XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2403
donor-acceptor bonds depends on many factors and spans
a continuum ranging from covalent to ionic. An important
point to note here is that in systems involving ore particles
(with significant complexity and heterogeneity) and dif-
ferent ligands, many of the LAB interactions may simply
involve a “redistribution” of electrons in the newly formed
complex upon overlap of the frontier donor and acceptor
orbitals. Such LAB interactions would not generate a cur-
rent peak in voltammetry since the ligand does not exhibit
any redox behavior. This contrasts with a widely expounded
electrochemical mechanism where the ligand itself oxidizes
(e.g., xanthate), allowing electron transfer beyond the
region of orbital overlap.
LAB interactions between ligands and mineral sur-
faces can be further rationalized using Pearson’s principle
of hard and soft acids and bases (HSAB) (Pearson, 1963).
Hard bases prefer to interact with hard acids, and soft bases
will interact with soft acids. Hard acids and bases are small,
have low polarizability, and tend to form complexes of a
more ionic character. Soft acids and bases are large, have
high polarizability, and tend to form covalent complexes.
A list of well-known HSAB is provided in Table 1, but it
must be noted that the classification of species as hard or
soft also falls along a spectrum. The significant point here
is to demonstrate that ligands which contain soft S donors
(bases) prefer to interact with soft acids in metal sulfides
and precious metals, while ligands which contain hard O
donors interact with hard acids as in non-sulfide mineral
systems (Nagaraj, 1988).
The ACADTCs are derived from the dialkyl dithiocar-
bamic acids (DADTCs), which are a versatile and impor-
tant class of ligands in coordination chemistry (Ajiboye
et al., 2022). The primary structural differences between
the ACADTCs and the DADTCs are: a) the −SH group
is replaced by a −S−R group and b) an alkoxycarbonyl
group (R−O−C(=O)−) is substituted on the N atom
(Figure 1). The replacement of−SH with −S−R improves
the hydrolytic stability and modulates the electron density
of the S donors the addition of the electron-withdrawing
alkoxycarbonyl group lowers the pKa and further tunes the
electron density on both S donors. These changes make the
ACADTCs far more selective than the DADTCs. Upon
deprotonation from the central NH group (which is a func-
tion of its pKa), the subsequent delocalization of electron
density across the −O−C(=O)−N−C(=S)−S− functional
group enhances the propensity of the soft S donor on the
C=S group to bond with soft acid sites (Cu+, Ag+, etc.) on
a mineral surface. In addition, the O atom on the C=O
group can act as an auxiliary donor while O is generally
classified as a hard donor, it becomes “softer” upon elec-
tron delocalization and can opportunistically interact with
soft acids by acting as a buttressing “anchoring point.” This
is especially as it is in a sterically favorable position to aid
the S donor in forming strong six-membered chelates and
even polynuclear complexes (Figure 2). The ACADTCs are
charge-neutral molecules which cannot ionize nor oxidize
under conditions relevant to those in industrial flotation
practice. It is highly unlikely that its adsorption involves a
means of electron transfer analogous to that of xanthates.
Rather, it is hypothesized that the behavior of ACADTC at
the mineral/solution interface is purely chemical, dictated
by electron redistribution which exemplifies LAB concepts.
Of the various thio- ligand families used in indus-
trial sulfide flotation, the ACADTCs are most structur-
ally similar to the alkoxycarbonyl alkyl thionocarbamates
(ACATCs) and alkoxycarbonyl alkyl thioureas (ACATUs).
These have been relatively well-studied at the fundamen-
tal level (Basilio, 1989 Fairthorne et al., 1998 Nagaraj et
al., 1989) and are recognized as powerful, selective collec-
tors for Cu sulfides. The only structural difference in the
ACADTCs is the −S−R, versus that of the −O−R and −N−R
in the respective thionocarbamate and thiourea analogues
(Figure 3). Despite this seemingly subtle change between S,
N, and O, it results in dramatic differences in their coordi-
nation chemistry.
At present, there is no existing literature that interprets
how ACADTCs interact with minerals/metals in the con-
text of interfacial and coordination chemistry. Ore flotation
Table 1. General classification of HSAB (Nagaraj, 1988)
Bases
(functional groups, ligands)
Acids
(metals, minerals)
Hard:
H2O, OH–, PO43–, SO42–,
ROH, RO–, R2O, NH3, O, N
Hard:
H+, Li+, Na+, K+, Mg2+,
Ca2+, Fe3+, Ti4+
Borderline:
C6H5NH2, C5H5N, N3–, NO2–
Borderline:
Fe2+, Ni2+, Cu2+, Zn2+,
Pb2+
Soft:
R
2 S, RSH, RS–, SCN, S
Soft:
Cu+, Ag+, Au+, Pd2+, Pt2+,
zero-valent metals
Figure 1. Structures of a) alkoxycarbonyl alkyl
dithiocarbamate and b) dialkyl dithiocarbamic acid.
donor-acceptor bonds depends on many factors and spans
a continuum ranging from covalent to ionic. An important
point to note here is that in systems involving ore particles
(with significant complexity and heterogeneity) and dif-
ferent ligands, many of the LAB interactions may simply
involve a “redistribution” of electrons in the newly formed
complex upon overlap of the frontier donor and acceptor
orbitals. Such LAB interactions would not generate a cur-
rent peak in voltammetry since the ligand does not exhibit
any redox behavior. This contrasts with a widely expounded
electrochemical mechanism where the ligand itself oxidizes
(e.g., xanthate), allowing electron transfer beyond the
region of orbital overlap.
