Preparation of controlled particulate mixtures with glass beads of different sizes

A method of mixing/preparing binary and ternary mixtures of glass beads was developed using a viscous solution of glycerol in water. After mixing, the mixture was transferred to a prismatic vessel and glycerol was washed out. The different sized beads were differently coloured and digital pictures taken from each face were automatically treated by image analysis to determine the coloured fraction present in each face. Statistical analysis showed that no significant deviation existed in the colour distribution of each of the four faces. A chi-square test showed that a uniform distribution could be accepted for the beads, no segregation of bead size nearby the edges was observed and that no wall effect was present. The two-dimensional picture obtained by image analysis was converted to the corresponding three-dimensional distribution, from which the expected bed porosity was inferred. The porosity previously estimated was compared with the experimental porosity determined by gravimetry directly on the bed. No significant deviations were found, thereby proving that the mixing method developed was reliable. Hundreds of experiments were done showing a very high reproducibility. The developed method was further used for studies on mixing of binary and ternary mixtures. In certain conditions (mixtures enriched with large size particles and having a significant difference in size) a segregation layering effect took place and the bottom layer presented a composition corresponding to the packing with the smallest porosity possible. (C) 2003 Elsevier B.V. All rights reserved.