*Article* **Chemical Recycling of WEEE Plastics—Production of High Purity Monocyclic Aromatic Chemicals**

**Tobias Rieger 1, Jessen C. Oey 1, Volodymyr Palchyk 1, Alexander Hofmann 1, Matthias Franke 1,\* and Andreas Hornung 1,2**


**Abstract:** More than 200 kg real waste electrical and electronic equipment (WEEE) shredder residues from a German dismantling plant were treated at 650 ◦C in a demonstration scale thermochemical conversion plant. The focus within this work was the generation, purification, and analysis of pyrolysis oil. Subsequent filtration and fractional distillation were combined to yield basic chemicals in high purity. By means of fractional distillation, pure monocyclic aromatic fractions containing benzene, toluene, ethylbenzene, and xylene (BTEX aromatics) as well as styrene and α-methyl styrene were isolated for chemical recycling. Mass balances were determined, and gas chromatography–mass spectrometry (GC-MS) as well as energy dispersive X-ray fluorescence (EDXRF) measurements provided data on the purity and halogen content of each fraction. This work shows that thermochemical conversion and the subsequent refining by fractional distillation is capable of recycling WEEE shredder residues, producing pure BTEX and other monocyclic aromatic fractions. A significant decrease of halogen content (up to 99%) was achieved with the applied methods.

**Keywords:** WEEE; chemical recycling; pyrolysis; recovery of aromatics; oil upgrading; dehalogenation

### **1. Introduction**

Waste electrical and electronic equipment (WEEE) represents a significant source of almost all precious and critical metals, but their recovery potential is far from being fully exploited as things stand today. At the end of state-of-the-art WEEE treatment processes, one or more output fractions are left behind, which are usually sent to landfills or to energetic utilization in waste incinerators ([1] pp. 131–133), [2,3]. With those fractions, remaining metals get lost, irretrievable for material recovery ([1] pp. 209–212), [4]. At the same time, WEEE and its output fractions contain high-quality plastics like HIPS, ABS, epoxy resins, PS, PE, PP, and PVC [5–7]. However, these plastics show high concentrations of flame retardants (FR) as TBBPA, DDO, HBCD, and DDE [3], resulting in bromine and chlorine concentrations of 0.6–4.0 wt.% [8].

As mechanical recycling recovers plastics in their given polymeric composition, stateof-the-art processes are not able to remove or to eject flame retardants from FR-containing WEEE plastics effectively. The recovery of unpolluted plastics by means of mechanical recycling is thus mostly limited to FR-free fractions ([1] pp. 209–212), [2,3,9]. Against this background, pyrolysis seems to offer a promising solution to complement established mechanical recycling processes, especially regarding highly chlorinated and brominated WEEE-plastics ([1] pp. 131–133), [3,5,8,10].

Due to the importance of effectively recycling and decontaminating WEEE plastics [1–3,9,11–13], numerous researchers have investigated chemical recycling of WEEE or

**Citation:** Rieger, T.; Oey, J.C.; Palchyk, V.; Hofmann, A.; Franke, M.; Hornung, A. Chemical Recycling of WEEE Plastics—Production of High Purity Monocyclic Aromatic Chemicals. *Processes* **2021**, *9*, 530. https://doi.org/10.3390/pr9030530

Academic Editor: Daniel Vollprecht

Received: 15 February 2021 Accepted: 9 March 2021 Published: 16 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of the flame retarded plastics present in such [14–24]. Most studies on novel recycling approaches regarding WEEE focus on the recovery of metals [4,25–27], whereas investigations on recycling of nonmetal fractions of real WEEE are limited [12].

Different chemical recycling technologies for WEEE plastics like thermal pyrolysis [4,21,22,28–32], co-pyrolysis [33–36], two step pyrolysis [33,37], catalytic pyrolysis [23,28,38–45], microwave-assisted pyrolysis [28,46,47], and combinations thereof [24,44,47,48] have been studied.

A very recent review by Charitopoulou et al. [12] on recycling of WEEE plastics comes to the conclusion that pyrolysis is considered a favorable technology to recycle FRcontaining plastics from WEEE. The formation of PXDD/F is theoretically suppressed due to the absence of oxygen. However, the oxygen already present in the nonmetal fraction of WEEE in practice leads to PXDD/F [11,41,48].

