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The influence of pyrite pre-oxidation on gold recovery by cyanidation
Minerals Engineering 19 (2006) 883–895 This article is also available online at: www.elsevier.com/locate/mineng
The influence of pyrite pre-oxidation on gold recovery by cyanidation J. Li a, B. Dabrowski a, J.D. Miller a
a,*
, S. Acar b, M. Dietrich b, K.M. LeVier b, R.Y. Wan
c
Department of Metallurgical Engineering, University of Utah, 135 S. 1460 E. Room 412, Salt Lake City, UT 84112-0114, United States b Newmont Mining Co., 10101 E. Dry Creek Road, Englewood, CO 80112, United States c Metallurgy Consultant, 9634 Kalamere Court, Highlands Ranch, CO 80126, United States Received 11 April 2005; accepted 18 September 2005 Available online 17 November 2005
Abstract The influence of pyrite pre-oxidation in alkaline solutions on gold recovery by cyanidation from Twin Creek refractory gold ore in which pyrite was identified as the major sulfide mineral has been investigated with the aid of electrochemical measurements, leaching experiments, and direct analysis of reaction products for selected residues. It was found that gold recovery by cyanidation in bottle roll experiments mainly depended on the extent of pyrite pre-oxidation. The rate of pyrite oxidation in alkaline solutions measured by electrochemical measurements, including chronoamperometry and linear sweep voltammetry, increased with an increase in pH, potential, and temperature. All alkaline reagents used for the electrochemical measurements, NaOH, NH4OH, Na2CO3 and Ca(OH)2, showed a similar effect on pyrite oxidation kinetics. However, the results of alkaline pre-oxidation for pyrite of the Twin Creek refractory gold ore suggested that NaOH and Na2CO3/Ca(OH)2 were superior to Ca(OH)2. Without pre-oxidation, cyanide leachable gold was found to be only 20% which could be increased to 70% under appropriate pre-oxidation conditions. At the same time, cyanide consumption decreased from 2.5 kg/t ore to 1.5 kg/t ore. Selected residues after pre-oxidation and cyanidation were examined by X-ray diffraction. Backscattered electron images of pyrite particles in these residues were taken. The reaction products at the surface of pyrite particles were found to be iron-, silicon-, and calcium-bearing compounds with variable amounts of sulfur as determined by X-ray energy dispersion analysis. Additionally, some mineral fines, such as aluminum and/or potassium-bearing minerals, were found to be present at the partially oxidized pyrite surface. 2005 Elsevier Ltd. All rights reserved. Keywords: Sulfide ores; Gold ores; Hydrometallurgy; Oxidation; Cyanidation
1. Introduction With depletion of the oxidized free-milling gold reserves close to the earthÕs surface, most of the important new deposits being mined today do not respond to direct cyanidation. It is found that the gold is very finely disseminated and encapsulated in host matrices that are inert and/or impermeable to the cyanide solution. In this case, it is necessary to first break down the host matrices and then it is possible to leach the liberated gold. In many cases, the host matrices are sulfide minerals, which exhibit a strong association with finely disseminated gold particles in many ore *
Corresponding author. Tel.: +1 801 581 5160; fax: +1 801 581 4937. E-mail addresses: [email protected] (J. Li), [email protected] (J.D. Miller). 0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.09.052
bodies. Of these sulfides, pyrite is one of the most important minerals. Therefore, a knowledge of pyrite leaching behavior is very important for the development of improved technology for the treatment of refractory gold ores. The electrochemistry of pyrite and the kinetics of pyrite oxidation in different systems have been extensively studied. The anodic dissolution behavior of pyrite has been investigated by various techniques (Brion, 1980; Hamilton and Woods, 1981; Buckley and Woods, 1987; Buckley et al., 1988; Ahlberg et al., 1990). It was found that pyrite is the most noble sulfide mineral and a good electrocatalyst for oxygen reduction (Biegler et al., 1975) and for hydrogen evolution (Peters, 1976). This property can affect the behavior of other sulfide minerals through galvanic coupling, in which pyrite serves as a cathode and thus enhances the oxidation of other sulfide minerals, as might occur
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during pre-oxidation processes for exposure of gold particles from sulfides matrices. For pressure oxidation in alkaline solutions, the rate of pyrite oxidation was found to be surface-reaction controlled (Lowson, 1982; Brown and Jurinak, 1989; Ciminelli and Osseo-Asare, 1995a,b). Some authors observed that the oxidation rate increases with an increase of pH, reagent concentration, and temperature (Brown and Jurinak, 1989; Nicholson et al., 1988, 1990; Ciminelli and Osseo-Asare, 1995a,b; Envangelou et al., 1998). With more extensive alkaline oxidation of pyrite, higher gold recovery is generally observed (Souza and Ciminelli, 1992). The kinetics of pyrite oxidation in alkaline solutions have been found to be affected by the nature of the reagents used to control the pH of the systems. Among the reagents, sodium carbonate/bicarbonate was claimed to be particularly effective in enhancing pyrite decomposition in aerated solutions (Wheelock, 1981; Guay, 1981a,b; Souza and Ciminelli, 1992). However, there still is some controversy regarding this position. For instance, Warren (1956) found that the addition of small amount of calcium carbonate for pH adjustment caused a drastic reduction in the pyrite decomposition rate. Warren (1956) explained the reduction in rate as a consequence of the formation of a growing iron-oxide film on pyrite particles, the oxidation rate then becoming controlled by diffusion in the product layer. In contrast, Guay (1981a,b) suggested that sodium carbonate enhanced the rate of pyrite oxidation by favoring the removal of the iron-oxide coating, thus increasing the exposure of fresh surfaces to oxidation. Caldeira et al. (2003) recently reported the product layer constituents and morphology from pyrite oxidation by molecular oxygen in alkaline solutions. In sodium hydroxide solutions, hematite was the main phase identified in the product layer. In carbonate solutions, the main constituent was ferrihydrite. In calcium hydroxide solutions, only calcium carbonate was detected on the surface of oxidized pyrite in an amount that increased when the system was opened to the atmosphere. The morphology of the product layers demonstrated that in NaOH solutions the pyrite particles were initially covered by a thin oxide layer and then the oxide layer fractured after longer reaction times. Pyrite oxidation in Na2CO3/NaHCO3 solutions resulted in particles that were initially covered by a discontinuous oxide coating that grew with reaction time, thus increasing the overall pyrite surface coverage. Upon review of the literature, the kinetics and products of pyrite decomposition in alkaline solutions under oxidative conditions are dependent on many variables, such as oxidation potential, temperature, solution composition, whether the system is open to the atmosphere, etc. (Buckley and Woods, 1987; Buckley et al., 1988; Ahlberg et al., 1990; Kathe et al., 1993; Caldeira et al., 2003). Generally, it is accepted that the oxidation of pyrite is described by the following overall reactions, in which the ferric ion precipitates on the pyrite surface or reports to the solution in suspension as ferric hydroxide
FeS2 + 3H2 O = Fe(OH)3 + 2S0 + 3Hþ + 3e or FeS2 + 3OH = Fe(OH)3 + 2S0 + 3e
ð1Þ ð2Þ
and at higher potentials þ FeS2 þ 11H2 O ¼ FeðOHÞ3 þ 2SO2 4 þ 19H þ 15e
ð3Þ
or FeS2 þ 19OH ¼ FeðOHÞ3 þ 2SO2 4 þ 8H2 O þ 15e
ð4Þ
Besides the aforementioned phenomena, other issues have also been considered: (1) whether elemental sulfur is involved as an intermediate during formation of sulfate; and (2) which component of pyrite is preferentially oxidized (Sato, 1960; Peters and Majima, 1968; Biegler and Swift, 1979; Hamilton and Woods, 1981; Goldhaber, 1983; Buckley and Woods, 1985; Buckley and Woods, 1987; Chander and Briceno, 1987; Kathe et al., 1993). It is due to the system complexity that a detailed mechanism of pyrite oxidation has not been well established (Hayes et al., 1987; Ahlberg et al., 1990; Caldeira et al., 2003). Consequently, in the current research, the behavior of pyrite oxidation in alkaline solutions was investigated by electrochemical measurements under conditions for industrial applications at ambient pressure. The variables tested, in experiments open to the atmosphere, included pH, potential, temperature, and alkaline reagent. Based on these fundamental results as a point of reference, other experiments were initiated for alkaline pre-oxidation of the Twin Creek refractory gold ore, and to determine the influence of pre-oxidation conditions on sulfide oxidation and gold recovery by subsequent cyanidation from the oxidized residues. In addition, characteristics of the solid products after alkaline pre-oxidation and cyanidation for gold recovery were made for selected samples by X-ray diffraction (XRD), backscattered electron (BSE) images, and X-ray energy dispersion (EDS) analysis. 2. Electrochemical experiments Pyrite is a semiconductor and known to support electrochemical reactions. Therefore, some electrochemical measurement techniques can be utilized to determine the rate of anodic pyrite oxidation. In this respect, it was decided to do a systematic electrochemical research on the kinetics of pyrite oxidation in alkaline solutions in order to provide supporting information for the alkaline pre-oxidation experiments. The electrochemical techniques used in the current research included chronoamperometry and linear sweep voltammetry. The rate of anodic pyrite oxidation in alkaline solutions under certain conditions and the influence of important factors on the rate were determined by electrochemical measurements. The experimental methods and results are presented in the following sections. 2.1. Pyrite electrode preparation A natural massive specimen of pyrite of unknown source was provided by Newmont Mining Corporation.
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Fig. 1. Effect of applied potential (EAP) on the rate of pyrite oxidation by chronoamperometry at pH 11 and ambient temperature with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm), and different alkaline reagents: NaOH, NH4OH, Na2CO3, and Ca(OH)2.
