Decomposition characteristics of metal cyanide complexes by wet oxidation at lower temperatures

Transcription

1 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) Decomposition characteristics of metal cyanide complexes by wet oxidation at lower temperatures Naoto Mihara 1, Shinji Agari 1, Tadashi Fukuta Dalibor Kuchar 1, Mitsuhiro Kubota 1 and Hitoki Matsuda 1 1. Department of Energy Engineering and Science, Nagoya University, Nagoya, Japan. 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 1 and the reaction temperature was changed from 33 K to 3 K. The cyanide decomposition was evaluated by determining the residual concentrations of cyanide in the reaction solution. As a result, in alkaline ph of 1, it was found that cyanide decomposition increased with an increase in the temperature and cyanide decompositions of 99.9 % and 9.1 % were achieved after hours at 3 K for Na [Zn(CN) ] and Na 3 [Cu(CN) ], respectively. In the case of Na [Zn(CN) ], mainly HCOO - and NH were identified as the decomposition products. By contrast, in the case of Na 3 [Cu(CN) ], carbon of was transformed either to H CO 3 or HCOO -, and nitrogen was converted to NH, N or NO -. 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 Cu 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 ( K), while oxygen was used as an oxidant. The reaction time varied from about.5 hour to about 3.75 hours according to cyanide source, cyanide initial concentration and operation conditions used [,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, mg/dm 3 ) 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 1 and the reaction temperature was changed from 33 K to 3 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.. EXPERIMENTAL.1. Materials In this study, copper cyanide (CuCN) and zinc cyanide (Zn(CN) ) of reagent grade were used to prepare cyanide aqueous solutions of Na 3 [Cu(CN) ] and Na [Zn(CN) ] by mixing CuCN with NaCN at a ratio of 1:3, and Zn(CN) with NaCN at a ratio of 1:, respectively. The initial concentration of cyanide complex was adjusted to 1, mg/dm 3 (as CN-concentration). In addition, the ph of the reaction solution was set to 1.5 or 1 with H SO or NaOH... Experimental set-up The wet oxidation of metal cyanides was carried out in a ml Teflon-coated reactor, shown in Figure 1, at temperatures of 33 K, 3 K and 3 K and oxygen atmosphere (pressure was set to 1. MPa). A volume of 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 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, n-mihara@ees.nagoya-u.ac.jp 933

2 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) reaction products P Oxygen Nitrogen 3 Needle valve 3- way valve 5 Liquid sampling Impeller 7 Motor T P Pressure gauge 9 Teflon vessel Heating jacket 11 Thermocouple 1 Temperature controller 13 Sulfuric acid 1 Injector Fig.1 Experimental set-up.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 concentration and NH concentration, the filtrate was distilled (distillation method JIS K) and subsequently, the residual and NH concentrations were determined using respective ion selective electrodes (DKK-TOA Corp.). Subsequently, the decomposition of was determined as a ratio of residual amount of and initial amount of. 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 - NO - 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 N in gas phase was analyzed using a gas chromatograph (GC-TCD, GC-1 B, Shimadzu). Finally, the filter cake/precipitate was dried and then subjected to an X-ray powder diffraction analysis (XRD; RINT- 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 33 K to 3 K under the alkaline conditions (ph=1). Figure 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 3 K, it was found that the cyanide decomposition rapidly proceeded for the first hours and the values of cyanide decomposition obtained were 7.5% and.5% for Exhaust gas Na 3 [Cu(CN) ] and Na [Zn(CN) ], respectively. Further, when the reaction time was extended up to hours, the decomposition increased to 9.1% for Na 3 [Cu(CN) ] and 99.9% for Na [Zn(CN) ]. Cyanide decomposition /% Reaction time /h (a) 3 K (b) 3 K (c) 33 K :Na [Zn(CN) ] :Na 3 [Cu(CN) ] Fig. Decomposition of Na [Zn(CN) ] and Na 3 [Cu(CN) ] by wet oxidation at alkaline ph of 1 Meanwhile, the decomposition of Na [Zn(CN) ] was found to be higher than that of Na 3 [Cu(CN) ], irrespective of the reaction temperature, which was attributed to a higher stability of Na 3 [Cu(CN) ] complex. The complex stability constant of Zn(CN) and Cu(CN) are reported to be 19. and 3.1 []. In the subsequent step, the experiments were conducted at a temperature of 3 K and the cyanide decomposition mechanism was thoroughly investigated. 3.. Mechanism of zinc cyanide complex decomposition in alkaline ph range (ph=1). Figure 3 shows the results of analysis of cyanide decomposition by-products and it can be seen that mainly HCOO - and NH were formed in the case of Na [Zn(CN) ]. In details,.3% of carbon of was converted to HCOO - within first hours and this value increased to % after hours of wet oxidation. Meanwhile, 9.% of nitrogen of was converted to NH within the first hours and the value increased to % after hours. Regarding the reaction mechanism, the zinc cyanide complex of Na [Zn(CN) ] was assumed to be mainly decomposed to ammonia and sodium formate. Further, based on the XRD analysis of reaction precipitates, it was concluded that Zn 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 H CO 3 as shown in 93

