336 Chiang Mai J. Sci. 2012; 39(2)

Chiang Mai J. Sci. 2012; 39(2) : 336-345http://it.science.cmu.ac.th/ejournal/Contributed Paper

Electrochemistry of Pentlandite in the Absence andPresence of Sodium SulfideJie Deng, Qiming Feng, Leming Ou, Guofan Zhang, Yiping Lu and Kun Liu*School of Minerals processing and Bioengineering, Central South University, Changsha, 410083, China.*Author for correspondence; e-mail: kliu@csu.edu.cn

Received: 21 October 2010Accepted: 3 July 2011

ABSTRACTThe collectorless flotation contains self-induced flotation and sodium sulfide-induced

flotation. In this paper, we studied the electrochemical performance of pentlandite in bothcircumstance. In moderate oxidation conditions (viz. 0 mV<Eh<500 mV vs. SHE), pentlanditewas oxidized spontaneously with the main oxidation product of S0 on the surface, and thehighest amount of the S0 could be found at about 300 mV. With the potential of higher than500 mV, however, the S0 on the surface was oxidized to sulfate which resulted in the hydrophilicsurface. Sodium sulfide would increase the hydrophobicity of pentlandite by forming moreS0 on the mineral surface. The extra S0 might come from the oxidation of hydrogen sulfideions by the ferric ions which dissolved from the mineral during the oxidation process. Theseresults show that the collectorless flotation of pentlandite would be achieved in the presenceof sodium sulfide with the Eh of 300 - 500 mV.

Keywords: pentlandite, sodium sulfide, electrochemistry.

1. INTRODUCTIONPentlandite, the chief source of nickel, is

usually separated from gangues by flotation.With the persistent decrease of rich nickelsources, the utilization of low-grade ore isurgent. Unfortunately, it’s hard to process thelow-grade ore by current flotation technology.As a result, more attention should be paid tothe characteristic that influence the flotabilityof sulfide minerals, and development ofnovel technologies to make full use of mineralresource.

It is well known that the flotability ofsulfide mineral is depressed by excessiveoxidation. Former researchers indicate thatsodium sulfide (Na2S) would improve the

flotability of many minerals. This processcalled as “sulfidization process” is used mostlyin the flotation of oxidized sulfide mineraland oxide mineral. Sodium sulfide has beenused to improve the flotability of copperoxide [1-3] since 1970’s. Some years later,Yoon [4] found that the sulfide minerals treatedwith sodium sulfide can be floated withoutcollector. Trahar [5] found in oxygen-freesystem the chalcopyrite was not floatable,which pointed out the importance of oxygenin chalcopyrite flotation. Luttrell [6, 7] showedthat the sodium sulfide was effective onlyunder oxidizing conditions, in addition, thesodium sulfide could remove the hydrophilic

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oxidation products. Researchers [8, 9] alsorealized that the sulfidization process couldonly be achieved under proper potentials.These researches displayed that the pulppotential played an important role in thesulfidization process, and the Na2S-induced-lk flotation would be carried out if the pulppotential was controlled in a certain range andsomething beneficial had been done [10].

Recently, Newell [11, 12] successfullyuses the sulfidization process to restore theflotability of one kind of oxidized copper-nickel sulfide by controlling the potential in aproper range. XPS analyses showed that aftersulfidization process the oxidized sulfideminerals surface was converted and thencomposed by the metal-deficient sulfide andpolysulfides or elemental sulfur. They alsofound the addition of base-metal ionsespecially ferric during sulfidization improvedthe flotation recovery of pentlandite.

The sulfidization process would bestrongly affected by the dosage of sodiumsulfide, time of conditioning, procedures ofmixing and other variables [13], whichsuggested that the sulfidization mechanismshould be recruited. Further, it is well knownthat the flotation process of sulfide mineralsis an electrochemical process, as a result it issignificant to study the sulfidization processby electrochemical method.

In the present work, the electrochemicalbehavior of pentlandite was evaluated usingCV and EIS. X-ray diffraction (XRD) wasused to analyze the pentlandite electrodesurface composition. The combanation ofthese methods can provide useful physicaland chemical information at the electrode/electrolyte (sulphide/culture medium)interface in the presence and absence of Na2S.

