- Email: [email protected]
Leaching behaviour of sulphides in ammoniacal thiosulphate systems
Hydrometallurgy 63 (2002) 189 – 200 www.elsevier.com/locate/hydromet
Leaching behaviour of sulphides in ammoniacal thiosulphate systems D. Feng, J.S.J. Van Deventer* Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia Received 23 July 2001; received in revised form 27 September 2001; accepted 23 November 2001
Abstract Systematic studies have been conducted to understand the leaching behaviour and dissolution mechanisms when common gold-host sulphides such as chalcopyrite, pyrite, arsenopyrite and pyrrhotite are treated by oxidative ammoniacal thiosulphate leaching. Leach solution composition strongly influenced the sulphide leaching, and the presence of sulphides also enhanced the decomposition of thiosulphate. The XRD patterns of the binary mixtures before and after leaching indicated that the relative leaching rates of the sulphides in the ammoniacal thiosulphate system were in the order chalcopyrite > pyrrhotite > arsenopyrite > pyrite, which was in accordance with the observations made from the leaching tests. SEM analysis with the aid of EDAX indicated the formation of iron oxide at the chalcopyrite, pyrrhotite and pyrite surfaces and the formation of iron arsenate at the arsenopyrite surface after leaching. SEM analysis also demonstrated that the high-energy defect sites and crystal boundaries favoured the sulphide leaching. Raman spectroscopy indicated that haematite was formed during the leaching of chalcopyrite. Iron and arsenic concentrations in the leach solutions were very low due to the formation of iron oxide and iron arsenate during the leaching reactions. Pyrite enhanced chalcopyrite and sphalerite dissolution. Chalcopyrite and sphalerite also enhanced pyrite dissolution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulphide; Oxidation; Leaching; Thiosulphate; Ammonia; Galvanic effect
1. Introduction Thiosulphate, as an alternative lixiviant for gold, has received much attention in recent years due to the growing environmental and public concerns over the use of cyanide. Acceptable gold leaching rates using thiosulphate are achieved in the presence of ammonia with cupric ion acting as the oxidant (Abbruzzese et
*
Corresponding author. Tel.: +61-3-9344-6620; fax: +61-39344-4153. E-mail address: [email protected] (J.S.J. Van Deventer).
al., 1995; Cao et al., 1992; Gong et al., 1993; Langhans et al., 1992; Jiang et al., 1993; Tozawa et al., 1981; Zipperian et al., 1988). Despite the multitude of papers in this area, the industrial applications where thiosulphate is used instead of cyanide are apparently limited to a silver ore plant in Mexico (Wan, 1997). Research has shown that some ores are well suited to thiosulphate leaching, while others show hardly any extraction. An important reason for this lack of acceptance into the industry is that the solution chemistry and the mineralogical factors affecting the effectiveness of ammoniacal thiosulphate systems are not understood adequately. Since modern gold and silver leaching has
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 5 - 0
190
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
been directed to very complex ores, the successful application of thiosulphate leaching depends not only on the dissolution behaviour of gold and silver, but also critically on the behaviour of the associated minerals. Surprisingly, there is hardly any publication that deals with mineralogical aspects of ores during thiosulphate leaching. Pyrite, chalcopyrite, chalcocite, bornite, pyrrhotite and arsenopyrite are the main sulphide minerals locking gold in refractory sulphide gold ores. The leaching behaviour of these minerals could play an important role in the thiosulphate leaching of gold sulphide ores. However, the leaching behaviour of the sulphides still remains unknown in such a leaching system. Sarveswara Rao and Ray (1998) conducted systematic studies on the oxidation behaviour and dissolution reaction mechanisms operating while pure copper, zinc and lead sulphide minerals and mixtures, and single concentrates are treated by oxidative ammonia leaching. The sequence of sulphide mineral dissolution from a quaternary mixture of CuS, ZnS, PbS and FeS2 follows the order PbS, CuS and ZnS. FeS2 does not react. In ammonia leaching, the oxidation of sulphur in sulphide minerals is complicated. For various reasons, the order of oxidation as estimated from the reaction potential does not agree with those determined experimentally. Majima and Peters (1966) studied oxidation rates and compared the oxidation order of single sulphide minerals in terms of decreasing order of oxidisability in ammonia at elevated temperatures as: chalcocite > bornite > chalcopyrite > antimonite > galena >pyrrhotite = pyrite = sphalerite. Subsequently, Tozawa et al. (1976) made attempts to describe the order of oxidation reaction for a complex sulphide bulk concentrate. Without referring to pyrite and galena minerals, they indicated the order to be bornite > chalcocite >chalcopyrite> sphalerite >horbachite > argentite. There appears to be some ambiguity about the oxidation of pyrite in ammoniacal medium. The oxidation rate of pyrite is stated to be lower than other minerals initially but, subsequently, it perhaps reacts at the same rate as pyrrhotite and sphalerite (Tozawa et al., 1976). Based on these results, pyrite is one of the least attacked minerals during ammonia leaching. However, it is also reported that pyrite does become leached to some extent (Tozawa et al., 1976), or dissolves significantly in the presence of chalcocite (Williams and Light,
1978). Sarveswara Rao et al. (1991, 1992, 1993) examined the role of galvanic interaction during ammonia leaching of multi-metal sulphides whereby pyrite enhances the dissolution of chalcopyrite, sphalerite, and galena minerals and is itself nearly inert. For ammonia –ammonium sulphate leaching, the oxidation of minerals in bulk concentrate follows an order of galena > sphalerite > chalcoprite, in comparison with the order pyrrhotite > galena > sphalerite > chalcopyrite > pyrite > chalcocite >bornite > argentite in sulphuric acid leaching of multi-metal sulphides (Forward and Veltman, 1961). The chemistry of the ammoniacal thiosulphate system is very complicated due to the simultaneous presence of complexing ligands such as ammonia and thiosulphate, the cupric and cuprous redox couple, and the possibility of oxidative decomposition reactions of thiosulphate involving the formation of additional sulphur compounds such as tetrathionate (Kerley, 1983). The presence of sulphides could affect the above series of reactions. Therefore, the ammoniacal thiosulphate leaching system is more complicated than the oxidative ammonia one. It is the objective of this paper to investigate the leaching behaviour of pyrite, chalcopyrite, pyrrhotite and arsenopyrite in ammoniacal thiosulphate leaching systems. Also examined is the galvanic interaction of these sulphides during the leaching of binary sulphide mixtures. An interdisciplinary approach comprising chemical phase analysis, X-ray diffraction, SEM and Raman spectroscopy is used to characterise partially leached residue and the products formed during leaching. Likely reaction pathways are proposed for the leaching processes.
2. Experimental work 2.1. Minerals and reagents Pyrite (py), chalcopyrite (cp), pyrrhotite (pr) and arsenopyrite (ar) samples were obtained from Geological Specimen Supplies, Australia. The sulphides were crushed, milled to 100% under 25 mm and stored in air-tight plastic bags in a refrigerator. Quantitative XRD was used to determine the mineralogy of the samples. The analytical results are shown in Table 1. The pyrite sample was very pure. The pyrrhotite and
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200 Table 1 Quantitative XRD analysis of the sulphide samples (mass %) Mineral
Pyrite Arsenopyrite Sphalerite Chalcopyrite Quartz Pyrrhotite
Sample Pyrite
Chalcopyrite
Arsenopyrite
Pyrrhotite
100 0.0 0.0 0.0 0.0 0.0
31.2 0.0 3.5 65.0 0.0 0.0
0 93.5 0.0 0.0 6.5 0.0
0.0 0.0 0.0 0.0 1.6 98.4
arsenopyrite samples contained a small amount of quartz, while the chalcopyrite sample contained pyrite and a small amount of sphalerite. Quartz is inert in ammoniacal thiosulphate leaching solutions (Abbruzzese et al., 1995). Therefore, the presence of quartz does not affect the leaching behaviour of pyrrhotite and arsenopyrite. However, the presence of other sulphides does affect the chalcopyrite leaching behaviour due to the galvanic interactions in the leaching process. Consequently, the leaching behaviour of the chalcopyrite sample is the bulk effect of chalcopyrite, pyrite and sphalerite. Laboratory-grade ammonium thiosulphate, ammonium sulphate and ammonia water (25%) were provided by Westlab Chem Supply, Australia. Analytically pure cupric sulphate, hydrogen peroxide (30% w/v), and hydrochloric acid were obtained from Merck. Distilled water was used in the experiments. 2.2. Analytical techniques Elemental concentrations of Cu, Zn, Fe, S, and As in solutions were determined by ICP-OES, after oxidising the sulphur species to stable sulphates prior to the analysis. After oxidation by hydrogen peroxide, solutions were acidified by HCl and boiled to ensure complete conversion of the metal species to the chloride form. The thiosulphate concentration was determined by iodometric methods. In order to eliminate the effect of the cupric ammonia complex on iodine titration, a certain amount of acetic acid (10% solution) was added prior to the titration with the indicator Vitex. To determine the products of oxidation of sulphide minerals present in leach residues, the samples were analysed by SEM coupled with EDAX (Philips XL30), and the surface morphology data were
191
also generated at the same time. Raman spectroscopy was also attempted to identify the surface species. Chemical phase analyses of the samples were carried out with a powder diffractometer (Philips, PC1800) in a 2h range of 10– 75 using a copper target. 2.3. Leaching procedure Experiments were performed in a 1-L baffled opentop reactor using a magnetic stirrer; 500 mL of solution was added to 10 g of the sulphide minerals. The stirrer was maintained at a speed of 800 min 1. All experiments were performed at a room temperature of about 25 C. Samples were taken continuously at a certain interval during a total contact time of 24 h. The samples were centrifuged and filtered for the subsequent iodine titration and oxidation for ICP analysis. The leaching of binary mixtures was conducted by using 10 g of mixtures with two different sulphide samples at the same percentage, and the mixtures were characterised before and after leaching by XRD. The dissolution of chalcopyrite was calculated based on the difference of copper concentrations in solutions; the dissolution of sphalerite (sp) was calculated based on the difference of zinc concentrations, and the other sulphide dissolution was calculated based on the difference of sulphur concentrations. The increase in Cu and Zn concentrations in solutions can be converted to the dissolution of chalcopyrite (CuFeS2) and sphalerite (ZnS) based on the stoichiometric chemical equations, respectively. Similarly, the increase in S concentrations in solutions can be converted to the dissolution of pyrite (FeS2), pyrrhotite (FeS) and arsenopyrite (FeAsS) based on the stoichiometric chemical equations, respectively. The pyrite dissolution in the chalcopyrite sample was calculated on the basis of mass balance of sulphur.
