size reduction

1. Crushing and grinding

2. Introduction
 The terms crushing and grinding usually signifies
subdividing to greater or less content.
 Few materials exist in optimum size and most
materials must be comminuted or size reduced at
some stage during production of dosage form
 Milling is the mechanical process of reducing the
particle size of solids
 Milling equipment is usually classified as coarse,
intermediate or fine according to the size of the
milled product

6.  Size is conventionally expressed in terms of mesh
(number of openings per linear inch of a screen)
 Coarse milling produces particles larger than 20
mesh, intermediate milling produces particles
from 200-20 mesh (74-840 microns) and fine
milling produces particles less than 200 mesh
 A given mill may operate in more than one class:
a hammer mill may be used to prepare a 16 mesh
granulation and to mill a crystalline material to
120 mesh powder

7. Pharmaceutical applications
 The surface area per unit weight known as the
specific surface is increased by size reduction
 This affects the therapeutic efficacy of drugs that
have low solubility in body fluids; eg- control of
fineness of griseofulvin led to an oral dosage
regimen half that of the original product
 Control of particle size and specific surface
influences the duration of adequate serum
concentration, rheology and product syringeability
of a suspension of penicillin G procaine for
intramuscular injection

8.  The rectal absorption of aspirin from theobroma
oil suppository is related to particle size
 Increased antisepetic action has been observed
for calomel ointment when the particle size of
calomel is reduced
 Size of particles used in inhalation aerosols
determines the position and retention of the
particles in the pulmonary system
 Size may affect texture, taste, rheology of oral
suspensions besides absorption

9.  The time required for extraction is shortened
when the drug is size reduced due to increase in
surface area due to increased area of contact
between the solvent and solid and reduced
distance the solvent needs to penetrate into the
material.
 The time required for dissolution of chemicals to
prepare solutions is shortened by use of smaller
particles
 Drying may also be facilitated by milling which
increases the surface area and reduces the
distance the moisture must travel within the
particle to reach the outer surface

10. Theory of comminution
 Strain and stress:
 When any solid body is subjected to opposing forces,
there is a finite change in its geometry depending
upon the nature of the applied load
 The relative amount of deformation produced by such
forces is called strain
 The ratio of the forces F necessary to produce this
strain to the area over which it acts is called the
stress
 The three types of strain:
 Tensile strain
 Compressive strain
 Shear strain

11.  The behaviour of solids which under stress are
strained and deformed as shown in the stress-
strain curve
 The initial linear portion is defined by Hooke’s law
(stress is proportional to strain) and Young’s
modulus (slope of linear portion) expresses the
stiffness or softness in dynes per sq cm
 The curve becomes non linear at the yield point
which is a measure of the resistance to
permanent deformation

13.  With still greater stress the region of irreversible
plastic deformation is reached.
 The area under the curve represents the energy
of fracture and is an approximate measure of
impact strength of the material
 Size reduction begins with the opening of any
small cracks that were initially present
 Thus larger particles with numerous cracks
fracture more readily than smaller particles with
fewer cracks

14.  If the force of impact does not exceed the elastic
limit (Hooke’s law region) the material is
reversibly deformed or stressed
 A force that exceeds the elastic limit fractures the
particle
 A flaw in a particle is any structural weakness that
may develop into a crack under strain
 Any force of milling produces a small flaw in the
particle

15.  The useful work in milling is proportional to the
length of new cracks formed
 A particle absorbs strain energy and is deformed
under shear or compression until the energy
exceeds the weakest flaw and causes fracture of
the particle
 The strain energy required for fracture is
proportional to the length of the crack formed
since the additional energy required to extend the
crack to fracture is supplied by the strain energy
to the crack

16.  Griffith theory of cracks and flaws assumes that all
solids contain flaws and cracks which increase with
applied force according to the crack length and focus
the stress at the atomic bond of the crack apex
 where T is tensile stress, Y is Young’s modulus, Ꞓ is
the surface energy of the crack wall and c is the
critical crack depth required for fracture
 The immediate objective of milling is to form cracks
that spread through the deformed particles at the
expense of strain energy and produce fracture

