A method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide

Abstract

This application provides a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide, belonging to the field of comprehensive utilization of solid waste. The method includes: reacting waste sodium salt with calcium oxide and a dispersant in an alkaline recovery reaction, followed by liquid-solid separation to obtain a first alkaline solution and a first calcium slag; slurrying the first calcium slag with water, then introducing concentrated carbon dioxide and adding a nucleating agent to conduct a carbonation reaction, followed by liquid-solid separation to obtain a circulating liquid and a second calcium slag; mixing the second calcium slag with the first alkaline solution and adding a supplementary agent to conduct a dissolution reaction, followed by liquid-solid separation to obtain a dissolved aluminum solution and wet calcium carbonate; sending the dissolved aluminum solution into an alumina production system to crystallize and precipitate aluminum hydroxide, and drying the wet calcium carbonate to obtain the calcium carbonate product. This application achieves full-process resource utilization of sodium, carbon, calcium, and aluminum from waste sodium salt by using a dispersant to inhibit aluminum loss and a nucleating agent to promote calcium-aluminum separation, with no waste residue or waste liquid discharged. The purity of the calcium carbonate product is higher than 98%, and the impurities of alumina and sodium oxide are both lower than 0.01%.

Description

Technical Field

[0001] This application relates to the field of solid waste comprehensive utilization technology, and in particular to a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide. Background Technology

[0002] In the aluminum industry's aluminum hydroxide production process, due to differences in raw materials and process conditions, a mixed system of solid phase and mother liquor with high sodium salt content is formed in the production system. This type of byproduct is mainly alkali metal carbonates, accompanied by various soluble impurities. Long-term accumulation can easily lead to system alkali imbalance, decreased heat exchange efficiency, and increased material loss. For waste sodium salt rich in sodium carbonate, some companies use lime to react with the waste sodium salt to achieve effective alkali regeneration and reuse. However, due to limitations in reaction conditions, sodium aluminate in the waste sodium salt is lost, forming a calcium aluminate phase, resulting in reduced alkali recovery efficiency and increased alumina loss. At the same time, alkali recovery reactions are costly and generate large amounts of solid waste. To avoid these problems, some companies directly outsource the treatment of waste sodium salt.

[0003] In existing technologies, some methods prepare light calcium carbonate using sintering carbonation mother liquor. This involves deep carbonation treatment of the alumina sintering carbonation mother liquor to obtain a mother liquor with low impurity content, followed by the addition of graded lime slurry for causticization. The causticized liquor is then returned to the Bayer process for batching, simultaneously producing light calcium carbonate. Other methods accumulate sodium carbonate concentration in the Bayer process alumina decomposition mother liquor to a certain range, then add slaked lime for complete causticization, yielding calcium carbonate after treatment. However, these methods directly add calcium oxide to the mother liquor. While this can recover some sodium hydroxide, the introduction of calcium oxide leads to alumina loss in the system, and some impurities enter the calcium carbonate product, affecting its purity. Furthermore, these methods still present solid waste discharge issues, failing to achieve full-process resource utilization of waste sodium salts. Therefore, how to achieve full-process resource utilization of waste sodium salts and achieve zero-waste treatment is a pressing technical problem to be solved in this field. Summary of the Invention

[0004] This application provides a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide, in order to solve the following technical problem: how to achieve the full-process resource utilization of waste sodium salt and achieve zero-waste treatment.

[0005] This application provides a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide, the method comprising: An alkali recovery reaction is carried out on a mixture of waste sodium salt, calcium oxide, and dispersant to obtain a mixed slurry; the dispersant includes at least one of hydroxy polycarboxylic acids, amino polycarboxylic acids, hydroxyphosphonic acids, and polysaccharides; The mixed slurry is subjected to a first liquid-solid separation to obtain a first alkaline solution and a first calcium slag. The first calcium slag is mixed with water to obtain a calcium slag slurry; The captured and concentrated flue gas containing carbon dioxide is introduced into the calcium slag slurry, and a nucleating agent is added to the calcium slag slurry to carry out a carbonization reaction to obtain a carbonized slurry; the nucleating agent includes at least one of aluminum hydroxide, calcium hydroxide, calcium oxide and calcium carbonate; The carbonized slurry is subjected to a second liquid-solid separation to obtain a circulating liquid as makeup water and a second calcium slag. Add a supplement to the mixture of the second calcium slag and the first alkaline solution to carry out an alkaline leaching reaction and obtain an aluminum recovery slurry; The aluminum recovery slurry is subjected to a third liquid-solid separation to obtain a dissolved aluminum solution and wet calcium carbonate. The molten aluminum solution is fed into an alumina production system to crystallize aluminum hydroxide, and the wet calcium carbonate is dried and dehydrated to obtain the calcium carbonate product.

[0006] Optionally, the waste sodium salt has a sodium oxide mass fraction > 45% and a moisture content < 30%.

[0007] Optionally, the mass ratio of the waste sodium salt to the calcium oxide is 1:(1.2~1.6); The alkali recovery reaction includes the following parameters: liquid-to-solid ratio of 2.5 to 6, reaction temperature of 80℃ to 100℃, and reaction time of 2h to 5h.

[0008] Optionally, the mass of the dispersant is 0.2% to 0.8% of the total mass of the reaction system of the alkali recovery reaction.

[0009] Optionally, the concentration of sodium hydroxide in the first alkaline solution is 100 g / L to 200 g / L; In the first calcium slag, the mass fraction of calcium carbonate is 80%~95%.

[0010] Optionally, the mass of the nucleating agent is 0.03% to 0.05% of the mass of calcium aluminate in the first calcium slag.

[0011] Optionally, the carbonization reaction includes the following parameters: flue gas temperature of 25~45℃, solid content of calcium slag slurry of 8%~25%, and pH of carbonization slurry of 9~10.

[0012] Optionally, the supplement is water and sodium carbonate.

[0013] Optionally, in the aluminum recovery slurry, the concentration of sodium hydroxide is 100g / L~150g / L, and the concentration of sodium carbonate is 20g / L~35g / L.

