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Environ Eng Res > Volume 29(1); 2024 > Article
Wan, Jiang, Zhang, Yang, and Wang: Enhancement of resource recovery from retired tires: Extraction of copper and zinc resources


The disposal of scrap rubber, represented by retired tires, has achieved industrial achievements, but the handling of brass-coated steel wire as a structural reinforcement in retired tires has received little attention. A feasible and efficient method of acquiring economical resources is proposed to extract copper and zinc (Cu-Zn) from brass-coated steel wire. With 20% ammonia, 10% hydrogen peroxide, and a liquid-solid ratio of 10, the brass coating almost completely dissolved in 5 minutes. This technique may remedy the challenge of weak dissolving efficiency brought on by the dual phase microstructure of brass. Electrochemical measurements and density functional theory (DFT) simulations assist in verifying that hydrogen peroxide decreases the resistance of the corrosion and that OH− and O-2 in solution are the main contributors to the dissolution. In addition, the AH effectively enhances the corrosion of the α phase in brass, which is notably evident in comparable works. This effective strategy broadens the application of retired tires and provides a new method for extracting Cu-Zn resources.

1. Introduction

Together with steel, coal, and oil, rubber is regarded as one of the world's four primary industrial raw materials and a crucial strategic resource for the defense industry [1, 2]. Tires are the most common rubber products. At the moment, global tire consumption has surpassed 1.5 billion (around 17 million tons), with only China's annual tire consumption reaching 400 million [35]. The amount of retired tires that have resulted from the rapid replacement of vehicles speaks for itself. During the natural degradation process, the products of stockpiled retired tires are complex and diverse, which poses a potential threat to the environment. One illustration of this is the 2016 fire at a tire landfill in Seseña (Toledo, Spain). Even from 700 meters away, the school near the fire site could still detect the carbon black that the burning had created, and the total particle number concentrations had reached 3.8×105 particles/cm3 (10 min resolution) [6]. Therefore, the massive amount of retired tires makes it impossible to disregard resource and environmental concerns [7].
Disposal methods for the purpose of producing high value-added products, such as refurbishment, reclaimed rubber, pyrolysis products, building materials, thermal insulation materials, leachate treatment media, etc., have become the main utilization strategies for retired tires [8, 9]. Particularly, carbon black made from the pyrolysis of retired tires has the potential to substitute commercial materials in fuel cell anodes and multipurpose adsorbents [1012]. However, the tire structure is diverse, in addition to rubber, tires also contain about 15 wt% of steel wire used to reinforce the structure. Steel wire used within tires, also known as brass-coated steel wire (brass: Cu-Zn alloy), varies from ordinary wire [13].
However, the majority of research has concentrated on the high-value uses of rubber. The brass-coated steel wires are often recycled as scrap by steel producers, in addition to being utilized as concrete additives or wear-resistant steel sand after crushing [14]. The elimination of brass-coated steel wires on a large scale seems to be best accomplished through remelting, although it is impossible to ignore the troubles of steel's properties decreasing due to Cu enrichment [15, 16]. If the steel wire and coating can be peeled, pure recycled steel and Cu-Zn secondary resources will be obtained, which will enhance the application range of retired tires.
For brass-coated steel wires, the choice of room temperature and environmentally acceptable disposal methods is a suitable solution. Subsequently, one of the methods for the widespread elimination of brass-coated steel wires could be the development of environmentally friendly cleaning solutions. First and foremost, cleaning agents for organic systems are excluded for environmental and economic reasons. The following issue is that the detergent for brass-coated steel wire may corrode the steel wire substrate in addition to dissolving Cu-Zn. Brass-coated steel wire's corrosion has been found to be extremely pH-sensitive, and the galvanic couple formed by brass and steel causes rapid corrosion after brass dissolves [13]. In various corrosion systems (NaCl, NaOH, Na2SO4, etc.), Zn in brass is preferentially oxidized, which is caused by the difference in the microstructure of brass [17]. Recent research results show that the corrosion resistant layer of brass is composed of ZnO and CuxO, and the protective layer hinders the charge transfer and mass transport of cations[18]. Moreover, the redox potential of Zn/Zn2+ is lower than that of Cu/Cu2+ and Fe/Fe2+, which proves that the dissolution process of brass-coated steel wire begins with zinc removal [19, 20]. The enrichment of Cu is subsequently encouraged by Zn consumption, and the Cu-Fe galvanic pair speeds up the dissolution of Fe [21]. The interference of other ions should be minimized as a technique for the extraction of Cu-Zn. As a result, using inorganic acid or salt as a cleaning solution is almost impractical.
