A method for preparing sodium aluminate based on alumina waste lye

Abstract

The application relates to the technical field of solid waste resource utilization, and discloses a preparation method of sodium aluminate based on alumina waste alkali, which comprises the following steps: feeding macro-micro-environment raw material balls prepared by mixing fly ash, secondary aluminum ash, industrial waste alkali containing sodium oxalate, bauxite and limestone according to a specific ratio into a countercurrent rotary kiln in an oxidizing atmosphere for roasting; using sodium oxalate pyrolysis to generate through pores in situ in the preheating section; using the mass transfer resistance generated by the particle size to construct a micro-reduction environment in the internal reduction section, promoting the silicon-aluminum bond breaking and desulfurization reaction through the heat release of residual carbon and secondary aluminum ash; completing the solidification of sulfur elements on the particle surface layer and the reconstruction of the sodium aluminate crystal phase in the external oxidation section; after the clinker is discharged from the kiln, air blowing, quenching and crushing, the clinker is added into a caustic alkali solution for mechanical stirring and dissolution, and sodium aluminate is obtained through separation. The application utilizes the gradient reaction inside and outside the particles, realizes the collaborative treatment of multiple solid wastes, improves the conversion rate of sodium aluminate and controls the waste gas emission.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization technology, specifically to a method for preparing sodium aluminate based on alumina waste alkali. Background Technology

[0002] The waste alkali generated during alumina production is rich in organic matter such as sodium oxalate and sulfur-containing impurities. Currently, this type of waste alkali is often used in combination with aluminum-rich solid wastes such as fly ash and secondary aluminum ash to extract sodium aluminate through a high-temperature roasting process in order to achieve resource recovery.

[0003] Existing mixed roasting processes are mostly carried out in a homogeneous powder state. The dense internal structure of the material restricts the gas-solid reaction. At the same time, conventional processes not only fail to make reasonable use of the physical effects brought about by the thermal decomposition of sodium oxalate in waste alkali, but also fail to properly guide the oxidation exothermic reaction of residual metals in secondary aluminum ash. Local disordered exothermic reaction can lead to the melting and ring formation of materials and the generation of insoluble complex salts, which reduces the subsequent leaching rate of aluminum.

[0004] Furthermore, single-atmosphere roasting in complex solid waste systems cannot simultaneously meet the reaction requirements of different components. While roasting in a reducing atmosphere is beneficial for breaking the silicon-aluminum-oxygen bonds in fly ash, sulfur-containing impurities will be converted into gases such as sulfur dioxide and discharged, increasing the burden of tail gas treatment. On the other hand, while an oxidizing atmosphere can promote sulfur fixation and the formation of sodium aluminate crystal phase, it will inhibit the deep decoupling of the silicon-aluminum mineral structure, resulting in the inability of aluminum elements to be rationally released. Current processes cannot reconcile the contradiction between deep silicon-aluminum bond breaking and in-situ solidification of sulfur elements within the same system, resulting in a low conversion rate of the target product sodium aluminate and causing environmental problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing sodium aluminate based on alumina waste alkali. This method solves the problems in existing mixed solid waste roasting processes where a single atmosphere cannot simultaneously achieve deep decoupling of silicon and aluminum minerals and in-situ solidification of sulfur elements, as well as the limitations of homogeneous powder material reactions leading to low conversion rates of the target product sodium aluminate and the generation of sulfur-containing waste gas pollution.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: a method for preparing sodium aluminate based on alumina waste alkali, the method comprising the following steps: continuously feeding macro-microenvironment raw material balls into a single-cylinder countercurrent rotary kiln, and controlling the main airflow of the rotary kiln to be an oxidizing atmosphere; As the kiln rotates, the raw material balls in the macro-micro environment enter the preheating and in-situ pore-forming section, causing the sodium oxalate in the waste alkali to thermally decompose and generate gas, thus creating interconnected mesopores in situ inside the raw material balls. The preheated raw material balls with in-situ pore formation enter the internal reduction and bond breaking decoupling section. The mass transfer resistance caused by the macroscopic size of the particles is used to construct a micro-reduction environment inside, and the in-situ reduction and bond breaking reaction is promoted by the residual carbon of fly ash and secondary aluminum ash. After internal reduction and bond breaking decoupling, the clinker precursor enters the external oxidation and phase reconstruction sintering section to complete the oxidation interface capture of gaseous sulfur elements on the particle surface and the reconstruction synthesis of the main crystalline phase sodium aluminate, thus obtaining clinker. The clinker is discharged, rapidly cooled and crushed, and then fed into a reaction device. A caustic alkali solution is added and mechanically stirred to dissolve the clinker. The filtrate is then separated and collected to obtain sodium aluminate.

[0007] By adopting the above technical solution, the present invention utilizes macroscopic particles to initiate gradient reactions in different temperature zones within the kiln. Specifically, the process of material propulsion is divided into three main reaction stages: Step 1: Preheating and Pore Formation Stage. The sodium oxalate contained in the waste alkali undergoes thermal decomposition when heated, transforming into sodium carbonate and releasing carbon monoxide gas. The carbon monoxide generated escapes outward, forming mesopores that penetrate from the inside to the outside within the raw material ball matrix, thereby increasing the specific surface area of ​​the particles and establishing diffusion channels for subsequent gas-solid reactions.

[0008] Step 2: Internal micro-reduction stage. The macroscopic pellet size creates resistance to the inward diffusion of oxygen in the main gas flow of the rotary kiln, resulting in a decrease in the oxygen partial pressure inside the raw material pellets and the formation of a local micro-reduction environment. Under these conditions, the residual carbon in the fly ash breaks the aluminum-oxygen bonds and silicon-oxygen bonds of the aluminosilicate minerals through carbothermic reduction. The parallel reaction is that the residual metal particles inside the secondary aluminum ash undergo oxidation to convert into alumina and release heat. At the same time, the concentrated heat released by the aluminothermic reduction promotes the decoupling of silicon and aluminum. Sulfur-containing compounds are converted into gaseous hydrogen sulfide or sulfur dioxide in the slightly reducing atmosphere and migrate to the surface.

[0009] Step 3: Surface oxidation and reconstruction stage. The surface of the raw material pellets is exposed to the high-temperature oxidizing main gas flow of the rotary kiln. The gaseous sulfides overflowing from the inside react with external oxygen and calcium oxide at the gas-solid interface on the surface of the particles to form calcium sulfate. The sulfur fixation reaction seals the sulfur element in the clinker lattice to control the emission of waste gas. At the same time, under the high-temperature oxidation environment on the surface, sodium carbonate and alumina components combine to form the target product sodium aluminate and release carbon dioxide gas.

[0010] Preferably, the firing step in a single-cylinder countercurrent rotary kiln includes: Adjust the kiln head blower to set the excess air coefficient to 1.15 to 1.25; When entering the preheating and in-situ hole-making section, the gas temperature inside the kiln is controlled to be constant at 650 to 800°C, and the material residence time is 20 to 40 minutes. When entering the internal reduction and bond breaking decoupling section, the gas temperature inside the kiln is controlled at 820 to 1000℃, and the material residence time is 25 to 50 minutes. When entering the external oxidation and phase reconstruction sintering section, the gas temperature inside the kiln is controlled at 1150 to 1250°C, and the material residence time is 45 to 60 minutes.

