The copper–ethanediamine–thiosulphate leaching of gold ore containing limonite with cetyltrimethyl ammonium bromide as the synergist

Hydrometallurgy 150 (2014) 178–183

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The copper–ethanediamine–thiosulphate leaching of gold ore containing limonite with cetyltrimethyl ammonium bromide as the synergist Hong Yu a, Futing Zi b, Xianzhi Hu b,⁎, Jin Zhong a, Yanhe Nie a, Pengzhi Xiang a a b

Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China Faculty of Science, Kunming University of Science and Technology, Kunming 650500, China

a r t i c l e

i n f o

Article history: Received 26 May 2014 Received in revised form 2 October 2014 Accepted 13 October 2014 Available online 23 October 2014 Keywords: Gold Thiosulphate Ethanediamine Cetyl trimethyl ammonium bromide

a b s t r a c t The leaching of thiosulphate gold for a gold ore containing limonite was investigated. The results indicate that the copper ion–ethanediamine(en)–thiosulphate system was more suitable than the copper ion–ammonia– thiosulphate system for gold extraction of the gold ore. The use of cetyltrimethyl ammonium bromide (CTAB) as an effective synergistic additive in the gold extraction process was demonstrated to increase the gold extraction rate and to reduce the thiosulphate consumption for the gold ore. One possible reason for this improvement may be that CTAB hydrolyses to [CTA+] and subsequently attracts the negative charged slime, thereby causing a change in the slurry rheology, which results in the gold surface's being exposed and thus easily leached in the thiosulphate leaching system. Another reason may be the formation of ion pairs, [CTA+]3[Au(S2O3)3− 2 ] •nH2O 3− and [CTA+][AuBr− 2 ]•nH2O, which leads to the stabilisation of [ Au(S2O3)2 ] in the leaching system. The two effects of CTAB increase the gold extraction rate remarkably and sharply reduce the thiosulphate consumption. The gold dissolution rate of the gold ore could reach 94.3% and the thiosulphate consumption could be reduced to 1.12 kg/t ore in the thiosulphate gold leaching system containing 0.1 mol/L of sodium thiosulphate, 0.06 M ethanediamine, 0.005 M copper ion and 1.5 kg/t of CTAB and operated at a rate of agitation of 150 rpm. © 2014 Elsevier B.V. All rights reserved.

1. Introduction For many years, thiosulphate gold leaching has received considerable attention as an alternative technology to the cyanidation of gold ores due to environmental reasons and the reduction of the amount of easily extracted gold ore. (Abbruzzesse et al., 1995; Aylmore and Muir, 2001; Senanayake, 2007) (Li and Miller, 2006). Thiosulphate leaching is also effective for the treatment of refractory ores, especially for carbonaceoustype gold ores, through the prevention of preg-robbing (Alymore, 2001; Aylmore and Muir, 2001; Feng and van Deventer, 2010a; Muir and Aylmore, 2004; Senanayake, 2007, 2011; Xia, 2001). Adding copper ions and ammonia into the thiosulphate solution of the mixture is the most widely used thiosulphate gold leaching liquor. One of the major problems in the thiosulphate gold leaching process is the high consumption of thiosulphate; this consumption is mainly due to thiosulphate decomposition, which is catalysed by the copper ammonia complex ions. To reduce the thiosulphate consumption, sulphite was added to the leaching solution to stabilise the thiosulphate (Kerley, 1981, 1983). Several researchers introduced additives, such as ethylene diamine tetraacetic acid to stabilise copper in solution at low reagent concentrations of ammonia (Feng and van Deventer, 2010b). Feng and van Deventer (2010b, 2011a,b) used orthophosphate, polyphosphate and

⁎ Corresponding author. E-mail address: [email protected] (X. Hu).

http://dx.doi.org/10.1016/j.hydromet.2014.10.008 0304-386X/© 2014 Elsevier B.V. All rights reserved.

