XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2161
trithiocarbonates, alkyl hydroxamates and many more
(Nagaraj and Farinato, 2016). One of the key criteria
for collector design specifically targeted at sulfide miner-
als, typically base metal sulfides such as chalcopyrite and
galena, is the semi-conducting capability of such minerals.
A charge transfer mechanism between the collector species,
the mineral and an electron acceptor is a key process that
enables successful attachment of the collector to mineral
surface. This attachment occurs either by chemisorption or
physisorption and is necessary for sustained hydrophobic-
ity on the particle. Detailed descriptions of electrochemical
adsorption mechanisms for semi-conducting minerals can
be found in the literature (Fuerstenau et al., 2007 Nagaraj
and Farinato, 2016 Rao and Leja, 2004 Woods, 2003).
Platinum group elements occur in nearly 300 dif-
ferent minerals (Cabri 1992) which can be grouped into
classes that will be discussed further in this paper. One of
the classes is the sulfide category which renders the elec-
trical conductivity and subsequent reactivity of this class
of PGMs to be very similar to that of base metal sulfides.
Other classes of PGMs include the tellurides, bismuthotel-
lurides, arsenides and ferrous PGMs amongst others. The
inclusion of either Pt and Pd in the mineral structure ren-
ders these minerals relatively good conductors due to the
electronic structure of the platinum group element which
belongs to the transition element block in the period table
(Gunn, 2014 Berlincourt et al., 1981). As a consequence
the study of PGM interactions with collector molecules
in froth flotation has largely taken place on the premise
that the interactions fundamentally involve charge transfer
reactions.
Fundamental Electrochemistry of Collector Adsorption
A fundamental way to view the interactions of collectors
with minerals surfaces from an electrochemical and semi-
conductor/conductor perspective is to consider the model
of donors and acceptors participating in the oxidation/
reduction (redox) reaction. Semiconductors and conduc-
tors have free electrons within the conduction band of their
electronic structure. The designated n-type semiconductors
have excess electrons whilst p-type semi-conductors have
what are termed holes or deficiencies of electrons within
the conduction band (Shuey, 2012). These characteristics
affect the way that such conductors interact with charged
species in solution. For collector adsorption to take place
the direct injection of electrons model can be applied. This
model indicates that due to the conduction band of con-
ductors, external species can directly transfer electrons to
the conductor or mineral in a donor action with the elec-
trons being transferred to an acceptor species in solution
(Xu and Schoonen, 2000). Direct injection by a donor spe-
cies occurs when an aqueous species has a redox potential
higher in energy than the conduction band of the mineral
with the reaction occurring because such a transfer is ener-
getically possible. Collector species oxidation from xanthate
to dixanthogen is one such example of a direct injection
reaction. The reaction has been shown to be kinetically slow
in and of itself in solution but catalysed by a mineral sur-
face to result in subsequent hydrophobicity of the mineral
surface with a dixanthogen covering (Majima and Takeda,
1968). Figure 1 shows a schematic representation of the
direct injection model as applied to the electrochemical
collector adsorption mechanism that is reported in litera-
ture (Fuerstenau et al., 2007 Rao and Leja, 2004 Woods,
2003). In Figure 2 electrons from the highest occupied
molecular orbital (HOMO) in the electronic structure of
the collector are transferred to the conduction band (CB)
of the mineral. These electrons are subsequently transferred
from the conduction band to the lowest occupied molecular
orbital (LUMO) of the acceptor species which in the sys-
tem representing collector adsorption is typically dissolved
oxygen. The chemical reaction represented in Figure 1 can
be represented according to either reactions (1), (2) or (3)
coupled with reaction (4) where X represents a charge trans-
ferring collector species e.g., xanthates, and M represents a
metal species within a mineral
Electrochemical Potential Control in Flotation
The electrochemical process resulting in hydrophobicity
discussed in the previous section has been considered by
researchers and flotation practitioners as the possible basis
of a control strategy for hydrophobicity in the froth flota-
tion cell. A number of studies on base metal sulfides have
considered the relationship between redox potential and
floatability of minerals (Lotter et al., 2016 Ralston, 1991
Rand and Woods, 1984 Woods, 2003). Correlations have
been drawn between the redox potential of the pulp and
the successful hydrophobicity of sulfide minerals, more
notably by the thiol collectors. Oxidizing pulp potentials
have been related to the ability of collectors to oxidize and
therefore transfer electrons as per the model outline in
Figure 1 resulting in hydrophobicity of the mineral spe-
cies. In a seminal review by Lotter et al., (2016) much of
the work presented in the literature is brought together to
show that the degree of oxidation in the pulp corresponds
to different speciation of collector and therefore floatabil-
ity occurs due to different species of collector forming on
the surface. The work of Lotter et al., (2016) highlights
the probability of a floatability region that can be defined
according to redox potential Eh and pH which is another
trithiocarbonates, alkyl hydroxamates and many more
(Nagaraj and Farinato, 2016). One of the key criteria
for collector design specifically targeted at sulfide miner-
als, typically base metal sulfides such as chalcopyrite and
galena, is the semi-conducting capability of such minerals.
