Solid and Fluid – Solid
Operations
Size reduction
Examples:
Crude ore crushed to small & workable size Synthetic chemicals are grounded into powder Plastic sheets are cut into tiny cubes
Commercial requirements to meet specific size and
shape
Reduced particle size increases the reactivity of
solids
Reduction can enhance the separation of unwanted
Methods of size reduction
1. Compression (nutcrackers)
For coarse reduction of hard solids to give fines
2. Impact (hammer)
Gives fine, medium or coarse products
3. Attrition or rubbing (file)
Yields very fine particles from soft, nonabrasive materials
Size reduction can occur from attrition of one particle by one or more other particles
4. Cutting (pair of shears)
Definite size and shape of particles with very few fines
Force Principle Example
Compressive Nutcracker Crushing rolls Impact Hammer Hammer mill Attrition File Disc attrition mill Cut Scissors Rotary knife cutter
Characteristics of comminuted products
Crushing or grinding is to produce small particles
because of their large surface or their shape, size and
number
In mechanical separations, the energy required to
create new surface is a measure of efficiency
Irrespective of uniformity of feed, most actual crushers
or grinders does not yield uniform product the
product particle size distribution is very wide
Some grinders can control the magnitude of largest
particles in their product but not the fines
Some grinders may minimize fines, but can not
eliminate them
If the feed is uniform (both physical and chemical structure), then shapes of individual units in the product may be quite uniform
Ratio between diameters of largest and smallest particles in a comminuted product is of the order of 104
relationships adequate for uniform sizes must be modified when applied to such mixtures
Because of extreme variation in sizes of the individual particles,
After crushing, unless the particles are smoothed by abrasion, comminuted particles resemble to polyhedrons with nearly plane faces and sharp edges and corners
These particles may be compact, with length, breadth and
Energy and power requirements in
comminution
Major expense in crushing and grinding is the power cost Factors that control power cost are very important
During size reduction, the particles of feed material are first distorted and strained
Work necessary to strain the particles is stored temporarily in the solids as mechanical energy of stress (as in coiled spring)
Then additional force is applied to the stressed particles so that they distorted beyond their ultimate strength and suddenly ruptured into fragments and new surfaces are generated
Since a unit area of solid has a definite amount of surface energy, the creation of new surfaces requires work
This work is supplied by release of energy of stress when the particles break
Efficiency
Size reduction is one of the least energy-efficient of
all the unit operations
Studies reveal that < 1% of energy applied to the
solids is used to create new surface
Rest is dissipated as heat
In operating size reduction machines, energy must
also be supplied to overcome friction in the bearings
and moving parts
Mechanical efficiency is the ratio of the energy
delivered to the solids to the total energy input to the
machine
Crushing laws and work index
Various laws and theories are proposed for predicting power requirements for size reduction of solids do not apply well in practice
Approximate calculations give actual efficiencies of
about 0.1-2%
Assumption: the energy required to produce a change
dDp in a particle of size Dp is a power function of Dp:
Dp: particle size, mm P: Power required, kW
m: mass flow rate, tons per hour
1
n
p p
dP
Rittinger’s law: n = 2
This law implies that the same energy required to produce a material from 100 mm to 50mm as is needed to reduce the same material from 50 mm to 33.3 mm
Kick’s law: n = 1
This law implies that the same energy required to produce a material from 100 mm to 50 mm as is required to
reduce the same material from 50 mm to 25mm
1
1
R pb pap
K
m
D
D
ln
pa K pbD
p
K
m
D
Bond’s law
More realistic way of estimating power required
for crushing and grinding of material
This law postulates that the work required to form
particles of size D
pfrom a very large feed
the
square root of the surface-to-volume ratio of
product (S
p/V
p)
KB is a constant depends on the type of the machine and on the material crushed
6 p p s p B p S p p m V m D K p m D
Work index is defined as the gross energy
requirement in kilowatthour per ton of feed needed to
reduce a very large feed to such a size that 80% of
the product passes a 100µm screens
Dp in mm
P in kW
m in tons per hour
3
100 10
0.3162
B ip
K
W
m
If 80% of feed passes a mesh size of D
pa(mm) and
80% of product passes a mesh of D
pb(mm)
W
iincludes the friction in the crusher and the power
achieved by above equation is the gross power
W
iis available for many standard solid materials (both
wet grinding and dry crushing) such as bauxite, coal,
coke, cement clinker, clay, granite, limestone, etc.