carboxymethyl cellulose as an additive in the leaching liquor, which can reduce the leaching passivation and improve the leach slurry rheology. Ethylenediamine (en) and copper can form more stable complexes than that of the cupric ammine complex in solution. The cupric-en complexes could lower the cupric/cuprous redox equilibrium potentials and thus reduce the catalytic oxidation ability of Cu (II), which can reduce the amount of thiosulphate consumption. Ethylenediamine was also used as an additive by Xia (2008) for the leaching of gold from composite ores containing various sulphide minerals. The consumption of thiosulphate was largely reduced. In this paper, the copper ion –ethanediamine(en)–thiosulphate liquor was used in the leaching of a gold ore containing limonite. To improve the leaching rate, cetyl trimethyl ammonium bromide (CTAB) was introduced as a synergistic additive in the leaching system. This study investigated the effects of the thiosulphate concentration, ethylenediamine addition, the presence of cupric ions, and several other variables affecting gold leaching. 2. Experiments 2.1. Materials Ethylenediamine was mixed with distilled water to prepare 0.1 M of in the form of sodium thiosulphate, Cu2+ in the stock solution. S2O2− 3 form of copper sulphate, and NH3/NH+ 4 in the form of ammonia water

H. Yu et al. / Hydrometallurgy 150 (2014) 178–183

179

3.1. Effect of copper ion concentration

Table 1 The multi-element analysis of the raw ore. Element

Au(g/t)

Ag(g/t)

As

Sb

Fe

Mn

S

C

Content,%

2.10

59.36

0.14

0.033

38. 10

3.60

0.024

0.66

Element

SiO2

Al2O3

CaO

MgO

K2O

Cu

Pb

Zn

Content,%

13.07

5.44

1.79

2.20

0.82

0.57

2.00

0.48

were added in the leaching solution and other reagents are analytical grade. The gold ore sample was obtained from the Yunnan Gold Mine Group in China. Currently, the cyanidation of gold leaching has been halted due to environmental problems in the actual production. The multi-element analysis of the raw ore is presented in Table 1. Using an optical microscope, a scanning electron microscope (SEM), and X-ray diffraction methods, the phase constitutions of gold and iron in raw ore were obtained, as presented in Tables 2 and 3, respectively. The raw ore contains many fine grained particles that form the slime in the gold leaching process, which makes the extraction of gold difficult. The data in Table 2 indicates that the gold in the ore primarily exists in the form of native gold with a distribution rate of 95.72%; the secondary forms of gold exist in sulphide and iron mineral with distribution rates of only 1.90% and 0.76%, respectively. The gold particle size is less than 0.030 mm in general. The natural gold enclosed in the limonite mainly is fissure gold, followed by the intergranular gold, generally in the size range of 0.010–0.030 mm. Table 3 indicates that 67.22% of the iron in the ore exists in the form of hematite and limonite and 23.65% exists in the form of magnetite. As is known, limonite easily undergoes argillisation; gold that is mainly enriched in limonite is difficult to extract.

2.2. Extraction of gold All experiments were performed at room temperature (17 °C– 22 °C), the gold leaching test was performed in a 500-mL conical flask using a mechanical stirrer. 150 mL of a leaching solution was added to 100 g of the gold ore. A natural pH of approximately 10 was used in the experiments. Gold leaching tests were performed at a rotation speed of 300 rpm. Samples were taken continuously at set intervals during a total leaching time of 24 h. The samples were subjected to iodine immediately, and titration to determine the concentration of S2O2− 3 the gold concentration was detected by ICP-AES analysis. The absorwas determined at 550 nm using a UV-722 apparabance of Cu(en)2+ 2 tus. The gold extraction rate of the ore was calculated by the gold concentration dissolved in the leaching liquor and the gold content in the ore.

