Dense Phase Milling - Is It For
You?
Guest article by Don Erkkila & Charlie Regenhard, CCE Technologies
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The terminology "dense
phase" was selected as being a more descriptive term of this
fluid energy milling process than fluidized bed. The principles
involved relating to particle acceleration and breakage in the
milling process are no different than those used in other types of
free-jet pulverizers, the only difference being the degree of solids
loading in the comminution zone, hence dense phase.
Operational Benefits
The primary benefit of the dense
phase mill is the increased probability of collision due to the high
solids loading. The incorporation of a forced vortex classifier
within the milling chamber allows utilization of the high degree of
particle dispersion for improved separation. This combination
results in narrow product distributions with close top size control
at lower specific energy levels in comparison to other fluid energy
milling concepts. Fluidized bed (Dense phase) micronization utilizes
turbulent free jets in combination with a high efficiency
centrifugal classifier in a common housing as shown in Figure 1.
 Feed
is introduced into the common housing through either a double
flapper valve or injector. Flooding the pulverizing zone to a level
above the grinding nozzles forms the mill load. Turbulent free jets
are used to accelerate the particles for impact and breakage. After
impact the fluid and size reduced particles leave the bed and travel
upwards to the centrifugal classifier where rotor speed will define
what size will continue with the fluid through the rotor and which
will be rejected back to the particle bed for further size
reduction.
The high degree of particle
dispersion leaving the pulverizing zone aids in the efficient
removal of fine particles by the classifier. Operating parameters of
rotor speed, nozzle pressure, and bed level allows for optimizing
productivity, product size, and distribution shape (slope). A
low-pressure air purge is used to seal the gap between the rotor and
the outlet plenum eliminating particles bypassing the rotor and
allowing for close top size control.
Philosophy of Dense Phase Mill
Operation
Product Top Size Control:
The high efficiency classifier incorporated into the fluid bed
allows for prediction of product size using the general theory for
particle separation in a centrifugal field.
-
Where D50 is the "cut
size", k is a constant dependent on the flow pattern
(determined experimentally). Do is the diameter of the
separation zone, H is the height of the separation zone, µ is fluid
viscosity, Q is volumetric airflow rate, N is rotor rotational speed
and Þp is true particle density.
The above expression defines the
effects of the operating parameters on product size. Given a unit
size and feed material, an empirical definition of this relationship
can be derived for product size prediction. For example, assuming
the classifier cut size is equivalent to the 95 percentile of the
product size distribution, Figure 2 shows the comparison of
theoretical and actual, using a dense phase mill processing silica.
Control Operators
Rotor Speed: Primary product
size control, the usual increase/decrease.
Mill load: Mill load will
define the maximum throughput. This load will also be the point at
which the minimum average particle size will be obtained given the
same rotor speed and nozzle pressure. Slope of the distribution
maybe steeper at a mill load above or below this optimum.
Nozzle Pressure: Airflow rate
can be decreased or increased by reducing or increasing pressure
accordingly. Slope of the distribution will increase with reduced
pressure. (Some data has been published with pressure as low as 45
psig, obviously at the sacrifice of throughput.) Obviously, reduced
pressure will allow for finer products.
Nozzle Size: Larger nozzles at
high pressure extend the particle size range upward in a given unit
size (with increased product rate).
Feed Size: Reduced feed stock
size will have a definite effect on productivity and is feed
material dependent.
Dense Mill Features
- Mill load controlled through use
of load cells to optimize performance at maximum productivity
and /or slope of product distribution.
- Choice of introducing feed using
an injector or by gravity drop using an airlock or double
flapper valve.
- Rotor purge to optimize product
top size control.
- Low-pressure purge air is used
instead of compressed air.
- Rotor purge air gap is adjustable
from the exterior.
- Wear resistant coatings for
abrasive applications.
Summary
When should you consider using dense
phase milling? When you want to mill to a narrow product size
distribution, when you need to have top size control or when you are
processing temperature-sensitive materials. For example, dense
phase mills are ideal for processing abrasives, toners, powder
coatings and other fine materials. Their simple, cost-effective
design coupled with low maintenance might be the solution for your
milling requirements.
About Our Author
For more information you can contact
our authors at:
-
Mr. Don Erkkila & Charlie Regenhard
CCE Technologies, Inc.
7555 95th Street South
Cottage Grove, MN 55016
Telephone: 651-688-2656
Fax: 651-688-8989
Web site:
http://www.ccetechnologies.com/
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