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Copper recovery and spent ammoniacal etchant regeneration based on hollow fiber supported liquid membrane technology: From bench-scale to pilot-scale tests
Journal of Membrane Science 286 (2006) 301–309
Copper recovery and spent ammoniacal etchant regeneration based on hollow fiber supported liquid membrane technology: From bench-scale to pilot-scale tests Qian Yang a,∗ , N.M. Kocherginsky a,b a
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Division of Bioengineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 15 July 2006; received in revised form 30 September 2006; accepted 5 October 2006 Available online 10 October 2006
Abstract One of the key steps in printed circuit board (PCB) production is etching of a thin copper layer. Ammoniacal etching solutions are widely used for this purpose. Earlier we have developed a supported liquid membrane based method to treat wastewater containing ammonia and copper (up to 1 g/L), where the membrane is stable for at least 1 month and can be easily regenerated if necessary [1]. Now we are describing an effective hollow fiber supported liquid membrane (HFSLM) based technology for copper recovery from spent ammoniacal etching solutions, where copper is present in much higher concentrations. A bench-scale HFSLM system with 1.4 m2 effective membrane surface area was firstly used to screen out the optimal hydrodynamic and other operation conditions for potentially practical spent ammoniacal etching solutions treatment. It was found that the excess of ammonia in spent etching solutions had negative effect on copper removal, especially when copper concentration became low in the feed solutions as the result of treatment. Different methods were employed to control the ammonia level and their efficiencies were compared. Finally, successful pilot-scale experiments were conducted on a hollow fiber membrane contactor with a surface area of 130 m2 . The process results in copper removal by a factor of ∼3000 from spent etching solution through the membrane and formation of nearly saturated copper sulfate solution in the sulfuric acid, used as a striping phase. Compositions of the regenerated etching solution and purity of CuSO4 ·5H2 O crystals formed in the striping phase were comparable or even better than their commercial analogues. The stability of the pilot-scale system is promising for further industrial scale-up. © 2006 Elsevier B.V. All rights reserved. Keywords: Spent ammoniacal etching solution; Copper removal; Ammonia removal; Supported liquid membrane; Hollow fiber membrane contactor
1. Introduction Printed circuit boards (PCBs) are important components of modern electronic products, which have generated billions of US dollars globally [2]. The manufacture of PCBs involves several technical processes and etching is one of the most important steps. According to PCBs’ design [3], part of the copper thin layer on the silicon base surface is first covered with photo resistant plastics. This permits the unmasked copper to be dissolved chemically into the etchant and the desired circuit pattern is pro-
∗
Corresponding author. Tel.: +65 65166433; fax: +65 67791936. E-mail address: [email protected] (Q. Yang).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.10.012
duced. With the rapid growth of PCBs industry, the total volume of generated spent etchant keeps increasing. Based on a market analysis in Singapore conducted by MacDermid in 2002, it was estimated that 70,000 L of spent ammoniacal etchant were produced every month by local PCBs plants [4]. These spent etching solutions are usually stored in drums or tanks and are ultimately shipped to an off-site treatment plant for copper recovery before disposal. Nowadays, two commonly copper recovery methods employed by PCBs industry are neutralization and solvent extraction. In neutralization, acid and alkaline spent etchant are mixed together to form Cu(OH)2 , which can be decomposed to CuO upon heating. In this process, generated CuO is a low value-added product and the resulting wastewater requires
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further treatment before disposal. To date, the most economically successful method for regeneration of spent etchant is the Mecer® system, developed by Sigma Metallextraktion AB, Sweden [5,6]. But a market survey conducted in 2001 has revealed that the sales record of Mecer® system is becoming worse. It has been evaluated that the payback time of Mecer® system is 2.5–3 years [6]. As we know nowadays the economic recycle time is becoming shorter and no PCB manufacturers would like to purchase a system with a payback time more than 2 years, especially the system is used only for wastewater treatment. In addition, Mecer® system is a complex process with extraction and re-extraction in different unit operations which used a lot of organic solvent but it can remove only 33% of the copper before the regenerated etchant being returned back to etching process [7]. Therefore, it is quite clear that the current market needs technology breakthrough to supply a more effective spent etchant treatment system. One of possible economical alternatives to treat this waste is to use supported liquid membrane (SLM) technology. In SLM, relatively small volume usage of organic extractant as the carrier impregnated in the micropores of polymeric membrane matrix allows possible usage of expensive carriers. Simultaneous extraction and re-extraction in one technological step offers a high mass transfer flux and it is easy to operate. LIX54, a commercial product from Cognis Corporation (Tucson, AZ, USA) is widely used as the carrier for selective removal of copper from alkaline solutions. It is a -diketone extractant with an active ingredient 1-phenyl-1,3-decanedione. In recent years [7–11] it has been extensively proved that LIX54 is an excellent extractant for copper recovery from ammoniacal solutions due to its ease of stripping, low or no ammonia loading, high selectivity of Cu2+ over Zn2+ and Ni2+ , fast transfer kinetics, good stability, etc. In addition, removal of copper ions through liquid membranes is possible even against of their concentration gradient due to H+ chemical potential difference between the feed and stripping solutions. In this case it is not necessary to use transmembrane pressure or voltage. The common problem is that though pH of the feed copper solution is initially much less acidic, it decreases with time due to electroneutral 2H+ /Cu2+ exchange through the liquid membrane and simultaneously Cu transport decreases. Without ammonia copper transfer process stops [12] when [Cu2+ ]f = [Cu2+ ]s
[H+ ]f [H+ ]s
2
Feasibility studies of copper separation from aqueous solutions by HFSLM have also been conducted worldwide [13–16], which is determined both by simplicity of the process and its practical potential and importance for industry. Nevertheless, the previous researches were mostly focused on the treatment of model copper solution in which copper species is existed as the form of Cu2+ with low concentration and most of them were constrained in the lab-scale. Now we describe an effective HFSLM system developed successfully from bench-scale to pilot-scale for the treatment of industrial spent etchant with copper content around 160 g/L. The objective of this work is to develop an economic and efficient way to regenerate spent etchant for further reuse and simultaneously to recover copper product. The bench-scale investigations were conducted to screen out the optimal conditions on a basis for potentially practical waste treatment process. Finally, successful pilot tests were accomplished with two valuable products generated from the industrial waste and without secondary waste. 2. Experimental 2.1. Reagents LIX54 was kindly supplied by Cognis Corporation (Tucson, AZ, USA). It was diluted in kerosene (Aldrich, Singapore) and used for preparation of liquid membrane phase. Copper sulfate pentahydrate (Nacalai Tesque, Japan), sulfuric and hydrochloric acids, sodium hydroxide (Merck, Singapore) were of reagent grade. The copper-containing spent ammoniacal etching solution was kindly supplied by a local PCB company in Singapore. Its composition is given in Table 1. 2.2. Hollow fiber supported liquid membrane (HFSLM) setups 2.2.1. Bench-scale setup A microporous polypropylene hollow fiber membrane contactor (Liqui-Cel® Extra-Flow 2.5 in. × 8 in. membrane contactor, Hoechst Celanese, USA) was employed to prepare a benchscale HFSLM system. Detailed specifications of the hollow fiber membrane contactor are given in Table 2 [17,18]. The membrane contactor was operated in a cross-flow mode. Fiber walls were
(1)
where subscripts ‘s’ and ‘f’ indicate species in the feed and stripping solution, respectively. In contrast to this situation we have demonstrated that in copper-containing ammoniacal solutions during their treatment, pH was almost constant due to the buffer effect of NH3 /NH4 + [1]. It is not necessary to adjust pH in the feed side to keep large H+ gradient and the process can be conducted till very low copper concentrations in the feed wastewater. Furthermore, hollow fiber membrane contactor as the support matrix for liquid membrane preparation represents a very attractive system with high throughputs and economical efficiency.
