Sponsored by: LMD Aluminum Committee
Program Organizer: Julian V. Copenhaver, Technical/Quality Manager, NSA [[arrowhorizex]] A Division of Southwire, PO Box 500, Hawesville, KY 42348
Thursday, AM Room: A10
February 8, 1996 Location: Anaheim Convention Center
Session Chairperson: Mr. Ray T. Richter, Aluminum Company of America, Alcoa Laboratories, Alcoa Center, PA 15069
8:30 am
A COMPUTER MODEL FOR ALKALI REMOVAL FROM MOLTEN ALUMINIUM: Dr. Christian J. Simensen, SINTEF Materials Technology, PO Box 124, Blindern, N-0314 Oslo, Norway; Dr. Madhu Nilmani, University of Melbourne, Department of Chemical Engineering, Parkville, Victoria 3052 Australia
A computer model has been developed for alkali removal from molten aluminium by reactive gas purging. The model takes into account the bubble size distribution and the bubble expansion as it ascends through the melt. Both bath furnace fluxing and fluxing in continuous series of in-line units are modeled. It was found that the following average bubble diameter could be introduced in the calculations. Where di0 is the initial diameter of the bubble in size class i and Gn and Gni are the total amount of gas entering the refining unit per second and the amount of gas in size class i respectively. The diffusion coefficients of alkali metals were determined by holding aluminium melts and measuring the decay of the alkali metals, in addition to using published values. The computer model was validated using oxygen desorption from water and measuring gas bubble distribution simultaneously. It was then applied to alkali removal from a 4.2-tonne holding furnace. The experimentally determined values and the computed values from the model for the effective average bubble diameter are in reasonable agreement.
8:50 am
THE ALCAN COMPACT DEGASSER [[arrowhorizex]] A TROUGH-BASED ALUMINUM TREATMENT PROCESS. Part I: Metallurgical Principles and Performance: Peter Waite, Alcan International Limited, Arvida Research and Development Centre, 1955 Mellon Boulevard, PO Box 1250, Jonquière, Québec, Canada G7S 4K8; Roger Thiffault, Alcan Smelters and Chemicals Ltd., Grande Baie Works, La Baie, Québec, Canada
In-line degassing processes, employing rotary gas injectors, are used extensively in the aluminum industry for removing unwanted impurities from the liquid metal just prior to casting. Although these processes are efficient, and provide the required level of metal treatment, a significant quantity of between 1 and 3 tons of metal is retained within the degassing unit between casts. Metal retention is a serious drawback for multi-alloy cast shops, due to the scrap metal produced by draining the degassing unit at alloy changes. The Alcan Compact Degasser is a new multi-stage in-line degassing process which treats the liquid metal directly in the trough. Following the initial development period in the laboratory, two years of process optimization in the plant has proven that the Alcan Compact Degasser provides metal treatment efficiencies equivalent to or better than existing technologies, while eliminating metal retention between casts. The metallurgical principles of this process are described, and quantitative plant data are presented which characterize the metallurgical performance with respect to degassing, metal cleanliness, and alkali removal.
9:10 am
THE ALCAN COMPACT DEGASSER [[arrowhorizex]] A TROUGH-BASED ALUMINUM TREATMENT PROCESS. Part II: Equipment Description and Plant Experience: Serge Lavoie, Alcan International Limited, Arvida Research and Development Centre, 1955 Mellon Boulevard, PO Box 1250, Jonquière, Québec, Canada G7S 4K8; Éric Pilote, Marc-André Thibault, STAS, 1846 Outarde, Chicoutimi, Québec, Canada G7K 1H1; Jean-Claude Pomerieau, Alcan Wire & Cable, Lapointe Works, Jonquière, Québec, Canada
This Alcan Compact Degasser is an efficient trough-based molten metal treatment unit designed as an alternative to the existing in-line "heated-box"-based degassers. It was shown in Part I of this paper that in spite of the Alcan Compact Degasser's small size, no concessions were made on its metallurgical performances. The Alcan Compact Degasser is installed directly in the casting trough, which eliminates the metal hold-up between casts. This has several significant advantages. Firstly, no heating system is required, which reduces the equipment and maintenance cost of the machine and simplifies it operation, as compared to the heated-box alternatives. Secondly, the floor space requirement is reduced, making it easier to retrofit in existing plants. Finally, the absence of hold-up eliminates a significant source of metal loss when frequent alloy changes are required. Another key feature of the Alcan Compact Degasser is it modularity. This allows the casting flow rate capability to be accurately adapted to the need and assures extremely good reliability. Various versions of the Alcan Compact Degasser have been used in Alcan for the last two years. This paper firstly describes the industrial unit and its main characteristics. The operational experience with the Alcan Compact Degasser in two Alcan plants is then presented.
9:30 am
HYDROGEN AND INCLUSION CONTENT IN RECYCLED ALUMINIUM AT HOLMESTRAND ROLLING MILL: Dr. Frede Frisvold, Eivind Øvrelid, SINTEF Materials Technology, N-7034 Trondheim, Norway; Per Bakke, Norsk Hydro a.s., Research Centre, PO Box 2560, N-3901 Porsgrunn, Norway; Niels Erik Hald, Thorvald A. Engh, The Norwegian Institute of Technology, Division of Metallurgy, N-7034 Trondheim, Norway
Recycling of aluminium by remelting of lacquered scrap in Norway is only carried out at Hydro Aluminium's Holmestrand Rolling Mill plant. A study has been carried out in order to determine if recycled metal contains higher than normal contents of carbide inclusions and dissolved hydrogen. The inclusion number size distribution was determined using filter samplers and employing automatic image analysis. An exponentially decreasing number size distribution was obtained. It was found that carbide inclusion contents were at the same levels as from primary producers. Dissolved hydrogen [[arrowhorizex]] before degassing [[arrowhorizex]] was, on the other hand, found to be slightly higher than in primary aluminium. Metal refining by a two-stage Hycast degassing unit ensured that both inclusion and hydrogen content were at acceptably low levels before casting.
