Research Progress of Secondary Aluminum Dross Resource Utilization
Secondary aluminum drossis produced by extracting aluminum from slag produced by aluminum smelting,which is com-posed of aluminum,aluminum oxide,aluminum nitride and salt refining agent.Secondary aluminum dross is classified as HW48 haz-ardous waste because of its reactivity and toxicity,and its harmless disposal and resource utilization are urgent problems.In this pa-per,the physical and chemical properties,environmental hazards and resource recovery technology of secondary aluminum dross were reviewed systematically.The development direction and industrialization of secondary aluminum dross were put forward.Secondary alu-minum dross had the dual characteristics of resource and pollution.On the one hand,secondary aluminum dross contained 40％～70％alumina,which as a valuable secondary resource.On the other hand,secondary aluminum dross contained toxic and dangerous com-pounds,such as fluoride,cyanide,Al4C3,AlN and heavy metals.When AlN,Al4C3 and other active components were exposed to the air,they reacted with water and release NH3,CH4,H2S and other toxic and explosive gases,heavy metals and toxic ions were easy to be released into the soil during the stacking process,damaging the ecological environment.At present,the disposal technology of sec-ondary aluminum dross could be divided into hydrometallurgy and pyrometallurgy.In the hydrometallurgy process,active alumina was the main product.After leaching,purification,precipitation,calcination,active alumina was manufactured from secondary aluminum dross.According to the different of leaching media,the hydrometallurgy process was divided into acid leaching and alkali leaching.Compared with alkali leaching,the acid leaching had a high leaching rate of Al3+,but there were many impurities in the filtrate.How-ever,regardless of acid leaching or alkali leaching,the leaching rate of Al34 was affected by the concentration of leaching medium,leaching temperature,leaching time and solid-liquid ratio.In addition,AlN and salts in secondary aluminum dross could restrict the leaching process.Therefore,the washing process was carried out to remove AlN and salts.However,NH3 would be released from AlN hydrolysis when secondary aluminum dross came into touch with water,and NH3 could cause air pollution.In addition,a lot of salts containing wastewater would be discharged after washing process.Therefore,the hydrometallurgy process required a large amount of acid or alkali,and the recovery of harmful gases such as NH3 and the disposal of filtrate were also considered,which made the process complicated.Compared with the hydrometallurgy process,the pyrometallurgy process was a non-polluting method to dispose of second-ary aluminum dross,AlN in secondary aluminum dross was converted to Al2O3 and N2 after calcination,meanwhile,the salt refining agent was evaporated,achieving zero emissions.In the pyrometallurgy process,building materials and refractories were the main prod-ucts.Secondary aluminum dross could replace part of cement in the production of concrete,but the addition of secondary aluminum dross was low and the consumption of secondary aluminum dross was small.The preparation of glass ceramics was one of the common methods for the reuse of solid waste.However,due to the oxidation reaction of AlN in the secondary aluminum dross during sintering,the surface of AlN was covered with Al2O3,which hindered the subsequent reaction,making it difficult to completely remove AlN,which would lead to large and uneven phase separation of the sample and reduce the performance of the product.At the same time,the high content of Al2O3 in the secondary aluminum dross increased the melting point of the mixture system,resulting in high energy con-sumption.Aluminate cement had the advantages of fast setting and hardening speed and high initial strength,which was mainly used in projects with urgent construction period,such as national defense,roads and special emergency repair projects.Due to the large market of aluminate cement and the high demand for alumina,the prospect of preparing aluminate cement with secondary aluminum ash was broad.At the same time,in the calcination process of aluminate cement,the self-heating characteristics of secondary alumi-num dross could be used,that was,after igniting the secondary aluminum ash,no additional heat source was required,which could reduce the production energy consumption.However,the national standard GB/T 201-2015 had certain limits on the content of SiO2,Fe2O3 and alkali of aluminate cement.But SiO2 and Fe2O3 in secondary aluminum dross could not be removed by calcination.Based on the conductivity difference of the mixture of metal aluminum and oxide salt,alumina and metal aluminum in secondary aluminum ash could be enriched through three stages:preliminary separation,pulse charging enhancement and secondary concentration.The electro-static separation process had certain advantages in terms of economic and environmental costs compared with hydrometallurgy and py-rometallurgy processes,but there were problems such as low enrichment rate and incomplete separation of impurities such as salt.Fi-nally,challenges and opportunities of resource reuse of secondary aluminum ash were proposed and discussed.On the premise of large-scale treatment of secondary aluminum ash,high-value utilization should also be considered.
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