a review of pyrrhotite flotation chemistry in the processing of pgm ores

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Minerals Engineering 18 (2005) 855–865

A review of pyrrhotite flotation chemistry in the processingof PGM ores

J.D. Miller a,*, J. Li a, J.C. Davidtz b, F. Vos b

a Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112, USAb Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria 0002, South Africa

Received 20 January 2005; accepted 26 February 2005

Abstract

The chemistry of pyrrhotite flotation using xanthate collectors is reviewed with respect to the processing of PGM ores and therecent results from captive bubble contact angle measurements at the University of Utah are presented. In some cases a low flotationrecovery of PGMmay be due to the surface state of pyrrhotite particles under conventional flotation conditions (open to air and pH9.0).

Thermodynamically pyrrhotite is not stable and reacts relatively quickly with its environment. Natural/collectorless flotation ofpyrrhotite is observed only under a low oxidation potential in acidic solution. Its surface is easily oxidized to ferric hydroxide/oxideunder conventional flotation conditions, creating a hydrophilic state at the pyrrhotite surface and low flotation recovery eventhough xanthate collectors can be adsorbed. Under these conditions, activation by copper is not easily achieved. These observationsreported in the literature have been confirmed by captive bubble contact angle measurements. Based on the analysis of previousresearch, conditions for improved pyrrhotite flotation and increased PGM recovery are suggested.� 2005 Published by Elsevier Ltd.

Keywords: Pyrrhotite; PGM; Flotation; Xanthate; Oxidation; Activation

1. Introduction

Pyrrhotite is one of the most abundant iron sulfideminerals. It is found in nature to be commonly associ-ated with pentlandite, quartz, ankerite (CaFe(CO3)2),pyrite, chalcopyrite, and other sulfide minerals. In manyflotation plants, pyrrhotite is rejected to the flotationtailings as a waste product such as in the case of Cu–Ni ores and massive nickel ores (Fornasiero et al.,1995; Bozkurt et al., 1998; Chanturiya et al., in press;Khan and Kelebek, 2004). However, a strong interestin pyrrhotite recovery arises in certain instances. Specif-ically, the importance of pyrrhotite flotation is evident in

0892-6875/$ - see front matter � 2005 Published by Elsevier Ltd.doi:10.1016/j.mineng.2005.02.011

* Corresponding author. Tel.: +1 801 581 5160; fax: +1 801 5814937.

E-mail address: [email protected] (J.D. Miller).

the processing PGM ores from the Bushveld Complex inSouth Africa.

The Bushveld complex is the world�s largest source ofplatinum group minerals (PGM�s). Within this deposit,two important horizons occur known as the Merenskyreef and the UG2 reef which contain the valuablePGM minerals. The PGMs are mainly associated withbase metal sulfides and to a lesser extent with oxidesand silicate minerals. Typically the PGM ore containsabout 1% base metal sulfide minerals. The nature ofthe PGM association determines the overall flotationefficiency.

There are three primary base metal sulfides in theMerensky and UG2 reefs, namely pentlandite ((Fe,Ni)9S8), chalcopyrite (CuFeS2), pyrrhotite (Fe1�xS),with lesser amounts of pyrite (FeS2). The main sulfidemineral in the Merensky reef is found to be pyrrhotite,

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856 J.D. Miller et al. / Minerals Engineering 18 (2005) 855–865

and consequently it is important to achieve high pyrrho-tite recoveries during flotation.

In general, under conventional flotation conditionsopen to air at pH 9.0, good chalcopyrite and pentlanditerecoveries are obtained with thiol collectors. However,pyrrhotite recoveries are not always satisfactory evenunder conditions where xanthate adsorption is expectedto take place (Buswell et al., 2002a). This may be due tothe fact that significant pyrrhotite oxidation occurs dur-ing milling and the fact that pyrrhotite is a very poorcatalyst for oxygen reduction as may be required forthe electrochemical adsorption of xanthate (Buswelland Nicol, 2002b).

With more and more attention being given to pyrrho-tite flotation in order to improve PGM recovery (Gathjeand Simmons, 2004), a review of the flotation chemistryliterature is presented together with some recent contactangle measurements.

Fig. 1. Contact angle at pyrrhotite surface as a function of pH atambient temperature in 0.05 M Na2SO4 solution and in the absence ofcollector (University of Utah, 2004).

