Sándor Poncsák

Thermal conductivity of halide solid solutions: measurement and prediction.

The composition dependence of the lattice thermal conductivity in NaCl-KCl solid solutions has been measured as a function of composition and temperature. Samples with systematically varied compositions were prepared and the laser flash technique was used to determine the thermal diffusivity from 373 K to 823 K. A theoretical model, based on the Debye approximation of phonon density of state (which contains no adjustable parameters) was used to predict the thermal conductivity of both stoichiometric compounds and fully disordered solid solutions. The predictions obtained with the model agree very well with our measurement. A general method for predicting the thermal conductivity of different halide systems is discussed.

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.

Modeling the Behavior of Alumina Agglomerate in the Hall‐Héroult Process

During the feeding of the Hall-Heroult cell, cold alumina comes into contact with the electrolyte bath and tends to agglomerate due to the formation of a frozen bath layer that holds physically the alumina together. This agglomeration, producing alumina agglomerates, called also lumps or sludge, affects both the rate of alumina dissolution and the stability of the cell. In this paper, all the physical phenomena (heat and mass transfer) between the formation and the complete dissolution of agglomerate will be described and are defined by a set of equations. This mathematical model allows observing the strong coupling between the mechanisms of heat and mass transfer e.g. the solidification with diffusion of chemical species, the infiltration of the bath in the porous agglomerate and the dissolution of sintered alumina. A parametric study using this model might identify the most important factors related to the lifespan and behavior of alumina agglomerates.

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.

Structural Characterisation and Thermophysical Properties of the Side Ledge in Hall-Héroult Cells

In the modern Hall-Heroult cells, a frozen bath layer — the side ledge — protects the sidewalls from the very corrosive liquid bath. This frozen bath layer has a significant impact on the heat balance of the cells as well as on the bath chemistry. For this reason, the geometry, the structure, the distribution of the chemical composition and certain physical properties of the side ledge must be studied. Those characteristics are important to the development of a mathematical model for the Hall-Heroult cells. Despite of all the research efforts invested, only a few results are available in the published literature. This paper presents a few results and observations, obtained by the analysis of side ledge samples, extracted from post-mortem cells. The results show a very inhomogeneous structure and a strong dependence of the thermophysical properties on material structure and temperature.

Effect of the Bubble Growth Mechanism on the Spectrum of Voltage Fluctuations in the Reduction Cell

The spectrum of the voltage oscillations in the aluminum reduction cell depends on the size of the anode. Small laboratory cells and industrial size cells exhibit different fluctuation patterns. The growth of bubbles can be divided into two periods controlled by different physical mechanisms. The detachment frequency of the bubbles from the nucleation sites depends on the current density and detachment size, the latter being influenced by the material, microstructure and shape of the anode bottom. After detachment the main factor causing growth of the traveling bubbles is coalescence. A mathematical model that keeps track of each and every individual gas bubbles generated under the anode was developed and used to analyze the character of the fluctuations of the cell voltage. It is shown that in the case of industrial size anodes the voltage oscillations are dominated by the dynamics of the bubble interactions (coalescence) in the two-phase layer, while in small size laboratory systems the frequency of nucleation of the individual bubbles can be observed.

Simulator of Non-homogenous Alumina and Current Distribution in an Aluminum Electrolysis Cell to Predict Low-Voltage Anode Effects

Perfluorocarbons are important contributors to aluminum production greenhouse gas inventories. Tetrafluoromethane and hexafluoroethane are produced in the electrolysis process when a harmful event called anode effect occurs in the cell. This incident is strongly related to the lack of alumina and the current distribution in the cell and can be classified into two categories: high-voltage and low-voltage anode effects. The latter is hard to detect during the normal electrolysis process and, therefore, new tools are necessary to predict this event and minimize its occurrence. This paper discusses a new approach to model the alumina distribution behavior in an electrolysis cell by dividing the electrolytic bath into non-homogenous concentration zones using discrete elements. The different mechanisms related to the alumina distribution are discussed in detail. Moreover, with a detailed electrical model, it is possible to calculate the current distribution among the different anodic assemblies. With this information, the model can evaluate if low-voltage emissions are likely to be present under the simulated conditions. Using the simulator will help the understanding of the role of the alumina distribution which, in turn, will improve the cell energy consumption and stability while reducing the occurrence of high- and low-voltage anode effects.

Study of the Impact of Anode Slots on the Voltage Fluctuations in Aluminium Electrolysis Cells, Using Bubble Layer Simulator

There is a constant effort from aluminium producers to reduce energy consumption of the Hall-Heroult cells in order to decrease cost and environmental fingerprint. Among others, slotted anodes were introduced in order to promote faster evacuation of the electrically isolating anode gas bubbles and thus diminish their contribution to the total cell voltage. A bubble layer simulator was developed to reproduce cell voltage fluctuations, caused by the dynamics of anode bubbles. Results of simulations show that the slots cut in the right position and direction can reduce both the amplitude of fluctuations and the average cell voltage. This impact is even higher for new, almost horizontal flat anode bottoms. It is also revealed that the slots are acting mainly as a simple bubble sink, but they also contribute to the acceleration of the bubble layer as well and thus their role in the momentum exchange between liquid and gas must be taken into account.

Spreading of Alumina and Raft Formation on the Surface of Cryolitic Bath

It is a well-known phenomenon in aluminum industry that alumina powder, fed into the electrolyte, tends to stay afloat on the surface of the bath. This hinders dispersion and direct contact of most of the powder with the electrolyte, therefore delays dissolution. In addition, large clusters of alumina particles sintered together during raft formation might lead to the agglomerate piercing through the bath-aluminum interface, which should be avoided. Since the interference between raft formation and alumina dissolution is significant, it deserves more attention. Several experiments were conducted in a small carbon crucible in which cryolitic bath was melted and alumina of different properties was fed. The injections were recorded by a video camera above the bath. The spreading of the powder on the surface, the infiltration of powder by the bath as well as the disintegration or the sinking of the raft was observed and the results analyzed.

New Approach for Quantification of Perfluorocarbons Resulting from High Voltage Anode Effects

The methodology used for accounting PFC emissions resulting from aluminum smelting have been developed nearly 20 years ago. There has not been any update on this methodology since 2008 and recently published research demonstrated possible avenues for improvements resulting in increased accuracy. Current models used in the aluminum industry quantify PFC emissions linearly, based on the monthly average polarized anode effect duration for entire pot lines. To potentially increase the precision of the quantification process, other approaches, composed of non-linear models, were used to estimate the emissions of PFC during individual high voltage anode effects. Gas monitoring was performed in different smelters and the efficiency of these methods was evaluated and discussed. The potential benefits and drawbacks related to those quantification methodologies were evaluated by extrapolating results for significant operation periods. Additionally, with this analysis, it was possible to assure time-consistency between the original quantification methodology and the new approaches explored in this paper.