- Created: Thursday, 13 December 2012 18:03
- Published: Thursday, 13 December 2012 18:03
By Carol Wasson
While everyone wants more tons per hour across the screen, the key to optimum screening is maximizing capacity without losing quality and efficiency. Vibrating screens must be properly selected, designed and applied – and must be operated under the correct parameters. If not, screening circuits may become big bottlenecks.
Do the math. A screen operating at 75-percent of total efficiency allows 25-percent of material within the desired product size range to be rejected with the oversize material. Bottom line, that means that production goals and profit margins are falling through the cracks.
When Callanan Industries faced the recent relocation of its Ravena, N.Y., crushing operation to South Bethlehem, the company seized an opportunity to make key upgrades to an 18-year-old plant. Now dubbed as Callanan’s South Bethlehem Plant 2, in South Bethlehem, N.Y., the relocated facility is on track to produce up to one million tpy of crushed stone with its beefed-up crushing, screening and washing operations.
“We needed to eliminate some bottlenecks, increase efficiencies, and continually improve the quality and consistency of our products,” said Mark Clemente, Callanan vice president of operations.
Screening circuit upgrades were planned in consultation with Deister Machine Co., a manufacturer which is currently celebrating its 100th year of industry service. Onboard with Deister is its East Schodack, N.Y.-based representative, Dibble Equipment.
Owner Rod Dibble worked closely with Clemente and Plant Manager Steve Akers to specify the most cost-effective solutions, while meeting stringent specifications. Ultimately, the team decided to capitalize on the reliability of five existing screens, while adding four new screens in strategic locations.
The first initiative was the replacement of a secondary 8- x 20-ft. double-deck scalping screen with an 8- x 24-ft. double-deck screen. “The 8- x 20-ft. screen was good in its day, but after we upgraded our secondary crushing circuit (from a cone/VSI to a cone/cone setup), we needed more screening capacity,” said Akers. “We also replaced our double-deck wash screen with a new 8- x 20-ft. triple-deck wash screen so that we could achieve three product sizes.”
However, the most important upgrade, said Akers, was the replacement of older single-deck high-speed screens with two Deister 6- x 12-ft. double-deck high-speed screens. “These high-speed screens allow us to be steps ahead by cleaning our material before it reaches our finishing screens. In the end, it’s about quality, not quantity,” said Akers who adds that the operation runs all the screens uphill for more efficient stratification. “Uphill is for quality and downhill is for quantity,” he stressed.
According to Dibble, the high-speed screens pull out the 1/8-in.-minus sand product before the material is conveyed to dual 8- x 20-ft. triple-deck finishing screens. “The high-speed screens run in excess of 1200 rpm, and operate with a very short ¼-in. stroke and at a steep incline of up to 30 degrees. Dibble further explains that the double-deck approach on the high-speed screens allows greater stratification between the decks. “The load is reduced on the bottom deck, which in turn allows more open area for greater throughput,” he said. As to this integral circuit, Dibble tapped the expertise of Joe Schlabach, vice president of marketing and sales at Deister. “Together we determined the exact operating parameters, and we also conducted the necessary screen cloth calculations to determine the optimal size and type of openings needed,” he said.
Proper Depth of Bed
When specifying screening circuits for operations such as the Callanan South Bethlehem Plant, Schlabach explained that maximum screening efficiency results from proper adjustments in speed, stroke, rotation (or throw) direction, and angle of inclination. Each of these parameters affects one of the most important facets in screening – proper depth of bed.
As feed material is a mixture of varying sizes, said Schlabach, oversize material will restrict the passage of undersize material, which results in a build-up or bed depth of material on the surface of the screen. Bed depth diminishes as the undersize material passes through the screen openings.
For efficient screening, the material bed should not reach a depth that prevents undersize from stratifying before it is discharged. He says that the industry rule of thumb is this: “Depth of bed (in dry screening) should not exceed four times the opening size at the discharge end of the screen.” Consequently, with a ½-in. opening, the depth of bed at the discharge end should not exceed 2-in., for example. Schlabach stresses that loading screens too heavily is a common practice, and one which leads to a carryover problem and less screening efficiency.
Boosting Product Quality
“Anytime we invest in upgrades, quality is our number-one goal,” said Clemente. “For example, in replacing a double-deck with a triple-deck wash screen, we will certainly produce a cleaner product – and we have added some additional scrubbing equipment to loosen silt and clay from the material before we wash it. We always look for additional ways to produce a consistent, high-quality product.”
As to working with Deister, Clemente said that it’s all about longevity and results. “With Deister screens, we can zero in on what is going to give us a good product. Also, they know our history and they know what we need.”
Although relocating a plant presents many challenges, the Callanan management team is proud of the resulting upgrades. “We’re excited about this season. Sometimes in the aggregate industry, the tail wags the dog, and with these improvements, we can be far more in control of our operation. We have reliability, predictability, and we can proactively plan our maintenance,” said Clemente.
Lastly, Clemente said, “Operations need to start with well-designed screens and high-capacity decks. If not, there will be bottlenecks. For us, this is a great time to watch all the material flow off our screens and be in spec.”
Carol Wasson is a Fort-Wayne, Ind.-based freelancer.