Preparation process of high-molecular-ratio cryolite based on purified fly ash

Abstract

The invention belongs to the technical field of resource utilization of industrial solid wastes, and particularly relates to a preparation process of high-molecular-ratio cryolite based on purified fly ash. Comprising the following steps: 1) purification pretreatment of fly ash: performing physical separation and sodium salt roasting on the raw material fly ash to obtain roasted clinker, and performing staged leaching and solution refining to obtain a high-purity sodium aluminate solution; 2) preparation of high-purity aluminum hydroxide: carrying out carbonation decomposition on the high-purity sodium aluminate solution to obtain the high-purity aluminum hydroxide; and 3) cryolite synthesis and crystallization: carrying out a step-by-step fluorination synthesis reaction on the high-purity aluminum hydroxide, a fluorinating agent and a sodium modifying agent, and then carrying out programmed temperature control crystallization to obtain a high-molecular-ratio cryolite solid. The invention provides an integrated solution for converting waste fly ash into cryolite with high added value and high molecular ratio, and simultaneously solves a plurality of technical problems of aluminum source purification, molecular ratio accurate control, product crystallization quality and the like.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization technology, specifically relating to a preparation process of high molecular weight cryolite based on purified fly ash. Background Technology

[0002] Cryolite, scientifically known as sodium hexafluoroaluminate, is an irreplaceable flux in modern aluminum electrolysis. Its main functions are to dissolve alumina, lower the electrolysis temperature, improve the conductivity of the electrolyte, and maintain the electrochemical balance during the electrolysis process. Among them, high molecular weight cryolite has become the preferred choice for large prebaked aluminum electrolysis cells due to its advantages such as less hydrogen fluoride volatilization loss during electrolysis, less erosion of the carbon lining of the electrolytic cell, and effective reduction of the primary crystallization temperature of the electrolyte.

[0003] Currently, the raw materials for industrial cryolite production mainly rely on natural fluorite, fluorosilicic acid, or fluorosilicic acid derived from the phosphate fertilizer industry. This production chain is constrained by upstream mineral resources and the chemical industry, resulting in significant cost fluctuations and environmental pressures. On the other hand, the large quantities of fly ash generated by coal-fired power plants pose serious environmental problems due to their storage and landfilling. Fly ash typically contains 15% to 40% alumina, especially high-alumina fly ash from some regions, where the alumina content can exceed 40%, making it a valuable non-traditional aluminum resource. Therefore, combining these two industrial demands with environmental issues—that is, efficiently converting the aluminum in fly ash into high-value cryolite products—has become a research direction with significant economic and social value.

[0004] Existing technologies have explored the preparation of cryolite from aluminum-containing materials. A common route involves the direct reaction of fluorosilicic acid with aluminum hydroxide or sodium aluminate. However, this route requires extremely high purity of the aluminum source; using industrial-grade aluminum hydroxide is costly, while using low-purity aluminum extracted from inexpensive raw materials like fly ash easily introduces impurities such as silicon, iron, and calcium into the product, severely affecting the quality of the cryolite. Another route involves treating fluorine-containing waste from the electrolytic aluminum industry, recovering cryolite through double salt decomposition and acid-base treatment. However, this method has limited raw material sources and processing scale. For the direct utilization of fly ash, some technical solutions propose acid or alkaline methods to extract aluminum, followed by synthesis with fluoride salts. For example, sulfuric acid can be used to directly leach aluminum from fly ash, followed by reaction with sodium fluoride. However, the acid method simultaneously dissolves large amounts of impurities such as iron and calcium, making subsequent separation extremely difficult, and achieving both high aluminum yield and product purity is challenging. Although the alkaline process is more selective for aluminum, in the traditional process, a large amount of silicon in fly ash enters the alkaline solution to form sodium silicate, which coexists with sodium aluminate and forms a sodium aluminosilicate complex that is difficult to separate, resulting in serious aluminum loss and insufficient purity of the extracted sodium aluminate solution.

[0005] Existing technologies, even after obtaining a sodium aluminate solution, have limitations in the cryolite synthesis stage. Most processes focus on producing ordinary cryolite with a molecular weight ratio of 1.0 to 2.0. When producing high molecular weight cryolite, to maintain a stable molecular weight ratio above 2.5, a strongly alkaline environment with a high sodium excess is usually required for the reaction. However, this easily leads to the regeneration of aluminum hydroxide or a decrease in fluorine utilization, making the reaction process difficult to control and resulting in uneven molecular weight distribution in the product. Furthermore, the crystallization process often employs rapid or natural cooling, resulting in fine crystal particles with low strength and low bulk density. These crystals tend to float and dissolve slowly during subsequent electrolysis, generating a large amount of dust. Therefore, a process for preparing high molecular weight cryolite based on purified fly ash is needed. Summary of the Invention

[0006] To address the above problems, the present invention aims to provide a preparation process for high molecular weight cryolite based on purified fly ash.

