A ball mill also known as pebble mill or tumbling mill is a milling machine that consists of a hallow cylinder containing balls; mounted on a metallic frame such that it can be rotated along its longitudinal axis. The balls which could be of different diameter occupy 30 – 50 % of the mill volume and its size depends on the feed and mill size. The large balls tend to break down the coarse feed materials and the smaller balls help to form fine product by reducing void spaces between the balls. Ball mills grind material by impact and attrition.
The degree of milling in a ball mill is influenced by;
a. Residence time of the material in the mill chamber.
b. The size, density, and number of the balls.
c. The nature of the balls (hardness of the grinding material)
d. Feed rate and feed level in the vessel.
e. Rotation speed of the cylinder.
Several types of ball mills exist. They differ to an extent in their operating principle. They also differ in their maximum capacity of the milling vessel, ranging from 0.010 liters for planetary ball mills, mixer mills, or vibration ball mills to several 100 liters for horizontal rolling ball mills.
1. The small and average capacity ball mills are used for the final grinding of drugs or for grinding suspensions.
2. The maximum capacity ball mills are used for milling ores prior to manufacture of pharmaceutical chemicals.
1. It produces very fine powder (particle size less than or equal to 10 microns).
2. It is suitable for milling toxic materials since it can be used in a completely enclosed form.
3. Has a wide application.
4. It can be used for continuous operation.
5. It is used in milling highly abrasive materials.
1. Contamination of product may occur as a result of wear and tear which occurs principally from the balls and partially from the casing.
2. High machine noise level especially if the hollow cylinder is made of metal, but much less if rubber is used.
3. Relatively long milling time.
4. It is difficult to clean the machine after use.