COMMINUTION

Size reduction by particle breakage
▪ liberation
• Create particles in certain size and
shape
• Increase surface area available for next
process

Comminution usually accounts for more than ____ total power

40 % to 50%

Only about ____ of the energy input is used to comminute

3% – 5 %

Problems associated with comminution:

➢ It is mechanically a wasteful process.
➢ Most of the energy input is lost to a lot of factors.
▪ Absorbed by the equipment
▪ Water resistance in pulp
▪ Deformation (not breakage) of plastic particles
➢ Heat is unavoidably produced.

An empirical relationship between energy consumed during size reduction and particle size.

Rittinger’s Law

energy of breakage ∝ amount
of new surfaces produced

Kick’s Law

energy required ∝ reduction in volume of the particles concerned

Bond’s Law

energy required ∝ new crack
tip length produced in particle
breakage

Work Index

Energy required to reduce one ton of ore from a very large size to 100µm

Jaw Crusher

1) Blake

2) Dodge Type

BlakeS

Movable jaw is pivoted at the top

Dodge Type

Movable jaw is fixed at the bottom

Relative motion of the crushing faces is
due to gyration motion of an
eccentrically mounted cone

gyratory crusher

Comminution by impact; sharp blows on free-falling rock

IMPACTORS

Modified gyratory crusher that yields smaller product

CONE CRUSHERS

Consists of two horizontal cylinders
revolving towards each other

ROLL CRUSHERS

GRINDING

Last stage in comminution
process

Grinding Size Reduction

impact + abrasion

What is used when grinding
medium is present

balls or rods

Green

• Parabolic path taken by charge as
projected by lifting of the mill
• Coarse grinding by impact

Cataracting

Black

• Rolling motion of the charge down to
the toe
• Leads to finer grinding by abrasion

Cascading

Red

Charge is carried around in a fixed
position

Centrifuging

Uses cast iron rods
as grinding medium

Rod Mill

Uses forged steel
balls as grinding
medium

Ball Mill

The action of the ore grinding upon itself with small amount of ball charge

Semi-autogenous Mill

SIZING

Selection of materials based on their size

SCREENING (Industrial Screening)

Use of screening equipment (screens)

• Extensively used for size separations from 300 mm down to roughly 40 µm, efficiency decreases rapidly with fineness

Screen equipment are classified as

stationary and moving

SCREENING objectives

To optimize the feed of the comminution stage by preventing the
materials which are already smaller, not to enter this stage
• To ensure that the desired product is according to the size
application
• To ensure that the feed to succeeding process does not exceed
size limitation of receiving area

Sizing/Classifying

To separate particles by size

Scalping

To remove coarsest size fractions in the feed material

Grading

To prepare a number of products within specified size ranges

Media recovery

For washing magnetic media from ore in dense medium circuits; or to retain grinding media inside grinding mills

Dewatering

To drain free moisture from a wet sand slurry

De-sliming/De-dusting

To remove fine material, generally below 0.5 mm, from a wet or dry feed

Trash removal

Usually to remove coarse wood fibers or tramp material from a slurry stream

Static Grizzlies

• 35 – 50° deck angle
• Uses heavy parallel bars
• Used with coarse crushing
(scalping operations)

Sieve Bends

• >50μm, 180m3/hr
• Curved screen w/ horizontal
wedge bars
• Slurry enters tangentially to
the bend
• Peeling action of the slurry
bed

stationary screens

1. Static Grizzlies
2. Sieve Bends

moving screens

Grizzly Screens
Trommels

Grizzly Screens

• Usually screens very coarse
material
• Characterized by parallel steel
bars on rails set at a fixed
distance apart and installed in
line with the flow of ore

Trommels

• Comprised of a cylindrical screen
• Can be made to deliver sized
products by using a series from
finest to coarsest or using
concentric trommels with the
coarsest mesh being innermost

CLASSIFICATION

LESS Than 1mm

Method of separating mixtures of minerals into 2 or more products based on the velocity of the particles as the grains fall
through a fluid medium (liquid or gas)

Wet classification

usually uses water, generally applied to
mineral particles considered too fine to be sorted efficiently by screening

CLASSIFIERS

• used for particles 1mm or less

CLASSIFIERS categorized by force field applied to unit

either gravitational or centrifugal

Hydrocyclone

Utilizes centrifugal force to accelerate settling rate of particless

gravitational classifiers

Spiral Classifier
Rake Classifier

Spiral Classifier

A continuously revolving spiral moves the sands up the slope

Rake Classifier

Uses rakes actuated by an eccentric motion, which causes them to dip into the settled material and move up the incline for a short distance

GRAVITY CONCENTRATION

• Most simple and economical of all concentration methods
• Particles are separated by their differences in density
• Greater density difference = easier separation

