Phosphoric acid concentration fluorine recovery process based on silicon dioxide assistance

Abstract

The invention relates to the technical field of phosphorus chemical industry and three-waste resourceful treatment, and discloses a silicon dioxide-assisted phosphoric acid concentration fluorine recovery process, which comprises the following steps: mixing anhydrous hydrogen fluoride byproduct silicon slag with dilute phosphoric acid to prepare slurry with the solid content of 9.0-9.5%; after being homogenized by a buffer tank, the slurry is premixed and diluted on line through a dilute phosphoric acid branch, and is injected into a concentration system; in the concentration process, silicon dioxide is used for promoting hydrogen fluoride to be converted into volatile silicon tetrafluoride to escape and be recycled. In order to solve the scaling problem in the conveying process, a double-pipeline switching mechanism is established, and 3%-5% dilute sulphuric acid is used for conducting periodic acid pickling maintenance on a shutdown pipeline. Through dynamic flow control, the fluorine recovery rate is greatly improved while the solid content of the concentrated phosphoric acid product is ensured to be qualified, the problems that pipelines are easy to block and fluorine resources are wasted in the traditional process are solved, and efficient resource utilization of the silicon slag is realized.

Description

Technical Field

[0001] This invention relates to the fields of phosphorus chemical industry and waste resource utilization technology, specifically a phosphoric acid concentration and fluorine recovery process based on silica-assisted phosphoric acid treatment. Background Technology

[0002] Wet-process phosphoric acid production is currently the main industrial route for obtaining phosphoric acid. This process generates a large amount of dilute phosphoric acid, which typically needs to be concentrated to industrial-grade phosphorus pentoxide concentration. A certain amount of fluorine is present in the dilute phosphoric acid; under the heating and negative pressure conditions during concentration, some of this fluorine escapes as fluorine-containing gas. Recovering this fluorine to produce byproducts such as fluorosilicic acid not only improves economic efficiency but also reduces environmental pollution. Studies have shown that adding active silica to the phosphoric acid concentration system can alter the phase equilibrium of fluorine, promoting the conversion of hydrogen fluoride to volatile silicon tetrafluoride, thereby increasing fluorine recovery. Simultaneously, the production of anhydrous hydrogen fluoride generates a large amount of byproduct silicon slag, whose main component is active silica. Introducing this as a defluorinating agent into the phosphoric acid concentration system would achieve resource utilization through waste treatment.

[0003] However, in existing industrial practices, utilizing silica slag to assist in phosphoric acid concentration and defluorination faces severe technical challenges, making long-term stable operation difficult. Firstly, the wet-process phosphoric acid system has a complex composition, containing various metallic impurity ions such as magnesium, sodium, iron, and aluminum. When exogenous silica slag is introduced, the concentration of fluorosilicate ions in the solution increases, readily combining with sodium and magnesium ions in the system to form a dense and hard fluorosilicate crystalline scale layer in the slurry conveying pipelines and nozzles. This scale layer has extremely strong adhesion, and conventional process water flushing cannot dissolve or remove it, leading to a rapid reduction in the flow cross-section of the conveying pipeline, increased conveying resistance, and even complete pipeline blockage, forcing frequent shutdowns for cleaning and severely restricting production efficiency.

[0004] Furthermore, existing technologies are relatively crude in controlling the addition of silica slag slurry, lacking refined process management. Directly adding solids or high-concentration slurry can easily lead to localized agglomeration at the feed inlet, resulting in uneven dispersion and reduced reaction efficiency. On the other hand, it is difficult to accurately balance the defluorination requirements with product quality, often resulting in excessive addition leading to severely excessive levels of insoluble matter (solid content) in the final concentrated phosphoric acid product, affecting the quality of downstream products. Currently, the industry lacks a systematic process solution that can efficiently utilize silica slag to improve fluorine recovery rates while effectively solving pipeline scaling and blockage and ensuring the quality of phosphoric acid products. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a phosphoric acid concentration and fluorine recovery process based on silica-assisted phosphoric acid concentration. This process solves the problems of low fluorine resource recovery rate in existing wet phosphoric acid concentration processes, and the ease with which fluorosilicate scaling can cause blockages in transport pipelines, making it difficult to achieve long-term continuous production and resulting in product quality damage when using silica slag to assist in defluorination.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A silica-assisted phosphoric acid concentration and fluorine recovery process includes the following steps:

[0008] The silicon slag, a byproduct of anhydrous hydrogen fluoride production, is mixed with dilute phosphoric acid in a slurry tank to prepare a silicon slag slurry with a predetermined solid content and density.

