Behaviour of zinc during the process of leaching copper from WPCBs by typical acidic ionic liquids

Abstract

Waste printed circuit boards (WPCBs) have attracted more and more attention, which is mainly focused on the recovery of valuable metals, especially copper. Few studies have been reported on the behaviour of heavy metals during the process of recycling copper from WPCBs. Hence, we selected zinc to represent the heavy metals and examined its behaviour in a typical acidic ionic liquid (IL) leaching system. The factors that affect the zinc leaching rate, such as particle size, temperature, ionic liquid concentration, volume of H₂O₂ added and solid to liquid ratio, were examined in detail. The result showed that zinc could be leached out successfully in five typical acidic ILs and the zinc leaching rate was significantly impacted by the volume of H₂O₂ added, solid to liquid ratio and temperature. In addition, the zinc leaching rate by [BSO₃HMIm]OTf was almost the same as [BSO₃HPy]OTf. Moreover, the acidic IL with CF₃SO₃⁻ was less efficient than the acidic IL with HSO₄⁻. Although zinc could restrain the leaching of copper because of the substitution reaction between metallic zinc and Cu²⁺, the two show almost the same tendency. The results of the zinc leaching kinetics analysis indicated that diffusion played a more important role than the surface reaction, which was the same as copper, but different from inorganic acids, which are usually controlled by the surface reaction.

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Introduction

Recently, with the development of technology, more and more electronic devices are proliferated wildly. Printed circuit boards (PCBs) is an essential part of almost all electric and electronic products, from personal computers (PC), TV sets, to electronic toys. The manufacture of PCBs has been developed dramatically with the rapid development of the information industry. It is reported that the average increasing rate of PCB manufacture is 14.4% in China, which is much higher than the rest of the world (8.7%),¹ leading to a huge quantity of waste printed circuit boards (WPCBs). WPCBs contains not only abundant valuable substances, i.e., copper, gold, and silver, but also environmentally harmful matter, i.e., halogenated flame retardants and heavy metals. For example, the copper and gold content in a typical WPCB from a PC is about 20 wt% and 250 g per ton, respectively, which is 20–40 fold or 25–250 fold higher than that of copper or gold ore.² However, zinc (1.5%), cadmium (0.015%) and chromium (0.05%) greatly threaten the environment and human health if they are not disposed properly.³ Hence, the understanding that we should build for recyclable WPCBs should not only be with respect to the recovery of valuable materials but also be considered from the angle of the environment.⁴

In the past few years, plenty of studies on recycling copper from WPCBs have been carried out. Among them, mechanical, pyrometallurgical, bioleaching and hydrometallurgical approaches were widely investigated,^5–10 especially, the hydrometallurgical methods because of their higher metal recovery rate. The hydrometallurgical process is harmful to the environment due to the large amounts of acid and alkali used, which contribute to the formation of waste water. Therefore, green extractants need to be developed to avoid these adverse effects. In recent years, ionic liquids (ILs) have been regarded as the most promising extractants for leaching and new green hydrometallurgical methods using ionic liquids will replace the conventional methods, which consume acid or alkali.¹¹ Ionic liquids (ILs) that include an organic cation with an inorganic or organic anion, also called room temperature ionic liquids (RTILs), are basically liquid at low temperatures. Their unique properties, such as negligible volatility, vapour pressure, thermal stability, low toxicity, high conductivity and wide electrochemical window, allow them to be widely used.¹² In addition, the price of ionic liquids is low. Thus, leaching using ionic liquids could efficiently avoid environmental, health, economic, and safety issues, which occur due to the use of the conventional methods. Previous studies reported the use of ILs to leach chalcopyrite, and the results indicated that a pure IL and its aqueous solutions were more effective than the conventional acid solutions.¹³ For example, A. Kilicarslan et al.¹¹ found that 82% of copper was leached out from brass waste by [bmim]HSO₄. For ILs used as extractants to leach copper from WPCBs, Huang et al.¹⁴ first reported that copper could be successfully leached out by acidic IL, 1-butyl-3-methylimidazolium hydrogen sulfate ([bmim]HSO₄), from WPCBs with a leaching rate of up to 99%. However, almost all the previous research are focused on the recovery of valuable resources, mainly copper, without considering the heavy metals, for example, zinc.

