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Electrochemical oxidation of cyanide in the hydrocyclone cell
Pergamon
Waste Management, Vol. 16, No. 4, pp. 257~61, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0956-053X/96 $15.00 + 0.00
PII: S0956-053X(96)00051-7
ORIGINAL CONTRIBUTION
ELECTROCHEMICAL OXIDATION OF CYANIDE IN THE HYDROCYCLONE CELL N. Dhamo Institut fiir Metallurgie, Metallhfittenkunde, TU Berlin, Germany
A B S T R A C T . A diluted electroplating cyanide rinse water has been used to test the use of the hydrocyclone cell (HCC) in
batch recycle mode of operation for the simultaneous oxidation of cyanide during the electrodeposition of silver. The results obtained in this work with regard to the final products, current efficiency and the number of transferred electrons per CN helped to establish a probable reaction scheme. According to this, the process occurs mainly with one-electron transfer, through cyanate and cyanogen as intermediate species. Meanwhile, under conditions where the electrolyte circulates in an open bath and flows successively through the cathodic and the anodic compartments, as in the case of the HCC system, the cyanate could be produced by the direct oxidation through air and/or generated peroxide and CN could be lost as HCN (g). Copyright © 1996 Published by Elsevier Science Ltd
but it is still not completely elucidated. In the opinion of many authors, the anodic oxidation of cyanide ions proceeds by steps, through cyanide radicals
1. INTRODUCTION The electrochemical oxidation of cyanide is especially attractive in the treatment of industrial effluents arising from the electroplating or metal/mineral processing operations, since it offers the possibility of simultaneous recovery of dissolved metals, which could pay for the process) 8 The present paper deals with the simultaneous oxidation of cyanide at the graphite anode during the electrodeposition of silver from diluted electroplating cyanide rinse waters by means of the hydrocyclone cell (HCC) - an electrochemical reactor developed recently. 9A°
C N - e -~ CN*
(1)
CN* + 2OH- - e ~ CNO +H20
(2)
to the cyanate
and/or the cyanogen CN* + C N - e ~ (CN)2
(3)
CN* + CN* -~ (CN)2
(4)
depending on the composition o f the effluent and the material of the anode. In the case of graphite anode and in the presence of hydroxide ions, the cyanate may react further electrolytically
2. COMMENTS ON THE OXIDATION REACTION SCHEMES Several papers j~ 20 have been dedicated to the mechanism of the electrochemical oxidation of cyanide,
2CNO + 4 O H - 6e --) 2 C O J +N2 "~+ 2H20 (5) according to which the cyanide is oxidized by fiveelectron transfer, or by hydrolysis
RECEIVED 17 APRIL 1995; ACCEPTED 12 JUNE 1996. Permanent address: Department of Chemistry, Faculty of Natural Sciences, University o f Tirana, Tirana, Albania. Acknowledgements--This research work was initiated under a grant from the Alexander von Humboldt Foundation which is very gratefully acknowledged. The author would like also to thank Prof. Dr Ing. R. Kammel from the Institut o f Metallurgy/TU Berlin for helpful discussions, Dr U. Landau from OTB-GambH & Co (Berlin) for the facilities offered and D. Kaiser (TU Berlin) for the chemical analyses.
CNO + 2H20 --) CO32 + NH4 +
(6)
during which the number of electrons per CN is two. The cyanogen may also react electrolytically (CN)2 + 4OH- - 2e - ) 2 C N O + 2H20
(7)
according to which the cyanide is oxidized by twoelectron transfer, or by hydrolysis: 257
258
N. D H A M O
(CN)2+ H20 --> HCN + HOCN
(8)
HCN --->CN- + H +
(8a)
nHCN --> (HCN),
(8b)
HOCN + OH --> CNO + H20
(8c)
(CN)2 + 4H20
-->
(COO)22- q- 2NH4 +
(9)
during which, depending on the subsequent paths, the number of electrons per CN- may be one or two. Experiments with a graphite anode ~6have shown that the oxidation of cyanide occurs with one- or twoelectron transfer depending on the concentration of the hydroxide ions. There are also data ~5 according to which, with the decrease of cyanide concentration from 2.5 to 0.5 g dm -3, the process occurs with oneelectron transfer and in lower concentrations with two or more electrons per CN-. Meanwhile, the above schemes show that the oxidation of cyanide through one-electron transfer may occur only through the cyanogen, namely through paths (8), (8b), (8c) and (6) which lead to the formation of azulmin ~3,2° and carbonate ions or through path (9) which leads to the formation of oxalate ions. The above schemes show only the possible paths for the cyanide oxidation, but other ways (the oxidation through generated peroxide ~5or through building of intermediate species on the electrode surface 1~14,16,18.19) and other final products 2~ cannot be excluded. Any attempt to settle a reaction scheme depends strongly on the chemical analysis, which may differentiate the reaction paths and assess their contributions.
