Study On Separation And Extraction Of Valuable Components In Clean Disposal Of Spent Carbon Cathodes From Aluminum Electrolysis

As the largest and continuously discharged hazardous solid waste from the aluminum electrolysis industry,spent carbon cathodes pose a serious threat to the ecological environment and are rich in high-quality graphite carbon and high-value fluoride salt.However,the current disposal technology has disadvantages:high energy consumption complex process,incomplete separation,and serious secondary pollution risk.Therefore,this paper focuses on the safe disposal and resource recycling of spent carbon cathodes and constructs a treatment process based on graphite carbon resource extraction,efficient dissociation of toxic substances,and recovery of non-carbon valuable elements.In addition,the comprehensive utilization path of recovered carbon to prepare carbon anode for the aluminum electrolysis was also explored.The main research results are as follows:（1）The effects of formation conditions on mineralogical characteristics and safe utilization properties of spent carbon cathodes from four different sources were studied.Results show that the spent carbon cathodes from different sources have similar phase composition,element types,element distribution law and thermal behavior.The formation conditions（service time,operating conditions,and shutdown reasons）are highly related to the corrosion and penetration degree of electrolyte components into cathode blocks.With the prolongation of service time,the content of electrolyte components of spent carbon cathodes gradually increased,the graphitization degree decreased,and the leaching toxicity tended to be significant.In addition,the stable coexistence of insoluble aluminosilicates and carbonaceous materials seriously hinders the separation and purification of graphite carbon from spent carbon cathodes.（2）The optimal parameters and strengthening mechanism of mechanical activation-assisted molten salt roasting treatment of spent carbon cathodes were explored.Results show that with the increase of mechanical activation conditions,the particle size of the mixture decreases,the specific surface area increases,the crystal structure tends to be amorphized,and the thermal stability decreases.These changes are beneficial to improve the degree of physical separation of non-carbon phases from spent carbon cathodes,the uniformity of the mixture,and the conversion efficiency of sodium carbonate to sodium oxide with high reactivity.Thus,the efficient separation of carbonaceous materials and inorganic salt components is promoted.However,excessive mechanical activation is beneficial for the formation of large-scale aggregates,and this phenomenon weakens the strengthening effect of mechanical activation on molten salt roasting treatment.The optimal mechanical activation conditions:the grinding combination is polyurethane tank with agate balls,the milling speed is 250r·min^-1,the mass ratio of balls to material is 6:1,and the activation time is 4h.（3）The preliminary technical route of the molten salt roasting-water leaching process for the treatment of spent carbon cathodes was constructed by coupling and matching mechanical activation,molten salt roasting,water leaching,carbonation decomposition,evaporative crystallization and multi-stage filtration.Under the parameters of alkali-to-material mass ratio of 1.25:1,roasting temperature of 950℃,reaction time of 4 h,heating rate of 3.75℃·min^-1,leaching temperature of 60℃,leaching time of 60 min and the liquid-solid ratio of 14:1,the recovered carbon purity was 95.41wt.%,the fluoride leaching rate was 96.63%,and the decomposition rate of cyanide was 99.12%.Based on the solution equilibrium calculation and the principle of carbonation decomposition,the cryolite product was successfully recovered from the water leaching solution by using the exhaust gas with a high concentration of CO/CO₂,and the separation and recovery of sodium fluoride and sodium carbonate were realized.（4）The recovered carbon was deeply purified via Na OH-Na₂CO₃ mixed molten salt roasting combined with hydrothermal acid leaching.The principle process of spent carbon cathode treatment was established.Under the optimal mixed molten salt roasting parameters,the recovered carbon purity and the fluoride leaching rate increased to 96.98 wt.%and 97.35%.The leaching concentrations of F^-and CN^-were 7.58 mg·L^-1and 0.12mg·L^-1,respectively.The decomposition,transformation and dissolution removal of complex insoluble substances such as Ca₄Si₂O₇F₂ and Na Ca₂FSi O₄ are realized through structural activation combined with hydrothermal acid leaching.Under the optimum hydrothermal acid leaching conditions,the carbon content of purified recovered carbon reached 99.63 wt.%.The principle molten salt roasting-water leaching process has a good treatment effect and raw material compatibility for spent carbon cathodes from different sources.（5）The raw material characteristics,blending conditions and impurity catalytic mechanism of spent carbon cathodes and recovered carbon to prepare carbon anodes were studied.Results show significant differences in raw material properties between spent carbon cathode and its recovered carbon compared with calcined petroleum coke.With the increase of the blending ratio,the performance of the carbon anode gradually decreased.Under the same blending ratio,the blending effect of purified graphite carbon is better than that of other blended carbon materials.The ash content,bulk density,compressive strength,resistivity and CO₂/air reactivity of carbon anode prepared by purified graphite carbon at a low mixing ratio（≤4 wt.%）are better than those of full petroleum coke anode.Based on molecular dynamics simulation and experimental verification,the reactivity of carbon anode is catalyzed or inhibited by Na and Ca elements through accelerating/decelerating the migration process of oxidizing gas to carbon anode matrix.