Preparation method of fluorine removal agent based on modified red mud

Abstract

The invention provides a preparation method of a fluorine removal agent based on modified red mud, which comprises the following steps: modifying industrial red mud, adding a certain amount of activated carbon, and mixing. The method for modifying the industrial red mud comprises the following steps: drying the industrial red mud and limestone until free water does not exist, and respectively grinding and sieving; mixing the ground industrial red mud and limestone according to a certain proportion, and adding a small amount of purified water to infiltrate to obtain a mixture of the industrial red mud and the limestone; adding an activating agent into the mixture for acidification activation, adjusting the pH value to be less than 1, stirring and standing to obtain an activated mixture; and evaporating the activated mixture to dryness, and calcining to obtain the modified red mud and limestone mixture. The industrial red mud is used as the raw material, the process is simple, the cost is low, waste resources are recycled, and the method has a good application prospect.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment, and more specifically, to a method for preparing a modified red mud defluorinating agent. Background Technology

[0002] Industrial red mud is the main waste residue discharged after refining alumina from bauxite. It is a typical representative of bulk industrial solid waste, typically generating about 1-2 tons of red mud for every ton of alumina produced. Depending on the alumina preparation process, it is divided into Bayer process red mud, sintering process red mud, and combined process red mud. The main components of industrial red mud from different sources are basically the same, with Al₂O₃, Fe₂O₃, SiO₂, CaO, Na₂O, and TiO₂ accounting for about 90% of the total mass. In addition, industrial red mud also contains caustic soda components, rare earth elements, and radioactive elements such as gallium, rhenium, yttrium, scandium, tantalum, niobium, thorium, uranium, and lanthanides.

[0003] Rapid industrial development has generated a large amount of fluoride-containing wastewater. If discharged directly without proper treatment, this wastewater poses a significant threat to the ecological environment and human health. Excessive fluoride levels not only severely corrode industrial pipes and equipment but can also accumulate in organisms through the food chain. Long-term ingestion can lead to diseases such as dental fluorosis and skeletal fluorosis, causing irreversible damage. Therefore, reducing the concentration of fluoride ions in wastewater from thousands of mg / L in industrial emissions to below the national emission standard of <10 mg / L is a crucial environmental protection task. In this process, defluorinating agents, as functional materials specifically designed to capture and fix fluoride ions, play a core detoxifying role, becoming a key link between industrial production and environmental protection.

[0004] Currently, based on their core components and physical forms, mainstream defluoridators can be mainly divided into five categories, each with its own characteristics and limitations. Aluminum salt-based defluoridators, represented by polyaluminum chloride, remove fluoride through the adsorption and co-precipitation of their hydrolysis products. They have the advantage of rapid reaction, but generate a large amount of chemical sludge after addition, resulting in high disposal costs and the potential risk of aluminum residue. Calcium-based materials, such as hydroxyapatite or lime, primarily remove fluoride by reacting calcium ions with fluoride ions to form calcium fluoride precipitate. This method is simple and low-cost, but it has specific pH requirements, and the precipitate is difficult to separate, potentially increasing the hardness of the effluent. Rare earth materials, such as lanthanum-modified activated alumina, utilize the strong affinity of rare earth ions for fluoride ions for adsorption and locking, exhibiting significant adsorption capacity and the ability to be repeatedly regenerated. However, their high cost limits their large-scale application. Iron-based materials, such as iron-manganese oxide composites, can simultaneously remove fluoride and adsorb heavy metals such as arsenic and lead, achieving multi-functional purification and solidifying heavy metals in sludge. However, they are mostly in powder form, and their solid-liquid separation efficiency needs improvement. Biomaterials, including modified chitosan or specific engineered bacteria, are environmentally friendly and particularly suitable for degrading organofluorine compounds; however, treatment efficiency, stability, and the economic viability of large-scale applications remain challenges. The working principles of these defluorinators profoundly reflect the physicochemical nature of removing fluoride ions from water. They separate fluoride ions from solution by forming insoluble compounds such as calcium fluoride; immobilize fluoride ions through coordination or ion exchange at active sites on the material surface; and degrade organofluorine compounds through microbial or enzymatic reactions.

