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Screens & Feeders, Plant & Equipment



The interior of a mobile Nirox trommel (rotary) screen.
The interior of a mobile Nirox trommel (rotary) screen.
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Choosing screens and feeders

Although they have their similarities, screens and feeders are distinctly different and have specialised applications.
Any consideration of this subject needs to be split into two major headings, one for screens and the other for feeders, with sub-groups to describe the various types and applications. This paper focuses on screens and feeders generally in use in quarries.

SCREENS
Screening devices have been around for as long as human beings have sought to extract clays and minerals. Some records dating back to 150BC describe methods where coarse sieves of hides with holes punched in them, woven horse hair, reeds or timbers were used to separate particle sizes. Screening efficiency was, at best, questionable! The first recorded use of woven wire meshes was by the Germans in the 15th century.

The first mention of a mechanically shaken screen was in the 1775 book John Smeaton’s Diary of his Journey to the Low Countries. Smeaton, a civil engineer who built the Eddystone Lighthouse in the UK, found that the Dutch were pulverising stone in stamp batteries and separating the product with a sieve tied to the head end of the stamper. As the stamper hammers rose, the feed end of the screen lifted 150mm and fell back as the heads dropped. The screen was in closed circuit, with the oversize being shovelled back manually into the stamper for reprocessing.

Since these early attempts, many types of screen have been tried; while some were far from successful, several still exist today. Existing quarry screens can be split into three main categories: inclined, horizontal and rotary.
Rotary screens

Rotary screens are used least today, but were popular in times past. They are also known as trommel screens (trommel being German for “drum”) and consist of a series of punched plate or wire mesh screening sections, starting with the smallest aperture at the feed end. The sections are rolled into a circle and joined to form a long barrel, with a central shaft supported at a number of points along the length of the barrel, with supporting bearings at each end of the shaft, thrust rollers at the discharge end and a bevel gear or ring gear drive. Standard sizes range from 600mm (24 inches) to 1800mm (72 inches) in diameter, with average feed rates from 17 tonnes per hour to 150 tonnes per hour. However, the length and aperture of each screening section determines the capacity of the units, and capacities may be listed in tonnes per hour per unit of length. Rotary screens are rarely used now, but there are still some manufacturers producing mobile units.

Inclined screens
Most quarry personnel will recognise ­inclined screens, also known as circular motion screens. They consist of a steel “box” with one or more screening decks and a vibrating mechanism. The mechanism, in the simplest form, comprises a shaft running in bearings, either above the screen box or between the decks, with counterweights at the ends of the shaft to provide the vibrating force. Counterweights can be either fixed or moving, depending on the screen design. Larger screens may have two counterweighted shafts, with a motor driving each shaft. The screen box is inclined between 15° and 22.5° and mounted on springs (either steel or rubber).

The screening action of the mechanism is elliptical, and the screening efficiency should be 85 to 90 per cent if the screen is correctly selected. The screen throw is set at optimum for the application by the manufacturer, but can be altered by adding or removing counterweights or varying the speed of the mechanism. However, alterations should only be done in consultation with the manufacturer.
The direction of the elliptical motion (and the direction of rotation of the mechanism shaft) is generally “with feed”, that is, the motion throws down the screen in the direction of the material flow. However, for better efficiency (albeit a lower capacity), the direction can be ‘contra-flow’, which throws the feed material back towards the feed end, giving longer time for the near size material to pass through the screen aperture.

Most screen manufacturers have inclined screens in their product line. These units come in widths from 900mm to 2400mm and lengths from 1800mm to 6100mm, although some smaller and larger units do exist. They can have between one and four screening decks. Among the more common units employed in the quarry industry are the Jaques Jetflo and Torrent screens and the Allis Ripl-Flo. Inclined screens are also fitted to Powerscreen, Finlay and similar portable screens.

Horizontal screens
Horizontal screens are seen in quarries when fitted to mobile crushing plants. The advantage over the inclined screen is that they require lower headroom for installation. Although similar in design to the inclined screen, most horizontal screens are installed flat or on a shallow angle (around 5°) and rely on the linear motion of the mechanism to throw the material forward. The exceptions to the flat installation are banana screens, which are larger and capable of higher tonnages. These are used more in mining, but some are now appearing in larger quarry plants.

