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Impact Crusher
Utilizes impactrather than compression Impact from sharp blows applied at high speed
to free-falling ores
The beaters transfer the kinetic to ore particles.The internal stress of ore increase and finally
break the ores into finer pieces.
Widely used in quarry(improved productshape)
Common impact crushers : hammer mill and
impact mill
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Hammer Mill The hammers are pivoted to
move out of the path of
oversize material.
The pivoted hammers exertless force than rigidly
attached.
The exit from the mill is
perforated, so that theoversize materials are retainedfor further impacting.
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This mill is designed to give the particlesvelocities of the order of that of the hammers.
Fracture is either due to the severity ofimpact
with the hammers or to the subsequent impactwith the casing or grid.
Much of the size reduction is by attrition (high
proportion of fines in product). Widely used in non-abrasive ores such as
limestone and coal.
Hammer Mill
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Impact Mill
The ore falls tangentially on to a rotor receiving aglancing impulse, which sends it spinning towards theimpact plates.
The velocity imparted is deliberately restricted to afraction of the velocity of the rotor to avoid enormousstress and probable failure of the rotor bearings.
The fractured pieces which can pass between theclearances of the rotor and breaker plate enter asecond chamber created by another breaker plate,
This grinding path is designed to reduce flakiness andgives very good cubicparticles.
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This impact mill gives a much better control of product
size than does the hammer mill and saves energy.
The blow bars are reversible to even out wear, and can
easily be removed and replaced. It can reduce 1.5 m top size run-of-mine ore to 20cm, at
capacities of around 1500 t/h or up to 3000 t/h.
It should not be used on ores containing over 15% silica
since it can provide high reduction ratio (40:1).
It is suitable with non-abrasive ores such as limestone.
Impact Mill
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Rotary Coal Breaker
It is often used with large tons of coal.
It consists of a cylinder of 1.8 3.6 m in diameter and
length 1.5-2.5 times.
It is massively constructed withperforated walls
(perforating size = size of ore to be broken).
The machine utilizes differential breakage, the coal
being much more friable than the associated stones
and shales, and rubbish such as wood & steel . The small particles of coal and shale quickly fall
through the holes, while the larger lumps are retained,
and are lifted by longitudinal lifters.
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Rotary Coal Breaker
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Crushing Circuits and Control
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Grinding Mills
Grinding - the last stage in the process ofcomminution is a combination ofimpact, chipping
and abrasion.
It can be done dry or in suspension in water.
It is performed in rotating cylindrical steel vessels
which contain a charge of loose crushing bodies - the
grinding medium- which is free to move inside the
mill, thus comminuting the ore particles.
According to the ways by which motion is imparted
to the charge, grinding mills are generally classified
into two types: tumbling mills andstirred mills.
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The mill shell is rotated and motion is impartedto the charge via the mill shell.
The grinding medium may be steel rods, balls,
or ore itself. Tumbling mills are typically employed in the
mineral industry for coarse-grinding processes,in which particles between 5 and 250 mm are
reduced in size to between 40 and 300 m. Common tumbling mills : ball mills, rod mills
and autogenous mills
Tumbling Mills
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Stirred Mills
The mill shell with either a horizontal or avertical orientation is stationary and motion is
imparted to the charge by the movement ofan
internal stirrer.
Fine grinding media inside the mill are agitated
or rotated by a stirrer, which typically comprises
a central shaft to which are attached pins or
discs of various designs.
Stirred mills find application in fine (15-40 m)
and ultra-fine (
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Correct grinding (an economic optimumparticle size) is the key to good mineral
processing.
Under-grindingof ore degree of liberation istoo low for economic separation
Over-grinding of ore wasting too much
energy to needless size reduction Grinding consume most energy in
comminutionthe greatest operation cost.
Grinding Mills
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There is a special case where over-grinding can beadvantage.
In case of gold concentration, over-grinding increasethe surface area, thus enhancing the efficiency of
gold leaching by cyanide.
The increased energy consumption can be offset bythe increase gold recovery.
This can be an advantage due to the fact thatleaching is much more efficient on particles withlarge surface area relation to the mass.
Advantage of Over-grinding
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Gold Processing Flow
1. Gyratory crusher
2. Conveyor
3. Stockpile
4. Feed
5. Primary cyclone
6. SAG mill
7. Ball mill8. Secondary cyclone
9 - 19 are chem. proc.
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Grinding Mechanisms
Impact or
CompressChipping Abrasion
These mechanisms distort the particles and
change their shape beyond certain limits determined by
their degree of elasticity, which causes them to break.
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Tumbling Mills
In tumbling mills, grinding is influenced bythe size, quantity, the type ofmotion and space
between the pieces of ores and medium in the
mill. It is a random process.The degree of grinding depends on
1) The probability of ore entering a zone
between the medium shapes2) The probability of some occurrence taking
place after entry
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The Motion of Charge in Tumbling Mills
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The Motion of Charge in Tumbling Mills
Surging
Cataracting Centrifuging
Cascading
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Centrifuging in Tumbling MillsIn practice, centrifuging occurs and the medium is
carried around in an essentially fixed position against theshell.
mg = mv2
R-r
v = 2 (R-r) N
N = 1 g 1/2
2 R-r
R>>r :N = 54.2/R1/2
R Radius of shell
r- radius of mediumN Critical speed
(min. required for
centrifuging)
V-linear velocity
Critical Speed = The speed at which medium cataract
down the toe instead of following a circular path
At critical speed, the mill behave like centrifuge.
