Glass recycling is the process of collecting, processing, and re-manufacturing waste glass into new products. Glass is ideal for recycling because it is not degraded by normal use. Scrap glass material, known as cullet, is used in glass manufacturing to reduce the consumption of energy and new raw materials. There are two types of cullet:
Internal cullet comprises waste glass generated by glass production processes themselves, including quality control rejects, material from production transitions (such as colour or specification changes), and manufacturing offcuts. External cullet consists of post-industrial and post-consumer waste glass collected through organised recycling programmes. External cullet requires more extensive processing and quality control because of potential contamination from consumer use and collection processes. Prior to recycling, glass waste must be purified and cleaned of contamination. Then, depending on the intended use and local processing capabilities, it might also have to be separated into different sizes and colours. Many recyclers collect different colours of glass separately, because glass tends to retain its colour after recycling; collection points often have separate bins for clear (flint), brown (amber) and green waste glass. Separation by colour at the collection point may not be required if the glass re-processor uses optical sorting equipment. Heat-resistant glass, such as borosilicate glass (including Pyrex), must not enter the glass recycling stream, because even a small piece will alter the viscosity of the fluid in the furnace at remelt.
Processing of external cullet To use external cullet in production, as much contamination should be removed as possible. Typical contaminations are:
Organics: Paper labels, and corks Inorganics: Plastic caps and rings, metal caps, stones, ceramics, porcelains, PVB (Polyvinyl butyral) and EVA (Ethylene-Vinyl Acetate) foils in flat/laminated glass Metals: Ferrous and non-ferrous metals Heat resistant (ex: Pyrex dishes) and lead glass (ex: crystal with lead content) Manpower or machinery can be used in different stages of purification. Since they melt at higher temperatures than glass, separation of inorganics, the removal of heat resistant glass and lead glass is critical. In the modern recycling facilities, dryer systems and optical sorting machines are used. The input material should be sized and cleaned for the highest efficiency in automatic sorting. More than one free fall or conveyor belt sorter can be used, depending on the requirements of the process. Different colors can be sorted by optical sorting machines.
Recycling into glass containers
Glass bottles and jars are infinitely recyclable. The use of recycled glass in manufacturing conserves raw materials and reduces energy consumption. Because the chemical energy required to melt the raw materials has already been expended, the use of cullet can significantly reduce energy consumption compared with manufacturing new glass from silica (SiO2), soda ash (Na2CO3), and calcium carbonate (CaCO3). Soda lime glass from virgin raw materials theoretically requires approximately 2.671 GJ/tonne compared to 1.886 GJ/tonne to melt 100% glass cullet. As a general rule, every 10% increase in cullet usage results in an energy savings of 2–3% in the melting process, with a theoretical maximum potential of 30% energy saving. Every metric ton (1,000 kg) of waste glass recycled into new items saves 315 kilograms (694 lb) of carbon dioxide from being released into the atmosphere during the manufacture of new glass. But recycling glass does not avoid the remelting process, which accounts for 75% of the energy consumption during production.
Other products The use of the recycled glass as aggregate in mortar (masonry) and concrete has become popular, with large-scale research on that application being carried out at Columbia University in New York. Recycled glass greatly enhances the aesthetic appeal of the concrete. Recent research has shown that concrete made with recycled glass aggregates have better long-term strength and better thermal insulation, due to the thermal properties of the glass aggregates. Glass which is not recycled, but crushed, reduces the volume of waste sent to landfill. Waste glass may also be kept out of landfill by using it for roadbed aggregate. Glass aggregate, a mix of colors crushed to a small size, is substituted for pea gravel, crushed rock, or sand in many construction and utility projects, reducing costs to a degree that varies depending on the size of the project. Glass aggregate is not sharp to handle. In many cases, the state Department of Transportation has specifications for use, size and percentage of quantity for use. Common applications are as pipe bedding—placed around sewer, storm water or drinking water pipes, to transfer weight from the surface and protect the pipe. Another common use is as fill to bring the level of a concrete floor even with a foundation. Foam glass gravel provides a lighter aggregate with other useful properties. Other uses for recycled glass include:
Fiberglass insulation products Ceramic sanitary ware production As a flux in brick manufacture Astroturf Agriculture and landscape applications, such as top dressing, root zone material or golf bunker sand Recycled glass countertops As water filtration media Abrasives Mixed waste streams may be collected from materials recovery facilities or mechanical biological treatment systems. Some facilities can sort mixed waste streams into different colours using electro-optical sorting units.
In construction The alternative markets for recycled glass waste include the construction sector (for pavement construction, as an aggregate in asphalt, pipe bedding material, drainage or filler aggregate), the production of cement and concrete (using glass waste as aggregate), as partial replacement to cement, partial replacement for cement and aggregate in the same mixture or raw material for cement production, as well as decorative aggregate, abrasives, or filtration media.
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