Physical modelling of slag foaming in two-phase and three-phase systems in the churn-flow regime S.A.C. Stadler, J.J. Eksteen * , C. Aldrich Department of Process Engineering, University of Stellenbosch, Private Bag X1, Matieland, 7602 Stellenbosch, South Africa Received 2 May 2005; accepted 25 May 2005 Available online 10 August 2005 Abstract Although the principal factors determining the stability of slag foams have been identified, there is little agreement as to the rel- ative importance of these factors. In this paper the fluid properties, the influence of solid precipitates and the significance of bubble size distributions in the churn flow regime are considered. The slag viscosity appeared to be the most important factor in two-phase systems, while the addition of solids in three-phase systems was observed to stabilise the foam and increase its volume. Investigation of bubble size distributions suggested that liquids with more stable foam have smaller average bubble sizes than liquids with less stable foam. Ó 2005 Elsevier Ltd. All rights reserved. 1. Introduction In slag-based, smelting–reduction processes, the foaming and consequent overflow of slag, owing to either gas injection or reaction-driven gas generation, pose major limitations to productivity. For example, in plas- ma arc furnaces, smelting might cause the cathode to respond wildly to maintain constant arc resistance, lead- ing to operating instabilities. In lance-based applications, slag foaming requires dynamic lance movement to main- tain a constant lance-to-bath distance. Likewise, in steel- making applications, the maintenance of stable foam is important to prevent unwanted heat loss and refractory damage arising from flares from three-phase arcs. The severity of foaming is strongly dependent on the slag chemistry, operating conditions and the rate of gas addition and/or formation. When bubbles start to form foam, they loose their spherical character and share the same films. Two types of foam are often observed (Zhang and Fruehan, 1995). Spherical foam (e.g. beer foam) is classified as wet foam (kugelschaum), while polyhedral foam, with thinner liquid films and a larger gas volume, is classified as dry foam (polyederschaum). The most important factor in the formation of bub- bles is the velocity of the gas or vapour passing through the fluid. If dry foam forms, the surface tension is ex- pected to play a crucial role in foam behaviour, but if wet foam forms, the movement of the liquid layers rela- tive to each other, that is the film drainage, is expected to play a more important role. The stability of foam therefore depends on the entrainment and drainage of fluid in the foam. The fluid viscosity primarily deter- mines this relative movement of the liquid layers. As the viscosity increases from a low value, the slag drain- age from the foam is lessened, which causes foaming to increase (Jiang and Fruehan, 1992; Utigard and Zamal- loa, 1993; Roth et al., 1993). This increase in foaming continues with increasing viscosity up to a critical point, where the gas starts to channel through the slag, so that no further foaming takes place. Bubble breakage occurs when the stresses applied to the film (bubble wall) are higher than the intermolecular 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.05.018 * Corresponding author. Tel.: +27 82 376 6055; fax: +27 21 808 2059. E-mail address: jeksteen@ing.sun.ac.za (J.J. Eksteen). This article is also available online at: www.elsevier.com/locate/mineng Minerals Engineering 19 (2006) 237–245