unit 1 - screening

8/2/2019 Unit 1 - Screening

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Unit 1 - Screening

By

Evelyn R. Laurito


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6/9/2011 ChE 211 Course Notes by ERLaurito 2

What is Screening?

Screening is a method of separatingparticles according to size alone.

Screening Surface Interwoven Wire Mesh (carbon or stainless

steel, phosphor bronze)

Cloth (silk, plastic, nylon, fiberglass)

Perforated Plates Bars


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Separation Theory

Screen

Feed

Oversize(Tails)

Undersize(Fines)


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Particle Size

Equidimensional particles: In general "diameter, Dp may be specified

Non-Equidimensional particles: Dp is the second longest major dimension

Units of Dp depend on the size of particles Coarse particles: inches or millimeters

Fine particles: screen size

Very fine particles: micrometers or nanometers

Ultra fine particles: surface area per unit mass,m2/g


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Mesh

Mesh Number Number of openingsper linear inch

Example:Mesh 4 Mesh 200

1

1

1

1
http://www.espi-metals.com/tech/meshmed.jpghttp://www.espi-metals.com/tech/meshsmall.JPG


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http://www.espi-metals.com/tech/meshlarge.JPGhttp://www.espi-metals.com/tech/meshmed.jpghttp://www.espi-metals.com/tech/meshsmall.JPG


7/306/9/2011 ChE 211 Course Notes by ERLaurito7

Sieve Scale

A sieve scale is a series of testing sieveshaving openings in a fixed succession(Largest opening to smallest)

3 Types:

Tyler Standard Sieve Series(MSH)

US Sieve Series(Table 19-6/HB)

International Test Sieve Series
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8/306/9/2011 ChE 211 Course Notes by ERLaurito 8

Common Sized Solids



Arrangement of Mesh Screens


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Modern Particle Size

Measurement While historically mainly methods using mechanical,aerodynamic, or hydrodynamic properties for discriminationand particle sizing have been used the particle is allowed to settle in a viscous fluid,

then the calculated diameter of a sphere of the same substance that

would have the same falling speed in the same fluid (i.e., the Stokes

diameter) is taken as the appropriate size parameter of the particle.

Since the Stokes diameter for the rod-shaped particle will obviously

differ from the rod diameter, this difference represents added infor-

mation concerning particle shape. The ratio of the diameters mea-

sured by two different techniques is called the shape factor. today methods based on the interaction of the particles with

electromagnetic waves (mainly light), ultrasound, or electricfields dominate.



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Laser Diffraction method

Lorenz-Mie theory is based on the assumption of spherical, isotropic, andhomogenous particles and that all particles can be described by a common complexrefractive index m = n i. Index m has to be precisely known for the evaluation

It is now ranging from below 0.1 m to about 1 cm. Laser diffraction is currently thefastest method for particle sizing at highest reproducibility. In combination withdry dispersion it can handle large amounts of sample, which makes this method wellsuited for process applications.



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Image Analysis Methods The extreme progress in image capturing and exceptional increase of

the computational power within the last few years haverevolutionized microscopic methods and made image analysismethods very popular for the characterization of particles, especiallysince, in addition to size, relevant shape information becomes available

by the method. Currently, mainly instruments creating a 2D image ofthe 3D particles are used. Two methods have to be distinguished.



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Methods of Indicating Particle Size

Unsized Particles Oversize 4 Mesh, +4, +4.76 mm

Undersize 4 Mesh, -4, -4.76 mm Sized Particles

Through 4 on 6

-4+6 4/6

-4.76 mm + 3.36 mm


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Particle Size Distribution

Differential Screen Analysis: Data consist ofMesh No,n vs Wt frac or % retained on thescreen(n)

Cumulative Screen Analysis (larger than Dp):Data consist of n vs Cumulative fractionlarger than n (n)

Cumulative Screen Analysis (smaller thanDp): Data consist of n vs Cumulative fractionsmaller than n (1-n)


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Screen Analysis

Mesh 8

Mesh 6

Mesh 10

Mesh 14

Mesh 20

50 kg

5 kg

20 kg

12 kg

6 kg

4 kg3 kg

Differential Screen Analysis

n n6 0.10

8 0.40

10 0.24

14 0.12

20 0.08pan 0.06

1.00


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Screen Analysis

Mesh 8

Mesh 6

Mesh 10

Mesh 14

Mesh 20

50 kg

5 kg, 0.10

20 kg, 0.40

12 kg, 0.24

6 kg, 0.12

4 kg, 0.083 kg0.06

Cumulative Screen AnalysisLarger than Dp

n n6 0.10

8 0.50

10 0.74

14 0.86

20 0.94pan 1.00


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Screen Analysis

Mesh 8

Mesh 6

Mesh 10

Mesh 14

Mesh 20

50 kg

5 kg, 0.10

20 kg, 0.40

12 kg, 0.24

6 kg, 0.12

4 kg, 0.083 kg0.06

Cumulative Screen AnalysisSmaller than Dp

n 1- n6 0.90

8 0.50

10 0.26

14 0.14

20 0.06pan 0


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Screening Equipment

Grizzly Screens

Rotating Screens

Shaking Screens

Vibrating Screens

Oscillating Screens


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Material Balance

F = P + R

FxF = PxP+ RxR

RP

RF

xx

xx

F

P

RP

FP

xxxx

FR

Screen

Feed

Oversize

Undersize

F, xFR, xR

P, xP


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Example No.1

It is desired to remove small particles from acrushed stone mixture by screening througha 10-mesh screen. The screen analysis of

