Lec No.5 Screening Rev

SCREENING

Introduction

The processing of mixed particulate solids entails treatment of the bulk solids for mixing, sampling, sizing and classification. Also important are operations for selective separation or concentration of solids from a mixture. The effectiveness of size classification or of size reduction will depend on the determination of the amount of material of different sizes. The separation of materials based on size is frequently important as means of preparing a product for sale or for a subsequent operation.

SCREENING  Separates a mixture of various sizes of particles into two or more portions, each of which is more uniform in size of particle than the original mixture. 

Passing a material over a surface provided with openings of the desired size.

Limitation: Particles should be within the size range measurable by the available screens

Importance of Screening   Means of preparing a product for sale or for subsequent operation(as in marketing of coal where the size is the basis for its classification for sale) 

It is a means of of analysis, either to control or gage the effectiveness of another unit operation such as crushing or grinding grinding ( to control rates of reactions involving solids)



To determine the value or applicability of a product for some specific application

Types of Screening 1. Dry Screening - refers to the treatment of a material containing a natural amount of moisture or a material that has been dried before screening 2. Wet Screening- refers to an operation when water is added to the material being treated for the purpose of washing the fine material through the screen. Advantages of Wet Screening a. Material is washed-off evenly over the screen

1 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye

b. Clogging of screens is prevented c. Small particles are washed-off the surface of the large

particles

d. Improved screen efficiency feed dilution and efficiency e. Assist in dust suppression Disadvantages of Wet Screening a. Require drying of material after screening b. Increased corrosion if wire screens are used When to use Wet Screening: a. Feed material has high moisture content (39%) b. Feed is mostly composed of small particle size c. Feed contains fine particle agglomerates (i.e. clay, balls)

Types of Screening Operations 1. Scalping

Used in the removal of oversize from a feed that is mostly fines Mostly applied in trash removal 2. Coarse Separation Used when making size separation greater than mesh 4 (i.e. mm)

  >

4.76

 Applied in dewatering operations (i.e. removal of free water from a solidwater mixture) 3. Fine Separation Used when making size separation smaller than mesh 4 but larger than mesh 48 (0.297 <  < 4.76 mm)  Applied in de-sliming operations (i.e. removal of extremely fine particles from a wet mixture)

2 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye

4. Ultrafine Separation

Used when making size separation smaller than mesh 48 (  < 0.297 mm) Methods of Indication Particle Size Distribution

First

Second

Third

Oversize, 1/4 in.

+¼

+¼

Oversize, 20 mesh

+20 mesh

+20 mesh

Through 10 mesh on 20 mesh

-10 + 20

10/20

1 1  + 4 8

Through 1/4 in on 1/8 in Undersize, 200 mesh

-200

1 1 / 4 8 -200

Terminologies 1. Screen Aperture 

 Aka “Screen Opening”



Clear space between the individual wires of the screens



Usually reported in mm or inch

Kinds of Screen Aperture a. Square Apertures  The most commonly-used shape  Offer accurate sizing, good wear life with reasonable open area

b. Round Apertures  Provides strong deck surface

3 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye

   

Usually used in heavy-duty applications where crushing likely Lower open area than square Deck surface prone to blinding

and wear is

c. Rectangular Apertures  Usually used in cases wherein blinding of screens is most likely occur  Problem in accuracy in sizing 2. Mesh  

Number of apertures/openings per linear inch The higher the mesh number, the smaller the screen opening

3. Open Area  The percentage of screen area that is aperture  The percentage of the actual openings vs total screen area  Depends on the shape of the aperture 4. Oversize or Plus Material  Material which fails to pass through the screen 5. Undersize or Minus Material  That which passes through the screen 6. Intermediates   Material passing one screening surface and retained on a subsequent surface 7. Particle Size Distribution  Shows the relative percentages (usually by weight) of each of the size fractions of the mixture being tested  Determined by conducting a complete sieve analysis using testing sieves 8. Testing Sieves   Used to determine the efficiency of screening devices, crushing and grinding operations  Series of standardized woven wire screens Types of Testing Sieves a. Tyler Standard Sieve Series  Based on 200 Mesh screen 4 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye

b. US Sieve Series  Based on No. 18 screen c. International Test Sieve Series  Improvement done by ISO  Added screens to the existing US Sieve Series

Screen Efficiency  Measure of how successful the screen is in separating the components of the mixture  Ratio of the % material actually passing through the screen to the % material capable of passing  Industrial screens are typically designed to be 90  – 95% efficient

Screen Capacity  Measured by the mass of material that can be fed per unit time to a unit area of the screen  Indicates how much material a screen can handle

Screen Analysis Screening is accomplished by passing the material successively over a series of “screens” or “sieves” having progressively smaller openings or apertures. The size of material which has passed through one screen and has been retained on a screen having openings of a smaller size is usually considered to be the arithmetic average of the two (2) screen openings and is called the “average diameter” represented by the symbol Dave.

SCREENING PRINCIPLE

Screening is defined as the mechanical separation of a mixture of various sizes of particles into two or more portions by means of a screening surface, each of which is more uniform in size of particle than the original mixture. It is the separation of a mixture of various sizes of grains into two or more portions by means of a screening surface, the screening surface acting as a multiple gono-go gauge and the final portions consisting of grains of more uniform size than those

5 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye

of the original mixture.

