Industrial screening is extensively used for separations from 300mm down to around 40µm, although the efficiency decreases rapidly with fineness. Dry screening is generally limited to material about 5mm in size, while wet screening down to around 250µm is common. Although there are screen types that are capable of efficient size separations down to 40µm, sizing below 250µm is also undertaken by classification is influenced by the fact that finer separations demand large areas of screening surface and therefore can be expensive compared with classification for high-throughput applications.
Objectives of screening:
a. Sizing or classifying: To separate particles by size.
b. Scalping: To remove coarsest size fractions in feed material.
c. Grading: To prepare a number of products within a specified size ranges.
d. Media Recovery: For washing magnetic media from ore in dense medium circuits.
e. Dewatering: To drain pre moisture from wet sand slurry.
f. De-sliming or De-dusting: To remove fine material, generally below 0.5mm from a wet or dry feed.
g. Trash removal: To remove wood fibres from a fine slurry stream.
Factors affecting screen performance:
Particle Size: Screen performance affected by factors that influence the probability of particle passage, and factors that influence the number of opportunities the particles are given to pass through the screen mesh.
Feed rate: The principle of sieve sizing analysis is to use a low feed rate and a very long screening time to effect an almost complete separation.
Screen angle: If a particle approaches the aperture at a shallow angle, it will “see” a narrower effective aperture dimension and near mesh particles are less likely to pass. The screen angle affects the speed at which particles are conveyed along the screen.
Particle shape: Most granular materials processed on screens are non-spherical. Spherical particles pass with equal probability in any orientation. Irregular shaped near-mesh particles must orient themselves in an attitude that permits them to pass. Elongated and slabby particles will present a small cross-section for passage in some orientations and a large cross-section in others. The extreme particle shapes therefore have a low screening efficiency.
Open area: The chance of passing through the aperture is proportional to the percentage of open area in the screen material, which is defined as the ratio of the net area of the apertures to the whole area of the screening surface. Open area decreases with the fineness of the screen aperture.
Vibration: Screens are vibrated in order to throw particles off the screening surface so that they can again be presented to the screen and to convey the particles along the screen.
Moisture: The amount of surface moisture present in the feed has a marked effect on screening efficiency.
Mathematical models of screens: Screen models aim to predict the size distribution and flow of the screen products. Models can classified as
- Phenomenological models that incorporate a theory of the screening process.
- Empirical models based on empirical data.
- Numerical models based on computer solutions of Newtonian mechanics.
There are different types of industrial screens. The dominant screen type in industrial application is the vibrating screen.
Vibrating screens of most types can be manufactured with more than one screening deck. On multiple-deck systems, the feed is introduced to the top coarse screen. The under-sized fractions falls to the lower screen decks, thus producing a range of sized fractions from a single screen. Some of the types of vibrating screens are as explained below.
Inclined screens: Inclined or circular motion screens are widely used as sizing screens. A vertical circular or elliptical vibration is induced mechanically by the rotation of unbalanced weights or flywheels attached usually to a single drive shaft.
Horizontal screens: They have a horizontal or near-horizontal screening surface, and therefore need less headrooms than inclined screens. Horizontal screens must be vibrated with a linear or an elliptical vibration produced by a double or triple shaft vibrator. The accuracy of particle sizing on horizontal screen is superior to that on inclined screens. Horizontal screens are used in sizing applications where screening efficiency is critical.
Resonance screens: They are a type of horizontal screen consisting of screen frame connected by rubber buffers to a dynamically balanced frame having a natural resonance frequency which is same as that of vibrating screening body.
Banana screens: They are used in high-tonnage sizing applications where both efficiency and capacity are important. Banana screens typically have a variable slope of around 40-300 at the feed end of the screen, reducing to around 0-150 increments of 3.5-50.
Dewatering screens: They are a type of vibrating screen that are fed thick slurry and produce a drained sand product.
Other screen types:
- Static screens.
- Trommels screens.
- Rotaspiral screens.
- Flip-flow screens.
- Roller screens.
- Linear screens.
- Circular screens.
- Mineral Processing Technology by B.A.Wills.
Question and answers:
Q. What are the objectives of industrial screening?
A. Sizing, scalping, grading, desliming, trash removal.
Q. What are the factors affecting screen performance?
A. Particle size, feed rate, vibration, screen angle, particle shape, open area, moisture.
Q. What are the different types of vibrating screens?
A. Inclined screens, Horizontal screens, Resonance screens, Dewatering screens, Banana screens.
Q. Mention the screening sizes that are generally limited to a material in dry screening and wet screening.
A. Dry screening is generally limited to material above about 5 mm in size, while wet screening down to around 250µm.