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Use of amino acids for gold dissolution
Hydrometallurgy 177 (2018) 79–85
Contents lists available at ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Use of amino acids for gold dissolution a,⁎
C.G. Perea , O.J. Restrepo a b
T
b
Group of Environmental Geochemistry, Pedagogical and Technological University of Colombia, Sogamoso, Colombia Institute Of Minerals CIMEX, School of Mines, Universidad Nacional de Colombia, Medellin, Colombia
A R T I C L E I N F O
A B S T R A C T
Keywords: Gold leaching Cyanide alternatives Organic solvent
As oxidants under alkaline conditions. To analyze the kinetics of leaching, tests were performed using electrical contacts pins and sheets of pure gold to avoid the effects of pore diffusion. The efficiency of each oxidant was evaluated, for concentrations of 0.004, 0.01 and 0.03 M, at pH values of 9.4 and 11. At the end of each test, the solid residue was analyzed to determine the total grade and percentage of metal extraction. The experimental results indicated that the percentage of gold extraction was affected by the oxidant concentration and the pH. Furthermore, it improves with the presence of Cu2+ ions, increasing by 100% for glycine and approximately 6 fold for glutamate as compared to glycine solutions. In assessing the percentage of gold extraction in the different conditions at 25 and 40 °C, it was observed that the kinetics of extraction is sensitive to temperature change, which suggests that the leach is chemically controlled.
1. Introduction
In this study, a method was evaluated for the recovery of gold from computer contact pins and a sheet of pure gold, using alkaline solutions of amino acids such as glycine and monosodium glutamate. The selection of these reagents was based on a review of scientific literature (Aylmore, 2005; Eksteen and Oraby, 2015a,b,c), where the former conducted a review of different methods of gold leaching as an alternative to cyanidation, mentioning the dissolution with amino acids and using as potassium permanganate oxidizing agent. Meanwhile Eksteen and Oraby performed the leaching of metallic gold using amino acids such as glycine, alanine and histidine at low concentrations, in alkaline solutions with hydrogen peroxide (H2O2), evaluating the influence of reagent concentration, temperature and presence of sulphide minerals in the rate of leaching. This methodology could represent an alternative to cyanide for gold recovery and could diminish the environmental impacts generated by the inadequate handling of the compounds used for gold beneficiation. Oraby and Eksteen (2014) also evaluated the extraction of copper and gold from a copper concentrate employing glycine; their results show a copper extraction of nearly 96%. However, a very low content of gold was found in solution, using hydrogen peroxide at pH values between 8 and 11. The reactions below indicate a strong affinity of copper with glycine, both in the cuprous as well as cupric oxidation states:
The extraction of gold from its minerals is achieved through hydrometallurgical processes, where the traditional process of cyanidation has dominated. Since the beginning of the 20th Century, the acceptance of cyanidation by gold producers is a tribute to its effectiveness (Avraamides, 1982). As a process, it has resisted economic difficulties imposed by the fall in the price of gold, and the introduction of Carbon in pulp (CIP), has tended to make it even more attractive as a leaching system. The main disadvantage of cyanidation is its toxicity. The environmental problems associated with the leakage of cyanide into groundwater systems and the health risks inherent in the use of this on any scale are a strong drawback (Avraamides, 1982). Furthermore, its instability, its high affinity with other elements, the decrease easily extracted gold and the presence of refractory minerals that are not susceptible to cyanide, sometimes cause relatively high operating costs. (Poisot, 2010). For that reason, there is a clear need to implement clean technologies for the extraction of silver and gold, that increase productivity by avoiding the generation of waste contaminants (The corral, 2003). The scientific community has proposed alternative hydrometallurgical processes for the dissolution of these precious metals using thiosulphate, thiourea, chlorine, thiocyanate, among others. An alternative route could employ other organic ligands, that can compete with the cyanidation in cost, performance and effectiveness (Guzmán, 2013; Galindo, 2013). ⁎
Cu2 + + 2(H2 NCH2 COO)− ↔ Cu(NH2 CH2 COO)2 , Log10K = 15.6 Cu+
Corresponding author. E-mail address: [email protected] (C.G. Perea).
https://doi.org/10.1016/j.hydromet.2018.03.002 Received 28 August 2017; Received in revised form 22 February 2018; Accepted 1 March 2018 0304-386X/ © 2018 Published by Elsevier B.V.
+ 2(H2 NCH2
COO)−
↔
Cu(NH2 CH2 COO)−2 ,
Log10K = 10.1
(1) (2)
Therefore, further study is warranted to evaluate the ability of
Hydrometallurgy 177 (2018) 79–85
C.G. Perea, O.J. Restrepo
amino acids in general, not only to enhance, but to completely replace cyanide as a ligand for gold extraction. In the present work, two amino acids, glycine and glutamate, were employed.
H 2 (Gly)+
1.0
Gly−
H(Gly)
0.8
2. Materials and methods All leaching experiments were carried out using pins from computers and thin gold sheets, to avoid the effects of pore diffusion, common in mineral, and to evaluate gold dissolution with these selected amino acids. The pins are electrical contacts, composed of gold-plated metal alloys. Therefore, these pins, due to its characteristics and its relatively low cost, became the ideal material to assess the conditions in which gold dissolves in aqueous solutions of these organic ligands (amino acids). The results obtained with the pins were compared with two tests performed on samples cut from a pure gold sheet (99.95%, SigmaAldrich). The organic ligands (glycine and glutamate) and oxidizing agents (hydrogen peroxide (30% w/v) and potassium permanganate) employed in this work were of analytical grade. For pH adjustment, NaOH was used. The pins were characterized by scanning electron microscopy SEMEDX (ZEISS EVO MA10), in order to estimate the chemical elements present and their distribution. A sample of these was digested in a mixture of nitric acid and hydrochloric acid, later the elemental composition of this solution was determined by using atomic absorption spectrophotometry (Varian SpectrAA 220fs). The pins were subjected to a pre-treatment, which consisted in the placement of 5 g of pins into 200 ml of 1.8 M H2SO4 and 7.8 M H2O2 aqueous solution for 3 h, mixing at 300 rpm, to eliminate the presence of other metals that might affect the gold leaching process. The filtered residue contained mainly gold. Leaching experiments of the residue were performed in aqueous solutions of 0.5 M glycine or glutamate, with potassium permanganate or hydrogen peroxide as oxidizing agents (Eksteen and Oraby, 2015a,b,c). Liquid samples of 0.5 ml were drawn at 1, 2, 4, 6, 23 and 24 h. Each of the samples was filtered and analyzed to determine the gold content in solution by the atomic absorption spectrophotometer. In Table 1 is shown the conditions of the leach test, where the pH value of 9.4 was selected based on the thermodynamic analysis (see below), while the pH 11 and the initial concentration of oxidizing agent to the 0.03 M was based on previous work (Eksteen and Oraby, 2015a,b,c) (Aylmore, 2005). Once the leach time was completed, the solution was filter and the leach residue was digested using a mixture nitric and sulfuric acids, which was analyzed by atomic absorption spectrophotometry, to determine the final gold content and to carry out the mass balance.
