Modeling Operational Parameters of a Reactive Electro-Dialysis Cell for Electro-Refining Anodic Scrap Copper *

Transcription

1 Amercan Journal of Analytcal Chemstry, 2014, 5, Publshed Onlne November 2014 n ScRes. Modelng Operatonal Parameters of a Reactve Electro-Dalyss Cell for Electro-Refnng Anodc Scrap Copper * Gerardo Cfuentes, José Hernández, Jorge Manríquez, Ncolas Guajardo Department of Metallurgcal Engneerng, Unversdad de Santago de Chle, Santago, Chle Emal: gerardo.cfuentes@usach.cl Receved 20 August 2014; revsed 5 October 2014; accepted 22 October 2014 Copyrght 2014 by authors and Scentfc Research Publshng Inc. Ths work s lcensed under the Creatve Commons Attrbuton Internatonal Lcense (CC BY). Abstract Ths work wll create an electro-dalyss cell model that has the purpose of refnng anodc scrap copper an element that currently must be returned to the copper converson process. The cell modelng s based on Ohm s Law, whle the resultng copper depost morphology s studed through the thckness of the layer deposted on the surface and the electrc current lnes traced from the anode to the cathode. The use of the model demonstrated that t s possble to effectvely predct the specfc energy consumpton requred for the refnement of the anodc scrap copper, and the morphology of the cathode obtaned, wth a margn of error of 9%. Keywords Electro-Refnng, Modelng, Copper Scrap, Reactve Electro-Dalyss, Specfc Consumpton 1. Introducton The objectve of copper electro-refnng process s the electrochemcal dssoluton of an mpure anode (98% Cu) to obtan a cathode wth 99.99% pure copper [1]-[3]. After a number of electro-refnng processes copper anodes should be removed from the electrolytc refnng cells wthout completely dssolvng for 2 reasons. Frst, the anode secton that s dssolved s only the one mmersed n the electrolyte. The anode cannot be completely mmersed due to the electrcal connecton wth the power source. If the latter s mmersed, t would dssolve along wth the anode and the process would stop. The second one that prevents the scrap from dssolvng s the qualty of the copper depost: f the cathode s not morphologcally regular, ts mechancal propertes (hardness, resstance, etc.) wll be very low, and therefore * Modelng operatonal parameters of a reactve electro-dalyss cell for electro-refnng anodc scrap copper. How to cte ths paper: Cfuentes, G., Hernández, J., Manríquez, J. and Guajardo, N. (2014) Modelng Operatonal Parameters of a Reactve Electro-Dalyss Cell for Electro-Refnng Anodc Scrap Copper. Amercan Journal of Analytcal Chemstry, 5,

2 wll not be capable of contnung wth the followng processes. To obtan a hgh-qualty cathode, the anode s surface must be as homogeneous as possble so t does not produce preferental deposts on the cathode. Because the scrap copper anode s not morphologcally regular, the cathode resultng of ths process would have a bad qualty surface. Consequently, the result of an eventual electro-refnng usng scrap would result n a cathode unft for the followng stages. In practcal terms, ths problem represents a percentage of copper whch cannot leave the crcut; that s, a crculatng charge wthn the process. The authors [4]-[6] have worked wth an electro-dalyss system to mnmze the scrap re-crculaton to the smeltng process. Stanless steel baskets are used as supportng electrodes to provde electrcal connecton of the system because, as n the case of scrap copper anodes, morphologcally rregular metals are usually used. Usng a basket results n hgh-qualty surface, regardless of the morphology of the metal put nsde. Therefore, ths electrode support shall be used to dssolve scrap copper anode. In parallel they use an electro-dalyss cell to separate both electrodes (anode and cathode). The reason for ths s the constant generaton of the anode slme nsde the stanless steel basket. The accumulaton of slme decreases the electrcal contact nsde the anode, ncreasng the potental system consumpton gradually to the pont of corrodng the stanless steel and stoppng the process. The modelng of process s a very mportant tool of knowledge; wth ths tool t s possble to present several scenaros to study the realty. In ths work, expermental data compled for the authors are used; all the data above-mentoned are necessary lke nput to the equatons for characterzng the system. Then t works wth COMSOL Multphyscs program to modelng the operatonal parameters of the electro-dalyss cell used for electrorefnng of anodc scrap copper. 2. Theory 2.1. Nernst-Planck Equaton The equaton whch models the flow of ons along the electrolyte s the Nernst-Planck equaton, showng that the flow may be due to 3 dfferent mechansms: dffuson, mgraton or convecton (Equaton (1)) [7]. dc F dϕ N= D + zc + cv dx RT dx (1) where N : total flow of ons n a soluton by unt of area and tme. dc D : on flow by dffuson. dx F dϕ D z c : on flow by mgraton. RT dx cv: on flow by convecton. The Nernst-Planck shows that the on flux n any of the systems may be due to any of the 3 aforementoned drvng forces: dffuson, convecton and mgraton. In normal workng condtons, these workng cells have a constant concentraton of copper throughout the soluton. The cell s agtaton s also very low. Consequently t can be assumed wth a certan degree of precson that the ons n the soluton are moved by the sole dfference of potental appled, that s, mgraton. In ths way the equaton Nernst-Plank s reduced to the Ohm Law, Equaton (2): = σ ϕ (2) where σ s onc conductvty of the soluton, s the current densty that s conducted through the cell, and s the dfference n appled potental usng the DC power source. In ths way, the decrease n the electrolyte potental and the cell s membrane wll be gven by the value of ther conductvty n S/m Electrode Knetcs Electrode knetcs allows determnng the over potental, µ, that must be appled to the electrodes for a gven current densty. 1012

