surface energy and are prone to forming hydrated colloidal aggregates. When traditional water pumps transport the slurry in a high-shear manner, they disrupt the internal hydration balance, causing rare earth carbonate crystals to accumulate disorderly at the feed front of the filter press and forming a filter cake skin layer with high fluid resistance on the filter cloth surface. The resulting secondary mass transfer resistance restricts the outward migration of interstitial water and adsorbed coordinated water in the deeper layers of the filter cake, resulting in excessive residual water inside the filter cake and a prolonged single filtration cycle. If the conveying pressure is increased... The force exceeds the mechanical tolerance limit of the sealing surface of the reinforced polypropylene filter plate and causes slurry leakage, thus creating a technical problem of mutual constraint between the dewatering driving force, the sealing limit of the filter plate, and the unobstructed flow of the internal pores of the filter cake. In specific implementation, compressed air is continuously introduced into the bottom of the mixing reaction system by an air compressor for aeration and stirring, and the reaction time is controlled at 60 minutes. Under this condition, rare earth mother liquor with a mass concentration of 0.2 g / L to 0.5 g / L is mixed with sodium bicarbonate aqueous solution with a mass concentration of 6% to 8% and reacted for 60 minutes to produce rare earth carbonate slurry. The rare earth carbonate slurry is sent to a sedimentation tank for sedimentation, and the bottom slurry after sedimentation is piped to a flow rate of 10 m³ / h to 1 A feed flow rate of 2 m³ / h is pumped into the product tank and allowed to settle for 3 hours. The upper layer of clear water is then drained through a siphon. Drainage is stopped when the liquid level drops to 5 cm below the bottom slurry surface to form a clear water protective layer. Clear water is added to the product tank to adjust the solid content of the bottom slurry to 120 g / L to 150 g / L, and the pH to 6.7 to 6.9 to suspend and disperse the solid particles. The average grain size of the solid particles in the rare earth carbonate slurry is controlled to be 5 μm to 10 μm, and the absolute value of the interfacial potential corresponding to the solid particles in the pretreated slurry is less than or equal to 15 mV. Thus, a pH of 6.7 to 6.9 and a 5 cm clear water protective layer are used to construct a clear water layer above the bottom rare earth carbonate slurry. The hydrostatic equilibrium interface, under physicochemical equilibrium, isolates air to prevent localized carbonation, hydrolysis, or oxidative amorphization of the surface rare earth carbonates. Simultaneously, it maintains the ion activity balance on the slurry surface, preventing secondary uncontrolled crystallization of rare earth carbonate grains due to abrupt changes in surface supersaturation. This stabilizes the surface interface potential and double-layer thickness of the rare earth carbonate crystals, maintaining them in a weakly repulsive suspension state with a regular spatial topological structure. This provides a spatial topological basis for eliminating the filter cake skin effect. Specifically, the technical path to stabilize the absolute value of the interface potential at less than or equal to 15 mV involves adjusting the pH of the slurry in the product tank to approximately 6.7 to 6, close to the isoelectric point of the rare earth carbonate particles.In the 9-range, the net charge density on the grain surface can be reduced to its intrinsic trough. Based on this, by adding water to dilute and maintain the solid content of the bottom slurry at 120 g / L to 150 g / L, the intrinsic strength of the non-specifically adsorbed anti-sign ions in the liquid phase is effectively controlled and maintained. Utilizing the active compression effect of the electric double layer, the absolute value of the Zeta potential corresponding to the solid particles in the final pretreated slurry is kept below 15 mV. Furthermore, the pretreated slurry is pumped via an 11 kW feed pump with a flow rate of 18 m³ / h to a 10 m³ transfer tank equipped with a stirring impeller. The rotation speed of the stirring impeller is controlled at 15 rpm to 20 rpm. The tank volume of the transfer vessel provides a volumetric buffer environment, reducing the flow rate and shear rate of the slurry. Since the pretreated slurry experiences instantaneous high shear stress during feed pumping, causing inelastic deformation of the monomer particles with elastic hydrated shells and accumulating internal rheological shear stress, low-shear stirring and buffering pressure stabilization within the transfer vessel allows the accumulated deformation stress within the slurry to undergo rheological relaxation during the residence time. This drives the hydrated crystal network structure, which had been tightly bound together by the high flow rate during pipeline transport, to dissociate back into suspended particles with intrinsic solvated shells. This restores the spatially uniform suspension and dispersion of the solid particles, ensuring that the rheological characteristics of the material remain constant during subsequent high-pressure feeding.