Fraction
0.6
0.4
0.2
0.0 0
2
4
6
8
10
12
14
pH Fig. 1. Species distribution diagram as a function of pH for glycine at 25 °C, [Gly] = 0.5 M.
Oraby (2015a,b,c). No value was found for the gold-glutamate complex; given its chemical similarity to glycine (glutamate has an additional carboxylic group), similar behavior can be assumed. In Fig. 1 is presented the species distribution diagram for 0.5 M glycine as a function of pH. At alkaline pH, the dissociation of the glycine increases, thus achieving the highest concentration of glycinate available to form complexes with gold or any other metal. Aylmore (2005) reported a log10 (stability constant) of 18 for the gold (I) - glycine complex. In Fig. 2 is shown the diagram of thermodynamic stability Eh-pH for the gold-glycine at room temperature in 0.5 M of glycine. As may be noted, the elemental gold is the species stable at all values of pH and potential under 0.5 V; for higher values, there is a zone of stability for the formation of the complex gold-glycine, although the lowest oxidation potential is reached at pH 9.4. Meanwhile, Fig. 3 shows the diagram of thermodynamic stability Eh-pH for the gold-glycine at temperature of 40 °C in 0.5 M of glycine. As may be noted, the elemental gold is the species stable at all values of pH and potential under 0.5 V; for higher values, there is a zone of stability for the formation of the complex gold-glycine, although the lowest oxidation potential is reached at pH 9.4. However, at higher values of potential of 0.5 V and pH 11, the predominant species is Au(OH)2−, which AuOH 2+ Au(OH) 2 +
1.5
Au(OH) 3 (c)
3. Thermodynamic analysis The thermodynamic analysis was performed with the Hydra – Medusa software suite, where the effect of the concentration of oxidizing agent and pH, to determine the potentially adequate conditions and interpretations of the kinetics for gold leaching. The stability constant for the complex gold-glycine complex was given by Aylmore (2005) and the reagent concentrations to be evaluated by Eksteen and
Au(OH) 4 − Au(OH) 5 2 −
1.0
ESHE / V
Au(Gly) −
0.5
Table 1 Conditions of the computer pin leaching experiments. Reagents
Oxidizing agents (concentration)
PH
Glutamate Glutamate Glutamate Glutamate Glycine Glycine Glycine
KMnO4 (0.03 M) H2O2 (0.03 M) KMnO4 (0.01 M) KMnO4 (0.04 M) H2O2 (0.03 M) KMnO4 (0.01 M) KMnO4 (0.04 M)
9.4 9.4 11 11 9.4 11 11
Au(OH) 2 − Au(c)
0.0 0
2
4
6
8
10
12
14
pH Fig. 2. Eh-pH diagram for the Au-glycine system at 25 °C, [Au] = 10−5 M, [Gly] = 0.5 M.
80
Hydrometallurgy 177 (2018) 79–85
C.G. Perea, O.J. Restrepo
1.5
1.5
A u(O H ) 2+ A uO H 2+ A u (O H ) 3 (c) A u(O H ) 4− A u(O H ) 52−
A u(G ly) −
Cu 2+ Cu(Gly) +
SH E
ESHE / V
/ V
1.0
1.0
0.5
E
A u (O H ) 2 −
Cu(OH) 42−
Cu(Gly) 2
0.5
0.0
Au(c)
Cu(cr)
Cu(Gly) 2−
Cu(OH) 2−
-0.5
0.0 0
2
4
6
8
10
12
0
14
Fig. 3. Eh-pH diagram for the Au-glycine system at 40 °C, [Au] = 10−5 M, [Gly] = 0.5 M.
12
14
The analysis of the pins employed in the leach testing was performed using Scanning Electron Microscopy (SEM), with EDX. The cross-section of the sample showed the existence of three layers in each of the pins: a core composed mainly of Cu and with small amounts of Sn (Fig. 6), a coating in the central zone consisting of Al, Cu, Fe, and Ni (Figs. 7 and 8), with an average thickness of 80 μm and finally, an outer layer of a very thin gold film, with a thickness of less than 10 μm (Fig. 9). The results of the semi-quantification in general reveal variation of the elements in the pins in terms of content and proportions. For example, in the core, the Cu concentrations are at least 97% and the Sn less than 3%, in atomic percentage. The composition of the first coating (which borders the core) varies: 6–41% Fe, 3–17% Ni, 13–34% Cu and 5–71% Al; gold is less than 4%. In the outer-most layer, the atomic percentages of gold are 83–91%, while Cu varies between 4 and 9%, Ni is less than 7% and Al less than 3%. Because the SEM results are non - quantitative and only show point values, they are not adequate to determine the gold composition of the sample. For that reason, Regia digestions were performed on several samples, which were subsequently analyzed by atomic absorption spectrophotometry. A sizeable disparity in the gold and copper content is presented in Table 2. Because of this variability, a metallurgical balance (gold extracted + gold remaining in the residue) was calculated for each leaching experiment and results are reported as % extraction.
