Investigation of options to extend cathode life using TiB2 granules in aluminium electrowinning cells

Abstract

Previous research showed that Refractory Hard Metals (RHM) such as titanium diboride (TiB2) are good for use as cathodes in aluminium smelting cells since these have extremely high resistance to attack by the electrolyte and metal. However, it has not been practical to construct a cathode directly with TiB2, while attempts to use the material as bonded composites had economic and practical drawbacks. With the greater emphasis on productivity and energy reduction, the aluminium smelting industry has moved from using anthracitic materials to graphitized carbon as cathodes. This has resulted in a reduction in achievable cell life from in excess of 3000 days to typically less than 2000 days, with the accompanying increase in generation of considerable toxic waste. This thesis is aimed at exploring the potential of inexpensively combining the properties of TiB2 with graphitized carbon to achieve longer cell life to about 3000 days. Two options were isolated as possible paths. The first was the in situ formation of TiB2 from TiO2 and B2O3 precursors through laboratory experiments. The second was based on direct spreading of TiB2 granules onto the surface of graphitized carbon cathode block. The latter was expected to provide a stable layer that would be formed due to the wetting of the granules with aluminium, acting to immobilise the particles. The option of in situ formation failed since formation of a coherent, thick and stable layer was not possible. However, the second option was successful and slurry was formed on the cathode surface. Comparative measurements demonstrate that it resulted in a reduction in the wear rate of the graphitized cathodes from 50 mm/year conservatively to 26 mm/year. The research to establish this was carried out in three phases. The first phase was the preliminary experiments to optimise method of addition and type of TiB2. The second phase was to add titanium diboride granules 3-5 mm screen size into a new industrial operating cell to obtain a surface coverage of 19 kg/m2 cathode area. This was followed by periodic detailed measurements of the cathode wear versus a reference cell of identical design and operating parameters at approximately six monthly intervals for about two years. Other routine measurements such as freeze profile, metal velocity, metal purity were also carried out. The investigation also involved a detailed cathode surface analysis and examination after cutting the cell out of operation. This was to give both visual confirmation of the behaviour of the slurry, cross check measurements of wear, and to ensure no other abnormalities had occurred (a partial autopsy). As a last phase, periodically collecting and filtering cell metal samples through Porous Disc Filtration Analyzer (PoDFA) and subsequently using Scanning Electron Microscope (SEM) to determine whether there were significant inclusions in the metal. Additional samples were collected through different stages of metal processing till the final billet production in Cast house in order to study metal cleanliness. A successful wear reduction was achieved to the extent that the average cathode lives is expected to be extend to approximately 3000 days if quality standards are maintained. There was also a substantial reduction in aluminium carbide in the product metal. The latter provides a potential explanation of the wear reduction mechanism. This is in line with observations of earlier researchers that a viscous layer formed due to wetting of the TiB2 by aluminium prevents sodium absorption into the cathode as well as access of electrolyte to the aluminium metal thus inhibiting formation and reactions of aluminium carbide. Preliminary calculations indicate a beneficial payback due to potential life extension, reduction in waste generation and increased cell availability. However, since this was a trial of a single cell, it is recommended that further trials be conducted to validate the findings from this study and determine the best granulometry of the TiB2to be added for achieving the optimum packing density. The loss of TiB2 due to continuous dissolution in metal and periodical metal tapping should also be accommodated by increasing the amount of addition.