A method and system for silica acidification and aluminum reduction of fluosilicic acid for production of aluminum fluoride

Abstract

The present application belongs to the field of phosphorus chemical by-product resource utilization, and particularly relates to a method and system for producing aluminum fluoride from fluosilicic acid by silica gel acidification and aluminum reduction. The method prepares composite reinforced acid, which is delivered to a neutralization reactor to react with aluminum hydroxide slurry. After the reaction is completed, solid-liquid separation is performed, the obtained aluminum fluoride liquid is sent to a crystallization process, and the obtained by-product silica gel is delivered to an acidification reactor. Dilute fluosilicic acid is added to the acidification reactor, stirring is started after preheating and warming, and acidification and aluminum reduction reactions are performed. The slurry after acidification is subjected to solid-liquid separation, the obtained acidic filtrate is refluxed to prepare composite reinforced acid, and the obtained silica gel solid is subjected to multi-stage countercurrent water washing, and the collected washing water is refluxed to prepare aluminum hydroxide slurry. The present application removes microcrystalline silicon in fluosilicic acid raw materials, destroys the reaction layer of aluminum hydroxide wrapped by silica gel in the reaction process, further improves the rate of the main reaction of aluminum fluoride, realizes efficient recovery of fluorine and silicon resources, reduces waste liquid discharge, and improves the quality of aluminum fluoride products and the utilization rate of raw materials.

Description

Technical Field

[0001] This invention belongs to the field of resource utilization of phosphorus chemical by-products, and specifically relates to a silica gel acidification method and system for producing aluminum fluoride from fluorosilicic acid. Background Technology

[0002] In the phosphate fertilizer industry, fluorine-containing gases (mainly SiF4 and HF) are absorbed by water to form fluorosilicic acid (H2SiF6) with a mass fraction of 8-22%. As the world's largest phosphate fertilizer producer, my country generates a huge amount of purified fluorosilicic acid as a byproduct annually. Traditional neutralization and precipitation methods not only pose environmental risks but also waste fluorine and silicon resources. Currently, the mainstream route for resource recovery is to react fluorosilicic acid with aluminum hydroxide to prepare anhydrous aluminum fluoride (AlF3) for electrolytic aluminum production. The reaction formula is as follows: 3H2SiF6 + 2Al(OH)3 → 2AlF3↓+ 3SiO2·nH2O↓ + 6H2O The main bottleneck of this process lies in the byproduct silica. Due to the rapid precipitation rate, the resulting hydrated amorphous silica gel has a large specific surface area, which traps a large amount of Al through physical encapsulation and surface adsorption. 3+ F - and aluminum fluoride complex ions (such as AlF6) 3- Analysis shows that the Al2O3 content in this type of by-product silica is as high as 1.5%~5.0%, and the F content is 3~8%, which is far higher than the purity index of Al2O3 (≤0.3%) and F (≤0.1%) for precipitated silica used in rubber, coatings and other materials. As a result, it can only be used as a low-end filler or stored as solid waste.

[0003] Existing technologies generally suffer from technical defects such as incomplete removal of impurities and introduction of secondary pollution (acid radical residues and saline wastewater) and lack of resource closure (loss of fluoride and aluminum from the washing liquid).

[0004] Therefore, there is an urgent need to develop a new method that can efficiently remove aluminum fluoride impurities, avoid the introduction of foreign anions, and allow the valuable elements in the washing liquid to be recycled back to the main process in a closed loop, so as to achieve true "waste treatment" and maximize resource utilization. Summary of the Invention

[0005] The purpose of this invention is to provide a silica acidification and dealuminization method and system for producing aluminum fluoride from fluorosilicic acid. Specifically, it is a method and its supporting industrial equipment for purifying and dealuminizing silica (amorphous silica) produced by reacting fluorosilicic acid (H2SiF6), a byproduct of the phosphate fertilizer industry, with aluminum hydroxide to improve the quality of silica.

[0006] This invention aims to solve the technical problems in the existing technology of high impurity content of silica gel by-products in the preparation of aluminum fluoride from fluorosilicic acid, large water washing consumption, easy introduction of foreign anion contamination, and inability to utilize resources in a closed loop. It provides a purification method and device that uses dilute fluorosilicic acid medium for acidification to remove aluminum, enhances the acidification by mixing acid and hydrofluoric acid, and recycles the washing liquid, so as to realize the high-value conversion of by-product silica gel into high-quality white carbon black.

[0007] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a method for reducing aluminum content by silica gel acidification during the production of aluminum fluoride from fluorosilicic acid, comprising the following steps: Step S1. Prepare aluminum hydroxide raw materials into aluminum hydroxide slurry and transport it to the neutralization reactor; Step S2. Prepare a composite reinforcing acid and transport it to a neutralization reactor to react with aluminum hydroxide slurry. The composite reinforcing acid is hydrofluoric acid and fluorosilicic acid, or a mixture of hydrofluoric acid and acidified acid separated by subsequent acidification and filtration. Step S3. The aluminum hydroxide slurry and the composite reinforced acid are synthesized in a neutralization reactor. After the reaction is completed, solid-liquid separation is performed. The resulting aluminum fluoride liquid is sent to the crystallization process, and the resulting by-product silica gel is sent to the acidification reactor. Step S4. Add dilute fluorosilicic acid to the acidification reactor, preheat and start stirring to carry out the acidification and aluminum reduction reaction; Step S5. The acidified slurry is subjected to solid-liquid separation. The resulting acidic filtrate is returned to step S2 for the preparation of composite enhanced acid. The resulting silica gel solid is subjected to multi-stage countercurrent water washing. The collected washing water is returned to step S1 for the preparation of aluminum hydroxide slurry.

