Recovery of Copper Layer from Waste Mobile Phone Printed Circuit Board Without Crushing

Sample WMPCBs

From the secondary market, three WMPCBs, each double-sided, are randomly collected, and an identity name is assigned to each sample: WMPCB 1 side A, WMPCB 1 side B, and so on. The source of these WMPCBs or the IMEI numbers of the discarded mobile phones were unknown to the shopkeeper. Figures 1(a) and (b) show the photographs of sample WMPCB 1 side A and WMPCB 1 side B, respectively, where a painting layer or mask of dark blue/green color is clearly visible on both sides of WMPCB 1. The other two samples WMPCB 2 and WMPCB 3 are presented in supplementary figures (Figures 4 and 5). However, under the paint, a thin layer of copper material is expected to exist on both sides of the WMPCBs. To make it visible and to use it as an electrode in an electrochemical process as discussed in Section 2.3, the paint must be removed, which is detailed in Section 2.2.

Chemical removal of paint

A common practice of using caustic soda (NaOH) for stripping away different kinds of organic coating is chosen for the removal of paint from the WMPCBs. The NaOH solution is alkaline, which produces soft blisters by breaking the bonds between polymer chains present in the organic paint. A sufficient amount of NaOH (98.7%, origin Merck, India) solution of 15 mol dm⁻³ was prepared, and WMPCB 1 was immersed in the solution for approximately 45 h. Then, a small cleaning brass was used to scratch over the blisters for complete removal of the paint. Figures 1(c) and (d) show the photographs of WMPCB 1 side A and WMPCB 1 side B, respectively, after removal of the paint, where the shiny copper layer is now clearly visible on each side of WMPCB 1. Note that the other solid components (ICs, resistors, capacitors, etc.) are neither dismantled nor crushed. Exactly in the same way, the paint from the other two samples (WMPCB 2 and WMPCB 3) are chemically removed. For the sake of brevity, the photographs of these two samples are presented as supplementary figures (Figures 4–5), which may be available in the online depository of the journal. Therefore, the three samples WMPCB 1, WMPCB 2, and WMPCB 3, each double-sided, are ready for electrochemical recovery of the shiny copper layer after minimal sample preparation.

Electrochemical process for recovery of copper layer from WMPCBs

Figure 2(a) shows the sketch of an electrochemical cell that is used for the recovery of the copper layer from the WMPCBs. Figure 2(b) shows the experimental setup of the electrochemical cell, where a sufficient amount of the copper sulfate (CuSO₄⋅5H₂O, 98%, origin Merck, India) electrolyte solution is taken. The cell is prepared in our laboratory, and the chemicals are taken from our departmental store. These chemicals are purchased through a scientific shop from a different country of origin. Note that, to maximize the highest purity of recovered copper, the concentrations of H₂SO₄ (98%, origin Merck, Germany), NaCl (99.7%, origin Merck, India), and CuSO₄⋅5H₂O are maintained at 118 g/L, 40 g/L, and 90 g/L, respectively, as reported by Liu et al. [11]. Each WMPCB is immersed in the cell one after the other, where the WMPCB acts as the anode of the cell. As shown in Figure 2(c), two other reference copper plates act as cathodes, which are also immersed in the cell. Based on Faraday’s laws of electrolysis, the principle of the method can be explained in brief as follows. When sufficient potential difference is applied across the cell, then due to the Coulomb force the Cu²⁺ ions from the solution are attracted towards the reference cathodes and are deposited as copper atoms on the cathodes. The SO ions are attracted towards the anode, WMPCB, where copper atoms from the open copper layer of the WMPCB (from both sides) dissolve into the solution by means of electrochemical reaction with the SO ions. In this way, an ionic current flows through the solution, which remains maximum in the beginning and gradually decreases towards a minimum value when the recovery process almost ceases and no more copper is left on the immersed WMPCB.

