Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
Decomposition characteristics of metal cyanide complexes by wet oxidation at lower temperatures Naoto Mihara1, Shinji Agari1, Tadashi Fukuta2 Dalibor Kuchar1, Mitsuhiro Kubota1 and Hitoki Matsuda1 1. Department of Energy Engineering and Science, Nagoya University, Nagoya, Japan 2. Sanshin Mfg. Co., Ltd, Inuyama, Japan Abstract: The present study is concerned with the characteristics of wet oxidation of zinc and copper cyanide complexes in a lab-scale hydrothermal reactor, under oxygen atmosphere of which the pressure was set to 1 MPa. The pH of the reaction solution was set either to 1.5 or 12 and the reaction temperature was changed from 383 K to 423 K. The cyanide decomposition was evaluated by determining the residual concentrations of cyanide in the reaction solution. As a result, in alkaline pH of 12, it was found that cyanide decomposition increased with an increase in the temperature and cyanide decompositions of 99.9 % and 96.1 % were achieved after 8 hours at 423 K for Na2[Zn(CN)4] and Na3[Cu(CN)4], respectively. In the case of Na2[Zn(CN)4], mainly HCOO- and NH4+ were identified as the decomposition products. By contrast, in the case of Na3[Cu(CN)4], carbon of CNwas transformed either to H2CO3 or HCOO-, and nitrogen was converted to NH4+, N2 or NO2-. Further, the decomposition of metal cyanide complexes was carried out in an acidic pH of 1.5, and lower decompositions ratios of zinc cyanide complex were obtained in acidic pH range than in alkaline pH range. In contrast with zinc cyanide complex, a significant increase in decomposition ratios was observed for copper cyanide complex in acidic pH compared to alkaline pH, which was attributed to a catalytic function of Cu2+ ions. Keywords: electroplating wastewater, copper cyanide, zinc cyanide, complex, wet oxidation 1. INTRODUCTION The copper and zinc cyanide complexes used in plating industry are known to be highly toxic and therefore an effective treatment is required to remove cyanide complexes from plating wastewaters. Presently, the cyanide decomposition is carried out using alkaline chlorination, and oxidation with hydrogen peroxide or ozone [1-3]. However, several drawbacks have been reported for these treatment methods and therefore new methods for cyanide decomposition are still being investigated. More precisely, a wet oxidation treatment has attracted many researchers, since cyanide decomposition as high as 99.99% was achieved in a hydrothermal reactor under elevated pressure (3-5 MPa) and temperature (473-573 K), while oxygen was used as an oxidant. The reaction time varied from about 0.5 hour to about 3.75 hours according to cyanide source, cyanide initial concentration and operation conditions used [4,5]. Nevertheless, from an energy savings point of view, process energy requirements are still high and further reduction in the operation pressure and temperature is essential to achieve more environmentally friendly system. Therefore, in this study, the wet oxidation of zinc and copper cyanide complexes (cyanide concentration of 1,000 mg/dm3) was conducted in a lab-scale hydrothermal reactor, under oxygen atmosphere of which the pressure was set to 1 MPa. The pH of the reaction solution was set either to 1.5 or 12 and the reaction temperature was changed from 383 K to 423 K. In the experiments, the effect of reaction time on the cyanide decomposition was mainly investigated. The cyanide decomposition was evaluated by determining the residual concentrations of
cyanide in the reaction solution. Finally, to evaluate the reaction mechanism, the reaction products such as cyanate, ammonium, formic acid, carbonic acid among the others were analyzed. 2. EXPERIMENTAL 2.1. Materials In this study, copper cyanide (CuCN) and zinc cyanide (Zn(CN)2) of reagent grade were used to prepare cyanide aqueous solutions of Na3[Cu(CN)4] and Na2[Zn(CN)4] by mixing CuCN with NaCN at a ratio of 1:3, and Zn(CN)2 with NaCN at a ratio of 1:2, respectively. The initial concentration of cyanide complex was adjusted to 1,000 mg/dm3 (as CN-concentration). In addition, the pH of the reaction solution was set to 1.5 or 12 with H2SO4 or NaOH. 2.2. Experimental set-up The wet oxidation of metal cyanides was carried out in a 200 ml Teflon-coated reactor, shown in Figure 1, at temperatures of 383 K, 403 K and 423 K and oxygen atmosphere (pressure was set to 1.0 MPa). A volume of 100 ml distilled water was poured into the reactor, and air in the reactor was purged by oxygen (purity: 99.9%). Then, the reactor operation temperature was raised to the prescribed temperature, whereupon the wet oxidation experiment was started by injecting a sample solution into the reactor. The reaction period was varied in the range up to 8 hours. At the end of the wet oxidation experiment, the reactor was allowed to cool down. The solution was withdrawn from the reactor and subjected to analyses of residual cyanide, pH, metal concentrations as well as the
Corresponding author: N. Mihara,
[email protected]
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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
Na3[Cu(CN)4] and Na2[Zn(CN)4], respectively. Further, when the reaction time was extended up to 8 hours, the decomposition increased to 96.1% for Na3[Cu(CN)4] and 99.9% for Na2[Zn(CN)4].