[0008] As a preferred option, in step S1, the solid content of the prepared aluminum hydroxide slurry is 20-40%, the slurry preparation process is carried out at room temperature, the stirring speed is 150-250 rpm, and the stirring is carried out for 10-20 minutes until the slurry is uniform and free of lumps. The solvent for slurry preparation is pure water or washing water generated in the washing process.

[0009] As a preferred embodiment, in step S2, the hydrofluoric acid has a mass concentration of 25-40%, the fluorosilicic acid is a dilute fluorosilicic acid solution with a mass concentration of 15-25%, and either the acidifying acid or the fluorosilicic acid is mixed with the hydrofluoric acid at a mass ratio of (3-15):1. The mixing process is carried out in an atmospheric pressure mixing tank at room temperature, with a stirring speed of 200-300 rpm, and stirring for 5-15 minutes until the system is homogeneous.

[0010] As a preferred embodiment, in step S3, the reaction temperature of the aluminum fluoride synthesis reaction is 80~95℃, the reaction time is 10~40min, the stirring speed is 200~300rpm, and the molar ratio of aluminum to fluorine in the reaction system is 1:(3.0~3.2).

[0011] As a preferred embodiment, in step S4, the solid-liquid mass ratio of the material in the acidification reactor is 1:(1.5~2.5), the fluorosilicic acid is a dilute fluorosilicic acid solution with a mass concentration of 15~25%, the acidification reactor is preheated to 75~95℃ and then the stirring device is started, the stirring speed is 250~350rpm, and the acidification reaction time is 20~40min.

[0012] As a preferred embodiment, in step S5, the acidified slurry is introduced into a plate and frame filter press for solid-liquid separation; the obtained silica gel solid is used to wash the filter cake with warm distilled water at 40~60℃ in multiple countercurrent stages, and the washing is stopped when the pH of the discharged filtrate reaches 6.5~7.5; the obtained silica gel solid is then washed with multiple countercurrent stages until the pH of the discharged filtrate reaches neutral, at which point the washing is stopped.

[0013] As a preferred option, in step S5, the multi-stage countercurrent washing adopts a three-stage countercurrent washing process: the first stage washing uses the washing water discharged from the second stage washing as the washing water, the second stage washing uses clean water, and the third stage washing uses deionized water.

[0014] As a preferred option, step S6 is also included. The wet filter cake that has been washed and qualified in step S5 is dried, cooled, weighed and packaged to obtain the finished precipitated silica. The drying is carried out by constant temperature drying or flash drying, with a drying temperature of 105~120℃ and a drying time of 2~4h, until the moisture content of the material is ≤5%.

[0015] In a second aspect, the present invention provides a system for reducing aluminum content by acidification of silica gel during the production of aluminum fluoride from fluorosilicic acid, comprising: The mixing tank has an aluminum hydroxide inlet and a washing water inlet at the top, a slurry outlet at the bottom, and a stirring device inside for mixing aluminum hydroxide slurry. The mixed acid reactor has its inlet connected to the hydrofluoric acid storage tank, the fluorosilicic acid storage tank, and the silica gel acidification filtration filtrate outlet, respectively. The outlet of the mixed acid reactor is connected to the inlet of the aluminum fluoride neutralization reactor. It is used to prepare composite acids. The neutralization reactor has an aluminum hydroxide slurry inlet and a composite acid inlet at the top, which are connected to the slurry preparation vessel and the mixed acid vessel, respectively. The reaction slurry outlet at the bottom of the reactor is connected to the first solid-liquid separation device for neutralization via a feed pump. The liquid phase outlet of the neutralization solid-liquid separation device is connected to the aluminum fluoride crystallization process, and the solid phase outlet is connected to the feed inlet of the acidification vessel to complete the neutralization synthesis reaction. The acidification reactor has a silica gel inlet and an acid inlet at the top and a slurry outlet at the bottom. An agitator is installed inside for silica gel acidification and aluminum reduction. The inlet of the second solid-liquid separation device is connected to the slurry outlet pipe of the acidification reactor. The filtrate outlet of the second solid-liquid separation device is connected to the mixing acid reactor for reflux preparation of composite enhanced acid. The solid phase outlet is connected to the silica gel washing machine for silica gel washing and deacidification. The washing water is reused in aluminum hydroxide slurry preparation. Attached Figure Description

[0016] Figure 1 This is a process flow diagram of one embodiment of the present invention; Figure 2 This is a schematic diagram of the system structure according to one embodiment of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased.

[0018] Unless otherwise stated, all percentages in this invention represent mass fractions. Ratios are mass percentages, and concentrations are mass concentrations.

[0019] Unless otherwise specified, all materials, instruments, and equipment used below are conventional materials, instruments, and equipment or obtained through commercial channels; all testing methods used are existing methods unless otherwise specified.

[0020] For the removal of impurities from silica, current technologies mainly employ the following approaches: Inorganic strong acid leaching method: This method mainly utilizes inorganic acids such as sulfuric acid or hydrochloric acid to dissolve aluminum and fluorine under acidic conditions. Although it can increase the SiO2 content to over 97%, it introduces SO4... 2- Cl - The acid radicals are difficult to completely wash away, resulting in salt residue in the product and generating a large amount of high-concentration acidic wastewater with high treatment costs.

[0021] Fluoride salt circulation washing method: This method uses ammonium fluoride solution for washing, relying on the common ion effect to suppress silicon dissolution. However, this method requires the purchase of large quantities of ammonium fluoride and ammonia, resulting in high operating costs and the introduction of NH4. + -N needs to be removed additionally in subsequent wastewater treatment, which is not conducive to large-scale continuous production.