gl~
3. EXPERIMENTAL 3.1. Apparatus
The experimental set up and the HCC concept are shown in Fig. 1. As depicted, the standardized cyclone, 78 mm diameter, 22 is modified: the stainless steel cyclone wall (a) serves as cathode and in the central exit a graphite block 100 mm in length and 50 mm in diameter, as an anode (b), is situated concentrically and is surrounded by the diaphragm (c), a cation exchange membrane type Thomapor 50382, which acts as "vortex-finder". The open reservoir (i), coupled with the cell via a sliding-vane type rotary pump (j), enables batch recycle or single-pass mode experiments to be performed. The apex-valve (k) was kept closed and all tangentially entering (d) solution left the cell upwards, via the helical free cross-section created between the anode and the diaphragm due to the helical wings (m) on the anode surface. The free volume of the cell is 0.78 dm 3. The device allows an easy dismantling of the cell-top (f) and inspection. Regarding pattern flow, the HCC provides two new features: an accelerated downward helical flow (n) along the cathode surface due to the conical shape of the cathodic compartment and a slightly decelerated upward helical flow in the form of a thin layer (3-4 ram) along the anode surface (p), due to the slightly increasing upwards helical free cross-section of the anodic compartment. The first plays an important role in metal recovery) whereas the second enables a good contact with
_.--4
f e
h
b
rnc
a
II II II II ii Ii Ii
r?
li ii
g P
i
F
i
FIGURE 1. Schematic arrangement of the experimental set up and the HCC concept, a, cathode; b, anode; c, diaphragm/vortex-finder; d, electrolyte inlet; e, Teflon gasket; f, transparent perspex cell-top; g, vent; h, electrolyte outlet; i, electrolyte reservoir (open bath); j, recirculating pump; k, apex-valve; m, helical wings; n, helical downward flow (inlet); p, helical upward flow (outlet).
ELECTROCHEMICAL OXIDATION OF CYANIDE IN THE HCC
259
TABLE 1 Experimental Results Concerning Cyanide Oxidation by Means of HCC
No.
1 2 3 4 5
Cell voltage (V)
1.8 1.9 2.0 2.2 2.4
Flow rate (1 rain ])
17 17 17 17.5 17
Electrolysis Electrical time conductivity (h) (S m l)
6.5 72 6.5 7 22
1.10 2.08 1.16 1.20 2.02
Current density (range) (A dm 2),
Initial Final Cumulative current cyanide cyanide efficiency (%) concentration concentration (g 1-1) (g l-J) One-electron Two-electron transfer transfer
0.31-0.40 0.48-0.55 0.33-0.46 0.424).58 0.67
1.550 2.444 1.420 1.530 l. 122
1.460 1.122 1.342 1.424 0.692
104 98.3 80.4 76 78.4
208 196.5 160.8 152 196.8
*Relative to the diaphragm area, = 1.36 dm2.
anode surface and offers more time to an anodic reaction. 3.2. Procedure The electrolyte, containing K[Ag(CN)2], KCN, K2CO3 and additives, is prepared by diluting with tap water an electroplating rinse water (of the first rinsing stage) containing initially 2.35 g dm 3- Ag + and 49.6 g dm 3- C N , and having electrical conductivity 4.22 S m 1 and pH = 11.5. The experiments were performed at room temperature (17-23°C) and in batch recycle mode of operation. The volume of the solution to be handled in each run amounted to ca. 35 d m 3. T h e d e p l e t i o n o f c y a n i d e was f o l l o w e d b y a r g e n t o m e t r i c titration, 6J2"]3 t a k i n g i n t o a c c o u n t the q u a n tity o f silver d e t e r m i n e d b y a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y . T h e m e t h o d d e t e r m i n e s the total TABLE 2 Experimental Results Concerning Cyanide Oxidation by Means of HCC, Under the Best Conditions for Silver Removal
quantity of cyanide and gave satisfactory results under the present conditions. 4. R E S U L T S A N D D I S C U S S I O N
Table 1 presents a summary of the experimental conditions and the results. As may be seen, it is very difficult to interpret the "high" current efficiencies (last column in Table 1) with the above-mentioned reaction schemes. With a view to explaining the results concerning the current efficiencies, a prolonged electrolysis was undertaken (Table 1, No. 2 and 5) under the best experimental conditions for the electrodeposition of silver. 9 The following discussion will be focused on this case. Table 2 presents the experimental conditions and the results obtained regarding cyanide depletion and the carbonate and oxalate ions produced. The concentrations of CO32 and (C00)22- were determined by standard volumetric methods. 23 Figure 2 shows the variation with time of the cyanide concentration for two applied cell voltages. As may be seen, the cell voltage has
Cell voltage (V)
Initial cyanide concentration (g dm -3) Final cyanide concentration (g dm 3) Quantity of the destroyed cyanide (g) Initial silver concentration (ppm)* Initial concentration of CO32 (g dm -3) Final concentration of CO32 (g dm -3) Initial concentration of (COO)22 (g dm 3) Final concentration of (COO)22 (g dm -~) Electrolysis time (h)
1.9
2.4
2500 1.122 46.27 104 4.056
1.122 0.692 15.05
2.50 ----"
8
2 . 0 0 ~
Initial pH value Overall electrolysis time (h) Overall quantity of cyanide destroyed (g) Overall quantity of carbonate ions produced (g) Overall quantity of oxalate ions produced (g) Final pH value
11.45 94 61.32 71.40 2.835 11.4
*After approx. 20 h the silver was almost completely removed; the experiments concerning metal removal are described elsewhere. 9
U=Z4v~
-0.0006"~.
