外文原文-氰化物渗滤液中元素银和金的电化学沉积.pdf

Electrochemical deposition of silver and gold from cyanide leaching solutions V Reyes Cruz C Ponce de Leo n I Gonza lez M T Oropeza Departamento de Qu mica Universidad Auto noma Metropolitana Iztapalapa Apdo Postal 55 534 C P 09340 Me xico D F Mexico Received 20 March 2001 received in revised form 14 November 2001 accepted 4 June 2002 Abstract A systematic voltammetric study developed in this work allows the determination of the potential range at which the selective deposition of gold and silver is carried out in the presence of a high content of copper As a first approach laboratory solutions prepared with a high content of cyanide and copper and low values of gold and silver are used later the methodology is applied to leaching solutions of industrial origin The chemical speciation and microelectrolysis studies showed that copper deposition occurs at more negative potentials than deposition of gold and silver Also the voltammetric study of a cyanide solution containing low concentrations of Au I and Ag I free of and with high concentration of Cu I was carried out The study shows the potential range at which Au I and Ag I are reduced despite the high concentration of the Cu I ions The deposition of gold and silver was not interfered with by the high concentration of Cu I ions when the leaching solution was electrolyzed in a laboratory electrochemical reactor FM01 LC with a reticulated vitreous carbon RVC cathode D 2002 Elsevier Science B V All rights reserved Keywords Silver cyanide Gold cyanide Copper cyanide Electrochemical recovery Cyanide leaching RVC electrode Electrodeposition 1 Introduction Cyanide leaching processes have been used by the mining industry for over 100 years in the extraction of noble metals Kordosky et al 1992 Fleming 1992 Minerals from which most of the noble metals are obtained are increasingly poorer gold and silver concentrations in the order of 1 5 ppm so the leaching solutions from these processes do contain Au I Ag I and Cu I with copper in concentrations typically over a hundred times higher than the other two metals This high concentration of Cu I might interfere during the selective recovery of gold and silver Jha 1984 and the use of ion exchange resins or activated carbon to remove the Cu I ion is often necessary to obtain a concentrated solution of Au I and Ag I The following stage after the removal of interfering ions is the recovery of noble metals Two traditional methods have been used cementation and electrolysis Fleming 1992 Jha 1984 The electrol ysis method has been used in concentrated solutions in electrochemical reactors with plane electrodes e g electrodeposition of copper or zinc however with dilute solutions the efficiency is low Jha 1984 Reyes 1998 Works reported in the literature have shown that the high surface area found in porous 0304 386X 02 see front matter D 2002 Elsevier Science B V All rights reserved PII S0304 386X 02 00083 X Corresponding author Fax 52 5804 4666 E mail address igm xanum uam mx I Gonza lez Hydrometallurgy 65 2002 187 203 electrodes increases the efficiency when used with diluted solutions Bennion and Newman 1972 Coeuret 1976 Coeuret et al 1976 Trainham and Newman 1977 Olive and Lacoste 1980 Coeuret and Storck 1981 Most gold plants actually use stainless steel wool as cathode The recovery of Au I and Ag I at concentrations between 11 and 30 ppm has been recently studied Waterman et al 1984 Reyes et al 2002 Stavart et al 1999 However the recovery of Au I and Ag I from solutions containing high concentration of Cu I in cyanide solutions has received less attention Since the metal with the highest concentration would be expected to be the first deposited the presence of large amounts of Cu I is likely to influence the selective recovery of gold or silver Preliminary studies performed on stainless steel 304 vitreous carbon and Ti showed that vitreous carbon is the most adequate material for performing Ag I deposition from a diluted silver solution 11 ppm with a high content of copper Reyes 1998 The solution used in this preliminary study 0 1 M Cu I 0 5 M CN and 10 4 M Ag I differs significantly from the solutions from industrial leaching processes with cyanides nevertheless the conditions of high copper and cyanide concentra tions allowed the realization of an electrochemical study on a vitreous carbon electrode where the recovery of silver was shown to be possible without the deposition of copper despite the fact that the