Online exclusive: Seeing through the screen

Online exclusive: Seeing through the screen

Disc screens are proving effective in processing variable loads of mixed C&D debris.

November 9, 2017
Doug Logan
Crushing & Screening Equipment Equipment & Products Mixed C&D

What will you find in a construction and demolition (C&D) debris container? Every load may contain a surprise, making it more difficult to recycle. Educating the public takes time and expense. Creating guidelines and supervision of incoming waste is essential. Most try to keep it out, but nonacceptable waste has become a common problem for many facilities. With ever changing types of waste ending up in C&D loads, it’s vital to select the proper recycling equipment. 

Waste composition, collection and transportation methods may vary across the country. Transfer stations tend to compound the problem and clients are usually more concerned with construction schedules than taking time to recover packaging materials from the job site. Everyone has deadlines and budgets to keep, but the effectiveness of recycling programs and seasonal changes may limit the ability to capture certain materials.
No matter how careful you are, the value of clean cardboard is diminished when exposed to other waste. Can it be sorted and kept clean? Is it a struggle to process different waste streams with the same system? Looking to add or eliminate presort stations? Does the primary screen capture all the fines material or do fines ride on top of larger items and surf across the screen? Regardless of the material being processed, screening solutions have become an important component of every recycling system. 

Screen performance and material recovery rate
Screen performance has a tremendous impact on the recovery rate of C&D debris. Conventional screening methods like trommel or star screens have limitations, while new dynamic disc screens prevent clogging and wrapping issues that occur with conventional screens. Dynamic disc screens make it easy to adjust the screen size for different types of materials. In terms of versatility, capacity and footprint, a dynamic disc screen is a much more efficient solution. 

Vibratory and rotating drums that have the same capacity will require two to three times the footprint. The Eco-Star screen by Aggregate Equipment Inc., Leola, Pennsylvania, has rotating shafts that constantly flip the material several times and capture the smaller items. Film and plastics tend to float above the flat discs. The remaining items discharge off the end of the screen in a metered flow to enhance further separation. The product sizing and efficiency of the screen remain constant.

It’s been noted that successful recycling operations handle debris fewer times and use effective screening methods to separate the stream by size. The quality of the screening process determines the efficiency and performance of subsequent processes such as ferrous metals recovery, density separation, manual sorting/inspection and volume reduction. If the fines aren’t removed by the primary screen, chances are the sorted products are contaminated. Any remaining fines and debris on the final sort belt typically end up in a landfill as waste. In that case, the fines content doesn’t matter. 

But things are changing, and many C&D recyclers are finding ways to divert the residuals from the landfill by creating an alternate fuel product. Getting all the fines out becomes a higher priority. The sorters and automated processes focus on capturing recyclable materials and removing contaminants to improve the quality of the fuel product. With the negative sort process, all materials that remain on the sort belt are transformed with size reduction and screens to produce alternative fuel. If the fines remain on the belts, the contamination level may exceed alternate fuel quality requirements. The efficiency and performance of the screening process is more crucial than ever. Proper sizing and recognition of material is vital to improve the effectiveness of automated and manual sorting techniques. 

When markets tighten up, suppliers of quality products will outperform the competition. Contamination and purity levels are directly related to screens and secondary sorting methods. To produce high quality, low impurity commodities, the equipment must be properly selected. Recovering materials from the waste stream is an expensive proposition. The markets fluctuate, and all too often the intrinsic value of recovered items are less than the dollars spent capturing them.

It’s risky to stockpile large quantities of sorted materials and hope the markets will improve. That’s why it’s important to capture the items that are profitable. Constantly evaluate the performance and properly maintained equipment can reduce down time and improve bottom line.

Doug Logan is the applications specialist at Aggregates Equipment Inc., a manufacturer of material processing equipment for demanding screening applications, including the Eco-Star dynamic disc screen, based in Leola, Pennsylvania. Logan has 20 years of experience in design, sales and operating C&D recycling equipment and systems.