Introduction

According to the World Copper Factbook 2023, the world's refined copper production reached 25.7 million tons in 2022, of which 80.4% was through copper electrolytic refining routes [1]. Copper anode slime is an inescapable by-product produced during the copper electrolytic refining process, accounting for approximately 0.2–0.8% of refined copper production [2]. Copper anode slime contains scattered and precious metals (SPMs) such as Au, Ag, Pt, Pd, Se, Te, and various base metals such as Pb, As, Sb, Bi, Sn, Cu, etc. [3, 4]. SPMs are not only critical materials for modern industry, national defense, and strategic emerging industries but also indispensable resources for national socioeconomic development and national defense construction [5,6,7,8,9]. In China, over 40% of Au, Ag, Se, and Te come from copper anode slime [10]. Globally, the demand for SPMs is growing faster than the demand for other metals [11].

Recently, various technologies have been developed to separate SPMs from base metals in copper anode slime. Liu [2] gradually separated base metals such as Cu, Ni, As, Pb, and Sn, with Ag and Au enriched in the residue. Khanlarian [12] achieved efficient and low-consumption leaching of Se, Cu, and Ag using a low-temperature mineral phase reconstruction method. In addition, research on wet craftsmanship, such as oxide acid immersion [13], oxygenic acid immersion [14], and mineral selection [15], is increasingly prosperous. Wet chlorination enables the formation of soluble base metal components (like As, Sb, and Bi) chlorides in copper anode slime, which are detrimental to copper ore [16]. As in the process of tellurium extraction, the impurity influencing factors are Se > Sb > As > Cu [17]. Meanwhile, in the extraction of Au from chlorine solution, Pb2+ will deteriorate the reduction efficiency of Au [18]. In summary, major existing SPMs processes aim to recycle a single metal. Generally, problems such as uneven metal types, low leaching efficiency, or narrow application range of raw materials are the limitations, as difficulty lies in the significant interference of base metals such as, Sb, and Bi on the separation and extraction of SPMs.

Our previous studies [10, 19] have clarified the selective separation and precipitation behavior of As, Sb, and Bi and revealed the selective separation and volatilization kinetics mechanism of Se. Regrettably, the occurrence and leaching behavior of base metals (such as Pb and Sn) and platinum group metals (Pt and Pd) was not studied. Herein, the objective of this study is to systematically investigate the leaching behavior of the main elements, achieve targeted enrichment of the Au, Te, Pt, and Pd, and form a short process technology for efficient separation of the SPMs from copper anode slime.

Experimental

Experimental Materials

The experimental raw material is the decopperized anode slime from a copper smelting enterprise in southern China. This enterprise includes directional removal of As, Sb, and Bi by composite acid leaching, recovery of Se by sulfuric acid roasting, and separation of Cu by washing the roasted slag with water. The washed residue is the decopperized anode slime. Particle size analysis of the decopperized anode slime was performed using a laser particle size analyzer (Mastersizer3000). The parameters and results of the laser particle size analysis are presented in Table 1, and the particle size distribution curve is shown in Fig. 1. The particle size of 100% of the decopperized anode slime is below 74 μm. The particle size distribution curve shows a normal distribution, which is favorable for the sampling uniformity.

Table 1 Particle size analysis parameters and the results of the decopperized anode slime
Full size table
Fig. 1
figure 1

Particle size distribution curve of the decopperized anode slime

Full size image

The chemical compositions of the decopperized anode slime are presented in Table 2. The copper anode slime contains 8.91% Ag, 0.60% Au, 4.81% Te, 45.05 g/t Pt and 182.85 g/t Pd, whereas the Ba, Pb, Sn, Sb, As, Bi, Cu, and Se contents are 22.14%, 18.93%, 7.18%, 2.03%, 0.54%, 0.41%, and 0.09%, respectively, indicating that this decopperized anode slime has an urgent recovery value. From the X-ray diffraction pattern of the raw materials, several diffraction peaks are related to SnO2, AgCl, BaSO4, (Ba0.69Pb0.31)SO4, and TeO2, as shown in Fig. 2. Because of the low content of Sb, Au, Pd, and so on, no diffraction peaks of each phase were observed.

