Proceedings SWEMP 2002, R. Ciccu (Ed.), Cagliari, Italy, October 7-10,2002

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1 Prceedings SWEMP 2002, R. Ciccu (Ed.), Cagliari, Italy, Octber 7-,2002 COPPER ELECTROWINNING FROM THE AMMONIACAL ETCHING EFFLUENTS OF PRINTED CIRCUIT BOARDS INDUSTRY D. Panias, I. Giannpulu, I. Paspaliaris Natinal Technical University f Athens (NTUA), Schl f Mining and Metallurgical Engineering, Sectin f Metallurgy and Materials Technlgy, Labratry f Metallurgy, Athens, Greece. SUMMARY: Ammniacal cupric chlride slutin is widely used at the chemical etching prcess in the printed circuit bard (PCB) industry t remve a metallic cpper prtin frm the cpper clad laminate. Spent etchant has a typical cpper cncentratin in the range f g/l, which makes the metal recvery envirnmentally required and ecnmically interesting. The present wrk aims at studying the electrlytic recvery f cpper frm the ammniacal etching effluent in a simple cell, wrking in batch mde. The effluent is pre-treated with a strng aninic exchange resin fr the reductin f the chlride cncentratin t lw level allwing cpper electrwinning withut any crrsin prblems created due t andic xidatin f chlride ins, The effects f electrde arrangement, electrlyte agitatin rate, current density and cpper initial cncentratin n current efficiency and cpper recvery are investigated in this study. The results are used t determine the ptimum cnditins fr ecnmically feasible cpper electrrecvery frm the ammniacal etching effluent. 1. INTRODUCTION The demand f printed cir<;uitbards (PCBs) as part f the electrnic cmpnents has increased quite rapidly fr the last tw decades, increasing cnsequently the prductin f PCBs. The printed circuit bard (PCB) industry has t deal with a large vlume f waste effluents cntaminated with dangerus and heavy metals and ther substances harmful t the human health. Thus, the treatment f these effluents is imprtant and necessary befre their discharge int the envirnment, accrding t the envirnmental legislatin requirements. Furthermre, the high metal cncentratin f certain PCBs prductin stages waste streams creates an additinal ecnmic interest and fr that reasn, the treatment f the PCBs prductin waste streams was recently recnsidered and special treatment techniques f islated waste streams were develped, cmbining envirnmental requirements and ecnmic benefits. The mst interesting waste stream in a PCB industry regarding the metal recvery is that generated frm the stage f chemical etching, This prcess is used in the PCBs prductin t remve the unwanted cpper frm the cpper clad bard in rder t be frmed the desired circuit pattern [1]. The amunt f the cpper remved depends upn the design and type f bard; it may be in the range -% f that riginally present n the bard, 50-70% being cmmn [21.As the etching prcess prceeds and the cpper cncentratin in the slutin increases, the rate f etching gradually drps and a part f the slutin has t be replaced with fresh etchant. Spent etching slutin is classified as hazardus material and has t be treated befre it is discharged int the envirnment. The etching slutin mst widely used in the PCB industry is the ammniacal cupric chlride, as it has the advantages f peratin in clsed lp systems, cmpatibility with mst metallic and rganic resists, high capacity f disslved cpper and fast etching rates [II. Spent ammniacal etching slutins are alkaline slutins at a ph , having a typical cpper cncentratin f gll, a chlride cncentratin f g/l and a mlar rati f ammnia t cpper 4:1[1,3].Usually these slutins cntain als trace quantities f rganic and/r inrganic chemical additives required fr the etching prcess. 1137