[0022] Pure water multi-stage washing method: This method reduces soluble impurities in silica gel through repeated washing and pressing. However, since impurities in the by-product silica gel often exist in complexed or micro-phase encapsulated states, such as the silica gel encapsulation and surface adsorption of large amounts of Al in aluminum fluoride, this method is problematic. 3+ F - and aluminum fluoride complex ions (such as AlF6) 3- AlF4 -(etc.) Deep purification cannot be achieved solely through physical diffusion and hydraulic shearing. To meet the purity standards for silica, an extremely high water washing ratio is required, resulting in enormous water consumption and low production efficiency.

[0023] In existing technologies, sulfuric acid or hydrochloric acid is typically used to wash the by-product silica gel. However, the aluminum and fluorine impurities in the by-product silica gel do not simply adhere in the form of free ions, but rather as aluminum fluoride complex ions (such as AlF6). 3- ), surface precipitates and microphases and encapsulated states exist within micropores. Although traditional strong acid washing can disrupt some complexation equilibrium by lowering the pH value, it introduces acid radicals that are difficult to wash off, and it is difficult to remove deeply encapsulated impurities by relying solely on physical diffusion, resulting in extremely high water washing ratios and product purity still not meeting standards.

[0024] Furthermore, in the first aspect, such as Figure 1 As shown, this embodiment of the invention provides a method for reducing aluminum content by silica gel acidification during the production of aluminum fluoride from fluorosilicic acid, comprising the following steps: Step S1. Prepare aluminum hydroxide raw materials into aluminum hydroxide slurry and transport it to the neutralization reactor; Step S2. Prepare a composite reinforcing acid and transport it to a neutralization reactor to react with aluminum hydroxide slurry. The composite reinforcing acid is hydrofluoric acid and fluorosilicic acid, or a mixture of hydrofluoric acid and acidified acid separated by subsequent acidification and filtration. Step S3. The aluminum hydroxide slurry and the composite reinforced acid are synthesized in a neutralization reactor. After the reaction is completed, solid-liquid separation is performed. The resulting aluminum fluoride liquid is sent to the crystallization process, and the resulting by-product silica gel is sent to the acidification reactor. Step S4. Add dilute fluorosilicic acid to the acidification reactor, preheat and start stirring to carry out the acidification and aluminum reduction reaction; Step S5. The acidified slurry is subjected to solid-liquid separation. The resulting acidic filtrate is returned to step S2 for the preparation of composite enhanced acid. The resulting silica gel solid is subjected to multi-stage countercurrent water washing. The collected washing water is returned to step S1 for the preparation of aluminum hydroxide slurry.

[0025] In the wet process for preparing aluminum fluoride from fluorosilicic acid, the recycled acidified acid or fluorosilicic acid after silica gel acidification is mixed with added hydrofluoric acid in a room temperature mixing tank to form a reinforced acid system. This process simultaneously removes microcrystalline silicon from the fluorosilicic acid raw material, breaks down the aluminum hydroxide layer encapsulated in the silica gel, and allows the acidified acid to be recycled back into the main reaction in a closed loop.

[0026] In step S2, the composite reinforcing acid prepared is transported to the aluminum fluoride synthesis section in step S3 for reuse. The composite reinforcing acid is used to remove microcrystalline silicon impurities from the fluorosilicic acid raw material and to destroy the aluminum hydroxide reaction layer wrapped with silica gel in the reaction, thereby increasing the main reaction rate.

[0027] This invention constructs a resource recycling system that "treats waste with waste," using dilute fluorosilicic acid to replace traditional strong inorganic acids, thus avoiding SO4 emissions. 2- Cl - The introduction of heterogeneous anions will also transport the removed aluminum and fluorine elements to the raw materials for the synthesis of aluminum fluoride at the front end, realizing the transformation from pollution control to resource regeneration.

[0028] In one embodiment, in step S1, the solid content of the prepared aluminum hydroxide slurry is 20-40%. The slurry preparation process is carried out at room temperature, with a stirring speed of 150-250 rpm for 10-20 minutes until the slurry is uniform and free of lumps. The solvent for slurry preparation is pure water or washing water generated from the washing process. A solid content of 20-40% ensures the fluidity and reaction efficiency of the slurry, avoiding excessive thinness which would result in high energy consumption, and excessive thickness which would result in high slurry viscosity. Stirring at 150-250 rpm for 10-20 minutes ensures that the slurry is uniform and free of lumps, while also avoiding excessive particle refinement that could lead to silica gel formation and a tendency for the aluminum hydroxide to react.

[0029] In one embodiment, in step S2, the hydrofluoric acid has a mass concentration of 25-40%, and the fluorosilicic acid is a dilute fluorosilicic acid solution with a mass concentration of 15-25%. One of the acidifying acid or fluorosilicic acid is mixed with the hydrofluoric acid at a mass ratio of (3-15):1, which can be (5-10):1. The mixing process is carried out in an atmospheric pressure mixing tank at room temperature, with a stirring speed of 200-300 rpm, and stirring for 5-15 minutes until the system is homogeneous, without the need for external temperature control.

[0030] The composite enhanced acid system also has a dual synergistic effect: First, the addition of hydrofluoric acid significantly increases the concentration of free HF in the system, which can efficiently dissolve the microcrystalline silicon impurities (SiO2) entrained in the fluorosilicic acid raw material and prevent them from being converted back into silica gel in subsequent reactions; Second, in the aluminum fluoride synthesis reaction, this enhanced acid can penetrate and destroy the aluminum hydroxide passivation layer wrapped on the surface of the silica gel particles, breaking the stability of the aluminum-fluorine complex, thereby greatly improving the main reaction rate and the primary conversion rate of fluorine, solving the problems of reaction lag and excess aluminum caused by the coating layer in the traditional process.