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Mills are driven, in practice, at speeds of50-90%of critical speed, the choice being influenced by
economic considerations.
Increase in speed increases capacity, but there islittle increase in efficiencyabove about 40-50%
of the critical speed.
Centrifuging in Tumbling Mills
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Construction of Mill
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Mill Shell Liners
Mill liners are a major cost in mill operation.
It is very important to prolong the life of liners
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Mill End
Grate / Pulp lifter / Discharge trunnion / Discharge cone
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Various Section of Mills
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Conventional Designs of Pulp Lifter
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Rod Mills Fine crusher or coarse
grinding machine
Capable of reducing 50 mm
to 300 m
Reduction ratio 15:1 to 20:1
The length of cylindrical shell is between 1.5 and 2.5 timesits diameter (to prevent the rods become wedged across
the diameter).
The rod > 6m will bend, so the length of mill is limited at
6.4 m. The diameter should not be over 4.57 m.
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Rod mills are rated by power (rather than capacity)which can be estimated by Bonds equation:
W = Wi 10 - 10
Rod mills are classified by the nature of discharge:
- Centre peripheral discharge mill
- End peripheral discharge mill
- Overflow mill
Rod Mills
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Centre Peripheral Discharge Mill
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End Peripheral Discharge Mill
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Overflow Mill
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Rod Mills
Rod consumption varies widely with thecharacteristics of the mill feed, mill speed, rod
length, and product size; it is normally in the
range 0.1-1.0 kg of steel per tonne of ore for wet
grinding.
Optimum grinding rates are obtained with new
rods when the volume is 35% of that of the shell.
This reduces to 20-30% with wear and is
maintained at this figure by substitution of new
rods for worn ones.
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Rod Mills
The grinding action results from line contact of
the rods on the ore particles; the rods tumble in
essentially aparallel alignment, and also spin,
thus acting rather like a series of crushing rolls.
This increases the tendency for grinding to takeplace preferentially on the larger particles,
thereby producing a minimum amount of
extremely fine material.
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Factors on Efficiency of Rod Mill
Pulp density should be maintain at 65 85 %to reduce metal to metal contact
Grinding surface area - medium charge with
rods of different diameters (25 - 150 mm) tomaximize the area. A charge with smaller rodsalso increase the area.
Rods should be made of high carbon steel
Mill charge volume should be maintained at45% of volume (The optimum grind with newrods 35%)
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This selective grinding gives a product ofrelatively narrow size range, with little oversize
or slimes.
Rod mills are therefore suitable for preparationof feed to gravity concentrators, certain flotation
processes with slime problems, magnetic
cobbing, and ball mills.
They are nearly always run in open circuit
because of this controlled size reduction.
Rod Mills
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Ball Mills
The final stage in comminution are performed inball mill.
Balls have a greater surface area per unit weight
than rodsbetter for fine grinding. Length to diameter ratio (L/D) is 1.5 to 1 and less.
If L/D is 3-5, called tube millorpebble mill. The
grinding mediums are pebble or flints. It is widely
used in ceramic raw material. It runs with 75-85%
of critical velocity.
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Since the weight of pebbles is less than steel balls,the power input and capacity of pebble mills are
correspondingly lower.
In a grinding circuit, a pebble mill would be muchlarger than a ball mill, with correspondingly higher
operating cost.
The higher operating cost can partially offset bythe lower cost of the grinding medium and energy
costper tonne of finished product.
Ball Mills
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Mechanism of Grinding in Ball Mill
Random impactcrushing of ore particles
Balls impact on ore particles at the toe
Due to the random grinding, the product
would have wide distribution of size.
The problem ofover-grinding often occurs.
Ball mill is generally controlled in a closed-circuit grinding with short resident time, in
order to control over-grinding.
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- It is fitted with discharge grates between the cylindrical
mill body and the discharge trunnion.- It has lower pulp level than overflow mills.
- Very little over-grinding takes place and the product
contain a large fraction of coarse material.
Ball Mills Type
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Ball Mills Type
- Simple trunnion overflow mills
- Most widely used, especially for find grinding andregrinding.
- Less energy consumption (15%) than grate discharge
mill
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Ball Mills
Ball mills are rated by power rather than capacity. Today the largest ball mill in operation is 7.3 m in
diameter with a corresponding motor power of
more than 11 MW.
Grinding in a ball mill is effected bypoint contact of
balls and ore particles and, given time, any degree of
fineness can be achieved.
The process is completely random- the probability of
a fine particle being struck by a ball is the same asthat of a coarse particle.