3. Results and discussion 3.1. Effect of thiosulphate concentration on sulphide leaching Fig. 1 shows a plot of percent sulphide dissolution vs. thiosulphate concentration in a leach solution of 1.0 M ammonia water, 6 mM Cu2+ and 0.25 M sulphate. It can be seen that the sulphide dissolution
192
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
decreased with an increase in the thiosulphate concentration up to 0.15 M, after which there was no further decrease in the sulphide dissolution. In the case of all the sulphides, the dissolution was more in the absence of thiosulphate than in the presence of it. This depression effect appeared to be the most prominent in the case of chalcopyrite. In all cases, chalcopyrite had the highest dissolution rate among the four sulphides investigated, followed by pyrrhotite and arsenopyrite. Pyrite was nearly inert with the lowest dissolution rate. Iron and arsenic concentrations remained very low in the leaching of sulphides as shown in Fig. 2. The leach solution originally contained 6 mM Cu2+ , 1.0 M ammonia, 0.25 M thiosulphate and 0.25 M sulphate. It can be seen that iron concentrations in all the sulphide leaching systems remained constantly low. However, the arsenic concentration in the arsenopyrite leaching system remained the highest at the very beginning of the leaching process, then gradually levelled off. This could be attributed to the formation of iron and arsenic precipitates in the ammoniacal thiosulphate leaching systems. Similar to the ammonia – ammonium sulphate leaching systems, iron would precipitate as hydrated ferric oxide (Sarveswara Rao et al., 1991) while arsenic would precipitate as iron arsenate (Errington and Pattinson, 1991). These leaching products were also verified with SEM coupled with EDAX analysis and Raman spectroscopy, which are discussed below. In the presence of dissolved oxygen, the oxidation reactions of pyrite, sphalerite, chalcopyrite, and pyrrhotite during the ammoniacal thiosulphate leaching
Fig. 2. Variation of iron and arsenic concentrations with contact time.
can be represented by the following simplified reactions: 2FeS2 þ ð13=2ÞO2 þ 4CuðNH3 Þ2þ 4 þ ð6 þ nÞH2 O gFe2 O3 nH2 O þ 4ðNH4 Þ2 SO4 þ þ4CuðNH3 Þþ 2 þ 4H
ð1Þ
ZnS þ 2CuðNH3 Þ2þ 4 þ ð3=2ÞO2 þ H2 O þ gZnðNH3 Þ4 SO4 þ 2CuðNH3 Þþ 2 þ 2H
ð2Þ
2CuFeS2 þ ð13=2ÞO2 þ 6CuðNH3 Þ2þ 4 þð6 þ nÞH2 Og 4ðNH4 Þ2 SO4 þ Fe2 O3 nH2 O þ þ8CuðNH3 Þþ 2 þ 4H
ð3Þ
2FeS þ 4O2 þ 2CuðNH3 Þ2þ 4 þ ð3 þ nÞH2 O gFe2 O3 nH2 O þ 2ðNH4 Þ2 SO4 þ þ2CuðNH3 Þþ 2 þ 2H
ð4Þ
The oxidation reaction of arsenopyrite in such a leaching system can be expressed as the following three steps: 4FeAsS þ ð23=2ÞO2 þ 6CuðNH3 Þ2þ 4 þ 9H2 O g4ðNH4 Þ3 AsO4 þ 4FeSO4 Fig. 1. Effect of thiosulphate concentration on sulphide dissolution. Solution: ammonia —1.0 M, Cu2+ — 6 mM, sulphate — 0.25 M.
þ þ6CuðNH3 Þþ 2 þ 6H
ð5Þ
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
FeSO4 þ CuðNH3 Þ2þ 4 þ 3H2 O gFeðOHÞ3 þ CuðNH3 Þþ 2 þðNH4 Þ2 SO4 þ Hþ
ð6Þ
ðNH4 Þ3 AsO4 þ FeðOHÞ3 gFeAsO4 þ 3NH4 OH
ð7Þ
It should be noted that some intermediate sulphur products such as polysulphur species and elemental sulphur could also form in the oxidative leaching of the sulphides. However, these intermediate sulphur species could be further oxidised to sulphate. For the sake of simplicity, all the sulphur species are assumed to be completely oxidised to sulphate. All the above equations are just the overall oxidative reactions of the sulphides. The actual oxidative reactions are far more complicated, and have not been reported before in the literature. In the presence of free thiosulphate, the Cu(NH3)2+ complex will be converted to be Cu(S2 O3)35 . Thiosulphate can be oxidised in the presence of the oxidants cupric tetra-ammonia complex and O2, with the reactions involved being: þ 2 2CuðNH3 Þ2þ 4 þ 2S2 O3 þ 4H2 O g2CuðNH3 Þ2
þ 4NH4 OH þ S4 O2 6
ð8Þ
2 2 2S4 O2 6 þ 3OH gð5=2ÞS2 O3 þ S3 O6
þ ð3=2ÞH2 O 2 2 3S2 O2 þ 3H2 O 3 þ 6OH g 4SO3 þ 2S
ð9Þ ð10Þ
2 2CuðNH3 Þ2þ 4 þ SO3 þ 2OH þ 3H2 O þ g SO2 4 þ 2CuðNH3 Þ2 þ 4NH4 OH
ð11Þ
2CuðNH3 Þþ 2 þ 4NH4 OH þ ð1=2ÞO2 g 2CuðNH3 Þ2þ 4 þ 2OH þ 3H2 O
ð12Þ
The leaching of arsenopyrite first formed soluble ammonium arsenate and then precipitated as ferric arsenate. This could contribute to the higher levels of
193
arsenic concentrations at the beginning of the leaching process. It could be expected that ferric oxide formed simultaneously along with the dissolution of iron-bearing sulphides dissolved in the leach solutions. In the ammoniacal thiosulphate leaching systems, both the cupric tetra-ammonia complex and O2 acted as oxidants. Thiosulphate was not stable and could easily be oxidised especially in the presence of the cupric tetra-ammonia complex, following the reaction pathways shown in Eqs. (8) –(12). The oxidation of thiosulphate would consume O2, which served as the oxidant itself for sulphide oxidation as well as the oxidant for the conversion of the Cu+ to Cu2+ ammonia complexes. Thiosulphate could be considered as the O2 scavenger, resulting in an insufficient O2 content in the leach solutions for sulphide oxidation. Therefore, thiosulphate depressed the oxidative dissolution of sulphides. The leaching of chalcopyrite generated a large amount of copper ions, which combined with ammonia to form more of the cupric ammonia complex for chalcopyrite oxidation. The formation of a large amount of the cupric ammonia complex would enhance chalcopyrite dissolution, contributing to the highest dissolution rate among the four sulphides. On the other hand, the high dissolution rate of chalcopyrite also demanded a high oxygen supply. Consequently, the presence of thiosulphate had the most significant depressing effect on chalcopyrite dissolution (Fig. 1). 3.2. Effect of ammonia concentration on sulphide leaching Ammonia water was used as the slurry pH regulator as well as the ligand for the formation of the cupric tetra-ammonia complex. In addition, ammonia itself could also act as the ligand for the leached heavy metal ions such as iron in the leaching process. The iron ammonia complex so formed was quickly converted to FeOOH and then to Fe2O3nH2O in such an oxidative basic environment. Therefore, the ammonia concentration would play an important part in the sulphide leaching. Fig. 3 shows the effect of ammonia concentration on the sulphide leaching. The leach solutions originally contained 6 mM Cu2+ , 0.25 M thiosulphate and 0.25 M sulphate. It can be seen that the sulphide dissolution rates increased with an increase in the ammonia concentration as expected. The ammonia concentration had the most significant
194
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
effect on chalcopyrite dissolution. The leach rates of pyrrhotite, arsenopyrite and pyrite showed only a marginal increase with an increase in the ammonia concentration. The ammonia will complex copper in the leaching of chalcopyrite as shown in Eq. (3). The high ammonia concentration would shift the equilibrium to the right, enhancing the chalcopyrite dissolution. 3.3. Effect of cupric concentration on sulphide leaching Fig. 4 shows the effect of cupric concentration on sulphide leaching. The leach solutions originally contained 0.25 M thiosulphate, 1.0 M ammonia and 0.25 M sulphate. Fig. 4 shows that the sulphide leaching rates were much lower in the absence of the cupric ion. The sulphide leaching rates increased with an increase in the cupric concentration. However, a further increase in the cupric concentration beyond about 6.3 mM would no longer increase the sulphide dissolution rates. Without the cupric ion, the leaching system was very similar to ammonia – ammonium sulphate leaching and only oxygen acted as the oxidant. The presence of the cupric ion would shift the oxidative dissolution reactions of the sulphides Eqs. (1) –(5) to the right. In the leaching processes, the stirring speed remained constant at all conditions without an extra addition of oxygen. It could be expected that the oxygen supply in the leach solutions remained constant in all cases. The leaching
Fig. 3. Effect of ammonia concentration on sulphide leaching. Solution: Cu2+ — 6 mM, thiosulphate — 0.25 M, sulphate — 0.25 M.
