Investigation of the Influence of Heat Balance Shifts on the Freeze Microstructure and Composition in Aluminum Smelting
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IN the Hall-Heroult smelting process, some of the electrolytic bath (termed as bath in this work) solidifies on cell side walls, forming a cryolite-based freeze lining (referred to as side ledge). A proper control of side ledge is essential for aluminum electrolysis process and cell life as well to achieve high current efficiency. The ledge thickness must be maintained in a certain range to protect the side wall from the corrosive bath and metal. However, it should not be so thick as to hinder the
JINGJING LIU is with the Department of Chemical & Materials Engineering, The University of Auckland, 1023, Auckland, New Zealand. ATA FALLAH-MEHRJARDI, DENIS SHISHIN, and EVGUENI JAK are with the School of Chemical Engineering, The University of Queensland, St Lucia, Brisbane, 4072, Queensland, Australia. MARK DORREEN is with the Light Metals Research Centre, The University of Auckland, 1023, Auckland, New Zealand. MARK TAYLOR is with the Department of Chemical & Materials Engineering, The University of Auckland and also with the NZ Product Accelerator, The University of Auckland, 1023, Auckland, New Zealand. Contact e-mail: [email protected] Manuscript submitted May 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
electrolysis operations. Both the ledge thickness and heat loss through side walls are determined by the heat transfer across the frozen ledge, which is driven by the heat flux from the molten bath to the bath/freeze interface.[1] The freezing/melting process of side ledge is hitherto assumed to be reversible and self-adjusted at a mean thickness through a thermal equilibrium.[2,3] A stable heat balance is critical to maintain the stability of side ledge. However, global electricity grid and aluminum market are driving dynamically varying economic conditions for aluminum production. To meet these demands, a new cell technology has been developed.[4–6] This new technology aims to maintain the heat balance at different steady states to extend the amperage input range. To maintain the heat balance, heat loss from a cell (output) is regulated in concert with changes in amperage (input) using the Shell Heat Exchanger technology (SHE).[7,8] Thus, shifting the heat balance from one steady state to another is achieved. During this heat balance shifting, the change in heat loss may disrupt the self-adjusting process of the ledge, giving rise to variation in ledge thickness. The microstructure and composition of the freeze lining are important in its thermochemical and mechanical integrity in face of the heat flow variation, which has
been investigated both on the industrial ledge and in laboratory.[9–11] A layered structure has been observed in both occasions and it is found that the frozen ledge mainly consists of cryolite with bath additives according to the bath composition. This layered structure was also investigated on other cooled probes immersed in hot slags and melts.[11–20] These layers are typically observed as (1) a quenched layer, adjacent to the cooling probe when the cooling rate is ve
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