Hydrometallurgy 156 (2015) 81–88

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Gold leaching in cyanide-starved copper solutions in the presenceof glycine

E.A. Oraby, J.J. Eksteen ⁎Western Australian School of Mines, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

⁎ Corresponding author.E-mail address: jacques.eksteen@curtin.edu.au (J.J. Eks

http://dx.doi.org/10.1016/j.hydromet.2015.05.0120304-386X/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 March 2015Received in revised form 23 May 2015Accepted 26 May 2015Available online 1 June 2015

Keywords:Copper–cyanide complexesGoldGlycine

In the cyanidation of copper–gold ores, the presence of copper minerals can lead to soluble gold losses, the pro-duction of weak acid dissociable (WAD) cyanide, as well as a number of operational challenges in CIP/CIL circuitswith regard to competitive adsorption, and subsequent difficulties associated with elution, electrowinning andsmelting. In addition, copperminerals are significant cyanide consumers, leading to higher cost in ore treatment.This paper presents a process to enhance the dissolution of gold using copper–cyanide solutions in the presenceof glycine where the solution is cyanide starved. The effect of glycine addition on gold leaching kinetics in cop-per–cyanide solutions under different leaching conditions was studied. The results show that, in the presenceof glycine, gold dissolution rate increases significantly in solutions containing copper–cyanide species at a verylow, or zero, free cyanide concentration. In the presence and absence of glycine, gold dissolution rates in solutionscontaining 10mMCu(CN)32−were 11.1 μmol/m2·s and 0.65 μmol/m2·s, respectively. It is shown that the averagegold dissolution rate in a Cu–CN−-glycine system is about 6.5 times higher than the gold dissolution rate in theconventional cyanidation, the presence of cyanide being similar in each system. Kinetic and electrochemicalstudies were conducted to evaluate the effects of glycine concentration, pH, CN/Cu ratio, and initial copper con-centrations on the dissolution of gold. It has also been shown that the gold dissolution rate increases by increasingthe glycine concentration up to 1 g/L and any further glycine addition has a negative effect on the dissolution ofgold. Increasing leaching pH up to 12 enhances gold dissolution in the copper–cyanide–glycine solutions. How-ever, increasing leaching pH to 13 has an adverse effect on the dissolution of gold.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

In the cyanidation of copper–gold ores to recover precious metals,the presence of copper minerals can cause a number of serious prob-lems, which include, but are not limited to:

(1) Copperminerals consume about 30 kg/t NaCN for every 1% of re-active copper present (Muir, 2011) or about 2.3 kg of NaCN forevery kilogram of Cu leached (Stewart and Kappes, 2012). Thehigh cyanide consumption could render the conventionalcyanidation of high copper–gold ores or concentrates uneco-nomic, unless other additional processes are added to recovereither cyanide or both copper and cyanide.

(2) The presence of copper minerals has a negative effect on theextraction of gold; leading to various gold losses and challenges(Muir et al., 1993; Nguyen et al., 1997) such as:

– Native copper dilutes gold gravity concentrate or/and provides areducing surface for gold cementation during the leaching;

– High free cyanide consumption leading to low gold recovery due toless free cyanide available;

teen).

– Competitive adsorption with gold leading to lower gold recovery;and

– Gold losses during electrowinning, smelting and refining.

(3) Additional processes are required to recover the copper and/or cyanide such as AVR (Acidification, Volatilization andReneutralization) and SART (Sulphidization, Acidification,Recycling and Thickening), adsorption onto activated carbonor ion exchange resins, electrowinning and solvent extrac-tion. Even then, the cyanide that deports to Fe(CN)64− andthiocyanate cannot be economically recovered (Dai et al.,2012; Estay et al., 2012).

(4) The presence of copper in the final tail solutions significantlycontributes to weak acid dissociable (WAD) cyanide specieswhich should be kept below 50 mg/L before the tail water isdischarged to the environment (Botz and di Parodi, 1997;Donato et al., 2007; Mudder and Botz, 2001) and, consequently,requires additional cyanide detoxification/destruction processes.

In leaching gold from copper-containing gold ores by cyanidation,some copper minerals dissolve in cyanide solutions to form different

Fig. 1. Potential-pH diagram for the copper–water–glycine system at 25 °C and 1 atm,0.01 M glycine and 10−6 M cupric nitrate (after Aksu and Doyle, 2001).