Fig. 4. Effect of cupric concentration on sulphide dissolution. Solution: thiosulphate — 0.25 M, ammonia — 1.0 M, sulphate — 0.25 M.
processes were dependent not only on the cupric concentration but also on the dissolved oxygen content, which converted Cu+ to Cu2+. At a fixed level of oxygen content, the sulphide dissolution rates would no longer increase with increase in the cupric concentration beyond a certain value, as shown in Fig. 4. 3.4. Effect of sulphate on sulphide leaching In the ammoniacal thiosulphate leaching system, sulphate is used to stabilise the thiosulphate in the leach solutions. Based on the oxidative reactions of thiosulphate shown in Eqs. (8) – (11), the final oxidation product of thiosulphate is sulphate. The presence of sulphate would shift the equations back to the left hand side, hence stabilising thiosulphate. However, the oxidative dissolution of the sulphides would generate sulphate as shown in Eqs. (1) –(5). Therefore, it is expected that the presence of sulphate in the leach solutions would depress the sulphide dissolution. Fig. 5 shows the effect of sulphate on sulphide leaching. The leach solutions originally contained 6 mM Cu2+ , 0.25 M thiosulphate and 1.0 M ammonia. As indicated in Fig. 5, sulphide dissolution decreased with the addition of sulphate. Fig. 5 shows that the leaching kinetics of chalcopyrite were almost linear, while the leaching of the other three sulphides was fast at the beginning and became linear with time. The formation of elemental sulphur, iron oxide and iron arsenate precipitates would cover the reaction sites at the sulphide particle surfaces as the sulphide leaching proceeds.
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
195
composition was different for different sulphides. Pyrite induced the highest thiosulphate decomposition (28%), followed by arsenopyrite (18%), chalcopyrite (12%) and pyrrhotite (10%). 3.6. Leaching of binary artificial mixtures
Fig. 5. Effect of sulphate on sulphide leaching. 0 M denotes no sulphate and 0.25 M denotes 0.25 M sulphate. Solution: Cu2+ — 6 mM, thiosulphate — 0.25 M, ammonia — 1.0 M.
This could be the reason why the sulphide leaching kinetics slowed down with time. 3.5. Effect of sulphides on thiosulphate decomposition Fig. 6 shows the thiosulphate decomposition in the presence of sulphides. The leach solutions originally contained 0.25 M thiosulphate, 6 mM Cu2+ , 1.0 M ammonia and 0.25 M sulphate. Fig. 6 shows that the presence of sulphides in the leaching systems enhanced the decomposition of thiosulphate. In the absence of sulphides, the thiosulphate decomposition was only about 3% during 24 h of contact. However, the degree of thiosulphate de-
Fig. 6. Effect of sulphides on thiosulphate decomposition. Blank denotes no sulphides in the leach solutions. Solution: thiosulphate — 0.25 M, Cu2+ — 6 mM, ammonia — 1.0 M, sulphate — 0.25 M.
The chalcopyrite sample was a mixture of chalcopyrite, pyrite and sphalerite. Therefore, the mechanical mixture of chalcopyrite and pyrite contained a higher percentage of pyrite and lower percentages of chalcopyrite and sphalerite than the natural chalcopyrite sample. Fig. 7 shows the leaching of the chalcopyrite and pyrite mixture at a ratio of 1:1. Fig. 7 shows that the chalcopyrite and sphalerite leaching was enhanced by the addition of the extra pyrite. The pyrite leaching was also enhanced in the presence of chalcopyrite and sphalerite in comparison with the pure pyrite leaching. Sphalerite had the highest leaching rate in the bulk leaching of sphalerite, chalcopyrite and pyrite, followed by chalcopyrite. Pyrite had the lowest leaching rate among the three minerals. Sphalerite and chalcopyrite leaching kinetics were approximately linear, while the leaching of pyrite gradually levelled off, similar to the observation in the pure pyrite leaching. The leaching behaviour of pure chalcopyrite and sphalerite was unknown in the ammoniacal thiosulphate leaching system, so that the galvanic effects of pyrite– chalcopyrite, pyrite – sphalerite and sphalerite –
Fig. 7. Leaching of the artificial mixture of pyrite and chalcopyrite. (cp) means the chalcopyrite sample, (py) the pyrite sample and (cp + py) the mixture of pyrite and chalcopyrite samples. Solution: Cu2+ — 6 mM, thiosulphate — 0.25 M, sulphate — 0.25 M, ammonia — 1.0 M.
196
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
chalcopyrite coupling could not be quantified in the experiments. However, it can be inferred that the sphalerite and chalcopyrite leaching was enhanced in the presence of pyrite, based on their higher leaching rates at higher pyrite ratios in the mix-ture. The pyrite leaching was also much improved in the presence of chalcopyrite and sphalerite, although the overall leaching rate of pyrite was still very low. This is in accordance with the findings of Sarveswara Rao et al. (1992), where pyrite enhanced the dissolution of chalcopyrite and sphalerite, and chalcopyrite and sphalerite improved the dissolution of pyrite as well, due to the galvanic coupling between sulphide minerals. Fig. 8 shows the leaching behaviour of chalcopyrite and sphalerite in the binary mixtures of chalcopyrite and the other three sulphide samples. It can be seen that both chalcopyrite and sphalerite leaching rates increased in the presence of pyrite, pyrrhotite and arsenopyrite. The effect of pyrite, arsenopyrite and pyrrhotite on chalcopyrite leaching enhancement was in the order pyrite > arsenopyrite > pyhrrotite. Similarly, the effect of pyrite, arsenopyrite and pyrrhotite on sphalerite leaching enhancement was in the order pr> py >ar. Because of the presence of pyrite in the chalcopyrite sample, it was impossible to quantify the leaching behaviour of arsenopyrite and pyrrhotite in the mixtures, which will be discussed qualitatively in the following section by use of XRD analysis.
Fig. 8. Leaching behaviour of chalcopyrite and sphalerite in the binary mixtures of chalcopyrite and the other three sulphide samples. (cp) means the chalcopyrite sample, (cp + py) the mixture of pyrite and chalcopyrite samples, (cp + ar) the mixture of chalcopyrite and arsenopyrite samples, and (cp + pr) the mixture of chalcopyrite and pyrrhotite samples. Solution: Cu2+ — 6 mM, thiosulphate — 0.25 M, sulphate — 0.25 M, ammonia — 1.0 M.
3.7. XRD diffraction patterns for binary sulphide mixtures before and after leaching XRD diffraction patterns were recorded for the binary sulphide mixtures of cp– py, cp – pr, cp – ar, py– pr, py– ar and pr –ar before and after 24 h leaching, respectively. The leach solution originally contained 6 mM Cu2+ , 0.25 M thiosulphate, 0.25 M sulphate and 1.0 M ammonia water. In the binary system of pyrite and chalcopyrite, the pyrite intensity increased markedly after leaching, while the chalcopyrite intensity decreased correspondingly. The change for the sphalerite intensity was very hard to identify due to its minor amount. Although a small amount of pyrite was dissolved in the leaching process, the pyrite intensity still increased after leaching because the relative percentage of pyrite in the leach residue increased due to the sharp decrease in the chalcopyrite amount. The chalcopyrite intensity shown in the binary system of chalcopyrite and pyrrhotite decreased noticeably after leaching, while the pyrrhotite intensity remained almost the same. Therefore, chalcopyrite dissolved to a larger extent and the pyrrhotite dissolved to a lesser degree. In the binary system of chalcopyrite and arsenopyrite, the chalcopyrite intensity decreased to some extent after leaching, while the arsenopyrite intensity decreased to a lesser extent. It could be expected that chalcopyrite dissolved more than arsenopyrite. The pyrite intensity in the binary system of pyrite and pyrrhotite increased markedly after leaching, while the pyrrhotite intensity decreased slightly. It could be expected that pyrrhotite dissolved more than pyrite. In the binary system of pyrite and arsenopyrite, the pyrite intensity increased slightly and the arsenopyrite intensity decreased slightly. Furthermore, the quartz intensity also increased slightly. It can be inferred that arsenopyrite dissolved more than pyrite. The quartz intensity in the binary system of pyrrhotite and arsenopyrite greatly increased after leaching and the arsenopyrite intensity also increased noticeably, while the pyrrhotite intensity decreased correspondingly. It could be inferred that pyrrhotite dissolved more than the arsenopyrite. In summary, the XRD patterns of the binary mixtures before and after leaching indicated that the leaching of the sulphides in the ammoniacal thiosul-
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
phate system was in the order cp > pr> ar > py, which was in accordance with the observations on the leaching tests. 3.8. Topological studies In order to study the reaction products at the leached particle surfaces as well as the surface morphology, the leached sulphide lumps were subjected to SEM analysis coupled with EDAX. The sulphide lumps (about 10 mm in diameter) were polished with a 1200 grade sand paper for the removal of the oxidation layer prior to the 24-h leach. The leached sulphide lumps were rinsed with distilled water and dried under vacuum overnight. Figs. 9 –12 show the SEM images of chalcopyrite, pyrite, pyrrhotite and arsenopyrite, respectively. There was a large area of erosion at the chalcopyrite surface (Fig. 9) and the surface was very rough and loose, approaching a porous structure. The EDAX analysis indicated the presence of iron oxide at the erosion surfaces, especially at the surface defect sites and the crystal boundaries. This may be because the leaching of chalcopyrite started from the high-energy defect sites and the crystal boundaries. Also illustrated in Fig. 9, there was some localised erosion at the
197
surface of the small pyrite particle surrounded by the chalcopyrite matrix. Erosion was also observed at the boundaries between the pyrite and chalcopyrite crystals. This phenomenon could explain the galvanic effect between chalcopyrite and pyrite in the ammoniacal thiosulphate leaching system. The pure pyrite surface was still very smooth after leaching except for some minor erosion at the surface defects (Fig. 10). The EDAX analysis demonstrated that only a small amount of oxygen associated with the sulphur and iron was present. It can be inferred that only a thin layer of iron oxide formed on the pyrite surface, which would hinder the further oxidative leaching of pyrite. This observation was in accordance with the above experimental results, i.e. the pyrite leaching rate was higher at the beginning and gradually levelled off. A large area of erosion was observed at the leached pyrrhotite surface (Fig. 11). The surface also appeared like a porous structure with very fine pores evenly distributed across it. There was some erosion at the large defect sites at the surface. The EDAX analysis indicated that a large amount of iron oxide phase was present at the eroded pyrrhotite surface. Similarly, the layer of iron oxide formed at the pyrrhotite surface would hinder the further oxidative leaching of pyrrho-
Fig. 9. SEM image of leached chalcopyrite.
