Glass Sorting Systems by Mark Henricks

—View a list of manufacturers at the bottom of the page

Municipalities are handing recyclers a double-edged sword as more of them shift to single-stream glass recycling. Single-stream makes it easier for households to set aside glass recyclables, increasing the amount of raw materials for sale to glass manufacturers. But the resulting commingled mess has created new challenges for recyclers.

“Even though single-stream is on the rise,” says Felix Hottenstein, sales director at MSS, Inc., a glass sorter manufacturer in Nashville, Tennessee, “there may not be quite as much glass out there, because much of it gets lost in the processing.”

The problem also lies with compactor trucks that pick up and cart the glass. Some vehicle types are more likely to crush the glass, making it harder to separate the three different glass types. Commingled glass produces uneven color quality while metal and ceramic create weakness. Trucks can also cause cross-contamination with other waste, including plastic and paper.

Consumers who put heat-resistant glass such as Pyrex and Visionware in recycling bins give recyclers another issue. "This glass causes big trouble once it’s fed back to the glass furnace as it does not melt and therefore clogs the glass production machines," says Peter Mayer, manager of sorting technologies at S+S Separation and Sorting Technology GmbH, a manufacturer in Schoenberg, Germany. S+S makes the Varisort X, which helps recyclers keep such dangerous offenders out of the waste stream. The company also produces Spektrum, a camera-equipped color sorter designed for mixed glass.

Austin AI Inc., a manufacturer in Laurel, Maryland, is another company that specializes in handling heat-resistant glass. “The problem is growing, and is showing up more and more as costly product rejects, and has the potential to damage furnaces and extruding equipment,” says Julie Villescas, sales manager at Austin AI. The company’s QXR-G uses X-rays to pull recyclable glass away from metals and other unwanted glass types, including ceramics, crystal and plate glass.

The QXR-G works on glass before or after crushing equipment, and detects glass contaminants as the materials move onto slide-ways or conveyors and then diverts them away from the processing line. The task takes about 0.14 of a second, according to the company.

The more glass is processed with products like the QXR-G, Villescas says, “the fewer the product rejects and the higher the customer satisfaction.” Newer models of the company’s machine incorporate more and smaller diverters to pull off contaminates. “This will minimize the amount of good cullet material that is pulled off to insure the contaminant was removed,” she says.

Cullet, which is recycled glass ready for the furnace, is the highest value end product in glass recycling. Glass manufacturers like it because it has lower melting temperatures and requires less energy to process than raw materials. For every 10 percent of cullet used, manufacturers save up to 3 percent in energy costs and 15 percent in material savings, according to Austin AI.

How satisfied recyclers are with the sorting process also depends on what happens to glass before it gets to the sorters. “Generally all sorting equipment works best with uniformly sized materials,” Villescas says. “This is one reason why they are typically placed downstream of a crusher. Generally some screening to remove fine, or dust material is performed.” Items can’t be too large or too small, and vacuuming must be used to remove labels and plastic caps.

As the quality of recovered glass improves, the larger the market will become and the more post-consumer recycled glass will enter the marketplace. "Green glass production is up to 100% of recycled glass being reused," Mayer says. "This is a real closed loop recycling process."

Other technologies in glass sorting include computer-operated cameras, light emitting diodes (LEDs) and laser detection. Metals and color contaminants can be sorted away from the waste stream via magnetic separators, eddy current systems and optical inspection equipment. Glass, ceramic and crystal requires separate solutions such as LED and spectrometry identification and sorting technologies in addition to newer applications such as X-ray.

Metal separators use high-frequency detectors down to 0.6 mm. A directed air stream blows out impurities to preserve as much of the glass as possible. Lasers work by identifying nontransparent materials such as ceramic, stone, porcelain and labeled glass. Unwanted materials are then ejected by air nozzles.

CCD cameras equipped with colored filters watch for glass of varying colors and link to machine settings that separate the glass into batches of clear, green or brown. S+S's Spektrum runs with programmable software that will sort one type of glass from the rest or create three separate piles for each major color group.

MSS’s GlassSort and Glass ColorSort systems were recently upgraded from halogen to LED technology to boost accuracy. High-intensity LEDs flash through the glass to highlight color differences. The Glass ColorSort modules remove ceramic and metal at 15 tons per hour and separates colored glass up to 6 tons per hour.

MSS also manufactures a glass bottle sorter that handles up to 75,000 bottles per hour. Bottles travel along individual slides fed by a vibrating feeder. As bottles exit the slide, computerized sensors assess color. A blast of compressed air ejects the bottle to its appropriate color batch.

New sensor technologies and more precise image processing continue to advance. New markets may also be on their way. California currently sponsors a program that encourages brick and tile manufacturers to include low-value mixed glass in their products. Faster, more efficient machines aim to increase the amount of recovered glass from municipal solid waste.

As glass manufacturers forge new markets, new sorting technologies pay off fasters. Recycled glass can now be found in roadbed material, filter sand for septic systems and pools, playground sand, sidewalks and driveways, backfill, drainage systems, landscaping, paint, hybridized cement, aggregate in stucco finishes, flux in ceramic tiles, sand blasting material, fiberglass, marbles, glass beads, lawn ornaments, stepping stones and retaining wall blocks.