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University of Saskatchewan
Geological EngineeringGEOE 498.3
Introduction to Mineral Engineering
Lecture 9 Mineral Processing 2
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Lecture 9Comminution MethodsCrushingGrinding
Classification MethodsHydrocycloneScreening
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These course notes are a compilation of work conducted by many
people.Notes for this lecture have been taken from the following
Edumine courses:Grinding 1 FundamentalsGrinding 2 Unit
OperationsGrinding 3 CircuitsGyratory crushingThe Mill Operating
Resource 1
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Mineral Processing - Review
The goals of mineral processing are to:
separate economic mineral particles from waste or ganguesubject
minerals to processes in order to concentrate them or to extract
metals from them
First step: comminutionSecond step: classification
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What is comminution?
Comminution is another word for size reduction. It is the
process of breaking large particles into small particles.Objective:
liberation
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ReviewWe mine rocks but we concentrate minerals.Gangue minerals
also importantUnderstanding mineralogy allows design of
processesImportant for feasibility studies
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Mineralogy determines recoverability
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Mineralogy studies include grain sizeDetermines particle size to
achieve liberationComminution equipment choices determined by
liberation requirements
Liberation
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Liberation Particles can be:
Fully liberated
Partially liberated: middlings
Gangue: waste
Degree of liberation : percentage of a mineral occurring as free
particles in relation to the total amount of that mineral present
in ore
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Fractures can occur at grain boundaries or across grains
depending on the strength of the grain boundaries. Liberation
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Liberation and Separation
To recover and concentrate valuable minerals, product from the
size reduction step is separated into two streams: valuable mineral
(concentrate), and gangue (tailings).
Middlings are often recycled internally. Flotation, magnetic
separation, and gravity methods are typical separation processes.
Ideally, concentrate should contain 100% valuable minerals and
tailings should contain 100% gangue. In reality, concentrate
contains some gangue and tailings contains some valuable minerals.
Mineral processing combines liberation and separation to
concentrate valuable minerals.
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Terminology % Recovery:
The fraction of valuable metal present in the ore that refers to
the concentrate.
Calculated by dividing the amount of metal in the concentrate by
the amount of metal fed to the mill.
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Terminology Grade: Purity of the product. It is the percentage
(by mass) of a metal in the solids.
Maximum achievable grade depends on chemical composition.
Examples:
Copper grade of a chalcopyrite concentrate is usually between
22% and 32%, vs. pure chalcopyrite contains 34.6% copper. Zinc
concentrate is between 55% and 62% vs. pure sphalerite is about 67%
zinc.
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Liberation and Separation
Grade and recovery are interdependent. In a well-run separation
unit, there is a trade-off between grade and recovery. If the grade
of a product increases, recovery usually drops. BUT Grade and
recovery can both increase when liberation is improved. By
improving liberation, we reduce the quantity of middlings in which
grains of valuable mineral are locked with gangue. Grade and
recovery targets are adjusted to maximize profits.
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Liberation and Separation
Particles can be described
as:roundedslabbyacicularflakyangularParticle shape affects
processing. Usually particles that are not rounded are harder to
grind, to classify, and to pump.
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Liberation and Separation
Density: calculate by dividing its mass by its volume g/cc or
kg/L or tonne/m3
Specific gravity: calculate by dividing the density of the
material by the density of water.
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Liberation and Separation
When the particle does not have a regular shape, or when the
sample contains particles of mixed sizes, diameter is not an
adequate measure.Determine particle size by screen (sieve)
analysis. Screen is made of a woven wire mesh. The size of the
screen is given by the width of one opening, or by the number of
openings in one linear inch. Particle size reported in mesh size or
in microns: 1000 microns (m) per millimeter.Example: A 65-mesh
Tyler screen has 65 holes per inch, and each opening measures 212
m,
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Liberation and Separation
Particle size measurements are used to determine the extent of
liberation. They are also used to evaluate equipment
performance.Screen analysis is the most common way of measuring
particle size. Two common references for particle size measurements
are 80% passing and % passing a specific size.
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Liberation and Separation
From grinding onward, ore is usually handled as a slurry. Slurry
= mixture of water and ore particles, aka pulp.The two most
important slurry properties are:Density Viscosity Strictly
speaking, slurry density should refer to the mass of the slurry per
unit volume. However, in the mineral processing industry, slurry
density is usually reported as % solids by mass.