82 E.A. Oraby, J.J. Eksteen / Hydrometallurgy 156 (2015) 81–88

soluble copper–cyanide complexes such as Cu(CN)2−, Cu(CN)32− andCu(CN)43−. The distribution of these complexes in the leach solution isnormally a function of the cyanide concentration and pH (Dai et al.,2005). Recently, the recovery and re-use of the copper–cyanide com-plexes from the final leaching solution streams have received someattention. Therefore, research investigations have been conducted toeconomically treat copper containing gold ores by either reducing thecyanide consumption using a cyanide–ammonia system in the leachingstage (Costello et al., 1992; Jeffrey et al., 2002; La Brooy et al., 1991;Muiret al., 1991, 1993), or by recovering copper and recycling the cyanideafter leaching (Adams et al., 2008; Alonso-Gonzalez et al., 2009;Alonso-González et al., 2013; Gonen et al., 2004; Xie and Dreisinger,2009a,b).

A fundamental study by Jeffrey et al. (2002) has shown that the ad-dition of ammonia to solutions containing cuprous cyanide species in-creases the dissolution of gold. The presence of copper(II) was alsoshown by the authors to increase the gold leach rate. However, thereare some issues regarding the stability of the cyanide–ammonia systemand the environmental considerations of ammonia which need atten-tion as the threshold limiting value (TLV) for ammonia gas, in air, is14 mg/m3 (Gos and Rubo, 2000).

There are also increasing environmental concerns and restrictionson the discharge of cyanide and copper–cyanide complexes to tailingdams. It is well known that copper–cyanide species are more difficultto destroy naturally than free cyanide although they are less toxicthan the free cyanide (Mudder and Botz, 2001). Destruction of the cya-nide and copper cyanides before discharge is adding an additional costto the gold production process and the treatment cost also increasesin the presence of high copper concentrations (Dai and Breuer, 2009).

Some research studies have also focused on the concept of leachingcopper–gold ores in cyanide deficient copper solutions. The outcomes ofthese studies show that gold leaching in air-saturated Cu(CN)32− solu-tions occurs at amuch slower rate than that in solutionswith significantfree cyanide (Breuer et al., 2005; Nugent, 1991). The Cu(CN)32− speciestend to predominate under gold leaching conditions, even though theCu(CN)2− species tend to predominantly adsorb on to carbon. As was re-ported by Fleming and Nicol (1984) and recently by Dai et al. (2010),the adsorption of copper cyanide species on carbon is following theorder of Cu(CN)2− N Cu(CN)32− N Cu(CN)4−3.

Therefore, the main aim of this work is to investigate the effect ofadding glycine on the enhancement of gold dissolution in solutions con-taining different copper–cyanide species, based on earlier research bythe authors looking at the solubility of copper minerals and gold in analkaline glycine–peroxide system (Eksteen and Oraby, 2015; Orabyand Eksteen, 2014, 2015). Glycine is one of the simplest and cheapestamino acids among the available 20 amino acids; it can be easily pro-duced industrially or derived as a by-product of different micro-organisms (González-lópez et al., 2005). The presence of glycine canalso enhance the solubility of gold in aqueous solutions as the mainlixiviant of dissolving gold due to its complexing action with gold(Brown and Smith, 1982; Aylmore, 2005; Eksteen and Oraby, 2015).Glycine also increases copper solubility at higher pH and lowerpotentials, and the stability constant (logβ) of glycine with copper, ac-cording to Aliyu and Na'aliya (2012), is 18.9. As shown in Fig. 1, afterAksu and Doyle (2001), the stability regions of CuO and Cu2O in acopper–water–glycine system decrease by increasing the glycineconcentration. Glycine also forms soluble complexes with both cupric[Cu(H3NCH2COO)+2], Cu(H2NCH2COO)+, Cu(H2NCH2COO)2 and cu-prous ions (Cu(H2NCH2COO)−2). These species are referred to in Fig. 1as CuHL2+, CuL+, CuL2 for Cu(II) and CuL2− for Cu(I). The large Eh–pHstability region of the neutral cupric ligand (CuL2) should be noted(Fig. 1), as well as the transition points from ligand to oxide stabilityfields at pHs around 12.5, giving the pH ceiling for a practical interactionwith glycine.