2 S far, many electrchemical methds were develped fr the treatment f the spent ammniacal etching slutin in a way t minimize cpper cncentratin at the required level accrding the relevant envirnmental legislatin and t make use f the high cpper cncentratin. Electrrecvery, caustic bil, slvent extractin in cmbinatin with cpper crystallizatin r cpper electrwinning and in exchange with catin exchange resins in cmbinatin with cpper electrlytic recvery, are the mst used methds [3-7]. Althugh these methds are effective, they present significant disadvantages. Whereas these methds lead t high purity prducts they have elevated investment and peratinal csts due t utilizatin f cmplex equipment and special materials, chemical cnsumptin, tight peratin and maintenance requirements and high-energy demand. In many cases, these methds have als an envirnmental impact as they create new and hazardus effluent discharges. The electrlytic methd seems t be the mst interesting methd fr the treatment f the spent ammniacal etchant because it reduces cpper cncentratin t lw levels that cntribute t envirnmental prtectin and recvers cpper as a pure metal with an additinal ecnmic benefit. Electrlysis fr cpper recvery may be applied either; t the spent etchant r t the stripping slutin resulted frm slvent extractin r in exchange techniques. In the frmer case, it is necessary t have an electrchemical reactr divided int ande and cathde cmpartments by a semi-permeable in exchange membrane, as sn as the spent etchant has a chlride cncentratin in the range f 175-1gll. These membranes are nt cmpletely effective in preventing the passage f chlride ins frm cathlyte t anlyte, resulting t a sensible chlride cncentratin in the anlyte that limits the chice f ande materials due t the severe crrsin prblems created frm chlride evlutin [5.7].Mrever, membranes have uncertain life, as membrane failure is frequent, increasing the perating cst f the methd. In the latter case, the slutins are dilute making the applicatin f electrlysis limited frm the viewpint f energy efficiency [6,8]and thus, they require evapratin t increase their metal cncentratin. Besides, a large quantity f chemicals is lst as in exchange methd necessitates excess resin regeneratin [8]. In the present wrk the electrlytic recvery f cpper frm the ammniacal etching effluents f the PCB industry is studied. The spent ammniacal etchant is cmbined in the same stream with the wastewater resulted frm the rinse stages that fllw the etching prcess and an effluent with the fllwing cmpsitin is prduced; cpper 30.8g1l, chlride 38.5g1l and ttal ammnia 2.09ml/l. Cpper electrwinning takes place in a simple, undivided cell with cpper cathdes and 316 stainless steel andes. A pre-treatment f the waste effluent with a strng basic in exchange resin is suggested, in rder t reduce chlride cncentratin t an acceptable fr the electrlysis applicatin level aviding andic crrsin due t the chlride ins xidatin. The majr parameters affecting the electrlytic recvery f cpper frm the ammniacal etching waste effluent are investigated in rder t determine the ptimum cnditins f cpper recvery frm bth an envirnmental and an ecnmic view. 2. EXPERIMENTALDETAILS A batch-type electrlytic cell was used in all the experiments. The cell was designed particularly fr the experiments f the present wrk and made f an acrylic material, with the ability t carry five electrdes. The electrlyte vlume in all the experiments was 350 ml. The cathdes used were rectangular cpper plates with the fllwing dimensins, length 7.5 cm, width 6 cm and thickness 2 mm. The andes were f perfrated 316 stainless steel plates and had the same dimensins as thse f the cathde. The electrdes were ttally immersed int the slutin and placed alternatively with a 2.5 and 1.2 cm gap between them. A regulated pwer supply unit (0-5V, DC; 0-40A) was used as DC surce fr the electrlytic tests. The pwer supply wrked in galvanstatic mde; cell vltage was allwed t vary s that electrlysis experiments were carried ut at a cnstant current. All experiments were carried ut at rm temperature; during electrlysis slutins were neither heated nr cled (except heating by the current flw thrugh the slutin). The electrlyte was agitated with a magnetic stirrer. 1138

