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Dissolution kinetics of galena in acetic acid solutions with hydrogen peroxide
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Hydrometallurgy 89 (2007) 189 – 195 www.elsevier.com/locate/hydromet
Dissolution kinetics of galena in acetic acid solutions with hydrogen peroxide Salih Aydoğan a,⁎, Ali Aras a , Gökhan Uçar a , Murat Erdemoğlu b a b
Selçuk University, Department of Mining Engineering, 42075 Konya, Turkey · Inönü University, Department of Mining Engineering, 44280 Malatya, Turkey
Received 23 May 2007; received in revised form 12 July 2007; accepted 13 July 2007 Available online 20 July 2007
Abstract The kinetics of leaching lead from galena in acetic (ethanoic) acid solutions with hydrogen peroxide are investigated with regard to stirring speed, temperature and concentration of HAc and H2O2 concentration. Oxidation of galena with H2O2 to produce lead sulphate which dissolves by complexing Pb2+ with acetate anion (PbCH3COO+ and Pb(CH3COO)2). Results indicate that the rate of galena dissolution is controlled by a surface chemical reaction with an apparent activation energy is 65.6 kJ mol− 1 in the temperature range 30–70 °C. Both HAc and H2O2 affect the rate of extraction of lead as an acetate complex. The order of reaction was 0.79 and 0.31 for H2O2 and HAc concentrations, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Galena; Leaching; Kinetics; Acetic acid; Hydrogen peroxide; Shrinking core model
1. Introduction Short-chain or low molecular weight carboxylic acids (LMWCA) having 1–5 C atoms per molecule are usually referred to as an aliphatic mono-carboxylic acids. These may affect mineral weathering rates by at least three mechanisms (Drever and Stillings, 1997): by changing the dissolution rate far from equilibrium through decreasing solution pH or forming complexes with cations at the mineral surface; by affecting the saturation state of the solution with respect to the mineral; and by affecting the speciation in the solution of ions.
⁎ Corresponding author. E-mail address: [email protected] (S. Aydoğan). 0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2007.07.004
In hydrometallurgical processes, carboxylic acids have been proposed as an alternative, and probably less expensive, leaching agent (Panias et al., 1996; Ambikadevi and Lalithambika, 2000). Organic acids such as oxalic, citric, ascorbic, acetic, fumaric and tartaric acid, have been used for their ability to solubilize iron and other metal oxides. Of the above organic acids, oxalic (Baumgartner et al., 1983; Cornell and Schindler, 1987; Blesa et al., 1987; Taxiarchou et al., 1997; Ubaldini et al., 1996) citric (Waite and Morel, 1984) and ascorbic acids (Afonso et al., 1990; Parida et al., 1997) are the most used carboxylic acids, due to their effectiveness as solvent reagents. Generally, the studies carried out were mainly focused on the dissolution of iron oxides and aluminum oxide; together with those on the dissolution of non-oxide minerals such as feldspar (Drever and Stillings, 1997) hornblende (Zhang and Bloom, 1999) and apatite (Goyne et al., 2006).
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S. Aydoğan et al. / Hydrometallurgy 89 (2007) 189–195
There are hardly any studies aimed at leaching sulphide minerals using organic acids. Evans and Masters (1976) developed a method for leaching finely divided galena bearing material to convert lead to soluble lead acetate with concurrent conversion of sulphur to elemental state. The method involves forming slurry consisting of the material dispersed in an aqueous medium containing free acetate ions and having a pH below 5.1, at a temperature of 60 to 120 °C under oxygen pressure. Geisler and Puddington (1996) is described a method for dissolving galena by in-situ leaching with acetic acid and acetate solution in the presence of an oxidant such as oxygen gas. Accordingly, the experiments conducted by Greet and Smart (2002) demonstrated that EDTA readily dissolves lead hydroxide, lead carbonate, lead sulphate, and lead hydroxy-carbonate — all typical galena oxidation products. This present study aims to examine the effectiveness of acetic acid to leach galena in aqueous oxidizing conditions and dissolve lead. For this purpose, galena was leached with acetic acid in the presence of hydrogen peroxide, and effects of some parameters such as stirring speed, temperature, acetic acid and hydrogen peroxide concentrations on the kinetic parameters of galena dissolution were investigated. 2. Materials and methods A galena concentrate sample obtained from the Koyulhisar complex sulphide ore concentrator in Sivas province of Turkey was used. The sample was sieved into three size fractions but experiments were performed only with the 45–75 μm fraction which contained 79.0% Pb, 14.61% S, 2.97% Fe, 1.90% Zn and 0.50% Cu. The dissolution experiments were carried out in a glass reactor of 1 L, equipped with a Teflon coated mechanical stirrer, maintained in a constant temperature bath. The ranges of dissolution parameters chosen were 0–600 min − 1 for stirring speed, 303–343 K for temperature, 0.1–2.0 mol/L for hydrogen peroxide concentration and 0.5–5.0 mol/L for acetic acid concentration. For each experiment, 1 g of galena was stirred in 500 mL of H2O2/HAc mixture for up to 90 min and sampled at various time intervals. Lead in the solution was determined using flame atomic absorption spectrophotometer (GBC Scientific Equipment, SensAA Model, Australia). For calculation of the fraction of lead extracted, the equation incorporating correction factors to account for volume losses during sampling was used. Distilled water and reagent grade chemicals were used to prepare all the solutions.