The effects of copper ion concentration on the gold extraction rate and the thiosulphate consumption in the copper ion–ammonia– thiosulphate system and the copper ion–ethanediamine(en)– thiosulphate system were investigated. The results for the gold extraction rate and the thiosulphate consumption are shown in Figs. 1 and 2, respectively. Copper ion concentration was varied from 0 to 0.05 M whilst the ethylenediamine concentration was kept at 0.06 M; the concentration of sodium thiosulphate was 0.1 M, and the concentration of NH3/NH+ 4 was 0.5 M (Feng and van Deventer, 2010b, 2011b). In the Cu2+–ammonia–thiosulphate system, increasing copper(II) enhances the redox reaction via the mixed complexes on the basis of the equilibrium shown in Eq. (1) and the rate determining steps in formed Eqs. (2) and (3) (Senanayake and Zhang, 2012). Whilst S2O2− 3 in Eq. (2) is dimerised to S4O2− 6 , the latter is hydrolysed to other products, as discussed later. In addition, excessive copper ions with S2 − may be from CuS, which covered the gold surface and hindered the dissolving of gold. CuðNH3 Þm

2þ

−2ðn−1Þ

2−

þ nS2 O3

¼ CuðNH3 Þp ðS2 O3 Þn

þ ðm−pÞNH3 :

First order reaction: −2ðn−1Þ

CuðNH3 Þp ðS2 O3 Þn

−ð2n−1Þ

þ S2 O3 :

ð2Þ

−ð2n−1Þ

þ S4 O6

2−

ð3Þ

¼ CuðNH3 Þp ðS2 O3 Þn−1

2−

Second order reaction: −2ðn−1Þ

CuðNH3 Þp ðS2 O3 Þn

¼ CuðNH3 Þp ðS2 O3 Þn−1

where m = 4, n = 1, 2, and p = 2, 3. In the Cu2+–ethanediamine–thiosulphate system, Xia (2008) preby mixing copper sulphate with ethylenediamine at a pared Cu(en)2+ 3 molar ratio of 1:3, and copper ions involved in the reactions described by Eqs. (4)–(9). However, according to Huheey, tri-ethylenediamine copper(II) is labile in water and dissociates to di-ethylenediamine copper(II) (Huheey et al., 1983). As a result, the primary species of copand Cu(en)2+ per in the leaching system were Cu(S2O3)5− 3 2 . 2þ

2þ

ð4Þ

2þ

þ 2OH →CuðOHÞ2

ð5Þ

2þ

þ 4OH →CuðOHÞ4

2en þ Cu

Cu

Cu

→ CuðenÞ2

−

−

2þ

2−

ð6Þ

−

CuðenÞ2 þ OH →CuðOHÞ2 þ 2en

3. Results and discussion The copper and ammonia were widely used in the systems; however, we also used ethylenediamine to replace ammonia and cetyltrimethyl ammonium bromide as the synergistic additive in the leaching liquor. Therefore, two gold thiosulphate leaching systems were used in this investigation simultaneously.

ð1Þ

þ

ð7Þ

þ

en þ Cu →½CuðenÞ

ð8Þ


5− þ 2− ½CuðenÞ þ 3S2 O3 → CuðS2 O3 Þ3 þ en:

ð9Þ

Table 2 The phase constitution of gold in the raw ore. Phase constitution

Bare gold

Gold enclosed by sulphite

Gold enclosed by iron ore

Gold enclosed by carbonate

Gold enclosed by silicate

Total

Content, g/t Distribution,%

2.01 95.72

0.016 0.76

0.04 1.90

0.004 0.19

0.03 1.43

2.10 100.00

180

H. Yu et al. / Hydrometallurgy 150 (2014) 178–183

Table 3 The phase constitution of iron in the raw ore. Phase constitution

Hematite and limonite

Magnetite

Pyrrhotite

Ferric sulphide

Iron carbonate

Ferrosilite total

Total

Content, g/t Distribution,%

25.61 67.22

9.01 23.65

0.08 0.21

0.06 0.16

0.60 1.57

2.74 7.19

38.10 100.00


3−
5− 2− þ Cu þ S2 O3 →Au þ CuðS2 O3 Þ3 AuðS2 O3 Þ2

þ

2−

þ 2S2 O3 →Cu þ AuðS2 O3 Þ2 :