Therefore, considering themerits of copper–glycine species stability,this research work presents a potential process to treat copper–gold

ores in the presence of glycine. The proposed work aims to significantlyenhance the leaching of gold in copper–cyanide solutions containinglow free cyanide by the addition of glycine.

2. Experimental

All experiments were carried out using solutions prepared from an-alytical grade reagents and distilled water. An analytical grade reagent(AR) of glycine has been used in all the conducted experiments. Cop-per–cyanide complexes were prepared by the complete dissolution ofan analytical grade CuCN powder (5.5 mM Cu) in sodium cyanide solu-tions (5.5 mM CN−). Leaching experiments were carried out using apure gold rotating disc electrode. The gold discwas polished using awa-terproof silicon carbide paper (FEPA # 2400) and 3-micron aluminiumoxide powder prior to each experiment. The pure gold disc was madefrom pure gold foil (99.99% Au). All discs are 17 mm in diameter andhave a surface area of 2.27 cm2. Gold and copper in solutions were de-termined by atomic absorption spectrophotometry (AAS) and induc-tively coupled plasma optical emission spectrometry (ICP-OES).

The electrochemical experiments (linear sweep voltammetry) wereconducted using a Pine WaveNow potentiostat. The reaction cell was athree-electrode system in which a platinumwire was used as a counterelectrode. All potentials were measured relative to the Ag/AgCl elec-trode (+0.199 V vs. SHE), but reported relative to the standard hydro-gen electrode (SHE). A gold disc electrode of 5 mm in diameter wasused to evaluate the anodic reaction of gold in the copper–cyanide solu-tions, in the presence and absence of glycine. Linear sweep voltammetryexperiments were performed at 22 °C, with a rotation rate of 400 rpmand a scan rate of 1 mV/s. The oxidation current (i) was measured asμA and reported as A/m2 at different potential ranges.

3. Results and discussions

In the following sections, kinetic and electrochemical studies wereconducted to evaluate and demonstrate the fundamental aspects ofgold leaching in cuprous–cyanide solutions in the presence and absenceof glycine under different leaching conditions. Gold dissolution ratesand linear sweep voltammograms for the anodic gold dissolution havebeen conducted. The effects of glycine concentration, solution pH, initialcopper concentration and glycine to coppermolar ratio on gold dissolu-tion rate and the anodic reaction of gold in copper–cyanide–glycine sys-tem are presented below. A comparison of using different complexingagents of copper such as glycine, ammonia and EDTA will also be pre-sented in the following sections.

83E.A. Oraby, J.J. Eksteen / Hydrometallurgy 156 (2015) 81–88

3.1. Gold leaching in cuprous cyanide–glycine solutions

Preliminary experimentswere conducted to evaluate the leaching ofgold in solutions containing cuprous cyanide at a very lowor nearly zerofree cyanide (starved cyanide solutions). This was done using thefollowing steps and leaching conditions; (step 1) leaching at naturalpH (pH 10.5) of the of cuprous cyanide solutions; (step 2) increasingthe pH of the leach solutions to 12.5 using NaOH; (step 3) adding0.85 g/L glycine to the leach solution and re-adjusting pH to 11.5. Thegold dissolution from leaching a pure gold disc (2.27 cm2) after 2 h,for each step, is shown in Fig. 2. It can be seen that increasing the pHof cuprous cyanide solutions (step 2) enhances the dissolution of goldusing Cu(CN)2− solutions at a rate two times higher than at pH 10.5. Atthis step (step 2), a blue precipitate was observed in the leach solution.The blue precipitate from Cu–CN solutions, at a low free cyanide, lowcyanide–copper ratio (b3.0) and high pH, can be attributed to the pre-cipitation of copper as Cu(OH)2 as shown in Eq. (1) (Vukcevic, 1997).

Cu CNð Þ−2 þ 14O2 þ 1

2H2Oþ OH−→Cu OHð Þ2 sð Þ↓þ 2CN− ð1Þ

The enhancement of gold dissolution at a higher pH can be attribut-ed to the released free cyanide ions (Eq. (1)). However, at this condition,CuCN can also be precipitated as shown in Eq. (2). The precipitation ofCuCN and Cu(OH)2 can passivate the gold surface and, hence, the golddissolution rate decreases (Breuer et al., 2005).

