Kinetics of the electrochemical process of galena electrodes in the diethyldithiocarbamate solution

International Journal of Minerals, Metallurgy and MaterialsVolume 17, Number 6, December 2010, Page 669DOI: 10.1007/s12613-010-0372-yKinetics of the electrochemical process of galena electrodesin the diethyldithiocarbamate solutionLi-li Cheng", Ti-chang Sun", Xian ping Luo2), and Dian-zuo Wang3)1) School of Civil and Environmental Engineering, University of Science and Technology Beijing, Bejing 100083, China2) School of Resources and Environmental Engineering. Jiangxi University of Science and Technology, Ganzhou 341000, China3) Chinese Academy of Engineering, Beiing 100038, China(Received: 1 Dcember 2009; revised: 15 January 2010; accepted: 7 February 2010)Abstract: The electrochemical process of galena in a pH 12.8 buffer solution was investigated using chronoamperometry and chronopoten-tiometry. To establish kinetic parameters on the surface of galena in the diethyldithiocarbamate solution, the exchange current density and thedependence of current density on reaction time were determined. Experimental results demonstrate that the exchange current density of ga-lena is 1.585x 10-2 A/m2 in the diethyldithiocarbamate-ftee solution. In the diethyldithiocarbamate solution, the thickness of lead diethyldi-thiocatbamate adsortbed on the surface of galena is 3.28 molecular layers, the diffusion cofficient of diethyldithiocarbamate on the surface ofgalena electrodes is 1.13x 10 " m'/s, and the exchange current density of galena is 0.45 A/m'. Lead diethyldithiocarbamate on the surface ofgalena is firmly adsorbed.Keywords: galena; electrochemistry; flotation; kinetics[This work was financially supported by the National Nature Science Foundation of China (No.50704018) and the Natural Science Founda-tion of Jiangxi Province, China (No. 2007GQC0643)]face of galena, determine its diffusion coefficient, and estab-1. Introductionlish the dynamic equation. In this study, the kinetics of elec-Pulp potential has been proved to play an important roletrochemical behavior of the galena electrode in the DDTCin the flotation of sulfide minerals. The nature of hydropho-solution was investigated, and some advanced electro-bic products on the surface of galena is affected by the an-chemical techniques were employed.odic process [1-2]. Therefore, flotation recovery has a close2. Materials and methodsrelationship with pulp potential in the flotation system. It isimportant to establish kinetic parameters of galena in the di-2.1. Materials and reagentsethyldithiocarbamate (DDTC) solution in view of the neces-The galena sample with a purity of 96.2% was from Huilisity in theoretical research and practical application [3-4].Lead-Zinc Mine of Sichuan Province, China. The sample,Although DDTC has extensive applications in lead-zinc sul-cut as a rectangular prism, is connected to a copper wire byfide mineral flotation [5], very lttle work has been done insilver cement and mounted in a pyrex tube with an electro-the electrode process of galena. To enhance the separationchemically inert epoxy resin. The mineral electrode is con-efficiency of lead-zinc sulfide mines in the DDTC flotationnected to an electrochemical test system (Princeton Appliedsystem, it is essential to find out the mechanism of the sur-Research PARSTAT 2263) through a copper wire shown inCorresponding author: Li-li Cheng E-mail: cegil2000@ 163.com◎University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2010中国煤化工包SpringerMYHCNM HG.670Int J. Miner. Metall. Mater, Vol.17, No.6, Dec 2010rig. 1 [6]. Also, the exposed surface area of the electrodewas 1x10 4 m2. The surface of the galena electrode was pol-. Pyrex tubeished before each experiment by wet abrading with 600grade sand paper, rinsed with deoxygenated distilled water,- Copper wireand then polished with 3x107 m alumina [7]. Before eachelectrochemical measurement, the working electrode wasimmersed in the electrolyte for 4 h to atain equilibrium.The purity of DDTC, which was used as a collector in theEpoxy resin- Silver cementflotation of galena, was 94%. The buffer solution with a pH. Mineralvalue of 12.8 and consisting of disodium hydrogen phos-phate and sodium hydroxide was used in the electrochemis-Fig. 1. Schematic drawing of the electrodes.try experiments as an electrolyte to prevent the change ofpH values in solution [8]. All the chemicals used for bufferE: Open circuit