Au(OH) 3 (c)
1.0
ESHE / V
10
4.1. Characterization of the computer pins
pH 9.4
Au(Gly) −
0.5
4.2. Leaching test
Au(c)
Log
8
4. Results and discussion
1.5
-3
6
waste, such as computer pins. For that reason, these were pretreated in an oxidizing medium to remove the other metals, especially copper.
can generate a passivation that affects the leaching process. Based on this analysis, the chosen testing conditions were pH 9.4 and 11, which is within the range in which the oxidation reaction could occur with 0.5 M glycine. On the other hand, at lower glycine concentrations, the oxidation is more difficult, as may be observed in Fig. 4; at 0.5 M (log10[Gly] = −0.3), the oxidation potential is approximately 0.5 V, while at 0.01 M, the potential is 150 mV higher. For that reason, 0.5 M glycine was chosen for the leaching solution. However, the most important disadvantage associated with the use of glycine is its great affinity for copper ions. At conditions, similar to those used to leach gold (0.5 M glycine and pH 9.4 and 11), copper is oxidized at potentials approximately 800 mV lower (Fig. 5). This implies that if metallic copper is present in the material to be leached, it will be preferentially dissolved, consuming oxidant as well as glycine. For that reason, any metallic copper should be removed from the solid, before leaching gold. This may not be the case in refractory copper sulfide minerals, but is particularly important in the treatment of e-
-4
4
Fig. 5. Eh-pH diagram for the Cu-glycine system at 25 °C, [Cu] = 10−5 M, [Gly] = 0.5 M.
pH
0.0 -5
2
-2 [Gly− ]
-1
4.2.1. Preparation of the material (pretreatment) As was mentioned previously, thermodynamic predictions and the results found by Eksteen and Oraby (2015a,b,c) show the great affinity of copper for glycine. Consequently, pre-treatment of the pins with sulfuric acid and hydrogen peroxide performed to remove the copper present in these, since this element constitutes the largest percentage of composition and so as not to affect the process of leaching of gold with glycine and glutamate. As a result of the pretreatment, the weight of the final residue was only 20–25% that of the original sample.
0
TOT
Fig. 4. Eh-log10 [glycine] diagram at pH 9.4 and 25 °C for the Au-glycine system, [Au] = 10−5 M.
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Hydrometallurgy 177 (2018) 79–85
C.G. Perea, O.J. Restrepo
Fig. 6. Electron micrograph SEM and EDX of the pin core.
4.3. Effect of the oxidizing agent To study the effect of the oxidizing agent in the leaching of gold in solution of monosodium glutamate, is added hydrogen peroxide (H2O2) and potassium permanganate (KMnO4) both to a concentration of 0.03 M to 0.5 M glutamate solution at room temperature (Fig. 10). The results show that oxidizing with hydrogen peroxide is less effective in glutamate solution in comparison with KMnO4. This is due to the fact that H2O2 favors the reaction of the amino acid with copper, given that under these conditions high concentrations of copper were obtained in solution, especially at the second hour, where the greatest decrease in the percentage of gold extraction occurs (data not shown here). 4.4. Effect of oxidizing agent concentration The effect of the concentration of potassium permanganate on the dissolution of gold in solutions of glycine and glutamate is shown in Figs. 11 and 12, respectively. It is clear that by increasing the concentration of KMnO4, the extraction of gold increases slightly. In general terms, the dissolution of gold depends on the concentration of KMnO4, which would be limiting reagent in solutions of potassium permanganate-glycine and glutamate‑potassium permanganate. However, excess oxidant has been shown to promote degradation of the reagents affecting the extraction of gold. In the results shown, low concentrations of potassium
Fig. 7. SEM micrograph of the average thickness of the core.
4.2.2. Leaching Each leaching test employed 39 mg of the residue in 100 ml of the corresponding solution. Listed below are some of the most effective leaching tests, with their respective conditions and results:
Fig. 8. Electron micrograph SEM and EDX central zone (core coating).
82
Hydrometallurgy 177 (2018) 79–85
C.G. Perea, O.J. Restrepo
Fig. 9. SEM and EDX electron micrograph of the pin surface (gold layer).
2.50
Table 2 Percentage of elemental composition of the pins.
Au Fe Cu Ni Zn Ag Pb
Average
Uncertainty
(%)
(+/−)
0.64 0.115 68.1 6.365 0.005 0.05 0.245
0.24 0.085 4.3 5.735 0.005 0.05 0.245
2.00 Au Extracon, %
Element
1.50 0.01 M KMnO4
1.00
0.004 M KMnO4
0.50 0.00 0
12.00 Au extracon, %
10
20
30
Time (h)
14.00
Fig. 12. Effect of the concentration of oxidizing agent on the dissolution of gold: [Glut] = 0.5 M, pH 11.
10.00
permanganate were employed in order to avoid reagent degradation. At low concentrations, reaction rates are much lower and a stagnation of the reaction occurs, although it presents a greater stability in the formation of the complex since there is no re-precipitation of gold in solution. This phenomenon of stagnation can be due to the fact that under these conditions of pH and low values of potential the predominant species would be (AuOH) auroso hydroxide and its presence might generate passivation of the gold surface, preventing the reaction with amino acids.
8.00 KMNO4
6.00
H2O2
4.00 2.00 0.00 0
5
10
15
20
25
30
Time (h) Fig. 10. Effect of oxidizing agent on the dissolution of gold: [Glut] = 0.5 M, [H2O2] and [KMnO4] = 0.03 M, pH 9.44.
4.5. Effect of solvent type In Fig. 13, the reaction of both amino acids with the gold is possible, achieving a greater extraction in the case of glycine. There is re-precipitation of gold with both ligands after 2 h and with glycine after 20 h. The oxidizing agent favors the reaction of both amino acids with copper, given that in these conditions high concentrations of copper in solution were obtained. According to Eksteen and Oraby (2015a,b,c), the evaluation the gold dissolution with these amino acids should be only after 150 h. In Fig. 14 is used in low concentrations of potassium permanganate, showing a slightly greater extraction with glycine, but are observed reaction rates much lower and a slight stagnation of the reaction, although it presents a greater stability in the formation of the complex as it is not re-precipitation of gold in solution. In the end, however, the degradation of both reagents due to the oxidizing agent is very strong.