[0007] The technical content of this invention is as follows: This invention provides a process for preparing high molecular weight cryolite based on purified fly ash, the process comprising the following steps: 1) Pretreatment for purification of fly ash: Physical sorting and sodium roasting of raw fly ash to obtain roasted clinker, followed by stage leaching and solution refining to obtain high-purity sodium aluminate solution. 2) Preparation of high-purity aluminum hydroxide: The high-purity sodium aluminate solution is subjected to carbonation decomposition to obtain high-purity aluminum hydroxide; 3) Cryolite Synthesis and Crystallization: The high-purity aluminum hydroxide is synthesized by stepwise fluorination reaction with fluorinating agent and sodium fluorinating agent, and then crystallized by temperature control to obtain high molecular weight cryolite solid.

[0008] The physical sorting and sodium roasting described in step 1) involve air classification or wet gravity settling of the raw fly ash to remove >80% of unburned carbon and <45μm fly ash particles (enriching inert phases such as mullite and quartz); the physically sorted fly ash is then ground to a specific surface area of ​​650-850 m². 2 / kg; then fly ash and sodium-modifying agent are mixed evenly at a mass ratio of 100:(15-30), wherein the sodium-modifying agent includes one or more of sodium carbonate, sodium hydroxide, and sodium sulfate; the mixture is calcined at 800-950℃ for 1.5-4 hours to obtain calcined clinker.

[0009] In step 1), the staged leaching and solution purification includes the following steps: a) Primary acid leaching: Using a sulfuric acid solution with a mass concentration of 5%-15%, the roasted clinker is leached at 50-80℃ for 30-90 minutes, and the primary leaching residue is obtained after solid-liquid separation. b) Secondary alkaline leaching: Using a sodium hydroxide solution with a concentration of 150-250 g / L and calculated as sodium oxide, the primary leaching residue is leached for 1-3 hours at a liquid-to-solid mass ratio of (4-6):1 and a temperature of 95℃ to 100℃. After hot filtration, a crude sodium aluminate solution and a secondary leaching residue are obtained. c) Heat the crude sodium aluminate solution and maintain it at 70-90°C, add a purification agent, keep it warm and stir for 0.5-2 hours, then filter to obtain a high-purity sodium aluminate solution.

[0010] In step 1), the impurity removal agent includes hydrated iron oxide, and the amount added is 0.1%-0.5% of the mass of the crude sodium aluminate solution.

[0011] The carbonation decomposition method described in step 2) is as follows: under continuous stirring, carbon dioxide gas is introduced into a high-purity sodium aluminate solution, the reaction temperature is 70-90℃, the reaction endpoint is controlled to be 10.5-11.5 by monitoring the pH value, then solid-liquid separation is performed, the solid is washed, and high-purity aluminum hydroxide is obtained.

[0012] The stepwise fluorination synthesis reaction described in step 3) includes the following steps: First step fluorination: The high-purity aluminum hydroxide is slurried and dispersed in water, and a fluorinating agent is added under stirring. The pH of the reaction system is 1.5-3.0, the reaction temperature is 60-85℃, and the reaction time is 0.5-2 hours to generate a fluoroaluminate intermediate solution. The second step, sodium formation, involves adding a sodium-forming agent to the fluoroaluminate intermediate solution, adjusting and maintaining the pH of the reaction system at 4.5-6.0, and continuing the reaction for 1-3 hours to obtain cryolite synthesis slurry.

[0013] The fluorinating agent includes one or more of hydrofluoric acid, fluorosilicic acid, and ammonium fluoride; the sodium-containing agent includes one or more of sodium hydroxide, sodium carbonate, and sodium fluoride.

[0014] The fluorinating agent is calculated based on fluorine, and the sodium-containing agent is calculated based on sodium. The molar ratio of fluorine to aluminum in high-purity aluminum hydroxide satisfies the following: the molar ratio of fluorine to aluminum is (5.8-6.3):1, and the molar ratio of sodium to aluminum is (2.9-3.2):1.

