POWDER BLENDING, BLEND UNIFORMITY, AND SEGREGATION
Segregation results
when particles separate due to differences in their size, shape,
or density. Blending, on the other hand, is the opposite of
segregation. The process of blending occurs when a collection
of particles is homogenized or multiple ingredients are mixed
to obtain a uniform product. Many industrial processes rely
upon blending of powder or granular ingredients; a few examples
include:
- Pharmaceutical direct compression formulation
- Plastic compounding
- Dry food blending
Equipment, such as ribbon blenders, tumblers, and tube blenders are well-used in industry, and in many cases, blend uniformity is regularly achieved.
Unfortunately, segregation can readily occur during blender discharge. Additionally, the phenomenon of segregation (also called de-mixing) can result upon routine handling of a blended product or even single component bulk material. Segregation is a common problem that can be observed everyday in nature. For example, you will notice the large rocks concentrate at a rock pile's periphery, while the finer rocks are located under the pile's fill point. With industrial processes involving blend handling, segregation can occur upon material transfer, and negative effects resulting can include:
- Incorrect particle size distribution
- Decreased flowability
- Reduced performance
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- Increased dusting
- Change in reactivity
- Change in color, taste, or appearance
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SEGREGATION
The high cost of segregation is apparent in the pharmaceutical industry where a powder formulation not meeting uniformity requirements stipulated by the US Food & Drug Administration can lead to disposal of the batch that may be worth millions of dollars.
Segregation patterns due to different mechanisms
Granular mixtures can segregate by a variety of mechanisms. Three common segregation mechanisms include sifting, fluidization, and dusting. With sifting segregation, the fine particles concentrate under the point of impact in a pile, while the larger, more coarse particles roll off the pile and locate at the periphery yielding a side-to-side segregation.
Fluidization segregation results when fine particles in an aerated powder locate towards the top of a container, while the coarse particles deaerate quickly and settle to the bottom of a container. This phenomenon results in a top-to-bottom segregation, and is very common with fine powder blends (average particle size 100 US mesh or 150 µm) handled in the pharmaceutical and foods industries.
Dusting segregation is also common with fine powder mixtures. The fine, dusty component of the blend concentrates in particular locations within a silo or container, such as at the walls. In many cases the dust does not discharge from the vessel as does the powder, and consequently, chemical variability can result.
The sifting and fluidization segregation tendencies can be estimated via bench-scale tests described in ASTM protocols D-6940 and D-6941. Equipment is available from Jenike & Johanson for measuring these characteristics, and samples collected from these testers can be analyzed for particle size distribution by Jenike & Johanson or returned to your laboratory for chemical or proprietary physical analysis.
BLENDING
Blending or mixing of powders can also present challenges.
Some materials require gentle tumble blending in a controlled
batch mode while others require high shear to continuously blend highly cohesive and tough to mix ingredients.
To properly evaluate blend performance, sampling is required. Sampling can be performed via point location collection (such as with a "thief") or via multiple samples gathered during discharge of a blender. Jenike & Johanson engineers have the technical capabilities to diagnose the intricate differences between a blending, sampling, and segregation problem. We have the capability in our laboratories to run scale-blending models with batch and continuous blenders. Correct sampling is also part of the study protocol to ensure blend homogeneity is achieved and to help ascertain what factors (such as blender speed, percent fill, number of rotations) affect the blending process.
Jenike & Johanson is prepared to assist you with solving or preventing segregation and blending problems. Our team of experienced consulting engineers and laboratory technicians have the knowledge, necessary skills, and state-of-the-art tools available for addressing your unique applications.
Whether your product must meet required performance based on chemistry, particle size, reactivity, color, or even taste, Jenike & Johanson can apply proven, practical technology to ensure success with your powder formulation and handling system. Act now to prevent costly segregation and/or blending mistakes that may occur later on.
