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An air-based automated material recycling system for postconsumer footwear products

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The increased availability of cheap produced in higher quantities goods, in conjunction with rapidly changing consumer fashion trends has resulted in a sharp increase in the intake of products in many industrial sectors. The world-wide per capita usage of shoes has improved considerably, from 1 pair of shoes per yr for every person on the planet in 1950 to nearly 2.6 pairs of shoes in 2005. In the EU, it's estimated that the quantity of waste due to postconsumer shoes could reach 1.2 million tonnes each year. The eyesight of 'No Waste to Landfill' therefore remains as one of the main difficulties of 21st century for the footwear sector. This target is quite ambitious as presently significantly less than 5% from the 20 billion pairs of sneakers produced worldwide each year are recycled or reused. However, increased raw material costs, producer-responsibility problems and forthcoming environmental legislations are expected to challenge the way the footwear industry handles its end-of-life products.

It really is argued that in lots of situations, material recycling is seen as the utmost suitable method of coping with discarded sneakers. However, for long-term sustainability of such footwear recovery activities an viable material recycling system should be established economically. In the electric/digital and motor vehicle sectors, where European Manufacturer Responsibility directives, like the End-of-life Vehicles directive and the Waste Electronic and Electrical Equipment directive have already been presented, a number of materials recycling value chains have now been established. This has been feasible because these products usual contain a huge percentage of conveniently recoverable metallic components to facilitate an economically sustainable value chain. Nevertheless, shoes products typically contain a large combination of components, such as for example rubbers, polymers, leather and textiles that have low recycled value relatively.

Therefore understanding and developing methods for footwear recycling is of main concern towards the footwear sector and this paper will discuss the development of an automated material recycling system for mixed postconsumer footwear waste. The first part of the paper starts by introducing the many EoL options for shoes and outlines the challenges of EoL shoes recycling. The paper then describes the recycling approach that has been created, provides a simple economic evaluation and outlines some potential applications for recovered components. The later area of the paper then presents the outcomes of experimental studies with three common forms of shoes products. Finally further work is definitely talked about and conclusions are drawn.

As discussed by Staikos and Rahimifard there are four main EoL options that may be considered for postconsumer footwear products, while illustrated in Fig. 1, they are: landfill, incineration/gasification, recycling and reuse. For each from the EoL options there are many environmental impacts, economic benefits and technical requirements that must definitely be considered.

Land-filling is definitely the most undesirable option, because of the obvious negative environmental effect, depletion of assets, increasing landfill taxes and in some national countries the limited availability of landfill space. Incineration is still considered a controversial technology with environmental concerns over the launch of polluting emissions. Reuse entails the assortment of put on or undesired shoes or boots for distribution primarily within developing countries. Charitable organisations such as the Salvation Army Trading Company Ltd. (SATCOL) and Oxfam, together with local municipalities and authorities are the main supporters of reuse strategies in the UK. However, it is argued that because the economic power of developing countries grows the demand for second hand shoes may begin to fall. Furthermore, not absolutely all shoes which are collected could be reused, because of the poor circumstances, and in such situations material recycling is seen as the most suitable option.

Nike is currently the only shoes manufacturer which is engaged in postconsumer footwear recycling on a commercial scale. Their scheme continues to be labelled the Nike 'reuse-a-shoe' programme and has been developed to recycle put on and defective athletic shoes. Customers can come back any brand of unwanted shoes via Nike's world-wide network of collection points placed within retail stores. The collected sneakers result in 1 of 2 central recycling vegetation - in the USA or in Belgium. In these vegetation the sneakers are shredded and subjected to some mechanical recycling processes to separate them into three materials streams: Nike Nike Fluff, Grind and Nike Foam. These materials are then useful for various sports related applications such as for example running track underlay, playground surfacing and golf ball courtroom underlay. The Nike 'reuse-a-shoe' system has been operating for over a decade and Nike statements to get recycled around 25 million pairs of shoes to date. However, the scheme is not designed to deal with the recycling of additional nonathletic varieties of postconsumer footwear waste. Therefore, a more common recycling strategy as outlined with this paper must deal with numerous kinds and styles of shoes products.