[0015] The temperature-controlled crystallization method described in step 3) is as follows: the cryolite synthesis slurry is cooled from the reaction end temperature to 25-40℃ at a rate of 0.5-2℃ / min, and aged at the endpoint temperature for 1-4 hours.

[0016] During the cooling process, when the slurry temperature drops to 55-65℃, cryolite seed crystals are added. The amount of seed crystals added is 0.01%-0.1% of the expected cryolite product quality.

[0017] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. This invention uses physical sorting to remove light impurities such as unburned carbon particles, reducing interference from subsequent chemical treatments. The subsequent sodium roasting, conducted at a set temperature and time, disrupts the aluminosilicate structure in the fly ash and transforms it into a more manageable form. The staged leaching is designed in an acid-to-alkali sequence. Acidic leaching preferentially dissolves some acid-soluble impurities, while alkaline leaching selectively extracts aluminum as sodium aluminate. This sequence facilitates the initial separation of aluminum from impurities such as iron and calcium, and may also preferentially dissolve some amorphous silica (especially the aluminosilicate glass commonly found in high-alumina fly ash). This significantly reduces aluminum loss and solution contamination caused by the formation of sodium aluminosilicate during subsequent alkaline leaching. The subsequent solution purification step involves introducing hydrated iron oxide as a purification agent and controlling the treatment temperature and time. In an alkaline sodium aluminate solution at 70-90℃, the surface hydroxyl groups of the hydrated iron oxide can undergo ligand exchange adsorption with anions such as silicate and phosphate in the solution, forming inner-layer complexes, thereby achieving deep desilication and dephosphorization. This series of purification steps overcomes the problem of incomplete impurity separation when extracting aluminum from complex fly ash, laying the foundation for obtaining the required high-purity sodium aluminate solution.

[0018] 2. This invention prepares high-purity aluminum hydroxide by controlling the reaction temperature and endpoint pH value of carbonation decomposition. In this step, the introduction of carbon dioxide gas decomposes sodium aluminate, and precise control of the endpoint pH value within a narrow range is crucial. This condition allows aluminum to precipitate sufficiently and selectively as aluminum hydroxide, while impurities such as silicon remain largely in the mother liquor, thus achieving deep separation of aluminum and silicon. This method avoids aluminum loss caused by the formation of sodium aluminosilicate complexes in traditional alkaline methods, thereby ensuring the conversion rate and product purity from sodium aluminate solution to solid aluminum hydroxide. The resulting high-purity aluminum hydroxide provides a reliable precursor for the synthesis of high-quality cryolite.

[0019] 3. In the cryolite synthesis stage, this invention employs a stepwise fluorination synthesis reaction. The first step, under controlled pH and temperature, allows high-purity aluminum hydroxide to fully react with a fluorinating agent to generate a fluoroaluminate intermediate, ensuring complete fluorination of aluminum. The second step introduces a sodium fluorinating agent and reacts within a different pH range, allowing sodium ions to combine with fluoroaluminate and crystallize. This stepwise approach, coupled with precise molar ratios of fluorine, sodium, and aluminum, makes the entire reaction process more controllable. It avoids the problems of aluminum hydroxide re-dissolution or low fluorine utilization that can occur with single-feeding or in strongly alkaline environments, thus facilitating the formation of high-molecular-weight cryolite with stable molecular ratios and uniform structure, solving the difficulties of inaccurate molecular ratio control and uneven product distribution in direct synthesis methods.

[0020] 4. This invention improves the physical properties of the final product through a programmed temperature-controlled crystallization process. Slow cooling promotes the continuous and orderly growth of crystal particles, while the introduction of seed crystals provides more nucleation sites for crystallization, guiding the directional growth of crystals. This controlled crystallization environment helps to form cryolite solids with more concentrated particle size distribution, more complete crystal morphology, and higher particle strength. The resulting product has a higher bulk density, better solubility in subsequent aluminum electrolysis applications, and less dust generation, overcoming the defects of small crystals and poor physical properties caused by rapid cooling or natural cooling. Detailed Implementation

[0021] The present invention will be further described in detail below through specific implementation examples. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope of the appended claims.

[0022] Unless otherwise specified, all raw materials and reagents used in this invention are from the conventional market.

