ELECTROCHEMICAL TREATMENT OF SPENT AMMONIACAL COPPER ETCHANTS Ioanna Giannopoulou 1 and Dr. Dimitrios Panias 1 1 National Technical University of Athens, School of Mining and Metallurgical Engineering, Laboratory of Metallurgy 9, Heroon Polytechniou, 1578 - Zografos Campus, Athens, Greece E-mail address: panias@metal.ntua.gr Abstract Ammoniacal etchants are widely used in the printed circuit boards (PCBs) manufacture to etch the metallic copper from the copper clad laminates. The efficiency of the etching process is optimal for a concentration of copper in the solution ranges between 13 to 16 g/l and declines for greater copper concentrations. Thus, the copper concentration of the solution must be restored once it becomes greater than 16 g/l by the simultaneous removal of spent etchant and feeding by fresh solution. The disposal of spent etchant is environmentally prohibitive, while it involves a loss of copper resources. In the present work the recovery of copper by electrowinning from the spent ammoniacal copper etchant is investigated as a function of the electrolyte agitation rate and the current density. The influence of these variables on current efficiency and energy consumption is studied. The results are used in order to determine the optimum process operating conditions for the electrowinning of copper from the spent ammoniacal etching solutions.
Introduction Etching is one of the major steps in the chemical process of the printed circuit boards manufacture. It is used to remove chemically the unwanted copper from the copper clad laminate as to be achieved the desired circuit patterns. Among the etching solutions that are used in the process currently, alkaline ammoniacal solutions find increasing acceptance owing to the advantages they present; they are utilized in continuous operation systems, are compatible with the most metallic and organic resists, reduce circuit s undercut, present high capacity of dissolved copper and provide fast etch rates [1]. The concentration of copper in the etching solution is critical for the process. Considering the quality of etching, the etching rate and the stability of the solution the etching process is optimal when the copper content in the etchant ranges from 13 to 16 g/l [1]. As the etching process proceeds, the concentration of copper increases and it is essential to remove a part of the etchant and to replace it simultaneously by a copper free replenishing solution that contains ammonium hydroxide and ammonium salts, until the lower allowed concentration of copper is reached. The removed spent ammoniacal etching solution is an alkaline solution at a ph 8.5-9., having a concentration of copper higher than 16 g/l, a chloride concentration between 17-19 gr/l and a copper to ammonia molar ratio of 1:4 [1]. This composition classifies the spent ammoniacal etchant to the hazardous wastes according to the relevant Greek and European environmental legislation and therefore, it is essential its treatment before its discharge into the environment. Besides, the high copper concentration of this waste effluent creates an additional economic interest to isolate the waste stream of the etching process and treat the effluent with an appropriate technique that could return an additional financial benefit from the recovery of copper. Towards this direction the electrochemical treatment methods of metal bearing waste effluents gain increasing attention during the last decades, combining environmental requirements for safe effluent discharge and economic benefits from the recovered metal values [2, 3]. These methods can be applied successfully for in-site effluent treatment, achieve the selective recovery of metals as pure materials, have low to medium labor and maintenance costs, produce small volumes of discharged effluents avoiding the production of secondary wastes and offer the possibility of water or solutions recycling. Among these methods, electrolysis is the most common applied for the recovery of copper from the spent etching solutions. Electrolysis can be used in the treatment of the spent etchants for either, total recovery of copper and safe discharge of the resulted effluent into the environment [4] or partial recovery of copper and recycling of the etchant back in the etching process [5]. In the present work, the electrolytic recovery of copper from the waste effluent, which is produced combining in the same stream the spent ammoniacal etchant with the wastewaters resulted from the rinse stages that follow the etching process, is studied. Considering a medium size manufacture of printed circuit boards with a monthly production of 11 m 2 of board, this waste effluent is an alkaline solution with ph values between 9 and 1 and the following composition: copper 3.8 g/l, chloride 38.5 g/l and total ammonia 2.9 mol/l. Copper recovery takes place in a simple, undivided electrolytic cell with copper plate cathodes and 316 stainless steel plate anodes. The high chloride concentration of the waste effluent lead to a partial oxidation of the chloride