Table 1 Typical composition of spent ammoniacal etching solution pH Cu(II) (M) Total NH3 (M) Cl− (M) Na+ (M) Zn(II) (M) Hg(II) (M) Ni(II) (M) Ca(II) (M) Total Fe (M) Cd(II) (M)
10 ∼2.5 10 5 2.5 × l0−2 1.7 × l0−3 4.7 × l0−4 3.4 × l0−4 9.3 × l0−5 4.7 × l0−5 1.8 × l0−6
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Table 2 Specifications of bench-scale hollow fiber membrane contactor (Liqui-Cel® Extra-Flow 2.5 in. × 8 in.) [17,18]
Table 3 Specifications of pilot-scale hollow fiber membrane contactor (Liqui-Cel® Extra-Flow 10 in. × 28 in.) [19]
Fiber type Number of fibers (N) Fiber internal radius (ri ) (m) Fiber outer radius (ro ) (m) Effective module outer diameter (da ) (cm) Effective module inner diameter (di ) (cm) Effective pore size (rp ) (m) Porosity (ε) (%) Tortuosity (τ) Effective fiber length (L) (cm) Effective surface area (Sm ) (m2 )
Water flow range
Polypropylene X50 fibers 9950 120 150 4.67 2.2 0.03 40 2.5 15 1.4
impregnated by pumping 33 v/v% LIX54 in kerosene as the liquid membrane phase through the contactor in a recycling mode using a peristaltic pump (Cole-Parmer, USA) for about 1 h. The extra oil was washed out with de-ionized water. In the subsequent experiments, feed spent etchant solutions were pumped through the tube (lumen) side of hollow fiber membrane contactor and acidic stripping solutions were pumped co-currently in the shell side unless otherwise specified. The cocurrent flow mode was adopted in order to provide a more stable SLM system. The hollow fiber with micropores impregnated by organic solutions (LIX54 in kerosene) acts as a barrier to forbid the direct contact of the feed etchant solutions and the stripping acid. Unless otherwise specified only one membrane contactor was used. In the case of spent etchant treatment with additional control of NH3 content the second membrane contactor was used for this purpose (Fig. 1). 2.2.2. Pilot-scale setup For a pilot-scale test, a bigger membrane contactor with 130 m2 effective surface area (Liqui-Cel® Extra-Flow 10 in. × 28 in. membrane contactor, Hoechst Celanese, USA) was used
Housing characteristics Material
Priming volume Weight (liquid full in shell side) Seal material Cartridge characteristics Membrane (X50 fibers)
Potting material
10–48 m3 /h Fiber reinforced plastic (FRP) housing with PVDF for all wetted surfaces Shell side: 26.1 L; lumen side: 10.6 L FRP and PVDF: 57 kg EPDM Material: polypropylene; porosity: ∼40%; OD/ID: 300/220 m; surface area: 130 m2 Epoxy
with specifications listed in Table 3 [19]. The pilot-scale setup is schematically depicted in Fig. 2. The HCl addition method was employed to control the excess NH3 in the feed etchant. 2.3. Analytical methods Samples of the bulk feed and stripping solutions were collected at predetermined time intervals (normally every 1 h) and concentrations of metal ions were measured using an inductively coupled plasma-atomic emission spectroscopy (ICPAES) (Perkin-Elmer, USA). pH value of the solutions was measured with a Cyberscan 200 digital pH meter (Eutech Instrument, Singapore) and combined pH glass electrode. Total ammonia content or total alkalinity in the feed was found by back titration using excess of H2 SO4 to convert all NH3 into NH4 + in the solution before titrating the rest of the acid with NaOH. Chloride concentration was measured with
Fig. 1. Bench-scale experimental setup for spent etchant treatment (the grey part is only for NH3 control with another membrane module). T and S indicate the tube and shell side of membrane contractor, respectively.
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Fig. 2. Pilot-scale experimental setup for spent etchant treatment.
Cl− ion selective electrode (Metrohm, Singapore). The specific gravity was measured using a hydrometer. These parameters of the regenerated etchant were analyzed and compared with the product specification for the commercial replenisher PlusCu-Etch (Plaschem, Singapore) which is used as an additive to dilute spent etchant in etching process. The replenisher’s main components are ammonia and ammonium chloride and it is used to maintain the concentration level of ammonium chloride but also help to keep a constant specific gravity of etching solution. As such, a stable etching operation could be achieved. The purity of CuSO4 ·5H2 O crystals from the pilot tests was characterized by measuring concentrations of various possible contaminants using ICP-AES and they were compared with commercial CuSO4 ·5H2 O crystals. Wide-angle X-ray diffraction (XRD) measurements of these crystals were carried out on a Shimadzu XRD-6000 X-ray diffractometer at 40 kV and 30 mA and at a scan speed of 2◦ /min. 3. Results and discussion
• At stripping side: CuR2 + H2 SO4 ⇔ 2HR + CuSO4
(3)
where overbar represents species in the organic phase. pKa for the weak acidic extractant LIX54 is 9.73 [22] and acidic stripping is easy with this chelating extractant. Kinetics of Cu transfer from 600 mL spent etchant for two different volumes of 2 M H2 SO4 used as the striping solution is shown in Fig. 3. It is found when 3 L 2 M H2 SO4 was used as a stripping solution, it results in a faster copper removal rate than 0.6 L 2 M H2 SO4 . This is understandable when a smaller volume of the stripping acid solution with the same molarity is used, due to Cu2+ /2H+ exchange an acid concentration in the stripping solution decrease faster than in the case of larger stripping volume used. Simultaneously, increase of copper concentration in the stripping solution is higher. Both these factors shift the reaction (2) backward and make the whole copper transport process slower. It is interesting to note that the initial rate of copper removal from the feed side is slightly higher than the
3.1. The effect of stripping acid on copper removal from spent etchant It is well known that Cu2+ in aqueous solutions with ammonia is forming octahedral complexes from Cu(NH3 )(H2 O)5 2+ to Cu(NH3 )4 (H2 O)2 2+ and the fifth and sixth ammonia do not bind strongly with Cu2+ because of Jahn–Teller effect [20,21]. The predominance of Cu(NH3 )4 2+ species is expected because of its high formation equilibrium constant over other ammine complexes and high ammonia content in spent etching solution. The chemical reactions occurring at the feed and stripping sides are presented below: • At feed side: Cu(NH3 )4 Cl2 + 2HR ⇔ CuR2 + 2NH4 Cl + 2NH3
(2)
Fig. 3. Effect of the stripping solution volume on copper removal rate from the bench-scale experiments. Feed: 600 mL spent etchant; flow rate: 70 mL/min both in tube and shell side. Exp. I—stripping: 0.6 L 2 M H2 SO4 ; Exp. II—stripping: 3 L 2 M H2 SO4 .
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Fig. 4. Effect of sulfuric acid molarity on copper transfer flux from the benchscale experiments. Feed: 1 L spent ammoniacal etchant; strip: 3 L H2 SO4 with different concentrations. Flow rate: 70 mL/min in both tube and shell side of membrane contactor.
corresponding increase in the stripping side. This is based on the fact that most of copper is transferred from feed to stripping phases but some still stay as the complex with carrier in the membrane phase. This is further confirmed by the initial drops of total copper mass in the aqueous solutions then keeping almost constant. The steady state is usually reached after 10 min (Fig. 3). The initial copper flux (mol cm−2 s−1 ) can be calculated based on the slope of copper concentration as a function of time in the feed solution: dC V (4) J =− dt Sm where dC is the change in concentration in the feed over time interval dt, V the feed solution volume and Sm is the effective membrane surface area. A similar equation could be used for the measurements in the stripping side but without minus sign. Fig. 4 shows that an increase of a stripping acid concentration facilitates the initial copper flux in the stripping side. However, an increase from 2 to 3 M results only in 20% increase of the flux but higher SO4 2− reduces the level of copper solubility in the stripping solution. If the spent etchant treatment is carried out in an extended period of time, it is possible to reach saturation in the stripping solution where crystals of copper sulfate are formed. This could spoil the membrane contactor when it is recirculated in the membrane contactor without notice. Taking a compromise between flux and solubility, we chose 2 M H2 SO4 as the stripping solution in the following experiments. 3.2. The effect of flow rates on copper removal from spent etchant It is very important to find an optimal hydrodynamic condition in the HFSLM system for better copper recovery and further scaling-up of the process. In this experiment, the volumetric flow rate of one stream was varied while the other was kept fixed at 70 mL/min and the initial flux was calculated based on the increase of copper concentration in the stripping solution. Fig. 5 demonstrates that an optimal flow rate exists both for the feed and stripping streams and its value in both cases is around 70 mL/min. In our previous work [12], we found that the rate of copper transfer reaches maximum when the carrier is fully loaded with
305
Fig. 5. Effect of volumetric flow rate on initial copper flux into the stripping side from the bench-scale experiments. Feed: 600 mL of spent etchant recirculated in the tube side of HFSLM. Stripping: 3 L of 2 M H2 SO4 recirculated in the shell side.