[0011] By adopting the above technical solution, the excess air coefficient is set in the range of 1.15 to 1.25, mainly to provide a suitable oxygen partial pressure for the macroscopic atmosphere in the kiln, so as to support the surface sulfur capture and residual carbon combustion. The set stepped temperature and residence time correspond to the reaction kinetic conditions of different stages: the range of 650 to 800℃ is suitable for the concentrated pyrolysis temperature of sodium oxalate; the range of 820 to 1000℃ basically meets the requirements of metal ignition point and carbothermic reduction activation energy; the range of 1150 to 1250℃ is in line with the thermodynamic conditions of dicalcium silicate and sodium aluminate co-crystallization, reducing the risk of raw material balls melting and sticking due to excessive temperature causing local liquid phase increase.

[0012] Preferably, the steps of rapidly cooling and crushing the clinker and mechanically stirring to dissolve it include: When the clinker is discharged and rapidly cooled and crushed, it is discharged into a single-cylinder cooler, where cold air is blown in for rapid cooling. The upper limit of the final cooling temperature is controlled to be 60 to 100℃. A jaw crusher is used for crushing, and the upper limit of the particle size of the crushed clinker is controlled to be 5 to 10mm. When mechanically stirring and dissolving, the crushed clinker is put into the reactor at a liquid-to-solid mass ratio of 3.0 to 5.0, and a caustic alkali solution with a concentration of 120 to 150 g / L is added. The mixture is then mechanically stirred and dissolved at 70 to 90°C for 30 to 60 minutes.

[0013] By adopting the above technical solution, the blast cooling operation after clinker exits the kiln is mainly used to maintain the highly active crystalline phase of sodium aluminate formed at high temperature, reduce the probability of crystal transformation or secondary reaction to form insoluble complex salts during the slow cooling process, control the upper limit of the crushed clinker particle size to 5 to 10 mm, and combine it with the specified liquid-solid mass ratio, alkali concentration and leaching temperature, which is conducive to expanding the contact interface between sodium aluminate and alkali. This, to a certain extent, avoids the local supersaturation of the leaching solution from causing sodium aluminate hydrolysis to form secondary precipitation of aluminum hydroxide, thereby improving the aluminum leaching rate.

[0014] Preferably, the reaction steps carried out in the rotary kiln include: By subjecting the raw material pellets to a preheating and in-situ pore-forming stage, the gas generated by the thermal decomposition of sodium oxalate is used to generate interconnected mesopores in situ inside the raw material pellets, thus producing a clinker precursor with an interconnected mesopore structure. In the internal reduction and bond-breaking decoupling stage, the self-balancing heating of the in-situ aluminothermic reaction of secondary aluminum ash is used to achieve an interference-resistant in-situ reduction and bond-breaking reaction.

[0015] By adopting the above technical solution, the heat released by the in-situ reaction of the secondary aluminum ash generated inside the raw material ball can provide self-generated heat compensation when the local temperature drops due to material fluctuations or unstable kiln conditions, maintain internal thermal balance, and the anti-interference mechanism helps maintain the energy conditions required for the breaking of silicon-aluminum-oxygen bonds, so as to promote the conversion of aluminum phase and silicon phase into sodium aluminate and calcium silicate according to the set molar ratio.

[0016] Preferably, the macro-microenvironment raw material pellets are formulated from raw materials comprising the following parts by weight based on the total dry weight of the raw materials: Fly ash: 10.0 to 20.0 parts by weight; Secondary aluminum ash: 3.0 to 8.0 parts by weight; Industrial waste alkali, monohydrate gibbsite bauxite and industrial limestone: balance the remaining mass to 100.0 parts by weight.

[0017] By adopting the above technical solution, fly ash and secondary aluminum ash are selected as auxiliary raw materials. Relying on the residual carbon brought in by fly ash and the residual metal particles in secondary aluminum ash, a built-in reducing agent and heat source are provided for the internal reduction desulfurization and bond breaking reconstruction of raw material pellets. Multi-source solid waste mixed treatment can be carried out without relying on the addition of coke.

[0018] Preferably, in the raw material mixing system, the total number of moles of calcium is equal to the sum of 2.0 to 2.2 times the total number of moles of silicon and 3.5 to 4.0 times the total number of moles of sulfur; Furthermore, the molar ratio of sodium to aluminum in the mixed system, excluding bound silicon, is 1.0 to 1.15.

[0019] By adopting the above technical solution, the total calcium molar amount is set to meet the consumption required for the reconstruction of the dicalcium silicate phase and the calcium sulfate phase. The 2.0 to 2.2 times calcium content of the silicon phase is mainly to promote the transformation of amorphous silicon into proto-calcium silicate crystals and reduce the formation of soluble sodium silicate. The 3.5 to 4.0 times calcium content of the sulfur phase provides sufficient sulfur-fixing agent to promote the transformation of surface gaseous sulfur dioxide into solid calcium sulfate. The sodium-aluminum molar ratio of 1.0 to 1.15 is conducive to the transformation of aluminum source into sodium aluminate in the system. The trace excess of sodium ions can inhibit the reverse decomposition of sodium aluminate to a certain extent.

[0020] Preferably, the industrial waste alkali used in the raw materials contains sodium oxalate, and the mass fraction of sodium oxalate in the industrial waste alkali is 10.0% to 15.0%.

[0021] By adopting the above technical solution, the mass fraction of sodium oxalate is limited to control the reaction scale of the pore-forming agent. When the sodium oxalate content is less than 10.0%, the gas production will be low, the development of internal through pores will be limited, and the gas-solid connection between the inside and outside will be affected. When the content is higher than 15.0%, the pyrolysis reaction rate will be accelerated. If the internal gas pressure exceeds the yield limit of the material structure, it will cause the raw material balls in the preheating section to be pulverized and fall off.

[0022] Preferably, the macro-microenvironment raw material balls are powder-formed granules, and the mass fraction of the powder inside the granules that passes through a 200-mesh standard sieve accounts for 80.0% to 90.0%.

[0023] By adopting the above technical solution, the internal powder particle size distribution range is controlled to adjust the solid phase contact area and channel spacing of the material. The fineness of the powder in a specific range provides a suitable initial packing density after being pressed into balls, so that the bonding force between the powder particles during the pyrolysis reaction is sufficient to cope with the internal stress generated by gas expansion and maintain the structural stability of the raw material balls.

[0024] Preferably, the moisture content of the macro-microenvironment raw material pellets after drying is 0.5% to 1.5%.

[0025] By adopting the above technical solution, the free moisture content before entering the kiln is controlled within a limited range. If the moisture content of the raw material pellets is too high, the material will disintegrate due to the instantaneous vaporization of free water when entering the 650℃ preheating section. Retaining 0.5% to 1.5% moisture content, combined with the pelletizing process, helps maintain the toughness of the material skeleton and reduces the risk of mechanical wear and breakage during the feeding and conveying process.

[0026] Preferably, the particle size of the macro-microenvironment raw material balls is 5 to 15 mm.

