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Screening, Classifying & Dewatering
Screening, classifying and dewatering are processes which aim to improve downstream separation and materials (particularly product) handling
Most separation processing benefit from a closely sized feed, since it will reduce the effect of particle mass and allow whichever physical charactoristic being utilised to be the dominant physical force involved.
Most products require dewatering for product handling or ahead of drying if dry processing or products are required.
SCREENING
Screening involves physical sizing using openings through which particles will either pass though or not and therefore is based on geometrical size and shape only. As particle and screen size gets smaller, screening generally becomes more difficult and throughput capacity is lower.
Examples of screens are:
Grizzly screens
The coarsest screens are grizzlies which often consist of tilted, fixed heavy duty steel bars and are typically used for downstream feed protection from excessively large boulders or removing tramp oversize such as tree stumps.
Trommel screens
Trommels are wet screens that rotate in a similar fashion to ball mills on trunnions, where feed enters one end, and oversize discharges out the other end, with screen undersize discharging along the length of the trommel. they can be fed dry from a conveyor and then mixed with water in a feed box into a slurry, or fed directly as a slurry (e.g. from a dredge). Typical aperture size is 3mm with 12mm longtitudinal slots with panels made of polyeurathane. An internal flighted screw is often employed to break slurry velocity and aid oversize discharge transportation. High flow and pressure wash water is normally used to keep the screens from blinding.
DSM screens
DSM screens (or sieve bends) are a fixed screen commonly used for dewatering. They are a fine screen and are able to process particles sized down to 500um due to their shape. The screen is fed from the top with slurry where the slope is practically vertical, while the deck changes in an elliptical fashion such the discharge is almost horizontal. The screen is normally made of wedge wire stainless steel with horizontal openings across the deck. As the screen leading edges wear, dewatering efficiency is reduced, and the screen should be reversed. the screens are sometimes fitted with rappers to prevent blinding.
Standard vibrating screens
For most wet screening applications, mechanical vibration is usually required to assist screening and removal of oversize, in conjunction with spray water and slope. they can be suppied in single or double deck configuration. There are numerous manufacturers of vibrating screens including Schenck.
Banana screens
These vibrating screens are curved as the name suggests, and because of this have a high level of energy, manufacturers include Schenck.
Derrick screens
These screens are particularly suitable for fine particle screening (down to 75u) due to their high frequency vibration and unique flexible urethane decks which are self cleaning, although spray waters are often fitted.They can be supplied in single and triple feed point configuration, and also as five parallel units as "stacksizers". The manufacturer is Derrick
Kason & Sweco screens
These fine particle screens are normally used for dry feeds. They have a circular shape and high frequency vibratory motion, causing the feed which enters centrally to spread out to the edge and discharge over a lip. They can be supplied in single and double deck configuration, and can have bouncing balls below the bottom deck to reduce blinding. Kason and Sweco
CLASSIFYING
Classification is also a sizing process which involves particle size as the primary physical parameter, but also is affected by particle specific gravity. It is a wet process (although dry classification exists) and involves a moving fluid medium which transports the particles to separate products based on size (and SG) accordingly.
Examples of classifiers are:
Hydrocyclones
These separators produce a size and SG separation of feed slurry into overflow and underflow components, with the water and fine low SG solids flowing preferentially to the former and coarse solids high SG to the latter. This is achieved primarily by the geometry of cyclones the feed pressure applied to them.
As such cyclones are often used to dewater as well as size. Cyclones are normally fitted with "breathers" on their overflow lines, when they discharge below the spigot, in order to control the suction effect produced. They can be fitted with "fish tails" on their spigots to increase underflow density, and these are used in conjunction with the "breathers". A cyclone is normally operated with an underflow "flare" discharge; if the "flare" becomes to heavy a "rope" discharge may occur which will mean the cyclone is choking with resultant loss of separation, solids in the overflow and shaking.
Cyclones can also be used in conjunction with a dense medium (such as fine magnetite) which is a common application in coal washing plants.
Cyclones vary in size from large high capacity units down to small low capacity ones. Larger cyclones produce a coarser cut size, and vice versa, with the cut being the mean particle size that a given mass percentage of the feed reports to either underflow or overflow, expressed as d50, d75 etc. The efficiency of a cyclone can be plotted on a chart by size analysis of the feed and products.
