Comminution in Mineral Processing

8/11/2019 Comminution in Mineral Processing

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After crushing, the next stage of mineral processing

is grinding it is last stage of comminution.

Particle size reduction is achieved by a combination

of impact and abrasion.

Grinding/milling is performed in rotating cylindrical

vessels called tumbling mills.

They contain loose, crushing bodies (grinding

medium) which are large, hard and heavy relative tothe ore particles, but small in relation to the mill

volume.

G.Dombo UNAM 2014TMLE3692 Lectures 5 and 6: Chapter 2;

Mineral Processing


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Grinding is usually performed wet although dry

operations are also possible. When the mill is

rotated the mixture of medium, ore and water (the

mill charge) is intimately mixed, the medium

comminuting the particles

Mechanisms of comminution:

impact/compression

chipping

Abrasion

G.Dombo UNAM 2014

TMLE3692 Lectures 5 and 6:

Chapter 2; Mineral Processing


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Tumbling Mills

A mill is basically a horizontal, cylindrical shell with

renewable wearing liners and a charge of grindingmedium.

The cylinder is supported so that it rotates on its

axis on hollow trunnions attached to the end walls.

Feed material is fed to the mill continuously through

one end trunnion, the ground product leaving via

the other trunnion, although some ports on the

cylinder periphery may be used

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At relatively low speeds the media tends to roll

down to the toe of the mill and abrasive

comminution occurs.

media is said to be cascading and this leads to

finer grinding but increased liner wear

At higher speed the medium is projected clear ofthe charge parabolically before landing on the toe

of the shell

This is called cataracting and leads to

comminution by impact and a coarser endproduct with reduced liner wear.

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optimum mill speed should result in both cascading

and cataracting

cr i t ical speed of the mill is speed at whichcentrifuging occurs and the medium is carried

around in essentially a fixed position against the

shell

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Suppose a rod or ball of radius r (or d/2) is lifted up

the shell of a mill of radius Rmetres (or D/2),

revolving at a speed of N rev/min.

Let mbe the mass of rod or ball and V be its linear

velocity.

At a pointpwhen the ball abandons circular path

for parabolic path, the gravitational force of the ball

just balances the centrifugal force acting on it.

Thus;

Or

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But

Therefore

The effective radius R = (D-d)/2

i.e.

critical speed of the mill occurs when = 0 i.e. cos

= 1

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Therefore critical speed of tumbling mill is given by

Mills usually operate at 50-90% of the critical speed

to achieve optimum grinding and liner wear There are 3 basic types of grinding millsrod mill,

ball mill and autogenous mill: based on the grinding

media

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ideal for coarse grinding. Steel rods are used as grinding media

capable of taking feed as large as 50mm and

making a product as fine as 300m

Reduction ratios of rod mills range from 15-20 : 1

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The length of the cylindrical shell is 1.5 to 2.5 x

diameter and the rods are only a few cm shorter

than the shell length to prevent them fromturning so that they become wedged across the

diameter of the cylinder.

They produce a narrow size range which is

suitable feed to gravity concentrators, magneticseparators and ball mills

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Used for final stage of comminution

steel balls are used as grinding media. Balls have

a greater surface area/unit weight than rods and

therefore are capable of fine grinding.

length of the cylindrical shell is 1 to 1.5 x diameter. If ratio goes up to 3-5, then the mill is called a tube

mill.

Tube mills are usually used in dry operations to

grind cement, gypsum and phosphates and oftenpebbles are used as the grinding medium

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The discharge system of a ball mill can either be

grate discharge where pulp flows through theopenings of the grate and is lifted to the trunnion

opening. It produces a coarser product

mill overflow discharge where pulp overflows

through trunnion. Results in finer product but has

less energy efficiency

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Ball mills are normally operated at 65-80% solids.

The average ball diameter,

Where dp average particle size

Grinding balls are usually made of forged or rolled

high C steel and consumption ranges between

0.1 to 1kg/t of ore depending on hardness of oreand fineness of grinding required.

Medium consumption is one of the largest costs in

milling operations with up to 40% of total costs

and therefore good grinding operation is acompromise between medium wear rate and

grinding efficiency.

0.5

b pd kd

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Comminution is effected by the action of ore

particles on each other.

Autogenous mills are normally used for uniform ore

which is neither too hard nor too soft.

A variation of AG is semi autogenous mill (SAG) inwhich the grinding medium is a combination of the

ore and a reduced load of steel balls.

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Advantages of SAG over conventional grinding

(on suitable ores) include:

lower capital costs

ability to handle wet and sticky material

lower manpower requirements

minimal grinding media expenses.

This method is the one used in marined iamonds process ing. Why not high load of

steel balls???

Other mills include vibratory mills (vibration ofshells), centrifugal mills (centrifugal force) and

tower mills (screwing effect)

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Feed in grinding operations can be wet or dry

Advantages of wet (over dry) grindinglower power consumption per tonne of product

(why?)

higher capacity per unit mill volume (why?)

wet screening/classification is possible, for

close product control

environmentally friendly as dust is eliminated

handling and transportation is made simple byuse of pumps, pipes, launders

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Advantages of dry (over wet) grinding

reduced lining wear

does not alter the chemical nature of the oree.g. in the case of cement!

Suitable for minerals like NaCl that easily

dissolve in water

higher proportion of fines

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Grinding circuits are classified as open or closed

Open ckt: grinding material is fed into the mill ata rate calculated to produce the correct product

size in one pass.

The feed rate must be low enough to ensure that

every particle spends enough time in the mill to bereduced to required product size.

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TMLE3692 Lectures 5 and 6:Chapter 2; Mineral Processing


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closed circuit:

no effort is made to effect all the size reduction

in a single pass.

Material returned to the mill by the classifier is

known as the circulating load.

Grinding in the minerals industry is almostalways in closed circuit

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Advantages:

reduces residence time of particles in each

pass.

Decreases over grinding and therefore

increases energy available for useful grinding.

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Comminution in Mineral Processing

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