[0023] In the specific embodiments of this application, the sources of various main raw materials are briefly described as follows: Sodium carbonate: Puyang Shengkai Environmental Protection New Material Technology Co., Ltd.; Sodium hydroxide, hydrofluoric acid: Yunnan Shenhuo New Materials Technology Co., Ltd.; Sodium sulfate: Sichuan Shenhong Chemical Co., Ltd.; Sulfuric acid: Guizhou Dafang Runfeng Chemical Co., Ltd.; Fluorosilicic acid: Wuhan Jiyesheng Chemical Co., Ltd.; Ammonium fluoride: Do-Fluoride Chemicals Co., Ltd.; Sodium fluoride: Binzhou Aoxiang Chemical Co., Ltd.; Hydrated iron oxide: Hubei Shuaiyan Ligao Biomedical Co., Ltd.; Cryolite seed crystal: Shandong Botao Group Co., Ltd.

[0024] Example 1 A preparation process for high molecular weight cryolite based on purified fly ash 1) Pretreatment for Fly Ash Purification: 1 kg of high-alumina fly ash from a power plant in Guangdong was taken and first removed by air classification to remove unburned carbon particles and some hollow microspheres. The sorted fly ash was then ground to achieve a specific surface area of ​​850 m² / kg. 100 parts by weight of the ground fly ash were mixed with 30 parts by weight of sodium carbonate as a sodium-fixing agent, ensuring uniform mixing. The mixture was placed in a muffle furnace and calcined at 950℃ for 1.5 hours. After natural cooling, calcined clinker was obtained. The calcined clinker underwent staged leaching. In the first step, it was leached with a 15% sulfuric acid solution at 80℃ for 30 minutes, followed by solid-liquid separation. The leachate was discarded, yielding primary leaching residue. The second step involves leaching the primary leaching residue for 1 hour using a sodium hydroxide solution with a concentration of 250 g / L (calculated as sodium oxide) at a liquid-to-solid mass ratio of 6:1 and a temperature of 100°C. This is followed by hot filtration to obtain a crude sodium aluminate solution and a secondary leaching residue. Finally, the solution is purified by heating the crude sodium aluminate solution to 90°C, adding 0.5% (by mass) of hydrated iron oxide as a purification agent, maintaining the temperature and stirring for 0.5 hours, and then filtering while hot to obtain a clear, high-purity sodium aluminate solution.

[0025] 2) Preparation of high-purity aluminum hydroxide: The obtained high-purity sodium aluminate solution was placed in a reaction vessel, and carbon dioxide gas was introduced into the vessel at 90°C with continuous stirring to carry out carbonation decomposition. The pH was monitored online with a pH meter, and when the pH value of the solution dropped to 10.5, the gas introduction was stopped, and the mixture was kept at the same temperature and stirred for another 30 minutes for maturation. The slurry was then filtered, and the filter cake was thoroughly washed with 80°C hot water to obtain high-purity aluminum hydroxide.

[0026] 3) Cryolite Synthesis and Crystallization: Weigh the above-mentioned high-purity aluminum hydroxide and disperse it in deionized water. Add hydrofluoric acid, a fluorinating agent, to the slurry at 85℃ with stirring, controlling the addition rate to maintain the pH of the reaction system at 1.5. React for 0.5 hours to obtain a fluoroaluminate intermediate solution. Subsequently, slowly add sodium fluoride, a sodium-modifying agent, to this intermediate solution to adjust and maintain the pH of the reaction system at 6.0, and continue the reaction for 1 hour to obtain a cryolite synthesis slurry. Throughout the synthesis reaction, the molar ratio of fluorine to aluminum is controlled at 6.3:1, and the molar ratio of sodium to aluminum is controlled at 3.2:1. Finally, perform temperature-controlled crystallization, cooling the synthesis slurry to 25℃ at a rate of 2.0℃ / min. When the temperature drops to 65℃, add 0.1% (by weight of the expected cryolite product) of cryolite powder with a molecular ratio of 3.2 as seed crystals. After the slurry cools to 25℃, age for 1 hour. After aging, the filter cake was filtered, washed three times with 40°C pure water, and dried at 120°C for 2 hours to obtain a high molecular weight cryolite solid product.