[0031] Regarding the mass ratio, when the mass ratio is too low, the hydrofluoric acid concentration is insufficient, which cannot effectively remove the microcrystalline silicon and destroy the coating layer, resulting in a low aluminum fluoride yield. When the mass ratio is too high, although it can increase the reaction rate, the excessive amount of hydrofluoric acid leads to a significant increase in raw material costs and easily causes excessive dissolution of the silica matrix, resulting in poor economic efficiency.

[0032] In one embodiment, in step S3, the reaction temperature of the aluminum fluoride synthesis reaction is 80-95℃, the reaction time is 10-40 min, and the stirring speed is 200-300 rpm; the molar ratio of aluminum to fluorine in the reaction system is 1:(3.0-3.2). When the reaction temperature is 80-95℃, the reaction rate and the stability of fluorosilicic acid reach a suitable range. If the temperature is too low, the reaction will be incomplete; if it is too high, aluminum fluoride will precipitate and remain in the silica gel, causing product yield loss and silica gel contamination. The reaction time of 10-40 min ensures sufficient reaction; too short a time results in low yield, and too long a time decreases efficiency. The stirring speed of 200-300 rpm enhances mass transfer; too low a speed results in partial precipitation of aluminum hydroxide and incomplete reaction, while too high a speed causes splashing of the reaction liquid and increased energy consumption. The aluminum-fluorine molar ratio of 1:(3.0-3.2) is based on a stoichiometric ratio of 1:3, with a slight excess of fluorine to ensure complete aluminum reaction. Excessive fluorine will form aluminum-fluorine complex ions that are encapsulated by the silica gel, increasing the burden on subsequent aluminum removal.

[0033] In one embodiment, in step S4, the solid-liquid mass ratio of the material in the acidification reactor is 1:(1.5~2.50, the fluorosilicic acid is a dilute fluorosilicic acid solution with a mass concentration of 15~25%, the acidification reactor is preheated to 75~95°C and then the stirring device is started, the stirring speed is 250~350 rpm, for example 300 rpm, so that the silica gel particles are in a suspended state and no eddy splashing is generated, and the acidification reaction time is 20~40 min.

[0034] This invention uses dilute fluorosilicic acid (H₂SiF₆) as the acidification medium. The principle is as follows: On the one hand, by utilizing the common ion effect, high concentrations of SiF6 in the solution 2- It can inhibit the reaction SiO2 + 6HF → H2SiF6 + 2H2O, thereby preventing excessive dissolution and structural collapse of the silica matrix in an acidic environment; on the other hand, high concentrations of H... + This disrupts the charge balance on the silicone surface, destroys the electrostatic adsorption of aluminum ions and silanol groups (Si-OH), and forces AlF6... 3- Plasmon ions dissociate from the silica micropores and enter the liquid phase. Stirring speed ensures the silica particles remain completely suspended, enhancing mass transfer efficiency and preventing silica caking caused by localized overheating.

[0035] In one implementation method, in step S5, the acidified slurry is introduced into a plate and frame filter press for solid-liquid separation. The obtained silica gel solid is used to wash the filter cake in multiple stages of countercurrent water flow with distilled water at 40-60°C. Washing is stopped when the pH of the discharged filtrate reaches 6.5-7.5. The obtained silica gel solid is then washed in multiple stages of countercurrent water flow until the pH of the discharged filtrate reaches neutral. Using distilled water at 40-60°C for washing enhances the diffusion coefficient of ions, significantly reducing the washing frequency required to reach neutral pH and saving water resources.

[0036] In one implementation method, in step S5, the multi-stage countercurrent washing adopts a three-stage countercurrent washing process: the first stage washing uses the washing water discharged from the second stage washing as the washing water, the second stage washing uses clean water, and the third stage washing uses deionized water.

[0037] The washing process employs a three-stage countercurrent washing technique: the first stage uses recycled washing water, the second stage uses clean water, and the third stage uses deionized water. This process utilizes the concentration gradient difference to maximize the aluminum resource recovery rate while ensuring that the residual Al2O3 content is reduced to below 0.3%.

[0038] As one implementation method, it also includes step S6. The wet filter cake that has been washed and qualified in step S5 is dried, cooled, weighed and packaged to obtain the finished white carbon black product. The drying is carried out by constant temperature drying or flash drying, with a drying temperature of 105~120℃ and a drying time of 2~4h, until the moisture content of the material is ≤5%.

[0039] Secondly, embodiments of the present invention also provide a system for reducing aluminum content by acidification of silica gel during the production of aluminum fluoride from fluorosilicic acid, comprising: The mixing tank has an aluminum hydroxide inlet and a washing water inlet at the top, a slurry outlet at the bottom, and a stirring device inside for mixing aluminum hydroxide slurry. The mixed acid reactor has its inlet connected to the hydrofluoric acid storage tank, the fluorosilicic acid storage tank, and the silica gel acidification filtration filtrate outlet, respectively. The outlet of the mixed acid reactor is connected to the inlet of the aluminum fluoride neutralization reactor. It is used to prepare composite acids. The neutralization reactor has an aluminum hydroxide slurry inlet and a composite acid inlet at the top, which are connected to the slurry preparation vessel and the mixed acid vessel, respectively. The reaction slurry outlet at the bottom of the reactor is connected to the first solid-liquid separation device for neutralization via a feed pump. The liquid phase outlet of the neutralization solid-liquid separation device is connected to the aluminum fluoride crystallization process, and the solid phase outlet is connected to the feed inlet of the acidification vessel to complete the neutralization synthesis reaction. The acidification reactor has a silica gel inlet and an acid inlet at the top and a slurry outlet at the bottom. An agitator is installed inside for silica gel acidification and aluminum reduction. The inlet of the second solid-liquid separation device is connected to the slurry outlet pipe of the acidification reactor. The filtrate outlet of the second solid-liquid separation device is connected to the mixing acid reactor for reflux preparation of composite enhanced acid. The solid phase outlet is connected to the silica gel washing machine for silica gel washing and deacidification. The washing water is reused in aluminum hydroxide slurry preparation.

