XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1627
The argument that the decay of current-time curve for
chalcopyrite provides electrochemical evidence for the pas-
sivating model is not supported because the surface reacti-
vates during periods when the current is off. This is shown
in Figure 1a, in which the current peak returns to a high
value after a period of being switched off. Such reactivation
does not happen with passivated metals – once the layer
is present, it does not immediately dissolve – and is not
expected for sulphide minerals.
The inability to define the composition of the passive
layer other than the discredited polysulphides makes it dif-
ficult to support the passivating-layer model.
SEMICONDUCTOR MODEL
Many sulphide minerals are semiconductors, with natural
crystals being used in “crystal” radio sets prior to the devel-
opment of solid-state devices (Roberts and Adams, 1922
Wherry, 1925). Our research work has led us to propose
that these properties also influence their rate of dissolution
(Crundwell, 1988a 2013). Crundwell (1988a, 1988b)
argued that the key to understanding the dissolution of
these minerals is to understand their electronic structure.
In order to dissolve a mineral oxidatively an elec-
tron must be removed from a bonding orbital at the sur-
face, and to dissolve a mineral reductively, an electron
must be inserted into an anti-bonding orbital at the sur-
face (Crundwell, 1988a 1988b 2013). This is shown in
Figure 2. The upper valence band of a semiconductor is
occupied by electrons and is usually composed of bonding
orbitals. The lower conduction band is usually composed
of anti-bonding orbitals. Electrons transfer without an
increase or decrease of energy, which means that the energy
level of the oxidant in solution must be relatively close to
that of the bonding orbital. However, the energy levels of
the oxidant in solution fluctuates significantly because of its
interactions with water, making it statistically possible for
adiabatic electron transfer to occur.
The electronic structure of minerals is more complex
because the upper valence band might have non-bonding
characteristics. This is particularly the case for sulphides
with d-band elements like iron. If the upper valence band
has non-bonding characteristics, then the removal of elec-
trons from these orbitals will not result in bond-breaking,
and hence dissolution will not occur. Such a mineral will
display noble characteristics. Sphalerite has an upper
valence band composed of bonding orbitals (Vaughan and
Tossell, 1980), while pyrite and chalcopyrite have upper
valence bands that are non-bonding and anti-bonding in
character (Vaughan and Rosso, 2006 Tossell et al., 1982).
Given the semiconductor model proposed by Crundwell
(1988a), this means that sphalerite will dissolve oxidatively
at a faster rate than pyrite or chalcopyrite, which is indeed
the case.
Crundwell and coworkers have expanded this model
by applying it to the specific cases of sphalerite, chalcopy-
rite and pyrite (Crundwell, 1988a, 2015 Crundwell, 2013
Figure 2. (a) The formation of molecular orbitals from the atomic orbitals, showing σ-bonding molecular orbitals and σ*- anti-
bonding molecular orbitals. (b) The formation of valence and conduction bands of bonding and anti-bonding character. (c)
Oxidative dissolution requires the breaking of bonds at the surface by the removal of electrons from bonding orbitals
The argument that the decay of current-time curve for
chalcopyrite provides electrochemical evidence for the pas-
sivating model is not supported because the surface reacti-
vates during periods when the current is off. This is shown
in Figure 1a, in which the current peak returns to a high
value after a period of being switched off. Such reactivation
does not happen with passivated metals – once the layer
is present, it does not immediately dissolve – and is not
expected for sulphide minerals.
The inability to define the composition of the passive
layer other than the discredited polysulphides makes it dif-
ficult to support the passivating-layer model.
SEMICONDUCTOR MODEL
Many sulphide minerals are semiconductors, with natural
crystals being used in “crystal” radio sets prior to the devel-
opment of solid-state devices (Roberts and Adams, 1922
Wherry, 1925). Our research work has led us to propose
that these properties also influence their rate of dissolution
(Crundwell, 1988a 2013). Crundwell (1988a, 1988b)
argued that the key to understanding the dissolution of
these minerals is to understand their electronic structure.
In order to dissolve a mineral oxidatively an elec-
tron must be removed from a bonding orbital at the sur-
face, and to dissolve a mineral reductively, an electron
must be inserted into an anti-bonding orbital at the sur-
face (Crundwell, 1988a 1988b 2013). This is shown in
Figure 2. The upper valence band of a semiconductor is
occupied by electrons and is usually composed of bonding
orbitals. The lower conduction band is usually composed
of anti-bonding orbitals. Electrons transfer without an
increase or decrease of energy, which means that the energy
level of the oxidant in solution must be relatively close to
that of the bonding orbital. However, the energy levels of
the oxidant in solution fluctuates significantly because of its
interactions with water, making it statistically possible for
adiabatic electron transfer to occur.
The electronic structure of minerals is more complex
because the upper valence band might have non-bonding
characteristics. This is particularly the case for sulphides
with d-band elements like iron. If the upper valence band
has non-bonding characteristics, then the removal of elec-
trons from these orbitals will not result in bond-breaking,
and hence dissolution will not occur. Such a mineral will
display noble characteristics. Sphalerite has an upper
valence band composed of bonding orbitals (Vaughan and
Tossell, 1980), while pyrite and chalcopyrite have upper
valence bands that are non-bonding and anti-bonding in
character (Vaughan and Rosso, 2006 Tossell et al., 1982).
Given the semiconductor model proposed by Crundwell
(1988a), this means that sphalerite will dissolve oxidatively
at a faster rate than pyrite or chalcopyrite, which is indeed
the case.
Crundwell and coworkers have expanded this model
by applying it to the specific cases of sphalerite, chalcopy-
rite and pyrite (Crundwell, 1988a, 2015 Crundwell, 2013
Figure 2. (a) The formation of molecular orbitals from the atomic orbitals, showing σ-bonding molecular orbitals and σ*- anti-
bonding molecular orbitals. (b) The formation of valence and conduction bands of bonding and anti-bonding character. (c)
Oxidative dissolution requires the breaking of bonds at the surface by the removal of electrons from bonding orbitals