[0027] By adopting the above technical solution, the macroscopic particle size of the raw material pellets determines the length of the oxygen diffusion mass transfer path. By controlling the particle size within this range, the diffusion rate of oxygen from the surface to the core is lower than the oxygen consumption rate of the reducing substances in the core. This is conducive to forming a redox gradient field in the radial direction of the particles, providing a basis for the decoupling reaction between internal reduction and external oxidation. If the particle size is too small, the particles will be in an oxidized state as a whole; if the particle size is too large, there is a risk of insufficient core heat conduction, which will cause incomplete clinker reaction inside.

[0028] This invention provides a method for preparing sodium aluminate based on alumina waste alkali. It has the following beneficial effects: 1. This invention prepares raw materials into macroscopic microenvironmental raw material balls with specific particle sizes. Combined with the internal interconnected pores generated by the thermal decomposition of sodium oxalate in waste alkali, the mass transfer resistance of the particles in a rotary kiln is used to create a micro-reducing environment inside the raw material balls, while the surface maintains an oxidizing environment. This spatial environmental difference allows for silicon-aluminum bond breaking and desulfurization reactions to occur inside the material. The gaseous sulfides that overflow are directly captured and solidified by calcium oxide on the particle surface. At the same time, sodium carbonate and aluminum oxide are reconstructed on the surface to generate sodium aluminate. This process changes the situation in traditional homogeneous powder calcination processes where it is difficult to balance the redox atmosphere. It improves the conversion rate of sodium aluminate while controlling the emission of sulfur-containing waste gas.

[0029] 2. This invention introduces fly ash and secondary aluminum ash into the raw material pellet formula, directly utilizing the residual carbon brought in by the fly ash as an endogenous reducing agent, and relying on the exothermic oxidation reaction of residual metal particles in the secondary aluminum ash to provide local heat. The heat released in situ during the reaction promotes silicon-aluminum decoupling and, to a certain extent, compensates for the local temperature drop that may occur in the rotary kiln. This batching method eliminates the need to add conventional reducing agents such as coal, which not only reduces the external energy consumption of the roasting process, but also realizes the synergistic utilization of various complex industrial solid wastes such as alumina waste alkali, fly ash, and secondary aluminum ash.

[0030] 3. This invention uses a pre-defined molar ratio of calcium, silicon, sulfur, sodium, and aluminum for batching, and employs a forced-air quenching process with specific leaching conditions after the clinker exits the kiln. The precise ratio promotes the conversion of amorphous silicon and free sulfur into stable calcium orthosilicate and calcium sulfate solid phases, limiting the entry of impurity ions into the leaching solution. The quenching operation after exiting the kiln maintains the active crystalline phase of sodium aluminate, reducing the risk of crystal transformation caused by slow cooling. Combined with mechanical stirring leaching with a limited liquid-to-solid ratio and alkali concentration, the possibility of secondary hydrolysis and precipitation of sodium aluminate is reduced, thereby increasing the actual leaching rate of aluminum in the target product. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the thermogravimetric analysis (TGA) and carbon monoxide emission mass spectrometry (CMS) curves of the initial state raw material pellets of the present invention. Figure 2 This is a schematic diagram of the nitrogen adsorption and desorption isotherms of the calcined raw material balls of the present invention; Figure 3 This is a schematic diagram of the high-resolution X-ray photoelectron spectroscopy of the inner and outer S2p layers of the clinker particles of the present invention. Figure 4 This is a schematic diagram of the X-ray diffraction patterns of the inner and outer layers of the clinker particles of the present invention; Figure 5 This is a schematic diagram of the linear shrinkage rate of the test sample of the present invention as a function of temperature. Figure 6 This is a schematic diagram showing the alumina dissolution rate of different clinker samples of the present invention as a function of time. Figure 7 This is a schematic diagram of the total chromium leaching concentration in the leaching tailings as a function of the pH value of the leaching agent according to the present invention. Figure 8 This is a schematic diagram of the infrared absorption spectrum of a local region of the leaching tailings of the present invention. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0033] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0034] Industrial waste alkali mainly consists of sodium carbonate (CAS No. 497-19-8), sodium sulfate (CAS No. 7757-82-6), and sodium oxalate (CAS No. 62-76-0). The specification is that the total sodium oxide mass fraction is 31.5%, of which the mass fraction of sodium carbonate is 40.0%, the mass fraction of sodium sulfate is 10.0%, the mass fraction of sodium oxalate is 8.0%, and the balance is water. Fly ash, whose main components include silicon dioxide (CAS No. 14808-60-7), alumina (CAS No. 1344-28-1) and carbon (CAS No. 7440-44-0), has the following specifications: silicon dioxide mass fraction 40.0%, alumina mass fraction 11.0%, elemental carbon mass fraction 12.0%, chloride ion mass fraction 0.8%, fluoride ion mass fraction 5.0%, and the balance being amorphous aluminosilicate glass. Secondary aluminum ash, whose main components include aluminum (CAS No. 7429-90-5) and alumina (CAS No. 1344-28-1), has the following specifications: the mass fraction of metallic elemental aluminum is 8.0%, the mass fraction of alumina is 65.0%, and the balance is non-combustible ash containing aluminum nitride (CAS No. 24304-00-5). This is a monohydrate gibbsite-type bauxite, whose main components include hydrated α-alumina (CAS No. 1318-23-6) and silicon dioxide (CAS No. 14808-60-7), with an effective alumina mass fraction of 55.0% and a silicon dioxide mass fraction of 10.0%. Industrial limestone, whose main component is calcium carbonate (CAS No. 471-34-1), has a calcium carbonate mass fraction greater than or equal to 95.0%.

[0035] In this invention, the units of measurement for all materials and solvents are uniformly referred to as parts by weight.

[0036] Preparation Example 1: This preparation example provides a method for preparing macroscopic microenvironment raw material pellets, the method including the following steps: Based on the total dry weight of the raw materials, weigh out 15.0 parts of fly ash and 5.0 parts of secondary alumina ash, and add the calculated amounts of industrial waste alkali with a sodium oxalate mass fraction of 10.0%, gibbsite-type bauxite, and industrial limestone. Control the amount of industrial limestone added to ensure that the total number of moles of calcium in the system is equal to the sum of 2.1 times the total number of moles of silicon and 3.8 times the total number of moles of sulfur. Control the amount of industrial waste alkali and gibbsite-type bauxite added to ensure that the sodium-aluminum molar ratio (excluding bound silicon) in the mixed system is 1.1. Balance the remaining mass to 100.0 parts and mix evenly to prepare the mixed raw materials. The mixed raw materials are fed into a ball mill for mixing and grinding, and the grinding is controlled until the mass fraction of the material passing through a 200-mesh standard sieve is 85.0%, thus obtaining homogeneous raw meal powder. Homogeneous raw meal powder is fed into a disc granulator, sprayed with deionized water for granulation, and the moisture content of the pellets is controlled to be 10.0% by mass. The pellets are then rolled to obtain the initial raw meal pellets. After granulation, the raw material balls in their initial state are placed in a drying device and dried at 120°C until the moisture content is 1.0%. Then, they are sieved to obtain particles with a diameter of 10 mm for later use. The macro-microenvironment raw material balls here are named raw material balls A.

