Study On Mechanisms Of Aluminum Electrolysis Creep Damage And Microstructure Evolution Of Graphite Cathode Materials

Modern large-scall aluminum electrolytic cell is a large industrial equipment with massive capital and intensive technology.The cathode is mainly made of graphite materials,which is the key component to determine the service life and production efficiency of the cell.Prolonging cathode life and reducing spend potlinings carbon waste are of great significance for promoting aluminum industry upgrading and its carbon neutrality.According to the classical theory in more than half a century,the damage of carbon/graphite cathode(semi-graphitic,graphitic and graphitized,etc.)is mainly related to Na/salts infiltration and sodium expansion.In recent ten years,more attention has been paid to the creep behavior of cathode materials under applied pressure in the design and engineering simulation of advanced electrolytic cell.However,the electrolysis expansion and the creep behavior of cathodes were usually investigated as two independent processes in previous studies.Thus,the coupling mechanismes of deformation,mechanical property deterioration and microstructure evolution of cathode materials were still lacking in knowledge pool.Based on this,the aim of this work to study the changing of elastic-plastic and microstructure,respectively,of high purity graphite cathode at different stages in the whole process of electrolysis expansion-creep and clear about the mechanismes microstructure evolution,elastic-plastic changing and damage of graphite cathode in the whole process of electrolysis expansion-creep.Moreover,the coal tar pitch binder of graphite cathode are modified and a method of estimating the remaining life span after creep damage is proposed for different graphite cathode materials.The main results are as follows:(1)The changing characteristics and rules of whole process of electrolysis expansion and creep for graphite cathode material were studie by modified Rapoport-samoilenko method.The electrolysis expansion-creep curves of graphite cathode during electrolysis process at 940? under different constant pressure(0-4 MPa)were in-suit measured accurately.The results show that the electrolysis expansion will accelerate to the rapid creep stage with load pressure increasing.Three typical stages of the whole process of cathode deformation can be determined from the deformation-time characteristics in strain curve under constant pressure:electrolysis expansion,rapid creep and steady creep atages.(2)The paramaters of elastic-plastic proporty changing of graphite cathode at different stages of electrolysis expansion-creep were measured by nanoindentation method,and crack propagation rules of graphite cathode were analyzed by finite element method.The results show that with the increase of electrolysis time under constant pressure,hardening effect will generate in graphite matrix,the elastic modulus increases from 15 GPa to 27.4 GPa,and the hardness increases from 0.29 GPa to 0.55 GPa.A new stress concentration zone can form on the surface of salt infiltration-uninfiltration interface at expansion stage and the increasing equivalent stress will promote cracks propagating mainly along the radial direction.In the rapid creep stage,the stress concentration zone diffuses to the center which will lead to the radial crack trend to propagate along the axial direction.Surface cracks on 45° and radical direction are more like to propagate.The simulation results are in good agreement with experimental observation.(3)The electrolysis expansion-creep behavior,mechanisms of elastic-plastic changing and damage of high purity graphite cathode material were studied from the perspective of microstructure evolution by XRD,Ranman spectrum and HRTEM,etc..The results show that at electrolysis expansion stage,the crystallite size along c-axis of graphite lattice,Lc,increases from 21.8?26.2 nm to 31.2?33.6 nm with the number of graphite stacking layers increasing.At rapid creep stage,the crystallite size along a-axis,La,decreases form 19.0?21.0 nm to 14.7?14.9 nm with the increasing of dislocation density which indicats that fractures may occur in graphite(002)planes.In the whole process of electrolysis expansion-creep,interplanar spacing of graphite(002)planes d002 gradually decreases from 0.33707 nm to 0.33575 nm with the graphitization degree increasing.The transformation from the expansion stage to the creep stage is related to the fracture of(002)planes,which may release a large degree of plastic deformation with dislocation slip under external stress.The hardness and elastic modulus of graphite matrix gradually increase with the evolution of microstructure during this process with deformationhardening and pinning effects.(4)Coal tar pitch(CTP)mixtures were modified with additions of graphene oxide(GO)and TiB₂ particles.The mechanismes of microstructure and mechanical properties changing with the two different types of addtives for CTP carbonation products were studied.The results indicate that 1 wt.%GO addition can increase the crystallite size,La,of pyrolysis and carbonization products by promoting the formation of planar aromatic macromolecules in mesophase.The additions of 3 or 5 wt.%TiB₂ can promote the stacking order of(002)planes with stacking height Lc increasing in CTP pyrolysis and carbonization products.The combined additions of TiB₂ with GO can generate a synergistic effect on La and Lc increasing for carbonization products,with the elastic modulus,limit strain and compressive strength inceasing to 3.24 GPa,6.6%and 194.7 MPa,respectively.TiB₂ can play a synergistic roles with GO for additional La increasing by reducing the oxidation of carbon-carbon bonds in carbonization products.(5)Graphitic,GO-TiB₂ modified graphitic,TiB₂-graphite composite cathodes were prepared.The anti-electrolysis expansion/creep performance and mechanical properties of the three types of graphite cathodes were compared after creep.An estimating method for the creep damage residual life span of cathode material was proposed according to the changing rules of steady creep strain curve with taking the residual limit strain after steady creep as the end of creep strain.The results show that GO-TiB₂ modified pitch can strengthen the carbonized binder phase of graphitic cathode.The anti-electrolysis expansion/creep properties and mechanical properties of modified graphitic cathode are improved to a certain extent.The life span evaluation results of different materials are TiB₂-graphite composite cathode>GO-TiB₂ modified graphitic cathode>Graphitic cathode>Pure graphite cathode.The results are agree with the empirical expectations.The residual creep damage life span of cathode material is mainly determined by steady creep rate and residual limit strain of cathode material after creep under electrolysis.The characteristics,microstructure evolution and elastic-plastic variation at various stages of electrolysis-expansion-creep process of the grapgite cathodes have important theoretical significance for better understanding of deformation and damage mechanism of graphite cathode materials in aluminum electrolysis.The obtained performance data,simulation of crack propagation and evaluation method of life span after creep damage for cath ode materials can provide important reference values for optimizing composition and structure design of cathodes,screening and comparing quality and service life of cathode products,prolonging the service life of electrolytic cells.