[0009] The silicon slag slurry is transported to a buffer tank for homogenization and temporary storage. Then, the slurry is injected into the feed pipeline of the phosphoric acid concentration system through a conveying device and mixed with the mainstream dilute phosphoric acid to be concentrated.

[0010] The mixed material enters the concentration unit. Under heating and concentration conditions, the silicon dioxide in the silicon slag reacts with the hydrogen fluoride in the dilute phosphoric acid to generate silicon tetrafluoride gas, which escapes with water vapor.

[0011] The escaped silicon tetrafluoride-containing gas is introduced into a washing system, where it is absorbed, converted, and recovered into fluorosilicic acid products.

[0012] During the process, the pipelines conveying the silicon slag slurry are periodically cleaned and maintained, including acid washing of the pipelines with dilute sulfuric acid solution.

[0013] By adopting the above technical solution, this invention achieves efficient recovery of fluorine resources and synergistic resource utilization of silicon-containing solid waste in wet-process phosphoric acid production. The specific process mechanism and beneficial effects are as follows:

[0014] First, regarding the chemical reaction and defluorination mechanism: This process introduces highly active amorphous silica (silicon slag, a byproduct of anhydrous hydrogen fluoride), disrupting the gas-liquid equilibrium of fluorides in the wet-process phosphoric acid system. Under concentrated heating and negative pressure, silica acts as a reactant, combining with the non-volatile hydrofluoric acid in phosphoric acid to generate fluorosilicic acid (H2SiF6). Subsequently, the fluorosilicic acid rapidly decomposes under heating conditions into volatile silicon tetrafluoride (SiF4) gas and hydrofluoric acid, with the gaseous SiF4 escaping from the system with water vapor. This process increases the partition coefficient of fluorine in the gas phase, thereby raising the fluorine recovery rate to over 68%, while simultaneously reducing fluorine residue in the concentrated phosphoric acid product.

[0015] Secondly, regarding the mechanism for inhibiting scale formation and long-term operation in pipelines: Wet-process phosphoric acid typically contains impurities such as magnesium, sodium, iron, and aluminum ions. During the transport of slurries containing silica slag, these impurity ions readily combine with fluorosilicate ions, forming a layer of insoluble fluorosilicate crystals (such as sodium fluorosilicate and magnesium fluorosilicate) on the pipe wall. Traditional industrial water flushing is insufficient to dissolve this dense scale layer, leading to a reduction in pipeline flow cross-section, increased transport resistance, and even blockage. This invention innovatively establishes a periodic acid washing mechanism using dilute sulfuric acid. Utilizing the strongly acidic environment of dilute sulfuric acid, it converts insoluble fluorosilicates into soluble sulfates or acid salts, thereby completely removing the scale layer from the pipe wall, restoring the pipeline's transport capacity, and solving the industry problem of easy scaling in phosphoric acid transport systems containing impurities.

[0016] Third, regarding the co-treatment of solid waste: This process directly disposes of the hazardous solid waste silicon slag from the production of anhydrous hydrogen fluoride, converting it into valuable fluorosilicic acid byproducts, thus achieving waste-to-waste treatment and zero solid waste discharge throughout the entire process.

[0017] Preferably, the specific operation of the preparation is as follows: adjust the mass ratio of silicon slag to dilute phosphoric acid, control the mass percentage of solid substances in the silicon slag slurry to be 9.0%-9.5%, and maintain the density of the slurry in the range of 1.25-1.30 g / mL.

[0018] By adopting the above technical solution, the physical properties of the slurry are strictly limited. The lower limit of 9.0% ensures that the slurry contains sufficient silica reactant per unit volume, avoiding the introduction of excessive water and increasing the consumption of concentration steam; the upper limit of 9.5% and density control ensure that the slurry has good rheological properties, preventing sedimentation stratification or pumping difficulties caused by excessive solid content.

[0019] Preferably, the injection method adopts an online premixing mode: branch dilute phosphoric acid is drawn from the main dilute phosphoric acid pipeline of the phosphoric acid concentration system, and the flow rate of the branch dilute phosphoric acid is controlled at 30-40 m³ / h; the branch dilute phosphoric acid is first mixed with the silicon slag slurry to form a diluted mixture, and then fed into the feed pump inlet of the phosphoric acid concentration system.

[0020] By adopting the above technical solution, high-flow-rate dilution and premixing of the slurry before it enters the main system effectively avoids agglomeration or instantaneous scaling caused by local oversaturation when the high-concentration slurry directly enters the main pipeline. At the same time, premixing improves the dispersion of silica particles in the liquid phase and increases the solid-liquid reaction contact area, thereby improving the defluorination reaction rate.

