- Email: [email protected]
Gold losses by cementation and thermal reduction in the gold recovery circuits
International Journal of Mineral Processing 98 (2011) 24–29
Contents lists available at ScienceDirect
International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / i j m i n p r o
Gold losses by cementation and thermal reduction in the gold recovery circuits B. Klein a,⁎, N.E. Altun b a b
Norman B. Keevil Institute of Mining Engineering, University of British Columbia, 6350 Stores Road, Vancouver, BC, Canada, V6T 1Z4 Muğla University, Department of Mining Engineering, 48000, Muğla, Turkey
a r t i c l e
i n f o
Article history: Received 21 July 2010 Received in revised form 6 September 2010 Accepted 17 September 2010 Available online 1 October 2010 Keywords: Gold Carbon-in-pulp cyanidation Cementation Thermal reduction Heterocoagulation
a b s t r a c t In this study, gold losses in a carbon-in-pulp (CIP) cyanidation gold recovery process and potential sources of these losses were investigated. Gold was found in samples from different streams through the CIP-cyanidation process, pointing to incidental losses. Mineralogical studies showed that gold losses occurred in two main forms, either as attached to larger entities or in the form of dendritic precipitates. SEM and EDS studies revealed that iron bearing minerals acted as the major media in cases when gold associations were observed as losses. The highly alkaline pH (≈13), elevated process temperature (≈145 °C), and high cyanide concentration (≥ 250 ppm) in the elution column along with a fine iron bearing material implied that gold attachment occurred through an electrochemical cementation mechanism. It was anticipated that the presence of iron in the process, which facilitated gold cementation, relied on the oxidative breakdown of the iron bearing minerals in the ore and/or due to the formation of porous iron oxides due to the roasting of iron sulfides in the regeneration kiln. In the elution column some part of the auro–cyanide complexes would remain non-eluted and be discharged into the carbon generation kiln and the carbon generation kiln was another section promoting gold losses. The high temperature condition in the carbon regeneration kiln (N 500 °C) caused thermal reduction of the non-eluted auro–cyanide complexes to metallic gold, leading to the formation of dendritic gold precipitates and their eventual loss. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the carbon-in-pulp cyanidation process to extract and recover gold, several known mineralogical factors contribute to incomplete recovery. Well established practice is applied to reduce or eliminate the refractoriness of ores. While there are numerous publications describing approaches to address the leaching process relatively little information exists on potential losses resulting from stripping gold from carbon and recovery in subsequent electrowinning. Losses during recovery may occur due to physical, physicochemical and/or electrochemical interaction of gold with other metallic and nonmetallic minerals as well as the physical and chemical process conditions such as temperature and pH. Studies showed that mutual coagulation of colloidal gold and iron oxides would take place under suitable conditions (Enzweiler and Joekes, 1991, 1992) at various phases of the carbon-in-pulp (CIP) process, leading to iron-mediated gold losses. In addition, electrochemical reactions between gold and other metallic minerals in the solution can occur. These reactions are well-understood, particularly between gold and zinc, and have been used for the recovery of gold (Merrill–Crowe process). However, redox reactions between gold and other metallic minerals during the ⁎ Corresponding author. University of British Columbia, NBK Institute of Mining Engineering, 6350 Stores Road Vancouver, V6T 1Z4 BC, Canada. Tel.: + 1 604 822 3986; fax: + 1 607 822 5599. E-mail address: [email protected] (B. Klein). 0301-7516/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2010.09.001
CIP process and subsequent steps are undesirable as they would result in the release of gold from the auro–cyanide complex and eventually its loss (Kenna et al., 1990; Dasovich et al., 1999). High process temperature also results in the release of gold from the auro–cyanide complex (Adams et al., 1995) leading to its loss through the process. This paper will examine possible losses of gold in a gold recovery circuit and identify potential sources for these losses and related factors. Samples from various streams of the recovery process were analyzed to identify the existence of free gold particles or gold associations that were likely to report as losses. The objective was to identify the mechanisms/reactions that led to the deficiencies during extraction and recovery of gold and locations of such reactions.
2. Materials and methods For the study the gold recovery process of the Mussel-White Mine, (previously Placer Dome) in Northwestern Ontario was investigated. A simplified flowsheet of the process is shown in Fig. 1. The process consists of conventional gravity separation, cyanide leaching and carbon-in-pulp (CIP) recovery of gold. The loaded carbon is eluted using the pressurized Zadra technique in an elution column at high pH, high temperature conditions with a hot aqueous solution containing sodium hydroxide and sodium cyanide. The solution from the elution column is passed to an electrowinning cell. The stripped carbon is sent to a dewatering screen and carbon
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
Gravity Concentration
Comminution
25
Concentrate
Tailings Cyanide Leaching
Carbon-in-pulp Process O/F CarbonElution Column
Stripped Carbon
Pregnant Solution Electrowinning
Dewatering Screen
O/F
Carbon Regeneration Kiln
Screen U/F
U/F Kiln
Carbon Fines Settling Tank
Gold
Fig. 1. Mussel-White gold recovery process.
regeneration kiln (650 °C). The gold sludge from electrowinning was pressure-washed, filtered, dried and melted into bullion-bars. The mineralogical composition of the Mussel-White ore is given in Table 1. The ore is described as a granurite-garnet-amphibole-chert iron formation. Gold is mainly associated with chalcopyrite, pyrite, pyrrhotite, arsenopyrite, magnetite, carbonates, hornblende and silicates. A hand magnet was placed in the overflow stream of the carbon fine settling tank and fine-magnetic particles were collected. It was found that the collected samples contained 2000 to 7000 g Au/ton, indicating losses of gold through the process. Samples from different points of the gold recovery process were collected to identify potential losses and investigate underlying mechanisms. The sampling points for the circuit are listed in Table 2. The collected samples were dried and subjected to mineralogical investigations using SEM (scanning electron microscopy) and EDS (energy dispersion spectrometer) analyses. Mineralogical investigations aimed at determining the occurrence, size and type of gold particles and associations in the samples.