[0027] Example 2 In this embodiment, the similarities to those in Embodiment 1 will not be repeated, and the differences are as follows: This embodiment provides another preparation process for high-molecular-weight cryolite based on purified fly ash: 1) Fly ash purification and pretreatment: 1 kg of fly ash from the same source as in Example 1 was taken and physically separated using wet gravity sedimentation. The separated fly ash was ground to a specific surface area of ​​650 m² / kg. 100 parts by weight of the ground fly ash were mixed with 15 parts by weight of sodium hydroxide, a sodium-modifying agent. The mixture was calcined at 800℃ for 4 hours to obtain calcined clinker. The staged leaching process was as follows: The calcined clinker was leached with a 5% sulfuric acid solution at 50℃ for 90 minutes, and a primary leaching residue was obtained after solid-liquid separation. The primary leaching residue was leached with a 150 g / L sodium hydroxide solution (calculated as sodium oxide) at a liquid-to-solid mass ratio of 4:1 and a temperature of 95℃ for 3 hours, and a crude sodium aluminate solution was obtained after hot filtration. The solution purification steps are as follows: heat the crude solution to 70°C, add 0.1% of its mass of hydrated iron oxide, keep warm and stir for 2 hours, and then filter to obtain a high-purity sodium aluminate solution.

[0028] 2) Preparation of high-purity aluminum hydroxide: Sodium aluminate solution was carbonated and decomposed by passing carbon dioxide through it at 70°C, with the final pH value controlled at 11.5. After the reaction was completed, the solution was filtered and washed to obtain high-purity aluminum hydroxide.

[0029] 3) Synthesis and Crystallization of Cryolite: High-purity aluminum hydroxide was slurried, and fluorosilicic acid, a fluorinating agent, was added at 60°C. The reaction pH was controlled at 3.0, and the reaction was carried out for 2 hours. Subsequently, sodium hydroxide, a sodium fluorinating agent, was added to maintain the pH of the system at 4.5, and the reaction was continued for 3 hours. During this process, the molar ratio of aluminum hydroxide to fluorine was controlled at 5.8:1, and the molar ratio of aluminum hydroxide to sodium hydroxide was controlled at 2.9:1. The crystallization process was as follows: The slurry was cooled to 40°C at a rate of 0.5°C / min. When the temperature dropped to 55°C, 0.01% of cryolite seed crystals with a molecular ratio of 2.8 (by mass of the expected product) was added. After cooling to 40°C, the mixture was aged for 4 hours, then washed, and dried at 100°C for 6 hours to obtain the final product.

[0030] Example 3 In this embodiment, the similarities to those in Embodiment 1 will not be repeated, and the differences are as follows: This embodiment provides yet another process for preparing high-molecular-weight cryolite based on purified fly ash: 1) Fly ash purification pretreatment: After air classification, fly ash is ground to a specific surface area of ​​750 m² / kg. 100 parts by weight are mixed with 22 parts by weight of a mixed sodium alkalizing agent composed of sodium carbonate and sodium sulfate in a mass ratio of 1:1. The mixture is calcined at 875℃ for 2.8 hours to obtain calcined clinker. Stage leaching: First, leaching is performed with a 10% sulfuric acid solution at 65℃ for 60 minutes, followed by solid-liquid separation; then, leaching is performed with a 200 g / L sodium hydroxide solution at a liquid-to-solid ratio of 5:1 and at 98℃ for 2 hours, followed by hot filtration to obtain a crude sodium aluminate solution. Refining steps: The crude solution is heated to 80℃, 0.3% by weight of hydrated iron oxide is added, and after stirring for 1 hour, it is filtered to obtain a high-purity sodium aluminate solution.

[0031] 2) Preparation of high-purity aluminum hydroxide: Sodium aluminate solution was carbonated and decomposed at 80℃, and the final pH value was controlled at 11.0. After filtration and washing, high-purity aluminum hydroxide was obtained.

[0032] 3) Synthesis and crystallization of cryolite: Aluminum hydroxide was slurried, and ammonium fluoride was added as a fluorinating agent at 72°C, maintaining the pH at 2.2, and reacting for 1.2 hours. Subsequently, a sodium fluoride agent composed of sodium carbonate and sodium fluoride was added, maintaining the pH at 5.2, and reacting for 2 hours. The molar ratio of aluminum fluoride to sodium fluoride was 6.0:1, and the molar ratio of sodium fluoride to aluminum fluoride was 3.0:1. Crystallization process: The temperature was lowered to 32°C at a rate of 1.2°C / min, and seed crystals with a molecular ratio of 3.0 (0.05% by weight of the expected product mass) were added at 60°C. After cooling to 32°C, the crystals were aged for 2.5 hours, washed, and dried at 110°C for 4 hours to obtain the finished product.

[0033] Comparative Example 1 In this comparative example, the similarities with Example 3 will not be repeated, and the differences are as follows: The physical sorting step of fly ash is omitted, and the raw fly ash is directly ground and all subsequent processing is carried out.