4.50 4.00 Au Extracon, %
3.50 3.00 2.50
0.01 M KMnO4
2.00
0.004 M KMnO4
1.50 1.00 0.50 0.00 0
5
10
15
20
25
30
Time (h) Fig. 11. Effect of the concentration of oxidizing agent on the dissolution of gold: [Gly] = 0.5 M, pH 11.
4.6. Effect of copper ions To study the behavior of the gold in the absence and presence of 83
Hydrometallurgy 177 (2018) 79–85
3.50
0.80
3.00
0.70
2.50 2.00
Au extracon, %
Au Extracon, %
C.G. Perea, O.J. Restrepo
Glycine
1.50
Glutamate
1.00
0.60
with copper ions
0.50
without copper ions
0.40 0.30 0.20 0.10
0.50
0.00
0.00 0
5
10
15
20
25
0
30
5
10
15
20
25
30
Time (h)
Time (h)
Fig. 16. Percentage of extraction of gold in the [Gly] = 0.5 M, [Cu2+/Cu+] = 50 ppm at pH = 10.1 and [KMnO4] = 0.01 M.
Fig. 13. Effect of the type of solvent on the dissolution of gold: [Gly] = 0.5 M, [Glut] = 0.5 M, [H2O2] = 0.03 M, pH 9.44.
0.7
5
0.6 Au Extracon, %
Au Extracon, %
4 3 Glycine
2
Glutamate
1
0.5 0.4 25°C
0.3
40°C
0.2 0.1 0
0 0
5
10
15
20
25
0
30
5
10
20
25
30
Fig. 17. Effect of temperature on the rate of extraction of gold, [Gly] = 0.5 M, [KMnO4] = 0.01 M, pH = 11.
Fig. 14. Effect of the type of solvent on the dissolution of gold: [Gly] = 0.5 M, [Glut] = 0.5 M, [KMnO4] = 0.01 M, pH 11.
4.7. Effect of temperature
1.80 1.60
Eksteen and Oraby (2015a,b,c) evaluated the effect of temperature on the kinetics of dissolution of gold in a 0.1 M glycine solution with hydrogen peroxide at temperatures of 23, 30, 40, 60 and 75 °C. They found that the dissolution of gold increases with temperature. This result was verified by a test comparing the leaching behavior of gold foil with potassium permanganate to 0.004 M and 0.5 M glycine with pH 11, at 25 °C and 40 °C (Fig. 17).
1.40 Au ExtracƟon, %
15 Time (h)
Time (h)
1.20 1.00 with copper ions
0.80
without copper ions
0.60 0.40 0.20
5. Conclusions
0.00 0
5
10
15
20
25
It has been shown that the type of oxidizing agent and its concentration affect gold leaching kinetics. The highest percentage of extraction was achieved with glutamate using 0.03 M potassium permanganate as an oxidant at pH 9.4. The addition of Cu2+ systems, together with potassium permanganate, using either glycine or glutamate improves the gold extraction. The great affinity between the amino acids and copper negatively affects the process of gold leaching. The present study showed gold leaching with these reagents has very slow kinetics in comparison with that of cyanide, which renders their application inconvenient, as reagents for agitated leach systems in the gold industry. In the glycine‑potassium permanganate system, the kinetics of leaching is strongly affected by the temperature, which indicates that the reaction mechanism is chemically controlled.
30
Time (h) Fig. 15. Percentage of extraction of gold in the [Glut] = 0.5 M, [Cu2+/Cu+] = 100 ppm at pH = 11 and [KMnO4] = 0.01 M.
added copper ions, additional two leaching test were performed using 5 mg of gold samples with 99.95% purity in selected conditions, based on the thermodynamic analysis, the above results and a review of the literature. In Figs. 15 and 16 it can be noted that the percentage of gold extraction in a solution of glycine with copper ions (Cu2+/Cu+) is greater than that obtained without the added copper ions; after 24 h, the extraction is doubled. Similar behavior may be observed in glutamate solutions, achieving 6 times the extraction with copper ions in 24 h. This indicates that possibly the copper ions avoids the passivation of the gold.
References Avraamides, J., 1982. Prospects for alternative leaching systems for gold: a review. In: Carbon-in-Pulp Seminar. The Aus.I.M.M. Perth and kaigooriie branches and murdoch
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Hydrometallurgy 177 (2018) 79–85
C.G. Perea, O.J. Restrepo
National University of Colombia, Medellín, Antioquia, Colombia. Guzmán, E., 2013. Gold Using Multiple Paths from a Metallurgical Material Concentrated in the Municipality of Abejorral, Antioquia. National University of Colombia, Medellín, Antioquia, Colombia. Oraby, E., Eksteen, J., 2014. The selective leaching of copper from a gold–copper concentrate in glycine solutions. Hydrometallurgy 14–19. Poisot, M., 2010. Study of Factors Affecting the Process of Electrodeposito Gold and Silver from Solutions of Thiourea in a Reactor Type Filter Press. Federal Distro of the Iztapalapa, Mexico. Cambridge University Press. The corral, G., 2003. Assessment of the Role and the Potential of the Mining Foundations and their Interaction with Local Communities. ECLAC, Santiago de Chile.
university. Aylmore, M., 2005. Alternative lixiviants to cyanide for leaching gold ores. Dev. Min. Process. 501–539. Eksteen, J., Oraby, E., 2015a. Gold leaching in cyanide-starved copper solutions in the presence of glycine. Hydrometallurgy 81–88. Eksteen, J., Oraby, E., 2015b. The leaching and adsorption of gold using low concentration amino acids and hydrogen peroxide: effect of catalytic ions, sulphide minerals and amino acid type. Miner. Eng. 36–42. Eksteen, J., Oraby, E., 2015c. The leaching of gold, silver and their alloys in alkaline glycine-peroxide solutions and their adsorption on coal. Hydrometallurgy 199–203. Galindo, L., 2013. Alternative Methods to the Cyanide Leaching of Precious Metals.
