Physical Fields Of The Aluminum Reduction Cell With Novel Cathodes

Worldwide,large commercial aluminum cells are being developed with capacities of 600 kA.Although the Hall-Heroult process has undergone many refinements and considerable improvements in the last few decades,the energy consumption of aluminum electrolysis process is still around 13.2 kWh/kg-Al,with energy efficiency less than 50%.With ongoing research and development efforts,the aluminum industry is constantly searching for ways to maximize productivity and efficiency while reducing energy requirements and costs.One effective method is improving the cathode design to achieve a more stable bath-metal interface.In the present study,electrolytic cells with novel cathodes(flat cathodes with rectangular protrusions,cubic protrusions,cylindrical protrusions and inclined surface)operating at 300 kA line current were developed in Pro/Engineer and simulated in ANSYS to calculate the physical fields and to estimate the impact of novel cathodes on the multi-fields.Through simulation the cathode structure could be optimized.When compared to the conventional cell with flat cathodes,simulated results show that in the conventional cell with flat cathodes,current density in the metal was relatively uniform in the longitudinal direction(y direction)while in the novel cell with modified cathodes,the current flowed around the rectangular protrusions of high electrical resistivity and converged in the grooves between the rectangular protrusions,which made the current density in the grooves larger.Current density in the grooves between the cubic and cylindrical protrusions were also larger than that above the protrusions.The magnetic field calculation adopted the three-dimensional edge element method of higher accuracy and considering the effect of the ferromagnetic components.The strong current in the busbars determined the magnetic flux density of the cells.Consequently,the magnetic distribution in the metal was the same for the conventional cell with flat cathodes and novel cells with modified cathodes.In the metal,Bx was antisymmetric to transversal central axis(y=0)with the maximum and minimum located at the downstream corners.By was antisymmetric to longitudinal central axis(x=0)with the maximum and minimum located at the longitudinal sides.Bz was antisymmetric to transversal central axis(y=0)with the maximum and minimum located at the downstream corners.SST k-? turbulence models were employed to solve the velocity field.SST k-co turbulence models can not only calculate the fully developed turbulence,but also be better in predicting adverse pressure gradient boundary layer flows and separation.By using these turbulence models,near wall velocity around the protrusions could be obtained.In the conventional cell with flat cathodes,two symmetrical vortices were present at different metal levels.In the novel cells with modified cathodes,the flow pattern was consistent with that in the conventional cell at the top metal level whilst at the lower metal level,the two vortices were smashed by the protrusions.Small eddies formed between the protrusions.The velocity at the lower metal level was more evenly distributed over the entire cell than that in the conventional cell.The maximum metal velocity located at the immediate vicinity of the longitudinal side,which was close to downstream vertical busbars and cathode busbars.The more uniform metal velocity reduced the metal wave.Amongst all four types of novel cathodes,the maximum metal velocity and bath-metal deformation were found in the cell using the cathodes with rectangular protrusions,followed by the cathodes with cubic protrusions and cylindrical protrusions.The industrial test results showed that the cathodes with rectangular protrusions,cubic protrusions and cylindrical protrusions could reduce the energy consumption effectively.In addition,the industrial test cell using the cathodes with inclined surface operated at DC energy consumption of 12.724 kWh/kg-Al over 17 months,which was 0.476 kWh/kg-Al less than that of the conventional cells(13.2 kWh/kg-Al).Combined with the production experience of the aluminum reduction cells with novel cathodes,the metal velocities at different metal levels were calculated by coupling the flow field and thermal field with the consideration of the thickness of sideledge and ledge toe extension.The reasonable metal levels were obtained by analyzing the influence of the metal level on the metal velocity.The sodium expansion was taken into account by converting the sodium expansion coefficient to thermal expansion coefficient in the thermal-electro calculation.The cathode block moved upwards and towards the periphery due to sodium expansion and thermal expansion.The periphery was restrained by the steel shell.30 days after start-up the upwards maximum z-direction displacement occurred in the cathode blocks at the cell center.The maximum x-direction displacement of the whole cell was located at the longitudinal side center of the steel shell,while the maximum y-direction displacement of the whole cell was located at the transversal side center of the steel shell.Both the displacements in the x and y directions were less than those in the vertical direction.The steel shell constrained the cathode blocks expansion,so for the four types of the cathode block,stress concentration was present at the cathode corner,which was beyond the mechanical strength of the cathode carbon,and could cause the crack in the cathode block.Large 1st principal stress also existed around the collector bar slots.Equivalent strain of the four steel shells was less than 0.16%,which was not beyond the yield strength 0.2%.The steel shells were in the elastic phase under the current loading conditions.The maximum calculated velocities of 350 kA aluminum reduction cells using flat cathodes,cathodes with 7 cubic protrusions,with 10 cylindrical protrusions and with 12 cylindrical protrusions were presented in decreasing order,which agreed to the industrial test results.Moreover,the most energy-saving cathode with cylindrical protrusions tested in the industrial cells was modified to cubic protrusions based on the larger drag coefficient.At the same arrangement and dimensions(same height 15 cm,edge length equals to circle diameter 22 cm),the cubic protrusions were calculated to perform better in slowing down the metal velocity than the cylindrical protrusions because the drag coefficient of the flat frontal area was larger than that of the round one.The maximum calculated velocities for cylinder62 and cubic62 were 12.4 and 9.6 cm·s-1 respectively,whereas the wave crests were 1.6 and 1.4 cm respectively.