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Density functional theory study of cyanide adsorption on the sphalerite (1 1 0) surface
Accepted Manuscript Full Length Article Density functional theory study of cyanide adsorption on the sphalerite (110) surface Tingsheng Qiu, Qingmin Nie, Yuanqing He, Qinzhi Yuan PII: DOI: Reference:
S0169-4332(18)32431-0 https://doi.org/10.1016/j.apsusc.2018.09.020 APSUSC 40324
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
9 May 2018 29 August 2018 3 September 2018
Please cite this article as: T. Qiu, Q. Nie, Y. He, Q. Yuan, Density functional theory study of cyanide adsorption on the sphalerite (110) surface, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.09.020
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Density functional theory study of cyanide adsorption on the sphalerite (110) surface Tingsheng Qiua*, Qingmin Nieb, Yuanqing Hec, Qinzhi Yuana a
Faculty of Resource and Environmental Engineering, Jiangxi University of Science and
Technology, Ganzhou, Jiangxi, 341000, China b
School of Chemical Engineering and Technology, China University of Mining and Technology,
Xuzhou, Jiangsu, 221116, China c
School of Materials Science and Engineering, Central South University, Changsha, Hunan,
410083, China
Abstract: Cyanidation tailings in gold plants often contain lots of sphalerites that are difficult to recycle by flotation because of the strongly depressive effect of the residual cyanide. To reveal the depression mechanism of cyanide on sphalerite, the adsorption of isolated cyanide molecule (CN) at different coverages on the sphalerite (110) surface were studied by means of density functional theory. The calculated results show that the adsorption energy of per CN molecule drops as the increase of adsorbed CN molecule coverage, indicating that the adsorption structure becomes more thermodynamically stable. Cyanide molecule adsorption on sphalrite (110) surface prior occurs on the atop site of surface Zn atom, in which the Zn 3d orbital donates electrons to the 2p orbital of C forming a d-p back donating bond, leading to the production of hydrophilic zinc cyanide complexes that serve as strong obstacles to the flotation recovery of sphalerite in cyanidation tailings. Electron transfer from mineral surface to the adsorbed CN molecules takes place during the process of CN adsorption, and the surface atoms lose more electrons when the coverage of adsorbed CN molecules is high, weakening the reactive activity of Zn and S atoms on the sphalerite (110) surface. Key words: Sphalerite; Cyanidation tailings; Surface adsorption; Density functional theory
1. Introduction With high leaching rate and relatively low cost, cyanidation is the primary process for the extraction of precious metals in gold and silver mines nowadays, especially when the successful application of carbon-in-pulp technology is achieved [1-2].
* Corresponding author. E-mail address: [email protected] (T. Qiu)
However, this method would produce a large amount of cyanidation tailings that contain toxic substance such as CN and heavy metal ions Cu 2 , Pb 2 and Zn2 , causing a series of problems to environment as well as the survival of mankind and wildlife [3-4]. In addition, the residues of cyanidation are often relatively rich in copper, lead and zinc minerals, which have a great potential for utilization under the background that mineral resources become poorer and more complicated [5]. In many gold and silver mines, pre-flotation is adopted to enrich the gold and silver contents of feed ores for the cyanide leaching operations. As a consequence, the grade of copper, lead and zinc sulfide minerals in cyanidation tailings get a rise. Therefore, enhancing the recovery of valuable minerals from these secondary resources has a great significance for environment protection as well as resources comprehensive utilization. Sphalerite (ZnS) is a kind of typical sulfide mineral, which is the main phase of zinc in cyanidation tailings. Flotation process is the most common method for the recovery of this sulfide mineral from ores and some tailings. However, the cyanide, an effective depressant for the sphalerite flotation, generated an intense inhibition effect on the ZnS during cyanide leaching operation, seriously hindering the flotation recovery of this mineral. Accordingly, it is necessary to make clear the depression mechanism of cyanide for the subsequent activation and flotation reclaim of sphalerite. Cyanide acts as a depressant in the separation flotation of lead-zinc sulfide ores to suppress the zinc floating, and the depression mechanism have gotten a great many attentions of researchers for a long time. Wark [6] found that the content suppressing the flotation of ZnS in pulp was Zn(CN)2 that were adsorbed on the mineral surface and improved the hydrophilicity of sphalerite. Though whether the Zn(CN)2 lead to the depressive effect have not been proven, these zinc cyanide complexes were generated in the circuit indeed [7-8]. Nevertheless, the X-ray photoelectron spectroscopy (XPS) analysis results of species on the cyanide treated sphalerite indicated that cyanide did not interact with zinc or sulfur on the mineral surface [9] or zinc cyanide compositions remained in solution rather than on the sphalerite surface [10]. On the basis of stability of cyano-zinc complexes and the reducibility of cyanide, some researchers thought that the depression effect of free cyanide on sphalerite were complicated, which depends on the ore properties and flotation pulp environments [11-12].
Not only could cyanide dissolve the oxides of zinc and zinc-xanthate
compounds, it also consumes a great deal of oxygen in pulp, suppressing the oxidation of xanthate to dixanthogen [13]. Moreover, the cyanide concentration played a significant role in the depression effect. In relatively low levels, the cyanides were adsorbed onto sphalerite surface making the hydrophilicity of mineral resemble to that of insoluble Zn(CN) 2 , and with the cyanide concentration increase, the generated Zn(CN) 2 became Zn(CN) 24 gradually, exhibiting a stronger inhibition effect on sphalerite [14]. Even though lots of researches have been conducted, the mechanism by which cyanide depress sphalerite flotation is not well understood yet. In recent years, with the rapid development of quantum chemistry and computer technology, density functional theory simulation is more and more widely used to study the structure, properties, reactivity etc. of materials. An enormous amount of researches have been carried out on the mineral crystal structures and defects [15-17], surface properties [18-20], and the interaction between reagent molecules and mineral surface [21-23], which demonstrated the mineral flotation performance on the molecular or atomic level. However, the first-principle studies on the depression mechanism of cyanide on sphalerite were relatively scarce. Ye Chen et al [24-25] investigated the adsorption of CN on sphalerite (110) surface and the influence of lattice impurity on mineral surface adsorption by density functional theory, and the calculated results indicated that CN adsorbed strongly on the perfect sphalerite (110) surface with hollow site adsorption showing the highest stability, where the C and N atoms of the CN molecule bond with two Zn atoms, respectively. As for lattice defects of sphalerite, the impurity of Fe and Mn could strengthen the adsorption of CN, while Cd weaken it. Moreover, the activation of copper atom enhanced the interaction between sphalerite surface and CN molecule, which adsorbed on the mineral surface via C atom bonding with the Cu atom perpendicularly on the Cuactivated sphalerite surface. These researches revealed the depression mechanism of cyanide on sphalerite to a certain extent, but only the low coverage adsorption of cyanide was focused on, which cannot reflect the heavy suppression of cyanide on sphalerite after a prolonged and highly concentrated immersion during cyanidation of gold and silver ores. For the sake of a further understanding on depression mechanism of cyanide on sphalerite in the cyanidation tailings, the adsorption of isolated cyanide molecules at different coverage were investigated by using the density functional theory here.