[0040] The solid-liquid separation equipment can be a plate and frame filter press. A silica gel washer can also be connected to a wash water buffer tank. The plate and frame filter press is connected to the silica gel washer. The wash water buffer tank can also be equipped with a pH detection element, and its outlet is connected to a slurry mixing tank via a reflux pump for recycling the wash liquid. Valves can be installed on the connecting pipelines of the above devices as needed, such as existing metering valves, to control the amount added.

[0041] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a silica gel acidification method and system for producing aluminum fluoride from fluorosilicic acid provided by the present invention.

[0042] like Figure 2 As shown, the apparatus for reducing aluminum content by silica gel acidification in the production of aluminum fluoride from fluorosilicic acid in this embodiment includes an acidification kettle, a neutralization reaction kettle, a mixed acid kettle, a slurry preparation kettle, a silica gel washing machine, a first plate and frame filter press, a second plate and frame filter press, and a dryer.

[0043] The slurry mixing tank is used to prepare aluminum hydroxide slurry. It has an aluminum hydroxide inlet and a washing water inlet at the top, a built-in stirring device, and a slurry outlet with a delivery pump at the bottom. The washing water inlet is connected to the outlet of the silicone washing machine, and the slurry outlet is connected to the neutralization reaction tank.

[0044] The mixed acid reactor is used to prepare composite acids. It is equipped with hydrofluoric acid inlet, fluorosilicic acid inlet and acidified acid inlet at the top, and has a built-in stirring device. The bottom is equipped with a mixed acid and composite acid outlet with a delivery pump. The acidified acid inlet is connected to the filtrate outlet of the second plate and frame filter press, and the mixed acid outlet is connected to the neutralization reaction vessel.

[0045] The neutralization reactor is used to complete the neutralization synthesis reaction. The top is equipped with a slurry inlet and a composite acid inlet that connect to the slurry preparation vessel and the mixed acid vessel. It has a built-in stirring device and a reaction slurry outlet with a delivery pump at the bottom, which is connected to the first plate and frame filter press.

[0046] The first plate and frame filter press is used to separate aluminum fluoride mother liquor from crude silica gel. The feed inlet is connected to the slurry outlet of the neutralization reactor, the filtrate outlet is connected to the aluminum fluoride crystallization section, and the solid phase outlet is connected to the acidification reactor.

[0047] The acidification kettle is used for silica gel acidification to reduce aluminum content. It has a horizontal or vertical structure, with a silica gel inlet and a fluorosilicic acid inlet at the top, a built-in variable frequency speed-regulating stirrer, and an acidified slurry outlet with a delivery pump at the bottom that connects to a second plate and frame filter press.

[0048] The second plate and frame filter press is used to separate acidified acid and acidified silica gel, so as to realize the acid solution recycling. Its inlet is connected to the slurry outlet of the acidification kettle, the filtrate outlet flows back to the mixing kettle, and the solid phase outlet is connected to the silica gel washing machine.

[0049] The silica gel washing machine is used for washing and deacidifying silica gel and reusing washing water. It has an acidified silica gel inlet, an acidic washing water inlet, and a pure water inlet at the top. The washing water outlet on the side returns to the washing water buffer tank. The water in the washing water buffer tank is used for the initial washing of the initial silica gel and is returned to the washing water buffer tank and sent to aluminum hydroxide for slurry preparation. The silica gel is then washed a second time with pure water, and the washing water is returned to the buffer tank for the next silica gel washing. The bottom outlet of the washing machine is connected to the dryer via a screw conveyor.

[0050] The dryer is equipped with a hot air furnace, and the feed end is connected to the discharge end of the silica gel washing machine. The wet silica gel is dried to obtain the finished product, white carbon black.

[0051] The aluminum hydroxide raw material in this embodiment is a commercially available product with a purity of 99.9%.

[0052] Example 1

[0053] The silica gel acidification method for reducing aluminum content in the production of aluminum fluoride from fluorosilicic acid in this embodiment includes the following steps: Step S1. Feed the aluminum hydroxide raw material into the slurry mixing tank, and introduce the acidic washing water recovered from the washing process to prepare the slurry; stir at 150 rpm for 10 minutes at room temperature until the slurry is uniform and free of lumps, and obtain an aluminum hydroxide slurry with a solid content of 20%, which is then transported to the neutralization reaction tank.

[0054] In the initial stage, aluminum hydroxide slurry is prepared with pure water, and the slurry is then prepared with acidic washing water generated during the washing process.

[0055] Step S2. Introduce the acidified acid separated by acidification filtration into the mixed acid reactor and mix it with hydrofluoric acid with a mass concentration of 25% at a mass ratio of 10:1. Stir at 200 rpm for 5 minutes under normal pressure and temperature until the system is homogeneous to prepare the composite enhanced acid, which is then transferred to the neutralization reactor.

[0056] In the initial state, the composite reinforcing acid is a mixture of 25% hydrofluoric acid and 15% fluorosilicic acid at a mass ratio of 10:1, which is then mixed with the acidified acid separated by acidification filtration.

