XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1645
different overall dissolution rates, meaning dissolution is
incongruent and there is no single rate equation. In many
cases, as will be discussed below, some part of the mineral
does not dissolve at all.
Connecting these mineralogical models to metal-
lurgical effects is fraught. The metallurgical literature on
(phyllo)silicate dissolution usually starts from the practical
problems experienced in operations and works backward to
the mineral source. In one example, Queneau and Berthold
(1986) described phyllosilicate dissolution in the context
of silica gel formation in leaching, a problem whose nega-
tive effects they reviewed comprehensively. Ndlovu et al.
(2014) focused on the rheology of slurries created by dif-
ferent phyllosilicate minerals in leaching, and hundreds
of studies over the years have examined the extraction (or
inextractability) of valuable base metals from phyllosilicate
ores. Studies of phyllosilicates in particular, or mechanis-
tic approaches, are rare in the metallurgical literature. The
most mechanistic study available is a pair of papers by Terry
(1983a,b), which describe the crystal chemistry and acid
dissolution mechanisms of silicates in general. The volume
edited by Graefe et al. (2017) attempted to split the dif-
ference between mechanism and practical implications by
tracing the impact of clays through the mineral process-
ing value chain, with strong mineralogical control. In the
case of heap leaching, they found that the specific effects of
phyllosilicates were overwhelmed by the many other factors
and minerals involved in leaching (Li et al., 2017). Thus,
as in most of geometallurgy, there remains a gap in under-
standing between the mineral properties and metallurgical
effects.
PHYLLOSILICATE BREAKDOWN IN
LEACHING
Excluding solution-related factors such as Eh, pH, temper-
ature, and activity, silicates’ dissolution behavior depends
first on the strength of their bonding and second on their
degree of polymerization.
Bond Strength and Dissolution Behavior
In general, under acid attack the weakest bonds in phyl-
losilicates break first. For phyllosilicates with an interlayer,
any cations held in the interlayer will be exchanged for H+
if an alkali layer is present, it then dissolves. Dissolution of
these components, with bond strength well under 1/3, is
usually complete and fairly rapid.
The next-weakest bonding is in the octahedral layer,
where dissolution behavior becomes more complicated.
Here, both the average bond strength throughout the layer
and individual bond strength of cations matter. In the
simplest case, the octahedral layer is filled with divalent
cations such as Mg2+ or Fe2+ with a bond strength of 1/3.
These will dissolve completely though slowly. Leaching dis-
solves divalent cations from the octahedral layers in both
T-O and T-O-T phyllosilicates to about the same final
extent. In T-O-T phyllosilicates the octahedral layer may
be slower to dissolve as the stronger tetrahedral layer on
both sides protects it, but there is no evidence that it pre-
vents dissolution over the longer term. The observed higher
molar acid consumption of chlorite compared to biotite
with equivalent composition probably relates to the fact
that octahedral layers make up a higher proportion of each
mole of chlorite, not that chlorite is a T-O structure (Baum,
1999 Chetty, 2018).
When the octahedral layer contains higher-valent cat-
ions, the bond strength approaches the maximum that
standard leaching conditions can dissolve. Some trivalent
metals will almost certainly still dissolve, particularly if they
are anomalies in a mainly divalent octahedral layer. This may
be derived from the observation that molar acid consump-
tion is much lower in muscovite than in biotite (Baum,
1999). The two minerals have identical structures and simi-
lar compositions, but in muscovite the octahedral layer is
filled with Al3+ and charge-balancing vacancies (average
bond strength of ½) rather than Mg2+ and Fe2+ (average
bond strength of 1/3). Full dissolution of the octahedral
layer from either of these requires the same amount of H+
and thus should technically yield the same molar acid con-
sumption. The lower acid consumption is due to the fail-
ure of muscovite’s octahedral layer to dissolve completely.
Its average bond strength is close to ½, whereas in biotite
the octahedral layer’s average bond strength is closer to 1/3
and Al3+ does apparently dissolve to a large extent. In chlo-
rites, serpentine, and phengites and other micas of mixed
composition, acid consumption is considerably higher than
for completely Al-dominated equivalents and Al dissolves.
Probably the dissolution of the rest of the octahedral layer
from around the Al ions leaves little other option.
Conversely, high average bond strength also seems to
protect the minority components of the layer even when
their bonding is nominally weak. Extraction of Co, Ni,
and other divalent elements from phyllomanganates is low
(Tillotson, 2023) even though their bond strength, at 1/3,
is in theory breakable in leaching. Tetravalent cations in the
octahedral layer are even less likely to dissolve. They are
rare since they tend to fit better in tetrahedral coordination.
Limited evidence from the case of V suggests that they do
not dissolve in leaching from phyllosilicates (Drexler et al.,
2023). The data, however, are only available for minerals
in which V4+ is a substituting ion in an Al3+-dominated
different overall dissolution rates, meaning dissolution is
incongruent and there is no single rate equation. In many
cases, as will be discussed below, some part of the mineral
does not dissolve at all.
