Facile manufacture of high-purity CuSO₄ from waste Cu-containing paint residue using combined processes of H₂SO₄ leaching and extraction stripping

With the growth of China's maritime commerce and shipbuilding industry, ship coating has become a crucial and significant step in ship construction. It is also critical for safeguarding against corrosion in severe sea conditions and prolonging the vessel's lifespan (Maia et al. 2015; Wang & Dai 2020). At present, due to the existence of new antifouling agents such as poor antifouling effect, short antifouling time, and relatively single target marine pollution organisms, the commercial antifouling paint used for ship coating is still mainly copper (Cu₂O) oil-soluble paints, and its unique Cu₂O components can form copper complexes after contact with seawater, which react with oxygen in seawater to form divalent copper ions, thereby forming a thin toxic microlayer on the coating surface. The Cu ions in the microlayer can enter biological cells and poison dead organisms, thereby achieving antifouling effects (Blanca 2008; Qian et al. 2015; Mohamed et al. 2015; Chen et al. 2021). Therefore, Cu-containing antifouling paints will still be the most mainstream antifouling coatings on the market for a long time to come.

During the coating process of ships, the Cu-containing antifouling paint will not be fully attached to the hull, but a large amount of paint mist is generated due to splashing. Therefore, the coating rate is relatively low, even with advanced electrostatic spraying. Only about 60% of the coating process is effective, and most of the paints used in the spraying process are splashed in the spraying room (Dvořák & Jandová 2005; Fu 2012). Currently, most of the coating processes in China absorb the paint mist using the circulating water in the spraying room, causing it to coalesce into paint residue (Salihoglu & Salihoglu 2016). In order to keep good antifouling performance of the ship, it is necessary to repaint it regularly, thus making a large amount of waste Cu-containing paint residue (WCPR). At present, China alone produces about 100,000 tons of WCPR annually, which contains not only a large amount of organic matter but also a variety of heavy metals such as Zn, Fe, and copper (5–10%). This amount is much higher than the average grade of ores in China. According to the ‘National Hazardous Waste List of the Ministry of Environmental Protection,’ this residue belongs to HW21 dyes and paints hazardous waste (Fu 2012). If WCPR is not effectively treated, it will cause severe damage to both the ecological environment and human health. Therefore, it is of great significance to explore a reasonable disposal method for WCPR to solve the environmental pollution problem from the source.

As WCPR contains a large number of organic compounds and heavy metal ions, these components usually have irreversible toxic effects on the environment and the human body (Xing et al. 2018). In addition, due to the long-term coexistence of a large number of toxic substances in WCPR, there is a possibility of synergistic effects, so that it has a higher degree of toxicity, irritation, and hazard (Wang & Dai 2020). There is still a long way to go to achieve the reasonable disposal of WCPR. At present, the main disposal methods of WCPR are still landfilling (Song et al. 2005; Arce et al. 2010) and incineration (Saft 2007; Rosli et al. 2018). However, both not only fail to achieve the utilization of valuable components in WCPR, resulting in the loss of its resource value, but also cause serious environmental pollution. For example, the incineration method has a high cost and is prone to secondary environmental problems, and the landfill method not only requires a large amount of land but also has safety hazards like leachate leakage. Therefore, to maximize the resource utilization of WCPR, some scholars have started to study the recovery of valuable components, but their focus is still on the recycling and utilization of resin and organic matter (Gerace & Gerace 1992; Avci et al. 2017). However, there are few reports on the recovery of heavy metals from WCPR.

Aiming at the aforementioned problems, this article proposes a process of H₂SO₄ leaching–extraction–stripping to prepare CuSO₄, so that Cu of WCPR undergoes the processes of dissolution, extraction, and stripping in the reaction, and finally outputs in the form of regenerated CuSO₄. The H₂SO₄ leaching residue does not contain heavy metals and can be used in cement kiln incineration (Yuan & Wang 2015; Zhao et al. 2017; Wang et al. 2018), so as to realize the recovery and utilization of WCPR. Therefore, this method can maximize the economic value of the recycled products under the premise of environmental safety, simple operation, rapid leaching, and extremely short process flow, thus realizing the purpose of reduction, harmlessness, and resource utilization of WCPR. To further realize a precise and quantitative regulation of the whole process, five aspects have been carried out. (1) the optimal process conditions for leaching Cu from WCPR with H₂SO₄; (2) the optimal extraction and stripping conditions for Cu; (3) the change of shape, size, phase, and elemental composition of WCPR before and after H₂SO₄ leaching; (4) the utilization of the H₂SO₄ leaching residue of WCPR; (5) the H₂SO₄ leaching solution is applied to fabricate an important chemical product CuSO₄ by using precipitation to achieve the high-value utilization of Cu in WCPR.