Ammonia is an inorganic substance among the various acids and bases that has a simple structure and stability in the complexes formed with metal ions. Hence, a strategy for removing brass coating may employ an ammonia-based cleaning (corrosion) solution. Early thermodynamic calculations (E-pH) have demonstrated that both Cu-Zn can dissolve and form soluble complexes in ammonia solutions [22]. The results of subsequent research demonstrated that the dissolution of copper is dependent on the ammonia concentration, and that at low concentrations, copper produces a passivation layer dominated by Cu(OH)2 to impede the dissolving process [23]. The behavior of Cu in both passivation and dissolution can be clearly revealed by electrochemical measurements [24]. This mechanism also explains the failure of Cu-based alloys, particularly when exposed to ammonia-containing seawater [25, 26]. Furthermore, the application of an ammonia-ammonium chloride solution can efficiently clean the corroded Cu on the printed circuit board (PCB), but because Au is chemically inert, there is no need to address Au corrosion [27]. Nevertheless, because it exhibits a low rate of corrosion, ammonia is unsuitable for large-scale manufacturing processes. However, ammonia is still the best option due to the corrosion resistance of the steel and the introduction of impurity ions. To increase the low efficiency of ammonia, perhaps certain additional substances need to be added.
The simplest technique to increase the rate of corrosion is to add a certain amount of oxidant to the corrosion solution. Since hydrogen peroxide has been demonstrated to be a powerful oxidant, few alloys are able to resist its corrosive effects. In a successful example, rusty titanium parts can be cleaned more effectively by employing an alkaline hydrogen peroxide solution (KOH-H2O2 solution) instead of typical acidic cleaning agents [28]. Furthermore, the passivation coating rapidly breaks when tiny amounts of hydrogen peroxide are added to the corrosion solution, which effectively speeds up the corrosion behavior of Ni-based alloy [29]. While the corrosion behavior of the alloy in hydrogen peroxide solution alone is different from the above. For example, the carbide preferentially interacts with hydrogen peroxide, causing the carbide to dissolve and form pits on the alloy surface [30]. In the case of copper alloys, hydrogen peroxide forms a corrosion layer that is primarily composed of oxides and hydroxides rather than leaving pits on the surface [31]. This is valid and beneficial to titanium alloys, where the development of surface engineering for corrosion resistance employing this oxidation mechanism has been demonstrated to be a successful and economical strategy [32]. It demonstrates that, due to its excellent efficacy, economics, and environmental friendliness, hydrogen peroxide has the potential to be one of the components included in cleaning solutions for brass-coated steel wires. To summarize, based on the above theoretical calculations and practical experience, it is possible to combine the advantages of ammonia (dissolution capacity) and hydrogen peroxide (oxidation capacity), while decreasing or eliminating their disadvantages (corrosion of the steel substrate). Furthermore, the majority of studies on corrosion concentrate on electrochemical measurements and microscopic characterizations, and little is done about atomic-scale process exploration.
In this work, at room temperature, an ammonia-hydrogen peroxide solution (AH) was designed for eliminating Cu-Zn from brass-coated steel wires. The feasibility of the method was evaluated through a series of experiments and characterizations. To investigate the corrosion behavior of brass coating in AH corrosion solution, H62 brass sheet with the same chemical composite as the brass coating was employed as the control experiment. Furthermore, employing DFT calculations, the function of each ion in the solution was determined, and the distinctions in corrosion behavior of the dual phase structure of brass were elucidated. Adopting this strategy makes sense from a practical standpoint since it could lay the theoretical foundation for a large-scale industrialization of the resource cycle while compensating for the increasing depletion of Fe, Cu, and Zn mineral deposits. Theoretically, it provides a theoretical framework for the investigation of corrosion and environmental engineering.

