The impact of polymeric flocculants on the sedimentation, flotation and dewatering of radwaste suspensions

The use of polymeric flocculants is receiving greater interest as a cheap, easily
deployable and effective reagent to aggregate fine radwaste particulates, thus
enhancing contaminant recovery in nuclear decommissioning. Particularly in the UK,
where a significant volume of Mg(OH)2 based sludge has developed as part of the UK
waste inventory in in situ environments. Removal and treatment of this radionuclide
impregnated sludge is crucial for current decommissioning strategies. This sludge is
a polydisperse distribution of particle sizes, from fuel fragments to colloidal
Mg(OH)2. Current strategies are dominated by the use of gravity driven operations
such as sedimentation and thickening, technologies which have low reliance on
mechanical parts being impervious to failure, which are difficult and expensive to
address in radioactive environments. Whilst coarser particle sizes settle with ease, the
colloidal and fine material is more problematic to remove through sedimentation.
Whilst it is known, and was shown in this work that polymeric flocculants can
improve suspension residence times, the role of floc structure is not fully understood
and was probed in depth in this thesis, where due to the lack of inter-aggregate
spacing, larger flocs made from poly(acrylic acid)-co-polyacrylamide settling aids
were most influential on sedimentation rates when scrutinised with fractal modified
hindered settling models.

As the decommissioning of facilities which hold the Mg(OH)2 sludge inventory is
time critical due to degradation of the building structures, an alternative to the use of
gravity driven sedimentation is dispersed air flotation. Flotation was investigated as a
rapid alternative to remove suspended Mg(OH)2 material using traditional anionic
surfactant collectors, sodium dodecyl sulphate and sodium lauroyl isethionate. In
order to optimise the process, the optimum dose of hydrophobic surface modifier,
known as a collector, was found to be determined to be a function of collector surface
coverage. A common observation of surfactant collector driven flotation is that there
is a hydrodynamically limited operational envelope for recoverable particle size
distributions. Where fines lack the inertia to overcome the slip streams generated by
rising bubbles and coarse particles have high bubble detachment energies. There was
also significant water entrainment during flotation using surfactants. Polymeric
flocculants were synthesised as block copolymers, poly(acrylic acid)-b-poly(n-butyl
acrylate), to display amphiphilic properties. These amphiphilic block copolymers
were deployed as dual flocculation-flotation aids to aggregate fines to possess the
inertia for bubble attachment, whilst lacking the interfacial surface energy reduction
to result in excess water entrainment. It was found that the relative hydrophobic chain
length of these macrosurfactants was highly influential on flotation and flocculation
performance, with greater collector efficiencies than traditional surfactants due to
their superior water retention.

The different approaches were compared by hypothesising five deployment strategies,
including sedimentation without polymers, sedimentation with the optimum dosages
for flocculation and sedimentation using both poly(acrylic acid)-co-polyacrylamide
and poly(acrylic acid)-b-poly(n-butyl acrylate). Finally, sedimentation rates were
compared pre and post-flotation using poly(acrylic acid)-b-poly(n-butyl acrylate).
The consolidated bed post flotation was also compared for the 5 scenarios and
additional considerations for selecting an optimum strategy were also discussed to
inform on possible future work.