[0021] Preferably, the injection flow rate of the silica slag slurry is dynamically controlled based on the processing load of the thickening unit: the dilute phosphoric acid processing capacity of a single thickening unit is set to 50-70 m³. 3 When the flow rate of the silicon slag slurry is / h, the injection flow rate is controlled between 2.21-6.5m.3 The total solids content of the material entering the concentration unit after mixing is controlled to be ≤3.0% between / h.

[0022] By adopting the above technical solution, a precise material balance control model was established. By limiting the total solid content entering the system to within 3.0%, it was ensured that the remaining solid impurities from the reaction could be controlled within the allowable range of the downstream concentrated phosphoric acid product (finished product solid content ≤2.0%), preventing the sacrifice of phosphoric acid product quality in pursuit of defluorination rate.

[0023] Preferably, the mass fraction of the dilute sulfuric acid solution is 3.0%-5.0%, the pickling time is 4-6 hours, the interval of the periodic cleaning and maintenance is 7-10 days, and the specific implementation of the periodic cleaning and maintenance is as follows: the pipeline for conveying silicon slag slurry is configured with a main pipeline and a backup pipeline; when the cleaning and maintenance cycle is reached or the flow rate is detected to decrease, the material is switched to the backup pipeline for conveying, and at the same time, the dilute sulfuric acid solution is introduced into the main pipeline that is not in operation for circulation cleaning. After the pickling is completed, industrial water is used for flushing and replacement.

[0024] By adopting the above technical solution, the dual-pipeline backup and online switching mechanism ensures production continuity, allowing maintenance to be completed without downtime. The 3.0%-5.0% sulfuric acid concentration and 4-6 hour cleaning time are optimal parameter windows determined based on dissolution kinetics, effectively removing fluorosilicate scale while avoiding corrosion of the pipe material by excessively high acid concentrations, and simultaneously controlling cleaning costs.

[0025] Preferably, the inner wall of the buffer tank is lined with acid-resistant rubber, and the volume is 500-700 m³. 3 The residence time of the silicon slag slurry in the buffer tank is sufficient to achieve homogenization and eliminate fluctuations in the upstream silicon slag material.

[0026] By adopting the above technical solution, the large-volume buffer tank acts as a water storage tank and homogenizer, which smooths out the impact of fluctuations in the amount of upstream silicon slag on the downstream concentration process and ensures that the feed properties are uniform and stable.

[0027] Preferably, the specific process of the chemical reaction is as follows: silicon dioxide reacts with hydrogen fluoride to generate fluorosilicic acid, and fluorosilicic acid decomposes into silicon tetrafluoride and hydrogen fluoride under heating and negative pressure conditions; through the reaction, the residual fluorine content in the concentrated phosphoric acid product is controlled between 0.50% and 0.65%.

[0028] By adopting the above technical solution, the reaction path and control objectives were clarified, and efficient defluorination within the preset range was achieved.

[0029] Preferably, the dynamic control further includes a feedback adjustment step: during process operation, the solid content of the concentrated phosphoric acid product at the outlet of the concentration unit is detected; when the detected solid content exceeds the range of 1.4%-2.0%, the injection flow rate of the silicon slag slurry is corrected and adjusted with this range as the target value.

[0030] This invention provides a silica-assisted phosphoric acid concentration and fluorine recovery process. It offers the following advantages:

[0031] 1. This invention introduces anhydrous hydrogen fluoride byproduct silicon slag into a wet phosphoric acid concentration system. By utilizing highly active silica to alter the gas-liquid balance of fluorides, it promotes the conversion of non-volatile hydrofluoric acid into easily volatile silicon tetrafluoride, which then escapes with water vapor. This not only improves the fluorine recovery rate during the phosphoric acid concentration process and reduces the fluorine content of concentrated phosphoric acid products, but also achieves the resource utilization of hazardous solid waste silicon slag, solving the dual problems of fluorine resource waste and solid waste disposal difficulties in traditional processes.

[0032] 2. This invention establishes a dual-pipeline switching maintenance mechanism based on dilute sulfuric acid cleaning, which effectively overcomes the problem of sodium and magnesium fluorosilicate scaling that is easily generated during the transportation of phosphoric acid containing impurities. It utilizes the dissolving properties of dilute sulfuric acid on stubborn crystalline scale layers to replace the traditional inefficient water flushing method, extending the continuous and stable operation cycle of the device, avoiding frequent shutdowns and flow attenuation caused by pipeline blockage, and ensuring production efficiency.