Several pretreatment technologies were studied to remove BFR from WEEE plastics prior to pyrolysis [31,49–56]. Examples of common pretreatment technologies are solvent extraction with isopropanol, toluene, or methanol, as well as supercritical fluid technologies with acetone, methanol, or ethanol as media. Such techniques were found to have good dehalogenation effectiveness. However, their high operational cost and energy consumption limit industrial implementation [12].

Patent research revealed several patent publications in the field of pyrolysis reactors or processes to treat mixed plastic wastes [57–64] as well as WEEE in particular [65–69]. Most reactor and process concepts are designed to either decompose WEEE in order to prevent landfilling of large volumes of hazardous wastes, dehalogenation, and/or to produce valuable chemicals or fuels at temperatures between 200 and 800 ◦C. Patented reactor concepts include screw reactors [58,70], U-shaped reactors [68], and fluidized bed reactors [61]. Most patents address thermal or catalytic pyrolysis. However, methods including vacuum pyrolysis [67] or the addition of other material, e.g., heavy oil or hydrocracking steams to a pyrolysis process [60,63], were published. Furthermore, methods comprising multistep processes are present. Examples are upstream technologies prior to pyrolysis [63,65,66] or downstream purification technologies to enhance product quality [57,59,60,67]. Flame retarded polymers sum up to on average 30% of WEEE plastics [71,72]. A crucial point in the thermochemical processing of FR-containing plastics is, however, the copper catalyzed formation of hazardous polyhalogenated dibenzo-*p*-dioxins and furans (PXDD/F) as precursors for PXDD/F, namely chlorine and bromine, are present in WEEE [2,73]. In order to prevent or to minimize their formation during thermochemical treatment, the critical temperature for PXDD/F formation between 200 and 600 ◦C [74,75] should be skipped quickly by rapidly heating up the plastics to >450 ◦C [4].

Consequently, Fraunhofer UMSICHT developed an innovative thermochemical conversion process for treatment of composite materials, including WEEE [70]. This process is based on an innovative auger reactor equipped with a unique heat exchanger (Section 2.1). The so-called iCycle® process (intelligent Composite Recycling) enables the conversion of WEEE fractions at very constant and controlled process conditions (heating up and retention time of feedstock, stability of process temperature). The current contribution deals with the recovery of chemicals from the liquid oil fraction generated during thermochemical treatment of shredder residues from a state-of-the art WEEE dismantling process. By use of iCycle® and downstream processing of the oil, the objectives of the current contribution are as follows:


### **2. Materials and Methods**

### *2.1. Thermochemical Conversion*

The residual WEEE fraction consisted of IT-appliances (collection group 5) and was provided by a manufacturer as shredder residue with a maximum particle size of 20 mm. This shredder residue contained around 40% metals and inorganics and 60% organics. 231 kg of the feedstock was treated in the continuous thermochemical demonstration plant (load capacity ~70 kg/h) illustrated in Figure 1. Prior to treatment, the plant was flushed with nitrogen overnight and heated up to an operating temperature of 650 ◦C.

**Figure 1.** Process flow diagram of the iCycle® pilot plant (thermochemical conversion process).

As seen in Figure 1, throughout the treatment, the feedstock material is fed by a screw conveyor (3) batch-wise from a receiver tank (2) into a lock, where it is flushed with N2 (1) to ensure that no ambient air enters the reactor. Thermochemical treatment and decomposition of the plastic fraction takes place in the reactor (4) at 650 ◦C. The reactor is a patented system with an innovative heat exchanger design. By use of an Archimedean screw, internal heating by externally preheated cycled spheres can be achieved. Along their way through the auger reactor, the spheres do not get in contact with the feedstock itself as they are moved forward in the inner section of the Archimedean screw. Thus, clogging of feedstock to the hot spheres is prevented. The system in combination with a relatively high temperature of 650 ◦C ensures a high heating rate of reaction media to avoid the formation of polyhalogenated dioxins and furans due to the presence of flame retardants [4,70]. In addition to the internal heat supply by cycled spheres, external heating is provided by heating sleeves covering the surface of the auger reactor.