[0005] Although there are many types of defluorinating agents available, they are still generally limited by factors such as cost, efficiency, sludge production, or applicable conditions.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The primary objective of this invention is to provide a composite defluorinating agent and its preparation method, which is formulated from industrial red mud as raw material through acidification activation, calcination modification, and compounding with calcium sources and activated carbon. This method not only achieves highly efficient purification of fluoride-containing wastewater, particularly the deep removal of low-concentration fluoride pollutants, but also significantly reduces treatment costs, promotes the high-value utilization of industrial red mud, and provides an economically feasible, environmentally friendly, and sustainable new technological approach for the treatment of industrial fluoride-containing wastewater.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: This invention provides a method for preparing a modified red mud defluorinating agent, comprising the following steps: Industrial red mud is modified to obtain a mixture of modified red mud and limestone; Activated carbon is added to a mixture of modified red mud and limestone, and the mixture is stirred to obtain the final product.

[0009] This invention systematically constructs a complete process framework combining industrial red mud modification and activation with functional compounding. This method positions industrial red mud as the core raw material, transforming it into an adsorption-active substrate material through initial modification treatment, and then introducing activated carbon in the later stage for enhanced compounding. This overall scheme not only clarifies the technological transformation path from waste to a highly efficient defluorinating agent, but more importantly, it strategically integrates and designs multiple defluorination mechanisms, including physical adsorption (from activated carbon and porous red mud matrix), chemical adsorption (from active metal sites on the modified red mud surface), and chemical precipitation (from limestone calcium source), laying a fundamental process foundation for achieving efficient, stable, and multifunctional defluorination performance.

[0010] Preferably, as a further specific embodiment, the modification treatment of industrial red mud includes the following steps: drying industrial red mud and limestone until no free water remains, and then grinding and sieving them separately; Grinded industrial red mud and limestone are mixed in a certain proportion, and a small amount of purified water is added to leach the mixture to obtain a mixture of industrial red mud and limestone. An activator is added to the mixture for acidification and activation. The pH is adjusted to <1, and the mixture is stirred and allowed to stand to obtain the activated mixture. The activated mixture is evaporated to dryness and then calcined to obtain a mixture of modified red mud and limestone.

[0011] The paper details the sequential operational steps necessary for modifying industrial red mud, including: drying industrial red mud and limestone until free water is removed and grinding and sieving them separately; mixing the two in a specific ratio and then wetting them with water; adding an activator for acidification activation until pH < 1, followed by stirring and standing; and finally evaporating the activated mixture to dryness and then calcining it. The practical technical effect lies in the fact that this series of interconnected and clearly defined steps constitutes the key transformation process for activating inert red mud into a high-performance adsorbent core. Specifically, drying and grinding are pretreatments aimed at eliminating moisture interference and increasing the reaction contact area; water wetting ensures a more uniform subsequent acid activation reaction; acid activation (pH < 1) utilizes a strong acid environment to dissolve the passivation layer on the red mud surface, clear pores, and expose more active sites of metals such as aluminum and iron through ion exchange or complexation, which is the core step in endowing it with strong chemical adsorption capabilities; evaporation is to fix the activated state and facilitate subsequent operations; calcination, at high temperatures, causes the acid-activated intermediate to undergo further phase transformation, removes residual volatile components, and ultimately forms a modified material with stable structure, suitable crystallinity, and robust active sites. The entire process design serves the dual purpose of chemical modification and physical structure optimization, ensuring the reproducibility of the modified product's performance and the industrial operability of the process.

[0012] Preferably, as a further specific embodiment, the mass ratio of the industrial red mud to limestone is (8:5) to (1:2).

[0013] The range of main raw material feeding ratios is limited. By controlling the amount of limestone added within this range, it is possible to ensure that sufficient calcium source is provided to assist chemical precipitation while avoiding excessive limestone dilution or coating of red mud active components, thereby ensuring that the composite material has basic adsorption-precipitation synergistic properties.

[0014] Preferably, as a further specific embodiment, the mass ratio of the industrial red mud to limestone is (2:1).

[0015] Further optimization of the feed mass ratio, which is the equilibrium point verified by experiments, enables the adsorption of industrial red mud and the precipitation of limestone to reach the best synergistic state, and is the key parameter for achieving the highest defluorination efficiency.

[0016] Preferably, as a further specific embodiment, the activator is one or more of oxalic acid, citric acid, aminosulfonic acid, sulfuric acid, or hydrochloric acid.

[0017] It provides a variety of feasible acidification activation routes, increasing the flexibility of the method and its adaptability to different raw materials and costs, while covering different activation mechanisms from inorganic strong acids to organic acids.

[0018] Preferably, as a further specific embodiment, the activator is prepared by mixing citric acid and aminosulfonic acid in a mass ratio of (1:4) to (4:1).

