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MILLING
Dr.Ibtihal O. Alkarim
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Dr.Ibtihal O. Alkarim
headlinesUnder this topic we’ll discuss:
1. What is milling?2. Pharmaceutical application3. Size distribution and
measurement4. Types of mills5. Selection of mills
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Dr.Ibtihal O. Alkarim
What is milling?Milling is mechanical process of reducing the
particle size of solids.Various terms has been used cursing,
disintegration, dispersion, grinding, and pulverization.
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Dr.Ibtihal O. Alkarim
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why milling?
Dr.Ibtihal O. Alkarim
Increase specific surface of the substance. Increase the free surface energy of milled
substance. significantly increase the speed of
substance and diffusion processes. strengthen the pharmacological activity of
substances
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Millingadvantages Mixing is more uniform if
ingredients are roughly the same size
Milling of wet granules can promote uniform and efficient drying
Increased surface area can improve dissolution rate and bioavailability
Improved content uniformity of dosage units
disadvantages:
Excessive heat generation can lead to degradation, change in polymorphic form
Increase in surface energy can lead to agglomeration
May result in excessive production of fines or overly broad particle size distribution
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Pharmaceutical application Specific surface
( surface area per unit weight) increase by size reduction which affect the therapeutic efficiency of medical substance.
specially if the material have low solubility.
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Examples: Griseofulvin Penicillin G procaine Aspirin suppository Calomel ointment Areosol suspension
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GriseofulvinControl of fineness half of the oral regimentWhy?Control particle size and Specific surface influences the duration of adequate serum concentration
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Penicillin GPenicillin G can be administered IMWhy?Product syringe-ability
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Aspirin suppository Increase rectal absorption
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Calomel ointment Increase of antiseptic action by reducing
the particle size
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Aerosols and Suspension Particle size affect the position and
retentionOf the particle on broncho-pulmonary system. Size affect texture, taste, and rheology of oral suspension in addition of adsorption.
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Potential Impact Processing Behavior
Product Quality Factors
Property Flow Blending Wetting Drying Mechanical Dissolution Stability
Particle Size X X X X X X XSurface Area X X X X X X X
Particle Shape X
Surface Energy X X X
Bulk Density X X X
Pore Size X X X
Internal Friction X X
Wall Friction X X
Hygroscopicity X X X
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Introduction Representation of data Measurement methods
Size distribution
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Measurement methodsThere are three method involved in size distribution measurement:a. Microscopyb. Sievingc. Sedimentation
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Microscopy An ordinary direct method. Depend of resolving power of the lens. If size close to the wavelength of light
can't resolve. Ordinary microscope lower limit is 0.4
micron Ultraviolet lower limit is 0.1 micron Ultra microscope lower limit is
0.01micron
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Sieving
Dr.Ibtihal O. Alkarim
Most widely used Inexpensive Rapid Little variation Lower limit is 50 micron (micromesh 10
micron)
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What is sieve? A pan with a bottom of wired cloth with
square openings In U.S, two standard of sieves are used. The procedure involve shaking, weighing
of portion retained, and taping
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sedimentation Used over size range from 1 to 200
microns. Based on the dependence of the rate of
sedimentation of the particle on their size expressed by stoke equation.
Pipet method is the simplest means of incremental particle size analysis.
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Types of mills
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A mill consist of three part:1. Feed chute2. Grinding mechanism3. Discharge chute
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Milling forces The principle of
operation depends on:
1. Cutting2. Attrition3. Compression4. Impact
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Continuing… Open-circuit and closed-circuit mills Rate of discharge equal rate of feed. Discharge chute designed at the bottom
of mills so that the force of gravity is sufficient to give free discharge.
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Continuing Comminution is characterized by the
symbol index –”i” - the degree of comminution of substances. “I” equal ratio of the diameter of pieces of material to the milling (D) to particles diameter after ones (d). i = D / d
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Types of milling in depend on degree of comminution in haw much times the particles size was reduced: 1. Great (large) – in 2-6 times; 2. Medium - up 6-10 times; 3. Fine - at 10-50 times; 4. Micro fine - in 50-100 times; 5. Super fine - thin (colloidal) – in 100-10000
times.
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Classification equipment for milling: By the way of milling: 1. Cutting machine 2. Attrition machine 3. Compression machine 4. Impact machine 5. Impact - Centrifugal machine
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Classification equipment for milling: By the degree of comminution:1. crushers:
for great, medium, fine milling.
2. mills:for micro fine, superfine (colloid) milling.