WCPR was obtained from a hazardous waste treatment company in Shandong Province. It was dried in an oven at 80 °C to constant weight and then ground through a 60-mesh sieve to obtain a powder with a particle size of 0.25 mm, which was finally stored in a sealed reagent bottle for subsequent experiments. The reagents used in the experiments were HCl, HNO₃, HF, HClO₄, H₂SO₄, Na₂SO₄, extractant (LIX984), and sulfonated kerosene (DT100), all of which were analytically pure, and all solutions were prepared with deionized water.

The H₂SO₄ leaching experiment was carried out in a conical flask. A certain amount of dried WCPR was placed in the H₂SO₄ solution that had been raised to a specified temperature and leached on a constant temperature water bath shaker for a certain period of time to investigate the effects of H₂SO₄ concentration, solid–liquid ratio (S/L), leaching temperature, stirring speed, and leaching time on the leaching rate of Cu. At the end of the leaching, the leaching solution and the residue were separated for the corresponding characterization.

The extraction experiment was carried out in a conical flask. A certain amount of leaching solution adjusted to the desired pH (adjusted by 1 mol/L Na₂SO₄ solution) was placed in the LIX984 diluted with DT100 that had been raised to a specified temperature and extracted on a constant temperature water bath shaker (50 r/min) for a certain period of time to investigate the effects of pH, the volumetric ratio of the organic phase to the aqueous phase (O/L), LIX984 concentration, and extraction time on the extraction rate of Cu. At the end of the reaction, a partition funnel was used to separate the two phases, the aqueous phase was used for iron removal and zinc precipitation (Davey & Scott 1976; Su et al. 2020), and the oil phase was used for the next step of the stripping experiment.

The stripping experiment (regeneration of CuSO₄) was carried out in a conical flask. A certain amount of extracted oil phase was placed in the H₂SO₄ solution that had been raised to a specified temperature and reacted on a constant temperature water bath shaker (50 r/min) for a certain period of time to investigate the effects of H₂SO₄ concentration, stripping time, and O/L on the copper stripping rate of Cu. At the end of the reaction, a partition funnel was used to separate the two phases: the aqueous phase was heated, evaporated, cooled, and crystallized to prepare CuSO₄ crystals, and the oil phase was reused for the extraction of Cu (Yi et al. 2019).

The leaching toxicity test was an important identification index of the toxicity properties of hazardous waste. The leaching toxicity for the leaching residue of WCPR was determined by using the method of Chinese standards (GB5085.3-2019 and GB168892008) as follows: deionized water was mixed with mixed acid (the mass ratio of H₂SO₄:HNO₃ was 2:1) to a pH of 3.20 ± 0.05, and the solid waste was added at S/L of 1:10 (kg/L). It was placed on an overturning oscillation device with an oscillation speed of 30 ± 2 r/min and a temperature of 23 ± 2 °C and leached for 18 ± 2 h. The metal concentrations in the supernatant were determined and compared with the national standard limits (Tian et al. 2022).

The content or concentration of various metal ions was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 8000, PerkinElmer, USA). The phase and structure of raw materials, various intermediates, and products were analyzed by X-ray diffraction (XRD; D8 Advance, Burker, Germany). The pH during leaching or reaction processes was detected and regulated by a pH meter (Mettler Toledo, S210, USA). The surface morphology and elemental distribution of the raw, intermediates, and products were investigated by scanning electron microscope (SEM) and mapping analysis (SU8020, Hitachi Ltd, Japan). The organic compounds of the raw, intermediates, and products were detected by Fourier transform infrared spectrometer (FT-IR; Nicolet iS50, Thermo Fisher, USA).

Figure 2(a) shows the effect of different H₂SO₄ concentrations (5–25%, v/v: 98 wt.% H₂SO_4/deionized water) on the leaching rate of Cu at S/L of 20%, leaching temperature of 30 °C, leaching time of 4 h, and stirring speed of 50 r/min. With the increase of H₂SO₄ concentration, the leaching rate of Cu increased steadily and reached a maximum of 92.86% when the H₂SO₄ concentration was 20%. However, when the H₂SO₄ concentration was 10%, the pH of the leaching solution was −0.22, indicating that there was an excess of H⁺ in the solution at that time. Moreover, the leaching rate of Cu at the H₂SO₄ concentration of 10% was 88.99%, which was not significantly different from the H₂SO₄ concentration was 20%. Therefore, to reduce the amount of Na₂CO₃ solution used in the subsequent adjustment of pH, the optimal H₂SO₄ leaching concentration was 10%.

