Abstract
A 2D transient mathematical model was established to separately describe the anode bubble dynamics and the bubble-induced electrolyte motion in the rare earth electrolysis cell with horizontal electrode. Results indicate that with the increase in the anode inclined angle, the maximum bubble thickness is increased gradually. Furthermore, compared with the conventional anode, the inclined and chamfered anodes are conductive to the bubble length reduction and the bubble velocity improvement. Meanwhile, the bubble-induced electrolyte motion in the electrolysis cell can improve the distribution and transport process of oxyfluorides, thereby enhancing the current efficiency. Finally, a novel feeding method based on the electrolyte flow is proposed.
Nowadays, molten salt electrolysis is an effective method to prepare rare earth metal (REM
Over the years, significant improvements and changes in the structure of electrolysis cells have been reported for the enhancement in performance and current efficienc

Fig.1 Rare earth electrolysis cell with horizontal electrode
expensive and can hardly be conducte
In this research, a 2D transient gas-liquid two-phase flow mathematical model was established to investigate the bubble dynamics in HEC. The bubble-induced electrolyte motion was quantified. The simulation results provide theoretical foundation for the research and development of HEC, which is conducive to the science and technology innovation for the rare earth electrolysis industry.
The bubble motion in the electrolysis cell involves a gas-liquid two-phase flow and two phases are not interpenetrating. Therefore, a 2D transient flow model was established to investigate the two-phase flow based on VOF method by solving the volume fraction continuity equation to capture the bubble shape. For each phase added to the model, a related variable is introduced to record the volume fraction of the phase in each control volume. The sum of volume fractions of all phases is 1 at each control volume. The fields for all variables and properties are shared by the phases and represent volume-averaged values, as long as the volume fraction of each phase is known at each location. Thus, the variables and properties in any given cell are representative of one specific phase or a mixture of phases, depending on the volume fraction values. Each phase agrees with the equation for mass conservatio
(1) |
where α, t, and v are the phase volume fraction, time, and velocity, respectively. The primary phase volume fraction can be computed on the basis of specific constrain
(2) |
where αg and αl are volume fractions of gas and liquid phases, respectively. As all variables and properties are shared by the phases and represent volume-averaged values, the corresponding equation for conservation of momentum can be obtaine
(3) |
where P is the pressure; Fs represents the surface tension force; ρ and μ are the mixture density and viscosity, respectively. The mixture density and viscosity are based on the volume averagin
(4) |
(5) |
where ρg and ρl are densities of gas and liquid phases, respectively; μg and μl are viscosities of gas and liquid phases, respectively. The simulation accuracy and efficiency strongly depend on the method of specific interpolation near the interfaces, which affects the convection and diffusion fluxes through the control volume fraction as well as the surface tension force Fs (included as a body force), as indicated in
In this research, the flow field was calculated by the commercial software FLUENT based on the finite volume method. The 2D simulation was used to investigate the motion of the bubble and electrolyte flow on account of computing efficiency and simulation accurac

Fig.2 2D simulation model of HEC (a); adaptive mesh for simulation model (b)
Parameter | Value |
---|---|
Cell size, Lc×Hc | 300 mm×270 mm |
Anode size, La×Ha | 200 mm×150 mm |
ACD/mm | 100 |
Electrolyte density/kg· | 3850 |
Electrolyte viscosity/kg·(m·s | 0.0049 |
Bubble density/kg· | 0.4 |
Bubble viscosity/×1 | 1.37 |
Bubble-electrolyte surface tension/N· | 0.4 |
The degassing momentum boundary condition was set for the outlet, which was considered as the wall for electrolyte and outlet for bubble. Other boundaries were assumed as no-slip wall for all fluids. During the simulation, the automatic mesh adaption method was used, allowing more accurate capture of the detailed interface between bubble and electrolyte and significantly reducing computing time. The global Courant number was set as 0.15 in this simulation. The flow was treated as laminar flow. The absolute convergence criteria for both continuity and velocity were set as 1

Fig.3 Predicted morphologies of bubble with 10 mm in diameter under anode with inclined angle of 2°: (a) t=1.0 s; (b) t=1.8 s; (c) t=2.3 s; (d) t=2.5 s

