Production and Electrolysis of Light Metals. Proceedings of the International Symposium on Production and Electrolysis of Light Metals, Halifax, August 20–24, 1989 PDF
Preview Production and Electrolysis of Light Metals. Proceedings of the International Symposium on Production and Electrolysis of Light Metals, Halifax, August 20–24, 1989
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Pergamon Press, Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A. U.K. Pergamon Press pic, Headington Hill Hall, Oxford 0X3 OBW, England PEOPLE'S REPUBLIC Pergamon Press, Room 4037, Qianmen Hotel, Beijing, OF CHINA People's Republic of China FEDERAL REPUBLIC Pergamon Press GmbH, Hammerweg 6, OF GERMANY D-6242 Kronberg, Federal Republic of Germany BRAZIL Pergamon Editora Ltda, Rua Ega de Queiros, 346, CEP 04011, Säo Paulo, Brazil AUSTRALIA Pergamon Press Australia Pty Ltd., P.O. Box 544, Potts Point, NSW 2011, Australia JAPAN Pergamon Press, 8th Floor, Matsuoka Central Building, 1-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160, Japan CANADA Pergamon Press Canada Ltd., Suite 271, 253 College Street, Toronto, Ontario M5T 1R5, Canada Copyright © 1989 by The Canadian Institute of Mining and Metallurgy All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. First edition 1989 Library of Congress Cataloging in Publication Data ISBN 0-08-037295-3 In order to make this volume available as economically and as rapidly as possible, the authors' typescripts have been reproduced in their original forms. This method unfortunately has its typographical limitations but it is hoped that they in no way distract the reader. Printed in the United States of America The paper used in this publication meets the minimum requirements of American National Standard for Information Sciences - Permanence of Paper for Printed Library Materials, ANSI Z39.48-1984 Session Chairmen Smelter Operations Electrolysis of Light Metals C. Bickert R. Guthrie Pechiney Corporation, McGill University Greenwich, Connecticut, U.S.A. Montreal, Quebec, Canada D. MacMillan P. Pinfold Alcan Rolled Products, Norsk Hydro Cleveland, Ohio, U.S.A. Becancour, Quebec, Canada Aluminum Casting Reduction and Production E. Ozberk of Light Metals Sherritt Gordon Ltd. P. Tremblay Fort Saskatchewan, Alcan International Ltd. Saskatchewan, Canada Jonquiere, Quebec, Canada P. Aylen M. Bouchard Alumax Company Universite du Quebec ä Chicoutimi Ferndale, Wisconsin, U.S.A. Chicoutimi, Quebec, Canada Aluminum Melt Treatment and Control M. Sahoo CANMET Ottawa, Ontario, Canada L. Larouche Canadian Reynolds Metals Co. Ltd. Baie-Comeau, Quebec, Canada 3 Performance prediction of the aluminum casting furnace R. Bui, A. Charette, G. Simard, A. Larouche, Y. Kocaefe Universita du Quabec ä Chicoutimi, Chicoutimi, Quebec, Canada, G7H 2B1 E. Dernedde, W. Stevens Alcan International limitae, Jonquiere, Quabec, Canada, G7S 4K8 ABSTRACT A comprehensive three-dimensional overall transient model has been built for the aluminum casting furnace. It results from the coupling of two models, repre- senting the combustion chamber and the metal respectively. The chamber model takes account of gas flow, combustion and heat transfer, mainly radiative. The metal model includes the melting of a solid charge. The overall model can readily accommodate the various operational procedures and can be used to study furnace performance as well as solve design problems. KEYWORDS Casting furnace, computer model, performance analysis, furnace design. INTRODUCTION The aluminum casting furnace plays a central role in the fabrication of primary aluminum. It receives liquid aluminum coming from the electrolytic cells, brings it up to a desired temperature and keeps it there while preparations are made (fluxing, alloying, skimming) before casting takes place. Also, a solid charge is introduced and melted. This is why the furnace is sometimes referred to as a melter-holder. Figure 1 presents the cutaway views of a typical casting furnace. It is composed essentially of two main parts: the chamber of combustion and the metal. The process going on in the casting furnace is quite complex. On the one hand, in the chamber there is the gas flow, combustion of the fuel (natural gas), heat transfer by convection and more importantly radiation, and conductive heat transfer through the roof refractories. On the other hand in the metal, there is the heat transfer by conduction and convection, both natural and forced. Forced convection is due to stirring and also to the various operational proce- dures such as the introduction of liquid metal siphoned in from the crucibles. The solid metal also undergoes a solid-to-liquid phase change. Each of these individual physical phenomena deserves on its own to be treated as a substantial research subject. The aim here, however, it to model all of these phenomena together and obtain a mathematical tool for the purpose of analysis and design of the furnace. What is more, the model must be reasonably thrifty in computer time in order to make sense in an industrial environment. The modelling work at hand is no simple task. However the alternative would be to conduct plant experiments, which are long, costly and sometimes risky, without giving all the answers that a mathematical model can give. When dealing with a complex process, it is often helpful to split it into two or more parts and model each of them separately. Intuition confirmed by experience shows that each such partial model is more