Segregation of powder mixtures at filling and complete discharge of silos

Abstract

This paper presents segregation of powder mixtures in silos of different scale on the basis of recent experimental findings with binary and ternary mixtures as well as commercial construction materials. Dimensional analysis is performed to reduce the number of variables and to bring forth dimensionless groups. Based on these experimental correlations (models) are proposed for describing the concentration of fine particles at the walls as a result of silo filling and segregation towards the end of complete discharge. Parameters of both models are fitted to experimental data using the method of least sum of squares. The filling model shows good agreement with experimental data and the description of segregation at discharge is in general agreement with experimental findings. Important variables for the distribution of particles at silo filling include the mass fraction of fines, particle size ratio (coarse/fine), particle solid density ratio (coarse/fine), free fall distance, silo diameter and inlet diameter. On the basis of the proposed model for silo filling, the concentration of fine particles at the silo walls increases with increasing fines content, size ratio and density ratio for the powder mixture, as well as the ratio for free fall distance and silo diameter and the ratio for silo diameter and inlet diameter. This result is in general agreement with the findings of other investigators, but the observation that segregation at silo discharge is, to a large extent, determined by the material distribution at filling is new and interesting. This confirms the qualitative conclusions of earlier work by the authors and shows that mass flow designs do not always entirely correct for side-to-side segregation patterns (induced during filling) at silo discharge. The findings of this paper should be of considerable interest to producers of construction materials and to other industries where segregation of powder mixtures in silos is a major concern.