ð11Þ

When the concentration of copper ion exceeds 0.005 M, first, the reaction of copper ions occurs according to Eq. (4) with ethylenediamine; next, excessive copper ions react according to Eq. (5) with ethylenediamine. The Cu(OH)2 covers the surface of the gold. Thereby, hindering the dissolving of gold. Fig. 2 shows that the concentration of thiosulphate decreases with the increase of the copper ion concentration. The concentration of thiosulphate decreased with longer leaching times under the same concentration of copper ion. This behaviour indicated that the consumption of thiosulphate increased with the increase of the amount of copper and the leaching time. The highest and lowest consumptions of thiosulphate are 11.16 kg/t and 6.70 kg/t, respectively. Surprisingly, compared with the ethylenediamine–thiosulphate system; Fig. 1 shows that the extraction of gold and consumption of thiosulphate exhibit an identical growth trend with the increase of copper in the Cu2+–ammonia–thiosulphate system: the highest extraction of gold was 40.03%; the highest and lowest consumptions of thiosulphate are 13.03 kg/t and 7.44 kg/t, respectively.

0 mol/l 0.001mol/l 0.005mol/l 0.01mol/l 0.05mol/l

45

The effect of ethylenediamine is shown in Fig. 3 in the Cu2+– ethanediamine–thiosulphate system. According to Xia (2008), the reactions amongst ethylenediamine, gold and thiosulphate are as follows (Eqs. (12)–(15)). þ

2þ

þ

Au þ CuðenÞ2 →Au þ CuðenÞ2

þ

þ

ð12Þ

þ

3þ

3Au þ CuðenÞ2 →Cu þ Au þ AuðenÞ2

3þ

2−

ð13Þ

3−

AuðenÞ2 þ 2S2 O3 þ 2e→AuðS2 O3 Þ2 þ 2en

ð14Þ

0 mol/l 0.001mol/l 0.005mol/l 0.01mol/l 0.05mol/l

70

0.100 60

0.100

0.095

0.095 0.090

35

0.085

30

0.080

25

0.075

20

[S2O32-](M)

Gold extraction(%)

40

3.2. Effect of ethylenediamine

Gold extraction(%)

2þ

Au þ Cu

ð10Þ

2+ Cu(NH3)2+ – 4 is easy to transfer into copper(II) hydroxide in the Cu ammonia –thiosulphate system, but Cu(en)22 + is more stable over a wide pH range in the Cu2+–ethylenediamine –thiosulphate system. Copper ethylenediamine complexes are stable without the precipitation of copper (II) hydroxide at pH = 6.0–11.0. Whilst in the Cu2+– ammonia–thiosulphate system, copper ammonia complexes are stable at only a narrow pH (pH 9.5–10.0) region (Aylmore and Muir, 2001). A broader pH range enables practical application and requires a lower complex agent dosage, which reduces the gold extraction cost in the Cu2+–ethylenediamine–thiosulphate system. Furthermore, because the higher concentration of copper also causes higher redox equilibrium potentials, which lead to passivation on the gold, the complexity of the system that consists of copper and ethylenediamine and the self-oxidation of the thiosulphate degraded the intermediate products (polythionate), which require a suitable concentration of copper ion. Considering the extraction of gold and the consumption of thiosulphate, the Cu2+–ethylenediamine –thiosulphate system was used in the following gold leaching test, and the copper ion dosage was immobilised to 0.005 M.

