Liquid-Liquid Separation (Solvent Extraction)

Mixer-Settler Settler Area and Mixer Volume Calculator

A mixer-settler is sized by two independent numbers: the settler area, which has to allow the dispersion to separate at your flows, and the mixer volume, which has to keep the phases in contact long enough for mass transfer. This calculator gives you both, plus the plan dimensions for a stated length-to-width ratio and a first estimate of the organic inventory sitting in the settlers.

Inputs

Aqueous flowm³/h

Aqueous flow through the circuit.

O/A ratio

Organic-to-aqueous volumetric flow ratio. Total flow = aqueous × (1 + O/A).

Settler specific throughputm³/m²·h

Design settling flux. For solvent extraction settlers this normally sits between 2 and 5 m³/m²·h depending on how fast the dispersion disengages — confirm it with a settling test on your own system.

Mixer residence timemin

Mixing (contact) time per stage. Typically 1 to 2 minutes for mixer-settlers; longer than the optimum buys no extra mass transfer but does make finer droplets.

Settler length : width

Plan aspect ratio. Longer settlers give the dispersion more distance to coalesce; short, wide settlers risk aqueous carry-over.

Dispersion band depthmm

Working dispersion (crud) band you want to allow for in the settler. Used only for the organic inventory estimate.

Organic fraction of total flow%

Share of the total flow that is organic — for a 1:1 O/A ratio this is 50%.

How to use it

Runs entirely in your browser — nothing you enter is sent anywhere. Defaults are typical lab/pilot values so you can see the arithmetic move; replace them with your own numbers before you rely on the result.

Results

Total flow —m³/h

Settler area (per stage) —m²

Settler width —m

Settler length —m

Mixer volume (per stage) —m³

Mixer volume incl. 20% freeboard —m³

Settler liquid volume (band only) —m³

Organic inventory in the band —L

The results appear here and update as you type. Each result is followed by the conditions under which it holds.

Read the assumptions before you use a number. A sizing figure without its conditions is not a result — it is a guess with a decimal point. The assumptions listed below are part of this calculator's output, not small print.

How to use it

Get a settling test first
Shake your actual organic and aqueous together, stop agitation, and time the separation. The specific throughput you enter must come from that test, not from a generic table — phase disengagement behaviour changes with the extractant, the diluent and anything surface-active in the feed.
Set the O/A ratio from the chemistry
Work out the ratio from the metal mass balance and the organic's loading capacity before you size anything.
Size the settler from total flow
Total flow is aqueous plus organic. Under-sizing the settler is the classic cause of a circuit that works at bench scale and floods at pilot scale.
Size the mixer from residence time
Mixer volume follows from total flow and the mixing time you want. Check that the resulting mixing energy density is enough to disperse the phases but not so high that it produces a stable emulsion.
Allow for the dispersion band
The band is where crud accumulates and where the interface moves when flows vary. Give it depth and give the settler a skimming point.
Check the organic inventory
Every litre of dispersion holds organic that you have paid for. Reducing band volume cuts inventory but also cuts your tolerance to flow upsets — that trade-off is a design decision, not an accident.

Formulas used

Q_total = Q_aqueous × (1 + O/A)
Settler area A = Q_total / specific throughput
W = sqrt(A / (L:W))            L = W × (L:W)
V_mixer = Q_total × (τ / 60)
V_band = A × band depth
Organic in band = V_band × organic fraction

Assumptions & applicability

Steady-state design flows. No allowance is made here for flow surges, start-up, or the temporary loss of circulation that a blocked line causes.
One settler per stage. Recycles and any internal aqueous recycle are not included in the total flow — add them before entering the figures.
Specific throughput is an input you must supply from your own settling test. It is the single most influential and least certain number in this calculation.
Settler area is based on plan area only; the effect of internals, picket fences, baffles and weirs on the effective settling area is not modelled.
Mixer volume includes no freeboard in the main figure; a separate line adds 20%. It also ignores the effect of impeller type on mixing intensity.
Organic inventory counts the band only. Organic held in the settlers' continuous layer, in the mixers, in pipework and in the crud layer is additional.

FAQ

What specific throughput should I use for a mixer-settler?

For solvent extraction settlers the usual design band is 2 to 5 m³/m²·h, with slower-settling systems at the low end. Treat that as a starting range and not a specification: the correct value comes from a settling test on your own organic and aqueous, because interfacial tension, temperature and any surface-active contamination all change how fast the dispersion separates.

Why does my settler flood even though the area looks adequate?

The most common cause is an effective area much smaller than the plan area — short-circuiting path, a badly placed feed distributor, or a weir arrangement that is not pulling evenly. Crud accumulation at the interface and increased organic viscosity at low temperature are the other frequent causes.

Is a longer mixer residence time always better?

No. Beyond the point where the phases reach equilibrium, extra mixing time buys no additional mass transfer but does break droplets into smaller ones, which slows disengagement and raises entrainment. Find the mixing time at which the outlet concentration stops improving, then allow a modest margin.