[0039] The slurry in the transfer tank is pumped to a reinforced polypropylene plate and frame filter press using a plunger pump. The plunger pump is started, and the outlet pressure is linearly increased from 0.1 MPa to 0.75 MPa within 5 minutes. The filter press is then maintained at an outlet pressure of 0.75 MPa for 25 minutes. The reinforced polypropylene plate and frame filter press has a filtration area of ​​80 m², a rated filtration pressure of less than or equal to 0.8 MPa, a total filter chamber volume of 1125 L, and uses polypropylene monofilament filter cloth with a permeability controlled at 0.5 m³ / (m²·min). The filtration accuracy of the filter cloth is controlled to be 1μm to 3μm, with a constant fluid pressure (n) ranging from 0.8m³ / (m²·min). A constant, pulse-free fluid pressure continuously pushes the material into the filtration chamber. The fluid dynamic pressure matches the critical yield stress of the rare earth carbonate crystal pores obtained under precipitation conditions of 6.7 to 6.9 pH, eliminating mechanical fatigue damage to the edge sealing surface of the reinforced polypropylene filter plate caused by fluid pressure pulses. This drives the rare earth carbonate particles to exhibit a regular, compact crystalline orientation on the filter cloth surface, constructing a uniformly distributed, interconnected structure across the entire cross-section. The filter cake utilizes a porous transport channel to shear and peel off the multi-layered hydrated shell adsorbed on the surface of rare earth crystals during the pressure-holding filtration period after the free water has been drained. This is achieved by using a constant pressure gradient to continuously shear and peel off the hydrated shell. This allows deep lattice-coordinated water to migrate and be discharged outwards, achieving ultimate dehydration of the filter cake without damaging the edge seal of the reinforced polypropylene filter plate. Specifically, the peeling off of the hydrated shell using a constant pressure gradient of 0.75 MPa refers to the process where, driven by this macroscopic hydrodynamic pressure, the fluid passes through a permeable microstructure constructed from 5-10 μm regularly oriented crystals. The fluid continuously flows through the pore channels, thereby generating a high-density microscopic shear flow field on the particle surface. The shear stress output by this shear flow field can effectively overcome the internal sliding resistance of the electrostatic multilayer hydration shell bound by weak chemical bonds or hydrogen bond networks on the surface of rare earth crystal particles, breaking the dynamic equilibrium of hydration in the micro-region. This causes the adsorbed and coordinated water molecules that were originally locked in the interstices of the crystal lattice and the particle surface to desorb and be continuously discharged outward along the fluid transport channel that runs through the entire cross-section. This achieves the ultimate dehydration of the deep-layer bound water in the filter cake without damaging the mechanical seal limit at the edge of the filter plate.