2. MATERIALS AND METHODSThe pentlandite used in this study was

obtained from Yunnan, China. Analytical work

on the samples indicated the the presenceof some pyrrhotite and pyrite, and thenickel grade was 28.35%. -lk The workingelectrode was made by carbon pasteelectrode (CPE), it was composed bygraphite (10%, SP), paraffin (10%) andpentlandite (80%) (< 0.074 mm). Freshsurface was created for each experiment bypolishing (using 800-grit silicon carbidepaper), treating with ultrasonic, rinsing theelectrode, drying in air and immediatelybeing inserted into electrochemical cell.Working electrode surface was 1 cm2 andthis value was used in calculation of currentdensity.

The electrochemical setup involved astandard 3-electrode cell using the CPE as theworking electrode, saturated Ag/AgClelectrode (197 mV vs. SHE) as the referenceelectrode and two graphite rods as the counterelectrodes. The reference electrode wasimmersed in a Luggin capillary filled withsaturated KCl, which was placed as close aspossible to the working electrode to minimizeohmic resistance. All potentials in this workrefer to the SHE scale. All the experimentswere conducted in buffer solution of pH 9.2(0.05 M Na2B4O7⋅10H2O). Measurementswere carried out with a Potentiostat/Galvanostat Model 283 (Princeton AppliedResearch, USA).

Cyclic voltammetry experiments werecarried out between potentials of -500 - 650 mVat a scan rate of 20 mV/s. The polarizationof the electrode was started from its restpotential and continued in the cathodicdirection up to the negative vertex potentialand the potential scan was then reversed inthe anodic direction and continued until itreached the positive vertex potential. This wasfollowed by a reversed potential scan to get acomplete cycle. For analytical purposes, theinitial scan between the rest potential and thenegative vertex potential was discarded

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because the current response in this regioncould be dominated by the reduction ofthe species produced during the polishingof the electrode [14]. EIS measurementswere performed potentiostatically in thefrequency range from 100 KHz to 0.5 mHzusing frequency response detector 1,025and M283, Potentiostat/Galvanostat withan AC voltage amplitude of ± 5 mV. Beforeeach experiment the electrode was polarizedat certain potential for 10 min.

The identification of polarizationproducts was conducted by the XRD analysis.The electrode was firstly polarized at certainpotential for 90 min and then transferedimmediately to perform the XRD analysiswhich was performed using an X-rayDiffractometer (Bruker D8 Advance) in therange of 2θ from 10o to 80o with Cu Kαradiation.

3. RESULTS AND DISCUSSION3.1 Cyclic Voltammetry

Figure 1 shows the cyclic voltammetryresult of pentlandite. On the anodic scan, twooxidation current peaks (A1 and A2) can benoted, on the cathodic scan, one prominentcathodic current peak (C1) is present. The peakA1 starts at about 0 mV and continues in awide potential range up to about 300 mV.This peak is attributed to the followingelectrochemical reaction:

(1)

where S0 could also be the metal deficient layer,and this sulfur-like layer makes the pentlanditehydrophobic [15, 16], whether the surfaceproduct is element sulfur or metal deficientlayer, which is decided by the extent ofoxidation. When the mineral starts to beoxidized, the metal ions dissolved from the

Figure 1. Cyclic voltammogram(20 mV⋅s-1) in a borate buffer solution of pH 9.2 atdifferent positive vertex potential for pentlandite.

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mineral surface to the solution, hence, thesurface is left as a metal deficient layer, withthe increasing of oxidation extent, all themetal ions of the outer side are dissolved,left an elemental sulfur entity; in the innerside of this sulfur layer, there is still a metaldeficient layer, which is also confirmed byCruz [17] and Buckley [18]. Because of thisreason, the formed sulfur is regarded as theformer skeleton of the mineral. In thealkaline solution, the Fe3+ and Ni2+

dissolved from the mineral would bechanged into hydroxides immediately andprecipitate on the pentlandite surface. Theseprecipitates, however, will be removedfrom pentlandite surface by strong stir inthe flotation process, which leads tochangeless recovery of pentlandite althoughwith the presence of these hydrophilic ferrichydroxides. The voltammogram of pentlanditereported by Kahn [14] did not indicate a peakin this region. In their work, there was onlyone anodic current peak appeared and the

commence potential was higher than 500mV. This was a surprising phenomenon butno further discussion was conducted. Inour experiment, an anodic current shoulder(A2) appears at about 600 mV, which isconsidered to involve the fomation ofsulfate and metal hydroxide. The overallreaction shows as follows:

(2)

The cathodic current peak C1 can onlybe noted when the anodic sweep potential ishigher than the commence potential of A1,and the current density increases significantlywith the increase of anodic vertex potential.According to the thermodynamic calculation,C1 corresponds to the reduction of S0 whichformed after A1.

