Solid Suspension Mixing: What It Takes to Keep Particles Moving

Solids suspension is one of the most common—and most easily misunderstood—industrial mixing duties. It appears in mineral processing, water and wastewater treatment, chemical production, food processing and many other applications where solid particles must remain moving in a liquid.

A successful design is not simply a matter of selecting a larger motor or running an impeller faster. The mixer must deliver the right amount of power and turbulence to the correct areas of the vessel, especially near the tank floor where solids are most likely to settle.

Better mixing starts with the process objective—not a catalogue number. Before selecting a mixer, the required degree of suspension, slurry properties, vessel geometry and operating conditions all need to be clearly defined.

What Does “Solid Suspension” Actually Mean?

The purpose of a solids-suspension mixer is to create enough liquid motion and turbulence to lift particles from the tank floor and distribute them through the vessel. However, not every process needs the same level of distribution.

Two common design targets are:

·        Off-bottom suspension: Particles remain in motion and do not rest on the tank floor for more than a brief period. This is often appropriate for storage, transfer or processes where preventing accumulation is the main goal.

·        Uniform suspension: Solids are distributed as evenly as practical throughout the liquid volume. This may be required when consistent concentration, reaction rate, heat transfer or product quality is important.

Uniform suspension can require substantially more power than simply keeping solids off the bottom. Specifying uniform suspension when the process only needs off-bottom movement can therefore increase equipment size, energy use and cost without improving the result that matters.

Free-Settling and Hindered-Settling Slurries

The way particles settle is a major factor in mixer design.

In a free-settling slurry, particles settle with relatively little interaction between them. This is more common at lower solid concentrations and with a narrower particle-size distribution.

In a hindered-settling slurry, particles interact as they fall, reducing and complicating their settling behaviour. These slurries often contain a higher concentration of solids, a wider range of particle sizes or a significant amount of fine material. At high concentrations, the slurry may also become viscous enough that the application needs to be treated as a viscous-blending problem rather than a conventional suspension duty.

This is why weight percentage alone cannot define the application. The full particle-size distribution, liquid properties and slurry rheology must also be understood.

Why Axial-Flow Impellers Are Commonly Used

Solid suspension depends heavily on conditions near the tank bottom. Axial-flow impellers are commonly selected because they direct liquid downward and promote circulation across the floor and back through the vessel.

By comparison, a radial-flow impeller sends much of its discharge toward the tank wall. It may consume power without delivering enough useful energy to the areas where particles collect.

Impeller diameter is equally important. For suspension duty, bigger is not automatically better. An impeller that is too large may redirect flow toward the wall before it effectively sweeps the tank floor. One that is too small may create strong motion directly below the impeller while leaving solids in the outer corners. The best design balances impeller diameter, speed, power and off-bottom clearance so energy is distributed effectively across the vessel floor.

The Process Information Needed for Proper Mixer Selection

Accurate sizing begins with accurate application data. The most important information includes:

·        Tank shape, diameter, height and working volume

·        Maximum and minimum liquid levels

·        Tank-bottom geometry

·        Existing or proposed baffle arrangement

·        Mixer mounting position

·        Liquid specific gravity and viscosity

·        Solid specific gravity—not bulk density

·        Solids concentration and its operating range

·        Particle-size distribution, including the design particle size

·        Required suspension level: off-bottom or uniform

·        Operating cycle, transfer method and expected shutdown conditions

Particle size affects settling velocity, while the difference between solid and liquid density affects the force driving the particle downward. Viscosity resists settling, but it can also change the entire mixing regime. For non-Newtonian slurries, viscosity should be considered at shear rates relevant to the actual mixer rather than relying only on a single high-shear laboratory reading.

Tank Geometry and Baffles Matter

Even a correctly selected impeller can underperform in a poorly configured vessel.

For a typical cylindrical tank, a vertical, centre-mounted mixer with properly designed wall baffles provides a strong foundation for solids suspension. Baffles limit swirling and help convert impeller discharge into productive top-to-bottom circulation.

Tank-bottom geometry also influences performance. A dished bottom generally helps guide particles back into circulation, while a flat bottom may allow a small solids fillet to develop in the corners. Steep cone-bottom tanks can be more challenging because solids may collect in the lower cone and the vessel walls can interfere with the main impeller’s discharge pattern.

Rectangular tanks require additional care. As the length-to-width ratio increases, a single mixer may no longer circulate the complete vessel effectively. Mixer quantity and placement should therefore be evaluated with the actual tank geometry—not selected from volume alone.

Impeller Position and Low-Liquid-Level Operation

The distance between the impeller and tank bottom can significantly affect required power. If the impeller is too high, less useful energy reaches the floor and more power may be needed to achieve the same suspension result.

Changing liquid level must also be considered. An axial-flow impeller needs adequate liquid coverage to pump properly. If the operating level falls too close to the impeller, pumping can collapse and suspended solids may begin to settle.

A small lower impeller, sometimes called a kicker, may help reduce stagnation or keep a bottom outlet clear. However, it should not automatically be treated as a substitute for the main suspension impeller. Its purpose and power must be included in the overall system design.

Do Not Overlook the Outlet

Keeping solids moving inside the vessel is only part of the job. They must also leave the tank reliably.

A low outlet is often preferred because heavier particles may never reach an overflow connection. Outlet location, suction arrangement and local flow around baffles can all affect plugging risk and solids carryover. When a riser pipe is used, its inlet elevation and internal velocity must be selected around the particles that need to be transferred—not simply the average slurry flow rate.

The mixer, tank and transfer system should therefore be treated as one process rather than separate pieces of equipment.

What Happens After a Shutdown?

If power is lost or a mixer is stopped, solids may settle around the lower impeller. Depending on the material, the settled bed may remain loose or compact into a dense layer.

This creates two separate questions:

1.        Can the mixer safely restart without excessive torque or mechanical damage?

2.        Can the process actually re-suspend the settled material?

A mechanically strengthened mixer can help manage restart torque, but it does not guarantee that a compacted slurry will return to full suspension. Where packing is possible, the design may need to consider alternate power, controlled dilution near the vessel bottom, operating procedures or laboratory testing with a representative sample.

Start With the Application

Reliable solids suspension depends on matching the mixer to the real process objective. Particle behaviour, suspension level, vessel geometry, impeller type, power, clearance, baffles, outlets and restart conditions all work together.

T.D. Rooke helps customers evaluate these details and select industrial mixing equipment around the application—not guesswork. With access to proven SPX FLOW Lightnin mixing technology and local technical support, we can help define the right approach for new installations, upgrades and troublesome existing systems.

Planning a solids-suspension application? Send us your tank dimensions, liquid properties, solids concentration, particle information, operating levels and required mixing result. Let’s define the right equipment together.