Postconsumer footwear products certainly are a largely untapped commodity with a substantial prospect of recycling. This shows the financial and environmental advantage that can be extracted from creating a sustainable shoe recycling chain. However, current materials recycling facilities and operators are either not capable of dealing with the precise material combine in shoes products or usually do not provide the most practical method of recovering maximum value from postconsumer sneakers waste. One of the main requirements for creating sustainable recycling methods within the shoes sector would be to investigate suitable recycling processes to successfully distinct postconsumer shoes into well-defined mono-fraction material streams. The evaluation of various postconsumer footwear waste has however shown the fact that materials recycling of combined shoes products can be an incredibly challenging problem. You can find two particular problems that present a substantial challenge to material recycling of sneakers, namely the varied range of footwear types with various construction techniques and the great number of different components used.

The footwear industry employs a multitude of materials to make a diverse range of different kinds and varieties of shoes. According to Weib there are around 40 different materials found in the developing of a shoe. Leather, rubber, foam, textile and plastics are amongst the simple materials most used in shoe produce generally, with each material possessing its own specific characteristics. There's also numerous metallic components within footwear products. These include visible metallic parts, such as metal eyelets, buckles and ornamental components and other metallic elements which are inlayed within the footwear for structural reasons frequently, such as metal steel high heel works with, shanks and metal feet caps. Removing these metal parts presents a substantial challenge for the materials recycling of shoes - the metals tend to be present as a small percentage of the total footwear by pounds and are generally highly entangled with other components and materials. At their most simple, shoes are made up of only two elements per pair, for example flip-flops, with foam exclusive and rubber strap, or could be complex constructions with 60 or more parts per pair, such as in many modern sports shoes. Nevertheless, most can be described as using a subset of parts and components which are generally common to all or any sorts of shoe. These include; upper grindery products, lower parts and parts. An average shoes product is going to be assembled from several components utilizing a selection of becoming a member of technologies, such as gluing, moulding and stitching. Previous analysis shows that because of the intricacy of shoe design and construction it is officially difficult and frustrating to manually disassemble and split footwear products into functional recycled material channels. It really is argued that due to the fairly low material beliefs manual processing this way would not be an economically lasting activity for large scale footwear recycling. In addition to complete manual disassembly, the authors also have explored the semi-automated parting of footwear parts based upon slicing or pulling/tearing. However, because of the huge selection of shoes designs and sizes these techniques have had just limited achievement with specific sub-categories of sneakers. Thus these technologies are not regarded as suitable for the large scale processing of the many tonnes of mixed footwear waste currently sent to landfill.

The complex material combination of modern shoes and the wide variety of construction techniques used necessitates the usage of an automated recycling process, based upon technologically feasible and commercially viable recycling technologies. Such extremely mechanised recycling systems are currently employed by various other industries because the primary method of recycling end-of-life products within an economically sustainable manner. Recycling products this way generally entails shredding or granulation, in a way that the product is usually put into different components and/or material types. After fragmentation following parting machines exploit the variations in materials properties to provide automated parting into different material streams. Generally speaking these systems are effective for separating components such as for example plastic and metal which have distinctly different properties. However, problems frequently arise when attempting to split up components with related properties, such as the different types of polymers and rubbers that are commonly found in footwear products.

