In many applications, the screening process is the key ingredient in transforming a waste material product into a highly sought after marketable material. Several styles of portable screening plants are available, including shaker screens, trommel screens, severe shake shaker screens, starscreens and box screens.
“Screening is simply the segregation of material by size,” said Rick Cohen, president of Screen USA Inc. located in Smyrna, Ga. “Although all screening plants are designed for the purpose of segregating material, different screening applications demand different screening technologies.
Shaker Screens
“Traditional shaker screens have been around for many years and are commonly used in screening aggregates and topsoil,” said Cohen. “The key to any screening plant is to have a nice consistent feed rate, and the shaker screens are no exception to this rule. Consistent feed rates allow the material to be evenly spread across the shaker screen to achieve the highest efficiency in screening. Many sand and gravel screening customers add spray bars to rinse the gravel during the screening operation. However, it’s important to understand that the addition of spray bars will rinse the rock, but not clean the sand. For optimal results, a fine material washer [sand screw] should be used in conjunction with the portable shaker screen to yield a quality washed sand product.
“To manufacture a sand product to meet a certain specification, many may require the assistance of a classifier, which will segregate different sizes of sand and blend them back together. Before purchasing any equipment, my recommendation is to take a few samples and send them out for a full sieve analysis. The results of the sieve analysis will dictate what additional screening accessories will be needed,” Cohen added.
Shaker screens also are widely used for screening topsoil. In clumpy soils, many manufacturers equip their screening plants with high-speed shredders, which break up the clumps before screening. This shredder is desirable when screening clay-based soils, but is not needed in sandy, loam topsoil, according to Cohen.
Trommel Screens
Trommel screens vary greatly from the shaker screens and utilize a spinning drum to screen material.
“Although both trommel screens and shaker screens do an excellent job of processing topsoil, trommel screens have a distinct advantage when screening topsoil contaminated with a great deal of organic debris. Trommel screens normally have a considerably larger screening area and incorporate a cleaning brush that assists in the screening of higher moisture material. Trommel screens are also very popular in screening organic materials, such as ground wood, mulch and compost. As one might expect, these applications require a fine screen size such as 3/8 inch and this complicates the screening process. The small screen opening equates to needing more screen area for efficient screening,” Cohen said.
Starscreeners
In high moisture materials with moisture levels of 35 to 45 percent, Cohen advocated the use of a starscreener.
“Starscreeners have been around for years screening peat and potatoes. At the present time, based on our customers’ demand, a newer version has been designed to screen high moisture organics such as compost. Over the past 20 years, the particle size of compost has changed from 1 inch to the current desired size of approximately 3/8 inch. High moistures and the smaller required sizes created a problem for some of the older models. Newer starscreens were reengineered to incorporate a unique continuous cleaning system, which has definitely changed the way the industry views starscreeners,” Cohen said.
Cohen does not recommend starscreeners for material that is contaminated with twine, banding wire or any other string like material.
Box Screens
One of the more simplistic screening products is the box screen, which is simply a shaker screen mounted on a portable chassis. Instead of having a feeder, a loader or excavator must “bucket sprinkle” material onto to shaker screen. The screened material either falls through the screen onto the ground or onto a built-on conveyor, with the oversized material falling directly off the end of the screening plant.
“These machines are excellent for coarser sizing, but are rated as fair to poor on precision screening. Since the loader or excavator bucket is being used as the feeder, it is difficult to provide a consistent feed. As a result, the machine is either overfed or is processing nothing at all. Over the past 10 years, many box screens have been introduced into the screening market. Most are extremely light duty, are marketed based on selling price, and work well for light duty screening work. However, for heavier applications, these light duty screening products will not hold up,” Cohen said.
In these cases he recommended choosing a heavy-duty box screen. For still other applications, the industry has developed an alternative called heavy-duty severe shake shaker screens.
Severe Shake Shaker Screens
As the name implies, heavy-duty severe shake shaker screens are used in heavy-duty applications such as construction and demolition. Instead of just having a shaker screen, the machines are equipped with a stout feeder, which utilizes a feeder belt with impact beds or a steel apron feeder. This allows the material to be fed at a consistent feed rate to achieve the highest level of production possible. Slabs of concrete, asphalt, quarry stone, etc. can be fed into these robust screening plants and they will typically screen into three fractions.
Many of these machines are track mounted allowing for complete mobility.
“Severe shake shaker screens are extremely efficient for log yard clean-up, screening root material and other tough applications. Many customers who own crushers purchase these screening plants as they can either screen crushed material into three sizes or use the same machine to scalp off the heavier materials to minimize unnecessary crushing,” Cohen said.
“In the most simplistic terms, once the specific screening technology is determined, we must look at the size of the loader bucket to select the proper sized screening plant to suit the customer’s product need,” added Cohen.
Screen USA Inc. specializes in manufacturing small to medium sized screening plants at its Smyrna, Ga, factory.
For more information, call 770/433-2440 or visit www.screenusa.net.
This story also appears on Aggregate Equipment Guide.
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