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.
Principle
A ball mill works on the principle
of impact: size reduction is done by impact as the balls drop from near the top
of the shell.
Construction
A ball mill consists of a hollow cylindrical shell rotating about its axis. The axis of the shell may be either horizontal or at a small angle to the horizontal. it is partially filled with balls. The grinding media is the balls, which may be made of steel (chrome steel), stainless steel or rubber. The inner surface of the cylindrical shell is usually lined with an abrasion-resistant material such as manganese steel or rubber. Less wear takes place in rubber lined mills. The length of the mill is approximately equal to its diameter.Working
In case of continuously operated ball mill, the material to be ground is fed from the left through 60° cone and the product is discharged through a 30° cone to the right. As the shell rotates, the balls are lifted up on the rising side of the shell and then they cascade down (or drop down on to the feed), from near the top of the shell. in doing so, the solid particles in between the balls are ground and reduced in size by impact.Applications
The ball mill is used for grinding materials such as coal, pigments, and felspar for pottery. Grinding can be carried out either wet or dry but the former is carried at low speed.
Description
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
the 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.
The ball mill is a key piece of
equipment for grinding crushed materials, and it is widely used in production
lines for powders such as cement, silicates, refractory material, fertilizer,
glass ceramics, etc. as well as for ore dressing of both ferrous non-ferrous
metals. The ball mill can grind various ores and other materials either wet or
dry. There are two kinds of ball mill, grate type and overfall type due to
different ways of discharging material. There are many types of grinding media suitable
for use in a ball mill, each material having its own specific properties and
advantages. Key properties of grinding media are size, density, hardness, and
composition.
- Size: The smaller the media particles, the smaller the particle size of the final product. At the same time, the grinding media particles should be substantially larger than the largest pieces of material to be ground.
- Density: The media should be denser than the material being ground. It becomes a problem if the grinding media floats on top of the material to be ground.
- Hardness: The grinding media needs to be durable enough to grind the material, but where possible should not be so tough that it also wears down the tumbler at a fast pace.
- Composition: Various grinding applications have special requirements. Some of these requirements are based on the fact that some of the grinding media will be in the finished product. Others are based in how the media will react with the material being ground.
- Where the color of the finished product is important, the color and material of the grinding media must be considered.
- Where low contamination is important, the grinding media may be selected for ease of separation from the finished product (i.e.: steel dust produced from stainless steel media can be magnetically separated from non-ferrous products). An alternative to separation is to use media of the same material as the product being ground.
- Flammable products have a tendency to become explosive in powder form. Steel media may spark, becoming an ignition source for these products. Either wet-grinding, or non-sparking media such as ceramic or lead must be selected.
- Some media, such as iron, may react with corrosive materials. For this reason, stainless steel, ceramic, and flint grinding media may each be used when corrosive substances are present during grinding.
The grinding chamber can also be
filled with an inert shield gas that does not react with the material being
ground, to prevent oxidation or explosive reactions that could occur with
ambient air inside the mill.
Advantages
of the ball mill
Ball milling boasts several
advantages over other systems: the cost of installation, power and grinding
medium is low; it is suitable for both batch and continuous operation,
similarly it is suitable for open as well as closed circuit grinding and is
applicable for materials of all degrees of hardness.
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