Recovery of copper cyanide from waste cyanide solution by LIX 7950

Minerals Engineering 22 (2009) 190–195

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Recovery of copper cyanide from waste cyanide solution by LIX 7950 Feng Xie *, David Dreisinger Department of Materials Engineering, University of British Columbia, 309-6350 Stores Road, Vancouver, BC, Canada V6T 1Z4

a r t i c l e

i n f o

Article history: Received 1 May 2008 Accepted 1 July 2008 Available online 9 August 2008 Keywords: Copper Cyanide effluent Solvent extraction Guanidine

a b s t r a c t The use of the guanidine extractant, LIX 7950, to extract copper cyanide from waste cyanide solution has been investigated. Copper extraction is favorable at low pH while a high cyanide to copper molar ratio tends to suppress copper loading. The extractant also strongly extracted zinc and nickel from cyanide solution, but the extraction of iron was poor. The presence of thiocyanate ion significantly depressed copper extraction, but thiosulfate ion produced negligible impact on copper extraction. The preferential extraction of metal cyanide species to free cyanide has been noticed. The potential application of the recovery technique as a pre-concentration step for the treatment of cyanide effluent has been suggested, by which copper can be extracted and concentrated into a small volume of solution and the barren cyanide solution recycled to the cyanidation process. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The treatment of the large volume of cyanide-contaminated effluents has long been a challenge for gold cyanidation process. Environmental constraints controlling the discharge of cyanide from gold mining industry are being tightened by the local governments. Detoxifying cyanide by traditional destruction methods, such as INCO SO2/Air, hydrogen peroxide or Caro’s acid, to levels that meet stringent environmental regulations significantly increases the operational cost (Goode et al., 2001; Botz et al., 2005). On the other hand, the common occurrence of copper minerals in a gold ore is also a matter of concern to gold cyanidation process. Except for chalcopyrite, copper minerals are easily and readily soluble in the cyanide solutions used in gold leaching, which usually results in high cyanide consumption and sometime even makes cyanidation process unprofitable (Jay, 2001; Sceresini, 2005). Subsequently, there has been growing interest in the recovery of valuable metals (principally copper) and cyanide from cyanide effluent arising from the gold mining industry. Acidification, volatilization, and recovery (AVR) and some modifications thereof have been developed and practiced in some gold operations, however the high consumption and cost of reagents has significantly limited their application (Barter et al., 2001). The indirect recovery of metals and cyanide with pre-concentration by activated carbon has been studied extensively (Adams, 1994; Williams and Petersen, 1997). Activated carbon has an affinity for many metal cyanide compounds, including the soluble cyanide species of copper, zinc, nickel and iron. However their low adsorption capability has severely hampered their wide application in practice. They are * Corresponding author. Tel.: +1 604 822 1357; fax: +1 604 822 3619. E-mail address: [email protected] (F. Xie). 0892-6875/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2008.07.001

more suitable for use as a polishing process to remove cyanide to low levels when initial cyanide concentration is already below about 1–5 mg/L (Fleming, 2005). The recovery of valuable metals and cyanide from cyanide effluent by strong-base resins has been proposed (Goldblatt, 1956, 1959; Leão et al., 1998). In recent years, the ion-exchange resin carrying guanidyl functionality was also developed (Kordosky et al., 1993). A typical ion-exchange resin technology is the augment process in which CuCN-precipitated resin is used for the adsorption of soluble copper cyanide from the leachate of copper-containing ore. The loaded copper cyanide species are eluted from the resin using a concentrated copper cyanide solution with a high molar ratio of cyanide to copper. The resin can be regenerated back to the CuCN form with sulfuric acid and the eluate is submitted to electrowinning to produce copper cathodes (Le Vier et al., 1997). One potential disadvantage of the processes is that the precipitated CuCN may block the resin pores decreasing the opportunity for additional metal cyanide complexes to be adsorbed into the resin. If cobalt is present in the effluent, the possible polymerization of adsorbed cobalt cyanide complexes under strongly acidic conditions will poison the resins (Goldblatt, 1959; Leão et al., 1998; Jay, 2001). Due to the potential high selectivity and loading capacity, and especially the relatively fast extraction rate, solvent extraction technology offers an alternative method for metal and cyanide recovery from waste cyanide solution. In the early research of Moore (1975) and Moore and Groenier (1976), the recovery of zinc and cadmium from highly alkaline solutions by quaternary amines has been reported. The uses of a solvent mixture of AliquatÒ 336 and nonylphenol to recover copper from cyanide solution were proposed by Davis et al. (1998). Some guanidine extractants has also been used for the recovery of metal and cyanide from gold mill