3 Electrode knetcs s represented based on the Butler-Volmer equaton (Equaton (3)) [8]. b v α n F µ β n F µ = 0 exp exp RT RT (3) where b v s the workng current densty value for a mxed knetc control, 0 s the exchange current densty, α and β are the knetc reacton parameters, n s the oxdaton state of the on, T s temperature and R s the unversal constant of gases. Nevertheless, t s known that n ndustral processes, knetcs of most electro-chemcal processes work wth mxed control, that s, they depend on the mass transfer as much as they do on the charge. Electrode knetcs are represented by the Equaton (4): m = lm lm + b v b v where lm s the system s lmt current densty and m s the workng current densty for mxed control knetcs. In the case of porous electrodes, the use of correcton factors s very common to consder the decrease n effectve conductvty nsde the porous electrode, due to the loss of ts real volume n electrode or electrolyte or because of the decreased convecton nsde the porous electrode due to ts nner ruggedness. The most common correcton s known as the Bruggeman coeffcent, whch consders the effectve volume of the supportng electrodes, n ths case, the stanless steel basket and the electrolyte nsde t. Ths way, the effectve conductvty of the supportng electrode and the electrolyte nsde the porous electrode wll be gven by the Equatons (5) and (6). = σ ϕ, σ = σ (5) 1.5 l, eff l l, eff l l = σ ϕ, σ = σ (6) 1.5 s s, eff s s, eff s s where the subndexes l and s are supportng electrolyte and electrode, whle σ l, eff and σ s, eff are ther effectve conductvtes and l and s ther volume, respectvely. Ths results n the value of the overpotental of the representatve electrode of the real case. 3. Cell Modellng The dagram of Fgure 1 represents the work system and the man components. The used work cell s shown n Fgure 2. Here t shows that the cathode corresponds to a vertcal stanless steel sheet, whereas for the anode a support basket was used to dssolve the anodc scrap copper t contans. The basket s consdered a porous electrode. The modelng of the cell wll be carred out usng the COMSOL Multphyscs program n ts verson 4.3b [9]. Frst, the propertes of the work cell for modelng must be defned. The cell wll be shown n Fgure 3. The work cell dmensons are the followng: Each half cell shown n Fgure 3 s cm 3 n dmenson. The actual work area s cm 2, whle the flow whch enters the electrolyte wthn each half-cell s 1.8 ml/s. Thus, the cell shown n Fgure 3 s represented n COMSOL Multphyscs accordng to Fgure 4. Havng defned the geometry of work (Fgure 4), we proceed to defne the sub-domans and boundary condtons: Electrolyte and membrane: cuprc ons are transported through them. Therefore, these sub-domans wll be defned through Ohm s law and ts onc conductvty. Cathode: n the cathode, the cathode current densty must be known, so that the thckness of copper deposted on the surface to be calculated usng Faraday s law, as well as the electrode knetcs, to determne the overpotental need to be appled to the electrode. Anode: the anode s made up of the stanless steel basket, the scrap copper to be dssolved, and the electrolyte theren. To consder the presence of all these elements, the anode wll be characterzed as a porous electrode, and ther respectve volume percentages shall decrease the effectve conductvty of the electrode. Fnally, ther knetc parameters obtaned from voltammetry curves are entered. (4) 1013

