An investigation into the precipitation of copper sulfide from acidic sulfate solutions

An investigation into the precipitation of copper sulfide from acidic sulfate solutions

Hydrometallurgy 192 (2020) 105288 Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet An i...

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Hydrometallurgy 192 (2020) 105288

Contents lists available at ScienceDirect

Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet

An investigation into the precipitation of copper sulfide from acidic sulfate solutions

T



Tao Honga, , Yan Weia, Linbo Lia, Kathryn A. Mumfordb, Geoffrey W. Stevensb a

School of Metallurgical Engineering, Shaanxi Engineering Research Center for Metallurgical Technology, Xi'an University of Architecture & Technology, Xi'an 710055, PR China b Department of Chemical Engineering, The University of Melbourne, Victoria 3010, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Heterogeneous metal sulfurization Elemental sulfur Sulfur dioxide Single factor experiment Reaction mechanism

Due to the local and global supersaturation of metal sulfides in aqueous solutions, the industrial metal sulfurization processes using soluble sulfur resources (Na2S/Na2S2O3/(NH4)2S/H2S) and elemental sulfur have insurmountable defects, including the unfilterable fine precipitates, weak chemical selectivity, high reagent toxicity, and generation of soluble polysulfides. In order to develop a cleaner and green sulfurization method, a surface/heterogeneous sulfurization system using wetted sulfur particles and sulfur dioxide was investigated through single-factor experiments and reaction mechanism tests. The results of disproportionation tests indicated that reaction parameters, such as temperature, powder contact angle, SO2 pressure and molar ratio of Cu-to-Sulfur, significantly affected the reaction efficiency. The reaction is mixed controlled by the temperature and powder contact angle. During the disproportionation‑sulfurization process, the total reaction rate for the generation of HS− ions is dependent upon: the liquid diffusion of SO2 absorption, the powder contact angle of wetted sulfur particles, and the nucleophilic reaction between SO3H− with sulfur. Meanwhile, the total reaction is a shrinking nuclear reaction which the particles sizes continuously increasing after the metal sulfides formed.

1. Introduction Transition metal sulfides have important applications in many industrial fields due to their unique physical and chemical properties. Their catalytic, electric, magnetic, and optical characteristics, and other attractive properties, including optical transmission (Qi et al., 2017), short-wavelength optoelectronic, photo-electronic and thermoelectric characteristics (Emadi et al., 2017), promote their use for catalysis (Bu, 2016; Burmistrov et al., 1983), lubrication, battery fabrication (Du et al., 2017), conductors for photo−/piezo−/opto−/mange-electronics (Atay et al., 2003, Jiang et al., 2005, Zou et al., 2007, Zhu et al., 2007 (Zou et al., 2007), cathode materials for high-energy-density battery and supercapacitors (La Porta et al., 2017) (Muñoz et al., 1998) (Lin et al., 2008). Meanwhile, metal sulfide precipitation technology is widely used in industrial processing, due to the materials low solubility and chemical stability in aqueous solution (Mokone et al., 2012) (Fu and Wang, 2011) (Wynn et al., 1998). This includes use in waste-water treatment, recovery for the multi-metal resources, and heavy metal hydrometallurgy (Bredol and Merikhi, 1998), (Janyasuthiwong et al., 2015) (Kononova et al., 2014) (Nicol and Tjandrawan, 2014) (Liu et al., 2019) (Sahinkaya et al., 2009), Harmandas and Koutsoukos, 1996).



For the preparation of functional micro−/nanoscaled transition metal sulfides, the hydrothermal synthesis route is the most important method, as it can be used to control the morphology and the size of the synthesis products. Soluble inorganic and organic sulfur agents are the main synthesis reagents, include sodium sulfide, sodium thiosulfate, carbon disulfide, thioacetamide, dodecanethiol, mercaptoethanol, thioglycolic acid, and L-cysteine. Sulfides with different valences have been prepared using the hydrothermal synthesis method. For industrial applications, the hydrothermal synthesis using organic sulfur agents has inherent deficiencies including a long reaction duration, high raw material costs, high reaction temperatures and energy consumption (Villa-Gomez et al., 2012) (Jones et al., 1992) (Oktaybaş et al., 1994) (Charerntanyarak, 1999) (Mersmann, 1999) (Stén and Forsling, 2000). Considering the chemical stability and floatability of the heavy metal sulfides, different inorganic sulfur sources (Na2S/(NH4)2S/ Na2S2O3/H2S/S) are used to precipitate the metal ions from aqueous solutions. Examples include the treatment of acidic mineral processing wastewater, purification of wash solution from zinc concentrate roasting processes and copper ion separation from nickel sulfate leach solutions. Sodium sulfide, ammonia sulfide, and hydrogen sulfide have good

