Fluid Flow and Bubble Behavior in the Aluminum Electrolysis Cell

Mr. Yufeng Wang, Prof. Lifeng Zhang, Mr. Xiangjun Zuo

Department of Materials Science and Engineering, Missouri University of Science & Technology, Rolla, MO 65401, USA,

Tel: +1-573-341-4776, Email: zhanglife@mst.edu

Keywords: Aluminum Electrolysis Cell, Fluid Flow, Bubbles, Water Modeling, LDV Measurement

Abstract

A full scale water model was established to investigate the

phenomena in aluminum reduction cells. The behavior of

bubbles under the anode is analyzed by both directly

observation and camera recording. Bubble under the anode

has a thick bubble front and a thin, long trail portion. With

0o tilted angle, hardly can the bubbles move forward, but

form a gas film under the anode. With non-zero tilted angle,

bubble motion under the anode is driven by the buoyancy

force, thus bubbles are easy to escape through the curved

end of the anode. LDV was used to investigate the fluid

flow pattern. The LDV measurements reveal a recirculation

flow pattern in side channel, similar to the observation of

the tracer dispersion. Larger tilted angle and larger gas flow

rate generate larger velocity and bigger turbulent energy,

especially in the region close to the end of the anode and

the top surface.

Introduction

Nowadays, most of the aluminum smelters are seeking

ways to increase the current and reduce energy

consumption. A transverse cross-sectional schematic of a

modern aluminum reduction cell is shown Figure 1. The

anodic gas bubbles in the Hall-Héroult cell are a very

important player in this process. The carbon anode is

consumed to give off approximately 2.5m3 of CO2 for

every kg of Al produced. 1)

It is generally accepted that gas

bubbles (predominantly CO2 with the basic chemical

reaction under the anode of Al2O3 + C = Al + CO2) formed

during normal operation provide some benefit by

contributing to electrolyte circulation and mixing. The

release of the anode gas from the horizontal anode surface

in a molten cryolitic electrolyte is not continuous, but

occurs in a cyclic fashion. The formation of the bubble

layer beneath the anode is one of the key factors for the

transport phenomena in the cell. The physics of bubble

dynamics includes its nucleation, departure, detachment,

coalescence, breakup, size and shape, sweeping, and escape

etc. Sloped anode must be also considered. This layer

contributes to the bath flow that is responsible for the

alumina dissolution and its transport into the interpolar

space. However, the bubble layer increases the voltage

drop and electric noise (voltage fluctuation) of the cell. The

bubble voltage could be as high as 300 mV depending on

the anode current density and alumina concentration. 2)

Also, the gas induces bath flow and bath turbulence, which

influence the current efficiency. The direct measurement of

the bubble layer 3)

is difficult due the harsh environment -

high temperature and corrosiveness of the bath. Physical

models have been used to study the phenomenon. 4)

There

are studies that have been made by numerical modeling

where only gas driven flow has been considered. 5)

.

Fig.1 Schematic phenomena in aluminum electrolysis

cell considering alumina addition 6)

Slotted anodes were first implemented in a Rio Tinto

Aluminium (RTA) managed plant in 1998. 7)

Slotted

anodes have been used in recent years by aluminum

smelters in order to reduce gas bubble resistance at the

anode/electrolyte interface.

Results of scientific investigations into aluminium

reduction cell gas bubbles and gas induced circulation were

made public over 30 years ago including, for example, see-

through cell studies 3)

and water modeling 4)

. A 2003 paper

by N. Richards et al. 8)

refers to at least 20 publications, by

a dozen different researchers, all focused on gas bubble

related phenomena. There are a few physical models of

slotted anodes 9, 10)

Water Model Experiments

The current study focuses on the water modeling of bubble

behavior and fluid flow in the reduction cell. The properties

of water and cryolite are shown in Table 1. Due to the

similar kinematic viscosity 11)

, the water-air system can be

used to model the aluminum electrolysis process (molten

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aluminum and CO2 bubbles). Since the passage of four

electron through the circuit is necessary to release one

molecule of CO2, the correlation between the current

density and gas flow rate is 12)

10 kAm – 2

= 2.71 L m – 2

s – 1

(1)

Table 1. Properties of water and cryolite 12)

Liquids Cryolite Water

Temperature, oC 950 25

Density, kg/m3 2.1 1.0

Surface tension, Dynes cm2/s 60 70

Kinematic viscosity (Poise cm3/g) 1.5 1.0

The water model system used in the current study is shown

in Figure 3. A Plexiglas sided tank is used as container.

