| Frothers are non ionic surfactants, commonly alcohols and polyglycols, used to provide two functions in flotation, namely to reduce bubble size and stabilize froth. Both functions imply an impact on the properties of the air-water interface (bubble surface) but there is no consensus on the mechanism, particularly with regard to bubble size reduction. Blending frothers is becoming common in flotation practice arguably enhancing performance by permitting independent control of the two frother functions. However, there have been no studies to determine this blend possible action. The blends used here focused on a small addition of polyglycol (F150 and DF250) to alcohols (1-pentanol and MIBC). The effect of blends on gas dispersion properties (bubble size and gas holdup) and froth properties (froth height and water overflow rate) was measured in three units, bubble column, 800 L and 5.5 L mechanical cells. Froth height and water overflow rate showed a strong blend effect, both increasing significantly compared to individual frothers. However, while the bubble size was decreased at blend concentration below the critical coalescence concentration (CCC) of the alcohol frother, bubble size was significantly larger above the alcohol CCC. Gas holdup data supported these effects on bubble size. This bubble size effect compromised testing the hypothesis of independent function control using blends. The thesis focussed on explaining the bubble size observations by designing coalescence and beak-up experiments.;Coalescence time of bubbles generated from two horizontal capillaries did not show a blend effect. Break-up tests for one-bubble-at-a-time and from an air stream conducted for F150 -- pentanol blends showed that the blend reduced bubble break-up compared to single frothers. The increase in bubble size above the base CCC therefore appears to due to decreased break-up. A mechanism based on the Marangoni effect is introduced to explain this phenomenon. |