Corresponding author. E-mail address: [email protected] (T. Hong).

https://doi.org/10.1016/j.hydromet.2020.105288 Received 28 August 2019; Received in revised form 18 January 2020; Accepted 31 January 2020 Available online 01 February 2020 0304-386X/ © 2020 Published by Elsevier B.V.

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system using wetted sulfur particles and sulfur dioxide at relatively low temperature (< 100 °C). Gaseous sulfur dioxide can use its mass transfer limitation to control the disproportionation reaction rate of sulfur, generation of supersaturation of metal sulfides, and the oxidation-reduction potential of the solution. Similarly, the interfacial diffusion for the wetted elemental sulfur particles to regulate the reaction temperature, sulfide ion concentration and generation of the metal sulfides at the surface of sulfur. In this paper, we investigated the influence of individual parameters on the recovery of copper from a synthetic copper sulfate solution using elemental sulfur and sulfur dioxide for the treatment of an acidic copper-containing solution originating from a heavy metal concentrate process. Furthermore, a kinetics study was comprehensively carried out to determine the heterogeneous sulfurization mechanism with the wetted sulfur particles and sulfur dioxides.

solubility in aqueous solutions, low cost, and wide range of industrial applications in heavy metal hydrometallurgy. However, in practice, there are four unavoidable problems. Firstly, it's difficult to filter fine (colloid) particles from the solution. Second, different metal ions may be forming the metal sulfides simultaneously, maybe therefore not achieving the desired separation. Meanwhile, impurity ions, such as Na+/NH4+, maybe brought into the solution during the reaction process. Hydrogen sulfide is also often used in the precipitation process posing a risk to operators and environment. Elemental sulfur and hydrogen sulfite ion originate from the dehydration of sodium thiosulfate in acidic solutions, and hydrogen sulfide is produced from the disproportionation reaction of elemental sulfur and hydrogen sulfite ions. The generated hydrogen sulfide ion at the surface of sulfur particles combines with metal ions to form metal sulfides. In our previous work, this method was used to recover copper and rhenium from acidic wash solutions from the copper smelting process. The industrial results showed copper and a proportion of arsenate were precipitated synchronously during the recovery of rhenate ions. Metal sulfide particles were found to be too fine for setting and filtration from solution. The composition of elemental sulfur and arsenic sulfide was too high to produce high-purity perrhenate using simple solvent extraction or ion exchange methods. Lewis et.al. (van Hille et al., 2005; Lewis and Swartbooi, 2006; Lewis and van Hille, 2006; Lewis, 2010; Mokone et al., 2010) investigated the influence of various factors on the removal efficiency of metal ions with soluble sulfide agents (Na2S/(NH4)2S/H2S/Na2S2O3) in acidic aqueous solutions. This included the solubility of metal sulfides, the solution chemistry, mass transfer limitations of gaseous sulfides, the local and global supersaturation. These results and our previous works (Hong et al., 2019a; Hong et al., 2019b) showed that the global and local supersaturation of sulfide ions in aqueous solution is too high to form large particles of metal sulfides through spontaneous homogenous nucleation. Due to the high surface area and surface energy of the formed fine (colloid) particles, the soluble polysulfide species and electrostatic particles generated under the action of the sulfide ion in solution. Increasing electrostatic repulsion between particles, makes it difficult for particles to grow spontaneously through agglomeration or through surface nucleation. Additionally, the displacement reaction between the metal sulfides with the metal ion in solution is inhibited. As previously mentioned, a surface/heterogeneous nucleation reaction method for the sulfurization reaction requires development before an efficient selective sulfurization technology may be established. The existing technical deficiencies in this method include: low precipitation efficiency, selective sulfurization of single metal ions, surface/heterogeneous nucleation and the formation of polysulfides. These may be solved via the investigation of the reaction system, i.e., control of the global and local supersaturation of sulfide ions in aqueous solution (regulating the concentration of sulfide ions), and the selection of metal sulfides with a lower solubility. As the solid-state sulfur resource, elemental sulfur particles were used in the investigation of the hydrothermal sulfurization processes that incorporate synthesis of Cu2S nanowires (Yu and An, 2010), recovery of heavy metals from the neutralization slags (Liang et al., 2012; Ke et al., 2015), and zinc sulfurization of hemimorphite (LI et al., 2013) and zinc‑lead ores(Li et al., 2014). The research indicated that sulfide products have the physicochemical characteristics and filtration performance. Nonetheless, there are some drawbacks in the reaction process, such as long reaction duration, high energy consumption, and poor chemical selectivity. The main reason for these problems is the inherent hydrophobicity of the elemental sulfur particles, the ions cannot diffuse to the surface of the sulfur particles at relatively low temperature (< 100 °C). Hydrothermal technology was used to promote the self-disproportionation of sulfur and ion diffusion from the solution to the surface of elemental sulfur particles. Based on this research, we proposed a heterogeneous sulfurization

