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Thermodynamic stability of pentlandite and violarite and new EH-pH diagrams for the iron-nickel sulphur aqueous system
hydrometallurgy Hydrometallurgy 41 (1996) 107-I 18
Thermodynamic stability of pentlandite and violarite and new E,-pH diagrams for the iron-nickel sulphur aqueous system T.E. Warner a, N.M. Rice a3*, N. Taylor b aDepartment of Mining and Mineral Engineering, Uniuersiry of Leeds, Leeds, UK b Department of Chemistry, University of Leeds, Leeds, UK
Received 15May 1995; accepted 2 August 1995
Abstract Thermodynamic data for ideal stoichiometric pentlandite (Fe,,Ni&,) and violarite (FeNi,S,) have been re-evaluated from arguments concerning relative phase stability, in conjunction with known values for binary and ternary sulphides. The values AGp Fe,.,Ni,.,S, = - 813 kJ mol- ’ S 4 = - 346 kJ mol- ’ (x = 0) at 298 K were obtained. These values were and AGf” Fe,_,Ni,+, used to calculate Nemst functions for the Fe-Ni-S aqueous system, which are displayed by a series of E,-pH diagrams. These diagrams reveal the co-existence of violarite and pentlandite in an aqueous system, which tallies with mineralogical observations. The value for violarite can plausibly account for the supergene nickel-enrichment process observed in nature.
1. Introduction E,--pH diagrams have proved useful for displaying equilibria for several binary metal sulphide phases in aqueous systems. Such diagrams have been extended to ternary metal sulphide phases, notably for chalcopyrite (CuFeS,) and bomite (Cu,FeS,) in the copper-iron-sulphur aqueous system reported by Peters [l]. To the best of our knowledge, there has been only one attempt to construct an E,-pH diagram for the iron-nickel-sulphur aqueous system (Thomber [2]), even though this system contains the commercially important nickel-iron sulphide minerals pentlandite (Fe,,Ni,,S8) and violarite (FeNi,S,).
* Corresponding author. Fax: + 44 113 246 7310. 0304-386X/96/$15.00 0 I996 Elsevier Science B.V. All rights reserved SSDI 0304-386X(95)00081-X
108
T.E. Warner et al./ Hydrometahrgy
41 (1996) 107-l 18
In this present work, the standard free energies of formation for pentlandite and violarite have been re-evaluated at 298 K. These values were then used to calculate the Nemst functions for the various electrochemical equilibria in the Fe-Ni-S aqueous system. These have then been portrayed as a series of En-pH diagrams representing both thermodynamically reversible and certain metastable equilibria. The information contained within these diagrams has been compared with geological observations whilst also complementing the results from electrochemical dissolution studies reported previously by us [3,51.
2. Discussion 2.1. Appraisal of thermodynamic data for pentlandite Fe,., Ni,,, S, Craig and Naldrett [6] derived the following free energy of formation reaction from the results of sulphur vapour pressure measurements over the assemblage: iron + nickel + pentlandite; in conjunction with an estimated value of 505.43 J mol-’ K- ’ for the entropy of pentlandite: l.l25Fe(c)
+ l.l25Ni(c)
+ S,(g) -+ 0.25Fe,,,Ni,,,S,(c)
with AG&..ction= - 338067 + 166.1 T/J mol- ’ (T = 673-773 K). Extrapolating the above function to 298 K, and taking AGf” (298 K) S,(g) = 79.73 kJ mol-’ [7], leads to AC,” (298 K) Fe,,Ni,,S, = -835.4 kJ mol-‘. This value is in close agreement with that similarly calculated by Thomber [2] (- 836.9 kJ mol-‘1, based on the same source reference of Craig and Naldrett [6] (correcting for the fact that Thomber unfortunately divided instead of multiplying by 4.184 for the conversion of calories to joules [8]). This extrapolation of high temperature thermodynamic data to 298 K ignores the effects of a second-order phase transition in naturally occurring pentlandite (which occurs between 323 and 473 K [9]), which is accompanied by an abrupt change in heat capacity and, thus, an associated change in entropy for the phase transition. Therefore, it is not surprising that calculations for the appropriate Nemst functions at 298 K, based on the thermodynamic data given by Craig and Naldrett [6], imply that violarite (FeNi,S,) is a metastable phase in an aqueous system, contrary to geological observations. More specifically, the equilibria: 4.5FeNi,S,(c)
+ llH+(aq)
+ lie-=
Fe 4.5Ni4,5Ss(c) + 4.5NiS(c) + S.SH,S(aq)
and: l.SFeS,(c)
+ Ni,S,(c)
+ 2H+(aq) + 2e-=
l.SFeNi,S,(c)
+ H,S(aq)
are metastable with respect to: 4.5FeS,(c)
