基于渗流理论的铝电解用氢气扩散阳极反应效率研究

In the current Hall-Heroult process the replacement of prebaked carbon anodes by hydrogen diffusion anodes can save energy and reduce emissions. One of the key points of the application of the new process is to increase the reaction efficiency of the hydrogen diffusion anode. In this project the increase of the reaction efficiency of the anode is carried out by utilizing displacement methods from percolation theory, to perform reciprocating movement of gas and liquid by applying periodic change of gas pressure in porous media, to increase the mass transfer and three-phase (hydrogen-bath-porous media) reaction boundary. In this project SnO2-based materials are used to prepare the porous anodes and molten KF-AlF3 is selected as the electrolyte. With this new anode, electrochemical techniques such as linear sweep voltammetry and electrochemical impedance spectroscopy are used to study the relationship between reciprocating movement of gas and liquid and anodic current density as well as mass transfer. Considering the effect of gas-liquid movement on three-phase boundary, lattice boltzmann method is applied to study the two-phase movement and saturation of hydrogen and molten salt in the porous anodes. It is summarized that the influence of periodic change of hydrogen pressure of hydrogen diffusion anodes on mass transfer and three-phase reaction boundary is explored to provide theoretical and technical basement of scale-up of laboratory experiments with application of the new hydrogen diffusion anode in aluminum electrolysis.

在现行铝电解工艺中采用氢气扩散阳极替代预焙碳阳极可实现电解铝工业的节能减排，新工艺应用的难点之一是提高氢气扩散阳极的反应效率。本项目借鉴渗流理论中运用多孔介质内气体压力周期性变化实现气液两相往复运动的驱替方法来加速传质并扩大“氢气-熔盐-多孔电极材料”三相反应界面，以提高阳极反应效率。本项目选择SnO2基材料制备多孔阳极，KF-AlF3为电解铝熔盐体系。在新型氢气扩散阳极电解铝试验装置上，采用线性电势扫描法、交流阻抗法等研究气液往复驱替运动对氢气扩散阳极电流密度和传质的影响规律。考虑到气液两相流动对传质与三相界面的影响，采用格子波尔兹曼法研究多孔电极内氢气和熔盐两相运动与饱和度的变化规律。综合以上研究，掌握氢气扩散阳极内氢气压力周期性变化对优化传质和扩大三相反应界面的作用机理，为新型氢气扩散阳极在铝电解中应用的放大实验提供理论与技术基础。

铝电解工艺中采用氢气扩散阳极替代预焙碳阳极新工艺应用的难点之一是提高氢气扩散阳极的反应效率。本项目提出采用具有分级孔径的多孔碳材料作为气体多孔阳极，以扩大“氢气-熔盐-多孔电极材料”三相反应界面，提高阳极反应效率。首先在Na3AlF6-AlF3-Al2O3体系中分别采用多孔SnO2基气体电极、Pt 气体电极和多孔碳气体电极作为阳极进行恒电流电解，电解过程中不通入气体、通入惰性气体氩气和通入氢气，观察各种条件下阳极电极电势的变化。实验结果表明，在多孔SnO2基气体阳极上，首先采用较大的电流对阳极进行较高电位的预极化（时间约为半小时），通入氢气后能观察到去极化现象，说明氢气成功参与了阳极三相界面反应；在多孔碳气体电极上通入氢气后能观察到较小的去极化现象，同时碳的消耗量相对于通入氩气减少约20 wt%。本研究进行了具有分级孔径的多孔碳材料的制备。选取710℃的NaCl-KCl-MnCl2熔盐体系，通过循环伏安法和方波伏安法研究了Mn2+的电化学行为，并进行恒电压电解制备金属锰。然后采用高碳锰铁作为阳极进行恒压电解，电解过程中高碳锰铁阳极中金属被氧化成离子进入熔盐，而后在阴极沉积，阳极剩下多孔碳。通过XRD，SEM，Raman以及TEM的表征分析，发现电解后的高碳锰铁阳极为同时具有微孔，介孔和大孔结构的高度石墨化的分级多孔碳化物衍生碳材料。本研究还选取710℃的NaCl-KCl熔盐体系，通过循环伏安，方波伏安和计时电流的方法，研究了Cr2+的电化学行为。然后采用高碳铬铁作为阳极进行恒压电解，在得到纯净铬铁的同时，电解后阳极可得到具有不同孔径的多孔碳。以上研究为新型氢气扩散阳极在铝电解中应用的放大实验提供理论与技术基础。