4 Catholyte Anolyte DC Power Source Flter (-) (+) Pump Sedmentaton Pond Pump Pump Fgure 1. Dagram of the system used. Fgure 2. Used work cell. Fgure 3. Work cell to be modeled. 1014

5 Electrc nsulaton: all outsde edges of the work cell are acrylc, whch s an nsulatng materal. Therefore, the current flowng to the outsde of the cell s zero. Mathematcally ths s expressed by the Equaton (7): n = 0 (7) where n s the vector correspondng to the normal drecton of the cell walls. Afterwards, the mportance of the exstng flow nsde the basket and the temperature of the system must be evaluated. For ths, expermental voltametry curves were drawn for dfferent workng condtons. The results are shown n Fgure 5. From Fgure 5 t can be seen that the best workng condton s when the agtaton of the cell and the temperature are at ther hghest value. Smlarly, the cathodc voltammetry at 40 C was also plotted, Fgure 6. The knetc parameters of each electrode are determned by the data obtaned n Fgure 5 and Fgure 6. Table 1 shows the expermental parameters of the work cell used for modelng. 4. Results The results obtaned of the model cell are as follows: Fgure 7 llustrates how the potental drops n the cell as the dstance toward the cathode decreases. From ths pont an mportant observaton s made: n general, t can be seen that the potental drop s constant throughout the cell, wth two exceptons: the frst occurs nsde the porous electrode. The reason for ths s the dffculty for the electrolyte to move nsde, n other words, a reducton n convectve transport mechansm occurs, whch must be compensated wth an ncrease of the potental dfference whch must be appled. The second pont where a sharp potental drop occurs s n the doman of the membrane [10]-[12]. As noted, the only dfference Fgure 4. Representaton of the cell n COMSOL Multphyscs 4.3b. Fgure 5. Expermental lnear voltametry drawn for the anode n dfferent workng condtons. 1015

6 Table 1. Expermental parameters used for modellng the cell [4]. Current ntensty 0.3 A Cathode area 15 cm² Cathode current densty 200 A/m² Bornes potental 0.82 V. Anode A/m². Cathode A/m² Anodc alpha 0.5 Cathodc alpha 1.5 Balance potental mv/enh Electrolyte conductvty 50 S/m Membrane conductvty 1.8 S/m Anode support conductvty 1.29E+06 S/m Anode effectve area /m Fgure 6. Expermental lnear voltammetry drawn for the cathode, 40 C. Fgure 7. Decrease n potental throughout the electrolyte. 1016

7 between ths and the electrolyte s each one s conductvty. As ths property s much lower n the membrane, t s necessary to apply a greater amount of energy compared to the rest of the electrolyte. To solve each of these mportant potental drops n the electrolyte, the strrng condtons wthn the porous electrode must mprove so that the on transport s not hndered by convecton. Regardng the membrane, the soluton s to desgn t as thn as possble and wth the largest cross-sectonal area, but not to the pont of fraglty, as these are made of polymers, or to mprove ts conductvty. Fgure 8 and Fgure 9 llustrate the anodc and cathodc overpotentals appled to the workng electrodes. These ponts are mportant because n many cases are very mportant fractons of the Bornes potental appled to the cell termnals. It can be seen that the cathode overpotental s the same throughout the electrode, whle ths parameter vares on the anode. Ths s due to the nature of the flow, whch s constant throughout the cross-sectonal area. For the anode, the system must provde energy as t moves away from the area over whch the electrcal connecton s made, due to the potental drop product of the tortuosty the porous electrode produces n the flow of electrolyte nsde. Fgure 10 shows 2 mportant characterstcs: The morphology of the copper deposts and the current lnes transported by the Cu 2+ ons. These 2 varables defne the morphology of the depost to be obtaned on the cathode. Recallng that the current always preferentally uses the paths that offer least resstance for transport, n ths type of testng ths effect wll be reflected n a hgher depost n some sectons than n others. The same fgure shows that the cathode surface s parallel to the porous electrode. As a result, the depost s practcally homogeneous n all ts dmensons. Addtonally, the mportance of usng support electrodes for the feld of electroplatng s also noted: thanks to them, the morphology of the metal to be dssolved for any type of coatng s rrelevant snce the supportng electrode s responsble for dstrbutng the current lnes evenly, as shown n Fgure 10. For ndustral applcaton, ths effect s very mportant, snce electro-refnng scrap has very rregular morphology, resultng n a very rregular cathodc deposton, however, ths problem s solved wth the anodc support. 5. Valdaton of Results The expermental data obtaned from the cell can be compared wth that predcted by the modelng n Table 2. Table 2 shows that t s possble to obtan representatve values of the data obtaned expermentally. 6. Conclusons It can be seen that modelng can be very useful to understand and mprove the operaton of electrochemcal systems. However, from unt 3 t can be concluded that the modelng of such systems cannot be smulated n any case wthout performng measurements to the system tself, snce most of the parameters used were measured after the system was underway. Fgure 8. Anodc overpotental. 1017