As reported by Huang et al.,¹⁴ [bmim]HSO₄ could successfully leach copper out from WPCBs, and we examined five other typical acidic ILs, [BSO₄HPy]HSO₄, [BSO₃HMIm]HSO₄, [BSO₃HMIm]OTf, [BSO₃HMIm]OTf and [BSO₃HPy]OTf, and found that these five acidic ILs could also successfully leach copper out from WPCBs. In this study, we try to investigate the behaviour of zinc during the process of leaching copper from WPCBs by these five ILs, with the purpose to examine if zinc could be leached out simultaneously with copper and to find out the regularity between copper, which is the target metal for valuable resource recycling, and zinc, which is the selected metal representative for heavy metals. According to the regularity, potential methods may be developed to make maximum amount recovery of valuable metals and minimum amount leaching of heavy metals in further studies. Factors, such as the WPCBs particle size, IL concentration, liquid to solid ratio and temperature, were studied in detail. Furthermore, the leaching kinetics was also analysed.

Experimental section

Sample preparation

WPCBs, without disassembled electronic components, used in this study were obtained from waste computers. The first step was to cut the WPCBs into small pieces, around 50 mm × 50 mm, by a cutting machine. Then, they were sent to be further comminuted by the Retsch SM-2000 Cutting Mill (Retsch, Germany), and then sieved into different fractions using standard sieves: F1 < 0.075 mm, 0.075 mm < F2 < 0.1 mm, 0.1 mm < F3 < 0.25 mm, 0.25 mm < F4 < 0.5 mm and F5 > 0.5 mm, which was described elsewhere.¹⁴ After that, they were dried at 105 °C for 24 h.

Characterization

A microwave aided HNO₃–H₂O₂–HF system¹⁵ was used to digest the obtained powders and the zinc concentration of the leached solutions was tested using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Thermo Scientific, iCAP 6500), and the results are shown in Table 1.

Table 1 Zinc contents in the WPCBs specimens

Particle size, mm	F1 (<0.075)	F2 (0.075–0.1)	F3 (0.1–0.25)	F4 (0.25–0.5)	F5 (>0.5)
Zn wt%	1.28	1.23	1.3	2.21	2.24

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As is shown in Table 1, the Zn content varied with the WPCBs particle size. For example, the Zn content decreased from 1.28% to 1.23%, when the WPCBs particle size increased from <0.075 mm to 0.075–0.1 mm. However, as the WPCBs particle size increased from 0.075–0.1 mm to >0.5 mm, the Zn content increased from 1.23% to 2.24%. It is reasonable that the bigger particle size contains more zinc. Moreover, this result is consistent with the results reported by Wang et al., in which the content of zinc reached 2.044% in WPCBs when the particle size of zinc was less than 200 mesh (about >0.075 mm) during the ammonia/ammonium leaching process.¹⁶

Reagents

[BSO₄HPy]HSO₄ (N-sulfobutylpyridinium hydrosulfate, 99%), [BSO₃HMIm]HSO₄ (1-sulfobuty-3-methylimidazolium hydrosulfate,99%), [BSO₃HMIm]OTf (N-sulfobutylpyridinium trifluoromethanesulfonate, 99%), [MIm]HSO₄ (methylimidazolium hydrosulfate, 98%) and [BSO₃HPy]OTf (1-sulfobutyl-3-methylimidazolium trifluoromethanesulfate, 98.5%) used in this study were provided by the Lanzhou Institute of Chemical Physics and their structures are shown in Table 2. Chemicals applied in all the experiments were analytical reagents except those specially mentioned and all aqueous IL leaching solutions were prepared using deionized water.

