Preventing Segregation

The Influence of Proper Product Design

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

However, smaller particles fill the voids between coarse particles, creating a shadow effect for the coarse particles. The fine particles within the voids occupy a proportionally greater percent of area than the volume fraction would suggest. This effect can be modeled by adding a weighting factor (Wi) to the linear combination of pure spectra:

The tester compares the computed intensity curve Fmixj(λ) with the actual measured mixture curve Factj(λ) and adjusts the weighting factors Wi and local component fractions down the pile xi,j to minimize the error between the two curves using a least squares approach. The result is a radial concentration profile. Now consider the effect of adding cohesion to the bulk material by comparing the segregation profiles given in Fig. 4.

A dimensionless radius value of 0.0 corresponds to the top of the pile; a value of 1.0 corresponds to the pile bottom. Consider the concentration profiles for the sand mixture without oil. The blue sand accumulates at the bottom of the pile while the yellow and white sands accumulate at the top of the pile.

When oil is added to the mixture then the cohesive forces between the particles prevent segregation resulting in a nearly uniform concentration as a function of radial distance. The standard deviation relative to the mean of each component concentration, or segregation intensity (SIi), is used to quantify the overall segregation potential of the mixtures:

Mitigating Segregation

Segregation intensity varies between 0 for uniform mixtures to 1.0 for completely segregated mixtures. Thus, a simple change in product properties can prevent segregation from occurring. Likewise, one could have changed the particle size or shape of key components and mitigated segregation. The trick, however, is having the appropriate means to easily measure segregation potential quickly and accurately. The new Spectester allows both accuracy and speed, making product design a very simple task. Since the tester requires only 10 to 15 minutes to run, many potential mixtures could be evaluated in just one day’s time, providing the necessary data to truly optimize product design.

Fig. 5 shows the reduction in segregation intensity as the oil content is increased. This interdependence demonstrates how a small change in formulation can mitigate segregation. Consider using this technique to help solve your next segregation problem, or to characterize your current materials.

Dr. Kerry Johanson, Material Flow Solutions, Inc.