What is a Screen?
In the mining perspective the screen is a surface with many apertures, or holes, usually with uniform dimensions. Particles of different sizes are presented to that surface so that undersize material will pass through and oversize materials will be retained. Thus we can sort the incoming material according to aperture size of the screen and further processing can be done depending on the size of ore.
What is the need for screening?
There are a wide range of screening objectives. The main purposes in the minerals industry are:
(a) Sizing or Classifying, to separate particles by size, usually to provide a downstream unit process with the particle size range suited to that unit operation;
(b) Scalping, to remove the coarsest size fractions in the feed material, usually so that they can be crushed or removed from the process;
(c) Grading, to prepare a number of products within specified size ranges. This is important in quarrying and iron ore, where the final product size is an important part of the specification;
(d) Media recovery, for washing magnetic media from ore in dense medium circuits;
(e) Dewatering, to drain free moisture from a wet sand slurry;
(f) De-sliming or de-dusting, to remove fine material, generally below 0.5 mm from a wet or dry feed; and
(g) Trash removal, usually to remove wood fibres from a fine slurry stream.
What factors affect the screen performance?
The efficiency of screening is determined by the degree of perfection of separation of the material into size fractions above or below the aperture size. However there are different factors affecting the efficiency and performance of the screen. A brief mention about them is given below.
i. Particle size – Taggart calculated the probabilities of passage related to particle size and has shown that as the particle size approaches the aperture size the chance of passage falls of rapidly. These near mesh size particles tend to peg or plug the apertures thereby reducing the screen efficiency.
ii. Feed rate – A low feed rate and a very long screening time can result in an almost complete separation. However industrial screening practice demands high feed rates, as a result dwell time for the particles on the screen is short. Thus there may not be sufficient time for the fine particles to reach the screen surface for getting an opportunity to pass through. So high capacity and high efficiency are opposing requirements and a compromise between the two is necessary.
iii. Screen angle – The chances of a particle passing through the screen is maximum when the particle approaches the aperture perpendicular to the aperture. 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 also affects the speed at which particles are conveyed along the screen, and therefore the dwell time on the screen and the number of opportunities particles have of passing the screen surface also changes.
iv. Particle shape – Most granular materials processed on screens are non-spherical. While spherical particles pass with equal probability in any orientation, irregular-shaped near-mesh particles must orient themselves in a direction that permits them to pass. Elongated and slab like 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.
v. 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. The smaller the area occupied by the screen deck construction material, the greater the chance of a particle reaching an aperture. Open area generally decreases with the fineness of the screen aperture. In order to increase the open area of a fine screen, very thin and fragile wires or deck construction must be used. This fragility and the low throughput capacity are the main reasons for classifiers replacing screens at fine aperture sizes.
vi. 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. The right type of vibration also induces stratification of the feed material, which allows the fines to work through the layer of particles to the screen surface while causing larger particles to rise to the top. The vibration must be sufficient to prevent pegging and blinding. However, excessive vibration intensity will cause particles to bounce from the screen deck and be thrown so far from the surface that there are very few effective presentations to the screen surface. Higher vibration rates can, in general, be used with higher feed rates, as the deeper bed of material has a “cushioning” effect which inhibits particle bounce. Vibrating screens typically operate with a vibration force of between 3 and 7 times the gravitational acceleration.
vii. Moisture – The amount of surface moisture present in the feed has a marked effect on screening efficiency, as does the presence of clays and other sticky materials. Damp feeds screen very poorly as they tend to agglomerate and “blind” the screen apertures. As a rule of thumb, screening at less than around 5 mm aperture size must be performed on perfectly dry or wet material, unless special measures are taken to prevent blinding. These measures may include using heated decks to break the surface tension of water between the screen wire and particles, ball-decks (a wire cage containing balls directly below the screening surface) to impart additional vibration to the underside of the screen cloth, or the use of non-blinding screen cloth weaves.
What are the different types of Screens?
There are different types of industrial screens available. The dominant screen type in industrial applications is the vibrating screen, of which there are many sub-types in use for coarse and fine-screening applications. There are also numerous other screen types in wide use for both coarse and fine screening applications. The names of some of the screens are: – Vibrating screen, Inclined screen, Grizzly screen, Horizontal screen, Resonance screen, Dewatering screen, Banana screen, Modular screen, Mogensen sizers, High frequency screen, Trommel etc.
What is Vibrating screens and how it works?
Vibrating screens are the most important and versatile screening machines for mineral processing applications. Vibrating screens have a rectangular screening surface with feed and oversize discharge at opposite ends. They perform size separations from 300mm in size down to 45 µm and they are used in a variety of sizing, grading, scalping, dewatering, wet screening, and washing applications. 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 undersize falling through to the lower screen decks, thus producing a range of sized fractions from a single screen.
What is Inclined screens and how it works?
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. The amplitude of throw can be adjusted by adding or removing weight elements bolted to the flywheels. The rotation direction can be contra-flow or in-flow. Contra-flow slows the material more and permits more efficient separation, whereas in-flow permits a greater throughput.
