Milling-Media Review: Part 4
by Dr. Paul Hassall
March 1, 2010
In
the last Milling-Media Review (PCI, March 2009), we introduced some of the parameters
and considerations necessary for operating agitator bead mills. For this issue,
we continue with the same theme, focusing essentially on the nature and
significance of the feed materials and their effects on the overall milling
process.
Operating Parameters
As
a first step, it is important to review the scope of activity for agitator bead
mills and detail, to some degree, the physical nature of the materials
processed. Figure 1 gives an overview of the general field of comminution. It
can be seen that agitator bead mills tend to operate in the range from
approximately a few hundred microns down to nanometer
sizings.
The
feed materials vary considerably from industry to industry, however some of the
basic concepts are common to all. Material hardness, size and density are all
critical considerations, but equally important is the nature of the particles
themselves – whether they are primary particles, aggregates or agglomerates
(Figure 2). All of these factors have significant bearing on the required
milling process.
Process Requirements
The
requirement to reduce particle size or disperse materials is necessary in many
industries to enhance and improve the final properties of particular products.
The actual targets can vary significantly from industry to industry. In the
paint and ink industry for example, it can be optical characteristics,
gloss/transparency, etc. In other industries, it may be a purely physical
consideration, such as the development of active surface area for ceramic
processing. A good example to review is the development of color intensity;
this is usually determined by the median particle size of the product. The
graph in Figure 3 details the intensity development of a particular color with
the median particle size D50.
The actual effects can be seen on Figure 4 where a
micrograph of the pigment particles size is shown next to the pigment color
produced. The level of achieved grind obviously greatly affects the final quality
of the product. The nature of the feed material and the target condition are
important considerations for the milling process.
Comminution and Deagglomeration
Comminution – True comminution is the grinding of primary
particles into smaller sizes. This process requires sufficient energy to break
the structural bond of the material itself.
Deagglomeration
– Deagglomeration is the breaking of agglomerates or aggregates into
primary particles in order to disperse the particles in the medium. This
requires only sufficient energy to break the binding forces (surface forces) of
the particles. Figure 5 illustrates these principles.
Feed Material Size, Hardness and Density
Size
The size of the largest
feed particles will particularly determine the size of the grinding media used.
There is a ‘Stress Energy’, SE, required to break feed particles; in general a
larger SE is required to break larger particles and therefore larger milling
media will be necessary. The media will normally be in the region of 20-40
times larger than the feed particles. Practically, the feed particles need to
fit easily between the voids of the milling media.
Hardness
The hardness of the
feed particles will also impact on the required energy levels. If the particles
are hard and difficult to break, then more energy will be required to break
them down. This may require larger and/or denser milling media or may result in
higher mill energy requirements and/or longer process times. Some typical
mineral hardness figures and densities are shown in Table
1.
The
hardness of the feed particles will also have an effect on the wear rates of
the milling media and the machine parts. Generally harder feed materials will
result in higher mill and bead wear (see Figure 6 for bead wear). For milling
media, the comparative hardness of the feed to the media is a critical factor.
The media ideally should have a surface hardness higher than that of the feed.
If the reverse situation occurs, there can be excessive wear on the media
resulting in rapid loss of milling efficiency. It is also important to consider
that the selection of mill lining materials, their hardness and elasticity
etc., will significantly affect the rate of overall wear.
Density
Feed
material density will also have an effect, although this is often overshadowed
by considerations of concentration and viscosity of the feed. Nonetheless, it
is preferable to use milling media of higher density than all of the target
product particles present in the formulation. It is important to consider that
many products contain a large number of raw materials; various fillers and
pigment types for example. All these materials have different characteristics,
yet they all need processing at the same time, through the same milling
operation. A paint formulation can contain five to 10 different solid
components as shown, for example, in Table 2.
Conclusion
The requirements of the milling process,
(comminution/deagglomeration) and the nature of the feed material, are
important factors when selecting milling media. The first consideration should
be process demand, to ensure that the bead is capable of transferring sufficient
energy to achieve the grind action required. This, however, needs to be
considered against the overall cost effectiveness of the process. The bead
properties, size, etc., need to be matched to process; if this is not achieved,
costs can escalate, particularly in terms of energy consumption and
machine/bead wear.
For further information e-mail paul.hassall@saint-gobain.com.
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