Storage and gravity flow of granular solids in silos Basics and theory

The storage and gravity flow of powders and granular materials in silos is a subject of study that has preoccupied both academic and industrial circles, particularly since the industrial revolutions and the increase in the quantities of powders to be stored, whether raw materials or finished products (ores, coal, sugar, cereals, flour, cement, etc.). Generally speaking, virtually all granular materials are stored in a silo at some stage in their life cycle.

From a technological point of view, a silo appears to be an extremely simple and basic piece of equipment for storing bulk solids. However, a number of criteria must be taken into account to ensure its smooth operation. The ideal silo should not only contain a maximum amount of product for a minimum floor area, but also ensure a regular flow (without chimney or vault formation) at the desired mass flow rate. Nevertheless, numerous problems (described in the article [J 2 255] ) can be observed during product storage and filling: formation of chimneys, dead zones and vaults (arches), irregular flow, submergence, segregation, heterogeneous storage time, caking, self-heating, explosion, and so on. Although the majority of these difficulties are associated with fine, cohesive powders, free-flowing granular solids (generally associated with the higher particle size materials discussed more specifically in this article) are also subject to a variety of problems. Indeed, even with free-flowing powders and low hopper angles, bulk flow is not always guaranteed, and chimney formation is possible. The result is heterogeneous storage times.

This article [J 2 258] and the following one [J 2 259] focus on the flow of "granular solids" during silo emptying. The term "granular solids" is used here to designate materials with a large particle size, for which gravitational forces (weight) predominate over inter-particle forces: van der Waals forces, electrostatic forces, etc. (detailed descriptions in the article