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Liberation and Separation
Slurry density can be measured with a Marcy pulp density scale.
A Marcy scale is a spring balance fitted with a one-litre container
Marcy scale converts this true slurry density (in kg / liter) to %
solids, given the particle specific gravity.
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Particle breakage
Rate of breakage, also referred to as grinding kinetics, depends
on the type of crushing or grinding equipment and on its operation
Small particles are more difficult to break because for the same
mass many more particles must be broken
a ten fold reduction in particle size requires 1000 times more
breakage events to maintain the rate of breakage).
Coarse - Impact breakage is a violent fracture that results from
striking or compression.
Fines - Attrition breakage is the result of abrasion and wear
caused by rubbing and chipping.
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Work index
Ore hardness refers to the ability to withstand penetration and
deformation. It requires more energy to crush or grind hard ores
than soft ores. The work index is the most common measure of ore
hardness. Ores that are difficult to grind have a high work index.
The work index gives the electrical energy required to grind one
tonne of ore to a specific size. The work index is given in
kilowatt-hours per tonne of material ground. Competency is a
qualitative term used to describe the structural integrity of ore.
If ore is highly fractured or flawed, has poor cohesion, crumbles
easily, it is said to not be competent.
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Beneficiation Terminology
Comminution: Reduction of particle sizeStarts at mine with
blasting Two basic types of equipment used:
Crushing
Grinding
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ShaftComminution Equipment
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Comminution
Primary crushing is the first stage in a circuit that may
include additional stages of crushing, depending on the type of
grinding mills being used.
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Crushing
Extreme duty: high large unit pressuresMaintenance intensive:
operated for 12-16 hours per 24 hour day Downstream interruption
avoided by stockpiling crusher product
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Comminution
On the crushing stroke, a lump of ore is shattered and on the
opening stroke those fragments that are larger than the discharge
gap are retained for further crushing. Those that are small enough
fall through and are no longer available to be broken. In this way
a process of size classification takes place simultaneously with
breakage in a crusher.
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ShaftComminution Equipment
Spider Cap - A heavy steel cap that protects the top of the
mainshaft.Spider - is the steady rest (fulcrum) point for the top
of the mainshaft, constrains itMainshaft - moving part in a
gyratory crusher. It is massive so as to remain rigid during the
crushing process. With the spider acting as the pivot (fulcrum) at
one end, the load in the middle, and the effort applied at the
other end, it acts as a lever to apply high crushing forces.Mantle
- covering of abrasion resistant steel that protects the mainshaft,
intended to wear. Concaves - abrasion resistant steel plates that
protect the crusher top shell bowl, intended to wear Eccentric - A
circular journal and bushing where the bottom of the mainshaft
moves in a circle around the crusher centerline.Pinion - transfers
motor power to the mainshaft Hydroset Piston - moves the main shaft
assembly up or down as oil is pumped into or released from the
cylinder.
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Comminution
Crusher sizing specified by feed opening sizeGape = largest feed
size that a crusher will acceptSet = largest product size it will
discharge, aka Open Size Setting (OSS)Throw = OSS - CSS Typical
reduction ratio is 6:1 to 8:1
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Comminution
Crushing action:When a large piece is broken the fragments fall
downward in the crusher until they are again nipped and once more
broken into smaller pieces throw of the mantle gets larger as the
particle moves down in the chamberdemand for power, becomes greater
as the center of gravity of the rock mass moves downward in the
crusher
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Comminution
Choke feed:Counteract slippage by maintaining a weight on top of
the rock in the crusher.Promote crushing of particles by other
particles. This reduces wear on mantle and concaves, produces more
fines (finished product), and increases the effective reduction
ratio.Even out the power demand.Maximize the machine's
capacity.Choking: stoppage in the downward flow of rock in the
crushing chamber.
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Comminution
Oversize: Blocking: a single rock is too big to enter the
crushing chamber Bridging: two or more rocks are small enough to
enter, but straddle the opening to prevent each other from falling
in Solutions: hydraulic rock breaker: jack hammers the rock into
smaller piecesRock hooklower the mantle with the hydroset
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Comminution
Jaw crusher is a much simpler piece of equipment. Its design
utilizes box frame construction to allow it to handle tougher ore
incorporates a flywheel to store energy
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Comminution
Gyratory crusher or a jaw crusher?
