Method for preparing calcium fluoride and co-producing nanoscale white carbon black from fluosilicic acid

Abstract

The invention relates to the field of preparation of inorganic chemical materials, in particular to a method for preparing calcium fluoride and co-producing nanoscale white carbon black from fluosilicic acid, which comprises the following steps: putting calcium carbonate slurry and fluosilicic acid as raw materials into a static mixer and then into a reaction kettle, and controlling the pH of a reaction endpoint; after the reaction, separating a nanoscale calcium fluoride coarse filter cake, carrying out size mixing on the coarse filter cake by hydrofluoric acid, filtering and washing by using a plate-and-frame filter press, and returning the obtained filtrate and washing liquor as a raw material fluosilicic acid; after adjusting the pH value of the filtrate of the belt filter, separating nanoscale white carbon black through an ultrahigh pressure laminated filter press, and freeze-drying the white carbon black to obtain a final product. Filtrate obtained after ultrahigh-pressure laminated filter pressing is used as process water to be recycled, and no solid waste is generated in the whole process.

Description

TECHNICAL FIELD

[0001] The application relates to the field of inorganic chemical material preparation, and particularly relates to a method for preparing calcium fluoride and co-producing nanoscale white carbon black from fluosilicic acid. BACKGROUND

[0002] Calcium fluoride (also known as fluorite or fluorite) is the core carrier of fluorine resources in nature, is a non-renewable strategic non-metallic mineral raw material, has stable chemical properties, strong corrosion resistance, and excellent characteristics such as high transmittance, low refractive index and strong ion conductivity, and plays an irreplaceable role in many fields. From the classification of grades, acid-grade fluorite (purity greater than or equal to 97%) is the core standard of fluorite for chemical industry, and is a key raw material for high-end industries. Different grades of fluorite have fully penetrated into various links of basic industries and high-end manufacturing: ordinary grade fluorite is a fluxing agent for the metallurgical industry, a mineralizing agent for the building materials industry, a color and fluxing agent for ceramic glaze, and an important skeleton material for the biomedical field, supporting the stable operation of basic industries such as metallurgy, building materials and ceramics; high-purity fluorite, with its unique physical and chemical properties, has become a core basic material in high-end fields such as national defense and optics, and is widely used in key components such as optical lithography lenses, optical instrument windows, laser crystals and inorganic scintillation crystals. It is also a potential solid-state electrolyte material, providing core support for the development of new energy and emerging fluorine chemical industries. Its application covers the whole industry chain from basic industry to high-end manufacturing, and is of great significance to the continuity of industrial production, breakthrough of high-end technology and safety of national strategic industries, and is a "necessary raw material" to ensure the sustainable development of global industries.

[0003] Although fluorite has outstanding strategic value and wide demand, the supply pattern of natural fluorite has been difficult to match the global market demand which continues to grow, and the development of artificial fluorite has become an inevitable choice. Under this background, artificial fluorite can break through the reserves limit of natural resources, accurately match the differentiated requirements of fluorite purity and performance in different industries, and avoid environmental pressure and policy control risks brought by natural mining, becoming a core solution to make up for resource shortage, solve the contradiction between "supply tightening and demand growth", ensure the stability of global industry chain and support the sustainable development of basic industries and strategic emerging industries. Its necessity has become an industry consensus.

[0004] As a key raw material for preparing artificial fluorite (calcium fluoride), fluosilicic acid has developed four core process routes, namely ammonia method, calcium fluosilicate thermal decomposition method, slaked lime method and calcium carbonate method, which have different characteristics in reaction principle, operation condition, product performance and economy.

[0005] Each of the four processes has its own advantages and disadvantages. The ammonia method and the calcium carbonate method have outstanding advantages in terms of product purity or cost control. The calcium fluorosilicate thermal decomposition method and the quicklime method need to overcome bottlenecks in terms of energy consumption reduction and separation technology optimization to provide more suitable options for the industrial production of artificial fluorite.

[0006] Meanwhile, existing fluorosilicic acid processing technologies have low resource utilization and insufficient added value, and traditional calcium fluoride preparation processes are unable to meet the demands of the high-end market, requiring additional purification processes that increase costs.

[0007] The co-production process of silica also has environmental shortcomings and economic defects, and cannot meet the needs of high-end rubber and coating fields. Summary of the Invention

[0008] In order to improve the resource utilization rate and added value of fluorosilicic acid treatment processes, the present invention aims to provide a method for preparing calcium fluoride from fluorosilicic acid and co-producing nano-sized silica. The present invention utilizes the reaction of fluorosilicic acid and calcium carbonate to simultaneously prepare nano-sized calcium fluoride and co-produce nano-sized silica, thereby realizing the full and high-value utilization of fluorosilicic acid resources and zero solid waste discharge.

