A phosphoric acid extraction hemihydrate-dihydrate alternate crystallization system and method

Abstract

The application discloses a kind of phosphoric acid extraction semi-water-two water alternate crystallization system and method, belong to the technical field of wet-process phosphoric acid production, the system is by the reaction tank unit of series connection and semi-water and two water reaction tank alternate arrangement, with the sulfuric acid dilution mixed heat exchanger group connected with reaction tank, simple heat exchanger group is arranged between tank to realize heat transfer.Construction.The method includes: material is temperature-adjusted after passing through sulfuric acid dilution mixed heat exchanger, in turn into the reaction tank sequence of alternate arrangement, heat circulation is carried out in adjacent tank by simple heat exchanger, under the control of PLC in 75~85 DEG C and 88~95 DEG C interval, realize the alternate crystallization of semi-water-two water calcium sulfate.The application replaces traditional flash evaporation external discharge with heat internal circulation between tank, solves the technical problems of high steam energy consumption, low phosphorus recovery rate, insufficient phosphoric acid concentration and high upgrading cost of stock device in traditional process, and has the comprehensive advantages of energy saving, yield, low-cost transformation and phosphogypsum resource.

Description

Technical Field

[0001] This invention belongs to the field of wet-process phosphoric acid production technology, specifically relating to a phosphoric acid extraction hemihydrate-dihydrate alternating crystallization system and method. Background Technology

[0002] Phosphoric acid extraction, a core step in the wet-process phosphoric acid production, directly impacts the utilization efficiency of phosphorus resources and significantly determines the energy consumption level and quality of the byproduct phosphogypsum. Currently, the industry commonly employs a multi-tank series dihydrate process, where calcium sulfate crystallizes out as a dihydrate (CaSO4·2H2O). Although this method is quite mature in terms of process and relatively stable in operation, a series of significant technical bottlenecks have gradually emerged in long-term industrial operation.

[0003] First, the reaction system for phosphoric acid extraction involves extremely complex heat sources, mainly including six major heat sources: sensible heat from the feed, heat released during the decomposition of phosphate rock, heat generated during the dilution of concentrated sulfuric acid, and latent heat associated with the phase transition during crystallization. Traditional processes often rely on external flash evaporation systems to directly release excess heat into the environment (see the specific process flow diagram). Figure 1 (As shown). This practice not only results in a steam consumption of up to 1.2 t / tP2O5, but also causes drastic fluctuations in the reaction temperature, which have an extremely adverse effect on the stability of the crystallization process.

[0004] Secondly, the single dihydrate crystallization mode tends to form a dense crystal structure during the crystallization process. This structure can encapsulate unreacted phosphate rock and the liquid phase of phosphoric acid, resulting in a phosphorus recovery rate that can only be maintained at 92% to 93%, and the obtained phosphoric acid concentration is relatively low. This phenomenon not only reduces the utilization rate of phosphorus resources, but also significantly increases the energy consumption burden of subsequent phosphoric acid concentration processes, further increasing production costs.

[0005] To overcome the aforementioned shortcomings of the traditional dihydrate process, various improvement schemes have been proposed in this field. For example, patent document CN112897490A discloses a method and system for producing wet-process phosphoric acid through multiple crystallization hemihydrate-dihydrate process (hereinafter referred to as "Prior Art 1"). This method improves product quality by adding a filter feed tank and feeding phosphate rock into the hemihydrate reaction step to consume sulfate. However, this process uses a sequential hemihydrate-dihydrate crystallization mode, which cannot achieve dynamic alternation of the crystallization process. Furthermore, heat management relies on an external flash evaporation system, resulting in high energy consumption, large temperature fluctuations, and phosphorus recovery rate remaining at 94% to 96%, making further breakthroughs difficult. At the same time, this process has poor adaptability to existing equipment modifications and incurs high investment costs.

[0006] Similarly, patent document CN115028148A relates to a method and system for increasing the production capacity of wet-process phosphoric acid (dihydrate-hemihydrate) and co-producing α-gypsum (hereinafter referred to as "Prior Art 2"). This method employs a recrystallization process involving secondary reaction and secondary filtration. While it can increase production capacity and produce α-gypsum as a byproduct, the process is lengthy, the equipment is complex, and heat recovery is insufficient, with steam consumption exceeding 1.0 t / tP2O5, making internal heat energy recycling impossible. Patent document CN107840317A provides a one-step wet-process phosphoric acid production process (hereinafter referred to as "Prior Art 3"). This process directly produces hemihydrate gypsum through leaching reaction and conversion crystal growth, but it relies on flash cooling for heat removal, resulting in low energy efficiency. Furthermore, the crystallization process is unstable, with temperature fluctuations exceeding ±5℃, affecting product consistency. Patent document CN111807340A discloses a hemihydrate-dihydrate phosphoric acid production apparatus and method (hereinafter referred to as "Prior Art 4"). It uses an integrated tank to integrate the reaction and crystal growth steps, but it is still a sequential process and cannot achieve the alternating cycle of hemihydrate and dihydrate crystallization. The heat utilization is not optimized and the improvement of phosphorus recovery rate is limited.