2. Experimental

2.1. Materials

The brass-coated steel wire was obtained from a retired tire recycling company in Inner Mongolia, China. The H62 brass was purchased from Wenghou Metal Materials Hefei, China. Ammonia (NH3·H2O, 25 wt.%) and hydrogen peroxide (H2O2, 30 wt.%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used in all experiments. All solutions are utilized right away after preparation to avoid the drop in ion concentration caused by volatilization. The chemical composition of brass-coated steel and H62 brass is listed in Table 1.

2.2. Corrosion Experiments

A series of experiments is designed to obtain a Cu-Zn ammonia-mixed aqueous solution without Fe in a short time in order to investigate the corrosion rate of the brass coating in the AH system. In the experiments, the solutions involved are expressed as volume fractions (e.g., 100 mL of 10% ammonia solution represents a mixture of 10 mL of concentrated ammonia and 90 mL of deionized water). Due to the low density of steel wire, 2±0.05 g of steel wire placed in a 50 mL beaker needs at least 20 mL of aqueous solution to be immersed, so the volume of solution in the experiment is fixed at 20 mL. The main factors considered in the experiment were the concentration of ammonia and the amount of hydrogen peroxide added, and the corrosion time was fixed at 5 min. The steel wire is removed from the corrosion solution using a vacuum filter once the set corrosion time has passed, and the resulting solution is then sealed in a 50 mL plastic tube for measurement of the concentration of Cu, Zn, and Fe ions.

2.3. Exploration of Corrosion Mechanism

Due to the small size of the brass-coated steel wire, the phase structure of the brass coating cannot be clearly identified, and the wire does not meet the strict requirements of the sample shape for the various measurement methods. It is a challenging project to explore the corrosion mechanism of brass coating. Therefore, H62 brass with the same chemical composition as the coating is employed instead of brass-coated steel wire when analyzing the phase structure, carrying out electrochemical measurements, and performing microscopic characterizations. The surface of the H62 brass sheet was ground in sequence with 600, 800, 1000, 1500, 3000, and 5000 mesh silicon carbide sandpaper, then polished with 1 μm alumina powder, and finally washed with absolute ethanol, and dried in cold air before use. The purpose of polishing was to remove everything that could interfere with the experiment results, such as oil, laser ablation layers, scratches, etc. Furthermore, the polished sheet guaranteed that the surface quality of the brass-coated steel wire and the H62 brass sheet were the same. The same corrosion solution was applied to the brass sheet, but the corrosion time was increased to 5 h in order to generate more recognizable microscopic images.

2.4. Electrochemical Measurements

To investigate the function of hydrogen peroxide and ammonia, the rate-limiting step in the corrosion process, and the quantified corrosion rate, electrochemical measurements are necessary. All electrochemical measurements were performed in a conventional three-electrode cell with a CHI760E (CH Instruments Ins.) electrochemical system. A 10×10mm platinum sheet was used as the counter electrode (CE), a 1M KOH Hg/HgO electrode was used as the reference electrode (RE), and the 10×20×1mm H62 brass sheet clamped by the electrode holder was used as the working electrode (WE). All electrochemical measurements were performed at room temperature, and the listed data were repeated three times to ensure accuracy and reproducibility. At the rate of 1 mV/s, the potential scanning interval of the potentiodynamic polarization curves was selected according to the open circuit potential (EOCP) of the sample. The electrochemical impedance spectroscopy was carried out in the frequency range of 100 kHZ~10 mHZ, using an amplitude signal of 10 mV.