ions at the anodes of the electrolytic cell creating distinct corrosion phenomena at these electrodes and influencing the electrolytic process. For that reason, a pre-treatment of the effluent with a strong basic ion exchange resin is suggested in order to reduce chloride concentration at an acceptable for the electrolysis application level, avoiding anodic corrosion phenomena. The major parameters affecting the efficiencies of electrolytic recovery of copper from the studied waste effluent are examined as to determine the optimum conditions for an environmentally and economically feasible method of treatment. Theoretical Approach of the System Cu(II)/Cu(I) NH 3 Cl - SO 4 2+ H 2 O Redox Equilibrium of the Cu NH 3 Cl - SO 4 2+ H 2 O system In the aqueous ammoniacal solutions of copper, ammonia ligands increase the solubility of copper by the formation of soluble ammine complexes. This behavior of ammonia as a complexing agent is mainly studied in aqueous systems that are used in hydrometallurgy for metal extraction processes [6, 7] and in metal industry for investigations of stress-corrosion cracking of metals and metal alloys [8]. In both cases, adequate Pourbaix-type diagrams were carried out depending on the different system conditions. According to these diagrams, at the region of the studied waste effluent, copper is presented as the cupric tetrammine complex ion, Cu(NH 3 ) 4 2+, the cuprous diammine complex ion, Cu(NH 3 ) 2 +, and metallic copper. The predominance area of each species depends on the redox potential conditions of the solution. The redox behavior of the species Cu(NH 3 ) 4 2+, Cu(NH 3 ) 2 + and Cu is predicted from the typical Frost diagram, which is shown in Figure 1. The standard free energies referred in this diagram are calculated according to the following equilibrium reactions (1) - (3): Cu + 2NH 3 Cu(NH 3 ) + 2 + e - E 1 = +.1215 V vs. SHE (1) Cu + 4NH 3 Cu(NH 3 ) 2+ 4 + 2e - E 2 = +.317 V vs. SHE (2) Cu(NH 3 ) 2+ 4 + e - + Cu(NH 3 ) 2 + 2NH 3 E 3 = +.138 V vs. SHE (3) Cu 2+ Cu(NH 3 ) 4 ΔG Cu(NH 3 ) 2 + -,317nF -,138nF -,1215nF 1 2 Oxidation states 3 Figure 1: Typical Frost diagram of elemental copper in ammoniacal aqueous solutions As it is evident from Fig. 1, when metallic copper is dissolved in aqueous ammoniacal solutions the formation of the cuprous diammine complex ion according to Eq. (1) and the formation of the cupric tetrammine complex ion according to Eq. (2) are thermodynamically favored. Regarding the standard potential of the two formation reactions, the cuprous diammine complex ion is
formed under a more positive standard potential than the respective one of the cupric tetrammine complex ion. Therefore, the reduction of the Cu(NH 3 ) 2+ 4 to Cu(NH 3 ) + 2 according to Eq. (3) is thermodynamically favored and thus, the Cu(NH 3 ) + 2 is the most stable ion in the aqueous ammoniacal solutions. In Figure 2 the log of concentration of the species presented in the studied waste effluent is plotted as a function of pe, at constant ph = 9.5. The thermodynamic data used in Fig. 2 were calculated by a model developed for the system Cu-NH 3 -Cl - -SO 4 2+ -H 2 O according to the Modified Bromely s Methodology, at 25 o C and 2M ionic strength. As it is seen in Fig. 2, the predominant soluble species of the studied waste effluent is Cu(NH 3 ) 4 2+ for pe values greater than 2, and Cu(NH 3 ) 2 + for pe values lower than 2. Metallic copper is presented and dominates at pe values lower than 3. -5 log [ ] -1 [Cu(NH3)2]2+ [Cu(NH3)3]2+ -15 [Cu(NH3)4]2+ [Cu(NH3)5]2+ -2 [Cu(NH3)]+ [Cu(NH3)2]+ Cu(s) CuO(s) -25-1 -8-6 -4-2 2 4 6 8 1 pe Figure 2: Stability diagram of copper species in ammoniacal aqueous solutions Thus, according to Fig. 2, at strong oxidative environments (pe > 2) the species Cu(NH 3 ) 4 2+ is stable, at mild oxidative to reductive conditions (-3 < pe < 2) the species Cu(NH 3 ) 2 + is stable and at strong reductive environments metallic copper, Cu (s), is deposited. Electrode Reactions Involved in the Electrolytic Cell Considering the chemical composition of the waste effluent, when a current was applied at the electrolytic cell, one of the following reduction reactions is possible to occur at the cathode: Cu(NH 3 ) 2+ 4 + 2e - Cu(s) + 4NH 3 E 4 = -,317 V vs. SHE (4) Cu(NH 3 ) 2+ 4 + e - Cu(NH 3 ) + 2 + 2NH 3 E 5 = +,138 V vs. SHE (5) Cu(NH 3 ) + 4 + e - Cu(s) + 2NH 3 E 6 = -,1215 V vs. SHE (6) 2H + + 2e - H 2 E 7 = V (7) and one of the following oxidation reactions at the anode: 4OH - O 2 + 2H 2 O + 4e - E 8 = -.41 V vs. SHE (8) 2Cl - Cl 2 + 2e - E 9 = - 1.3597 V vs. SHE (9)