3. Results and discussion 3.1. Dissolution of galena The oxidation and dissolution mechanisms of galena still remain uncertain with a wide variety of possible mechanisms having being proposed in the literature. Surface species found on galena during oxidation and dissolution have been studied using many techniques (Gerson and O'Dea, 2003). It has been assumed that complexation takes place at the surface of hydrous galena sulphides. The adsorption of H+ ions onto a surface of S atom in the aqueous phase is found to be favorable whereas the adsorption onto a surface Pb atom is not favorable. On exposure to air and aqueous solution, hydroxides form on galena surface, often with a lead deficient sulphide beneath the hydroxides. On oxidation of galena in air, a metal deficient sulphide is formed, covered with a layer of lead hydroxide, oxide and carbonate. On further exposure to air, basic sulphate and lead sulphate are also observed. An ion exchange process between protons and lead ions is assumed to occur at the surface resulting in the formation of a lead deficient surface. It has been reported by Hsieh and Huang (1989) that as the dissolution rate is much higher under acidic conditions as compared to alkaline conditions, a protonated surface, PbSH22+, initiates the dissolution process: In the presence of oxygen, 2þ 2þ PbSH2þ 2ðsolidÞ þ 2O2 →PbS2 d 2O2ðsolidÞ →Pb þ þ SO2− 4 þ 2H
Pb2þ þ SO2− 4 →PbSO4ðsolidÞ
ð1Þ
ð2Þ
Accordingly, Jennings et al. (2000) have stated that the formation of PbSO4 prevents Pb2+ from undergoing hydrolysis and generating acid, Pb2þ þ 2H2 O→PbðOHÞ2ðsolidÞ þ 2Hþ
ð3Þ
Instead, the overall reaction for galena weathering by H2O2 is presented as, PbS þ 4H2 O2 →PbSO4ðsolidÞ þ 4H2 O
ð4Þ
Hydrogen peroxide, H2O2, is a strong oxidant and environmentally safe reagent as apart from water no other reaction products generated during the dissolution of sulphide minerals. The oxidative action of hydrogen
S. Aydoğan et al. / Hydrometallurgy 89 (2007) 189–195
peroxide in acidic solutions is based on its reduction according to, H2 O2 þ 2Hþ þ 2e− →2H2 O E 0 ¼ 1:77V
ð5Þ
Hydrogen peroxide can also act as a reducing agent, undergoing oxidation: H2 O−2 →O2 þ 2Hþ þ 2ē
ð6Þ
Overall reaction can be written: 2H2 O2 →O2 þ 2H2 O
ð7Þ
By decomposition of hydrogen peroxide, oxygen is adsorbed on the mineral surface whereby electron transfer takes places in the solution. The potential value of 1.77 V is adequate to oxidize almost all of the metal sulphides (Aydoğan, 2006). The net results of the oxidation of sulphides, despite the consequences of details of the dissolution mechanism for galena, are (1) to get the metal ion into solution or into the form of an insoluble compound stable under surface conditions, (2) to convert the sulphur to sulphate ion, and (3) to produce relatively acid solutions. Discussion of such reaction for lead minerals can be refined by the use of Eh–pH diagrams (Fig. 1), which serves only to express the relationships quantitatively (Krauskopf and Bird, 1994). Of the four lead minerals shown in Fig. 1, galena is the most stable at low values of Eh, regardless
191
of activity. When conditions are oxidizing, anglesite is the most stable in an acid environment and cerussite in most alkaline conditions. Regardless of very low solubility of PbSO 4 (Ksp = 1.82 × 10− 8), acetic acid (CH3COOH) and acetate (CH3COO−) rapidly form complexes with lead ions. The lead acetate complexes formed have stoichiometries which can be described by the following generalized reaction; Pb2þ þ nAc− →PbðAcÞn 2−n
ð8Þ
where Ac is the acetate ion (CH3COO−) and n is the number of acetate ligands in the nth lead species. The degree of association of lead is defined in terms of average ligand number, nav, calculated from the following expressions: i n h P 2n n Pb ð Ac Þ n mHAc ½HAc ½Ac nav ¼ ¼ 1 ð9Þ mPb2þ mPb2þ where mHAc represents the stoichiometric concentration of acetic acid, [HAc] and [Ac−] are the calculated acetic acid and free acetate concentrations, respectively, and mPb2+ represents the total stoichiometric metal concentration (Bénézeth and Palmer, 2000). The leaching solutions after removal of un-dissolved solids were analysed for SO42− ion by adding 0.1 mol/L Ba (NO3)2 solution instead of BaCl2 which is generally used for sulphate precipitation. Nitrate salt was employed to avoid precipitation of PbCl2 (Ksp = 1.78 × 10− 5). It was observed that BaSO4 (Ksp = 1.08 × 10− 10), as identified by chemical analysis, precipitated without delay. In the literature, there are some studies both on the oxidation of galena (Wittstock et al., 1996) and the dissolution of lead sulphate in acetate medium (Giordano, 1989). The thermodynamic stability constants at 298 K for the complexes PbCH3COO+ and Pb(CH3COO)2 are log K1 = 2.4 and log K2 = 3.4, respectively, as calculated by Giordano (1989). He found that lead hydrolysis is significant in those experiments conducted at 343 and 353 K; however hydrolyzed lead is not detected at 298, 313 and 328 K. The author concludes that lead acetate complexes predominate over Pb2+ and PbOH+ at temperatures between 298 and 358 K in solutions which are slightly acid and have free acetate concentrations N 0.01 m. 3.2. Effects of parameters
Fig. 1. Eh–pH diagram for lead minerals at 298 K and 1 bar. Total carbonate = 10− 3 mol/L and total dissolved sulphur = 10− 2 mol/L, (Krauskopf and Bird, 1994).