copper at high feed copper concentration stage and the flux can be described as 0
Jmax,m =
εDCuR2 ,m [HR]m τδm 2
(5)
where DCuR2 ,m is the diffusion coefficient of copper-carrier complex in the organic membrane phase (8.9 × 10−7 cm2 /s); δm the thickness of hollow fiber membrane wall; ε and τ the porosity 0 and tortuosity of hollow fiber membranes, respectively; [HR]m is the initial carrier concentration (33 v/v% LIX54 in kerosene corresponding to 0.55 M). Substituting these parameters from Table 2 [17,18] into Eq. (5), we can get the theoretical maximum flux is around 1.31 × 10−8 mol cm−2 s−1 , which agrees with the experimental maximum flux at optimal volumetric flow rates of 70 mL/min both for feed and stripping streams. The existence of the optimal flow rate is probably because the process reaches the optimal residence time for copper contacting with the carrier and the minimal thickness of non-stirring layers are obtained in copper transport process through the HFSLM system. Further increase of feed and/or stripping flow rates will not further decrease the thickness of non-stirring layers but will result in the shorter contact time of copper with the carrier LIX54. Volumetric flow rates lower than 50 mL/min were not used in our experiment runs because of too small etchant volume purified in the unit time. On the other hand, at flow rates higher than 200 mL/min the supported liquid membrane could be unstable. Therefore, high flow rates were not employed. It is interesting to find when the solutions in the tube and shell sides were swapped but both were recirculated at 70 mL/min, the fluxes were practically the same. This is probably because the rate determining step in this case is not the transport in both aqueous solutions but in the liquid membrane phase. 3.3. The role of feed ammonia on copper removal from spent etchant and a comparison of the efficiency of different methods to control ammonia level In our previous work [12] it is shown that in the presence of ammonia and alkaline pH when major copper species is Cu(NH3 )4 2+ , equilibrium of the transmembrane Cu2+ /2H+
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exchange is described by an equation: + 2 [H ]f [Cu(NH3 )4 2+ ]f = K [NH3 ]4f s [Cu2+ ]s [H+ ]s
(6)
where Ks is the stability constant for Cu(NH3 )4 2+ ; the species in the bracket indicate their concentrations in aqueous solutions in equilibrium condition. It demonstrates that it is possible to transfer copper from ammoniacal solution against copper concentration gradient and thus to have an active transport. The simplest way to accumulate copper in the stripping solution is to have higher H+ concentration in the stripping than in the feed. As an example for ∼1 M NH3 and pHf 10 in the feed and pHs 0 in the stripping solutions enrichment of copper in the stripping over feed can be higher than 104 times. However, an increase of ammonia concentration in the feed makes copper extraction process less favorable due to its competition with carrier for copper. This could also lead to the formation of higher order copper-ammonia complexes like Cu(NH3 )5 2+ and Cu(NH3 )5 2+ and thus depress the extraction of copper. In the beginning of spent etchant treatment, when copper concentration is high, the carrier is fully loaded with copper and the hindrance effect of ammonia on copper removal should not be significant. Later, at low copper concentrations and a significant excess of ammonia present in the etchant, the competition between ammonia and carrier for copper can become stronger [7]. It can unfavorably change both the rates and final equilibrium state. Therefore, the excessive NH3 accumulation in the feed hinders the forward copper extraction and reduces the copper transfer flux. Several methods were used to reduce free ammonia concentration in the etching solution during its treatment. One way to reduce ammonia is simply to add hydrochloric acid into the feed etchant. Either concentrated HCl (37%) or 5 M HCl solution was used for this purpose. 5 M HCl was chosen based on the fact that the NH4 Cl concentration in the commercial replenisher is around this value. The addition was carried out gradually within the whole experiment in order to keep almost constant pH of feed solution around 8. When concentrated HCl was used, needle shape crystals of NH4 Cl were found in the feed solution. To avoid this, in further experiments only 5 M HCl was used to control ammonia level in the feed. Fig. 6 shows that HCl addition in the initial stage of treatment has no significant effect. However, faster removal rate could be obtained by this method when feed copper concentration is low. The problem is that HCl addition decreases NH3 content due to its conversion to NH4 Cl, thus decreasing the efficiency of the treated etchant for PCBs etching. Therefore, NH3 should be added to the treated etchant for further reuse as a replenisher. Recently, it was reported that membrane contactors can be used to remove 95% of ammonia with concentration of 1100 mg/L [23]. Ignited by this information we have tried to use another membrane contactor without impregnating to sweep excess NH3 in etchant treatment process. In this case, ammonia can diffuse through the air-filled membrane pores, while aqueous solution cannot penetrate through the pores from shell to tube side due to the hydrophobicity of the polypropylene membrane
Fig. 6. Comparison of the various methods for spent ammoniacal etchant treatment from the bench-scale experiments. Feed: 1 L spent ammoniacal etchant solution; strip: 4 L 2 M H2 SO4 . Flow rate: 70 mL/min in both tube and shell side of membrane contactor.
matrix and slightly higher pressure applied in the tube side. The swept NH3 later can be re-adsorbed and re-used to top-up NH3 in the treated etchant solution. The efficiency of three methods to treat spent ammoniacal etchant is compared quantitatively: initial copper flux J calculated from the linear part of feed copper concentration dependence versus time and also the apparent copper mass transfer coefficient Ka at low copper concentration stage are listed in Table 4. The apparent mass transfer coefficient is calculated based on Eq. (7) where Ct and Ct+t represent copper concentration at time t and t + t in the feed at low copper concentration stage; Q is the volumetric flow rate in the feed: Q Ct (7) ln Ka = Sm Ct+t Fig. 6 as well as Table 4 demonstrates that there is not much difference in copper removal rate in first few hours when feed copper concentration is more than 40 g/L no matter whether the control of feed ammonia is employed or not. Later, when copper concentration in the feed is low as the result of treatment, the control of feed ammonia level greatly facilitates copper removal. Inset in Fig. 6 presents it in the semi-log coordinates. For readers’ information, the feed volume was increased when HCl addition was employed. Therefore, for a proper comparison the copper concentration values shown in Fig. 6 were amended to eliminate the diluting effect. Role of ammonia cannot be reduced only to the complex formation with copper in the feed and also to the shift of extraction equilibrium. It is also necessary to investigate if ammonia can Table 4 Comparison of initial copper flux at high copper concentration stage and apparent mass transfer coefficient at low copper concentration stage of different benchscale spent etchant treatment processes
Initial copper flux (mol cm−2 s−1 ) Ka (cm/s)
HCl addition method
Hollow fiber membrane method
Control
5.20 × l0−9
5.07 × 10−9
4.98 × 10−9
1.27 × 10−5
1.45 × 10−5
7.23 × l0−6
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Table 5 Mass balance calculations for different bench-scale spent ammoniacal etchant solution treatment processes Control
5 M HCl addition to control excess NH3
Membrane contactor to control excess NH3
mol
%
mol
%
mol
%
Feed side A. Total Cu(II) removal B. Total NH3 loss C. Conversion to NH4 + D. Moved to stripping (=J) E. Due to HCl neutralization F. Due to air stripping G. Evaporated to air
2.54 8.62 5.08 1.75 – – 1.79
100 58.93 20.30 – – 20.77
2.49 9.14 4.98 0.19 3.60 – 0.37
100.0 54.49 2.08 39.39 – 4.05
2.43 9.25 4.86 0.12 – 4.27 –
100 52.54 1.30 – 46.16 –
Stripping side H. Total H+ consumption I. H+ consumption due to Cu/2H+ exchange J. Discrepancy (=H – I)
6.83 5.08 1.75
100 74.38 25.62
5.17 4.98 0.19
100 96.32 3.68
4.98 4.86 0.12
100 97.60 2.40
penetrate directly through the membrane into the acidic media. Simultaneously with Cu/2H+ exchange and ammonia transport there is also a chance that H+ can go directly through the membrane due to high pH difference. It was found that the carrier LIX54 did not extract ammonia in appreciable quantities in the absence of copper in aqueous solution, while copper loading led to an increase in ammonia content in the organic phase [24]. In addition, ammonia has around 3% mole fraction solubility in kerosene serving as the solvent for liquid membrane preparation at 30 ◦ C and 1 atm [25]. The quantities of ammonia carry-over from feed to stripping solutions could be significant over long periods’ operation and this is deleterious to the subsequent performance of electrowinning or crystallization steps. To characterize relative role of these three processes we have measured the concentration of H+ in the stripping sulfuric acid by titration and also total ammonia of feed etchant solution both at the beginning and in the end of each treatment process. For example in the control experiment (refer to Table 5), when amount of copper in the feed phase is decreased by 2.54 mol in whole treatment process, content of ammonia measured in the feed is decreased by 8.62 mol. Only part of it (5.08 mol) can be explained by its conversion into NH4 + based on reaction (2). Total consumption of H+ was also much higher (6.83 mol) than theoretical consumption (5.08 mol). The difference of 1.75 mol is probably due to direct ammonia diffusion through the organic membrane phase and neutralization of the acid in stripping side. The rest of ammonia decrease (1.79 mol) could be due to simple loss to an air. As mentioned before there is also a possibility of H+ direct transport from the stripping acid to feed etchant which could be accounted for the discrepancy between theoretical H+ consumption and experimental determination in the whole etchant treatment process. If it is the truth, the amount of H+ direct transportation over long periods of treatment should be almost the same for these three experimental runs. However, this is contradictory to the fact that when measures are taken to control and decrease feed NH3 level, total H+ consumption in the stripping is much less. Both with HCl addition and air stripping using second membrane contactor less NH3 was transferred directly into the
