SEPARATION, SCREENING AND CLASSIFICATION

By:

Waqas AhmedDated : 13th January, 2014

Separation

• Techniques used to separate one material from the other are called separation.

• Separations are extremely in chemical manufacture.

• In fact, much processing equipment is devoted to separate one phase or one material from the other

Types of Separation

• Diffusional Separation

• Mechanical Separation

Diffusional Separation

• Diffusional separation is a technique used for the separations of homogeneous mixtures.

• This separation involves the transfer of the material between the phases.

• Following are the well adopted methods for the diffusional separations.– Distillation – Crystallization – Absorption

Mechanical Separation

• The techniques used for the separation of the heterogeneous mixtures.

• These are based on the physical differences between the particles such as size, shape or density.

• It can be applied for separating solids from solids, solids from liquids and also solids from the gases.

• There are further two types of mechanical separations:– Classification – Screening

• Classification is applied for the separation of solids from the gases and solids from liquids

• Screening is used for the separation of solids from solids

Cont…

Screening

• Separation of mixture of particles of various sizes into two or more fractions by a screening surface is called screening.

• This method is only based on the size of particles.

General Terminologies

• Over size material Material that retain on the screening surface is

called over size material. • Under size material Material that passes through the screening

surface is called under size material.• Intermediate material When two screens are used for screening, the

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

Cont…

• 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.

• 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

Cont…

• Mesh Hole of the screen is called the mesh.

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

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

Cont…

• Clear and actual opening

Actual 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 opening

1/mesh# = clear opening + wire diameter

Screens

Construction of screens

• Woven wires

• Metal bars

• Perforated or slotted metal plates

• Silk or plastic clothes

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.

Cont…

• 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.

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.

Cont…

• Overall material balance:

F = D + B

• Material balance of component ‘a’

FxF = DxD + BxB

Cont…

• Overflow to feed ratio:

D = (xF – xB)

F (xD – xB)• Underflow to feed ratio:

B = (xF – xD)

F (xB – xD)

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.

Cont…

• Screen effectiveness based on the oversize

Ea = DxD/FxF

• Screen effectiveness based on the undersize

Eb = B(1 – xB)/F(1 – xF)

Cont…

• Combined overall efficiency

E = Ea × Eb

E = xD(1 – xB)(xF – xB) (xD – xF)

xF(1 – xF)(xD – xB)2

Factors Affecting the Effectiveness

• Mesh size and wire diameter• Capacity• Blinding• Moisture• Direction of approach of particle to

screen surface• Cohesion• Adhesion