Effect of baking temperature and anode current density on anode carbon consumption
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I.
INTRODUCTION
T H E primary electrochemical reaction in the Hall--H~ro u r cell can be written as 2
4 AlzO~ + C = ~ A1 + CO,
[1]
Accordingly, the theoretical carbon consumption (W,h) per second can be calculated by Faraday's law: W,~
M~'I = 4.F (g/s)
[2]
where M c is the molar weight of carbon (12) and I denotes the total current, In practice, we know that the carbon consumption (CC) is generally greater than theoretical. This may be due to an excess consumption in the form of excess carbon gasification (CG), as outlined subsequently, and the formation of carbon dust (CD). A certain disintegration of the working face of the anode occurs due to different rates of consumption of the two components in the carbon anodes,V-:,-~J since the binder coke, originating from the pitch binder, is chemically more reactive than the ag~egate coke. The CD consists of carbon particles which have been detached from the anode, mainly in the form of grains of aggregate coke. Selective wear of the anode is not predominant at high current densities.t~-'l Excess carbon gasification is mainly due to the Boudouard reaction: C + CO z = 2CO
[3]
It has been shownl4.~] that this reaction does not take place on the surface of the polarized anode, but CO_, can react
Z. KUANG, formerly Postdoctoral Student, Depar~ent of Electrochemistry. The Norwegian Institute of TecbJ~ology, is Student, Queen's University, Kingston, ON, Canada K7K 5S9. J. THONSTAD, Professor. Department of Electrochemisl~7, and S. ROLSETH. Group Leader, SINTEF Materials Technology, The Foundation tbr Scientific and Industrial Research, are wiflaThe Norwegian Institute of Technology, N-7034 Trondheim. Norway'. M. SORLIE, Concern Specialist. is with Elkem Carbon R&D, N--4062 Kristiansand, Norway. Manuscript submitted April 4, 1995. METALLURGICAL AND M,VFERIALSTRANSACTIONS B
inside the pores of the anode or with CD floating in the bath. Exposure of the anode to air during electrolysis also causes excess consumption in the forms of gasification (air burning) and dusting. It is possible that some primary CO may also be generated, according to 2/3A120~ + 2C = 4/3A1 + 2CO
[4]
The net results are the same as for Reaction [1] followed by Reaction [3], so it is not possible to distinguish unambiguously between primary CO and CO formed in the Boudouard reaction. However, there is strong evidencet~.s.-'-"~that Reaction [4] plays a very minor role, except at very low current densities. (
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