The massive pyrite specimen was cut into a parallelepiped with dimensions of 14 · 13 · 8 mm and then attached to a copper wire (2.0 mm in diameter) using silver conducting glue. The pyrite electrode was molded in epoxy, exposing one side to the solution. 2.2. Electrochemical procedures Before each experiment, the pyrite electrode was ground with silicon carbide papers (800 and 2400 mesh) successively, rinsed with ethanol and deionized water, respectively. The electrode was immediately put into the test solution in an electrochemical cell. During the experiment, the solution was open to the atmosphere. The volume of the test solution was 250 mL. The pH was adjusted by addition of concentrated solutions of different alkaline chemicals. The potential was measured and referenced against the saturated calomel electrode (SCE), E0SCE ¼ 0:242 V vs. SHE. Most experiments were performed at ambient temperature (24 C) with the stationary pyrite electrode, solution being stirred (500 rpm) with a magnetic bar (8.0 mm in diameter and 38.0 mm in length). Sodium sulfate was added as an indifferent electrolyte
(0.2 M). Variations from these conditions can be noted in the text and in the figure captions. A typical three-electrode system was employed for these measurements. The pyrite electrode was used as the working electrode, a graphite rod was used as the counter electrode, and the SCE was used as the reference electrode. Potential measurements between the working electrode and the reference electrode were made through a Luggin capillary. The current densities for anodic pyrite oxidation or decomposition were obtained with an EG&G Princeton Applied Research, Potential-Galvanostat (Model 273). 2.3. Experimental results and discussion The current density determined by chronoamperometry for pyrite oxidation under specific conditions in this paper is the average of anodic current densities within a certain experimental time, for example, after 5 min, during which time the current density reaches a relatively stable level. When the anodic current densities are available, the following equation can be used to calculate the rates (mol/ cm2 min) of pyrite oxidation from the corresponding anodic current densities.
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Fig. 2. Effect of pH on the rate of pyrite oxidation by chronoamperometry at applied potential (EAP) 0.258 V (SCE), and ambient temperature with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm), and different alkaline reagents: NaOH and Na2CO3.
Ia 60 ðs= minÞ ðA=cm2 Þ 96 490 ðC=molÞ na 1000 where R is the rate of pyrite oxidation (mol/min cm2), Ia is the anodic current density of pyrite oxidation (mA/cm2) and na is the number of electrons transferred for pyrite oxidation. However, it is expected that the products from anodic pyrite oxidation vary, for example, ferrous ion or ferric ion and elemental sulfur or sulfate etc., depending on experimental conditions. Because of the different reactions that can occur, the na value cannot be defined unambiguously. For this case, the anodic current densities for pyrite decomposition will be used as an indication of the rate for pyrite oxidation in order to compare the influence of some important variables on pyrite oxidation rate. R¼
2.3.1. Stirring speed In order to investigate the reaction characteristics for anodic pyrite oxidation, it is necessary to examine many different variables, including stirring speed. The influence of magnetically stirring speed on the anodic current density for pyrite oxidation was examined at potential 0.258 V (SCE) and pH 11 (Ca(OH)2). It was found that when the
Fig. 3. Effect of pH on the rate of pyrite oxidation by chronoamperometry at applied potentials (EAP) 0.258 V and 0.370 V (SCE), and ambient temperature with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm), and alkaline reagent Ca(OH)2.
Fig. 4. Examination of pH (Ca(OH)2) effect on the rate of pyrite oxidation by linear sweep voltammetry at ambient temperature and potential scanning rate 2 mV/s with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm), and alkaline reagent Ca(OH)2.
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solution was stationary, the current density was much lower than that at the stirring speed of 250 rpm; when the stirring speed was above 250 rpm, the current density was found to be almost independent of the stirring speed. These results indicate that when the stirring speed is above 250 rpm the reaction rate for pyrite oxidation is no longer limited by aqueous phase transport processes. Thus, the speed was kept at 500 rpm for all of the following experiments. 2.3.2. Effect of potential Examination of the influence of the oxidation potential on the rate of pyrite oxidation was made by chronoamperometry at the potentials of 0.258 V and 0.370 V (SCE) with different alkaline reagents to adjust the solution pH, including sodium hydroxide, ammonium hydroxide, sodium carbonate, and calcium hydroxide. These results are presented in Fig. 1, which clearly demonstrate that the rate of anodic pyrite oxidation is significant at these potentials. The rate at 0.370 V is much higher than at 0.258 V. A similar phenomenon can be observed from the results determined by linear sweep voltammetry presented in Fig. 4. For example, it is evident that the oxidation rate increases with an increase in the oxidation potential at each pH level. 2.3.3. Effect of pH The effect of pH on the rate of pyrite oxidation using different alkaline reagents for adjustment of the solution pH was measured by different electrochemical measurements. The results determined by chronoamperometry are shown in Fig. 2 using NaOH and Na2CO3 at potential of 0.258 V, and in Fig. 3 using Ca(OH)2 at potentials of 0.258 V and 0.370 V. The results determined by linear sweep voltammetry are shown in Fig. 4 using Ca(OH)2. These results suggest that the oxidation rate increases with
Fig. 5. Reaction order determination by plot of the logarithm of anodic current density (Ia) vs. pH. Data taken from Fig. 3 at potentials of 0.258 V and 0.370 V (SCE).
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an increase in pH, which is expected by examination of the reactions (1)–(4) for pyrite oxidation and is similar to the results reported by other authors (Brown and Jurinak, 1989; Nicholson et al., 1988, 1990; Ciminelli and OsseoAsare, 1995a,b; Envangelou et al., 1998). Plots of the logarithm of anodic current density vs. pH for pyrite oxidation at potentials of 0.258 V and 0.370 V are presented in Fig. 5. The data are taken from Fig. 3. The slopes of 0.584 for 0.258 V and 0.586 for 0.370 V presented in Fig. 5 demonstrate that the oxidation rate is approximately half order with respect to hydroxide ion concentration. The half order may indicate that the reaction for anodic pyrite decomposition or pyrite oxidation in alkaline solution would be a multiple-step process, which needs to be further studied in future research. In addition, the close values of the slopes indicate that the oxidation
Fig. 6. Examination of chemical effect on the rate of pyrite oxidation by chronoamperometry at applied potentials (EAP) of 0.258 V and 0.370 V (SCE), pH 11, and ambient temperature with a stationary pyrite electrode and magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
Fig. 7. Voltammogram for pH effect on the rate of pyrite oxidation using different alkaline reagents at ambient temperature and potential scanning rate 2 mV/s with a stationary pyrite electrode and magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
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reaction at the different potentials may have the same ratelimiting step and/or produce the same products under these circumstances. 2.3.4. Alkaline reagents It can be noted from review of the literature that the kinetics of pyrite oxidation in alkaline solutions are dependent not only upon pH, potential and whether the system is open to the atmosphere, but also upon the solution composition or reagents used to adjust the solution pH (Lowson, 1982; Brown and Jurinak, 1989; Nicholson et al., 1990; Ciminelli and Osseo-Asare, 1995a,b; Caldeira et al., 2003). In the current research, the influence of different alkaline reagents on pyrite oxidation kinetics, including sodium hydroxide, ammonium hydroxide, sodium carbonate, and calcium hydroxide, was examined by chronoamperometry
Fig. 8. Effect of temperature on the rate of pyrite oxidation by chronoamperometry at applied potential (EAP) of 0.258 V (SCE), and pH 11 (NaOH or Na2CO3) with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
Fig. 10. Effect of pH on the rate of pyrite oxidation by chronoamperometry at elevated temperature (45 C and 65 C), and applied potential 0.258 V (SCE) with a stationary pyrite electrode and magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
Fig. 11. Influence of purging N2 on the rate of pyrite oxidation by chronoamperometry at ambient temperature, applied potentials (EAP) of 0.258 V and 0.370 V (SCE), and pH 11 (Na2CO3) with a stationary pyrite electrode and magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
Fig. 9. Arrhenius plot of the natural logarithm of anodic current density (Ia) for pyrite oxidation vs. 1/T. Data taken from Fig. 8.
at potentials of 0.258 V and 0.370 V (Fig. 6) and by linear sweep voltammetry at pH values of 10, 11 and 12 (Fig. 7).
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These results suggest that the rate of pyrite oxidation under different conditions is independent of the alkaline reagents used under the conditions studied. However, it should be pointed out that, in the current research, the anodic current densities for pyrite decomposition as determined by these electrochemical measurements only represent the initial rates of pyrite oxidation. The effect of these alkaline reagents needs to be further examined by actual pre-oxidation tests. Especially, the oxidation rate may be affected by the reaction products formed at the pyrite surface with different alkaline reagents in actual pre-oxidation systems. 2.3.5. Temperature The effect of temperature on the anodic current density for pyrite oxidation was examined by chronoamperometry in NaOH and Na2CO3 solutions at pH 11 and potential 0.258 V (SCE). The results are presented in Fig. 8. A logaTable 1 Chemical analysis of Twin Creek refractory ore Au (g/t)
S-total (%)
S-sulfate (%)
S-sulfide (%)
C-total (%)
C-org. (%)
Fe (%)
As (%)
2.49
3.45
0.39
3.06
0.60
0.61
4.52
0.20
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rithm of the anodic current density is then plotted against reciprocal of the absolute temperature in Fig. 9. According to the Arrhenius theory (EAct/R = 4150), the apparent activation energy was found to be 8.20 kcal/mol (34.5 kJ/mol), which indicates that, under the conditions studied, the rate of pyrite oxidation is limited by surface reaction control or by mixed control. Additionally, the influence of pH on the kinetics of alkaline pyrite oxidation at elevated temperatures of 45 C and 65 C was investigated by chronoamperometry in sodium carbonate/sodium hydroxide solutions and the results are presented in Fig. 10. It can be seen from these results that the rate of pyrite oxidation significantly increases with an increase in pH value from pH 10 to pH 11, and that when the pH exceeds pH 11 the initial rate seems to become not significantly dependent of pH under the conditions studied. At the elevated temperatures and with the consideration of alkaline reagent consumption, control at pH 11 is suitable for pyrite oxidation for an actual pre-oxidation process. 2.3.6. Purging with nitrogen In the current research, all solutions were open to the atmosphere. In most fundamental studies by other authors, the systems generally were isolated from the atmosphere by
Table 2 Summary of baseline cyanide/CIL test results Test
Sulfide oxidation (%)
Au in residue (g/t ore)
Au recovery (%)
NaCN consumption (kg/t ore)
1 2
6.2 14.4
2.0 1.68
19.9 32.4
2.5 2.7
Table 3 Alkaline pre-oxidation tests at 60 C for 6 h followed by cyanide/CIL Test
Reagent
pH
Sulfide oxidation (%)
Au extraction (%)
3 4 5 6 7
NaOH – Ca(OH)2 – Na2CO3 Ca(OH)2 –
10 11 10 11 10
26.1 67.6 17.3 28.4 23.8
38.4 63.1 32.9 37.0 41.1
14.6 62.7 18.2 41.1 48
1.6 1.2 1.2 1.3 1.3
11
67.9
54.4
147.5
0.7
8
Alkaline reagent (kg/t)
NaCN (kg/t)
Fig. 12. Effect of the pre-oxidation slurry pH on gold extraction from the oxidized residues by cyanide/CIL (Test 2 as a reference: gold extraction 32% for the pre-oxidation without addition of alkaline reagent).