[0007] It can be seen that the common defects of the above-mentioned existing technologies 1 to 4 include: (1) the crystallization process is mostly sequential (hemihydrate first then dihydrate or dihydrate first then hemihydrate), which cannot achieve periodic reconstruction of crystals, making it difficult to further improve the phosphorus recovery rate and the phosphoric acid concentration is low; (2) heat management depends on an external flash evaporation system, a large amount of heat energy is directly discharged, steam consumption is high, steam consumption is as high as 1.0-1.2 t / tP2O5, and the temperature fluctuation is large, exceeding ±5℃, which affects the crystallization stability; (3) the system modification or new construction costs are high, and it is difficult to apply it economically to existing equipment.

[0008] Furthermore, to improve the efficiency of phosphoric acid extraction, if a hemihydrate-dihydrate alternating crystallization process is considered, a new dedicated reactor must be built, or the existing hexagonal or nonagonal grid series tank layout must be redesigned. However, such retrofitting projects often require investments of up to several billion yuan, making economically feasible upgrades almost impossible for existing facilities.

[0009] The aforementioned problems collectively hinder the advancement of phosphoric acid extraction technology towards higher efficiency, energy conservation, and green development. Therefore, a new technological approach is urgently needed. This approach should be able to precisely control the heat distribution between tanks and achieve dynamic alternation between hemihydrate and dihydrate crystallization processes without large-scale equipment modifications. This would effectively solve many bottleneck problems in the existing process and promote the continuous optimization and upgrading of phosphoric acid extraction technology. Summary of the Invention

[0010] The purpose of this invention is to provide a phosphoric acid extraction hemihydrate-dihydrate alternating crystallization system and method, which solves the problems of high energy consumption, low phosphorus recovery rate, large temperature fluctuation and high modification cost that cannot be overcome by existing technologies.

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] A phosphoric acid extraction hemihydrate-dihydrate alternating crystallization system includes:

[0013] The alternating reaction tank unit arranged in series consists of three or more reaction tanks connected in series. The reaction tanks start with a dihydrate reaction tank and are arranged alternately with a semihydrate reaction tank. The temperature of the dihydrate reaction tank is controlled at 75℃~85℃, and the temperature of the semihydrate reaction tank is controlled at 88℃~95℃.

[0014] The sulfuric acid dilution mixing heat exchanger group consists of at least one sulfuric acid dilution mixing heat exchanger connected to the alternating reaction tank unit, used to mix and exchange heat between phosphate rock slurry, 98% concentrated sulfuric acid and return acid, and control the outlet temperature between 40°C and 60°C.

[0015] A simple heat exchanger group and a sulfuric acid dilution mixing heat exchanger group are both connected to an alternating reaction tank unit. The simple heat exchanger group includes at least one simple heat exchanger, which is disposed between adjacent reaction tanks of the alternating reaction tank unit.

[0016] The feeding unit includes a phosphate rock slurry pipeline, a concentrated sulfuric acid pipeline, and a return acid pipeline, all of which are connected to the feed inlet of the sulfuric acid dilution and mixing heat exchanger. The simple heat exchanger is connected to the return acid pipeline through a pipeline and is used to introduce the heat after heat exchange into the return acid circulation system.

[0017] Furthermore, the present invention also includes a temperature control unit, which includes a temperature sensor, an electromagnetic flow meter, and a PLC control cabinet arranged at the outlet of each reaction tank. The signal output terminals of the temperature sensor and the electromagnetic flow meter are both electrically connected to the PLC control cabinet. The control signal output terminal of the PLC control cabinet is simultaneously electrically connected to the sulfuric acid dilution mixing heat exchanger group and the simple heat exchanger group. The temperature control unit is used to dynamically adjust the valve opening of each heat exchanger to maintain the system temperature fluctuation within ±2℃.

[0018] Specifically, the simple heat exchanger uses ceramic heat exchange tubes, with high-temperature slurry flowing through the tube side and low-temperature slurry flowing through the shell side. The heat exchange area is matched at a ratio of 1:1.2 to 1.5 of the capacity of a single reaction tank.

[0019] Furthermore, the sulfuric acid dilution mixing heat exchanger group includes multiple sulfuric acid dilution mixing heat exchangers, which are respectively connected to the feed ends of multiple reaction tanks in the alternating reaction tank unit, and the sulfuric acid dilution mixing heat exchangers are connected in series through pipelines.

[0020] Based on the above system, the present invention also provides a method for alternating crystallization of hemihydrate and dihydrate by phosphoric acid extraction, comprising the following steps:

[0021] Step S1: The phosphate rock slurry, 98% concentrated sulfuric acid and back acid are fed into the sulfuric acid dilution mixing heat exchanger for mixing and preliminary heat exchange, and the outlet temperature of the sulfuric acid dilution mixing heat exchanger is controlled at 40℃~60℃.

[0022] Step S2: The slurry treated in step S1 is fed into a dihydrate reaction tank and subjected to calcium sulfate dihydrate crystallization reaction at 75℃~85℃ for 2 h~3 h.

