Method for preparing high-purity superfine calcium fluoride based on dynamic pH control gradient crystallization

Abstract

This invention relates to the field of inorganic fine chemical material preparation technology, and discloses a method for preparing high-purity ultrafine calcium fluoride based on dynamic pH-controlled gradient crystallization. The method uses industrial-grade calcium carbonate and fluorosilicic acid as raw materials. Its core is based on dynamic pH gradient control of the reaction mechanism, precisely dividing the calcium fluoride crystallization process into three stages: primary nucleation, secondary growth, and tertiary purification. Through the coordinated matching of parameters such as pH, temperature, and stirring rate at each stage, complete separation and control of nucleation and growth are achieved, ultimately producing high-purity ultrafine calcium fluoride powder with a purity ≥99.9% and a particle size of 1–5 μm. This invention features low raw material costs, strong process controllability, and the main byproducts are CO2 and silicic acid, both of which can be recycled. No fluoride-containing wastewater is discharged during the process, achieving clean production. Furthermore, the process has high stability and is suitable for large-scale industrial production, offering significant economic and environmental benefits.

Description

Technical Field

[0001] This invention relates to the field of inorganic fine chemical material preparation technology, and in particular to a method for preparing high-purity ultrafine calcium fluoride based on dynamic pH-controlled gradient crystallization. Background Technology

[0002] Calcium fluoride, also known as fluorite or fluorspar, is an important non-metallic mineral raw material. Its main component is calcium fluoride (chemical formula CaF2, relative molecular mass 78.07), containing small amounts of Fe2O3, SiO2, and trace amounts of Cl, He, and other impurities. As a major source of fluorine resources in nature, calcium fluoride is a non-renewable resource, commonly found in colorless crystals or white powder. As an important inorganic functional material, calcium fluoride is widely used in high-end fields such as specialty glass, optical crystals, electronic components, and specialty ceramics due to its excellent optical properties, chemical stability, and mechanical properties. These applications place stringent requirements on the purity (≥99.9%) and particle size (1–5 μm ultrafine uniform particle size) of calcium fluoride.

[0003] Traditional methods for preparing calcium fluoride mainly fall into two categories: natural fluorite purification and chemical synthesis. While the natural fluorite purification method is relatively simple, it is limited by the grade and reserves of fluorite resources, making it difficult to obtain high-purity products and placing significant pressure on the environment. Chemical synthesis methods mainly include the hydrofluoric acid-calcium hydroxide method and the fluorosilicic acid-calcium carbonate method. The hydrofluoric acid-calcium hydroxide method is currently the most widely used industrial method. Its basic principle is the direct reaction of hydrofluoric acid (HF) with calcium hydroxide (Ca(OH)2) to produce calcium fluoride. The advantages of this method are its relatively simple process and high product purity, but it has significant technical bottlenecks: First, hydrofluoric acid, as a raw material, is extremely corrosive and toxic, requiring highly sophisticated equipment and posing significant safety risks; second, hydrofluoric acid is expensive, typically requiring electronic-grade or industrial-grade products with a concentration ≥40%. If impurities such as sulfuric acid or hydrochloric acid are added, calcium sulfate, calcium chloride, and other difficult-to-wash impurities will be generated during the reaction; third, this method consumes a large amount of fresh water, increasing wastewater discharge and significantly raising production costs.

[0004] The fluorosilicic acid-calcium carbonate method is another important preparation route, with the reaction equation: H₂SiF₆ + 3CaCO₃ = 3CaF₂↓ + 3CO₂↑ + SiO₂·nH₂O. This method uses inexpensive industrial-grade calcium carbonate and fluorosilicic acid as raw materials, theoretically giving it a cost advantage.