50

0.090

40

0.085

30

[S2O32-](M)

Fig. 2 shows that in the Cu2+–ethylenediamine–thiosulphate system, the gold extraction rate initially increases with increasing copper ion concentration. When the copper ion concentration is increased to 0.005 M, 65% of the gold in the ore was extracted. However, the gold extracted was only 30% without copper ions acting as catalyst for gold dissolution, which was an amount similar to that of the traditional ammonia system. In addition, the gold extraction rate decreases when the concentration of copper ion is over 0.005 M. According to literature's summary, the catalytic impact of copper ions in the ammonia system follows Eqs. (10)–(11) (Grosse et al., 2003; Guerra and D. B. D., 1999; Karavasteva, 2010):

0.080

20

0.075

0.070 15

10 0.070

0.065 5

10 5

10

15

20

25

10

15

20

25

Time(h)

Time(h) Fig. 1. The effect of the concentration of copper ion in the Cu2+–ammonia–thiosulphate 2− system (C+ NH3/NH4 = 0.5 M, CS2O3 = 0.1 M, pH = 10).

Fig. 2. The effect of the concentration of copper ion in the Cu2+–ethanediamine – thiosulphate system (Cen = 0.06 M, C2− S2O3 = 0.1 M, pH = 10). In Figs. 1 and 2, the blue line represents [S2O2− 3 ]; the black line represents the extraction rate of gold.

H. Yu et al. / Hydrometallurgy 150 (2014) 178–183 0.29 0.06M en

0.27

55 50

0.26

45

0.25

40

0.24

35

0.23

70

Gold extraction(%)

Gold extraction(%)

0.28

0.18M en

60

No additive 1.5kg/t NaBr 1.5kg/t DTAC 1.5kg/t CTAB 1.5kg/t bromine solvent 1.5kg/t Na2 SiO3

80

0.12M en

Absorbance of the [Cuen2]2+

65

181

60 50 40 30 20 10

30 0.22

0

25 5

10

15

20

5

25

10

Time(h) 2− Fig. 3. The effect of the en concentration (C2+ Cu = 0.005 M, CS2O3 = 0.1 M, pH = 10, the blue line represents the absorbance of [Cuen2]2−, the black line represents the extraction rate of gold).

2þ

Cu

15

20

25

Time(h)

2þ

þ 2en→CuðenÞ2 :

ð15Þ

2+ Fig. 5. The effect of additives on the Au dissolved (C2− S2O3 = 0.1 M, CCu = 0.005 M, Cen = 0.06 M, pH = 10).

In summary, the en concentration should be controlled to an appropriate concentration in the process of gold leaching.

3.3. Effect of stirring speed 3− Cu(en)2+ 2 and Au(S2O3)2 would be more stable when the pH varied from 5 to 14 and the oxidation potential of Cu (en)2+ 2 decreases with increasing pH. The oxidation potential decreases from 0.1 to 0 in the typical thiosulphate leaching, for pH in the range of 9 and 10. A high concentration of en would cause a higher pH. As a result, the high concentration of en is detrimental for gold leaching because of the lower is involved in the reaction electric potential of the solution. Cu(en)2+ 2 described by Eq. (16):

2þ

2−

3−

3−

CuðenÞ2 þ Au þ 2S2 O3 →CuðS2 O3 Þ2 þ AuðS2 O3 Þ2 þ 2en:

ð16Þ

The gold dissolution was also found to decrease with the increase of the en concentration up to 0.18 M. The highest gold leaching rate is 65.0% for the en concentration of 0.06 M. The concentration of Cu (en) 2+ would increase initially and then decrease for the concentration of 2 en at either 0.06 M or 0.12 M; however, the concentration of Cu (en) 2+ would increase persistently when the concentration of en is up to 2 occurs 0.18 M. The highest extraction of gold from the use of Cu(en)2+ 2 in a relatively short period of time.

It is well-known that higher rotation rate creates turbulence, which increases the oxygen content and accelerates the diffusion between reactants in the solution, which may be good for the gold leaching performance. In this test, the rotation speed of stirrer was varied from 150 to 600 rpm. Fig. 4 shows that the stirring speed has no difference in the gold leaching process after 24 h. Approximately 65% of the gold was extracted when the stirring speed varied from 150 to 600 rpm. This result indicated that the reaction of gold dissolving is not controlled by diffusion and that the oxygen content in the solution is not significant for the dissolving of gold. Nevertheless, according to Senanayake and Zhang (Senanayake and Zhang, 2012; Senanayake, 2005), a higher stirring speed causes a higher amount of dissolved oxygen in the solution, which accelerated oxidation of cuprous ion and decomposition of thiosulphate at the same time. Therefore, a low stirring speed was found to be beneficial for the leaching of gold.

