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dc.contributor.advisorGrande, Tor
dc.contributor.advisorRatvik, Arne Petter
dc.contributor.advisorWang, Zhaohui
dc.contributor.authorSenanu, Samuel
dc.date.accessioned2019-11-11T14:56:17Z
dc.date.available2019-11-11T14:56:17Z
dc.date.issued2019
dc.identifier.isbn978-82-326-4231-1
dc.identifier.issn1503-8181
dc.identifier.urihttp://hdl.handle.net/11250/2627792
dc.description.abstractThe 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 indicates a non-preferential wear all over the carbon-electrolyte interface. It was therefore concluded that cathode wear in these potlinings was not dominated by grain pull-out or a detachment mechanism. Moreover, a difference in pitting size between the pitting spotted at the cathode block ends and the centre channel areas of the graphitic cathode blocks point to a wear process facilitated by high current density and faster transport rates. Also, an important finding was the partly overlapping and clustered pitting observed in several of the potlinings. This observation suggests that new pittings evolve as the growth of older ones slow down. In all the spent potlinings a layer of electrolyte, covering the whole cathode surface, was observed. Al4C3 was often found below the layer of electrolyte on the cathode surface. The presence of electrolyte and Al4C3 between the carbon cathode and the metal pad points to limited wettability of the carbon cathode by the molten Al metal pad. The electrolyte found on the cathode surface was shown to consist mainly of phases such as CaF2, Na3AlF6 and Al2O3, while the bulk bath consisted mainly of phases like NaCaAlF6, Na5Al3F14 and Al2O3. These findings demonstrated that the electrolyte on the cathode surface was basic in composition relative to the composition of the bulk bath. The main phases found in the electrolyte within the pores of the cathode lining included NaF, implying a cryolite ratio higher than 3. This confirmed the basic nature of the electrolyte found on the cathode surface. The following hypothesis for the pitting of the cathode surface was proposed based on the autopsies and wetting experiments. Cathode wear is closely linked to the formation of Al4C3, AlF3 content in the electrolyte and the current density. Further, AlF3 is consumed due to the formation of Al4C3 on the cathode surface. The consumption of AlF3 results in a more basic bath which increases the liquidus temperature and results in partly or completely solidification of the molten bath close to the cathode surface. More basic bath also reduces the transport of Al4C3 away from the surface due to the lower solubility of Al4C3 in the basic bath. Frozen bath on the carbon cathode surface also slows down the formation of Al4C3 due to locally reduced current density and carbide dissolution. Supply of fresh molten bath with a higher AlF3 content will restart the wear process through re-melting of the frozen bath or supply of a new reactive liquid phase, causing new carbide formation, dissolution and transport. Increased cathode surface temperature due to hot running cells can also re-melt the frozen bath and restart the wear process. The hypothesis was supported by finite element method simulations of the current density for a situation with partly solidified bath at the cathode surface. The simulations demonstrated that freezing of electrolyte drastically reduces the local current density at the cathode surface, which will locally hinder the formation of Al4C3. The wettability of four different types of carbon cathode materials by molten electrolyte and aluminium was investigated by the immersion-emersion technique. The weight of a cylindrical sample was measured during an immersion-emersion cycle in the molten electrolyte or a combination of the electrolyte and molten aluminium. The electrolyte was contained in a graphite crucible also used as anode. A constant current was applied for 30 s at the final immersion position using the sample as the cathode. The wetting experiments carried out in molten electrolyte revealed that none of the carbon materials was wetted by the molten electrolyte. A change in wettability from non-wetting to wetting and the resulting electrolyte penetration into the carbon material occurred within seconds after cathodic polarization. The wettability remained even when the current was switched off but declined over time. The amount of electrolyte that penetrated the carbon materials can be ranked as Anthracitic > Graphitic > Graphitized > Graphite. It was also shown that all the carbon materials were wetted by the electrolyte in equilibrium with molten Al, but the amount of electrolyte that penetrated into the carbon samples were clearly lower than under cathodic polarization. Moreover, cathodic polarization did not change the wetting induced by molten Al significantly. No wettability was achieved between the carbon materials and the molten Al even with cathodic polarization. Electrolyte and Al4C3 were confirmed present between the carbon cathode and Al metal on the cathodically polarized carbon samplesnb_NO
dc.language.isoengnb_NO
dc.publisherNTNUnb_NO
dc.relation.ispartofseriesDoctoral theses at NTNU;2019:317
dc.titleCathode Wear in Aluminium Electrolysis Cellsnb_NO
dc.typeDoctoral thesisnb_NO
dc.subject.nsiVDP::Technology: 500::Materials science and engineering: 520nb_NO


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