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
Tuesday, AM Room: A9
February 6, 1996 Location: Anaheim Convention Center
Session Chairperson: Morten Sorlie, Elkem a/s Research, P.O. Box 40 Vagsbyd, N-4602 Kristiansand, Norway
8:30 am
USE OF OLIVINE AS A DRY MIX BARRIER FOR THE PRODUCTION OF ALUMINIUM REDUCTION CELL POT LINING REFRACTORIES AND METHOD FOR THE MINERALOGICAL IDENTIFYING OF CREATED PHASES: Ove Thornblad, Svenska Hoganas AB, Bruksgatan, S-263 83 Hoganas, Sweden; Egil Grotnes, Hoganas Eldfast AS, Fjellhamarveign 46, Postboks 54, N-1472, Fjellhamar, Norway
In recent years, fireclay bricks and low conductivity insulations have been used in the bottom of alumina reduction cells to save power energy. In order to protect the thermal insulation from attach by bath between the carbon cathode and bottom insulation, an olivine based refractory barrier is developed. Bath resistant tests have shown that the barrier has good resistance to bath attach, bath penetration and thermal conductivity. The material layer, instead of the fireclay bricks layer, was used in a Soderberg alumina reduction cell in Norway. After 16 months operation the bath was not penetrated through the barrier material layer and there was normal heat loss through the bottom. A petrological method, by microscopical examination of thin sections, has been used for the identifying of created phases.
9:00 am
A CRITICAL ANALYSIS OF SODIUM MEMBRANES TO PROTECT SODIUM RELATED CARBON CATHODE DAMAGE IN THE HALL- HEROULT CELL: J.A. Sekhar, J. Liu, J.J. Duruz, International Center for Micropyretics, University of Cincinnati, Cincinnati, OH, 45221-0012; V. de Nora, MOLTECH S.A., 9, Route de Troinex, 1227 Carouge, Geneva, Switzerland
During the past few years, cathodic membranes have become available which are designed for improved wettability and reduced sodium attack of the cathode. The microstructure and sodium trapping mechanisms of such membranes are discussed. The efficacy of the different microstructures in relation with their electrical and chemical properties towards the prevention of sodium attack is illustrated for various start-up conditions and current densities. Results from using such membranes in commercial cells will also be shown.
9:30 am
INFLUENCE OF TEMPERATURE AND CONTACT PRESSURE BETWEEN CAST IRON AND CATHODE CARBON ON CONTACT RESISTANCE: Frank Hiltmann, Jorg Mittag, SGL CARBON AG, Werk Griesheim, Stroofstrasse 27, D-65933 Frankfurt, Germany. Anne Store, Harald A. Oye, Institute of Inorganic Chemistry, The Norwegian Institute of Technology, N-7034 Trondheim, Norway
The contact resistance between cathode block and cast iron as collector bar casting agent is said to give a significant contribution to the cathode voltage drop (CVD) measured in aluminum electrolysis cells. Five typical cathode carbon qualities were examined in this respect from room temperature to 950deg.C. At cell operating temperature the contact resistance is found to be usually negligible in comparison to other resistance effects. However, inadequate casting parameters and cell ageing processes leading to an improper contact or contact pressure may impede the current flow between cast iron and cathode block considerably and hence result in an increasing CVD with cell age.
10:00 am BREAK
10:15 am
CHARACTERIZATION OF ANODE STUB CORROSION IN HALL REDUCTION CELLS: Xiangwen Wang and Ray D. Peterson, Manufacturing Technology Laboratory, Reynolds Metals Company, 3326 East 2nd Street, Muscle Shoals, AL 35661-1258
Mild steel is widely used as a structural material in the aluminum smelting industry. In prebaked-anode reduction cells, the stability of the steel used as an anode "stub" against high temperature oxidation and corrosion is very important with regard to its full service life and maintaining aluminum cleanliness. This paper deals with the accelerated corrosion of the steel material used as anode stubs in the presence of the sulfur-containing anode gases. Oxidized scale and the interface region of the oxidation reaction zone in a stub from a reduction cell were fully examined using scanning electron microscopy and Z-ray diffraction. The sulfur from the bath and the anode carbon, released as SO2, play an important role in accelerating the anode stub corrosion process. A sulfidation-oxidation corrosion mechanism is proposed to support the corrosion phenomena observed on the steel anode pieces.
10:45 am
CRITICAL ISSUES FOR THE DEVELOPMENT OF AN OXIDATION TREATMENT FOR CARBON ANODES: J.A. Sekhar, J. Liu, International Center for Micropyretics, University of Cincinnati, Cincinnati, OH, 45221-0012; V. de Nora, MOLTECH S.A., 9, Route de Troinex, 1227 Carouge, Geneva, Switzerland
The oxidation of the carbon anodes in the Hall-Heroult Cell, leads to an increase of the cost of aluminum on account of a consumption of more than 0.43kg of carbon per kg of aluminum produced instead of the theoretical amount which is 0.333kg. Several oxidation prevention treatments have been proposed which have worked in the laboratory, but have fallen short of the expected performance when the same treatments have been applied to a commercial anode. An investigation of this effect was carried out and a new theory is proposed to explain the anode degradation. It is found that a brittleness limit may be encountered in the cell prior to oxidation related loss. This limit is examined and a new treatment is proposed to improve the anode consumption by reducing the anode brittleness and further improving the oxidation resistance.
11:15 am
THE UTILISATION OF COMPOSITE CARBON-SILICON CARBIDE SIDEWALL BLOCKS IN CATHODES: Edward I. Curtis, Paul D. Mascieri, Alton Tabereaux, Reynolds Metals Company, Corporate Research and Development, 3326 East Second St, Muscle Shoals, AL 35661-1258
A recently developed carbon-silicon carbide composite sidewall block has
performed perfectly in the cathode lining of 180kA prebake reduction cells for
nearly two years. The composite block provides the opportunity to extend the
potlife of cells operating with conventional carbon sidewalls, or can offer
excellent cost saving while maintaining the desired operational results in
cells using full size silicon carbide bricks. The composite block consists of a
calcined anthracite carbon block, nitride-bonded silicon carbide and a glue or
carbonaceous cement. The exact thickness or dimensions of the carbon and
silicon carbide layers are optimised for each reduction cell sidewall design.
Considerable time and research was dedicated to the bonding of the composite
materials in order to develop the required strength to withstand the extremely
harsh environment in the reduction cell. The advantage of the silicon carbide
layer is to provide excellent resistance to erosion and oxidation of the
sidewall lining while the carbon provides the added flexibility in design
specification due to the case of machining. This paper discusses the research
work involved in developing the required composite bond strength, thermal
conductivity calculations for application of the composite block in cells, and
measured plant results of side ledge freeze profiles and cathode shell
temperatures to date.
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