2. Material and methods 2.1. Chemicals Copper sulfate pentahydrate (CuSO4·5H2O), elemental sulfur powder (S), sulfuric acid (H2SO4, 98.0 wt%), sodium hydroxide (NaOH), hydrochloric acid (HCl, 36.0 wt%), potassium chloride and sulfur dioxide (SO2) used in the experiments were analytical grade chemicals obtained from the Chemical Co. of Xi'an. Ammonium cetylbenzenesulphonate was provided by Sigma-Aldrich. All chemicals were used without further purification. The water used in all the experiments was purified using a Milli-Q instrument (Millipore Corp.). 2.2. The wetting treatment of elemental sulfur powder A baffled 1-L glass mixed-tank reactor equipped with a two-flatblade turbine mixer, and heated by a water jacket. The temperature was controlled using a band heater inside the reactor. Ammonium cetylbenzenesulphonate solutions were prepared before the wetting treatment of the elemental sulfur powder; solutions of different concentrations were prepared by dissolving predetermined amounts of ammonium cetylbenzenesulphonate in high-purity water. Wetted sulfur particles were prepared by adding elemental sulfur powders in the different concentration of ammonium cetylbenzenesulphonate solutions at the optimum treatment conditions (time of 1.0 h, the temperature of 40 °C, liquid to solid mass ratio of 4, stirring speed of 500 rpm, and sulfur particles diameter size of 0.140–0.160 mm). After the wetting process was completed, the sulfur particles were filtered, washed with high-purity water, and dried in a vacuum oven at 50 °C for 12 h. 2.3. The disproportionation reaction experiments Experimental acidic copper sulfate aqueous solutions of different copper or sulfuric acid were prepared by dissolving predetermined amounts of CuSO4·5H2O and H2SO4 in high-purity water. Disproportionation experiments were conducted in 1.5-L titanium autoclave (Weihai Autoclave Co. of China) which was baffled, equipped with a three-flat-blade turbine mixer, and heated by an electric jacket. The temperature was controlled using a temperature controller inside the autoclave. Wetted sulfur powders were added to pre-determined amounts of an acidic copper sulfate solution, then the solution containing solid was heated to a pre-set temperature in the sealed autoclave added and SO2 gas. After the reaction time was complete, the slurry was suction-filtered at 30 °C and the liquor was analyzed to determine the final copper concentration. The precipitate was filtered, washed with high-purity water, dried in a vacuum oven at 60 °C for 12 h, and analyzed to determine the chemical and phase compositions. 2

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2.4. Analytical methods 2.4.1. Copper and sulfuric acid concentration in aqueous solution The concentration of copper ions in the initial and reacted liquor was analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES, DV6300, Perkin Elmer). The percentage of copper sulfurization from the solution was calculated using Eq. (1):