1 1 0.3162 i pb pa p W m D D
Work indices for dry crushing and
grinding
S. No Material Specific gravity Work index, Wi
1 Bauxite 2.20 8.78 2 Cement clinker 3.15 13.45 3 Cement raw material 2.67 10.51
4 Clay 2.51 6.30
5 Coal 1.4 13.00
6 Coke 1.31 15.13
7 Granite 2.66 15.13 8 Gypsum rock 2.69 6.73 9 Hematite (iron ore) 3.53 12.84 10 Limestone 2.66 12.74 11 Phosphate rock 2.74 9.92 12 Quartz 2.65 13.57 For dry grinding, multiply by 4/3.
Ex. Calculate power required to crush 100 ton/h of
limestone if 80% of feed passes a 2” screen and
80% of the product passes a (1/8)” screen
i 100 ton/h 2" 2 25.4 50.8 1 " 3.175 8 1 1 0.3162 ; W for limestone is 12.74 169.6 pa pb i pb pa m D mm D mm p mW D D p kW Equipment for size reduction
Divided into four types: crushers, grinders, ultrafine grinders, cutting machines
Crushers employ compression;
grinders employ impact and attrition (sometimes combined with compression);
ultrafine grinders operate by attrition
Feed size Product size
Coarse crushers 1500-40mm 50-5mm Intermediate crushers 50-5mm 5-0.1mm Fine crushers 5-2mm 0.1mm
Crushers
Breaking large pieces of solid material into small lumps
Primary crusher operates on run-of-mine material, accepting anything that comes from the mine face and breaking it into 150-250mm lumps
Secondary crusher reduces these lumps to particles ≈ 6 mm in size
Grinders
Reduce crushed feed to powder
Product from an intermediate grinder might pass a 40-mesh screen
Product from a fine grinder may pass a 200-mesh with a 74µm opening
Ultrafine grinder
Accepts feed no larger than 6mm
Product size is approximately 1-50µm
Cutters
Size reduction machines used in food
processing engineering
Range of reduction Generic equipment name
Type of equipment
Coarse Crushers Jaw crushers
Gyratory crushers Crushing rolls Intermediate Grinders Roller mills
Hammer mills Tumbling mills Disc attrition mills Fine Ultrafine grinders Hammer mills with
internal classification Fluid-energy mills Agitation mills
Methods of operating crushers
Two methods of feeding material to a crusher
Free crushing: feeding the material at a comparatively low rate so that product can readily escapes
Residence time in the machine is short and production of appreciable quantities of undersize material is avoided
Choke feeding: machine is kept full of material and discharge of product is impeded so that the material remains in the crusher for a longer period
Higher degree of crushing but capacity is reduced
Energy consumption is high because of accumulated product inside machine
used only for small amounts of materials and when it is desired to
Crushers
Slow-speed machines for coarse reduction of large
quantities of solids
Types of crushers
Jaw crushers Gyratory crushers Smooth-roll crushers Toothed-roll crushers
Jaw, gyratory and smooth-roll crushers operate by
compression, for instance, primary and secondary
reduction of rocks and ores
Such primary crushers are often used in mining,
cement manufacture industries, etc.