2. Natural or collectorless flotation

The natural floatability of pyrrhotite was investigatedby Hodgson and Agar (1984). These researchers sug-gested that a stable intermediate surface state,Fe(OH)S2, formed in acidic solution for short condition-ing times and at low oxidation potential of 0–200 mV(SHE) accounting for the natural hydrophobicity ob-served under certain conditions. However, extensive oxi-dation during long conditioning times was found to leadto poor flotation. At the same time, it was reported byHeyes and Trahar (1984) that pyrrhotite displays self-induced flotation as a result of mild oxidation. Peters(1977) pointed out that pyrrhotite-collectorless flotationresults from elemental sulfur formation at the mineralsurface since sulfur is strongly hydrophobic and may re-main stable for a long time, even in alkaline solutions.Similar research (Hamilton and Woods, 1981, 1983;Heyes and Trahar, 1984; Buckley and Woods, 1985;Jones et al., 1992) regarding the natural hydrophobicityof pyrrhotite attributed its collectorless flotation to theformation of iron-deficient/sulfur-rich metastable inter-mediates at relatively low pH and under mild oxidationpotentials, which condition may be described by the fol-lowing reactions:

Fe1�xSþ 3yOH� ¼ Fe1�ðxþyÞSþ yFeðOHÞ3 þ 3ye� ð1Þ

and

Fe1�xSþ 3ð1� xÞH2O ¼ ð1� xÞFeðOHÞ3 þ S

þ 3ð1� xÞHþ þ 3ð1� xÞe�

ð2Þ

However, pyrrhotite surfaces are oxidized rapidly uponexposure to air, and with an increase in oxidation timethe surfaces are covered by an overlay of iron(III)

hydroxide (Smart et al., 2003; Buckley and Woods,1985; Heyes and Trahar, 1984):

Fe1�xSþ ð7� 3xÞH2O ¼ ð1� xÞFeðOHÞ3 þ SO2�4

þ ð11� 3xÞHþ þ ð9þ 3xÞe�

ð3Þ

Under these conditions, the natural flotation of pyrrho-tite does not occur.

Recently at the University of Utah (2004), the naturalhydrophobicity of pyrrhotite (provided by the GeologyCurator, College of Mines and Earth Sciences, Univer-sity of Utah; Pyrrhotite content >95%, unknown source)was examined in air from pH 3.0 to pH 9.2 based oncaptive bubble contact angle measurements. The resultsin Fig. 1 demonstrate that the pyrrhotite surface has astrong hydrophilic state at pH values above pH 4.5 (con-tact angle 0�). When the pH is less than pH 4.5, the nat-ural hydrophobicity of pyrrhotite increases with adecrease in pH, having a contact angle of 51� at pH3.0. These experimental results further confirm the flota-tion results reported by other investigators. It is evidentthat a hydrophilic surface state is stabilized at pH >4.5under an air atmosphere and room temperature.

Further, pyrrhotite is thermodynamically unstable inacidic solutions (pH � 4.5) without consideration of oxi-dation. Theoretical solution chemistry calculationregarding pyrrhotite stability by formation of H2S sug-gests that H2S should evolve (1 atm total pressure) un-der an assumption of a ferrous ion activity of 10�4

(M) at pH 6 7 and determined by Fe(OH)2 solubilityproduct at pH P 8. Thus, it is expected that such a reac-tion at the pyrrhotite surface may account for the natu-ral hydrophobicity in the absence of collector asobserved below pH 5 (see Fig. 1). Although the rate of

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J.D. Miller et al. / Minerals Engineering 18 (2005) 855–865 857

this reaction has been reported to involve a significantinduction period, additional research is warranted.

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6

Ethyl Xanthate Concentration (M)

Flot

atio

n R

ecoc

ery

(%)

pH 5.5

pH 7.0

pH 9.0

Fig. 3. Flotation recovery of pyrrhotite (conditioned 30 min) con-tacted with various concentrations of ethyl xanthate for 15 min at pH5.5 (s), pH 7.0 (m), and pH 9 (h), [FeS] = 3.3 g/l in 2 · 10�3 mol/lKNO3 (Montalti, 1994).

3. Flotation with thiocarbonate collectors

Thiocarbonates, particularly xanthates, are widelyused as collectors for selective and bulk flotation of sul-fide minerals from PGM ores. It has been found thatPGM recovery in bulk flotation is limited in some in-stances by the poor flotation response of pyrrhotite.With more and more attention being given to pyrrhotiterecovery, the use of thiocarbonate collectors has beenexamined in order to identify conditions for increasedrecovery.

