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Back to basics: Choosing a screen

By and |  January 6, 2015

PQ1501_screen-overview1Taking a look at horizontal versus inclined screens, as well as the effects of screening motion.

Screens are used throughout the aggregate processing flow to separate and size material prior to and following crushing stages. While most producers are aware of the overall process, following is refresher on the types and effects of screen angles and motion.

At the primary stage, large scalping screens remove fine material before the feed enters the primary crusher, helping to protect the crusher’s wear parts from abrasive stone or sand material that has already been sized. Without scalping, the primary crusher’s liners wear down faster, requiring more frequent changes and maintenance downtime.

Following the primary crushing stage, screens with two or three decks and different opening sizes are used to separate the aggregate material into different size categories – with conveyors transporting the sized material for further crushing or for stockpiling as a saleable product. Usually this screening is accomplished through dry screens. Wet screens may help to remove debris from material before stockpiling, as clean stone is often required for concrete and asphalt specifications.

Depending on the process stage, the material to be screened is fed to the screen either from a loading device such as a wheel loader (intermittent feed) or from a hopper or conveyor (continuous feed). The screen box vibrates via shafts with counterweights or exciters, causing the material bed to vibrate, as well.

Through the vibration, larger particles work their way up to the top of the material bed, while the smaller particles make contact with the screening surface. This effect is called “stratification.” If the particles are smaller than the openings in the screen surface, they will fall through. If they are larger than the screen openings, the particles will be retained on the surface and eventually carried off of the screen. This is called the “probability of separation.”

Horizontal or inclined

Screen choices begin with the determination of whether a horizontal or an inclined screen is best for the application. There are advantages and disadvantages to each.

■ Horizontal screens operate at 0 degrees, but may be adjustable to about 10 degrees in either direction, functioning at a lower overall height than inclined screens. They may have two or three shafts located within the screen box’s center of gravity, with timing gears and counterweights designed to provide either an oval or a linear motion.

Or they may provide linear motion via out-of-balance motors or geared exciters, within which eccentric weights are mounted onto both ends of a shaft located above the screen box’s center of gravity to provide vibratory motion. Horizontal screens provide high g-forces to vibrate and move the material down the length of the screen.

Horizontal screens typically offer better efficiency (accurate particle sizing) than inclined screens because the material is retained on the screen longer, with a slower travel rate to the end of the screen. This allows more properly sized material to fall through the screen openings. In a stationary plant setup, they are more often seen as tertiary (third stage) and/or finishing screens. Horizontal screens are also often specified for portable plants because of their low profile.

■ Inclined screens generally operate at angles ranging from 10 degrees to 35 degrees in one direction, and vibrate in a circular motion, offering better production capacity (processing rate) over horizontal screens because gravity helps move the material down the screen. Single or dual shafts with up to four bearings, located within the screen box’s center of gravity, provide the circular motion via weight elements mounted on the shaft’s flywheel. Inclined screens do not exert the same g-forces as horizontal screens.

Inclined screens can command a lower initial capital expenditure than horizontal screens because they typically are built with just one or two shafts; their frames do not have to be as robust as those of horizontal screens; and they do not require a higher-horsepower motor. Efficiency is not as high as it is with horizontal screens, however. Inclined screens are usually seen in stationary plants, and can be specified for just about any processing stage.

Screening motion

Circular-motion screens provide a high travel rate because they also are inclined screens. They generally accept a continuous feed very well. Screens using circular motion are best suited for larger material, as finer material tends to blind on this style of screen. Also, wet, sticky material does not screen well with this type of screen, unless water spray is also used.

Linear-motion horizontal screens typically generate less blinding and pegging of material on screen media because their straight-line motion, with high g-forces, can both dislodge material and also convey it forward across the screen. This motion can be more effective than circular- or elliptical-motion screens, resulting in a high-efficiency screen that also operates at a fairly high speed.

The operator is able to better control the material travel rate across the screen, further improving screening efficiency. Linear-motion screens also benefit producers through a lower installed cost because they require less headroom than circular or elliptical motion screens.

Elliptical-motion horizontal screens may offer some combined benefits of efficiency from linear-motion screens and the tumbling effect generated by inclined circular-motion screens. They also work to speed material travel rate at the feed end, while slowing it at the discharge end. However, this type of screen does not exert the high g-forces that linear-motion screens do.

Specification

Formulas exist to help select screens based on many factors, including tonnage of feed, screening area and desired efficiency. There are enough variables involved in the formula that it is best to work with manufacturers who understand the complete parameters of the application. It is important that the manufacturer knows the feed method, size, gradation, moisture content and rate.  Existing equipment and mounting structure, total plant production needs and efficiency requirements are also part of the equation. Manufacturers can help to specify not only the best screen unit for the application, but also the best screen media choice.

Take note

Horizontal screens are often specified for portable plants because of their low profile.

Information for this article courtesy of Linatex Corp. of America, a Weir Minerals company. Learn more at www.weirminerals.com.

Photo: Wier Minerals

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