Asbjørn Solheim

Liquidus temperatures for primary crystallization of cryolite in molten salt systems of interest for aluminum electrolysis

AbstractTemperatures for primary crystallization of Na3AlF6 in multicomponent electrolyte systems of interest for the aluminum electrolysis process were determined by thermal analysis. The results are presented as binary and quasibinary diagrams and discussed in view of the literature data. An empirical equation describing liquidus temperatures for primary crystallization of Na3AlF6 was derived:

Liquidus Temperature and Alumina Solubility in the System Na3AlF6-AlF3-LiF-CaF2-MgF2

\begin{gathered} t/(^\circ C) = 1011 + 0.50[AlF_3 ] - 0.13[AIF_3 ] - \frac{{3.45[CaF_2 ]}}{{1 + 0.0173[CaF_2 ]}} \hfill \\ + 0.124[CaF_2 ] \cdot [AlF_3 ] - 0.00542([CaF_2 ] \cdot [AlF_3 ])^{1.5} \hfill \\ - \frac{{7.93[Al_2 O_3 ]}}{{1 + 0.0936[Al_2 O_3 ] - 0.0017[Al_2 O_3 ]^2 - 0.0023[AlF_3 ] \cdot [Al_2 O_3 ]}} \hfill \\ - \frac{{8.90[LiF]}}{{1 + 0.0047[LiF] + 0.0010[AlF3]^2 }} - 3.95[MgF_2 ] - 3.95 \hfill \\ \end{gathered}

The Effect of Current Density on Cathode Expansion During Start-Up

wheret is the temperature in degree Celsius and the square brackets denote the weight percent of components in the system Na3AlF6-AlF3-CaF2-Al2O3-LiF-MgF2-KF. The composition limitations are [AlF3] ≈ [CaF2] ≈ [LiF] < 20 wt pct, [MgF2] ≈ [KF] < 5 wt pct, and [A12O3] up to saturation.

Concentration Gradients of Individual Anion Species in the Cathode Boundary Layer of Aluminium Reduction Cells

A literature review concerning the heat transfer coefficient between bath and sideledge (h) is given. Normally, the heat transfer is controlled mainly by the circulating bath motion due to gas drainage into the peripheral channel. After the introduction of slotted anodes that direct the gas towards the centre channel, it is likely that h will be determined by natural convection in some cases, and an equation is suggested to take this into account. The coupling between heat and mass transfer during melting and freezing of sideledge was studied in a numerical model involving multicomponent diffusion. During freezing, the concentration of bath components other than cryolite is higher at the ledge surface than in the bulk of the bath. The surface temperature of the ledge varies with the rate of freezing and melting, in such a way that the variation of the ledge thickness becomes slower than thought earlier.

Heat Recovery from Aluminium Reduction Cells

The liquidus temperature for primary crystallization of cryolite in the system Na3AlF6-AlF3-LiF-CaF2-MgF2 was determined by thermal analysis. The data were fitted to an empirical equation, valid from 1011 °C to approximately 800 °C. The alumina solubility was determined from the weight loss of a rotating sintercorundum disc. The investigated temperature range was 850–1050 °C, and the data were fitted to an empirical expression. The data are also presented in the form of quasi-binary phase diagrams.

High Frequency Measurements of Current through Individual Anodes: Some Results from Measurement Campaigns at Hydro

The service life time for high amperage aluminium reduction cells with graphitized cathodes is limited by cathode wear. The wear is normally very non-uniform, and it is commonly documented by photography and/or manual point measurements. In an attempt to record the wear pattern in a much more detailed way, a laser scanning procedure was developed. A laser scanner with a single point accuracy of 10 mm has been used to produce a 3D model based on three overlapping scans with an average resolution of about 1 cm. The same cathodes were also measured manually for comparison. The method developed gives detailed information regarding the wear at different positions within the cell, and it may become a valuable tool for investigating the influence of different parameters on the cathode wear.

INVESTIGATION OF THE CATHODE WEAR MECHANISM IN A LABORATORY TEST CELL

During start-up of aluminium reduction cells, sodium penetration causes expansion in the carbon cathode, which may influence the lifetime of the cathode lining. Traditionally, the sodium expansion has been measured at cathode current densities up to 0.75 A/cm2. However, it is well known that the current distribution in the cathode is non-uniform, and high local current densities may be experienced close to the sideledge, which commonly is associated with the W wear pattern. Hence, the sodium expansion may cause both local stresses in the cathode blocks, as well as in the total cell lining. The aim of this study is to determine the sodium expansion over a wider range of current densities. Typically, it is found that the sodium expansion starts to increase again above 0.7 A/cm2, after the plateau reached at 0.2 A/cm2. Apparently, this second increase continues outside the range of 1.5 A/cm2 applied in this work.

Current Efficiency in Aluminium Reduction Cells: Theories, Models, Concepts, and Speculations

It is well known that the system NaF-AlF3, which constitutes the “backbone” of the electrolyte used in primary aluminium manufacture, forms Na+, F-, and a number of fluoro-aluminate anion complexes in the molten state. Since mainly the Na+ ion carries electric current, the aluminium-containing complexes must diffuse towards the cathode, resulting in concentration gradients in the cathode boundary layer. Starting from a structural model for the melt containing five anion species, it was possible to calculate the concentration gradients of the individual ions using the Stefan-Maxwell equation for diffusion in a multi-component system. Generally, AlF4- and Al2F7- were transported towards the cathode, while F- and AlF63- moved away from the cathode. For NaF/AlF3 molar ratios higher than 2.0, AlF52- moved towards the cathode, while it diffused away from the cathode in more acid melts.