Ultrasonic velocity measurements in powders and their relationship to strength in particles formed by agglomeration

Abstract

Many industries produce pellets, powders, and granules as finished or intermediate products. The strength of these products is important to their marketability, as dusting and breakdown from weak product is undesirable and results in poor handling properties. Measurement of “strength” usually takes the form of some empirical test involving crushing or attrition. The possibility of using sound velocity measurements to replace these techniques (especially for composite materials such as pellets or agglomerates) is investigated by performing measurements on glass sphere powders and test industrial materials. The ultrasonic velocity measurements in glass spheres were shown to be described by effective medium theories of sound propagation and can be used to probe the packing structure of the powder. For coarser powders sensible values of the co-ordination number Z could be calculated during compaction of the material. For powders finer than 0.2 mm the sound velocity measurements are no longer repeatable and rapid changes of the configuration may occur though the packing density only changes slowly. Two industrially important materials, iron ore pellets and calcined alumina powders were studied. It was found that reproducible measurements can be made in these materials that correlate to standard industrial “strength” measurements. In the case of iron ore pellets the correlation coefficient was 0.94 and the variability of the sound velocity based “strength” measurement was a factor of five smaller than the particle breakage method. For alumina powders the standard attrition index measurements could be correlated with the ultrasonic velocity and bulk density for both hydrated and calcined alumina powders with correlation coefficients of up to 0.95. The sound velocity strength measurement technique has significant advantages in repeatability, sample size and provides direct measurement of a physical parameter related to strength.