η=

Cini − Cfin Cini

(1)

where Cini (g/L) and Cfin (g/L) donate the initial copper concentration of the solution and that of reacted liquor, respectively. The concentration of sulfuric acid in the initial solution was analyzed by the acid-base titration using 0.1 mol/L NaOH solution. 2.4.2. Surface characterization of wetted sulfur powders Zeta potential of original and wetted sulfur powder was measured by using a Zetasizer (Zetasizer Nano ZSP, Malvern-Panalytical Netherlands). About 0.1 g solid sample was diluted in 10 mL 0.1 mol/L KCl aqueous solution, then 1 ml of the diluted sample was injected into a flow cell. Zeta potential was measured within the pH range from 2.0 to 12.0. Manipulation of pH was performed via addition of 0.2 mol/L HCl and/or NaOH. All values were measured by repeated three times at a temperature of 30 °C. The multi-functional tensiometer (DCAT21, DataPhysics Germany) was used to measure the powder contact angle of the original and wetted sulfur particles. About 5.0 g solid sample was filled into a funnel, then powder contact angle was measured using 0.1 mol/L CuSO4 solution at a temperature of 60 °C.

Fig. 1. effect of duration on the separation efficiency of copper ions. Reaction temperature of 70 °C; the concentration of copper and sulfuric acid of 10 g/L and 20 g/L, respectively; SO2 pressure of 0.3 MPa; the diameter of initial sulfur particles of 0.100–0.140 mm, the powder contact angle of wetted sulfur of 72.7°; molar ratio of copper-to‑sulfur of 1:1.

120 min. Elemental sulfur is a rhombohedral compound, in which its atoms form cyclic octatomic molecules. In the disproportionation‑sulfurization process, the sulfur‑sulfur bonds in elemental sulfur perform nucleophilic degradation under the action of sulfurous acid (H2SO3) in solution, then copper ions react with sulfide ions originating from the nucleophilic degradation at the surface of the sulfur particles, ultimately affecting the separation efficiency. Fig. 2(a) and (b) displays SEM images of the wetted sulfur particles and the precipitate produced at 120 min reaction time. The particles exhibit an irregular spherical crystal morphology and the diameter of the crystal is over 0.100 mm. With an increase in reaction temperature, the diameter of the crystal increases by about 0.015–0.020 mm. This indicated that the copper sulfide may be formed at the surface of wetted sulfur particles during the separation process. Fig. 2(c) and (d) are the XRD patterns of the wetted sulfur particles and the precipitate produced at 120 min reaction time. The cuprous sulfide (Cu2S), copper sulfide (CuS), and sulfur (S) are crystalline compounds in the sulfurization product. As shown in Table 1., experiment 1 is the chemical composition of the precipitate. CuS is the main component in the product, whilst Cu2S/S/Cu are of relatively low concentrations. It is extremely important is the water content is much lower than that of conventional sulfide precipitation products for the reuse of the precipitate.

2.4.3. Phase characterization of precipitates X-ray diffraction (XRD) patterns of the precipitates were recorded on a powder diffractometer (X'pert Pro, Panalytical Netherlands) with Cu Kα (λ = 0.15408 nm) radiation at 25 °C. Field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan) was conducted to analyze the microstructure of the precipitates, which had been ultrasonicated and well-dispersed in absolute alcohol. X-ray photoelectron spectroscopy (XPS) technique (ESCALAB Xi+, Thermo Fisher Scientific, U·S) equipped with mono-energetic Al Kα X-rays (1486.6 eV) were used to analyze the valence state of the elements in the precipitates. The compositions of elemental sulfur, CuS, and Cu2S in the precipitates were analyzed by selective dissolution and volumetric methods. Solid samples were dissolved in CCl4 solution at 50 °C for 1.0 h, then filtered and washed with CCl4 at 30 °C. The leached sample was used to analyze Cu2S and CuS. The pregnant and washed CCl4 solution was dried to a constant weight at 80 °C, the residual solid is elemental sulfur. The leached solid was dissolved with 1:1 (Volume ratio) HCl solution at 70 °C for 0.5 h, and then filtered and washed with 2 wt% nitric acid. The leach solution was later analyzed by ICP-OES for the composition of Cu2S. The remaining solid was dissolved with 20.0 wt% nitric acid solution at boiling temperatures and diluted with 2 wt% nitric acid; they were later analyzed by ICP-OES for the composition of CuS.