[0023] Step S3: The material from the dihydrate reaction tank is heated by a simple heat exchanger group and then sent to the hemihydrate reaction tank for hemihydrate calcium sulfate crystallization reaction at 88℃~95℃.

[0024] Step S4: The material from the semi-aqueous reaction tank is cooled by a simple heat exchanger and then sent to the next dihydrate reaction tank for secondary dihydrate crystallization at 75℃~85℃.

[0025] Step S5: Heat transfer and temperature control between adjacent reaction tanks in the reaction tank unit are achieved sequentially through subsequent simple heat exchangers, forming a hemihydrate-dihydrate alternating crystallization sequence.

[0026] Preferably, in step S1, the molar ratio of phosphate rock slurry to concentrated sulfuric acid is controlled at 1:1.02 to 1.05, and the amount of acid added is 30% to 40% of the total feed.

[0027] Preferably, in step S3, the heat exchange area of ​​the simple heat exchanger is matched at a ratio of 1:1.2 to 1.5 of the reaction tank capacity.

[0028] In the above process, the operating conditions of each reaction tank are monitored in real time by temperature sensors and electromagnetic flow meters, and the opening degree of each heat exchanger is dynamically adjusted by the PLC control cabinet.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) This invention completely abandons the high-energy-consuming mode of relying on the external heat discharge of the traditional flash evaporation system. It constructs an efficient internal heat circulation network through a "sulfuric acid dilution mixing heat exchanger + simple heat exchanger group", and precisely controls the outlet temperature of the sulfuric acid dilution mixing heat exchanger at 40-60℃ and matches the heat exchange area and reaction tank capacity in a specific ratio of 1:1.2-1.5, realizing the accurate identification and efficient reuse of the six heat sources inside the reaction system. This structure not only realizes the multi-level coupling and cascade utilization of thermal energy, but also significantly improves the thermal management accuracy and energy utilization efficiency of the system. This design eliminates the steam consumption of up to 1.2 t / tP2O5 in the traditional wet phosphoric acid process, and at the same time greatly reduces the cooling water consumption and the operating load of the supporting equipment. More importantly, this invention transforms the reaction system from an "energy-consuming heat dissipation" system that originally needed to continuously rely on external energy supply and had large fluctuations in thermodynamic state into an "energy-saving heat preservation" system that can maintain thermal balance autonomously and has good stability. Compared with existing technologies, this invention avoids energy loss in flash evaporation systems by using internal heat circulation and parameter settings, thus significantly reducing energy consumption and providing technical support for the industry to promote green and low-carbon production.

[0031] (2) This invention drives the periodic reconstruction of calcium sulfate crystals between two crystal forms by using a forced alternating hemihydrate and dihydrate crystallization sequence and controlling the temperature of the dihydrate reaction tank within a specific range of 75–85°C and the hemihydrate reaction tank within a specific range of 88–95°C. This process forms a controllable crystal phase transition cycle in the reaction system, causing the crystal structure to repeatedly switch between α-hemihydrate and dihydrate forms. This dynamic conversion process can effectively break down and peel off the unreacted phosphate rock and phosphoric acid liquid phase encapsulated in the crystals of the traditional dihydrate method, significantly increasing the solid-liquid reaction interface, promoting further dissolution of phosphorus, and thus increasing the phosphorus recovery rate to over 98%. At the same time, since the hemihydrate crystallization stage can be carried out stably at higher temperatures, the system can operate in a higher temperature range, directly producing higher concentrations of phosphoric acid, avoiding the low thermal efficiency problem caused by large-scale low-temperature dilution in the traditional process, and greatly reducing the energy consumption and equipment load of the subsequent concentration process. This not only reduces steam consumption and equipment investment, but also shortens the process flow, achieving full-process cost optimization from extraction source to product output. Compared with the sequential crystallization mode of the prior art 1, the alternating crystallization sequence and specific temperature control of the present invention realize the periodic reconstruction of crystals, solving the problem that the prior art 1 cannot further improve the phosphorus recovery rate.

[0032] (3) This invention does not require large-scale structural modifications to existing phosphoric acid production facilities, nor does it overturn the currently prevalent multi-tank series layout. Furthermore, it avoids the high costs and time pressure associated with investing hundreds of millions of yuan in building new dedicated reactors. This invention only requires adding a series of high-efficiency heat exchangers to the existing pipeline system connecting the reaction tanks and simultaneously upgrading the existing control system to intelligently transform the traditional single dihydrate phosphoric acid production process into an advanced hemihydrate-dihydrate alternating crystallization system. This invention not only makes extensive use of existing equipment, greatly shortening the modification cycle, but also keeps the overall investment cost at a very low proportion of the cost of traditional demolition and reconstruction or new route modification. Therefore, compared with the high investment and modification costs of existing technologies 1-4, this invention provides a practical, low-investment, high-return upgrade path for large-scale existing production facilities in the industry that urgently need technological updates, possessing outstanding promotional value and practical significance.