Connecting these mineralogical models to metal-
lurgical effects is fraught. The metallurgical literature on
(phyllo)silicate dissolution usually starts from the practical
problems experienced in operations and works backward to
the mineral source. In one example, Queneau and Berthold
(1986) described phyllosilicate dissolution in the context
of silica gel formation in leaching, a problem whose nega-
tive effects they reviewed comprehensively. Ndlovu et al.
(2014) focused on the rheology of slurries created by dif-
ferent phyllosilicate minerals in leaching, and hundreds
of studies over the years have examined the extraction (or
inextractability) of valuable base metals from phyllosilicate
ores. Studies of phyllosilicates in particular, or mechanis-
tic approaches, are rare in the metallurgical literature. The
most mechanistic study available is a pair of papers by Terry
(1983a,b), which describe the crystal chemistry and acid
dissolution mechanisms of silicates in general. The volume
edited by Graefe et al. (2017) attempted to split the dif-
ference between mechanism and practical implications by
tracing the impact of clays through the mineral process-
ing value chain, with strong mineralogical control. In the
case of heap leaching, they found that the specific effects of
phyllosilicates were overwhelmed by the many other factors
and minerals involved in leaching (Li et al., 2017). Thus,
as in most of geometallurgy, there remains a gap in under-
standing between the mineral properties and metallurgical
effects.
PHYLLOSILICATE BREAKDOWN IN
LEACHING
Excluding solution-related factors such as Eh, pH, temper-
ature, and activity, silicates’ dissolution behavior depends
first on the strength of their bonding and second on their
degree of polymerization.
Bond Strength and Dissolution Behavior
In general, under acid attack the weakest bonds in phyl-
losilicates break first. For phyllosilicates with an interlayer,
any cations held in the interlayer will be exchanged for H+
if an alkali layer is present, it then dissolves. Dissolution of
these components, with bond strength well under 1/3, is
usually complete and fairly rapid.
The next-weakest bonding is in the octahedral layer,
where dissolution behavior becomes more complicated.
Here, both the average bond strength throughout the layer
and individual bond strength of cations matter. In the
simplest case, the octahedral layer is filled with divalent
cations such as Mg2+ or Fe2+ with a bond strength of 1/3.
These will dissolve completely though slowly. Leaching dis-
solves divalent cations from the octahedral layers in both
T-O and T-O-T phyllosilicates to about the same final
extent. In T-O-T phyllosilicates the octahedral layer may
be slower to dissolve as the stronger tetrahedral layer on
both sides protects it, but there is no evidence that it pre-
vents dissolution over the longer term. The observed higher
molar acid consumption of chlorite compared to biotite
with equivalent composition probably relates to the fact
that octahedral layers make up a higher proportion of each
mole of chlorite, not that chlorite is a T-O structure (Baum,
1999 Chetty, 2018).
When the octahedral layer contains higher-valent cat-
ions, the bond strength approaches the maximum that
standard leaching conditions can dissolve. Some trivalent
metals will almost certainly still dissolve, particularly if they
are anomalies in a mainly divalent octahedral layer. This may
be derived from the observation that molar acid consump-
tion is much lower in muscovite than in biotite (Baum,
1999). The two minerals have identical structures and simi-
lar compositions, but in muscovite the octahedral layer is
filled with Al3+ and charge-balancing vacancies (average
bond strength of ½) rather than Mg2+ and Fe2+ (average
bond strength of 1/3). Full dissolution of the octahedral
layer from either of these requires the same amount of H+
and thus should technically yield the same molar acid con-
sumption. The lower acid consumption is due to the fail-
ure of muscovite’s octahedral layer to dissolve completely.
Its average bond strength is close to ½, whereas in biotite
the octahedral layer’s average bond strength is closer to 1/3
and Al3+ does apparently dissolve to a large extent. In chlo-
rites, serpentine, and phengites and other micas of mixed
composition, acid consumption is considerably higher than
for completely Al-dominated equivalents and Al dissolves.
Probably the dissolution of the rest of the octahedral layer
from around the Al ions leaves little other option.
Conversely, high average bond strength also seems to
protect the minority components of the layer even when
their bonding is nominally weak. Extraction of Co, Ni,
and other divalent elements from phyllomanganates is low
(Tillotson, 2023) even though their bond strength, at 1/3,
is in theory breakable in leaching. Tetravalent cations in the
octahedral layer are even less likely to dissolve. They are
rare since they tend to fit better in tetrahedral coordination.
Limited evidence from the case of V suggests that they do
not dissolve in leaching from phyllosilicates (Drexler et al.,
2023). The data, however, are only available for minerals
in which V4+ is a substituting ion in an Al3+-dominated