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Effect of potential and ferric ions on lead sulfide dissolution in nitric acid
Hydrometallurgy 63 (2002) 171 – 179 www.elsevier.com/locate/hydromet
Effect of potential and ferric ions on lead sulfide dissolution in nitric acid Gennady L. Pashkov, Elena V. Mikhlina, Alexander G. Kholmogorov, Yuri L. Mikhlin* Institute of Chemistry and Chemical Technology of Siberian Branch of Russian Academy of Sciences, K. Marx Street, 42, Krasnoyarsk 660049, Russia Received 14 August 2001; received in revised form 9 November 2001; accepted 9 November 2001
Abstract The dissolution of the rotating disk electrode (RDE) of natural lead sulfide (galena) and the ground mineral in nitric acid solutions has been studied as a function of electrode potential, HNO3 concentration and temperature. The rate of dissolution producing hydrogen sulfide slowly increases as the potential varies from + 0.1 to 0.4 V (Ag/AgCl). The reaction order on nitric acid concentration has been found to be 1.2 F 0.15 at 0.2 V and 0.9 at 0.4 V (40 C), and the apparent activation energy is 35 kJ mol 1 in 1 M HNO3 at 0 V, suggesting that the process is controlled by a chemical or electrochemical reaction. At higher biases the RDE of PbS dissolves for the most part anodically, showing the highest rate at f 0.7 V, whereas the rate as a function of acid concentration is maximal in 1 M HNO3. The yield of sulfate increases with potential and is small for the leaching of both compact and ground galena, while it reaches 50% in the case of a flotation lead concentrate. Ferric ions catalyze the dissolution of compact and, especially, ground galena, with the peak rate at the potential of immersed platinum electrode of 0.4 – 0.5 V. The Fe3 + /Fe2 + couple is concluded to act as an intermediator for the electron transfer between nitrate ions and the solid, indicating that the dissolution is electrochemical in nature. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferric ions; Lead sulfide; Nitric acid
1. Introduction Nitric acid leaching of sulfide concentrates and ores is an attractive method for recovery of basic metals, because HNO3 is a strong oxidant that can be readily recycled by the addition of oxygen directly in the leach slurry or using the external oxidation of nitrous oxides. Several nitric acid-based methods for processing sulfide minerals and concentrates of non-ferrous metals and iron have been proposed and tested (Bjorling and Kolta, 1964: Prater et al., 1973; Habashi, 1973a,b; *
Corresponding author. Fax: +7-3912-238658. E-mail address: [email protected] (Y.L. Mikhlin).
Bjorling et al., 1976; Vizsolyi and Peters, 1980; Brennecke et al., 1981; Fair et al., 1986; Fuerstenau et al., 1987; Van Weert and Shang, 1993; Droppert and Shang, 1995; Kholmogorov et al., 1998; Novoselov and Makotchenko, 1999) but no commercial plants appear to be operating at present. An important weakness of the nitric acid leach is the oxidation of sulfide sulfur both to elemental sulfur and sulfate, in many cases in almost equal parts. This results in an increase in nitric acid consumption and necessity to utilize the sulfate, raising the costs of the solution treatment and the acid regeneration. In particular, although the lead – zinc sulfide concentrate was almost completely decomposed by 2 M HNO3 at 90 C, about 50% of lead re-
0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X ( 0 1 ) 0 0 2 2 1 - 3
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mained in the residue as PbSO4 (Kholmogorov et al., 1998). Hydrometallurgy of lead is complicated by a low solubility of the majority of lead salts, and the important advantage of nitrate electrolytes that lead nitrate is highly soluble is lost in the case of the conventional leaching of lead-containing products by nitric acid due to the lead sulfide conversion to the solid PbSO4. The kinetics and mechanism of metal sulfides dissolution in nitric acid media and the formation of elemental sulfur and sulfate are poorly understood. It seems to be established that the sulfate production grows with an increase in the nitric acid concentration and electrode potential of pyrite and other minerals, while elemental sulfur is oxidised to sulfate very slowly (Droppert and Shang, 1995; Flatt and Woods, 2000); however, conclusions about the effect of temperature on the sulfate yield are contradictory (Bjorling and Kolta, 1964; Flatt and Woods, 2000). Iron, copper, and silver ions were found to accelerate the dissolution of metal sulfides in nitric acid (Mulak, 1987; Fuerstenau et al., 1987), as well as in other oxidative media. The behavior of lead sulfide (galena) in HNO3 solutions has received only very little attention. Fuerstenau et al. (1987) reported the kinetics of galena leaching in ferric nitrate solutions. They found that the galena oxidation produced elemental sulfur and the rate was proportional to the square root of the concentration of Fe3 + ; the addition of more than 1 M sodium nitrate slightly decelerated the reaction. The activation energy was 47 kJ mol 1, and the authors assumed that the dissolution proceeded by the electrochemical mechanism with ferric ions as an oxidant and the rate was controlled by surface reactions. Holmes and Crundwell (1995) studied the voltammetry of the electrochemical oxidation of the rotating disk electrode (RDE) of PbS in 0.6 M NaNO3 + 0.5 M H + solution and arrived at the conclusion that the current was controlled by the rate of the electrochemical reaction, although the maximum or shoulder at 0.48 V vs. SCE was ascribed to the oxidation of the passive film of elemental sulfur. Mikhlin et al. (2002) compared the voltammetry and XPS and SEM data for PbS oxidised in nitric and perchloric acids and suggested that the anodic dissolution was affected by the alterations of the composition and semiconducting characteristics of the disordered reacted surface layer of galena rather than the formation of ele-
mental sulfur and lead sulfate or thiosulfate. It was also shown that the oxidation products, including sulfate, arise from the decay of the distorted non-stoichiometric layer. The objective of this research was to study the kinetics and mechanism of PbS decomposition in nitric acid solutions as a function of electrode potential and to correlate the behaviour of compact galena electrodes with that of the ground mineral and a sulfidic lead concentrate. The strong catalytic effect of ferric ions was discovered and considered as an important detail for explaining the mechanism of PbS interaction with acidic nitrate electrolyte.
2. Experimental The polycrystalline galena obtained from Geological Museum of Central Siberia (Krasnoyarsk, Russia) had less than 0.1% by mass of iron, zinc, copper and silver as the major impurities and no inclusions of other phases. The material was of n-type conductivity with f61017 cm3 electron density. The flotation lead concentrate from Gorevskaya concentrating plant contained (%) Pb 59.2, Zn 2.6, Fe 10.0 (mainly as FeCO3), Mn 0.27, S 20, and was used in leach experiments without additional treatment. Disk electrodes were cut from massive mineral specimens to obtain cylindrical samples of f8 mm diameter and were embedded in Teflon. Copper was electrochemically deposited from acidic sulfate solutions on the side opposite to a working surface of the electrode in order to fabricate the ohmic contact. Electrode surfaces were polished on silicon carbide paper, cleaned with wet filter paper to remove fine particles and rinsed with doubly distilled water. A three-electrode water-jacketed glass cell was used; a Pt counter electrode was separated from the working electrode compartment by a glass frit. The electrode potential was referenced against a saturated Ag/AgCl electrode. The experiments were performed with a rotating disk electrode installation SVA-1BM; the speed of disk rotation was normally 1900 min 1. In order to examine the kinetics of the RDE dissolution, 5– 8 aliquots were periodically extracted from the cell and analysed for lead using atomic absorption spectroscopy. The dissolution rate was calculated as a slope of the amount of dissolved lead vs. time curves for periods of 15 to
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180 min, maintaining a comparable degree of the PbS etching. The current through the electrode was registered by a recorder and integrated to determine the charge passed. The galena samples were ground in a jasper mortar to 70 mm directly before the leach experiments, which were conducted in a thermostated glass vessel with Pt or PbS indicator and Ag/AgCl reference electrodes introduced. The vessel was isolated from atmosphere using a water seal; no attempts were made to remove oxygen before or in the course of the tests. The slurry (usually 5% solid) was agitated by a magnetic stirrer. After each test, the concentration of lead in aqueous phase was determined, the solid residue was filtered, washed, dried and analysed for lead sulfate using conditioning in hot 20% sodium acetate and titration with EDTA. The same methodology was applied when processing the flotation concentrate. The solutions were prepared from analytical grade HNO3, Fe(NO3)3 and other chemicals and doubly distilled water.