[0014] Optionally, in the calcium carbonate product, the mass fraction of calcium carbonate is >98%, the mass fraction of alumina is <0.01%, the mass fraction of sodium oxide is <0.01%, and the moisture content is <0.1%.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing calcium carbonate and recovering aluminum hydroxide from waste sodium salt. The method involves converting sodium in the waste sodium salt into sodium hydroxide in a first alkaline solution for subsequent alkaline leaching reactions, converting carbon and calcium in the waste sodium salt into calcium carbonate products, and converting associated aluminum in the waste sodium salt into a soluble aluminum solution which is then fed into an alumina production system to crystallize and precipitate aluminum hydroxide. Simultaneously, the circulating liquid generated from the carbonation reaction is recycled back into the system as supplementary water. This ensures that all intermediate and final products are effectively utilized or recycled, with no waste discharged, thereby achieving full-process resource utilization of waste sodium salt and achieving zero-waste treatment. Attached Figure Description

[0016] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A process flow diagram of a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide, provided in an embodiment of this application; Figure 2 This application provides a technical flow chart of a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide. Figure label: 1-Alkali recovery tank; 2-First liquid-solid separation; 3-Carbonizer; 4-Second liquid-solid separation; 5-Aluminum recovery; 6-Third liquid-solid separation; 7-Drying; 201-First alkali solution; 202-First calcium slag; 301-Industrial water; 302-Nucleating agent; 401-Circulating liquid; 402-Second calcium slag; 601-Aluminum dissolving solution; 3011-Waste heat recovery; 3012-Carbon capture; 3013-Carbon dioxide; 101-Dispersant; 501-Supplement; 6011-Alumina production system. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0021] In aluminum production, the Bayer process is currently the mainstream technology for alumina production. Using bauxite as raw material, alumina is dissolved from the ore under high temperature and pressure with a caustic alkali solution, generating sodium aluminate solution. Insoluble impurities are separated and discharged as red mud. The purified sodium aluminate concentrate is then decomposed by adding aluminum hydroxide seed crystals, cooling, and stirring to precipitate aluminum hydroxide crystals. After washing and separation, the precipitate is calcined at high temperature to obtain the finished alumina. The mother liquor after decomposition is evaporated and concentrated before being returned to the leaching process to achieve alkali recycling. Simultaneously, evaporation removes impurities such as sodium carbonate and sodium oxalate, preventing their accumulation in the system and affecting production stability. The entire process forms a continuous closed loop, characterized by mature technology, low energy consumption, and suitability for large-scale production.

[0022] In the Bayer process of alumina production, the raw materials cause sodium carbonate to continuously accumulate in the circulating alkali solution. Excess sodium carbonate must be discharged as waste crystalline salt through processes such as evaporation concentration and cooling crystallization to avoid problems such as equipment scaling, increased solution viscosity, and reduced seed crystal decomposition efficiency. Traditional processes result in large volumes of waste sodium salt, which has a complex composition containing alkali, aluminum, and organic matter. Direct discharge not only leads to the loss of alkali and alumina resources but also creates a large amount of difficult-to-dispose-of solid waste, posing environmental pressures and high treatment costs.

[0023] Currently, some companies use lime to react with waste sodium salt to achieve effective alkali regeneration and reuse. However, due to limitations in reaction conditions, sodium aluminate in the waste sodium salt is lost, forming a calcium aluminate phase, resulting in low alkali recovery efficiency and significant alumina loss. Furthermore, alkali recovery reactions are costly and generate large amounts of solid waste. To avoid these problems, some companies outsource the treatment of waste sodium salt.

[0024] Due to significant drawbacks in the practical application of waste sodium salt, the reaction generates a large amount of solid waste residue containing valuable components such as aluminum and alkali, resulting not only in resource waste but also increased pressure and costs associated with solid waste disposal. Furthermore, the reaction process cannot completely convert waste sodium salt into sodium hydroxide, leaving some carbonate residue accumulating in the system, failing to fundamentally solve the problems of scaling and impurity accumulation. In actual production, the reaction residue particles are fine and have poor settling and filtration performance, easily leading to equipment scaling, separation difficulties, and increased energy consumption and operating load.

[0025] To address the technical challenges of difficult waste sodium salt treatment, the difficulty in recovering alkali and aluminum, and the waste residue generated during the recovery process, this application provides a method for preparing calcium carbonate from waste sodium salt and recovering alumina. The specific technical solution is as follows: Figure 1 This is a process flow diagram of a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide, provided in an embodiment of this application.

[0026] like Figure 1 As shown in the embodiment of this application, a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide is provided, the method comprising: S1. The mixture of waste sodium salt, calcium oxide, and dispersant is subjected to an alkali recovery reaction to obtain a mixed slurry; the dispersant includes at least one of hydroxy polycarboxylic acids, amino polycarboxylic acids, hydroxyphosphonic acids, and polysaccharides; S2. The mixed slurry is subjected to a first liquid-solid separation to obtain a first alkaline solution and a first calcium slag; S3. Mix the first calcium slag with water to obtain calcium slag slurry; S4. The captured and concentrated flue gas containing carbon dioxide is introduced into the calcium slag slurry, and a nucleating agent is added to the calcium slag slurry to carry out a carbonization reaction to obtain a carbonized slurry; the nucleating agent includes at least one of aluminum hydroxide, calcium hydroxide, calcium oxide and calcium carbonate; S5. The carbonized slurry is subjected to a second liquid-solid separation to obtain a circulating liquid as makeup water and a second calcium slag. S6. Add a supplement to the mixture of the second calcium slag and the first alkaline solution to carry out an alkaline leaching reaction and obtain an aluminum recovery slurry. S7. The aluminum recovery slurry is subjected to a third liquid-solid separation to obtain a dissolved aluminum solution and wet calcium carbonate. S8. The molten aluminum solution is fed into the alumina production system to crystallize and precipitate aluminum hydroxide, and the wet calcium carbonate is dried and dehydrated to obtain the calcium carbonate product.

