Cheng-Lin Liu1, Ze Sun1, Meng-Jie Luo2, Gui-Min Lu1, Xing-Fu Song1, and Jian-Guo Yu1,2,*
1 National Engineering Research Center for Integrated Utilization of Salt Lake Resource, East China University of Science and Technology, Shanghai, China.
2 State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, China.
Magnesium has found a variety of applications due to a number of advantages including low mass density and high specific strength (Brown, 2007; Eliezer et al., 1998). Like the Hall-Herault process of aluminum production (Perron et al., 2007), the electrolysis process for magnesium is one of the most energy intensive industrial processes (Eklund et al., 2014). Thus the energy consumption and current efficiency are crucially important indexes. Due to high temperature and strong corrosive conditions involved during the industry production, it is extremely challenging to obtain the experimental data. It is also expensive and impracticable to optimizate the structure by setting up new electrolysis cells. Over the last decade, lots of research efforts have been made on the use of commercial software package to model the flow field (Jain et al., 2008), thermoelectric field (Dupuis, 2013), Electrohydrodynamic field (Rezvanpour et al., 2012), thermoelectromechanical model (Richard et al., 2001) and magnetohydrodynamic model (Severo et al., 2005). However, much attention has been paid on the aluminum reduction cell and little effort has been made on the magnesium electrolysis cell. In summary, most of the reported studies of magnesium electrolysis cell only considered the mathematical model based on one or two physical fields. Little has been found on the thermo-electro-magneto-hydrodynamics coupling model.
This paper presents a thermo-electro-magneto-hydrodynamics model for the magnesium electrolysis cells to investigate the distributions of electric filed, temperature field, magnetic field and flow field simultaneously. The model mainly need to solve the following governing equations:
As shown in Figure 1, the voltage drop of cathode, anode and electrolyte are 0.175 V, 0.549 V, and 1.007 V respectively. Electrolyte temperature is about 700 ºC. The heat generation is 2.11°Á105 W and energy dissipation is 2.13°Á105 W. The flow field of mathematical model was validated by the cold model experiments of PIV in the previous research (Liu et al., 2015). In Figure 2, three circulations, circulation A under the metal separating compartment, circulation B closed to the cathode, and circulation C closed to the free surface of the electrolysis compartment, are existed in the cell. Circulation A and C are beneficial for the separation of magnesium droplets and the magnesium droplets in the circulation B will float to the free surface of the electrolysis compartment, which will increase the contact time of magnesium and chlorine. The effects of the Lorentz force on the motion of the electrolyte is investigated in the work.
Figure 1 120 kA magnesium electrolysis cell: (a) electric field, (b) temperature field, and (c) magnetic field.
Figure 2 Flow field distribution in the 120 kA magnesium electrolysis cell (a) velocity vectors, (b) flow field in the channels, and (c) velocity vectors in the cross section.
After generated at cathode, the magnesium droplets finally gather at the free surface of the electrolysis compartment and metal separating compartment. Those magnesium droplets floating on the free surface of metal separating compartment are sucked out of the electrolysis cell; and other magnesium droplets will be drawn into the circulation of electrolyte again. Most of the magnesium droplets finally flow into the metal separating compartment after multiple cycles. Therefore, we define the process that magnesium droplets first floating on the free surface as the primary circulation of magnesium droplet. The rest of the cycles is defined as the secondary circulation of magnesium droplet. The ratio of the quantity of magnesium droplets in the metal separating compartment during the primary circulation to the whole quantity of magnesium droplets generated at the cathode is defined as the primary separation rate of magnesium droplets (PSR). In order to improve the electrolysis efficiency, the cell needs to increase its primary separation rate of magnesium droplets.
Particle tracing module of COMSOL is used to simulate the trajectory path of magnesium droplets. The density distribution of particle is proportional to the normal current density. Movement of magnesium droplets at x-z plane in the electrolyte is shown in Figure 3. At t=50 s, particles exist all over the electrolyte. After 300 s, almost all the particles float on the electrolyte and the PSR is about 16.6%, which means that the original cell has a lot of room for improvement.
Through adjusting the structure and process parameters of magnesium electrolysis cell, the PSR has no significant improvement. Thus, two new-type of cathodes with hollow channel and separator are used in this work to farther increase the PSR. A new-type cathode with a 0.07 m hollow channel is used to gather the electrolyte and push it into the metal separating compartment. The PSR in the new type structure is about 30.1% under the same condition.Another optimized electrolysis cell, which has a separator between the electrolysis compartment and metal separating compartment, is used to guide the electrolyte flow. The separator is combined with cathode, which can increase the working area of cathode. The variation of the optimized cell with separator is 61.9%. Figure 4 shows the movement of magnesium droplets in the electrolyte from 0 s to 300 s. Unlike the original cell, most magnesium droplets in the optimized cell are involved into the circulation A at 20 s, and finally float on the free surface of electrolyte in the metal separating compartment. The separator has an obvious effect in the improvement of PSR.
Figure 4 Movement of magnesium droplets in the electrolyte from 0 s to 300 s
Keywords: magnesium electrolysis cell, multiphysical fields, primary separation rate
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