Introduction

Powder segregation is one of the most common challenges encountered when handling granular materials. When a powder blend is subjected to vibrations, tapping, conveying, or other mechanical solicitations, its different components may separate according to their size, density, or shape.

This phenomenon can have significant consequences in many industries. In pharmaceutical manufacturing, segregation may lead to dosage variability. In chemical processes, local composition changes may affect safety or product quality. In powder processing, segregation can also modify flowability, packing behavior, and downstream process performance.

For these reasons, quantifying the level of segregation is essential for monitoring blend stability and optimizing formulations.

Why Measure Powder Segregation?

A homogeneous blend contains all its components uniformly distributed throughout the material. When segregation occurs:

• Particle concentration varies from one location to another.
• Blend homogeneity decreases.
• Product consistency is reduced.
• Processability may change.

The ability to quantify segregation helps users:

• Compare formulations.
• Evaluate process stability.
• Monitor blend quality.
• Assess the performance of anti-segregation strategies.

The GranuPack provides a practical approach by comparing the measured tapped density of a blend with the theoretical density expected for a fully segregated material.

The Principle Behind the Method

The protocol is based on a simple concept. When a binary blend becomes fully segregated, each component forms a separate monodisperse phase. Each phase exhibits its own characteristic tapped bulk density. Knowing the tapped density of each pure component makes it possible to calculate the theoretical tapped density of a fully segregated blend, denoted: ρ*

The actual tapped density of the blend, measured with the GranuPack after 500 taps, can then be compared with this theoretical value.

The closer the measured density is to ρ*, the more segregated the blend is likely to be.

Determining the Reference Densities

The first step consists of measuring the tapped bulk density of each pure component separately. These measurements are performed using standard GranuPack conditions:

• 500 taps
• 1 Hz tapping frequency
• 1 mm free-fall distance

The resulting tapped densities are:

• ρ₁(500)
• ρ₂(500)

If pure materials are unavailable, the blend can be sieved to isolate sufficient quantities of each particle population for independent measurements.

Figure 1: Picture of a fully segregated blend.

Calculating the Density of a Fully Segregated Blend

Once the properties of the individual components are known, the theoretical density of a fully segregated blend can be calculated.

If the volume fractions are known:

ρ* = f₁^ᵛρ₁ + f₂^ᵛρ₂

If the mass fractions are known:

ρ* = 1 / (f₁^ᵐ/ρ₁ + f₂^ᵐ/ρ₂)

where:

• fᵛ represents volume fractions.
• fᵐ represents mass fractions.
• ρ₁ and ρ₂ are the tapped densities of the individual species.

This theoretical density serves as a reference corresponding to complete segregation.

Measuring the Segregation Index

After defining the theoretical density, a GranuPack measurement is performed on the blend itself.

The tapped density of the blend after 500 taps is noted:

ρ(500)

The Segregation Index (SI) is then calculated as:

SI = 1 / (ρ − ρ*)

This metric provides a quantitative assessment of segregation.

As the blend becomes increasingly segregated, its measured density approaches the theoretical density of the fully segregated state, causing the Segregation Index to increase.

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