Scaling in Bayer Refineries is a natural consequence of the
super-saturated liquors used in the process. The caustic liquor is
supersaturated not only with alumina but also silica and calcia at various
stages of the process. The basic scaling mechanisms are reviewed and
examples of the two basic types, growth and settled scales, are given.
Examples include gibbsite scales formed in the precipitators and silica
scales formed in heat exchangers. The main factors affecting the rate of
growth of growth scales are temperature and supersaturation and are
highlighted using a silica scale example. For growth scales chemical
cleaning rates can readily be predicted from kinetics and an example for a
gibbsite scale is given. For settled scales the growth rate cannot be
predicted and the hydrodynamics within the system have most effect.
There is no simple method to predict the potential for chemical cleaning
of settled scales; some of the relevant issues are discussed, in particular
methods to identify the cementing phase in the scale. An example of
design considerations for a scaling system, transport of slaked lime, is
Given.