[0009] The specific technical solution of the present invention is as follows: A method for preparing calcium fluoride and co-producing nano-sized silica from fluorosilicic acid includes the following steps: Step (1) Raw material preparation: Prepare calcium carbonate solid into calcium carbonate slurry, and obtain fluorosilicic acid clear liquid by plate and frame filter. Step (2) Preparation of calcium fluoride: The calcium carbonate slurry from step (1) and fluorosilicic acid are put into a static mixer and then into a reaction vessel for mixing and reaction. The pH value of the reaction system is monitored in real time, and the pH value at the end of the reaction is controlled to be 3.8~4.0. Step (3) First solid-liquid separation: The reaction product of step (2) is sent to a filtration device for solid-liquid separation to obtain nano-sized calcium fluoride filter cake and first filtrate; the calcium fluoride filter cake is washed with dilute hydrofluoric acid solution and then separated again by plate and frame filter press to obtain refined calcium fluoride filter cake and second filtrate. The second filtrate is returned as raw material fluorosilicic acid, and the refined filter cake is dried to obtain nano-sized calcium fluoride. Step (4) Co-production of silica: Add an alkaline agent to the first filtrate of step (3) to adjust the pH value of the first filtrate to 7.0±0.2, and obtain a mixture containing silica gel; Step (5) Third solid-liquid separation: The mixture from step (4) is sent to an ultra-high pressure stacked filter press for deep solid-liquid separation to obtain nano-sized silica filter cake and third filtrate; the nano-sized silica is freeze-dried to obtain high-quality white carbon black; Step (6) Process water circulation: The third filtrate from step (5) is used as process water and circulated for the preparation of calcium carbonate slurry in step (1), and for the washing of calcium fluoride filter cake in step (3) and silica filter cake in step (5).

[0010] The method of this invention optimizes resource utilization: it can utilize low-concentration fluorosilicic acid to fully utilize the fluorine (converted into nano-calcium fluoride) and silicon (converted into nano-silica) in the low-value-added fluorosilicic acid, thereby increasing the added value of the product by 5 to 8 times.

[0011] The method of this invention is environmentally friendly and produces no solid waste: no waste residue is generated throughout the entire process, process water is 100% recycled, and wastewater discharge is reduced to zero, which complies with national environmental protection policies.

[0012] The method of this invention has strong process controllability: by fixing the molar ratio and the pH at the reaction endpoint, the particle size of the product is ensured to be stable at the nanometer level, with a pass rate of >98%.

[0013] The method of this invention is economically superior: it eliminates the costs of solid waste treatment and fresh water procurement, reducing overall production costs by 30-40%, while ensuring process stability, making it suitable for large-scale industrial production. No solid waste is generated throughout the process, achieving high-value utilization of fluorine and silicon resources in fluorosilicic acid.

[0014] As a preferred option, in step (1), a calcium carbonate slurry with a mass concentration of 20-40% is prepared. During preparation, the stirring speed is 200-300 rpm to ensure that the slurry is uniform and there is no obvious particle sedimentation. At the same time, the original fluorosilicic acid is filtered through a plate and frame filter to obtain a fluorosilicic acid clear liquid with a mass concentration of 4-30% and a solid content of <0.05%.

[0015] Preferably, in step (2), the molar ratio of calcium carbonate to fluorosilicic acid solution is 3.05:1; 20-40% calcium carbonate slurry and 4-30% fluorosilicic acid are simultaneously added to a static mixer at a rate of 15.7-43.1 kg / min and then into a reaction vessel. The stirring speed in the reaction vessel is 250-300 rpm, the reaction temperature is controlled at 25-35℃, and the reaction time is 10-30 min.

[0016] Preferably, in step (2), when the pH drops to 3.8~4.0, the addition of fluorosilicic acid is stopped, and stirring is continued until the reaction is complete to ensure sufficient reaction.

[0017] Preferably, in step (3), the filtration equipment is a belt filter, and the filter cloth is cleaned with a high-pressure water gun at a working pressure of 0.8~1.2MPa to avoid filter cake clogging; the nano-grade calcium fluoride filter cake is washed with process water 2~3 times.

[0018] In step (3), the filter cake is washed with dilute hydrofluoric acid solution to remove residual silica, and then solid-liquid separation is performed by plate and frame filter press.

[0019] Preferably, in step (4), the alkali is a sodium hydroxide solution with a mass concentration of 10-15% or a sodium bicarbonate solution with a mass concentration of 8-12%, and the stirring speed is 150-200 rpm when adjusting the pH value to ensure that the silicate ions in the solution are converted into silica gel.

[0020] Preferably, in step (5), the operating pressure of the ultra-high pressure stacked filter press is 10~15MPa; the nano-sized silica filter cake needs to be washed with process water until the conductivity of the filter cake washing liquid is <50μS / cm, and the washed filter cake is freeze-dried to obtain nano-sized silica product with a purity >99.8%, a particle size of 30~80nm, and a specific surface area of ​​300~450m². 2 / g.

[0021] As a preferred option, in step (5), the freeze drying specifically involves: pre-freezing at -40~-60℃ for 2~2.5h, sublimation temperature at 10~20℃, desorption temperature at 30~40℃, and shortening the desorption time to 2~3h.

[0022] Preferably, in step (6), the conductivity of the process water needs to be tested before the process water is circulated to ensure that the conductivity is <80μS / cm; if the conductivity of the process water is ≥80μS / cm, it needs to be purified by ion exchange resin before being circulated for the next preparation of calcium carbonate slurry, calcium fluoride washing solution, and silica gel washing solution. Attached Figure Description

[0023] Figure 1 This is a process flow diagram of one embodiment of the present invention. Detailed Implementation

[0024] 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.

[0025] Unless otherwise stated, all percentages in this invention represent mass fractions. Ratios are mass percentages, and concentrations are mass concentrations.

[0026] 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.