3. Results 3.1. Dissolution kinetics of PbS electrode The rate of lead dissolution, W, as a function of the RDE potential that was kept constant over the experiments by using a potentiostat is given in Fig. 1. The rate slowly increases with decreasing steady-state potential in the range + 0.1 to 0.4 V where PbS dissolves by the non-oxidative mechanism PbS þ 2Hþ ¼ Pb2þ þ H2 S,
ð1Þ
and displays a small maximum at 0.4 V. The dissolution accelerates with an increase in HNO3 concentration; formal reaction order of 1.2 F 0.1 at 0.2 V reduces to 0.9 when the potential is shifted to 0.4 V (40 C). Apparent activation energy is 35 kJ mol 1 in 1 M HN03 at 0 V, being only slightly influenced by the electrode potential variation. This value is close to the gap width in PbS (0.4 eV 38.5 kJ mol 1) and is characteristic for the reactions with kinetic control. Such behaviour is typical for the non-oxidative leaching of galena in different acids, in which, however, the potential dependence of the rate commonly passes
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Fig. 1. Dissolution of lead as a function of potential of a rotating (1900 min 1) PbS disk electrode in 0.5 M HNO3 (1), 1 M HNO3 (2), 2 M HNO3 (3), 1 M HNO3 + 0.01 M Fe(NO3)3 (4) solutions. 40 C.
through a maximum at more positive biases (Scott and Nicol, 1977; Mikhlin et al., 2000). The lead release enhances due to oxidation of PbS as the potential becomes more positive starting from approximately 0.2 V. At the biases higher than 0.4 V, the anodic current sharply falls in a few initial seconds and exhibits several maxima afterwards; the mass of lead dissolved in the first time interval is also the largest (Fig. 2), implying that several stages occur in the course of the oxidation. The respective reactions may involve the formation of a passive film of elemental sulfur, the oxidation of that, and the passivation by lead sulfate or basic sulfate (Holmes and Crundwell, 1995). Alternatively, these stages have been suggested to be associated with alterations of reacted PbS surface, including the recurring development and decomposition of the defective layers (Mikhlin et al., 2000, 2002). The averaged rate shows a maximum at f 0.7 V and depends upon the nitric acid concentration in an irregular manner, having a maximum value in 0.5– 1 M HNO3 (Fig. 1), similar to the voltammetric curves (Mikhlin et al., 2000, 2002). Small addition of ferric nitrate promotes the lead dissolution over the whole potential scale (Fig. 1). The plots ln W vs. 1/T are usually non-linear at the anodic biases, indicating that the effective activation energy and, hence, the ratecontrolling stages change with the temperature elevation at a constant electrode potential.
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Fig. 2. Typical chronoamperograms and dissolved lead vs. time curves for the oxidation of the rotating (1900 min 1) PbS disk electrode in 1 M HNO3 (40 C): 1— at potential of 0.5 V, 2 — 0.8 V.
The ratio of the positive charge passed through the electrode and lead solubilised during the oxidation, i.e., a number of electrons released for 1 Pb atom, ne, approaches 2 and grows higher at the potentials 0.5 V, with the ne magnitude being essentially reduced in the presence of Fe3 + ions (Fig. 3). The figures vary to some extent under various experimental conditions; so ne was found to be of 2.3, 2.7, and 3.2 after PbS oxidation at 0.4, 0.6, and 0.8 V, respectively, in 1 M HNO3 quiescent solution at 20 C for 10 min, probably owing to lower contribution of chemical mechanisms under these conditions. Lead sulfide can be oxidised by HNO3 or Fe3 + by way of the direct ‘‘chemical’’ interaction
nitrate addition simultaneously accelerates the lead dissolution and reduces the ne, it can be concluded that a part of the ‘‘chemical’’ dissolution by reaction (3) becomes considerable. A correct quantitative determination of that part is, however, problematic because of possible involvement of the reactions (4), (5) and some others. In particular, the ferrous ions formed can be converted into Fe3 + electrochemically or be oxidised by nitrate ions. If one neglects the chemical dissolution of galena by (1) –(3), which is acceptable at high enough biases, a proportion of sulfide sulfur oxidised to sulfate can be calculated using Eqs. (4) and (5) and the experimental values of ne (mass of lead remaining on the electrode in the form which is soluble in 20% sodium acetate was found insignificant). The results demonstrate that the production of sulfate increases with potential and is restricted within 20%. This approach seems rather arbitrary, because thiosulfate and other intermediate oxysulfur species may be formed electrochemically and then oxidised to sulfate in aqueous phase or, alternatively, such products of chemical reactions as H2S, Fe2 + and others may be oxidised electrochemically. The formation of the defective reactions layers on galena also should influence the ne value. It is commonly accepted that aqueous oxidation of metal sulfides proceeds via the electrochemical mechanism, involving coupled counter-reactions of cathodic reduction of an oxidant (Table 1) and anodic oxidation
þ PbS þ ð2x þ 1ÞNO 3 þ 4H
¼ ð1 xÞPb2þ þ ð1 xÞS0 þ xPbSO4 þ ð2x þ 1ÞNO þ 2H2 O, PbS þ 2Fe3þ ¼ Pb2þ þ S0 þ 2Fe2þ ,
ð2Þ ð3Þ
or anodically by means of the applied voltage PbS ¼ Pb2þ þ S0 þ 2e,
ð4Þ
þ PbS þ 4H2 O ¼ Pb2þ þ SO2 4 þ 8H þ 8e:
ð5Þ
The ne number lower than 2 suggests a perceptible contribution from reactions (2) and (3). Since ferric
Fig. 3. Number of electrons consumed per Pb2 + ion released during dissolution of the RDE of PbS for 60 min at 40 C.
