Role of polyelectrolyte dispersant in the settling behaviour of barium sulphate suspension

Settling properties of barium sulphate suspensions have been systematically investigated as a function of particle volume fraction and polyelectrolyte dispersant concentration. The settling kinetics and the microstructure of sediments were examined by turbiscan measurement and scanning electron microscopy at low temperature (open-fractured samples). In the absence of dispersant, following the particle volume fraction, several types of behaviour were observed. The overall experimental results are well described by a model, which takes account of the coupling between aggregation and sedimentation. In the presence of a dispersant in excess, the particles are well dispersed, no aggregation occurs, and the settling kinetics follows classical hydrodynamic laws. The nature of suspension stabilisation was investigated by electrophoretic and sediment compactness measurements all along polyacrylic acid (PAA) and polystyrene sulphonate (PSS) isotherm adsorption at pH 9. The settling results show a sharp transition between the behaviour of aggregating and well-dispersed suspension. For the two polyelectrolyte dispersants used (PAA and PSS), at the transition the adsorbed amounts correspond to a same value of the electrokinetic potential, a value that agrees well with DLVO theory predictions.