Table 2 Acidic ionic liquids used in this study

[BSO₄HPy]HSO₄, C₉H₁₅NS₂O₇, N-butylsulfonate pyridinium hydrosulfate:

[BSO₃HMIm]HSO₄, C₈H₁₆N₂S₂O₇, 1-sulfobuty-3-methylimidazolium hydrosulfate:

[BSO₃HMIm]OTf, C₁₀H₁₄NS₂O₆F₃, N-sulfobutylpyridinium trifluoromethanesulfonate:

[MIm]HSO₄, C₄H₈N₂SO₄, N-methylimidazolium hydrogen sulfate:

[BSO₃HPy]OTf, C₉H₁₅N₂S₂O₆F₃, 1-sulfobutyl-3-methylimidazolium trifluoromethanesulfate:

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Leaching experiments

A constant temperature water bath oscillator using a constant oscillating frequency of 250 rpm in the temperature range of 40 °C to 70 °C was used to place a batch of 250 mL glass conical flasks, in which all the leaching experiment were carried out. Subsequently, the effect of WPCBs particle size, IL concentration (v/v, in liquid solution), concentration of acidic IL, volume of H₂O₂ (30 wt%) added and temperature on the zinc leaching rate by the five acidic ILs were investigated separately and the detailed experiment arrangements are given in Table 3. The data for copper leaching by these five ILs are provided in the (ESI†) for comparison.

Table 3 Experimental arrangement

Factors	Levels investigated
WPCBs particle size, mm	F1 (<0.075 mm), F2 (0.075–0.1 mm), F3 (0.1–0.25 mm), F4 (0.25–0.5 mm), F5 (>0.5 mm)
Temperature, °C	40, 50, 60, 70
Acidic IL concentration, v/v	10%, 20%, 40%, 60%, 80%
H2O2 volume, mL	0, 2, 5, 7, 10, 15
Solid/liquid, g mL^−1	1 : 1, 1 : 2.5, 1 : 5, 1 : 7.5, 1 : 10, 1 : 15
Time, min	5, 10, 20, 30, 60, 120, 240, 480

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According to a previous study on copper leaching,¹⁵ the leaching efficiency for zinc was also determined according to the following formula:

The relative standard deviations were within the limited range and mean values are given in the tables and figures without error bar.

Results and discussion

Particle size

It can be seen from Fig. 1, generally, as the particle size increased in the entire investigated range, the Zn leaching rate for [BSO₄HPy]HSO₄ and [MIm]HSO₄ increased from 67.2% and 41.7% to 93.0% and 99.0%, respectively. However, the Zn leaching rate for [BSO₃HMIm]OTf and [BSO₃HPy]OTf displayed a different tendency, which increased first and then decreased as the particle size increased, i.e., for [BSO₃HPy]OTf, the Zn leaching rate first increased from 39.9% to 56.5% when the particle size increased from <0.075 mm to 0.1–0.25 mm, then decreased from 56.5% to 16.7% when the particle size increased from 0.1–0.25 mm to 0.25–0.5 mm. [BSO₃HMIm]HSO₄ displayed a totally different trend, which decreased from 75.2% to 57.2% first and then increased from 57.2% to 99.3% when the particles size increased from <0.075 mm and 0.25–0.5 mm to 0.25–0.5 mm and 0.5–1 mm, respectively.

Fig. 1 Effect of particle size on zinc leaching rate by acidic ionic liquids (1 g WPCBs powder, 15 mL, 10% (v/v) ionic liquid, 5 mL hydrogen peroxide, leaching temperature 50 °C, leaching time 2 h).

Compared with copper (ESI, Fig. S1†), the zinc leaching rate was higher when the WPCBs particle was <0.075 mm. This is may be caused by the significant increase in particle–particle collisions when the particle size was reduced to a critical level,¹⁸ which was <0.075 mm in this study. In this condition, the leaching liquid is hard to permeate the fine WPCBs powder. Since the metallicity of zinc is stronger than that of copper, the zinc leaching rate would be slightly higher than copper.

