A ball mill is a typeof grinder used to grind and blend materialsfor use in mineral dressing processes, paints, pyrotechnics, ceramics and selective laser sintering.

Principle

A ball mill works on theprinciple of impact and attrition: size reduction is done by impact as theballs drop from near the top of the shell.

Construction

A ball mill consists of ahollow cylindrical shell rotating about its axis. The axis of the shell may beeither horizontal or at a small angle to the horizontal. It is partially filledwith balls. The grinding media is the balls, which may be made of steel (chromesteel), stainless steel, ceramic, or rubber. The inner surface of thecylindrical shell is usually lined with an abrasion-resistant material such asmanganese steel or rubber. Less wear takes place in rubber lined mills. Thelength of the mill is approximately equal to its diameter.

Working

In case of continuouslyoperated ball mill, the material to be ground is fed from the left through a60° cone and the product is discharged through a 30° cone to the right. As theshell rotates,the balls are lifted up on the rising side of the shell and thenthey cascade down (or drop down on to the feed), from near the top of theshell. In doing so, the solid particles in between the balls and ground arereduced in size by impact.

Applications

The ball mill is used forgrinding materials such as coal, pigments, and felspar for pottery. Grindingcan be carried out either wet or dry but the former is performed at low speed.Blending of explosives is an example of an application for rubber balls.Forsystems with multiple components, ball milling has been shown to be effectivein increasing solid-state chemicalreactivity.Additionally, ball milling has been shown effective for productionof amorphous materials. 

Description

A ballmill, a type of grinder, is a cylindrical device used ingrinding (or mixing) materials like ores,chemicals, ceramic raw materials and paints. Ball mills rotate around ahorizontal axis, partially filled with the material to be ground plus thegrinding medium. Different materials are used as media, including ceramic balls, flint pebbles and stainless steel balls.An internal cascading effect reduces the material to a fine powder. Industrialball mills can operate continuously, fed at one end and discharged at the otherend. Large to medium-sized ball mills are mechanically rotated on their axis,but small ones normally consist of a cylindrical capped container that sits ontwo drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler functionson the same principle. Ball mills are also used in pyrotechnics andthe manufacture of black powder, but cannot be used in thepreparation of some pyrotechnic mixtures such as flash powder becauseof their sensitivity to impact. High-quality ball mills are potentiallyexpensive and can grind mixture particles to as small as 5 nm,enormously increasing surface area and reaction rates. The grinding works on theprinciple of critical speed. The critical speed can be understood as that speedafter which the steel balls (which are responsible for the grinding of particles) start rotating along the direction of the cylindrical device; thuscausing no further grinding.

Ball mills are usedextensively in the mechanical alloying process in which they are not only used forgrinding but for cold welding as well, with the purpose of producing alloysfrom powders.