Effect of DO, Free Cyanide and Mineralogy on Gold Cyanidation Mechanism: An Electrochemical and Surface Analysis Study

Abstract

There are many factors which affect the gold cyanidation mechanism—mineral composition, free cyanide and dissolved oxygen (DO) concentrations, heavy metal additions, etc. This study combines electrochemistry and surface analysis methods to examine the effectiveness of the above factors on gold cyanidation process. Three gold ore samples were used in this study. Sample 1 was a pyrite concentrate, contained mercury and silver . Sample 2 also contained sulfide minerals , including galena . Sample 3 contained pyrite with sphalerite and galena . In the case of samples 1 and 2, the presence of heavy metals enhanced gold cyanidation kinetics, though the addition of higher DO was not effective on improving the kinetics. Sample 3 showed an opposite behavior: the kinetics was not enhanced by galena , because of its relatively small amount, however, the use of higher DO made improvement in the gold leaching kinetics, likely caused by iron species generated by pyrite oxidation .

Introduction

It is well known that more than 90% of gold can be selectively leached from a free milling gold ore via cyanidation process, which is known to be the most popular gold extraction technique. According to Elsner’s equation [1], gold forms cyanide complex [Au (CN)₂^–] and dissolves in cyanide solution (Eq. 1).

$4{\text{Au}} + 8{\text{C}}N^{ - } + O_{{2 \left( {aq} \right)}} + 2H_{2} O = 4Au(CN)_{2}^{ - } + 4OH^{ - }$

(1)

As an electrochemical reaction, gold cyanidation generally consists of the oxidation of metallic gold and reduction of oxygen. Thus, in previous studies, the characteristics of gold cyanidation were identified by electrochemical methods [2–6]. These studies have shown that 3 significant anodic peaks of gold cyanidation process. According to Kirk and Foulkes (1980) [7], all of the 3 peaks evolved by the reaction schemes shown in Eqs. 2–4, and the rate-limiting step of the first and second peaks was step 2 (Eq. 3) [7–9]. On the other hand, at the third, the step 3 (Eq. 4) was rate-limiting, and at this peak, the reaction was getting complicated, because of gold (III) oxidation .

${\text{Step}}\;1:\;{\text{Au}} + {\text{C}}N^{ - } = AuCN_{{\left( {ads} \right)}}^{ - }$

(2)

${\text{Step}}\;2:\;{\text{AuC}}N_{{\left( {ads} \right)}}^{ - } = AuCN_{{\left( {ads} \right)}} + e^{ - }$

(3)

${\text{Step}}\;3:\;{\text{AuC}}N_{{\left( {ads} \right)}} + CN^{ - } = Au(CN)_{2}^{ - }$

(4)

By the gold cyanidation process, as the chemistry shown in above, the extraction of gold from an oxide ore is rather easy. However, the grade of gold from most of the new mines is getting lower, besides, the presence of sulfide minerals difficult to handle in gold cyanidation process is increasing. Mostly, they interfere gold leaching reaction by consuming chemicals or making passivation on gold surface. Thus, the ore mineralogy should be carefully considered.

To prevent the problems caused by the sulfide minerals and improve the gold leaching efficiency , pure oxygen can be introduced to the process. By the utilization of pure oxygen in the sulfidic gold ore leaching process, the sulfide minerals in the ore can be oxidized by the pre-oxidation process to form sulfates [10–13]. Additionally, the gold leaching rate can be improved. As shown in Eq. 1, increasing the dissolved oxygen (DO) concentration accelerates the forward reaction rate, and more gold can dissolve and be turned to Au (CN)₂^–. However, high DO dosage is not always effective on gold cyanidation , but it is important to consider the relationship among DO and cyanide concentrations, and the ore mineralogy .

In this study, the gold oxidation behavior in the cyanide solutions was investigated with the presence of different sulfidic gold ores under various free cyanide and DO concentrations. The gold dissolution mechanism in cyanide solution was mainly studied by electrochemical methods, cyclic voltammetry (CV) and chronoamperometry. In addition, to observe surface change of the metallic gold , X-ray photoelectron spectroscopy (XPS) was employed.

