Modelling and Measurement of Bubble Formation and Growth in Electroflotation Processes

Shahjahan K A Sarkar,Geoffrey M Evans,Piotr M Machniewski

doi:10.2478/cpe-2013-0026

Abstract

Abstract Electroflotation is used in the water treatment industry for the recovery of suspended particles. In this study the bubble formation and release of hydrogen bubbles generated electrolytically from a platinum cathode was investigated. Previously, it was found that both the growth rate and detachment diameter increased with increasing wire diameter. Conversely, current density had little effect on the released bubble size. It was also found that the detached bubbles rapidly increased in volume as they rose through the liquid as a result of decreasing hydrostatic pressure and high levels of dissolved hydrogen gas in the surrounding liquid. The experimental system was computationally modelled using a Lagrangian-Eulerian Discrete Particle approach. It was revealed that desorption of gaseous solutes from the electrolyte solution, other than hydrogen, may have a significant impact on the diameter variation of the formed bubbles. The simulation confirmed that liquid circulation, either forced or induced by the rising bubble plume, influences both the hydrogen supersaturation (concentration) in the neighbourhood of the electrode and the size of the resulting bubbles.

Highlights

The recovery of fine particles is inversely related to bubble size and having bubble size smaller than that of conventional flotation device, and for these reasons electroflotation has the viability of being an efficient system for floating fine particles (Sarkar et al, 2010; Sarkar et al, 2011)
It can be observed that the dissolved hydrogen concentration field around the electrode wire was strongly asymmetric, which was the result of convection induced by the rising plume of bubbles and simultaneous mass transfer
Observed growth of bubble diameters after detachment from the electrode can be attributed to desorption of dissolved hydrogen and other gases from the liquid phase

Summary

Introduction

The recovery of fine particles is inversely related to bubble size and having bubble size smaller than that of conventional flotation device, and for these reasons electroflotation has the viability of being an efficient system for floating fine particles (Sarkar et al, 2010; Sarkar et al, 2011). It is very important to have a clear understanding of the factors that influence the size of bubbles produced by electrolysis of water while designing an electroflotation system. The uncertainty in influences of variables such as electrode curvature, surface preparation, and most importantly current density, on bubble size has made it difficult to effectively design efficient electroflotation systems for fine particle recovery. Recovery will be maximised if all of the electrolytically-produced gas results in the formation of gas bubbles. This may not always be the case since a portion of gases remains dissolved in solution (Müller et al, 1989; Sarkar et al, 2010; Vogt H., 1984a; Vogt H., 1984b)

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