IN DEPTH ANALYSIS OF ENERGY-SAVING AND CURRENT EFFICIENCY IMPROVEMENT OF

ALUMINUM REDUCTION CELLS

Yan Feiya¹, Marc Dupuis², Zhou Jianfei¹, Ruan Shaoyong¹

¹CHALIECO GAMI Guiyang Guizhou China 550081

²GéniSim Inc., 3111 Alger St., Jonquière, QC, Canada, G7S 2M9

Keywords: aluminum reduction pot, prebaked pot, pot voltage, energy consumption.

Abstract

In view of the existing aluminum overcapacity and lower

aluminum price in China, many companies took measures to

reduce the production cost and the energy consumption, but there

has been no normalized theory and method defined as yet.

To address these issues, this paper puts forward the evident effects

of energy-saving and current efficiency improvement in

aluminum reduction cells using new thermal insulation pot lining

design, application of optimal cathode structure and reduction of

horizontal current device. A proper application of new lining

materials and combination of relevant process parameters based

on the finite element software ANSYS^® and thermal field

simulation software as the calculation method combining the

actual production data are also used. Practice proves that the

above-mentioned method combining design, simulation and

experiment can become the effective and feasible way to achieve

low energy consumption, low cost and high profit.

Introduction

In recent years, the nonferrous metal industry sets off an upsurge

of scientific and technological innovation activities on quality and

capacity increase, energy-saving, consumption-reduction and

environment protection. The key technologies in aluminum

reduction area such as low temperature operation, intensifying

Figure 1. Energy balance relationship

[1]

current, on-line measurement of superheat, “3-variables” control

technology, anode slotting technology, irregular cathode

Figure 1 shows that the heat input/output may be divided into the

technology, improved thermal insulation lining design, cathode

above items based on pot energy balance principle. The object of

design that reduces metal pad horizontal current, application of

voltage reduction is the voltage combination in the heat input, the

new lining materials, inert anode etc. have been studied and are in

majority of which is voltage drop between anode-cathode.

the process of being implemented.

It should be pointed out that the high-temperature production

This will soon raises Chinese aluminum reduction technology to

during aluminum reduction mainly depends on the Joule heat

the world advanced level. Moreover, the consumption of energy

generated in the bath in which the current passes from the anode

and raw material for aluminum reduction production has been

to the cathode. The normal production shall be kept through the

very high in recent years, especially power consumption. With the

dynamic balance of heat output and heat input during operation. If

energy crisis, the aluminum reduction production cost must be

the Joule heat generated from heat input is not enough to maintain

reduced without delay. For this, the most efficient method is to

the heat output, the pot shall get cool gradually, and the process

reduce the DC consumption by increasing current efficiency (CE)

system shall be damaged.

and reducing pot voltage.

Therefore, the energy balance of the pot is maintained by reducing

Analysis of mechanism and nature of pot work voltage

the heat dissipation in heat output combination as well as the

reduction based on energy balance principle

voltage in heat input combination so as to reduce the voltage.

The pot energy balance was summarized by Warren Haupin as

shown in Figure 1.

Analysis of potentialities and approaches on voltage

Where:

reduction by voltage composition of heat input

i_a

is the Anodic Current Density (A/cm²);

is the Electrical Conductivity of the Bath (1/Ωcm);

is the Bubble Layer Thickness (cm);

t_a

is the Adhering Bubble Thickness (cm);

is the Gas Fraction in the Bath (0.02 · %Al₂O₃);

f_c

is the Fraction of the Anode Covered with Gas ().

From Equations 2 and 3, we can see that the mechanism of bubble

voltage drop reduction lies on improving the bubble release

capacity, hence reducing the bubble coverage fraction and

reducing the bubble thickness.

Until now, the measures taken by the industry mainly include the

slotting of anodes, the control the length-width ratio of anode, the

Figure

2. Heat input structure and approaches of

improvement of bath composition, etc.

voltage reduction

The slotted anode can make the bubble release from the anode

The total pot voltage is just the sum of the parts as follows^[2,3]:

bottom more efficient to reduce the bubble coverage fraction and

the bubble thickness, so as to reduce the bubble voltage drop of

Vpot = Vanode + |Ee| + ηsa + ηca

anode, thus reducing the pot voltage.

