Rigidity in Weak Clay Suspensions

Abstract

IN the course of work necessitating the purification of quantities of the smallest soil particles (the so-called clay fraction), a striking phenomenon was observed during the flocculation and sedimentation of the material in dilute hydrochloric acid. Many industrial and laboratory processes entail flocculation and sedimentation, so an account of our own observations may be of general interest. When the concentration of the suspension exceeds a certain critical value—the significance of which will appear later—a number of sharp ramifying fissures develop containing clear liquid. The density of this being less than that of the surrounding clay-laden liquid, a circulation is set up, clear liquid rising through the fissures while the remainder sinks. Some of the fissures form against the glass walls of the vessel, so the progress of sedimentation can be watched in detail. Near the bottom of the vessel the fissures tend to close, and to enlarge progressively towards the top of the column into conical chimneys, through which the motion of the ascending liquid can be traced by the movement of floccules detached from the walls of the fissures. The circulation is completed by the deposition of these floccules in a crater or ring around the exit of the chimney. There is no doubt that the suspension has acquired rigidity. The descending surface retains the initial form impressed on it by the curvature of the meniscus and by occasional air bubbles floating on the water. Marks deliberately made on the clay surface with a rod are also retained.