7: Handout Industrial Screening Handout
Screening is necessary to separate particle sizes before processing. Screens are used everywhere in the crushing/milling process, from ROM, in the form of grizzlies to pulp, in the form of sieve bends. Screening has three primary purposes: Prevent the undersize from entering the crushing machine. Thereby, increasing crusher capacity. Prevent oversize material from entering the next stage of the crushing/grinding process. Prepare closely sized products.
Grizzlies: The grizzly is used for screening ROM ore before it goes into the primary crusher. Oversized material will go into the primary crusher and undersize material will go to a secondary crusher or mill. Grizzlies consist of robust parallel bars or chains. Spacing between bars or chains s equal to the product size of the primary crusher. Inclined at 20 50 degrees. 300 mm 20 mm opening. Capacities up to 5000 tph. Sieve bends: The sieve bend is used wet and is effective for separating fine material (down to 50 micrometers). The sieve bend is a curved array of horizontal bars. As the slurry runs down the bend, layers of fines are striped off the slurry letting the fines flow through the grate and the coarse material to flow down the front of the grate (the inside of the curve).
Composed of horizontal wedge bars. Separations down to 50 micrometers. Capacities up to 180 m3/hr.
Prepared by Michael Anderson based on book by BA Wills
Revolving screens: Revolving screens are near-horizontal cylindrical screens that rotate with the material inside. They can be used to separate multiple particle sizes by varying the screen size as the material moves down slope inside the screen. Limited in use due to high wear rate. Poor capacities Easily blinded.
Shaking screens: Shaking screens are shaken at a rate of 60 to 800 strokes per minute. They are operated either horizontal or with a gentle slope. Shaking screens are commonly used with dry coal, but are not good for metalliferous ore due to high wear rates. 60-800 strokes per minute large feed sizes, down to 12 mm. Dry coal prep.
Reciprocating screens: Reciprocating screens sits near horizontal and have a circular motion on the horizontal plane, which decreases in amplitude as the particle progresses to the end of the screen. The circular motion of the screen has two important effects. Spreads the material. Stratifies the material by putting the fines on the bottom. Reciprocating screens are used for fine separation of, usually, dry material. 1000 rpm slightly inclined for dry, light materials 10 mm to 250 micrometers.
Prepared by Michael based Anderson on book by BA Wills
Gyratory screens: Gyratory screens work like the reciprocating screen above. A spinning off-center weight produces horizontal gyratory motion. However, it has multiple decks. The rotap you used is a small scale gyratory screen. Each deck has an opening on the side that allows oversize material to fall out as new feed material is fed in. Used wet or dry Effective down to 40 micrometers.
Vibrating Screens: The Most Important screen for mineral processing is the vibrating screen. They are mainly used in crushing circuits where they handle material from 25 cm down to 5 mm. Vibrating screens are available in multiple deck systems. Most vibrating screens are operated by mechanical means (an off balance weight). A few, such as the hummer use an electromagnet, like a speaker, for small amplitude, high frequency movements. Most important screen for mineral possessing. Typical use: 25cm to 5mm. Capable down to 250 micrometers Requires little headroom.
Efficiency: Screening is generally effective down to 250 micrometers. Although some sieve bends can achieve finer separation. Screen efficiency decreases with decreasing particle size due to blinding and because fine screens are easily damaged. Two ways to increase efficiency is to increase residence time, which decreases capacity. Or to increase screen area, which increases cost and needed space. You will see screen efficiency defined in two ways, either in terms of recovered oversize (EOS) or in terms of recovered undersize (EUS). Be sure to know which method is...