Glass

Glass recycling is the processing of waste glass into usable products. Glass that is crushed and ready to be remelted is called cullet. There are two types of cullet: internal and external. Internal cullet is composed of defective products detected and rejected by a quality control process during the industrial process of glass manufacturing, transition phases of product changes (such as thickness and colour changes) and production offcuts. External cullet is waste glass that has been collected and/or reprocessed with the purpose of recycling. External cullet (which can be pre- or post-consumer) is classified as waste. The word “cullet”, when used in the context of end-of-waste, will always refer to external cullet.

 

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 colors. Many recyclers collect different colors of glass separately since glass retains its color after recycling. The most common colours used for consumer containers are clear (flint) glass, green glass, and brown (amber) glass. Glass is ideal for recycling since none of the material is degraded by normal use.

Many collection points have separate bins for clear (flint), green and brown (amber). Glass re-processors intending to make new glass containers require separation by color, because glass tends to retain its color after recycling. 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 color at the collection point may not be required. Heat-resistant glass, such as Pyrex or borosilicate glass, must not be part of the glass recycling stream, because even a small piece of such material will alter the viscosity of the fluid in the furnace at remelt.

 

To be able to use external cullet in production, any contaminants should be removed as much as possible. Typical contaminations are:

 

  • Organics: Paper, plastics, caps, rings, PVB foils for flat glass
  • Inorganics: Stones, ceramics, porcelains
  • Metals: Ferrous and non-ferrous metals

 

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 lime (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.[5] 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.

 

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 or landfill cover.

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, saving municipalities, such as the City of Tumwater, Washington Public Works, thousands of dollars (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.

 

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.

Glass recycling process

 

As soon as the glass waste is delivered to glass recycling plants, its separation according to color starts while metal rings and bottle tops are being removed. The glass waste is crushed into small particles and the mixture is called ‘cullet’. The recycling process of glass depends entirely on the type of glass that will be produced .There are some basic rules to be followed during glass recycling, so glass should be free from metal tops, ceramics and stones and be sorted according to color. There should be a thorough removal of foreign materials, otherwise the produced glass might be defective. The quantity of ceramics left on the cullet should not be more than 25 g per tonne, while the metal particles should be less than 5 g per tonne. Therefore, the basic container glass recycling process steps are:

 

  1. initial rinsing, cap and lid removal
  2. color separation
  3. volume reduction by breaking or crushing
  4. packaging and shipping
  5. final treatment.

In many recycling programs, consumers sort glass containers according to color and remove labels, metal rings and bottle tops before delivery at waste disposal sites (Germany). In other cases, sorting according to color occurs in the glass recycling industry. Since colored glass wastes cannot be used in some applications (e.g. production of new clear glass), color sorting is important. Until recently, color separation was carried out manually, but nowadays specially designed laser equipment can sort out glass, rapidly and accurately. Color sorting based on transparency, employs light from different kinds of lamps and lasers. In laser applications, partition of light to the sensing device is guided very elegantly by a rotating mirror and optical fiber devices . As regards metal detection, theoretically, all non-ferrous and ferrous metals can be detected with a metal detection device and subsequently removed through a reject mechanism. The system sensitivity to non-ferrous metals depends on the conductivity, the shape and the size of individual metal particles. The design and construction of an effective and economically viable metal detector for a glass recycling industry is more difficult than it seems .However, prior to recycling the colored glass, stones and ceramic particles must be meticulously removed. The detection of ceramic materials in glass cullet usually makes use of the properties of transmission or reflection of light since a laser generated beam of light falls on to a sensing device. For every opaque particle this beam will be broken and a signal will be transmitted to the sensing device. This information will be further processed with an electronic device and one or more blasting valves will be activated to refuse the undesired particle. This type of sorting device is considered to be very rapid and reliable. The final product quality is usually determined by new microwave-based techniques.

Glass breaking is not desirable if it occurs before color separation. In certain mixed waste stream processing systems, the glass fraction of the waste stream simply becomes part of git residue, which is either land filled or just becomes a component of a composted waste product. In compost, glass particles usually prove to be beneficial because they have similar properties to sand. Where glass containers are to be recovered for usage in the glass manufacturing industry, color sorting has to occur before breakage, while metal neck rings, paper labels and food residues may be removed after initial breakage. After breakage, glass is packaged, stored or shipped to the market. The storage is an essential part of glass recycling, since only collection of a considerable amount of glass will ensure the economic viability of the recycling process and marketing. Broken glass storage should provide the conservation of glass in good condition until packaging occurs. Glass cullet of high quality is usually packaged in paperboard and employed for special applications. Occasionally, broken glass is shipped unpacked in containers. As soon as the glass cullet is delivered to industry, a second sorting takes place, in order to assure the desired quality of the final product. Then glass cullet is mixed with raw materials used in glass production. After mixing, the batch is melted in a furnace at 1425–1535°C, depending on the percentage of cullet contained in the batch. The more cullet used the less heating is required for melting. The melted glass is pumped into a forming (blowing) machine where it assumes its final shape.