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Classification equipment for milling:By the mechanism used to break the particles:
1. Disc 2. Boll 3. Rotor 4. Hammer 5. Rollers and others
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Requirements for the milling process: 1. Substances must has initial its composition and
pharmacological properties during the milling process;
2. Periodically to remove enough milled particles from the sphere of comminution;
3. Prevent formation of dust (sealed enclosures); 4. Appropriate moisture of material5. The use of sequence connected crushing-milling
machines.
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Mills selection: 1. Initial size of the material and
desirable size of final product 2. The total amount of material for
milling3. Physical and chemical properties of
material for milling
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Dr.Ibtihal O. Alkarim
Mills selection
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Cutting machine.are used to comminution herbal drug that has a fibers structure Grass cutting and straw cutting (grass, stalks). Root cutting with the guillotine knife - for grinding roots,
bark. Drum straw cutting - for grinding dry medicines. Knife mill - for milling volume materials, containing cellulose Motorized soft meat grinder - for fresh herbal drugs, animal
raw materials
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Crushes machineswhich are used for milling solid materials: Cheek and jaw mill
Rollers mill
Cone mill
Hammer mill
Disk mill (Excelsior)
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Hammer mills It’s an impact-centrifuge mill. Using high-speed rotor to which number of swining
hammers are fixed. The material is fed at the top or center Thrown out centrifugally Ground by impact of the hammers or against the plates
around the periphery of the casing. The material is retained inside the casing until it is small
enough to fall the the screen at lower portion of casing.
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Hammer mill1. rotor2. hamm
er3. Corps4. plates.
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Most popular Used almost at any type of size reduction Can be used for dry, wet, cakes, ointment,
slurries. Comminution is affected by impact of peripheral
hammer speed ( up to 7600m/minute). The size of product is controlled by speed of
hammers and type and size of the screen
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advantages Narrow size distribution Simple to install and operate Speed and screen can be rapidly
changed Easy to clean May be operate as closed system.
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Hammer mill
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Ball mills It’s combined of attrition and impact mills. Consist of horizontally rotating hollow
vessel of cylindric shape. The mills filled with balls of steel or
pebbles ( grinding medium). Pebbles, rods, and tube mills
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Work of Ball mill At low speed
attrition force At higher speed
impact force At sufficient
speed centrifug force
C
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Factor affecting milling process1. size of ball 2. Charge of balls in mills3. Amount of material to be
mille4. Weight of balls5. Wet an dry milling6. Viscosity7. Wetting agent
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Ball mill
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Advantages of ball mill Used either for dry or wet Batch or continuous operations Sterile grounding possible Installation, operation, and labor costs
are low Unsurpassed for fine grinding of abrasive
and hard material
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Fluid energy mill (Micronizer) At which material are
suspended and conveyed at high velocity by air or steam. Which pass through nozzle.
The violent turbulence of air or steam reduce the particle size by interparticular attrition
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Internal classification Pre-milling (20-
100mesh) is required. Reduction of particle
size to 1-20 microns Fluid energy source
and dust collector required.
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Cutting mills Used for tough
and fibrous material
Cutting and shearing
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Horizontal rotor with rotary knife cutter 2-12 knives, with cylindrical casing with stationary knives.
With a screen at the bottom
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The feed should not exceed the length of knife
Not more that 1 inch thick
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Roller mill Consist of two-five
smooth roller. Operating at
different speed Reduction power is
combination of compression and shearing forces
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Rollers mill
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Colloid mill Consist of high speed
rotor and stator with adjustable clearance range.
Rotor speed up to 20.000 rpm
Premilling is required
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Colloid mill Used to process suspension and
emulsion Not for dry material Pre mixing with liquid May be smooth or rough surface
regarding rotor and stator
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Colloid millUsed for suspension and emulsion
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Factor influence milling1. Properties of a solid (Hard (iodine), soft and
intermediate solid).2. Specification of the product.3. Physical nature of material (fibrous, friable)4. Water content5. Heat during milling (low melting point, heat
sensitive drug, pigment, unstable compound)
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1. Shape of milled particle (flow specifications)
2. Milling may alter Crystalline structure and causes chemical change in some material
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Dr.Ibtihal O. Alkarim
Selection of a mill The choose of the mill depend on:1. Product specification (size range, particle size
distribution, shape, moisture content, physical and chemical properties of material)
2. Capacity of the mill and product rate requirement3. Versatility of operation ( wet and dry milling,
rapid change of speed and screen, and safety requirement)
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1. Dust control (loss of costly drug, health hazard, contamination)
2. Sanitation (ease of cleaning, sterilization)
3. Auxiliary equipment4. Batch or continuous operation5. Economical factor
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Thanks….