The effect of stirring on the dissolution rate was determined at 323 K in solutions containing 3 mol/L HAc and 0.5 mol/L H2O2. Interestingly, for this reactive sulphide at low pulp density, there was no significant
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Fig. 2. Effect of temperature on galena dissolution (no stirring; HAc: 3 mol/L; H2O2: 0.5 mol/L).
Fig. 4. Effect of HAc concentration on galena dissolution (no stirring; H2O2: 0.5 mol/L; temperature: 323 K).
effect of stirring on the rate of dissolution. In previous studies, it was reported that oxygen derived from the decomposition of H2O2 caused self mixing with mineral particles (Aydoğan, 2006; Adebayo et al., 2006; Antonijević et al., 1997, 2004). But in this study, no oxygen gas emission was observed yet the dissolution rate was high without stirring except for simply shaking to homogenize the solution prior to the sampling. The effect of temperature on dissolution was carried out in the 303–343 K temperature range again in solutions containing 3 mol/L HAc and 0.5 mol/L H2O2. It was observed that fast leaching rates occurred within the first 20 min. (Fig. 2). The fraction of lead extraction after 20 min was 0.69, 0.77 and 0.85 at 323, 333 and 343 K, respectively. After that time, the leaching slows down or stops suggesting that the lead sulphate produced
immediately precipitates at the un-reacted galena surface, inhibiting further attack of hydrogen peroxide. In order to determine the effect of hydrogen peroxide concentration, the experiments were performed by varying the initial H2O2 concentration in the range of 0.1–2.0 mol/ L at 323 K in solutions containing 3 mol/L HAc. Only 40% of lead was extracted using 0.1 mol/L H2O2 within 90 min, while the extraction was almost completed by leaching with 0.5 mol/L H2O2. It was observed that as the H2O2 concentration increased, oxidation of galena together with dissolution of lead sulphate significantly accelerated (Fig. 3). However, the rate drastically decreased when using 1 and 2 mol/L H2O2 after 20 min of leaching, suggesting the same phenomenon observed during leaching at higher temperatures, e.g., fast precipitation of lead sulphate inhibits further oxidation of unreacted galena.
Fig. 3. Effect of H2O2 concentration on galena dissolution (no stirring; HAc: 3 mol/L; temperature: 323 K).
Fig. 5. Variation in surface chemical reaction model equation with time at various temperatures (symbols as in Fig. 2).
S. Aydoğan et al. / Hydrometallurgy 89 (2007) 189–195
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reaction products are crucial for a complete understanding of the system. In order to elucidate the reaction mechanism of galena dissolution with hydrogen peroxide in acetic acid solutions, the shrinking core model was utilized (Habashi, 1999). According to the shrinking core model, if the reaction of galena can be represented as, Afluid þ bBsolid ¼ Fluid products þ Solid products
ð10Þ
and the rate of the reaction is controlled by surface chemical reaction, then the integral rate equation is constituted as follows: 1 ð1 XÞ1=3 ¼ Fig. 6. Arrhenius plot of the data presented in Fig. 5.
To establish the effect of acetic acid, four different HAc concentrations were examined in the range of 0.5– 5.0 mol/L at 323 K in solutions containing 0.5 mol/L H2O2. Increasing the HAc concentration up to 3 mol/L increased the rate of sulphate dissolution (Fig. 4). But a further increase in HAc concentration caused a decrease in the dissolution rate. The slower kinetics with 5 mol/L acetic acid is likely due to the increased viscosity and lower mass transport of H2O2 through the solution and surface layer. This effect could also rationalize why the reaction order of HAc is unusually low. The change in the rate at higher reagent concentrations has been claimed to be due to the larger quantity of solid lead sulphate produced.
kc MB CA t ¼ kr t qB ar0
ð11Þ
where X is the fraction reacted, kc is the kinetic constant, MB is the molecular weight of the solid, CA is the concentration of the dissolved lixiviant A in the bulk of the solution, a is the stoichiometric coefficient of the reagent in the leaching reaction, r0 is the initial radius of the solid particle, t is the reaction time, ρB density of the solid and kr is the rate constant calculated from Eq. (11). Eq. (11) was applied to the data obtained from each temperature which gives a straight line (Fig. 5). The rate constants were calculated as slopes of the straight lines. By using these values, the Arrhenius equation k = Ae−E/RT, (plotted in Fig. 6), gives an activation energy of 65.6 kJ mol− 1 that supports the proposed surface chemical reaction as the rate controlling step. The activation energies of galena dissolution reported by various authors are summarized in Table 1 and compares favourably.
3.3. Dissolution kinetics Understanding the mechanism of a leaching system is the main consideration, while a knowledge of the kinetics of the rate controlling processes and solid Table 1 Reported activation energies for leaching of galena with different leaching media Leaching reagent
Activation energy (kJ mol− 1)
Reference
Ferric fluosilicate
62.1
Ferric chloride Cupric chloride Ferric nitrate
40–45 33 47
HCl HClO4 HBr
64.4 71.5 66.5
Chen and Dreisinger (1994) Dutrizac (1986) Dutrizac (1989) Fuerstenau et al. (1987) Nunez et al. (1988) Fig. 7. Determination of reaction order for galena dissolution with respect to H2O2 and HAc concentrations.