The motion can be provided by various means such as out of balance motors, gear driven mechanical exciters or counterweight mechanisms similar to those fitted to inclined screens. Most of these units have at least two mechanisms to provide the motion, either twin exciters or twin (even triple) counterweight shafts, often with a single drive motor.

The action imparted by the mechanism is treated as a straight line, although several manufacturers offer horizontal screens with an elliptical motion. The angle of the throw is a 40° to 60° incline to the screen deck; it is pre-set in the factory and should not be considered field adjustable. The lower angle will increase the speed the material passes over the deck, reducing screening efficiency. The greater angle will increase efficiency and can reduce ‘pegging’ (near size material caught in the mesh aperture) but will also reduce the screen capacity.
Standard sizes for horizontal screens are from 900mm to 2400mm wide and from 3600mm to 6100mm long. Larger and smaller units may be available. The most common horizontal screens are the Allis “low head” and ElJay “flat” screens.

Screening media
Screening media comes in several types, depending on the application. The most common and cheapest is the woven wire deck, available in several types of wire, depending on the abrasion resistance required. It can be supplied with standard square apertures from 1.25mm to 100mm as well as long slot for use when screening damp or unusual materials, although the finer apertures are seldom used in quarry applications. Most manufacturers will supply standard panels to suit the client’s requirements and can often produce special screens on request.

For heavier duty applications, such as scalping screens, decks can be made from steel plate with punched or hand-cut apertures to suit the requirements. Apertures can be square or round as required, but are generally fairly large.

Screens decks can also be supplied in rubber, such as Trellex, and polyurethane materials. These materials resist abrasion better than metals, but often require ­alterations to the supporting frames to allow them to be fitted. They are more ­expensive in the initial purchase price but, in many applications, can be more ­economical in the longer term. They are also quieter in operation and resist ­plugging and blinding of the deck.

For special applications, screens can be fitted with rod decks or wedge-wire decks, although these are uncommon in quarry applications.

Screen selection
Screen selection is complicated, and not to be treated lightly. To select the correct screen for any particular application, many factors need to be considered. If selecting a multi-deck screen, calculations need to be done for each deck as loadings vary from deck to deck.

Amongst the things to be considered are:
• Type of installation (mobile, fixed, adequate headroom).
• Type of material to be screened.
• Type of duty (scalping, sizing [dry or wet], dewatering, washing etc).
• Material characteristics (mass per cubic metre [m3], condition of feed [dry, damp, clayey, etc], surface moisture, particle shape [cubical, slabby, etc]).
• Preferred type of screening media (woven wire, polyurethane, rubber).
• Operating schedule (hours per day, days per week).
• Feed rate including recirculating load (if any).
• Feed analysis (which requires estimates of percentage undersize, percentage oversize and percentage half-size for each aperture under consideration).
• The required product size.

Consideration of these points will guide your choice of screen, such as selection of a horizontal screen if dewatering is required. Once the information is collated and additional factors such as deck location (eg top, deck, second deck, etc), screen media open area, media aperture shape (square, round, slotted) and screening efficiency (85 per cent or 90 per cent used for calculation) are applied to the selection formulae, the screening area of the screen deck is calculated. This will determine the size of screen. For instance, if the calculation indicates a screening area (worst case) of 4m3, the next nearest standard size screen is either 3m x 1.5m, 4.18m3 effective screen deck area (10 feet x 5 feet) or 4.2m x 1.2m, 4.55m3 effective deck area (14 feet x 4 feet). Note that the effective deck area is less than the actual area, due to the intrusion of the side frames and tensioning bars onto the screen area.

FEEDERS
Feeders are used in several quarry applications, from transferring material to crushers, screens and conveyors at a regulated rate from bins or stockpiles to protection of other equipment from impact loads.