[0027] In existing technologies, the ammonia process achieves efficient conversion of fluorine resources through a two-step reaction. The core reaction consists of a first reaction of fluorosilicic acid with ammonia gas or ammonia water, followed by a subsequent reaction with calcium hydroxide, specifically: 6NH3 + H2SiF6 + 2H2O = 6NH4F + SiO2, followed by: 3Ca (OH)2+6NH4F+3H2O=3CaF2+6NH3+9H2O The silica produced in the first step is insoluble in water and can be quickly separated by filtration. The second step produces the target product, calcium fluoride, and recyclable ammonia. The outstanding advantage of this process is that the fluoride recovery rate is high and the resulting calcium fluoride has excellent purity. However, it is limited by the long process flow, the high energy consumption caused by the ammonia distillation step, and the difficulty in separating calcium fluoride during the reaction, resulting in a relatively high overall production cost and limiting its application on a large industrial scale.

[0028] The thermal decomposition method for calcium fluorosilicate adopts a "reaction-high temperature decomposition-recycling" technical approach. First, the reaction CaO + H2SiF6 = CaSiF6 + H2O causes fluorosilicic acid to combine with calcium oxide to form calcium fluorosilicate precipitate. After filtration to obtain a solid filter cake, thermal decomposition is carried out at a high temperature of 200~600 ℃, resulting in the reaction CaSiF6 = CaF2↓ + SiF4↑, which generates calcium fluoride solid and silicon tetrafluoride gas. The latter can be recycled by regenerating fluorosilicic acid after being absorbed by water. Simultaneously, silica is produced as a byproduct. However, this process has the disadvantage of low calcium fluoride yield and relies on a large amount of silicon tetrafluoride gas to participate in the recycling process, resulting in high system energy consumption and significantly affecting the overall economic efficiency.

[0029] The quicklime process employs a simplified direct reaction route, where fluorosilicic acid reacts directly with calcium hydroxide to produce calcium fluoride and silicon dioxide via the reaction 3Ca(OH)₂ + H₂SiF₆ = 3CaF₂↓ + SiO₂↓ + 4H₂O. Its core advantages are rapid reaction and a simple process. However, the reaction environment is susceptible to OH⁻ introduced by calcium hydroxide. - The pH value is difficult to control precisely, and the separation of the calcium fluoride and silica mixture is also difficult. Although some literature suggests that the density difference between the two substances can be used to improve the separation technology, the problem of insufficient reaction between calcium hydroxide and fluorosilicic acid still exists. This results in low purity of the final calcium fluoride product, which often contains unreacted calcium hydroxide impurities. In addition, the utilization rate of the by-product silica is low due to the limitations of separation technology.

[0030] The calcium carbonate method is characterized by its low cost and ease of operation. It involves the reaction H₂SiF₆ + 3CaCO₃ = 3CaF₂↓ + 3CO₂↑ + SiO₂·nH₂O, using widely available and inexpensive limestone (primarily calcium carbonate) to neutralize fluorosilicic acid and prepare calcium fluoride. The resulting product can contain over 90% calcium fluoride. This process requires no complex equipment, the reaction can proceed smoothly at room temperature and pressure, and it is simple and easy to control. It also possesses good process control potential; adjusting reaction parameters such as pH can further improve product purity. However, its drawback lies in the relatively high impurity content of the product, which requires optimization and improvement through subsequent purification processes.

[0031] Meanwhile, existing fluorosilicic acid treatment processes have low resource utilization and insufficient added value. For example, the calcium fluoride production method disclosed in patent CN101134595A, although using fluorosilicic acid and calcium oxide as raw materials to co-produce silica, requires high-temperature decomposition of calcium fluorosilicate intermediates at 200~600℃, which not only consumes a large amount of energy, but also has a silicon resource recovery rate of only about 70% during the silicon tetrafluoride gas absorption hydrolysis process, leaving some silica gel residue. In addition, the fluorosilicic acid to anhydrous hydrogen fluoride technology adopted by Guizhou Phosphate Group, although achieving fluorine resource recovery, has high equipment investment, long construction period, and does not effectively utilize silicon resources, with a resource utilization rate of less than 50%, making it difficult to meet the needs of small and medium-sized enterprises.

[0032] Traditional calcium fluoride preparation processes are insufficient to meet the demands of the high-end market: Existing patents, such as CN101134595A, do not precisely control the reaction endpoint, but only adjust the process by fixing the reaction time, resulting in large fluctuations in the particle size of calcium fluoride products (generally 1~5μm). This fails to meet the purity (≥99%) and particle size uniformity requirements of high-end fields such as optical glass and lithium battery separators for 50~100nm nanoscale products. Furthermore, the process does not include dedicated washing and separation optimization schemes, and the residual fluoride ion content on the product surface often exceeds 500ppm, requiring additional purification treatment and increasing costs.

[0033] The co-production process of silica has environmental shortcomings and economic defects: The publicly disclosed co-production technologies generally lack a complete system water circulation design. For example, CN101134595A only achieves partial circulation of the fluorosilicic acid solution, and the silica washing filtrate is directly discharged, consuming 0.3 tons of water per ton of product. This not only wastes water resources but also requires additional treatment facilities due to the discharge of fluoride-containing wastewater. Furthermore, existing processes mostly use conventional plate and frame filter presses to separate silica, resulting in filter cake moisture content exceeding 70%, increasing subsequent drying energy consumption by 40%, and the product specific surface area is generally below 400 m². 2 / g cannot meet the needs of high-end rubber and coating industries.