[0033] (4) The present invention strictly controls the temperature fluctuation of the reaction system within a high-precision range of ±2℃. This near-constant temperature process environment is the key to ensuring efficient and directional conversion of hemihydrate and dihydrate crystallization, significantly improving the crystallization success rate and product quality consistency, and providing a solid guarantee for the long-term and stable operation of the system.

[0034] (5) Thanks to the effective improvement of crystal morphology and purity achieved by the alternating crystallization process, the phosphogypsum produced by the present invention has fewer impurities and a more regular crystal structure, resulting in a fundamental improvement in its quality. It can be directly used as a high-quality building mortar raw material. Therefore, compared with the phosphogypsum quality problems of prior art 2 and prior art 4, the present invention provides a more efficient resource utilization solution. It changes the predicament of phosphogypsum being stockpiled and landfilled as industrial solid waste, not only solving the environmental protection problem but also opening up new economic growth points, and powerfully promoting the green and sustainable development of the phosphate chemical industry. Attached Figure Description

[0035] Figure 1 This is a process flow diagram of a traditional phosphoric acid extraction system that uses a flash evaporation system.

[0036] Figure 2 This is a system process flow diagram in Embodiment 1 of the present invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments and accompanying drawings.

[0038] Example 1

[0039] See Figure 2The phosphoric acid extraction hemihydrate-dihydrate alternating crystallization system optimizes the acidolysis and crystallization process of phosphate rock through alternating reaction tank units arranged in series. This system features highly efficient heat utilization and crystal control characteristics. Specifically, it includes alternating reaction tank units, sulfuric acid dilution mixing heat exchanger groups, simple heat exchanger groups, a feeding unit, and a temperature control unit. These units are tightly connected by pipes and valves, forming a complete reaction and heat circulation system. The alternating reaction tank unit consists of six reaction tanks connected in series, alternating between dihydrate and hemihydrate reaction tanks. Reaction tanks 1, 3, and 5 are dihydrate reaction tanks, with a controlled temperature of 75–85℃, and the tank bodies are made of 316L stainless steel with a silicon carbide wear-resistant lining. Reaction tanks 2, 4, and 6 are hemihydrate reaction tanks, with a controlled temperature of 88–95℃, and the tank structure is designed as a flat-bottomed, jacketed form. The inner wall of the dihydrate reaction tank is equipped with a guide plate structure to extend the residence time to 2-3 hours by changing the slurry flow path, ensuring sufficient growth of calcium sulfate dihydrate crystals. The guide plates are spirally distributed with an inclination angle of 45°-60°. A mechanical agitator is installed at the bottom of the hemihydrate reaction tank, with the rotation speed controlled at 60-80 r / min. The shear force generated by the rotating blades enhances the crystal suspension and mass transfer process, preventing crystal deposition and agglomeration. The agitator is a double-layered inclined blade turbine type.

[0040] The sulfuric acid dilution and mixing heat exchanger assembly includes three heat exchangers: No. 1, No. 2, and No. 3, which are connected to the feed ends of reaction tanks No. 1, No. 2, and No. 3, respectively. These heat exchangers are connected in series via pipelines. The sulfuric acid dilution and mixing heat exchanger integrates a dual-function structure of mixing and heat exchange chambers. Concentrated sulfuric acid, back acid, and slurry enter the mixing chamber through a tangential inlet, achieving molecular-level mixing under turbulent conditions. The mixing intensity can be adjusted by the inlet pressure. The heat exchange chamber adopts a shell-and-tube structure. The mixed slurry flows through the tube side, while cooling water or a heat transfer medium is introduced into the shell side for temperature regulation. The outlet temperature is precisely controlled within the range of 40–60°C by regulating the medium flow rate. The tube-side design pressure is 0.6 MPa, and the shell-side design pressure is 1.0 MPa. A pressure balancing device, including a two-way pressure relief valve and a buffer chamber, is installed between the mixing chamber and the heat exchange chamber to ensure material flow stability and prevent cavitation and water hammer.

[0041] The simple heat exchanger group includes three simple heat exchangers: No. 1, No. 2, and No. 3, which are respectively located between reaction tanks No. 3 and No. 4, between reaction tanks No. 4 and No. 5, and between reaction tanks No. 5 and No. 6, forming a cascaded heat exchange network. The simple heat exchangers use ceramic heat exchange tubes with a wall thickness of 3 mm, capable of withstanding strongly acidic environments with pH values ​​of 0.5 to 3.0. High-temperature slurry releases heat as it flows through the tube side, while low-temperature slurry absorbs heat as it flows through the shell side, achieving bidirectional heat transfer between the semi-aqueous and diaqueous reaction tanks. The heat exchange process adopts a counter-current arrangement. The heat exchange area is matched to the reaction tank capacity at a ratio of 1:1.2 to 1.5, based on the effective volume of the reaction tank (m³). 3 ) and heat exchange area (m 2 The ratio of heat transfer efficiency to surface temperature is carefully controlled to ensure a heat transfer efficiency of 92%–95%. Each heat exchanger is equipped with a manhole and cleaning port for easy regular descaling and maintenance. Temperature monitoring points are installed at the inlet and outlet of the heat exchanger, using dual platinum resistance thermometers for redundant measurement, providing real-time feedback on heat transfer performance with a data refresh cycle of 5 seconds.

