Tuesday 30th Nov, 2021

Segregation and stratification in granular mixtures

Segregation issues can be complex, often requiring tailored solutions. Preventing segregation during vessel filling can be challenging. Jie Guo and Corin Holmes explain how mass flow can be established during discharge, and the associated problems can be mitigated or eliminated.

Segregation issues can be complex, often requiring tailored solutions. Preventing segregation during vessel filling can be challenging. Jie Guo and Corin Holmes explain how mass flow can be established during discharge, and the associated problems can be mitigated or eliminated.

Bulk solids segregation is a term used to describe the separation of a mixture of particles into regions that vary in particle size, density, shape or other distinguishing characteristic.

Segregation can occur naturally, under the force of gravity, or while external factors such as imposed air flow, vibration, or impact are present. Stratification is one of the phenomena often associated with segregation, for which, different materials overlay each other to form alternating layers depending on the physical properties of the particles.

Segregation can cause numerous problems, ranging from difficulty in material handling processes to variations in final product quality resulting in product rejections or even recalls if they have been released to market. Many industries such as food, mining, cosmetics, pharmaceuticals, chemicals, powdered metals, and glass have experienced segregation challenges.

Close attention should be paid to planning the processing plants and designing the handling equipment, as segregation and stratification can occur either within the handling devices, such as silo/bins, agglomerators and blenders, or at the transferring steps, such as from belt conveyors to stockpiles, and from hoppers to feeders.

Examples of Segregation

Segregation issues can be complex often requiring tailored solutions. Figure 1 depicts a common segregation mechanism seen in centrally filled stockpiles – a common storage form in the mining industry. As can be seen, the coarser particles (darker material) have rolled to the periphery of the pile, whereas the finer particles have concentrated in the middle, under the piles feed point.

Figure 2 illustrates some more complicated segregation patterns generated in two consecutive bins which are validated by sampling materials from different locations. Both bins operate in a funnel flow pattern of discharge where a flow channel is established above the outlet and stagnant material zones exist along the bin and hopper walls. In funnel flow, materials follow a first in – last out flow sequence. In Bin 1, a flat baffle is featured directly under the fill point. In this case, based on sampling results, an inverse segregation pattern was formed (see Figure 2) where the fines concentrated near the walls and the coarse particles fell into the flow channel.

Figure 2: Segregation pattern in Bin 1.

The material discharged from Bin 1 was then fed into Bin 2. The sampling results from this bin indicated that the coarse material flowed out first, followed by finer materials, and then coarse materials again resulting in a segregation pattern as shown in Figure 3.

Figure 3: Segregation pattern in Bin 2.

In this example, the segregation issues experienced occurred in a pharmaceutical process where stringent quality assurance regulations are applied resulting in millions of dollars loss due to product rejection.

Bulk solids particles often differ in their resilience, inertia, and other dynamic characteristics which can cause them to segregate, particularly when they are forming a pile such as when charged into a bin or discharged from a chute. The more resilient particles will tend to bounce to the outer area of the pile and the less resilient ones will stay in the centre. Since fines are usually less resilient than coarse particles, dynamic segregation can accentuate a central fines concentration. Regardless of the process, to avoid or mitigate segregation effects, an understanding of how and why segregation occurs in your process, is required to identify the options to improve the situation.

Segregation and stratification mechanism

Some early pioneering work regarding segregation was undertaken in the 1950s, but most of the research focused on differences in colour while the rest of the physical properties remained identical. In the following decades, attention has been given to the fact that materials can become segregated in a handling system if they have differences in particle size, density, shape and resilience.

J.C Williams stated in his paper, The segregation of particulate materials, “the most urgent need with regard to particle segregation is that engineers responsible for the design of solids handling plants should understand the causes of segregation and should be aware of the points in a process at which it is most likely to occur, so that in designing plant they can minimise its effects”.

There are several primary mechanisms that have been identified as being responsible for most segregation problems when handling bulk solid materials.

Sifting segregation occurs during filling when fine particles concentrate in the centre of a vessel and the coarse particles roll to the periphery (see Figure 1). For sifting segregation to occur, the following conditions must be met:

(1) A difference in particle size between individual components. In general, the larger the ratio of particle sizes, the greater the tendency for particles to segregate by sifting.

(2) A sufficiently large mean particle diameter. J.C. Williams’ study, The Mixing and Segregation of Particulate Solids of Different Particle Size, shows that the tendency to segregate by sifting for particles below 500 µm decreases substantially. This may be attributed to the fact that the attraction forces acting between finer particles become prominent compared to their weights, which hinders the mobility of the particles.

(3)  Free-flowing material. For sifting between different sized particles to occur, it is essential that no agglomerates are formed, either between particles of a given size or particles of varying size.

(4) Interparticle motion. There should be interparticle movements, in other words, there must exist a velocity gradient between smaller and larger particles.

Figure 4 illustrates the spontaneous stratification associated with sifting segregation. As can be seen, the smaller (darker) particles concentrate in the centre and the larger particles (lighter) flow further to the periphery. Meanwhile, the mixture spontaneously stratifies into alternating layers of small and large particles. It is noticed that the stratification in Figure 4 is more signification compared to the one in Figure 1. This is related to the difference in angles of repose between smaller and larger particles. The research conducted by Makse et. Al in Spontaneous Stratification in Granular Mixtures concluded that the stratification can only occur when the larger particles have larger angle of repose than the smaller particles. Otherwise, only segregation and not stratification is obtained.