Regarding the standard redox potentials of the cathodic reactions (4)-(7), the reduction of Cu(NH 3 ) 4 2+ to Cu(NH 3 ) 2 + according to the Eq. (5) is thermodynamically spontaneous and thus, the deposition of copper on the cathode of the electrolytic cell will occur mainly by the reduction of Cu(NH 3 ) 2 + on it. The standard redox potentials of the oxidation reactions presented by Eq. (8) and (9) predict that oxygen evolution according to Eq. (8) is expected at the anode of the electrolytic cell. In reality, the high chloride concentration of the waste effluent and the significant overvoltages created in the cell during the electrolysis from the applied currents made chloride oxidation the favorable anodic reaction. Chlorine formation at the anodes causes several corrosion problems on them and therefore, the removal of chloride ions before the application of electrolysis is necessary. For that reason, a pre-treatment of the waste effluent with a strong, basic, anionic exchange resin (AMBERLITE IRA 9) was suggested as to reduce chloride concentration at an acceptable for the application of electrolysis level, which was experimentally defined at 8g/l [9]. Sulfate ion was selected to replace chloride ion, as it contains the central atom of sulfur at its higher oxidation state and thus, it is impossible to participate in oxidation reactions at the anodes of the electrolytic cell. Electrolysis experiments of sulfate copper ammoniacal solutions have shown that oxygen evolution was the preferable anodic reaction and corrosion phenomena at the anode electrodes were not observed. Experimental Figure 3 illustrates the electrolytic cell where all the experiments were performed. It was a batch operation, undivided, laboratory-scale cell with three electrodes one cathode, two anodes placed vertically in the cell, at a distance of 1 cm from the bottom of the cell and kept face-to-face 1.2 cm apart. The cathode was a rectangular copper plate of length 7.5 cm, width 6 cm and thickness 2 mm, while the anodes were stainless steel perforated plates (316 SS) and had the same dimensions as those of the cathode. V A 3 1 1. DC power source 2 2. anodes 3. cathode 4. electrolytic cell 5 5. stirring bar 4 6 6. magnetic stirrer Figure 3: Schematic view of the experimental setup The experiments were carried out at room temperature; during electrolysis the solutions were neither heated nor cooled (except heating by the current flow through the solution). A constant power supply unit (-5V, DC; -4A) was used as DC source for the electrolytic cell. All experiments were carried out under galvanostatic conditions; cell voltage was allowed to vary so
that electrolysis was carried out at a constant current. A magnetic stirrer was used for the electrolyte agitation. Agitation increases the effective surface area of cathode and minimizes the thickness of the boundary layer in a manner to avoid electrodes polarization [1]. Synthetic solutions used for the study were prepared from analytical reagent grade copper sulfate pentahydrate, copper chloride, ammonia solution and distilled water. These solutions contained 3.8g/l copper, 35.7g/l sulfate, 8g/l chloride and 2.9mol/l total NH 3, in a way to simulate the waste effluent produced from the spent ammoniacal etching solutions after the suggested pretreatment. The following parameters were investigated in order to determine the optimum conditions for copper recovery: the electrolyte agitation rate and the applied current density. During the electrolysis experiments small samples of solution were withdrawn every 15 min for copper analysis by atomic absorption spectrophotometry (Perkin Elmer 21). The current efficiency was calculated from the reduction of copper concentration in the solution. Results and Discussion Effect of Electrolyte Agitation on Current Efficiency The electrode-solution interface is a crucial factor affecting the economic feasibility of the electrolytic treatment of metal bearing waste effluents. In the electrolytic cell, the passage of current across the electrode-solution interface results in an ever-increasing diffusion layer thickness that inhibit the mass transfer from the bulk solution to the electrode under natural conditions of diffusion control and since the maximum deposition current is inversely proportional to this thickness, the current steadily falls. Even though the effects of