Separation and Recovery of Cu from Industrial Dust via a Solvometallurgical Process

Abstract

In this study, a solvometallurgical process for the recovery of Cu from industrial dust by leaching with a lixiviant comprising hydrochloric acid (HCl) and ethylene glycol followed by extraction with trioctylphosphine oxide (TOPO) was developed. Copper, Ni, Mg, and Al contained in the dust were efficiently dissolved by using HCl in ethylene glycol, leaving most of the Ag in the residue. The parameters (concentration of the lixivant, reaction temperature, pulp density) that affect leaching efficiency were extensively investigated and optimized. TOPO was used to selectively extract Cu over other metal ions (Ni, Mg, Al, and Ag) from the obtained leachate. The Cu extraction mechanism was investigated by using the slope method, and 0.1 mol/L sulfuric acid was used to strip Cu from the Cu-loaded TOPO. McCabe–Thiele plots for Cu extraction and stripping were constructed to determine the number of counter-current stages along with the volumetric flow ratio of the two phases. Counter-current Cu extraction and stripping simulation tests were carried out to confirm its feasibility. Finally, a flow diagram of the proposed process for separation and recovery of Cu from industrial dust is provided.

1. Introduction

Copper plays an important role in both daily life and industry, such as being utilized in the manufacture of electrical materials and telecommunication devices, construction, power supply, transportation, petroleum refining, and electroplating [1,2,3,4,5]. However, Cu is toxic at high concentrations, so its presence in the environment should be controlled. Industrial dust that arises during the pyrometallurgy processing of spent camera modules contains Cu and some other valuable metals such as Ag, Ni, Mg, and Al. Thus, separation and recovery of Cu from this industrial dust is highly desirable, especially when considering the shortage of this material resource, environmental contamination, and the decline in the cost of waste treatment [1].

Pyrometallurgical and hydrometallurgical methods have been widely used for Cu recovery from both natural ores and secondary resources [6]. Whereas using pyrometallurgy is unattractive due to its high cost and energy consumption, hydrometallurgy is both highly efficient and straightforward, albeit the requirement for large amounts of chemicals and the generation of a lot of wastewater are two drawbacks that must be considered. On the other hand, solvometallurgical processes, including solvoleaching, non-aqueous solvent extraction, and non-aqueous electrodeposition, for recovering metals have attracted much interest. Producing large amounts of wastewater is mitigated by using polar solvents such as ethylene glycol, acetone, or methanol instead, and in some cases, significant improvements in leaching and extraction efficiency have been obtained [6,7]. Among them, ethylene glycol is considered to be the greenest, with desirable characteristics such as non-volatility, low flammability, and low toxicity. Ethylene glycol has typically been adopted as the organic solvent of choice in solvometallurgical processes [6,8,9]. Moreover, it can be generated from various catalytic and non-catalytic chemical systems and renewable resources such as cellulose [10].

Some of the reported solvometallurgical processes for the recovery/separation of Cu from various resources are summarized in

Table 1. A summary of previously reported Cu separation/recovery methods from various resources by using solvometallurgy.

Material	Lixivant/Extractant	Mark	Ref
Chalcopyrite	Iron(III) chloride (FeCl3) + ethylene glycol	Cu(I) was dissolved in a mixture of (FeCl3) and ethylene glycol and metallic Cu was obtained from the lixiviant via electrodeposition.	[6]
E-waste	Hydrogen sulfate [HSO4−] ionic liquid + H2O2	[HSO4−] ionic liquid as the solvent was used with H2O2 for Cu leaching from e-waste.Analysis of variance was used to investigate the significance of the factors affecting leaching.	[11]
Chalcopyrite	Ionic liquids based on imidazolium and ethyl ammonium cations and hydrogen sulfate, nitrate, acetate, or dicyanamide anions	The liquid, solid, and gas phases of the foremost systems were studied.	[12]
Chalcopyrite	1-Butyl-3-methyl-imidazolium [bmin]-[HSO4−] ionic liquid	Compared with leaching in acidic aqueous sulfate solutions, the pure ionic liquid and its aqueous solutions provided easier leaching of chalcopyrite at higher temperatures.	[13]
Chalcopyrite	Imidazolium- and ammonium-based ionic liquids	Ionic liquid-based lixiviants with more polar cations ([NH4+], K+, or ([C1Him]+) appear to be better for extraction than those with less polar cations ([C2C1im]+ or [C4C1im]+). However, none of them performed better than aqueous H2SO4 when corrected for the difference in PH.	[14]
Chalcopyrite	Acetone; ethylene glycol, 2-propanol, methanol, H2O2 + H2SO4	The use of organic solvents favors the oxidative leaching of chalcopyrite. However, solvent mineralization and extensive H2O2 consumption were detected in the case of acetone.	[15]
Chalcopyrite	Ethylene glycol + H2O2	Chalcopyrite dissolution kinetics in the presence of ethylene glycol are described by using the reaction-controlled shrinking particle model.	[16]
Chalcopyrite	Ethylene glycol + H2SO4 + H2O2	Ethylene glycol + H2SO4 did not increase chalcopyrite oxidation.Improved chalcopyrite dissolution could be due to a decrease in interfacial tension.	[17]
A Co/Ni/Cu Alloy	HCl + ethylene glycol/Aliquat336 in kerosene	62.3% Co2+ and 18.3% Cu2+ were loaded onto Aliquat336 and then stripped by using 2 mol/L H2SO4.Cu was separated by extraction with Cyanex 301 followed by stripping with 50% (v/v) aqua regia.	[18]