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To establish the order of reaction with respect to reagent concentration, the dissolution data obtained from Figs. 3 and 4 were used in Eq. (11) to determine rate constants. By constructing the plots of ln kr versus ln [H2O2] and ln [HAc], the order of reaction was found proportional to 0.79 and 0.31 power of H2O2 and HAc concentrations (Fig. 7). 4. Conclusions The dissolution of galena with hydrogen peroxide in acetic acid solutions assumes that galena oxidizes with hydrogen peroxide to form solid lead sulphate which further dissolves with acetate anion to give a lead acetate complex in solution. This study concludes that the leaching kinetics of galena follows the shrinking core model with the surface chemical reaction as the rate controlling step. Increasing the hydrogen peroxide and increasing the acetic acid concentration up to 3 mol/L accelerates galena dissolution. The activation energy was found to be 65.6 kJ mol−1 in the temperature range 303–343 K, supporting a surface chemical reaction controlled dissolution. Empirical orders of the galena dissolution reaction with respect to hydrogen peroxide and acetic acid concentrations are 0.79 and 0.31, respectively. Acknowledgement This study was supported by The Research Foundation of Selcuk University. References Adebayo, A.O., Ipinmoroti, K.O., Ajayi, O.O., 2006. Leaching of sphalerite with hydrogen peroxide and nitric acid solutions. Journal of Minerals and Materials Characterization and Engineering 5 (2), 167–177. Afonso, M.S., Morando, P.J., Blesa, M.A., Banwart, S., Stumm, W., 1990. The reductive dissolution of iron oxides by ascorbate. Journal of Colloid and Interface Science 138, 74–82. Ambikadevi, V.R., Lalithambika, M., 2000. Effect of organic acids on ferric iron removal from iron-stained kaolinite. Applied Clay Science 16, 133–145. Antonijević, M.M., Dimitrijević, M., Janković, Z., 1997. Leaching of pyrite with hydrogen peroxide in sulphuric acid. Hydrometallurgy 46, 71–83. Antonijević, M.M., Janković, Z., Dimitrijević, M., 2004. Kinetics of chalcopyrite dissolution by hydrogen peroxide in sulphuric acid. Hydrometallurgy, 71, 329–334. Aydoğan, S., 2006. Dissolution kinetics of sphalerite with hydrogen peroxide in sulphuric acid medium. Chemical Engineering Journal 123, 65–70. Baumgartner, E.C., Blesa, M.A., Marinovich, H.A., Maroto, A.J.G., 1983. Heterogeneous electron transfer as a pathway in the
dissolution of magnetite in oxalic acid solutions. Inorganic Chemistry 22, 2224–2226. Bénézeth, P., Palmer, D.A., 2000. Potentiometric determination of cadmium-acetate complexation in aqueous solutions to 250°C. Chemical Geology 167, 11–24. Blesa, M.A., Marinovich, H.A., Baumgartner, E.C., Maroto, A.J.G., 1987. Mechanism of dissolution of magnetite by oxalic acidferrous ion solutions. Inorganic Chemistry 26, 3713–3717. Chen, A.A., Dreisinger, D.B., 1994. The ferric fluosilicate leaching of lead concentrates. I. Kinetic studies. Metallurgical and Materials Transactions. B, Process Metallurgy and Materials Processing Science 25, 473–480. Cornell, R.M., Schindler, P.W., 1987. Photochemical dissolution of goethite in acid/oxalate solutions. Clays and Clay Minerals 35, 347–352. Drever, J.I., Stillings, L.L., 1997. The role of organic acids in mineral weathering. Colloids and Surfaces. A, Physicochemical and Engineering Aspects 120, 167–181. Dutrizac, J.E., 1986. The dissolution of galena in ferric chloride media. Metallurgical Transactions. B, Process Metallurgy 17, 5–17. Dutrizac, J.E., 1989. The leaching of galena in cupric chloride media. Metallurgical Transactions. B, Process Metallurgy 20, 475–483. Evans, D.J.I., Masters, I.M., 1976. Treatment of lead sulphide bearing material. United States Patent No 3,933,973. Fuerstenau, M.C., Nebo, C., Elango, B.V., Han, K.N., 1987. The kinetics of leaching galena with ferric nitrate. Metallurgical Transactions. B, Process Metallurgy 18, 25–30. Geisler, R.A., Puddington, I.E., 1996. Treatment of lead sulphide bearing minerals. United States Patent No 5,523,066. Gerson, A.R., O’Dea, A.R., 2003. A quantum chemical investigation of the oxidation and dissolution mechanisms of galena. Geochimica et Cosmochimica Acta 67, 813–822. Giordano, T.H., 1989. Anglesite (PbSO4) solubility in acetate solutions: the determination of stability constants for lead acetate complexes to 85°C. Geochimica et Cosmochimica Acta 53, 359–366. Goyne, K.W., Brantley, S.L., Chorover, J., 2006. Effects of organic acids and dissolved oxygen on apatite and chalcopyrite dissolution: implications for using elements as organo-markers and oxymarkers. Chemical Geology 234, 28–45. Greet, C., Smart, R.St.C., 2002. Diagnostic leaching of galena and its oxidation products with EDTA. Minerals Engineering 15, 515–522. Habashi, F., 1999. Kinetics of Metallurgical Processes, 2nd edition. Metallurgies Extractive Quebec, Quebec, Canada. Hsieh, Y.H., Huang, C.P., 1989. The dissolution of PbS(s) in dilute aqueous solutions. Journal of Colloid and Interface Science. 131, 537–549. Jennings, S.R., Dollhopf, D.J., Inskeep, W.P., 2000. Acid production from sulfide minerals using hydrogen peroxide weathering. Applied Geochemistry 15, 235–243. Krauskopf, K.B., Bird, D.K., 1994. Introduction to Geochemistry, 3rd edition. McGraw-Hill International Editions, New York, USA. Nunez, C., Espiell, F., Garcia-Zayas, J., 1988. Kinetics of nonoxidative leaching of galena in perchloric, hydrobromic, and hydrochloric acid solutions. Metallurgical Transactions. B, Process Metallurgy 19, 541–546. Panias, D., Taxiarchou, M., Paspaliaris, I., Kontopoulos, A., 1996. Mechanisms of dissolution of iron oxides in aqueous oxalic acid solutions. Hydrometallurgy 42, 257–265. Parida, K.M., Das, J., Data, P., 1997. Application of statistical design of experiments in the study of dissolution of goethite (α-FeOOH) in hydrochloric acid in the presence of ascorbic acid. Hydrometallurgy 46, 271–275.
