Article pubs.acs.org/IECR
Simultaneous Silver Recovery and Cyanide Removal from Electroplating Wastewater by Pulse Current Electrolysis Using Static Cylinder Electrodes Yixian Gao,†,‡,§ Yao Zhou,†,‡,§ Haitao Wang,†,‡,§ Wenshuang Lin,†,‡,§ Yuanpeng Wang,*,† Daohua Sun,† Jinqing Hong,† and Qingbiao Li*,†,‡,§ †
Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, ‡National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, and §Key Lab for Chemical Biology of Fujian Province, Xiamen University, Xiamen, 361005, People’s Republic of China ABSTRACT: An effective electrochemical approach for simultaneous silver recovery and cyanide removal from electroplating wastewater was presented. Accordingly, pulse current (PC) electrolysis with parameters including voltage (4.0 V), frequency (800 Hz), and duty cycle (50%) were settled using static cylinder electrodes. Then the influences of technological conditions on the electroplating wastewater treatment process were investigated, which manifested that the concentration of silver ions in electroplating wastewater could be reduced from 221 to 0.4 mg L−1 and cyanide could be simultaneously removed from 157 to 4.9 mg L−1 after 3.0 h of PC electrolysis at pH 9.5 ± 0.5, aeration rate of 100 L h−1, and stirring speed of 1000 rpm with NaCl addition of 0.05 mol L−1 at room temperature. The results of XRD and EDX analysis showed that the silver deposits on the cathode were crystalline in face centered cubic structure and had a high purity. Electrochemical methods including electrodialysis,17 electrocoagulation,18−20 electrooxidation,21−23 and electroreduction24−28 have drawn more and more attention. Gonzalez et al. confirmed the potential of electrochemical methods in treating electroplating wastewater through a series of studies.29−36 Compared with other methods, the advantages of electrochemical methods include lower operating costs and less usage of extra chemical reagents and simultaneous achievements of fairly pure metals and removal of organic pollutants. The efficiency of the electrolysis process in recovering metals depends on a variety of factors such as the hydrodynamic and mass transport characteristics, features of electrodes, and power supply. For batch electroplating, the mass transport at the solid−liquid interface is an important factor affecting the plating efficiency. However, the features of the power supply directly affect the driving force of the electrochemical reaction and the electrodes define where the reaction occurs. Therefore, they are of primary importance in determining the performance of the electroplating system and will be the focus of the present work. Direct current and planar static electrodes are employed in conventional electrochemical treatment. However, the direct current must be carefully tuned to avoid producing dendritic and spongy silver deposits.37 In contrast to direct current, pulse current (PC) generates a constant current density during the pulse on time (Ton), followed by a pause during the pulse off time (Toff). The major advantages of PC lie in the Toff, which provides spares time for complete deposition. It was first used as a novel means of controlling membrane ion transport selectivity in electrodialysis
1. INTRODUCTION Electroplated silver, owing to its well-known electrical and thermal conductivities and corrosion resistance, has important applications in the electronic industry as components of printed circuit systems including switches, connectors, and the like.1 Cyanide-free baths using phosphate, thiosulfate, 5-sulfosalicylic acid dehydrate, and ammonium salts as complexing agents could be applied in the plating industry. However, compared to cyanide baths, the costs of silver salts required for cyanide-free baths are very high. Therefore, for the stability in achieving excellent surface appearance and adhesion, silver cyanide (AgCN) and dicyanoargentate (KAg(CN)2) solutions are still broadly used in silver plating baths on a large scale.2,3 For example, around 4 billion tons of electroplating wastewater containing AgCN and KAg(CN)2 result from metal surface cleaning, rinsing, acid pickling, and silver stripping processes per year in China. Both cyanide and silver are toxic to animals and plants. Exposure to waterborne silver ions results in severe disturbance of branchial Na+ and Cl− regulation. For human health protection, free cyanide values must be 104). The stirring speed should be at least 860 rpm