[0037] Preparation Example 2: This preparation example provides a method for preparing macroscopic microenvironment raw material pellets, the method including the following steps: Based on the total dry weight of the raw materials, weigh out 10.0 parts of fly ash and 3.0 parts of secondary alumina ash, and add the calculated amount of industrial waste alkali with a sodium oxalate mass fraction of 15.0%, gibbsite-type bauxite, and industrial limestone. Control the amount of industrial limestone added to ensure that the total number of moles of calcium in the system is equal to the sum of 2.0 times the total number of moles of silicon and 3.5 times the total number of moles of sulfur. Control the amount of industrial waste alkali and gibbsite-type bauxite added to ensure that the sodium-aluminum molar ratio (excluding bound silicon) in the mixed system is 1.0. Balance the remaining mass to 100.0 parts and mix evenly to prepare the mixed raw materials. The mixed raw materials are fed into a ball mill for mixing and grinding, and the grinding is controlled until the mass fraction of the material passing through a 200-mesh standard sieve is 80.0%, thus obtaining homogeneous raw meal powder; Homogeneous raw meal powder is fed into a disc granulator, sprayed with deionized water for granulation, and the moisture content of the pellets is controlled to be 8.0% by mass. The pellets are then rolled to obtain the initial raw meal pellets. After granulation, the raw material balls in their initial state are placed in a drying device and dried at 100°C until the moisture content is 0.5%. Then, they are sieved to obtain particles with a diameter of 5 mm for later use. The macro-microenvironment raw material balls here are named raw material balls B.

[0038] Preparation Example 3: This preparation example provides a method for preparing macroscopic microenvironment raw material pellets, the method including the following steps: Based on the total dry weight of the raw materials, weigh out 20.0 parts of fly ash and 8.0 parts of secondary alumina ash, and add the calculated amounts of industrial waste alkali with a sodium oxalate mass fraction of 13.5%, gibbsite bauxite, and industrial limestone. Control the amount of industrial limestone added to ensure that the total number of moles of calcium in the system is equal to the sum of 2.2 times the total number of moles of silicon and 4.0 times the total number of moles of sulfur. Control the amount of industrial waste alkali and gibbsite bauxite added to ensure that the sodium-aluminum molar ratio (excluding bound silicon) in the mixed system is 1.15. Balance the remaining mass to 100.0 parts and mix evenly to prepare the mixed raw materials. The mixed raw materials are fed into a ball mill for mixing and grinding. The grinding is controlled until the mass fraction of the material passing through a 200-mesh standard sieve is 90.0%, thus obtaining homogeneous raw meal powder. Homogeneous raw material powder is fed into a roller briquetting machine, deionized water is sprayed for granulation, the moisture content of the briquettes is controlled at 15.0%, and the raw material briquettes are pressed into shape to obtain the initial raw material briquettes. After granulation, the raw material balls in their initial state are placed in a drying device and dried at 150°C until the moisture content is 1.5%. Then, they are sieved to obtain particles with a diameter of 15 mm for later use. The macro-microenvironment raw material balls here are named raw material balls C.

[0039] Preparation Example 4: This preparation example provides a method for preparing macroscopic microenvironment raw material pellets, the method including the following steps: Based on the total dry weight of the raw materials, weigh out 15.0 parts of fly ash and 5.0 parts of secondary alumina ash, and add the calculated amounts of industrial waste alkali with a sodium oxalate mass fraction of 12.0%, gibbsite bauxite, and industrial limestone. Control the amount of industrial limestone added to ensure that the total number of moles of calcium in the system is equal to the sum of 2.1 times the total number of moles of silicon and 3.8 times the total number of moles of sulfur. Control the amount of industrial waste alkali and gibbsite bauxite added to ensure that the sodium-aluminum molar ratio (excluding bound silicon) in the mixed system is 1.1. Balance the remaining mass to 100.0 parts and mix evenly to prepare the mixed raw materials. The mixed raw materials are fed into a ball mill for mixing and grinding, and the grinding is controlled until the mass fraction of the material passing through a 200-mesh standard sieve is 85.0%, thus obtaining homogeneous raw meal powder. Homogeneous raw meal powder is fed into a disc granulator, sprayed with deionized water for granulation, and the moisture content of the pellets is controlled at 12.0% by mass. The pellets are then rolled to obtain the initial raw meal pellets. After granulation, the raw material balls in their initial state are placed in a drying equipment and dried at 130°C until the moisture content is 1.2%. Then, they are sieved to obtain particles with a diameter of 12 mm for later use. The macro-microenvironment raw material balls here are named raw material balls D. Example 1

[0040] This embodiment provides a method for preparing sodium aluminate based on alumina waste alkali, the preparation method including the following steps: Raw material pellets A are continuously fed into a single-cylinder countercurrent rotary kiln. The main airflow of the rotary kiln is controlled to be an oxidizing atmosphere. The kiln head blower is adjusted to set the excess air coefficient to 1.20. As the kiln rotates, raw material ball A enters the preheating and in-situ pore-forming section, controlling the gas temperature inside the kiln to be constant at 750℃. The material residence time in this section is 30 minutes, causing the sodium oxalate in the waste alkali to thermally decompose and generate gas, thus creating interconnected mesopores in situ inside the raw material ball. The preheated material enters the internal reduction and bond breaking decoupling section, and the gas temperature inside the kiln is controlled at 900℃. The residence time of the material in this section is 40 minutes. The micro-reduction environment is constructed inside by utilizing the mass transfer resistance caused by the macroscopic size of the particles. The in-situ reduction and bond breaking reaction is promoted by the residual carbon of fly ash and secondary aluminum ash. After roasting, the material enters the external oxidation and phase reconstruction sintering section, and the gas temperature inside the kiln is controlled at 1200℃. The residence time of the material in this section is 50 minutes, which completes the capture of gaseous sulfur elements on the surface of the particles at the oxidation interface and the reconstruction and synthesis of the main crystalline phase sodium aluminate. After high-temperature roasting, the clinker is discharged into a single-cylinder cooler and cooled rapidly to below 80°C by blowing in cold air. It is then crushed into particles smaller than 8mm using a jaw crusher. Subsequently, the crushed clinker is fed into a reaction vessel at a liquid-to-solid mass ratio of 4.0, and a caustic alkali solution with a concentration of 135g / L is added. The mixture is mechanically stirred at 80°C for 45 minutes to dissolve the clinker. The filtrate is collected by centrifugation to obtain the sodium aluminate sample. Example 2