Most quarry operators will be familiar with the primary feeder, which moves the run of quarry material into the primary crusher. In small applications, these can be heavy duty vibrating screens fitted with punched plate decks, but more often these machines are specifically designed vibrating feeders, belt or apron feeders or reciprocating plate feeders. There are also less common machines such as bar, roll and chain feeders. This paper will only discuss the more common units.

Vibrating feeders
Vibrating feeders vary from lightweight, electromagnetically driven units such as the Syntron type through similar machines (but of heavier construction) to vibrating screens to heavy duty machines with gear-driven vibrating mechanisms. Applications for these units are as varied as the machines themselves. Lighter units are used under bins or stockpiles to regulate the discharge of materials onto conveyors or into secondary subsequent crushing stages, whereas the heavier machines are employed as primary crusher feeders.

The heavier feeders have a full deck of grizzly bars or may have a grizzly bar discharge section incorporated at the discharge end of the pan to remove finer material before it enters the crusher.

All units have a variable speed control to regulate the feed rate, although fixed speed drives are used as a cost-saving exercise. This can limit the versatility of the machine and lead to stop-start operation rather than regulated feed. The feeders can be used for most materials, although wet and sticky feeds can cause problems due to packing.

Apron feeders
Apron feeders have a heavy duty fabricated steel frame supporting overlapping steel or cast manganese pans mounted on heavy “caterpillar” chains operating on rails or rollers. The drive for this feeder is achieved by a variable speed hydraulic motor.

These feeders are the most robust of the common feeders and used for conveying heavy, lumpy or abrasive materials. They will handle wet and sticky feeds but can create a build-up of fines under the machine, both from material filtering through the gaps between pans and from sticky materials falling off the return side of the feeder. Apron feeders are often seen in primary feeder applications, although they have also been used as stockpile feeders in heavy duty applications.

Belt feeders
Belt feeders are similar to belt conveyors, albeit of heavier construction. The belt is flat(across the width) but may have a slight trough. The heavy duty rubber belt is carried on closely spaced carrying rollers.

Belt feeders are selected for smoothness of operation and can handle most materials except those containing very large lumps. They are good for sands and finer fraction materials.

Reciprocating plate feeders
Reciprocating plate feeders consist of a steel pan supported on rollers and driven in a reciprocating motion. They are seen as a lighter weight (and cheaper) alternative to vibrating pan feeders in smaller applications. They have a fixed speed and fixed stroke drive and are able to handle sticky feed materials.

Reciprocating pans require a head of material to work most effectively. The reciprocating motion of the pan imparts a pulsating forward flow of the material, and thus the feed discharge is more or less a stop-start action.

Feeder selection
Feeder selection is not as complicated as screen selection, but there are still several factors to consider when choosing a feeder for your application.

The following points need to be considered when selecting a feeder:
• Type of installation (what will the feeder be feeding, eg crusher, bin, conveyor?).
• Largest size of material in the feed.
• Approximate feed analysis.
• Type and description of material (shot rock, aggregate, sand, wet, sticky, etc).
• Required capacity.

Often more than one type or size of feeder will suit the requirements. In that case, the final selection will depend on:
• The size/type of crusher or width of conveyor/screen being fed.
• The size of the material being handled and loading on feeder.
• The hopper capacity required. The larger the feeder, the greater the hopper size.
• Available headroom. An inclined feeder will require more headroom.
• Future needs. Will the selected feeder allow for plant upgrades in the future?
• Price. The final selection must fall within budget parameters. However, selection purely on price is false economy as the unit chosen may not perform to the day to day needs of the operation.

Screens and feeders are vital pieces of equipment in any crushing plant. Incorrectly sized units can cause bottlenecks in processing plants, leading to higher recirculating loads or reduced crusher capacity. Plenty of manufacturers will be willing to assist quarry operators to select the most economical solution to screening or feeding problems. However, it must be noted that the most economical is often not the cheapest in initial purchase price!









ABOUT THE AUTHOR
Peter Mayo
Peter Mayo is the manager of Auscrush Services, a consultancy to the quarrying industry.








Saturday, 20 October, 2018 08:14pm
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