198
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
Fig. 10. SEM image of leached pyrite.
tite, which was in accordance with the above experimental observations. Like pyrrhotite, the leached arsenopyrite surface also showed a large area of erosion (Fig. 12). The leached surface also had fine pores on it. Severe ero-
sion was observed at the defect sites and the crystal boundaries. This was because the high-energy defect sites and crystal boundaries favoured the leaching process. The EDAX analysis demonstrated that a type of iron arsenate existed at the leached sites.
Fig. 11. SEM image of leached pyrrhotite.
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
199
Fig. 12. SEM image of leached arsenopyrite.
3.9. Characterisation of the leach product by Raman spectroscopy Attempts were made to use Raman spectroscopy to identify the reaction species on the leached sulphide lump surfaces. However, the only reaction product identified was haematite, which was formed at the leached chalcopyrite surface (Fig. 13). The failure of this method to identify other products is due to the small amounts of products in comparison with the
Fig. 13. Raman spectra of chalcopyrite surface before and after leaching.
unreacted matrices. Chalcopyrite dissolved substantially in the leaching process, and hence a high ratio of haematite was formed in the leach residue.
4. Conclusions The leaching of sulphides in an ammoniacal thiosulphate system is dependent on the solution composition and the mineral types. On the other hand, the leaching of sulphides influences the oxidation of thiosulphate. The following conclusions can be drawn from the results of this study: . The relative leaching rates of sulphides were in the order chalcopyrite > pyrrhotite > arsenopyrite > pyrite. The chalcopyrite leaching kinetics were nearly linear. However, the leaching of the other three sulphides was fast at the start and gradually levelled off. This was attributed to the formation of an iron oxide or arsenate layer at the particle surface, hindering further exposure of the sulphides to the leach solution. The iron and arsenic concentrations remained at low levels in the leaching process due to the formation of precipitates. . Thiosulphate depressed the leaching of sulphides due to its preferential oxidation over sulphides. Thiosulphate consumed the dissolved oxygen in the leach
200
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
solution, which was essential for the direct oxidation of sulphides as well as the conversion of the Cu+ to Cu2+ ammonia complexes. . Ammonia acted as the pH regulator and the ligand for Cu2+ to form the cupric ammonia complex. An increase in ammonia concentration up to a certain level resulted in an enhanced sulphide leaching. . An increase in Cu2+ concentration up to a certain level enhanced the sulphide leaching. . Because sulphate was one of the final leaching products for all the sulphide minerals, the presence of sulphate would shift the equilibrium back to the sulphide side. Therefore, the presence of sulphate depressed the sulphide leaching. . The presence of sulphides in the leach solution resulted in an increased oxidation rate of thiosulphate. Pyrite induced the highest thiosulphate decomposition, followed by arsenopyrite, chalcopyrite and pyrrhotite. . Pyrite enhanced chalcopyrite and sphalerite dissolution. In return, chalcopyrite and sphalerite also enhanced pyrite dissolution. This could be clearly observed in the SEM image, where pyrite revealed erosion at the pyrite– chalcopyrite boundaries. . SEM analysis with the aid of EDAX indicated the formation of iron oxide at the chalcopyrite, pyrrhotite and pyrite surfaces and the formation of iron arsenate at the arsenopyrite surface after leaching. In addition, SEM analysis demonstrated that the high-energy defect sites and crystal boundaries favoured the sulphide leaching. Raman spectroscopy indicated that haematite was formed during the leaching of chalcopyrite. Acknowledgements The financial support from Newcrest Mining Limited, Placer Dome Technical Services Limited and the Australian Research Council is gratefully acknowledged. Appreciation is also expressed to Hui Tan and Fay Lim for assistance in the experimental work. References Abbruzzese, C., Fornari, P., Massidda, R., Veglio, F., Ubaldini, S., 1995. Thiosulphate leaching for gold hydrometallurgy. Hydrometallurgy 39, 265 – 276.
Cao, C., Hu, J., Gong, Q., 1992. Leaching of gold by low concentration thiosulphate solution. Randol Gold Forum, Vancouver ’92. Randol International, Colorado, USA, pp. 293 – 298. Errington, M.T., Pattinson, D., 1991. Alkaline pressure oxidation leaching (APOL) of refractory gold bearing materials. Innovations in Precious Metals Recovery and Refining. SCI, London. Forward, F.A., Veltman, H., 1961. A process for direct leaching of zinc sulphide concentrates with sulphuric acid and oxygen under pressure. Physical Chemistry of Extractive Metallurgy, Part 2. Interscience Publishers, New York. Gong, Q., Hu, J., Cao, C., 1993. Kinetics of gold leaching from sulphide gold concentrates with thiosulphate solution. Trans. Nonferrous Met. Soc. China 3 (4), 30 – 36. Jiang, T., Chen, J., Xu, S., 1993. A kinetic study of gold leaching with thiosulphate. In: Hiskey, J.B., Warren, G.W. (Eds.), Hydrometallurgy, Fundamentals, Technology and Innovations. AIME, Littleton, Colorado, USA, pp. 119 – 126, Chap. 7. Kerley, B.J., 1983. Recovery of precious metals from difficult ores. US patent, 1983, no. 4369061. Langhans, J.W., Lei, K.P.V., Carnahan, T.G., 1992. Copper-catalyzed thiosulphate leaching of low-grade gold ores. Hydrometallurgy 29, 191 – 203. Majima, H., Peters, E., 1966. Oxidation rates of sulphide minerals by aqueous oxidation at elevated temperatures. Trans. Metall. Soc. AIME 236, 1409 – 1413. Sarveswara Rao, K., Ray, H.S., 1998. A new look at characterization and oxidative ammonia leaching behaviour of multimetal sulphides. Miner. Eng. 11 (11), 1011 – 1024. Sarveswara Rao, K., Das, R.P., Ray, H.S., 1991. Study of leaching of multimetal sulphides through an interdisciplinary approach. Min. Proc. Ext. Met. Rev. 7, 209 – 234. Sarveswara Rao, K., Paramguru, R.K., Das, R.P., Ray, H.S., 1992. The role of galvanic interaction during ammonia leaching of multimetal sulphides. Min. Proc. Ext. Met. Rev. 11, 21 – 37. Sarveswara Rao, K., Das, R.P., Mukunda, P.G., Ray, H.S., 1993. Use of X-ray diffraction in a study of ammonia leaching of multimetal sulphides. Metall. Trans. 24B, 937 – 945. Tozawa, K., Umetsu, Y., Sato, K., 1976. In: Yannopoulos, J.C., Agarwal, J.C. (Eds.), Extractive Metallurgy of Copper, Met. Soc. AIME, vol. II, pp. 706 – 721. Tozawa, K., Inliu, Y., Umetsu, Y., 1981. Dissolution of gold in ammoniacal thiosulphate solutions. 110th Annual meeting of AIME Meeting, Chicago, IL, TMS paper, A81-25. Wan, R.Y., 1997. Importance of solution chemistry for thiosulphate leaching of gold. World Gold ’97 Conference, Singapore. Society for Mining, Metallurgy and Exploration (SME), Littleton, Colorado, USA, pp. 159 – 162. Williams, R.D., Light, S.D., 1978. Copper concentrate dissolution chemistry and kinetics in an ammonia-oxygen environment. Fundamental Aspects of Hydrometallurgical Processes. AIChE Symp. Ser. 74 (174), 21 – 27. Zipperian, D., Raghavan, S., Wilson, J.P., 1988. Gold and silver extraction by ammoniacal thiosulphate leaching from a Rhyolite ore. Hydrometallurgy 19, 361 – 375.