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Thepulp density: should be as high as possibleto ease the flow through the mill
The balls must be coated with a layer of ore. (To
avoid metal to metal contact the wear of steelballs)
Ball mill should operate between 65 80% solid
by weight. Viscosity( fineness of ore) can lower pulp
density of fine grinding.
Factors on Efficiency of Ball Mills
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Surface area of medium (balls) High surface
area for fine grinding
Charge volume should be maintained at 40 -
50 % of mill volume
Mill speed should be run 70 80% of critical
speed (to achieve the cataracting of medium)
Factors on Efficiency of Ball Mills
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Variables for Optimization Ball Mill
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Autogenous Mills
An AG mill is a tumbling mill that utilizes the ore
itself as grinding media.
The ore must contain sufficient competent pieces
to act as grinding media.
An SAG mill is an autogenous mill that utilizes
steel balls in addition to the natural grinding
media.
The ball charges used in SAG have generally beenmost effective in the range of4-15% of the mill
volume (including voids).
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The length to diameter ratio is around 5. It runs with 80 85 % of critical velocity.
It can reduce the ore size of25 cm to 0.1 mm.
AG Mills are widely used with iron ores, whileSAG are preferable with non-ferrous ores.
AG and SAG are classified by the aspect ratio
of diameter to length.
Autogenous Mills
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AG/SAG Mills
The main advantages:
- Lower capital cost
- Wide range of ore (sticky, clay)
- Simple flowsheet
- Low manpower
- Reduce grinding media expense
SAG Mill
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SAG Mill
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Energy Consumed in Tumbling Mills
Energy Input VS Charge Volume
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Vibratory Mills
It is designed for continuous or batch grindingto give a very fine end product from a wide
variety of materials
It can be performed either wet or dry. Two tubes functioning as vibrating grinding
cylinders are located one above the other in a
plane inclined at 30 to the perpendicular. Between them lies an eccentrically supported
weight connected by a flexible universal joint toa 1000 -1500 rev/min.
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Rotation of the eccentric vibrates the tubes toproduce an oscillation circle of a few mms. Thecylinders are filled to about 60 -70% withgrinding medium, usually steel balls of diameter
10 50 mm. The material being ground passes longitudinally
through the cylinders like a fluid, in a complex
spinning helix, thus allowing the grindingmedium to reduce it by attrition.
The material is fed and discharged throughflexible bellow-type hoses.
Vibratory Mills
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Vibratory Mills
ill
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Tower Mills In Tower mills, the motion is imparted to the charge
by the movement of an internal stirrerwhile theshells are stationary.
It is an alternative for fine and ultra-fine grinding.
The feed enters at the top, with mill water, and isreduced in size by attrition and abrasion as it falls,the finely ground particles being carried upwards bypumped liquid and overflowing to a classifier.
Oversize particles are returned to the bottom of thechamber and classified by hydrocyclone, whichreturn underflow to the mill sump for furthergrinding.
T Mill
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Tower MillsAdvantages:
- Small installation area- Low noise
- Efficient energy usage
- Minimal overgrinding
- Low capital and operating cost- Product size can be 1 100 m
- Capacity up to 100 t/h- Widely used with limestone, silica,rock salt, coal and copper
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Stirred Mills
Despite the manufacturers claim that tower millscan achieve a product size of 1 m, they are
generally used at the coarse end of the fine grinding
spectrum due to the use of coarse media (around 6
mm), and operation of their stirrer at a relatively
slow velocity (less than 3 m/s tip speed).
Finer grinding is normally achieved with stirred
mills. It employs stirrers comprising a shaft with pins or
disks.
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It can be classified by their shell orientation:- Sala Agitate Mill (SAM) : vertical
- IsaMill : horizontal
IsaMill is widely used for before flotation or oreleaching due to the high efficiency in sharp cut
of ore by g-forces.
The worlds largest ultra-fine grinding mill is 8MW M 50000 IsaMill in Anglos Platinum, South
Africa.
Stirred Mills
I Mill
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IsaMill
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- Energy efficiency- Improved recovery
- High intensity
- Open circuit configuration- Simplicity in maintenance
- Large scale
- Horizontal layout- Low installation cost
- Accurate scale up
Advantages of IsaMill
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Roller Mill It is often used for dry grinding of medium
soft materials of up to 4-5 mohs hardness.
It can reduce the ore of 2 inches to 400 #
The grinding is a result from movement of the
roller and the pneumetic transport of fine to
air separator (size classification).
It is widely used in coal, gypsum, barite and
clinker.
R ll Mill
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Roller Mill
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Grinding CircuitsAdvantages of wet grinding:
- It consumes lower powerper tonne of product.
- It has higher capacityper unit mill volume.- It makes possible the use of wet screening or
classification for close product control.
- It eliminates the dust problem.
- It makes possible the use ofsimple handling
and transportmethods such as pumps, pipes,
and launders.
Wet or dry feed ?
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Grinding Circuits
Single-stage open circuit : Rod mill
Simple closed-grinding circuit : Ball mill
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Grinding Circuits
2 Stages grinding circuit
2 Stages grinding circuit: rod mill and ball mill