Cu CNð Þ−2 ↔CuCNþ CN− ð2Þ

The most interesting result observed in step 3 was that once glycinehas been added to the leach solution, the gold dissolution increases dra-matically. Gold dissolution in solutions containing cuprous cyanide spe-cies in the presence of glycine was more than twelve times higher thanin the absence of glycine. It was also observed that all the precipitatesfrom step 2 were completely re-dissolved after about 10 min of addingglycine. It is most likely that the effect of glycine on gold dissolution, insolutions containing copper–cyanide complexes, is similar to the effectof ammonia in the copper–cyanide solutions.

3.2. The mechanism of copper–cyanide–glycine system

The mechanism and the roles of glycine in copper–cyanide–glycinesystem are expected to be one or more of the following:

1. Replacing cyanide by complexing Cu(I) or any Cu(II) formed fromthe oxidation of Cu(I). It has been indicated, by Aksu and Doyle(2001), that glycine can form soluble complexes with both cupricand cuprous ions as shown in Eqs. (3)–(5). As a comparison, the

Fig. 2. Gold dissolution in cuprous cyanide solutions (500 mL) at room temperature (instep 3, 0.85 g/L glycine added after 4 h leaching time).

stability constants for copper and gold complexes with different li-gands of cyanide, glycine, ammonia and EDTA have been reportedby different authors as shown in Table 1.

Cu2þ þ H2NCH2COOð Þ−↔Cu NH2CH2COOð Þþ; ð3Þ

Cu2þ þ 2 H2NCH2COOð Þ−↔Cu NH2CH2COOð Þ2; ð4Þ

Cuþ þ 2 H2NCH2COOð Þ−↔Cu NH2CH2COOð Þ−2 ; ð5Þ

2. Dissolving the passivation layer of both Cu(OH)2 and CuCN on thegold surface, and re-generating free CN− ions from CuCN whichwill be involved in any further gold dissolution. Breuer et al.(2005), reported that when Cu(CN)32− reacts with gold or copperminerals to form Cu(CN)2−, which is rapidly dissociated wherethere is no free cyanide in the solution and forms a passivationlayer of CuCN on the surface of gold and copper minerals as shownin Eq. (6).

2Cu CNð Þ−2 ↔CuCNþ Cu CNð Þ2−3 ð6Þ

3. Glycine is acting asmain lixiviant for gold dissolution in thepresenceof peroxide according to Eq. (7) (Eksteen and Oraby, 2015).

4Auþ 8NH2CH2COOHþ 4NaOHþ O2 →H2O2 4Na Au NH2CH2COOð Þ2

� �

þ 6H2O ð7Þ

As shown in Fig. 2, in the presence of glycine, gold dissolution in-creases significantly. The rate of gold dissolution, in the presence of gly-cine, was 6.28 μmol/m2·s and, without glycine, was 0.54 μmol/m2·s.

3.3. Linear sweep voltammetry

To further investigate the effect of glycine on gold dissolution, linearsweep voltammetry studies were conducted in copper–cyanide solu-tions containing glycine. Fig. 3 shows the linear sweep voltammogramsof the anodic gold dissolution in a solution containing 5.5mMCu(I) and11 mMCN− in the presence and absence of 1 g/L glycine. It can be seenthat, in the absence of glycine, the current density generated from thegold oxidation was very small at the whole range of applied potentials(50–300 mV vs SHE). However, the current density was significantlyenhanced in the presence of 1 g/L glycine. These results confirm thoseobtained from the kinetic results of the gold leach rates shown inFig. 2. The current density for gold oxidation in the presence of glycineincreases linearly by increasing the potential from 75 to 200 mV(Fig. 3). It also can be seen that by increasing the applied potentialabove 200mV, the oxidisation current still increases but at a lower rate.

3.4. Kinetics of gold dissolution

Gold leaching kinetics in free cyanide (11 mM CN−) and copper–cyanide–glycine containing 5.5 mM Cu(I), 11 mM CN− and 1 g/L(13.3 mM) glycine solutions were conducted for 2 h at pH 11.5and the results are shown in Fig. 4. It is interesting to see that the av-erage gold dissolution rate in copper–cyanide–glycine solutions(4.78 μmol/m2·s) is about 6.5 times higher than the dissolutionrate of gold in free cyanide solutions (0.73 μmol/m2·s) in the pres-ence of a similar amount of cyanide (11 mM) in each system. Thepresence of glycine may play a major role in the enhancement ofgold dissolution as either a main lixiviant or as a complexing agentfor Cu(II), which can act as an additional oxidant for gold dissolution.As found by Jeffrey et al. (2002), the presence of Cu(II) increases thegold leach rate in the copper–cyanide–ammonia system in the

Table 1Stability constant of gold and copper with different ligands.

