Pin Mill Basics
A pin mill reduces dry bulk materials to medium fineness by impact
— that is, by hitting or striking particles with high energy to break them. The mill can accept feed materials with a particle size from 20 microns to 0.5 inch and a Mohs hardness up to about 4, reducing them to particles as small as 10 microns, depending on the mill type and feed size. This versatile machine handles a wide range of materials; some typical examples are hydrated lime, sugar, wet corn slurry, food ingredients, and fertilizers.
Pin mills are available in two basic configurations: horizontal or vertical. In a horizontal pin mill, as shown in Figure A, stationary pins are screwed or bolted onto a stationary disc (called a stationary mill plate
), which is located above a rotating disc (called the rotor
). Pins are also mounted on the rotor. In a vertical pin mill, two discs mounted with pins are oriented in a vertical plane: one disc is stationary and mounted on a swing door while the other rotates. In both pin mills, feed material enters at the center of the discs, where centrifugal force causes the particles to accelerate outward toward the disc edges and pass through the pins on the discs. The exiting particles impact a stationary outer ring (called an impact wall
) or liner, typically consisting of stator pins or a smooth or corrugated wall, which further reduces the particles. On the horizontal pin mill, the discharged material is collected in a cone hopper, which is removable to provide access to the rotor and pins.
Unlike many other impact mills, the pin mill operates without an outlet screen and thus isn't subject to screen blinding problems. Washing the mill is also easier and faster than with many other size reduction machines, a benefit for sanitary and other applications that require frequent washdown. To help convey on-size particles out of the mill or to cool a heat-sensitive material during grinding, the unit can be equipped with an air system that blows air straight through the mill at rates from 300 to 1,000 cfm.