Leaching Of Gold And Palladium With Ozone

Hydrometallurgy 81 (2006) 142 – 151 www.elsevier.com/locate/hydromet

Leaching of gold and palladium with aqueous ozone in dilute chloride media J. Viñals a,⁎, E. Juan a , M. Ruiz a , E. Ferrando a , M. Cruells a , A. Roca a , J. Casado b a

University of Barcelona, Department of Materials Science and Metallurgical Engineering, Martí i Franqués 1, E-08028 Barcelona, Spain b MATGAS Research Centre, Universitat Autónoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain Received 22 September 2005; received in revised form 6 December 2005; accepted 6 December 2005 Available online 24 January 2006

Abstract The recycling of gold and palladium from metallic scraps can be carried out by ozone-leaching at ambient temperature and low (∼0.1 M) H+ and Cl− concentrations. Rh and Pt remain un-reacted, whereas metals such as Cu, Ni, Ag, can be previously eliminated through O2/H+ and O2/O3/H+ leaching pretreatments. Gold and palladium are dissolved in O3/Cl−/H+ with formation of − AuCl−4 and PdCl2− 4 . Leaching studies showed a passive region, basically located at b 0.01 and b 0.05 M Cl for Au and Pd, + − respectively. In the non-passive region, rates were only slightly dependent on either H and Cl . Secondary formation of chlorine or hypochlorous acid was negligible at ≤ 0.1 M Cl−. Kinetics appeared to be controlled by mass transfer of O3(aq) to the solid–liquid interface, showing first order dependency with respect to [O3]aq. Rates increased with temperature up to about 40 °C, but decreased at higher temperatures due to the fall in the O3 solubility. The ozone mass transfer coefficients showed an activation energy b 20 kJ/ mol. Gold leaching rate gradually diminished for pH N 2, as consequence of the influence of the [H+] on transfer control. The electric power consumption associated with O3 generation was in the range 4–8 kWh/kg metal leached. © 2006 Elsevier B.V. All rights reserved. Keywords: Gold; Palladium; Ozone; Chloride; Recycling; Leaching; Kinetics

1. Introduction Aqueous ozone may be a reasonable alternative for the leaching of concentrates and wastes containing precious metals, mainly due to the formation of oxygen as reaction by-product. Another advantage is that the ozone can be used at very low aqueous concentration (∼10− 4 M) by injecting O2/O3 mixtures at low PO3 (b10 kPa). Major disadvantage is the electric power consumption, 12–18 kWh/kg O3 (Gottschalk et al., ⁎ Corresponding author. E-mail addresses: [email protected] (J. Viñals), [email protected] (J. Casado). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2005.12.004

2000). This fact would limit the ozone leaching to high price metals and/or when high costs for the detoxification of effluents are to be expected in the conventional processes. Concerning gold hydrometallurgy, O3 pretreatment was reported by Van Antwerp and Lincoln (1987) as a means for improving gold recoveries in the subsequent hypochlorite leaching. Patiño et al. (2003) studied a similar pretreatment for refractory gold ores, prior to conventional cyanidation. Direct O3 leaching for gold was also investigated in different media. Scheiner and Linstrom (1973) patented a process for the gold recovery from carbonaceous ores by O3-treatment in high (1–5 M) chloride media. However, the stoichiometry of the