2.5. Characterizations

Due to the special size of the brass-coated steel wire, the morphology of brass coating cannot be observed directly, so it is embedded in phenolic resin. Ten steel wires were embedded in the phenolic resin block and polished using the same grinding and polishing method as the H62 brass sheet. The microstructure and element distribution of the samples were observed by the scanning electron microscope (SEM, ZEISS Gemini 300) equipped with the energy dispersive spectrometer (EDS, OXFORD XPLORE 30). Before shooting, the surface of the phenolic resin samples was coated with gold-palladium alloy to obtain a clear image, and the acceleration voltage was kept at 15 kV during shooting. More detailed surface morphology and roughness were observed by atomic force microscopy (AFM, Bruker Dimension Icon) with a scanning range of 8×8 μm. The resulting data was post-processed using Gwyddion (v2.43) software. A rapid carbon and sulfur analyzer (HIR-944) was used to quantify C and S in brass-coated steel wire and H62 brass sheet. The inductively coupled plasma optical emission spectrometer (ICP-OES, Prodigy Plus) was employed for quantifying the chemical composition (without C and S) of the brass-coated steel wire, brass sheet, and corrosion solution. The sample preparation method and details for ICP-OES measurements are illustrated in the Supplementary Materials (Text S1).

2.6. Computational Methods

The DFT calculations as implemented in the QUANTUM ESPRESSO (v6.7) were performed [33]. The projector-augmented-wave (PAW) potentials and the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) were employed to represent the electron-ion interactions and electron exchange-correlation interactions, respectively. A plane-wave cutoff energy of 80 Ry with a threshold of less than 10−5 Ry on the total energy was used in all the computations, and the Brillouin zone was set to 3×3×1 Monkhorst-Pack k-points grids for structure relaxations and electronic structure computations, respectively. In order to avoid the negative effect of the period in the z direction, a vacuum layer of 15 Å was set in the z direction of the model.

3. Results and Discussion

3.1. AH System Corrosion of Brass

In order to investigate the optimal parameters (concentrations of ammonia and hydrogen peroxide) for the corrosion of the brass-coated steel wire, a series of single factor experiments were performed. In Fig. 1a and 1b, with the increase of ammonia or hydrogen peroxide concentration, the three ions (Cu, Zn, and Fe, same below) exhibit different trends. The trend curves of the three ions reach an extreme value when the ammonia concentration is 20% and the hydrogen peroxide concentration is 10% (Cu: 129 mg/L, Zn: 114 mg/L, Fe: 0.41 mg/L). When compared to Cu-Zn ions, the concentration of Fe ions is always less than 5 mg/L throughout all experiments. Fig. 1a and 1b exhibit cross-sectional SEM images of brass-coated steel wire before and after the corrosion, respectively. It is nearly impossible to spot the interface between the brass coating and the steel wire. However, the presence of a brass coating can be clearly identified based on the distribution of Fe, Cu, and Zn. After AH corrosion, Cu-Zn cannot be accurately detected, and the distribution of Cu-Zn is dispersed. It is worth noting that the corroded steel wire interface exhibits a rough microstructure, which is due to the fact that the AH solution is still slightly corrosive to the steel wire, so a low concentration of Fe is detected in the corrosion solution. Through the aforementioned investigations, it was found that the AH solution can rapidly dissolve the brass coating, and the short corrosion time prevents the steel wire substrate from corroding excessively. These analyses provide practical experience for effectively eliminating Cu-Zn from brass-coated steel wire. The controlled experiments are shown in Since the corrosion solution is nearly transparent, Fig. S2 illustrates the extremely poor efficiency of employing ammonia or hydrogen peroxide alone, further demonstrating the great efficiency of the AH corrosion solution. This supports the findings of previous investigations that copper alloy corrosion is significantly accelerated by the addition of oxidizing agents to ammonia solutions [18, 34]. Even though the results of the corrosion experiment are remarkable, the superiority of the AH solution needs further evaluation by micro-characterization.

