Fine grinding

In many technical processes it is necessary to divide solid material into fine particles and distribute them evenly within a liquid carrier. This process is generally known as ”dispersion”. During dispersion, the adhesive forces that act between the extremely fine solid matter powder particles must be overcome. When the requirements on fineness are high or the solid matter is difficult to disperse, a dispersion with the dissolver is often insufficient. Due to their ability to process a wide variety of solid matters that are difficult to disperse, high‒speed bead mills have gained particular acceptance.

In the dispersion process, three partial steps run in parallel:

• The wetting of the surface of the solid matter to be processed, by liquid components of the millbase.
• The mechanical division of agglomerates into smaller agglomerates and primary particles.
• The stabilisation of primary particles, agglomerates and aggregates against renewed attraction (= flocculation).

While the stabilisation against flocculation is primarily a property of a colloid chemical system, which depends on the interaction of the liquid components (in varnishes for instance: binders, solvents and additives) with the solid matter parts (e.g. pigments and fillers) or on that of the solid particles with each other, the dispersion machinery used plays a vital part in the mechanical division and more important aids the wetting process.

The actual dispersion system in a bead mill consists of a milling chamber and an agitator; the milling chamber is filled with the grinding beads (material e.g. glass, zircon oxide, steel) and the product to be dispersed. In the milling vessel, the grinding medium is kept moving by the agitator, which itself is driven by a motor. The dispersion process takes place between the grinding beads sliding on each other and between the rotor and/or the vessel sides and the grinding beads.