Inside the reactor, the auger unit moves the solid material from the feeding point to the discharge point, where remaining solid matter drops out of the reactor and is transferred by a conveyer screw (5) into a collection reservoir (6). The heated auger reactor has a length of 6000 mm and a diameter of 470 mm. Gaseous and vaporous decomposition products (at 650 ◦C) leave the reactor through a cyclone (7) and subsequently the cooling and condensation train (9, 10), consisting of two tube bundle heat-exchangers, connection pipes, and a pump that transfers the condensate into a collection reservoir (11). All material found in the cooling and condensation train after the experiment is referred to as "condensate". The remaining gas is cleaned in a NaOH-scrubber unit (12) and an electrostatic particle

filter (ESP) (13) and is subsequently burned on sight (14, 15) in accordance with German Federal Emission Control Act (BImSchG).

### *2.2. Pyrolysis Oil Pretreatment*

Prior to fractional distillation, the pretreatment of condensate was performed to remove solids and aqueous phase present. Two pretreatment methods were conducted in this research. The first method was a vacuum filtration process using a filter paper and a Buchner funnel to separate solids from the crude condensate produced throughout the iCycle® process. Filtration was performed within two steps with two different pore sizes of filter paper, i.e., 40 μm and 2 μm. The condensate was then poured through the funnel into a borosilicate flask, and the solids that were larger than the pore size of the filter paper were separated. A vacuum pump (KNF, model: N810FT.18; 100 kPa) was used to initiate the oil suction and enhance the filtration process. The filter paper was frequently replaced, as soon as the filter paper clogs in order to prevent performance decrease. The mass of the total solids that remained in filter papers was measured and prepared for analysis.

Subsequently, a separation of the observable aqueous phase from the filtrate of the second filtration was conducted using a separation funnel. In this way, the lower phase (aqueous) was released by gravitational force via the stopcock (tap) at the bottom of the funnel. The weight of the separated phase was measured and the water phase prepared for analysis.

### *2.3. Fractional Distillation*

For the fractional distillation of 5 kg of the pretreated condensate (oil), a batch distillation system (PILODIST-104) was operated. The plant's column features a stainless-steel wire mesh (20 theoretical stages), arranged with a head temperature sensor and a reflux divider. The column was insulated by a heating mantle, where the temperature was adjusted to maintain at 5 K below the column head temperature to provide an adiabatic condition during the distillation process. The main condenser with a subsequent distillate cooler at the top of the column ensures a sufficient condensation of ascending distillate vapors. The distillation flask at the bottom of the column, surrounded by an insulation mantle, has a volume of 20 L. The heating of the flask was controlled according to the temperature difference (ΔT) between the heating device and the oil temperature inside the heating flask. The reflux ratio was defined by adjusting the off-take time and the reflux time of the distillate fraction. A vacuum pump was connected to the top of the condenser and the fraction collector to maintain the desired pressure. The system was constantly flushed with nitrogen with a flow rate of 0.5 L/min in order to prevent oxidation reactions. A liquid nitrogen cold trap was used to protect the vacuum pump during the distillation with reduced pressure. The following parameters were operated in order to distill the oil generated:


Based on the boiling points of benzene, toluene, ethylbenzene, and xylene (BTEX aromatics) as well as phenolic compounds present in the operated oil, the temperature intervals and operating pressures depicted in Table 1 were selected to divide the oil into distillate fractions comprising high purities of BTEX-aromatics and monocyclic aromatic substances as styrene, α-methyl styrene, phenol and cresols.


**Table 1.** Fractional distillation operating parameters.

<sup>1</sup> Atmospheric equivalent temperature.

