Milling
Milling is a mechanical process that reduces particle size of solids. There are words like grinding ,Churning, crushing, dispersion etc. The size of solid particles is traditionally expressed in mesh size i.e.
Number of pores per inch screen.
Coarse Milling
produces particles size more than 20 mesh
Intermediate Milling 20-200 mesh
Fine milling. > 200 mesh
Milling is essential in pharmaceuticals and it plays a significant role
1. Small size, large surface area and will have rapid disruption.
Specific surface = Surface area / weight
Typical surface increases on the decrease in size, resulting in increased area of contact between the solid and dissociated fluid such as :griseofulvin
2. Increase in area contact due to reduction in size between solid and solvent, reduce time for extraction.
3. The particle size in inhalation aerosol determines the position and retention of the particles Bronchopulmonary system.
But milling can sometimes change the crystalline structure and cause chemical changes some materials such as Starch, amylopectin and povoidone structure breaks down with molecular weight polymers.
Pure C12 and C16 fatty acids can be decarboxylated and converted into hydrocarbons, in which a carbon is reduced by ball mill.
Methods of determining particle size
a. Sieving method
B. Microscopy
C. Sedimentation
D. Electrical conductance method
E. Light and x-ray scattering method.
1.Sieving method
Particle Size (Micron) Mesh Size
2000 10
850 20
600 30
400 40
300. 50
250. 60
200 70
175. 80
150 100
75 200
37. 400
B. Microscopy method: Can measures particle size up to 05 μm -150 μm and generally 625 particles are counted.
C. Sedimentation Method: Measures particle size of 1μm - 200 μm and uses Andreasen pipette.
Generally 1 % Suspension of powder has been placed and basis on the strokes law
x/t = dp2 (ρp- ρo).g /18Ș
x / t = sedimentation or settlement rate (distance / time)
dp = particle diameter, Ş = viscosity of the medium, (ρp- ρo) = density gradient, g = acceleration due to gravity
Most mills are used for size reduction
1. Hammer mill For almost all medicines
2. Roller attrition soft material
3. Fluid energy mill Moderately hard and friable ( like dried ) material .
4. Cutter Mill Fibrous Materials
5. Colloid Mill Emulsion
6. Ball Mill
• Hammer mill is used mostly for wet grains. The material is fed on top and thrown out Centrifugal and sieve material is collected after scratched screen is present Bottom of the cover. The particle of the discharged material is smaller than the screen hole.
• Ball mill: The optimum amount of balls should be 50% of the volume. Can make different size balls. The particles can be used for fine production. Small balls grind fine. Ball mill use both impact and attrition. The Speed of ball mill is very important because if the ball is moving at slow speed then balls are rolled up and provides a cascade action action on top of each other. As the speed is increased, the balls are taken up and falling freely on the material with an effect action which is responsible for most of the reduction in size. but If the speed is increased further, the balls are started centrifugging with the mill.
• Find fluid energy: uses effects and attraction both. Here micronized material is used to carry with high velocity of the air that follows an elliptical path in the cyclone separator, the fine particles pass through one discharge go to the outlet and bag collector. While large particles are taken in the perimeter by centrifugal force. Where they are exposed further attrition. It can produce the particle size 2-20μ.
• Colloid Mill: Used mostly for the production of emulsion and is not used for dry particles. colloid mill uses arotor with RPM 2000-20000. The space between the rotor and the stator is adjusted to get the desired size globules of Emulsion.
Milling is a mechanical process that reduces particle size of solids. There are words like grinding ,Churning, crushing, dispersion etc. The size of solid particles is traditionally expressed in mesh size i.e.
Number of pores per inch screen.
Coarse Milling
produces particles size more than 20 mesh
Intermediate Milling 20-200 mesh
Fine milling. > 200 mesh
Milling is essential in pharmaceuticals and it plays a significant role
1. Small size, large surface area and will have rapid disruption.
Specific surface = Surface area / weight
Typical surface increases on the decrease in size, resulting in increased area of contact between the solid and dissociated fluid such as :griseofulvin
2. Increase in area contact due to reduction in size between solid and solvent, reduce time for extraction.
3. The particle size in inhalation aerosol determines the position and retention of the particles Bronchopulmonary system.
But milling can sometimes change the crystalline structure and cause chemical changes some materials such as Starch, amylopectin and povoidone structure breaks down with molecular weight polymers.
Pure C12 and C16 fatty acids can be decarboxylated and converted into hydrocarbons, in which a carbon is reduced by ball mill.
Methods of determining particle size
a. Sieving method
B. Microscopy
C. Sedimentation
D. Electrical conductance method
E. Light and x-ray scattering method.
1.Sieving method
Particle Size (Micron) Mesh Size
2000 10
850 20
600 30
400 40
300. 50
250. 60
200 70
175. 80
150 100
75 200
37. 400
B. Microscopy method: Can measures particle size up to 05 μm -150 μm and generally 625 particles are counted.
C. Sedimentation Method: Measures particle size of 1μm - 200 μm and uses Andreasen pipette.
Generally 1 % Suspension of powder has been placed and basis on the strokes law
x/t = dp2 (ρp- ρo).g /18Ș
x / t = sedimentation or settlement rate (distance / time)
dp = particle diameter, Ş = viscosity of the medium, (ρp- ρo) = density gradient, g = acceleration due to gravity
Most mills are used for size reduction
1. Hammer mill For almost all medicines
2. Roller attrition soft material
3. Fluid energy mill Moderately hard and friable ( like dried ) material .
4. Cutter Mill Fibrous Materials
5. Colloid Mill Emulsion
6. Ball Mill
• Hammer mill is used mostly for wet grains. The material is fed on top and thrown out Centrifugal and sieve material is collected after scratched screen is present Bottom of the cover. The particle of the discharged material is smaller than the screen hole.
• Ball mill: The optimum amount of balls should be 50% of the volume. Can make different size balls. The particles can be used for fine production. Small balls grind fine. Ball mill use both impact and attrition. The Speed of ball mill is very important because if the ball is moving at slow speed then balls are rolled up and provides a cascade action action on top of each other. As the speed is increased, the balls are taken up and falling freely on the material with an effect action which is responsible for most of the reduction in size. but If the speed is increased further, the balls are started centrifugging with the mill.
• Find fluid energy: uses effects and attraction both. Here micronized material is used to carry with high velocity of the air that follows an elliptical path in the cyclone separator, the fine particles pass through one discharge go to the outlet and bag collector. While large particles are taken in the perimeter by centrifugal force. Where they are exposed further attrition. It can produce the particle size 2-20μ.
• Colloid Mill: Used mostly for the production of emulsion and is not used for dry particles. colloid mill uses arotor with RPM 2000-20000. The space between the rotor and the stator is adjusted to get the desired size globules of Emulsion.
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