Screening in Separation Process | Screening in Engineering

'A method of separating solid particles according to size alone is called screening. Screening is for separating solids based on size. It refers to the separation of solid materials based on size using screens of known openings. 

Screening is the separation of a mixture of solid particles of various sizes into two or more fractions using a screening surface.

In screening, a mixture of solid particles of various sizes is dropped on a screening surface/screen (a surface provided with suitable openings) which acts as a multiple-go and no-go gage. The material that passes through a given screen surface is called the under-size or minus (-) material while the material that remains on the screen surface is called oversize or plus (+) material.

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A single screen can make a single separation of the material charged into two fractions. These are called unsized fractions as only the upper or lower limit of the particle sizes they certain is known and the other limit is not known. The material can be separated into sized fractions i.e. the fractions in which both the maximum and minimum particle sizes are known, by passing it through a series of screens of different sizes.

Screening is much more commonly adopted for dry particulate solids and occasionally for wet particulate solids. In this topic, we will limit our discussion to screening dry particulate solids.

Made for Industrial Screen

Industrial screens are made from metal bars, woven wire cloth, silk bolting cloth, and perforated or slotted metal plates. Many varieties and types of screens are available for different purposes but we will discuss a few representative types.

(i) Remove the fines from feed material before a piece of reduction equipment such as a jaw crusher, ball mill, or rod mill.

(ii) Prevent an incompletely crushed material from entering into other unit operations.

(iii) Produce a commercial or process-grade material to meet specific particle size limits.

(iv) Remove the fines from a finished product before shipping.

Types of Screening Equipment and Uses 

Screening equipment can be classified based largely on the size of the material as the screens may be called upon to pass grains ranging from several mm in diameter down to 200 mesh.

1. Grizzlies (fixed inclined screens) are used for the coarse screening of large lumps.

2. Trommels (revolving Screens) are generally used for fairly large particles. 

3. Shaking and Vibrating screens are used in a coarse range and also down into fine meshes (fine sizing).

4. Oscillating screens are used for the finer meshes below 4-mesh.

In the screening operation, coarse particles drop easily through the large openings in a stationary surface but with fine particles, the screen must be agitated by shaking, gyrating, or Vibrating it mechanically or electrically. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane

Types of Screen Motions 

Shows in a picture of screen motions.

(a) Gyrations in a horizontal plane.

Gyrations in horizontal plane.

(b) Gyrations in vertical.

Screens motions

(c) Gyrations at one end and shaking at the other.

Screens motions

(d) Shaking.

Screens motions

(e) Mechanically vibrated.

Screens motions

(f) Electrically vibrated.

Screens motions


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