reaction products.
14 5 P
4
T 7
P
12
8
3
Exhaust gas
9 6 1
Cyanide decomposition /%
11
2 10 1 Oxygen 2 Nitrogen 3 Needle valve 4 3- way valve 5 Liquid sampling 6 Impeller 7 Motor
13
8 Pressure gauge 9 Teflon vessel 10 Heating jacket 11 Thermocouple 12 Temperature controller 13 Sulfuric acid 14 Injector
Fig.1 Experimental set-up 2.3. Product analysis After the experiments, the sample solutions were filtered using 1 µm pore size filter and the filtrates and filter cakes were subjected to further analyses. At first, in the determination of residual CN- concentration and NH4+ concentration, the filtrate was distilled (distillation method JIS K0102) and subsequently, the residual CN- and NH4+ concentrations were determined using respective ion selective electrodes (DKK-TOA Corp.). Subsequently, the decomposition of CN- was determined as a ratio of residual amount of CN- and initial amount of CN-. Then, the content of organic and inorganic carbon in filtrate was determined using a Total Organic Carbon (TOC) analyzer (TOC-VCSH, Shimadzu). Further, the concentrations of HCOO-, CNO- NO2- were analyzed using an ion chromatograph (Shimadzu), and the concentrations of zinc and copper in the filtrates were determined using ICP (Vista-MPX Simultaneous ICP-OES, Varian, Inc.). The concentration of N2 in gas phase was analyzed using a gas chromatograph (GC-TCD, GC-14 B, Shimadzu). Finally, the filter cake/precipitate was dried and then subjected to an X-ray powder diffraction analysis (XRD; RINT-2500 TTR, Rigaku Model) 3. RESULTS AND DISCUSSION 3.1. Effect of reaction temperature on wet oxidation The wet oxidation experiments of copper and zinc cyanide complexes were carried out at the temperature range of 383 K to 423 K under the alkaline conditions (pH=12). Figure 2 shows the results of the cyanide decomposition obtained and it can be seen that the cyanide decomposition significantly increased with an increase in the temperature. More precisely, at the reaction temperature of 423 K, it was found that the cyanide decomposition rapidly proceeded for the first 2 hours and the values of cyanide decomposition obtained were 74.5% and 90.5% for
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100 90 80 70 60 50 40 30 20 10 0
(a) 423 K
0
1
2
3
4
5
6
7
8
100 90 80 70 60 50 40 30 20 10 0
(b) 403 K
0
1
2
3
4
5
6
7
8
100 90 80 70 60 50 40 30 20 10 0
(c) 383 K
■：Na2[Zn(CN)4] ◇：Na3[Cu(CN)4] 0
1
2
3
4
5
6
7
8
Reaction time /h Fig. 2 Decomposition of Na2[Zn(CN)4] and Na3[Cu(CN)4] by wet oxidation at alkaline pH of 12 Meanwhile, the decomposition of Na2[Zn(CN)4] was found to be higher than that of Na3[Cu(CN)4], irrespective of the reaction temperature, which was attributed to a higher stability of Na3[Cu(CN)4] complex. The complex stability constant of Zn(CN)4 and Cu(CN)4 are reported to be 1019.6 and 1023.1 [6]. In the subsequent step, the experiments were conducted at a temperature of 423 K and the cyanide decomposition mechanism was thoroughly investigated. 