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1997 TMS Annual Meeting: Tuesday Session

CAST SHOP TECHNOLOGY: Session IV: Metal Treatment-Fluxing

Sponsored by: LMD Aluminum Committee
Program Organizer: Wolfgang A. Schneider, VAW aluminium AG, Research and Development, Georg-von-Boeselager-Str.25, D-53117 Bonn, Germany

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Room: 230B

Session Chairperson: Julian V. Copenhaver, NSA - A Division of Southwire, P.O. Box 500, Hawesville, KY 42348

2:00 pm

ON THE EFFICIENCY OF IN-LINE DEVICES TO CLEAN THE MELT: P. Pouly, E. Wuilloud, Alusuisse Technology and Management Ltd., Technology Center Chippis, 3965 Chippis, Switzerland

Rotating impellers for injecting gas into aluminium melt are an often used in-line device to remove impurities. However, the treatment efficiency is seldom investigated, as the precise effect of flotation is not well understood. Moreover, the interpretation of the results obtained by measuring the inclusion content is not always obvious. For these reasons, it is sometimes observed that rotors work in an erroneous way. This paper illustrates how the variation of some parameters can affect, either in a good or a bad way, the efficiency of rotating impellers. The problematic of measurement techniques with some of their limitations will also be exposed. A special emphasis will be laid on inclusion removal.

2:20 pm

THE FLUID MECHANICS IN THE HI10 HYCAST REACTOR: Bodil Hop, Frede Frisvold, Stein Tore Johansen, SINTEF Materials Technology, N-7034 Trondheim, Norway; Bjørn Rasch, Hydro Aluminium a.s R&D Centre, N-6600 Sunndalsøra, Norway

The flowfield in the HI10 HYCAST reactor is investigated both theoretically and experimentally. The paper demonstrates how fluid flow calculations can be used to interpolate between experimental velocities. The experiments are performed by a two component Laser Doppler-technique in a reactor which has one single baffle. In the 3-dimensional computation a rotating grid for the rotor is imposed on the stationary grid for the reactor body. The flow effects caused by the baffle are discussed in relation to metallurgical performance. The turbulent dissipation rate is translated into bubble sizes and the result is compared to experimental bubble sizes. The paper discusses the reasons for deviations between predicted and experimental velocities.

2:40 pm

IN-LINE FLUXING WITH HIGH SPEED MULTIPLE DISPERSER ROTORS: D.C. Chesonis, H. Yu, Aluminum Company of America, Alcoa Technical Center, 100 Technical Drive, Alcoa Center, PA 15069; M. Scherbak, Aluminum Company of America, Ingot Technology Group, 900 South Gay Street, Knoxville, TN 37902

The impurity removal efficiency of in-line fluxing equipment depends strongly on the interfacial area between the gas bubbles and the molten metal. High interfacial areas can be generated by direct shearing of the gas bubbles using rotating dispersers. Placing multiple dispersers on a single rotor shaft and injecting gas into the metal at each disperser increases this direct shearing, improving the efficiency of the in-line fluxing unit. Small dispersers can operate at high gas loading and high speed without vortexing. Small dispersers also allow smaller metal treatment units, reducing floor space requirements and metal holdup. This paper will summarize the theory behind the direct shearing approach and will present data illustrating the difference in trace element removal for single, double, and triple disperser rotors. Comparisons with other in-line fluxing equipment will also be presented.

3:00 pm

EVOLUTION OF THE ALPUR® ROTOR TOWARDS IMPROVED PERFORMANCE: Isabelle Ventre, Jean-Marie Chateau, Pechiney Aluminium Engineering, Centr'Alp-725 rue Aristide Bergès, F-38340 Voreppe, France

The ability of a rotary injector to efficiently reduce dissolved hydrogen and impurities in an in-line treatment box is directly related to its ability to create an intimate dispersion of tiny bubbles of treatment gases within the aluminum melt. This is achieved through a combination of shearing effect of the injected gas flow and strong pumping of the metal contained in the box. The pumping action of the rotary injector can however create some metal recirculation loops within the box and cause undesirable turbulences at the surface of the metal inside the box. To address this issue the design of the Alpur® rotor was improved and optimized which resulted in drastic improvement of the surface conditions without affecting the intrinsic performance of this injector. We review in this paper the basic criteria for this optimum rotor design and assess through water modelling and testing in aluminum the overall benefits for the operation.