[0034] Comparative Example 2 In this comparative example, the similarities with Example 3 will not be repeated, and the differences are as follows: The leaching sequence was changed, with a second alkaline leaching followed by an acidic leaching. Specifically, the roasted clinker was first leached with sodium hydroxide solution, and the leaching residue was then treated with sulfuric acid solution.

[0035] Comparative Example 3 In this comparative example, the similarities with Example 3 will not be repeated, and the differences are as follows: The solution purification step is omitted. That is, after obtaining the crude sodium aluminate solution, the carbonation decomposition is carried out directly without heating and adding hydrated iron oxide to remove impurities.

[0036] Comparative Example 4 In this comparative example, the similarities with Example 3 will not be repeated, and the differences are as follows: Modify the carbonation decomposition conditions. Control the final pH value of the decomposition at 12.5.

[0037] Comparative Example 5 In this comparative example, the similarities with Example 3 will not be repeated, and the differences are as follows: The cryolite synthesis steps were modified. The stepwise fluorination synthesis was changed to a one-step synthesis, in which high-purity aluminum hydroxide, hydrofluoric acid, and sodium hydroxide were added to the reactor simultaneously. The final reaction pH was controlled at 5.2, and the reaction temperature and time were the same as in Example 3. The molar ratio of the feed materials was also the same as in Example 3.

[0038] The cryolite products prepared in the examples and comparative examples were subjected to the following performance tests, and the results and analysis are as follows: To evaluate the effectiveness of the process of the present invention, the high-purity aluminum hydroxide intermediate and the final cryolite product obtained from the above three embodiments and five comparative examples were tested.

[0039] The testing method is as follows: The purity of alumina in high-purity aluminum hydroxide was determined by EDTA titration. The molecular ratio of cryolite products was calculated by determining their total fluorine, total aluminum, and total sodium content using chemical analysis. The total content of iron oxide and titanium dioxide in the product was determined by inductively coupled plasma atomic emission spectrometry. The bulk density of cryolite products was determined according to the national standard GB / T 1479.1-2011; The aluminum yield from fly ash to aluminum hydroxide was obtained by calculating the material balance of each step and the aluminum content in the aluminum hydroxide.

[0040] The specific test results are shown in Table 1 below: Table 1 Test Results of Aluminum Hydroxide Intermediates and Cryolite Products sample <![CDATA[Al2O3 purity (%) in aluminum hydroxide]]> Overall aluminum yield (%) <![CDATA[Cryolite molecular ratio (molar ratio of NaF / AlF3)]]> Density of cryolite in loose form (g / cm³) Example 1 99.5 78.2 3.4 1.25 Example 2 99.1 76.8 2.7 1.18 Example 3 99.3 77.5 3.0 1.22 Comparative Example 1 97.8 71.5 2.9 1.15 Comparative Example 2 98.2 68.4 2.8 1.10 Comparative Example 3 98.5 75.1 3.1 1.20 Comparative Example 4 94.3 65.2 2.5 1.05 Comparative Example 5 99.2 77.0 2.1 1.08 Table 2. Composition content and particle size distribution of cryolite products sample <![CDATA[Total content of Fe2O3 + TiO2 (%)]]> <![CDATA[Main content of Na3AlF6 (%)]]> <![CDATA[SiO2(%)]]> <![CDATA[P2O5(%)]]> <![CDATA[Particle size distribution D 50 (μm)]]> Example 1 0.12 98.5 0.08 0.005 45 Example 2 0.14 98.0 0.10 0.006 38 Example 3 0.09 98.8 0.07 0.004 42 Comparative Example 1 0.31 96.0 0.25 0.020 25 Comparative Example 2 0.28 96.5 0.30 0.015 22 Comparative Example 3 0.35 97.2 0.15 0.008 35 Comparative Example 4 0.11 94.5 0.50 0.010 15 Comparative Example 5 0.10 95.8 0.06 0.005 30 As can be seen from Tables 1 and 2, firstly, regarding aluminum source purification and aluminum yield, Examples 1 to 3 all obtained high-purity aluminum hydroxide with an alumina purity higher than 99%, and the overall aluminum element yield was above 76%. This indicates that the series of purification steps described in this invention—physical sorting, sodium roasting, acid-then-alkali staged leaching, and solution purification—are effective. Comparative Example 1, lacking physical sorting, allowed unburned carbon particles and other impurities to enter the subsequent high-temperature roasting, potentially affecting conversion efficiency and introducing more impurities, resulting in a significant decrease in both aluminum hydroxide purity and aluminum yield, and the highest impurity content in the product. As shown in Table 2, in the cryolite product of Comparative Example 1, not only was the total amount of iron and titanium impurities (0.31%) 2-3 times that of the Examples, but its SiO2 content (0.25%) was also significantly higher than that of the Examples (0.07-0.10%), causing its main content (Na3AlF6) to decrease to 96.0%. Comparative Example 2 altered the leaching sequence, with an alkali-then-acid process causing a large amount of silicon to dissolve along with aluminum in the initial alkali leaching stage. This silicon may co-precipitate with aluminum during subsequent acid treatment, leading to increased aluminum loss and a significant decrease in yield. Table 2 also demonstrates that the alkali-then-acid process results in deep cross-linking of silicon impurities in the aluminum extraction channel, making effective separation impossible and contaminating the product. Comparative Example 3 omitted solution purification. Although the aluminum yield was acceptable, the lack of a deep impurity removal step resulted in a final cryolite product with an iron-titanium impurity content as high as 0.35%, far exceeding that of the example. This verifies the crucial role of solution purification in obtaining a low-impurity sodium aluminate solution. Furthermore, the P2O5 content in Comparative Example 3 was also higher than that in the example, indicating that the hydrated iron oxide impurity remover also has a synergistic removal effect on phosphorus.