J. Viñals et al. / Hydrometallurgy 81 (2006) 142–151

reaction, the ozone consumption, and full kinetics were not determined. Grudeva and Grudev (1991) studied a combination of chemical and biochemical treatments for gold bearing polymetallic ores. The final step consisted of silver and gold thiourea leaching in presence of ozone. Reported extractions were 60% Au and 55% Ag but in 150 days. Nobre (1993) also patented a process of gold leaching with thiourea in presence of oxidants in which, among others, ozone was cited. Chtytan and Babayan (1977) studied the O3 leaching of gold and silver with acid thiocarbamide, and Chtytan et al. (1985) in polyacrylonitrile, in a medium of formamide and chloroform. Recently (Viñals et al., 2001, 2004), an O2–O3 leaching process (Metalozon) has been described. The process consists of three consecutive steps: (1) Cu and Ni are recovered in a first step by treatment in O2/ H2SO4(dil) (2) Ag is then selectively leached in O2/O3/ H2SO4(dil) (3) Au and Pd are recovered by treatment in O2/O3/H2SO4(dil) in presence of dilute Cl− (∼0.1 M), with Pt and Rh remaining in the final residue. The residual O2 from steps 2 and 3 is recycled to step 1. All steps are carried out at ambient temperature and pressure, with low reagent concentrations (∼0.1 M). The process was tested at laboratory-scale on gold bearing electronic scrap, gold gravimetric concentrates, anode slimes from copper electrorefining and waste radiographic plates. A kinetic study of the step 2 of the process has also been published (Viñals et al., 2005). The rate of the silver leaching was limited by mass transfer of the aqueous ozone with a first order dependency with respect to the PO3 and [O3]aq. The ozone consumption was about 1 mol O3/mol Ag. The present paper is a base study for optimizing step 3 of the Metalozon process. The kinetics of the reaction of gold and palladium with aqueous ozone, including stoichiometry and the effects of stirring speed, ozone, chloride and acid concentrations and temperature are described. Some preliminary experiments on Rh and Pt are also included in order to corroborate the selectivity of the Au and Pd leaching. 2. Materials and methods 2.1. Materials Metal powders (Au, Pd, Rh, Pt, N 99.9%) consisting of aggregates of spheroidal grains (0.5–5 μm), were used in most of the preliminary experiments and in the determination of the Au and Pd stoichiometry. Gold and palladium plates (N 99.9%) of 1 mm thickness and 21.1 cm2 (Au) and 19.6 cm2 (Pd) of

143

total surface, were used in the kinetic experiments for the measurement of specific rates and the mass transfer coefficients. 2.2. Leaching experiments The leaching apparatus was described in a previous paper (Viñals et al., 2005). It consisted of an ozone generator, a leaching reactor and an absorber of residual ozone containing conc. KI solution. The leaching reactor consisted of a stirred, thermostatted closed vessel, with a gas entrance to the solution through a fritted-glass diffuser, a gas exit connected to the KI absorber and a sampling device. Volumetric flow (q) and total pressure of the O2/O3 mixture were measured. The O3 mass flow (qO3) and PO3 in the input gas were regulated through the current intensity applied to the ozone generator. The solubility of O3 in the leach solutions was measured by pumping 50 cm3 solution into an absorber flask containing KI and further titration. Most of the experiments were carried out in the following conditions: q 21.5 L/h, qO3 0.42–2.1 g/h, PO3 1–5 kPa. The conditions for the preliminary tests on Au, Pd, Rh and Pd are reported with the leaching results. In some experiments, the very low extraction levels were determined by solution analysis in the ppb range, by using Inductively Coupled Plasma/Mass Spectroscopy (ICP-MS). Stoichiometry experiments for Au and Pd were performed as a follows: An excess of metal powder (0.200 g) was placed in 550 cm3 of a previously saturated O3/HCl solution of known concentration (O3 5.1 × 10− 4 M). The mixture was maintained hermetically closed and stirred for different time periods (up to ∼6 h) until ozone was completely consumed. Stoichiometry was determined by the ratio of the moles of Au or Pd dissolved and the initial moles of ozone in the reactor. Kinetic experiments on Au and Pd were conducted in 500 cm3 solutions of known O3 concentration, as described in the previous paper (Viñals et al., 2005). A plate of constant surface area of Au or Pd was suspended in the solution, and the leaching rate was measured by solution sampling and analysis by AAS (Au) or ICPAES (Pd). The O3 saturation was practically maintained during the experiments because the rates of metal leached (∼ 10 − 4 mol/h) were relatively small as compared with the rate of ozone injection (∼10− 2 mol O3/h). In the kinetic study, the effects of H+ and Cl− were separated by using H2SO4/NaCl solutions. The conditions of each group of tests are reported together with the leaching results. •