There are a number of cyclone manufacturers including Krebs, and Weir
Operating variables include:
Hydrosizers
These separators also produce a size and SG separation of feed slurry into overflow and underflow components, with the water and fine low SG solids flowing preferentially to the former and coarse solids high SG to the latter. This is achieved by the application of an upward flow of water which is injected into the hydrosizer, so that depending on their size and SG the grains move either in an upward or downward direction. A pressure sensor in the hydrosizer measures the head (which is a function of the slurry density) which controls the underflow valve to a set point.
The hydrosizer depicted below has a conical base, however it is also common to have a flat bottom with multiple controlled underflow discharge points.
Hydrosizer manufacturers include Outokumpu and Linatex
Operating variables include:
DEWATERING
Dewatering is a process, which as the name suggests involves the removal of water from solids (usually slurries), which may be necessary for subsequent processing or product handling.
As discussed in the above, hydrocyclones are commonly used for dewatering.
Other methods of dewatering are as follows:
Dewatering Stockpiles
This is the simplest form of dewatering sand sized solids, but usually needs to be preceded by a cyclone or dewatering cone to avoid wash outs due to excessive flow of water. The base of a stockpile needs to have sufficient drainage (ie porosity of the ground, or slope in the case of concrete bases) to prevent pooling. Time required for a stockplie to drain to a suitable solids content will depend on the particle size disribution, and weather (if not covered). The bottom of a stockpile will alwayus be the wettest and may require selective removal.
Dewatering screens
These are high frequency vibrating screens with small slotted apertures which only allow water and very fine solids to pass through them. They typically are fed from cyclone underflow and operate with a bed of solids that is transported slowly to discharge in a sufficiently dewatered state for further handling.
Thickeners
These devices are primarily used to settle and dewater fine solids, and produce a clean overflow to enable process water to be recycled. They consist of a large circular tank into which a feed slurry enters through a central well, with the water overflowing to a launder around the perimeter, and the solids settling to form a bed and discharged via a central underflow well assisted by a slow moving rake. In most cases either a diluted floculent or clarifier (or both) are added stagewise ahead of the thickener to assist in solids settling and overflow water clarity. The flocculent which attaches itself to the particles to form chains of larger particles which then have sufficient mass to settle.
A conventional thickener is depicted below. Other types of thickeners include high rate, deep cone and paste.
Thickener manufacturers include Outokumpu and Weir
Vacuum Belt Filters
Vacuum belt filters are commonly used where a consistent low moisture content product (such as feed to a dryer or conveyor) from sand sized feed slurry is required.
Slurry is typically fed from a cyclone underflow via a feed box to distribute across a slow moving porous belt to form a bed. A vacuum is maintained under the belt by the bed itself and the tray underneath and rubber sides in conjunction with a vacuum pump, which progressively removes water as the material moves along with the belt, to discharge off the end in a dewatered state. Spray water is often used at points along the belt to assist in spreading the bed evenly.
Belt filter manufacturers include Delkor
Operating variables include:
Pressure Filters
Pressure filters are used for very fine solids where sufficient dewatering is difficult to achieve by other means. Pressure filtering is a batch process whereby typically thicked fine particle slurry is pumped into multiple filters, which are then closed and subjected to positive pressure which forces the water through a cloth membrane, while preventing passage of soids. after sufficient time, the pressure is turned off and the cells are opened, to enable the solids cake to discharge.
Operating variables include:
Bucket Dewatering Wheels
Bucket dewatering wheels are generally used to dewater coarse sands (such as in the construction industry) and consist of a slowly rotating central wheel in a bath which has multiple specially shaped buckets on the outside, the bottom section of which is submerged in a bath of flowing slurry. The bath is fed with slurry from a central feed box so that the buckets collect most the slurry and as they rise out of the bath, the sand being dewatered though a combination of slots and a central pipe at the bottom of each bucket, the latter developing a suction effect as the water drains from it further enhancing dewatering, until the bucket passes over the top of the wheel and then discharges the dewatered sand as it moves downwards into a launder.
The water discharges from overflows at each the side of the bath, and any sand carried with it is returned to the centre of the bath by spiral scrapers.
The degree of dewatering possible depends primarily on the dewatering charactoristics of the sand, with fine sand being the most difficult. There are a few operating variables such a rotational speed.
Bucket dewatering wheel manufacturers include KISA
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Screening, Classifying & Dewatering