LAB interactions between ligands and mineral sur-
faces can be further rationalized using Pearson’s principle
of hard and soft acids and bases (HSAB) (Pearson, 1963).
Hard bases prefer to interact with hard acids, and soft bases
will interact with soft acids. Hard acids and bases are small,
have low polarizability, and tend to form complexes of a
more ionic character. Soft acids and bases are large, have
high polarizability, and tend to form covalent complexes.
A list of well-known HSAB is provided in Table 1, but it
must be noted that the classification of species as hard or
soft also falls along a spectrum. The significant point here
is to demonstrate that ligands which contain soft S donors
(bases) prefer to interact with soft acids in metal sulfides
and precious metals, while ligands which contain hard O
donors interact with hard acids as in non-sulfide mineral
systems (Nagaraj, 1988).
The ACADTCs are derived from the dialkyl dithiocar-
bamic acids (DADTCs), which are a versatile and impor-
tant class of ligands in coordination chemistry (Ajiboye
et al., 2022). The primary structural differences between
the ACADTCs and the DADTCs are: a) the −SH group
is replaced by a −S−R group and b) an alkoxycarbonyl
group (R−O−C(=O)−) is substituted on the N atom
(Figure 1). The replacement of−SH with −S−R improves
the hydrolytic stability and modulates the electron density
of the S donors the addition of the electron-withdrawing
alkoxycarbonyl group lowers the pKa and further tunes the
electron density on both S donors. These changes make the
ACADTCs far more selective than the DADTCs. Upon
deprotonation from the central NH group (which is a func-
tion of its pKa), the subsequent delocalization of electron
density across the −O−C(=O)−N−C(=S)−S− functional
group enhances the propensity of the soft S donor on the
C=S group to bond with soft acid sites (Cu+, Ag+, etc.) on
a mineral surface. In addition, the O atom on the C=O
group can act as an auxiliary donor while O is generally
classified as a hard donor, it becomes “softer” upon elec-
tron delocalization and can opportunistically interact with
soft acids by acting as a buttressing “anchoring point.” This
is especially as it is in a sterically favorable position to aid
the S donor in forming strong six-membered chelates and
even polynuclear complexes (Figure 2). The ACADTCs are
charge-neutral molecules which cannot ionize nor oxidize
under conditions relevant to those in industrial flotation
practice. It is highly unlikely that its adsorption involves a
means of electron transfer analogous to that of xanthates.
Rather, it is hypothesized that the behavior of ACADTC at
the mineral/solution interface is purely chemical, dictated
by electron redistribution which exemplifies LAB concepts.
Of the various thio- ligand families used in indus-
trial sulfide flotation, the ACADTCs are most structur-
ally similar to the alkoxycarbonyl alkyl thionocarbamates
(ACATCs) and alkoxycarbonyl alkyl thioureas (ACATUs).
These have been relatively well-studied at the fundamen-
tal level (Basilio, 1989 Fairthorne et al., 1998 Nagaraj et
al., 1989) and are recognized as powerful, selective collec-
tors for Cu sulfides. The only structural difference in the
ACADTCs is the −S−R, versus that of the −O−R and −N−R
in the respective thionocarbamate and thiourea analogues
(Figure 3). Despite this seemingly subtle change between S,
N, and O, it results in dramatic differences in their coordi-
nation chemistry.
At present, there is no existing literature that interprets
how ACADTCs interact with minerals/metals in the con-
text of interfacial and coordination chemistry. Ore flotation
Table 1. General classification of HSAB (Nagaraj, 1988)
Bases
(functional groups, ligands)
Acids
(metals, minerals)
Hard:
H2O, OH–, PO43–, SO42–,
ROH, RO–, R2O, NH3, O, N
Hard:
H+, Li+, Na+, K+, Mg2+,
Ca2+, Fe3+, Ti4+
Borderline:
C6H5NH2, C5H5N, N3–, NO2–
Borderline:
Fe2+, Ni2+, Cu2+, Zn2+,
Pb2+
Soft:
R
2 S, RSH, RS–, SCN, S
Soft:
Cu+, Ag+, Au+, Pd2+, Pt2+,
zero-valent metals
Figure 1. Structures of a) alkoxycarbonyl alkyl
dithiocarbamate and b) dialkyl dithiocarbamic acid.