Features Aggregates Technology
Screening issues: Case studies from the field

In the world of aggregate production, vibratory screens often face multiple challenges, with numerous potential solutions. While theoretical discussions are valuable, practical case studies provide concrete examples of how theory can be applied to solve real-world problems.

Case Study 1: Overcoming Bottlenecks in a Limestone Production Circuit
A limestone producer had been operating their circuit as designed for approximately 10 years, maintaining their equipment meticulously. However, increased sales volumes exposed bottlenecks in their system, particularly in the screen media selection on their classification screens and their recirculatory cone crusher loop.

A limestone producer sought out Polydeck’s expertise after experiencing bottlenecks in their system, particularly in the screen media selection on their classification screens and their recirculatory cone crusher loop. Photo: Polydeck

The primary issue was with the top deck media on their twin Simplicity 8×20 triple deck dry classification screens. Material rejected by the top deck would recirculate to a Sandvik 6800 cone crusher. Each recirculation increased the amount of fines processed and the bed depth on the top decks, quickly becoming the limiting factor in the circuit.

Initially, the site staff adjusted the screen motor polarities to rotate the screen “against flow” or “uphill,” which resulted in a significant number of pegged particles in the screen media openings, worsening the situation. At this point, they sought assistance from Polydeck.

Polydeck’s first step was to return the screens to “with flow” or “downhill” rotation, remove the pegged particles, and reduce the screen speed by approximately 50 RPM to retain some travel time without the pegging issue. Note: it’s crucial to understand and avoid a machine’s critical frequencies when making speed changes to prevent catastrophic damage. Consult with your screen OEM before making changes.

Next, Polydeck assessed the open area of the site’s current media selection. The media had 36.4 per cent open area, lasted approximately three to four seasons, and was the same media design that had been used for 10 years. Polydeck designed a new panel with the same aperture size but increased the open area to 55.3 per cent. These high open area panels were applied to 16-feet of the 20-foot long top decks, with the first 4 feet using the older, higher wear life design to handle the feed zone’s impact. The new panels had an average lifespan of two to three seasons, allowing the site to increase the production of saleable concrete and asphalt stone without recirculating to the cone crusher. This also reduced the fines produced since less crushing was required.

Noise reduction was another critical factor, as the site was near residential areas. The initial selection of rubber media was due to its noise reduction characteristics, with synthetic media successfully halving the noise emitted from screens. Additionally, the site was able to maintain its 1’x2’ modular screen media setup, easing maintenance and enhancing safety.

The photo of the team involved in solving a pegging issue on a vibrating screen at the Dufferin Aggregates Butler Pit (now depleted). From left: Alex Caruana, Brian Janssens, Alex Sedore, Jeremy Dingman and Matt Toth. Photo: Polydeck

Case Study 2: Addressing pegging issues in a new vibratory screen
The second case study involves a common issue with turnkey vibratory screens provided by OEMs. A customer purchased an 8×20 triple deck wash screen expected to perform at over 500 metric tonnes per hour, processing -1” crusher run into various products. This screen was also in direct circuit, meaning any top deck overs did not go to a recirculatory loop. However, after commissioning, the screen faced aggressive pegging issues, primarily on the top and middle decks.

The OEM’s initial response was to provide larger aperture sizes to avoid near-size issues, but this did not resolve the problem as all OEM panels used the same rigid polyurethane. Rotating the screen “with flow” or “downhill” helped with pegging but resulted in significant carry-over, reducing the running rate to 280 to 300 tonnes per hour, which was unacceptable.

Polydeck was then called in to provide a solution. They installed three decks of POLYDEX VR panels, offering flexibility and relieved openings to combat pegging. Dams were placed every four feet along the deck to aid material stratification. The screen was returned to “against flow” or “uphill” rotation, successfully resolving pegging but achieving efficient tonnage of only about 450 tonnes per hour.

Further investigation revealed that the slurry box, which wet the feed before screening, introduced the feed nearly two feet into the feed zone, effectively missing 10 per cent of the screen. Adjusting the slurry box backward and aligning each spray nozzle allowed the producer to exceed their efficient tonnage target.

The key takeaway is that while turnkey packages may sound appealing, the expertise of screen media specialists often becomes crucial after commissioning. This producer now exclusively requests Polydeck for their screens, regardless of the vibratory screen OEM’s offerings.

These case studies illustrate the importance of practical solutions and expert consultation in optimizing vibratory screen performance in aggregate production circuits.

Alex Caruana is a civil engineer with approximately 18 years of experience in the industry. With a serious passion for Screening, Alex has been with Polydeck for more than a decade and his current title is National Manager, Canadian Aggregates. Recognized as “Top 10 Under 40” by Rock to Road in 2014, and “Member of The Year” by the Ontario Stone, Sand & Gravel Association for 2020, Alex has grown to be a valuable resource to his customers any time they have process and screening issues.