[0041] This embodiment provides a method for preparing sodium aluminate based on alumina waste alkali, the preparation method including the following steps: Raw material pellets B are continuously fed into a single-cylinder countercurrent rotary kiln. The main airflow of the rotary kiln is controlled to be an oxidizing atmosphere. The kiln head blower is adjusted to set the excess air coefficient to 1.15. As the kiln rotates, raw material ball B enters the preheating and in-situ pore-forming section, controlling the gas temperature inside the kiln to remain constant at 650℃. The material residence time in this section is 20 minutes, causing the sodium oxalate in the waste alkali to thermally decompose and generate gas, thus creating interconnected mesopores in situ inside the raw material ball. The preheated material enters the internal reduction and bond breaking decoupling section, and the gas temperature inside the kiln is controlled at 850℃. The residence time of the material in this section is 30 minutes. The micro-reduction environment is constructed inside by utilizing the mass transfer resistance caused by the macroscopic size of the particles. The in-situ reduction and bond breaking reaction is promoted by the residual carbon of fly ash and secondary aluminum ash. After roasting, the material enters the external oxidation and phase reconstruction sintering section, and the gas temperature inside the kiln is controlled at 1150℃. The residence time of the material in this section is 45 minutes, which completes the capture of gaseous sulfur elements on the surface of the particles at the oxidation interface and the reconstruction and synthesis of the main crystalline phase sodium aluminate. After high-temperature roasting, the clinker is discharged into a single-cylinder cooler and cooled rapidly to below 60°C by blowing in cold air. It is then crushed into particles smaller than 5mm using a jaw crusher. Subsequently, the crushed clinker is fed into a reaction vessel at a liquid-to-solid mass ratio of 3.0, and a caustic alkali solution with a concentration of 120g / L is added. The mixture is mechanically stirred at 70°C for 30 minutes to dissolve the clinker. The filtrate is then collected by plate and frame filter press to obtain the sodium aluminate sample. Example 3

[0042] This embodiment provides a method for preparing sodium aluminate based on alumina waste alkali, the preparation method including the following steps: Raw material pellets C are continuously fed into a single-cylinder countercurrent rotary kiln. The main airflow of the rotary kiln is controlled to be an oxidizing atmosphere. The kiln head blower is adjusted to set the excess air coefficient to 1.25. As the kiln rotates, the raw material ball C enters the preheating and in-situ pore-forming section, and the gas temperature inside the kiln is kept constant at 800℃. The residence time of the material in this section is 40 minutes, which causes the sodium oxalate in the waste alkali to decompose and generate gas, and in-situ generates interconnected mesopores inside the raw material ball. The preheated material enters the internal reduction and bond breaking decoupling section, and the gas temperature inside the kiln is controlled at 1000℃. The residence time of the material in this section is 50 minutes. The micro-reduction environment is constructed inside by utilizing the mass transfer resistance caused by the macroscopic size of the particles. The in-situ reduction and bond breaking reaction is promoted by the residual carbon of fly ash and secondary aluminum ash. After roasting, the material enters the external oxidation and phase reconstruction sintering section, and the gas temperature inside the kiln is controlled at 1250℃. The residence time of the material in this section is 60 minutes, which completes the capture of gaseous sulfur elements on the surface of the particles at the oxidation interface and the reconstruction and synthesis of the main crystalline phase sodium aluminate. After high-temperature roasting, the clinker is discharged into a single-cylinder cooler and cooled rapidly to below 100°C by blowing in cold air. It is then crushed into particles smaller than 10mm using a jaw crusher. Subsequently, the crushed clinker is fed into a reaction vessel at a liquid-to-solid mass ratio of 5.0, and a caustic alkali solution with a concentration of 150g / L is added. The mixture is mechanically stirred at 90°C for 60 minutes to dissolve the clinker. The filtrate is collected by centrifugation to obtain the sodium aluminate sample. Example 4

[0043] This embodiment provides a method for preparing sodium aluminate based on alumina waste alkali, the preparation method including the following steps: Raw material pellets D are continuously fed into a single-cylinder countercurrent rotary kiln. The main airflow of the rotary kiln is controlled to be an oxidizing atmosphere. The kiln head blower is adjusted to set the excess air coefficient to 1.20. As the kiln rotates, the raw material pellets D enter the preheating and in-situ pore-forming section, and the gas temperature inside the kiln is kept constant at 750℃. The residence time of the material in this section is 30 minutes, which causes the sodium oxalate in the waste alkali to decompose and generate gas, and in-situ generates interconnected mesopores inside the raw material pellets. After preheating, the material enters the internal reduction and bond breaking decoupling section to simulate the kiln temperature fluctuation in actual production. The gas temperature inside the kiln is reduced to 820℃, and the residence time of the material in this section is shortened to 25 minutes to verify the anti-interference ability of the self-balanced heating of the in-situ aluminothermic reaction of secondary aluminum ash. After roasting, the material enters the external oxidation and phase reconstruction sintering section, and the gas temperature inside the kiln is controlled at 1200℃. The residence time of the material in this section is 50 minutes, which completes the capture of gaseous sulfur elements on the surface of the particles at the oxidation interface and the reconstruction and synthesis of the main crystalline phase sodium aluminate. After high-temperature roasting, the clinker is discharged into a single-cylinder cooler and cooled rapidly to below 90°C by blowing in cold air. It is then crushed to a particle size of less than 6 mm using a jaw crusher. Subsequently, the crushed clinker is fed into a reaction vessel at a liquid-to-solid mass ratio of 4.0, and a caustic alkali solution with a concentration of 135 g / L is added. The mixture is mechanically stirred at 80°C for 45 min to dissolve the clinker. The filtrate is collected by centrifugation to obtain the sodium aluminate sample.

[0044] Comparative Example 1: Compared with Example 1, the difference is that the disc granulation and pellet drying steps are omitted in the preparation stage, and the homogeneous raw material powder obtained after ball milling is directly and continuously fed into a single-cylinder countercurrent rotary kiln for calcination. All other aspects are the same.

[0045] Comparative Example 2: Compared with Example 1, the difference is that the preparation stage uses purified waste alkali that has undergone deep pretreatment and completely removed sodium oxalate and organic components as raw material, while the rest are the same.

[0046] Comparative Example 3: Compared with Example 4, the difference is that secondary aluminum ash is not added in the batching stage, but is replaced by conventional monohydrate gibbsite bauxite with an equal amount of effective alumina content for balancing, while the rest are the same.

[0047] Comparative Example 4: Compared with Example 1, the difference is that the calcium curing requirement of sulfides is not considered in the batching stage, and the amount of industrial limestone added is calculated only according to the total number of calcium moles in the system being equal to 2.1 times the total number of silicon moles. That is, the batching addition item for the calcium-sulfur molar ratio is removed, and the rest are the same.

[0048] Test Example 1: The raw material spheres A and B prepared in Example 1 and Example 2 were used as test objects. Physical samples were taken from the raw material spheres A and B. The sampled raw material spheres A and B were placed in alumina crucibles and then transferred to a simultaneous thermal analysis and mass spectrometry instrument.

[0049] The test atmosphere was set as simulated air with a flow rate of 50 mL / min and a volume ratio of oxygen to nitrogen of 1:4. The heating rate was set to 10 °C / min, and the scanning temperature range was from room temperature to 1000 °C. During the programmed heating process, the thermogravimetric curve was recorded, and the mass spectrometry channel was simultaneously turned on to monitor the change of ion current signal with a mass-to-charge ratio of 28, and the escape curve of carbon monoxide gas was obtained.

[0050] Take the raw material balls A prepared in Example 1 and the raw material balls B prepared in Example 2, place them in a muffle furnace, and calcine them at 800°C for 30 minutes at a rate of 10°C / min in air atmosphere. Then, let them cool naturally to room temperature to obtain the calcined raw material balls A of Example 1 and the calcined raw material balls B of Example 2.

[0051] Weigh 2g each of the initial state raw material ball A from Example 1, the initial state raw material ball B from Example 2, the calcined state raw material ball A from Example 1, and the calcined state raw material ball B from Example 2, and degas them under vacuum at 200℃ for 4 hours.

