Particle Bubble Interactions in Flotation Systems

Abstract

Flotation separation is an important process in mineral processing and paper recycling. Flotation is a separation process used to remove particles from a liquid stream. Bubbles are introduced into the liquid stream, and, as they rise, particles may adsorb to the bubble surface. The bubbles may then carry the particles to the surface of the liquid, where they are removed. This dissertation focuses on flotation deinking. Flotation deinking is used to remove ink particles and other contaminants from recycled paper streams. Flotation is a complicated combination of fluid mechanics, thin-film dynamics, and surface chemistry. The fundamental process in flotation, the adhesion of a particle to a bubble, was studied using high-speed and high-magnification imaging techniques. Facilities for the study of particle interactions with stationary and flowing bubbles in several different system chemistries are discussed. These techniques were used to study the interactions between toner ink particles and bubble surfaces. Toner ink particles were observed to adsorb to bubble surfaces. In the presence of calcium oleate surfactant, toner particle formed networks at the bubble surface. These networks were observed to be very stable. The role of particle size and system chemistry in the adhesion of toner particles to bubbles was examined. Calcium oleate resulted in a larger percentage of bubble coverage than sodium lauryl sulfate chemistry; however, the percentage of bubble surface covered with ink particles did not depend upon the particle size. Estimates were obtained for the volume and mass of toner ink particles attached to the bubble surface. For sodium lauryl sulfate chemistry, the mass of adsorbed toner ink increased from 3.0 mg of ink per bubble for particles less than 75 ?m in size to 6.0 mg of ink per bubble for particles with a size range of 250 to 475 ?m. When calcium chloride is added to sodium lauryl sulfate surfactant, the mass of adsorbed ink increased. For sodium oleate surfactant with calcium chloride, the mass of adsorbed toner ink increased from 5.9 mg of ink per bubble for particles less than 75 ?m in size to 12.5 mg of ink per bubble for particles with a size range of 250 to 475 ?m. When the calcium chloride was not used with sodium oleate surfactant, the mass of attached ink decreased. The mass of attached ink particles was largest for the calcium oleate chemistry and increased as particle size increased. No change was seen with the addition of calcium ions to a surfactant-free system. No particle agglomeration was observed in the absence of calcium, suggesting that the proposed “Calcium Bridge” mechanism for particle agglomeration does not occur in this system. Imaging of model glass beads was also used to study the fundamentals of particle / bubble interactions. The Stokes number (the ratio of inertial forces and viscous drag forces on a particle in a fluid) was used as a criteria to study the mechanism of particle to bubble collision. Particles with a high Stokes number (> 1) were observed to undergo impact collision at the bubble surface. Particles with lower Stokes numbers (< 1) were observed to follow the sliding collision mechanism. These observations confirm the Schulze prediction for the mechanism of collision between a particle and a bubble. Flotation models were examined for the toner deinking system. Estimates for the probability of particle to bubble collision, probability of particle adhesion, and probability of stable attachment were found experimentally and compared to modeling results. Experimental and data analysis methods were developed to directly measure the probabilities of the subprocesses from visualization measurements. Model predictions do not match experimental observations. Specifically, estimates for the probability of particle to bubble adhesion were very different from experimental observations.