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Bulk solids segregation is a hidden but critical issue in industrial powder and bulk material handling that can lead to inconsistent product quality, flow blockages, process upsets and costly downtime. It occurs when particles separate by size, density or shape during handling and is driven by mechanisms such as sifting, fluidisation and dusting. While blending is often effective, segregation frequently develops in downstream process steps such as transfer, storage and discharge.

The impact of segregation can include inconsistent material flow, changes in bulk density, equipment blockages and variations in final product quality across industries such as food, pharmaceuticals, cement and chemicals. Identifying and controlling segregation requires a combination of in-process sampling and laboratory testing to understand root causes.

Once identified, segregation can be reduced through changes to material properties, process design or equipment configuration. Effective strategies include minimising transfer steps, optimising vessel flow patterns and designing equipment to reduce air interaction and promote uniform discharge.

Introduction

Kurt Naugler - Bulk solids segregation expert

Bulk solids segregation is an issue that routinely impacts almost every industry where solids are handled. Many solids processes will have steps where the material being handled has a wide distribution of particles, or is a blend, and the uniformity of the material is critical to downstream processes or the resulting product. Segregation causing a lack of material uniformity can lead to nonconforming product, quality concerns, and even process upsets due to flow stoppages, or chemical imbalances.

Bulk solids segregation is a major challenge across industries including food processing, pharmaceuticals, chemicals, cement and minerals processing. Even well-designed solids handling systems can experience process instability, inconsistent product quality and material flow problems when segregation mechanisms are not properly understood or controlled.

When evaluating a process, it is important to consider what aspect of material uniformity is important. Is it critical for a material to have consistent particle size, particle shape, or density? Or is flowability, chemical composition or even colour the most important aspect?

The principal concern for each situation may be different, but engineering decisions should focus on the impact to product and process performance. Minimising the effect of segregation on these key factors can be vital to the success of a solids handling process.

Segregation of a material is defined as the separation of particles into distinct zones by particle size, shape, density, or other physical characteristic (Maynard, 2012). It is often looked at as a competing phenomenon with the common processes of mixing and blending. Where mixing and blending are used to achieve a uniform distribution of multiple components, segregation can have the opposite effect.

These competing effects often lead to difficulty in troubleshooting process problems that are observed as caused by non-uniform materials. When such an observation is made, the first area to investigate is often the bulk mixer or blender.

Experience has shown that while an insufficiently blended material will have negative consequences, the blending step of most processes is done well and can be quickly validated. Many organisations have robust blending design knowledge and capability to validate sufficient uniformity, but the capability to troubleshoot and pinpoint root cause of a segregation problem is not as common. This is due to a wide range of both process steps that can cause segregation, and the number of different segregation mechanisms that can have a negative impact on the material being handled.

“Segregation causing a lack of material uniformity can lead to nonconforming product, quality concerns, and even process upsets due to flow stoppages, or chemical imbalances.”

How bulk solids segregation impacts industrial processes

As a new process is designed, or changes are made to an existing facility, material or blend properties are selected to serve as a design basis to ensure systems and equipment operate as intended. When segregation occurs, resulting material properties may differ greatly from the original design basis and cause unforeseen challenges with achieving design production rates, avoiding need for manual intervention, or meeting product quality specifications.

Common process problems caused by solids segregation

Bulk solids segregation is often not immediately visible, but its effects become apparent through a range of operational and product-related issues across industrial processes. As materials separate by particle size, density or shape during handling and transfer, the resulting loss of uniformity can disrupt both process stability and product consistency.

One of the most common consequences is process instability. When segregated material reaches different stages of production at irregular intervals, it can cause fluctuations in feed rates, mixing performance and downstream processing conditions. This makes it difficult to maintain steady-state operation and can increase reliance on manual adjustments.

Segregation also leads to inconsistent material flow. Variations in particle size distribution or composition can change how the material behaves in hoppers, chutes and conveyors, resulting in uneven discharge rates or unpredictable flow behaviour. This inconsistency can have a knock-on effect on overall process control.

In more severe cases, segregation can contribute to production downtime. Fine or coarse particle build-ups may create flow restrictions or blockages within equipment, requiring unplanned stoppages for cleaning or intervention. These interruptions reduce overall plant efficiency and increase operational costs.

Product quality variation is another key impact, particularly in processes where blend uniformity is critical. Segregation can cause inconsistent chemical composition, colour variation or changes in physical properties, leading to off-specification product and potential batch rejection.

Finally, equipment blockages can occur when segregated fines concentrate in specific areas of the system, increasing cohesion and friction. This can result in material build-up at transfer points, vessel outlets or chutes, ultimately restricting flow and in some cases causing complete system blockage.

