Effect of mode of flocculation on flocs sedimentation behaviour in various regions of thickeners

Main portion of water in mineral processing plants is recovered in thickeners. The required surface area for thickeners is calculated based on the Coe and Clevenger theory where it is assumed, when no flocculants used, the settling velocity of particles in each layer in the free settling region is only a function of solids concentration in that layer. In this study, the effect of mode of flocculation, defined as the initial solids concentration where the floes first formed, on their settling velocity was investigated using batch settling tests. The pulps used for settling tests were prepared by two different methods to arrive at equal solids concentration. The methods were the standard procedure of adding solids to a known amount of liquid and decantation. The latter method made the change of mode of flocculation possible by decanting the dilute suspensions to obtain higher solids content pulps. Results of all settling tests (with and without flocculant) on copper, iron, lead and zinc ores, and coal suspensions showed that, unlike the Coe and Clevenger theory, the settling velocity of particles in any layer even in the free settling region is affected by solids content of the suspension where floes were first formed. For instance, the settling velocity of a layer of iron ore tailing with 22% solids in a flocculated (5 g/t) suspension varied from 1 .32 to 2. 73 em/min when the initial feed solids varied from 5 to 20%. Furthermore it was shown that the settling velocities of suspensions with equal solids content prepared by the two methods were different owing to the significant difference in their floes structures. It was concluded that the settling tests performed to determine the required surface area for thickening should be carried out etther on the single sample with the solids concentration of the thickener feed (after dilution if it is diluted) or on decanted samples. The conventional procedure of measuring settling velocities in various percent solids between the feed and the desired underflow is prone to large errors.