The document discusses various milling techniques used in industrial pharmacy including impact, cutting, compression, attrition, and combined methods. It describes different types of mills like hammer mills, roller mills, and ball mills; and factors that influence particle size reduction like speed, screen size, number of balls, and feed rate. The goal of milling is to break down materials into the desired particle size range using techniques that minimize heat generation and avoid damaging the material being milled.

1. Industrial pharmacy I
Lec 5:
milling
Pharmacist
Hussein kadhim alkufi
MSc in pharmaceutics
Collage of thi-qar
University of pharmacy

2. Theory of comminution

3. • The mechanical behavior of solid, which under
stress are strained and deformed, is shown in the
stress-strain curve.
• The initial linear portion of the curve is defined by
Hookes law ( stress is proportional to strain), and
Youngs modulus (slope of linear portion) expresses
the stiffness or softness in dynecm².
• The stress-strain curve becomes nonlinear at the
yield point, which is a measure of the resistance to
permanent deformation. i.e is the point that cracks
start to break the particles and persist beyond
which for milling occuring

4. • Wit still greater stress, the region of irreversible plastic
deformation is reached
• The AUC represents the energy of fracture and is an
approximate measure of the impact strength of the material.
• Note
1. In all milling processes, it is a random matter if and, when
a given particle is subjected to a sudden impact and is
fractured, it yields a few relatively large particles and a
number of fine particles, with relatively few particles of
intermediate size.
2. So with time as smaller particles are formed we need more
impact “ more energy is needed” for breaking up of
particles as particle size decrease, but also because more
energy is needed to initiate cracking.
3. The most efficient mills utilize less than 1 % of the energy
input to fracture the particles

5. The rest of energy is dissipated in
1. Elastic deformation of unfractured particles
2. Transport of material within the milling chamber
3. Friction between particle
4. Friction between particle and mill
5. Heat
6. Vibration and noise
7. Inefficiency of transmission and motor

6. Energy of comminution
The energy required to reduce the size of
particles is inversely proportional to the size
raised to some power. This may be expressed
mathematically:
Where dE is the amount of energy required to
produce a change in size, dD, of unit mass of
material, and where c and n are constants

7. Kicks hypothesis
In this we have direct relationship between
energy E and the reduction ratio D1D2

8. Rittinger hypothesis
• In which we have direct relationship between
the energy required and the increase in
surface area

9. Method of milling
Depending on the type of milling action or the
principle of operation: we have different type
1. the cutting method
2. The pressure or compression method
3. Impact method
4. Attrition method
5. Combined impact and attrition method

10. These method are different in
1. The milling action or principle of action
2. The type of mill used or the milling medium
3. Particle size range. In µm or in mesh
4. Type of material to be milled
Ex sticky or wetting, fibrous, friable, soft and
abrasive material

11. Note: each milling equipment or mill consist of 3 basic
part.
1. Feed chute, which delivers the material
2. Grinding mechanism, usually consisting of a rotor
and stator for milling action
3. A discharge chute or chamber and sometime
called selector.
HW: the feeding and discharge speed or rate can
effect the milling action and milling rate?

14. 1_Impact method
a. Hammer mill
b. Vibration mill

15. Hammer mill
For this type the grinding medium is the number of
swinging hammers fixed on a certain stator, this can
be rotated in different rotation speed may reach to
10000 rpm in presence high speed rotter
The material is retained until it is small enough to
fall through the screen that form the lower portion
of the casing.
This type can be used at most in all drug except the
abrasive material

18. Note:
The particle size is controlled by
1_ select the speed of rotation
2_ the size and type of screen
Q// Why speed of rotation hummer is crucial for
particle size? HW

19. What is the effect of hammer speed and the
thickness of screen on the particle size?
If you have high speed rotation mostly we can
get smaller particle size “ more decrease in
particle size” because there is smaller angle
under which the particles approach the screen
and smaller screen, opening appear to the
particles this will lead to increase the returning
of material within the mill.
If we have low speed “the reverse occur” the
open appear larger and less reduction in particle
size

20. Advantage:
1. Easy to setup and clean up
2. Minimal problems, wide range of particle
size, small space is required
Disadvantage
a. Potential clogging of the screen
b. The heat generated during milling may lead
to degeneration or damage of the material to
be milled
B_ Vibration mill type

21. 2_ Cutting mill
Size range 100 – 1000 µm
Used for tough, fibrous materials and provide a
successive cutting or shearing action rather than
attrition impact
The rotary knife cutter has a horizontal rotor
with two to 12 knives spaced uniformly on its
periphery turning from 200 to 900 rpm and a
cylindric casting having several stationary knives
attached to the milling casting

23. 3_ compression and compaction
pressure
• For small scale by use of mortal and pestle
• For large scale by use of roller mill which consist of
2-5 cylindric smooth rollers operating at different
speed.
• One of rollers rotate directly but the second rotated
by friction of another one.
• This mill can give compaction action
• Can be used for soft material
• 100 – 10000 µm

24. 4_ Attrition method
• Ex roller mill:. The material to be milled dispersion
in susp paste or ointment.
• The rollers used rotating at different speed so we
have slower and faster roller
• The material “after milling” can be removed by
scrapper
• Size range may reach to 1 µm

25. 5_ Combined impact and attrition
method
Ex: boll mill ( 1- 100 µm) ex pin mill
(10 – 10000 µm) ex fluid energy mill
(1- 10000 µm) boll similar vibration but
differ in capacity, rotation speed so site of balls differ
between them

26. Ball
Ball milling is a size reduction technique that uses
media in a rotating cylindrical chamber to mill
materials to a fine powder.

28. The balls are of different size:
1_ the large one balls can give breaking down for the
coarse particle
2_ small balls can make reduction for the fine particles
Factors influencing the size of the product in the ball
mill:
1. The rate of feed.
2. Weight of balls.
3. The diameter of the balls.
4. The slope of the mill.
5. Discharge freedom.
6. The speed.

29. Hw :
1_colloid mill
2_ special milling technique