Similarities:speeds are the same -- 100 to 200 revolutions per
minute both break ore by compression accept rock of up to sixty
inches across, discharge down to 7 inches Differences:gyratory
crusher - can be fed from two sides, handle ore that tends to slab,
more energy efficientjaw crusher smaller makes it a logical choice
for underground, can be used on tougher ores
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Comminution
Primary/Secondary Crusher Circuit:Standard installation in a
truck haulage operation consists of the stone box, gyratory
crusher, surge pocket, and feeder Grizzly screen: removes fines
from the crusher feed. consists of heavy steel bars deal with
coarse rock. Secondary crushing is usually done in a separate
crushing and screening plant utilizing cone crushers and vibrating
screens Typically used ahead of rod or ball grinding mills since
these will not accept feed greater than 2.5 cm (1 inch) size
range
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Comminution
Secondary and tertiary crushing circuits: purpose is to reduce
the size of ore to a uniform size, usually +/- 1 cm ( 3/8" ).
Cone crusher:
The crushing head rotates in the bowl with an eccentric motion.
As the head approaches the bowl, particles are nipped and broken
between the mantle and the bowl liner. Rotates at 500-600RPM,
causes hammering rather than squeezing like in gyratory very hard
for ore to pass through this zone without being hit at least
oncegreater angle of the cone crusher puts the pivot point below
the distributor plate
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Comminution
feed inlet: funnels ore into crusher.feed distributing plate:
spreads the feed uniformly around the cavity.bowl: forms the
outside of the crusher cavity. It can be moved up or down to adjust
gap between liner and mantle.bowl liner: protects bowl from
wear.Mantle: protects cone from wear.Cone (or head): forms the
inside of the crusher cavity. It is the moving part of the crusher
that effects crushing.Spring release: protects the crusher from
damage due to tramp metal or other non-crushable material.Crusher
cavity (aka feed pocket): the space where the ore inside the
crusher is located.Eccentric and Pinion: shaft that provides the
circular movement to the cone.Bowl adjustment ring: acts as a giant
nut into which the bowl is screwed. The bowl is raised or lowered
by turning it (like a screw).
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Comminution
Other crushing equipment:High Pressure Grinding Rolls
(HPGR)Newer technology Competes with SAG/AG millsMore energy
efficient
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Comminution
Other crushing equipment:Granulator (hammer mill)Minimize fines
creation Typically used on salt, potash, coalDischarge screen
determines product size
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Comminution
May be open or closed circuitGenerally, the harder the ore, the
more crushing stages Closed circuit ensures uniform size
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Ancillary Equipment
Dust collection required!Crushing produces very fine dust, can
be inhaled and presents a hazard. Dust Enclosures - confined where
it is produced so it can be withdrawn.Ducting - conveys the dust
laden air to the dust extraction equipment.Dry Cyclone - extracts
coarse dust particles from the air by centrifugal force - effective
when used to pre-treatFilter - bags that present a physical barrier
to the dust particles. As air is passed through, particulates
adhere to the cloth - periodic cleaning required.Wet Scrubber -
passes contaminated air through water sprays. The dust particles
adhere onto the fine water droplets, forming slurry with the
dust.Electrostatic Precipitator - passes the contaminated air
through an electric field. The particulates become electrically
charged and are drawn to an electrode of opposite charge. The
collector must be periodically cleaned. Water is typically used,
forming slurry.Slurry Disposal - Slurried dust is collected in
sumps and pumped to the wet grinding circuit.Fans - draws air into
the collection system and through the dust extraction equipment.
Cleaned air passes through fan and discharged.
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Ancillary Equipment
Stockpiles are used to store ore before further
processing.Buffer feed rate variations from upstream operations can
be on a pad, which is typical for coarse ores bins (silos), typical
for fine ores.
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Ancillary Equipment
Feeders: introduce material at a controlled rate. Apron feeder -
most common for run-of mine / crushed orePan feeder aka
hydra-strokeTypically located under stockpiles
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Ancillary Equipment
Screw or vibratory feeder for finer material Usually fines
stored in bins
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Ancillary Equipment
Belt conveyor: moves bulk solids from one area to another. Keep
away from a running conveyor!
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Comminution
Tramp metal is a serious problem in crushing circuits. One of
the most serious is crusher pluggingCan damage machineryCommon
practice to put a belt magnet over the crusher feed conveyor
beltAlso may place a metal detector after the primary crusher.