[0033] 3. This invention adopts a feeding strategy that combines online dilution and premixing with dynamic flow control. By fully mixing the high-concentration slurry with dilute phosphoric acid before feeding and strictly controlling the total solids content entering the concentration system to within 3.0%, local agglomeration is prevented and the reaction contact area is optimized. While maximizing defluorination efficiency, it ensures that the solids content of the final concentrated phosphoric acid product meets the industry standard of less than 2.0%, thus achieving a balance between process efficiency and product quality. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments and comparative examples. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] Preparation Examples 1-3:

[0036] Preparation Example 1: This preparation example provides a silica slag slurry for phosphoric acid concentration and defluorination.

[0037] The byproduct silicon slag generated during the anhydrous hydrogen fluoride production process is transported to a slurry tank equipped with a mechanical stirring device, while dilute phosphoric acid (P2O5 content approximately 24%, density approximately 1.30 g / cm³) from the wet-process phosphoric acid unit is continuously injected into the tank. 3 The agitator was activated for vigorous dispersion and mixing. The material ratio of the mixing system was controlled by adjusting the feed rate of the silica slag and the injection flow rate of dilute phosphoric acid. After uniform mixing, samples were taken to test the slurry properties, yielding a silica slag slurry with a solid content of 9.3% (mass fraction) and a density of 1.27 g / mL. This slurry was a grayish-white suspension with good fluidity and no obvious large particle sedimentation. It was temporarily stored in a buffer tank for later use.

[0038] Preparation Example 2: This preparation example provides a silicon slag slurry for phosphoric acid concentration and defluorination.

[0039] The preparation process was basically the same as in Preparation Example 1, except that the ratio of silicon slag to dilute phosphoric acid was adjusted, and the relative amount of dilute phosphoric acid added was appropriately increased. After being mixed evenly in the slurry tank, the resulting silicon slag slurry had a solid content of 9.0% (mass fraction) and a density of 1.25 g / mL. The viscosity of this slurry was slightly lower than that of Preparation Example 1, making it suitable for applications with long conveying distances or high pumping resistance. It was temporarily stored in a buffer tank for later use.

[0040] Preparation Example 3: This preparation example provides a silica slag slurry for phosphoric acid concentration and defluorination.

[0041] The preparation process was basically the same as in Preparation Example 1, except that the ratio of silicon slag to dilute phosphoric acid was adjusted, and the relative amount of dilute phosphoric acid added was appropriately reduced, thereby increasing the solid content. After being mixed evenly in the slurry tank, the resulting silicon slag slurry was found to have a solid content of 9.5% (mass fraction) and a density of 1.30 g / mL. This slurry has a high solid loading, making it suitable for applications where the concentration unit has excess capacity but a strong demand for defluorination. It was temporarily stored in a buffer tank for later use.

[0042] Examples 1-3:

[0043] Example 1: This example provides a silica-assisted phosphoric acid concentration and fluorine recovery process. This process is applied to the concentration section of a wet-process phosphoric acid production unit and includes the following steps:

[0044] (1) Slurry conveying and premixing: Solid silica slag (with a silica content of 92.5% as a byproduct of anhydrous hydrogen fluoride production) was mixed with dilute phosphoric acid in a slurry preparation tank to prepare a silica slag slurry with a solid content of 9.3% and a density of 1.27 g / mL. The slurry was placed in a 600 m³ / mL container. 3 The slurry is placed in a rubber-lined buffer tank. A corrosion-resistant centrifugal pump pumps the slurry out, controlling the flow rate at 6.5 m³ / s. 3 / h. Before the slurry enters the main feed line of the thickening unit, a flow rate of 40m³ is drawn from the main dilute phosphoric acid pipe. 3 The dilute phosphoric acid branch, with a flow rate of / h, is premixed online with the pumped silica slag slurry at the pipeline junction to form a well-flowing phosphoric acid-silica mixture.

[0045] (2) Concentration and defluorination reaction: The mixture obtained in step (1) is fed into a single unit with a dilute phosphoric acid treatment capacity of 60m³. 3 The feed system of the concentration unit operates at a rate of / h. The mixed material enters the concentration heater and flash chamber. Under the negative pressure evaporation conditions of the concentration process, the silica in the slurry reacts with the hydrogen fluoride in the dilute phosphoric acid to generate silicon tetrafluoride gas. The silicon tetrafluoride escapes from the liquid phase with water vapor. The concentrated phosphoric acid product (P2O5 concentration of approximately 47%-50%) is discharged from the bottom of the system.

[0046] (3) Fluorine resource recovery: The fluorine-containing gas that escaped in step (2) is introduced into the washing and absorption tower, and the circulating washing liquid is used for spray absorption. The generated fluorosilicic acid solution is exported as a by-product for recovery.

