Thiosulphate leaching of gold in the presence of orthophosphate and polyphosphate

Hydrometallurgy 106 (2011) 38–45

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Thiosulphate leaching of gold in the presence of orthophosphate and polyphosphate D. Feng ⁎, J.S.J. van Deventer Department of Chemical and Biomolecular Engineering, University of Melbourne, Victoria, 3010, Australia

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 10 September 2010 Received in revised form 21 November 2010 Accepted 22 November 2010 Available online 2 December 2010

The effects of sodium orthophosphate and hexametaphosphate, were investigated in ammoniacal thiosulphate leaching of pure gold and a sulphide gold ore. Hexametaphosphate and orthophosphate readily complexed with copper(II) ions and prevented the substitution of thiosulphate into the copper(II) inner coordination sphere, thereby stabilising thiosulphate against oxidation by copper(II). In pure gold leaching, the kinetics and overall gold dissolution increased with increasing hexametaphosphate and orthophosphate concentrations. In contrast, there was an optimal hexametaphosphate concentration of about 0.8 g/L for the sulphide ore leaching. The gold leaching was significantly retarded at a hexametaphosphate concentration beyond 1.6 g/L. Silver leaching followed a similar trend to gold leaching. Orthophosphate slightly increased the overall extent of gold and silver extractions. Thiosulphate consumption decreased with increasing hexametaphosphate and orthophosphate concentrations in the leaching of the sulphide ore. After 48 h leaching, the thiosulphate consumption decreased from 8.15 kg/t under the standard leach condition to 5.19 kg/t at 1.0 g/L orthophosphate, and to 5.56 kg/t at 1.0 g/L hexametaphosphate. The enhanced gold leaching by hexametaphosphate was attributed to stabilisation of thiosulphate, reduced interactions between gold/thiosulphate and the sulphide minerals, dispersion of the slurry system and improvement of the leach slurry rheology. © 2010 Elsevier B.V. All rights reserved.

Keywords: Gold leaching Sulphide gold ore Thiosulphate Copper(II) phosphate Rheology

1. Introduction The leaching of gold with ammoniacal thiosulphate solutions has been studied extensively as an alternative technology to the traditional cyanidation process (Aylmore and Muir, 2001; Muir and Aylmore, 2005). The reaction mechanisms, leaching kinetics, speciation, thiosulphate stability and mineralogical effects have been reviewed extensively by Senanayake (2004, 2005a,b, 2007). Economic and technical evaluation of pilot plant tests carried out by Newmont Mining (Bhakta, 2003) and by Barrick Gold (Fleming et al., 2003) showed great promise for thiosulphate leaching. The leaching of gold in ammoniacal thiosulphate solutions is an electrochemical reaction and is promoted by the presence of copper(II) ions. The role of copper(II) ions in the oxidation of metallic gold is shown in the following reaction: 2−

2þ

3−

5−

Au þ 5S2 O3 þ CuðNH3 Þ4 →AuðS2 O3 Þ2 þ 4NH3 þ CuðS2 O3 Þ3 :

⁎ Corresponding author. Tel.: +61 3 83449570; fax: +61 3 8344 4153. E-mail address: [email protected] (D. Feng). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.11.016

ð1Þ

In addition to gold oxidation, some thiosulphate degradation to tetrathionate occurs. The oxidation reaction is promoted by copper(II) ions as follows: 2þ

2−

5−

2−

2CuðNH3 Þ4 þ 8S2 O3 →2CuðS2 O3 Þ3 þ 8NH3 þ S4 O6 :

ð2Þ

One of the major problems in thiosulphate leaching is the high consumption of thiosulphate. Much work has been dedicated to the stabilisation of thiosulphate. Sulphite was added to the leaching solution to stabilise thiosulphate (Kerley, 1981, 1983). The presence of sulphite prevented the formation of any free sulphide ion and the precipitation of gold or silver from solution at the price of reduced leaching recovery. Desired additives should perform in such a manner that the oxidation of thiosulphate by copper(II) is reduced and the catalytic capacity of copper(II) for gold leaching is maintained. Thus, Hu and Gong (1991) suggested sulphate as an additive to stabilise thiosulphate. The beneficial effect of sulphate addition was observed in practical leaching systems with respect to percentage gold extraction (Xia et al., 2003). EDTA was shown to effectively stabilise thiosulphate via its complexation with copper(II) and the thiosulphate leaching of gold was substantially improved with the addition of EDTA in small quantities (Xia et al., 2003; Feng and van Deventer, 2010a). Michel and Frenay (1998) theoretically suggested amino acid as an additive for increasing thiosulphate stability in view of the fact that amino acid may form more stable copper complex and weaken