Figure 2(b) shows the effect of different S/L (10–50%) on the leaching rate of Cu at H₂SO₄ concentration of 10%, leaching temperature time of 30 °C, leaching time of 4 h, and stirring speed of 50 r/min. With the increase of S/L, the leaching of Cu was first stabilized and then decreased. The leaching rate of Cu reached a maximum of 85.15% when S/L was 20%. As S/L increased, the difficulty of contact between H⁺ and Cu in WCPR increased, and the reaction between H⁺ and impurity ions increased, leading to a decrease in the leaching rate of Cu. In addition, the increase of S/L led to an increase in solids in the solution, which was not conducive to the subsequent separation, so the optimal leaching S/L was 20%.

Figure 2(c) shows the effect of different times (1–5 h) on the leaching rate of Cu at H₂SO₄ concentration of 10%, S/L of 20%, leaching temperature time of 30 °C, leaching time of 4 h, and stirring speed of 50 r/min. The leaching rate of Cu increased with the increase of leaching time, which increased rapidly in 0–1 h and increased slowly in 1–4 h. The leaching rate of Cu reached a maximum of 92.76% when the leaching time reached 4 h. When the leaching time reached 4 h, the leaching rate of Cu was stable and unchanged, so the optimal leaching time was 4 h.

Figure 2(d) shows the effect of different temperatures (25–45 °C) on the leaching rate of Cu at an H₂SO₄ concentration of 10%, S/L of 20%, leaching time of 4 h, and stirring speed of 50 r/min. With the increase in temperature, the leaching rate of Cu remained basically unchanged. When the leaching temperature was 45 °C, the copper leaching rate of Cu increased to 95.66%, so the optimal leaching temperature was 45 °C.

Figure 2(e) shows the effect of different stirring speeds (0–200 r/min) on the leaching rate of Cu at an H₂SO₄ concentration of 10%, S/L of 20%, leaching time of 4 h, and leaching temperature of 45 °C. With the increase of the stirring speed, the leaching of Cu basically remained unchanged, so to release the generated gas as soon as possible, it was necessary to shake, and so the optimal stirring speed was 50 r/min.

Therefore, under the conditions of the H₂SO₄ concentration of 10%, S/L of 20%, 150 min, leaching time of 4 h, leaching temperature of 45 °C, and stirring speed of 50 r/min, the best leaching rate of Cu (94.31%) could be ensured. However, as shown in Table 2, in addition to Cu (leaching concentration was 18.02 g/L), the leach solution also contained a large amount of Fe, Zn, Al, and Mg impurities.

Figure 3(a) shows the effect of different pH values (1–4) on the extraction rate of Cu at LIX984 concentration of 20% (LIX984: DT100, v/v), O/L of 2:1, and extraction time of 10 min. When the pH of the solution was in the range of 1–2, the extraction rate of Cu increased rapidly; after the pH of the solution was higher than 2.0, the extraction rate of Cu remained basically unchanged. When the solution pH was 1.0, the extraction rate of Cu was 84.76%; when the pH of the solution was 2.0, the extraction rate of Cu was 99.00%. Therefore, the optimal extraction pH was 2.0.

Figure 3(b) shows the effect of different O/L (1:1–5:1) on the extraction rate of Cu at a pH of 2.0, LIX984 concentration of 20%, and extraction time of 10 min. Similar to the effect of pH on Cu extraction, the extraction rate of Cu was 93.72% when O/L was 1:1 and reached 98.64% when O/L was 2:1. The extraction rate of Cu was basically around 99% as O/L continued to increase. By increasing the amount of extractant, the extraction rate of Cu was less than 1%. Therefore, the optimal O/L of 2:1 was more economical.

Figure 3(c) shows the effect of different extraction times (5–15 min) on the extraction rate of Cu at pH of 2.0, LIX984 concentration of 20%, and O/L of 2:1. With the increase of extraction time, the extraction of Cu first increased and then stabilized, and the extraction rate of Cu was 78.38% at 5 min and then reached the maximum of 99.51% at 10 min. Therefore, the optimal extraction time was 10 min.

Figure 3(d) shows the effect of different LIX984 concentrations (10–30%) on the extraction rate of Cu at pH of 2.0, O/L of 2:1, and extraction time of 10 min. With the increase of LIX984concentration, the extraction rate of Cu fluctuated between 99 and 100%, meaning that a LIX984 concentration of 10% could already extract 99% of the Cu in the leach solution, so the optimal LIX984 concentration was 10%.

