P.T.L. Koh

A review of factors that affect contact angle and implications for flotation practice.

Contact angle and the wetting behaviour of solid particles are influenced by many physical and chemical factors such as surface roughness and heterogeneity as well as particle shape and size. A significant amount of effort has been invested in order to probe the correlation between these factors and surface wettability. Some of the key investigations reported in the literature are reviewed here. It is clear from the papers reviewed that, depending on many experimental conditions such as the size of the surface heterogeneities and asperities, surface cleanliness, and the resolution of measuring equipment and data interpretation, obtaining meaningful contact angle values is extremely difficult and such values are reliant on careful experimental control. Surface wetting behaviour depends on not only surface texture (roughness and particle shape), and surface chemistry (heterogeneity) but also on hydrodynamic conditions in the preparation route. The inability to distinguish the effects of each factor may be due to the interplay and/or overlap of two or more factors in each system. From this review, it was concluded that: Surface geometry (and surface roughness of different scales) can be used to tune the contact angle; with increasing surface roughness the apparent contact angle decreases for hydrophilic materials and increases for hydrophobic materials. For non-ideal surfaces, such as mineral surfaces in the flotation process, kinetics plays a more important role than thermodynamics in dictating wettability. Particle size encountered in flotation (10-200 microm) showed no significant effect on contact angle but has a strong effect on flotation rate constant. There is a lack of a rigid quantitative correlation between factors affecting wetting, wetting behaviour and contact angle on minerals; and hence their implication for flotation process. Specifically, universal correlation of contact angle to flotation recovery is still difficult to predict from first principles. Other advanced techniques and measures complementary to contact angle will be essential to establish the link between research and practice in flotation.

CDF simulation of bubble-particle collisions in mineral flotation cells

The efficiency of the flotation process depends highly on the initial contact between the air bubble and the mineral particle. To enhance this contact, flotation cells are designed to achieve good mixing between the suspending solids and the dispersing air. CFD simulation of flotation cells provides an opportunity to analyse the influence of variations in design features and operating conditions on the performance of flotation cells. A laboratory flotation cell designed by CSIRO Minerals and a cylindrical tank fitted with a Rushton turbine used as a flotation machine have been modelled. The impeller and cell geometries have been set up using multi-blocking and sliding mesh techniques. Complex two-phase flow fields within the cells are predicted. Three-dimensional profiles of the turbulence dissipation rates and volumetric fraction of the air are also obtained. These form the basis for determining the number of bubble-particle collisions per unit time and unit volume. These profiles are important for locating positions within flotation cells where the initial contacts between bubbles and particles are made. Collision rates in the CSIRO flotation cell and the stirred tank have been compared. As flotation machines, the CSIRO cell is superior because of the higher maximum collision rate in comparison to the stirred tank.

Modelling shear-flocculation by population balances

Abstract A discretized population-balance approach based on a coalescence model is presented in this paper. The population-balance equation was tested for its validity by applying it to two special cases: constant collision-rate function and orthokinetic aggregation of monosized particles. Numerical solutions obtained for these two cases were compared with simplified solutions available in the literature. The size distribution of flocculated suspensions as observed in shear-flocculation in stirred tanks can be predicted by this method. An empirical collision-efficiency function obtained from small-scale laboratory tests was incorporated into the orthokinetic mechanism to limit floc growth to a maximum stable size. The size distribution of flocculated suspensions in a larger tank of similar geometry can be calculated from this method which was found to be satisfactory for the scheelite-sodium oleate system used in laboratory tests.

Compartmental modelling of stirred tank for flocculation requiring a minimum critical shear rate

Abstract In uniform shear flow, a minimum critical shear rate is known to be present during flocculation of a partially destabilized suspension so that at shear rates below the critical value, the flocculation rate is zero. In a stirred tank with distributed shear rates, it is possible for the shear rates in some parts of the tank to be below the critical value. In this paper the flocculation kinetics are described by a compartmental model in which zones of low shear rate are treated as a separate compartment. The theoretical flocculation rate for a monosized suspension is obtained by numerical integration from the mean shear rate for the whole tank, or from the stirring speed in a given geometry.