Impact of solids segregation on flow behaviour, blockages and product quality

A common result of segregation is surges of material containing high concentration of fine or coarse particles reaching sensitive portions of the process train at varying times. Even if the material is a single component, and chemical composition does not change, fine and coarse fractions can have drastically different flow properties (Naugler, 2023).

If segregation causes a slug of fines to reach the outlet of a storage vessel, or a critical intersection of a transfer chute, higher cohesive strength and friction properties of the fines can lead to buildup and eventual blockages. Buildup alone can cause challenges with material processing, and blockages often result in complete process shutdowns and unplanned downtime.

In addition to the potential for complete process stoppages, fluctuations in the particle size of material being handled (due to segregation) can impact other bulk properties such as bulk density and permeability. Each of these will impact process control for mass flow rate uniformity and ultimately result in weight variations for downstream processes or even the final product.

When handling a mixture or blend of multiple components, segregation has a significant impact on bulk material composition, which is often a critical quality metric for both in process checks and analysis of each unit product.

Product failures have been attributed to segregation of blend components in industries ranging from pharmaceutical, nutraceutical, and food where recipes and resultant compositions are closely controlled by regulatory bodies, to industries such as cement, glass, roofing, etc. where variations lead to unacceptable fluctuations in product strength, clarity, longevity, or other quality metrics.

“Segregation of a material is defined as the separation of particles into distinct zones by particle size, shape, density, or other physical characteristic (Maynard, 2012).”

The most common bulk solids segregation mechanisms

Which mechanism a bulk material may segregate by is due to a variety of factors. First, it is a function of the material itself. The physical and chemical properties such as particle size distribution, particle shape, electrical charge, and cohesion have a direct impact on how a material will be impacted by segregation.

Next, the forces induced on the particles within the bulk material will dictate what mechanism of segregation is most prevalent. These forces could be the result of air flow, vibration, gravity, and impact of particles on equipment surfaces.

Finally, the fill and flow sequences of process vessels determines the opportunity for segregation to occur. The difference between continuous and batch processes, as well as how material flows into and out of vessels assists in determining the cause and mechanism of observed segregation.

While research has identified and documented numerous unique segregation mechanisms, they can be boiled down to or associated with the three most common: sifting, fluidisation, and dusting; see Figure 1.

Understanding the mechanism responsible for segregation is essential when troubleshooting solids handling systems. Different segregation behaviours require different engineering approaches to minimise their impact on production efficiency and product quality.

sifting fluidisation dusting — Figure 1 – The most common segregation mechanisms: Sifting, Fluidisation, and Dusting.

Sifting segregation in bulk solids handling, finer particles concentrate in the center of the pile while coarse particles avalanche to the outside or periphery. This creates a side-to-side form of segregation where the composition of material on the edges of a pile is different than that of the material in the middle, as shown in Figure 2.

Sifting Segration — Figure 2 – Sifting segregation from pile formation: stuffing mix

Fluidisation and dusting segregation mechanisms

Fluidisation and dusting segregation are related to how particles of different sizes, densities, or morphologies interact with air, or process gas, within a handling system. In fluidisation segregation, the finer, lighter particles raise to the top of a fluidised bed, while the heavier, coarser particles settle towards the bottom of the bed. Dusting segregation is characterised by fine and ultra-fine particles remaining entrained in air currents and settling towards the top and side of surge vessels.

It is important to note that depending on the material particle size differences, particle density, air currents, and processing steps, one or multiple segregation mechanisms can occur.

How to evaluate bulk solids segregation risk

Testing for segregation can be broken into two distinct approaches: in-process and laboratory testing. In-process testing requires the use of sampling at different points of the process to evaluate where and what level of segregation is occurring.

This type of testing also requires that a blending validation has been completed so the starting, or reference, level of blend uniformity is known. From there, sampling at different points of the process and careful evaluation of those samples will give the most accurate picture of segregation.

Accurate sampling is a science in and of itself, and not the focus of this discussion, but there are “golden rules” that should be adhered to where possible. First, sampling solids at rest should be avoided and samples should instead be taken from a moving stream of material. Second, the full moving stream should be sampled and obtaining material from only a portion of the moving stream should be avoided (Allen, 1990).

“The three most common mechanisms for segregation include sifting, fluidisation, and dusting.”

Following sampling, it is important to ensure that any division prior to analysis does not introduce bias. There are numerous approaches to sample division, with spin riffling being one common approach to minimise introduction of sample bias.

If a process is not yet in operation, is being scaled, or sufficient in-process sampling is not possible, laboratory testing is a valid approach to evaluating material segregation. It can also be coupled with in-process testing to assist in root cause analysis of existing segregation problems.

