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Silver electrowinning from photographic fixing solutions using zirconium cathode
Hydrometallurgy 54 Ž2000. 79–90 www.elsevier.nlrlocaterhydromet
Silver electrowinning from photographic fixing solutions using zirconium cathode M. Chatelut ) , E. Gobert, O. Vittori Laboratoire d’Electrochimie Analytique — LICAS, UniÕersite´ Lyon I, CPE, 43 Bd du 11 NoÕembre 1918, 69622 Villeurbanne Cedex, France Received 6 March 1998; accepted 3 February 1999
Abstract Suitable electrochemical conditions for using zirconium as a cathode in the electrochemical recovery of silver from photographic fixing solutions were studied by cyclic voltammetry. The influence of the thin, natural zirconium oxide layer was investigated. A flowing system was tested to simulate industrial electrolysis and determine the recovery efficiency, the compactness and purity of the recovered silver. The influence of deposition potential and volumetric flow rate influences were studied. The electrowinning of silver is possible with an applied potential lower than 0.9 V between the cathode and a carbon graphite anode. The final Ag concentration in the bath was 20 mgrl, and 98% of the total silver was recovered with an electrical efficiency close to 92%. Silver deposits were very easy to remove from the electrode. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Silver electrowinning; Photographic fixing solutions; Zirconium cathode; Cyclic voltammetry
1. Introduction Electroreduction appears to be one of the best processes for metal recovery from waste waters, at least for environmental protection, because no other polluting species are induced. In photographic baths, silver forms various complexes with the anions, such as S 2 O 32y, SO 32y, Bry or Cly, present in the solution. These complexes are principally AgŽS 2 O 3 . 23y and AgŽS 2 O 3 . 35y w1x. Electrowinning of silver from photographic fixing solutions has been studied from the beginning of the 20th century and, there are many industrial installations. Massive and porous cathodes have been studied, both have shown some disadvantages. )
Corresponding author. E-mail: [email protected]
0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 9 9 . 0 0 0 6 4 - X
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M. Chatelut et al.r Hydrometallurgy 54 (2000) 79–90
Massive cathodes have been principally made of stainless steel or graphite w2,3x. Silver plated copper has also been proposed w4x. The main disadvantage of massive electrodes was that of a final silver concentration larger than the recommended value for discharge Žtypically 0.1 to 1 mgrl depending on the country.. Porous or three-dimensional cathodes with various compositions have been described, for example carbon felt w5x, carbon fibres w6x, nickel-covered polymer w7x, metallic- or silver-covered glass balls w8x and grids of expanded stainless steel w9x. Recently, Gabe et al. w10x have discussed the performance of rotating cylinder electrodes applied to metal recovery and electrodeposition. The porous electrodes are at present the better way for total silver elimination from photographic wastes. The major problem encountered is the plugging up of the electrodes. Metal recovery generally needs further steps such as burning the substrate w7x, melting or acidic dissolution followed by a new electrolysis of the deposited silver w11x. The nature of the photosensitive film causes the presence in the baths of halide ions which are very corrosive for numerous materials. Carbon graphite electrodes are corrosion resistant, but have a poor mechanical resistance. Zirconium is always covered with an oxide layer insulating the metal from corrosion, but allowing electroreduction processes w12,13x. Previous works showed us that the electrodeposited metals are generally easy to remove w14,15x. So, it seems interesting to test its performance as cathode material for silver electrowinning. In this work, we first studied the electrochemical properties of a commercial fixing solution by cyclic voltammetry on zirconium and silver electrodes. The same study was performed after an addition of silver ions using freshly polished zirconium electrode and the efficiency was also tested with a more oxidized zirconium electrode. Finally a circulating flow cell and a spent fixing solution, available from a photographic laboratory, were used to simulate industrial conditions. Experimental conditions were studied in order to limit the reduction of the fixing solution and silver deposit contamination w15x. Characterizations of the deposits were made by scanning electron microscopy, energy dispersive spectroscopy ŽEDS. and X-ray diffraction.
2. Experimental details 2.1. Apparatus Rest potential determination, cyclic voltammetry and chronoamperometry were carried out using a PGP 20-1 three electrodes potentiostat ŽRadiometer, France. monitored by a PS-1 IBM computer with the ELCOM 201 software. The potential between working and auxiliary electrodes was recorded with an L 6512 device ŽLinseis, Germany.. Electrolyte was circulated using a centrifugal pump ŽEheim, Germany.. SEM examinations and EDS analysis, were carried out in the microscopy centre of the University Claude Bernard, Lyon I ŽCMEABG. using an S800 scanning electron microscope ŽHitachi, Japan. and a CAMEBAX ŽCameca, France.. X-ray diffraction was
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done with a D500 diffractometer ŽSiemens, Germany. in the centre Henri Longchambon of the University Claude Bernard, Lyon I. Determination of silver concentration in the fixing solutions was carried out by ICP using a JY 38-3 spectrophotometer ŽJobin-Yvon, France.. 2.2. Electrodes For voltammetric studies, the working electrode was an EDI rotating disc electrode ŽTacussel, France.. The silver and zirconium rods were purchased from Aldrich ŽUSA. and were 99% purity. They were embedded in a Teflon rod, the geometric electrode area was 0.0314 cm2 for silver and 0.196 cm2 for zirconium. Before each experiment they were carefully polished with a 3-mm grade diamond paste ŽEscil, France., then rinsed with distilled water, quickly wiped and immediately immersed in the solution under investigation. The auxiliary electrode was a Pt wire and the reference electrode was saturated calomel, with a salt bridge filled with 0.1 M Na 2 SO4 . For the experiments performed in the flow cell, the cathode was a 3.14 cm2 zirconium foil Ž99%, Aldrich. used as received, not polished but only cleaned with acetone. The anode was a 30-cm2 carbon graphite plate. During the preliminary studies, the rotating disc electrode was chosen rather than a rotating cylinder principally for three reasons: first it would be difficult to reproduce surface states by polishing a cylinder, second the deposits recovery was easier from disc and third the hydrodynamic conditions for rotating disc were closer than that obtained in the flow circulating system tested here. 2.3. Reagents The fixing reagent, purchased from Ilford ŽUK. ref. Hypam, was based on aqueous ammonium thiosulphate and, as suggested by the manufacturer, it was 1r5 vrv diluted. AgNO 3 was purchased from Johnson Matthey Chemicals ŽUK.. All solutions were made with distilled water. The spent fixing solution was kindly provided by the University Lyon I Centre of Microscopy ŽCMEABG..