85
Contents lists available at ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Use of amino acids for gold dissolution a,⁎
C.G. Perea , O.J. Restrepo a b
T
b
Group of Environmental Geochemistry, Pedagogical and Technological University of Colombia, Sogamoso, Colombia Institute Of Minerals CIMEX, School of Mines, Universidad Nacional de Colombia, Medellin, Colombia
A R T I C L E I N F O
A B S T R A C T
Keywords: Gold leaching Cyanide alternatives Organic solvent
As oxidants under alkaline conditions. To analyze the kinetics of leaching, tests were performed using electrical contacts pins and sheets of pure gold to avoid the effects of pore diffusion. The efficiency of each oxidant was evaluated, for concentrations of 0.004, 0.01 and 0.03 M, at pH values of 9.4 and 11. At the end of each test, the solid residue was analyzed to determine the total grade and percentage of metal extraction. The experimental results indicated that the percentage of gold extraction was affected by the oxidant concentration and the pH. Furthermore, it improves with the presence of Cu2+ ions, increasing by 100% for glycine and approximately 6 fold for glutamate as compared to glycine solutions. In assessing the percentage of gold extraction in the different conditions at 25 and 40 °C, it was observed that the kinetics of extraction is sensitive to temperature change, which suggests that the leach is chemically controlled.
1. Introduction
In this study, a method was evaluated for the recovery of gold from computer contact pins and a sheet of pure gold, using alkaline solutions of amino acids such as glycine and monosodium glutamate. The selection of these reagents was based on a review of scientific literature (Aylmore, 2005; Eksteen and Oraby, 2015a,b,c), where the former conducted a review of different methods of gold leaching as an alternative to cyanidation, mentioning the dissolution with amino acids and using as potassium permanganate oxidizing agent. Meanwhile Eksteen and Oraby performed the leaching of metallic gold using amino acids such as glycine, alanine and histidine at low concentrations, in alkaline solutions with hydrogen peroxide (H2O2), evaluating the influence of reagent concentration, temperature and presence of sulphide minerals in the rate of leaching. This methodology could represent an alternative to cyanide for gold recovery and could diminish the environmental impacts generated by the inadequate handling of the compounds used for gold beneficiation. Oraby and Eksteen (2014) also evaluated the extraction of copper and gold from a copper concentrate employing glycine; their results show a copper extraction of nearly 96%. However, a very low content of gold was found in solution, using hydrogen peroxide at pH values between 8 and 11. The reactions below indicate a strong affinity of copper with glycine, both in the cuprous as well as cupric oxidation states:
The extraction of gold from its minerals is achieved through hydrometallurgical processes, where the traditional process of cyanidation has dominated. Since the beginning of the 20th Century, the acceptance of cyanidation by gold producers is a tribute to its effectiveness (Avraamides, 1982). As a process, it has resisted economic difficulties imposed by the fall in the price of gold, and the introduction of Carbon in pulp (CIP), has tended to make it even more attractive as a leaching system. The main disadvantage of cyanidation is its toxicity. The environmental problems associated with the leakage of cyanide into groundwater systems and the health risks inherent in the use of this on any scale are a strong drawback (Avraamides, 1982). Furthermore, its instability, its high affinity with other elements, the decrease easily extracted gold and the presence of refractory minerals that are not susceptible to cyanide, sometimes cause relatively high operating costs. (Poisot, 2010). For that reason, there is a clear need to implement clean technologies for the extraction of silver and gold, that increase productivity by avoiding the generation of waste contaminants (The corral, 2003). The scientific community has proposed alternative hydrometallurgical processes for the dissolution of these precious metals using thiosulphate, thiourea, chlorine, thiocyanate, among others. An alternative route could employ other organic ligands, that can compete with the cyanidation in cost, performance and effectiveness (Guzmán, 2013; Galindo, 2013). ⁎
Cu2 + + 2(H2 NCH2 COO)− ↔ Cu(NH2 CH2 COO)2 , Log10K = 15.6 Cu+
Corresponding author. E-mail address: [email protected] (C.G. Perea).
https://doi.org/10.1016/j.hydromet.2018.03.002 Received 28 August 2017; Received in revised form 22 February 2018; Accepted 1 March 2018 0304-386X/ © 2018 Published by Elsevier B.V.
+ 2(H2 NCH2
COO)−
↔
Cu(NH2 CH2 COO)−2 ,
Log10K = 10.1
(1) (2)
Therefore, further study is warranted to evaluate the ability of
Hydrometallurgy 177 (2018) 79–85
C.G. Perea, O.J. Restrepo
amino acids in general, not only to enhance, but to completely replace cyanide as a ligand for gold extraction. In the present work, two amino acids, glycine and glutamate, were employed.
H 2 (Gly)+
1.0
Gly−
H(Gly)
0.8
2. Materials and methods All leaching experiments were carried out using pins from computers and thin gold sheets, to avoid the effects of pore diffusion, common in mineral, and to evaluate gold dissolution with these selected amino acids. The pins are electrical contacts, composed of gold-plated metal alloys. Therefore, these pins, due to its characteristics and its relatively low cost, became the ideal material to assess the conditions in which gold dissolves in aqueous solutions of these organic ligands (amino acids). The results obtained with the pins were compared with two tests performed on samples cut from a pure gold sheet (99.95%, SigmaAldrich). The organic ligands (glycine and glutamate) and oxidizing agents (hydrogen peroxide (30% w/v) and potassium permanganate) employed in this work were of analytical grade. For pH adjustment, NaOH was used. The pins were characterized by scanning electron microscopy SEMEDX (ZEISS EVO MA10), in order to estimate the chemical elements present and their distribution. A sample of these was digested in a mixture of nitric acid and hydrochloric acid, later the elemental composition of this solution was determined by using atomic absorption spectrophotometry (Varian SpectrAA 220fs). The pins were subjected to a pre-treatment, which consisted in the placement of 5 g of pins into 200 ml of 1.8 M H2SO4 and 7.8 M H2O2 aqueous solution for 3 h, mixing at 300 rpm, to eliminate the presence of other metals that might affect the gold leaching process. The filtered residue contained mainly gold. Leaching experiments of the residue were performed in aqueous solutions of 0.5 M glycine or glutamate, with potassium permanganate or hydrogen peroxide as oxidizing agents (Eksteen and Oraby, 2015a,b,c). Liquid samples of 0.5 ml were drawn at 1, 2, 4, 6, 23 and 24 h. Each of the samples was filtered and analyzed to determine the gold content in solution by the atomic absorption spectrophotometer. In Table 1 is shown the conditions of the leach test, where the pH value of 9.4 was selected based on the thermodynamic analysis (see below), while the pH 11 and the initial concentration of oxidizing agent to the 0.03 M was based on previous work (Eksteen and Oraby, 2015a,b,c) (Aylmore, 2005). Once the leach time was completed, the solution was filter and the leach residue was digested using a mixture nitric and sulfuric acids, which was analyzed by atomic absorption spectrophotometry, to determine the final gold content and to carry out the mass balance.