Two anodes are modeled by suspending two Plexiglas

boxes in the tank with a dimension of 150cm long, 17cm

wide and 43cm high. The distance between the anode and

the bottom of the tank, and the anode tilt angle are

adjustable in the current study. Gas evolution is simulated

by injecting air through 8 plastic pipes on each anode. In

low-temperature models, coalescence is the main growth

mechanism of bubbles. Small bubbles newly nucleating on

the anode surface do not significantly affect the motion of

large bubbles 13, 14)

, thus in the current study the pipes were

employed instead of porous plates under the anode.

Fig.3 Schematic of the water model

The operation parameters used in this study are listed in

Table 2. The anode cathode distance (ACD) is set to 40mm,

and the anode slope angle is various from 0 º to 1.1º. The

carbon anode is consumed to give off approximately 2.5m3

of CO2 for every kg of Al produced 1)

. It is generally

accepted that gas bubbles (predominantly CO2 with the

basic chemical reaction under the anode of Al2O3 + C = Al

+ CO2) formed during normal operation provide some

benefit by contributing to electrolyte circulation and mixing,

and there is a strict correlation between the current density

and gas flow rate15)

, as shown in Eq. (2). By the calculation,

the gas flow rate in the experiment was from 16.0 to 157

l/min, which equivalent to a current density of

0.2~1.0A/cm2.

4

iRTq

FP= 3 1 2[ ]m s m− − (2)

where i is the anodic current density, (Am-2

), R is gas, T is

Absolute temperature (K), F is Faraday’s constant, P is

pressure (Pa).

Table 2 Parameters in the water model

Items Value

Anode cathode distance (ACD) 40mm

Anode slope 0 º, 0.4 º, 1.1 º

Slot width 3mm

Side channel 390mm

Center channel 107mm

Gas flow rate 16.0~157l/min

Fifteen experiments were carried out (Table 3). The

different locations and views of the anode are recorded by

digital camera, as shown in Figure 4. The bubble behavior

at these locations is analyzed.

Table 3 Gas flow rate and anode tilted angle in the water

model experiments

Anode

slope

Current Density (A/cm2)

0.2 0.4 0.6 0.8 1.0

0º ▲ ▲ ▲ ▲ ▲

0.4 º ▲ ▲ ▲ ▲ ▲

1.1 º ▲ ▲ ▲ ▲ ▲

Location -1(Side)

Location -2(Side)

Location -3(Side)

Location -4(Bottom)

Location -5(Front)

Fig.4 Observation locations in the water experiments

Results and Discussion

The shape of bubbles

Every bubble below the anode has a thick bubble front and

a thin, long trail portion, as shown in Figure 5(a). If being

Anode

Anode

Anode

Anode

Anode

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viewed from the bottom, as shown in Figure 5(b), the

largest dimension is transversal to the motion direction.

With the increase of current density, the thickness of the

bubble will increase, as confirmed by reference 11)

.

(a) Observion from the side

(b) Observation from the bottom

Fig.5 The shape of the bubble

When the size of the anode is large enough, big gas pockets

are formed by coalescence and during their passage they

sweep away smaller bubbles. The shape of these very big

gas pockets is not circular anymore, having a nearly

straight leading and trailing edge. While the thickness of

the bubble laden layer is generally determined by the

maximal possible height of a bubble under a solid surface,

these big gas pockets have a “head” at the leading edge

which is about twice thicker than the overall thickness of

the bubble layer.16)

The effect of the tilted angle

The gas flow rate is set to 125L/min·m2, and the anode

tilted angle is various from 0º to 1.1º. The bubble motion

velocity increases with increasing tilted angle. With 0o tilt

angle, lots of bubbles escape from the slot but not the

curved end of the anode. If the tilt angle is larger than 0.4º,

bubbles are mainly escape from the curved end of the

anode, as shown in Figure 6. With zero tilted angle, the gas

film coverage ratio of the anode bottom is largest compared

to non-zero tilted angle, and the coverage ratio decreases

with the increasing tilted angle. The liquid motion is also

greatly affected by the tilted angle. In the current

experiments, the ink tracer is injected into the system, as

shown in Figures 7. With 0º tilted angle, the tracer

stagnates under the anode for a long time. If the tilted angle

is larger than 0.4º, the ink tracer quickly enters the side

channel (between the end of the anode and the side wall of

the tank) and generates a recirculation flow in the side

channel. With larger tilted angle, the ink enters the side

channel more quickly and generates stronger recirculation.