[0040] After discharging the filter cake and obtaining the rare earth carbonate filter press product, the product is transferred to a drying kiln. The heating rate inside the kiln is controlled at 5℃ / min to 8℃ / min. First, it is dried at a constant temperature of 105℃ to 110℃ for 3 to 4 hours to remove residual free water. Then, it is calcined at 850℃ to 900℃ for 2 to 3 hours. By controlling the emission rate of carbon dioxide and water vapor in the exhaust gas generated during calcination, the rare earth carbonate filter press product is converted into high-purity rare earth oxides. Testing shows its purity is greater than or equal to 99.99%, and the calcination yield is consistently within the range of 25% to 25.8%. No slurry leakage occurred between the reinforced polypropylene filter plates during continuous production. Throughout this conversion process, the purity remains greater than or equal to 99.9%. The level of non-volatile heterometallic impurities in 9% high-purity rare earth oxides is essentially determined by the pre-solid-liquid separation and high-purity purification process. By controlling the discharge rate of carbon dioxide and water vapor in the tail gas, its core function is to dynamically adjust the local atmospheric pressure environment and phase change mass transfer rate inside the drying kiln. On the one hand, it prevents secondary mechanical pollution caused by local airflow turbulence due to sudden and violent gas explosions, which would carry impurities from the refractory materials of the kiln wall. On the other hand, it ensures that the non-metallic carbon and sulfur volatile impurities generated by the thermal decomposition of rare earth carbonates can be completely extracted with the gas phase, blocking the microscopic encapsulation of residual amorphous carbon by local overheating agglomeration, so that the analytical purity of the main non-volatile rare earth oxide components finally detected reaches the theoretical upper limit of 99.99%.

[0041] Example 2: The method claimed in this invention is applicable to the solid-liquid separation process of rare earth carbonate slurry produced by the mixing and reaction of rare earth mother liquor and sodium bicarbonate aqueous solution in the manufacture of high-purity rare earth compounds. In the solid-liquid separation process, the solid particles have high surface energy and are prone to forming hydrated colloidal aggregates. When the pump delivers the slurry in a high-shear manner, it disrupts the internal hydration balance, causing rare earth carbonate crystals to accumulate at the feed front of the filter press and form a filter cake skin on the filter cloth surface. The mass transfer resistance generated by this structure restricts the migration of interstitial water and adsorbed coordinated water inside the filter cake to the outside, resulting in excessive residual water inside the filter cake and difficulty in single pressing. Extended filtration cycles, coupled with increased conveying pressure, exceed the mechanical tolerance limit of the sealing surface at the edge of the reinforced polypropylene filter plate, leading to slurry leakage. This creates a technical challenge where dewatering driving force, filter plate sealing limit, and the permeability of the filter cake's internal pores are mutually constrained. The high-purity rare earth precipitation and solid-liquid separation simulation device includes a 50L reactor with a temperature control accuracy of 0.5℃, a 120L product tank, a plunger pump with a pressure adjustment gradient of 0.05MPa, and a plate and frame filter press with a filtration area of ​​2m². The rotational speed of the stirring paddles in the intermediate tank is determined by the need to eliminate crystal precipitation and sedimentation. To maintain a rheological balance with preventing the accumulation of secondary shear stress, if the agitator blade rotation speed is below 15 rpm, high-density solid particles in the slurry will locally deposit at the bottom of the tank under gravity, causing pipeline blockage. If the agitator blade rotation speed is above 20 rpm, the high shear stress zone at the edge of the agitator blade will re-inject shear deformation internal stress into the slurry and destroy the hydration solvation shell on the grain surface, leading to a deterioration in material flow characteristics. When the solid content of the slurry changes from low to high, the spatial contact resistance of suspended particles inside the material increases, the rheological relaxation time of shear internal stress is prolonged, and the rotation speed of the agitator blade... The speed approaches the upper limit of the range, 20 rpm, to provide spatial disturbance and accelerate the release of rheological stress. For the working condition slurry with a solid content of 135 g / L, this control logic determines the rotational speed setpoint of the stirring blade to be 18 rpm. The control unit determines the rotational speed based on the rheological dynamics of the solid particles in the shear flow field. According to the correlation model between shear strain rate and shear stress in non-Newtonian fluids, the spatial contact resistance of solid particles in the slurry increases with the increase of solid content. It is necessary to simultaneously increase the shear rate to accelerate the relaxation of rheological stress. The formula for calculating the rotational speed of the stirring blade is as follows: ,in, The target rotational speed of the stirring blades. The lower limit operating speed constant of the agitator blades is 15. The value represents the actual solid content of the slurry, collected online by an electromagnetic density meter, ranging from 120 to 150. The preset lower limit constant for slurry solid content is 120. The dimensionless viscous rheological adjustment coefficient of the slurry is fixed at 0.2. The control unit will adjust the actual solid content. Substituting 135 into the formula, the output is the unique target rotational speed. The value is 18, which is converted into a frequency drive signal and sent to the motor controller to control the operation of the stirring blades. The dimensionless slurry viscous rheological adjustment coefficient k is fixed at 0.2, which is determined based on the non-Newtonian shear thinning and yield stress rheological sensitivity test curves of rare earth carbonate slurry with a specific particle size in the industrial solid content range of 120 g / L to 150 g / L. Within this specific solid content range, for every 10 g / L increase in slurry solid content, the apparent viscosity and local shear contact resistance of the slurry system increase linearly. The stirring blades need to synchronously compensate with a rotational speed of 2 rpm at the edge linear velocity to output sufficient mechanical disturbance shear force to complete the relaxation of shear internal stress and at the same time prevent the material from gravity deposition at the bottom of the tank. That is, it corresponds to satisfying the mapping relationship between the difference between the rotational speed change and the solid content change. If the coefficient is lower than 0.2, the speed compensation will be insufficient, and high-concentration materials will experience local laminar flow agglomeration and deposition. If it is higher than 0.2, the speed will easily spike excessively, causing the high-shear micro-zones at the edge of the stirring blades to destroy the intrinsic solvation shell of the particles themselves. In addition, in order to test the working stability of the system in a non-ideal industrial environment, a continuous mechanical power frequency disturbance with a frequency of 12Hz and an amplitude of 0.5mm generated by an external vibrator is superimposed on the base of the test platform.

