Lei Tian, Ao Gong, Xuan-gao Wu, Yan Liu, Zhi-feng Xu, and Ting-an Zhang, Cu2+-catalyzed mechanism in oxygen-pressure acid leaching of artificial sphalerite, Int. J. Miner. Metall. Mater., 27(2020), No. 7, pp. 910-923. https://doi.org/10.1007/s12613-019-1918-2
Cite this article as:
Lei Tian, Ao Gong, Xuan-gao Wu, Yan Liu, Zhi-feng Xu, and Ting-an Zhang, Cu2+-catalyzed mechanism in oxygen-pressure acid leaching of artificial sphalerite, Int. J. Miner. Metall. Mater., 27(2020), No. 7, pp. 910-923. https://doi.org/10.1007/s12613-019-1918-2
Research Article

Cu2+-catalyzed mechanism in oxygen-pressure acid leaching of artificial sphalerite

+ Author Affiliations
  • Corresponding authors:

    Zhi-feng Xu    E-mail: xzf_1@163.com

    Ting-an Zhang    E-mail: zta2000@163.net

  • Received: 14 July 2019Revised: 10 September 2019Accepted: 12 September 2019Available online: 28 October 2019
  • The potential autoclave was used to study the catalytic mechanism of Cu2+ during the oxygen pressure leaching process of artificial sphalerite. By studying the potential change of the system at different temperatures and the SEM–EDS difference of the leaching residues, it was found that in the temperature range of 363–423 K, the internal Cu2+ formed a CuS deposit on the surface of sphalerite, which hindered the leaching reaction, resulting in a zinc leaching rate of only 51.04%. When the temperature exceeds 463 K, the system potential increases steadily. The increase in temperature leads to the dissolution of the CuS, which is beneficial to the circulation catalysis of Cu2+. At this time, the leaching rate of Zn exceeds 95%. In addition, the leaching kinetics equations at 363–423 and 423–483 K were established. The activation energy of zinc leaching at 363–423 and 423–483 K is 38.66 and 36.25 kJ/mol, respectively, and the leaching process is controlled by surface chemical reactions.

  • Sphalerite, the most significant ore in zinc, has a covalent bond lattice structure, and its solubility is very low; thus, the O2-pressure leaching of sphalerite has very slow kinetics [14]. However, with the addition of a suitable catalyst, the leaching efficiency can be significantly improved [57]. Sphalerite is a typical sulfide ore and most sulfide minerals are semiconductors, and pressure leaching is essentially an electrochemical reaction process [89]. Li et al. [10] indicated that defects and impurities in the crystal lattice of sphalerite significantly affect the kinetics of the electrochemical processes, so it is very important for the role of impurities in solid solutions (including mineral phases and ions) on the leaching process.

    Under normal circumstances, the concentrates and diffusion rates of Fe2+, Cu2+, and Mn2+ in solution determine the leaching rate of Zn. These ions can be assumed to not only function as O2-carrying materials, but also to participate in the formation of the original battery, functioning as catalysts and oxidants [1115], promoting the leaching of the valence metal. Under the condition of sufficient O2 and acidity, Fe2+, Cu2+, Mn2+, and other cations reach a dynamic balance in the reaction system.

    Many researchers have investigated the catalysis of different types of metal ions on the leaching of sulfide minerals [1618]. Ghosh et al. [19] used Cu2+ as an oxidation catalyst to study the leaching kinetics of sphalerite in ammonia. The results have shown that the surface reaction was the reaction-controlling step, and the apparent activation energy was about 48.3 kJ·mol−1, and apparent reaction orders of 0.2, 0.3, and 0.3 were obtained for the oxygen partial pressure, concentrate of ammonia, and Cu2+ concentration, respectively. The catalytic action of Cu2+ has been attributed to the redox couple Cu2+/Cu+. Ballester et al. [20] investigated the catalytic effect of Cu2+ on the leaching of sphalerite. In the absence of Cu2+, the oxidative leaching of sphalerite is slow because of the dense layer of elemental S that forms on the mineral surface. However, in the presence of Cu2+, the leaching solution has a higher electrical conductivity, which accelerates the dissolution of the valuable metals.

