Research On The Mechanism Of Anode Carbon Slag Combustion And The Initial Crystallization Temperature Of Enriched Electrolyte

Anode carbon residue is an industrial hazardous waste that is produced during the aluminum electrolysis process.It is formed when pre-baked anode materials are exposed to high temperatures for a long time and fall into the electrolyte system.It contains approximately 80%electrolyte and 20%carbonaceous material.Recycling and reusing anode carbon residue can provide high economic and environmental benefits.However,the high-temperature combustion characteristics and kinetics of anode carbon residue in the disposal process of domestic enterprises using pyrometallurgical smelting technology are not clear.This study investigates the optimal process conditions for recovering electrolytes by high-temperature combustion of anode carbon residue and clarifies the combustion characteristics,mechanism,and kinetic conditions of anode carbon residue in the air using TG-DSC.It also reveals the impact of component changes on the initial crystallization temperature and phase of the electrolyte system through electrolyte enrichment and conditioning research,providing theoretical guidance and data support for enterprise production.The main results of this paper are as follows:(1)Research on the physicochemical properties of anode carbon slag shows that carbonaceous materials exist in the form of electrolyte encapsulation,which is not conducive to the combustion of anode carbon slag.Volatility studies on Na3AlF6,AlF3,and NaF show that their volatility performance is in the order of AlF3>NaF>Na3AlF6.At temperatures above 900℃,severe corrosion of the reaction vessel occurs,and impurities of silicates are introduced into the slag.(2)Study on the combustion characteristics,kinetic conditions,and ignition mechanism of anode carbon residue.The TG and DTG results of anode carbon residue at different heating rates show that with the increase of heating rate,the weight loss of anode carbon residue gradually decreases,and the temperature corresponding to the maximum weight loss increases.The dynamic function models of combustion process in each stage were calculated by CoatsRedfern integral method.The first stage of combustion temperature(476-713℃)is the combustion of surface carbon materials of anode carbon residue.The second stage of combustion temperature(713-886℃)is the partial combustion of electrolyte crystal transformation,volatilization,and partially encapsulated carbon materials.The calculation results are consistent with the chemical reaction function model g(α)=(1-α)-1/2.The third stage of combustion temperature(886-1100℃)is the improvement of electrolyte volatilization performance and the complete combustion of carbon materials.The calculation results are consistent with the one-dimensional diffusion function model g(α)=α2.The ignition mode of anode carbon residue is heterogeneous ignition,and its ignition mechanism is that the surface component is not synchronized with the particle as a whole.The ignition temperature depends on the ignition temperature of the active carbon material on the particle surface and is independent of the particle’s ignition temperature.(4)The impact of enriched electrolyte tempering on the initial crystal temperature of the system was investigated.The findings reveal that the inclusion of AlF3,LiF,and KF additives aids in the development of the first crystal temperature system of the electrolyte system.Specifically,an increase in AlF3 content triggers the appearance of the low-temperature LiNa2AlF6 phase in the material phase of the electrolyte system,while an increase in LiF content leads to the emergence of the low-temperature K2LiAlF6 phase in the system.Furthermore,an increase in KF content promotes the generation of the lowtemperature K2NaAlF6 phase.The calculation of Na3AlF6 lattice constant indicates that the tempering process of the enriched electrolyte triggers solid solution reactions:Na3AlF6+Li+→LixNa3-xAlF6,Li++K2NaAlF6→K2Na1xLixAlF6,Na3AlF6+2K+→KxNa3-xAlF6.The addition of additives facilitates the fusion of substances on the surface of the particles,which fills surface defects and forms a smooth and dense surface structure.EDS analysis reveals that the generation of low-temperature phase of the system concentrates the distribution of K and Ca and gradually reduces the content of Na.