[0005] However, the traditional fluorosilicic acid-calcium carbonate method faces numerous technical challenges in practical applications: 1. Difficult reaction control: The reaction between fluorosilicic acid and calcium carbonate is a complex multiphase reaction process involving multiple steps such as acid hydrolysis, ion dissociation, nucleation and growth, and impurity adsorption. Traditional processes use a single pH setting for control, which cannot meet the needs of each stage, resulting in large fluctuations in product performance. 2. Low product purity: The purity of calcium fluoride products prepared by traditional processes is typically only 94-97% (based on dry matter), with CaCO3 content below 1% and SiO2 content below 3%. This level of purity is insufficient to meet the stringent purity requirements of high-end applications, especially semiconductors and optical devices. 3. Uneven particle size distribution: Traditional processes struggle to achieve precise control over crystal nucleation and growth, resulting in a wide particle size distribution, severe agglomeration, and an inability to obtain ultrafine products with particle sizes of 1-5 μm. 4. Poor process stability: Traditional processes are sensitive to changes in reaction conditions. Even small fluctuations in parameters such as temperature, pH value, and stirring rate can lead to significant changes in product quality, affecting the stability of industrial production.

[0006] Therefore, developing a process for preparing high-purity, ultrafine, and uniform-sized calcium fluoride by dynamically controlling gradient crystallization with pH is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for preparing high-purity ultrafine calcium fluoride based on dynamic pH-controlled gradient crystallization. This invention dynamically adjusts the pH value in stages and coordinates the control of parameters such as temperature and stirring rate to achieve precise control of nucleation and crystal growth in stages. This solves the problems of low purity, uneven particle size, and severe agglomeration in traditional processes, and obtains ultrafine uniform calcium fluoride products with a purity ≥99.9% and a particle size of 1-5 μm.

[0008] The solution of the present invention is: A method for preparing high-purity ultrafine calcium fluoride based on dynamic pH-controlled gradient crystallization includes the following steps: S1. Raw material pretreatment: Add calcium carbonate powder to deionized water to form a suspension and ultrasonically disperse for 30-40 minutes; dilute 30% fluorosilicic acid to 5%-10%, with a molar ratio of calcium carbonate to fluorosilicic acid of 3:1.0-1.2. S2. Dynamic pH-controlled gradient crystallization: The calcium carbonate suspension is heated to 30–40°C, stirred at 300–350 r / min, and fluorosilicic acid solution is added dropwise, controlling the pH value to 4.5–5.0. The reaction is carried out for 30–40 min to complete the first-stage nucleation. Fluorosilicic acid solution is then added dropwise, adjusting the pH value to 3.5–4.0, and the temperature is raised to 45–55°C. The reaction is carried out for 40–60 min to complete the second-stage growth. Finally, a 0.1% polyacrylamide solution is added, adjusting the pH value to 4.0–4.5, and the mixture is aged for 10–15 min to complete the third-stage purification. S3. Post-treatment: Filtration and separation. Wash the precipitate with deionized water until the pH of the washing solution is 6-7. Dry at 105℃ for 12-15 hours and grind to obtain the product.

[0009] As a preferred technical solution, in step S1, the mass concentration of the calcium carbonate suspension is 10% to 20%, the purity of the calcium carbonate is ≥99.0%, and the particle size is 5 to 20 μm.

[0010] As a preferred technical solution, the dropping rate of fluorosilicic acid in the primary nucleation is 0.01 to 0.03 mL / min per gram of calcium carbonate.

[0011] As a preferred technical solution, the dropping rate of fluorosilicic acid in the secondary growth is 0.02-0.04 mL / min per gram of calcium carbonate.

[0012] As a preferred technical solution, the amount of polyacrylamide solution added is 0.5% to 1% of the total mass of the system, and the stirring rate during the aging process is 50 to 100 r / min.

[0013] As a preferred technical solution, in step S1, the ultrasonic dispersion power is 100-150W.

[0014] As a preferred technical solution, the calcium fluoride product has a purity of ≥99.9%, a particle size of 1-5 μm, and a particle size distribution variation coefficient of ≤15%.

