Method for enhanced denitrification, defluorination and resource utilization of secondary aluminum ash

Abstract

The invention discloses a method for enhanced denitrification, defluorination and resource utilization of secondary aluminum ash. According to the method, the secondary aluminum ash is subjected to enhanced denitrification and defluorination by adopting a leaching solution containing additional aluminum salts (such as aluminum chloride, polyaluminum chloride, aluminum sulfate and the like), and then is subjected to resource utilization. The core of the process lies in that passivation films on the surfaces of aluminum nitride particles are effectively corroded by utilizing in-situ acidification and complexation generated by aluminum salt hydrolysis, and the nitrogen removal kinetics hindering problem is solved; and meanwhile, the leaching efficiency of insoluble fluorides such as cryolite and calcium fluoride is improved by utilizing the strong complexing effect of aluminum ions and fluorine ions. Carrying out solid-liquid separation on the reacted slurry, adding alkali into the filtrate to precipitate fluorine so as to obtain alkaline aluminum fluoride precipitate, and carrying out evaporative crystallization to recover chlorine salt; and washing and drying the filter cake to obtain high-quality detoxified aluminum ash slag which is used as a raw material for producing aluminate, building materials or refractory materials. The method has the advantages of mild reaction, thorough detoxification, full-component recovery and the like, and resource utilization of the secondary aluminum ash is realized.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste treatment and resource utilization technology, specifically involving a wet process for efficient denitrification, defluorination and resource utilization of secondary aluminum ash. Background Technology

[0002] Secondary aluminum ash is the residue after primary aluminum ash has undergone aluminum extraction through mechanical and physical methods. It mainly contains alumina, aluminum nitride, metallic aluminum, and chlorides (NaCl, KCl), fluorides (NaF, MgF2, CaF2, Na3AlF6), etc. With the rapid development of my country's aluminum industry, the annual output of secondary aluminum ash has reached millions of tons. From an environmental safety perspective, secondary aluminum ash is hazardous waste, its harmfulness mainly stemming from its high chemical activity and complex composition: the aluminum nitride it contains releases ammonia gas with a strong pungent odor and toxicity when it comes into contact with water, causing air pollution; the fluorides, chlorides, and other components, if directly dumped without treatment, are easily leached into the soil and groundwater systems by rainwater, leading to serious pollution. However, from a resource perspective, secondary aluminum ash contains 40%–70% alumina and is rich in metallic aluminum, magnesium, silicon, and other elements, making it a secondary resource with significant utilization value. Achieving deep detoxification and full utilization of all components of secondary aluminum ash is not only key to solving environmental bottlenecks but also an inevitable choice for implementing a circular economy and achieving sustainable development of the aluminum industry.

[0003] Currently, wet processing is the mainstream method for treating secondary aluminum ash. Patent CN103060566A discloses a method for recovering alumina, chloride salts, and fluoride salts from aluminum ash, employing an aqueous solution leaching process to treat the aluminum ash slag. By mixing aluminum ash with water and stirring at a certain temperature, soluble salts such as sodium chloride, potassium chloride, and sodium fluoride in the aluminum ash slag are dissolved and removed. Denitrification is achieved through the hydrolysis reaction of aluminum nitride (AlN + 3H2O = Al(OH)3 + NH3). Although this reaction is thermodynamically spontaneous, its kinetics are severely limited. The hydrolysis process of aluminum nitride can be described by a typical "unreacted core model": In the initial stage of the reaction, water molecules undergo interfacial hydrolysis on the surface of AlN particles. The generated Al(OH)3 has extremely low solubility under neutral or weakly alkaline conditions, making it difficult to migrate into the liquid phase. Simultaneously, because the molar volume of Al(OH)3 is significantly larger than that of AlN, the interfacial reaction is accompanied by significant volume expansion, causing the generated aluminum hydroxide to continuously accumulate on the particle surface and undergo pore compaction, rapidly forming a continuous, low-porosity, and low-permeability product film. This aluminum hydroxide passivation film significantly hinders the diffusion of water molecules into the unreacted core, changing the reaction from being controlled by surface chemical reactions to being controlled by product layer diffusion. This leads to a sharp drop in the hydrolysis rate in the later stages, ultimately resulting in incomplete denitrification.

