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Pyrargyrite (Ag3SbS3): Silver and antimony dissolution by ozone oxidation in acid media
Hydrometallurgy 164 (2016) 15–23
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Pyrargyrite (Ag3SbS3): Silver and antimony dissolution by ozone oxidation in acid media C. Rodríguez-Rodríguez a, F. Nava-Alonso b,c,⁎, A. Uribe-Salas b, J. Viñals d a
Universidad de Guanajuato, Departamento en Ingeniería en Minas, Metalurgia y Geología, Ex Hacienda de San Matías s/n, San Javier, Guanajuato, Guanajuato 36000, Mexico CINVESTAV Unidad Saltillo, Industria Metalúrgica 1062, Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila 25900, Mexico On sabbatical leave at Department of Mining, Metallurgical and Materials Engineering, Laval University, Quebec, Qc G1K 7P4, Canada d Universidad de Barcelona, Departamento de Ingeniería Química y Metalurgia, Martí i Franqués 1, E-08028 Barcelona, Spain b c
a r t i c l e
i n f o
Article history: Received 4 October 2015 Received in revised form 22 April 2016 Accepted 30 April 2016 Available online 03 May 2016 Keywords: Pyrargyrite Ozone leaching Kinetics Refractory ores
a b s t r a c t Silver and antimony leaching from pyrargyrite (Ag3SbS3) by ozone in acid media was experimentally studied in order to evaluate this method as an alternative to the conventional cyanidation. When 1 g pyrargyrite was leached in 800 mL of 0.18 M sulfuric and 0.079 g O3/L gas was bubbled, it was observed that after 80% of metals dissolution the silver and antimony dissolution rates decreased until completely stopping. These results suggest the presence of a solid product layer onto the unreacted pyrargyrite cores. The SEM-EDS and XRD analysis showed that antimony oxides were formed as a layer covering the surface of pyrargyrite particles, hindering reactants transferring/diffusion to the unreacted surface. The chemical composition of the layer was identified as a silver/antimony oxide AgxSb2Oy (x = 0.51 ± 0.04 and y = 6.08 ± 0.43). For low solid/liquid ratios the pyrargyrite dissolution reached about 98%, however, for high solid/liquid ratios a maximum value of antimony concentration was found to be around 0.0008 mol/L. An activation energy of 27.88 kJ/mol was calculated, which is a typical value for mixed-control systems. The kinetic study showed that the controlling step changed as the reaction proceeds. The oxidation pretreatment with ozone is a promising alternative for pyrargyrite dissolution, currently under development. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The complex mineralogy of silver species represents a problem in the extractive metallurgy of precious metals. There are ores in which silver is associated with iron, arsenic, antimony, manganese or silicon; the silver recovery in these mineral is less than 80% when conventional cyanidation is used, and for this reason they are named refractory ores (Fraser et al., 1991). The methods currently available for the treatment of refractory ores are roasting, pressure oxidation and bacterial oxidation; however, environmental issues (e.g. SO2 generation), excessive costs and long operating times are associated with these methods. Pyrargyrite (Ag3SbS3) is one of the most important refractory silver ores due to its abundance; the silver recovery from this mineral species is low. The need of recovering gold and silver from refractory ores has encouraged the search of new approaches for the treatment of these minerals. One processing alternative is an oxidizing pre-treatment with ozone prior to the cyanide leaching of refractory ores. Besides the treatment of cyanidation effluents, ozone has played an increasing role in the extractive metallurgy in the last years; the
⁎ Corresponding author at: CINVESTAV Unidad Saltillo, Industria Metalúrgica 1062, Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila 25900, Mexico. E-mail address: [email protected] (F. Nava-Alonso).
http://dx.doi.org/10.1016/j.hydromet.2016.04.014 0304-386X/© 2016 Elsevier B.V. All rights reserved.
leaching of metals and its use in the gold hydrometallurgy have been the most significant applications suggested (Nava-Alonso et al., 2003; Ukasik and Havlik, 2005; Viñals et al., 2006). Due to its high oxidizing potential (E° = 2.07 V in acid solutions), ozone in acid media provides favorable thermodynamic conditions to oxidize all types of ores, mainly sulfide minerals (Beltrán, 2004). Some of the advantages of the use of ozone are its minimal impact on the environment, the oxygen formation as by-product and the possibility of working at atmospheric temperature and pressure (Roca et al., 2000). The use of ozone as oxidant for refractory ores has not been explained in detail, although lab-scale experiments indicate that ozone could be an alternative for this kind of ores. Several studies with industrial complex ores have demonstrated that gold and silver recoveries could increase after an ozone pretreatment (Elorza-Rodríguez et al., 2006; Carrillo-Pedroza et al., 2007; Li et al., 2009; González-Anaya et al., 2011). In previous works, Rodríguez-Rodríguez et al. (2014) and NavaAlonso et al. (2011) explored the pyrargyrite leaching with ozone in acid media. The authors defined the stoichiometry of the reaction and identified the variables that affect the dissolution (i.e., ozone concentration and acid concentration). It was also demonstrated that after 80% of silver dissolution, the silver and antimony dissolution rate decreased until completely stopping. These results suggest the presence of a solid product layer on the pyrargyrite particles.
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Table 1 Chemical characterization of pyrargyrite samples used in this work. Size fraction
d80, μm
% wt. Ag
Sb
S
Ag3SbS3a
Bulk Fines
No size particle control 13
54.47 53.8
20.75 20.89
17.3 17.0
91.2 90.1
a The balance of the mineral samples is quartz and minor amounts of lead and iron sulfides.
According to the above, the aim of this work is to identify the nature of the solid products of the pyrargyrite-ozone reaction and to determine the mechanism controlling the reaction. This knowledge would permit to increase the silver recovery in order to propose a method to process this refractory species. 2. Materials and methods Fig. 1. Silver and antimony dissolution as a function of ozone treatment time.