Materials and Methods

Sample Characteristics

Three different ore samples from unknown North American gold mines were used in this study. The mineralogy of the ore samples was analyzed by X-ray diffraction (XRD) and the quantitative evaluation of minerals by scanning electron microscopy (QENSCAN), and the results shown in Table 1. Sample 1 was a pyrite ore , and it also had the other sulfide minerals , such as enargite (Cu₃AsS₄), galena (PbS) and tennantite (Cu₁₂As₄S₁₃). Sample 2 consisted of mostly gangue minerals and contained several sulfide minerals , pyrite (FeS₂), chalcopyrite , chalcocite (Cu₂S), sphalerite (ZnS) and galena . Sample 3 had 5.4% pyrite, 0.14% sphalerite and 0.06% galena as significant sulfide minerals .

Table 1

Mineralogical composition of the samples

Sample 1		Sample 2		Sample 3
Mineral	wt%	Mineral	wt%	Mineral	wt%
Pyrite	24.3	Sphalerite	0.53	Pyrite	5.43
Enargite	0.36	Pyrite	0.28	Sphalerite	0.14
Galena	0.21	Chalcopyrite	0.26	Galena	0.06
Tennantite	0.08	Galena	0.25	Cu sulfide	0.01
Chalcopyrite	0.02	Chalcocite	0.07	Quartz	49.96
Acanthite	0.01	Other sulfides	0.05	Feldspar	24.03
Quartz	73.5	Quartz	47.20	Mica	14.45
Grinding media	0.61	K-Feldspar	16.70	Carbonate/oxides	4.24
Barite	0.43	Chlorite	12.70	Sulfide /silicate texture or sulfate	0.86
Alunite	0.27	Plagioclase	8.44	Kaolinite	0.30
Iron Oxide	0.05	Epidote	5.05	Other	0.29
Orthoclase	0.04	Clays	3.56	Chlorite	0.14
Chamosite	0.04	Micas	2.39	Ilmenite and Fe oxides	0.06
Andesine	0.02	Titanite	0.99	Barite	0.02
Albite	0.02	Oxides	0.74	Other silicates	0.01
Rutile	0.02	Carbonates	0.55
Silver	0.02	Other silicates	0.14
		Apatite	0.08
		Other	0.01

According to chemical composition of the samples (Table 2), all samples have Pb in different concentrations. Sample 1 also had 195 ppm mercury. For the sulfur content, sample 1 has the highest (13%). Sample 3 and sample 2 contained an amount of sulfur as 3.1 and 0.4%, respectively.

Table 2

Chemical composition of the samples

Sample	Au ppm	Ag ppm	As ppm	Cu ppm	Fe %	Pb ppm	S %	Sb ppm	Zn ppm	Hg ppm
Sample 1	11.71	640	3800	2100	12	1800	13	920	37	195
Sample 2	2.26	0.36	8	438	4.59	2060	0.42	12	4540	–
Sample 3	11.8	13.7	497	107	3.44	550	3.09	75	922	1

The samples were acquired as ground materials, then split by riffle and rotational micro splitter to obtain representative samples for electrochemical tests. The samples were always kept in the fridge to prevent further oxidation of the sulfide minerals .

Electrochemical Experiments

The cyanide solution was prepared by dissolving sodium cyanide (NaCN, Fisher scientific) in a sodium hydroxide (NaOH, Fisher scientific) solution, and all solutions were made from deionized (DI) water. Prior to testing , free cyanide (CN^–) concentration was measured by a titrator (916 Ti-Touch, Metrhom) with silver nitrate (0.0192 M AgNO₃, Ricca chemical company) as a titrant. The free CN^– concentrations were adjusted as 300, 600 and 900 mg/L. Then, the sample was put into the CN^– solution, to achieve a slurry with 50 g/L solids. The slurry pH was adjusted again, by NaOH or DI water addition, to 10.50 (±5%), after the solids were added. Then, air or oxygen gas was injected to the slurry with a flow rate of 1.38 L_gas/min/L_slurry.

Afterwards, the electrodes were immersed in the slurry. The electrochemical tests were conducted in a three-electrode cell. A 99.9986% pure gold rod (VWR) with 3 mm diameter (surface area 0.07 cm²) was used as a working electrode. The gold electrode was attached to the copper wire and covered by epoxy resin (LECO). One side of the gold rod was polished and exposed to the slurry. The working electrode was immersed in the slurry as a stationary electrode; only the slurry was magnetically stirred at 150 rpm, preventing vortex occurrence and ore particles settlement. After each test, the surface of the gold rod was polished using alumina and silicon carbide paper of a grit size 1200–2400 to regenerate fresh surface for the next test. A graphite rod was used as a counter electrode, and a glass-body Ag/AgCl/KCl electrode saturated with AgCl (0.199 V vs. SHE at 25°C) was used as a reference electrode. All potentials reported in this study were measured with respect to the reference electrode. Using a Gamry Reference 600 + potentiostat, electrochemical tests were conducted at 21 ± 1°C under air or oxygen injection. Open circuit potential (OCP) was measured for an hour prior to kinetics or polarization tests.

