Dynamics of anodic process of gold electrode in thiourea solution in the presence of sulfur dioxide

Article ID: 1003 - 6326(2003)03 - 0695- 04Dynamics of anodic process of gold electrodein thiourea solution in the presence of sulfur dioxideLONG Huai-zhong(龙怀中)，SHU Wan-yin(舒万艮)( School of Chemistry and Chemical Engineering, Central South University, Changsha 4 10083，China)Abstract: The electrochemical kinetics equation of the oxidation of gold in thiourea solution in the presence of sul-fur dioxide was given out.The kinetic parameters are determined as: the transmission coefficient=0.477; whenthe reaction time is zero, the initial over potential of the oxidation of gold in acid thiourea solution is 0.267 V. Thedynamics of anodic process of gold eletrode was studied by using Tafel plots. In the solution containing 5X 10-“mol/L sulfur dioxide the activated energy is 46.11 kJ/mol. W hen the electrode potential is higher than 270 mV, thegold is oxidized by thiourea. The addition of H2 SO3 can obviously improve the corrosion speed of gold.Key words: thiourea; gold; electrochemical dynamicsCLC number: TQ 150.1Document code: AINTRODUCTIONn a three- compartment cell， including a graphitecounter electrode, gold electrode and saturated cal-Cyanide free technology of gold extraction isomel reference electrode (SCE). The reference and .an important issue in hydrometallurgyl1.2 .Thiou-auxiliary electrodes were saturated calomel elec-rea was used as an important metallurgical reagenttrode (SCE) and graphite rods respectively. Alldue to its good selectivity, low poisonousness andpotential in this study were quoted in volts， withhigh efficiency~3.4]. Refs. [5- 7] showed that alrespect to a saturated calomel electrode (SCE).ode dissolving of gold will be accelerated if sulfu-An electronics potentiostat Model 273A wasrous acid or sulfur dioxide (H2SO3) is added into .used in all electrochemical measurement. Thethiourea system. The mechanism was consideredEG&_G corrosion measurement system ( Princetonthat sulfurous acid was involved into the electro-Applied Research Model 352) and Analysis Soft-chemistry reaction of the system, mainly accelera-ware (Model 270) were used for the analysis ofing deoxidization reaction of formamidine disulfideelectrochemical experimental results.T2U2，enhancing stability of thiourea, increasingvalid thiourea concentration in the solution. At the .3 RUSULTS AND DISCUSSIONsame time, catalysis could accelerate dissolvation3.1 Voltammogram of gold electrodeof gold8-0]. In the process of gold leaching， dy-In thiourea solution the half cell reaction fornamic parameters, such as concentration of thiou-rea，sulfurous acid and temperature, could influ-gold dissolution in acid thiourea solutions was giv-ence the leaching speed of gold. In this paper,en by Kazakov as :Au+ 2CS(NH2)--→Au (CS(NH2))2+2ethree techniques as potentiodynamic polarization ,chronopotentiometry and Tafel plot for measure-ment of corrosion rate of metals, were introducedφθ=0.107 V(vs SCE )to study the electrochemistry dynamics action ofFig.1 shows the voltammogram curves of goldthiourea-sulfurous acid system， which provideselectrode in thiourea solution when the concentra-theoretic base for right condition of gold leachingtion of sulfur dioxide is from 0. 5X 10-6 mol/L toby thiourea.1.0X 10-5 mol/L. Comparison of curves 1，2，3and 4 shows that with the increase of sulfur dioxide2 EXPERIMENTALconcentration，the anodic current density increa-ses. When the potential is in the range of 0. 10The purity of the gold used in this study was0.40 V, the first anode peak appears at about 270.99.9%. Two kinds of electrolyte were prepared u-mV._ Iudeed from_ the thermodynamics equilibri-sing analytic grade KCl and Naz B:Or with deion-um.中国煤化工(1), where the goldized water of resistivity more than 6. 