•

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3. Results and discussion

HCl 0.25M

40

H2SO4 0.25M

30

0.1

20

10

0 0

10

3.1. Preliminary experiments on Au, Pd, Rh and Pt 3.1.1. Gold No leaching of Au in O3/H2SO4 (25 °C, 1 h) was detected and no surface change was observed using SEM. However, it is well known that the action of gaseous O3 on gold surface produces nanolayers of Au2O3 (King, 1995; Krozer and Rodahl, 1997). In aqueous media, however, nanolayers of gold(III)hydroxyl species such as Au(OH)3 seem more probable, because these were reported during the anodic oxidation of gold in H2SO4(dil) (Gmelin Handbook of Inorganic and Organometallic Chemistry, 1992). Treatment in O3/ HCl removes these layers showing linear leaching rates (Fig. 1). No significant [HCl] effect was observed in a wide range of concentrations (0.1–2 M). 3.1.2. Palladium The behavior of Pd was similar to Au. However, in O3/H2SO4 some leaching was detected by ICP-MS solution analysis, although the levels were extremely low, even with 0.5–5 μm particles (∼0.05% in 1 h). These low values ensure a practical selectivity of the silver leaching in analogous conditions (Viñals et al., 2005). On the other hand, the curve obtained in O3/ H2SO4 (Fig. 2) suggests the formation of a passive surface. Although no product layer was observed using

20

30

40

50

60

0 70

Time (min)

Fig. 2. O3 leaching of Pd powder in different media. (12 mg Pd/300 cm3 solution, 20 °C, q: 21.5 L/h, PO3: 4.95 kPa, 700 min− 1).

SEM, PdO was reported as the most stable oxide during the anodic passivation of this metal (Gmelin Handbook of Inorganic and Organometallic Chemistry, 1989). The leaching in O3/HCl was N103 times faster than in O3/ H2SO4, showing linear rates. As in gold, no significant effect of the HCl concentration was observed in the range 0.25–1 M (Fig. 2). 3.1.3. Rhodium The behavior of Rh in O3 leaching was different from those of the Au and Pd. Rhodium reacts very slowly but at linear rates in O3/H2SO4, as observed through ICPMS. In addition, the presence of the Cl− does not significantly increases the leaching rate (Fig. 3). Moreover, temperature has a significant positive effect (Fig. 3), in spite of diminishing O3, for instance, by 50% from 20 to 40 °C. On the other hand, an increase of [H2SO4] leads to a continuous decrease in the leaching rate, which is nearly imperceptible at 5 M H2SO4, presumably as consequence of the fall in the O3 0.1

HCl 0.25M, 20°C H2SO4 0.25M, 20°C

50

H2SO4 0.25M, 40°C

30

% Extraction

HCl 0.1M HCl 1M HCl 2M

40 (gAu/m2)

0.2

HCl 1M % Extraction in H2SO4

Selected samples of affected powders and plates were observed using Scanning Electron Microscopy in conjunction with Energy Dispersive Spectrometry (SEM/EDS). For the detection of possible Cl2 or HClO in experiments in O3/Cl− media, UV spectra of selected solutions were recorded in the range 190–500 nm.

% Extraction in HCl

144

20

0.05

10 0

0 0

20

40

60

80

100

Time (min)

Fig. 1. O3/HCl leaching of Au plates. (21.1 cm2 plate/500 cm3 solution, 25 °C, q: 21.5 L/h, PO3: 4.95 kPa, 700 min− 1).

0

20

40

60

80

Time (min)

Fig. 3. O3 leaching of Rh powder in different media. (12 mg Rh/300 cm3 solution, q: 21.5 L/h, PO3: 4.95 kPa, 700 min− 1).

J. Viñals et al. / Hydrometallurgy 81 (2006) 142–151

3.1.4. Platinum No significant attack and surface change was detected (by SEM) on Pt in O3/H2SO4 (0.25 M H2SO4, 25 °C, 3 h), or in O3–HCl (0.1–2 M HCl, 25 °C, 3 h). Leaching in O3/H2O2/HCl showed significant rates, but only at high HCl concentration (6 M). However, chlorine evolved under these last conditions, and, consequently, no environmental advantage over the classical leaching processes was observed.