Therefore, as Table 3 shows, under the conditions of solution pH of 2.0, O/L of 2:1, extraction time of 10 min, and LIX984 concentration of 10%, the extraction rate of Cu reached 99.46%, and the Cu in the leaching solution was completely in the organic phase.

Figure 4(a) shows the effect of different H₂SO₄ concentrations (50–300 g/L) at the stripping rate of Cu at O/L of 1:1 and stripping time of 10 min. With the increase of H₂SO₄ concentration, the stripping rate of Cu increased and then remained basically unchanged when the H₂SO₄ concentration reached 200 g/L and above. When the concentration was 200 g/L, the stripping rate of Cu was 95.65%. The optimal H₂SO₄ concentration was 200 g/L.

Figure 4(b) shows the effect of different O/L (3:1–1:3) on the stripping rate of Cu at H₂SO₄ concentration of 200 g/L and stripping time of 10 min. As O/L decreased, the stripping rate of Cu first increased and then remained stable. When O/L was 3:1, the stripping rate of Cu was 75.90%, and when O/L was 1:1, the stripping rate of Cu increased to 95.65%. When O/L was lowered again, the stripping rate of Cu did not increase, so using an O/L of less than 1:1 to strip Cu would waste a part of the H₂SO₄ solution. Therefore, the optimal O/L was 1:1.

Figure 4(c) shows the effect of different stripping times (5–15 min) on the stripping rate of Cu at H₂SO₄ concentration of 200 g/L and O/L of 1:1. With the increase of the stripping time, the stripping rate of Cu was basically a straight line, and the stripping rate of Cu was basically around 92%. Therefore, the optimal stripping time was 5 min.

Cu was an indispensable basic material in modern industry, but the increasing depletion of copper mines had led to an increase in the production costs of Cu materials. As a result, the recovery of Cu from solid wastes even hazardous wastes containing Cu for the production of valuable Cu materials has attracted great concern around the world. Considering the enormous amount of WCPR and the high concentration of Cu contained in it, the efficient utilization of WCPR as a secondary resource to produce Cu materials was preferred, which had the benefits of eliminating Cu pollution and recovering the Cu resource. In this article, WCPR was taken as the research object, and CuSO₄ was prepared by the sulfuric acid leaching extraction stripping process, so as to provide a theoretical basis for the recovery of Cu in WCPR. The conclusions were as follows:

• (1)

  WCPR contained a large amount of Cu, and it mainly existed in the form of Cu₂O. However, due to its high dispersion and complex distribution, it was often chelated with organic compounds and was difficult to directly utilize. Under the optimal leaching conditions at H₂SO₄ concentration of 10%, S/L of 20%, leaching time of 4 h, leaching temperature of 45 °C, and stirring speed of 50 r/min, the leaching concentration of Cu was 18.02 g/L and the leaching rate was 94.31%.

• (2)

  Due to the large variety and quantity of metal ions in the leaching solution, it was not suitable to use the step-by-step precipitation method to remove impurity ions, so the solvent extraction method was selected to recover Cu from the leaching solution. Under the optimal extraction conditions at pH of 2.0, O/L of 2:1, extraction time of 10 min, and LIX984 concentration of 10%, the extraction rate of Cu reached 99.46% and Cu in the leaching solution completely entered the organic phase. The leaching solution of separated Cu could be mixed with the leaching solution of WCPR and then continued to remove iron and precipitate Zn. Under the optimal stripping conditions at an H₂SO₄ concentration of 200 g/L, O/L of 1:1, and stripping time of 5 min, the concentration of Cu was 6.14 g/L and the stripping rate was 95.32%. The CuSO₄ crystal was obtained by evaporating, cooling, and crystallizing the stripping solution.

• (3)

  The toxicity leaching concentration of Cu in the leaching residue was 50.23 mg/L, and the concentration of Zn was 21.38 mg/L, which was below the standard limit of 100 mg/L, indicating that the leaching residue of WCPR had no leaching toxicity. It mainly consisted of acrylic or methacrylic resin, SiO₂, and a small amount of Ca, Fe, and other substances. The leaching residue of WCPR could be used for cement kiln incineration, thereby achieving a high-value utilization of WCPR.

This work was supported by the National key research and development program (2018YFC1900304 and 2018YFC190030) and the National Natural Science Foundation of China (21777007 and 52200141).

Gongchu Shi: conceptualization, methodology, investigation, writing – original draft. Lingkai Wen: writing – review & editing. Shihao Zhang: resources. Jian Cheng: validation, visualization, and formal analysis. Xiaohui Chen: validation. Yanyu Zhou: formal analysis. Zhikai Xu: writing – review & editing. Baoping Xin: writing – review & editing and supervision.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.