stripping sulfuric acid. This is very important for the downstream electrowinning or crystallization unit operations. In large-scale treatment process, the stripping acidic solution has to be reused as long as possible and carry-over of ammonia through the organic phase should be avoided or at least minimized. 3.4. Pilot-scale HFSLM system for copper recovery and spent ammoniacal etchant regeneration Pilot plant experiment was carried out with a setup shown in Fig. 2. The HCl addition method was employed to control NH3 in the feed etchant. Within 55 h of operation 200 L of spent etchant have been treated and copper concentration was reduced from 150 g/L to 50 ppm, i.e. by a factor of 3000. Simultaneously, half a ton of almost saturated copper sulfate in sulfuric acid solution was formed. In the subsequent downstream process, crystallization of copper sulfate pentahydrate was conducted by refrigeration of the stripping acidic solution with concentration of copper sulfate near saturation rather than traditional electrowinning step. This is because the CuSO4 ·5H2 O produced from the same volume of stripping solution has around four times more weight than corresponding simple copper metal and higher market price per unit volume. The crystals in terms of cation composition (Table 6) are similar to the commercial analogues. The X-ray diffraction (XRD) spectra of the crystals are also similar to that of commercial crystals (not shown): four molecules of water are located in the square around the copper, and two oxygens form with them approximately octahedron. The Table 6 Comparison of CuSO4 ·5H2 O generated from the pilot-scale tests with commercial analogues Element
Crystals from pilot test (%)
Commercial crystals (%)
Al Fe K Mg Pb Zn
0.36 0.004 0.23 0.02 1.4 0.5
0.858 0 0.17 0.02 1.8 0.5
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Table 7 Comparison of treated spent etchant from the pilot-scale tests with commercial replenisher Treated etchant Cu concentration (ppm) NH4 C1 concentration (g/L) Total alkalinity (g/L) pH Specific gravity (20 ◦ C)
Regenerated etchant after NH3 top-up
Commercial replenisher
47
50
–
276
271
250–270
18 8.15 1.070
346 9.68 1.02
310–370 9.5–9.9 1.020–1.040
fifth water is not coordinated but is in contact with two oxygen atoms and two other water molecules [26]. However, in feed side the total alkalinity or ammonia content after the treatment is very low compared to the replenisher used as an additive to dilute spent etchant in etching process. This also leads to comparatively low pH in the treated etchant solution due to its conversion to NH4 + because of the gradual addition of HCl solution. Therefore, it is necessary to add NH3 into the treated etchant in order for it to be practically used as a replacement for the replenisher. The treated spent etchant with NH3 top-up is termed as the regenerated etchant in Table 7. A comparison of the regenerated etchant and the commercial replenisher Plus-CuEtch demonstrates that besides the small amount of remaining copper, the important chemical contents and properties of the regenerated etchant fall within acceptable range and it is ready to be used for etching by potential PCB manufacturers. As mentioned in Ref. [7], the etching efficiency is maximal when ammoniacal solution contains 110–130 g/L copper and when the copper concentration is 150–170 g/L in the etching solution, it does not have etching effect. We found that in our separated experiments all the copper transmembrane fluxes into the stripping side are higher at high feed copper concentration stage. In this case, the carrier is fully loaded with feed copper and the transmembrane flux is determined by the copper-carrier diffusion through the organic membrane phase. With the time feed copper concentration becomes lower as the result of treatment. While at low feed copper concentration stage, the flux is proportional to the copper concentration [12]. Therefore, if the pilot-scale HFSLM system is used for copper reduction in spent etchant solutions to 110–130 g/L, the treated volume per day and the yield of copper sulfate pentahydrate crystals would be much higher. After an inactivity period of 3 months, another two separate pilot tests were conducted. The treatment efficiency of these two runs is almost the same as the 1st run based on the terms of initial flux at high feed copper concentration stage and apparent mass transfer coefficient at low copper concentration stage. This demonstrates that the stability of this HFSLM system is promising for industrial-scale spent etchant treatment. 4. Conclusions Bench-scale investigations for spent etchant regeneration and copper recover were conducted in this work to screen out the
optimal operation conditions for the potential practical process. It is found that the accumulation of excess ammonia in the feed etchant solution has negative effect on the copper removal through a HFSLM system. The effect is more significant in the end of the process at lower copper concentrations. Ammonia removal from the feed improves the copper removal and also reduces the ammonia transfer to the acidic stripping solution. Both sweeping gas in an additional hollow fiber membrane contactor and HCl addition methods can be used to decrease excess ammonia in the feed etchant and minimize its carry-over to the stripping solution effectively. Successful pilot-scale tests were accomplished by employing a bigger membrane contactor with 130 m2 membrane area. Treated solution with very low copper concentration has chemical and physical properties, satisfying the specification of commercially available replenisher. Copper is recovered in the form of CuSO4 ·5H2 O, a value-added product with high purity. The pilot-scale setup has an operation capacity of ∼100 L spent etchant regeneration and ∼60 kg CuSO4 ·5H2 O recovery per day. The stability of the pilot tests is promising for further scale-up to industrial spent etchant treatment. Acknowledgements The authors would like to thank Agency for Science, Technology and Research (ASTAR), Singapore for funding this research work under grant number R-279-000-164-305. Yang Qian is currently under Prof. Neal Chung Tai-Shung supervision and he wants to take this opportunity to thank Prof. Chung for his great help to finalize this work and also to appreciate the financial top-up support for PhD study from Prof. Chung’s ASTAR grant (Grant no. R-279-000-184-112). Thanks are also given to Dr. Kostetski, Y.Y., Mr. Zhang, Y.K., Dr. Grishchenko, A.B., and Dr. Ragevan Chitra for their valuable help and participation on different stages of the project. References [1] N.M. Kocherginsky, A. Grishchenko, Method for metal recovery from aqueous solution. US Patent 6,521,117 B2 (2003). [2] R. Moore, Profile of the West European Printed Circuit board Industrymarket Prospects to 1999, Elsevier Science Ltd., Oxford, 1995. [3] R.S. Villanucci, A.W. Avtgis, W.F. Megow, Single-sided PCB processing: Print-and-Etch technique, in: S. Hella (Ed.), Electronic Techniques: Shop Practices and Construction, 7th ed., Wentworth Institute of Technology, Boston, 2002. [4] E.S. Robert Jr., Electronics Business Manager of MacDermid, Current etchant market in Singapore, personal communication, 2002. [5] Mecer process. http://www.sigma-mercer.com. [6] Electronic Computers and Printed Circuit Boards: SIC 3571 and 3672. http://www.moea.state.mn.us/publications/SIC3571.pdf. [7] G. Kyuchoukov, M.B. Bogacki, J. Szymanowski, Copper extraction from ammoniacal solutions with LIX84 and LIX54, Ind. Eng. Chem. Res. 37 (10) (1998) 4084–4089. [8] P.A. Ohara, M.P. Bohrer, Supported liquid membranes for copper transport, J. Membr. Sci. 44 (2–3) (1989) 273–287. [9] W. Mickler, E. Uhlemann, Liquid–liquid-extraction of copper from ammoniacal solution with beta-diketones, Sep. Sci. Technol. 27 (12) (1992) 1669–1674. [10] LIX 54, A New Reagent for Metal Extraction from Ammoniacal Solutions, Henkel Corporation, Tuczon, Arizona, 1975.
Q. Yang, N.M. Kocherginsky / Journal of Membrane Science 286 (2006) 301–309 [11] Q. Yang, J.W. Jiang, T.S. Chung, N.M. Kocherginsky, Experimental and computational studies of membrane extraction of Cu(II), AIChE J. 52 (9) (2006) 3266–3277. [12] N.M. Kocherginsky, Q. Yang, Big carrousel mechanism of copper removal from ammoniacal wastewater through supported liquid membrane, Sep. Purif. Technol., doi:10.1016/j.seppur.2006.08.019, in press. [13] P.R. Danesi, A simplified model for the coupled transport of metal ions through hollow-fiber supported liquid membranes, J. Membr. Sci. 20 (1984) 231–248. [14] G.R.M. Breembroek, A. van Straalen, G.J. Witkamp, G.M. van Rosmalen, Extraction of cadmium and copper using hollow fiber supported liquid membranes, J. Membr. Sci. 146 (2) (1998) 185–195. [15] J.C. Lee, J.K. Jeong, J.T. Park, I.J. Youn, H.S. Chung, Selective and simultaneous extractions of Zn and Cu ions by hollow fiber SLM modules containing HEH (EHP) and LIX84, Sep. Sci. Technol. 34 (8) (1999) 1689–1701. [16] F. Valenzuela, C. Basualto, C. Tapia, J. Sapag, Application of hollow-fiber supported liquid membranes technique to the selective recovery of a low content of copper from a Chilean mine water, J. Membr. Sci. 155 (1) (1999) 163–168. [17] P. Schoner, P. Plucinski, W. Nitsch, U. Daiminger, Mass transfer in the shell side of cross flow hollow fiber modules, Chem. Eng. Sci. 53 (13) (1998) 2319–2326.
