Side Ledge Thickness Research Articles

A Modeling Approach for Time-Dependent Geometry Applied to Transient Heat Transfer of Aluminum Electrolysis Cells

The thermal balance of aluminum electrolysis cells (AEC) have to be rigorously controlled in order to improve the efficiency and sustainability of this industrial process. A new modeling strategy is developed to consider the displacements of solid bodies and moving boundaries in finite element models. The transient thermal-electric modeling of the AEC demonstrates the effect of an increase in operating voltage on both the anode cover and the side ledge. With higher heat generation, the anode cover deteriorates and the side ledge thickness decreases. Since the anode cover is characterized by irreversible transformations, the top heat dissipation remains higher even when the operating voltage comes back to its typical value. For the first time, the transient temperature and electric fields throughout the anode life are simulated and validated by industrial measurements. The modeling predictions have been validated from instrumented anodes and manual measurements, all performed on operating AEC.

Improved heat transfer modeling of the top of aluminum electrolysis cells

Abstract A thermal-electric model is developed using finite elements and thoroughly validated against thermal measurements performed in industrial aluminum electrolysis cells (AEC). Knowledge about the geometries of anode cover material (ACM) and anode crust is improved by measuring their profiles in industrial cells. Moreover, the shape of the cavity formed by the melting of the anode crust is predicted with the numerical model, using a radiosity module combined to an iterative method. The thermal conductivity and the emissivity of both ACM and anode crust are also evaluated based on experimental measurements. The thermal-electric model accurately predicts the measurements obtained from heat flux sensors and thermocouples installed on industrial anodes. Modeling results show that increasing the ACM thickness reduces the top heat losses and increases the heat dissipation from the side, while the bottom losses remain constant. With thicker top insulation, the side ledge shrinks and the anode crust melts. The impact of a film of sludge under the liquid aluminum is quantified with the model. Accordingly, the sludge increases the cathode voltage drop (CVD), enlarges the cavity, reduces the side ledge thickness and amplifies the side heat dissipation. The thermal-electric modeling provides insights to improve the design and operation of AEC in order to reach higher efficiency.

Influence of Electrolyte Overheating and Composition on the Sideledge of an Aluminum Bath

The effect of the electrolyte chemical composition and overheating on the size of a sideledge formed in an aluminum-smelting bath is investigated theoretically. Three electrolyte compositions are chosen: sodium cryolite with the cryolite ratio (CR) = 2.7, cryolite (CR) = 2.7 + 5 wt % CaF2, and cryolite (CR) = 2.7 + 5 wt % CaF2 + 5 wt % Al2O3. The electrolyte liquidus overheating temperatures are 5, 10, 15 and 20oC. The calculations are performed using the finite-element method. A simplified design of an aluminum cell with a prebaked anode is used. To calculate the temperature field, a mathematical model in the Boussinesq approximation is used. The model contains the Navier–Stokes equation, the thermal conductivity equation, and the incompressibility equation. The key role of electrolyte overheating on the sideledge formation is established. The resulting sideledge profile depends on the heat transfer coefficients and thermal properties of materials. The smallest sideledge thickness with the same electrolyte overheating is observed in cryolite with CR = 2.7, 5 wt % CaF2, and 5% by weight of Al2O3, and formed sideledge profiles for cryolite with KO = 2.7 and cryolite with KO = 2.7 and 5 wt % CaF2 almost coincide. The thickness of the sideledge formed with overheating of 5 K is from 7 cm or more, and the difference in temperature between the sideledge touching the electrolyte and airborne block wall is 20–25 K. Almost complete sideledge disappearance occurs when the electrolyte liquidus is overheated by 20 K.

INFLUENCE OF ELECTROLYTE COMPOSITION AND OVERHEATING ON THE SIDELEDGE IN THE ALUMINUM CELL

The paper presents a theoretical study conducted to investigate the effect that the chemical composition of electrolyte and its overheating have on the size of sideledge formed in an aluminum smelting bath. Three electrolyte compositions were chosen: (1) sodium cryolite with the cryolite ratio CR = 2,7; (2) cryolite CR = 2,7 + 5 wt.% CaF2; (3) cryolite CR = 2,7 + 5 wt.% CaF2 + 5 wt.% Al2О3. The electrolyte liquidus overheating temperatures were 5, 10, 15 and 20 °C. Calculations were performed using the finite element method. A simplified design of an aluminum cell was used with a prebaked anode. The temperature field was calculated using a mathematical model based on the Boussinesq approximation, which contains the Navier–Stokes equation as well as thermal conductivity and incompressibility equations. The key role of electrolyte overheating in sideledge formation was established. The resulting sideledge profile depends on the heat transfer coefficients and thermophysical properties of materials. The smallest sideledge thickness with the same electrolyte overheating was observed in cryolite composition 3, and the profiles of the formed sideledge for samples 1 and 2 were nearly the same. The thickness of the sideledge formed with a 5 degree overheating exceeded 7 cm and the difference in temperature between the sideledge in contact with electrolyte and the side block wall was 20–25 degrees. It was found that the virtually total disappearance of the sideledge occurs at electrolyte liquidus overheating by 20 degrees.

