Over many years the continuous development of wet grinding mills has resulted in finer and finer particle size reduction with lower process times. It is well established that small-diameter grinding media (beads) produce finer product particle sizes. Bead sizes down to 0.05 mm are now available. The inherent high-density ceramic material used results in smaller media with higher kinetic energy potential.

However, these can only be used under certain conditions. The bead mill mechanical specification has to be engineered correctly, with particular reference to the separator type (gap/screen) and other rotating/stationary element tolerances (seals, bead eliminators, grinding zone limits, etc.).

Product viscosity is also an important factor as small beads typically have less mass, which in turn can result in a fall in bead velocity under certain rheological conditions.

Most horizontal/vertical mill mechanical aspects within the grinding chamber are not suitable for maintaining constant bead velocity; the grinding zones are typically too far apart. Most horizontal mills are unable to accommodate very small beads for prolonged usage, as eventual mechanical break down (bead entry into separators, seal housing, etc.) and separator blockage will occur. In addition, most bead mills generate the necessary grinding action in a fairly random manner, and cylinders must be packed with a large volume of beads. Generally, horizontal mills give unstable grinding media control as there is a tendency for the media to move continually toward the separator/outlet. In the case of a vertical mill, there is an uneven bead packing density, heavier at the bottom and lighter at the top. In order to maximize the efficiency of these mills, a media charge of up to 70–90% is required. Of course, efficient separation of the product and grinding media is essential, and large volumes of media will result in high process temperatures and pressures, separator damage, and mill wear (cylinders and agitators).

The agitation, by either discs or pegs, imparts shear and attrition forces to the bead mass, which in turn transfers some of this energy into the grinding action. However, most of this energy is lost in frictional heat build-up and it is necessary to incorporate an efficient cooling system. Controlling and managing this otherwise random bead action becomes paramount in order to use the energy being consumed more efficiently.

Whilst most mills will produce, in time, a dispersion where the majority of particles fall into the sub-micron territory, there is usually a ‘big end’ of micron-sized material still in evidence. Further size reduction of these particles is difficult to achieve under the above conditions.

The Microtron™

Now consider the Microtron, which has been developed with the specific task of “sub-micron grinding” in order to produce better-quality products within the coating industry. It is a misconception that grinding finer is simply a function of time when we consider how to quickly obtain 100% nanometer particle size reduction.

First consider the effect of replacing a 1-millimeter diameter bead as a contact object with one thousand 0.1-millimeter diameter beads occupying the same volume.

The contact ratio increases many times and, therefore, the grinding ability of a given mass of beads improves dramatically.

In order to achieve consistent nanometer particle sizes it was essential to specify the milling mechanics accordingly.

· Particular attention to the effective bead mass control.

· The ability to effectively separate product from media.

· Utilizing the smallest available grinding media (down to 0.05mm).

· Media charge volumes of no more than 45%, which result in 100% nanometer particle sizes reduction at low temperatures.

To achieve consistent sub-micron particle sizes it is essential to ensure that the physical properties of the premixed slurry fall into a specific range. Particular attention should always be given to product viscosity and initial particle size. Initial pre-mixes of 50 microns can be reduced to 300 nanometres and below consistently in minimum pass operations. Effective and reliable separation of the product and grinding media is achieved away from the outlet of the mill.

Utilizing both the smallest grinding media size available, along with specific grinding media management control and effective separation within the milling operation, 100% sub-micron sized particles will result. The large particle size end is removed altogether.

The bead velocity is maximized by the design of the main agitator. High-velocity beads moving only short distances are continually recaptured, forming a double spiralling opposing helix in and around the periphery of the main agitator, precisely directing the movement of the bead charge. Separation is also achieved at this stage. Centrifugal force separation is maximized by specific design where the grinding media and product separate accordingly.

The media is held in a controlled and continually interactive double spiralling mass where the centrifugal milling intensity is at its greatest. The product is subject to a consistent and intensely packed bead mass. These conditions do not alter during the milling operation. The product can only escape from the mill via this intense grinding zone.

For more information, e-mail info@cdcmicrotron.com or call (44) 1925 575678.