3. Results and discussion Table 2 summarizes the results of the mineralogical analyses of the samples taken from different sections of the gold recovery circuit. The SEM and EDS analyses showed that gold losses occurred in two main forms. Attachment of gold with other minerals was common at various sections of the recovery process. Attached gold was identified on iron oxides, iron silicates and iron sulfides. In addition, gold in the form of dendritic precipitates was observed. In all cases, the size of gold varied from 2 to 40 μm. The presence of Ag was also found, but to an insignificant extent. The SEM and EDS studies clearly showed that iron bearing materials acted as the major media that facilitated gold cementation during the recovery process.
Table 1 The composition of Mussel-White run-off-mine ore. Mineral
Amount
Gold, g/t Silver, g/t Chalcopyrite, % Pyrite, % Arsenopyrite, % Pyrrhotite, % Magnetite, % Carbonates, % Hornblende, % Silicates, %
58.1 2635 0–1 0–2 0–2 1–15 1–15 5–10 10–15 40–55
Figs. 2–5 show the “cemented” gold entities attached onto iron bearing particles. Gold associations on iron sulphides (Figs. 2 and 3), iron silicates (Fig. 4) and iron oxides (Fig. 5) were identified in samples taken from the carbon fine settling tank (S-6), the filter fines of the carbon settling tank (S-7) and carbon fine tank overflow that had been upgraded by panning (S-4) (Table 2). These findings show that gold was specifically attached onto iron bearing material. This observation and the presence of the auro–cyanide complex suggest “cementation” as one of the mechanisms underlying association of gold on iron bearing particles during the process. Cementation is a redox reaction in which the more noble metal is deposited from the solution while the less noble metal dissolves (Kenna et al., 1990). In addition to the electrochemical displacement, precipitation and crystallization of the noble metal occur on the substrate metal as the substrate progressively dissolves (Gould et al., 1984; Ritchie, 2003; Choo and Jeffrey, 2004; Gaur et al., 2006; Wang et al., 2007). In summary, cementation results in the deposition and association of the reduced metal on the reductant metal surface. Gold cementation has been well known for more than a century since zinc dust was used by Merril for the precipitation of gold through cementation with zinc, a less noble metal than gold (Dasovich et al., 1999). Previous investigations have shown that cementation of gold onto iron occurs once the necessary conditions were created (Kenna et al., 1990; Wang et al., 2007). Davidson et al. (1978) showed that gold dissolution was severely prohibited by the presence of tramp iron, causing gold cementation onto iron during leaching. Similar observations were reported by Kenna et al. (1990)
Table 2 Samples from different sections of the Mussel–White process and results of mineralogical analyses. Sample Source stream
Au Type of association occurrence
S-1
Electrowinning cell tail-filter-sludge Pregnant tank sludge Elution column discharge
–
Panning concentrates from carbon fine tank overflow Reactivated carbon at kiln discharge
Attached
Magnetic fines from carbon fine settling tank Filter fines from carbon fine settling tank
Attached, dendritic Attached, dendritic
S-2 S-3 S-4 S-5
S-6 S-7
– Attached
Attached
Fe sulphides, Fesilicate and SiO2 Fe sulphides and Fe oxides Fe sulphides, Fe oxides and tramp iron, Fe sulphides Fe sulphides, Fe oxide-silicates
Au grain size
2–35 μm 2–40 μm 2 μm
2–30 μm 2–25 μm
26
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
Au
Fe-S
Fe-S Au
Au
Au
Fe-S
Au
Au
S
Fe Au 0.0
5.0
10.0
Fe 15.0 0.0
5.0
10.0
keV
15.0
keV
Fig. 2. SEM View and EDS analysis of cemented gold on a Fe–S bearing particle from elution column discharge.
Fe-S
Au
Au
S
Fe Au 0.0
5.0
10.0
keV
15.0 0.0
5.0
10.0
15.0
keV
Fig. 3. SEM view and EDS analysis of cemented gold on a Fe–S bearing particle from panning concentrates of carbon fine tank overflow.
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
27
Au
Fe-Si
Au
Si Fe
Au Au 5.0
0.0
10.0
5.0
15.0 0.0
10.0
15.0
keV
keV
Fig. 4. SEM view and EDS analysis of cemented gold on a Fe–Si bearing particle from elution column discharge.
and the overall mechanism is:
and Dasovich et al. (1999) who found gold cementation onto iron during the elution (stripping) process. During cementation of gold onto iron, the iron replaces the gold in the auro–cyanide complex (Kenna et al., 1990). This reaction occurs as a result of two consecutive redox steps: −
−
−
AuðCNÞ2 þ e →Au þ 2CN
−
4−
Fe þ 6CN →FeðCNÞ6 þ 2e
−
ð2Þ
ð3Þ
Au
Fe-Ox
Au
4−
Through this electrochemical reaction, as iron dissolves and departs from the surface of the iron bearing particles, gold ions are reduced and incorporated on the surface of the iron bearing particles. This results in the simultaneous deposition of gold onto iron and the formation of gold attached iron bearing particles. Wang et al. (2007) studied cementation of gold onto iron from gold–cyanide complexes and observed appreciable rates of gold cementation onto iron. The process conditions for iron-mediated gold cementation should favor
ð1Þ
−
−
2AuðCNÞ2 þ Fe þ 2CN →2Au þ FeðCNÞ6
Fe-Ox Au
Au
Fe
Ag
O Au
0.0
5.0
10.0
keV
15.0 0.0
5.0
10.0
15.0
keV
Fig. 5. SEM view and EDS analysis of gold cemented on an ıron oxide particle from panning concentrates of carbon fine tank overflow.