100 min-1

20

300 min-1 500 min-1

16 (mole Au/m2)x100

solubility. However, even at 40 °C and in dilute H2SO4, the leaching rate of rhodium can be considered negligible as compared from the data of Ag in O3/ H2SO4, and the data obtained in the present study for Au and Pd in O3/Cl− media.

145

500 min-1 bis 700 min-1

12

700 min-1 bis 900 min-1

8

900 min-1 bis

4

0 0

20

40

60

80

100

Time (min)

Fig. 5. Effect of stirring speed on gold leaching. (25 °C, H2SO4 0.01 M, NaCl 0.05 M, PO3 4.95 kPa).

3.2. Kinetics of gold and palladium leaching 3.2.1. Stoichiometry Fig. 4 shows the results of the stoichiometry experiments. Results indicate a molar ratio of metal leached/O3 consumed, close to 0.67 and 1 for Au and Pd, respectively. Thus, the overall reactions can be written as: Au þ 3=2O3 þ 3Hþ þ 4Cl− →AuCl−4 þ 3=2O2 þ 3H2 O

ð1Þ

Pd þ O3 þ 2Hþ þ 4Cl− →PdCl2− 4 þ O2 þ H2 O

ð2Þ

Formation of Pd(II) was also reported when Pd reacts with H2O2 in HCl media (Zhang and Zheng, 2003), along with PdCl42− the predominant species for high Cl− / PdII ratio (Gmelin Handbook of Inorganic and Organometallic Chemistry, 1989). However, in absence

of the metal, the ozonization of the aqueous Pd(II) chloride complexes can produce oxidation to PdCl62−, although the kinetics is not instantaneous (Zamashchikov and Pryadko, 1985). 3.2.2. Effect of the stirring speed Figs. 5 and 6 show the results obtained for gold and palladium, respectively. Leaching showed practically linear rates, as expected for the constant surface area of the solid. For Pd plates, however, an initially slow rate was detected in all the leaching experiments. This was attributed to the presence of an initially passive surface due to contact with the atmospheric O2. Therefore, rate constants were computed from ≥ 5 min points. On the other hand, rates increase with increasing stirring speed, indicating a probable kinetic control due to transfer of aqueous ozone. Therefore, data treatment was similar to that used in the previous paper (Viñals et al., 2005). Assuming that

2 15 300 min-1

Pd

1.5

600 min-1

Pd

(mole Pd/m2)x100

mole metal / mole O3

Au

Pd(II)

1

0.5

Au

Au(III)

3

4

900 min-1

10

1000 min-1

5

0 0

1

2

5

6

Time (h)

0 0

10

20

30

40

Time (min)

Fig. 4. Plot of mole of metal leached/mole of O3 consumed at different times. (0.200 g metal powder, 550 cm3 HCl 1M, [O3]initial 5.1 × 10− M, 20 °C, 700 min− 1).

Fig. 6. Effect of stirring speed on palladium leaching. (25 °C, H2SO4 0.1 M, NaCl 0.1 M, PO3 4.95 kPa).

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J. Viñals et al. / Hydrometallurgy 81 (2006) 142–151 Au Pd

2.6x10-4 M 25

8

4

0 0

200

400

600

800

1000

1200

4.4x10-4 M 20

5.3x10-4 M

15

6.0x10-4 M

10 5

Stirring speed (min-1)

0

Fig. 7. Mass transfer coefficients of the aqueous ozone as a function of the stirring speed. (Conditions as in Figs. 5 and 6).

the reaction rate was basically limited by mass transfer in the aqueous ozone and considering the different stoichiometries for the two processes, the rates of Au and Pd leaching in fixed hydrodynamic conditions and temperature, can be written as: 1 dNAu − ¼ ð2=3Þkt ½O3
sat A dt −

