MINERALOGICAL CHARACTERIZATION OF SUDBURY PYRRHOTITE TAILINGS: EVALUATING THE BIOLEACHING POTENTIAL

Abstract

Flotation operations in Sudbury, Ontario, seeking to separate pentlandite and chalcopyrite from mixed sulphide deposits, produce pyrrhotite tailings that have been disposed of in tailings ponds for over 50 years. These reactive tailings are viewed as waste, despite containing ~0.8 wt% Ni, and represent an environmental liability because of their acid mine drainage (AMD) potential. Given the large quantities of pyrrhotite currently stored in Sudbury, and declining ore grades elsewhere, these tailings have shifted from being viewed as a waste to a potential resource. Recent advances in the bioleaching of low-grade sulphide ores have raised the prospect of bioleaching as a potential low-cost processing option. However optimization of this proposed process requires a thorough understanding of the mineralogy of the pyrrhotite tailings that would be leached. The work presented here addresses the characterization of two distinct Sudbury pyrrhotite tailings, namely from the Vale and Glencore operations, using electron microprobe, XRD and QEMSCAN analysis. Particular focus is given to how the mineralogy may affect bioleaching as a processing option, through the deportment of nickel and the various phases present.

Full text

MINERALOGICAL CHARACTERIZATION OF SUDBURY PYRRHOTITE TAILINGS:
EVALUATING THE BIOLEACHING POTENTIAL
*Douglass Duffy1, Srinath Garg1, Cheryl Washer1, Tassos Grammatikopoulos2 and
Vladimiros Papangelakis1
1Department of Chemical Engineering and Applied Chemistry
University of Toronto, 200 College Street
Toronto, ON, Canada M5S 3E5
(*Corresponding author: douglass.duffy@mail.utoronto.ca)
2SGS Canada Inc.
185 Concession Street
Lakefield, ON, Canada K0L 2H0
ABSTRACT
Flotation operations in Sudbury, Ontario, seeking to separate pentlandite and chalcopyrite from
mixed sulphide deposits, produce pyrrhotite tailings that have been disposed of in tailings ponds for over
50 years. These reactive tailings are viewed as waste, despite containing ~0.8 wt% Ni, and represent an
environmental liability because of their acid mine drainage (AMD) potential. Given the large quantities of
pyrrhotite currently stored in Sudbury, and declining ore grades elsewhere, these tailings have shifted from
being viewed as a waste to a potential resource. Recent advances in the bioleaching of low-grade sulphide
ores have raised the prospect of bioleaching as a potential low-cost processing option. However
optimization of this proposed process requires a thorough understanding of the mineralogy of the pyrrhotite
tailings that would be leached. The work presented here addresses the characterization of two distinct
Sudbury pyrrhotite tailings, namely from the Vale and Glencore operations, using electron microprobe,
XRD and QEMSCAN analysis. Particular focus is given to how the mineralogy may affect bioleaching as
a processing option, through the deportment of nickel and the various phases present.
KEYWORDS
Pyrrhotite, Nickel, Sudbury, Mineralogy, Characterization, Bioleaching
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INTRODUCTION
The mixed sulphide deposit in the Sudbury basin has been mined, predominantly for its nickel
content, for approximately 100 years. The main minerals of interest in this ore body are pentlandite,
chalcopyrite, pyrrhotite and pyrite. Over the course of this history, the different mineral fractions in the ore
have been processed or disposed of in varying ways. Historically, the pyrrhotite fraction was processed
pyrometallurgically along with the nickel concentrate, thus converting the sulphide content of the
pyrrhotite to SO2 during the roasting process and diverting the iron content to the slag (Peek et al., 2011).
This increased volume of slag limited smelter throughput and, coupled with environmental regulations
designed to reduce SO2 emissions, encouraged mills to maximize the rejection of pyrrhotite from the
nickel concentrate.
As pyrrhotite was being removed from the nickel concentrate, attempts were made to develop
processes to economically treat pyrrhotite for both its sulphur and iron content (Queneau et al., 1951;
Sproule et al., 1955). These processes were never commercialized and were eventually abandoned in the
1980’s due to low-cost iron ore coming from Brazil and Australia, and sulphur from sour-gas
oversupplying the sulphuric acid market (Peek et al., 2011). As a result of the lack of processing options
and the acid mine drainage environmental liability caused by its reactivity, pyrrhotite has subsequently
been deposited in shallow lakes or tailings ponds.