Table 5 XRD analysis of Twin Creek refractory gold ore and pre-oxidation residues (%)a
Table 4 Effect of lime addition in grinding circuit Test
Lime in grinding (g/kg)
pH
Sulfide oxidation (%)
CIL Au Extraction (%)
9 10 11 12 13a 14a
5 5 10 10 5 5
10 11 10 11 10 11
49.6 79.6 37.1 71.8 48.4 88.0
62.9 71.4 47.6 63.3 62.8 69.8
Alkaline reagent used: Na2CO3/Ca(OH)2 (molar ratio 1:1). a Addition of Pb(NO3)2 in cyanide/CIL.
Mineral
Formula
Anatase Calcite Muscovite/ Illite Pyrite Quartz
TiO2 CaCO3 KAl2(AlSi3O10)(OH)2/ K0.7Al2(Si,Al) O10(OH)2 FeS2 SiO2
Total a
Head
Test 4
Test 10
Test 12 1 12 22
2
1
25
26
1 13 23
6 67
2 71
2 62
2 63
100
100
100
100
All values were obtained by Rietveld analysis/whole pattern fitting.
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Fig. 13. BSE images of pyrite (py) in the Test 4 residue sample. Cyan arrows show EDS analysis points in rimming material, confirming S removal with Ca and Si addition. Upper image includes an arsenopyrite (asp) grain. Right-hand pyrite in lower image has attached muscovite/illite (labeled) that shows no effect from chemical treatment. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
purging with an inert gas. In order to identify the difference for alkaline pyrite oxidation kinetics under these different conditions, the influence of N2 and O2 purging on pyrite oxidation was examined by chronoamperometry, and comparison of these results with those open to the atmosphere is made in Fig. 11. It can be noted that when being isolated from oxygen in the atmosphere by purging with nitrogen, the anodic current densities or the rates of pyrite oxidation at potentials of 0.258 V and 0.370 V are lower than those open to the atmosphere, and that when purging with oxygen the oxidation rates are the same as those open to the atmosphere. These results indicate that oxygen may participate in the surface reaction during pyrite oxidation and
that more oxygen does not affect the oxidation kinetics under the conditions studied. These phenomena need to be investigated in future research. 3. Alkaline pre-oxidation and gold leaching by cyanidation A refractory sulfidic-carbonaceous ore from NewmontÕs Twin Creek Mine (a representative for one of refractory gold ore) was used for pre-oxidation and gold leaching tests. The received sample was crushed to minus 10-mesh and split into 500-g charges. Prior to each test, the crushed sample was wetfine ground in two stage grinding mills. Particle size distribu-
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Fig. 14. BSE images of pyrite (py) in the Test 10 (lime 5 g/kg ore) residue sample. Cyan arrows show EDS analysis points in the surrounding material, confirming S removal with Ca and Si addition. The upper-most EDS spectrum shows evidence for a small amount of Cl. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
tion analysis indicated that the particle size of the ground sample was 100% minus 45 lm and 80% minus 20 lm. A sample of 250 g of the fine ground sample was used for each oxidation test. The oxidation tests were performed in a 1 L reactor at about 60 C, 30% solids slurry with 5 scfpm (standard cubic feet per minute) oxygen flow rate for 6 h. The agitation speed was controlled at 600 rpm. The slurry pH for the pre-oxidation was adjusted by addition of selected alkaline reagents, such as NaOH, Ca(OH)2 and Ca(OH)2/Na2CO3 (molar ratio 1:1). After the pre-oxidation test, a solution sample was taken for analysis and the oxidized slurry was transferred into a one-gallon glass bottle for gold leaching by cyanidation/CIL. Both leach residue and activated carbon were analyzed. Gold extraction was calculated based on mass balance, leach residue and feed gold assay. 3.1. Ore sample characteristics The Twin Creek ore is highly refractory due to both gold locking in pyrite matrix and preg-robbing activity. The mineral constituents of the ore were identified and deter-
mined by semiquantitative XRD analysis, which indicated that the ore is comprised of 6% pyrite, 2% anatase, 25% muscovite/illite and about 67% quartz. Fire assay and chemical analysis of the head sample are given in Table 1. The ore sample contains 2.5 g Au/ton ore. S-sulfide content is about 3%. The cyanide leachable gold assay of this ore (AuCN) is 0.39 ppm, which indicates that only 15.7% of the gold in the ore is cyanide leachable. The preg-robbing ability of the ore was tested. Results from samples spiked with gold solution and cyanide leaching returned a spike recovery only 62%, which indicates the ore has a moderate preg-robbing characteristic. The elevated organic carbon, 0.6%, may be one important factor causing preg-robbing. This matter needs to be investigated in future research. To find maximum gold recovery after complete sulfide oxidation, a series of hot nitric acid pretreatment tests were performed on the fine-ground samples and followed by cyanide/CIL. After the nitric acid pretreatment, the sulfide oxidation was over 99.2%. Even with this high sulfide oxidation, gold recoveries by cyanide/CIL only ranged from 78.2% to 83.7%. Results suggested that even with the oxi-
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Fig. 15. BSE image of pyrite (py) in the Test 10 (lime 5 g/kg ore) residue sample surrounded in this case by mineral fines. The pyrite appears to be disintegrating. Cyan arrows show EDS analysis points. The upper-most spectrum indicates mostly quartz with a trace amount of muscovite/illite. The other spectra show higher quantities of Ca and Fe. The S contents at the analysis points are less than the limit of detection (0.4%). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
dation pretreatment, the maximum achievable gold recovery would only be about 80% due to the preg-robbing activity of the ore sample. 3.2. Alkaline cyanide/CIL tests for gold leaching An alkaline cyanide/CIL test was performed directly on the fine-ground samples without pre-oxidation at ambient temperature with 20 g/L carbon for 24 h and an initial sodium cyanide concentration of 1 g/L. The alkaline cyanide/CIL test result (Test 1) is given in Table 2. Table 2 also includes a pre-oxidation test followed by cyanide/CIL (Test 2). The pre-oxidation was carried out with oxygen at 60 C for 6 h without the addition of alkaline chemical reagent. Alkaline cyanide/CIL testing resulted in gold recovery of 20%. After aerated with oxygen for 6 h at the elevated temperature, the subsequent gold extraction by alkaline cyanide/CIL increased to 32%. 3.3. Comparison of alkaline pre-oxidation at pH 10 and pH 11 Results of alkaline pre-oxidation with selected alkaline reagents and gold leaching by cyanidation/CIL are listed
in Table 3. The alkaline reagents used were NaOH, Ca(OH)2 and a mixture of Na2CO3/Ca(OH)2 (molar ratio of 1:1). The pre-oxidation tests were examined at pH 10 and pH 11 for each reagent tested. It was intended to determine the pH effect on sulfide oxidation instead of minimizing the reagent dosage. The results in Table 3 suggest that gold extraction increases with an increase in the degree of sulfide oxidation and that the effect of pH on the pre-oxidation reaction is evident. Both sulfide oxidation and gold extraction are greater at pH 11 than at pH 10. The highest gold extraction is 63%, about 40% greater than the gold extraction without pre-oxidation though the ore has some prerobbing activity. Additionally, the cyanide consumption is significantly reduced after sulfide oxidation. 3.4. Effect of lime addition in grinding process The influence of lime addition during fine grinding on the extent of sulfide oxidation and gold extraction was examined. Test results, shown in Table 4, demonstrate that the lime addition obviously increases the extent of sulfide oxidation and the level of gold extraction. Under similar pre-oxidation condition, an additional 15% of gold extrac-
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Fig. 16. BSE image and EDS spectra from two pyrite (py) grains in the Test 12 (lime 10 g/kg ore) residue sample. Cyan arrows show analysis points. The upper-right pyrite grain contains arsenic (lower-right EDS spectrum). The darker gray along the right side of this grain may be quartz, with the iron and sulfur of the EDS spectrum from the nearby pyrite. The upper-most and lower-most EDS spectra are typical of most of the pyrite coatings observed during this study, showing increased iron and lower sulfur in comparison to pyrite, with the addition of calcium and silicon. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
tion is achieved with addition of 5 g/kg lime during fine grinding (compare Tests 8 and 10). It can be noted that the results of Tests 11 and 12 indicate that an excess of lime addition reduces sulfide oxidation as well as gold extraction. Further research is required to explain the phenomenon. The encapsulation of the sulfide particle in Tests 11 and 12 residues was observed by X-ray energy dispersion spectroscopy. The cyanide/CIL gold extractions from the residues after the alkaline pre-oxidation tests are illustrated in Fig. 12. Fig. 12 demonstrates that the alkaline oxidation is an effective pretreatment for this refractory sulfide ore. The pH effect of pre-oxidation slurry on sulfide oxidation and gold extraction is significant. Substantial increase in gold extraction with the pretreatment is realized when compared to direct cyanide/CIL without pre-oxidation. 4. Reaction products after pre-oxidation and cyanidation The reaction products of selected residues after alkaline pre-oxidation and cyanidation from Tests 4, 10, and 12 were examined by X-ray diffraction (XRD). Also, backscattered electron (BSE) images of the pyrite particles in
these residues were taken. Finally, reaction products and/ or coatings at the surface of pyrite particles were analyzed by X-ray energy dispersion (EDS) spectroscopy. Quantitative treatment of the XRD spectra was accomplished by Rietveld analysis and whole pattern fitting methods. The Rietveld analysis refined the crystal structures (lattice parameters, etc.) of the minerals using the powder diffraction data, beginning with close approximation from previous results. The whole pattern fitting adjusted mineral percentages until a best fit occurred between a calculated spectrum and the chemical analysis. The results determined by the quantitative analysis are presented in Table 5. According to the results presented in Table 5, it is obvious that the XRD analysis for the Twin Creek refractory gold ore head sample agrees quite well with the chemical analysis presented in Section 3.1. The level of sulfide oxidation from Tests 4, 10, and 12 determined by the XRD analysis in Table 5 matches roughly with the results of chemical analysis presented in Tables 3 and 4, in which the level of sulfide oxidation from Tests 4, 10, and 12 reaches about 70.0%. It is further confirmed with these results that gold extraction (up to 70%) by cyanidation mainly depends on the level of pyrite oxidation (up to 70%). Table 5 also