[0042] The experimental groups included Experimental Group 1, Experimental Group 2, Experimental Group 3, Control Group 1, Control Group 2, Control Group 3, and Control Group 4. Initially, the rare earth mother liquor concentration was 0.35 g / L, and the sodium bicarbonate aqueous solution concentration was 7.2%. The rare earth carbonate slurry produced by the reaction was sequentially fed into each group's equipment. After the reaction and sedimentation steps were completed, the intermediate process data obtained from the tests showed that in Experimental Group 1, with a solid content set at 120 g / L and a pH set at 6.8, the measured average grain size of the solid particles was 5.8 μm, and the absolute value of the interface potential was 14.2 mV. In Experimental Group 2, with a solid content set at 135 g / L and a pH set at 6.85, the measured average grain size of the solid particles was 7.5 μm, and the absolute value of the interface potential was... The measured value was 11.5 mV; in test group three, the solid content was set at 150 g / L and the pH was set at 6.9. The measured average grain size of the solid particles was 9.2 μm, and the absolute value of the interfacial potential was 13.8 mV; in control group one, no water protection layer was provided and the material was directly exposed to air. The pH was set at 6.85 and the solid content was set at 135 g / L. At the downstream end of the product tank, infrared spectroscopy showed a non-uniform abrupt change in the supersaturation of the solid particle surface. The measured absolute value of the interfacial potential increased to 28.4 mV, and the average grain size decreased to 2.3 μm, accompanied by colloidal agglomerates; in control group two, the material was directly pumped to the filter press without buffering and relaxation in the transfer tank. The non-Newtonian fluid apparent viscosity of the material measured at the transfer input end remained at 42.5. The pH was set at 6.5, and the measured absolute value of the interface potential was 22.1 mV. The pH was set at 7.5, and the measured absolute value of the interface potential increased to 24.6 mV. The materials of each group were pumped into a filter press and filtered under pressure for 25 min, and then sent to a drying kiln and drying equipment to be heated at a rate of 5 °C / min. They were dried at 105 °C for 3.5 h and calcined at 850 °C for 2.5 h. The final quantitative output indicators showed that the moisture content of the filter cake obtained by the first experimental group was 11.45%, the single filter press cycle was 29.2 min, no slurry leakage occurred at the edge of the filter plate, the purity after conversion to high-purity rare earth oxides was 99.991%, and the final filter press calcination yield was 25.12%. The filter cake obtained in Experiment Group 2 had a moisture content of 10.23%, a single filtration cycle of 28.5 min, no slurry leakage, a purity of 99.994% for the high-purity rare earth oxides, and a filtration yield of 25.54%. The filter cake obtained in Experiment Group 3 had a moisture content of 11.12%, a single filtration cycle of 29.8 min, no slurry leakage, a purity of 99.992% for the high-purity rare earth oxides, and a filtration yield of 25.28%. The filter cake in Control Group 1 had a moisture content of 18.56%, and the single filtration cycle was extended to 49.3 min. Due to the excessive moisture content, high-temperature sintering entrainment occurred during drying and calcination, resulting in a final filtration yield of 20.12%. Control Group 2 achieved a filtration yield of 20.12% when the plunger pump feed pressure reached 0.At 75 MPa, the instantaneous impact pressure in the pipeline rose to 0.84 MPa. This value exceeded the mechanical tolerance limit of the reinforced polypropylene filter plate sealing surface, causing slurry leakage between the filter plates. The final filter cake moisture content was 17.34%, the single filtration cycle was 46.2 min, and the filtration yield decreased to 21.05%. In contrast, control group three had a filter cake moisture content of 15.89% and a filtration yield of 21.84%. Control group four had a filter cake moisture content of 16.12% and a final filtration yield that dropped to 21.43%. This confirms that the range of process parameters falls within the working window for synergistically achieving high-yield dehydration of rare earth carbonate slurry.