    Several mechanisms for explaining catalytic activity have been proposed. One mechanism is thought to be due to changes in surface reactivity, while the other proposes copper ions as "oxygen carriers" to transfer electrons from S to oxidants. The mechanism and electrochemistry of sphalerite pressure leaching in the Cu2+ catalyst system needs further investigation. In this study, the catalytic effect of a Cu2+ catalyst system on the dissolution of sphalerite was investigated using a potential autoclave. Experiments were performed to elucidate the catalytic mechanism and the potential-variation rule of the leaching system, and the kinetic influences of several variables on the pressure-leaching process were considered. The research in this paper should be beneficial toward improving the recovery of valuable metals from sphalerite and developing appropriate methods for achieving the effective use of complex sulfide minerals.

    Zinc sulfide in sphalerite is α-type; therefore, to obtain artificial sphalerite (α-ZnS), analytical reagent (β-ZnS) was placed in a high-temperature pipe furnace. The preparation of α-ZnS was conducted in an inert-gas atmosphere. The β-ZnS was sintered in a solid-state at 1223 K for 120 min firstly, and then it was homogenized at 1123 K for 60 min. The XRD pattern of artificial sphalerite is shown in Fig. 1.

    The pressure-leaching experiment was performed using an FCFD 2-1.0 potential autoclave (the volume of 2000 mL). The body of the autoclave was made of Zr, and the maximum pressure and temperature achievable by the autoclave were 6.0 MPa and 573 K, respectively. A diagrammatic sketch and physical chart of the FCFD 2-1.0 potential high-pressure autoclave are shown in Fig. 2.

    Fig. 1.  XRD pattern of artificial sphalerite.
    Fig. 2.  Potential autoclave. 1—Electrode measuring instrument; 2—Reference electrode; 3—Measuring electrode; 4—Feed inlet; 5—Discharge port; 6—Motor; 7—Thermocouple.

    First, 800-mL aqueous solution, 20.0-g artificial sphalerite, and 0.3-g calcium lignosulfonate were added to the potential autoclave and heated to the set temperature. Next, 200-mL sulfuric acid solution and a CuSO4 solution were pressed into the Zr autoclave via O2; the potential change was recorded by the potentiometer during the reaction.

    5-mL slurry was then withdrawn, and the content of Zn was detected by an inductively coupled plasma (ICP) emission spectrometer (Leeman, USA). The experimental flowchart is shown in Fig. 3.

    Fig. 3.  Flowchart of Cu2+ catalyzed in the O2-pressure acid leaching of the artificial sphalerite.

    The leaching rate α of valuable elements was calculated as follows:

    $$\alpha = \frac{{{C_1} \times {V_1}}}{{{m_0} \times {x_0}}} \times 100\text% $$ (1)

    where α is the leaching ratio of zinc (%), C1 is the concentrate of elements in the leaching solution (g·L−1), V1 is the volume of leaching solution (L), m0 is the quantity of the sample (g), and x0 is the mass content of elements in the artificial sphalerite (wt%).

    The O2-pressure leaching of artificial sphalerite particles in catalytic systems with different cations is shown in Fig. 4. During the pressure leaching of zinc sulfide particles without a cation catalyst, H2SO4 first reacted with ZnS, producing H2S and Zn2+, and H2S was then oxidized, generating elemental S owing to the [O]solution. H+ was then released, continuing the reaction with ZnS. However, the O dissolution into the leaching solution was very slow, significantly reducing the rate of the leaching reaction.

