Froth flotation is a widely used process for concentrating valuable minerals from their ores by virtue of their differing surface properties. Flotation occurs in two phases; a pulp phase that is responsible for the collection of minerals via attachment to bubbles, and a froth phase that is responsible for concentrating the bubble-particle aggregates generated in the pulp phase. Ore characteristics, machine design characteristics, operating characteristics and flotation chemistry all affect the efficiency of the separation achieved. The subject of this thesis, frother evaluation, is an example of a flotation reagent contributing toward the overall flotation chemistry. Through their addition to a flotation cell or circuit, flotation frothers have the potential to modify pulp phase bubble size, to alter collector-frother or frother-mineral interactions, to assist in the dissolution of collectors to enhance their effectiveness, and to alter froth phase stability.
Given their importance in flotation, over the years, a great deal of research has been dedicated toward understanding and characterising frother behaviour. The research environment, however, has largely been restricted to laboratory studies where the use of a two-phase environment has dominated. Further, no comprehensive efforts into the isolation and evaluation of frother impacts in a three-phase industrial environment are reported in the literature. This thesis serves to address the identified gap in three-phase frother knowledge by providing a methodology for industrial scale frother evaluation.
To this end, the Savassi equation, Equation 1, (Savassi, 1998) was selected to provide the underlying structure for the developed methodology for industrial scale, three-phase frother evaluations. Its selection for this purpose was on the basis of the equation's explicit identification of key pulp and froth sub-processes and their contribution toward overall recovery (R), the measurable nature of the included sub-processes, and the fact that the model derivation and validation has been carried out with a view toward the industrial case. Thus, commercial frother blends may be evaluated as a function of their measurable impacts on the flotation sub-processes, which are identified in the Savassi equation as being relevant to industrial scale, three-phase overall metallurgical performance. The measurable parameters representing these sub-processes include bubble surface area flux (Sb), ore mineral floatability (Pi), froth recovery of attached particles (Rf ), water recovery (Rw) and the degree of entrainment (ENT).
Ri =[ PiSbRfiτ* (1 - Rw) + ENT * Rw ] / [ (1 + PiSbRfiτ ) (1 - Rw) + ENT * Rw ] (1)
A case study was conducted utilising the derived methodology in order to evaluate the effects of a selection of commercial frother blends on overall performance in a three-phase, semi-industrial scale 3 m3 flotation cell. Results from the case study used in this thesis demonstrated that the frothers produced a measurable impact on pulp phase bubble size, consistent with their inferred surface activity from two-phase, laboratory scale surface tension measurements. This relationship was quantified by demonstrating that a linear correlation exists between the surface tension and pulp phase bubble size for this system. The observed correlation was independent of the frother type or concentration utilised to produce the change in surface tension. The correlation will, however, be system specific, owing to the variable impact of the solution chemistry of each flotation process e.g. the impact of frother on bubble size is overridden in a hyper-saline solution environment (Laskowksi et al., 2003a).
The contribution of frothers to overall flotation performance was demonstrated through their impact on bubble size, and therefore bubble surface area flux, and the subsequent impact of bubble surface area flux on collection zone rate constant, overall flotation rate constant, overall recovery and water recovery. Each of the correlations was demonstrated to be independent of the frother type and concentration used to produce the change in bubble surface area ux. Thus, whilst frother type and concentration were considered to be variables utilised to allow movement up and down existing performance curves, in this case they were not considered to be operating variables that could be utilised to alter these correlations, i.e. there was no evidence to suggest a move from one performance curve to another curve that represented either improved or worsened performance as a result of the frother type or concentration.
No change in ore floatability was observed as a result of changes to frother type and concentration tested in this case study. Again, this is a system specific observation which is in contrast to findings reported by other researchers in this field e.g. Fuerstenau and Yamada (1962); Muki et al. (1972); Lekki and Laskowski (1975); Malysa (1981); Subrahmanyan and Forssberg (1988); Liu and Peng (1999); El-Shall et al. (2000). Consequently, this finding should not be taken as a generic conclusion applicable to all flotation systems. The value of the derived methodology for frother evaluation is that it allows for the evaluation of frother impacts on ore floatability and is able to accommodate these contrasting findings.
It has been demonstrated, in this case study, that froth recovery and water recovery are interdependent. It then follows that froth recovery and entrainment are interconnected owing to the known relationship existent between water recovery and entrainment. Whilst these are logical conclusions, they highlight that one cannot increase froth recovery without accepting the deleterious impacts of higher water recovery resulting in an increased dilution of the flotation concentrate by non-valuable gangue minerals.
The impact of frother type on the degree of entrainment, ENT, was clear and deemed to be statistically significant at the 95% confidence interval. A correlation between air hold-up and ENT was demonstrated, thus supporting the theory of bubble swarming being a key contributing factor toward entrained material entering the froth phase. In this investigation, the two Alcohol-glycol-glycol ether frother blends of the polyglycol family were demonstrated to produce a significant increase in the measured air hold-up, and subsequently ENT. Consequently, flotation selectivity was deleteriously altered as a result of the increase in ENT.
The two-phase laboratory findings of Gelinas and Finch (2005a) and Gelinas et al. (2005) demonstrated that a change in water content in the froth phase results as a consequence of the direct association between water molecules and constituent polar groups in frother molecules. Contrarily, the commercial frother blends used in this case study produced no measurable change to the operating relationship between the water recovered and mass recovered to the concentrate stream in the three-phase environment investigated. This finding demonstrated the overriding impact of the solids phase in dictating the froth phase water hold-up and is consistent with the findings of Hemmings (1980, 1981); Flynn and Woodburn (1987).
For the first time, a novel, systematic framework has been provided for the evaluation of frother behaviour in the three-phase, industrial environment in which they are typically used. This methodology is sufficiently flexible to accommodate the varying frother behaviour encountered in differing three-phase, industrial environments and in some cases, such as this study, three-phase results may be linked to two-phase laboratory surface tension measurements.
The derived methodology is not limited to the evaluation of frother behaviour. It may be utilised to evaluate the effects that any other flotation reagents of interest have had on the key sub-processes of flotation. In so doing, it is possible to adopt a more holistic approach to reagent evaluation and selection, and realise the full benefits of reagent modifications; for example: a collector may have a positive impact on ore floatability, resulting in an increase in collection zone recovery. The increase in particle hydrophobicity as a result of this change may, however, result in an unstable froth phase. Application of the methodology will decouple and identify the competing effects. It is then possible to alter the frother in order to address the deleterious effects that the collector has had on the froth phase and, in so doing, improve overall recovery.