[0043] The reaction products of rare earth mother liquor and sodium bicarbonate solution are placed under a protective layer of acidity / alkalinity and clear water to control the interfacial potential. Combined with the volume buffer of the transfer tank, a rheological relaxation environment is provided to eliminate the internal stress of shear stress during transport. Under the filtration precision constraints of polypropylene monofilament filter cloth, and with linear pressure increase, the absolute value of the surface potential of the solid particles in the slurry is stabilized below 15mV, and a uniform suspension state is maintained in space. This creates a fully open fluid transmission channel in the filter chamber of the plate and frame filter press. Without damaging the edge seal of the reinforced polypropylene filter plate, the multi-layer hydrated shell on the surface of the particles is sheared and peeled off, achieving a filter cake moisture content of less than 12% and a single filtration cycle controlled within 30 minutes. This ensures that the purity of the final high-purity rare earth oxide is maintained above 99.99%, and the filtration yield during the process is stably maintained within the physical range of 25% to 25.8%.

[0044] Example 3: When a high-purity rare earth compound manufacturing system faces a significant fluctuation in the initial mass concentration of the upstream rare earth mother liquor between 0.15 g / L and 0.60 g / L, the fluctuation in the feed composition will disturb the crystallization kinetic equilibrium within the reactor, causing the local pH during the precipitation process to deviate from the control window of 6.7 to 6.9. This results in the formation of a highly hydrophilic amorphous hydroxide secondary phase. This amorphous secondary phase encapsulates and carries impurity ions, causing the absolute value of the interfacial potential on the surface of solid particles to be higher, which in turn triggers disordered colloid formation between solid particles. Aggregation of materials leads to flow accumulation of material at the front end of the cloth chamber of the subsequent plate and frame filter press, forming a dense filter cake skin of uneven thickness on the filter cloth surface. The mass transfer resistance caused by this structure restricts the outward migration of interstitial water and adsorbed coordinated water inside the filter cake, resulting in excessive residual water inside the filter cake and excessively long single filtration cycle. If the conveying pressure is increased unilaterally, it will exceed the mechanical tolerance limit of the edge sealing surface of the reinforced polypropylene filter plate. This creates a technical problem of mutual constraint between the dewatering driving force, the sealing limit of the filter plate, and the unobstructed flow of pores inside the filter cake.

