Cullet
Advantages Disadvantages Less raw materials use.
Energy consumption (cullet processing).
Less CO2 and thus lower melting energy
Indirect CO2 emission (cullet collection).
for processing raw materials. Less direct CO2 emission from raw materials
Glass quality issues (redox, metals etc.).
provides lower transportation costs. Less energy for melting.
Higher abrasive wear on some systems.
Less CO2 emissions from melting. Less indirect energy consumption and CO2 emissions. (extractions).
� Table 1. Pros and cons of cullet recycling.
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�Fig 2. One possible process designed to make clean cullet. In current practice, cullet represents a large part of a batch charge, reaching up to 90% of the batch in some European glass processes. Its use gives many energy and environmental advantages, and it has become an irreplaceable component in the majority of large-scale glass production. The glass industry has extensive cumulative experience in the use of cullet, overcoming several difficulties occurred since its first use in the eighties[2], by finding suitable technological solutions or working with cullet suppliers for a cleaner and more stable product. This paper describes a new analytical tool for the control and evaluation of cullet effects on the glass final redox state. A new method developed by SSV to determine the Inorganic Redox is described and its application to some real cases illustrated.
Research context Cullet is one of the main raw materials for the glass industry in Europe. More than 60% of glass cullet is recycled to produce new glass packaging, with some furnaces producing green colour glasses having more than a 90% recycling rate (Fig 1). Cullet use is advantageous for the glass industry. The main pros and cons are listed in Table 1.
Treatment process The treatment process consists of cleaning of the waste from any contaminants such as plastic, ceramic, stones and metals in order to obtain almost clean cullet. This is not only required for the recycling of the cullet in the glass furnace, but also to abide by the End of Waste European regulation. This regulation establishes the criteria that define how a material ceases to be a waste, in order to be stored, shipped and used as the secondary raw material. The treatment process consists of a
series of steps, conceived to efficiently clean and remove any non-glassy material from the recyclables. The details of the process may vary between different companies: the schematic in Fig 2 is a general description of one possible process designed to make clean cullet. For some companies the order of the operation can vary, depending on the type of incoming material. The input material comes from the municipal waste dry recyclables, and the output can either be mixed cullet, or flint cullet produced by colour separation plus the remaining cullet. If managed correctly, such treatment results in a final mixed cullet product that is virtually clean from ceramic, stone, plastic and metallic contaminants. The introduction of colour sorting machines had a positive impact on the amount of cullet recycled, however some unexpected contraindications have been registered. The redox of the cullet fed depends on the relative amounts of the different colours. Usually mixed cullet containing flint glass is no longer widely available in the market, and the coloured cullet obtained after flint cullet separation is a mixture of green and amber cullet. This change in colour composition leads to a different product. For example, when used in the production of coloured glass, small variation can modify the redox of the batch, introducing instability on the glass colour, especially in case of reduced glasses (amber and UVAG).
Inorganic Redox Description of the methodology The following paragraph describes the procedure employed to determine the cullet redox ratio, that is the Inorganic Redox, starting from the material properly sampled in the glass plant. A sample of at least 50kg of cullet is ground to a size below 3mm. The ground materials then evenly divided by a sampler divider, to obtain a sample of 1kg and submitted for melting. The melting is performed by first keeping the material in a ceramic crucible for three hours at 550°C, in order to burn out the remaining organic materials (i.e. wood, paper or plastic). After organic removal, the cullet is further heated and melted at 1350°C, until a bubble-free glass is obtained, then Continued>>
42 Glass International December/January 2018
Cullet SsV.indd 2
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