Tracking the Flow of Resources in Electronic Waste - The Case of End

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Tracking the End-of-Life Flow of Resources in Electronic Waste - The Case of Computer Hard Disk Drives Komal Habib, Keshav Parajuly, and Henrik Wenzel Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02264 • Publication Date (Web): 09 Sep 2015 Downloaded from http://pubs.acs.org on September 13, 2015

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Environmental Science & Technology

Tracking the End-of-Life Flow of Resources in Electronic Waste – The Case of Computer Hard Disk Drives

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Komal Habib*, Keshav Parajuly, Henrik Wenzel

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Department of Chemical Engineering, Biotechnology and Environmental Technology. University of

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Southern Denmark. Campusvej 55, DK-5230, Odense M, Denmark.

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*Corresponding author

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Komal Habib: e-mail: [email protected]; Phone: +45 24409707

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Total word count: 5700, plus 2 figures and 1 table

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ABSTRACT

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Recovery of resources, in particular, metals from waste flows is widely seen as a prioritized option

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to reduce their potential supply constraints in future. The current waste electrical and electronic

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equipment (WEEE) treatment system is more focused on bulk metals, where the recycling rate of

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specialty metals such as rare earths is negligible compared to their increasing use in modern

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products such as electronics. This study investigates the challenges in recovering these resources in

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the existing WEEE treatment system. It is illustrated by following the material flows of resources in

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a conventional WEEE treatment plant in Denmark. The computer hard disk drives (HDDs)

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containing neodymium-iron-born (NdFeB) magnets were selected as the case product for this

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experiment. The resulting output fractions were tracked until their final treatment in order to

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estimate the recovery potential of rare earth elements (REEs) and other resources contained in

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HDDs. The results further show that out of the 244 kg of HDDs treated, 212 kg comprising mainly

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of aluminum and steel can be finally recovered from the metallurgic process. The results further

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demonstrate the complete loss of rare earths in the existing shredding-based WEEE treatment

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processes. Dismantling and separate processing of NdFeB magnets from their end-use products can

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be a more preferred option over shredding. However, it remains as technological and logistic

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challenge for the existing system.

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Abstract art

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KEY WORDS: resource recovery, rare earths, hard disk drives, material flow analysis, WEEE treatment


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INTRODUCTION

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Mineral resources, in particular, metals have played the crucial role in the development of modern

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society. Metals are chosen due to their specific properties, imperative for the functioning of a

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product. For a big part of human history, only a few metals were commonly used such as iron,

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copper, lead, and tin. With the advancement of technology, especially during the last century, more

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and more metals were brought into use. Today, almost all of the stable elements of periodic table

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are used in the modern products1, 2. The unprecedented growth in metals production during the last

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century has raised the concerns regarding long-term availability of metals to meet the future

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generation’s demand. In this context, the concept of urban mining – recovery of resources from the

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waste streams is widely seen as an apt solution to deal with the depletion issue of resources3.

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However, enhancing the recovery of resources from the waste flows such as waste electrical and

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electronic equipment (WEEE) is not straightforward due to the immense complexity of modern

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products, in both design and material aspects4.

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Though the overall recycling rate of WEEE has improved as a result of improved collection and

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processing, it mainly accounts for the base metals such as steel, aluminum and copper. It does not

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necessarily claim the proportional improvement in the recovery of other valuable resources.

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Complexity in design features and material composition including hazardous substances, lack of

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technically and economically efficient processing techniques, and uncertainties in the market of

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recycled resources are contributing to the loss of resources. It has resulted into the down cycling of

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specialty metals5, which are used in small amounts in the final products to achieve the specific

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functionalities such as rare earth elements (REEs)6, 7.

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REEs have been classified as critical resources by a number of governing bodies and research

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institutes due to their high supply risk and increasing importance in the modern applications8-13. The

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current recycling rate of REEs from the End-of-Life products is reported to be less than 1%

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This is mainly because of the long lifetimes of their major end-use products such as wind turbines

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and passenger vehicles. Another reason for the current negligible recycling rate of REEs is their

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small amount in the existing waste flows16. During recent years, the focus on enhancing the

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recovery of REEs from different waste streams has increased exponentially17. Several studies have

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focused on estimating the recovery potential of rare earths from NdFeB magnets contained in

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various end-use products16,

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commercial scale is yet to be realized. This is mainly because of the low concentrations of REEs in

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.