3.1. Ethyl xanthate

Montalti (1994) systematically investigated pyrrhotitefloatability using ethyl xanthate. The pyrrhotite samplewas from North Bend, Washington and was analyzedby XRD to be only one phase b-Fe1�xS (Fe 47.0%and S 32.6%) having hexagonal crystal structure withsome impurities (C 1.45% and Zn 4.64%). The zeta po-tential of this pyrrhotite sample was measured under se-lected conditions and the results are presented in Fig. 2.The results show that the point of zero charge (PZC)for this pyrrhotite mineral (sample 2 g/l, KNO3

2 · 10�3 mol/l) is around pH 6.5 and that the electroki-netic response is not significantly affected by the pres-ence of ethyl xanthate at low concentration. Theseresults might be expected if the surface is oxidized andFe(OH)3 is stabilized at the pyrrhotite surface sinceFe(OH)3 has a PZC of about pH 6.5.

Fig. 2. Zeta potential–pH plot for pyrrhotite in the presence of differ-ent concentrations of ethyl xanthate of 0 mol/l (s), 5.0 · 10�4 mol/l(m), 1.0 · 10�3 mol/l (h), [FeS] = 2.0 g/l in 2 · 10�3 mol/l KNO3(Montalti, 1994).

In this study by Montalti, the effects of pH, ethylxanthate concentration, and collector reaction time, onflotation recovery were determined. It can be seenfrom Fig. 3 that the influence of pH and collector con-centration is significant. The flotation recovery at1 · 10�4 mol/l ethyl xanthate significantly increasesfrom 50% to 82% with a pH decrease from pH 9.0 topH 5.5. When the collector concentration is above1 · 10�4 mol/l, the flotation recovery was found to beinsensitive to variations in concentration. For theseexperiments, the conditioning time was 30 min, andthe reaction time for collector adsorption was 15 min.

The influence of ethyl xanthate adsorption time onflotation recovery for different ethyl xanthate concentra-tions was examined and the flotation recovery wasfound to reach a maximum at 15 min. Of course, the ex-tent of recovery is dependent on pH. A maximum flota-tion recovery of 90% at pH 5.5 was achieved with aninitial concentration of 5 · 10�4 mol/l ethyl xanthate.

3.2. Butyl xanthate

Sodium n-butyl xanthate (1 · 10�4 mol/l) was used toinvestigate the flotation selectivity between pentlanditeand pyrrhotite at pH 9.0 to 9.5 under potential control(Khan and Kelebek, 2004). The results are shown inFig. 4. The samples employed in these tests originatedfrom process streams consisting mainly of pyrrhotite–pentlandite middlings processed in a nickel–copperplant in the Sudbury region of Canada. The middlingswere first reground in order to liberate pentlandite frompyrrhotite using mild steel grinding media in the pHrange of 9.0–9.5 (nitrogen atmosphere). The case for

0

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40

50

60

70

80

0 10 20 30 40 50 60 70 80

Pyrrhotite Recovery %

Pent

land

ite R

ecov

ery

%

Potential Controlled: -0.095 to - 0.005 V/SHE

Potential Uncontrolled:0.25 to 0.30 V/SHE

Fig. 4. Flotation selectivity between pentlandite and pyrrhotite at pH9.0 to 9.5; potential controlled: �0.095 to �000.5 V (SHE), potentialuncontrolled: 0.25–0.3 V (SHE); sodium isobutyl xanthate 1 · 10�4 M(Khan and Kelebek, 2004).

Fig. 5. Contact angle at pyrrhotite surface as a function of pH in theabsence/presence of SIBX at ambient temperature and 0.05 M Na2SO4

(University of Utah).


potential control refers to a condition whereby the po-tential of the pulp was initially kept at relatively low lev-els (�0.095 to �0.055 V/SHE) by sparging nitrogen andthen at about �0.005 to 0 V/SHE for subsequent stages.In the case of uncontrolled potential, the potential dur-ing flotation with air continuously increased up to alevel 0.25–0.3 V as in usual practice.

Fig. 4 demonstrates that under potential control theselective flotation of pentlandite from pyrrhotite oc-curs with sodium n-butyl xanthate. For uncontrolledpotential less flotation selectivity was observed. Theseresults suggest that the relatively high potential (opento air during conditioning) favors pyrrhotite flota-tion in the system studied. A similar phenomenonwas observed in Alekseeva�s research (1965), in whichthe surface oxidation did not apparently depresspyrrhotite flotation. In the presence of xanthate, usinghigh levels of aeration, flotation recoveries of pyrrho-tite were improved as compared to low levels ofaeration.

The floatability of pyrrhotite has recently beenconsidered from contact angle measurements at the Uni-versity of Utah (2004) using sodium isobutyl xanthate(SIBX) as collector for solution pH values from pH4.5 to 9.2. During these measurements, the system wasopen to air. Fig. 5 suggests that the addition of collectorimproves the hydrophobicity when compared with thatin the absence of collector, but the improvement de-pends on the system pH. When the pH is above pH 5the hydrophobicity of the pyrrhotite surface is relativelylow (contact angle �30�) and not very sensitive to vari-ation in collector concentration. At pH 4.5, the contactangle increases greatly from 30� to 84� with the additionof collector (1 · 10�4 M).