[0027] It should be noted that step S1 involves an alkali recovery reaction of a mixture of waste sodium salt, calcium oxide, and dispersant to obtain a mixed slurry. The core function of this step is to achieve alkali recovery from the waste sodium salt, specifically by using calcium oxide to convert the sodium salt in the waste sodium salt into sodium hydroxide, while simultaneously converting carbonate ions into calcium carbonate. The dispersant is used to selectively inhibit the unavoidable reaction between sodium aluminate and calcium hydroxide in the waste sodium salt to form calcium aluminate precipitate, thereby reducing aluminum loss and improving the recovery efficiency of sodium hydroxide.

[0028] Step S2 involves a first liquid-solid separation of the mixed slurry to obtain a first alkaline solution and a first calcium slag. The purpose of this step is to separate the first alkaline solution, which is rich in sodium hydroxide (for subsequent aluminum leaching), and the first calcium slag, which is mainly composed of calcium carbonate, calcium aluminate, and unreacted calcium oxide.

[0029] Step S3 involves mixing the first calcium slag with water to obtain a calcium slag slurry. The purpose of this step is to redisperse the solid first calcium slag into a homogeneous slurry, providing a suitable solid-liquid mixing state for the subsequent carbonization reaction.

[0030] In step S4, the concentrated carbon dioxide-containing flue gas is passed into the calcium slag slurry, and a nucleating agent is added to initiate a carbonation reaction, resulting in a carbonized slurry. The purpose of this step is to utilize high-concentration carbon dioxide to convert free calcium oxide in the calcium slag slurry into calcium carbonate, and to induce crystallization through the nucleating agent, promoting the separation of calcium aluminate and calcium carbonate, as well as the agglomeration of aluminum hydroxide, thereby improving the purity of subsequent products.

[0031] Step S5 involves a second liquid-solid separation of the carbonization slurry to obtain a circulating liquid used as makeup water and a second calcium slag. The purpose of this step is to recover the liquid phase generated during the carbonization reaction for recycling as makeup water, while simultaneously obtaining a second calcium slag containing aluminum hydroxide and calcium carbonate after carbonization treatment.

[0032] Step S6 involves adding a supplement to the mixture of the second calcium slag and the first alkaline solution to conduct an alkaline leaching reaction, resulting in an aluminum recovery slurry. The purpose of this step is to utilize the sodium hydroxide in the first alkaline solution and the sodium carbonate and water in the supplement to dissolve the aluminum hydroxide in the second calcium slag, generating soluble sodium aluminate (aluminum solution), while simultaneously allowing calcium to remain in the solid phase as calcium carbonate.

[0033] Step S7 involves a third liquid-solid separation of the aluminum recovery slurry to obtain a soluble aluminum solution and wet calcium carbonate. The purpose of this step is to separate the soluble aluminum solution rich in sodium aluminate from the wet calcium carbonate solid.

[0034] Step S8 involves feeding the molten aluminum solution into the alumina production system to crystallize aluminum hydroxide, and then drying and dehydrating the wet calcium carbonate to obtain the calcium carbonate product. The purpose of this step is to ultimately recover the aluminum hydroxide product and prepare high-purity calcium carbonate.

[0035] In some embodiments, the waste sodium salt has a sodium oxide mass fraction > 45% and a moisture content < 30%.

[0036] Limiting the mass fraction of sodium oxide in waste sodium salt to >45% and the moisture content to <30% ensures that the waste sodium salt has sufficient sodium resource value, enabling each unit mass of waste sodium salt to provide enough sodium for the alkali recovery reaction. Simultaneously, the lower moisture content improves the thermal efficiency of the alkali recovery reaction, reduces heat consumption during the heating process, and facilitates accurate control of the liquid-solid ratio of the reaction system. For example, the mass fraction of sodium oxide can be 46%, 48%, 50%, 52%, 55%, 58%, 60%, 65%, etc.; and the moisture content can be 28%, 25%, 22%, 20%, 18%, 15%, 12%, 10%, etc.

[0037] In some embodiments, the mass ratio of the waste sodium salt to the calcium oxide is 1:(1.2~1.6); The alkali recovery reaction includes the following parameters: liquid-to-solid ratio of 2.5 to 6, reaction temperature of 80℃ to 100℃, and reaction time of 2h to 5h.

[0038] A controlled mass ratio of waste sodium salt to calcium oxide of 1:1.2~1.6 ensures that calcium oxide is in excess relative to the sodium salt in the waste sodium salt, thus completely converting the sodium salt in the waste sodium salt into sodium hydroxide and calcium carbonate. A controlled liquid-to-solid ratio of 2.5~6 ensures good fluidity and uniform mixing of the mixed slurry, allowing for sufficient contact of the reactants. A controlled reaction temperature of 80℃~100℃ accelerates the alkali recovery reaction rate, allowing the reaction to reach equilibrium within a reasonable time. A controlled reaction time of 2h~5h ensures that the alkali recovery reaction proceeds fully, resulting in near-complete conversion of the convertible sodium salt in the waste sodium salt. For example, the mass ratio of waste sodium salt to calcium oxide can be 1:1.2, 1:1.25, 1:1.3, 1:1.35, 1:1.4, 1:1.45, 1:1.5, 1:1.55, etc.; the liquid-solid ratio can be 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 5.8, etc.; the reaction temperature can be 80℃, 85℃, 90℃, 95℃, 100℃, etc.; and the reaction time can be 2h, 2.5h, 3h, 3.5h, 4h, 4.2h, 4.5h, 4.8h, etc.

[0039] In some embodiments, the mass of the dispersant is 0.2% to 0.8% of the total mass of the reaction system for the alkali recovery reaction.

[0040] Limiting the mass of the dispersant to 0.2% to 0.8% of the total mass of the reaction system ensures that the dispersant reaches an effective concentration in the reaction system, fully utilizing its selective inhibition function. The dispersant is limited to at least one selected from hydroxy polycarboxylic acids, amino polycarboxylic acids, hydroxyphosphonic acids, and polysaccharides. It utilizes the carboxyl, hydroxyl, and phosphonic acid groups in its molecular structure to form stable chelates with calcium ions, and forms a dense adsorption layer and steric hindrance layer on the surface of calcium hydroxide particles. This directionally inhibits the formation of calcium aluminate precipitate from sodium aluminate and calcium hydroxide, while not affecting the normal reaction of carbonate and lime, achieving the technical effect of reducing alkali consumption and aluminum loss. For example, the mass ratio of the dispersant to the total mass of the alkali recovery reaction system can be 0.2%, 0.3%, 0.4%, 0.5%, 0.55%, 0.6%, 0.7%, 0.75%, etc.