    Fig. 4.  Schematic of O2-pressure leaching with artificial sphalerite particles.
    $${\rm{ZnS}} + 2{{\rm{H}}^ + } \to {\rm{Z}}{{\rm{n}}^{2 + }} + {{\rm{H}}_2}{\rm{S}}$$ (2)
    $${\rm{C}}{{\rm{u}}^{2 + }} + {{\rm{H}}_2}{\rm{S}} \to {\rm{CuS}} + 2{{\rm{H}}^ + }$$ (3)
    $${\rm{CuS}} + 2{\rm{H}^+} + {\left[ {\rm{O}} \right]_{{\rm{solution}}}} \to {\rm{C}}{{\rm{u}}^{2 + }} + {\rm{S}} \downarrow + \,{\rm{H}_2{\rm{O}}}$$ (4)

    Using artificial sphalerite as a raw material, at the basic conditions with a temperature of 423 K, an H2SO4 solution concentration of 140 g/L, an oxygen partial pressure of 1.0 MPa, and a CuSO4 solution concentration of 0.01 mol/L, the Zn extraction and content of Cu2+ under different conditions were investigated, as shown in Figs. 5 and 6.

    Fig. 5.  Effects of different conditions on Zn extraction from the artificial sphalerite: (a) concentrate of CuSO4; (b) temperature; (c) concentrate of H2SO4; (d) oxygen partial pressure.
    Fig. 6.  Effects of different conditions on the Cu2+ leached from the artificial sphalerite: (a) temperature; (b) concentrate of H2SO4; (c) oxygen partial pressure.

    As shown in Figs. 5(b)5(d), with the increase of temperature, H2SO4 concentration, oxygen partial pressure, and leaching time, the leaching rate of Zn exhibited a relatively stable increase. However, as can be seen from Fig. 5(a), the zinc leaching rate during sphalerite oxygen pressure leaching decreases with increasing copper sulfate concentration, and the maximum zinc leaching rate was only 51.04% when the amount of CuSO4 was minimum of 0.01 mol/L, indicating that Cu2+ had a poor catalytic effect. This experiment results were inconsistent with the analysis results for the Cu2+ catalytic leaching mechanism predicted in Section 3.1.

    As shown in Fig. 6, under the different conditions, the Cu2+ content in the leaching solution decreased as the leaching time increased. However, most of the Cu still existed in the leaching solution, indicating that the Cu2+ reacted with H2S to a small extent. The CuS deposited on the mineral surface only minimally reacted with the [O]solution.

    The relative potential of the O2-pressure leaching system was investigated at a temperature of 403 K, an H2SO4 concentration of 110 g/L, an oxygen partial pressure of 0.8 MPa, and a CuSO4 concentration of 0.05 mol/L. Results are shown in Fig. 7.

    Fig. 7.  Potential change in the O2-pressure leaching process.

    As shown in Fig. 7, the potential change in the O2-pressure leaching system could be divided into the following two regions:

    (1) With the addition of sulfuric acid, Cu ions, and O, the rapid increase in the electric potential was mainly due to the addition of a large number of cations, such as Cu2+, H+, and [O]solution. The acid leaching consumption of H+ reduced the potential.

    (2) The potential dropped gently until the end of the leaching experiment, because Cu2+ gradually reacted with the H2S generated by acid leaching, forming CuS. That is, throughout the leaching cycle, the Cu2+ and H2S replacement reaction was continuous and rapid compared with the oxidation rate of CuS, Thus, macroscopically, there appeared to be a continuous decrease in the concentration of Cu2+; that is, the potential of the system decreased continuously.

    Two experiments were performed at 423 K. In the first, the reaction was stopped immediately after the material was added for 5 min (cooling water was connected, and O2 pressure was released). In the second, the reaction was completed after the addition of materials for 90 min. The O2-pressure-leached slag obtained from the two reactions was subjected to SEM analysis to compare the changes in the leaching residue at different reaction times. Results are shown in Figs. 810.

    Fig. 8.  SEM image showing the morphology of the sphalerite.
    Fig. 9.  SEM morphology of the leaching residue with the Cu2+ catalytic process (5 min, 423 K).
    Fig. 10.  SEM morphology of the leaching residue with the Cu2+ catalytic process (90 min, 423 K).

    As shown in Fig. 9, the mineral surface was still smooth after 5 min of leaching, but pits indicating corrosion were formed on the mineral surface. This indicated that there were not many CuS deposits on the mineral surface at this time; that is, most of the Cu elements still existed in the leaching solution, and no lattice replacement was achieved through the reaction with H2S. According to the regional energy-spectrum analysis, most of the leaching residue contained Zn, S, and a few Cu2+ deposits.

