646
W. Zhang et al.
Additionally, zeta potential results show the same trend as contact angle measurements: there are obvious negative changes of zeta potential values of sphalerite and
galena but slight changes of pyrite in CSC-1 test system. Using EX as the collector,
however, the changes of zeta potential values of three kinds of minerals is minor
and similar. Zeta potential results further confirm the inferences from contact angle:
CSC-1 has a stronger adsorption on sphalerite and galena but weaker adsorption on
pyrite, compared with EX, consistent with the experimental results shown in Figs. 3,
4, 5, and 6.
On the basis of the above discussions, CSC-1 molecule prefers to absorb on the
surface of sphalerite and galena rather than pyrite surface. This is the reason for the
efficient and selective flotation separation for sphalerite and galena against pyrite by
using CSC-1 collector. As a comparison, EX collector mediates these three minerals
poorly and non-selectively.
EPS and NCP studies of collectors confirm that the bonding groups of CSC1with metal ions are conjugated thiocarbonyl and diazole structure, while that of
EX is tautomeric thiocarbonyl and sulfydryl. Noteworthy, sulphur atom is a softer
Lewis base and better electron-donating than nitrogen atom, which binds with most
transition metal ions rapidly and stably. As a consequence, the “all-sulfur-composed”
bidentate ligand (EX collector) is not performing as selective as the aza-collector
CSC-1. In addition, negatively charged N2 occupy a considerable proportion in the
HOMO orbital of CSC-1 molecule; this is the supplementary reason for the excellent
selectivity of CSC-1 collector.
Lone pair electron is the key factor for selective chelation. ELF study reveals that
there is an evident angle between the location lobes of electron pairs in EX molecule
(S1 and S2), while the located electron pairs in CSC-1 molecule are almost parallel
(S1’ and N1). This suggests that CSC-1 molecule is more likely to bind to Pb
2+ with
larger radius, while EX molecule prefers to Pb
2+ with smaller radius, because of the
match relationship between the located electron pairs of collectors and the empty
orbital of metal ions. The result of ELF is consistent with flotation tests, EPS, and
NCP studies.
The optimized Gibbs free energy of molecular geometries demonstrates that
CSC-1–Pb
2+ coordination is the most stable chelation modes of CSC-1–M
2+ complexes. This is the primary reason why the flotation recovery of galena is the highest
compared with sphalerite and pyrite in batch-flotations tests. And this point also provides us with one question: why does Pb
2+ become the decisive factor for the stabilization of EX–M
2+ and CSC-1–M
2+ The answer is ring tension. EX–M
2+ chelation
forms a monocyclic complex, and CSC-1–M
2+ chelation incorporates two atoms
(C and N) into different rings, enforcing monocyclic and fused ring tension, respectively. As shown in Fig. 10, Pb
2+ , Zn
2+ , and Fe
2+ take their turn to hold the position in
the collector–M
2+ complexes, but Fe
2+ is not favorable to be the coordinative metal
center because there are obvious distortions in the bidentate angles of EX–Fe
2+
(S–C = S, 111.6
o ) and CSC-1–Fe
2+ (S–C = N, 110.7
o ). According to distortion
energy, Pb
2+ is the optimum metal ion to reduce fused ring tension of collector–M
2+
W. Zhang et al.
Additionally, zeta potential results show the same trend as contact angle measurements: there are obvious negative changes of zeta potential values of sphalerite and
galena but slight changes of pyrite in CSC-1 test system. Using EX as the collector,
however, the changes of zeta potential values of three kinds of minerals is minor
and similar. Zeta potential results further confirm the inferences from contact angle:
CSC-1 has a stronger adsorption on sphalerite and galena but weaker adsorption on
pyrite, compared with EX, consistent with the experimental results shown in Figs. 3,
4, 5, and 6.
On the basis of the above discussions, CSC-1 molecule prefers to absorb on the
surface of sphalerite and galena rather than pyrite surface. This is the reason for the
efficient and selective flotation separation for sphalerite and galena against pyrite by
using CSC-1 collector. As a comparison, EX collector mediates these three minerals
poorly and non-selectively.
EPS and NCP studies of collectors confirm that the bonding groups of CSC1with metal ions are conjugated thiocarbonyl and diazole structure, while that of
EX is tautomeric thiocarbonyl and sulfydryl. Noteworthy, sulphur atom is a softer
Lewis base and better electron-donating than nitrogen atom, which binds with most
transition metal ions rapidly and stably. As a consequence, the “all-sulfur-composed”
bidentate ligand (EX collector) is not performing as selective as the aza-collector
CSC-1. In addition, negatively charged N2 occupy a considerable proportion in the
HOMO orbital of CSC-1 molecule; this is the supplementary reason for the excellent
selectivity of CSC-1 collector.
Lone pair electron is the key factor for selective chelation. ELF study reveals that
there is an evident angle between the location lobes of electron pairs in EX molecule
(S1 and S2), while the located electron pairs in CSC-1 molecule are almost parallel
(S1’ and N1). This suggests that CSC-1 molecule is more likely to bind to Pb
2+ with
larger radius, while EX molecule prefers to Pb
2+ with smaller radius, because of the
match relationship between the located electron pairs of collectors and the empty
orbital of metal ions. The result of ELF is consistent with flotation tests, EPS, and
NCP studies.
The optimized Gibbs free energy of molecular geometries demonstrates that
CSC-1–Pb
2+ coordination is the most stable chelation modes of CSC-1–M
2+ complexes. This is the primary reason why the flotation recovery of galena is the highest
compared with sphalerite and pyrite in batch-flotations tests. And this point also provides us with one question: why does Pb
2+ become the decisive factor for the stabilization of EX–M
2+ and CSC-1–M
2+ The answer is ring tension. EX–M
2+ chelation
forms a monocyclic complex, and CSC-1–M
2+ chelation incorporates two atoms
(C and N) into different rings, enforcing monocyclic and fused ring tension, respectively. As shown in Fig. 10, Pb
2+ , Zn
2+ , and Fe
2+ take their turn to hold the position in
the collector–M
2+ complexes, but Fe
2+ is not favorable to be the coordinative metal
center because there are obvious distortions in the bidentate angles of EX–Fe
2+
(S–C = S, 111.6
o ) and CSC-1–Fe
2+ (S–C = N, 110.7
o ). According to distortion
energy, Pb
2+ is the optimum metal ion to reduce fused ring tension of collector–M
2+