concentration of Cu I was 100 times higher than the concentration of silver Reyes et al 2002 The study showed that the Cu I deposited at potentials more negative than Ag I and that the cyanide species of Cu I modified the capacitive current densities of the media CN making favorable the deposition of silver On the other hand in this study a systematic methodology is established to determine the potential range at which the selective silver deposition takes place For that reason it confronts the problem of faradaic current density response from the reduction process of a low silver concentration 11 ppm masked by the capacitive current of cyanide Thus a part of information of a previously performed study Reyes et al 2002 is taken up again and the same methodology is applied in order to study the model solutions containing a Au I and b Au I with Ag I in the presence and absence of Cu I laboratory solutions This allows later analysis validation of the results obtained in the study of an industrial leaching solution containing low concentrations of copper and cyanide and much lower concentrations of noble metals of the order of 1 5 ppm Firstly speciation and voltammetric studies of a cyanide solution with a high concentration of Cu I and low concentrations of Au I laboratory solution were carried out The purpose of this study was to find out whether the high concentration of copper inter feres with the selective deposition of gold After this experiment a voltammetric study of the cyanide solution containing the three metallic ions high Cu I concentration and low concentrations of Au I and Ag I laboratory solutions was carried out in order to determine if the interaction between the three metal ions changes the potential range at which the selective reduction of Au I and Ag I occurs After wards the same methodology was applied to the industrial leaching solution Once the potential range of the Au I and Ag I reduction was established an electrolysis of the cya nide leaching solution containing the three metallic ions was carried out in an electrochemical FM01 LC laboratory reactor 2 Experimental A 100 ml Pyrex cell with a three electrode system and nitrogen inlet was used in the voltammetric study The working electrode was a vitreous carbon rod with geometric area of 0 07 cm2 The electrode surface was polished with 0 3 Am alumina powder water and rinsed with deionised water followed by ultrasonic bath for 5 min to remove traces of alumina grease and a final rinse with deionised water The reference and counter electrodes were saturated calomel and graph ite respectively The solutions were Laboratory solutions for methodology validation a Cyanide solution0 5 M KCN b Cyanide solution of Cu I 0 1 M CuCN and 0 5 M KCN c Cyanide solution of Au I 10 4M KAuCN2and 0 5 M KCN V Reyes Cruz et al Hydrometallurgy 65 2002 187 203188 All solutions except the industrial leaching solution were prepared with deionised water 18 MV 1 cm 1 and analytical grade reagents The pH was held constant at 10 for laboratory solutions and the solu tions were deoxygenated with nitrogen for 10 min prior to cyclic voltammetry experiments A flux of nitrogen previously saturated in a CN solution was held on the surface of the solution during the experi ments without disturbing the electrolyte The electrolysis of the selective reduction of Au I and Ag I in the presence of high concentrations of Cu I was carried out using an industrial leaching solution on a filter press electrochemical reactor FM01 LC A full description of this reactor has been outlined in the literature Brown and Plecher 1992 Brown and Plecher 1993 Brown and Plecher 1994 A cationic NafionR membrane NX550 divided the anodic and cathodic reactor compartments The cath ode used was a reticulated vitreous carbon RVC electrode with 60 pores per inch ppi filling a volume of 31 cm3 Electrosynthesis Copper plates glued by conductive carbon paste made the electrical contact with the RVC electrode The anode was an A304 stainless steel plate The industrial solution was re circulated through the anolyte and catholyte compartments using two centrifugal pumps obtained from Cole Palmer A potentiostat EG and G model PARC 273A with a Booster KEPCO 0 20 A and 0 20 V and the M270 software were used in the voltammetric and macro electrolysis experiments The determination of total Au I Ag I and Cu I concentrations was carried out by atomic absorption spectroscopy AAS Varian model Spectr AA 20 3 Results and discussion 3 1 