A laboratory study was performed to investigate the influence of cathodic current density, bath composition, temperature, interpolar distance and electrolyte impurity levels on the current efficiency with respect to aluminium (CE) in cryolitic melts. The laboratory cell was designed specifically to ensure an evenly distributed current density on the aluminium cathode surface, and to ensure good and reproducible convective mass transport conditions in the bulk phase of the electrolyte. The CE was determined from the weight gain of the cathode metal, with an estimated uncertainty in a single determined CE value of (+ or -)0.7%. Initial studies of CE and cathodic overvoltage as functions of cathodic current density, indicated good performance of the cell, with high and reproducible CE's and reproducible mass transport conditions. The cathodic overvoltage in the present laboratory cell was of the same order of size as previously found in commercial cells for comparable operating conditions, indicating that the mass transport conditions in the laboratory cell are representative for conditions in commercial cells. An existing CE model, based on theory of electrochemistry and mass transport was modified, and unknown model parameters were determined from the results of CE as a function of NaF/AlF(sub 3) ratio and temperature. The model parameters were determined with the assumption of bath composition independent mass transfer coefficients. This assumption may not be entirely true, but an estimate showed that the assumption does not lead to any serious error in the validity of the model. 182 refs., 50 figs., 19 tabs.
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