Despite the importance and usefulness of basicity in metallurgical slags, the
optimum quantitative expression of it has remained elusive, partly due to the
fact that the precise concept of basicity has not been properly clarified. The
two basic approaches to the concept of basicity are the Lux-Flood, which
perceives basicity in terms of the activity of the free oxide ion, a
O2 , and the Lewis, which perceives basicity in terms of the ability
to donate negative charge. The former has been almost universally adopted in the
area of metallurgical slags, while the latter has been increasingly accepted in
the field of glass chemistry.
In this paper, some difficulties of the Lux-Flood approach are pointed out,
including the need for indirect measures of a O2 , which have
proliferated in recent years. Accordingly, it is suggested that the Lewis
approach has a good deal to commend it, and that the quantification of it in
terms of optical basicity is the best measure of basicity available, which
should therefore be more widely applied in metallurgical slags than it currently
is. The fact that there are intrinsic problems in evaluating the optical
basicities for the transition metal oxides is acknowledged, and a new scale is
recommended. The patterns of iso-optical basicity lines in ternary systems are
shown to agree well with the pattern of iso-activity and iso-capacity lines
under appropriate conditions.
* Originally published in the Proceedings of MINPREX 2000, published by The
AusIMM.