[0015] The mechanism by which this invention prepares high-purity ultrafine calcium fluoride through gradient crystallization controlled by dynamic pH is as follows: This invention uses industrial-grade calcium carbonate (CaCO3) and fluorosilicic acid (H2SiF6) as raw materials. Based on the multi-step reaction characteristics of the calcium carbonate-fluorosilicic acid system, it achieves precise separation of nucleation and growth through gradient pH dynamic control: 1. Primary nucleation stage (pH 4.5–5.0): Within this pH range, the dissociation rate of fluorosilicic acid is relatively slow, releasing H2SiF6. + It reacts mildly with calcium carbonate, Ca 2+ Dissolution rate is controllable; at the same time, F -The concentration is maintained at a low level, and the supersaturation of the system is stabilized between 1.2 and 1.8 (the optimal supersaturation range for nucleation), ensuring that the formed critical nuclei have uniform particle size, good dispersion, and no agglomeration. 2. Secondary growth stage (pH 3.5–4.0): The lower pH value accelerates the dissociation of fluorosilicic acid and the acidolysis of calcium carbonate, increasing the free F - With Ca 2+ The concentration and supersaturation of the system are stabilized in the range of 1.8 to 2.5 (the optimal range for growth). The temperature is raised to 45 to 55°C to further accelerate ion diffusion. Ions are directionally attached to the surface of the crystal nucleus through surface reaction and grow in an orderly manner according to the face-centered cubic structure of fluorite, while inhibiting the formation of new crystal nuclei. 3. Third-stage purification (pH 4.0 to 4.5): The pH value is adjusted appropriately to reduce the ion reactivity and reduce the impurity ions adsorbed on the crystal surface. Low-speed stirring avoids crystal agglomeration and promotes the flocculation and separation of silica colloids adsorbed on the crystal surface. Defects on the crystal surface are eliminated through aging, the crystal morphology is optimized, and the purity and dispersibility of the product are improved.

[0016] The dynamic pH control of this invention can achieve precise control through the following methods: a. Online monitoring system: A high-precision pH sensor is used to monitor the pH value of the reaction system in real time, with an accuracy of ±0.01.

[0017] b. Automatic control system: Based on a preset pH curve, the PLC control system automatically adjusts the amount of acid or alkali added to achieve precise pH control.

[0018] c. Buffer system design: Add an appropriate amount of buffer to the reaction system to improve pH stability and reduce fluctuations.

[0019] d. Segmented control strategy: Automatically switch pH control parameters at different stages according to the reaction process to achieve true dynamic control.

[0020] Compared with the prior art, the advantages of the present invention are: 1. Excellent product performance: Through dynamic pH gradient regulation, the nucleation and growth process is precisely controlled, the product purity is ≥99.9%, the particle size is concentrated in 1~5μm, the particle size distribution is uniform (coefficient of variation ≤15%), and there is no obvious agglomeration phenomenon, which meets the application needs of high-end optical materials, special ceramics and other fields. 2. Strong process controllability: The staged pH control strategy is adapted to the multi-step reaction characteristics of the calcium carbonate-fluorosilicic acid system, which effectively solves the problems of difficult reaction rate matching and incomplete impurity removal in traditional processes. The process parameters are easy to adjust and are suitable for large-scale production. 3. Low raw material cost: Industrial-grade calcium carbonate and fluorosilicic acid are used as raw materials. The raw materials are widely available and inexpensive. Compared with soluble calcium salt and fluoride salt systems, the raw material cost is reduced by 30% to 40%, and no harmful by-products are generated, which is in line with the trend of green chemical development. 4. Safe and environmentally friendly operation: Fluorosilicic acid is added dropwise after dilution to avoid excessive local acidity and violent escape of HF gas. The reaction process only produces byproducts such as carbon dioxide and silicic acid. Carbon dioxide can be collected and utilized, and silicic acid can be recovered after flocculation and separation, so there is no wastewater pollution problem. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a SEM image of calcium fluoride, the product of Example 1 of the present invention. Figure 3 This is a particle size analysis diagram of calcium fluoride, the product of Example 1 of the present invention. Detailed Implementation

[0022] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0023] 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.

