Automated glass colour-sorting technology improves the efficiency and economics of established commingled container collection systems. The typical system breaks approximately 50 per cent of the glass containers collected. Breakage occurs at several points in the system: when glass containers are emptied into the collection truck, during transfer to the MRF floor, and when the containers are loaded onto in-feed conveyors. The vast majority of this broken glass is lost in the screening process with plastic, paper, and other debris.
The degree to which these other contaminants affect the marketability of the mixed-colour broken glass varies from facility to facility. The best it can bring–due to the commingled nature of the glass colours (providing that contamination is removed)–is a low value aggregate or sand blasting material. Where these low value markets do not exist, this screened fraction represents a negative value because of disposal costs.
Glass separation system
Today, technology exists to recover 80 to 90 per cent of this “lost glass,” automatically colour-sort it, and return it to the bottle recycling loop. MSS, Inc. in Schaumberg, Illinois, develops and supplies glass-sorting equipment.
The glass-sorting process requires specific equipment. Glass first enters the system through a surge hopper. The hopper then feeds the first metering conveyor which provides a steady, even flow of glass to the rest of the system. Magnetic separation is used to remove ferrous contaminants; the glass is then screened to remove glass particles below 3/8 inch (9.5 mm) in size. (These smaller particles can’t be effectively sorted.) The screening mechanism also removes glass particles larger than 2 inches (51 mm) in size. These large particles are then routed through a glass crusher that reduces the glass to the proper size for sorting. Following the screen, removal of light non-glass materials such as paper and plastic is achieved with a vacuum system. At this point the glass enters the Electro Pneumatic Aluminum Concentrator (ELPAC) which removes aluminum and other non-ferrous contaminants.
The mixed color glass now has the majority of contamination removed and has been appropriately sized. Glass then enters the vibrating feeder which provides a single layer of glass to the ColorSort module. The single layer of glass travels down a slide which gravity feeds the glass through the identification array. High-speed optical scanners and a series of digital signal processors distinguish different glass colors. An array of 96 precision air jets then separates the selected item from the glass stream. The first module removes ceramic and other opaque contaminants. The second module removes green glass while the final module removes brown glass. All sorted fractions are of 95 per cent or greater purity. Total system capacities begin at 5 tonnes per hour. Additional capacity is attained by paralleling the modules. A touch-screen provides users with sort selection, system diagnostics, sorting statistics, and parameter access for fine tuning
Decrease costs and increase revenue
Alliance Waste Services in Schaumburg, Illinois, installed the MSS Glass sort system in the spring of 1998. The company has realized a 70 per cent reduction in residue disposal costs and has doubled revenue generated from the sale of colour-sorted glass.
Plant manager Mike Tisdale explains, “By reducing disposal costs and increasing glass recovery, this system will pay for itself very quickly.”
Written by Josh Bickman, marketing director for MSS, Inc. in Schaumberg, Illinois.