3.2. Corrosion Behavior of H62 Brass

Further microscopic characterization of brass-coated steel wire is exceedingly challenging due to its size restriction. To investigate the corrosion mechanism, H62 brass sheet, which has the same chemical composition as brass coating, is employed. In Fig. 2, H62 brass is a typical dual-phase copper alloy with α (Cu 60 at%, Zn 40 at%) and β (Cu 50 at%, Zn 50 at%) phases at room temperature. The identification method of the dual phase structure is illustrated in the Supplementary Materials (Text S2, Fig. S1). Ammonia solution alone does have a corrosive reaction on brass, similar to the phenomenon described in most literature, the corrosion ability of ammonia solution is limited, and only β phase is corroded. In other words, since Fig. 2a and 2b are so comparable, ammonia is more of a colorant. However, the AH corrosion solution is different in that both the α and β phases are extensively corroded and have struggled to be identified, but no corrosion products have been deposited on the brass surface. Fig. S3 provides evidence for this since no peaks of oxides or hydroxides were detected in the XRD pattern. Furthermore, it demonstrates that the concentration of the corrosive solution chosen for this work will not result in the formation of oxides or hydroxides that prevent corrosion on the surface of brass. Most significantly, this also demonstrates that hydrogen peroxide can strengthen the corrosive behavior of ammonia and get rid of the difficulty of the α phase, which is difficult to corrode.
The above experiments and characterizations verify the success of the AH solution, but the corrosion rate issue is still left without any quantitative metrics. Considering that the corrosion process is complex, it is important to evaluate the function of the corrosive liquid from more than just a two-dimensional perspective. The surface morphology of corroded brass can be further observed using AFM, allowing for a more accurate characterization of the function of the AH corrosion solution. The AFM images in Fig. 2f, compared to Fig. 2d and 2e, significantly illustrate the function of the hydrogen peroxide since the longitudinal depth of the brass surface has exceeded 2 μm after AH corrosion. The roughness (Rq) of the sample, which can be determined using Gwyddion software to analyze the images, can be found to be 4.77 nm, 28 nm, and 231 nm, demonstrating once again that the presence of hydrogen peroxide speeds up the corrosion of brass. However, the analysis in this section can only prove the feasibility of the AH corrosion solution, which is far from enough for evaluating the function. The properties of the solution and the corrosion behavior of brass are still uncertain, and further exploration by electrochemical measurements is necessary. The general principle of brass corrosion in ammonia is confirmed by the fact that the β phase will be preferentially corroded, which is consistent with the conclusions in the literature [3537].
Electrochemical measurements can be used to determine the nature of the solution and the limiting step of the corrosion reaction, as well as the corrosion behavior of brass in the AH corrosive solution. The typical active-to-passive transition is not detected in Fig. 3a, but active behavior is observed in ammonia systems, implying that anodic active dissolution occurs. This demonstrates that the concentration of ammonia used for this work is appropriate, in contrast to the passivation behavior caused by low concentrations that has been discussed in the literature [38, 39]. The corrosion current density (icorr), polarization resistance (Rp), corrosion rate (CR), and anodic/cathodic Tafel slopes (βa and βc, respectively) are calculated and listed in Table S1. After adding hydrogen peroxide, icorr increased significantly, indicating that hydrogen peroxide increased the corrosion tendency of brass. According to the corrosion rate calculated by icorr, it is obvious that hydrogen peroxide doubles the corrosion rate of brass [40].
In order to further clarify the corrosion characteristics of brass, EIS data are analyzed (Fig. 3b~3d). Since the corrosion of brass in the AH system is accompanied by the escape of gas, there are a few bad data (pink curve, Z’=~120 ohm) on the curve during measurements. The fitted equivalent circuit model by Nyquist plot is similar to the results obtained by most researchers, which include electrolyte resistance (R1), film resistance (R2), charge transfer resistance (R3), and constant phase elements (CPE1, CPE2, QPE, L) [41]. All the equivalent circuits proposed in this paper can be considered to be composed of an electrolyte resistance and several R-CPE/Q/L couples. For the ammonia system, R2-CPE1 represents the insulating layer, which can be understood as the oxide layer due to the oxidizability of the copper alloy [42]. R3-QPE represents the corrosion behavior of brass, where QPE represents the two-layer capacitance due to the non-ideal capacitance behavior of non-uniform electrodes. The low frequency loop corresponding to R4-L represents the diffusion process of corrosion products. R5-CPE2 represents Faraday resistance and Faraday capacitance, which means that the resulting corrosion products will be further transformed. However, after adding hydrogen peroxide, the above equivalent circuit cannot accurately describe the corrosion behavior of brass. This could be because the addition of hydrogen peroxide makes the corrosion products more stable, and almost no redox transformation occurs again [43]. Furthermore, corrosion products enter the electrolyte more easily, making the diffusion process almost without obstacles. The equivalent circuit parameters listed in Table S2 prove this view, that is, the electrolyte resistance (R1) decreases but the hindrance of the oxide film (R2) is almost unchanged, and the charge transfer resistance (R3) decreases sharply [44]. The above evidence shows that the addition of hydrogen peroxide can indeed accelerate the corrosion of brass, and EIS is consistent with the conclusion of potentiodynamic polarization, which is consistent with the inference of microscopic characterization. The analysis is consistent with the finding in most literature, that is, the corrosion of the Cu alloy in the ammonia-based solution is controlled by charge transfer. However, the corrosive behavior of each active component in the AH corrosion solution on brass cannot be characterized through electrochemical measurements. In order to more precisely describe the corrosion behavior of brass, DFT calculations will be employed in the following sections to investigate the contribution of each active component in the AH to corrosion.