[0034] Furthermore, this invention provides a method for preparing calcium fluoride and co-producing nano-sized silica from fluorosilicic acid, comprising the following steps: Step (1) Raw material preparation: Prepare calcium carbonate solid into calcium carbonate slurry, and obtain fluorosilicic acid clear liquid by plate and frame filter. Step (2) Preparation of calcium fluoride: The calcium carbonate slurry from step (1) and fluorosilicic acid are put into a static mixer and then into a reaction vessel for mixing and reaction. The pH value of the reaction system is monitored in real time, and the pH value at the end of the reaction is controlled to be 3.8~4.0. Step (3) First solid-liquid separation: The reaction product of step (2) is sent to a filtration device for solid-liquid separation to obtain nano-sized calcium fluoride filter cake and first filtrate; the calcium fluoride filter cake is washed with dilute hydrofluoric acid solution and then separated again by plate and frame filter press to obtain refined calcium fluoride filter cake and second filtrate. The second filtrate is returned as raw material fluorosilicic acid, and the refined filter cake is dried to obtain nano-sized calcium fluoride. Step (4) Co-production of silica: Add an alkaline agent to the first filtrate of step (3) to adjust the pH value of the first filtrate to 7.0±0.2, and obtain a mixture containing silica gel; Step (5) Third solid-liquid separation: The mixture from step (4) is sent to an ultra-high pressure stacked filter press for deep solid-liquid separation to obtain nano-sized silica filter cake and third filtrate; the nano-sized silica is freeze-dried to obtain high-quality white carbon black; Step (6) Process water circulation: The third filtrate from step (5) is used as process water and circulated for the preparation of calcium carbonate slurry in step (1), and for the washing of calcium fluoride filter cake in step (3) and silica filter cake in step (5).

[0035] In this embodiment of the invention, calcium carbonate slurry and fluorosilicic acid are used as raw materials. After being added to a static mixer, calcium carbonate and fluorosilicic acid enter the reaction vessel, and the pH value at the final reaction point is controlled. First, a coarse filter cake of nano-sized calcium fluoride is separated by a belt filter. The coarse filter cake of calcium fluoride is then slurried with hydrofluoric acid and filtered and washed by a plate and frame filter press. The filtrate and washing liquid are returned as raw material fluorosilicic acid. The filtrate from the belt filter is adjusted to pH ≈ 7 with sodium hydroxide and then separated into nano-sized silica by an ultra-high pressure stacked filter press. The nano-sized silica is freeze-dried to obtain the final product. The filtrate after ultra-high pressure stacked filter press is recycled as process water, and no solid waste is generated throughout the process.

[0036] The reaction mechanisms involved in this invention mainly include: H2SiF6+3CaCO3=3CaF2↓+3CO2↑+SiO2.nH2O The main technical challenges of this invention are the separation of calcium fluoride and orthosilicic acid, and the residual calcium carbonate in the system reaction. In the laboratory, effective separation is achieved by controlling the molar ratio of the two, the simultaneous addition rate of both, and the final pH. Industrially, a static mixer and optimized filtration equipment, employing a belt filter and plate and frame filter press, are used to ensure a complete reaction and rapid, efficient separation of calcium fluoride and orthosilicic acid. Considering the properties of orthosilicic acid, a freeze-drying centrifugal drying process combined with an ultra-high pressure laminated filter press is used to produce high specific surface area nano-sized silica, ensuring system water circulation. This invention does not generate wastewater or waste, thus enabling the engineering application of this process.

[0037] In one implementation method, in step (1), a calcium carbonate slurry with a mass concentration of 20-40% is prepared. During preparation, the stirring speed is 200-300 rpm to ensure that the slurry is uniform and there is no obvious particle sedimentation. At the same time, the original fluorosilicic acid is filtered through a plate and frame filter to obtain a fluorosilicic acid clear liquid with a mass concentration of 4-30% and a solid content of <0.05%. The fluorosilicic acid is then pressure filtered to remove the silica gel inside, and the solid content is controlled to be <0.05%.

[0038] In one embodiment, in step (2), the molar ratio of calcium carbonate to fluorosilicic acid slurry is 3.05:1; 20-40% calcium carbonate slurry is added simultaneously to a static mixer at a rate of 11.5-22.7 kg / min and 4-30% fluorosilicic acid at a rate of 15.7-43.1 kg / min, and then the mixture is introduced into a reaction vessel. The stirring speed in the reaction vessel is 250-300 rpm, the reaction temperature is controlled at 25-35℃, and the reaction time is 10-30 min. In step (2), a certain amount of water is first added to the reaction vessel, and the stirring is started after the water has submerged the stirring paddle.

[0039] The addition of calcium fluoride at the same time is to strictly control the molar ratio, ensure thorough mixing, and generate the highest purity calcium fluoride. It also facilitates the effective separation of calcium fluoride from silicon dioxide later.

[0040] Rotation speed control is used to keep the calcium fluoride stirred and prevent it from settling to the bottom. The overflow system, connected in series, ensures a stable overall calcium fluoride concentration, which helps control the reaction residence time. Temperature and time control are both essential to ensure the longest possible time for liquid silica to be filtered, allowing for rapid filtration within a confined space to separate calcium fluoride from liquid silica, thus ensuring the purity of calcium fluoride and improving the economics of the process.

[0041] In one implementation method, in step (2), when the pH drops to 3.8~4.0, the addition of fluorosilicic acid is stopped, and stirring continues until the reaction is complete, ensuring a sufficient reaction. The pH value at the reaction endpoint can be monitored using an online pH sensor with an accuracy of ±0.05pH, ensuring that the control deviation at the reaction endpoint is ≤0.1pH.