Furthermore, influences of adsorbed CN molecules on the electronic properties of sphalerite (110) surface were discussed. Our works can provide theoretical references for the understanding of cyanide depression mechanism on sphalerite as well as the subsequent activation and flotation recovery of sphalerite in cyanidation tailings.
2. Computational methods All calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) developed by Payne et al [26], which is a plane-wave pseudopotential method based on the density functional theory (DFT). The interaction between ionic core and valence electrons were represented by ultra-soft pseudopotential. Based on the results of convergence tests, the generalized gradient approximation (GGA) developed by Perdew, Burke and Ernzerhof [27-28] was used to compute the exchange correlation effect of different electrons, and the kinetic energy cutoff of plane wave basis was 420 eV, which was used throughout the investigation. The brillouin zone integration was approximated by a sum over special k points that employed a 2×3×1 grid of the Monkhorst-Pack scheme for structure calculations. The valence electron configurations considered in this study were Zn 3d104s2, S 3s23p4, Cu 3d104s1, C 2s22p2, and N 2s22p3. A convergence tolerance of 2.0×10-6eV/atom was employed for the self-consistent electronic minimization, and an energy tolerance of 2.0×10-5eV/atom, a maximum force tolerance of 0.05 eV/Å, and a maximum displacement tolerance of 0.002 Å were employed for the geometry optimization. The crystal of sphalerite (β-ZnS) belongs to the isometric system, whose space group is F - 43m , and cell lattice are a b c 5.409 Å and 90 [29]. After geometry optimization with above calculation parameters, the computed lattice of sphalerite are 5.442 Å, being only 0.61% larger than the experimental value, which is close to the calculation results of A. Sarvaramini (5.47 Å) [23] and X. H. Long (5.471 Å) [21], and indicates that the simulation results coincide with experimental values. Sphalerite (110) surface is the predominant cleavage surface, of which the surface energy is relatively low [29], and it has been used as the principal cleavage surface of sphalerite by many researchers for surface simulation [21,24,30-32]. Therefore, the cyanides are mainly adsorbed onto the sphalerite (110) surface that is selected for the DFT calculations. In consideration of the influence of thickness of slab and numbers of fixed atomic layers, convergence tests were carried out, based on
the results of which the sphalerite (110) surface slab model with 10 layers were used for the simulation, where the top four layers were allowed to relax and the bottom six layers were fixed in their bulk position, which is sufficient for the study of surface properties. In addition, a vacuum layer of 15
along the c-parameter was added on
the atomic layers to eliminate the interaction between different slabs [16,21,33], and a 2 2 1 supercell was employed in the simulation. The bulk sphalerite and its (110) surface model are shown in Fig. 1. For the calculation of adsorption energy of per CN molecule, the following equation can be utilized.
1 E slabnCN E slab nE CN (1) n represents the total energy of sphalerite (110) surface
E ads / CN In the equation, the E slabnCN
model with n cyanide molecules adsorbed, the E slab is the energy of perfect surface, E CN involves the energy of a free CN molecule calculated in a cubic cell, and n is the
number of adsorbed CN molecules. The coverage of CN molecule ( CN ) is represented by the number of adsorbed CN molecules over the total number of exposed Zn and S atoms on the first layer of sphalerite (110) surface [34].
3. Results and discussion The configuration of sphalerite (110) surface is cleaved from optimized bulk sphalerite crystal, and before the calculations of CN adsorption, the relaxation of sphalerite (110) surface was conducted for the surface free energy minimization. Then, in order to simulate the intense depression effect of cyanide on sphalerite, the CN adsorption on sphalerite (110) surface at different coverage of 1/10, 1/5 and 2/5 ML were calculated using the method of density functional theory. 3.1 CN adsorption at CN of 1/10 ML There are 6 sulfur atoms and 4 zinc atoms exposed on the sphalerite (110) surface, and the CN coverage of 1/10 ML indicates that the number of adsorbed CN is one. In this studies, the top of sulfur and zinc, bridge and hollow sites were selected as the active adsorption sites on the relaxed surface. Therefore, five adsorption modes were calculated to simulate the interaction between CN and sphalerite (110) surface. The adsorption modes with bond lengths are shown in Fig. 2, and the adsorption energy for different modes are given in Table 1.
Table 1 CN adsorption energy on different sites of sphalerite (110) surface at CN of 1/10 ML Adsorption site
Adsorption energy/(kJ/mol）
Zn-Zn hollow Top S-C
-184.01 -106.70
Top Zn-N Top Zn-C
-148.54 -186.75
Zn-S bridge
-111.48
As shown in Table 1, the adsorption energy of CN on different sites of sphalerite (110) surface are all negative, which indicates that the interaction between CN and sphalerite is exothermic and cyanide can be adsorbed onto the mineral surface easily. Among the adsorption sites, the adsorption energy is relatively low when CN is adsorbed on the top Zn-C site (186.75 kJ/mol) and Zn-Zn hollow site (184.01 kJ/mol), indicating that the CN adsorption on perfect sphalerite (110) surface mainly occurs on the top Zn-C site and Zn-Zn hollow site when the coverage of CN is 1/10 ML. In the reference [24], the adsorption energy of CN molecule on the sphalerite (110) surface is lowest when the molecule was adsorbed on the Zn-Zn hollow site, which is different from the calculation results in this paper. And from the Table 1, it can be known that both C atom and N atom of CN molecule can react with the surface atoms. However, when the cyanide was adsorbed on the Zn-Zn hollow site, the calculation results showed that the anti-bonds could be formed between the C atom, N atom of CN molecule and the S atom facing the CN molecule on the mineral surface, which can strengthen the repulsion between cyanide and mineral surface atoms and weaken the stability of the adsorption configuration. Therefore, the adsorption energy of CN on Zn-Zn hollow site was a little high than that when the CN was adsorbed on the ZnC site. In top Zn-C site (Fig. 2D), the CN molecule locate at the top of zinc atom on the sphalerite (110) surface via the interaction between C atom of CN and surface Zn atom, with a distance of 0.1940 nanometers, which is shorter than the sum of covalent radius of C and Zn atom (0.2100nm), suggesting that a chemical bond formed between C atom of CN and Zn atom of spahlerite (110) surface after the interaction.