Experimental Determination of the Thermal Diffusivity of α-Cryolite up to 810 K and Comparison with First Principles Predictions.

In aluminum electrolysis cells, a ledge of frozen electrolyte is formed on the sides. Controlling the side ledge thickness (a few centimeters) is essential to maintain a reasonable life span of the electrolysis cell, as the ledge acts as a protective layer against chemical attacks from the electrolyte bath used to dissolve alumina. The numerical modeling of the side ledge thickness, by using, for example, finite element analysis, requires some input data on the thermal transport properties of the side ledge. Unfortunately, there is a severe lack of experimental data, in particular, for the main constituent of the side ledge, the cryolite (Na3AlF6). The aim of this study is twofold. First, the thermal transport properties of cryolite, not available in the literature, were measured experimentally. Second, the experimental data were compared with previous theoretical predictions based on first principle calculations. This was carried out to evaluate the capability of first principle methods in predicting the thermal transport properties of complex insulating materials. The thermal diffusivity of a porous synthetic cryolite sample containing 0.9 wt % of alumina was measured over a wide range of temperature (473–810 K), using the monotone heating method. Because of limited computational resources, the first principle method can be used only to determine the thermal properties of single crystals. The dependence of thermal diffusivity of the Na3AlF6 + 0.9 wt % Al2O3 mixture on the microstructural parameters is discussed. A simple analytical function describing both thermal diffusivity and thermal conductivity of cryolite as a function of temperature is proposed.

Impact of the Solidification Rate on the Chemical Composition of Frozen Cryolite Bath

Solidification of cryolite (Na3AlF6)-based bath takes place at different rates along the sideledge, and around alumina rafts and new anodes. The solidification rate has a significant impact on the structure and the chemical composition that determine the thermal conductivity and thus the thickness of sideledge, or the duration of the existence of the temporary frozen bath layers in other cases. Unfortunately, samples that can be collected in industrial cells are formed under unknown, spatially and temporally varying conditions. For this reason, frozen bath samples were created under different heat flux conditions in a well-controlled laboratory environment using the so-called cold finger technique. The samples were analyzed by X-ray Diffractometer (XRD) and Scanning Electron Microscope (SEM) in Back Scattering (BS) mode in order to obtain spatial distribution of chemical composition. Results were correlated with structural analysis. XRD confirmed our earlier hypothesis of recrystallization of cryolite to chiolite under medium heat flux regime. Lower α-alumina, and higher γ-alumina content in the samples obtained with very high heating rate suggest that fast cooling reduces α–γ conversion. In accordance with the expectation, SEM-BS revealed significant variation of the Na/Al ratio in the transient sample.

Thermal conductivity of the sideledge in aluminium electrolysis cells: Experiments and numerical modelling

During aluminium electrolysis, a ledge of frozen electrolytes is generally formed, attached to the sides of the cells. This ledge acts as a protective layer, preventing erosion and chemical attacks of both the electrolyte melt and the liquid aluminium on the side wall materials. The control of the sideledge thickness is thus essential in ensuring a reasonable lifetime for the cells. The key property for modelling and predicting the sideledge thickness as a function of temperature and electrolyte composition is the thermal conductivity. Unfortunately, almost no data is available on the thermal conductivity of the sideledge. The aim of this work is to alleviate this lack of data. For seven different samples of sideledge microstructures, recovered from post-mortem industrial electrolysis cells, the thermal diffusivity, the density, and the phase compositions were measured in the temperature range of 423 K to 873 K. The thermal diffusivity was measured with a laser flash technique and the average phase compositions by X-ray diffraction analysis. The thermal conductivity of the sideledge is deduced from the present experimental thermal diffusivity and density, and the thermodynamically assessed heat capacity. In addition to the present experimental work, a theoretical model for the prediction of the effective thermal transport properties of the sideledge microstructure is also proposed. The proposed model considers an equivalent microstructure and depends on phase fractions, porosity, and temperature. The strength of the model lies in the fact that only a few key physical properties are required for its parametrization and they can be predicted with a good accuracy via first principles calculations. It is shown that the theoretical predictions are in a good agreement with the present experimental measurements.