2370 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
of metal xanthate complexes, and (iii) formation of dixan-
thogen (Ralston, 1991). Each of the xanthate adsorption
steps is an oxidation process which is coupled by oxygen
reduction during flotation.
Due to the high demand for base metals and the rapid
depletion of more easily processed sulfide ore deposits, oxi-
dized sulfide ores (formation of metal oxides on the sur-
faces of sulfide minerals which occur as low-grade ores and
stockpiled in mine operations to allow priority processing
of higher grade sulfide ores) and metal oxide ores also need
to be processed to extract base metals. However, the flo-
tation of these types of ores is problematic because metal
oxides do not respond efficiently to thiol collectors dur-
ing flotation owing to their nonconductivities. In order to
effectively float oxidized sulfide ores and oxide ores, sulfidi-
zation, a process which converts a metal oxide to a metal
sulfide on the mineral surface in a sulfide solution has been
widely applied in industry (Orwe et al., 1998 Kappes and
Meikle, 2013). The formed metal sulfide enables the flota-
tion of oxidized sulfide ores and metal oxides by applying
thiol collectors to impart surface hydrophobicity. The clas-
sical sulfidization process is shown in Reaction (1) where
M2+ and A2– represent the cation and anion on the sur-
face of oxidized sulfides or oxides, respectively (Clark et al.,
2000). However, recent studies have shown that the sulfidi-
zation process also involves electrochemical reactions and
that it is not a simple anionic exchange (Huai et al., 2020).
2 2Na M A Na S M S A 2 2- 2 2+ 2 2- "++++++--+(1)
Cao et al. (2018) sulfidized and floated pyrite (FeS2)
which had been oxidized and depressed in copper flotation
for a flotation plant where gold was floated first with cop-
per sulfides and then with pyrite. These authors found that
sulfidization at 480 g/t Na2S achieved 74% pyrite recovery
in subsequent flotation even without the addition of collec-
tor in comparison with only 5% pyrite recovery achieved
without the sulfidization treatment. Cyro-XPS (X-ray pho-
toelectron spectroscopy) analyses in this work indicated the
formation of hydrophobic elemental sulfur and polysulfide
on the pyrite surface after being treated at 480 g/t Na2S and
the authors attributed the presence of elemental sulfur and
polysulfide to being responsible for pyrite flotation after
sulfidization. Although the pyrite surface was rendered
hydrophobic to some extent in this work, a new metal sul-
fide phase was not formed and therefore the sulfidization in
the work by Cao et al. (2018) may be classified as “superfi-
cial sulfidization”.
In other work, Huai and Peng (2020) investigated the
formation of a new metal sulfide phase on the surface of
oxidized pyrite during sulfidization with potential control.
By using electrochemical analyses, the authors identified
that a new iron sulfide phase was formed at a potential at
and below −300 mV (SHE). However, in order to allow
the newly formed iron sulfide phase to react with potas-
sium amyl xanthate (PAX), which is a necessity for effec-
tive pyrite flotation, the pulp potential had to be increased
to 100 mV (SHE) after sulfidization. Unfortunately, the
newly formed iron sulfide phase oxidized at a potential
of about −50 mV (SHE). Therefore, these studies on the
sulfidization of oxidized pyrite indicate the limitations of
the conventional sulfidization process. Although “super-
ficial sulfidization” generates hydrophobic sulfur oxida-
tion products, the flotation efficiency is low compared to
collector-induced flotation. On the other hand, although
the sulfidization at a low pulp potential can generate a new
iron sulfide phase to potentially promote collector-induced
flotation, the new iron sulfide phase is too reactive to be
sustained in an oxidizing environment where xanthate col-
lectors can adsorb on the surface.
In fact, pyrite flotation requires the highest potential
to induce xanthate adsorption compared to the flotation of
other sulfide minerals including chalcocite (Cu2S), born-
ite (Cu5FeS4) and chalcopyrite (CuFeS2) (Richardson and
Walker, 1985). Richardson and Walker (1985) found that
chalcocite can float well at pH 9.2 in the presence of ethyl
xanthate even at a negative potential. Additionally, pyrite
flotation in alkaline conditions is poor in general and the
addition of Cu(II) ions during pyrite flotation to activate
pyrite is a normal practice. With the addition of Cu(II),
the surface of pyrite may be activated with the formation
of a new Cu(I)S phase which enhances pyrite flotation
(Chandra et al., 2012). It is documented that copper acti-
vation on the surface of pyrite involves the reduction of
Cu(II) to Cu(I) and the oxidation of sulfide from the sur-
face lattice (von Oertzen et al., 2007). Peng et al. (2012)
found that a reducing environment is favorable for copper
activation on the surface of pyrite since the reduction of
Cu(II) to Cu(I) is promoted.
The studies on the sulfidization of oxidized pyrite and
copper activation on the surface of pyrite suggest that the
addition of copper ions during the sulfidization of oxi-
dized pyrite will enhance the efficiency of sulfidization and
subsequent pyrite flotation. While the reducing environ-
ment with the addition of Na2S during sulfidization will
enhance the reduction of Cu(II) and then copper activa-
tion on the surface of newly formed iron sulfide phase, the
formation of the Cu(I)S phase will be sustained at a lower
pulp potential for xanthate adsorption. It is important to
note that the reduction of Cu(II) during sulfidization will
be coupled to the oxidation of the sulfidizer (S2–). It is
of metal xanthate complexes, and (iii) formation of dixan-
thogen (Ralston, 1991). Each of the xanthate adsorption
steps is an oxidation process which is coupled by oxygen
reduction during flotation.