18-20

. Nevertheless, development of efficient REEs recovery plants at



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the products, which are further diluted into the outgoing recyclates from WEEE treatment facilities.

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Two recent studies have highlighted this issue21, 22. REEs contained in the NdFeB magnets stick to

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the ferrous surfaces due to their magnetism. Therefore, they become part of the outgoing ferrous

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fraction. Bandara et al.23 have shown the traces of neodymium in the slag from steel mills, where

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the input material for these mills has been the output ferrous fraction of the electronic waste

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shredder.

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This study tracks the EoL flow of resources (metals, alloys, plastics and others) in electronic waste,

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demonstrated with the help of REEs based permanent magnets i.e. neodymium-iron-boron (NdFeB)

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magnets contained in the computer hard disk drives (HDDs). The motivation behind it is to

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ascertain the final fate of resources in the existing WEEE handling and treatment system. The main

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reasons for selecting the computer HDDs as a case study are: a. HDDs are often reported to be the

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largest end-user of NdFeB magnets(~30%)

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WEEE streams contain the NdFeB magnets; and c. HDDs represent the maximum amount of rare

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earths present in the existing waste flows16. According to Sprecher et al.19, HDDs offer the most

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feasible option for the large-scale recovery of neodymium. In general, the HDDs contain two

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different kinds of NdFeB magnets: a high performance sintered magnet found in the voice coil

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actuator assembly of the HDDs, and a low-quality NdFeB epoxy bonded magnet found in the

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spindle motor 26, 27. Due to the low REEs content in the bonded magnets, only the sintered magnets

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are considered in this study.

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To the best of authors’ knowledge, this is the first study presenting a detailed flow of resources

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contained in the HDDs such as rare earths, aluminum, steel, copper, and mix plastics in a WEEE

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treatment facility. The study shows the empirical results regarding the composition of various

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output fractions of the WEEE treatment facility. Another innovative element of the study is that it

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further tracks the resulting output fractions of the WEEE treatment facility until the smelting plants

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in order to estimate the recovery potential of rare earths as well as the other materials present in

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HDDs. This contribution presents the detailed flow of resources from treatment of EoL HDDs to the

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final recovery of resources, whereas the figures related to the experiments as well as the detailed

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material composition of the output fraction can be found in the Supporting Information (SI).

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; b. almost all the HDDs found in the existing


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METHODS

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Dismantling of HDDs and material composition analysis of components

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To define the general material composition of computer HDDs, a sample of 20 HDDs, representing

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different manufacturers and age groups (see table S1 & S2 in SI), were collected from the WEEE

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stream and analyzed. This sample included 10 HDDs from desktop computers (sized 3.5’’) and

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another 10 from laptops (sized 2.5’’), randomly collected from the local WEEE stream. These

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HDDs were manually disassembled with the help of hand tools and the individual components were

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weighed. The elemental composition analysis of the dismantled components was carried out with

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the help of X-ray fluorescence (XRF) spectroscopy system28. The detailed composition of NdFeB

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magnets was derived from Habib et al.16

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Material Flow Analysis

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Material Flow Analysis (MFA)29 is used as a methodological approach to track the flow of rare

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earths present in the NdFeB magnets, along with other resources contained in the computer HDDs.

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MFA is a widely used tool to balance the incoming and outgoing flows of resources within defined

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spatial and temporal boundaries. A conventional WEEE treatment facility located in Denmark was

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chosen to track the end-of-life flow of resources contained in the electronic waste. Figure 1 shows

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the process flow layout of the WEEE treatment plant, where all the WEEE processing steps can be

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seen in an order.

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The WEEE processed at this facility is first unloaded in a fully covered area, from where it is

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passed on to an input conveyer belt. The WEEE reaches the first manual pre-sorting section, where

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hazardous and/or components containing valuable materials such as screens, batteries and Printed

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Circuit Boards (PCBs) are disassembled from the products. The remaining flow then enters the

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rotating chain shredder via conveyer belt. This shredder has a size adjustable exit hole, which is

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used to optimize the size of material coming out of the shredder. As visible from Figure 1, the

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shredder is equipped with air suction extractors which extract the dust and light material that enters

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the three step air filtering system. The output material from the shredder passes through the first

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over-belt magnet where the ferrous fraction is sorted from the rest. The remaining fraction after this

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magnetic separator enters the size sorting section, where the size sorting equipment separates the

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incoming material into two streams, larger and smaller than 10 cm2. The first magnetic separator

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and the size sorting equipment are attached to the same air filtration system as the chain shredder.



