Settling systems

Brot Tiian movement becomes appreciable for particles under 3 microns and predominates when the particle size reaches 0.1 micron [13]. This motion usually has little effect in the average industrial process settling system except for the very fine fogs and dusts. However, this does not mean that problems are not present in special applications. 

The settling characteristics of hindered settling systems differ significantly from those of freely settling particles in several ways ... 

Treatment of a suspension with the necessary chemical will produce a ffster settling system with a more bulky and more permeable sediment, but the final liquid content of the shidge or filter cake may well be significantly hi er than if a nontreated suspension had been settled. The benefits therefore tend to be found in flie enhanced settling rate and increased permeabiUty, and sometimes the sediment is of such bulk as to prove troublesome and expensive in disposal Other solid-liquid separation operations such as dissolved air flotation and centrifiigation have different requirements that must be met by the pretreatment system. 

The behaviour of the settling system is characterised by the batch settling curve as described in Chapter 3 and design calculations are based on that data u g the Kynch [1951] theory. 

The table gives some values for A and R for a number of common settling systems. Most of the values for A are within the desired range, but those for R are usually too great which will indicate that some instability may be present. The effect of this can be countered by providing additional settling area as shown in the example below. 

Example 14-7 Limestone slurry (236 kg limestone per m of slurry) is tested in a single-batch settling system. The data from the test are as follows ... 

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