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A linear pipe reactor (LPR) system was used to study the flocculation of mineral suspensions with commercial flocculants under a variety of operating conditions. Incorporation of a Lasentec focused beam reflectance measurement (FBRM) probe into the system provided real-time, in situ characterisation of the aggregate dimensions during the flocculation process. At constant flow rate, variation of the distance between the flocculant addition point and detection by the FBRM probe enabled control of the reaction time for flocculation. Once flocculant is mixed with the slurry, the size of the aggregates at a given point downstream in the LPR is determined by the relative rates of the competing processes of aggregate formation and rupture. These processes depend upon, amongst others, upon the mixing conditions, flocculant dosage and solids concentration of the slurry. The mixing intensity present within the LPR is determined by the pipe diameter and the slurry flow rate. Both a calcite slurry and a slimes fraction from a sand-processing operation were studied to assess their flocculation performance under different operating conditions (flow rate, flocculant dosage). Following a short induction period, aggregate size increased rapidly to a peak, with the rate of change indicative of the net aggregation kinetics. Rupture processes then dominated with aggregate size decreasing at a slower rate. The flow rate and hence the shear conditions had a marked effect on the aggregate structure. The effective (net) aggregation rate increased as shear increased, but the ultimate aggregate size decreased reflecting the fragility of large aggregates. For both systems, increasing the flocculant dosage increased the maximum aggregate size, but the effective aggregation rate only increased for the sand slimes, while reaching a constant rate for the calcite system. It is proposed that this reflects the degree to which the particle surface is effectively covered by the adsorbed flocculant. |
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