+ 1.5Ni3S4(c) + 14H+(aq) + 14e-= Fe,.,Ni,,,S,(c)
+ 7H,S(aq)
T.E. Warner et al./Hydrometallurgy
41 (1996) 107-I 18
109
Likewise, when considering the equilibria where hydrogen sulphide and sulphate are neither consumed nor produced, adopting the standard free energy data given by Craig and Naldrett [5] implies that the following equilibria: 2FeNi,S,(c)
+ OSNi’+(aq)
+ 2.5Fe2+(aq) -t 6e-= Fe,,,Ni,,,S,(c)
and: FeS,(c)
+ NiS,(c) + Ni2+(aq) + 2e-= FeNi,S,(c)
are metastable with respect to: 4FeS,(c)
+ OSFe’+(aq)
+ 4.5Ni2+(aq) + lOe-= Fe,,,Ni,,,S,(c)
Consequently, Thomber [2] suggested a manipulation of the free energy data to fit the mineralogical evidence; thus, he obtained a self-consistent set of data by using pyrite (AC,” (298 K) FeS, = - 160.2 kJ mol- ‘) as an ‘anchor value’, although he did not describe the exact criteria he used in selecting such data. More specifically, he obtained a value for AGf” (298 K) Fe,,Ni,,S, = -770.0 kJ mol-‘, which is inconsistent with the value for the following inequality where troilite (FeS), pyrrhotite (Fe& and heazlewoodite (Ni,S,) (whose free energy data are known with a reasonable degree of accuracy) are the phases presumed to be metastable with respect to pentlandite at 298 K: FeS + 0.5 Fe,S, + 1.5 Ni,S, --) Fe,.SNi,.5S, For Fe,,Ni,,S, to be a thermodynamically stable phase, AGgeacrionmust be less than zero; hence, using standard free energy data for the above binary phases (Table 1) leads to the inequality: AGfO(298K)Fe4.5Ni4,5Ss < -790.1 kJmol_’ Thus, faced with the lack of more reliable experimental data, the arithmetic mean between the value of this inequality (i.e., - 790.1 kJ mol- ’> and the value previously derived from Craig and Naldrett [6] (i.e., -835.2 kJ mol-‘), was adopted as the standard free energy of the formation for pentlandite at 298 K. Hence, a value of AGP (298 K) Fe,,Ni,,S,(c) = - 813 kJ mol-’ was used in this work. 2.2. Appraisal of thermodynamic data for violarite FeNi, S, Craig [lo] derived the following free energy of formation reaction from the results of various sulphur vapour pressure measurements in conjunction with an estimated value of 192.46 J mol-’ K- ’ for the entropy of violarite: Fe(c) + 2Ni(c) + 2S,(g) -+ FeNi,S,(c) AGL3ction
=
-
582204 + 350.6 T/Jmol-’
Taking AC,” (298 K) S,(g) = 79.73 kJ mol-’ [7] whilst solving the above function at 298 K leads to AC,” (298 K) FeNi,S,(c) = - 318.3 kJ mol- ’ (which is the same as that calculated by Thomber ( - 3 19.0 kJ mol- ’) 121,based on the same data). Craig’s estimate for the entropy of violarite (192.46 J mol-’ K- ‘> is derived from the average value for the two reactions [lo]: FeS,(c)
+ 2NiS(s) + FeNi,S,(c)
110
T.E. Warner et al./Hydrometallurgy
Table 1 Standard free energy of formation
41 (1996) 107-118
data at 298 K
Species
AC$‘/kJ mol-
&0(l)
-237.19 - 27.86 12.59 -741.99 - 752.87 - 742.0 - 78.87 - 4.6 - 45.6 - loo.7 - 748.5 - 160.2 -210.1 - 559.8 -91.5 -312.7 - 126.2 - 1015.3 - 742.34 - 488.0 -211.60 -712.1 - 469.9 - 447.3
H,S(aq) HS - (aq> H 2 SO&aq> HSO; (aq) SO:- (aq) Fe2’ (aq) Fe3+ (aq) Ni2+ (aq) FeS(c) (troilite) Fe,&(c) (pyrrhotite) Fe&(c) (pyrite) N&S,(c) (heazlewoodite) Ni,S,(c) (godlevskite) a-NiS(c) (millerite) Ni,S,(c) (polydymite) NiS,(c) (vaesite)
Fe,O,(c) Fe,O,(c) cy-FedOHXc) NiO(c) Ni ,0,(c)
Ni,O,(c) Ni(OH),(c)
’
Reference
1181 [71
[191 1191 [I91 it91 ml DO1 DOI [71 1201 171 [71 [71 171 [71 [71 DOI I201 1211 DOI iI91 WI I201
and: FeS(c)
+ NiS(c)
+ NiS,(c)
+ FeNi,S,(c)
using available entropy data for the binary phases, whilst assuming that the entropy change for these reactions is zero. In the present work, it is suggested that a more accurate estimate of AGP (298 K) FeNi,S, is obtainable if one considers that the change in standard free energy rather than the change in standard entropy is assumed to be zero for the particular reaction: FeS,(c)
+ 2NiS(c)
+ FeNi,S,(c)
at 298 K; by exploiting circumstantial evidence of certain phase phenomena presumed at temperatures near 298 K as described in detail below. The phase equilibria studied by Craig [lo], together with mineralogical observations and the results from experimental synthesis as reviewed by Warner [3], suggest that violarite of ideal stoichiometry (FeNi,S,) is thermodynamically unstable at 298 K with respect to the assemblage: Ni-rich violarite (Fe,_,NiZ+XS,, 1 2 x 2 O), pyrite (Fe&), millerite (NiS) and sulphur. Since FeNi,S, exists as a stable phase at 473 K [lo], it is assumed that FeNi,S, is only just unstable at 298 K (at least as an approximation).