[6,11,12,13,14,15,16,17,18]. The Cu leaching efficiency from chalcopyrite ore has been enhanced by using ethylene glycol as the polar solvent in a lixiviant containing sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) as an oxidant [15,16,17]. The dissolution kinetics in the presence of ethylene glycol have been elucidated by using the reaction-controlled shrinking particle model [16]. The role of ethylene glycol in the solvoleaching of Cu and improving chalcopyrite ore dissolution could be due to a decrease in interfacial tension [17]. When ethylene glycol was used in place of water in leaching of metals (Co, Ni and Cu), the conversion from Co(H₂O)₆²⁺ to CoCl₄²⁻ was preferred to an aqueous solution due to a decrease in water activity. The stabilized metal complex led to an improvement in leaching efficiency [18]. In non-aqueous solvent extraction, the presence of ethylene glycol removes the necessity of using water and results in increased extraction of metal ions in the extractant [18,19]. However, the mechanism involved in non-aqueous solvent extraction has so far remained elusive.

In this study, a solvometallurgical process for recovering Cu from industrial dust produced during the pyrometallurgy of spent camera modules was developed. In solvoleaching experiments, hydrogen chloride (HCl) in ethylene glycol was used as the lixiviant to dissolve Cu from the industrial dust. Optimal leaching conditions were identified by investigating the parameters that affect leaching efficiency. Subsequently, to recover Cu from the obtained leachate, the mechanism of non-aqueous solvent extraction of Cu using trioctylphosphine oxide (TOPO) was investigated by using the slope method. H₂SO₄ was used to strip Cu from the Cu-loaded TOPO. The effect of the phase ratio on extraction and stripping was also investigated. Moreover, the feasibility of the proposed process was verified by performing batch simulation counter-current extraction and stripping experiments.

2. Materials and Methods

2.1. Materials

Industrial dust generated during the pyrometallurgy of spent camera modules was received from a Korean refinery (Figure 1). The powder was homogenized by thorough mixing to produce a size distribution lower than 0.1 mm. Samples were kept in a desiccator before use. The chemical composition of the dust was determined by using energy dispersive X-ray (EDX) and inductively coupled plasma-optical emission spectrometry (ICP-OES; Optima 8300, PerkinElmer, Waltham, MA, USA) analyses. The results are given in Figure 2 and

Table 2. Composition of the industrial dust used in the study.

Ag	Cu	Ni	Mg	Al
4.08	15.6	5.1	0.27	0.96

Unit: wt%.

, respectively.

2.2. The Leaching Procedure

Lixiviants were prepared by diluting the required amount of HCl (35%; Daejung, Korea) in ethylene glycol (99.5%; Daejung, Korea). Leaching experiments were carried out in a glass hermetic container (500 mL) under magnetic agitation of 300 rpm. The temperature was controlled by using a heating mantle (MS-DM605, Misung, Daejeon, Korea).

In the leaching process, 200 mL of lixiviant was first placed in a glass hermetic container, to which 2 g of powdered dust was added when the temperature of the lixiviant reached 60 °C (except for when investigating the effect of reaction temperature). After 2 h, the residue was immediately separated from the leaching solution by using vacuum filtration. These optimal experimental conditions were obtained from several preliminary experiments. The concentrations of metals in the leachate were measured by using ICP-OES after serial dilution with 3% nitric acid (HNO₃) solution. Each leaching experiment was carried out three times, and the leaching percentage of metals varied within ±3%.

