Effect of the concentration of NaCl and cylinder height on the sedimentation of flocculated suspension of Na-montmorillonite in the semi-dilute regime

Abstract

In this study, we focused on the effect of the degree of floc growth on the moving velocity of the interface between sediment of flocculated montmorillonite and supernatant in the semi-dilute suspension, which is characterized by the start of an extremely slow movement of interface followed by an abrupt settling and ending in consolidation. Na-montmorillonite suspension coagulated under different ionic strengths, ranging from 0.5 M to 1.5 M of NaCl, was placed in a settling cylinder. The initial height of suspension was varied from 13 cm to 50 cm. The flocculated suspension was left to settle in the cylinder after the manual mixing of end-over-end. Changes in the height of the interface between the flocculated sediment and the transparent supernatant were measured as a function of elapsed time. It was confirmed that the maximum settling velocity increased with an increase in the height of cylinder within the range of our measurement. This tendency was found to be more significantly pronounced by the growth of flocs. This result indicates the presence of a feed-forward mechanism to enhance the upward motion of fluid or downward motion of flocculated sediment, or both. That means, sedimentation is accelerated by the growth of big flocs, and the growth of big flocs is accelerated by the sedimentation. These motions will eventually induce the generation of an upward plume or channel of water. The formed plume flutters slowly with rather a large scale. We term this phenomenon the "sedimentation turbulence."

Rheology and Sedimentation of Aqueous Suspension of Na-montmorillonite in the Very Dilute Domain

“…30. It appears that the maximum settling velocity was varied depending on the height of initial slurry and the value was increased steadily with an increase of initial slurry height (Ghazali M.E.B. et al, 2019).…”

“…Owing to the specif ic characteristics of Namontmorillonite to form enormously big flocs irrespective of pH value (Miyahara K. et al, 1998) under the condition of high concentration of NaCl, we carefully observed the sedimentation behavior of strongly f locculated Namontmorillonite in semi-dilute regime. In our recent studies, we measured the changes of interface between sediment and supernatant in varied initial height of suspension as a function of elapsed time for different levels of salt concentration (Wu M. and Adachi Y., 2017;Ghazali M.E.B. et al, 2019).…”

“…The observed pattern of fluctuation was similar to a turbulent flow (Pope S.B, 2000). We term this phenomenon as the "sedimentation turbulence" (Ghazali M.E.B. et al, 2019).…”

“…Schematic representation explaining the progress of sedimentation of flocculated materials in the semi-dilute regime(Ghazali M.E.B. et al, 2019).…”

“…Maximum velocity as function of cylinder height at different levels of ionic strength(Ghazali M.E.B. et al, 2019).…”