A pellet that is made under compaction is essentially a
composite which is made of a solid phase and a gaseous
(air) phase. During compaction, the raw material is pushed
together under a high pressure. When the volume of the
powder bed is reduced, the corresponding bulk density of
the pellet increases and the porosity is reduced. This volume reduction brings the particles into close proximity
with one another and facilitates the forming of bonds. The
porosity of the pellet depends on particle size, compaction
speed, moisture content, density, and applied pressure and
has a significant impact on material strength and handling
aspects [12, 13]. Studies have found that as the porosity of
the powder bed reduces, the strength of a pellet increases
[13–15]. In addition, it has been observed that beyond a
certain pressure during compaction, the strength of pellets
does not increase with the compaction pressure. The
maximum strength is thus achieved which corresponds to
no or near-zero porosity in the pellets [14, 16]