1997 TMS Annual Meeting: Wednesday Session

ALUMINIUM REDUCTION TECHNOLOGY: Session V: Fundamentals

8:30 am

UNDERSTANDING BOUNDARY-LAYERS: Warren Haupin, 2820 Seventh Street Road, Lower Burrell, PA 15068

Understanding the physics and chemistry of boundary-layers, the thin stagnant zones that form at each bath interface, clarifies many cell phenomena. A non-wetting boundary at the anode causes the anode effect. The effect of bath velocity on the boundary-layer thickness and its effect on the heat transfer coefficient coupled with bath temperature and bath composition explains the formation and loss of frozen ledge and its relationship to heat losses from the cell walls. Similarly, the boundary-layer at the bath-aluminum interface ex plains cathodic overvoltage, the sodium content of the aluminum, current efficiency and the relationship of each to metal pad stability (magneto-hydrodynamic stability).

8:55 am

ON THE COMPOSITION OF SOLID DEPOSITS FROZEN OUT FROM CRYOLITIC MELTS: Asbjørn Solheim, Lisbet I. R. Støen, SINTEF Materials Technology, N-7034 Trondheim, Norway

Thermodynamically, the primary crystallization product formed by cooling a melt containing cryolite, AlF3, Al2O3 and CaF2 is expected to be cryolite with minor amounts of CaF2 and AlF3 in solid solution. In practice, however, any deposit frozen out from such melts may contain considerable amounts of bath constituents other than cryolite. The factors governing the composition of the freeze are studied theoretically and by experiment. Besides solid solution, the composition depends on two factors, 1) diffusion of bath constituents from the surface of the deposit towards the bulk of the melt, and 2) dendritic crystal growth with subsequent trapping of bath between the crystals. The condition which gives dendrite formation is formulated. Based on this criterion, a deposit frozen out from melts containing excess AlF3 and alumina would contain more AlF3 than a deposit frozen out from a melt containing excess AlF3 alone. This was confirmed by experiment.

9:20 am

THE GAS UNDER ANODES IN SMELTING CELLS. Part I: MEASURING AND MODELLING BUBBLE RESISTANCE UNDER HORIZONTAL DE-ORIENTED ELECTRODES: T. M. Hyde, B. J. Welch, Department of Chemical Engineering, University of Auckland, New Zealand

With a continuing drive to reduce energy and the tighter operating tolerances of modern aluminium smelting cells, there is an ever increasing need for a precise cell voltage equation. Precise correlation exists for the electrical conductivity in smelting cells, the reverse is not the case for the added resistance effect of gas bubbles. Where it has been allowed for, the models used have been based on those proposed for vertically oriented electrodes. Furthermore the measurements on which the model is based can be influenced by other electrochemical changes while the shape and dispersion and gas bubble differ from those expected in aluminium smelting cells based on model studies. An apparatus and high speed switching data acquisition technique have been developed to allow the ohmic resistance of a molten salt electrolyte to be isolated from all other contributors. Ceramic objects of known geometry, size and volume, rest in the electrolyte immediately under the upper electrode (anode) thus simulating gas bubbles. The technique developed enables the measurement of the direct increase in resistance due to the introduced volume of non-conducting simulated gas. The bubble parameters examined included volume, electrode coverage, depth and shape. The resulting data have been tested against existing models.

9:45 am

THE GAS UNDER ANODES IN SMELTING CELLS. Part II: GAS VOLUME AND BUBBLE LAYER CHARACTERISTICS: R. Aaberg, V. Ranum, The Norwegian University of Science and Technology, 7034 Trondheim, Norway; K. Williamson, B. J. Welch, Department of Chemical & Materials Engineering, The University of Auckland, New Zealand

Recent physical models have suggested that the gas bubbles formed under anodes in smelting cells tend to coalesce and release as large bubbles predominantly. Whilst it is well established that the gas bubbles increase the cell resistance, and correlations proposed include the gas volume or bubble layer depth and surface coverage. However, the volume and degree of coverage of the electrodes have been subject to speculation rather than accurate measurements. In this investigation, using a larger than normal laboratory cell, it has been confirmed that the gas released is consistent with the physical models with most of the gas being evolved in discreet large bubbles. The release frequency is similar to the dominant frequency found in operating cells. The average gas volume under an anode prior to release is between 0.4 and 0.5 cm3 per cm2 of electrode. From combined resistance studies it has been calculated that approximately two thirds of the anode is covered with gas at release with an average bubble thickness of 5 mm.