[0047] (4) Pipeline scale prevention and maintenance: Main and backup pipelines (both DN150 steel-reinforced composite pipes) are installed in the slurry conveying pipeline and premixing pipeline. During continuous operation, pipeline switching is performed every 7 days. After switching, a 3% dilute sulfuric acid solution is circulated into the shut-down pipeline for cleaning. The cleaning time lasts for 4-6 hours. After the crystalline scale layer on the pipe wall is dissolved, it is flushed and replaced with process water and drained, serving as the backup pipeline for the next cycle.

[0048] Example 2: This example provides a silica-assisted phosphoric acid concentration and fluorine recovery process. This process is applied to the concentration section of a wet-process phosphoric acid production unit and includes the following steps:

[0049] (1) Slurry conveying and premixing: Solid silica slag (with a silica content of 94.8% as a byproduct of anhydrous hydrogen fluoride production) was mixed with dilute phosphoric acid in a slurry preparation tank to prepare a silica slag slurry with a solid content of 9.5% and a density of 1.30 g / mL. The slurry was placed in a 600 m³ / mL container. 3 The slurry is placed in a rubber-lined buffer tank. A corrosion-resistant centrifugal pump pumps the slurry out, controlling the flow rate at 2.21 m³ / s. 3 / h. Before the slurry enters the main feed line of the thickening unit, a flow rate of 30m³ is drawn from the main dilute phosphoric acid pipe. 3 The dilute phosphoric acid branch, with a flow rate of / h, is premixed online with the pumped silica slag slurry at the pipeline junction to form a well-flowing phosphoric acid-silica mixture.

[0050] (2) Concentration and defluorination reaction: The mixture obtained in step (1) is fed into a single unit with a dilute phosphoric acid treatment capacity of 60m³.3 The feed system of the concentration unit ( / h) receives the mixed material into the concentration heater and flash chamber. Under the negative pressure evaporation conditions of the concentration process, the silica in the slurry reacts with the hydrogen fluoride in the dilute phosphoric acid to generate silicon tetrafluoride gas, which escapes from the liquid phase with water vapor. The concentrated phosphoric acid product is discharged from the bottom of the system.

[0051] (3) Fluorine resource recovery: The fluorine-containing gas that escaped in step (2) is introduced into the washing and absorption tower, and the circulating washing liquid is used for spray absorption. The generated fluorosilicic acid solution is exported as a by-product for recovery.

[0052] (4) Pipeline scale prevention and maintenance: Main and backup pipelines are set up in the slurry conveying pipeline and premixing pipeline. During continuous operation, pipeline switching is performed every 10 days. After switching, a 3% dilute sulfuric acid solution is circulated into the shut-down pipeline for cleaning. The cleaning time lasts for 4-6 hours. After the crystalline scale layer on the pipe wall is dissolved, it is flushed and replaced with process water and drained, serving as the backup pipeline for the next cycle.

[0053] Example 3: This example provides a silica-assisted phosphoric acid concentration and fluorine recovery process. This process is applied to the concentration section of a wet-process phosphoric acid production unit and includes the following steps:

[0054] (1) Slurry delivery and premixing: The silica slag slurry (containing 9.5% solids and 1.30 g / mL) obtained in Preparation Example 3 was placed in a container with a volume of 600 m³. 3 The slurry is placed in a rubber-lined buffer tank. A corrosion-resistant centrifugal pump pumps the slurry out, controlling the flow rate at 4.5 m³ / s. 3 / h. Before the slurry enters the main feed line of the thickening unit, a flow rate of 35m³ is drawn from the main dilute phosphoric acid pipe. 3 The dilute phosphoric acid branch, with a flow rate of / h, is premixed online with the pumped silica slag slurry at the pipeline junction to form a well-flowing phosphoric acid-silica mixture.

[0055] (2) Concentration and defluorination reaction: The mixture obtained in step (1) is fed into a single unit with a dilute phosphoric acid treatment capacity of 60m³. 3 The feed system of the concentration unit ( / h) receives the mixed material into the concentration heater and flash chamber. Under the negative pressure evaporation conditions of the concentration process, the silica in the slurry reacts with the hydrogen fluoride in the dilute phosphoric acid to generate silicon tetrafluoride gas, which escapes from the liquid phase with water vapor. The concentrated phosphoric acid product is discharged from the bottom of the system.

[0056] (3) Fluorine resource recovery: The fluorine-containing gas that escaped in step (2) is introduced into the washing and absorption tower, and the circulating washing liquid is used for spray absorption. The generated fluorosilicic acid solution is exported as a by-product for recovery.

