Choosing screens and feeders - Quarry Magazine

When discussing the selection of screens and feeders, it is essential to categorize the topic into two primary elements: screens and feeders. Each of these categories can then be divided into subcategories that describe the different types and their respective applications. This guide aims to provide insights into the screens and feeders that are most commonly utilized in quarry operations.

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SCREENS
Screening devices have been integral to the extraction of materials like clay and minerals since ancient times. Historical references, some dating back to 150 BC, indicate that early screening was conducted using rudimentary tools made from hides with perforations, woven horsehair, and even reeds or timber. The effectiveness of these early screening methods was limited. The practice of employing woven wire meshes first appeared in Germany during the 15th century.

The concept of mechanically shaken screens was recorded in John Smeaton’s Diary during his travels to the Low Countries. Smeaton, a notable civil engineer responsible for constructing the Eddystone Lighthouse in the UK, observed that the Dutch used a stamp battery to pulverize stones while separating the crushed product with a sieve that was affixed to the stamper. As the stampers lifted, the screen's feed end rose slightly and fell again as the hammers descended, allowing materials to be processed in a closed circuit—oversized materials were manually returned to the stamper for reprocessing.

Since those early efforts, the evolution of screens has led to a range of types, with some enduring in modern applications. Current quarry screening systems fall into three major categories: inclined, horizontal, and rotary.
Rotary screens

Although they have fallen out of favor in recent years, rotary screens—also known as trommel screens—were once quite popular. These screens consist of a series of punched plate or wire mesh sections arranged in a cylindrical shape, starting with the smallest openings at the feed side. The complete assembly forms a lengthy barrel with a central shaft that is supported at several points along its length, complete with bearings at each end, thrust rollers at the discharge side, and a drive mechanism, typically a bevel or ring gear. While standard diameters for these screens range from 600mm (24 inches) to 1800mm (72 inches), their feed rates vary based on specific features, generally between 17 to 150 tonnes per hour. However, capacities are also influenced by the length and aperture of each screening section. Despite their historical significance, rotary screens are rarely produced today, although a few manufacturers still offer mobile variants.

Inclined screens
Quarry professionals will likely be well-acquainted with inclined screens, often referred to as circular motion screens. These setups involve a robust steel “box” housing one or more screening decks, along with a vibrating mechanism. Typically, this mechanism includes a shaft that runs on bearings, either positioned above the screen box or located between the decks, complete with counterbalances to generate vibration. Depending on the screen configuration, counterweights may be either fixed or adjustable, with larger screens featuring dual counterweighted shafts driven by separate motors. Positioned at an incline between 15° and 22.5°, the overall configuration utilizes springs for support (either rubber or steel).

The vibrating mechanism produces an elliptical screening action, and under optimal conditions, the screening efficiency ranges from 85% to 90%. While manufacturers preset the screen throw for specific applications, adjustments can be made by modifying counterweights or changing the mechanism speed—with prior consultation from the manufacturer recommended. Typically, the elliptical motion’s direction follows the feed, propelling material in the same direction as the flow. However, a “contra-flow” configuration can enhance efficiency for finer materials, allowing more time for them to pass through the screen apertures.

Most manufacturers include inclined screens in their offerings, providing units that generally vary in width from 900mm to 2.4m, and lengths from 1.8m to 7.5m. Common models utilized within the quarry industry include the Jaques Jetflo and Torrent screens, as well as the Allis Ripl-Flo. Many inclined screens are also integrated into mobile units like Powerscreen and Finlay.

Horizontal screens
Horizontal screens are often employed in quarries with mobile crushing systems, providing the benefit of requiring less headroom than their inclined counterparts. While they share a similar design to inclined screens, horizontal screens are typically installed flat or at a slight angle (about 5 degrees) and use linear motion to advance materials across the deck. One notable exception includes banana screens, which are larger and can handle greater throughput, despite being more commonly associated with mining rather than quarrying operations.

Various drive mechanisms can create the necessary movement, including unbalanced motors, gear-driven exciters, or counterbalancing methods akin to those utilized in inclined screens. Many of these installations incorporate dual or even triple mechanisms to optimize performance. The mechanism imparts linear motion, although some manufacturers offer horizontal screens with elliptical motion capabilities. Factory settings establish the angle of throw, typically between a 40° and 60° incline to the screening deck, which should not be altered during field operations. A lower angle might accelerate material flow but hinder screening efficiency, while a steeper angle could increase efficiency but may also reduce capacity.
Standard dimensions for horizontal screens tend to be between 900mm and 2.4m in width, and from 1.8m to 7.5m in length, with other sizes also available. Common models include the Allis "low head" and ElJay "flat" screens.

Screening media
There are several types of screening media available, tailored to specific applications. The go-to option is the woven wire deck, which offers several wire types based on abrasion resistance needs. This screening media comes in various standard square aperture sizes, ranging from 1.25mm to 100mm, along with elongated slots for managing damp or unconventional materials; fine apertures are rare in quarry settings. Most manufacturers provide standard panel configurations, with capabilities to create custom screens upon request.

For heavier applications, like scalping screens, decks may consist of steel plates with punched or hand-cut openings. These apertures can take on either square or round shapes per the application's requirements.

Decks can also be composed of alternative materials such as rubber or polyurethane. These options may offer superior abrasion resistance compared to metal but might necessitate modifications to supporting frames during installation. They involve a higher initial investment, yet often provide longer-term economic benefits due to reduced operational noise and decreased plugging or blinding of the screen.

In specific scenarios, screens may be outfitted with rod decks or wedge-wire configurations, although these are not commonly utilized in quarry settings.