manageable. Also, each model can be conceived as a self-containing simulator that can be used separately for partial design problems. Finally, in the context of today's fast moving 4 PRODUCTION AND ELECTROLYSIS OF LIGHT METALS ® Figure 1- Cutaway views of the casting furnace (A) longitudinal (B) cross section 1,2: roof and its refractories 9: melt level and insulation 10: solid metal 3: exit duct (to stack) 11: syphon 4: burner 12: pouring spout 5: doors 13: thermocouples 6,7 ,8 : floor and its refractories and insulation computer technology, parallel computing allows these models to be run simulta- neously and their outputs coupled interactively, thus considerably shortening the simulation time. For these reasons, the casting furnace has been modelled in two parts: the chamber and the metal. In a previous publication (Bui, Charette, Dernedde 1988), these two models were presented together with the analysis and validation of their respective outputs. The next step was to couple the two models by an appropriate technique to obtain the overall model. This paper shows how this overall model is built and how the physical and simulation parameters are handled. The overall model is next validated on a real furnace, then model capabilities in terms of improving the operation or the design of the furnace are discussed. THE OVERALL MODEL The model of the chamber and that of the metal are built separately but based on a common computational tool, namely the general-purpose fluid-flow code PRODUCTION AND ELECTROLYSIS OF LIGHT METALS 5 PHOENICS, an acronym for Parabolic, Hyperbolic or Elliptic Numerical Integration Code Series (Rosten and Spalding 1986). In the chamber model, the differential equations that describe the transport of momentum, energy and concentration in 3D are solved by PHOENICS. The radiative heat transfer is calculated by the Imaginary Planes (IP) method (Charette, Erchiqui, Kocaefe 1987), a simplified version of the classical zone method that yields a computer time reduced by a factor of 20 compared to the classical method of zones. A model of combustion based on the combustion kinetics, incorporated in PHOENICS, is used for the simulation of the flame. In the metal model, the solid-to-liquid phase change is treated by the effective thermal properties (ETP) method (Simard, Bui, Potocnik 1987), while the effect of natural and forced convection on the heat transfer is accounted for by the use of an augmented conductivity in the liquid metal (Bui et al 1989). The overall model of the furnace is obtained by a coupling of the model of the chamber and that of the metal. The primary aim of coupling is to ensure the continuous heat transfer between chamber and metal, but it also has to take account of the fact that the geometry of the metal changes with time due to the addition of the liquid metal from the crucibles and the gradual melting of the solid blocks. Strictly speaking, that part of the metal space still unoccupied by the liquid metal should be seen as part of the chamber, and as a consequence, chamber geometry should change with time. But it would be unrealistic to recalculate the gas flow and heat transfer for a new chamber geometry at each time step. The solution is to choose a fixed parallelepiped geometry for the chamber, let the metal geometry change with time, and perform the coupling through an "equiv- alent plane" separating chamber and metal and corresponding to the bottom plane of the chamber parallelepiped. This equivalent plane represents all parts of the furnace underneath itself, namely the metal and the empty space filled with the gas before being gradually filled with liquid metal. In the coupling process, the information exchange at the equivalent plane level takes place once every coupling time step known as the period of interaction between the two models. A 5-minute interaction period was used with success. The chamber model is run for 5 minutes and its heat flow outputs are transferred to the metal model which then runs for 5 minutes before sending its emissivity and tempera- ture outputs back to the chamber model, and the recurrent process continues. The effective emissivities are calculated by successive steps to represent two or more surfaces by one equivalent surface until the equivalent plane is finally reached. For each step, the effective emissivities are determined from the radiative heat balances making use of the concept of interchange areas (Hottel, Sarofim 1967). Figure 2 illustrates the method. It represents a two- dimensional case obtained with a cross section of the metal made at a location where a solid block exists. In element II we have: S S + S S k 2 ) *\ * < 1 2 7 3 7 > I l < where S .S . = total exchange area between areas i and j Cm ] A = area of equivalent surface 7 Cm 1 ε' = effective emissivity Similarly for element III: 6 PRODUCTION AND ELECTROLYSIS OF LIGHT METALS S + (2) = < A W ' \ Note that in this case both surfaces 4 and 5 are refractories and if ε - ε , relation (2) simplifies further. Finally for element I: ε' δ + A (3) l < Λ W / l and ε' is the final effective emissivity to be sent to the chamber model. EQUIVALENT PLANE- S5.