[0042] In this invention, calcium carbonate and fluorosilicic acid solution are simultaneously added to a static mixer in a specific ratio, rather than one solution being added to another. The pH is maintained at 3.8-4, effectively separating calcium fluoride and silica. Within this range, silica remains in a "liquid silica" state for the longest time. pH ≈ 3.8 is the dividing point, where the specific surface area is largest, the degree of polymerization is smallest, and the monodispersity is best, facilitating subsequent solid-liquid separation and ensuring the purity of calcium fluoride while maintaining the economic efficiency of the process. When the pH value is below the dividing point, the specific surface area increases with increasing pH, meaning the degree of polymerization gradually decreases. When the pH value exceeds 3.8, the specific surface area gradually decreases, the degree of polymerization gradually increases, and eventually plateaus.

[0043] In one implementation method, in step (3), the filtration equipment is a belt filter, and the filter cloth is cleaned with a high-pressure water gun at a working pressure of 0.8~1.2MPa to avoid filter cake clogging; the nano-grade calcium fluoride filter cake is washed with process water 2~3 times.

[0044] The calcium fluoride filter cake after washing in this invention is purified with dilute hydrofluoric acid, and then dried by a second solid-liquid separation using a plate and frame filter press.

[0045] In step (3), the filter cake is washed with a dilute hydrofluoric acid solution to remove residual silica, and then solid-liquid separation is performed by plate and frame filtration. The concentration of dilute hydrofluoric acid can be 5-15%, and the liquid-to-solid ratio is 1:2.

[0046] The filtrate obtained from calcium fluoride separation and washing is returned as raw material fluorosilicic acid, which can improve the overall fluoride yield of the process, with a fluoride yield >98%. After washing with a small amount of circulating process water to remove residual fluoride ions, the dried calcium fluoride particles have a size of 50~100nm, a purity ≥97%, silica <0.5%, and calcium carbonate <0.5%.

[0047] The liquid silica separation time in this invention is approximately 4 hours. After 4 hours, the liquid silica gradually transforms into a gel state, making it difficult to effectively separate calcium fluoride from silica, thus affecting the purity of calcium fluoride and reducing the economic efficiency of the process. A belt filter can ensure rapid separation of calcium fluoride from silica while it is in the liquid silica state.

[0048] In one embodiment, in step (4), the alkali is a sodium hydroxide solution with a mass concentration of 10-15% or a sodium bicarbonate solution with a mass concentration of 8-12%. When adjusting the pH value, the stirring speed is 150-200 rpm to ensure that the silicate ions in the solution are converted into silica gel. This stirring speed and mass concentration, under the most economical conditions (low reagent consumption), rapidly converts silicate ions into silica gel, thereby improving the pressure filtration efficiency and reducing the floor space and investment.

[0049] In one embodiment, in step (5), the operating pressure of the ultra-high pressure stacked filter press is 10~15MPa; the nano-sized silica filter cake needs to be washed with process water until the conductivity of the filter cake washing liquid is <50μS / cm, and the washed filter cake is freeze-dried to obtain nano-sized silica product with a purity >99.8%, a particle size of 30~80nm, and a specific surface area of ​​300~450m². 2 / g. Deep solid-liquid separation is performed using an ultra-high pressure multilayer filter press.

[0050] Due to its inherent properties, nano-sized silica is difficult to keep below 80% moisture content. However, by using an ultra-high pressure stacked filter press for deep solid-liquid separation, the moisture content of the material can be reduced to 80%.

[0051] In one implementation method, step (5) specifically involves pre-freezing at -40 to -60°C for 2 to 2.5 hours, sublimation temperature at 10 to 20°C, and desorption temperature at 30 to 40°C, thus shortening the desorption time to 2 to 3 hours. Based on the properties of nano-sized silica, the freeze-centrifugal drying time can be shortened.

[0052] In one implementation, in step (6), the conductivity of the process water needs to be tested before circulation to ensure that the conductivity is <80μS / cm; if the conductivity of the process water is ≥80μS / cm, it needs to be purified by ion exchange resin before circulation for use in the next preparation of calcium carbonate slurry, calcium fluoride washing solution, and silica gel washing solution. The process water circulation is used in two stages: Prepare a 40% calcium carbonate slurry to replace fresh water and reduce water consumption; wash the calcium fluoride and silica filter cake to reduce wastewater discharge and ensure a 100% recycling rate.

[0053] The water used in this process can be largely reused. However, due to the special nature of the phosphate chemical industry, the conductivity of the demineralized water in the sulfuric acid plant needs to be controlled. Therefore, for the demineralized water process, ion exchange resins are required to control the conductivity of the water below 80 μS / cm.

[0054] The filtrate treated by this invention has P, F, Ca, and S all <100ppm, making it fully usable in the field of phosphorus chemical industry.

[0055] For ion exchange resin, you can choose Lanxiao Technology LXC-108Fe (for high fluorine and high phosphorus applications) and purify it using conventional methods.

[0056] When recycling process water, the filtrate (i.e., process water) from the ultra-high pressure stacked filter press is collected and stored in a circulating water tank.

[0057] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a method for preparing calcium fluoride and co-producing nano-sized silica from fluorosilicic acid provided by the present invention.

[0058] In the embodiment, the static mixer SV-5-20 / 200 is made of 316L material.

[0059] The belt filter is Jingjin Environmental Protection's DU-50 model.

