A novel probe for detecting gas bubbles in liquid metals
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I. INTRODUCTION
THERE are several metallurgical processing operations where gasses are injected into liquid metals. Argon degassing and ladle metallurgy of steel, chlorine fluxing of aluminum, and deoxidation of blister copper are representative examples. The bubbles provide gaseous reactants and an interfacial area at which heterogeneous reactions or mass transport occur. The bubbles also bring about both localized motion of the liquid (in their wakes, for example) as well as large scale circulation driven by effective density differences between regions rich and poor in bubbles. All these phenomena are governed by the distribution of the bubbles within the liquid volume and by the size, or size distribution, of the bubbles. There is therefore value in learning about bubbles in liquid metals at the laboratory scale through commercial scale. Unfortunately, the means for investigating bubbles are imperfect. They include X-raying the melt, as done by Iguchi et al.,[1] for example, and using microphones to detect bubble detachment at a submerged nozzle and subsequent arrival at a melt surface, as exemplified by Andreini and co-workers[2] or by Fu and Evans.[3] Neither of these methods can readily be extended beyond the laboratory scale. The latter technique also provides no information on the distribution of the bubbles in the melt and, for that, a probe that can be moved around in the melt to study the arrival of bubbles at various positions is required. Such a probe is the electroresistivity probe that appears to have been first used in the 1960s to study bubbles in fluidized beds.[4,5] This probe consists of an insulated wire with an exposed tip that makes contact with the liquid metal. A second contact consists of a second wire with an exposed tip adjacent to the first, or another electrical connection to the melt. The electrical circuit formed by the contacts is broken as a bubble passes. Brimacombe’s group[6,7] made use of this probe in studies of bubbles in both mercury and water. Iguchi and colleagues[8,9,10] have used the probe in their investigations of bubbles in liquid metals, including investigations in iron at 1600 °C. A disadvantage of the electroresistivity probe is the sensitivity of the conducting tip, exposed to the liquid metal, to chemical M. SCHNEIDER, Mechanical Engineer, is with Engenuity Systems, Inc., Fremont, CA 94538. J.W. EVANS, Professor of Metallurgy and P. Malozernof Chair, is with the Department of Materials Science and Engineering, University of California, Berkeley CA, 94720. Contact e-mail: evans@ berkeley.edu Manuscript submitted November 30, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
attack or dissolution by the melt. Fu and Evans[11] describe a probe where this difficulty can be avoided by measuring capacitance between an immersed insulated wire and the melt, rather than resistance. The wire can then be enclosed completely in an inert material (e.g., alumina for contact with liquid aluminum). Such a probe has been used by Fjeld et al.[12] for measurement of bubbles in liquid alum
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