Dense Media
Separation

Particles are placed in a
liquid of known density.
Denser particles sink
while lighter particles float

Vertical Current
Separation

Stratification of mineral
layers based on their
densities on a particle
bed, fluidized by a stream
of water

Jigging

Shaking Table

Separation via altering surface-chemical properties of minerals

bubbles

FROTH FLOTATION

• Uses the difference in magnetic
properties between minerals

Magnetic Separation

Uses the difference in electrical
conductivities between minerals

High Tension Separation

DEWATERING

Solid – Liquid Separation
• Produces a relatively dry
concentrate (less shipping
volume)
• For certain processes that
requires low H2O content

Dewatering (combination of the
three)

Sedimentation
Filtration
Thermal Drying

Sedimentation

Increases concentration
of suspensions by
settling solids

Filtration

Uses a porous medium that blocks solids (filter cake) and allows liquids (filtrate) to pass through.

Thermal Drying

Final operation in a mineral processing
plant. Moisture content is lowered to ~5 wt%

Sedimentation

Coagulation
Flocculation

Coagulation

Neutralizes charged particles so that they
will settle and not continuously repel each other

Flocculation

Uses long-chain polymers to bridge particles until they areheavy enough to settle

THERMAL DRYING | rotary dryer

ORE HANDLING

Costs account for 30 – 60% of total delivered price of raw materials

• Covers the processes of transportation, storage, feeding, washing, and sampling of the ore en route to, or during its various
stages of treatment in the mill

ORE TRANSPORTATION

Minimum upward and horizontal movement
• Maximum practicable pulp density

ORE TRANSPORTATION (Basic Philosophy)

• The use of gravity should be maximized.
• There should be continuous movement.
• Shortest possible distances in between processing units should be
utilized.

ORE TRANSPORTATION | dry ore

Chutes
(Standard Rubber) Belt Conveyor
Gravity Bucket Elevators

Chutes

(Standard Rubber) Belt Conveyor

Gravity Bucket Elevators

ORE STORAGE

Necessity arises from successive processing units performing at
different rates
▪ Intermittent vs. continuous, for repairs, and for successive batch and
continuous processes

ORE STORAGE Depends on

▪ The equipment in the mill / method of operation
▪ Frequency of maintenance work required

ORE STORAGE Accomplished by having

stockpiles, and using bins or tanks

ORE STORAGE

Stockpiles
Ore Bins
Slurry Tanks / Conditioning Tanks

Can be reclaimed by front-end
loaders or bucket-wheel
reclaimers

Stockpiles

• Used as intermediate between
crushing and grinding circuits
• They allow the steady discharge of
ore w/o segregation or choking
• Sloping-bottom bins for easily
oxidized ore

Ore Bins

• For storing suspensions of fine
solids with provisions for agitation
for suspension of solids or
prerequisite chemical reactions

Slurry Tanks/Conditioning Tanks

ORE FEEDING

Desired for delivery of uniform stream of ore

• Unnecessary for cases when succeeding operations have same
rates of flow

• Used for smooth control of bin discharge

• Consist of curtain of heavy loops of chain which when moves, the ore begins to
slide

A. Chain Feeders

• Essentially a conveyor with abrasion-resistant steel pans attached in series
• Most widely used feeder for coarse materials

B. Apron Feeders

• Remove fines from coarse
particles before crushing
• “Scalping” the ore to prevent
consolidation of fines

C. Vibrating Grizzlies

• Long elliptical bars in alternate
vertical and horizontal positions
all rotating in one direction
• Rocking action sifts the ore; the
fines fall through to a conveyor,
the coarsest tumbles across to
crushe

D. Elliptical Bar Feeder

Hand Sorting

used when in abundance of cheap manual labor, or additional equipment installation is not economically justified

Magnetic Separators

situated above conveyors to pick up large pieces of tramp iron and steel; used w/ metal detectors

Vibrating Scalping Screens

removal of wood from pulp (wood causes choking)

Washing

▪ removal of slimes (very fine particles)
• Uses high-pressure water jets over vibrating screens

Measures of Efficiency

DEGREE OF LIBERATION
MESH-OF-GRIND
PARTICLE SIZE ANALYSIS

DEGREE OF LIBERATION

The percentage of a mineral or phase occurring as free particles in relation to the total of the minerals in the ore

MESH-OF-GRIND

The size up to which the ore is ground to ensure sufficient
liberation of wanted mineral
• Normally taken as 80% passing product
• Balance between liberation and economics (longer grinding duration =
higher costs)

PARTICLE SIZE ANALYSIS

Control of quality of grinding
• Measure of degree of liberation
• Determining the optimum feed size
• Establishing the grinding limit to reduce losses