3:20 pm

QUANTIFIED QUALITY AND EFFICIENCY IMPROVEMENTS AT SAPA, LTD.: Stephen J. Rose, SAPA Ltd., Saw Pit Lane, Tibshelf Derbyshire DE55 5NH, England; David W. Busch, Foseco Inc., 777 Old Saw Mill River Road, Tarrytown, NY 10591-6729

SAPA, Ltd. is a 3 press aluminium extrusion operation specializing in quality flat and hollow product with a small remelt facility for in house generated scrap. In 1994 a decision was made to improve the overall quality of the in plant remelt facility. The first capital improvement was the installation of a SNIF Sheer P-60U two nozzle degassing system with immersion heaters. This unit was started up in December of 1994. A new melter with an exhaust collection system was brought on line in September of 1995. This paper will describe how improved melt treatment produced measurable economic benefits downstream.

3:40 pm BREAK

3:50 pm

CHLORINATION OF TiB2 GRAIN REFINED ALUMINIUM: T. Gudmundsson, G. Saevarsdottir, T.I. Sigfusson, Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavik, Iceland; D.G. McCartney, Department of Materials Engineering & Materials Design, University of Nottingham, United Kingdom

The effects of fluxing grain refined molten aluminium with Ar/Cl2 gas mixtures have been studied both in the laboratory and during experimental casthouse trials. Samples taken during gas fluxing were subjected to chemical analysis and examined in the scanning electron microscope (SEM). Detailed chemical analysis of agglomerates was undertaken in the SEM using energy dispersive X-ray analysis and the elements Na, K and F were found to be associated with TiB2 clusters. Results will be presented in detail and discussed in the context of previously published work on grain refiner characterization and wettability of TiB2 in liquid Al/salt mixtures.

4:10 pm

COALESCENCE BEHAVIOUR OF ALUMINUM DROPLETS UNDER A MOLTEN SALT FLUX COVER: K.J. Friesen, T.A .Utigard, Department of Metallurgy and Materials Science, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 1A4; C. Dupuis, J.P .Martin, Arvida Laboratories and Development Centre, Alcan International Ltd., 1955 Mellon Blvd., Jonquière, Québec, Canada G7S 4K8

The formation of a suspension of aluminum droplets in salt fluxes used during the refining and recycling of aluminum, leads to decreased aluminum recovery. To enhance the coalescence and recovery, it is required to remove the oxide layer formed on the surface of these small aluminum drops. Coalescence behaviour of aluminum droplets under a molten salt flux cover was investigated using a hot stage microscope at a temperature of 740°C. Two salt systems were studied: i) an equal weight mixture of NaCl-KCl with chloride and fluoride additives and ii) the MgCl2-KCl system with fluoride additives. Chloride salts did nothing to enhance coalescence but fluoride additions to both salt systems removed the oxide film from the metal. NaF, KF and Na3AlF6 additives were the most effective salts at stripping away the oxide layer and promoting coalescence. As the concentration of MgCl2 increased, the ability of the droplets to coalesce decreased significantly.