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The material smaller in size than 10 cm2 passes through two magnetic separators. The first one is an

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over-belt magnet whereas the second is a drum magnet. At this point, most of the ferrous material is

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sorted from the rest. The remaining material passes through an eddy-current separator to separate

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aluminum from other materials. The material larger in size than 10 cm2 enters the fourth magnetic

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separator, which is an over-belt magnet separator. Here again, the ferrous materials are separated

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from the rest, which then passes through the second eddy-current separator to sort aluminum from

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the residual fraction. In this experiment, the second eddy-current was not used because of the high

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aluminum content of the residual fraction coming from the fourth magnetic separators.

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For this experiment, a total of 244 kg of HDDs, representing 700 2.5’’ HDDs and 350 3.5’’ HDDs

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were processed in the WEEE treatment facility. The weight of HDDs was selected considering the 1

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ton hour-1 operating capacity of the WEEE treatment plant, where this experiment took 15 minutes.

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Moreover, 244 kg of HDDs were considered an appropriate sample size due to the logistics, time and

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economic constraints regarding fine sorting of different output material fractions of the WEEE treatment

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plant. The experiment was performed on 3rd of April, 2014. The PCBs were taken out of the HDDs

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through manual sorting before the HDDs were fed into the shredder due to their high economic

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value, and thus are not represented in the above mentioned total weight of HDDs. The input to the

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shredder and the output material fractions were weighted within the WEEE treatment plant, using a

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weighting scale with ± 1 kg precision. The resulting 10 various output fractions were collected in large

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bags, and transported back to the sorting lab situated at the University of Southern Denmark (SDU).

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It is worthwhile to mention here that the Fraction 3 shown in Figure 1 is not an outcome of the size

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sorting equipment, but originates as a result of designed leak to avoid any hindrance to the normal

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flow of materials through different sections in the plant. The first two fractions resulting from the

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filters were not sorted any further because they mainly comprised dust apart from small plastic and

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rubber pieces. The remaining eight output fractions were further sorted manually to visualize the

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material composition of each fraction (see section S1 and S2 in SI for figures of output fractions).

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As shown in Figure 1, the magnetic dust containing rare earths was present in five different

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fractions. This dust was collected and analyzed further to reveal the chemical composition (see the

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next section for more detail).

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The fine sorting of eight different fractions led to creating a mass balance for all the components

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found in HDDs throughout the process flow, representing different material compositions. After

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performing this initial mass balance, the subsequent processing, i.e. the final treatment of these


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output fractions, was also taken into account. The final fate of these fractions was traced until the

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smelting step, based on the data obtained from WEEE recycling industry, in order to estimate the

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recovery potential of different metals and alloys contained in the HDDs. Based on the expert advice

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obtained from the WEEE recycling industry regarding the material composition of the output

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fractions, a 5% material processing loss was assumed for the smelting process in order to calculate

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the maximum recoverable amount of metals and alloys, e.g. aluminum and steel (see Figure 1).

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Chemical characterization of magnetic dust

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The magnetic dust was manually isolated from the various shredded components with the help of a

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brush. This dust found in four ferrous fractions resulting from the magnetic separators plus fraction

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3 was collected. The screws and other metal components were taken out from the collected dust.

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Five representative samples were collected from each fraction. Each homogenized sample was

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pressed into a tablet (size: 32 mm: pressure: 30 tons for 10 sec). The elemental composition of the

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tablet was estimated using wavelength dispersive X-ray fluorescence spectrometer (WD-XRF)30,

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with a detection limit of 0.001 – 0.01 % (w/w) and ± 15% precision.

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RESULTS

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The aim of this research was to explore the recovery challenges of critical resources such as rare

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earths from the urban mines by highlighting the drawbacks in the existing WEEE treatment system.

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This was accomplished by tracking the flow of computer HDDs containing rare earths in the form

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of NdFeB magnets in a conventional WEEE treatment facility. The results are divided into four

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main sections: material composition HDDs, overall mass balance of HDDs with a focus on material

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sorting efficiency of plant, final treatment of the output fractions, and finally the flow of rare earths

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in the WEEE treatment plant.

