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Cullet Quality and Furnace Efficiency

Post Time:Feb 18,2013Classify:Company NewsView:533

Around the world almost without exception in the hollow glass field, cullet is used to supplement raw materials in the batch. In fact nearly all modern furnaces require cullet to be present in order to function at all. Cullet is therefore one of the most important ingredients in the batch, and can have a profound effect, either positive or negative, on glass production. Many production lines rely to a degree on so-called “internal cullet” to provide this important glass content, and on occasion if other sources cannot be found it may be necessary to deliberately over produce containers to be used as internal cullet. Of course in Europe it is very common for glass manufactures to obtain “external cullet” as a direct raw material replacement, that is, cullet sourced from government or industry administered Post Consumer recycling programs. This post consumer glass packaging must then be rigorously processed and cleaned in order to be certified as suitable for use in container production.

The main attraction of cullet for glass manufacturers it that its presence in the batch allows lower furnace operating temperatures, which in turn reduces energy consumption. For example, increasing the use of cullet from 40% of batch to 70% could provide more than EUR200,000 of energy savings per year on a 300 ton furnace.
Other valuable benefits include reduced use of raw materials, reduced NOx and SOx emissions, increased furnace capacity and extended furnace life.

The main factor effecting external cullet use in any particular furnace has traditionally been local availability, however in recent years as glass manufactures have become more aware of the benefits of increasing cullet use, they have been casting further and further afield to source this useful material. Today the biggest challenge is the quality of the available cullet. The benefits of cullet are clear, however there are also risks. The quality of so-called “furnace ready” cullet varies massively from one supplier to the next. The negative impact of poor quality material cannot be underestimated, damage to shear blades and former blockages due to Ceramic, Stones or Glass Ceramic contamination is a common complaint, not to mention inclusions in the product resulting in high return rates. Therefore it is vital that any manufacture desirous of increasing cullet usage, source high quality material from reliable glass processing facilities. Many glass manufactures obtain unprocessed post consumer bottles and jars and operate an internal processing facility in an attempt to control the cullet quality themselves. However the recycled glass industry is currently experiencing major upheaval, as the collection methods for municipal recycling programs change, so the quality of the material they produce has worsened dramatically, it is now very common for all recyclate to be collected mixed together, not separated at source. This resulting co-mingled stream of recyclable materials is transported to large MRF’s (Materials Recycling Facilities) for separation. This method is increasingly popular as it represents huge cost savings for Municipalities when compared to collecting each material separately. Unfortunately the biggest loser in this development is the Glass Industry, this is because the recycled glass produced from this process contains very high levels of contamination, for example CSP (ceramic, stones and porcelain) levels are on average five times higher than normal. This has very serious repercussions on the amount of traditional unprocessed glass available to feed Europe’s processing facilities and furnaces.

This material cannot be processed to sufficient standards using traditional sorting methods and equipment, any cullet user who relies on existing supply arrangements will be competing for an ever decreasing pool of high quality unprocessed feedstock. This issue is most easily noticed in the UK where the co-mingled collection system is most prevalent, and which in the past has been a good supplier of unprocessed glass into other European countries.

However that has given glass re-processors in the UK a “head start” in developing processes that can efficiently treat MRF derived material to an acceptable standard. One of the most difficult challenges to overcome is the sacrificial glass losses that result from removing such high levels of CSP contaminates. Most of the available technology has been designed with a certain feedstock quality in mind and attempting to force that equipment to handle very dirty glass, results in yields of usable clean cullet somewhere in the region of 30-40%. This will prove uneconomical for most of the existing “bolt on” processing plants attached to the front end of furnaces across Europe.

Recresco has been able to overcome these issues by using a spread of different technologies, some from traditional suppliers but other borrowed from other industries, and filling in the gaps with proprietary equipment. Another challenge is associated with the higher than normal quantities of mineral stones present, these contaminants are particularly difficult to remove as most detection equipment uses a “slider plate” arrangement to present the material to the detection and removal zone, however stones are very unstable when travelling along this plate and as a result are often not detected and even if they are identified, can subsequently be missed by the removal system. Specific shapes such as cup handles, can also be very difficult to remove as they present a small cross-section to the air jet used to eject defects.
When using a multitude of different equipment suppliers in this way it is very difficult to obtain performance guarantees for the finished product, as each manufacturer will only certify their own technology, in this case it is necessary for an “umbrella” entity, in this case the reprocessor, to take responsibility for the whole process from start to finish thus giving the cullet user the protection they require and recourse in the case of production problems caused by contamination in the furnace or hot end. By developing such a supply arrangement, more cullet can and will be made available to the market.

The demand for higher quality cullet coupled with a worsening standard of unprocessed glass places pressure on the cullet supply model that has been popular for the last few decades. Glass processing is a highly specialized industry requiring unique technology and processes that have little in common with hollow glass production techniques. Also, sourcing sufficient quantities of raw material to ensure that this expensive process is financially viable often requires negotiating with governmental organizations to secure feedstock contracts, this is in itself a specialist activity.

In order to guarantee a reliable source of very high quality cullet, manufactures need to partner with specialist re-processors who can then be held accountable for the quality and continuity of supply. Another important factor to consider is that quality is very much linked to price, if extremely high quality cullet is used, for example, less than 10 grams per tonne CSP, it is possible to use 80 or even 90 percent cullet in the batch, under those circumstances the energy savings are sufficient to justify even a cullet price premium over and above what would normally be paid for raw materials.

The processing of glass is a highly complex business, perhaps overlooked due to the “dirty” nature of the material, but the use of cullet can be enough to “make or break” a container production financial model, being able to source top quality cullet in high quantities should be one of the main priorities when seeking to increase profit margins.

Source: Recresco Ltd.Author: shangyi

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