Sponsored by: LMD Aluminum Committee
Program Organizer: Ms Fiona J Stevens, Comalco Research and Technology, Comalco Research Centre, PO Box 316, Thomastown, Victoria 3074, Australia
Monday, PM Room: A9
February 5, 1996 Location: Anaheim Convention Center
Session Chairperson: Alton Tabereaux, Reynolds Metal Company, Corporate Research and Development, 3326 East Second St, Muscle Shoals, AL 35661-1258
2:00 pm
AN IMPROVED METHOD FOR CURRENT EFFICIENCY DETERMINATION IN A LABORATORY ALUMINUM CELL: M.R. Dorreen, M. Hyland and B.J. Welch, Department of Chemical and Materials Engineering, The University of Auckland, Auckland New Zealand; J.M. Purdie, Comalco Research and Technology, PO Box 316, Thomastown, Victoria, Australia
Obtaining rapid, continuous current efficiency measurement is important when using a laboratory cell to investigate the disputed relationship between current efficiency and alumina concentration. An improved method of measuring continuous current efficiency in a laboratory cell has been developed. The method uses mass spectrometry to perform a total mass balance of oxygen in the cell. The cell is flushed with argon, carrying the anode gases through solid state mass flow meters to a previously calibrated quadrupole mass spectrometer. An algorithm corrects the C0 concentration by subtracting the "shadow" C0 resulting from fragmentation of C02 in the mass spectrometer. The calibration algorithm also accounts for mass spectrometer drift during the course of the experiment. Cell data and gas composition are recorded, so that with a known starting bath composition the current efficiency can be calculated with respect to cell temperature and aluminum concentration.
2:30 pm
PROCESSING SILICON, SILUMIN AND ALUMINIUM FROM FELDSPARS - A METHOD TO REGULATE THIS Si, AlSi ALLOYS AND AL AMOUNTS BY A CONTINUOUS WAY: Jan R. Stubergh, Oslo College, School of Engineering, Section of Chemistry, Cort Adelers gate 30, N-0254 Oslo, Norway
Feldspars are mixed with cryolite and electrolyzed at about 1000deg.C. In the first bath silicon "metal" is deposited in a high purity state and in a desired chosen amount. A carbon cathode is placed at the top of the bath and the carbon anode in the bottom of the bath. By electrolysis CO2 is formed at the anode and bubbles through the bath in good contact with the silicon crystals.Si4+ diss + 4e -> Si(s). In the second bath the rest of deposited silicon "metal" and of Si(IV) from the first bath is reduced by aluminium metal at the same temperature. Silicon is deposited in aluminium metal as a silumin alloy and removed from the bottom of the bath. 3Si4+ diss + 4Al(l) -> 3Si (l) +4Al3+ diss Si(s) + Al(l) -> AlSi(l). In the third bath the Si(IV) poor electrolyte from the second bath is electrolyzed by using aluminium metal as a cathode. The Al metal is deposited in the bottom of the bath. Al3+ diss + 3e -> Al(l).
3:00 pm BREAK
3:10 pm
INERT ANODES FOR THE PRIMARY ALUMINIUM INDUSTRY: AN UPDATE: R.P. Pawlek, Technical Info Services and Consulting, Avenue du Rothorn 14, CH-3960 Sierre, Switzerland
The recent development of inert anodes for the primary aluminium industry is reviewed. The types of high temperature conducting oxide and metal electrodes are considered as well as the developments and research under various problems. Of special interest are the stability and corrosion resistance, the metal contamination, mechanical integrity and practical and economic fabrication.
3:40 pm
THE BEHAVIOUR OF NICKEL FERRITE CERMET MATERIALS AS INERT ANODES: Espen Olsen and Jomar Thonstad, Department of Electrochemistry, The Norwegian Institute of Technology, N-7034, Trondheim, Norway
Various compositions of nickel ferrite cermets were tested in a laboratory cell. The materials were based on the copper-containing cermet material (NiFe204+Ni0+Cu) originally developed by Alcoa. Electrical and chemical properties were studied as function of the content of excess nickel oxide. A new method of powder preparation was used, giving smaller grain size and somewhat higher electrical conductivity than before. Unusual electrical properties were found for a material containing no excess nickel oxide. The materials were tested as anodes in a conventional electrolyte for up to 50 hours at a current density of 0.8 A/cm2. The contamination of anode components in the deposited metal varied for the different materials, but typically 2200 ppm Fe, 400 ppm Ni and 450 ppm Cu were found. The corrosion rates were very low, corresponding to 0.12 cm/year. Mass transfer of impurities from the bath into the metal was slow with mass transfer coefficients of the order of 10-7 m/s.
4:10 pm
TIN DIOXIDE-BASED CERAMICS AS INERT ANODES FOR ALUMINIUM SMELTING: A LABORATORY STUDY: A.M. Vecchio-Sadus, D.C. Constable, R. Dorin and E.J. Frazer,CSIRO Division of Minerals, PO Box 124, Port Melbourne, Victoria 3207, Australia; M.B. Trigg, G.S. Neal, S. Lathabai and I. Fernandez, CSIRO Division of Materials Science and Technology, Locked Bag 33, Clayton, Victoria 3168, Australia
The behaviour of tin dioxide-based ceramics as inert anodes was examined in a
laboratory-scale aluminium smelting cell over a range of electrolyte
compositions with operating temperatures between 830-975[[ring]]C. Anodes of a
nominal composition Sn02 (96 wt%), Sb203 (2 wt%) and Cu0 (2 wt%) were
electrolyzed for 90 min at a current density of 1Acm-2. The
corrosion rate was determined by total tin and copper analyses of the recovered
electrolyte, metal and fume. The corrosion rates were 12.5, 1.6 and 6.5 mg
(Ah)-1 in electrolytes with bath ratios 1.5 (975[[ring]]C), 0.89
(903[[ring]]C) and 0.74 (830[[ring]]C), respectively. A four-fold increase in
corrosion rate was obtained at open circuit demonstrating the protection
provided by oxygen evolution during electrolysis. Scanning electron microscopy
coupled with EDS anlaysis revealed a depletion of copper from the anode and a
build-up of an alumina-rich surface layer under certain conditions. A
part-factorial experimental design was employed to assist in the optimization
of the additive levels in the anode.
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