    As shown in Fig. 10, as the leaching time increased, villi deposits were generated on the mineral surface, and the amount of CuS deposited on the mineral surface gradually increased, making the surface rough. Because the CuS had difficulty interacting with the [O]solution, once sulfide was formed on the mineral surface, it was almost impossible to reproduce the oxidation to achieve catalyst regeneration. Additionally, the regional energy-spectrum analysis showed that the atomic percentages of Zn, S, and Cu changed after the leaching. In particular, the Cu content increased significantly, indicating that the Cu2+ reacted slowly with H2S as the leaching reaction proceeded. The continuous slow reaction of Cu2+ and H2S and the adhesion and coating on the mineral surface hindered the leaching reaction.

    Using artificial sphalerite as a raw material, at the basic conditions with a temperature of 463 K, an H2SO4 solution concentration of 110 g/L, an oxygen partial pressure of 0.8 MPa, and a CuSO4 solution concentration of 0.05 mol/L, the Zn extraction under different conditions was investigated, as shown in Figs. 11 and 12.

    Fig. 11.  Effects of different conditions on Zn extraction from the artificial sphalerite: (a) concentrate of CuSO4; (b) temperature; (c) concentrate of H2SO4; (d) oxygen partial pressure.
    Fig. 12.  Effects of different conditions on the Cu2+ leached from the artificial sphalerite: (a) temperature; (b) concentrate of H2SO4; (c) oxygen partial pressure.

    As shown in Fig. 11, with the increase of temperature, acid concentration, oxygen partial pressure, and leaching time, the leaching rate of Zn increased gradually. The leaching rate of Zn reached 95.71% with a temperature of 463 K, an acidity of 110 g/L, an oxygen partial pressure of 0.8 MPa, and a CuSO4 solution concentrate of 0.05 mol/L, and the reaction reached equilibrium. As shown in the Fig. 11(a), the Zn leaching rate increased with the amount of Cu added at 463 K, which is in contrast to Fig. 5(a). It was considered that Cu2+ had a better catalytic effect when the temperature was ≥463 K. Therefore, temperature was an important factor affecting the Cu2+ catalytic reaction.

    As shown in Fig. 12, when the temperature was ≥463 K, Cu2+ content in the leaching solution exhibited a slight decreasing trend within 0–10 min of the leaching time, and then almost entered the leaching solution. When the temperature was 463 K, the sulfuric acid concentration and oxygen partial pressure changed, and this phenomenon, again, occurred. This indicated that the Cu2+ participated in the initial stage of the reaction according to Eq. (6). First, it reacted with H2S, forming CuS. However, this reaction was soon replaced by that in Eq. (7). The CuS deposited on the mineral surface reacted with [O]solution rapidly and released Cu2+ for the cyclic reaction. This is consistent with the predicted Cu2+ catalytic leaching mechanism in Section 3.1.

    The relative potential of the O2-pressure leaching system was investigated with a temperature of 463 K, H2SO4 concentration of 110 g/L, oxygen partial pressure of 0.8 MPa, and CuSO4 concentration of 0.05 mol. Results are shown in Fig. 13.

    Fig. 13.  Potential change in the O2-pressure leaching process

    In Fig. 13, the curve representing the relative potential of the artificial sphalerite is divided into three areas, and the following observations were made. (1) After the addition of sulfuric acid and O, the system potential increased rapidly and then decreased. This phenomenon was due to the addition of considerable quantities of H+, Cu2+, and [O]solution, which rapidly increased the potential of the leaching system. Subsequently, the sphalerite in the pyrolysis activation state caused preliminary and rapid acid dissolution. A certain amount of H2S was generated, which caused the sulfuric acid to be consumed. Additionally, the Cu2+ gradually reacted with the H2S to form CuS, which further reduced the potential. (2) At the beginning of the leaching process, the system potential exhibited a significant overall increasing trend (as shown in Region (2) in Fig. 13) caused by [O]solution and Cu2+. In particular, when the temperature exceeded 463 K, CuS was effectively destroyed by [O]solution and H2SO4, and Cu2+ was re-released. Thus, the Cu2+ and [O]solution improved the elimination and oxidation of the H2S gas film. (3) During the middle stage of the leaching process, the potential fluctuations were smaller than those in the previous stage because the sphalerite had been consumed; therefore, a larger amount of S2− was oxidized, increasing the system potential. The reduction of the S2− ions gradually weakened the fluctuations in the electric potential, and the curve showed a smooth upward trend. In the final stage of the leaching process, because the basic leaching reaction reached equilibrium and a Zn leaching rate of >90% was achieved in 70 min, no significant change was observed in the potential of the system.