3.3. Corrosion Mechanism

There is an apparent connection between the corrosion of the alloy and the active components in the solution, understanding the corrosion behavior requires more than just focus on the properties of the solution [45, 46]. In view of the complex corrosion behavior of brass in the AH system, the behavior of various active components in the solution should be considered in the exploration of corrosion mechanisms. In the AH corrosion solution, the presence of at least NH3, OH and O- 2 ions may be related to the corrosion of brass. Furthermore, the dual phase structure of brass should also be represented in the calculations. As for modeling, both the α and β phases are cubic, so the densest packing direction ([111]) is chosen as the simulation object. The β phase in brass is a solid solution based on the CuZn compound, so it is only necessary to consider whether the upper and lower surfaces are symmetrical when modeling. In summary, there are at least two models of β phase, that is, the outermost atoms are all Cu (β-Cu) or all Zn (β-Zn). According to the Eq. (1), the surface energies of β-Cu and β-Zn are −0.99 and −1.45 Ry, respectively [47]. The lower energy represents a more stable surface, so the β-Cu surface will no longer be considered in this paper.
where Esurf is the calculated surface energy, Eslab is the total energy of the model, Ebulk is the bulk energy per atom, n is the number of atoms in the slab, and A is the surface area [48]. This supports the conclusion obtained in the literature that the corrosion of brass usually starts with the dissolution of Zn, because the outer surface of the β phase is composed of Zn. The α phase is a structure with Cu as the matrix and Zn as the solid solution element. The solid solution position of Zn needs to be considered when modeling. Unless the model is large enough, the α phase cannot be constructed by replacing atoms. Therefore, in order to describe the relationship between Cu-Zn in the α phase, a model is constructed as shown in Fig. 4, where the central atom on the outer surface of Cu is replaced by Zn to save computing power [49].
Fig. 4 shows the binding of different adsorbates on different phases after energy minimization, and the statistical results of bond length are listed in Table S3. The surface of the phase, in particular, is composed of Zn atoms, but O- 2 ions can bind to Cu atoms on the subsurface, promoting charge transfer between Cu-Zn-O and accelerating the corrosion reaction [50]. Because the distance between the adsorbate and the α and β phases is not greatly exaggerated, the energy relationship must be calculated further to determine the difference in corrosion.
Under the influence of the three ions, the adsorption energy of β phase is weaker than that of α phase, which proves that the formation of the adsorption system is exothermic and that the more negative energy value represents the more stable structure (Fig. 5g). Furthermore, the effect of OH is the strongest, followed by O- 2, and NH3 is the weakest, indicating that in the actual corrosion system, the alkaline environment provided by ammonia is a critical factor in corrosion behavior, which cannot be clearly detected in the experiment. The differential charge density in Fig. 5h can support this view. The electron transfer between the binding site and NH3 is weak in both phases, indicating that Zn-N and Cu-N have almost no interaction. On the contrary, there is a strong electron transfer between Zn-O and Cu-O, and in the β phase, the Cu atoms on the subsurface also participate in the bonding. The α and β phases have something in common, that is, the two oxygen atoms of O- 2 bind to Cu-Zn, respectively. The O-O bond (α: 1.47 Å, β: 1.48 Å) is nearly unchanged when compared to oxygen molecules (O2: 1.48 Å). However, unlike oxygen, O- 2 is fixed on the surface of brass, and the transition metal's d-orbital electrons are transferred to the p-orbital of the O- 2 ion, which is easier to obtain electrons from than oxygen, forming the M-O oxide structure and causing brass corrosion. The effect of OH is similar, as both cause the surface to form the M-OH hydroxide structure via electron transfer, destroying the stability of brass [51].