S. Aydoğan et al. / Hydrometallurgy 89 (2007) 189–195 Taxiarchou, M., Panias, D., Paspaliaris, I., Douni, I., Kontopoulos, A., 1997. Removal of iron from silica sand by leaching with oxalic acid. Hydrometallurgy 46, 215–227. Ubaldini, S., Piga, L., Fornari, P., Massida, R., 1996. Removal of iron from quartz sands: a study by column leaching using a complete factorial design. Hydrometallurgy 40, 369–379. Waite, T.D., Morel, F.M.M., 1984. Photo-reduction dissolution of colloidal iron oxide: effect of citrate. Journal of Colloid and Interface Science 102, 121–137.
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Wittstock, G., Kartio, I., Hirsch, D., Kunze, S., Szargan, R., 1996. Oxidation of galena in acetate buffer investigated by Atomic Force Microscopy and Photoelectron Spectroscopy. Langmuir 12, 5709–5721. Zhang, H., Bloom, P.R., 1999. Dissolution of hornblende in organic acid solutions. Soil Science Society of America Journal 63, 815–822.
Hydrometallurgy 89 (2007) 189 – 195 www.elsevier.com/locate/hydromet
Dissolution kinetics of galena in acetic acid solutions with hydrogen peroxide Salih Aydoğan a,⁎, Ali Aras a , Gökhan Uçar a , Murat Erdemoğlu b a b
Selçuk University, Department of Mining Engineering, 42075 Konya, Turkey · Inönü University, Department of Mining Engineering, 44280 Malatya, Turkey
Received 23 May 2007; received in revised form 12 July 2007; accepted 13 July 2007 Available online 20 July 2007
Abstract The kinetics of leaching lead from galena in acetic (ethanoic) acid solutions with hydrogen peroxide are investigated with regard to stirring speed, temperature and concentration of HAc and H2O2 concentration. Oxidation of galena with H2O2 to produce lead sulphate which dissolves by complexing Pb2+ with acetate anion (PbCH3COO+ and Pb(CH3COO)2). Results indicate that the rate of galena dissolution is controlled by a surface chemical reaction with an apparent activation energy is 65.6 kJ mol− 1 in the temperature range 30–70 °C. Both HAc and H2O2 affect the rate of extraction of lead as an acetate complex. The order of reaction was 0.79 and 0.31 for H2O2 and HAc concentrations, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Galena; Leaching; Kinetics; Acetic acid; Hydrogen peroxide; Shrinking core model
1. Introduction Short-chain or low molecular weight carboxylic acids (LMWCA) having 1–5 C atoms per molecule are usually referred to as an aliphatic mono-carboxylic acids. These may affect mineral weathering rates by at least three mechanisms (Drever and Stillings, 1997): by changing the dissolution rate far from equilibrium through decreasing solution pH or forming complexes with cations at the mineral surface; by affecting the saturation state of the solution with respect to the mineral; and by affecting the speciation in the solution of ions.
⁎ Corresponding author. E-mail address: [email protected] (S. Aydoğan). 0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2007.07.004
In hydrometallurgical processes, carboxylic acids have been proposed as an alternative, and probably less expensive, leaching agent (Panias et al., 1996; Ambikadevi and Lalithambika, 2000). Organic acids such as oxalic, citric, ascorbic, acetic, fumaric and tartaric acid, have been used for their ability to solubilize iron and other metal oxides. Of the above organic acids, oxalic (Baumgartner et al., 1983; Cornell and Schindler, 1987; Blesa et al., 1987; Taxiarchou et al., 1997; Ubaldini et al., 1996) citric (Waite and Morel, 1984) and ascorbic acids (Afonso et al., 1990; Parida et al., 1997) are the most used carboxylic acids, due to their effectiveness as solvent reagents. Generally, the studies carried out were mainly focused on the dissolution of iron oxides and aluminum oxide; together with those on the dissolution of non-oxide minerals such as feldspar (Drever and Stillings, 1997) hornblende (Zhang and Bloom, 1999) and apatite (Goyne et al., 2006).