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Material composition of 3.5’’ and 2.5’’ HDDs

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Table 1 presents the detailed composition of 3.5’’ and 2.5’’ HDDs, where it becomes evident that

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Al is the dominant constituent of both types of HDDs. Nearly 50 % of 3.5’’ HDDs weight and 40 %

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of 2.5’’ HDDs weight is represented by Al alloy, out of which almost 80% is consumed by a single

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component, i.e. the base casting. Another component made of Al alloy is the top cover of HDDs,

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though the majority of the 3.5’’ HDDs showed steel as a basic material for their cover (see table 1).

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Furthermore, the platters (data storage discs) were also found to be made of Al coated with

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magnetic layers. The majority of the platters found in 2.5’’ HDDs are manufactured of glass and



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ceramics. The spindle motor and the voice coil actuators mainly consist of Al, along with the small

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amount of copper coils and steel (see table 1 for more details).

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Table 1: Material composition of 3.5’’ and 2.5’’ HDDs as per component* Component

3.5’’

Weight (g) 108.14

Share of total weight (%) 19.3

2.5’’

9.04

9.4

Base casting

3.5’’ 2.5’’

228 29.44

40.7 30.7

Platters

3.5’’

42.53

7.59

Magnets

2.5’’ 3.5’’

8.16 13.35

8.5 2.4

2.5’’

2.75

2.9

3.5’’ 2.5’’ 3.5’’

51.13 8.87 19.13

9.12 9.25 3.41

2.5’’

7.07

7.4

3.5’’ 2.5’’ 3.5’’

47.18 11.15 32.69

8.4 11.64 5.8

2.5’’

11.83

12.34

3.5’’ 2.5’’

18.02 7.52

3.22 7.84

Cover

Magnet carrying plates Voice coil actuators

Spindle motor Printed Circuit Boards (PCBs) Others

HDD

Material composition Mostly steel comprising >80% Fe and 15-18% Cr. The rest is Mn, V, Co and Ni. Aluminum alloy with a mixture of 90-97% Al, and small amounts of Ni, Fe, Mn, Cr, Co, V. Aluminum alloy with a mixture of 93-96% Al, and small amount of other alloying elements such as Fe, Cu, Zn, Ni, Cr, Mn and V. Pure Al core coated with thin layers containing Fe, Cr, Co, Ni, Zn and V. Mixture of glass and ceramics NdFeB magnets with a composition of 4.6% Pr, 30.8% Nd, 1.6% Dy. The remaining is mainly iron. NdFeB magnets with a composition of 4% Pr, 30.4% Nd, 2.4% Dy. The remaining is mainly iron Steel coated with Ni Actuator coil consists of Cu, axis mainly comprises of steel and the arms are made of Al alloy. Mainly consists of Al with small amount of Fe, Cu and Mn. 25 – 30% are screws made of different steel alloys, and remaining are mix plastic fraction containing paper, plastic and foam.

Total weight

3.5’’ 560.16 100 2.5’’ 95.829 100 *The detailed data regarding source and age of HDDs, and weight of different components found in the computer HDDs can be found in the Supporting Information, Table S1 and S2.

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Overall Material Flow Analysis

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Figure 1 displays on the overall mass balance of HDDs throughout the treatment plant, as well as

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the performance of the treatment facility with respect to the separation of various components and

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materials into the different output fractions. The first thing to be noticed is the difference of nearly

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7.3 kg in the output amount compared to the input amount. A big share of this mass difference are

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the materials that did not come out of the shredder (stayed at the bottom of shredder). This missing

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weight of HDDs in shredder is expected to come out of shredder with the next batch of WEEE


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processed in the plant. The remaining loss may have happened due to some components being stuck

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at various points in the plant.

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The largest output fraction was Fraction 8 equivalent to 90.8 kg, resulting from the Eddy-current

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separator. This fraction mostly consisted of aluminum, which is no surprise as aluminum dominates

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the total mass of HDDs (see Table 1). The second largest fraction was Fraction 10 originating from

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the magnetic separator. This fraction weighed almost 56.3 kg and comprised mainly steel followed

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by aluminum. The major source of steel in this fraction is the top cover of HDDs and the plates used

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as base for magnets. Fraction 4 was the third biggest output fraction that resulted from the first

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magnetic separator. The total weight of this fraction was 34.4 kg. As visible from Figure 1, this

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fraction mainly consisted of steel coming from the top and bottom plates for magnets and the top

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cover of HDDs. Nearly 25% of this fraction consisted of the partially broken HDDs and the residue

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consisting of spindle motors, screws, spacer rings, broken glass and ceramics from the platters.