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[18] S.H. Lin, R.S. Juang, Mass-transfer in hollow-fiber modules for extraction and back-extraction of copper(II) with LIX64N carriers, J. Membr. Sci. 188 (2) (2001) 251–262. [19] Liqui-Cel® Membrane Contactors, Membrana, http://www.liqui-cel.com. [20] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York, 1988. [21] N.S. Akhmetov, General and Inorganic Chemistry, Victor Kamkin, Moscow, 1988. [22] M.J. Zapatero, M.P. Elizalde, J.M. Castresana, Acid–base equilibria and metal extraction properties of 1-phenyl-1,3-decanedione, Anal. Sci. 7 (6) (1991) 935–940. [23] B.A. Kitteringham, German Manufacturing Facility Uses Liqui-Cel® Membrane Contactors for Ammonia Removal from Wastewater. http://www.liqui-cel.com/technical-resources/case-studies.cfm. [24] D.S. Flett, J. Melling, Extraction of ammonia by commercial copper chelating extractants, Hydrometallurgy 4 (1979) 135–146. [25] U. Messow, D. Pape, Comparison between observed and calculated solubilities of gases in oil fractions, Pol. J. Chem. 54 (1980) 2001– 2009. [26] C.A. Beevers, H. Lipson, The crystal structure of copper sulphate pentahydrate, Proc. Roy. Soc. Lond. A (1934).
Copper recovery and spent ammoniacal etchant regeneration based on hollow fiber supported liquid membrane technology: From bench-scale to pilot-scale tests Qian Yang a,∗ , N.M. Kocherginsky a,b a
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Division of Bioengineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 15 July 2006; received in revised form 30 September 2006; accepted 5 October 2006 Available online 10 October 2006
Abstract One of the key steps in printed circuit board (PCB) production is etching of a thin copper layer. Ammoniacal etching solutions are widely used for this purpose. Earlier we have developed a supported liquid membrane based method to treat wastewater containing ammonia and copper (up to 1 g/L), where the membrane is stable for at least 1 month and can be easily regenerated if necessary [1]. Now we are describing an effective hollow fiber supported liquid membrane (HFSLM) based technology for copper recovery from spent ammoniacal etching solutions, where copper is present in much higher concentrations. A bench-scale HFSLM system with 1.4 m2 effective membrane surface area was firstly used to screen out the optimal hydrodynamic and other operation conditions for potentially practical spent ammoniacal etching solutions treatment. It was found that the excess of ammonia in spent etching solutions had negative effect on copper removal, especially when copper concentration became low in the feed solutions as the result of treatment. Different methods were employed to control the ammonia level and their efficiencies were compared. Finally, successful pilot-scale experiments were conducted on a hollow fiber membrane contactor with a surface area of 130 m2 . The process results in copper removal by a factor of ∼3000 from spent etching solution through the membrane and formation of nearly saturated copper sulfate solution in the sulfuric acid, used as a striping phase. Compositions of the regenerated etching solution and purity of CuSO4 ·5H2 O crystals formed in the striping phase were comparable or even better than their commercial analogues. The stability of the pilot-scale system is promising for further industrial scale-up. © 2006 Elsevier B.V. All rights reserved. Keywords: Spent ammoniacal etching solution; Copper removal; Ammonia removal; Supported liquid membrane; Hollow fiber membrane contactor
1. Introduction Printed circuit boards (PCBs) are important components of modern electronic products, which have generated billions of US dollars globally [2]. The manufacture of PCBs involves several technical processes and etching is one of the most important steps. According to PCBs’ design [3], part of the copper thin layer on the silicon base surface is first covered with photo resistant plastics. This permits the unmasked copper to be dissolved chemically into the etchant and the desired circuit pattern is pro-
∗
Corresponding author. Tel.: +65 65166433; fax: +65 67791936. E-mail address: [email protected] (Q. Yang).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.10.012
duced. With the rapid growth of PCBs industry, the total volume of generated spent etchant keeps increasing. Based on a market analysis in Singapore conducted by MacDermid in 2002, it was estimated that 70,000 L of spent ammoniacal etchant were produced every month by local PCBs plants [4]. These spent etching solutions are usually stored in drums or tanks and are ultimately shipped to an off-site treatment plant for copper recovery before disposal. Nowadays, two commonly copper recovery methods employed by PCBs industry are neutralization and solvent extraction. In neutralization, acid and alkaline spent etchant are mixed together to form Cu(OH)2 , which can be decomposed to CuO upon heating. In this process, generated CuO is a low value-added product and the resulting wastewater requires
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further treatment before disposal. To date, the most economically successful method for regeneration of spent etchant is the Mecer® system, developed by Sigma Metallextraktion AB, Sweden [5,6]. But a market survey conducted in 2001 has revealed that the sales record of Mecer® system is becoming worse. It has been evaluated that the payback time of Mecer® system is 2.5–3 years [6]. As we know nowadays the economic recycle time is becoming shorter and no PCB manufacturers would like to purchase a system with a payback time more than 2 years, especially the system is used only for wastewater treatment. In addition, Mecer® system is a complex process with extraction and re-extraction in different unit operations which used a lot of organic solvent but it can remove only 33% of the copper before the regenerated etchant being returned back to etching process [7]. Therefore, it is quite clear that the current market needs technology breakthrough to supply a more effective spent etchant treatment system. One of possible economical alternatives to treat this waste is to use supported liquid membrane (SLM) technology. In SLM, relatively small volume usage of organic extractant as the carrier impregnated in the micropores of polymeric membrane matrix allows possible usage of expensive carriers. Simultaneous extraction and re-extraction in one technological step offers a high mass transfer flux and it is easy to operate. LIX54, a commercial product from Cognis Corporation (Tucson, AZ, USA) is widely used as the carrier for selective removal of copper from alkaline solutions. It is a -diketone extractant with an active ingredient 1-phenyl-1,3-decanedione. In recent years [7–11] it has been extensively proved that LIX54 is an excellent extractant for copper recovery from ammoniacal solutions due to its ease of stripping, low or no ammonia loading, high selectivity of Cu2+ over Zn2+ and Ni2+ , fast transfer kinetics, good stability, etc. In addition, removal of copper ions through liquid membranes is possible even against of their concentration gradient due to H+ chemical potential difference between the feed and stripping solutions. In this case it is not necessary to use transmembrane pressure or voltage. The common problem is that though pH of the feed copper solution is initially much less acidic, it decreases with time due to electroneutral 2H+ /Cu2+ exchange through the liquid membrane and simultaneously Cu transport decreases. Without ammonia copper transfer process stops [12] when [Cu2+ ]f = [Cu2+ ]s
[H+ ]f [H+ ]s
2
Feasibility studies of copper separation from aqueous solutions by HFSLM have also been conducted worldwide [13–16], which is determined both by simplicity of the process and its practical potential and importance for industry. Nevertheless, the previous researches were mostly focused on the treatment of model copper solution in which copper species is existed as the form of Cu2+ with low concentration and most of them were constrained in the lab-scale. Now we describe an effective HFSLM system developed successfully from bench-scale to pilot-scale for the treatment of industrial spent etchant with copper content around 160 g/L. The objective of this work is to develop an economic and efficient way to regenerate spent etchant for further reuse and simultaneously to recover copper product. The bench-scale investigations were conducted to screen out the optimal conditions on a basis for potentially practical waste treatment process. Finally, successful pilot tests were accomplished with two valuable products generated from the industrial waste and without secondary waste. 2. Experimental 2.1. Reagents LIX54 was kindly supplied by Cognis Corporation (Tucson, AZ, USA). It was diluted in kerosene (Aldrich, Singapore) and used for preparation of liquid membrane phase. Copper sulfate pentahydrate (Nacalai Tesque, Japan), sulfuric and hydrochloric acids, sodium hydroxide (Merck, Singapore) were of reagent grade. The copper-containing spent ammoniacal etching solution was kindly supplied by a local PCB company in Singapore. Its composition is given in Table 1. 2.2. Hollow fiber supported liquid membrane (HFSLM) setups 2.2.1. Bench-scale setup A microporous polypropylene hollow fiber membrane contactor (Liqui-Cel® Extra-Flow 2.5 in. × 8 in. membrane contactor, Hoechst Celanese, USA) was employed to prepare a benchscale HFSLM system. Detailed specifications of the hollow fiber membrane contactor are given in Table 2 [17,18]. The membrane contactor was operated in a cross-flow mode. Fiber walls were
(1)
where subscripts ‘s’ and ‘f’ indicate species in the feed and stripping solution, respectively. In contrast to this situation we have demonstrated that in copper-containing ammoniacal solutions during their treatment, pH was almost constant due to the buffer effect of NH3 /NH4 + [1]. It is not necessary to adjust pH in the feed side to keep large H+ gradient and the process can be conducted till very low copper concentrations in the feed wastewater. Furthermore, hollow fiber membrane contactor as the support matrix for liquid membrane preparation represents a very attractive system with high throughputs and economical efficiency.