17.  The useful work is directly proportional to the new
surface area
 Since the crack length is proportional to the
square root of the new surface area produced,
the useful work is inversely proportional to the
square root of the product diameter minus the
feed diameter.
 The energy E’ required to produce new surface
area is:
 Where D1 is the diameter of the feed material, D2
is the diameter of the product discharged from the

19. Types of mills
 A mill consists of three basic parts:
 Feed chute which delivers the material
 Grinding mechanism usually with a rotor and a
stator
 Discharge chute
 The principle of operation depends on direct
pressure, impact from a sharp blow, attrition and
cutting
 In most mills the grinding effect is a combination
of these actions

22. Classification of crushing and
grinding machinery
 1. coarse crushers
 (a) Jaw crushers- i) Blake ii) Dodge
 2. Intermediate
 (a) Rolls
 (b) Disc crushers
 (c ) Edge runners
 (d)Disintegrators
 (e ) Hammer mills
 3. Fine grinders
 (a) Centrifugal- i) Raymond
 (b) Buhrstones
 (c ) Roller mills
 (d) Ball mills and tube mills
 (e ) Ultrafine grinders

23.  If the milling operation is such that the material is
reduced to the desired size by passing it once
through the mill, the process is called open-circuit
milling
 A closed-circuit milling is one in which the
discharge from the mill is passed through a size
separation device or classifier and the oversize
particles are returned to the mill for further
reduction in size

25. Hammer mill
 It is an impact mill using a high speed rotor (upto
10,000 rpm) to which a number of swinging
hammers are fixed
 The material is fed at the top or centre, thrown
out centrifugally and ground by impact of the
hammers or against the plates of the casing
 The clearance between the housing and
hammers cause size reduction

28.  The material is retained until it is small enough to
fall through the screen that forms the lower
portion of the casing
 Use: it is used to mill almost any type of material,
like dry materials, wet filter cakes, ointments, and
slurries
 Brittle materials are best fractured by impact from
blunt hammer while fibrous materials are best
reduced in size by cutting edges

29. Advantages
 Hammer mills are compact with a high capacity
 Size reduction of 20-40 microns may be achieved
 Hammer mills are simple to install and operate
 The speed and screen can be rapidly changed
 They are easy to clean and may be operated as a
closed system to reduce dust and explosion
hazards

30. Ball mill
 It consists of a horizontal rotating hollow vessel of
cylindrical shape with the length slightly greater
than its diameter
 The mill is partially filled with balls of steel or
pebbles which act as the grinding medium
 If pebbles are used then it is called pebble mill
 If rods/bars are used then it is called rod mill
 Rod mill is suitable for sticky materials which
would hold the balls together because greater
weight of the rods causes them to pull apart

31.  The tube mill is a modified ball mill in which the length
is about four times that of the diameter and the balls
are smaller than in a ball mill
 As the material remains in the longer tube mill for a
greater length of time the tube mill grinds more finely
than the ball mill
 Operation of ball mill
 In a ball mill, rotating at a slower speed the balls roll
and cascade over one another providing attrition
 As the speed is increased, the balls are carried up the
sides of the mill and fall freely onto the material with
an impact action which is responsible for most of the
size reduction

33.  Ball milling is thus a combination of impact and
attrition
 If the speed is increased sufficiently, the balls are
held against the mill casing by centrifugal force
and revolve with the mill
 The critical speed of the ball mill is the speed
at which the balls just begin to centrifuge with the
mill
 Thus at critical speed, the centrifugal force is
equal to the weight of the ball and the critical

34.  At and above the critical speed, no significant
size reduction occurs
 Nc= 76.6/√D where D is the diameter of the mill in
feet
 Ball mills are rotated from 60-85% of the critical
speed
 For optimum speed, the empirical rule is: n=57-
40 logD where n is the speed in revolutions per
minute and D is the inside diameter of the mill in
feet

35.  The smaller balls give slower but finer grinding
 The optimum diameter of a ball is app
proportional to the sq root of the size of the feed:
Dball
2=kD where, Dball and D are the diameters
of the ball and the feed particles, respectively
 If the diameters are expressed in inches then k
may be the grindability constant varying from 55
for hard materials to 35 for soft materials
 To operate effectively, a ball charge of 30-50% of
the volume of the mill is required