Jaw crushers
Feed is admitted between two
jaws, set to form a V open at top
One jaw is stationary; the other
driven by eccentric, reciprocates in a horizontal plane and crushes
lumps caught between jaws
Advantages:
high and constant capacity,
high operational reliability,
long lifetime,
easy replacement of wear and spare parts,
Gyratory grinders
Conical crushing head
gyrates inside a
funnel-shaped casing, open at
the top
Eccentric drives the
shaft carrying the
crushing head
Solids caught between
the head and the casing
are broken and
re-broken until they pass
out the bottom
Smooth-roll crushers
These are secondary
crushers, producing a
product 1-12mm in size
Limited by the size of
particle that can be
nipped by the rolls to
feed that range in size
from 12-75mm
Toothed-roll crushers
Roll faces carry
corrugations, breaker bars or teeth
May contain two rolls, or
only one roll working against a stationary curved breaker plate
Not limited by the problem of nip inherent with smooth
rolls
Operate by compression, impact and shear, not by compression alone
Handle softer materials such as coal, bone and soft shale
Grinders
Size reduction machines for intermediate duty
Crusher products are often fed to grinder for further
reduction
Commercial grinders
Haller mills and impactors
Rolling-compression machines
Attrition mills
Hammer mills
Contain high-speed rotor turning inside a cylindrical casing
Feed dropped into the top of the
casing is broken and falls out through a bottom opening
Particles are broken by sets of swing
hammers pinned to a rotor disk
Particle entering the grinding zone cannot escape being struck by the hammers
Particle shatters into pieces, which fly against a stationary plate inside the casing and break into still smaller fragments
These in turn are rubbed into powder by hammers and pushed through a grate or screen that covers the
Several rotor disks, 150-450 mm in diameter and
each carrying four to eight swing hammers, are often mounted on the same shaft
Hammers may be straight bars of metal with plain or enlarged ends or with ends sharpened to a cutting edge
Intermediate hammer mills yield a product 25mm to 20-mesh in particle size
Hammer mills for fine production, the peripheral speed of the hammer tips may reach 110m/s and reduce 0.1-15 tons/h to sizes finer than 200mesh (74µm)
Hammer mills can grind anything – tough fibrous
solids like bark or leather, steel turnings, soft wet pastes, sticky clays, hard rock
For fine production, they are limited to softer materials Capacity and power requirements of a hammer mill
vary greatly with the nature of the feed and cannot be estimated with confidence from theoretical
considerations
They may be found from small-scale or full-scale tests
of the mill with a sample of the actual material to be ground
Commercial mills typically reduce 60 – 240 kg of solid
Impactors
Impactors resembles s heavy-duty hammer mill except that contains no grate or screen
Particles are broken by impact alone, without the rubbing action
characteristics of hammer mill
These are often primary
reduction machines for rock and ore, processing up to 600 tons/h
Rotor in an impactor may be run in either direction to
Roller mills
Solids are caught and crushed between vertical cylindrical rollers and a stationary anvil ring or bull ring
Rollers are driven at moderate speeds in a circular path
Plows lift the solid lumps from the floor of the mill and direct them between the ring and the rolls where the reduction takes place
Product is swept out of the mill by a stream of air to a classifier separator, from which oversize particles are
returned to the mill for further reduction
Application: in reduction of limestone, cement clinkers and coal
They pulverize up to 50tons/h
If the classifier is used, the product may be as fine as 99% through a 200-mesh
Attrition mills
Particles of soft solids are
rubbed between the grooved flat faces of rotating circular disks
In a single runner mill one disk is stationary and one rotates
In double runner machine both disks are driven at high speed in opposite directions
Feed enters through an opening in the hub of one of the disks
Then feed passes outward
though narrow gap between the disks and discharges from the periphery into a stationary
casing
At least one grinding plate is spring-mounted so that
the disks can separate if unbreakable material gets into the mill
Mills with different patterns of grooves, corrugations,
or teeth on the disks perform a variety of operations, including grinding, cracking, granulating, shredding and sometimes blending
Attrition mills grind from 0.5 to 8 tons/h to products
that will pass a 200-mesh screen
Energy required depends strongly on the nature of the
feed and the degree of reduction accomplished and is much higher than in any other crushers and grinders considered so far
Energy requirement is typically between 8 – 80 kWh
Single runner
attrition mill
Double runner attrition mill
Contain disks of
buhrstone or rock emery for reducing solids such as clay and talc, or metal disks for solids such as wood, starch, insecticide powders, and carnauba wax
Metal disks are usually of white iron, although for corrosive materials disks of stainless steel are
necessary
Disks are 250-1400mm in diameter turning at 350-700 rpm
In general, grind to finer