[0041] Examples 1, 2, and 3, through stepwise fluorination synthesis and precise feed ratios, successfully stabilized the product molecular ratios at 3.4, 2.7, and 3.0, respectively, achieving flexible control within the range of 2.5-3.5. Correlation with the data in Table 2 shows a strong correlation between the polymer ratio and the high main content (98.0-98.8%), indicating that the product is a highly pure cryolite phase with a near-stoichiometric polymer ratio. Comparative Example 5 employed a one-step synthesis method. Despite using aluminum hydroxide of the same purity and a precise total feed ratio, the product molecular ratio was only 2.1, falling within the range of ordinary cryolite. This result clearly demonstrates that the stepwise synthesis strategy of this invention—first completing fluorination under acidic conditions, then completing sodiumization under near-neutral conditions—is crucial for avoiding the hydrolysis of intermediate products and ensuring that sodium fluoroaluminate crystallizes in the predetermined polymer ratio form. One-step synthesis is difficult to control the instantaneous changes in the reaction microenvironment, easily generating products with non-uniform molecular ratios. In Table 2, the cryolite content of Comparative Example 5 is only 95.8%, the lowest among all samples. This indicates that the one-step method leads to a low molecular ratio, resulting in a decrease in the effective Na3AlF6 content, confirming the necessity of stepwise synthesis from the perspective of product chemical composition. In Comparative Example 4, due to raising the final pH value of carbonation to 12.5, the excessively alkaline environment caused more impurities such as sodium silicate to be encapsulated in the aluminum hydroxide precipitate. This not only reduced the purity of the aluminum hydroxide itself but may also affect the accuracy of the subsequent fluorination reaction, resulting in a low final cryolite molecular ratio and extremely severe SiO2 contamination in this sample, causing the main content to plummet to 94.5%. This indicates that the silicon-contaminated precursors generated at high pH alter the molecular ratio and also damage the chemical purity of the product.

[0042] Regarding the physical properties of the products, the loose bulk density of cryolite in all three embodiments was higher than 1.18 g / cm³, thanks to the temperature-controlled crystallization and seed crystal addition technology. Slow cooling and seed crystal induction created favorable conditions for crystal growth, resulting in particles with uniform size and dense structure. Table 2 shows the particle size data (D...). 50 The crystal size (38-45 μm) corresponds to this, indicating that the crystal achieved sufficient and ordered growth. In contrast, the D crystals in Comparative Examples 1, 2, and 5... 50 All samples had a particle size less than 30 μm, with Comparative Example 4 even reaching as low as 15 μm. Although Comparative Examples 4 and 5 used different process routes, both suffered from insufficient control during the crystallization process. Comparative Example 4's low product density and deteriorated crystal physical properties were due to issues with precursor purity, while Comparative Example 5's low density was likely influenced by the different synthesis routes. This further confirms the necessity of a controlled crystallization process for improving product application performance.

[0043] In summary, this invention provides an integrated solution for converting waste fly ash into high-value-added cryolite with a high molecular weight ratio, while simultaneously solving several technical challenges such as aluminum source purification, precise control of molecular weight ratio, and product crystallization quality.

