Collapse of house-of-cards clay structures and corresponding tailings dewatering induced by alternating electric fields

Oil sands tailings are composed of charged clay colloidal solutions that are stabilized due to their double layer repulsions. Due to these repulsions, the microstructural arrangement of clay platelets in tailings is observed to be a stable house-of-cards like pattern with dominant edge-to-edge or edge-to-face attractions. The microstructural gaps from this arrangement tend to trap significant amounts of water. Scanning electron microscopy (SEM) was conducted on freeze dried samples to determine these gaps to be around 7 μm in control tailings with 40 wt.% clays and 30 μm in control tailings with 30 wt. % clays. In this study, we investigated the role of alternating current (AC) in inducing dewatering by collapsing this stable house-of-card structure. AC frequency caused localized charge displacements leading to formation of induced dipoles around clay double layers. These dipoles then underwent dielectrophoresis allowing face-to-face aggregation, which reduced microstructural gaps and caused dewatering. Depending on the electrical impedance of tailings-electrode load, decided by electrical properties of tailings and electrode configuration, optimal AC electric fields were identified and applied. AC frequencies between 10 kHz to 1 MHz were tested and a signature electric field between 500 and 1600 V/m was identified that resulted in the most desirable aggregation and dewatering effect. Control tailings with 30 and 40 wt.% solids content were tested and observed, respectively, to transform to 40–55 wt.% solids content on loose aggregation and 50 to 65 wt.% on strong aggregation. The dewatering effect arising from both aggregation cases resulted in about 12% water recovery, observed through gravity settling tests. Structural transformations of tailings post-AC treatment observed by SEM further prove the collapse of the stable house-of-card microstructural arrangement.