    Two experiments were performed at 463 K, and the experimental conditions were the same as those for the experiment conducted at 423 K (Section 3.2.3). The O2-pressure leached slag obtained from the two reactions was subjected to SEM analysis to compare the changes in the leaching residue at different reaction times. Results are shown in Figs. 14 and 15.

    Fig. 14.  SEM morphology of the leaching residue with the Cu2+ catalytic process (5 min, 463 K)

    As shown in Fig. 14, the mineral surface was still relatively smooth after a leaching time of 5 min. According to regional energy-spectrum analysis, CuS was deposited on the mineral surface, indicating that Cu2+ and H2S reacted, yielding lattice replacement, and the increase in temperature was conducive to the reaction between Cu2+ and S2−.

    As shown in Fig. 15, the morphology of the product became a flocculant polymer with increasing leaching time. Owing to the temperature increase, the CuS and sphalerite deposited on the surface of the minerals interacted with [O]solution, forming Cu2+ and S. The Cu2+ and H2S reacted continuously, and the product was continuously dissolved, yielding the cycle regeneration of Cu2+. Additionally, the regional energy-spectrum analysis showed that the atomic percentages of Zn, S, and Cu were changed after the leaching. In particular, the significant increase in the content of S indicated that with the progress of the leaching reaction, ZnS and CuS were continuously leached out, and Cu2+ and [O]solution continued to react with H2S, yielding elemental S as the final product.

    Fig. 15.  SEM morphology of the leaching residue with the Cu2+ catalytic process (90 min, 463 K).

    In order to determine the control steps of Zn leaching in the Cu catalyst system at different temperature ranges, kinetic experiments were conducted during the leaching process. The kinetic equations [3] were fitted (leaching data from Figs. 5 and 11), and the interface equation was proven to have the best linear correlation by the correlation coefficient. Owusu et al. [2122] found that the dense solid product layer of S was effectively destroyed by adding calcium lignosulfonate in the leaching experiment. Therefore, compared with the other steps, the resistance of the layer could be considered to be relatively small. When the leaching process was controlled by the chemical reaction or by a hybrid control, which is explained in detail later in this section, the equation can be expressed as follows [23]:

    $${(1 - \alpha )^{1/3}} = \frac{{K \times C_2^{{n_1}} \times C_3^{{n_2}} \times {P^{{n_3}}} \times {{\rm{e}}^{ - E/RT}}}}{{{r_0} \times \rho }} \times t = kt$$ (5)

    where k is the synthesis rate constant, K is the rate constant, t is the leaching time (min), C2 is the concentrate of CuSO4 (mol·L−1), C3 is the concentrate of H2SO4 (g·L−1), P is the O2 pressure (MPa), r0 is the original radius of the particles (μm), T is the reaction temperature (K), ρ is the density of the particles (kg·m−3), R = 8.314 J·mol−1·K−1 is the universal gas constant, E is the apparent activation energy (J·mol−1), and ni is the order of the reaction.

    To determine the activation energy in the leaching process, the Zn leaching rates in Fig. 5(b) at different temperatures were plotted and fitted with a regression equation, as shown in Fig. 16. The result shows that the chemical reaction and unreacted shrinkage model had good linear correlation ($[1 - {(1 - \alpha )^{1/3}}]$t).

    Fig. 16.  Plot of the data fitting for the leaching rate of Zn and the leaching time for various temperatures (data from Fig. 5(b)).

