An innovative dry cleaning process offers advantages to glass recyclers - and, by extension, glass producers - through waste reduction, energy savings and increased product quality.
G.R.L.-Glasrecycling in Lummen, Belgium processes glass like no one else. At the heart of its operation is a newly installed fluidised bed dryer which has a range of applications in other industries but here is being used to produce furnace-ready `cullet' - waste glass that is crushed, sorted, cleaned and ready to be remelted.
There are two ways to produce new glass: one is to create virgin glass purely from raw ingredients, like sand, lime and soda, and the other is to melt down cullet as a substitute material. The second approach saves energy - about 1% for every 4% of cullet used - and is gentler on the environment because fewer raw materials require extraction: each tonne of cullet used can save 1.2 tonnes in raw materials.
But the challenge of glass recycling is that it can be hard to know exactly what is in the cullet. Glass producers must rely on the recycler's ability to separate out other non-glass materials that can damage their furnaces and increase the energy needed to run them.
Standard industry practice now is to pass cullet through a series of operations which, in turn, detect and remove any foreign objects: through handpicking, screen filters, magnets, eddy currents for non-ferrous or non-iron-bearing metals, air suction and optical sorting.
Optical sorting uses digital video cameras equipped with sensors that can distinguish glass from non-transparent materials such as ceramic, porcelain and stone. If detected, these are ejected from the material stream using quick and precise bursts of compressed air.
Unfortunately, though, these sensors often become obstructed by organic residues on the glass - for instance, the sticky marmalade people neglect to wash from the bottom of jars.
To solve this problem, G.R.L. settled on a new combination of technologies. Before undergoing optical separation, the glass is sent through two additional units. The first is the fluidised bed dryer, which pumps large quantities of hot air through the glass. This process completely dries out any organic material present in the batch, causing it to cling to the glass.
This may seem counterproductive, but it allows the glass to be more easily cleaned in the second unit - essentially, a large drum in which the desiccated organic matter is polished from the glass.
While this process does add slightly to operating costs, the reduction in glass waste is well worth it, according to Raf Vanswartenbrouck, G.R.L's managing director - after all, recyclers get paid only for what is not thrown away. For glass producers themselves, the benefits of fluidised dry cleaning are also very clear. "In the long and medium term, it's only quality which counts," says Vanswartenbrouck.