Types of Grinding Mills
A ball mill uses centrifugal force and many metal balls to reduce substances into a fine powder. It is an important tool for grinding work and is used mainly on materials such as cement, glass and chemical fertilizer. Its primary design is a large horizontal metal cylinder with two warehouses, or compartments, containing metal balls and scale-boarded, or grooved, edges. The cylinder is then rotated to create centrifugal force to lift the balls and drop them on the material, grinding them. The first warehouse is made to start the grinding process and when the material is made fine enough, it enters through specified holes into the second warehouse, which--with different specification balls and scale boards--grinds the material into a finer powder.
Like a ball mill, a rod mill is a large horizontal cylinder that uses centrifugal force to grind material. However, instead of balls, it uses long metal rods that are placed almost parallel to each other and the cylinder. The walls of the grinder are also scale-boarded, so when the rods tumble, they crush the material against them. A rod mill creates a relatively unified gradient, and is great for reducing very coarse material into a size manageable by ball mills.
This mill does not use a cylinder principle; instead, it uses a pair of discs held close and parallel to each other to create a grinding chamber out of the resulting gap. One disc is held stationary while the other spins at great speed to grind the material until it is fine enough to exit from the gap at the edges of the disc. Some attrition mills have both discs spin at counter rotations to quicken this process. A hole is made in the center of the disc to provide a feed for the material and the gap is made smaller towards the disc's edge, making the material finer and finer before it exits the chamber.
In most cases, industries size reduction falls into two general categories: crushing and milling. Crushing typically means reducing large chunks to sizes about ½ in. to ¾ in. in diameter or smaller. Milling usually means reducing material to sizes in the low micron or even nan-size range. Crushing is mostly done dry while attrition milling maybe done wet or dry. Grinding milling technology plays a significant role in three major areas of chemical processing; particle size reduction of chemicals, mixing and milling of several chemicals to form new chemical compounds and activation or liberation of chemical raw materials. Proper selection of grinding mill equipment is necessary for the success in all three areas.