A dilute phosphoric acid concentration device and a concentration process thereof

Abstract

This invention provides a dilute phosphoric acid concentration device and its concentration process, relating to the field of non-metallic elements and dilute phosphoric acid compound concentration technology. The invention utilizes a concentric reactor with a coaxially arranged shell, membrane element, and water collection pipe forming an annular crystallization zone. This integrates calcium sulfate crystallization, membrane separation, and sulfuric acid dilution heat recovery into the same container, reducing equipment footprint and external piping. The double-layered jacket of the shell allows the flowing dilute phosphoric acid to absorb the sulfuric acid dilution heat within the annular crystallization zone, achieving in-situ preheating of the feed and reducing external preheating energy consumption.

Description

Technical Field

[0001] This invention relates to the field of non-metallic elements and dilute phosphoric acid compound concentration technology, specifically to a dilute phosphoric acid concentration device and its concentration process. Background Technology

[0002] With the continuous development of the phosphate chemical industry, wet-process phosphoric acid, as a basic raw material for downstream products such as phosphate fertilizers, feed-grade calcium phosphate, and industrial-grade phosphoric acid, is experiencing continuous expansion in production scale. The wet-process phosphoric acid production process typically involves the decomposition of phosphate rock with sulfuric acid, resulting in crude phosphoric acid with a concentration generally between 20% and 30% P2O5 (i.e., dilute phosphoric acid), containing large amounts of calcium sulfate, fluorosilicates, and metallic impurities such as iron, aluminum, and magnesium. To meet the requirements of subsequent processing and utilization, the dilute phosphoric acid must be purified and concentrated. Currently, in the industry, the concentration of dilute phosphoric acid mainly relies on multi-effect evaporation or direct contact thermal evaporation, typically represented by single-effect / multi-effect vacuum evaporation concentration units and submerged combustion evaporation units. These traditional methods suffer from prominent problems such as extremely high energy consumption (steam consumption can reach 1.5–2.5 tons per ton of P2O5), severe equipment scaling, frequent cleaning, and the generation of large amounts of acidic waste gas. Especially against the backdrop of increasingly mature membrane separation technology and increasingly stringent requirements for energy conservation and emission reduction, the inherent defects of the traditional pure thermal concentration route in terms of energy consumption and environmental impact are becoming increasingly prominent.

[0003] For example, invention patent CN114870631B discloses an integrated two-stage membrane purification device and method for wet-process phosphoric acid, relating to the field of wet-process phosphoric acid purification technology. The device includes a pretreatment unit, an ultrafiltration unit, a three-stage nanofiltration unit, and a product tank connected in sequence. After pretreatment to remove fluorine, silicon, sulfur, arsenic, heavy metals, and TOC from the wet-process phosphoric acid, it undergoes sequential purification via ultrafiltration and three-stage nanofiltration membranes, producing purified phosphoric acid products of different grades. This device utilizes membrane separation technology to achieve deep purification of wet-process phosphoric acid, providing a low-pollution technical means for phosphoric acid purification.

[0004] For example, utility model patent CN222411265U discloses a treatment device for removing sulfate ions from water by combining membrane concentration with a calcium sulfate crystallization device, relating to the field of concentrated brine treatment technology. This device includes a calcium sulfate crystallization device, a filter, and a membrane concentration device. Raw water first enters the calcium sulfate crystallization device to precipitate calcium and sulfate ions in the form of calcium sulfate crystals. After two stages of filtration, it enters the membrane concentration device for salt separation. The concentrated water from the membrane concentration is then recycled back to the calcium sulfate crystallization device for further treatment. This device combines crystallization and membrane separation, achieving the dual goals of sulfate removal and volume reduction of the concentrated water from the membrane concentration device.

[0005] For example, the invention patent with publication number CN119185973B discloses a system for comprehensive heat utilization in a phosphoric acid concentration device, relating to the fields of phosphoric acid concentration and heat recovery technology. This system includes inlet and outlet heat exchangers, a raw material acid preheater, a first-stage evaporator separator, a circulating pump, and a circulating water cooler. Through a heat exchange network, the waste heat from steam condensation during the evaporation and concentration process is recovered and utilized in stages for feed preheating and circulating water cooling, reducing external steam consumption and circulating cooling water load, thus providing an effective means for energy saving and consumption reduction in the phosphoric acid concentration process.

[0006] Currently, even after fully utilizing membrane separation technology for purification and pre-concentration, the dilute phosphoric acid concentration and purification system in the industry still suffers from the following shortcomings: When removing scale-causing cations such as calcium from calcium sulfate crystallization, the crystallization unit and the membrane separation unit are usually independent devices, relying on external pipeline connections. This not only increases the equipment footprint and the number of external pipeline connections but also prevents the synergistic utilization of crystallization, membrane separation, and reaction heat. The large amount of dilution heat released during the mixed crystallization process of nanofiltration concentrate and sulfuric acid is not effectively recovered and utilized for feed preheating, resulting in energy waste and increased external steam consumption. Scaling problems are severe on the inner wall of the crystallizer and the surface of the membrane elements. Existing technologies mostly employ mechanical scrapers or periodic chemical cleaning. Mechanical scrapers require additional motor drives, increasing energy consumption and maintenance costs, while chemical cleaning requires shutdown operations and generates secondary waste liquid. The particle size control of existing crystallizers relies on external grading equipment, making it impossible to achieve self-grading and selective discharge of crystals by particle size within the crystallizer using fluid dynamics principles. Furthermore, due to prolonged immersion in high-solids-content slurry, the membrane elements inside the crystallizer are prone to crystal deposition and adhesion, leading to a rapid decline in membrane flux. Existing backwashing methods require the introduction of external flushing water, and the flushing efficiency is limited. These shortcomings restrict the overall integration, operational continuity, and comprehensive energy consumption optimization of wet-process dilute phosphoric acid concentration and purification, making it difficult to meet the demands of the modern phosphate chemical industry for high integration, low energy consumption, and long-term stable operation.

