6. What is Mesh Size? U.S. Mesh Size (or
U.S. Sieve Size) is defined as the number of
openings in one square inch of a screen.
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25. THEORIES OF SIZE REDUCTION /
MILLING
• A number of theories have been proposed to
establish a relationship between energy input
andthe degreeof sizereduction produced.
• Rittinger’s theory :
• Bond’stheory
• Kick’stheory
• Walker’s theory
26. Rittinger’s theory
Rittinger’s theory suggests that energy
required in a sizereduction process is
proportionalto the new surfacearea
produced.
where, E=energyrequired for sizereduction
KR=Rittinger’s constant
Si=initial specificsurfacearea
Sn=final specificsurfacearea
Application:It ismostapplicablein sizereducingbrittle
materials undergoingfine milling.
27. Bond’stheory
• Bond’s theory states that the energy used in crack
propagation is proportional to the new crack length
produced.
• where, E=energyrequired for size reduction
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KB=Bond’swork index
di =initial diameter of particles
dn=final diameter of particles
• Application:Thislaw isusefulin roughmill sizing.Thework
indexisusefulin comparing the efficiency of milling
operations.
28. Kick’stheory
• Kick’s theory states that the energy
used in deforming (or fracturing) a set of
particles ofequivalent shape is proportional to
the ratio of change of size/diameter
• where, E=energyrequired for size reduction
•
•
•
KK=Kick’s constant
di =initial diameter of particles, dn=final diameter of particles
• Application:Forcrushingof largeparticlesKick’stheory most
useful.
29. Walker’s theory
• Walker proposed a generalized differential form of the
energy-size relationship:
Where E=amount of energy(work done) required to producea change
D=sizeof unit mass K=Constant n=constant
For
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n=1.0 Walker equation becomesKick’stheory
It isusedfor coarseparticles >1 um.
n=1.5 Walker equation becomesBond’s theory.
Thistheory isusedwhenneither Kick’snor Rittinger’s law isapplicable.
n=2.0 Walker equation becomesRittinger’s theory
Usedfor fine particles<1 um size.
35. ADVANTAGESOFHAMMERMILL
• It is rapid in action, and is capable of grinding many
different types of materials.
• They are easy to install and operate, the operation
iscontinuous.
• There is little contamination of the product with
metal abraded from the mill as no surface move
againsteach other.
• The particle size of the material to be reduced can
be easily controlled by changing the speed of the
rotor, hammertype, shapeandsizeof the screen.
36. DISADVANTAGES
• Heat buildup during milling is more,
therefore, product degradationispossible.
• Hammer mills cannot be employed to mill
sticky,fibrousandhard materials.
• Thescreensmayget clogged.Wearing of mill
andscreenismore with abrasive materials.
37. BALLMILL
• Theseare alsoknowsastumbling mills.
• Principle: The ball mill works on the principle of
impact between the rapidly moving balls and the
powder material, both enclosedin a hollow cylinder.
• At low speeds, the ball roll over each other and
attrition (rubbing action) will be the predominate
mode of action. Thus, in the ball mill, impact or
attrition or both are responsible for the size
reduction.