0X1052●m.is oYHCNMH GThe electrochemical experiments were carried outD Foundation item: Project(20047) supported by the Foundation of National Excellent Doctoral Dissertation of ChinaReceived date: 2003 -02 - 15; Accepted date: 2003 -04 -06Correspondelos第NG Huai zhong, Tel: + 86-731-8877197; E -mail: hzlong@ hotmail. comTrans. Nonferrous Met. Soc. ChinaJun.20031.00.950.8 I0.60.80.4-0.5X 10-6 mol/L0.2-20X 10-6 mol/L0.7- 1.0X 10-6 molL0.1 0.20.30.50.67φ (VS SCE)/VFig. 1 Voltammogram curves of gold electrodeFig. 2 Potential vs time in response toin thiourea solution with different concentrationsgalvanostatic step for gold electrodeof sulfur dioxide at scan rate of(c(H2SO3)=5.0X10-6 mol/L;30 mV/s and pH=2. 0c(TU)=5.0X10-6 mol/L)3.2 Chronopotentiometry of gold electrodetential of reaction(1) can be calculated by the equa-In chronopotentiometry， the potentiostat ap-tion as: η=φ→Po. Fig. 3 shows the resulting curveplies a constant current for a specified duration andof overpotential η vs time in response to 0.8 mA/monitors the resulting potential, which can be usedcm2 galvanostatic step， it gives the plot of overpo-for many different types of analyses or investigate-tential (》) vs lg[1- (t/t)°5].ing electrode kineticl11,12]. The half cell reactionfor gold dissolution in acid thiourea solutions wasgiven as reaction (1). For this reaction, the fol-0.50lowing suppositions are given as:1) the electrochemigration and convection are0.45negligible;2) the variation of TU concentration is caused bydiffusion of TU ion. .0.35The relationship between overpotential of re-action (1) and the reaction time can be described0.30by= 2.303RTIg(二)-0.25-3.5-2.5-1.5-0.5βnFIg[1-(1τ )0.s]2. 303RT1g[1-(与)ns](2)Fig. 3 Relationship between overpotential andwhere η is the overpctential of reaction (1)，T islg[1- (t/r)9.5] of gold electrodetemperature, Ja is the galvanostatic current densi-ty，J。is the exchange current density， τ is theThen the relationship between overpotential ηtransition time, t is the time of reaction，β is theand lg[1- (t/t)0.5] can be expressed bytransmission coefficient.η= 0.267一0.062 lg[1- (t/t)°.5](3)Fig. 2 shows the resulting curve of chronopo-Eqn.(3) is the electrochemical dynamics equa-tentiometry for a gold electrode in thiourea solu-tion (5X 10 -6 mol/L) when the concentration oftion of reaction(1). Because the values of R，T,sulfur dioxide is 5X 10-6 mol/L. The plot of poF，J。，τ are all fixed, the electrochemical kineticsparameters can be calculated by Eqns.(2) and (3)tential vs time is given in response to 0. 8 mA/ cm2galvanostatic step for electrode. From Fig. 2，weas:477 From_ Fqn.(3)， when the re-can get the value of the transition time(t)， whichacti中国煤化Iinitial overpotential ofis about 6. 9 s. The thermodynamic potential(qo )reac:MYHCNM HGof reaction (1) is calculated by Nernst equilibrium.When the concentration of TU is 5X 10-6 mol/L,3.3 Tafel plots of gold electrode in thiourea solu-tionφo=0.448 V(vs SCE ).From Fig. 2, the electrode potential(p) can beThe Tafel plots of gold electrode in thioureaobtained死互黎据ne from 0 to 6.9 s. The overpo-solution are shown in Fig. 4. The correspond cor-Vol.13 No. 3Dynamics of anodic process of gold electrode in thiourea solutionrosion current density J is listed in Table 1, show-ing that the concentration of H2SO3 have an obvi-1.8 |ous effect on the corrosion of gold electrode. As1.6H2 SO3 concentration is enhanced, the current den-sity will increase. Based on electrochemistry theo-.4y, we know that.2 tJor=K●c"(4)1.0where K is a constant，c is the concentration ofH2SO3，n represents the grade of reaction. As0.8shown in Fig.5， a straight line can be gotten,画0.6which shows that reaction is a one- grade reaction.0.4 tTable 1 Corrosion current density of gold-6.4-5.6-5.2 -4.8electrode in sulfur dioxide solutionlg[/(molL-1)]c(H2SO.)/(10-* mol. L-I) Joon/(10~ °A. cm-2)0.52.51Fig.5 Relationship between lgc and lgJ corr of2.09. 59gold electrode in thiourea solution5.022. 5110.043. 