size reduction and role in pharmacy

1. Size reductionalsotermedascoominution,diminutionorpulverization.
It can be achivedbyprecipitation(byadditionof othersolvent) orbymechanical processes.
Name of mill Action Usedfor Notusedfor Special points
Rotary cutter
mill
cutting Tough and
fibrous
materials
Friable
materials
Rollermill compression Softtissues Abrasive
materials
Hammermill impact For all
materials
Sticky,abrasive
material
Hammerhas
basicallyrwo
shapes-a)bar
b)stirrup.Barshaped
extensivelyusedin
tablet
granulation.Hammer
mainlycomposedof
haystellite or
carbaloy.
Ball mill or
pebble mill or
tumblingill
Impact and
attrition.Attrition
predominantat
lowspeeds.
Brittle drugs Soft
tacky,fibrous
material
Can b usedforboth
wetand dry grinding
processes.
Fluidenergy
mill orjetmill
or micronizer
or ultrafine
grinders
Impact and
attrition
Moderately
hard and
fibrous
material
Soft,tackyand
fibrous
material
Usedwhenstrict
qualitycontrol is
desirable forthe
purpose of
absorption
Colloidmill Shearforce For all
material
Notfor dry
milling
Roughsurface used
for fibrous
materials.
Edge runner
mill
Crushingand
shearing
Tough and
hard drugs
Sticky
materials
End runner
mill
Crushingand
shearing
Mainlyall
materials
Sticky
materials
Openand closedcircuit condition:openmeansoperationcarriedoutonlyonce andclosedmeans
operationcarriedoutagainand againto get desiredparticle size.
Theoryrelatedto size reductionis:
Griffiththeory:
Accordingto it,force applieddependsoncracklengthandfocusof stressat the atomicbondof the
crack apex
T=(Y£/c)1/2
T=tensile strength

2. Y=Young modulus
C=critical pathlength
£=surface energyof wall crack.
Theoriesrelatedtoenergyforcomminution:
1)Rittinger`stheory:Energyrequired forsize reductionof unitmassisdirectlypropotionaltonew
surface produced.
E=K(Sn-Si)
E=amount of energy(Kwh)
K=Rittinger`sconstant(kWh/m3
)
Si=initial specificsurface area
Sn=newspecificsurface area
Most applicable tobrittle material undergofinemillingandthistheoryignores particledeformation
before fracture.
2)Bond`stheory:It saysthat energyusedincrack propagationispropotional tothe new crack length
produced.
E=2K(1/(dn)1/2
-1/(di)1/2
) where Kis bond`sworkindex anddi and dn are initial andfinal
diameters.
BOND WORKINDEX:workrequiredtoreduce aunitweightfroma theoretical infinite sizeto80%
passing 100m.
3)Kick`stheory:Energyusedindeformingasetof particle of equivalentshape ispropotionaltothe
ratioof change in size.
E=K lnDi/Dn K iskick`sconstant and Di and Dn are the initial andfinal
diameter.