D. Feng, J.S.J. van Deventer / Hydrometallurgy 106 (2011) 38–45

the activity of copper(II) ion in the redox reactions. Anions especially phosphate were shown to significantly reduce the oxidation of thiosulphate by copper(II) ions, due to the competition with thiosulphate anions to complex with copper(II) ions at the axial coordinate sites (Breuer and Jeffrey, 2003). It is yet to be investigated whether phosphate has any impact on thiosulphate leaching of gold. Phosphate and polyphosphates have been used extensively in the mineral industry to disperse oxide and alumino-silicate minerals (Huynh et al., 2000). Phosphate readily complexes with copper(II) ions and prevents the substitution of thiosulphate into the copper(II) inner coordination sphere. This will lower the catalytic effect of the copper(II) tetra-ammine complex on the oxidative decomposition of thiosulphate. Polyphosphates may disperse siliceous mineral particles as dominant phases in the leaching of gold ores with ammoniacal thiosulphate solutions. It is the purpose of this study to investigate thiosulphate leaching of gold in the presence of orthophosphate and polyphosphates. Sodium orthophosphate and sodium hexametaphosphate as a typical polyphosphate were artificially added in the leaching of pure gold and a sulphidic gold ore with ammoniacal thiosulphate solution. 2. Experimental 2.1. Materials Gold foils (99.99% Au, thickness 0.2 mm) with a surface area of about 20 mm2 were used in the leaching of pure gold. The gold foils were polished with 0.1 μm mono-crystalline diamond paste (Electron Microscopy Sciences), washed with acetone twice, rinsed with distilled water and swept with lint-free paper. A new gold foil was used for each leaching test. It should be noted that gold in general exists in the form of Au/Ag alloys with about 5% Ag in gold ores. According to previous experience, the leaching behaviour of gold followed a similar trend for pure gold and Au/Ag alloys. Sodium hexametaphosphate ((NaPO3)x, Chem-Supply) and sodium orthophosphate (APS Ajax Finechem) were used to prepare fresh solutions with the addition of distilled water. All other chemicals are of analytical or reagent grade. A sulphide gold ore was obtained from a Newcrest gold mine with a particle size of 80% passing 75 μm. A rotary splitter was used to obtain representative samples of the ore for experimental use. Quantitative XRD was used to determine the mineralogy of the sulphide ore. An elemental analysis was performed by digestion of the ore. The mineralogical and elemental analysis results are shown in Table 1, which lists only the elements considered most important in the study. 2.2. Analytical techniques Elemental concentrations in solutions were determined by ICPOES, involving the oxidation of sulphur species as stable sulphates prior to the analysis. After oxidation by hydrogen peroxide, solutions were acidified by HCl and HNO3 and boiled to ensure complete con-

39

version of the metal species to the chloride form. For silver analysis, the oxidised solutions were only acidified by HNO3. The thiosulphate concentration was determined by iodometric method with the addition of acetic acid (10% solution) for eliminating the interference of the copper(II) tetra-ammine complex with the titration. The concentration of the copper(II) ammonia complex was monitored at 618 nm using UV–Vis spectrophotometry (Varian). Solutions of the copper(II) ammonia complex present a blue coloration, which is the key of the colorimetric determination. Solutions of the copper(I) thiosulphate present colourless. Other species gave zero absorbance at this wavelength. A platinum electrode (M21Pt, Radiometer) was used to measure the mixed solution potential with a double-junction reference electrode (Ag/AgCl, saturated KCl, Orion) to avoid the interference of thiosulphate with the reference electrode. All potentials were given with respect to SHE. Particle zeta potential was measured in a Colloidal Dynamics instrument (Model Acoustosizer II). Samples were ground to a particle size of −20 μm and sonicated for 1 min with an ultrasonic probe (UP 200S, dr. Hielscher GMBH) for surface cleaning. A cleaned sample of 10 g was added to 200 ml of 0.01 M NaCl electrolyte solution prepared with deionised water. Readings started from about pH 12 to 2 with the addition of NaOH or HCl. The slurry viscosity was determined with a rotational viscometer (Haake VT550). Slurry samples were shaken and placed in the viscometer as quickly as possible, in order to keep the solids from settling on the bottom of the container. 2.3. Leaching tests Leaching of pure gold was performed in a 250 ml reactor with a sampling port using a magnetic stirrer. Leach solution of 200 ml was used with desired concentrations of sodium hexametaphosphate and orthophosphate. The gold foils were suspended in the upper part of the leaching reactor with a nylon thread, ensuring no contact with the reactor wall during leaching. The stirring speed was maintained at 250 min− 1. The gold dissolution was calculated based on the dissolved gold mass per m2 of the gold foil surface. Leaching of the sulphide ore was conducted in a 1.5 L baffled PVC reactor using an overhead flat-bladed impeller. The reactors were open to the air through the sampling ports. Leach solution of 1.0 L was added to the sulphide ore of 400 g in the reactors. A natural pH of about 10.3 was maintained, due to the buffering effect of NH+ 4 /NH3. Leaching tests were conducted at a rotation speed of 250 min− 1 and a temperature of 25 °C in a water bath. Samples were taken continuously at certain intervals during a total retention time of 48 h. The samples were immediately subjected to the subsequent iodine titration and oxidation for ICP analysis. Duplicate tests were conducted with only average results reported, due to the standard deviations of all the tests being within 3%. 3. Results and discussion 3.1. Leaching of pure gold