to generate the fully turbulent flow. However, the speed is not the higher the better. As shown in Figure 8, the silver recovery rate decreased when the stirring speed reached 2000 rpm. That might be due to the adverse effects on silver deposits caused by the overly high stirring speed. Moreover, no evident changes were observed for RCN with increasing stirring speed, as shown in Figure 8a, indicating that the diffusion of reactants toward the anode surface was not the rate-determining step for cyanide removal. Consistently, according to the results in the previous sections, the reaction temperature, pH value of the solution, and aeration rate had strong influences on the cyanide removal rate. Such results indicated that the rate-determining step for the cyanide removal process was the chemical reaction on the surface of the anodes, i.e., the oxidation of cyanide species. Taking the above discussion into consideration, the best preferable stirring speed was 1000 rpm. 3.2.5. Effect of NaCl Addition. As discussed above, the removal of cyanide was controlled by the oxidation of the cyanide on the surface of the anode. Meanwhile, the conductivity of the solution is important in the electrochemical process. NaCl could be a supporting electrolyte, and it could also promote cyanide removal.23 Hence the effect of NaCl addition on the electrolysis process was also investigated. As shown in Figure 9, the addition of NaCl could enhance silver recovery and cyanide removal rates. With the addition of 0.05 mol L−1 NaCl, the time to achieve RAg ≥ 95.0% was dramatically reduced, dropping from 2.5 to 1.0 h (Figure 9b). The increase of the silver recovery rate is due to the addition of Na+ and Cl− ions which could enhance the conductivity of the solution. For removal of cyanide, before addition of NaCl, RCN after electrolysis for 3.0 h was 57.5% whereas it became larger than 99.0% within 3.0 h when the NaCl concentration was 0.05 mol L−1. 5876
dx.doi.org/10.1021/ie301731g | Ind. Eng. Chem. Res. 2013, 52, 5871−5879
Industrial & Engineering Chemistry Research
Article
3.4. Surface Morphology of Silver Deposits. To have an insight into the quality of silver deposits, the morphology of the silver deposits obtained at different stages during the treatment of the electroplating wastewater was investigated. The XRD patterns in Figure 10 indicate that silver deposits and powders
Such a result demonstrated that addition of NaCl could enhance the oxidation of cyanide on the surface of the anode. As depicted in eqs 7−9, Cl− could be oxidized into Cl2 and then transformed into HClO and ClO−.23 2Cl− = Cl 2 + 2e
(E° = + 1.36 V)
(7)
Cl 2 + H 2O = HClO + H+ + Cl−
(8)
Cl 2 + 2OH− = ClO− + Cl− + H 2O
(9)
−
The formed ClO was a main oxidant which turned cyanide into CO2 and N2 through eqs 10−12.23 In fact, it is the alkaline chlorination−oxidation process that is commonly adopted in treatment of cyanide contaminated wastewater.15 ClO− + CN− + H 2O = CNCl + 2OH−
(10)
CNCl + 2OH− = CNO− + Cl− + H 2O
(11)
−
−
−
3ClO + CNO = CO2 + N2 + 3Cl
(12)
All in all, the above investigations about the influences of the factors showed that the efficiency of the electrolysis process in terms of silver recovery ratio and cyanide removal ratio was determined by the intrinsic kinetics of electrochemical reactions on the surface of the electrodes and the mass transfer between the bulk solution and the solid−liquid interface. The temperature, pH value, and NaCl addition significantly influenced the intrinsic electrochemical reactions, and the mass transfer was mainly determined by the stirring speed and the aeration. According to the results above, the appropriate technological conditions were 0.05 mol L−1 NaCl, 9.5 ± 0.5 pH, 100 L h−1 aeration rate, and 1000 rpm stirring speed at room temperature. 3.3. Verification Experiments with Appropriate Technological Conditions. Under the appropriate conditions, four verification experiments were carried out. As shown in Table 3,
Figure 10. XRD patterns of silver deposits on copper sheet after PC electrolysis (U = 4.0 V, f = 800 Hz, and r = 50%) using SCE under 0.05 mol L−1 NaCl, pH 9.5 ± 0.5, 100 L h−1 aeration rate, and 1000 rpm stirring speed at room temperature for (a) 1.0, (b) 2.0, (c) 3.0, and (d) 4.0 h, and of (e) silver powders scraped from cathode after PC electrolysis for 4.0 h. Diffraction peaks were assigned to silver and copper according to PDF File No. 040783 (Ag) and No. 040836 (Cu), respectively.
