Separation of copper and polyvinyl chloride from thin waste electric cables: A combined PVC-swelling and centrifugal approach
Graphical abstract
Introduction
Electric cables are newsworthy components since they are widely used in transport, construction, communication, and consumer goods, especially in automobiles and electrical and electronic equipment (EEE) (Conesa et al., 2013). With the significant worldwide growth of end-of-life vehicles (ELVs) and waste EEE, the sheer volume of waste electric cables has emerged as a significant problem in the terms of environment protection and resource recycling (Janajreh et al., 2015, Saleh and Gupta, 2014). A common copper-containing cable typically consists of a conductive copper core and a polyvinyl chloride (PVC) insulating layer (Suresh et al., 2017). Copper is the third most important industrial metal, just behind iron and aluminum in terms of consumption, due to its resistance to corrosion and its thermal and electrical conductivities. In 2010, about 25 million metric tons of copper were produced globally, and 35% of which originated from recycled copper resources (Glöser et al., 2013). Moreover, the demand for copper is estimated to increase by between 275 and 350% by 2050 (Elshkaki et al., 2016). On the other hand, PVC combined with plasticizers, flame-retardants, fillers, or stabilizers is a globally popular commodity. The consumption of PVC reached 38.5 million tons in 2013 and it should rise by 3.2% per year up to 2021 (Yu et al., 2016). However, about 70,000 tons of waste cables are disposed of in landfills each year, which wastes copper and PVC resources, and creates local environmental burdens (Nunome et al., 2018). Therefore, the recycling and reuse of copper and PVC in waste cables by using effective methods and techniques is very important.
Methods for waste-cable recycling are classified into mechanical-recycling, energy-recovery, and chemical-recycling techniques (Li et al., 2017, Xiao et al., 2016). For mechanical recycling, thick (cm-order-diameter) waste cables are easily stripped or crushed into small scrap nuggets and then sorted. However, thin (mm-order-diameter) waste cables cannot be directly disposed of via nugget-forming processes or peeling techniques, but are treated by freezing (You, 2015), ultrasound (Ghaedi et al., 2015, Li et al., 2011), hot-water (Sheih and Tsai, 2000), or plasticizer-extraction (Xu et al., 2018) processes, which embrittle the PVC coverings. The pretreated thin waste cables are then blended or ball milled into small pieces that are separated by density-separation (sink-swim) (Hagstrom et al., 2006) or electrostatic-separation (Chagnes et al., 2016, Jeon et al., 2009, Park et al., 2015) techniques that provide high-purity copper and PVC. Energy-recovery processes include incineration and thermal decomposition. Incineration of waste cables only provides oxidized copper, undesirable gases, and heat energy, however it is the easiest and most widespread method (Bigum et al., 2017). Hydrocarbons can be recovered from PVC (Kumagai and Yoshioka, 2016) and unoxidized copper by thermal decomposition. Pyrolysis of PVC-containing waste plastics could provide alternatives that replace natural gas and propane (Honus et al., 2018a, Honus et al., 2018b, Honus et al., 2016a, Honus et al., 2016b, Saravanan et al., 2013); however, pyrolysis is associated with high-energy costs required for heating (Conesa et al., 2013, Zablocka-Malicka et al., 2015).
In recent years, researchers have focused on chemical recycling methods involving the chemical dissolution of metals by oxidants or by leaching solvents in order to recycle the residual copper from waste electric cables. Lambert et al. employed leaching methods to recycle copper from waste electric cables; 45–55-g samples of various plastic fragments and copper wires (containing 4.9% copper) were placed in a double jacket reactor with 450–500 mL of solutions that were then tested under abiotic, chemical-leaching conditions (in which thymol was added as a bactericide and ferric iron sulfate was added as the oxidant), and under biotic, bio-leaching conditions (in which Acidithiobacillus ferrooxidans was added as the catalyst). More than 90% of the copper was extracted by either the chemical- or the bio-leaching method (Lambert et al., 2015). Kameda et al. reported a method for the recovery of metal from separated cable waste using chloride volatilization. Cable scrap was placed on quartz wool and heated in a furnace at 500–900 °C; HCl was fed into the reactor for 60 min and the products were collected in a 5 wt% solution of HNO3. About 80% of the Cu was recovered as CuCl at 900 °C (Kameda et al., 2013). Anastassakis et al. reported a method that combining mechanical and chemical separation methods to recovery copper from low-copper-content waste-cable tailings. Copper from 4 to 5-mm cable samples (with <5% copper content) was concentrated by sieving with a Weston screen machine, after which they were chemically dissolved in a mixture of hydrogen peroxide and sulfuric acid (or hydrochloric acid). Metallic copper was recovered from the solution by cementation or electrowinning after washing the copper solution (Anastassakis et al., 2015). As has been reported, PVC can be swollen or dissolved by submersion in some organic solvents (Grause et al., 2015). Solvay et al. reported a process, referred to as ‘vinyloop’, for recycling PVC materials, in which PVC from scrap electric cables is dissolved in a solvent, but metals and other polymers do not dissolve and are separated (Hagstrom et al., 2006). Diegmann et al. developed a process for recycling automotive wire harnesses, in which PVC is softened using an organic solvent to the consistency of yogurt instead of being dissolved; the PVC separates from the Cu wire and is then granulated (Diegmann et al., 2000). Some methods recycle residual copper from the separated cable waste in high purities and with high separation rates. However, the copper-handling capacities of these methods were low and the recycled copper was in liquid or gaseous form, rather than the desired metallic form. In addition, chemical recycling methods are not suited to the recovery of the covering plastics, and problems associated with the disposal of impure organic solvents, as well as the risks of environmental pollution by these solvents, remain.