In the period since 1991, Glencore (formerly Xstrata and Falconbridge) has disposed of
approximately eight million dry metric tons of pyrrhotite, while Vale (formerly Inco) has disposed of about
four times that amount. As both companies have been operating flotation circuits for over 50 years, a
conservative estimate of the total stored pyrrhotite would be 50-100 million dry metric tons (Peek et al.,
2011). Considering that these pyrrhotite tailings contain ~0.8 wt% Ni (relatively high for pyrrhotite), there
is significant potential value to be recovered, as shown in Table 1 (Index Mundi, 2015).
Table 1 – Pyrrhotite (Po) tailings contained nickel value
Estimated Po Tailings
100 Mt
wt% Ni
0.80%
Nickel Mass
800,000 tonnes
Nickel Value
(80% recovery, 18,000
CAD/tonne Ni)
$11.5 Billion CAD
Given the general trend of declining ore grades, and the fact that current operations at Talvivaara
attempt to mine and process nickel sulphide of even lower grade (0.27 wt% Ni), Sudbury pyrrhotite should
be considered a resource as opposed to a waste (Riekkola-Vanhanen, 2007; Puhakka et al., 2007).
Furthermore, the pyrrhotite tailings have the additional benefit of being already ground and readily
available for reclamation through surface mining.
Considering the success that bioleaching has found in processing low-grade copper sulphides, a
growing number of attempts are being made to process low-grade nickel sulphides via bioleaching
(Watling, 2006). The most notable project among these is the aforementioned Talvivaara project in
Sotkamo, Finland, which underwent decades of design work before encountering environmental issues
after start-up.
Bioleaching (i.e., biologically-assisted leaching) relies on a group of microorganisms that derive
the energy for their metabolism from the oxidation of inorganic compounds. Many species of organisms
have evolved to oxidize iron and/or sulphur at temperatures ranging from 30 °C to 80 °C, with some of the
more notable being Acidothiobacillus ferrooxidans, Leptospirillum ferrooxidans and Sulfolobus metallicus
(Hallberg and Johnson, 2003). In the bioleaching of iron sulphides, the microorganisms are capable of
oxidizing the ferrous iron dissolved in solution, provided the right microbial conditions exist. This ferric
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iron in turn is capable of oxidatively leaching additional iron sulphide minerals, such as pyrrhotite, as
depicted in both reaction (1) and Figure 1 below.
FeS (s) + 2Fe3+ (aq) → 3Fe2+ (aq) + S° (s) (1)
Figure 1 – Schematic of the bacterial role during bioleaching (Fowler et al., 2001)
In reality, however, both the reaction mechanism and stoichiometry are more complicated than
illustrated above. Depending upon the oxidation conditions, the amount of sulphide oxidized fully to
sulphate varies, with the remainder being only partly oxidized to elemental sulphur and various thiosalts.
Pyrrhotite has an additional leaching reaction to consider in the acidic ferrous/ferric sulphate solutions used
in bioleaching: namely, the non-oxidative acid dissolution. These two considerations are illustrated below
in reactions (2) and (3), respectively, where α is the fraction of sulphide oxidized to sulphate.
FeS(s) + (2+6α)Fe3+(aq) + 4αH2O(l) → (3+6α)Fe2+(aq) + (1-α)S°(s) + αSO42-(aq) + 8αH+(aq) (2)
FeS(s) + H2SO4(aq) → FeSO4(aq) + H2S(g) (3)
In the bioleaching of pyrite, it has been shown that microorganisms increase the oxidation rate
over and above that observed under the same Fe3+/Fe2+ ORP conditions. Fowler et al. (2001) provide data
indicating that the mechanism behind this increase is related to microorganisms changing the local surface
chemistry of the dissolving mineral by naturally adhering to the mineral surface, not by fundamentally
changing the reaction mechanism (Fowler et al., 2001). For the bioleaching of pyrrhotite, the mechanism
is less clear, but there is some evidence that the presence of bacteria increases the reaction rate compared to
similar abiotic conditions (Belzile et al., 2004). For pyrrhotite, oxidation with ferric ions appears to be
about an order of magnitude faster than either atmospheric oxidation or aqueous oxidation with oxygen,
10-8 mol m-2s-1 vs 10-9 mol m-2s-1 (Belzile et al., 2004; Steger, 1982; Janzen et al., 2000). The acid
dissolution reaction rate for pyrrhotite at pH=2.75 was found to be another order of magnitude slower, at
10-10 mol m-2s-1.
