Preaeration of Feed

optimal aeration condition for efficient particle collection and a suitable hydrodynamic environment for effective transfer of bubble/particle aggregates from the remaining pulp. Unfortunately, conflicting hy-drodynamic environments are usually required for these two sub-processes. It is often difficult - if not impossible - to evaluate theoretically the relative contributions of individual collection mechanisms in a particular flotation device. The limited understanding of the aeration mechanisms in flotation processes is partly responsible for the development of more than 200 flotation cell designs over the years. Many of these designs are not subtle variations in basic hardware, but variations in design principles. Therefore, knowing where and how collection occurs and which aeration method is suitable for a particular application is an important step in a more scientific approach to flotation cell design.

Aeration methods used in flotation can be conveniently categorized as air dispersion and air dissolution. In the air dispersion approach, a stream of air is dispersed into slurry to achieve suitable sizes and population of bubbles. This is accomplished by shearing the air stream into bubbles under mechanical agitation as in mechanical flotation machines, or using in-line static mixers as in Microcels and packing materials as in packed columns. Air can also be dispersed through porous spargers, as used in pneumatic flotation machines or conventional Canadian flotation columns.

With the air dissolution method, on the other hand, the air is dissolved under a pressure of 3-5 atm into slurry for subsequent gas nucleation (or gas precipitation) and cavitation. Bubble formation is then achieved by either releasing gas-supersaturated slurry to atmospheric pressure as in dissolved air flotation, or decreasing the pressure of slurry by aspiration as in vacuum flotation.

Dispersed air flotation is widely used in minerals processing with relatively coarse particles (larger than 20 |im) and high slurry densities (greater than 30% solids). Other areas of applications include solid cleaning, de-inking from recycled paper and bitumen recovery from oil sands. Dissolved air flotation is suitable for municipal water and industrial effluent treatment, due to its capability of generating relatively fine bubbles of less than 100 |im required for recovering particles finer than 10 |im at a slurry density of less than 0.5% solids.

An emerging trend is to integrate the useful features of dissolved air flotation into dispersed air flotation. The combination of the two bubble-generating mechanisms has led to a new flotation cell design. Traditionally, slurry aeration and flotation separation are performed in the same vessel. Feed aeration followed

Figure 1 Schematic illustration of the concept of a flotation system consisting of a reactor and a separator.

by flotation separation in a separation vessel (a reactor-separator design), has been evolved with demonstrated higher flotation kinetics. The concept of a flotation system consisting of a reactor and a separator is illustrated in Figure 1. The reactor is a vigorous bubble/particle contacting device where particle collection takes place with bubbles formed by both air dispersion and nucleation/cavitation mechanisms. The separator is a quiescent bubble/pulp separation device where the hydrodynamics favour the separation of bubble/particle aggregates from the pulp with essentially no or little turbulence.

With continuing improved understanding of particle/bubble collection mechanisms and the role of aeration in flotation, it is anticipated that pre-aeration of feed will become an important component in modern flotation circuits as a means of increasing flotation kinetics and improving selectivity of fine particles. This article focuses on the fundamentals and recent developments in pre-aeration of feed used in mineral flotation.

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