[0041] It should be noted that due to limitations in the dehydration process of waste sodium salt, sodium aluminate will inevitably be present in the waste sodium salt. This sodium aluminate will participate in the alkaline recovery reaction between the waste sodium salt and calcium oxide, producing stable calcium aluminate, resulting in a loss of alumina. By using a mixture of hydroxy polycarboxylic acids, amino polycarboxylic acids, hydroxyphosphonic acids, and polysaccharides as dispersants, the carboxyl, hydroxyl, and phosphonic acid groups in their molecular structures can react with Ca²⁺. + It forms a stable chelate and a dense adsorption layer and steric hindrance layer on the surface of Ca(OH)2 particles, which directionally inhibits the formation of calcium aluminate precipitate from sodium aluminate and calcium hydroxide, while not affecting the normal reaction of carbonate and lime, thereby reducing alkali consumption.

[0042] Since the alkali recovery reaction is a high-alkali reaction, suppressing the reaction of sodium aluminate and reducing the formation of calcium aluminate is a major challenge in this process technology. Only when the dispersant molecule contains multiple carboxyl groups and ortho / homogeneous hydroxyl groups can it stably chelate Ca under high alkali conditions. 2+ It also forms an adsorption layer on the lime surface, preventing Ca from being absorbed. 2+ With Al(OH)4 ~ Bonding. A dense chelate adsorption film is formed on the surface of Ca(OH)₂ via hydroxy polycarboxylic acids, reducing the reactivity of lime and directionally inhibiting the nucleation and crystal growth of 3CaO·Al₂O₃·6H₂O without affecting the reaction between carbonate and lime. Dispersants containing amino polycarboxylic acids exhibit selective inhibitory effects in highly alkaline systems, selectively coordinating free Ca. 2+ It reduces the supersaturation of calcium and aluminum, inhibits the precipitation of calcium aluminate, but does not complex Na in the solution. + Al(OH)4 - CO3² -Meanwhile, some amino polycarboxylic acids are biodegradable and will not affect the alumina production process. Dispersants containing hydroxyphosphonic acids form a strongly adsorbed calcium phosphonate layer on the Ca(OH)₂ surface, preventing aluminate ions from diffusing to the lime surface, thereby directionally inhibiting calcium aluminate formation and improving the decomposition efficiency of waste sodium salts. Dispersants containing long-chain polysaccharides adsorb onto the lime particle surface, forming a polymeric barrier that physically blocks Al(OH)₄. - By being close to the source, calcium aluminate is suppressed due to steric hindrance. Through the compounding of hydroxypolycarboxylic acids, hydroxyphosphonic acids, and anionic polysaccharides, it exhibits strong selectivity in inhibiting calcium-aluminum side reactions, thus avoiding the loss of sodium aluminate during the reaction of waste sodium salt and calcium oxide.

[0043] In some embodiments, the concentration of sodium hydroxide in the first alkaline solution is 100 g / L to 200 g / L; In the first calcium slag, the mass fraction of calcium carbonate is 80%~95%.

[0044] Limiting the sodium hydroxide concentration in the first alkaline solution to 100 g / L to 200 g / L ensures sufficient alkalinity for subsequent alkaline leaching reactions. This concentration range also matches the concentration requirements of the subsequent aluminum recovery slurry, facilitating process integration. Limiting the calcium carbonate mass fraction in the first calcium slag to 80% to 95% indicates that the alkali recovery reaction has effectively converted most of the carbonate ions in the waste sodium salt into calcium carbonate, providing a good raw material basis for the subsequent preparation of high-purity calcium carbonate products. For example, the sodium hydroxide concentration can be 100 g / L, 115 g / L, 130 g / L, 145 g / L, 160 g / L, 175 g / L, 185 g / L, 195 g / L, etc.; the calcium carbonate mass fraction can be 80%, 85%, 88%, 90%, 92%, 95%, etc.

[0045] In some embodiments, the mass of the nucleating agent is 0.03% to 0.05% of the mass of calcium aluminate in the first calcium slag.

[0046] The nucleating agent is limited to 0.03% to 0.05% of the mass of calcium aluminate in the first calcium slag. This ensures the introduction of sufficient nucleation centers into the carbonization reaction system, effectively inducing the separation and nucleation of the intercalated structures of calcium aluminate and calcium carbonate, as well as the agglomeration of aluminum hydroxide during carbonization. The nucleating agent is limited to at least one selected from aluminum hydroxide, calcium hydroxide, calcium oxide, and calcium carbonate. These substances, having the same or similar crystal structure as the carbonization reaction products, provide an interface for heterogeneous nucleation, promoting the crystallization and growth of the target product. For example, the mass ratio of the nucleating agent to the mass of calcium aluminate in the first calcium slag can be 0.03%, 0.032%, 0.035%, 0.038%, 0.04%, 0.042%, 0.045%, 0.048%, etc.

[0047] In some embodiments, the carbonization reaction includes the following parameters: flue gas temperature of 25~45℃, solid content of calcium slag slurry of 8%~25%, and pH of carbonization slurry of 9~10.

[0048] Limiting the flue gas temperature to 25-45℃ maintains high solubility of carbon dioxide in the liquid phase while ensuring a suitable carbonation reaction rate. Limiting the solid content of the calcium slag slurry to 8%-25% ensures sufficient contact between carbon dioxide gas and solid particles in the slurry, while maintaining the slurry's pumpability. Limiting the pH of the carbonation slurry to 9-10 ensures the carbonation reaction reaches its endpoint, allowing free calcium oxide and calcium components in the calcium slag slurry to be fully converted into calcium carbonate. For example, the flue gas temperature can be 25℃, 28℃, 30℃, 33℃, 35℃, 38℃, 40℃, 43℃, etc.; the solid content of the calcium slag slurry can be 8%, 10%, 12%, 15%, 18%, 20%, 22%, 24%, etc.; and the pH of the carbonation slurry can be 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.7, 9.9, etc.

