Bubble Formation at Nozzles in Pig Iron G. A. IRONS AND R. I. L. GUTHRIE An experimental study was undertaken to determine how several variables affect the size of gas bubbles formed at nozzles in liquid pig iron. The frequency of bubble formation was measured by an acoustic device, which could detect the vibrations produced by the bubble release. Accurate knowledge of the gas flow rate then enabled the calculation of bubble volumes. The use of large baths (60 Kg), melted by induction heating, permitted a wide range of experimental parameters: gas flow rate (0.5 to 1000 cc/s), outside nozzle diameter (0.64 to 5.1 cm), inside diameter (0.16 to 0.64 cm), chamber volume (23 to 2200 cc), nozzle depth (7.6 to 20 cm), surface tension (700 to 1500 dynes/cm) and nozzle orientation (up, down and sideways). The resulting bubble volumes were between 0.5 and 100 cc. The bubbles were found to form at the outer diameter of the nozzles due to the nonwettability of the nozzles. Furthermore, the bubbles were of a uniform size at low flow rates, but increased in volume with the flow rate, so that a constant frequency was established. In addition, the bubble volume was strongly dependent on the chamber vol- ume upstream from the nozzle. This is known as a "capacitance" effect and is due to compressibility of the gas. !'Doublets" or "double bubbles" at small chamber volumes and bubble "pairs" at large chamber volumes were also observed. These phenomena re- sult in smaller bubbles, which make precise predictions of bubble size difficult. The re- sults are compared with those obtained by other investigators in aqueous and metallic systems. THE injection of gas from submerged nozzles or ori- fices into liquid metal baths is an integral feature of many metallurgical processing and refining opera- tions. Familiar examples include the mixing of liquid metals by inert gas bubbling through porous plugs or submerged lances, as well as tuyere-based injection processes for producing steel, blister copper or Bes- semer matte. Any successful understanding of process kinetics under such circumstances requires a detailed know- ledge of the fluid mechanic phenomena involved. The present paper is concerned specifically with a study of those factors determining bubble sizes when the gas is injected at low and moderate flow rates through submerged nozzles. Although much data have been accumulated on aqueous systems, direct extrapolation to liquid metal systems can often be misleading and yield erroneous conclusions. Consequently the work now described has been largely experimental, due to the paucity of phenomenological data on liquid metal systems. Such studies should precede the development of appropriate theoretical models, which are neces- sarily mechanistic in approach. The experimental system chosen involved graphite lances submerged in carbon-saturated pig iron at 1250~ It should also be noted that the study made use of pilot-scale, induction-melting facilities, and this enabled reasonably large gas flow rates and noz- zle dimensions to be used in the work. THEORY Although several aspects of bubble formation have received successful theoretical interpretation, and G. A. IRONS and R. I. L. GUTHRIE are Graduate Student and Associate Professor, respectively,in the Department of Mining and Metallurgical Engineering,McGfllUniversity, Montreal, Canada. Manuscript submitted May 25, 1977. METALLURGICALTRANSACTIONS B these are treated below, many observations still lack quantitative explanations, particularly those at high flow rates. Constant Volume Regime It has been found that at low flow rates of gas through submerged nozzles, the volume of the bubble at release, Vbm , is determined by a balance between its buoyancy force in the liquid and those surface tension forces constraining it to the inner circumfer- ence of the nozzle, ~'2 hence Vbm = 7rDn';~ [1] Pg These predicted bubble volumes apply to aqueous-type systems in which the liquid wets the nozzle material. However, for nonwetting, metallic-type systems, the bubbles generally tend to form at the outer circum- Fig. lmSchematic representation of bubble formation in aqueous wetting and metallic nonwetting systems. ISSN 0350-2141/78/0310-0101500.75/0 9 1978AMERICANSOCIETYFOR METALSAND VOLUME 9B, MARCH 1978-101 THE METALLURGICAL SOCIETYOF AIME