natural convection of charged species under the influence of an electric field or under a gradient of chemical potential are included, the diffusion layer is still relatively thick, so that the maximum current is quite small [11]. Therefore, in order to achieve high current efficiencies in the electrolytic cell, the improvement of mass transfer conditions under proper hydrodynamic conditions is necessary. The electrolyte or the electrodes movement are usually employed in achieving high mass transfer conditions in a simple electrolytic cell, which is operating in batch mode. At the present study, the electrolyte agitation by a magnetic stirrer was selected in order to establish a well-defined diffusion layer, sufficiently thin to give high rates of mass transfer and hence maximize the current efficiency for metal deposition. Different agitation rates in the range of 6min -1 to 5min -1 were studied for a current density of 11Am -2. The effect of stirring rate on copper recovery is presented in Figure 4. As it is shown in Fig. 4, the rate of copper recovery is independent of stirring rates within the range of 6min -1 to 255min -1 and approaches the theoretical line, which represents the theoretically calculated rate of copper recovery according to the Faraday s law, at the initials stages of electrolysis. Stirring rates higher than 255min -1 affect negatively the copper recovery rates, especially at initial stages of electrolysis. In this case, the curves of copper recovery versus time appear concave curvature, which is typical of kinetic inhibitions and the current efficiencies decrease substantially, although the current density remains constant. Therefore, the stirring rate of the electrolyte is an essential parameter of the operation of the electrolytic cell regarding the energy consumption, as it determines the rate at which reactants arrive at the electrode surface
from the bulk of the electrolyte and the rate at which the reaction occurs. At low stirring rates the charged ions approach with difficulty to the electrode surface due to the thickness of the boundary layer, while at high stirring rates the charged ions have to overcome the kinetic inhibitions created from the turbulent flow of the electrolyte so as to remain to the electrode surface the appropriate time for their reduction and deposition [11]. 1 Copper recovery, % 8 6 4 2 theoretical line 514 min-1 37 min-1 255 min-1 123 min-1 6 min-1 3 6 9 12 time, min Figure 4: The effect of stirring rate on copper recovery at a current density 11Am -2 For the electrolytic cell used in the present work, stirring rates in the range 123-255 min -1 seem to provide the proper hydrodynamic conditions for the optimum mass transfer rate of copper ions to the cathode and hence for the maximum current efficiency of copper electrodeposition. Moreover, smooth and compact copper depositions were obtained at stirring rates in the range 123-255 min -1, while granulated depositions were observed for the higher agitation rates and jelly type loose depositions for the lower ones. Effect of Current Density on Current Efficiency and Energy Consumption A series of electrolysis experiments at different currents were conducted, in order to specify the optimum current density for the recovery of copper from the waste effluent, with respect to the energy consumption. Five current intensities were imposed, 15A, 12A, 1A, 7.5A and 5A corresponding respectively to current densities of 1666 Am -2, 1388 Am -2, 11 Am -2, 833 Am -2 and 555 Am -2. In all experiments the electrolyte agitation rate was kept constant at 123min -1. The experimental results are shown in Figure 5, where the copper recovery is plotted as a function of the quantity of electricity and current density. As the current density increases the recovery of copper tends to follow the theoretical line, which presents the amount of copper recovered calculated by the Faraday s law, during the initial stages of electrolysis. Especially the curves corresponding to a current density higher than 11 Am -2 seem to follow the theoretical line until a copper recovery of about 4%. During this period, the current efficiency under the mentioned current densities is higher than 9%, as it is shown in Figure 6. After this period, the obtained current efficiency under the lower applied current densities (833 Am -2 and 555 Am -2 ) is gradually improved and at copper concentration lower than 15 g/l becomes higher than the respective ones of 1666-11 Am -2 current densities, as it is shown in Figure 6. Finally, the obtained current efficiency under the lower studied current density (555 Am -2 ) is the best one. Current inefficiencies were attributed mainly to the hydrogen
