In solid–liquid extraction, the agitator isn’t “a motor that spins.” It controls:
whether solids stay suspended (no dead zones)
mass transfer rate (cycle time)
temperature uniformity (no hot spots)
fines generation risk (over-shear)
downstream filtration behavior (settling vs carryover)
Many extraction projects underperform because the tank is sized correctly—but the agitator is not.
If you’re sizing the tank first, start here: working volume and headspace sizing
1) Start with the Real Requirement (Not “RPM”)
Before selecting an impeller, define the mixing objective:
Common solid–liquid extraction objectives
Off-bottom suspension: keep solids from settling
Complete suspension: uniform solids concentration throughout tank
Rapid wetting: eliminate floating solids and dry pockets
Heat transfer mixing: prevent hot spots during heating
Gentle mixing: avoid breaking fragile solids into fines
Your objective determines impeller type and speed range.
2) The Two Variables That Drive Impeller Choice
2.1 Solids behavior
Key inputs:
solids loading (% or g/L)
particle size window (coarse vs fines)
density (settling speed)
swelling/gelling tendency
tendency to float (hydrophobic powders)
2.2 Apparent viscosity (and how it changes)
Extraction viscosity is often non-linear:
low at start, then rises as gums/resins dissolve
rises sharply when S/L ratio is low
rises when temperature drops
If viscosity rises mid-batch, choose an agitator that can handle peak viscosity, not only starting viscosity.
Related process lever:
3) Impeller Selection Map (By Solids + Viscosity)
Below is a practical, buyer-friendly map. (Exact sizing requires geometry and test data, but this prevents the most common wrong choices.)
3.1 Low to moderate viscosity, solids need suspension (most botanical slurries)
Typical goal: off-bottom suspension + uniformity
Common impeller families (function-focused):
Axial-flow impellers (strong vertical circulation)
Hydrofoil-style impellers (efficient axial pumping)
Pitched-blade turbines (PBT) (versatile, stronger shear than hydrofoils)
Why they work: axial flow creates top-to-bottom circulation that keeps particles moving and improves mass transfer.
Typical buyer pitfall: using a small high-speed impeller that creates a vortex but does not lift settled solids.
3.2 Higher viscosity (resinous extracts, gum release)
Typical goal: move viscous bulk + prevent dead zones
Common impeller families:
Anchor / gate style (wall sweeping)
Helical ribbon (strong bulk movement for viscous fluids)
Combination systems (e.g., anchor + central axial) for wide viscosity ranges
Why they work: viscous systems need wall-wiping and bulk displacement rather than only axial circulation.
Common pitfall: using only an axial impeller in a high-viscosity paste → mixing stalls, hot spots appear, solids settle in corners.
3.3 Very fragile solids or fines-sensitive products
Typical goal: gentle wetting + avoid particle breakup
Approach:
lower tip speed
larger impeller diameter at lower RPM (more pumping, less shear)
avoid aggressive high-shear heads unless required
Why this matters: fines generation can destroy filtration throughput.
4) Baffles, Geometry, and “Why My Agitator Didn’t Work”
Even the right impeller fails if the geometry is wrong.
4.1 Baffles (or anti-vortex features)
In many tanks, baffles are needed to:
break vortexing
convert rotation into vertical circulation
improve suspension at lower RPM
4.2 Impeller diameter and placement
too small diameter → poor pumping
too high placement → solids settle below impeller zone
multi-impeller stages may be needed for tall tanks or wide viscosity ranges
4.3 Bottom shape and drain
Solids often accumulate around:
flat bottoms and corners
poorly designed drains
areas with low flow
If you plan high solids loading, bottom design and drainability matter.
5) Torque vs Power: What to Tell Suppliers (So they don’t under-design)
Buyers often specify “0–60 rpm” or “0–200 rpm,” but speed alone is meaningless without torque.
Practical RFQ guidance
Provide:
maximum viscosity (or “viscosity rises significantly”)
solids loading and particle size
whether complete suspension is required
temperature range (viscosity changes with temperature)
whether shear-sensitive
Ask suppliers to specify:
motor power and gearbox torque
impeller type, diameter, stages
seal type (single/double) if solvent is used
If you’re preparing a full line quote:
6) Agitation and Extraction Cycle Time (Why it shortens or lengthens batches)
Agitation improves cycle time when it:
eliminates dead zones
increases effective mass transfer area
stabilizes temperature
maintains consistent solvent contact
But over-agitation can:
generate fines (hurts filtration)
increase emulsions (hurts separation)
increase oxidation if air is entrained
If cycle time is your focus, see:
7) Troubleshooting (Fast diagnosis)
Symptom A: Solids settle at bottom even at high RPM
Likely causes:
impeller too small or too high
wrong impeller type (poor axial pumping)
no baffles (vortex without circulation)
Fix:larger diameter axial impeller, lower placement
add baffles or anti-vortex design
consider multi-stage
Symptom B: Mixing looks strong but extraction is inconsistent
Likely causes:
dead zones and short-circuiting
poor temperature uniformity
Fix:improve flow pattern (axial circulation)
add temperature sensors and adjust agitation strategy
Symptom C: Filtration suddenly became slow after improving mixing
Likely causes:
fines generation from shear
Fix:reduce tip speed
use larger diameter lower RPM
refine milling strategy and staged filtration
FAQ (with answers)
1. What is the best impeller for herbal extraction?
There is no single best. For many slurries, axial-flow/hydrofoil or pitched-blade designs work well. Resinous/high-viscosity systems often need anchor or helical ribbon styles.
2. Should I choose higher RPM to mix better?
Not necessarily. Higher RPM increases shear and can generate fines. Often a larger diameter impeller at lower RPM gives better pumping with less shear.
3. Do I need baffles in an extraction tank?
Often yes, especially for low-viscosity liquids where vortexing occurs. Baffles improve circulation and suspension efficiency.
4. Why does my agitator work at the start but fail later?
Viscosity often increases during extraction as gums/resins dissolve or solids load rises. Design for peak viscosity and consider combination agitators for wide viscosity range.
5. What should I send in an RFQ to get the right agitator?
Solids loading, particle size, viscosity range, temperature range, mixing objective (off-bottom vs complete suspension), and whether shear-sensitive.
Want a full extraction tank spec (mixing + heating + reflux interface)?
Want a full extraction tank spec (mixing + heating + reflux interface)?