3 Agitatin increased the effective surface area f cathde and minimized the thickness f the bundary layer in a manner t avid electrdes plarizatin [9]. The experimental apparatus is schematically illustrated in Figure electrlytic cell 2. electrdes 3. magnetic stirrer 4. pwer supply Figure 1. Experimental apparatus The electrlyte slutin was prepared by labratry reagent grade cpper sulfate pentahydrate, cpper chlride, ammnia slutin and distilled water. As it is mentined abve, the waste effluent was pre-treated with a strng basic resin t reduce chlride cncentratin in such a level that permitted the applicatin f electrlysis withut andes crrsin. The maximum allwed chlride cncentratin fr the applicatin f electrlysis was experii)lentally defined at Sgn. The cmpsitin f the synthetic electrlyte slutin was 30.Sgn cupric in, 35.7g11sulfate in, sgn chlride in and 2.09mIn ttal NH3,in a way t simulate the waste effluent generated frm the alkaline etching stage f a PCB prcess. Three parameters were investigated in rder t determine the ptimum cnditins fr cpper electrwinning frm the effluent: inter-electrde distance, electrlyte agitatin rate and current density. During electrlysis small samples f slutin were taken every 15 min fr cpper analysis by atmic absrptin spectrphtmetry (Perkin Elmer 20). Current efficiency was calculated frm the reductin f cpper cncentratin in the slutin. 3. RESULTS AND DISCUSSION 3.1 Effect f inter-electrde distance The electrde arrangement that was emplyed in all the experiments f the present wrk was ande-cathde-ande. At this arrangement the cathdic electrde was placed between tw andic electrdes and thus, the cathde surfaces were lcated int the electric field that was develped between ande and cathde. Thereby, chemical disslutin f cpper that culd take place t the cathde surface lcated ut f the electric field and expsed t the alkaline etching slutin, in the case f an ande-cathde arrangement, was avided. In the ande-cathde arrangements, cpper chemical disslutin tk place alng with cpper recvery. Fr the ande-cathde-ande electrde arrangement, tw pssible distances between electrdes, 2.5 and 1.2cm were investigated. Figure 2 shws the cpper recvery as a functin f time fr the tw inter - electrde distances at a current density f 10Am-2.In that Figure, the theretical line crrespnding t cpper recvery at llooam-2accrding t the Faraday's law is als given. As the distance decreased, the curve f cpper recvery versus time apprached the theretical line. As the inter-electrde distance decreased frm 2,5 t 1,2 cm, the cell vltage drpped frm 6.17V t 4.0SV as this has been experimentally cnfirmed. Therefre, the cell resistance decreased whereas the current density remained cnstant. Cell resistance is the summatin f andic, cathdic and bulk slutin resistances. Thus, the andic and cathdic resistances decreased as far as the cmpsitin f the bulk slutin remained cnstant and fr that reasn, the current efficiency increased as the inter-electrde distance decreased. Mrever, when the inter-electrde distance decreases the intensity f the electric field develped between the electrdes increases and cnsequently, the electrstatic frce that drives the charged cupric ins t the cathde electrde

4 surface becmes higher. As a result. the surface charge densities at the bundary layer and the active electrde surface increase and thus. the ptential gradient at duble layer decreases and the cpper recvery rate becmes higher. 0 # 80 c ~ 50 '" R. 4) tune.min -thereuca11ine <> 2.5cm c 1.2cm lj() 180 Figure 2. Effect f the ande - cathde distance n cpper recvery 3.2 Effect f electrlyte slutin agitatin In rder t achieve the peratin f an electrwinning cell at lw energy cnsumptins. prper hydrdynamic cnditins shuld be maintained t imprve the mass transfer cnditins in the cell. The electrlyte r the electrdes mtin are usually emplyed in rder t achieve high mass transfer cnditins in a simple electrlytic batch mde cell. In the present wrk the electrlytic cell was perated with the electrlyte agitated with a magnetic stirrer. The studied agitatin rates were in the range f 60min-1t 500min-1at a cnstant current density f llooam-2.the effect f the stirring rate n cpper recvery and n the quality f cpper depsitin n cathde was investigated. The results are presented in Figure 3. 