Table 1 Chemical and metallurgical analysis of the sulphide ore. Quantitative XRD analysis

Elemental analysis

Mineral

Content (mass %)

Element

Content

Albite Arsenopyrite Calcite Chalcopyrite Dolomite Pyrite Quartz

23.5 0.1 1.1 0.1 21.2 5.4 41.3

Cu (%) Fe (%) S (%) As (mg/kg) Au (mg/kg) Ag (mg/kg) Ni (mg/kg)

0.07 5.27 4.35 383 4.3 2.0 65

Fig. 1 illustrates gold dissolution in the presence of hexametaphosphate and orthophosphate at varied concentrations. Both leaching kinetics and overall gold dissolution substantially improved with the addition of hexametaphosphate and orthophosphate. This beneficial effect became more significant with an increase in the hexametaphosphate and orthophosphate concentrations within the tested ranges. Compared to hexametaphosphate, orthophosphate enhanced the gold dissolution to a much lesser extent at a similar dosage (Fig. 1). The copper(II) tetra-ammine concentration increased with the addition of orthophosphate at lower concentrations (Fig. 2). Hexametaphosphate increased the copper(II) tetra-ammine concentration to

40

D. Feng, J.S.J. van Deventer / Hydrometallurgy 106 (2011) 38–45

36

Gold dissolution, g/m2

O P - 0.4 g/L

30

P - 1.0 g/L P - 8.2 g/L

24

HP - 0.4 g/L HP - 1.0 g/L

18

HP - 1.8 g/L

12 6 0 0

5

10

15

20

25

Time, h Fig. 1. Leaching of pure gold in the presence of hexametaphosphate and orthophosphate. Leach solution — 0.1 M (NH4)2S2O3, 0.5 M NH3 and 50 mg/L Cu2+; P — orthophosphate; HP — hexametaphosphate; and O — standard condition.

a much lesser extent than orthophosphate at the same concentration. A further increase in the orthophosphate concentration to 8.2 g/L resulted in the decrease of the copper(II) tetra-ammine concentration. This is due to the precipitation of copper(II) phosphate species in the presence of orthophosphate at high concentrations based on the 3− thermodynamic analysis of the Cu–NH3–S2O2− 3 –PO4 system in Fig. 3. Construction of Eh–pH diagrams was carried out using Outokumpu HSC Chemistry for Windows (Roine, 1994). Thermodynamic data were obtained from Smith et al. (1998). The ammoniacal thiosulphate leaching took place in the pH range of 8–11 and Eh range of 0.1–0.35 V. The Eh–pH diagram of the Cu– 3− NH3–S2O2− system (Fig. 3) indicates that the copper(I) 3 –PO4 thiosulphate complex could be the predominant copper species in the leaching region and that the copper(II) tetra-ammine complex could still be present in solution as the catalyst for the oxidation of gold. At low concentrations, orthophosphate could form colloidal copper(II) phosphate complexes with the copper(II) ion in the lower pH region. With the increasing phosphate concentration, the stability region of the copper(II) phosphate complexes would extend with the stability region of the copper(II) tetra-ammine complex becoming narrower (Fig. 3). The colloidal copper(II) phosphate species could precipitate out of solution when their concentrations reached critical levels. This can explain the fact that the total copper concentration decreased from 50.0 mg/L to 44.5 mg/L at 8.2 g/L orthophosphate. Similarly, hexametaphosphate could likely form colloidal copper