In Zn-Zn hollow site (Fig. 2A), the CN is adsorbed parallel onto the surface, with the C and N atom reacting with two zinc atoms of sphalerite (110) surface respectively. The distance between C and Zn1 is 0.2025 nanometers, and the distance between N and Zn2 is 0.2122 nanometers, which are all in the scope of chemical effect. Moreover, the distance between C and Zn1 is shorter than that between N and Zn2, indicating that the interaction between C and Zn1 are stronger. 3.2 CN adsorption at CN of 1/5 and 2/5 ML The results of CN adsorption at CN of 1/10 ML indicates that the top Zn-C and ZnZn hollow are the preferred sites for the adsorption of CN molecule on sphalerite (110) surface. Thereby, the CN adsorption at CN of 1/5 ML (two CN molecules) was conducted on these active sites. Because of the hexatomic ring structure (Fig. 1B) constituted by the sulfur and zinc atoms from first layer and second layer of sphalerite (110) surface slab model, there are two configurations for the Zn top site CN adsorption at the CN of 1/5 ML, including two CN molecules adsorbed on different hexatomic ring and on same hexatomic ring respectively. Thus, three adsorption modes were studied when the coverage of CN was 1/5 ML, seen in the Fig. 3(A-C), and the adsorption energy for different modes are presented in Table 2. Table 2 CN adsorption energy on different sites of sphalerite (110) surface at CN of 1/5 and 2/5 ML Adsorption sites 2 CN on Zn-Zn hollow site 2 CN on Zn top of different ring 2 CN on Zn top of same ring 4 CN on Zn top site
Adsorption energy per molecule/(kJ/mol） -182.78 -188.88 -176.89 -196.81
As shown in Table 2, the adsorption energy of per CN molecule varies from -188.88 kJ/mol to -176.89kJ/mol when the coverage of adsorbent is 1/5 ML. In comparison with the CN adsorption energy at 1/10 ML, it is indicated that the coverage of adsorbed CN would affect the interaction between CN molecules and sphalerite (110) surface. When two CN molecules are adsorbed on the Zn-Zn hollow sites (Fig. 3A) and Zn top sites of same hexatomic ring (Fig. 3C), the adsorption energy of per CN molecule is higher than that when the coverage of CN is 1/10 ML, suggesting that the adsorption structure becomes more unstable with the increase of adsorbed CN molecule coverage, which may be caused by the mutual exclusion of two CN
molecules in a close distance. However, when the two CN molecules are adsorbed on the Zn top site of different hexatomic ring (Fig. 3B), the adsorption energy of per CN molecule is -188.88kJ/mol, lower than that (186.75kJ/mol) when a single CN molecule is adsorbs on the Zn top site, which indicates that the CN molecules are mainly adsorbed on the Zn top site of different hexatomic ring when the coverage of CN molecules increase to 1/5 ML. In addition, the distance between C and Zn is shorter when the CN molecules are adsorbed on the Zn top site, implying the Zn top sites are the most stable adsorption site on the sphalerite (110) surface. The CN adsorption at the coverage of 2/5 ML (four CN molecules) was calculated to investigate the reaction of cyanide with sphalerite under the condition of high cyanide concentration. The adsorption configuration is shown in Fig. 3D, and the adsorption energy is given in Table 2. It can be observed from Table 2 that the adsorption energy of per CN molecule is -196.81 kJ/mol when the coverage of CN molecules is 2/5 ML, which is obviously lower than that when the coverage of CN is 1/10 or 1/5 ML, indicating that with the increase of CN molecule coverage the adsorption models become more stable and the depression effect of cyanide goes stronger. As shown in Fig. 3D, the distance between C atom of CN and Zn atom on surface is about 0.1596 nanometers, shorter than the sum of covalent radius of C and Zn atom, suggesting that the intensive interaction takes place between the CN molecules and sphalerite. From above discussion, it can be known that the zinc cyanide complexes were generated on the sphalerite (110) surface after CN molecules adsorption, which could enhance the hydrophilicity of sphalerite surface in flotation. 3.3 Electronic properties To gain more insight into how CN molecules interact with spahlerite (110) surface, the electronic properties of adsorption model were studied. The charge density and charge density difference maps of CN adsorption on Zn top site are shown in Fig. 4. From Fig. 4A, it can be seen that there is an overlap of electron clouds between C and Zn atom, indicating that a C-Zn covalent bond was formed after the CN adsorption. In Fig. 4B, the red area represents obtaining electrons while the blue area expresses losing electrons. Therefore, some electrons transfer from the sphalerite surface to the CN molecule during the interaction. The DOS (density of states) of CN molecule and sphalerite (110) surface before and after adsorption are shown in Fig. 5 and Fig. 6 respectively, and the numbers in the brackets represent the number of adsorbed CN molecules. From the molecular orbital
theory, it is known that the electronic configuration of CN molecule is 2 2 4 1 0 3 4 1 5 2 . As shown in Fig. 5, the DOS of free CN molecule
consists of four parts, of which the electronic states near the Fermi level are the full 1π orbital and semi-full 5σ orbital. After adsorption, The DOS of CN molecule changes greatly, especially near the Fermi level. Almost the same changes of CN molecule DOS take place when the adsorbed molecule coverage is 1/10 ML and 1/5 ML. The DOS at Fermi level and -2.9eV are distributed from -5eV to 0eV with a high non-locality, and the non-occupied states above Fermi level decrease, which indicates that CN molecule obtains electrons. Furthermore, a hybrid peak is formed at -6.0eV, which is attributed to the interaction of C 2s, C 2p with Zn 3d orbitals. When the coverage of CN molecules is 2/5 ML, the state peaks of 2p orbitals of N and C atom decrease obviously and the non-locality become stronger, which show that the 1π orbital of CN molecule interact with atoms of sphalerite (110) surface as well. As shown in Fig. 6, the DOS of sphalerite (110) surface changes apparently after CN molecule adsorption. At the CN molecule coverage of 1/10 ML, the states peak of Zn 3d orbital reduces and a new hybrid peak appears around -5.1eV, which probably caused by the interaction between 3d orbital of Zn with 2s and 2p orbital of C atom. In addition, the S 3p states decrease and move to the conduction band, suggesting that the S 3p orbital loses electrons. With the coverage of CN molecules increasing to 1/5 ML, the hybrid peak of Zn 3d orbital around -5.1eV rises, indicating the interaction between Zn atom and CN molecule becomes more intensive. When the CN molecule coverage is 2/5 ML, it can be seen that the states peak of Zn 3d orbital decreases remarkably and the whole states of S atom move to the conduction band, indicating that the sphalerite (110) surface loses more electrons with the coverage of CN molecules increases. Notice that the S atoms on sphalerite (110) surface lose electrons during the CN adsorption. However, the distance between C atom of CN molecule and S atom of surface is longer than 0.3 nanometers, and there is no chemical bond formed between C and S. Therefore, the electrons of S atom do not transfer from S atom to CN molecule directly, but through the Zn atom that interacts with CN molecule. The Table 3, 4 and 5 show the Mulliken population analysis of CN molecule and Zn, S atom on sphalerite (110) surface before and after CN adsorption at the CN of 1/10, 1/5 and 2/5 ML respectively. As shown in Table 3 and Table 4, the changes of