28
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
the simultaneous electrochemical displacement–deposition reaction. Kenna et al. (1990) found that cementation of gold onto iron occurred either at slightly alkaline (pH 9) conditions with a low cyanide concentration or highly alkaline conditions (pH 14) with a high cyanide concentration. Kenna added that the reaction was promoted at temperatures above 80 °C. Gaur et al. (2006) also reported that an increase of temperature from 60 °C to 95 °C improved kinetics of gold cementation onto iron. Table 3 lists the process conditions in the elution column. The elution column, provided an optimum environment for gold cementation to occur; in addition to suitable solution pH, process temperature and sufficient cyanide concentration (N250 ppm), fine iron bearing material existed in the elution column and dissolved oxygen concentration was minimum. According to the Elsner reaction, the reduced gold may re-dissolve if oxygen concentration is sufficiently high (Davidson et al., 1978; Dasovich et al., 1999) and the auro–cyanide complex may re-form according to the following reaction; −
1
1
−
Au þ 2CN þ =2 H2 O þ =4 O2 →AuðCNÞ2 þ OH
Au
Au Au
Au
ð4Þ
Thus, the low oxygen concentration in the elution column promotes the formation of stable cemented gold onto iron particles. Hetero-coagulation can also result in the attachment of gold onto iron bearing particles. Colloidal adsorption of ultrafine gold particles (b1 μm) onto iron oxides like goethite, and hematite has been reported in several studies (Hogg et al., 1966; Hansen and Matijevic, 1980; Enzweiler and Joekes, 1991, 1992). This process, which is also referred to as hetero-coagulation, relies on the electrostatic adsorption of negatively charged colloidal gold particles onto positively charged iron oxide particles. This interaction is pH dependent and positive surface charge of iron oxides is required. For heterocoagulation of gold onto iron oxide surfaces, acidic to neutral pH conditions are required so that oppositely charged surfaces exist (Enzweiler and Joekes, 1991). It was found that at a neutral pH, the adsorption of gold onto goethite was significant, but gold desorption occurred as the pH was increased to 10.5. Desorption was due to the decay of the positive surface charge of iron oxides with the increase in the concentration of OH− ions. In view of these findings and the highly alkaline process conditions, particularly in the elution column (Table 3), hetero-coagulation and colloidal adsorption of gold onto iron oxides are not likely to occur during the cyanidation leaching and CIP recovery of gold. In addition to gold attached onto iron, characteristic dendritic gold particles were identified in the samples taken from the carbon fine settling tank and filter fines of the carbon fine settling tank (Table 2, Figs. 6 and 7). The size of the dendritic gold particles was between 2 and 35 μm. The dendritic structure implied that the gold was not in native form and was the result of a forced reaction. Depending on the temperature, release of gold from adsorbed materials is likely to occur (Adams et al., 1995) and desorption of auro–cyanide from activated carbon particles would take place as the temperature reached high levels during the process. Gold–cyanide complexes could remain stable up to 240 °C at neutral and alkaline conditions. At temperatures higher than 240 °C, and particularly above 300 °C, thermal reduction of auro–cyanide to metallic gold occurs, accompanied by the release of HCN(g). The thermal reduction of the residual gold–cyanide com-
Au 5.0
0.0
10.0
15.0
keV Fig. 6. SEM view and EDS analysis of a dendritic gold grain from carbon fine settling tank.
plexes was shown by SEM and XRD and the following mechanism was suggested (Adams et al., 1995);
300 0C
--OH + Au(CN)-2
--O- + HCN + AuCN ð5Þ
Au
Au
Au
Au
Table 3 Elution column parameters.
Au
Cyanide concentration
≥250 ppm
Temperature pH Dissolved O2 concentration
≈ 145 °C ≈ 13 b1.0 ppm
0.0
5.0
10.0
15.0
keV Fig. 7. SEM view and EDS analysis of a dendritic gold grain from carbon fine settling tank.