[0019] A composite system of two specific organic acids was employed, with a wide range of ratios provided. A synergistic effect between citric acid and aminosulfonic acid was discovered in the activation of red mud. This composite system can more effectively modify the surface of industrial red mud and create more active sites. This ratio range represents the basic boundary for producing a significant synergistic effect.

[0020] Preferably, as a further specific embodiment, the activator is prepared by mixing citric acid and aminosulfonic acid in a mass ratio of 3:2.

[0021] The optimal ratio was further determined within the composite range. This specific ratio represents the optimal balance point for the synergistic activation of the two organic acids, maximizing the synergistic effect and thus producing a modified material with the most suitable surface properties and the strongest adsorption capacity for fluoride ions.

[0022] Preferably, as a further specific embodiment, the calcination temperature is 450℃-600℃, and the calcination time is 1-2h.

[0023] This temperature range ensures the full decomposition of organic components and the effective transformation and stabilization of industrial red mud, thereby forming materials with a stable structure and abundant active surfaces, while avoiding incomplete activation due to excessively low temperatures or sintering deactivation due to excessively high temperatures.

[0024] Preferably, as a further specific embodiment, the calcination temperature is 600°C and the calcination time is 2 hours.

[0025] The optimal conditions specified within the calcination window. These conditions represent the best point within the process window, enabling the most effective activation and crystal transformation of the material, maximizing the adsorption performance of the modified red mud, and are critical process parameters ensuring the high performance of the final product.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention provides a method for preparing a modified red mud defluorinating agent. This method uses red mud from the aluminum industry as the main raw material, and through a systematic activation and calcination modification process combined with activated carbon compounding technology, successfully prepares a high-performance modified red mud defluorinating agent. This method achieves high-value resource utilization of industrial hazardous waste, significantly reducing the production cost and raw material dependence of the defluorinating agent. The preparation process involved is simple, mild, and energy-efficient, making it easy to implement on a large scale. By selecting specific composite activators and controlling key calcination conditions, the surface activity and adsorption sites of the material are effectively improved; the introduction of activated carbon further enhances the synergistic effect of physical adsorption and chemical action, giving the material excellent deep defluorination capability and concentration adaptability. The final product can stably achieve the standard purification of fluoride-containing wastewater, effectively controlling pollution while embodying the green circular concept of treating waste with waste, and has significant environmental, economic, and social benefits. Detailed Implementation

[0027] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] To more clearly illustrate the technical solutions in this invention, specific embodiments are described below.

[0029] Example 1 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Prepare an activator by mixing citric acid and aminosulfonic acid in a mass ratio of 3:2. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add activator until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0030] Example 2 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Prepare an activator by mixing citric acid and aminosulfonic acid in a mass ratio of 1:4. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add activator until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0031] Example 3 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Prepare an activator by mixing citric acid and aminosulfonic acid in a mass ratio of 4:1. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add activator until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0032] Example 4 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add oxalic acid until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and placed in a muffle furnace and calcined at 550°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0033] Example 5 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add citric acid until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and placed in a muffle furnace and calcined at 450°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0034] Example 6 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add citric acid until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0035] Example 7 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add aminosulfonic acid until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 1 hour. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0036] Example 8 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add aminosulfonic acid until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and placed in a muffle furnace and calcined at 500°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0037] Example 9 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Accurately measure 4.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add sulfuric acid until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and placed in a muffle furnace and calcined at 450°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0038] Example 10 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.400g (±0.005g) of limestone, add a small amount of pure water to soak, then add hydrochloric acid until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0039] Example 11 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add hydrochloric acid until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 1.5 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0040] Example 12 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Prepare an activator by mixing citric acid and aminosulfonic acid in a mass ratio of 2:3. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add activator until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0041] Example 13 The preparation process of this modified red mud defluorinating agent is as follows: Industrial red mud and limestone were dried until no free water remained, and then ground separately through a 60-mesh sieve. Prepare an activator by mixing citric acid and aminosulfonic acid in a 1:1 mass ratio. Accurately measure 1.000g (±0.01g) of industrial red mud and 0.500g (±0.005g) of limestone, add a small amount of pure water to soak, then add activator until pH < 1, stir thoroughly, and let stand for 2 hours to activate; The activated industrial red mud and limestone mixture was transferred to an evaporating dish and the moisture was evaporated. The dried modified red mud and limestone mixture was transferred to a crucible and calcined in a muffle furnace at 600°C for 2 hours. Accurately measure 0.500g (±0.005g) of activated carbon and mix it again with the modified red mud and limestone mixture to obtain the final product.

[0042] Comparative Example 1 The specific preparation steps for this comparative example are the same as those for Example 1, except that the mass of limestone weighed is changed to 1.000g (±0.01g).

