The making of kaolinite suspension time-dependent in the structural rejuvenation mode – the face charge hypothesis

Abstract The physical property requirements for kaolinite suspensions to display time-dependent structural rebuilding or rejuvenation behavior are the same as required by smectite gels such as hectorite and Na-montmorillonite (NaMnt), a rare discovery linking the two types of clay. A holistic framework for predicting clay gel behavior based on the clay physical properties such as charge, crystal structure, and mineralogy, may emerge and was the ultimate goal of this research. A structural rejuvenation process during both ageing and stepdown shear rate modes was postulated to require that the silica and alumina faces of the kaolinite platelets be negatively charged to produce a strong electric double layer (EDL) repulsion in all face-face configurations. This is needed to slow down the (+)edge to (–)face bonding process to produce the time-dependent behavior. Currently, the unlike charge attraction between the silica and alumina faces makes the structural rebuilding process of a pre-sheared kaolinite suspension too fast to be observable. Two methods of making the alumina face negatively charged, i.e. the use of adsorbed P 2 O 7 4– and high pH, did indeed produce the time-dependent behavior for both KGa-1b and KGa-2 suspensions, thus validating the proposed hypothesis. The KGa-1b with a lower content of octahedral positive layer charge required less P 2 O 7 4– and a lower pH to achieve the desired outcome. Addition of 0.002 M NaCl to the high pH-treated KGa-2 suspension hastened the structural rejuvenation process and increased the aged gel strength. The oscillatory behavior in the stepdown shear stress at low pH was due to layer agglomerates formed by (+)face to (–)face attraction. Bulky layered agglomerates were reduced markedly by both the P 2 O 7 4– and high pH treatments. The knowledge gained was applied successfully to make clay-rich iron ore tailings time-dependent in both the ageing and stepdown shear rate modes.