T.E. Wumer et cd./ Hydrometallurgy 41 (1996) 107-118
111
Thus, the above decomposition reaction can be considered as being near to equilibrium, such that for the reaction: Fe,-,Ni,+, S 4(c) -P (1 - x)FeS,(c) + (2 + x)NiS(c) AG”Reaction= 0 (for a certain small value of ‘x’ at 298 K). Since the bonding in violarite of ideal composition, FeNizS4, is similar to that in Ni-rich violarite [ 111 and, in particular, since both these phases share an almost identical crystal structure, with the only significant differences being the ratio and ordering of Fe/Ni within the sulphur substructure, it is reasonable to assume that each has a similar enthalpy and entropy of formation at 298 K. Furthermore, if ‘x’ is relatively small at 298 K (as is proposed here), then the approximate value for AGf FeNi,S, at 298 K can be derived from the following reaction: FeNi,S,(c)
+ FeS,(c)
+ 2NiS(c)
where AGLaction (298 K) = 0, (which is presumed exact for a particular temperature between 298 and 473 K). This leads to the following inequalities at 298 K: AG,“FeNi,S, > AGPFeS, + 2AGFNiS AGPFe,_,Ni,+,S,
< AGPFeS, + 2AGPNiS
The thermodynamic data for pyrite and millerite as cited by Mills [7] (Table 11, yields the following speculative inequalities at 298 K: AG,“FeNi,S, > -343.1
Wmol-’
AGPFe,_,Ni2+XS4 < -343.1
kJmol-’
This result is comparable with the value ( - 348.4 kJ mol- ’> deduced by Thomber [2], yet takes into consideration the metastability of nominal violarite vis a vis stability of Ni-rich violarite at 298 K. Since the above result is expressed as an inequality, it is considered appropriate to take the mean between this inequality and the value estimated by Thomber [2], thus arriving at: AGP( 298 K) Fe, _ x Ni 2+XS4(c) = -346 kJmol-’ (x > 0) although the actual value may be more negative. Most encouragingly, when this value (- 346 kJ mol-’ ) is adopted in conjunction with the respective standard free energy data at 298 K (Table 1) for the following reaction: Fe,_,Ni 2+XS4(c) + (1 - x)Ni2+(aq)
+ Ni,S,(c)
+ (1 - x)Fe2+(aq)
and upon taking x = 0, it yields AG&actiOn = 0. This is consistent with such an ion-exchange equilibrium existing during supergene enrichment just beneath the water table [12,13], whereby the extent of the nickel enrichment of violarite would appear to be strongly influenced by the ratio of the Ni2+/Fe2’ aqueous activities in the surrounding environment. 2.3. Chemical equilibria (E,-pH
diagrams) for the Fe-Ni-S
aqueous system
E,-pH diagrams are a useful portrayal of the equilibria involved in the leaching of metal sulphides. The dissolution equilibria of the Fe-Ni-S aqueous system can be
112
T.E.Warner et al./Hydrometallurgy
41 (1996) 107-118
-0.6
-0.8
-1.0 0
2
4
6
8
IO
12
PH Fig. 1. E, -pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 0.1 mol dm - 3. Activities of aqueous iron and nickel species = 0.1 mol dm-‘. All reactions reversible.
viewed in a way that is analogous to the Pourbaix diagrams for the M-H,0 system [ 141, although the thermodynamic treatment involves a further two components (Figs. l-5). A comprehensive list of the Nernst functions for the various equilibria in the Fe-Ni-S aqueous system, together with the thermodynamic data on which the calculations are based, has been given by Warner [3] and the data are provided in Table 1. For violarite (where AGP (298 K) Fe, _ xNi2+x S 4 = - 346 kJ mol- ’>, since ‘x’ is assumed to be small and its precise value at 298 K unknown, the value x = 0 was adopted as an approximation, in order to simplify the stoichiometry and computation for the relevant Nemst functions. It is important to realise that equilibria more anodic than the oxygen/water equilibrium and more cathodic than the water/hydrogen equilibrium are thermodynamically
T.E. Warner et al./ Hydrometalhrgy 41 (1996) 107-l 18
’
113
0
P
‘\ . . *. -.
::::1 *.
‘.
-..
..b
-0.6
4%..
‘.
-.
-.
‘.
-0.8
m-
-1.0
0
2
4
6
I
,
a
10
’
I
12
PH Fig. 2. E,-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 1O-6 mol dme3. Activities of aqueous iron and nickel species = 0.1 mol dm- 3 (bold line) and 1O-6 mol drne3 (fine line). Metastable equilibria: with Fe,s Ni,,,S, as the only metal sulphide phase. H,S(aq) not consumed. n-FeO(OH) and Ni(OH), as metastable products.
meaningless, since the aqueous phase which includes H+(aq) does not exist under these circumstances. Nonetheless, equilibria drawn outside the aqueous domain, particularly in the cathodic region, are useful, since they give an indication of which reactions may occur at appropriate hydrogen over-potentials, and/or hydrogen over-pressures. The latter need to be fairly high in order to achieve this objective, so that changes in free energy of formation of the various species with respect to pressure should also be calculated in this situation. For the Eu-pH diagrams that describe ternary systems, it is particularly important to appreciate that the lines drawn on these diagrams refer to specific reactions, whilst the concept of a field of stability for a predominating phase has to be treated within the
114
T.E. Warner et al./Hydrometallurgy
41 (1996) 107-I 18
-0.6
-0.8
-1.0
c
,
I
,
0
2
4
r
I
1
6
8
’
I
I
IO
I2
PH Fig. 3. E,-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 0.1 mol dm-3. Activities of aqueous iron and nickel species = 0.1 mol dm- 3 (bold line) and 10e6 mol dm-3 (fine line). Metastable equilibria: with Fe,,Ni &s as the only metal sulphide phase. H,S(aq) not consumed. cu-FedOH) and Ni(OH), as metastable products.
context of these reactions. This is clearly illustrated in the example for the equilibrium (in strong alkali) between the two ternary phases: pentlandite and violarite (Fig. 11, which incorporates a third phase, millerite (NiS), and has a pH dependence as given by Nemst function: 4SFeNi,S,(c)
+ llH+(aq)
= Fe,,,Ni,.,S,(c)
+ lie-
+ 5SH,S(aq)
+ 4.5NiS(c)
En = -0.167 - 0.0591 pH - O.O296log[H,S(aq)] Nonetheless, the practical advantage of labelling such domains (mainly for improved clarity) was exploited for the diagrams given in this work.
T.E. Warner et al./HydrometaNurgy
41 (1996) 107-118
1 IS
0.6
-0.6
-0.8
-1.0 0
2
4
6
8
10
I2
PH Fig. 4. E,-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 10m6 mol dm- 3. Activities of aqueous iron and nickel species = 0.1 mol dmm3 (bold line) and 10e6 mol dmd3 (fme line). Metastable equilibria: with Fe,,Ni &Ss and FeNi,S, as the only metal sulphide phases. H,S(aq) not consumed. cu-FedOH) and Ni(OH), as metastable products.
The activities of aqueous ionic species are often treated by other workers, for example, Peters [I], as though they are independent variables in the Nemst function for the metal-sulphide aqueous system; that is, they are arbitrarily ascribed various values for the portrayal of En-pH diagrams. However, they are quite often interdependent, through complex formation equilibria; for example, iron(II1) sulphate complex ions, which involve the following equilibria [15]: Fe3+ (aq) + SOi- (aq) = FeSO: (aq) FeSO:(aq)
+ SO:-(aq)
= Fe(S0,);
log,oKl = 2 (aq) log,,K,
= 1
116
T.E. Warner et al./ Hydrometallurgy
41 (1996) 107-l 18
0.8
0.6
-0.6
-0.8 -
I
-1.o!