0 # 80 c3 70 ~ 60 <> ~ 50 i 4J 8"30 20 time,min - tb..ticlline c 514 min-i 11 3 min min-1 In min.i <> 60 min-i Ij() Figure 3. Effect f stirring rate n cpper recvery at a current density 1JOOAm-2 As it is seen in Fig.3 the cpper recvery was independent f the stirring rate in the range f min-1and apprached the theretical line at the initial stage f electrlysis. Stirring rates higher than 255min-1affected negatively the cpper recvery rates. In that case. the curves f cpper recvery appeared cncave curvature that is typical f kinetic inhibitins. Furthermre. in the absence f electrlyte agitatin. cpper reductin was nly ccurred in the area between electrdes under the influence f the applied electric field. Thus. a cpper cncentratin gradient between this area and the rest f the electrlytic cell was created. Despite this cncentratin gradient. cupric ins have a very lw tendency fr diffusin due t their very lw mlar diffusivity in the electrlyte slutin. The ammnia mlecules are highly plar and bind water mlecules with hydrgen bnding. creating a three dimensin pseud-lattice in the electrlyte slutin that renders the mlar diffusin f large cupric ins very difficult. Therefre. the stirring rate was an essential parameter f the electrlytic cell regarding the energy cnsumptin. as it determined the rate at which the reactants arrived at the electrde surface frm

5 the bulk f the electrlyte. Fr the electrlytic cell f the present wrk, stirring rates in the range min-1seem t create the prper hydrdynamic cnditins that enhance the mass transfer f the cupric ins and the depsitin f Cu nt the cathde. Mrever, unifrm and smth cpper depsitins were btained at stirring rates in the range min-i while granulated depsitins were bserved fr higher agitatin rates and jelly type lse depsitins fr lwer agitatin rates. 3.3 Effect f current density Five current intensities were investigated, 15A, 12.5A, loa, 7.5A and 5A crrespnding t current densities f Am-2, Am-2, 96.5 Am-2,822.4 Am-2and Am-2.The agitatin rate was kept cnstant at 123min-1in all the experiments. The results are shwn in Figure 4 where the cpper recvery is pltted as a functin f quantity f electricity and current density. As the current density increased, the curves tended t fllw the theretical line during the initial stages f electrlysis. In particular, the curves crrespnding t high current densities ( Am-2, Am-2and 96.5 Am-2),seemed t fllw the theretical line till the cpper recvery f abut 40%, which is achieved at the first 30 minutes f electrlysis. During that time, the cpper depsitin rate was high fr the mentined current densities and the current efficiency was greater than %. 0 #- 80 ~ 70 ~ ~ 4) g 30 u theretic<llme AIm2 -a aim2 --96,5AIm2 ~ 822,4Ahn ,2AJm2 5 7, ,5 20 Figure 4. Effect f current density n cpper recvery at stirring rate 123min-1 Quantity f electricity, Ah After that perid, the rates f cpper depsitin under current densities lwer than 96.5 Am-2 became higher and fr that reasn the btained current efficiencies were better than the nes at current densities higher than 96.5 Am-2,as it is shwn in Figure 5. Figure 5 shws the current efficiency as a functin f cpper cncentratin in the slutin fr different current densities. The current efficiency reached 0% fr the highest current density applied ( Am-2),until the pint where cpper cncentratin in the slutin was abut 18g1l. As the cpper cncentratin reduced belw that value, current efficiency decreased sharply and became lwer and thus, the electrlysis under lwer current density was mre efficient frm an energetic pint f view. At the final stages f electrlysis, where the cpper cncentratin was substantially reduced, the btained current efficiency under the lwer studied current density (548.2 Am-2) shwed the best results regarding the cpper recvery. Current efficiency values belw 0% indicate either; that the back reactin ccurs t sme extent t the cell, r that byprducts are frmed [2].In