Cathode Wear in Aluminium Electrolysis Cells

The increasing global demand for aluminium has resulted in growing production of aluminium metal through increased production capacity and improvement of the electrolysis technology. This has caused a development towards larger cells and higher current densities, which have resulted in the use of more graphitized cathode blocks and larger anodes. These changes have also resulted in reduced cathode life, which has become one of the major challenges in the aluminium industry. Carbon cathode wear is the major cause of reduced cathode life. The main objective of this thesis was to investigate possible mechanisms behind carbon cathode wear, specifically the role of pitting on the carbon cathodes. The work consisted of two parts. Autopsies and characterizations of spent potlinings constituted the first part of the work. The second part focused on laboratory studies of wetting of carbon cathode materials since wetting has been pointed out as one important factor related to cathode wear. Autopsies of six spent potlinings were conducted during the course of the study. The cells were operated at three different smelters with different cell technologies and with cathode blocks of different carbon types and properties. Macro wear patterns were documented by a combination of visual observations, photography and laser interferometry. Samples of the electrolyte at the cathode surface and cylindrical samples drilled out of the cathodes were collected and characterized by a combination of X-ray tomography, X-ray diffraction, optical and scanning electron microscopy as well as different computer software programs to characterize the micro wear. Typical W and WW wear patterns were identified for the spent potlinings with prebaked anodes, while a relatively uniform wear was found for the Søderberg spent potlining. Closer inspections of all the cathode surfaces revealed a wear pattern characterized by pitting, resembling pitting corrosion of metals. X-ray tomography, as well as optical microscopy, revealed that the pitting observed did not correlate with the distribution of aggregates within the carbon matrix of the cathode blocks. The characteristic size of the pitting, measured by interferometry, did not correlate with the average aggregate size in the cathode blocks determined by X-ray tomography. No other evidence of aggregate or grain detachment was found in any of the spent potlinings. Furthermore, a uniform wear surface across the aggregates and the binder matrix indica