Fig.4 Bubble thickness curves with different bubble diameters under anode with inclined angle of 2° (a), 4° (b), 6° (c), 8° (d), and 10° (e)

Fig.5 Effect of anode inclined angle on bubble length under different bubble diameters

Fig.6 Effect of anode inclined angle on maximum bubble thickness under different bubble diameters
on the maximum bubble thickness for the bubble with 5 mm in diameter. The bubble presents a flat shape with the maximum bubble thickness of about 5.61 mm under different inclined angle conditions. As the bubble diameter exceeds 10 mm, the maximum bubble thickness is increased significantly with the increase in anode inclined angle. For example, when the anode inclined angle increases from 2° to 10°, the maximum bubble thickness with the bubble diameter of 20 mm increase from 7.36 mm to 9.14 mm. With the increase in inclined angle, the component of force perpendicular to the anode bottom is decreased and that parallel to the anode bottom is increased. The perpendicular component of the mass vector causes the variation in thickness. Thus, the greater the inclined angle, the larger the bubble thickness. The maximum bubble thickness ranges from 5.25 mm to 9.14 mm under all simulation conditions. Small bubbles coalesce to form big bubbles and then detach from the anode during industrial electrolysis process. The bubble may touch the bottom molten metal when ACD decreases to a certain threshold value, which consumes the molten metal through the secondary reaction. Because of the secondary reaction, the efficiency of the electrochemical reaction decreases, therefore affecting the current efficiency. To prevent the secondary reaction, ACD must be kept at a specific value in the usual operation of the electrolytic process, which is however detrimental to the energy saving. Hence, the maximum bubble thickness of large bubble plays an important role in the selection of ACD.

Fig.7 Effect of anode inclined angle on average bubble velocity under different bubble diameters

Fig.8 Comparison of bubble velocity with bubble diameter of 5 mm (a), 10 mm (b), 15 mm (c), and 20 mm (d) under conventional and chamfered anode

Fig.9 Velocity fields of electrolyte with different bubble diameters under anode with inclined angle of 2° (a), 4° (b), 6° (c), 8° (d), and 10° (e)

Fig.10 Relationship between maximum electrolyte velocity and inclined angle under different bubble diameters: (a) near the bubble and (b) near the bottom of electrolysis cell

Fig.11 Velocity fields of electrolyte during bubble motion process with bubble diameter of 10 mm under inclined angle of 2°: (a) beneath anode; (b) at the anode edge; (c) in the center channel

Fig.12 Schematic diagrams of bubble, electrolyte, and ReOF trajectory during electrolysis process
The electrolyte located in the upper part between the anode and cathode moves upward along the slope of anode to the top side under the action of bubble. According to the direction of electrolyte flow, two flow modes of electrolyte can be distinguished. Partial electrolyte reaches the top of electrolysis cell through the gap between anodes and then returns to the zone between anode and cathode. Other electrolyte enters the region between anodes and flows back to the region between anode and cathode directly. These electrolytes merge and then flow to the bottom surface of anode. Therefore, the bubble motion leads to the circulating movement in molten electrolyte, which can accelerate the dissolution and diffusion of ReOF, thereby contributing to the transport process of
1) The increase in anode inclined angle can decrease bubble length and increase bubble thickness in HEC, particularly when the bubble diameter is 10–20 mm. The maximum bubble thickness ranges from 5.25 mm to 9.14 mm under all simulation conditions.
2) Bubble velocity is promoted by the anode inclined anode: the larger the anode inclined angle, the faster the bubble velocity. Moreover, the chamfered anode can further increase bubble velocity at the anode edge.
3) The electrolyte motion is promoted by bubble motion in HEC. Bubble motion can cause a local vortex in molten electrolyte, thereby accelerating the transportation of ReOF from the feeding region at the top of electrolysis cell to the reaction region located between anode and cathode.
4) A novel feeding method based on the electrolyte motion is developed, which includes feeding oxide on the side of electrolysis cell at the early electrolysis stage to form crust and then feeding oxide at the center of electrolysis cell to achieve the uniform distribution of ReOF.
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