For aggregate producers, high efficiency “shaped” wire screens are the way to the future
For aggregate producers, efficient and accurate sizing of material during processing is essential to profitability. One renowned screening expert has termed plant screening stations as the “cash box” of the operation. Square opening, woven wire screen is still the media of choice in most aggregate operations. However, carryover of prime size material can become a costly production bottleneck when excessive undersize particles fail to pass through the screen openings.
The most common method of addressing carryover is to slightly decrease the wire gauge (diameter) being used to create more open screening area. However, in doing so, wear life declines due to abrasion if traditional round wire is used. For decades, this has been the accepted tradeoff: increased sorting efficiency means shortened screen life.
“Dealing with the tradeoff between media throughput and wear life has been a constant battle in our industry,” says a quarry production supervisor in charge of crushing, blending, screening and load out of mostly course grained granite at a US plant producing construction aggregate. “Every rock that rides over is rock that we are not making money on, because I have to re-crush it and send it back across the screen.”
Previously, when his operation started wearing through screens, the quarry supervisor would use larger diameter wire. Instead of .250-gauge wire, he would use .312-gauge wire or heavier. “But the heavier the wire, the fewer holes per square foot, and the less open area to fall through. Basically, we were shaking rock with limited production,” explains the production supervisor.
Although conventional round wire screen has been the industry standard for generations, the geometry of the wire itself contributes to ride over. When rock particles contact the round wire, they tend to deflect up and over the screen, lessening production.
Even though many producers of wire screen simply import semi-finished product and finish it domestically, at least one US-based manufacturer has gone to the root of the problem. The solution, it turns out, is to engineer “shaped” wire that is designed to provide more open area in wire screen of a given diameter, enabling significantly greater flow and production with less ride over. On top of this, some of the designs actually increase wear life by thickening the screen’s vertical contact surface to offer greater abrasion resistance.
As an example, one engineered woven wire screen called Hi-Rise from Hoyt Wire Cloth, compresses and expands wire so it is taller than it is wide with flat sides. The result of using shaped, rather than round wire, is that up to 15% more screen openings can be fit into a given area. The flat surface of the shaped screen wire allows a greater quantity of smaller diameter crushed rock to fall through rather than bounce over the screen, causing quick stratification. Because the material separates readily, more screen area is available for remaining material to be sized. The result is that near size material moves through the screen earlier allowing for a higher feed rate.
“With shaped wire, you have more open area in the panel. The wire is elongated. The shape acts like a dam to water: the rock hits the flat area and bounces through instead of continuing to ride over the top of it,” explains the quarry production supervisor.
In an effort to improve efficiency and production, he has used and tested several types of shaped wire, along with traditional round wire, in the past few years. “If you run production with shaped, wire such as Hi-Rise you can capture a lot more material,” the quarry supervisor says. “Our studies have shown that with such high efficiency wire screen, instead of conventional round woven top, you can get 25% to 30% more through.” He notes that he ran at least 300,000 tons of course grained granite using the shaped high efficiency wire screen so far.
As a manager whose sole focus is on safely and efficiently increasing production, the supervisor translates what such an efficiency boost actually means for the industry. “In my plant, right now, if I run only conventional, old school round wire screen, I would probably have to crush rock 12 to 14 hours a day, where I can now achieve the same production in eight hours with high-efficiency, shaped wire screen.”
According to the production supervisor, the increased production efficiency also reduces employee fatigue and crusher wear and tear. “If you run a crusher 12 hours to make the same tonnage you could in eight, you are putting much greater wear on the machine for no reason.”
Hi-Rise wire screen case study
For a limestone quarry in Western New York specialising in crushed limestone, hot mix asphalt, landscape boulders, rip rap, and high calcium agricultural lime, reducing production downtime and was the main issue, according to their quarry superintendent. “We were not getting the wear life that we expected out of the traditional round wire screen. If we wanted a greater open area percentage on our screen deck, we needed to go to a smaller diameter wire, so wear and production downtime was an issue.” When the limestone quarry switched to shaped wire screen, the wear and production issue was resolved.
In the case of Hi-Rise, the shaped wire extends wear life with greater height and wear surface to abrading rock material, which results in a longer lasting screen panel. In addition, the flat sides of the shaped wire wear more slowly than standard round wire because it presents less surface area to such abrasion. As a result, the screen panel holds accurate sizing longer, further prolonging usable life.
“With the Hi-Rise wire screen, we can use a heavy diameter wire, but still get the same open area percentage so we do not have to change the screens as much. In fact, we are probably getting 20% to 30% longer life span from our wire screens now and spend much less on maintenance,” the superintendent says.
As far as production, the limestone quarry superintendent estimates that the switch to shaped wire screen has enabled producing considerably more. “We also have more open area and more screen efficiency with the shaped wire screen. So, we are producing about 20 tons more per hour with it than our previous round wire screen,” mentions the superintendent.
While the cost of the shaped wire is slightly more than traditional round wire, the gainsin production and wear life make ROI extremely quick. “In terms of increased production as well as reduced downtime, screen maintenance and labour, I would say our ROI is essentially immediate. This screen innovation is definitely the way of the future.”