6.1 Water and ITC-Computational adsorptions energy comparison
6.1.1 Pyrite adsorptions
Figure 6.1: The adsorption configurations of DTP, DTC and xanthate on the pyrite (100) surfaces : (a) before and (b) after adsorption in the absence of water.
Figure 6.2: The adsorption configurations of DTP, DTC and xanthate on the pyrite (100) surfaces in the presence of water.
Since the surface free energy of FeS2 can be expressed as a function of the Wulff- constructions facets, the obtained construction should also be related to the three- dimensional equilibrium shapes of FeS2. The exposed surfaces are labelled accordingly. In chapter 2, Fig 3.10 the shape of the FeS2 crystal is shown to be is a truncated octahedron, covered by the (100) and (111) surfaces. Furthermore, the (100) terminating at the apices of the octahedron has a higher surface ratio than (111) at the surface of the octahedron. There is no visible appearance of other surfaces. On this basis, the (100) and (111) terminated surface was be used as the pyrite working surface.
The adsorption configurations and energies of xanthate, DeDTP and DeDTC on the pyrite surfaces in the absence and presence of water are presented in Figure 6.1, Figure 6.2 and Table 6.1. We found that in the absence of water the distances between DTP/ DTC/ xanthate S atom and the pyrite Fe atom are in the order: DeDTP (S1−Fe1: 2.455 Å, S2−Fe2:2.411 Å) > xanthate (S1−Fe1: 2.440 Å, S2−Fe2:2.360 Å)
> DeDTC (S1−Fe1: 2.349 Å, S2−Fe2:2.377 Å), which are close to the sum of atomic radius of Fe and S of 2.310 Å. This implied that there was a relatively strong interaction between these three collectors and pyrite surface. In the presence of water, it was perceived that the Δd values are much larger than the sum of the atomic radii of Fe-S (d2), which inferred that their bonds are weaker. Moreover, this indicated that the collector are able to do displace the water molecules on the surface, a required effect during flotation.
The calculated adsorption energies strength for collector adsorption on FeS2 (100) followed the order as: DeDTP (406 kJ/mol) > eX (387.6 kJ/mol) > and DeDTC (380.9.
kJ/mol), respectively (see Figure 6.3). It could be concluded from the calculated results that DeDTP, DeDTC and eX can float pyrite. We observed that the order does not follow the anticipated experimental results, after water adsorptions as shown in Figure 6.1 and 6.2, We found that the adsorption energies of DeDTP, DeDTC and eX decrease dramatically: 119.3 kJ/mol, 56.3 kJ/mol and 61.4 kJ/mol, respectively as shown in Figure 6.3. This suggested that the chemisorption occurs on the water pre- adsorbed pyrite surface and the presence of water molecule will not hinder the adsorption of the collector.
Figure 6.3:The adsorption energies of DTP, DTC and xanthate on the pyrite (100) surfaces in the absence and presence of water.
DeDTP DeDTC eX
Computational [100] 406,4 380,9 387,6 360,0
370,0 380,0 390,0 400,0 410,0
Heat of Adsoption (‐kJ/mol)
Asorbates
Computational (100)
DeDTP DeDTC eX
[100] with water 119,3 56,3 61,4
0,0 20,0 40,0 60,0 80,0 100,0 120,0 140,0
Heat of adsoption (‐kJ/mol)
Adsorbates
Computational (100) with water
Figure 6.4: The adsorption configurations of DTP, DTC and xanthate on the pyrite (111) surfaces in the absence of water.
Figure 6.5: The adsorption configurations of DTP, DTC and xanthate on the pyrite (111) surfaces in the presence of water.
The FeS2 (111) adsorptions energy strength followed the order as: DeDTC (272.6 kJ/mol) > eX (252.7 kJ/mol) > DeDTP (232.4 kJ/mol), which follows the same order of experimental heats of adsorption (DeDTC > SEX > DeDTP) as shown in Figure 6.4 and 6.5, This suggested that the FeS2 (111) adsorption behaviour emulate those of the ITC experiments.
Figure 6.6: The adsorption energies of DTP, DTC and xanthate on the pyrite (111) surfaces in the absence and presence of water.
It was notable that the presence of water molecules decreased the adsorption energy for DeDTP and eX, while adsorption energy increased for DeDTC. which suggests that the water molecule could enhance the adsorption of DTC on (111) surface.
suggesting that the chemisorption occurs on the water pre-adsorbed pyrite surface.
DeDTP DeDTC eX
Computational [111] 232,4 272,6 259,7 210,0
220,0 230,0 240,0 250,0 260,0 270,0 280,0
Heat of adsorption (‐kJ/mol)
Adsorbates
Computational (111)
DeDTP DeDTC eX
[111] with water 49,0 288,9 119,4
0,0 50,0 100,0 150,0 200,0 250,0 300,0 350,0
Heat of adsorption (‐kJ/mol)
Adsorbates
Computational (111) with water
Figure 6.7: The adsorption energies of DTP, DTC and xanthate on the pyrite. Microflotation recoveries for pyrite using DeDTC, DeDTP and ethyl xanthates as collectors (pH 9.2; the relative standard error was always < ±2% hence error bars are too small to be visible on the graphs) [10].
The computational results showed that the calculated adsorption energies were more exothermic than the experimentally determined ones [10]. It must be emphasised that this study focussed more on the trends observed between the calculated and the experimental enthalpies of adsorption, and not necessarily their absolute values. The results showed that the trends in the calculated and experimental energies of
DeDTP DeDTC eX
Experimental 6,0 62,3 57,1
0,0 10,0 20,0 30,0 40,0 50,0 60,0 70,0
Heat of adsorption (kJ/mol)
Adsorbates
ITC
DeDTP DeDTC eX
Experimental 11,6 10,1 10,8
9 9,5 10 10,5 11 11,5 12
Cumulative Recovery (%)
Adsorbates
Flotation
adsorption were similar only in the case of FeS2 (111). The adsorption of water molecules was found to have little influence on the DTC adsorption on (111) surfaces.
Table 6.1: Mulliken bond populations of the interaction between DeDTP, DispDTP and DbDTP pyrite surface.
collector Mineral surface
Average interactive distance d1 (Å)
Sum of the atomic radius d2 (Å)
Strength of interaction Δd = d1 - d2 (Å)
In the absence of water
DTP FeS2 (100) 2.412 2.310 0.102
PbS (100) 2.902 2.840 0.062
DTC FeS2 (100) 2.416 2.310 0.106
PbS (100) 2.853 2.840 0.013
Xanthate FeS2 (100) 2.398 2.310 0.088
PbS (100) 2.872 2.840 0.032
In the presence of water
DTP FeS2 (100) 2.429 2.310 0.119
PbS (100) 2.955 2.840 0.115
DTC FeS2 (100) 2.402 2.310 0.092
PbS (100) 3.041 2.840 0.201
Xanthate FeS2 (100) 2.393 2.310 0.083
PbS (100) 3.069 2.840 0.229