36. Advantages
 The ball mill may be used for dry or wet milling
 May be used for batch or continuous operation: for
unstable or explosive materials in batch operation, the
milling chamber may be sealed with an inert
atmosphere and ground
 Ball mills may be sterilized and sealed for sterile
milling for production of ophthalmic and parenteral
products
 The installation, operation and labor costs are low
 It is unmatchable for fine grinding of hard, abrasive

37. Fluid energy mill/micronizer
 The material is suspended and conveyed at high
velocity by air or steam, which is passed through
nozzles at 100-150 psi
 The violent turbulence of the air and steam
reduces the particle size by interparticular attrition
 Air is used as most drugs are thermo labile or
have low melting points
 As compressed air expands at the orifice the
cooling effect counteracts the heat generated by
milling

39.  The material is fed near the bottom of the mill through
a venturi injector
 As the compressed air passes through the nozzles,
the material is thrown outward against the wall of the
grinding chamber and other particles
 The air moves at high speed in an elliptical path
carrying with it fine particles that pass out of the
discharge outlet into a cyclone separator and a bag
collector
 The large particles are carried by centrifugal force to
the periphery where they are exposed to attrition
action

40.  The mill provides internal classification which
permits the finer and lighter particles to be
discharged and the heavier oversized particles
under the effect of centrifugal force to be retained
until reduced to a small size
 The mill reduces the particle to 1-20 microns
 The feed should be pre-milled to app 20-100
mesh size to facilitate milling

41. Cutter mill
 Cutting mills are used for tough, fibrous materials
and provide a successive cutting or shearing
action rather than attrition or impact
 The rotary knife cutter has a horizontal rotor with
2-12 knives spaced uniformly on its periphery
turning from 200-900 rpm and a cylindrical casing
having several stationary knives
 The bottom of the casing holds a screen that
controls the size of the material discharged from
the milling zone

43.  The feed size should be less than 1 inch thick
and should not exceed the length of the cutting
knife
 The size limit of a rotary cutter is 80 mesh
 Roller mills
 They consist of 2-5 smooth rollers operating at
different speeds , thus size reduction is effected
by a combination of compression and shearing
action

44. Colloid mill
 A colloid mill consists of a high speed rotor (3000-
20,000 rpm) and stator with conical milling
surfaces between which is an adjustable
clearance ranging from 0.002-0.03 inches
 The rotor speed is 3000-20,000 rpm
 The material to be ground should be pre-milled as
finely as possible to prevent damage to the
colloid mill
 Use: to process suspensions and emulsions, not
used for dry materials

46.  The premilled solids are mixed with liquid vehicle
before being introduced into the colloid mill
 Interfacial tension causes part of the material to
adhere to and rotate with the rotor
 Centrifugal force throws part of the material across
the rotor onto the stator
 At a point between the rotor and the stator, the motion
imparted by the rotor ceases and hydraulic shearing
force exceeds the particle-particle attractive forces
which cause aggregates

47.  In emulsification, a clearance of 75 microns may
produce a dispersion with average particle size of
3 microns
 The milled liquid is discharged from an outlet in
the periphery of the housing and may be recycled

48. Edge runner mill

52. Factors for selection of mill
 The choice of a mill is based on:
 Product specifications (size range, particle size
distribution, shape, moisture content, physical
and chemical properties)
 Capacity of the mill and production rate
requirements
 Versatility of operation (wet and dry milling;
change of speed, screen, safety features, etc)

53.  Dust control (loss of costly drugs, health hazards,
plant contamination)
 Sanitation (ease of cleaning, sterilization)
 Auxiliary equipment (cooling system, dust
collectors, forced feeding, stage reduction)
 Batch or continuous operation
 Economical factors (cost, power consumption,
space occupied, labor cost)