[0060] BYTS Ultra-High Pressure Filter Press

[0061] Example 1 like Figure 1 As shown in this embodiment, the method for preparing calcium fluoride and co-producing nano-sized silica from fluorosilicic acid includes the following steps: Step (1) Raw material preparation: Prepare calcium carbonate solid into calcium carbonate slurry with a mass concentration of 40% (CaCO3 content 400g / L). During preparation, the stirring speed is 250rpm. At the same time, fluorosilicic acid is filtered through plate and frame to obtain fluorosilicic acid clear liquid with a mass concentration of 10% and a solid content of <0.01% (H2SiF6 content 100g / L).

[0062] Step (2) Preparation of calcium fluoride: The calcium carbonate to fluorosilicic acid solution is fed at a molar ratio of 3.05:1. 40% calcium carbonate slurry and 10% fluorosilicic acid are added simultaneously to the static mixer at a rate of 15 kg / min and 20 kg / min, and then into the reactor. The stirring speed in the reactor is 250 rpm, the reaction temperature is controlled at 30℃, and the reaction time is 10 min. When the pH drops to 3.9, the addition of fluorosilicic acid is stopped, and stirring is continued until the reaction is complete.

[0063] Step (3) First solid-liquid separation: The reaction product from step (2) is fed into a belt filter for solid-liquid separation. The filter cloth is cleaned with a high-pressure water gun at a working pressure of 1.0 MPa. Nanoscale calcium fluoride filter cake and first filtrate (mainly composed of orthosilicic acid) are obtained. The calcium fluoride filter cake is slurried and washed with dilute hydrofluoric acid (concentration 10%) solution (the washing liquid is filtered to form a second washing liquid). Then, a plate and frame filter press is used for secondary separation to obtain refined calcium fluoride filter cake and second filtrate (mainly composed of dilute fluorosilicic acid solution). The second filtrate is returned as raw material fluorosilicic acid. The refined filter cake is dried to obtain nanoscale calcium fluoride. The nanoscale calcium fluoride filter cake is washed 3 times with process water. After drying, the filter cake has a particle size of 65 nm, a purity of 97.6%, and a silica content of 0.42%.

[0064] The dilute hydrofluoric acid is refined. The process water or silica pressure filter return water is used on the plate and frame filter.

[0065] Step (4) Co-production of silica: Add 15% sodium hydroxide solution to the first filtrate of step (3), adjust the pH of the first filtrate to 7.0, and react for 10 min to form a gel, thus obtaining a mixture containing silica gel; the stirring speed is 180 rpm when adjusting the pH.

[0066] Step (5) Third solid-liquid separation: The mixture from step (4) is fed into an ultra-high pressure stacked filter press for deep solid-liquid separation to obtain a nano-sized silica filter cake and a third filtrate (the main components of which are P, F, Ca, and S, all <100ppm, in recycled water); the nano-sized silica is freeze-dried to obtain high-quality silica. The operating pressure of the ultra-high pressure stacked filter press is 12MPa; the nano-sized silica filter cake is washed with process water until the conductivity of the filter cake washing liquid is 42μS / cm.

[0067] The freeze-drying process specifically involves: pre-freezing at -50℃ for 2 hours, sublimation temperature at 15℃, desorption temperature at 36℃, and desorption time of 2 hours.

[0068] Step (6) Process water circulation: The third filtrate from step (5) is used as process water and circulated for the preparation of calcium carbonate slurry in step (1), and for washing the calcium fluoride filter cake in step (3) and the silica filter cake in step (5). The conductivity of the process water is 42 μS / cm.

[0069] Example 2 The method for preparing calcium fluoride and co-producing nano-sized silica from fluorosilicic acid in this embodiment includes the following steps: Step (1) Raw material preparation: Prepare calcium carbonate solid into calcium carbonate slurry with a mass concentration of 20% (CaCO3 content 200g / L). During preparation, the stirring speed is 300rpm. At the same time, fluorosilicic acid is filtered through plate and frame to obtain fluorosilicic acid clear liquid with a mass concentration of 4% and a solid content of <0.01% (H2SiF6 content 40g / L).

[0070] Step (2) Preparation of calcium fluoride: The calcium carbonate to fluorosilicic acid solution is added at a molar ratio of 3.05:1. 20% calcium carbonate slurry and 4% fluorosilicic acid are added simultaneously to the static mixer at a rate of 20 kg / min and 30 kg / min, and then into the reactor. The stirring speed in the reactor is 250 rpm, the reaction temperature is controlled at 25℃, and the reaction time is 15 min. When the pH drops to 3.9, the addition of fluorosilicic acid is stopped, and stirring is continued until the reaction is complete.

[0071] Step (3) First solid-liquid separation: The reaction product from step (2) is fed into a belt filter for solid-liquid separation. The filter cloth is cleaned with a high-pressure water gun at a working pressure of 1.2 MPa, resulting in a nano-sized calcium fluoride filter cake and a first filtrate. The calcium fluoride filter cake is washed with a dilute hydrofluoric acid solution and then separated a second time using a plate and frame filter press to obtain a refined calcium fluoride filter cake and a second filtrate. The second filtrate is returned as raw material fluorosilicic acid, and the refined filter cake is dried to obtain nano-sized calcium fluoride. The nano-sized calcium fluoride filter cake is washed twice with process water. After drying, the filter cake has a particle size of 65 nm, a purity of 99.4%, and a silica content of 0.32%.