[0049] In addition to calcium carbonate, the first calcium slag also contains incompletely reacted calcium oxide and calcium aluminate, a byproduct of the alkali recovery reaction. To obtain high-quality calcium carbonate, the impurities need to be treated. This invention utilizes the flue gas from an industrial kiln. By capturing and concentrating the flue gas, the original flue gas with a carbon dioxide content of 5-12% is reduced to a carbon dioxide content of 50-99.9%.

[0050] The first calcium slag is slurried and then subjected to a contact carbonization reaction with high-concentration flue gas. The carbonization reaction temperature is 25-45℃, the solid content of the calcium slag slurry is 8-25%, and the pH of the carbonization slurry is controlled at 9-10. During the carbonization reaction, to improve the crystallization effect of the reaction products and enhance the dehydration and separation effect, nucleating agents such as aluminum hydroxide, calcium hydroxide, calcium oxide, and calcium carbonate are added. These agents induce crystallization reactions, promoting the agglomeration of aluminum hydroxide and the separation and nucleation process of the embedded structures of calcium aluminate and calcium carbonate during the carbonization of the first calcium slag. The amount of nucleating agent added is 0.03-0.05% of the mass of calcium aluminate in the first calcium slag. Adding too little will not promote the separation and nucleation of calcium aluminate, while adding too much will waste the nucleating agent and increase operating costs.

[0051] In some embodiments, the supplement is water and sodium carbonate.

[0052] The supplementary agents are limited to water and sodium carbonate. Water can be used to adjust the liquid-solid ratio and viscosity of the aluminum recovery slurry, while sodium carbonate provides additional carbonate ions, which work synergistically with sodium hydroxide in the first alkaline solution to jointly promote the alkaline dissolution reaction of calcium aluminate in the second calcium slag.

[0053] In some embodiments, the concentration of sodium hydroxide in the aluminum recovery slurry is 100 g / L to 150 g / L, and the concentration of sodium carbonate is 20 g / L to 35 g / L.

[0054] Limiting the concentration of sodium hydroxide in the aluminum recovery slurry to 100 g / L~150 g / L and the concentration of sodium carbonate oxide to 20 g / L~35 g / L ensures that calcium aluminate fully dissolves to form sodium aluminate during the alkaline leaching reaction. This concentration range also matches the feed requirements of the seed crystal decomposition process in the alumina production system, which is beneficial for the subsequent crystallization and precipitation of aluminum hydroxide. For example, the concentration of sodium hydroxide can be 100 g / L, 108 g / L, 115 g / L, 122 g / L, 130 g / L, 138 g / L, 142 g / L, 148 g / L, etc.; and the concentration of sodium carbonate oxide can be 20 g / L, 22 g / L, 24 g / L, 26 g / L, 28 g / L, 30 g / L, 32 g / L, 34 g / L, etc.

[0055] In some embodiments, the calcium carbonate product has a calcium carbonate mass fraction >98%, an aluminum oxide mass fraction <0.01%, a sodium oxide mass fraction <0.01%, and a moisture content <0.1%.

[0056] The method of this application not only realizes the recycling of all resources in waste sodium salt, but also reduces the emission of carbon dioxide in flue gas, and at the same time, it makes product use of the waste heat resources in the flue gas.

[0057] By using dispersants to inhibit calcium aluminate formation, nucleating agents to promote aluminum-calcium separation, alkaline leaching reactions to selectively dissolve aluminum, multi-stage liquid-solid separation to remove sodium-containing liquid phases, and final drying and dehydration to remove moisture, the synergistic effect of each step enables the calcium carbonate product to achieve a high purity quality with a calcium carbonate content of over 98%, an alumina and sodium oxide content of less than 0.01%, and a moisture content of less than 0.1%.

[0058] To address the removal of alumina impurities, this application employs synergistic control across multiple stages. In the alkali recovery reaction step, a dispersant is added to selectively inhibit the unavoidable reaction between sodium aluminate and calcium hydroxide in waste sodium salts to form calcium aluminate precipitate, thus reducing the amount of aluminum entering the first calcium slag at the source. For the calcium aluminate already formed and entering the first calcium slag, this application adds a nucleating agent in the carbonation reaction step to promote the separation of calcium aluminate and calcium carbonate embedded structures and the agglomeration of aluminum hydroxide during carbonation, making the calcium aluminate more easily selectively dissolved in subsequent steps. Subsequently, in the alkaline leaching reaction step, the second calcium slag is mixed with the first alkaline solution and a supplement is added. Utilizing the sodium hydroxide in the first alkaline solution and the sodium carbonate and water in the supplement, the calcium aluminate in the second calcium slag is fully dissolved to form soluble sodium aluminate which enters the liquid phase, while calcium carbonate remains in the solid phase. Through a third liquid-solid separation, the aluminum-rich solution containing sodium aluminate is completely separated from the wet calcium carbonate, thereby achieving highly efficient removal of alumina impurities.

[0059] To remove sodium oxide impurities, this application employs multi-stage liquid-solid separation and liquid-phase displacement. After the alkali recovery reaction, the mixed slurry undergoes a first liquid-solid separation to obtain a first alkali solution and a first calcium slag. At this stage, the sodium-containing solution entrained in the first calcium slag is initially separated. The carbonized slurry after the carbonation reaction undergoes a second liquid-solid separation to obtain a circulating liquid and a second calcium slag. This step further washes away the residual sodium-containing liquid phase in the second calcium slag. In the alkaline leaching reaction, the second calcium slag is mixed with the first alkali solution. Although a sodium-containing solution is introduced, the subsequent third liquid-solid separation separates the dissolved aluminum solution (containing sodium) from the wet calcium carbonate, leaving only a small amount of adhering liquid on the surface of the wet calcium carbonate. Finally, through a drying and dehydration step, the residual water in the wet calcium carbonate is evaporated, and the amount of sodium salt dissolved in the adhering liquid remaining on the solid surface is extremely low as the water is removed, ensuring that the sodium oxide mass fraction in the calcium carbonate product is less than 0.01%.