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Fig. 17. BSE image showing two pyrite grains (light gray, upper-left and lower-right) in the Test 12 (lime 10 g/kg ore) residue sample. Cyan arrows show EDS analysis points in coatings around pyrite. All of the coatings contain calcium and silicon, and higher Fe:S ratios than pyrite. Aluminum and potassium of the lower-right EDS spectrum indicates muscovite/illite. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
shows that the residues from tests where Ca(OH)2 was used contain 12–13% calcite in addition to the quartz, muscovite/illite, pyrite, and anatase. The pyrite particles and the reaction products at the surface of pyrite particles after the alkaline pre-oxidation and cyanidation were examined in these three residue samples by BSE images and EDS spectroscopy. Fig. 13 shows the BSE micrographs of pyrite particles and the EDS spectra of the reaction products at the surface of pyrite in the residue sample from Test 4. Figs. 14 and 15 show the results in the residue sample from Test 10, and Figs. 16 and 17 show the results in the residue sample from Test 12. These BSE images and EDS spectra clearly show that iron-, silicon-, and calcium-bearing compounds, with variable amounts of sulfur, commonly surround the pyrite particles. Some mineral fines were found in the product layer, such as arsenic, aluminum, and/or potassium-bearing minerals. Additionally, it can be noted that the EDS spectrum intensity for elemental calcium at the surface of pyrite particles is significantly greater for lime addition 10 g/kg ore (Figs. 16 and 17) than the calcium intensity for a lime addition is 5 g/kg ore (Figs. 14 and 15). This result supports the hypothesis that when lime is added in excess (10 g/kg) the
lime may physically encapsulate mineral particles, causing low levels of sulfide oxidation and low gold extraction. However, this conclusion needs to be confirmed by more evidences in future research. Based on the fact that gold extraction mainly depends on the extent of sulfides or pyrite oxidation, it appears that the product layers at pyrite surface are porous under the conditions studied. However, the results from Test 6 with the addition of lime (Ca(OH)2) show that the extent of pyrite oxidation and gold leaching are much less than those from Tests 4, 10 and 12. In this regard, the morphology of the coatings on pyrite after alkaline pre-oxidation under different conditions needs to be studied further in future research. 5. Summary and conclusions In this investigation, the kinetics of anodic pyrite oxidation in alkaline solutions was studied by electrochemical measurements, including chronoamperometry and linear sweep voltammetry. Based on these electrochemical measurements, some experiments were designed for alkaline pre-oxidation of Twin Creek refractory gold ore in which the major sulfide mineral was pyrite. Gold extraction from
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the pre-oxidized auriferous pyrite ore was accomplished by cyanidation. From the electrochemical measurements, it was found that the rate of anodic pyrite oxidation increased with an increase in oxidation potential, pH, and temperature. However, when temperature was above 45 C and pH was above 11, the pH effect from pH 11 to pH 12 was not significant. The alkaline reagents, NaOH, NH4OH, Na2CO3, and Ca(OH)2, had a similar effect on the oxidation rate under the conditions studied. The reaction kinetics for pyrite oxidation were found to be under surface reaction control or under mixed control, which was supported by the independence on stirring speed and by the relatively high apparent activation energy of 8.20 kcal/mol (or 34.5 kJ/ mol). Purging with nitrogen reduced the oxidation kinetics and Purging with oxygen did not affect the oxidation rate when compared with those open to the atmosphere. The results of alkaline pre-oxidation and gold leaching by cyanidation suggested that the alkaline pre-oxidation was an effective treatment for the refractory gold ore. Gold extraction after pre-oxidation under appropriate conditions increased from 20% of direct cyanide leaching to 70.0%. Cyanide consumption decreased from 2.5 kg/t ore for direct cyanide leaching to 1.5 kg/t ore with the pre-oxidation. It was found that gold extraction mainly dependent on the extent of sulfide oxidation under the conditions studied though the ore had a moderate pre-robbing activity. Sulfide oxidation increased with an increase in pH. Upon comparison of the alkaline reagents used, NaOH and Na2CO3/Ca(OH)2 were superior to Ca(OH)2 for the sulfide oxidation. Appropriate addition of lime (5 g/kg ore) during the fine grinding improved the sulfide oxidation (up to 70–80%). Additionally, the residues at the surface of pyrite particles after alkaline pre-oxidation and cyanidation examined with selected samples were by XRD, BSE images, and EDS spectroscopy. It was observed that an iron-, silicon-, and calcium-bearing compound, with variable amounts of sulfur, commonly surrounded pyrite grains. Mineral fines also were found in the product layer including arsenic, aluminum, and/or potassium-bearing minerals. At the same time, it was revealed that when the lime addition was 10 g/kg ore for the ultra-fine grinding the lime physically encapsulated pyrite grains, which caused low sulfide oxidation and low gold extraction. References Ahlberg, E., Forssberg, K.S.E., Wang, X., 1990. The surface oxidation of pyrite in alkaline solutions. Journal of Applied Electrochemistry 20, 1033–1039. Biegler, T., Rand, D.A.J., Woods, R., 1975. Oxygen reduction on sulfide minerals, Part I, Kinetics and mechanism at rotated pyrite electrodes. Journal of Electroanalytical Chemistry—Interfacial Electrochemistry 20, 151–162. Biegler, Y., Swift, D.A., 1979. Anodic behavior of pyrite in acid solutions. Electrochimica Acta 24, 415–420.
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Brion, D., 1980. Etude par spectroscopie de photoelectrons de la degradation superficielle de FeS2, CuFeS2, ZnS ET PbS a lÕair et dans lÕeae. Applied Surface Sciences 5, 133–152. Brown, A.B., Jurinak, J.J., 1989. Mechanism of pyrite oxidation in aqueous mixtures. Journal of Environmental Quality 18, 545–550. Buckley, A.N., Woods, R., 1985. X-ray photoelectron spectroscopy of oxidized pyrrhotite surfaces. Applications of Surface Science 22/23, 280–287. Buckley, A.N., Woods, R., 1987. The surface oxidation of pyrite. Applied Surface Science 27, 437–452. Buckley, A.N., Hamilton, I.C., Woods, R., 1988. In: Richardson, P.E., Woods, R. (Eds.), Proceedings of International Symposium, Electrochemistry in Mining and Metal Processing II. The Electrochemistry Society, Pennington, New York, p. 234. Caldeira, C.L., Ciminelli, V.S.T., Dias, A., Osseo-Asare, K., 2003. Pyrite oxidation in alkaline solutions: nature of the product layer. International Journal of Mineral Processing 72, 373–386. Chander, S., Briceno, A., 1987. Kinetics of pyrite oxidation. Minerals and Metal Processing 4, 171–176. Ciminelli, V.S.T., Osseo-Asare, K., 1995a. Kinetics of pyrite oxidation in sodium carbonate solutions. Metallurgical and Materials Transactions, B 26, 209–218. Ciminelli, V.S.T., Osseo-Asare, K., 1995b. Kinetics of pyrite oxidation in sodium hydroxide solutions. Metallurgical and Materials Transactions, B 26, 677–685. Envangelou, V.P., Seta, A.K., Holt, A., 1998. Potential role of bicarbonate during pyrite oxidation. Environmental Science and Technology 32 (12), 2084–2091. Goldhaber, M.B., 1983. Experimental study of metastable sulfur oxyanion formation during pyrite oxidation at pH 6–9 and 30 C. American Journal of Science 283 (3), 193–217. Guay, W.J., 1981a. Gold and silver leaching. In: Schlitt, W.J. et al. (Eds.), Recovery and Economics. AIME, New York, pp. 17–22. Guay, W.J., 1981b. How carlin treats gold ore by double oxidation. World Mining 33 (3), 47–49. Hamilton, I.C., Woods, R., 1981. An investigation of surface oxidation of pyrite and pyrrhotite by linear potential sweep voltammetry. Journal of Electroanalytical Chemistry 118, 327–343. Hayes, R.A., Price, D.M., Ralston, J., Smith, R.W., 1987. Collectorless flotation of sulfide minerals. Mineral Processing and Extractive Metallurgy Review 2, 203–234. Kathe, S., Szargan, R., Suoninen, E., 1993. Oxidation of pyrite surface: a photoelectron spectroscopic study. Applied Science Surface 72, 157–170. Lowson, R.T., 1982. Aqueous oxidation of pyrite by molecular oxygen. Chemical Review 82, 461–497. Nicholson, R.V., Gillham, R.W., Reardon, E.J., 1988. Pyrite oxidation in carbonate-buffered solutions: 1. Experimental kinetics. Geochimica et Cosmochimica Acta 52, 1077–1085. Nicholson, R.V., Gillham, R.W., Reardon, E.J., 1990. Pyrite oxidation in carbonate-buffered solutions: 2. Rate control by oxide coatings. Geochimica et Cosmochimica Acta 54, 395–402. Peters, E., 1976. The electrochemistry of sulfide minerals. In: Bockris, J.OÕM., Rand, D.A.J., Welch, B.J. (Eds.), Trends in Electrochemistry. Plenum Press, New York and London, pp. 267–290. Peters, E., Majima, H., 1968. Electrochemical reaction of pyrite in acid perchlorate solutions. Canadian Metallurgical Quarterly 7 (3), 111– 117. Sato, M., 1960. Oxidation of sulfide ore bodies, II. Oxidation mechanism of sulfide minerals at 25 C. Economic Geology 55, 1202–1231. Souza, A.V., Ciminelli, V.S.T., 1992. Alkaline pressure oxidation for treatment of refractory gold concentrate. In: Ciminelli, V.S.T., Salum, M.J.G. (Eds.), Gold Extraction: Fundamentals, Practice and Environment. ABTM, Belo Horizonte, Brazil, pp. 177–196. Warren, I.H., 1956. The generation of sulphuric acid from pyrite by pressure leaching. Australian Journal of Applied Science 7, 346–358. Wheelock, T.D., 1981. Oxydesulfurization of coal in alkaline-solutions. Chemical Engineering Communications 12 (1–3), 137–159.