3 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) Eq. (). Distribution of C /% Distribution of N /% Na [Zn(CN) ] NaOH 7H O NH 3 HCOONa ZnO (1) HCOOH 1/O H CO 3 () Time /h HCOO - H CO 3 NH Fig. 3 Distribution of C, N elements during wet oxidation of zinc cyanide Mechanism of copper cyanide complex decomposition in alkaline ph range (ph=1) Figure shows the cyanide decomposition in terms of formation of decomposition by-products. Distribution of C/% Distribution of N/% HCOO - CNO - H CO 3 NH CNO - NO - N Fig. Distribution of C, N elements during wet oxidation of copper cyanide It can be seen that 5.% of carbon of was transformed to H CO 3 and the remaining carbon was distributed among HCOO - and un-decomposed, within the wet oxidation for hours. Meanwhile, the nitrogen of was converted to NH (7.3%) and to N (19.9%) within the same period. The presence of various decomposition by-products suggested more difficult decomposition mechanism of Na 3 [Cu(CN) ] than that proposed for Na [Zn(CN) ] and it was considered that Na 3 [Cu(CN) ] decomposition proceeded via two sets of simultaneous reactions. The first set of reactions is given by Eqs. (3) and () and it was considered that Na 3 [Cu(CN) ] was decomposed to NH 3 and HCOONa and HCOOH was then partially converted to H CO 3. The metal ion of Cu was oxidized to Cu and subsequently precipitated as CuO. Na 3 [Cu(CN) ] NaOH 15H O 1/O NH 3 HCOONa CuO (3) COOH 1/O H CO 3 () The second set of reactions, expressed by Eqs. (5)-(7), involved an intermediate product of CNO -, which was subsequently oxidized to H CO 3 and NO -. The NO - ions then most likely reacted with NH to form N detected by gas chromatography. O CNO - (5) CNO - 3O H O H CO 3 NO - NH NO - () N H O (7) 3.. Decomposition of zinc cyanide complexes in acidic ph range (ph=1.5) In the sections , decomposition of copper and zinc cyanide complexes in alkaline ph of 1 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 : Na [Zn(CN) ] ph=1.5 : Na [Zn(CN) ] ph=1 : ph ph Fig.5 Decomposition of zinc cyanide complex by wet oxidation at acidic ph of 1.5 Figure 5 shows the results of zinc cyanide decomposition carried out at ph of 1.5, temperature of 3 K and pressure of 1 MPa. As a reference, the results obtained at ph value of 1 are also shown in this figure. It can be seen that zinc cyanide decomposition ratio in acidic ph 935