Ligand Gold Copper

Complex Log ᵦ Reference Complex Log ᵦ Reference

Cyanide Au(CN)2− 38.3 Senanayake (2004) Cu(CN)2− 16.3 Smith and Martell (1976)Au(CN)4− 85.0 Sharpe (1976) Cu(CN)32− 21.7

Cu(CN)43− 23.1Glycine Au(NH2CH2COO)2− 18.0 Michel and Frenay (1999) Cu(NH2CH2COO)+ 8.6 Aksu and Doyle (2001)

Cu(NH2CH2COO)2 15.6 Aksu and Doyle (2001)Cu(NH2CH2COO)2− 10.1 Aksu and Doyle (2001)

Ammonia Au(NH3)2+ 19.5 Senanayake (2004) Cu(NH3)42+ 12.52 Hogfeldt (1982)Au(NH3)43+ 46.0 Senanayake (2004)

EDTA Cu(EDTA)2− 18.7 Smith et al. (1998)

84 E.A. Oraby, J.J. Eksteen / Hydrometallurgy 156 (2015) 81–88

absence of free cyanide. The slow rate of gold dissolution in a solu-tion containing low cyanide concentration can be attributed to thefact that gold dissolution is diffusion limited by the cyanide concen-tration at these conditions. It is well known in the cyanidationprocess that both cyanide concentration and dissolved oxygen influ-ence the dissolution rate of gold. The dissolution rate is either a func-tion of cyanide concentration at low cyanide concentrations or afunction of oxygen concentration at high cyanide concentrations(Habashi, 1967).

3.5. Effect of glycine

The kinetics of gold dissolution in solutions containing 5.5 mMCu(I) and 11 mM CN− in the presence of different glycine concentra-tions (at pH 11.95 and room temperature) were studied and the resultsare shown in Fig. 5. From the results shown in Fig. 5, the gold dissolutionrate and the amounts of gold in the leach solution increase by increasingthe glycine concentration up to 1 g/L. The results also show that golddissolution is enhanced by the presence of glycine at very low concen-trations, even as low as 3.33 mM (0.25 g/L).

The gold leaching rates in the copper–cyanide–glycine (Gly) systemat different ratios of Gly/Cu are shown in Fig. 6. By increasing the molarratio of Gly/Cu up to 2.4, the gold leach rate increases linearly. However,any further increase in the glycine concentration has a negative effect onthe dissolution of gold (also shown in Fig. 5). Increasing glycine from1 g/L (Gly/Cu molar ratio 2.4:1) to 2 g/L (Gly/Cu molar ratio 4.8:1)leads to an increase of the free glycine in the leach solution which con-sequently retards the gold dissolution as it will be shown later in Fig. 8.It can also be seen that the optimum glycine addition should be slightlyhigher than the essential stoichiometric amount of glycine that is re-quired to complex the amount of copper (i.e., Gly/Cu= 2.0) in the solu-tion as shown in Eqs. (3)–(5). Glycine can also dissolve gold and forms a

Fig. 3. Linear sweepvoltammograms showing the oxidation of gold in solutions containing5.5 mM Cu(I) and 11 mM CN−, in the presence and absence of 1 g/L (13.3 mM) glycine.

strong complex Au(NH2CH2COO)2− with gold(I) as shown in Eq. (8)(Eksteen and Oraby, 2015).