+ Vbub + VACD + ηcc + Vcath + Vext

(1)

There is deeper research on the slotted anode technology abroad,

Where:

including slotting location, slotting width, slotting depth, slotting

process, etc. At present, the more mature slotted anode structure

in China consists of 2 slots (at trisection location), slot width

Vpot

is the Pot Voltage (V);

around 1~1.5 cm, slot depth or height of half of anode consumed

Vanode is the Anode Voltage (V);

in anode change cycle as per the anode height generally. The

|Ee|

is the Equilibrium Potential (V);

slotting process is generally vibrating forming plus slot cleaning.

As per a lot of on site tests in China, using slotted anode can

ηsa

is the Anode Surface Overvoltage (V);

reduce the pot voltage by 30~60 mV.

ηca

is the Anode Concentration Overvoltage (V);

Vbub

is the Bubble Voltage (V);

Bath voltage drop^[3]

VACD is the Voltage Across the ACD (V);

ηcc

is the Cathode Concentration Overvoltage (V);

Vcath is the Cathode Voltage (V);

Vext

is the External Voltage (V).

Where:

Figure 2 shows that if the design dimensions of the pot are

determined, the object of pot voltage reduction is mainly the

ACD is the Anode to Cathode Distance (cm).

voltage drop reduction between the anode and the cathode

(industry term: active voltage). If the CE is fixed, the object of

Equation 4 shows that the mechanism of ACD voltage drop

voltage reduction is mainly the voltage drop of bubble (Vbub) and

reduction lies on changing the bath composition and reducing the

ACD itself.

in the bath across the ACD (VACD).

Perspective of changing the bath composition

Bubble voltage drop^[3]

Up to now, the measures taken in industry are as follows: the bath

conductivity is increased by adding the additive, in which the

most effective method is to add the LiF, and there will be

significant effect combining with low bath ratio technology.

From the present calculation and statistical data, it is seen that for

every increase of LiF by 1%, the voltage drop of unit ACD (cm)

will be reduced by about 3~5 mV. For a pot with an ACD of 5 cm

for example, every 1% LiF addition can reduce the voltage by

15~25 mV, and every 3% LiF addition can reduce it by 45~75

mV, which is a considerable effect.

The main designs tried in the aluminium industry in China have

Perspective of ACD reduction

been a stepped surface cathode, a sloped surface cathode and a

flow resistance block.

The ACD reduction is theoretically divided into: (1) effective

ACD reduction; (2) non effective ACD reduction (see Figure 3).

Stepped surface cathode metal flow model

Figure 4. Stepped surface cathode design

Figure 3. 3-layers structure model of ACD^[4]

As shown in Figure 3, the ACD consists of 3 parts including a- a

bubble layer, b- an effective ACD layer and c- a non effective

ACD layer. Zone a depends on the width of the anode, the specific

gravity and viscosity of liquid bath, the surface tension of bath to

carbon dioxide gas, the alumina concentration, etc.; zone b is a

heating area for maintaining the high temperature production of

the pot, as well as an isolation layer for making the wave crest of

metal away from the lower edge of bubble to avoid the back

reaction; zone c depends on the MHD cell stability.

For the conventional pot, if the ACD is

5 cm, as per the

calculation and averaged measurement, generally zone a (bubble

Figure 5. Model of metal flow velocity of stepped

layer) is about 0.5cm, zone c (non effective ACD) is 1.5~4 cm (it

surface cathode

has relationship with the pot stability), hence, zone b (effective

ACD) is 0.5~3 cm. The irregular cathode technology and the

Sloped surface cathode metal flow model

horizontal current reduction technology are decreasing the height

of zone c (non effective ACD) to reduce the pot voltage; and the

current intensification technology is decreasing the height of zone

b (effective ACD) to reduce the pot voltage, i.e. the lowest voltage

of current intensification selected in order to satisfy the heat

balance, thus obtaining the lowest height of zone b assuming no

CE loss. Therefore, if the pot with bad stability has current

intensification to reduce the voltage, it is highly possible that it

will reduce the height of zone c and bring about more back

Figure 6. Sloped surface cathode design

reaction, thus the pot will experience CE loss and overheating.

