Size Reduction

Content 

- Size reduction
- Objectives of size reduction
- Mechanism of size reduction
- Hammer mill
- Ball mill
- Fluid energy mill

Size reduction :- 

what is size reduction? 

Size reduction is a process of reducing large solid unit masses - vegetables or chemical substances into small unit masses, coarse particles or fine particles. 

Size reduction is commonly employed in

pharmaceutical industries. Size reduction process is also known  as Comminution and Grinding.

Objectives of size reduction :-

1. Size reduction leads to increase of surface area.

2. Pharmaceutical capsules, insufflations (i.e. powders inhaled directly into the lungs), suppositories and ointments require particles size to be below 60 mm size.

3. To increase the therapeutic effectiveness certain drugs by reducing the particle size.

4. Size reduction produces particles in narrow size range. Mixing of powders with narrow size range is easier and uniform. 

5. Pharmaceutical suspensions require finer particle size. It reduces rate of sedimentation. 

Mechanism of size reduction :-

1. Impact – this involve hammer or bar at high speed (hammer mill).

2. Compression- particle crushed b/w rollers by the application of force (roller mill).

3. Cutting – the material cut by a sharp blade (cutter mill)

4. Attrition - arising from particles scraping against one another or rubbing action (fluid energy mill)

Factors affecting Size Reduction :-

Selection of mill - It is related to feed, milled product, safety and economics

Factors related to nature of raw materials affecting size reduction

1.Hardness - It is easier to break soft material than hard materials. Ex: For iodine hammer 

mill is used.

2.Fibrous - These are tough in nature. A soft, tough material has more difficulty than a hard, 

brittle substance. Ex: Rauwolfia, Ginger. Here cutters can be used.

3.Elastic / Sticky - Become soft during milling. Ex: synthetic gums, waxes, resins. Low 

melting substances should be chilled before milling. These are milled using hammer, 

colloid or fluid energy mill.

4.Melting point - Waxy substances, fats and oils are softened during size reduction due 

to heat generated. This is avoided by cooling the mill and the substance.

5.Hygroscopic - Certain substances absorb moisture content rapidly. This wet mass 

hampers the milling process. Ex: Potassium carbonate. Closed system such as porcelain 

ball mill is used.

1. Hammer Mill

 Principle of Hammer mill :-

It operates on the principle of impact between rapidly moving hammers mounted on rotor and the stationary powder material.

Parts of Hammer mill :-

Consists of a metal casing, enclosing a central shaft, to which 4 or more swinging hammers are attached. Lower part of casing consists of a screen, through which material can pass and collected in a suitable receiver.

Construction & Working of hammer mill:- 

1. A hammer mill is essentially a steel drum containing a vertical or horizontal rotating shaft or drum on which hammers are mounted.

2.The hammers swings on the ends or fixed to the central rotor.

3.The rotor is rotates at a high speed inside the drum while material is fed into a feed hopper.

4.The material is put into the hopper which is connected with the drum.

5.The material is powdered to the desired size due to fast rotation of hammers and is collected under the screen

6.This are mainly operated at 1000 to 2500 rpm for the reduction of the large sized particles. High speed rotor uses 10000 rpm speed.

Uses of Hammer Mill :- 

1.Brittle material is best fractured by impact from blunt hammers.

2.Fibrous material is best reduced by cutting edges

Advantages of Hammer Mill :- 

1.It is rapid in action, and is capable of grinding many different types of materials.

2.They are easy to install and operate, the operation is continuous.

 

3.There is little contamination of the product with metal abraded from the mill as surface move against each other.

4.The particle size of the material to be reduced can be easily controlled by changing the speed of the rotor, hammer type, shape and size of the screen.

Disadvantages of Hammer Mill :-

1.Heat build up during milling is more, therefore, product degradation is possible.

 

2.Hammer mills cannot be employed to mill sticky, fibrous and hard materials.

3.The screens may get clogged.

2. Ball Mill

Principle of Ball Mill :-

It operates on the principle of impact and attrition between the rapidly moving balls and the powder material, both enclosed in a hollow cylinder.

Parts of Ball Mill :-

Consists of a hollow cylinder mounted on a metallic frame such that it can be rotated along its longitudinal axis. Cylinder contains balls(they made of metal or rubber) occupying 30–50 % of mill volume. Weight of ball is constant; Size depends on the feed quantity and diameter of mill.

Working of Ball Mill :- 

1.At low speeds, the ball roll over each other and attrition (rubbing action) will be the predominate

mode of action.

2.Thus, in the ball mill, impact or attrition or both are responsible for the size reduction.

Advantages of Ball Mill :-

1.It can produce very fine powder.

2.Ball mill is used for both wet and dry grinding processes.

3.Toxic substances can be ground, as the cylinder is closed system.

4.installation, operation and labour costs are low.

Disadvantages of Ball Mill :-

1.The ball mill is a very noisy machine.

2.Ball mill is a slow process.

3.Soft, fibrous material cannot be milled by ball mill.

Uses of Ball Mill :-

1.Stainless steel are preferred in the production of ophthalmic & parentral products.

2.ball mill at low speed is used for milling dyes, pigments & insecticides.

3. Fluid Energy Mill

Principle of Fluid Energy Mill :- 

It wirking on the principle of impact and attrition

construction of Fluid Energy Mill :- 

1.Consists of a loop of pipe with diameter 20-200 mm. The overall height of the pipe is 2 m.

2.Inlet for feed and a series of nozzles for air, inert gas. Outlet with classifier which prevents the

particles to pass until they become sufficiently fine.

3.There is an inlet for the feed and a series of nozzles for the inlet of air or an inert gas.

4.It also has an outlet with a classifier which allow the air to escape but prevents the particles to pass until they become sufficiently fine.

Working of Fluid Energy Mill :-

The air or inert gas is introduced with a very high pressure through the nozzles .Solids are introduced into air steam through inlet .due to high degree of turbulence, impact and attrition forces occurs between the particles.The fine particles are collected through a classifier .Fluid energy mill reduces the particles to 1 to 20 micron. To get a very fine powder ,even upto five micron, the material is pretreated to reduce the particle size to

the order of 100 mesh and then passed through fluid energy mill.

Advantages of Fluid Energy Mill :- 

1.The mill is used to grind the material to fine powder.

2.The particle size of powder can be controlled due to the use of a classifier.

3.There is no wear of the mill and hence there is no contamination of the product.

4.It is useful for grinding heat sensitive substances such as sulphonamides, vitamins and antibiotics.

Disadvantages of Fluid Energy Mill :-

5.Tendency of forming aggregates or agglomerates after milling.

6.Generation of amorphous content due to high energy impact.

7.Formation of ultra-fine particles

8.High energy consumption.

Uses of Fluid Energy Mill :- 

1.The mill is used to grind heat sensitive material to fine powder.

2.The mill is used to grind those drugs in which high degree of purity is required.

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