8 Table 2. Comparson between expermental data and modelng. Potental drop mv Expermental data Modelng wth correcton Anodc overpotental Cathodc overpotental Electrolyte and membrane Bornes potental Relatve error 9% Fgure 9. Anodc overpotental. Fgure 10. Anodc overpotental. From modelng and system smulaton work t s concluded that the best way to reduce energy consumpton would be to mprove the condtons of agtaton nsde the basket, snce the loss of convecton was caused by the very presence of copper scrap n the stanless steel basket, forcng the system to apply great anode overpotental and potental drop wthn the electrolyte. Fnally, another effcent alternatve s to use a membrane that has better conductvty than that used n ths experment, snce the latter n partcular represented a sgnfcant decrease n potental. 1018

9 Acknowledgements Support of ths work by DICYT of the Unversdad de Santago de Chle s gratefully acknowledged. References [1] Cfuentes, G. (2000) Theory and Praxs of Electrometallurgy (Teoría y práctca de la electrometalurga). Class Notes, Metallurgcal Department at Unversty of Santago of Chle, Santago. [2] Urra, C. (2003) Electrolytc Refnng of Partculate Anodc Scrap (Refnacón electrolítca de scrap anódco partculado). Ttulaton Work, Metallurgcal Department, Unversty of Santago of Chle, Santago. [3] Davenport, G., Kng, M., Schlesnger, M. and Bswas, A.K. (2002) Extractve Metallurgy of Copper. 3rd Edton, Elsever, Oxford. [4] Cfuentes, G., Hernández, J. and Guajardo, N. (2014) Recoverng Scrap Anode Copper Usng Reactve Electrodalyss. Amercan Journal of Analytcal Chemstry, 5, 9. [5] Hernández, J. (2014) Anodc Scrap Recoverng Usng Reactve Electrodalyss (Recuperacón de Scrap anódco por electrodálss reactva). M.Sc. Thess, Metallurgcal Department, Unversty of Santago of Chle, Santago. [6] Cfuentes, G., Smpson, J., Lobos, F., Brones, L. and Morales, A. (2009) Copper Electrownnng Based on Reactve Electrodalyss. Journal of the Chlean Chemcal Socety, 54, [7] Walsh, F. (1999) A Frst Course of Electrochemcal Engneerng (Un prmer curso de Ingenería Electroquímca). Edtoral Club Unverstaro, San Vcente, España. [8] Bockrs, J. and Reddy, A. (1977) Modern Electrochemstry: An Introducton to an Interdscplnary Area, Volume 2. 3th Edton, Plenum Rosetta Edton, New York. [9] Introducton to COMSOL Multphyscs. [10] Xu, T.W. (2005) Ion Exchange Membranes: State of Ther Development and Perspectve. Journal of Membrane Scence, 263, [11] Baker, R.W. (2004) Membrane Technology and Applcatons. 2nd Edton, Membrane Technology and Research, Inc., Menlo Park, Calforna. [12] Davs, S.M. (2006) Electrochemcal Splttng of Sodum Sulfate. M.Sc. Thess, Georga Insttute of Technology, Atlanta, Georga. 1019

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