Table 1 Typical composition of spent ammoniacal etching solution pH Cu(II) (M) Total NH3 (M) Cl− (M) Na+ (M) Zn(II) (M) Hg(II) (M) Ni(II) (M) Ca(II) (M) Total Fe (M) Cd(II) (M)
10 ∼2.5 10 5 2.5 × l0−2 1.7 × l0−3 4.7 × l0−4 3.4 × l0−4 9.3 × l0−5 4.7 × l0−5 1.8 × l0−6
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Table 2 Specifications of bench-scale hollow fiber membrane contactor (Liqui-Cel® Extra-Flow 2.5 in. × 8 in.) [17,18]
Table 3 Specifications of pilot-scale hollow fiber membrane contactor (Liqui-Cel® Extra-Flow 10 in. × 28 in.) [19]
Fiber type Number of fibers (N) Fiber internal radius (ri ) (m) Fiber outer radius (ro ) (m) Effective module outer diameter (da ) (cm) Effective module inner diameter (di ) (cm) Effective pore size (rp ) (m) Porosity (ε) (%) Tortuosity (τ) Effective fiber length (L) (cm) Effective surface area (Sm ) (m2 )
Water flow range
Polypropylene X50 fibers 9950 120 150 4.67 2.2 0.03 40 2.5 15 1.4
impregnated by pumping 33 v/v% LIX54 in kerosene as the liquid membrane phase through the contactor in a recycling mode using a peristaltic pump (Cole-Parmer, USA) for about 1 h. The extra oil was washed out with de-ionized water. In the subsequent experiments, feed spent etchant solutions were pumped through the tube (lumen) side of hollow fiber membrane contactor and acidic stripping solutions were pumped co-currently in the shell side unless otherwise specified. The cocurrent flow mode was adopted in order to provide a more stable SLM system. The hollow fiber with micropores impregnated by organic solutions (LIX54 in kerosene) acts as a barrier to forbid the direct contact of the feed etchant solutions and the stripping acid. Unless otherwise specified only one membrane contactor was used. In the case of spent etchant treatment with additional control of NH3 content the second membrane contactor was used for this purpose (Fig. 1). 2.2.2. Pilot-scale setup For a pilot-scale test, a bigger membrane contactor with 130 m2 effective surface area (Liqui-Cel® Extra-Flow 10 in. × 28 in. membrane contactor, Hoechst Celanese, USA) was used
Housing characteristics Material
Priming volume Weight (liquid full in shell side) Seal material Cartridge characteristics Membrane (X50 fibers)
Potting material
10–48 m3 /h Fiber reinforced plastic (FRP) housing with PVDF for all wetted surfaces Shell side: 26.1 L; lumen side: 10.6 L FRP and PVDF: 57 kg EPDM Material: polypropylene; porosity: ∼40%; OD/ID: 300/220 m; surface area: 130 m2 Epoxy
with specifications listed in Table 3 [19]. The pilot-scale setup is schematically depicted in Fig. 2. The HCl addition method was employed to control the excess NH3 in the feed etchant. 2.3. Analytical methods Samples of the bulk feed and stripping solutions were collected at predetermined time intervals (normally every 1 h) and concentrations of metal ions were measured using an inductively coupled plasma-atomic emission spectroscopy (ICPAES) (Perkin-Elmer, USA). pH value of the solutions was measured with a Cyberscan 200 digital pH meter (Eutech Instrument, Singapore) and combined pH glass electrode. Total ammonia content or total alkalinity in the feed was found by back titration using excess of H2 SO4 to convert all NH3 into NH4 + in the solution before titrating the rest of the acid with NaOH. Chloride concentration was measured with
Fig. 1. Bench-scale experimental setup for spent etchant treatment (the grey part is only for NH3 control with another membrane module). T and S indicate the tube and shell side of membrane contractor, respectively.
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Fig. 2. Pilot-scale experimental setup for spent etchant treatment.
Cl− ion selective electrode (Metrohm, Singapore). The specific gravity was measured using a hydrometer. These parameters of the regenerated etchant were analyzed and compared with the product specification for the commercial replenisher PlusCu-Etch (Plaschem, Singapore) which is used as an additive to dilute spent etchant in etching process. The replenisher’s main components are ammonia and ammonium chloride and it is used to maintain the concentration level of ammonium chloride but also help to keep a constant specific gravity of etching solution. As such, a stable etching operation could be achieved. The purity of CuSO4 ·5H2 O crystals from the pilot tests was characterized by measuring concentrations of various possible contaminants using ICP-AES and they were compared with commercial CuSO4 ·5H2 O crystals. Wide-angle X-ray diffraction (XRD) measurements of these crystals were carried out on a Shimadzu XRD-6000 X-ray diffractometer at 40 kV and 30 mA and at a scan speed of 2◦ /min. 3. Results and discussion
• At stripping side: CuR2 + H2 SO4 ⇔ 2HR + CuSO4
(3)
where overbar represents species in the organic phase. pKa for the weak acidic extractant LIX54 is 9.73 [22] and acidic stripping is easy with this chelating extractant. Kinetics of Cu transfer from 600 mL spent etchant for two different volumes of 2 M H2 SO4 used as the striping solution is shown in Fig. 3. It is found when 3 L 2 M H2 SO4 was used as a stripping solution, it results in a faster copper removal rate than 0.6 L 2 M H2 SO4 . This is understandable when a smaller volume of the stripping acid solution with the same molarity is used, due to Cu2+ /2H+ exchange an acid concentration in the stripping solution decrease faster than in the case of larger stripping volume used. Simultaneously, increase of copper concentration in the stripping solution is higher. Both these factors shift the reaction (2) backward and make the whole copper transport process slower. It is interesting to note that the initial rate of copper removal from the feed side is slightly higher than the
3.1. The effect of stripping acid on copper removal from spent etchant It is well known that Cu2+ in aqueous solutions with ammonia is forming octahedral complexes from Cu(NH3 )(H2 O)5 2+ to Cu(NH3 )4 (H2 O)2 2+ and the fifth and sixth ammonia do not bind strongly with Cu2+ because of Jahn–Teller effect [20,21]. The predominance of Cu(NH3 )4 2+ species is expected because of its high formation equilibrium constant over other ammine complexes and high ammonia content in spent etching solution. The chemical reactions occurring at the feed and stripping sides are presented below: • At feed side: Cu(NH3 )4 Cl2 + 2HR ⇔ CuR2 + 2NH4 Cl + 2NH3
(2)
Fig. 3. Effect of the stripping solution volume on copper removal rate from the bench-scale experiments. Feed: 600 mL spent etchant; flow rate: 70 mL/min both in tube and shell side. Exp. I—stripping: 0.6 L 2 M H2 SO4 ; Exp. II—stripping: 3 L 2 M H2 SO4 .
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Fig. 4. Effect of sulfuric acid molarity on copper transfer flux from the benchscale experiments. Feed: 1 L spent ammoniacal etchant; strip: 3 L H2 SO4 with different concentrations. Flow rate: 70 mL/min in both tube and shell side of membrane contactor.
corresponding increase in the stripping side. This is based on the fact that most of copper is transferred from feed to stripping phases but some still stay as the complex with carrier in the membrane phase. This is further confirmed by the initial drops of total copper mass in the aqueous solutions then keeping almost constant. The steady state is usually reached after 10 min (Fig. 3). The initial copper flux (mol cm−2 s−1 ) can be calculated based on the slope of copper concentration as a function of time in the feed solution: dC V (4) J =− dt Sm where dC is the change in concentration in the feed over time interval dt, V the feed solution volume and Sm is the effective membrane surface area. A similar equation could be used for the measurements in the stripping side but without minus sign. Fig. 4 shows that an increase of a stripping acid concentration facilitates the initial copper flux in the stripping side. However, an increase from 2 to 3 M results only in 20% increase of the flux but higher SO4 2− reduces the level of copper solubility in the stripping solution. If the spent etchant treatment is carried out in an extended period of time, it is possible to reach saturation in the stripping solution where crystals of copper sulfate are formed. This could spoil the membrane contactor when it is recirculated in the membrane contactor without notice. Taking a compromise between flux and solubility, we chose 2 M H2 SO4 as the stripping solution in the following experiments. 3.2. The effect of flow rates on copper removal from spent etchant It is very important to find an optimal hydrodynamic condition in the HFSLM system for better copper recovery and further scaling-up of the process. In this experiment, the volumetric flow rate of one stream was varied while the other was kept fixed at 70 mL/min and the initial flux was calculated based on the increase of copper concentration in the stripping solution. Fig. 5 demonstrates that an optimal flow rate exists both for the feed and stripping streams and its value in both cases is around 70 mL/min. In our previous work [12], we found that the rate of copper transfer reaches maximum when the carrier is fully loaded with
305
Fig. 5. Effect of volumetric flow rate on initial copper flux into the stripping side from the bench-scale experiments. Feed: 600 mL of spent etchant recirculated in the tube side of HFSLM. Stripping: 3 L of 2 M H2 SO4 recirculated in the shell side.