Even though most researchers claim that mild oxida-tion and/or low oxidation potential favors natural and/or collector flotation of pyrrhotite, it was reported (Raoand Finch, 1991; Hodgson and Agar, 1991) that under anitrogen atmosphere pyrrhotite flotation with additionof xanthate is not possible. In some cases, when xan-thate is not oxidized to dixanthogen, the collectoradsorption density and degree of hydrophobicity maybe reduced.

4. Activation

In order to improve recovery in the xanthate flotationof sulfide minerals, copper, lead, silver, and other metalions can be used as activators. Chang et al. (1954) con-ducted a detailed experimental investigation of the acti-vation and flotation of pyrrhotite with xanthates andcopper sulfate at pH 5.1. It was found that copper sul-fate significantly improves pyrrhotite recovery. In theseexperiments, the cupric ions directly exchange with fer-rous ions at the pyrrhotite surface, the activated stateaccounting for the improved flotation of pyrrhotite:

Cu2þ þ FeS ¼ CuSþ Fe2þ ð4ÞThe concentration of copper, iron, and sulfur during

activation of pyrrhotite was measured and the resultspresented in Fig. 6 confirm the activation reaction givenin Eq. (4). Buswell and Nicol (2002b) electrochemicallymeasured the anodic decomposition rate of CuS at apyrrhotite surface. When the potential is above 0.4 V(SHE), the anodic rate was found to become significant.The standard equilibrium potential for CuS oxidationwas calculated to be 0.548 V (SHE). In the absence ofcopper, one of the anodic reactions for pyrrhotite is

Fig. 6. Concentration of Fe, Cu, and S in solution during the reactionof pyrrhotite with copper sulfate solution at pH 5.1 (Chang et al.,1954).

Fig. 7. Contact angle at pyrrhotite surface with and without copperactivation as a function of pH in the presence of SIBX (1 · 10�4 M) atambient temperature and 0.05 M Na2SO4 (University of Utah, 2004).

Fig. 8. Contact angle at pyrrhotite surface with and without leadactivation as a function of pH in the presence of SIBX (1 · 10�4 M) atambient temperature and 0.05 M Na2SO4 (University of Utah, 2004).


found at a much lower potential of about 0.1 V (SHE).Based on these results it seems that copper stabilizes thepyrrhotite surface.

In contrast, the role and effectiveness of copper ionsfor the activation of pyrrhotite in alkaline solutions isnot fully understood, and the conclusions are controver-sial. Fundamental research (Nicol, 1984) demonstratedthat copper activation is not possible since copper isessentially insoluble above pH 8 and thus not availablefor reaction at the pyrrhotite surface. Furthermore, pyr-rhotite particles are likely to be well oxidized and cov-ered with ferric hydroxide/oxide, which may inhibitany reaction with the underlying mineral surface. Incontrast, other studies (Kelebek et al., 1996; Leppinen,1990; Senior et al., 1995) have shown that pyrrhotiterecoveries are significantly improved in alkaline solu-tions with the addition of copper sulfate. Copper wasdetected on concentrate particles from actual flotationcircuits operating at pH 9 (Yoon et al., 1995), indicatingthat pyrrhotite was effectively activated.

Recent experiments for activation of pyrrhotite bycupric ions (sulfate) and lead(II) ions (nitrate) at an ini-tial concentration of 1 · 10�4 m sodium isobutyl xan-thate in 0.05 M Na2SO4 have been carried out withcontact angle measurements at the University of Utah(2004), Figs. 7 and 8. These results show that whenthe pH is greater than pH 7 the presence of these metalions does not influence the contact angle at the pyrrho-tite surface. When the pH is reduced to pH 5, these me-tal ions significantly increase the contact angle, from 28�to 84� for copper and from 28� to 51� to 73� for lead,depending on concentration. It should be noted thatcopper activation is insensitive to concentration varia-tions under the conditions studied and that the contactangle is greater than that obtained with lead activation.

5. Other factors

In an actual flotation system, it is expected that thesituation becomes much more complicated since pyrrho-tite particles coexist and/or are in contact with otherminerals/materials. Under the various conditions, a pyr-rhotite particle may act as an anode or cathode, whichmay lead to hydrophobic or hydrophilic surface states.From a review of the literature, there are such other fac-tors influencing pyrrhotite floatability, including:

Pyrrhotite-grinding media: the contact of pyrrhotitewith mild steel when being ground was found to havea deleterious effect on its floatability (Admas et al.,1984). The reduced floatability was attributed to the


formation of an iron hydroxide coating on the pyrrho-tite particles, which is not only hydrophilic, but alsoinhibits xanthate adsorption.