Table 2 Main chemical components of the decopperized anode slime
Full size table
Fig. 2
figure 2

X-ray diffraction pattern of the decopperized anode slime

Full size image

Figure 3 illustrates the distribution maps of the main elements present in the raw materials. There are irregular large particles, rod-shaped particles, amorphous flocculent small particles, and bright spots, as shown in Fig. 3. Based on the distribution of elements, irregular large particles consist of a core layer and a shell layer, in which the shell layer was (Ba, Pb)SO4 and the core phase was BaSO4. The rod-shaped particles are tin oxide. Silver was distributed in flocculent small particles, with a small portion embedded in irregular large particles. Arsenic coexisted with Ag and O. Antimony was present in addition to Ag with O and can also be present together with Sn. Bismuth was present with the (Ba, Pb)SO4 shell layer. The occurrence of partial tellurium is like that of Sb and partial tellurium had a highly overlapping distribution of Ag and Au, respectively. This indicates that tellurium oxide, silver telluride, and gold telluride were presented in the raw materials. Copper was embedded in tin oxide. There was no enrichment of selenium or platinum. Bright spots consisted of Au and Pd. To determine the exact composition of the decopperized anode slime, several points (as shown in Fig. 4) were selected for quantitative analysis using EPMA. Table 3 presents the results.

Fig. 3
figure 3

EPMA maps of the main metals in the decopperized anode slime

Full size image
Fig. 4
figure 4

BSE maps and analysis points for the decopperized anode slime, X and Y represent the horizontal position of the electron probe lens, and D represents the height of the lens

Full size image
Table 3 Element content for the selected points in Fig. 4 quantitatively determined by EPMA (at%)
Full size table

According to the quantitative data in Table 3 and the distribution of elements in Fig. 3, the point 1 particle was identified as Ag2Te, AgCl, Ag2SO4, TeO2, Te, (BaxPby)SO4, SnO2, CuO, and a few As2O3, Bi2O3, Pt, or PtO2. The floc at point 2 was identified as AgCl, Ag2SO4, TeO2, (BaxPby)SO4, SnO2, CuO, As2O3, Bi2O3, Pt, or PtO2. The bright spot at point 3 was identified as Au, Pt, Pd, and Ag. The presence of (BaxPby)SO4 is due to the selection area encompassing a small portion of the dark matrix, as shown in Fig. 4a. The fine particle in point 4 was mainly composed of AgCl, Ag2SO4, Ag2O, TeO2, SnO2, CuO, and a small amount of As2O3, Bi2O3, Au, and Pt. Point 5 particle mainly contained (BaxPby)SO4, AgCl, TeO2, Ag2SO4, and a few Au. Point 6 was identified as SnO2. In addition to Cu, SnO2, Cu2Te, and Sn, there were also small amounts of (BaxPby)SO4, Ag2Te, AgCl, and Bi2O3 in the blocky particles of point 7. Point 8 particle comprised AgCl, TeO2, Sb2O3, Ag2O, CuSO4, (BaxPby)SO4, and small amounts of As2O3, Bi2O3, Pt, Pd, or PtO2 and PdO. It was indicated that part of the Ag was wrapped in BaSO4 core phase and that a small amount of Sb2O3, CuO, As2O3, Bi2O3, Pt, Pd, Ag, and Ag2Te were mixed in the (BaxPby)SO4 shell phase from points 9 and 10. Point 11 particle was composed of AgCl, Ag2Te, Ag2SO4, Sn, SnO2, TeO2, (BaxPby)SO4, and small amounts of CuO, As2O3, Bi2O3, and PtO2. Point 12 was mainly composed of Au, AgCl, (BaxPby)SO4, AuTe2, Ag2SO4, and a small amount of Cu, SeO2, Pt, Pd, As2O3, and Bi2O3. Compared with points 3 and 12, point 13 can better explain the enrichment phenomenon of SPMs, which mainly consisted of Au, Pd, and Ag2Se. The occurrence state and embedding characteristics of the main metal element in the decopperized anode slime are summarized in Table 4. To verify the EPMA analysis result, chemical analysis was used to identify the mineral phases of Cu, Ag, and Te in the decopperized anode slime, as shown in Table 5.

