Interfacial chemistry and particle interactions and their impact upon the dewatering behaviour of iron oxide dispersions

The influence of interfacial chemistry and particle interactions on sedimentation and electroosmosis (EO) of coagulated iron oxide dispersions has been investigated. Both pH and ionic strength of NH4Cl electrolyte had a profound effect on the particle electrokinetic zeta potential and dispersion rheological behaviour (shear yield stress), impacting strongly on the dewatering behaviour. In the absence of NH4Cl, the zeta potential decreased with increasing dispersion pH, with an isoelectric point (iep) observed at pH 5.6. Upon NH4Cl addition, specific adsorption of NH4+ ions onto particles at pH>6 occurred and led to a significant reduction in particle zeta potential, accompanied by a shift in iep to higher pH (>7.5) and/or charge reversal. The shear yield stress was large where the zeta potential was low and small where the zeta potential was high. Fast dewatering rate was achieved by pH modification and NH4Cl addition, however, these had no substantial impact upon the dispersion consolidation behaviour. Dewatering rates correlated strongly with both particle zeta potential and shear yield stress, the latter being highest at pH where the zeta potential was low and, hence, interparticle attraction was at a maximum (repulsion minimized). The consolidation of concentrated dispersions (60 wt.% solid) to a “spadeable” paste consistency (85 wt.% solid) by electroosmosis was achieved for moderately charged particles at 5×10−3 M NH4Cl and pH values away from the iep using moderate power consumption. The results exemplify clear links existing between interfacial chemistry, particle interactions and dewaterability of iron oxide tailings, which may be optimized to facilitate tails compaction.