Fragmentation - Theoretical aspects

Fragmentation is the operation by which we seek to reduce the size and/or increase the developed surface area of the mass unit (specific surface area) of solid particles. Its effectiveness is always assessed by measuring the increase in fineness. Mechanical stress increases the free energy of materials, which can be converted into different forms. Elastic stress energy is thus converted into the elastic energy of lattice defects, whether punctual (on an atomic scale), linear (dislocations, macles), planar (stacking defects, grain boundaries) or volumetric (structural disorders). The conversion of larger quantities of free energy into surface energy leads to fracturing. Other modes of energy dissipation are manifested by mechanochemical effects such as amorphization (mass or surface), agglomeration, polymorphic transitions and so on.

Fragmentation can serve a variety of purposes:

• reduce dimensions, either to facilitate handling, packaging or use, or to release components prior to a separation operation;

• eliminate potential fracture zones (release of quasi-single-crystal units) before processing;

• increase reactivity to processes whose kinetics depend on the fineness or degree of disorder;

• homogenize (mixtures, solid dilutions, dosages) ;

• provide specifications for shape, texture and granular distribution;

• modify functionality, either through mechanochemical activation or by taking advantage of the creation of new surfaces to implant the desired functional groups.

Fragmentation always seeks to satisfy requirements relating to subsequent uses, and is generally aimed at at least one of the above-mentioned priority goals. The other effects, which are generally unavoidable, will be penalizing if they give rise to undesirable behavior, but rewarding if they can be used to improve the substance's properties for use.

This dossier is the first in a series. Subsequent issues will cover :

• technology;

• applications in the metal ore industry ;

• applications in industrial minerals and various manufacturing processes.