The froth zone in froth flotation is important because it helps to refine the flotation separation. From the mass transport point of view, the performance of the froth zone depends on the froth movement (vertical and horizontal) and the bubble stability (coalescence and bursting rates) which in turn are affected by the use of chemical reagents, froth depth, and physical effects such as the cell geometry (e. g. use of booster plates) and the addition (or not) of wash water. If chemistry is assumed to be constant, the physical effects can be studied.
Froth zone recovery (Rf) quantifies flotation froth performance. To date, most studies of the flotation process have considered the flotation environment as a single rheological phase, i. e, froth performance efficiency 100%, because of the difficulty of estimating Rf. This thesis work seeks to understand the froth phase and to establish methodologies to study it.
Previous studies of flotation froths reported in the literature, using either laboratory batch or laboratory column cells, have looked at the froth phase in conditions very far removed from the full scale situation. In order to study the froth phase at conditions similar to those observed at full scale, the material arriving at the pulp-froth interface should be delivered at rates equivalent to those measured at the industrial scale. However, the bubble surface area flux (Sb), which is associated with the process mass transfer rate, produced at the industrial scale is much greater than that at the laboratory scale. This provided the impetus to develop a laboratory high Sb flotation cell (16 L) to study the froth phase. Hydrodynamic characterisation of the new high Sb laboratory flotation cell confirmed that the range of Sb values produced by the new cell overlaps the range of values observed in industrial-scale cells.
With the use of this new laboratory scale high Sb flotation cell, a methodology to measure froth performance (in terms of Rf) was developed which consists of carrying out flotation experiments at different froth depths (FD). From these tests, flotation rate constants (k) are obtained, and related to the froth depth. Froth zone recovery is calculated from the relationship between k and FD. This relationship allows Rf to be determined at different froth depths, and also provides collection zone rate constants (kc).
With the aid of the new equipment and the new methodology, this thesis set out to study the effect of different operating conditions on Rf for a Mount Isa Mines copper ore (AG mill discharge, feed ore containing chalcopyrite, pyrite and silica). The results showed that air flow rate, impeller speed, wash water and frother dose have an important impact on froth zone recovery values, as does particle size, i. e. Rf decreased with increasing particle size. The findings also revealed that the froth zone did not exhibit selectivity for the minerals entering it by true flotation. Last of all, it was observed that events in the pulp affected the collection zone rate constant (kc), and events in the froth affected the froth zone recovery (Rf) — supporting the assumptions inherent in the methodology to determine Rf.
Further experiments were carried out using copper rougher feed obtained directly from the Copper Concentrator of Mount Isa Mines Ltd. This material was much finer than the AG mill discharge sample and had a very high solids concentration (48.5% vs 20%). In addition to determining kc and Rf, water recovery and entrainment were also evaluated. The effect of feed flow rate (mean residence time) on the collection zone rate constant and froth zone recovery was also investigated. Results showed that overall recoveries increased with increasing pulp mean residence time. Due to the feed characteristics, a significant entrainment contribution was observed, thus the concept of "dirty" froth zone recovery (Rf*) was introduced. Rf* for chalcopyrite was found to be independent of water recovery while Rf* for pyrite was found to increase with increasing water recovery.
This dissertation also examined froth performance scale-up methodologies. Rf values obtained using the specially developed high Sb cell (16 L) were compared with those measured in an industrial unit (a 2.8-m3 mechanical flotation machine). And then, results were analysed using two approaches: (1) a froth plug flow model, and (2) an empirical model.
Finally, the thesis proposes a way of estimating froth zone recovery (Rf) by relating it to the ultimate practically attainable enrichment ratio (γ). The approach was based on the well-known recovery-grade (enrichment ratio, γ) curve for which an empirical model was developed. This allowed an opportunity to uncover some of the secrets of the froth phase. Briefly stated, froth zone recovery was related to the froth upgrading process through the parameters of the recovery-enrichment ratio curve.