The influence of pyrite pre-oxidation on gold recovery by cyanidation J. Li a, B. Dabrowski a, J.D. Miller a
a,*
, S. Acar b, M. Dietrich b, K.M. LeVier b, R.Y. Wan
c
Department of Metallurgical Engineering, University of Utah, 135 S. 1460 E. Room 412, Salt Lake City, UT 84112-0114, United States b Newmont Mining Co., 10101 E. Dry Creek Road, Englewood, CO 80112, United States c Metallurgy Consultant, 9634 Kalamere Court, Highlands Ranch, CO 80126, United States Received 11 April 2005; accepted 18 September 2005 Available online 17 November 2005
Abstract The influence of pyrite pre-oxidation in alkaline solutions on gold recovery by cyanidation from Twin Creek refractory gold ore in which pyrite was identified as the major sulfide mineral has been investigated with the aid of electrochemical measurements, leaching experiments, and direct analysis of reaction products for selected residues. It was found that gold recovery by cyanidation in bottle roll experiments mainly depended on the extent of pyrite pre-oxidation. The rate of pyrite oxidation in alkaline solutions measured by electrochemical measurements, including chronoamperometry and linear sweep voltammetry, increased with an increase in pH, potential, and temperature. All alkaline reagents used for the electrochemical measurements, NaOH, NH4OH, Na2CO3 and Ca(OH)2, showed a similar effect on pyrite oxidation kinetics. However, the results of alkaline pre-oxidation for pyrite of the Twin Creek refractory gold ore suggested that NaOH and Na2CO3/Ca(OH)2 were superior to Ca(OH)2. Without pre-oxidation, cyanide leachable gold was found to be only 20% which could be increased to 70% under appropriate pre-oxidation conditions. At the same time, cyanide consumption decreased from 2.5 kg/t ore to 1.5 kg/t ore. Selected residues after pre-oxidation and cyanidation were examined by X-ray diffraction. Backscattered electron images of pyrite particles in these residues were taken. The reaction products at the surface of pyrite particles were found to be iron-, silicon-, and calcium-bearing compounds with variable amounts of sulfur as determined by X-ray energy dispersion analysis. Additionally, some mineral fines, such as aluminum and/or potassium-bearing minerals, were found to be present at the partially oxidized pyrite surface. 2005 Elsevier Ltd. All rights reserved. Keywords: Sulfide ores; Gold ores; Hydrometallurgy; Oxidation; Cyanidation
1. Introduction With depletion of the oxidized free-milling gold reserves close to the earthÕs surface, most of the important new deposits being mined today do not respond to direct cyanidation. It is found that the gold is very finely disseminated and encapsulated in host matrices that are inert and/or impermeable to the cyanide solution. In this case, it is necessary to first break down the host matrices and then it is possible to leach the liberated gold. In many cases, the host matrices are sulfide minerals, which exhibit a strong association with finely disseminated gold particles in many ore *
Corresponding author. Tel.: +1 801 581 5160; fax: +1 801 581 4937. E-mail addresses: [email protected] (J. Li), [email protected] (J.D. Miller). 0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.09.052
bodies. Of these sulfides, pyrite is one of the most important minerals. Therefore, a knowledge of pyrite leaching behavior is very important for the development of improved technology for the treatment of refractory gold ores. The electrochemistry of pyrite and the kinetics of pyrite oxidation in different systems have been extensively studied. The anodic dissolution behavior of pyrite has been investigated by various techniques (Brion, 1980; Hamilton and Woods, 1981; Buckley and Woods, 1987; Buckley et al., 1988; Ahlberg et al., 1990). It was found that pyrite is the most noble sulfide mineral and a good electrocatalyst for oxygen reduction (Biegler et al., 1975) and for hydrogen evolution (Peters, 1976). This property can affect the behavior of other sulfide minerals through galvanic coupling, in which pyrite serves as a cathode and thus enhances the oxidation of other sulfide minerals, as might occur
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during pre-oxidation processes for exposure of gold particles from sulfides matrices. For pressure oxidation in alkaline solutions, the rate of pyrite oxidation was found to be surface-reaction controlled (Lowson, 1982; Brown and Jurinak, 1989; Ciminelli and Osseo-Asare, 1995a,b). Some authors observed that the oxidation rate increases with an increase of pH, reagent concentration, and temperature (Brown and Jurinak, 1989; Nicholson et al., 1988, 1990; Ciminelli and Osseo-Asare, 1995a,b; Envangelou et al., 1998). With more extensive alkaline oxidation of pyrite, higher gold recovery is generally observed (Souza and Ciminelli, 1992). The kinetics of pyrite oxidation in alkaline solutions have been found to be affected by the nature of the reagents used to control the pH of the systems. Among the reagents, sodium carbonate/bicarbonate was claimed to be particularly effective in enhancing pyrite decomposition in aerated solutions (Wheelock, 1981; Guay, 1981a,b; Souza and Ciminelli, 1992). However, there still is some controversy regarding this position. For instance, Warren (1956) found that the addition of small amount of calcium carbonate for pH adjustment caused a drastic reduction in the pyrite decomposition rate. Warren (1956) explained the reduction in rate as a consequence of the formation of a growing iron-oxide film on pyrite particles, the oxidation rate then becoming controlled by diffusion in the product layer. In contrast, Guay (1981a,b) suggested that sodium carbonate enhanced the rate of pyrite oxidation by favoring the removal of the iron-oxide coating, thus increasing the exposure of fresh surfaces to oxidation. Caldeira et al. (2003) recently reported the product layer constituents and morphology from pyrite oxidation by molecular oxygen in alkaline solutions. In sodium hydroxide solutions, hematite was the main phase identified in the product layer. In carbonate solutions, the main constituent was ferrihydrite. In calcium hydroxide solutions, only calcium carbonate was detected on the surface of oxidized pyrite in an amount that increased when the system was opened to the atmosphere. The morphology of the product layers demonstrated that in NaOH solutions the pyrite particles were initially covered by a thin oxide layer and then the oxide layer fractured after longer reaction times. Pyrite oxidation in Na2CO3/NaHCO3 solutions resulted in particles that were initially covered by a discontinuous oxide coating that grew with reaction time, thus increasing the overall pyrite surface coverage. Upon review of the literature, the kinetics and products of pyrite decomposition in alkaline solutions under oxidative conditions are dependent on many variables, such as oxidation potential, temperature, solution composition, whether the system is open to the atmosphere, etc. (Buckley and Woods, 1987; Buckley et al., 1988; Ahlberg et al., 1990; Kathe et al., 1993; Caldeira et al., 2003). Generally, it is accepted that the oxidation of pyrite is described by the following overall reactions, in which the ferric ion precipitates on the pyrite surface or reports to the solution in suspension as ferric hydroxide
FeS2 + 3H2 O = Fe(OH)3 + 2S0 + 3Hþ + 3e or FeS2 + 3OH = Fe(OH)3 + 2S0 + 3e
ð1Þ ð2Þ
and at higher potentials þ FeS2 þ 11H2 O ¼ FeðOHÞ3 þ 2SO2 4 þ 19H þ 15e
ð3Þ
or FeS2 þ 19OH ¼ FeðOHÞ3 þ 2SO2 4 þ 8H2 O þ 15e
ð4Þ
Besides the aforementioned phenomena, other issues have also been considered: (1) whether elemental sulfur is involved as an intermediate during formation of sulfate; and (2) which component of pyrite is preferentially oxidized (Sato, 1960; Peters and Majima, 1968; Biegler and Swift, 1979; Hamilton and Woods, 1981; Goldhaber, 1983; Buckley and Woods, 1985; Buckley and Woods, 1987; Chander and Briceno, 1987; Kathe et al., 1993). It is due to the system complexity that a detailed mechanism of pyrite oxidation has not been well established (Hayes et al., 1987; Ahlberg et al., 1990; Caldeira et al., 2003). Consequently, in the current research, the behavior of pyrite oxidation in alkaline solutions was investigated by electrochemical measurements under conditions for industrial applications at ambient pressure. The variables tested, in experiments open to the atmosphere, included pH, potential, temperature, and alkaline reagent. Based on these fundamental results as a point of reference, other experiments were initiated for alkaline pre-oxidation of the Twin Creek refractory gold ore, and to determine the influence of pre-oxidation conditions on sulfide oxidation and gold recovery by subsequent cyanidation from the oxidized residues. In addition, characteristics of the solid products after alkaline pre-oxidation and cyanidation for gold recovery were made for selected samples by X-ray diffraction (XRD), backscattered electron (BSE) images, and X-ray energy dispersion (EDS) analysis. 2. Electrochemical experiments Pyrite is a semiconductor and known to support electrochemical reactions. Therefore, some electrochemical measurement techniques can be utilized to determine the rate of anodic pyrite oxidation. In this respect, it was decided to do a systematic electrochemical research on the kinetics of pyrite oxidation in alkaline solutions in order to provide supporting information for the alkaline pre-oxidation experiments. The electrochemical techniques used in the current research included chronoamperometry and linear sweep voltammetry. The rate of anodic pyrite oxidation in alkaline solutions under certain conditions and the influence of important factors on the rate were determined by electrochemical measurements. The experimental methods and results are presented in the following sections. 2.1. Pyrite electrode preparation A natural massive specimen of pyrite of unknown source was provided by Newmont Mining Corporation.
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Fig. 1. Effect of applied potential (EAP) on the rate of pyrite oxidation by chronoamperometry at pH 11 and ambient temperature with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm), and different alkaline reagents: NaOH, NH4OH, Na2CO3, and Ca(OH)2.