4:30 pm

KINETICS OF MAGNESIUM REMOVAL FROM ALUMINUM ALLOYS BY CHLORINE FLUXING: Qian Fu, J.W .Evans, Dept. of Materials Science and Mineral Engineering, University of California, Berkeley CA 94720

Chlorine fluxing (sparging chlorine-argon mixtures into the melt) is a standard procedure for eliminating magnesium from Al-Mg alloys. The kinetics of the reactions/mass transport involved have not been extensively studied in the past. The paper describes an ongoing investigation of those kinetics. Much of the work to date has been using laboratory-scale melts with magnesium removal measured by analysis of samples. Techniques have been developed to measure both bubble frequency and bubble residence time in the melt, enabling estimation of the total interfacial area and, consequently, the intrinsic kinetics of the phenomena involved. It is observed that, above the melting point of MgCl2, there exists a "critical Mg concentration". Above this critical concentration the reactions are sufficiently fast that all entering chlorine is converted to MgCl2 and there are negligible chlorine emissions. Below this concentration, Mg mass transport in the alloy appears to be rate-controlling. The results are interpreted in terms of a mathematical model for fluxing. The paper also describes the preliminary results of experiments on a larger scale (few tens kg alloy) in which bubble probes are used to detect the dispersion of gas injected into the melt and measure the interfacial area.

4:50 pm

INTERFACIAL TENSION BETWEEN ALUMINUM AND CHLORIDE-FLUORIDE MELTS: A. Silny, Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-84236 Bratislava, Slovakia; T.A. Utigard, Department of Metallurgy and Materials Science, University of Toronto, M5S 3E4 Toronto, Canada

A technique was developed for the measurement of the interfacial tension between liquid metals and molten salts at elevated temperatures. The technique is based on the measurement of the capillary depression ocurring when a capillary, which is moved vertically down through the molten salt layer, passes through the salt/metal interface. The depression is measured by simultaneous video recording of the immersion height of the alumina capillary and the position of a liquid meniscus in a horizontal tube connected to the alumina capillary. The interfacial tension was measured in the following systems: aluminium and an equimolar melt of NaCl and KCl with several salt additions at 1000 K, aluminium and NaCl-NaF at 1123 K, and aluminium and NaCl-KF at 1123 K and aluminium and equimolar melt of NaCl-KCl with additions of NaF, KF, LiF, BaF2, SrF2, CaF2, Na3AlF6, MgF2 and AlF3. It was found that the interfacial tension decreases with increasing NaF, KF, LiF, CaF2, BaF2 and SrF2 additions, remains unchanged with AlF3 additions and slightly decreases with MgF2 and Na3AlF6 additions. Aluminium and equimolar melt of NaCl-KCl with additions of LiCl, AlCl3, BaCl2, CaCl2, MgCl2. All above chlorides slightly increase the interfacial tension in the system. Obtained data are compared with the ones found in literature and some conclusions for the aluminium refining and recycling were derived.

5:10 pm

A COMPARATIVE STUDY ON THE EFFICIENCIES OF Na2SiF6 AND AlF3 FOR DEMAGGING MOLTEN ALUMINUM BY SUBMERGED POWDER INJECTION: A. Flores-Valdéz, M.A. Hinojosa-SanMiguel, A.H. Castillejos-Escobar, E. Macias-Avila, F.A. Acosta-González, Centro de Investigación y de Estudios Avanzados del IPN, Unidad Saltillo, P.O. Box 663, 25000 Saltillo, Coahuila, México

A comparative study on the efficiencies of Na2SiF6 and AlF3 for demagging molten aluminum by submerged powder injection is reported. Experimental trials carried out at a scale of 250 Kg molten metal furnace capacity showed that the higher efficiencies, ~ 70%, were attained by the use of the Na2SiF6 powder. The basic parameter to establish the comparison between the two demagging agents was the powder size, maintaining constant the other important parameters, e.g., initial magnesium content, carrier gas flow-to-powder flow ratio, pressure of the carrier gas, temperature of the bath, the dimensions of the reactor and the shape and dimensions of the injection lance. The removal efficiency was monitored analyzing the magnesium content in both the metal and the slag. The fumes produced were also analyzed, having found that the Na2SiF6 powders have the inconvenience of giving place to the generation of highly toxic SiF4 fumes.

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