Study On The Micro Formation Mechanism And Capture/Recycle Technologies Of Greenhouse Gases In Aluminum Electrolysis

Aluminum electrolysis,as a non-ferrous metal industry with the largest carbon emissions,is key to achieve the China’s"double carbon"target.Greenhouse gases including CO₂in normal electrolysis and perfluorocarbon（PFC）gas in abnormal cell conditions,are generated at the electrolysis reaction interface between carbon anode and high-temperature electrolyte.By far,systematic research on the generation mechanism at micro-scale and exploration on suitable treatment technologies of these two types of greenhouse gases is still lacking.Herein,based on in-depth analysis of the dominant discharge ions on the anode surface at different stages,the micro formation mechanism of greenhouse gas in aluminum electrolysis is carefully studied by means of theoretical calculation combined with experiment.Meanwhile,treatment methods of CO₂and PFC are also investigated to provide technical reference for the end treatment and recycling of electrolytic exhaust gas.The main conclusions are shown following:（1）Through theoretical calculation,the relationship between the concentration of Al₂O₃in molten salt and the dominant discharge ions on the anode surface is clarified,laying a theoretical basis for furtherly investigating the formation mechanism of different anode gases.The results show that in the molten salt system,all O exists in the form of bridged oxygen O_b（Al-O-Al）,while most F exists in the form of terminal fluorine（F-Al）.When Al₂O₃concentration in molten salt gradually decreases to 0,oxygen-containing complex ions in molten salt decrease significantly until they disappear.Therefore,aluminum-fluorine complex ions(mainly[Al F₅]^2-)and free fluorine（single F^-）are more favorable to discharge on the anode surface at this stage.When Al₂O₃is sufficient in molten salt（Al₂O₃>2 wt%）,the number,type and proportion of oxygen-containing complex ions increase obviously,which is more conducive to the discharge of Al-F complex ions such as[Al₂OF₇]^3-and[Al₃O₂F₈]³at the anode reaction interface.（2）Based on the theoretical calculation and the experimental detection results of anode reaction interface,the types of intermediate products in anode electrochemical reaction are confirmed,and the formation mechanism of different gaseous products in anode is revealed.The calculation and detection results show that CO₂and CO are obtained from different C-O intermediates in the O^2-preferential discharge stage,which corresponds to the C-O-C bridge structure at the anode interface when Al₂O₃content is sufficient.In the F^--dominant discharge stage,CF₄and C₂F₆are obtained by the reaction of different C-F intermediates,corresponding to the different types of F-C bonds and-CF_xgroups at the anode interface when Al₂O₃content is insufficient.In addition,when O^2-and F^-discharge together,unstable COF₂gas will be formed.It is speculated that the low concentration of CF₄in the non-anodic stage of aluminum electrolysis is generated by the pyrolysis of COF₂.（3）A suitable modified graphene material for PFC adsorption and a novel electrochemical desorption method for ethanolamine CO₂absorption solution is discovered through theoretical calculation and experimental exploration,thus an anode gas capture and recovery scheme can be designed to deal with greenhouse gases.The results show that compared with the n-type graphene represented by single vacancy and As doped graphene,and the p-type graphene represented by Al doped graphene,the Cr doped graphene has better chemisorption effect for various anode gases that mainly composed of PFC.Additionally,for the electrochemical desorption method,an optimized electrochemical desorption effect can be achieved at room temperature（16℃）by using the membrane combination of bipolar membrane+proton exchange membrane（BPM+PEM）,MEA solution with a concentration of 30%（volume fraction）and a desorption voltage of-5V.The electrochemical desorption energy consumption is as low as 2.133 k Wh/kg-CO₂,significantly lower than the traditional thermal desorption energy consumption（3.126 k Wh/kg-CO₂）.110 figures,53 tables,169 references...