Due to the high demand for base metals and the rapid
depletion of more easily processed sulfide ore deposits, oxi-
dized sulfide ores (formation of metal oxides on the sur-
faces of sulfide minerals which occur as low-grade ores and
stockpiled in mine operations to allow priority processing
of higher grade sulfide ores) and metal oxide ores also need
to be processed to extract base metals. However, the flo-
tation of these types of ores is problematic because metal
oxides do not respond efficiently to thiol collectors dur-
ing flotation owing to their nonconductivities. In order to
effectively float oxidized sulfide ores and oxide ores, sulfidi-
zation, a process which converts a metal oxide to a metal
sulfide on the mineral surface in a sulfide solution has been
widely applied in industry (Orwe et al., 1998 Kappes and
Meikle, 2013). The formed metal sulfide enables the flota-
tion of oxidized sulfide ores and metal oxides by applying
thiol collectors to impart surface hydrophobicity. The clas-
sical sulfidization process is shown in Reaction (1) where
M2+ and A2– represent the cation and anion on the sur-
face of oxidized sulfides or oxides, respectively (Clark et al.,
2000). However, recent studies have shown that the sulfidi-
zation process also involves electrochemical reactions and
that it is not a simple anionic exchange (Huai et al., 2020).
2 2Na M A Na S M S A 2 2- 2 2+ 2 2- "++++++--+(1)
Cao et al. (2018) sulfidized and floated pyrite (FeS2)
which had been oxidized and depressed in copper flotation
for a flotation plant where gold was floated first with cop-
per sulfides and then with pyrite. These authors found that
sulfidization at 480 g/t Na2S achieved 74% pyrite recovery
in subsequent flotation even without the addition of collec-
tor in comparison with only 5% pyrite recovery achieved
without the sulfidization treatment. Cyro-XPS (X-ray pho-
toelectron spectroscopy) analyses in this work indicated the
formation of hydrophobic elemental sulfur and polysulfide
on the pyrite surface after being treated at 480 g/t Na2S and
the authors attributed the presence of elemental sulfur and
polysulfide to being responsible for pyrite flotation after
sulfidization. Although the pyrite surface was rendered
hydrophobic to some extent in this work, a new metal sul-
fide phase was not formed and therefore the sulfidization in
the work by Cao et al. (2018) may be classified as “superfi-
cial sulfidization”.
In other work, Huai and Peng (2020) investigated the
formation of a new metal sulfide phase on the surface of
oxidized pyrite during sulfidization with potential control.
By using electrochemical analyses, the authors identified
that a new iron sulfide phase was formed at a potential at
and below −300 mV (SHE). However, in order to allow
the newly formed iron sulfide phase to react with potas-
sium amyl xanthate (PAX), which is a necessity for effec-
tive pyrite flotation, the pulp potential had to be increased
to 100 mV (SHE) after sulfidization. Unfortunately, the
newly formed iron sulfide phase oxidized at a potential
of about −50 mV (SHE). Therefore, these studies on the
sulfidization of oxidized pyrite indicate the limitations of
the conventional sulfidization process. Although “super-
ficial sulfidization” generates hydrophobic sulfur oxida-
tion products, the flotation efficiency is low compared to
collector-induced flotation. On the other hand, although
the sulfidization at a low pulp potential can generate a new
iron sulfide phase to potentially promote collector-induced
flotation, the new iron sulfide phase is too reactive to be
sustained in an oxidizing environment where xanthate col-
lectors can adsorb on the surface.
In fact, pyrite flotation requires the highest potential
to induce xanthate adsorption compared to the flotation of
other sulfide minerals including chalcocite (Cu2S), born-
ite (Cu5FeS4) and chalcopyrite (CuFeS2) (Richardson and
Walker, 1985). Richardson and Walker (1985) found that
chalcocite can float well at pH 9.2 in the presence of ethyl
xanthate even at a negative potential. Additionally, pyrite
flotation in alkaline conditions is poor in general and the
addition of Cu(II) ions during pyrite flotation to activate
pyrite is a normal practice. With the addition of Cu(II),
the surface of pyrite may be activated with the formation
of a new Cu(I)S phase which enhances pyrite flotation
(Chandra et al., 2012). It is documented that copper acti-
vation on the surface of pyrite involves the reduction of
Cu(II) to Cu(I) and the oxidation of sulfide from the sur-
face lattice (von Oertzen et al., 2007). Peng et al. (2012)
found that a reducing environment is favorable for copper
activation on the surface of pyrite since the reduction of
Cu(II) to Cu(I) is promoted.
The studies on the sulfidization of oxidized pyrite and
copper activation on the surface of pyrite suggest that the
addition of copper ions during the sulfidization of oxi-
dized pyrite will enhance the efficiency of sulfidization and
subsequent pyrite flotation. While the reducing environ-
ment with the addition of Na2S during sulfidization will
enhance the reduction of Cu(II) and then copper activa-
tion on the surface of newly formed iron sulfide phase, the
formation of the Cu(I)S phase will be sustained at a lower
pulp potential for xanthate adsorption. It is important to
note that the reduction of Cu(II) during sulfidization will
be coupled to the oxidation of the sulfidizer (S2–). It is