Effects of sulfurous acid on anodic process of gold electrode in thiourea solution

Vol. 10 Nu.2 June 2003J. CENT. SOUTH UNIV. TECHNOL.Article ID: 1005- 9784( 2003) 02-0126-04Effects of sulfurous acid on anodic process ofgold electrode in thiourea solutionLO)NG Huai-zhong(龙怀中) ,SHU Wangyin(舒万艮)(School of Chemistry and Chemical Engineering ,Central South University , Changsha 410083, China)Abstract: The electrochemistry behaviors of gold electrode in thiourea solution were studied by using electrochemi-cal techniques, such as potentiodynamic. voltammogram and current step. A catalytical electrochemical- reductionmechanisnl was proposed to0 identify the anodic oxidaion of gold in the thiourea solution. The results indicate thatthe decomposition of thiourea occurs when the electrode potential is higher than 640 mV. The addition of sulfurousacid presents a very positive effect on the anodic process of gold electrode. The anodic oxidation rate of gold is 5times faster than that without sulfurous acid. The passivity of gold electrode is attributed to the accumulation of ele-mental sulfur on the surface of gold. The sulfurous acid reacts with the oxidation product ((SC(NH)NH2)2) of thi-ourea, which can decrease the decomposition of thiourea and improves its stability.Key words: thiourea; gold; electrochemnistryCLC number: TQI50. 1Document code: A1 INTRODUCTIONis about 1 cm2. The electrochemical experimentswere carried out in a three-compartment cell, in-Cyanide is an important agent of the extrac-cluding a graphite counter electrode, gold electrodetion of gold in hydrometallurgy, but it has someand saturated calomel reference electrode ( SCE).harmful effects on environmentl-.2. It is an impor-The reference and auxiliary electrodes were satu-tant research topic to find an effective leaching goldrated calomel electrode and graphite rods, respec-reagent instead of cyanide. Thiourea was reportedtively. All potentials in this study are quoted inas a hopeful agent for gold dissolution [3.4)， be-volt, with respect to a saturated calomel referencecause it has many characteristics including toxin-electrode.free, high velocity of leaching and special selectivi-An electronics potentiostat model 273A wasty. At the present day, considerable research hasused in all electrochemical measurement. Thebeen directed towards the use of thiourea as an al-EG&G corrosion measurement system ( Princetonternative reagent to cyanide for gold leaching 5.6i.Applied Research Model 352 ) and analysis soft-The reason why thiourea cannot be put into largeware ( Model 270 ) were used for the analysis ofscale practice is its large consumption and not be-electrochemical results. During the experiments, aing recycled during leaching gold. It is regarded asnitrogen atmosphere was maintained over the elec -an important method in industry practice to reducetrolyte to keep oxygen free.consumption of thiourea efectively-9. In this pa-er，the electrochemical process of gold electrode3 RESULTS AND DISCUSSIONis studied by adding sulfurous acid into the thioure-a solution, and the interaction of the sulfurous acid3. 1 Electrochemical reactions on gold electrode inwith anode oxidation of gold electrode is discussed.thiourea solutionThe half cell reaction for gold dissolution in a-2 EXPERIMENTALcidic thiourea solutions was given by Kazakov asfollows.The purity of the gold electrode usediAu+2CS(NH2)-一+ Au(CS(NH2))片+e99.9%. The exposed surface area of the electrode?=0.352V中国煤化工Foundation item: The Special Fund for the Narional Exellet Doetoral :.MHCNMHGBlograpby of the lirst aothor: L0NG Huai-zhong, born in July 1970, majoring in applied chenistry.Recived date; Ot.18, 2002LONG Hui-zhong, etul: Effectof sulfurous acid o anodic process of gold electrode in thioureu solution127Thiourea itself is unstable under an appropri-addition of sulfurous acid improves the anodic oxiate oxidizing condition and can be oxidized. Thedation of gold in thiourea solution.oxidation products of thioutea depend on the oxi-dants, such as hydrogen peroxide, ferric iron. I5.0was reported that with ferric iron etc, thiourea de-4.0composed as followsf'].2SC(NH2)2一→+(SC(NH)NH2)2+2H+ +2e(2)potentiodynamicpolarization curvesshown in Fig. I were obtained in 0.05 mol/L thiou-1.0-rea(TU) solution for a freshly ground stationarygold electrode starts from 0 to 1. 20 V vs SCE.From Fig. I it can be seen that the first anodic0.20 0.40 0.60 0.80 1.00 T.20shoulder a is close to the peak at 270 mV, and an-φ(vs SCE)/Vother anodic peak b appears when the potential is640 mV. Judging from the thermodynamics equi-1-<(HSO3)=5.0X10-6 mol/L. c(TU)=0;librium, peak a is assigned to reaction (1) where2-c(H2 SO,)= 1.0X 10-s mol/L, c(TU)=0. 05 mol/L;gold is oxidized, and peak b is assigned to reaction3 -c(H2SO3)= 5.0X 10-' mol/I. c(TU)=0.05 mol/L.Fig. 2 Potentiodynamic polarization curves of(2) where thiourea is oxidized.gold electrode in thiourea solution in thepresence of H2SO3 at scan rate of0.3mV/s，pH2.0and298K3-g3. 