[0007] Therefore, a dilute phosphoric acid concentration device and its concentration process are provided. Summary of the Invention

[0008] To address the problems mentioned in the background art, the present invention provides the following technical solution: a dilute phosphoric acid concentration device, comprising a concentric reactor, wherein the concentric reactor includes:

[0009] The outer shell has a double-layer jacket structure to form a heat exchange channel, which is configured to preheat the feed of the nanofiltration purification unit using the sulfuric acid dilution heat; the outer shell is equipped with a jacket inlet pipe and a jacket outlet pipe, and the top of the outer shell is sealed with a flange cover plate.

[0010] The annular crystallization zone, located inside the outer shell, is an annular space that receives nanofiltration concentrate and sulfuric acid, where calcium sulfate crystallizes. The annular crystallization zone is equipped with a nanofiltration concentrate inlet pipe, a sulfuric acid inlet pipe, a crystal slurry outlet pipe, and a drain outlet pipe. A spiral scraper assembly is installed within the annular crystallization zone. The spiral scraper assembly guides the liquid flow to form a spiral flow channel within the annular space. At the same time, the centrifugal force of the spiral flow is used to classify the calcium sulfate crystals according to their particle size: large crystals migrate to the outer wall and are discharged through the bottom, while small crystals migrate to the inner wall.

[0011] The membrane element is located in the center of the annular crystallization zone. The membrane element separates the liquid phase into permeate and concentrate. The permeate permeates into the water collection pipe located in the center of the membrane element and is then discharged through the permeate outlet pipe fixed to the bottom of the outer shell at the bottom of the water collection pipe.

[0012] Among them, the small crystals migrating towards the inner wall in the annular crystallization zone and the liquid flow continuously scour the outer surface of the membrane element, achieving self-cleaning of the membrane surface;

[0013] The spiral scraper assembly includes three to five spiral plates, two fixed rings, multiple balls, two linkage rings, and multiple blades. The two ends of each spiral plate are fixedly connected to the two fixed rings. An annular groove is formed on the outer end face of each fixed ring, and the balls are rotatably positioned within the annular groove. The spiral scraper assembly is rotatably supported between the inner wall of the annular crystallization zone and the outer surface of the membrane element via the balls. The two linkage rings are fixedly connected to the spiral plates, and their positions are aligned with the nanofiltration concentrate inlet pipe and the sulfuric acid inlet pipe, respectively. The blades are evenly distributed on the outer wall of the linkage rings. When liquid is fed into the nanofiltration concentrate inlet pipe and the sulfuric acid inlet pipe, it pushes the blades to rotate, causing the spiral plates to rotate around the axis of the concentric reactor.

[0014] A process for concentrating dilute phosphoric acid, using a dilute phosphoric acid concentration device, includes the following steps:

[0015] Step S1: The wet-process dilute phosphoric acid is pretreated to remove suspended solids and colloids;

[0016] Step S2: The pretreated dilute phosphoric acid is sent into the nanofiltration purification unit to be separated into nanofiltration permeate and nanofiltration concentrate;

[0017] Step S3: The nanofiltration permeate is fed into the reverse osmosis pre-concentration unit and separated into reverse osmosis concentrate and reverse osmosis desalinated water;

[0018] Step S4: The nanofiltration concentrate and sulfuric acid are fed tangentially into the annular crystallization zone of the concentric reactor to form a spiral flow. Calcium sulfate crystallizes in the annular crystallization zone and is separated by the membrane element. Large crystals are discharged through the crystal slurry outlet pipe under centrifugal force, while small crystals wash the membrane surface of the membrane element. The heat of sulfuric acid dilution is recovered through the jacket of the outer shell and used to preheat the feed of the nanofiltration purification unit.

[0019] Step S5: The reverse osmosis concentrate is sent to the thermal evaporation concentration unit for concentration to obtain concentrated phosphoric acid product;

[0020] Step S6: The crystal slurry discharged from the bottom of the concentric reactor is sent to the solid-liquid separation unit. The separated calcium sulfate dihydrate crystal product is discharged, and the supernatant is returned to the feed side of the nanofiltration purification unit. Part of the supernatant is discharged from the system as a tributary.

[0021] The present invention has the following beneficial effects:

[0022] 1. This invention uses a concentric reactor with a coaxially arranged shell, membrane element and water collection pipe from the outside to the inside to form an annular crystallization zone. This integrates calcium sulfate crystallization, membrane separation and sulfuric acid dilution heat recovery in the same container, reducing equipment footprint and external piping. The double-layer jacket of the shell allows the flowing dilute phosphoric acid to absorb the sulfuric acid dilution heat in the annular crystallization zone, realizing in-situ preheating of the feed and reducing external preheating energy consumption.

[0023] 2. In this invention, the nanofiltration concentrate inlet pipe and the sulfuric acid inlet pipe extend tangentially into the annular crystallization zone at different axial heights, with the pipe ends obliquely cut at 45°. After the two fluids enter the annular space tangentially at different heights, they form a stable spiral flow, which can be uniformly mixed and drive the liquid flow to rotate without the need for an additional water distribution device. The spiral plate guides the liquid flow in an axial spiral flow, prolonging the crystallization residence time and improving the calcium sulfate precipitation rate.

[0024] 3. This invention utilizes the acute angle between the blade surface and the radial direction of the linkage ring in the spiral scraper assembly to convert the impact force of the spiral flow into mechanical energy that drives the assembly to rotate around its axis, eliminating the need for a separate drive motor and reducing operating power consumption and maintenance costs. The assembly rotates continuously under the impetus of the liquid flow, forcing the nanofiltration concentrate and sulfuric acid to mix evenly, avoiding explosive nucleation caused by excessively high local acid concentrations, and ensuring a uniform distribution of supersaturation in the crystallization zone, thereby improving the crystallization efficiency of calcium sulfate and improving the crystal particle size distribution. The zirconia ceramic balls ensure the long-term flexible rotation of the scraper assembly in the acidic crystal slurry environment through rolling contact, extending the continuous operation time of the equipment.