Auþ þ 2 H3NCH2COOð Þ−↔Au NH2CH2COOð Þ−2 ; log K ¼ 18:0 ð8Þ

The effect of glycine concentrations (0.25–2 g/L) on the anodic golddissolution, in solutions containing 5.5 mM Cu and 11 mM CN−, wasstudied by using the linear sweep voltammetry technique. The differentvoltammograms obtained in the presence of different glycine concen-trations are shown in Fig. 7. It can be seen that the current density gen-erated from the anodic oxidation of gold increases, and the goldoxidation occurs at a lower potential, by increasing the glycine concen-tration. The voltammograms also show that there is an increase in thecurrent density of gold dissolution, at the presence of 2 g/L glycine, byincreasing the potential to 225 mV. However, any further increase inthe applied potential, above 225 mV, leads to a decrease in the anodiccurrent density. These results are consistent with the results shown inFig. 6, as it was shown that the gold dissolution rate decreases byextending the leaching time from 0.5 h to 2 h only in the solution con-taining 2 g/L glycine. However, this phenomenon was not observed atlower glycine concentrations.

The effect of free glycine on the anodic gold dissolution, in solutionscontaining 6.5 mM free cyanide and 6.5 mM free glycine in the absenceof copperwas studied by linear sweep voltammetry in the presence andabsence of glycine. The results are shown in Fig. 8. It can be seen that theoxidation of gold is hindered by the presence of glycine and absence ofcopper. This phenomenon was also observed by a study conducted byJeffrey et al. (2002) in the cyanide–ammonia system. These authorsfound that, in solutions containing free cyanide and no copper, the pres-ence of ammonia decreases the rate of gold leaching. The decreasinggold dissolution in the presence of ammonia was also observed byZheng et al. (1995).

Fig. 4. Gold dissolution versus leaching time in solutions containing: (a) 11 mM free cya-nide and (b) 5.5 mM Cu, 11 mM CN−, 1 g/L (13.3 mM) glycine at pH 11. 5.

Fig. 5. Gold dissolution versus leaching time in a solution containing 5.5 mM Cu(I) and11 mM CN− at pH 11.95 in the presence of different glycine concentrations.

Fig. 7. Linear sweep voltammograms showing the oxidation of gold in solutions containing5.5 mM Cu and 11 mM CN− in the presence of different glycine concentrations.

85E.A. Oraby, J.J. Eksteen / Hydrometallurgy 156 (2015) 81–88

3.6. Effect of pH

The effects of pH on the gold dissolution are shown in Fig. 9. It can beseen that gold dissolution increases significantly by increasing theleaching pH from 10 to 12. Increasing the pH to 13 slightly decreasesthe initial gold dissolution relative to pH 12, and the gold leach rate de-creases further by extending the leaching time. Fig. 10 shows the goldleach rate in solutions containing 1 g/L glycine at different leaching pHvalues. Gold leaching rates at pH 10 and 12, after 2 h of leaching, were3.36 and 7.22 μmol/m2·s, respectively. The results also show that thegold leaching rate increases linearly by increasing the leaching pH inthe range from 10 to 12. These results are consistentwith the results ob-tained by Vukcevic (1997) in the cyanide–copper–ammonia system.The author reported that the selective leaching of gold over copper inthe presence of ammonia at pH values between 10 and 12 occurs bythe reaction shown in Eq. (9). It can be emphasised that glycine in thecopper–cyanide solutions, in the presence OH− ions, may play a similarrole as ammonia does with the advantage of its high chemical stabilityand lower environmental concerns.

Auþ Cu CNð Þ−2 þ 4NHþ4 þ 2OH− þ 1

2O2↔Au CNð Þ−2 þ Cu NH3ð Þ2þ4 þ 2H2O ð9Þ

To demonstrate the effects of pH on the anodic gold dissolution insolutions containing 5.5 mM Cu and 11mM CN−, linear sweep voltam-mograms were obtained at different leaching pH values as shown inFig. 11. It can be seen that the anodic oxidation of gold is enhancedand occurs at a lower potential by increasing the leaching pH. The volt-ammograms also show that there is no increase in the current density ofgold dissolution at pH 13 above 130 mV. These results are consistentwith the gold kinetic leach rates shown in Fig. 10.

Fig. 6. Gold dissolution rates after 0.5 and 2 h of leaching gold disc in solutions containing5.5 mM Cu and 11 mM CN− at different Gly/Cu molar ratios.