recovered from electroplating wastewater were crystalline in face centered cubic (fcc) crystal nature. The copper peaks in Figure 10a−d might be ascribed to copper sheets. Therefore, it shows that silver powders scraped from the cathode had a very high purity without copper peaks (Figure 10e). The relative intensity of silver and copper peaks reveals that silver deposits became denser within 3.0 h. The results could be confirmed by the SEM observation of silver deposits. The SEM images in Figure 11 show that the surface of silver deposits became smoother and their particle size grew larger and more uniform with increasing electrolysis time within 3.0 h. That was because PC could lessen the electrode passivation and concentration polarization caused by the increasing electrolysis time. During the electrolysis interval time Toff, the silver ions in the bulk solution could supplement silver ions near the cathode, which were diminished during the electrolysis working time Ton. However, as shown in Figure 11d, the surface of silver deposits attained at 4.0 h was coarse and the purity reduced from 90.2% at 3.0 h to 72.4%. It can be seen from Figure 2 that RAg increased very slightly when it was larger than 95.0%. With low silver ion concentration in the solution, there might be other subsidiary reactions such as hydrogen evolution or water electrolysis. Hence, to prevent excess electrical energy consumption and damage of the formed silver deposits, the electrolysis process should be terminated in an appropriate reaction time, which was 3.0 h in this study.
Table 3. Results of Verification Experimentsa no.
CAg,0 (mg L−1)
CCN,0 (mg L−1)
CAg,t (mg L−1)
CCN,t (mg L−1)
RAg (%)
RCN (%)
1 2 3 4
240 221 219 213
189 157 158 143
0.4 0.4 0.5 0.5
9.8 7.0 1.6 4.7
99.8 99. 8 99.8 99.8
94.8 95.6 99.0 96.8
a
Power supply parameters: 4.0 V voltage, 800 Hz pulse frequency, and 50% duty cycle. Technological conditions: 0.05 mol L−1NaCl, pH 9.5 ± 0.5, aeration rate 100 L h−1, and stirring speed 1000 rpm at room temperature. Electrolysis time: 3.0 h.
within 3.0 h of PC electrolysis, silver was recovered with RAg = 99.8% and cyanide was removed with RCN > 95.0%, simultaneously. The remaining silver was below 0.50 mg L−1, which reached the Chinese National Standard in Emission Standard of Pollutants for Electroplating (GB 21900-2008). Removal of cyanide using electrochemical methods has been reported previously. For instance, 93% cyanide could be removed by the photoelectrocatalytic detoxification technique,22 or cyanide could be reduced from 1000 to 30 mg L−1 by anodic oxidation.23 It was also reported that cyanide could be reduced from 250 to 7.9 mg L−1 with simultaneous recovery of copper, but it required usage of the expensive Ti/Pt anode.25 It could be observed that our result was comparable to or even better than those reported in those preexisting electrochemical methods.
4. CONCLUSIONS The parameters of power supply and technological conditions in treating silver electroplating wastewater through an electrochemical process were determined. Specifically, SCE combining a stainless steel cylinder cathode and a porous graphite anode with parameters of PC electrolysis determined as U = 4.0 V, f = 800 Hz, and r = 50% were selected. The effects of technological 5877
dx.doi.org/10.1021/ie301731g | Ind. Eng. Chem. Res. 2013, 52, 5871−5879
Industrial & Engineering Chemistry Research
Article
Figure 11. SEM images of silver deposits on copper sheet after PC electrolysis (U = 4.0 V, f = 800 Hz, and r = 50%) using SCE under 0.05 mol L−1 NaCl, pH 9.5 ± 0.5, 100 L h−1 aeration rate, and 1000 rpm stirring speed at room temperature for (a) 1.0, (b) 2.0, (c) 3.0, and (d) 4.0 h.
and Testing Centre of Xiamen University for helping us with SEM and EDS analyses.
conditions (including pH, temperature, stirring speed, aeration rate, and NaCl addition) on the silver recovery and cyanide removal rate were investigated. The results showed that the appropriate conditions were 0.05 mol L−1 NaCl, pH 9.5 ± 0.5, 100 L h−1 aeration rate, and 1000 rpm stirring speed at room temperature. Under such conditions, high ratios of silver recovery (99.8%) and cyanide removal (>95.0%) could be achieved simultaneously within 3.0 h, with the remaining silver of