Fe, N
, 0
I
(
2
1
,
4
I
,
6
,!
,
/
8
10
I
,
12
PH
Fig. 5. E,-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 0.1 mol dmm3. Activities of aqueous iron and nickel species = 0.1 mol dm-’ (bold line) and 10e6 mol dme3 (fine line). Metastable equilibria: with Fe,,Ni 4,5Ss and FeNi,S, as the only metal sulphide phases. H,S(aq) not consumed. cr-FeO(OH) and Ni(OH), as metastable products.
It is only for the sake of convenience that these equilibria are ignored, but this can be minimised by choosing values which are consistent with these reactions. Although conventional E,-pH diagrams show reversible equilibria reactions in which all the species as determined by thermodynamic criteria are present, in reality, they yield little practical information with regard to hydrometallurgical leaching and supergene alteration processes (especially for ternary systems) for the reasons described below. Equilibrium between three solid phases (which is a requirement of Gibbs Phase Rule for an invariant electrode potential in the ternary system [16]), is a far slower process in
T.E. Warner et al./HydrometaNurgy
41 (1996) 107-118
117
comparison with the equilibrium between just two such phases. Therefore, it is more suitable to consider pseudo two-solid phase equilibria. However, many of these are still unlikely to occur for kinetic reasons, therefore reactions which are known to occur (either from mineralogical or experimental observations) present a more practical choice; for example, pentlandite + violarite; pentlandite + orthorhombic sulphur; and violarite + orthorhombic sulphur. Similar reasoning can be used in the selection of the metastable phases: a-FeO(OH) and Ni(OH), versus Fe,O, and NiO, respectively. These reveal the co-existence of violarite and pentlandite in an aqueous system, which tallies with mineralogical evidence, and suggests that the pentlandite/violarite supergene alteration [ 171represents a near-equilibrium process. The standard free energy of formation for violarite at 298 K is consistent with the supergene cation-exchange nickel-enrichment process: FeNi,S,(c)
+ xNi2+(aq) + Fe,_,Ni,+,S,(c)
+ xFe’+ (aq)
These diagrams are useful for interpreting the results from hydrometallurgical leaching reactions where one common product of oxidation of pentlandite (or violarite) is elemental amorphous sulphur [4,5]. The lack of formation of violarite as an intermediate phase in the oxidative dissolution of pentlandite implies a state of metastability, corresponding to a kinetically irreversible oxidative process.
Acknowledgements
T.E.W would like to thank the Science and Engineering Research Council of the United Kingdom for the provision of a studentship in the form of a Research Quota Award.
References [I] Peters, E., Direct leaching of sulphides: Chemistry and applications. Metall. Trans. B, 7 (1976): 505-517. [2] Thomber, M.R., J. Appl. Electrochem., 13 (1983): 253-267. [3] Warner, T.E., An electrochemical study of the oxidative dissolution of synthetic nickel-iron sulphide minerals in aqueous media. Ph.D. Thesis, Univ. Leeds (1988). [4] Warner, T.E., Rice, N.M. and Taylor, N., Electrochemical study of the dissolution of pentlandite. Hydrometallurgy 3 1 (1992): 5.5-90. [5] Warner, T.E., Rice, N.M. and Taylor, N., An electrochemical study of the oxidative dissolution of synthetic violarite in aqueous media. In: Conf. Proc. Hydrometallurgy ‘94, on Environmentally sustainable Technology (Cambridge, UK, July 1 l-15), IMM and SC1 (1994). pp. 273-287. [6] Craig, J.R. and Naldrett, A.J., GAL-MAC Conference, (Sudbury, 1971) p. 16. [7] Mills, K.C., Thermodynamic Data for Inorganic Sulphides, Selenides and Tellurides. Butterworths, London ( 1974). [8] Thomber, M.R., Pers. commun. (August 1986). [9] Rajamani, V. and Prewitt, C.T., Am. Mineral., 60 (1975): 39-48. [lo] Craig, J.R., Am. Mineral., 56 (1971): 1303-1311. [ll] Vaughan, D.J. and Craig, J.R., Am. Mineral., 70 (1985): 1036-1043. [12] Nickel, E.H., Aus. IMM Conf. (Perth, Western Australia, May, 1973), pp. 11 l-l 16. [13] Thomber, M.R., Chem. Geol., 15 (1975): l-14.
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41 (1996) 107-118
Equilibria in Aqueous Solutions. Pergamon, Oxford (1966). [14] Porbaix, M., Atlas of Electrochemical [15] Sillen, L.G. and Martell, A.E., Stability constants of metal-ion complexes. Chem. Sot. Spec. Publ., 17 (1964) and 25 (1971). [16] Peters, E., The electrochemistry of sulphide minerals. In: J.O’M. Bockris, D.A.J. Rand and B.J. Welch (Editors), Trends in Electrochemistry. Plenum Press, New York (1977). pp. 267-290. [17] Nickel, E.H., Ross, J.R. and Thomber, M.R., Econ. Geol., 69 (1974): 93-107. [18] Pankratz, L.B., Thermodynamic properties of elements and oxides. U.S. Bur. Mines Bull., 672 (1982). [19] Latimer, W.M., The Oxidation States of the Elements and their Potentials in Aqueous Solutions. Prentice-Hall, Englewood Cliffs, N.J., 2nd ed. (1952). [20] Wagman, D.D., Evans, W.H., Parker, V.B., Halow, I., Bailey, SM. and Schumm, R.H., Selected values of chemical thermodynamic properties. Natl. Bur. Standards Techn. Note 270-4 (1969). [21] Barry, T.I., Coexistence or predominance area diagrams. DCS Internal Rep. 2/78, Natl. Physical Lab. Dept. of Industry (March, 1978). 14 pp.