«5 Figure 2- Principle for determining the effective emissivities The temperature distribution on the equivalent plane is done by projection, as illustrated by Figure 3 where T , T , T are the average emerged solid surface temperature, average liquid surface temperature and average refractory surface temperature respectively. The chamber model thus receives the effective emissivities and temperatures coming from the metal model. It must return a heat flow distribution to the metal model. Figure 3 illustrates the simple two-dimensional case where heat flow distribution is done by projection. Q , Qn»CL are the heat flows calcu- lated by the chamber model for the sections of tne equivalent plane facing the solid, the liquid and the refractories respectively. They are applied directly to each of these three surfaces. SECTIONS: SOLID LIQUID i REFRACTORIES Figure 3- Convention used for assigning temperatures and heat flows PRODUCTION AND ELECTROLYSIS OF LIGHT METALS 7 For those furnace cross sections falling in between two solid blocks and thus containing no solid metal, the situation is more complex due to the radiative exchanges between the two blocks. The recipe for determining the effective emissivities and the heat flow distribution is then slightly more complicated arithmetically but still based on the same principle. The usefulness of such coupling process cannot be overemphasized. Due to it, the overall model not only can account for all the physical phenomena taking place in the chamber (gas flow, combustion, heat transfer) and in the metal (melting, heat transfer) but also can accommodate the various operating conditions such as different heel levels and temperatures, different solid loading techniques, preheating of the metal or the refractories, different time schedules of the arrivals of crucibles, different crucible liquid metal tempera- tures. The casting furnace operational procedures are so varied and flexible that a model that cannot accommodate them sees its usefulness seriously reduced. PHYSICAL AND SIMULATION PARAMETERS Figure 4 shows the handling of the physical parameters and the simulation parameters. Figure 4a explains the preparation of the data files. For the chamber, three different cartesian grids are used, one for the heat transfer and fluid flow calculations by PHOENICS, another, coarser, for radiative heat transfer calculations. The latter consume more CPU time due to the need to determine the shape factors and therefore must be done on a coarser grid. Still a third grid, this one one-dimensional, is used to calculate the transient heat conduction through the refractories. For the metal, body-fitted coordinates are used to fit the non-cartesian geometry. However the depthwise discretization is kept horizontal to accommodate the successive arrivals of crucibles and the ensuing changes in liquid metal level. The mesh generation is done with PATRAN, a finite element pre- and post-processing software interfaced with PHOENICS through a software called PAPH, or its improved version NEWPAPH. The various details of the operational procedure such as preheating, crucibles arrivals, heating... are spelled out in the code PIL, an acronym for PHOENICS Input Language. Figure 4b is the continuation of Figure 4a. It shows the chamber model in part A, the metal model in part B and the coupling in between. In the coupling, the metal model sends the effective values of emissivities and temperatures to the chamber model and in return, receives from it the heat flow distribution. VALIDATING SIMULATION In this section, the overall model is run to simulate a complete batch using the data from a 1987 plant test conducted on an operating castshop furnace of Alcan Smelters and Chemicals, Jonquiere, Quebec, Canada. The purpose of the simu- lation is to validate the model against the plant test data, and also to show the model capabilities in terms of operation and design improvements. Physical parameters of the batch The batch starts with a 20-ton liquid heel at 750°C. The refractories have been preheated to a 800°C initial inner surface temperature, and 100°C for outer surface. Three 4-ton blocks of solid metal are introduced at equidistance along the furnace and leaning against the walls on the door side (Figure 8). Their initial temperature is 100°C. The liquid metal coming from the crucibles is pure aluminum with 660°C melting point. The solid blocks are an alloy of average melting point 642°C. PRODUCTION AND ELECTROLYSIS OF LIGHT METALS PARAMETERS OF THE BATCH IPHOENJCSI -loading of scrap metal: -number of blocks -position of blocks heat transfer (combustion,radiation, -kind of alloy conduction,convection) in unsteady -level and mass of the heel state -distribution of the arrivals of crucibles -characteristics of the burner (fuel,mass convection : velocity field pre-calcu flow) lated -characteristics of stirring (continuous or intermittent, intensity) 1 tciavelc ulaetme istshiev itiefsfe c- |^—|i tleuefrnfet sc ptialvaten eth te mepqueirva-, ,1' cfhaoecnaet rofoluf xv moaelut tmathl e sur- »,' input of data and preparation generation of the mesh with of the meshes : PATRAN et NEWPAPH radiation and refractories i geometrical, [ parameters ' calculation of the shape |PIL codes| factors (Monte-Carlo) 1 preheating 1 arrival of crucible #1 |PHOENICS| 1 arrival of crucible #2 JPHOENICSJ fluid flow with heat transfer heat transfer by conduction with c(coonmvebcutisotnio)inn, rasdteiatdioy n,sctoanted uction, arrival of crucible #n prheasseen tc hacnognev eacntdio na ugmented-k to re- heating 1 verify of the setting of the blocks simplify model to add liquid metal T from crucibles Θ Θ Figure 4a- Preparation of the data f i l e s Figure 4b- The recurrent coupling process
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