Paradoxically, the above inference seems to prove that the corrosion process of α and β phases is the same, but the experimental phenomenon clearly observes the corrosion difference between α and β phases. This is due to the completely different binding behavior of OH in the and phases, which is reflected in the energy, that is, the adsorption energy difference between the α and β phases is much larger than that of other ions. Cu-Zn atoms on the surface of α phase combine with O, while only Zn atoms on the surface of β phase combine with O, and the distance is closer, which aggravates the electron exchange. This view can be supported by evidence on the DOS, that is, compared with the α phase, the DOS of the β phase is significantly positive shifted, and the DOS of Zn-3d and O-2p are highly overlapped (−4~−6 eV in Fig. 5f), which aggravates the hybridization and enhances the bond strength [52]. The bader charge listed in Table S4 supports this view. The simultaneous gain or loss of electrons by the N atom and the Zn atom to which it is bonded indicates the instability of the Zn-N bond [53]. In addition, for other corrosion solutions, the phenomenon of metals losing electrons and adsorbing molecules gaining electrons is exhibited. The metal atoms on the surface and subsurface of the β phase are more likely to lose electrons, which is consistent with the conclusions of DOS and differential charge analysis [54].
The above calculations can be further clarified to show that the oxygen atoms (OH and O- 2) in the solution cause the oxidation of the brass due to electron transfer, but the brass is little damaged by the NH+ 4. This is why brass can still corrode in ammonia alone, due to the alkaline environment provided by the ammonia (Fig. 2b). These conclusions are in line with the experimental observation that ammonia alone can cause corrosion in brass and that hydrogen peroxide can accelerate corrosion by increasing the concentration of oxygen atoms in solution, and the active O- 2 enhances the electron transfer between the metal and oxygen atoms.
However, in a practical corrosion solution, the effect of ions on the surface is complex, and different ions may promote each other. The OH-O2 co-adsorption model listed in Fig. 6 can realistically simulate the corrosion behavior of the brass surface in a practical system. The results shown in Fig. 6c prove that OH-O2 co-adsorption does not change the fact that the β phase preferentially corrodes, which is attributed to the lower adsorption energy of the β phase. In comparison to single ion adsorption, the phase transforms the initially stable top adsorption into a complex co-adsorption under the action of multiple ions, and the electron transfer phenomenon is intensified, making OH and O2 easier to bond with brass. The Bader charge (Table S5) confirms this phenomenon, that is, the number of co-adsorption transferred electrons is larger and the subsurface atoms are also involved in bonding. It is worth noting that the number of transferred electrons does not change significantly whether it is single-ion adsorption or co-adsorption. This proves that the coexistence of multiple ions on the brass surface promotes each other, so that the unsteady adsorption sites can also achieve the effect of steady adsorption. After co-adsorption, DOS exhibits more pronounced overlap, and electron hybridization intensifies, forming more stable chemical bonds. In conclusion, this co-adsorption phenomenon in AH solution has a synergistic effect that enables rapid corrosion to dissolve the brass coating. The AH corrosion solution proposed in this work is an effective cleaning solution for extracting Cu-Zn from brass-coated steel wire. Compared with the previous work, the AH corrosion solution is more efficient, further completing the corrosion mechanism of brass, clarifying the interaction between the corrosion solution and brass, and providing a theoretical basis for the design of more advanced brass-coated steel wire cleaning solutions [18, 45].