Thermodynamic stability of pentlandite and violarite and new E,-pH diagrams for the iron-nickel sulphur aqueous system T.E. Warner a, N.M. Rice a3*, N. Taylor b aDepartment of Mining and Mineral Engineering, Uniuersiry of Leeds, Leeds, UK b Department of Chemistry, University of Leeds, Leeds, UK
Received 15May 1995; accepted 2 August 1995
Abstract Thermodynamic data for ideal stoichiometric pentlandite (Fe,,Ni&,) and violarite (FeNi,S,) have been re-evaluated from arguments concerning relative phase stability, in conjunction with known values for binary and ternary sulphides. The values AGp Fe,.,Ni,.,S, = - 813 kJ mol- ’ S 4 = - 346 kJ mol- ’ (x = 0) at 298 K were obtained. These values were and AGf” Fe,_,Ni,+, used to calculate Nemst functions for the Fe-Ni-S aqueous system, which are displayed by a series of E,-pH diagrams. These diagrams reveal the co-existence of violarite and pentlandite in an aqueous system, which tallies with mineralogical observations. The value for violarite can plausibly account for the supergene nickel-enrichment process observed in nature.
1. Introduction E,--pH diagrams have proved useful for displaying equilibria for several binary metal sulphide phases in aqueous systems. Such diagrams have been extended to ternary metal sulphide phases, notably for chalcopyrite (CuFeS,) and bomite (Cu,FeS,) in the copper-iron-sulphur aqueous system reported by Peters [l]. To the best of our knowledge, there has been only one attempt to construct an E,-pH diagram for the iron-nickel-sulphur aqueous system (Thomber [2]), even though this system contains the commercially important nickel-iron sulphide minerals pentlandite (Fe,,Ni,,S8) and violarite (FeNi,S,).
* Corresponding author. Fax: + 44 113 246 7310. 0304-386X/96/$15.00 0 I996 Elsevier Science B.V. All rights reserved SSDI 0304-386X(95)00081-X
108
T.E. Warner et al./ Hydrometahrgy
41 (1996) 107-l 18
In this present work, the standard free energies of formation for pentlandite and violarite have been re-evaluated at 298 K. These values were then used to calculate the Nemst functions for the various electrochemical equilibria in the Fe-Ni-S aqueous system. These have then been portrayed as a series of En-pH diagrams representing both thermodynamically reversible and certain metastable equilibria. The information contained within these diagrams has been compared with geological observations whilst also complementing the results from electrochemical dissolution studies reported previously by us [3,51.
2. Discussion 2.1. Appraisal of thermodynamic data for pentlandite Fe,., Ni,,, S, Craig and Naldrett [6] derived the following free energy of formation reaction from the results of sulphur vapour pressure measurements over the assemblage: iron + nickel + pentlandite; in conjunction with an estimated value of 505.43 J mol-’ K- ’ for the entropy of pentlandite: l.l25Fe(c)
+ l.l25Ni(c)
+ S,(g) -+ 0.25Fe,,,Ni,,,S,(c)
with AG&..ction= - 338067 + 166.1 T/J mol- ’ (T = 673-773 K). Extrapolating the above function to 298 K, and taking AGf” (298 K) S,(g) = 79.73 kJ mol-’ [7], leads to AC,” (298 K) Fe,,Ni,,S, = -835.4 kJ mol-‘. This value is in close agreement with that similarly calculated by Thomber [2] (- 836.9 kJ mol-‘1, based on the same source reference of Craig and Naldrett [6] (correcting for the fact that Thomber unfortunately divided instead of multiplying by 4.184 for the conversion of calories to joules [8]). This extrapolation of high temperature thermodynamic data to 298 K ignores the effects of a second-order phase transition in naturally occurring pentlandite (which occurs between 323 and 473 K [9]), which is accompanied by an abrupt change in heat capacity and, thus, an associated change in entropy for the phase transition. Therefore, it is not surprising that calculations for the appropriate Nemst functions at 298 K, based on the thermodynamic data given by Craig and Naldrett [6], imply that violarite (FeNi,S,) is a metastable phase in an aqueous system, contrary to geological observations. More specifically, the equilibria: 4.5FeNi,S,(c)
+ llH+(aq)
+ lie-=
Fe 4.5Ni4,5Ss(c) + 4.5NiS(c) + S.SH,S(aq)
and: l.SFeS,(c)
+ Ni,S,(c)
+ 2H+(aq) + 2e-=
l.SFeNi,S,(c)
+ H,S(aq)
are metastable with respect to: 4.5FeS,(c)
+ 1.5Ni3S4(c) + 14H+(aq) + 14e-= Fe,.,Ni,,,S,(c)
+ 7H,S(aq)
T.E. Warner et al./Hydrometallurgy
41 (1996) 107-I 18
109
