As appeared in May 2017 | PBEI Copyright CSC Publishing www.pbeinternational.com
How grinding can improve your blend
quality
Peter R. Holman Holman Engineering
This article explains how size reduction can help
you achieve a good-quality powder blend and
prevent it from segregating.
mélange de poudres de bonne qualité et à prévenir
la ségrégation.
Triturar para mejorar la calidad de una
mezcla
El siguiente artículo explica cómo la reducción de
partículas puede contribuir a lograr una mezcla en
polvo de alta calidad evitando que se disgregue.
Améliorer la qualité de votre mélange par le
broyage
Cet article explique comment la réduction de
granulométrie peut vous aider à atteindre un
W
hile size reduction is an important operation in
many bulk solids processes, it can be particularly
useful in improving the quality of powder blends. Grinding
one or more of the powders in the batch can ensure that you
not only meet the required blend quality but keep the blend
from segregating before it reaches your customer.
The next time you make pancakes from a mix, look
closely at the powder blend in the package. Notice how narrow the particle size distribution is. We know that segregation is less likely when all the powders in the mix are under
10 microns. Such a blend is cohesive and doesn’t flow well;
therefore it tends not to segregate. So if the blend you’re producing includes powders whose particles are larger than 10
microns, you might solve the segregation problem by grinding everything down to less than 10 microns.
There are several ways to approach this. For example,
you can grind each ingredient down to 10 microns or less,
Wie mahlen die Mischqualität verbessern kann
Dieser Artikel beschreibt, wie die Größenreduktion
der Bestandteile ihnen eine hohe Mischqualität des
Puders geben können, und damit eine Trennung
dieser Bestandteile verhindern können.
then blend all of the ingredients together. Or you can specify
that each ingredient you purchase has a particle size of 10 microns or less. Either way will work. Once you blend the ingredients, you’ll have a nonsegregating blend. (Before going this
route, be sure that your customer can use such a fine blend.)
If it’s not ideal to grind your ingredients down to a
10-micron particle size, you could produce a nonsegregating
blend by using ingredients whose particles are roughly the
same size. For example, if the smallest particles in your batch
are 40 microns, try grinding the others down to 40 microns
or purchasing all of your ingredients at a 40-micron particle
size. Using particles with a tight particle size distribution
usually results in a cohesive blend.
When to grind
When it comes to size reduction, you can grind your ingredients before you blend, while you blend, or after you
blend. Grinding at any of these points is going to help give
Copyright CSC Publishing your product a tighter particle size distribution, making it
more cohesive and less likely to segregate.
I’ve had clients who followed a complex route to get a
quality, nonsegregating product. One client weighed up the
batch, blended it, ground it, and then blended it again. This
seems like a lot of work, but the client eliminated segregation
and kept its customer. It turned out that even with the extra
steps, this was the company’s lowest-cost way to solve its segregation problem.
What to consider when choosing a grinder
If you can’t buy all of your ingredients at the ideal particle size, you’ll have to invest in an appropriate grinder or
other size reduction machine and do the grinding in-house.
While buying grinding equipment can be a significant expense, it can cost less in the long run than frequently reworking blends that segregate or sending your customers
segregated or inconsistent blends.
Different ingredients grind differently. One ingredient
will grind well in one grinder, while another ingredient will
grind well in another. Yet purchasing several grinders to reduce ingredients with diverse characteristics can be costly. In
this case, it may be more economical to grind some ingredients in-house and buy others at the desired particle size.
When you select a grinder, you’ll have to consider your
ingredients’ properties, including toughness, abrasiveness,
particle size, cohesiveness, particle structure, softening point,
fat content, aromatic content, toxicity, flammability, and
bulk density. While this is a lot to consider, all of these properties have an effect on how the ingredient will behave in a
grinder. They also influence how effectively your ingredients
will blend in your batch: The more diverse they are, the less
cohesive your batch is likely to be.
What grinder to choose
Two common ways to break a particle are stressing it between two solid surfaces and stressing it against a solid surface.
Stressing a particle between two solid surfaces is also called
crushing. Materials that break down favorably with crushing
include medium-hard to hard materials, such as fluorite, feldspar, quartz, topaz corundum, and diamond. Several types of
equipment will crush these materials, including the following:
• A jaw crusher operates similarly to a human or animal
jaw. The material to be crushed is placed between two plates
connected on one side by a hinged jaw. Pressure is exerted to
close the jaw, crushing the material.
• A gyratory crusher has a rotating cone-shaped spindle
that fits inside a stationary inverted-cone-shaped bowl. The
material is placed in the bowl and is crushed between the
bowl and spindle.
• A roll crusher generally has two smooth, corrugated, or
toothed counter-rotating rollers. The material to be crushed
is drawn into the small gap between the rollers and crushed.
Stressing a particle against a solid surface is also called
milling. Materials that break down favorably in this type of
equipment include soft to medium-hard materials, such as
talcum, gypsum, salt, calcite, fluorite, and feldspar. Many
types of milling equipment are available, including these:
• Impact mills include rotor breakers, hammermills, and
cage impactors. These mills use a hammer or other device to
strike the material, or they cause the material to be flung at a
breaker plate or other hard surface.
• A jet mill produces very fine particles by using a highspeed, turbulent air jet to cause the particles to impact and
grind against each other.
• A classifier mill is a jet mill that separates the particles
into size fractions during the milling process.
• A ball mill or media mill has a stirring device and a
chamber filled with tiny ceramic or metal balls (media). The
material enters the chamber and is ground by friction between the moving media.
Make sure the grinder you choose can handle the hardness range of your ingredients. For advice on which unit will
help you produce a nonsegregating mixture, consult an independent consultant or equipment supplier.
If customers are rejecting a lot of your product batches
because of segregation problems, adding size reduction to
your process might be the solution. This method isn’t for everyone, but it may work for you. The greatest benefit is that
once this method fixes your bad process, your technical people will be available to develop new products to help your
company grow.
PBEI
For further reading
Find more information on this topic in articles listed
under “Mixing and blending” and “Size reduction” in Powder
and Bulk Engineering/International’s comprehensive article
index at PBE/I’s website, www.pbeinternational.com, and in
publications available in the website’s Bookstore. Additional
articles and other resources can be found in the Article Archive on Powder and Bulk Engineering’s website, www.powderbulk.com. (All articles in PBE’s Article Archive are
available free to registered users.)
Peter R. Holman, retired
Holman Engineering
Ocala, FL USA
[email protected]