3.1.2. Effect of the pressure of SO2 on disproportionation reaction Due to the multi-valence characteristics of copper ions and the principle of sulfurization using sulfur and SO2, the valence of copper ions and the separation efficiency of copper are influenced by the pressure of SO2 in the reactor. The effect of the pressure of SO2 on the separation efficiency and the phase characterization of the precipitates were investigated, as shown in Fig. 3. Fig. 3(a) illustrates the variation in the separation percentage of copper with reaction time at different pressures of SO2. It can be seen that the precipitation efficiency of copper increased with an increase in the reaction time and SO2 pressure. The efficiency increased slowly with time at a SO2 pressure of 0.1 MPa; whilst the efficiency increased rapidly at pressures above 0.2 MPa. The maximum separation percentage was about 65.38% at 0.3 MPa and reaction time of 120 min. Fig. 3(b) shows the influence of SO2 pressure of on the separation percentage of copper ions at a reaction time of 120 min. The percentage

3. Results and discussion 3.1. The sulfurization experiments 3.1.1. Effect of duration on disproportionation reaction The effect of duration on the separation efficiency of copper ions and the composition characteristics of the resulting precipitate was studied. As shown in Fig. 1., the reaction duration had a considerable effect on disproportionation‑sulfurization efficiency. With an increase in the reaction time, the precipitation percentage of copper ions rapidly increased in 15 min, then increased slowly to a reaction time of 3

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Fig. 2. Image of precipitate characteristics (a) SEM image of wetted sulfur, (b) SEM of the precipitate, (c) XRD pattern of wetted sulfur, (d) XRD pattern of the precipitate.

reaction process, due to surface diffusion at the wetted elemental sulfur particles. With the extent of reaction time, the separation efficiency of copper ions rapid increased, especially when the temper-ature exceeds 80 °C. At the same time, it can be observed that the extent of decopperization increased with an increase in temperature. Typically for the temperatures, the percentage of decopperization increased to 71.6% at 60 °C, then slowly rising with increased temperature. The reaction efficiency reaches 97.5% at 90 °C. It indicated that the optimum temperature for sulfurization is 90 °C, and the better reaction time is 120 min. In order to study the effect of temperature on the morphology of the precipitates, products at different temperatures were analyzed by SEM. As shown in Fig. 5, the products exhibit a near-spherical morphology. With an increase in temperature, the diameter of products slowly reduces and forming loosen structure, especially at reaction temperatures above 80 °C.

increased with an increase in the SO2 pressure and then decreased when the pressure over 0.30 MPa. For investigate the reason for this phenomenon, the phase composition of the precipitates at different SO2 pressure was analyzed, the results are shown in Table 1. Experiment 1 to 4 is the phase composition of the different products. The composition of CuS increased with an increase in SO2 pressure, and that of the elemental sulfur decreased. It indicated that the sulfur disproportionation and metal sulfurization reaction were enhanced by the increase in the SO2 pressure. In the same time, the component of Cu2S decreased and then increased, while that of copper increased continuously. It may be explained that the sulfurization and self-disproportionation of the cuprous ion perform in the aqueous solution due to the decrease in the reduction potential of the solution (for the chemical reducibility of SO2). 3.1.3. Effect of temperature on disproportionation reaction Fig. 4 illustrates the separation rate of copper and the extent of decopperization as functions of reaction temperature. There may be an induction period of disproportionation within half an hour during the Table 1 the composition of precipitates during the study. Experimental number

1 2 3 4

Reaction time (min)

120 120 120 120

Reaction temperature (°C)

70 70 70 70

The pressure of SO2 (MPa)

0.10 0.20 0.30 0.35

4

The composition of precipitates (%) Cu2S

CuS

Cu

Sulfur

H2 O

1.23 1.01 2.11 3.02

51.51 62.16 71.82 66.41

0.57 0.64 0.65 0.73

40.80 33.24 17.52 26.88

5.89 2.95 8.10 2.96

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Fig. 3. effect of pressure of SO2 on the separation efficiency of copper ions (a) reaction rate, (b) separation efficiency. The concentration of copper and sulfuric acid of 10 g/L and 20 g/L, respectively; the temperature of 70 °C; the diameter of original sulfur particles of 0.100–0.140 mm, the powder contact angle of wetted sulfur of 72.7°; molar ratio of copper-to‑sulfur of 1:1; reaction time of 120 min (b).