Likewise, when considering the equilibria where hydrogen sulphide and sulphate are neither consumed nor produced, adopting the standard free energy data given by Craig and Naldrett [5] implies that the following equilibria: 2FeNi,S,(c)
+ OSNi’+(aq)
+ 2.5Fe2+(aq) -t 6e-= Fe,,,Ni,,,S,(c)
and: FeS,(c)
+ NiS,(c) + Ni2+(aq) + 2e-= FeNi,S,(c)
are metastable with respect to: 4FeS,(c)
+ OSFe’+(aq)
+ 4.5Ni2+(aq) + lOe-= Fe,,,Ni,,,S,(c)
Consequently, Thomber [2] suggested a manipulation of the free energy data to fit the mineralogical evidence; thus, he obtained a self-consistent set of data by using pyrite (AC,” (298 K) FeS, = - 160.2 kJ mol- ‘) as an ‘anchor value’, although he did not describe the exact criteria he used in selecting such data. More specifically, he obtained a value for AGf” (298 K) Fe,,Ni,,S, = -770.0 kJ mol-‘, which is inconsistent with the value for the following inequality where troilite (FeS), pyrrhotite (Fe& and heazlewoodite (Ni,S,) (whose free energy data are known with a reasonable degree of accuracy) are the phases presumed to be metastable with respect to pentlandite at 298 K: FeS + 0.5 Fe,S, + 1.5 Ni,S, --) Fe,.SNi,.5S, For Fe,,Ni,,S, to be a thermodynamically stable phase, AGgeacrionmust be less than zero; hence, using standard free energy data for the above binary phases (Table 1) leads to the inequality: AGfO(298K)Fe4.5Ni4,5Ss < -790.1 kJmol_’ Thus, faced with the lack of more reliable experimental data, the arithmetic mean between the value of this inequality (i.e., - 790.1 kJ mol- ’> and the value previously derived from Craig and Naldrett [6] (i.e., -835.2 kJ mol-‘), was adopted as the standard free energy of the formation for pentlandite at 298 K. Hence, a value of AGP (298 K) Fe,,Ni,,S,(c) = - 813 kJ mol-’ was used in this work. 2.2. Appraisal of thermodynamic data for violarite FeNi, S, Craig [lo] derived the following free energy of formation reaction from the results of various sulphur vapour pressure measurements in conjunction with an estimated value of 192.46 J mol-’ K- ’ for the entropy of violarite: Fe(c) + 2Ni(c) + 2S,(g) -+ FeNi,S,(c) AGL3ction
=
-
582204 + 350.6 T/Jmol-’
Taking AC,” (298 K) S,(g) = 79.73 kJ mol-’ [7] whilst solving the above function at 298 K leads to AC,” (298 K) FeNi,S,(c) = - 318.3 kJ mol- ’ (which is the same as that calculated by Thomber ( - 3 19.0 kJ mol- ’) 121,based on the same data). Craig’s estimate for the entropy of violarite (192.46 J mol-’ K- ‘> is derived from the average value for the two reactions [lo]: FeS,(c)
+ 2NiS(s) + FeNi,S,(c)
110
T.E. Warner et al./Hydrometallurgy
Table 1 Standard free energy of formation
41 (1996) 107-118
data at 298 K
Species
AC$‘/kJ mol-
&0(l)
-237.19 - 27.86 12.59 -741.99 - 752.87 - 742.0 - 78.87 - 4.6 - 45.6 - loo.7 - 748.5 - 160.2 -210.1 - 559.8 -91.5 -312.7 - 126.2 - 1015.3 - 742.34 - 488.0 -211.60 -712.1 - 469.9 - 447.3
H,S(aq) HS - (aq> H 2 SO&aq> HSO; (aq) SO:- (aq) Fe2’ (aq) Fe3+ (aq) Ni2+ (aq) FeS(c) (troilite) Fe,&(c) (pyrrhotite) Fe&(c) (pyrite) N&S,(c) (heazlewoodite) Ni,S,(c) (godlevskite) a-NiS(c) (millerite) Ni,S,(c) (polydymite) NiS,(c) (vaesite)
Fe,O,(c) Fe,O,(c) cy-FedOHXc) NiO(c) Ni ,0,(c)
Ni,O,(c) Ni(OH),(c)
’
Reference
1181 [71
[191 1191 [I91 it91 ml DO1 DOI [71 1201 171 [71 [71 171 [71 [71 DOI I201 1211 DOI iI91 WI I201
and: FeS(c)
+ NiS(c)
+ NiS,(c)
+ FeNi,S,(c)
using available entropy data for the binary phases, whilst assuming that the entropy change for these reactions is zero. In the present work, it is suggested that a more accurate estimate of AGP (298 K) FeNi,S, is obtainable if one considers that the change in standard free energy rather than the change in standard entropy is assumed to be zero for the particular reaction: FeS,(c)
+ 2NiS(c)
+ FeNi,S,(c)
at 298 K; by exploiting circumstantial evidence of certain phase phenomena presumed at temperatures near 298 K as described in detail below. The phase equilibria studied by Craig [lo], together with mineralogical observations and the results from experimental synthesis as reviewed by Warner [3], suggest that violarite of ideal stoichiometry (FeNi,S,) is thermodynamically unstable at 298 K with respect to the assemblage: Ni-rich violarite (Fe,_,NiZ+XS,, 1 2 x 2 O), pyrite (Fe&), millerite (NiS) and sulphur. Since FeNi,S, exists as a stable phase at 473 K [lo], it is assumed that FeNi,S, is only just unstable at 298 K (at least as an approximation).