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effluent. Solvent extraction of metal cyanide species by LIXÒ 79 (a tri-alkylguanidine extractant developed by Cognis) showed that it has a high selectivity for gold and silver than other base metals and can efficiently extract gold and silver from cyanide solutions (Virning and Wolfe, 1996; Kordosky et al., 1992; Satre et al., 1999; Aguayo et al., 2007). The recovery of copper and cyanide by using XI 7950 (now named LIXÒ 7950, also based on formulation of a tri-alkylguanidine but with a higher concentration than LIXÒ 79), was reported by Dreisinger et al. (1996). The initial investigation on distribution isotherm for copper extraction has shown that copper can be effectively extracted by the extractant from gold leachate and copper extraction was influenced by both the initial copper concentration and the molar ratio of copper to cyanide. In this work, continuous laboratory tests on the use of reagent LIXÒ 7950 for copper and cyanide extraction from waste cyanide solution has been conducted. Some fundamental aspects on copper cyanide extraction chemistry have been examined, including the influence cyanide to copper molar ratio on both copper and cyanide extraction under different equilibrium pH and the effects of temperature and the presence of non-copper species on copper cyanide recovery. The co-extraction of multiple-metal cyanide complexes was also investigated in order to elucidate their selectivity and potential effects in practice. The potential application of the extractant for pre-concentration of copper cyanide from waste cyanide solution was discussed.

glass impellers. All tests were conducted at a phase ratio of unity. Equilibrium pH was adjusted by direct addition of concentrated H2SO4 solution (50%, v/v) or NaOH solution (2 M). When equilibrium was reached, phase separation was conducted in a separatory funnel. Samples of the aqueous solution were separated and filtered to remove any entrained organic before analysis. The organic samples for stripping were also filtered (1 PS Phase Separation paper) to remove the entrained aqueous solution. The metal content in the aqueous sample was analyzed by atomic absorption spectroscopy (AAS) and metal in the organic phase was calculated by mass balance. Total cyanide and thiocyanate content in the aqueous solution was determined by the distillation method (Csikai and Barnard, 1983) and their content in organic was calculated by mass balance. The metal and total cyanide content in some selected organic samples were also analyzed. Sodium hydroxide solution (1 M) was used to thoroughly strip the loaded copper and cyanide (by three times contact at a phase ratio of unity) and then the same procedure as above was used to determine the metal and total cyanide content in the resulting strip solution. It was confirmed that good agreement between analytical and calculated results could usually be obtained.

2. Experimental

Initial investigations showed that the equilibrium between the two phases could be established rapidly. Both solution pH and copper extraction amount are constant after 3 min of mixing, indicating the extraction kinetics was relatively fast. In subsequent experiments, 10 min contact time was arbitrarily chosen for equilibrium establishment. The effect of initial molar ratio of cyanide to copper (CN/Cu) on copper extraction was examined at different equilibrium pH (Fig. 2). The results show that copper could be effectively extracted from cyanide solution by the extractant over a wide pH range (up to 11). However, copper extraction begins to decrease with an increase of the equilibrium pH when pH is higher than 10. Copper loading was also influenced by the initial CN/Cu ratio – higher cyanide levels tend to suppress copper extraction. The extraction of total cyanide by the extractant was also examined (Fig. 3). Comparing cyanide extraction with copper