2.1. Materials The pyrargyrite sample used in this work was obtained from Zacatecas, Mexico, and was hand-concentrated after crushing and grinding a massive crystal. The two particle sizes employed were termed as the Bulk sample and the Fines fraction. The chemical composition of the samples is presented in Table 1; a characterization by X-ray diffraction (Philips X-Pert) showed that the only significant phase present is pyrargyrite. Silver and antimony in the solids were analyzed by atomic absorption spectrometry (Thermo Elemental Instruments SOLAAR S4), while sulfur was analyzed with a simultaneous carbon and sulfur analyzer (LECO CS230). The % of pyrargyrite was estimated from these data. The d80 of the size fraction was estimated from the particle size distribution measured with a laser diffraction particle size analyzer (Coulter LS-100Q). Dissolved silver and antimony in the liquid samples were analyzed by atomic absorption spectrometry (Varian SpectrAA220), and the partially reacted solids were characterized by SEM-EDS (JEOL JSM-840 equipped with INCA 250-Analyzer). 2.2. Leaching procedure The oxidative pyrargyrite leaching was performed in a one-liter cylindrical glass reactor mechanically stirred at 800 rpm (Lightnin L1U08) and hermetically closed with an acrylic cover which accommodates the pH, ORP and dissolved ozone electrodes. The reactor was equipped with a jacket through which water at constant temperature was circulated. Ozone was produced from dry oxygen with a Pacific Ozone Technology generator (L22), and was fed into the reactor through a stainless steel cylindrical sparger (5 μm pore diameter). Online measurements of pH (Orion 720A), redox potential (Orion 720A, platinum electrode against Ag/AgCl), dissolved ozone (ATI A15/64) and ozone concentration in the gas (ultraviolet monitor BMT 963) were registered. Signals were acquired with a Keithley KPCMCIA board and processed with ExceLINX software. For each leaching test, 800 mL of deionized water with the desired concentration of sulfuric acid were poured into the reactor and the solution temperature was set to 25 °C by means of the circulating water. Once the ozone concentration in the gas phase and the temperature were maintained constant at the desired value, the ozone containing gas stream was switched to
enter the reactor and the pyrargyrite sample was added to it. The data acquisition and the test began at this moment. At the end of the test, the remaining solids were washed with deionized water, filtered, dried at room temperature (ca. 28 °C) and weighed. The experimental setup has been described in detail before (Rodríguez-Rodríguez et al., 2014).
2.3. Methodology The work is divided into three sections: the leaching tests for the characterization of the partially reacted solids, the effect of the solids concentration on the silver and antimony dissolution, and the kinetic study to determine the controlling step of the reaction.
2.3.1. Leaching tests for characterization of reacted solids Two test of pyrargyrite oxidation with ozone in acid media were performed in order to evaluate the silver and antimony dissolution as well as to determine the nature of the solid products for two reaction times: 25 and 60 min (tests A and B). As the visual aspect of the solids after 60 min oxidation was different from that of the 25 min test, a third test with coarser particles and longer oxidation time (test C, 420 min) was conducted with the aim of characterizing the partially reacted particles. Table 2 shows the experimental conditions used in these tests.
2.3.2. Effect of the solids concentration on the silver and antimony dissolution It was observed from the leaching tests that the silver and antimony dissolution rates are not comparable; the antimony concentration in solution reaches a maximum value and then remains constant, whereas silver continues dissolving but at a smaller rate. In order to evaluate if the solids concentration has an effect on the metals dissolution, four leaching test were performed; the pyrargyrite amounts used were 0.25, 0.5, 1.0 and 1.5 g per 800 mL of 0.18 M sulfuric acid solution. In all the test the stirring speed was 800 rpm, the temperature 25 °C, the gas flowrate 1.2 L/min and the ozone concentration 0.079 L/min.
Table 2 Experimental conditions for solids characterization tests (0.18 M sulfuric acid concentration, 25 °C and 800 rpm). Test
Pyrargyrite weight, g
Volume of solution, mL
Ozone concentration, g/L
Gas flowrate, L/min
Particle size sample
A (25 min) B (60 min) C (420 min)
1 1 0.5
800 800 400
0.079 0.079 0.045
1.2 1.2 0.75
Fines Fines Bulk
C. Rodríguez-Rodríguez et al. / Hydrometallurgy 164 (2016) 15–23
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Fig. 2. X-ray diffraction patterns of pyrargyrite treated with ozone for tests A and B (25 and 60 min) compared with the initial pyrargyrite sample.
2.3.3. Kinetic study A kinetic study was performed in order to determine the controlling step of the pyrargyrite-ozone reaction. Leaching tests at 10, 25, 40 and 55 °C were carried out for the following experimental conditions: 1 g pyrargyrite (Fines) in 800 mL deionized water containing 0.18 M sulfuric acid; 800 rpm and 1.2 L/min of gas containing 0.079 g O3/L. The shrinking core model for heterogeneous reactions (Levenspiel, 1997) was used in order to identify any of the three controlling mechanisms of the liquid-solid heterogeneous reaction system:
t ¼ XB τ
a) Diffusion through the boundary layer on the solid-liquid interface, Eq. (1).
t ¼ 1−3ð1−X B Þ2=3 þ 2ð1−X B Þ τ
ð1Þ
b) Chemical reaction on the surface of the unreacted pyrargyrite core, Eq. (2). t ¼ 1−ð1−X B Þ1=3 τ
ð2Þ
c) Diffusion through the layer of solid products of the reaction, Eq. (3).
Fig. 3. X-Ray diffraction patterns of initial pyrargyrite sample and the two types of remaining solids for test C at 420 min.
ð3Þ
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Fig. 4. SEM-EDS elemental mapping of Ag, Sb and S conducted on the cross-section of a partially reacted pyrargyrite particle after ozone treatment (test C, 420 min).
where XB is the pyrargyrite fraction reacted at time t (s), and τ is the time for complete reaction of the particle (s). For each test, data were plotted using the three equations for identifying the controlling step, which corresponds to the equation that better fits to a straight line. From this equation, the slope (1 / τ), permits to obtain the required time for complete reaction (τ). For processes controlled by chemical reaction, it is possible to estimate the reaction rate constant ks, (cm/s) for each temperature, by means of Eq. (4): τ¼
ρB R bks C O3 g
ð4Þ
where ρB is the particle molar density (0.0107 mol/cm3), R is the initial
particle radius (0.00065 cm), b is the stoichiometric factor (1/14.5, based on pyrargyrite-ozone stoichiometry estimated by RodríguezRodríguez et al. (2014)) and CO3g is the ozone concentration in the bulk solution (mol/cm3). This model assumes that the ozone concentration in the bulk solution is constant; however, in the studied system the ozone concentration in solution varies as the reaction proceeds and therefore, to approximate the actual system, an average ozone concentration during the test was considered. The activation energy was obtained from Arrhenius equation (Eq. (5)): ks ¼ A0 e
−Ea RT
Ln ks ¼ −
Fig. 5. Morphology of partially reacted particles at the end of test C.
Ea 1 þ Ln A0 R T
ð5Þ
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Fig. 6. EDS spectrum and microanalysis of a) unreacted pyrargyrite particle core, and b) solid product layer.