[0025] 4. This invention maintains a 5mm to 15mm gap between the outer surface of the membrane element and the inner edge of the spiral plate, and between the inner wall of the annular crystallization zone and the outer edge of the spiral plate. This avoids rigid contact between the spiral plate and the membrane element and the shell wall during rotation, protecting the membrane element and ensuring smooth rotation. It also increases the local flow velocity in the narrow gap, creating a strong scouring effect on the wall surface, flushing away the not-yet-firmly-attached crystal nuclei and microcrystals, thus achieving non-contact dynamic anti-scaling and self-cleaning. At the same time, this gap serves as a channel for crystal grading by particle size, allowing large calcium sulfate crystals subjected to centrifugal force to settle through the outer gap and be discharged from the crystal slurry outlet pipe, avoiding blockage and breakage.

[0026] 5. This invention uses a nanofiltration purification unit to remove Ca... ²+ Mg²+ Fe ³+ Al ³+ The phosphate is retained in the nanofiltration concentrate and sent to a concentric reactor to crystallize and remove it in the form of calcium sulfate, which greatly reduces the concentration of scaling cations in the nanofiltration permeate entering the reverse osmosis pre-concentration unit, reduces the risk of scaling on the reverse osmosis membrane surface, and extends the cleaning cycle and service life of the reverse osmosis membrane; the membrane permeate is returned to the feed side of the nanofiltration purification unit through a one-way valve to recycle and recover phosphoric acid, thereby improving the overall yield.

[0027] 6. This invention features a three-way valve on the outlet pipeline of the reverse osmosis pre-concentration unit. One outlet connects to the inlet of the recovery unit, and the outlet of the recovery unit connects to the feed pipeline of the nanofiltration purification unit. This allows the pressure energy released by depressurizing part of the reverse osmosis concentrate to be directly transferred to the nanofiltration feed liquid, reducing the external power consumption of the nanofiltration feed pump. The three-way valve allows for flexible adjustment of the flow ratio, achieving an optimized balance between evaporation load and energy recovery. The reverse osmosis pre-concentration concentrates the nanofiltration permeate from approximately 5% to 10% P2O5 to 15% to 25% P2O5, significantly reducing the evaporation load of subsequent thermal evaporation and concentration, and lowering overall energy consumption.

[0028] 7. This invention uses a three-way valve installed on the supernatant outlet pipeline of the solid-liquid separation unit. One outlet returns 85% to 95% of the supernatant to the nanofiltration purification unit for recycling, reducing the total amount of dilute phosphoric acid to be treated and increasing system capacity. The other outlet discharges 5% to 15% of the supernatant as a tributary out of the system, carrying away accumulated impurities such as fluoride ions, iron ions, and aluminum ions. This prevents impurities from having excessively high concentrations in the closed-loop circulation, which could affect crystallization and membrane separation effects, and ensures long-term stable operation of the system.

[0029] 8. This invention uses fresh water produced by the reverse osmosis pre-concentration unit as the water source in the flushing unit, and performs osmotic backwashing cleaning on the membrane element through pulse mode, without the need for external flushing water, thus avoiding secondary pollution; the instantaneous pressure impact of the pulse is more effective in removing deposits on the membrane surface than continuous backwashing, and uses less water; the pulse control valve opens and closes at timed intervals, which can restore membrane flux without stopping the machine, and extend the continuous operation time and replacement cycle of the membrane element.

[0030] 9. The present invention uses an MVR evaporator in the thermal evaporation concentration unit, which utilizes secondary steam compression cycle heating. After startup, only a small amount of steam needs to be added, which greatly reduces external steam consumption and lowers the operating cost of evaporation concentration. The solid-liquid separation unit uses a horizontal screw centrifuge to continuously separate the crystal slurry, obtaining calcium sulfate dihydrate crystal by-products that can be sold externally, reducing solid waste emissions and improving economic efficiency.

[0031] Of course, any product implementing this invention does not necessarily need to achieve all of the above advantages at the same time. Attached Figure Description

[0032] Figure 1This is a schematic diagram of the overall invention.

[0033] Figure 2 This is a front view of the concentric reactor of the present invention.

[0034] Figure 3 This is an axial sectional view of the concentric reactor of the present invention.

[0035] Figure 4 This is an isometric view of the spiral scraper assembly of the present invention.

[0036] In the diagram: Pretreatment unit 1, Nanofiltration purification unit 2, Concentric reactor 3, Outer shell 31, Jacket inlet pipe 311, Jacket outlet pipe 312, Annular crystallization zone 32, Nanofiltration concentrate inlet pipe 321, Sulfuric acid inlet pipe 322, Crystal slurry outlet pipe 323, Drain outlet pipe 324, Membrane element 33, Water collection pipe 34, Permeate outlet pipe 341, Spiral scraper assembly 35, Spiral plate 351, Fixed ring 352, Ball bearing 353, Linkage ring 354, Blade 355, Flange cover plate 36, Reverse osmosis pre-concentration unit 4, Thermal evaporation concentration unit 5, Solid-liquid separation unit 6, Recovery unit 7, Washing unit 8. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1:

[0039] Please see Figures 1 to 4 The present invention provides a technical solution: a dilute phosphoric acid concentration device, comprising a pretreatment unit 1, a nanofiltration purification unit 2, a concentric reactor 3, a reverse osmosis pre-concentration unit 4, a thermal evaporation concentration unit 5, a solid-liquid separation unit 6, a recovery unit 7, and a rinsing unit 8.