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In general, it can be seen that none of the output fractions consist of a single material. For example,

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the purpose of installing the magnetic separators in the WEEE treatment facility is to isolate the

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ferrous metals from the rest. However, all the four fractions originating from the magnetic

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separators comprised various materials apart from iron and steel, such as platters. The majority of

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the platters are made of aluminum coated with thin magnetic layers containing chromium, cobalt,

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iron, nickel and zinc to enhance the magnetic storage capacity of these platters. Due to these thin

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layers, the platters are attracted by the magnetic separators and end up in the ferrous stream. The

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same is the case with voice coil actuators and the spindle motors which were not fully liberated

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from the adjoining components, and thus appeared in the ferrous fractions (see section S3 in SI for

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detailed material composition of non-liberated components and miscellaneous categories mentioned

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in Figure 1).



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Figure 1: Composition of different output fraction resulting from treating the HDDs in a WEEE treatment facility. The pie charts represent the component composition (%) and the bar charts present the material composition of various output fractions (kg). The figure presents the treatment plant layout accompanied with the flow of HDDs throughout the plant. The width of arrows with respect to the output fractions is representative of the share of a particular fraction in the total weight of the output fractions. The dashed arrows represent the subsequent processes for the different output fractions resulting from the WEEE treatment plant.

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Final treatment of output fractions

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The ten different output fractions are sent to the final material and/or energy recovery facilities

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depending on their material composition. These facilities may be located within or outside the

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country, for example the energy recovery facilities are mostly situated inside Denmark. However,

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there are no metal smelters present in Denmark to recover the pure metals or the alloys from the

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different material fractions resulting from WEEE treatment facilities. Figure 1 reveals the fate of

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different output fractions with the help of dashed arrows and boxes. Figure 2 shows the simplified

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flow of different materials present in HDDs from the WEEE treatment plant to the final treatment of

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resulting output fractions. Figures 1& 2 show that the Fractions 1 & 2 are directly sent to the local

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incinerator because they mainly consist of dust and some mix plastics. The remaining fractions are

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sent either directly to smelters such as Fractions 3 & 8, or are first sent to the local scrap dealers for

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fine sorting of different materials and then sent to the smelters sited outside Denmark.

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Figure 2: The mass flow of different materials contained in the computer HDDs along with their

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estimated recovery amounts.

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As shown in Figure 1, the output Fractions 3 & 8 were mixed together because almost 86% and

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99% of their weight was dominated by Al respectively. Due to the high material purity of these

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fractions, it is not required to send them to further shredding and sorting processes, and thus they



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were directly sent to the Al smelters outside Denmark. The total amount of material entering the

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smelters was 97.8 kg, where 96.5 kg is Al and the remaining impurities consisted of steel, Cu, mix

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plastics and the magnetic dust (see Figures 1& 2 for details). Rigamonti et al.31 have documented

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the smelter efficiency for Aluminum as 83.5% considering the aluminum contained in the

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Municipal Solid Waste (MSW). However, based on the high material quality of output fractions and

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the information collected from the experts in the WEEE treatment and final processing (smelters)

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industry, we have assumed 5% processing loss for the Al and steel smelters. Thus, the total amount

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of Al that can be recovered from Fractions 3 & 8 is equivalent to 91.67 kg.

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Fraction 10 – the second largest output – mainly consisted of Al (47%) and steel (52%). This

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fraction is first sent to the local scrap dealer where 0.26 kg of pure Cu from the voice coil actuators

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and the spindle motors is separated and sent to the copper smelter. The processing loss for copper

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smelter is 1%32. The remaining amount is then sent to a local shredding (jaw crushing) plant to

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crush the material into small pieces, hence making it convenient to liberate materials from each

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other such as steel and aluminum. These fine-sorted fractions are then sent to the Al and steel

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smelters located outside Denmark. The finally recovered amount of Al and steel is estimated to be

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24.95 kg and 28.04 kg respectively, assuming 5% processing loss in the Al and steel smelters.

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As shown in Figure 2, Fraction 4 is a ferrous stream resulting from the first magnetic separator. The

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total weight of this fraction was 34.4 kg, out of which nearly 31 kg was steel. The remaining

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consisted of Al, Cu, plastics and magnetic dust. This fraction is also sent to the local shredding and

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sorting plant, and eventually reaches the final smelters. At this point, there lies an opportunity to

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separate 0.64 kg of Cu and 2 kg of Al from the steel. The rest is sold to the steel smelters located

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outside Denmark, where 29 kg of steel can be finally recovered.