Mulliken population of each atom after adsorption are similar when the coverage of CN molecule is 1/10 ML and 1/5 ML. The populations of C 2p orbital increase remarkably, and the charge of C atom shifts from positive to negative. No obvious change takes place to the populations of N atom, with the 2s orbital gains a small amount of electrons merely. The populations of Zn 4s and 3d orbitals decrease, but the Zn 4p orbital obtains some more electrons, resulting in the slight reduction of Zn charge. As for the S atom on sphalerite (110) surface, the populations of 3p orbital drops, causing the decline of negative charge of S atom. When the CN is 2/5 ML in Table 5, the populations of C 2s orbital decrease from 1.34e to 1.32e and that of 2p orbital increase from 3.71e to 4.14e. The populations of Zn 4s and 3d as well as S 3p decrease. It is known that the electronic configuration of CN molecule is 2 2 4 1 0 3 4 1 5 2 , and there are a hemi-full 5σ orbital and an empty 2π
anti-bonding orbital in the molecule. Therefore, the Zn 3d orbitals donate electrons to the C 2p orbital, forming a d-p back-donation bond. Meanwhile, the electrons of S 2p orbital transfer to Zn atom, weakening the reducibility of S atom on the sphalerite (110) surface. When the coverage of adsorbed CN molecules increases, sphalerite (110) surface loses more electrons, reducing the reactive ability of both Zn and S atoms on the surface. In addition, from Figure 2(D), Figure 3(B) and Figure 3(D), it can be found that the adsorption configuration changed remarkably with the increase of CN molecule coverage. When the coverage is 1/10 ML and 1/5 ML, the adsorption positions of cyanide molecule are analogous, in which the CN molecules are adsorbed nearly perpendicularly and the holistic distance between CN molecule and sphalerite (110) surface is far. When the coverage increases to 2/5 ML, the cyanide molecules are adsorbed nearly horizontally, and both C atom and N atom of CN molecule approach to the mineral surface. Therefore, with the coverage of CN molecule increasing, more electrons transfer from sphalerite surface to the adsorbed CN molecules. Table 3 Mulliken population of CN molecule and Zn, S atom on surface ( CN =1/10 ML). atoms C N Zn
status before after before after before
s 1.34 1.28 1.69 1.73 0.89
p 2.37 2.91 3.60 3.60 0.78
d 0.00 0.00 0.00 0.00 9.98
Total 3.71 4.19 5.29 5.33 11.65
charge 0.29 -0.19 -0.29 -0.33 0.35
S
after before after
0.86 1.82 1.86
0.87 4.67 4.58
9.96 0.00 0.00
11.69 6.49 6.44
0.31 -0.49 -0.44
Table 4 Mulliken population of CN molecule and Zn, S atom on surface ( CN =1/5 ML). atoms C N Zn S
status before after before after before after before after
s 1.34 1.28 1.69 1.73 0.89 0.85 1.82 1.86
p 2.37 2.91 3.60 3.57 0.78 0.87 4.67 4.56
d 0.00 0.00 0.00 0.00 9.98 9.96 0.00 0.00
Total 3.71 4.19 5.29 5.30 11.65 11.67 6.49 6.41
charge 0.29 -0.19 -0.29 -0.30 0.35 0.33 -0.49 -0.41
Table 5 Mulliken population of CN molecule and Zn, S atom on surface ( CN =2/5 ML). atoms C N Zn S
status before after before after before after before after
s 1.34 1.32 1.69 1.71 0.89 0.76 1.82 1.86
p 2.37 2.82 3.60 3.61 0.78 0.85 4.67 4.45
d 0.00 0.00 0.00 0.00 9.98 9.95 0.00 0.00
Total 3.71 4.14 5.29 5.32 11.65 11.56 6.49 6.31
charge 0.29 -0.14 -0.29 -0.32 0.35 0.44 -0.49 -0.31
The mulliken charges of the first layer atoms on the sphalerite (110) surface before and after CN adsorption are shown in Fig. 7, and the tagged Zn atoms are the atoms that interact with CN molecules. From Fig. 7A, it can be seen that the mulliken charges of Zn and S atom on the perfect sphalerite (110) surface are 0.35e and -0.49e respectively. When the adsorbed CN molecule coverage is 1/10 ML (Fig. 7B), the charges of Zn atom bonding with CN change from 0.35e to 0.31e, indicating that the Zn atom obtains a few electrons, while the S atoms around it lose electrons, of which the negative charges decrease. Analogous to the changes of mulliken charges of surface atoms at the CN of 1/10 ML, the positive charges of Zn atom decline while the negative charges of S atoms increase when the adsorbed CN coverage is 1/5 ML (Fig. 7C). With the coverage of adsorbed CN molecules rising to 2/5 ML (Fig. 7D),
the electronic charges of Zn atoms bonding with CN change from 0.35e to 0.43e, and that of S atoms near the reacted Zn atoms change from -0.49e to -0.34e, suggesting that both the Zn and S atoms on the sphalerie (110) surface lose more electrons. From the mulliken charges of Zn and S atoms on the sphalerie (110) surface before and after CN adsorption, it can be found that the adsorbed CN molecule not only affect the electronic properties of surface Zn atom, but also cause the electrons transfer takes place from S atoms to the Zn atoms that have lost electrons. In addition, with the coverage of adsorbed CN molecules increasing, the sphalerite (110) surface loses more electrons, resulting in the decline of surface reducibility.
4. Conclusions In this work, the adsorption of CN molecule on sphalerite (110) surface at different coverage were calculated adopting density functional theory, and the effects of adsorbed CN molecules on the electronic properties of mineral surface were discussed. The simulated results show that CN molecule can be adsorbed onto sphalerite (110) surface easily, and the most stable site for CN adsorption is the top of surface Zn atoms, where a chemical bond was formed between C atom of CN molecule and surface Zn atom. With the adsorbed CN molecule coverage increasing, the adsorption energy of per CN molecule decreases, and the adsorption configuration becomes more stable. The adsorption mechanism of CN molecule on the top of surface Zn atom is that Zn 3d orbital donates electrons to the C 2p orbital, forming a back-donation bond, and at the same time the electrons of S atoms transfer to the Zn atoms that bond with CN molecules, abating the reducibility of surface S atoms. When the coverage of adsorbed CN molecules is high, both Zn and S atoms on the mineral surface lose electrons, which weakens the reactive activity of sphalerite surface in the flotation process.
Acknowledgements The authors gratefully acknowledge the financial support of this work from the Project of National Natural Science Foundation of China (No. 51474114). Notes Declarations of interest: none.
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Figure captions:
Fig. 1 Bulk sphalerite (A), and its (110) surface model (B)
Fig. 2 CN adsorption on different sites of sphalerite (110) surface at CN of 1/10 ML: (A) Zn-Zn hollow site; (B) top S-C; (C) top Zn-N; (D) top Zn-C; (E) bridge site
Fig. 3 CN adsorption on different sites of sphalerite (110) surface at CN of 1/5: (A) hollow site; (B) on different rings; (C) on same ring; and at CN of 2/5 ML: (D) 4 CN Zn top site
Fig. 4 Charge density (A), and charge density difference (B) maps of CN adsoroption on Zn top site
Fig. 5 DOS of CN before and after adsorption at different coverage
Fig. 6 DOS of sphalerite (110) surface before and after CN adsorption at different coverage
Fig. 7 Mulliken charges of sphalerite (110) surface before and after CN adsorption: (A) perfect surface; (B) CN =1/10 ML; (C) CN =1/5 ML; (D) CN =2/5 ML
Research Highlights of “Density functional theory study of cyanide adsorption on the sphalerite (110) surface” • The adsorption configuration becomes more stable when the coverage of adsorbed CN on sphalerite (110) surface increases. • Zinc cyanide complexes formed through the d-p donating bond after the CN adsorption on sphalerite (110) surface. • Both zinc and sulfide atoms lose electrons when the coverage of adsorbed CN on sphalerite (110) surface is high.
