Ball mills are devices for grinding materials into powders. It is how many ores are processed, and it is how many common materials–like concrete and gunpowder–are made. The basic idea is to put the materials to be ground into a cylinder that contains steel balls and then rotate the cylinder. A screen on the bottom lets out the powder when it is fine enough.
The outer cylinder that rotates is called the barrel. The bigger it is, the more material you can process at one time. Sometimes, in industrial applications, there will be a series of barrels in decreasing sizes–each one grinding the material into a finer and finer powder. There must be two openings in the barrel–one where new material comes into the barrel and one where the powder goes out. Small ball mills–like those used in a shop or laboratory–solve these problems by having the barrel vertical, with an open top and a screened bottom. The operator adds material when the level goes down and gravity moves the powder out of the barrel and into a collection pan. In industrial applications the barrels are horizontal and sensors detect when the input material is low and the barrel is stopped while an automatic mechanism loads in new material. The output powder is often removed by a vacuum process that takes the powder out through a hollow axis.
The balls must be tougher than the material you are grinding. The size of the balls is a major consideration–the smaller the balls are the finer the powder will be, but the balls must be larger than the average size of the input material in order to quickly reduce it. There is also another reason why the size of the balls is important–there is an important relationship between the size of the balls and the rotation speed of the drum. The ball size limits the rotation speed, because at some speed (depending on ball size relative to the barrel diameter) the balls will all go to the inner surface of the drum and be held there by centrifugal force. When this happens, no grinding takes place. On the other hand, you want the barrel to spin as fast as possible because barrel rotation speed determines how fast the grinding process proceeds.
Chambered Ball Mills
For large industrial applications–with outputs measured in tons per hour–chambered ball mills are used. From the outside, these look like one enormous–hundreds of feet long–rotating barrel. Inside, the barrel is divided up into a series of chambers. There are screens of the appropriate size separating the chambers and a screen separating the last chamber from the output duct. Within each chamber are balls of a different size. The screens keep the balls in the proper chamber. The ball size decreases from the input chamber to the output chamber. Air and clinker (the input material) come in one end and dust goes out the other end.