On the Electrocatalysis of the Carbon Anode in

Aluminium Electrolysis

 

Yexiang Liu¹⁾ and Jomar Thonstad²⁾

¹⁾ Department of Metallurgy, Central South University, Changsha, China,

²⁾ Department of Materials Technology and Electrochemistry, Norwegian University of Science and Technology, Trondheim, Norway.

 

 

The specific energy consumption, (P) in aluminium electrolysis can be expressed as:

  [kWh per kg Al]            

where CE is the current efficiency and V is the cell voltage. This expression demonstrates that the energy consumption can be reduced by either lowering the cell voltage or by increasing the current efficiency. Typical values are V = 4.2V, CE = 0.94, and P = 13.30 kWh/kg Al. One can see that a 50 mV reduction in V would reduce P by 1.2%.

 

The cell reaction in aluminium electrolysis is:

1/2Al₂O₃ + 3/2C = Al+3/4CO₂               

where a cryolite (Na₃AlF₆ - AlF₃ - CaF₂) melt at ~970 ºC serves as the solvent for the alumina reactant. The standard reversible potential for the cell reaction  is –1.187 V at 970 ºC. The anodic reaction is associated with considerable overvoltage (0.4~0.6V), which obviously represents a significant contribution to the overall energy consumption. Hence it is of interest to find ways to reduce this overvoltage.

 

Over the years several attempts have been made to reduce the anodic overvoltage by adding catalytic substances to the anode. Such studies have been conducted on a laboratory scale as well as in the industry. Much of this work has been carried out in China. A review of published data is available, covering work up until 1995 [1].

 

The catalytic substances that have been tested have been LiF, AlF₃ and a variety of oxides. Anodes are made of a mixture of petroleum coke aggregate and a pitch binder phase. The catalyst is normally added to the aggregate coke in the form of a fine powder, but in laboratory tests also impregnation of aqueous solutions of catalysts have been tested. Both techniques have been found to yield similar results, so the simpler procedure, i.e. addition of powder, seems to be adequate. An important issue is that the catalytic substance must not contaminate the aluminium produced in any deleterious way.

 

Claims have been made that addition of catalytic substances can reduce the anodic overvoltage with as much as several hundred millivolts [1]. Careful laboratory studies carried out by the present authors using AlF₃ or MgO + Al₂O₃ as dopants, have shown that the effect is more modest. Typically, a reduction in overvoltage of 40-80 mV has been found. The exact mechanism by which the electrocatalytic substances work, is under investigation, and results will be reported at the conference.

 

[1] Y.-X. Liu, J. Thonstad and J.-H. Yang, Aluminium 72, 836-841 (1996).