This paper describes an experimental investigation of the effect of various commercial anionic polyelectrolyte thinners on the stability of bentonite dispersions contaminated by mono and divalent salts. Combined adsorption and electrophoresis studies lead to the definition of a model for the mechanism of interaction between the polyelectrolyte and clay. The data presented suggest that, with the exception of FeCr-lignosulphonate, the thinners adsorb preferentially onto the edges of the clay particles. In the case of FeCr-lignosulphonate significant adsorption onto the faces of the bentonite is attributed to the bridging effect of the polyvalent metal ions. The different adsorption mechanisms have consequences for the amount of polyelectrolyte required to deflocculate and stabilise the dispersions. Flocculation tests confirm that polymers adsorbing selectively on the "edge" sites are more effective in stabilising the dispersion against flocculation than those which adsorb less selectively onto the bentonite surface. Interestingly, the rheological properties of bentonite dispersions improve even where the thinner produces a scarce deflocculation effect At least two mechanisms appear to describe the rheology-reducing properties of the thinners studied. Thinners like FeCr-lignosulphonate, which do not deflocculate the dispersions, can be distinguished from others such as the polyacrylates, which are true dispersing agents for bentonite in the presence of high electrolyte concentration, by means of scaling analysis of the rheological results.

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