Fig. 7. Brown flocs separated by decantation at the end of leaching test C; a) morphology and b) EDS spectrum and chemical analysis.
where A0 is the frequency factor, Ea is the apparent activation energy (kJ/mol) and R is the universal gas constant (kJ/mol K−1). 3. Results 3.1. Leaching tests for reacted solid characterization The behavior of silver and antimony dissolution for the leaching tests A and B is presented in Fig. 1, where it is observed that after 10 Table 3 Atomic percentage, atomic ratios and atomic number obtained from 10 measurements conducted on the product layer of solids from test C (SEM-EDS). Measurement
Atomic percentage
Atomic ratios
Atomic number For Sb = 2
1 2 3 4 5 6 7 8 9 10 Average Standard deviation
O
S
Ag
Sb
Sb/Ag
O/Sb
Ag
O
69.63 69.56 66.03 70.88 68.84 70.74 68.18 71.06 68.49 69.44 69.29 1.52
2.02 1.87 2.15 2.12 1.91 1.75 2.11 2.07 2.48 2.45 2.09 0.23
6.22 5.78 6.88 5.44 6.55 5.02 5.86 5.64 6.07 5.64 5.91 0.54
22.13 22.79 24.94 21.50 22.69 22.49 23.85 21.23 22.96 24.47 22.91 1.21
3.56 3.94 3.63 3.95 3.46 4.48 4.07 3.76 3.78 4.34 3.90 0.33
3.15 3.05 2.65 3.30 3.03 3.15 2.86 3.35 2.98 2.84 3.04 0.22
0.56 0.51 0.55 0.51 0.58 0.45 0.49 0.53 0.53 0.46 0.51 0.04
6.29 6.10 5.30 6.59 6.07 6.29 5.72 6.69 5.97 5.68 6.07 0.43
to 15 min the silver dissolution rate starts to decrease while that of the antimony is practically nil. The maximum silver dissolution was ca. 65% for test A, and 81% for test B (25 and 60 min respectively), whereas the maximum antimony dissolution was ca. 42% for both tests. The morphology and appearance of the reacted solids after 25 min leaching were similar to the initial pyrargyrite (e.g., dark and dense), while in the solids reacted 60 min, besides the remaining pyrargyrite, some brown and light flocs are visually observed. Elementary mapping obtained by SEM-EDS of the initial and reacted solids of test A, showed no difference in chemical composition and morphology, while those of test B revealed that the reacted solids consisted of particles with a higher concentration of antimony, compared to the initial sample. Fig. 2 presents the X-ray diffraction patterns obtained for the initial pyrargyrite sample and the solids for the test A y B after 25 and 60 min of oxidation, where it is observed that the diffraction peaks they are practically identical to those of the initial mineral. No reaction products of crystalline nature were detected, most likely because the reaction products are scarce or amorphous in nature. Test C, performed with coarser particles and longer oxidation time (420 min), was carried out in order to increase the amount of the new solid formed, and to permit its identification. At the end of this test, the remaining product consisted of two types of solids, as judged by visual inspection: a) pyrargyrite coarse particles of dark appearance, and b) brown flocs suspended in the bulk solution. These two types of particles were separated for characterization. Fig. 3 presents the X-ray diffraction patterns of both solids. The dark solid was identify as pyrargyrite (As3SbS3, JCPDS 077-0329, main pics at 27 and 32°), while
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Fig. 8. Silver and antimony dissolution as a function of time, for different solids concentrations.
Table 4 Silver and antimony molar concentrations at different moments during the leaching tests. Pyrargyrite in 800 mL
t, min
Ag
Sb
% 0.25 g 0.5 g 1.0 g
1.5 g
15 45 15 45 15 60 120 15 60 120 240
– 96.9 80.4 91.9 59.9 83.8 86.6 42.6 68.7 75.5 83.4
Ag
Sb
Table 5 Mass fraction of pyrargyrite dissolution (X) along the tests conducted at 10, 25, 40 and 55 °C. Data obtained from silver dissolution. Time, min
mol/L 89.4 99.5 65.2 72.2 44.7 47.6 49.7 25.6 27.5 25.2 22.9
– 0.0017 0.0027 0.0032 0.0040 0.0057 0.0060 0.0041 0.0067 0.0076 0.0086
0.0005 0.0005 0.0007 0.0008 0.0009 0.0011 0.0011 0.0008 0.0009 0.0008 0.0008
in the brown flocs, some antimony oxides were identified besides pyrargyrite (probably with low crystallinity due the nanometric size of the particles). The most significant peaks of the diffraction pattern of the brown flocs (at 15, 30, 50 and 59°), coincide well with the peaks of the antimony oxides (+ 5) (Sb2O5, PDF-00-011-0690, and Sb6O13, PDF 01-071-1091) as well as with the peaks of Ag0.5 Sb2O4.83 (PDF 00-052-0262). Oxides such as Sb2O3 and Sb2O4 were not detected. SEM-EDS characterization of the partially reacted solids obtained in test C is presented in Fig. 4, where a cross-section of a reacted pyrargyrite particle is presented (polished sample), showing the unreacted core surrounded by a layer of solid product. The chemical composition of this layer is mainly antimony, with traces of silver and sulfur.
5 10 15 20 25
Ag3SbS3 dissolution, X 10 °C
25 °C
40 °C
55 °C
0.189 0.410 0.529 0.623 0.658
0.309 0.539 0.658 0.712 0.735
0.277 0.525 0.668 0.720 0.752
0.253 0.551 0.682 0.730 0.765
Fig. 5 presents the morphology of the partially reacted particles and Fig. 6 the EDS spectra of the unreacted core (Fig. 6a) and of the product layer (Fig. 6b). The presence of oxygen in the external layer and its higher antimony content confirms the presence of antimony oxides. Fig. 7a shows the morphology of flocs of brown color decanted from the leach solution and Fig. 7b presents the corresponding EDS spectrum, showing the high atomic percent of Sb and O measured. This information was not taken into account in the analysis performed to identify the antimony oxide due to the presence of unreacted pyrargyrite fines. With the aim of identifying the antimony oxide species formed, several microanalysis were performed in fragments of the layer completely separated from the pyrargyrite particles (to avoid its interference due to the penetration of the electron beam). Table 3 summarizes the results of the microanalysis, together with the atomic Sb/Ag and O/Sb ratios obtained. The data obtained from this analysis confirmed the presence of the species AgxSb2Oy, with x = 0.51 ± 0.04 and y = 6.08 ± 0.43, stoichiometric coefficients close to those of the antimony oxide identified in the X-ray diffraction analysis presented in Fig. 3, namely Ag0.5Sb2O4.83.
Fig. 9. Effect of the temperature on a) kinetics of pyrargyrite dissolution and b) profiles of ozone dissolved in the leaching solution.
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Fig. 10. Comparison between the rate controlling steps of the shrinking core model for a) 10 °C b) 25 °C c) 40 °C and d) 55 °C.