2.3. Extraction and Stripping

The required quantities of copper(II) chloride CuCl₂ (95%; Daejung), nickel(II) chloride (NiCl₂·6H₂O) (96%; Junsei), magnesium dichloride (MgCl₂·6H₂O) (97%; Junsei), and aluminum trichloride (AlCl₃·6H₂O) (97%; Daejung) were dissolved in the desired volume of ethylene glycol (99.5%, Daejung) to prepare the synthetic leachate. Hydrochloric acid (HCl) (35%; Daejung) was used to adjust the acidity of the solution. Hydrochloric acid (HCl) (35%; Daejung) and H₂SO₄(95%; Daejung) dissolved in distilled water were used to prepare the stripping solution.

The extractant, TOPO (Cognis), was used without further purification. Kerosene (90% distillation at 265 °C; Daejung) was used as the diluent. 15%(v/v) 1-Octanol (98%; Junsei) was employed to prevent the formation of a third phase during the extraction process.

In batch extraction experiments, equal volumes (20 mL) of leachate and extractant were mixed in a sealed bottle (100 mL) and shaken for 30 min using a wrist-action shaker (Model 75, Burrell, Chicago, IL, USA) at ambient temperature (24 ± 1 °C). Afterward, the two phases were separated by using a separation funnel. Stripping experiments were performed through the same procedure by mixing the loaded extractant and stripping solution. Subsequently, the concentrations of metal ions in the ethylene glycol and stripping solution were measured through ICP-OES after being diluted with 3% HNO₃ solution, after which the concentrations of metal ions in the extractant were determined by using mass balance.

3. Results and Discussions

3.1. Leaching of Metals from the Industrial Dust

3.1.1. The Effect of HCl Concentration on the Leaching Efficiency of Metal Ions

The effect of HCl concentration on the leaching efficiency of metal ions was investigated by varying the concentration of HCl in ethylene glycol from 0.1 to 4 mol/L. The experiments were carried out at 60 °C for 2 h while the pulp density (the ratio of solid to liquid) was fixed at 10 g/L.

Over the entire studied HCl concentration range, Figure 3 shows that the leaching efficiency of Ni from the dust was higher than 99.99% while that of Ag was less than 2%. The dissolution of Mg and Al was evidently affected by the concentration of HCl in the lixiviant. The leaching percentage of Cu increased with increasing concentration of HCl from 0.1 to 3 mol/L and then remained constant. When the concentration of HCl in ethylene glycol was higher than 3 mol/L, most of the metals (except for Ag) were completely dissolved. In our previous study [20], we reported that Ag in the industrial dust was easily recovered by leaching with HNO₃. For convenience in the subsequent procedure, 3 mol/L of HCl was selected for the leaching experiments.

3.1.2. The Effect of Reaction Temperature on the Leaching Efficiency of Metal Ions

The effect of the reaction temperature was investigated by varying the reaction temperature from 30 to 70 °C. The concentration of HCl in the lixiviant and the pulp density were fixed at 3 mol/L and 10 g/L, respectively. The values of the other variables were as reported in Section 2.

Figure 4 reveals that the leaching percentages of most of the metal ions (except for Ag) increased slightly when the temperature was increased from 30 to 60 °C. However, that of Ag did not change (<2%). Complete dissolution of Cu, Al, Mg, and Ni occurred at a reaction temperature of 60 °C and increasing the temperature further did not affect the leaching efficiency. Thus, to completely retrieve Cu from the industrial dust, 60 °C was selected as the reaction temperature in the subsequent experiments.

3.1.3. The Effect of Pulp Density on the Leaching Efficiency of Metal Ions

The effect of pulp density on the dissolution of Cu from the industrial dust was investigated by varying the pulp density from 5 to 40 g/L. The concentration of HCl in the lixiviant and the reaction temperature were controlled to 3 mol/L and 60 °C, respectively, while the other variables were as reported in Section 2.

