Impurities and Current Efficiency in Aluminum Electrolysis

Abstract

Different factors can contribute to current efficiency loss in aluminum electrolysis and one of them is due to the presence of impurities. Concerns on impurities have been raised since decline in raw material quality and installation of dry-scrubbers. The most well-known example of an impurity with detrimental effect 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 for 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 (1.5 A/cm2) conditions 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 which are of particular interest for the industry (0-220 ppm). The effect of current density on current efficiency up to high current densities was also investigated prior to the experiments with impurities. Also 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) was collected and analyzed for correlations 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 done by adding Na2SO4 directly into 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 phosphorous and demonstrated a slightly higher decrease 1.1 % per 100 ppm of phosphorus. Performing regression analysis for lower 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 showed 1.1 % decrease in current efficiency per 100 ppm sulfur in the electrolyte. The experiments with increased current density up to 2 A/cm2 illustrated that although the current efficiency increases with increasing current density up to 1.5 A/cm2 the trend reverses at that levels and at higher current densities the current efficiency is reduced. These findings are valid for the specific cell design under investigation but similar mechanisms may apply in other settings. Industrial data analysis showed strong correlations between high phosphorus in the metal with lower superheat and higher bath height, as well as older cell age.