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
OH
½
O
+ AuCN
OH
½
+ Au + HCN
O ð6Þ
The kinetics of this reaction is favored by an increase in temperature and desorption of the auro–cyanide complex from activated carbon into metallic gold happens more rapidly at elevated temperatures. In the elution column some part of the auro–cyanide complexes may possibly remain non-eluted and be discharged into the carbon generation kiln with the activated carbon. In most CIP circuits the temperature in the carbon regeneration kiln is maintained above 500 °C and for the studied circuit, it was even higher (≈ 650 °C). It is anticipated that the noneluted gold was thermally converted into metallic gold in the carbon regeneration kiln. This confirms Davidson et al. (1978) such that in the case of the incidental presence of loaded carbon in the carbon regeneration kiln, the reduction of the adsorbed gold into the metallic state occurred as the temperature was increased to 350 °C. After thermal reduction, the structural form of the metallic gold depends on pH. Neutral conditions led to the formation of colloidal gold particles and alkaline pH levels resulted in the formation of dendritic precipitates (Adams and Fleming, 1989; Adams et al., 1995). The highly alkaline condition during the recovery process explains the formation of the dendritic gold precipitates after thermal reduction of the non-eluted gold in the regeneration kiln. It was noted that incidental gold losses were mainly stimulated by the iron bearing entities. This makes iron or iron bearing compounds a critical issue in the gold recovery processes, as they act as the major media resulting in the formation of gold precipitates. The presence of iron in such process mainly relies on two mechanisms, which are either the oxidative breakdown of the iron bearing minerals and/or the formation of porous magnetite and hematite as a result of the roasting of iron sulfides in the regeneration kiln. The ore in this study consists of iron bearing minerals such as pyrite, chalcopyrite, and arsenopyrite, similar to most gold deposits (Table 1). The oxidative breakdown of these minerals may easily result in the release of iron in ferrous form (Rees and Van Deventer, 1999). The presence of cyanide then leads to the formation of Fe(CN)4− according to the reaction 6 given in Eq. 3, accompanied by the release of gold from the auro– cyanide complex. Operating conditions in the elution column provide
Fig. 8. SEM view of a roasted pyrite grain with porous structure.
29
a suitable environment for the decomposition of the iron bearing minerals and the successive cementation reaction, as stated above. The formation of porous magnetite and hematite due to the roasting of iron sulfides in the regeneration kiln would be another source of iron. These particles would either serve as a source of iron for cementation reactions or as a porous media for the physical attachment of auro–cyanide complexes into the micropores. This claim is supported by the scanning electron micrographs of the samples taken at the kiln discharge, showing a roasted iron bearing grain with a porous structure (Fig. 8). It was also seen that the centre of the roasted porous particle is pyrite. 4. Conclusions Various streams in a gold recovery circuit were sampled and analyzed to identify gold occurrences. Gold was found in two main forms, as attached to iron bearing particles and in the form of dendritic particles. The findings suggest that such losses are likely in the CIP-cyanidation gold recovery circuits and therefore close control over the process should be maintained to minimize these losses. The elution column and carbon regeneration kiln were the most critical sections in the process in terms of losses, providing suitable conditions for cementation of gold and thermal reduction of gold, respectively. In this respect, in the CIP-leaching circuits, it is of crucial importance to avoid or minimize iron bearing particles to prevent iron-mediated gold cementation. Also, close monitoring of the elution column efficiency should be ensured to avoid short-circuiting of noneluted gold into the carbon regeneration kiln and eventual formation of dendritic gold precipitates. References Adams, M.D., Fleming, C.A., 1989. The mechanism of adsorption of aurocyanide onto activated carbon. Metall. Trans. B 20B (970), 315–325. Adams, M.D., Friedl, J., Wagner, F.E., 1995. The mechanism of adsorption of aurocyanide on to activated carbon, 2. Thermal stability of the adsorbed species. Hydrometallurgy 37, 33–45. Choo, W.L., Jeffrey, M.I., 2004. An electrochemical study of copper cementation of gold (I) thiosulfate. Hydrometallurgy 71, 351–362. Dasovich, A., Vickell, G., Taschereau, C., 1999. Iron-mediated gold cementation during the pressure elution of carbon. Technical Report. Omai Gold Mines Ltd. Davidson, R.J., Brown, G.A., Scmidt, C.G., Hanf, N.W., Duncanson, D., Taylor, J.D., 1978. The intensive cyanidation of gold-plant gravity concentrates. J. S. Afr. Inst. Min. Metall. 146–165 January. Enzweiler, J., Joekes, I., 1991. Adsorption of colloidal gold on colloidal iron oxides. J. Geochem. Explor. 40, 133–142. Enzweiler, J., Joekes, I., 1992. Hetero- and homocoagulation of colloidal gold and iron oxides. J. Colloid Interface Sci. 50 (2), 559–566. Gaur, R.P.S., Braymiller, S.A., Wolfe, T., Pierce, M.R., Houck, D.L., 2006. Electrochemical displacement–deposition method for making composite metal powders. United States Patent, US 7, 041, 151, B2. Gould, J.P., Masingale, M.Y., Miller, M., 1984. Recovery of silver and mercury from COD samples by iron cementation. Water Environ. Fed. 56 (3–1), 280–286. Hansen, F.K., Matijevic, E., 1980. Heterocoagulation. Part 5, adsorption of a carboxylated polymer latex on monodispersed hydrated metal oxides. J. Chem. Soc. Faraday Trans. 1 (76), 1240–1262. Hogg, R., Healy, T.W., Fuerstenau, D.W., 1966. Mutual coagulation of colloidal dispersions. Trans. Faraday Soc. 62, 1638–1651. Kenna, C., Ritchie, I.M., Singh, P., 1990. The cementation of gold by iron from cyanide solutions. Hydrometallurgy 23, 263–279. Rees, K.L., Van Deventer, J.S.J., 1999. The role of metal–cyanide species in leaching gold from a copper concentrate. Miner. Eng. 12 (8), 877–892. Ritchie, I.M., 2003. Some aspects of cementation reactions. Hydrometallurgy 2003: Proceedings of the 5th International Symposium, pp. 1179–1194. Wang, Z., Chen, D., Chen, L., 2007. Gold cementation from thiocyanate solutions by iron powder. Miner. Eng. 20, 581–590.