[0057] Step S3. The composite reinforced acid is introduced into the neutralization reactor to react with the aluminum hydroxide slurry. The molar ratio of aluminum to fluorine in the system is controlled to be 1:3. The reaction is carried out at 80°C and 200 rpm for 10 min. After the reaction, the solid and liquid are separated by the first plate and frame filter press. The resulting aluminum fluoride liquid is sent to the crystallization process, and the by-product silica gel is sent to the acidification reactor.

[0058] Step S4. Add a 15% (w / w) dilute fluorosilicic acid solution to the acidification reactor to react with the by-product silica gel, and control the solid-liquid mass ratio of the materials to be 1:1.5; after preheating to 75°C, start stirring at 250 rpm to suspend the silica gel particles without eddy splashing, and keep the reaction at this temperature for 20 min.

[0059] Step S5. After acidification, the slurry is subjected to solid-liquid separation through a second plate and frame filter press. The resulting acidic filtrate, i.e., acidified acid, is returned to step S2 for the preparation of composite enhanced acid. The obtained silica gel solid is subjected to multi-stage countercurrent water washing in a silica gel washing machine using 40°C distilled water until the pH of the discharged washing liquid reaches 6.5. The washing is then stopped, and the acidic washing water is collected through a washing water buffer tank. The collected acidic washing water is returned to step S1 for slurry conditioning.

[0060] Step S6. The wet filter cake that has passed the washing in step S5 is sent to the dryer by a screw conveyor and dried at 105℃ for 2 hours until the moisture content of the material is less than 5.0%. After cooling, it is weighed and packaged to obtain the finished white carbon black product.

[0061] Example 2

[0062] The silica gel acidification method for reducing aluminum content in the production of aluminum fluoride from fluorosilicic acid in this embodiment includes the following steps: Step S1. Feed the aluminum hydroxide raw material into the slurry preparation tank, and introduce the acidic washing water recovered from the washing process to prepare the slurry; stir at 200 rpm for 15 minutes at room temperature until the slurry is uniform and free of lumps, and obtain an aluminum hydroxide slurry with a solid content of 30%, which is then transported to the neutralization reaction tank.

[0063] In the initial stage, aluminum hydroxide slurry is prepared with pure water, and the slurry is then prepared with acidic washing water generated during the washing process.

[0064] Step S2. Introduce the acid separated by acidification filtration into the mixed acid reactor and mix it with hydrofluoric acid with a mass concentration of 32% at a mass ratio of 7.5:1. Stir at 250 rpm for 10 min under normal pressure and temperature until the system is homogeneous to prepare the composite enhanced acid, and then transfer it to the neutralization reactor.

[0065] In the initial state, the composite reinforcing acid is a mixture of hydrofluoric acid with a mass concentration of 32% and fluorosilicic acid with a mass concentration of 20% at a mass ratio of 7.5:1, which is then mixed with the acidified acid separated by acidification filtration.

[0066] Step S3. The composite reinforced acid is introduced into the neutralization reactor to react with the aluminum hydroxide slurry. The molar ratio of aluminum to fluorine in the system is controlled to be 1:3.1. The reaction is carried out at 90°C and 250 rpm for 25 min. After the reaction, the solid and liquid are separated by the first plate and frame filter press. The resulting aluminum fluoride liquid is sent to the crystallization process, and the by-product silica gel is sent to the acidification reactor.

[0067] Step S4. Add by-product silica gel and a 20% (w / w) dilute fluorosilicic acid solution to the acidification reactor to react with the by-product silica gel, controlling the solid-liquid mass ratio of the materials to be 1:2; after preheating to 85°C, start stirring at 300 rpm to suspend the silica gel particles without eddy splashing, and keep the reaction at this temperature for 30 min.

[0068] Step S5. After acidification, the slurry is subjected to solid-liquid separation through a second plate and frame filter press. The resulting acidic filtrate, i.e., acidified acid, is returned to step S2 for the preparation of composite enhanced acid. The resulting silica gel solid is subjected to multi-stage countercurrent water washing in a silica gel washing machine using 50°C distilled water until the pH of the discharged washing liquid reaches 7.0. The washing water is then collected through a washing water buffer tank and returned to step S1 for slurry conditioning.

[0069] Step S6. The qualified wet filter cake from step S5 is sent to the dryer by a screw conveyor and dried under hot air at 110℃ until the moisture content is less than 5.0%. After cooling, it is weighed and packaged to obtain the finished white carbon black product.

[0070] Example 3

[0071] The silica gel acidification method for reducing aluminum content in the production of aluminum fluoride from fluorosilicic acid in this embodiment includes the following steps: Step S1. Feed the aluminum hydroxide raw material into the slurry preparation tank, and introduce the acidic washing water recovered from the washing process to prepare the slurry; stir at 250 rpm for 20 minutes at room temperature until the slurry is uniform and free of lumps, and obtain an aluminum hydroxide slurry with a solid content of 35%, which is then transported to the neutralization reaction tank.

[0072] In the initial stage, aluminum hydroxide slurry is prepared with pure water, and the slurry is then prepared with acidic washing water generated during the washing process.

[0073] Step S2. Introduce the acidified acid separated by acidification filtration into the mixed acid reactor and mix it with hydrofluoric acid with a mass concentration of 40% at a mass ratio of 5:1. Stir at 300 rpm for 15 minutes under normal pressure and temperature until the system is homogeneous to prepare the composite enhanced acid, which is then transferred to the neutralization reactor.

[0074] In the initial state, the composite reinforcing acid is a mixture of 40% hydrofluoric acid and 25% fluorosilicic acid at a mass ratio of 10:1, which is then mixed with the acidified acid separated by acidification filtration.

