Electrode process of diethyldithiocarbamate on surface of pyrrhotite

Article ID : 1005 - 9784( 2005 )04 -0416 -04Electrode process of diethyldithiocarbamate on surface of pyrrhotite'LI Wei-zhong(黎维中) , QIN Wen-qing( 覃文庆) , QIU Guan-zhou( 邱冠周) , DONG Qing-hai(董清海尸( 1. School of Resources Processing and Bioengineering ,Central South University , Changsha 410083 , China ;2. General Research Institute for Nonferrous Metals , Beijing 100088 , China )A bstract : The electrode process of diethyldithiocarbamate on the surface of pyrrhotite was studied using systematic elec-trochemical analysis , including cyclic voltammetry , chronopotentiometry and galvanostatic. Experimental results show thattetraethylthioram disulphid( TETD ) is electrodeposited on pyrthotite electrode surface in the presence of 1.0x10-* mol/Ldiethyldithiocarbamate when the electrode potential is higher than 0.25 V. The electrochemical kinetics parameters of the e-lectrode process of diethyldithiocarbamate on surface of pyrrhotite are calculated as follows : the exchange current density is2. 48 μA/cm2 , and the transmission coeffcient is 0. 46. The electrodeposition includes two steps electrochemical reaction.The first reaction is electrochemical adsorption of diethyldithiocarbamate ion , then the adsorbed ion associates with a dieth-yldithiocarbamate ion from the solution and forms tetraethylthioram disulphide on the surface of prhotite.Key words : electrode process ; prhotite ; diethyldithiocarbamateCLC number : TD923Document code : A1 INTRODUCTIONwere reported9. HU et al471 considered that the sur-face reaction product of DDTC on galena was lead di-In the past decade , much progress has been madeethyldithiocarbamate ( PbD2 ) and the reaction ofin understanding the reactions of a sulfide mineral sur-DDTC on galena was characterized by fast rate , strongface with xanthate agents. Many electrochemical tech-adsorption and high critical pH value. And the rela-niques have been employed to study the reaction mech-tionship between the coverage of adsorbed products andanism of sulfide minerals ( such as pyrite , galena andinterfacial resistance and the electrode potential waschalcopyrite ) with xanthate reagents 7. These inves-derived mathematically. Bhaskar and Forsling "0 J stud-tigations indicate that the oxidation of both the mineralied the adsorption of DDTC on covellite , cuprite andand the collector plays an important role in the flotationtenorite. Depending on the concentration of DDTCprocess. It is generally believed that the reactions pro-two types of copper complexes ,i.e. [ Cu( DTC )]' atduce the hydrophobic particle surfaces required in flo-low concentration and Cu( DTC )2 at higher concentra-tation. It is important in flotation to identify the differ-ent anodic reactions which are coupled to a cathodiction , are noted on copper( II ) substrates. The electrodeprocess that is generally represented by the reduction ofprocess of pyrite in diethyldithiocarbamate solution aloxygen'56]. Since the anodic reaction gives rise to thepH 11.4 was investigted.hydrophobic character of the mineral surface , a clearThe electrochemical reaction on the surface of py-relationship between the potential and flotation recoveryrite during flotation has been studied well , but thereexists in various flotation systems.are only a few references available on the kinetics andDiethyldithiocarbamate( DDTC ) is one of the mostmechanism of the chemical oxidation reaction on sur-frequently used collector for flotation of heavy-metalface of pyrrhotite 12131.sulfide minerals , such as galena , chalcopyrite , jame-It is important and essential to determine the ki-sonite etc，and it shows strong selectivity. In a pre-netics parameters of electrode process of DDTC on sul-vious paper , DDTC was shown to be used in highly al-fide mineral surface from both scientific and practicalkaline( pH > 11.4 ) to separate galena from sphaler-point of view. To improve the efficiency of DDTC useite , pyrrhotite and pyrite8. It was showed that it wasin flotation and