T.E. Wumer et cd./ Hydrometallurgy 41 (1996) 107-118
111
Thus, the above decomposition reaction can be considered as being near to equilibrium, such that for the reaction: Fe,-,Ni,+, S 4(c) -P (1 - x)FeS,(c) + (2 + x)NiS(c) AG”Reaction= 0 (for a certain small value of ‘x’ at 298 K). Since the bonding in violarite of ideal composition, FeNizS4, is similar to that in Ni-rich violarite [ 111 and, in particular, since both these phases share an almost identical crystal structure, with the only significant differences being the ratio and ordering of Fe/Ni within the sulphur substructure, it is reasonable to assume that each has a similar enthalpy and entropy of formation at 298 K. Furthermore, if ‘x’ is relatively small at 298 K (as is proposed here), then the approximate value for AGf FeNi,S, at 298 K can be derived from the following reaction: FeNi,S,(c)
+ FeS,(c)
+ 2NiS(c)
where AGLaction (298 K) = 0, (which is presumed exact for a particular temperature between 298 and 473 K). This leads to the following inequalities at 298 K: AG,“FeNi,S, > AGPFeS, + 2AGFNiS AGPFe,_,Ni,+,S,
< AGPFeS, + 2AGPNiS
The thermodynamic data for pyrite and millerite as cited by Mills [7] (Table 11, yields the following speculative inequalities at 298 K: AG,“FeNi,S, > -343.1
Wmol-’
AGPFe,_,Ni2+XS4 < -343.1
kJmol-’
This result is comparable with the value ( - 348.4 kJ mol- ’> deduced by Thomber [2], yet takes into consideration the metastability of nominal violarite vis a vis stability of Ni-rich violarite at 298 K. Since the above result is expressed as an inequality, it is considered appropriate to take the mean between this inequality and the value estimated by Thomber [2], thus arriving at: AGP( 298 K) Fe, _ x Ni 2+XS4(c) = -346 kJmol-’ (x > 0) although the actual value may be more negative. Most encouragingly, when this value (- 346 kJ mol-’ ) is adopted in conjunction with the respective standard free energy data at 298 K (Table 1) for the following reaction: Fe,_,Ni 2+XS4(c) + (1 - x)Ni2+(aq)
+ Ni,S,(c)
+ (1 - x)Fe2+(aq)
and upon taking x = 0, it yields AG&actiOn = 0. This is consistent with such an ion-exchange equilibrium existing during supergene enrichment just beneath the water table [12,13], whereby the extent of the nickel enrichment of violarite would appear to be strongly influenced by the ratio of the Ni2+/Fe2’ aqueous activities in the surrounding environment. 2.3. Chemical equilibria (E,-pH
diagrams) for the Fe-Ni-S
aqueous system
E,-pH diagrams are a useful portrayal of the equilibria involved in the leaching of metal sulphides. The dissolution equilibria of the Fe-Ni-S aqueous system can be
112
T.E.Warner et al./Hydrometallurgy
41 (1996) 107-118
-0.6
-0.8
-1.0 0
2
4
6
8
IO
12
PH Fig. 1. E, -pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 0.1 mol dm - 3. Activities of aqueous iron and nickel species = 0.1 mol dm-‘. All reactions reversible.
viewed in a way that is analogous to the Pourbaix diagrams for the M-H,0 system [ 141, although the thermodynamic treatment involves a further two components (Figs. l-5). A comprehensive list of the Nernst functions for the various equilibria in the Fe-Ni-S aqueous system, together with the thermodynamic data on which the calculations are based, has been given by Warner [3] and the data are provided in Table 1. For violarite (where AGP (298 K) Fe, _ xNi2+x S 4 = - 346 kJ mol- ’>, since ‘x’ is assumed to be small and its precise value at 298 K unknown, the value x = 0 was adopted as an approximation, in order to simplify the stoichiometry and computation for the relevant Nemst functions. It is important to realise that equilibria more anodic than the oxygen/water equilibrium and more cathodic than the water/hydrogen equilibrium are thermodynamically
T.E. Warner et al./ Hydrometalhrgy 41 (1996) 107-l 18
’
113
0
P
‘\ . . *. -.
::::1 *.
‘.
-..
..b
-0.6
4%..
‘.
-.
-.
‘.
-0.8
m-
-1.0
0
2
4
6
I
,
a
10
’
I
12
PH Fig. 2. E,-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 1O-6 mol dme3. Activities of aqueous iron and nickel species = 0.1 mol dm- 3 (bold line) and 1O-6 mol drne3 (fine line). Metastable equilibria: with Fe,s Ni,,,S, as the only metal sulphide phase. H,S(aq) not consumed. n-FeO(OH) and Ni(OH), as metastable products.
meaningless, since the aqueous phase which includes H+(aq) does not exist under these circumstances. Nonetheless, equilibria drawn outside the aqueous domain, particularly in the cathodic region, are useful, since they give an indication of which reactions may occur at appropriate hydrogen over-potentials, and/or hydrogen over-pressures. The latter need to be fairly high in order to achieve this objective, so that changes in free energy of formation of the various species with respect to pressure should also be calculated in this situation. For the Eu-pH diagrams that describe ternary systems, it is particularly important to appreciate that the lines drawn on these diagrams refer to specific reactions, whilst the concept of a field of stability for a predominating phase has to be treated within the
114
T.E. Warner et al./Hydrometallurgy
41 (1996) 107-I 18
-0.6
-0.8
-1.0
c
,
I
,
0
2
4
r
I
1
6
8
’
I
I
IO
I2
PH Fig. 3. E,-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 0.1 mol dm-3. Activities of aqueous iron and nickel species = 0.1 mol dm- 3 (bold line) and 10e6 mol dm-3 (fine line). Metastable equilibria: with Fe,,Ni &s as the only metal sulphide phase. H,S(aq) not consumed. cu-FedOH) and Ni(OH), as metastable products.
context of these reactions. This is clearly illustrated in the example for the equilibrium (in strong alkali) between the two ternary phases: pentlandite and violarite (Fig. 11, which incorporates a third phase, millerite (NiS), and has a pH dependence as given by Nemst function: 4SFeNi,S,(c)
+ llH+(aq)
= Fe,,,Ni,.,S,(c)
+ lie-
+ 5SH,S(aq)
+ 4.5NiS(c)
En = -0.167 - 0.0591 pH - O.O296log[H,S(aq)] Nonetheless, the practical advantage of labelling such domains (mainly for improved clarity) was exploited for the diagrams given in this work.
T.E. Warner et al./HydrometaNurgy
41 (1996) 107-118
1 IS
0.6
-0.6
-0.8
-1.0 0
2
4
6
8
10
I2
PH Fig. 4. E,-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 10m6 mol dm- 3. Activities of aqueous iron and nickel species = 0.1 mol dmm3 (bold line) and 10e6 mol dmd3 (fme line). Metastable equilibria: with Fe,,Ni &Ss and FeNi,S, as the only metal sulphide phases. H,S(aq) not consumed. cu-FedOH) and Ni(OH), as metastable products.
The activities of aqueous ionic species are often treated by other workers, for example, Peters [I], as though they are independent variables in the Nemst function for the metal-sulphide aqueous system; that is, they are arbitrarily ascribed various values for the portrayal of En-pH diagrams. However, they are quite often interdependent, through complex formation equilibria; for example, iron(II1) sulphate complex ions, which involve the following equilibria [15]: Fe3+ (aq) + SOi- (aq) = FeSO: (aq) FeSO:(aq)
+ SO:-(aq)
= Fe(S0,);
log,oKl = 2 (aq) log,,K,
= 1
116
T.E. Warner et al./ Hydrometallurgy
41 (1996) 107-l 18
0.8
0.6
-0.6
-0.8 -
I
-1.o!