0.062 0.143 22
U:t.Sv~
•
1.50
6.12
72
•
slope ( tool/ I hour) ~ k
l 1.00
%,
0.500.00 0.00
'
I ' I 40.00 80.00 time(hour)
'
120.00
FIGURE 2. Variation with time of cyanide concentration.
N. D H A M O
260
little influence on the rate of cyanide depletion, at least in this concentration range. Current efficiency, space-time-yield and specific electrical energy have been calculated assuming oneelectron transfer. The variation of current efficiency with cyanide depletion is shown in Fig. 3, while the results concerning the space-time-yield, the specific electrical energy and the number of electrons per CN- (corresponding to 100% current efficiency) are summarized in Table 3. Though no attempt was made to analyse all species in the solution before and after electrolysis, some deductions can be made. The results obtained for current efficiency (Table 1 and Fig. 3) and for the number of transferred electrons per CN (Table 3) show that the oxidation of cyanide occurs almost entirely with one-electron transfer, i.e. referring to the above schemes, through cyanogen, following paths (8), (8b), (8c) and (6) or following path (9). Oxidation by two or more electron transfer leads to absurd current efficiencies (> 180%); therefore it seems to be excluded or to make only a minor contribution to the process. Meanwhile, azulmin (8b) may be produced in only a very small quantity, 2° whereas the quantity of the oxalate ions produced in this case amounted only to 3% and that of the carbonate ions to 52% of the quantities corresponding to the destroyed cyanide (Table 2). The rest of the cyanide must be destroyed by other methods. One could be direct oxidation by air, which is always present in the open baths: 2CN + 02 ~ 2CNO
(10)
2CNO- + 02 - 2e ~ 2CO21" + N2~
(11)
Another possible way could be oxidation through the peroxide, which is generated in the presence of air, even at low electrode potentials ~5 02 + 2H20 + 2e -o H202 + 2OH-
(12)
It reacts homogeneously with cyanide CN- + H202 --~ C N O + H20
(13)
and then electrochemically with the cyanate 2 C N O + 2H202 - 2e ~ 2CO2r + N2r + H20 (14) 120.
100.
8o
60
0.50
•o
oo
"0 &&
'
&&
!
~
'
I
'
U=1.9volt !
I
1.00 1.,50 2.00 cyanid concentration (g/l)
'
2.50
F I G U R E 3. Current efficiency as a function of cyanide depletion (assuming one-electron transfer).
TABLE 3 Summary of the Results Concerning Cyanide Oxidation by Means of HCC, Under the Best Conditions for Silver Recovery
Cell voltage (V) 1.9
2.444-1.122 Cyanide depletion (g dm 3) Space-time-yield (g s J m 3) 0.31-0.20 Specific electrical energy (kWh/kg) 1.82-2.08 0.93-1.06 Number of electrons per CN-
2.4
1.122-0.692 = 0.25 ~- 3.05 = 1.25
The batch recycle mode of operation in the HCC system (Fig. 1) enables direct oxidation through air as well as the formation of the peroxide and its reaction with cyanide and cyanate, because the electrolyte is circulated through an open reservoir and flows successively through both cathodic and anodic compartments of the cell; this mode of operation may lead to the loss of cyanide as HCN (g) as well. The contribution of paths (10) to (14) and the loss of cyanide as HCN may explain the one-electron transfer mechanism for the oxidation of cyanide in the HCC. At higher cell voltages, the paths (5) and (7) begin to contribute to the cyanide oxidation; this explains the increase in the number of electrons per CN- (see Tables 1 and 3). The pure graphite, type EK 586, used in those experiments proved to be a suitable material for the anode in such alkaline solutions: even after more than 230 hours' electrolysis, no changes were observed on the anode's surface. 5. CONCLUSIONS Experiments performed using HCC for the simultaneous oxidation of cyanide during the electrodeposition of silver from a diluted electroplating rinsewater demonstrated the following. (a) In the concentration range 2.5-0.69 g d m 3 cyanide destruction occurs at an almost constant rate -- 0.73 g h -~ (-- 8 × 104 mol la ~) and with good performance indexes (Figs 2 and 3). (b) The process occurs mainly by one-electron transfer, through cyanate and cyanogen as intermediate species. (c) Under the conditions where the electrolyte circulates in an open bath and flows successively through the cathodic and the anodic compartment the cyanate could be produced by the direct oxidation through air and/or generated peroxide. (d) The loss of cyanide as HCN cannot be excluded, neither can it be prevented. (e) Oxidation by two or more electrons per CNseems to be almost excluded, at least in this concentration range and at low cell voltages.