evolution occurred at the cathode of the electrolytic cell according to Eq. (7) and partially to the reduction of Cu(NH 3 ) 4 2+ to Cu(NH 3 ) 2 + occurred to some extent at the same electrode according to Eq. (5) [12]. 1 Copper recovery, % 8 6 4 2 theor.line 11 A/m2 1666 A/m2 833 A/m2 1388 A/m2 555 A/m2 5 1 15 2 Q, Ah Figure 5: Copper recovery as a function of quantity of electricity and current density at stirring rate 123 min -1 1 Current Efficiency, % 8 6 4 2 1666 A/m2 1388 A/m2 11 A/m2 833 A/m2 555 A/m2 5 1 15 2 25 3 Copper concentration, g/l Figure 6: Current efficiency as a function of copper concentration for the applied current densities In Figure 7, the specific energy consumption is plotted as a function of copper concentration in the waste effluent and of the applied current densities. As it follows from Fig. 7, the specific energy consumption increases when the applied current density increases, although the current efficiency increases too. That was expected since the energy converted to heat into the electrolytic cell according to the Joule heating effect is proportional to the square of the electrical current, which is passed through it. Thus, the portion of energy that was converted to heat during electrolysis was greater at the higher applied currents than at the lower ones, as it was confirmed from the increasing of the electrolyte temperature that was observed during electrolysis under the current densities within the range of 11 to 1666A/m 2. At the low current densities (833Am -2-555Am -2 ) the quality of copper deposition was well adhered at the cathode surface, compact and smooth. At the high current densities (11Am -2-1666Am -2 ) the copper deposition was granulated, non-uniform, and with no coherence.
Specific energy consumption, kwh/kg 7 6 1666 A/m2 1388 A/m2 11 A/m2 5 833 A/m2 555 A/m2 4 3 2 1 5 1 15 2 25 3 Solution copper concentration, g/l Figure 7: Specific energy consumption as a function of copper concentration and current density Consequently, from the point of view of current efficiency, the electrowinning of copper from the waste effluent is favourable under the higher applied current densities and only at the last stages of electrolysis it is favourable under the lower applied ones (Figures 5 and 6). On the contrary, from the point of view of energy consumption, Figure 7 indicates that the copper electrowinning is all over favourable under the lower applied current densities. Moreover, the duration of electrolysis under the lower examined current densities is longer, leading to increased operation s electrolysis cost, mainly due to the increased energy consumption for the electrolyte agitation. Therefore, a compromise of current efficiency and specific energy consumption is necessary in order to be economically feasible the electrolytic treatment of the studied waste effluent. The present results have shown that the optimum conditions of copper electrowinning from the waste effluent could be achieved under gradually decreasing current density, so that the current be used as far as possible for the reaction of interest and the cell voltage be made as low as practicable. Conclusions The electrolytic recovery of copper from the spent ammoniacal etching solutions was studied. The investigated waste effluent was an alkaline solution with a ph ranged from 9. to 1. that contained 3.8g/l copper, 2.9mol/l total NH 3, 35.7g/l sulfate and 8g/l chloride. Electrolyte agitation was proved as a crucial parameter for the efficiency of the electrolytic copper recovery. Electrolyte agitation rates in the range of 255 to 123min -1 created the proper conditions for copper recovery rates and produced uniform, smooth and compact copper depositions. Current efficiencies greater than 9% were achieved under current densities higher than 11Am -1, during the initial stages of electrolysis where the 4% of the initial copper amount in the solution was recovered. After that period, the current efficiency was higher for the lower applied current densities of 833Am -1 and 555Am -1. The specific energy consumption decreased as the electrolysis current density was decreased. Therefore, the electrolytic process has to be performed under gradually decreasing current densities in order to be optimised from energetic point of view. The quality of copper depositions was strongly depended on current density, and uniform, fine and compact deposits of copper were obtained at current densities 833Am -1-555Am -1 with current efficiency between 75% - 85% and specific energy consumption ranged within 3 to 3.5 kwh/kg of deposited Cu.
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