2015 AIChE Annual Meeting Proceedings
(242e) Optimization of Primary Separation Rate of Magnesium Electrolysis Cell Based on Thermo-Electro-Magneto-Hydrodynamics Coupling Model
Authors
Optimization of primary separation rate of magnesium electrolysis
cell based on thermo-electro-magneto-hydrodynamics coupling model
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.
Figure
3 Movement of magnesium droplets in the electrolyte from 0 s to 300 s.
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
References
Brown,
R.E., 2007. Magnesium Industry Review 2006/2007. Light Metals 65,(1), 59.