1.3x10-4 M

30

(mole Au/m2)x100

k (m/s)x105

12

ð3Þ

1 dNPd ¼ kt ½O3
sat A dt

ð4Þ

Where A is the surface area (m2), Ni the mole of metal, [O3]sat the ozone concentration in the bulk solution (mol/m3), and kt (m/s) the mass transfer coefficient for aqueous ozone. In this formulation, the effects of Cl− and H+ ions are considered to have little significance over a wide range, as shown in the preliminary experiments. Fig. 7 shows a plot of the mass transfer coefficients, calculated from the specific rates and Eqs. (3) and (4), under different stirring conditions. For both metals, the values obtained for the O3 mass transfer coefficients were close, increasing with the stirring speed. This is

0

50

100

150

Time (min)

Fig. 9. Gold leaching data for different ozone concentrations. (25 °C, H2SO4 0.01 M, NaCl 0.05 M, 900 min− 1).

consistent with the kinetic control assumed to be present. 3.2.3. Effect of ozone concentration The effect of ozone concentration was studied at 900 min− 1, 25 °C and in the range [O3] 1.3 × 10− 4– 6.0 × 10− 4 M. These leach solutions were prepared by saturation at different PO3. The measured O3 solubility in dilute H2SO4/NaCl media and in dilute H2SO4 were similar (Fig. 8). Figs. 9 and 10 show the leaching data and Fig. 11 is a plot of the specific rates against the ozone concentration. Rates were practically proportional to the [O3], in agreement with the expected first order dependency. 3.2.4. Effect of chloride concentration The effect of the chloride concentration on the leaching rate is shown in Figs. 12 and 13. The plot of the specific rates vs chloride concentration is presented in Fig. 14. For both metals, there is a threshold [Cl−] at which there is a

10 5.3x10-4 M 5.2x10-4 M 3.3x10-4 M 2.0x10-4 M 1.5x10-4 M

8

8

H2SO4 0.1M, NaCl 0.1M

6

H2SO4 0.25M (Viñals et al., 2005)

(mole Pd/m2)x100

[O3] (mole/L)x104

H2SO4 0.01M, NaCl 0.05M

4

6 4 2

2

0 0

0 0

1

2

3

4

5

6

5

10

15

20

25

30

35

40

45

Time (min)

PO3 (kPa)

Fig. 8. Ozone solubility for different partial pressures. (25 °C).

Fig. 10. Palladium leaching data for different ozone concentrations. (25 °C, H2SO4 0.1 M, NaCl 0.5 M, 900 min− 1).

J. Viñals et al. / Hydrometallurgy 81 (2006) 142–151

indicated that chlorine can be formed in ozone/chloride solutions in acid media, according to:

4 Au

3

Pd

2

1

0 0

2

4

6

8

[O3] (mole/L)x104

Fig. 11. Plot of the specific rates against the ozone concentration. (25 °C, 900 min− 1) (Au: H2SO4 0.01 M, NaCl 0.05 M. Pd: H2SO4 0.1 M, NaCl 0.1 M).

change from passive to non-passive behavior. Under the conditions studied (25 °C, H2SO4 0.01 M) the passive region is located at b0.01 M Cl− for gold and b0.05 M for palladium. However, the passive behavior of Pd in the O3/Cl− / + H system is more complex than that observed for gold, and further studies using XPS-Auger would be necessary to investigate the nature of the oxide layers and their growth rates. Whereas for gold the [Cl−] was the only determinant parameter, for palladium the passive region also depends on the temperature and possibly on the [H+], for [H+] b 0.01 M. The borders of the passive region for Pd were not extensively studied. Fortunately, the passive region is located at highly diluted concentrations and low temperatures. Conditions such as T ≥ 20 °C, [Cl− ] ≥ 0.05 M and [H+ ] ≥ 0.01 ensure reactive palladium during the ozone leaching. In the non-passive region, the leaching rate for both metals was not very sensitive to Cl− concentration, although a maximum rate was found on about 0.05 M Cl−, with a slight decrease at higher concentrations. This latter effect can mainly be attributed to a slight decrease in ozone solubility (From 5.9 × 10− 4 mol/L at 0.01 M Cl− to 4.9 × 10−4 mol/L at 0.5 M Cl−). The use of HCl solutions instead of the H2SO4/ NaCl solutions was studied for gold (Fig. 1). The specific rates obtained at 0.1, 1 and 2M HCl were in the same range (3 ± 0.3 × 10− 5 mol/m2 s) that in H2SO4/NaCl (Fig. 14). On the other hand, the secondary formation of chlorine or hypochlorous acid during the O3/Cl − leaching was also checked. Yeatts and Taube (1949)