[0045] To control process uncertainties caused by fluctuations in raw material concentration, a rare earth mother liquor replenishment and adjustment step based on online material density sensing is adopted. An electromagnetic densitometer installed on the main feed pipeline collects fluid density data of the rare earth mother liquor. The initial mass concentration value is calculated based on the monotonically increasing relationship between fluid density and solute content, and this value is used as a pre-compensation variable and input to the control unit in real time. To quantify the double-layer stability of suspended solid particles in the reactor after material concentration disturbances, the corresponding double-layer passivation index is obtained by multiplying the absolute value of the solid particle interface potential by a preset correction coefficient. The control unit establishes a quantitative evaluation mechanism based on the mapping law between the electrostatic repulsion of the colloidal chemielectric double layer and the particle interface potential. The formula for calculating the double-layer passivation index is as follows: ,in, The passivation index of the electric double layer of materials in the reactor. This is a dimensionless double-layer charge shielding correction factor, fixed at a value of 1.0. The absolute value of the solid particle interface potential acquired by the online electrochemical sensor is greater than 0. The control unit periodically calculates the double-layer passivation index. As a physical constraint indicator of downstream material conveying status, when When the value falls below the preset stable critical threshold of 15.0, the control unit sends a passage permission signal to the control valve of the underlying delivery pipeline; if... When the reaction exceeds the stable critical threshold, the control unit automatically reduces the output power of the feed pump to prolong the reaction residence time and guide the reaction. Returning to below 15.0, the control steps for extending the reaction residence time by reducing the output power of the feed pump are as follows: The control unit reduces the discharge rate of the feed pump, resulting in a decrease in the material propulsion velocity in the interstage main pipeline. This reduced flow rate acts as a chain feedback signal, synchronously driving the connecting discharge control valve at the bottom of the upstream crystallization reactor to proportionally reduce its opening. By limiting the rate at which the current batch of nucleating slurry is continuously discharged from the reactor, the actual reaction and agitation aging time of subsequent materials that have not yet been discharged is forcibly increased inside the reactor, ensuring that they obtain a more sufficient double-layer passivation growth time, thereby guiding... The passivation index of the double layer of solid particles in the next time-series output actively returns to below the preset stable critical threshold. When the electromagnetic density meter detects that the initial mass concentration of the rare earth mother liquor deviates from the normal median value of 0.35 g / L to the upper limit critical value of 0.55 g / L, the control unit calculates the target volumetric flow rate increase correction amount of the sodium bicarbonate aqueous solution based on the preset mass conservation neutralization reaction material ratio relationship, and outputs a pulse signal to drive the regulating valve opening, increasing the feed flow rate of the 8% sodium bicarbonate aqueous solution from 2.5 m³ / h to 3.8 m³ / h. The solution flow rate was controlled to stabilize the pH value at 6.85 during the instantaneous mixing in the reactor. The absolute value of the interface potential of the solid particles obtained from sampling was controlled to be 12.4 mV, and the corresponding double-layer passivation index output value was 12.4. This value is lower than the preset stability critical threshold of 15.0, thus blocking the hydroxyl complexation side reaction caused by the local excessive supersaturation of rare earth ions. This kept the average grain size of the solid particles in the newly precipitated rare earth carbonate crystals stable at 7.5 μm, thereby maintaining the weakly repulsive spatial suspension dispersion of the solid particles. Rare earth slurry is sent to a sedimentation tank for sedimentation. The resulting bottom slurry is pumped to a product tank via a pipeline pump and allowed to settle for 3 hours. The upper clear water is then discharged through a siphon pipe. When the liquid level drops to 5 cm from the surface of the bottom slurry, drainage is stopped to form a clear water protective layer. The control unit calls the pH sensor located in the product tank to collect pH data online. If the pH deviates from the target window due to material dilution, adjusting water is added to the product tank via a metering pump to keep the pH of the pretreated slurry stable within the range of 6.7 to 6.9 and the solid content within the range of 120 g / L to 150 g / L.