ELECTROCHEMICAL
O X I D A T I O N O F C Y A N I D E IN T H E H C C
The results obtained in this work help to establish other reaction schemes, (11) and (14), which lead to the formation of carbon dioxide and nitrogen. REFERENCES 1. Wiaux, J. P. Die Abwasserreinigung durch Elektrolyse. Oberflgiche-Surface 24:16 (1988). 2. Kirch, R. and Schnell, M. Elektrolytische Entgiftung verbrauchter cyanidischer Entfettungsb~ider im Galvanikbetrieb. Galvanotechnik 78/4:964 (1987). 3. Ramirez, E. R. and D'Alessio, O. F. Innovative design and engineering of a wastewater pretreatment facility for a metal finishing operation. Proc. 39'h Ind. Waste Conf., Purdue University, p. 545 (1985). 4. Kammel, R. Recent developments in the design of electrochemical cells for the recovery of metals. Keynote Paper, MIN1TEK 50, Johannesburg, p. 443 (1984). 5. Silman, H. Electrolytishe Verfahren zur Metallr~ckgewinnung aus Abwasser. Galvanotechnik 73/6:589 (1982). 6. EI-Ghaoui, E. A. and Jansson, R. E. W. Application of the trickle tower to problems of pollution control. III. Heavymetal cyanide solutions. J. AppL Electrochem. 12:75 (1982). 7. Zabban, W. and Helwick, R. Cyanide waste treatment technology - the old, the new and the practical. Plat. Surf. Fin. 67:56 (1980). 8. Demidov, V. I. Extraction of Au and Cu during electrochemical purification of discard cyanide water. Tsvetnye Metally/ Non-ferrous Metals 12:57 (1974). 9. Dhamo, N. and Kammel, R. Electrochemical hydrocyclone cell for metal recovery from dilute solutions. METALL 46/9: 912 (1992). 10. Dhamo, N. An electrochemical hydrocyclone cell for the treatment of dilute solutions: approximate plug-flow model for electrodeposition kinetics. J. Appl. Electrochem. 24:745 (1994).
261
11. Gulbas, W. and Gotzelmann, W. Die elektrolytische Zerst6rung yon Cyanid. Metalloberflgiche 43/1:7 (1989). 12. Kelsall, G. H., Savage, S. and Brandt, D. Cyanide oxidation at nickel anodes. II Voltammetry and coulometry of Ni/CNH20 systems. J. Electroehem. Soc. 138:117 (1991). 13. Hine, F., Yasuda, M., Iida, T. and Ogata, Y. On the oxidation of cyanide solutions with lead dioxide coated anode. Electrochim. Acta 31:1389 (1986). 14. Katagiri, A., Yoshimura, S. and Yoshizawa, S. Formation constant of the tetracyanocuprate (II) ion and the mechanism of its decomposition. Inorg Chem. 20:4143 (1981). 15. Kuhn, A. T. and Biddle, K. Die anodische Zerst6rung von Cyaniden in wassrigen L6sungen. Oberfldche-Surface 18/7: 182 (1977). 16. Arikado, T., lwakura, C., Yoneyama, H. and Tamura, H. Anodic oxidation of potassium cyanide on the graphite electrode. Electrochim. Acta 21:1021 (1976). 17. Knorre, H. Erkenntnisse und Erfahrungen bei der Entgiftung cyanidischer Abwasser. Galvanotechnik 66/5:374 (1975). 18. Tamura, H., Arikado, T., Yoneyama, H. and Matsuda, Y. Anodic oxidation of potassium cyanide on platinum electrode. Electrochim. Acta 19:273 (1974). 19. Sawyer, D. T. and Day, R. J. Electrochemical oxidation of cyanide ion at platinum electrodes. J. Electroanal. Chem. 5: 195 (1963). 20. Krusenstjern, v.A. and Mussinger, W. Dicyan-Bildung in cyanidischen Silberbadern. Metalloberflgiche 16/9:263 (1962). 21. EI-Ghaoui, E. A., Jansson, R. E. W. and Moreland, C. Application of the trickle tower to problems of pollution control. II. The direct and indirect oxidation of cyanide..L Appl. Electrochem. 12:69 (1982). 22. Bradley, D. The Hydrocychme. Pergamon Press, New York (1965). 23. Belcher, R., Nutten, A. J. and Macdonald, A. M. G. Quantitative Inorganic Analysis, 3ra edn. Butterworths, London (1970).