Recovery of copper layer from WMPCBs

The experimental recovery of the copper layer from the WMPCBs is conducted using the prescribed setup as shown in Figure 2. Each WMPCB is separately used for recovery one after the other. A potential difference is applied from a direct current power supply (model MCP-M10-SP3003L), and the resulting current and time duration are recorded. In the beginning of the electrochemical process, the initial applied voltage and current are 5 V and 1.5 A, respectively, and the process continues for a long time approximately about 180 min. During the process, the voltage across the cell and the current through gradually decrease. After 180 min, the current and the voltage reduce to a minimum value around 0.04 A and 2.6 V, respectively, i.e., at this time, the ionic current density that is responsible for copper recovery decreases to approximately zero value, indicating that the recovery process has almost ended.

To analyze the mass of the recovery process, each time the WMPCBs and the cathodes are taken out and dried in open air, and the respective masses of the cathodes are measured. Figures 3(a) and (b) show the photographs of the two damaged sides of WMPCB 1. The other two WMPCBs (WMPCB 2 and WMPCB 3) are shown as supplementary figures in Figure 6. However, the respective masses of the reference cathodes before and after the electrochemical process are measured. The copper atoms from the WMPCBs are pulled out by means of electrochemical reaction so that the WMPCBs lose their weight and are damaged (compare Figures 1 and 3). On the other hand, the cathodes become thicker by gaining copper atoms, which is the recovered copper from the WMPCBs. By mass analyses, the amount of recovered copper w₅ from both sides of each WMPCB is calculated and presented in Table 1. The gross amount of recovered copper, w₅, and their average are found to be 5.94 g and 1.98 g, respectively.

Discussion

The photographs of damaged WMPCB 1 as presented in Figure 3 showed that mostly the copper content area was eliminated due to the extraction of the copper layer from there. Furthermore, the amount of recovered copper from the reference cathodes was not the same. There can be a couple of reasons for this. First, the immersed WMPCB with respect to the two reference cathodes was not exactly in the middle, which was very unintentional during the experimental phase, resulting in an unequal electric field across the WMPCBs. Second, the surface area of the physical copper layer on both sides of the WMPCBs was not the same, which also resulted in unequal copper recovery from the cathodes. Third, there might be a chance that some other contaminants from the WMPCBs could have dissolved in the solution. This could reduce the strength of the solution gradually, which in turn might affect the recovery process. If this happens, it can be investigated further and can be resolved by continuous monitoring and adding an extra reagent accordingly to the cell for maintaining the required strength of the solution.

However, after the recovery of the copper layer from the WMPCBs, another suitable technique could be applied for the next material recovery one after the other, which could reduce the complexities of the recovery processes and ensure minimum draining of materials as compared to the crushing-based technique. Material recycling without crushing is useful and advantageous over traditional primitive technology, which can also be compared with other works from a social and an environmental point of view. As an example, a case study in China (Guangdong Province and Zhejiang Province) can be considered here [5]. There, it was found that the use of primitive e-waste recycling technique led to several health risks to society especially for children. It also caused contamination of the local soil, air, sediment, and water by heavy toxic metals (e.g., Pb, Cu) and pollution of the environment by emission of dioxins, polybrominated diphenyl ethers, cyanide, etc. The case study with adequate findings reported that the e-waste recycling needs advanced technology for economic, social, and environmental benefits. Contemporarily, another work can be cited where the same kind of crude activities as in the primitive recycling technique of e-waste processing caused significant contamination of heavy metals (Pb and Cu) into the dust of the local environment, which posed a potential health risk to children [12]. Therefore, the innovative idea or method without crushing of e-waste can be useful for material recovery in a quite suitable manner.

Another issue regarding the removal of the painting mask from the WMPCBs can be of prime interest, where more environment-friendly methods or alternative chemicals instead of NaOH can be investigated in the future, which will add more value to the noncrushing technique for recovering copper from WMPCBs. Besides, it can also be recommended that the manufacturer must take some sort of responsibility during e-product design and manufacturing so that material recovery from e-wastes becomes easier and eco-friendly. From this point of view, a light paint or eco-friendly organic paint can be used instead of such hard paint to meet the extended producer responsibility (EPR) requirement  [5, 14–16].