3.2. Effect of solids concentration on silver and antimony dissolution The maximum antimony dissolution found in the leaching tests of Fig. 1, appears to indicate that no further antimony can be dissolved, which could correspond to the equilibrium concentration for the antimony oxide precipitation. In order to find out the maximum silver and antimony dissolution for different solids concentrations, four tests were performed under the experimental conditions reported in Section 2.3.2. Fig. 8 shows the metals dissolution obtained in these tests, where it can be observed that for the lowest solid concentration (0.25 g/800 mL), the silver and antimony dissolutions reached 98%, reflected by the complete dissolution of the initial pyrargyrite sample.
For the other solids concentrations (0.5, 1 and 1.5 g/800 mL), the antimony dissolution stopped after 15 min, while the silver dissolution rate decreased with time, but it did not stop. Table 4 presents the molar concentration of silver and antimony at different leaching times. It may be noted that the silver concentration always increases along the leaching tests; on the contrary, after 15 min the antimony concentration remains practically constant around a value between 0.0008 and 0.0011 mol/L. These facts indicate that for low solids concentrations, the mineral can be completely dissolved, whereas for high solids concentration, the antimony concentration in the solution will reach a maximum value, determined by the saturation concentration.
3.3. Kinetic study Tests of acid leaching of pyrargyrite with ozone at 10, 25, 40 and 55 °C were performed under the experimental conditions presented in Section 2.3.3. Fig. 9a shows the silver dissolution (pyrargyrite oxidation), while Fig. 9b presents the profiles of the ozone dissolved in the leaching solution. Similar silver dissolution kinetics was obtained for 25, 40 and 55 °C tests, but the lower temperature (10 °C) showed a lower dissolution. With regard to ozone solubility, it is corroborated
Table 6 Reaction rate constants obtained from the fitting of the data to the equation that considers the chemical reaction as the rate limiting step. Temperature, °C
Fig. 11. Experimental data adjusted to the equation of chemical reaction controlled kinetics, Eq. (2).
10 25 40 55
CO3,g, mol/cm3 −7
1.9 × 10 1.8 × 10−7 1.5 × 10−7 4.9 × 10−8
m ¼ 1τ −4
2.412 × 10 3.517 × 10−4 3.497 × 10−4 3.611 × 10−4
τ, s
ks, cm/s
4146 2843 2859 2769
0.122 0.192 0.233 0.712
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tion rate depends on ozone transfer rate from the gas to the liquid phase. When ozone is available in the solution there is a change in the controlling step: the kinetics of the reaction is then controlled by the chemical reaction; finally, when the antimony oxides appear on the surface of the pyrargyrite and the dissolution rate decreases due to the growing of the product layer, the controlling step changes and the pyrargyrite oxidation is controlled by the diffusion of reactants through the ash layer. Fig. 13 presents these controlling mechanisms during the pyrargyrite-ozone reaction. 4. Conclusions
Fig. 12. Arrhenius plot for the pyrargyrite oxidation.
by the data presented in Fig. 9b that this decreases as the temperature increases, as stated by Henry's law. Nevertheless, even if the dissolved ozone is significantly smaller for temperatures over 25 °C, the pyrargyrite oxidation rate observed at 25, 40 and 55 °C is practically the same. Table 5 shows the evolution of pyrargyrite dissolution as a function of the mass fraction with time for the different temperatures tested. Fig. 10 shows the data analyzed under the light of the shrinking core model. The highest correlation coefficients for the complete tests (1500 s) were obtained when the diffusion through the ash layer equation was used. Nevertheless, if the analysis is based on the data of the first minutes of pyrargyrite oxidation (900 s) the controlling step would be the chemical reaction as can be seen in Fig. 11. It may be suggested that the process is controlled by the chemical reaction at the beginning of the test, and subsequently the controlling step changes to diffusion through the ash layer due to antimony oxide precipitation. The reaction rate constants were calculated from the kinetic data adjusted to the chemical reaction equation of the model (see Table 6 and Fig. 11). From the Arrhenius plot shown in Fig. 12, the activation energy was calculated to be 27.8 kJ/mol; this value confirms the contribution of a mixed-control kinetics: diffusion through the product layer and chemical reaction (Habashi, 1996). These findings suggest that the controlling step for the kinetics of pyrargyrite oxidation with ozone changes as the reaction proceeds. Initially, when there is not ozone in the solution, the pyrargyrite dissolu-
Ozone oxidation can be considered as a promising alternative for the treatment of refractory ores such as pyrargyrite. This work presents an explanation to the deceleration of pyrargyrite dissolution when leached with ozone in acid media. The pyrargyrite was submitted to an acid oxidative leaching for 25 and 60 min, where it was observed that the silver dissolution rate substantially decreased after 15 min, while the antimony dissolution completely stopped. The reacted solids were analyzed by XRD and SEM-EDS; it was found that after 25 min of ozone treatment, the mineral morphology and the chemical composition were similar to the initial mineral sample; nevertheless, after 60 min the solids were notably different. The presence of a layer of solid product was observed on the surface of pyrargyrite unreacted cores, which hindered the transferring − of the reactants and products (ozone, H+, Ag2+, SbO− 3 and HSO4 ). This product layer is rich in antimony and oxygen and contains small amounts of silver, with concentrations that were consistent with the stoichiometry of a silver/antimony oxide of the type AgxSb2Oy (x = 0.51 ± 0.04 and y = 6.08 ± 0.43). The low antimony solubility (ca. 0.0008 mol/L) at the high redox potential (1500 mV vs SHE), is the reason of the decrease in the antimony dissolution rate; further dissolution leads to precipitation of antimony oxide. Different solid/liquid ratios experiments demonstrated that pyrargyrite can be completely dissolved at low solid/liquid ratios. The controlling step of the reaction changes as the pyrargyrite-ozone reaction proceeds according to the experimental behavior observed and the shrinking core model analysis performed. Diffusion of ozone gas through the liquid-gas interface is the initial controlling step, followed by the chemical reaction and the diffusion through the ash layer. An activation energy of 27.88 kJ/mol was obtained for the chemical reaction, a value typical of processes governed by mixed-control kinetic. Acknowledgements The authors gratefully acknowledge Conacyt for the financial support received through the scholarship granted to Rodríguez-Rodríguez, to the sabbatical support to Nava-Alonso, and through the research project CB2010/153885. References
Fig. 13. Graphical representation of the rate controlling steps and reaction products during the pyrargyrite-ozone reaction.