Table 4 The occurrence state and embedding characteristics of the main metal element in the decopperized anode slime
Full size table
Table 5 Mineral phase composition of decopperized anode slime
Full size table

Summarily, tellurium was identified as five occurrence states: Ag2Te, TeO2, Te, AuTe2, and Cu2Te. In addition to Ag2Te, silver also was identified as AgCl, Ag2SO4, Ag2O, Ag, and Ag2Se. Gold was identified as two types of objects: Au and AuTe2. Most platinum and palladium were present in the metallic phase, but there may still be a small amount of their oxides coexisting with Ag-enrichment. Interestingly, most SPMs, such as Te, Au, Pt, and Pd were distributed in active regions, such as the surface of particles with a large specific surface area and alloy phase. On the contrary, base metals, such as Pb, Ba, and Sn, were distributed in stable particles, such as sulfate blocks and rod-shaped particles.

Experimental Principle and Method

According to the phase analysis results, Table 6 lists the possible reactions of the existing phases during chlorination leaching. The Gibbs free energy changes of reactions (1)–(17) were calculated using the Reaction module in Factsage 8.1 [20] and plotted as a ∆Grθ−T diagram, as shown in Fig. 5. Thermodynamic data are obtained from the FactPS database and the aqueous solutions [21, 22]. As shown in Fig. 5, the ∆Grθ of reactions (1)–(17) are all negative, indicating that the SPMs phases will undergo reactions during the chlorination leaching process. Furthermore, tellurium, gold, platinum, and palladium will present in the solution in the form of TeCl62−, AuCl4−, PtCl62−, and PdCl42−, respectively, while silver will exist in the residue in the form of AgCl.

Table 6 The possible reaction during the chlorination leaching of the decopperized anode slime
Full size table
Fig. 5
figure 5

The Gibbs free energy variation curve of the possible reaction during the chlorination leaching process of the decopperized anode slime with temperature

Full size image

The specified concentration of the mixture of H2SO4 and NaCl was added to the beaker, a certain mass of the decopperized anode slime and NaClO3 were added and stirred well, then transferred to a heating magnetic stirrer (DF-101S) and set to a temperature ranging from 5 °C to 95 °C, with stirring at 350 rpm. During leaching, the beaker was sealed with plastic wrap to reduce the evaporation of the solution.

Analytical Method

The main phases were identified by X-ray diffraction (XRD, PANalytical X’Pert Pro Powder, Almelo, the Netherlands) using a CuKα radiation source with an acceleration potential of 40 kV and a current of 40 mA. XRD diffractograms were analyzed using HighScore Plus software. The concentrations of Pb, Sn, Ag, Sb, Se, Cu, Bi, Te, Au, Pt, and Pd were determined by inductively coupled plasma optical emission spectroscopy (ICP-AES, Thermo Electron IRIS Intrepid II XSP). The leaching rate was calculated using the following formula:

$$\eta =\frac{\rho \cdot V}{m\cdot w\cdot 1000}\times 100\%$$

where η (%) represents the leaching rate; ρ (μg/mL) and m (g) are the concentration of various elements in the leaching solution and the quantity of the decopperized anode slime, respectively; and V (mL) and w (%) are the leaching solution volume and the content of the corresponding elements in the decopperized anode slime before leaching, respectively.

The phase composition of the main metals in the decopperized anode slime was determined using JXA 8200 EPMA with wavelength dispersion. When analyzing the element surface distribution, EPMA worked at an accelerating voltage of 20 kV and a probe current of 50 nA. When quantitatively analyzing the content of the elements, EPMA worked at an accelerating voltage of 20 kV and a probe current of 10 nA. The ZAF correction procedure was applied. BaSO4 was the standard for Ba, S, and O, while metallic substances were the standards for Pb, Sn, Ag, Sb, Se, Cu, Bi, Te, Au, Pt, and Pd. GeAs and Na4AlBeSi4O12Cl (Tugtupite) were used as the standard for As and Cl, respectively. X-ray fluorescence spectrometry (Philips PW2400) was used to determine the chemical composition of the leaching residue.