3.7. Effect of CN/Cu ratio

In the cyanidation of gold ores containing oxides and secondary sul-phide copper minerals, copper dissolves to form different Cu(I) cyanidecomplexes such as Cu(CN)4−3, Cu(CN)3−2 and Cu(CN)2−. The effect ofadding glycine on gold leaching in the presence of Cu(CN)2− andCu(CN)32− complexes was studied and the results are shown in Fig. 12.It is not surprising to observe that, by increasing the molar CN/Curatio, the gold leach rate increases and the rates after 2 h, atmolar ratiosof CN/Cu of 3 and 2, are 11.1 and6.36 μmol/m2·s, respectively. However,in the absence of glycine, the gold leach rate increases and the rates atmolar ratios of CN/Cu of 3 and 2 are 0.65 and 0.53 μmol/m2·s, respec-tively. The increase in the gold leach rate at a high CN/Cu ratio can beattributed to the fact that the free cyanide ions generated fromCu(CN)3−2 are higher than those generated from Cu(CN)2− in the pres-ence of glycine, as shown in Eqs. (10) and (11). The generated free cya-nide ions (CN−) can then lead to further gold dissolution as shown inEq. (12).

2Cu CNð Þ−2 þ 2 H2NCH2COOð Þ−↔Cu NH2CH2COOð Þ−2 þ 3CN−

þ CuCN ð10Þ

2Cu CNð Þ2−3 þ 2 H2NCH2COOð Þ−↔Cu NH2CH2COOð Þ−2 þ Cu CNð Þ−2þ 4CN− ð11Þ

4Auþ 8CN− þ O2 þ 2H2O→4Au CNð Þ−2 þ 4OH− ð12Þ

Fig. 13 shows the gold dissolution rate in copper–cyanide deficientsolutions in the absence of glycine. The gold dissolution in solutionscontaining no free cyanide can be attributed to gold dissolution byCu(CN)2− and Cu(CN)3−2 according to Eqs. (13) and (14), in which thecopper–cyanide complexes act as a lixiviant for gold dissolution by

Fig. 8. Linear sweep voltammograms showing the oxidation of gold in solutions containing6.5 mM free CN− in the presence and absence of 0.42 g/L (6.5 mM) glycine.

Fig. 9.Golddissolution versus leaching time in solutions containing 5.5mMCu and11mMCN− in the presence of 1 g/L glycine at different leaching pH values. Fig. 11. Linear sweep voltammograms showing the oxidation of gold in solutions contain-

ing 5.5 mM Cu and 11 mM CN− in the presence of 1 g/L glycine at different pH values.

86 E.A. Oraby, J.J. Eksteen / Hydrometallurgy 156 (2015) 81–88

providing the cyanide ions needed to form the Au(CN)2− complex(Jeffrey et al., 2002).

4Auþ 8Cu CNð Þ−2 þ O2 þ 2H2O→4Au CNð Þ−2 þ 8CuCNþ 4OH− ð13Þ

4Auþ 8Cu CNð Þ2−3 þ O2 þ 2H2O→4Au CNð Þ−2 þ 8Cu CNð Þ−2 þ 4OH− ð14Þ

3.8. Effect of initial Cu concentration

The effect of different initial copper concentrations on leaching goldfrom a rotating gold disc electrode has been studied. In solutions con-taining a CN:Cu:Gly molar ratio of 2:1:2, at an initial copper concentra-tion of 4, 5.5 and 15 mM Cu (added as CuCN) at pH 11.5, the golddissolution rates from these solutions are calculated and shown inFig. 14. The results show that by increasing the initial copper concentra-tion, the gold leach rate increases. This can be attributed to that at thehigh initial copper concentration, more free cyanide ions are generated,as shown in Eqs. (10) and (11).

3.9. Effect of complexing reagent

As it has been discussed earlier in the mechanism of copper–cyanide–glycine system (Section 3.2), glycine plays a major roleof complexing copper and release cyanide ions for further gold leaching.In this section, the effect of well-known complexing agents of coppersuch as ammonia anEDTAhas been studied and evaluated. The gold dis-solution rates in solutions containing 5.5 mM Cu and 11 mM CN− atpH 11.5 in the presence of 1.1 g/L (14.5 mM) glycine, 11 mM EDTA,and 200 mM of ammonia are shown in Fig. 15. The results show thatgold dissolves rapidly in the copper–glycine and copper–ammonia sys-tem, whereas the gold dissolution in solutions containing EDTA is very

Fig. 10. Gold dissolution rates after 0.5 and 2 h of leaching in solutions containing 5.5 mMCu and 11 mM CN− in the presence of 1 g/L glycine at different leaching pH values.

slow. The results also show that the gold dissolution rate in the presenceof glycine increases by extending the leaching time, however, the golddissolution rate decreases with time in the presence of ammonia. Thedecrease of gold dissolution in the presence of ammonia can be attribut-ed to the volatilization of ammonia from the leach solution.