Leaching behaviour of sulphides in ammoniacal thiosulphate systems D. Feng, J.S.J. Van Deventer* Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia Received 23 July 2001; received in revised form 27 September 2001; accepted 23 November 2001
Abstract Systematic studies have been conducted to understand the leaching behaviour and dissolution mechanisms when common gold-host sulphides such as chalcopyrite, pyrite, arsenopyrite and pyrrhotite are treated by oxidative ammoniacal thiosulphate leaching. Leach solution composition strongly influenced the sulphide leaching, and the presence of sulphides also enhanced the decomposition of thiosulphate. The XRD patterns of the binary mixtures before and after leaching indicated that the relative leaching rates of the sulphides in the ammoniacal thiosulphate system were in the order chalcopyrite > pyrrhotite > arsenopyrite > pyrite, which was in accordance with the observations made from the leaching tests. SEM analysis with the aid of EDAX indicated the formation of iron oxide at the chalcopyrite, pyrrhotite and pyrite surfaces and the formation of iron arsenate at the arsenopyrite surface after leaching. SEM analysis also demonstrated that the high-energy defect sites and crystal boundaries favoured the sulphide leaching. Raman spectroscopy indicated that haematite was formed during the leaching of chalcopyrite. Iron and arsenic concentrations in the leach solutions were very low due to the formation of iron oxide and iron arsenate during the leaching reactions. Pyrite enhanced chalcopyrite and sphalerite dissolution. Chalcopyrite and sphalerite also enhanced pyrite dissolution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulphide; Oxidation; Leaching; Thiosulphate; Ammonia; Galvanic effect
1. Introduction Thiosulphate, as an alternative lixiviant for gold, has received much attention in recent years due to the growing environmental and public concerns over the use of cyanide. Acceptable gold leaching rates using thiosulphate are achieved in the presence of ammonia with cupric ion acting as the oxidant (Abbruzzese et
*
Corresponding author. Tel.: +61-3-9344-6620; fax: +61-39344-4153. E-mail address: [email protected] (J.S.J. Van Deventer).
al., 1995; Cao et al., 1992; Gong et al., 1993; Langhans et al., 1992; Jiang et al., 1993; Tozawa et al., 1981; Zipperian et al., 1988). Despite the multitude of papers in this area, the industrial applications where thiosulphate is used instead of cyanide are apparently limited to a silver ore plant in Mexico (Wan, 1997). Research has shown that some ores are well suited to thiosulphate leaching, while others show hardly any extraction. An important reason for this lack of acceptance into the industry is that the solution chemistry and the mineralogical factors affecting the effectiveness of ammoniacal thiosulphate systems are not understood adequately. Since modern gold and silver leaching has
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 5 - 0
190
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
been directed to very complex ores, the successful application of thiosulphate leaching depends not only on the dissolution behaviour of gold and silver, but also critically on the behaviour of the associated minerals. Surprisingly, there is hardly any publication that deals with mineralogical aspects of ores during thiosulphate leaching. Pyrite, chalcopyrite, chalcocite, bornite, pyrrhotite and arsenopyrite are the main sulphide minerals locking gold in refractory sulphide gold ores. The leaching behaviour of these minerals could play an important role in the thiosulphate leaching of gold sulphide ores. However, the leaching behaviour of the sulphides still remains unknown in such a leaching system. Sarveswara Rao and Ray (1998) conducted systematic studies on the oxidation behaviour and dissolution reaction mechanisms operating while pure copper, zinc and lead sulphide minerals and mixtures, and single concentrates are treated by oxidative ammonia leaching. The sequence of sulphide mineral dissolution from a quaternary mixture of CuS, ZnS, PbS and FeS2 follows the order PbS, CuS and ZnS. FeS2 does not react. In ammonia leaching, the oxidation of sulphur in sulphide minerals is complicated. For various reasons, the order of oxidation as estimated from the reaction potential does not agree with those determined experimentally. Majima and Peters (1966) studied oxidation rates and compared the oxidation order of single sulphide minerals in terms of decreasing order of oxidisability in ammonia at elevated temperatures as: chalcocite > bornite > chalcopyrite > antimonite > galena >pyrrhotite = pyrite = sphalerite. Subsequently, Tozawa et al. (1976) made attempts to describe the order of oxidation reaction for a complex sulphide bulk concentrate. Without referring to pyrite and galena minerals, they indicated the order to be bornite > chalcocite >chalcopyrite> sphalerite >horbachite > argentite. There appears to be some ambiguity about the oxidation of pyrite in ammoniacal medium. The oxidation rate of pyrite is stated to be lower than other minerals initially but, subsequently, it perhaps reacts at the same rate as pyrrhotite and sphalerite (Tozawa et al., 1976). Based on these results, pyrite is one of the least attacked minerals during ammonia leaching. However, it is also reported that pyrite does become leached to some extent (Tozawa et al., 1976), or dissolves significantly in the presence of chalcocite (Williams and Light,
1978). Sarveswara Rao et al. (1991, 1992, 1993) examined the role of galvanic interaction during ammonia leaching of multi-metal sulphides whereby pyrite enhances the dissolution of chalcopyrite, sphalerite, and galena minerals and is itself nearly inert. For ammonia –ammonium sulphate leaching, the oxidation of minerals in bulk concentrate follows an order of galena > sphalerite > chalcoprite, in comparison with the order pyrrhotite > galena > sphalerite > chalcopyrite > pyrite > chalcocite >bornite > argentite in sulphuric acid leaching of multi-metal sulphides (Forward and Veltman, 1961). The chemistry of the ammoniacal thiosulphate system is very complicated due to the simultaneous presence of complexing ligands such as ammonia and thiosulphate, the cupric and cuprous redox couple, and the possibility of oxidative decomposition reactions of thiosulphate involving the formation of additional sulphur compounds such as tetrathionate (Kerley, 1983). The presence of sulphides could affect the above series of reactions. Therefore, the ammoniacal thiosulphate leaching system is more complicated than the oxidative ammonia one. It is the objective of this paper to investigate the leaching behaviour of pyrite, chalcopyrite, pyrrhotite and arsenopyrite in ammoniacal thiosulphate leaching systems. Also examined is the galvanic interaction of these sulphides during the leaching of binary sulphide mixtures. An interdisciplinary approach comprising chemical phase analysis, X-ray diffraction, SEM and Raman spectroscopy is used to characterise partially leached residue and the products formed during leaching. Likely reaction pathways are proposed for the leaching processes.
2. Experimental work 2.1. Minerals and reagents Pyrite (py), chalcopyrite (cp), pyrrhotite (pr) and arsenopyrite (ar) samples were obtained from Geological Specimen Supplies, Australia. The sulphides were crushed, milled to 100% under 25 mm and stored in air-tight plastic bags in a refrigerator. Quantitative XRD was used to determine the mineralogy of the samples. The analytical results are shown in Table 1. The pyrite sample was very pure. The pyrrhotite and
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200 Table 1 Quantitative XRD analysis of the sulphide samples (mass %) Mineral
Pyrite Arsenopyrite Sphalerite Chalcopyrite Quartz Pyrrhotite
Sample Pyrite
Chalcopyrite
Arsenopyrite
Pyrrhotite
100 0.0 0.0 0.0 0.0 0.0
31.2 0.0 3.5 65.0 0.0 0.0
0 93.5 0.0 0.0 6.5 0.0
0.0 0.0 0.0 0.0 1.6 98.4
arsenopyrite samples contained a small amount of quartz, while the chalcopyrite sample contained pyrite and a small amount of sphalerite. Quartz is inert in ammoniacal thiosulphate leaching solutions (Abbruzzese et al., 1995). Therefore, the presence of quartz does not affect the leaching behaviour of pyrrhotite and arsenopyrite. However, the presence of other sulphides does affect the chalcopyrite leaching behaviour due to the galvanic interactions in the leaching process. Consequently, the leaching behaviour of the chalcopyrite sample is the bulk effect of chalcopyrite, pyrite and sphalerite. Laboratory-grade ammonium thiosulphate, ammonium sulphate and ammonia water (25%) were provided by Westlab Chem Supply, Australia. Analytically pure cupric sulphate, hydrogen peroxide (30% w/v), and hydrochloric acid were obtained from Merck. Distilled water was used in the experiments. 2.2. Analytical techniques Elemental concentrations of Cu, Zn, Fe, S, and As in solutions were determined by ICP-OES, after oxidising the sulphur species to stable sulphates prior to the analysis. After oxidation by hydrogen peroxide, solutions were acidified by HCl and boiled to ensure complete conversion of the metal species to the chloride form. The thiosulphate concentration was determined by iodometric methods. In order to eliminate the effect of the cupric ammonia complex on iodine titration, a certain amount of acetic acid (10% solution) was added prior to the titration with the indicator Vitex. To determine the products of oxidation of sulphide minerals present in leach residues, the samples were analysed by SEM coupled with EDAX (Philips XL30), and the surface morphology data were
191
also generated at the same time. Raman spectroscopy was also attempted to identify the surface species. Chemical phase analyses of the samples were carried out with a powder diffractometer (Philips, PC1800) in a 2h range of 10– 75 using a copper target. 2.3. Leaching procedure Experiments were performed in a 1-L baffled opentop reactor using a magnetic stirrer; 500 mL of solution was added to 10 g of the sulphide minerals. The stirrer was maintained at a speed of 800 min 1. All experiments were performed at a room temperature of about 25 C. Samples were taken continuously at a certain interval during a total contact time of 24 h. The samples were centrifuged and filtered for the subsequent iodine titration and oxidation for ICP analysis. The leaching of binary mixtures was conducted by using 10 g of mixtures with two different sulphide samples at the same percentage, and the mixtures were characterised before and after leaching by XRD. The dissolution of chalcopyrite was calculated based on the difference of copper concentrations in solutions; the dissolution of sphalerite (sp) was calculated based on the difference of zinc concentrations, and the other sulphide dissolution was calculated based on the difference of sulphur concentrations. The increase in Cu and Zn concentrations in solutions can be converted to the dissolution of chalcopyrite (CuFeS2) and sphalerite (ZnS) based on the stoichiometric chemical equations, respectively. Similarly, the increase in S concentrations in solutions can be converted to the dissolution of pyrite (FeS2), pyrrhotite (FeS) and arsenopyrite (FeAsS) based on the stoichiometric chemical equations, respectively. The pyrite dissolution in the chalcopyrite sample was calculated on the basis of mass balance of sulphur.
