Learn size reduction in pharmacy with methods, mechanism, equipments. It is used at pharmaceutical field to reduce size of compound.
What is Size reduction in pharmacy?
Size reduction is a reducing process by which solid large particles splits into smaller fine particles.
Normally pharmaceutical powders available in different sizes and it can create difficulties in the production of ideal dosage forms. Particles with same size may produce ideal dosage form.
Size reduction help to reduce large sized particles into smaller particles. But only size reduction is not sufficient to obtain same size particles, that’s why size reduction and size separation both should applied together to obtain ideal sized particles.
Size reduction methods
- Precipitation method.
- Mechanical method.
a) Precipitation method
This method generally used to obtain raw materials and bulk drugs. Inorganic compound like calcium carbonate, magnesium carbonate are produced by this method. In this method chemical substance get dissolved in an appropriate solvent, the chemical substance is soluble in the solvent. Then one more solvent is added, but here the chemical substance is insoluble and form precipitates.
b) Mechanical method
In this method force is applied by various mechanical instruments for size reduction. This method generally used for preparation of capsule, tablets, emulsions etc. Instruments example: Hammer mill, Ball mill etc.
Size reduction mechanism
The mechanism of size reduction depends on the nature of materials. Mechanism follows few modes, those are:
1) Cutting: Material size is reduced by cutting by using a sharp blade. Equipment: Cutter mill.
2)Compression or crushing: Material sized is reduced by using roller. Materials placed in between the roller and crushed. Equipment: Roller mill.
3)Impact: Material size is reduced by followings:
- High speed
- Hammers.
High speed: In this process particles moving at high speeds collide each other and produce smaller particles. Equipment: Fluid energy mill.
Hammers: Material size is reduced by using hammers. When a large particle introduces with rotating hammers then particles split in various part. This process is continued until same sized particles are obtained. Equipment: Hammers mill, Fluid energy mill.
4)Attrition: Size reduce by rubbing. Equipment: Fluid energy mill.
Classification size reduction equipment
- Cutting machine.
- Crusher.
- Grinders.
- Ultrafine grinders.
1) Cutting machine
For cutting mechanism in size reduction in pharmacy cutter mill is used
Cutter mill principle:
Cutter mill contains blades, by the help cutter sharp knives material size is reduced.
Construction:
- A Milling chamber consists of two types of sharp knives, rotating knives and stationary knives.
- A rotator disc is horizontally mounted in the mill chamber, and this disc contain 2-12 sharp knives, these knives is known as rotating knives.
- In mill case, cutter mill contains stationary knives.
- One hopper is adjusted above the case of mill.
- At bottom, this mill contain a screen which help to control the size of materials..
- For discharge of materials, it contains discharge outlet.
Cutter mill Working:
- The disc will rotate in between 200 to 900 revolutions per minute.
- Feed materials will be loaded through the hopper, automatically materials will reach to mill chamber by gravitational force.
- During rotation, materials will introduce with stationary knives and rotating knives.
- Those knives will cut the materials and form smaller fine materials.
- Desired smaller materials will discharge through the screen and collect into a receiver.
Cutter mill uses:
- Cutter mill is used for size reduction of fibrous materials.
- Size reduction of roots, woods of medicinal plants this mill is used.
- This equipment is also used in rubber industry.
- In plastic industry and paper industry this equipment is widely used for recycling the waste like plastic and paper.
2) Crusher
a) Roller mill principle:
The material is crushed or compressed by the application of stress. Stress is applied by rotating heavy wheels or rollers.
Construction:
- Roller mill consists of two cylindrical rollers, made of stone or metal, which are mounted horizontally.
- Rollers can have a diameter ranging from a few millimeters up to a meter.
- Rollers are capable of rotating on their longitudinal axes.
- One of the rollers driven directly by using a motor, while the second one runs freely.
- The gap between the rollers can be controlled to obtain the desired particle size.
Roller mill working:
- Rotate the rollers.
- From the hopper, the material is fed into the gap between the rollers.
- The material is crushed through the rollers under high pressure. The clearance or gap between the rollers can be adjusted to control the degree of size reduction.
