XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1631
present of surface states. However, the lack of this effect
does not negate the model, it enhances its sophistication.
In addition to the detailed work of Bryson and
Crundwell (2014), numerous other researchers showed
that a pyrite electrode responds to light and increases the
rate of dissolution (Chen et al., 1991 Ennaouni et al.,
1985, 1986 Mishra and Osseo-Asare, 1992 Schubert and
Tributsch, 1990).
Chalcopyrite, Its Color and Its Rate of Dissolution
Chalcopyrite has received a large amount of attention, par-
ticularly in the search for a passivating layer. The band gap
of chalcopyrite is low, about 0.6 eV. Usually, the absorption
of light by a semiconductor with such a small band gap
would be black or dark grey in colour. Germanium and
galena have a similar band gaps and are grey. Chalcopyrite
is brassy, suggesting a more complex electronic structure.
The band structure of chalcopyrite based on the
interpretation of adsorption spectra is shown in Figure 7
(Oguchi, 1980). Light excites electrons from the valence
band to three different levels, resulting in a richer spec-
trum than expected simply from the band gap. Bryson et
al. (2016) reported that the electrochemical current was
affected by irradiation with light across the visible spec-
trum, but that this effect was stronger from the red end to
the violet end of the spectrum. The effect of potential on
the photocurrent and the effect of the colour (energy) of
the light is shown in Figure 8. This is in accordance with
the band diagram. Nicol’s (2016) results support the semi-
conductor model rather than his objections.
The significant feature of this band structure is that
the highest occupied orbitals (that is, the valence band) has
antibonding characteristics (Tossel et al., 1982). This means
that the removal of an electron from the valence band in
an oxidative reaction will not result in bond-breaking – to
cause bond-breaking an electron should be injected into
antibonding orbital. In other words, a reductive step might
assist in the dissolution, particularly at lower redox poten-
tials when the energy levels of the oxidant overlap weakly
with the deeper bonding orbitals. This feature explains some
of the results that are more difficult to understand, such the
work of Hiroyoshi (2000) that argued for a ferrous-pro-
moted mechanism, and redox-potential window promoted
in the bioleaching literature (Pinches et al., 1997).
CONCLUSIONS
This paper has provided further exposition of the semicon-
ductor model proposed by Crundwell and co-workers for
the dissolution of sulphide minerals. The iron impurity of
sphalerite has a major effect on its colour and rate of dis-
solution, for similar reasons: the iron impurity provides an
energy level within the band gap which absorbs light in the
blue end of the visible spectrum and allows electrons to be
removed more easily from bonding orbitals. The dissolu-
tion of pyrite is facilitated by surface states. However, but
because the upper valence band of pyrite is of non-bond-
ing character, the rate of dissolution is slow. The colour of
Figure 6. Band structure of pyrite, with upper valence band from non-bonding t2g orbitals of iron
present of surface states. However, the lack of this effect
does not negate the model, it enhances its sophistication.
In addition to the detailed work of Bryson and
Crundwell (2014), numerous other researchers showed
that a pyrite electrode responds to light and increases the
rate of dissolution (Chen et al., 1991 Ennaouni et al.,
1985, 1986 Mishra and Osseo-Asare, 1992 Schubert and
Tributsch, 1990).
Chalcopyrite, Its Color and Its Rate of Dissolution
Chalcopyrite has received a large amount of attention, par-
ticularly in the search for a passivating layer. The band gap
of chalcopyrite is low, about 0.6 eV. Usually, the absorption
of light by a semiconductor with such a small band gap
would be black or dark grey in colour. Germanium and
galena have a similar band gaps and are grey. Chalcopyrite
is brassy, suggesting a more complex electronic structure.
The band structure of chalcopyrite based on the
interpretation of adsorption spectra is shown in Figure 7
(Oguchi, 1980). Light excites electrons from the valence
band to three different levels, resulting in a richer spec-
trum than expected simply from the band gap. Bryson et
al. (2016) reported that the electrochemical current was
affected by irradiation with light across the visible spec-
trum, but that this effect was stronger from the red end to
the violet end of the spectrum. The effect of potential on
the photocurrent and the effect of the colour (energy) of
the light is shown in Figure 8. This is in accordance with
the band diagram. Nicol’s (2016) results support the semi-
conductor model rather than his objections.
The significant feature of this band structure is that
the highest occupied orbitals (that is, the valence band) has
antibonding characteristics (Tossel et al., 1982). This means
that the removal of an electron from the valence band in
an oxidative reaction will not result in bond-breaking – to
cause bond-breaking an electron should be injected into
antibonding orbital. In other words, a reductive step might
assist in the dissolution, particularly at lower redox poten-
tials when the energy levels of the oxidant overlap weakly
with the deeper bonding orbitals. This feature explains some
of the results that are more difficult to understand, such the
work of Hiroyoshi (2000) that argued for a ferrous-pro-
moted mechanism, and redox-potential window promoted
in the bioleaching literature (Pinches et al., 1997).
CONCLUSIONS
This paper has provided further exposition of the semicon-
ductor model proposed by Crundwell and co-workers for
the dissolution of sulphide minerals. The iron impurity of
sphalerite has a major effect on its colour and rate of dis-
solution, for similar reasons: the iron impurity provides an
energy level within the band gap which absorbs light in the
blue end of the visible spectrum and allows electrons to be
removed more easily from bonding orbitals. The dissolu-
tion of pyrite is facilitated by surface states. However, but
because the upper valence band of pyrite is of non-bond-
ing character, the rate of dissolution is slow. The colour of
Figure 6. Band structure of pyrite, with upper valence band from non-bonding t2g orbitals of iron