3. Results and discussion 3.1. Effect of thiosulphate concentration on sulphide leaching Fig. 1 shows a plot of percent sulphide dissolution vs. thiosulphate concentration in a leach solution of 1.0 M ammonia water, 6 mM Cu2+ and 0.25 M sulphate. It can be seen that the sulphide dissolution
192
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
decreased with an increase in the thiosulphate concentration up to 0.15 M, after which there was no further decrease in the sulphide dissolution. In the case of all the sulphides, the dissolution was more in the absence of thiosulphate than in the presence of it. This depression effect appeared to be the most prominent in the case of chalcopyrite. In all cases, chalcopyrite had the highest dissolution rate among the four sulphides investigated, followed by pyrrhotite and arsenopyrite. Pyrite was nearly inert with the lowest dissolution rate. Iron and arsenic concentrations remained very low in the leaching of sulphides as shown in Fig. 2. The leach solution originally contained 6 mM Cu2+ , 1.0 M ammonia, 0.25 M thiosulphate and 0.25 M sulphate. It can be seen that iron concentrations in all the sulphide leaching systems remained constantly low. However, the arsenic concentration in the arsenopyrite leaching system remained the highest at the very beginning of the leaching process, then gradually levelled off. This could be attributed to the formation of iron and arsenic precipitates in the ammoniacal thiosulphate leaching systems. Similar to the ammonia – ammonium sulphate leaching systems, iron would precipitate as hydrated ferric oxide (Sarveswara Rao et al., 1991) while arsenic would precipitate as iron arsenate (Errington and Pattinson, 1991). These leaching products were also verified with SEM coupled with EDAX analysis and Raman spectroscopy, which are discussed below. In the presence of dissolved oxygen, the oxidation reactions of pyrite, sphalerite, chalcopyrite, and pyrrhotite during the ammoniacal thiosulphate leaching
Fig. 2. Variation of iron and arsenic concentrations with contact time.
can be represented by the following simplified reactions: 2FeS2 þ ð13=2ÞO2 þ 4CuðNH3 Þ2þ 4 þ ð6 þ nÞH2 O gFe2 O3 nH2 O þ 4ðNH4 Þ2 SO4 þ þ4CuðNH3 Þþ 2 þ 4H
ð1Þ
ZnS þ 2CuðNH3 Þ2þ 4 þ ð3=2ÞO2 þ H2 O þ gZnðNH3 Þ4 SO4 þ 2CuðNH3 Þþ 2 þ 2H
ð2Þ
2CuFeS2 þ ð13=2ÞO2 þ 6CuðNH3 Þ2þ 4 þð6 þ nÞH2 Og 4ðNH4 Þ2 SO4 þ Fe2 O3 nH2 O þ þ8CuðNH3 Þþ 2 þ 4H
ð3Þ
2FeS þ 4O2 þ 2CuðNH3 Þ2þ 4 þ ð3 þ nÞH2 O gFe2 O3 nH2 O þ 2ðNH4 Þ2 SO4 þ þ2CuðNH3 Þþ 2 þ 2H
ð4Þ
The oxidation reaction of arsenopyrite in such a leaching system can be expressed as the following three steps: 4FeAsS þ ð23=2ÞO2 þ 6CuðNH3 Þ2þ 4 þ 9H2 O g4ðNH4 Þ3 AsO4 þ 4FeSO4 Fig. 1. Effect of thiosulphate concentration on sulphide dissolution. Solution: ammonia —1.0 M, Cu2+ — 6 mM, sulphate — 0.25 M.
þ þ6CuðNH3 Þþ 2 þ 6H
ð5Þ
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
FeSO4 þ CuðNH3 Þ2þ 4 þ 3H2 O gFeðOHÞ3 þ CuðNH3 Þþ 2 þðNH4 Þ2 SO4 þ Hþ
ð6Þ
ðNH4 Þ3 AsO4 þ FeðOHÞ3 gFeAsO4 þ 3NH4 OH
ð7Þ
It should be noted that some intermediate sulphur products such as polysulphur species and elemental sulphur could also form in the oxidative leaching of the sulphides. However, these intermediate sulphur species could be further oxidised to sulphate. For the sake of simplicity, all the sulphur species are assumed to be completely oxidised to sulphate. All the above equations are just the overall oxidative reactions of the sulphides. The actual oxidative reactions are far more complicated, and have not been reported before in the literature. In the presence of free thiosulphate, the Cu(NH3)2+ complex will be converted to be Cu(S2 O3)35 . Thiosulphate can be oxidised in the presence of the oxidants cupric tetra-ammonia complex and O2, with the reactions involved being: þ 2 2CuðNH3 Þ2þ 4 þ 2S2 O3 þ 4H2 O g2CuðNH3 Þ2
þ 4NH4 OH þ S4 O2 6
ð8Þ
2 2 2S4 O2 6 þ 3OH gð5=2ÞS2 O3 þ S3 O6
þ ð3=2ÞH2 O 2 2 3S2 O2 þ 3H2 O 3 þ 6OH g 4SO3 þ 2S
ð9Þ ð10Þ
2 2CuðNH3 Þ2þ 4 þ SO3 þ 2OH þ 3H2 O þ g SO2 4 þ 2CuðNH3 Þ2 þ 4NH4 OH
ð11Þ
2CuðNH3 Þþ 2 þ 4NH4 OH þ ð1=2ÞO2 g 2CuðNH3 Þ2þ 4 þ 2OH þ 3H2 O
ð12Þ
The leaching of arsenopyrite first formed soluble ammonium arsenate and then precipitated as ferric arsenate. This could contribute to the higher levels of
193
arsenic concentrations at the beginning of the leaching process. It could be expected that ferric oxide formed simultaneously along with the dissolution of iron-bearing sulphides dissolved in the leach solutions. In the ammoniacal thiosulphate leaching systems, both the cupric tetra-ammonia complex and O2 acted as oxidants. Thiosulphate was not stable and could easily be oxidised especially in the presence of the cupric tetra-ammonia complex, following the reaction pathways shown in Eqs. (8) –(12). The oxidation of thiosulphate would consume O2, which served as the oxidant itself for sulphide oxidation as well as the oxidant for the conversion of the Cu+ to Cu2+ ammonia complexes. Thiosulphate could be considered as the O2 scavenger, resulting in an insufficient O2 content in the leach solutions for sulphide oxidation. Therefore, thiosulphate depressed the oxidative dissolution of sulphides. The leaching of chalcopyrite generated a large amount of copper ions, which combined with ammonia to form more of the cupric ammonia complex for chalcopyrite oxidation. The formation of a large amount of the cupric ammonia complex would enhance chalcopyrite dissolution, contributing to the highest dissolution rate among the four sulphides. On the other hand, the high dissolution rate of chalcopyrite also demanded a high oxygen supply. Consequently, the presence of thiosulphate had the most significant depressing effect on chalcopyrite dissolution (Fig. 1). 3.2. Effect of ammonia concentration on sulphide leaching Ammonia water was used as the slurry pH regulator as well as the ligand for the formation of the cupric tetra-ammonia complex. In addition, ammonia itself could also act as the ligand for the leached heavy metal ions such as iron in the leaching process. The iron ammonia complex so formed was quickly converted to FeOOH and then to Fe2O3nH2O in such an oxidative basic environment. Therefore, the ammonia concentration would play an important part in the sulphide leaching. Fig. 3 shows the effect of ammonia concentration on the sulphide leaching. The leach solutions originally contained 6 mM Cu2+ , 0.25 M thiosulphate and 0.25 M sulphate. It can be seen that the sulphide dissolution rates increased with an increase in the ammonia concentration as expected. The ammonia concentration had the most significant
194
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
effect on chalcopyrite dissolution. The leach rates of pyrrhotite, arsenopyrite and pyrite showed only a marginal increase with an increase in the ammonia concentration. The ammonia will complex copper in the leaching of chalcopyrite as shown in Eq. (3). The high ammonia concentration would shift the equilibrium to the right, enhancing the chalcopyrite dissolution. 3.3. Effect of cupric concentration on sulphide leaching Fig. 4 shows the effect of cupric concentration on sulphide leaching. The leach solutions originally contained 0.25 M thiosulphate, 1.0 M ammonia and 0.25 M sulphate. Fig. 4 shows that the sulphide leaching rates were much lower in the absence of the cupric ion. The sulphide leaching rates increased with an increase in the cupric concentration. However, a further increase in the cupric concentration beyond about 6.3 mM would no longer increase the sulphide dissolution rates. Without the cupric ion, the leaching system was very similar to ammonia – ammonium sulphate leaching and only oxygen acted as the oxidant. The presence of the cupric ion would shift the oxidative dissolution reactions of the sulphides Eqs. (1) –(5) to the right. In the leaching processes, the stirring speed remained constant at all conditions without an extra addition of oxygen. It could be expected that the oxygen supply in the leach solutions remained constant in all cases. The leaching
Fig. 3. Effect of ammonia concentration on sulphide leaching. Solution: Cu2+ — 6 mM, thiosulphate — 0.25 M, sulphate — 0.25 M.