Temperature

Fig. 2 shows that the Zn leaching rate decreased with the increase of temperature for [BSO₄HPy]HSO₄, [BSO₃HMIm]HSO₄ and [MIm]HSO₄ in the entire range investigated. For example, the Zn leaching rate for [BSO₃HMIm]HSO₄ decreased from 51.4% to 15.9% when the temperature increased from 40 °C to 70 °C. However, for [BSO₃HMIm]OTf and [BSO₃HPy]OTf, the Zn leaching rate slightly increased as the temperature increased, i.e., for [BSO₃HPy]OTf, the Zn leaching rate increased from 6.1% to 11.1% when the temperature increased from 40 °C to 60 °C and then as the temperature increased from 60 °C to 70 °C, the Zn leaching rate remained almost the same with a change from 11.1% to 10.3%.

Fig. 2 Effect of leaching temperature on the zinc leaching rate by acidic ionic liquids (5 g WPCBs powder, 75 mL, 10% (v/v) ionic liquid, 25 mL hydrogen peroxide, leaching time 2 h).

Considering the effect of temperature on both copper (ESI, Fig. S2†) and zinc, the two almost presented the same trend. It is logical that both the copper and zinc leaching rate decreased finally as the temperature increased because hydrogen peroxide would decompose and the copper leaching rate as well as the zinc leaching rate would decrease relatively as the temperature increased. Furthermore, it can be seen that the zinc and copper leaching rate for [BSO₃HMIm]OTf and [BSO₃HPy]OTf changed more than the other three ionic liquids. It also can be found that temperature showed a much stronger effect on the copper leaching rate than zinc.

Ionic liquid concentration

As shown in Fig. 3, Zn leaching rate increased with the acidic IL concentration increased for [BSO₃HMIm]HSO₄, [BSO₃HPy]OTf and [BSO₃HMIm]OTf. Taking [BSO₃HPy]OTf as an example, the zinc leaching rate increased from 16.7% to 74.9% as the acidic ionic liquid concentration increased from 10% to 80%. However, for [BSO₄HPy]HSO₄, the Zn leaching rate was almost constant with a relatively higher leaching rate of about 90% in the entire investigated range. In addition, the zinc leaching rate for [MIm]HSO₄ decreased from 73.7% to 17.3% as the ionic liquid concentration increased from 10% to 80%. It is uncommon that the zinc leaching rate decreases with an increase in ionic liquid concentration because it should display an increasing trend as the ionic liquid concentration increases, and should be studied further.

Fig. 3 Effect of acidic ionic liquid concentration on zinc leaching rate by acidic ionic liquids (1 g WPCBs powder, 15 mL, 10% (v/v) ionic liquid, 5 mL hydrogen peroxide, leaching temperature 50 °C, leaching time 2 h).

For the effect of IL concentration, it is the same as temperature: it showed almost the same effect on the both copper (ESI, Fig. S3†) and zinc leaching rate. Furthermore, it can also be found that the zinc leaching rate is higher than the copper leaching rate when the acidic ionic liquid is at a lower concentration. For these five acidic ILs, they can instantly release H⁺ into aqueous solution owing to their strong acidity. Obviously, at a lower concentration, acidic ILs are not sufficient for leaching out both zinc and copper. The metallicity of zinc is stronger than that of copper such that zinc is leached by these five ionic liquids first. In addition, substitution between metallic zinc and the copper ion leads to the reduction of copper ion in the liquid to metallic copper, which also restrains the leaching of copper.

Effect of H₂O₂ dosage

Fig. 4 indicates that the zinc leaching rate increased as the volume of H₂O₂ added increased, until it reached 5 mL, and then decreased when the volume of H₂O₂ added was higher than 5 mL. Taking [BSO₃HMIm]HSO₄ as an example, the zinc leaching rate increased from 11.9% to 57.2% when the H₂O₂ volume added increased from 0 mL to 5 mL, and then decreased from 57.2% to 11.6% when the H₂O₂ volume increased from 5 mL to 15 mL.

Fig. 4 Effect of H₂O₂ adding amount on the zinc leaching rate by acidic ionic liquids (1 g WPCBs powder, 15 mL, 10% (v/v) ionic liquid, leaching temperature 50 °C, leaching time 2 h).