[0075] Step S3. The composite reinforced acid is introduced into the neutralization reactor to react with the aluminum hydroxide slurry. The molar ratio of aluminum to fluorine in the system is controlled to be 1:3.2. The reaction is carried out at 95°C and 300 rpm for 40 min. After the reaction, the solid and liquid are separated by the first plate and frame filter press. The resulting aluminum fluoride liquid is sent to the crystallization process, and the by-product silica gel is sent to the acidification reactor.

[0076] Step S4. Add by-product silica gel and a 25% (w / w) dilute fluorosilicic acid solution to the acidification reactor to react with the by-product silica gel, controlling the solid-liquid mass ratio of the materials to be 1:2.5; after preheating to 95°C, start stirring at 350 rpm to suspend the silica gel particles without eddy splashing, and keep the reaction at this temperature for 40 min.

[0077] Step S5. After acidification, the slurry is subjected to solid-liquid separation through a second plate and frame filter press. The resulting acidic filtrate, i.e., acidified acid, is returned to step S2 for the preparation of composite enhanced acid. The obtained silica gel solid is subjected to multi-stage countercurrent water washing in a silica gel washing machine using 60°C distilled water until the pH of the discharged washing liquid reaches 7.5. The washing water is then collected through a washing water buffer tank and returned to step S1 for slurry conditioning.

[0078] Step S6. The qualified wet filter cake from step S5 is sent to the dryer by a screw conveyor and dried under hot air at 110℃ until the moisture content is less than 5.0%. After cooling, it is weighed and packaged to obtain the finished white carbon black product.

[0079] Comparative Example 1 The same process conditions and parameters as in Example 2 were used, except that sulfuric acid was used for acidification.

[0080] Comparative Example 2 Using a similar process to Example 1 of CN102311123A as a control: Wet silica gel from the same batch was taken, and without adding any acid, deionized water was directly added to form a slurry. After stirring at 80°C for 30 minutes, the slurry was filtered. This process was repeated five times (i.e., five water washes) until the filtrate showed a neutral pH.

[0081] Comparative Example 3 Using the exact same process parameters as in Example 2, the acidified acid produced in step S3 is directly transported to the front-end aluminum fluoride synthesis section for reuse, without being mixed with hydrofluoric acid or subjected to enhanced acidification treatment (i.e., only fluorosilicic acid).

[0082] To clarify the effect of fluorosilicic acid acidification on the purity of silica gel during the acidification step, other operating parameters were fixed, and the following standards were followed: GB / T32678-2016 "Determination of Silica Content in Precipitated Hydrated Silica in Rubber Compounding Agents - Gravimetric Method", GB / T 32679-2016 "Determination of Aluminum, Iron and Titanium Content in Precipitated Hydrated Silica in Rubber Compounding Agents - Inductively Coupled Plasma Atomic Emission Spectrometry", GB / T 32680-2016 "Determination of Fluorine Content in Precipitated Hydrated Silica in Rubber Compounding Agents - Ion Selective Electrode Method", GB / T 32681-2016 "Determination of Sulfate Content in Precipitated Hydrated Silica in Rubber Compounding Agents - Turbidity Method", and GB / T32682-2016 "Determination of Loss on Ignition of Precipitated Hydrated Silica in Rubber Compounding Agents".

[0083] The silica samples prepared according to the above standards were tested, and the test results are shown in Table 1 below: Table 1. Experimental results of purity of silica products under different processing conditions

[0084] Raw silica gel: refers to the original by-product after the reaction of fluorosilicic acid and aluminum hydroxide to prepare aluminum fluoride, without any acidification or washing treatment (i.e., silica gel by-product of wet aluminum fluoride process).

[0085] As shown in Table 1, this invention reduces Al2O3 in by-product silica gel to below 0.35% and F to below 0.1% through the chemical dissociation and common ion effect of fluorosilicic acid medium, while maintaining a stable SiO2 purity of over 99%. This overcomes the problem that traditional pure water washing cannot remove encapsulated fluorine impurities. Compared to the sulfuric acid method, this process avoids SO42-. 2- The introduction of heterogeneous anions results in higher product purity and no salt residue. At the same time, the removed aluminum and fluorine impurities are directly reused as aluminum fluoride raw material liquid, realizing a closed-loop resource system of "treating waste with waste" and achieving significant economic and environmental benefits.

[0086] To clarify the effect of the addition of a strengthening acid in the reaction steps on the silica gel structure and reaction process, other operating parameters were fixed and determined according to GB / T 23656-2016 (Determination of specific surface area of ​​hydrated silica precipitated in rubber compounding agents by CTAB method), GB / T32698-2016 (Determination of particle size distribution of hydrated silica precipitated in rubber compounding agents by laser diffraction method), and the conversion rate of aluminum fluoride main reaction was determined by gravimetric method.

[0087] The silica samples and aluminum fluoride prepared according to the above standards were tested, and the test results are shown in Table 2 below: Table 2. Experimental results of silica product structure and conversion rate under different processing conditions.

[0088] As shown in Table 2, the combination of hydrofluoric acid and a stronger acid is key to improving the activity of silica gel and the yield of aluminum fluoride. Comparative Example 3 (without HF) had a specific surface area of ​​only 75.6 m² due to the inability to remove microcrystalline silicon. 2 / g, the aluminum fluoride yield drops to 85.5%; while when the hydrofluoric acid ratio is too low (<5:1), although acid consumption can be reduced, the yield improvement is limited (about 90%) due to incomplete removal of microcrystalline silicon; the preferred ratio of this invention is (5~10):1, especially 7.5:1, which ensures complete removal of microcrystalline silicon (specific surface area increases to 129.4~147.5m²). 2 / g), and stabilized the aluminum fluoride yield at around 95%. If the ratio is too high (>10:1), although the specific surface area can be further increased, the consumption of hydrofluoric acid will surge, and it will cause excessive dissolution of silica gel, resulting in an increase in solid waste, which does not meet the cost control requirements of industrial production.