[0040] Pretreatment unit 1 includes a multi-media filter and an ultrafiltration membrane module. The outlet of the multi-media filter is connected to the inlet of the ultrafiltration membrane module via a pipeline; this sequentially removes suspended solids and colloids, reducing the fouling load on subsequent membrane modules and extending membrane lifespan. The multi-media filter is internally filled with anthracite filter layers, quartz sand filter layers, and garnet filter layers from top to bottom; the combination of filter layers with different densities allows for staged retention, improving suspended solids removal efficiency. The ultrafiltration membrane module is either a tubular ultrafiltration membrane module or a hollow fiber ultrafiltration membrane module.

[0041] Nanofiltration purification unit 2 is equipped with an acid-resistant nanofiltration membrane module. This module is a spiral-wound membrane module made of a layer-by-layer self-assembled polyelectrolyte composite membrane. This material is stable in acidic environments, effectively retaining polyvalent cations while allowing phosphate molecules to permeate. Nanofiltration purification unit 2 has a permeate outlet and a concentrate outlet.

[0042] The concentric reactor 3 is composed of an outer shell 31, a membrane element 33, and a water collection pipe 34 arranged sequentially from the outside to the inside. The outer shell 31 and the membrane element 33 form an annular crystallization zone 32. This concentric arrangement integrates crystallization, membrane separation, and heat recovery into the same container, reducing the equipment footprint and external pipeline connections.

[0043] The outer shell 31 has a double-layer jacket structure, with the gap between the inner and outer cylinder walls forming a heat exchange channel; this jacket gap is 20mm to 50mm; the upper part of the outer shell 31 is provided with a jacket inlet pipe 311, and the lower part is provided with a jacket outlet pipe 312, both of which are connected to the jacket gap; the dilute phosphoric acid flowing through the jacket can absorb the dilution heat released by the dilution of sulfuric acid in the annular crystallization zone, realizing in-situ heat recovery and reducing external preheating energy consumption. The inner and outer cylinder walls are both made of 316L stainless steel, and the outer wall of the outer shell 31 is provided with a heat insulation layer made of heat insulation material: foam;

[0044] The upper sidewall of the annular crystallization zone 32 is equipped with a nanofiltration concentrate inlet pipe 321 and a sulfuric acid inlet pipe 322. The nanofiltration concentrate inlet pipe 321 is located above the sulfuric acid inlet pipe 322, with an axial distance of 100mm to 200mm between them. This distance ensures that the two fluids enter tangentially at different axial heights, which is beneficial for uniform mixing and the formation of a stable spiral flow field. The nanofiltration concentrate inlet pipe 321 and the sulfuric acid inlet pipe 322 each extend into the annular crystallization zone 32 after passing through the outer and inner cylinder walls of the outer shell 31, respectively. The inlets of both pipes are beveled at 45°. The beveled design allows the fluid to enter tangentially along the annular space, directly driving the liquid flow to rotate without the need for an additional water distribution device. A spiral scraper assembly 35 is provided inside the annular crystallization zone 32.

[0045] The spiral scraper assembly 35 includes 3 to 5 spiral plates 351, two fixed rings 352, multiple balls 353, two linkage rings 354, and multiple blades 355; the spiral plates 351 have a pitch of 150 mm to 300 mm, an inclination angle of 15° to 30°, are made of 316L stainless steel, and have a polytetrafluoroethylene coating on their surface; the spiral plates guide the liquid flow in an axial spiral flow, increasing the crystallization residence time, while their surface coating can reduce crystal adhesion and reduce scaling. The spiral plate 351 is welded and fixed to two fixed rings 352 at both ends. An annular groove is formed on the outer end face of the fixed ring 352. A portion of the ball bearing 353 is embedded in the annular groove, and the other portion extends out from the groove opening, making rolling contact with the inner wall of the bottom end face of the outer shell 31 or the lower surface of the flange cover plate 36. The ball bearing 353 is made of zirconia ceramic or silicon carbide ceramic. Zirconia ceramic balls are corrosion-resistant in phosphoric acid and calcium sulfate crystal slurries, and the rolling contact method significantly reduces rotational resistance, ensuring long-term flexible rotation of the scraper assembly. The spiral scraper assembly 35 is supported within the annular crystallization zone 32 by the ball bearing 353 and can rotate around the axis of the concentric reactor 3. Two linkage rings 354 are welded and fixed to the outer edge of the spiral plate 351, and their axial positions are aligned with the nanofiltration concentrate inlet pipe 321 and the sulfuric acid inlet pipe 322, respectively. The linkage rings enable the blades to rotate synchronously with the spiral plate, ensuring that the liquid flow impact force is effectively transmitted to the entire spiral structure. The blade 355 is a rectangular thin plate made of 316L stainless steel with a polytetrafluoroethylene coating. Multiple blades 355 are evenly distributed on the outer wall of the linkage ring 354. One end of each blade 355 is welded and fixed to the outer wall of the linkage ring 354, and the other end extends towards the inner wall of the annular crystallization zone 32. The plate surface direction of the blade 355 forms an acute angle with the radial direction of the linkage ring 354. The blade generates rotational torque due to the impact of the liquid flow, converting the fluid kinetic energy into mechanical rotation. No external drive motor is required, reducing operating power consumption.

[0046] Membrane element 33 is disposed at the center of the annular crystallization zone 32. It is either a spiral wound membrane element or a hollow fiber membrane element, selected from either a layer-by-layer self-assembled acid-resistant nanofiltration membrane or an acid-resistant reverse osmosis membrane. The membrane element is directly immersed in the crystallization zone, allowing for simultaneous solid-liquid separation during the crystallization process, thereby improving treatment efficiency. Membrane element 33 covers the outer wall of water collection pipe 34. The outer wall of water collection pipe 34 has a mesh-like structure with multiple through holes. The bottom end of water collection pipe 34 passes through the bottom of outer shell 31 and extends outside the outer shell 31, with its bottom end forming the permeate outlet pipe 341.