[0042] The feeding unit includes pipelines for phosphate rock slurry from the boundary area, 98% concentrated sulfuric acid, and a 40–60℃ return acid pipeline, which are connected to the A, B, and C inlets of sulfuric acid dilution and mixing heat exchangers No. 1, 2, and 3, respectively. Each pipeline is equipped with a filter, check valve, and safety valve. The solid content of the phosphate rock slurry is controlled at 65%–70%, and the particle size distribution requires 80% to pass through a 200-mesh sieve. The conveying pump is a PTFE-lined centrifugal pump, with a flow rate fluctuation range not exceeding ±5%. The concentrated sulfuric acid concentration is maintained at 98% ± 0.5%, the conveying pressure is 0.3–0.5 MPa, and the pipeline heating temperature is maintained at 50–60℃ to prevent crystallization. The return acid is system-produced phosphoric acid, with a concentration of 40% ± 2%, and the temperature is controlled within the range of 40–60℃, and it is circulated and conveyed by a titanium pump. The flow ratio of the three materials is precisely controlled by a metering pump. The molar ratio of phosphate rock slurry to concentrated sulfuric acid is controlled at 1:1.02 to 1.05. The amount of acid return added is 30% to 40% of the total feed. Each flow signal is connected to the DCS system for ratio adjustment.

[0043] The temperature control unit comprises temperature sensors, electromagnetic flow meters, and a PLC control cabinet located at the outlets of each reaction tank, forming a distributed control system. The temperature sensors utilize platinum resistance temperature sensing elements, with a measurement range of 0–150℃ and an accuracy of ±0.5℃. They are installed at the center of the discharge pipe of the reaction tank, with an insertion depth of 1 / 3 to 1 / 2 of the pipe diameter. The electromagnetic flow meters are corrosion-resistant, with a measurement error ≤1.5%. Flange connections ensure sealing performance, and the inner lining is made of perfluoroethylene propylene material. The signal output terminals of both the temperature sensors and electromagnetic flow meters are electrically connected to the PLC control cabinet. Signal transmission uses a standard 4–20 mA current signal, and the cable shielding level reaches IP67. The control signal output terminals of the PLC control cabinet are electrically connected to the regulating valves of each heat exchanger. The control signal is a pulse width modulation signal, and the regulating valves employ intelligent electric actuators with a positioning accuracy of 0.5%.

[0044] The system connections are as follows: Reactors 1-6 are connected in series via pipelines; three sulfuric acid dilution mixing heat exchangers and three simple heat exchangers are connected in series via pipelines; sulfuric acid dilution mixing heat exchangers 1, 2, and 3 are connected to reactors 1, 2, and 3 via pipelines, respectively; simple heat exchangers 1, 2, and 3 are located between reactors 3 and 4, between reactors 4 and 5, and between reactors 5 and 6, respectively; reactor 1 is a dihydrate reactor, and reactor 2 is a hemihydrate reactor. The No. 3 reaction tank is a dihydrate reaction tank, and the specific type is determined according to the alternating arrangement rule. The feed ends of the No. 1, 2, and 3 reaction tanks are all connected to a sulfuric acid dilution mixing heat exchanger. The phosphate rock slurry pipeline, the 98% concentrated sulfuric acid pipeline, and the 40-60℃ acid return pipeline are connected to the A, B, and C feed ports of the No. 1, 2, and 3 sulfuric acid dilution mixing heat exchangers, respectively. The No. 1, 2, and 3 simple heat exchangers are connected to the acid return pipeline through pipelines. The three simple heat exchangers can exchange a small amount of heat with the external environment through pipelines.

[0045] In this embodiment, the specific steps of the phosphoric acid extraction hemihydrate-dihydrate alternating crystallization method are as follows:

[0046] In step S1, the phosphate rock slurry, 98% concentrated sulfuric acid, and 40–60°C return acid are respectively introduced into sulfuric acid dilution mixing heat exchangers No. 1, 2, and 3 for mixing and preliminary heat exchange. The phosphate rock slurry is delivered by a PTFE-lined centrifugal pump at a flow rate of 12–15 m³ / h, with the solid content controlled at 65%–70% and the inlet pressure at 0.3–0.5 MPa. The 98% concentrated sulfuric acid is injected by a metering pump at a flow rate of 8–10 m³ / h, with a pump stroke adjustment accuracy of 0.1%, and the pipeline heating temperature is maintained at 50–60°C. The return acid is added by a titanium circulating pump at a flow rate of 15–20 m³ / h, with the addition amount being 30%–40% of the total feed volume, and a circulation return branch is set in the pipeline to stabilize the flow rate. The three materials form turbulent mixing in the mixing chamber through the tangential inlet, with the mixing time controlled at 45–60 s, the pressure loss maintained at 0.05–0.08 MPa, and the mixing intensity adjusted by the inlet pressure. The mixture then enters the heat exchange chamber. The mixed slurry flows through the tube side, while cooling water or a heat transfer medium flows through the shell side. The cooling water inlet temperature is 25–30℃, and the outlet temperature is 40–45℃. The medium flow rate is controlled by adjusting the valve opening, precisely controlling the outlet temperature of the mixed slurry within the range of 40–60℃, with a temperature control deviation not exceeding ±1.5℃. The sulfuric acid dilution mixing heat exchanger is designed with a tube side pressure of 0.6 MPa and a shell side pressure of 1.0 MPa. A two-way pressure relief valve and a buffer chamber are installed between the mixing chamber and the heat exchange chamber to balance the pressure.