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for reducing aluminum content by silica gel acidification during the production of aluminum fluoride from fluorosilicic acid, characterized in that, Includes the following steps: Step S1. Prepare aluminum hydroxide raw materials into aluminum hydroxide slurry and transport it to the neutralization reactor; Step S2. Prepare a composite reinforcing acid and transport it to a neutralization reactor to react with aluminum hydroxide slurry. The composite reinforcing acid is hydrofluoric acid and fluorosilicic acid, or a mixture of hydrofluoric acid and acidified acid separated by subsequent acidification and filtration. Step S3. The aluminum hydroxide slurry and the composite reinforced acid are synthesized in a neutralization reactor. After the reaction is completed, solid-liquid separation is performed. The resulting aluminum fluoride liquid is sent to the crystallization process, and the resulting by-product silica gel is sent to the acidification reactor. Step S4. Add dilute fluorosilicic acid to the acidification reactor, preheat and start stirring to carry out the acidification and aluminum reduction reaction; Step S5. The acidified slurry is subjected to solid-liquid separation. The resulting acidic filtrate is returned to step S2 for the preparation of composite enhanced acid. The resulting silica gel solid is subjected to multi-stage countercurrent water washing. The collected washing water is returned to step S1 for the preparation of aluminum hydroxide slurry.

2. The method according to claim 1, characterized in that, In step S1, the solid content of the prepared aluminum hydroxide slurry is 20-40%. The slurry preparation process is carried out at room temperature, with a stirring speed of 150-250 rpm, and stirring for 10-20 minutes until the slurry is uniform and free of lumps. The solvent for slurry preparation is pure water or washing water generated in the washing process.

3. The method according to claim 1, characterized in that, In step S2, the hydrofluoric acid has a mass concentration of 25-40%, and the fluorosilicic acid is a dilute fluorosilicic acid solution with a mass concentration of 15-25%. One of the acidifying acid or fluorosilicic acid is mixed with the hydrofluoric acid at a mass ratio of (3-15):

1. The mixing process is carried out in an atmospheric pressure mixing tank at room temperature, with a stirring speed of 200-300 rpm, and stirring for 5-15 minutes until the system is homogeneous.

4. The method according to claim 1, characterized in that, In step S3, the reaction temperature of the aluminum fluoride synthesis reaction is 80~95℃, the reaction time is 10~40min, and the stirring speed is 200~300rpm; the molar ratio of aluminum to fluorine in the reaction system is 1:(3.0~3.2).

5. The method according to claim 1, characterized in that, In step S4, the solid-liquid mass ratio of the material in the acidification reactor is 1:(1.5~2.5), the fluorosilicic acid is a dilute fluorosilicic acid solution with a mass concentration of 15~25%, the acidification reactor is preheated to 75~95℃ and then the stirring device is started, the stirring speed is 250~350rpm, and the acidification reaction time is 20~40min.

6. The method according to claim 1, characterized in that, In step S5, the acidified slurry is introduced into a plate and frame filter press for solid-liquid separation; the obtained silica gel solid is washed with distilled water at 40~60℃ in multiple countercurrent stages, and washing is stopped when the pH of the discharged filtrate reaches 6.5~7.

5. The obtained silica gel solid was subjected to multi-stage countercurrent water washing until the pH of the effluent reached neutral, at which point the washing was stopped.

7. The method according to claim 6, characterized in that, In step S5, the multi-stage countercurrent washing adopts a three-stage countercurrent washing process: the first stage washing uses the washing water discharged from the second stage washing as the washing water, the second stage washing uses clean water, and the third stage washing uses deionized water.

8. The method according to claim 1, characterized in that, The process also includes steps S6 and S5. After the wet filter cake that has been washed and qualified is dried and cooled, it is weighed and packaged to obtain the finished precipitated silica. The drying process adopts constant temperature drying or flash drying, with a drying temperature of 105~120℃ and a drying time of 2~4h, until the moisture content of the material is ≤5%.

9. A system for reducing aluminum content by silica gel acidification during the production of aluminum fluoride from fluorosilicic acid, characterized in that, include: The mixing tank has an aluminum hydroxide inlet and a washing water inlet at the top, a slurry outlet at the bottom, and a stirring device inside for mixing aluminum hydroxide slurry. The mixed acid reactor has its inlet connected to the hydrofluoric acid storage tank, the fluorosilicic acid storage tank, and the silica gel acidification filtration filtrate outlet, respectively. The outlet of the mixed acid reactor is connected to the inlet of the aluminum fluoride neutralization reactor. It is used to prepare composite acids. The neutralization reactor has an aluminum hydroxide slurry inlet and a composite acid inlet at the top, which are connected to the slurry preparation vessel and the mixed acid vessel, respectively. The reaction slurry outlet at the bottom of the reactor is connected to the first solid-liquid separation device for neutralization via a feed pump. The liquid phase outlet of the neutralization solid-liquid separation device is connected to the aluminum fluoride crystallization process, and the solid phase outlet is connected to the feed inlet of the acidification vessel to complete the neutralization synthesis reaction. The acidification reactor has a silica gel inlet and an acid inlet at the top and a slurry outlet at the bottom. An agitator is installed inside for silica gel acidification and aluminum reduction. The inlet of the second solid-liquid separation device is connected to the slurry outlet pipe of the acidification reactor. The filtrate outlet of the second solid-liquid separation device is connected to the mixing acid reactor for reflux preparation of composite enhanced acid. The solid phase outlet is connected to the silica gel washing machine for silica gel washing and deacidification. The washing water is reused in aluminum hydroxide slurry preparation.

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