Open for discussion until 28 February 1997
Waste Management, Vol. 16, No. 4, pp. 257~61, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0956-053X/96 $15.00 + 0.00
PII: S0956-053X(96)00051-7
ORIGINAL CONTRIBUTION
ELECTROCHEMICAL OXIDATION OF CYANIDE IN THE HYDROCYCLONE CELL N. Dhamo Institut fiir Metallurgie, Metallhfittenkunde, TU Berlin, Germany
A B S T R A C T . A diluted electroplating cyanide rinse water has been used to test the use of the hydrocyclone cell (HCC) in
batch recycle mode of operation for the simultaneous oxidation of cyanide during the electrodeposition of silver. The results obtained in this work with regard to the final products, current efficiency and the number of transferred electrons per CN helped to establish a probable reaction scheme. According to this, the process occurs mainly with one-electron transfer, through cyanate and cyanogen as intermediate species. Meanwhile, under conditions where the electrolyte circulates in an open bath and flows successively through the cathodic and the anodic compartments, as in the case of the HCC system, the cyanate could be produced by the direct oxidation through air and/or generated peroxide and CN could be lost as HCN (g). Copyright © 1996 Published by Elsevier Science Ltd
but it is still not completely elucidated. In the opinion of many authors, the anodic oxidation of cyanide ions proceeds by steps, through cyanide radicals
1. INTRODUCTION The electrochemical oxidation of cyanide is especially attractive in the treatment of industrial effluents arising from the electroplating or metal/mineral processing operations, since it offers the possibility of simultaneous recovery of dissolved metals, which could pay for the process) 8 The present paper deals with the simultaneous oxidation of cyanide at the graphite anode during the electrodeposition of silver from diluted electroplating cyanide rinse waters by means of the hydrocyclone cell (HCC) - an electrochemical reactor developed recently. 9A°
C N - e -~ CN*
(1)
CN* + 2OH- - e ~ CNO +H20
(2)
to the cyanate
and/or the cyanogen CN* + C N - e ~ (CN)2
(3)
CN* + CN* -~ (CN)2
(4)
depending on the composition o f the effluent and the material of the anode. In the case of graphite anode and in the presence of hydroxide ions, the cyanate may react further electrolytically
2. COMMENTS ON THE OXIDATION REACTION SCHEMES Several papers j~ 20 have been dedicated to the mechanism of the electrochemical oxidation of cyanide,
2CNO + 4 O H - 6e --) 2 C O J +N2 "~+ 2H20 (5) according to which the cyanide is oxidized by fiveelectron transfer, or by hydrolysis
RECEIVED 17 APRIL 1995; ACCEPTED 12 JUNE 1996. Permanent address: Department of Chemistry, Faculty of Natural Sciences, University o f Tirana, Tirana, Albania. Acknowledgements--This research work was initiated under a grant from the Alexander von Humboldt Foundation which is very gratefully acknowledged. The author would like also to thank Prof. Dr Ing. R. Kammel from the Institut o f Metallurgy/TU Berlin for helpful discussions, Dr U. Landau from OTB-GambH & Co (Berlin) for the facilities offered and D. Kaiser (TU Berlin) for the chemical analyses.
CNO + 2H20 --) CO32 + NH4 +
(6)
during which the number of electrons per CN is two. The cyanogen may also react electrolytically (CN)2 + 4OH- - 2e - ) 2 C N O + 2H20
(7)
according to which the cyanide is oxidized by twoelectron transfer, or by hydrolysis: 257
258
N. D H A M O
(CN)2+ H20 --> HCN + HOCN
(8)
HCN --->CN- + H +
(8a)
nHCN --> (HCN),
(8b)
HOCN + OH --> CNO + H20
(8c)
(CN)2 + 4H20
-->
(COO)22- q- 2NH4 +
(9)
during which, depending on the subsequent paths, the number of electrons per CN- may be one or two. Experiments with a graphite anode ~6have shown that the oxidation of cyanide occurs with one- or twoelectron transfer depending on the concentration of the hydroxide ions. There are also data ~5 according to which, with the decrease of cyanide concentration from 2.5 to 0.5 g dm -3, the process occurs with oneelectron transfer and in lower concentrations with two or more electrons per CN-. Meanwhile, the above schemes show that the oxidation of cyanide through one-electron transfer may occur only through the cyanogen, namely through paths (8), (8b), (8c) and (6) which lead to the formation of azulmin ~3,2° and carbonate ions or through path (9) which leads to the formation of oxalate ions. The above schemes show only the possible paths for the cyanide oxidation, but other ways (the oxidation through generated peroxide ~5or through building of intermediate species on the electrode surface 1~14,16,18.19) and other final products 2~ cannot be excluded. Any attempt to settle a reaction scheme depends strongly on the chemical analysis, which may differentiate the reaction paths and assess their contributions.
