Keeping Your Solids in Suspension

Optimized Solids Suspension Applications

Achieving a uniform dispersion of solids in a product
is critical to your product’s quality.

In processes that mix solids in fluid, particulates that cannot dissolve in liquids will either sink to the bottom of the tank and collect in the corners, or stick together and agglomerate. This compromises product uniformity (product quality), and in extreme cases, can cause a system to malfunction or fail.

The main objectives of solid and liquid solutions are:

1. Avoiding solids accumulation in a stirred tank
2. Maximizing the contact area between solids in a liquid
3. Ensuring the solid particles are uniformly distributed throughout the tank

The goal is to bring the mixture into specification and keep it there. This means keeping the solids evenly dispersed throughout the product, without damage.

Good Mixing Begins with the Impeller

All impellers are designed to “pump” fluid and create turbulence within a mixing vessel. Both of these effects are essential to mixing. Pumping and turbulence produce fluid velocity and fluid shear respectively.

Key Factors

Two key factors to consider during mixer configuration are settling rate and suspension quality.

Settling Rate

When sizing a mixer for applications that require solids suspension, we need to calculate the settling rate of the particles. Two components that affect the settling rate are:

1. Particle size
2. Solid concentration

The solid settling rate is crucial as it affects the amount of pumping required for the appropriate level of suspension to be achieved, therefore impacting the configuration of the mixer.

Suspension Quality

The level of suspension, or suspension quality, required in an application is generally determined by how uniform the product needs to be. Suspension quality is achieved using the degree of pumping necessary. The higher the pumping the higher the mixing intensity, which is usually categorized as mild, medium, and vigorous. As the pumping level increases so does the level of uniformity.

Let’s take a closer look at the three levels of suspension quality that can be achieved in mixing:

	Off Bottom: When the degree of pumping achieves 2x the settling rate, it raises the solids off the bottom and prevents a settled bed of particulates from collecting at the bottom of the tank. This is ideal when pumping the slurry out is required, but the consistency is not important.
	Moderate Uniformity: When the degree of pumping achieves 3x the settling rate, it produces moderate product uniformity that is suitable for most applications.
	Top Draw-Off: When the degree of pumping achieves 6x the settling rate, it engages the entire tank and allows for consistent top draw-off. This is ideal in applications where the product is drawn from the top of the tank or in overflow applications.

Each level of suspension quality depends on complex interactions of impeller generated flow, turbulence, and solid loading. Although there are multiple factors to consider in solid/liquid systems, mixer configuration for this process always begins with impeller selection.

Mixer Configuration

Correct impeller design is crucial to reaching and maintaining the quality of an application. Impellers that create an axial flow deliver a superficial velocity and an upward pumping action necessary to ensure that solids remain in suspension.The rate of the flow in a tank needs to be greater than the solid settling rate. And the level of superficial velocity achieved will determine the suspension quality created in the tank. For more on impeller types and their flow patterns, see our article on The Basic Principles of Mixing and Impellers If you’re interested in learning more about mixing solutions for solid-liquid applications, post a comment or email us at info@dynamixinc.com.