Result and Discussion

The Effect of Leaching Time

The curve of the leaching rate over time is shown in Fig. 6. The leaching efficiencies of Au, Pt, Pd, and Se reached equilibrium earlier due to differences in occurrence states. It can also be seen from Fig. 5 that the ∆Grθ of reactions (5) to (17) were relatively more negative. This indicates that the substances of the metallic phase will react preferentially. The leaching efficiencies of platinum and palladium remained stable at 80% and 90% with increasing reaction time, respectively, because PtO2 and PdO were encapsulated by Ag-enrichment. In addition, the maximum leaching rate of Bi remained stable at approximately 57% with increasing leaching time. The reasons are as follows: Bi2O3 was encapsulated by (BaxPby)SO4; Bi3+ can form an insoluble substance such as Bi(OH)SO4 [23] in SO42− medium, and the reaction is as follows:

Fig. 6
figure 6

Effect of leaching time on the leaching rate of each element (T = 80 °C , L/S = 4 mL/g, H2SO4 = 3 mol/L, NaCl = 100 g/L, NaClO3 = 0.2 g)

Full size image
$${{\text{Bi}}}^{3+}+{{\text{SO}}}_{4}^{2-}+{{\text{H}}}_{2}{\text{O}}={\text{Bi}}\left({\text{OH}}\right){{\text{SO}}}_{4}+{{\text{H}}}^{+}$$
(18)

With the extension of the leaching time, the leaching efficiencies of Cu, As, and Sb increased gently, whereas those of Sn, Pb, and Ag appeared unaffected. Comprehensively, the leaching time was selected to be 60 min.

The Effect of Liquid–Solid Ratio

As an integral factor in the leaching process, the importance of the liquid–solid ratio is undeniable. In this study, the leaching efficiencies of the main metals were observed in the liquid–solid ratio of 1 mL/g to 8 mL/g, and the results are shown in Fig. 7. Notably, after the solid–liquid separation process, it was found that there were still insoluble substances of AgCl and PbSO4 in the filtrate, as shown in Fig. 8. Due to the ppm concentration of lead and silver ions in the solution and the high concentrations of SO42− and Cl−, which formed extremely fine particles of lead sulfate and silver chloride. The leaching efficiencies of Se, Te, Cu, and Bi were less affected by the liquid–solid ratio when excessive leaching reagents were used. In comparison, with an increase in the liquid–solid ratio, the leaching efficiencies of Pb and Ag rose, while those of Au, Pt, and Pd decreased. The possible reasons are as follows: first, as the liquid content increased, the Pb and Ag phases came into fuller contact with the leaching reagent, generating AgCl and PbSO4, which enhanced the encapsulation of Au, Pt, and Pd; second, the decrease in NaClO3 concentration led to a decline in the leaching efficiencies of Au, Pt, and Pd. To exist stably in chlorinated solution, Au, Pt, and Pd need to be oxidized to high valence states of AuCl4− [18, 24, 25], PtCl62− [18, 25], and PdCl42− [24], respectively. The reduction of the liquid–solid ratio can be favored for the subsequent enrichment of SPMs and wastewater treatment. Comprehensively, the liquid–solid ratio was selected to be 1 mL/g.

Fig. 7
figure 7

Effect of L/S on the leaching rate of each element (T = 80 °C , Time = 60 min, H2SO4 = 3 mol/L, NaCl = 100 g/L, NaClO3 = 0.2 g)

Full size image
Fig. 8
figure 8

The XRD pattern of insoluble substances in leaching solution

Full size image

The Effect of H2SO4 Concentration

Figure 9 shows the effect of H2SO4 concentration on the leaching of Pb, Sn, Ag, Sb, Se, Cu, Bi, Te, Au, Pt, and Pd. It is worth noting that Ag will exist in the solution as AgHSO4 [26] or AgCln+1n− [21] in the case of excessive H2SO4 and low acidity, which led to an increase in Ag leaching efficiency with the increase in H2SO4 concentration.

