Bead Mill – Grinding & Dispersing

1. General knowledge of bead mill

The bead mill is an apparatus that agitates grinding media (beads) in a cylindrical vessel to grind or disperse minute particles in slurry. The rotor of the mill generates bead motion, which induces intense shear force and impact on the particles. The bead mill is applied to a wide range of applications, such as grinding relatively large particles and dispersing nano particles. Particles of the size of microns or sub micron are effectively reduced into finer particles as fine as tens of nano meters. Energy and impact force that materialize the best condition for each material are adjusted by optimizing the media size, rotor speed, and type of the rotor.

2. The importance of the size of the beads to be selected

The bead size is the most important factor for milling practices. Large beads, bigger than 0.5 mm, are adequate for grinding micron-size particles into sub micron-size ones. Small beads, 0.3 mm or finer, are applied to grinding or dispersing sub micron- or nano meter-size particles. 

For dispersing, a large impact is not necessarily required, and, in addition, smaller beads provide faster processing rates since frequency of contact between a bead and a particle increases. In particular: 

1) The adequate impact energy for milling, which is controlled by the bead size, rotor speed, and mass of the beads charged in the mill, is determined according to the target size and hardness of the particles.

2) The frequency of the impact between a bead and a particle, which is controlled by the rotor speed and bead size, affects the processing rate.

3) The inter-bead space affects the final size of particles after milling. The size of the inter-bead space is proportional to that of beads, so that smaller beads provide more chances for contacting the finer particles. Note that the inter-bead space is the space surrounded by beads when beads are closely packed

Bead selection in grinding processing

In case of grinding large or hard particles, it is necessary to apply high impact energy to the particles. Since the intensity of energy is determined by the mass and speed of beads, large beads with a high speed are needed for grinding large and/or hard particles. When grinding hard particles, such as silica, alumina, and hard natural ores, relatively large beads—larger than 0.3 mm—are used as a grinding medium.

However, large beads are not always suitable for grinding processing. The size of the beads affects the final size of the particles. In case of grinding minute particles, as of 200 nm or finer, beads of the size of 0.1 or 0.3 mm are applied for grinding processing.
In addition, smaller beads shorten the processing time because their wider surface area increases the frequency of impact between beads and particles. Accordingly, if the particles are soft and then the required impact energy is low, milling with smaller beads results in a faster processing rate. Examples of such soft particles are calcium carbonate, iron oxide, agro chemicals, and pharmaceutical chemicals. In case of grinding fine particles of the size of 200 nm or smaller, the selection of beads’ size is quite important. Since the inter-bead space among larger beads is larger, opportunities for the particles to come into contact with the beads are fewer, resulting in poor grinding efficiency. Therefore, processing practice using small beads is required for grinding fine particles.

The size of the beads required for milling particles is 10 to 30 times the maximum particle size of the raw material and 1,000 to 3,000 times the mean particle size after milling. For example, when grinding calcium carbonate of 10 μm as the largest size of raw material to final-size particles of 100 nm, it is recommended to use 0.1 mm beads.

Bead selection in dispersing processing
The basic idea of selecting bead size for dispersing processing is similar to that for grinding processing. Some other conditions being considered are, however, different. The most important among them is the damage on primary particles by the impact of beads. The impact energy required for dispersion is controlled so as not to damage the primary particles and, at the same time, be enough to tear a cluster into primary particles.

Since each particle is made in the intended size that provides appropriate properties designed for the final products, damage on the particles is not desirable. In addition, in case that the impact energy is too high, primary particles get damaged and fine fractions are generated. The fine fractions act as a binder for re-agglomeration of the particles, so that dispersion is not completed even by a long time processing. The impact energy is, therefore, controlled not too intense and not too weak. Applying low-impact energy through small beads is, therefore, important for dispersing processing for obtaining lowly damaged nano particles. In addition, small beads provide high frequency of impacts between the particle and the bead, resulting in a high processing rate.

Generally, it is necessary to select smaller beads in dispersion processing as the primary particles become smaller. Desirable bead size is also 1,000 to 2,000 times the primary particles’. In many cases of modern dispersing practices, the primary particle is mainly 10 to 200 nm in size, so that the bead size to be selected is generally smaller than that for grinding processing. For example, when the primary particle is 100 nm in size, beads of 100 μm or finer are commonly selected. Primary particles that have been recently processed for modern nano materials are 50 nm or finer in size.

