Effect of agitation on the settling behaviour of electrocoagulated silica

In conventional thickening operations, chemical reagents are added as coagulants or flocculants to improve the settling behaviour of suspended and colloidal solids. The effectiveness of these reagents depends on the nature and particle size of the solids. Poor settling may necessitate impoundment of the entire suspension, making land requirements exorbitant and imposing severe constraints on the plant water balance. The demand for fresh water increases as recycle of supernatant water from the thickening operation is curtailed. A bottleneck is created and plant capacity is effectively limited.In light of these considerations, alternating current electrocoagulation (AC/EC) came to be viewed as a potential alternative to chemical coagulation for treating suspensions produced in the mineral, coal and petroleum industries. Consequently, a detailed program of fundamental study was undertaken on the application of AC/EC to the enhancement of the settling behaviour of fine silica, which served as the model tailings solid.This paper reports on the study of the influence of agitation on the settling behaviour of silica that was treated by the AC/EC technique. The technique enhanced the initial settling rate of silica (1.6 × 10− 3 cm/min) by over three orders of magnitude. Agitation was beneficial, but not essential, to this enhancement. Over the impeller tip speed range of 0 to 4.97 m/s, the initial settling rate of electrocoagulated silica was observed to be modestly sensitive to agitation rate, from which it may be deduced that silica formed strong flocs. Enhanced settling behaviour of electrocoagulated fine particles is manifested by their aggregation into flocs, as well as by their adsorption onto larger particles. In comparison tests, all of the aluminum added in chemical coagulation was precipitated in the sweep floc mechanism. This was not the case in electrocoagulation, which produced less voluminous sediment for the same Al dosage. In coagulation by specific adsorption, such as in electrocoagulation, zeta potential cannot be relied upon to track the progress of particle destabilization.

Research Highlights
► Agitation is beneficial, but not essential, to electrocoagulation.
► Agitation shortened electrocoagulation time for a given initial settling rate.
► Cationic hydroxoaluminum and oxyhydroxoaluminum complexes were identified.
► Flocs were formed by interparticle bridging during electrocoagulation.
► Aggregation and adsorption onto larger particles help fine particles to settle.