Batch Screening

8/3/2019 Batch Screening

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Batch Screening


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CHEMICAL ENGINEERS..

Must characterize

a particulate solids

Must predict the

solids properties

Crushing DryingFiltering

Crystallization

etc.


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Methods of Determining Particle Size

SOLIDS

Microscopicmethod

Sedimentationmethod

Screeningmethod


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particles are enlarged under themicroscope and are directly measured

Microscopicmethod

a sample of solid is mixed with water andis shaken. The size is determined byutilizing the settling velocity equationswhich is a function of Dp, (particlediameter)

Sedimentationmethod

particle size is measured by mountingscreens on a vibrator (eitherhorizontal/vertical screening) or byshaking using the hands

Screeningmethod


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SCREENING

a separation of a mixture of various sizes of grains into twoor more portions by a screening surface acting as a multiple

go-no-go gauge

the final portions consisting of grains of more uniform sizethan those of the original mixture

The material will pass over a surface that has openings ofdesired size;

particle size is measured by mounting screens on a vibrator(either horizontal/vertical screening) or by shaking using thehands.


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Two ways of conducting screening

--term used whenthe materials thatare being screenedcontain only theirnatural moistureor they were

deliberately driedprior to thescreening

DRYscreening

--when water isadded to thematerial so thatsmall particles arewashed off thesurface of large

particles.--also preventsclogging.

WETScreening


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Types of Screening operations

Scalping

Used in removal of a small amount of oversize from a feed that is mostly fines

Coarse Separation

Used when making a size separation at mesh 4 or larger. Also applied indewatering operations (i.e. removal of free water from a solid-water mixture)

Fine Separation

Used when making a size separation smaller than Mesh 4 but larger than mesh48. Applied in desliming operations (i.e. removal of extremely fine particles

from a wet material)

Ultrafine Separation

Used when making a size separation smaller than 48 mesh


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Screening is significant

Because it is one way ofproduct preparation

to determine the valueof a product for somespecific application.

a means of analysis-tocontrol or gauge the

effectiveness ofanother operation


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Terms used in Screening

Consist of wire mesh cloth wherein the wire diameters and thespaces between the wires are both speified

usual units: inch or mmScreens

Aperture= (1/Mesh#) - Diameterwire

Screen Aperture(a.k.a. screen

opening)

number of apertures or openings per linear inchMesh

percentage of actual openings versus total screen area depends on shape of the screen apertureOpen Area

relationship between the successive sizes of screen openings inseries

depends on shapeof the screen

aperture


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material that remains on top or the ones that did not passthrough the screen

Oversize or (+)material

materials that passed thru the screen; sometimes called finesUndersize or (-)

material

material that passed through one screen and retained on thenext screen

Intermediatematerial

relative percentages (by weight) of each of the size fractions of

the sample usually determined by a complete size analysis ofthe sample using testing sieves

Particle Size

Distribution (PSD)

series of testing sieves having openings in a fixed successionSieve scale


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Particle size distribution

a.) Mass fraction retained (xi) vs. Dpi

b.) Mass fraction retained (Xi) vs. Dpi,mean

Fractional or differential plot

does not require computation of theaverage screen diameter but theaddition of fractions passing throughthe screens.

a. cumulative mass fractions largerthan Dpi vs. Dpi.

b.) cumulative mass fractions smallerthan Dpi vs. Dpi.

Cumulative plot

Shows the proportionate fraction of each size of individual particles in mixture

Assumes that density and size of the particle is constant per fraction


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Screen efficiency

a measure of the success of a screen inseparating the components of a

material

A common measure of screen

efficiency is the ration of oversizematerial that is actually in the overflowto the amount entering with the feed


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Mass Balance

screen

FEED,F

XF

OVERSIZE, D

XD

UNDERSIZE, B

XB


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Pertinent Equations

MASS BALANCE:

F = D + B

Component Bal:Fxf = DxD + BxB

RATIO:

D/F = ?

B/F = ?


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Fxf= Dxd + Bxb

Fxf Bxb = Dxd

B = F D

Fxf (F-D)xb = Dxd

Fxf Fxb + Dxb = Dxd

F(xf xb) = D(xd-xb)Oversize to feed ratio

D = (XD - XB)

F (XFXB)


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While,

Undersize to

feed ratio

B = (XD XF)

F (XD

XB)


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Based on oversize and undersize

if product is oversized.Effectiveness= Overall screen effectiveness ()= (EA) (EB)

Where:

(EA)= recovery of desired material in oversize (EB)=recovery of undesired material in undersize

= amount of desired material in oversize = amount of undesired material in undersize

amount of desired product in feed amount of undesired material in feed

E= DXD B[1- XB]

FXF F [1- XF]

EA EB


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if product is undersized. = (EA) (EB)Where:(EA)= recovery of undesired material in oversize (EB)=recovery of desired material in undersize

= amount of undesired material in oversize = amount of undesired material in undersize

amount of undesired material in feed amount of desired material in feed

(EA)= D (1-XD) (EB)= BXB

F (1-XF) FXF

= D (1-XD) BXB

F (1-XF) FXF


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Based on recovery and rejection

= Recovery(R1 x R2)

Where:

R1= amount desired material in productamount desired material in feed

R2= 1 (1-XD)D

(1-XF) F

R1= DXpFXF

R2= 1- recovery of the undesired materialin product


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Substitution of Values for R1 and R2:

Effectiveness= DXD 1 (1-XD)DFXF (1-XF) F


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OBJECTIVES

3. Calculate thescreen effectiveness

of an unknownscreen.

2. Construct andanalyze fractional

and cumulativeplots for the

screen analysis ofthe sample

1. Conduct andinterpret a

screen analysisof a givensample.


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MATERIALS

sample

pan

balance

Standard

screens


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METHODS

ASSEMBLY

screens will be stacked on top of one another, from the screen with thelargest opening down to the screen with the smallest opening, and apan/basin at the bottom for the smallest particles

INITIALSCREENING

sample will be weighed and fed to the first screen screens will be agitated mechanically or manually for about 10 to 20 minutes

the weight of the fraction from each screen and the pan will be determined andused in making differential and cumulative plots

SCREENANALYSIS

separated fractions will be mixed sample will be screened using a screen of specific mesh and the undersize andoversize fractions determined for use in determining the effectiveness

same procedure will be repeated to other screens


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DATA BLANKS

Mesh

Number

Screening

Opening,

mm

Dpi, mm Feed/ Undersize/

Oversize

Cumulativ

e Fraction

Larger

than Dpi

Cumulativ

e Fraction

Smaller

than Dpi

Mass, g Mass

Fraction----

----

----

----

Pan

Total

Documents

Batch Screening