[0047] The gap between the outer surface of the membrane element 33 and the inner edge of the spiral plate 351 is 5 mm to 15 mm, and the gap between the inner wall of the annular crystallization zone 32 and the outer edge of the spiral plate 351 is 5 mm to 15 mm. This gap serves two purposes: firstly, it prevents rigid contact between the spiral plate and the membrane element and the inner wall of the outer casing during rotation, protecting the membrane element from mechanical damage and ensuring smooth operation of the rotating assembly; secondly, this gap forms a high-velocity slit shear zone at the edge of the spiral plate, effectively disrupting the laminar boundary layer near the wall and generating strong turbulent scouring action on the wall, thereby peeling off and flushing away unattached crystal nuclei and microcrystals back into the main fluid, achieving non-contact dynamic scale prevention and self-cleaning. Simultaneously, this gap serves as a channel for the crystal's size-based classification, allowing large calcium sulfate crystals subjected to centrifugal force to smoothly pass through the outer gap area and settle downwards, preventing crystal blockage and breakage.

[0048] The bottom of the annular crystallization zone 32 is equipped with a crystal slurry outlet pipe 323 and a drain outlet pipe 324. The crystal slurry outlet pipe 323 is located at the bottom of the concentric reactor 3 and is arranged tangentially, with one end connected to the bottom of the annular crystallization zone 32. The tangential arrangement of the crystal slurry outlet facilitates the smooth discharge of high-concentration crystal slurry in the spiral flow, preventing blockage. The drain outlet pipe 324 is located at the lowest point of the bottom of the concentric reactor 3, with one end connected to the lowest point of the bottom of the annular crystallization zone 32, and the other end equipped with a valve. The drain outlet is used to completely empty the reactor when the reactor is shut down, facilitating maintenance and cleaning.

[0049] The reverse osmosis pre-concentration unit 4 is equipped with an acid-resistant reverse osmosis membrane module, which is a spiral wound membrane module; the reverse osmosis pre-concentration unit 4 is equipped with a concentrate outlet and a freshwater outlet;

[0050] The thermal evaporation concentration unit 5 is an MVR evaporator, which includes an evaporation chamber, a compressor, a heating chamber, and a condensate outlet. The MVR evaporator utilizes a secondary steam compression cycle, which can significantly reduce external steam consumption and lower the operating costs of evaporation concentration.

[0051] The solid-liquid separation unit 6 is a horizontal screw centrifuge, which includes a rotating drum, a screw conveyor, and a differential gear. The horizontal screw centrifuge can continuously separate crystal slurry to obtain high-purity calcium sulfate crystal products and achieve efficient separation of liquid and solid phases.

[0052] The recovery unit 7 is any one of a rotary pressure exchanger, a hydraulic turbine, or a twin-cylinder piston pressure exchanger; this recovery unit can recover the pressure energy of the reverse osmosis concentrate and feed it back to the nanofiltration feed, reducing the power consumption of the high-pressure pump.

[0053] The flushing unit 8 is a pressure storage tank with a pulse control valve, including a tank body, a pulse control valve and a pressure regulator; the pulse flushing method can generate a high-impact backwash wave in a short time, effectively removing contaminants from the membrane surface and restoring membrane flux.

[0054] The piping connections between the units are as follows:

[0055] The outlet of pretreatment unit 1 is connected to the jacket inlet pipe 311 via a pipeline; the jacket outlet pipe 312 is connected to the inlet of nanofiltration purification unit 2 via a pipeline; this allows dilute phosphoric acid to first flow through the jacket to absorb dilution heat before entering the nanofiltration unit, achieving cascade utilization of heat. The concentrate outlet of nanofiltration purification unit 2 is connected to the nanofiltration concentrate inlet pipe 321 via a pipeline; the permeate outlet of nanofiltration purification unit 2 is connected to the inlet of reverse osmosis pre-concentration unit 4 via a pipeline; the permeate outlet pipe 341 is connected to the feed side pipeline of nanofiltration purification unit 2 via a pipeline, and this connecting pipeline is equipped with a one-way valve, with the flow direction of the one-way valve being from the permeate outlet pipe 341 to the feed side of nanofiltration purification unit 2; by returning the permeate from the concentric reactor to the nanofiltration inlet, phosphoric acid can be recovered, improving the overall process yield.

[0056] The outlet of the concentrate from the reverse osmosis pre-concentration unit 4 is connected to the inlet of the thermal evaporation concentration unit 5 via a pipeline, and a three-way valve is installed on this pipeline. One outlet of the three-way valve is connected to the inlet of the recovery unit 7 via a pipeline. The outlet of the recovery unit 7 is connected to the feed side of the nanofiltration purification unit 2 via a pipeline, and a one-way valve is installed on this connecting pipeline. The flow direction of the one-way valve is from the outlet of the recovery unit 7 to the feed side of the nanofiltration purification unit 2. By diverting the flow through the three-way valve, the proportion of concentrate entering the thermal evaporation and pressure energy recovery processes can be flexibly adjusted to achieve an optimized balance between energy consumption and concentration. The freshwater outlet of the reverse osmosis pre-concentration unit 4 is connected to the inlet of the flushing unit 8 via a pipeline. The outlet of the flushing unit 8 is connected to the permeate outlet pipe 341 via a pipeline, and a pulse control valve is installed on this pipeline. Using reverse osmosis permeate as the flushing water source eliminates the need for additional external water, and the water quality is excellent, avoiding secondary pollution.

[0057] The crystal slurry outlet pipe 323 of the concentric reactor 3 is connected to the inlet of the solid-liquid separation unit 6 via a pipeline; the supernatant outlet of the solid-liquid separation unit 6 is connected to the feed side pipeline of the nanofiltration purification unit 2 via a pipeline, and a three-way valve is installed on the pipeline. One outlet of the three-way valve is connected to the outside of the system via a pipeline to form a discharge branch, and the other outlet is connected to the feed side pipeline of the nanofiltration purification unit 2 via a pipeline; most of the supernatant is recycled to improve the utilization rate, and a small portion of the discharge can remove the impurity ions accumulated in the system to maintain the stable operation of the system.