gl~
3. EXPERIMENTAL 3.1. Apparatus
The experimental set up and the HCC concept are shown in Fig. 1. As depicted, the standardized cyclone, 78 mm diameter, 22 is modified: the stainless steel cyclone wall (a) serves as cathode and in the central exit a graphite block 100 mm in length and 50 mm in diameter, as an anode (b), is situated concentrically and is surrounded by the diaphragm (c), a cation exchange membrane type Thomapor 50382, which acts as "vortex-finder". The open reservoir (i), coupled with the cell via a sliding-vane type rotary pump (j), enables batch recycle or single-pass mode experiments to be performed. The apex-valve (k) was kept closed and all tangentially entering (d) solution left the cell upwards, via the helical free cross-section created between the anode and the diaphragm due to the helical wings (m) on the anode surface. The free volume of the cell is 0.78 dm 3. The device allows an easy dismantling of the cell-top (f) and inspection. Regarding pattern flow, the HCC provides two new features: an accelerated downward helical flow (n) along the cathode surface due to the conical shape of the cathodic compartment and a slightly decelerated upward helical flow in the form of a thin layer (3-4 ram) along the anode surface (p), due to the slightly increasing upwards helical free cross-section of the anodic compartment. The first plays an important role in metal recovery) whereas the second enables a good contact with
_.--4
f e
h
b
rnc
a
II II II II ii Ii Ii
r?
li ii
g P
i
F
i
FIGURE 1. Schematic arrangement of the experimental set up and the HCC concept, a, cathode; b, anode; c, diaphragm/vortex-finder; d, electrolyte inlet; e, Teflon gasket; f, transparent perspex cell-top; g, vent; h, electrolyte outlet; i, electrolyte reservoir (open bath); j, recirculating pump; k, apex-valve; m, helical wings; n, helical downward flow (inlet); p, helical upward flow (outlet).
ELECTROCHEMICAL OXIDATION OF CYANIDE IN THE HCC
259
TABLE 1 Experimental Results Concerning Cyanide Oxidation by Means of HCC
No.
1 2 3 4 5
Cell voltage (V)
1.8 1.9 2.0 2.2 2.4
Flow rate (1 rain ])
17 17 17 17.5 17
Electrolysis Electrical time conductivity (h) (S m l)
6.5 72 6.5 7 22
1.10 2.08 1.16 1.20 2.02
Current density (range) (A dm 2),
Initial Final Cumulative current cyanide cyanide efficiency (%) concentration concentration (g 1-1) (g l-J) One-electron Two-electron transfer transfer
0.31-0.40 0.48-0.55 0.33-0.46 0.424).58 0.67
1.550 2.444 1.420 1.530 l. 122
1.460 1.122 1.342 1.424 0.692
104 98.3 80.4 76 78.4
208 196.5 160.8 152 196.8
*Relative to the diaphragm area, = 1.36 dm2.
anode surface and offers more time to an anodic reaction. 3.2. Procedure The electrolyte, containing K[Ag(CN)2], KCN, K2CO3 and additives, is prepared by diluting with tap water an electroplating rinse water (of the first rinsing stage) containing initially 2.35 g dm 3- Ag + and 49.6 g dm 3- C N , and having electrical conductivity 4.22 S m 1 and pH = 11.5. The experiments were performed at room temperature (17-23°C) and in batch recycle mode of operation. The volume of the solution to be handled in each run amounted to ca. 35 d m 3. T h e d e p l e t i o n o f c y a n i d e was f o l l o w e d b y a r g e n t o m e t r i c titration, 6J2"]3 t a k i n g i n t o a c c o u n t the q u a n tity o f silver d e t e r m i n e d b y a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y . T h e m e t h o d d e t e r m i n e s the total TABLE 2 Experimental Results Concerning Cyanide Oxidation by Means of HCC, Under the Best Conditions for Silver Removal
quantity of cyanide and gave satisfactory results under the present conditions. 4. R E S U L T S A N D D I S C U S S I O N
Table 1 presents a summary of the experimental conditions and the results. As may be seen, it is very difficult to interpret the "high" current efficiencies (last column in Table 1) with the above-mentioned reaction schemes. With a view to explaining the results concerning the current efficiencies, a prolonged electrolysis was undertaken (Table 1, No. 2 and 5) under the best experimental conditions for the electrodeposition of silver. 9 The following discussion will be focused on this case. Table 2 presents the experimental conditions and the results obtained regarding cyanide depletion and the carbonate and oxalate ions produced. The concentrations of CO32 and (C00)22- were determined by standard volumetric methods. 23 Figure 2 shows the variation with time of the cyanide concentration for two applied cell voltages. As may be seen, the cell voltage has
Cell voltage (V)
Initial cyanide concentration (g dm -3) Final cyanide concentration (g dm 3) Quantity of the destroyed cyanide (g) Initial silver concentration (ppm)* Initial concentration of CO32 (g dm -3) Final concentration of CO32 (g dm -3) Initial concentration of (COO)22 (g dm 3) Final concentration of (COO)22 (g dm -~) Electrolysis time (h)
1.9
2.4
2500 1.122 46.27 104 4.056
1.122 0.692 15.05
2.50 ----"
8
2 . 0 0 ~
Initial pH value Overall electrolysis time (h) Overall quantity of cyanide destroyed (g) Overall quantity of carbonate ions produced (g) Overall quantity of oxalate ions produced (g) Final pH value
11.45 94 61.32 71.40 2.835 11.4
*After approx. 20 h the silver was almost completely removed; the experiments concerning metal removal are described elsewhere. 9
U=Z4v~
-0.0006"~.
