Glass recycling


Glass recycling is the comprehensive process of collecting, processing, and re-manufacturing waste glass into new products.
The recycled glass material, known as cullet,
serves as a crucial raw material in glass manufacturing, reducing energy consumption and environmental impact in glass-manufacturing operations Cullet consists of recycled material prepared for remelting in glass furnaces. This material exists in two distinct categories based on its origin and processing pathway:
  • Internal cullet comprises manufacturing waste generated during glass-production processes, including quality-control rejects, material from production transitions, and manufacturing offcuts that never reach consumer markets.
  • External cullet represents post-industrial and post-consumer waste glass collected through organized recycling programs, encompassing both pre-consumer cullet from glass-product manufacturers and post-consumer cullet from used containers and products collected from end-users.
The distinction between these categories is crucial for waste classification and end-of-waste criteria, as external cullet requires more extensive processing and quality-control measures due to potential contamination from consumer use and collection processes.
To be recycled, glass waste needs to be purified and cleaned of contamination. Then, depending on the end-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, since glass tends to retain its colour after recycling. The most common colours used for consumer containers are clear glass, green glass, and brown glass. Glass is ideal for recycling since none of the material is degraded by normal use.
Many collection-points have separate bins for clear, green and brown waste. Glass re-processors intending to make new glass containers require separation by colour. If the recycled glass is not going to be made into more glass, or if the glass re-processor uses newer optical sorting equipment, separation by colour at the collection point may not be required. Heat-resistant glass, such as Pyrex or borosilicate glass, must not enter the glass-recycling stream, because even a small piece of such material 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 and EVA foils in flat/laminated glass
  • Metals: Ferrous and non-ferrous metals
  • Heat resistant and lead glass
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, soda ash, and calcium carbonate. 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 of waste glass recycled into new items saves 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.

Recycling into other products

The use of the recycled glass as aggregate in 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 or crushed rock 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:
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.

Recycled glass in construction

The alternative markets for recycled glass waste include the construction sector, the production of cement and concrete, 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.

Recycled glass in road pavement

Three different samples of recycled glass with different gradation curves produced from residential and industrial waste glass streams in Victoria were studied in this research to investigate their usage as a construction material in geotechnical applications. The Fine Recycled Glass and Medium Recycled Glass were classified as well-graded, while Coarse Recycled Glass was poorly graded according to the Unified Soil Classification System. The specific gravity of recycled glass was approximately 10% lower than that of natural aggregate. MRG exhibited higher maximum dry unit weight and lower optimum water content compared to FRG. LA abrasion tests showed FRG and MRG to have abrasion resistance similar to construction and demolition material, while CRG had higher abrasion values. Post-compaction analysis indicated stability for FRG and MRG, but CRG displayed poor compaction behavior due to particle shape and moisture absorption issues. CBR and direct shear tests revealed MRG's superior shear resistance and slightly higher internal friction angle compared to FRG. Consolidated drained triaxial shear tests confirmed these findings, suggesting FRG and MRG behave similarly to natural sand and gravel mixtures in geotechnical applications. Hydraulic conductivity tests demonstrated medium permeability and good drainage characteristics for FRG and MRG. Compliance with EPA Victoria requirements for fill material was also confirmed. Overall, the study supports using recycled glass in various geotechnical engineering applications.

Recycled glass in bricks

, a material commonly used in industrial flooring, uses polymers, typically resins, to replace lime-type cements as a binder. Researchers have found that ground recycled glass can be used as a substitute for sand when making polymer concrete.
According to research, using recycled glass instead of sand produces a high strength, water-resistant material suitable for industrial flooring and infrastructure drainage, particularly in areas subject to heavy traffic such as service stations, forklift operating areas and airports.

Challenges

Despite all the improvement in the waste and recovery processes, challenges include:
  • Lack of incentive to recycle when inconvenient; opt-in and subscription models lead to low participation
  • Rising material recovery facility fees and pressure from the waste management industry have caused some municipalities to remove glass from curbside recycling
  • Broken and crushed glass causes a very high level of wear to vehicles and machinery used to transport and sort it
  • Lack of recycling mandates and high levels of contamination cause a significant portion of materials to be disposed of in landfills.
  • Low landfill tip fees for many MRFs incentivize sending glass to the landfill.
  • Lack of capacity in certain areas hinders the ability to meet the market demand and reduces the incentive to invest in materials recovery facilities.
  • In some regions, strong demand for cullet from other end markets reduces potential supply for glass containers.
  • Distance between the sources of and markets for cullet requires long-haul shipping.
  • Virgin materials are often cheaper than cullet, sometimes by as much as 20%.

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