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Fractions 5, 6, 7 and 9 consist of mixed materials, mainly the non-liberated and the miscellaneous

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components. These fractions are mixed together and then sold to a local scrap dealer, who then

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sends the material to a local shredding and sorting plant. This process allows the partially broken

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HDDs to be shredded into smaller pieces, leading to enhanced liberation of various components

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from each other. At this step, 1.9 kg of Cu can be separated as a pure Cu fraction. The fine sorted

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fractions are then sent to Al and steel smelters situated outside Denmark. The total amount of steel

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and Al entering these smelters is 17.4 kg and 17.7 kg respectively, where the finally recovered

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amount of steel and Al is estimated to be 16.53 kg and 16.81 kg respectively. Almost 10 kg of

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residue material from these fractions is sent to the local incinerator. 12 ACS Paragon Plus Environment

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Finally, the results presented in Figure 2 make it clear that 212 kg of metals and alloys are

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recovered from an input stream of 244 kg of HDDs. Out of these 212 kg of refined metals and

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alloys, 135.4 kg are Al, 73.6 kg are steel and 2.87 kg are Cu. This implies that nearly 32 kg are lost

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throughout the process chain – from EoL HDDs to the refined materials. Out of this, nearly 7.3 kg

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did not appear in the output fractions of WEEE treatment facility, and the rest is lost during the

279

subsequent processes such as additional shredding and sorting followed by smelting.

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Tracking the flow of rare earths

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Figure 1 shows a detailed process flow of rare earths contained in the NdFeB magnets present in

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computer HDDs across the WEEE treatment facility. The NdFeB magnets found in HDDs are

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brittle in nature, and convert into powder when they pass through the shredder. This powder retains

284

its magnetism and is prone to stick to the ferrous surfaces. Results presented in Figure 1 reveal that

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all the four output fractions originating from the magnetic separators and Fraction 3 do contain

286

considerable amount of this powder sticking to the surfaces of shredded ferrous stream. This dust

287

like powder forms lumps due to the attraction of dust particles caused by the magnetic nature of

288

particles forming this dust (see figure S20 in SI). The total amount of collected dust from the four

289

output fractions was only 2.5 kg. Comparing this amount to the average weight of NdFeB magnets

290

in the input HDDs, which is equivalent to 6.6 kg, highlights that almost two third of the input

291

magnet fraction was lost during the processing of HDDs. This missing amount of dust containing

292

rare earths has high potential to stick to the internal walls of different processing equipment (e.g.

293

chain shredder, pipes, sides of the conveyer belts and the collection containers) as they are mainly

294

made of ferrous metals. Apart from this, nearly 0.28 kg of intact NdFeB magnets was found in the

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partially shredded HDDs in the non-liberated component of different output fractions (see Figure S5

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& Table S3 in SI). This corresponds to 0.09 kg of neodymium and 0.006 kg of dysprosium.

297

A detailed elemental analysis of the rare earths containing dust collected from five different

298

fractions (see Figure 1) is presented in Table S4 of the SI. The results impart that the amount of rare

299

earths present in this dust fraction was negligible, where neodymium and dysprosium made only 0.9

300

and 0.1% of the total dust weigh respectively. This translates into 0.02 kg of neodymium and 0.003

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kg of dysprosium in the dust fraction. The estimated amount of neodymium and dysprosium in the

302

NdFeB magnets contained in the input stream of HDDs was equivalent to 1.98 kg of neodymium

303

and 0.13 kg of dysprosium. Comparing this to the amount of neodymium and dysprosium in the

304

output dust fractions reveals that almost 99% of the input rare earths are lost while processing of



305

HDDs in the WEEE treatment facility. This entire loss of rare earths can be attributed to two main

306

reasons:

307

•

First, the NdFeB magnets convert into powder in the shredding process and this powder

308

retains the magnetism of the magnet. Due to this magnetic nature, the powder sticks to the

309

ferrous surfaces of different equipment of the treatment plant. Already at this point almost

310

90% of the rare earths are lost, and the remaining amount ends up in the ferrous fractions.

311

•

Second, during the shredding process, apart from the NdFeB magnets different other easily

312

breakable components, mainly plastic and ceramics, also convert into powder depending on

313

the retention time in the shredder. This resulting powder further dilutes the rare earths

314

containing dust and disperses over various output fractions, and thus rare earths are

315

completely lost during the process.