to take effective steps for depressinga powerful collector for galena and very selective a-gainst pyrite or pyrrhotite in high alkaline conditions.pyrrhotite in DDTC system ,it is essential to understandthe. decomposition of DDTCThe oxidation and decomposition mechanisms of dieth-on I中国煤化工beifically , to calculateyldithiocarbamate on different sulfide minerals in thepresence of commonly used oxidants such as oxygen ,:TYHC N M H Gameters of the electrodechlorite , hypo-chlorite and iodine at different pH levelsprocess of diethyldithiocarbamate on surface of pyrrho-①Foundation item : Prjec( 50204013 )supported by the National Natural Science Foundation of ChinaReceived date : 2005 -02 -28 ; Accepted date :2005 -04 -20Corresponden&ttIN Wen-qing , Prfessor ;Tel : + 86-731 8830346 ; E-mail :QWQ@ mail. csu. edu. cnLI Wei-zhong , et al : Electrode process of diethyldithiocarbamate on surface of pyrrhotitetite and establish the kinetics equilibrium.electrode is shown in Fig. 1. The electrode potentialstarts from -0.9 V to0.7 V in the positive direction.2 EXPERIMENTALThe first anodic shoulder is close to the peak at -0.4V ,which is assigned to the reaction( 1 ). From Fig.The pyrrhotite used in this experiment consisted of1 ,it can be seen that the current density increases ab-57.2%Fe,38.6%S,0.4%Pb,3.1%SiO2(massruptly when the potential is higher than 0 V. It there-fraction ) and was from Dachang Mine of Guangxi Prov-fore provides evidence that the second anodic peak isince in China. The crystal form of the pyrrhotite wasclose to 0.2 V , and it is assigned to the oxidation ofmonoclinic. Sections cut out from the highly mineral-DDTC on pyrrhotite surface as reaction( 6 ) that prized pyrrhotite were fashioned into form of electrodesduces tetraethylthioram disulphide( TETD ). Fig. 1 alsofor electrochemical measurements. The cut out sectionshows that TETD can be reduced at negative potentialof mineral was mounted on the tip of a perspex tubuleof about -0.4 V.of 7 mm in diameter using epoxy resin and the exposedouter surface was well polished. The exposed surface8.8area of the electrode was about 1 cm- 。The referenceandauxiliary .electrodes were saturated calomelelectrode( SCE ) and graphite rods , respectively. All4.4potentials in this study are quoted in volts , with respectto a standard hydrogen electrode( SHE ).An electronics potentiostat Model 273A and Model636 electrode rotator were used in all electrochemicalmeasurement. The EG&G corrosion measurement sys--4.4tem ( Princeton Applied Research Model 352 ) and a--0.6 -0.20.20.61.0nalysis software( Model 270 ) were used for the analysisE(vs SHE)/Vof electrochemical experimental results. The electro-chemical experiments were carried out in a three-com-Fig.1 Cyclic voltammetry curve of pyrrhotitepartment cell，including a graphite counter electrodeelectrode in presence of diethyldithiocarbamatepyrite electrode and saturated calomel reference elec-( ( DDTC)=1x10-4 mol/L ,pH=9.18 ,25°C )trode , assumed to have a potential of 0. 245 V( vsSHE ). The studies were carried out using the pyrrho-3.2 Chronopotentiometry of pyrrhotite electrodetite as electrode in 0.1 mol/L Na2B O, solution in theIn chronopotentiometry , the potentiostat applies apresence of diethyldithiocarbamate.constant current for a specified duration and monitorsthe resulting potential , which can be used to investi-3 RESULTS AND DISCUSSIONgate electrode kinetics. The oxidation of diethyldithio-carbamate ion on pyrrhotite surface is an irreversibleIn DDTC solution , the anodic reactions occurringreaction with the following assumptions.on pyrrhotite surface may take place as follows :1 ) The electromigration and convection of diethyl-Fe2+ +3H20-→Fe( 0H); +3H* +e(1)dithiocarbamate are neglected.1/16Fe,S. +H20-→5/16Fe( OH ) +2 ) The variations of concentration of diethyldithio-1/8Fe( OH )+1/2s0 +H+ +e(2)carbamate are caused by the diffusion of diethyldithio-1/21Fe,S. +H20一+1/3Fe( 0H); +2/7S0 +H+ +ecarbamate ion.