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S. Aydoğan et al. / Hydrometallurgy 89 (2007) 189–195
There are hardly any studies aimed at leaching sulphide minerals using organic acids. Evans and Masters (1976) developed a method for leaching finely divided galena bearing material to convert lead to soluble lead acetate with concurrent conversion of sulphur to elemental state. The method involves forming slurry consisting of the material dispersed in an aqueous medium containing free acetate ions and having a pH below 5.1, at a temperature of 60 to 120 °C under oxygen pressure. Geisler and Puddington (1996) is described a method for dissolving galena by in-situ leaching with acetic acid and acetate solution in the presence of an oxidant such as oxygen gas. Accordingly, the experiments conducted by Greet and Smart (2002) demonstrated that EDTA readily dissolves lead hydroxide, lead carbonate, lead sulphate, and lead hydroxy-carbonate — all typical galena oxidation products. This present study aims to examine the effectiveness of acetic acid to leach galena in aqueous oxidizing conditions and dissolve lead. For this purpose, galena was leached with acetic acid in the presence of hydrogen peroxide, and effects of some parameters such as stirring speed, temperature, acetic acid and hydrogen peroxide concentrations on the kinetic parameters of galena dissolution were investigated. 2. Materials and methods A galena concentrate sample obtained from the Koyulhisar complex sulphide ore concentrator in Sivas province of Turkey was used. The sample was sieved into three size fractions but experiments were performed only with the 45–75 μm fraction which contained 79.0% Pb, 14.61% S, 2.97% Fe, 1.90% Zn and 0.50% Cu. The dissolution experiments were carried out in a glass reactor of 1 L, equipped with a Teflon coated mechanical stirrer, maintained in a constant temperature bath. The ranges of dissolution parameters chosen were 0–600 min − 1 for stirring speed, 303–343 K for temperature, 0.1–2.0 mol/L for hydrogen peroxide concentration and 0.5–5.0 mol/L for acetic acid concentration. For each experiment, 1 g of galena was stirred in 500 mL of H2O2/HAc mixture for up to 90 min and sampled at various time intervals. Lead in the solution was determined using flame atomic absorption spectrophotometer (GBC Scientific Equipment, SensAA Model, Australia). For calculation of the fraction of lead extracted, the equation incorporating correction factors to account for volume losses during sampling was used. Distilled water and reagent grade chemicals were used to prepare all the solutions.
3. Results and discussion 3.1. Dissolution of galena The oxidation and dissolution mechanisms of galena still remain uncertain with a wide variety of possible mechanisms having being proposed in the literature. Surface species found on galena during oxidation and dissolution have been studied using many techniques (Gerson and O'Dea, 2003). It has been assumed that complexation takes place at the surface of hydrous galena sulphides. The adsorption of H+ ions onto a surface of S atom in the aqueous phase is found to be favorable whereas the adsorption onto a surface Pb atom is not favorable. On exposure to air and aqueous solution, hydroxides form on galena surface, often with a lead deficient sulphide beneath the hydroxides. On oxidation of galena in air, a metal deficient sulphide is formed, covered with a layer of lead hydroxide, oxide and carbonate. On further exposure to air, basic sulphate and lead sulphate are also observed. An ion exchange process between protons and lead ions is assumed to occur at the surface resulting in the formation of a lead deficient surface. It has been reported by Hsieh and Huang (1989) that as the dissolution rate is much higher under acidic conditions as compared to alkaline conditions, a protonated surface, PbSH22+, initiates the dissolution process: In the presence of oxygen, 2þ 2þ PbSH2þ 2ðsolidÞ þ 2O2 →PbS2 d 2O2ðsolidÞ →Pb þ þ SO2− 4 þ 2H
Pb2þ þ SO2− 4 →PbSO4ðsolidÞ
ð1Þ
ð2Þ
Accordingly, Jennings et al. (2000) have stated that the formation of PbSO4 prevents Pb2+ from undergoing hydrolysis and generating acid, Pb2þ þ 2H2 O→PbðOHÞ2ðsolidÞ þ 2Hþ
ð3Þ
Instead, the overall reaction for galena weathering by H2O2 is presented as, PbS þ 4H2 O2 →PbSO4ðsolidÞ þ 4H2 O
ð4Þ
Hydrogen peroxide, H2O2, is a strong oxidant and environmentally safe reagent as apart from water no other reaction products generated during the dissolution of sulphide minerals. The oxidative action of hydrogen
S. Aydoğan et al. / Hydrometallurgy 89 (2007) 189–195
peroxide in acidic solutions is based on its reduction according to, H2 O2 þ 2Hþ þ 2e− →2H2 O E 0 ¼ 1:77V
ð5Þ
Hydrogen peroxide can also act as a reducing agent, undergoing oxidation: H2 O−2 →O2 þ 2Hþ þ 2ē
ð6Þ
Overall reaction can be written: 2H2 O2 →O2 þ 2H2 O
ð7Þ
