Impurities and Current Efficiency in Aluminum Electrolysis

This is an experimental project run as a collaboration between Alcoa Fjarðaál, HRV engineering, Reykjavik University and the Norwegian University of Science and TechnologyCurrent efficiency loss in aluminum electrolysis happens due to a variety of factors. One such factor is the presence of impurities. Concerns about impurities have been voiced since the quality of raw materials has declined and dry-scrubbers were installed. The most talked about impurity with detrimental effects on aluminum electrolysis is phosphorus, which can reduce current efficiency by about 1% per each 100 ppm of added phosphorus. The growing trend towards high amperage in aluminum reduction cells may require higher current densities. Therefore, it is of interest to industry to know if the phosphorus effect persists at high current densities. The main purpose of the thesis was to study the impact of phosphorus under high current density conditions (1.5 A/cm2) by adding AlPO4 into the electrolyte. Low current density (0.8 A/cm2) experiments were performed as a control. This study also included experiments at low concentrations of phosphorus (0-220 ppm), which are of particular interest for the industry. The effect of current density on current efficiency up to high current densities was also investigated prior to the experiments with impurities. Additionally, a data analysis campaign was run on operational data from Alcoa Fjarðaál. Daily measurements and operational data (from January 2011 to December 2013 at Alcoa Fjarðaál) were collected and analyzed to improve the understanding of effects of various process parameters (bath height, temperature, superheat, and age) on the concentration of phosphorus in the metal. A second objective of the thesis was to investigate the effect of sulfur in the bath on current efficiency. This was achieved by adding Na2SO4 directly into the electrolyte. Sodium sulfate was found to be very reactive under electrolyzing conditions and required frequent additions. The results of the current efficiency experiments for phosphorus support previous findings that phosphorus contributes to a decrease in current efficiency by 0.67% at 0.8 A/cm2. Current efficiency measurements at a high current density of 1.5 A/cm2 showed a clear negative effect of phosphorus and demonstrated a slightly higher decrease in current efficiency (1.1% per 100 ppm of phosphorus). Regression analysis of data obtained for lower phosphorus concentrations (0-230 ppm) revealed a pronounced current efficiency reduction of 0.92% per 100 ppm of phosphorus at 0.8 A/cm2 and 2.41% at 1.5 A/cm2. The detrimental effect of sulfur on current efficiency was confirmed and revealed a 1.1% decrease in current efficiency per 100 ppm of sulfur in the electrolyte. Experiments with current densities of up to 2 A/cm2 showed that, although current efficiency increases with increasing current density of up to 1.5 A/cm2, at higher current densities, the trend reverses and current efficiency is reduced instead. These findings are valid for the specific cell design under investigation, but similar mechanisms may apply in other settings. Industrial data analysis revealed strong correlations between high phosphorus in the metal and lower superheat, higher bath height, and increased cell age.Alcoa Fjarðaál and HRV engineeringThe thesis contains 6 papers that have been published in different places. Copyright was signed for all of these papers, therefore please contact publishers to get permission