Ball mill || Pharmaceutics || Trekpharma || definition or Working principle or Construction or Advantages or Disadvantages or Application

 

BALL MILL

CONSTRUCTION:

The ball mill consists of a hollow cylinder rotated on its horizontal axis.

Inside the cylinder balls or pebbles are placed.

Cylinder:

Cylinder may be made up of metal, porcelain or rubber.

Rubber reduces the abrasion.

Diameter of the cylinder ranges from 1 to 3 m in pharmaceutical practice.

Balls:

Balls occupy about 30 to 50% of the volume of the cylinder.

Diameter of the balls depends on the feed size and diameter of the cylinder.

The diameter of balls ranges from 2 cm to 15 cm.

Balls may be of metal, porcelain or pebbles.

Working Principle:

Works on the principle of Impact and Attrition.

Larger particles are fed through an opening of the cylinder.

The opening is closed.

The cylinder is rotated at the critical speed of ball mill.

The optimum size reduction in a ball mill depends on the following factors:

Feed quantity:

Too much feed will produce cushioning effect and too little feed will produce loss of efficiency of the mill.

Speed of rotation of the cylinder:

At low speed the mass of balls will slide or roll over each other and only a negligible amount of size will be reduction will take place.

At high speeds, balls will be thrown out to the wall of the cylinder due to centrifugal force and no grinding will occur.

At 2/3rd speed at which centrifugation just occurs is called the critical speed of the ball mill.

At this speed the balls are carried almost to the top of the mill and then fall in a cascade across the diameter of the mill.

By this means the maximum size reduction is obtained by impact of the particles between the balls and by attrition between the balls.

Generally, it is 0.5 cycles per seconds (cps).

ADVANTAGES

It is capable of grinding a wide variety of materials of differing hardness.

It can be used in completely enclosed form, which makes it suitable for use with toxic materials.

It can produce very fine powders.

It is suitable both for dry and wet milling. Wet milling is required for preparation of pharmaceutical suspensions.

DISADVANTAGES

Wear occurs from the balls and the inside surface of the cylinder hence there is possibility of contamination of product with mill material.

Abrasive materials increase wear.

Soft or sticky materials may cause problems by caking on the sides of the mill or by holding the balls in aggregates.

The ball mill is a very noisy machine, particularly if the cylinder is made of metal.

APPLICATIONS

Large ball mills are used to grinding ores prior to manufacture of pharmaceutical chemicals.

Smaller ball mills are used for grinding of drugs or excipients or for grinding suspensions.

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