Contents lists available at ScienceDirect
International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / i j m i n p r o
Gold losses by cementation and thermal reduction in the gold recovery circuits B. Klein a,⁎, N.E. Altun b a b
Norman B. Keevil Institute of Mining Engineering, University of British Columbia, 6350 Stores Road, Vancouver, BC, Canada, V6T 1Z4 Muğla University, Department of Mining Engineering, 48000, Muğla, Turkey
a r t i c l e
i n f o
Article history: Received 21 July 2010 Received in revised form 6 September 2010 Accepted 17 September 2010 Available online 1 October 2010 Keywords: Gold Carbon-in-pulp cyanidation Cementation Thermal reduction Heterocoagulation
a b s t r a c t In this study, gold losses in a carbon-in-pulp (CIP) cyanidation gold recovery process and potential sources of these losses were investigated. Gold was found in samples from different streams through the CIP-cyanidation process, pointing to incidental losses. Mineralogical studies showed that gold losses occurred in two main forms, either as attached to larger entities or in the form of dendritic precipitates. SEM and EDS studies revealed that iron bearing minerals acted as the major media in cases when gold associations were observed as losses. The highly alkaline pH (≈13), elevated process temperature (≈145 °C), and high cyanide concentration (≥ 250 ppm) in the elution column along with a fine iron bearing material implied that gold attachment occurred through an electrochemical cementation mechanism. It was anticipated that the presence of iron in the process, which facilitated gold cementation, relied on the oxidative breakdown of the iron bearing minerals in the ore and/or due to the formation of porous iron oxides due to the roasting of iron sulfides in the regeneration kiln. In the elution column some part of the auro–cyanide complexes would remain non-eluted and be discharged into the carbon generation kiln and the carbon generation kiln was another section promoting gold losses. The high temperature condition in the carbon regeneration kiln (N 500 °C) caused thermal reduction of the non-eluted auro–cyanide complexes to metallic gold, leading to the formation of dendritic gold precipitates and their eventual loss. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the carbon-in-pulp cyanidation process to extract and recover gold, several known mineralogical factors contribute to incomplete recovery. Well established practice is applied to reduce or eliminate the refractoriness of ores. While there are numerous publications describing approaches to address the leaching process relatively little information exists on potential losses resulting from stripping gold from carbon and recovery in subsequent electrowinning. Losses during recovery may occur due to physical, physicochemical and/or electrochemical interaction of gold with other metallic and nonmetallic minerals as well as the physical and chemical process conditions such as temperature and pH. Studies showed that mutual coagulation of colloidal gold and iron oxides would take place under suitable conditions (Enzweiler and Joekes, 1991, 1992) at various phases of the carbon-in-pulp (CIP) process, leading to iron-mediated gold losses. In addition, electrochemical reactions between gold and other metallic minerals in the solution can occur. These reactions are well-understood, particularly between gold and zinc, and have been used for the recovery of gold (Merrill–Crowe process). However, redox reactions between gold and other metallic minerals during the ⁎ Corresponding author. University of British Columbia, NBK Institute of Mining Engineering, 6350 Stores Road Vancouver, V6T 1Z4 BC, Canada. Tel.: + 1 604 822 3986; fax: + 1 607 822 5599. E-mail address: [email protected] (B. Klein). 0301-7516/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2010.09.001
CIP process and subsequent steps are undesirable as they would result in the release of gold from the auro–cyanide complex and eventually its loss (Kenna et al., 1990; Dasovich et al., 1999). High process temperature also results in the release of gold from the auro–cyanide complex (Adams et al., 1995) leading to its loss through the process. This paper will examine possible losses of gold in a gold recovery circuit and identify potential sources for these losses and related factors. Samples from various streams of the recovery process were analyzed to identify the existence of free gold particles or gold associations that were likely to report as losses. The objective was to identify the mechanisms/reactions that led to the deficiencies during extraction and recovery of gold and locations of such reactions.
2. Materials and methods For the study the gold recovery process of the Mussel-White Mine, (previously Placer Dome) in Northwestern Ontario was investigated. A simplified flowsheet of the process is shown in Fig. 1. The process consists of conventional gravity separation, cyanide leaching and carbon-in-pulp (CIP) recovery of gold. The loaded carbon is eluted using the pressurized Zadra technique in an elution column at high pH, high temperature conditions with a hot aqueous solution containing sodium hydroxide and sodium cyanide. The solution from the elution column is passed to an electrowinning cell. The stripped carbon is sent to a dewatering screen and carbon
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
Gravity Concentration
Comminution
25
Concentrate
Tailings Cyanide Leaching
Carbon-in-pulp Process O/F CarbonElution Column
Stripped Carbon
Pregnant Solution Electrowinning
Dewatering Screen
O/F
Carbon Regeneration Kiln
Screen U/F
U/F Kiln
Carbon Fines Settling Tank
Gold
Fig. 1. Mussel-White gold recovery process.
regeneration kiln (650 °C). The gold sludge from electrowinning was pressure-washed, filtered, dried and melted into bullion-bars. The mineralogical composition of the Mussel-White ore is given in Table 1. The ore is described as a granurite-garnet-amphibole-chert iron formation. Gold is mainly associated with chalcopyrite, pyrite, pyrrhotite, arsenopyrite, magnetite, carbonates, hornblende and silicates. A hand magnet was placed in the overflow stream of the carbon fine settling tank and fine-magnetic particles were collected. It was found that the collected samples contained 2000 to 7000 g Au/ton, indicating losses of gold through the process. Samples from different points of the gold recovery process were collected to identify potential losses and investigate underlying mechanisms. The sampling points for the circuit are listed in Table 2. The collected samples were dried and subjected to mineralogical investigations using SEM (scanning electron microscopy) and EDS (energy dispersion spectrometer) analyses. Mineralogical investigations aimed at determining the occurrence, size and type of gold particles and associations in the samples.