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Table 1 Standard electrode potentials, E0, for some reactions at 25 C (Lurye, 1989) No. 1 2 3 4 5 6 7
Reaction 3+
E0, V 2+
Fe + e = Fe + NO 3 + 4H + 3e = NO + 2H2O + NO 3 + 3H + 2e = HNO2 + H2O + NO + 2H + e = NO2 + H2O 3 HNO2 + H + + e = NO + H2O S0 + 2H + + 2e = H2S SO24 + 10H + + 8e = H2S + 4H2O
0.57 0.76 0.74 0.60 0.78 0.03 0.11
of PbS (Eqs. (4) and (5)). For the system under investigation, the role of oxidants (Fe3 + , NO3 ) appears to be more complex as Fe3 + ions affect the kinetics even at the constant electrode potentials supported with the potentiostat. Whereas the Pt electrode attains the potential meanings of 0.7 –0.8 V, close to the equilibrium potentials for nitrate and nitrous acid reduction (Table 1), the stationary potential of the galena electrode ranged from 0.2 to 0.2 V is considerably lower than the equilibrium value, probably due to low exchange currents for the nitrogenous species at the semiconducting PbS. 3.2. Leaching of ground galena Similar to the compact samples at open-circuit potentials, the dissolution of powdered galena is rather slow, produces H2S and becomes faster as HNO3
Fig. 4. Effect of the initial concentration of HNO3 on lead recovery from ground galena for 60 min at different temperatures (5% solid).
Fig. 5. Dissolution of galena in various nitric acid solutions: 1—1 M HNO3, 2 — 3 M HNO3, 3 — 5 M HNO3, 4 —1 M HNO3 + 0.01 M Fe(NO3)3, 5 — potential of Pt electrode in the test 4. 50 C, 5% solid.
concentration increases (Figs. 4 and 5). The rest potential of PbS electrode immersed into the slurry is about 0.2 V, while that of the Pt electrode varies in the range from 0.1 to + 0.9 V depending on the acid content and H2S evolution in an intricate way. The recovery of lead is less than 60% in 3 M HNO3 at 40 C and merely 30% in 5 M HNO3 at 20 C, but it tends to completeness in more concentrated acid and at elevated temperatures. A proportion of lead converted into lead sulfate in the solid residue is less than 1%; the yield of sulfate increases with the rise of nitric acid concentration and process duration up to f3%. No thiosulfate or other oxysulfur salts of lead were detected by XRD in the residue. The dissolution of galena sharply accelerates in the presence of ferric nitrate, with almost all sulfur of PbS converting into S0 (Fig. 5). PbS dissolves in 1 M HNO3 + 0.1 M Fe(NO3)3 at 50 C for the most part in the initial period when the potential of the Pt electrode was 0.4– 0.5 V (compare plots 4 and 5, Fig. 5). The PbS dissolution percentage against initial Fe3+ content shows a marked threshold at the concentration that is equivalent to the 6% amount required by reaction (3) (Fig. 6). A longer leach time and higher concentration of Fe3+ is necessary to reach a high solubilization at 20 C. Hydrogen ions also affect the kinetics, facilitating the lead recovery and suppressing the sulfate formation (Fig. 7). These facts favour a conclusion that
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was noticed to result in a reduction of the mineral breakdown. Similar effects are known to be distinguishing characteristics of a chain-radical reaction mechanism and may be assigned to active (radical) particles deactivation on the wall. 3.3. Leaching of lead concentrate
Fig. 6. Recovery of lead from ground galena in 1 M HNO3 as a function of added Fe(NO3)3 concentration. 50 C, 60 min, 5% solid.
nitric acid is a principle oxidant while ferric and/or ferrous ions act as catalysts. It should be admitted that the specific percentages are dependent on such experimental conditions as reaction vessel volume, insertion of the glass capillaries with electrodes and so on, especially in the case of ironfree solutions, although the regularities reported above remain valid. The extension of the inert surface area
Fig. 7. Effect of the initial concentration of HNO3 on the leaching of ground galena in 0.025 M Fe(NO3)3 solutions, total NO 3 concentration was adjusted to 1 M with NaNO3. 50 C, 60 min, 5% solid.
The lead recovery from the lead concentrate by nitric acid solutions without Fe3 + addition is generally larger than from pure galena under identical conditions. The rate alters with time, and the correlation between the quantity of dissolved lead and the potential measured by the Pt electrode immersed in the slurry is evident; a typical example is given in Fig. 8. At the start of the leach, the rate is not large and the potential is about 0.3 – 0.35 V. After about 30 min, the potential increases to 0.4 – 0.5 V, the dissolution sharply accelerates, and the recovery of lead reaches more than 90% in several minutes. Then the potential surges up to 0.7– 0.8 V, the dissolution slows down, with a portion of aqueous lead precipitating as PbSO4. It is fairly easy to obtain almost total decomposition of galena but nearly equal distribution of lead between the liquor and the solid residue cannot be prevented. For example, more than 95% breakdown of lead sulfide takes place in 2 M HNO3 at 50 C for 15 min, and about 50% of lead turns into insoluble sulfate. Small
Fig. 8. Recovery of lead and variation of the potential of Pt electrode for leaching of the flotation lead concentrate. Initial concentration of nitric acid was 1 M, 50 C, 5% solid.
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additions of ferric iron have minor influence on the kinetics, probably because the concentrate contains siderite, FeCO3, readily soluble in acid, and some amounts of Fe2 + and Fe3 + occur in the electrolyte from the beginning of the process, with Fe3 + concentration rising with time. Moreover, the sudden increase in the dissolution rate (after f 30 min) may be due to a certain Fe3 + /Fe2 + ratio reached, when the nitrate starts to oxidize Fe2 + to Fe3 + at high speed, as suggested by Van Weert and Shang (1993).
4. Discussion The above results make clear that the interaction of pure lead sulfide with nitric acid proceeds largely by non-oxidative mechanism (Eq. (1)), releasing H2S and dictating the low rest potential of PbS electrode. Hydrogen sulfide slowly reduces nitrate; this entails a gradual accumulation of NO, NO2 in gas phase and HNO2 and other nitrous species in the electrolyte þ 0 3H2 S þ 2NO 3 þ 2H ¼ 3S þ 2NOðgÞ þ 4H2 O,
ð6Þ 2NOðgÞ þ O2 ðgÞ ¼ 2NO2 ðgÞ,
ð7Þ
þ 2NO2 ðgÞ þ H2 O ¼ HNO2 þ NO 3 þH :
ð8Þ
Nitrite (nitrous acid) is known to be a much more active oxidant than nitrate (Van Weert and Shang, 1993; Novoselov and Makotchenko, 1999), and its involvement promotes the oxidation of sulfides, including the sulfate formation PbS þ ð8 6xÞHNO2 þ 2xHþ ¼ xPb2þ þ xS0 þ ð1 xÞPbSO4 ðsÞ þð8 6xÞNOðgÞ þ ð4 2xÞH2 O,
ð9Þ
and indicates that the whole process occurs by the oxidation route. The reactions of NO3 , HNO2 and possible intermediate species are far from full comprehension, at least their role is obviously greater than a depolarizer in the cathodic half-reactions listed in Table 1. The production of oxysulfur species by the