0.20

70

95 150r/min

0.18

300r/min

90

450r/min

Gold extraction(%)

Gold extraction(%)

600r/min

50

40

30

85

0.16

80

0.14

75

[S2O32-](M)

60

0.12

70 0.10

20

65

10

60

0.08 0.0

5

10

15

20

25

0.5

1.0

1.5

2.0

The dosage of CTAB(g)

Time(h) 2+ Fig. 4. The effect of the stirring speed (C2− S2O3 = 0.1 M, C Cu = 0.005 M, Cen = 0.06 M, pH = 10).

Fig. 6. The effect of CTAB on the Au dissolved and the S2O2− consumption (C2− 3 S2O3 = 0.1 M, = 0.005 M, C = 0.06 M, pH = 10; the blue line represents the [S2O2− C2+ Cu en 3 ]; the black line represents the extraction rate of gold).

182

H. Yu et al. / Hydrometallurgy 150 (2014) 178–183

Table 4 The consumption of thiosulphate in different conditions. Au(g/t)

C2− S2O3

C2+ Cu

NH3/NH+ 4

en

Adjuvant

pH

The gold extraction

The consumption of thiosulphate (kg/t)

4.3 4.3 4.3 2.10

0.1 0.1 0.1 0.1

50 mg/L 50 mg/L 50 mg/L 5 mmol/L

0.5 M 0.5 M 0.5 M –

– – – 0.06 M

Orthophosphate/polyphosphate CMC EDTA CTAB

10.3 10.3 10.3 10.0

100% 100% 100% 94.3%

5.19/5.56a 17.37b 3.85c 1.12

a b c

M M M M

From D. Feng, J.S.J. van Deventer (thiosulphate leaching of gold in the presence of orthophosphate and polyphosphate). From D. Feng, J.S.J. van Deventer D. Feng, J.S.J. van Deventer (thiosulphate leaching of gold in the presence of carboxymethyl cellulose (CMC)). From D. Feng, J.S.J. van Deventer (Thiosulphate leaching of gold in the presence of ethylenediaminetetraacetic acid (EDTA)).

2−

3.4. Effect of additives The particulates of less than 5 μm in diameter cover the surface of purpose mineral and lead to the formation of colloid in the gold leaching system, which hinder the reaction between the active mineral and reagent in the solution. However, the colloid particles have high specific surface energy, which can lead to the adsorption of leaching reagents, resulting in a large amount of consumption of thiosulphate. Therefore, the adverse impact of the fine particles in the thiosulphate gold leaching system must be eliminated in favour of gold dissolved from the ore. The particle sizes of gold and gangue mineral in the gold ore, such as limonite and goethite, as well as slime are very fine, which prevent the gold from being dissolved effectively. The effect of these fine particles is the most important reason that the gold extraction rate cannot improve further. To further improve the gold extraction rate, additives, such as sodium silicate, cetyltrimethyl ammonium bromide (CTAB), sodium bromide, bromine solvent, and cetyltrimethyl ammonium chloride (DTAC), were added to the gold leaching system. The results are shown in Fig. 5. As shown in Fig. 5, almost all the additives except CTAB do not have a significantly positive effect on increasing the gold leaching. In fact, NaBr even inhibited gold leaching. CTAB has a flocculation effect on the slime produced by limonite in pulp, which could eliminate the adsorption of slime on the gold particle surface; therefore, the gold could be dissolved easily. In addition, the fine particles flocculated by CTAB could also reduce the adsorption of reagents on the surfaces of the fine particle, thereby reducing the consumption of thiosulphate. CTAB can hydrolyse to produce [CTA+] and Br− in solution, enabling gold to be easily dissolved. In Fig. 5, comparing CTAB to NaBr and bromine solvent, the results indicated that only the Br− in the solution has no promoting effect to the gold dissolved; for DTAC or Na2SiO3, the addition of flocculant (Na2SiO3), did not improve the leaching rate of gold. [CTA+] has rod micelle structure, which acts as shape-directing agents and stabiliser in preparing gold nanoparticles (Kai-Zhong et al., 2003; Khan et al., 2013). According to the application of CTAB in the field of nanoparticle preparation (Kai-Zhong et al., 2003; Khan et al., 2013; Li et al., 2014; Weican et al., 2000) and the gold leaching test results, we propose a synergistic mechanism of CTAB to the gold thiosulphate leaching: First, CTAB hydrolysed to [CTA+] and Br−; next, [CTA+] combined 3− + with [Au(S2O3)3− 2 ] to form an ion pair [CTA ]3[Au(S2O3)2 ] •nH2O(Yang et al., 2009), as in Eq. (19). Simultaneously, part of − generated [CTA+][AuBr− [CTA+]3[Au(S2O3)3− 2 ]•nH2O and Br 2 ] •nH2O, as in Eq. (20). In the gold leaching process, CTAB and Na2S2O3 played a collaborative role in the gold leaching process to substantially improve gold dissolved. First: h i þ − CTAB→ CTA þ Br