[0047] In step S2, the slurry treated by the No. 1 sulfuric acid dilution and mixing heat exchanger is fed into the No. 1 dihydrate reaction tank for calcium sulfate dihydrate crystallization at 75–85℃. The effective volume of the reaction tank is 50–60 m³. The residence time of the slurry is extended to 2–3 h by using a spiral guide plate (tilt angle 45°–60°). The liquid level is controlled by adjusting the overflow weir plate, and the slurry flow rate is maintained at 0.3–0.5 m / s. During the reaction, the pH value of the slurry is monitored and maintained at 1.5–2.0, with real-time feedback provided by an online pH meter with a combination of antimony electrode and reference electrode. The growth rate of calcium sulfate dihydrate crystals is controlled at 0.5–1.0 mm / h, and the crystal morphology is needle-like or plate-like, with an average aspect ratio maintained between 3:1 and 5:1. The temperature of the slurry at the reaction tank outlet is monitored by a temperature sensor, and the signal is transmitted to the PLC control cabinet to ensure that the temperature fluctuation does not exceed ±2℃.

[0048] In step S3, the effluent from tank 1 (dihydrate) is heated by a sulfuric acid dilution and mixing heat exchanger (tank 2) and then fed into tank 2 (hemihydrate). The hemihydrate calcium sulfate crystallization reaction takes place at 88–95°C. During the heating process in the heat exchanger, the high-temperature slurry flows through the tube side, while the heat medium is introduced through the shell side. The inlet temperature is 75–85°C, and the outlet temperature is 88–92°C, with a temperature rise of 13–17°C. The effective volume of the hemihydrate reaction tank is the same as that of tank 1. A double-layered inclined-blade turbine mechanical agitator is installed at the bottom, with the rotation speed controlled at 60–80 r / min. The shear force generated by the rotating blades enhances the crystal suspension and mass transfer process, preventing crystal deposition and agglomeration. During the reaction, the solid content of the slurry is monitored and maintained at 25%–30%. The crystal suspension is maintained by adjusting the stirring rate, and the average particle size of the hemihydrate calcium sulfate crystals is controlled at 50–80 μm.

[0049] In step S4, the effluent from the No. 2 semi-aqueous reaction tank is cooled by the No. 2 sulfuric acid dilution and mixing heat exchanger and then fed into the No. 3 diaqueous reaction tank for secondary diaqueous crystallization at 75–85°C. The heat exchanger uses ceramic tubes (3 mm wall thickness). The tube side flows with the high-temperature slurry (inlet temperature 88–95°C, outlet temperature 78–85°C, temperature drop 10–15°C), while the shell side flows with the low-temperature slurry (inlet temperature 40–60°C, outlet temperature 70–80°C, temperature rise 30–35°C). The heat exchange area is matched to the tank capacity at a ratio of 1:1.2–1.5 (approximately 60–75 m²), and a counter-current arrangement is used, achieving a heat transfer efficiency of 92%–95%. The No. 3 diaqueous reaction tank has the same structure as the No. 1 tank, but the residence time is extended to 2–3 hours via guide plates to ensure sufficient growth of calcium sulfate dihydrate crystals, achieving a crystal conversion rate of over 98% and controlling the free water content at 28%–32%.

[0050] Step S5 involves sequentially using subsequent simple heat exchangers to achieve heat transfer and temperature control between adjacent reaction tanks in the reaction tank unit, forming a hemihydrate-dihydrate alternating crystallization sequence. The material from reaction tank No. 3 (dihydrate) is heated by the sulfuric acid dilution mixing heat exchanger No. 3 and then enters reaction tank No. 4 (hemihydrate) (temperature 88–95℃). The material from tank No. 4 is cooled by the simple heat exchanger No. 1 (located between tanks 3 and 4) and then enters reaction tank No. 5 (dihydrate) (75–85℃). The material from tank No. 5 is heated by the simple heat exchanger No. 2 (located between tanks 4 and 5) and then enters reaction tank No. 6 (hemihydrate) (88–95℃). Finally, the material from reaction tank No. 6 (hemihydrate) undergoes a small amount of heat exchange with the external environment via the simple heat exchanger No. 3 (located between tanks 5 and 6), with the solid content controlled at 30%–35%, and is then sent to the external phosphogypsum treatment unit. Each individual heat exchanger is connected to the acid return pipe via a pipeline, and the heat after heat exchange is introduced into the internal circulation of the system through the acid return pipe. The heat exchange area is strictly matched according to the capacity of the reaction tank, and the average heat transfer efficiency reaches 92% to 95%.