    Fig. 17 was fitted for the reciprocal of the reaction temperature (1/T) versus the natural logarithm of the apparent rate constant (ln k). The apparent activation energy is expressed as follows:

    Fig. 17.  Relationship between ln k and T1 for the Zn leaching rates at 363–423 K.
    $$k = {A_0}{{\rm{e}}^{ - E/RT}}$$ (6)

    where A0 is the pre-exponential factor.

    The activation energy of the Zn leaching at 363–423 K was determined to be 38660 J·mol−1, indicating that the process was controlled by surface chemical reactions [23]. The kinetic equation describing the effect of the temperature (363–423 K) on the Zn leaching is as follows:

    $$\ln k = 5.06 - 4.65 \times {10^3}/T$$ (7)

    The addition of CuSO4 had a significant effect on the leaching of Zn. Fig. 18 shows $[1 - {(1 - \alpha )^{1/3}}]$ (data from Fig. 5(a)) versus leaching time at different CuSO4 concentrations, and in Fig. 19, these results were used to plot the natural logarithm of the CuSO4 concentration [ln c(CuSO4)] versus the natural logarithm of the apparent rate constant (ln k). The reaction order of zinc leaching, as calculated from Fig. 19, was −0.16, indicating that the increase in the Cu2+ concentration hindered the leaching reaction [3]. The kinetic equation describing the effect of the CuSO4 concentration on the Zn leaching rate is as follows:

    Fig. 18.  Plot of 1 − (1 − α)1/3 versus t for the leaching rate of Zn at different CuSO4 concentrations (data from Fig. 5(a)).
    Fig. 19.  Relationship between ln k and ln c(CuSO4) for Zn leaching.
    $$\ln k = - 6.77 - 0.16\ln c\left( {{\rm{CuS}}{{\rm{O}}_4}} \right)$$ (8)

    Fig. 20 shows$[1 - {(1 - \alpha )^{1/3}}]$ (data from Fig. 5(c)) versus leaching time at different H2SO4 concentrations, and Fig. 21, the relationship between the natural log of the apparent rate constant (ln k) and the natural log of the concentrate of H2SO4 [(ln c(H2SO4)]. The reaction order for zinc leaching, as calculated from Fig. 21, was 0.8. Thus, the kinetic equation describing the effect of the H2SO4 concentration on the Zn leaching rate is as follows:

    Fig. 20.  Plot of 1 − (1 − α)1/3 versus t for the leaching rate of Zn at different H2SO4 concentrations (data from Fig. 5(c)).
    Fig. 21.  Relationship between ln k and ln c(H2SO4) for Zn leaching.
    $$\ln k = - 10.04 + 0.8\ln c\left( {{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}} \right)$$ (9)

    Fig. 22 shows $[1 - {(1 - \alpha )^{1/3}}]$ (Data from Fig. 5(d)) versus leaching time at oxygen partial pressure, and Fig. 23 shows the natural logarithm of the natural logarithm of the oxygen partial pressure (ln PO2) and the apparent rate constant (ln k). The reaction order for Zn leaching, as calculated from Fig. 23, was 0.66. Thus, the kinetic equation describing the effect of the oxygen partial pressure on the Zn leaching rate is as follows:

    Fig. 22.  Plot of 1 − (1 − α)1/3 versus t for the leaching rate of Zn at different oxygen partial pressures (data from Fig. 5(d)).
    Fig. 23.  Relationship between ln k and ln ${ P}_{\bf O_2} $ for Zn leaching.
    $$\ln k = - 6.11 + 0.66\ln {P_{{{\rm{O}}_2}}}$$ (10)

    To determine the activation energy in the leaching process, the Zn leaching rates in Fig. 11(b) at different temperatures were plotted and fitted with a regression equation, as shown in Fig. 24.

    According to the fitting results in Fig. 24, Fig. 25 shows the natural logarithm of the apparent rate constant (ln k) versus the reciprocal of the reaction temperature (1/T). The apparent activation energy was calculated by Eq. (6).

    Fig. 24.  Plot of the data fitting for the leaching rate of Zn and the leaching time for various temperatures (data from Fig. 11(b)).
    Fig. 25.  Relationship between ln k and T1 for the Zn leaching rate at 423–483 K.

