Viscoelasticity and yielding properties of flocculated kaolinite sediments in saline water

Abstract

The effect of salinity on the yielding properties and viscoelastic behaviour of flocculated kaolinite sediments was studied through stress growth and creep-recovery tests. Turbulent flocculation was performed on a detachable cylinder using the plunger method to promote efficient and controlled mixing. The suspensions were prepared at natural pH (between 5.8–6.0) with salinities in the range 0–0.5M NaCl. Once a suspension was prepared, it was allowed to settle for 2.5h, during which time the variation in sediment height was small. The sediment was removed with minimal disturbance and subjected to stress growth or creep-recovery tests on a rheometer operated in control rate and control stress modes, respectively. Kaolinite suspensions in the presence of flocculant and salt in concentrations larger than a small although critical concentration (NaCl 0.001M) exhibit an inverse relationship between yield stress and settling rate with salt concentration, and direct relationship between shear strain and salt concentration. It is concluded that the particle network becomes denser and the crosslinking weaker with increasing salt. At the critical concentration of 0.001M NaCl, flocculation is best, the initial settling rate and yield stress are the highest, and shear strain the lowest. The response of the floc network to salinity changes is the result of two mechanisms competition: (i) charge screening of both anionic particles and flocculant by salt counterions, and (ii) shielding of the active sites on the flocculant by salt cations that causes the polymer to fold into compact structures adopting balled-up conformations. The first mechanism promotes flocculation while the second supresses flocculation.