[0004] Secondly, regarding the removal of fluorides, secondary aluminum ash typically contains a large amount of structurally stable fluoride salts in the form of cryolite (Na3AlF6) and calcium fluoride (CaF2). These fluorides have low solubility products and high chemical stability, and hardly dissolve under conventional water immersion conditions, resulting in low defluorination rates in traditional water washing processes. To address this issue, patent CN201910636850.3 proposes using strong alkali leaching to dissolve sparingly soluble fluoride salts after water washing to remove chloride salts, and further adjusting the solution pH to neutral by adding dilute acid to precipitate and recover fluoride salts. However, this technical route still has significant shortcomings in practical industrial applications: on the one hand, the "strong alkali leaching-dilute acid neutralization" operation results in high consumption of acid and alkali reagents and high processing costs; on the other hand, the alternating effects of strong alkali and dilute acid conditions place high demands on the corrosion resistance of the reaction vessel, pipelines, and solid-liquid separation equipment, significantly increasing equipment investment and maintenance costs. In addition, to improve the fluoride leaching efficiency, existing processes usually require high-temperature operation and multiple chemical addition and pH adjustment steps, resulting in lengthy processes, high energy consumption, and complex processes, making it difficult to achieve economical and efficient industrial applications.

[0005] Therefore, developing a new wet process with mild reaction conditions, short process flow, and the ability to simultaneously achieve deep removal of nitrogen and fluorine has become a key problem that urgently needs to be solved in the field of secondary aluminum ash resource utilization. Summary of the Invention

[0006] The purpose of this invention is to provide a secondary aluminum ash enhanced denitrification, defluorination and resource utilization process, which aims to solve the problems of incomplete aluminum nitride hydrolysis and low removal rate of sparingly soluble fluorides in existing wet processes.

[0007] The technical solution of this invention is as follows:

[0008] A method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash, characterized by comprising the following steps: S1: Raw material pretreatment: The secondary aluminum ash is crushed, ground and sieved in sequence to obtain secondary aluminum ash powder with a particle size distribution within a set range; S2: Leaching reaction: The aluminum ash powder is added to a leaching solution containing added aluminum salt, and a leaching reaction is carried out under set temperature and stirring conditions; wherein, the added aluminum salt is selected from one or a combination of aluminum chloride, polyaluminum chloride, and hydrated aluminum sulfate; after the reaction is completed, solid-liquid separation is performed to obtain a primary filtrate and a primary filter cake, respectively. S3: Filtrate precipitation and defluorination: The primary filtrate is heated to a preset temperature, and a pH adjuster is added to adjust the pH value of the solution to a preset range, so that the aluminum-fluorine complex ions in the solution precipitate out in the form of basic aluminum fluoride. After solid-liquid separation, secondary filter residue and secondary filtrate are obtained. S4: Evaporation and crystallization of filtrate: The secondary filtrate is subjected to evaporation and crystallization treatment, and the crystallized chloride salts are collected; the condensate generated during evaporation is reused for the preparation of the leachate in step S2; S5: Resource utilization: The primary filter cake obtained in step S2 is washed and dried to obtain detoxified aluminum ash residue; the secondary filter residue obtained in step S3 is washed and dried and used as raw material for the preparation of aluminum fluoride.

[0009] Furthermore, the sieve mesh number used in the sieving process in step S1 is ≥50 mesh; Furthermore, in step S2, the mass percentage concentration of aluminum salt in the leachate is 1% to 50%, preferably 5% to 20%; Furthermore, the liquid-to-solid ratio of the leaching reaction in step S2 is 3:1 to 6:1; Furthermore, the leaching temperature in step S2 is 25℃~90℃, preferably 60℃~85℃; the leaching reaction time is 1~10 hours. Furthermore, the heating temperature of the primary filtrate in step S3 is 25℃~90℃; Furthermore, the pH adjuster mentioned in step S3 is selected from one or more of NaOH, Na2CO3, and NH4OH solutions; Furthermore, in step S3, the pH value of the solution is adjusted within the range of 3 to 8, preferably within the range of 4 to 6; Furthermore, the evaporation temperature of the secondary filtrate in step S4 is 100~120℃, and the evaporation time is 4~8 hours; Furthermore, the main phase of the detoxified aluminum ash slag described in step S5 is alumina, which can be used as a raw material for the production of aluminates, building materials or refractory materials. The beneficial effects of this invention are:

[0010] A method for enhanced denitrification, defluorination, and resource utilization of aluminum ash using a secondary process involves introducing exogenous aluminum salts into the wet leaching system of aluminum ash, utilizing the unique chemical properties of aluminum salts to achieve "dual enhancement." 1) Disrupt the interfacial passivation film to enhance hydrolysis and denitrification (specifically for denitrification): To address the issue of dense aluminum hydroxide passivation film formation on the surface during aluminum nitride hydrolysis, aluminum chloride is added. Aluminum chloride exhibits a strong tendency to hydrolyze in aqueous solution, and its hydrolysis process continuously releases H₂. + This adjusts the reaction system from a neutral or weakly alkaline environment to a weakly acidic one, significantly increasing the interfacial solubility of aluminum hydroxide thermodynamically. Simultaneously, the introduced Cl⁻ in the system affects the solubility of Al. 3+ It has a certain complexing effect (AlCl) x 3−xThis can lower the energy barrier for the dissociation and migration of aluminum species from the aluminum hydroxide lattice, thereby inhibiting the continuous deposition and densification of aluminum hydroxide on the particle surface. Under these conditions, the pore compaction effect induced by volume expansion at the spatially confined interface can be significantly weakened, making it easy for local dissolution, defect accumulation, or non-uniform coverage to occur, and making it difficult for the product layer to form a continuous, low-porosity, and complete covering structure.