[0072] Step (4) Co-production of silica: Add 10% sodium bicarbonate solution to the first filtrate of step (3), adjust the pH of the first filtrate to 7.0, and react for 10 min to form a gel, thus obtaining a mixture containing silica gel; the stirring speed is 200 rpm when adjusting the pH.

[0073] Step (5) Third solid-liquid separation: The mixture from step (4) is fed into an ultra-high pressure stacked filter press for deep solid-liquid separation to obtain a nano-sized silica filter cake and a third filtrate; the nano-sized silica is freeze-dried to obtain high-quality silica. The operating pressure of the ultra-high pressure stacked filter press is 12 MPa; the nano-sized silica filter cake is washed with process water until the conductivity of the filter cake washing liquid is 38 μS / cm.

[0074] The freeze-drying process specifically involves: pre-freezing at -55℃ for 2.2 hours, sublimation temperature at 12℃, desorption temperature at 30℃, and desorption time of 3 hours.

[0075] Step (6) Process water circulation: The third filtrate from step (5) is used as process water and circulated for the preparation of calcium carbonate slurry in step (1), and for washing the calcium fluoride filter cake in step (3) and the silica filter cake in step (5). The conductivity of the process water is 38 μS / cm.

[0076] Example 3 The method for preparing calcium fluoride and co-producing nano-sized silica from fluorosilicic acid in this embodiment includes the following steps: Step (1) Raw material preparation: Prepare calcium carbonate solid into calcium carbonate slurry with a mass concentration of 30% (CaCO3 content 300g / L). During preparation, the stirring speed is 280rpm. At the same time, fluorosilicic acid is filtered through plate and frame to obtain fluorosilicic acid clear liquid with a mass concentration of 7% and a solid content of <0.01% (H2SiF6 content 70g / L).

[0077] Step (2) Preparation of calcium fluoride: The calcium carbonate to fluorosilicic acid solution was fed at a molar ratio of 3.05:1. 30% calcium carbonate slurry and 7% fluorosilicic acid were added simultaneously to the static mixer at a rate of 18 kg / min and 25 kg / min, and then into the reactor. The stirring speed in the reactor was 260 rpm, the reaction temperature was controlled at 28℃, and the reaction time was 12 min. When the pH dropped to 3.9, the addition of fluorosilicic acid was stopped, and stirring was continued until the reaction was complete.

[0078] Step (3) First solid-liquid separation: The reaction product from step (2) is fed into a belt filter for solid-liquid separation. The filter cloth is cleaned with a high-pressure water gun at a working pressure of 1.1 MPa, resulting in a nano-sized calcium fluoride filter cake and a first filtrate. The calcium fluoride filter cake is washed with a dilute hydrofluoric acid solution and then separated a second time using a plate and frame filter press to obtain a refined calcium fluoride filter cake and a second filtrate. The second filtrate is returned as raw material fluorosilicic acid, and the refined filter cake is dried to obtain nano-sized calcium fluoride. The nano-sized calcium fluoride filter cake is washed three times with process water. After drying, the filter cake has a particle size of 62 nm, a purity of 97.2%, and a silica purity of 0.38%.

[0079] Step (4) Co-production of silica: Add 12% sodium hydroxide solution to the first filtrate of step (3), adjust the pH of the first filtrate to 7.0, and react for 10 min to form a gel, thus obtaining a mixture containing silica gel; the stirring speed is 190 rpm when adjusting the pH.

[0080] Step (5) Third solid-liquid separation: The mixture from step (4) is fed into an ultra-high pressure stacked filter press for deep solid-liquid separation to obtain a nano-sized silica filter cake and a third filtrate; the nano-sized silica is freeze-dried to obtain high-quality silica. The operating pressure of the ultra-high pressure stacked filter press is 18 MPa; the nano-sized silica filter cake is washed with process water until the conductivity of the filter cake washing liquid is 40 μS / cm.

[0081] The freeze-drying process specifically involves: pre-freezing at -52℃ for 2.1 hours, sublimation temperature at 13℃, desorption temperature at 33℃, and desorption time of 2.5 hours.

[0082] Step (6) Process water circulation: The third filtrate from step (5) is used as process water and circulated for the preparation of calcium carbonate slurry in step (1), and for washing the calcium fluoride filter cake in step (3) and the silica filter cake in step (5). The conductivity of the process water is 40 μS / cm.

[0083] Comparative Example 1: Change in the method of adding raw materials The difference from Example 1 is that step (2) is changed to "first add 10% fluorosilicic acid (20 kg / min) to the reactor, and then slowly add 40% calcium carbonate slurry (15 kg / min)".

[0084] Comparative Example 2: Feed rate exceeds the limit range The difference from Example 1 is that step (2) is changed to "40% calcium carbonate slurry is added at a rate of 30 kg / min and 10% fluorosilicic acid is added at a rate of 50 kg / min".

[0085] Comparative Example 3: Using conventional hot air drying instead of freeze drying The difference from Example 1 is that step (5) drying is changed to "120℃ hot air drying for 4 hours".

[0086] Comparative Example 4: Freeze-drying parameters deviate from the specified range The difference from Example 1 is that the freeze-drying parameters in step (5) are changed to "pre-freeze at -20℃ for 2 hours, sublimation temperature at 25℃, desorption temperature at 25℃, and desorption time at 1 hour".