[0024] Unless otherwise stated, all percentages in this invention represent mass fractions. Ratios are mass percentages, and concentrations are mass concentrations.

[0025] 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. Example 1

[0026] Raw material pretreatment: Take 30g of calcium carbonate powder (purity 99.2%, particle size 5-15μm) and add 200mL of deionized water to prepare a calcium carbonate suspension with a mass concentration of about 13.0%. Disperse the suspension by ultrasonication at 120W for 35min to obtain a calcium carbonate suspension. In a fume hood, dilute 45mL of 30% fluorosilicic acid to 100mL to obtain an 8% fluorosilicic acid solution. The molar ratio of calcium carbonate to fluorosilicic acid is 3:1.1.

[0027] Gradient crystallization: Add calcium carbonate suspension to a 1000 mL crystallization reactor, stir at 320 r / min, and heat to 35 °C; add fluorosilicic acid solution dropwise at a rate of 0.8 mL / min, control the pH value to 4.8, and react for 35 min (primary nucleation); continue to add fluorosilicic acid solution dropwise (rate 1.0 mL / min), adjust the pH value to 3.8, heat to 50 °C, and react for 50 min (secondary growth); add 5 mL of 0.1% polyacrylamide solution (the amount of this solution added accounts for 0.6% of the total mass of the system), adjust the pH value to 4.2, and stir at 100 r / min for aging for 12 min (tertiary purification).

[0028] Post-processing: The precipitate was separated by filtration, washed 6 times with deionized water until the pH of the washing solution was 6.5; dried at 105℃ for 14 hours, and then lightly ground to obtain the calcium fluoride product.

[0029] The product was tested and found to have a purity of 99.93%, a particle size of 2.3–4.7 μm, an average particle size of 3.5 μm, and a particle size distribution coefficient of variation of 12.8% (coefficient of variation CV = standard deviation / average particle size × 100%, tested using a laser particle size analyzer). The particles are spherical and uniform, with no obvious agglomeration. Example 2

[0030] Raw material pretreatment: Take 40g of calcium carbonate powder (purity 99.0%, particle size 10-20μm) and add 250mL of deionized water. Disperse the powder by ultrasonication at 150W for 30min to obtain a calcium carbonate suspension. In a fume hood, dilute 60mL of 30% fluorosilicic acid to 150mL to obtain a 6% fluorosilicic acid solution. The molar ratio of calcium carbonate to fluorosilicic acid is 3:1.05.

[0031] Gradient crystallization: Add calcium carbonate suspension to a 1000 mL crystallization reactor, stir at 350 r / min, and heat to 30 °C; add fluorosilicic acid solution dropwise at a rate of 0.5 mL / min, control the pH value to 4.5, and react for 40 min (primary nucleation); continue to add fluorosilicic acid solution dropwise (rate 0.8 mL / min), adjust the pH value to 3.5, heat to 45 °C, and react for 60 min (secondary growth); add 8 mL of 0.1% polyacrylamide solution (the amount of this solution added accounts for 0.8% of the total mass of the system), adjust the pH value to 4.0, and stir at 80 r / min for aging for 15 min (tertiary purification).

[0032] Post-processing: The precipitate was separated by filtration, washed 5 times with deionized water until the pH of the washing solution was 6.2; dried at 100℃ for 15 hours, and then lightly ground to obtain the calcium fluoride product.

[0033] The product was tested and found to have a purity of 99.91%, a particle size of 1.8–4.2 μm, an average particle size of 3.0 μm, and a particle size distribution coefficient of variation of 14.5% (coefficient of variation CV = standard deviation / average particle size × 100%, tested using a laser particle size analyzer), indicating good particle dispersibility.