Fig. 4. Effect of cupric concentration on sulphide dissolution. Solution: thiosulphate — 0.25 M, ammonia — 1.0 M, sulphate — 0.25 M.
processes were dependent not only on the cupric concentration but also on the dissolved oxygen content, which converted Cu+ to Cu2+. At a fixed level of oxygen content, the sulphide dissolution rates would no longer increase with increase in the cupric concentration beyond a certain value, as shown in Fig. 4. 3.4. Effect of sulphate on sulphide leaching In the ammoniacal thiosulphate leaching system, sulphate is used to stabilise the thiosulphate in the leach solutions. Based on the oxidative reactions of thiosulphate shown in Eqs. (8) – (11), the final oxidation product of thiosulphate is sulphate. The presence of sulphate would shift the equations back to the left hand side, hence stabilising thiosulphate. However, the oxidative dissolution of the sulphides would generate sulphate as shown in Eqs. (1) –(5). Therefore, it is expected that the presence of sulphate in the leach solutions would depress the sulphide dissolution. Fig. 5 shows the effect of sulphate on sulphide leaching. The leach solutions originally contained 6 mM Cu2+ , 0.25 M thiosulphate and 1.0 M ammonia. As indicated in Fig. 5, sulphide dissolution decreased with the addition of sulphate. Fig. 5 shows that the leaching kinetics of chalcopyrite were almost linear, while the leaching of the other three sulphides was fast at the beginning and became linear with time. The formation of elemental sulphur, iron oxide and iron arsenate precipitates would cover the reaction sites at the sulphide particle surfaces as the sulphide leaching proceeds.
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
195
composition was different for different sulphides. Pyrite induced the highest thiosulphate decomposition (28%), followed by arsenopyrite (18%), chalcopyrite (12%) and pyrrhotite (10%). 3.6. Leaching of binary artificial mixtures
Fig. 5. Effect of sulphate on sulphide leaching. 0 M denotes no sulphate and 0.25 M denotes 0.25 M sulphate. Solution: Cu2+ — 6 mM, thiosulphate — 0.25 M, ammonia — 1.0 M.
This could be the reason why the sulphide leaching kinetics slowed down with time. 3.5. Effect of sulphides on thiosulphate decomposition Fig. 6 shows the thiosulphate decomposition in the presence of sulphides. The leach solutions originally contained 0.25 M thiosulphate, 6 mM Cu2+ , 1.0 M ammonia and 0.25 M sulphate. Fig. 6 shows that the presence of sulphides in the leaching systems enhanced the decomposition of thiosulphate. In the absence of sulphides, the thiosulphate decomposition was only about 3% during 24 h of contact. However, the degree of thiosulphate de-
Fig. 6. Effect of sulphides on thiosulphate decomposition. Blank denotes no sulphides in the leach solutions. Solution: thiosulphate — 0.25 M, Cu2+ — 6 mM, ammonia — 1.0 M, sulphate — 0.25 M.
The chalcopyrite sample was a mixture of chalcopyrite, pyrite and sphalerite. Therefore, the mechanical mixture of chalcopyrite and pyrite contained a higher percentage of pyrite and lower percentages of chalcopyrite and sphalerite than the natural chalcopyrite sample. Fig. 7 shows the leaching of the chalcopyrite and pyrite mixture at a ratio of 1:1. Fig. 7 shows that the chalcopyrite and sphalerite leaching was enhanced by the addition of the extra pyrite. The pyrite leaching was also enhanced in the presence of chalcopyrite and sphalerite in comparison with the pure pyrite leaching. Sphalerite had the highest leaching rate in the bulk leaching of sphalerite, chalcopyrite and pyrite, followed by chalcopyrite. Pyrite had the lowest leaching rate among the three minerals. Sphalerite and chalcopyrite leaching kinetics were approximately linear, while the leaching of pyrite gradually levelled off, similar to the observation in the pure pyrite leaching. The leaching behaviour of pure chalcopyrite and sphalerite was unknown in the ammoniacal thiosulphate leaching system, so that the galvanic effects of pyrite– chalcopyrite, pyrite – sphalerite and sphalerite –
Fig. 7. Leaching of the artificial mixture of pyrite and chalcopyrite. (cp) means the chalcopyrite sample, (py) the pyrite sample and (cp + py) the mixture of pyrite and chalcopyrite samples. Solution: Cu2+ — 6 mM, thiosulphate — 0.25 M, sulphate — 0.25 M, ammonia — 1.0 M.
196
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
chalcopyrite coupling could not be quantified in the experiments. However, it can be inferred that the sphalerite and chalcopyrite leaching was enhanced in the presence of pyrite, based on their higher leaching rates at higher pyrite ratios in the mix-ture. The pyrite leaching was also much improved in the presence of chalcopyrite and sphalerite, although the overall leaching rate of pyrite was still very low. This is in accordance with the findings of Sarveswara Rao et al. (1992), where pyrite enhanced the dissolution of chalcopyrite and sphalerite, and chalcopyrite and sphalerite improved the dissolution of pyrite as well, due to the galvanic coupling between sulphide minerals. Fig. 8 shows the leaching behaviour of chalcopyrite and sphalerite in the binary mixtures of chalcopyrite and the other three sulphide samples. It can be seen that both chalcopyrite and sphalerite leaching rates increased in the presence of pyrite, pyrrhotite and arsenopyrite. The effect of pyrite, arsenopyrite and pyrrhotite on chalcopyrite leaching enhancement was in the order pyrite > arsenopyrite > pyhrrotite. Similarly, the effect of pyrite, arsenopyrite and pyrrhotite on sphalerite leaching enhancement was in the order pr> py >ar. Because of the presence of pyrite in the chalcopyrite sample, it was impossible to quantify the leaching behaviour of arsenopyrite and pyrrhotite in the mixtures, which will be discussed qualitatively in the following section by use of XRD analysis.
Fig. 8. Leaching behaviour of chalcopyrite and sphalerite in the binary mixtures of chalcopyrite and the other three sulphide samples. (cp) means the chalcopyrite sample, (cp + py) the mixture of pyrite and chalcopyrite samples, (cp + ar) the mixture of chalcopyrite and arsenopyrite samples, and (cp + pr) the mixture of chalcopyrite and pyrrhotite samples. Solution: Cu2+ — 6 mM, thiosulphate — 0.25 M, sulphate — 0.25 M, ammonia — 1.0 M.
3.7. XRD diffraction patterns for binary sulphide mixtures before and after leaching XRD diffraction patterns were recorded for the binary sulphide mixtures of cp– py, cp – pr, cp – ar, py– pr, py– ar and pr –ar before and after 24 h leaching, respectively. The leach solution originally contained 6 mM Cu2+ , 0.25 M thiosulphate, 0.25 M sulphate and 1.0 M ammonia water. In the binary system of pyrite and chalcopyrite, the pyrite intensity increased markedly after leaching, while the chalcopyrite intensity decreased correspondingly. The change for the sphalerite intensity was very hard to identify due to its minor amount. Although a small amount of pyrite was dissolved in the leaching process, the pyrite intensity still increased after leaching because the relative percentage of pyrite in the leach residue increased due to the sharp decrease in the chalcopyrite amount. The chalcopyrite intensity shown in the binary system of chalcopyrite and pyrrhotite decreased noticeably after leaching, while the pyrrhotite intensity remained almost the same. Therefore, chalcopyrite dissolved to a larger extent and the pyrrhotite dissolved to a lesser degree. In the binary system of chalcopyrite and arsenopyrite, the chalcopyrite intensity decreased to some extent after leaching, while the arsenopyrite intensity decreased to a lesser extent. It could be expected that chalcopyrite dissolved more than arsenopyrite. The pyrite intensity in the binary system of pyrite and pyrrhotite increased markedly after leaching, while the pyrrhotite intensity decreased slightly. It could be expected that pyrrhotite dissolved more than pyrite. In the binary system of pyrite and arsenopyrite, the pyrite intensity increased slightly and the arsenopyrite intensity decreased slightly. Furthermore, the quartz intensity also increased slightly. It can be inferred that arsenopyrite dissolved more than pyrite. The quartz intensity in the binary system of pyrrhotite and arsenopyrite greatly increased after leaching and the arsenopyrite intensity also increased noticeably, while the pyrrhotite intensity decreased correspondingly. It could be inferred that pyrrhotite dissolved more than the arsenopyrite. In summary, the XRD patterns of the binary mixtures before and after leaching indicated that the leaching of the sulphides in the ammoniacal thiosul-
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
phate system was in the order cp > pr> ar > py, which was in accordance with the observations on the leaching tests. 3.8. Topological studies In order to study the reaction products at the leached particle surfaces as well as the surface morphology, the leached sulphide lumps were subjected to SEM analysis coupled with EDAX. The sulphide lumps (about 10 mm in diameter) were polished with a 1200 grade sand paper for the removal of the oxidation layer prior to the 24-h leach. The leached sulphide lumps were rinsed with distilled water and dried under vacuum overnight. Figs. 9 –12 show the SEM images of chalcopyrite, pyrite, pyrrhotite and arsenopyrite, respectively. There was a large area of erosion at the chalcopyrite surface (Fig. 9) and the surface was very rough and loose, approaching a porous structure. The EDAX analysis indicated the presence of iron oxide at the erosion surfaces, especially at the surface defect sites and the crystal boundaries. This may be because the leaching of chalcopyrite started from the high-energy defect sites and the crystal boundaries. Also illustrated in Fig. 9, there was some localised erosion at the
197
surface of the small pyrite particle surrounded by the chalcopyrite matrix. Erosion was also observed at the boundaries between the pyrite and chalcopyrite crystals. This phenomenon could explain the galvanic effect between chalcopyrite and pyrite in the ammoniacal thiosulphate leaching system. The pure pyrite surface was still very smooth after leaching except for some minor erosion at the surface defects (Fig. 10). The EDAX analysis demonstrated that only a small amount of oxygen associated with the sulphur and iron was present. It can be inferred that only a thin layer of iron oxide formed on the pyrite surface, which would hinder the further oxidative leaching of pyrite. This observation was in accordance with the above experimental results, i.e. the pyrite leaching rate was higher at the beginning and gradually levelled off. A large area of erosion was observed at the leached pyrrhotite surface (Fig. 11). The surface also appeared like a porous structure with very fine pores evenly distributed across it. There was some erosion at the large defect sites at the surface. The EDAX analysis indicated that a large amount of iron oxide phase was present at the eroded pyrrhotite surface. Similarly, the layer of iron oxide formed at the pyrrhotite surface would hinder the further oxidative leaching of pyrrho-
Fig. 9. SEM image of leached chalcopyrite.