products than single
runner mills but process softer feeds
Air is often drawn
through the mill to
remove the product and prevent choking
Disks may be cooled
with water or
refrigerated brine
Turn faster at
Tumbling mills
A cylindrical shell slowly
turning about a horizontal axis and filled to about one-half its volume with a solid grinding medium forms a tumbling mill Shell is usually steel, lined
with high carbon steel plate, porcelain, silica rock or rubber Grinding medium is metal
rods in a rod mill, lengths of chain or balls of metal, rubber, or wood in a ball mill, flint
pebbles or porcelain or
zirconia spheres in a pebble mill
For intermediate and fine reduction of abrasive materials, tumbling mills are unequalled
Unlike other mills seen so far (require continuous feed), tumbling mills can be continuous or batch
In a batch machine, a measured quantity of solid to be ground is loaded into the mill through an opening in the shell
The opening is then closed and the mill turned on for several hours; it is then stopped, and the product is discharged
In a continuous mill, the solid flows steadily through the revolving shell
In a tumbling mill, the grinding elements are carried up the side of the shell nearly to the top, from whence they fall on the particles underneath
Rod mill
Much of the reduction is done by rolling compression and by attrition as the rods slide downward and roll over one another
Grinding rods are usually steel, 25-125mm in diameter with several
sizes present at all times in any given mill
Rod mills are intermediate grinders, reducing a 20mm feed to perhaps 10-mesh
Often preparing product from a crusher for final reduction in a ball mill
They yield a product with little oversize and a minimum of fines
Ball mill or pebble mill
Most of the reduction
done by impact as the
balls or pebbles drop
from the top of the shell
In a large ball mill the
shell might be 3m in
diameter and 4.25m
long
Balls are 25-125mm in
diameter; the pebbles in
pebble mill are
50-Tube mills and compartment mills
Tube mill is a continuous mill with a cylindrical shell, in
which material is ground for 2-5 times as long as in the shorter ball mill
Tube mills are excellent for grinding to very fine
powders in a single pass where the amount of energy consumed is not of primary importance
Putting slotted transverse partitions in a tube mill converts it into a compartment mill
One compartment mill may contain large balls, another
small balls, and a third pebbles
This segregation of the grinding media into elements of
different size and weight aids considerably in avoiding wasted work, for the large, heavy balls break only the large particles, without interference by the fines
Critical speed of rotating mills
Faster the mill is rotated, the farther the balls are carried up inside the mill and greater the power consumption and the capacity of the mill
If the speed is too high, the balls are carried over and the mill is said to be centrifuging
The speed at which centrifuging occurs is called the critical speed
From a balance between the gravitational and centrifugal forces, the critical speed nc may be founds as below
g is the acceleration of gravity R is the radius of the mill
r is the radius of the grinding elements
Operating speed must be less than nc
Tumbling mills run at 65-80% of the critical speed, with lower
1 2 c g n R r
Ultrafine grinders
Many commercial powders must contain particles
averaging 1-20µm with substantially all particles
passing 325-mesh screen that has opening 44µm
Ultrafine grinders can reduce particles to such fine
size
Ultrafine grinding of dry powder is done by high
speed hammer mills, provided with internal or
external classification, and by fluid-energy or jet mills
Classifying hammer mills
In a hammer mill with internal classification a set of
swing hammers is held between two rotor disks as in
a conventional machine
But in addition to the hammers the rotor shaft carries
two fans, which draw air through the mill inward
toward the drive shaft and then discharge into ducts
leading to collectors for the product
On the rotor disks, there are short radial vanes for
separating oversize particles from the required
Fine particles are carried past the radial vanes as
product
Particles which are too large are thrown back for
further reduction in the grinding chamber
Maximum particle size of the product is varied by
changing the rotor speed or by the size and no. of
separator vanes
Capacity: 1 – 2 tons/h to an average size of 1 – 20
µm
Fluid energy mills
Particles are suspended in a high velocity gas stream
Gas may flow in a circular or elliptical path
Gas flow may act as jets which rigorously agitate a
fluidized bed
Some reduction may occur when particles strike or rub against the walls of the confining chamber
But most of reduction is caused by interparticle attrition Internal classification keeps the larger particles in the mill
until they are reduced to desired size
Suspending gas is usually compressed air or super heated steam admitted at 7atm through energizing nozzles
Grinding chamber is an oval loop
of pipe 25-200mm
Feed enters near the bottom of
the loop through a venturi injector
Classification of ground particles
takes place at the upper bend of the loop
As gas stream flows around this