4 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) was lower than that in alkaline ph. As the reaction by-products, Zn 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. (). Na [Zn(CN) ] H SO Na SO ZnSO HCN () 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 H O HCOOH NH 3 (9) 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 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. As a reference, the results obtained at ph value of 1 are also shown in the same figure : Na 3 [Cu(CN) ] ph=1.5 : Na 3 [Cu(CN) ] ph=1 : ph ph Fig. 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., over the reaction time of hours. Regarding the by-products, mainly formic acid and ammonia were detected in the reaction solution, while N and CO 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 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). Intensity Intensity (a) Reaction Time 1h (b) Reaction Time h CuCN 3 Diffraction angle θ / deg [CuSO 3Cu(OH) ] 3 Diffraction angle θ / deg Fig. 7 XRD analysis of sediments obtained after wet oxidation of copper cyanide complex The results of XRD analysis showed that the white sediment was identified as CuCN, while the greenish sediment was identified as CuSO 3Cu(OH). 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 H SO (Eq. ()). Na 3 [Cu(CN) ] 3H SO 3Na SO CuCN HCN () Then, since Cu ions were identified in the precipitate as CuSO 3Cu(OH), it was assumed that Cu ions were oxidized to Cu ions, when the reaction period was extended beyond two hours as shown in Eq. (11). CuCN 1/O 3H Cu HCN OH - (11) The oxidation of cuprous ions to cupric ions also explained the ph increase shown in Figure, since OH - ions were generated by Eq. (11). Further, Cu ions were simultaneously precipitated as CuSO 3Cu(OH) by a reaction of Cu with SO - and OH - (Eq. (1)). Cu SO - OH - CuSO 3Cu(OH) (1) As for the decomposition of HCN formed by Eqs. ()- (11), it was assumed that HCN was partially decomposed 93

5 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) to formic acid and ammonia as shown in Eq. (9). Additionally, cyanide oxidation with O according to Eq. (13) could also take place leading to the generation of nitrogen and carbon dioxide identified in the gas phase. HCN 5O H O CO N (13) The higher decomposition ratios obtained for cyanide decomposition in acidic ph range compared to alkaline ph range, were attributed to a catalytic function of Cu, reported also in the work of Morisaki et al. [7]. In order to confirm the catalytic effect of Cu on decomposition, the wet oxidation experiments of zinc cyanide complex were carried out in the presence of CuSO (addition of or mg) at the 3 K and ph of CuSO mg CuSO mg CuSO mg Fig. Effect of CuSO addition on wet oxidation of zinc cyanide complex Figure shows the cyanide decomposition ratio obtained after hours. It can be seen that the zinc cyanide decomposition ratio was increased by addition of CuSO. Hence, it was confirmed that Cu ion had catalytic ability on the cyanide decomposition by wet oxidation.. CONCLUSIONS The decomposition of Na [Zn(CN) ] and Na 3 [Cu(CN) ] was carried out in a hydrothermal reactor under oxygen atmosphere (pressure of 1. MPa). The ph of the reaction solution was set either to 1.5 or 1 and the reaction temperature was changed from 33 K to 3 K. 1) In alkaline ph range (ph=1), it was found that cyanide decomposition increased with an increase in the temperature and cyanide decompositions of 99.9 % and 9.1 % were achieved after hours at 3 K for Na [Zn(CN) ] and Na 3 [Cu(CN) ], respectively. ) In the case of Na [Zn(CN) ], mainly HCOO - and NH were identified as the decomposition products. By contrast, in the case of Na 3 [Cu(CN) ], carbon of was transformed either to H CO 3 or HCOO -, and nitrogen was converted to NH, N or NO -. 3) In acidic ph range (ph=1.5), lower decomposition ratios were obtained for zinc cyanide complex compared to alkaline range (ph=1). 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 Cu ions. ) Finally, catalytic function of Cu ions was further confirmed by adding CuSO to the reaction solution of zinc cyanide complex. It was found that the cyanide decomposition increased from about % to more than % when CuSO addition exceeded mg and the wet oxidation was carried out for two hours. REFERENCES 1. J. D. Desai, C. Ramakrishna, P. S. Patel and S.K. Awasthi, Chem. Eng. World, 33 (199), pp M. Sarla, M. Pandit, D. K. Tyagi and J. C. Kapoor, J. Hazard. Mater., 11 (), pp F. Barriga-Ordonez, F. Nava-Alonso and A. Uribe-Salas, Miner. Eng.,19 (), pp H. L. Robey, Plat. Surf. Finish., (193), pp J. Kalman, Z. Izsaki, L. Kovacs, A. Grofscik and I. Szebenyi, Wat. Sci. Tech., 1 (199), pp The Chemical Society of Japan, Chemistry Handbook, Volume II, th Edition, Maruzen, Tokyo (1993), p.3 (in Japanese) 7. S. Morisaki, K. Komamiya and M. Naito, Nippon Kagaku Kaishi, 7(19), pp