S0169-4332(18)32431-0 https://doi.org/10.1016/j.apsusc.2018.09.020 APSUSC 40324
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
9 May 2018 29 August 2018 3 September 2018
Please cite this article as: T. Qiu, Q. Nie, Y. He, Q. Yuan, Density functional theory study of cyanide adsorption on the sphalerite (110) surface, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.09.020
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Density functional theory study of cyanide adsorption on the sphalerite (110) surface Tingsheng Qiua*, Qingmin Nieb, Yuanqing Hec, Qinzhi Yuana a
Faculty of Resource and Environmental Engineering, Jiangxi University of Science and
Technology, Ganzhou, Jiangxi, 341000, China b
School of Chemical Engineering and Technology, China University of Mining and Technology,
Xuzhou, Jiangsu, 221116, China c
School of Materials Science and Engineering, Central South University, Changsha, Hunan,
410083, China
Abstract: Cyanidation tailings in gold plants often contain lots of sphalerites that are difficult to recycle by flotation because of the strongly depressive effect of the residual cyanide. To reveal the depression mechanism of cyanide on sphalerite, the adsorption of isolated cyanide molecule (CN) at different coverages on the sphalerite (110) surface were studied by means of density functional theory. The calculated results show that the adsorption energy of per CN molecule drops as the increase of adsorbed CN molecule coverage, indicating that the adsorption structure becomes more thermodynamically stable. Cyanide molecule adsorption on sphalrite (110) surface prior occurs on the atop site of surface Zn atom, in which the Zn 3d orbital donates electrons to the 2p orbital of C forming a d-p back donating bond, leading to the production of hydrophilic zinc cyanide complexes that serve as strong obstacles to the flotation recovery of sphalerite in cyanidation tailings. Electron transfer from mineral surface to the adsorbed CN molecules takes place during the process of CN adsorption, and the surface atoms lose more electrons when the coverage of adsorbed CN molecules is high, weakening the reactive activity of Zn and S atoms on the sphalerite (110) surface. Key words: Sphalerite; Cyanidation tailings; Surface adsorption; Density functional theory
1. Introduction With high leaching rate and relatively low cost, cyanidation is the primary process for the extraction of precious metals in gold and silver mines nowadays, especially when the successful application of carbon-in-pulp technology is achieved [1-2].
* Corresponding author. E-mail address: [email protected] (T. Qiu)
However, this method would produce a large amount of cyanidation tailings that contain toxic substance such as CN and heavy metal ions Cu 2 , Pb 2 and Zn2 , causing a series of problems to environment as well as the survival of mankind and wildlife [3-4]. In addition, the residues of cyanidation are often relatively rich in copper, lead and zinc minerals, which have a great potential for utilization under the background that mineral resources become poorer and more complicated [5]. In many gold and silver mines, pre-flotation is adopted to enrich the gold and silver contents of feed ores for the cyanide leaching operations. As a consequence, the grade of copper, lead and zinc sulfide minerals in cyanidation tailings get a rise. Therefore, enhancing the recovery of valuable minerals from these secondary resources has a great significance for environment protection as well as resources comprehensive utilization. Sphalerite (ZnS) is a kind of typical sulfide mineral, which is the main phase of zinc in cyanidation tailings. Flotation process is the most common method for the recovery of this sulfide mineral from ores and some tailings. However, the cyanide, an effective depressant for the sphalerite flotation, generated an intense inhibition effect on the ZnS during cyanide leaching operation, seriously hindering the flotation recovery of this mineral. Accordingly, it is necessary to make clear the depression mechanism of cyanide for the subsequent activation and flotation reclaim of sphalerite. Cyanide acts as a depressant in the separation flotation of lead-zinc sulfide ores to suppress the zinc floating, and the depression mechanism have gotten a great many attentions of researchers for a long time. Wark [6] found that the content suppressing the flotation of ZnS in pulp was Zn(CN)2 that were adsorbed on the mineral surface and improved the hydrophilicity of sphalerite. Though whether the Zn(CN)2 lead to the depressive effect have not been proven, these zinc cyanide complexes were generated in the circuit indeed [7-8]. Nevertheless, the X-ray photoelectron spectroscopy (XPS) analysis results of species on the cyanide treated sphalerite indicated that cyanide did not interact with zinc or sulfur on the mineral surface [9] or zinc cyanide compositions remained in solution rather than on the sphalerite surface [10]. On the basis of stability of cyano-zinc complexes and the reducibility of cyanide, some researchers thought that the depression effect of free cyanide on sphalerite were complicated, which depends on the ore properties and flotation pulp environments [11-12].
Not only could cyanide dissolve the oxides of zinc and zinc-xanthate
compounds, it also consumes a great deal of oxygen in pulp, suppressing the oxidation of xanthate to dixanthogen [13]. Moreover, the cyanide concentration played a significant role in the depression effect. In relatively low levels, the cyanides were adsorbed onto sphalerite surface making the hydrophilicity of mineral resemble to that of insoluble Zn(CN) 2 , and with the cyanide concentration increase, the generated Zn(CN) 2 became Zn(CN) 24 gradually, exhibiting a stronger inhibition effect on sphalerite [14]. Even though lots of researches have been conducted, the mechanism by which cyanide depress sphalerite flotation is not well understood yet. In recent years, with the rapid development of quantum chemistry and computer technology, density functional theory simulation is more and more widely used to study the structure, properties, reactivity etc. of materials. An enormous amount of researches have been carried out on the mineral crystal structures and defects [15-17], surface properties [18-20], and the interaction between reagent molecules and mineral surface [21-23], which demonstrated the mineral flotation performance on the molecular or atomic level. However, the first-principle studies on the depression mechanism of cyanide on sphalerite were relatively scarce. Ye Chen et al [24-25] investigated the adsorption of CN on sphalerite (110) surface and the influence of lattice impurity on mineral surface adsorption by density functional theory, and the calculated results indicated that CN adsorbed strongly on the perfect sphalerite (110) surface with hollow site adsorption showing the highest stability, where the C and N atoms of the CN molecule bond with two Zn atoms, respectively. As for lattice defects of sphalerite, the impurity of Fe and Mn could strengthen the adsorption of CN, while Cd weaken it. Moreover, the activation of copper atom enhanced the interaction between sphalerite surface and CN molecule, which adsorbed on the mineral surface via C atom bonding with the Cu atom perpendicularly on the Cuactivated sphalerite surface. These researches revealed the depression mechanism of cyanide on sphalerite to a certain extent, but only the low coverage adsorption of cyanide was focused on, which cannot reflect the heavy suppression of cyanide on sphalerite after a prolonged and highly concentrated immersion during cyanidation of gold and silver ores. For the sake of a further understanding on depression mechanism of cyanide on sphalerite in the cyanidation tailings, the adsorption of isolated cyanide molecules at different coverage were investigated by using the density functional theory here.