Standardised test procedures and equipment have been developed to evaluate the propensity for materials to segregate by the mechanisms described above. ASTM D6940 describes a test procedure for evaluating sifting segregation and ASTM D6941 focuses on fluidisation segregation. The test setup for these methods is shown in Figure 3.

Each of these methods applies forces to the material being tested in order to provide the opportunity for segregation to occur. Following application of these forces, the sample is divided to evaluate the extent of segregation. Results from testing can be correlated to an existing, or proposed, process to identify the risk of segregation and where the most impactful changes can be made (Pittenger, Purutyan, & Barnum, 2000).

Figure 3 – ASTM D6940, Sifting Segregation (left) and ASTM D6941, Fluidisation Segregation (right) test setups.

Engineering solutions for bulk solids segregation

Evaluating a segregation concern in your process should be approached like any other bulk solid handling problem. First, if the cause of segregation is not known, root cause analysis will be important in pinpointing the areas of the process or pieces of equipment that will have the most impact.

Root cause analysis should include a robust sampling plan in addition to evaluation of segregation potential through laboratory testing. Ability to couple results from in-process sampling with test results will be important in defining segregation mechanism and thus, the best approach for minimising the impact of segregation.

Once root cause has been identified, there are three general areas that can be adjusted to solve the segregation problem: the material, the process, and the equipment.

Material-based approaches to reducing segregation

Material:

While chemical composition of a material or blend being handled is often set and cannot be adjusted, there are some processes where a simple change to the material can minimise segregation. Most highly segregating materials are free flowing.

This characteristic is helpful in ensuring reliable flow of the material through process equipment but can contribute to higher segregation potential. Adding moisture, or making other adjustments, to increase the cohesive nature of a blend may minimise how much segregation occurs without any changes to the process or equipment.

Other material changes that can assist in minimising segregation include adjusting the particle size of the components or granulating the blend. A mono-size blend is much less likely to segregate by the mechanisms that rely on particle size differences between components, and granulation can be used to ensure that each individual particle has the desired composition of the blend (Williams, 1976).

Process:

Each time material is in motion or is transferred between pieces of equipment is an opportunity for segregation to occur. Designing a robust process with the minimum number of transfer steps of a blended material will reduce the number of opportunities for segregation to occur. To accomplish this, it is important to locate the blending operation as far downstream as possible.

Utilising in-bin blending or other processes to move blending closer to the end-product or packaging operation can assist in eliminating transfer steps where segregation occurs. While not often possible, dosing a blend during the packaging operation guarantees that each unit package will have the desired composition of the blend (assuming this process is in control).

“Once the root cause is understood, most segregation problems can be solved through careful changes to the material, the process, or the equipment.”

Preventing stagnant zones in continuous processes reduces the opportunity for a region of high fines to concentrate at the boundary between the flow channel and stagnant material. If funnel flow cannot be avoided, ensure sufficient level is maintained in the vessel so that the fines layer does not enter the process except for in an upset condition where variations are acceptable.

Splitting of streams to different downstream processes or between multiple lines should be done in a way such that there are no differences created between the material concentrations in each stream. For example, a stream of material on a chute surface may segregate with the fines percolating to the bottom of the stream and coarse particles rising to the top. If such a stream is split horizontally, the bottom layer will have a high concentration of fines.

Equipment:

When changing the material or process is not practical, adjustments can often be made to the equipment itself to minimise the impact of segregation. Side-to-side segregation can often be overcome with use of a vessel that operates in mass flow by ensuring material from all locations within the vessel are discharged whenever the outlet is activated.

In a similar vein, further decreasing friction of the material on the hopper surface or using a vessel with steeper walls can decrease any velocity gradients that exist. Providing uniform velocity across a full material bed further ensures that the material is recombined at the outlet and any side-to-side segregation within the vessel is minimised.

Similarly, avoid using eccentric hoppers because they create large velocity gradients and will preferentially discharge from specific locations within the material bed. Symmetric hoppers and chutes will reduce this potential for segregation.

Care must also be taken when designing a piece of equipment with multiple outlets. For hoppers, each outlet should be located at the same radial distance from the hopper centerline. To minimise segregation, the system should only be operated when all outlets are open and discharging material at similar rates.

Equipment should also be designed to minimise the interaction of fine powders with air. Drop heights should be reduced along with volume of displaced air. For powders that are sensitive to fluidisation or dusting segregation, proper ventilation of the equipment is vital.