Fe, N
, 0
I
(
2
1
,
4
I
,
6
,!
,
/
8
10
I
,
12
PH
Fig. 5. E,-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulphur species = 0.1 mol dmm3. Activities of aqueous iron and nickel species = 0.1 mol dm-’ (bold line) and 10e6 mol dme3 (fine line). Metastable equilibria: with Fe,,Ni 4,5Ss and FeNi,S, as the only metal sulphide phases. H,S(aq) not consumed. cr-FeO(OH) and Ni(OH), as metastable products.
It is only for the sake of convenience that these equilibria are ignored, but this can be minimised by choosing values which are consistent with these reactions. Although conventional E,-pH diagrams show reversible equilibria reactions in which all the species as determined by thermodynamic criteria are present, in reality, they yield little practical information with regard to hydrometallurgical leaching and supergene alteration processes (especially for ternary systems) for the reasons described below. Equilibrium between three solid phases (which is a requirement of Gibbs Phase Rule for an invariant electrode potential in the ternary system [16]), is a far slower process in
T.E. Warner et al./HydrometaNurgy
41 (1996) 107-118
117
comparison with the equilibrium between just two such phases. Therefore, it is more suitable to consider pseudo two-solid phase equilibria. However, many of these are still unlikely to occur for kinetic reasons, therefore reactions which are known to occur (either from mineralogical or experimental observations) present a more practical choice; for example, pentlandite + violarite; pentlandite + orthorhombic sulphur; and violarite + orthorhombic sulphur. Similar reasoning can be used in the selection of the metastable phases: a-FeO(OH) and Ni(OH), versus Fe,O, and NiO, respectively. These reveal the co-existence of violarite and pentlandite in an aqueous system, which tallies with mineralogical evidence, and suggests that the pentlandite/violarite supergene alteration [ 171represents a near-equilibrium process. The standard free energy of formation for violarite at 298 K is consistent with the supergene cation-exchange nickel-enrichment process: FeNi,S,(c)
+ xNi2+(aq) + Fe,_,Ni,+,S,(c)
+ xFe’+ (aq)
These diagrams are useful for interpreting the results from hydrometallurgical leaching reactions where one common product of oxidation of pentlandite (or violarite) is elemental amorphous sulphur [4,5]. The lack of formation of violarite as an intermediate phase in the oxidative dissolution of pentlandite implies a state of metastability, corresponding to a kinetically irreversible oxidative process.
Acknowledgements
T.E.W would like to thank the Science and Engineering Research Council of the United Kingdom for the provision of a studentship in the form of a Research Quota Award.
References [I] Peters, E., Direct leaching of sulphides: Chemistry and applications. Metall. Trans. B, 7 (1976): 505-517. [2] Thomber, M.R., J. Appl. Electrochem., 13 (1983): 253-267. [3] Warner, T.E., An electrochemical study of the oxidative dissolution of synthetic nickel-iron sulphide minerals in aqueous media. Ph.D. Thesis, Univ. Leeds (1988). [4] Warner, T.E., Rice, N.M. and Taylor, N., Electrochemical study of the dissolution of pentlandite. Hydrometallurgy 3 1 (1992): 5.5-90. [5] Warner, T.E., Rice, N.M. and Taylor, N., An electrochemical study of the oxidative dissolution of synthetic violarite in aqueous media. In: Conf. Proc. Hydrometallurgy ‘94, on Environmentally sustainable Technology (Cambridge, UK, July 1 l-15), IMM and SC1 (1994). pp. 273-287. [6] Craig, J.R. and Naldrett, A.J., GAL-MAC Conference, (Sudbury, 1971) p. 16. [7] Mills, K.C., Thermodynamic Data for Inorganic Sulphides, Selenides and Tellurides. Butterworths, London ( 1974). [8] Thomber, M.R., Pers. commun. (August 1986). [9] Rajamani, V. and Prewitt, C.T., Am. Mineral., 60 (1975): 39-48. [lo] Craig, J.R., Am. Mineral., 56 (1971): 1303-1311. [ll] Vaughan, D.J. and Craig, J.R., Am. Mineral., 70 (1985): 1036-1043. [12] Nickel, E.H., Aus. IMM Conf. (Perth, Western Australia, May, 1973), pp. 11 l-l 16. [13] Thomber, M.R., Chem. Geol., 15 (1975): l-14.
118
T.E. Warner et al./Hydrometallurgy
41 (1996) 107-118
Equilibria in Aqueous Solutions. Pergamon, Oxford (1966). [14] Porbaix, M., Atlas of Electrochemical [15] Sillen, L.G. and Martell, A.E., Stability constants of metal-ion complexes. Chem. Sot. Spec. Publ., 17 (1964) and 25 (1971). [16] Peters, E., The electrochemistry of sulphide minerals. In: J.O’M. Bockris, D.A.J. Rand and B.J. Welch (Editors), Trends in Electrochemistry. Plenum Press, New York (1977). pp. 267-290. [17] Nickel, E.H., Ross, J.R. and Thomber, M.R., Econ. Geol., 69 (1974): 93-107. [18] Pankratz, L.B., Thermodynamic properties of elements and oxides. U.S. Bur. Mines Bull., 672 (1982). [19] Latimer, W.M., The Oxidation States of the Elements and their Potentials in Aqueous Solutions. Prentice-Hall, Englewood Cliffs, N.J., 2nd ed. (1952). [20] Wagman, D.D., Evans, W.H., Parker, V.B., Halow, I., Bailey, SM. and Schumm, R.H., Selected values of chemical thermodynamic properties. Natl. Bur. Standards Techn. Note 270-4 (1969). [21] Barry, T.I., Coexistence or predominance area diagrams. DCS Internal Rep. 2/78, Natl. Physical Lab. Dept. of Industry (March, 1978). 14 pp.