Settler design

The General Mills mixer—settler (117), shown in Figure 13b, is a pump—mix unit designed for hydrometaHurgical extraction. It has a baffled cylindrical mixer fitted in the base and a turbine that mixes and pumps the incoming Hquids. The dispersion leaves from the top of the mixer and flows into a shallow rectangular settler designed for minimum holdup. 

Besides looking at just the mixing, it is important at this time to also consider the settling time of the phases after mixing since this will impact on the settler design. Higher intensity of mixing may decrease the residence time for mass transfer, but at the same time create fine dispersions which are difficult to settle. 

In stagewise equipment, the design and scale-up is simple and is often determined from bench data. The stage efficiency is usually high, and the capacity is determined by the settler design necessary to achieve coalescence of the dispersed phases. For differential contactors, such as columns, the flow capacity is determined by the droplet size and the type of internals (see... 

Settler design (entry to dispersion band, flow rate design, baffling)... 

In one mixer-settler design, the mixed phases flow down a shallow trough placed over the settler, which gives them an opportunity to coalesce and separate before entering the settler. In this way, the capacity of the settler is markedly increased, with a concomitant reduction in the inventory of solvent required for a given duty. 

For a practical settler design, the height H of the dense dispersion, which is below the principal interface and has to find space in the settler, is evaluated by a simple estimation formula ... 

Hooper and Jacobs (in Schweitzer, 1979, 1.343-1.358). Numerical examples of settler design also are given in that work. For especially difficult separations or for space saving, centrifuges are applied. Liquid hydrocyclones individually have low efficiencies, but a number in series can attain 80-85% efficiency overall. Electrical coalescence is used commonly for separation of brine from crude oil the subject is treated by Waterman (Chem. Ens . Pros . 61(10), 51 1965). 

Godfrey and Slater10 suggest the following empirical relationships for mixer-settler design, For batch deta, the efficiency may be exprereed as the approach to equilibrium ... 

Floe doesn t settle out] floe formed is too loose/see settler design Section 5.8. [Floe forms hut breaks up] local turbulence > shear strength of floe. 

Use mass transfer analysis for extraction mixer-settler design... 

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