- The product is collected is to a receiver.
Roller mill uses:
- Before extraction of fixed oils, roller mill is used to crushing and cracking of seeds.
- It is also used to crush soft tissue to help in the penetration of solvent during extraction process.
b) Edge runner mill principle
The size reduction is done by crushing or compression by the help of heavy weight stones. During the movement of the stones, shearing force is also applied.
Construction:
- Edge runner mill consists of two heavy rollers, the weight of stones may be several tons.
- The rollers move on a bed which is made of stone or granite.
- The rollers are mounted horizontally which can easily move around the bed.
- Each roller has a central shaft and revolve on its axis.
Edge runner Working:
- The material is placed in the path of stone wheel on the bed by the help of a scrapper.
- Placed the stone wheel on its axis.
- The outer part of the stone has to travel more than the inner, so that size reduction is achieved by shearing as well as crushing.
- Leave the materials for certain period of time.
- For required size of powder, it passes through a sieve and collected.
Edge runner mill uses:
- Edge runner mill is used to reduce size of tough solid materials.
- For extraction of medicinal plants this mill can be used.
Advantages of edge runner mill:
- During processing, this mill doesn’t need any attention.
Disadvantages edge runner mill:
- This mill cannot be used for sticky materials.
- This operation is very time consuming.
- More space is required for edge runner mill.
c) End runner mill principle:
Size reduction is done by using heavy weight of steel pestle. It follows the crushing or compression mechanism of size reduction. Shearing stress is also applied during the movement of motor and pestle.
End runner mill construction:
- It consists of a mechanical mortar and pestle, which are made up of steels. The pestle is dumbbell shaped, but the bottom of pestle is flat.
- A steel mortar is adjusted to a flanged plate.
- A scrapper is horizontally placed.
- Under the flanged plate, a beveled cog fitting is horizontally attached and beveled cog fitting contain a pully.
- The pestle contains an arm, which help the pestle rise and fall over the material in the motor.
End runner mill Working:
- The material is placed to ground in the mortar.
- The scrapper pushes the material in the path of pestle.
- The mortar runs at high speed.
- The pestle is placed in mortar.
- The running mortar force the pestle to move faster.
- During this process, size reduction is achieved by shearing as well as crushing.
- The material is collected and passed through a sieve to get the powder of desired size.
Uses of end runner mill
- Fine powder of material can be obtain by this end runner mill.
- For preparation of ointments and semi solids drug this equipment can be used.
- Used to reduce fibrous crude drug to fine sized crude drugs.
Advantages end runner mill
- End runner mill follows the uniform distribution of materials.
- This equipment is good for fine grinding of materials.
Disadvantages of end runner mill
End runner mill is not suitable for slightly broken drugs.
3) Grinders:
a) Hammer mill principle:
The material is crushed or compressed by the application of stress. Stress is applied by rotating heavy wheels, mullers or rollers.
Hammer mill Construction:
- The hammer mills available in horizontal or the-vertical shaft type.
- Hammers are usually made of hard steel or stainless steel.
- The impact surface made of an extremely grinding or rubbing resistant material such as Haystellite and Carbaloy.
- Normally, stainless steel hammers are used for pharmaceutical purpose.
- Hammers may have several shapes, like stirrup shape and the bar shape.
- Bar-shaped hammers are used for production of tablets.
- The hammer blades can be with flat edges or sharp edges or both.
- The Hammers can be swing type or rigid.
- Free swinging of hammers can increase the clearance between hammers and screen.
- The chamber of hammer mill contains a removable screen through which materials can easily pass.
- The screen is prepared using metal sheets which varying thickness with perforated holes or slots.
Hammer mill working:
- Rotate the hammers at 8000 to 15000 revaluations per minute.
- Feed material is placed into the hopper.
- Feed materials flows down vertically and the horizontally.
- The rotating hammer forced the materials to yield smaller particles.
- Then the particles pass through the screen.
- For tangential exit, the size of the product is considerably smaller than the aperture of the screen.
- The screen is interchangeable, so we can get any kind of find powder.