[0034] Comparative Example 1 The traditional single pH control process was adopted, and all parameters except for the pH control method were the same as in Example 1: the calcium carbonate suspension was heated to 50°C, the stirring rate was 320 r / min, fluorosilicic acid solution was added dropwise at a rate of 0.8 mL / min, the pH value was controlled at 4.0, and the reaction was carried out for 85 min. The three-stage purification steps were the same as in Example 1 (5 mL of 0.1% polyacrylamide solution was added, the pH value was adjusted to 4.0, and the mixture was stirred at 100 r / min and aged for 12 min), and the post-treatment steps were the same as in Example 1.

[0035] The product was tested and found to have a purity of 99.3%, a particle size of 0.5–8.2 μm, an average particle size of 4.1 μm, and a particle size distribution variation coefficient of 35.2%, indicating significant agglomeration.

[0036] Comparative Example 2 This comparative case aims to examine the impact of a low primary nucleation pH on product quality. Based on the process parameters of Example 2, the pH values ​​at each stage were lowered overall.

[0037] Raw material pretreatment: Same as in Example 2.

[0038] Gradient crystallization: Calcium carbonate suspension was added to a 1000 mL crystallization reactor, stirred at 350 r / min, and heated to 30 °C; fluorosilicic acid solution was added dropwise at a rate of 0.5 mL / min, controlling the pH value to 3.8 (0.7 lower than in Example 2), and the reaction was carried out for 40 min (primary nucleation); fluorosilicic acid solution was continued to be added dropwise (at a rate of 0.8 mL / min), adjusting the pH value to 3.8 (0.7 lower than in Example 2), and the temperature was raised to 45 °C, and the reaction was carried out for 60 min (secondary growth); 8 mL of 0.1% polyacrylamide solution was added, adjusting the pH value to 3.8 (0.7 lower than in Example 2), and the mixture was stirred at 80 r / min and aged for 15 min (tertiary purification).

[0039] Post-processing: Same as in Example 2.

[0040] Testing revealed that the product had a purity of 92.58%, a particle size of 1.5–5.0 μm, an average particle size of 3.2 μm, and a particle size distribution variation coefficient of 23.4%. The product surface showed slight corrosion, irregular particle morphology, and poor dispersibility. The results indicated that a low pH (<4.5) during the primary nucleation stage led to high supersaturation, resulting in excessively rapid and uneven crystal nucleus formation. A low pH (<4.0) during the tertiary purification stage weakened the impurity flocculation and separation effect, significantly reducing product purity and particle size uniformity.

[0041] Comparative Example 3 This comparative case aims to examine the impact of severely low primary nucleation pH and malfunctioning drop rate on product quality. Based on the process parameters of Example 2, the primary nucleation pH was further reduced and the drop rate was exchanged.

[0042] Raw material pretreatment: Same as in Example 2.

[0043] Gradient crystallization: Calcium carbonate suspension was added to a 1000 mL crystallization reactor, stirred at 350 r / min, and heated to 30 °C; fluorosilicic acid solution was added dropwise at a rate of 0.8 mL / min, controlling the pH value to 2.8 (1.7 lower than in Example 2), and the reaction was carried out for 40 min (first-stage nucleation); fluorosilicic acid solution was continued to be added dropwise (at a rate of 0.5 mL / min), adjusting the pH value to 3.8 (0.7 lower than in Example 2), and the temperature was raised to 45 °C, and the reaction was carried out for 60 min (second-stage growth); 8 mL of 0.1% polyacrylamide solution was added, adjusting the pH value to 3.8 (0.7 lower than in Example 2), and the mixture was stirred at 80 r / min and aged for 15 min (third-stage purification).

[0044] Post-processing: Same as in Example 2.

[0045] Testing revealed that the product had a purity of 89.47%, a particle size of 1.5–5.0 μm, an average particle size of 3.2 μm, and a particle size distribution variation coefficient of 23.4%. The product surface showed significant corrosion, irregular particle morphology, and severe agglomeration. The results indicated that the severely low pH value (2.8) during the primary nucleation stage led to a vigorous reaction, and Ca… 2+ The uncontrolled dissolution rate led to the formation of numerous fine crystal nuclei and agglomeration; at the same time, the imbalance in the fluorosilicic acid droplet acceleration rate further exacerbated the inhomogeneity of the reaction; and the low pH value during the three-stage purification stage resulted in a significant decrease in product purity.

