A ball mill is, essentially, a large cylinder filled with steel balls. Milling is accomplished by rotation of the cylinder, which causes the balls to drop onto materials placed inside. Rotation of the mill is between 4 to 20 RPM (
revolutions per minute) depending on the mill's
diameter, larger mills will have a slower RPM than smalelr mills. As the ball rotates the fine particles come out the end while the larger particles remain in the ball to continue being crushed.
A ball mills
critical speed is the point where the RPM becomes so great that it ends up acting like a
centrifuge, thus the balls no longer fall back but stay on the outside perimeter of the mill. Ball mills operate at 65 to 75 percent of their critical speed.
In English? If the mill spins too fast, the balls will not drop onto the materials but instead will go round-and-round, make sense? Good.
Ball mills can include a
breaker plate, or no breaker plate, those with a breaker plate grinde more efficiently for particles larger than 1 mm. Ball mills range in size from 500 gallon industrial mills with tons of
throughput per minute to
laboratory mills with a gram or less capacity. To produce
submicron particles (less than 1 micron)
vibratory, tumbling, or
high-speed agitation mills are used.
Typical advantages of a ball mill:
- loss-free grinding
- easy to operate and clean
- wet and dry grinding
- visible grinding process
- choice of rinding vessel materials
- continuously adjustable vibration