presentation 14 che-section-b(1)

04/15/20231

Presentation #01

Communication Skills

Usama Iqbal (12) Usman Bhatia (18) Abdul Rehman (14) Muhammad Shakeel (16) Muhammad Atif (20)

Screening

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Screening

There are two types of Mechanical Separation.

It is the separation of solids It is the separation of solids from liquids or gases. from solids.

Classification

Screening

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Screening

The sorting on screens of bulk materials according to particle size.

Screening is used to separate coal, ore. building materials, and other bulk materials into fractions or to sort out particles of a given size.

For example, in the production of crushed gravel mass of raw sand and gravel is sorted out into five fractions (70–40 mm. 40–20 mm, 20–10 mm, 10–5 mm, and less than 5 mm). During screening, the material is separated into layers as it moves along the sieve of the screen: the larger the particles, the higher the layer along which they move.

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Screening

Particles that are smaller than the screen openings (so-called lower grade) fall through the openings upon reaching the screen’s surface (screen underflow); larger particles (so-called upper grade) slide along the sieve and form the screen overflow. Because of limitations on the length of the screen, not all particles that are smaller than the sieve openings fall through; some remain in the screen overflow, contaminating it and reducing the quantity of the underflow.

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General Terminologies

Over size material Material that retain on the screening

surface is called over size material. In it the fines don’t pass through the screen opening.

Under size material Material that passes through the screening

surface is called under size material. In it the fines pass through the screen opening.

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Intermediate material When two screens are used for screening,

the material that retain on the second screen is called the intermediate material.

Unsized function A single screen can make a single

separation into two fractions i.e. under size and over size. Such type of functions is called the unsized function.

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Sized function When the material is passed through the

series of screens then it is divided into many fractions. Such type of function is known as sized function.

Mesh Hole of the screen is called the mesh.

Mesh numberIt is defined as the numbers of holes per linear inch.

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Aperture of screenAperture is the maximum clear space between the edges of the screen opening. It is usually given in inches or mm.

Clear and actual openingActual opening of the screen is always smaller than corresponding mesh number because wire diameter is also included.There is a formula for expressing the mesh number and clear opening1/mesh# = clear opening + wire diameter

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Screens

Construction of screens

Woven wires Metal bars Perforated or slotted metal plates Silk or plastic clothes

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Standard Screens

U.S. TylerThese are the U.S standard sieves and are available in mesh number 4 – 325.

U.S. ASTMIt is “American Society of Testing Materials” standard sieves series and are available in mesh number 4 – 325.

B.S.SBritish standard sieve as available in mesh number 5 – 300.

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Standard Screens

I.M.M.S“Institute of Mining and Metallurgy” standard sieves are available in mesh number 5 – 200.

F.S.S“French standard sieves” are available in mesh number 17 – 38.

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Screening Equipment

Different types of screens are available for different purposes.

In most screens, the particle drop through the opening by gravity.

In some screens centrifugal screens or brush is used.

Coarse particle easily dropped.Fine particles must be agitated on the screen

surface in some i.e; by shaking and vibrating it.

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Screening Equipment

Some types of the screen equipment are follows.

Stationary Screens and GrizzliesVibrating ScreensCentrifugal Sifter

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Screening Equipment

Stationary Screens and Grizzlies:Grid of parallel metal bar set.The slope and path of material is parallel to

length of bar.The top of each bar is wider than bottom.The spacing between the bar is 2-8 inch (50-

200mm)Effective for very coarse free flowing solids

containing few fine particles.

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Screening Equipment

Vibrating Screens:Vibrating rapidly with small amplitude than

gyrating screensVibration generated by mechanically or

electrically.Mechanical vibrations are transmitted from

high speed eccentric to the casing of unit.Electrical vibrations are transmitted from

heavy duty solenoid to the casing of unit.

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Screening Equipment

Centrifugal Sifters:Screen is horizontal cylinder of woven metal

or plastic. Central shaft contains helical pedals which

impel the solids into the stationary screens.Fine passes through, oversize is conveyed to

discharge.Brushes attach to paddles assist the

centrifugal action in pushing solids through the screen.

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Material Balance Over Screen

Consider two materials ‘a’ & ‘b’ is feed for screening. Let F, D and B be the mass flow rates of feed, overflow and underflow respectively. Let xF, xD and xB be the mass fractions of material ‘a’ in three streams, therefore mass fractions of the material ‘b’ are 1 – xF, 1 – xD and 1 – xB in the three streams.

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Overall material balance:

F = D + B Material balance of component ‘a’

FxF = DxD + BxB

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Overflow to feed ratio:

D = (xF – xB)F (xD – xB)

Underflow to feed ratio:

B = (xF – xD)F (xB – xD)

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Screen Effectiveness / Screen Efficiency

Effectiveness of screenIt is the measure of the success of the screen in closely separating the materials.

Consider two materials ‘a’ & ‘b’ is feed for screening. Let F, D and B be the mass flow rates of feed, overflow and underflow respectively. Let xF, xD and xB be the mass fractions of material ‘a’ in three streams, therefore mass fractions of the material ‘b’ are 1 – xF, 1 – xD and 1 – xB in the three streams. It is supposed that ‘a’ is larger in size than ‘b’ material.

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Screen effectiveness based on the oversize

Ea = DxD/FxF

Screen effectiveness based on the undersize

Eb = B(1 – xB)/F(1 – xF)

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Combined overall efficiency

E = Ea × Eb

E = xD(1 – xB)(xF – xB) (xD – xF)

xF(1 – xF)(xD – xB)2

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Factors Affecting the Effectiveness

Mesh size and wire diameterCapacityBlindingMoistureDirection of approach of particle to

screen surfaceCohesionAdhesion

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“THE END”

presentation 14 che-section-b(1)

Education