[0046] The rare earth carbonate slurry, in a slightly repulsive suspended dispersion state, was diverted to a 10 m³ transfer tank equipped with a stirring impeller. The stirring impeller was controlled to maintain low-shear flow at a rotation speed of 16 rpm, allowing the inelastic shear stress accumulated in the material during pipeline transport due to flow velocity shear to undergo rheological relaxation. A high-pressure plunger pump was used to pump the material into the filter chamber of a reinforced polypropylene plate and frame filter press with a total volume of 1125 L. The feed power mechanism was controlled to linearly increase the outlet pressure from 0.1 MPa to 0.75 MPa within 5 minutes, and the pressure was maintained at the rated pressure of 0.75 MPa for 25 minutes. Within the filter chamber of the plate and frame filter press, the materials were uniformly arranged across the entire cross-section. The crystal particles construct a full-section through-flow fluid transport channel on the surface of the filter cloth. With the help of the pressure gradient inside the filter chamber, the hydrated shell adsorbed on the surface of the solid particles is sheared and peeled off. The residual moisture content of the filter cake after the fluid is discharged is measured by an online microwave moisture sensor and is stably reduced to 10.45%. This allows the subsequent solid material to be heated in the drying kiln at a heating rate of 6℃ / min and dried at 108℃ for 3.5h, followed by calcination at 880℃. The high-purity rare earth oxide produced has a purity greater than or equal to 99.992%, and the pressure filtration calcination yield during the process is stably maintained at 25.4%. This eliminates the hidden danger of slurry leakage caused by pressure pulses at the edge sealing surface of the reinforced polypropylene filter plate.

[0047] Example 4: When the system faces the situation of replacing a new batch of rare earth mother liquor and the pH sensor experiencing zero-point drift, in order to eliminate the interference of measurement deviation on the grain size regularity in the crystallization reaction, a pre-calibration step is initiated before feeding. A control system with constant pH of 4.00 and 7.00 is prepared using standard buffer solutions. The pH sensors are sequentially immersed in the control system, and the corresponding original potential difference scalar is read by the control chip. The reference conversion parameters in the memory are corrected based on the slope of the change in the original potential difference scalar and the bias voltage to control the measurement deviation within 0.02, maintaining a constant state of alkalinity control in the reaction. The calibrated rare earth mother liquor is then injected into the reactor, and the reaction is carried out for 60 minutes under constant aeration and stirring conditions maintained by the air compressor. The pH trend of the system is monitored in real time based on the modified reference conversion parameters. When the instantaneous pH value exceeds the control range of 6.7 to 6.9, the control unit calculates the flow rate adjustment operator of the sodium bicarbonate solution based on the pH deviation, drives the valve to change the flow rate of the sodium bicarbonate solution with a mass concentration of 7.2%, so that the internal chemical supersaturation returns to the equilibrium state. The average grain size of the solid particles in the produced rare earth carbonate slurry is 8.2 μm, and the absolute value of the interface potential corresponding to the solid particles is maintained at 12.6 mV. After solid-liquid separation and calcination, high-purity rare earth oxides with a purity greater than or equal to 99.992% are obtained. The yield of the entire process is kept constant at 25.2%, and there is no slurry leakage on the sealing surface of the reinforced polypropylene filter plate.

[0048] The above description is only a few preferred embodiments of the present invention and an explanation of the technical principles used. Those skilled in the art should understand that the scope of the invention involved in the embodiments of the present invention is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the above-mentioned inventive concept. For example, technical solutions formed by replacing the above-mentioned features with the technical features with similar functions disclosed in the embodiments of the present invention.

Claims

1. A method for improving the yield of rare earth carbonates by pressure filtration and calcination, characterized in that, Includes the following steps: Step S1: Under the condition of aeration and stirring using an air compressor, a rare earth mother liquor with a mass concentration of 0.2 g / L to 0.5 g / L is mixed with a sodium bicarbonate aqueous solution with a mass concentration of 6% to 8% and reacted for 60 min to generate a rare earth carbonate slurry. The rare earth carbonate slurry is sent to a sedimentation tank for sedimentation. The sedimented bottom slurry is pumped through a pipeline to a product tank and allowed to stand for 3 h. The upper clear water is discharged, and the bottom slurry and a 5 cm clear water protective layer above the bottom slurry are retained. Clear water is added to the product tank to adjust the solid content of the bottom slurry to 120 g / L to 150 g / L and the pH value to 6.7 to 6.9, so that the solid particles are suspended and dispersed to obtain a pretreated slurry. Step S2: The pretreated slurry is pumped into a transfer tank with a volume of 10m³ and equipped with a stirring blade via a feed pump with a power of 11kW and a flow rate of 18m³ / h. The rotation speed of the stirring blade is controlled at 15rpm to 20rpm. The pressure is buffered and stabilized in the transfer tank to reduce the slurry flow rate, dissociate the crystal agglomerates, and restore the suspension state of solid particles in the slurry. Step S3: The slurry in the transfer tank is pumped to the reinforced polypropylene plate and frame filter press by a plunger pump. The plunger pump is started and the outlet pressure is linearly increased to 0.75 MPa within 5 minutes. After maintaining the outlet pressure at 0.75 MPa for 25 minutes, the filter cake is discharged to obtain rare earth carbonate filter press product.