[0052] The degassed sample was transferred into a surface area and porosity analyzer, and nitrogen adsorption and desorption isotherm tests were performed at 77 K. The specific surface area of ​​the sample was calculated using the BET method, and the total pore volume was calculated using the BJH model.

[0053] Table 1: Pore structure data before and after pore formation

[0054] According to Table 1 and Figure 1 and Figure 2 It can be seen that the specific surface area of ​​the initial raw material ball A in Example 1 is 2.14 m². 2 / g, total pore volume is 0.012cm³ 3 / g; The specific surface area of ​​the initial raw material pellet B in Example 2 was 1.83m². 2 / g, total pore volume is 0.009cm³ 3 / g, after calcination at 800℃, the specific surface area of ​​the calcined raw material pellets A in Example 1 was measured to be 38.67m². 2 / g, total pore volume measured as 0.145cm³ 3 / g; The specific surface area of ​​the calcined raw material pellets B in Example 2 was measured to be 29.54 m². 2 / g, total pore volume measured as 0.112cm³ 3 / g.

[0055] Combination Figure 1 The curve test results showed that no gas escape was detected at room temperature, as shown in Table 1; however, within the programmed heating range of 650℃ to 800℃, the thermogravimetric curves of raw material ball A in Example 1 and raw material ball B in Example 2 showed obvious weight loss steps, and the mass spectrometry channel recorded an ion current escape peak with a mass-to-charge ratio of 28, with corresponding peak temperatures of 731.4℃ and 728.9℃, respectively.

[0056] The adsorption and desorption isotherms in the figure exhibit a hysteresis loop, demonstrating the physical adsorption properties of the mesoporous material. The adsorption amounts at the adsorption branches logically correspond to the total pore volume data in Table 1.

[0057] Based on the combined data of the exponential increase in specific surface area, porosity data, and the CO main emission peak temperatures of 731.4℃ and 728.9℃ detected by MS, the CO emission peak temperatures detected in Table 1 accurately fall within the temperature range of 650℃ to 800℃ set in this application for the preheating and in-situ pore-forming section.

[0058] This fully demonstrates that the sodium oxalate component in the waste alkali material undergoes a violent thermal decomposition reaction near this specific peak temperature, releasing a large amount of carbon monoxide gas. With the concentrated release of gas at the peak temperature, the gas flow breaks through and escapes from the inside of the raw material ball to the outside, etching in situ in the solid material matrix and generating interconnected mesoporous channels. This in-situ pore-forming mechanism changes the physical microstructure inside the raw material ball, providing not only the necessary microenvironment and gas mass transfer channels for the subsequent internal reduction and bond breaking decoupling reactions, but also a huge penetration contact area for the final alkali solution dissolution.

[0059] Test Example 2: The clinker particles prepared in Example 1 and Example 3 were used as test objects. Undamaged clinker particles were selected from the high-temperature roasting products of Examples 1 and 3. The selected clinker particles were fixed on conductive adhesive and moved into an X-ray photoelectron spectrometer. A monochromatic aluminum target X-ray source was used to first perform a broadband scan on the outer surface area of ​​the clinker particles, and then a high-resolution narrow-spectrum scan of the S2p orbital was performed in the binding energy range of 158 eV to 174 eV to obtain the sulfur binding energy data of the outer shell layer.

[0060] After the outer surface region was tested, the argon ion beam etching gun configured on the X-ray photoelectron spectrometer was activated to perform in-situ deep etching on the test area. The etching depth was set to penetrate the outer surface and reach the core layer region inside the particle. After the etching stopped, the exposed core layer region was scanned again with high-resolution narrow spectrum of S2p orbitals to record the elemental valence state binding energy data of the core layer region.

[0061] Clinker particles prepared in Example 1 and Example 3 were collected respectively. The outer layer of the clinker particles was peeled off by mechanical grinding and peeling method to obtain the outer shell powder. The remaining internal particles were ground to obtain the core layer powder, thus obtaining the outer shell powder of Example 1, the core layer powder of Example 1, the outer shell powder of Example 3, and the core layer powder of Example 3.

[0062] The outer shell powder of Example 1, the core layer powder of Example 1, the outer shell powder of Example 3, and the core layer powder of Example 3 were respectively placed in the sample stage of an X-ray diffractometer. A CuKα radiation source was used, with the operating voltage set to 40kV, the operating current set to 40mA, the scanning step size set to 0.02 degrees, the scanning speed set to 5 degrees / min, and the scanning range of the diffraction angle 2θ set to 10 degrees to 70 degrees. The X-ray diffraction patterns of each powder sample were recorded to obtain the crystal phase diffraction characteristics of the powder samples.

[0063] Table 2: Phase and Elemental Characteristics of Different Spatial Regions of Clinker Particles

[0064] According to Table 2 and Figure 3 and Figure 4 It can be seen that in Example 1, the relative area ratio of sulfate sulfur in the outer shell layer of clinker particles is 93.42%, and the relative area ratio of sulfide sulfur is 2.15%. In the core layer after argon ion etching, the relative area ratio of sulfide sulfur is 88.61%, and the relative area ratio of sulfate sulfur is 4.87%. The clinker particles in Example 3 show a similar elemental distribution. The small proportion of sulfur substances in intermediate transition valence states, such as sulfite, that could not be completely collected in the high-resolution energy spectrum of each region are sulfite.

[0065] Combination Figure 3The photoelectron spectroscopy distribution curves show that the outer shell layer exhibits a characteristic peak representing sulfate ions near the binding energy of 168.5 eV, while the core layer exhibits a characteristic peak representing sulfides near the binding energy of 162.0 eV. The aforementioned data indicate that during the calcination process, the inside and outside of the raw material pellets are in different reaction states. A relatively reducing environment is formed inside the raw material pellets, which promotes the reduction reaction of sodium sulfate and its conversion into low-valence sulfur-containing substances. After the generated sulfur-containing substances diffuse to the surface, they undergo an oxidation reaction under an oxygen-containing atmosphere to form sulfates.

[0066] In Example 1, the main characteristic diffraction peak of sodium aluminate was recorded in the core layer powder of clinker particles, while no diffraction peak of sodium sulfate with conventional intensity was observed. In the outer shell powder, the main characteristic diffraction peak of calcium sulfoaluminate solid solution was recorded. The test results reflect that there are differences in the crystal phase composition of different spatial regions of clinker particles. The reduction reaction inside the raw material ball consumes sodium sulfate and forms the target product sodium aluminate. Sulfur-containing substances react on the surface of the raw material ball to form the calcium sulfoaluminate mineral phase. The structural characteristics of the inside and outside are different, reflecting that the reduction reaction and the oxidation and sulfur fixation process in the waste alkali treatment process are distributed in different spatial levels, providing conditions for phase separation and conversion of the target product.

[0067] X-ray photoelectron spectroscopy reflects the relative valence distribution of sulfur in the test micro-region. Most of the low-valence gaseous sulfur-containing substances generated during the reduction inside the raw material ball have diffused to the outer layer through the mesoscopic pores generated in the previous step and have been oxidized and solidified on the oxygen-containing surface layer. As the core layer has been almost completely volatilized, the residual absolute sulfur content is low. The relative distribution of elements in the above micro-region further confirms the microscopic kinetic mechanism of decoupling desulfurization and phase separation between the inner and outer layers of the raw material ball, avoiding contamination of the core target product sodium aluminate by soluble sulfides.