Pyrrhotite–pyrite: when pyrrhotite particles are con-tacted/associated with pyrite particles during flotation,pyrrhotite recovery can be improved. This phenomenonis attributed to a shift of oxygen reduction from sites onpyrrhotite to pyrite, which consequently reduces ironhydroxide formation on pyrrhotite and promotes its flo-tation (Nakazawa and Iwasaki, 1985, 1986).

Pyrrhotite–chalcopyrite: similar to pyrite influence,chalcopyrite acts as the cathode and pyrrhotite acts asthe anode. The presence of chalcopyrite accelerates theoxidation reaction of xanthate on pyrrhotite (i.e. effec-tive collector adsorption) and increases pyrrhotite float-ability. At the same time, a lower floatability ofchalcopyrite was observed either due to iron hydroxideformation on its surface (as a cathode for oxygen reduc-tion) or to a shift of increased xanthate adsorption bypyrrhotite. An XPS study confirmed that the increasedfloatability of pyrrhotite is not due to activation of cop-per ions but rather the absence of iron hydroxide on thepyrrhotite surface (Cheng and Iwasaki, 1992, 1994).

Fig. 9. Stability regions of iron oxides and sulfides in water at 25 �C,1 atm. with Sum S = 1 · 10�6 mol/l and Sum Fe = 1 · 10�6 mol/l(Garrels and Christ, 1990).

6. Discussion

6.1. Pyrrhotite characteristics

Pyrrhotite, Fe1�xS, is nonstoichiometric and of vari-able composition with a density which varies from4.58 to 4.65. Accordingly, it has some unusual charac-teristics. First, the amount of sulfur varies from 50 to55 atoms of sulfur per 50 atoms of iron. That is, the x

value varies from 0 to 0.2 (when x = 0, the pyrrhotitemineral is called troilite). Secondly, it has two symme-tries for its crystal structure: when pyrrhotite is relativelylow in sulfur or x is close to 0, the mineral structure ishexagonal or prismatic; when pyrrhotite is high in sulfurits structure is monoclinic. Thirdly, pyrrhotite magne-tism is very low when x is 0 (hexagonal) at 20 �C, onlyhaving a magnetic susceptibility of 1 · 10�5 e.m.u./g.When x is close to 0.2 (monoclinic), its magnetism in-creases to 13.1 e.m.u./g. So, next to magnetite, the sulfurrich monoclinic pyrrhotite is the most common mag-netic mineral. The bonding difference between troiliteand monoclinic pyrrhotite mineral samples has recentlybeen discussed based on XPS measurements (Skinneret al., 2004).

In nature, pyrrhotite minerals occur with significantvariation in structure and/or composition. Generally, amixture of hexagonal (relatively iron-rich, h-Po) andmonoclinic (relatively sulfur-rich, m-Po) phases is pre-dominant according to the literature. Arnold (1967)made the analysis of 82 terrestrial pyrrhotite samples.Seventy percents were a mixture of h-Po and m-Po,

10% contained hexagonal pyrrhotite only, and 9% con-tained monoclinic pyrrhotite only. The remaining sam-ples, 8% contained troilite, stoichiometric FeS.

6.2. Stability of pyrrhotite

Thermodynamically, pyrrhotite is not stable in aque-ous solutions under conventional flotation conditions(pH 9.0, open to the air atmosphere, and 25 �C), whichis evident from the Eh–pH diagram in Fig. 9 (Garrelsand Christ, 1990). At low levels of dissolved sulfur andiron pyrrhotite is stable between pH 7 and pH 9 at a po-tential less than �0.35 V (SHE). It is expected that pyr-rhotite is oxidized finally to hematite and sulfate underconventional flotation conditions.

Kinetically, Montalti (1994) examined the dissolvediron concentration from pyrrhotite, chalcopyrite, andpyrite in a neutral solution (pH 6.9) with variation inthe oxidation potential as shown in Fig. 10. It is evidentthat the rate and extent of iron release from pyrrhotite ismuch greater than that from chalcopyrite and pyrite. Itwas found that the rate of pyrrhotite dissolution at�200 mV (SHE) decreases with an increase in reactiontime and that the dissolution rate becomes quite slowafter 3 h. It is expected that a portion of the dissolvediron precipitated from the solution as Fe(OH)3, coveringthe sample surface, and contributing to the decrease indissolution rate.