    The activation energy of the Zn leaching rates at 423–483 K was determined to be 36249 J·mol−1, indicating that the process was controlled by surface chemical reactions [23]. The kinetic equation describing the effect of the temperature (423–483 K) on the Zn leaching rate is as follows:

    $$\ln k = 6.01 - 4.36 \times {10^3}/T$$ (11)

    The addition of CuSO4 had a significant effect on the leaching of Zn. Fig. 26 shows $[1 - {(1 - \alpha )^{1/3}}]$ (data from Fig. 11(a)) versus leaching time at different CuSO4 concentrations, and in Fig. 27, these results are used to plot the natural logarithm of the CuSO4 concentration [ln c(CuSO4)] versus the natural logarithm of the apparent rate constant (ln k). The reaction order for zinc leaching, as calculated from Fig. 27, was 0.59, indicating that the increase in the Cu2+ concentration promoting the leaching reaction. The kinetic equation describing the effect of the CuSO4 concentration on the Zn leaching rate is as follows.

    $$\ln k = - 3.04 + 0.59\ln c\left( {{\rm{CuS}}{{\rm{O}}_4}} \right)$$ (12)
    Fig. 26.  Plot of 1 − (1 − α)1/3 versus t for the leaching rate of Zn at different CuSO4 concentrations (data from Fig. 11(a)).
    Fig. 27.  Relationship between ln k and ln c(CuSO4) for Zn leaching.

    Fig. 28 shows $[1 - {(1 - \alpha )^{1/3}}]$ (data from Fig. 11(c)) versus time at different H2SO4 concentrations, and in Fig. 29, these results were used to plot the natural logarithm of the H2SO4 concentration [ln c(H2SO4)] versus the natural logarithm of the apparent rate constant (ln k). The reaction order for Zn leaching, as calculated from Fig. 29, was 0.8. Thus, the kinetic equation describing the effect of the H2SO4 concentration on the Zn leaching rate is as follows:

    $$\ln k = - 10.93 + 1.31\ln c\left( {{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}} \right)$$ (13)

    Fig. 30 shows $[1 - {(1 - \alpha )^{1/3}}]$ (data from Fig. 11(d)) versus leaching time at different partial pressures of O2, and in Fig. 31, the natural log of the oxygen partial pressure (ln PO2) and the natural log of the apparent rate constant (ln k) are plotted. The reaction order for Zn leaching, as calculated from Fig. 31, was 1.54. Thus, the kinetic equation describing the effect of the oxygen partial pressure on the Zn leaching rate is as follows:

    Fig. 28.  Plot of 1 − (1 − α)1/3 versus t for the leaching rate of Zn at different H2SO4 concentrations (data from Fig. 11(c)).
    Fig. 29.  Relationship between ln k and ln c(H2SO4) for Zn leaching.
    Fig. 30.  Plot of 1 − (1 − α)1/3 versus t for the leaching rate of Zn at different oxygen partial pressures (data from Fig. 11(d)).
    Fig. 31.  Relationship between ln k and ln ${ P}_{\bf O_2} $ for Zn leaching.
    $$\ln k = - 4.54 + 1.54\ln {P_{{{\rm{O}}_2}}}$$ (14)

    According to Eq. (5), the kinetic equation can be expressed as follows:

    $$1 \!- {(1 \!-\! \alpha )^{1/3}} \!=\! K \times c{({\rm{CuS}}{{\rm{O}}_4})^{{n_1}}} \times c{({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4})^{{n_2}}} \times P_{{{\rm{O}}_2}}^{{n_3}} \times {{\rm{e}}^{ - E/RT}} \times t$$ (15)

    where K is the pre-exponential factor.

    The relationships between $[1 - {(1 - \alpha )^{1/3}}]$ and ${K_0} \times c{({\rm{CuS}}{{\rm{O}}_4})^{{n_1}}} \times c{({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4})^{{n_2}}} \times P_{{{\rm{o}}_2}}^{{n_3}} \times {{\rm{e}}^{ - E/RT}} \times t$ for the artificial sphalerite O2-pressure leaching at 363–423 and 423–483 K are shown in Fig. 32. Although the points in the plot show scattering, the straight lines could be fitted to the data with a correlation coefficient of > 0.97. Thus, according to Fig. 32, K0 was determined to be 0.053 and 0.028 for 363–423 and 423–483 K, respectively.

