• Table of Contents
• Introduction
• Plant Data
• Electrolyte Properties
• Concentration Converter
• Cell Voltage
• Anode Table Layout
• Alumina Feeding

• Table of Contents
• Introduction
• How to Use AlPrg
• Plant Data
• Alumina Feeding
• Electrolyte Properties
• Cell Voltage

• Table of Contents
• Introduction
• ATab Version 2
• Layout-View
• Evaluation-View
• 3D-View
• Floating Windows
• Optimization Window
• Examples
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• Table of Contents
• Principles Hall-Héroult
• Mass Balance
• Electrolyte Properties
• Cell Voltage
• Energy Balance

• Current Efficiency

• Raw Materials

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• Measurements

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• Examples, Exercises
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4.7 Overvoltages

In addition to the reversible decomposition voltage and to the voltages to overcome the electrical resistance a supplementary voltage must be applied to execute the electrolysis process. Electrochemistry distinguishes two types of overvoltage:

4.7.1 Charge Transfer Overvoltage (Reaction Overvoltage):

The excess applied voltage makes the reactions especially the electron transfer at the electrodes proceed at an appropriate speed.

Reaction Overvoltage on the Anode:

AlWeb and AlPrg calculate the Anodic Reaction Overvoltage (η_AR) according to the relations given by Warren Haupin [Lit. Equ. 24 to 27]:

(4.7.1A)

with

(4.7.1B)

4.7.2 Concentration Overvoltage (Polarization):

Due to the slow mass transport of the reacting species a concentration gradient is built up around the electrodes. The additional concentration overvoltage is necessary to make the electroactive species migrate against this concentration gradient.

Concentration Overvoltage on the Anode:

Concentration gradients in the electrolyte close to the anode produce concentration overvoltage. The concentration overvoltage is normally small about η_AC = 0.05-0.1V. At low alumina content however the concentration overvoltage increases until fluoride ions discharge (anode effect):

(4.7.2)

AlWeb and AlPrg calculate the critical current density (j_crit) according the Equ. 19 of [Lit.]. You find a similar relation in [Lit.] Equ. 13, p.145.

(4.7.3)

Concentration Overvoltage on the Cathode:

Similar to the anode the concentration gradients at the metal pad cathode produce the cathodic concentration overvoltage (η_CC). The relation is taken from [Lit.], Equ. 28:

(4.7.4)