198
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
Fig. 10. SEM image of leached pyrite.
tite, which was in accordance with the above experimental observations. Like pyrrhotite, the leached arsenopyrite surface also showed a large area of erosion (Fig. 12). The leached surface also had fine pores on it. Severe ero-
sion was observed at the defect sites and the crystal boundaries. This was because the high-energy defect sites and crystal boundaries favoured the leaching process. The EDAX analysis demonstrated that a type of iron arsenate existed at the leached sites.
Fig. 11. SEM image of leached pyrrhotite.
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
199
Fig. 12. SEM image of leached arsenopyrite.
3.9. Characterisation of the leach product by Raman spectroscopy Attempts were made to use Raman spectroscopy to identify the reaction species on the leached sulphide lump surfaces. However, the only reaction product identified was haematite, which was formed at the leached chalcopyrite surface (Fig. 13). The failure of this method to identify other products is due to the small amounts of products in comparison with the
Fig. 13. Raman spectra of chalcopyrite surface before and after leaching.
unreacted matrices. Chalcopyrite dissolved substantially in the leaching process, and hence a high ratio of haematite was formed in the leach residue.
4. Conclusions The leaching of sulphides in an ammoniacal thiosulphate system is dependent on the solution composition and the mineral types. On the other hand, the leaching of sulphides influences the oxidation of thiosulphate. The following conclusions can be drawn from the results of this study: . The relative leaching rates of sulphides were in the order chalcopyrite > pyrrhotite > arsenopyrite > pyrite. The chalcopyrite leaching kinetics were nearly linear. However, the leaching of the other three sulphides was fast at the start and gradually levelled off. This was attributed to the formation of an iron oxide or arsenate layer at the particle surface, hindering further exposure of the sulphides to the leach solution. The iron and arsenic concentrations remained at low levels in the leaching process due to the formation of precipitates. . Thiosulphate depressed the leaching of sulphides due to its preferential oxidation over sulphides. Thiosulphate consumed the dissolved oxygen in the leach
200
D. Feng, J.S.J. Van Deventer / Hydrometallurgy 63 (2002) 189–200
solution, which was essential for the direct oxidation of sulphides as well as the conversion of the Cu+ to Cu2+ ammonia complexes. . Ammonia acted as the pH regulator and the ligand for Cu2+ to form the cupric ammonia complex. An increase in ammonia concentration up to a certain level resulted in an enhanced sulphide leaching. . An increase in Cu2+ concentration up to a certain level enhanced the sulphide leaching. . Because sulphate was one of the final leaching products for all the sulphide minerals, the presence of sulphate would shift the equilibrium back to the sulphide side. Therefore, the presence of sulphate depressed the sulphide leaching. . The presence of sulphides in the leach solution resulted in an increased oxidation rate of thiosulphate. Pyrite induced the highest thiosulphate decomposition, followed by arsenopyrite, chalcopyrite and pyrrhotite. . Pyrite enhanced chalcopyrite and sphalerite dissolution. In return, chalcopyrite and sphalerite also enhanced pyrite dissolution. This could be clearly observed in the SEM image, where pyrite revealed erosion at the pyrite– chalcopyrite boundaries. . SEM analysis with the aid of EDAX indicated the formation of iron oxide at the chalcopyrite, pyrrhotite and pyrite surfaces and the formation of iron arsenate at the arsenopyrite surface after leaching. In addition, SEM analysis demonstrated that the high-energy defect sites and crystal boundaries favoured the sulphide leaching. Raman spectroscopy indicated that haematite was formed during the leaching of chalcopyrite. Acknowledgements The financial support from Newcrest Mining Limited, Placer Dome Technical Services Limited and the Australian Research Council is gratefully acknowledged. Appreciation is also expressed to Hui Tan and Fay Lim for assistance in the experimental work. References Abbruzzese, C., Fornari, P., Massidda, R., Veglio, F., Ubaldini, S., 1995. Thiosulphate leaching for gold hydrometallurgy. Hydrometallurgy 39, 265 – 276.
Cao, C., Hu, J., Gong, Q., 1992. Leaching of gold by low concentration thiosulphate solution. Randol Gold Forum, Vancouver ’92. Randol International, Colorado, USA, pp. 293 – 298. Errington, M.T., Pattinson, D., 1991. Alkaline pressure oxidation leaching (APOL) of refractory gold bearing materials. Innovations in Precious Metals Recovery and Refining. SCI, London. Forward, F.A., Veltman, H., 1961. A process for direct leaching of zinc sulphide concentrates with sulphuric acid and oxygen under pressure. Physical Chemistry of Extractive Metallurgy, Part 2. Interscience Publishers, New York. Gong, Q., Hu, J., Cao, C., 1993. Kinetics of gold leaching from sulphide gold concentrates with thiosulphate solution. Trans. Nonferrous Met. Soc. China 3 (4), 30 – 36. Jiang, T., Chen, J., Xu, S., 1993. A kinetic study of gold leaching with thiosulphate. In: Hiskey, J.B., Warren, G.W. (Eds.), Hydrometallurgy, Fundamentals, Technology and Innovations. AIME, Littleton, Colorado, USA, pp. 119 – 126, Chap. 7. Kerley, B.J., 1983. Recovery of precious metals from difficult ores. US patent, 1983, no. 4369061. Langhans, J.W., Lei, K.P.V., Carnahan, T.G., 1992. Copper-catalyzed thiosulphate leaching of low-grade gold ores. Hydrometallurgy 29, 191 – 203. Majima, H., Peters, E., 1966. Oxidation rates of sulphide minerals by aqueous oxidation at elevated temperatures. Trans. Metall. Soc. AIME 236, 1409 – 1413. Sarveswara Rao, K., Ray, H.S., 1998. A new look at characterization and oxidative ammonia leaching behaviour of multimetal sulphides. Miner. Eng. 11 (11), 1011 – 1024. Sarveswara Rao, K., Das, R.P., Ray, H.S., 1991. Study of leaching of multimetal sulphides through an interdisciplinary approach. Min. Proc. Ext. Met. Rev. 7, 209 – 234. Sarveswara Rao, K., Paramguru, R.K., Das, R.P., Ray, H.S., 1992. The role of galvanic interaction during ammonia leaching of multimetal sulphides. Min. Proc. Ext. Met. Rev. 11, 21 – 37. Sarveswara Rao, K., Das, R.P., Mukunda, P.G., Ray, H.S., 1993. Use of X-ray diffraction in a study of ammonia leaching of multimetal sulphides. Metall. Trans. 24B, 937 – 945. Tozawa, K., Umetsu, Y., Sato, K., 1976. In: Yannopoulos, J.C., Agarwal, J.C. (Eds.), Extractive Metallurgy of Copper, Met. Soc. AIME, vol. II, pp. 706 – 721. Tozawa, K., Inliu, Y., Umetsu, Y., 1981. Dissolution of gold in ammoniacal thiosulphate solutions. 110th Annual meeting of AIME Meeting, Chicago, IL, TMS paper, A81-25. Wan, R.Y., 1997. Importance of solution chemistry for thiosulphate leaching of gold. World Gold ’97 Conference, Singapore. Society for Mining, Metallurgy and Exploration (SME), Littleton, Colorado, USA, pp. 159 – 162. Williams, R.D., Light, S.D., 1978. Copper concentrate dissolution chemistry and kinetics in an ammonia-oxygen environment. Fundamental Aspects of Hydrometallurgical Processes. AIChE Symp. Ser. 74 (174), 21 – 27. Zipperian, D., Raghavan, S., Wilson, J.P., 1988. Gold and silver extraction by ammoniacal thiosulphate leaching from a Rhyolite ore. Hydrometallurgy 19, 361 – 375.