54. Size separation
 Separation of solids on the basis of size
 It deals with methods of determining size
distribution of sample by screen analysis followed
by methods of for separating dust from gas
streams
 Standard screens
 Various standard screens have been proposed in
which both the diameter of the wire and the
number of meshes per inch (mesh=openings
across one linear inch of screen) are specified
so as to give a definite ratio between the
openings in one screen and the next succeeding
screen in the series

55.  One common set of standard screens is the Tyler
standard screen scale
 This is based on a 200-mesh screen with wire
0.0021 in. in diameter giving a clear opening
0.0029 in. square
 Succeeding coarser screens have their mesh and
wire diameter so adjusted that the area of the
opening in one screen is approximately twice the
area of the opening in the next finer screen

56.  This means that the linear sizes of the openings
in any two successive screens have the ratio of
1:√2
 Normally 200 mesh is the smallest screen
 Another common specification for standard
screens is the US standard
 This uses the Tyler 200-mesh standard as a basis
but differs slightly in other sizes

57. Screen analysis
 Screen analysis of a material is carried out by
placing a sample on the coarsest of a set of
standard screens
 Below the coarsest screen are placed a series of
screens in the order of decreasing size of mesh
 The pile of screens with the sample on the top
screen is shaken in a definite manner, either
manually or mechanically for a definite length of
time and the material collected on each screen is
removed and weighed

60.  A sample screen analysis is given in the table
 The first and fourth columns are the experimental
data
 The second column gives the nominal screen
opening in microns
 The third column gives the average particle size
of the fraction retained on each screen,
calculated as the arithmetic mean of the two
screen openings used to obtain the fraction
 Eg- the material which passed through 14 mesh
screen (1168 microns) and retained on 20-mesh
screen (833 microns) has an average particle size

61.  The graphical presentation of the screen analysis
may be made in a variety of ways, like:
 The weight percent of material retained plotted vs
the average particle size called a frequency-size
distribution plot
 Fraction of total weight of particles having a size
greater than or less than a given screen opening
called the cumulative size distribution curves
 Plotting the log of screen aperture vs cumulative
percent undersize/oversize we get the log-
probability plot

62. Wire screen
 Screen may be obtained in a variety of meshes in
a variety of weights for any given mesh
 In most screens, the wire is a double crimp that
helps to preserve the allignment of the wires
 The ordinary screen usually has the same
number of meshes per inch in both directions

63. Types of screening equipment
 Since screens may be called upon to pass grains
ranging from several inches in diameter to 200
mesh various types of screening equipment have
been developed differing in ruggedness, method
of moving the material across them and materials
of construction
 Based on size of material, we have:
 Grizzlies: used for coarse screening of large
lumps and are of rugged construction
 Trommels: rotating screens used for fairly large
particles
 Shaking and vibrating screens used for fine sizing

64.  Grizzly: it is a simple device consisting of a
grating made of bars usually built on a slope
across which material is passed
 The slope and the path of the material is parallel
to the length of the bars
 The bar is shaped such that the top is wider than
the bottom so that the bar can be made fairly
deep for strength without being choked by
particles
 It is often constructed in the form of a short
endless belt so that the oversize is dumped over
the end while the sized material passes through
 It is used for only coarsest and roughest
separations

66. Trommels
 It consists of a rotating cylinder of perforated sheet
metal or wire screen
 It is open at one or both ends and the axis of the
cylinder is horizontal or slightly inclined so that the
material is advanced by the rotation of the cylinder
 It is best suited for relatively coarse material (1/2 in
over)
 Shaking screen
 Many size separations in which product may be from
½ in down to almost the finest sizes that can be
handled by screens, may be performed by means of
flat or slightly inclined screens that are given a
reciprocating motion

67. Trommel

69.  The frame is of channel irons suspended by hanger
rods so that it can move freely
 It is shaken by means of an ordinary eccentric on a
rotating shaft
 The screen cloth may be riveted directly to the frame
or it may be soldered over a light removable frame
bolted into place
 Vibrating screens
 In some cases instead of giving the screen a shaking
or a reciprocating motion, the screen is vibrated to
keep the particles moving and to prevent blinding
 This vibration may be accomplished by attaching to
the screen cloth, pins that pass through the screen