[0011] 2) Promotes fluoride dissolution and enhances leaching for defluorination (specifically for defluorination): For fluorides present in secondary aluminum ash in the form of CaF2, Na3AlF6, etc., whose solubility is limited under conventional leaching conditions, the addition of aluminum chloride can significantly enhance their leaching behavior. Aluminum chloride dissolves and hydrolyzes in aqueous solution to produce a high concentration of Al2+. 3+ Its relationship with F - It has a very strong affinity and can form a stable aluminum-fluorine complex (AlF) with the fluoride ions released from the dissociation of fluoride in solution. x 3-x This significantly reduces the activity of free F⁻ in the solution, thereby continuously promoting the dissolution reaction of fluorides such as CaF₂ and Na₃AlF₆, thus significantly increasing the leaching rate and removal degree of fluoride. Detailed Implementation Example 1

[0012] First, the secondary aluminum ash is mechanically crushed and ground. The ground material is then sieved through a 100-mesh sieve. The aluminum enriched in the material on the sieve is removed, and the material under the sieve is used as the raw material for subsequent leaching.

[0013] 1 kg of pretreated aluminum ash powder was added to a reaction vessel, along with 4 L of 10% aluminum chloride solution, maintaining a liquid-to-solid ratio of 4:1. The system was heated under continuous stirring, and the reaction was maintained at 80°C for 6 h.

[0014] After the reaction was completed, the reaction slurry was subjected to solid-liquid separation to obtain a primary filter cake and a primary filtrate. The obtained primary filtrate was heated to 60°C, and a 10% sodium hydroxide solution was slowly added dropwise under stirring to adjust the pH of the solution to 6.0. After stirring for 2 hours, the solution was allowed to stand, allowing fluorine in the solution to precipitate out as basic aluminum fluoride. After standing, the solution was filtered to obtain a secondary filter residue and a secondary filtrate, wherein the secondary filter residue was basic aluminum fluoride.

[0015] The resulting secondary filtrate was placed in an evaporation device and subjected to evaporation and crystallization treatment at 110°C for 6 hours. The condensate generated during the evaporation process was reused in the aforementioned leaching process to achieve water resource recycling.

[0016] Compositional analysis of the treated solid products showed that the residual aluminum nitride content in the primary filter cake was 0.18%, and the residual fluorine content was 0.12%. Example 2

[0017] First, the secondary aluminum ash is crushed and then sieved through a 150-mesh sieve. The material passing through the sieve is used as a raw material for subsequent reactions, while the coarse particles on the sieve are discarded or disposed of separately.

[0018] Take 1 kg of sieved aluminum ash powder and add it to the reactor. Add 5 L of 15% polyaluminum chloride leaching solution to control the liquid-solid ratio at 5:1. Heat the reaction system under continuous stirring. When the temperature reaches 90℃, maintain the temperature for 4 h.

[0019] After the reaction, the resulting slurry was subjected to solid-liquid separation to obtain a primary filter cake and a primary filtrate. The primary filtrate was heated to 70°C, and sodium carbonate solution was slowly added under stirring to adjust the pH of the solution to 5.5, causing fluorine in the solution to precipitate out as aluminum fluoride. After complete precipitation, the solution was filtered to separate the liquid and solid phases.

[0020] The resulting primary filter cake was washed with deionized water until neutral and then dried. Composition analysis showed a significantly increased alumina content and a substantial decrease in nitrogen and fluorine impurities, making it suitable for resource utilization as aggregate in high-alumina refractory materials.

[0021] Analysis of the treatment effect showed that the residual aluminum nitride content in the treated solid was 0.25% and the residual fluorine content was 0.15%. Example 3

[0022] First, the secondary aluminum ash is crushed and then sieved through a 150-mesh sieve. The material passing through the sieve is used as raw material for the subsequent leaching reaction.

[0023] Take 1 kg of sieved aluminum ash powder and add it to the reactor. Add 3 L of 20% aluminum sulfate solution to control the liquid-solid ratio at 3:1. Heat the reaction system under continuous stirring. When the temperature reaches 50℃, maintain the temperature for 8 h.

[0024] After the reaction was completed, the reaction slurry was subjected to solid-liquid separation to obtain a primary filter cake and a primary filtrate. The primary filtrate was kept at room temperature (approximately 25°C), and ammonia was slowly added under stirring to adjust the pH of the solution to 6.5, causing fluoride in the solution to precipitate out as basic aluminum fluoride. After complete precipitation, the solution was filtered to obtain a fluoride-containing precipitate.