3. Results and discussion 3.1. Initial oxide surface characterization In a previous work on zirconium electrodes we have shown that oxide forms instantaneously in air after abrasion, and we estimated the minimum layer thickness to be 1.9 nm w16x. Assuming a uniform coverage with ZrO 2 , we have shown that the oxide layer thickness on the zirconium surface could be estimated from electrochemical oxidation in acidic media w15x. Using the described procedure, we calculated the oxide thickness made during 3 days and a week on a polished electrode just exposed to air. We found identical values close to 4 nm. So a 6-nm thickness ‘‘natural oxide’’ layer was a minimum which was always present in experiments when zirconium was not freshly abraded.
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Fig. 1. Determination of the zirconium rest potential: ` polished zirconium and I natural oxide-covered zirconium in diluted fixing solution; v polished zirconium and B natural oxide-covered zirconium after 7 g ly1 Ag ions added in the diluted fixing solution ŽRDE electrode, N s 0 miny1 , dilutions1r5 vrv..
3.2. Rest potential of zirconium in fixing solution In order to avoid any uncontrolled electrochemical oxidation of zirconium by starting cyclic voltammetry from too high potentials, it was necessary to know the instantaneous rest potentials of polished and natural zirconium in the various solutions. Measurements were done in the diluted fixing solution Ž1r5 vrv. and in the same solution after 7 g ly1 Agq addition. Results are reported in Fig. 1. It can be noticed that equilibria was quickly reached and that, as expected, the rest potential was less negative when zirconium was covered with the natural oxide layer. In both cases, the presence of silver shifted these rest potentials towards less cathodic values. 3.3. Electrochemical behaÕiour of the fixing solution Because during silver electrowinning the cathode became progressively a silver electrode, the fixing solution reduction was studied on both zirconium and silver. All the experiments were done with a sweep rate of 500 mV miny1 , starting from the rest potential of the working electrode. Working electrodes were freshly polished and stationary. The voltammograms, reported on Fig. 2, show that the fixing solution is weakly reduced at the zirconium electrode, for potentials ranging from y1.2 to y1.6 V vs. SCE. But, at the silver electrode two reduction waves are observed before the water reduction. The half-wave potentials of the fixing solution reductions are y0.62 and y1.1 V vs. SCE. The water reduction occurs significantly for a potential lower than y1.3 V vs. SCE. Giron-Palacios et al. w16x have proposed the following reactions for the thiosulphate and bisulphite ions reduction. S 2 O 32y q 8Hqq 8ey™ 2HSyq 3H 2 O y y 6Hqq HSOy 3 q 6e ™ HS q 3H 2 O
M. Chatelut et al.r Hydrometallurgy 54 (2000) 79–90
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Fig. 2. Cyclic voltammetry in diluted fixing solution: v polished zirconium; I polished silver ŽRDE electrode, N s 0 miny1 , scan rates 500 mV miny1 , dilutions1r5 vrv..
3.4. Cyclic Õoltammetry of silÕer in the fixing solution The voltammogram of freshly polished zirconium in the fixing solution containing 7 g ly1 Ag, was obtained between y0.35 and y1.00 V vs. SCE. On Fig. 3 we observe that the silver reduction, according to the reaction w16x, occurs from y0.35 V vs. SCE. Ag Ž S 2 O 3 .
Ž2 xy1 .
q x Hqq ey™ x HS 2 Oy 3 q Ag
The further reductions are due, as mentioned above, to the reduction of the fixing solution on the freshly formed silver deposit. On Fig. 4 are reported the successive cyclic voltammograms obtained with zirconium 3 days after polishing. During the first cycle no noticeable reduction current can be observed, but progressively, as the number of cycles increased, the shape of the voltammograms became similar to those obtained with freshly polished zirconium. In a few minutes, i.e., three to four cycles Ž6 min. the role of the oxide layer disappeared because the silver deposit became more important and the nature of the electrode changed from zirconium to silver. Consequently, the performances of this electrode would be strongly influenced by the deposit adherence to the electrode surface. Since the reduction of the fixing solution occurred on the silver electrode for potentials lower than y0.55 V vs. SCE, the deposition potential must be chosen between y0.43 and y0.55 V vs. SCE to obtain pure silver with a high current efficiency.