potentialsolution were of analytical grade. The buffer solution wasE: Applied potentialdeoxygenated with an intensive bulling of high-purity ni-Etrogen gas (99.998vol%) before each run for 15 min toeliminate the effects of dissolved oxygen on the experimen-2tal results. During the experimentation, the flow of nitrogenwas stopped and the experimental condition was completelysealed to prevent the diffusion of atmospheric oxygen intothe cell.2.2. Electrochemical measurementsTime/sA conventional thre-electrode system was used for elec-Fig. 2. Applied potential vs. time in chronoamperometry ex-trochemical measurements, in which a saturated calomelperiments.electrode (SCE), a platinum foil electrode with 1x10+ m2area, and a galena electrode were used as reference, auxil-iary, and working electrodes, respectively. All potentials inthis study were quoted in volts with respect to a standardhydrogen electrode (SHE).The study on the variation of current response with timepotentiostatic control was chronoamperometry. Thcurrent resulting from a potential step was shown in Fig. 2.The purposes of chronoamperometry experiments were todetermine the exchange current density io of galena in the :DDTC-free buffer solution, the thickness of lead diethyldi-thiocarbamate (PbD2), and the diffusion coefficient (D) ofFig. 3. Applied current vs. time in chronopotentiometry ex-DDTC on the surface of galena electrodes in a 1x10 7mol/m3 DDTC.3. Results and discussionFig. 3 shows that a current applied to an electrode pro-vokes a change in its potential. The study on the variation of3.1. Determination of the exchange current density ofpotential with time is chronopotentiometry [9]. The purposegalenaof chronopotentiometry experiments was to investigate thestability of the product on the surface of galena electrodes inunder potentiostatic control in the DDTC-free buffer solu-a 1x10-7 mol/m' DDTC solution.tion was done中国煤化工ing chronoam-HCNMHG.LL. Cheng et al, Kinetics of the electrochemical process of galena electrodes in the diethy ldithiocarbamate solution671perometry experimentation, a constant potential was applied0.by an electrochemical test system to a special durationStep potential is0.25V Vws. SHEwhich monitored the resulting current censity. The current: 0.4-3: Step potential is 0.35 V vs. SHEresulting from a potential step in chronoamperometry ex-三0.3-periments is shown in Fig. 4. Chronoamperometry experi-ments were carried out to investigate the electrode oxidationprocess.To investigate the exchange current density io of the ga-0.1-ena electrode in the pH 12.8 buffer solution, the followingequation is introduced [9-10]:0.0Time/sn= -2.303RTlgin -2.303RT-lgi-o(1Fig.4. Current density vs. reaction time in the DDTC-freeβnFbuffer solution.where η is the overpotential, R the gas constant, T the.30-absolute temperature, β the transfer coefficient， n the.25 tnumber of transfer electrons in reaction, F the Faradayconstant, and i; =0 the current density when the concentra-0.20tion polarization of reaction cannot exist.0.15|According to Eq. (1), the value of in can be obtained0.10|from the intercept ofthe plotof η Vs. lgi=0. Therefore, it0.05is necessary to study the relationship between η and i=o.0.00As is shown in the following equation [9], the values ofi=0 indifferent potentials can be acquired from the inter-000.050.10.150.200.250.300.355cept of the plot of i vs.- t" 2 through chronoamperome--105/(-1s05)try experiments:Fig.5. Current density of the galena electrode vs. -45 in re-sponse to potentiostatic steps.i=i=o-i=o-22Jt(2/π~ir=0 can be extrapolated in different potentiostatic stepsfrom Fig. 5, so the relationship between the overpotentialwhere t is the time, and(η) and lgi=ocan be obtained as Fig. 6 according to Eq.(1).a_K。,.R(3Do"/2From Fig6, the slope of the curve can be determined as1.560, and the intercept can be obtained as 7.523. Accordingwhere K。is the rate constant for reduction, K。the rate0.39constant for oxidation, Do the diffusion coefficient 0oxidized form, and Dp the diffusion coefficient of reduced里0.36-form.? 0.33In chronoamperometry experiments， E is the opencircuit potential at which there is no current, and the E2values are 0.25, 0.30, and 0.35 V, respectively. The results0.27-of chronoamperometry experiments in the pH 12.8 buffersolution are displayed in Fig. 4, which shows the plot of cur-6 0.24-rent density vs. time for the galena electrode in response to-4.68 -4.66-4.64-4.62-4.60 -4.58different potentiostatic steps. Fig. 5, derived from Fig. 4,lgi./(Am)]shows a linear correlation between current density i, andFig. 