3− Fig. 3. Eh–pH diagram for the Cu–NH3–S2O2− 3 –PO4 system at 25 °C. Condition: NH3 — — 0.1 M; and PO3− — 0.01 M. 0.5 M; Cu — 7 × 10− 4 M; S2O2− 3 4

particles through complexing the copper(II) ions at high concentrations. Below 1.8 g/L hexametaphosphate, no clear drop of the total copper concentration in solution was observed. Thiosulphate decomposition decreased with the addition of orthophosphate and hexametaphosphate, and this beneficial effect became more pronounced at higher orthophosphate and hexametaphosphate concentrations (Fig. 4). Orthophosphate reduced the thiosulphate decomposition to a greater extent than hexametaphosphate at the same dosage. The reaction between copper(II) and thiosulphate was shown to occur via coordination of thiosulphate to copper(II) (Brown et al., 2003; Zhang and Jeffrey, 2008). Clearly, orthophosphate readily complexes with copper(II) ions and prevents the substitution of thiosulphate into the copper(II) inner coordination sphere. The negative overall charge of the copper(II) phosphate complexes likely caused repulsion towards other anions such as thiosulphate. Thus, the presence of orthophosphate and hexametaphosphate stabilised thiosulphate against oxidation by copper(II). The minimisation of the thiosulphate decomposition could reduce the extent of leaching passivation at the gold surfaces. It was believed that sulphur and copper sulphide layers could form at gold surfaces in the thiosulphate leaching system, and the layers prevented thiosulphate from diffusing to the gold surfaces and hence, inhibiting the gold dissolution (Bagdasaryan et al., 1983; Pedraza et al., 1988). Both 102

50 98

40

[S2O32-], mM

[Cu(NH3)42+], mg/L

45

35 30

94

90

25 86 20 15

O

P - 0.4 g/L

P - 1.0 g/L

P - 8.2 g/L

HP - 1.0 g/L

HP - 1.8 g/L

O

P - 0.4 g/L

P - 1.0 g/L

P - 8.2 g/L

HP - 0.4 g/L

HP - 1.0 g/L

HP - 1.8 g/L

82 0

5

10

15

20

25

Time, h Fig. 2. Variation of Cu(NH3)2+ concentration with time in the presence of 4 orthophosphate and hexametaphosphate. Leach solution — 0.1 M (NH4)2S2O3, 0.5 M 2+ NH3 and 50 mg/L Cu ; orthophosphate (P) — 0.4, 1.0 and 8.2 g/L; hexametaphosphate (HP) — 1.0 and 1.8 g/L; and O — standard condition.

0

5

10

15

20

25

Time, h Fig. 4. Thiosulphate decomposition in the presence of orthophosphate and hexametaphosphate. Leach solution — 0.1 M (NH4)2S2O3, 0.5 M NH3 and 50 mg/L Cu2+, hexametaphosphate (HP) — 0.4, 1.0 and 1.8 g/L; orthophosphate (P) — 0.4, 1.0 and 8.2 g/L; and O — standard condition.

D. Feng, J.S.J. van Deventer / Hydrometallurgy 106 (2011) 38–45

elemental and sulphide sulphur can be provided by the decomposition of thiosulphate in alkaline solutions. Orthophosphate and hexametaphosphate increased the copper(II) tetra-ammine concentration, resulting in higher leaching kinetics of gold. In the meantime, thiosulphate was stabilised with the addition of orthophosphate and hexametaphosphate. Thus, the leaching kinetics and overall gold dissolution increased in the presence of orthophosphate and hexametaphosphate. However, orthophosphate and hexametaphosphate should be added at relatively low concentrations for the improvement of gold leaching, as precipitation of the copper(II) phosphate species occurred at high concentrations. 3.2. Leaching of the sulphide ore Fig. 5a shows the effect of hexametaphosphate on thiosulphate leaching of the sulphide ore at varied concentrations. Leach curves appeared to give higher initial leaching kinetics when hexametaphosphate was added. This was possibly because the exposed gold, being readily accessible by the ammoniacal thiosulphate solutions, was quickly leached out at the start. The gold extraction was improved over 48 h with the addition of hexametaphosphate and this beneficial effect became more pronounced at a higher hexametaphosphate concentration below 0.8 g/L (Fig. 5a). Near 100% gold extraction was achieved over an extended leaching time of 48 h at a hexametaphosphate concentration of 0.8 g/L (Fig. 5a). With a further increase in hexametaphosphate concentration to around 1.0 g/L, the extent of the improvement of the gold extraction was substantially reduced (Fig. 5a). When the hexametaphosphate concentration was over about 1.6 g/L,