Fig. 3. Cyclic voltammetry of the freshly polished zirconium in diluted fixing solution with 7 g ly1 Ag ions added ŽRDE electrode, N s 0 miny1 , scan rates 500 mV miny1 ..
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Fig. 4. Cyclic voltammetry in diluted fixing solution after 7 g ly1 Ag ions added of the ‘‘natural oxide’’-covered zirconium: ` first scan; v third scan; B fourth scan; I sixth; ^ seventh scan ŽRDE electrode, N s 0 miny1 , scan rates 500 mV miny1 ..
3.5. Deposits characterization Depositions were performed using the rotating disc electrode for 22 h in 75 cm3 of the spent fixing solution. The initial silver concentration was 3.2 g ly1 Ži.e., 240 mg in the cell.. Influences of surface oxide thickness, deposition potential and electrode rotation speed were studied. Freshly polished Ž2 nm oxide-covered. and ‘‘natural oxidized’’ zirconium electrodes Ž6 nm oxide layer. were tested, rotation speed was 2000 or 500 miny1 , and depositions were performed at y0.500 and y0.800 V vs. SCE. According to deposition conditions, silver deposits appeared powdery or as pellets more or less compact. On Fig. 5 are reported typical chronoamperometric curves obtained with freshly abraded zirconium electrode during deposition performed at y0.500 VrECS when 500 and 2000 miny1 rotations were applied. We first observed a large reduction in current increase due to the progressive transformation of the zirconium electrode into a plated silver one. When rotation was 2000 miny1 , numerous
Fig. 5. Chronoamperometric curves of the freshly polished zirconium in the spent fixing solution: B N s 500 miny1 ; I N s 2000 miny1 ŽRDE electrode, applied potential Esy0.500 V vs. SCE, silver concentration 3.2 g ly1 , volume 75 cm3 ..
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Table 1 Aspect, recovered percentage and electrical efficiency of silver deposits as a function of the zirconium oxide layer thickness Ž e ., applied potential Ž E . and rotation speed Ž N .. RDE electrode in 75 cm3 of the spent fixing solution containing 3.2 g ly1 of silver, 22 h deposit time ernm
E vs. SCErV
Nrminy1
Deposits characteristics
Ž%. Silver recovered
Ž%. Current efficiency
2
y0.500
2000
76.0
89.5
2
y0.500
500
89.5
81.2
6
y0.500
2000
75.0
98.9
6
y0.500
500
61.6
95.7
2
y0.800
500
129.7 mg as two inegal silver-grey and brittle pellets with one shinning side and dendritic growth at the circle of the other side; 49.3 mg as darkish particles 214.74 mg as one pale grey, very brittle pellet with two dull sides and dendritic growth, it broke when taking it 180 mg as one compact, silver-grey pellet, one shinning side and the other rough 147.9 mg as darkish powder and silver-grey pieces 232.55 mg as darkish particles and pieces of a very brittle pellet with one side dark grey and the other silver-grey and shinning
96.9
17.7
discontinuities can be observed at the beginning of deposition. During this first period it was observed that silver particles fell down and suddenly some parts of the zirconium electrode were in contact with the solution. On this less reactive electrode the silver reduction rate abruptly decreased, then increased again when silver was deposited. Progressively the zirconium surface was chemically oxidized by the solution and silver gripped enough to form adherent pellet-like deposits. After 22 h of deposition, all pellets fell in the cell due to an insufficient adherence. Silver powder was recovered by filtration, total deposit was rinsed, dried and then weighed in order to determine the percents of silver recovered from the initial solution. The electrical efficiency was calculated from the electrical charges computed by integration of the I s f Ž t . curves using the ELCOM 201 software. Results are summarized in Table 1. X-rays diffraction patterns showed only Ag rays for all deposits. From these experiments it appeared that: Ža. as expected, a too negative potential was detrimental to the obtaining compact deposits and satisfying electric efficiency, Žb. on freshly abraded zirconium, 500 miny1 rotation speed seemed better, but deposits were always brittle. Silver recovery and electrical efficiency were acceptable, and Žc. electrowinning with ‘‘natural’’ oxidized zirconium cathode was quantitatively less efficient but allowed, using a larger rotation speed, the formation of a compact silver deposit with a very satisfying electrical efficiency.
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Rotation speed could be related to a linear flow rate by the relation: Õ s p dN where Õ is the linear flow rate, d is the electrode diameter and N is the rotation speed. Applied to a 5-mm diameter electrode with 2000 miny1 rotations we obtained 524 mm sy1 for the flow rate at the periphery of the electrode Žfor comparison, see Section 3.6.. 3.6. Efficiency of electrowinning from spent fixing solution This study was done using the flow circuit cell ŽFig. 6.. Raw zirconium foil was not polished but only cleaned with acetone before use. The discharge of the pump was calculated as follows: Õ s Ž 4rp . Vfy2 where f is the interior diameter of the feed pipe, V is the volume yield per unit of time. In the present case, f was 8 mm, so in order to obtain a similar flow rate the discharge of pump must be close to 100 l hy1 Ž Õ s 552 mm sy1 .. A typical experiment was run as follows. Ža. A 0.9l unknown spent solution was placed in the holder. Previous determination by ICP indicated that silver concentration was 3.2 g ly1 Ži.e., 2.88 g of silver in all..