4. Conclusions

The effect of glycine addition on gold leaching kinetics in copper(I)–cyanide solutions at different leaching conditions has been studied.Kinetic and electrochemical studies have also been conducted to studythe effects of glycine concentration, pH, CN/Cu ratio, and initial copperconcentrations on the dissolution of gold. In solutions containingcopper–cyanide complexes with no free cyanide (or only enoughcyanide to just dissolve CuCN), the presence of glycine significantly in-creases the rate of gold dissolution. The roles of glycine on gold dissolu-tion, in solutions containing copper–cyanide complexes, can include(i) replacing cyanide by complexing Cu(I) or any Cu(II) formed fromthe oxidation of Cu(I), as glycine can form soluble complexes withboth cupric and cuprous ions; (ii) dissolving the passivation layer ofboth Cu(OH)2 and Cu2O, formed at zero, or a very low free cyanide con-centration; and (iii) dissolving gold as themain lixiviant in the presenceof oxygen and Cu(II) as oxidants.

From the results and findings that have been shown in thiswork, thefollowing observations can be made:

• In the presence and absence of glycine, gold dissolution rates insolutions containing 10 mM Cu(CN)32− were 11.1 μmol/m2·s and0.65 μmol/m2·s, respectively.

• The addition of glycine was found to decrease the dissolution rate ofgold in free cyanide solutions. However, in the absence of free cyanide

Fig. 12.Golddissolution rates in solutions containing5.5mMCuanddifferent CN/Cu ratiosin the presence of 0.83 g/L (11 mM) glycine at pH 11.5.

Fig. 13.Golddissolution rates in solutions containing5.5mMCuanddifferent CN/Cu ratiosin the absence of glycine at pH 11.5.

Fig. 15. Gold dissolution rates of leaching a rotating gold electrode in solutions containing5.5 mM Cu and 12 mM CN− at pH 11.5 in the presence of 1.1 g/L (14.5 mM) glycine,11 mM EDTA, and 200 mM of ammonia.

87E.A. Oraby, J.J. Eksteen / Hydrometallurgy 156 (2015) 81–88

and the presence of copper–cyanide complexes, glycine additionimproves the rate of gold dissolution.

• The average gold dissolution rate in a copper–cyanide–glycine systemis about 6.5 times higher than the gold dissolution rate in a freecyanide system in the presence of 11 mM CN− in each system.

• Gold dissolution rates increase by increasing the glycine concentra-tion, pH, CN/Cu ratio and initial copper concentrations.

• Gold dissolution rate in Cu–CN system, in the presence of glycine,increases by extending the leaching time. However, the gold dissolu-tion rate decreases in the presence of ammonia.

The outcomes from this research could lead to the development of anew process in which glycine could be used to enhance the gold disso-lution in copper–cyanide solutions. In the cyanidation of differentcopper–gold ores, in order to improve the dissolution of gold and de-crease the cyanide consumption, glycine can be added to the copper–cyanide solutions, either in the leaching stage after the copper–cyanidespecies have been formed, or into the final barren solutions containingcopper–cyanide species after the gold has been loaded onto the activat-ed carbon. In order to make this work feasible for any proposedfuture industrial applications, some future stages of this researchhave to be considered. These stages include: (a) the application ofcopper–cyanide–glycine system in leaching different sources of goldand copper–gold deposits; (b) the effects of glycine addition on thedifferent gold recovery processes such as Merrill–Crowe and gold ad-sorption on activated carbon. Research on the possible methods of cop-per recovery has also to be evaluated to avoid the copper accumulationfrom the barren solution recycling. The competitive adsorption of Cu–glycinate complex versus Au–CN complex on activated carbon has also

Fig. 14. Gold dissolution rates in solutions containing a CN:Cu:Gly molar ratio of 2:1:2 atinitial copper concentrations of 4, 5.5 and 15 mM Cu and 12 mM CN− at pH 11.5.

to be tested. In addition to that, a selective solvent extraction processof Cu-glycinate from leach solution may also be considered.

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