A charge transfer mechanism between the collector species,
the mineral and an electron acceptor is a key process that
enables successful attachment of the collector to mineral
surface. This attachment occurs either by chemisorption or
physisorption and is necessary for sustained hydrophobic-
ity on the particle. Detailed descriptions of electrochemical
adsorption mechanisms for semi-conducting minerals can
be found in the literature (Fuerstenau et al., 2007 Nagaraj
and Farinato, 2016 Rao and Leja, 2004 Woods, 2003).
Platinum group elements occur in nearly 300 dif-
ferent minerals (Cabri 1992) which can be grouped into
classes that will be discussed further in this paper. One of
the classes is the sulfide category which renders the elec-
trical conductivity and subsequent reactivity of this class
of PGMs to be very similar to that of base metal sulfides.
Other classes of PGMs include the tellurides, bismuthotel-
lurides, arsenides and ferrous PGMs amongst others. The
inclusion of either Pt and Pd in the mineral structure ren-
ders these minerals relatively good conductors due to the
electronic structure of the platinum group element which
belongs to the transition element block in the period table
(Gunn, 2014 Berlincourt et al., 1981). As a consequence
the study of PGM interactions with collector molecules
in froth flotation has largely taken place on the premise
that the interactions fundamentally involve charge transfer
reactions.
Fundamental Electrochemistry of Collector Adsorption
A fundamental way to view the interactions of collectors
with minerals surfaces from an electrochemical and semi-
conductor/conductor perspective is to consider the model
of donors and acceptors participating in the oxidation/
reduction (redox) reaction. Semiconductors and conduc-
tors have free electrons within the conduction band of their
electronic structure. The designated n-type semiconductors
have excess electrons whilst p-type semi-conductors have
what are termed holes or deficiencies of electrons within
the conduction band (Shuey, 2012). These characteristics
affect the way that such conductors interact with charged
species in solution. For collector adsorption to take place
the direct injection of electrons model can be applied. This
model indicates that due to the conduction band of con-
ductors, external species can directly transfer electrons to
the conductor or mineral in a donor action with the elec-
trons being transferred to an acceptor species in solution
(Xu and Schoonen, 2000). Direct injection by a donor spe-
cies occurs when an aqueous species has a redox potential
higher in energy than the conduction band of the mineral
with the reaction occurring because such a transfer is ener-
getically possible. Collector species oxidation from xanthate
to dixanthogen is one such example of a direct injection
reaction. The reaction has been shown to be kinetically slow
in and of itself in solution but catalysed by a mineral sur-
face to result in subsequent hydrophobicity of the mineral
surface with a dixanthogen covering (Majima and Takeda,
1968). Figure 1 shows a schematic representation of the
direct injection model as applied to the electrochemical
collector adsorption mechanism that is reported in litera-
ture (Fuerstenau et al., 2007 Rao and Leja, 2004 Woods,
2003). In Figure 2 electrons from the highest occupied
molecular orbital (HOMO) in the electronic structure of
the collector are transferred to the conduction band (CB)
of the mineral. These electrons are subsequently transferred
from the conduction band to the lowest occupied molecular
orbital (LUMO) of the acceptor species which in the sys-
tem representing collector adsorption is typically dissolved
oxygen. The chemical reaction represented in Figure 1 can
be represented according to either reactions (1), (2) or (3)
coupled with reaction (4) where X represents a charge trans-
ferring collector species e.g., xanthates, and M represents a
metal species within a mineral
Electrochemical Potential Control in Flotation
The electrochemical process resulting in hydrophobicity
discussed in the previous section has been considered by
researchers and flotation practitioners as the possible basis
of a control strategy for hydrophobicity in the froth flota-
tion cell. A number of studies on base metal sulfides have
considered the relationship between redox potential and
floatability of minerals (Lotter et al., 2016 Ralston, 1991
Rand and Woods, 1984 Woods, 2003). Correlations have
been drawn between the redox potential of the pulp and
the successful hydrophobicity of sulfide minerals, more
notably by the thiol collectors. Oxidizing pulp potentials
have been related to the ability of collectors to oxidize and
therefore transfer electrons as per the model outline in
Figure 1 resulting in hydrophobicity of the mineral spe-
cies. In a seminal review by Lotter et al., (2016) much of
the work presented in the literature is brought together to
show that the degree of oxidation in the pulp corresponds
to different speciation of collector and therefore floatabil-
ity occurs due to different species of collector forming on
the surface. The work of Lotter et al., (2016) highlights
the probability of a floatability region that can be defined
according to redox potential Eh and pH which is another