The massive pyrite specimen was cut into a parallelepiped with dimensions of 14 · 13 · 8 mm and then attached to a copper wire (2.0 mm in diameter) using silver conducting glue. The pyrite electrode was molded in epoxy, exposing one side to the solution. 2.2. Electrochemical procedures Before each experiment, the pyrite electrode was ground with silicon carbide papers (800 and 2400 mesh) successively, rinsed with ethanol and deionized water, respectively. The electrode was immediately put into the test solution in an electrochemical cell. During the experiment, the solution was open to the atmosphere. The volume of the test solution was 250 mL. The pH was adjusted by addition of concentrated solutions of different alkaline chemicals. The potential was measured and referenced against the saturated calomel electrode (SCE), E0SCE ¼ 0:242 V vs. SHE. Most experiments were performed at ambient temperature (24 C) with the stationary pyrite electrode, solution being stirred (500 rpm) with a magnetic bar (8.0 mm in diameter and 38.0 mm in length). Sodium sulfate was added as an indifferent electrolyte
(0.2 M). Variations from these conditions can be noted in the text and in the figure captions. A typical three-electrode system was employed for these measurements. The pyrite electrode was used as the working electrode, a graphite rod was used as the counter electrode, and the SCE was used as the reference electrode. Potential measurements between the working electrode and the reference electrode were made through a Luggin capillary. The current densities for anodic pyrite oxidation or decomposition were obtained with an EG&G Princeton Applied Research, Potential-Galvanostat (Model 273). 2.3. Experimental results and discussion The current density determined by chronoamperometry for pyrite oxidation under specific conditions in this paper is the average of anodic current densities within a certain experimental time, for example, after 5 min, during which time the current density reaches a relatively stable level. When the anodic current densities are available, the following equation can be used to calculate the rates (mol/ cm2 min) of pyrite oxidation from the corresponding anodic current densities.
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Fig. 2. Effect of pH on the rate of pyrite oxidation by chronoamperometry at applied potential (EAP) 0.258 V (SCE), and ambient temperature with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm), and different alkaline reagents: NaOH and Na2CO3.
Ia 60 ðs= minÞ ðA=cm2 Þ 96 490 ðC=molÞ na 1000 where R is the rate of pyrite oxidation (mol/min cm2), Ia is the anodic current density of pyrite oxidation (mA/cm2) and na is the number of electrons transferred for pyrite oxidation. However, it is expected that the products from anodic pyrite oxidation vary, for example, ferrous ion or ferric ion and elemental sulfur or sulfate etc., depending on experimental conditions. Because of the different reactions that can occur, the na value cannot be defined unambiguously. For this case, the anodic current densities for pyrite decomposition will be used as an indication of the rate for pyrite oxidation in order to compare the influence of some important variables on pyrite oxidation rate. R¼
2.3.1. Stirring speed In order to investigate the reaction characteristics for anodic pyrite oxidation, it is necessary to examine many different variables, including stirring speed. The influence of magnetically stirring speed on the anodic current density for pyrite oxidation was examined at potential 0.258 V (SCE) and pH 11 (Ca(OH)2). It was found that when the
Fig. 3. Effect of pH on the rate of pyrite oxidation by chronoamperometry at applied potentials (EAP) 0.258 V and 0.370 V (SCE), and ambient temperature with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm), and alkaline reagent Ca(OH)2.
Fig. 4. Examination of pH (Ca(OH)2) effect on the rate of pyrite oxidation by linear sweep voltammetry at ambient temperature and potential scanning rate 2 mV/s with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm), and alkaline reagent Ca(OH)2.
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solution was stationary, the current density was much lower than that at the stirring speed of 250 rpm; when the stirring speed was above 250 rpm, the current density was found to be almost independent of the stirring speed. These results indicate that when the stirring speed is above 250 rpm the reaction rate for pyrite oxidation is no longer limited by aqueous phase transport processes. Thus, the speed was kept at 500 rpm for all of the following experiments. 2.3.2. Effect of potential Examination of the influence of the oxidation potential on the rate of pyrite oxidation was made by chronoamperometry at the potentials of 0.258 V and 0.370 V (SCE) with different alkaline reagents to adjust the solution pH, including sodium hydroxide, ammonium hydroxide, sodium carbonate, and calcium hydroxide. These results are presented in Fig. 1, which clearly demonstrate that the rate of anodic pyrite oxidation is significant at these potentials. The rate at 0.370 V is much higher than at 0.258 V. A similar phenomenon can be observed from the results determined by linear sweep voltammetry presented in Fig. 4. For example, it is evident that the oxidation rate increases with an increase in the oxidation potential at each pH level. 2.3.3. Effect of pH The effect of pH on the rate of pyrite oxidation using different alkaline reagents for adjustment of the solution pH was measured by different electrochemical measurements. The results determined by chronoamperometry are shown in Fig. 2 using NaOH and Na2CO3 at potential of 0.258 V, and in Fig. 3 using Ca(OH)2 at potentials of 0.258 V and 0.370 V. The results determined by linear sweep voltammetry are shown in Fig. 4 using Ca(OH)2. These results suggest that the oxidation rate increases with
Fig. 5. Reaction order determination by plot of the logarithm of anodic current density (Ia) vs. pH. Data taken from Fig. 3 at potentials of 0.258 V and 0.370 V (SCE).
887
an increase in pH, which is expected by examination of the reactions (1)–(4) for pyrite oxidation and is similar to the results reported by other authors (Brown and Jurinak, 1989; Nicholson et al., 1988, 1990; Ciminelli and OsseoAsare, 1995a,b; Envangelou et al., 1998). Plots of the logarithm of anodic current density vs. pH for pyrite oxidation at potentials of 0.258 V and 0.370 V are presented in Fig. 5. The data are taken from Fig. 3. The slopes of 0.584 for 0.258 V and 0.586 for 0.370 V presented in Fig. 5 demonstrate that the oxidation rate is approximately half order with respect to hydroxide ion concentration. The half order may indicate that the reaction for anodic pyrite decomposition or pyrite oxidation in alkaline solution would be a multiple-step process, which needs to be further studied in future research. In addition, the close values of the slopes indicate that the oxidation
Fig. 6. Examination of chemical effect on the rate of pyrite oxidation by chronoamperometry at applied potentials (EAP) of 0.258 V and 0.370 V (SCE), pH 11, and ambient temperature with a stationary pyrite electrode and magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
Fig. 7. Voltammogram for pH effect on the rate of pyrite oxidation using different alkaline reagents at ambient temperature and potential scanning rate 2 mV/s with a stationary pyrite electrode and magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
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reaction at the different potentials may have the same ratelimiting step and/or produce the same products under these circumstances. 2.3.4. Alkaline reagents It can be noted from review of the literature that the kinetics of pyrite oxidation in alkaline solutions are dependent not only upon pH, potential and whether the system is open to the atmosphere, but also upon the solution composition or reagents used to adjust the solution pH (Lowson, 1982; Brown and Jurinak, 1989; Nicholson et al., 1990; Ciminelli and Osseo-Asare, 1995a,b; Caldeira et al., 2003). In the current research, the influence of different alkaline reagents on pyrite oxidation kinetics, including sodium hydroxide, ammonium hydroxide, sodium carbonate, and calcium hydroxide, was examined by chronoamperometry
Fig. 8. Effect of temperature on the rate of pyrite oxidation by chronoamperometry at applied potential (EAP) of 0.258 V (SCE), and pH 11 (NaOH or Na2CO3) with a stationary pyrite electrode, magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
Fig. 10. Effect of pH on the rate of pyrite oxidation by chronoamperometry at elevated temperature (45 C and 65 C), and applied potential 0.258 V (SCE) with a stationary pyrite electrode and magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
Fig. 11. Influence of purging N2 on the rate of pyrite oxidation by chronoamperometry at ambient temperature, applied potentials (EAP) of 0.258 V and 0.370 V (SCE), and pH 11 (Na2CO3) with a stationary pyrite electrode and magnetically stirred solution (0.2 M Na2SO4, 500 rpm).
Fig. 9. Arrhenius plot of the natural logarithm of anodic current density (Ia) for pyrite oxidation vs. 1/T. Data taken from Fig. 8.
at potentials of 0.258 V and 0.370 V (Fig. 6) and by linear sweep voltammetry at pH values of 10, 11 and 12 (Fig. 7).
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These results suggest that the rate of pyrite oxidation under different conditions is independent of the alkaline reagents used under the conditions studied. However, it should be pointed out that, in the current research, the anodic current densities for pyrite decomposition as determined by these electrochemical measurements only represent the initial rates of pyrite oxidation. The effect of these alkaline reagents needs to be further examined by actual pre-oxidation tests. Especially, the oxidation rate may be affected by the reaction products formed at the pyrite surface with different alkaline reagents in actual pre-oxidation systems. 2.3.5. Temperature The effect of temperature on the anodic current density for pyrite oxidation was examined by chronoamperometry in NaOH and Na2CO3 solutions at pH 11 and potential 0.258 V (SCE). The results are presented in Fig. 8. A logaTable 1 Chemical analysis of Twin Creek refractory ore Au (g/t)
S-total (%)
S-sulfate (%)
S-sulfide (%)
C-total (%)
C-org. (%)
Fe (%)
As (%)
2.49
3.45
0.39
3.06
0.60
0.61
4.52
0.20
889
rithm of the anodic current density is then plotted against reciprocal of the absolute temperature in Fig. 9. According to the Arrhenius theory (EAct/R = 4150), the apparent activation energy was found to be 8.20 kcal/mol (34.5 kJ/mol), which indicates that, under the conditions studied, the rate of pyrite oxidation is limited by surface reaction control or by mixed control. Additionally, the influence of pH on the kinetics of alkaline pyrite oxidation at elevated temperatures of 45 C and 65 C was investigated by chronoamperometry in sodium carbonate/sodium hydroxide solutions and the results are presented in Fig. 10. It can be seen from these results that the rate of pyrite oxidation significantly increases with an increase in pH value from pH 10 to pH 11, and that when the pH exceeds pH 11 the initial rate seems to become not significantly dependent of pH under the conditions studied. At the elevated temperatures and with the consideration of alkaline reagent consumption, control at pH 11 is suitable for pyrite oxidation for an actual pre-oxidation process. 2.3.6. Purging with nitrogen In the current research, all solutions were open to the atmosphere. In most fundamental studies by other authors, the systems generally were isolated from the atmosphere by
Table 2 Summary of baseline cyanide/CIL test results Test
Sulfide oxidation (%)
Au in residue (g/t ore)
Au recovery (%)
NaCN consumption (kg/t ore)
1 2
6.2 14.4
2.0 1.68
19.9 32.4
2.5 2.7
Table 3 Alkaline pre-oxidation tests at 60 C for 6 h followed by cyanide/CIL Test
Reagent
pH
Sulfide oxidation (%)
Au extraction (%)
3 4 5 6 7
NaOH – Ca(OH)2 – Na2CO3 Ca(OH)2 –
10 11 10 11 10
26.1 67.6 17.3 28.4 23.8
38.4 63.1 32.9 37.0 41.1
14.6 62.7 18.2 41.1 48
1.6 1.2 1.2 1.3 1.3
11
67.9
54.4
147.5
0.7
8
Alkaline reagent (kg/t)
NaCN (kg/t)
Fig. 12. Effect of the pre-oxidation slurry pH on gold extraction from the oxidized residues by cyanide/CIL (Test 2 as a reference: gold extraction 32% for the pre-oxidation without addition of alkaline reagent).