[0058] Example 2:

[0059] The concentration of wet-process dilute phosphoric acid using the dilute phosphoric acid concentration apparatus as described in Example 1 includes the following steps:

[0060] Step S1: Preprocessing

[0061] Wet-process dilute phosphoric acid is fed into pretreatment unit 1, first passing through a multi-media filter, then sequentially through anthracite, quartz sand, and garnet filter layers to remove suspended solids. This process removes most particulate impurities, preventing them from clogging subsequent ultrafiltration and nanofiltration membranes. It then enters the ultrafiltration membrane module to remove colloids; ultrafiltration further removes fine colloids, reducing membrane fouling tendency. The pretreated dilute phosphoric acid is discharged from the ultrafiltration membrane module outlet.

[0062] Step S2: Nanofiltration purification

[0063] Pretreated dilute phosphoric acid is fed from the outlet of pretreatment unit 1 into the jacket inlet pipe 311 of the outer shell 31 via a pipeline. Within the jacket gap, it flows axially downwards along the outer shell 31, absorbing the heat of sulfuric acid dilution conducted through the inner cylinder wall from the annular crystallization zone 32, raising its temperature by 10°C to 20°C. This heat recovery process requires no additional energy, directly utilizing the heat released during crystallization, significantly reducing steam consumption for feed heating. The absorbed dilute phosphoric acid flows out from the jacket outlet pipe 312 and is then fed into the inlet of nanofiltration purification unit 2 via a pipeline.

[0064] Under operating pressures of 5 to 15 bar, dilute phosphoric acid flows through the surface of an acid-resistant nanofiltration membrane module. H3PO4 molecules and water molecules permeate through the membrane to form the nanofiltration permeate, while Ca... ²+ Mg ²+ Fe ³+ Al ³+ The retained material forms nanofiltration concentrate; nanofiltration enriches scale-causing cations such as calcium ions in the concentrate, which are then removed as calcium sulfate in the concentric reactor, thereby protecting the downstream reverse osmosis membrane from scaling damage. The nanofiltration permeate is sent from the permeate outlet to the reverse osmosis pre-concentration unit 4, and the nanofiltration concentrate is sent from the concentrate outlet to the nanofiltration concentrate inlet pipe 321 of the concentric reactor 3.

[0065] Step S3: Reverse osmosis pre-concentration and pressure energy recovery

[0066] The nanofiltration permeate enters the reverse osmosis pre-concentration unit 4, flowing through the acid-resistant reverse osmosis membrane surface under operating pressures of 30 to 80 bar. Water molecules permeate through the membrane to form reverse osmosis permeate, while H3PO4 is retained to form reverse osmosis concentrate. Reverse osmosis pre-concentration can increase the phosphoric acid concentration from approximately 5% to 10% of the nanofiltration permeate to 15% to 25%, significantly reducing the subsequent evaporation load. The reverse osmosis concentrate is divided into two parts from the concentrate outlet via a three-way valve: one part is sent to the thermal evaporation concentration unit 5, and the other part is sent to the recovery unit 7. The reverse osmosis permeate is sent from the permeate outlet to the flushing unit 8 for temporary storage.

[0067] The recovery unit 7 recovers the pressure energy released during the depressurization process of the incoming reverse osmosis concentrate and applies it to the reverse osmosis concentrate flowing out of the outlet of the recovery unit 7. The reverse osmosis concentrate after pressure energy recovery flows back to the feed side pipeline of the nanofiltration purification unit 2 through a one-way valve. This pressure energy recovery and utilization can reduce the external energy required by the nanofiltration feed pump, resulting in significant energy saving, especially suitable for high-pressure reverse osmosis operation.

[0068] Step S4: Crystallization and membrane separation in a concentric reactor

[0069] Nanofiltration concentrate enters the annular crystallization zone 32 through a 45° oblique opening of the nanofiltration concentrate inlet pipe 321 along the tangential direction. Sulfuric acid enters the annular crystallization zone 32 through a 45° oblique opening of the sulfuric acid inlet pipe 322 along the tangential direction. The two enter at different axial heights along the tangential direction and mix. The sulfuric acid is diluted and releases heat of dilution. This heat of dilution is transferred through the inner wall of the outer shell 31 to the dilute phosphoric acid in the jacket gap. This step simultaneously completes heat recovery, crystallization precipitation and membrane separation in the same equipment, which simplifies the process and reduces equipment investment.

[0070] The mixture flows spirally downwards along the annular space, and the impact blades 355 drive the spiral scraper assembly 35 to rotate around its axis. As the spiral plate 351 rotates, it disturbs the liquid boundary layer on the inner wall surface and the outer surface of the membrane element 33. This disturbance effectively prevents crystals from adhering to the wall and membrane surfaces, reducing the frequency of manual cleaning and extending the continuous operation cycle. Calcium sulfate crystallizes out of the mixture, and the centrifugal force of the spiral flow causes larger crystals to migrate towards the outer wall of the annular crystallization zone 32 and settle, then be discharged through the bottom crystal slurry outlet pipe 323. Large crystal particles are thrown towards the outer wall by centrifugal force, enabling self-classification of crystals by size and increasing the solid content of the discharged crystal slurry. Smaller crystals migrate towards the inner wall and are carried by the liquid flow to wash the outer surface of the membrane element 33. The washing of the membrane surface by small crystals provides a gentle physical cleaning effect, helping to maintain membrane flux.

[0071] Driven by operating pressure, membrane element 33 separates the liquid phase in the annular crystallization zone 32 into permeate and concentrate. The permeate passes through membrane element 33, enters the interior via water collection pipe 34, and exits from permeate outlet pipe 341. It then returns to the feed side of nanofiltration purification unit 2 via a one-way valve, mixes with preheated dilute phosphoric acid in the jacket, and re-enters nanofiltration purification unit 2. This permeate reflux allows unpermeated phosphoric acid and some dissolved substances to be recycled, improving the overall phosphoric acid recovery rate.