Figure 4: Sift segregation and
spontaneous stratification.

J.R Johanson also pointed out the correlation between angle of repose and segregation. He stated in Particle Segregation and What to do About it that even uniformly sized particles of different materials in a mixture can segregate significantly during pilling if the materials have different angles of repose. The material having a larger angle of repose (measured from horizontal) tends to concentrate in the centre, whereas the one having a smaller angle of repose will settle on the periphery. In this case, the segregation is no longer caused by the sift mechanism.


Air entrainment (fluidisation)
segregation generally occurs in powders with an average particle size less than 100 µm. It is very likely to occur when fine materials are fed into a storage container via pneumatic conveying or when air is flowing counter to the flow of solids. In fluidisation segregation the finer, lighter particles rise to the top of the fluidised bed while the larger, heavier particles concentrate at the bottom (see Figure 5).

Figure 5: Fluidisation segregation.


Dusting segregation
occurs when dust generated during filling becomes entrained and settles in areas away from the incoming feed stream. The finer the particle size, the longer it may remain suspended in an air stream thus, secondary air currents can carry airborne particles away from a fill point into outer areas of a bin. Dusting segregation starts to become pronounced with particle sizes of 50 µm and is very common with particles below 10 µm. Dusting segregation concentrates the ultrafine and fine particles at a container’s walls or at points farthest from the incoming stream (see Figure 6).

Figure 6: Dusting segregation.


Particle velocity differences while sliding on a surface

If there are variations in particle size or shape, the smaller particles and/or those that are more irregular in shape will typically have a higher frictional drag on a surface. This higher drag results in a lowered particle velocity. This effect can be accentuated due to stratification on the chute surface because of the sifting mechanism described above. Concentrations of smaller particles close to the chute surface and larger particles at the top of the bed of material, combined with the typically higher frictional drag of finer particles, often result in a concentration of fine particles falling close to the end of the chute, with coarse particles falling further away as shown in Figure 7. This can be particularly detrimental to operations if portions of the pile go to different processing points, as is often the case with multiple outlet bins.

Figure 7: Segregation after chute transfer.

Mitigate against segregation

Since segregation can be quite costly in terms of rejected materials, attention should be paid to prevent or mitigate its occurrence during the design phase of a bulk material handling system. A holistic approach can be followed:

• Determine the bulk material’s flow properties

• Understand how the bulk material flows through bins, hoppers, chutes, feeders, conveyors, and other material handling equipment based on its flow properties

• Understand segregation mechanisms and analyse the potential for the material to segregate through each process step

Discharge pattern affects segregation. Figure 8 illustrates funnel flow and mass flow patterns that can develop during discharge. In funnel flow, a first-in last-out flow sequence, an active flow channel forms above the outlet, with non-flowing material at the periphery. As the level of material in the vessel decreases, layers of the non-flowing material may or may not slide into the flowing channel, which can result in the formation of stable ratholes. In mass flow, a first-in-first-out flow sequence, all material is in motion during discharge. Material from the centre and periphery moves toward the hopper outlet uniformly thus reducing segregation effects.

Corrective actions

Consider the following techniques if segregation problems are identified. Note that these techniques can also be applied if segregation problems are present in an existing process/system.

(1)  Change the material

Generally, an easy flowing material is more prone to segregation than a cohesive material. Consider adding liquid or binders to a material to increase its cohesive strength and decrease its segregation tendencies. Note that the change needs to be balanced between segregation and other flow issues (such as ratholing and arching) which may be caused by the increased cohesiveness of the material.

Altering the bulk materials’ particle size distribution such as narrowing the particle size range and decreasing the mean diameter of the particles below 100 µm can also be beneficial for improving segregation.

(2) Change the process

If the mixture being handled consists of several materials which are uniform in themselves but vary distinctly from one another, consider handling them separately and blending immediately prior to the final processing step.

The material stream discharged from a conveyor may be subject to vertical or side to side segregation. Free-fall chutes should not be used to transfer segregating materials unless there is a mixing device downstream of the chute.

Maintaining a high material level in a mass flow bin during processing can influence the potential for segregation tendency. For example, the top surface of the material should be at least three quarters of a bin diameter above the transition between the cylindrical section and converging hopper section.

(3)  Change of design of the equipment

The use of mass-flow hoppers can alleviate segregation induced problems significantly as they promote the material flow of the entire cross-section within the bins. They will not necessarily eliminate segregation within the bins, but fines and coarse particles will be discharged simultaneously and the proportion of each can be maintained.

In some instances, a properly designed distributor at the inlet of a bin can be an effective way of reducing segregation. In effect, the distributor generates multiple filling points and creates multiple small piles. The segregation that occurs during pile formation is allowed at a smaller scale.


Conclusion

Segregation issues can be complex often requiring tailored solutions. Segregation is a function of the material, equipment, and the process itself. Preventing segregation during vessel filling can be challenging however, when mass flow can be established during discharge, the associated problems can be mitigated or eliminated. 

Do you have a bulk solids handling question? Jenike & Johanson has developed the science of bulk solids flow and specialises in applying it to solving the most challenging bulk solids handling problems. So why not put them to the test with your question? 

Note: The advice here is of a general nature. Specific solutions are very sensitive to their circumstances. You should consult with a specialist in the area before proceeding.