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Rodríguez-Rodríguez, C., Nava-Alonso, F., Uribe-Salas, A., 2014. Silver leaching from pyrargyrite oxidation by ozone in acid media. Hydrometallurgy 149, 168–176. Ukasik, M., Havlik, T., 2005. Effect of selected parameters on tetrahedrite leaching by ozone. Hydrometallurgy 77, 139–145. Viñals, J., Juan, E., Ruiz, M., Ferrando, E., Cruells, M., Roca, A., Casado, J., 2006. Leaching of gold and palladium with aqueous ozone in dilute chloride media. Hydrometallurgy 81, 142–151.
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Pyrargyrite (Ag3SbS3): Silver and antimony dissolution by ozone oxidation in acid media C. Rodríguez-Rodríguez a, F. Nava-Alonso b,c,⁎, A. Uribe-Salas b, J. Viñals d a
Universidad de Guanajuato, Departamento en Ingeniería en Minas, Metalurgia y Geología, Ex Hacienda de San Matías s/n, San Javier, Guanajuato, Guanajuato 36000, Mexico CINVESTAV Unidad Saltillo, Industria Metalúrgica 1062, Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila 25900, Mexico On sabbatical leave at Department of Mining, Metallurgical and Materials Engineering, Laval University, Quebec, Qc G1K 7P4, Canada d Universidad de Barcelona, Departamento de Ingeniería Química y Metalurgia, Martí i Franqués 1, E-08028 Barcelona, Spain b c
a r t i c l e
i n f o
Article history: Received 4 October 2015 Received in revised form 22 April 2016 Accepted 30 April 2016 Available online 03 May 2016 Keywords: Pyrargyrite Ozone leaching Kinetics Refractory ores
a b s t r a c t Silver and antimony leaching from pyrargyrite (Ag3SbS3) by ozone in acid media was experimentally studied in order to evaluate this method as an alternative to the conventional cyanidation. When 1 g pyrargyrite was leached in 800 mL of 0.18 M sulfuric and 0.079 g O3/L gas was bubbled, it was observed that after 80% of metals dissolution the silver and antimony dissolution rates decreased until completely stopping. These results suggest the presence of a solid product layer onto the unreacted pyrargyrite cores. The SEM-EDS and XRD analysis showed that antimony oxides were formed as a layer covering the surface of pyrargyrite particles, hindering reactants transferring/diffusion to the unreacted surface. The chemical composition of the layer was identified as a silver/antimony oxide AgxSb2Oy (x = 0.51 ± 0.04 and y = 6.08 ± 0.43). For low solid/liquid ratios the pyrargyrite dissolution reached about 98%, however, for high solid/liquid ratios a maximum value of antimony concentration was found to be around 0.0008 mol/L. An activation energy of 27.88 kJ/mol was calculated, which is a typical value for mixed-control systems. The kinetic study showed that the controlling step changed as the reaction proceeds. The oxidation pretreatment with ozone is a promising alternative for pyrargyrite dissolution, currently under development. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The complex mineralogy of silver species represents a problem in the extractive metallurgy of precious metals. There are ores in which silver is associated with iron, arsenic, antimony, manganese or silicon; the silver recovery in these mineral is less than 80% when conventional cyanidation is used, and for this reason they are named refractory ores (Fraser et al., 1991). The methods currently available for the treatment of refractory ores are roasting, pressure oxidation and bacterial oxidation; however, environmental issues (e.g. SO2 generation), excessive costs and long operating times are associated with these methods. Pyrargyrite (Ag3SbS3) is one of the most important refractory silver ores due to its abundance; the silver recovery from this mineral species is low. The need of recovering gold and silver from refractory ores has encouraged the search of new approaches for the treatment of these minerals. One processing alternative is an oxidizing pre-treatment with ozone prior to the cyanide leaching of refractory ores. Besides the treatment of cyanidation effluents, ozone has played an increasing role in the extractive metallurgy in the last years; the
⁎ Corresponding author at: CINVESTAV Unidad Saltillo, Industria Metalúrgica 1062, Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila 25900, Mexico. E-mail address: [email protected] (F. Nava-Alonso).
http://dx.doi.org/10.1016/j.hydromet.2016.04.014 0304-386X/© 2016 Elsevier B.V. All rights reserved.
leaching of metals and its use in the gold hydrometallurgy have been the most significant applications suggested (Nava-Alonso et al., 2003; Ukasik and Havlik, 2005; Viñals et al., 2006). Due to its high oxidizing potential (E° = 2.07 V in acid solutions), ozone in acid media provides favorable thermodynamic conditions to oxidize all types of ores, mainly sulfide minerals (Beltrán, 2004). Some of the advantages of the use of ozone are its minimal impact on the environment, the oxygen formation as by-product and the possibility of working at atmospheric temperature and pressure (Roca et al., 2000). The use of ozone as oxidant for refractory ores has not been explained in detail, although lab-scale experiments indicate that ozone could be an alternative for this kind of ores. Several studies with industrial complex ores have demonstrated that gold and silver recoveries could increase after an ozone pretreatment (Elorza-Rodríguez et al., 2006; Carrillo-Pedroza et al., 2007; Li et al., 2009; González-Anaya et al., 2011). In previous works, Rodríguez-Rodríguez et al. (2014) and NavaAlonso et al. (2011) explored the pyrargyrite leaching with ozone in acid media. The authors defined the stoichiometry of the reaction and identified the variables that affect the dissolution (i.e., ozone concentration and acid concentration). It was also demonstrated that after 80% of silver dissolution, the silver and antimony dissolution rate decreased until completely stopping. These results suggest the presence of a solid product layer on the pyrargyrite particles.
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Table 1 Chemical characterization of pyrargyrite samples used in this work. Size fraction
d80, μm
% wt. Ag
Sb
S
Ag3SbS3a
Bulk Fines
No size particle control 13
54.47 53.8
20.75 20.89
17.3 17.0
91.2 90.1
a The balance of the mineral samples is quartz and minor amounts of lead and iron sulfides.
According to the above, the aim of this work is to identify the nature of the solid products of the pyrargyrite-ozone reaction and to determine the mechanism controlling the reaction. This knowledge would permit to increase the silver recovery in order to propose a method to process this refractory species. 2. Materials and methods Fig. 1. Silver and antimony dissolution as a function of ozone treatment time.