To investigate the kinetics of the gold electrodissolution in the cyanide slurry in the presence of the samples, CV method was utilized, and i-E plots were developed under different conditions. The CV tests were conducted in the potential range of –0.8 to +1.0 V, and the sweep rate was 10 mV/s (which was found to be the most stable scan rate). To check any change of the working electrode surface, 3 cycles were repeated in each test. From the plots, the anodic gold oxidation peak and peak current density were shown in most cases, and some of full range scan results were also introduced for an extra explanation.

X-Ray Photoelectron Spectroscopy Analysis

To observe the passivation of the gold electrode surface, X-ray photoelectron spectroscopy (XPS) analysis was conducted. After picking a potential at which showed unusual behavior from the CV tests, the gold electrode was polarized at that constant potential for 1 h. Then, the electrode was rinsed with DI water, and sealed until the XPS analysis.

XPS analysis was conducted by ThermoFisher Scientific K-Alpha XPS spectrometer with a monochromated Al Kα X-ray source excitation (1486.6 eV). The system uses a microfocused X-ray spot, ranging in size from 30 to 400 μm, and the analysis in this study was done with a 400 μm spot. The analysis chamber vacuum was in the range of ~10⁻⁹ mbar. The survey spectrum was typically acquired in a high pass energy (200 eV), low point-density (1 point/eV) scanned mode. Regional spectra, used to determine relative atomic composition as well as for determination of chemical information, were acquired in a low pass energy (50 eV), high point-density (0.1 eV spacing) scanned mode. The energy scale of the instrument was established using sputter-cleaned Au , Cu and Ag. Accepted binding energy (BE) value for the Au 4f (Au 4f7/2 = 84.0 eV), Cu2p (Cu 2p3/2 = 932.67 eV) and Ag3d (Ag 3d5/2 = 368.26 eV) transitions were used to establish absolute energy scale and linearity. After acquiring XP-spectra, XPSPeak 4.1 software was used for fitting XPS peaks.

Results and Discussion

Cyanidation Behavior Without Mineral Addition

The electrochemical study results in this section will be used to compare and explain the different cyanidation behavior derived from the presence of foreign materials in following sections. As shown in Fig. 1, three oxidation peaks of gold cyanidation were observed without any minerals . At the potential between –0.3 and –0.1 V, the current density was 0 to 0.1 mA/cm² in CN^– concentrations from 300 to 900 mg/L, under 8.30 mg/L DO. It was consistent with several previous studies [14–17], which have found that current density was not so significant at low overpotential region, caused by a protective AuCN layer on the gold surface. With increasing DO concentration to 37 mg/L, the current density increases from 0.02 to 0.15 mA/cm², in 300–900 mg/L CN^– solution. Although the current densities were relatively low even under 37 mg/L of DO, the current density, i.e. gold cyanidation kinetics, was definitely improved with increasing DO concentration.

../images/468727_1_En_140_Chapter/468727_1_En_140_Fig1_HTML.gif — Fig. 1
CVs in cyanide solution of different free cyanide concentrations: a 8.30, and b 37 mg/L DO

The second peak increased from –0.1 V. At 8.30 mg/L DO, the second peak was only obvious in 900 mg/L CN^– solution, while the peak was observed and increased in all CN^– concentrations at 37 mg/L DO. Thus, it can be noted that the second peak of gold oxidation in cyanide solution was improved with higher DO concentration.

The third peak was observed at around 0.55 V, and the current density was the highest among the three oxidation peaks. At this potential, the peak current density and potential was not changed much depending on the DO concentration, but more affected by CN^– concentration.

Cyanidation Behavior in Ore Slurry

Sample 1

Figure 2 shows the current-potential curve of gold oxidation in cyanide solution in the presence of sample 1. This sample generated considerably different plots compared with those of the cyanide solutions without any minerals under all of conditions. In 300 mg/L CN^– solution under both 8.30 and 37 mg/L of DO, the first peak was overlapped with the second peak in the potential region from –0.4 to 0.4 V. The first peak current density was lower in 37 mg/L DO. Then, the current density was noticeably decreased at 0.55 and 0.50 V in 8.30 and 37 mg/L DO, respectively. It seemed that the heavy metals in sample 1, Hg and Ag, improved gold oxidation kinetics in the first and second peaks, and then they blocked the gold surface, which caused the reduction in current density after the second peak.

