Ball Mill Principle, Construction, Uses, Advantage, Disadvantage, and Question Answer for GPAT, GATE, Pharmacist, Drug Inspector & NIPER JEE Exam

Principle

The ball mill operates on the principle of impact and attrition. In this, impact occurs between fast moving balls and the powder material while the attrition mode comes in action when the hollow cylinder rotates on its longitudinal axis. All is occurs at low speed and the balls roll over each other.

Construction

Ball mill consist of a hollow cylinder mounted/fitted on a metallic frame and rotates around its longitudinal axis. The cylinder is made up of metal and lined with chrome but in most of the pharmaceutical industries, cylinder is lined with rubber and porcelain. The balls occupy about 30-50% of mill volume. These balls are made of steel, iron or stoneware. They act as grinding medium and their weight is kept constant.

Working

• The feed material is introduced in the cylinder in such a quantity that it occupies 60% of the volume of cylinder.
• A fixed no of balls are introduced and the cylinder is closed and the mill starts rotating
• The speed of the rotation is important. At low speed, the balls will roll over each other and mostly only attrition will occur.
• At high speed, balls are kind of thrown out to the walls due to the centrifugal force and hence grinding will not occur. And in this case, only compression by the balls against the wall will takes place which is not sufficient for size reduction.
• At critical/ optimum speed, the centrifugal force will occur and as the result of this, the balls are picked by the walls and carried to the top and then they fall to the bottom. In this way both impact and attrition occur and size reduction hence becomes more efficient.

Uses

1. Fine grinding with particle size 100-5 micro-meters or less
2. Can be used particularly for hard and abrasive materials
3. Stainless steel balls are preferred for the production of ophthalmic and parental products
4. At low speed, ball mill is used for milling dyes, pigments and insecticides

Advantages

1. It produces very fine powder
2. It is suitable for both wet and dry grinding
3. Since it is a close system, toxic substances can be ground
4. Sterility is achieved due to closed cylinder
5. The milling process can be done in an inert atmosphere if the oxygen sensitive substances are to be milled.

Disadvantages

1. It is a very noisy machine
2. It is a slow process
3. Soft, tacky, fibrous material cannot be milled by ball mill

Multiple Choice Questions (MCQs)

1. Pebble mill works on which of the following mechanism?
A. Compression
B. Attrition
C. Impact
D. Both B and C

2. At which speed centrifugal force occurs in ball mill?
A. Low speed
B. High speed
C. Optimum Speed
D. All of the above

3. The cylindrical part of the ball mill rotates around which axis?
A. Latitudinal axis
B. Longitudinal axis
C. Diagonal axis
D. Both A and C

4. Which of the following material lines the metallic hollow cylinder of the mill?
A. Rubber
B. Chrome
C. Both
D. None of the above

5. Match the following-
A. At high speed           1. Ball mill can be used for milling
B. At low speed            2. Ball mill cannot be used for milling
C. Sticky material        3. Only compression by balls occur
D. Brittle drugs            4. Only attrition takes place

6. The balls used in pebble mill are made up of which type of material?
A. Steel
B. Iron
C. Stoneware
D. All of the above

7. Which type of grinding can tumbling mill perform?
A. Dry grinding
B. Wet grinding
C. Both
D. None

8. Which type of balls is preferred for the production of ophthalmic and parental products?
A. Rubber
B. Iron
C. Porcelain
D. None of the above

9. Which of the following statement is NOT true?
A. Hardinge mill, the variant of ball mill consist of a hollow cylinder with conical end
B. Rods and bars can also be used in place of balls as grinding medium in ball mill
C. Ball mill is an open system, hence sterility is a question
D. Fibrous materials cannot be milled by ball mill

10. What particle size can be obtained through ball mill?
A. 20 to 80 mesh
B. 4 to 325 mesh
C. 20 to 200 mesh
D. 1 to 30 mm

ANSWERS:-
1. Both B and C
2. Optimum speed
3. Longitudinal axis
4. Both
5. A – 3 B – 4 C – 2 D – 1
6. All of the above
7. Both
8. None of the above
9. Ball mill is an open system, hence sterility is a question
10. 20 to 200 mesh

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