Sizing Methods

• There are two methods of industrial sizing.

i. Screening

ii. Classification

• Screening is generally carried out on relatively coarse

material, as the efficiency decreases rapidly with fineness.

Screening is generally limited to materials above about 250

microns in size, finer sizing normally being undertaken by

classification.

• Classification: Classification is defined as a method of

separating mixtures of mineral particles into two or more

products according to their settling velocities in water, in air or

in other fluids as given in below figure.

Industrial classification may be carried out in different

types of classifiers and these classifiers are; hydraulic

classifiers, mechanical classifiers and cyclones. Basically they

all work according to the principle that the particles are

suspended in water which has a slight upward movement

relative to the particles. Particles below a certain size and

density are carried away with the water-flow, whereas the

coarser and heavier particles will settle.

Size control

Size

con

trol

Size control – IntroductionWith size control we understand the process of separating solids into two or more products on basis of their size. This can be done dry or wet.

As mentioned earlier neither crushers nor grinding mills are too precise in their size reduction job and a lot of size fractions are misplaced. By using optimum size control the result can be improved both regarding capacity, size and particle shape.

Size control by duties

• Classificationusing particle motion for size control.

Size control by methodsIn mineral processing practices we have two methods dominating size control processes:

• Screeningusing a geometrical pattern for size control.

Toprevent undersize in the feed from blocking the next size reduction stage (scalping)

Toprevent oversize from moving into the next size reduction or operation stage (circuit sizing)

Toprepare a sized product (product sizing)

Bars Wire Circle

Square Rectangle Rectangle

BASICS IN MINERAL PROCESSING 1

Size control

Size

con

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ScreensPerformance of screens will fall back on three main parameters: Motion–Inclination–Screeningmedia

Screen motions

Screening by stratificationBy building up a material bed on a screen deck the material will stratify when the motion of the screen will reduce the internal friction in the material. This means that the finer particles can pass between the larger ones giving a sharp separation.

Screening by free fallIf we use the double inclination used for stratification (from 10-15 up to 20-30 degrees) we are in free fall, meaning that no particle layer can build up on the screen deck. The particles will now be sized directly via the screening media, giving a higher capacity, (or a more compact installation), but also less sharpness in separation. Optimal use when a large amount of fines shall be removed fastly.

Stratification

Separation

Inclined

Circularmotion

Horizontal

Ellipticalmotion

Straightline motion

Horizontal

Straightline throw Inclined

2 BASICS IN MINERAL PROCESSING

Size control

Size

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Single inclination• Stratificationscreen• Circular(15deg.)• Linear0 – 5 (deg.)• Stilltheleaderinselective screeningData sheet, see 4:6

Triple inclination• Combinecapacityandselectivity• Typicalcontrolscreenfor advanced product fractions Data sheets see 4:8

Multiple inclination (’’banana screen’’)

• Effective”Thin-layer”screen• Popularincoalandmetallic mining

Double inclination• Freefall• Compact-highcapacitypaidfor by lower selectivity•TypicalincircuitscreeningData sheet, see 4:7

Screen types There are many types of screens, but they can be reduced to the four types shown below. Of these types approx. 80 % used worldwide are of typesingleinclination, stratification screens. The other are of type double,triple or multiple inclination,wherescreeningbystratificationandfreefallarecombinedfordifferentapplications.

BASICS IN MINERAL PROCESSING 3

Size control

Size

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trol

Rubber or polyurethane?Feedsize Select Because

>35 mm dry Rubber 60 sh Absorbes impact Resistant to sliding abrasion <0-50 mm wet Polyurethane Very good against sliding abrasion Accurate separation<40 mm dry/moist Rubber 40 sh (soft) Very flexible Prevents blinding Lookoutfor: Oil in rubber applications Hot water or acids in PU-applications

Selection of screening mediaSelectionofthecorrectsizeandtypeofscreenisimportant.Equallyimportantis the selection of the screening media. This refers not only to a correct aperture relatedtothe”cutsize”,butalsotothewearinoperationofthesescreens.Belowashort selection guide to screening media can be found.

THINNER THICKER

+ Capacity – + Accuracy – – Service life + – Blinding/Pegging + TendencyN.B.: Thickness should not exceed required product size

What happens if we go...?

General rule for min. thickness

Max feed size = Panel thickness 4

What thickness?

What type of panel

Tension mats with hooks fits all screens designed with cambered decks and tensioning rails.

Bolt down panels, pre-tensioned for easy installation and guaranteed screening performance.

Self supporting panels, for screens of open frame design for tough applications .

Modular systems provide flexibility in wear material/hole configuration combinations.

Wire mesh panels offer superior open area and are quickly available.

4 BASICS IN MINERAL PROCESSING

Size control

Size

con

trol

Generalguidelineforwiremesh:

“Requiredproductsizeplus5–10%”

Generalguidelineforrubberpanels:

“Requiredproductsizeplus25–30%”

GeneralguidelineforPUpanels:

“Requiredproductsizeplus15–20%”

What hole size? (Inclined deck)

The standard choice

Forimprovedservicelife(coarsescreening)

Forimprovedcapacity

Forimprovedaccuracyanddewatering

What type of hole?

mesh* micron mesh micron mesh micron 2½ 8000 14 1180 80 180 3 6700 16 1000 100 150 3½ 5600 20 850 115 125 4 4750 24 710 150 106 5 4000 28 600 170 90 6 3350 32 500 200 75 7 2800 35 425 250 63 8 2360 42 355 270 53 9 2000 48 300 325 45 10 1700 60 250 400 38 12 1400 65 212 500 25

*Taylor serie (US)

Mesh number = the number of wires per inch or the number of square apertures per inch

4000micron

12345

1”

Particle size – Mesh or Micron?