Because of the variation of pyrrhotite in compositionand crystal structure, each pyrrhotite sample may have adifferent nature, which is supported by the following re-sults. Khan and Kelebek (2004) and Buswell and Nicol(2002b) examined the kinetics of pyrrhotite decomposi-tion by electrochemical measurements—cyclic voltam-

Fig. 10. Concentration of iron in solution (pH 6.9) as a function oftime for various sulfide minerals at several Eh: chalcopyrite Eh+200 mV (s), pyrite +200 mV (n), pyrite Eh +0 mV (j), pyrite Eh�200 mV (d), pyrrhotite Eh �200 mV (h) (Montalti, 1994).

Fig. 12. Cyclic voltammograms for FeS electrode in deoxygenatedNa2B4O7 (0.05 M) solution at pH 9.3, a scan rate 20 mV/s (Buswell,2002).


metry. Khan�s results for a Sudbury pyrrhotite, as pre-sented in Fig. 11, demonstrate that when the potentialis below 0.5 V (SHE) the initial rate of pyrrhotite oxida-tion (anodic current density) is quite slow even thoughthere is a peak for the oxidation in the potential regionfrom �0.05 V to 0.18 V (SHE). The maximum currentdensity for this peak is 0.2 mA/cm2 at pH 9.2 and oxy-gen concentration of 0.1 mg/l in the absence of collector.This peak was attributed to the following anodicreaction:

Fe1�xSþ 3yOH� ¼ Fe1�ðxþyÞSþ yFeðOHÞ3 þ 3ye� ð1Þ

When the positive potential scan reaches 0.5 V (SHE), anew anodic current peak appears and reaches its maxi-

Fig. 11. Cyclic voltammogram of FeS electrode at pH 9.2, a scan rate10 mv/s, DO = 0.1 mg/l, X = 0.0 (Khan and Kelebek, 2004).DO = dissolved oxygen, X = sodium n-butyl xanthate.

mum current density at 0.75 V (SHE). This anodicprocess is considered to involve oxidation of pyrrhotiteto sulfate in addition to the formation of ferrichydroxide:

Fe1�xSþ ð7� 3xÞH2O ¼ ð1� xÞFeðOHÞ3 þ SO2�4

þ ð11� 3xÞHþ þ ð9þ 3xÞe�

ð3Þ

In contrast, the results from electrochemical measure-ments (pH 9.3 and 0.05 M Na2B4O7) by Buswell and Ni-col (2002b) show that pyrrhotite from an RSA mineraldealer is much more active than the Sudbury pyrrhotitesample used by Khan and Kelebek (2004), see Fig. 12.These results suggest that when the potential exceeds�0.4 V (SHE), the initial rate of pyrrhotite oxidation(anodic current density) becomes significant. The anodiccurrent density varies from 1.0 to 2.0 mA/cm2 at a po-tential 0–0.2 V (SHE).

6.3. Flotation response

It is evident that pyrrhotite is susceptible to oxidationin aqueous solution, an important factor which influ-ences its flotation recovery since the oxidation productsat particle surfaces inevitably influences surface hydro-phobicity. The kinetics of pyrrhotite oxidation and theoxidation products at the surface depend on:

(i) surface area of pyrrhotite;(ii) oxidation potential/oxygen pressure;(iii) solution pH;(iv) temperature;(v) time.

As a consequence all these factors will influence thenatural/collectorless flotation of pyrrhotite.

Fig. 13. FTIR spectra of pyrrhotite at xanthate concentration of5 · 10�5 mol/l and pH 9.2 in the cases of single mineral (A), mixedminerals (B, solution contact with pentlandite), and mixed minerals


As shown in Figs. 9–12, pyrrhotite tends to be ther-modynamically and kinetically unstable. It is generallyaccepted that during the flotation process in acidic solu-tion under low oxidation potentials, such as spargingnitrogen, the surface products of pyrrhotite oxidationare rich sulfur intermediates and/or elemental sulfur,which accounts for the naturally hydrophobic pyrrhotitesurface. In alkaline solutions and under relatively highoxidation potentials—open to the air atmosphere, thesurface is covered by ferric oxides and/or ferric hydrox-ide, resulting in a hydrophilic surface state.