72.  Rotating shafts on the outside of the casing carry
hinged hammers that strike these pins
 Another method is to place one or two light
channels or other form of bearing surface on the
underside of the screen frame
 These channels rest on cams attached to rotating
shafts
 One of the well known type is the Hum-mer,
another example is Rotex

73. Air separation methods- Cyclone
separators
 Cyclones are mainly used for the separation of
solids from fluids and utilize centrifugal force to
effect the separation
 Such a separation depends not only on particle
size but also on particle density so that cyclones
may be used to effect a separation on the basis
of particle size or density or both
 It consists of a short vertical cylinder closed by a
flat or dished plate on top and by a conical bottom
 The air with its load of solid is introduced
tangentially at the top of the cylindrical portion
 Centrifugal force throws the solid particles out
against the wall and they drop into the hopper

75.  The outlet for the air is usually in the centre of the
top and is also usually provided with a duct that
extends inwardly into the separator to prevent the
air short-circuiting directly from the inlet to the
outlet
 Such separators are widely used for collecting of
wood chips; heavy and coarse dusts and all
manner of separations in which the material to be
removed is not too fine
 They may also be used for separating heavy or
coarse materials from fine dust

76. Air separators
 The cyclone alone cannot carry size separations on
fine materials
 For such separations a current of air combined with
centrifugal force is used
 The feed enters at A and falls on to the rotating plate
B.
 Driven by the same shaft is a set of fan blades C
which produce a current of air as shown by arrows
 Fine particles are picked up by the draft and carried
into the space D where the air velocity is sufficiently
reduced so that they are dropped and removed at E
 Particles too heavy to be picked up by the air stream
fall to the bottom and are removed at F

78. Size separation by settling
 When size separations are to be carried out on
particles too small to screen effectively, or where
very large tonnages are to be handled methods
involving differences in the rates of settling of
particles of different sizes and of different
materials are used
 Eg: two particles of different settling rates in water
are placed in an upward-flowing water stream; if
the velocity of the water is adjusted, the slower
particle will be carried upward, the faster particle
will move downward and a separation is attained

80.  The settling rate of particle also depends on
shape and density besides size
 Classification apparatus may involve simple
settling or settling aided by mechanical devices
 It may operate only on water entering with the
pulp or additional water may be supplied, called
hydraulic water
 If the apparatus is to settle out all the solids
introduced and give a clear overflow, the process
is called sedimentation
 The Dorr thickener is an equipment for
sedimentation

81. General laws of settling-free settling
 Say a spherical particle of density ρs and
diameter D, starting from rest and settling in a
stagnant fluid of density ρ and viscosity μ.
 The bulk of the fluid with respect to the particle is
assumed to be large- the distance of the particle
from the vessel wall or any other solid must be
atleast 10-20 particle diameters
 These conditions define the process called free
settling
 The particle will accelerate under gravity
 As it accelerates the fluid offers a greater
frictional resistance
 At a point of time, the resisting force=gravitational
force and the acceleration is zero

82.  From this time onwards, the particle will settle at a
definite constant velocity
 Let this velocity be ut, called the terminal settling
velocity
 Now, the resistance offered by a fluid to a solid
body in motion relative to the fluid is given by:
 Where, F=total resisting force; A= Area of solid in
contact with the fluid; u= velocity of fluid past
body; ρ =density of fluid; μ = viscosity of fluid; φ’=
a function which is determined experimentally

83.  Since the area of a sphere projected on a plane
normal to the direction of motion is πD2/4
 When the velocity u reaches the terminal value,
the resisting force F=force of gravity
 Since the volume of the particle is proportional to
the cube of diameter

84.  Equating the force of gravity to the resisting force,
 Viscous resistance; Stoke’s law
 Stokes derived a relationship for the resistance
offered to the motion of a sphere in a fluid under
such conditions that the entire resistance is
caused by the internal friction of the fluid and
inertial effects are negligible
 This relationship applies to the laminar-flow
region and may be expressed as:

85.  Substituting Eq
 This relationship is called Stoke’s law has been
shown to be valid for Reynold’s numbers less
than 0.1