the case f metal depsitin frm aqueus slutins, the mst significant side reactin is the hydrgen gas evlutin at the cathde. Fr a given current density, a decrease f the metal cncentratin in the electrlyte reduced the current efficiency because metal ins culd nt be diffused t the cathde fast enugh and thus, hydrgen ins were reduced at high rates leading t a decrease in current efficiency. Therefre, there is a range f ptimal current densities that depend n the cpper cncentratin in the slutin, which must be applied fr the electrrecvery f cpper frm the ammniacal etching effluents in rder t achieve bth; high cpper depsitin rates and high current efficiencies. 1141

6 0 ;f. 80 ~ ~ 50 a 4J 30 u A/mJ 131O.6A/mJ ,5A/mJ ,4AJm ,2AJm S Cpper cncenlntin, gi1 Figure 5. Current efficiency as a functin f cpper cncentratin and current density. 4. CONCLUSIONS The majr parameters that affect the efficiencies f cpper electr winning frm the ammniacal etching effluents f the PCB industry, in an undivided electrlytic cell with cpper cathde and 316 stainless steel andes, were investigated. The waste slutin cntained 30.8 gil cpper, 2.09 mlji NH3 and 38.5 gil chlride and had a ph in the range The high chlride cncentratin in the slutin caused serius crrsin prblems n 316 stainless steel andes and fr that reasn a pre-treatment f spent etching slutin with a strng basic in exchange resin was suggested in rder t reduce the chlride cncentratin frm 38.5g1l t less than 8g1l. The latter value f chlride cncentratin was determined experimentally as the maximum ne allwed fr the applicatin f electrlysis withut andic crrsin. Cpper electrwinning prcess was prved t be mre efficient as the inter-electrde distance was decreased; a distance f 1.2cm between electrdes was fund as an ptimal fr the electrlytic cell. Under that cnditin high cpper electrrecvery mtes and better quality f cpper depsitin were achieved. The agitatin mte f the electrlyte was prved as an essential parameter fr the electrchemical reactr. Agitatin mtes in the range f 123 t 255min-1 created the prper hydrdynamic cnditins that maximized cpper recvery mtes and prduced unifrm, smth and cmpact cpper depsitins. High current efficiency was achieved during the initial stages f electrlysis under the higher current densities that were applied. After a perid, where abut 40% f the initial cpper amunt in the slutin was recvered, the current efficiency was higher fr the lwer applied current densities. Therefre, in rder t btain an ecnmically acceptable current efficiency and a high cpper recvery, the electrlytic prcess has t be perfrmed under gradually decreasing current densities. The ttal cpper electrwinning in all the experiments was in the range f % and the Cu(II) in final cncentratin was lwer than 19l1, a value that is required frm the Greek and the Eurpean legislatin fr safe and acceptable effluent dispsal at the envirnment. REFERENCES [I] e. F/ Cmbs: Printed Circuits Handbk, McGraw - Hill Bk Cmpany, 1988, Chap. 14. [2] D. Pletcher and F.e. Walsh: Industrial Electrchemistry, New Yrk, Chapman and Hall, 19, 2ndeditin, Chap. 2 & 9. [3] W. Schwab and R Kehl: "Cpper recycling frm spent ammniacal etching slutins and their regeneratin by slvent extractin", Prceedings f the XX IMPC - Aachen, September 1997,pp [4] T. Kawashima: "Recvering Cpper and Cpper Chemicals frm Spent Alkalin Etchant", M, Octber 1997, p. 40. [5] RM. Meyyappan, N. Sathaiyan and P. Adaikkalam: "Recvery f cpper frm ammniacal cpper etchantsusing exchange membranes", B. Electrchem5(2), February 1989,pp [6] R. Kammel: "Electrrecvery f metals frm dilute prcess slutins in electrplating", The minerals, Material & Materials Sciety, 1991, [7] M.R Hills: "The applicatin f the Capenhurst electrlytic etchant regeneratin (CEER) prcess t cupric chlride and ammniacal etchants, Trans. Inst. Metal Finishing, 62 Part I, (1984) pp [8] G.E. Hunt: "Waste Reductin in the Metal Finishing Industry", JAPCA, v1.38,n.5 (1988), pp [9] F. Gdridge,K. Sctt:ElectrchemicalPrcessEngineering,NewYrk,PlenumPress,1995,pp