Gravitational settling of glass fibers on an air bubble

Froth flotation is a separation process in which air bubbles are introduced in a water tank to separate the valuable commodities from the valueless material. Based on their relative affinity to water the valuable particles attach to the bubble surface and are carried to the top of the flotation tank to form the froth layer. The resulting froth layer is eventually collected to produce the concentrate. Froth flotation has been used for more than a century in mining operations to separate valuable materials such as rare earth metals from excavated ores. More recently, froth flotation has been employed for the treatment of contaminated water. In the present study, the effect of the particle elongation on the attachment mechanism is investigated in great detail. Using an in-house optical micro-bubble sensor the attachment of micron glass fibres on the surface of a stationary air bubble immersed in stagnant water is investigated. The attachment mechanism is here defined as three successive events: the approach of the particle near the bubble upstream pole, the collision of the solid particle with the gas-liquid interface and the particle sliding on the gas bubble surface. The translational particle velocities together with the particle orientation during entire attachment process are measured and compared with a theoretical model. For the first time the existence of two types of attachment is shown. Upon collision near the upstream pole of the gas bubble the major axis of the fibre aligns with the local bubble surface. If collision occurs at least 30° further downstream the contact is likely to take a punctual form, i.e. the head of the fibre is in contact with the gas-liquid interface.