3. Results and discussion Table 2 summarizes the results of the mineralogical analyses of the samples taken from different sections of the gold recovery circuit. The SEM and EDS analyses showed that gold losses occurred in two main forms. Attachment of gold with other minerals was common at various sections of the recovery process. Attached gold was identified on iron oxides, iron silicates and iron sulfides. In addition, gold in the form of dendritic precipitates was observed. In all cases, the size of gold varied from 2 to 40 μm. The presence of Ag was also found, but to an insignificant extent. The SEM and EDS studies clearly showed that iron bearing materials acted as the major media that facilitated gold cementation during the recovery process.
Table 1 The composition of Mussel-White run-off-mine ore. Mineral
Amount
Gold, g/t Silver, g/t Chalcopyrite, % Pyrite, % Arsenopyrite, % Pyrrhotite, % Magnetite, % Carbonates, % Hornblende, % Silicates, %
58.1 2635 0–1 0–2 0–2 1–15 1–15 5–10 10–15 40–55
Figs. 2–5 show the “cemented” gold entities attached onto iron bearing particles. Gold associations on iron sulphides (Figs. 2 and 3), iron silicates (Fig. 4) and iron oxides (Fig. 5) were identified in samples taken from the carbon fine settling tank (S-6), the filter fines of the carbon settling tank (S-7) and carbon fine tank overflow that had been upgraded by panning (S-4) (Table 2). These findings show that gold was specifically attached onto iron bearing material. This observation and the presence of the auro–cyanide complex suggest “cementation” as one of the mechanisms underlying association of gold on iron bearing particles during the process. Cementation is a redox reaction in which the more noble metal is deposited from the solution while the less noble metal dissolves (Kenna et al., 1990). In addition to the electrochemical displacement, precipitation and crystallization of the noble metal occur on the substrate metal as the substrate progressively dissolves (Gould et al., 1984; Ritchie, 2003; Choo and Jeffrey, 2004; Gaur et al., 2006; Wang et al., 2007). In summary, cementation results in the deposition and association of the reduced metal on the reductant metal surface. Gold cementation has been well known for more than a century since zinc dust was used by Merril for the precipitation of gold through cementation with zinc, a less noble metal than gold (Dasovich et al., 1999). Previous investigations have shown that cementation of gold onto iron occurs once the necessary conditions were created (Kenna et al., 1990; Wang et al., 2007). Davidson et al. (1978) showed that gold dissolution was severely prohibited by the presence of tramp iron, causing gold cementation onto iron during leaching. Similar observations were reported by Kenna et al. (1990)
Table 2 Samples from different sections of the Mussel–White process and results of mineralogical analyses. Sample Source stream
Au Type of association occurrence
S-1
Electrowinning cell tail-filter-sludge Pregnant tank sludge Elution column discharge
–
Panning concentrates from carbon fine tank overflow Reactivated carbon at kiln discharge
Attached
Magnetic fines from carbon fine settling tank Filter fines from carbon fine settling tank
Attached, dendritic Attached, dendritic
S-2 S-3 S-4 S-5
S-6 S-7
– Attached
Attached
Fe sulphides, Fesilicate and SiO2 Fe sulphides and Fe oxides Fe sulphides, Fe oxides and tramp iron, Fe sulphides Fe sulphides, Fe oxide-silicates
Au grain size
2–35 μm 2–40 μm 2 μm
2–30 μm 2–25 μm
26
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
Au
Fe-S
Fe-S Au
Au
Au
Fe-S
Au
Au
S
Fe Au 0.0
5.0
10.0
Fe 15.0 0.0
5.0
10.0
keV
15.0
keV
Fig. 2. SEM View and EDS analysis of cemented gold on a Fe–S bearing particle from elution column discharge.
Fe-S
Au
Au
S
Fe Au 0.0
5.0
10.0
keV
15.0 0.0
5.0
10.0
15.0
keV
Fig. 3. SEM view and EDS analysis of cemented gold on a Fe–S bearing particle from panning concentrates of carbon fine tank overflow.
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
27
Au
Fe-Si
Au
Si Fe
Au Au 5.0
0.0
10.0
5.0
15.0 0.0
10.0
15.0
keV
keV
Fig. 4. SEM view and EDS analysis of cemented gold on a Fe–Si bearing particle from elution column discharge.
and the overall mechanism is:
and Dasovich et al. (1999) who found gold cementation onto iron during the elution (stripping) process. During cementation of gold onto iron, the iron replaces the gold in the auro–cyanide complex (Kenna et al., 1990). This reaction occurs as a result of two consecutive redox steps: −
−
−
AuðCNÞ2 þ e →Au þ 2CN
−
4−
Fe þ 6CN →FeðCNÞ6 þ 2e
−
ð2Þ
ð3Þ
Au
Fe-Ox
Au
4−
Through this electrochemical reaction, as iron dissolves and departs from the surface of the iron bearing particles, gold ions are reduced and incorporated on the surface of the iron bearing particles. This results in the simultaneous deposition of gold onto iron and the formation of gold attached iron bearing particles. Wang et al. (2007) studied cementation of gold onto iron from gold–cyanide complexes and observed appreciable rates of gold cementation onto iron. The process conditions for iron-mediated gold cementation should favor
ð1Þ
−
−
2AuðCNÞ2 þ Fe þ 2CN →2Au þ FeðCNÞ6
Fe-Ox Au
Au
Fe
Ag
O Au
0.0
5.0
10.0
keV
15.0 0.0
5.0
10.0
15.0
keV
Fig. 5. SEM view and EDS analysis of gold cemented on an ıron oxide particle from panning concentrates of carbon fine tank overflow.
