Published on March 9, 2014
SEPARATION, SCREENING AND CLASSIFICATION By: Waqas Ahmed Dated : 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
Cont… • 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
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 number It is defined as the numbers of holes per linear inch. • Aperture of screen Aperture 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. Tyler These are the U.S standard sieves and are available in mesh number 4 – 325. • U.S. ASTM It is “American Society of Testing Materials” standard sieves series and are available in mesh number 4 – 325. • B.S.S British 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 screen It 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 = E a × 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
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