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Tumbling millsGrinding is usually wet, vs. crushing is usually
dryThe four basic types of tumbling mills are classified by the
type of grinding media used. They are:rod millsball millsautogenous
(AG) millssemi-autogenous (SAG) millsAG and SAG mills are used to
grind very coarse material (up to 30 cm). Rod mills are used for
material up to 3 cm. Ball mills are used for fine grinding. They
are often preceded by an AG, SAG or rod mill.
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Comminution
AG/SAG mills accept a coarser feed than do rod/ball mills.
Typical AG/SAG feed particle sizes range up to 30 cm (12 inches)
which corresponds with the product size of primary crushing. They
do require primary crushing so that the randomly sized run-of-mine
ore is reduced to a uniformly distributed feed size acceptable to
the AG/SAG mill. AG/SAG circuits do not require secondary and
tertiary crushing stages between primary crushing and grinding.
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Comminution
Motion in a tumbling mill Cascading: produces attrition breakage
which leads to fine particle grinding.
Cataracting: produces impact breakage which leads to coarse
particle grinding.
As ore particles become smaller they become less susceptible to
breakage by impact; this means that ore must be reduced by
attrition
Speed: critical speed is when the grinding media are pinned to
the shell by centrifugal force
Normally, mill speed is between 55% and 80% of critical speed.
Mill speed is usually fixed, but some mills have variable speed
drives.
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Comminution
Feed chute: introduces ore into the mill. A seal between the
stationary feed chute and the rotating mill prevents
leaking.Lifters: promote the tumbling action of grinding media.
Liners: protect the shell from wear. Shell: holds liners and
lifters.Trunnions: provide entry and discharge points for slurry.
Usually lined with spiral flights. Normally support the mill.
Trommel screen: prevents large rocks, tramp metal, or grinding
media from leaving with the ground product. Grinding media: loose
objects that move freely inside the mill.
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Comminution
Drive assembly: bull gear/ring gear: transmits the motion of the
pinion to the mill. The mill rotates as the pinion gear meshes with
the bull gearTrunnion bearings: support the mill at either
end.Pinion bearings: support the pinion and motor shaft.Electric
motor: supplies energy to rotate the mill. Motor shaft: transfers
the energy supplied by the motor to the pinion.Air clutch connects
the motor shaft to the pinion. The air clutch protects the motor
from overload during startup: the motor is brought up to full speed
before the clutch is engagedPinion gear: transfers the motion of
the shaft to the bull gear.
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Comminution
Lifters: High profile promotes cataracting and impact grinding,
low profile or beveled lifters promote cascading and attrition
grinding.
Lifter wear: grinding efficiency is affected
rubber liner is typically used in ball and SAG mills and is
light, long wearing, easily replaced and quiet
steel liner is typically used in rod mills where abrasion and
impact factors are high.
As liners wear out, the lifting portion of the liner will be
reduced
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Comminution
Grinding media wear down. Steel consumption is somewhere between
0.2 kg and 1 kg of steel per tonne of mill feed. To maintain
grinding efficiency, new grinding media must be added periodically.
Mill power and other factors are used to determine when to add new
grinding media.
The rate of breakage inside a mill is directly affected by the
size of the grinding media. Grinding efficiency is poor if the
grinding media are too large or too small for the ore.
small media offers more, but lower energy, collisions per unit
of time than larger media.
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Comminution Wet vs. dry grindBecause of the dust problems
associated with grinding solids (health, explosion, and fire
hazard, mechanical losses, etc), grinding is usually carried out in
water. Presence of water in the product does not harm subsequent
separation processes, since most of these operations are carried
out in water. Wet grinding advantageous - requires less power per
ton of material ground than dry grinding. Dry grinding consumes
more energy because the fine particles adhere to the balls, forming
a layer that causes the solids to occasionally slide between the
balls without fracture. The disadvantage of wet grinding, however,
is that there is more wear.
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Comminution
% Solids Optimization Trade-off between: increasing % solids to
maximize the number of particles in the slurry thereby increasing
breakage events decreasing % solids to ensure flow through the mill
and grinding media collide with high energy Operating at the
optimum % solids can have a large impact on grinding efficiency. AG
and SAG mills - 65% to 75% range provides enough water flowing
through the mill to remove ground ore.
