Egil Skybakmoen

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:

Current efficiency in the Hall–He´roult process for aluminium electrolysis: experimental and modelling studies

\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}

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

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.

Measurement of Cathode Surface Wear Profiles by Laser Scanning

The influence of electrolyte impurity species on current efficiency with respect to aluminium (CE) was studied in a specially designed laboratory cell at 980∘C, with a graphite anode and a cathodic current density of 0.85Acm−2. The electrowinning was performed in a base melt of Na3AlF6 with a NaF/AlF3 molar ratio of 2.5 and with 4–6wt% Al2O3 and 5wt% CaF2. Impurity species, probably present in only one valence state in the electrolyte, Mg, Ba and B, had no measurable effect on CE for low impurity concentrations. Sn, added to the electrolyte as SnO2, also did not affect current efficiency, probably due to its low solubility. The results show a linear decrease in CE with increasing electrolyte concentration of the polyvalent impurity species from the elements, Fe, P, V, Si, Zn, Ti and Ga. The decrease was found to be within the range 0.1 to 0.7% in C E per0.01wt% of impurity cations present in the electrolyte, with phosphorus ions as the most detrimental. The effects of the individual impurity species on CE appear to be roughly additive for electrolytes containing more than one impurity species. The results obtained cannot be explained by a simple codeposition mechanism or a single reduction to a soluble species of a lower valency. The most likely mechanism explaining the observed decrease in CE for a large number of impurity species is cyclic redox reactions in the cathode and anode/CO2 interfacial boundary layers. Such a mechanism may also be the dominant one in commercial cells, since the impurity levels are of the same size as in the laboratory cell.

Quality Evaluation of Nitride Bonded Silicon Carbide Sidelining Materials

Results are presented from a laboratory study of the influence of electrolyte composition, temperature, cathodic current density and interpolar distance on the current efficiency with respect to aluminium (CE). The current efficiency was determined from the weight gain of metal, in a laboratory cell designed to attain good and reproducible convective conditions, and with a flat cathode surface which ensures uniform cathodic current distribution. The cell is believed to more closely represent conditions in industrial cells than traditional small-scale cells, and is a good basis for an experimental study of the influence of isolated variable parameters on the current efficiency with respect to aluminium. The results show a nonlinear decrease of CE with increasing electrolyte temperature, a close to linear decrease of CE with increasing NaF/AlF3 ratio in the electrolyte, a slight increase of CE with increasing electrolyte CaF2 concentration, and no influence of electrolyte Al2O3 concentration on CE. A current efficiency model, based on previous work and theory of electrochemistry and mass transport, shows good agreement with the obtained results.

INVESTIGATION OF THE CATHODE WEAR MECHANISM IN A LABORATORY TEST CELL

The present work deals with the investigation of an electrolytic method for titanium production that uses TiO2 enriched titania slag as raw material. The process involves two steps: i) carbothermal reduction of the slag to form titanium oxycarbide powder; and ii) electrolysis in a molten chloride-based electrolyte using a titanium oxycarbide consumable anode. Electrochemical studies show the stability of the different Ti species in the equimolar NaCl-KCl melt at 850oC. These results, together with previous work about the anodic oxidation mechanism of a consumable titanium oxycarbide anode in molten chlorides, allow us to optimize the anode and cathode voltages in the electrolysis experiments. The results show that best quality titanium deposits are obtained when the reduction occurs in a single electrochemical step, i.e. directly from di-valent titanium species to Ti metal. Then, the complete conversion of the Ti(III) ions released from the consumable oxycarbide anode to Ti(II) species by adding Ti sponge to the electrolyte, must be fulfilled.

Design and performance of a laboratory cell for determination of current efficiency in the electrowinning of aluminium

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.

Interaction of Sodium Vapor and Graphite Studied by Thermogravimetric Analysis

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.