Clay Fabric in Electrokinetic Dewatering

Electrokinetic dewatering, a promising way to separate the clay particles from water in suspensions, has been researched for many years on topics like equipment design, parametric optimization, and ion migration.¹ However, little work has been done on the clay fabric which affects the performance to a considerable degree during electrokinetic dewatering since different fabrics have different electrokinetic properties. Therefore, if we could figure out fabric and control it intentionally, a breakthrough in improving the productivity and lowering the energy consumption will be possible.

Suspensions come from four corners mining plant in central Florida, which contains approximately one-third phosphate, one-third sand and one- third clay. Clay, mainly smectite here, plays the key role in dewatering due to the relatively small particle size and large surface area. Smectite plate is a 2:1 structure with permanent negative charge on the two faces, and pH-dependent charge on the edge.² It is this property that makes the behavior of smectite complex in electrokinetic dewatering under the effect of both pH and electrical force simultaneously. To dissociate the effect of the two mechanisms, a natural settling experiment at different pH and an electrokinetic dewatering experiment were conducted.

In the natural settling experiment, suspensions at acid environment reach stable state very fast and show a “gel” like behavior. However, suspensions in neutral and alkaline environment remains to be liquid like ones. The SEM results (Fig 1) of the sediments samples prepared by freeze-drying show that smectite particles in the presence of acid show edge-face fabric and flocculate into “honeycomb” structure as a result of the edge-face attraction and face-face repulsion (Fig 1a), while particles in neutral suspensions mainly display edge-edge, and in alkaline environment show all fabrics (edge-edge, edge-face, face-face) without dominating part.

In the electrokinetic dewatering experiment, an electric field of 4V/cm was applied for 13 minutes. The SEM results (Fig 1b) elucidates that honeycomb forms at lower layer near anode where H⁺ exists due to electrolysis of water during the separation process, but appears to be more compressed compared with that of natural settling (Fig. 1a), which should be due to the electrical force. The middle and upper layer with a slightly alkaline environment looks much more disordered than the lower layer.

Intuitively, we could infer that the rigid “gel” like honeycomb is not desirable for separation of particles compared with “soft” suspensions. In addition, the cells of the honeycomb are likely to trap water, which weakens electro-osmosis. The significance of this work exists in the difference of the electrokinetic properties between disordered plates and flocculated honeycomb. If the difference is quantified, then measures could be taken to control the fabric intentionally. Thus, we can predict the process more accurately and achieve a better performance.

References

1. A. Mahmoud, J. Olivier, J. Vaxelaire, A. F.  Hoadley, Electrical field: a historical review of its application and contributions in wastewater sludge dewatering,Water research, 44(2010), 8.
2. G. Lagaly, S. Ziesme . Colloid chemistry of clay minerals : the coagulation of montmorillonite dispersions. Advances in Colloid and Interface Science, 100-102(2003), 105–128.