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anodic oxidation of PbS is small, and a considerable share of sulfate forms probably in the liquid but not on the electrode surface (Mikhlin et al., 2002). The yield of sulfate is minor for unmixed powdered galena and is essentially higher for the sulfidic concentrate, with some PbSO4 visibly precipitating from the solution, albeit elemental sulfur has been reported to be stable in hot nitric acid solutions (Droppert and Shang, 1995). These facts imply that the surface formation occurs via a ‘‘chemical’’ mechanism, i.e. specific interaction of sulfur of lead sulfide with nitrite and/or some other intermediates rather than with nitrate. Moreover, sulfate ions may originate from extraneous mineral sulfides rather than from galena. The rate of galena dissolution and the lead sulfate formation strongly depends upon the potential. The difference in the potentials measured using Pt and PbS electrodes is due to semi-conducting properties and a limited density of free charge carriers, first of all holes in the surface layer of galena. Furthermore, these properties are thought to vary as a result of time- and potential-dependent modifications of the composition and structure of the reacted near-surface PbS, being responsible, in particular, for retarding the anodic oxidation at the potentials more positive than f0.7 V (Mikhlin et al., 2002). Nevertheless, the anodic current is still large at these potentials (Fig. 2), and the comparably slow oxidation of powdered galena appears to be caused by small currents of the cathodic halfreactions (Table 1) and rather slow chemical reactions (2) and (9) involving nitrate and nitrous species. The fastest release of lead both from mineral samples and the concentrate takes place in Fe-bearing leach media in the short period when the potential of Pt electrode is in the range of 0.4 –0.5 V (Figs. 5 and 8). The galena dissolution rate is not extremal at these biases (Fig. 1) corresponding with the potential of ferric/ferrous iron couple with Fe3 + /Fe2 + ratio less than 0.1 (Table 1). Mulak (1987) explained the catalytic effect of silver, cupric and ferric ions on the dissolution of heazlewoodite, Ni3S2, in nitric acid by the formation of intermediate sulfide products (CuS, Cu2S, FeS, FeS2), which were believed to be oxidised faster than hydrogen sulfide. In our opinion, the surprisingly strong effect of ferric nitrate on the PbS oxidation should be rationalized as follows. The couple Fe3 + /Fe2 + is well known to possess very high exchange current, so Fe3 + is rapidly reduced on the
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galena surface, injecting holes into the valence band (or extracting electrons from the conduction band) and facilitating the anodic oxidation of PbS. Note that the ferric nitrate addition accelerates the lead dissolution even at steady-state electrode potentials (Fig. 1). On the other hand, arising ferrous ions are oxidised by nitrate þ 3þ 3Fe2þ þ NO þ NOðgÞ þ 2H2 O: 3 þ 4H ¼ 3Fe
ð10Þ The excessive concentrations of NO3 provide instantaneous regeneration of ferric iron in the immediate proximity to the PbS surface, especially as lead ions, both aqueous and in solid, catalyze this reaction (Ottley et al., 1997). So, the Fe3 + /Fe2 + couple acts as an electron transfer mediator, which changes the dissolution mechanism to the electrochemical one, enormously accelerating cathodic and anodic counter-reactions and the process as a whole. This offers interesting opportunities for development of a method of processing of sulfide lead concentrates. 5. Conclusions Galena dissolves in nitric acid media largely via the non-oxidative pathway producing hydrogen sulfide; the process is controlled by a chemical or electrochemical stage and becomes somewhat faster, in contrast with other acids, when the electrode potential is shifted in the negative direction. At higher biases, the RDE of PbS dissolves for the most part anodically, showing the highest rate at f 0.7 V, whereas the rate as a function of acid concentration is maximal in 1 M HNO3. Such behaviour is apparently due to a passivation, although the oxidation remains rapid under the ‘‘passive’’ conditions. Ferric ions catalyze the dissolution of galena, acting as intermediators of the electron transfer between nitrate ions and the solid and making the process electrochemically controlled. References Bjorling, G., Kolta, G.A., 1964. Oxidizing leach of sulphide concentrates and other materials catalyzed by nitric acid. VII Int. Mineral Processing Congr., Part III. Gordon and Breach Sci. Publ., New York, pp. 127 – 138.
Bjorling, G., Faldt, I., Lindgren, E., Toromanov, I., 1976. A nitric acid route in combination with solvent extraction for hydrometallurgical treatment of chalcopyrite. In: Yannopoulos, T.C., Agarwal, T.C. (Eds.), AIME Extractive Metallurgy of Copper, Las Vegas, NV, Feb. 22 – 26, 1976, pp. 726 – 737. Brennecke, H.M., Bergmann, O., Ellefson, R.R., Davies, D.S., Lueders, R.E., Spitz, R.A., 1981. Nitric – sulfuric leach process for recovery of copper from concentrate. Mining Eng. 33, 1259 – 1266. Droppert, D.J., Shang, Y., 1995. The leaching behaviour of nickeliferous pyrrhotite concentrate in hot nitric acid. Hydrometallurgy 39, 169 – 182. Fair, K.J., Schneider, J.C., Van Weert, G., 1986. Prochem’s NITROX process. CIM Bull. 79, 84 – 85. Flatt, J.R., Woods, R., 2000. Oxidation of pyrite in nitric acid solutions: relation to treatment of refractory gold ores. In: Woods, R., Doyle, F. (Eds.), Electrochemistry in Mineral and Metal Processing V. The Electrochem. Soc., Pennington, NJ, pp. 152 – 163. Fuerstenau, M.C., Nebo, C.O., Elango, B.V., Han, K.N., 1987. The kinetics of leaching of galena with ferric nitrate. Metall. Trans. B 18, 25 – 30. Habashi, F., 1973a. Action of nitric acid on chalcopyrite. Trans. Soc. Min. Eng. AIME 254, 224 – 228. Habashi, F., 1973b. Treatment of low-grade nickel – copper sulfide concentrate by nitric acid. Trans. Soc. Min. Eng. AIME 254, 228 – 230. Holmes, P.R., Crundwell, F.K., 1995. Kinetic aspects of galvanic interaction between minerals during dissolution. Hydrometallurgy 39, 353 – 375. Kholmogorov, A.G., Mikhlina, E.V., Pashkov, G.L., Patrushev, V.V., Drozdov, S.V., Zryachikh, L.V., 1998. Effect of temperature and nitric acid concentration on the breakdown of lead concentrate. Russ. J. Appl. Chem. 71, 363 – 366. Lurye, Y.Y., 1989. Handbook of Analytical Chemistry. Khimiya, Moscow, pp. 276 – 293. Mikhlin, Yu., Tomashevich, Ye., Asanov, I., Okotrub, A., 2000. Effect of surface non-stoichiometry on the dissolution of metal sulfides. In: Woods, R., Doyle, F.M. (Eds.), Electrochemistry in Mineral and Metal Processing V. The Electrochemical Society, Pennington, NJ, pp. 282 – 293. Mikhlin, Yu.L., Kuklinskiy, A.V., Mikhlina, E.V., Kargin, V.F., Asanov, I.P., 2002. Electrochemical behaviour of lead sulfide in nitric and perchloric acid solutions: voltammetric, SEM, and XPS characterization and application of the disordered semiconductor concept. In press. Mulak, W., 1987. The catalytic action of cupric and ferric ions in nitric acid leaching of Ni3S2. Hydrometallurgy 17, 201 – 214. Novoselov, R.I., Makotchenko, E.V., 1999. Oxygen as an ecologically compatible agent for oxidation of non-ferrous and noble metals and sulfide minerals. Chem. Sustainable Dev. 7, 321 – 330. Ottley, C.J., Davison, W., Edmunds, W.M., 1997. Chemical catalysis of nitrate reduction by iron (II). Geochim. Cosmochim. Acta 61, 1819 – 1828. Prater, J.D., Queneau, P.B., Hudson, T.J., 1973. Nitric acid route to processing copper concentrates. Trans. Soc. Min. Eng. AIME 254, 117 – 122.
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lurgy by autoclaving hydrolysis of iron (III) nitrate. Hydrometallurgy 33, 273 – 290. Vizsolyi, A., Peters, E., 1980. Nitric acid leaching molybdenite concentrates. Hydromettallurgy 6, 103 – 119.