[0072] Step S5: Thermal evaporation and concentration

[0073] The reverse osmosis concentrate fed into the thermal evaporation and concentration unit 5 enters the heating chamber of the MVR evaporator, where it absorbs heat and evaporates to remove water. The secondary steam generated by evaporation is compressed and heated by a compressor and then recycled as a heat source. The MVR technology only requires a small amount of external steam during startup; during normal operation, it relies on compressed secondary steam for heating, resulting in significant energy savings. The concentrated liquid phase is discharged from the heating chamber, yielding a concentrated phosphoric acid product with a concentration of 40% to 54% P2O5. The condensate generated by evaporation is discharged from the condensate outlet.

[0074] Step S6: Solid-liquid separation and discharge

[0075] The slurry discharged from the bottom slurry outlet pipe 323 of the concentric reactor 3 is fed into the rotating drum of the solid-liquid separation unit 6. Under centrifugal force, the solid phase is deposited on the inner wall of the drum and discharged from the crystal product outlet by the screw conveyor, yielding calcium sulfate dihydrate crystals. The separated calcium sulfate crystals can be sold as a by-product, reducing solid waste emissions and improving economic efficiency. The liquid phase is discharged from the supernatant outlet.

[0076] The discharged supernatant is diverted via a three-way valve, with 85% to 95% returned to the feed side of nanofiltration purification unit 2. There, it mixes with pretreated dilute phosphoric acid and re-enters nanofiltration purification unit 2. This recycling of most of the supernatant significantly reduces the total amount of dilute phosphoric acid to be treated, increasing system capacity. 5% to 15% of the supernatant is discharged from the system via a discharge branch. This small portion of discharge effectively removes accumulated impurities such as fluoride ions and iron / aluminum ions, preventing excessive concentrations in the closed-loop cycle that could negatively impact crystallization and membrane separation.

[0077] When the flux of membrane element 33 decreases, the feed pump of nanofiltration purification unit 2 and the high-pressure pump of reverse osmosis pre-concentration unit 4 are shut down. The pulse control valve between flushing unit 8 and permeate outlet pipe 341 is opened, allowing temporarily stored reverse osmosis permeate to enter the water collection pipe 34 in a pulsed manner through permeate outlet pipe 341. This permeate then flows in reverse from the permeate side to the feed side through membrane element 33, backwashing the membrane element 33. The backwashed cleaning solution is discharged through nanofiltration concentrate inlet pipe 321 or crystal slurry outlet pipe 323. The transient pressure difference generated by pulse backwashing effectively removes deposits from the membrane surface, providing better results than conventional continuous backwashing with less water consumption. The pulse control valve opens and closes periodically at set time intervals. Periodic flushing can restore membrane flux without shutting down the system, extending the membrane element replacement cycle and ensuring long-term stable operation of the unit.

[0078] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0079] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A dilute phosphoric acid concentration device, characterized in that, The concentric reactor (3) includes: a shell (31) having a double-layer jacket structure to form a heat exchange channel, wherein the heat exchange channel is configured to preheat the feed of the nanofiltration purification unit (2) using sulfuric acid dilution heat; the shell (31) is provided with a jacket inlet pipe (311) and a jacket outlet pipe (312), and a flange cover plate (36) is sealed on the top of the shell (31); An annular crystallization zone (32) is located inside the outer shell (31) and is an annular space that receives nanofiltration concentrate and sulfuric acid, in which calcium sulfate crystallizes. The annular crystallization zone (32) is provided with a nanofiltration concentrate inlet pipe (321), a sulfuric acid inlet pipe (322), a crystal slurry outlet pipe (323), and a drain outlet pipe (324). The annular crystallization zone (32) is provided with a spiral scraper assembly (35). The spiral scraper assembly (35) guides the liquid flow to form a spiral flow channel in the annular space. At the same time, the centrifugal force of the spiral flow is used to classify the calcium sulfate crystals according to their particle size: large crystals migrate to the outer wall and are discharged through the bottom, while small crystals migrate to the inner wall. The membrane element (33) is located in the center of the annular crystallization zone (32). The membrane element (33) separates the liquid phase into permeate and concentrate. The permeate permeates into the water collection pipe (34) located in the center of the membrane element (33) and is discharged through the permeate outlet pipe (341) fixed to the bottom of the outer shell (31) at the bottom of the water collection pipe (34). Among them, the small crystals and liquid flow that migrate towards the inner wall in the annular crystallization zone (32) continuously wash the outer surface of the membrane element (33) to achieve self-cleaning of the membrane surface; The spiral scraper assembly (35) includes three to five spiral plates (351), two fixing rings (352), multiple balls (353), two linkage rings (354), and multiple blades (355); the two ends of the spiral plates (351) are respectively fixedly connected to the two fixing rings (352), and the outer end face of the fixing rings (352) is provided with an annular groove. The balls (353) are rotatably disposed in the annular groove. The spiral scraper assembly (35) is rotatably supported on the inner wall surface of the annular crystallization zone (32) and the membrane element (353) through the balls (353). Between the outer surfaces of 33); the two linkage rings (354) are respectively fixedly connected to the spiral plate (351), and the positions of the two linkage rings (354) are respectively aligned with the nanofiltration concentrate inlet pipe (321) and the sulfuric acid inlet pipe (322), and the blades (355) are evenly distributed on the outer wall surface of the linkage rings (354); when liquid is fed into the nanofiltration concentrate inlet pipe (321) and the sulfuric acid inlet pipe (322), the blades (355) are pushed to rotate, which drives the spiral plate (351) to rotate around the axis of the concentric reactor (3).