[0057] (4) Pipeline scale prevention and maintenance: Main and backup pipelines are set up in the slurry conveying pipeline and premixing pipeline. During continuous operation, pipeline switching is performed every 8 days. After switching, a 3% dilute sulfuric acid solution is circulated into the shut-down pipeline for cleaning. The cleaning time lasts for 4-6 hours. After the crystalline scale layer on the pipe wall is dissolved, it is flushed and replaced with process water and drained, serving as the backup pipeline for the next cycle.

[0058] Comparative Examples 1-4:

[0059] Comparative Example 1: This comparative example is used to verify the necessity of adding silicon slag for defluorination.

[0060] Compared with Example 1, the difference is that this comparative example is a traditional wet phosphoric acid concentration process, without the preparation, transportation and addition of silicon slag slurry in step (1), and the concentration device directly processes the original dilute phosphoric acid. The other steps and parameters are the same.

[0061] Comparative Example 2: This comparative example is used to verify the impact of silicon slag slurry addition and solid content control on product quality.

[0062] Compared with Example 1, the difference is that the solid content of the silicon slag slurry used in step (1) is adjusted to 15.0% (mass fraction), which causes the total solid load entering the concentration system to exceed the design threshold (>3%) significantly. The other process parameters and steps are the same.

[0063] Comparative Example 3: This comparative example is used to verify the key role of the dilute sulfuric acid cleaning mechanism in the long-term stable operation of the system.

[0064] Compared with Example 1, the difference is that in the pipeline anti-scaling maintenance in step (4), after the pipeline is switched, only process water is used to circulate and flush the shut-down pipeline, and a 3% dilute sulfuric acid solution is not used for acid washing. All other aspects are the same.

[0065] Comparative Example 4: This comparative example is used to verify the effect of the online premixing step of dilute phosphoric acid on the reaction effect and delivery stability.

[0066] Compared with Example 1, the difference is that in step (1), the online premixing operation of the dilute phosphoric acid branch is not performed, and the silicon slag slurry is directly fed into the main feed pipeline of the concentration device through the conveying pump. The rest are the same.

[0067] Test Example 1-2:

[0068] Test Example 1: Process Feasibility and Product Quality Test

[0069] Test Method Description: To verify the actual operating effect of this process in industrial production and the quality indicators of the produced phosphoric acid product, concentrated phosphoric acid samples were collected at the product outlet of the concentration system after the processes described in Examples 1 to 3 had been running stably for 24 hours. Simultaneously, the fluorosilicic acid production and raw material consumption data during system operation were recorded to calculate the fluorine recovery rate. Specific detection indicators and analysis methods are as follows:

[0070] Determination of phosphorus pentoxide (P2O5) content: The determination was carried out according to the GB / T2091-2008 standard "Industrial Phosphoric Acid", using the quinoline phosphomolybdate gravimetric method.

[0071] Fluorine (F) content determination: According to HG / T4072-2008 standard, the fluorine content in the distillate was first determined by distillation and then by the thorium nitrate volumetric method.

[0072] Solid content determination: The gravimetric method was used. The solids were filtered, washed, dried and weighed in a G4 glass frit crucible.

[0073] Fluorine recovery rate calculation: Based on the principle of material balance, the total amount of fluorine entering the system per unit time (carried in by dilute phosphoric acid + carried in by silicon slag) and the amount of fluorine-reduced fluorosilicic acid recovered by the washing system are statistically analyzed, and the percentage is calculated.

[0074] Test results:

[0075] Table 1. Process operation data and product index test results for Examples 1-3

[0076]

[0077] Results analysis:

[0078] Based on the data in Table 1 and the technical mechanism of this invention, the analysis is as follows:

[0079] Data from Examples 1 to 3 show that this process, by introducing a pre-proportioned silica slurry into the dilute phosphoric acid concentration system, alters the gas-liquid equilibrium of fluorides in the phosphoric acid solution. According to the principle of chemical reaction equilibrium, silica combines with non-volatile hydrogen fluoride to form fluorosilicic acid, which rapidly decomposes into silicon tetrafluoride gas under the concentrated high temperature and negative pressure environment.

[0080] Data shows that in Example 1, at the maximum addition amount (6.5m³), 3Under the condition of / h), the residual fluorine in concentrated phosphoric acid was reduced to 0.58%, the fluorine recovery rate reached 70.08%, and the solid content of the product (1.98%) was controlled within the industry standard limit of 2.0%. In Example 2, under the condition of minimum addition, although the fluorine recovery rate decreased slightly (68.95%), it was still higher than the general level of existing conventional processes (about 62%), and the solid content was lower (1.45%), indicating that this process can maintain good defluorination effect within a wide flow rate adjustment range.