3−

Au þ 2S2 O3 →AuðS2 O3 Þ2 þ e

ð17Þ

ð18Þ

Second: h i 2− þ AuðS2 O3 Þ2 þ 3 CTA h i h i þ 3− þ nH2 O→ CTA AuðS2 O3 Þ2 •nH2 O: 3

ð19Þ

Alternatively, a small number of [CTA+]3[Au (S2O3)3− 2 ] •nH2O may be formed according to the following reaction: h

i h i 2− − þ − AuðS2 O3 Þ2 •nH2 O þ 2Br → CTA ½AuBr2 •nH2 O h3 i þ 2− þ 2 CTA þ 2S2 O3 : þ

CTA

i h

ð20Þ

3.5. Effect of CTAB The effect of the CTAB dosage on the increase of the gold extraction rate was tested; the results are shown in Fig. 6. From Fig. 6, the gold extraction rate was found to increase with increasing CTAB dosage, and the gold extraction rate was up to 94.3% when the CTAB dosage was 1.5 kg/t ore. The gold extraction rate did not continue to increase with the increasing of CTAB dosage after the 1.5-kg/t ore dosage. Examining the gold leaching process results in Table 4, the CTAB was found to not only improve the gold extraction rate but also reduce the thiosulphate consumption.

4. Conclusions The copper ion–ethanediamine(en)–thiosulphate system was found to be more suitable than the copper ion–ammonia–thiosulphate system for gold extraction of the gold ore containing limonite. The cetyltrimethyl ammonium bromide (CTAB) can hydrolyse to [CTA+] to attract the negative charged slime, thereby changing the slurry rheology, which causes the gold surface to be exposed and thus be easily leached in the thiosulphate gold leaching system. In addition, + the formation of the ion pair [CTA+]3[Au(S2O3)3− 2 ] •nH2O and [CTA ] 3− ] •nH O leads to the stabilisation of [Au(S O ) ] in the leaching [AuBr− 2 2 2 3 2 system. The two effects of CTAB enable the gold extraction rate to be remarkably increased and the thiosulphate consumption to be sharply reduced. The gold extraction rate of the gold ore was determined to reach 94.3%, whilst the thiosulphate consumption was reduced to 1.12 kg/t ore in the thiosulphate–gold leaching system containing 0.1 mol/L of sodium thiosulphate, 0.06 M of ethanediamine, 0.005 M of copper ion, and 1.5 kg/t of CTAB operated at a rate of agitation of 150 rpm.

H. Yu et al. / Hydrometallurgy 150 (2014) 178–183

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