We can divide the type of applications used to reduce the ACD in

three categories: (1) irregular cathode technology; (2) horizontal

current reduction technology; (3) current intensification.

Irregular cathode technology

In 1994, Vittorio de Nora put forward the thinking of the irregular

cathode structure. The irregular cathode structure is adopted to

change the metal and bath flow state and reduce the melt flow

velocity and the interface wave range of metal surface (reduce the

non effective ACD), thus improving the pot stability in order to

gain the option to reduce the ACD. Such technology is a kind of

method to reduce the non effective ACD.

Figure 7. Model of metal flow velocity of sloped surface

cathode

Comparison of results obtained

Special insulation^[5] between cathode carbon block and collector

bar

Table

1 is the comparison of calculation and measurement

between irregular cathode and standard cathode in a plant in

This new kind of design is a cathode assembly which reduces the

China.

horizontal current by adding an electrically insulated region

between the cathode carbon block and the collector bar, as shown

Max. flow velocity

Max deformation of

in Figure 8.

(cm/s)

metal surface (cm)

Calcula-

Measure-

Calcula-

Measure-

tion

ment

tion

ment

Standard

15.73

14.99

1.82

1.97

pot

Irregular

7.47

8.24

0.65

0.51

cathode

Variation

52.50%

45%

64.30%

74.10%

percentage

Table 1. Comparison of calculation and measurement

between irregular cathode and standard

cathode in a plant

Compared to the standard cathode, for the irregular cathode the

flow velocity is reduced by about 50%, the maximum deformation

of metal surface by 65~75% and noise by 10~15%. At present,

the voltage of the most of irregular cathode pots in China is about

Figure 8. Cathode assembly for restraining the

3.7~3.9 V, based on the calculation of 2~3% of CE loss. The

horizontal current ( JY)

power consumption can be reduced by 560 kWh/T Al compared

to the standard cathode pots.

This design has been modeled using a 3D generic parametric

whole pot model, based on ANSYS^® 13.0, as shown in Figure 9.

Horizontal current reduction technology

It has been proved by the long-term practice that the fluctuation of

metal liquid layer and bath liquid layer has close relationship with

the horizontal current and the vertical magnetic field, which

combined brings about the pot voltage fluctuation. So for a given

vertical magnetic field, a reduction of the horizontal current in the

metal can make possible a significant reduction of the height in

the metal pad, reducing the cell heat loss and so provide an

opportunity for pot voltage reduction, while maintaining the pot

production and increasing the CE, all for the purpose of reducing

the specific energy consumption.

The horizontal current has relationship with the following factors:

1) Geometric dimensions, such as width and length of cathode,

Figure 9. Geometry of the ANSYS^® based 3D generic

width and height of collector bars;

parametric whole pot model

2) Material of cathode, such as material of cathode carbon

block, connection method between the cathode carbon block

The comparisons of simulation results between the cathode

and the collector bar;

assembly without restraining horizontal current and that with

3) Geometric dimensions of the pot, such as dimension of

restraining horizontal current are shown in Figures 10 and 11

thermally insulating pier, hence the position of the ledge toe;

respectively.

4) The location of the collector bars exit out of the pot (side

wall or otherwise).

From the above analysis and comparisons it shows that the

cathode assembly with insulation has a good effect on the

The up to date prototype tests were designed to:

reduction of the horizontal current; from the curve distribution, it

shows that the curve of no sloping pasting presents the raised

1) Increase the electrical insulation between cathode carbon

parabola

(Figure

10) with a maximum value of 0.26 A/cm².

block and collector bar;

However, the curve of the anode bottom middle of sloping pasting

2) Try a cathode design with bottom exit collector bar.

presents a leveled curve (Figure 11) at a value of 0-0.04 A/cm².