[0044] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A preparation process for high molecular weight cryolite based on purified fly ash, characterized in that, Includes the following steps: 1) Pretreatment for purification of fly ash: Physical sorting and sodium roasting of raw fly ash to obtain roasted clinker, followed by stage leaching and solution refining to obtain high-purity sodium aluminate solution. 2) Preparation of high-purity aluminum hydroxide: The high-purity sodium aluminate solution is subjected to carbonation decomposition to obtain high-purity aluminum hydroxide; 3) Cryolite Synthesis and Crystallization: The high-purity aluminum hydroxide is synthesized by stepwise fluorination reaction with fluorinating agent and sodium fluorinating agent, and then crystallized by temperature control to obtain high molecular weight cryolite solid.

2. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 1, characterized in that, The physical sorting and sodium roasting mentioned in step 1) involve air classification or wet gravity settling of the raw fly ash; the physically sorted fly ash is then ground to a specific surface area of ​​650-850 m². 2 / kg; then fly ash and sodium-modifying agent are mixed evenly at a mass ratio of 100:(15-30), wherein the sodium-modifying agent includes one or more of sodium carbonate, sodium hydroxide, and sodium sulfate; the mixture is calcined at 800-950℃ for 1.5-4 hours to obtain calcined clinker.

3. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 1, characterized in that, In step 1), the staged leaching and solution purification includes the following steps: a) Primary acid leaching: Using a sulfuric acid solution with a mass concentration of 5%-15%, the roasted clinker is leached at 50-80℃ for 30-90 minutes, and the primary leaching residue is obtained after solid-liquid separation. b) Secondary alkaline leaching: Using a sodium hydroxide solution with a concentration of 150-250 g / L and calculated as sodium oxide, the primary leaching residue is leached for 1-3 hours at a liquid-to-solid mass ratio of (4-6):1 and a temperature of 95℃ to 100℃. After hot filtration, a crude sodium aluminate solution and a secondary leaching residue are obtained. c) Heat the crude sodium aluminate solution and maintain it at 70-90°C, add a purification agent, keep it warm and stir for 0.5-2 hours, then filter to obtain a high-purity sodium aluminate solution.

4. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 1, characterized in that, In step 1), the impurity removal agent includes hydrated iron oxide, and the amount added is 0.1%-0.5% of the mass of the crude sodium aluminate solution.

5. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 1, characterized in that, The carbonation decomposition method described in step 2) is as follows: under continuous stirring, carbon dioxide gas is introduced into a high-purity sodium aluminate solution, the reaction temperature is 70-90℃, the reaction endpoint is controlled to be 10.5-11.5 by monitoring the pH value, then solid-liquid separation is performed, the solid is washed, and high-purity aluminum hydroxide is obtained.

6. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 1, characterized in that, The stepwise fluorination synthesis reaction described in step 3) includes the following steps: First step fluorination: The high-purity aluminum hydroxide is slurried and dispersed in water, and a fluorinating agent is added under stirring. The pH of the reaction system is 1.5-3.0, the reaction temperature is 60-85℃, and the reaction time is 0.5-2 hours to generate a fluoroaluminate intermediate solution. The second step, sodium formation, involves adding a sodium-forming agent to the fluoroaluminate intermediate solution, adjusting and maintaining the pH of the reaction system at 4.5-6.0, and continuing the reaction for 1-3 hours to obtain cryolite synthesis slurry.

7. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 6, characterized in that, The fluorinating agent includes one or more of hydrofluoric acid, fluorosilicic acid, and ammonium fluoride; the sodium-containing agent includes one or more of sodium hydroxide, sodium carbonate, and sodium fluoride.

8. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 6, characterized in that, The fluorinating agent is calculated based on fluorine, and the sodium-containing agent is calculated based on sodium. The molar ratio of fluorine to aluminum in high-purity aluminum hydroxide satisfies the following: the molar ratio of fluorine to aluminum is (5.8-6.3):1, and the molar ratio of sodium to aluminum is (2.9-3.2):

1.

9. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 1, characterized in that, The temperature-controlled crystallization method described in step 3) is as follows: the cryolite synthesis slurry is cooled from the reaction end temperature to 25-40℃ at a rate of 0.5-2℃ / min, and aged at the endpoint temperature for 1-4 hours.

10. The preparation process of high molecular weight cryolite based on purified fly ash according to claim 9, characterized in that, During the cooling process, when the slurry temperature drops to 55-65℃, cryolite seed crystals are added. The amount of seed crystals added is 0.01%-0.1% of the expected cryolite product quality.