[0060] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0061] Example 1 Figure 2 This is a technical flow diagram illustrating a method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide, provided as an embodiment of this application. Figure 2As shown, waste sodium salt with a sodium oxide content of 46% emitted from the aluminum industry is sent to an alkali recovery tank 1 containing a calcium oxide solution. The mass ratio of waste sodium salt to calcium oxide in the alkali recovery tank 1 is 1:1.2, the liquid-to-solid ratio is 3, the reaction temperature is 98℃, and 0.21% of a dispersant 101 to promote the alkali recovery reaction is added. After the alkali recovery reaction, a mixed slurry is obtained. The mixed slurry undergoes a first liquid-to-solid separation 2 to obtain a first alkali solution 201 with a sodium hydroxide concentration of 180 g / L and a first calcium slag 202 with a calcium carbonate content of 85%. The first calcium slag 202 is mixed with water to obtain a calcium slag slurry. Carbon dioxide-containing flue gas is concentrated by a carbon capture 302, and carbon dioxide 303 with a concentration of 65% is sent to a carbonizer 3 containing the calcium slag slurry. The temperature of the carbonization reaction flue gas is 25℃, the solid content of the calcium slag slurry is 20%, and the pH of the carbonization slurry is controlled at 9.2. The nucleating agent is added at a rate of 0.03% of the mass of calcium aluminate in the first calcium slag. After the carbonization reaction is completed, a carbonization slurry is obtained. The carbonization slurry is subjected to a second liquid-solid separation 4 to obtain a circulating liquid 401 and a second calcium slag 402. The circulating liquid 401 is sent to the carbonizer 3 as makeup water, and the industrial water 301 is used as makeup water for reaction loss. The second calcium slag 402 is mixed with the first alkaline solution 201, and water and sodium carbonate are added. After the aluminum recovery 5 reaction, an aluminum recovery slurry with sodium hydroxide at 110 g / L and sodium hydroxide at 25 g / L is obtained. The aluminum recovery slurry is subjected to a third liquid-solid separation 6 to obtain a dissolved aluminum solution 601 and wet calcium carbonate 602. The dissolved aluminum solution is sent to the alumina production system 6011 for alumina recovery. The heat from the waste heat recovery 301 is sent to the drying 7. The wet calcium carbonate is dehydrated by the drying 7 to obtain the calcium carbonate product. The calcium carbonate product contains 98.6% calcium carbonate, 0.008% aluminum oxide, 0.005% sodium oxide, and 0.06% moisture.

[0062] Example 2 Waste sodium salt with a sodium oxide content of 47.3% emitted from the aluminum industry is sent to an alkali recovery tank 1 containing a calcium oxide solution. The mass ratio of waste sodium salt to calcium oxide in the alkali recovery tank 1 is 1:1.6, the liquid-to-solid ratio is 5, the reaction temperature is 80℃, and 0.75% of a dispersant 101 to promote the alkali recovery reaction is added. After the alkali recovery reaction, a mixed slurry is obtained. The mixed slurry undergoes a first liquid-to-solid separation 2 to obtain a first alkali solution 201 with a sodium hydroxide concentration of 186 g / L and a first calcium slag 202 with a calcium carbonate content of 90%. The first calcium slag 202 is mixed with water to obtain a calcium slag slurry. Carbon dioxide-containing flue gas is concentrated by a carbon capture 302, and carbon dioxide 303 with a concentration of 99% is sent to a carbonizer 3 containing the calcium slag slurry. The carbonization reaction flue gas temperature is 40℃, the calcium slag slurry solid content is 8%, and the pH of the carbonized slurry is controlled at 9.5. The amount of nucleating agent added accounts for 0.043% of the mass of calcium aluminate in the first calcium slag. After the carbonization reaction is completed, a carbonization slurry is obtained. The carbonization slurry is subjected to a second liquid-solid separation 4 to obtain a circulating liquid 401 and a second calcium slag 402. The circulating liquid 401 is sent to the carbonizer 3 as makeup water, and the industrial water 301 is used as makeup water for reaction loss. The second calcium slag 402 is mixed with the first alkaline solution 201, and water and sodium carbonate are added. After the aluminum recovery 5 reaction, an aluminum recovery slurry with sodium hydroxide at 145 g / L and sodium hydroxide at 30 g / L is obtained. The aluminum recovery slurry is subjected to a third liquid-solid separation 6 to obtain a dissolved aluminum solution 601 and wet calcium carbonate 602. The dissolved aluminum solution is sent to the alumina production system 6011 for alumina recovery. The heat from the waste heat recovery 301 is sent to the drying 7. The wet calcium carbonate is dehydrated by the drying 7 to obtain the calcium carbonate product. The calcium carbonate product contains 99.0% calcium carbonate, 0.006% aluminum oxide, 0.005% sodium oxide, and 0.02% moisture.

[0063] Example 3 Waste sodium salt with a sodium oxide content of 47.3% emitted from the aluminum industry is sent to an alkali recovery tank 1 containing a calcium oxide solution. The mass ratio of waste sodium salt to calcium oxide in the alkali recovery tank 1 is 1:1.4, the liquid-to-solid ratio is 4.5, the reaction temperature is 100℃, and 0.31% of a dispersant 101 to promote the alkali recovery reaction is added. After the alkali recovery reaction, a mixed slurry is obtained. The mixed slurry undergoes a first liquid-to-solid separation 2 to obtain a first alkali solution 201 with a sodium hydroxide concentration of 100 g / L and a first calcium slag 202 with a calcium carbonate content of 93.6%. The first calcium slag 202 is mixed with water to obtain a calcium slag slurry. Carbon dioxide-containing flue gas is concentrated by a carbon capture 302, and carbon dioxide 303 with a concentration of 60% is sent to a carbonizer 3 containing the calcium slag slurry. The carbonization reaction flue gas temperature is 35℃, the calcium slag slurry solid content is 10%, and the pH of the carbonization slurry is controlled at 9. 4. The nucleating agent is added at 0.05% of the mass of calcium aluminate in the first calcium slag. After the carbonization reaction is completed, a carbonization slurry is obtained. The carbonization slurry is subjected to a second liquid-solid separation 4 to obtain a circulating liquid 401 and a second calcium slag 402. The circulating liquid 401 is sent to the carbonizer 3 as makeup water, and the industrial water 301 is used as makeup water for reaction loss. The second calcium slag 402 is mixed with the first alkaline solution 201, and water and sodium carbonate are added. After the aluminum recovery 5 reaction, an aluminum recovery slurry with sodium hydroxide at 120 g / L and sodium hydroxide at 31 g / L is obtained. The aluminum recovery slurry is subjected to a third liquid-solid separation 6 to obtain a dissolved aluminum solution 601 and wet calcium carbonate 602. The dissolved aluminum solution is sent to the alumina production system 6011 for alumina recovery. The heat from the waste heat recovery 301 is sent to the drying 7. The wet calcium carbonate is dehydrated by the drying 7 to obtain calcium carbonate product. The calcium carbonate product contains 98.9% calcium carbonate, 0.006% aluminum oxide, 0.004% sodium oxide, and 0.03% moisture.

