DETACHMENT OF PARTICLES FROM BUBBLES IN A STIRRED CELL

摘要

It has been conjectured that a reason for the poor recovery of coarse particles in flotation, is the detachment of large particles from bubbles in the turbulent shear flow that is present in mechanical flotation cells. The aim of this study is to observe the behaviour of particle-laden bubbles in a turbulent shear flow. An agitated vessel was constructed in which particle-laden bubbles could be introduced beneath the impeller. The bubbles were generated in a liquid-fluidised bed in a special compartment beneath the cell. The bed was composed of fine silica particles 110 to 250 μm in diameter, with an upflow of water containing DDA as collector. Bubbles were formed at the tip of a capillary tube within the bed, and particles were collected as the individual bubbles rose through the fluidised bed. These bubbles then rose one by one into the base of the flotation cell, into the path of the rotating impeller. Some of the particles became detached by the action of the impeller and settled to the base of the cell. Others remained attached to the bubble and rose to the top of the cell, where they were collected as the product. The detached particles, which settled in the base of the cell, and the particles that flowed over the lip of the cell, were collected and weighed. Experiments were conducted at various impeller speeds. Observed values of the fractional detachment were related to the mechanical energy dissipation rate in the region swept by the impeller, in the light of the classical theory of Schulze. According to this theory, a particle will become detached from a bubble in the centre of a rotating eddy, when the centrifugal force due to the rotation exceeds the capillary force holding the particle to the bubble. The theory predicts that a particle of given diameter will detach from a bubble of a given size, when Bo >1, where Bo is the Bond number. It was found that the theory overpredicts the Bond number for detachment by an order of magnitude. Approximately 80 per cent of the particles will detach at a Bond number of 0.08.