Some Observations on the Draining of CaO-SiO_2-Al_2O_3 Slag Bubble Films

摘要

1. No clear relationship between the draining rate and melt viscosity could be established, as viscosity could not be independently changed without decreasing the slag surface tension. Although the film draining rate increased with increasing temperature, a complicated trend was observed for the draining of CaO-SiO_2-Al2O_3 films as a function of SiO_2 concentration. The addition of up to 10 wt % Fe_2O_3 to the slags was expected to increase the film draining rate, if bulk viscosity were to strongly influence their draining behaviour. However, iron-oxide additions were found to dramatically decrease the draining rate of the films, irrespective of temperature. Clearly, further work is required to explain any likely influence of slag viscosity on the draining rate of bubble films. 2. Drainage of the liquid within the film 'Plateau borders' themselves showed a very strong temperature dependence, ie., the calculated activation energy for border thinning (～ 150 kJ/mol) was very close to that calculated for viscous flow within the slag system studied (～ 160 kJ/mol). This suggests that, as within aqueous foam films, bulk viscosity of the continuous phase can play an important role in foam drainage, once the liquid reaches the Plateau borders under capillarity. Surface tension was shown to exert a very strong influence on the draining rate of CaO-SiO_2-Al_2O_3 slag bubble films. Addition of dilute concentrations of strong surface-active agents (such as P_2O_5 and Na_2O) to lower the surface tension of the slags, resulted in a profound decrease in their draining rate, irrespective of temperature. The above results suggest the physics of liquid film drainage within silicate slag films and Plateau borders to be similar to those encountered in typical aqueous systems. 3. A crude correlation has shown that the rate of draining of liquid within CaO-SiO_2-Al_2O_3 slag bubble films at 1673 K is strongly influenced by the (estimated) rate of surface tension lowering, per unit concentration change of surfactant (dγ/dC). That is, the higher the estimated value of dγ/dC for a given surface-active agent, the higher the resulting "retardatory index," ie., the greater the magnitude of the decrease in film draining rate after surfactant addition. Limited results obtained by the authors for an independent Na_2O-B_2O_3 'model' system at 1233 K also agreed with this trend. Further data treatment is necessary in order to quantify the true physical meaning of the proposed "retardatory index." 4. The slag bubble films in this study appear to show thinning characteristics similar to those observed in aqueous bubble films, ie., at the well-drained stages, the films tend to become uniform in thickness, prior to rupture at thicknesses of δ ～ 0.1 - 0.2 μm.