Study On The Bubble Behaviors During The Process Of Aluminum Electrolysis

Primary aluminum production is a basic and signifigant industry for the national economy. However, aluminum electrolysis industry consumes aboundant energy resource when it provides a great deal of aluminum for the national defense industry and economic development. The energy consumption of primary aluminum is about of 13000-14000kWh/t-Al, therefore energy-saving is always the hot topic of aluminum electrolysis technology and development. The energy consumption of primary aluminum per tonne functions the cell voltage and current efficiency. At present, the common cell voltage is in an interval of 3.7-4.2V, and the highest current efficiency can reach 95%, while the energy efficiency of aluminum electrolysis is less than 50% becourse of the very lower voltage efficiency. Beside the ohmic resistance of electrolyte in the cell, the gas bubbles covering on the anode surface also contributes to a higher cell voltage.It is imperative that we have to investigate the gas bubble behaviors on the anode surface during the cource of electrolysis with a purpose of developing an energy-saving large-scale prebaked anode aluminum reduction cell. Nevertheless, the distribution and movement of bubbles are very difficult to measure directly in commercial cells. Therefore, the simulation of bubble behavior in aluminum reduction cell is of great significance to the design and production of primary aluminum reduction cell.In this paper, aiming at investigation of the anode bubble motion, mathematical model is established and the bottom of anode bubble flow is simulated using the commercial software ANSYS-Fluent. Specifically, the main work includes:1. Based on the real situation of commercial aluminum reduction cell, in current study, the air-water system is used to modeling the bubble motion during the electrolysis process. Hence, bases on the physical model cell the bubble distribution under the anode and around the slot between the anode and the cell-wall are both observed and measured.It was found out that, along the rising of the bubble, the gas diffusion range in horizontal is increased. The gas diffusion region is approximately the shape of an inverted triangle in the edge of the anode. And the position with maximum time-average gas volume fraction is almost round the midline of the triangle. As the input speed of the gas is increased, the bubble diffusion region is larger, and the local time-average gas volume fraction also risen obviously.2. Applying the basic fluid dynamics law, mathematical model of aluminum reduction cell is derived to simulate the single bubble movement in the cell Volume Fraction (VOF) free surface tracking method model is employed to capture the detail bubble interface changes. And via varying the parameters, bubble size on bubble behavior are clearly investigated.The results demonstrate that both systems show a similar trend of bubble dynamics:the sliding velocity in the anode-cathode gap increases as the bubble size increases; smaller bubbles can escape from the anode edge as a whole, while larger bubbles tend to break following the release of part of the bubble. Quantitatively, there are some differences between the two systems, which can be reflected in the three stages of the bubble release processes:for a fixed size, the carbon dioxide-cryolite system leads to a larger bubble sliding velocity and a smaller bubble thickness than in the air-water system. For an air-water system, the bubble continuously detaches from the vertical anode wall, while for a carbon dioxide-cryolite system, the bubble mainly attaches the vertical wall. Consequently, the momentum exchanges occur at different regions of the cell channel.3. Finite element electric field of aluminum reduction cell model is also established in current work by using the destitution information derived from the physical cold model cell. Therefore, the electric field distribution of aluminum reduction cell can be studied with detail of voltage drop around each bubble. Therefore, the electric field distribution of each part of cell is studied and the relationship between the current density and bubble has been obtained. The main findings from this study are:The presence of bubbles does not greatly affect the global current flow in the whole cell, but it does significantly affect the local current flow at the anode bath interface. Local peaks in current flow occur at the bubble and liquid boundary on the anode. The predicted bubble induced resistances are within the range of published empirical correlations, but does not fit in any particular expression. The predicted increasing in the rate of resistance as a function of the bubble coverage is higher than given by empirical predictions.