[0043] Comparative Example 2 The specific preparation steps for this comparative example are the same as those in Example 1, except that the ratio of citric acid to aminosulfonic acid is changed to 1:5.

[0044] Comparative Example 3 The specific preparation steps for this comparative example are the same as those for Example 1, except that the ratio of citric acid to aminosulfonic acid is changed to 5:1.

[0045] Comparative Example 4 The specific preparation steps of this comparative example are the same as those of Example 1, except that the calcination temperature is changed to 300℃ and the calcination time is changed to 2.5h.

[0046] Comparative Example 5 The specific preparation steps of this comparative example are the same as those of Example 1, except that the calcination temperature is changed to 700℃ and the calcination time is changed to 0.5h.

[0047] Experimental Example 1: Determination of Fluoride Ion Absorption Rate Accurately weigh 1.000 g (±0.01 g) of the modified red mud defluorinating agent prepared in Examples 1-13 and Comparative Examples 1-5. The weighed modified red mud defluorinating agent was placed into a 10 mg / L fluoride ion solution (1 L solution volume) and stirred at 20 °C until the fluoride ion concentration no longer decreased. The modified red mud defluorinating agent was separated using 0.22 mm qualitative filter paper, and the supernatant was collected. Determine the fluoride ion absorption rate; Fluoride ion concentration was determined according to the "Determination of Fluoride in Water - Ion Selective Electrode Method" (GB / T 748).

[0048] The final data is shown in Table 1: Table 1

[0049] Experimental Example 2: Determination of Maximum Absorbable Concentration per Unit Mass 1.000 g (±0.01 g) of the modified red mud defluorinating agent prepared in Example 1 was precisely measured. The weighed modified red mud defluorinating agent was placed into fluoride ion solutions of 10 mg / L, 100 mg / L, and 1000 mg / L (solution volume 1L), and stirred at 20℃ until the fluoride ion concentration no longer decreased. The modified red mud defluorinating agent was separated using 0.22 mm qualitative filter paper, and the supernatant was collected. The maximum absorption concentration per unit mass of the modified red mud defluorinating agent was determined.

[0050] Fluoride ion concentration was determined according to the "Determination of Fluoride in Water - Ion Selective Electrode Method" (GB / T 7484).

[0051] The final data is shown in Table 2 below:

[0052] The data above shows that Example 1, as the optimal embodiment, used a composite activator prepared with citric acid and aminosulfonic acid at a mass ratio of 3:2, and industrial red mud to limestone at a mass ratio of 2:1, and was calcined at 600°C for 2 hours. This combination achieved the highest defluorination efficiency (99.37%). This indicates that the composite organic acid activator at this specific ratio, combined with a sufficiently high calcination temperature, can most effectively stimulate the adsorption potential of industrial red mud and produce optimal synergy with the calcium source provided by limestone.

[0053] Examples 2 and 3 also used a composite activator (citric acid: sulfamic acid 1:4 and 4:1, respectively) and calcination at 600°C, achieving defluorination rates of 97.65% and 99.12%, respectively. Although the performance remained excellent, it was slightly lower than the 3:2 ratio (Example 1). This indicates that there is an optimal ratio for the composite activator, and deviations from this value may prevent the synergistic effect of the two acids from reaching its peak.

[0054] Example 4 used oxalic acid as a single activator and calcined at 550°C, achieving a defluorination rate of 86.15%. This performance is significantly lower than the example using a composite activator at a higher temperature (600°C). This highlights that single oxalic acid may be inferior to a composite acid system with a specific ratio in terms of activation capacity and final material properties.

[0055] Both Examples 5 and 6 used citric acid for activation, but the calcination temperatures were 450°C and 600°C, respectively. Their defluorination rates were 80.22% and 96.54%, respectively. This comparison clearly shows that, with the same activator, calcination temperature is the key parameter determining material properties. The higher temperature (600°C) significantly improved the activation degree and defluorination capability of the material.

[0056] Both Examples 7 and 8 used aminosulfonic acid as a single activator, but the calcination conditions were different (600℃ / 1h and 500℃ / 2h). The defluorination rates were 90.11% and 95.61%, respectively. The results show that aminosulfonic acid performs well as a single activator, and high performance can be obtained under appropriate calcination conditions (such as 500℃ for 2 hours).