3 Passivation of anodic dissolution of gold in thi-2ourea solutionThe voltammogram curves shown in Fig. 3尼1were obtained in thiourea solution when the con-centration of sulfurous acid ranges from 5.0X 10mol/L to 5.0X10-5 mol/I. Comparison of curves1, 2, 3, shows that the anodic current density in .150.20 0.400.600.801.00 T.20creases with increasing concentration of sulfurousφ (Vs SCEYVacid, also indicating the increase of dissolution ve-Fig. 1 Potentiodynamic polarization curves oflocity of gold. From Fig. 3, the production of a-nodic oxidation can represent a stable phase on thegold electrode in thiourea solution atelectrode surface forming a passive film， because298 K and scan rate of 0. 3mV/s, pHthe anodic current decreases when the electrode po-2. 0 and c(TU)=0. 05 mol/Ltential is 0.27-0. 36 V.3.2 Anodic oxidation of gold in thiourea solution3.4 Effect of sulfurous acid on anodic dissolution'heof goldshown in Fig. 2 were obtained in thiourea solutionThe voltammogram curves shown in Fig. 4with different sulfurous acid concentrations. Com-were obtained at different electrode potential scanpared with Fig. 1, the velocity of the anodic disso-rates in thiourea solution when the concentration oflution of gold becomes quicker， the first peak issulfurous acid is 5. 0X 10-6 mol/I. . The relation-close to the negative direction at about 250 mVship of the peak current density and the scan rate isshown in Table 1, where U, Jp stand for scan ratewhen the concentration of sulfurous acid is 1. 0X-ively. .10-' mol/1, and another anodic peak appears中国煤化工eofJ。/on decre-when the electrode potential is 450 mV during the|YHC N M H Ge，which indicatesanodic oxidation process of gold, showing that sul-that ueie 1 acurrespululung Irreversible chemicalfurous acid is oxidized. Fig. 2 also shows that thereaction.128Journal CSUT Vol. 10 No. 22003ses obviously. When the concentration is 1. 0 X10-5 mol/L, the dissolution rate is 5 times faster.1.0-]t indicates that the anode oxidation of gold can becatalyzed.2.4-0.6-~12.00.4-1.6-”0.2， 1.2-3|2|0.8-0.100.300.40φ(vs SCE)/V0.41-<(112S02)= 5.0X 10-* mol/L; .2 c(H2S0)= 1.0X 10- mol/I;00.10 0.200.40 0.503 (H;S0))=5. 0x 10~ mol/1.φ(vs SCE)VFig. 3 Voltammograms of gold electrode1-c(H2S0,)=0; 2- -r(H2SO,)=0.5X10。 mol/L;in 0.05 mol/l. thiourea solution with3 c(H2S0O3)=2. 0X 10~“no/L:;different concentrations of sulfurous4-c(H2SO,)=5.0X10。mol/L:acid at scan rate of 30mV/s and pH 2.05-(H2S04)一1. 0X 10~“mol/1.Fig.5 Voltammograms of gold electrode inTable 1 Relationship of peak current density5X10 6 mol/I. thiourea solution inand the scan ratethe presence of H2SO3 at scan ratew/《mV●s1)J/(10-6A.cm°2)Jp/ui2 .of8mV/s，pH2.0and298K1530883.32015712.43.5 Electrochemical Mechanism of anodic dissolu-2310577. 5tion of gold in thiourea solution in the presence3(3010.549.5of sulfurous acidWhile the potentiosiat applies a constant cur-.5-rent to the gold electrode for a specified duration ..0-the overall consumed charge (Q) can be calculatedby the equation; 2.Q=Q+Q.目2.0-where Q. is the charge consumed by the diffusion.5of SO; on the gold electrode surface, Qo is thecharge consumed by the electrochemical adsorption.0叶of SO;.stAccording to the Sand formula: .oQ=Qo+n2 F°rDc: /(4J)(4)where n is the number of electron of reaction, D is0.05 0.10 0.15~ 0.200.25 0.30 0.35 0.40 0.45 0.50the diffusion coefficient of SO;- ，Co is the initialφ(Vs SCE)/Vconcentration of SOg， F is the Farady constant, Jis the current density.1 30 mV/si 2- 16 mV/s; 3-8mV/s; 4 -3 mV/sFig. 4 Voltammograms of gold electrode infer reaction can be verified using galvanostaticthiourea solution at different scan ratestechnique 1011. Table 2 shows the results of gal-and c(TU) 0. 05 mol/L, c(H2SO3)vanostatic experiments for gold electrode in thiou-5.0X10~* mol/L. pH 2.0 and 298 Krea solution in the presence of sulfurous acid. Thecor中国煤化工5 mol/I. ,and sul-Fig.5 shows the voltammogram curves of goldur-The value of Qelectrode in thiourea solution. It shows that whenar I|YHC N M H Gstatic technique.the concentration of sulfurous acid is 5. 0X 10 - 6If the electrochemical adsorption of SOF~ ionmol/L. the anodic dissolution rate of gold increa-on gold surface could not occur, the charge con-L0NG Huai-zhong. etal: Effect of sulfurous acid on anodic process of gold electrode in thioureu solution29sumed by the eletrochemical adsorption of SO-ion (Qo) is zero. Based on the results in Table 2,the plot of the consumed charge Q as a function ofthe reciprocal of current density (1/J) is shown in一23.0+Fig.6, where Q is linearly related to 1/J. It isknown that the charge consumed by the electro-chemical adsorption of SO; ion (Q) is zero,e 2.5twhich means that there is no adsorption of sulfu-.0rous acid..sL25p/h1-<(HyS0h)=0; 2-e(HzS0)=5,0X10-5 mol/L20Fig. 7 Relationship between concentrationof thiourea and reaction time in the目1presence or absence of sulfurousacidatpH2.0REFERENCES5卜[1] Jeffrey M I, Breuer P L, Choo W L. A kinetic study0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18that compares the leaching of gold in the cyanide, thio-J-/(10*em2.A |)sulfate, and chloride systems J]. Process Metallurgyand Materials Processing Science ( Metallurgical andFig. 6 Relationship of current density andMaterials Transactions B) , 2001 ,32(6) :979-986.quantity of electricity in galvanostatic[2] Drok K. Ritchie I. 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