| China is the world’s largest producer and consumer of primary and recycled aluminum,generating millions of tons of solid hazardous waste aluminum ash every year.The complex composition of aluminum ash poses great difficulty in disposal and utilization,and most of the current technological processes are only at the laboratory research stage.Aluminum ash has the dual characteristics of hazardous waste and valuable resources,and improper handling not only causes environmental hazards but also wastes precious resources.The harmless disposal and high-value resource utilization of aluminum ash have become major technical requirements for the sustainable development of China’s aluminum industry.Based on the analysis of the existence state and physicochemical properties of valuable and toxic substances in aluminum ash generated in different processes,this paper proposes innovative ideas for the resource utilization of aluminum electrolytic ash(AEA)and aluminum alloy recycling ash(ARA),respectively.The most valuable element to recover from aluminum ash is Al,with a content of 35%to 45%.The main aluminum-containing phases include alumina(α-Al2O3,γAl2O3,and β-Al2O3),metallic aluminum,AlN,and MgAl2O4.The minerals of Si,Fe,Mg mainly exist in the form of quartz,hematite,and magnesium-aluminum spinel.The elements of Na,K,Ca mainly exist in the form of chloride(NaCl and KCl)and fluoride(NaF,KF,Na3AlF6,and CaF2).Aluminum ash is characterized as a hazardous waste with reactivity and leaching toxicity,and the main toxic components are AlN and soluble fluoride(NaF and KF).The AEA has a high content of Al2O3 and fluoride,and almost no alloy elements.It is the best way to achieve short-process and high-value resource utilization by returning it to the aluminum electrolysis process for recycling.However,the ARA contain a relatively high amount of chloride and small amounts of alloying elements such as Cu,Zn,Mn,and Ti.They are not suitable for returning to the aluminum electrolysis process for producing primary aluminum,and new methods need to be explored for removing harmful substances and extracting valuable components.For the resource utilization of AEA,this paper innovatively designs a technological route of AEA strengthening denitrification,phase transition,preparation of anode protection rings,protection of anode steel stubs,and return to the electrolysis bath as covering material after crushing for recycling.The key to the harmless treatment and resource utilization of AEA is the removal of AlN.By strengthening hydrolysis and phase transition,AlN can be selectively converted into active Al2O3 required in the electrolysis process.The thermodynamic properties and kinetic behavior of AlN hydrolysis in AEA has been investigated in this paper.And the influence of various factors on the removal rate of AlN in AEA has been examined,which reveals the reaction mechanism of alkaline additives in strengthening denitrification,explores the phase transition path of denitrified AEA during calcination,and clarifying the activation mechanism of denitrified AEA.The experimental results show that temperature and alkaline additives are key factors affecting the removal rate of AlN.The hydrolysis process of AlN in AEA follows the unreacted core model with diffusion control.The addition of alkaline additives can erode and destroy the product layer of Al(OH)3,exposing the surface of AlN again.Under the conditions of strengthened denitrification,the removal rate of AlN can reach 96.24%.The phase transition path of denitrified AEA is Bayerite(?)Boelmite(?)γ-Al2O(?)α-Al2O3.Controlling the temperature between 600℃ to 800℃ can direct the Al(OH)3 in denitrified AEA to transform into active γ-Al2O3 that is soluble in the aluminum electrolyte.Based on the Tyndall effect and high-speed photography,the dissolution behavior of denitrified phase-transformed AEA in the electrolyte system was studied using transparent observation,and it was proven that AlN is the key factor affecting the solubility of AEA.Combined with the characteristics of severe corrosion of the anode steel stubs and the temperature near the steel stub being about 650℃ during the aluminum electrolysis production,this study innovatively proposes to make the denitrified AEA into the protection rings of anode steel stubs.Firstly,the residual heat of the aluminum electrolysis bath can be used to completely remove the crystallization water from the Al(OH)3 and convert it into active γ-alumina.Secondly,it can protect the anode steel stubs from electrolyte corrosion and extend the service life of the steel stubs.A set of equipment for preparing protection rings of anode steel stubs from AEA was designed,and industrial experiments were carried out to verify the protective effect of AEA protection rings on the anode steel stubs,as well as the impact of adding denitrified phase-transformed AEA on the Fe and Si impurity content of aluminum liquid in the returned electrolysis bath.The experimental results show that the dissolution rate of denitrified phase-transformed AEA in the electrolyte system is significantly improved.At 950℃,with a molecular ratio of 2.5,in the Na3AlF6-AlF3-CaF2-MgF2-Al2O3 electrolyte system,the average dissolution rate was increased by 7.33 times compared to the original AEA.In the industrial experiments of three anode cycles,the average corrosion rate