Fig. 6. Scheme of the circulating flow cell.
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Fig. 7. Variation during the potentiostatic electrolysis of: B reduction current density J; ' Eanode y Ecathode Žcirculating flow cell, ‘‘natural oxide’’-covered zirconium, cathode areas 3.14 10y4 m2 , applied potential Esy0.500 V vs. SCE, graphite anode area s 3=10y3 m2 , flow rates100 l hy1 , initial silver concentration 3.2 g ly1 , volume 0.9 l..
Žb. The anode, a large plate of graphite Žabout 30 cmy2 immersed., was placed parallel to the cathode, as near as possible Ž30 mm.. Žc. The reference electrode was located between anode and cathode. Žd. Temperature was 258C and an increase of 18C in the spent solution was observed during electrolysis. Že. The cathode was raised to y0.600 V vs. SCE during 1r2 h to start the deposition process and then the potential was fixed at y0.500 V vs. SCE. The time variations of the reduction current and of the potential difference between anode and cathode during deposition with y0.5 V vs. SCE applied, are reported on Fig. 7. The mass transport coefficient k m was evaluated according to the expression w10x: k m s I LrAZFC
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Fig. 8. SEM photographs of a silver deposit obtained in the circulating flow cell on the ‘‘natural oxide’’-covered zirconium, using the spent fixing solution. Deposition conditions: potential y0.600 V vs. SCE for 30 min followed by 128 h at y0.500 V vs. SCE, flow rate 100 l hy1 . Ža. Side in contact with the zirconium electrode, Žb. side in contact with the solution.
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where I L is the initial current ŽA., A is the electrode area Žcm2 ., Z is the number of electron change, F is the Faraday’s constant, C is the concentration Žmol cmy3 .. It was found to be 0.0013 cm sy1 . This low value indicated that our cell geometry has to be improved, but our objective, in this preliminary work, was only to test the zirconium electrode at low current densities, in order to prevent side reactions. Electrolysis was stopped after 128 h, when the reduction current reached a weak value, which was constant. Silver concentration in the final solution was found to be 0.020 g ly1 . The electrical charge calculated with the ELCOM software was 2765 C. The deposit was easily removed from the zirconium foil according to the very weak adherence, carefully rinsed and dried. It appeared as a pellet of 3.14 cm2 area, 0.13 cm depth and 2.83 g weight. The calculated specific weight was 6.9 g cmy3 , a value to compare to that of the metallic silver Ž10.5 g cmy3 .. This value showed that the deposit was porous, and the large electrode area could explain the low silver final level. X-ray emission analysis ŽEDS. showed that silver was 99.2% pure and that sulphur concentration in the obtained deposit was always lower than 0.13%. Four identical experiments were run according to the same procedure. It was found that silver recovery reached 98 " 1%, the electrical efficiency 93 " 1% and that the specific weight of the silver deposit was 6.9 " 0.1 g cmy3 . It was noticed that the last 20 h introduced a loss of the electrical efficiency because the reduction of the thio-compounds became predominant when silver concentration reached low values. On Fig. 8 are reported the scanning electron microscopy photographs of such a deposit. On the side which was in contact with the electrode we can notice that only few parts were really attached to the electrode surface, explaining the ease of recovery ŽFig. 8a. and dendritic structure clearly appeared. The row structure observed on the side in contact with the solution confirmed the porous nature expected from the calculated apparent specific weight ŽFig. 8b..
4. Conclusions From these experiments it appeared that zirconium was an interesting cathode material for silver electrowinning from spent fixing solutions. The natural oxide layer, far from being a disadvantage, allowed weakly sticking and sufficiently compact silver deposits, easily removed from the electrode. Although the final silver concentration was about 20 mg ly1 Ža value higher than the norm. the zirconium electrode could be successfully used when the major purpose is metal recovery. Moreover, the cell voltage was lower than 0.9 V. The current efficiency obtained in this work, despite a non-optimized system, was very encouraging because it certainly could be improved. It was demonstrated that a carefully chosen potential avoids any side reaction, i.e., sulphide formation, even at low silver concentration. Despite a long recovery time, 98% efficiency after 100 h, no passivation phenomenon was observed and the purity of silver remained above 99%. The high cost is certainly a serious handicap for using zirconium, but it could be warranted by its great resistance to corrosion especially when chloride or bromide ions are present, as occurs in spent fixing solution.
