A: Stephen Miranda, Netzsch Premier Technologies, says:
Reducing particles to nanosize involves one or both of these processes: mild dispersion (disaggregating or deagglomerating — that is, separating particles from one another without changing their primary size or structure) and real comminution (reducing particles below their primary size by grinding and fracturing). Reducing particles to nanosize is challenging and is most efficiently done with the coarse particles in a suspension. Not every mill can do this job. The mill commonly used to reliably reduce coarse particles to nanosize — whether by dispersion or comminution — is the agitator bead mill, as shown in Figure 1.
An agitator bead mill consists of a grinding chamber, an agitator consisting of a rotating shaft equipped with agitator elements, a drive motor, and a media separator (located at the mill’s discharge). The agitator elements are typically disks or pins. The grinding chamber is filled with 9-millimeter to 30-micron grinding media up to 95 percent of the mill volume. The grinding media can be made from materials such as stainless steel and glass as well as advanced ceramic materials such as yttrium-stabilized zirconium oxide and cerium-stabilized zirconium oxide.
In operation, the suspension containing the coarse material is pumped into the mill from a feed tank. The material flows into the grinding chamber and downward into the spaces between the grinding media. As the agitator rotates (at typical tip speeds between 4 and 20 m/s), the media move around the chamber and impart compression and shear forces to the suspended particles, fracturing or dispersing them. Particles reduced to the required fineness discharge through the media separator to a product tank.
The milling process is conducted in one of four modes, as shown in Figure 2. In the single-pass mode the suspension passes through the mill once and is collected in a product tank (Figure 2a). In the pendular (or multipass) mode the suspension passes through the same mill multiple times, traveling from the feed tank to the product tank repeatedly (Figure 2b). In the cascade mode the suspension passes through two connected mills (Figure 2c). In the circulation mode the suspension can be continuously pumped through the mill multiple times, each with a short residence time, until the desired particle size is reached (Figure 2d).
Each mode has advantages and disadvantages. The main advantage of the single-pass mode is simplicity for those applications where the end particle size can be reached in a single pass. However, there’s no guarantee that every particle passes through the mill’s highest-energy zones; therefore, the final particle size distribution (PSD) may be wider than desired.
The pendular mode ensures that more of the particles pass through the mill’s highest-energy zones. Using a high flowrate and two or more passes, the required particle size and a steeper PSD may be reached with a lower total residence time. This mode’s higher flowrate also results in less material heating, but the material is handled two or more times, which is undesirable in some applications.
The cascade mode allows the use of two mills with different grinding media sizes — a larger size in the first mill takes a coarse feed material to a size that allows the next mill to use finer media to reach the final desired particle size. In this way two-step grinding is accomplished in a single process.
If the material requires more than two or three passes, the circulation mode may be the best option. In this mode, all particles ultimately pass through the mill’s highest-energy zones and achieve the steepest PSD and finest particle size.
The circulation mode’s high flowrate also gives the material a short residence time, keeping both the material and the mill cooler and allowing accurate control of the material temperature.
Netzsch Premier Technologies, Exton, PA, manufactures wet grinding and dispersing equipment.