0.062 0.143 22
U:t.Sv~
•
1.50
6.12
72
•
slope ( tool/ I hour) ~ k
l 1.00
%,
0.500.00 0.00
'
I ' I 40.00 80.00 time(hour)
'
120.00
FIGURE 2. Variation with time of cyanide concentration.
N. D H A M O
260
little influence on the rate of cyanide depletion, at least in this concentration range. Current efficiency, space-time-yield and specific electrical energy have been calculated assuming oneelectron transfer. The variation of current efficiency with cyanide depletion is shown in Fig. 3, while the results concerning the space-time-yield, the specific electrical energy and the number of electrons per CN- (corresponding to 100% current efficiency) are summarized in Table 3. Though no attempt was made to analyse all species in the solution before and after electrolysis, some deductions can be made. The results obtained for current efficiency (Table 1 and Fig. 3) and for the number of transferred electrons per CN (Table 3) show that the oxidation of cyanide occurs almost entirely with one-electron transfer, i.e. referring to the above schemes, through cyanogen, following paths (8), (8b), (8c) and (6) or following path (9). Oxidation by two or more electron transfer leads to absurd current efficiencies (> 180%); therefore it seems to be excluded or to make only a minor contribution to the process. Meanwhile, azulmin (8b) may be produced in only a very small quantity, 2° whereas the quantity of the oxalate ions produced in this case amounted only to 3% and that of the carbonate ions to 52% of the quantities corresponding to the destroyed cyanide (Table 2). The rest of the cyanide must be destroyed by other methods. One could be direct oxidation by air, which is always present in the open baths: 2CN + 02 ~ 2CNO
(10)
2CNO- + 02 - 2e ~ 2CO21" + N2~
(11)
Another possible way could be oxidation through the peroxide, which is generated in the presence of air, even at low electrode potentials ~5 02 + 2H20 + 2e -o H202 + 2OH-
(12)
It reacts homogeneously with cyanide CN- + H202 --~ C N O + H20
(13)
and then electrochemically with the cyanate 2 C N O + 2H202 - 2e ~ 2CO2r + N2r + H20 (14) 120.
100.
8o
60
0.50
•o
oo
"0 &&
'
&&
!
~
'
I
'
U=1.9volt !
I
1.00 1.,50 2.00 cyanid concentration (g/l)
'
2.50
F I G U R E 3. Current efficiency as a function of cyanide depletion (assuming one-electron transfer).
TABLE 3 Summary of the Results Concerning Cyanide Oxidation by Means of HCC, Under the Best Conditions for Silver Recovery
Cell voltage (V) 1.9
2.444-1.122 Cyanide depletion (g dm 3) Space-time-yield (g s J m 3) 0.31-0.20 Specific electrical energy (kWh/kg) 1.82-2.08 0.93-1.06 Number of electrons per CN-
2.4
1.122-0.692 = 0.25 ~- 3.05 = 1.25
The batch recycle mode of operation in the HCC system (Fig. 1) enables direct oxidation through air as well as the formation of the peroxide and its reaction with cyanide and cyanate, because the electrolyte is circulated through an open reservoir and flows successively through both cathodic and anodic compartments of the cell; this mode of operation may lead to the loss of cyanide as HCN (g) as well. The contribution of paths (10) to (14) and the loss of cyanide as HCN may explain the one-electron transfer mechanism for the oxidation of cyanide in the HCC. At higher cell voltages, the paths (5) and (7) begin to contribute to the cyanide oxidation; this explains the increase in the number of electrons per CN- (see Tables 1 and 3). The pure graphite, type EK 586, used in those experiments proved to be a suitable material for the anode in such alkaline solutions: even after more than 230 hours' electrolysis, no changes were observed on the anode's surface. 5. CONCLUSIONS Experiments performed using HCC for the simultaneous oxidation of cyanide during the electrodeposition of silver from a diluted electroplating rinsewater demonstrated the following. (a) In the concentration range 2.5-0.69 g d m 3 cyanide destruction occurs at an almost constant rate -- 0.73 g h -~ (-- 8 × 104 mol la ~) and with good performance indexes (Figs 2 and 3). (b) The process occurs mainly by one-electron transfer, through cyanate and cyanogen as intermediate species. (c) Under the conditions where the electrolyte circulates in an open bath and flows successively through the cathodic and the anodic compartment the cyanate could be produced by the direct oxidation through air and/or generated peroxide. (d) The loss of cyanide as HCN cannot be excluded, neither can it be prevented. (e) Oxidation by two or more electrons per CNseems to be almost excluded, at least in this concentration range and at low cell voltages.