3.2. Mechanism of zinc cyanide complex decomposition in alkaline pH range (pH=12). Figure 3 shows the results of analysis of cyanide decomposition by-products and it can be seen that mainly HCOO- and NH4+ were formed in the case of Na2[Zn(CN)4]. In details, 86.3% of carbon of CN- was converted to HCOO- within first 2 hours and this value increased to 100% after 8 hours of wet oxidation. Meanwhile, 89.6% of nitrogen of CN- was converted to NH4+ within the first 2 hours and the value increased to 100% after 8 hours. Regarding the reaction mechanism, the zinc cyanide complex of Na2[Zn(CN)4] was assumed to be mainly decomposed to ammonia and sodium formate. Further, based on the XRD analysis of reaction precipitates, it was concluded that Zn2+ ions were precipitated as ZnO, in alkaline pH range. The overall reaction mechanism of cyanide decomposition is expressed by Eq. (1). Further, HCOOH was gradually converted to H2CO3 as shown in
Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
Eq. (2). (1)
HCOOH + 1/2O2 →
(2)
100 90 80 70 60 50 40 30 20 10 0
CNHCOOH2CO3
Distribution of N /%
0 100 90 80 70 60 50 40 30 20 10 0
H2CO3
1
2
3
4
5
6
7
8
2
3
5 4 Time /h
6
7
Cyanide Decomposition /%
Distribution of C/% Distribution of N/%
HCOOCNOH2CO3 1
2
3
4
5
6
7
8
CNNH4+ CNONO2-
1
2
3
4 5 Time/h
6
7
2H2CO3 + 2NO2-
(6)
N2 + 2H2O
(7)
100 90 80 70 60 50 40 30 20 10 0
14 12 10 8 6 4 2 0 0
1
2
3
4 5 Time/h
6
7
8
◇： Na2[Zn(CN)4] pH=1.5 ■： Na2[Zn(CN)4] pH=12 ▲： pH
N2 0
(5)
3.4. Decomposition of zinc cyanide complexes in acidic pH range (pH=1.5) In the sections 3.1-3.3, decomposition of copper and zinc cyanide complexes in alkaline pH of 12 was discussed. It was found that long time was necessary to achieve high decompositions of copper and zinc cyanide complexes in the alkaline pH, and therefore cyanide decomposition in acidic pH range was also investigated.
CN-
0
2CNO-
NH4+ + NO2- →
3.3. Mechanism of copper cyanide complex decomposition in alkaline pH range (pH=12) Figure 4 shows the cyanide decomposition in terms of formation of decomposition by-products.
100 90 80 70 60 50 40 30 20 10 0
(4)
2CNO-+ 3O2 + 2H2O →
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Fig. 3 Distribution of C, N elements during wet oxidation of zinc cyanide.
100 90 80 70 60 50 40 30 20 10 0
COOH + 1/2O2 → H2CO3
2CN- + O2 →
NH4+ 1
(3)
The second set of reactions, expressed by Eqs. (5)-(7), involved an intermediate product of CNO-, which was subsequently oxidized to H2CO3 and NO2-. The NO2- ions then most likely reacted with NH4+ to form N2 detected by gas chromatography. CN-
0
2Na3[Cu(CN)4] + 2NaOH + 15H2O + 1/2O2 → 8NH3 + 8HCOONa + 2CuO
pH
Distribution of C /%
Na2[Zn(CN)4] + 2NaOH + 7H2O → 4NH3 + 4HCOONa + ZnO
The presence of various decomposition by-products suggested more difficult decomposition mechanism of Na3[Cu(CN)4] than that proposed for Na2[Zn(CN)4] and it was considered that Na3[Cu(CN)4] decomposition proceeded via two sets of simultaneous reactions. The first set of reactions is given by Eqs. (3) and (4) and it was considered that Na3[Cu(CN)4] was decomposed to NH3 and HCOONa and HCOOH was then partially converted to H2CO3. The metal ion of Cu+ was oxidized to Cu2+ and subsequently precipitated as CuO.
