Elsevier

Journal of Cleaner Production

Volume 186, 10 June 2018, Pages 490-498
Journal of Cleaner Production

Separation efficiency of valuable and critical metals in WEEE mechanical treatments

https://doi.org/10.1016/j.jclepro.2018.03.112Get rights and content

Highlights

  • The fate of critical metals during WEEE mechanical treatments was investigated.

  • The dust fraction could turn from waste stream to alternative source of REEs.

  • Technical aspects related to potential recovery of critical metals were discussed.

Abstract

The high demand of rare earth elements (REEs) in electronics industry and their high supply risk owing to the dependence on limited source countries have increased the interest towards Waste Electrical and Electronic Equipment (WEEE) as a potential secondary source of these elements, identified as critical metals. Although REEs are present in a wide range of high-tech products in relative low concentrations, the recycling of critical metals from WEEE is regarded as an important opportunity for promoting the conservation of primary resources and preventing waste production. However, the existing systems for WEEE collection and treatment mainly focus on the recovery of base and precious metals while the fate of REEs has not been addressed yet. To this end, the present study aims at evaluating the substance flows of critical metals in WEEE mechanical pre-treatments as these processes, preceding the metallurgical treatments of refining, determine the amount of metals entering the further recovery chain. The separation efficiency of a full scale mechanical process, including a sequential shredding and selection treatment of small WEEE, was investigated assessing the mass balance of both base metals and special metals as well as the quality of the output products. The mass flow analysis revealed that after pre-treatments only a third of precious metals entering the treatment process was conveyed to the target output destined to further recovery and less than 2% of REEs was concentrated in the potentially recyclable metallic fractions, while approximately 80% of these elements was distributed in the dust stream actually destined to landfill. Furthermore, the study pointed out that the fine fraction of the metallic outputs from the sorting process is characterized by a major degree of purity, indicating that both the dust stream and the fine grain fraction could be regarded as secondary sources for the recovery of valuable and critical metals from WEEE.

Introduction

Due to the increasing exploitation of resources and the scarcity of native raw materials, the strategies of waste management have been turned from the linear model of “take-use-waste” to a circular approach based on the prevention of waste as well as the re-introduction of the materials into the economic “loop”. In place of virgin materials, compounds and elements are desirably reclaimed from anthropogenic stock resources, such as waste which acts as “urban mines” (Cossu and Williams, 2015). In this regard, great attention has been focused on Waste Electrical and Electronic Equipment (WEEE) as this waste stream is characterized by the highest growth rate per year and by the most wide-ranging source of materials (Tanskanen, 2013, Widmer et al., 2005).

As a result of the continuous expansion of the electronic market and the reduction of the lifespan of many electronic devices, around 20–50 million tons of WEEE are annually generated worldwide (UNEP, 2013) and this trend was expected to reach 65.4 million tons in 2017 (Environment OREP, 2014). Such large volume of produced waste contributes to make WEEE management a challenge, especially when considering its extremely variable and complex composition, coupling valuable materials with harmful substances (Baldé et al., 2015, Cui and Zhang, 2008). If the presence of hazardous components in WEEE can deal with the potential risk for both human health and environment as a result of an improper handling of this waste (Kiddee et al., 2013, Tsydenova and Bengtsson, 2011), the relatively high concentrations of metals can provide a promising reserve of these materials (Jadhav and Hocheng, 2012, Lee and Pandey, 2012, Oguchi et al., 2011, Tuncuk et al., 2012).

Both base and precious metals are largely used in the industry of electrical and electronic devices. Their economic value, along with the limited reserves and the environmental impacts related to their primary production (Behrendt et al., 2007), represents a relevant incentive for resource recovery from waste materials. This aspect is especially evident for rare earth elements (REEs), since this group of metals, which are widely used in many field of high-tech applications (Schüler et al., 2011), has been identified by the European Commission as the most critical: its scarce worldwide production, mainly limited to China, and the current low recycling rate entail its high supply risk (Binnemans et al., 2013, European Commission, 2014).

Due to their magnetic, luminescent and chemical properties, REEs are highly demanded in electronics industry as they make possible the miniaturization of information technology (IT) components and batteries. Rare earths are used in several parts or components of electronic devices, such as fluorescent lamps, magnets, accumulators, semi-conductors, capacitors and batteries. The higher contents of these critical elements can be found in products containing phosphors (fluorescent lamps, cathode ray tubes, LED and plasma display panels), neodymium-iron-boron (NdFeB) magnets (hard disk drives, speakers, headphones, mobile phones) and nickel-metal hybrid batteries (NiMH) (Binnemans et al., 2013, Tunsu et al., 2015). Focusing on these keys applications, Binnemans et al. (2013) roughly estimated a REE potential recycling in 2020 ranging from 5600 to 10700 tons, which could significantly contribute to the overall supply of rare earth elements. In this regard, the recovery of REEs from end-of-life products is beneficial in both economic and environmental terms: the extraction of rare earths from native minerals is indeed extremely challenging as they are often found dispersed and mixed with radioactive elements (Jha et al., 2016).