[0051] In the above process, the operating conditions of each reaction tank are monitored in real time using temperature sensors and electromagnetic flowmeters. The temperature sensors are platinum resistance type (measuring range 0–150℃, accuracy ±0.5℃), collecting data every 0.5 seconds, and the signal is transmitted to the PLC control cabinet via a shielded cable. The electromagnetic flowmeters are corrosion-resistant type (measuring error ≤1.5%), collecting flow data every 1 second and uploading it via RS485 communication protocol. The PLC control cabinet has a built-in PID control algorithm (control cycle 2 seconds, proportional band 20%, integral time 45 seconds) to dynamically adjust the opening of the heat exchanger valves (adjustment accuracy 1%, response time <0.5 seconds), controlling the temperature fluctuation of the reaction system within ±2℃. The system ensures the stability of the crystallization process through closed-loop control. In Example 1, the phosphorus recovery rate reached 98.5%, the product phosphoric acid concentration reached 40% ±2%, and the fluctuation did not exceed ±0.5%.

[0052] Example 2

[0053] In this embodiment, the alternating reaction tank unit is expanded to eight reaction tanks, four each of dihydrate and hemihydrate reaction tanks, arranged in an alternating layout to enhance the continuity of the reaction and the cascade utilization of heat. The sulfuric acid dilution mixing heat exchanger group is increased from three to four, with each connected to the feed end of reaction tanks 1, 2, 3, and 4 respectively, achieving more precise material pre-conditioning and temperature control. The simple heat exchanger group is increased from three to four, respectively located between reaction tanks 4 and 5, 5 and 6, 6 and 7, and 7 and 8, forming a more robust heat recovery network. All heat exchangers are made of reinforced ceramic material, possessing excellent corrosion resistance and mechanical strength. Their heat exchange area is strictly matched to the reaction tank capacity at a ratio of 1:1.5, increasing the average heat transfer efficiency to 97% and significantly reducing system heat loss.

[0054] The temperature control unit is a distributed intelligent control system, with a local control station set up in each reaction tank area. It connects to the central control room in real time via a high-speed industrial Ethernet, enabling high-frequency data acquisition and centralized monitoring. All temperature sensors have been replaced with dual-plasma resistance type sensors, significantly improving measurement accuracy to ±0.2℃ and providing redundancy verification. High-precision electromagnetic flowmeters are selected to ensure measurement errors are no greater than 0.5%. The control system employs a self-tuning PID algorithm based on fuzzy rules, dynamically processing temperature deviations and their rates of change through membership functions. This shortens the control cycle to 1 second, further reducing the temperature fluctuation range of each reaction tank to ±1℃, greatly improving the stability and response speed of thermal control.

[0055] The feeding unit is equipped with a high-precision online component analyzer, which can monitor changes in phosphorus pentoxide content, concentrated sulfuric acid concentration, and backflow acid composition in the phosphate rock slurry in real time. The analytical data is seamlessly transmitted to the control system via the OPC protocol and participates in the dynamic adjustment of the proportions of various materials. The molar ratio of phosphate rock slurry to concentrated sulfuric acid is optimized in real time based on the slurry composition, with a control accuracy of ±0.5%. The amount of backflow acid added is automatically adjusted based on the real-time heat balance of the system, with the adjustment range precisely controlled within ±2%, effectively avoiding heat load shocks caused by material fluctuations.

[0056] Both the sulfuric acid dilution mixing heat exchanger group and the simple heat exchanger group adopt an innovative series-parallel hybrid arrangement: Heat exchangers 1, 2, 3, and 4 are connected in parallel to form a high-temperature heat exchange network, primarily responsible for heat recovery from the semi-aqueous reactor; while heat exchangers 1, 2, 3, and 4 are connected in parallel to form a low-temperature heat exchange network, mainly meeting the cooling needs of the dihydrate reactor. An intelligent balancing valve is installed between the two networks, automatically distributing the circulation flow according to the real-time heat load of the system, ensuring that each heat exchanger always operates at its optimal operating point, thereby improving overall heat exchange efficiency and equipment utilization.

[0057] In terms of monitoring the crystallization process, a new online particle size analyzer has been added, which can monitor the crystal size distribution of calcium sulfate dihydrate and calcium sulfate hemihydrate in real time. The particle size data is processed in real time using image analysis algorithms to keep the average crystal size stably controlled within the ideal range of 50–100 μm. Once the crystal size deviates from the set range, the system automatically adjusts the stirring rate of the reaction tank or the material residence time to maintain the stability of the crystallization quality. At the same time, turbidity monitoring points have been added at key nodes. The solid content of the slurry is measured in real time using the principle of laser scattering, and the solid content is strictly controlled within the process range of 25%–35%, further ensuring the consistency of the product and its filtration performance.