10:10 am BREAK

10:30 am

MICROPYRETICALLY SYNTHESIZED POROUS NON-CONSUMABLE ANODES IN THE Ni-Fe-Cu-Al SYSTEM: J. A. Sekhar, H. Deng, J. Liu, International Center for Micropyretics, Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012; V. de Nora, MOLTECH S.A., 9, Route de Troinex, 1227 Carouge, Geneva, Switzerland

A micropyretically synthesised porous Ni-Fe-Cu-Al intermetallic composite electrode has been developed for use as a non-consumable anode in the Hall Héroult cell. The oxidation behavior in air and under electrolysis conditions have been studied. The electrode is noted to be resistant to the corrosive conditions encountered during electrolysis for up to 300 hour tests in 10 Amp and 100 Amp cells. The synthesis procedures for the manufacture of the electrodes and the beneficial influence of the pores are discussed. Preliminary results on the metal contamination during electrolysis are also presented.

10:55 am

APPLICATION OF NONEQUILIBRIUM THERMODYNAMICS TO THE LEDGE SURFACE OF ALUMINIUM ELECTROLYSIS CELLS: Ellen Marie Hansen, Signe Kjelstrup, Department of Physical Chemistry, The Norwegian University of Science and Technology, N-7034 Trondheim, Norway

Nonequilibrium thermodynamics for surfaces has been applied to the interface ledge-electrolyte in aluminium electrolysis cells. The method describes the interface as a layer of finite thickness , having a bath-like part and a ledge-like part, with properties differing from the bulk properties. The method can explain the heat transfer coefficient for the interface in a new way, namely as a combination of the thermal conductivities in the two parts of the interface. Examples are given to show how the temperature "jump" between the two bulk phases may either be located in the ledge-like part of the interface, in the bath-like part, or in both. In analysis of ledge growth, the possibility of different surface temperatures should be taken into account, because this temperature may, together with the temperatures close to the interface on both sides, determine the rate of freezing or melting. It is also shown how the simple steady state model can be expanded into a dynamic one.

11:20 am

PHYSICO-CHEMICAL PROPERTIES OF Na3AlF6-AlF3-LiF-CaF2 SYSTEM UNDER THE SAME SOLVABILITY OF Al2O3: Li Dexiang, Chen Jianshe, Li Guohua, Department of Nonferrous Metallurgy, Northeastern University, Shenyang, 110006, China; Ma Xiufang, Beijing Nonferrous Metal Research Institute, Beijing, China

The concept of properties of electrolyte melts under the same solvability of alumina at nonequal temperature, that is, the properties of melts under equal solvability of alumina dissolving in the electrolyte at the temperature which is 20°C above the liquidus, were put forward. Based on this theory, the physico-chemical properties, such as the initial crystallization temperature, solvability of alumina, conductivity, density of (2.23-3.0) NaF·AlF3 - (0-5%) LiF - (0-15 %) CaF2 system were studied. The results will provide more scientific basis for choosing the optimum electrolyte composition in aluminium electrolysis.

11:45 am

MEASUREMENTS OF HF IN STACKS AND POT ROOMS USING A REMOTE SENSING EYE-SAFE LASER INSTRUMENT: H.I. Schiff, Unisearch Associates, Inc., 222 Suidercroft Rd., Concord, Ontario, Canada L4K 1B5

A remote sensing system for continuous measurement of HF and other gases has been developed based on eye-safe turnable gases. The laser, its controls and data acquisition system are contained in a small instrument which can be located anywhere in the plant, such as the control room. The optical beam is transported to one or more stacks or ducts by fibre optics which may be kilometres in length. It is then directed across the stack and returned by a retroreflector to the same optical fibre and retroreflector combination for pot-room or roof-top monitoring. The use of optical multiplexing permits a single instrument to make simultaneous measurements at a number of locations providing a very cost effective system which can operate in any environment. Systems have been installed in four Canadian smelters and one in the UK. Examples of some of the measurements made with these systems will be presented.