**1. Introduction**

According to the literature data, in the period 2016–2030, the quantity of waste electrical and electronic equipment (WEEEs) will increase at world level from 44.7 million tonnes to 110 million tonnes [1–4]. Thus, increasing the quantity of WEEEs requires the urgent development of environmentally friendly and low cost recycling technologies [5]. Waste printed circuit boards (WPCBs) represents an important part of WEEEs (3–5 wt.%) with more than 30% metals (of which ~30% copper) and 70% non-metals [6–8]. For this reason, the recycling of WPCBs can constitute a potential secondary source of raw materials for different industrial sectors [9], contributing to preservation of natural resources [10–12]. With this regard, a lot of research has been done that has established different techniques for WPCBs recycling based on physico-mechanical [13,14], pyrometallurgical [15], pyrolytic [11,16,17], and hydrometallurgical approaches [18,19]. One of the major issues that needs to be solved regarding the recycling of WPCBs is related to its dismantling into different material fractions that can be further processed into value added products [20,21]. Some studies so far conducted to WPCBs dismantling involve the thermal processing of WPCBs, but this operation is not eco-friendly because it occurs with the release of toxic gases [22],

**Citation:** Fogarasi, S.; Imre-Lucaci, Á.; Imre-Lucaci, F. Dismantling of Waste Printed Circuit Boards with the Simultaneous Recovery of Copper: Experimental Study and Process Modeling. *Materials* **2021**, *14*, 5186. https://doi.org/10.3390/ma14185186

Academic Editors: Rossana Bellopede and Lorena Zichella

Received: 31 July 2021 Accepted: 6 September 2021 Published: 9 September 2021

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high energy consumption, and the burning of components that could be reused [23,24]. On the other hand, hydrometallurgical recovery techniques of valuable materials from WPCBs are achieved by grinding followed by acid or basic leaching of base metals from the obtained powder with the generation of other unusable or toxic residues; at the same time, they are non-selective processes [25–27]. Typically, the approach does not allow the full recovery of metallic copper, which is the main base metal of the EC found on the motherboards. Considering that traditional approaches are highly energy-demanding and environmentally dangerous [28,29], our research group has been carrying out the development of chemical and electrochemical processes for the recovery of metals from WPCBs, which allow the oxidative dissolution of metals with the simultaneous electrochemical regeneration of the oxidant, leading to the minimization of several drawbacks [30–33]. It was found that the metallic parts that hold together the different parts of WPCBs can be dissolved in the combined chemical–electrochemical processes, leading to the disassembly of WPCBs into different material fractions, such as plastics, printed circuit boards without EC, chips and small EC, and sludge [31,33–35]. Considering the success achieved with particular types of WPCBs [31], in this work, it is our intention to prove the applicability of this method for the dismantling of other types of WPCBs simultaneously with the electrochemical regeneration of the leaching agent and the partial electrodeposition of dissolved copper. In addition, to evaluate the contribution of all stages and equipment to the technical–environmental performance of the process, besides the ones used in the experimental studies, the process was extended, modeled, and simulated using ChemCAD process flow modelling software for a higher productivity.

In order to accomplish the abovementioned aims, the following sections are discussed after the materials and methods section: (i) *Theoretical background for the dissolution of metals* to better understand the processes that occur in the case of WPCBs disassembly; (ii) *experimental dismantling process of WPCBs* meant to provide the necessary background for the scale up of the process; (iii) *the scaled-up dismantling process* in order to evaluate the contribution of all steps and equipment to the performance of the overall process and to provide the necessary data for the environmental assessment; (iv) *environmental assessment of the scaled-up dismantling process* to assess the environmental impact of the process in the early phase of development using the Biwer–Heinzle method.

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

The WPCB dismantling experiments were performed using four types of motherboards, which differ in terms of technical details (Table 1) and metal content (Table 2) due to the different periods of production. For each case, before inserting the WPCBs into the rotating drum of the chemical reactor (CR), large pieces of aluminum and stainless steel were removed from the surface of the WPCBs, because they can be easier recycled this way. Additionally, it is not justified, due to the high power consumption, to bring the metals from these parts into the solution. Next, the WPCBs were cut into 40–100 cm2 pieces to fit into the rotating drum.


**Table 1.** WPCBs samples used in the experimental studies.


**Table 2.** Concentration (wt.%) of the most important metals in the WPCBs samples.

The experimental setup used for the dismantling of the WPCBs samples consisted of two reactors connected in series, including a 2L CR with perforated rotating drum and a 3L divided electrochemical reactor (ER) by a ceramic separator. In all experiments, 5L of 0.3 M FeCl3 in 0.5 M HCl solution was recirculated between the two reactors using two Medorex TC200 pumps (Medorex, Nörten-Hardenberg, Germany). The solution was evacuated at the bottom of the CR and supplied at the bottom of the cathode compartment of the ER. From the top of the cathode compartment, the solution was transported to the bottom of the anode compartment, and with the help of the second pump, the solution was pumped from the top of the anode compartment back into the CR. In consequence, a cross flow of electrolyte between the two reactors is achieved. The rectangular cathode and anode were made of copper and graphite, respectively, each with an area of 570 cm2. Two Ag/AgCl/KClsat reference electrodes were used to measure the cathodic and anodic potentials. The experiments were carried out in the optimal operating conditions identified for the combined chemical-electrochemical processes in previous studies [31,32,36]: drum rotation speed (30 rpm), solid/liquid ratio (1/8), constant current density 4 mA/cm2, initial electrolyte composition 0.3 M FeCl3 in 0.5 M HCl, and a flow rate of 400 mL/min. An atomic absorption spectrometer was used to determine the metal content and different material fractions of the solutions, while the surface morphology and chemical composition of the cathodic deposits were characterized with a scanning electron microscope (SEM, Thermo Fisher Scientific, Waltham, MA, USA) equipped with an energy dispersive X-ray spectrometer (SEM/EDAX, FEI QUANTA 3D).