Recycling plastic pelletizer technologies regarded as technically and economically simple for footwear products include: shredding and granulation systems; air-based parting devices; liquid-based density parting; and, for recovery from the metallic components, eddy and magnetic current separation and basic sensor based 'detect and eject' chutes. Other commercially obtainable recycling technologies such as electrostatic parting products and advanced sensor centered sorters have also been considered for shoes recycling. However, there has to be further research in to the technical and financial feasibility of such recycling systems for mixed shoes products. At present materials separation based on particle size and weight is probably the most cost-effective, high-capacity process that could be used to automate the parting of shoes waste on an industrial range. A recycling system based on fragmentation and air-based parting technologies has hence been created for the material recovery of footwear products. The procedure is layed out in Fig. 3 and has been made to procedure the vast majority of shoes types and styles we.e. sports activities leather and shoes or boots based sneakers with rubber soles. Along the way there are three main measures, these are: sorting, metal removal and materials parting. Experimental research have got produced the normal mass balance and purity of the primary recoverable materials fractions.

It is envisaged a business footwear recycling system will include a sorting stage to split up shoes and boots into different types that may then end up being processed in batches. With this true method the yield and purity of the prospective materials types can be improved. For example, to reclaim foam components in the correct manner shoes which have high foam content, such as sports shoes, should be recycled separately from leather centered sneakers. It is because the parting of low density foams from leathers is present a significant problem using the proposed air-based technology.

There are several options that are becoming considered for removing the metallic parts in postconsumer footwear waste. The very first involves removing metal using a manual removal process. For example, shoes or boots could be pre-shredded to expose the inserted metal parts, which would then become delivered to a selecting line for manual sorting and removal of metallic items. However, initial experimentation shows that depending upon the labour price this manual intervention may possibly not be an economical lasting activity.

The next option is mechanical separation using specialist metallic separation equipment i.e. shredding accompanied by magnetic, eddy current and induction sensor centered'detect and ejects' chutes. When handling metal parts, shedding is generally required because granulators tend to be unable to process metals without incurring economically unsustainable wear and damage. The shredding process does of course add further complexity and cost to the footwear recycling process plan.

Initial experiments have already been conducted with an over-band magnetic separator during shredding trials with commercially obtainable equipment. Although no detailed analysis from the separation was conducted, preliminary visual inspection from the waste materials streams showed great recovery of the ferrous metals when shoes had been shredded to 20-30 mm. As shoes consist of both ferrous and nonferrous metals you will see a particular percentage of non-ferrous metals still present after magnetic separation. A subsequent separation stage is certainly therefore needed to remove these non-magnetic steel particles. This may be finished with an eddy current separator - however, it really is argued these separators usually do not provide the most technically or economically feasible means to remove the small percentages of nonferrous metals present in the waste materials stream. An inexpensive means to individual the rest of the metals after magnetic parting is to use a sensor structured 'detect and eject' chute such as those employed to protect plastic procedure equipment from foreign metals parts. However, with this technology, a degree of extra materials will be ejected along with the metallic parts, which may decrease the overall yield of recycled components.

Apart from specialised metallic parting processes there are other technologies that might be used to remove the metallic parts from shredded footwear waste. Initial tests using a basic sink-float liquid1 based density parting process have proven that it is possible to successfully independent metals from rubber/foam/leather and showcase the potential of utilizing a commercial dense mass media separator such as a hydrocyclone to remove the metallic content present in shredded footwear waste materials.

However, you may still find concerns on the technical feasibility of removing all metallic content with the above mentioned technologies completely. As metallic contamination can reduce the value of the additional recycled components considerably, it really is argued that there surely is a dependence on the reduction as well as eradication of metallic elements at the footwear design stage.

The next stage of separation aims to liberate rubber granulates from your PU and EVA based foams from sports shoes, or for leather based shoes the rubber from leather. The right means to offer this separation is really a vibrating air-table. As depicted in Fig. 4b, the air-table uses surroundings and vibration to split up the heavier rubber that techniques in the table through the lighter material that stratifies at the top and slides down the desk. Separation efficiency depends upon optimisation of various procedure parameters extremely, which include: the angle of the vibrating deck; the vibration frequency; the air speed; and the surface characteristics from the deck. To ensure maximum separation efficiency the authors have developed a customised air-table that is specifically designed and optimised for the parting of the granulated rubber from foam and leather materials in shoes products.

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