Fig.6 Bubble motion viewed from the front of the anode:

upper- tilted angle 0º; lower- tilted angle 1.1º

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Fig.7 Tracer dispersion at different time (tilted angle

0.4º)

Anode tilt dramatically affects the behavior of the bubble

layer. A tilt of a fraction of a degree is sufficient to induce

significant buoyancy driven motion of the gas bubbles. It

was found that 12)

the ACD has no effect on gas bubble

behavior. An increase in current density increased the

bubble size and thickness of the bubble front as well as gas

coverage of the anode face and bubble velocity. Current

density has no effect on bubble release frequencies. An

increase in electrolyte velocity decreased the bubble size

and the gas coverage and increased the bubble velocity and

release frequency. An increase in anode tilt decreased the

bubble size and gas coverage and increased the bubble

release frequency. Tilt has no effect on bubble velocity.

Behavior of the gas layer on a horizontal anode was

different than on an inclined anode. On a horizontal anode,

the process of bubble nucleation, growth, coalescence and

release involved no bubble motion and led to as gas layer

thickness of approximately 5 mm. 12)

On the inclined

surface, gas behavior was dominated by the motion of large

bubbles across the anode surface. Hydrodynamic effects

increase the maximum thickness of the bubbles to more

than 2cm. Figure 8 shows a possible mechanism of bubble

detachment from the bottom of a plate. When the anode

was slightly tilted(a fraction of a degree), large bubbles

formed and flowed beneath the anode with a leading edge

penetrating up to 2 cm into the electrolyte and a long tail as

seen in Figure 5a.

Fig.8 Possible mechanism of bubble detachment from

the bottom of a plate 17)

Figure 9 shows a picture taken with the PIV system for a

bubble moving on a liquid layer with a volume of 1.7 cm3

and an inclination of 5°. 17)

The PIV allows visualizing the

flow in two-dimensions around a moving bubble. In the

front of the bubble, the liquid is pushed away in the

direction normal to the interface. The depression in the rear

part of the bubble creates a counter flow to respect the

continuity. 17)

Fig.9 Flow around a moving bubble obtained with a PIV

system 17)

The effect of the gas flow rate

The tilted angle was set to 1.1º, and the gas flow rates

varies from 16.0-60.2l/min, which equals to a current

density of 0.2-1.0A/cm2. The gas release frequency

increases with increasing gas flow rate. The velocity of

recirculation flow in the side channel is accelerated with

increasing gas flow rate. So does the turbulence intensity

(Figure 10). Under 70L/min·m2 (equivalent 0.5A/cm

2) gas,

it takes ~ 6s for the tracer to move from the bottom to the

side, and under 157L/min·m2 (equivalent 1A/cm

2) gas, only

4s is needed.

(a) Flow rate of 70L/min·m

2, t=0s

(b) Flow rate of 70L/min·m

2, t=6s

(c) Flow rate of 157L/min·m

2, t=0s

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(d) Flow rate of 157L/min·m

2, t=4s

Fig.10 Tracer dispersion (Tilted angle 1.1º)

LDV Measurement

In this current chapter, the fluid flow in the water model of

the aluminum electrolysis cell is characterized using a two-

component Laser Doppler Velocimetry (LDV) - TSI Model

9833 two-component LDV. Through LDV, the local

instantaneous velocity (u and v) at some points can be

measured. For each point measurement, the mean velocity

( u and v ) can be obtained by averaging the velocities of

1000 particles, approximately during the period of 5-8

minutes. The turbulent velocity fluctuation ( u′ and v′ )

can be calculated by

u u u′ = − v v v′ = − (4)

It is important to know the local value of the turbulent

energy because it provides useful information of the local

mixing intensity. The turbulent energy per unit mass, k

(m2/s

2), is calculated according to:

( )2 21 2 vk u′ ′= + (5)

The fluid flow features at two different locations was

measured: the region at side channel and the region under

the anode. At the side channel region more than 300 points

were measured and at the region under the anode more than

100 points were collected. LDV measurement is through

points to points. If it takes 6 minutes, then measuring 400

points needs at least 40 hours of pure measurement time

except for other experimental preparation time. Thus, in the

current study, only three groups of experiments were

carried out. Experimental details were listed in Table 4.