[0064] Example 4 Waste sodium salt with a sodium oxide content of 46.8% emitted from the aluminum industry is sent to an alkali recovery tank 1 containing a calcium oxide solution. The mass ratio of waste sodium salt to calcium oxide in the alkali recovery tank 1 is 1:1.5, the liquid-to-solid ratio is 2.8, the reaction temperature is 95℃, and 0.75% of a dispersant 101 to promote the alkali recovery reaction is added. After the alkali recovery reaction, a mixed slurry is obtained. The mixed slurry undergoes a first liquid-to-solid separation 2 to obtain a first alkali solution 201 with a sodium hydroxide concentration of 186 g / L and a first calcium slag 202 with a calcium carbonate content of 89%. The first calcium slag 202 is mixed with water to obtain a calcium slag slurry. Carbon dioxide-containing flue gas is concentrated by a carbon capture 302, and carbon dioxide 303 with a concentration of 99% is sent to a carbonizer 3 containing the calcium slag slurry. The temperature of the carbonization reaction flue gas is 40℃, the solid content of the calcium slag slurry is 8%, and the pH of the carbonized slurry is controlled at 9.5. The nucleating agent is added at 0.0-45% of the mass of calcium aluminate in the first calcium slag. After the carbonization reaction is completed, a carbonization slurry is obtained. The carbonization slurry is subjected to a second liquid-solid separation 4 to obtain a circulating liquid 401 and a second calcium slag 402. The circulating liquid 401 is sent to the carbonizer 3 as makeup water, and the industrial water 301 is used as makeup water for reaction loss. The second calcium slag 402 is mixed with the first alkaline solution 201, and water and sodium carbonate are added. After the aluminum recovery 5 reaction, an aluminum recovery slurry with sodium hydroxide at 145 g / L and sodium hydroxide at 30 g / L is obtained. The aluminum recovery slurry is subjected to a third liquid-solid separation 6 to obtain a dissolved aluminum solution 601 and wet calcium carbonate 602. The dissolved aluminum solution is sent to the alumina production system 6011 for alumina recovery. The heat from the waste heat recovery 301 is sent to the drying 7. The wet calcium carbonate is dehydrated by the drying 7 to obtain the calcium carbonate product. The calcium carbonate product contains 99.4% calcium carbonate, 0.007% aluminum oxide, 0.006% sodium oxide, and 0.08% moisture.

[0065] Example 5 Waste sodium salt with a sodium oxide content of 47.6% emitted from the aluminum industry is sent to an alkali recovery tank 1 containing a calcium oxide solution. The mass ratio of waste sodium salt to calcium oxide in the alkali recovery tank 1 is 1:1.45, the liquid-to-solid ratio is 6, the reaction temperature is 99℃, and 0.41% of a dispersant 101 to promote the alkali recovery reaction is added. After the alkali recovery reaction, a mixed slurry is obtained. The mixed slurry undergoes a first liquid-to-solid separation 2 to obtain a first alkali solution 201 with a sodium hydroxide concentration of 105 g / L and a first calcium slag 202 with a calcium carbonate content of 88%. The first calcium slag 202 is mixed with water to obtain a calcium slag slurry. Carbon dioxide-containing flue gas is concentrated by a carbon capture 302, and carbon dioxide 303 with a concentration of 75% is sent to a carbonizer 3 containing the calcium slag slurry. The temperature of the carbonization reaction flue gas is 28℃, the solid content of the calcium slag slurry is 10%, and the pH of the carbonized slurry is controlled at 9.6. The nucleating agent is added at a rate of 0.036% of the mass of calcium aluminate in the first calcium slag. After the carbonization reaction is completed, a carbonization slurry is obtained. The carbonization slurry is subjected to a second liquid-solid separation 4 to obtain a circulating liquid 401 and a second calcium slag 402. The circulating liquid 401 is sent to the carbonizer 3 as makeup water, and the industrial water 301 is used as makeup water for reaction loss. The second calcium slag 402 is mixed with the first alkaline solution 201, and water and sodium carbonate are added. After the aluminum recovery 5 reaction, an aluminum recovery slurry with sodium hydroxide at 134 g / L and sodium hydroxide at 29 g / L is obtained. The aluminum recovery slurry is subjected to a third liquid-solid separation 6 to obtain a dissolved aluminum solution 601 and wet calcium carbonate 602. The dissolved aluminum solution is sent to the alumina production system 6011 for alumina recovery. The heat from the waste heat recovery 301 is sent to the drying 7. The wet calcium carbonate is dehydrated by the drying 7 to obtain the calcium carbonate product. The calcium carbonate product contains 99.1% calcium carbonate, 0.007% aluminum oxide, 0.004% sodium oxide, and 0.03% moisture.

[0066] The performance of the calcium carbonate products obtained in Examples 1-5 is summarized in Table 1.

[0067] Table 1. Performance of calcium carbonate products from Examples 1-5

[0068] As shown in Table 1, in the calcium carbonate products of Examples 1 to 5, the mass fraction of calcium carbonate is >98%, the mass fraction of aluminum oxide is <0.01%, the mass fraction of sodium oxide is <0.01%, and the moisture content is <0.1%.

[0069] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages: (1) High-value utilization of waste sodium salt has been realized. This application uses industrial waste sodium salt as production raw material, and converts the sodium salt in it into sodium hydroxide for aluminum leaching through a series of reactions. At the same time, the carbonate ions are converted into high-purity calcium carbonate products, realizing the dual resource utilization of sodium and carbon elements in waste sodium salt.

