either directly, by simply increasing the bulk viscosity, or by polymer–
surfactant complexation effects, that change the surface viscosity and
surface elasticity, so contributing to changes in foaming characteristics.
It was stated, that increasing bulk viscosity decreases the rate of
drainage in the froth contributing to longer foam lifetimes. In this
system this effect is not as much influencing the foam lifetime, but
rather the foaming characteristics, leading to an increasing foam height,
a more wet foam and probably less bubble coalescence in case of the
more viscous frother–CMC mixtures. The complexation effects that take
place at high frother (50 ppm) and low depressant (100 ppm)
concentrations decrease the surface viscosity that leads to faster
drainage of the thin liquid films, causing faster froth collapse. High
Depramin 267 concentrations, on the other hand seem to increase
surface viscosity that has a froth stabilizing effect.
In case of the Stypress 504 frother the increase in bulk viscosity with
increasing depressant concentration is not as dramatic as with the CMC
and the depressant seems to influence more the surface viscosity that
mainly has its contribution to the drainage of thin liquid films. This is in
good accordance with the observation that this frother-depressant
blend forms quite stable Polyedershaum type of foam structures.
However, the colloidal impurities present in the guar might also
contribute to the altered foam structure and stability.
It was also shown, that the depressants affected the froth formation
in three phase system as well. However, in this case the effect is much
more complex, as the depressants not only change the bulk and surface
rheology, but also adsorb at the solid/liquid interfaces, decreasing the
hydrophobicity of minerals. Therefore in three phase systems the
affinity of the polymer towards the minerals has also to be taken into
consideration.
The three-phase froth stability study has shown, that if very
hydrophobic particles are present in the guar containing solution, the
froth stability decreases dramatically compared with the case when the
particles are less hydrophobic or when there are no particles in the
system. As an opposite to this effect, the CMC does not affect the froth
stability when the particles are less hydrophobic, but it increases froth
stability with hydrophobic particles present. The suggested mechanism
is that the more dry, guar froth composed mainly of thin films, is easily
destabilized with the addition of hydrophobic particles, as these
particles break the balance in the thin film, and therefore the bubbles
burst. On the other hand, the hydrophobic particles added to the CMC
froth contribute to the stabilization by attaching to the air/liquid
interfaces and preventing the fast drainage of the liquid from the films.