2. The dilute phosphoric acid concentration apparatus according to claim 1, characterized in that, The ball bearing (353) is made of zirconium oxide ceramic or silicon carbide ceramic; the spiral plate (351) is made of 316L stainless steel and coated with polytetrafluoroethylene, with a pitch of 150mm to 300mm and an inclination angle of 15° to 30°.

3. The dilute phosphoric acid concentration apparatus according to claim 1, characterized in that, The upper sidewall of the annular crystallization zone (32) is provided with the nanofiltration concentrate inlet pipe (321) and the sulfuric acid inlet pipe (322). The nanofiltration concentrate inlet pipe (321) is located above the sulfuric acid inlet pipe (322), and the distance between the two is 100mm to 200mm. The pipe ends of the nanofiltration concentrate inlet pipe (321) and the sulfuric acid inlet pipe (322) extend into the annular space and are obliquely cut at 45°, so that the feed enters the annular space along the tangential direction to form a spiral flow.

4. The dilute phosphoric acid concentration apparatus according to claim 1, characterized in that, The membrane element (33) is a spiral wound membrane element or a hollow fiber membrane element, selected from any one of layer-by-layer self-assembled acid-resistant nanofiltration membrane or acid-resistant reverse osmosis membrane; the membrane element (33) covers the outer wall of the water collection pipe (34), the outer wall of the water collection pipe (34) has a mesh structure, and the mesh structure has multiple through holes.

5. The dilute phosphoric acid concentration apparatus according to claim 1, characterized in that, The jacket gap of the outer shell (31) is 20mm to 50mm. The jacket inlet pipe (311) and the jacket outlet pipe (312) are located at the upper and lower parts of the concentric reactor (3), respectively. The crystal slurry outlet pipe (323) is located at the bottom of the concentric reactor (3) and arranged along the tangential direction. The drain outlet pipe (324) is located at the lowest point of the bottom of the concentric reactor (3).

6. The dilute phosphoric acid concentration apparatus according to any one of claims 1 to 5, characterized in that, Also includes: Pretreatment unit (1) receives wet dilute phosphoric acid and removes suspended solids and colloids; The nanofiltration purification unit (2) receives pretreated dilute phosphoric acid and separates it into nanofiltration permeate and nanofiltration concentrate using an acid-resistant nanofiltration membrane. The nanofiltration concentrate is then fed into the nanofiltration concentrate inlet pipe (321) of the concentric reactor (3). The reverse osmosis pre-concentration unit (4) receives the nanofiltration permeate and separates it into reverse osmosis concentrate and reverse osmosis desalinated water using an acid-resistant reverse osmosis membrane; The thermal evaporation and concentration unit (5) receives the reverse osmosis concentrate, evaporates and concentrates it to the target concentration, and obtains concentrated phosphoric acid product. The solid-liquid separation unit (6) receives the crystal slurry discharged from the bottom crystal slurry outlet pipe (323) of the concentric reactor (3) and separates it into calcium sulfate dihydrate crystal product and supernatant. The recovery unit (7) is connected in series with the reverse osmosis concentrate return pipeline to recover the pressure energy during the depressurization process of the reverse osmosis concentrate and use it to increase the feed pressure of the nanofiltration purification unit (2). The flushing unit (8) injects the reverse osmosis fresh water into the permeate side of the membrane element (33) in a pulse manner to perform permeate backwashing cleaning on the membrane element (33); The permeate discharged from the permeate outlet pipe (341) of the concentric reactor (3) is returned to the feed side of the nanofiltration purification unit (2).

7. The dilute phosphoric acid concentration apparatus according to claim 6, characterized in that, The supernatant obtained by the solid-liquid separation unit (6) is returned to the feed side of the nanofiltration purification unit (2) by 85% to 95%, and discharged as a tributary by 5% to 15%; the recovery unit (7) is any one of a rotary pressure exchanger, a hydraulic turbine or a double-cylinder piston pressure exchanger.

8. A dilute phosphoric acid concentration process, using the dilute phosphoric acid concentration apparatus of claim 6, characterized in that, Includes the following steps: Step S1: The wet-process dilute phosphoric acid is pretreated in unit (1) to remove suspended solids and colloids; Step S2: The pretreated dilute phosphoric acid is sent into the nanofiltration purification unit (2) and separated into nanofiltration permeate and nanofiltration concentrate; Step S3: The nanofiltration permeate is fed into the reverse osmosis pre-concentration unit (4) to separate it into reverse osmosis concentrate and reverse osmosis desalinated water; Step S4: The nanofiltration concentrate and sulfuric acid are fed tangentially into the annular crystallization zone (32) of the concentric reactor (3) to form a spiral flow. Calcium sulfate crystallizes in the annular crystallization zone (32) and is separated by the membrane element (33). Large crystals are discharged through the crystal slurry outlet pipe (323) under centrifugal force, and small crystals wash the membrane surface of the membrane element (33). The heat of sulfuric acid dilution is recovered through the jacket of the outer shell (31) and used to preheat the feed of the nanofiltration purification unit (2). Step S5: The reverse osmosis concentrate is sent to the thermal evaporation concentration unit (5) for concentration to obtain concentrated phosphoric acid product; Step S6: The crystal slurry discharged from the bottom of the concentric reactor (3) is sent to the solid-liquid separation unit (6), the separated calcium sulfate dihydrate crystal product is discharged, the supernatant is returned to the feed side of the nanofiltration purification unit (2), and part of the supernatant is discharged from the system as a tributary.

9. The dilute phosphoric acid concentration process according to claim 8, characterized in that, In step S2, the operating pressure of the nanofiltration purification unit (2) is 5 bar to 15 bar; in step S3, the operating pressure of the reverse osmosis pre-concentration unit (4) is 30 bar to 80 bar; in step S4, the sulfuric acid dilution heat raises the feed temperature of the nanofiltration purification unit (2) by 10°C to 20°C through the jacket of the outer shell (31); in step S5, the reverse osmosis concentrate is concentrated to 40% to 54% P2O5 by thermal evaporation.

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