What is Grizzly screens and how it works?
Very coarse material is usually screened on an inclined screen called a grizzly screen. Grizzlies are characterised by parallel steel bars or rails set at a fixed distance apart and installed in line with the flow of ore. The gap between grizzly bars is usually greater than 50 mm and can be as large as 300 mm, with feed top size as large as 1 m. Vibrating grizzlies are usually inclined at an angle of around 20 degrees and have a circular throw mechanism. The bars are typically made from wear-resistant manganese steel, and are usually tapered to create gaps that become wider towards the discharge end of the screen to prevent rocks from wedging between the bars. Domed or peaked profiles on the tops of the bars give added wear protection and prevent undersized rocks from “tiding” along the bars and being misplaced.
What is Horizontal screens and how it works?
Horizontal low-head or linear vibrating screens have a horizontal or near-horizontal screening surface, and therefore need less headroom 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 screens is superior to that on inclined screens; however because gravity does not assist the transport of material along the screen they have lower capacity than inclined screens. Horizontal screens are used in sizing applications where screening efficiency is critical, and in drain-and-rinse screens in heavy medium circuits.
What is Resonance screens and how it works?
Resonance screens are a type of horizontal screen consisting of a screen frame connected by rubber buffers to a dynamically balanced frame having a natural resonance frequency which is the same as that of the vibrating screen body. The vibration energy imparted to the screen frame is stored up in the balancing frame, and re-imparted to the screen frame on the return stroke. The energy losses are reduced to a minimum, and the sharp return motion produced by the resonant action imparts a lively action to the deck and promotes good screening.
What is Dewatering screens and how it works?
Dewatering screens are a type of vibrating screen that are fed a thick slurry and produce a drained sand product. Dewatering screens are often installed with a slight up-hill incline to ensure that water does not flow over with the product. A thick bed of particles forms, trapping particles finer than the screen aperture.
What is Banana screens and how it works?
Banana or Multi-slope screens have become widely used in high-tonnage sizing applications where both efficiency and capacity are important. Banana screens typically have a variable slope of around 40-30 degree at the feed end of the screen, reducing to around 0-15 degree in increments of 3.5-5 degree. Banana screens are usually designed with a linear-stroke vibrator. The steep sections of the screen cause the feed material to flow rapidly at the feed end of the screen. The resulting thin bed of particles stratifies more quickly and therefore has a faster screening rate for the very fine material than would be possible on a slower moving thick bed. Towards the discharge end of the screen, the slope decreases to slow down the remaining material, enabling more efficient screening of the near-size material. The capacity of banana screens is significantly greater and is reported to be up to three or four times that of conventional vibrating screens.
What is Modular screens and how it works?
Modular Screens consist of two or more independent screen modules arranged in series, effectively making a large screen from a number of smaller units. A key advantage of this arrangement is that each screen module can be separately configured with a unique screen slope, screen surface type, vibration stroke, and frequency. This allows screening performance to be optimised separately on different sections of the screen. The individual screen sections being smaller and lighter are mechanically more robust compared with a single screen with an equivalent total size. Modular screens are frequently installed in a multi-slope configuration.
What is Mogensen sizers screen and how it works?
The Mogensen Sizer is a vibrating screen that uses the principle that particle smaller than the aperture statistically requires a certain number of presentations to the screen in order to pass. The Mogensen Sizer consists of a system of oscillating and sloping screens of decreasing aperture size, the smallest of which has a mesh size up to twice the size of the desired separation size. This arrangement allows particles very much finer than the screens to pass through quickly, but causing larger particles to be rejected by one of the screen surfaces. A thin layer of particles on each screen surface is maintained, enabling high capacity such that a particular screening duty can be met with a machine occupying less floor space than a conventional screen, and blinding and wear are reduced.
What is High frequency screen and how it works?
Efficient screening of fine particles requires a vibration with small amplitude and high frequency. Frequencies up to 3600 rpm are used to separate down to 100 microns compared with vibrating screens for coarser applications that are vibrated at around 700- 1200rpm. The vibration of the screening surface can be created by electric motors or with electrical solenoids. In some screen, the vibrators are mounted above and connected by rods directly to the screening surface so that energy is not wasted in vibrating the entire screen body.
What is Trommel and how it works?
This is one of the oldest screening devices, which is a cylindrical screen typically rotating at between 35 and 45% critical speed. It is a type of revolving screen. Trommels are installed on a small angle to the horizontal or use a series of internal baffles to transport material along the cylinder. Trommels can be made to deliver several sized products by using trommel screens in series from finest to coarsest; or using concentric trommels with the coarsest mesh being innermost. Trommels can handle material from 55 mm down to 6 mm, and even smaller sizes can be handled under wet screening conditions. Although trommels are cheaper, vibration free, and mechanically robust; they typically have lower capacities than vibrating screens since only part of the screen surface is in use at any one time, and they can be more prone to blinding.