Preventing Segregation

The Influence of Proper Product Design

09.09.2010 | Author / Deskman: K. Johanson, USA / Marcel Dröttboom

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Segregation can be caused by several effects.

Segregation causes a number of effects which may not only influence the product quality but also the efficiency of a production process. Segregation data can therefore be used to optimise product design and minimise its effects on production. A reliable means of measuring segregation tendencies is the key to this optimisation.

Segregation is responsible for a significant amount of lost product due to poor quality. Predicting and controlling segregation is critical to optimise product design or mitigate quality issues with bulk powders and granules. Segregation can be caused by several effects. Fine particles sift down through a matrix of coarse particles.

Differences in repose angle can cause separation during pile formation. Air currents may carry fine particles, depositing them preferentially in different parts of the bin. In general, segregation is caused by a difference in a particle scale property. Thus, the segregation driving force is some function of these key flow properties. Cohesion can often be used to mitigate segregation. However, the effect of cohesion is different depending on the segregation mechanism.

To solve a segregation problem we need a reliable means of measuring segregation tendencies in bulk materials. Engineers use segregation data to optimise product design, creating a product with minimal segregation, or to modify the process and minimize the effect of segregation in their plant packaging process or handling facility. In either case, the segregation pattern, segregation mechanism, and magnitude of segregation are key parameters in process or product design.

Basics of Measurement

Any measurement of segregation must relate to the process. Segregation occurs due to differences in key particle scale properties and almost all materials will segregate if exposed to an external stimulus that induces different behaviours based on a key property. For example, fines can be carried by air currents. However, if the process has minimal air current events, then segregation will not be prominent.

The real question is: will the mixture segregate when exposed to a feed behaviour similar to that present in the process? Therefore, any measurement of segregation tendency should have three key elements.

First, feed must be controlled to allow the measured segregation to be scalable to process conditions. Second, the segregation pattern must be included as part of the measurement to predict the expected concentration leaving process equipment. Finally, the magnitude of segregation must be quantified. We can form a pile at conditions similar to what might exist in the process and then measure the concentration of key components down the pile using a slice box similar to that shown in Fig. 1.

The back of this slice model is made of glass, allowing access to a view of the segregation pattern in the bulk material. Observation view ports are chosen and reflectance spectroscopic methods employed to measure subtle colour differences in a user-defined view port. Since segregation measurement is a scale issue, the view port size must be large enough to contain a representative number of particles, yet small enough that local composition differences are not lost in the averaging scheme (Fig 2).

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