The most common method for obtaining a homogeneous sample is grinding of comminution. Grinding samples has many benefits for sample preparation since it increases homogeneity, increases surface area and decreases particle size which can improve extraction efficiency. Some of the negatives regarding grinding samples are: potential contamination, increase in moisture, evaporation, loss or alteration of volatile compounds or labile constituents, and safety issues regarding grinding.
Sample size reduction is accomplished by either crushing or grinding using forces of impact, attrition, shearing, or compression (Figure 1).
- Impact Force: the striking of one object or material against another. One object may either be stationary or both may be in motion.
- Attrition Force: created by materials rubbing against each other usually in opposite directions or planes.
- Shearing force: cleaving or cutting of a material by some cutting implement or blade.
- Compression force: slow application of a force against a solid to crush it into smaller pieces usually between two solid surfaces.
Figure 1. Grinding forces.
Particle reduction of solids occurs in multiple stages starting with the accumulation of defects or stresses in a concentrated location increasing the strain on a solid or particle. The stress forms microcracks and in crystal lattices, it will disrupt the crystal lattice in several cells or locations. The microcracks then join to form a larger major disruption or crack which ultimately divides the solid into pieces (Figure 2).
Figure 2. Attrition: Smaller particles break off larger particles through rubbing against a surface or each other. Impact and Compression: Faults or microcracks within larger particles increase with increasing force causing the larger particle to break apart.
Different applications, quantity of throughput and final end products often designate the method employed to grind materials. In our next blog post we will look at the different types of equipment used in grinding and homogenizing materials.