Duration of initial flocculation stage in the sedimentation of sodium montmorillonite suspension in the semi-dilute regime

In the duration of initial flocculation stage, required time for waiting prior to the gravitational collapse of Na montmorillonite suspension in the semi-dilute coagulated regime (with solid volume fraction of 1.097 × 10−4) was analyzed as a function of ionic strength for the two different pH values (pH = 4.0 (acid) and pH = 9.5(alkaline)). The obtained result clearly demonstrated that the duration time correlates the coagulation rate of clay particles. That is, the duration time is shorter (but not to be zero) for the system undergoing rapid coagulation which takes place under sufficiently high ionic strength. In this region, the effect of pH is shielded by the complete compression of electric double layer. However, the duration becomes longer with a decrease of ionic strength in accordance with the decrease of the rate of coagulation. In addition, under the condition approaching to the region near critical coagulation concentration, further longer duration is observed for the case of high pH value as compared to that obtained for the case of lower pH value. The reason for the additional increase of duration was ascribed to the elimination of positive charge along the edge of clay sheet under alkaline condition. In the case of low pH, on the contrary, the effect of the oppositely charged edge(+)-to-face(−) interaction is considered to induce attraction between clay particles which contributes to induce heterocoagulation. This effect will result in the shorter duration time for the case of low pH than that of high pH.