feed, overflow and underflow are given inthe table.a) Calculate the mass ratios of the overflow and

underflow to feed.

b) Plot Dp vs Screen analysis for the feed, overflowand underflow

c) Find the effectiveness and capacity of thescreen if the feed rate is 100 tons/hr


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Mesh Dp

(mm) Feed Overflow Underflow

4 4.699 0 0

6 3.327 0.025 0.071

8 2.362 0.125 0.43 1

10 1.651 0.32 0.85 0.805

14 1.168 0.26 0.97 0.42

20 0.833 0.155 0.99 0.17

28 0.589 0.055 1.0 0.09

35 0.417 0.02 0.0665 0.208 0.02 0.025

Pan 0.02 0

Screen Analysis


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Screen Analysis

4 4.699 0 0

6 3.327 0.025 0.071

8 2.362 0.125 0.43 110 1.651 0.32 0.85 0.805

14 1.168 0.26 0.97 0.42

20 0.833 0.155 0.99 0.17

28 0.589 0.055 1 0.09

35 0.417 0.02 0.06

65 0.208 0.02 0.025

Pan 0.02 0

CSA Larger CSA SmallerDSA

xF=.47 xP=.85 xR=.195


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Solution

From the table, xF=0.47, x

P=0.85, x

R=0.195

RP

RF

xx

xx

F

P

195.085.0

195.047.0

= 0.42

RP

FP

xx

xx

F

R

F

P1 = 0.58

)x1)(xx(

)x1)(xx(E

FRP

RFP

FRP

PRF

x)xx(

x)xx(

=0.669


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Screen Effectiveness(E)EA = Recovery of desired material in the product

F

P

Fx

Px

EB = Recovery of undesired material in the reject

)x1(F

)x1(R

F

R

E=EAEBF

P

Fx

Px

)x1(F

)x1(R

F

R

FRP

PRF

x)xx(

x)xx(

)x1)(xx(

)x1)(xx(

FRP

RFP

)x1)(xx(

)x1)(xx(E

FRP

RFP

FRP

PRF

x)xx(

x)xx(


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Screen Capacity

Capacity=mass of feed/area=F/A

Throughflow method: Matthews Equation

A=0.4Ct/CuFoaFs (19-23/HB)F=100

Ct= 58

Cu=.46

Foa=100a2/(a+d)2

Fs= 1

a=.0661 d=.0319


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Screen Series


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Screen SeriesUS Sieve

Size

Tyler

Equivalen

t

Opening

mm in

- 2 Mesh 8.00 0.312- 3 Mesh 6.73 0.265

No. 3 3 Mesh 5.66 0.233

No. 4 4 Mesh 4.76 0.187

No. 5 5 Mesh 4.00 0.157

No. 6 6 Mesh 3.36 0.132

No. 7 7 Mesh 2.83 0.111

No. 8 8 Mesh 2.38 0.0937

No.10 9 Mesh 2.00 0.0787

No. 12 10 Mesh 1.68 0.0661

No. 14 12 Mesh 1.41 0.0555

No. 16 14 Mesh 1.19 0.0469

No. 18 16 Mesh 1.00 0.0394

No. 20 20 Mesh 0.841 0.0331

No. 25 24 Mesh 0.707 0.0278

No. 30 28 Mesh 0.595 0.0234

No. 35 32 Mesh 0.500 0.0197

No. 40 35 Mesh 0.420 0.0165No. 45 42 Mesh 0.354 0.0139

No. 50 48 Mesh 0.297 0.0117

No. 60 60 Mesh 0.250 0.0098

No. 70 65 Mesh 0.210 0.0083

No. 80 80 Mesh 0.177 0.0070

No.100 100 Mesh 0.149 0.0059

No. 120 115 Mesh 0.125 0.0049

No. 140 150 Mesh 0.105 0.0041

No. 170 170 Mesh 0.088 0.0035

No. 200 200 Mesh 0.074 0.0029

No. 230 250 Mesh 0.063 0.0025

No. 270 270 Mesh 0.053 0.0021

No. 325 325 Mesh 0.044 0.0017

No. 400 400 Mesh 0.037 0.0015

US Sieve

Size

Tyler

Equivalen

t

Opening

mm in

unit 1 - screening

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