copper at high feed copper concentration stage and the flux can be described as 0
Jmax,m =
εDCuR2 ,m [HR]m τδm 2
(5)
where DCuR2 ,m is the diffusion coefficient of copper-carrier complex in the organic membrane phase (8.9 × 10−7 cm2 /s); δm the thickness of hollow fiber membrane wall; ε and τ the porosity 0 and tortuosity of hollow fiber membranes, respectively; [HR]m is the initial carrier concentration (33 v/v% LIX54 in kerosene corresponding to 0.55 M). Substituting these parameters from Table 2 [17,18] into Eq. (5), we can get the theoretical maximum flux is around 1.31 × 10−8 mol cm−2 s−1 , which agrees with the experimental maximum flux at optimal volumetric flow rates of 70 mL/min both for feed and stripping streams. The existence of the optimal flow rate is probably because the process reaches the optimal residence time for copper contacting with the carrier and the minimal thickness of non-stirring layers are obtained in copper transport process through the HFSLM system. Further increase of feed and/or stripping flow rates will not further decrease the thickness of non-stirring layers but will result in the shorter contact time of copper with the carrier LIX54. Volumetric flow rates lower than 50 mL/min were not used in our experiment runs because of too small etchant volume purified in the unit time. On the other hand, at flow rates higher than 200 mL/min the supported liquid membrane could be unstable. Therefore, high flow rates were not employed. It is interesting to find when the solutions in the tube and shell sides were swapped but both were recirculated at 70 mL/min, the fluxes were practically the same. This is probably because the rate determining step in this case is not the transport in both aqueous solutions but in the liquid membrane phase. 3.3. The role of feed ammonia on copper removal from spent etchant and a comparison of the efficiency of different methods to control ammonia level In our previous work [12] it is shown that in the presence of ammonia and alkaline pH when major copper species is Cu(NH3 )4 2+ , equilibrium of the transmembrane Cu2+ /2H+
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exchange is described by an equation: + 2 [H ]f [Cu(NH3 )4 2+ ]f = K [NH3 ]4f s [Cu2+ ]s [H+ ]s
(6)
where Ks is the stability constant for Cu(NH3 )4 2+ ; the species in the bracket indicate their concentrations in aqueous solutions in equilibrium condition. It demonstrates that it is possible to transfer copper from ammoniacal solution against copper concentration gradient and thus to have an active transport. The simplest way to accumulate copper in the stripping solution is to have higher H+ concentration in the stripping than in the feed. As an example for ∼1 M NH3 and pHf 10 in the feed and pHs 0 in the stripping solutions enrichment of copper in the stripping over feed can be higher than 104 times. However, an increase of ammonia concentration in the feed makes copper extraction process less favorable due to its competition with carrier for copper. This could also lead to the formation of higher order copper-ammonia complexes like Cu(NH3 )5 2+ and Cu(NH3 )5 2+ and thus depress the extraction of copper. In the beginning of spent etchant treatment, when copper concentration is high, the carrier is fully loaded with copper and the hindrance effect of ammonia on copper removal should not be significant. Later, at low copper concentrations and a significant excess of ammonia present in the etchant, the competition between ammonia and carrier for copper can become stronger [7]. It can unfavorably change both the rates and final equilibrium state. Therefore, the excessive NH3 accumulation in the feed hinders the forward copper extraction and reduces the copper transfer flux. Several methods were used to reduce free ammonia concentration in the etching solution during its treatment. One way to reduce ammonia is simply to add hydrochloric acid into the feed etchant. Either concentrated HCl (37%) or 5 M HCl solution was used for this purpose. 5 M HCl was chosen based on the fact that the NH4 Cl concentration in the commercial replenisher is around this value. The addition was carried out gradually within the whole experiment in order to keep almost constant pH of feed solution around 8. When concentrated HCl was used, needle shape crystals of NH4 Cl were found in the feed solution. To avoid this, in further experiments only 5 M HCl was used to control ammonia level in the feed. Fig. 6 shows that HCl addition in the initial stage of treatment has no significant effect. However, faster removal rate could be obtained by this method when feed copper concentration is low. The problem is that HCl addition decreases NH3 content due to its conversion to NH4 Cl, thus decreasing the efficiency of the treated etchant for PCBs etching. Therefore, NH3 should be added to the treated etchant for further reuse as a replenisher. Recently, it was reported that membrane contactors can be used to remove 95% of ammonia with concentration of 1100 mg/L [23]. Ignited by this information we have tried to use another membrane contactor without impregnating to sweep excess NH3 in etchant treatment process. In this case, ammonia can diffuse through the air-filled membrane pores, while aqueous solution cannot penetrate through the pores from shell to tube side due to the hydrophobicity of the polypropylene membrane
Fig. 6. Comparison of the various methods for spent ammoniacal etchant treatment from the bench-scale experiments. Feed: 1 L spent ammoniacal etchant solution; strip: 4 L 2 M H2 SO4 . Flow rate: 70 mL/min in both tube and shell side of membrane contactor.
matrix and slightly higher pressure applied in the tube side. The swept NH3 later can be re-adsorbed and re-used to top-up NH3 in the treated etchant solution. The efficiency of three methods to treat spent ammoniacal etchant is compared quantitatively: initial copper flux J calculated from the linear part of feed copper concentration dependence versus time and also the apparent copper mass transfer coefficient Ka at low copper concentration stage are listed in Table 4. The apparent mass transfer coefficient is calculated based on Eq. (7) where Ct and Ct+t represent copper concentration at time t and t + t in the feed at low copper concentration stage; Q is the volumetric flow rate in the feed: Q Ct (7) ln Ka = Sm Ct+t Fig. 6 as well as Table 4 demonstrates that there is not much difference in copper removal rate in first few hours when feed copper concentration is more than 40 g/L no matter whether the control of feed ammonia is employed or not. Later, when copper concentration in the feed is low as the result of treatment, the control of feed ammonia level greatly facilitates copper removal. Inset in Fig. 6 presents it in the semi-log coordinates. For readers’ information, the feed volume was increased when HCl addition was employed. Therefore, for a proper comparison the copper concentration values shown in Fig. 6 were amended to eliminate the diluting effect. Role of ammonia cannot be reduced only to the complex formation with copper in the feed and also to the shift of extraction equilibrium. It is also necessary to investigate if ammonia can Table 4 Comparison of initial copper flux at high copper concentration stage and apparent mass transfer coefficient at low copper concentration stage of different benchscale spent etchant treatment processes
Initial copper flux (mol cm−2 s−1 ) Ka (cm/s)
HCl addition method
Hollow fiber membrane method
Control
5.20 × l0−9
5.07 × 10−9
4.98 × 10−9
1.27 × 10−5
1.45 × 10−5
7.23 × l0−6
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307
Table 5 Mass balance calculations for different bench-scale spent ammoniacal etchant solution treatment processes Control
5 M HCl addition to control excess NH3
Membrane contactor to control excess NH3
mol
%
mol
%
mol
%
Feed side A. Total Cu(II) removal B. Total NH3 loss C. Conversion to NH4 + D. Moved to stripping (=J) E. Due to HCl neutralization F. Due to air stripping G. Evaporated to air
2.54 8.62 5.08 1.75 – – 1.79
100 58.93 20.30 – – 20.77
2.49 9.14 4.98 0.19 3.60 – 0.37
100.0 54.49 2.08 39.39 – 4.05
2.43 9.25 4.86 0.12 – 4.27 –
100 52.54 1.30 – 46.16 –
Stripping side H. Total H+ consumption I. H+ consumption due to Cu/2H+ exchange J. Discrepancy (=H – I)
6.83 5.08 1.75
100 74.38 25.62
5.17 4.98 0.19
100 96.32 3.68
4.98 4.86 0.12
100 97.60 2.40
