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Capture of mineral particles by rising bubbles

Final Report Summary - CAPTURE (Capture of mineral particles by rising bubbles)

Froth flotation is a versatile separation process and plays a major role in the mining industry. It is employed to recover a vast array of different valuable commodities such as copper, zinc, nickel, phosphate and rare earth minerals essential to the manufacture of high-tech products. In mineral froth flotation the separation can be accomplished in a flotation cell, which is essentially a tank fitted with an impeller. The impeller disperses air into fine gas bubbles and agitates the slurry. It provides a favourable environment in the cell for the promotion of bubble collision with the finely ground ore. The desired hydrophobic mineral particles attach to the surface of the rising bubbles. The particle-bubble aggregates are then conveyed to the top of the flotation cell to form a rich mineral-laden froth layer, which eventually overflows into a launder as a separate product. The commercially valueless hydrophilic material exits the flotation cell as slurry. The recovery of the valuable minerals by froth flotation is largely determined by the capture of the mineral particles by the rising air bubbles. Aiming at an efficient capture of hydrophobic mineral particles by rising gas bubbles, the research project pursued the following three objectives: 1. development of a three-phase flow model to directly simulate the particle capture, 2. microscale experiments on the attachment of particles to the surface of a gas bubble, and 3. consolidation of the European excellence in multiphase flow applications. In an attempt to develop numerical tools, which will find future applications in the flotation process, the simulation of colloidal mineral particles interacting with a fluidic interface was performed. The fluidic interface corresponded to the gas-liquid interface of the bubbles rising in the mineral suspension. The “Smooth Profile Method”, a numerical method originally developed at the University of Kyoto (outgoing host) for the direct numerical simulation of colloidal particles in monophasic fluids, was combined with a newly-defined two-fluid model. This extended method, here referred to as the Extended Smooth Profile Method, essentially involved the replacement of the fluidic boundary and the two fluid-particle boundaries with smoothly spreading interfaces. In a first published paper (Lecrivain et al., Physics of Fluids, 2016) the attachment of spherical particles to an immersed gas bubble was simulated. The method was found capable of reproducing the three microprocesses associated with the particle attachment. The change in the trajectory as the particle approaches the fluidic interface, the collision process, and the sliding down the bubble surface were all simulated and compared remarkably well with literature data and on-site microscale experiments. In a second stage (Lecrivain et al., Physical Review E, 2017) the detachment of non-spherical mineral particles from the fluidic interface of an immersed bubble was investigated numerically. It was found that plate-like mineral particles attach more rapidly to the fluidic interface and are then subsequently harder to dislodge when subject to an external force. A small-scale facility was also conceived to visualise the attachment of mineral particles to an air bubble immersed in water (Lecrivain et al., International Journal of Multiphase flows, 2015). The set-up consisted of water tank in which a needle was placed in a horizontal position. The precision syringe, attached to the needle, allowed the blowing of stable and stationary air bubbles in the water. Using the in-house optical micro-bubble sensor developed at the Helmholtz-Zentrum Dresden-Rossendorf (Return host), the sliding and the adhesion of micron milled glass fibres on the surface of a stationary air bubble immersed in stagnant water was investigated. It was found that the orientation of fibre-like mineral particles during the sliding phase largely depended on the collision area. Upon collision near the upstream pole of a gas bubble, the major axis of the fibre aligned with the local bubble surface (tangential fibre alignment). If collision occurred at least 30° further downstream, only the head of the fibre was in contact with the gas–liquid interface (radial fibre alignment).

Flotation underpins many aspects of our everyday life. Many minerals form important resources that are needed for the European industry. Without them, it would be impossible to build solar panels and electrical engines for cars and wind turbines. CAPTURE contributes very well to the European excellence. The extended smooth-profile method developed in Japan and the experimental facility built in Germany were used to investigate the key microprocesses involved in mineral flotation. The use of computational fluid dynamics, particularly the use of the direct numerical, to fully resolve the attachment of mineral particles to rising bubbles is new. In the longer run, the obtained results will help build larger-scale simulation models of the entire flotation cell. The numerical results were also supported by small-scale experimental data on particle-bubble interactions.

Flotation has also seen widespread applications in the non-mining fields. It is for example widely used for cleaning coals, de-inking recycled paper fibres and waste water treatment. Reinforcement of the collaboration between the University of Kyoto and the Helmholtz-Zentrum Dresden-Rossendorf continues and currently involves further experiments on deformable microfibres interacting with gas bubbles. Preliminary results already show promise in the recycling of textile and paper fibres.

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