[0087] The product specifications of calcium fluoride in the examples and comparative examples are shown in Tables 1 and 2 below: Table 1 Comparison of calcium fluoride product indicators between the examples and comparative examples. Table 2 Comparison of silica product indicators between the examples and comparative examples In Table 1, the detection limit for calcium carbonate content is 0.1%. The detection results for Examples 1 and 2 are less than 0.1%. The result less than 0.1 has a large error, so it is expressed as less than 0.1.

[0088] In Comparative Example 1, adding fluorosilicic acid first, followed by the dropwise addition of calcium carbonate, resulted in excessive local pH fluctuations (initial pH < 1, then a sudden rise to 4.5). This made it difficult to effectively separate calcium fluoride from silica, with some silica coating the surface of the calcium fluoride. Therefore, simultaneous feeding is crucial.

[0089] In Comparative Example 2, the feed rate was too fast, the materials in the static mixer were not mixed sufficiently, the local molar ratio in the reactor was unbalanced, some calcium carbonate did not react completely, and the concentrated exothermic reaction caused the local temperature to rise to 45°C, which damaged the stability of "liquid silica".

[0090] In the conventional hot air drying process of Comparative Example 3, the nano-sized silica particles agglomerate due to high temperature, resulting in a significant decrease in specific surface area and a reduction in oil absorption value.

[0091] Comparative Example 4: The pre-freezing temperature was too high (-20℃ > -40℃), resulting in insufficient freezing of the material. The sublimation and desorption temperatures were mismatched, the time was too short, and the moisture was not completely removed. The residual moisture in the silica product caused particle agglomeration.

[0092] 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 preparing calcium fluoride and co-producing nano-sized silica from fluorosilicic acid, characterized in that, Includes the following steps: Step (1) Raw material preparation: Prepare calcium carbonate solid into calcium carbonate slurry, and obtain fluorosilicic acid clear liquid by plate and frame filter. Step (2) Preparation of calcium fluoride: The calcium carbonate slurry from step (1) and fluorosilicic acid are put into a static mixer and mixed in a reaction vessel. The pH value of the reaction system is monitored in real time, and the pH value at the end of the reaction is controlled to be 3.8~4.

0. Step (3) First solid-liquid separation: The reaction product of step (2) is sent to a filtration device for solid-liquid separation to obtain nano-sized calcium fluoride filter cake and first filtrate; the calcium fluoride filter cake is washed with dilute hydrofluoric acid solution and then separated again by plate and frame filter press to obtain refined calcium fluoride filter cake and second filtrate. The second filtrate is returned as raw material fluorosilicic acid, and the refined filter cake is dried to obtain nano-sized calcium fluoride. Step (4) Co-production of silica: Add an alkaline agent to the first filtrate of step (3) to adjust the pH value of the first filtrate to 7.0±0.2, and obtain a mixture containing silica gel; Step (5) Third solid-liquid separation: The mixture from step (4) is sent to an ultra-high pressure stacked filter press for deep solid-liquid separation to obtain nano-sized silica filter cake and third filtrate; the nano-sized silica is freeze-dried to obtain high-quality white carbon black; Step (6) Process water circulation: The third filtrate from step (5) is used as process water and circulated for the preparation of calcium carbonate slurry in step (1), and for the washing of calcium fluoride filter cake in step (3) and silica filter cake in step (5).

2. The method according to claim 1, characterized in that, In step (1), a calcium carbonate slurry with a mass concentration of 20-40% is prepared. During preparation, the stirring speed is 200-300 rpm. At the same time, the original fluorosilicic acid is filtered through a plate and frame filter to obtain a fluorosilicic acid clear liquid with a mass concentration of 4-30% and a solid content of <0.05%.

3. The method according to claim 1, characterized in that, In step (2), calcium carbonate slurry with a mass concentration of 20-40% is added to the static mixer at a rate of 11.5-22.7 kg / min and fluorosilicic acid with a mass concentration of 4-30% at a rate of 15.7-43.1 kg / min, and then the mixture is added to the reactor. The stirring speed in the reactor is 250-300 rpm, the reaction temperature is controlled at 25-35℃, and the reaction time is 10-30 min.

4. The method according to claim 1, characterized in that, In step (2), when the pH drops to 3.8~4.0, stop adding fluorosilicic acid and continue stirring until the reaction is complete.

5. The method according to claim 1, characterized in that, In step (3), the filtration equipment is a belt filter, and the filter cloth is cleaned with a high-pressure water gun at a working pressure of 0.8~1.2MPa; the nano-grade calcium fluoride filter cake is washed with process water 2~3 times.

6. The method according to claim 1, characterized in that, In step (4), the alkali is a sodium hydroxide solution with a mass concentration of 10-15% or a sodium bicarbonate solution with a mass concentration of 8-12%, and the stirring speed is 150-200 rpm when adjusting the pH value.

7. The method according to claim 1, characterized in that, In step (5), the operating pressure of the ultra-high pressure stacked filter press is 10~15MPa; the nano-sized silica filter cake is washed with process water until the conductivity of the filter cake washing liquid is <50μS / cm, and the washed filter cake is freeze-dried to obtain nano-sized silica product.

8. The method according to claim 1, characterized in that, In step (5), the freeze drying process specifically involves: pre-freezing at -40~-60℃ for 2~2.5h, sublimation temperature at 10~20℃, desorption temperature at 30~40℃, and desorption time at 2~3h.

9. The method according to claim 1, characterized in that, In step (6), before recycling the process water, ensure that the conductivity is <80μS / cm; if the conductivity of the process water is ≥80μS / cm, it needs to be purified by ion exchange resin before recycling.

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