It is generally thought that thiocarbonate collectorsadsorb at sulfide mineral surface due to an electrochem-ical reaction involving the formation of a dimer such asshown by the following half cell reaction for thedixanthogen formation:

2ROCðSÞS� ¼ ROCðSÞS–SðSÞCORþ 2e� ð5ÞThe standard potential of the xanthate/dixanthogen pairin reaction (5) was reported to be �0.128 V/SHE and�0.06 V/SHE (Buswell et al., 2002a; Buswell and Nicol,2002b; Winter and Woods, 1973). Of course, the stan-dard potential depends on the length of the hydrocarbonchain of the collector molecule (Plessis, 2004). Such areaction is expected at the pyrrhotite surface coupledwith reduction of oxygen:

1=2O2 þH2Oþ 2e� ¼ 2OH� ð6ÞFor the pyrrhotite–xanthate system under practical flo-tation conditions, the pyrrhotite surface can be oxidizedto hydroxides and/or oxides which inhibit pyrrhotite flo-tation. However, in some cases, the pyrrhotite wasfound to be floatable. Thus, Khan and Kelebek (2004)suggested that xanthate can form thermodynamicallystable Fe(II)/Fe(III) hydroxyl xanthates in alkaline solu-tions at potentials lower than the stability domain ofdixanthogen (Harris and Finkelstein, 1975; Wanget al., 1989). During the initial stage of adsorption, xan-thate electrostatically interacts with ferrous ion on thesurface.

Po½FeðOHÞ�þ þX� ! Po½FeðOHÞ�þX� ! Po½FeðOHÞX�ð7Þ

This species is considered to be metastable and has alow level of hydrophobicity, but it is considered to be afavorable site to accommodate more xanthate and/ordixanthogen. More stable iron compounds can follow:

Po½FeðOHÞX� þOH� ! Po½FeðOHÞ2X� þ e� ð8ÞFurther adsorption of xanthate can lead to dixantho-

gen formation:

Po½FeðOHÞ2X� þX� ! Po½FeðOHÞ2X–X� þ e� ð9Þ

Po½FeðOHÞ2X–X� þX� ! Po½FeðOHÞ2X–X2� þ e�

ð10Þ

Bozkurt et al. (1998) examined the IR spectra of pyrrho-tite with isobutyl xanthate adsorption (1 · 10�5 M, 9.2)(see Fig. 13). The bands in the spectra are well defined at1265, 1048, and 1026 cm�1, which are characteristic ofcoupled S–C–S and C–O–C stretching vibrations in dix-anthogen. These results are similar to those reported byLeppinen et al. (1989), Leppinen (1990), and Valli andPersson (1994). It appears from Fig. 13 that the adsorp-tion density of dixanthogen at the pyrrhotite surface is alittle greater for pyrrhotite alone (A) than for the pyr-rhotite in direct contact with pentlandite (C). Additionalstudies by Rao and Finch (1991) established that the to-tal xanthate adsorption was slightly greater in the pres-ence of air than with nitrogen. At the same time, theamount of adsorbed dixanthogen was found to be greaterthan adsorbed xanthate in the presence of air. Impor-tantly, these researchers observed that dixanthogen wasnot produced in the nitrogen environment. These resultsmay help explain the fact that at a relatively low potentialthe selective flotation of pentlandite from pyrrhotite ispossible, as presented in Fig. 4, or that a nitrogenenvironment (very low potential) inhibits pyrrhotite flo-atability with xanthates. Also noting a high level of aera-tion or oxidation potential for improved pyrrhotiteflotation (Khan and Kelebek, 2004; Alekseeva, 1965), itis expected that the oxide or hydroxide product layerfrom pyrrhotite oxidation may have a high electric resis-tance (average resistivity for natural pyrrhotite: 1 ·10�5 X/m, average resistivity for hematite: 1 · 10�1 X/m, Shuey, 1975) and/or that xanthate oxidationmay havesignificant over-potentials on the surface of hydroxide–oxide products. The high oxidation potential might beneeded to oxidize xanthate to dixanthogen and wouldfavor flotation of oxidized pyrrhotite particles. In con-

(C, direct contact with pentlandite) (Bozkurt et al., 1998).


trast, it was reported that pyrrhotite was effectively de-pressed and selective flotation of pentlandite and chalco-pyrite from pyrrhotite was achieved (Khan and Kelebek,2004; Kelebek, 1993) as a result of selective oxidation ofpyrrhotite at relative high levels of potentials throughpre-aeration of slurries.

Even though adsorption of xanthate and/or dixanth-ogen can occur on the oxidized pyrrhotite particles, thehydrophobicity is not great, and a low flotation recoveryfor pyrrhotite is frequently found under the traditionalflotation conditions as used in the PGM industry. Thesephenomena indicate that adsorbed collector covers onlya part of the oxidized pyrrhotite particles, a state whichneeds to be examined in future research.

