Glass Menagerie

NEW MARKETS. The plant employs two basic processes: a glass-breaking and grinding system made by Glass Aggregate Systems, Faribault, Minn., and classifying equipment from Kason Corp., Millburn, N.J. The classifying system, which separates the glass into the desired sizes, is a key element because it allows the plant to make relatively valuable products for the local market, says David MacLeod, senior recycling officer for the Western Isles Council.

Previously, Lewis has been landfilling all its glass containers because the cost of shipping the glass to the mainland for recycling was about double the market price of the material. Graham notes that about 70 percent of the glass that is recycled in the U.K. is used as aggregate in road making and the rest is re-melted for bottle making. Glass sold for these applications sells for as little as $9 per metric ton.

On the other hand, landfilling was an unsatisfactory disposal method in Lewis because landfill space had become increasingly expensive and the dumping cost had increased to $77 per metric ton from about $52 per metric ton less than two years ago. "We expect the cost to go to about $112 per metric ton by 2010," says MacLeod.

The new plant can produce not just aggregate and re-melt material, but also various higher-value products for other uses. Initially, the plant has been producing decorative glass for landscaping, which sells for about $90 to $110 per metric ton. Other product grades are being test marketed as shot-blasting material and as filtration media for potable water treatment plants. Glass used for these purposes sells for as much as $185 per ton or more, says Graham.

The plant cost approximately $110,000, and the total operating cost ranges from $11 to $13 per ton, says MacLeod. "We haven’t done a detailed cost-benefit analysis, but we expect the plant will pay for itself in a few years," he says. "Our main interest is to avoid sending material to a landfill, but this process also gives us an opportunity to make saleable products."

THE STORNOWAY SET-UP. Incoming bottles are loaded into a hopper, from which they drop onto a vibrating tray that controls the flow of bottles onto a conveyor belt. Although the system is designed for processing bottles that have been separated from other municipal waste, it can also remove metal cans, plastics and other material accidentally mixed with the feed.

First, ferrous metal is picked off the belt by a magnetic drum and sent to a Dumpster. The bottles and extraneous material are conveyed to a "popper unit," which uses rotating hammers to shatter the glass into pieces no larger than 2 to 3 inches in diameter. Next, a shearing unit (a rotating drum with a fixed blade) liberates the glass from paper labels and foil, after which the material flows onto a revolving cylindrical trommel screen. The glass particles drop through the screen into a sanding unit, and the rest of the non-glass material continues across the screen and is ejected into a bin.

In the sander, the glass is ground into particles that have no sharp edges and range in size from about 0.75 inches to dust that is less than 0.12 inches in diameter.

Finally, the Kason classifier does the critical job of separating the glass particles into the necessary size fractions for the desired applications.

The Vibroscreen classifier, as it is called, is a vibratory screener that consists of up to five circular, horizontal screens located one above the other in a vertical, cylindrical housing. Mounted to the bottom of the unit is a high-torque, one-horsepower gyratory motor with a double-extension shaft at the top and the bottom ends of the motor. Eccentric weights are fitted to each extension to give the motor a gyratory motion. The unit is supported on a circular base by springs, allowing the screen assembly to vibrate freely without transmitting vibration to the plant floor.

Material to be separated is fed onto the center of the top screen, which has the largest mesh. Oversize particles are moved to the screen periphery in a spiral pathway and discharged through an outlet, while undersize particles drop through to the next screen. The mesh sizes become progressively smaller toward the bottom of the unit.

Separation is enhanced by the three-dimensional motion generated by the unit’s gyratory motor. The motor’s top eccentric weight generates a horizontal flow to the screen assembly, while the bottom weight creates a high-frequency tilt on the screens. The combination of the horizontal and vertical movements creates a tangential action that moves oversize material laterally across a screen and encourages undersized particles to flow through the screen. The machine can be optimized for a particular application by varying the mass of either eccentric weight.

In the Lewis installation, the Vibroscreen classifier is a 48-inch-diameter unit with four screens that can process up to three metric tons per hour.

A DIVERSION TACTIC. The equipment used in the Lewis plant has proved to be "robust and reliable," says MacLeod. "I think our only downtime has been when we have had an odd bit of stone mixed with the glass and we have had to stop the operation to clean the stones from the conveyor."