Particle size analysis of bulk powders using mathematical morphology

The objective of the work was to determine the particle size of powders displayed in bulk by using image analysis. As image data treatment, two basic operations of mathematical morphology, grey-level erosion and dilation, were studied for the characterization of the particle size. The procedure was tested on two products: steel marbles taken as spherical model particles and ground pea kernels. Five fractions (A, B, C, D and E) of particle size varying from 0.300 to 1 mm were collected and mixtures were made from the fractions A, C and E. Monochrome images of a thin layer of the particles in bulk were acquired. Successive steps of erosion and dilation were applied to the images. The variations of grey level observed in the images at each step were measured and granulometric curves were built. The curves were compared by principal component analysis. In the case of marbles, the finest fractions were overestimated because projected surfaces were visualized while the mixtures were made by weighting the fractions. In the case of ground peas, some demixing of the particles occurred that led to an underestimation of the finest fraction. Despite this, the correlation coefficient between the median diameter measured by laser diffraction and the first principal component was −0.95.