2.1. Materials The extractant LIXÒ 7950 was used as supplied by the manufacturer and the basic structure is shown in Fig. 1, where R represents alkyl groups (Kordosky et al., 1992). Dodecane was used as the diluent and dodecanol (50 g/L) was used as the modifier. Synthetic copper cyanide solutions were made up from CuCN and NaCN. A mixed solution of metal cyanides was prepared from NaCN and the corresponding metal cyanides (Zn(CN)2, Ni(CN)24H2O, CuCN and K4Fe(CN)63H2O, respectively) and the content of metal species is shown in Table 1. All the chemicals were reagent grade. 2.2. Procedure The extraction and stripping tests were carried out in a sealed beaker and mixing was provided by a mechanical agitator with

3. Results and discussion 3.1. Effect of cyanide to copper molar ratio

100

80

N R1

R3 N

C

R2

N R4

Fig. 1. The schematic structure of the extractant (LIXÒ 7950, Kordosky et al., 1992).

Table 1 The major component in the mixture solution

Cu extraction,%

R5 60

40

20

0 9. 5

Component

Concentration, mg/L

Cu (I) Zn (II) Ni (II) Fe (II) CNT

250.0 255.7 230.1 220.3 2045.6

10

10.5

11

11.5

12

12.5

Equilibrium pH CN/Cu=3

CN/Cu=5

CN/Cu=10

Fig. 2. Copper extraction vs. equilibrium pH at different initial CN/Cu ratios (org.: 10%, v/v LIX 7950 in dodecane, 50 g/L dodecanol; initial [Cu] = 3.93  103 M).

F. Xie, D. Dreisinger / Minerals Engineering 22 (2009) 190–195

100

CN extraction, %

80

60

40

20

0 9.5

10

10.5

11

11.5

12

12.5

Equilibrium pH CN/Cu=3

CN/Cu=5

CN/Cu=10

Fig. 3. Cyanide extraction vs. equilibrium pH at different initial CN/Cu ratios (org.: 10 %, v/v LIX 7950 in dodecane, 50 g/L dodecanol; initial [Cu] = 3.93  103 M).

extraction, it was found that the molar ratios of loaded cyanide to loaded copper in organic phase were all close to three even when the initial CN/Cu ratio was as high as 10, indicating that most of the loaded copper and cyanide occurred as CuðCNÞ2 3 . The analysis of the stripping solutions of the loaded organic samples further confirmed the result. It is believed that these results are associated with copper speciation in the cyanide solutions. In the aqueous phase, the equilibrium speciation of Cu (I) in cyanide solution can be represented by reactions (1)–(6).

½CuCN ½Cuþ ½CN  ½CuðCNÞ2  K2 ¼ ½CuCN½CN 

Cuþ þ CN ¼ CuCN K 1 ¼ CuCN þ CN ¼ CuðCNÞ2

ð1Þ ð2Þ 

CuðCNÞ2 þ CN ¼ CuðCNÞ2 3 CuðCNÞ2 3

CuðCNÞ3 4



þ CN ¼

CuCN ¼ Cuþ þ CN þ

K3 ¼

½CuðCNÞ23  ½CuðCNÞ2 ½CN 

K4 ¼

 ½CuðCNÞ34   ½CuðCNÞ23 ½CN  

K sp5 ¼ ½Cuþ ½CN 

Cu2 O þ H2 O ¼ 2Cu þ 2OH

K sp6 ¼ ½Cuþ ½OH 

ð3Þ

3.2. Effect of temperature The tests for temperature effect on the extraction of copper cyanide were carried out with a dilute copper cyanide solution ([Cu] = 103 mol/L CN/Cu = 3). Equilibrium pH varied in the range is the dominant anion in the of 10–11. In this case, CuðCNÞ2 3 solution (account for about 98% of total copper cyanide). The equilibrium reaction for copper extraction by the extractant can be generally expressed by the following equation:  2RGorg þ 2H2 O þ CuðCNÞ2 3 ¼ 2ðRGHÞCuðCNÞ3org þ 2OH