Material Balance Single Screen Analysis

Feed, F XF Oversize or tails, R XR

Undersize or fines, P, X

P

Let x = mass fraction of the desired material (in this case the undersize)

 Recovery =  Rejection = 1 – Recovery of Undesired Material =

1 

1  1  

Effectiveness = Recovery x Rejection

 1 E=  x (1  1 )   In terms of mass fraction: F = P + R

 Component Material Balance:   =  +  ;  Recovery =

=

 −  −

  −   −  6 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye

Rejection =

1   

1  1   





E=

 −   −−   x 1   (−)(− )  − 

Industrial Screening Equipment

1. Grizzlies- are widely used for screening large sizes, particularly of 1 in. and over. The material enters at the top left and works its way downward to the right. The large and oversize particles are discharged over the lower right end and the smaller particles pass through the slots between the bars into the hopper directly below. 2. Stationary Screens- are made of punched metal plate or woven wire mesh, usually set an angle with the horizontal up to about 60 degrees. Suitable for small scale operation such as screening sand, gravel or coal. 3. Vibrated Screens- are used where large tonnages are to be treated. They are particularly useful in the chemical and fertilizer industry. They handle very successfully many light, fine, dry materials and metal powders. Most of this screens has intense, high speed (1500-7200 vibrations/min) 4. Oscillating Screens- are characterized by low speed (300-400 rpm) oscillations. Screens in this group are usually used from ½ in. to 60 mesh. Silk cloths are often used. 5. Reciprocating Screens- these screens are used extensively and are standard equipment in many chemical and processing plants for handling fine separations even down to 300 mesh. Used to handle a variety of chemicals usually dry, light or bulky materials, light metal powders, powdered foods and granular materials. 6. Trommels or Revolving Screens- consists of a cylindrical screen rotating about its axis. Revolves at very low speed of 15-20 rpm. Efficiency is relatively low. 7. Gyratory Screens- are box like machines either round or square with a series of screen cloths nested atop one another. 8. Testing Sieve Shakers: a. Ro-Tap Testing Sieve- this is equipped from 1-13 sieves at a time b. End-Strak Testing Sieve Shaker c. Dynamic Sieve Shaker

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Motions of Screens :(a) gyrations in horizontal plane; (b) gyrations in vertical plane; (c) gyrations at one end, shaking at other;(d) shaking;(e) mechanically vibrated; (f)electrically vibrated

(a)Heavy Duty Vertically Gyrated Screen :( b) Horizontally Gyrated Screen 8 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye

Capacity of Screens:

The screen capacity indicates how much material a screen can handle.

Expressions of Screen Capacity: Mass of feed/(time)(area):

  −

Mass of feed/(time)(area)(aperture):

  − −

PROBLEM SOLVING ON SCREENING

Problem #1

It is desired to separate a mixture of sugar crystals into two fractions, a coarse fraction retained on an 8-mesh screen, and a fine fraction passing through it. Screen analysis of feed, coarse and fine fractions show Mass fraction of +8 particles in feed Mass fraction of +8 particles in coarse fraction Mass fraction of +8 particles in fine fraction

= 0.46 = 0.88 = 0.32

The overall efficiency of the screen used for the separation purpose per 100 kg of feed is?

Problem #2

If the total percentage of particles larger than the screen opening in the feed, product, and undersize are 36%, 89% and 3% respectively. What is the effectiveness of the screen? Ans: 88.61%

9 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye

Problem #3

Limestone is crushed by six units operating in parallel and the products separated by six 35  –mesh screens also in parallel, into two fractions. The effective dimension of each screen is 6ft x 20ft. the common undersize from the screen comes out at the rate of 50 tons/hour. Assume no losses. Mesh Size

Feed Size

Oversize

Undersize

6/8

0.075

0.08

0.02

8/10

0.125

0.145

0.055

10/20

0.1

0.17

0.09

20/28

0.125

0.15

0.085

28/35

0.125

0.28

0.1

35/48

0.175

0.175

0.15

48/65

0.225

0.15

65/100

0.05

0.25

100/150

0.1

Determine: a. Efficiency of screening operation b. Capacity of each screen in lb/24 hr per sq.ft

Ans. 52.82%

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Problem #4

 A quartz mixture having a screen analysis below is screened through standard 10-mesh screen. Calculate the mass ratio of overflow to feed, underflow to feed and overall effectiveness of screen. Cumulative Screen Analysis

Mesh

Dp, mm

Feed

Overflow

4

4.699

0

0

6

3.327

0.025

0.071

8

2.362

0.15

0.43

0

10

1.651

Xf=0.47

Xo= 0.89

Xu=0.199

14

1.168

0.73

0.97

0.58

20

0.833

0.885

0.99

0.83

28

0.589

0.94

1

0.91

35

0.417

0.96

0.94

65

0.208

0.98

0.975

1

1

Pan

Underflow

11 Separation Processes & Introduction to Particle Technology Engr. Rejie C. Magnaye