Ball mill

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Ball mill

A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.

Description

Bench top ball mill

High-energy ball milling

A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding 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. Industrial ball mills can operate continuously, fed at one end and discharged at the other end. Large to medium-sized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the manufacture of black powder, but cannot be used in the preparation of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. High-quality ball mills are potentially expensive and can grind mixture particles to as small as 5 nm, enormously increasing surface area and reaction rates. The grinding works on principle of critical speed. The critical speed can be understood as that speed after which the steel balls (which are responsible for the grinding of particles) start rotating along the direction of the cylindrical device; thus causing no further grinding.

Ball mills are used extensively in the Mechanical alloying process in which they are not only used for grinding but for cold welding as well, with the purpose of producing alloys from powders.

Grinding media

Lead antimony grinding media with aluminum powder.

A ball mill inside the Mayflower Mill near Silverton, Colorado.

There are many types of grinding media suitable for use in a ball mill, each material having its own specific properties and advantages. Common in some applications are stainless steel balls. While usually very effective due to their high density and low contamination of the material being processed, stainless steel balls are unsuitable for some applications, including:

• Black powder and other flammable materials require non-sparking lead, antimony, brass, or bronze grinding media
• Contamination by iron of sensitive substances such as ceramic raw materials. In this application ceramic or flint grinding media is used. Ceramic media are also very resistant to corrosive materials.

High density alumina media (90–95% alumina) is widely used in the ceramic industry to grind clay bodies, frits, glazes and other ingredients. It is more expensive than silica / silex media but is more efficient

Varieties

Aside from common ball mills there is a second type of ball mill called Planetary Ball Mill. Planetary ball mills are smaller than common ball mills and mainly used in laboratories for grinding sample material down to very small sizes. A planetary ball mill consists of at least one grinding jar which is arranged eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to that of the grinding jars (ratio: 1:-2 or 1:-1 or else). The grinding balls in the grinding jars are subjected to superimposed rotational movements, the so-called Coriolis forces. The difference in speeds between the balls and grinding jars produces an interaction between frictional and impact forces, which releases high dynamic energies. The interplay between these forces produces the high and very effective degree of size reduction of the planetary ball mill.

History

Devices for shaking materials along with hard balls might be old, but it was not until the industrial revolution and the invention of steam power that a machine could be built. It is reported to have been used for grinding flint for pottery in 1870 ^[1].

See also

• Cement mill
• Tumble finishing

References

External links