Advances in Pin Mill Technology
Advances in Pin Mill Technology
Advances in Pin Mill Technology
F
or decades, the ultimate per-
formance in fine mechanical
milling performance has been
provided by the air-swept
classifying mill (ASCM). Perfectly
suited to a broad range of non-
abrasive milling applications, the
air-swept classifying mill is a single
machine used to carry out a two-
step 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 perfor-
mance advantages over other op-
tions, 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 classify-
ing mill has been widely held as the more sizable ongoing investment. a red blood cell). Despite being rela-
gold standard in milling technology. Historically, while pin mills have al- tively costly to operate and requir-
Today, those processors seeking ways used less energy, been simpler ing an experienced operator, users
to upgrade or purchase new mill- to operate, and been more economi- throughout the chemical process in-
ing equipment have several different cal overall, they were not as effec- dustries (CPI) have tended to gravi-
types of impact milling technolo- tive in terms of their ability to reach tate toward them, thanks to several
gies to select from. The pin mill is the same levels of fine-particle size. unique features beyond fineness.
one option that has seen major ad- A primary goal of mill manufactur- Such desirable attributes include the
vancements over the last decade, ers over the last decade has been following:
enabling performance that rivals that to close this gap in particle-size ca- The air-sweeping feature can cool
of the air-swept classifying mill. Until pabilities, to take advantage of the materials
recently, significant differences be- improvement in cost-per-ton figures These systems have the ability to
tween air-swept classifying mills and that a pin mill can yield. To help read- hold and control particles through
pin mills were obvious. In general, ers better understand a comparison mechanical classification
air-swept classifying mills provided of the economics, a brief introduc- The design allows operators to
the ultimate milling performance, but tion to each of these two technolo- remove oxygen from the process
also carried a large capital invest- gies is presented below. using inert gases, such as nitrogen
ment, as they required the purchase or argon, thereby reducing the risk
of a baghouse filter (to separate the Air-swept classifying mills of spark or ignition
milled material from the air stream), Air-swept classifying mills (Figure 1) Air-swept classifying mills can pro-
a high-static-pressure blower and are impact mills that can grind ma- duce particles in small sizes that
auxiliaries. These auxiliaries and the terials into very fine powders. The other mechanical mills cannot
operation of the mill increased the products range in size from about How air-swept classifying mills
cost per ton of material processed 100 m (the diameter of a human work. Air-swept classifying mills per-
by the mill, and contributed to a hair), down to 5 m (the diameter of form both milling and air-classifying
60 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM FEBRUARY 2017
of spinning blades called the air- of air-swept classifying mills offer a
classifying wheel. The wheel acts as standard industrial version and cus-
a screen and only lets the particles tomized versions. Customization
that are small enough through. The options typically include specific,
larger particles remain in the mill for duty-purpose innovations to suit in-
additional grinding. In its simplest dividual markets.
form, fine particles exit the mill and One manufacturer offers a variant
are collected in a baghouse product of the traditional air-swept classifying
collector. Particle size is adjusted by mill that is specifically designed for
increasing or decreasing the clas- food ingredients. It presents mate-
sifier speed. Airflow rate and feed rial directly to the classifying wheel
rate affect the residence time in first, to reduce the energy used for
the chamber, and the action of the grinding on specification mate-
sweeping air prevents heat buildup. rial, and then uses hammer blocks
Types of air-swept classifying to grind the material. The standard
mills. Air-swept classifying mills materials of construction change to
vary by size, speed and air flow. Dif- stainless steel and the fasteners are
ferent systems use various impact changed to no-tool access (this fea-
FIGURE 2. Pin mills can be configured in many
ways to suit the needs of the application (for elements, materials of construc- ture has helped the design to gain
instance, using vertical or horizontal shaft, pneu- tion, impact walls, impact area ge- U.S. Dept. of Agriculture (USDA)
matic or gravity-feed options, varying materials of ometries and directions of mate- acceptance). Another manufacturer
construction and pin configurations) and more rial flow. These options are used offers a variant that is specifically de-
in combination to produce smaller signed for grit reduction of low-den-
functions in a single step. As the inlet particles, or a product with greater sity materials, such as carbon black.
product comes into contact with uniformity, less contamination, or The feed stream is introduced below
a series of spinning impact blocks, any such characteristics that may (rather than above) the rotor, and the
the material is smashed into fine be deemed desirable for a particu- mill incorporates an external coarse
particles that pass through a cage lar application. Most manufacturers recycling system.
Food Ingredients
Size Reduction
Herbicides Minerals
Wet & Dry Size Reduction
Steel & Ceramic Lined Mills Nutraceuticals Pesticides
Jars & Jar Rolling Mills Pharmaceuticals Pigments
Quality &
Innovation Since 1911
5
speeds (Figure 2). Gentle handling,
4 ASCM performance such as mixing and blending, hap-
Pin mill performance pens at lower speeds so more of
3
the energy can be efficiently directed
2 into the product without degrading
1 it. Individual pin mill designs feature
different types of rotors and rotor
0
0 10 20 30 40 50 60 70 80
diameters, varying types, numbers,
Particle size, m and orientations of pins, different ca-
pacities, feed options and liners to
FIGURE 3. Shown here is a comparison of the particle-size distribution for a pin mill versus an air- produce different results.
classifying mill handling sodium bicarbonate. As shown, the pin mill was not able to achieve the same
maximum fineness as the final product produced by the air-swept classifying mill
Types of pin mills
Pin mills to provide controlled atmospheres. The type of pin mill to consider for
Modern impact pin mill technol- Constructed from stainless steel, an application depends on the size,
ogy. Pin mills use varying degrees carbon steel, and many other ma- density and type of product that is
of impact and agitation to process terials, they are available for applica- supplied and desired, and the envi-
many types of materials. They per- tions in the chemical process, food, ronment the mill will be working in.
form a variety of functions, helping pharmaceutical, grain milling and Pin mills can be customized to fit
users to reduce the size of bulk sol- mineral industries. with pressure-pneumatic or gravity-
ids, deagglomerate materials, break How pin mills work. There are sev- feed systems, and they can be con-
apart fibers in natural materials, eral types of pin mills, but all of them structed from different materials ac-
densify, fluff, mix, blend, disperse achieve the same end: producing cording to the application.
and homogenize the product. They homogeneous products. They work Horizontal and vertical pin mills.
provide a range of speeds. Rela- by introducing the feed product onto The name indicates the orienta-
tively slow mixing and blending typi- various spinning rotors with different tion of the rotor. Some applications
cally has rotor speeds on the order configurations of pins or blocks that have special requirements so they
of 6,800 ft/min; high-impact grind- act as impactors. The number and can only use one or the other. For
ing may have speeds up to 36,000 orientation of the pins determines instance, those that need a screen
ft/min. Todays designs are sealed how material is processed, and the at the bottom require a vertical mill.
Centrifugal impact pin mills.
These mills are used to reduce me-
dium- to low-density materials to
fine, uniform sizes of 50325 mesh
(30044 microns, m), for applica-
tions where low wear is critical. They
are widely used in many low-wear
markets, including food-product
processing, such as corn fraction-
ation, flours, starch densification,
corn slurry, sugar and sweeteners,
and semi-abrasive material applica-
tions, such as aluminum sulfate, fer-
tilizer, gypsum, soda ash, tungsten
and other powder de-agglomera-
tion (including tungsten), talc, clay
and kaolin.