Effect of potential and ferric ions on lead sulfide dissolution in nitric acid Gennady L. Pashkov, Elena V. Mikhlina, Alexander G. Kholmogorov, Yuri L. Mikhlin* Institute of Chemistry and Chemical Technology of Siberian Branch of Russian Academy of Sciences, K. Marx Street, 42, Krasnoyarsk 660049, Russia Received 14 August 2001; received in revised form 9 November 2001; accepted 9 November 2001
Abstract The dissolution of the rotating disk electrode (RDE) of natural lead sulfide (galena) and the ground mineral in nitric acid solutions has been studied as a function of electrode potential, HNO3 concentration and temperature. The rate of dissolution producing hydrogen sulfide slowly increases as the potential varies from + 0.1 to 0.4 V (Ag/AgCl). The reaction order on nitric acid concentration has been found to be 1.2 F 0.15 at 0.2 V and 0.9 at 0.4 V (40 C), and the apparent activation energy is 35 kJ mol 1 in 1 M HNO3 at 0 V, suggesting that the process is controlled by a chemical or electrochemical reaction. At higher biases the RDE of PbS dissolves for the most part anodically, showing the highest rate at f 0.7 V, whereas the rate as a function of acid concentration is maximal in 1 M HNO3. The yield of sulfate increases with potential and is small for the leaching of both compact and ground galena, while it reaches 50% in the case of a flotation lead concentrate. Ferric ions catalyze the dissolution of compact and, especially, ground galena, with the peak rate at the potential of immersed platinum electrode of 0.4 – 0.5 V. The Fe3 + /Fe2 + couple is concluded to act as an intermediator for the electron transfer between nitrate ions and the solid, indicating that the dissolution is electrochemical in nature. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferric ions; Lead sulfide; Nitric acid
1. Introduction Nitric acid leaching of sulfide concentrates and ores is an attractive method for recovery of basic metals, because HNO3 is a strong oxidant that can be readily recycled by the addition of oxygen directly in the leach slurry or using the external oxidation of nitrous oxides. Several nitric acid-based methods for processing sulfide minerals and concentrates of non-ferrous metals and iron have been proposed and tested (Bjorling and Kolta, 1964: Prater et al., 1973; Habashi, 1973a,b; *
Corresponding author. Fax: +7-3912-238658. E-mail address: [email protected] (Y.L. Mikhlin).
Bjorling et al., 1976; Vizsolyi and Peters, 1980; Brennecke et al., 1981; Fair et al., 1986; Fuerstenau et al., 1987; Van Weert and Shang, 1993; Droppert and Shang, 1995; Kholmogorov et al., 1998; Novoselov and Makotchenko, 1999) but no commercial plants appear to be operating at present. An important weakness of the nitric acid leach is the oxidation of sulfide sulfur both to elemental sulfur and sulfate, in many cases in almost equal parts. This results in an increase in nitric acid consumption and necessity to utilize the sulfate, raising the costs of the solution treatment and the acid regeneration. In particular, although the lead – zinc sulfide concentrate was almost completely decomposed by 2 M HNO3 at 90 C, about 50% of lead re-
0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X ( 0 1 ) 0 0 2 2 1 - 3
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mained in the residue as PbSO4 (Kholmogorov et al., 1998). Hydrometallurgy of lead is complicated by a low solubility of the majority of lead salts, and the important advantage of nitrate electrolytes that lead nitrate is highly soluble is lost in the case of the conventional leaching of lead-containing products by nitric acid due to the lead sulfide conversion to the solid PbSO4. The kinetics and mechanism of metal sulfides dissolution in nitric acid media and the formation of elemental sulfur and sulfate are poorly understood. It seems to be established that the sulfate production grows with an increase in the nitric acid concentration and electrode potential of pyrite and other minerals, while elemental sulfur is oxidised to sulfate very slowly (Droppert and Shang, 1995; Flatt and Woods, 2000); however, conclusions about the effect of temperature on the sulfate yield are contradictory (Bjorling and Kolta, 1964; Flatt and Woods, 2000). Iron, copper, and silver ions were found to accelerate the dissolution of metal sulfides in nitric acid (Mulak, 1987; Fuerstenau et al., 1987), as well as in other oxidative media. The behavior of lead sulfide (galena) in HNO3 solutions has received only very little attention. Fuerstenau et al. (1987) reported the kinetics of galena leaching in ferric nitrate solutions. They found that the galena oxidation produced elemental sulfur and the rate was proportional to the square root of the concentration of Fe3 + ; the addition of more than 1 M sodium nitrate slightly decelerated the reaction. The activation energy was 47 kJ mol 1, and the authors assumed that the dissolution proceeded by the electrochemical mechanism with ferric ions as an oxidant and the rate was controlled by surface reactions. Holmes and Crundwell (1995) studied the voltammetry of the electrochemical oxidation of the rotating disk electrode (RDE) of PbS in 0.6 M NaNO3 + 0.5 M H + solution and arrived at the conclusion that the current was controlled by the rate of the electrochemical reaction, although the maximum or shoulder at 0.48 V vs. SCE was ascribed to the oxidation of the passive film of elemental sulfur. Mikhlin et al. (2002) compared the voltammetry and XPS and SEM data for PbS oxidised in nitric and perchloric acids and suggested that the anodic dissolution was affected by the alterations of the composition and semiconducting characteristics of the disordered reacted surface layer of galena rather than the formation of ele-
mental sulfur and lead sulfate or thiosulfate. It was also shown that the oxidation products, including sulfate, arise from the decay of the distorted non-stoichiometric layer. The objective of this research was to study the kinetics and mechanism of PbS decomposition in nitric acid solutions as a function of electrode potential and to correlate the behaviour of compact galena electrodes with that of the ground mineral and a sulfidic lead concentrate. The strong catalytic effect of ferric ions was discovered and considered as an important detail for explaining the mechanism of PbS interaction with acidic nitrate electrolyte.
2. Experimental The polycrystalline galena obtained from Geological Museum of Central Siberia (Krasnoyarsk, Russia) had less than 0.1% by mass of iron, zinc, copper and silver as the major impurities and no inclusions of other phases. The material was of n-type conductivity with f61017 cm3 electron density. The flotation lead concentrate from Gorevskaya concentrating plant contained (%) Pb 59.2, Zn 2.6, Fe 10.0 (mainly as FeCO3), Mn 0.27, S 20, and was used in leach experiments without additional treatment. Disk electrodes were cut from massive mineral specimens to obtain cylindrical samples of f8 mm diameter and were embedded in Teflon. Copper was electrochemically deposited from acidic sulfate solutions on the side opposite to a working surface of the electrode in order to fabricate the ohmic contact. Electrode surfaces were polished on silicon carbide paper, cleaned with wet filter paper to remove fine particles and rinsed with doubly distilled water. A three-electrode water-jacketed glass cell was used; a Pt counter electrode was separated from the working electrode compartment by a glass frit. The electrode potential was referenced against a saturated Ag/AgCl electrode. The experiments were performed with a rotating disk electrode installation SVA-1BM; the speed of disk rotation was normally 1900 min 1. In order to examine the kinetics of the RDE dissolution, 5– 8 aliquots were periodically extracted from the cell and analysed for lead using atomic absorption spectroscopy. The dissolution rate was calculated as a slope of the amount of dissolved lead vs. time curves for periods of 15 to
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180 min, maintaining a comparable degree of the PbS etching. The current through the electrode was registered by a recorder and integrated to determine the charge passed. The galena samples were ground in a jasper mortar to 70 mm directly before the leach experiments, which were conducted in a thermostated glass vessel with Pt or PbS indicator and Ag/AgCl reference electrodes introduced. The vessel was isolated from atmosphere using a water seal; no attempts were made to remove oxygen before or in the course of the tests. The slurry (usually 5% solid) was agitated by a magnetic stirrer. After each test, the concentration of lead in aqueous phase was determined, the solid residue was filtered, washed, dried and analysed for lead sulfate using conditioning in hot 20% sodium acetate and titration with EDTA. The same methodology was applied when processing the flotation concentrate. The solutions were prepared from analytical grade HNO3, Fe(NO3)3 and other chemicals and doubly distilled water.