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Comminution
To design a circuit there are some factors that have to be
known:the hardness of the orethe tonnage to be processedhow fine
the ore has to be ground
Work index: determined by the electrical output (measured in
Kilowatt Hours consumed) required to reduce one short ton (2000
lb.) of ore to the point that 80% will pass through a 100 micron
screen.
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Comminution
Energy input: Comminution is generally most energy intensive
circuit
Exponentially higher energy input as grind becomes finer
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Comminution
Principles of operation of a tumbling mill: motion of material
inside a mill is described in terms of the changes in center of
gravity.
These changes affect torque and the power required to keep the
mill turning.
Torque: The distance, or arm, is measured from the point where
the force is applied, in this case, the center of gravity of the
load, to the mill center line. Power is torque multiplied by
angular velocity
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Comminution
Autogenous and Semi-Autogenous Grinding mills fed directly into
the mill from either the primary crusher or the mine itself
In a (semi-)autogenous mill the diameter is greater than the
length. The diameter can be as much as 11 m (36')
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AG vs. SAG
Autogenous self-breakingAG mill fully autogenous SAG mill
semi-autogenousAG and SAG mills, coarse particles (ideally about 20
% of 10 cm to 25 cm) are very important since they are part of the
grinding media In SAG mills large balls (10 cm to 15 cm) are added
(typically 6 to 12 % volume loading) to enhance the grinding
action, especially for critical sized material.
Other common option is to combine AG mill with screens and cone
crushers to break critical size in the circuit
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Comminution
AG/SAG:grate discharge assembly serves two purposes: prevent
coarse material from leaving the mill "pump" slurry out of the
mill. Grate sections: prevent coarse material from leaving the mill
while letting fine slurry pass through
Pulp lifters: carry slurry from the grate to the cone as the
mill turns. Slurry leaves the mill by this pumping action.
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Comminution Inside a SAG/AG mill
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Comminution
AG and SAG Mill Overload Potential for material to accumulate in
the mill due to the grate discharge.
If there is too much material in the mill, the mill will not
grind properly, the load will increase further
To avoid this situation mill power and mill load are
monitored.
Stopping the feed for a few minutes is a quick method to check
if the mill is an overload condition. If power increases, then the
mill had been overloaded.
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Comminution
Critical Size
In a grate discharge mill there can be a buildup of what is
known as critical size material (typically 2.5 cm to 7.5 cm). The
rate of breakage of critical size is not fast enough and causes
buildup. In ball mills this occurs for material that is too large
for the balls. In AG and SAG mills it occurs for material that is
too small to act as grinding media but is too large to be ground at
a sufficient rate.
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Comminution
Rod mill: used to grind the product from a crushing circuit
(typical particle size of 3 cm) and grinds it to a size fine enough
for a ball mill to handle (0.5 cm).grinding takes place
preferentially on coarser particles produce less fines than ball
millsMedia: steel rods almost as long as the mill and can weigh
400- 600 lbs
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Comminution
Ball mill :
Takes the product from a rod mill or AG/SAG circuit (typical
particle size of 0.5 cm and finer)
grinds it to a finer size (0.1 cm or finer) usually final size.
usually connected to a classifier to form a closed circuit - coarse
particles are recycled to the mill for further grinding
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Comminution
Other grinding equipment:Ultra fine grinding uses internal
stirrerTower mill vertical cylinder, small media Isamill ultra high
intensity mixing
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Comminution
Operating a grinding circuit
the most important areas to monitor are ... the tonnage coming
into the circuit the grind leaving the circuit
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Beneficiation Terminology
Classification : Separation based on particle sizeBehavior
affected by size, shape, and density of the particlesTwo common
types of classifiers:Screens - mechanical sortingGenerally for
larger particlesStationary or vibratoryWet or dry feed
Hydrocyclones centrifugal forceGenerally smaller particles
(final sizing)Slurry feed
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Classification
Classification is the process of separating a mixture containing
particles of different sizes into two streams: coarse and fine
particles. Perfect classifier: all coarse particles report to the
coarse stream, and all fine particles report to the fine stream.
The line that separates the two is called the cut size.
In practice, classification efficiency is poorer. Some fine
particles leave with the coarse stream and some coarse particles
leave with the fine stream.
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Classification
Partition Curve
When classification is not perfect, the cut size represents the
size at which particles have an equal chance of going to either the
underflow or overflow. Sharpness of separation: indicated by the
slope of the curve. For a perfect classifier, the line is vertical.