O3 þ 2Cl− þ 2Hþ →Cl2 þ H2 O þ O2

ð5Þ

d½Cl2
=dt ¼ k 1 ½O3
½Cl−
þ k 2 ½O3
½Cl−
½Hþ
:

ð6Þ

From the reported kinetic constants at 0 °C and 9.5 °C, the values at 25 °C can be estimated, resulting in: k1 ∼ 10− 3 L/mol s and k2 ∼ 10− 2 L2/mol2 s. Reaction rate at 0.05 M Cl− and 0.01 M [H+] would be very small (∼10− 8 mol/L s) in the leach solutions of the present paper. But, for instance, at 0.5 M Cl− and 0.5 M H+ the rate could reach values of about 10− 6 mol/L s. More recent works (Hoigne et al., 1985; Levanov et al., 2002, 2003, 2005) confirm the ∼10− 3 L/mol s second order rate constant as well as the catalytic effect of the H+. In order to verify the possible formation of chlorine, UV spectra of different leach solutions were recorded (Fig. 15). For dilute Cl− concentration (≤0.1 M), only the peaks of ozone (260 nm) (Alder and Hill, 1950) and Cl− (195–200 nm) were detected. However, on increasing Cl− concentration to 0.5 M, a peak of hypochlorous acid (230 nm) (Zimmerman and Strong, 1957) appeared together with the peak of ozone. No molecular chlorine (310 nm) was found even at 0.5 M Cl−, because in acid medium and for very dilute (total) chlorine solutions, the hydrolysis equilibrium to hypochlorous acid is strongly displaced. Thus, the reaction of the ozone under the conditions tested was of the type: O3 þ Cl− þ Hþ →HClO þ O2

ð7Þ

Formation of some hypochlorous acid was not a serious inconvenient for the leaching, because it was

30

0.001 M 0.01 M 0.05 M 0.05 M bis 0.1 M 0.5 M

25 (mole Au/m2)x100

Rate (mole/m2.s)x105

5

147

20 15 10 5 0 0

50

100

150

Time (min)

Fig. 12. Gold leaching data for different chloride concentrations. (25 °C, H2SO4 0.01 M, PO3 4.95 kPa, 900 min− 1).

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J. Viñals et al. / Hydrometallurgy 81 (2006) 142–151 0.01 M 0.03 M 0.05 M 0.05 M bis 0.1 M 0.1 M bis 0.2 M 0.3 M 0.4 M 0.4 M bis 0.5 M

(mole Pd/m2)x100

9

6

3

0 0

10

20

30

40

Time (min)

Fig. 13. Palladium leaching data for different chloride concentrations. (25 °C, H2SO4 0.01 M, PO3 4.95 kPa, 900 min− 1).

Fig. 15. UV spectra for different ozone solutions (25 °C, H2SO4 0.1 M, O3 5.4 × 10− 4 M).

also consumed during the attack of the metals. Therefore, the global leaching rate was practically unaffected (Fig. 14). However, from the environmental point of view, optimum conditions are achieved operating at low [Cl−] and [H+], which minimize this secondary reaction.

For both metals, the specific rates (Fig. 16) increase with increasing temperature up to a maximum rate at about 40 °C, decreasing at higher temperatures. These results can be attributed to two overlapped and contrary effects: The increase of the rate constants and the diminution of the O3 solubility when temperature increases. In order to determine the mass transfer coefficients at different temperatures, the O3 solubility in the leaching media was measured. Fig. 17 shows a plot of the ln [O3]sat vs the reciprocal of the absolute temperature. Solubility decreases exponentially from 7.5 × 10− 4 M at 12 °C to 1.6 × 10− 4 M at 80 °C, and is nearly identical at

3.2.5. Temperature effect Temperature effect was studied in the range 12– 80 °C, [H2SO4] 0.01 M, at a fixed PO3 4.95 kPa. [Cl−] was 0.05 M for gold and 0.1 M for palladium, except in the experiment at 12 °C in which Pd was passive under the former conditions. At 12 °C, the test for Pd was performed in [H2SO4] 0.05 M, [Cl−] 0.5 M.