The cyclic voltammograms of pentlanditeobtained in different concentration of Na2Sbuffered at pH 9.2 are shown in Figure 2.These it clearly show that no additional peak

Figure 2. Cyclic voltammogram (20mV⋅s-1) in a borate buffer solution of pH 9.2 atdifferent sodium sulfide dosage for pentlandite.

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appears in the presence of Na2S, but themaximum current density of A1 decreaseswith the increasing of the concentration ofNa2S. The Na2S does not influence thecommence potential of A1.

The present study indicates that in thepresence of Na2S, the current flow corres-ponding to the oxidation of pentlanditesurface is somewhat inhibited. These effectsare evidently caused by the presence of Na2S.If it is caused in an electrochemical way, thecurrent density in the voltammogram shouldbe higher than that of without Na2S, becausemore reaction proceeded at the interface. Butin this case, the current flow is lower than thatof without Na2S. Considering the strongoxidizing capability of Fe3+ released from theoxidation of pentlandite, the Na2S can beoxidized to elemental sulfur and precipitateon the surface of pentlandite, increase theresistance of the interface, which explains thereduction of current density, enhancement offlotability. This reaction will happen only whenthe Fe3+ is released from the electrode. This isconfirmed by the changing in Figure 2. Beforethe commence potential of A1, few Fe3+ isreleased to the solution resulting inundetectable changes of current density byCV. When the potential reaches the initialpotential of A1, the Fe3+ starts to dissolve intothe solution preferentially [19]. As well thesodium sulfide is oxidized to elemental sulfurand precipitate on the electrode, whichincreases the resistance ofthe interface and lowers the current density.It reacts as:

2Fe3+ + HS − + OH − → 2Fe2+ S 0 + H2O (3)

In our dosage of Na2S, we detect thatthe higher concentration of Na2S results in thelower current density, which probably becauseof the low content of Na2S relative to thedissolved Fe3+ content. Although the

oxidation process includes charge transferbut the Fe3+ can contact the Na2S only whenit is diffuse to the outer face of the electrode,the following reaction can not be detectedby CV.

It also can be seen that some small peaks(A3) appear at about 0 mV, and the commencepotential shift positively with the increasingof Na2S concentration. By thermodynamiccaculation, these peaks are attributed to theoxidation of H2S, it reacts as follows:

H2S → S0 + 2H +

+ 2e − (4)

3.2 Electrochemical Impedance Spectro-scopy

The surface layer formed during polari-zation would affect the impedance. So, by theEIS spectra we can identify the informationthat formed on the surface of electrode. Atypical Nyquist plot recorded for pentlanditeelectrode at different applied potential inweakly alkaline is shown in Figure 3, it can beseen that with the potential changing towardpositive potentials the semicircle increases.

When the potential is lower than 200 mV,there is one semicircle in the high frequencyrange and a line which is formed an anglenearly 45o with the real axis in the lowfrequency range. The semicircle of the Nyquistplot indicates that there is a kinetic limited stepoccurred on the surface of electrode. The linerpart of the Nyquist plot in the low frequencyregion is the typical feature of Warburgimpedance and represents the diffusion-limitedelectron-transfer process [20]. The factors,such as the roughness of electrode surface andthe asymmetry of current distri-bution couldresult in the formation of dispersion effect.In addition, the diffusing of electrolytes in theinner of the electrode might also be one ofthe reasons cause the line part of the EISspectra in the low frequency region.

When the applied potential is increased

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to above 300 mV, only one semicircleappears in the Nyquist plot. Comparingthe semicircle at 300 mV with that of at200 mV, it can be found that the ohmresistance on the surface of electrode isincreased unproportionately, and theWarburg impedance line disappeares.From the CV diagram, it suggests that thesulfur reaches its highest amount when thepotential is about 300 mV, which could bethe reason why the ohm resistance increases.