8
Fig. 4 Distribution of C, N elements during wet oxidation of copper cyanide
Fig.5 Decomposition of zinc cyanide complex by wet oxidation at acidic pH of 1.5
It can be seen that 58.0% of carbon of CN- was transformed to H2CO3 and the remaining carbon was distributed among HCOO- and un-decomposed CN-, within the wet oxidation for 8 hours. Meanwhile, the nitrogen of CN- was converted to NH4+ (76.3%) and to N2 (19.9%) within the same period.
Figure 5 shows the results of zinc cyanide decomposition carried out at pH of 1.5, temperature of 423 K and pressure of 1 MPa. As a reference, the results obtained at pH value of 12 are also shown in this figure. It can be seen that zinc cyanide decomposition ratio in acidic pH
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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
greenish sediment was observed. In order to identify the composition of these sediments, the sediments were subjected to XRD analysis and the results are shown in Figure 7 (a: white sediment, b: greenish sediment).
was lower than that in alkaline pH. As the reaction by-products, Zn2+ ions, formic acid and ammonia were detected in the reaction solution. In addition, the presence of HCN was detected in the gas phase. Then, based on the analyses of reaction by-products, the zinc cyanide decomposition was considered to proceed according to Eq. (8).
CuCN
Intensity
Na2[Zn(CN)4] + 2H2SO4 → Na2SO4 + ZnSO4 + 4HCN
(a) Reaction Time 1h
(8)
Subsequently, HCN generated by cyanide decomposition was supposedly hydrolyzed in water yielding formic acid and ammonia detected in the reaction solution. The reaction is given in Eq. (9). HCN + 2H2O → HCOOH + NH3
10
(9)
20 30 40 50 60 70 Diffraction angle 2θ / deg
80
(b) Reaction Time 2h
However, since the majority of cyanide was detected in the gas phase as a HCN gas, it was concluded that the hydrolysis of cyanide in the liquid phase had not proceeded efficiently.
100 90 80 70 60 50 40 30 20 10 0
14 10 8 6 2 1
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4 5 Time/h
6
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8
20
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Diffraction angle 2θ / deg
The results of XRD analysis showed that the white sediment was identified as CuCN, while the greenish sediment was identified as CuSO4 ･3Cu(OH)2. Subsequently, the reaction mechanism was proposed based on the by-products identified after wet oxidation. At first, the presence of CuCN suggested that copper cyanide complex was dissociated to CuCN and HCN by addition of H2SO4 (Eq. (10)).
4
0
10
Fig. 7 XRD analysis of sediments obtained after wet oxidation of copper cyanide complex
12
pH
Cyanide Decomposition /%
3.5. Decomposition of copper cyanide complexes in acidic pH range (pH=1.5) Decomposition of copper cyanide complex was conducted at pH of 1.5 and the results are shown as Figure 6. As a reference, the results obtained at pH value of 12 are also shown in the same figure.
Intensity
[CuSO4･3Cu(OH)2]
0
2Na3[Cu(CN)4] + 3H2SO4 → 3Na2SO4 + 2CuCN + 6HCN
◇： Na3[Cu(CN)4] pH=1.5 ■： Na3[Cu(CN)4] pH=12 ▲： pH
(10)
Then, since Cu2+ ions were identified in the precipitate as CuSO4･3Cu(OH)2, it was assumed that Cu+ ions were oxidized to Cu2+ ions, when the reaction period was extended beyond two hours as shown in Eq. (11).