In this article, we introduce a novel method for the recycling of both copper and PVC from hard-to-recycle thin waste electric cables by the combination of swelling and stirring in an organic or a solvent mixture for the first time. We focused on the ability of PVC swelling in adequate organic solution. We expected that the swelling generates gaps between the copper and the PVC coverings and facilitates the detachment of the copper from the PVC coverings by centrifugal motion. This method was applied in two kinds of solvent systems. As shown in Fig. 1, the first involved stirring in an organic solvent, which enabled the individual recycling of the plasticizer, which originally formed the PVC coverings, as well as the copper. PVC devoid of a plasticizer is rigid PVC (RPVC) and is widely used in the construction of pipes, floorings, and window frames. Furthermore, the other method involves stirring in a mixture of an organic solvent and water, which facilitated the recycling of the copper and plasticizer-imbedded PVC coverings, because the plasticizer was prevented from leaching from the PVC coverings by the water. PVC that containing a plasticizer is flexible (FPVC) and is readily reused for wire coatings, raincoats, and the soles of shoes, among others (Kameda et al., 2010, Yu et al., 2016). As expected, these methods are suitable for dealing with large amounts of waste cables; both recycled copper and PVC coverings are in their original forms and simple to reuse and remanufacture. Herein, we investigated these methods with two kinds of solvent systems from the perspectives of the relationship between the solubility parameters of PVC and the organic solvents, organic-solvent suitability in two-solvent systems, stirring conditions and cable conditions, and for achieving a high waste-cable separation rate and high purity recycled PVC coverings, Cu, and plasticizers.
Section snippets
Materials
Commercial electric cables (KV-0.75-W-100, diameter 2.1 mm) were purchased from MISUMI Group Inc. (Tokyo, Japan) and cut into 1-, 3-, 5-, and 8-cm-long pieces. A total of 12 g of cables was used in each experiment. The cable samples were composed of 63 wt% Cu wire and 37 wt% PVC. The elemental composition of the PVC covering was: 35.8% C, 4.5% H, 0.2% N, 25.2% Cl, and 34.3% other components (balance), which includes oxygen and ash. The PVC covering was found to contain 18 wt% diisononyl
Investigating the swelling nature of PVC coverings
In our previous work on the separation of electric cables by Soxhlet extraction and ball milling (Grause et al., 2015, Xu et al., 2018), we found that PVC coverings swelled when submerged in organic solvents. The swelling rate (Rswel) is an important factor for centrifugal separation because it is responsible for creating the gap between the copper and the PVC covering that results in separation of the electric cable. Hence, some common organic solvents were selected for PVC-swelling
Conclusions
In summary, we developed a simple and effective chemical recycling method, which combines PVC swelling and centrifugal separation, for the recovery of high-purity PVC coverings and copper from waste thin electric cables. Copper, PVC coverings, and plasticizer could individually be recycled from waste cables using organic solvents. Acetone demonstrated the best ability to swell PVC and extract the plasticizer, with Rsep and Yext values of 100% and 97%, respectively. On the other hand, copper and
Acknowledgements
This research was supported by the Environment Research and Technology Development Fund [3RF1701] of the Environmental Restoration and Conservation Agency of Japan, and partially supported by JSPS KAKENHI grant number 17H00795, and JST grant number J170002403. Jing Xu is supported by the Chinese Scholarship Council (CSC).
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