(a) hexametaphosphate Gold extraction, %

100 80 60 O 0.2 g/L

40

41

both the leaching kinetics and overall gold extraction over 48 h were reduced and this detrimental effect became more marked with a further increase in the hexametaphosphate concentration (Fig. 5a). The optimal hexametaphosphate concentration appeared around 0.8 g/L in the leaching of the sulphide ore. Orthophosphate also enhanced the gold extraction slightly and this effect became more pronounced at a higher orthophosphate concentration (Fig. 5b). The leaching of the sulphide species in the sulphide ore was significantly enhanced in the presence of orthophosphate and hexametaphosphate, as indicated by an increase in the total sulphur concentration in Fig. 6. The sulphide species were leached to a greater extent at higher orthophosphate and hexametaphosphate concentrations. Compared to hexametaphosphate, orthophosphate gave higher leaching of the sulphur species at the same dosage. The leaching of ferrous sulphides such as pyrite, chalcopyrite and arsenopyrite released iron into the solution, which could precipitate on the sulphide surfaces in alkaline solutions and hence hinder further leaching (Feng and Van Deventer, 2002). The Eh–pH diagram of the Fe–NH3–PO3− 4 system (Fig. 7) indicates that under the experimental conditions iron could be present as Fe (OH)3 in a phosphate free solution. With the addition of phosphate, the stability region of Fe(OH)3 became narrower with the stability region of FePO4·2H2O getting wider. This trend became more significant at a higher phosphate concentration. As a result, the predominant iron species would be colloidal FePO4·2H2O in the presence of phosphate. Therefore, phosphate could prevent iron from precipitating back on the sulphide surfaces by stabilising iron via the formation of the colloidal iron phosphate. The increased leaching of the sulphide matrices would reduce the interactions between gold/ thiosulphate and the sulphide minerals, and hence improving the gold leaching. The leaching of the sulphur species increased with an increase in the hexametaphosphate concentration. However, the beneficial effect of hexametaphosphate on the gold extraction percentage decreased at a hexametaphosphate concentration over 0.8 g/L, and the gold leaching was retarded with a further increase in the hexametaphosphate concentration over about 1.6 g/L. This indicates that the interactions between gold/thiosulphate and the sulphide minerals were not the only factor governing the leaching of the sulphide gold ore.

0.4 g/L 0.8 g/L 1.0 g/L

20

1.6 g/L 1.8 g/L

0 0

10

20

30

40

50

3.2.1. Effect of phosphates on copper concentration in solution Fig. 8 shows the variation of copper concentration in solution with time in the leaching of the sulphide ore. The copper concentration in solution increased in the presence of hexametaphosphate and became

Time, h

(b) orthophosphate

13000 12000

75

11000

[S], mg/L

Gold extraction, %

90

60 O

45

10000 O HP - 0.8 g/L HP - 1.6 g/L P - 0.4 g/L P - 8.2 g/L

9000

0.4 g/L

8000

1.0 g/L 8.2 g/L

30

HP - 0.4 g/L HP - 1.0 g/L HP - 1.8 g/L P - 1.0 g/L

7000

15

6000 0

10

20

30

40

50

Time, h Fig. 5. Leaching of gold from the sulphide ore in the presence of (a) hexametaphosphate and (b) orthophosphate. Leach solution — 0.1 M (NH4)2S2O3, 0.5 M NH3 and 50 mg/L Cu2+; O — standard condition; hexametaphosphate — 0.2, 0.4, 0.8, 1.0, 1.6 and 1.8 g/L; and orthophosphate — 0.4, 1.0 and 8.2 g/L.

0

10

20

30

40

50

Time, h Fig. 6. Variation of sulphur concentration in solution in the leaching of the sulphide ore. Leach solution — 0.1 M (NH4)2S2O3, 0.5 M NH3 and 50 mg/L Cu2+; hexametaphosphate (HP) — 0.4, 0.8, 1.0, 1.6 and 1.8 g/L; orthophosphate (P) — 0.4, 1.0 and 8.2 g/L; and O — standard condition.

42

D. Feng, J.S.J. van Deventer / Hydrometallurgy 106 (2011) 38–45

Cu2+ ions on sulphide minerals could be caused by the chemical adsorption of Cu2+ ions on the sulphide surfaces with the reduction of Cu2+ to Cu+ and the oxidation of sulphur species (Finkelstein, 1997; Voigt et al., 1994; Weisener and Gerson, 2000). Orthophosphate and hexametaphosphate readily formed complexes with copper(II) ions and hence stabilised copper(II) ions in solution. However, excessive orthophosphate and hexametaphosphate resulted in the precipitation of copper(II) phosphate species.