Fraction
0.6
0.4
0.2
0.0 0
2
4
6
8
10
12
14
pH Fig. 1. Species distribution diagram as a function of pH for glycine at 25 °C, [Gly] = 0.5 M.
Oraby (2015a,b,c). No value was found for the gold-glutamate complex; given its chemical similarity to glycine (glutamate has an additional carboxylic group), similar behavior can be assumed. In Fig. 1 is presented the species distribution diagram for 0.5 M glycine as a function of pH. At alkaline pH, the dissociation of the glycine increases, thus achieving the highest concentration of glycinate available to form complexes with gold or any other metal. Aylmore (2005) reported a log10 (stability constant) of 18 for the gold (I) - glycine complex. In Fig. 2 is shown the diagram of thermodynamic stability Eh-pH for the gold-glycine at room temperature in 0.5 M of glycine. As may be noted, the elemental gold is the species stable at all values of pH and potential under 0.5 V; for higher values, there is a zone of stability for the formation of the complex gold-glycine, although the lowest oxidation potential is reached at pH 9.4. Meanwhile, Fig. 3 shows the diagram of thermodynamic stability Eh-pH for the gold-glycine at temperature of 40 °C in 0.5 M of glycine. As may be noted, the elemental gold is the species stable at all values of pH and potential under 0.5 V; for higher values, there is a zone of stability for the formation of the complex gold-glycine, although the lowest oxidation potential is reached at pH 9.4. However, at higher values of potential of 0.5 V and pH 11, the predominant species is Au(OH)2−, which AuOH 2+ Au(OH) 2 +
1.5
Au(OH) 3 (c)
3. Thermodynamic analysis The thermodynamic analysis was performed with the Hydra – Medusa software suite, where the effect of the concentration of oxidizing agent and pH, to determine the potentially adequate conditions and interpretations of the kinetics for gold leaching. The stability constant for the complex gold-glycine complex was given by Aylmore (2005) and the reagent concentrations to be evaluated by Eksteen and
Au(OH) 4 − Au(OH) 5 2 −
1.0
ESHE / V
Au(Gly) −
0.5
Table 1 Conditions of the computer pin leaching experiments. Reagents
Oxidizing agents (concentration)
PH
Glutamate Glutamate Glutamate Glutamate Glycine Glycine Glycine
KMnO4 (0.03 M) H2O2 (0.03 M) KMnO4 (0.01 M) KMnO4 (0.04 M) H2O2 (0.03 M) KMnO4 (0.01 M) KMnO4 (0.04 M)
9.4 9.4 11 11 9.4 11 11
Au(OH) 2 − Au(c)
0.0 0
2
4
6
8
10
12
14
pH Fig. 2. Eh-pH diagram for the Au-glycine system at 25 °C, [Au] = 10−5 M, [Gly] = 0.5 M.
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Hydrometallurgy 177 (2018) 79–85
C.G. Perea, O.J. Restrepo
1.5
1.5
A u(O H ) 2+ A uO H 2+ A u (O H ) 3 (c) A u(O H ) 4− A u(O H ) 52−
A u(G ly) −
Cu 2+ Cu(Gly) +
SH E
ESHE / V
/ V
1.0
1.0
0.5
E
A u (O H ) 2 −
Cu(OH) 42−
Cu(Gly) 2
0.5
0.0
Au(c)
Cu(cr)
Cu(Gly) 2−
Cu(OH) 2−
-0.5
0.0 0
2
4
6
8
10
12
0
14
Fig. 3. Eh-pH diagram for the Au-glycine system at 40 °C, [Au] = 10−5 M, [Gly] = 0.5 M.
12
14
The analysis of the pins employed in the leach testing was performed using Scanning Electron Microscopy (SEM), with EDX. The cross-section of the sample showed the existence of three layers in each of the pins: a core composed mainly of Cu and with small amounts of Sn (Fig. 6), a coating in the central zone consisting of Al, Cu, Fe, and Ni (Figs. 7 and 8), with an average thickness of 80 μm and finally, an outer layer of a very thin gold film, with a thickness of less than 10 μm (Fig. 9). The results of the semi-quantification in general reveal variation of the elements in the pins in terms of content and proportions. For example, in the core, the Cu concentrations are at least 97% and the Sn less than 3%, in atomic percentage. The composition of the first coating (which borders the core) varies: 6–41% Fe, 3–17% Ni, 13–34% Cu and 5–71% Al; gold is less than 4%. In the outer-most layer, the atomic percentages of gold are 83–91%, while Cu varies between 4 and 9%, Ni is less than 7% and Al less than 3%. Because the SEM results are non - quantitative and only show point values, they are not adequate to determine the gold composition of the sample. For that reason, Regia digestions were performed on several samples, which were subsequently analyzed by atomic absorption spectrophotometry. A sizeable disparity in the gold and copper content is presented in Table 2. Because of this variability, a metallurgical balance (gold extracted + gold remaining in the residue) was calculated for each leaching experiment and results are reported as % extraction.