[0081] In summary, this process utilizes anhydrous hydrogen fluoride byproduct silicon slag as an auxiliary defluorinating agent, effectively promoting the release and recovery of fluorine resources from phosphoric acid without introducing new impurities. It achieves the dual goals of resource utilization of silicon slag and preparation of high-quality, low-fluoride phosphoric acid. The process parameters are reasonably designed, and the industrial application is highly feasible.

[0082] Test Example 2: Comparative Test

[0083] Test Method Description: To verify the technical effectiveness of the process parameter settings and key steps (such as slurry concentration control, online premixing, and acid washing maintenance) of this invention, Example 1 and Comparative Examples 1 to 4 were selected for long-term continuous operation comparative tests. All tests were conducted on industrial-grade thickening devices of the same specifications, with a designed operating cycle of 30 days (720 hours).

[0084] The following parameters will be monitored during the test:

[0085] Fluorine recovery rate: Samples are taken daily, and the average value over the entire cycle is used.

[0086] Solid content of the product: The content of insoluble matter in concentrated phosphoric acid is tested daily, and the average value over the entire cycle is taken.

[0087] Pipeline transport stability: The outlet pressure and flow rate of the slurry transport pump are recorded by the DCS system. When the flow rate decreases by more than 20% of the set value or the pump outlet pressure exceeds the rated working pressure by 15%, it is determined that the pipeline is blocked or severely scaled, and the operation is stopped and the continuous operating time is recorded.

[0088] Defluorination volatility: The standard deviation of fluorine recovery rate during operation is calculated to assess process stability.

[0089] Test results:

[0090] Table 2. Overall performance comparison test results of Example 1 and Comparative Examples 1-4

[0091]

[0092] Results analysis:

[0093] Based on the data in Table 2 and the preset conditions of each comparative example, the analysis is as follows:

[0094] Effectiveness of silica-assisted defluorination: Comparing Example 1 and Comparative Example 1 (blank control), without the addition of silica slag, the fluorine recovery rate of Comparative Example 1 was only 62.15%, while that of Example 1 was increased to 70.12%. This confirms that introducing active silica into the concentration system can break the dissociation equilibrium of fluorosilicic acid in dilute phosphoric acid, and convert the non-volatile hydrofluoric acid into the volatile silicon tetrafluoride through a chemical reaction, thereby improving the recovery efficiency of fluorine resources.

[0095] Impact of slurry parameter control on product quality and system throughput: Comparative Example 2 used a slurry with a high solids content of 15%. Although the fluorine recovery rate (71.88%) was slightly higher than that of Example 1, it caused the solids content of the finished concentrated phosphoric acid to surge to 7.42%, severely exceeding the 2.0% limit required by downstream products. Furthermore, due to the excessive solids load, the system was forced to shut down after only 112 hours of operation due to severe sludge accumulation and blockage in the concentration heater tube bundle and conveying pipeline. In contrast, Example 1, through precise calculation and control of slurry addition within the system's maximum allowable solids content (3%), ensured both effective defluorination and long-term stable operation of the unit and a high product qualification rate.

[0096] The necessity of the dilute sulfuric acid cleaning mechanism: Comparative Example 3 only used water rinsing during the maintenance phase without dilute sulfuric acid pickling. Data shows that its indicators were similar to those of Example 1 in the initial stage of operation, but after 185 hours (approximately 7.7 days) of continuous operation, the slurry delivery flow rate decreased drastically, leading to a system cascading shutdown. This indicates that impurity ions such as magnesium and sodium in dilute phosphoric acid form a water-insoluble fluorosilicate crystal scale layer on the pipe wall during transportation, which cannot be removed by simple water rinsing. Example 1 used 3% dilute sulfuric acid for periodic cleaning, utilizing the acidic medium to destroy the crystal structure, effectively maintaining the flow cross-section of the pipeline, and achieving more than 30 days of trouble-free operation.

[0097] The impact of online premixing on reaction stability: Comparative Example 4 eliminated the online premixing step of dilute phosphoric acid and directly injected high-concentration slurry. Although the system did not experience complete blockage, data showed that its fluorine recovery rate was only 66.35%, lower than that of Example 1. This is because the high-concentration slurry was unevenly dispersed upon entering the main pipeline, and the local solid-liquid ratio imbalance limited the reaction contact area. Simultaneously, the lack of premixing and dilution led to deterioration of the delivery pump's operating conditions, causing delivery pulsation and fluctuations in defluorination efficiency. Example 1 optimized the rheological and dispersible properties of the slurry through a premixing process, ensuring the consistency and efficiency of the reaction.