### *2.4. Analysis of Pyrolysis Oil and Fraction Characterization*

### 2.4.1. Gas Chromatography–Mass Spectrometry (GC-MS) Analysis

The composition of oil was analyzed on a gas chromatograph coupled with a mass spectrometer Shimadzu GCMS-QP2020. The chromatograph was equipped with a 30 m nonpolar 0.25 mm inner diameter (i. d.), 0.25 μm film thickness DB-5ms, and 2.5 m middle polar 0.15 mm i. d., 0.15 μm film thickness VF-17ms column set from Agilent Technologies. Helium with 5.0 purity was used as carrier gas for all experiments. The injection volume was set to 1 μL. Dilution was conducted with 1 mg of sample in 1 mL of DCM. The measurements were performed at a constant linear velocity 40 cm/min of carrier gas. The temperature of the GC oven was programmed using the settings starting at 40 ◦C, 3 min hold to 320 ◦C, 3 min hold at 10 ◦C/min. The temperatures of the injector, the MS-interface, and MS were set to 250, 280 and 200 ◦C, respectively. The quadrupole MS detector was operated at scan speed of 5000 Hz using a mass range of 35–500 m/z. Solvent cut time was 3 min, with the MS start at 3.2 min. Total analysis time was 34 min. BTEX, styrene, α-methyl styrene, phenol, cresols and naphthalene were identified using standards solutions of pure chemicals. NIST-17 Mass Spectral Library was used for all other substance identification. Only the substances detected with the similarity index (SI) of more than 70 were identified. The proportion of each substance in the sample was given in percentage area. Only the proportions of the substances of particular interest in this research were shown in the graph, while other substances were not included and were referred to as "others".

### 2.4.2. Energy Dispersive X-ray Fluorescence (EDXRF) Analysis

A quantitative halogen analysis of all the samples was made with an energy dispersive X-ray fluorescence spectrometer (EDXRF) from Shimadzu (EDX 720). The concentrations for chlorine and bromine in oils were calculated based on the calibration curve and the Cl/Br peak intensities measured by EDXRF. Standard solutions were made with 1,3,5 trichlorobenzene, 98% from Alfa Aesar™ for chlorine and with 1,3,5-tribromobenzene, and 98% from Alfa Aesar™ for bromine. Toluene at 99.8% from Merck was used as a solvent. Each sample was measured three times in order to get statistical data.

### 2.4.3. Water Content Analysis

Water content in pyrolysis oil and distillate fractions was measured using a Karl-Fischer volumetric titration method. HYDRANAL™—Composite 5 from Honeywell Fluka™ was used as one component reagent titrant and methanol ACS Reagent, while ≥99.8% from Honeywell Riedel-de Haën™ was used as a titration medium. All measurement were performed on Metrohm 915 KF Ti-Touch and leaned on DIN 51777. Same as in case of EDXRF, measurements were performed three times.

### **3. Results**

Real WEEE shredder residues were thermally converted by means of pyrolysis at 650 ◦C with a residence time of 30 min. Solid, liquid, and gaseous products were yielded from this process; these products will be termed solid residue, condensate, and gas hereinafter, respectively. The obtained condensate underwent two pretreatment steps, namely solids filtration and aqueous phase separation. The obtained product, termed "oil", has thereafter been divided into eight fractions (including residue) by a fractional batch distillation process. The aim of the processing of the WEEE shredder residues was to produce highly enriched monocyclic aromatic mixtures with reduced content of solids, aqueous phase, and halogen concentrations to make them valuable for industrial reuse. The results of the individual process steps are presented in the following section.

### *3.1. Thermochemical Conversion*

The weights of the treated feedstock material, the collected solid product, and the condensate were determined at 231, 74, and 67 kg. The weight of the gaseous products was determined by a difference at 89 kg. Hence, the products yields were approximately 32, 29, and 39 wt.% for the solid product, the condensate, and the gaseous product, respectively. The results are illustrated in Table 2.


**Table 2.** Mass balance of the thermochemical conversion process.

The GC-MS analysis of the condensate showed a composition of 6.26 area% benzene, 22.05 area% toluene, 8.39 area% ethylbenzene, 0.73 area% xylenes, 26.63 area% styrene, 6.94 area% phenol, 3.90 area% α-methyl styrene, and 1.91 area% cresols. It follows that the condensate consisted of 72.16 area% of monocyclic aromatic substances to be recovered potentially. The composition is also illustrated in Figure 2 in Section 3.3.