28
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
the simultaneous electrochemical displacement–deposition reaction. Kenna et al. (1990) found that cementation of gold onto iron occurred either at slightly alkaline (pH 9) conditions with a low cyanide concentration or highly alkaline conditions (pH 14) with a high cyanide concentration. Kenna added that the reaction was promoted at temperatures above 80 °C. Gaur et al. (2006) also reported that an increase of temperature from 60 °C to 95 °C improved kinetics of gold cementation onto iron. Table 3 lists the process conditions in the elution column. The elution column, provided an optimum environment for gold cementation to occur; in addition to suitable solution pH, process temperature and sufficient cyanide concentration (N250 ppm), fine iron bearing material existed in the elution column and dissolved oxygen concentration was minimum. According to the Elsner reaction, the reduced gold may re-dissolve if oxygen concentration is sufficiently high (Davidson et al., 1978; Dasovich et al., 1999) and the auro–cyanide complex may re-form according to the following reaction; −
1
1
−
Au þ 2CN þ =2 H2 O þ =4 O2 →AuðCNÞ2 þ OH
Au
Au Au
Au
ð4Þ
Thus, the low oxygen concentration in the elution column promotes the formation of stable cemented gold onto iron particles. Hetero-coagulation can also result in the attachment of gold onto iron bearing particles. Colloidal adsorption of ultrafine gold particles (b1 μm) onto iron oxides like goethite, and hematite has been reported in several studies (Hogg et al., 1966; Hansen and Matijevic, 1980; Enzweiler and Joekes, 1991, 1992). This process, which is also referred to as hetero-coagulation, relies on the electrostatic adsorption of negatively charged colloidal gold particles onto positively charged iron oxide particles. This interaction is pH dependent and positive surface charge of iron oxides is required. For heterocoagulation of gold onto iron oxide surfaces, acidic to neutral pH conditions are required so that oppositely charged surfaces exist (Enzweiler and Joekes, 1991). It was found that at a neutral pH, the adsorption of gold onto goethite was significant, but gold desorption occurred as the pH was increased to 10.5. Desorption was due to the decay of the positive surface charge of iron oxides with the increase in the concentration of OH− ions. In view of these findings and the highly alkaline process conditions, particularly in the elution column (Table 3), hetero-coagulation and colloidal adsorption of gold onto iron oxides are not likely to occur during the cyanidation leaching and CIP recovery of gold. In addition to gold attached onto iron, characteristic dendritic gold particles were identified in the samples taken from the carbon fine settling tank and filter fines of the carbon fine settling tank (Table 2, Figs. 6 and 7). The size of the dendritic gold particles was between 2 and 35 μm. The dendritic structure implied that the gold was not in native form and was the result of a forced reaction. Depending on the temperature, release of gold from adsorbed materials is likely to occur (Adams et al., 1995) and desorption of auro–cyanide from activated carbon particles would take place as the temperature reached high levels during the process. Gold–cyanide complexes could remain stable up to 240 °C at neutral and alkaline conditions. At temperatures higher than 240 °C, and particularly above 300 °C, thermal reduction of auro–cyanide to metallic gold occurs, accompanied by the release of HCN(g). The thermal reduction of the residual gold–cyanide com-
Au 5.0
0.0
10.0
15.0
keV Fig. 6. SEM view and EDS analysis of a dendritic gold grain from carbon fine settling tank.
plexes was shown by SEM and XRD and the following mechanism was suggested (Adams et al., 1995);
300 0C
--OH + Au(CN)-2
--O- + HCN + AuCN ð5Þ
Au
Au
Au
Au
Table 3 Elution column parameters.
Au
Cyanide concentration
≥250 ppm
Temperature pH Dissolved O2 concentration
≈ 145 °C ≈ 13 b1.0 ppm
0.0
5.0
10.0
15.0
keV Fig. 7. SEM view and EDS analysis of a dendritic gold grain from carbon fine settling tank.
