Basic Research On Application Of Wet Denitrification And Defluorination Process For Secondary Aluminum Dross

Two million tons of toxic aluminum dross are produced every year in the production and processing of electrolytic aluminum in china.Due to the lack of large-scale disposal technology,the historical storage has exceeded ten million tons.Because of the high content of toxic components such as aluminium nitride and fluoride,aluminium dross has been listed in the National Hazardous Waste List,and it is of great significance to make it harmless.According to the metal aluminium content and recovery times of aluminium dross,it is divided into primary aluminium dross and secondary aluminium dross(SAD).Primary aluminium dross has high aluminium content and obvious economic value.Slag recovered from aluminium metal by roasting ash method,tilting rotary kiln method and screening method has changed into SAD,in which aluminium nitride and fluoride are further enriched,posing a serious threat to the environment.Therefore,the key to the comprehensive treatment of aluminium ash is to realize harmless disposal of the harmful components caused by SAD poisoning.Acid and alkali medium are mainly used to activate SAD in the existing wet treatment methods.However,solid phase layers of aluminum nitride hydrolysate encapsulate unreacted aluminum nitride active sites,and micro/insoluble cryolite crystal is stable,resulting in incomplete hydrolysis of aluminum nitride and incomplete dissociation of fluoride,causing prominent environmental pressure.Aiming at the above problems,the key scientific issues of interphase migration transfer and enhanced dissociation regulation of toxic components in the process of SAD reaction-detoxification are studied in this thesis,via the method of theoretical analysis and experimental research.The process and mechanism of high efficiency hydrolysis of aluminum nitride and alkali leaching of fluoride in SAD are investigated to realize the deep detoxification of SAD.(1)Aluminum nitride was deeply removed by wet grinding pretreatment-alkali leaching process.The effects of Na OH concentration,reaction temperature,liquid-solid ratio and reaction time on the removal rate of aluminum nitride were investigated.The hydrolysis rate of aluminum nitride reached 94.67%at the wet grinding time of 15min,the hydrolysis temperature of 75?,the Na OH concentration of 5%,L/S of 2,and the hydrolysis time of 120 min.The wet grinding process enhances the decomposition of SAD and exposes the active sites of unreacted aluminium nitride particles,thus promoting their deep hydrolysis.The kinetics study showed that the hydrolysis of aluminum nitride conformed to the mixed control model.The kinetics equation was k_mt=[ln(1-x)]/3-1+(1-x)^-1/3 and the apparent activation energy was 31.00 k J/mol.(2)The effect of Na OH concentration,leaching temperature,liquid to solid ratio and leaching time on fluoride removal rate were investigated by alkali leaching process.At the alkali concentration of 10%,the leaching temperature of 80?,the L/S of 4 and the leaching time of 20 min,the removal rate of fluorine reached 78.29%as the highest.The kinetic study showed that the leaching of fluorine was in accordance with the mixed control model.The kinetic equation was k_mt=[ln(1-x)]/3-1(1-x)^-1/3 and the apparent activation energy was 39.64 k J/mol.After alkali leaching defluorination,the leaching toxicity of SAD reached the national standard.(3)The field-microwave-assisted defluorination leaching was explored on the basis of conventional leaching,and the effect of microwave power on the leaching rate of fluorine in SAD was discussed.The results showed that microwave-assisted leaching can effectively shorten the reaction time,and the removal rate of fluorine can reach the effect of conventional leaching for 20 min in 5 min.(4)After wet denitrification and defluorination,the alkali leaching solution contains a lot of fluorine ions,which must be solidified.The defluorination rate could reach 99.91%by using Ca O to precipitate fluoride ions in alkali liquor,and the alkali liquor could be recycled.