Ball Mill

A ball mill is an example of a comminution method which produces size reduction by both impact and attrition of particles. The ball mill consists of a horizontally rotating hollow vessel of cylindrical shape with the length slightly greater than its diameter.

The mill is partially filled with balls of steel or pebbles, which act as the grinding medium. If pebbles are used, it is known as a pebble mill. Conversely, if rods or bars are used, it is known as a rod mill. The rod mill is particularly useful with sticky materials that would hold the balls together because the greater weight of the rods causes them to pull apart.

The tube mill is a modified ball mill in which the length is about four times that of the diameter and in which the balls are somewhat smaller than in a ball mill. Because the material remains in the longer tube mill for a greater length of time, the tube mill grinds more finely than the ball mill.

The ball mill may be modified to a conical shape and tapered at the discharge end. If balls of different sizes are used in a conical mill, they segregate according to size. This segregation provides progressively finer grinding as the material flows axially through the mill.

Ball mill in operation showing correct cascade action.
Fig.1: Ball mill in operation showing correct cascade action.

Description of Ball mill

In a ball mill rotating at a slow speed, the balls roll and cascade over one another, creating an attrition action. As the speed increases, the balls are carried up the sides of the mill and fall freely onto the material, producing an impact action that is responsible for most of the size reduction (see Fig. 1). Ball milling thus combines both impact and attrition. When the speed is increased sufficiently, the balls are held against the mill casing by centrifugal force and revolve with the mill. The critical speed is the speed at which the balls just begin to centrifuge with the mill. At this critical speed, the centrifugal force equals the weight of the ball, and the critical angular velocity, ωc, can be expressed as:

$$ \omega_c=\sqrt{\frac gr} $$

where, r is the radius of the ball mill. At and above the critical speed, no significant size reduction occurs. The critical speed nc is given by the equation:

$$ n_c=\frac{76.6}{\sqrt D} $$

where D is the diameter of the mill.

A larger mill reaches its critical speed at a slower revolution rate than a smaller mill. Ball mills are operated at from 60 to 85% of the critical speed. Over this range, the output increases with the speed; however, the lower speeds are for fi ner grinding. An empiric rule for the optimum speed of a ball mill is:

$$ n=57-40\;\log(D) $$

where n is the speed in revolutions per minute and D is the inside diameter of the mill in feet. In practice, the calculated speed should be used initially in the process and modified as required.

Advantages and Disadvantages of Ball mill

In addition to being used for either wet or dry milling, this method has the advantage of being used for batch or continuous operation. In a batch operation, unstable or explosive materials may be sealed within an inert atmosphere and satisfactorily ground. Ball mills may be sterilized and sealed for sterile milling in the production of ophthalmic and parenteral products. The installation, operation, and labor costs involved in ball milling are low. Finally, the ball mill is unsurpassed for fine grinding of hard and abrasive materials.


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