2. The method for improving the yield of rare earth carbonates by pressure filtration calcination according to claim 1, characterized in that, In step S1, the average grain size of the solid particles in the rare earth carbonate slurry is 5 μm to 10 μm.

3. The method for improving the yield of rare earth carbonates by pressure filtration calcination according to claim 1, characterized in that, In step S1, the method of pumping the rare earth carbonate slurry to the product tank for static settling after sedimentation in the sedimentation tank includes the following sub-steps: Step S11, pumping the bottom slurry after sedimentation in the sedimentation tank to the product tank through a pipeline, controlling the feed flow rate to be 10 m³ / h to 12 m³ / h; Step S12, after static settling in the product tank for 3 hours, draining the upper layer of clear water through a siphon pipe, stopping the drainage when the liquid level drops to 5 cm from the surface of the bottom slurry, forming a clear water protective layer.

4. The method for improving the yield of rare earth carbonates by pressure filtration calcination according to claim 1, characterized in that, In step S1, the absolute value of the Zeta potential corresponding to the solid particles in the pretreated slurry is less than or equal to 15mV.

5. The method for improving the yield of rare earth carbonates by pressure filtration calcination according to claim 1, characterized in that, In step S2, the stirring blades inside the transfer tank are made of stainless steel with low shear properties.

6. The method for improving the yield of rare earth carbonates by pressure filtration calcination according to claim 1, characterized in that, In step S3, the method of pumping the slurry in the transfer tank to the reinforced polypropylene plate and frame filter press by a plunger pump includes the following sub-steps: Step S31, start the plunger pump and linearly increase the outlet pressure from 0.1MPa to 0.75MPa within 5 minutes to push the slurry into the reinforced polypropylene plate and frame filter press; Step S32, maintain the pressure at the outlet pressure of 0.75MPa for 25 minutes and then discharge the filter cake.

7. The method for improving the yield of rare earth carbonates by pressure filtration calcination according to claim 1, characterized in that, In step S3, the filter cloth used in the reinforced polypropylene plate and frame filter press is a polypropylene monofilament filter cloth, and the air permeability of the filter cloth is controlled to be 0.5 m³ / m²·min to 0.8 m³ / m²·min, and the filtration accuracy of the filter cloth is controlled to be 1 μm to 3 μm.

8. The method for improving the yield of rare earth carbonates by pressure filtration calcination according to claim 1, characterized in that, After obtaining the rare earth carbonate filter press product, the rare earth carbonate filter press product is transferred into a drying kiln for drying and calcination. First, it is dried at a constant temperature of 105°C to 110°C for 3 to 4 hours to remove residual free water. Then, it is calcined at 850°C to 900°C for 2 to 3 hours to convert the rare earth carbonate filter press product into high-purity rare earth oxides. The final filter press yield is controlled to be 25% to 25.8%.

9. A method for improving the yield of rare earth carbonates by pressure filtration calcination according to claim 8, characterized in that, During calcination, the heating rate inside the drying kiln is controlled at 5°C / min to 8°C / min. By controlling the discharge rate of carbon dioxide and water vapor in the exhaust gas generated during calcination, high-purity rare earth oxides with a purity greater than or equal to 99.99% are obtained.

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