4.5 5

4 3.5 Rate (mole/m2.s)x105

Rate (mole/m2.s)x105

4

3

2 Au Pd

1

3 2.5 2 1.5 Au

1

Pd

0.5 0

0 0

0.1

0.2

0.3

0.4

0.5

[Cl-] (mole/L)

Fig. 14. Plot of the specific rates against the chloride concentration. (25 °C, PO3 4.95 kPa, H2SO4 0.01 M, 900 min− 1).

0

20

40

60

80

100

Temperature (°C)

Fig. 16. Specific rates at different temperatures. (PO3 4.95 kPa, [H2SO4] 0.01 M, [Cl−] 0.05 M for Au and 0.1 M for Pd, 900 min− 1).

J. Viñals et al. / Hydrometallurgy 81 (2006) 142–151 -6.5 2.6

2.8

3

3.2

3.4

3.6

3.8

-7

ln [O3] (mole/L)

y = 2.0365x - 14.369 -7.5

-8 [Cl-] 0.05M [Cl-] 0.1M -8.5

-9 1000/T (K-1)

Fig. 17. O3 solubility in H2SO4 0.01 M at PO3 4.95 kPa. o 0.05 and 0.1 M Cl−. From this plot, the ΔHdis of the ozone can be evaluated, resulting in − 17 kJ/mol. These data agree with those in the literature in pure water (− 16.3 kJ/mol, Briner and Perrottet, 1939), indicating that there was no significant effect of the added ions in the solution enthalpy of ozone. Since this value corresponds to an intrinsic thermodynamic property, ozone as the major oxidant species was confirmed in these diluted chloride media. Fig. 18 is the Arrhenius plot of the mass transfer coefficients obtained from the specific rates and the measured O3 solubility. In the same figure, the data of the silver leaching in aqueous ozone but in the absence of Cl− were also plotted. The non-specific behavior of the ozone for these metals is clearly evident, as expected from the non-specific character of the mass transfer kinetics. In the range 25–80 °C, activation energies of 11 and 17 kJ/mol (3–4 kcal/mol) were obtained for Au and Pd, respectively, which confirmed the rate control by transfer of the aqueous ozone. However, the

149

Arrhenius plot was not completely linear indicating some mixed transfer-chemical control at low temperatures, but in any case, with a predominant transfer component. At b25 °C, the rough activation energy of ∼35 kJ/mol (∼8 kcal/mol) for both Au and Pd would be consistent with this. 3.2.6. Effect of H2SO4 concentration This effect was studied at 25 °C, PO3 4.95 kPa, Cl− 0.1 M, 900 min− 1, and in the range 0.01–1 M H2SO4 for both metals. Specific rates were not particularly sensitive in this concentration interval. Therefore, mass transfer coefficients very close to those presented in the previous figures were obtained. However, the leaching of gold at very low [H+] was also investigated. Fig. 19 shows the specific rates at different pH values. For pH N 2, leaching rate progressively diminishes and is nearly insignificant at pH 6. This response was nearly identical to that observed in silver leaching under analogous conditions, but in absence of Cl− (Fig. 19). Therefore, the decrease of the leaching rate when pH increases, should be attributed to a gradual change of the kinetic control from the O3 transfer to the H+ transfer. The effect of [H+] when pH increases, can be explained by means of a classical corrosion mechanism, through an electrochemical process with separate cathodic (+) and anodic (−) areas: ðþÞO3 þ H2 O þ 2e→O2 þ 2OH−

