Recovery of fine particles in turbulent flotation systems

Contact Person: O. Oikonomidou [], Paulina Tsave []

The overall aim of the present research project is to create new scientific knowledge in order to enhance the recovery of fine sized mineral particles with the process of flotation.

Viscoelastic interfacial properties of liquid-gas systems involving in a flotation process

The first part of this work aims to investigate the mobility of bubbles boundary in the liquid phase of a flotation system, so as to incorporate the proper equations for process characterization. Bubble boundaries mobility in flotation liquids can be indicated by the viscoelastic behavior of the generated air/liquid interface. For this, interfacial viscosity flow sweep runs under constant shear rate, elastic and loss moduli measurements under strain and frequency sweep oscillations, as well as creep compliance tests, are performed with a Physica MCR 301 Anton Paar rheometer equipped with a Bicon bob, at 15°C and 30°C. Several water-frother or water-frother-collector solutions of different concentrations (5-50ppm) consist the liquid phase, while the gas phase (air) resembles the presence of bubbles. BASF Petrochemical surfactants (PPG200, PPG400, PPG600, BTEG, BDEG, BDPG and BTPG) are utilized as flotation frothers, while Sodium Oleate (NaOl) is used as a collector at its remaining concentration upon conditioning of mineral particles.

Rheological study of flotation systems: a) Temperature sweep viscosity curves of Newtonian non ionic surfactants (frothers), b) Interfacial viscosity of non ionic frother (PPG600) and anionic collector (NaOl) layers on water/air interface. The synergistic effect of non ionic co-collectors (i.e. Dodexyl Alkoxylate 54) on the formation of robust surfactant layers.

Bubble-particle collision frequency in turbulent mineral flotation

In another aspect, this work aims to develop a generalized framework for the bubble-particle collision frequency in the turbulent flows of flotation systems. For this, the already existing collision frequency models are unified, to examine and assess properly all the relevant physical principles and fundamental theories. The developed generalized framework, models the pulp phase from device scale to thin film scale separating bubbles and particles. The core of this model is the term describing the collision frequency between bubbles and particles. All existing approaches are examined in detail and critically commented demonstrating several inconsistencies. A unified and consistent approach for deriving this collision frequency term is described, overcoming all the inconsistencies of previous approaches. The obtained results refer to the case of dispersed air flotation of fine particles, being practically the only case for which a collision frequency expression of algebraic complexity can be derived. Furthermore, the development of a robust mean field model (based on the existing population balance models of flotation) aims to simulate flotation in turbulent flow field.

General structure for the flotation cell model focused on pulp phase. Bubbles are shown in white and particles in black (not to scale for clarity of presentation).

Fine minerals recovery due to hybrid mechanical-electrolytic flotation

The third part of the project focuses on the development of an innovative flotation technology to intensify the attachment between fine mineral particles and gas bubbles and hence, enhance flotation performance. The proposed technology is based on existing experimental evidence showing that bubble–particle collision efficiency and fine particles flotation recovery increases with the decrease of bubbles size. For this, the design and operation of two hybrid flotation systems, able to produce bubbles with different diameters, is accomplished. A micro-bubble generator (electrolysis unit) is adapted on a mechanical flotation device and on a flotation column and the resulting hybrid devices operate by combining dispersed-air bubbles and micro-bubbles from water electrolysis. The significance of this process is that micro-bubbles attached on the hydrophobized surface of fine particles assist the attachment of conventional-sized bubbles and subsequently increase the flotation recovery of mineral particles.

Custom hybrid flotation devices: a) Hybrid mechanical-electrolytic flotation device, b) Hybrid dispersed-electroflotation column

The computed recovery of the particles after each flotation experiment is an indicator of the process efficiency. Experimental flotation results of the hybrid devices, so far indicate the enhancement of fine particles recovery by approximately 10%, due to the addition of micro-bubbles.

Effect of bubble size on the recovery fine mineral particles

Furthermore, gas phase involved in the flotation process is characterized using both optical and electrical diagnostics. The optical diagnostics (high resolution still digital camera & high speed camera, light scattering method) are employed to determine the bubble size distributions, bubble trajectories, velocities and collisions and moreover the turbidity of the flotation medium. A unique, patented, electrical impedance spectroscopy technique, equipped with a multiplexer, is applied for the determination of the volumetric gas fraction distribution along the flotation column.

Flotation experimental set up equipped with optical and electrical diagnostics for gas phase characterization

Bubble size distributions of dispersed air bubbles and electrolytic bubbles involved in the gas phase