Talvivaara, like many of the copper-sulphide bioleaching processes that came before it, relies on
the relatively simple heap bioleach strategy, where the ore is stacked in large piles that are irrigated with
the lixiviant solution and aerated (Watling, 2008). An alternative strategy to heap bioleaching that has also
received increasing attention is the use of tank bioleaching, inspired by successful previous techniques
such as the BIOXTM and BACOX processes for the pre-treatment of pyritic gold concentrates; furthermore,
the BioNICTM and BacTech/Mintek Process have each been demonstrated at the pilot scale using various
organisms and operating temperatures (Watling, 2008; Dew et al., 1999; Gilbertson, 2000; Coram and
Rawlings, 2002; van Aswegen et al., 2007). As the Sudbury pyrrhotite is already ground to a fine particle
size (<100 μm), it is likely more amenable to tank bioleaching than to heap leaching (which requires much
larger particles for mechanical stability of the pile and permeability).
As part of a larger ongoing research program conducted at the University of Toronto into the
bioleaching of Sudbury pyrrhotite tailings, the current work focuses on mineralogical characterization,
through the use of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron microprobe
analysis. Pyrrhotite tailings produced by both Vale and Glencore were examined, with particular emphasis
on investigating how mineralogy may affect processing via bioleaching. The overall goal was to optimize
the kinetics and maximum nickel extraction of the process. More specifically, the scope of the present
work was to identify the various phases present in the pyrrhotite tailings, and elucidate the composition of
major nickel bearing phases. This enables the determination of the deportment of nickel between the
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various phases, primarily pyrrhotite and pentlandite. Knowledge of the gangue phases present might also
aid in elucidating the acid consuming behavior of the pyrrhotite tailings, over and above the acid consumed
by the non-oxidative dissolution of pyrrhotite. Finally, QEMSCAN analysis provides information on the
liberation and exposure degree of the various phases present, offering knowledge of how “available” each
of the mineral phases is to the leaching medium.
METHODOLOGY
Electron Microprobe
An analysis of the pyrrhotite tailings by SEM Microprobe was performed at the Geology
Department of the University of Toronto. Compositional analyses were acquired with an electron
microprobe (Cameca SX-50/51 (DCI 1300 DLL)) equipped with 3 tunable wavelength dispersive
spectrometers. The operating conditions were 40 degrees takeoff angle, and the beam diameter was 1 μm,
with energy of 20 keV and a current of 20 nA. Elemental spectra were acquired using analyzing crystals of
LiF for Fe Kα, Ni Kα, Cu Kα, Co Kα, and PET for S Kα. The calibration standards were: synthetic
pyrrhotite for Fe Kα, S Kα, cobaltite (Astimex) for Co Kα, pentlandite for Ni Kα, and chalcopyrite for Cu
Kα. The counting time was 20 seconds for Fe Kα, S Kα, Co Kα, 30 seconds for Cu Kα, and 80 seconds for
Ni Kα. The off peak counting time was 20 seconds for Fe Kα, S Kα, Co Kα, 30 seconds for Cu Kα, and 80
seconds for Ni Kα. Off Peak correction method was linear for Cu Kα, average for Ni Kα, high only for Co
Kα, and slope (Hi) for S Kα, Fe Kα. Unknown and standard intensities were corrected for dead-time and
standard intensities were corrected for standard drift over time.
XRD and QEMSCAN Analysis
The pyrrhotite tailings were also delivered to SGS Lakefield, ON, Canada to undergo Rietveld X-
ray Diffraction (XRD) and QEMSCAN analysis. The pyrrhotite tailings sample was screened into two size
fractions, +25 μm and -25 μm, prior to analysis by QEMSCAN. The XRD analysis was performed on the
head sample. The QEMSCAN analysis was performed in a Particle Mineral Analysis (PMA) mode (SGS
Canada Inc., 2013; SGS Canada Inc., 2015).
