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