Table 4 Experimental parameters

Case 1 Case 2 Case 3

ACD, mm 40 40 40

Anode slope, ° 0 1.1 1.1

Gas flow rate, L/min 64.2 16 64.2

The measured mean velocity vector and the turbulent

energy for all the three cases in Table 4 are shown in

Figures 11-13. For all these three cases, the high

turbulence is located at the gas plume region near the end

of the anode and close to the liquid surface. The high

turbulent kinetic energy near the end of the anode is mainly

generated by the momentum transfer from rising bubbles to

the liquid. The turbulent kinetic energy close to the liquid

surface is due to energy dissipation of the waves, which is

induced by bubbles reaching the top surface.

With zero degree tilted angle (Fig.11), bubbles are not easy

to escape from the end of the anode, so the gas film

expands under the anode and rise up through the side wall

of the anode. the liquid follows the gas bubbles. Thus, the

velocity under the anode is very random and big at some

locations (Fig.11b). Occasionally there are bubbles rising

up the end of the anode, which generate a recirculation

flow pattern at the side channel (Fig.11a). With zero degree

tilt angle, larger gas flow rate does not always generate

larger velocity below the anode because the movement of

bubbles is driven by the expansion of the gas film instead

of buoyancy. The similar flow pattern is observed by water

model experiment before and validated by the stagnation of

tracer under the anode. Also the distribution of the

turbulent kinetic energy is different from those cases of

non-zero degree tilt angle. The larger turbulent kinetic

energy under the anode is the result of the generation of

dissipation of bubbles.

With 1.1o tilt angle (Figures 12-13), the fluid flow pattern

can be characterized as follows:

- Stable horizontal flow under the anode since almost all

bubbles escape through the end side of the anode;

- Similar but slight larger velocity in the side channel

region, especially near the anode and the top surface,

compared to the case of zero degree tilt angle;

- Eddy position is lower than the case of zero tilt angle;

- Larger gas flow rate generates larger turbulent energy,

especially around the top end of the anode, and

generates larger velocity under the anode, which is due

to the fact that large gas flow generates more bubbles.

a) Side Channel

b) Under Anode

Fig.11 Measured velocity and turbulent energy (Anode

tilted angle, 0°, Gas flow rate, 64.2L/min)

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a) Side Channel

b) Under Anode

Fig.12 Measured velocity and turbulent energy (Anode

tilted angle, 1.1°, Gas flow rate, 16L/min)

a) Side Channel

b) Under Anode

Fig.13 Measured velocity and turbulent energy (Anode

tilted angle, 1.1°, Gas flow rate, 64.2L/min)

Summary

A full scale water model was established to investigate the

phenomena in aluminum reduction cells. The behavior of

bubbles under the anode is analyzed by both directly

observation and camera recording. Bubble under the anode

has a thick bubble front and a thin, long trail portion. With

0o tilted angle, hardly can the bubbles move forward, but

form a gas film under the anode. Although occasionally

bubbles escape from the curved end of the anode, they

mainly escape from the side wall of the anode. The large

gas film expands, which means high resistance and energy

consuming in aluminum reduction processes. With non-

zero tilted angle, bubble motion under the anode is driven

by the buoyancy force, thus bubbles are easy to escape

through the curved end of the anode. LDV was used to

investigate the fluid flow pattern. The LDV measurements

reveal a recirculation flow pattern in side channel, similar

to the observation of the tracer dispersion. With 0o degree

tilted angle, the recirculation flow pattern also exists.

However, larger tilted angle and larger gas flow rate

generate larger velocity and bigger turbulent energy,

especially in the region close to the end of the anode and

the top surface. The flow pattern under the anode is also

measured using LDV. With 0o degree tilted angle, the flow

pattern and turbulent kinetic energy are more random than

the cases with non-0o degree tilted angle.

The investigation can go further to optimize the operation,

such as the micro scale behavior of bubbles, the effect of

tilted angle, anode curve or slots in more detail. CFD

simulation of the multiphase flow is also necessary to

extend the results of the physical modeling to a whole cell.

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