Fig. 9
figure 9

Effect of sulfuric acid concentration on the leaching rate of each element (T = 80 °C , Time = 60 min, NaCl = 100 g/L, L/S = 1 mL/g, NaClO3 = 0.2 g)

Full size image
$${{\text{Ag}}}_{2}{{\text{SO}}}_{4}+{{\text{H}}}_{2}{{\text{SO}}}_{4}=2{{\text{AgHSO}}}_{4}$$
(19)
$${\text{AgCl}}+{{\text{nCl}}}^{-}={{\text{AgCl}}}_{{\text{n}}+1}^{{\text{n}}-}$$
(20)

Interestingly, the leaching efficiencies of Cu and Se were 35.08% and 18.43%, respectively, at 0 mol/L H2SO4, confirming the presence of water-soluble phases such as CuSO4 and SeO2. The leaching efficiencies of Au, Pt, Pd, Te, and Se significantly increased with the increase of H2SO4 concentration, and all tended to be stable after the H2SO4 concentration was ≥ 1 mol/L. When the concentration of H2SO4 concentration increased from 1 mol/L to 2 mol/L, the leaching efficiencies of As, Sb, and Bi increased from 18.22%, 0.93%, and 51.19% to 41.83%, 1.61%, and 56.31% respectively. Both As, Sb, and Bi are the main impurities in high-purity tellurium [17]. Comprehensively, the H2SO4 concentration was chosen to be 1 moL/L.

The Effect of NaCl Concentration

The leaching behaviors of Pb, Sn, Ag, Sb, Se, Cu, Bi, Te, Au, Pt, and Pd were studied in the NaCl concentration range of 0–200 g/L, as shown in Fig. 10. When the NaCl concentration increased from 0 g/L to 100 g/L, the leaching efficiencies of Te, Au, Pt, and Pd increased significantly, from 27.41 to 93.80%, from 0.29 to 99.10%, from 7.84 to 95.11% and from 4.39 to 99.62%, respectively. The cause of this phenomenon is that Te, Au, Pt, and Pd can complex chloride ions to form multi-coordination compounds in an acidic system. When the NaCl concentration increased from 60 g/L to 80 g/L, Bi leaching efficiency enhanced obviously. It can be explained that Bi2O3 was soluble in hydrochloric acid [27, 28], and may reacted by following the formula in the H2SO4 + NaCl system,

Fig. 10
figure 10

Effect of NaCl concentration on the leaching rate of each element (T = 80 °C , time = 60 min, H2SO4 = 1 mol/L, L/S = 1 mL/g, NaClO3 = 0.2 g)

Full size image
$$B{{\text{i}}}_{2}{{\text{O}}}_{3}+{{\text{H}}}_{2}{{\text{SO}}}_{4}+{\text{NaCl}}=2{{\text{BiCl}}}_{3}+3{{\text{Na}}}_{2}{{\text{SO}}}_{4}+3{{\text{H}}}_{2}{\text{O}}$$
(21)

In addition, when the NaCl concentration exceeded 30 g/L, the leaching efficiency of as was stable at 27% around. The reason is that the leaching of As was affected by the concentration of H+, as shown in Fig. 9. Comprehensively, the NaCl concentration was chosen to be 100 g/L.

The Effect of the Dosage of NaClO3

The reagent consumption is a statistical index in the production process. After calculation, the theoretical dosage of NaClO3 required for 5 g of the decopperized anode is 0.12 g. This section discussed the influence of 0 to 1.75 times the theoretical dosage, as shown in Fig. 11. Without adding NaClO3, the leaching efficiencies of Te, Se, Pt, and Pd were 42.51%, 60.85%, 20.34%, and 7.44% respectively. This indicates that some Te, Se, Pt, and Pd existed in the form of oxides. These oxides formed multi-coordination compounds with chloride ions, which dissolved in the leachate under acidic conditions and Se presented in the solution as H2SeO3. As the dosage of NaClO3 increased, the leaching of Te, Se, Au, Pt, and Pd was promoted, while that of Cu was affected imperceptibly. By contrast, with the increase in NaClO3 dosage, the Bi leaching efficiency increased from 42.43 to 48.40% and then decreased to 44.12%. In addition, the As leaching efficiency decreased with the increase in oxidant dosage. The reasons for the phenomena of As and Bi are as follows: as was oxidized to a high valence state, such as AsO43−, which is difficult to dissolve in the sulfuric acid system [29]; moreover, Bi3+ combined with AsO43− to insoluble substances such as BiAsO4 [30, 31], and the reactions involved are as follows:

Fig. 11
figure 11

Effect of the usage of NaClO3 on the leaching rate of each element (T = 80℃, time = 60 min, H2SO4 = 1 mol/L, L/S = 1 mL/g, NaCl = 100 g/L)

Full size image
$$B{{\text{i}}}^{3+}+{{\text{AsO}}}_{4}^{3-}={{\text{BiAsO}}}_{4}$$
(22)

Comprehensively, the multiple of the theoretical dosage of NaClO3 was chosen to be 1.25 times, which is the 3% by mass of the decopperized anode slime.

The Effect of Leaching Temperature

The reaction temperature is an important technical parameter in the production process, and its impact on the leaching process is shown in Fig. 12. When the temperature increased from 5 °C to 80 °C, the leaching efficiencies of Au, Te, Pt, Pd, and Se increased from 0.12%, 32.68%, 20.34%, 2.09%, and 54.66% to 100.79%, 97.69%, 94.47%, 100.38%, and 81.67%, respectively. High temperatures can promote the diffusion between ions and reduce the activation energy required for the reaction [3]. However, Cl− volatilized in the form of chlorine, and the concentration of Cl− decreased, which led to a slight decrease in the leaching rate of Te, Au, Pt, and Pd. Interestingly, the leaching efficiency of as declined most significantly. Therefore, the phase composition of leaching residue with the reaction temperature of 95 °C was analyzed. This result shows that As in the residue was mainly present in PbBi6O4(AsO4)4. This indicates that high temperature was conducive to the oxidation of as into a high valence state of AsO43−, as shown in Fig. 13. Comprehensively, 80℃ was the appropriate leaching temperature.

Fig. 12
figure 12

Effect of the leaching temperature on the leaching rate of each element (Time = 60 min, H2SO4 = 1 mol/L, L/S = 1 mL/g, NaCl = 100 g/L, NaClO3 = 0.15 g)

Full size image
Fig. 13
figure 13

The XRD pattern of the leaching residue at Temperature = 95 °C

Full size image

The main elements content of leaching residue is shown in Table 7. The leaching residue contained 15.55% Ba, 14.79% Pb, 8.37% Ag, 8.52% Sn, 2.01% Sb, 1.23% Cu, and 0.26% Te, without Au. The leaching residue mainly contains phases such as SnO2, BaSO4, (Ba0.69Pb0.31)SO4, AgCl, PbSO4, Sb2O3 and PbBi6O4(AsO4)4, as shown in Fig. 13.

Table 7 Main chemical components of the leaching residue at Temperature = 95 °C
Full size table

In summary, the optimal reaction conditions for chlorination leaching of the decopperized anode slime were a reaction time of 60 min, H2SO4 concentration of 1 mol/L, L/S = 1 mL/g, NaCl concentration of 100 g/L, and NaClO3 addition of 3% by the mass of the decopperized anode slime. The leaching efficiencies of Pb, Sn, Ag, Sb, Se, Cu, Bi, Te, Au, Pt, and Pd under this condition were 0.07%, 2.10%, 0.00%, 0.89%, 81.67%, 66.56%, 45.52%, 97.69%, 100.79%, 94.47%, 100.38%, respectively.

Conclusions

Effects of leaching time, liquid-solid ratio, H2SO4 concentration, the dosage of NaClO3, and leaching temperature on the leaching behavior of the main elements were experimentally investigated. In the H2SO4+NaCl system, increasing the H2SO4 concentration, NaCl concentration, or temperature can promote the leaching of Te, Au, Pt, and Pd. Addicting NaClO3 scarcely affected the leaching of Se, Cu, and Bi while impacting that of Te, Au, Pt, and Pd. The leaching efficiencies of Te, Au, Pt, and Pd reached 97.69%, 100.79%, 94.47%, and 100.38%, respectively. The possible reasons were discussed to explain the leaching differences between the SPMs and base metals. The SPMs and base metals can be separated by one-step chlorination oxidation leaching. This process provides an efficient and short technology for industrial production.