According to the results in Figure 5, the leaching percentage of Ag remained lower than 2% while those of other metal ions (Cu, Ni, Mg, and Al) decreased with increasing pulp density. The increasing molar ratios of metal ions to acid in the lixiviant and a high pulp density resulted in a decrease in leaching efficiency. When the pulp density was lower than 10 g/L, most of the Cu, Ni, Mg, and Al along with 1.67% of Ag became dissolved, leaving most of the Ag in the residue.

Based on the above leaching experimental results, the optimal conditions for the dissolution of Cu from the industrial dust are 3 mol/L of HCl in ethylene glycol as the lixiviant with a pulp density of 10 g/L and a reaction temperature of 60 °C. In the obtained leachate, the concentration of Cu, Ni, Mg, Al, and Ag was 0.024, 0.007, 0.001, 0.003, and 6.31 × 10⁻⁵ mol/L, respectively.

3.2. Separation and Recovery of Cu from the Leachate

3.2.1. The Effect of HCl Concentration on the Extraction of Metal Ions

To investigate the effect of HCl concentration on extraction of metal ions, the concentration of HCl in the synthetic leachate was varied from 0.5 to 5 mol/L, the concentrations of metal ions were kept the same as those in the real leachate, the concentration of TOPO was maintained at 0.5 mol/L, and the volume ratio of leachate to TOPO was fixed at 1:1.

Figure 6 shows the effect of HCl concentration in the feed solution on the extraction percentages of metal ions. TOPO selectively extracted Cu over the other metal ions (Ni, Mg, Al, and Ag) over the whole studied HCl concentration range. The extraction percentage of Cu increased slightly with increasing concentration of HCl in the feed solution from 0.5 to 4 mol/L. When the HCl concentration was higher than 3mol/L, the extraction percentage of Cu was higher than 70% while those of the other metals were negligible, thereby indicating the effective separation of Cu from the other metals.

To the best of our knowledge, the mechanism of the non-aqueous solvent extraction of Cu with TOPO has not previously been reported, and thus, it was further investigated by using the slope method. The results for Cu extraction for a concentration range of HCl from 2 to 5 mol/L are presented as a plot of logD versus log[H⁺] in Figure 7; the slope is 2.177, which indicates the presence of two hydrogen ions in the extracted Cu complex.

3.2.2. The Effect of TOPO Concentration on the Extraction of Metal Ions

Effect of TOPO concentration on the extraction of metals was investigated by varying the TOPO concentration in the extractant from 0.1 to 1 mol/L. The concentration of HCl in the feed solution was maintained at 3 mol/L, and the metal ion concentrations were the same as those in the leachate (Section 3.1.3).

The results indicate that the extraction percentage of Cu increased sharply from 2.4 to 85.1% while leaving the other metal ions in the raffinate by increasing the concentration of TOPO from 0.1 to 1 mol/L. The extraction percentages of the other metal ions were ~0% and did not change with varying of the TOPO concentration. The results of extracting Cu with TOPO are presented as a logD versus log[TOPO] plot in Figure 8; the slope is 2.4046 which suggests that two TOPO molecules are associated with the extracted Cu species.

3.2.3. The Effect of Chloride Ion (Cl⁻) Concentration on the Extraction of Metal Ions

To investigate the dependence of Cu extraction on chloride ions, the Cl⁻ concentration was varied by preparing various concentrations of NaCl (0.02 to 0.1 mol/L) with the same HCl concentration (3 mol/L) in the feed solution. With 1 mol/L TOPO, the extraction percentage of Cu increased slightly from 82.48 to 84.56% when increasing the Cl⁻ concentration from 0.02 to 0.1 mol/L. This proves the presence of Cl⁻ in the extracted Cu complex. As before, the extraction percentages of the other metal ions did not change (~0%). The results of extraction of Cu with TOPO in the presence of varying Cl⁻ concentration are presented as a plot of logD versus log[Cl⁻] in Figure 9; the slope of the linear plot is 4.0348, which indicates four Cl⁻ in the extracted Cu species.

In conclusion, the extraction mechanism could be as follows:

$${\mathrm{Cu}}^{2+}+{2\mathrm{H}}^{+}+{4\mathrm{Cl}}^{-}+2\mathrm{TOPO}\iff {\mathrm{H}}_2{\mathrm{CuCl}}_4\cdot 2\mathrm{TOPO}$$

(1)

This result is similar to previously reported Cu extraction with a neutral extractant (a mixture of trialkyl phosphine oxides, Cyanex923) from HCl aqueous solutions [21].

