Sorting it out: Matching screens to the job at hand
In many applications, the screening process is the key ingredient in taking a waste material and transforming it into a highly sought after marketable material. When researching portable screening plants recyclers will find numerous styles of machines including: shaker screens, trommel screens, severe shake shaker screens, starscreeners, and box screens.
“Screening is simply the segregation of material by size,” explains Rick Cohen, president of Screen USA. “Although all screening plants are designed with such a purpose, different screening applications demand different screening technologies.”
“In the marketplace, many commissioned salesmen try to sell the customer what they have in stock, not necessarily what the customer needs.”
Traditional shaker screens have been around for many years and are commonly found in aggregates and topsoil applications. The key to any screening plant is to have a nice, consistent feed rate. The shaker screen is no exception. A consistent feed allows material to spread evenly across the shaker screen to achieve the highest efficiency. Many sand and gravel customers add spray bars to rinse gravel during the screening operation. It is important to know that while adding spray bars will rinse rock, it will not clean sand. A fine material washer (sand screw) is needed in addition to a portable shaker screen to yield a washed sand product.
To manufacture sand to meet a certain specification, many customers may need the assistance of a classifier which will segregate different sizes of sand and blend them back together. Before the purchase of equipment, take a few samples and send them out for a full sieve analysis. The results of your sieve analysis will dictate what additional screening accessories will be needed.
Shaker screens are also ideal for screening topsoil. In clumpy soils, many manufacturers equip their screening plants with high-speed shredders which will break up the clumps before screening. Pre-shredding is very desirable when screening clay-based soils, but is not needed in sandy, loam topsoil.
Trommel Screens (shown top) vary a great deal from shaker screens and utilize a spinning drum to screen the material. Although both trommel screens and shaker screens do an excellent job processing topsoil, trommels 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 a cleaning brush that assists when screening higher moistures. Trommels are also very popular in screening ground wood such as 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 the need for more screen area for efficiency. Secondly in high moisture materials with moisture levels of 35 percent to 45 percent, a starscreener will perform better than a trommel screen.
Starscreeners have been around for years, mainly used in peat and potato applications. However, a newer version has been designed to screen high moisture organics such as compost. Over the past 20 years, the desired particle size of compost has changed from one-inch to the current size of approximately 3/8-inch.
“Due to high levels of moisture in compost, and the smaller required sizes, SCREEN USA was forced to find a solution as our prior technologies were less effective due to the moisture content in compost,” says Cohen.
“We have perfected starscreeners to screen high moisture compost. Our unique continuous cleaning system has definitely changed the way the industry views starscreeners,” he continues.
“Although starscreens are excellent for high moisture, they are not recommended when the material is contaminated with twine, banding wire, or any other stringlike material.”
Box Screens are simply shaker screens mounted on a portable chassis. Instead of having a feeder, the loader bucket must sprinkle the material onto to shaker screen. The screened material either falls through the screen onto the ground or onto a built-on conveyor. The oversized falls directly off the end of the screening plant. These machines are excellent for coarser sizing, but are fair to poor on precision screening. Since the loader 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. During the past 10 years, many box screens have been introduced into the screening market.
“Most are extremely light-duty and are marketed mainly on the internet,” says Cohen. “Many of the manufacturers do not demonstrate and are selling based on price.”
“For light-duty screening these machines work,” he continues. “However, for heavier duty applications, choose a heavy-duty box screen. You cannot buy a heavy-duty box screen for $35,000. You will need to pay two to three times this amount.”
Severe shaker screens
As the name implies, heavy-duty severe shake shaker screens are used in heavy-duty applications such as construction & demolition. Instead of just having a shaker screen, machines in heavy-duty applications are equipped with an extremely stout feeder, which utilizes a feeder belt with impact beds or a steel apron feeder. This allows the material to be fed a consistent feed to achieve the most 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. Log yard clean-up, screening root mat, and other tough applications find these severe shake shaker screens to be most efficient, and 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 material to minimize unnecessary crushing.
“There are a variety of decisions to make when purchasing screening equipment,” concludes Cohen. “First you must decide which of the screening machines fits your applications. Secondly, you must decide whether you want American Made with Standard over the Counter parts or Foreign machines with metric parts. Lastly, you must decide what size machine will fill your production needs. Normally the size of your bucket loader dictates the size of machine you will need.”