Mixer-Settlers :
A mixer-settler is a single-stage extraction device comprising of a mixture for contacting the two liquid phases to effect mass transfer and a settler for mechanical separation of the phases.
For extraction operations carried out in a batchwise fashion/manner, the mixer and settler may be the same unit (i.e., mixing and settling is carried out in the same piece of equipment).
Mixer Settlers consists of a vertical vessel incorporating a turbine or propeller agitator. It is provided with charging nozzles at the top and a discharge connection provided with a sight glass at the bottom. The feed solution to be extracted is taken into the agitated vessel, required amount of the solvent is added, and whole mass is agitated for a predetermined time. At the end of mixing cycle, agitation is stopped and settling is applied for the phase separation. Afterwards, the raffinate and extract phases are withdrawn from the bottom discharge connection into separate receivers.
Also Read: Rotameter Working Principal
Also Read: Venturi Meter Working Principal
(a) Mixer settler for batch operation
(Mechanically agitated vessel)
(b) Continuous single-stage
Mixer-settler
For continuous extraction operation, the mixer and settler are separate units. [Fig. (b)]. The mixer is a small baffled-agitated tank provided with inlet-outlet connections and the settler is often a continuous gravity decanter (simple or settler with coalescer for emulsifying liquids).
In this extractor, the two phases are continuously in contact with each other in the mixer under thorough agitation before flowing to the settler for the phase separation.
One phase is usually dispersed into the other in the form of small droplets in order to produce large interfacial areas resulting in faster extraction.
When several contacts are required, a battery/train of mixer settlers operated in a current fashion is used. Generally the liquids are pumped from one stage to the next, but if sufficient headroom is available, gravity flow is arranged.
Mixer settler battery for counter current extraction
A feed solution is introduced in the first mixer and a fresh solvent is fed to the last mixer so that the raffinate from each settler becomes the feed to the next settler, and the extract from the last settler acts as the solvent to the second last mixer and so on. Final extract leaves the first settler, while the final raffinate is taken out from the last settler.
Gravity settlers (a) simple and (b) with coalescer
In case of liquids which emulsify easily and having nearly the same density it is necessary to pass a mixer discharge through a pad of glass fibre to coalesce the droplets of the dispersed phase before gravity settling is used.
Since pumps and piping systems are involved in the installation of a train of mixer settlers, initial investment on equipment is high.
Advantages of mixer-settlers :
(i) High stage efficiency, (ii) good flexibility (iii) capacity to handle liquids of high viscosity and (iv) high capacity.
These units are used in the petrochemical, fertilisers and metallurgical industries.