[0068] Test Example 3: The raw material spheres A prepared in Example 1, the mixed raw material powder prepared in Comparative Example 1, and the sodium oxalate-free raw material spheres prepared in Comparative Example 2 were used as test objects. The mixed raw material powder prepared in Comparative Example 1 was pressed into cylindrical test blocks with the same size as the raw material spheres A in a tablet press. The raw material spheres A, the cylindrical test blocks, and the sodium oxalate-free raw material spheres were placed on the alumina test platform of the thermomechanical analyzer, and a quartz probe was applied to the top of the test samples.

[0069] The static load of the thermomechanical analyzer was set to 0.05 N, the test atmosphere was air, the heating rate was set to 10 °C / min, and the temperature scan range was from room temperature to 1000 °C. The dimensional changes of the initial raw material ball A, the cylindrical test block, and the sodium oxalate-free initial raw material ball in the Z-axis direction were recorded during the heating process. The linear shrinkage curves of different samples were calculated based on the dimensional change data.

[0070] Collect the clinker particles obtained from roasting in Example 1, the powdered clinker obtained from roasting in Comparative Example 1, and the non-porous clinker particles obtained from roasting in Comparative Example 2. Crush the clinker particles obtained from roasting in Example 1, the powdered clinker obtained from roasting in Comparative Example 1, and the non-porous clinker particles obtained from roasting in Comparative Example 2, and sieve them to a particle size range of 100 to 120 mesh. Accurately weigh equal masses of the sieved clinker particles from Example 1, the powdered clinker from Comparative Example 1, and the non-porous clinker particles from Comparative Example 2, and place them in a constant temperature water bath reactor equipped with a mechanical stirring device.

[0071] A standard circulating mother liquor with a concentration of 135 g / L was added to a constant temperature water bath reactor at a liquid-to-solid ratio of 4:1. The leaching reaction temperature was set to 80℃ and the stirring speed was set to 300 r / min. Quantitative slurries were extracted at 15, 30, 45, 60, 75, and 90 minutes of the leaching reaction. The quantitative slurries extracted at different time points were vacuum filtered, and the filter cake was washed with deionized water. The combined filtrates at the corresponding time points were collected, and the concentrations of aluminum and sodium ions in the collected combined filtrate samples were determined using inductively coupled plasma atomic emission spectrometry. The alumina dissolution rate and effective alkali recovery rate at different time points were calculated.

[0072] Table 3: Data on Macroscopic Thermal Shrinkage and Dissolution Performance

[0073] According to Table 3 and Figure 5 and Figure 6 It can be seen that the linear shrinkage rates of the raw material balls A in Example 1 and the raw material balls without sodium oxalate in Comparative Example 2 when heated to 900℃ were 8.64% and 7.92%, respectively, and the curves changed smoothly. However, the linear shrinkage rate of the cylindrical test block in Comparative Example 1 increased sharply to 36.81% in the range of 880℃ to 900℃, and macroscopic structural softening and collapse occurred. The reason is that the melting point of sodium sulfate is about 884℃. The powdered material in Comparative Example 1 was directly exposed to the oxidizing gas flow. The sodium sulfate that did not undergo the reduction reaction turned into a liquid phase after reaching the melting point. In continuous industrial production, this would cause the material to stick to the wall and form rings.

[0074] The granulation process in Example 1 creates a restricted oxidation environment inside the raw material pellets, which causes sodium sulfate to be converted into high-melting-point sulfur-containing substances before reaching its melting point, controls the formation of the liquid phase, and maintains the morphological stability of the material in the high-temperature section.

[0075] After leaching for 90 minutes, the alumina dissolution rate of the clinker particles in Example 1 reached 92.17%, and the rate of increase was relatively fast in the first 30 minutes. The alumina dissolution rates of the powdered clinker in Comparative Example 1 and the non-porous clinker particles in Comparative Example 2 were 71.53% and 65.34%, respectively, and the curves changed relatively slowly. The sodium oxalate in the formulation of Example 1 decomposed to produce gas during the heating stage, and constructed a porous structure in situ inside the raw material balls.

[0076] The porous structure increases the specific surface area of ​​the clinker particles, providing a channel for the penetration of the circulating mother liquor and improving the mass transfer conditions of the internal solid-liquid reaction. In Comparative Example 2, due to the lack of a pore-forming process, the clinker particles after roasting have a dense structure, and there is physical resistance to the penetration of the circulating mother liquor into the interior, which makes it difficult for the generated sodium aluminate to be fully contacted and dissolved, thus reducing the dissolution index of the target product.

[0077] Test Example 4: The leaching tailings obtained from Example 1 after leaching reaction and solid-liquid separation, Example 4 after leaching reaction and solid-liquid separation, Comparative Example 1 after leaching reaction and solid-liquid separation, Comparative Example 3 after leaching reaction and solid-liquid separation, and Comparative Example 4 after leaching reaction and solid-liquid separation were collected as test objects. Mixed buffer solutions of nitric acid and sodium hydroxide with pH values ​​of 2.0, 4.0, 6.0, 7.0, 8.0, 10.0, and 12.0 were prepared as leaching agents.

[0078] Accurately weigh the leaching tailings from Example 1, Example 4, Comparative Example 1, Comparative Example 3, and Comparative Example 4, and add them to the extraction agents at different pH values ​​according to a liquid-to-solid ratio of 10:1. Place the mixture in a vortex extractor, set the operating temperature of the vortex extractor to 25°C, the vortex speed to 30 r / min, and the continuous vortex time to 18 hours.

[0079] After the oscillation process was completed, the extracted solid-liquid mixture was filtered through a microporous membrane with a pore size of 0.45 micrometers. The separated filtrate samples were collected, and the total chromium concentration in each filtrate sample under different pH conditions was determined by inductively coupled plasma mass spectrometry.

[0080] The leaching tailings from Example 1, Example 4, Comparative Example 1, Comparative Example 3, and Comparative Example 4 were dried in a vacuum oven at 80°C for 12 hours.

[0081] The dried leaching tailings of Example 1, Example 4, Comparative Example 1, Comparative Example 3, and Comparative Example 4 were ground in an agate mortar until they all passed through a 200-mesh sieve, respectively, to obtain tailings powder of Example 1, Example 4, Comparative Example 1, Comparative Example 3, and Comparative Example 4.

[0082] Equal masses of tailings powder from Example 1, Example 4, Comparative Example 1, Comparative Example 3, and Comparative Example 4 were weighed and mixed with spectrally pure potassium bromide at a mass ratio of 1:100 and then ground.

[0083] Infrared transparent test films were pressed into thin films using a tablet press. These films were then placed in the test chamber of a Fourier transform infrared spectrometer, and the scanning wavenumber range of the spectrometer was set to 400 cm⁻¹. -1 Up to 4000cm -1 The spectral resolution was set to 4cm. -1 The number of scans for both the background and the sample was set to 32.

[0084] Infrared absorption spectral data of tailings powder from Example 1, Example 4, Comparative Example 1, Comparative Example 3, and Comparative Example 4 were obtained, and 800 cm⁻¹ infrared absorption spectra were extracted. -1 Up to 1200cm -1 The infrared spectral signals within the wavenumber range are analyzed.