Furthermore, influences of adsorbed CN molecules on the electronic properties of sphalerite (110) surface were discussed. Our works can provide theoretical references for the understanding of cyanide depression mechanism on sphalerite as well as the subsequent activation and flotation recovery of sphalerite in cyanidation tailings.
2. Computational methods All calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) developed by Payne et al [26], which is a plane-wave pseudopotential method based on the density functional theory (DFT). The interaction between ionic core and valence electrons were represented by ultra-soft pseudopotential. Based on the results of convergence tests, the generalized gradient approximation (GGA) developed by Perdew, Burke and Ernzerhof [27-28] was used to compute the exchange correlation effect of different electrons, and the kinetic energy cutoff of plane wave basis was 420 eV, which was used throughout the investigation. The brillouin zone integration was approximated by a sum over special k points that employed a 2×3×1 grid of the Monkhorst-Pack scheme for structure calculations. The valence electron configurations considered in this study were Zn 3d104s2, S 3s23p4, Cu 3d104s1, C 2s22p2, and N 2s22p3. A convergence tolerance of 2.0×10-6eV/atom was employed for the self-consistent electronic minimization, and an energy tolerance of 2.0×10-5eV/atom, a maximum force tolerance of 0.05 eV/Å, and a maximum displacement tolerance of 0.002 Å were employed for the geometry optimization. The crystal of sphalerite (β-ZnS) belongs to the isometric system, whose space group is F - 43m , and cell lattice are a b c 5.409 Å and 90 [29]. After geometry optimization with above calculation parameters, the computed lattice of sphalerite are 5.442 Å, being only 0.61% larger than the experimental value, which is close to the calculation results of A. Sarvaramini (5.47 Å) [23] and X. H. Long (5.471 Å) [21], and indicates that the simulation results coincide with experimental values. Sphalerite (110) surface is the predominant cleavage surface, of which the surface energy is relatively low [29], and it has been used as the principal cleavage surface of sphalerite by many researchers for surface simulation [21,24,30-32]. Therefore, the cyanides are mainly adsorbed onto the sphalerite (110) surface that is selected for the DFT calculations. In consideration of the influence of thickness of slab and numbers of fixed atomic layers, convergence tests were carried out, based on
the results of which the sphalerite (110) surface slab model with 10 layers were used for the simulation, where the top four layers were allowed to relax and the bottom six layers were fixed in their bulk position, which is sufficient for the study of surface properties. In addition, a vacuum layer of 15
along the c-parameter was added on
the atomic layers to eliminate the interaction between different slabs [16,21,33], and a 2 2 1 supercell was employed in the simulation. The bulk sphalerite and its (110) surface model are shown in Fig. 1. For the calculation of adsorption energy of per CN molecule, the following equation can be utilized.
1 E slabnCN E slab nE CN (1) n represents the total energy of sphalerite (110) surface
E ads / CN In the equation, the E slabnCN
model with n cyanide molecules adsorbed, the E slab is the energy of perfect surface, E CN involves the energy of a free CN molecule calculated in a cubic cell, and n is the
number of adsorbed CN molecules. The coverage of CN molecule ( CN ) is represented by the number of adsorbed CN molecules over the total number of exposed Zn and S atoms on the first layer of sphalerite (110) surface [34].
3. Results and discussion The configuration of sphalerite (110) surface is cleaved from optimized bulk sphalerite crystal, and before the calculations of CN adsorption, the relaxation of sphalerite (110) surface was conducted for the surface free energy minimization. Then, in order to simulate the intense depression effect of cyanide on sphalerite, the CN adsorption on sphalerite (110) surface at different coverage of 1/10, 1/5 and 2/5 ML were calculated using the method of density functional theory. 3.1 CN adsorption at CN of 1/10 ML There are 6 sulfur atoms and 4 zinc atoms exposed on the sphalerite (110) surface, and the CN coverage of 1/10 ML indicates that the number of adsorbed CN is one. In this studies, the top of sulfur and zinc, bridge and hollow sites were selected as the active adsorption sites on the relaxed surface. Therefore, five adsorption modes were calculated to simulate the interaction between CN and sphalerite (110) surface. The adsorption modes with bond lengths are shown in Fig. 2, and the adsorption energy for different modes are given in Table 1.
Table 1 CN adsorption energy on different sites of sphalerite (110) surface at CN of 1/10 ML Adsorption site
Adsorption energy/(kJ/mol）
Zn-Zn hollow Top S-C
-184.01 -106.70
Top Zn-N Top Zn-C
-148.54 -186.75
Zn-S bridge
-111.48
As shown in Table 1, the adsorption energy of CN on different sites of sphalerite (110) surface are all negative, which indicates that the interaction between CN and sphalerite is exothermic and cyanide can be adsorbed onto the mineral surface easily. Among the adsorption sites, the adsorption energy is relatively low when CN is adsorbed on the top Zn-C site (186.75 kJ/mol) and Zn-Zn hollow site (184.01 kJ/mol), indicating that the CN adsorption on perfect sphalerite (110) surface mainly occurs on the top Zn-C site and Zn-Zn hollow site when the coverage of CN is 1/10 ML. In the reference [24], the adsorption energy of CN molecule on the sphalerite (110) surface is lowest when the molecule was adsorbed on the Zn-Zn hollow site, which is different from the calculation results in this paper. And from the Table 1, it can be known that both C atom and N atom of CN molecule can react with the surface atoms. However, when the cyanide was adsorbed on the Zn-Zn hollow site, the calculation results showed that the anti-bonds could be formed between the C atom, N atom of CN molecule and the S atom facing the CN molecule on the mineral surface, which can strengthen the repulsion between cyanide and mineral surface atoms and weaken the stability of the adsorption configuration. Therefore, the adsorption energy of CN on Zn-Zn hollow site was a little high than that when the CN was adsorbed on the ZnC site. In top Zn-C site (Fig. 2D), the CN molecule locate at the top of zinc atom on the sphalerite (110) surface via the interaction between C atom of CN and surface Zn atom, with a distance of 0.1940 nanometers, which is shorter than the sum of covalent radius of C and Zn atom (0.2100nm), suggesting that a chemical bond formed between C atom of CN and Zn atom of spahlerite (110) surface after the interaction.