2.1. Materials The pyrargyrite sample used in this work was obtained from Zacatecas, Mexico, and was hand-concentrated after crushing and grinding a massive crystal. The two particle sizes employed were termed as the Bulk sample and the Fines fraction. The chemical composition of the samples is presented in Table 1; a characterization by X-ray diffraction (Philips X-Pert) showed that the only significant phase present is pyrargyrite. Silver and antimony in the solids were analyzed by atomic absorption spectrometry (Thermo Elemental Instruments SOLAAR S4), while sulfur was analyzed with a simultaneous carbon and sulfur analyzer (LECO CS230). The % of pyrargyrite was estimated from these data. The d80 of the size fraction was estimated from the particle size distribution measured with a laser diffraction particle size analyzer (Coulter LS-100Q). Dissolved silver and antimony in the liquid samples were analyzed by atomic absorption spectrometry (Varian SpectrAA220), and the partially reacted solids were characterized by SEM-EDS (JEOL JSM-840 equipped with INCA 250-Analyzer). 2.2. Leaching procedure The oxidative pyrargyrite leaching was performed in a one-liter cylindrical glass reactor mechanically stirred at 800 rpm (Lightnin L1U08) and hermetically closed with an acrylic cover which accommodates the pH, ORP and dissolved ozone electrodes. The reactor was equipped with a jacket through which water at constant temperature was circulated. Ozone was produced from dry oxygen with a Pacific Ozone Technology generator (L22), and was fed into the reactor through a stainless steel cylindrical sparger (5 μm pore diameter). Online measurements of pH (Orion 720A), redox potential (Orion 720A, platinum electrode against Ag/AgCl), dissolved ozone (ATI A15/64) and ozone concentration in the gas (ultraviolet monitor BMT 963) were registered. Signals were acquired with a Keithley KPCMCIA board and processed with ExceLINX software. For each leaching test, 800 mL of deionized water with the desired concentration of sulfuric acid were poured into the reactor and the solution temperature was set to 25 °C by means of the circulating water. Once the ozone concentration in the gas phase and the temperature were maintained constant at the desired value, the ozone containing gas stream was switched to
enter the reactor and the pyrargyrite sample was added to it. The data acquisition and the test began at this moment. At the end of the test, the remaining solids were washed with deionized water, filtered, dried at room temperature (ca. 28 °C) and weighed. The experimental setup has been described in detail before (Rodríguez-Rodríguez et al., 2014).
2.3. Methodology The work is divided into three sections: the leaching tests for the characterization of the partially reacted solids, the effect of the solids concentration on the silver and antimony dissolution, and the kinetic study to determine the controlling step of the reaction.
2.3.1. Leaching tests for characterization of reacted solids Two test of pyrargyrite oxidation with ozone in acid media were performed in order to evaluate the silver and antimony dissolution as well as to determine the nature of the solid products for two reaction times: 25 and 60 min (tests A and B). As the visual aspect of the solids after 60 min oxidation was different from that of the 25 min test, a third test with coarser particles and longer oxidation time (test C, 420 min) was conducted with the aim of characterizing the partially reacted particles. Table 2 shows the experimental conditions used in these tests.
2.3.2. Effect of the solids concentration on the silver and antimony dissolution It was observed from the leaching tests that the silver and antimony dissolution rates are not comparable; the antimony concentration in solution reaches a maximum value and then remains constant, whereas silver continues dissolving but at a smaller rate. In order to evaluate if the solids concentration has an effect on the metals dissolution, four leaching test were performed; the pyrargyrite amounts used were 0.25, 0.5, 1.0 and 1.5 g per 800 mL of 0.18 M sulfuric acid solution. In all the test the stirring speed was 800 rpm, the temperature 25 °C, the gas flowrate 1.2 L/min and the ozone concentration 0.079 L/min.
Table 2 Experimental conditions for solids characterization tests (0.18 M sulfuric acid concentration, 25 °C and 800 rpm). Test
Pyrargyrite weight, g
Volume of solution, mL
Ozone concentration, g/L
Gas flowrate, L/min
Particle size sample
A (25 min) B (60 min) C (420 min)
1 1 0.5
800 800 400
0.079 0.079 0.045
1.2 1.2 0.75
Fines Fines Bulk
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Fig. 2. X-ray diffraction patterns of pyrargyrite treated with ozone for tests A and B (25 and 60 min) compared with the initial pyrargyrite sample.
2.3.3. Kinetic study A kinetic study was performed in order to determine the controlling step of the pyrargyrite-ozone reaction. Leaching tests at 10, 25, 40 and 55 °C were carried out for the following experimental conditions: 1 g pyrargyrite (Fines) in 800 mL deionized water containing 0.18 M sulfuric acid; 800 rpm and 1.2 L/min of gas containing 0.079 g O3/L. The shrinking core model for heterogeneous reactions (Levenspiel, 1997) was used in order to identify any of the three controlling mechanisms of the liquid-solid heterogeneous reaction system:
t ¼ XB τ
a) Diffusion through the boundary layer on the solid-liquid interface, Eq. (1).
t ¼ 1−3ð1−X B Þ2=3 þ 2ð1−X B Þ τ
ð1Þ
b) Chemical reaction on the surface of the unreacted pyrargyrite core, Eq. (2). t ¼ 1−ð1−X B Þ1=3 τ
ð2Þ
c) Diffusion through the layer of solid products of the reaction, Eq. (3).
Fig. 3. X-Ray diffraction patterns of initial pyrargyrite sample and the two types of remaining solids for test C at 420 min.
ð3Þ
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Fig. 4. SEM-EDS elemental mapping of Ag, Sb and S conducted on the cross-section of a partially reacted pyrargyrite particle after ozone treatment (test C, 420 min).
where XB is the pyrargyrite fraction reacted at time t (s), and τ is the time for complete reaction of the particle (s). For each test, data were plotted using the three equations for identifying the controlling step, which corresponds to the equation that better fits to a straight line. From this equation, the slope (1 / τ), permits to obtain the required time for complete reaction (τ). For processes controlled by chemical reaction, it is possible to estimate the reaction rate constant ks, (cm/s) for each temperature, by means of Eq. (4): τ¼
ρB R bks C O3 g
ð4Þ
where ρB is the particle molar density (0.0107 mol/cm3), R is the initial
particle radius (0.00065 cm), b is the stoichiometric factor (1/14.5, based on pyrargyrite-ozone stoichiometry estimated by RodríguezRodríguez et al. (2014)) and CO3g is the ozone concentration in the bulk solution (mol/cm3). This model assumes that the ozone concentration in the bulk solution is constant; however, in the studied system the ozone concentration in solution varies as the reaction proceeds and therefore, to approximate the actual system, an average ozone concentration during the test was considered. The activation energy was obtained from Arrhenius equation (Eq. (5)): ks ¼ A0 e
−Ea RT
Ln ks ¼ −
Fig. 5. Morphology of partially reacted particles at the end of test C.
Ea 1 þ Ln A0 R T
ð5Þ
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Fig. 6. EDS spectrum and microanalysis of a) unreacted pyrargyrite particle core, and b) solid product layer.
