Gas Dispersion

One of the most important hydrodynamic conditions in a mechanical flotation cell is dispersion of gas into fine bubbles. The bubble generation mechanism in

Figure 3 Schematic diagram of formation of bubbles in mechanical cells (after Grainger Allen, 1970; courtesy of Transactions of the Institute of Metallurgy, UK).

a mechanical cell is a two-stage process. Firstly, air cavities are formed at the trailing edge of the impeller blades, which is the low pressure region. Thereafter, bubbles form by shedding of vortices from the tail of the cavity, as shown in Figure 3.

The dispersion of air into bubbles can be characterized by three properties: bubble size, gas hold-up and superficial gas velocity. Mean bubble size in industrial mechanical cells varies, in general, from 0.5 to 2 mm; gas hold-up varies from 5 to 15% and superficial gas velocity varies from 0.6 to 1.5 cm s_1, depending on cell operating conditions (impeller speeds and air rates) and the cell duty in plant operation - roughers, scavenging, cleaners, etc. Recent studies have shown that bubble size, gas hold-up and superficial gas velocity cannot describe the gas dispersion in a mechanical cell adequately when taken individually; but when taken together the gas dispersion properties determine the bubble surface area flux Sb in the cell, which has been shown to characterize gas dispersion very well. Typical Sb values in industrial cells vary from 30 to 60 s_1. The concept of Sb has been found to be useful in metallurgical scale-up and cell optimization, design and selection.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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