When sharpness of separation is poor, the line is closer to
horizontal. Bypass: the percentage of fine particles brought into
the underflow by water.
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Classification Equipment
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Screens
Separates the feed into two or more streams, each containing a
different size range of particles.Separation takes place by letting
fine particles fall through openings in the screen deck.Screen
shape: Rectangular or slotted openings offer more open area and
less blinding for most ores. However, square and round openings
produce a higher sharpness of separation.
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Grizzly Screen
Scalping: removing any material that may slow down production.
May be rock that is too big for the equipment to effectively
handleMay be fine material that is taking up valuable space and
will consume precious energy if it is handled further some
grizzlies are placed on an incline, others flat Slabby rock may sit
on top
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Vibratory Screen
The screen deck has openings to let the smaller material flow
through it. Screen vibration keeps coarse material moving on the
deck. A screen can have several decks, each with a different size
opening.
feed box: (aka feed pan) distributes the feed across the width
of the screen.Counter weight: balances the screen weight to control
vibration more easily.Springs: isolate the floor from screen
vibration.Discharge lip: directs the flow out of the
screen.Tensioning plates: keep the deck secure.
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Classification
Design considerations:
Vibration: amplitude and frequency - promotes
stratificationScreen load: bed of an overloaded screen is too thick
to allow fine particles through Screen slope: must be steep enough
to ensure that the oversize solids will flow across the deck Area:
capacity is proportional to the screen width, while efficiency is
proportional to screen length Water sprays: used to clean coarse
particles and prevent agglomerations of particles
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Classification
Other common screen types:
Derrick screen:
Fine vibratory screen, down to 200 mesh
Alternative to cyclones
Urethane construction
Beware holes in screening!
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Classification
Other common screen types:
Sieve bend (DSM):Wedge wire screenStatic or vibratoryTypically
used for scalping trash from cyclone or gravity concentrator
feed1-5mm opening typical
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Hydrocyclone - Principles
Large particles settle faster than small particles of the same
ore or mineral. Dense particles settle faster than light particles
of the same size and shape - allows us to separate individual
particles. To speed up settling, we can create an artificial
gravitational force, called centrifugal force. a cyclone uses a
rotating motion to create a centrifugal force.
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Classification
Size Separation. The tangential inlet shape of the cyclone
forces feed to travel in a rapid circular path. The circular motion
of the slurry creates the centrifugal force necessary for particle
settling.Larger and heavier particles, shown in blue, which have a
higher settling rate, are thrown against the cyclone wall and flow
down towards the apex.Because of the cyclone design, the conical
bottom in the vortex finder being larger than the apex, most water
moves to the outflow stream, dragging the lighter particles, shown
in yellow, with it. These fines and water form an inner spiral,
which leaves through the vortex finder. During normal operation, an
air core at the center of the cyclone extends from the apex to the
vortex.
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Classification
Inlet: directs the feed into the cyclone - creates a circular
motion.Vortex finder: collects fine material near the top of the
cyclone - called overflow. Most of the water in the feed leaves
with the overflow. The vortex finder extends into the cylindrical
section to prevent the feed from short-circuiting to the
overflow.Cylindrical section: where classification takes
place.Conical section: guides coarse material towards the bottom of
the cyclone.Apex: at the bottom, discharges coarse or heavy
material, called underflow. On some cyclones, the size of the apex
can be adjusted.
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Classification
Cyclones are grouped together in a compact arrangement to
increase capacity. The number of cyclones on-stream can be changed
to adjust capacity. The valves also allow cyclones to be switched
for maintenance.The central feed distributor directs the feed to
each cyclone.The cyclone inlet valves are used to isolate the
cyclones.The cyclones separate fine or light particles from coarse
or heavy particles.The common underflow launder collects the
underflow from individual cyclones.The common overflow launder
collects overflow from individual cyclones.
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Classification
Cyclone variables:
feed % solids is the most important operating variableWhen water
is added, the cut size becomes smaller. A higher feed rate produces
a slightly finer overflow. particles with high specific gravity
have finer cut size than particles of the same size but with a
lower specific gravity.
A smaller vortex produces finer overflow
If the apex capacity is exceeded, the air core inside the apex
collapses and the spiraling motion is almost lost. The discharge
looks like a rope. If there is a surge in the feed rate, quite
often coarse material incorrectly reports to the overflow.
