Bubble-particle detachment in a turbulent vortex I: Experimental

The mechanics of the detachment of particles from bubbles in the flotation process in a turbulent environment are unclear. The traditional hypothesis assumes a bubble particle aggregate is trapped inside an eddy of equivalent size, and the attached particles rotate at the same speed as the eddy. The rotational movement subjects the attached particles to a centrifugal force. It is theorised that particles detach when the centrifugal force is greater than the capillary force, but this hypothesis has not yet been experimentally proven. This work is an experimental study of bubble particle detachment in a rotating eddy. A special experiment was designed to obtain a strong confined vortex, and bubble particle aggregates were introduced into the cavity without destroying the vortex structure. This newly developed method, which provides a realistic analogue of the turbulent conditions in a flotation cell, is well suited to the study of an important sub-process of flotation in a turbulent field, namely, the stability of single bubble particle aggregates. Particles can detach from bubbles by a number of ways, including inertial actions induced by rapid changes in direction, and disruption due to coalescence of colliding bubbles. In this paper, we focus on a particular mechanism, in which bubbles are observed to rotate in a turbulent vortex. Particles can be held on the surface of the bubble by surface tension, and the radial centripetal force induced by the rotation is sufficiently high, particles may detach. Experiments are described in which the process of particle detachment due to centrifugal movement, was captured by a high-speed video camera, and the necessary physical parameters, especially the rotational velocity of the particles, were extracted. For the first time, centrifugal movement of the particle on the bubble surface inside a vortex was observed, and the theory of detachment due to centrifugal forces in the turbulent field was experimentally proven. (C) 2016 Elsevier Ltd. All rights reserved.