SCREEN PLAY

When it comes to screening efficiency, common culprits of lost production are blinding and pegging, says Gary Pederson, vice president of sales for Major Wire Industries, Ltd., based in Candiac, Quebec. “Usually, the bottleneck of the plant is the screen,” he explains. “It’s not the crusher. It’s not the conveyor. It’s often the screens, because you can’t get enough material through them.”

Al Grove, quarry superintendent, with Doylestown, Pa.-based Plumstead Materials, agrees. Several years ago, one of his sites was putting blast feed into the primary crusher, then scalping modified material and selling it right away. “The quickest return on investment is to produce a saleable product with the least refinement,” Grove says. “The less process the material has to go through, the less cost it creates.” In this case, the site was skipping the back end of the production process to get a quicker return.

The challenge Grove faced, however, was blinding. “If there’s moisture, it creates a blinding issue, no matter what kind of screen you are using,” he says. “If it rained for a day, we’d have to wait four or five days for the material to dry out enough to screen it effectively.” One trick he learned along the way was to decrease the volume of material that was being screened based on the moisture levels.

A splitter system was installed at the site that allowed the operator to control the feed flow in 20-percent increments. “The splitter system allows me to control the volume of modified material going to the screens,” he says. “That worked great.” The plant operator running the scalper screen was trained to eyeball it. If the operator saw the material becoming drier, he could adjust the splitter door. “It’s trial and error,” Grove notes. “Controlling the volume is the best way to control blinding.”

The selection of screen media can also reduce blinding, Pederson notes: “The operator may decide to run a stainless-steel wire mesh instead of a tempered wire mesh because the stainless steel is a little smoother and slicker and can pass some of that sticky material that would stick to a tempered wire.”

Many manufacturers suggest a self-cleaning screen cloth for applications prone to blinding, Pederson adds. For example, polyurethane strips in the place of cross wires can promote vibration in the wire that allows it to throw material off.

“In addition to the eccentric motion of the screen, you’re getting a vibration through the wire of the screen cloth,” he says. “The eccentric motion, coupled with the vibration on the screen cloth, helps prevent blinding and pegging. That makes a significant difference.”

1. Configure conveyor setup

Evaluate how the feed is hitting the screen. If rock coming off the conveyor hits only a portion of the screen, the entire screen cloth is not being used, and it will wear prematurely. Consider either repositioning the conveyor or installing a pant-leg chute or deflector that will spread the material evenly across the screen.

2. Beware blinding and pegging

Blinding and pegging are the two most common challenges operators face while screening. Control the flow of feed to ensure the screen is not overloaded, then work with various combinations of screen media to determine which will best meet the need for production volume, cost, and ease of maintenance.

3. Keep it clean

If the material needs to be rinsed, set the spray bars to disperse water up the deck, opposite of the material flow. Nozzles should shoot a wide fan of water across the rock to ensure that it doesn’t create pegging problems or “burn” holes through the screen media. Avoid setting nozzles so they spray straight down or in narrow arcs onto the material and the screen media.

4. Positive tension

For longer screen media life, make sure it has proper tension. Examine items such as the clamp rails and the crown rubber to make sure the tension is evenly distributed across the screen media and that no gaps appear between the media and its supports. Such gaps will allow wire cloth to move and, eventually, break.