3. How to choose the type of bead mills

As explained in the previous section, it is very important to choose proper bead size according to the purpose of processing. It is, therefore, important to select the best method of bead separation, which depends on the bead size.
Selection of the bead separation methods is, therefore, one of the most important factors for choosing the type of bead mills. Bead separation methods are generally grouped into slit, screen, and centrifugal methods, each of which has its preferable bead size. The three methods are explained in the following sections.

Slit separator
This is a method of separating beads from the slurry by a narrow slit gap at the slurry exit. The gap width is about one-third the bead diameter. A bead mill equipped with a slit separator functions with beads of the size of 0.3 mm or larger. This is because when the slit gap is narrower than 0.1 mm, for beads smaller than 0.3 mm, the slit clogs due to entrapment of relatively coarse particles, and the required pressure for slurry to pass through the slit rises largely. This method allows a stable operation without leaking beads even for a highly viscous slurry.

Screen separator
This separator, similar to the slit separator, utilizes the geometry of narrow passes of a screen at the slurry exit. The screen has a mesh with a texture of metal wires that provide narrow gaps for separation of the beads from the slurry. A bead mill equipped with this method functions with beads of size of 0.1 mm or larger. This method can use smaller beads than the slit method, because the screen has a larger opening area for the slurry passage. However, in case of using beads smaller than 0.1 mm, a finer screen is needed, and it tends to clog due to coarse particles in the slurry. Clogging with this method occurs as likely as with the slit method. This method is less favorable for a high viscous slurry.

Centrifugal separator
This is a separator that utilizes centrifugal force for separating beads from the slurry. The separator has plates that line up in a circular manner. The plates induce a rotation of fluid, which is favorable for bead separation. Since the bead has a density 4 to 5 times that of the slurry, the centrifugal force effectively acts as gravitational screening. An important feature of this method is that this method has no narrow gap. There is no need for slurry to pass through a narrow gap: therefore, no clogging occurs even with beads as small as 15 or 30 μm. Thus, this is the best bead separator for micro bead processing applied to 3. How to choose the type of bead mills dispersion of sub micron- and nano meter-size particles. It is also applied to grinding particles of the size of sub micron or nano-meters.

Comparison of the types of separators

When planning grinding or dispersing operation of the bead mill, one should first determine an adequate bead size. The bead size is, as mentioned before, selected according to properties of materials and purpose of the processing. After selecting the bead size, the type of separators is to be selected for the best fit for the bead selected.

Since each of the bead separators has its own strong and weak points, the type of separator is to be selected according to its characteristics and the selected bead size. In the case of grinding particles of micron or sub micron in the final size, slit and screen separators are adopted because the grinding practice requires relatively large beads. In the case of dispersing particles of sub micron in the final size, screen and centrifugal separators are adopted because the favorable bead size is less than 0.3 mm. In the case of dispersing nano particles, a centrifugal separator is selected since the bead size required for dispersion is less than 100 μm.

4. Comparison of horizontal and vertical types

The orientation of the mill is an item for selection of the type of bead mill. There are two types of bead mills: horizontal and vertical. Since the centrifugal force acting on the beads is tens to hundreds times the gravity, the influence of gravity is small enough to be ignored. There is, therefore, no big difference in the processing performance between the two types. However, in terms of operability and maintenance, there are the following advantages and disadvantages:

5. Applications of bead mills

A bead mill is used for various applications such as grinding of food materials, metal oxide for glaze, iron oxide for magnetic tape, dispersing barium titanate for MLCC, and titanium oxide for UV care liquids.
In recent years, especially this decade, needs for nano particle dispersion have increased largely. In addition to conventional applications, bead mills are being applied to dispersing particles of 20 to 100 nm. Such applications are dispersion of organic pigments for LC color filters, zirconia for hard coatings, and barium titanate for advanced MLCCs. These applications need dispersing conditions with low impact energy. Thus, requirements for bead mills that employ micro beads have been increasing.
Another upcoming technology is the grinding of pharmaceutical nano particles (APIｄs). Required is an efficient grinding with less contamination of metal elements, such as zirconium, aluminum, and chromium.