3. Results 3.1. Dissolution kinetics of PbS electrode The rate of lead dissolution, W, as a function of the RDE potential that was kept constant over the experiments by using a potentiostat is given in Fig. 1. The rate slowly increases with decreasing steady-state potential in the range + 0.1 to 0.4 V where PbS dissolves by the non-oxidative mechanism PbS þ 2Hþ ¼ Pb2þ þ H2 S,
ð1Þ
and displays a small maximum at 0.4 V. The dissolution accelerates with an increase in HNO3 concentration; formal reaction order of 1.2 F 0.1 at 0.2 V reduces to 0.9 when the potential is shifted to 0.4 V (40 C). Apparent activation energy is 35 kJ mol 1 in 1 M HN03 at 0 V, being only slightly influenced by the electrode potential variation. This value is close to the gap width in PbS (0.4 eV 38.5 kJ mol 1) and is characteristic for the reactions with kinetic control. Such behaviour is typical for the non-oxidative leaching of galena in different acids, in which, however, the potential dependence of the rate commonly passes
173
Fig. 1. Dissolution of lead as a function of potential of a rotating (1900 min 1) PbS disk electrode in 0.5 M HNO3 (1), 1 M HNO3 (2), 2 M HNO3 (3), 1 M HNO3 + 0.01 M Fe(NO3)3 (4) solutions. 40 C.
through a maximum at more positive biases (Scott and Nicol, 1977; Mikhlin et al., 2000). The lead release enhances due to oxidation of PbS as the potential becomes more positive starting from approximately 0.2 V. At the biases higher than 0.4 V, the anodic current sharply falls in a few initial seconds and exhibits several maxima afterwards; the mass of lead dissolved in the first time interval is also the largest (Fig. 2), implying that several stages occur in the course of the oxidation. The respective reactions may involve the formation of a passive film of elemental sulfur, the oxidation of that, and the passivation by lead sulfate or basic sulfate (Holmes and Crundwell, 1995). Alternatively, these stages have been suggested to be associated with alterations of reacted PbS surface, including the recurring development and decomposition of the defective layers (Mikhlin et al., 2000, 2002). The averaged rate shows a maximum at f 0.7 V and depends upon the nitric acid concentration in an irregular manner, having a maximum value in 0.5– 1 M HNO3 (Fig. 1), similar to the voltammetric curves (Mikhlin et al., 2000, 2002). Small addition of ferric nitrate promotes the lead dissolution over the whole potential scale (Fig. 1). The plots ln W vs. 1/T are usually non-linear at the anodic biases, indicating that the effective activation energy and, hence, the ratecontrolling stages change with the temperature elevation at a constant electrode potential.
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Fig. 2. Typical chronoamperograms and dissolved lead vs. time curves for the oxidation of the rotating (1900 min 1) PbS disk electrode in 1 M HNO3 (40 C): 1— at potential of 0.5 V, 2 — 0.8 V.
The ratio of the positive charge passed through the electrode and lead solubilised during the oxidation, i.e., a number of electrons released for 1 Pb atom, ne, approaches 2 and grows higher at the potentials 0.5 V, with the ne magnitude being essentially reduced in the presence of Fe3 + ions (Fig. 3). The figures vary to some extent under various experimental conditions; so ne was found to be of 2.3, 2.7, and 3.2 after PbS oxidation at 0.4, 0.6, and 0.8 V, respectively, in 1 M HNO3 quiescent solution at 20 C for 10 min, probably owing to lower contribution of chemical mechanisms under these conditions. Lead sulfide can be oxidised by HNO3 or Fe3 + by way of the direct ‘‘chemical’’ interaction
nitrate addition simultaneously accelerates the lead dissolution and reduces the ne, it can be concluded that a part of the ‘‘chemical’’ dissolution by reaction (3) becomes considerable. A correct quantitative determination of that part is, however, problematic because of possible involvement of the reactions (4), (5) and some others. In particular, the ferrous ions formed can be converted into Fe3 + electrochemically or be oxidised by nitrate ions. If one neglects the chemical dissolution of galena by (1) –(3), which is acceptable at high enough biases, a proportion of sulfide sulfur oxidised to sulfate can be calculated using Eqs. (4) and (5) and the experimental values of ne (mass of lead remaining on the electrode in the form which is soluble in 20% sodium acetate was found insignificant). The results demonstrate that the production of sulfate increases with potential and is restricted within 20%. This approach seems rather arbitrary, because thiosulfate and other intermediate oxysulfur species may be formed electrochemically and then oxidised to sulfate in aqueous phase or, alternatively, such products of chemical reactions as H2S, Fe2 + and others may be oxidised electrochemically. The formation of the defective reactions layers on galena also should influence the ne value. It is commonly accepted that aqueous oxidation of metal sulfides proceeds via the electrochemical mechanism, involving coupled counter-reactions of cathodic reduction of an oxidant (Table 1) and anodic oxidation
þ PbS þ ð2x þ 1ÞNO 3 þ 4H
¼ ð1 xÞPb2þ þ ð1 xÞS0 þ xPbSO4 þ ð2x þ 1ÞNO þ 2H2 O, PbS þ 2Fe3þ ¼ Pb2þ þ S0 þ 2Fe2þ ,
ð2Þ ð3Þ
or anodically by means of the applied voltage PbS ¼ Pb2þ þ S0 þ 2e,
ð4Þ
þ PbS þ 4H2 O ¼ Pb2þ þ SO2 4 þ 8H þ 8e:
ð5Þ
The ne number lower than 2 suggests a perceptible contribution from reactions (2) and (3). Since ferric
Fig. 3. Number of electrons consumed per Pb2 + ion released during dissolution of the RDE of PbS for 60 min at 40 C.
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Table 1 Standard electrode potentials, E0, for some reactions at 25 C (Lurye, 1989) No. 1 2 3 4 5 6 7
Reaction 3+
E0, V 2+
Fe + e = Fe + NO 3 + 4H + 3e = NO + 2H2O + NO 3 + 3H + 2e = HNO2 + H2O + NO + 2H + e = NO2 + H2O 3 HNO2 + H + + e = NO + H2O S0 + 2H + + 2e = H2S SO24 + 10H + + 8e = H2S + 4H2O
0.57 0.76 0.74 0.60 0.78 0.03 0.11
of PbS (Eqs. (4) and (5)). For the system under investigation, the role of oxidants (Fe3 + , NO3 ) appears to be more complex as Fe3 + ions affect the kinetics even at the constant electrode potentials supported with the potentiostat. Whereas the Pt electrode attains the potential meanings of 0.7 –0.8 V, close to the equilibrium potentials for nitrate and nitrous acid reduction (Table 1), the stationary potential of the galena electrode ranged from 0.2 to 0.2 V is considerably lower than the equilibrium value, probably due to low exchange currents for the nitrogenous species at the semiconducting PbS. 3.2. Leaching of ground galena Similar to the compact samples at open-circuit potentials, the dissolution of powdered galena is rather slow, produces H2S and becomes faster as HNO3
Fig. 4. Effect of the initial concentration of HNO3 on lead recovery from ground galena for 60 min at different temperatures (5% solid).