Fig.6 Decomposition of copper cyanide complex by wet oxidation at acidic pH of 1.5 In contrast with zinc cyanide complex decomposition, higher values of copper cyanide decomposition were obtained in acidic pH range than in alkaline pH range. In this figure, it can be seen that pH increased from the initial value of 1.5 to about 8.2, over the reaction time of 8 hours. Regarding the by-products, mainly formic acid and ammonia were detected in the reaction solution, while N2 and CO2 were detected in the gas phase. Besides, formation of precipitate was observed in the reaction solution and the nature of the precipitate was observed to change with the reaction time. At first, white sediment was obtained within the reaction time of one hour. Then, when the reaction time exceeded two hours, a formation of
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2CuCN + 1/2O2 + 3H＋ → 2Cu2+ + 2HCN + OH-
(11)
The oxidation of cuprous ions to cupric ions also explained the pH increase shown in Figure 6, since OHions were generated by Eq. (11). Further, Cu2+ ions were simultaneously precipitated as CuSO4 ･3Cu(OH)2 by a reaction of Cu2+ with SO42- and OH- (Eq. (12)). 4Cu 2+ + SO42- + 6OH- → CuSO4･3Cu(OH)2
(12)
As for the decomposition of HCN formed by Eqs. (10)(11), it was assumed that HCN was partially decomposed
Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
to formic acid and ammonia as shown in Eq. (9). Additionally, cyanide oxidation with O2 according to Eq. (13) could also take place leading to the generation of nitrogen and carbon dioxide identified in the gas phase. 4HCN + 5O2 → 2H2O + 4CO2 + 2N2
(13)
Cyanide Decomposition /%
The higher decomposition ratios obtained for cyanide decomposition in acidic pH range compared to alkaline pH range, were attributed to a catalytic function of Cu2+, reported also in the work of Morisaki et al. [7]. In order to confirm the catalytic effect of Cu2+ on CN- decomposition, the wet oxidation experiments of zinc cyanide complex were carried out in the presence of CuSO4 (addition of 100 or 200 mg) at the 423 K and pH of 1.5. 100 90 80 70 60 50 40 30 20 10 0 pH:1.5 CuSO 4 0 mg
pH:1.5 CuSO 4 100 mg
1) In alkaline pH range (pH=12), it was found that cyanide decomposition increased with an increase in the temperature and cyanide decompositions of 99.9 % and 96.1 % were achieved after 8 hours at 423 K for Na2[Zn(CN)4] and Na3[Cu(CN)4], respectively. 2) In the case of Na2[Zn(CN)4], mainly HCOO- and NH4+ were identified as the decomposition products. By contrast, in the case of Na3[Cu(CN)4], carbon of CN- was transformed either to H2CO3 or HCOO-, and nitrogen was converted to NH4+, N2 or NO2-. 3) In acidic pH range (pH=1.5), lower decomposition ratios were obtained for zinc cyanide complex compared to alkaline range (pH=12). However, significantly higher decomposition ratios of copper cyanide complex were obtained in acidic pH range than in alkaline pH range, which was attributed to catalytic function of Cu2+ ions. 4) Finally, catalytic function of Cu2+ ions was further confirmed by adding CuSO4 to the reaction solution of zinc cyanide complex. It was found that the cyanide decomposition increased from about 48% to more than 80% when CuSO4 addition exceeded 100mg and the wet oxidation was carried out for two hours.
pH:1.5 CuSO 4 200 mg
REFERENCES
Fig. 8 Effect of CuSO4 addition on wet oxidation of zinc cyanide complex Figure 8 shows the cyanide decomposition ratio obtained after 2 hours. It can be seen that the zinc cyanide decomposition ratio was increased by addition of CuSO4. Hence, it was confirmed that Cu2+ ion had catalytic ability on the cyanide decomposition by wet oxidation. 4. CONCLUSIONS The decomposition of Na2[Zn(CN)4] and Na3[Cu(CN)4] was carried out in a hydrothermal reactor under oxygen atmosphere (pressure of 1.0 MPa). The pH of the reaction solution was set either to 1.5 or 12 and the reaction temperature was changed from 383 K to 423 K.
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1. J. D. Desai, C. Ramakrishna, P. S. Patel and S.K. Awasthi, Chem. Eng. World, 33 (1998), pp.115-124. 2. M. Sarla, M. Pandit, D. K. Tyagi and J. C. Kapoor, J. Hazard. Mater., 116 (2004), pp.49-56. 3. F. Barriga-Ordonez, F. Nava-Alonso and A. Uribe-Salas, Miner. Eng.,19 (2006), pp.117-122. 4. H. L. Robey, Plat. Surf. Finish., 70 (1983), pp.79-82. 5. J. Kalman, Z. Izsaki, L. Kovacs, A. Grofscik and I. Szebenyi, Wat. Sci. Tech., 21 (1989), pp.289-295. 6. The Chemical Society of Japan, Chemistry Handbook, Volume II, 4th Edition, Maruzen, Tokyo (1993), p.326 (in Japanese) 7. S. Morisaki, K. Komamiya and M. Naito, Nippon Kagaku Kaishi, 7(1980), pp.1191-1193.