4. Conclusions

This work demonstrates that it is possible to dissolve the brass-coated steel wire from retired tires employing AH solutions through microscopic characterizations and corrosion experiments. The concentrations of Cu, Zn, and Fe ions are 129 mg/L, 114 mg/L, and 0.41 mg/L, respectively, at 20% ammonia concentration, 10% hydrogen peroxide concentration, and a corrosion time of 5 min. According to electrochemical measurements, charge transfer and diffusion regulate the corrosion of brass in the ammonia solution alone. In the AH corrosion solution, the corrosion of brass is controlled by charge transfer, and the addition of hydrogen peroxide accelerates the mass transfer rate and corrosion rate. DFT calculations show that ammonia ions in solution have the least contribution to corrosion, while alkaline environments and peroxide ions contribute the most to corrosion. In addition, the calculation shows that β phase is preferentially corroded, which is consistent with the microscopic characterization results. The reason is that different from α phase, Cu atoms on the subsurface of β phase also participate in bonding, which accelerates the corrosion of brass. The DFT calculations are in line with the experimental results. The obtained data provides an enhancement strategy for the low utilization rate of brass-coated steel wire from retired tires. It provides theoretical support and practical significance for the recycling of retired tires, and the obtaining of Cu-Zn resources.

Supplementary Information


This research was funded by the National Natural Science Foundation of China (U1702253, U2102213), the Fundamental Research Funds for Central Universities (N2225012, N2224001-9), the National Science Fund for Distinguished Young Scholars (52204419), the Natural Science Foundation of Liaoning (2022-BS-076), and the National Science and Technology Major Project of Guangxi (2021AA12013).


Conflict-of-Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

X. W. (PhD) conducted all the experiments and wrote the manuscript. K. J. (Professor) visualized and reviewed the original draft. T. Z. (Professor) visualized, edited and reviewed the original draft. H. Y. (PhD) edited and the manuscript. K. W. (PhD) edited and reviewed the original draft.


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Fig. 1
Contents of Cu, Zn, and Fe ions at (a) various ammonia (fix 5% hydrogen peroxide) and (b) various hydrogen peroxide concentrations (fix 20% ammonia). Distribution of Cu, Zn and Fe (c) before and (d) after AH corrosion.
Fig. 2
(a) Dual phase microstructure of H62 brass (After coloring). H62 brass surface after (b) ammonia, (c) AH corrosion, respectively. AFM image of (d) polished H62 brass, (e) after ammonia, (f) after AH corrosion.
Fig. 3
(a) Potentiodynamic polarization and (b) EIS curves of H62 brass at different corrosion solutions. Equivalent circuits for EIS in (c) ammonia and (d) AH corrosion solution.
Fig. 4
The α phase after optimization: (a) α-NH3, (b) α-OH, and (c) α-O2, respectively. The β phase after optimization: (d) β-NH3, (e) β-OH, and (f) β-O2, respectively.
Fig. 5
DOS of (a) α-NH3, (b) α-O2, and (c) α-OH, respectively. DOS of (d) β-NH3, (e) β-O2, and (f) β-OH, respectively. (g) Adsorption energy of different ions at different phases. (h) Differential charge density of different ions on different phases.
Fig. 6
DOS of (a) α-OH-O2 and (b) β-OH-O2. (c) Adsorption energy of different complex corrosion systems. (d) Differential charge density of different complex corrosion systems.
Table 1
The chemical composition of brass-coated steel wire and H62 brass
Steel wire Element C P S Mn Si Cu Zn Ni Mo Cr Fe
Content/wt% 0.94 0.02 0.02 0.57 0.14 0.11 0.06 <0.01 <0.01 <0.01 Bal.

H62 brass Element C P S Cu Fe Pb Sb Bi Al Zn
Content/wt% <0.01 <0.01 <0.01 63.21 0.02 0.04 <0.01 <0.01 0.02 Bal.
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