The Effect of Particle Size and Shape on Transport through Confined Channels in three-phase Froths

Multiphase systems (containing solid, liquid and gas) are increasingly common in a number of industries, with the most complex manifestation being three-phase froth. The interstitial suspension has to navigate tortuous channels and its transport is affected by drag, capillary and gravitational forces. Particle properties such as wettability, size, shape, and morphology results in a number of different types of interactions with the liquid-air interface and can have a significant effect on froth composition and stability. The effect of particle size and shape on its transport through these confined channels is thus of great interest for a number of industrial applications and is the focus of this work. This transport behavior is studied using a three phase transient froth that is produced in the froth flotation process for mineral separation. In this system, hydrophilic non-value particles present in the interstitial liquid phase do not attach to air bubbles, and their removal is desirable. The original hypothesis was that as particles become more anisotropic in shape, there is an increase in the froth interstitial viscosity, which results in reduced drainage rate of particles through the froth. Flotation experiments, froth sampling experiments, and rheological experiments were conducted to test this hypothesis. Froth zone sampling experiments were conducted using mixtures of sized platy mica, needle-like wollastonite, and fibrous chrysotile, all mixed with low aspect ratio silica in varying amounts. The froth zone suspension compositions were then used to prepare the froth interstitial suspension ex-situ, and bulk rheological measurements were conducted on the suspensions. The data showed that while the relative viscosities of the suspensions were much higher at even low concentrations of the fibrous ore in the mixture, there was no significant difference when mica was substituted for silica in the mixture at high concentrations (~50 wt%) at the solids volume fraction of interest (~7.5%). The bulk rheological measurements thus could not fully account for the difference in transport behavior between mica and silica. Flotation experiments were conducted with a copper mineral-containing ore augmented with additional hydrophilic minerals mica, silica (low aspect ratio), wollastonite or chrysotile. The results suggest increasing aspect ratios of the added non-value particles result in increased net transport (transport accounting for loss due to drainage) through the froth zone; mica transport is faster than silica. Froth zone sampling experiments (using pure mixtures of above minerals) confirmed that mica net transport was greater than that of silica. It was then hypothesized that this increase was due to increased drag experienced by high aspect ratio mica compared to low aspect ratio silica. The doped ore flotation data also suggested a decrease in transport as size of added platy mica increased until a local transport minimum was reached, beyond which another increase in transport was observed. It was further hypothesized that this was related to confinement of coarse mica particles in the plateau borders when the size of the constriction was comparable to particle size. Froth sampling experiments under high drag (upward flow dominated) conditions were compared with those under conditions where drag and drainage were more balanced (steady state froths). Under high drag conditions, mica mixtures showed more hydrophilic mineral mass in the froth zone compared to silica mixtures. Under drag and drainage-balanced conditions when the size of mica approached the size of the measured channel size, platy mica was found to be accumulating in the froth. This was not the case for silica particles with settling being more efficient for silica than for mica. The key parameters driving transport of particles through the froth are the bulk rheology of the interstitial suspension (driven by particle size and shape distributions and solids concentration), the size of constrictions in the plateau borders and vertices and the resulting confinement effects, and the mobility or elasticity of the interfaces (driven largely by the hydrophobic particles attached at the interface).