Effect of Potassium Oxide Addition on Viscosity of Calcium Aluminosilicate Melts at 1673–1873 K

Abstract

We investigated the changes induced in the viscosities of CaO-SiO2-Al2O3-K2O melts (CaO/SiO2 molar ratio = 0.68 ± 0.04, Al2O3 content = 13.4 ± 0.6 mol%) with the addition of K2O in amounts of 0-17.4 mol% for temperatures of 1 673-1 873 K using the rotating crucible method. The viscosity increased with an increase in the K2O content when K2O/Al2O3 molar ratio < 0.7. On the other hand, the viscosity decreased with the addition of K2O for K2O/Al2O3 molar ratio > 0.9. The maximum values of the viscosities were noticed in the range of 0.7 < K2O/Al2O3 molar ratio < 0.9. This behavior could not be explained on the basis of the polymerization degree of the aluminosilicate network structure. The 17 O magic angle spinning nuclear magnetic resonance (MAS NMR) spectrum of the 31.6CaO-44.2SiO2-13.4Al2O3-10.8K2O (mol%) glass suggested that K + ions preferentially compensate the negative charge of AlO4, while the Ca 2+ ions primarily create non-bridging oxygens (NBOs). The viscosity data and the 17 O MAS NMR spectrum also suggested that the increase in viscosity for K2O/Al2O3 molar ratio < 0.7 was because of the increase in the average bond strength of the aluminosilicate framework, which was attributable to the substitution of Ca 2+ ions by K + ions at the charge compensator sites. The activation energies for viscous flow indicated that the NBOs bonded with K + ions, forming NBO-K species, for K2O contents greater than those corresponding to the viscosity maxima. Thus, the viscosities of aluminosilicate melts are indicative of the average bond strengths of the melts.KEY WORDS: viscosity; K-Ca aluminosilicate melts and glasses;17 O MAS NMR spectroscopy.

Structure and Properties of Slags Used in the Continuous Casting of Steel: Part 2 Specialist Mould Powders

“…However, additions of CaO or Na 2 O result (shown in Eqs. (7) and (8) 42) ) in the formation of structural units with both 48) This was found to be accompanied by a slight increase in viscosity followed by a decrease in viscosity when (X K /X A ) > 0.9 (Fig. 3).…”

“…However, some workers have made systematic studies on the effect of specific additions (e.g. B 2 O 3 , TiO 2 or Al 2 O 3 ) on the properties (usually viscosity and T br ) and some workers have represented their viscosity data in the form of modified constants 48,50,51) for the Riboud viscosity model. 49) However, at the present time the reader will have to use a number of models to calculate the viscosities of different slag types.…”

“…3). 48) This behaviour was attributed to the competition between the average bond strength of the NBOs (i.e. O-Ca and O-K) and the number of NBOs.…”

“…In some cases, these data have been processed to give in the form of modified constants for the Riboud equation 49) low (C/S) 50,51) and CA-type slags for casting high Al steels. 48) Although such calculated values are useful, the data would have wider applicability if they were incorporated into a more global model. The model to calculate viscosities of slags due to Zhang et al 52) covers the compositional range of the different types of mould slags covered here but does not contain B 2 O 3 at the present time.…”

“…O-Ca and O-K) and the number of NBOs. 48) Calcium aluminate type mould slags usually contain some SiO 2 (5-10%) …”