Au(OH) 3 (c)
1.0
ESHE / V
10
4.1. Characterization of the computer pins
pH 9.4
Au(Gly) −
0.5
4.2. Leaching test
Au(c)
Log
8
4. Results and discussion
1.5
-3
6
waste, such as computer pins. For that reason, these were pretreated in an oxidizing medium to remove the other metals, especially copper.
can generate a passivation that affects the leaching process. Based on this analysis, the chosen testing conditions were pH 9.4 and 11, which is within the range in which the oxidation reaction could occur with 0.5 M glycine. On the other hand, at lower glycine concentrations, the oxidation is more difficult, as may be observed in Fig. 4; at 0.5 M (log10[Gly] = −0.3), the oxidation potential is approximately 0.5 V, while at 0.01 M, the potential is 150 mV higher. For that reason, 0.5 M glycine was chosen for the leaching solution. However, the most important disadvantage associated with the use of glycine is its great affinity for copper ions. At conditions, similar to those used to leach gold (0.5 M glycine and pH 9.4 and 11), copper is oxidized at potentials approximately 800 mV lower (Fig. 5). This implies that if metallic copper is present in the material to be leached, it will be preferentially dissolved, consuming oxidant as well as glycine. For that reason, any metallic copper should be removed from the solid, before leaching gold. This may not be the case in refractory copper sulfide minerals, but is particularly important in the treatment of e-
-4
4
Fig. 5. Eh-pH diagram for the Cu-glycine system at 25 °C, [Cu] = 10−5 M, [Gly] = 0.5 M.
pH
0.0 -5
2
-2 [Gly− ]
-1
4.2.1. Preparation of the material (pretreatment) As was mentioned previously, thermodynamic predictions and the results found by Eksteen and Oraby (2015a,b,c) show the great affinity of copper for glycine. Consequently, pre-treatment of the pins with sulfuric acid and hydrogen peroxide performed to remove the copper present in these, since this element constitutes the largest percentage of composition and so as not to affect the process of leaching of gold with glycine and glutamate. As a result of the pretreatment, the weight of the final residue was only 20–25% that of the original sample.
0
TOT
Fig. 4. Eh-log10 [glycine] diagram at pH 9.4 and 25 °C for the Au-glycine system, [Au] = 10−5 M.
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Hydrometallurgy 177 (2018) 79–85
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Fig. 6. Electron micrograph SEM and EDX of the pin core.
4.3. Effect of the oxidizing agent To study the effect of the oxidizing agent in the leaching of gold in solution of monosodium glutamate, is added hydrogen peroxide (H2O2) and potassium permanganate (KMnO4) both to a concentration of 0.03 M to 0.5 M glutamate solution at room temperature (Fig. 10). The results show that oxidizing with hydrogen peroxide is less effective in glutamate solution in comparison with KMnO4. This is due to the fact that H2O2 favors the reaction of the amino acid with copper, given that under these conditions high concentrations of copper were obtained in solution, especially at the second hour, where the greatest decrease in the percentage of gold extraction occurs (data not shown here). 4.4. Effect of oxidizing agent concentration The effect of the concentration of potassium permanganate on the dissolution of gold in solutions of glycine and glutamate is shown in Figs. 11 and 12, respectively. It is clear that by increasing the concentration of KMnO4, the extraction of gold increases slightly. In general terms, the dissolution of gold depends on the concentration of KMnO4, which would be limiting reagent in solutions of potassium permanganate-glycine and glutamate‑potassium permanganate. However, excess oxidant has been shown to promote degradation of the reagents affecting the extraction of gold. In the results shown, low concentrations of potassium
Fig. 7. SEM micrograph of the average thickness of the core.
4.2.2. Leaching Each leaching test employed 39 mg of the residue in 100 ml of the corresponding solution. Listed below are some of the most effective leaching tests, with their respective conditions and results:
Fig. 8. Electron micrograph SEM and EDX central zone (core coating).
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Fig. 9. SEM and EDX electron micrograph of the pin surface (gold layer).
2.50
Table 2 Percentage of elemental composition of the pins.
Au Fe Cu Ni Zn Ag Pb
Average
Uncertainty
(%)
(+/−)
0.64 0.115 68.1 6.365 0.005 0.05 0.245
0.24 0.085 4.3 5.735 0.005 0.05 0.245
2.00 Au Extracon, %
Element
1.50 0.01 M KMnO4
1.00
0.004 M KMnO4
0.50 0.00 0
12.00 Au extracon, %
10
20
30
Time (h)
14.00
Fig. 12. Effect of the concentration of oxidizing agent on the dissolution of gold: [Glut] = 0.5 M, pH 11.
10.00
permanganate were employed in order to avoid reagent degradation. At low concentrations, reaction rates are much lower and a stagnation of the reaction occurs, although it presents a greater stability in the formation of the complex since there is no re-precipitation of gold in solution. This phenomenon of stagnation can be due to the fact that under these conditions of pH and low values of potential the predominant species would be (AuOH) auroso hydroxide and its presence might generate passivation of the gold surface, preventing the reaction with amino acids.
8.00 KMNO4
6.00
H2O2
4.00 2.00 0.00 0
5
10
15
20
25
30
Time (h) Fig. 10. Effect of oxidizing agent on the dissolution of gold: [Glut] = 0.5 M, [H2O2] and [KMnO4] = 0.03 M, pH 9.44.
4.5. Effect of solvent type In Fig. 13, the reaction of both amino acids with the gold is possible, achieving a greater extraction in the case of glycine. There is re-precipitation of gold with both ligands after 2 h and with glycine after 20 h. The oxidizing agent favors the reaction of both amino acids with copper, given that in these conditions high concentrations of copper in solution were obtained. According to Eksteen and Oraby (2015a,b,c), the evaluation the gold dissolution with these amino acids should be only after 150 h. In Fig. 14 is used in low concentrations of potassium permanganate, showing a slightly greater extraction with glycine, but are observed reaction rates much lower and a slight stagnation of the reaction, although it presents a greater stability in the formation of the complex as it is not re-precipitation of gold in solution. In the end, however, the degradation of both reagents due to the oxidizing agent is very strong.
