Sorted: The Comingled Glass Conundrum

Credit: Peter Van den Bossche The use of comingled recycling collections has risen significantly over recent years. For many materials sorting techniques have proved adept at creating high quality recyclates from these collections. But in the case of glass there have been wide variations in the quality and quantity recovered. However, recent PHD research has identified methods to recover increased high quality volumes of glass from mixed recycling. By Ben Eule Figures from the British Glass Manufacturers Confederation claim that the average UK family uses around 330 glass bottles and jars every year and, according to the Waste & resources Action Programme (WRAP), in 2012 the UK met its EU target to recycle 60% of that glass. This proves that the message to recycle is getting through, at least where glass is concerned. The processing of comingled recyclables with glass has been practised across the UK for over ten years, and according to WRAP's Glassflow 2012 report, the proportion of comingled glass being collected has increased from 27% in 2009/10 to 40% in 2011/12. This indicates an increase in mixed glass that requires colour sorting if it is to be used in the remelt sector, particularly for container manufacturing. The story around glass is a positive one. According to trade association, British Glass, the energy saving from recycling a single bottle will power a 100 watt light bulb for almost an hour or a colour TV for 20 minutes. It is an infinitely recyclable product and the benefits of recycling are measurable and easily portrayed - people can relate to this and it fulfils our need to feel that we doing something to support recycling. Yet, from an industry point of view, it is still surprising to see the variety in quality and quantity of glass from the comingled material and there is agreement that there needs to be more emphasis on the processing of the recyclate to make it more useable and attractive to the end markets and to develop those markets in the longer term. This is particularly relevant when figures show that the amount of glass being collected through kerbside recycling is increasing, whilst collection through alternatives such as bottle banks - which are single product and only require colour sorting - is decreasing. The energy saved by recycling a single bottle can power a 100 watt light bulb for nearly an hour Credit: James Cridland Challenges With reprocessors almost totally reliant on the product coming out of the recycling centres, it is essential to be able to guarantee quantity and quality. Due to the level of glass now being collected in comingled waste in the UK, and the quality coming through from the recycling plants, many recyclers/processors, aggregate processors and manufacturers felt that they would need to invest in, or be able to access, additional glass sorting technology to raise the quality. However, this would require considerable investment, which would have to be added to the cost of the glass cullet or aggregate. There are some well-known challenges for processing of comingled recyclate material with glass. Its bulk density ranges usually between 0.18 and 0.25t/m3. As a consequence, the relation between known percentages and expected volumes of materials has to be considered. In addition material segregation requires new solutions such as the segregation of paper and plastic film. Primary and secondary screening of comingled recyclate material will be important to enrich required product categories whilst reducing potential cross contamination within products. In principal, screening efficiency is depended on screen size available, the size difference and particle size distribution of the material and the required screen sizes. As the required product categories are usually a high quality paper fraction (News and Pams or N&P), food & beverage containers and bottles and a lesser quality mixed paper, screening equipment is required to differentiate material by size and by form. It is therefore important to select the correct screening size. It is also very important to successfully isolate the N&P fraction from all low grade paper, cardboard and plastic containers and films. Therefore, after the primary and secondary screening the four main product streams should be successfully generated ready for further processing either mechanically or by hand. An additional challenge is the general segregation of plastic bags and paper. Whereas historically air knifes and wind shifters performed an important task in the clean-up of plastic bags and film, their use in the comingled recyclate is limited, due to too many similar characteristics of the film and paper which are present in the mix. Contamination issues occur on the N&P line as, although of smaller size, plastic bags end up in the trommel screen oversize and on the mixed paper line as a consequence of ballistic separation. Optical sorting is an important process which is increasingly widespread reprocessing Optical Sorting Optical sorting in general is an important process which is finding increasingly widespread applications in waste reprocessing. In some cases, due to its initial high capital investment, the use of optical sorters is a limiting factor. For the N&P line the primary use is detecting plastic film and oversize container contamination. Cardboard can be targeted at the same time, and all segregated material can be further optically separated to recover the valuable cardboard and accidentally segregated paper and newspaper. The challenge is in removing as much contamination as possible without accidentally removing good product. A two stage process is therefore imperative. Very wide conveyor belts of up to 2500mm - 2800mm running at sufficient speed are required to spread out the material on the belt prior to detection. As optical sorters work best with material stationary on the belt, it is of importance that the increase in width and speed of conveyor belts occurs gradually and that fast air ejected from the optical sorter ejection valves has sufficient expansion area in the hoods to reduce turbulence. Due to the inevitability of contamination it is a necessity to protect the process plant Contamination and the need for education A further challenge is input contamination. It is of little surprise that waste has always got an element of input inconsistency. This is, however, more evident when discussing contamination. There are two reasons for