../images/468727_1_En_140_Chapter/468727_1_En_140_Fig2_HTML.gif — Fig. 2
CVs at different free cyanide concentrations in the presence of sample 1: a 8.30, and b 37 mg/L DO

The effect of heavy metals was more noticeable in higher CN^– and lower DO concentrations. In Fig. 2a, in 600 and 900 mg/L CN^– solution at 8.30 mg/L DO, the high first peak was observed at –0.2 V. This gold oxidation kinetics improvement was due to the action of heavy metals via Eq. 5 [3, 18]. In sample 1, 195 ppm of Hg and 640 ppm of Ag seemed to give the activation effect on gold cyanidation . However, the heavy metals were not fully acted at higher DO concentration, 37 mg/L, as shown in Fig. 2b, which shows the lower first peak than in 8.30 mg/L DO.

${\text{Me}}^{2 + } + 2Au + 4CN^{ - } = Me + 2Au\left( {CN} \right)_{2}^{ - } \quad \left( {{\text{Me}} = {\text{Pb}},\;{\text{Hg}},\;{\text{Tl}},{\text{etc}}.} \right)$

(5)

The act of Hg and Ag was examined by XPS . After treating the working electrode in a 900 mg/L CN^– solution in the presence of sample 1 for an hour at –0.2 V, the treated electrode was analyzed by XPS. Clear Hg (Hg 4f 7/2 = 99.85 eV, Δ = 4.0 eV) and Ag (Ag 3d 5/2 = 368.26, Δ = 6.0 eV) peaks were observed as shown in Fig. 3; the peaks of the metallic Hg (99.97 and 103.99 eV in Fig. 3a) [19], HgS (100.22 and 104.21 eV in Fig. 3a) [20] and Ag₂S (368.02 and 374.02 in Fig. 3b) [21] were detected on the gold surface. This result showed that Hg and Ag deposition was occurred on the gold surface after activating gold oxidation kinetics . By applying higher anodic potentials to the gold electrode Hg and Ag deposited, the deposition seemed to be oxidized and passivate the gold surface. Therefore, to enhance gold oxidation kinetics in low overpotential region, 8.30 mg/L DO was better in terms of the activation role of Hg and Ag on gold cyanidation with 600 or higher CN^– concentration.

../images/468727_1_En_140_Chapter/468727_1_En_140_Fig3_HTML.gif — Fig. 3
a Hg 4f and b Ag 3d XP-spectra of the gold electrode surface after treating in 900 mg/L CN^– and 8.30 mg/L DO at –0.20 V in the presence of sample 1

Sample 2

Sample 2 did not make change much on the gold cyanidation behavior in 300 mg/L CN^– solution (Fig. 4a and b). The first peak was insignificant, which observed at between –0.3 and –0.1 V. The second and third peaks were slightly shifted to anodic direction, 0.30 and 0.69 V, respectively, but the change of current density was not so significant compared to the peaks observed in cyanide solution without minerals .

../images/468727_1_En_140_Chapter/468727_1_En_140_Fig4_HTML.gif — Fig. 4
CVs at different free cyanide concentrations in the presence of sample 2: a 8.30, and b 37 mg/L DO

On the other hand, in Fig. 4a, a high first peak appeared at –0.40 V in 600 and 900 mg/L CN^– solutions at 8.30 mg/L DO. Like sample 1, this high peak was caused by the heavy metal in the sample; sample 2 had 0.25% galena , thus some lead dissolved from the galena and affected the gold oxidation kinetics improvement. After the first peak, the second and third peaks were less shifted to an anodic direction than the peaks in 300 mg/L CN^–. However, the reduction in second peak was observed, compared with the peak in the cyanide solution without minerals .