The mode of QEMSCAN analysis used was the Particle Mineral Analysis mode (PMA). This is a
two-dimensional mapping analysis aimed at resolving liberation and locking characteristics of a generic set
of particles. A pre-defined number of particles are mapped at a point spacing selected to spatially resolve
and describe mineral textures and associations. This mode is often selected to characterize concentrate
products, as both gangue and value minerals report in statistically abundant quantities to be resolved. The
PMA scans the entire polished section and provides a statistically robust population of mineral
identifications based on the X-ray chemistry of minerals. It should be noted that the energy dispersive X-
ray characteristics for magnetite and hematite are nearly identical and that these two minerals cannot
reliably be distinguished by QEMSCAN. Light elements such as Li, B, C, Be and H cannot be
discriminated by the QEMSCAN analysis (SGS Canada Inc., 2013; SGS Canada Inc., 2015).
RESULTS AND DISCUSSION
Electron Microprobe Analysis
The composition of the important nickel bearing phases in the pyrrhotite tailings was determined
by SEM microprobe. Generally speaking, Sudbury pyrrhotite tailings are known to contain nickel both in
solid solution inside the pyrrhotite matrix, and as fine-grained pentlandite phases (Peek et al., 2011).
Compositions were determined for both the pyrrhotite and pentlandite phases, for both Vale and Glencore
pyrrhotite tailings. In addition, measurements were made of the chalcopyrite phase for the Glencore
sample, and magnetic and non-magnetic enrichments of the Vale pyrrhotite, with the data summarized
below in Table 2. These data are then combined with the QEMSCAN analysis to determine the deportment
of nickel.
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Table 2 – Composition of nickel-bearing phases in Sudbury pyrrhotite tailings
Weight Percent (wt%)
Sample Mineral Phase Fe Ni S Co Cu
Glencore
Pyrrhotite
59.74
0.83
39.42
0.00
0.00
Pentlandite
31.80
34.25
33.45
0.50
0.00
Chalcopyrite
30.57
0.02
35.26
0.00
34.15
Vale
Pyrrhotite 60.17 0.72 39.12 N/A N/A
Mag. Pyrrhotite 59.98 0.64 39.38 N/A N/A
Non-Mag
Pyrrhotite
60.55 0.76 38.69 N/A N/A
Pentlandite 30.15 36.00 33.85 N/A N/A
Note: 0.00 denotes a value below detection limit. N/A denotes no measurement.
Table 2 indicates that the pyrrhotite phase in each of the tailings, from Vale and Glencore, has a
similar composition, roughly 60 wt% Fe, 39 wt% S and 0.7-0.8 wt% Ni. The magnetic and non-magnetic
enriched fractions of the Vale pyrrhotite sample do not display significantly different compositions
compared to the bulk sample. The composition of the pentlandite phase in each sample is also fairly
consistent, though the Vale sample has a slightly higher Ni:Fe ratio. Cobalt measurements taken for the
Glencore sample indicate that the pentlandite phase also contains approximately 0.5 wt% Co. For the
Glencore sample, the chalcopyrite phase was found to also contain a small but detectable quantity of trace
nickel, in addition to the expected Fe, Cu and S contents.
X-Ray Diffraction Analysis
Both the Vale and Glencore pyrrhotite tailings were submitted to SGS Lakefield for Rietveld
XRD analysis. These results are summarized below in Table 3.
It can be observed that the Glencore and Vale pyrrhotite tailings are both predominantly
pyrrhotite, though the Vale tailings contain notably more pyrrhotite. In addition, two distinct crystal
structures of pyrrhotite were identified, monoclinic and hexagonal. The pyrrhotite in the Glencore tailings
is mainly monoclinic, present at a ratio of nearly 20:1, while the Vale tailings contains more similar
amounts of the two phases, present in a monoclinic to hexagonal ratio of approximately 1.5:1.
