STAGE-WISE EQUIPMENT FOR EXTRACTION

As discussed in Volume 1, Chapter 7, the two key parameters determining power are the Reynolds number for the agitator and the power number. The Reynolds number should exceed 104 for optimum agitation. This gives a power number of about 6 for a fully baffled tank. It is important to note that the power number and the tip speed cannot both be kept constant in scale-up. 

The settler. In this unit, gravitational settling frequently occurs and, in addition, coalescence of droplets must take place. Baffles are fitted at the inlet in order to aid distribution. The rates of sedimentation and coalescence increase with drop size, and therefore excessive agitation resulting in the formation of very small drops should be avoided. The height of the dispersion band ZB is influenced by the throughput since a minimum residence time is required for coalescence to occur. This height Zb is related to the dispersed and continuous phase superficial velocities, //,/ and uc by  

Pilot tests may be necessary to achieve satisfactory design, although the sizing of the settler is a difficult problem when the throughputs are large. 

Recent work has emphasised the need to consider the combined mixer-settler operation, particularly in metal extraction systems where the throughput may be very large. Thus 

In the case of spray towers it has been shown by Thornton 10 that ur is well represented by Hod — j) where u0 is the velocity of a single droplet relative to the continuous phase, and is termed the droplet characteristic velocity. The term (1 — j) is a correction to m0 which takes into account the way in which the characteristic velocity is modified when there is a finite population of droplets present, as opposed to a single droplet. It must be seen therefore that for very dilute dispersions, that is as j - 0, w0(l — j) o- On the other hand, as the fractional hold-up increases, the relative velocity of the dispersed phase decreases due to interactions between the droplets. Substituting for ur, equation 13.32 may be written as  

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