Chemical Engineering November 2013 - 47

Solids Processing
Actual rupture strength
Ultimate strength
Yield point
Selecting the
Proper Mill for
Your Product
Understanding breakage behavior is crucial in mills
Kerry Johanson
Material Flow Solutions, Inc.
M
illing is a mechanistically
driven process that breaks
solid material into smaller
pieces.
Selection of the
optimal mill is a daunting task that
often leaves engineers relying on vendor
suggestions to decide which mill
is right for their job. This article addresses
a methodology to score and
rank the various mills for use with a
particular material.
In milling, a materials are broken
up due to a few basic breakage mechanisms.
Some particles tend to split,
creating smaller particles that are
one-half to one-third the size of the
original particles. This type of breakage
is termed fracture. In some cases,
particles break as corners are knocked
off or subsurface cracks form particles
that are considerably smaller than the
original particles. This type of breakage
is termed abrasion. Some materials
require many impacts or stressstrain
events to induce breakage. This
is termed fatigue and can result in
the formation of both small and large
particles. The rate of breakage always
changes during a fatigue-driven event.
Some particles require large strains
before breakage occurs. These large
strains may be due to plastic deformations,
such as in the case of plastic,
rubber and ductile metals, separation
of fibers and internal structures as
in biomass, or due to non-linear elastic
effects, such as with tissues and
complex polymers.
The premise for this article's mill
ranking comes from the fact that each
mill induces a unique set of impact or
stress-strain events, or both. If a particular
material is sensitive to breakage
due to stress-strain events, then
mills exploiting stress-strain events
will be the most efficient choice for
particle breakage unit operations.
Likewise, if the material is sensitive
to breakage due to impact, then mills
that exploit impact events are the best
choice for this material. Materials
sensitive to fatigue require mills that
have repeated stress-strain events or
repeated impact events to break particles.
Thus, an analysis of the number
of breakage events in a particular mill
has some merit in choosing the best
mill. Finally, materials that require
large strains to break must be coupled
with mills that can induce large
strains to effectively break particles.
For the purpose of this article, this will
be described as breakage due to shear.
We will start by describing typical
breakage events in various mill types
and assign each mill a score on a scale
of 1 to 10 based on breakage due to impact,
stress-strain, fatigue, or shear. A
score of 1 indicates a low propensity
of breakage due to this event. A score
of 10 indicates a high probability of
breakage due to this type of event.
Following this scoring and ranking
system, the article will present some
tests that can be used to quantify a
particular material's propensity to
break due to a particular mechanism,
and then suggest which mills would
be best for use with these materials.
It should be noted that general classes
of mills will be considered for this article.
The breakage in a specific mill will
be due to the details of operation for
that particular mill. This article simply
provides a guide, and the engineer
may need to adjust these rankings
based on a unique mill of a given type.
Likewise, some materials are anisotroRupture
strength
Elastic
limit
Strain
FIGURE 1. Resilience equals the area
under the stress-strain curve up to the
elastic limit
Ultimate strength
Elastic limit
Rupture
strength
Toughness
Strain
FIGURE 2. Toughness equals the area
under the stress-strain curve up to the
rupture strength
pic and have different breakage tendencies
in different directions, making
them subject to breakage events due
to several mechanisms depending on
how the particles are introduced to the
mill. Again, this article is to be used
as a guide, and the exact behavior will
depend on the material of interest. In
any case, much of the breakage behavior
depends on the detailed structure
of the particle. Some of these effects
will be discussed, but this article is not
intended to be a comprehensive analysis
on the subject.
Material properties
For the purpose of this discussion, it
is useful to define some engineering
terms common to the study of material
mechanics. For a material to break, it
must pass through all of the stages on
a stress-strain diagram. When material
is either compressed or stretched,
it undergoes an elastic transition.
Relieving stresses under this condition
causes a complete rebound in the
strain imposed on the material. At
some point, the stress becomes great
enough that the material cannot supCHEMICAL
ENGINEERING WWW.CHE.COM NOVEMBER 2013 47
Stress
Stress
Resilience
Resilience
http://WWW.CHE.COM
Chemical Engineering November 2013

Table of Contents for the Digital Edition of Chemical Engineering November 2013
