• Table of Contents
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4.2 Components of the Cell Voltage

The Cell Voltage is the entire voltage applied to an electrolysis cell. The cell voltmeter shows this value which is measured from the entrance to the exit of the cell busbar system.

Measurement of the Cell Voltage.

The cell voltage is the entire voltage applied to an electrolysis cell. It is measured from the entrance to the exit of the busbar system.

The Cell voltage is divided into several components:

Components of the Cell Voltage.

The entire Cell Voltage is the Sum of the Entry to Bath Voltage, Bath Voltage, Bottom - Exit Voltage.

Entry - Bath: the Entry - Bath value checks the electrical connections between the different parts of the anode system (carbon anodes, iron yoke, aluminum rod, anode clamp, and anode beam).
Bath Voltage: the bath voltage contains a resistance and an electrochemical part. The measured value of the bath voltage determines the set value of the cell voltage.
Bottom - Exit: Bottom - Exit contains the cathode voltage or bottom voltage that is determined by the amount of bottom sludge or bottom crust, the quality and the ageing of the carbon bottom blocks. It includes also the¨voltage drop in the external bus bar system. The Figures 4.2 shows the definition of the different voltage drops. It does not indicate, however, how these voltage drops are actually determined e.g. measured practically during pot operation.

4.3 Components of the Bath Voltage

The bath voltage contributes with 3.5-4.0 V the greatest part to the cell voltage. The bath voltage is measured in the anode to cathode distance (ACD) e.g. between the carbon anodes and the aluminum metal cathode. The next figure shows the components of the bath voltage:

Components of the Bath Voltage.

The bath voltage is the sum of resistance com-ponents (U_Ω: ohmic bath voltage, U_bub: bubble voltage) and electrochemical components (E₀: reversible electromotoric force, η_AC, η_AR: anodic concentration and reaction overvoltage, η_CC: cathodic concentration overvoltage). ACD is the distance between anode and cathode.

E₀ Reversible Decomposition Voltage: the minimum voltage which is necessary to produce aluminum according to the Principal Equation in an ideal electrolysis cell.

U_Ω Ohmic Bath Voltage: the voltage loss due to the ohmic resistance of the electrolyte.

U_bub Bubble Voltage: according to the Electrolysis Equation the anodes produce gas which forms a layer in the anode to cathode distance. The layer is an insulator for the electrolysis current. It increases the resistance of the electrolyte e.g. the bath voltage.

η_AC Anodic Concentration Overvoltage: this voltage is necessary to overcome the concentration gradient of the reacting species at the anode.

η_AR Anodic Reaction Overvoltage: an extra voltage has to be applied to make the reaction at the anode proceed with an appropriate speed.

η_CC Cathodic Concentration Overvoltage: this voltage is necessary to overcome the concentration gradient of the reacting species at the metal pad cathode.

4.4 Reversible Decomposition Voltage

Electrochemistry uses the concept of an ideal electrochemical cell. When this cell is in equilibrium, i.e. no electric current is flowing in the system, the measured voltage is called the reversible electromotive force. Applied to the Principal Equation of the Hall-Héroult-Process this Reversible Electromotive Force (EMF) or Reversible Decomposition Voltage is the minimum voltage to produce aluminum in an ideal electrolysis cell.
In reality, however, when an electric current is flowing through the cell and aluminum is produced additional voltages have to be applied namely

Ohmic Voltage Drop: this voltage is necessary to overcome the electrical resistance in the busbar system and in the electrolytic cell.

Overvoltages:: due to the electrochemical processes on the electrodes additional electrical energy i.e. voltage is necessary to transport the electroactive species to the electrodes and to keep them reacting at a reasonable speed.

According to the Principal Reaction Equation

the Reversible Decomposition Voltage (E₀) is written as:

(4.2)

n_e = 4·3 because 4Al³⁺ ions are involved in reaction 4-1. Since aluminum, carbon dioxide and carbon are close enough to their standard states unit activity is assigned. The relation for the activity of alumina is taken from the literature [Lit. Equ. 9 and 10]:

(4.3)

4.4.1 Standard Potential

The Nernst Equation combines the Gibbs Free Energy (ΔG) with the Standard Potential (E⁰) where n_e are the number of electrons exchanged in the chemical reaction and F is the Faraday Constant.

(4.4)

Haupin and Kvande [Lit. Equ. 6 and 7] use the following expressions:

(4.5A)

(4.5B)

Using Equ. 4.5A one calculates for 950°C E⁰ = 1.197 V for carbon anodes and with Equ. 4.5B E⁰ = 2.223 V for inert anodes. The carbon anodes reduce the standard potential by about 1V e.g. using inert oxygen producing anodes increase the electrochemical voltage by about 1V. This is called the Energy Penalty. However this value is reduced because of the small reaction overvoltage on inert andodes.