(3)According to the theory of electrochemical , the re-1/32Fe,Sg +7/8H20- +5/32Fe( 0H ) +lationship between the overpotential( η ) of the anodic1/16F( OH ) +1/8S,0- +5/4H+ +e (4)reaction and the reaction time( t ) can be expressed as :1/37Fe,S。+33/37H20一→7/37Fe( OH ); +η=[ 2.303RT( nβF )]lg( J./Jo) -[ 2.303RT/( nβF )]s[ 1-(1/T )"2] (8)4/37S205- +45/37H* +e(5)where T is the temperature , J。is the galvanostatic2DDTC-- >TETD +2e(6)current density ,Jo is the exchange current density , τIn solution , the concurrent cathodic reduction ois th_transmission cofficient ,oxygen would occur on the surface of pyrrhotite elec-Fi中国煤化工trode :1/202 +H20 +2e-- -20H(7)TYHC N M H Gential versus time in re-sponse to 200 μA/ cm2 galvanostatic step for electrode.3.1 Cyclic VoltammetryFrom Fig.2 ,we can get the value of the transition timeThe Cyclic voltammetry curve of the pyrrhotite( τ ) ,which is about 5.6 s. The thermodynamie， 418.Journal CSUT Vol.12 No.4 2005potential( E。) of reaction( 6 ) can be calculated by(8 )and(9 )as follows :Jo =2.48 μA/cm2 ,n=2 ,βNernst Equilibrium while the concentration of DDTC is .=0.462. From Eqn.(9 ) , when the reaction time( t )0| 0-mol/L.E。=is zero ,the initial overpotential of oxidation of DDTC is-0.015 - 0.059lg( DDTC )=0.211 V.0.122 V. The results demonstrate that it is easy forDDTC adsorbed on pyrrhotite surface to be oxidized.So the pyrrhotite can be floated well in DDTC solution-0.2 Fat pH9. 18. This result of electrochemical experimentis accordant with the result of flotation of pyrrhotite in-0.1DDTC solution.3.3 Galvanostatic experimentforpyrrhotite0.1electrodeWhile the potentiostat applies a constant current0.2 Con the pyrrhotite electrode for a specified duration , the-0.50.5 1.5overall consumed charge( Q ) can be calculated by thet/sfollowing equation :Fig.2 Relationship between potential and timeQ=Q。+Q.( 10)in response togalvanostatic step forwhere Q。 is the charge consumed by the diffusion ofpyrhotite electrodediethyldithiocarbamate ion at the surface of pyrrhotite( d( DDTC)=1x10- mol/L ,pH=9.18 ,25 C )Q。is the charge consumed by the electrochemical ad-sorption of diethyldithiocarbamate ion.From Fig.2 , the electrode potential( E ) can beAccording to the Sand formula ,obtained at anytime from 0 to 5.6 s. The overpotentialQ=Q。+n2 FP2 πDCj/4J(11 )of reaction( 6 ) can be calculated by the following equa-where n is the number of electron of reaction , D istionthe diffusion cofficient of diethyldithiocarbamate ion,η=Eo-EnC。is the initial concentration of diethyldithiocarbamateFig. 3 shows the curve of overpotential versus lg[ 1ion ,J is the current density.-( t/τ )'2 ] in response to 200 μA/ cm2 galvanostaticIf the electrochemical adsorption of diethyldithio-step. So the relationship between the η and lg[ 1 -( t/carbamate ion on the surface of pyrrhotite did not oC-τ )" ]can be got as follows :cur , the charge consumed by the electrochemical ad-η=0.122 -0.0641[ 1 -( t/T)'2 ](9)sorption of diethyldithiocarbamate ion( Q。) would bezero.The value of Q can be verified using galvanostatie0.30technique. Table 1 lists the results of galvanostatic ex-0.26periments for a pyrrhotite electrode at different currentdensities. And the value of Q can be calculated by the .0.220.18Q=Jτ0.14Table 1 Results of galvanostatic experiments0.10 ITransition Chronocoulometry0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.01.(mA cmemr." mA-1)time/s(μCcm-2)lg[1-(/t)|2]0.205.005.601 1200.254.003.74935Fig.3 Relationship between overpotential of3.332.70809pyrrhotite electrode and l[ 1 -( l/τ )2 ]0. 352.862.03710中国煤化工，60640It is the electrochemical dynamics equation of theoxidation of DDTC on pyrrhotite surface. Because theMYHCNMHGThe Q value is linearly related to 1/J. Based on .values of R ,T ,F ,J。,T are certain , so the electro-the results in Table 1 , the plot of the consumed chargechemical kinetics parameters are calculated from Eqns.Q as a function of the reciprocal of current density( 1/LI Wei-zhong , et al : Electrode process of diethyldithiocarbamate on surface of pyrrhotite， 419.J )is shown in Fig. 4. 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