The working of hammer mill can be depending upon few factors like rotor speed, feed rate, Clearence between hammers and grinding, number and type of hammers, plates and size of the discharge opening (screen).
Hammer mill uses:
- Fine particles (10 mm to 400 mm) can be obtained by this hammer mill equipment.
- For size reduction of dry materials, slurries this equipment is use.
Advantages of Hammer mill
- Hammer mill is easy to install.
- Various types of feed can be handled.
- Hammer mill occupies small area.
- Speed of hammers and screen can be change rapidly and easily.
Disadvantages of Hammer mill:
- During operation excessive heat can be generate, which can affect the product.
- For sticky and hard materials size reduction, this equipment cannot be used.
b) Ball mill Principle:
Ball mill is also known as tumbling mill. For size reduction ball mill follows the impact principle, where the particles move at high speeds and collide each other to produce smaller particles.
For ball mill, rapidly moving balls and the powder materials enclosed in a hollow cylinder, at low speed the balls roll over each other and follow rubbing mechanism (attrition).
That’s why ball mill follows impact and attrition(rubbing) both mechanism for size reduction.
Ball mill Construction:
- Ball mills consist of a hollow cylinder made of metal. The cylinder is mounted on a metaled frame in such way that it can be rotate on its longitudinal axis. Length of cylinder is greater than the diameter of cylinder. In pharmaceutical industry, sometimes the cylinder lined with rubber or porcelain.
- Cylinder contains balls made of steel, which occupy 30%-40% of the volume cylinder. Weight of balls is constant. Balls size depends on the feed materials. A fixed number of balls is kept in the cylinder and rotated on its axis.
Ball mill Working:
- The drug is put into cylinder in such quantity, that is 60% of the cylinder volume.
- Introduce a fixed number of balls(small) and rotate the cylinder through correct speed.
- At correct speed, centrifugal force occurs into cylinder, as a result the balls are picked up by the mill wall and carried to the top, where they break contact with the wall and fall to the bottom.
- Apply impact stress which will be effective for size reduction.
Uses of Ball mill:
- To obtain fine powder in range between 5mm to 100mm ball mill is used.
- Preparation of ophthalmic and parenteral products Ball mill can be used.
- Grinding of hard materials.
- For Preparation of dyes and insecticides.
Advantages of Ball mill:
- Produce very fine powders.
- Dry and wet grinding can be done.
- Available in low cost.
- Installation process is easy.
Disadvantages of Ball mill:
- Equipment produces noisy sound.
- Slow process
- Soft and fibrous materials cannot be milled.
4) Ultrafine grinders:
Fluid energy mill principle:
Fluid energy mill follows the mechanism of impact and attrition. In this equipment feed materials are evaluated by the help of air streamer. Milling takes place because of high velocity collision between the feed materials.
Construction:
- Fluid energy mill consist of an elliptical pipe (height 2 meters and diameter in between 20 mm to 200 mm).
- Mill surface made of stainless steel.
- Grinding nozzles (2 to 6) may place in opposite to the initial flow path of powder.
- For size reduction, Compressed air is used at 600 kilopascals to 1 megapascal.
- Venturi feed is attached in the path of air flow.
- For discharging of particles, a filter bag or cyclone separator is adjusted to outlet.
Fluid energy mill working:
- Powder is introduce to mill through venturi.
- Air will enter from grinding nozzles and transport the powder in the elliptical pipe (circular).
- For stormy stream of air, the powder colloid with each other and break into smaller fine powder.
- Smaller powders are carried to outlet and removed by the filter bag or cyclone separator.
- And larger powder again goes for recirculation.
Fluid energy mill uses:
- Preparation of antibiotics and vitamins.
- Moderate hard materials can be processed for size reduction.
Advantages fluid energy mill:
- It is a rapid process helps to reduce powder size below 30mm.
- Contamination is not possible.
- Heat will not produce during milling.
Disadvantage fluid energy mill:
- Is not suitable for fibrous and soft materials.
- Expensive equipment.
Let’s read Derivation of centrifugation with theories, procedure.