ð7Þ

ð−ÞAu þ 3H2 O→½AuðOHÞ3
s þ 3Hþ þ 3e

ð8Þ

½AuðOHÞ3
s þ 3Hþ þ 4Cl− →AuCl−4 þ 3H2 O

ð9Þ

6 Au Ag (Viñals et al., 2005)

-8.1 2.8

3

3.2

3.4

3.6

ln k (m/s)

-8.6 -9.1 -9.6 -10.1

Au Pd Ag (Viñals et al., 2005)

-10.6

Rate (mole/m2.s)x105

5 4 3 2 1 0

1000/T (K-1)

0

2

4

6

pH

Fig. 18. Arrhenius plot of the rate constants. (PO3 4.95 kPa, Au: H2SO4 0.01 M, Cl− 0.05 M, 900 min− 1. Pd: H2SO4 0.01 M, Cl− 0.1 M, 900 min− 1. Ag: H2SO4 0.25 M, 700 min− 1).

Fig. 19. Gold and silver specific rates at different pH values. (25 °C, PO3 4.95 kPa. Au: Cl− 0.1 M, 900 min− 1. Ag: 700 min− 1).

150

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The transfer of Cl− cannot be a controlling step, since the non passive region for gold appeared at [Cl−] ≥ 10− 2 M, and the typical [O3] was about 10− 4 M. Therefore, under stationary state conditions, the rate of gold leaching can be related to the rates of the O3 and H+ transfer, as:

2. The global leaching reactions can be written as:

Rate ¼ 2=3k O3 ½O3
Aþ ¼ 1=3k Hþ ½Hþ
A−

However, gold and palladium present a region of passive behavior, located at Cl− b 0.01 M for Au and Cl− b 0.05 M for Pd. 3. In the non-passive region, rates are only slightly dependent on either H + (for pH b 2) and Cl − concentrations. Kinetics is controlled by mass transfer of O3(aq) to the solid–liquid interface, showing first order dependency with respect to [O3] aq and PO 3. Gold leaching rate gradually decreases for pH N 2, as consequence of the influence of the [H+] in the mass transfer control. 4. Secondary formation of chlorine or hypochlorous acid was not significant for Cl− ≤ 0.1 M, but for 0.5 M Cl− , ozone reacts partially in these leaching media forming hypochlorous acid:

ð10Þ

Where kO3 and kH+ are the mass transfer coefficients for O3 and H+, and A+, A−, the cathodic and anodic surface areas, respectively. Considering that the total area, A, is the sum of the cathodic and anodic areas, the rate equation can be written as: Rate ¼

2 A kO3 kHþ ½O3
½Hþ
: 6 kO3 ½O3
þ 3kHþ ½Hþ


ð11Þ

For pH ≤ 2, [H+] H [O3], and the rate depends basically on the [O3], as was confirmed from the leaching data. But for pH N 2 rates would also depend on the [H+], as was also experimentally observed (Fig. 14).

3.3. Electric power consumption The power consumption during ozone production was determined for different ozone partial pressures. For the used laboratory-scale generator (Ervin–Sander), the specific consumption was only slightly dependent of the PO3, showing values of 14–17 kWh/kg O3 in the range 2–5 kPa (Viñals et al., 2004). Data of similar magnitude were reported for the industrial generators (12–18 kWh/ kg O3, Gottschalk et al., 2000), which can operate up to about 9 kPa. Considering the kinetic characteristics of the O3 leaching of the studied metals, the most effective variable for maximizing the leaching productivity is the ozone concentration. Consequently, optimal conditions are at the maximum technically feasible PO3. For suspended metallic plates, the specific rates at 9 kPa can be extrapolated to about 45 g/m2 h for gold and about 25 g/m2 h for palladium and silver. The associated power consumption would be in the range of 4–8 kWh/kg of leached metal. 4. Conclusions 1. Gold and palladium can be leached in aqueous ozone at ambient temperature and low H + and Cl − concentrations (∼0.1 M). Under these conditions, rhodium and platinum remain stable.

Au þ 3=2O3 þ 3Hþ þ 4Cl− →AuCl−4 þ 3=2O2 þ 3H2 O Pd þ O3 þ 2Hþ þ 4Cl− →PdCl2− 4 þ O2 þ H2 O

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