Impeller choice for solids suspension: which impeller is preferred when strong vertical currents are needed to keep heavy particles suspended?

Introduction / Context:
Solid–liquid mixing often requires reliable suspension of dense or large particles to prevent settling and to ensure uniform contact between reactants. The hydrodynamic pattern created by the impeller determines whether solids lift off the bottom and remain in circulation.

Given Data / Assumptions:

• Particles are “heavily” settling and need vertical lift.
• Tank is baffled and mechanically agitated.
• Objective is strong axial circulation.

Concept / Approach:
Axial-flow impellers such as marine propellers generate pronounced top-to-bottom circulation loops that sweep the tank floor, lifting solids into suspension. Radial-flow turbines discharge sideways, which is less effective for vertical lifting unless combined with multiple stages and careful placement. Paddles produce weak flow and are better for gentle blending at low viscosities. Helical ribbons suit very viscous fluids, not typical solids-laden, low-viscosity broths seeking strong vertical currents.

Step-by-Step Solution:
1) Identify the needed flow pattern: vertical axial currents to scour the bottom.2) Match impeller: marine propeller (axial) is designed for strong axial throughput.3) Conclude propeller is the preferred choice for suspending heavy particles.

Verification / Alternative check:
Correlations for just-suspended speed (Njs) show lower Njs for axial impellers than for radial turbines at comparable conditions, confirming better suspension efficiency.

Why Other Options Are Wrong:
Paddle mixing is weak for suspension; radial turbines emphasize radial jets, not vertical lift; helical ribbons target laminar, viscous mixing and are not optimal for solids suspension in low-viscosity systems; “None” is incorrect because propellers fit the need.

Common Pitfalls:
Confusing gas–liquid dispersion needs with solids suspension; axial flow is especially beneficial for keeping particles off the bottom.

Final Answer:
Propeller