Investigation of Freeze Linings in Copper-Containing Slag Systems: Part II. Mechanism of the Deposit Stabilization
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TO date, the designs of freeze-lining for pyrometallurgical furnaces have been based almost exclusively on heat-transfer considerations.[1–9] There is increasing evidence, however, to show that optimum practice should also take into account the effects of bath chemistry. In part I of this series,[10] a range of microstructures in freeze-lining deposits formed from copper-containing slags was described. The freeze-lining deposits in general have been found to consist of several different layers. Starting from the cold wall, these layers consist of 1) glass, 2) glass with microcrystalline precipitates, 3) multiphase subliquidus material containing solid delafossite and cuprite and high-silica liquid that is separated from the bulk liquid (closed crystalline layer), and 4) a complex phase assemblage containing ATA FALLAH-MEHRJARDI, Ph.D. Student, PETER C. HAYES, Xstrata Professor of Metallurgical Engineering, and EVGUENI JAK, Professor, are with the PYROSEARCH, School of Chemical Engineering, The University of Queensland, Brisbane, Australia. Contact e-mail: [email protected] Manuscript submitted July 27, 2012. Article published online February 16, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
delafossite and cuprite crystals and a high-silica liquid phase that is connected to the bulk liquid (open crystalline layer). A dense layer of primary phase crystals of bulk bath—the fifth layer called the primary phase sealing layer—has not been observed in any of the steady-state deposits in the current study.[10] This is very different from the anticipated structure of stationary deposit-bath interface at steady-state conditions—it was expected that the deposit interface would consist solely of the primary phase crystals, delafossite. The reasoning behind the expectation of the presence of the primary phase sealing layer is that at steady-state conditions, the crystal phases are already present at the stationary deposit so there is no nucleation barrier to overcome, and crystallization on the outer surface of the interface with the bath would take place due to continuous heat removal through the stationary deposit as well as continuous supply of fresh chemicals from the bulk bath to the interface by relatively fast mass transfer through the agitated layers of liquid until equilibrium is reached at liquidus temperature of the bulk bath (Tliquidus). These observations reported in the Part I of this series indicate the need to further investigate the mechanisms of freeze-lining formation and the effects of chemically VOLUME 44B, JUNE 2013—549
relevant parameters on the microstructure, stability, thickness, and overall heat transfer of freeze lining particularly at steady-state conditions.
II.
EXPERIMENTAL METHODOLOGY
The details of the experimental methods and techniques used in the current study have been described in part I of this series.[10] Briefly, the current laboratory studies have been undertaken using an air-cooled probe, or so-called ‘‘cold finger’’ technique. The probe is immersed in a molten slag bath
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