A Study Of Multi-physical Fields Coupled Modelling And Structure Optimization Of Large-scale Energy-saving Aluminum Reduction Cells

Aluminum reduction cell is the core equipment for producing aluminum in industry, and the distributions of its multi-physical fields have remarkable effect on the technical and economic indexes of the production. At present aluminium reduction cell is developing fast toward large-capacity and energy-saving type. The existing method of the physical field modeling for aluminium reduction cell cannot satisfy the need of technology development. In this paper, aiming at the defects in theory and applicability of the traditional models, a systematic modelling study for the multi-physical fields in the large-capacity energy-saving aluminum reduction cells was carried out and the structure optimization of the cell was also studied in theory on the basis of the developed models, which could provide the necessary technical support and theoretical basis for the development of the large-capacity energy-saving cell and its operating technology. The main contents and achievements in this study are as follows:(1) A three-dimensional transient three-phase (metal-bath-anode gas bubble) magnetohydrodynamics (MHD) model, applicable for large energy-saving aluminum reduction cells, was built up. Several MHD models were compared and demonstrated combining with a practical simulation case. It is proved that neglecting anode gas bubbles or adopting the assumption of steady state in modeling will lead to the loss of the important informations in the calculated results. Meanwhile, the significant influence of anode gas phase on the fluid behavior is obtained based on the transient three-phase MHD study:gas bubbles change the bath flow pattern remarkably and weaken the metal circle flow, driven by electromagnetic forces(EMFs), through the interphase force between metal and bath; due to the movement of the bubbles, the projection of the anode bottoms presents on the metal-bath interface, and the interface behavior transforms from long-period wave to relative high-frequency perturbations.(2) A strongly coupled model of thermal-electrical field was established and an application case of the model is given. It is pointed out that not setting the temperature of the melt is the premise to realize the fully coupling of the electrical field and thermal field, and that roundly introducing the magnitude and distribution of each exactly described heat source, including positive source and negative source, into the coupling model is key to obtain reliable results for the thermal-electrical field calculation.(3) It was proposed in this paper that the mass transfer nearby the metal-bath interface should be described with surface renewal model. The global calculation model of current efficiency (CE) associated with the melt movement was established, regarding the magnitude and distribution of the turbulence eddy dissipation on the metal-bath interface as the key indicator. The model was effectively applied to the analysis of the global and local CE in a large-capacity aluminum reduction cells.(4) The magnetic fluid behaviors and the corresponding CE of the same large-capacity aluminum reduction cells with different cathode structures were analyzed by using the three-dimensional transient three-phase MHD model and the CE calculation model. The results show that adopting the special-shaped cathode, the cathode with new conductive structure or the collector bar with high conductivity proposed in this paper can decrease both the metal and bath velocities in varying degree for this cell, compared to adopting normal cathode. However, the bath velocity in the cell with special-shaped cathode becomes too small, which is unfavorable to the dispersion of alumina in electrolyte. The metal-bath interface perturbations are reduced with each of the three cathode schemes. The special-shaped cathode scheme is relatively effective for reducing the interfacial regions with large deformation. For the other two schemes, their effects on dropping interface perturbations are proportional to their effects on decreasing horizontal current in metal. The scheme of special-shaped cathode make the global CE of the cell decrease to a small degree, while, the global CE doesn’t change visibly when using the other two schemes.(5) The effect of anode structure on facilitating the escape of anode gas from electrolyte was studied by using a bath-bubble flow model, and the shape evolution and current distribution change in electrolysis process of different structure anodes were also studied by developing a transient model combining anode consumption with electrical field based on secondary current distribution. The results show adopting the anode with chamfer, the anode with sloped bottom or the slotted anode is favorable for reducing the content of gas bubble in the electrolyte. Comparing the effect of promoting release of the gas, the slotted anode occupied the first place, and the anode with chamfers takes the second place. The position of slots, the size of chamfer and the gradient of slopped bottom all have influence on the content of gas bubble in the electrolyte. Compared to normal anode, it takes less time to reach the steady shape through reactions for the anode with sloped bottom and the anode with chamfer, both of which can improve the uniformity of anode current distribution to some degree in the early stage of electrolysis.