bend at high speed, coarser particles are thrown outward
against the outer wall while fines congregate at the inner wall
Discharge opening in the inner wall at this point leads
to a cyclone separator and a bag collector for product
Classification is aided by the complex pattern of swirl
generated in the gas stream at the bend in the loop of pipe
Fluid energy mills can accept feed particles as large
as 12mm but more effective when the feed particles are no larger than 100-mesh screen
They reduce up to 1 ton/h of nonsticky solid to
particles of average size 0.5-10µm in diameter using 1 – 4 kg of steam or 6 – 9 kg of air per kg of product
Agitated mills
Small batch non-rotating mills containing solid grinding
medium are available
Grinding medium consists of hard solid elements such as
balls, pellets, or sand grains
These mills are vertical vessel 4 – 1200l in capacity, filled
with liquid in which the grinding medium is suspended
The charge is agitated with multiarmed impellers
Also reciprocating central column vibrates the vessel contents at about 20 Hz
Concentrated feed slurry is admitted at the top and
product (with some liquid) is withdrawn through a screen at the bottom
Colloid mills
Intense fluid shear in a high velocity stream is used
to disperse particles or liquid droplets to form a
stable suspension or emulsion
Final size of particles or droplets is usually < 5µm
Often there is a little actual size reduction in the mills
Principal action is the disruption of lightly bonded
clusters or agglomerates
Syrups, milk, purees, ointments, paints, and greases
are typical products using colloid mills
Chemical additives are often useful for stabilizing
suspensions
The feed liquid is pumped between closely spaced surfaces, one of which is moving relative to the other at speeds of 50m/s or more In some design, liquid
passes through the narrow spaces between a disk-shaped rotor and its casing This clearance are
adjustable down to 25µm Often cooling is required to
remove the heat generated Capacity is relatively low up
to 2-3 l/min for small mills and up to 440 l/m for largest mill
Cutting machines
In some size reduction problems, the feed stocks are too
resilient to be broken by compression, impact or attrition
In other problems the feed must be reduced to particles
of fixed dimensions
These requirements can be met by machines known as
granulators which yield more or less irregular pieces
Other machines can meet these requirements are cutter which produces cubes, thin squares or diamonds
These devices find application in many manufacturing
processes but are especially well adapted to size reduction problems in making rubber and plastics
They find important applications in recycling paper and
Rotary knife cutters
Contain a horizontal rotor turning at 200 – 900 rpm in a cylindrical chamber On the rotor 2 – 12 flying
knives with edges of
tempered steel , passing with close clearance over 1 to 7 stationary bed
knives
Feed particles entering from above may be cut several times before they are small enough to pass through a bottom screen with 5 – 8 mm openings
Criteria for size reduction process
A crusher, grinder or cutter cannot be expected to
perform satisfactorily unless
The feed is of suitable size and enters at a uniform rate
The product is removed asap after the particles are of desired size achieved
Unbreakable material is kept out of machine
In the reduction of low melting or heat sensitive products, the heat generated in the mill is removed
Therefore heaters and coolers, metal separators,
pumps and blowers and constant-rate feeders are
important adjuncts to the size reduction unit
Open- and close-circuit operations
In many mills, the feed is broken into particles of satisfactory size by passing it once through the mill
When no attempt is made to return oversize particles to the machines for further reduction, the mill is said to be
operating in open-circuit
This may require excessive amounts of power, for much of the energy is wasted in regrinding particles that are already fine enough
Thus it is often economical to remove partially ground
materials from the mill and pass it through a size separation device
The undersize becomes the product and the oversize is returned to be grounded
The separation device is sometimes inside the mill, as in ultrafine grinders; more commonly it is outside the mill
Close-circuit operation is applied to the action of a mill and separator
connected so that the oversize particles are returned to the mill
Energy must be supplied to drive the conveyors and separators in a
closed-circuit system
But despite this, the
reduction in total energy requirement over open-circuit grinding often reaches 25%
Preliminary guide for selecting size
reduction equipment
Equipment Max. Feed size (mm) Min. Prod. size (mm) Capacity (ton/day) Applications examples
Jaw crushers 1500 150 <1 - >103 Metallic and nonmetallic
minerals Gyratory
crushers
2000 300 >103 Metallic and nonmetallic
minerals
Roller mill 30 1 1 - >103 Cereals, vegetables,
calcite, kaolin
Hammer mill 40 0.01 <1 - <103 Phosphates, pigments,
dried fruits Disc attrition
mill
12 0.07 <1 - 103 Cellulose, asbestos,
rubber
Ball mill 4 0.3 10 - >103 Calcite, kaolin, ceramics
Fluid-energy mill
30 0.001 <1 - 102 Ceramics, pesticides,
pigments