ð10Þ

ð4Þ ð5Þ ð6Þ

where K1–K4 are the equilibrium constants (here they are selected as 3.16  1019, 3.39  104, 4.17  104 and 50.1, respectively, Lu et al., 2002) and Ksp-5 and Ksp-6 are solubility product constants and are 1020 and 1029.5, respectively at 25 °C (Weast et al., 1998). Cyanide occurs as free cyanide (HCN and CN, Ka = 109.21, Weast et al., 1998) and complexed cyanide in the solution and their concentrations depend on temperature, pH and total copper and cyanide content, etc. The speciation of copper cyanide complexes was calculated through a spread sheet program based on the following mass and charge balances:******

½Cu ¼ ½Cuþ  þ ½CuCN þ ½CuðCNÞ2  þ ½CuðCNÞ2 3  þ ½CuðCNÞ3 4 

The formation of Cu2O, CuCN and CuðCNÞ 2 is negligible for the copper cyanide solutions used for the tests (at pH range of 9–13 and CN/Cu higher than three). From the calculated speciation diagram (Fig. 4), it can be seen that higher CN/Cu ratio favors the for2 mation of CuðCNÞ3 4 . About 98% of copper occurs as CuðCNÞ3 when CN/Cu ratio is three and when CN/Cu ratio increases up to 10, and 53% of copper as about 47% of copper occurs as CuðCNÞ2 3 CuðCNÞ3 4 . The preferential extraction of CuðCNÞ2 to CuðCNÞ3 4 can be confirmed based on copper and cyanide extraction (Fig. 2 and 3). The charge density effect and the geometrical factors are playing important roles. It is believed that the ion that has a lower charge density (lower charge and/or larger size) will be preferentially extracted by the amine extractants (Mooiman and Miller, 1986; has one more coordinated CN Riveros, 1990). Though CuðCNÞ3 4 ligands than CuðCNÞ2 and potentially has a larger size, CuðCNÞ2 3 3 will exhibit a lower overall charge density than CuðCNÞ3 since 4 the former has two negative charge and the later three. Moreover, 2 CuðCNÞ3 4 has a tetrahedral shape and CuðCNÞ3 is triangular planar 3 (Sharpe, 1976). The extraction of CuðCNÞ4 would require three molecules of the extractant per molecule of copper while CuðCNÞ2 3 would only require two, therefore the former would be poorly extracted by the extractant. Free cyanide may occur as HCN and CN in the solution. HCN will not be extracted by the extractant. Though CN has only one charge and a linear shape, its small size leads to its poor extraction by the extractant which is a relative large molecule. As a result, the loaded copper cyanide mainly occur as CuðCNÞ2 3 and most of the free cyanide (HCN and CN) remains in the aqueous solution.

1.0

Cu(CN)320.8

mol fraction

192

0.6

0.4

Cu(CN)43-

0.2

ð7Þ Cu(CN)2-

½CN ¼ ½CN  þ ½HCN þ 2½CuðCNÞ2  þ 3½CuðCNÞ2 3  þ 4½CuðCNÞ3 4 

ð8Þ

½Naþ  þ ½Cuþ  þ ½Hþ  ¼ ½CN  þ 2½CuðCNÞ2  þ 3½CuðCNÞ2 3   þ 4½CuðCNÞ3 4  þ ½OH 

0.0 3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

CN/Cu molar ratio ð9Þ

Fig. 4. The calculated speciation of copper cyanide complexes vs. CN/Cu molar ratio (pH = 11, [Cu] = 3.93  103 M, 25 °C).

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and the extraction equilibrium constant, Kex, is defined by

100

 2

½2ðRGHÞCuðCNÞ3 org ½OH 

ð11Þ

2 ½CuðCNÞ2 3 ½RGorg

Here ideal behavior for all the related species in both organic and aqueous phase has been hypothesized. Otherwise the activity rather than the concentration value should be considered. The concentration of free extractant ([RG]org) can be calculated from the mass balance equation:

½RGorg ¼ ½RGT-org  2½2ðRGHÞCuðCNÞ3 org

ð12Þ

where [RG]T-org is the total extractant concentration (10%, v/v of LIX 7950 is equivalent to 0.013–0.0135 mol/L). The distribution coefficient of copper D is defined as