[0025] Compositional analysis of the treated solid products showed that the residual aluminum nitride in the filter cake decreased to 0.15%, and the residual fluorine was 0.18%. Example 4

[0026] First, the secondary aluminum ash is simply crushed and then sieved through a 100-mesh sieve. The material passing through the sieve is used as the raw material for the subsequent leaching reaction.

[0027] Take 1 kg of sieved aluminum ash and add it to the reactor. Add 3 L of 45% aluminum chloride solution to control the liquid-solid ratio at 3:1. Heat the reaction system under rapid stirring. When the temperature reaches 85 °C, maintain the temperature for 1.5 h.

[0028] After the reaction is completed, the reaction slurry is subjected to solid-liquid separation, and the pH of the resulting filtrate is adjusted to 7.0 so that the fluoride in the solution precipitates out in the form of aluminum fluoride, thus achieving simultaneous fluoride removal.

[0029] Compositional analysis of the treated solid products showed that the residual aluminum nitride in the filter cake decreased to 0.25%, and the residual fluorine was 0.17%.

[0030] Comparative Example 1

[0031] Take 1 kg of the same batch of secondary aluminum ash as in Example 1, add 4 L of pure water directly, control the liquid-solid ratio to be 4:1, and do not add any aluminum salts or other chemical additives to the system.

[0032] The reaction system was heated to 80°C under continuous stirring and maintained at this temperature for 6 hours, with other operating conditions remaining consistent with Example 1. After the reaction, the reaction slurry was subjected to solid-liquid separation to obtain a primary filter cake and a primary filtrate. The compositional analysis of the obtained primary filter cake showed that the solid contained up to 4.2% aluminum nitride and 2.6% fluorine.

[0033] Therefore, it is evident that without the introduction of aluminum salts, the hydrolysis reaction of aluminum nitride in secondary aluminum ash is incomplete. Simultaneously, fluorides are difficult to dissolve under conventional water immersion conditions, resulting in low defluorination efficiency. This comparative example fully demonstrates the necessity and significant technical effect of this invention in synergistically enhancing denitrification and defluorination by introducing aluminum salts.

Claims

1. A method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash, characterized in that, Includes the following steps: S1: Raw material pretreatment: The secondary aluminum ash is crushed, ground and sieved in sequence to obtain secondary aluminum ash powder with a particle size distribution within a set range; S2: Leaching reaction: The aluminum ash powder is added to a leaching solution containing added aluminum salt, and a leaching reaction is carried out under set temperature and stirring conditions; wherein, the added aluminum salt is selected from one or a combination of aluminum chloride, polyaluminum chloride, and hydrated aluminum sulfate; after the reaction is completed, solid-liquid separation is performed to obtain a primary filtrate and a primary filter cake, respectively. S3: Filtrate precipitation and defluorination: The primary filtrate is heated to a preset temperature, and a pH adjuster is added to adjust the pH value of the solution to a preset range, so that the aluminum-fluorine complex ions in the solution precipitate out in the form of basic aluminum fluoride. After solid-liquid separation, secondary filter residue and secondary filtrate are obtained. S4: Evaporation and crystallization of filtrate: The secondary filtrate is subjected to evaporation and crystallization treatment, and the crystallized chloride salts are collected; the condensate generated during evaporation is reused for the preparation of the leachate in step S2; S5: Resource utilization: The primary filter cake obtained in step S2 is washed and dried to obtain detoxified aluminum ash residue; the secondary filter residue obtained in step S3 is washed and dried and used as raw material for the preparation of aluminum fluoride.

2. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: The sieve mesh number used in the sieving process in step S1 is ≥50 mesh.

3. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: In step S2, the mass percentage concentration of aluminum salt in the leachate is 1% to 50%, preferably 5% to 20%.

4. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: The liquid-to-solid ratio of the leaching reaction in step S2 is 3:1 to 6:

1.

5. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: The leaching temperature in step S2 is 25℃~90℃, preferably 60℃~85℃; the leaching reaction time is 1~10 hours.

6. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: The heating temperature of the primary filtrate in step S3 is 25℃~90℃.

7. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: The pH adjuster mentioned in step S3 is selected from one or more of NaOH, Na2CO3, and NH4OH solutions.

8. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: In step S3, the pH value of the solution is adjusted within the range of 3 to 8, with a preferred range of 4 to 6.

9. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: The evaporation temperature of the secondary filtrate in step S4 is 100~120℃, and the evaporation time is 4~8 hours.

10. The method for enhanced denitrification, defluorination, and resource utilization of secondary aluminum ash according to claim 1, characterized in that: The main phase of the detoxified aluminum ash slag described in step S5 is alumina, which can be used as a raw material for the production of aluminates, building materials or refractory materials.