In European countries the recycling of WEEE is a legal obligation and the recovery of metals from WEEE is currently achieved through mechanical and metallurgical treatments: the formers are mainly used as pre-processing in order to physically upgrade the material contents, while the latter ones, relying on techniques coming from the metallurgical sector namely pyro- and hydrometallurgy, act as refining processes (Cui and Zhang, 2008, Khaliq et al., 2014). However, the existing technologies for the recovery of metals from WEEE primarily focus on precious and base metals. In the last years several research efforts have addressed the recycling of REEs from batteries, phosphors and magnets through hydrometallurgical treatments (Binnemans et al., 2013, Tunsu et al., 2015). Moreover, a number of European research projects related to the recovery of REEs from end-of-life products are currently ongoing (Tunsu et al., 2015) and an increasing patenting trend in this area has been reported as well (Tsamis and Coyne, 2015). Nevertheless, the industrial scale applications are still limited (Binnemans et al., 2013, Tsamis and Coyne, 2015, Tunsu et al., 2015).

Over the recycling chain, the mechanical treatments play a key role as their separation efficiency ensures the material concentration in the output streams destined to further recovery processes (Chancerel et al., 2009, Meskers and Hagelüken, 2009, Meskers et al., 2009). During mechanical processes WEEE is selectively dismantled in order to separate re-usable parts and hazardous components. Then, well-established selection technologies, such as shredding, magnetic separation, eddy current and density separation, are used with the main aim of separating metals from non-metals (Khaliq et al., 2014). The effectiveness of the sorting process is affected by the type and the combination of the adopted selection techniques as well as by the physical characteristics of the waste material (Cui and Forssberg, 2003, Meskers and Hagelüken, 2009, Sun et al., 2015, Veit et al., 2002). Mechanical treatments are recognized to efficiently recover common metals such as iron and copper while precious metals are often lost (Bachér et al., 2015, Chancerel et al., 2009, Cui and Zhang, 2008, Lu and Xu, 2016, Oguchi et al., 2012, Veit et al., 2002). Although the effectiveness of the mechanical processes in separating metal fractions from polymers as well as in obtaining metal-concentrated fractions has been widely investigated, the fate of rare earth metals in WEEE pre-treatment stages has not been addressed yet. The comprehensive understanding of this aspect is, however, essential to pursue the effective recovery of these elements from WEEE, as the novelty that marks the concept of critical raw materials has not yet allowed the development of established methods (Van Eygen et al., 2016).

This study aims at evaluating the separation efficiency of valuable and critical metals in WEEE mechanical treatments. The distribution and the concentration of both base and special metals in the output fractions of mechanical pre-processing are discussed and the quality of the obtained materials is pointed out in the view of their further refining. Wider considerations on technical implications are finally underlined.

Section snippets

Sampled materials

For the purposes of this study a full-scale WEEE treatment plant operating in the South of Italy was considered. The plant is designed to treat 2 tons per hour of waste coming from small electronic equipment, IT and consumer appliances. The input WEEE is processed through a mechanical treatment line that mainly enables the separation of recyclable metals from plastic fractions.

The process line consists of a two-stage shredding pre-treatment in order to reduce the particle size of the incoming

Material characterization

The average metal content detected in the investigated fractions is summarised in Table 1.

Metals are recognized to be the dominant fraction by weight in WEEE (Widmer et al., 2005). Results of the present study confirmed that iron (Fe), copper (Cu) and aluminium (Al) are the prevalent metals contained in WEEE as extensively reported in other studies (Meskers and Hagelüken, 2009, Widmer et al., 2005). The average content of these metals determined in the input WEEE (around 504 g/kg Fe, 60 g/kg Cu

Technical aspects

This study confirmed that conventional mechanical treatments enabled an efficient separation of base metals, whereas lower recovery yields were provided in terms of both precious metals and especially REEs. The latter were observed to be subjected to significant losses as they were mainly conveyed in the dust stream originating from mechanical units. These losses can be mainly related to the shredding technology that gathered the liberated materials into the small particles (Bachér et al., 2015

Conclusions

Nowadays the recycling of WEEE is regarded as a challenge due to its heterogeneity in terms of components as well as the presence of both hazardous substances and critical metals, whose recovery represents a charming driver for WEEE recycling. Mechanical treatments cover the first step of the WEEE recycling chain and therefore their effectiveness has a key role for further refining processes.

The present study addressed the recovery of rare earth elements from WEEE during the mechanical

Acknowledgements

Research activities were partially funded by the FARB project of the University of Salerno.

The authors would like to express their gratitude to the plant manager and the staff of the WEEE treatment facility for the kind support provided during the sampling campaigns. The technical support of Giuseppina Oliva as well as the analytical assistance at the SEED laboratory of Anna Farina and Paolo Napodano were deeply appreciated.

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