Screen selection
Selecting the appropriate screen is a nuanced process that should not be underestimated. A multitude of factors must be accounted for to ensure optimal performance for a particular application. Particularly when choosing multi-deck screens, distinct calculations are necessary for each deck due to varying load conditions.

Key considerations include:
• Type of installation (mobile, stationary, headroom availability).
• Nature of material being screened.
• Intended purpose (scalping, sizing—whether dry or wet, dewatering, washing, etc.).
• Characteristics of the material (mass per cubic meter, moisture condition, shape of particles).
• Preferred screening media type (woven wire, polyurethane, rubber).
• Operational schedule (daily hours, weekly days).
• Anticipated feed rates, including recirculating flows.
• Feed material analysis (percentages of undersize, oversize, and half-size materials using appropriate apertures).
• Desired end product size.

Assessing these aspects will help identify the optimal screen type. For instance, if dewatering is a requirement, a horizontal screen may be most suitable. Once all pertinent information has been gathered, including details about deck locations (top, second, etc.), open area of screen media, media aperture shapes (square, round, slotted), and anticipated screening efficiency (typically between 85% and 90%), calculations can proceed to determine the necessary screen deck area. This will dictate the size requirement. For instance, if calculations suggest a necessary screening area of 4m², the closest standard screen size would be either 3m x 1.5m (with an effective area of 10ft x 5ft, or 4.18m²) or 4.2m x 1.2m (approximately 4.55m² effective area, or 14ft x 4ft). It's essential to remember that the effective deck area is always less than the actual dimension due to the encroachment of side frames and tension bars into the screening realm.

FEEDERS
In quarry operations, feeders play a crucial role in multiple applications, facilitating the controlled transfer of materials to crushers, screens, and conveyors from bins or stockpiles, thereby safeguarding other equipment from impact loads.

Typically, quarry operators are familiar with the primary feeder, responsible for moving raw quarry resources into the primary crusher. In smaller settings, these may consist of robust vibrating screens fitted with punched plate decks; however, more frequently, these are purpose-built vibrating feeders, belt or apron feeders, or reciprocating plate feeders. Less common configurations include bar, roll, and chain feeders—this discussion will focus on the more prevalent options.

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Vibrating feeders
Vibrating feeders are available in a spectrum of designs, from lightweight electromagnetic units, like the Syntron type, to more robust configurations that include vibrating screens and heavy-duty machines with gear-driven mechanisms. Applications for these feeders are as diverse as the machines themselves. Lightweight units often regulate material discharge from bins or stockpiles onto conveyors and into secondary crushing stages, while heavier models typically serve as primary feeders in crushing operations.

Heavier feeders may feature a full deck of grizzly bars or incorporate a grizzly section at the discharge end to eliminate finer materials before entry into the crusher.

All units typically come with variable-speed controls to adjust the feed rate, though fixed-speed drives may be used as a means to save costs. However, this can limit machine versatility and lead to stop-start operation rather than a smoothly regulated feed. While feeders can process a wide variety of materials, feeding wet or sticky substances can lead to complications due to material buildup.

Apron feeders
Apron feeders boast a heavy-duty fabricated steel frame that supports overlapping steel or cast manganese pans, driven by heavy-duty “caterpillar” chains that operate on rails or rollers. The drive mechanism of this feeder is typically powered by a variable-speed hydraulic motor.

Recognized for their durability, apron feeders excel in conveying heavy, uneven, or abrasive materials. While they are capable of managing wet and sticky feeds, they may also experience a buildup of fines beneath the machinery due to materials filtering through the spaces between pans and sticky substances dropping off the feeder’s return side. Commonly, apron feeders find their place in primary feeding applications but can also serve as stockpile feeders in demanding environments.

Belt feeders
Belt feeders resemble belt conveyors but are designed with more robust construction. The belts are flat, with some having a slight trough shape, and they rest on closely spaced carrying rollers for support.

These feeders are chosen for their operational smoothness and can accommodate most materials, although they are less suitable for handling very large lumps. They are particularly effective for moving finer fraction materials such as sands.

Reciprocating plate feeders
Reciprocating plate feeders consist of a steel pan supported on rollers and move in a reciprocating action. Viewed as a lighter and cost-effective alternative to vibrating pan feeders, they are especially practical for smaller applications. They operate at a fixed speed and stroke, capable of managing sticky feed materials effectively.

These feeders tend to require a head of material for optimal performance. The reciprocating action generates a pushing motion that results in a somewhat stop-start feed discharge.

Feeder selection
While the selection of feeders is somewhat less complex than screens, several factors remain vital when narrowing down choices for specific applications.

Consider the following aspects when selecting a feeder:
• Type of installation (what is it feeding, e.g., crusher, bin, or conveyor?).
• Largest size of material being fed.
• Approximate analysis of the feed material.
• Type and description of materials (shot rock, aggregate, sand, wet, sticky, etc.).
• Capacity requirements.

In many instances, multiple feeder types or sizes could meet operational needs. In such cases, the final decision often hinges on:
• The specifications of the crusher or width of the conveyor/screen being fed.
• Size of materials being handled and the load on the feeder.
• Required hopper capacity—larger feeders typically necessitate larger hopper sizes.
• Available headroom; inclined feeders require additional space.
• Future considerations—will the chosen feeder accommodate potential upgrades?
• Price—while it is essential for the selected option to meet budget constraints, prioritizing cost alone may result in poor performance over time.

Both screens and feeders serve critical roles in any crushing operation. Mis-sizing these units can lead to processing bottlenecks, adversely impacting crusher capacity or leading to increased recirculation loads. Numerous manufacturers are ready to assist quarry operators in selecting economically viable solutions for screening and feeding challenges. It’s crucial to remember that the best economical choice is not always represented by the lowest initial price!

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