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References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x
F.C. Hickman, W. Weyert, O.E. Goehler, Ind. Eng. Chem. 25 Ž1933. 202–212. J.M. Grau, J.M. Bisang, J. Chem. Tech. Biotechnol. 53 Ž1992. 105–110. N. Goto, S. Koboshi, N. Takabayashi, Y. Makida, EP 0 387 907 A1Ž19r09r1990. 19 pp. J.R. Fyson, WO 91r09159 Ž04r12r1990. 22 pp. V. Tricolini, N. Vatistas, P.F. Marconi, J. Appl. Electrochem. 23 Ž1993. 390–392. G.I.P. Levenson, C.J. Sharpe, J. Photogr. Sci. 29 Ž1981. 16. C.R. Bertorelli, P. Bernard, Fr 2 681 079-B1 Ž09r09r1994. 21 pp. P. Bernard, Y. Eugene, C.R. Bertorelli, A. Marchand, Fr 2 676 465-B1 Ž17r12r1993. 16 pp. G. Valentin, M. Giron, A. Stork, J.P. Guerlet, Rev. Gen. Electr. 3 Ž1992. 47–55. D.R. Gabe, G.D. Wilcox, J. Gonzalez-Garcia, F.C. Walsh, J. Appl. Electrochem. 28 Ž1998. 759–780. N. Takabayashi, N. Goto, Y. Makida, Y. Kawakami, EP 0 329 275 A1 Ž23r08r1994. 33 pp. L. Young, Anodic Oxide Film, Academic Press, London, 1961. M.J. Dignam, The kinetic of the growth of oxides, in: J.O’M. Bockris, B.E. Conway, E. Yeager, R.E. White ŽEds.., Comprehensive Treatise of Electrochemistry, Vol. 4, Plenum, New York, 1981. E. Gobert, PhD Thesis, Univ. Lyon, No. 285-94, 1994. M. Chatelut, E. Gobert, O. Vittori, Hydrometallurgy 43 Ž1996. 287–298. M. Giron-Palacios, G. Valentin, A. Stork, J.P. Guerlet, Entropie 132 Ž1986. 61.
Silver electrowinning from photographic fixing solutions using zirconium cathode M. Chatelut ) , E. Gobert, O. Vittori Laboratoire d’Electrochimie Analytique — LICAS, UniÕersite´ Lyon I, CPE, 43 Bd du 11 NoÕembre 1918, 69622 Villeurbanne Cedex, France Received 6 March 1998; accepted 3 February 1999
Abstract Suitable electrochemical conditions for using zirconium as a cathode in the electrochemical recovery of silver from photographic fixing solutions were studied by cyclic voltammetry. The influence of the thin, natural zirconium oxide layer was investigated. A flowing system was tested to simulate industrial electrolysis and determine the recovery efficiency, the compactness and purity of the recovered silver. The influence of deposition potential and volumetric flow rate influences were studied. The electrowinning of silver is possible with an applied potential lower than 0.9 V between the cathode and a carbon graphite anode. The final Ag concentration in the bath was 20 mgrl, and 98% of the total silver was recovered with an electrical efficiency close to 92%. Silver deposits were very easy to remove from the electrode. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Silver electrowinning; Photographic fixing solutions; Zirconium cathode; Cyclic voltammetry
1. Introduction Electroreduction appears to be one of the best processes for metal recovery from waste waters, at least for environmental protection, because no other polluting species are induced. In photographic baths, silver forms various complexes with the anions, such as S 2 O 32y, SO 32y, Bry or Cly, present in the solution. These complexes are principally AgŽS 2 O 3 . 23y and AgŽS 2 O 3 . 35y w1x. Electrowinning of silver from photographic fixing solutions has been studied from the beginning of the 20th century and, there are many industrial installations. Massive and porous cathodes have been studied, both have shown some disadvantages. )
Corresponding author. E-mail: [email protected]
0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 9 9 . 0 0 0 6 4 - X
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M. Chatelut et al.r Hydrometallurgy 54 (2000) 79–90
Massive cathodes have been principally made of stainless steel or graphite w2,3x. Silver plated copper has also been proposed w4x. The main disadvantage of massive electrodes was that of a final silver concentration larger than the recommended value for discharge Žtypically 0.1 to 1 mgrl depending on the country.. Porous or three-dimensional cathodes with various compositions have been described, for example carbon felt w5x, carbon fibres w6x, nickel-covered polymer w7x, metallic- or silver-covered glass balls w8x and grids of expanded stainless steel w9x. Recently, Gabe et al. w10x have discussed the performance of rotating cylinder electrodes applied to metal recovery and electrodeposition. The porous electrodes are at present the better way for total silver elimination from photographic wastes. The major problem encountered is the plugging up of the electrodes. Metal recovery generally needs further steps such as burning the substrate w7x, melting or acidic dissolution followed by a new electrolysis of the deposited silver w11x. The nature of the photosensitive film causes the presence in the baths of halide ions which are very corrosive for numerous materials. Carbon graphite electrodes are corrosion resistant, but have a poor mechanical resistance. Zirconium is always covered with an oxide layer insulating the metal from corrosion, but allowing electroreduction processes w12,13x. Previous works showed us that the electrodeposited metals are generally easy to remove w14,15x. So, it seems interesting to test its performance as cathode material for silver electrowinning. In this work, we first studied the electrochemical properties of a commercial fixing solution by cyclic voltammetry on zirconium and silver electrodes. The same study was performed after an addition of silver ions using freshly polished zirconium electrode and the efficiency was also tested with a more oxidized zirconium electrode. Finally a circulating flow cell and a spent fixing solution, available from a photographic laboratory, were used to simulate industrial conditions. Experimental conditions were studied in order to limit the reduction of the fixing solution and silver deposit contamination w15x. Characterizations of the deposits were made by scanning electron microscopy, energy dispersive spectroscopy ŽEDS. and X-ray diffraction.