[0046] Comparative Case Analysis and Conclusions By comparing Example 2 with Comparative Examples 2 and 3, it can be seen that the dynamic pH gradient crystallization technology of the present invention has the following key functions: pH value during nucleation (4.5–5.0): A suitable pH range ensures that the supersaturation of the system is stable within the optimal nucleation range of 1.2–1.8, resulting in uniform critical nuclei with good dispersion. A pH value that is too low (<4.5) will lead to uncontrolled supersaturation, resulting in excessively rapid and uneven nuclei formation.

[0047] During the growth stage, the pH value (3.5–4.0) is synergistically matched with the temperature (45–55℃) to accelerate ion diffusion and surface reactions, achieving orderly crystal growth. A pH value deviating from the optimal range will disrupt the matching relationship between ion deposition rate and surface reaction.

[0048] pH value during purification (4.0-4.5): Appropriately adjusting the pH value can reduce ionic reactivity and impurity adsorption, and combined with low-speed stirring, promote the flocculation and separation of silica colloids. A pH value that is too low (<4.0) will weaken the impurity removal effect.

[0049] Multi-parameter synergistic matching: The technical effectiveness of this invention relies not only on the precise control of pH value at each stage, but also on the synergistic matching of parameters such as temperature, droplet acceleration rate, and stirring rate. Deviation of a single parameter (such as in Comparative Example 1) or a comprehensive deviation of multiple parameters (such as in Comparative Examples 2 and 3) will lead to a significant decrease in product quality, verifying the scientific nature and necessity of the technical solution of this invention.

[0050] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-purity ultrafine calcium fluoride by gradient crystallization based on dynamic pH control, characterized in that, Includes the following steps: S1. Raw material pretreatment: Add calcium carbonate powder to deionized water to form a suspension and ultrasonically disperse for 30-40 minutes; dilute 30% fluorosilicic acid to 5%-10%, with a molar ratio of calcium carbonate to fluorosilicic acid of 3:1.0-1.

2. S2. Dynamic pH-controlled gradient crystallization: The calcium carbonate suspension is heated to 30–40°C, stirred at 300–350 r / min, and fluorosilicic acid solution is added dropwise, controlling the pH value to 4.5–5.

0. The reaction is carried out for 30–40 min to complete the first-stage nucleation. Fluorosilicic acid solution is then added dropwise, adjusting the pH value to 3.5–4.0, and the temperature is raised to 45–55°C. The reaction is carried out for 40–60 min to complete the second-stage growth. Finally, a 0.1% polyacrylamide solution is added, adjusting the pH value to 4.0–4.5, and the mixture is aged for 10–15 min to complete the third-stage purification. S3. Post-treatment: Filtration and separation. Wash the precipitate with deionized water until the pH of the washing solution is 6-7. Dry at 105℃ for 12-15 hours and grind to obtain the product.

2. The method of claim 1, wherein: In step S1, the mass concentration of the calcium carbonate suspension is 10% to 20%, the purity of the calcium carbonate is ≥99.0%, and the particle size is 5 to 20 μm.

3. The method of claim 1, wherein: The dropping rate of fluorosilicic acid in the primary nucleation process is 0.01–0.03 mL / min per gram of calcium carbonate.

4. The method of claim 1, wherein: The dropping rate of fluorosilicic acid during the secondary growth was 0.02–0.04 mL / min per gram of calcium carbonate.

5. The method of claim 1, wherein: The amount of polyacrylamide solution added is 0.5% to 1% of the total mass of the system, and the stirring rate during the aging process is 50 to 100 r / min.

6. The method of claim 1, wherein: In step S1, the ultrasonic dispersion power is 100-150W.

7. The method of any one of claims 1-6, wherein: The calcium fluoride product has a purity of ≥99.9%, a particle size of 1-5 μm, and a particle size distribution variation coefficient of ≤15%.

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