By decomposition of hydrogen peroxide, oxygen is adsorbed on the mineral surface whereby electron transfer takes places in the solution. The potential value of 1.77 V is adequate to oxidize almost all of the metal sulphides (Aydoğan, 2006). The net results of the oxidation of sulphides, despite the consequences of details of the dissolution mechanism for galena, are (1) to get the metal ion into solution or into the form of an insoluble compound stable under surface conditions, (2) to convert the sulphur to sulphate ion, and (3) to produce relatively acid solutions. Discussion of such reaction for lead minerals can be refined by the use of Eh–pH diagrams (Fig. 1), which serves only to express the relationships quantitatively (Krauskopf and Bird, 1994). Of the four lead minerals shown in Fig. 1, galena is the most stable at low values of Eh, regardless
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of activity. When conditions are oxidizing, anglesite is the most stable in an acid environment and cerussite in most alkaline conditions. Regardless of very low solubility of PbSO 4 (Ksp = 1.82 × 10− 8), acetic acid (CH3COOH) and acetate (CH3COO−) rapidly form complexes with lead ions. The lead acetate complexes formed have stoichiometries which can be described by the following generalized reaction; Pb2þ þ nAc− →PbðAcÞn 2−n
ð8Þ
where Ac is the acetate ion (CH3COO−) and n is the number of acetate ligands in the nth lead species. The degree of association of lead is defined in terms of average ligand number, nav, calculated from the following expressions: i n h P 2n n Pb ð Ac Þ n mHAc ½HAc ½Ac nav ¼ ¼ 1 ð9Þ mPb2þ mPb2þ where mHAc represents the stoichiometric concentration of acetic acid, [HAc] and [Ac−] are the calculated acetic acid and free acetate concentrations, respectively, and mPb2+ represents the total stoichiometric metal concentration (Bénézeth and Palmer, 2000). The leaching solutions after removal of un-dissolved solids were analysed for SO42− ion by adding 0.1 mol/L Ba (NO3)2 solution instead of BaCl2 which is generally used for sulphate precipitation. Nitrate salt was employed to avoid precipitation of PbCl2 (Ksp = 1.78 × 10− 5). It was observed that BaSO4 (Ksp = 1.08 × 10− 10), as identified by chemical analysis, precipitated without delay. In the literature, there are some studies both on the oxidation of galena (Wittstock et al., 1996) and the dissolution of lead sulphate in acetate medium (Giordano, 1989). The thermodynamic stability constants at 298 K for the complexes PbCH3COO+ and Pb(CH3COO)2 are log K1 = 2.4 and log K2 = 3.4, respectively, as calculated by Giordano (1989). He found that lead hydrolysis is significant in those experiments conducted at 343 and 353 K; however hydrolyzed lead is not detected at 298, 313 and 328 K. The author concludes that lead acetate complexes predominate over Pb2+ and PbOH+ at temperatures between 298 and 358 K in solutions which are slightly acid and have free acetate concentrations N 0.01 m. 3.2. Effects of parameters
Fig. 1. Eh–pH diagram for lead minerals at 298 K and 1 bar. Total carbonate = 10− 3 mol/L and total dissolved sulphur = 10− 2 mol/L, (Krauskopf and Bird, 1994).
The effect of stirring on the dissolution rate was determined at 323 K in solutions containing 3 mol/L HAc and 0.5 mol/L H2O2. Interestingly, for this reactive sulphide at low pulp density, there was no significant
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Fig. 2. Effect of temperature on galena dissolution (no stirring; HAc: 3 mol/L; H2O2: 0.5 mol/L).
Fig. 4. Effect of HAc concentration on galena dissolution (no stirring; H2O2: 0.5 mol/L; temperature: 323 K).
effect of stirring on the rate of dissolution. In previous studies, it was reported that oxygen derived from the decomposition of H2O2 caused self mixing with mineral particles (Aydoğan, 2006; Adebayo et al., 2006; Antonijević et al., 1997, 2004). But in this study, no oxygen gas emission was observed yet the dissolution rate was high without stirring except for simply shaking to homogenize the solution prior to the sampling. The effect of temperature on dissolution was carried out in the 303–343 K temperature range again in solutions containing 3 mol/L HAc and 0.5 mol/L H2O2. It was observed that fast leaching rates occurred within the first 20 min. (Fig. 2). The fraction of lead extraction after 20 min was 0.69, 0.77 and 0.85 at 323, 333 and 343 K, respectively. After that time, the leaching slows down or stops suggesting that the lead sulphate produced
immediately precipitates at the un-reacted galena surface, inhibiting further attack of hydrogen peroxide. In order to determine the effect of hydrogen peroxide concentration, the experiments were performed by varying the initial H2O2 concentration in the range of 0.1–2.0 mol/ L at 323 K in solutions containing 3 mol/L HAc. Only 40% of lead was extracted using 0.1 mol/L H2O2 within 90 min, while the extraction was almost completed by leaching with 0.5 mol/L H2O2. It was observed that as the H2O2 concentration increased, oxidation of galena together with dissolution of lead sulphate significantly accelerated (Fig. 3). However, the rate drastically decreased when using 1 and 2 mol/L H2O2 after 20 min of leaching, suggesting the same phenomenon observed during leaching at higher temperatures, e.g., fast precipitation of lead sulphate inhibits further oxidation of unreacted galena.
Fig. 3. Effect of H2O2 concentration on galena dissolution (no stirring; HAc: 3 mol/L; temperature: 323 K).
Fig. 5. Variation in surface chemical reaction model equation with time at various temperatures (symbols as in Fig. 2).