The reason of the second peak reduction was identified by XPS analysis. The electrode was treated in 900 mg/L CN^– solution with sample 2 at 0.4 V ahead of the analysis. XPS peaks of Pb 4f (Pb 4f 7/2 = 136.9 eV, Δ = 4.86 eV) [22] showed that metallic lead (137.15 and 141.96 eV in Fig. 5) and PbO (138.87 and 143.70 eV in Fig. 5) as dominant species and the weak peak of PbS (137.77 and 142.65 eV in Fig. 5) were detected. It can be noted that during the electrochemical test, lead species were deposited onto the gold surface, and hindered the gold oxidation kinetics at high potential by lead oxidation . According to Tshilombo and Sandenbergh (2001) [23], the presence of lead is only effective for gold leaching in cyanide solution when the potential is in the range of –0.5 and –0.2 V (vs. SCE), where the lead itself is not passivated. In their study, the lead species did not improve the gold oxidation kinetics with increasing the potential. This was along with the observations of the present study, that in the presence of lead the highly oxidizing condition should be avoided.

../images/468727_1_En_140_Chapter/468727_1_En_140_Fig5_HTML.gif — Fig. 5
Pb 4f XPS spectrum of the gold electrode surface after treating at 0.4 V in 900 mg/L CN^– and 8.30 mg/L DO in the presence of sample 2

The high first peak was not observed in any of the CN^– concentrations, when the DO was increased to 37 mg/L (Fig. 4b). Under the highly oxidizing conditions, like high DO, lead can be oxidized and precipitated [24]. However, lead should be present as aqueous species to enhance gold oxidation kinetics by Eq. 5. Therefore, the gold cyanidation kinetics was better under lower DO and higher CN^– concentrations in the presence of sample 2, by maximizing the role of lead contained in the ore sample itself.

Sample 3

The CV curves of gold oxidation in cyanide slurry of sample 3 were similar under the both DO conditions of 8.30 and 37 mg/L with all CN^– concentrations applied (Fig. 6). The first and second peaks showed improvements in their current densities under the both DO concentrations, and more improvement was observed with a higher DO concentration. This is because iron species dissolved from pyrite in sample 3 can improve the gold cyanidation kinetics. In previous studies, the gold cyanidation rate was enhanced by iron species in the presence of pyrite under 8.30 mg/L DO condition [25].

../images/468727_1_En_140_Chapter/468727_1_En_140_Fig6_HTML.gif — Fig. 6
CVs at different free cyanide concentrations in the presence of sample 3: a 8.30, and b 37 mg/L DO

Generally, sphalerite did not give a negative effect on the gold cyanidation kinetics under any CN^– or DO concentrations with its concentration up to 8 g/L in slurry [25]. In fact, the gold leaching kinetics was increased with its addition in 8 mg/L DO [26] and higher DO concentration [25].

Galena in sample 3, unlike sample 2, did not give any effect on gold cyanidation , under all of conditions studied. It seemed that the amount of galena in sample 3 was relatively lower than that of pyrite and sphalerite , and they may interfere the dissolution of galena to make aqueous lead species. Consequently, the gold oxidation kinetics enhanced with a higher DO concentration in the presence of sample 3 by the act of pyrite and sphalerite , although there was no significant effect derived from the lead species from galena in sample 3.

Conclusions

In this study, the electrochemical behavior of gold cyanidation was investigated in the presence of three different sulfidic gold ore under several CN^– and DO concentrations. The different mineralogy of the ore samples made different gold cyanidation behavior.

Sample 1 highly improved the gold kinetics at –0.2 V in 600 and 900 mg/L CN^– solutions under 8.30 mg/L DO, which was caused by the act of Hg and Ag species. Under higher DO concentration (37 mg/L DO), the kinetics was less improved because of Hg oxidation . Thus, in the presence of sample 1, gold cyanidation can be more effective at 8.30 mg/L DO with 600 mg/L or higher CN^– concentration.

The gold oxidation was enhanced by galena in the presence of sample 2, at –0.4 V in 600 and 900 mg/L CN^–, and 8.30 mg/L DO. However, the kinetics was decreased at highly oxidizing conditions, both high DO and high potential, because of lead oxidation . As similar as sample 1, gold cyanidation was more effective at 8.30 mg/L DO with CN^– concentrations higher than 600 mg/L in the presence of sample 2.

Sample 3 made improvements on gold cyanidation kinetics in the region of the first and second peak potentials in cyclic voltammetry at 37 mg/L of DO. Galena did not give any improvement effect because its amount was relatively less than pyrite and sphalerite . Nevertheless, iron species dissolved from pyrite activated gold cyanidation , and the kinetics was more improved with increasing CN^– and DO concentrations.

Acknowledgements

The authors would like to thank MITACS (Ref. IT07117) and Air Liquide for the financial support of this study.

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