Entropic Heat Effects in Aluminum Electrolysis Cells with Inert Anodes
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UCTION
ALL major aluminum companies apply elaborate mathematical models for cell design, cell control, and operation. Due to the need of protecting the cell lining by a layer of frozen electrolyte (side ledge), it is very important to control the overall energy balance of the cell. Therefore, the heat losses from the cell are being mapped in detail by computation as well as by measurements. In the author’s opinion, the electrolysis process is generally well understood, and, in particular, the distribution of heat losses. However, the fundamental understanding is scarce when it comes to non-ohmic heat sources. Even in advanced mathematical models used by the industry, the internal heat sources and sinks are not separated into different terms and distributed to their correct locations in the cell. The overall entropic heat effect in the cell reaction can easily be calculated from thermodynamic data, e.g., JANAF.[1] However, the distribution of the heat to the anode, the cathode, and the electrolyte is not trivial. It has been shown by several authors that the entropy associated with the cell reaction leads to relatively strong cooling of the anode in traditional aluminum electrolysis cells with carbon anodes.[2–9] Several methods have been used for the assessment of the cooling intensity. Mozhaev et al.[2,3] were probably the first to describe heat effects on individual electrodes in aluminum electrolysis cells. Their work was based on direct temperature measurements,[2] and the findings were later confirmed by the measured Seebeck coefficient.[3] Ratkje[4] distributed the total entropy to the individual electrodes by using irreversible thermodynamics.
ASBJØRN SOLHEIM, Chief Scientist, is with SINTEF Materials and Chemistry, P.O. Box 4760 Sluppen, NO-7465 Trondheim, Norway. Contact e-mail: [email protected] Manuscript submitted August 17, 2015. Article published online January 5, 2016. 1274—VOLUME 47B, APRIL 2016
Ødega˚rd et al.[5,6] based their treatment on estimated thermodynamic data for the anion species assumed to be present, whereas Flem et al.[7] derived the entropic heat effects from experimentally determined Seebeck coefficients. The work by Kjelstrup et al.[8] was based on measuring temperature changes at the electrodes, following steps in the electrolysis current. Solheim’s treatment[9] was based on the same principle as used by Ødega˚rd et al.,[6] but formal activity data for NaF, AlF3, and Al2O3 derived by Solheim and Sterten[10] were used instead of estimated thermodynamic data for the anions present. So far, there have been no attempts to assess the entropic heat effects in cells with inert anodes, although all methods mentioned above can be applied. The present work can be regarded as a direct continuation of recent work concerning a cell with carbon anodes,[9] inasmuch as the same method was applied.
II.
CELL REACTION: OVERALL THERMODYNAMIC DATA
The cell reaction in aluminum electrolysis with inert anodes is simply 1 3 3F Al2 O3 ða; sÞ ¼ Al(lÞ þ O2 ðgÞ: 2 4
½1
The energy changes related to any rea
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