1626 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
products that inhibits further dissolution, was by Ammou-
Chokroum et al. (1977). Although this was criticized by
authors at the time, such as Dutrizac (1978), the idea of
passivation gained traction without much supporting evi-
dence (see O’Connor 2019 for a clear exposition). Several
different layers have been proposed, but none positively
observed.
The most compelling evidence for passivation comes
from electrochemistry, where the current is seen to decay
slowly following either switching the current on (at a set
voltage) or performing a step change in voltage. The results
from Parker et al. (1981) are shown in Figure 1. The decay
in current has been interpreted as an indication that a layer
of material is accumulating on the surface of the chalcopy-
rite (Parker et al., 1981 McMillan et al., 1982 Biegler and
Horne, 1985).
The current-voltage curve has a low current region
that is mostly independent of voltage followed by region
in which the current is strongly dependent on voltage (see
Figure 1b). This curve has been interpreted in terms of a
passive region followed by an active region. Because the
current-voltage curve does not follow the classical active-
passive-transpassive for valve metals, it is hypothesized that
the surface is self-passivating, which should make the iden-
tification of the composition of the passivating layer easier
because it does not need to be specially prepared.
Hackl et al. (1995) identified polysulphides on the sur-
face of chalcopyrite, and proposed that were the cause of
passivation, while others have proposed different chemis-
tries. This work is summarized in Crundwell (2015) and
O’Connor (2019).
The rate of dissolution of pyrite is also slow, and passiv-
ation theories have been advanced for its dissolution. Peters
(1984) used a version of the Pilling-Bedworth ratio to
argue that pyrite is passivated by sulphur because its molar
density is higher than that of the unreacted pyrite result-
ing in a dense sulphur layer. Similar arguments have been
presented informally at recent conferences. In the field of
metal passivation, the concepts underpinning the Pilling-
Bedworth ratio have not been supported by experimental
work. In other work, layers of polysulphides were proposed
as passivating layer (Hackl et al., 1996).
The rate of dissolution of sphalerite is relatively fast
compared with pyrite and chalcopyrite. However, the rate
of dissolution of sphalerite has also been interpreted as
being passivated, with layers of polysulphides implicated as
the cause (Wiesener et al., 2003).
Thus, the passivating layer model argues that all sul-
phide minerals form passivating layers which inhibit their
dissolution, and that the formation of polysulphides on the
surface were the probable cause of this passivation.
However, the evidence for the passivating model has
been weakened by several studies. Holmes and Crundwell
(2013) measured the thickness of the polysulphide layer at
different voltages at a pyrite electrode. Because the current
showed no sign of slowing with voltage or polysulphide
thickness, they concluded that polysulphides are not the
cause of passivation, at least on pyrite, and by implication,
on other minerals. Because the passivating-layer model
relied on the hypothesis that it is composed of polysul-
phides, the work of Holmes and Crundwell (2013) calls
the passivating model into question.
Figure 1. (a) Chronoamperometry curves at 0.5 V SCE with rests at the OCP for times indicated (Parker et al., 1981). (b) The
current–voltage curve for chalcopyrite. Data from Warren et al. (1982) for ‘Transvaal’ chalcopyrite in 1M H
2 SO
4 at 25°C
products that inhibits further dissolution, was by Ammou-
Chokroum et al. (1977). Although this was criticized by
authors at the time, such as Dutrizac (1978), the idea of
passivation gained traction without much supporting evi-
dence (see O’Connor 2019 for a clear exposition). Several
different layers have been proposed, but none positively
observed.
The most compelling evidence for passivation comes
from electrochemistry, where the current is seen to decay
slowly following either switching the current on (at a set
voltage) or performing a step change in voltage. The results
from Parker et al. (1981) are shown in Figure 1. The decay
in current has been interpreted as an indication that a layer
of material is accumulating on the surface of the chalcopy-
rite (Parker et al., 1981 McMillan et al., 1982 Biegler and
Horne, 1985).
The current-voltage curve has a low current region
that is mostly independent of voltage followed by region
in which the current is strongly dependent on voltage (see
Figure 1b). This curve has been interpreted in terms of a
passive region followed by an active region. Because the
current-voltage curve does not follow the classical active-
passive-transpassive for valve metals, it is hypothesized that
the surface is self-passivating, which should make the iden-
tification of the composition of the passivating layer easier
because it does not need to be specially prepared.
Hackl et al. (1995) identified polysulphides on the sur-
face of chalcopyrite, and proposed that were the cause of
passivation, while others have proposed different chemis-
tries. This work is summarized in Crundwell (2015) and
O’Connor (2019).
The rate of dissolution of pyrite is also slow, and passiv-
ation theories have been advanced for its dissolution. Peters
(1984) used a version of the Pilling-Bedworth ratio to
argue that pyrite is passivated by sulphur because its molar
density is higher than that of the unreacted pyrite result-
ing in a dense sulphur layer. Similar arguments have been
presented informally at recent conferences. In the field of
metal passivation, the concepts underpinning the Pilling-
Bedworth ratio have not been supported by experimental
work. In other work, layers of polysulphides were proposed
as passivating layer (Hackl et al., 1996).
The rate of dissolution of sphalerite is relatively fast
compared with pyrite and chalcopyrite. However, the rate
of dissolution of sphalerite has also been interpreted as
being passivated, with layers of polysulphides implicated as
the cause (Wiesener et al., 2003).
Thus, the passivating layer model argues that all sul-
phide minerals form passivating layers which inhibit their
dissolution, and that the formation of polysulphides on the
surface were the probable cause of this passivation.
However, the evidence for the passivating model has
been weakened by several studies. Holmes and Crundwell
(2013) measured the thickness of the polysulphide layer at
different voltages at a pyrite electrode. Because the current
showed no sign of slowing with voltage or polysulphide
thickness, they concluded that polysulphides are not the
cause of passivation, at least on pyrite, and by implication,
on other minerals. Because the passivating-layer model
relied on the hypothesis that it is composed of polysul-
phides, the work of Holmes and Crundwell (2013) calls
the passivating model into question.
Figure 1. (a) Chronoamperometry curves at 0.5 V SCE with rests at the OCP for times indicated (Parker et al., 1981). (b) The
current–voltage curve for chalcopyrite. Data from Warren et al. (1982) for ‘Transvaal’ chalcopyrite in 1M H
2 SO
4 at 25°C