2. Experimental details 2.1. Apparatus Rest potential determination, cyclic voltammetry and chronoamperometry were carried out using a PGP 20-1 three electrodes potentiostat ŽRadiometer, France. monitored by a PS-1 IBM computer with the ELCOM 201 software. The potential between working and auxiliary electrodes was recorded with an L 6512 device ŽLinseis, Germany.. Electrolyte was circulated using a centrifugal pump ŽEheim, Germany.. SEM examinations and EDS analysis, were carried out in the microscopy centre of the University Claude Bernard, Lyon I ŽCMEABG. using an S800 scanning electron microscope ŽHitachi, Japan. and a CAMEBAX ŽCameca, France.. X-ray diffraction was
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done with a D500 diffractometer ŽSiemens, Germany. in the centre Henri Longchambon of the University Claude Bernard, Lyon I. Determination of silver concentration in the fixing solutions was carried out by ICP using a JY 38-3 spectrophotometer ŽJobin-Yvon, France.. 2.2. Electrodes For voltammetric studies, the working electrode was an EDI rotating disc electrode ŽTacussel, France.. The silver and zirconium rods were purchased from Aldrich ŽUSA. and were 99% purity. They were embedded in a Teflon rod, the geometric electrode area was 0.0314 cm2 for silver and 0.196 cm2 for zirconium. Before each experiment they were carefully polished with a 3-mm grade diamond paste ŽEscil, France., then rinsed with distilled water, quickly wiped and immediately immersed in the solution under investigation. The auxiliary electrode was a Pt wire and the reference electrode was saturated calomel, with a salt bridge filled with 0.1 M Na 2 SO4 . For the experiments performed in the flow cell, the cathode was a 3.14 cm2 zirconium foil Ž99%, Aldrich. used as received, not polished but only cleaned with acetone. The anode was a 30-cm2 carbon graphite plate. During the preliminary studies, the rotating disc electrode was chosen rather than a rotating cylinder principally for three reasons: first it would be difficult to reproduce surface states by polishing a cylinder, second the deposits recovery was easier from disc and third the hydrodynamic conditions for rotating disc were closer than that obtained in the flow circulating system tested here. 2.3. Reagents The fixing reagent, purchased from Ilford ŽUK. ref. Hypam, was based on aqueous ammonium thiosulphate and, as suggested by the manufacturer, it was 1r5 vrv diluted. AgNO 3 was purchased from Johnson Matthey Chemicals ŽUK.. All solutions were made with distilled water. The spent fixing solution was kindly provided by the University Lyon I Centre of Microscopy ŽCMEABG..
3. Results and discussion 3.1. Initial oxide surface characterization In a previous work on zirconium electrodes we have shown that oxide forms instantaneously in air after abrasion, and we estimated the minimum layer thickness to be 1.9 nm w16x. Assuming a uniform coverage with ZrO 2 , we have shown that the oxide layer thickness on the zirconium surface could be estimated from electrochemical oxidation in acidic media w15x. Using the described procedure, we calculated the oxide thickness made during 3 days and a week on a polished electrode just exposed to air. We found identical values close to 4 nm. So a 6-nm thickness ‘‘natural oxide’’ layer was a minimum which was always present in experiments when zirconium was not freshly abraded.
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Fig. 1. Determination of the zirconium rest potential: ` polished zirconium and I natural oxide-covered zirconium in diluted fixing solution; v polished zirconium and B natural oxide-covered zirconium after 7 g ly1 Ag ions added in the diluted fixing solution ŽRDE electrode, N s 0 miny1 , dilutions1r5 vrv..
3.2. Rest potential of zirconium in fixing solution In order to avoid any uncontrolled electrochemical oxidation of zirconium by starting cyclic voltammetry from too high potentials, it was necessary to know the instantaneous rest potentials of polished and natural zirconium in the various solutions. Measurements were done in the diluted fixing solution Ž1r5 vrv. and in the same solution after 7 g ly1 Agq addition. Results are reported in Fig. 1. It can be noticed that equilibria was quickly reached and that, as expected, the rest potential was less negative when zirconium was covered with the natural oxide layer. In both cases, the presence of silver shifted these rest potentials towards less cathodic values. 3.3. Electrochemical behaÕiour of the fixing solution Because during silver electrowinning the cathode became progressively a silver electrode, the fixing solution reduction was studied on both zirconium and silver. All the experiments were done with a sweep rate of 500 mV miny1 , starting from the rest potential of the working electrode. Working electrodes were freshly polished and stationary. The voltammograms, reported on Fig. 2, show that the fixing solution is weakly reduced at the zirconium electrode, for potentials ranging from y1.2 to y1.6 V vs. SCE. But, at the silver electrode two reduction waves are observed before the water reduction. The half-wave potentials of the fixing solution reductions are y0.62 and y1.1 V vs. SCE. The water reduction occurs significantly for a potential lower than y1.3 V vs. SCE. Giron-Palacios et al. w16x have proposed the following reactions for the thiosulphate and bisulphite ions reduction. S 2 O 32y q 8Hqq 8ey™ 2HSyq 3H 2 O y y 6Hqq HSOy 3 q 6e ™ HS q 3H 2 O
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Fig. 2. Cyclic voltammetry in diluted fixing solution: v polished zirconium; I polished silver ŽRDE electrode, N s 0 miny1 , scan rates 500 mV miny1 , dilutions1r5 vrv..
3.4. Cyclic Õoltammetry of silÕer in the fixing solution The voltammogram of freshly polished zirconium in the fixing solution containing 7 g ly1 Ag, was obtained between y0.35 and y1.00 V vs. SCE. On Fig. 3 we observe that the silver reduction, according to the reaction w16x, occurs from y0.35 V vs. SCE. Ag Ž S 2 O 3 .