of the steel stubs was slowed down by 67.7%,and the service life of the steel stubs was extended by about three times,significantly reducing the maintenance cost of the steel stubs.In the 5-month industrial comparative experiments,due to the slowing down of the corrosion of the steel stubs and the impurities falling into the electrolysis bath,the average Fe content in the aluminum liquid decreased by about 18.5%,and the average Si content decreased by about 4.7%,effectively improving the quality of the aluminum liquid.This technology fully recovers Al,O,F,Na,and N elements from AEA,and the hydrolysis of AlN yields ammonia for recycling,while alumina and fluoride all enter the electrolysis for recycling with the waste AEA protection ring.Long-term industrial application has shown that the consumption of alumina per ton of aluminum is reduced by 8-10 kg,achieving the shortprocess and high-value utilization and industrial application of AEA.In addition,a technology route for detoxification and Al element extraction by dry-pressing,roasting and preparing sodium aluminate(NaAlO2)or calcium aluminate(C12A7)clinkers in a coordinated manner was innovatively developed for the resource utilization of ARA.The phase transformation and toxic element migration behavior in the ARA-Na2O roasting system,the ARA-Na2O-CaO roasting system,the ARA-CaO roasting system,and the ARA-CaO alkali-doped roasting system were systematically studied,as well as the optimal leaching system and kinetics for different clinkers and the mechanism of alkaline-doping activation of C12A7 clinker.By constructing the ARA-Na2O-CaO roasting system and defining alkalinization coefficient of Al extraction(n(N/AF)=n(Na)/n(Al)+n(Fe))and calcification coefficient of impurity removal(n(CF/S)=n(Ca)/2n(Si)+0.5n(F)),the ratio of each component in the roasting system can be effectively controlled to generate soluble NaAlO2 through the directional reaction of the Al-containing component with the alkaline additive,and the extracted Al source can be used to prepare high-purity Al(OH)3.The impurity components,such as Si,Fe,and Mg,are directed to react with the alkaline additive to generate insoluble Ca2SiO4,Fe(OH)3,and MgO,which enter the residue phase.The impurity alloy elements,such as Mn,Ni,and Cu,have not entered the solution and are all enriched in the leaching residue.Although Zn and Cr elements have entered the liquid phase in small amounts,they are far below the hazardous waste standard limit.During the roasting process,the toxic components in the ARA are efficiently removed.AlN is oxidized and released into harmless nitrogen gas,and the chloride is volatilized and recovered in the flue gas,while the soluble fluoride is solidified into harmless CaF2.Under the optimal roasting conditions,the dissolution rates of Al and Na elements are both above 95%,and the removal rates of N,Cl,and soluble F elements are all above 95%.Furthermore,by constructing the ARA-CaO alkali-doped calcination system and defining the calcification coefficient of Al extraction and impurity removal(n(C/R)=n(Ca)/7/6n(Al)+2n(Si)+0.5n(F)+n(Ti)and alkali doping coefficlent(m(N/CA)=m(Na2CO3)/m(C12A7),the ratio of each component in the roasting system can be effectively controlled to generate C12A7 that is soluble in Na2Oc solution through the directional reaction of the aluminum-containing component with the calcium additive,and the alkaline additive doping enhances the C12A7 leaching performance.The extracted Al source can be used to prepare high-purity Al(OH)3,while impurity components such as Si,Mg,and Ti enter the residue as insoluble Ca2SiO4,MgO,and CaTiO3.The alkaline doping enhances the crystal structure activity and free energy of C12A7 clinker,reducing the activation energy of leaching from 18.05 kJ·mol-1 to 15.50 kJ·mol-1.The leaching temperature and Na2Oc concentration are key factors affecting the leaching performance of Ca12A7 clinker.Excess Na2Oc must be maintained in the leaching solution to ensure that the hydrolysis product of C12A7 is CaCO3 and to avoid the generation of hydrated calcium aluminate.Under the optimal calcination conditions,the dissolution rate of Al element is above 94%,and the removal rates of N,Cl,and soluble F elements are all above 99%.In summary,this study proposed an innovative approach for the efficient detoxification and Al extraction of aluminum ash.For AEA,the study developed a technique of strengthening denitrification,phase transition,and preparation of anode protection rings,protection of anode steel stubs,and return to the electrolysis bath as covering material after crushing for recycling.This approach not only fully recovers the Al,O,F,Na,and N elements from AEA,but also effectively reduces the corrosion rate of the steel stubs and improves the quality of the aluminum liquid.For the complex composition of ARA,the study developed a dry-pressing and roasting method for the preparation of NaAlO2 and C12A7 clinkers,which facilitates the detoxification and Al extraction of the ARA.Both techniques achieve clean and efficient resource utilization of aluminum ash,with significant socioeconomic benefits.This study provides a green,clean,efficient,energy-saving,and scalable implementation approach for the harmless disposal and resource utilization of aluminum ash in China. |
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