2. SIZE REDUCTION
Reduction of particle size is an important operation in many chemical
and other industries.
The important reasons for size reduction are:
• Easy handling
• Increase in surface area per unit volume
• Separation of entrapped components
3. laws governing size reduction
Rittinger’s law
The energy expended during comminution is proportional to the area of
the new surface produced as a result of particle fragmentation.
Kick’s law
The energy is proportional to the size reduction ratio.
Bond’s law
The total work input represented by a given weight of crushed or
ground product is inversely proportional to the square root of the
diameter of the product particles.
4. MECHANISM OF PULVERIZATION
Pulverizer
A pulverizer or grinder is a mechanical device for the grinding of many
different types of materials.
BASED ON PRINCIPLES OF PARTICLE SIZE REDUCTION
-IMPACT
-ATTRITION
-CRUSHING
PULVERISERS USE ONE, TWO OR ALL THE THREE PRINCIPLES.
5. TYPES OF PULVERIZERS
Speed Low
10 to 20 RPM
Medium
40 to 70 RPM
High
900 to 1000
RPM
Type Ball tube mills
(16 rpm in
Vindhyachal)
Bowl Mill,
MPS Mill,
Ball and Race
mill
Impact or
Hammer
Mill.
Beater Mill
or Fan Mill.
6. TYPES OF MILL
• Tube or ball mills: These are low-speed machines that grind the coal with
steel balls in a rotating horizontal cylinder.
• Vertical spindle mill: These are medium-speed machines that include
bowl mills, ring roll mills, and ring and ball mills. The bowl mills are
further divided into deep bowl or shallow bowl mills.
• Impact mill: These are high-speed impact machines that use beater
wheels to crush the coal.
7. Why we pulverizing pigments?
production of finely divided low pigments by an improved milling
process.
It has long been recognized in the art that pigments must be in a
relatively finely divided condition in order to' be useful in the
manufacture of such commodities as paints, paper, rubber, oilcloth
coatings, and the like.
It is necessary to reduce particle size in order to disperse the pigments
in different products.
8. DISPERSING PROCESS
High quality coatings of high brilliance and color strength are characterized by
a perfect pigment dispersion, optimal pigment particle size, and long-term
stabilization of the dispersed particle in the formulation.
The dispersion of a pigment in liquid coatings, paints or inks to produce stable
suspension, can be divided into the following three processes:
9. •Pigment wetting:
• All of the air and moisture is displaced from the surface and between the particles of the
pigment aggregates and agglomerates (clusters) and is replaced by the resin solution. The
solid/gaseous interface ( pigment/air) is transformed into a solid/liquid interface (pigment/resin
solution).
•Grinding stage:
•Through mechanical energy (impact and shear forces), the pigment agglomerates are broken
up and disrupted into smaller units and dispersed (uniformly distributed).
•Stabilization of pigment suspension:
•The pigment dispersion is stabilized by dispersing agents in order to prevent the formation of
uncontrolled flocculates. The resultant suspension is stabilized due to the adsorption of binder
species or molecules at the pigment surface.
10. Some Methods used for dispersing pigments are:
Ball milling
Ball mills are used for heavily agglomerated pigments
or for maximum dispersion.
A ball mill is essentially a large horizontal drum
suspended by axles at each end.
The pigments and resins or liquids are added to the
drum along with heavy round balls made of ceramic,
metal, or stone.
As the drum rotates, the pigment, liquid, and balls
cascade over each other, effectively mixing and
crushing the pigment agglomerates.
11. Stone Mill
The through-put speed is greater than the ball mill, but
it requires constant monitoring and adjustment from an
operator.
A mill base is fed by gravity or pump into an inlet at the
top of the mill.
The mill base is drawn down into a chamber where a
high speed motor is turning a dome-shaped stone
placed at a close range to a fixed stone.
The mill base is forced through the gap between the
two stones, which then grind the pigment.
12. Sand or Bead Mill
High speed media (sand or bead) mills became popular
around 1948 and are still widely used today for wet
grinding and dispersion of solids in liquids.
Bead mills operating with 250 micron grinding media
are effective for converting crude pigments to finished
pigment dispersions.
