Air Velocity in the Cell

The superficial gas velocity is the upward superficial velocity of air in a flotation cell, calculated by dividing the downcomer air rate (cm3 s-1) by the cross-sectional area (cm2) of the riser part of the cell. The cell is normally circular or rectangular in section, and the appropriate cross-sectional area is simply the area normal to the direction of the flow of the froth, excluding the area occupied by the downcomer(s). The superficial velocity Jg is conveniently expressed in units of cms-1 because values typically range from 0.5 to 4 cm s_1 in practice.

The recovery and concentrate carrying rate (g min- cm-2) tend to increase with increasing Jg. As in conventional columns, there is a limiting upward flux of bubble surface through the pulp above which froth flooding occurs, resulting in the loss of froth-pulp interface, a very wet froth and total loss of selectivity. There is consequently a maximum air rate Jgmax defined by this limiting flux and bubble size. In flooding, the entire cell fills with froth as the only stable phase, and there is no pulp phase.

The operating Jg used in the sizing of the Jameson cell depends strongly on the application, and on the residual reagent concentrations from any upstream processes. Generally speaking, low values (Jg = 0.4-0.8 cm s-1) are employed in cleaning applications, and high values (Jg = 1.0-2.0 cm s-1) are employed in roughing or scavenging applications.

In cleaning operations, a high proportion of the feed reports to the concentrate, and the froth loading tends to be high. Consequently, the bubbles are well coated with particles, which tend to stabilize the froth by reducing the froth coalescence rate. The drainage rate of the interstitial liquid in the froth is retarded by the relatively high concentration of particles, which has the effect of increasing the apparent viscosity of the interstitial liquid. Accordingly, it is necessary to design for lower values of Jg to allow time for the gangue to drain from the froth to obtain the required high grade. In roughing applications, however, only a small fraction of the feed reports to the concentrate and the froths formed tend to be less stable as a consequence. Also, gangue entrainment is not such a serious problem, because it can be dealt with in the downstream cleaning circuit. As a consequence of the higher coalescence rate the froth bed is shallower than that of the cleaners and a lower froth residence time will give good drainage. It is therefore usual to design a Jameson cell for a roughing application with a higher Jg than in the cleaners.

In some circumstances, high residual concentration of reagents in the feed necessitates the use of low values of Jg to avoid froth flooding. Although frother concentration is of primary importance to bubble size and hence the advent of froth flooding, circumstances have arisen where collector and frother interaction has been observed. In such a case, the frother dose should be decreased if collector dose is increased, and vice versa. Too high a frother or collector concentration can lead to froth flooding while too low a dose can lead to loss of froth stability.

Particle size can also have an influence on the maximum Jg, due to its effect on froth stability through bubble-bridging. Small particles (less than 100 |im) are easily collected at low gas rates, while recovery of coarser particles may be assisted by higher rates.

A complex system of liquid and air recirculation patterns forms in the bottom of the cell. The cell design is based on downcomer flows and downcomer placement to optimize this system to produce best grade and recovery. There is no limit on cell volume, providing the net downwards velocity of pulp, Jl, is sufficiently low to avoid the entrainment of bubbles in the underflow. When the froth and disengagement zones have the same cross-sectional area, the two important velocities are the rate of rise of the bubbles in the pulp, and the rate of drainage of liquid in the froth. The former is usually greater than the latter, so that a column sized to give the correct Jg will also give the correct JL, and bubble entrainment in the downward flow will not be a problem.

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.

Get My Free Ebook

Post a comment