Laboratory solutions 3 1 1 Chemical speciation study A chemical speciationstudy was performed in order to establish the nature of the electroactive species of gold silver and copper in cyanide solutions A Pour baix type diagram Fig 1 was constructed using the thermodynamic data for soluble and insoluble species of gold silver and copper reported in the literature Ringbom 1963 Zhang et al 1997 Caroli and Sharma 1978 taking into account the laboratory solution conditions pCNV log CNV 0 222 pAuV log AuV I 4 pAgV log AgV I 4 and pCuV I log CuV I 1 using the method proposed by Rojas et al 1991 1993 The MV I corresponds to the generalized chemical species of the metallic ion in solution Rojas et al 1991 1993 The full lines in Fig 1 represent the conditional potential change of the system MV I M 0 with pH The dotted lines represent the limit of the pH range where a particular metal ion species predominates high molar fraction with respect to the other cyanide species of the same metallic ion generalized chemical species or in the other case the solubility limit for CuCN s Fig 1 shows that under the chemical conditions of leaching solutions there are four copper two silver and one gold species of cyanide Fig 1 shows that at any pH the redox potential of the couple CuV I Cu 0 has more negative values with respect to the potential of the couples AuV I Au 0 and AgV I Ag 0 These results indicate that under thermodynamic considerations the copper species should not interfere with the selective reduc tion of Au I and Ag I under experimental condi tions of laboratory solution used to validate the methodology d Cyanide solution of Au I and Cu I 0 1 M CuCN 10 4M KAuCN2 and 0 5 M KCN e Cyanide solution of Au I Ag I and Cu I 0 1 M CuCN 10 4M KAuCN2 10 4M AgNO3 and 0 5 M KCN Industrial leaching solution f Industrial leaching solution 0 025 M 400 ppm free CN 8 10 3M 500 ppm Cu I 5 10 6M 1 ppm Au I 3 10 5M 3 ppm Ag I 0 015 M 615 ppm Ca 0 025 M 575 ppm Na and 3 10 4M 17 ppm Zn at pH 11 V Reyes Cruz et al Hydrometallurgy 65 2002 187 203189 The calculated potentials associated with the elec trochemical reactions AuV I Au 0 AgV I Ag 0 and CuV I Cu 0 at pH 10 pCNV 0 22 pAuV 4 pAgV 4 and pCuV 1 are Au CN 2 1e Au0 2CN Ej 1060 mV vs SCE 1 Ag CN 2 3 1e Ag0 3CN Ej 1198 mV vs SCE 2 Cu CN 3 4 1e Cu0 4CN Ej 1546 mV vs SCE 3 It is important to note that under experimental conditions of the laboratory solution the difference in Ej for gold and silver is very small this would indicate that the selective deposition of these two metals is not easy 3 1 2 Voltammetric study of gold A voltammetric study of cyanide solutions with and without metal ions was carried out on a plane vitreous carbon electrode within the range of 100 to 1800 mV vs SCE In this study the influence of Cu I and cyanide on the electrochemical deposition of gold was determined Fig 2 shows a comparison of voltammetric behav iors of the following solutions cyanide solution free of metallic ions curve a cyanide solution with Au I curve b cyanide solution with Cu I curve c and cyanide solution with both Cu I and Au I curve d In Fig 2a the cyanide solution free of metallic ions has a high capacitive current density at potentials lower than or equal to 1400 mV vs SCE The current density is attributed to the adsorption of cyanide on the surface of the vitreous carbon electrode Reyes et al 2000 2001 Vilchis Carbajal et al 2000 see inset in Fig 2 Towards more negative Fig 1 Pourbaix type diagrams of soluble and insoluble chemical species involved in the redox couples AuV I Au 0 n AgV I Ag 0 E and CuV I Cu 0 y The diagrams were constructed with thermodynamic data from the literature considering as non variable the following concentrations 10 4M AuV I 10 4M AgV I and 0 1 M CuV I in 0 5 M CN The concentrations of these chemical species represents the composition found in cyanide laboratory solutions V Reyes Cruz et al Hydrometallurgy 65 2002 187 203190 potentials the reduction of the medium increases due to the water reduction The presence of Au I ions in the cyanide solution shifts the current density associated with the reduction of water towards more negative potentials with respect to the cyanide solution free of metallic ions Fig 2b and a respectively Since the reduction of water does not have an associated peak the influence of the electroactive species in this process was measured by the potential at which the current density was 85 AA cm 2 In each case this potential E 85 AA cm 2 was used as a parameter