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reaction products are crucial for a complete understanding of the system. In order to elucidate the reaction mechanism of galena dissolution with hydrogen peroxide in acetic acid solutions, the shrinking core model was utilized (Habashi, 1999). According to the shrinking core model, if the reaction of galena can be represented as, Afluid þ bBsolid ¼ Fluid products þ Solid products
ð10Þ
and the rate of the reaction is controlled by surface chemical reaction, then the integral rate equation is constituted as follows: 1 ð1 XÞ1=3 ¼ Fig. 6. Arrhenius plot of the data presented in Fig. 5.
To establish the effect of acetic acid, four different HAc concentrations were examined in the range of 0.5– 5.0 mol/L at 323 K in solutions containing 0.5 mol/L H2O2. Increasing the HAc concentration up to 3 mol/L increased the rate of sulphate dissolution (Fig. 4). But a further increase in HAc concentration caused a decrease in the dissolution rate. The slower kinetics with 5 mol/L acetic acid is likely due to the increased viscosity and lower mass transport of H2O2 through the solution and surface layer. This effect could also rationalize why the reaction order of HAc is unusually low. The change in the rate at higher reagent concentrations has been claimed to be due to the larger quantity of solid lead sulphate produced.
kc MB CA t ¼ kr t qB ar0
ð11Þ
where X is the fraction reacted, kc is the kinetic constant, MB is the molecular weight of the solid, CA is the concentration of the dissolved lixiviant A in the bulk of the solution, a is the stoichiometric coefficient of the reagent in the leaching reaction, r0 is the initial radius of the solid particle, t is the reaction time, ρB density of the solid and kr is the rate constant calculated from Eq. (11). Eq. (11) was applied to the data obtained from each temperature which gives a straight line (Fig. 5). The rate constants were calculated as slopes of the straight lines. By using these values, the Arrhenius equation k = Ae−E/RT, (plotted in Fig. 6), gives an activation energy of 65.6 kJ mol− 1 that supports the proposed surface chemical reaction as the rate controlling step. The activation energies of galena dissolution reported by various authors are summarized in Table 1 and compares favourably.
3.3. Dissolution kinetics Understanding the mechanism of a leaching system is the main consideration, while a knowledge of the kinetics of the rate controlling processes and solid Table 1 Reported activation energies for leaching of galena with different leaching media Leaching reagent
Activation energy (kJ mol− 1)
Reference
Ferric fluosilicate
62.1
Ferric chloride Cupric chloride Ferric nitrate
40–45 33 47
HCl HClO4 HBr
64.4 71.5 66.5
Chen and Dreisinger (1994) Dutrizac (1986) Dutrizac (1989) Fuerstenau et al. (1987) Nunez et al. (1988) Fig. 7. Determination of reaction order for galena dissolution with respect to H2O2 and HAc concentrations.
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To establish the order of reaction with respect to reagent concentration, the dissolution data obtained from Figs. 3 and 4 were used in Eq. (11) to determine rate constants. By constructing the plots of ln kr versus ln [H2O2] and ln [HAc], the order of reaction was found proportional to 0.79 and 0.31 power of H2O2 and HAc concentrations (Fig. 7). 4. Conclusions The dissolution of galena with hydrogen peroxide in acetic acid solutions assumes that galena oxidizes with hydrogen peroxide to form solid lead sulphate which further dissolves with acetate anion to give a lead acetate complex in solution. This study concludes that the leaching kinetics of galena follows the shrinking core model with the surface chemical reaction as the rate controlling step. Increasing the hydrogen peroxide and increasing the acetic acid concentration up to 3 mol/L accelerates galena dissolution. The activation energy was found to be 65.6 kJ mol−1 in the temperature range 303–343 K, supporting a surface chemical reaction controlled dissolution. Empirical orders of the galena dissolution reaction with respect to hydrogen peroxide and acetic acid concentrations are 0.79 and 0.31, respectively. Acknowledgement This study was supported by The Research Foundation of Selcuk University. References Adebayo, A.O., Ipinmoroti, K.O., Ajayi, O.O., 2006. Leaching of sphalerite with hydrogen peroxide and nitric acid solutions. Journal of Minerals and Materials Characterization and Engineering 5 (2), 167–177. Afonso, M.S., Morando, P.J., Blesa, M.A., Banwart, S., Stumm, W., 1990. The reductive dissolution of iron oxides by ascorbate. Journal of Colloid and Interface Science 138, 74–82. Ambikadevi, V.R., Lalithambika, M., 2000. Effect of organic acids on ferric iron removal from iron-stained kaolinite. Applied Clay Science 16, 133–145. Antonijević, M.M., Dimitrijević, M., Janković, Z., 1997. Leaching of pyrite with hydrogen peroxide in sulphuric acid. Hydrometallurgy 46, 71–83. Antonijević, M.M., Janković, Z., Dimitrijević, M., 2004. Kinetics of chalcopyrite dissolution by hydrogen peroxide in sulphuric acid. Hydrometallurgy, 71, 329–334. Aydoğan, S., 2006. Dissolution kinetics of sphalerite with hydrogen peroxide in sulphuric acid medium. Chemical Engineering Journal 123, 65–70. Baumgartner, E.C., Blesa, M.A., Marinovich, H.A., Maroto, A.J.G., 1983. Heterogeneous electron transfer as a pathway in the
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