Table 5 XRD analysis of Twin Creek refractory gold ore and pre-oxidation residues (%)a
Table 4 Effect of lime addition in grinding circuit Test
Lime in grinding (g/kg)
pH
Sulfide oxidation (%)
CIL Au Extraction (%)
9 10 11 12 13a 14a
5 5 10 10 5 5
10 11 10 11 10 11
49.6 79.6 37.1 71.8 48.4 88.0
62.9 71.4 47.6 63.3 62.8 69.8
Alkaline reagent used: Na2CO3/Ca(OH)2 (molar ratio 1:1). a Addition of Pb(NO3)2 in cyanide/CIL.
Mineral
Formula
Anatase Calcite Muscovite/ Illite Pyrite Quartz
TiO2 CaCO3 KAl2(AlSi3O10)(OH)2/ K0.7Al2(Si,Al) O10(OH)2 FeS2 SiO2
Total a
Head
Test 4
Test 10
Test 12 1 12 22
2
1
25
26
1 13 23
6 67
2 71
2 62
2 63
100
100
100
100
All values were obtained by Rietveld analysis/whole pattern fitting.
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Fig. 13. BSE images of pyrite (py) in the Test 4 residue sample. Cyan arrows show EDS analysis points in rimming material, confirming S removal with Ca and Si addition. Upper image includes an arsenopyrite (asp) grain. Right-hand pyrite in lower image has attached muscovite/illite (labeled) that shows no effect from chemical treatment. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
purging with an inert gas. In order to identify the difference for alkaline pyrite oxidation kinetics under these different conditions, the influence of N2 and O2 purging on pyrite oxidation was examined by chronoamperometry, and comparison of these results with those open to the atmosphere is made in Fig. 11. It can be noted that when being isolated from oxygen in the atmosphere by purging with nitrogen, the anodic current densities or the rates of pyrite oxidation at potentials of 0.258 V and 0.370 V are lower than those open to the atmosphere, and that when purging with oxygen the oxidation rates are the same as those open to the atmosphere. These results indicate that oxygen may participate in the surface reaction during pyrite oxidation and
that more oxygen does not affect the oxidation kinetics under the conditions studied. These phenomena need to be investigated in future research. 3. Alkaline pre-oxidation and gold leaching by cyanidation A refractory sulfidic-carbonaceous ore from NewmontÕs Twin Creek Mine (a representative for one of refractory gold ore) was used for pre-oxidation and gold leaching tests. The received sample was crushed to minus 10-mesh and split into 500-g charges. Prior to each test, the crushed sample was wetfine ground in two stage grinding mills. Particle size distribu-
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Fig. 14. BSE images of pyrite (py) in the Test 10 (lime 5 g/kg ore) residue sample. Cyan arrows show EDS analysis points in the surrounding material, confirming S removal with Ca and Si addition. The upper-most EDS spectrum shows evidence for a small amount of Cl. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
tion analysis indicated that the particle size of the ground sample was 100% minus 45 lm and 80% minus 20 lm. A sample of 250 g of the fine ground sample was used for each oxidation test. The oxidation tests were performed in a 1 L reactor at about 60 C, 30% solids slurry with 5 scfpm (standard cubic feet per minute) oxygen flow rate for 6 h. The agitation speed was controlled at 600 rpm. The slurry pH for the pre-oxidation was adjusted by addition of selected alkaline reagents, such as NaOH, Ca(OH)2 and Ca(OH)2/Na2CO3 (molar ratio 1:1). After the pre-oxidation test, a solution sample was taken for analysis and the oxidized slurry was transferred into a one-gallon glass bottle for gold leaching by cyanidation/CIL. Both leach residue and activated carbon were analyzed. Gold extraction was calculated based on mass balance, leach residue and feed gold assay. 3.1. Ore sample characteristics The Twin Creek ore is highly refractory due to both gold locking in pyrite matrix and preg-robbing activity. The mineral constituents of the ore were identified and deter-
mined by semiquantitative XRD analysis, which indicated that the ore is comprised of 6% pyrite, 2% anatase, 25% muscovite/illite and about 67% quartz. Fire assay and chemical analysis of the head sample are given in Table 1. The ore sample contains 2.5 g Au/ton ore. S-sulfide content is about 3%. The cyanide leachable gold assay of this ore (AuCN) is 0.39 ppm, which indicates that only 15.7% of the gold in the ore is cyanide leachable. The preg-robbing ability of the ore was tested. Results from samples spiked with gold solution and cyanide leaching returned a spike recovery only 62%, which indicates the ore has a moderate preg-robbing characteristic. The elevated organic carbon, 0.6%, may be one important factor causing preg-robbing. This matter needs to be investigated in future research. To find maximum gold recovery after complete sulfide oxidation, a series of hot nitric acid pretreatment tests were performed on the fine-ground samples and followed by cyanide/CIL. After the nitric acid pretreatment, the sulfide oxidation was over 99.2%. Even with this high sulfide oxidation, gold recoveries by cyanide/CIL only ranged from 78.2% to 83.7%. Results suggested that even with the oxi-
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Fig. 15. BSE image of pyrite (py) in the Test 10 (lime 5 g/kg ore) residue sample surrounded in this case by mineral fines. The pyrite appears to be disintegrating. Cyan arrows show EDS analysis points. The upper-most spectrum indicates mostly quartz with a trace amount of muscovite/illite. The other spectra show higher quantities of Ca and Fe. The S contents at the analysis points are less than the limit of detection (0.4%). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
dation pretreatment, the maximum achievable gold recovery would only be about 80% due to the preg-robbing activity of the ore sample. 3.2. Alkaline cyanide/CIL tests for gold leaching An alkaline cyanide/CIL test was performed directly on the fine-ground samples without pre-oxidation at ambient temperature with 20 g/L carbon for 24 h and an initial sodium cyanide concentration of 1 g/L. The alkaline cyanide/CIL test result (Test 1) is given in Table 2. Table 2 also includes a pre-oxidation test followed by cyanide/CIL (Test 2). The pre-oxidation was carried out with oxygen at 60 C for 6 h without the addition of alkaline chemical reagent. Alkaline cyanide/CIL testing resulted in gold recovery of 20%. After aerated with oxygen for 6 h at the elevated temperature, the subsequent gold extraction by alkaline cyanide/CIL increased to 32%. 3.3. Comparison of alkaline pre-oxidation at pH 10 and pH 11 Results of alkaline pre-oxidation with selected alkaline reagents and gold leaching by cyanidation/CIL are listed
in Table 3. The alkaline reagents used were NaOH, Ca(OH)2 and a mixture of Na2CO3/Ca(OH)2 (molar ratio of 1:1). The pre-oxidation tests were examined at pH 10 and pH 11 for each reagent tested. It was intended to determine the pH effect on sulfide oxidation instead of minimizing the reagent dosage. The results in Table 3 suggest that gold extraction increases with an increase in the degree of sulfide oxidation and that the effect of pH on the pre-oxidation reaction is evident. Both sulfide oxidation and gold extraction are greater at pH 11 than at pH 10. The highest gold extraction is 63%, about 40% greater than the gold extraction without pre-oxidation though the ore has some prerobbing activity. Additionally, the cyanide consumption is significantly reduced after sulfide oxidation. 3.4. Effect of lime addition in grinding process The influence of lime addition during fine grinding on the extent of sulfide oxidation and gold extraction was examined. Test results, shown in Table 4, demonstrate that the lime addition obviously increases the extent of sulfide oxidation and the level of gold extraction. Under similar pre-oxidation condition, an additional 15% of gold extrac-
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Fig. 16. BSE image and EDS spectra from two pyrite (py) grains in the Test 12 (lime 10 g/kg ore) residue sample. Cyan arrows show analysis points. The upper-right pyrite grain contains arsenic (lower-right EDS spectrum). The darker gray along the right side of this grain may be quartz, with the iron and sulfur of the EDS spectrum from the nearby pyrite. The upper-most and lower-most EDS spectra are typical of most of the pyrite coatings observed during this study, showing increased iron and lower sulfur in comparison to pyrite, with the addition of calcium and silicon. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
tion is achieved with addition of 5 g/kg lime during fine grinding (compare Tests 8 and 10). It can be noted that the results of Tests 11 and 12 indicate that an excess of lime addition reduces sulfide oxidation as well as gold extraction. Further research is required to explain the phenomenon. The encapsulation of the sulfide particle in Tests 11 and 12 residues was observed by X-ray energy dispersion spectroscopy. The cyanide/CIL gold extractions from the residues after the alkaline pre-oxidation tests are illustrated in Fig. 12. Fig. 12 demonstrates that the alkaline oxidation is an effective pretreatment for this refractory sulfide ore. The pH effect of pre-oxidation slurry on sulfide oxidation and gold extraction is significant. Substantial increase in gold extraction with the pretreatment is realized when compared to direct cyanide/CIL without pre-oxidation. 4. Reaction products after pre-oxidation and cyanidation The reaction products of selected residues after alkaline pre-oxidation and cyanidation from Tests 4, 10, and 12 were examined by X-ray diffraction (XRD). Also, backscattered electron (BSE) images of the pyrite particles in
these residues were taken. Finally, reaction products and/ or coatings at the surface of pyrite particles were analyzed by X-ray energy dispersion (EDS) spectroscopy. Quantitative treatment of the XRD spectra was accomplished by Rietveld analysis and whole pattern fitting methods. The Rietveld analysis refined the crystal structures (lattice parameters, etc.) of the minerals using the powder diffraction data, beginning with close approximation from previous results. The whole pattern fitting adjusted mineral percentages until a best fit occurred between a calculated spectrum and the chemical analysis. The results determined by the quantitative analysis are presented in Table 5. According to the results presented in Table 5, it is obvious that the XRD analysis for the Twin Creek refractory gold ore head sample agrees quite well with the chemical analysis presented in Section 3.1. The level of sulfide oxidation from Tests 4, 10, and 12 determined by the XRD analysis in Table 5 matches roughly with the results of chemical analysis presented in Tables 3 and 4, in which the level of sulfide oxidation from Tests 4, 10, and 12 reaches about 70.0%. It is further confirmed with these results that gold extraction (up to 70%) by cyanidation mainly depends on the level of pyrite oxidation (up to 70%). Table 5 also
