Chemical Engineering February 2017 - 60

Solids Processing
Advances in Pin Mill Technology
Improvements provide finer grinding at lower energy costs compared to air-swept
classifying mills that have long reigned supreme in particle-size-reduction efforts
Sam Rajkovich
Sturtevant
F
or decades, the ultimate performance
in fine mechanical
milling performance has been
provided by the air-swept
classifying mill (ASCM). Perfectly
suited to a broad range of nonabrasive
milling applications, the
air-swept classifying mill is a single
machine used to carry out a twostep
process. The process both
reduces particle size and limits the
output of particles using mechanical
classification - only delivering those
particles of a predetermined, easily
controlled, fine size as the finished
product exiting the mill. Historically,
this technology has offered performance
advantages over other options,
in terms of its ability to deliver
highly controlled particle sizes for a
broad range of applications, from
powder coatings to paper fillers.
Until recently, the air-swept classifying
mill has been widely held as the
gold standard in milling technology.
Today, those processors seeking
to upgrade or purchase new milling
equipment have several different
types of impact milling technologies
to select from. The pin mill is
one option that has seen major advancements
over the last decade,
enabling performance that rivals that
of the air-swept classifying mill. Until
recently, significant differences between
air-swept classifying mills and
pin mills were obvious. In general,
air-swept classifying mills provided
the ultimate milling performance, but
also carried a large capital investment,
as they required the purchase
of a baghouse filter (to separate the
milled material from the air stream),
a high-static-pressure blower and
auxiliaries. These auxiliaries and the
operation of the mill increased the
cost per ton of material processed
by the mill, and contributed to a
60
FIGURE 1. The air-swept classifier mill carries out size
reduction and classification in a single process, and
particle size can be controlled by adjusting such variables
as airflow rate, feed rate, classifier speed and
residence time in the chamber.
more sizable ongoing investment.
Historically, while pin mills have always
used less energy, been simpler
to operate, and been more economical
overall, they were not as effective
in terms of their ability to reach
the same levels of fine-particle size.
A primary goal of mill manufacturers
over the last decade has been
to close this gap in particle-size capabilities,
to take advantage of the
improvement in cost-per-ton figures
that a pin mill can yield. To help readers
better understand a comparison
of the economics, a brief introduction
to each of these two technologies
is presented below.
Air-swept classifying mills
Air-swept classifying mills (Figure 1)
are impact mills that can grind materials
into very fine powders. The
products range in size from about
100 µm (the diameter of a human
hair), down to 5 µm (the diameter of
a red blood cell). Despite being relatively
costly to operate and requiring
an experienced operator, users
throughout the chemical process industries
(CPI) have tended to gravitate
toward them, thanks to several
unique features beyond fineness.
Such desirable attributes include the
following:
* The air-sweeping feature can cool
materials
* These systems have the ability to
hold and control particles through
mechanical classification
* The design allows operators to
remove oxygen from the process
using inert gases, such as nitrogen
or argon, thereby reducing the risk
of spark or ignition
* Air-swept classifying mills can produce
particles in small sizes that
other mechanical mills cannot
How air-swept classifying mills
work. Air-swept classifying mills perform
both milling and air-classifying
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
FEBRUARY 2017
http://WWW.CHEMENGONLINE.COM
Chemical Engineering February 2017

Table of Contents for the Digital Edition of Chemical Engineering February 2017