Compared with copper (ESI, Fig. S4†), the volume of H₂O₂ added shows the same tendency as the zinc leaching rate. Hydrogen peroxide could decompose and release oxygen, which combines with H⁺ and copper or zinc as reactants to form Cu²⁺ or Zn²⁺, resulting in an increase in the copper and zinc leaching rate when the volume of H₂O₂ added was less than 5 mL. However, the overdosed hydrogen peroxide could decompose greatly and release a great deal of oxygen into the ionic liquid, which may cause oxidation of the acidic ionic liquids. Therefore, all of them showed a decreasing trend when the volume of H₂O₂ added was higher than 5 mL. It also can be seen that the peak value of the zinc leaching rate appeared with a lower volume of H₂O₂ added for [BSO₃HMIm]OTf and [BSO₃HPy]OTf than copper leaching rate. As is known to us, the acidity of [BSO₃HMIm]OTf and [BSO₃HPy]OTf is lower than the other three ionic liquids and the metal activity of zinc is stronger than copper. Hence, it is reasonable that the peak value of the zinc leaching rate appeared with a lower volume of H₂O₂ added for [BSO₃HMIm]OTf and [BSO₃HPy]OTf than the copper leaching rate.

Effect of solid to liquid ratio

It can be seen from Fig. 5 that the Zn leaching rate increased first and then decreased as the solid/liquid ratio decreased for [BSO₃HMIm]HSO₄, [BSO₄HPy]HSO₄ and [MIm]HSO₄. For example, for [MIm]HSO₄, when the solid/liquid ratio decreased from 1 : 1 to 1 : 5, the Zn leaching rate increased from 11.6% to 73.7% and then the Zn leaching rate decreased from 73.7% to 24.3% as the solid/liquid ratio decreased from 1 : 5 to 1 : 15. However, for [BSO₃HPy]OTf and [BSO₃HMIm]OTf, the zinc leaching rate increased as the solid/liquid decreased in the entire investigated range.

Fig. 5 Effect of solid/liquid ratio on zinc leaching rate by acidic ionic liquids (1 g WPCBs powders, 15 mL, 10% (v/v) ionic liquid, 5 mL hydrogen peroxide, leaching temperature 50 °C, leaching time 2 h).

The effects of the solid to liquid ratio on both the copper (ESI, Fig. S5†) and zinc leaching rate were almost the same: the copper and zinc leaching rate increased first when the value of the solid to liquid ratio was larger and then decreased when the value of the solid to liquid ratio was less than a critical value. It is reasonable that the reaction of the WPCBs powder with an ionic liquid is more severe with the decrease of the solid to liquid ratio and the overdosed WPCBs powder makes it difficult to adequately react with the acidic ionic liquids. Perhaps, zinc was also an important factor that affected the copper leaching rate because of the substitution reaction as mentioned previously. It also can be seen that the zinc leaching rate was higher than copper when the solid to liquid value was higher. This could also explained by metallicity, as mentioned previously. Moreover, the acidity of [BSO₃HMIm]OTf and [BSO₃HPy]OTf is weaker than [BSO₄HPy]HSO₄, [BSO₃HMIm]HSO₄ and [MIm]HSO₄. Hence, it is logical that the peak values of the copper leaching rate and zinc leaching rate by [BSO₃HMIm]OTf and [BSO₃HPy]OTf were lower than the other three ionic liquids when the value of the solid to liquid ratio was at a lower level.

Kinetic analysis

Fig. 6 presents the zinc leaching rate by the investigated five acidic ILs with time. For further study, a kinetic analysis was conducted based on the results of the zinc leaching rate by leaching versus time.

Fig. 6 Effect of time on the zinc leaching rate by acidic ionic liquids (5 g WPCBs powder, 75 mL, 10% (v/v) ionic liquid, 25 mL hydrogen peroxide, leaching temperature 50 °C).

