Mixing and homogenizing divided solids

Mixing divided solids (powders, granular media) is a key operation in many industrial fields (pharmaceuticals, food, cement, plastics, etc.). By regulating compositional homogeneity at the required scale, which is often that of packaging, it is primarily responsible for ensuring that formulated products meet their specifications and properties of use. But the homogeneity of a mixture of solids, inseparable from the scales of observation and segregation, remains difficult to measure. In most cases, it has to be estimated by taking samples, which poses technical and statistical problems. However, the current development of non-intrusive on-line measurement methods should soon enable us to better define and control homogeneity.

A certain mixing quality can be achieved by playing on dynamic aspects, themselves linked to the mechanisms that set the particles in motion: convection, shear and diffusion. These mechanisms depend not only on the flow properties of the products, but also on the technological capabilities of the mixing equipment. From this point of view, there are three main types of mixer used in industry, depending on whether agitation is produced by an internal mobile (convective mixers), by rotation of the vessel (drum mixers), or by the material's own flow (static mixers). Both continuous and discontinuous mixing processes can be used, the choice depending on conventional production constraints and usage in different sectors. However, the sizing rules for these devices are essentially based on empiricism and remain, to a large extent, the prerogative of equipment manufacturers.

As with other unit operations involving divided solids, basic scientific knowledge is still insufficient to clearly explain the phenomenology of mixing, probably due to the mesoscopic nature of these media. Within the framework of a systemic approach to this operation, however, it is possible to take into account certain classical dynamic aspects (mixing kinetics, residence time distribution, etc.) in "process engineering" and to model the operation globally. Finally, it is important to consider that the notion of mixture quality must be integrated at the level of the whole process, as the presence of segregation-inducing stages (transport, storage, etc.) can affect the homogeneity of a mixture after it leaves a mixer.