[0058] This invention, though seemingly simple, is actually quite sophisticated. Only through in-depth exploration and research into the wet-process phosphoric acid production process can the limitations of existing technologies be overcome in a simple way, achieving superior technical results. Therefore, compared with existing technologies, this invention has outstanding substantive features and significant progress.

[0059] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All modifications made based on the design principles of the present invention, and modifications made without creative effort, should fall within the scope of protection of the present invention.

Claims

1. A phosphoric acid extraction hemihydrate-dihydrate alternating crystallization system, characterized in that, include: The alternating reaction tank unit arranged in series consists of three or more reaction tanks connected in series. The reaction tanks start with a dihydrate reaction tank and are arranged alternately with a semihydrate reaction tank. The temperature of the dihydrate reaction tank is controlled at 75℃~85℃, and the temperature of the semihydrate reaction tank is controlled at 88℃~95℃. The sulfuric acid dilution mixing heat exchanger group consists of at least one sulfuric acid dilution mixing heat exchanger connected to the alternating reaction tank unit, used to mix and exchange heat between phosphate rock slurry, 98% concentrated sulfuric acid and return acid, and control the outlet temperature between 40°C and 60°C. A simple heat exchanger group and a sulfuric acid dilution mixing heat exchanger group are both connected to an alternating reaction tank unit. The simple heat exchanger group includes at least one simple heat exchanger, which is disposed between adjacent reaction tanks of the alternating reaction tank unit. The feeding unit includes a phosphate rock slurry pipeline, a concentrated sulfuric acid pipeline, and a return acid pipeline, all of which are connected to the feed inlet of the sulfuric acid dilution and mixing heat exchanger. The simple heat exchanger is connected to the return acid pipeline through a pipeline and is used to introduce the heat after heat exchange into the return acid circulation system.

2. The phosphoric acid extraction hemihydrate-dihydrate alternating crystallization system according to claim 1, characterized in that, It also includes a temperature control unit, which comprises temperature sensors, electromagnetic flow meters, and a PLC control cabinet arranged at the outlets of each reaction tank. The signal output terminals of the temperature sensors and electromagnetic flow meters are electrically connected to the PLC control cabinet, and the control signal output terminal of the PLC control cabinet is simultaneously electrically connected to the sulfuric acid dilution mixing heat exchanger group and the simple heat exchanger group. The temperature control unit is used to dynamically adjust the valve opening of the sulfuric acid dilution mixing heat exchanger and the simple heat exchanger to maintain the system temperature fluctuation within ±2℃.

3. The phosphoric acid extraction hemihydrate-dihydrate alternating crystallization system according to claim 2, characterized in that, The simple heat exchanger uses ceramic heat exchange tubes, and the heat exchange area is matched to the reaction tank capacity at a ratio of 1:1.2 to 1.

5.

4. The phosphoric acid extraction hemihydrate-dihydrate alternating crystallization system according to claim 1, characterized in that, The sulfuric acid dilution mixing heat exchanger group includes multiple sulfuric acid dilution mixing heat exchangers, which are respectively connected to the feed ends of multiple reaction tanks in the alternating reaction tank unit. The sulfuric acid dilution mixing heat exchangers are connected in series through pipelines.

5. A method for alternating hemihydrate-dihydrate crystallization by phosphoric acid extraction, implemented using the system described in any one of claims 1 to 4, characterized in that, Includes the following steps: Step S1: The phosphate rock slurry, 98% concentrated sulfuric acid and back acid are fed into the sulfuric acid dilution mixing heat exchanger for mixing and preliminary heat exchange, and the outlet temperature of the sulfuric acid dilution mixing heat exchanger is controlled at 40℃~60℃. Step S2: The slurry treated in step S1 is fed into a dihydrate reaction tank and subjected to calcium sulfate dihydrate crystallization reaction at 75℃~85℃ for 2 h~3 h. Step S3: The material from the dihydrate reaction tank is heated by a simple heat exchanger and then sent to the hemihydrate reaction tank for hemihydrate calcium sulfate crystallization reaction at 88℃~95℃. Step S4: The material from the semi-aqueous reaction tank is cooled by a simple heat exchanger and then sent to the next dihydrate reaction tank for secondary dihydrate crystallization at 75℃~85℃. Step S5: Heat transfer and temperature control between adjacent reaction tanks in the reaction tank unit are achieved sequentially through subsequent simple heat exchangers, forming a hemihydrate-dihydrate alternating crystallization sequence.

6. The phosphoric acid extraction hemihydrate-dihydrate alternating crystallization method according to claim 5, characterized in that, In step S1, the molar ratio of phosphate rock slurry to concentrated sulfuric acid is controlled at 1:1.02 to 1.05, and the amount of acid added is 30% to 40% of the total feed.

7. The phosphoric acid extraction hemihydrate-dihydrate alternating crystallization method according to claim 5 or 6, characterized in that, In step S3, the heat exchange area of ​​the simple heat exchanger is matched according to a ratio of 1:1.2 to 1.5 of the reaction tank capacity.

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