850.240.180.060.200.16 ! ?0.12|-0.060.08了40.04-0.129-8-7-6-5-4-3-2-10Ig[J/(A.cm-2)]-0.04-0.08-8 -7--4-3-2Fig.6 Tafel plots of gold electrode in thioureaand H2 SO3 solution at different temperatures1- -298K; 2- 303K; 3- -313K; 4- -323 KFig.4 Tafel plots of gold electrode inTable 2 Corrosion current density ofthiourea solution with different concentrationsgold electrode at different temperaturesof sulfur dioxideT/KJ./(10-6 A *cm-2)1-0.5X10-6 mol/L; 2- -2.0X10-' mol/L;3-5.0X10 -6 mol/L; 4一1.0X107 5 mol/L2983033. 49The Tafel plots of gold electrode under differ-:nt temperatures in H2SO3 (5X 10-“mol/L) and3136. 45thiourea(5 X 10-6 mol/L) solution are shown in32310.94.Fig.6. It can be seen that in a certain temperaturerange, the higher the temperature is, the faster thegold electrode oxidation. When the temperature isship中国煤化工-equation， the relation-E，corrosion currenthigher than 323 K，this anode process will be in-dens:MYHCNMHGcanbedescribedashibited. The corrosion current density of gold elec-trode at different temperatures is listed in Table 2.In Jor= K。E.1Using Table 2 as a data source, we plot lnJcorr aswhere K。 is a constant， R is Prank constant， Efunction of T- 1，and get a straight line as shown inrepresents activity energy，T stands for absoluteFig. 7. T序方数据of the line is一5. 55.temperature.Trans. Nonferrous Met. Soc. ChinaJun.2003ourea solutions [J]. Minerals Engineering， 2000, 132.(10):1083 - 1096.2.4[3] CHAI Li-yuan, Okido M. Dissolution theory of gold in2.2alkaline thiourea solution (I) anodice behavior on gold in al-kaline thiourea solution containing Na:SO3 [J]. Trans2.0Nonferrous Met Soc China, 1999, 9 (1) : 145- 151.[4] LI Ding xin. Extraction and Refining of Costly Metals[M]. Changsha: Central South University of Technol-ogy Press, 1989. (in Chinese)E 1.2[5] van Deventer J s，Reuter M A，Lorenzen L，et al.1.0Galvanic interactions during the dissolution of gold in0.3.05 3.10 3.15 3.20 3.2 53.30 3.35 3.40cyanide and thiourea solutions[J]. Minerals Engineer-ing，1990， 3 (6): 589 - 597.T-1/10-3K-1[6] Salinas S A，Romero M A E, Gonzalez I. Electro-chemical dissolution of calaverite( AuTer) in thioureaFig. 7 Relationship between T1 and In Jorracidic solutions[J]. Journal of Applied Electrochemis-try, 1998， 28<4): 417- 422.[7] Carlos M J, Dutra AchillesJ B. Investigation on goldThe slope of the lineis - 5. 55，then the activ-electrowinning from acidic thiourea solutions Authority energy can be calculated as 46. 11 kJ/mol.Affiliation[ A]. TMS Annual Meeting[C]. TMS Min-erals, Metals &. Materials Scc, 1997. 231 - 245.4 CONCLUSIONS[8] Wojciech C, Jolanta M. Use of thiourea dioxide as re-ducing agent in the application of sulphur dyes [J].1) When the electrode potential is higher thanDyes and Pigments，1994， 26 (2): 77-81.[9] Rabai G, Wang R T, Kustin K. Kinetics and mecha-0.270 V, the gold is oxidized by thiourea.nism of the oxidation of thiourea by chlorine dioxide2) The electrochemical dynamics equation of[J]. International Journal of Chemical Kinetics, 1993，oxidation of gold is ascertained as η= 0. 267-25(1): 53 -62.0.062 lg[1- (t/t)°.5] . The electrochemical kinet-[10] LIJ， Millr J D. Reaction kinetics for gold dissolu-ics parameters are calculated as n=2，β=0. 477.tion in acid thiourea solution using formamidine disul-When the reaction time (t) is zero, the initial over .fide as oxidant[J]. Hydrometallurgy, 2002, 63 (3):potential of reaction(1) is 0. 267 V.215 - 223.3) The addition of H2SO3 can obviously im-[11] QIN Wei-qing， QIU Guan-zhou， HU Yue-hua. Ki-netics of electrochemical process of galena electrode inprove the corrosion speed of gold.diethyldithiocarbamate solution[J]. Trans Nonfer-rous Met Soc China, 2001，11(4): 587- 590.REFERENCES[12] QIN Wen-qing，QIU Guan- zhou. Dynamics of elec-trodeposition of tetraethylthioram disulfide on pyrite1] Jeffrey M I, Breuer P L, Choo W L. A kinetic studysurface[J]. Journal of Central South University ofthat compares the leaching of gold in the cyanide, thio-Technology, 2001, 8(3): 164- 168.sulfate. and chloride systems[J]. Metall Mater Trans(Edited by YUAN Sai-qian)B, 2001, 32(6): 979 - 986.[2] Juarez C M, Dutra A J. Gold electrowinning from thi-中国煤化工MHCNMHG