Mathematical Modeling of Aluminum Reduction Cell MHD Stability

The paper describes a mathematical model of magnetic hydrodynamics and heat transfer in an aluminum electrolyzer. The model takes into account three phases: gas, electrolyte and metal, and investigates their interaction. Mathematical modeling of the dynamics of the aluminumelectrolyte interface is carried out depending on the potential distribution over the anode for the Soderberg electrolyzer and the multi-anode electrolyzer. A numerical study made it possible to conclude that the Soderberg electrolyzer is less MHD-stable than a multi-anode electrolyzer with burnt anodes. Calculations of MHD stability are carried out when changing the shape of the working space of the bath for various forms of accretion and skull. The interface between the electrolyte-metal media and the boundary of the reverse oxidation zone, which is determined by the spatial distribution of the gas phase, were calculated. The calculations make it possible to accurately predict the development of MHD instability in the bath under various conditions of the process, which minimizes the loss of metal current efficiency

Formation of ledge in aluminum electrolyzer

A model unit simulating the actual conditions of electrolytic aluminum production was used to conduct an experimental study of ledge to determine its dynamic behavior (formation/dissolution) depending on the electrolyte overheating temperature, lining thermal resistance and cryolite-alumina electrolyte composition. A window was mounted in the front wall of the unit housing to change the lining material. Ledge is formed due to the heat flow generated by the temperature difference between the electrolyte and electrolyzer walls. The electrolyte cryolite ratio (CR) varied in the range of 2.1–2.5. The alumina concentration in the electrolyte did not exceed 4.5 wt.%. Shape change in the electrolyzer working space during electrolysis was determined by the thickness of the formed ledge on the walls and bottom. The dynamic ledge formation in the experimental cell begins at the overheating of 3–4 degrees. It was found that with a decrease in the thermal resistance of the lining material from 16 to 14 m2/W at the same overheating temperature, the side ledge with a greater thickness was formed, however, the decrease in the thermal resistance hardly affected its thickness when the ledge has been already formed. As in the industrial electrolyzer, the ledge profile formed in the experimental cell can be conditionally divided into three zones: bottom ledge, metal/electrolyte interface ledge and side ledge. The dynamic behavior of the side ledge was different from the bottom ledge: the higher the CR, the thicker the side ledge and the thinner the bottom ledge. Chemical analysis of components in the dry knockout showed that the CR and Al2O3 concentration increase throughout the cell height from top to bottom. It was concluded that the side ledge has a heterogeneous composition depending on the electrolyte composition and cooling rate.

Development and industrial tests of composite material based on TiB2 for repair of local destructions in electrolyzer bottom blocks

The paper presents the developed composition and technology for obtaining a repair mixture consisting of lumped corundum with a TiB2–C composite coating wettable with aluminum for restoration of local bottom block fractures without electrolyzer stops. The proposed technical solution made it possible to reduce bottom wear and increase aluminum electrolyzer service life by 6 months.A mixture of titanium diboride powder and a refractory powder-like binder in a ratio of 50 : 50 (wt.%) was used to obtain the repair mixture with an optimal composition. Then the lumped corundum was coated with the obtained mixture, dried at 150 °C and after that heat-treated under a carbon-bed at t= 700÷900 °C. As a result of reducing firing the TiB2–C composite material with a carbon content of 15–20 wt.% was formed on the surface of lumped corundum. A qualitative evaluation of the properties of the developed composite coating shows that the coating has a sufficiently high hardness, wear resistance and adhesion to the substrate after the heat treatment. For pilot testing, the repair mixture was covered with molten aluminum to obtain an Al–TiB2–C repair mass in the form of plates. The pilot testing of the repair mass on the 400 kA operating electrolyzer in the RUSAL-Sayanogorsk pilot shop showed that the bottom wear have slowed down 3 months after the local fractures were restored without electrolysis bath stops. This fact is evidenced by a 13 % decrease in the average depth of fractures with a stable current value of 4,7–4,8 kA/bloom after repair. Thus, the locallyused repair mass slowed that the overall wear of the cathode surface and allowed to extend the electrolyzer life.