½Cuorg ½2ðRGHÞCuðCNÞ3 org ¼ ½Cuaq ½CuðCNÞ2 3 

log K ex ¼ log D  2 log ½RGorg þ 2ðpH  14Þ

ð14Þ

The extraction constant of copper cyanide was determined at various temperatures under the same conditions (Fig. 5). It was found that the copper extraction constant decreased as temperature was increased. It shows that the extraction reaction is exothermic and the enthalpy change of the reaction was evaluated from the slope of log Kex versus 1/T (DH° = 2.303R  1000  slope = 191 kJ/mol). 3.3. Effect of non-copper species The effect of two sulfur species, thiocyanate (SCN) and thiosulfate (S2 O2 3 ) on copper cyanide extraction is shown in Fig. 6. The efis negligible while copper extraction decreased fect of S2 O2 3 significantly with an increase in SCN concentration. The singly charged SCN has a larger size compared with CN and its lower charge density promotes its extraction by the extractant. From Fig. 7, it can be seen that the extractant exhibits a much higher extraction affinity for SCN than CN. This is quite similar to the selectivity characteristics for the strong-base ion-exchange resins (Fleming, 2005). As a result, when SCN ion is present in copper

0 0

0.1

0.2

0.3

NaSCN/Na2S2O3, mol/L Fig. 6. The effect of thiocyanate and thiosulfate ions on copper extraction (org.: 10%, v/v LIX 7950 in dodecane, 50 g/L dodecanol; initial aq: [Cu] = 3.93  103 M, CN/Cu = 3; equilibrium pH = 10.50 ± 0.05).

0.008

SCN

0.006

CN 0.004

0.002

0

0

0.01

0.02

0.03

0.04

0.05

0.06

SCN/CN in aq, mol/L Fig. 7. The extraction isotherm of SCN and CN by LIX 7950 (org.: 10 %, v/v LIX 7950 in dodecane, 50 g/L dodecanol; equilibrium pH, 10.50 ± 0.05).

-1

log Kex

-2

y = 10.00x - 34.84 -3

-4 3.1

40

NaSCN

ð13Þ

where [Cu]org and [Cu]aq are the molar concentration of copper in the organic phase and in aqueous phase, respectively. Combining equation (11) and (13) can lead to the following expression:

60

20

SCN/CN in org, mol/L

D¼

Na2S2O3

80

Cu extraction, %

K ex ¼

3.2

3.3

1000/T,K

3.4

-1

Fig. 5. The effect of temperature on copper extraction by LIX 7950 (10%, v/v LIX 7950 in dodecane, 50 g/L dodecanol; initial [Cu] = 103 M, CN/Cu = 3).

cyanide solution, it will compete for the available extractant with copper cyanides species. The mass balance calculation showed that, during co-extraction with copper cyanide species ([Cu] = 3.93  103 M, CN/Cu = 3; equilibrium pH = 10.50 ± 0.05), when initial concentration of SCN was 0.05 mol/L, the loaded SCN in the organic phase would consume more than 90% of the by the extractant is extractant. However the extraction of S2 O2 3 negligible even when the initial concentration of Na2S2O3 was as ion has a much higher high as 0.25 mol/L. This is because S2 O2 3 charge density (two negative charge and with a relative small size) than SCN and CuðCNÞ2 3 and tends to have a tighter solvation shell. It prefers staying in the aqueous phase rather than forming complexes with the extractant and therefore exhibits little effect on the extraction of copper cyanide. Solvent extraction of a mixture of metal cyanide solution (see Table 1) by LIXÒ 7950 was also examined and the results are shown in Fig. 8. Initially all metals in the aqueous solution were at the same concentration of 3.93  103 M – equal to the molarity of a 250 mg/L copper solution. The speciation calculations indicated

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F. Xie, D. Dreisinger / Minerals Engineering 22 (2009) 190–195 Table 2 Stripping of copper and cyanide from the loaded extractant by NaOH solution (10%, v/v LIX 7950 in dodecane, 50 g/L dodecanol, initial loaded Cu and total CN are 3.68  103 M and 1.11  102 M, respectively, unity O/A ratio)