ELECTROCHEMICAL
O X I D A T I O N O F C Y A N I D E IN T H E H C C
The results obtained in this work help to establish other reaction schemes, (11) and (14), which lead to the formation of carbon dioxide and nitrogen. REFERENCES 1. Wiaux, J. P. Die Abwasserreinigung durch Elektrolyse. Oberflgiche-Surface 24:16 (1988). 2. Kirch, R. and Schnell, M. Elektrolytische Entgiftung verbrauchter cyanidischer Entfettungsb~ider im Galvanikbetrieb. Galvanotechnik 78/4:964 (1987). 3. Ramirez, E. R. and D'Alessio, O. F. Innovative design and engineering of a wastewater pretreatment facility for a metal finishing operation. Proc. 39'h Ind. Waste Conf., Purdue University, p. 545 (1985). 4. Kammel, R. Recent developments in the design of electrochemical cells for the recovery of metals. Keynote Paper, MIN1TEK 50, Johannesburg, p. 443 (1984). 5. Silman, H. Electrolytishe Verfahren zur Metallr~ckgewinnung aus Abwasser. Galvanotechnik 73/6:589 (1982). 6. EI-Ghaoui, E. A. and Jansson, R. E. W. Application of the trickle tower to problems of pollution control. III. Heavymetal cyanide solutions. J. AppL Electrochem. 12:75 (1982). 7. Zabban, W. and Helwick, R. Cyanide waste treatment technology - the old, the new and the practical. Plat. Surf. Fin. 67:56 (1980). 8. Demidov, V. I. Extraction of Au and Cu during electrochemical purification of discard cyanide water. Tsvetnye Metally/ Non-ferrous Metals 12:57 (1974). 9. Dhamo, N. and Kammel, R. Electrochemical hydrocyclone cell for metal recovery from dilute solutions. METALL 46/9: 912 (1992). 10. Dhamo, N. An electrochemical hydrocyclone cell for the treatment of dilute solutions: approximate plug-flow model for electrodeposition kinetics. J. Appl. Electrochem. 24:745 (1994).
261
11. Gulbas, W. and Gotzelmann, W. Die elektrolytische Zerst6rung yon Cyanid. Metalloberflgiche 43/1:7 (1989). 12. Kelsall, G. H., Savage, S. and Brandt, D. Cyanide oxidation at nickel anodes. II Voltammetry and coulometry of Ni/CNH20 systems. J. Electroehem. Soc. 138:117 (1991). 13. Hine, F., Yasuda, M., Iida, T. and Ogata, Y. On the oxidation of cyanide solutions with lead dioxide coated anode. Electrochim. Acta 31:1389 (1986). 14. Katagiri, A., Yoshimura, S. and Yoshizawa, S. Formation constant of the tetracyanocuprate (II) ion and the mechanism of its decomposition. Inorg Chem. 20:4143 (1981). 15. Kuhn, A. T. and Biddle, K. Die anodische Zerst6rung von Cyaniden in wassrigen L6sungen. Oberfldche-Surface 18/7: 182 (1977). 16. Arikado, T., lwakura, C., Yoneyama, H. and Tamura, H. Anodic oxidation of potassium cyanide on the graphite electrode. Electrochim. Acta 21:1021 (1976). 17. Knorre, H. Erkenntnisse und Erfahrungen bei der Entgiftung cyanidischer Abwasser. Galvanotechnik 66/5:374 (1975). 18. Tamura, H., Arikado, T., Yoneyama, H. and Matsuda, Y. Anodic oxidation of potassium cyanide on platinum electrode. Electrochim. Acta 19:273 (1974). 19. Sawyer, D. T. and Day, R. J. Electrochemical oxidation of cyanide ion at platinum electrodes. J. Electroanal. Chem. 5: 195 (1963). 20. Krusenstjern, v.A. and Mussinger, W. Dicyan-Bildung in cyanidischen Silberbadern. Metalloberflgiche 16/9:263 (1962). 21. EI-Ghaoui, E. A., Jansson, R. E. W. and Moreland, C. Application of the trickle tower to problems of pollution control. II. The direct and indirect oxidation of cyanide..L Appl. Electrochem. 12:69 (1982). 22. Bradley, D. The Hydrocychme. Pergamon Press, New York (1965). 23. Belcher, R., Nutten, A. J. and Macdonald, A. M. G. Quantitative Inorganic Analysis, 3ra edn. Butterworths, London (1970).
Open for discussion until 28 February 1997