Pelletising
Pelletising is an agglomeration process in which fine powder particles are formed into larger, stable agglomerates. The aim is to improve the material’s handling properties and achieve defined mechanical properties. The process is also referred to as agglomeration or granulation. In this process, fine-grained or powdered materials are combined to form approximately spherical, dense and stable pellets.
Pelletising is based on the relative movement of the particles, the addition of liquid or a binder, and mechanical forces. Binders form liquid bridges between the particles. These generate capillary forces that lead to agglomeration. At the same time, shear, impact and rolling movements act to compact and round off the pellets.
The formation process takes place in several phases. First, nuclei form through the accumulation of fine particles. These then continue to grow through coalescence or layer formation. During the compaction phase, the packing density increases and the mechanical strength of the pellets rises.
Key process parameters include the amount of liquid, the particle size distribution, wettability, mixing intensity, residence time and surface properties. Too low a liquid content leads to unstable agglomerates. Conversely, too high a liquid content can lead to a pasty consistency or the formation of undesirable large agglomerates.
Pelletising serves to compact powders and improve flowability. Dust formation is reduced, segregation is avoided and solubility is specifically influenced. Typical fields of application include the chemical industry, the food and pharmaceutical industries, fertiliser production and powder metallurgy. With amixon® equipment, powder mixtures and dusts can be efficiently pelletised, although the process should first be tested individually.
In industrial practice, the term ‘pelletising’ is also used to describe the mechanical compaction of dry bulk materials. In this process, pellets are formed under pressure, for example using tablet presses, roller presses, extruders, sleeving presses or briquetting presses.
A homogeneous mixture of the raw material is required to ensure consistent pellet quality. The mixing process is therefore a preliminary step, particularly when several powder components are involved. Heat is generated during the pressing process. The pellets must therefore be sufficiently cooled before storage or packaging.
Further information on the related term ‘agglomeration’ can be found in the amixon® glossary.