316

The shredding-based treatment of HDDs is not efficient to recover the small quantity of REEs from

317

the magnet. A better approach for this could be to separate the magnets from HDDs before they

318

enter the shredder. Currently, manual disassembly of HDDs is performed to take out the PCBs due

319

to their high economic value. The NdFeB magnets can also be recovered manually from the HDDs

320

at the same time. However, it is not practiced due to the difficulty in removing these magnets

321

manually due to their strong magnetism. The manual separation may not be always economically

322

feasible considering the current low concentration of REEs and high labor costs in Denmark16.

323

DISCUSSION

324

The overall material flow analysis of computer HDDs highlights a number of obstacles in the

325

efficient recovery of different resources. One of them is the design of products which hinders the

326

effective recovery of various materials contained in the products. This issue can be better explained

327

with the results presented in this study. The comprehensive data regarding the composition of 8

328

output fractions resulting from the WEEE treatment plant shows that all of these fractions contain

329

unintended materials. For example, the fractions resulting from the magnetic separators consist of

330

aluminum and copper apart from their purposive output product, i.e. ferrous materials. The complex

331

design of modern products hinders the complete liberation of components, and thus the materials

332

from each other. In case of HDDs the screws and other fixtures are the main obstacles in separation

333

of components like top cover and the base casting from each other. This results in aluminum


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334

contamination in the ferrous fraction and vice versa. The WEEE treatment technologies are not yet

335

matured enough to recover the elements from the complex mixture of different materials in the

336

products that are not designed for the EoL material recovery.

337

The current WEEE treatment system is focused on material-centric recycling

338

recycling of bulk materials (metals and alloys) such as aluminum, copper, iron and steel, and the

339

precious metals such as gold, silver and platinum group metals. The critical raw materials such as

340

rare earths are not on the priority list of materials recycling, due to both, their current volatile

341

market price, and small and dispersed amounts in existing waste flows. The complexity of modern

342

products in terms of material composition and design features cannot be addressed by the existing

343

manner of WEEE recycling. This holds more true during the initial processing of WEEE, where

344

different types of products are shredded together to generate material streams that follow the

345

material-centric recycling chain. The product-centric approach is seen as a potential solution to

346

maximize the material recovery from WEEE in the attempt of closing the material cycle. The

347

increasing focus on design for recycling, design for EoL, design for metallurgy, design for

348

sustainability and similar approaches seem to be promising in order to ensure increased resource

349

efficiency in future.

350

Regarding rare earths recovery, the semi-quantitative data collected from the WEEE treatment

351

companies in Denmark show that in 2014 nearly 60 Mg of HDDs were received at the WEEE

352

treatment plants in Denmark. Out of these, almost 85% were 3.5’’ and the remaining were 2.5’’

353

HDDs. Taking an average weight of 13.35 g and 2.75 g of NdFeB magnets found in the 3.5’’ and

354

2.5’’ HDDs respectively, this amount translates into a total of 1.6 Mg of NdFeB magnets present in

355

the computer HDDs reaching the WEEE treatment plants in Denmark in 2014. However, in Habib

356

et al. (2014)16, it has been shown that the current maximum theoretical recovery potential of NdFeB

357

magnets contained in the computer HDDs was estimated to be 4.5 Mg. This implies that in reality

358

only 35% of the EoL computer HDDs succeed in reaching WEEE treatment facilities in Denmark.

359

The remaining amount of the EoL computers does not enter the local WEEE treatment chain and is

360

transported to WEEE treatment facilities located outside Denmark. This means that almost two third

361

of the recovery potential of rare earths is lost at the beginning of the whole recycling process chain.

362

The next challenge, as highlighted in Habib et al. (2014)16, is the very small amount of REEs

363

present in the different end-use products. The weight and composition of NdFeB magnets vary

364

between different product types within the same product category, such as between computer HDDs 15 ACS Paragon Plus Environment

33

that aims at


365

and the cell phones within the IT & telecommunication category. Even within the same product

366

type, the NdFeB magnets weight and composition varies depending on the size and manufacturer of

367

the end-use products. This does not allow the existing WEEE processing technology to effectively

368

concentrate the recyclates suitable for REEs recovery. Furthermore, these magnets are not easy to

369

disassemble due to the design features and connections. Their strong magnetism is another obstacle

370

in separating these magnets from their end-use products and transporting them for further

371

processing. These challenges lead the recyclers to treat the EoL products containing NdFeB

372

magnets in a traditional WEEE treatment process i.e. shredding. The shredding process, however,

373

results in the complete loss of NdFeB magnets and the REEs contained in them.