Fig. 7. Brown flocs separated by decantation at the end of leaching test C; a) morphology and b) EDS spectrum and chemical analysis.
where A0 is the frequency factor, Ea is the apparent activation energy (kJ/mol) and R is the universal gas constant (kJ/mol K−1). 3. Results 3.1. Leaching tests for reacted solid characterization The behavior of silver and antimony dissolution for the leaching tests A and B is presented in Fig. 1, where it is observed that after 10 Table 3 Atomic percentage, atomic ratios and atomic number obtained from 10 measurements conducted on the product layer of solids from test C (SEM-EDS). Measurement
Atomic percentage
Atomic ratios
Atomic number For Sb = 2
1 2 3 4 5 6 7 8 9 10 Average Standard deviation
O
S
Ag
Sb
Sb/Ag
O/Sb
Ag
O
69.63 69.56 66.03 70.88 68.84 70.74 68.18 71.06 68.49 69.44 69.29 1.52
2.02 1.87 2.15 2.12 1.91 1.75 2.11 2.07 2.48 2.45 2.09 0.23
6.22 5.78 6.88 5.44 6.55 5.02 5.86 5.64 6.07 5.64 5.91 0.54
22.13 22.79 24.94 21.50 22.69 22.49 23.85 21.23 22.96 24.47 22.91 1.21
3.56 3.94 3.63 3.95 3.46 4.48 4.07 3.76 3.78 4.34 3.90 0.33
3.15 3.05 2.65 3.30 3.03 3.15 2.86 3.35 2.98 2.84 3.04 0.22
0.56 0.51 0.55 0.51 0.58 0.45 0.49 0.53 0.53 0.46 0.51 0.04
6.29 6.10 5.30 6.59 6.07 6.29 5.72 6.69 5.97 5.68 6.07 0.43
to 15 min the silver dissolution rate starts to decrease while that of the antimony is practically nil. The maximum silver dissolution was ca. 65% for test A, and 81% for test B (25 and 60 min respectively), whereas the maximum antimony dissolution was ca. 42% for both tests. The morphology and appearance of the reacted solids after 25 min leaching were similar to the initial pyrargyrite (e.g., dark and dense), while in the solids reacted 60 min, besides the remaining pyrargyrite, some brown and light flocs are visually observed. Elementary mapping obtained by SEM-EDS of the initial and reacted solids of test A, showed no difference in chemical composition and morphology, while those of test B revealed that the reacted solids consisted of particles with a higher concentration of antimony, compared to the initial sample. Fig. 2 presents the X-ray diffraction patterns obtained for the initial pyrargyrite sample and the solids for the test A y B after 25 and 60 min of oxidation, where it is observed that the diffraction peaks they are practically identical to those of the initial mineral. No reaction products of crystalline nature were detected, most likely because the reaction products are scarce or amorphous in nature. Test C, performed with coarser particles and longer oxidation time (420 min), was carried out in order to increase the amount of the new solid formed, and to permit its identification. At the end of this test, the remaining product consisted of two types of solids, as judged by visual inspection: a) pyrargyrite coarse particles of dark appearance, and b) brown flocs suspended in the bulk solution. These two types of particles were separated for characterization. Fig. 3 presents the X-ray diffraction patterns of both solids. The dark solid was identify as pyrargyrite (As3SbS3, JCPDS 077-0329, main pics at 27 and 32°), while
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Fig. 8. Silver and antimony dissolution as a function of time, for different solids concentrations.
Table 4 Silver and antimony molar concentrations at different moments during the leaching tests. Pyrargyrite in 800 mL
t, min
Ag
Sb
% 0.25 g 0.5 g 1.0 g
1.5 g
15 45 15 45 15 60 120 15 60 120 240
– 96.9 80.4 91.9 59.9 83.8 86.6 42.6 68.7 75.5 83.4
Ag
Sb
Table 5 Mass fraction of pyrargyrite dissolution (X) along the tests conducted at 10, 25, 40 and 55 °C. Data obtained from silver dissolution. Time, min
mol/L 89.4 99.5 65.2 72.2 44.7 47.6 49.7 25.6 27.5 25.2 22.9
– 0.0017 0.0027 0.0032 0.0040 0.0057 0.0060 0.0041 0.0067 0.0076 0.0086
0.0005 0.0005 0.0007 0.0008 0.0009 0.0011 0.0011 0.0008 0.0009 0.0008 0.0008
in the brown flocs, some antimony oxides were identified besides pyrargyrite (probably with low crystallinity due the nanometric size of the particles). The most significant peaks of the diffraction pattern of the brown flocs (at 15, 30, 50 and 59°), coincide well with the peaks of the antimony oxides (+ 5) (Sb2O5, PDF-00-011-0690, and Sb6O13, PDF 01-071-1091) as well as with the peaks of Ag0.5 Sb2O4.83 (PDF 00-052-0262). Oxides such as Sb2O3 and Sb2O4 were not detected. SEM-EDS characterization of the partially reacted solids obtained in test C is presented in Fig. 4, where a cross-section of a reacted pyrargyrite particle is presented (polished sample), showing the unreacted core surrounded by a layer of solid product. The chemical composition of this layer is mainly antimony, with traces of silver and sulfur.
5 10 15 20 25
Ag3SbS3 dissolution, X 10 °C
25 °C
40 °C
55 °C
0.189 0.410 0.529 0.623 0.658
0.309 0.539 0.658 0.712 0.735
0.277 0.525 0.668 0.720 0.752
0.253 0.551 0.682 0.730 0.765
Fig. 5 presents the morphology of the partially reacted particles and Fig. 6 the EDS spectra of the unreacted core (Fig. 6a) and of the product layer (Fig. 6b). The presence of oxygen in the external layer and its higher antimony content confirms the presence of antimony oxides. Fig. 7a shows the morphology of flocs of brown color decanted from the leach solution and Fig. 7b presents the corresponding EDS spectrum, showing the high atomic percent of Sb and O measured. This information was not taken into account in the analysis performed to identify the antimony oxide due to the presence of unreacted pyrargyrite fines. With the aim of identifying the antimony oxide species formed, several microanalysis were performed in fragments of the layer completely separated from the pyrargyrite particles (to avoid its interference due to the penetration of the electron beam). Table 3 summarizes the results of the microanalysis, together with the atomic Sb/Ag and O/Sb ratios obtained. The data obtained from this analysis confirmed the presence of the species AgxSb2Oy, with x = 0.51 ± 0.04 and y = 6.08 ± 0.43, stoichiometric coefficients close to those of the antimony oxide identified in the X-ray diffraction analysis presented in Fig. 3, namely Ag0.5Sb2O4.83.
Fig. 9. Effect of the temperature on a) kinetics of pyrargyrite dissolution and b) profiles of ozone dissolved in the leaching solution.