In Zn-Zn hollow site (Fig. 2A), the CN is adsorbed parallel onto the surface, with the C and N atom reacting with two zinc atoms of sphalerite (110) surface respectively. The distance between C and Zn1 is 0.2025 nanometers, and the distance between N and Zn2 is 0.2122 nanometers, which are all in the scope of chemical effect. Moreover, the distance between C and Zn1 is shorter than that between N and Zn2, indicating that the interaction between C and Zn1 are stronger. 3.2 CN adsorption at CN of 1/5 and 2/5 ML The results of CN adsorption at CN of 1/10 ML indicates that the top Zn-C and ZnZn hollow are the preferred sites for the adsorption of CN molecule on sphalerite (110) surface. Thereby, the CN adsorption at CN of 1/5 ML (two CN molecules) was conducted on these active sites. Because of the hexatomic ring structure (Fig. 1B) constituted by the sulfur and zinc atoms from first layer and second layer of sphalerite (110) surface slab model, there are two configurations for the Zn top site CN adsorption at the CN of 1/5 ML, including two CN molecules adsorbed on different hexatomic ring and on same hexatomic ring respectively. Thus, three adsorption modes were studied when the coverage of CN was 1/5 ML, seen in the Fig. 3(A-C), and the adsorption energy for different modes are presented in Table 2. Table 2 CN adsorption energy on different sites of sphalerite (110) surface at CN of 1/5 and 2/5 ML Adsorption sites 2 CN on Zn-Zn hollow site 2 CN on Zn top of different ring 2 CN on Zn top of same ring 4 CN on Zn top site
Adsorption energy per molecule/(kJ/mol） -182.78 -188.88 -176.89 -196.81
As shown in Table 2, the adsorption energy of per CN molecule varies from -188.88 kJ/mol to -176.89kJ/mol when the coverage of adsorbent is 1/5 ML. In comparison with the CN adsorption energy at 1/10 ML, it is indicated that the coverage of adsorbed CN would affect the interaction between CN molecules and sphalerite (110) surface. When two CN molecules are adsorbed on the Zn-Zn hollow sites (Fig. 3A) and Zn top sites of same hexatomic ring (Fig. 3C), the adsorption energy of per CN molecule is higher than that when the coverage of CN is 1/10 ML, suggesting that the adsorption structure becomes more unstable with the increase of adsorbed CN molecule coverage, which may be caused by the mutual exclusion of two CN
molecules in a close distance. However, when the two CN molecules are adsorbed on the Zn top site of different hexatomic ring (Fig. 3B), the adsorption energy of per CN molecule is -188.88kJ/mol, lower than that (186.75kJ/mol) when a single CN molecule is adsorbs on the Zn top site, which indicates that the CN molecules are mainly adsorbed on the Zn top site of different hexatomic ring when the coverage of CN molecules increase to 1/5 ML. In addition, the distance between C and Zn is shorter when the CN molecules are adsorbed on the Zn top site, implying the Zn top sites are the most stable adsorption site on the sphalerite (110) surface. The CN adsorption at the coverage of 2/5 ML (four CN molecules) was calculated to investigate the reaction of cyanide with sphalerite under the condition of high cyanide concentration. The adsorption configuration is shown in Fig. 3D, and the adsorption energy is given in Table 2. It can be observed from Table 2 that the adsorption energy of per CN molecule is -196.81 kJ/mol when the coverage of CN molecules is 2/5 ML, which is obviously lower than that when the coverage of CN is 1/10 or 1/5 ML, indicating that with the increase of CN molecule coverage the adsorption models become more stable and the depression effect of cyanide goes stronger. As shown in Fig. 3D, the distance between C atom of CN and Zn atom on surface is about 0.1596 nanometers, shorter than the sum of covalent radius of C and Zn atom, suggesting that the intensive interaction takes place between the CN molecules and sphalerite. From above discussion, it can be known that the zinc cyanide complexes were generated on the sphalerite (110) surface after CN molecules adsorption, which could enhance the hydrophilicity of sphalerite surface in flotation. 3.3 Electronic properties To gain more insight into how CN molecules interact with spahlerite (110) surface, the electronic properties of adsorption model were studied. The charge density and charge density difference maps of CN adsorption on Zn top site are shown in Fig. 4. From Fig. 4A, it can be seen that there is an overlap of electron clouds between C and Zn atom, indicating that a C-Zn covalent bond was formed after the CN adsorption. In Fig. 4B, the red area represents obtaining electrons while the blue area expresses losing electrons. Therefore, some electrons transfer from the sphalerite surface to the CN molecule during the interaction. The DOS (density of states) of CN molecule and sphalerite (110) surface before and after adsorption are shown in Fig. 5 and Fig. 6 respectively, and the numbers in the brackets represent the number of adsorbed CN molecules. From the molecular orbital
theory, it is known that the electronic configuration of CN molecule is 2 2 4 1 0 3 4 1 5 2 . As shown in Fig. 5, the DOS of free CN molecule
consists of four parts, of which the electronic states near the Fermi level are the full 1π orbital and semi-full 5σ orbital. After adsorption, The DOS of CN molecule changes greatly, especially near the Fermi level. Almost the same changes of CN molecule DOS take place when the adsorbed molecule coverage is 1/10 ML and 1/5 ML. The DOS at Fermi level and -2.9eV are distributed from -5eV to 0eV with a high non-locality, and the non-occupied states above Fermi level decrease, which indicates that CN molecule obtains electrons. Furthermore, a hybrid peak is formed at -6.0eV, which is attributed to the interaction of C 2s, C 2p with Zn 3d orbitals. When the coverage of CN molecules is 2/5 ML, the state peaks of 2p orbitals of N and C atom decrease obviously and the non-locality become stronger, which show that the 1π orbital of CN molecule interact with atoms of sphalerite (110) surface as well. As shown in Fig. 6, the DOS of sphalerite (110) surface changes apparently after CN molecule adsorption. At the CN molecule coverage of 1/10 ML, the states peak of Zn 3d orbital reduces and a new hybrid peak appears around -5.1eV, which probably caused by the interaction between 3d orbital of Zn with 2s and 2p orbital of C atom. In addition, the S 3p states decrease and move to the conduction band, suggesting that the S 3p orbital loses electrons. With the coverage of CN molecules increasing to 1/5 ML, the hybrid peak of Zn 3d orbital around -5.1eV rises, indicating the interaction between Zn atom and CN molecule becomes more intensive. When the CN molecule coverage is 2/5 ML, it can be seen that the states peak of Zn 3d orbital decreases remarkably and the whole states of S atom move to the conduction band, indicating that the sphalerite (110) surface loses more electrons with the coverage of CN molecules increases. Notice that the S atoms on sphalerite (110) surface lose electrons during the CN adsorption. However, the distance between C atom of CN molecule and S atom of surface is longer than 0.3 nanometers, and there is no chemical bond formed between C and S. Therefore, the electrons of S atom do not transfer from S atom to CN molecule directly, but through the Zn atom that interacts with CN molecule. The Table 3, 4 and 5 show the Mulliken population analysis of CN molecule and Zn, S atom on sphalerite (110) surface before and after CN adsorption at the CN of 1/10, 1/5 and 2/5 ML respectively. As shown in Table 3 and Table 4, the changes of