Ž2 xy1 .
q x Hqq ey™ x HS 2 Oy 3 q Ag
The further reductions are due, as mentioned above, to the reduction of the fixing solution on the freshly formed silver deposit. On Fig. 4 are reported the successive cyclic voltammograms obtained with zirconium 3 days after polishing. During the first cycle no noticeable reduction current can be observed, but progressively, as the number of cycles increased, the shape of the voltammograms became similar to those obtained with freshly polished zirconium. In a few minutes, i.e., three to four cycles Ž6 min. the role of the oxide layer disappeared because the silver deposit became more important and the nature of the electrode changed from zirconium to silver. Consequently, the performances of this electrode would be strongly influenced by the deposit adherence to the electrode surface. Since the reduction of the fixing solution occurred on the silver electrode for potentials lower than y0.55 V vs. SCE, the deposition potential must be chosen between y0.43 and y0.55 V vs. SCE to obtain pure silver with a high current efficiency.
Fig. 3. Cyclic voltammetry of the freshly polished zirconium in diluted fixing solution with 7 g ly1 Ag ions added ŽRDE electrode, N s 0 miny1 , scan rates 500 mV miny1 ..
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Fig. 4. Cyclic voltammetry in diluted fixing solution after 7 g ly1 Ag ions added of the ‘‘natural oxide’’-covered zirconium: ` first scan; v third scan; B fourth scan; I sixth; ^ seventh scan ŽRDE electrode, N s 0 miny1 , scan rates 500 mV miny1 ..
3.5. Deposits characterization Depositions were performed using the rotating disc electrode for 22 h in 75 cm3 of the spent fixing solution. The initial silver concentration was 3.2 g ly1 Ži.e., 240 mg in the cell.. Influences of surface oxide thickness, deposition potential and electrode rotation speed were studied. Freshly polished Ž2 nm oxide-covered. and ‘‘natural oxidized’’ zirconium electrodes Ž6 nm oxide layer. were tested, rotation speed was 2000 or 500 miny1 , and depositions were performed at y0.500 and y0.800 V vs. SCE. According to deposition conditions, silver deposits appeared powdery or as pellets more or less compact. On Fig. 5 are reported typical chronoamperometric curves obtained with freshly abraded zirconium electrode during deposition performed at y0.500 VrECS when 500 and 2000 miny1 rotations were applied. We first observed a large reduction in current increase due to the progressive transformation of the zirconium electrode into a plated silver one. When rotation was 2000 miny1 , numerous
Fig. 5. Chronoamperometric curves of the freshly polished zirconium in the spent fixing solution: B N s 500 miny1 ; I N s 2000 miny1 ŽRDE electrode, applied potential Esy0.500 V vs. SCE, silver concentration 3.2 g ly1 , volume 75 cm3 ..
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Table 1 Aspect, recovered percentage and electrical efficiency of silver deposits as a function of the zirconium oxide layer thickness Ž e ., applied potential Ž E . and rotation speed Ž N .. RDE electrode in 75 cm3 of the spent fixing solution containing 3.2 g ly1 of silver, 22 h deposit time ernm
E vs. SCErV
Nrminy1
Deposits characteristics
Ž%. Silver recovered
Ž%. Current efficiency
2
y0.500
2000
76.0
89.5
2
y0.500
500
89.5
81.2
6
y0.500
2000
75.0
98.9
6
y0.500
500
61.6
95.7
2
y0.800
500
129.7 mg as two inegal silver-grey and brittle pellets with one shinning side and dendritic growth at the circle of the other side; 49.3 mg as darkish particles 214.74 mg as one pale grey, very brittle pellet with two dull sides and dendritic growth, it broke when taking it 180 mg as one compact, silver-grey pellet, one shinning side and the other rough 147.9 mg as darkish powder and silver-grey pieces 232.55 mg as darkish particles and pieces of a very brittle pellet with one side dark grey and the other silver-grey and shinning
96.9
17.7
discontinuities can be observed at the beginning of deposition. During this first period it was observed that silver particles fell down and suddenly some parts of the zirconium electrode were in contact with the solution. On this less reactive electrode the silver reduction rate abruptly decreased, then increased again when silver was deposited. Progressively the zirconium surface was chemically oxidized by the solution and silver gripped enough to form adherent pellet-like deposits. After 22 h of deposition, all pellets fell in the cell due to an insufficient adherence. Silver powder was recovered by filtration, total deposit was rinsed, dried and then weighed in order to determine the percents of silver recovered from the initial solution. The electrical efficiency was calculated from the electrical charges computed by integration of the I s f Ž t . curves using the ELCOM 201 software. Results are summarized in Table 1. X-rays diffraction patterns showed only Ag rays for all deposits. From these experiments it appeared that: Ža. as expected, a too negative potential was detrimental to the obtaining compact deposits and satisfying electric efficiency, Žb. on freshly abraded zirconium, 500 miny1 rotation speed seemed better, but deposits were always brittle. Silver recovery and electrical efficiency were acceptable, and Žc. electrowinning with ‘‘natural’’ oxidized zirconium cathode was quantitatively less efficient but allowed, using a larger rotation speed, the formation of a compact silver deposit with a very satisfying electrical efficiency.