374

In order to tackle these issues related to REEs recovery, it is imperative to separate magnets form

375

their EoL products. This will allow concentrating the target elements, such as REEs, present in the

376

EoL product, leading to their efficient recovery. However, as shown in Habib et al. (2014)16, the

377

small amount of NdFeB magnets contained in the computer HDDs is not economically attractive for

378

the WEEE treatment plants in Denmark. The additional labor and time required to separate magnets

379

from the EoL products is seen as a key hindrance with respect to the current volatile market price of

380

REEs. Nevertheless, the number of EoL products containing NdFeB magnets is likely to increase in

381

future16, 34. Moreover, the future developments regarding commercial scale recycling technologies

382

can make it possible to separate the NdFeB magnets, and the final recovery of REEs from these

383

magnets an economically efficient process.

384

To enhance the recovery of critical resources from WEEE, we suggest the improvement on both

385

ends of the value chain, beginning with the design of products to the handling and treatment of EoL

386

products. Designing the products in a way to enhance the liberation of different components and

387

materials contained in the products is the key for maximizing the resource efficiency.

388

Simultaneously, the improvement of existing WEEE handling and treatment system is necessary to

389

ensure the efficient recovery of critical resources. A solution here could be installing robots for

390

automatic disassembly of products followed by separation of components containing the critical

391

resources. This could be a technically feasible solution in case of HDDs due to the almost identical

392

design of HDDs, where it becomes easier for the robots to cut and open the specific part of HDDs

393

containing the NdFeB magnets. These parts can be then heat treated35 to demagnetize the magnets

394

for further sorting and processing.

395

REEs mixture from the magnets is another promising solution36.

Treating the computer HDDs with hydrogen to release the


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396

Finally, the authors are aware of the data uncertainties associated with the material composition of

397

input stream to the WEEE treatment facility. The exact composition of 244 kg of computer HDDs

398

was not analyzed, because disassembly and sorting of various components and materials would

399

have been a pre-requisite for such an analysis. This could have manipulated the actual performance

400

of the WEEE treatment plant. For this reason, the composition of the incoming flow was

401

generalized based on detailed composition analysis of 20 HDDs. As the computer HDDs often

402

follow the same design configuration and material composition, the sample size of 20 HDDs from

403

different manufacturers and age groups was considered a sufficient sample size for this kind of

404

study. Furthermore, the model uncertainties related to the processing of HDDs such as the

405

difference of 7.3 kg between the input and output of the treatment facility are not assessed

406

extensively in this work. More empirical data is required in order to address these issues, which is

407

an ambition for future research.

408

SUPPORTING INFORMATION

409

A description of the detailed material composition of the output fractions, the chemical

410

characterization results for the magnetic dust, and the pictures related to the experiment. This

411

material is available free of charge via the internet at http://pubs.acs.org/.

412

ACKNOWLEDGEMENTS

413

We would like to thank the projects INNOSORT (http://innosort.teknologisk.dk/) and TOPWASTE

414

(www.topwaste.dk) for providing us the opportunity to carry out this research and facilitating our access to

415

the necessary data. The support provided by Ciprian Cimpan and Peter Klausen Schibye regarding the

416

experiment and sorting is highly acknowledged. Similarly, the authors are thankful to Tom Ellegaard for

417

facilitating the experiment. The authors pay their deepest gratitude to Lorie Hamelin and the two anonymous

418

reviewers for providing with valuable comments.



419

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Figure 1: Composition of different output fraction resulting from treating the HDDs in a WEEE treatment facility. The pie charts represent the component composition (%) and the bar charts present the material composition of various output fractions (kg). The figure presents the treatment plant layout accompanied with the flow of HDDs throughout the plant. The width of arrows with respect to the output fractions is representative of the share of a particular fraction in the total weight of the output fractions. The dashed arrows represent the subsequent processes for the different output fractions resulting from the WEEE treatment plant. 379x219mm (96 x 96 DPI)


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Figure 2: The mass flow of different materials contained in the computer HDDs along with their estimated recovery amounts. 254x139mm (96 x 96 DPI)


Tracking the Flow of Resources in Electronic Waste - The Case of End

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