[0057] Examples 9, 10, and 11 used inorganic acids such as sulfuric acid and hydrochloric acid as activators, respectively. Their defluorination rates ranged from 77.84% to 86.59%, generally lower than those of the examples using organic acids, especially complex organic acids. This indicates that inorganic acids are less effective than organic acids in functionalizing this type of red mud, because the complexation effect of organic acids is more effective in modifying the surface properties of the material.

[0058] Examples 12 and 13 further verified the excellent performance of the composite activator of citric acid and aminosulfonic acid under calcination at 600°C, with defluorination rates reaching 98.12% and 98.88%, respectively. This further consolidates the advantages of the "citric acid-aminosulfonic acid" composite system over most single acid systems.

[0059] In Comparative Example 1, under optimal activation and calcination conditions, simply changing the ratio of industrial red mud to limestone from 2:1 to 1:1 (increasing the amount of limestone) caused the fluoride removal rate to plummet to 72.88%. This demonstrates that the raw material ratio is crucial; excessive limestone is not only useless but may also dilute the active ingredients or hinder the adsorption process.

[0060] Comparative Examples 2 and 3, under optimal calcination conditions, showed significant deviations in the ratio of the composite activator to 1:5 and 5:1 (beyond the scope of the claims), respectively. The defluorination rate decreased to approximately 72%, indicating severe performance degradation. This strongly supports the necessity of the ratio range (1:4 to 4:1) stated in the claims, beyond which the synergistic effect fails.

[0061] Comparative Examples 4 and 5 were designed with optimal activator and formulation, but the calcination temperature was varied. In Comparative Example 4 (300℃), the temperature was too low, potentially leading to insufficient activation and crystal transformation; in Comparative Example 5 (700℃), the temperature was too high, which could cause material sintering and a decrease in specific surface area. Both examples showed a defluorination rate of only about 70%, indicating that the calcination temperature must be strictly controlled within a specific range (e.g., 450-600℃), as temperatures that are too low or too high can lead to material failure.

[0062] Experiment 2: Multiple concentration tests were conducted on the modified red mud defluorinator prepared under optimal conditions. It achieved removal rates of 99.37%, 99.2%, and 95.34% for fluoride solutions of 10 mg / L, 100 mg / L, and 1000 mg / L, respectively, and the maximum absorption capacity per unit mass was measured. This fully demonstrates that this optimal product is not only suitable for deep purification but also possesses strong treatment capabilities for high-concentration wastewater, showcasing its broad application potential.

[0063] As can be seen from the item-by-item analysis of the embodiments, in the preparation method of the present invention, the use of activator, sufficient calcination temperature, and appropriate proportion of core raw materials are four indispensable and interrelated elements for obtaining high-performance modified red mud defluorinating agent. Deviation from any one of these will lead to a significant decrease in the performance of the final product. Example 1 embodies the combination of all these optimal conditions, thus achieving excellent defluorination effect.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a modified red mud defluorinating agent, characterized in that, Includes the following steps: Industrial red mud is modified to obtain a mixture of modified red mud and limestone; Activated carbon is added to a mixture of modified red mud and limestone, and the mixture is stirred to obtain the final product.

2. The preparation method of the modified red mud defluorinating agent according to claim 1, characterized in that, The modification treatment of industrial red mud includes the following steps: drying industrial red mud and limestone until no free water remains, and then grinding and sieving them separately; Grinded industrial red mud and limestone are mixed in a certain proportion, and a small amount of purified water is added to soak them to obtain a mixture of industrial red mud and limestone. An activator is added to the mixture for acidification and activation. The pH is adjusted to <1, and the mixture is stirred and allowed to stand to obtain the activated mixture. The activated mixture is evaporated to dryness and then calcined to obtain a mixture of modified red mud and limestone.

3. The preparation method according to claim 2, characterized in that, The mass ratio of industrial red mud to limestone is (8:5)-(5:2).

4. The preparation method according to claim 3, characterized in that, The mass ratio of industrial red mud to limestone is 2:

1.

5. The preparation method according to claim 2, characterized in that, The activator is one or more of oxalic acid, citric acid, aminosulfonic acid, sulfuric acid, or hydrochloric acid.

6. The preparation method according to claim 5, characterized in that, The activator is prepared by mixing citric acid and aminosulfonic acid in a mass ratio of (1:4) to (4:1).

7. The preparation method according to claim 6, characterized in that, The activator is prepared by mixing citric acid and aminosulfonic acid in a mass ratio of 3:

2.

8. The preparation method according to claim 2, characterized in that, The calcination temperature is 450℃-600℃, and the calcination time is 1-2 hours.

9. The preparation method according to claim 8, characterized in that, The calcination temperature is 600℃ and the calcination time is 2 hours.

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