From the Nyquist plot we can see thatthe ohm resistance at 300 mV is almost 1.6times than that of 200 mV. This clearly showsthat more sulfur accumulate on the surfaceof electrode when the potential is up to 300mV. From this result, it can be conclude thatthe flotability of pentlandite would beenhanced when the potential is higher than200 mV. It also can be seen in this plot thatthe diffusing control step disappeares and turnsto be an electrochemical polarization processwhen the applied potential is above 300 mV.It was reported that the sulfur formed on

the electrode surface is porous [21], whichallows the dissolved metal ions to the bulksolution. This may be the reason why thediffusion control step disappears.

When sodium sulfide is present in theelectrolyte, the Nyquist plot is present inFigure 4. Comparing the Nyquist plot withand without sodium sulfide the impedancewhen the electrolyte with Na2S is higher thanthat without Na2S at the same applied potential,and no Warburg impedance can be identifiedin the plot.

As we discussed above, these phenomenonsuggest that plenty of porous sulfur depositon the electrode surface. At the same appliedpotential, it can be concluded that with theaddition of sodium sulfide more sulfur woulddeposit on the electrode, as well as extra sulfurmust come from the sodium sulfide, whichconfirms the previous test results.

For all the conditions the impedancespectra can be described by the same type ofequivalent circuit model, shown in Figure5. It contains a solution resistance (Rs) in

Figure 3. Nyquist plot of pentlandite in a borate buffer solution at different appliedpotential.

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series with the parallel network of chargetransfer resistance (Rp), and a constantphase element (CPE) substituted for thedouble layer capacitance, which is used tomodel the experimental data: ZCPE =

. The corresponding transferfunction of the total impedance, Ztotal, forthis circuit is given by:

where Q is the capacitance parameter, n is the

parameter which characterizes the deviationof the system from ideal capacitive (n = 1)behavior, ω is the angular frequency and j isthe imaginary constant. When it is used insteadof the double layer capacitance, the CPEcauses a rotation of the centre of thecapacitive semicircle below the real axes bya frequency independent constant phaseangle φ = (1 − n)π / 2. For a perfectcapacitor, φ = 0 (n = 1), and for a perfectresistor, φ = π / 2 (n = 0).

Figure 5. Schematic models for pentlandite electrode in borate buffer solution.

Figure 4. Nyquist plot of pentlandite in a borate buffer solution containing sodiumsulfide (4×10-4mol⋅L-1 )at different applied potential.

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3.3 Surface AnalysisThe XRD patterns of the pentlandite

electrode surface at different conditions arepresented in Figure 6. It is obvious that theoriginal surface of the electrode is composedby the pentlandite and graphite. After theelectrode was polarized at 300 mV for 90min in pH 9.2 buffer solution, the peaks ofpyrrhotite and elemental sulfur appears in theXRD dirgram, this implies that the pentlanditeis oxidized and the metal-deficient sulfide isformed, part of this sulfide is further oxidizedto elemental sulfur. When the electrode ispolarized in sodium sulfide solution, nopyrrhotite peak is noticed on the dirgram butthe peaks of elemental sulfur are moresignificant. If part of the sulfur generated bythe oxidize of pentlandite, according to thediscussion above, metal-deficiency sulfideshould have trace, but, there are only sulfurpeaks in the diagram, which confirms our

previous discussion, in sodium sulfidesolution the elemental sulfur is the mainhydrophbic product and it comes from thesodium sulfide in solution.

4. CONCLUSIONSThe electrochemistry property of pen-

tlandite was tested by the cyclic voltammentray,electrochemical impedance spectroscopyand XRD analysis, from which it could beconcluded that the pentlandite starts to oxidizeat 0 mV, and the hydrophobic product iselemental sulfur which reached the highestamount at about 300 mV. Adding sodiumsulfide is benificial for the self-flotation ofpentlandite because more elemental sulfurwould formed on the surface of mineral. Theferric ions, which played an important role inthe Na2S-induced flotation, were dissolvedfrom the oxidation of mineral and oxidizedthe sodium sulfide to elemental sulfur. This

Figure 6. The composition analysis of original pentlandite electrode surface (a),polarization product of electrode in pH 9.2 (b) and in the presence of Na2S solution (c).

√ - pentlandite, N - elemental sulfur, o - graphic, - pyrrhotite

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kind of sulfur would precipitate on thesurface of pentlandite, as a result, increasethe hydro-phobicity of pentlandite.

ACKNOWLEDGEMENTSThe authors are greatful to the funding

support of National Basic Research Programof China ( 973 Program ) No.2007CB613602,and this research was also supportedsupported by the Graduate degree thesisInnovation Foundation of Central SouthUniversity.

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