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Fig. 10. Comparison between the rate controlling steps of the shrinking core model for a) 10 °C b) 25 °C c) 40 °C and d) 55 °C.
3.2. Effect of solids concentration on silver and antimony dissolution The maximum antimony dissolution found in the leaching tests of Fig. 1, appears to indicate that no further antimony can be dissolved, which could correspond to the equilibrium concentration for the antimony oxide precipitation. In order to find out the maximum silver and antimony dissolution for different solids concentrations, four tests were performed under the experimental conditions reported in Section 2.3.2. Fig. 8 shows the metals dissolution obtained in these tests, where it can be observed that for the lowest solid concentration (0.25 g/800 mL), the silver and antimony dissolutions reached 98%, reflected by the complete dissolution of the initial pyrargyrite sample.
For the other solids concentrations (0.5, 1 and 1.5 g/800 mL), the antimony dissolution stopped after 15 min, while the silver dissolution rate decreased with time, but it did not stop. Table 4 presents the molar concentration of silver and antimony at different leaching times. It may be noted that the silver concentration always increases along the leaching tests; on the contrary, after 15 min the antimony concentration remains practically constant around a value between 0.0008 and 0.0011 mol/L. These facts indicate that for low solids concentrations, the mineral can be completely dissolved, whereas for high solids concentration, the antimony concentration in the solution will reach a maximum value, determined by the saturation concentration.
3.3. Kinetic study Tests of acid leaching of pyrargyrite with ozone at 10, 25, 40 and 55 °C were performed under the experimental conditions presented in Section 2.3.3. Fig. 9a shows the silver dissolution (pyrargyrite oxidation), while Fig. 9b presents the profiles of the ozone dissolved in the leaching solution. Similar silver dissolution kinetics was obtained for 25, 40 and 55 °C tests, but the lower temperature (10 °C) showed a lower dissolution. With regard to ozone solubility, it is corroborated
Table 6 Reaction rate constants obtained from the fitting of the data to the equation that considers the chemical reaction as the rate limiting step. Temperature, °C
Fig. 11. Experimental data adjusted to the equation of chemical reaction controlled kinetics, Eq. (2).
10 25 40 55
CO3,g, mol/cm3 −7
1.9 × 10 1.8 × 10−7 1.5 × 10−7 4.9 × 10−8
m ¼ 1τ −4
2.412 × 10 3.517 × 10−4 3.497 × 10−4 3.611 × 10−4
τ, s
ks, cm/s
4146 2843 2859 2769
0.122 0.192 0.233 0.712
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tion rate depends on ozone transfer rate from the gas to the liquid phase. When ozone is available in the solution there is a change in the controlling step: the kinetics of the reaction is then controlled by the chemical reaction; finally, when the antimony oxides appear on the surface of the pyrargyrite and the dissolution rate decreases due to the growing of the product layer, the controlling step changes and the pyrargyrite oxidation is controlled by the diffusion of reactants through the ash layer. Fig. 13 presents these controlling mechanisms during the pyrargyrite-ozone reaction. 4. Conclusions
Fig. 12. Arrhenius plot for the pyrargyrite oxidation.
by the data presented in Fig. 9b that this decreases as the temperature increases, as stated by Henry's law. Nevertheless, even if the dissolved ozone is significantly smaller for temperatures over 25 °C, the pyrargyrite oxidation rate observed at 25, 40 and 55 °C is practically the same. Table 5 shows the evolution of pyrargyrite dissolution as a function of the mass fraction with time for the different temperatures tested. Fig. 10 shows the data analyzed under the light of the shrinking core model. The highest correlation coefficients for the complete tests (1500 s) were obtained when the diffusion through the ash layer equation was used. Nevertheless, if the analysis is based on the data of the first minutes of pyrargyrite oxidation (900 s) the controlling step would be the chemical reaction as can be seen in Fig. 11. It may be suggested that the process is controlled by the chemical reaction at the beginning of the test, and subsequently the controlling step changes to diffusion through the ash layer due to antimony oxide precipitation. The reaction rate constants were calculated from the kinetic data adjusted to the chemical reaction equation of the model (see Table 6 and Fig. 11). From the Arrhenius plot shown in Fig. 12, the activation energy was calculated to be 27.8 kJ/mol; this value confirms the contribution of a mixed-control kinetics: diffusion through the product layer and chemical reaction (Habashi, 1996). These findings suggest that the controlling step for the kinetics of pyrargyrite oxidation with ozone changes as the reaction proceeds. Initially, when there is not ozone in the solution, the pyrargyrite dissolu-
Ozone oxidation can be considered as a promising alternative for the treatment of refractory ores such as pyrargyrite. This work presents an explanation to the deceleration of pyrargyrite dissolution when leached with ozone in acid media. The pyrargyrite was submitted to an acid oxidative leaching for 25 and 60 min, where it was observed that the silver dissolution rate substantially decreased after 15 min, while the antimony dissolution completely stopped. The reacted solids were analyzed by XRD and SEM-EDS; it was found that after 25 min of ozone treatment, the mineral morphology and the chemical composition were similar to the initial mineral sample; nevertheless, after 60 min the solids were notably different. The presence of a layer of solid product was observed on the surface of pyrargyrite unreacted cores, which hindered the transferring − of the reactants and products (ozone, H+, Ag2+, SbO− 3 and HSO4 ). This product layer is rich in antimony and oxygen and contains small amounts of silver, with concentrations that were consistent with the stoichiometry of a silver/antimony oxide of the type AgxSb2Oy (x = 0.51 ± 0.04 and y = 6.08 ± 0.43). The low antimony solubility (ca. 0.0008 mol/L) at the high redox potential (1500 mV vs SHE), is the reason of the decrease in the antimony dissolution rate; further dissolution leads to precipitation of antimony oxide. Different solid/liquid ratios experiments demonstrated that pyrargyrite can be completely dissolved at low solid/liquid ratios. The controlling step of the reaction changes as the pyrargyrite-ozone reaction proceeds according to the experimental behavior observed and the shrinking core model analysis performed. Diffusion of ozone gas through the liquid-gas interface is the initial controlling step, followed by the chemical reaction and the diffusion through the ash layer. An activation energy of 27.88 kJ/mol was obtained for the chemical reaction, a value typical of processes governed by mixed-control kinetic. Acknowledgements The authors gratefully acknowledge Conacyt for the financial support received through the scholarship granted to Rodríguez-Rodríguez, to the sabbatical support to Nava-Alonso, and through the research project CB2010/153885. References
Fig. 13. Graphical representation of the rate controlling steps and reaction products during the pyrargyrite-ozone reaction.
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