Entrainment of fine particles in froth flotation

Abstract

Entrainment has become a significant issue nowadays with fine grinding being used to liberate valuable minerals from low grade and complex ores to increase “true flotation”. Entrainment results in large amounts of fine gangue materials reporting to flotation concentrates along with recovered valuable minerals and reduces the final product quality. In froth flotation, entrainment is the dominant recovery mechanism for fine gangue mineral particles, and it is influenced by a number of factors in both the pulp and froth phase, including particle properties (e.g. particle size and density) and flotation operational variables (e.g. gas flowrate, froth depth, impeller speed and reagent dosage). A number of models incorporating different factors have been developed to estimate entrainment recovery. Most of these models relate entrainment recovery to two parameters: the water recovery to concentrate, and the degree of entrainment, a classification factor to account for the degree of drainage of entrained particles with respect to the water in the pulp and froth phases of a flotation cell. However, there are still significant barriers to be overcome before they can be applied for routine estimation of entrainment in industrial applications. There is no consensus as to the variables which significantly affect the entrainment process and should therefore be included in the model as well as to the mechanisms involved. This thesis investigated the effect of flotation operational factors and particle properties on the entrainment parameters (i.e. degree of entrainment and water recovery) and the mechanisms underpinning the observed effects. The objective of the thesis was to develop a comprehensive understanding of the key operating conditions and particle attributes influencing the entrainment of gangue minerals in flotation, which should enable the development of a more sophisticated predictive entrainment model by more accurately predicting the degree of entrainment and water recovery. A two level factorial experimental program was adopted to investigate the effect of impeller speed, gas flowrate, froth height and specific gravity of gangue minerals on the degree of entrainment. Batch flotation experiments were performed in a 3.5 L conventional Agitair flotation cell using a feed which was a mixture of liberated chalcopyrite and one of two liberated gangue minerals, quartz and hematite. Results show that there was no direct correlation between the degree of entrainment and water recovery, indicating that the key drivers of water recovery were not the same as those that affected the degree of entrainment. The degree of entrainment was significantly influenced by particle size, particle density, frother and the interaction between gas flowrate and froth height. The mechanisms underpinning the effect of these significant influencing factors were investigated. Results suggest that the classification of gangue mineral particles mainly occurs at the pulp/froth interface region. Whether a particle moves upward into the froth and subsequently follows the water phase depends on whether the drag force acting on the solids is greater than the apparent immersed weight of the particles which promotes particle settling. Particle size and density are two variables that affect apparent immersed weight and drag force. Frother, gas flowrate and froth height can alter the liquid velocity at the interface and hence the drag force, and therefore also have a significant impact on the degree of entrainment. The results from the two level factorial experiment were also analysed with respect to water recovery. It was found that water recovery was significantly affected by (1) froth height, (2) gas flowrate, (3) impeller speed, (4) the interaction between impeller speed and particle density, and (5) the interaction between gas flowrate and froth height (in the order of reducing effects). These effects resulted from these factors affecting the water motion in a flotation cell: water drainage in the froth phase and water flow into the froth from the pulp phase. Moreover, the results show that water recovery could be modelled effectively using an exponential function, incorporating water drainage and water entering into the froth. The outcomes of this work can provide plant operators with information they can use to minimise entrainment in their flotation operations as well as providing relationships that could be used to develop an improved predictive entrainment model.