2106511 - 05 - crusher - grinding mills

7/31/2019 2106511 - 05 - Crusher - Grinding Mills

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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

2106511 - 05 - crusher - grinding mills

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