Figure 12. Voltage results for the cathode with bottom

exit collector bar

Figure 10. Cathode assembly without restraining

horizontal current (JY)

Figure 13. Current density results for the cathode with

bottom exit collector bar

Figure 11. Cathode assembly with restraining

horizontal current (JY)

Current intensification

From the curve distribution and values it shows that the sloping

This technology reduces the voltage and maintains the relative

pasting technology has obvious effect for restraining the JY. At

constant heat input by the current intensification, thus obtaining a

the moment in China, the different insulated materials for this

way to maintain the stable thermal balance. Such way is a method

technology are adopted for test and engineering applications.

to reduce the effective ACD, the premise of which is that the pot

has good MHD stability.

Cathode design with bottom exit collector bar

The development trend of current intensification for advanced cell

This kind of design is the cathode assembly which reduces the

technology outside of China at present is as follows:

horizontal current by changing the collector bar design and cell

exit location. Figures 12 and 13 show that the cathode voltage

1) Rio Tinto Alcan (Pechiney): pot capacity: 300 kA → 400

drop is 194 mV (anode current density is 0.73 A/cm²) which is a

kA, anode current density: >0.98 A/cm² , pot voltage: <4.02

reduction of 70~100 mV compared to that of traditional cathode

V, CE: 95%-96%, DC consumption: 12800 kWh/T Al;

based on the same anode current density. From the horizontal

current reduction analysis, it shows that the vertical current

2) Hydro Aluminium: pot capacity: 300 kA → 420 kA, anode

density in the cathode carbon block increases by about 0.2 A/cm²

current density: > 0.99 A/cm², pot voltage: 4.08 V, CE: 94%-

due to bottom exit. This technology is currently only in test phase,

95%, DC consumption: 12800kWh/T Al;

and the potential of voltage reduction needs to be proven.

3) Dubal: DX type pot capacity: 340 kA → 370 kA, anode

current density: >0.99 A/cm², pot voltage: 4.15 V, CE: 95%-

96%, DC consumption: < 13000 kWh/T Al.

Today, the development condition of advance representative pot

type in China is: an anode current density of current

intensification for pots operating from 200 kA to 400 kA already

that reaches 0.8~0.83 A/cm², and a voltage of 3.85~4.05 V.

Conclusions

In summary, through lots of prototype tests, mathematical

modeling and comparison, the main effective approaches for

reducing the pot voltage are as follows at present in China:

· Change of bath composition

· Sloped surface cathode

· Cathode assembly technology for restraining JY

· Current intensification

· Optimization of anode design

· Cast iron rodding for cathode

For the pot with the above technologies, for example in a plant in

China the voltage is 3.75~3.85 V and the CE is above 94% [6][7].

Compared to the traditional pot with voltage being 4.1~4.2 V and

CE being 93%, the energy consumption can be reduced to about

1250 kWh/T Al, and reduced by about 62.5*10⁷ kWh per year

based on an annual production capacity of 500 thousand tons.

The operation cost savings are about 312 million Yuan per year

based on power price being at 0.5 Yuan per kWh.

References

[1]

Warren Haupin, Halvor Kvande,

“Thermodynamics of

Electrochemical Reduction of Alumina”, TMS Light Metals,

2000, 379-384.

[2]

Jay Bruggeman, “Pot Heat Balance Fundamentals”, Proc 6^th

Aust Al Smelting Workshop 1998, 167-189.

[3]

Grjotheim, K., Kvande, H.

(eds.), Introduction to

Aluminium Electrolysis, Aluminium-Verlag, Dusseldorf,

1993, 144-145.

[4]

Tian Yingpu, Wang Hang, “Pot anode cathode distance

composing model and process energy consumption”, Light

Metals, 2011, 567-568.

[5]

Zhou Jianfei, “A kind of cathode assembly to improve the

pot stability, increase CE, reduce the energy consumption

and prolong the pot life”, Chinese Patent: 201020504034.

[6]

Chen Cairong, “Production index of 80KA and 300KA pot

in Yunnan Aluminum Plant”

(internal data), Guiyang

Aluminum Magnesium Design & Research institute

Corp.Ltd (GAMI), 2011.

[7]

Zhou Jianfei, “Thermal field test report of 80KA pot in

Yunnan Aluminum Plant”

(internal data), Guiyang

Aluminum Magnesium Design & Research Institute

Corp.Ltd (GAMI), 2011.