Mulliken population of each atom after adsorption are similar when the coverage of CN molecule is 1/10 ML and 1/5 ML. The populations of C 2p orbital increase remarkably, and the charge of C atom shifts from positive to negative. No obvious change takes place to the populations of N atom, with the 2s orbital gains a small amount of electrons merely. The populations of Zn 4s and 3d orbitals decrease, but the Zn 4p orbital obtains some more electrons, resulting in the slight reduction of Zn charge. As for the S atom on sphalerite (110) surface, the populations of 3p orbital drops, causing the decline of negative charge of S atom. When the CN is 2/5 ML in Table 5, the populations of C 2s orbital decrease from 1.34e to 1.32e and that of 2p orbital increase from 3.71e to 4.14e. The populations of Zn 4s and 3d as well as S 3p decrease. It is known that the electronic configuration of CN molecule is 2 2 4 1 0 3 4 1 5 2 , and there are a hemi-full 5σ orbital and an empty 2π
anti-bonding orbital in the molecule. Therefore, the Zn 3d orbitals donate electrons to the C 2p orbital, forming a d-p back-donation bond. Meanwhile, the electrons of S 2p orbital transfer to Zn atom, weakening the reducibility of S atom on the sphalerite (110) surface. When the coverage of adsorbed CN molecules increases, sphalerite (110) surface loses more electrons, reducing the reactive ability of both Zn and S atoms on the surface. In addition, from Figure 2(D), Figure 3(B) and Figure 3(D), it can be found that the adsorption configuration changed remarkably with the increase of CN molecule coverage. When the coverage is 1/10 ML and 1/5 ML, the adsorption positions of cyanide molecule are analogous, in which the CN molecules are adsorbed nearly perpendicularly and the holistic distance between CN molecule and sphalerite (110) surface is far. When the coverage increases to 2/5 ML, the cyanide molecules are adsorbed nearly horizontally, and both C atom and N atom of CN molecule approach to the mineral surface. Therefore, with the coverage of CN molecule increasing, more electrons transfer from sphalerite surface to the adsorbed CN molecules. Table 3 Mulliken population of CN molecule and Zn, S atom on surface ( CN =1/10 ML). atoms C N Zn
status before after before after before
s 1.34 1.28 1.69 1.73 0.89
p 2.37 2.91 3.60 3.60 0.78
d 0.00 0.00 0.00 0.00 9.98
Total 3.71 4.19 5.29 5.33 11.65
charge 0.29 -0.19 -0.29 -0.33 0.35
S
after before after
0.86 1.82 1.86
0.87 4.67 4.58
9.96 0.00 0.00
11.69 6.49 6.44
0.31 -0.49 -0.44
Table 4 Mulliken population of CN molecule and Zn, S atom on surface ( CN =1/5 ML). atoms C N Zn S
status before after before after before after before after
s 1.34 1.28 1.69 1.73 0.89 0.85 1.82 1.86
p 2.37 2.91 3.60 3.57 0.78 0.87 4.67 4.56
d 0.00 0.00 0.00 0.00 9.98 9.96 0.00 0.00
Total 3.71 4.19 5.29 5.30 11.65 11.67 6.49 6.41
charge 0.29 -0.19 -0.29 -0.30 0.35 0.33 -0.49 -0.41
Table 5 Mulliken population of CN molecule and Zn, S atom on surface ( CN =2/5 ML). atoms C N Zn S
status before after before after before after before after
s 1.34 1.32 1.69 1.71 0.89 0.76 1.82 1.86
p 2.37 2.82 3.60 3.61 0.78 0.85 4.67 4.45
d 0.00 0.00 0.00 0.00 9.98 9.95 0.00 0.00
Total 3.71 4.14 5.29 5.32 11.65 11.56 6.49 6.31
charge 0.29 -0.14 -0.29 -0.32 0.35 0.44 -0.49 -0.31
The mulliken charges of the first layer atoms on the sphalerite (110) surface before and after CN adsorption are shown in Fig. 7, and the tagged Zn atoms are the atoms that interact with CN molecules. From Fig. 7A, it can be seen that the mulliken charges of Zn and S atom on the perfect sphalerite (110) surface are 0.35e and -0.49e respectively. When the adsorbed CN molecule coverage is 1/10 ML (Fig. 7B), the charges of Zn atom bonding with CN change from 0.35e to 0.31e, indicating that the Zn atom obtains a few electrons, while the S atoms around it lose electrons, of which the negative charges decrease. Analogous to the changes of mulliken charges of surface atoms at the CN of 1/10 ML, the positive charges of Zn atom decline while the negative charges of S atoms increase when the adsorbed CN coverage is 1/5 ML (Fig. 7C). With the coverage of adsorbed CN molecules rising to 2/5 ML (Fig. 7D),
the electronic charges of Zn atoms bonding with CN change from 0.35e to 0.43e, and that of S atoms near the reacted Zn atoms change from -0.49e to -0.34e, suggesting that both the Zn and S atoms on the sphalerie (110) surface lose more electrons. From the mulliken charges of Zn and S atoms on the sphalerie (110) surface before and after CN adsorption, it can be found that the adsorbed CN molecule not only affect the electronic properties of surface Zn atom, but also cause the electrons transfer takes place from S atoms to the Zn atoms that have lost electrons. In addition, with the coverage of adsorbed CN molecules increasing, the sphalerite (110) surface loses more electrons, resulting in the decline of surface reducibility.
4. Conclusions In this work, the adsorption of CN molecule on sphalerite (110) surface at different coverage were calculated adopting density functional theory, and the effects of adsorbed CN molecules on the electronic properties of mineral surface were discussed. The simulated results show that CN molecule can be adsorbed onto sphalerite (110) surface easily, and the most stable site for CN adsorption is the top of surface Zn atoms, where a chemical bond was formed between C atom of CN molecule and surface Zn atom. With the adsorbed CN molecule coverage increasing, the adsorption energy of per CN molecule decreases, and the adsorption configuration becomes more stable. The adsorption mechanism of CN molecule on the top of surface Zn atom is that Zn 3d orbital donates electrons to the C 2p orbital, forming a back-donation bond, and at the same time the electrons of S atoms transfer to the Zn atoms that bond with CN molecules, abating the reducibility of surface S atoms. When the coverage of adsorbed CN molecules is high, both Zn and S atoms on the mineral surface lose electrons, which weakens the reactive activity of sphalerite surface in the flotation process.
Acknowledgements The authors gratefully acknowledge the financial support of this work from the Project of National Natural Science Foundation of China (No. 51474114). Notes Declarations of interest: none.
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Figure captions:
Fig. 1 Bulk sphalerite (A), and its (110) surface model (B)
Fig. 2 CN adsorption on different sites of sphalerite (110) surface at CN of 1/10 ML: (A) Zn-Zn hollow site; (B) top S-C; (C) top Zn-N; (D) top Zn-C; (E) bridge site
Fig. 3 CN adsorption on different sites of sphalerite (110) surface at CN of 1/5: (A) hollow site; (B) on different rings; (C) on same ring; and at CN of 2/5 ML: (D) 4 CN Zn top site
Fig. 4 Charge density (A), and charge density difference (B) maps of CN adsoroption on Zn top site
Fig. 5 DOS of CN before and after adsorption at different coverage
Fig. 6 DOS of sphalerite (110) surface before and after CN adsorption at different coverage
Fig. 7 Mulliken charges of sphalerite (110) surface before and after CN adsorption: (A) perfect surface; (B) CN =1/10 ML; (C) CN =1/5 ML; (D) CN =2/5 ML
Research Highlights of “Density functional theory study of cyanide adsorption on the sphalerite (110) surface” • The adsorption configuration becomes more stable when the coverage of adsorbed CN on sphalerite (110) surface increases. • Zinc cyanide complexes formed through the d-p donating bond after the CN adsorption on sphalerite (110) surface. • Both zinc and sulfide atoms lose electrons when the coverage of adsorbed CN on sphalerite (110) surface is high.