6.4. Summary

It can be noted from review of the literature and re-cent results at the University of Utah that the pyrrhotiteflotation system is very complicated, resulting from var-iation in the pyrrhotite composition/structure, pyrrho-tite and thiocarbonate oxidation, Eh/pH, mineralscontacted/associated with it, and activation reactions.The following chemical factors appear to have a signif-icant influence on pyrrhotite hydrophobicity and itsrecovery by flotation.

Oxidation potential: it has been seen in the above sec-tions that pyrrhotite is thermodynamically and kineti-cally unstable and is easily oxidized to ferrichydroxide/oxide, or intermediate compounds, depend-ing on the oxidation potential level and solution pH.If the flotation process is operated under conventionalconditions, it is unavoidable that the product from pyr-rhotite oxidation is ferric hydroxide/oxide precipitatingat particle surfaces, rendering the pyrrhotite surfacehydrophilic and causing its low flotation recovery eventhough adsorption of xanthates and formation of di-xanthogen occurs. In this regard, appropriate controlof the oxidation potential should be important duringflotation process for improved flotation recovery. First,the oxidation potential/oxygen level during millingshould be controlled to protect pyrrhotite from oxida-tion. Second, in order to assure formation of dixantho-gen, relatively high oxidation potential (0–0.2 V/SHE)should be maintained during a short conditioning stageprior to flotation.

pH: the pH effect has been shown to be very impor-tant as is evident from the results for pyrrhotite naturalfloatability, collector flotation, and activation. These re-sults demonstrate that under the traditional flotationconditions, such as at pH 9–9.5, no natural pyrrhotitefloatability is observed (contact angle is zero); pyrrhotitefloatability with collector is low; and activators (copperand lead) lose their effectiveness in most cases, precipi-tating as hydroxides. Significantly improved pyrrhotiteflotation can be achieved at pH < 5.

Activation: according to the literature, the effective-ness of copper for pyrrhotite activation in alkaline solu-tions is controversial. Some researchers claim thatcopper activation is not possible since copper ions areessentially insoluble in alkaline solution and thus notavailable for reaction. Furthermore, pyrrhotite particlesare likely to be well oxidized and covered with ferrichydroxide/oxide, which may inhibit any activation reac-tion with the underlying mineral surface.

Other studies have shown that pyrrhotite flotationrecovery is significantly improved with addition of cop-per sulfate in alkaline solutions, and that copper hasbeen detected on concentrate particles from actual flota-tion circuits. These results obviously indicate that pyr-rhotite may be activated by copper ions.

The recent results of contact angle measurement atthe University of Utah support the first observation.Addition of copper ions or lead ions did not improvethe hydrophobicity of pyrrhotite in neutral and alkalinesolution, although some positive effect was observed be-low pH 7.

7. Conclusions

The flotation recovery of PGMs is dependent onthe pyrrhotite flotation recovery since PGMs are nat-urally associated with this sulfide mineral. Pyrrhotiteis not stable and is easily oxidized to ferric hydrox-ide/oxide under conventional flotation conditions,open to air at pH 9.0. The products from pyrrhotiteoxidation cover the pyrrhotite particles, renderingthe particle surface hydrophilic and causing alower flotation recovery of pyrrhotite under mostcircumstances.

Under low oxidation potential in acidic solutions,pyrrhotite displays a distinct natural floatability. In thepresence of xanthate, however, if the potential is toolow to oxidize xanthate to dixanthogen, pyrrhotite flota-tion is inhibited. It is evident that appropriate control ofoxidation potential should be important during the flo-tation process.

Generally, activation of pyrrhotite by copper andlead is significant in acidic solutions, but in some caseshas been found to be negligible in neutral and alkalinesolutions.

Based on this review of the literature, it seems thatimproved pyrrhotite flotation to increase PGM recoverymight be achieved in at least two ways:

• Development of new collectors for improved pyrrho-tite hydrophobicity in air at pH 9.0.

• Control of Eh/pH to minimize oxidation of pyrrho-tite and still achieve collector adsorption with activa-tion as might be necessary.


Research in both these areas is in progress. The use oftrithiocarbonates for pyrrhotite flotation is one exampleof collector development as an alternative to the tradi-tional xanthate collectors. With regard to oxidation po-tential control, the use of inert gas flotation for theprocessing of PGM ores has been demonstrated (Gathjeand Simmons, 2004) and is being evaluated further.

Acknowledgments

Support for the PGM flotation research program atthe University of Utah and the University of Pretoriaby the National Science Foundation (NSN GrantINT—0352807) and industrial participants, ImpalaPlatinum, Newmont, BOC, and Bateman, is recognizedwith appreciation.

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a review of pyrrhotite flotation chemistry in the processing of pgm ores

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