Preventing downtime and product quality issues caused by segregation

Preventing the operational and product quality impacts of bulk solids segregation begins with recognising its potential early in the process design or optimisation stage. Early segregation analysis allows engineers to identify where separation of particles is most likely to occur and to implement control strategies before these issues become embedded in routine production. By understanding material behaviour at each stage of handling, more robust and reliable systems can be designed from the outset.

One of the most significant benefits of addressing segregation early is improved operational efficiency. When material flow remains consistent and uniform, downstream processes can operate at stable conditions with fewer interruptions. This reduces variability in feed rates, improves process predictability and helps maintain continuous production.

Reducing maintenance interventions is another key advantage. Segregation-related issues such as build-up, blockages or uneven flow often lead to increased cleaning requirements and unplanned maintenance activity. By minimising segregation, equipment operates more reliably, reducing the need for operator intervention and lowering overall maintenance burden.

Improved product consistency is also a major outcome of effective segregation control. When blend uniformity is maintained throughout the process, variations in particle size distribution, composition or density are minimised. This ensures that final product quality remains within specification, which is particularly critical in regulated industries such as pharmaceuticals, food and construction materials.

Minimising unplanned shutdowns is another important consideration. Segregation can lead to sudden blockages or flow disruptions that force production to stop unexpectedly. By controlling segregation mechanisms, manufacturers can significantly reduce the risk of these costly interruptions and maintain more predictable production schedules.

Supporting reliable process performance is closely linked to all of the above factors. Stable, predictable material behaviour allows equipment to operate as designed, improving overall process control and reducing variability across production batches.

Finally, proper hopper and chute design plays a critical role in preventing segregation. Equipment designed with appropriate geometry, mass flow characteristics and controlled material discharge helps maintain uniformity and reduce the formation of segregated zones. Careful design of transfer points, in particular, is essential for maintaining consistent flow and avoiding the reintroduction of segregation within the system.

Conclusions

Many bulk processes involve handling of materials with wide distributions of particles or components. While initial blending steps are often performed adequately, material segregation in subsequent process steps can have a negative impact on downstream processes or the end product. The three most common mechanisms for segregation include sifting, fluidisation, and dusting.

When approaching a problem related to segregation, it is important to first understand the root cause of segregation through both in-process and laboratory testing. Once the root cause is understood, most segregation problems can be solved through careful changes to the material, the process, or the equipment.

Works Cited

Allen, T. (1990). Particle Size Measurement (Vol. 4th Edition). Chapman & Hall.

Maynard, E. (2012, April). Avoid Bulk Solids Segregation Problems. AIChE – CEP.

Naugler, K. (2023). Critical Flow Parameters of Bulk Solids: A review and study on the impact of particle size. IBA Conference. Salt Lake City: The Institute for Briquetting and Agglomeration.

Pittenger, B. H., Purutyan, H., & Barnum, R. A. (2000). Reducing/Eliminating Segregation Problems in Powdered Metal Processing. P/M Science Technology Briefs, 5-9.

Williams, J. (1976). The Segregation of Particulate Materials: A Review. Powder Technology, pp. 245-254.

FAQ's

What is bulk solids segregation?

Bulk solids segregation is the separation of particles within a material mixture based on differences in size, shape, density or other physical properties. It commonly occurs during handling, transfer or storage of powders and granules and can lead to non-uniform material composition and process inconsistencies.

What causes bulk solids segregation?

Segregation is caused by a combination of material properties, process conditions and equipment design. Key drivers include particle size distribution, density differences, vibration, air flow, drop heights and the way materials are loaded or discharged from equipment such as hoppers and chutes.

Why is bulk solids segregation a problem in industrial processes?

Segregation can lead to inconsistent product quality, unstable process performance, equipment blockages and unplanned downtime. It can also affect bulk material properties such as flowability, density and composition which directly impacts downstream production efficiency and product conformity.

What are the main mechanisms of segregation?

The most common mechanisms are sifting segregation where smaller particles move downward through voids fluidisation segregation where lighter particles rise in an air stream and dusting segregation where fine particles become entrained in air and redistribute unevenly within a system.

How can bulk solids segregation be reduced?

Segregation can be reduced through material modifications such as particle size control process improvements such as reducing transfer steps and equipment design changes such as using mass flow hoppers minimising drop heights and controlling air movement within systems.

How does hopper design affect segregation?

Poorly designed hoppers can encourage segregation by allowing material to separate during filling and discharge. Proper hopper design promotes mass flow ensuring that all particles move uniformly towards the outlet which helps maintain blend consistency and reduces the risk of segregation.

Can segregation be eliminated completely?

In most industrial processes segregation cannot be completely eliminated but it can be significantly reduced and controlled. By combining good material handling practices with optimised process design and suitable equipment the impact of segregation can be minimised to acceptable levels.