penetrate directly through the membrane into the acidic media. Simultaneously with Cu/2H+ exchange and ammonia transport there is also a chance that H+ can go directly through the membrane due to high pH difference. It was found that the carrier LIX54 did not extract ammonia in appreciable quantities in the absence of copper in aqueous solution, while copper loading led to an increase in ammonia content in the organic phase [24]. In addition, ammonia has around 3% mole fraction solubility in kerosene serving as the solvent for liquid membrane preparation at 30 ◦ C and 1 atm [25]. The quantities of ammonia carry-over from feed to stripping solutions could be significant over long periods’ operation and this is deleterious to the subsequent performance of electrowinning or crystallization steps. To characterize relative role of these three processes we have measured the concentration of H+ in the stripping sulfuric acid by titration and also total ammonia of feed etchant solution both at the beginning and in the end of each treatment process. For example in the control experiment (refer to Table 5), when amount of copper in the feed phase is decreased by 2.54 mol in whole treatment process, content of ammonia measured in the feed is decreased by 8.62 mol. Only part of it (5.08 mol) can be explained by its conversion into NH4 + based on reaction (2). Total consumption of H+ was also much higher (6.83 mol) than theoretical consumption (5.08 mol). The difference of 1.75 mol is probably due to direct ammonia diffusion through the organic membrane phase and neutralization of the acid in stripping side. The rest of ammonia decrease (1.79 mol) could be due to simple loss to an air. As mentioned before there is also a possibility of H+ direct transport from the stripping acid to feed etchant which could be accounted for the discrepancy between theoretical H+ consumption and experimental determination in the whole etchant treatment process. If it is the truth, the amount of H+ direct transportation over long periods of treatment should be almost the same for these three experimental runs. However, this is contradictory to the fact that when measures are taken to control and decrease feed NH3 level, total H+ consumption in the stripping is much less. Both with HCl addition and air stripping using second membrane contactor less NH3 was transferred directly into the
stripping sulfuric acid. This is very important for the downstream electrowinning or crystallization unit operations. In large-scale treatment process, the stripping acidic solution has to be reused as long as possible and carry-over of ammonia through the organic phase should be avoided or at least minimized. 3.4. Pilot-scale HFSLM system for copper recovery and spent ammoniacal etchant regeneration Pilot plant experiment was carried out with a setup shown in Fig. 2. The HCl addition method was employed to control NH3 in the feed etchant. Within 55 h of operation 200 L of spent etchant have been treated and copper concentration was reduced from 150 g/L to 50 ppm, i.e. by a factor of 3000. Simultaneously, half a ton of almost saturated copper sulfate in sulfuric acid solution was formed. In the subsequent downstream process, crystallization of copper sulfate pentahydrate was conducted by refrigeration of the stripping acidic solution with concentration of copper sulfate near saturation rather than traditional electrowinning step. This is because the CuSO4 ·5H2 O produced from the same volume of stripping solution has around four times more weight than corresponding simple copper metal and higher market price per unit volume. The crystals in terms of cation composition (Table 6) are similar to the commercial analogues. The X-ray diffraction (XRD) spectra of the crystals are also similar to that of commercial crystals (not shown): four molecules of water are located in the square around the copper, and two oxygens form with them approximately octahedron. The Table 6 Comparison of CuSO4 ·5H2 O generated from the pilot-scale tests with commercial analogues Element
Crystals from pilot test (%)
Commercial crystals (%)
Al Fe K Mg Pb Zn
0.36 0.004 0.23 0.02 1.4 0.5
0.858 0 0.17 0.02 1.8 0.5
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Table 7 Comparison of treated spent etchant from the pilot-scale tests with commercial replenisher Treated etchant Cu concentration (ppm) NH4 C1 concentration (g/L) Total alkalinity (g/L) pH Specific gravity (20 ◦ C)
Regenerated etchant after NH3 top-up
Commercial replenisher
47
50
–
276
271
250–270
18 8.15 1.070
346 9.68 1.02
310–370 9.5–9.9 1.020–1.040
fifth water is not coordinated but is in contact with two oxygen atoms and two other water molecules [26]. However, in feed side the total alkalinity or ammonia content after the treatment is very low compared to the replenisher used as an additive to dilute spent etchant in etching process. This also leads to comparatively low pH in the treated etchant solution due to its conversion to NH4 + because of the gradual addition of HCl solution. Therefore, it is necessary to add NH3 into the treated etchant in order for it to be practically used as a replacement for the replenisher. The treated spent etchant with NH3 top-up is termed as the regenerated etchant in Table 7. A comparison of the regenerated etchant and the commercial replenisher Plus-CuEtch demonstrates that besides the small amount of remaining copper, the important chemical contents and properties of the regenerated etchant fall within acceptable range and it is ready to be used for etching by potential PCB manufacturers. As mentioned in Ref. [7], the etching efficiency is maximal when ammoniacal solution contains 110–130 g/L copper and when the copper concentration is 150–170 g/L in the etching solution, it does not have etching effect. We found that in our separated experiments all the copper transmembrane fluxes into the stripping side are higher at high feed copper concentration stage. In this case, the carrier is fully loaded with feed copper and the transmembrane flux is determined by the copper-carrier diffusion through the organic membrane phase. With the time feed copper concentration becomes lower as the result of treatment. While at low feed copper concentration stage, the flux is proportional to the copper concentration [12]. Therefore, if the pilot-scale HFSLM system is used for copper reduction in spent etchant solutions to 110–130 g/L, the treated volume per day and the yield of copper sulfate pentahydrate crystals would be much higher. After an inactivity period of 3 months, another two separate pilot tests were conducted. The treatment efficiency of these two runs is almost the same as the 1st run based on the terms of initial flux at high feed copper concentration stage and apparent mass transfer coefficient at low copper concentration stage. This demonstrates that the stability of this HFSLM system is promising for industrial-scale spent etchant treatment. 4. Conclusions Bench-scale investigations for spent etchant regeneration and copper recover were conducted in this work to screen out the
optimal operation conditions for the potential practical process. It is found that the accumulation of excess ammonia in the feed etchant solution has negative effect on the copper removal through a HFSLM system. The effect is more significant in the end of the process at lower copper concentrations. Ammonia removal from the feed improves the copper removal and also reduces the ammonia transfer to the acidic stripping solution. Both sweeping gas in an additional hollow fiber membrane contactor and HCl addition methods can be used to decrease excess ammonia in the feed etchant and minimize its carry-over to the stripping solution effectively. Successful pilot-scale tests were accomplished by employing a bigger membrane contactor with 130 m2 membrane area. Treated solution with very low copper concentration has chemical and physical properties, satisfying the specification of commercially available replenisher. Copper is recovered in the form of CuSO4 ·5H2 O, a value-added product with high purity. The pilot-scale setup has an operation capacity of ∼100 L spent etchant regeneration and ∼60 kg CuSO4 ·5H2 O recovery per day. The stability of the pilot tests is promising for further scale-up to industrial spent etchant treatment. Acknowledgements The authors would like to thank Agency for Science, Technology and Research (ASTAR), Singapore for funding this research work under grant number R-279-000-164-305. Yang Qian is currently under Prof. Neal Chung Tai-Shung supervision and he wants to take this opportunity to thank Prof. Chung for his great help to finalize this work and also to appreciate the financial top-up support for PhD study from Prof. Chung’s ASTAR grant (Grant no. R-279-000-184-112). Thanks are also given to Dr. Kostetski, Y.Y., Mr. Zhang, Y.K., Dr. Grishchenko, A.B., and Dr. Ragevan Chitra for their valuable help and participation on different stages of the project. References [1] N.M. Kocherginsky, A. Grishchenko, Method for metal recovery from aqueous solution. US Patent 6,521,117 B2 (2003). [2] R. Moore, Profile of the West European Printed Circuit board Industrymarket Prospects to 1999, Elsevier Science Ltd., Oxford, 1995. [3] R.S. Villanucci, A.W. Avtgis, W.F. Megow, Single-sided PCB processing: Print-and-Etch technique, in: S. Hella (Ed.), Electronic Techniques: Shop Practices and Construction, 7th ed., Wentworth Institute of Technology, Boston, 2002. [4] E.S. Robert Jr., Electronics Business Manager of MacDermid, Current etchant market in Singapore, personal communication, 2002. [5] Mecer process. http://www.sigma-mercer.com. [6] Electronic Computers and Printed Circuit Boards: SIC 3571 and 3672. http://www.moea.state.mn.us/publications/SIC3571.pdf. [7] G. Kyuchoukov, M.B. Bogacki, J. Szymanowski, Copper extraction from ammoniacal solutions with LIX84 and LIX54, Ind. Eng. Chem. Res. 37 (10) (1998) 4084–4089. [8] P.A. Ohara, M.P. Bohrer, Supported liquid membranes for copper transport, J. Membr. Sci. 44 (2–3) (1989) 273–287. [9] W. Mickler, E. Uhlemann, Liquid–liquid-extraction of copper from ammoniacal solution with beta-diketones, Sep. Sci. Technol. 27 (12) (1992) 1669–1674. [10] LIX 54, A New Reagent for Metal Extraction from Ammoniacal Solutions, Henkel Corporation, Tuczon, Arizona, 1975.
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