Partech Prelims (SCREENING)

Screening

• a method of separating particles according by size

the opening of the 200 mesh screen which is established at 0.074 mm.

Where is the set of standard screens in the lab based on?

sqrt of 2 or 1.41

What is the ratio of the actual mesh dimension of any screen to the next smaller screen?

fourth root of two (ex: 45, 53, 63, 75, 90 mm)

What is the most common modern sieve size?

• woven wire

• silk or plastic cloth

• metal bars

• perforated or slotted metal plates

• wire

What are industrial screens made out of?

Stainless steel

What is the most common used metal material for screens?

• shaking

• gyrating

• vibrating (mechanically/electrically)

How are screens agitated?

Stationary grizzly

• a grind of parallel metal bars set in an inclined stationary frame

• slope and path of material are usually parallel to the length of the bars

2 to 8 inch or 50 to 200 mm

What is the spacing between the bars of stationary grizzlies?

Gyratory or Vibratory Screen

• two screens one above the other are held in a casing inclined at an angle between 16 to 30 degrees with the horizontal

• casing and screens are gyrated in a vertical plane about a horizontal axis by an eccentric with a rate between 600 to 1800 rpm and dimensions (0.5 to 1.2 m) by (1.5 to 4.3 m)

Centrifugal shifter

• screen is a horizontal cylinder of woven metal or plastic

• high speed helical paddles on a central shaft impel the solids against the inside of the stationary screen

• fines pass through and oversize is conveyed to the discharge

Homogeneous / Uniform Mixture

It has constant particle density, sphericity or uniform shape, and the particle diameter can be represented as average, number of particle, and total surface area.

Heterogeneous Mixture

Mixture of particles having various sizes and densities where the properties are described by range of values.

• Dry screening

• Wet screening

Types of screening

Dry screening

the treatment of a material containing a natural amount of moisture or a material that has been dried before screening

commonly used when material is not prone to sticking or clumping and can easily pass through the screen opening without assistance

Wet screening

An operating when water is added to the material being treated for the purpose of washing the fine material through the screen.

This technique is particularly useful when dealing with fine particles or sticky materials that would otherwise clog the screen openings

• Scalping

• Separation

• Dewatering

• Desliming

Types of screening operations

Scalping

Removing a small amount of oversize material from a feed predominantly composed of fines (ex: trash removal)

Separation

screening coarse, fine, and ultrafine particles

Coarse

Size separation at mesh 4 and/or larger (ex: media recovery)

Fine

Size separation smaller than mesh 4 but larger than mesh 48 (ex: de-slimming)

Ultrafine

Size separation smaller than mesh 48 (ex: ultrafine solid treatment)

Dewatering

Removing of free water from a solids-water mixture, generally limited to mesh 4 and above (ex: washed sand before stockpiling)

Desliming

Removing extremely fine particles from a wet material by passing it over a screen surface (ex: eliminating clay fines from iron ore slurry)

Mesh

number of opening per linear inch given by the formula M(A+D) = 1”

Aperture

clear space between the individual wires of the screen or simply the screen opening given by the formula A = (L/M) - D

Pitch

sum of the aperture or opening size and the wire’s diameter of gauge size

Open Area

percentage of actual openings versus total screen area given by the formula %open area = (A/[A+D]) squared x 100%

Screen Interval

Sizes of screen openings in a series or the relationship between successive sizes

Oversize

material that does not pass through the screen

Undersize

material that passes through the screen

Intermediate

for two or more screens, material passing through one screen and retained on the subsequent surfacce

Sized fractions

intermediates with known upper limit or lower limit diameter

Unsized fractions

a part with only a known upper or lower limit

• in terms of aperture

• in terms of mesh number

What are the methods for indicating sized fractions?

• oversize, ¼ in

• through ¼ in on 1/8 in

• through 1/8 in on 1/16 in

• undersize, 1/16 in

Example of aperture-based sized fraction indication

-3” +4” or 3”/4”

Example of mesh number - based sized fraction indication

• Differential or fractional analysis

• Cumulative analysis for (Dp) greater than the mesh

• Cumulative analysis for (Dp) less than the mesh

What are the methods of indicating screen analyses?

Differential or Fractional analysis

based on the mass fraction retained on each screen (ex: 100 g basis)

Cumulative Analysis for (Dp) greater than the mesh

based on the mass fraction of particles larger than the screen opening (ex: 100 g basis and On Screen)

Cumulative Analysis for (Dp) less than the mesh

based on the mass fraction of particles smaller than the screen opening (ex: 100 g basis and Through Screen)

Screen Effectiveness of Efficiency

a mesure of the success of the screen in closely separating the materials A and B, target is that all material A would be in the overflow and B in the underflow

Material Balance

What can be done over a screen to calculate its effectiveness or efficiency (formula: F = D(overflow) + B(underflow))

by the mass of material that can be fed per unit time to a unit area of the screen

mass flux

How is the capacity of a screen measured?

TRUE

TRUE OR FALSE: The capacity and effectiveness are opposite factors since to obtain effectiveness, the capacity must be small

by varying the rate of feed to the unit

How do you control the capacity of a screen?

TRUE

TRUE OR FALSE: The effectiveness obtained for a given capacity depends on the nature of the screening operation

• the fraction of total surface represented by openings

• the ratio of the diameter of the particle to the width of an opening

• the number of contacts between the particle and the screen surface

the probability of passage of a particle through a screen depends on:

1/dpc squared

In terms of no. of openings per screen area, mesh number is directly proportional to? (smaller mesh means more openings per area)

dpc cubed

In terms of mass of one particle, the mesh size is directly proportional to? (bigger particles are much heavier)

dpc

In terms of the capacity of screen, the mesh number is directly proportional to? (the higher mesh number, the larger opening, more capacity)

grizzlies - 0.05 to 0.2

vibrating screens - 0.2 - 0.8

What are the capacities of actual screens in normal ranges? (ton/ft² - h - mm)

Critical Particle Size (dpc)

It is the size of the largest particle which just pass through the screen and taken equal to the width of a screen opening.