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Classification
Other types of classifiers:
Rake classifierSpiral classifierBoth convey free-settling solids
from the bottomAllow fines to overflow the launder
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Ancillary Equipment
Pumps: transfer slurry from one point to another. increases the
pressure of a fluid to give it the driving force required for
flow.In a grinding circuit, usually centrifugal pumpspump box
provides the pump with surge capacity
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Circuits
Comminution and Classification Circuits
Building blocks of grinding circuits: Tumbling millscyclones or
other classification devicespump boxes, pumpsconveyors, Stockpiles
and feedersThe three basic types of grinding circuits are: Open
circuit Closed circuit Reversed closed circuitThe circuits differ
in the way their components are put together.
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Circuits
Open circuit: the simplest circuit. An open circuit has: a
feedera conveyora millDry ore is fed to the mill by the feed
conveyor.Water is added to the feed at the mill inlet to form
slurry.The mill discharge flows out of the mill to the next process
operation.
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Circuits
Closed circuit: some of the ground product is recycled to the
mill. Water is added to the mill discharge pump box.The cyclone
feed is pumped from the pump box to the cyclone.The cyclone
underflow returns to the mill.The cyclone overflow goes to the next
process operation.Because of this recycle loop returning coarse
particles to the mill, closed loop has higher circuit capacity and
a more uniform product size distribution.
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Circuits
Reversed closed circuit: classification takes place before
grindinghas a higher capacity than a normal closed circuit because
fines are removed before grinding Often used when the circuit feed
comes from a first grinding stage (rod mill or a SAG mill for
example) to remove the fines before they reach the ball mill.
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Circuits
Circulating load: The quantity of coarse material recycled to
the mill in a closed circuit.A circulating load of 300% means that
for every tonne of solids fed to the circuit, three tonnes are
recycled from the cyclone to the mill.Roughly indicates how many
passes particles make through the mill. Calculate by dividing the
recycle rate by the circuit feed (or discharge) rate:Circulating
load = Recycle solids rate / Fresh feed solids rateGiven as a
percentage.Circulating loads for ball mill circuits usually range
from 100% to 400% and from 10% to 60% for AG/SAG mill circuits.
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Circuits
Typical SAG mill/ball mill circuit configuration
Feed to an AG or SAG circuit is stockpiled from the mine or from
a coarse crushing stage with a typical top size of between 15 cm
and 25 cm The grate discharge acts both to retain the grinding
media and to effect classification to the desired size.
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Circuits
Typical AG mill / ball mill circuitScreen oversize is mainly
composed of critical sized material that does not grind readily.
The crusher prevents a build-up of critical sized material in the
AG or SAG circuit, thereby increasing circuit capacity.splitter is
used to regulate the flow of ore to the crusher
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Circuits
Typical rod mill / ball mill circuitFeed is nomally from
secondary/tertiary crushing. The rod mill is open circuit, ball
mill is closed circuit for size control
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Circuits
In general, the most common low-level control loops for grinding
circuits are:Tonnage Water flows Mill % solids Pump box level
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Assignment / Tutorial #9 Tutorial / AssignmentComplete Review
questions on Edumine:
The Mill Operating Resource - 1: Ore PreparationPart 3 -
Secondary and Tertiary Crushing, Review #3 Part 4 - Storage and
Grinding, Review #4
Equipment Nomenclature
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Gold
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Oil Sands
This is an overhead photo of the surface operations of Kidd Mine
and shows the key ventilation infrastructure for the mine. Greater
detail is given in the following slides.
Kidd Mine employs the use of a predominantly exhaust-only type
of ventilation system whereby the primary flows through the mine
are enabled through the operation of a surface exhaust fan system.
The two exhaust shafts are as follows:NVS (North Vent Shaft): The
primary exhaust route for the mine air.SVR (South Vent Raise): The
secondary route of mine exhaust air. The fresh air is drawn into
the mine through four main routes:#2 Shaft: Primary intake. Note
that the air is heated during the winter to prevent freeze-up of
the shaft conveyances.#1 Shaft: Main fresh air supply for Mine D.
This air is refrigerated in the summer using the Refrigeration
Plant and BAC (Bulk Air Cooler)Portal: Fresh air for upper part of
the mine. Air is heated during winter to prevent icing of the
ramp.Open Pit: Air is drawn through the bottom of the pit for the
Cold Stope.