6. Overpot中国煤化工o potentiostatic-、t in the specified time area.steps.YHCNM HG.672Int J. Miner. Metall. Mater, Vol.17, No.6, Dec 2010to Eq. (1), the oxidation dynamic equation of the galenaAccording to Eq. (5) and Fig.7, the relationship betweenelectrode at pH 12.8 is established as follows:current density and time can be obtained as follows:η= 7.523 + 1.560lgi=0(4i;= 1.9065x10-4 + 2.72381x10-3/0.5(6)Also, the exchange current density in the DDTC-freeTherefore, the diffusion coefficient DpDrc of DDTCbuffer solution can be calculated as io = 1.585x10 2 A/m2on the surface of galena can be determined by Eqs. (5) and3.2. Determination of the diffusion coefficient of DDTC(6), which is about 1.13x 10-10 m2/s. The floating rapidity ofon the surface of galenagalena is quantified by this result.The electrode process and the diffusion coefficient of theAlso, the consumed charge ( Q ) can be calculated bygalena electrode can be investigated by chronoamperometryexperiments. To determine the diffusion coffcient DpDrcQ=. j"ir=j21.90645x10-4 + 2.72381x10-30.sdt=of DDTC on the surface of galena, Eqs. (1) and (5) were1772 μC/cm2=17.72 C/m2(7utilized [10-11]. The validity of Eq. (5) was verified in detailby the classic experiments of Kolthoff and Laitinen, whoTo calculate the thickness of PbD2 adsorbed on themeasured or controlled all parameters.surface of galena (θ), the following equation should be em-ployed [11]:nFA、DpDTc CDDTC(5;√πt0=_ 2(8)nqNAwhere A is the galena electrode area and CpDTC thbulk concentration of DDTC.where q is the electric charge of an electron and N thenumber of atoms per unit area of galena electrode.In Eq. (5), the values ofn, π,F,A, and CpDTC were 2,3.1415926, 96500 C/mol, 1x10 4 m2, and 1x10-7 mol/m',The value of N can be calculated as 1.685x1019 mi 2respectively.from the crystal parameter of galena, which is 0.5934 nmWhen a system conforms to Eq. (5), aplotof i7l vs.Therefore, the thickness of PbD2 adsorbed on the sur-√元i0.5 is a straight line with a slope offace of galena can be obtained from Eqs. (7) and (8).nFAVDoprc CDDTC__Q17.72The chronoamperometry experiment was carried out onnqNA= 2x(1.602x10-19 )x(1.685 x109)x13.28the galena electrode, and the potential value was set as 0.25V vs. SHE to avoid excessive oxidation [12]. The current(9)resulting from a potential step in the chronoamperometryThe thickness of PbD2 adsorbed on the surface of ga-experiment is shown in Fig. 7.lena, calculated through Eq. (9), is about 3.28 molecular0.06layers at pH 12.8. The minimum usage of DDTC can be de-pH 12.8termined based on this result.0.053.3. Stability of the product on the surface of the galena0.04-。electrode in the DDTC solution0.03Hagihara et al. [14], using electron diffraction and X-raymethods, proved that the product was PbD2 on the galenasurface, which was reacted with DDTC in aqueous solutions.0.01Furthermore, Eq. (10) deduced by Wang Dianzuo et al. inRef. [15] showed that the product on the surface of galena0.00洲0.0.5.0was PbD2. As is known, the stability of the product plays anTime/simportant role in the flotation process. However, the stabil-rig. 7. Current density vs. time in the buffer solution includ-ity of PbD2 on中国煤化工TC solution ising the 1x10-7 mol/m3 DDTC.still not fully uper carried outYHCN MH G'.L.L. Cheng et al, Kinetics of the electrochemical process of galena electrodes in the diethyldithiocarbamate solution673chronopotentiometry experiments to investigate the stabilitydetermined, according to Eq. (12). The relationship betweenof PbD2.By applying constant potential (E= 0.2 V) on the galenane and(门can be obtained as follows:electrode, the product PbD2 was formed on the electrodesurface in the pH 12.8 buffer solution including 1x107mol/m’DDTC. Then, the chronopotentiometry experimentn。