Jet mill

Compressed gas is forced into the mill through nozzles tangent to the cylinder wall, creating a vortex.

The gas leaves the mill through a tube along the axis of the cylinder.

Solid particles in the mill are subject to two competing forces: The drag on small particles is less than large particles, according to the formula derived from Stokes' law, where V is the flow settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf ), g is the gravitational acceleration (m/s2), ρp is the mass density of the particles (kg/m3), ρf is the mass density of the fluid (kg/m3), μ is the dynamic viscosity (kg /m*s), and R is the radius of the spherical particle (m).

The formula shows that particles will be pulled toward the wall of the mill according to the square of their radius or diameter.

Large particles will continue the comminution process, until they are small enough to stay in the center of the mill where the discharge port is located.

Illustration of a jet mill. Red arrow shows material entering the mill. Blue arrows show compressed air entering and circulating in the mill. Green arrow shows small particles leaving mill.
Flow past a solid particle reacting to centrifugal force: streamlines , drag force F d and force by gravity or centrifugal force F g .
F d points toward the center of the mill, and F g points to the wall.