An Introduction to the Impact of High-Shear Blending

In a number of industries, powders are combined with other powders, or binders, in high-shear blending operations, to form homogeneous mixtures for further downstream processing.

The wide scope of blender sizes, methodologies, and geometries means that performance can differ between machines, creating mixtures with different flow characteristics which perform differently in subsequent operations.

It is vital to understand the effect of any given unit operation on downstream performance, as incompatibility can result in a product which is out of specification and not fit for purpose. An understanding of how the powder and the process interact with each other is needed to make sure that downstream performance has not been influenced detrimentally by upstream processes. The process can be optimized to supply a product with desirable properties once this is understood. This supplies major commercial advantages in terms of reduced wastage and higher productivity.

The Influence of Processing Powders

For the production of refractory bricks, three blends of Magnesium Oxide (MgO) and Graphite, together with a resin binder, were utilized. The blends were prepared using different mixers (Mixer 1, Mixer 2 and Mixer 3) but were from the same feedstock, and differences in the quality of the final product were noted depending on the mixture employed.

Mixture 1 exhibited optimum performance, Mixture 2 represented intermediate performance, and Mixture 3 showed the worst behavior. The three mixtures were assessed by employing an FT4 Powder Rheometer® to look at the dynamic flow, bulk and shear properties.

FT4 Powder Rheometer®

FT4 Powder Rheometer®

Image Credit: Freeman Technology

The FT4 Powder Rheometer is a universal powder tester which supplies reliable, automated, and comprehensive measurement of bulk material characteristics. To increase processing efficiency and aid quality control, this information can be correlated with process experience.

It specializes in the quantification of dynamic flow properties, the FT4 also incorporates a shear cell, and the ability to measure bulk properties such as compressibility, density, and permeability, allowing comprehensive characterization of a powder in a context which is process relevant.

Dynamic testing uses a unique measurement method to establish a powder’s resistance to flow. A specially shaped blade traverses along a prescribed path through a precise volume of the powder. As it moves axially and rotationally, the force and torque acting on the blade are combined to supply a value for flow energy.

Results of the Study

Dynamic Testing: Basic Flowability Energy (BFE)

Mixtures 2 and 3 produced BFE values which were comparable, but the BFE of the Mixture 1 was notably higher. High BFE shows a better resistance to flow, which is likely due to a more uniform particle packing structure in this case.

Dynamic Testing: Basic Flowability Energy (BFE)

Image Credit: Freeman Technology

Bulk Testing: Permeability

A clear trend was seen which correlated with the reported process performance. Mixture 1 produced the lowest Pressure Drop across the powder bed (highest permeability), and Mixture 3 produced the highest. Low Pressure Drop (high permeability) suggests that the powder can transmit air more easily, which promotes uniform filling and aids gravitational flow.

Bulk Testing: Permeability

Image Credit: Freeman Technology

Shear Cell Testing

Small differences were seen in the shear stress values, but the trend does not reflect the observed performance. In addition, the differentiation between Mixtures 1 and 3 is minimal and does not reflect the significant differences in process performance.

This lack of correlation with the process behavior shows how Shear Cell testing, which assesses how a powder transitions from a static to dynamic state after consolidation, may not be relevant to the more dynamic, low-stress conditions present in conveying, drying, and filling operations.

Shear Cell Testing

Image Credit: Freeman Technology


The FT4 Powder Rheometer has identified repeatable and clear differences between three blends of powder which rationalize the observed differences in process performance.

Permeability was identified to be highly differentiating, indicating that the release of entrained air during drying and filling operations affected final product quality significantly. BFE also reinforced that in the manufacturing process, more uniformly packed blends perform most effectively.

The Shear Cell test supplied limited information, indicating that shear properties did not have much effect on the performance overall. Therefore, the dynamic flow and bulk parameters may be utilized to predict downstream process performance, allowing mixing operations to be optimized to target desirable properties of the resulting blend.

Powder flowability is more about the ability of powder to flow in a desired manner in a specific set of processes, it is not an inherent material property. The powder and the process must be well-matched to achieve successful processing, and it is not uncommon for the same powder to perform poorly in one process but well in another.

This means that multiple characterization techniques are needed, the results from which can be correlated with process ranking to find the most influential parameters on process behavior.

The FT4’s multivariate approach simulates a range of unit operations instead of relying on single number characterization to illustrate behavior over all processes, enabling the direct investigation of a powder’s response to various process and environmental conditions.

This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.

For more information on this source, please visit Freeman Technology.


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