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Rotation speed could be related to a linear flow rate by the relation: Õ s p dN where Õ is the linear flow rate, d is the electrode diameter and N is the rotation speed. Applied to a 5-mm diameter electrode with 2000 miny1 rotations we obtained 524 mm sy1 for the flow rate at the periphery of the electrode Žfor comparison, see Section 3.6.. 3.6. Efficiency of electrowinning from spent fixing solution This study was done using the flow circuit cell ŽFig. 6.. Raw zirconium foil was not polished but only cleaned with acetone before use. The discharge of the pump was calculated as follows: Õ s Ž 4rp . Vfy2 where f is the interior diameter of the feed pipe, V is the volume yield per unit of time. In the present case, f was 8 mm, so in order to obtain a similar flow rate the discharge of pump must be close to 100 l hy1 Ž Õ s 552 mm sy1 .. A typical experiment was run as follows. Ža. A 0.9l unknown spent solution was placed in the holder. Previous determination by ICP indicated that silver concentration was 3.2 g ly1 Ži.e., 2.88 g of silver in all..
Fig. 6. Scheme of the circulating flow cell.
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Fig. 7. Variation during the potentiostatic electrolysis of: B reduction current density J; ' Eanode y Ecathode Žcirculating flow cell, ‘‘natural oxide’’-covered zirconium, cathode areas 3.14 10y4 m2 , applied potential Esy0.500 V vs. SCE, graphite anode area s 3=10y3 m2 , flow rates100 l hy1 , initial silver concentration 3.2 g ly1 , volume 0.9 l..
Žb. The anode, a large plate of graphite Žabout 30 cmy2 immersed., was placed parallel to the cathode, as near as possible Ž30 mm.. Žc. The reference electrode was located between anode and cathode. Žd. Temperature was 258C and an increase of 18C in the spent solution was observed during electrolysis. Že. The cathode was raised to y0.600 V vs. SCE during 1r2 h to start the deposition process and then the potential was fixed at y0.500 V vs. SCE. The time variations of the reduction current and of the potential difference between anode and cathode during deposition with y0.5 V vs. SCE applied, are reported on Fig. 7. The mass transport coefficient k m was evaluated according to the expression w10x: k m s I LrAZFC
88
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Fig. 8. SEM photographs of a silver deposit obtained in the circulating flow cell on the ‘‘natural oxide’’-covered zirconium, using the spent fixing solution. Deposition conditions: potential y0.600 V vs. SCE for 30 min followed by 128 h at y0.500 V vs. SCE, flow rate 100 l hy1 . Ža. Side in contact with the zirconium electrode, Žb. side in contact with the solution.
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where I L is the initial current ŽA., A is the electrode area Žcm2 ., Z is the number of electron change, F is the Faraday’s constant, C is the concentration Žmol cmy3 .. It was found to be 0.0013 cm sy1 . This low value indicated that our cell geometry has to be improved, but our objective, in this preliminary work, was only to test the zirconium electrode at low current densities, in order to prevent side reactions. Electrolysis was stopped after 128 h, when the reduction current reached a weak value, which was constant. Silver concentration in the final solution was found to be 0.020 g ly1 . The electrical charge calculated with the ELCOM software was 2765 C. The deposit was easily removed from the zirconium foil according to the very weak adherence, carefully rinsed and dried. It appeared as a pellet of 3.14 cm2 area, 0.13 cm depth and 2.83 g weight. The calculated specific weight was 6.9 g cmy3 , a value to compare to that of the metallic silver Ž10.5 g cmy3 .. This value showed that the deposit was porous, and the large electrode area could explain the low silver final level. X-ray emission analysis ŽEDS. showed that silver was 99.2% pure and that sulphur concentration in the obtained deposit was always lower than 0.13%. Four identical experiments were run according to the same procedure. It was found that silver recovery reached 98 " 1%, the electrical efficiency 93 " 1% and that the specific weight of the silver deposit was 6.9 " 0.1 g cmy3 . It was noticed that the last 20 h introduced a loss of the electrical efficiency because the reduction of the thio-compounds became predominant when silver concentration reached low values. On Fig. 8 are reported the scanning electron microscopy photographs of such a deposit. On the side which was in contact with the electrode we can notice that only few parts were really attached to the electrode surface, explaining the ease of recovery ŽFig. 8a. and dendritic structure clearly appeared. The row structure observed on the side in contact with the solution confirmed the porous nature expected from the calculated apparent specific weight ŽFig. 8b..
4. Conclusions From these experiments it appeared that zirconium was an interesting cathode material for silver electrowinning from spent fixing solutions. The natural oxide layer, far from being a disadvantage, allowed weakly sticking and sufficiently compact silver deposits, easily removed from the electrode. Although the final silver concentration was about 20 mg ly1 Ža value higher than the norm. the zirconium electrode could be successfully used when the major purpose is metal recovery. Moreover, the cell voltage was lower than 0.9 V. The current efficiency obtained in this work, despite a non-optimized system, was very encouraging because it certainly could be improved. It was demonstrated that a carefully chosen potential avoids any side reaction, i.e., sulphide formation, even at low silver concentration. Despite a long recovery time, 98% efficiency after 100 h, no passivation phenomenon was observed and the purity of silver remained above 99%. The high cost is certainly a serious handicap for using zirconium, but it could be warranted by its great resistance to corrosion especially when chloride or bromide ions are present, as occurs in spent fixing solution.
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