1646 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
octahedral layer, which would have a high average bond
strength and thus resist dissolution in any case. Whether
V4+ would dissolve from an octahedral layer dominated by
divalent cations is unknown based on the current data.
Moving on to the tetrahedral layer, bond strengths are
between ¾ and 1. The average bond strength depends on
how much Al and Si respectively occupy the tetrahedral sites.
Other high-valent cations, such as Ti4+ and V4+, can also be
present, but substitutions by lower-valent cations are rare.
This makes the tetrahedral layer’s average bond strength
far too high to dissolve in conventional leaching. Higher
temperatures and/or more concentrated acids appear able
to leach tetrahedrally coordinated Al with a bond strength
of ¾, but fail to dissolve tetrahedral Si (McDonald and
Whittington, 2008 and references therein). This constrains
the maximum Pauling bond strength that acid leaching can
break to somewhere between 0.75 and 1.0. Further dissolu-
tion is possible with a fluorinated leaching adjuvant, which
can dissolve Si and other tetravalent cations from the tetra-
hedral layer (Zheng et al., 2019), but its use is mainly con-
fined to the laboratory scale due to safety issues with HF.
Silica Polymerization and Dissolution Behavior
What happens to the tetrahedral layer, and in the end to
the phyllosilicate’s fundamental structure, depends on
the fate of the octahedral layer. If it fails to dissolve, the
mineral retains much of its fundamental structure and
can compensate for underbonding by adsorbing charges
out of solution. In this case much of the crystalline lattice
structure remains intact. However, if most of the octahe-
dral layer dissolves, the dangling bonds left behind are too
strong for adsorption to compensate and the tetrahedral
layer cannot maintain electrical neutrality in its original lat-
tice configuration.
When this happens, it presents a problem, since the
polymerized silica bonding is far too strong to break in
standard leaching conditions. The layer thus cannot dis-
solve and remains solid, but is forced to crumple and lose
its crystalline lattice structure to maintain electrical neutral-
ity (Figure 3). Experimental data reveal that the formerly
tetrahedral layer rearranges into a 3-dimensional tetrahe-
dral framework with close to 100% polymerization (Okada
et al., 2005). This neutralizes the dangling charges, but
forms a hydrous gel that can be highly deleterious to leach-
ing (Queneau and Berthold, 1986 Lu et al., 2017).
It is not clear how or whether Al substitution in the
tetrahedral layer affects silica gel formation during leach-
ing. If tetrahedral Al remains undissolved in a phyllosilicate
that has lost its octahedral layer, it will be only slightly less
underbonded than the tetrahedral Si and will have to com-
pensate by joining in the gel. If Al dissolves from the tet-
rahedral layer, as happens with elevated temperatures and
concentrated acids, it enters solution rather than becoming
incorporated in the gel (Okada et al., 2005).
Figure 3. Schematic presentation of phyllosilicate decrepitation during acid leaching using a T-O-T
example. If the octahedral layer is strongly bonded (aluminous) and resists leaching, the phyllosilicate
retains most of its crystal structure and does not form a gel (top). The destruction of more weakly bonded
(magnesian or ferrous) octahedral layers causes the tetrahedral layer to crumple (bottom)
octahedral layer, which would have a high average bond
strength and thus resist dissolution in any case. Whether
V4+ would dissolve from an octahedral layer dominated by
divalent cations is unknown based on the current data.
Moving on to the tetrahedral layer, bond strengths are
between ¾ and 1. The average bond strength depends on
how much Al and Si respectively occupy the tetrahedral sites.
Other high-valent cations, such as Ti4+ and V4+, can also be
present, but substitutions by lower-valent cations are rare.
This makes the tetrahedral layer’s average bond strength
far too high to dissolve in conventional leaching. Higher
temperatures and/or more concentrated acids appear able
to leach tetrahedrally coordinated Al with a bond strength
of ¾, but fail to dissolve tetrahedral Si (McDonald and
Whittington, 2008 and references therein). This constrains
the maximum Pauling bond strength that acid leaching can
break to somewhere between 0.75 and 1.0. Further dissolu-
tion is possible with a fluorinated leaching adjuvant, which
can dissolve Si and other tetravalent cations from the tetra-
hedral layer (Zheng et al., 2019), but its use is mainly con-
fined to the laboratory scale due to safety issues with HF.
Silica Polymerization and Dissolution Behavior
What happens to the tetrahedral layer, and in the end to
the phyllosilicate’s fundamental structure, depends on
the fate of the octahedral layer. If it fails to dissolve, the
mineral retains much of its fundamental structure and
can compensate for underbonding by adsorbing charges
out of solution. In this case much of the crystalline lattice
structure remains intact. However, if most of the octahe-
dral layer dissolves, the dangling bonds left behind are too
strong for adsorption to compensate and the tetrahedral
layer cannot maintain electrical neutrality in its original lat-
tice configuration.
When this happens, it presents a problem, since the
polymerized silica bonding is far too strong to break in
standard leaching conditions. The layer thus cannot dis-
solve and remains solid, but is forced to crumple and lose
its crystalline lattice structure to maintain electrical neutral-
ity (Figure 3). Experimental data reveal that the formerly
tetrahedral layer rearranges into a 3-dimensional tetrahe-
dral framework with close to 100% polymerization (Okada
et al., 2005). This neutralizes the dangling charges, but
forms a hydrous gel that can be highly deleterious to leach-
ing (Queneau and Berthold, 1986 Lu et al., 2017).
It is not clear how or whether Al substitution in the
tetrahedral layer affects silica gel formation during leach-
ing. If tetrahedral Al remains undissolved in a phyllosilicate
that has lost its octahedral layer, it will be only slightly less
underbonded than the tetrahedral Si and will have to com-
pensate by joining in the gel. If Al dissolves from the tet-
rahedral layer, as happens with elevated temperatures and
concentrated acids, it enters solution rather than becoming
incorporated in the gel (Okada et al., 2005).
Figure 3. Schematic presentation of phyllosilicate decrepitation during acid leaching using a T-O-T
example. If the octahedral layer is strongly bonded (aluminous) and resists leaching, the phyllosilicate
retains most of its crystal structure and does not form a gel (top). The destruction of more weakly bonded
(magnesian or ferrous) octahedral layers causes the tetrahedral layer to crumple (bottom)