[0098] In summary, the process solution provided by this invention has technical advantages in improving fluorine recovery rate, ensuring product quality, and maintaining long-term stable operation of the system.

Claims

1. A phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction, characterized in that, Includes the following steps: The silicon slag, a byproduct of anhydrous hydrogen fluoride production, is mixed with dilute phosphoric acid in a slurry tank to prepare a silicon slag slurry with a predetermined solid content and density. The silicon slag slurry is transported to a buffer tank for homogenization and temporary storage. Then, the slurry is injected into the feed pipeline of the phosphoric acid concentration system through a conveying device and mixed with the mainstream dilute phosphoric acid to be concentrated. The mixed material enters the concentration unit. Under heating and concentration conditions, the silicon dioxide in the silicon slag reacts with the hydrogen fluoride in the dilute phosphoric acid to generate silicon tetrafluoride gas, which escapes with water vapor. The escaped silicon tetrafluoride-containing gas is introduced into a washing system, where it is absorbed, converted, and recovered into fluorosilicic acid products. During the process, the pipelines conveying the silicon slag slurry are periodically cleaned and maintained, including acid washing of the pipelines with dilute sulfuric acid solution.

2. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 1, characterized in that, The specific preparation steps are as follows: Adjust the mass ratio of silicon slag to dilute phosphoric acid to control the mass percentage of solid substances in the silicon slag slurry to 9.0%-9.5%, and maintain the density of the slurry within the range of 1.25-1.30 g / mL.

3. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 1, characterized in that, The injection method specifically employs an online premixing mode: A branch dilute phosphoric acid is drawn from the main dilute phosphoric acid pipeline of the phosphoric acid concentration system, and the flow rate of the branch dilute phosphoric acid is controlled at 30-40 m³ / h. 3 / h; The dilute phosphoric acid in the branch is first mixed with the silicon slag slurry to form a diluted mixture, and then fed into the feed pump inlet of the phosphoric acid concentration system.

4. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 1, characterized in that, The injection flow rate of the silica slag slurry is dynamically controlled based on the processing load of the thickening unit. The capacity of a single concentration unit for dilute phosphoric acid is set at 50-70 m³. 3 When the flow rate of the silicon slag slurry is / h, the injection flow rate is controlled between 2.21-6.5m. 3 The total solids content of the material entering the concentration unit after mixing is controlled to be ≤3.0% between / h.

5. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 1, characterized in that, The mass fraction of the dilute sulfuric acid solution is 3.0%-5.0%, and the pickling time is 4-6 hours; The interval for the periodic cleaning and maintenance is 7-10 days.

6. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 5, characterized in that, The specific implementation method for the periodic cleaning and maintenance is as follows: The pipeline for conveying silicon slag slurry is configured as a main pipeline and a backup pipeline; When the cleaning and maintenance cycle is reached or flow attenuation is detected, the material is switched to the backup pipeline for transportation. At the same time, the dilute sulfuric acid solution is circulated into the main pipeline that is out of service for cleaning. After the acid washing is completed, industrial water is used for rinsing and replacement.

7. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 1, characterized in that, The inner wall of the buffer tank is lined with acid-resistant rubber, and its volume is 500-700 m³. 3 ; The residence time of the silicon slag slurry in the buffer tank is sufficient to achieve homogenization and eliminate fluctuations in the upstream silicon slag supply.

8. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 1, characterized in that, The specific process of the chemical reaction is as follows: Silicon dioxide reacts with hydrogen fluoride to produce fluorosilicic acid, which decomposes into silicon tetrafluoride and hydrogen fluoride under heat and negative pressure conditions. The reaction process controls the residual fluoride content in the concentrated phosphoric acid product to between 0.50% and 0.65%.

9. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 1, characterized in that, The dilute phosphoric acid is derived from the filtrate of a wet-process phosphoric acid production system, and the mass fraction of P2O5 in the dilute phosphoric acid is 20%-26%. The silicon slag is a solid waste residue discharged from the bottom of an anhydrous hydrogen fluoride reactor, and the silicon dioxide content in the silicon slag is ≥90%.

10. The phosphoric acid concentration and fluorine recovery process based on silica-assisted extraction according to claim 4, characterized in that, The dynamic control also includes a feedback adjustment step: During the process operation, the solid content of the concentrated phosphoric acid product at the outlet of the concentration unit should be checked regularly; When the detected solid content exceeds the range of 1.4%-2.0%, the injection flow rate of the silicon slag slurry is adjusted to the target value within this range.

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