Fig. 7. Eh–pH diagram for the Fe–NH3–PO3− system at 25 °C. Condition: NH3 — 0.5 M; 4 Fe — 2 × 10− 4 M; and PO3− — 0.05 M. 4

more significant with increasing the hexametaphosphate concentration up to 1.6 g/L, but then decreased above this concentration (Fig. 8a). Similarly, the presence of up to 1 g/L orthophosphate increased the copper concentration in solution (Fig. 8b) but 8.2 g/L orthophosphate had little effect. The increase in the copper concentration in the presence of hexametaphosphate and orthophosphate was likely attributed to the enhanced leaching of copper minerals and the reduced adsorption of copper(II) ions on the sulphide minerals in the ore. The adsorption of

75

[Cu], mg/L

68 61 54 O

HP - 0.4 g/L

HP - 0.8 g/L

HP - 1.0 g/L

HP - 1.6 g/L

HP - 1.8 g/L

40 0

10

20

30

40

Thiosulphate consumption, kg/t

(a) hexametaphosphate

(a) hexametaphosphate

47

3.2.2. Effect of phosphate ions on thiosulphate consumption Thiosulphate consumption decreased with the addition of hexametaphosphate and orthophosphate, and this beneficial effect became more significant at higher hexametaphosphate and orthophosphate concentrations (Fig. 9). After 48 h leaching of the sulphide gold ore, the thiosulphate consumption decreased from 8.15 kg/t under the standard leach conditions to 7.41, 5.19 and 4.08 kg/t with the addition of orthophosphate at 0.4, 1.0 and 8.2 g/L, respectively (Fig. 9b). Similarly, the thiosulphate consumption was 7.04, 5.93, 5.56, 4.45 and 3.71 kg/t in the presence of hexametaphosphate at 0.4, 0.8, 1.0, 1.6 and 1.8 g/L, respectively (Fig. 9a). Again, the decreased decomposition of thiosulphate in the presence of orthophosphate and hexametaphosphate is likely due to the formation of the copper(II) phosphate species and decreased substitution of thiosulphate into the copper(II) inner coordination sphere. The negative overall charge of the copper(II) phosphate complexes likely caused repulsion towards other anions such as thiosulphate, inhibiting the catalytic oxidation of thiosulphate. In comparison with the pure gold leaching, the sulphide gold ore leaching consumed more thiosulphate likely due to the presence of semi-conductive minerals such as sulphides (Xu and Schoonen, 1995;

9 8 7

O

6

HP - 0.4 g/L

5

HP - 0.8 g/L HP - 1.0 g/L

4

HP - 1.6 g/L

3

HP - 1.8 g/L

2 1 0 0

50

10

20

Time, h Thiosulphate consumption, kg/t

75

[Cu], mg/L

68 61 54 47

O

P - 0.4 g/L

P - 1.0 g/L

P - 8.2 g/L

40 10

40

50

30

40

50

(b) orthophosphate

(b) orthophosphate

0

30

Time, h

20

30

40

50

Time, h Fig. 8. Variation of copper concentration with time in the leaching of the sulphide ore with (a) hexametaphosphate and (b) orthophosphate. Leach solution — 0.1 M (NH4)2S2O3, 0.5 M NH3 and 50 mg/L Cu2+; hexametaphosphate (HP) — 0.4, 0.8, 1.0, 1.6 and 1.8 g/L; orthophosphate (P) — 0.4, 1.0 and 8.2 g/L; and O — standard condition.

9 8 7

O

6

P - 0.4 g/L

5

P - 1.0 g/L

4

P - 8.2 g/L

3 2 1 0 0

10

20

Time, h Fig. 9. Thiosulphate decomposition in the leaching of the sulphide ore with (a) hexametaphosphate and (b) orthophosphate. Leach solution — 0.1 M (NH4)2S2O3, 0.5 M NH3 and 50 mg/L Cu2+; hexametaphosphate (HP) — 0.4, 0.8, 1.0, 1.6 and 1.8 g/L; orthophosphate (P) — 0.4, 1.0 and 8.2 g/L; and O — standard condition.

