Novel features of smectite settling

Aqueous suspensions of Ca or Mn montmorillonite were left to settle in cylinders ranging in diameter, D, from 1.0 to 7.7 cm after magnetic stirring and/or manual tumbling. Changes in height, H, of the interface between the turbid suspension and the clear supernatant were recorded visually as a function of time, T. In the absence of added electrolytes the H/T plots were approximately linear, but after addition of electrolytes they assumed an inverse S shape. An initial latency period was followed by a time of rapid settling and, finally, by slow compaction of the sediment until no further changes occurred. Within a limited range of concentration the latency period increased exponentially with decreasing D. It increased with increasing height of the settling columns, H_i, and with increasing concentration of the suspensions. With Mn montmorillonite the rate of fast settling also depended on D. The parameters of the H/T plots depended on the prehistory of the samples. Fairly reproducible results were obtained in duplicates within any one series of experiments, but changes in the pretreatment altered the parameters. Treatment of the suspensions with an ultrasonic tip changed the settling process drastically. Sedimentation became very slow, the effects of D and H_i on the latency period were reversed and the final sediment volume increased. The effects of vessel diameter on sedimentation observed are much greater than any wall effects described in the literature. The changes observed after ultrasonication of the suspensions, particularly the reversal of the effects of D and H_i on the settling process, are novel features.