B. Klein, N.E. Altun / International Journal of Mineral Processing 98 (2011) 24–29
OH
½
O
+ AuCN
OH
½
+ Au + HCN
O ð6Þ
The kinetics of this reaction is favored by an increase in temperature and desorption of the auro–cyanide complex from activated carbon into metallic gold happens more rapidly at elevated temperatures. In the elution column some part of the auro–cyanide complexes may possibly remain non-eluted and be discharged into the carbon generation kiln with the activated carbon. In most CIP circuits the temperature in the carbon regeneration kiln is maintained above 500 °C and for the studied circuit, it was even higher (≈ 650 °C). It is anticipated that the noneluted gold was thermally converted into metallic gold in the carbon regeneration kiln. This confirms Davidson et al. (1978) such that in the case of the incidental presence of loaded carbon in the carbon regeneration kiln, the reduction of the adsorbed gold into the metallic state occurred as the temperature was increased to 350 °C. After thermal reduction, the structural form of the metallic gold depends on pH. Neutral conditions led to the formation of colloidal gold particles and alkaline pH levels resulted in the formation of dendritic precipitates (Adams and Fleming, 1989; Adams et al., 1995). The highly alkaline condition during the recovery process explains the formation of the dendritic gold precipitates after thermal reduction of the non-eluted gold in the regeneration kiln. It was noted that incidental gold losses were mainly stimulated by the iron bearing entities. This makes iron or iron bearing compounds a critical issue in the gold recovery processes, as they act as the major media resulting in the formation of gold precipitates. The presence of iron in such process mainly relies on two mechanisms, which are either the oxidative breakdown of the iron bearing minerals and/or the formation of porous magnetite and hematite as a result of the roasting of iron sulfides in the regeneration kiln. The ore in this study consists of iron bearing minerals such as pyrite, chalcopyrite, and arsenopyrite, similar to most gold deposits (Table 1). The oxidative breakdown of these minerals may easily result in the release of iron in ferrous form (Rees and Van Deventer, 1999). The presence of cyanide then leads to the formation of Fe(CN)4− according to the reaction 6 given in Eq. 3, accompanied by the release of gold from the auro– cyanide complex. Operating conditions in the elution column provide
Fig. 8. SEM view of a roasted pyrite grain with porous structure.
29
a suitable environment for the decomposition of the iron bearing minerals and the successive cementation reaction, as stated above. The formation of porous magnetite and hematite due to the roasting of iron sulfides in the regeneration kiln would be another source of iron. These particles would either serve as a source of iron for cementation reactions or as a porous media for the physical attachment of auro–cyanide complexes into the micropores. This claim is supported by the scanning electron micrographs of the samples taken at the kiln discharge, showing a roasted iron bearing grain with a porous structure (Fig. 8). It was also seen that the centre of the roasted porous particle is pyrite. 4. Conclusions Various streams in a gold recovery circuit were sampled and analyzed to identify gold occurrences. Gold was found in two main forms, as attached to iron bearing particles and in the form of dendritic particles. The findings suggest that such losses are likely in the CIP-cyanidation gold recovery circuits and therefore close control over the process should be maintained to minimize these losses. The elution column and carbon regeneration kiln were the most critical sections in the process in terms of losses, providing suitable conditions for cementation of gold and thermal reduction of gold, respectively. In this respect, in the CIP-leaching circuits, it is of crucial importance to avoid or minimize iron bearing particles to prevent iron-mediated gold cementation. Also, close monitoring of the elution column efficiency should be ensured to avoid short-circuiting of noneluted gold into the carbon regeneration kiln and eventual formation of dendritic gold precipitates. References Adams, M.D., Fleming, C.A., 1989. The mechanism of adsorption of aurocyanide onto activated carbon. Metall. Trans. B 20B (970), 315–325. Adams, M.D., Friedl, J., Wagner, F.E., 1995. The mechanism of adsorption of aurocyanide on to activated carbon, 2. Thermal stability of the adsorbed species. Hydrometallurgy 37, 33–45. Choo, W.L., Jeffrey, M.I., 2004. An electrochemical study of copper cementation of gold (I) thiosulfate. Hydrometallurgy 71, 351–362. Dasovich, A., Vickell, G., Taschereau, C., 1999. Iron-mediated gold cementation during the pressure elution of carbon. Technical Report. Omai Gold Mines Ltd. Davidson, R.J., Brown, G.A., Scmidt, C.G., Hanf, N.W., Duncanson, D., Taylor, J.D., 1978. The intensive cyanidation of gold-plant gravity concentrates. J. S. Afr. Inst. Min. Metall. 146–165 January. Enzweiler, J., Joekes, I., 1991. Adsorption of colloidal gold on colloidal iron oxides. J. Geochem. Explor. 40, 133–142. Enzweiler, J., Joekes, I., 1992. Hetero- and homocoagulation of colloidal gold and iron oxides. J. Colloid Interface Sci. 50 (2), 559–566. Gaur, R.P.S., Braymiller, S.A., Wolfe, T., Pierce, M.R., Houck, D.L., 2006. Electrochemical displacement–deposition method for making composite metal powders. United States Patent, US 7, 041, 151, B2. Gould, J.P., Masingale, M.Y., Miller, M., 1984. Recovery of silver and mercury from COD samples by iron cementation. Water Environ. Fed. 56 (3–1), 280–286. Hansen, F.K., Matijevic, E., 1980. Heterocoagulation. Part 5, adsorption of a carboxylated polymer latex on monodispersed hydrated metal oxides. J. Chem. Soc. Faraday Trans. 1 (76), 1240–1262. Hogg, R., Healy, T.W., Fuerstenau, D.W., 1966. Mutual coagulation of colloidal dispersions. Trans. Faraday Soc. 62, 1638–1651. Kenna, C., Ritchie, I.M., Singh, P., 1990. The cementation of gold by iron from cyanide solutions. Hydrometallurgy 23, 263–279. Rees, K.L., Van Deventer, J.S.J., 1999. The role of metal–cyanide species in leaching gold from a copper concentrate. Miner. Eng. 12 (8), 877–892. Ritchie, I.M., 2003. Some aspects of cementation reactions. Hydrometallurgy 2003: Proceedings of the 5th International Symposium, pp. 1179–1194. Wang, Z., Chen, D., Chen, L., 2007. Gold cementation from thiocyanate solutions by iron powder. Miner. Eng. 20, 581–590.