D. Feng, J.S.J. van Deventer / Hydrometallurgy 106 (2011) 38–45

Benedetti and Boulëgue, 1991). The catalysis of sulphides in the thiosulphate decomposition was believed to originate from its strong affinity for aqueous sulphur species and their semi-conducting properties. Orthophosphate and hexametaphosphate may modify the sulphide surface properties, reducing the catalytic decomposition of thiosulphate. 3.2.3. Effect of phosphate ions on iron concentration in solution Iron oxides and hydroxides are omnipresent in mixed sulphide ore pulps, originating from the mild steel grinding media, iron sulphide minerals and non-sulphide gangue. A recent study demonstrated that the presence of colloidal iron oxide coatings at mineral surfaces retarded thiosulphate leaching of the gold ores (Feng and Van Deventer, 2010b). Iron oxide or hydroxide slime coatings likely existed in the leaching of the sulphide ore. The presence of orthophosphate and hexametaphosphate could convert iron hydroxide to colloidal iron phosphate, which was beneficial for reducing the detrimental effect of iron oxide slimes on gold leaching. However, when the colloidal iron phosphate concentration reached a critical level at a high orthophosphate and hexametaphosphate concentration due to the leaching of iron minerals, the colloidal iron phosphate would precipitate as iron phosphate slimes. The variation of the iron concentration in solution with the hexametaphosphate concentration reflects these views. In the absence of hexametaphosphate, no iron was detectable in solution. However, after 3 h of leaching, the iron concentration in the solution increased to 1.5 mg/L with up to 0.8 g/L hexametaphosphate, and thereafter steadily decreased to 0.3 mg/L with increasing hexametaphosphate up to 1.8 g/L. Thus, the presence of hexametaphosphate over 0.8 g/L likely formed iron phosphate slimes, retarding gold leaching. 3.3. Rheological study Fig. 10 exhibits the apparent zeta potential of mineral particles present in the sulphide ore as a function of pH, in the background of 0.01 M NaCl. The slurry pH was initially increased to about pH 12 by NaOH, and subsequently brought down to around pH 2 by HCl. The particle zeta potentials reported here are nett values for all the particles of diverse mineralogy, as indicated in Table 1. The observed values, therefore, are the averages as determined by surface area weighted contributions of the various mineral phases present. Quartz was the predominant gangue mineral in the sulphide ore, followed by albite and dolomite (Table 1). The zeta potential curve showed an isoelectric point (iep) of ~4.5. The iep was about 2.9 for quartz (Tschapek and Wasowski, 1986), 9.5 for dolomite (Sadowski and Laskowski, 1980) and 2.0–2.4 for albite

150

Zeta potential, mV

Error bar 0

-150

HP - 0.4 g/L HP - 1.6 g/L P - 0.4 g/L

-450 1

3

(Mukhopadhyay and Walther, 2001). The result suggests that the overall interfacial chemistry was predominantly defined by quartz, albite and dolomite. Clearly, the presence of hexametaphosphate increased the negativity of the surface charges of the mineral particles and shifted the iep of the particles to a lower pH, and this tended to be more pronounced at a higher hexametaphosphate concentration. Orthophosphate also increased the negative charges of the particles, but to a lesser extent at a similar dosage in comparison with hexametaphosphate (Fig. 10). Under the standard leaching condition without hexametaphosphate and orthophosphate, some minerals appeared positively charged such as dolomite while the others were negatively charged such as quartz and albite. The mineral surface charges would change due to the selective adsorption of cations and precipitation of secondary phases during the leaching. The overall zeta potential was fairly low under the standard leach condition, so that the particle interactions may be mainly van der Waals forces. This is due to low electrical double layer repulsion between the particles across the aqueous phase. In such a case, coagulation or aggregation of various mineral particles existed in the absence of hexametaphosphate and orthophosphate. Hexametaphosphate adsorbed onto the mineral particles such as quartz (Lu et al., 1992), albite (Karagüzel and Çobanoğlu, 2010) and dolomite (Matis et al., 1988), increasing surface negative charge and inducing a very strong steric repulsion between particles. Thus, coagulation or aggregation of the various mineral particles was reduced due to electrostatic repulsion and steric repulsion forces occurring among the particles. The highly dispersed slurry system could allow the leach solution to readily access gold surfaces, improving gold leaching with the addition of hexametaphosphate. The addition of hexametaphosphate substantially reduced the slurry viscosity in the leaching of the sulphide ore, as shown in Table 2. This beneficial effect became more pronounced with an increase in the hexametaphosphate concentration within the range of 1.8 g/L. Likewise, orthophosphate reduced the slurry viscosity, but to a much lesser extent in comparison with hexametaphosphate at the same dosages (Table 2). The addition of hexametaphosphate and orthophosphate prevented particle coagulation in the leach slurry via rendering the particle surfaces more negatively charged. The improved rheology of the leach slurries with the addition of hexametaphosphate and phosphate would enhance the mass transfer processes occurring in the leaching.