[0085] Table 4: Environmental Stability and Microstructure Data of Leaching Tail

[0086] According to Table 4 and Figure 7 and Figure 8 It can be seen that within the test range of pH 2 to 12, the total chromium leaching concentration of the tailings in Examples 1 and 4 remained at a low level, with only 0.048 mg / L and 0.063 mg / L at pH 2, respectively; while Comparative Examples 1, 3 and 4 all exhibited a U-shaped desorption characteristic with high values ​​at both ends, reaching 2.651 mg / L, 1.892 mg / L and 2.215 mg / L at pH 2, respectively.

[0087] The examples demonstrate that by using a matched pelletizing and roasting system, heavy metal chromium is steadily introduced into the crystal lattice of crystalline minerals through isomorphous substitution, achieving excellent environmental safety. In particular, Example 4 still performs well under simulated kiln temperature fluctuations, verifying that the spontaneous aluminothermic heat release effect of secondary aluminum ash can compensate for local enthalpy deficiency and ensure complete crystal phase development. In contrast, Comparative Example 3 suffers from incomplete mineral phase development and heavy metal ionization due to the lack of this thermodynamic compensation.

[0088] Furthermore, infrared spectral data show that the main peak of the Si-O asymmetric stretching vibration in the tailings powder of Examples 1 and 4 shifts significantly to a lower wavenumber, reaching 986.3 cm⁻¹. -1 and 989.1cm -1 The corresponding main peak in the control group remained at 1012.4 cm⁻¹. -1 Up to 1024.7cm -1 The high position and significant shift of the main peak indicate that the highly polymerized three-dimensional silicon-oxygen network in the embodiment system has undergone depolymerization, with a large number of bonds broken and transformed into isolated silicon-oxygen tetrahedra. This thermodynamically metastable structure endows the leaching tailings with potential hydration and cementitious activity, laying the microstructural foundation for its subsequent resource utilization as a cement admixture.

[0089] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing sodium aluminate based on alumina waste alkali, characterized in that, The method includes the following steps: The macro-microenvironment raw material pellets are continuously fed into a single-cylinder countercurrent rotary kiln, and the main airflow of the rotary kiln is controlled to be an oxidizing atmosphere. As the kiln rotates, the macro-microenvironment raw material balls enter the preheating and in-situ pore-forming section, causing the sodium oxalate in the waste alkali to thermally decompose and generate gas, thus creating interconnected mesopores in situ inside the raw material balls. The preheated raw material balls with in-situ pore formation enter the internal reduction and bond breaking decoupling section. The mass transfer resistance caused by the macroscopic size of the particles is used to construct a micro-reduction environment inside, and the in-situ reduction and bond breaking reaction is promoted by the residual carbon of fly ash and secondary aluminum ash. After internal reduction and bond breaking decoupling, the clinker precursor enters the external oxidation and phase reconstruction sintering section to complete the oxidation interface capture of gaseous sulfur elements on the particle surface and the reconstruction synthesis of the main crystalline phase sodium aluminate, thus obtaining clinker. The clinker is discharged, rapidly cooled and crushed, and then fed into a reaction device. A caustic alkali solution is added and mechanically stirred to dissolve the clinker. The filtrate is then separated and collected to obtain sodium aluminate.

2. The method for preparing sodium aluminate based on alumina waste alkali according to claim 1, characterized in that, The roasting step in the single-cylinder countercurrent rotary kiln includes: Adjust the kiln head blower to set the excess air coefficient to 1.15 to 1.25; When entering the preheating and in-situ hole-making section, the gas temperature inside the kiln is controlled to be constant at 650 to 800°C, and the material residence time is 20 to 40 minutes. When entering the internal reduction and bond breaking decoupling section, the gas temperature inside the kiln is controlled at 820 to 1000°C, and the material residence time is 25 to 50 minutes. When entering the external oxidation and phase reconstruction firing section, the gas temperature inside the kiln is controlled at 1150 to 1250°C, and the material residence time is 45 to 60 minutes.

3. The method for preparing sodium aluminate based on alumina waste alkali according to claim 1, characterized in that, The steps of rapidly cooling, crushing, and mechanically dissolving the clinker include: When the clinker is discharged and rapidly cooled and crushed, it is discharged into a single-cylinder cooler, where cold air is blown in for rapid cooling. The upper limit of the final cooling temperature is controlled to be 60 to 100°C. A jaw crusher is used for crushing, and the upper limit of the particle size of the crushed clinker is controlled to be 5 to 10 mm. During the mechanical stirring dissolution process, the crushed clinker is added to the reactor at a liquid-to-solid mass ratio of 3.0 to 5.0, and a caustic alkali solution with a concentration of 120 to 150 g / L is added. The mixture is then mechanically stirred at 70 to 90 °C for 30 to 60 minutes.

4. The method for preparing sodium aluminate based on alumina waste alkali according to claim 1, characterized in that, The steps of carrying out the reaction in the rotary kiln include: By subjecting the macro-microenvironment raw material balls to the preheating and in-situ pore-forming section, the gas generated by the thermal decomposition of sodium oxalate is used to generate interconnected mesopores in situ inside the raw material balls, thus obtaining a clinker precursor with an interconnected mesopore structure; and in the internal reduction and bond-breaking decoupling section, the self-balancing heating of the in-situ aluminothermic reaction of secondary aluminum ash is used to achieve an interference-resistant in-situ reduction and bond-breaking reaction.

5. The method for preparing sodium aluminate based on alumina waste alkali according to claim 1, characterized in that, The macro-microenvironment raw material pellets are formulated from the following raw materials in parts by weight based on the total dry weight of the raw materials: Fly ash: 10.0 to 20.0 parts by weight; Secondary aluminum ash: 3.0 to 8.0 parts by weight; Industrial waste alkali, monohydrate gibbsite bauxite and industrial limestone: balance the remaining mass to 100.0 parts by weight.

6. The method for preparing sodium aluminate based on alumina waste alkali according to claim 5, characterized in that, In the mixture of the raw materials, the total number of moles of calcium is equal to the sum of 2.0 to 2.2 times the total number of moles of silicon and 3.5 to 4.0 times the total number of moles of sulfur; Furthermore, the molar ratio of sodium to aluminum in the mixed system, excluding the bound silicon, is 1.0 to 1.

15.

7. The method for preparing sodium aluminate based on alumina waste alkali according to claim 5, characterized in that, The industrial waste alkali used in the raw materials contains sodium oxalate, and the mass fraction of sodium oxalate in the industrial waste alkali is 10.0% to 15.0%.

8. The method for preparing sodium aluminate based on alumina waste alkali according to claim 5, characterized in that, The macro-microenvironment raw material ball is a powder-formed particle, and the mass fraction of the powder inside the particle that passes through a 200-mesh standard sieve is 80.0% to 90.0%.

9. The method for preparing sodium aluminate based on alumina waste alkali according to claim 5, characterized in that, The moisture content of the macro-microenvironment raw material balls after drying is 0.5% to 1.5%.

10. The method for preparing sodium aluminate based on alumina waste alkali according to claim 5, characterized in that, The particle size of the macro-microenvironment raw material balls is 5 to 15 mm.

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