100

Extraction, %

80

60

40

Cu stripping, %

CN stripping, %

0.05 0.1 1 5a 10a

89.9 90.8 93.9 79.4 2.8

90.5 91.2 93.3 81.2 3.1

a

20

0 9.5

NaOH, M

10

10.5

11

11.5

12

12.5

13

Equilibrium pH Cu

Zn

Ni

Fe

CN

Fig. 8. The co-extraction of metal and cyanide at different equilibrium pH (30%, v/v LIX 7950 in dodecane, 50 g/L dodecanol; initial aqueous solution as in Table 1).

that metal species in the mixture mainly occurred as ZnðCNÞ2 4 , 2 NiðCNÞ2 4 , CuðCNÞ3 (accounting for about 87% of total copper and 4 about 13% CuðCNÞ3 4 and FeðCNÞ6 . The extractant exhibited selectivity order as follows: 2 2 4 ZnðCNÞ2 4 > NiðCNÞ4 > CuðCNÞ3 > FeðCNÞ6

Again charge density and the geometric shape are key factors. and NiðCNÞ2 are lower than The charge densities for ZnðCNÞ2 4 4 2 for CuðCNÞ3 as being with the same charge, the former two complexes have four coordinated CN ligands and CuðCNÞ2 3 has three. Due to the high charge density and the octahedral shape, FeðCNÞ4 6 exhibited much poorer extraction compared to the other three species. The selectivity order is similar to those results for LIX 79 reported by Virning and Wolfe (1996) and Satre et al. (1999). Similar selectivity order was also reported by Riveros (1990) for quaternary amine and Mooiman and Miller (1986) for modified amines. An erroneous selectivity order for Cu and Zn was given by Aguayo et al. (2007) since obviously Zn shows much higher extraction than Cu based on their calculations on the distribution coefficients and the selectivity for Cu and Zn. The preferential 2 over NiðCNÞ4 is probably due to the fact extraction of ZnðCNÞ2 4 2 2 that ZnðCNÞ4 has a tetrahedral shape while NiðCNÞ4 is square pla2 nar. The distribution of charge over ZnðCNÞ4 is more uniform than 2 NiðCNÞ4 and favors the combination with the extractant molecules. The lower stability of zinc cyanide complexes may be another factor as part of zinc may occur as ZnðCNÞ 3 which is much easier to extract by the extractant according to the charge density effect. The analysis of total cyanide extraction by the extractant showed that the extracted cyanide mainly occurred as complexes and that most of the free cyanide remains in the aqueous phase. 3.4. Stripping of copper and cyanide It is desirable to recover copper and cyanide from the loaded extractant using a simple technique so that the solvent can be reused. Fig. 2 and 3 show that the extraction of copper cyanide is significantly depressed as equilibrium pH increases from 9 to 12, indicating the extracted copper cyanide can be gradually stripped to the aqueous phase with an increase of equilibrium pH. Table 2 shows the results of stripping copper cyanide from the organic phase under different NaOH concentrations (at room temperature).

Third phase formed.