The system of a reaction between a solid and a fluid can be described as a heterogeneous model. In a heterogeneous solid/liquid reaction system, it is extremely difficult to express the overall leaching rate because of the complicated interaction between the physical and chemical processes. To better explain the process of zinc dissolution, the metal particles are considered as spherical particles and the leaching process is described as the shrinking core model. According to this model, the following steps are considered to occur in succession during the dissolution:¹⁷

(I) Lixiviant diffuses from the solution to the fluid film surrounding the solid.

(II) Lixiviant diffuses from fluid film to solid surface.

(III) Chemical reaction occurs on the surface of unreacted particle cores between the lixiviant and the solid.

(IV) Formation of products at the surface of unreacted particle cores and the products diffuse from the interface into the fluid film.

(V) Products diffuse from fluid film to solution.

The reaction rate primarily lies on the step with the highest resistance, and if the reaction is controlled by the surface chemical reaction, the kinetic equation is as follows:¹⁸

1 − (1 − x)^1/3 = kt (2)

If the reaction is controlled by diffusion, the kinetic equation is as follows:¹⁸

1 − 2/3x − (1 − x)^2/3 = kt (3)

where “x” is the zinc leaching rate, “t” is the reaction time (min or h) and “k” is the apparent rate constant (min⁻¹ or h⁻¹).

Eqn (2) and (3) show that if the leaching reaction is controlled by diffusion through the product layers or the surface reaction, there must be a linear relationship between the left side of equation and time. For eqn (2), the fit data shows a weak linear relationship with time, which is not given. Hence, this leaching zinc reaction from WPCBs could not be controlled by a surface reaction. For eqn (3), in which the reaction is controlled by diffusion, a better linear relationship is obtained for the fit data, which is shown in Fig. 7. It can be seen from Fig. 7 that the two acidic ILs, [BSO₄HPy]HSO₄ and [MIm]HSO₄, present a better linear relationship, and the values of R² are 0.9734 and 0.9469, respectively. For [BSO₃HMIm]OTf and [BSO₃HMIm]HSO₄, the values of R² are 0.8092 and 0.8489, respectively. For [BSO₃HPy]OTf, it does not fit well. Thus, one conclusion that can be drawn from these results is that diffusion plays a more important role than the surface reaction.

Fig. 7 Plots of shrinking core model for diffusion control.

Clearly, for [BSO₃HMIm]HSO₄, [BSO₄HPy]HSO₄ and [MIm]HSO₄, the linear relationships of zinc for the reaction controlled by diffusion are better than copper (ESI, Fig. S6†), the R² of which were 0.5986, 0.8759 and 0.3108, respectively. In addition, both zinc and copper showed a weak linear relationship for [BSO₃HPy]OTf. This could be attributed to the weak acidity of [BSO₃HPy]OTf, in which the surface chemical reaction step is of the highest resistance. However, for [BSO₃HMIm]OTf, it is strange that a better linear relationship is obtained for copper than zinc, and the reason for this needs further study.

Conclusions

Zinc in WPCBs was leached out successfully in [BSO₄HPy]HSO₄, [BSO₃HMIm]HSO₄, [BSO₃HMIm]OTf, [MIm]HSO₄ and [BSO₃HPy]OTf, and the zinc leaching rate was greatly affected by the volume of hydrogen peroxide added, solid to liquid ratio and temperature. In addition, the tendency of the zinc leaching rate in [BSO₃HMIm]OTf was almost the same as [BSO₃HPy]OTf. Moreover, the acidic IL with CF₃SO₃⁻ was less efficient than the acidic IL with HSO₄⁻.

The behaviour of zinc almost presents a same tendency as copper during the leaching process in the five acidic ionic liquids. The leaching of zinc restrains the leaching of copper to an extent. However, the detailed interaction between copper and zinc during the leaching process needs to be studied further. In addition, for acidic ILs, diffusion plays a more important role than the surface reaction during the zinc leaching process, which is the same as copper, but different from inorganic acids, usually controlled by the surface reaction.

Acknowledgements

This research was supported by a grant from the National Natural Science Foundation of China (21377104) and by the Outstanding Young Scientists Support Program of Southwest University of Science and Technology (13zx9110).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S6. See DOI: 10.1039/c5ra02655e

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