    Fig. 32.  Relationships between 1 − (1 − α)1/3 and ${{{ K} \times { c}({\bf{CuS}}{\bf{O}_{ 4}})^{{{ n}_{ 1}}} \times { c}{({{\bf{H}}_2}{\bf{S}}{{\bf{O}}_{ 4}})^{{{\bf n}_2}}} \times { P}_{{{\bf{O}}_{ 2}}}^{{{ n}_3}} \times {{\bf{e}}^{-{ {E/RT}}}} \times { t} }}$ for the artificial sphalerite O2-pressure leaching at (a) 363–423 K and (b) 423–483 K.

    According to the activation energy, reaction order, the kinetic equations of the artificial sphalerite O2-pressure leaching at 363–423 and 423–483 K could be expressed as Eqs. (16) and (17), respectively:

    $$\begin{aligned} &1 -\! {(1 - \alpha )^{1/3}} =\! 0.053 \times c{({\rm{CuS}}{{\rm{O}}_4})^{ - 0.16}} \times c{({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4})^{0.80}} \times \\ & \quad P_{{{\rm{O}}_2}}^{{0.66}} \times {{\rm{e}}^{ - 38660/RT}} \times t \end{aligned}$$ (16)
    $$\begin{aligned} & 1 - {(1 - \alpha )^{1/3}} = 0.028 \times c{({\rm{CuS}}{{\rm{O}}_4})^{0.59}} \times c{({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4})^{1.31}} \times \\ & \quad P_{{{\rm{O}}_2}}^{1.54} \times {{\rm{e}}^{ - 36249/RT}} \times t \end{aligned}$$ (17)

    In this paper, the catalytic mechanism of Cu2+ in the oxygen pressure acid leaching process of artificial sphalerite was studied, and the following conclusions can be drawn.

    (1) The zinc leaching behavior of artificial sphalerite in two different temperature ranges of 363–423 and 423–483 K was different. At 363–423 K, the zinc leaching efficiency gradually increased with the increase of temperature, acidity, and oxygen partial pressure, but decreased with the increase of CuSO4 concentration, the highest leaching rate was only 51.04%. However, the zinc leaching efficiency increased with increasing temperature, acidity, oxygen partial pressure, and CuSO4 concentration at 423–483 K, and the zinc leaching rate can reach 95.71%.

    (2) When the temperature was lower than 423 K, SEM–EDS showed that with the increase of leaching time, copper content on mineral surface gradually increased and villous sediments were formed. It was because the Cu2+ react with H2S generates CuS, and deposited on the surface of the mineral to hinder the leaching reaction, at this time, with the increased of leaching time, the system potential gradually decreases.When the temperature exceeds 423 K, CuS on the surface of the mineral reacts with [O]solution to form Cu2+ and S, Cu2+ can act as a catalyst to promote zinc ion leaching. At this time, with the increase of leaching time, the system potential shows a steady rise trend.

    (3) The activation energy of the O2-pressure acid leaching of the artificial sphalerite at 363–423 and 423–483 K was 38660 and 36230 J·mol−1, respectively. The apparent reaction orders of Cu2+, sulfuric acid, and oxygen partial pressure at 368–432 K were determined to be 0.6, 0.8, and 0.66, respectively, and at 423–483 K were 0.59, 1.31, and 1.54, respectively. Finally, the kinetic equation for fitting the results was formulated.

    This work was financially supported by the Joint Funds of the National Natural Science Foundation of China (Nos. 51804136, U1402271, 51764016), Jiangxi Province Nature Science Foundation, China (No. 20181BAB216017), Jiangxi Science and Technology Landing Project, China (No. KJLD13046), and Research Fund Program of State Key Laboratory of Rare Metals Separaten and Comprehensive Utilization, Guangzhou, China (No. GK-201803).

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