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On the mechanism of the anode effect in aluminum electrolysis

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Abstract

The dependency of the critical current density in aluminum electrolysis on the bulk concentration of alumina has been reported in various forms. Some workers found different relationships in restricted ranges of the alumina content and concluded on possible changes of the reaction mechanisms and various types of anode effect. A previously developed mathematical model could show that the anode effect is initiated as the actual current density equals the limiting one. The model is now applied to check some of the available theories. Comparison with experimental data shows that the varying effect of the alumina concentration can be described by a single relationship taking account of the combined action of mass transfer, fluid dynamics of gas release, and wettability for all values of the alumina content. The results suggest that there is every reason for the view that only one process occurs. A distinction of various types of mechanisms provoking the anode effect is unnecessary.

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Abbreviations

A :

electrode surface area (m2)

c :

concentration (mol m−3)

C :

multiplier, Eqs. [1] and [2] (A m−2)

C 1 :

constant, Eq. [9] (A s m−3)

C 2 :

constant, Eq. [12] (m2 A−0.5 s−1)

C 3 :

constant, Eq. [14] (A m−2)

d :

final equivalent bubble diameter (m)

D :

diffusion coefficient (m2 s−1)

F :

Faraday constant, F=96,487 A s mol−1

f G :

gas evolution efficiency (—)

f I :

fraction of the current passing through the electrode side walls (—)

I :

total current (A)

I*:

maximum current, Eq. [16] (A)

j :

actual current density (A m−2)

k :

overall mass-transfer coefficient (m s−1)

k 0 :

microconvective mass-transfer coefficient (m s−1)

k v :

macroconvective mass-transfer coefficient (m s−1)

K :

multiplier, Eq. [21] (—)

L :

length of electrode edge crossed by bubbles (m)

M :

molar mass (kg mol−1)

n :

charge number (—)

p :

pressure (kg m−1 s−2)

R:

universal gas constant, R = 8.3143 kg m2 s−2 mol−1 K−1

Sc:

Schmidt number, Sc ≡ η L DA −1 ρ L −1

T :

temperature (°C)

ν max :

velocity (m s−1)

w :

alumina mass fraction (—)

ɛ :

current efficiency (—)

η L :

liquid viscosity (kg m−1 s−)

ϑ :

contact angle

Θ*:

fractional shielding of the electrode surface by large bubbles (—)

Θ**:

fractional shielding of the electrode surface by small bubbles (—)

v :

stoichiometric number

ρ L :

liquid density (kg m−3)

A :

oxygen-containing ion

B :

dissolved gas

c :

critical

G :

gas

w :

electrode

∞:

liquid bulk

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  1. Laboratory of Reaction Technology, TFH Berlin, University of Applied Sciences, D-13353, Berlin, Germany

    H. Vogt (Professor)

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Vogt, H. On the mechanism of the anode effect in aluminum electrolysis. Metall Mater Trans B 31, 1225–1230 (2000). https://doi.org/10.1007/s11663-000-0009-z

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  • DOI: https://doi.org/10.1007/s11663-000-0009-z

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