= 0.301- 0.2891g(13)川一周of the galena electrode was carried out under the conditionof current density i= -4.50 A/m2. Fig. 8 shows that the po-tential varies with time in the reduction process.0.0.1-0.2-0.0管0.0一-0.2--0.4-0.8-1.2-1.6lg[1-(t/z)°]-0.4- CFig. 9. Overpotential(7)rs. lg[1. (/i>"].Time/ sBecause the values of R, T,F, ic and τ are certain, the .Fig.8. Potential vs. time in response to galvanostatic steps onexchange current density can be calculated as i =0.45the galena electrode (pH 12.8, 25"C, [DDTC]=1x10 : mo/m).A/m2.The transition time () can be determined as 3.8 s fromAccording to the chronopotentiometry experiment, theFig 8. Also, the reduction of PbD2 is determined by the fol-passing charge Q' [11] on the galena surface can be ob-lowing reaction [15]: .tained as follows:PbS+2D =PbD2 +S+2e(10)Q' =iq.r=1710 uC/cm2=17.10 C/m2(14)E= -0.301 -0.059.1g[D~](11)On the basis of the above electrochemical experiment,the overall consumed charge Q is about 17.72 C/m2.According to Eq. (11), the value of the thermodynamicTherefore,potential E can be obtained as about -0.065 V with the con-centration of DDTC is 1x107 mol/m'. The oxidation of2'、x100% = 96.5%(15)DDTC ions on the galena surface is an ireversible reaction.QAs a consequence, there exists overpotential ηc in Eq. (10),which can be determined by the above electrochemical ex-Because 96.5% of PbD2 can be released by Faradayperiment. The relationship between the overpotential (η。 )pattern, the oxidation product ( PbD2 ) on the surface of ga-of the cathodic reaction and reaction time can be expressedlena is firmly adsorbed. In other words, PbD2 adsorbed onas[11]the surface of galena is stable. Therefore, galena can befloated well in the DDTC solution and has a good floata-2.303RT lg(ic/i)_ 2.303RTlg[I-(t/r)°5]bility. This result agrees with the flotation practice of galena(12)(1- β)nF(1- 3)nFin the DDTC solution.where i。 is the cathodic current density.4. ConclusionsFig. 9 shows that the overpotential is linearly related to(1) Theexchange current density of galena in theDDTC-free bufl中国煤化工as 1.585*102Also, the slope and intercept can beA/m? by the chrcYHCNMH G.674Int J. Miner. Metall. Mater, Vol.17, No.6, Dec 2010(2) In the pH 12.8 buffer solution including 1x10-[6] J.A. Muioz, C. Gomez, A. Ballester, et al, Electrochemicalmol/m' DDTC, the thickness of PbD2 adsorbed on thebehaviour of chalcopyrite in the presence of silver and Sul-surface of galena is determined as about 3.28 molecular lay-folobus bacteria, J. Appl. Electrochem., 28(1998), p.49.ers, the diffusion cofficient of DDTC on the surface of ga-7] 1. Lazaro and M. J. Nicol, A rotating ring-disk study of thelena electrode is about 1.13x10-"0 m2/s, and the exchangenitial stages of the anodic dissolution of chalcopyrite incurrent density of galena is determined as 0.45 A/m2 by theacidic solutions, J. Appl. Electrochem, 36(2006), p.425.chronoamperometry method.8] T. Giuler, C. Hicyllmaz, G. Gokagac, and Z. Ekmekci, Elec-(3) The oxidation product ( PbD2 )on the surface of ga-trochemical behaviour of chalcopyrite in the absence andlena is firmly adsorbed, about 96.5% of which can be re-presence of dithiophosphate, Int. J Miner: Process, 75(2005),leased by Faraday pattern.p.217.I9] D.C. Cai, Electrochemical Research Methods, ElectronicReferencesScience and Technology University Press, Chengdu, 2005,p.83.[1] Q. Li, G.Z. Qiu, and W.Q. Qin, Kinetics of electrochemicalprocess of pyrite electrode in diethyldithiocarbamate solution,[10] A.J. Bard and L.R. Faulkner, Electrochemical Methods, 2ndEd, John Wiley & Sons Inc., New York, 2001, p.102.Min. Metall. Eng, 21(2001), No.2, p.30.[11] Z. Gu, C.S. Dai, and L. Chen, Electrochemical Measurements,2] I. Chermyshova, Anodic processes on a galena (PbS) elec-trode in the presence of n-butyl xanthate studiedChemical Industry Press, Beijing, 2006. p.106.FTIR-spectroelectrochemically, J. Phys. Chem. B, 105(2001),[12] I. Cisneros-Gonzalez, M.T. Oropeza-Guzman, and I. Gon-No.34, p.8185.zilez, Cyclic voltammetry applied to the charaterization of3] R. Woods, Electrochemical potential contolling flotation, Int.galena, Hydromallurgy, 53(999), p.133.J. Miner. Process, 72(2003), p.151.[13] Y. Noda, s. Ohba, s. Sato, and Y. Saito, Charge distribution4] R. Herrera-Urbina, FJ Sotillo, and D.W. Fuerstenau, Effectand atomic thermal vibration in lead chalcogenide crystals,of sodium sulfide additions on the pulp potential and amylActa Crystallogr: B, 39(1983), p.312.xanthate flotation of cerussite and galena, Int. J. Miner.[14] H. Iwasaki and H. Hagihara, The crystal structure of lead (II)Process, 55(1999), p.157.diethyldithiocarbamate, Acta Crystallogr. B, 28(1972), p.507.5] G.H. Gu, Y.H. Hu, G.Z. Qiu, et al, Origin potential flotation[15] D.Z. Wang, GH Gu, R.Y. Liu, Potential adjustment flotationof galena and its industrial application, Min. Metall. Eng.of galena-lime-diethyldithiocarbamate system, Chin. J. Non-22(2002), No.4, p.30.ferrous Met, 8(1998), No.2, p.322.中国煤化工MYHCNMH G.