contamination being present within the input material. The main one is not, as may be expected, the result of people maliciously contaminating their wheelie bin with waste (ie. garden or general household waste). On the contrary, it is as a result of consumers being too effective with their recycling, so that items such as copper pipe, microwaves, hand held tools, plant pots etc. end up in the input material, because they are perceived as recyclable material. This is where households need to receive the correct education. Intentionally introduced contamination can be higher in areas where householders' general waste is charged by weight and bin size. The consumer transfers waste into the wrong bin in an attempt to save costs. Another source of contamination derives from refuse collection vehicles (RCVs) not being used solely for recycling collection rounds. If they collect recyclate on alternate weeks, with general waste or organics collected in the other week, they introduce additional fines or remaining 'waste' into the recycling collection. As glass is abrasive in nature it is important to minimise its exposure to moving parts Protecting the line Although there is only limited possibility in predicting the type of contamination, there is no question that it will occur. There is therefore a requirement to protect the process plant, the quality of the product and the safety of the workforce. This can be achieved by input quality control and having the infrastructure available to isolate contaminated input loads from RCVs. In addition, the loading shovel operator's main task, after making sure that the plant is continuously fed with material, is to watch for possible contamination. Due to the complexity of the previously stated tasks, the main control point in any process could be a manual picking pre-sort station, capable of removing large contamination which could block or damage the process plant. There are additional benefits of a pre-sort station which we will examine later. The remaining challenge is to achieve the required purity for mixed paper whilst keeping the contamination and manual picking required to a minimum. As the particle size distribution within this category is typically between 70mm and 180mm it does generally contain a high percentage of unwanted material, such as plastic bags, crisp bags, flattened plastic trays, textiles, glass etc. The quality of glass being recovered from comingled collections varies widely Credit: Orin Zebest Early extraction If the comingled glass can be extracted earlier in the process, and more can be extracted and lower contamination levels achieved, it could have a profound impact on the financial success of this valuable recycled commodity. This also addresses the concerns of the end users who believed they may have to incur additional costs to get the purity of product they need. As glass has a very abrasive nature, the aim is to keep not only its exposure to moving parts to a minimum, but also to avoid impact movements as much as possible. An important part is conveyor design, which requires the use of an aggregate type of conveyor to keep the material as centred as possible to avoid it spilling over the sides of the rubber belt and fines material entering shafts and bearings. The inclusion and processing of glass within the comingled material increases the abrasion of all exposed equipment, resulting in increased maintenance requirements. However if this is communicated and addressed via preventative maintenance rather than the 'repair when broken' approach, such increased wear and tear can be successfully combated and plant availability can be kept high. Whereas in a non-glass MRF the fraction smaller than 40mm to 45mm is generally deemed to be waste material, a glass MRF has to take a different approach as the majority of the glass will end up within this particle size group. The challenge is therefore to recover as much glass as possible from the fines fraction without catching too much contamination in the form of shredded paper, organics or other small combustible materials. The first stage in any glass recycling process is to reduce the size of the input material and then effectively separate it from the more valuable commodities such as paper, card and rigid plastics. This approach retains the value of the MRF products and, at the same time, reduces the wear and tear on the sorting and conveying equipment further up the processing chain. The downside of this process is that it calls for more aggressive screening to separate the glass from the other recyclable commodities. Interestingly, this aggressive approach to maximise glass diversion increases the non-glass content in the isolated glass mix, which leads to increased challenges to clean up it up. The research conducted at Aachen University of Technology confirms that the removal of standard contaminants of recovered MRF glass, such as organics, shredded paper and plastics, can be achieved with a combination of screening, air density separation and optical sorting. Results and conclusions Through research and field work at UK MRFs, data was collected and the results led to the development of an improved process for the handling of comingled recyclate containing glass. This integrated treatment is capable of achieving glass purities in excess of 98%. It gives the processor the opportunity to focus on the production of a mixed glass cullet for the remelt market rather than having to downgrade it to glass sand to be used in the aggregate industry. The positive aspect of the research is that any changes required to the processing lines to achieve the improved glass recovery and purity rates can be implemented with very little disruption to the continuation of the operation. In most cases, the equipment required can be installed as a separate line with the minimum impact on the operation of the MRF overall. As a result of the ongoing reduction of higher value recyclables, such as paper, in the fully comingled mix, the levels of all other commodities will increase - by mass - over time. Consequently, glass will become an even more important commodity which, if treated properly, will provide an increased income stream for processors and local authorities. Ben Eule is global technical manager with Stadler Engineering and attained his doctorate from Aachen University in Germany for his work examining the 'Processing of Comingled Recyclate Material at UK Material Recycling Facilities'.