Fig. 5. Dissolution of galena in various nitric acid solutions: 1—1 M HNO3, 2 — 3 M HNO3, 3 — 5 M HNO3, 4 —1 M HNO3 + 0.01 M Fe(NO3)3, 5 — potential of Pt electrode in the test 4. 50 C, 5% solid.
concentration increases (Figs. 4 and 5). The rest potential of PbS electrode immersed into the slurry is about 0.2 V, while that of the Pt electrode varies in the range from 0.1 to + 0.9 V depending on the acid content and H2S evolution in an intricate way. The recovery of lead is less than 60% in 3 M HNO3 at 40 C and merely 30% in 5 M HNO3 at 20 C, but it tends to completeness in more concentrated acid and at elevated temperatures. A proportion of lead converted into lead sulfate in the solid residue is less than 1%; the yield of sulfate increases with the rise of nitric acid concentration and process duration up to f3%. No thiosulfate or other oxysulfur salts of lead were detected by XRD in the residue. The dissolution of galena sharply accelerates in the presence of ferric nitrate, with almost all sulfur of PbS converting into S0 (Fig. 5). PbS dissolves in 1 M HNO3 + 0.1 M Fe(NO3)3 at 50 C for the most part in the initial period when the potential of the Pt electrode was 0.4– 0.5 V (compare plots 4 and 5, Fig. 5). The PbS dissolution percentage against initial Fe3+ content shows a marked threshold at the concentration that is equivalent to the 6% amount required by reaction (3) (Fig. 6). A longer leach time and higher concentration of Fe3+ is necessary to reach a high solubilization at 20 C. Hydrogen ions also affect the kinetics, facilitating the lead recovery and suppressing the sulfate formation (Fig. 7). These facts favour a conclusion that
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was noticed to result in a reduction of the mineral breakdown. Similar effects are known to be distinguishing characteristics of a chain-radical reaction mechanism and may be assigned to active (radical) particles deactivation on the wall. 3.3. Leaching of lead concentrate
Fig. 6. Recovery of lead from ground galena in 1 M HNO3 as a function of added Fe(NO3)3 concentration. 50 C, 60 min, 5% solid.
nitric acid is a principle oxidant while ferric and/or ferrous ions act as catalysts. It should be admitted that the specific percentages are dependent on such experimental conditions as reaction vessel volume, insertion of the glass capillaries with electrodes and so on, especially in the case of ironfree solutions, although the regularities reported above remain valid. The extension of the inert surface area
Fig. 7. Effect of the initial concentration of HNO3 on the leaching of ground galena in 0.025 M Fe(NO3)3 solutions, total NO 3 concentration was adjusted to 1 M with NaNO3. 50 C, 60 min, 5% solid.
The lead recovery from the lead concentrate by nitric acid solutions without Fe3 + addition is generally larger than from pure galena under identical conditions. The rate alters with time, and the correlation between the quantity of dissolved lead and the potential measured by the Pt electrode immersed in the slurry is evident; a typical example is given in Fig. 8. At the start of the leach, the rate is not large and the potential is about 0.3 – 0.35 V. After about 30 min, the potential increases to 0.4 – 0.5 V, the dissolution sharply accelerates, and the recovery of lead reaches more than 90% in several minutes. Then the potential surges up to 0.7– 0.8 V, the dissolution slows down, with a portion of aqueous lead precipitating as PbSO4. It is fairly easy to obtain almost total decomposition of galena but nearly equal distribution of lead between the liquor and the solid residue cannot be prevented. For example, more than 95% breakdown of lead sulfide takes place in 2 M HNO3 at 50 C for 15 min, and about 50% of lead turns into insoluble sulfate. Small
Fig. 8. Recovery of lead and variation of the potential of Pt electrode for leaching of the flotation lead concentrate. Initial concentration of nitric acid was 1 M, 50 C, 5% solid.
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additions of ferric iron have minor influence on the kinetics, probably because the concentrate contains siderite, FeCO3, readily soluble in acid, and some amounts of Fe2 + and Fe3 + occur in the electrolyte from the beginning of the process, with Fe3 + concentration rising with time. Moreover, the sudden increase in the dissolution rate (after f 30 min) may be due to a certain Fe3 + /Fe2 + ratio reached, when the nitrate starts to oxidize Fe2 + to Fe3 + at high speed, as suggested by Van Weert and Shang (1993).
4. Discussion The above results make clear that the interaction of pure lead sulfide with nitric acid proceeds largely by non-oxidative mechanism (Eq. (1)), releasing H2S and dictating the low rest potential of PbS electrode. Hydrogen sulfide slowly reduces nitrate; this entails a gradual accumulation of NO, NO2 in gas phase and HNO2 and other nitrous species in the electrolyte þ 0 3H2 S þ 2NO 3 þ 2H ¼ 3S þ 2NOðgÞ þ 4H2 O,
ð6Þ 2NOðgÞ þ O2 ðgÞ ¼ 2NO2 ðgÞ,
ð7Þ
þ 2NO2 ðgÞ þ H2 O ¼ HNO2 þ NO 3 þH :
ð8Þ
Nitrite (nitrous acid) is known to be a much more active oxidant than nitrate (Van Weert and Shang, 1993; Novoselov and Makotchenko, 1999), and its involvement promotes the oxidation of sulfides, including the sulfate formation PbS þ ð8 6xÞHNO2 þ 2xHþ ¼ xPb2þ þ xS0 þ ð1 xÞPbSO4 ðsÞ þð8 6xÞNOðgÞ þ ð4 2xÞH2 O,
ð9Þ
and indicates that the whole process occurs by the oxidation route. The reactions of NO3 , HNO2 and possible intermediate species are far from full comprehension, at least their role is obviously greater than a depolarizer in the cathodic half-reactions listed in Table 1. The production of oxysulfur species by the
177
anodic oxidation of PbS is small, and a considerable share of sulfate forms probably in the liquid but not on the electrode surface (Mikhlin et al., 2002). The yield of sulfate is minor for unmixed powdered galena and is essentially higher for the sulfidic concentrate, with some PbSO4 visibly precipitating from the solution, albeit elemental sulfur has been reported to be stable in hot nitric acid solutions (Droppert and Shang, 1995). These facts imply that the surface formation occurs via a ‘‘chemical’’ mechanism, i.e. specific interaction of sulfur of lead sulfide with nitrite and/or some other intermediates rather than with nitrate. Moreover, sulfate ions may originate from extraneous mineral sulfides rather than from galena. The rate of galena dissolution and the lead sulfate formation strongly depends upon the potential. The difference in the potentials measured using Pt and PbS electrodes is due to semi-conducting properties and a limited density of free charge carriers, first of all holes in the surface layer of galena. Furthermore, these properties are thought to