9:50 am
ALLOYING OF LIQUID ALUMINUM: A. Shafyei, R.I.L. Guthrie, Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street, Montréal, Québec, Canada, H3A 2A7
The addition of high melting point alloying elements is an essential step in the processing of molten aluminum. In this research, the kinetics of dissolution of Mn and Fe particles, freely dispersed in turbulent-liquid aluminum, has been investigated on a laboratory scale. First, the suspension behaviour of alloying elements in liquid aluminum was studied through water modelling experiments as well as dimensional analysis and similarity techniques. Second, mass transfer coefficients from solid Mn and Fe particles into stirred liquid aluminum were measured. Results of high temperature experiments showed that the measured mass transfer coefficients of Mn and Fe particles are strongly dependent on the intensity of the mixing, until the particles became fully suspended. Further increases in the rate of mixing produced negligible changes in mass transfer coefficients. Therefore, from a practical point of view, a very high rate of mixing of the melt is not recommended during alloying process.
10:10 am BREAK
10:20 am
EXPERIMENTAL DETERMINATION OF BUBBLE SIZES IN MELT REFINING REACTORS: Stein T. Johansen, S. Grådahl, Ø. Dahle, I.-R. Johansen, SINTEF Materials Technology, N-7034 Trondheim-NTH, Norway; R. Tangen, MESTECH as, Fyrstikkalleen 21, 0661 Oslo, Norway; Erling Myrbostad, Hydro Aluminium, Metallurgical R & D Centre, N-6601, Sunndalsøra, Norway
An intrusive optical probe for experimental determination of bubble sizes in water models has been designed and tested. The system is using automatic image processing and gives information about bubble size distribution and bubble shapes. Using a standard in-line melt refining rotor, the distribution of bubble sizes has been determined. The paper will discuss the experimental findings for this system. The bubbles close to the rotor are very small and grow due to coalescence towards te reactor surface. The effects of rotor frequency, gas purging flowrate and melt flow-rate will be discussed. Removal rates of dissolved oxygen will be presented and theoretical mass transfer theories will be discussed. Finally, the accuracy of this new bubble probe, compared to resistance probes, will be addressed.
10:40 am
IMPROVEMENT OF IN-LINE MELT CLEANING SYSTEMS AT HOOGOVENS ALUMINIUM NV [[arrowhorizex]] SIDAL: Jan M. Rabenberg, R. Boterman H.B.M. Schulte, Corporate Research Laboratorium, Hoogovens Groep BV, IJmuiden, The Netherlands; D. Van Nieuwerburgh, Hoogovens Aluminium NV, (A Company of the Hoogovens Groep), A. Stocletlaan 87, B-2570 Duffel, Belgium
The cast shop of Hoogovens Aluminium NV [[arrowhorizex]] Sidal in Duffel, Belgium, annually produces 180,000 tonnes of slab and 30,000 tonnes of billets. The casting rate of the large EMC unit goes up to 40 tonnes per hour. A proprietary in-line system, called Sidafil, cleans the melt by means of a purge gas, diffused by rotating nozzles. In view of the increasing quality demands, Hoogovens Aluminium initiated a research project to improve the efficiency of the Sidafil. Degassing of oxygen from water in a full scale water model established the influence of main factors like melt flow rate, purge gas flow rate, rotor size, rotor speed, and baffle positions. Dimensionless groups describe bubble flow regime and rotor mixing characteristics. The general degassing theory, for example as formulated by Sigworth and Engh, provides insight in water model results and cast shop experiments.
11:00 am
EFFECT OF POROUS PLUGS ON METAL QUALITY: Ronald Murga, Casting Quality Process Control Engineer, Ravenswood Aluminum Corporation, P. O. Box 98, Ravenswood, WV 26164
An initial look at what the effect of porous plugs have on molten metal quality in a tilting holding furnace. From a quality viewpoint, how do porous plugs compare to conventional non-porous plug technology in melt preparation? How is the quality of a typical charge of molten metal impacted by the use of porous plugs in the holding furnace for the introduction of process gas?
11:20 am
OPERATING PRINCIPLES AND CAST SHOP APPLICATION OF THE NOTORP HYDROGEN ANALYZER: Luiz C. B. Martins, Leonard S. Aubrey, SELEE Corporation, 700 Shepherd Street, Hendersonville, NC 28792; George M. Grendell, TYK Refractories Company, 301 Brickyard Road, Clairton, PA 15025
The NOTORP Hydrogen Analyzer was developed recently to monitor dissolved
hydrogen content in molten aluminum alloys. The Analyzer uses an
electrochemical cell mounted inside a sampling tube, with one side of the cell
exposed to the molten aluminum and the other side in equilibrium with a
reference gas of known hydrogen content. The electrochemical potential measured
across a solid state proton conducting ceramic element is proportional to the
difference in hydrogen activity. The Analyzer was tested in several cast shops,
demonstrating ease of use, reliability, and rugged design. Comparison of the
"NOTORP" Analyzer with other techniques was also done during these tests, and
the results are presented here. Hydrogen content results were consistent with
other techniques. Advantages of the NOTORP Analyzer were ease of use,
continuous readout capability, and longer probe life.
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