To select an air classifier mill for your application, you’ll need to share some basic information about your feed
material and your final product requirements with mill suppliers. The information includes your feed material’s particle
size and other characteristics (such as particle shape, bulk density, cohesiveness, Mohs hardness, moisture content,
friability, and softening or melting point) and your final product’s required particle size distribution.
A typical air classifier mill — called avertical air classifier mill because of its classifier wheel’s orientation — is shown in The mill has a round vertical housing enclosing an internal classifier wheel,which has multiple closely spaced vanes (or blades), and an impact rotor that’s mounted in a horizontal position and driven by a motor with from 1 to 600 horsepower, depend ing on the mill size. Impact tools (usually hammers) are mounted around the edge of a rotor disc located below the
classifier wheel; the rotor disc is mounted on a bearing housing with a drive separate from the classifier wheel
drive. A ring-shaped liner, typically a multiple-deflector liner with vertical grooves, surrounds the rotor disc and
hammers. A shroud-and-baffle assembly is usually lo cated above the impact rotor between the liner and the
classifier. The space between the shroud-and-baffle as sembly and the liner forms the grinding zone, and the
space between the assembly and the classifier wheel forms the classification zone.
The mill housing, liner, impact rotor, and shroud-and-baffle assembly together form the grinding chamber. A feed
material inlet is located at one side of the grinding chamber, an air inlet is located below the rotor disc, and a com-
bined product-and-air outlet is located at the classifier wheel’s discharge side.
In operation, air enters through the air inlet in the grinding chamber’s bottom and flows upward from
under the rotor disc, entraining the entering feed material and directing it into the grinding zone. The rotating ham-
mers impact the particles and deflect them into the liner, where the liner’s vertical grooves slow the particles’cir-
cumferential speed and deflect them back into the hammers’ path to maximize the impact force on the particles.
The reduced particles are carried upward by the airflow, and the baffles in the shroud-and-baffle assembly help di-
rect the particles into the classification zone. Fine particles pass through the slots between the classifier wheel’s vanes
and flow with the air out of the product-and-air outlet, while coarse particles that can’t pass through the classifier wheel
are returned to the grinding zone for further reduction
Separate drives for the classifier wheel and impact rotor allow you to adjust each component’s rotational speed independently. Both can be variable-speed drives, but the impact rotor is usually provided with a fixed-speed drive.
The mill in has a coaxialdrive assembly, basically consisting of a drive shaft within
a drive shaft, which reduces the mill’s overall height.