BASICS IN MINERAL PROCESSING 5

Physical Concentration Methods 1. Separation dependent on optical and radioactive

properties of minerals, i.e. hand pickling, optical sorting,

radioactive sorting, etc.

2. Separation dependent on specific gravity (density)

difference of minerals, i.e. heavy-media separation,

gravity concentration by use of tables, jigs, cones, etc.

3. Separation utilizing the different surface properties (i.e.

surface chemistry) of the minerals, i.e. froth flotation, etc.

4. Separation dependent on magnetic properties of the

minerals, i.e. low and high, dry and wet magnetic

separation, etc.

5. Separation dependent on electrical conductivity

properties of the minerals, i.e. electrostatic separation, etc.

1- Gravity separation

A- Gravity separation involves feeding grounded ore into a pulsating body of water, which serves to settle out the heavy material while floating away the light material. If the heavy material is what you wishes to keep, then it is taken from the bottom. If the light material is what you wishes to keep, then it is taken from the top.

B- Cyclone :

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.

2- Electrostatic separator An electrostatic beneficiator works because different

minerals have different electrostatic affinities -- will absorb

different amounts of charge depending upon their

composition, and hence are deflected different amounts by

an electric field. After grains are sieved by size, they are

placed through a beneficiator. After a few passes through

beneficiators, we have separated different minerals fairly

well. (There's no change in physical or chemical identity;

there's only separation of minerals.)

3- Magnetic separator

Magnetic separation is most commonly used to separate

natural magnetic iron ore (magnetite) from a variety of less-

magnetic or nonmagnetic material. Today, magnetic separation

techniques are used to beneficiate over 90 percent of all domestic

iron ore.

4- Floatation Froth flotation is a highly versatile method for physically

separating particles based on differences in the ability of air

bubbles to selectively adhere to specific mineral surfaces in a

mineral/water slurry. The particles with attached air bubbles are

then carried to the surface and removed, while the particles that

remain completely wetted stay in the liquid phase. Froth flotation

can be adapted to a broad range of mineral separations, as it is

possible to use chemical treatments to selectively alter mineral

surfaces so that they have the necessary properties for the

separation. It is currently in use for many diverse applications,

with a few examples being: separating sulfide minerals from silica

gangue (and from other sulfide minerals); separating potassium

chloride (sylvite) from sodium chloride (halite); separating coal

from ash-forming minerals; removing silicate minerals from iron

ores; separating phosphate minerals from silicates; and even non-

mineral applications such as de-inking recycled newsprint. It is

particularly useful for processing fine-grained ores that are not

amenable to conventional gravity concentration.

Fig : The flotation system includes many interrelated

components, and changes in one area will produce compensating effects in other areas.

Flotation Reagents

Collectors. These are used primarily to make solids

hydrophobic and promote adhesion to air bubbles or oil droplets.

Common examples are fatty acids, sulfonates and amines.

Frothers. Frothers promote the formation of a metastable

froth phase that facilitates the removal of particles carried by air

bubbles to the top of the flotation cell. Examples of frothers are

pine oil, long-chain alcohols.

Auxiliary Reagents. These reagents include depressants,

which are used to prevent solids from becoming hydrophobic, and

activators, which promote the adsorption of reagents onto selected

solids.

FLOTATION PRQCESS

Flotation is a method of separating an ore species froth

another, based on its hydrophobia surface characteristics either

natural or induced when present as a suspension in water with air

bubble. Due to the affinity of the desired mineral to adhere to air

bubbles it is floated out ' of the ore slurry. Schematics of various

sub processes controlling flotation system is given in fig.

Fig. : Various sub-processes controlling flotation system

The successful industrial practice of flotation involves

knowledge and optimisation of four important components of

flotation process namely,

1- Mineraiogical characteristics of the ore (mineral

association, liberation size, presence of slime particles and soluble

species contributed by the ore).

2. Surface colloid and reagent chemistry which determines

selectivity of separation (colllectors, frothers, activators,

depressant, modifier, dispersants etc.)

3. Process engineering (feed preparation that is size reduction ,

cell design, control system etc..

4. Operating parameters such as aeration rate, temperature, Eh/

pH, ionic strength and flotation circuit configuration.

Overall separation efficiency in flotation is dependent on

1. Surface chemistry factors such as particle bubble

attachment, mineral reagent interactions, reagent chemistry etc.

These factors are related to equilibrium considerations

contributing selectivity to separation.

2. Hydrodynamics factors which contribute to the kinetics of

flotation such as agitation, air flow rate, dispersion and cell design

etc. control recovery of minerals.

Important physico-chemical variables in flotation are :

(a) Rote of mineral/ water interface.

(b) Surface charge on the minerals.

(c) Effect of hydrocarbon chain length of the collector.

(d) Effect of neutral molecules.

(e) Rote of polar functional group of the collector.

(f) Role of solution chemistry of the collector.

(g) Role of inorganic ions (activator and depressant).

(h) Effect of temperature

(i) Ore properties i.e. grade, minerology, degree of oxidation,

liberation of minerals.

Flotation cell