It shows that the stripping of copper cyanide increases as NaOH concentration increase from 0.05 M up to 1 M. At unity volume ratio of organic phase to aqueous phase (O/A), about 93% of copper and cyanide can be stripped by 1 M NaOH solution indicating sodium hydroxide is an efficient stripping reagent. However when NaOH concentration further increases (above 5 M), an obvious third phase formed between two clear phases (organic and aqueous). This phenomenon happens when the blank extractant solution (10%, v/v LIX 7950 and 50 g/L dodecanol in dodecane) was contacted with concentrated NaOH solution (5 M or higher). In both cases, the volume of the aqueous phase decreases significantly (from initially 25 mL down to 22 mL) and the change of the volume of the organic phase is negligible. The change of NaOH concentration in the aqueous phase was insignificant. The calculation of the mass balance for copper and cyanide showed that part of copper and cyanide occurred in the third phase. It was also found that the clear organic phase exhibited a much poorer extraction capability on copper cyanide when re-contacted with copper cyanide solution. These facts indicated that the third phase was probably formed by sodium hydroxide, water and the organics including the extractant and part of the copper cyanide will be extracted into the third phase if it is present. As a result, the stripping of copper and cyanide decreased significantly. Since the formation of third phase is intolerable in practice, the moderate strong NaOH solution (<5 M) is proposed for copper cyanide stripping. 4. Summary The recovery of copper cyanide from cyanide solution by a guanidine extractant (LIX 7950) has been investigated and the results show that copper cyanide can be extracted effectively from the alkaline cyanide solution. A high cyanide to copper ratio tends to suppress copper cyanide loading and the preferential extraction over CuðCNÞ3 and free cyanide (CN) was noticed. of CuðCNÞ2 3 4 The extraction of copper cyanide by the extractant is an exothermic reaction (DH° = 191 kJ/mol). The extractant exhibits higher extraction affinity for SCN than  CN due to the charge density effect. The presence of SCN significantly suppressed the extraction of copper cyanide while thiosulfate ion exhibited negligible effect. The extractant exhibits a 2 2 selective extraction sequence as ZnðCNÞ2 4 > NiðCNÞ4 > CuðCNÞ3 > FeðCNÞ4 . The effective stripping of copper cyanide can be accom6 plished using moderate strong sodium hydroxide solutions (1 M). The extractant can be used in the SX circuit for pre-concentrating copper cyanide into a small volume of strip solution which can be further treated by electrowinning, AVR, or similar processes to recover copper products and cyanide. Due to the preferential extraction of metal cyanide complexes, most of the free cyanide will remain in the aqueous phase which allows for the potential recycling of the barren solution to the cyanidation process. If zinc and nickel cyanide complexes are present in the waste solution, they will be preferentially extracted into the organic phase and their potential effect on the subsequent copper recovery process

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should be considered. Iron (II) cyanide complexes will probably accumulate in the recycling water if a closed circuit system is applied in practice and their potential effects on the cyanidation process should be considered. The need for clarified feed solution for solvent extraction will not be a limitation while targeting heap leaching solutions, overflow stream or dam return water from tailings. For operations using carbon-in-pulp (CIP) for the recovery of gold, it will be necessary to thicken and wash the solids in order to produce a clarified feed solution for SX circuit. Acknowledgements The authors thank Dr. Berend Wassink for his generous help on the project and Cognis is thanked for providing laboratory samples of reagents. References Adams, M.D., 1994. Removal of cyanide from solution using activated carbon. Minerals Engineering 7 (9), 1165–1177. Aguayo, S., Valenzuela, J., Parga, J.R., Lewis, R.G., Cruz, M., 2007. Continuous laboratory gold solvent extraction from cyanide solutions using LIX 79 reagent. Chemical Engineering and Technology 30 (11), 1532–1536. Barter, J., Lane, G., Mitchell, D., Kelson, R., Dunne, R., Trang, C., Dreisinger, D., 2001. Cyanide management by SART. In: Young, A.A., Twidwell, L.G., Anderson, C.G. (Eds.), Cyanide: Social, Industrial and Economic Aspects. TMS, Warrendale, PA, pp. 49–562. Botz, M.M., Mudder, T.I., Akcil, A.U., 2005. Cyanide treatment: physical, chemical and biological process. In: Adams, M.D. (Ed.), Advances in Gold Ore Processing. Elsevier, pp. 672–702. Csikai, N.J., Barnard, A.J., 1983. Determination of total cyanide in thiocyanate-containing waste water. Journal of Analytical Chemistry 55, 1677–1682. Davis, M.R., MacKenzie, M.W., Sole, K.C., Virnig, M.J., 1998. A proposed solvent extraction route for the treatment of copper cyanide solutions produced in leaching of gold ores. Alta Copper Hydrometallurgy Forum. Brisbane, Austrilia. Dreisinger, D.B., Wassink, B., De Kock, F.P., West-Sells, P., 1996. Solvent extraction and electrowinning recovery of copper and cyanide – recent developments. Randol Gold ’96. Golden, Colorado. Fleming, C.A., 2005. Cyanide recovery. In: Adams, M.D. (Ed.), Advances in Gold Ore Processing. Elsevier, pp. 703–727.

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