vary as a result of time- and potential-dependent modifications of the composition and structure of the reacted near-surface PbS, being responsible, in particular, for retarding the anodic oxidation at the potentials more positive than f0.7 V (Mikhlin et al., 2002). Nevertheless, the anodic current is still large at these potentials (Fig. 2), and the comparably slow oxidation of powdered galena appears to be caused by small currents of the cathodic halfreactions (Table 1) and rather slow chemical reactions (2) and (9) involving nitrate and nitrous species. The fastest release of lead both from mineral samples and the concentrate takes place in Fe-bearing leach media in the short period when the potential of Pt electrode is in the range of 0.4 –0.5 V (Figs. 5 and 8). The galena dissolution rate is not extremal at these biases (Fig. 1) corresponding with the potential of ferric/ferrous iron couple with Fe3 + /Fe2 + ratio less than 0.1 (Table 1). Mulak (1987) explained the catalytic effect of silver, cupric and ferric ions on the dissolution of heazlewoodite, Ni3S2, in nitric acid by the formation of intermediate sulfide products (CuS, Cu2S, FeS, FeS2), which were believed to be oxidised faster than hydrogen sulfide. In our opinion, the surprisingly strong effect of ferric nitrate on the PbS oxidation should be rationalized as follows. The couple Fe3 + /Fe2 + is well known to possess very high exchange current, so Fe3 + is rapidly reduced on the
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galena surface, injecting holes into the valence band (or extracting electrons from the conduction band) and facilitating the anodic oxidation of PbS. Note that the ferric nitrate addition accelerates the lead dissolution even at steady-state electrode potentials (Fig. 1). On the other hand, arising ferrous ions are oxidised by nitrate þ 3þ 3Fe2þ þ NO þ NOðgÞ þ 2H2 O: 3 þ 4H ¼ 3Fe
ð10Þ The excessive concentrations of NO3 provide instantaneous regeneration of ferric iron in the immediate proximity to the PbS surface, especially as lead ions, both aqueous and in solid, catalyze this reaction (Ottley et al., 1997). So, the Fe3 + /Fe2 + couple acts as an electron transfer mediator, which changes the dissolution mechanism to the electrochemical one, enormously accelerating cathodic and anodic counter-reactions and the process as a whole. This offers interesting opportunities for development of a method of processing of sulfide lead concentrates. 5. Conclusions Galena dissolves in nitric acid media largely via the non-oxidative pathway producing hydrogen sulfide; the process is controlled by a chemical or electrochemical stage and becomes somewhat faster, in contrast with other acids, when the electrode potential is shifted in the negative direction. At higher biases, the RDE of PbS dissolves for the most part anodically, showing the highest rate at f 0.7 V, whereas the rate as a function of acid concentration is maximal in 1 M HNO3. Such behaviour is apparently due to a passivation, although the oxidation remains rapid under the ‘‘passive’’ conditions. Ferric ions catalyze the dissolution of galena, acting as intermediators of the electron transfer between nitrate ions and the solid and making the process electrochemically controlled. References Bjorling, G., Kolta, G.A., 1964. Oxidizing leach of sulphide concentrates and other materials catalyzed by nitric acid. VII Int. Mineral Processing Congr., Part III. Gordon and Breach Sci. Publ., New York, pp. 127 – 138.
Bjorling, G., Faldt, I., Lindgren, E., Toromanov, I., 1976. A nitric acid route in combination with solvent extraction for hydrometallurgical treatment of chalcopyrite. In: Yannopoulos, T.C., Agarwal, T.C. (Eds.), AIME Extractive Metallurgy of Copper, Las Vegas, NV, Feb. 22 – 26, 1976, pp. 726 – 737. Brennecke, H.M., Bergmann, O., Ellefson, R.R., Davies, D.S., Lueders, R.E., Spitz, R.A., 1981. Nitric – sulfuric leach process for recovery of copper from concentrate. Mining Eng. 33, 1259 – 1266. Droppert, D.J., Shang, Y., 1995. The leaching behaviour of nickeliferous pyrrhotite concentrate in hot nitric acid. Hydrometallurgy 39, 169 – 182. Fair, K.J., Schneider, J.C., Van Weert, G., 1986. Prochem’s NITROX process. CIM Bull. 79, 84 – 85. Flatt, J.R., Woods, R., 2000. Oxidation of pyrite in nitric acid solutions: relation to treatment of refractory gold ores. In: Woods, R., Doyle, F. (Eds.), Electrochemistry in Mineral and Metal Processing V. The Electrochem. Soc., Pennington, NJ, pp. 152 – 163. Fuerstenau, M.C., Nebo, C.O., Elango, B.V., Han, K.N., 1987. The kinetics of leaching of galena with ferric nitrate. Metall. Trans. B 18, 25 – 30. Habashi, F., 1973a. Action of nitric acid on chalcopyrite. Trans. Soc. Min. Eng. AIME 254, 224 – 228. Habashi, F., 1973b. Treatment of low-grade nickel – copper sulfide concentrate by nitric acid. Trans. Soc. Min. Eng. AIME 254, 228 – 230. Holmes, P.R., Crundwell, F.K., 1995. Kinetic aspects of galvanic interaction between minerals during dissolution. Hydrometallurgy 39, 353 – 375. Kholmogorov, A.G., Mikhlina, E.V., Pashkov, G.L., Patrushev, V.V., Drozdov, S.V., Zryachikh, L.V., 1998. Effect of temperature and nitric acid concentration on the breakdown of lead concentrate. Russ. J. Appl. Chem. 71, 363 – 366. Lurye, Y.Y., 1989. Handbook of Analytical Chemistry. Khimiya, Moscow, pp. 276 – 293. Mikhlin, Yu., Tomashevich, Ye., Asanov, I., Okotrub, A., 2000. Effect of surface non-stoichiometry on the dissolution of metal sulfides. In: Woods, R., Doyle, F.M. (Eds.), Electrochemistry in Mineral and Metal Processing V. The Electrochemical Society, Pennington, NJ, pp. 282 – 293. Mikhlin, Yu.L., Kuklinskiy, A.V., Mikhlina, E.V., Kargin, V.F., Asanov, I.P., 2002. Electrochemical behaviour of lead sulfide in nitric and perchloric acid solutions: voltammetric, SEM, and XPS characterization and application of the disordered semiconductor concept. In press. Mulak, W., 1987. The catalytic action of cupric and ferric ions in nitric acid leaching of Ni3S2. Hydrometallurgy 17, 201 – 214. Novoselov, R.I., Makotchenko, E.V., 1999. Oxygen as an ecologically compatible agent for oxidation of non-ferrous and noble metals and sulfide minerals. Chem. Sustainable Dev. 7, 321 – 330. Ottley, C.J., Davison, W., Edmunds, W.M., 1997. Chemical catalysis of nitrate reduction by iron (II). Geochim. Cosmochim. Acta 61, 1819 – 1828. Prater, J.D., Queneau, P.B., Hudson, T.J., 1973. Nitric acid route to processing copper concentrates. Trans. Soc. Min. Eng. AIME 254, 117 – 122.
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lurgy by autoclaving hydrolysis of iron (III) nitrate. Hydrometallurgy 33, 273 – 290. Vizsolyi, A., Peters, E., 1980. Nitric acid leaching molybdenite concentrates. Hydromettallurgy 6, 103 – 119.