AGITATION AND RESIDENCE TIME EFFECTS DURING THE FLOCCULATION OF MINERAL SUSPENSIONS

The use of flocculants to enhance the settling rate of fine solids is an important operational aspect of all Bayer refineries. Traditionally the effectiveness of a given flocculant and the conditions for its application have been evaluated using batch cylinder settling tests. Although valuable as a general screening procedure, cylinder tests are highly operator dependent and cannot give reliable information on the effect of important performance factors such as the intensity and duration of agitation during the flocculation process.

A new procedure is outlined for conducting flocculation test work. The flocculation process is conducted in a continuous mode using a mixing device (termed a " Shear Vessel") based upon Couette geometry (concentric rotating cylinders). It readily allows the impact of factors such as agitation intensity, residence time, flocculant type and dosage, dual addition procedures, etc on the flocculation state (settling rate, supernatant clarity, etc) of mineral suspensions to be determined without operator bias. It also produces a reliable indication of the strength of aggregates formed under different flocculation conditions.

This paper gives details on the operational procedures associated with the Shear Vessel technique. Results from flocculation studies with the Shear Vessel at a number of mineral processing operations, including Bayer refineries are presented. These results illustrate the critical importance of agitation intensity and residence time on flocculation processes. It is also demonstrated that different commercial flocculants have different agitation and residence time requirements for optimal performance, a factor usually not recognised from batch settling tests. Implications of the Shear Vessel results for plant use of flocculants is outlined.