for comparison In the metal Fig 2 Typical cyclic voltammograms obtained from laboratory solutions containing 0 5 M CN at pH 10 on a vitreous carbon electrode at 25 mV s 1 The curves show the influence of different ions a free of metallic ions b 10 4M Au CN 2 c 0 1 M Cu CN 4 3and d 10 4M Au CN 2 and0 1MCu CN 4 3 Theinsetshowsthevoltammetricresponsesatlowcurrentdensities ThecurrentdensityE 85 AA cm 2chosenfor analysis of water reduction is indicated V Reyes Cruz et al Hydrometallurgy 65 2002 187 203191 free solution E 85 AA cm 2 1506 mV vs SCE while in the solution containing Au I E 85 AA cm 2 1658 mV vs SCE The presence of gold reduces the capacitive current density which was attributed to the complexes of gold with the adsorbed cyanide Mac Arthur 1972 Kirk et al 1980 Mughogho and Crundwell 1996 Chrzanowski et al 1996 Bindra et al 1989 In the reverse potential scan for the solution containing Au I and CN the current den sity was higher than in the forward scan This is a typical behavior of an interface modified by the presence of a gold deposit In the forward scan the gold and water were reduced on the vitreous carbon while in the reverse scan they were reduced on the gold Due to the low concentration of Au I in the cyanide solution the cathodic current density associ ated with the reverse scan is directly attributed to the reduction of water which notably increases the inter facial pH In the inset of Fig 2b it can be seen that during the reverse scan the curve has an associated constant current density in the potential range of 1050 to 400 mV vs SCE This observation suggests the formation of a passive film does not allow the gold dissolution to be seen as it has been reported in the literature Mac Arthur 1972 Kirk et al 1980 Mughogho and Crundwell 1996 The said passive film was enhanced by the pH modification at the interface This fact does not allow the gold deposition to be clearly evidenced however the above descrip tion of voltammetric behavior suggests the presence of deposited gold The influence of the Cu I ion in the cyanide solution Fig 2c can be seen in the voltammetric response as a shift of the reduction potential of water towards more negative potentials E 85 AA cm 2 1664 mV vs SCE and a modification of the capacitive current density The latter effect can be attributed to the fact that Cu I is adsorbed on the vitreous carbon surface Reyes et al 2000 2001 and competes with the adsorbed cyanide species Reyes et al 2000 2001 Vilchis Carbajal et al 2000 On the other hand there are no peaks asso ciated with the reduction and oxidation of copper in the range of the potential scan studied In a previous work it was shown that the reduction of Cu I on a vitreous carbon electrode from a cyanide solution begins at a potential of 2200 mV vs SCE Reyes et al 2002 From Fig 2d it can be seen that the presence of Au I and Cu I in the cyanide solution causes a more negative potential for the reduction of water E 85 AA cm 2 1728 mV vs SCE with respect to Fig 2a b and c This indicates that Au I and Cu I ions in the cyanide solution have an influence on the capacitive current densities and the reduction of water since both metals involve species adsorbed on the vitreous carbon surface Reyes et al 2000 2001 Mac Arthur 1972 Kirk et al 1980 Mughogho and Crundwell 1996 Chrzanowski et al 1996 Bindra et al 1989 The competition between different adsorbates on the vitreous carbon causes a modifica tion of the voltammetric response In this work we associated such behavior with the modification of the capacitive current densities A more detailed study could consider the evaluation of these capacitive currents however it is out of the scope of this work During the reverse scan in Fig 2d there are two oxidation processes process I at 900 mV vs SCE and process II at 550 mV vs SCE These processes can be attributed to the oxidation of gold as reported in the literature Mac Arthur 1972 Kirk et al 1980 Mughogho and Crundwell 1996 or to the dissolution of the codeposit Cu Au as shown below When Cu I is in solution it is possible to observe the oxidation process of gold which indicates that the passive process seen in alkaline media has been modified by the Cu I because the pH modification in the interface is notably diminished This effect is similar to that reported for the presence of lead in the gold oxidation process From the above study it was considered necessary to identify the potential range at which the selective deposition of gold occurs