component of wetted sulfur particles in aqueous solution increased with a decrease in the molar ratio of copper-to‑sulfur. It means that the concentration of hydrogen sulfide ions (HS−) at the surface of sulfur particles and in overall solution continuously increased during the same change. Copper ions combined with the hydrogen sulfides ions to form CuS, and the excess copper ions form Cu2S and Cu(0) when the molar ratio of copper-to‑sulfur is above 1:1. This causes the actual separation efficiency to become higher than the theoretical calculation. For the molar ratio of copper-to‑sulfur less than 1:1, an excess of HS− was generated in solution. Under the control of local and global supersaturation of metal sulfides in solution, the soluble copper polysulfides formed from the reaction of CuS/Cu2S with HS−. It indicated that the separation percentage decreased with a decrease in the molar ratio, and it is why the real separation percentage is lower than the calculation. Fig. 7. is the morphology of precipitation products at different molar ratio of copper-to‑sulfur. It showed the molar ratio has a significant effect on the morphology of precipitates. The diameter of the precipitates increased, and the surface of particles became smooth during the reduction of the molar ratio. It illustrates that the reaction between copper ions with the formed HS− weakened with the change of the molar ratio. Meanwhile, the composition of elemental sulfur in the particles and the diameter of the particles gradually increased.

3.1.4. Effect of the molar ratio of copper-to‑sulfur on disproportionation reaction As another component of the heterogeneous disproportionation‑sulfurization system, the molar ratio of copper-towetted sulfur has an important influence on the reaction rate. The reaction rate and separation efficiency as the function of the molar ratio of Cu-to-S were examined and monitored, as shown in Fig. 6. It indicated in Fig. 6(a) that the heterogeneous sulfurization rate is first increased and then decreased with a decrease in the molar ratio of Cuto-S. At a molar ratio of Cu-to-S values was 1, the reaction rate is the fastest. The results of the effect of the SO2 pressure show the main composition of the precipitates are copper sulfide (CuS). It was assumed that the reaction product is CuS, and the difference between the experimental results and the theoretical calculation under different molar ratio at the reaction time of 120 min was compared. It can be seen from Fig. 6(b) that the separation percentage first increased and then decreased with an increase in the molar ratio of Cu-to-S, the maximum value appears in the molar ratio of 1:1. Comparing the theoretical value with experimental results, the experimental results are higher than the calculation and then gradually becomes consistent, and the experimental value is lower than the calculation after the molar ratio less than 1:1. With the other factors of the reaction process unchanged, the

Fig. 4. effect of temperature on the disproportionation reaction (a) reaction rate, (b) separation efficiency. The concentration of copper and sulfuric acid of 10 g/L and 20 g/L, respectively; SO2 pressure of 0.3 MPa; the diameter of original sulfur particles of 0.100–0.140 mm, the powder contact angle of wetted sulfur of 72.7°; molar ratio of copper-to‑sulfur of 1:1; reaction time of 120 min (b). 5

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Fig. 5. SEM images showing the morphologies of reaction products at different temperatures.

undergo the disproportionation sulfurization in aqueous solution. It indicates that the chemical reactivity of the hydrophobic sulfur particles was improved by the hydrophilic surface treatment using surfactants. In the same time, the copper separation percentage increased with a decrease in powder contact angle. This denotes the ion absorption and sulfurization at the surface of the sulfur particles were promoted through the improvement of the hydrophilicity for the sulfur powders, ant the control of the selective separation can be achieved by the same method.

3.1.5. Effect of powder contact angle on disproportionation reaction The external diffusion coefficient of copper ions for the heterogeneous sulfurization system and the absorption of sulfur particles for copper ions are improved through the wetting treatment of elemental sulfur. Therefore, as a quantitative index of the wetting treatment, the powder contact angle has an important influence on the heterogeneous sulfurization efficiency. Fig. 8 demonstrated the effect of the powder contact angle on the separation efficiency. In can be seen that the original sulfur powders (88.03°) did not react in the multi-phase system, nevertheless the wetted sulfur particles

Fig. 6. effect of the molar ratio of Cu-to-S on the disproportionation reaction (a) reaction rate, (b) separation efficiency. The concentration of copper and sulfuric acid of 10 g/L and 20 g/L, respectively; SO2 pressure of 0.3 MPa; the diameter of original sulfur particles of 0.100–0.140 mm, the powder contact angle of wetted sulfur of 72.7°; temperature of 90 °C; reaction time of 120 min (b). 6