4.50 4.00 Au Extracon, %
3.50 3.00 2.50
0.01 M KMnO4
2.00
0.004 M KMnO4
1.50 1.00 0.50 0.00 0
5
10
15
20
25
30
Time (h) Fig. 11. Effect of the concentration of oxidizing agent on the dissolution of gold: [Gly] = 0.5 M, pH 11.
4.6. Effect of copper ions To study the behavior of the gold in the absence and presence of 83
Hydrometallurgy 177 (2018) 79–85
3.50
0.80
3.00
0.70
2.50 2.00
Au extracon, %
Au Extracon, %
C.G. Perea, O.J. Restrepo
Glycine
1.50
Glutamate
1.00
0.60
with copper ions
0.50
without copper ions
0.40 0.30 0.20 0.10
0.50
0.00
0.00 0
5
10
15
20
25
0
30
5
10
15
20
25
30
Time (h)
Time (h)
Fig. 16. Percentage of extraction of gold in the [Gly] = 0.5 M, [Cu2+/Cu+] = 50 ppm at pH = 10.1 and [KMnO4] = 0.01 M.
Fig. 13. Effect of the type of solvent on the dissolution of gold: [Gly] = 0.5 M, [Glut] = 0.5 M, [H2O2] = 0.03 M, pH 9.44.
0.7
5
0.6 Au Extracon, %
Au Extracon, %
4 3 Glycine
2
Glutamate
1
0.5 0.4 25°C
0.3
40°C
0.2 0.1 0
0 0
5
10
15
20
25
0
30
5
10
20
25
30
Fig. 17. Effect of temperature on the rate of extraction of gold, [Gly] = 0.5 M, [KMnO4] = 0.01 M, pH = 11.
Fig. 14. Effect of the type of solvent on the dissolution of gold: [Gly] = 0.5 M, [Glut] = 0.5 M, [KMnO4] = 0.01 M, pH 11.
4.7. Effect of temperature
1.80 1.60
Eksteen and Oraby (2015a,b,c) evaluated the effect of temperature on the kinetics of dissolution of gold in a 0.1 M glycine solution with hydrogen peroxide at temperatures of 23, 30, 40, 60 and 75 °C. They found that the dissolution of gold increases with temperature. This result was verified by a test comparing the leaching behavior of gold foil with potassium permanganate to 0.004 M and 0.5 M glycine with pH 11, at 25 °C and 40 °C (Fig. 17).
1.40 Au ExtracƟon, %
15 Time (h)
Time (h)
1.20 1.00 with copper ions
0.80
without copper ions
0.60 0.40 0.20
5. Conclusions
0.00 0
5
10
15
20
25
It has been shown that the type of oxidizing agent and its concentration affect gold leaching kinetics. The highest percentage of extraction was achieved with glutamate using 0.03 M potassium permanganate as an oxidant at pH 9.4. The addition of Cu2+ systems, together with potassium permanganate, using either glycine or glutamate improves the gold extraction. The great affinity between the amino acids and copper negatively affects the process of gold leaching. The present study showed gold leaching with these reagents has very slow kinetics in comparison with that of cyanide, which renders their application inconvenient, as reagents for agitated leach systems in the gold industry. In the glycine‑potassium permanganate system, the kinetics of leaching is strongly affected by the temperature, which indicates that the reaction mechanism is chemically controlled.
30
Time (h) Fig. 15. Percentage of extraction of gold in the [Glut] = 0.5 M, [Cu2+/Cu+] = 100 ppm at pH = 11 and [KMnO4] = 0.01 M.
added copper ions, additional two leaching test were performed using 5 mg of gold samples with 99.95% purity in selected conditions, based on the thermodynamic analysis, the above results and a review of the literature. In Figs. 15 and 16 it can be noted that the percentage of gold extraction in a solution of glycine with copper ions (Cu2+/Cu+) is greater than that obtained without the added copper ions; after 24 h, the extraction is doubled. Similar behavior may be observed in glutamate solutions, achieving 6 times the extraction with copper ions in 24 h. This indicates that possibly the copper ions avoids the passivation of the gold.
References Avraamides, J., 1982. Prospects for alternative leaching systems for gold: a review. In: Carbon-in-Pulp Seminar. The Aus.I.M.M. Perth and kaigooriie branches and murdoch
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National University of Colombia, Medellín, Antioquia, Colombia. Guzmán, E., 2013. Gold Using Multiple Paths from a Metallurgical Material Concentrated in the Municipality of Abejorral, Antioquia. National University of Colombia, Medellín, Antioquia, Colombia. Oraby, E., Eksteen, J., 2014. The selective leaching of copper from a gold–copper concentrate in glycine solutions. Hydrometallurgy 14–19. Poisot, M., 2010. Study of Factors Affecting the Process of Electrodeposito Gold and Silver from Solutions of Thiourea in a Reactor Type Filter Press. Federal Distro of the Iztapalapa, Mexico. Cambridge University Press. The corral, G., 2003. Assessment of the Role and the Potential of the Mining Foundations and their Interaction with Local Communities. ECLAC, Santiago de Chile.
university. Aylmore, M., 2005. Alternative lixiviants to cyanide for leaching gold ores. Dev. Min. Process. 501–539. Eksteen, J., Oraby, E., 2015a. Gold leaching in cyanide-starved copper solutions in the presence of glycine. Hydrometallurgy 81–88. Eksteen, J., Oraby, E., 2015b. The leaching and adsorption of gold using low concentration amino acids and hydrogen peroxide: effect of catalytic ions, sulphide minerals and amino acid type. Miner. Eng. 36–42. Eksteen, J., Oraby, E., 2015c. The leaching of gold, silver and their alloys in alkaline glycine-peroxide solutions and their adsorption on coal. Hydrometallurgy 199–203. Galindo, L., 2013. Alternative Methods to the Cyanide Leaching of Precious Metals.
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