[0070] (2) Achieved efficient recovery of aluminum. This application reduces the loss of aluminum by adding a dispersant to the alkaline recovery reaction to inhibit the conversion of sodium aluminate into calcium aluminate precipitate from the source; at the same time, the calcium aluminate in the second calcium slag is fully dissolved through alkaline leaching reaction to generate a soluble aluminum solution, which is finally sent to the alumina production system to crystallize and precipitate aluminum hydroxide. This achieves efficient recovery of associated aluminum in waste sodium salt.

[0071] (3) The prepared calcium carbonate product has high purity and extremely low impurity content. This application achieves high purity and low impurity content through multi-step synergistic control, including dispersant inhibition, nucleating agent-induced calcium-aluminum separation, alkaline leaching selective dealuminization, multi-stage liquid-solid separation desodiumization, and drying dehydration. This results in a calcium carbonate product with a mass fraction greater than 98%, an alumina mass fraction less than 0.01%, a sodium oxide mass fraction less than 0.01%, and a moisture content less than 0.1%, which meets the stringent purity requirements of high-end applications.

[0072] (4) Resource utilization of carbon dioxide in flue gas has been realized. This application uses high-concentration carbon dioxide gas obtained by capturing and concentrating industrial kiln flue gas for carbonation reaction, converting free calcium oxide in the first calcium slag and calcium components in calcium aluminate into calcium carbonate, thus treating solid waste and realizing the fixation and utilization of greenhouse gas carbon dioxide.

[0073] (5) Process water is recycled, reducing water consumption and waste liquid discharge. In this application, the circulating liquid obtained after the second liquid-solid separation of carbonized slurry is used as makeup water and reused in the system, which reduces the amount of fresh water to be added and at the same time reduces the discharge of alkaline waste liquid.

[0074] (6) The dispersant has a selective inhibitory effect and does not affect the main reaction. The hydroxy polycarboxylic acids, amino polycarboxylic acids, hydroxyphosphonic acids and polysaccharides used in this application can directionally inhibit the formation of calcium aluminate precipitate from sodium aluminate and calcium hydroxide, but do not affect the normal reaction of carbonate and lime, thereby ensuring the efficiency of alkali recovery reaction while reducing aluminum loss.

[0075] (7) The nucleating agent is used in small quantities and has a significant effect. The amount of nucleating agent added in this application is only 0.03% to 0.05% of the mass of calcium aluminate in the first calcium slag, which can effectively promote the separation and nucleation of calcium aluminate and calcium carbonate embedded structures and the agglomeration of aluminum hydroxide during carbonation, thereby greatly improving the subsequent aluminum-calcium separation effect at a lower cost.

[0076] (8) The process conditions are mild and the operation is controllable. The alkali recovery reaction temperature of this application is 80℃~100℃, and the carbonization reaction flue gas temperature is 25~45℃. Both are operated at atmospheric pressure or close to atmospheric pressure, without the need for high temperature and high pressure equipment, which is conducive to industrial promotion and safe production.

[0077] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing calcium carbonate from waste sodium salt and recovering aluminum hydroxide, characterized in that, The method includes: An alkali recovery reaction is carried out on a mixture of waste sodium salt, calcium oxide, and dispersant to obtain a mixed slurry; the dispersant includes at least one of hydroxy polycarboxylic acids, amino polycarboxylic acids, hydroxyphosphonic acids, and polysaccharides; The mixed slurry is subjected to a first liquid-solid separation to obtain a first alkaline solution and a first calcium slag. The first calcium slag is mixed with water to obtain a calcium slag slurry; The captured and concentrated flue gas containing carbon dioxide is introduced into the calcium slag slurry, and a nucleating agent is added to the calcium slag slurry to carry out a carbonization reaction to obtain a carbonized slurry; the nucleating agent includes at least one of aluminum hydroxide, calcium hydroxide, calcium oxide and calcium carbonate; The carbonized slurry is subjected to a second liquid-solid separation to obtain a circulating liquid as makeup water and a second calcium slag. Add a supplement to the mixture of the second calcium slag and the first alkaline solution to carry out an alkaline leaching reaction and obtain an aluminum recovery slurry; The aluminum recovery slurry is subjected to a third liquid-solid separation to obtain a dissolved aluminum solution and wet calcium carbonate. The molten aluminum solution is fed into an alumina production system to crystallize aluminum hydroxide, and the wet calcium carbonate is dried and dehydrated to obtain the calcium carbonate product.

2. The method of claim 1, wherein, The waste sodium salt has a sodium oxide mass fraction >45% and a moisture content <30%.

3. The method of claim 1, wherein, The mass ratio of the waste sodium salt to the calcium oxide is 1:(1.2~1.6); The alkali recovery reaction includes the following parameters: liquid-to-solid ratio of 2.5 to 6, reaction temperature of 80℃ to 100℃, and reaction time of 2h to 5h.

4. The method of claim 1, wherein, The mass of the dispersant is 0.2% to 0.8% of the total mass of the reaction system for the alkali recovery reaction.

5. The method of claim 1, wherein, In the first alkaline solution, the concentration of sodium hydroxide is 100 g / L to 200 g / L; In the first calcium slag, the mass fraction of calcium carbonate is 80%~95%.

6. The method of claim 1, wherein, The mass of the nucleating agent is 0.03% to 0.05% of the mass of calcium aluminate in the first calcium slag.

7. The method according to claim 1, characterized in that, The carbonization reaction includes the following parameters: flue gas temperature of 25~45℃, solid content of calcium slag slurry of 8%~25%, and pH of carbonization slurry of 9~10.

8. The method according to claim 1, characterized in that, The supplement consists of water and sodium carbonate.

9. The method according to claim 1, characterized in that, In the aluminum recovery slurry, the concentration of sodium hydroxide is 100g / L~150g / L, and the concentration of sodium carbonate is 20g / L~35g / L.

10. The method of claim 1, wherein, The calcium carbonate product has the following composition: calcium carbonate mass fraction > 98%, aluminum oxide mass fraction < 0.01%, sodium oxide mass fraction < 0.01%, and moisture content < 0.1%.

Images