The biggest distinction between the two pyrrhotite tailings is in the gangue phases present, with
the Glencore tailings showing a diverse mixture of silicates and the Vale tailings displaying only quartz,
calcite and szomolnokite. Magnetite, pentlandite and chalcopyrite are present in the Glencore tailings in
minor amounts, 6.8, 1.2 and 0.2 wt% respectively, with assorted silicate gangue phases accounting for
about 30 wt%. The Vale tailings contain magnetite, pentlandite and chalcopyrite as well, though in smaller
amounts of 3.3, 0.6 and 0.2 wt%, respectively. The presence of szomolnokite, FeSO4*H2O, is likely
indicative of some surface oxidation of the Vale sample prior to analysis.
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Table 3 – Summary of XRD analyses
Mineral/Compound
Glencore
Pyrrhotite
(wt%)
Vale Pyrrhotite
(wt%)
Quartz 4.5 1.0
Albite 12.8
Microcline 3.7
Chlorite 6.5
Muscovite 0.0
Biotite 1.3
Actinolite 2.2
Chalcopyrite 0.2 0.2
Pentlandite 1.2 0.6
Magnetite 6.8 3.3
Pyrrhotite (Hexagonal) 3.3 35.9
Pyrrhotite (Monoclinic) 57.5 55.8
Diopside 0.0
Calcite N/A 0.3
Szomolnokite N/A 2.9
Total 100
Note: N/A denotes no measurement.
QEMSCAN Analysis
Samples of both Vale and Glencore pyrrhotite tailings were submitted to SGS Lakefield to
undergo QEMSCAN analysis, using the Particle Mineral Analysis (PMA) mode described above. Each of
these samples was screened prior to analysis into two fractions, +25 μm and -25 μm. The mineral
abundance data for each fraction as well as the calculated abundance of the head, or combined, sample can
be seen in Table 4.
The mineral abundance results largely confirm the XRD analysis, but provide clearer data with
regards to size fraction. As QEMSCAN relies on energy dispersive X-ray measurements, not X-ray
diffraction, it is incapable of distinguishing the two crystal structures of pyrrhotite. It is important to note
the widely different size distributions of the two tailings, with the Glencore sample having approximately
two-thirds of its mass in the fine fraction (-25 μm), while the situation is reversed for the Vale pyrrhotite
tailings.
Table 4 indicates that the Glencore pyrrhotite tailings contain 61.3 wt% pyrrhotite, 7.4 wt%
magnetite, 1.2 wt% pentlandite and 0.2 wt% chalcopyrite, all closely agreeing with the Rietveld XRD
analysis. The silicate gangue phases account for a total of ~30 wt%, again matching the total from XRD;
however, the distribution among the silicate phases varies considerably.
The relative purity of the Vale pyrrhotite tailings is indicated by the higher pyrrhotite fraction
(86.2 wt%) and total silicates of ~7 wt%. Magnetite, pentlandite and chalcopyrite are also present in
quantities of 4.5, 1.2, and 0.6 wt%, respectively.
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Table 4 – Summary of mineral abundance by QEMSCAN
Sample Glencore Vale
Size Fraction
Combined
+25um
-25um
Combined
+25um
-25um
Mass Size Distribution (%)
100.0
32.9
67.1
100
64.6
35.4
Mineral
Mass
(%)
Chalcopyrite 0.19 0.13 0.06 0.62 0.49 0.13
Other Cu
Sulphides
0.03 0.02 0.01 0.06 0.05 0.01
Pentlandite 1.17 0.30 0.87 1.19 0.76 0.43
Pyrite 0.58 0.06 0.52 0.41 0.23 0.18
Pyrrhotite 61.3 23.4 37.9 86.2 56.6 29.5
Other Sulphides 0.01 0.01 0.00 0.08 0.04 0.04
Fe-Oxides 7.39 1.28 6.12 4.45 1.90 2.55
Other Oxides 0.24 0.05 0.19 0.11 0.06 0.05
Quartz 3.36 1.04 2.33 0.34 0.20 0.13
K-Feldspar 3.89 1.10 2.79 0.18 0.12 0.06
Plagioclase 7.61 2.37 5.24 1.5 1.06 0.43
Orthopyroxene 0.24 0.06 0.17 0.55 0.38 0.17
Clinopyroxene 1.39 0.34 1.05 0.16 0.12 0.04
Mica 1.94 0.29 1.65 0.69 0.44 0.25
Chlorite 1.93 0.48 1.45 0.85 0.44 0.41
Amphibole 7.76 1.49 6.27 1.54 1.09 0.45
Epidote 0.55 0.42 0.13 N/A N/A N/A
Other Silicates 0.24 0.07 0.17 0.56 0.47 0.09
Other 0.21 0.05 0.15 0.56 0.13 0.44
Total 100.0 32.9 67.1 100 64.6 35.4
Note: N/A denotes no measurement.