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Fig. 17. BSE image showing two pyrite grains (light gray, upper-left and lower-right) in the Test 12 (lime 10 g/kg ore) residue sample. Cyan arrows show EDS analysis points in coatings around pyrite. All of the coatings contain calcium and silicon, and higher Fe:S ratios than pyrite. Aluminum and potassium of the lower-right EDS spectrum indicates muscovite/illite. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
shows that the residues from tests where Ca(OH)2 was used contain 12–13% calcite in addition to the quartz, muscovite/illite, pyrite, and anatase. The pyrite particles and the reaction products at the surface of pyrite particles after the alkaline pre-oxidation and cyanidation were examined in these three residue samples by BSE images and EDS spectroscopy. Fig. 13 shows the BSE micrographs of pyrite particles and the EDS spectra of the reaction products at the surface of pyrite in the residue sample from Test 4. Figs. 14 and 15 show the results in the residue sample from Test 10, and Figs. 16 and 17 show the results in the residue sample from Test 12. These BSE images and EDS spectra clearly show that iron-, silicon-, and calcium-bearing compounds, with variable amounts of sulfur, commonly surround the pyrite particles. Some mineral fines were found in the product layer, such as arsenic, aluminum, and/or potassium-bearing minerals. Additionally, it can be noted that the EDS spectrum intensity for elemental calcium at the surface of pyrite particles is significantly greater for lime addition 10 g/kg ore (Figs. 16 and 17) than the calcium intensity for a lime addition is 5 g/kg ore (Figs. 14 and 15). This result supports the hypothesis that when lime is added in excess (10 g/kg) the
lime may physically encapsulate mineral particles, causing low levels of sulfide oxidation and low gold extraction. However, this conclusion needs to be confirmed by more evidences in future research. Based on the fact that gold extraction mainly depends on the extent of sulfides or pyrite oxidation, it appears that the product layers at pyrite surface are porous under the conditions studied. However, the results from Test 6 with the addition of lime (Ca(OH)2) show that the extent of pyrite oxidation and gold leaching are much less than those from Tests 4, 10 and 12. In this regard, the morphology of the coatings on pyrite after alkaline pre-oxidation under different conditions needs to be studied further in future research. 5. Summary and conclusions In this investigation, the kinetics of anodic pyrite oxidation in alkaline solutions was studied by electrochemical measurements, including chronoamperometry and linear sweep voltammetry. Based on these electrochemical measurements, some experiments were designed for alkaline pre-oxidation of Twin Creek refractory gold ore in which the major sulfide mineral was pyrite. Gold extraction from
J. Li et al. / Minerals Engineering 19 (2006) 883–895
the pre-oxidized auriferous pyrite ore was accomplished by cyanidation. From the electrochemical measurements, it was found that the rate of anodic pyrite oxidation increased with an increase in oxidation potential, pH, and temperature. However, when temperature was above 45 C and pH was above 11, the pH effect from pH 11 to pH 12 was not significant. The alkaline reagents, NaOH, NH4OH, Na2CO3, and Ca(OH)2, had a similar effect on the oxidation rate under the conditions studied. The reaction kinetics for pyrite oxidation were found to be under surface reaction control or under mixed control, which was supported by the independence on stirring speed and by the relatively high apparent activation energy of 8.20 kcal/mol (or 34.5 kJ/ mol). Purging with nitrogen reduced the oxidation kinetics and Purging with oxygen did not affect the oxidation rate when compared with those open to the atmosphere. The results of alkaline pre-oxidation and gold leaching by cyanidation suggested that the alkaline pre-oxidation was an effective treatment for the refractory gold ore. Gold extraction after pre-oxidation under appropriate conditions increased from 20% of direct cyanide leaching to 70.0%. Cyanide consumption decreased from 2.5 kg/t ore for direct cyanide leaching to 1.5 kg/t ore with the pre-oxidation. It was found that gold extraction mainly dependent on the extent of sulfide oxidation under the conditions studied though the ore had a moderate pre-robbing activity. Sulfide oxidation increased with an increase in pH. Upon comparison of the alkaline reagents used, NaOH and Na2CO3/Ca(OH)2 were superior to Ca(OH)2 for the sulfide oxidation. Appropriate addition of lime (5 g/kg ore) during the fine grinding improved the sulfide oxidation (up to 70–80%). Additionally, the residues at the surface of pyrite particles after alkaline pre-oxidation and cyanidation examined with selected samples were by XRD, BSE images, and EDS spectroscopy. It was observed that an iron-, silicon-, and calcium-bearing compound, with variable amounts of sulfur, commonly surrounded pyrite grains. Mineral fines also were found in the product layer including arsenic, aluminum, and/or potassium-bearing minerals. At the same time, it was revealed that when the lime addition was 10 g/kg ore for the ultra-fine grinding the lime physically encapsulated pyrite grains, which caused low sulfide oxidation and low gold extraction. References Ahlberg, E., Forssberg, K.S.E., Wang, X., 1990. The surface oxidation of pyrite in alkaline solutions. Journal of Applied Electrochemistry 20, 1033–1039. Biegler, T., Rand, D.A.J., Woods, R., 1975. Oxygen reduction on sulfide minerals, Part I, Kinetics and mechanism at rotated pyrite electrodes. Journal of Electroanalytical Chemistry—Interfacial Electrochemistry 20, 151–162. Biegler, Y., Swift, D.A., 1979. Anodic behavior of pyrite in acid solutions. Electrochimica Acta 24, 415–420.
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Brion, D., 1980. Etude par spectroscopie de photoelectrons de la degradation superficielle de FeS2, CuFeS2, ZnS ET PbS a lÕair et dans lÕeae. Applied Surface Sciences 5, 133–152. Brown, A.B., Jurinak, J.J., 1989. Mechanism of pyrite oxidation in aqueous mixtures. Journal of Environmental Quality 18, 545–550. Buckley, A.N., Woods, R., 1985. X-ray photoelectron spectroscopy of oxidized pyrrhotite surfaces. Applications of Surface Science 22/23, 280–287. Buckley, A.N., Woods, R., 1987. The surface oxidation of pyrite. Applied Surface Science 27, 437–452. Buckley, A.N., Hamilton, I.C., Woods, R., 1988. In: Richardson, P.E., Woods, R. (Eds.), Proceedings of International Symposium, Electrochemistry in Mining and Metal Processing II. The Electrochemistry Society, Pennington, New York, p. 234. Caldeira, C.L., Ciminelli, V.S.T., Dias, A., Osseo-Asare, K., 2003. Pyrite oxidation in alkaline solutions: nature of the product layer. International Journal of Mineral Processing 72, 373–386. Chander, S., Briceno, A., 1987. Kinetics of pyrite oxidation. Minerals and Metal Processing 4, 171–176. Ciminelli, V.S.T., Osseo-Asare, K., 1995a. Kinetics of pyrite oxidation in sodium carbonate solutions. Metallurgical and Materials Transactions, B 26, 209–218. Ciminelli, V.S.T., Osseo-Asare, K., 1995b. Kinetics of pyrite oxidation in sodium hydroxide solutions. Metallurgical and Materials Transactions, B 26, 677–685. Envangelou, V.P., Seta, A.K., Holt, A., 1998. Potential role of bicarbonate during pyrite oxidation. Environmental Science and Technology 32 (12), 2084–2091. Goldhaber, M.B., 1983. Experimental study of metastable sulfur oxyanion formation during pyrite oxidation at pH 6–9 and 30 C. American Journal of Science 283 (3), 193–217. Guay, W.J., 1981a. Gold and silver leaching. In: Schlitt, W.J. et al. (Eds.), Recovery and Economics. AIME, New York, pp. 17–22. Guay, W.J., 1981b. How carlin treats gold ore by double oxidation. World Mining 33 (3), 47–49. Hamilton, I.C., Woods, R., 1981. An investigation of surface oxidation of pyrite and pyrrhotite by linear potential sweep voltammetry. Journal of Electroanalytical Chemistry 118, 327–343. Hayes, R.A., Price, D.M., Ralston, J., Smith, R.W., 1987. Collectorless flotation of sulfide minerals. Mineral Processing and Extractive Metallurgy Review 2, 203–234. Kathe, S., Szargan, R., Suoninen, E., 1993. Oxidation of pyrite surface: a photoelectron spectroscopic study. Applied Science Surface 72, 157–170. Lowson, R.T., 1982. Aqueous oxidation of pyrite by molecular oxygen. Chemical Review 82, 461–497. Nicholson, R.V., Gillham, R.W., Reardon, E.J., 1988. Pyrite oxidation in carbonate-buffered solutions: 1. Experimental kinetics. Geochimica et Cosmochimica Acta 52, 1077–1085. Nicholson, R.V., Gillham, R.W., Reardon, E.J., 1990. Pyrite oxidation in carbonate-buffered solutions: 2. Rate control by oxide coatings. Geochimica et Cosmochimica Acta 54, 395–402. Peters, E., 1976. The electrochemistry of sulfide minerals. In: Bockris, J.OÕM., Rand, D.A.J., Welch, B.J. (Eds.), Trends in Electrochemistry. Plenum Press, New York and London, pp. 267–290. Peters, E., Majima, H., 1968. Electrochemical reaction of pyrite in acid perchlorate solutions. Canadian Metallurgical Quarterly 7 (3), 111– 117. Sato, M., 1960. Oxidation of sulfide ore bodies, II. Oxidation mechanism of sulfide minerals at 25 C. Economic Geology 55, 1202–1231. Souza, A.V., Ciminelli, V.S.T., 1992. Alkaline pressure oxidation for treatment of refractory gold concentrate. In: Ciminelli, V.S.T., Salum, M.J.G. (Eds.), Gold Extraction: Fundamentals, Practice and Environment. ABTM, Belo Horizonte, Brazil, pp. 177–196. Warren, I.H., 1956. The generation of sulphuric acid from pyrite by pressure leaching. Australian Journal of Applied Science 7, 346–358. Wheelock, T.D., 1981. Oxydesulfurization of coal in alkaline-solutions. Chemical Engineering Communications 12 (1–3), 137–159.