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Fig. 7. SEM images showing the morphologies of reaction products at different molar ratio of Cu-to-S.

original and wetted sulfur particles was studied. Fig. 9 a) and b) are the influence of the pH of the aqueous solution on the zeta potential of the original and wetted sulfur particles, respectively. As shown in Fig. 9, the zeta potential of untreated particles decreased with an increase in the basicity of the solution, and the surface of particles shows a negative charge state after the isoelectric point (pH ≈ 4.7). This phenomenon indicated that the cation ion should be absorbed on the particles in the neutral and alkaline aqueous solutions. If the untreated sulfur powder is used for the sulfur resource, the sulfurization reaction may be performed in the alkaline solutions. This result is consistent with the hydrothermal sulfurization using elemental sulfur in the alkaline aqueous solution used in the work of Chai(Liang et al., 2012)(Ke et al., 2015) and Li at.al(LI et al., 2013). In contrast, the wetted sulfur particles that were treated with the anionic surfactant exhibit a stronger negative electrostatic state than the original sulfur particles. The isoelectric point changes from pH ≈ 4.7 to pH ≈ 3.0, it implies that the wetted sulfur particles can absorb the metal ion in the acidic solution, and the disproportionation‑sulfurization for the wetted sulfur particles performed in the acidic solutions.

Fig. 8. Effect of the powder contact angle on the separation efficiency. The concentration of copper and sulfuric acid of 10 g/L and 20 g/L, respectively; SO2 pressure of 0.3 MPa; the diameter of original sulfur particles of 0.100–0.140 mm, the temperature of 90 °C; reaction time of 120 min.

3.2.2. The valence of the elements in precipitate The spectrum for the precipitate showed typical features of a solid surface as can be seen by the uppermost trace in Fig. 10. The sulfur spectrum shows typical peaks for monosulfide species, there is no appreciable indication of disulfide and polysulfides or an energy loss peak. Meanwhile, copper is present as Cu (II). It indicated the compound at the precipitate surface is copper sulfide (CuS), i.e., CuS is the main product in the heterogeneous disproportionation‑sulfurization process

3.2. Disproportionation reaction mechanism 3.2.1. Electrostatic characteristics of elemental sulfur powder For investigate the charging of the electrostatic situation during the wetting treatment of elemental sulfur powder, the zeta potential of the 7

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Fig. 9. Diagram of Zeta Potential of (a) original sulfur powder and (b) wetted sulfur powder. The diameter of original sulfur particles of 0.100–0.140 mm, the powder contact angle of wetted sulfur of 64.8°.

and the diameter of the products constantly increased. Cu2+, Cu+, SO3H−, formed SO4H− ions reach or leave the reaction interface of the unreacted sulfur with metal sulfides through the external and internal diffusion, respectively. That is, the total reaction is the shrinking nuclear reaction which the particles continuously increased after the metal sulfides formed. In this complex system, excessive Cu+ ions generated under the action of the higher SO2 pressure. If Cu+ cannot quickly combine with HS− due to the diffusion control, the elemental Cu(0) originated from the self-disproportionation in acidic solution. In another case, the soluble copper polysulfides formed from the reaction of CuS/Cu2S with HS− under the control of local and global supersaturation of metal sulfides in solution when the lower molar ratio of Cu-to-S. The main reactions and related side reactions are as follows: Disproportion reaction:

with the wetted sulfur powder and SO2. 3.2.3. Disproportionation reaction Fig. 11 illustrated the disproportionation‑sulfurization mechanism using wetted elemental sulfur with SO2 and copper ions. The nucleophilic reaction between the hydrogen sulfite ions with the wetted sulfur, the SeS bond-breaking reaction for the octasulfane mono− sulphonic acid ions (Sn-S-SO3H− ions, , n < 7), the generation of HS and the metal sulfurization at the surface of the sulfur particles are the main processes in the acidic sulfate solution. Consider the SeS bond-breaking reaction and the generation of HS− ions are fast, the total reaction rate for the generation of HS− ions is mainly controlled by the liquid film diffusion for the SO2 absorption and the nucleophilic reaction. Due to the nucleophilic reaction rate is affected by the wetting characteristics of the sulfur particles, the total reaction rate for the generation of HS− ions is controlled mixed step: the liquid diffusion of SO2 absorption, the powder contact angle of wetted sulfur particles, and the nucleophilic reaction between SO3H− with sulfur. Therefore, there is an induction period of 15 min in singlefactor experiments. In the same time, a portion of Cu2+ ions was reduced to Cu+ ions under the action of the SO3H−. The copper and cuprous ions combined with the HS− to form the production layer (CuS/Cu2S) at the face of the elemental sulfur particles. Since the higher density of metal sulfides and the lower crystallinity of the metal sulfides, the diameter of unreacted sulfur particles continuously reduced, a porous layer that contains the metal sulfides formed on the surface of the unreacted sulfur particles,

SO2 (g) + H2 O ⇌ H2 SO3 ⇌ H+ + SO3 H−

(2)

S + SO3 H− + H2 O ⇌ S2 O3 H− + H2 O ⇌ HS − + SO4 H−

(3)

Metal sulfurization reaction:

Cu2 + + HS − ⇌ CuS ↓ + H+

(4)

Side reaction:

2Cu2 + + SO2 + 2H2 O ⇌ 2Cu+ + SO4 H− + 3H+

(5)

2Cu+ + HS − ⇌ Cu2 S ↓ + H+

(6)

2Cu+

⇌ Cu +

Cu2 +

(7)

Fig. 10. XPS spectra of precipitate at optimum reaction conditions (a) copper and (b) sulfur. The precipitate originates from the optimum sulfurization conditions: the initial concentration of copper ion of 10.0 g/L, reaction temperature of 80 °C, duration of 2.0 h, the powder contact angle of wetted sulfur of 64.8°, the diameter of initial sulfur particles of 0.100–0.140 mm, SO2 pressure of 0.30 MPa. 8

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Fig. 11. Schematic diagram of the disproportionation reaction mechanism using elemental sulfur with SO2.

CuS + nHS − ⇌ (CuSn + 1)−2n + nH+

(8)

References

Cu2 S + nHS − ⇌ (Cu2 Sn + 1)−2n + nH+

(9)

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4. Conclusion In this investigation, the sulfurization of copper ion from acidic sulfate solution by disproportionation reaction with elemental sulfur and sulfur dioxide was studied. The results of disproportionation tests indicated that reaction parameters, such as temperature, powder contact angle, SO2 pressure and molar ratio of Cu-to-Sulfur, significantly affected the reaction efficiency, especially the temperature and powder contact angle are the mixed control factor for the metal sulfurization. When the synthetic acidic copper solution (Cu2+ 10.0 g/L, free sulfuric acid 20.0 g/L) was reacted with the temperature at 90 °C for 120 mins, the molar ratio of Cu-to-S of 1.0, the pressure of SO2 of 0.30 MPa, the powder contact angle of 69.8°,the diameter of original sulfur particles of 0.100–0.140 mm, the separation percentage of copper ions was found to be 97.12%. Furthermore, the precipitates consist of CuS, Cu2S, Cu and unreacted elemental sulfur, most of them are CuS. During the disproportionation‑sulfurization process, the total reaction rate for the generation of HS− ions is controlled mixed step: the liquid diffusion of SO2 absorption, the powder contact angle of wetted sulfur particles, and the nucleophilic reaction between SO3H− with sulfur. Meanwhile, the total reaction is the shrinking nuclear reaction which the particles continuously increased after the metal sulfides formed. In this complex system, excessive Cu+ ions generated under the action of the higher SO2 pressure. If Cu+ cannot quickly combine with HS− due to the diffusion control, the elemental Cu(0) originated from the self-disproportionation in acidic solution. In another case, the soluble copper polysulfides formed from the reaction of CuS/Cu2S with HS− under the control of local and global supersaturation of metal sulfides in solution when the lower molar ratio of Cu-to-S.

Note The authors declare no competing financial interest.

Acknowledge This work was financially supported by the Natural Science Foundation of Shaanxi Province of China (Grant No. 2016JM5025) and the Basic Research Foundation of Xi'a University of Architecture & Technology (Grant No. QN1724). Tao Hong was financially supported by the China Scholarship Council (No. 201707835003). 9

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