During a bioleach operation on either tailing, the chemistry of the system will be dictated largely
by pyrrhotite. However, the silicate gangue phases, particularly for the Glencore tailings, may also affect
the chemistry of the bioleach operation, especially in relation to acid consumption.
Combining the mineral abundance data provided by QEMSCAN (Table 4) and the phase
compositions for the nickel bearing phases acquired using the electron microprobe (Table 2), allows the
calculation of the deportment of nickel between phases; these data are presented below in Table 5. It
should be noted that magnetite likely accounts for a small fraction of the nickel in the Glencore tailings as
well, but measurements of its phase composition were not made. Based on the assumed composition of 0.2
wt% Ni, magnetite would account for approximately 1-2% of the nickel in the Glencore tailings and ~1%
in the Vale tailings.
It can be seen that while pyrrhotite makes up the majority of the mass of the tailings, pentlandite
with its higher stoichiometric nickel content and smaller mass accounts for nearly half of the overall nickel
content. Therefore, any bioleach operation should dissolve both mineral phases to gain reasonable nickel
recoveries.
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Table 5 – Nickel deportment
Mass % Ni
Glencore Vale
Pentlandite
45
40
Pyrrhotite
55
59
The cumulative grain size distributions of the mineral phases, as well as the particle size, of each
pyrrhotite tailings sample are shown in Figure 2. Overall, the two tailings show quite similar grain size
distributions; however, the Vale pyrrhotite tailings consistently display slightly larger grain sizes,
particularly for the pentlandite and silicate phases, except for fine silicates. The particle sizes d50 for the
Vale and Glencore pyrrhotite tailings are 21 μm and 28 μm, respectively.
Figure 2 – Cumulative grain size distribution by phase
Liberation and exposure data are also provided by the QEMSCAN analysis, summarized below in
Table 6. The liberation percentage indicates the mass fraction of the mineral that is free and liberated, the
threshold being that an individual mineral grain accounts for greater than 80% of the 2D particle area. The
exposed percentage indicates the mass fraction of the mineral having an exposed surface greater than 80%.
Table 6 – Liberation and exposure
Glencore Vale
Liberated Exposed Liberated Exposed
Pentlandite 75% 73% 48% 45%
Pyrrhotite 94% 92% 97% 96%
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The pyrrhotite in each tailings sample is well liberated and exposed, indicating that the majority of
the surface area is amenable to leaching. Pentlandite is less well liberated and exposed in both tailings
samples, particularly in the Vale tailings, and this fact may obstruct the leaching of the mineral.
CONCLUSIONS
The mineralogy of two distinct pyrrhotite tailings from Sudbury, supplied by Vale and Glencore,
has been investigated. Each tailings sample contains a majority of pyrrhotite, with pyrrhotite accounting for
86 wt% of the Vale tailings and 61 wt% of the Glencore tailings. The pyrrhotite in each of the tailings
contains approximately 0.7-0.8 wt% nickel in solid solution in the pyrrhotite structure. Pentlandite, though
accounting for only 1.2 wt% of each of the tailings, contains nearly half of the nickel, 40 and 45% for Vale
and Glencore samples, respectively. The balance of each of the tailings is magnetite, traces of other
sulphides, and various silicate phases. This gangue content, particularly for the Glencore tailings, where it
accounts for approximately 30 wt%, may significantly affect the chemistry of the bioleach.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge Vale Canada Ltd., Glencore Canada Co. and NSERC for
financial assistance, as well as SGS Canada Inc. for the analyses they performed.
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COM 2015 | THE CONFERENCE OF METALLURGISTS
hosting AMCAA | America's Conference on Aluminum Alloys
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