3.2.4. The Effect of Varying the Phase Ratio on Cu Extraction

To determine the theoretical number of stages needed for the complete extraction of Cu from the leachate, extraction experiments with 1 mol/L TOPO while varying the volume ratio of the extractant (TOPO) to the leachate from 1:2 to 5:1 were conducted. The results are presented as a McCabe–Thiele plot for the extraction of Cu in Figure 10; the extraction efficiencies of the other metal ions were ~0% and so are not included in the figure. The results suggest that two stages are needed to completely extract Cu from the leachate with 1 mol/L of TOPO at an extractant (TOPO) to leachate ratio of 1:1.

A two-stage counter-current batch simulation test was carried out at an extractant (TOPO) to leachate volume ratio of 1:1. The concentration of Cu in the raffinate after two extraction stages was <0.01 mg/L, corresponding to an extraction efficiency of 99.999%. The concentrations of the other metal ions (Ni, Mg, Al, and Ag) in the leachate were not changed by the extraction process.

3.2.5. Stripping the Cu from the Loaded TOPO

Experiments for stripping of Cu from the loaded TOPO were performed using various concentration of HCl and H₂SO₄ solutions. Synthetic leachate with 3 mol/L HCl in ethylene glycol was used as the feed solution. Copper-loaded TOPO was prepared by mixing the leachate and 1 mol/L TOPO at the unit phase ratio. The results in Figure 11 show that the Cu stripping percentage decreased slightly with increasing HCl/H₂SO₄ concentration from 0.1 to 0.5 mol/L. Copper stripping of 76.6% was obtained by using 0.1 mol/L H₂SO₄, which was selected for subsequent stripping experiments.

3.2.6. The Effect of Varying the Phase Ratio on Cu Stripping

Effect of phase ratio on the stripping of Cu was investigated by varying the volume ratio of H₂SO₄ to Cu-loaded TOPO from 5:1 to 1:3. A two-stage counter-current extraction process at the unit phase ratio was performed to prepare the Cu-loaded TOPO. Synthetic leachate with 3 mol/L HCl in ethylene glycol was used as the feed solution, and 1 mol/L TOPO was used as the extractant. The concentration of Cu in the Cu-loaded TOPO was 0.024 mol/L, and accordingly, 0.1 mol/L H₂SO₄ was used as the stripping solution.

The stripping percentage of Cu increased from 38.7% to 97.5% with an increase in the volume ratio of H₂SO₄ to Cu-loaded TOPO from 1:3 to 5:1, which is presented as a McCabe–Thiele plot in Figure 12; the extraction efficiencies of the other metal ions were ~0%, and so they are not included in the figure. The results indicate that two stages are required for the quantitative stripping of Cu from Cu-loaded TOPO using 0.1 mol/L H₂SO₄ at an H₂SO₄ to Cu-loaded TOPO ratio of 3:1.

A two-stage counter-current batch simulation stripping experiment was carried out to verify the results obtained from the McCabe–Thiele diagram in Figure 12, in which 99.96% of Cu was stripped after two-stage counter-current simulation stripping. The concentration of Cu in the obtained stripping solution was 7.999 × 10⁻³ mol/L.

4. Conclusions

A solvometallurgical process was developed to separate and recover Cu from industrial dust generated from the pyrometallurgy processing of spent camera modules. Optimal leaching conditions were obtained. 100% Cu, Ni, Mg, and Al and 1.76% Ag were dissolved with 3 mol/L HCl in ethylene glycol as the lixiviant with a pulp density of 10 g/L and a reaction temperature of 60 °C.

Copper was selectively extracted from the leachate by using TOPO as an extractant while leaving Ni, Mg, Al, and Ag in the raffinate. The extraction mechanism was investigated by using the slope method. The extracted species was concluded as H₂CuCl₄∙2TOPO. 0.1 mol/L H₂SO₄ could strip Cu from Cu-loaded TOPO. From McCabe–Thiele plots for Cu extraction and stripping, we predicted that complete extraction of Cu from the leaching solution could be obtained with 1 mol/L TOPO at the unit ratio of TOPO to leachate in two stages, while complete stripping of Cu from Cu-loaded TOPO could be obtained with 0.1 mol/L H₂SO₄ at an H₂SO₄ to Cu-loaded TOPO ratio of 3:1 in two stages. The integrated leaching and extraction process for the separation and recovery of Cu are presented in Figure 13.