che_november-2024 - 16

Facts At Your Fingertips
Milling for Particle Size Control
Department Editor: Scott Jenkins
illing is commonly used for
particle-size reduction and
control of particle-size distribution
(PSD) for crystalline solids.
Mills are effective for generating consistent
solids properties, through size
reduction
M
and
surface-modification
mechanisms [1]. This one-page reference
outlines the mechanical forces
involved in size reduction and the important
material properties for selecting
milling equipment.
Forces for size reduction
In comminution, solids are fractured
by mechanical force delivered to particles
by the milling machine. This is
achieved in a myriad of ways, depending
on the mill type. Seven different
types of stresses can be imparted
to achieve size reduction, as
listed below [2]:
1. Compression between two rigid
surfaces
2. Compression between two rigid
surfaces and against an adjacent bed
of solids
3. Shearing (tearing, cutting, shredding
and cleaving) mechanically
4. Shearing forces due to surrounding
media
5. High-velocity impact against a rigid
surface (particle impacting a stationary
surface or a moving surface impacting
a particle)
6. Particle-particle impact causing
breakage and shattering
7. Abrasion during particle-wall and
particle-particle impacts
Desired performance of size reduction
can be achieved by matching
the machine design with particle
properties and mode of operation.
Key material properties
The following properties of the material
must be evaluated for suitable
selection of equipment and to understand
its size-reduction behavior.
* PSD in the feed material
* Particle shape
* Bulk density
* Flowability, cohesiveness or adhesiveness
*
Corrosivity and composition
* Moisture content
16
* Hardness, brittleness,
friability
* Toxicity
* Abrasiveness
* Shock sensitivity
or explosivity
* Elasticity, plasticity
ductility
* Dust-explosion
characteristics
* Temperature sensitivity
- degradation, stickiness,
phase change
* Fibrous morphology
* Oil or fat content, especially
those released during grinding
* Reactivity or release of gases
* Stickiness
Milling equipment
The choice of milling equipment for
an application depends on the performance
capabilities relative to the
properties of the feed material, as well
as constraints of the mill type.
Broadly, mills can be separated
into wet and dry mills, and these two
categories are each further subdivided
by how comminution is achieved,
such as rotor-stator mills, jet mills, pin
mills, hammer mills, media (ball) mills
and cavitation mills. Wet mills use
liquid as their working fluid, whereas
dry mills use gas. Figure 1 (adapted
from Ref. 2) shows how various milling
techniques are categorized. Each
type of mill is characterized by an energy
input mechanism.
Effects on solids properties
Milling processes have several general
effects on solid particles. Several
are outlined here:
Particle size reduction. The mechanical
forces imparted by the mill
reduce particle size.
Particle-size distribution changes.
Generally, milling results in narrower
PSDs, and is intended to provide normalization
(a similar size distribution
of product is obtained for various size
distributions of feed materials). Theoretically,
a milled product is predicted
to have a log-normal or Weibull distribution,
a probability distribution best
fit by a function with two or more paFIGURE
1. Milling technologies are
divided into wet and dry methods
and impart a range of forces on solid
particles
rameters [1]. The function is related to
the probability of a particle fracturing
and the nature of the fragment as a
function of its original size. Practically
speaking, this means that milling does
not typically give symmetrical Gaussian
distributions. Skewness and tails
are common.
Increased surface area. Milling generally
increases total surface area as
solid particles are broken.
Surface property changes. Particle
surfaces are altered as solids are
milled, and this can affect how particles
behave with regard to mixing,
compacting, dissolving, aerosolizing
and interacting with moisture.
Particle shape changes. Milling also
impacts particle shape, or morphology,
particularly for high-aspect-ratio
" needle- " or " rod- " type particles. The
comminution fracture mechanism often
operates along the length of these
crystals, effectively chopping them
along the length scale into shapes
with lower aspect ratios. This breakage
gives rise to consequent change
in the physical attributes, such as bulk
density, as well as formulation behavior,
such as flow properties.
n
References
1. Lee, I., Sirota, E., and Moment, A., Milling in the Pharmaceutical
Industry, Chem. Eng., October 2022, pp. 27-33.
2. Dhodapkar, S. and Theuerkof, J., Maximizing Performance in Size
Reduction, Chem. Eng., June 2011, pp. 45-48.
3. Rajkovich, S., Advances in Pin Mill Technology, Chem. Eng.,
February 2017, pp. 60-63.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
NOVEMBER 2024
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