MEASURING AND MODELLING BUBBLE COALESCENCE AND FROTH STABILITY TO INCREASE MINERAL FLOTATION AND REDUCE ENVIRONMENTAL IMPACT

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Mineral ores contain only a small fraction of valuable metal. Enormous tonnages of waste material are therefore generated as a result of mining. These waste dumps contain a huge amount of residual sulphur and metals (e.g. Cu, Fe, As, Pb and Hg), but at such low concentrations that they cannot be recovered economically. Over time, the residual sulphur leaches out as sulphuric acid causing acid mine drainage and mobilising the heavy metals; a huge and long-term environmental problem.Froth flotation is the prime separation process used for collecting selectively the small fraction of valuable mineral from a mined ore, and is based on differences in particle hydrophobicity. A flotation froth is similar to the foam of a beer being poured into a glass; it is generated from the bottom and bubbles overflow the edge or burst on the surface. In flotation, the bubbles carry the valuable, hydrophobic mineral with them. Current limits on flotation efficiency mean that approximately 95% of the metal and sulphur can be recovered economically, the remainder is discarded onto the waste dumps.The unintentional collection of waste, non-valuable minerals by entrainment from the pulp into the froth is the key flotation inefficiency and places an economic limit on the fraction of valuable mineral recovered. If entrainment can be reduced, the sulphur and metals recovered economically can be increased. This will increase the sustainability of the mineral resource by producing more metal from each ton of ore mined, while also reducing the potential environmental impact of the discarded minerals.Froth physics determines the flotation efficiency. Physics-based models of flotation have been very successful at interpreting flotation performance and predicting the effect of process changes on entrainment. However, appropriate physical models do not currently exist for the change in bubble size in the froth between the pulp and the froth surface (bubble coalescence), and the fraction of air entering the froth that leaves by bursting on the surface rather than overflowing the flotation tank edge (froth stability). Bubble coalescence and froth stability determine to a significant extent the mineral collection rate and the inefficiency due to entrainment in the separation. This severely limits the utility of existing froth simulations as it does not allow either confident equipment design or process modification.This project will improve flotation efficiency by developing measurement techniques and models of bubble coalescence and froth bursting. Techniques will be developed to measure experimentally the bubble size change between the bottom and top of the froth, and the loading of particles on the bubble lamellae. The models will be based on particle-stabilised film physics. The measurements will be used to verify and test the models. The models will then be incorporated into an existing flotation simulator.Equipment designs and process modifications that reduce entrainment will be produced by simulation. Appropriate designs and modifications will be implemented on a mining operation and evaluated. The goal is to reduce the sulphur and metal discarded by at least 50%. This will significantly reduce the environmental impact of mining, and enhance the sustainability of metal production.