3.4. Silver extraction Fig. 11 shows silver extraction in the leaching of the sulphide ore at varied hexametaphosphate concentrations. Similar to the gold extraction, the silver extraction from the sulphide ore also increased with increasing hexametaphosphate concentration up to 0.8 g/L and thereafter decreased at higher concentrations. Indeed the silver extraction markedly decreased at a hexametaphosphate concentration of 1.8 g/L. Orthophosphate also showed a marginal increase in the silver extraction with the dosage of 8.2 g/L (not shown here).

Table 2 Slurry viscosity of the sulphide ore at varied hexametaphosphate and orthophosphate concentrations. Conditions: solid concentration 28.6 wt.%, pH 9.3 and stirring speed 250 min− 1.

0

-300

43

5

7

9

11

13

pH Fig. 10. Particle zeta potential of the sulphide ore as function of pH in the presence of sodium hexametaphosphate or orthophosphate. Electrolyte — 0.01 M NaCl; hexametaphosphate — 0, 0.4 and 1.6 g/L; and orthophosphate — 0.4 g/L.

Hexametaphosphate (g/L)

Viscosity (mPa s)

Orthophosphate (g/L)

Viscosity (mPa s)

0 0.2 0.4 0.8 1.0 1.6 1.8

39 35 29 25 24 20 19

0 0.4

39 38

1.0

34

8.2

29

D. Feng, J.S.J. van Deventer / Hydrometallurgy 106 (2011) 38–45

Silver extraction, %

44

90

References

75

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60 45 O 0.4 g/L

30

0.8 g/L 1.0 g/L

15

1.6 g/L 1.8 g/L

0 0

10

20

30

40

50

Time, h Fig. 11. Silver extraction from the sulphide ore in the presence of hexametaphosphate. Leach solution — 0.1 M (NH4)2S2O3, 0.5 M NH3 and 50 mg/L Cu2+; hexametaphosphate — 0.4, 0.8, 1.0, 1.6 and 1.8 g/L; and O — standard condition.

The leaching mechanism for silver by ammoniacal thiosulphate solutions with copper was similar to that for gold, as silver generally existed in the form of Ag/Au alloys in gold ores. Therefore, hexametaphosphate and orthophosphate are expected to follow similar mechanisms in enhancing both gold and silver leaching.

4. Conclusions Hexametaphosphate and orthophosphate readily complexed with copper(II) ions and prevented the substitution of thiosulphate into the copper(II) inner coordination sphere, stabilising thiosulphate against oxidation by copper(II) ions in the ammoniacal thiosulphate leaching of gold. The copper(II) tetra-ammine concentration increased in the presence of hexametaphosphate and orthophosphate, leading to higher kinetics and overall gold dissolution in pure gold leaching. At a similar dosage, orthophosphate enhanced the gold dissolution to a lesser extent than hexametaphosphate. The gold leaching kinetics and overall gold extraction improved with increasing hexametaphosphate concentration up to 0.8 g/L and near 100% gold extraction was achieved at 0.8 g/L hexametaphosphate. This beneficial effect decreased at a hexametaphosphate concentration over ~ 0.8 g/L. The gold leaching was retarded at a hexametaphosphate concentration over ~ 1.6 g/L. Orthophosphate marginally increased the gold extraction within a concentration of 8.2 g/L. The silver leaching followed a similar trend to gold extraction in the presence of hexametaphosphate and orthophosphate. Thiosulphate consumption decreased with increasing hexametaphosphate and orthophosphate concentrations in the leaching of the sulphide gold ore. After 48 h leaching, the thiosulphate consumption decreased from 8.15 kg/t under the standard leach condition to 5.19 kg/t at 1.0 g/L orthophosphate, and to 5.56 kg/t at 1.0 g/L hexametaphosphate. The presence of hexametaphosphate and orthophosphate also increased the negativity of the mineral particles and reduced the slurry viscosity. The enhanced gold leaching by hexametaphosphate was attributed to stabilisation of thiosulphate, decrease of interactions between gold/thiosulphate and the sulphide minerals, dispersion of the slurry system and improvement of the leach slurry rheology.

Acknowledgements The financial support from Newcrest Mining Limited, former Placer Dome Technical Services Limited, and the Australian Research Council is gratefully acknowledged.

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